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Sample records for abundant mantle xenoliths

  1. Superplume Metasomatism: Evidence from Siberian mantle xenoliths

    NASA Astrophysics Data System (ADS)

    Taylor, L. A.; Howarth, G. H.; Barry, P. H.; Pernet-Fisher, J. F.; Baziotis, I. P.; Pokhilenko, L. N.; Bodnar, R. J.; Pokhilenko, N. P.

    2013-12-01

    The Siberian craton has been subjected to numerous stages of Superplume-related magmatism, including several pre- and post-temporal stages of kimberlite emplacement relative to the extrusion of the Siberian Flood basalts (SFB; 250 Ma). The primary objective of this study is to characterize the metasomatic imprints rendered on the sub-continental lithospheric mantle (SCLM) by percolating Superplume related fluids. Mantle xenoliths brought to the surface by kimberlites provide rare windows into the SCLM. Here, we present major- and trace-element mineral data for peridotite xenoliths of the Late Devonian Udachnaya (360 Ma) and Jurassic Obnazhennaya (180 Ma) kimberlites. These xenoliths were selected in order to better characterize the temporal evolution of metasomatic processes affecting the SCLM over the life cycle of the Siberian Superplume; they represent sections of SCLM that bracket the SFB climax of activity. This work presents an initial model as part of a larger study focusing on the chemical effects of Superplume related metasomatism on the Siberian SCLM, which also include; Re/Os systematics [1] and noble gas geochemistry [2]. Garnet compositions have two distinct trends in CaO-Cr2O3 space: 1) increasing CaO at constant Cr2O3 within the harzburgite field, and 2) decreasing CaO and Cr2O3 within the lherzolite field, moving from ultramafic compositions of Udachanaya toward mafic compositions of Obnazhennaya. Distinct-zoned garnet grains have sinusoidal-REE patterns within cores and display a gradational change to flat MREE-HREE profiles at the rims. Clinopyroxenes typically are LREE-enriched and have high Ti/Sr. Re-constructed melts in equilibrium with garnet REE chemistry indicate that Obnazhennaya garnets were over-printed by plume-derived basaltic fluids, whereas Udachnaya garnets were over-printed mainly by kimberlite fluids. The ubiquitous plume signatures in the younger Obnazhennaya garnets are clear evidence for extensive metasomatism by mafic fluids

  2. Mantle Samples Included in Volcanic Rocks: Xenoliths and Diamonds

    NASA Astrophysics Data System (ADS)

    Pearson, D. G.; Canil, D.; Shirey, S. B.

    2003-12-01

    , 1989).Even within either "group" of volcanic rocks the variety of possible xenolith types is great. Table 1 presents a summary of the most common mantle xenolith groups that are found in kimberlitic hosts and within the alkalic and potassic mafic magmas. The significance and abundance of these groups will be discussed below. Table 1. Major groups of mantle xenoliths in kimberlite-related and alkali basalt series volcanic rocks (after Harte and Hawkesworth, 1989). Textural classification follows that of Harte (1977). Terminology for phlogopite-rich mafic mantle xenoliths from Gregoire et al. (2002). For supplementary data and classifications see Nixon (1987), table 62 TypeCharacteristicsExamplesMg# olivine (A) Cratonic/circum-cratonic xenoliths erupted by Kimberlite-related volcanics AI: Coarse Mg-rich, low-T peridotitesOften abundant. Mostly harzburgites and lherzolites with varying but low modal diopside and garnet. Wide range of orthopyroxene abundance, Kaapvaal examples notably enriched. Crystals typically 0.2 mm with equant or tabular shapes, irregular grain boundaries, rarely granoblastic (Harte, 1977). Bulk compositions typically highly depleted in Fe, Ca, and Al, enriched in Mg. Mineralogy: Cr-rich pyrope, Cr-diopside. Orthopyroxene in garnet facies characterized by >1.0 wt.% Al2O3. Cr-spinel sometimes evident. Minor phlogopite common grading into type VIII phlogopite peridotites. Phlogopite often surrounds garnet and is strongly correlated with the presence of diopside. Estimated equilibration temperatures less than 1,100 °C. Equilibration pressures can vary widely within a pipe and range from c. 2 GPa to >6 GPa. Rarely diamondiferous (e.g., Dawson and Smith, 1975), more commonly contain graphite ( Pearson et al., 1994).N. Lesotho (Nixon and Boyd, 1973a), Kaapvaal craton ( Gurney and Harte, 1980; Boyd and Nixon, 1978; Boyd and Mertzman, 1987; Nixon, 1987), Siberia ( Sobolev, 1974; Boyd et al., 1993); Jericho Slave craton ( Kopylova et al., 1999)Av 92.8 (91

  3. Petrology of lower crustal and upper mantle xenoliths from the Cima Volcanic Field, California

    USGS Publications Warehouse

    Wilshire, H.G.; McGuire, A.V.; Noller, J.S.; Turrin, B.D.

    1991-01-01

    Basaltic rocks of the Cima Volcanic Field in the southern Basin and Range province contain abundant gabbro, pyroxenite, and peridotite xenoliths. Composite xenoliths containing two or more rock types show that upper-mantle spinel peridotite was enriched by multiple dike intrusions in at least three episodes; the mantle was further enriched by intergranular and shear-zone melt infiltration in at least two episodes. Because of their high densities, the gabbros and pyroxenites can occupy the zone immediately above the present Moho (modeled on seismic data as 10-13 km thick, with Vp 6.8 km/s) only if their seismic velocities are reduced by the joints, partial melts, and fluid inclusions that occur in them. Alternatively, these xenoliths may have been derived entirely from beneath the Moho, in which case the Moho is not the local crust-mantle boundary. -from Authors

  4. Ultradeep (greater than 300 kilometers), ultramafic upper mantle xenoliths.

    PubMed

    Haggerty, S E; Sautter, V

    1990-05-25

    Geophysical discontinuities in Earth's upper mantle and experimental data predict the structural transformation of pyroxene to garnet and the solid-state dissolution of pyroxene into garnet with increasing depth. These predictions are indirectly verified by omphacitic pyroxene exsolution in pyropic garnet-bearing xenoliths from a diamondiferous kimberlite. Conditions for silicon in octahedral sites in the original garnets are met at pressures greater than 130 kilobars, placing the origin of these xenoliths at depths of 300 to 400 kilometers. These ultradeep xenoliths support the theory that the 400-km seismic discontinuity is marked by a transition from peridotite to eclogite. PMID:17745405

  5. Ultradeep (greater than 300 kilometers), ultramafic upper mantle xenoliths.

    PubMed

    Haggerty, S E; Sautter, V

    1990-05-25

    Geophysical discontinuities in Earth's upper mantle and experimental data predict the structural transformation of pyroxene to garnet and the solid-state dissolution of pyroxene into garnet with increasing depth. These predictions are indirectly verified by omphacitic pyroxene exsolution in pyropic garnet-bearing xenoliths from a diamondiferous kimberlite. Conditions for silicon in octahedral sites in the original garnets are met at pressures greater than 130 kilobars, placing the origin of these xenoliths at depths of 300 to 400 kilometers. These ultradeep xenoliths support the theory that the 400-km seismic discontinuity is marked by a transition from peridotite to eclogite.

  6. The Fidelity of Xenoliths in Recording Mantle Water Concentrations

    NASA Astrophysics Data System (ADS)

    Plank, T. A.; Ferriss, E.; Lloyd, A. S.; Hauri, E.

    2014-12-01

    Mantle xenoliths bear the nominally anhydrous minerals olivine and pyroxene which may contain significant quantities of water and potentially provide information on the water contents (and therefore solidus and strength) of the mantle. Very high diffusivities of water (as H) in olivine and pyroxenes, however, will promote rapid equilibration of water between xenoliths and their host magma. If magma ascends from the mantle (> 40 km) on the order of 10 m/s (an upper bound), minimum timescales available for interaction between melt and xenoliths are on the order of hours. Assuming the fastest diffusivities measured in olivine and clinopyroxene [1,2], water could exchange on the mm to cm lengthscales that are relevant to xenoliths during ascent. In this case, xenoliths would no longer convey reliable information about the water concentration of the upper mantle. However, multiple diffusive mechanisms may operate in olivine [3] and clinopyroxene [4]; some of these mechanisms are orders of magnitude slower than others, and may prevent widespread diffusive exchange [3]. To explore the potential for water exchange between mantle xenoliths and their host magma, we report here data on xenolith-host magma pairs from two cinder cones in the Western US: MO1016 from the Mojave Desert, CA and GCB from the north rim of the Grand Canyon, AZ). The host magma at MO1016 is relatively dry, while the magma at GCB is relatively wet, with 1.3 vs. 3.2 wt% H2O as measured by SIMS in olivine-hosted melt inclusions. The water concentration of mantle xenolith clinopyroxenes (cpx) in MO1016 also contain lower H2O (< 225 ppm, by SIMS) than those in GCB (< 560 ppm, from [5]). If we apply an Al-dependent partition coefficient [6] to the cpx water concentrations, the predicted melt concentrations are nearly identical to those recorded in the host magma melt inclusions. This relationship is suggestive of complete diffusive exchange of xenolith cpx with the host magma. Future work will explore grain

  7. Mantle xenoliths, seismic tomography, and the tectosphere beneath southern Africa

    NASA Astrophysics Data System (ADS)

    James, D.; Boyd, F. R.; Schutt, D.; Bell, D.; Carlson, R.

    2003-04-01

    We impose geologic constraints on seismic 3-D images of the upper mantle beneath southern Africa by calculating seismic velocities and rock densities from approximately 100 geothermobarometrically calibrated mantle xenoliths from the Archean Kaapvaal craton and adjacent Proterozoic mobile belts. Velocity and density estimates are based on the elastic and thermal moduli of constituent minerals at the P-T conditions in the mantle source. Results are consistent with tomographic evidence that cratonic mantle samples are higher in velocity by 0.5-1.5% and lower in density by about 1% relative to off-craton samples at comparable depths. Seismic velocity variations in the xenoliths are controlled dominantly by temperature, with compositional effects playing a secondary role. Composition remains the dominant control for density. At depths of 100-150 km in the upper mantle, the cratonic xenoliths are consistently lower density and higher seismic velocities than non-cratonic xenoliths. Low-T (granular) cratonic xenoliths exhibit a positive velocity-depth curve, rising from about 8.1 km/s at uppermost mantle depths to about 8.25 km/s at 200 km depth. S-wave velocities decrease slightly over the same depth range from about 4.7 km/s in the uppermost mantle to 4.65 km/s at 180 km depth. Velocities for high-T (sheared) lherzolites are widely scattered, ranging from highs close to those of the low-T nodules to velocities as low as 8.05 and 4.47 at depths in excess of 200 km. These low velocities, while not asthenospheric, are inconsistent with seismic tomographic images that indicate high velocity root material extending to depths of at least 250 km. We suggest that the high-T xenoliths may therefore have originated in halos around kimberlite embayments into the tectospheric root. Seismic velocities and densities for the cratonic sample differ significantly from those predicted for both pyrolite (fertile) and eclogite mantle materials. The xenoliths and petrologic models we have

  8. Constraints on kimberlite ascent mechanisms revealed by phlogopite compositions in kimberlites and mantle xenoliths

    NASA Astrophysics Data System (ADS)

    Giuliani, Andrea; Phillips, David; Kamenetsky, Vadim S.; Goemann, Karsten

    2016-01-01

    Kimberlite magmas are of economic and scientific importance because they represent the major host to diamonds and are probably the deepest magmas from continental regions. In addition, kimberlite magmas transport abundant mantle and crustal xenoliths, thus providing fundamental information on the composition of the sub-continental lithosphere. Despite their importance, the composition and ascent mechanism(s) of kimberlite melts remain poorly constrained. Phlogopite is one of the few minerals that preserves a history of fluid migration and magmatism in the mantle and crust and is therefore an invaluable petrogenetic indicator of kimberlite magma evolution. Here we present major and trace element compositional data for phlogopite from the Bultfontein kimberlite (Kimberley, South Africa; i.e. the kimberlite type-locality) and from entrained mantle xenoliths. Phlogopite macrocrysts (~ > 0.3-0.5 mm) and microcrysts (between ~ 0.1 and 0.3 mm) in the Bultfontein kimberlite display concentric compositional zoning patterns. The cores of these phlogopite grains exhibit compositions typical of phlogopite contained in peridotite mantle xenoliths. However, the rims of some grains show compositions analogous to kimberlite groundmass phlogopite (i.e. high Ti, Al and Ba; low Cr), whereas other rims and intermediate zones (between cores and rims) exhibit unusually elevated Cr and lower Al and Ba concentrations. The latter compositions are indistinguishable from matrix phlogopite in polymict breccia xenoliths (considered to represent failed kimberlite intrusions) and from Ti-rich overgrowth rims on phlogopite in other mantle xenoliths. Consequently, it is likely that these phlogopite grains crystallized from kimberlite melts and that the high Ti-Cr zones originated from earlier kimberlite melts at mantle depths. We postulate that successive pulses of ascending kimberlite magma progressively metasomatised the conduit along which later kimberlite pulses ascended, producing

  9. Mantle metasomatism above subduction zones: Trace-element and radiogenic isotope characteristics of peridotite xenoliths from Batan Island (Philippines)

    SciTech Connect

    Vidal, Ph. ); Dupuy, C. ); Maury, R.; Richard, M. )

    1989-12-01

    Trace-element abundances and radiogenic isotope ratios have been determined for a suite of upper mantle-derived xenoliths collected from Pliocene-Quaternary andesitic lavas on Batan Island, northernmost Philippines. The xenoliths exhibit mineralogical changes and large ion lithophile enrichment indicative of metasomatism involving H{sub 2}O-rich fluids. Strontium and neodymium isotopes in the xenoliths are not totally consistent with those in host lavas, but a common signature is indicated by the fact that all samples plot below the mantle array. The flux of fluids in the mantle wedge probably occurred over a long period of time. The flux induced large but variable changes in mineral and trace and isotopic compositions, and ultimately resulted in the melting of the peridotites and production of island-arc lavas.

  10. Li Isotope Studies of Olivine in Mantle Xenoliths by SIMS

    NASA Technical Reports Server (NTRS)

    Bell, D. R.; Hervig, R. L.; Buseck, P. R.

    2005-01-01

    Variations in the ratio of the stable isotopes of Li are a potentially powerful tracer of processes in planetary and nebular environments [1]. Large differences in the 7Li/6Li ratio between the terrestrial upper mantle and various crustal materials make Li isotope composition a potentially powerful tracer of crustal recycling processes on Earth [2]. Recent SIMS studies of terrestrial mantle and Martian meteorite samples report intra-mineral Li isotope zoning [3-5]. Substantial Li isotope heterogeneity also exists within and between the components of chondritic meteorites [6,7]. Experimental studies of Li diffusion suggest the potential for rapid isotope exchange at elevated temperatures [8]. Large variations in 7Li, exceeding the range of unaltered basalts, occur in terrestrial mantle-derived xenoliths from individual localities [9]. The origins of these variations are not fully understood.

  11. Constraints from Xenoliths on the Rheology of the Mojave Lower Crust and Lithospheric Mantle

    NASA Astrophysics Data System (ADS)

    Bernard, R. E.; Behr, W. M.

    2015-12-01

    We use xenoliths from young (3 Ma to present) cinder cones in the tectonically active Mojave region of southern California to characterize the rheological properties of the lower crust and upper mantle beneath the Eastern California Shear Zone. The xenoliths, which include spinel and plagioclase facies peridotites and lower crustal rocks (representing a depth range of ~25-60 km), were collected from two localities ~80 km apart: the Cima and Dish Hill volcanic fields. We document how stress, temperature, water content, deformation mechanism, lattice preferred orientation (LPO), and style of localization vary spatially and with depth. Key findings include the following: (1) Both xenolith suites exhibit a wide range of deformation textures, ranging from granular, to protogranular, to porphyroclastic and mylonitic. Higher strain fabrics show no evidence for static annealing, thus are likely reflecting youthful deformation and strain gradients at depth. (2) Both xenolith suites show abundant dynamic recrystallization and other evidence for dislocation creep as the dominant deformation mechanism. (3) A- and E-type olivine LPOs occur in both xenolith suites. In general, E-type LPO is associated with higher strain fabrics than A-type. (4) Water contents—found using Fourier transform infrared spectroscopy (FTIR) and Secondary Ion Mass Spectrometry (SIMS)—range from 115-254 ppm for clinopyroxene, 35-165 ppm for orthopyroxene, and less than 10 ppm for olivine. We have found no correlation between water content and olivine LPO, despite experimental work associating higher water content with the development of E-type LPO, compared to A-type. (5) Deformation in most lower crustal gabbros is weak, but some show strong fabrics associated with plagioclase-rich zones. Water content from clinopyroxene in one highly-deformed gabbro is <1 ppm. (6) Paleopiezometers for olivine and plagioclase indicate stress magnitudes of 16-21 MPa for the uppermost mantle, and 0.1 MPa for the

  12. Experimental study bearing on the absence of carbonate in mantle-derived xenoliths

    SciTech Connect

    Canil, D. )

    1990-10-01

    Experimentation at high pressures in peridotite + CO{sub 2} systems has suggested that some CO{sub 2}-rich, Si-undersaturated magmas originate in mantle source regions consisting of carbonated peridotite. However, a caveat to the petrological application of such experiments has been the virtual absence of carbonate in mantle xenoliths. Decompression experiments were undertaken to resolve this problem and to test the hypothesis that carbonated peridotite exists in the upper mantle, but decomposes during ascent as xenoliths in its host magma. The author demonstrate that carbonate coexisting with silicates in mantle-derived xenoliths could not survive entrainment even in the fastest ascending magmas due to rapid decarbonation upon decompression. The author conclude that carbonate may indeed exist in the upper mantle, and that the paucity of this phase in mantle-derived xenoliths does not require its absence in the mantle source regions of many primary, alkalic magmas.

  13. Mantle xenoliths from Marosticano area (Northern Italy): a comparison with Veneto Volcanic Province lithospheric mantle

    NASA Astrophysics Data System (ADS)

    Brombin, Valentina; Bonadiman, Costanza; Coltorti, Massimo

    2016-04-01

    The Tertiary Magmatic Province of Veneto, known as Veneto Volcanic Province (VVP), in the North-East of Italy, represents the most important volcanic distric of Adria Plate. It is composed by five volcanic bodies: Val d'Adige, Marosticano, Mts. Lessini, Berici Hills and Euganean Hills. Most of the volcanic products are relatively undifferentiated lavas and range in composition from nephelinites to tholeiites. Often VVP nephelinites and basanites carry mantle xenoliths (mainly harzburgites and lherzolite). This study reports petrological comparison between Marosticano xenoliths (new outcrop) and xenoliths from the Lessinean and Val d'Adige areas already studied by many Authors (Siena & Coltorti 1989; Beccaluva et al., 2001, Gasperini et al., 2006). Mineral major elements analyses show that the Marosticano lherzolites and harzburgites reflect "more restitic" composition than the mantle domain beneath the other VVP districts (Lessini Mts. and Val d'Adige). In fact, olivine and pyroxene of Marosticano xenoliths have the highest mg# values of the entire district (Marosticano→90-93; literature→86-92). At comparable mg# (45-85 wt%) Marosticano spinels tend to be higher in Cr2O3 (23-44 wt%) contents with respect to the other VVP sp (7-25 wt%). It is worth noting that, Ni contents of Marosticano olivines in both harzburgites and lherzolites are higher (2650-3620 ppm) than those of the Lessinean xenoliths (1500- 3450 ppm), and similar to that of Val d'Adige lherzolites (3000-3500 ppm), approaching the contents of Archean cratonic mantle (Kelemen, 1998). In turn, Lessinean olivines properly fall in the Ni-mg# Phanerozoic field. At fixed pressure of 15 kbar, the equilibration temperature of Marosticano xenoliths are similar (Brey & Köhler: 920-1120°C) to those of Lessini (O'Neill & Wall: 990-1110°C; Beccaluva et al., 2007), but higher than those of Val d'Adige (Wells: 909-956°C; Gasperini et al., 2006). Finally, Marosticano mantle fragment show similar relatively high

  14. Sulfur and selenium systematics of the subcontinental lithospheric mantle: Inferences from the Massif Central xenolith suite (France)

    NASA Astrophysics Data System (ADS)

    Lorand, Jean-Pierre; Alard, Olivier; Luguet, Ambre; Keays, Reid R.

    2003-11-01

    Selenium has been analyzed in addition to S in 58 spinel peridotite xenoliths collected in Cenozoic alkali basalts from the Massif Central (France). The S concentration range now available for this suite, calculated from 123 samples, is the largest ever reported for alkali basalt-hosted xenoliths (<3-592 ppm). Likewise, the Se concentrations range between 0.2 and 67 ppb. No partial melting signature can be identified from the S and Se systematic. Half of the analyzed xenoliths have lost S during supergene weathering. By contrast, neither surficial alteration, nor loss of chalcophile elements during eruption can explain the regional-scale variations of S and Se concentrations. A first group of lherzolite xenoliths sampled in Southern Massif Central, from volcanic centers older and spatially unrelated to the Massif Central plume that triggered the Cenozoic volcanism, contains between 20 and 250 ppm S (with occasional S concentrations up to 592 ppm) and 12-67 ppb Se. It is clear that the highest S values, originally interpreted as representing S abundances in the primitive mantle, were in fact enriched by metasomatism. Highly variable S and Se contents (<5-360 ppm; 9-52 ppb) have also been observed in peridotite xenoliths collected in the Northern Massif Central, from volcanic centers mostly older than the plume. Like Group I xenoliths, these Group II xenoliths were strongly metasomatized by volatile-rich carbonated/silicated melts which precipitated Cu-rich sulfides. A third group of xenoliths from Plio-Quaternary basalts spatially related to the Massif Central Plume are uniformly poor in S (10-60 ppm) and Se (9-29 ppb). In this Group III, poikiloblastic textured xenoliths have lost most of their S and Se budget by peridotite-melt interactions at high melt/rock ratios. Taken as a whole, the Massif Central xenolith suite provides further evidence for strong heterogeneities in the S and Se budget of the sub-continental lithospheric mantle. However, the few LREE

  15. Carbonates in mantle xenoliths from French Massif Central: inference for carbonatite-related metasomatism.

    NASA Astrophysics Data System (ADS)

    Wagner, Christiane; Deloule, Etienne

    2016-04-01

    -richer, particularly the type 2 and 3 carbonates (XCa = 0.88 - 0.91; 3 - 5 wt. % MgO), a composition similar to that of the carbonates from the vesicles in the basanite (XCa = 0.86 - 0.88; 4 - 5 wt. % MgO). In both xenoliths, the carbonates have low REE abundances (mostly below the detection limit except La and Ce), similar to those reported for carbonates from mantle xenoliths. Moreover, the carbonate globules in the basanite have the same REE composition. Although the presence of rounded vesicles of calcite was originally interpreted as an evidence for silicate-carbonate liquid immiscibility, experimental studies have shown that alkali-free immiscible carbonates cannot be almost pure calcite. Textural features and composition (high XCa, low alkali contents and low REE abundances) of carbonates rule out their origin as quenched carbonatitic melts or immiscible carbonate liquids and favor, thus, an origin as crystal cumulates from mantle-derived carbonate-rich melts (e.g. alkali-carbonate melts). A possible scenario is the injection of small amounts of a carbonate-rich melt at mantle level shortly before the eruption to preserve the calcite crystals. Carbonate-rich melt or emanated fluids may have permeated the xenoliths (MC2) during the ascent and precipitated calcite crystals in the xenolith as well as in the entraining basanitic magma.

  16. The Mantle Beneath Southern Africa: Insights from Seismic Tomography and Mantle Xenoliths

    NASA Astrophysics Data System (ADS)

    James, D. E.; Bell, D. R.; Boyd, F.; Schutt, D.; Carlson, R. W.

    2004-05-01

    We impose geologic constraints on computed tomographic structure of the upper mantle beneath southern Africa by calculating seismic velocities and rock densities from approximately 120 geothermobarometrically calibrated mantle xenoliths from the Archean Kaapvaal craton and adjacent Proterozoic mobile belts. Velocity and density estimates are based on the elastic and thermal moduli of constituent minerals under equilibrium P-T conditions at the mantle source. The largest sources of error in the velocity estimates derive from inaccurate thermo-barometry and, to a lesser extent, from uncertainties in the elastic constants of the constituent minerals. We show that seismic velocity variations between cratonic and non-cratonic xenoliths are controlled dominantly by differences in calculated temperatures, with compositional effects secondary. Different temperature profiles between cratonic and non-cratonic regions have a relatively minor influence on density, where composition remains the dominant control. Low-T cratonic xenoliths exhibit a positive velocity-depth curve, rising from about 8.13 km/s at uppermost mantle depths to about 8.25 km/s at 180-km depth. S-wave velocities decrease slightly over the same depth interval, from about 4.7 km/s in the uppermost mantle to 4.65 km/s at 180-km depth. P and S-wave velocities for high-T lherzolites are highly scattered, ranging from highs close to those of the low-T xenoliths to lows of 8.05 km/s and 4.5 km/s at depths in excess of 200 km. These low velocities, while not asthenospheric, are inconsistent with seismic tomographic images that indicate high velocity root material extending to depths of at least 250 km. The high-T mantle xenoliths, most of which were erupted ca 80-90 Ma, appear to have been affected by localized non-equilibrium thermal perturbations occurring around the time of the kimberlite eruption as well as by compositional modifications associated with emplacement of metasomatic fluids into the deep

  17. Aqueous fluids and sedimentary melts as agents for mantle wedge metasomatism, as inferred from peridotite xenoliths at Pinatubo and Iraya volcanoes, Luzon arc, Philippines

    NASA Astrophysics Data System (ADS)

    Yoshikawa, Masako; Tamura, Akihiro; Arai, Shoji; Kawamoto, Tatsuhiko; Payot, Betchaida D.; Rivera, Danikko John; Bariso, Ericson B.; Mirabueno, Ma. Hannah T.; Okuno, Mitsuru; Kobayashi, Tetsuo

    2016-10-01

    Mantle xenoliths entrained in subduction-zone magmas often record metasomatic signature of the mantle wedge. Such xenoliths occur in magmas from Iraya and Pinatubo volcanoes, located at the volcanic front of the Luzon arc in the Philippines. In this study, we present the major element compositions of the main minerals, trace element abundances in pyroxenes and amphiboles, and Nd-Sr isotopic compositions of amphiboles in the peridotite xenoliths from Pinatubo volcano. The data indicate enrichment in fluid-mobile elements, such as Rb, Ba, U, Pb, and Sr, and Nd-Sr isotopic ratios relative to those of mantle. The results are considered in terms of mixing of asthenospheric mantle and subducting oceanic crustal components. The enrichments observed in the Pinatubo mantle xenoliths are much less pronounced than those reported for the Iraya mantle xenoliths. This disparity suggests differences in the metasomatic agents contributing to the two suites; i.e., aqueous fluids infiltrated the mantle wedge beneath the Pinatubo volcano, whereas aqueous fluids and sediment-derived melts infiltrated the mantle wedge beneath the Iraya volcano.

  18. Nanodiamond finding in the hyblean shallow mantle xenoliths.

    PubMed

    Simakov, S K; Kouchi, A; Mel'nik, N N; Scribano, V; Kimura, Y; Hama, T; Suzuki, N; Saito, H; Yoshizawa, T

    2015-01-01

    Most of Earth's diamonds are connected with deep-seated mantle rocks; however, in recent years, μm-sized diamonds have been found in shallower metamorphic rocks, and the process of shallow-seated diamond formation has become a hotly debated topic. Nanodiamonds occur mainly in chondrite meteorites associated with organic matter and water. They can be synthesized in the stability field of graphite from organic compounds under hydrothermal conditions. Similar physicochemical conditions occur in serpentinite-hosted hydrothermal systems. Herein, we report the first finding of nanodiamonds, primarily of 6 and 10 nm, in Hyblean asphaltene-bearing serpentinite xenoliths (Sicily, Italy). The discovery was made by electron microscopy observations coupled with Raman spectroscopy analyses. The finding reveals new aspects of carbon speciation and diamond formation in shallow crustal settings. Nanodiamonds can grow during the hydrothermal alteration of ultramafic rocks, as well as during the lithogenesis of sediments bearing organic matter. PMID:26030133

  19. Nanodiamond finding in the hyblean shallow mantle xenoliths.

    PubMed

    Simakov, S K; Kouchi, A; Mel'nik, N N; Scribano, V; Kimura, Y; Hama, T; Suzuki, N; Saito, H; Yoshizawa, T

    2015-06-01

    Most of Earth's diamonds are connected with deep-seated mantle rocks; however, in recent years, μm-sized diamonds have been found in shallower metamorphic rocks, and the process of shallow-seated diamond formation has become a hotly debated topic. Nanodiamonds occur mainly in chondrite meteorites associated with organic matter and water. They can be synthesized in the stability field of graphite from organic compounds under hydrothermal conditions. Similar physicochemical conditions occur in serpentinite-hosted hydrothermal systems. Herein, we report the first finding of nanodiamonds, primarily of 6 and 10 nm, in Hyblean asphaltene-bearing serpentinite xenoliths (Sicily, Italy). The discovery was made by electron microscopy observations coupled with Raman spectroscopy analyses. The finding reveals new aspects of carbon speciation and diamond formation in shallow crustal settings. Nanodiamonds can grow during the hydrothermal alteration of ultramafic rocks, as well as during the lithogenesis of sediments bearing organic matter.

  20. Asphaltene-bearing mantle xenoliths from Hyblean diatremes, Sicily

    NASA Astrophysics Data System (ADS)

    Scirè, Salvatore; Ciliberto, Enrico; Crisafulli, Carmelo; Scribano, Vittorio; Bellatreccia, Fabio; Ventura, Giancarlo Della

    2011-08-01

    Microscopic blebs of sulfur-bearing organic matter (OM) commonly occur between the secondary calcite grains and fibrous phyllosilicates in extensively serpentinized and carbonated mantle-derived ultramafic xenoliths from Hyblean nephelinite diatremes, Sicily, Italy. Rarely, coarse bituminous patches give the rock a blackish color. Micro Fourier transform infrared spectra (μ-FTIR) point to asphaltene-like structures in the OM, due to partially condensed aromatic rings with aliphatic tails consisting of a few C atoms. X-ray photoelectron spectroscopy (XPS) analysis indicates the occurrence of minor S═O (either sulphonyl or sulphoxide) functional groups in the OM. Solubility tests in toluene, thermo-gravimetric (TGA) and differential thermal (DTA) analyses confirm the presence of asphaltene structures. It is proposed that asphaltenes derive from the in situ aromatization (with decrease in H/C ratio) of previous light aliphatic hydrocarbons. Field evidence excludes that hydrocarbon from an external source percolated through the xenolith bearing tuff-breccia. The discriminating presence of hydrocarbon in a particular type of xenolith only and the lack of hydrocarbon in the host breccia matrix, are also inconsistent with an interaction between the ascending eruptive system and a supposed deep-seated oil reservoir. Assuming that the Hyblean unexposed basement consists of mantle ultramafics and mafic intrusive rocks having hosted an early abyssal-type hydrothermal system, one can put forward the hypothesis that the hydrocarbon production was related to hydrothermal activity in a serpentinite system. Although a bacteriogenesis or thermogenesis cannot be ruled out, the coexisting serpentine, Ni-Fe ores and hydrocarbon strongly suggest a Fischer-Tropsch-type (FTT) synthesis. Subsequent variations in the chemical and physical conditions of the system, for example an increase in the water/rock ratio, gave rise to partial oxidation and late carbonation of the serpentinite

  1. Constraining the Composition of the Subcontinental Lithospheric Mantle Beneath the East African Rift: FTIR Analysis of Water in Spinel Peridotite Mantle Xenoliths

    NASA Technical Reports Server (NTRS)

    Erickson, Stephanie Gwen; Nelson, Wendy R.; Peslier, Anne H.; Snow, Jonathan E.

    2014-01-01

    The East African Rift System was initiated by the impingement of the Afar mantle plume on the base of the non-cratonic continental lithosphere (assembled during the Pan-African Orogeny), producing over 300,000 kmof continental flood basalts approx.30 Ma ago. The contribution of the subcontinental lithospheric mantle (SCLM) to this voluminous period of volcanism is implied based on basaltic geochemical and isotopic data. However, the role of percolating melts on the SCLM composition is less clear. Metasomatism is capable of hybridizing or overprinting the geochemical signature of the SCLM. In addition, models suggest that adding fluids to lithospheric mantle affects its stability. We investigated the nature of the SCLM using Fourier transform infrared spectrometry (FTIR) to measure water content in mantle xenoliths entrained in young (1 Ma) basaltic lavas from the Ethiopian volcanic province. The mantle xenoliths consist dominantly of spinel lherzolites and are composed of nominally anhydrous minerals, which can contain trace water as H in mineral defects. Eleven mantle xenoliths come from the Injibara-Gojam region and two from the Mega-Sidamo region. Water abundances of olivines in six samples are 1-5ppm H2O while the rest are below the limit of detection (<0.5 ppm H2O); orthopyroxene and clinopyroxene contain 80-238 and 111-340 ppm wt H2O, respectively. Two xenoliths have higher water contents - a websterite (470 ppm) and dunite (229 ppm), consistent with involvement of ascending melts. The low water content of the upper SCLM beneath Ethiopia is as dry as the oceanic mantle except for small domains represented by percolating melts. Consequently, rifting of the East African lithosphere may not have been facilitated by a hydrated upper mantle.

  2. The Homestead kimberlite, central Montana, USA: mineralogy, xenocrysts, and upper-mantle xenoliths

    NASA Astrophysics Data System (ADS)

    Carter Hearn, B.

    2004-09-01

    The Homestead kimberlite was emplaced in lower Cretaceous marine shale and siltstone in the Grassrange area of central Montana. The Grassrange area includes aillikite, alnoite, carbonatite, kimberlite, and monchiquite and is situated within the Archean Wyoming craton. The kimberlite contains 25-30 modal% olivine as xenocrysts and phenocrysts in a matrix of phlogopite, monticellite, diopside, serpentine, chlorite, hydrous Ca-Al-Na silicates, perovskite, and spinel. The rock is kimberlite based on mineralogy, the presence of atoll-textured groundmass spinels, and kimberlitic core-rim zoning of groundmass spinels and groundmass phlogopites. Garnet xenocrysts are mainly Cr-pyropes, of which 2-12% are G10 compositions, crustal almandines are rare and eclogitic garnets are absent. Spinel xenocrysts have MgO and Cr 2O 3 contents ranging into the diamond inclusion field. Mg-ilmenite xenocrysts contain 7-11 wt.% MgO and 0.8-1.9 wt.% Cr 2O 3, with (Fe +3/Fe tot) from 0.17-0.31. Olivine is the only obvious megacryst mineral present. One microdiamond was recovered from caustic fusion of a 45-kg sample. Upper-mantle xenoliths up to 70 cm size are abundant and are some of the largest known garnet peridotite xenoliths in North America. The xenolith suite is dominated by dunites, and harzburgites containing garnet and/or spinel. Granulites are rare and eclogites are absent. Among 153 xenoliths, 7% are lherzolites, 61% are harzburgites, 31% are dunites, and 1% are orthopyroxenites. Three of 30 peridotite xenoliths that were analysed are low-Ca garnet-spinel harzburgites containing G10 garnets. Xenolith textures are mainly coarse granular, and only 5% are porphyroclastic. Xenolith modal mineralogy and mineral compositions indicate ancient major-element depletion as observed in other Wyoming craton xenolith assemblages, followed by younger enrichment events evidenced by tectonized or undeformed veins of orthopyroxenite, clinopyroxenite, websterite, and the presence of phlogopite

  3. The Homestead kimberlite, central Montana, USA: Mineralogy, xenocrysts, and upper-mantle xenoliths

    USGS Publications Warehouse

    Carter, Hearn B.

    2004-01-01

    The Homestead kimberlite was emplaced in lower Cretaceous marine shale and siltstone in the Grassrange area of central Montana. The Grassrange area includes aillikite, alnoite, carbonatite, kimberlite, and monchiquite and is situated within the Archean Wyoming craton. The kimberlite contains 25-30 modal% olivine as xenocrysts and phenocrysts in a matrix of phlogopite, monticellite, diopside, serpentine, chlorite, hydrous Ca-Al-Na silicates, perovskite, and spinel. The rock is kimberlite based on mineralogy, the presence of atoll-textured groundmass spinels, and kimberlitic core-rim zoning of groundmass spinels and groundmass phlogopites. Garnet xenocrysts are mainly Cr-pyropes, of which 2-12% are G10 compositions, crustal almandines are rare and eclogitic garnets are absent. Spinel xenocrysts have MgO and Cr2O3 contents ranging into the diamond inclusion field. Mg-ilmenite xenocrysts contain 7-11 wt.% MgO and 0.8-1.9 wt.% Cr2O3, with (Fe+3/Fetot) from 0.17-0.31. Olivine is the only obvious megacryst mineral present. One microdiamond was recovered from caustic fusion of a 45-kg sample. Upper-mantle xenoliths up to 70 cm size are abundant and are some of the largest known garnet peridotite xenoliths in North America. The xenolith suite is dominated by dunites, and harzburgites containing garnet and/or spinel. Granulites are rare and eclogites are absent. Among 153 xenoliths, 7% are lherzolites, 61% are harzburgites, 31% are dunites, and 1% are orthopyroxenites. Three of 30 peridotite xenoliths that were analysed are low-Ca garnet-spinel harzburgites containing G10 garnets. Xenolith textures are mainly coarse granular, and only 5% are porphyroclastic. Xenolith modal mineralogy and mineral compositions indicate ancient major-element depletion as observed in other Wyoming craton xenolith assemblages, followed by younger enrichment events evidenced by tectonized or undeformed veins of orthopyroxenite, clinopyroxenite, websterite, and the presence of phlogopite

  4. Isotopic and trace element compositions of upper mantle and lower crustal xenoliths, Cima volcanic field, California: Implications for evolution of the subcontinental lithospheric mantle

    USGS Publications Warehouse

    Mukasa, S.B.; Wilshire, H.G.

    1997-01-01

    Ultramafic and mafic xenoliths from the Cima volcanic field, southern California, provide evidence of episodic modification of the upper mantle and underplating of the crust beneath a portion of the southern Basin and Range province. The upper mantle xenoliths include spinel peridotite and anhydrous and hydrous pyroxenite, some cut by igneous-textured pyroxenite-gabbro veins and dikes and some by veins of amphibole ?? plagioclase. Igneous-textured pyroxenites and gabbros like the dike rocks also occur abundantly as isolated xenoliths inferred to represent underplated crust. Mineral and whole rock trace element compositions among and within the different groups of xenoliths are highly variable, reflecting multiple processes that include magma-mantle wall rock reactions, episodic intrusion and it filtration of basaltic melts of varied sources into the mantle wall rock, and fractionation. Nd, Sr, and Pb isotopic compositions mostly of clinopyroxene and plagioclase mineral separates show distinct differences between mantle xenoliths (??Nd = -5.7 to +3.4; 87Sr/86Sr = 0.7051 - 0.7073; 206Pb/204Pb = 19.045 - 19.195) and the igneous-textured xenoliths (??Nd = +7.7 to +11.7; 87Sr/86Sr = 0.7027 - 0.7036 with one carbonate-affected outlier at 0.7054; and 206Pb/204Pb = 18.751 - 19.068), so that they cannot be related. The igneous-textured pyroxenites and gabbros are similar in their isotopic compositions to the host basaltic rocks, which have ??Nd of+5.1 to +9.3; 87Sr/86Sr of 0.7028 - 0.7050, and 206Pb/204Pb of 18.685 - 21.050. The igneous-textured pyroxenites and gabbros are therefore inferred to be related to the host rocks as earlier cogenetic intrusions in the mantle and in the lower crust. Two samples of peridotite, one modally metasomatized by amphibole and the other by plagioclase, have isotopic compositions intermediate between the igneous-textured xenoliths and the mantle rock, suggesting mixing, but also derivation of the metasomatizing magmas from two separate and

  5. Iddingzitized olivine in mantle xenoliths: evidence for (really) early alteration

    NASA Astrophysics Data System (ADS)

    Low, P. C.; Schultz, L.; Stier, N.

    2011-12-01

    olivine. Some of the partially altered olivine crystals exhibiting the later texture contain deformational kink bands that, given that the young eruption age of the host basalt, post-dates any crustal activity that could be responsible for such a fabric at this location, much have occurred in the lithospheric mantle. Textural evidence that the iddingzitized areas of these olivine crystals pre-date the kink banding suggests that the alteration of these olivines occurred in the mantle. Post-entrainment alteration is unlikely particularly given the close proximity of altered and unaltered samples with similar post-entrainment history and the fact that the host basalts are not particularly hydrous. The variety of alteration in xenoliths at this location suggest that the entraining flows are either sampling small regions of the mantle that contains very high gradients with regard to water content or oxygen fugacity or are sampling relatively large areas. Quantitative analysis of volatiles in altered and unaltered olivine crystals planned in order to better constrain these gradients.

  6. Mantle xenoliths from Bondoró Volcanic Complex

    NASA Astrophysics Data System (ADS)

    Janisch, Astrid; Mundl, Andrea; Ntaflos, Theodoros; Harangi, Szabolcs

    2014-05-01

    The late Miocene Bondoró Volcanic Complex belongs to the Bakony-Balaton-Highland-Volcanic-Field (BBHVF) in the western part of the Pannonian Basin on the northern shore of Lake Balaton, Hungary. The volcanic field includes approximately 150 erosional remnants of maars, tuff rings, scoria cones and shield volcanoes. The Pannonian Basin is an extensional back-arc basin of the Carpathian Arc formed between Miocene and Pleistocene. Mantle xenoliths brought to the surface by alkali basalts comprise sp-lherzolites, sp-harzburgites and pyroxenites. Two distinct types of textures have been found: fine-grained, equigranular textures that are predominantly foliated and coarse grained, protogranular textures which often exhibit layering of pyroxenes. A striking feature of some harzburgites is a noticeably high modal composition of orthopyroxenes. While phlogopite was only found in one sample, intergranular, percolating melt and melt pockets are common in Bondoró mantle xenoliths. Whole rock Al2O3 and CaO contents range from 1.01 to 3.93 wt% and 0.71 to 3.20 wt%, respectively. Mineral analyzes of primary ol reveal Fo contents of 89.4 to 91.4. Cpx are predominantly Cr-Diopsides with En48.1-51.9-Wo43.4-47.7-Fs3.2-6.1 and Mg# of 0.89 to 0.93. Opx compositions are in the range of En87.8-90.3-Wo0.8-2.3-Fs8.1-9.9 with Mg# between 90 and 91.8. While Cr# in primary sp range from 12 to 21, secondary sp in melt pockets and melt intrusions reveal higher Cr# of 41 to 55. The (La/Yb)N ratios in clinopyroxenes vary from 0.35 to 2.63 with an exception of a sample with a by far higher ratio of 17.9. Equilibration temperatures calculated using two-pyroxene-thermometer of Brey and Koehler (1990) are estimated to be in the range of 950 to 1100°C at 1.5 GPa pressure. According to the clinopyroxene incompatible trace element composition three groups have been identified, which reflect different kinds of metasomatic overprint: (a) hydrous silicate metasomatism, (b) basaltic melt percolation, (c

  7. Mapping a Mantle Xenolith Using Micro X-ray Diffraction

    NASA Astrophysics Data System (ADS)

    Harwood, B. P.; Flemming, R. L.; Stachel, T.

    2009-05-01

    fairly accurate estimates of the modal mineralogy (when compared to point counts), with the added benefit of recording the spatial distribution. Some areas of the thin sections were not assigned to any mineral, due to the orientation of the mineral not satisfying Bragg's law, or the mineral being highly altered in that area of the thin section. Further development of this technique will focus on refining the quality of the final maps. This technique could potentially be used to provide information about preferred orientation of minerals in mantle xenoliths and other rocks, or to distinguish minerals which exist in different hydrated states.

  8. Fluids in mantle xenoliths related to multiple metasomatism from the Rio Grande Rift, New Mexico

    NASA Astrophysics Data System (ADS)

    Park, M.; Berkesi, M.; Jung, H.; Kil, Y.; Szabo, C.

    2012-12-01

    Mantle-derived volatile-rich fluid inclusions can give important information on chemical features and physical condition on fluid regimes in the upper mantle. These volatiles may also play important role in understanding the fluid/mantle rock interaction in the lithospheric mantle causing mantle metasomatism associated with shallow subduction process. Spinel peridotite xenoliths, hosted in alkali basalts (~15 Ma age), were collected from Adam's Diggings in the Rio Grande Rift (RGR), New Mexico, USA. We selected five representative spinel peridotite xenoliths which are abundant in fluid inclusions (FIs). Based on fluid inclusion petrography, three kinds of orthopyroxene-hosted FIs were identified: Type IA (healed fracture-related, large, negative crystal shape; 10 - 25 μm), Type IB (containing opaque mineral, small, negative crystal shape; 5 - 10 μm), and Type IC (exsolved spinel-related spherical shape; 5 - 10 μm). We studied the FIs system by using heating-freezing stage (microthermometry), high resolution Raman spectroscopy and FIB-SEM (Focused Ion Beam-Scanning Electron Microscopy) techniques. These FIs are characterized as CO2-dominated with other minor components (visible melting occurred at -58.0 ~ -56.8 ± 0.2 °C). The calculated CO2 density for Type IC, IB and IA turned out to be 1.05 - 1.12 g/cm3, 0.98 - 1.08 g/cm3, and 0.69 - 0.86 g/cm3, respectively. In addition to the CO2-rich liquid, Type IA, IB and IC FIs contain magnesite as step-daughter phase proved by Raman spectroscopy. Raman spectroscopy also provided clear evidence the presence of H2O in Type IA fluid inclusions and of N2 both in Type IB and Type IC FIs. The former FIs have hydrous solid (amphibole?) and the latter ones contain Fe-rich sulfide minerals which were confirmed by FIB-SEM technique. A thin glass film with vesicles on the wall of the FIs generally occurs in Type IA. Furthermore, in Type IA FIs anhydrite as step-daughter mineral were also identified by FIB-SEM technique. Based on

  9. Lithospheric mantle evolution in the Afro-Arabian domain: Insights from Bir Ali mantle xenoliths (Yemen)

    NASA Astrophysics Data System (ADS)

    Sgualdo, P.; Aviado, K.; Beccaluva, L.; Bianchini, G.; Blichert-Toft, J.; Bryce, J. G.; Graham, D. W.; Natali, C.; Siena, F.

    2015-05-01

    Detailed petrological and geochemical investigations of an extensive sampling of mantle xenoliths from the Neogene-Quaternary Bir Ali diatreme (southern Yemen) indicate that the underlying lithospheric mantle consists predominantly of medium- to fine-grained (often foliated) spinel-peridotites (85-90%) and spinel-pyroxenites (10-15%) showing thermobarometric estimates in the P-T range of 0.9-2.0 GPa and 900-1150 °C. Peridotites, including lherzolites, harzburgites and dunites delineate continuous chemical, modal and mineralogical variations compatible with large extractions of basic melts occurring since the late Proterozoic (~ 2 Ga, according to Lu-Hf model ages). Pyroxenites may represent intrusions of subalkaline basic melts interacting and equilibrated with the host peridotite. Subsequent metasomatism has led to modal changes, with evidence of reaction patches and clinopyroxene and spinel destabilization, as well as formation of new phases (glass, amphibole and feldspar). These changes are accompanied by enrichment of the most incompatible elements and isotopic compositions. 143Nd/144Nd ranges from 0.51419 to 0.51209 (εNd from + 30.3 to - 10.5), 176Hf/177Hf from 0.28459 to 0.28239 (εHf from + 64.4 to - 13.6), and 208Pb/204Pb from 36.85 to 41.56, thus extending from the depleted mantle (DM) towards the enriched OIB mantle (EM and HIMU) components. 3He/4He (R/RA) ratios vary from 7.2 to 7.9 with He concentrations co-varying with the most incompatible element enrichment, in parallel with metasomatic effects. These metasomatic events, particularly effective in harzburgites and dunites, are attributable to the variable interaction with alkaline basic melts related to the general extensional and rifting regime affecting the East Africa-Arabian domain during the Cenozoic. In this respect, Bir Ali mantle xenoliths resemble those occurring along the Arabian margins and the East Africa Rift system, similarly affected by alkaline metasomatism, whereas they are

  10. Actualistic models of mantle metasomatism documented in a composite xenolith from Dish Hill, California

    USGS Publications Warehouse

    Nielson, J.E.; Budahn, J.R.; Unruh, D.M.; Wilshire, H.G.

    1993-01-01

    infiltrated beyond that zone. The small distance over which variations occur is due to the small amount of liquid that infiltrated. Only in the contact zone was peridotite wallrock saturated by a liquid composition similar to the dike. Comparison of the Ba-2-1 data with those of another xenolith from Dish Hill suggests that the compositional variations of mantle metasomatism result from both the compositional contrast between the metasomatizing liquid and wallrock and the relative abundances of each. Compositional and volumetric variations of mantle partial melts and their fractionates, and repeated events of melting and reaction in contiguous mantle, can create broad ranges of metasomatic "signatures" from the same process. ?? 1993.

  11. In situ SIMS oxygen isotope analysis of olivine in the Tibetan mantle xenoliths

    NASA Astrophysics Data System (ADS)

    Zhao, Zhidan; Zhu, Di-Cheng; Liu, Dong; Mo, Xuanxue

    2016-04-01

    Although the mantle-derived xenoliths from Lhasa terrane provide a means of directly investigating the mantle underlying the southern part of the plateau, they were rarely found in the region. The only case of mantle xenoliths came from the Sailipu ultrapotassic volcanic rocks, erupted at ˜17 Ma, which have indicated that the subcontinental mantle of southern Tibetan Plateau is hot and strongly influenced by metasomatism (Zhao et al., 2008a, b; Liu et al., 2011). A further study by Liu et al.(2014) of in-situ oxygen isotope of olivine crystals in Sailipu mantle xenoliths identify a metasomatized mantle reservoir that interpreted as the sub-arc lithospheric mantle, with anomalously enriched oxygen isotopes (δ18O=8.03). Here we present oxygen isotopes data on the Sailipu mantle xenolith olivines, using different method of sample preparation. Mantle xenoliths (less than 1 cm in diameter) together originally with their host volcanic rocks were prepared in epoxy adjacent to grains of a San Carlos olivine intralaboratory standard and then polished to a flat and smooth surface. Oxygen isotope compositions of olivines occurs both in mantle xenolith and as phenocryst in the host rock, were analyzed in situ using CAMECA SIMS-1280 ion microprobe at the Institute of Geology and Geophysics, Chinese Academy of Sciences. We also performed traditional oxygen isotope analysis on three olivine phenocrysts separates from the host lava. Our new data show: (1) The mantle xenolith olivines have typical mantle oxygen isotopic composition (δ18O=4.8-8.0‰ with average of 5.5±0.2‰ n=105) with variety Fo#(78-90), (2) Oxygen isotopes of situ olivine phenocrysts in the Sailipu lavas (δ18O=7.1-9.2‰ Fo#=70-84, n=66), are similar to that of the whole rock (δ18O=7.0-9.4‰ Fo#=64-74, n=8, Zhao et al., 2009), and three olivine phenocryst grains (δ18O=7.2-7.8); (3) The intralaboratory standard of San Carlos olivine can be a suitable standard using for analyzing olivines with Fo not only

  12. Petrology and geodynamical interpretation of mantle xenoliths from Late Cretaceous lamprophyres, Villány Mts (S Hungary)

    NASA Astrophysics Data System (ADS)

    Nédli, Zsuzsanna; M. Tóth, Tivadar; Downes, Hilary; Császár, Géza; Beard, Andrew; Szabó, Csaba

    2010-06-01

    A Late Cretaceous lamprophyre dyke in the Villány Mts (S Hungary), situated in the Tisza unit, contains abundant spinel lherzolite xenoliths with porphyroclastic textures. Mineral chemistry suggests a relatively fertile mantle, which experienced only 5-7% melt extraction. Differences in porphyroclast and neoblast chemistry and thermobarometric calculations suggest that the mantle section represented by the xenoliths experienced recrystallization at lower PT as it was transported to shallow mantle depths close to the plagioclase stability field, followed by later relaxation. Based on volcanological and sedimentological constraints from the Villány Mts and the neighboring Mecsek Mts, we suggest that the uprise of the subcontinental mantle material was related to a Cretaceous rifting event and lithospheric deformation of the southwestern part of the Tisza unit. Mantle upwelling and formation of lamprophyre melts can be related to generation or reactivation of deep fractures of the lithosphere, during a period of lithospheric extension between the major nappe emplacements (Albian-Cenomanian and Paleocene) of the region.

  13. Crustal evolution at mantle depths constrained from Pamir xenoliths

    NASA Astrophysics Data System (ADS)

    Kooijman, E.; Hacker, B. R.; Smit, M. A.; Kylander-Clark, A. R.; Ratschbacher, L.

    2012-12-01

    Lower crustal xenoliths erupted in the Pamir at ~11 Ma provide an exclusive opportunity to study the evolution of crust at mantle depths during a continent-continent collision. To investigate, and constrain the timing of, the petrologic processes that occurred during burial to the peak conditions (2.5-2.8 GPa, 1000-1100 °C; [1]), we performed chemical- and isotope analyses of accessory minerals in 10 xenoliths, ranging from eclogites to grt-ky-qtz granulites. In situ laser ablation split-stream ICPMS yielded 1) U-Pb ages, Ti concentrations and REE in zircon, 2) U/Th-Pb ages and REE in monazite, and 3) U-Pb ages and trace elements in rutile. In addition, garnet, and biotite and K-feldspar were dated using Lu-Hf and 40Ar/39Ar geochronology, respectively. Zircon and monazite U-(Th-)Pb ages are 101.9±1.8, 53.7±1.0, 39.1±0.8, 21.7±0.4, 18.2±0.5, 16.9±0.8, 15.1±0.3 (2σ) and 12.5-11.1 Ma; most samples showed several or all of these populations. The 53.7 Ma and older ages are xenocrystic or detrital. For younger ages, zircon and monazite in individual samples recorded different ages-although zircon in one rock and monazite in another can be the same age. The 39.1 Ma zircon and monazite mostly occur as inclusions in minerals of the garnet-bearing assemblage that represents the early, low-P stages of burial. Garnet Lu-Hf ages of 37.8±0.3 Ma support garnet growth at this time. Spinifex-like textures containing 21.7-11.1 Ma zircon and monazite record short-lived partial melting events during burial. Aligned kyanite near these patches indicates associated deformation. Zircons yielding ≤12.5 Ma exhibit increased Eu/Eu* and markedly decreased HREE concentrations, interpreted to record feldspar breakdown and omphacite growth during increasing pressure. Rutile U-Pb cooling ages are 10.8±0.3 Ma in all samples. This agrees with the weighted mean 40Ar/39Ar age of eight biotite, K-feldspar and whole rock separates of 11.00+0.16/-0.09 Ma. Rutile in eclogites provides Zr

  14. Geochemical and Sr-Nd isotopic characteristics of mantle xenoliths from NE Spain

    NASA Astrophysics Data System (ADS)

    Galán, G.; Oliveras, V.

    2012-04-01

    Mantle xenoliths in alkaline mafic lavas and pyroclasts from the Neogene-Quaternary volcanism in NE Spain are studied using major, trace element geochemistry and Sr-Nd isotopes, to compare the lithospheric mantle of this area with that of other European zones, and to assess the different processes that conditioned its evolution. The xenoliths mostly come from two volcanoes and are mainly formed of anhydrous spinel lherzolites and harzburgites, in approximately equal proportion. Accessory amphibole and phlogopite are occasional, as it happens with plagioclase, which appears in corona textures around lherzolite spinel. Much subordinated cumulate pyroxenite xenoliths (olivine wbsterite, clinopyroxenite types) are also found. Textures are mostly protogranular, but there are also porphyroclastic, transitional between protogranular and porphyroclastic, and equigranular forms among lherzolites. Pyrometamorphic textures are observed in a few xenoliths. Co-variation diagrams for basaltic components and MgO concentrations in whole rock analyses show gradation from lherzolites to harzburgites. This is also the case for compatible and mildly incompatible trace elements, but not for the most incompatible ones. This gradual variation is also confirmed by mineral compositions, most of which correspond to off-craton xenoliths. REE patterns for lherzolites and for their clinopyroxene are LREE and MREE depleted, whereas for harzburgites are LREE and MREE enriched. U-shaped REE patterns are rarely observed in lherzolite clinopyroxene that also shows more significant negative anomalies at Zr and Ti. Clinopyroxene from harzburgites is also remarked by more significant negative anomalies at Nb, Ti and Zr, and by higher Th and U abundances, than lherzolite clinopyroxene . Sr and Nd isotopic compositions for clinopyroxene define a continuous and inverse trend from DMM lherzolites to enriched harzburgites (87Sr/86Sr: 0.702486-0.709772; 143Nd/144Nd: 0.513359-0.512411). Harzburgite

  15. Crystal chemical constraints on inter-mineral Fe isotope fractionation and implications for Fe isotope disequilibrium in San Carlos mantle xenoliths

    NASA Astrophysics Data System (ADS)

    Macris, Catherine A.; Manning, Craig E.; Young, Edward D.

    2015-04-01

    The origin of variations in iron isotope compositions of mantle minerals is uncertain, and predictions of equilibrium inter-mineral iron isotope fractionation conflict. This hinders interpretation of the petrologic and geochemical implications of Fe isotope data from mantle lithologies. To address this, we present a revised ionic model for predicting equilibrium iron isotope fractionation between mantle minerals and use it to interpret measured inter-mineral iron isotopic fractionation from five distinct mantle xenolith lithologies from San Carlos, Arizona. The samples represent a broad range of modal abundances and include lherzolite, harzburgite, dunite, clinopyroxenite, and websterite. The xenoliths exhibit Fe-isotopic variation between minerals in a single sample, and between samples. In all cases where spinel and olivine coexist, the 57Fe/54Fe of spinel is greater than that of the corresponding olivine, agreeing with expectations for equilibrium fractionation from theory (ionic model), but disagreeing with predictions based on Mössbauer data. The 57Fe/54Fe values of clinopyroxenes from the xenoliths show no clear systematic differences. We interpret this to be a result of varying degrees of metasomatism, perhaps involving interaction with a melt. The spinel peridotite samples (lherzolite, harzburgite, and dunite) are partially melted residual mantle that exhibit a decrease in whole-rock 57Fe/54Fe with increasing olivine abundance. This is consistent with progressive extraction of a 57Fe-rich partial melt. The clinopyroxenite has the highest whole-rock 57Fe/54Fe, consistent with its origin as a cumulate from an unrelated magma possessing elevated 57Fe/54Fe. The websterite sample is transitional to Group II type xenoliths, has the lowest whole-rock 57Fe/54Fe of the investigated samples, and likely experienced a more complex metasomatic history. This study demonstrates that the Fe isotope compositions of San Carlos xenoliths and their component minerals record

  16. Early mantle heterogeneities in the Réunion hotspot source inferred from highly siderophile elements in cumulate xenoliths

    NASA Astrophysics Data System (ADS)

    Peters, Bradley J.; Day, James M. D.; Taylor, Lawrence A.

    2016-08-01

    Ultramafic cumulate rocks form during intrusive crystallization of high-MgO magmas, incorporating relatively high abundances of compatible elements, including Cr and Ni, and high abundances of the highly siderophile elements (HSE: Os, Ir, Ru, Pt, Pd, Re). Here, we utilize a suite of cumulate xenoliths from Piton de la Fournaise, La Réunion (Indian Ocean), to examine the mantle source composition of the Réunion hotspot using HSE abundances and Os isotopes. Dunite and wherlite xenoliths and associated lavas from the Piton de la Fournaise volcanic complex span a range of MgO contents (46 to 7 wt.%), yet exhibit remarkably homogeneous 187Os/188Os (0.1324 ± 0.0014, 2σ), representing the Os-isotopic composition of Réunion hotspot primary melts. A significant fraction of the xenoliths also have primitive upper-mantle (PUM) normalized HSE patterns with elevated Ru and Pd (PUM-normalized Ru/Ir and Pd/Ir of 0.8-6.3 and 0.2-7.2, respectively). These patterns are not artifacts of alteration, fractional crystallization, or partial melting processes, but rather require a primary magma with similar relative enrichments. Some highly olivine-phyric (>40 modal percent olivine) Piton de la Fournaise lavas also preserve these relative Ru and Pd enrichments, while others preserve a pattern that is likely related to sulfur saturation in evolved melts. The estimate of HSE abundances in PUM indicates high Ru/Ir and Pd/Pt values relative to carbonaceous, ordinary and enstatite chondrite meteorite groups. Thus, the existence of cumulate rocks with even more fractionated HSE patterns relative to PUM suggests that the Réunion hotspot samples a yet unrecognized mantle source. The origin of fractionated HSE patterns in Réunion melts may arise from sampling of a mantle source that experienced limited late accretion (<0.2% by mass) compared with PUM (0.5-0.8%), possibly involving impactors that were distinct from present-day chondrites, or limited core-mantle interactions. Given the

  17. 3-D X-ray tomography of diamondiferous mantle eclogite xenoliths, Siberia: A review

    NASA Astrophysics Data System (ADS)

    Howarth, Geoffrey H.; Sobolev, Nikolay V.; Pernet-Fisher, John F.; Ketcham, Richard A.; Maisano, Jessica A.; Pokhilenko, Lyudmila N.; Taylor, Dawn; Taylor, Lawrence A.

    2015-04-01

    Diamonds form over billions of years, hundreds of kilometers beneath the Earth's surface, and in combination with inclusions trapped within, provide important constraints on the evolution of the mantle over geological time. Diamonds are generally studied as individual crystals sourced from highly explosive kimberlite pipes, which entrain and subsequently disaggregate mantle fragments (xenoliths) en route to the surface. This has resulted in a general absence of robust textural descriptions of diamonds relative to their hosting mantle protolith. The textural associations of diamonds within their mantle host rocks are reviewed here on the basis of a compilation of X-ray tomographic data for 17 diamondiferous eclogite xenoliths from Siberian kimberlites. This review represents a comprehensive description of diamonds relative to their host silicates. The lack of such descriptions in previous studies is largely due to the rarity of these xenoliths, the difficulty in preparing petrographic thin sections containing diamonds, and their high-monetary value. High-resolution computed X-ray tomography (HRCXT) produces up to 1200 sequential 2-D slices through individual xenoliths, each of which represents a 'pseudo thin-section' with a resolution on the order of 5-20 μm. The improved resolution of X-ray imaging in recent studies allows for the identification of not only primary minerals, but metasomatic minerals assemblages, including: 'spongy' textured clinopyroxene, phlogopite/K-richterite, and hercynitic spinel, allowing for the delineation of distinct metasomatic pathways through the xenoliths and their relationship to diamonds. Diamonds are observed in three distinct textural settings, potentially representing several temporally distinct diamond growth events, these setting includes: (1) diamonds completely enclosed in garnet; (2) diamonds associated with highly embayed silicate grain boundaries; and (3) diamonds contained within distinct metasomatic 'plumbing

  18. Preliminary data on mantle xenoliths from the Feldstein basalt (Thuringia, Germany)

    NASA Astrophysics Data System (ADS)

    Kukuła, Anna; Puziewicz, Jacek; Ntaflos, Theodoros; Matusiak-Małek, Magdalena; Milke, Ralf

    2014-05-01

    Feldstein is an isolated outcrop of columnar basaltic rock nearby Themar, located 60 km south-west of Erfurt (Thuringia, Germany). The Feldstein alkali basalt (ca. 16.3 Ma) belongs to the Heldburger Gangschar subset of the Central European Volcanic Province (Abratis et al. 2007). The Feldstein alkali basalt contains peridotitic xenoliths, which were the subject of our study. Two groups of spinel peridotite xenoliths occur in the Feldstein basalt. Group A spinel peridotite (2 xenoliths) is characterized by protogranular texture with typical grain size of 2 - 3 mm (max 8 mm). It consists of olivine (90.28 - 91.36 % Fo, 0.35 - 0.45 wt. % NiO), orthopyroxene (mg# 0.91 - 0.92, Al 0.09 - 0.18 a pfu), clinopyroxene (mg# 0.93 - 0.95, Al 0.06 - 0.21 a pfu) and spinel (cr# 0.20 - 0.41, mg# 0.66 - 0.78). The mg# and Al content in clinopyroxene are negatively correlated following the depletion trend after variable degrees of partial melting of the same source. One of the studied samples contains clinopyroxene that does not plot on the general depletion trend but has significantly higher Al (0.15 - 0.21 a pfu) for similar mg # 0.93 - 0.94 with clinopyroxenes from this trend. However the primitive mantle normalized clinopyroxene REE patterns (concave upwards with LaN/YbN=0.11) indicate that they are the residues after elevated degrees of partial melting. The most magnesian clinopyroxene that is Ca-rich and Al-poor has REE abundances, typical for strongly depleted spinel peridotites. It has concave upwards primitive mantle normalized pattern and LaN/YbN=0.61. A slight increase of LaN and CeN with inflection point at PrN has been observed as well. The group B spinel peridotites have protogranular texture (3-4 mm, max 7 mm grains) and some of them contain several melt pockets of basaltic composition. It consists of olivine (88.95 - 91.32 % Fo, 0.34 - 0.47 wt.% NiO), orthopyroxene (mg# 0.90 - 0.93, Al 0.04 - 0.16 apfu) and clinopyroxene (mg# 0.90 - 0.93, Al 0.10 - 0.20 a pfu). The

  19. Xenolith constraints on seismic velocities in the upper mantle beneath southern Africa

    NASA Astrophysics Data System (ADS)

    James, D. E.; Boyd, F. R.; Schutt, D.; Bell, D. R.; Carlson, R. W.

    2004-01-01

    We impose geologic constraints on seismic three-dimensional (3-D) images of the upper mantle beneath southern Africa by calculating seismic velocities and rock densities from approximately 120 geothermobarometrically calibrated mantle xenoliths from the Archean Kaapvaal craton and adjacent Proterozoic mobile belts. Velocity and density estimates are based on the elastic and thermal moduli of constituent minerals under equilibrium P-T conditions at the mantle source. The largest sources of error in the velocity estimates derive from inaccurate thermo-barometry and, to a lesser extent, from uncertainties in the elastic constants of the constituent minerals. Results are consistent with tomographic evidence that cratonic mantle is higher in velocity by 0.5-1.5% and lower in density by about 1% relative to off-craton Proterozoic samples at comparable depths. Seismic velocity variations between cratonic and noncratonic xenoliths are controlled dominantly by differences in calculated temperatures, with compositional effects secondary. Different temperature profiles between cratonic and noncratonic regions have a relatively minor influence on density, where composition remains the dominant control. Low-T cratonic xenoliths exhibit a positive velocity-depth curve, rising from about 8.13 km/s at uppermost mantle depths to about 8.25 km/s at 180-km depth. S velocities decrease slightly over the same depth interval, from about 4.7 km/s in the uppermost mantle to 4.65 km/s at 180-km depth. P and S velocities for high-T lherzolites are highly scattered, ranging from highs close to those of the low-T xenoliths to lows of 8.05 km/s and 4.5 km/s at depths in excess of 200 km. These low velocities, while not asthenospheric, are inconsistent with seismic tomographic images that indicate high velocity root material extending to depths of at least 250 km. One plausible explanation is that high temperatures determined for the high-T xenoliths are a nonequilibrium consequence of

  20. Rock Magnetic Mineral Assemblage in Mineral Separates from Xenoliths of Continental Lithospheric Mantle

    NASA Astrophysics Data System (ADS)

    Khakhalova, E.; Feinberg, J. M.; Ionov, D. A.; Ferre, E. C.; Friedman, S. A.; Hernandez, F. M.; Neal, C. R.; Conder, J. A.

    2014-12-01

    Studies of aeromagnetic anomalies suggest that the lithospheric mantle may contribute to long wavelength features. Examination of unaltered mantle xenoliths may reveal the mineralogical sources of these aeromagnetic anomalies. Prior work has reported microscopic inclusions of magnetic minerals in mantle silicates. Here we explore the magnetism of pure olivine, clinopyroxene, orthopyroxene, and spinel separated from peridotite xenoliths from the Dariganga and Tariat localities in Mongolia that sample the lithospheric mantle. All separates were leached with HF and HCl to remove secondary minerals adhering to the surface of the grains or in cracks. Separates were then mounted in cement to create monomineralic specimens for investigation using hysteresis loops, first order reversal curves (FORC), alternating field and thermal demagnetization of a 1T IRM, and low-temperature magnetometry. All specimens showed trace concentrations of ferromagnetic inclusions with Ms values of ~10-3 Am2kg-1. Thermal demagnetization showed a range of unblocking temperatures with median destructive temperatures of 300-400°C. Two specimens showed a dramatic demagnetization at 585°C, consistent with pure magnetite (Mt). The presence of Mt was confirmed by observations of the Verwey transition at 100-120K and by backfield remanence acquisition curves that plateau at ~300 mT. The median destructive alternating field was ~20 mT and 40-80 mT for specimens from Dariganga and Tariat, respectively. FORC diagrams show single-domain-like behavior with a median Hc of ~20 mT. The demagnetization experiments suggest that Mt inclusions in the lattice of olivine, opx, cpx and spinel carry magnetic remanence. Thus, the lithospheric mantle may exhibit in-situ ferromagnetism carried by Mt below 585°C. The magnetization of separates varies between xenolith localities but is consistent amongst minerals of the same locality. Future work will address whether the Mt formed before or during xenolith ascent.

  1. Petrofabric and seismic properties of lithospheric mantle xenoliths from the Calatrava volcanic field (Central Spain)

    NASA Astrophysics Data System (ADS)

    Puelles, P.; Ábalos, B.; Gil Ibarguchi, J. I.; Sarrionandia, F.; Carracedo, M.; Fernández-Armas, S.

    2016-06-01

    The microstructural and petrofabric study of peridotite xenoliths from the El Aprisco (Neogene Calatrava Volcanic Field) has provided new information on deformation mechanisms, ambient conditions and seismic properties of the central Iberian subcontinental mantle. Olivine, orthopyroxene, clinopyroxene, amphibole and spinel constitute the mineral assemblage in equilibrium. Their microstructure indicates that they accommodated crystal-plastic deformation under high water fugacity conditions. Crystallographic preferred orientation patterns of key minerals were determined with the EBSD technique. The xenoliths exhibit B, C and A olivine fabrics. B-type fabrics, involving the (010)[001] slip system, may develop in domains where deformation occurs under comparatively lower temperature, higher water-content and faster strain rates. They are interpreted here as the result of deformation in a suprasubduction mantle setting triggered by changing conditions imposed by a cooler subducting slab that incorporated fluids into the system. Xenoliths with olivine C-type fabrics involve activation of the dominant (100)[001] slip system, denote intracrystalline slip at higher temperatures and water-contents. They are here interpreted to sample lithospheric mantle domains where the impact of those new conditions was not so strong. Finally, the A-type fabrics, characteristic of the (010)[100] slip system, are frequent in the mantle under moderate to high temperature. These fabrics are considered here as characteristic of the mantle prior to subduction. The olivine fabrics constrain heterogeneous seismic properties. Propagation orientation of P waves (8.27-8.51 km/s) coincides with olivine [100] axis concentrations, whereas the fastest S1 waves (5.13-5.22 km/s) propagate parallel to [010] axis minima. The maximum shear wave birefringence (VS1-VS2 = 0.17-0.37 km/s) is close to the direction of the macroscopic lineation. Heterogeneity of calculated seismic properties would concur with

  2. Microscale effects of melt infiltration into the lithospheric mantle: Peridotite xenoliths from Xilong, South China

    NASA Astrophysics Data System (ADS)

    Lu, Jianggu; Zheng, Jianping; Griffin, William L.; O'Reilly, Suzanne Y.; Pearson, Norman J.

    2015-09-01

    Melting and reactions between minerals and melts are important processes in the evolution of the lithospheric mantle, and are usually inferred from their geochemical fingerprints in mantle samples. However, a suite of mantle-derived peridotite xenoliths from the Xilong area, South China, records the reaction of successive silicate melts of different compositions with mineral assemblages in the mantle, preserved by quenching during entrainment. These xenoliths form two groups and record a compositionally layered mantle. Group 1 has olivine Mg# ~ 91, (and is thus relatively refractory), is derived from depths of ~ 50-65 km, and shows the trace-element geochemical signature of "old" carbonatitic metasomatism. Group 2 is more fertile with olivine Mg# mainly ~ 89-90, is derived from ~ 40 to 55 km and has ubiquitous modal spinel. Xenoliths of both groups then show sequential infiltration by two compositionally distinct melts (Na-rich and K-rich) not long before eruption. The Na-rich melts are enclosed in spongy clinopyroxene and spinel rims and are inferred to have triggered the reactions that formed the spongy rims, which have lower Al2O3, Na2O and Mg#, but higher FeO, TiO2 and Cr# than the primary phases. The undersaturated Na-rich mafic melts were probably formed in the asthenosphere by low-degree melting. The K-rich melts occur mainly in reaction zones around orthopyroxene and in reaction patches containing fine-grained secondary olivine, clinopyroxene and minor spinel. The melts have high contents of SiO2, K2O (mean 14.3 wt.%), Rb, Ba, and LREE but very low Na2O/K2O (0.01-0.29), positive anomalies in Eu and Sr, and variable HFSE anomalies. These compositional characteristics are consistent with an origin as low-degree partial melts of pre-existing phlogopite-bearing rocks. The K-rich melts also react with primary olivine, and the spongy-textured secondary clinopyroxene and spinel inferred to have formed by reaction with the Na-rich melts, yielding secondary olivine

  3. Origin of the DUPAL anomaly in mantle xenoliths of Patagonia (Argentina) and geodynamic consequences

    NASA Astrophysics Data System (ADS)

    Mazzucchelli, Maurizio; Cipriani, Anna; Hémond, Christophe; Zanetti, Alberto; Bertotto, Gustavo Walter; Cingolani, Carlos Alberto

    2016-04-01

    The sub-continental lithospheric mantle of South America has been known for some time to carry the DUPAL isotope anomaly as seen in volcanics from the Paraná volcanic province. However, this has not allowed discriminating whether the DUPAL anomaly is a primary feature of the mantle source or acquired during the upwelling and emplacement of the primary magmas. We discovered mantle xenoliths from the Tres Lagos location in Patagonia that carry evidence of percolation by metasomatic melts that imparted the DUPAL isotope anomaly signature. We discuss a model that requires four isotope components (LCC, EM2, HIMU and DM) to account for the Sr, Nd and Pb isotope variability of our samples. We propose that upwelling of hot astenosphere during the Miocene could have triggered the melting of the LCC and EM2 components carrying the DUPAL anomaly, previously entrained in the subcontinental mantle by subduction. These ascending melts would have then metasomatised the local SCLM characterised by DMM and HIMU geochemical affinity generating the hybrid DUPAL-bearing mantle sampled by the Tres Lagos xenoliths.

  4. Variations on the Lower Silesian (SW Poland) lithospheric mantle - the Grodziec xenoliths case study

    NASA Astrophysics Data System (ADS)

    Matusiak-Małek, Magdalena; Ćwiek, Mateusz; Puziewicz, Jacek; Ntaflos, Theodoros

    2016-04-01

    The lithospheric mantle underlying the northern margin of Bohemian Massif (Lower Silesia, SW Poland) is in general characterized by presence of two ultramafic lithologies, both of mostly harzburgitic composition. The group A harzburgites are strongly depleted and record variable metasomatic events, which are however always related to reactions with mixed alkaline-carbonatite melts. The group B harzburgites also record reaction with mafic melts, but in this case the reaction resulted in enrichment of the peridotites in Fe ("Fe-metasomatism"). The xenoliths suites from Grodziec (this study), Krzeniów (Matusiak-Małek et al., 2014, JoP) and Wilcza Góra (Matusiak-Małek et al., submitted), all in the Złotoryja volcanic complex, follow the "A" and "B" lithological characteristics. The Grodziec suite contains, however, numerous lherzolitic xenoliths. The group A xenoliths from Grodziec are anhydrous lherzolites, scarcely harzburgites. The Fo content in olivine varies from 90.7 to 91.8%, Mg# in ortho-and clinopyroxene is 0.91-0.92. Al content in orthopyroxene is 0.05-0.14 a pfu (0.70 to 3.44 wt.%), which makes them one of the highest in region. Few lherzolites are characterized by slightly lower Fo content in olivine (89.16-90.10%) and are therefore classified as A- group. The Mg# of pyroxenes in this group varies from 0.89 to 0.90, but orthopyroxene is generally characterized by low Al content (< 0.08 a pfu, corresponding to <2 wt.% in majority of xenoliths). Group B xenoliths are orthopyroxene - free dunites, and wehrlite. Olivine contains from 85.14 to 86.14 % of Fo, the Mg# of clinopyroxene varies from 0.84 to 0.88. Clinopyroxene in all the groups is LREE enriched and has negative Sr, Zf-Hf and Ti anomalies, but the enrichment decreases from group A to B and so are the depths of negative anomalies. Temperatures of major element equilibration of group A and A- pyroxenes are from approximately 1010 to 1100°C with no specific differences between the groups. So high

  5. Metasomatic Control of Water in Garnet and Pyroxene from Kaapvaal Craton Mantle Xenoliths

    NASA Technical Reports Server (NTRS)

    Peslier, Anne H.; Woodland, Alan B.; Bell, David R.; Lazarov, Marina; Lapen, Thomas J.

    2012-01-01

    Fourier transform infrared spectrometry (FTIR) and laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) were used to determine water, rare earth (REE), lithophile (LILE), and high field strength (HFSE) element contents in garnet and pyroxene from mantle xenoliths, Kaapvaal craton, southern Africa. Water enters these nominally anhydrous minerals as protons bonded to structural oxygen in lattice defects. Pyroxene water contents (150-400 ppm in clinopyroxene; 40-250 ppm in orthopyroxene) correlate with their Al, Fe, Ca and Na and are homogeneous within a mineral grains and a xenolith. Garnets from Jagersfontein are chemically zoned for Cr, Ca, Ti and water contents. Garnets contain 0 to 20 ppm H2 Despite the fast diffusion rate of H in mantle m inerals, the observations above indicate that the water contents of mantle xenolith minerals were not disturbed during kimberlite entrainment and that the measured water data represent mantle values. Trace elements in all minerals show various degrees of light REE and LILE enrichments indicative of minimal to strong metasomatism. Water contents of peridotite minerals from the Kaapvaal lithosphere are not related to the degree of depletion of the peridotites. Instead, metasomatism exerts a clear control on the amount of water of mantle minerals. Xenoliths from each location record specific types of metasomatism with different outcomes for the water contents of mantle minerals. At pressures . 5.5 GPa, highly alkaline melts metasomatized Liqhobong and Kimberley peridotites, and increased the water contents of their olivine, pyroxenes and garnet. At higher pressures, the circulation of ultramafic melts reacting with peridotite resulted in co-variation of Ca, Ti and water at the edge of garnets at Jagersfontein, overall decreasing their water content, and lowered the water content of olivines at Finsch Mine. The calculated water content of these melts varies depending on whether the water content of the peridotite

  6. Nature and evolution of lithospheric mantle beneath the southern Ethiopian rift zone: evidence from petrology and geochemistry of mantle xenoliths

    NASA Astrophysics Data System (ADS)

    Alemayehu, Melesse; Zhang, Hong-Fu; Sakyi, Patrick Asamoah

    2016-06-01

    Mantle xenoliths hosted in Quaternary basaltic lavas from the Dillo and Megado areas of the southern Ethiopian rift are investigated to understand the geochemical composition and associated processes occurring in the lithospheric mantle beneath the region. The xenoliths are comprised of predominantly spinel lherzolite with subordinate harzburgite and clinopyroxenite. Fo content of olivine and Cr# of spinel for peridotites from both localities positively correlate and suggest the occurrence of variable degrees of partial melting and melt extraction. The clinopyroxene from lherzolites is both LREE depleted (La/Sm(N) = 0.11-0.37 × Cl) and LREE enriched (La/Sm(N) = 1.88-15.72 × Cl) with flat HREEs (Dy/Lu(N) = 0.96-1.31 × Cl). All clinopyroxene from the harzburgites and clinopyroxenites exhibits LREE-enriched (La/Sm(N) = 2.92-27.63.1 × Cl and, 0.45 and 1.38 × Cl, respectively) patterns with slight fractionation of HREE. The 143Nd/144Nd and 176Hf/177Hf ratios of clinopyroxene from lherzolite range from 0.51291 to 0.51370 and 0.28289 to 0.28385, respectively. Most of the samples define ages of 900 and 500 Ma on Sm-Nd and Lu-Hf reference isochrons, within the age range of Pan-African crustal formation. The initial Nd and Hf isotopic ratios were calculated at 1, 1.5, 2 and 2.5 Ga plot away from the trends defined by MORB, DMM and E-DMM which were determined from southern Ethiopian peridotites, thus indicating that the Dillo and Megado xenoliths could have been produced by melt extraction from the asthenosphere during the Pan-African orogenic event. There is no significant difference in 87Sr/86Sr ratios between the depleted and enriched clinopyroxene. This suggests that the melts that caused the enrichment of the clinopyroxene are mainly derived from the depleted asthenospheric mantle from which the xenoliths are extracted. Largely, the mineralogical and isotopic compositions of the xenoliths show heterogeneity of the CLM that could have been produced from various

  7. Old mantle beneath the Avalon terrane: evidence from Osmium isotopes in spinel peridotite xenoliths

    NASA Astrophysics Data System (ADS)

    Minarik, W. G.; Hermes, O. D.; Walker, R. J.

    2002-05-01

    Spinel lherzolite xenoliths from mid-Jurassic lamprophyre dikes near Ashaway, R.I. have been analyzed for Os isotopic ratios and Re and Os concentrations. The xenoliths were collected from freshly exposed outcrops, and are minimally serpentinized. Most contain evidence of carbonate metasomatism. Two grams of powdered peridotite was dissolved in aqua regia in sealed Carius tubes, purified using solvent extraction (Os) and anion resin exchange (Re) and analyzed using NTIMS. The bulk xenoliths contain approx. 4 ppb Os, and have 187Re/188Os ratios of 0.04 to 0.2. These low Re concentrations result in minimal correction back to an initial Jurassic 187Os/188Os ratio of 0.1165, which corresponds to a sub-chondritic initial γ Os of -7.4. The minimum model age (TRD) of melt extraction and separation of these lherzolites from the convecting upper mantle using these 187Os/188Os ratios is 1.5 to 1.7 Ga (after Meisel et al., 2001). Major and trace element compositions will be used to estimate the degree of melt depletion in order to refine the constraints on formation of this subcontinental lithospheric mantle. Avalonian supercrustal rocks range from the Neoproterozoic to Permian in age. These preliminary model ages imply that Mesoproterozoic (or older) mantle remains under the Hope Valley subterrane of Avalonia in Rhode Island. This supports evidence from Archean-age zircon cores found in the surrounding Permian-age granites that significantly older lithosphere exists at depth. This lithosphere could be either an old lithosphere core to the Avalonian arc or underthrust West African craton. Models that call for the introduction of asthenospheric mantle during the Alleghanian orogeny or the initiation of Atlantic rifting are not supported by these data.

  8. Characterization of the sub-continental lithospheric mantle beneath the Cameroon volcanic line inferred from alkaline basalt hosted peridotite xenoliths from Barombi Mbo and Nyos Lakes

    NASA Astrophysics Data System (ADS)

    Pintér, Zsanett; Patkó, Levente; Tene Djoukam, Joëlle Flore; Kovács, István; Tchouankoue, Jean Pierre; Falus, György; Konc, Zoltán; Tommasi, Andréa; Barou, Fabrice; Mihály, Judith; Németh, Csaba; Jeffries, Teresa

    2015-11-01

    We carried out detailed petrographic, major and trace element geochemical, microstructural and FTIR analyses on eight characteristic ultramafic xenoliths from Nyos and Barombi Mbo Lakes in the continental sector of the Cameroon Volcanic Line (CVL). The studied xenoliths are spinel lherzolites showing lithologies similar to the other xenoliths reported previously along the CVL. They have protogranular and porphyroclastic textures. One of the Barombi xenolith contains amphibole, which had not been previously reported in this locality. Amphibole is common in the Nyos xenoliths suite. Peridotite xenoliths from both localities show some chemical heterogeneity, but Barombi xenoliths generally are less depleted in basaltic elements with respect to Nyos xenoliths. Trace element compositions of Nyos spinel lherzolites show a moderately depleted initial (premetasomatic) composition and variable enrichment in REE. Evidence for both modal and cryptic metasomatism is present in Nyos xenoliths. Rare earth element patterns of clinopyroxene suggest that interaction between mafic melts and the upper mantle occurred beneath the Nyos locality. Barombi Mbo xenoliths, on the other hand, record a small degree of partial melting. The Barombi Mbo xenoliths have weak, dominantly orthorhombic olivine crystal preferred orientations, whereas Nyos ones have strong axial-[010] patterns, which may have formed in response to transpression. Nominally anhydrous mantle minerals (NAMs) of the Barombi Mbo xenoliths show generally higher bulk concentrations of 'water' (70-127 ppm) than Nyos xenoliths (32-81 ppm). The Barombi Mbo xenoliths could originate from a juvenile segment of the lithospheric mantle, which had been originally part of the asthenosphere. It became a part of the lithosphere in response to thermal relaxation following the extension, forming a weakly deformed lower lithospheric mantle region along the CVL. The Nyos xenoliths, however, represent a shallow lithospheric mantle bearing

  9. Alteration of Mantle Sulfides: the Effects of Oxidation and Melt Infiltration in a Kilbourne Hole Harzburgite Xenolith

    NASA Astrophysics Data System (ADS)

    Barrett, T. J.; Harvey, J.; Warren, J. M.; Klein, F.; Walshaw, R.

    2013-12-01

    Sulfides, while commonly present in volumetrically minor amounts (< 0.1 modal %; e.g.[1]) in the mantle, impart a strong control over many of the highly siderophile and strongly chalcophile elements. The mass balance of some elements, such as Os, are almost completely controlled by heterogeneously distributed sulfide grains[2][3]. Hence, processes that re-distribute sulfides and / or alter their composition can have profound effects on the information preserved within them regarding primary mantle processes. Different generations of interstitial sulfide may partly or completely re-equilibrate with one another or may be exposed to open-system processes that mobilize and / or precipitate sulfides[4]. In mantle xenoliths in particular, supergene weathering at the Earth's surface can oxidize sulfide to soluble sulfate, and its removal affects highly siderophile and strongly chalcophile element abundances [6]. Here we present the initial results from a study of interstitial mantle sulfides (n = 24) recovered from a single harzburgitic xenolith from Kilbourne Hole, NM. Large compositional differences are observed in the sulfides even at the scale of a single xenolith. Mono-sulfide solid solution has exsolved into two Fe-Ni-rich phases, one with a significantly larger Ni content for a given Fe abundance. Occurrences of Cu-rich sulfides are rare, but where present Cu can account for up to 22 weight % of the sulfide. Critically, no fresh, unaltered sulfides were recovered and in all of the sulfides there is evidence for at least two secondary processes. EDS mapping of the sulfides reveals pervasive, but incomplete, oxidation in all of the grains; Raman spectroscopy reveals this oxide to be goethite. In addition, there is also evidence for the interaction of many of the sulfides with a volatile-rich silicate melt. Silicate melt veins cross-cut the original sulfide mineralogy in some areas of the sulfide grain, while leaving other areas virtually untouched. The degree of

  10. Mantle xenoliths from Central Vietnam: evidence for at least Meso-Proterozoic formation of the lithospheric mantle

    NASA Astrophysics Data System (ADS)

    Proßegger, Peter; Ntaflos, Theodoros; Ackerman, Lukáš; Hauzenberger, Christoph; Tran, Tuan Anh

    2016-04-01

    Intraplate Cenozoic basalts that are widely dispersed along the continental margin of East Asia belong to the Western Pacific "diffuse" igneous province. They consist mainly of alkali basalts, basanites,rarely nephelinites, which are mantle xenolith-bearing, potassic rocks and quartz tholeiites. The volcanism in this area has been attributed to the continental extension caused by the collision of India with Asia and by the subduction of the Pacific Ocean below Asia. We studied a suite of 24 mantle xenoliths from La Bang Lake, Dak Doa district and Bien Ho, Pleiku city in the Gia Province, Central Vietnam. They are predominantly spinel lherzolites (19) but spinel harburgites (3) and two garnet pyroxenites are present as well. The sizes of the xenoliths range from 5 to 40 cm in diameter with medium to coarse-grained protogranular textures. Whole rock major and trace element analyses display a wide range of compositions. The MgO concentration varies from 36.0 to 45.8 wt% whereas Al2O3 and CaO range from 0.63 to 4.36 wt% and from 0.52 to 4.21 wt% (with one sample having CaO of 6.63 wt%) respectively. Both CaO and Al2O3 positively correlate with MgO most likely indicating that the sampled rocks were derived from a common mantle source experienced variable degrees of partial melting. Mineral analyses show that the rock forming minerals are chemically homogeneous. The Fo contents of olivine vary between 89.2 and 91.2 and the Mg# of orthopyroxene and clinopyroxene range from 89 to 92 and 89 to 94 respectively. The range of Cr# for spinel is 0.06-0.26. Model calculations in both whole rock and clinopyroxenes show that lithospheric mantle underneath Central Vietnam experienced melt extractions that vary between 2-7, 12-15 and 20-30%. The majority of the primitive mantle-normalized whole rock and clinopyroxene REE patterns are parallel to each other indicating that clinopyroxene is the main repository of the trace elements. Clinopyroxenes are divided into two groups: group A

  11. Metasomatism vs Refertilisation: New Insights from Northern Victoria Land mantle xenoliths (Antarctica)

    NASA Astrophysics Data System (ADS)

    Pelorosso, Beatrice; Bonadiman, Costanza; Coltorti, Massimo; Ntaflos, Theodoros; Gregoire, Michel

    2016-04-01

    The petrology of anhydrous (Greene Point, Handler Ridge) and amphibole-bearing (Harrow Peaks, Baker Rocks) xenolith populations from Cenozoic volcanics of Northern Victoria Land (NVL), Antarctica, provide new geochemical/geodynamic constraints on the nature and evolution of the NVL lithospheric mantle. Based on mineral major and trace element modelling applied on the various xenolith suites, this mantle domain is supposed to represent a residuum after 10 to 20% of partial melting, with Handler Ridge showing the most fertile character. Moreover, melting models and isotopic results evidence the large geochemical contribution of melt infiltration acting in different times, from at least Jurassic to Cenozoic. The close correlation between Greene Point clinopyroxene trace element contents and those from phenocrysts of Ferrar tholeiites, allows to ascribe the first refertilisation event to the Jurassic Ferrar magmatism; this asthenospheric melt was also able to transfer a garnet signature to some NVL mantle segment. The rare presence of glassy patches and related secondary phases in Greene Point and Handler Ridge, as well as the amphibole presence in Harrow Peaks and Baker Rocks xenoliths prove that alkaline metasomatism, probably related to the West Antarctic Rift System opening, heterogeneously affected the NVL lithospheric domain. At a fixed P of 15 Kbar, T and fO2 were calculated; data reveal that the presence of amphibole (always < 3% modal content) does not influence the ambient redox conditions (comparable in amphibole-bearing and anhydrous Greene Point peridotites [ Δlog fO2 (QFM) -0.7)] but the anhydrous suite presents systematic higher temperature (950-1050 °C) than those amphibole bearing (850 °C).

  12. Tychite in mantle xenoliths from kimberlites: The first find and a new genetic type

    NASA Astrophysics Data System (ADS)

    Sharygin, I. S.; Golovin, A. V.; Korsakov, A. V.; Pokhilenko, N. P.

    2016-03-01

    Tychite Na6Mg2(CO3)4(SO3) is a rare natural Na and Mg sulfatocarbonate. It is found only as minor mineral in deposits of saline lakes in the United States, Canada, Uganda, and China. In these continental evaporites tychite has sedimentary genesis. In this study, we report the first occurrence of tychite as a crystal phase in the melt inclusions in olivine from mantle xenoliths of the Udachnaya-East kimberlite pipe. This find provides an evidence for the probability of tychite crystallization from melts; i.e., this rare sulfatocarbonate may have a magmatic origin as well.

  13. Hf-Zr anomalies in clinopyroxene from mantle xenoliths from France and Poland: implications for Lu-Hf dating of spinel peridotite lithospheric mantle

    NASA Astrophysics Data System (ADS)

    Downes, Hilary; de Vries, Caja; Wittig, Nadine

    2014-09-01

    Clinopyroxenes in some fresh anhydrous spinel peridotite mantle xenoliths from the northern Massif Central (France) and Lower Silesia (Poland), analysed for a range of incompatible trace elements by laser ablation inductively coupled plasma mass spectrometry, show unusually strong negative anomalies in Hf and Zr relative to adjacent elements Sm and Nd, on primitive mantle-normalised diagrams. Similar Zr-Hf anomalies have only rarely been reported from clinopyroxene in spinel peridotite mantle xenoliths worldwide, and most are not as strong as the examples reported here. Low Hf contents give rise to a wide range of Lu/Hf ratios, which over geological time would result in highly radiogenic ɛHf values, decoupling them from ɛNd ratios. The high 176Lu/177Hf could in theory produce an isochronous relationship with 176Hf/177Hf over time; an errorchron is shown by clinopyroxene from mantle xenoliths from the northern Massif Central. However, in a review of the literature, we show that most mantle spinel peridotites do not show such high Lu/Hf ratios in their constituent clinopyroxenes, because they lack the distinctive Zr-Hf anomaly, and this limits the usefulness of the application of the Lu-Hf system of dating to garnet-free mantle rocks. Nevertheless, some mantle xenoliths from Poland or the Czech Republic may be amenable to Hf-isotope dating in the future.

  14. Hf-Zr anomalies in clinopyroxene from mantle xenoliths from France and Poland: implications for Lu-Hf dating of spinel peridotite lithospheric mantle

    NASA Astrophysics Data System (ADS)

    Downes, Hilary; de Vries, Caja; Wittig, Nadine

    2015-01-01

    Clinopyroxenes in some fresh anhydrous spinel peridotite mantle xenoliths from the northern Massif Central (France) and Lower Silesia (Poland), analysed for a range of incompatible trace elements by laser ablation inductively coupled plasma mass spectrometry, show unusually strong negative anomalies in Hf and Zr relative to adjacent elements Sm and Nd, on primitive mantle-normalised diagrams. Similar Zr-Hf anomalies have only rarely been reported from clinopyroxene in spinel peridotite mantle xenoliths worldwide, and most are not as strong as the examples reported here. Low Hf contents give rise to a wide range of Lu/Hf ratios, which over geological time would result in highly radiogenic ɛHf values, decoupling them from ɛNd ratios. The high 176Lu/177Hf could in theory produce an isochronous relationship with 176Hf/177Hf over time; an errorchron is shown by clinopyroxene from mantle xenoliths from the northern Massif Central. However, in a review of the literature, we show that most mantle spinel peridotites do not show such high Lu/Hf ratios in their constituent clinopyroxenes, because they lack the distinctive Zr-Hf anomaly, and this limits the usefulness of the application of the Lu-Hf system of dating to garnet-free mantle rocks. Nevertheless, some mantle xenoliths from Poland or the Czech Republic may be amenable to Hf-isotope dating in the future.

  15. The origin of spongy texture in minerals of mantle xenoliths from the Western Qinling, central China

    NASA Astrophysics Data System (ADS)

    Su, Ben-Xun; Zhang, Hong-Fu; Sakyi, Patrick Asamoah; Yang, Yue-Heng; Ying, Ji-Feng; Tang, Yan-Jie; Qin, Ke-Zhang; Xiao, Yan; Zhao, Xin-Miao; Mao, Qian; Ma, Yu-Guang

    2011-03-01

    Spongy textures are observed in mantle peridotite xenoliths hosted in Cenozoic kamafugites from the Western Qinling, central China. These textures are mainly developed in clinopyroxenes and spinels, and occur as spongy rims consisting of low-Na clinopyroxene, ilmenite, and bubbles, enclosing nonspongy cores. The ilmenites and bubbles exhibit shapes and sizes that vary with the width of the spongy rims. The spongy-textured minerals preserve primary shapes and well-defined grain boundaries and do not show apparent interaction with contact minerals or observed melts except the subsequent melts forming melt pockets. The xenocrysts display reactive zoning textures with host magmas rather than spongy textures. Compositionally, the spongy rims are enriched in Ca, Ti, and most trace elements, have high Cr#, and are depleted in Na, Al, Fe, AlVI, and AlIV/AlVI compared with the cores. These observations suggest that melts/host magmas did not play any significant role in the formation of the spongy textures. We therefore propose that spongy-textured clinopyroxenes and spinels in Western Qinling peridotite xenoliths developed from a decompression-induced partial melting event prior to formation of melt pockets and xenolith entrainment in host magmas.

  16. Origin of melt pockets in mantle xenoliths from southern Patagonia, Argentina

    NASA Astrophysics Data System (ADS)

    Aliani, Paola; Ntaflos, Theodoros; Bjerg, Ernesto

    2009-12-01

    Peridotite mantle xenoliths collected north of Gobernador Gregores, Patagonia, affected by cryptic and modal metasomatism bear melt pockets of unusually large size. Melt pockets consist of second generation olivine (ol2), clinopyroxene (cpx2) and spinel (sp2) ± relict amphibole (amph) immersed in a yellowish vesicular glass matrix. Amphibole breakdown was responsible for melt pocket generation as suggested by textural evidence and proved by consistent mass-balance calculations: amph → cpx2 + ol2 + sp2 + melt. Composition of calculated amphibole in amphibole-free melt pockets is very similar to that measured in amphibole-bearing melt pockets from the same xenolith, i.e. amphibole was consumed in the melt pocket generation process. In melt pockets devoid of relict amphibole, mass-balance calculations show remarkable differences between the calculated amphibole and the measured amphibole compositions in melt pockets from the same xenolith. The participation of minor proportions of a consumed reactant phase could be a reasonable explanation. In some samples the calculated phase proportion of glass is in excess compared to modal estimations based on backscattered electron images, probably because a portion of the generated melt was able to migrate out of the melt pockets. Compositional inhomogeneity of cpx2 and variable Ti Kd in cpx2 vs. glass in the same melt pocket reflect fast nucleation and growth and disequilibrium crystallisation, respectively. This and the difference between forsterite content in calculated equilibrium olivine and second generation olivine, suggest that mineral equilibrium was inhibited by rapid quenching of melt pockets.

  17. Metasomatic Enrichment of Oceanic Lithospheric Mantle Documented by Petit-Spot Xenoliths

    NASA Astrophysics Data System (ADS)

    Pilet, S.; Abe, N.; Rochat, L.; Hirano, N.; Machida, S.; Kaczmarek, M. A.; Muntener, O.

    2015-12-01

    Oceanic lithosphere is generally interpreted as mantle residue after MORB extraction. It has been proposed, however, that metasomatism could take place at the interface between the low-velocity zone and the cooling and thickening oceanic lithosphere or by the percolation of low-degree melts produced in periphery of Mid Ocean Ridges. This later process is observed in slow spreading ridges and ophiolites where shallow oceanic lithospheric mantle could be metasomatized/refertilized during incomplete MORB melt extraction. Nevertheless, direct evidence for metasomatic refertilization of the deep part of the oceanic lithospheric mantle is still missing. Xenoliths and xenocrysts sampled by petit-spot volcanoes interpreted as low-degree melts extracted from the base of the lithosphere in response to plate flexure, provide important new information about the nature and the processes associated with the evolution of oceanic lithospheric mantle. Here, we report, first, the presence of a garnet xenocryst in petit-spot lavas from Japan characterized by low-Cr, low-Ti content and mostly flat MREE-HREE pattern. This garnet is interpreted as formed during subsolidus cooling of pyroxenitic or gabbroic cumulates formed at ~1 GPa during the incomplete melt extraction at the periphery of the Pacific mid-ocean ridge. It is the first time that such processes are documented in fast spreading context. Second, we report petit-spot mantle xenoliths with cpx trace element "signatures" characterized by high U, Th, relative depletion in Nb, Pb, Ti and high but variable LREE/HREE ratio suggesting equilibration depth closed to the Gt/Sp transition zone. Such "signatures" are unknown from oceanic settings and show unexpected similarity to melt-metasomatized gt-peridotites sampled by kimberlites. This similarity suggests that metasomatic processes are not restricted to continental setting, but could correspond to a global mechanism at the lithosphere-asthenosphere boundary. As plate flexure

  18. Major and trace element whole rock and mineral chemistry of Southern Patagonian Mantle Xenoliths

    NASA Astrophysics Data System (ADS)

    Mundl, Andrea; Ntaflos, Theodoros; Bjerg, Ernesto; Hauzenberger, Christoph

    2013-04-01

    The Pali Aike Volcanic Field (PAVF) situated in the back-arc tectonic setting of southern Patagonia is one of the two Patagonian localities where garnet and spinel peridotites are brought to the surface by alkaline basalts. The other locality is Prahuaniyeu in northern Patagonia. The xenoliths from the PAVF were collected at Salsa, El Ruido and Potrok Aike mare and are spinel- and spinel-garnet-lherzolites, followed by spinel- and spinel-garnet harzburgites. Textures are protogranular to protogranular-equigranular with some samples slightly foliated. None of the studied xenoliths contain hydrous phases such as phlogopite and/or amphibole but few contain secondary interstitial clinopyroxenes and melt pockets. Inclusions of spinel in some garnets suggest transition from spinel to garnet peridotite stability field. Frequently, Opx show exsolution lamellae of Cpx (+/- Sp) and vice versa indicating subsolidus cooling. Occasionally, Cpx show spongy rims suggesting either decompression or reaction with infiltrating melt. Whole rock Al2O3 and CaO contents range from 0.63 to 3.54 wt% and 0.24 to 2.90 wt%, respectively. The variation diagrams of CaO and Al2O3 versus MgO exhibit a linear correlation that could be interpreted as residuals after extraction of melts with different degrees of partial melting from a common mantle source. Modeled fractional melting degrees for Salsa, El Ruido and Potrok Aike Sp-peridotites are 6.5 - 12%, 4 - 17% and 5.5 - 9%, respectively. Olivine compositions vary between Fo89.51 - Fo92.04. However, one El Ruido Sp-harzburgite and one Gt-harzburgite have lower Fo-contents (Fo88.88 -Fo89.11) indicating either a cumulate nature of the xenoliths or a change in chemical composition of the rocks due to reaction with infiltrating melts. Primary Cpx are Cr-diopsides with mean compositions En47-50Fs4-5Wo48-45 in lherzolites, En49-53Fs6-5Wo44-41 in harzburgites, and En47Fs3Wo50 in the El Ruido dunite. Cpx Al2O3 and Cr2O3 contents in all rock types vary

  19. Sapphirine and fluid inclusions in Tel Thanoun mantle xenoliths,Syria

    NASA Astrophysics Data System (ADS)

    Bilal, Ahmad

    2016-04-01

    Volcanoes along the Syrian rift, which extend a distance of about 1000 km, brought to the surface mantle xenoliths within erupted basalts, during multiples periods of volcanic activity. Xenoliths in early Cretaceous volcanoes originate in the garnet peridotite field of the subcontinental mantle, whereas those in recent Cenozoic volcanoes, the prime object of this study, are issued from shallower levels (spinel peridotite field). The recent discovery of sapphirine-bearing websterite in Tel Thanoun, a small volcanic diatreme inside the larger Quaternary volcanic field (Djebel Al Arab), allows us to estimate the P-T evolution and fluid-rock interaction at the volcanic source. Harzburgites and lherzolites are equilibrated at a temperature of about 1000 °C at a depth of 35-40 km. Sapphirine appears to have formed during cooling, at depth at a temperature of about 900 °C, at a time where spinel exsolution occurred in harzburgite and lherzolite pyroxenes. This occurred in the presence of a high-density pure CO2 fluid phase, still present in primary fluid inclusions. The highly-aluminous sapphirine-bearing protolith might be former garnet websterite (possibly uplifted during cretaceous magmatism), which resided and cooled in the spinel peridotite stability field, and was then dragged and brought to the surface by quaternary basalts.

  20. Garnet and chromite- bearing mantle peridotite xenoliths from Komsomolskaya pipe, Alakit field, Yakutia

    NASA Astrophysics Data System (ADS)

    Ashchepkov, Igor; Logvinova, Alla; Valdykin, Nikolai; Makovchuk, Igor; Karpenko, Mikhail; Spetsius, Zdislav; Khmelnikova, Olga; Palessky, Stanislav

    2014-05-01

    Fresh xenoliths in Alakit field in Siberian platform are rare. In the xenoliths from Komsomolskaya pipe there were found 30 xenoliths with fresh Cpx, Gar as well as chromites, phlogopites and ilmenites which allows to construct the Cpx - based geotherm which was before constructed for the Yubileynaya pipe (Ashchepkov et al., 2004). The comparison of the garnets from the breccias and porphyric kimberlites show more depleted and Cr- rich varieties of garnets as it is common for the other pipes. Only relatively fresh material from the dark - grey breccia good relatively fresh xenoliths could be used for the mineral thermobarometry. Large xenolths from the Komsomolskaya pipe belong mainly to the Gar harzburgite or refertilized lherzolite types as also detected on the Cr2O3 - CaO diagram where they belong mainly to the 5-11% Cr2O3 interval. The low Cr varieties are mainly referring to the Fe- enriched pyroxenites or to Phl metasomatites. In SCLM beneath Komsomolskaya pipe is essentially more heated then those beneath Yubileynaya and Sytykanskaya pipes and in lower part they are close to the PTXFO2 are closer in conditrions to the ilmenites which determined the . Peridotites from the lithosphere base (7-6GPa) are enriched in Fe and belong to the porphyroclustic or deformed type by chemistry Fe# =0.14-0.15. the relatively HT conditions were determined also for the peridotites from the 5.0-4. GPa. The most of the Cpx- refertilized varieties give the conditions of the middle part of the mantle section. Their garnest are enriched in CaO probably reflection reactions with the Ca- rich protokimberlites. The Na- richterite bearin xenoliths are from the same PT interval The cold clot in the 60-5.5 GPa (34 mwm-2) are represented by the peridotites of low Fe# 7-9 Fe- low peridotites with the garnets of sub-Ca types. But there are also varieties of reduced Cr and the Fe-enriched which are closer to the pyroxenites or Phl metasomatites which in Pt are from the upper part of mantle

  1. The redox conditions of anhydrous and hydrous xenoliths of suprasubduction and intraplate lithospheric mantle

    NASA Astrophysics Data System (ADS)

    Bonadiman, C.; Coltorti, M.

    2012-12-01

    The oxidation state of the upper mantle, its relationship with C-H-O fluids speciation and tectonic settings has been debated for decades and the various modelling have considered the prevalent role of the hydrous minerals over nominally anhydrous minerals (and the opposite) as well as the dissolution of silicate minerals (as providers of Fe3+ to the system) as directly related to water activity and oxygen fugacity. Each of these modelling has different implications for mantle rheology, seismic structure, and the evolution of the lithosphere (i.e.: Karato and Jung, 1998, Hirshmann, 2006). Upper mantle is the only part of the Earth's mantle where the oxygen fugacity can be directly measured, its values/variation being dependent on various processes such as partial melting and metasomatism often operating in time and space without solution of continuity. Recent general reviews of oxygen thermobarometry measurements (Forst & McCammon, 2008; Foley, 2011) indicate that the oxygen fugacity at the top of the upper mantle falls within ±2 log units of the fayalite-magnetite-quartz (FMQ) oxygen buffer. There is also a general consensus in considering H2O as the strongest oxidizing agent in mantle metasomatic fluids, its activity leading to the formation of amphibole and raising the mantle redox state. This contribution presents fO2 and water activity results from three spinel-bearing mantle xenolith localities and distinct geodynamic settings: Ichinomegata (Japan) amphibole-bearing peridotites entrained in calc-alkaline basalts and Cerro Fraile (South Patagonia, Argentina), mostly anhydrous lherzolites and pirossenites brought up to the surface by alkaline basalts representing fragments of sub-arc mantle and Baker Rocks, Victoria Land (Antarctica), amphibole-bearing lherzolites representing portion of intraplate subcontinental lithospheric mantle. The three mantle sectors records fO2 values in the range of -1.9 to +0.8 log units of the FQM buffer. and low to very low aH2O

  2. Geochemistry and petrology of spinel lherzolite xenoliths from Xalapasco de La Joya, San Luis Potosi, Mexico: Partial melting and mantle metasomatism

    NASA Astrophysics Data System (ADS)

    Liang, Yan; Elthon, Don

    1990-09-01

    Spinel Iherzolite xenoliths from Xalapasco de La Joya, San Luis Potosi, Mexico, are divided into two distinct groups according to their major element and trace element characteristics. Group Ia xenoliths are characterized by light rare earth element (LREE) depletion ((La/Lu)N = 0.10-0.77 in clinopyroxene) and linear major and compatible trace element relationships. Group Ib xenoliths are characterized by FeO and Na2O enrichment and higher (La/Lu)N ratios (0.80-4.1 in clinopyroxene) and complex major element relationships. These samples, which have a range of equilibrium temperatures of 910°-1070°C, exhibit protogranular textures and typical orthopyroxene+clinopyroxene+spinel clusters. Modal abundances and chemical compositions of the group Ia xenoliths vary from primitive (15.2% clinopyroxene, 38.5% MgO, 1824 ppm Ni) to moderately depleted (6.4-8.7% clinopyroxene, 43.8-44.1% MgO, 2192 ppm Ni). Systematic variations of major elements and compatible trace elements in the group Ia xenoliths are interpreted to result from various degrees (<25%) of partial melting and melt extraction, followed by subsolidus equilibration and annealing. The extracted melts have a range of compositions similar to picritic basalts. Abundances of moderately incompatible trace elements, Sc and Cr, in the group Ia minerals have been substantially redistributed during subsolidus equilibration. In a few of these xenoliths there appears to be vestiges of incipient metasomatism, but metasomatism has not substantially influenced the group as a whole. Group Ib xenoliths have been substantially influenced by metasomatic processes. The ∑FeO and Na2O contents of the cores of clinopyroxenes in group Ib xenoliths are higher than clinopyroxenes in group Ia samples. The higher La contents and La/Lu ratios in group Ib clinopyroxenes (compared to group Ia), together with this FeO and Na2O enrichment, suggest that equilibration of basanites with residual mantle has been a major process in the evolution

  3. Sedimentary halogens and noble gases within Western Antarctic xenoliths: Implications of extensive volatile recycling to the sub continental lithospheric mantle

    NASA Astrophysics Data System (ADS)

    Broadley, Michael W.; Ballentine, Chris J.; Chavrit, Déborah; Dallai, Luigi; Burgess, Ray

    2016-03-01

    Recycling of marine volatiles back into the mantle at subduction zones has a profound, yet poorly constrained impact on the geochemical evolution of the Earth's mantle. Here we present a combined noble gas and halogen study on mantle xenoliths from the Western Antarctic Rift System (WARS) to better understand the flux of subducted volatiles to the sub continental lithospheric mantle (SCLM) and assess the impact this has on mantle chemistry. The xenoliths are extremely enriched in the heavy halogens (Br and I), with I concentrations up to 1 ppm and maximum measured I/Cl ratios (85.2 × 10-3) being ∼2000 times greater than mid ocean ridge basalts (MORB). The Br/Cl and I/Cl ratios of the xenoliths span a range from MORB-like ratios to values similar to marine pore fluids and serpentinites, whilst the 84Kr/36Ar and 130Xe/36Ar ratios range from modern atmosphere to oceanic sediments. This indicates that marine derived volatiles have been incorporated into the SCLM during an episode of subduction related metasomatism. Helium isotopic analysis of the xenoliths show average 3He/4He ratios of 7.5 ± 0.5 RA (where RA is the 3He/4He ratio of air = 1.39 × 10-6), similar to that of MORB. The 3He/4He ratios within the xenoliths are higher than expected for the xenoliths originating from the SCLM which has been extensively modified by the addition of subducted volatiles, indicating that the SCLM beneath the WARS must have seen a secondary alteration from the infiltration and rise of asthenospheric fluids/melts as a consequence of rifting and lithospheric thinning. Noble gases and halogens within these xenoliths have recorded past episodes of volatile interaction within the SCLM and can be used to reconstruct a tectonic history of the WARS. Marine halogen and noble gas signatures within the SCLM xenoliths provide evidence for the introduction and retention of recycled volatiles within the SCLM by subduction related metasomatism, signifying that not all volatiles that survive

  4. Age, origin and significance of SKS splitting in SE Iberia: insights from mantle xenoliths from Neogene alkaline basalts

    NASA Astrophysics Data System (ADS)

    Konc, Z.; Garrido, C. J.; Tommasi, A.; Padron Navarta, J. A.; Hidas, K.; Bosch, D.; Marchesi, C.; Szabó, Cs.

    2012-04-01

    The volcanic activity in the SE Iberian Volcanic Province (SE IVP) is the surface expression of magmatism in a complex geodynamic setting during the Cenozoic development of a Mediterranean-type back-arc basin in the Alboran realm. The late stage of this geodynamical evolution was characterized by Neogene alkaline basalt volcanism erupted at 2-3 Ma in the Tallante and Los Perez (Murcia) volcanic centers. This volcanism entrained numerous mantle xenoliths that provide a recent snapshot of the structure of the lithospheric mantle beneath this region. Xenoliths are spinel (± plagioclase ± amphibole) lherzolite, and minor harzburgite and wehrlite showing porphyroclastic to fine- to medium-grained granoblastic textures. Mantle xenoliths display a marked axial [100] pattern olivine Crystal Preferred Orientation (CPO) characterized by a strong alignment of olivine [100] axes near or parallel to the peridotite lineation and a girdle distribution of [010] axes with a maximum normal to the peridotite foliation. This CPO is consistent with dominant activation of the high temperature [100]{0kl} slip systems of olivine formed under a deformation regime dominated by simple shear or combinations of simple shear and pure shear with a transtensional component. The age of this deformation event is constrained by syn-tectonice composite xenoliths formed by reactive percolation of Si-rich melt/fluids with the lithospheric mantle during middle Miocene subduction/delamination of the paleo-iberian margin beneath the SE IVP. In order to investigate whether SKS anystropy recently measured beneath this region can be accounted by the olivine CPO of the lithospheric mantle, we have computed the theoretical seismic anisotropy of mantle xenoliths from their olivine CPOs and modal compositions. The averaged seismic properties of SE-IVP mantle xenoliths are characterized by fast propagation of P-waves and polarization of fast S-waves parallel to the peridotite lineation. The computed highest S

  5. Geochemical and isotopic characterization of mantle xenoliths from the back arc region of north Patagonia

    NASA Astrophysics Data System (ADS)

    Mundl, Andrea; Ntaflos, Theodoros; Lukás, Ackerman; Bizimis, Michael; Bjerg, Ernesto; Hauzenberger, Christoph

    2015-04-01

    Mantle xenoliths brought to the surface by alkali basalts in north Patagonia, Argentina, were studied for their geochemical and isotopic compositions. Samples were collected at four different outcrops within the North Patagonian Massif in Rio Negro Province. The studied sample suite from Comallo (COM), Puesto Diaz (PD), Cerro Chenque (CH) and Prahuaniyeu (PRA) comprises anhydrous spinel-harzburgites, spinel-dunites, as well as one PRA spinel-garnet-lherzolite. Bulk rock Al2O3 and CaO compositions of the studied samples lie within a narrow range of 0.68 to 1.31 wt. % and 0.36 to 1.25 wt. %, respectively. Al2O3 and CaO decrease with increasing MgO ranging from 44.7 to 48.0 wt. %. The single spinel-garnet lherzolite has a more fertile composition with CaO, Al2O3 and MgO of 2.78 wt. %, 2.35 wt. % and 42.2 wt. %, respectively. With the exception of the PRA region, the studied mantle xenoliths represent an overall highly depleted lithospheric mantle underneath northern Patagonia (up to 25% melt extraction). The rock forming minerals show concentrations typical for depleted peridotites with mg# that range from 0.91 to 0.92 for olivine and orthopyroxene and mg# from 0.91 to 0.95 for clinopyroxene. Variably elevated clinopyroxene trace element compositions in all studied north Patagonian mantle samples indicate reactions with a chromatographically fractionated melt after the initial melt depletion event. A percolating melt reacts with a strongly depleted mantle peridotite as it propagates through the wall rock and subsequently changes its composition. These melt-rock reactions lead to a compositionally evolving melt and variably altered mantle sections, which is strongly reflected in primitive mantle normalized clinopyroxene trace element patterns. Bulk rock and mineral compositions, on the other hand, suggest that this melt percolation event has not significantly affected major element systematics. Contrary, isotope and HSE systematics also indicate melt-rock reactions. In

  6. Petrological and geochemical studies of mantle xenoliths from La Palma, Canary Islands

    NASA Astrophysics Data System (ADS)

    Janisch, Astrid; Ntaflos, Theodoros

    2015-04-01

    La Palma is the second youngest island, after El Hierro, of the Canary archipelago. The archipelago consists of seven large islands, forming an east-west-trending island chain, and several seamounts. All together they form a volcanic belt of around 800 km length and 450 km width, which presumably comprises roughly the Canary hotspot. The islands are located off the western coast of Morocco, Africa. The distance ranges from 100 km to 500 km. Concurrently with the distance, subaerial volcanism age progresses from the oldest lava in the east to the youngest in the west of the archipelago. Presently, La Palma is in the shield building stage of growth (alongside with El Hierro and Tenerife) and is furthermore the fastest growing island of the Canary archipelago. Historical volcanic eruptions are restricted on the younger islands, La Palma and El Hierro, with the last eruption at the south end of La Palma in 1971. Mantle xenoliths described in this work were collected at the slopes of San Antonio Volcano, Fuencaliente, brought to the surface during the 1677/1678 eruption. The mantle xenolith collection comprises sp-lherzolites, sp-harzburgites and pyroxenites. The texture can be distinguished between coarse-grained matrix and fine-grained veins in various thicknesses, mostly with olivine and pyroxene but also with amphibole, phlogopite as well as apatite. Mineral analyses reveal the existence of primary and secondary ol, cpx and opx. Primary ol has Fo contents of 89.2 to 91.7 and NiO ranging from 0.3 to 0.45 wt.%, whereas secondary ol show Fo values of 78.4 to 91.9 but with NiO below 0.3 wt.%. Primary cpx are predominantly Cr-Diopsides with En48.7-51.9-Wo43.5-44.3-Fs4.1-4.9 and Mg# of 91.5 to 92.4. Secondary cpx, primarily Ti-Augit, display En36.7-44.4-Wo47.7-49.6-Fs6.7-13.0 and Mg# of 75.3 to 90.8. Primary opx compositions are in range of En89.3-90.6-Wo1.3-1.5-Fs8.1-9.3 with Mg# between 90.7 and 92.0. Secondary opx exhibit En88.7-89.2-Wo1.7-1.9-Fs9.1-9.5 and Mg# of 90

  7. Application of thermodynamic modelling to natural mantle xenoliths: examples of density variations and pressure-temperature evolution of the lithospheric mantle

    NASA Astrophysics Data System (ADS)

    Ziberna, L.; Klemme, S.

    2016-02-01

    In this paper, we show how the results of phase equilibria calculations in different mantle compositions can be reconciled with the evidence from natural mantle samples. We present data on the response of bulk rock density to pressure (P), temperature (T) and compositional changes in the lithospheric mantle and obtain constraints on the P-T evolution recorded by mantle xenoliths. To do this, we examine the mantle xenolith suite from the Quaternary alkali basalts of Pali-Aike, Patagonia, using phase equilibria calculation in six representative compositions. The calculations were done subsolidus and in volatile-free conditions. Our results show that the density change related to the spinel peridotite to garnet peridotite transition is not sharp and strongly depends on the bulk composition. In a depleted mantle composition, this transition is not reflected in the density profile, while in a fertile mantle it leads to a relative increase in density with respect to more depleted compositions. In mantle sections characterized by hot geothermal gradients (~70 mW/m2), the spinel-garnet transition may overlap with the lithosphere-asthenosphere boundary. Phase equilibria calculations in peridotitic compositions representative of the Pali-Aike mantle were also used to constrain the origin and evolution of the mantle xenoliths. Our results indicate that the mineral modes and compositions, and the mineral zonation reported for the low-temperature peridotites (spinel and spinel + garnet harzburgites and lherzolites), are linked to a cooling event in the mantle which occurred long before the eruption of the host basalts. In addition, our phase equilibria calculations show that kelyphitic rims around garnets, as those observed in the high-temperature garnet peridotites from Pali-Aike, can be explained simply by decompression and do not require additional metasomatic fluid or melt.

  8. Argon isotopic studies of minerals in kimberlites, mantle xenoliths and diamonds, from selected southern African localities

    SciTech Connect

    Phillips, D.

    1989-01-01

    The occurrence, composition, behavior and origin of excess argon components, in mantle phases hosted by southern African kimberlites, is evaluated using furnace step-heating and laser-probe analytical techniques. Laser-probe {sup 40}Ar/{sup 39}Ar analyses of phlogopite from the swartruggens kimberlite dyke (145 Ma), and Premier diatreme ({approximately}1200 Ma) lherzolite xenoliths, yielded apparent ages decreasing from high ages at grain centres to values approaching the age of kimberlite intrusion, along grain margins. The old apparent ages are attributed excess radiogenic argon, with high {sup 40}Ar/{sup 39}Ar ratios (> 15,000), incorporated prior to kimberlite intrusion under conditions of locally high argon partial pressure. The preservation of the excess argon components is dependent on the timing of melt devolatilization, temperature, cooling rate and the characteristic radius for argon diffusion. Swartruggens phlogopite grains also display chlorine zonations, measured by a neutron activation technique and the laser probe. Fluorine contents, determined by electron microprobe were uniform. Halogen analyses of Premier xenolith phlogopite revealed minor variations. {sup 40}Ar/{sup 39}Ar laser-probe analyses of eclogitic suite clinopyroxene inclusions in diamonds from the Premier kimberlite yielded an age of 1198 {plus minus} 6 Ma, indistinguishable from the inferred time of intrusion of the host kimberlite ({approximately}1200 Ma). This implies diamond formation synchronous with, or no more than {approximately}20 Ma before kimberlite generation. The associated initial {sup 40}Ar/{sup 39}Ar ratio of 334 {plus minus} 50 is similar to the present day atmospheric composition. It is suggested that late-stage equilibration with {sup 36}Ar-rich fluids, derived either from primordial mantle, or from subducted atmospheric argon, is the most likely explanation for this low {sup 40}Ar/{sup 39}Ar value.

  9. Geochemistry of peridotite xenoliths in basalt from Hannuoba, eastern China: Implications for subcontinental mantle heterogeneity

    SciTech Connect

    Yan Song; Frey, F.A. )

    1989-01-01

    Based on geochemical studies of six anhydrous spinel peridotite xenoliths in basanite, the upper mantle beneath Hannuoba, eastern China is compositionally heterogeneous. These samples range in Sr and Nd isotopic ratios from MORB-like to near bulk-earth estimates. The low {sup 87}Sr/{sup 86}Sr and high {sup 143}Nd/{sup 144}Nd samples contain the largest amount of a basaltic component, but they are relatively depleted in light rare earth elements compared to chondrites. Other samples have U-shaped chondrite-normalized REE patterns. Trace element and radiogenic isotopic data require enrichment processes acting on depleted mantle. Constraints on these processes are: (a) inverse correlations between basaltic constituents, such as CaO and Al{sub 2}O{sub 3}, and La/Sm; and, (b) samples most depleted in CaO and Al{sub 2}O{sub 3} have the highest {sup 87}Sr/{sup 86}Sr and lowest {sup 143}Nd/{sup 144}Nd. These trends can be explained by a model whereby garnet peridotite zoned in isotopic composition undergoes partial melting. Because of a gradient in degree of melting, e.g., from the wall-rock contact to hotter interior, or as a function of depth in a diapir, melts initially segregate from regions where the degree of melting is high. Subsequently, the recently created residues are infiltrated by slower segregating incipient melts. Preferential mixing of these incipient melts with residues from high degrees of melting can explain the observed complex geochemical trends seen in Hannuoba and many other peridotite xenolith suites. Clinopyroxene-rich veins in some of the peridotites may reflect pathways of ascending melt.

  10. Geochemical and isotopic characteristics of lithospheric mantle beneath West Kettle River, British Columbia: Evidence from ultramafic xenoliths

    SciTech Connect

    Xue, Xianyu; Baadsgaard, H.; Scarfe, C.M. ); Irving, A.J. )

    1990-09-10

    A group of spinel peridotite xenoliths from West Kettle River, British Columbia, represents essentially undepleted to moderately depleted lithospheric mantle rocks in terms of major and compatible trace elements. Whole rock Sr isotopic composition for most of these xenoliths, and whole rock Sm-Nd isotopic composition and LREE contents for some of them, seem to have been perturbed by near-surface processes. Sr and Nd isotopic results for acid-cleaned clinopyroxenes separated from these spinel peridotites reveal an isotopically mid-ocean ridge basalt (MORB)-like mantle. Seven spinel lherzolites gave Nd model ages of 1.5-3.6 Ga, similar to MORB, and on a Sm-Nd isotope diagram plot close to a reference Nd isochron with an age of 0.7 Ga and an initial {var epsilon}{sub Nd} of +7. These features likely resulted from multiple mantle depletion. The isotopic similarities of these xenoliths with MORB suggest that this area is underlain by oceanic lithospheric mantle, possibly accreted to North America during the mid-Jurassic. The Nd isochron age could record the time when the oceanic lithosphere was isolated from the asthenosphere. Recent enrichment event may have acted on such a depleted mantle, as indicated by the low Sm/Nd ratios of two spinel harzburgites.

  11. The role of CO2-rich fluids in trace element transport and metasomatism in the lithospheric mantle beneath the Central Pannonian Basin, Hungary, based on fluid inclusions in mantle xenoliths

    NASA Astrophysics Data System (ADS)

    Berkesi, Márta; Guzmics, Tibor; Szabó, Csaba; Dubessy, Jean; Bodnar, Robert J.; Hidas, Károly; Ratter, Kitti

    2012-05-01

    Upper mantle peridotite xenoliths from the Tihany Maar Volcanic Complex, Bakony-Balaton Highland Volcanic Field (Central Pannonian Basin, Hungary) contain abundant pyroxene-hosted negative crystal shaped CO2-rich fluid inclusions. The good correlation between enrichment of the clinopyroxenes in Al2O3, TiO2, Na2O, MREE and Zr, and the presence of fluid inclusions in the xenoliths provide strong evidence for fluid-related cryptic metasomatism of the studied xenoliths. The FIB-SEM (focused ion beam-scanning electron microscopy) exposure technique revealed a thin glass film, covering the wall of the fluid inclusions, which provides direct evidence that the silicate components were formerly dissolved in the CO2-rich fluid phase. This means that at upper mantle conditions CO2-rich fluids are capable of transporting trace and major elements, and are the agents responsible for cryptic metasomatism of the peridotite wall rock. Several daughter phases, including magnesite, quartz and sulfide, were identified in the fluid inclusions. Magnesite and quartz are the products of a post entrapment carbonation reaction, whereby the reactants are the CO2-rich fluid and the host orthopyroxene. It is likely that the thin glass film prevented or arrested further growth of the magnesite and quartz by isolating the fluid from the host orthopyroxene, resulting in the preservation of residual CO2 in the fluid inclusions.

  12. Helium isotopes of the Siberian sub-continental lithospheric mantle: Insights from eclogite xenoliths

    NASA Astrophysics Data System (ADS)

    Barry, P. H.; Hilton, D. R.; Day, J. M.; Pernet-Fisher, J.; Howarth, G. H.; Taylor, L. A.

    2014-12-01

    Helium isotopes (3He/4He) have been extensively used to define distinct segments of Earth's mantle and characterize its chemical structure. Specifically, they have been used to illustrate the long-term isolation and preservation of high-3He/4He (≥50 RA; [1]) plume-derived materials from the well-mixed and more-extensively degassed depleted MORB mantle (DMM) (8 RA; [2]). However, the He-isotope signature of the sub-continental lithospheric mantle (SCLM) remains relatively poorly characterized (6.1 RA; [3]). The Siberian craton hosts >1000 kimberlite intrusions, which carry mantle-derived xenoliths - of varying compositions (i.e., peridotites, dunites, and eclogites) - to the Earth's surface, making it an ideal setting for investigating the chemical evolution of the SCLM. Here, we report new He-isotope and concentration data for a suite of eclogitic xenoliths (n=10) from the Udachnaya pipe, Siberia. He-isotopes and [He] contents were determined by crushing garnet and pyroxene mineral separates from 2.7-3.1 Ga Siberian eclogites. 3He/4He values ranged from 0.11 to 1.0 RA, displaying predominantly radiogenic (i.e., low 3He/4He) He-isotope values. In contrast, Siberian flood basalt values extend up to ~13 RA [4]. Helium concentrations span ~4 orders of magnitude from 60 to 569,000 [4He]C ncm3STP/g. The radiogenic nature of Udachnaya eclogites indicate that they have been largely isolated from basaltic metasomatic fluxes over geological time due to position within the lithosphere and/or lithospheric age. Further, low 3He/4He values may reflect the addition of high U-Th material into the lithosphere by accretion of ancient island-arc terrains. These new data add to the growing He-isotope database [5,6] for the Siberian SCLM, and reveal the heterogeneous nature of this region with respect to He-isotopes, as well as the potential importance of crustal recycling and metasomatic processes. [1] Stuart et al., 2003. Nature. [2] Graham, 2002. Reviews in Mineralogy and

  13. Metasedimentary and igneous xenoliths from Tallante (Betic Cordillera, Spain): Inferences on crust-mantle interactions and clues for post-collisional volcanism magma sources

    NASA Astrophysics Data System (ADS)

    Bianchini, Gianluca; Braga, Roberto; Langone, Antonio; Natali, Claudio; Tiepolo, Massimo

    2015-04-01

    The deep seated xenolith association exhumed in the Pliocenic volcano of Tallante (Betic Cordillera, Spain) includes protogranular mantle peridotites, felsic (metasedimentary) crustal rocks, as well as cumulus igneous rocks such as norites and amphibole (± phlogopite)-clinopyroxenites. The whole xenolith suite equilibrated at the same pressure (0.7-0.9 GPa) representing the local crust-mantle boundary (MOHO) characterized by extreme lithological heterogeneity. This heterogeneity resulted from orogenic processes that induced the juxtaposition of crustal rocks (variably depleted in fusible components) within mantle domains including metasomes, as it is commonly observed in orogenic mantle massifs of the Mediterranean area. In this contribution, we report new mineral compositions of igneous parageneses recorded in these xenoliths, and we present Sr-Nd isotope data on both igneous and metasedimentary xenoliths that integrate those from the literature. Sr-Nd isotopes coherently indicate a restitic character of the metasedimentary xenoliths, which according to model ages were affected by partial melting in Paleozoic times. Sr-Nd isotopic errorchrons on the igneous xenoliths, on the other hand, qualitatively indicate Tertiary ages, which are corroborated by U-Pb zircon datings of one norite xenolith and two composite xenoliths having zircon-bearing norite veinlets. The new data are discussed proposing that MOHO lithologies of Tallante could provide significant source compositions for the genesis of the Neogene volcanics of the Betic area, which included calcalkaline lavas as well as more potassic products such as lamproites.

  14. Rheological properties of the lower crust and upper mantle beneath Baja California: a microstructural study of xenoliths from San Quintin

    NASA Astrophysics Data System (ADS)

    Van der Werf, Thomas F.; Chatzaras, Vasileios; Tikoff, Basil; Drury, Martyn R.

    2016-04-01

    Baja California is an active transtensional rift zone, which links the San Andreas Fault with the East Pacific Rise. The erupted basalts of the Holocene San Quintin volcanic field contain xenoliths, which sample the lower crust and upper mantle beneath Baja California. The aim of this research is to gain insight in the rheology of the lower crust and the upper mantle by investigating the xenolith microstructure. Microstructural observations have been used to determine the dominant deformation mechanisms. Differential stresses were estimated from recrystallized grain size piezometry of plagioclase and clinopyroxene for the lower crust and olivine for the upper mantle. The degree of deformation can be inferred from macroscopic foliations and the deformation microstructures. Preliminary results show that both the lower crust and the upper mantle have been affected by multiple stages of deformation and recrystallization. In addition the dominant deformation mechanism in both the lower crust and the upper mantle is dislocation creep based on the existence of strong crystallographic preferred orientations. The differential stress estimates for the lower crust are 10-29 MPa using plagioclase piezometry and 12-35 MPa using clinopyroxene piezometry. For the upper mantle, differential stress estimates are 10-20 MPa. These results indicate that the strength of the lower crust and the upper mantle are very similar. Our data do not fit with the general models of lithospheric strength and may have important implications for the rheological structure of the lithosphere in transtensional plate margins and for geodynamic models of the region.

  15. Water in the Lithospheric Mantle Beneath a Phanerozoic Continental Belt: FTIR Analyses of Alligator Lake Xenoliths (Yukon, Canada)

    NASA Technical Reports Server (NTRS)

    Gelber, McKensie; Peslier, Ann H.; Brandon, Alan D.

    2015-01-01

    Water in the mantle influences melting, metasomatism, viscosity and electrical conductivity. The Alligator Lake mantle xenolith suite is one of three bimodal peridotite suites from the northern Canadian Cordillera brought to the surface by alkali basalts, i.e., it consists of chemically distinct lherzolites and harzburgites. The lherzolites have equilibration temperatures about 50 C lower than the harzburgites and are thought to represent the fertile upper mantle of the region. The harzburgites might have come from slightly deeper in the mantle and/or be the result of a melting event above an asthenospheric upwelling detected as a seismic anomaly at 400-500 km depth. Major and trace element data are best interpreted as the lherzolite mantle having simultaneously experienced 20-25% partial melting and a metasomatic event to create the harzburgites. Well-characterized xenoliths are being analyzed for water by FTIR. Harzburgites contain 29-52 ppm H2O in orthopyroxene (opx) and (is) approximately140 ppm H2O in clinopyroxene (cpx). The lherzolites have H2O contents of 27-150 ppm in opx and 46-361 ppm in cpx. Despite correlating with enrichments in LREE, the water contents of the harzburgite pyroxenes are low relative to those of typical peridotite xenoliths, suggesting that the metasomatic agents were water-poor, contrarily to what has been suggested before. The water content of cpx is about double that of opx indicating equilibrium. Olivine water contents are low ((is) less than 5 ppm H2O) and out of equilibrium with those of opx and cpx, which may be due to H loss during xenolith ascent. This is consistent with olivines containing more water in their cores than their rims. Olivines exclusively exhibit water bands in the 3400-3000 cm-1 range, which may be indicative of a reduced environment.

  16. Metasomatic processes in the mantle beneath the Arkhangelsk province, Russia: evidence from garnet in mantle peridotite xenoliths, Grib pipe

    NASA Astrophysics Data System (ADS)

    Kargin, Alexei; Sazonova, Lyudmila; Nosova, Anna; Kovalchuk, Elena; Minevrina, Elena

    2015-04-01

    The Arkhangelsk province is located in the northern East European Craton and includes more than 80 bodies of kimberlite, alkaline picrite and other ultramafic and mafic rocks. They erupted through the Archean-Early Proterozoic basement into the Riphean-Paleozoic sedimentary cover. The Grib kimberlite pipe is located in the central part of the Arkhangelsk province in the Verkhotina (Chernoozerskoe) kimberlite field. The age of the Grib kimberlite is 376+-3 Ma (Rb-Sr by phlogopite). The Grib kimberlite pipe is the moderate-Ti kimberlites (TiO2 1-2 wt %) with strongly fractionated REE pattern , (La/Yb)n = 38-87. The Nd isotopic composition of the Grib pipe ranges epsilon Nd from -0.4 to + 1.0 and 87Sr/86Sr(t) from 0.7042 to 0.7069 (Kononova et al., 2006). Geochemical (Jeol JXA-8200 electron microprobe; SIMS; LA-ICP-MS) composition of clinopyroxene and garnet from mantle-derived xenoliths of the Grib kimberlite pipe was studied to provide new insights into metasomatic processes in the mantle beneath the Arkhangelsk province. Based on both major and trace element data, five geochemical groups of peridotitic garnet were distinguished. The partial melting of metasomatic peridotite with crystallization of a garnet-clinopyroxene association, and orthopyroxene assimilation by protokimberlitic melts was simulated and a model of garnet and clinopyroxene metasomatic origin was proposed. The model includes three stages: 1. Mantle peridotite was fertilized by subduction-derived sediment partial melts/fluids at the lithosphere-asthenosphere boundary to yield a CO2-bearing mantle peridotite (source I). 2. The partial melting of the carbonate-bearing mantle source 1 produced carbonatite-like melts (a degree of partial melting was 1,5 %), which could form the carbonatite-kimberlite rocks of the Mela River (Arkhangelsk province, 50 km North-West of Grib kimberlite) and also produce the metasomatic reworking of (carbonate-bearing) mantle peridotite (mantle source II) and form type-1

  17. Exploring the Moon's surface for remnants of the lunar mantle 1. Dunite xenoliths in mare basalts. A crustal or mantle origin?

    NASA Astrophysics Data System (ADS)

    Shearer, Charles K.; Burger, Paul V.; Bell, Aaron S.; Guan, Yunbin; Neal, Clive R.

    2015-08-01

    Remotely sensed observations from recent missions (e.g., GRAIL, Kaguya, Chandrayaan-1) have been interpreted as indicating that the deep crust and upper mantle are close to or at the lunar surface in many large impact basins (e.g., Crisium, Apollo, Moscoviense). If this is correct, the capability of either impact or volcanic processes to transport mantle lithologies to the lunar surface should be enhanced in these regions. Somewhat problematic to these observations and interpretations is that examples of mantle lithologies in the lunar sample collection (Apollo Program, Luna Program, lunar meteorites) are at best ambiguous. Dunite xenoliths in high-Ti mare basalt 74275 are one of these ambiguous examples. In this high-Ti mare basalt, olivine occurs in three generations: olivine associated with dunite xenoliths, olivine megacrysts, and olivine microphenocrysts. The dunite xenoliths are anhedral in shape and are generally greater than 800 μm in diameter. The interior of the xenoliths are fairly homogeneous with regard to many divalent cations. For example, the Mg# (Mg/Mg + Fe × 100) ranges from 82 to 83 in their interiors and decreases from 82 to 68 over the 10-30 μm wide outer rim. Titanium and phosphorus X-ray maps of the xenolith illustrate that these slow diffusing elements preserve primary cumulate zoning textures. These textures indicate that the xenoliths consist of many individual olivine grains approximately 150-200 μm in diameter with low Ti, Al, and P cores. These highly incompatible elements are enriched in the outer Fe-rich rims of the xenoliths and slightly enriched in the rims of the individual olivine grains. Highly compatible elements in olivine such as Ni exhibit a decrease in the rim surrounding the xenolith, an increase in the incompatible element depleted cores of the individual olivine grains, and a slight decrease in the "interior rims" of the individual olivine grains. Inferred melt composition, liquid lines of descent, and zoning profiles

  18. Abundances of Ag and Cu in mantle peridotites and the implications for the behavior of chalcophile elements in the mantle

    NASA Astrophysics Data System (ADS)

    Wang, Zaicong; Becker, Harry

    2015-07-01

    Silver abundances in mantle peridotites and the behavior of Ag during high temperature mantle processes have received little attention and, as a consequence, the abundance of Ag in the bulk silicate Earth (BSE) has been poorly constrained. In order to better understand the processes that fractionate Ag and other chalcophile elements in the mantle, abundances of Ag and Cu in mantle peridotites from different geological settings (n = 68) have been obtained by isotope dilution ICP-MS methods. In peridotite tectonites and in a few suites of peridotite xenoliths which display evidence for variable extents of melt depletion and refertilization by silicate melts, Ag and Cu abundances show positive correlations with moderately incompatible elements such as S, Se, Te and Au. The mean Cu/Ag in fertile peridotites (3500 ± 1200, 1s, n = 38) is indistinguishable from the mean Cu/Ag of mid ocean ridge basalts (MORB, 3600 ± 400, 1s, n = 338) and MORB sulfide droplets. The constant mean Cu/Ag ratios indicate similar behavior of Ag and Cu during partial melting of the mantle, refertilization and magmatic fractionation, and thus should be representative of the Earth's upper mantle. The systematic fractionation of Cu, Ag, Au, S, Se and Te in peridotites and basalts is consistent with sulfide melt-silicate melt partitioning with apparent partition coefficients of platinum group elements (PGE) > Au ⩾ Te > Cu ≈ Ag > Se ⩾ S. Because of the effects of secondary processes, the abundances of chalcophile elements, notably S, Se, but also Cu and the PGE in many peridotite xenoliths are variable and lower than in peridotite massifs. Refertilization of peridotite may change abundances of chalcophile and lithophile elements in peridotite massifs, however, this seems to mostly occur in a systematic way. Correlations with lithophile and chalcophile elements and the overlapping mean Cu/Ag ratios of peridotites and ocean ridge basalts are used to constrain abundances of Ag and Cu in the BSE

  19. Nature of the lithospheric mantle beneath the Arabian Shield and genesis of Al-spinel micropods: Evidence from the mantle xenoliths of Harrat Kishb, Western Saudi Arabia

    NASA Astrophysics Data System (ADS)

    Ahmed, Ahmed H.; Moghazi, Abdel Kader M.; Moufti, Mohamed R.; Dawood, Yehia H.; Ali, Kamal A.

    2016-01-01

    The Harrat Kishb area of western Saudi Arabia is part of the Cenozoic volcanic fields in the western margin of the Arabian Shield. Numerous fresh ultramafic xenoliths are entrained in the basanite lava of Harrat Kishb, providing an opportunity to study the nature and petrogenetic processes involved in the evolution of the lithospheric mantle beneath the Arabian Shield. Based on the petrological characteristics and mineralogical compositions, the majority of the mantle xenoliths (~ 92%) are peridotites (lherzolites and pyroxene-bearing harzburgites); the remaining xenoliths (~ 8%) are unusual spinel-rich wehrlites containing black Al-spinel micropods. The two types of mantle xenoliths display magmatic protogranular texture. The peridotite xenoliths have high bulk-rock Mg#, high forsterite (Fo90-Fo92) and NiO (0.24-0.46 wt.%) contents of olivine, high clinopyroxene Mg# (0.91-0.93), variable spinel Cr# (0.10-0.49, atomic ratio), and approximately flat chondrite-normalized REE patterns. These features indicate that the peridotite xenoliths represent residues after variable degrees of melt extraction from fertile mantle. The estimated P (9-16 kbar) and T (877-1227 °C) as well as the oxidation state (∆logfO2 = - 3.38 to - 0.22) under which these peridotite xenoliths originated are consistent with formation conditions similar to most sub-arc abyssal-type peridotites worldwide. The spinel-rich wehrlite xenoliths have an unusual amount (~ 30 vol.%) of Al-spinel as peculiar micropods with very minor Cr2O3 content (< 1 wt.%). Olivines of the spinel-rich wehrlites have low-average Fo (Fo81) and NiO (0.18 wt.%) contents, low-average cpx Mg# (0.79), high average cpx Al2O3 content (8.46 wt.%), and very low-average spinel Cr# (0.01). These features characterize early mantle cumulates from a picritic melt fraction produced by low degrees of partial melting of a garnet-bearing mantle source. The relatively high Na2O and Al2O3 contents of cpx suggest that the spinel-rich wehrlites

  20. Continuation of the San Andreas fault system into the upper mantle: Evidence from spinel peridotite xenoliths in the Coyote Lake basalt, central California

    NASA Astrophysics Data System (ADS)

    Titus, Sarah J.; Medaris, L. Gordon; Wang, Herbert F.; Tikoff, Basil

    2007-01-01

    The Coyote Lake basalt, located near the intersection of the Hayward and Calaveras faults in central California, contains spinel peridotite xenoliths from the mantle beneath the San Andreas fault system. Six upper mantle xenoliths were studied in detail by a combination of petrologic techniques. Temperature estimates, obtained from three two-pyroxene geothermometers and the Al-in-orthopyroxene geothermometer, indicate that the xenoliths equilibrated at 970-1100 °C. A thermal model was used to estimate the corresponding depth of equilibration for these xenoliths, resulting in depths between 38 and 43 km. The lattice preferred orientation of olivine measured in five of the xenolith samples show strong point distributions of olivine crystallographic axes suggesting that fabrics formed under high-temperature conditions. Calculated seismic anisotropy values indicate an average shear wave anisotropy of 6%, higher than the anisotropy calculated from xenoliths from other tectonic environments. Using this value, the anisotropic layer responsible for fault-parallel shear wave splitting in central California is less than 100 km thick. The strong fabric preserved in the xenoliths suggests that a mantle shear zone exists below the Calaveras fault to a depth of at least 40 km, and combining xenolith petrofabrics with shear wave splitting studies helps distinguish between different models for deformation at depth beneath the San Andrea fault system.

  1. The mantle lithosphere beneath the Hangay dome in central Mongolia : Microstructures and seismic properties from mantle xenoliths from the Tariat alkali basalts

    NASA Astrophysics Data System (ADS)

    Tommasi, A.; Ionov, D. A.; Higgie, K.; Carlson, R. W.

    2013-12-01

    The Hangay region in Mongolia displays a high-elevation, low relief topography, underlain by a low velocity zone in the upper mantle at depths between 100 to 200 km beneath central Mongolia. The relation between this active intra-continental uplift and the mantle dynamics are, however, still poorly understood. To constrain the deformation and thermal evolution and estimate the seismic properties of the mantle lithosphere beneath the Hangay, we analyzed the microstructures and crystal preferred orientations (CPO) of olivine, pyroxenes, and hydrous phases, when present, of 40 mantle xenoliths carried up by the Shavaryn-Tsaram, Zala, and Haer volcanics, which erupted between 8 and <1m.y. along the northeastern boundary of the dome. All xenoliths are medium to coarse-grained spinel-lherzolites. Some xenoliths contain rare amphibole, phlogopite and silicate glass. Coarse-granular, highly annealed microstructures predominate. They are characterized by equiaxed polygonal olivine crystals with rare, widely-spaced subgrains and either sinuous or straight grain boundaries and coarse irregularly-shaped pyroxenes. Tabular microstructures, marked by a weak shape preferred orientation of olivine crystals flattened normal to [010], but clear of internal deformation features (except for rare subgrains), are also common. Porphyroclastic microstructures recording a recent deformation related to the surface uplift are not present among the studied xenoliths. The annealed microstructures are associated with well-developed olivine and pyroxenes CPO, typical of deformation under high temperature, moderate pressure, dry conditions. This association indicates that ductile deformation was followed by static recrystallization, which annealed the deformation microstructure but preserved the CPO and, hence, the anisotropy of physical properties in the Hangay mantle lithosphere. This static recrystallization might be due to a long quiescence episode since the last deformation episode or to

  2. Mathiasite-loveringite and priderite in mantle xenoliths from the Alto Paranaíba Igneous Province, Brazil: genesis and constraints on mantle metasomatism

    NASA Astrophysics Data System (ADS)

    Almeida, Vidyã; Janasi, Valdecir; Svisero, Darcy; Nannini, Felix

    2014-12-01

    Alkali-bearing Ti oxides were identified in mantle xenoliths enclosed in kimberlite-like rocks from Limeira 1 alkaline intrusion from the Alto Paranaíba Igneous Province, southeastern Brazil. The metasomatic mineral assemblages include mathiasite-loveringite and priderite associated with clinopyroxene, phlogopite, ilmenite and rutile. Mathiasite-loveringite (55-60 wt.% TiO2; 5.2-6.7 wt.% ZrO2) occurs in peridotite xenoliths rimming chromite (˜50 wt.% Cr2O3) and subordinate ilmenite (12-13.4 wt.% MgO) in double reaction rim coronas. Priderite (Ba/(K+Ba)< 0.05) occurs in phlogopite-rich xenoliths as lamellae within Mg-ilmenite (8.4-9.8 wt.% MgO) or as intergrowths in rutile crystals that may be included in sagenitic phlogopite. Mathiasite-loveringite was formed by reaction of peridotite primary minerals with alkaline melts. The priderite was formed by reaction of peridotite minerals with ultrapotassic melts. Disequilibrium textures and chemical zoning of associated minerals suggest that the metasomatic reactions responsible for the formation of the alkali-bearing Ti oxides took place shortly prior the entrainment of the xenoliths in the host magma, and is not connected to old (Proterozoic) mantle enrichment events.

  3. Interaction of extended mantle plume head with ancient lithosphere: evidence from deep-seated xenoliths in basalts and lamprophyre diatremes in Western Syria

    NASA Astrophysics Data System (ADS)

    Sharkov, Evgenii

    2016-04-01

    The Middle Cretaceous lamprophyric diatremes of the Jabel Ansaria Ridge contain xenoliths of ancient lower crustal rocks mainly represented by the suite of partly altered garnet granulite and eclogite-like rocks, which were formed at the expense of ferrogabbros and ferroclinopyroxenites most likely in the course of underplating of Fe-Ti basalt. Garnet (Alm26Grs11Py63) megacrysts and coarse-granular garnet-clinopyroxene intergrowths are most likely the varieties of rocks of this series. Garnet megacrysts are represented by large (up to 10 cm in diameter) round "nodules," often molten from the surface. Garnet is usually fractured, and the kelyphite material similar to that in rocks of the eclogite-granulite series occurs in fractures. In addition, we found several intergrowths of garnet with large (up to 3-5 cm in length) crystals of high-Al augite with the low of Ti and Na contents like in rocks of the eclogite-granulite suite. Coarse-grained garnet-clinopyroxene-hornblende rocks with spinel, as well as megacrysts of Al-Ti augite with kaersutite, form the second group in prevalence. This group is close to mantle xenoliths of the "black series" in alkali Fe-Ti basalt worldwide. Kaersutite in these rocks contains gaseous cavities, which provides evidence for the origin of rocks at the expense of a strongly fluidized melt/fluid. In contrast to rocks of the eclogite-granulite series, these rocks did not undergo alteration. Garnet Alm19-26Grs12-13.5Py59-67.5 usually associates with dark opaque spinel. In contrast, the Late Cenozoic plateaubasalts of the region practically do not contain lower crustal xenoliths, whereas xenoliths of mantle spinel lherzolite (fragments of the upper cooled rim of the plume head) are widely abundant. According to data of mineralogical thermobarometry, rocks of the eclogite-granulite suite were formed at 13.5-15.4 kbar (depths of 45-54 km) and 965-1115°C. Rocks of this suite are typical representatives of the continental lower crust

  4. High Water Contents in the Siberian Cratonic Mantle: An FTIR Study of Udachnaya Peridotite Xenoliths

    NASA Technical Reports Server (NTRS)

    Doucet, Luc S.; Peslier, Anne H.; Ionov, Dimitri A.; Brandon, Alan D.; Golovin, Alexander V.; Ashchepkov, Igor V.

    2013-01-01

    Water is believed to be a key factor controlling the long-term stability of cratonic lithosphere, but mechanisms responsible for the water content distribution in the mantle remain poorly constrained. Water contents were obtained by FTIR in olivine, pyroxene and garnet for 20 well-characterized peridotite xenoliths from the Udachnaya kimberlite (central Siberian craton) and equilibrated at 2-7 GPa. Water contents in minerals do not appear to be related to interaction with the host kimberlite. Diffusion modeling indicates that the core of olivines preserved their original water contents. The Udachnaya peridotites show a broad range of water contents in olivine (6.5 +/- 1.1 to 323 +- 65 ppm H2O (2 sigma)), and garnet (0 - 23 +/- 6 ppm H2O). The water contents of olivine and garnet are positively correlated with modal clinopyroxene, garnet and FeO in olivine. Water-rich garnets are also rich in middle rare earth elements. This is interpreted as the result of interaction between residual peridotites and water rich-melts, consistent with modal and cryptic metasomatism evidenced in the Siberian cratonic mantle. The most water-rich Udachnaya minerals contain 2 to 3 times more water than those from the Kaapvaal craton, the only craton with an intact mantle root for which water data is available. The highest water contents in olivine and orthopyroxene in this study (>= 300 ppm) are found at the bottom of the lithosphere (> 6.5 GPa). This is in contrast with the Kaapvaal craton where the olivines of peridotites equilibrated at > 6.4 GPa have < 1 ppm H2O. The latter "dry" olivine may make the base of the Kaapvaal cratonic root strong and thus protects it from erosion by the convective mantle The calculated viscosity for water-rich Udachnaya peridotites at > 6 GPa is lower or similar (8.4× 10(exp 16) to 8.0× 10(exp 18) Pa./s) to that of the asthenosphere (<= 3.7x10(exp 18) Pa./s ). Such lithologies would not be able to resist delamination by the convecting asthenosphere

  5. Water Contents of the Mantle Beneath the Rio Grande Rift: FTIR Analysis of Kilbourne Hole Peridotite Xenoliths

    NASA Technical Reports Server (NTRS)

    Schaffer, Lillian A.; Peslier, Anne; Brandon, Alan

    2013-01-01

    Although nominally anhydrous mantle minerals contain only trace amounts of water, they are the main reservoir of water in the mantle. Added up at the scale of the Earth's mantle, these trace amounts of water represent oceans worth in mass]. Mantle xenoliths from Kilbourne Hole in southern New Mexico are ideal to study mantle water distribution in a rift tectonic setting as they come from a recently-erupted maar in the middle of the Rio Grande Rift. Eleven lherzolites, one harzburgite, and one dunite are being analyzed for water contents by FTIR. The xenoliths will also be analyzed for major and trace element composition, Fe3+/Summation (Fe) ratios, and characterized petrologically. Olivines exhibit variable water contents with less water at the rims compared to the cores. This is probably due to H loss during decompression and xenolith transport by the host magma. Mantle water contents appear to have been primarily preserved in the core of the olivines, based on diffusion modeling of the typically plateau-shaped water content profiles across these grains. Water concentrations are in equilibrium between clino- and orthopyroxene, but olivine concentrations are typically not in equilibrium with those of either pyroxene. Lherzolites analyzed so far have water contents of 2-12 ppm H2O in olivines, 125-165 ppm H2O in orthopyroxenes, and 328-447 ppm H2O in clinopyroxenes. These water contents are similar to, but with a narrower range, than those for the respective minerals in other continental peridotite xenoliths. The lherzolites have bulk-rock (BR) Al2O3 contents that range between 3.17 and 3.78 wt%, indicating similar degrees of partial melting, which could explain the narrow range of their pyroxene water contents. Primitive mantle normalized rare earth element (REE) profiles of the bulk lherzolites vary from light REE depleted to flat, with no significant differences between, nor relation to, their mineral water contents. Consequently, the metasomatic agents that

  6. Petrological features of mantle xenoliths from Handler Ridge, Northern Victoria Land (NVL) , Antarctica

    NASA Astrophysics Data System (ADS)

    Pelorosso, Beatrice; Bonadiman, Costanza; Coltorti, Massimo; Giacomoni, Pier Paolo; Ntaflos, Theodoros; Grégoire, Michel

    2015-04-01

    A petrological study of ultramafic xenoliths from Handler Ridge has been carried out, in order to characterize the lithospheric mantle domain of the Western Antarctic Rift System, from Mt Melbourne (74°21'S 164°42'E) to Handler Ridge (72°31'167°18'E). Samples are mainly anhydrous spinel (sp)-bearing lherzolites, but few wehrlites also occur. Two textures were recognized: i) medium to coarse grained and ii) fine grained types. Evidences of melt/rock interaction (secondary clinopyroxene cpx2, cpx spongy and cloudy rims, glassy patches) can be also observed. Olivine (ol) is forsteritic in composition with Fo varying from 87.5 to 91.0. Within lherzolites a more fertile group can be recognized with Fo ranging between 87.5 and 88.6. In wehrlites ol varies from Fo 84.5 to 86.1. NiO ranges from 0. 28 to 0.44 wt% for lherzolites, while it has a lower content for wehrlites (0.20-0.40 wt%) As for ol, orthopyroxene (opx) in the most fertile lherzolites presents mg# from 88.1 to 88.3, while it varies from 88.3 to 91 in the residual lherzolitic group. None differences in Al2O3 and TiO2 contents between the two groups were recognized. In chondrite-normalized incompatible element patterns opx is depleted in light REE (LREE), with remarkable Ti and Zr positive anomalies. In lherzolites mg# of primary clinopyroxene (cpx1) varies between 87.6 and 92.1. As for ol and opx, two groups can be recognized, although some overlap exists for two samples. Al2O3 and TiO2 vary from 3.68-6.51 wt% and from 0.19 to 0.71 wt% respectively. Secondary cpx (cpx2) is generally richer in FeO, TiO2 and N2O with respect to cpx1. In wehrlites cpx is characterized by very low mg# (84.3-88.6) and higher TiO2 contents (0.69-1.39 wt%) than lherzolites. Irrespective to the lithology, chondrite-normalized incompatible trace element cpx1 patterns are variable enriched in Th, U, Nb, and Ta with negative Sr, Zr and Hf anomalies. Two trends can be recognized. The first one with (La/Yb)N varying from 1.28 to 9

  7. Thermal history of the upper mantle beneath a young back-arc extensional zone: ultramafic xenoliths from San Luis Potosí, Central Mexico

    NASA Astrophysics Data System (ADS)

    Heinrich, Wilhelm; Besch, Thomas

    1992-06-01

    At the San Luis Potosí (SLP) volcanic field (Central Mexico), Quaternary basanites and tuff breccias have sampled a suite of ultramafic xenoliths, predominately spinel lherzolites, spinel-olivine websterites, spinel pyroxenites, and hornblende-rich pyroxenites. Spinel lherzolites from the La Ventura maars have protogranular to equigranular textures, those from the Santo Domingo maars are strongly sheared. Both spinel-lherzolite types show similar whole-rock major and trace-element abundances. They are fertile to slightly depleted with mineralogical and geochemical heterogeneities induced by partial melting processes. Pyroxenites with either magmatic or metamorphic textures are high-pressure cumulates. Hornblende-rich pyroxenites are genetically linked to the host basanites. Most of the protogranular spinel lherzolites contain veinlets of glass along grain boundaries. These glasses are chemically homogeneous and have trachybasaltic to trachyandesitic compositions. Mg- and Fe2+-partitioning between olivine and glass suggests chemical equilibrium between the melts represented by the glasses and the spinel-lherzolite mineral assemblage at about 1,000°C and 10 to 15 kbar. The melts are interpreted to be of upper mantle origin. They may have been formed by in-situ partial melting in the presence of volatiles or represent percolating melts chemically buffered by the spinel-lherzolite mineral assemblage at uppermost mantle conditions. Mineral chemistry in all rock types of the whole xenolith suite reveals distinct disequilibrium features reflecting partial re-equilibration stages towards lower temperatures estimated to be from 1,050°C to 850°C at 9 to 15 kbar. The presence of similar zoning and exsolution features mainly documented in pyroxenes along with similar maximum and minimum temperatures requires all sampled xenoliths to have undergone the same temperature regime within the upper mantle. The sheared spinel lherzolites from the Sto. Domingo field are interpreted

  8. Calcium isotopic composition of mantle xenoliths and minerals from Eastern China

    NASA Astrophysics Data System (ADS)

    Kang, Jin-Ting; Zhu, Hong-Li; Liu, Yu-Fei; Liu, Fang; Wu, Fei; Hao, Yan-Tao; Zhi, Xia-Chen; Zhang, Zhao-Feng; Huang, Fang

    2016-02-01

    This study presents calcium isotope data for co-existing clinopyroxenes (cpx), orthopyroxenes (opx), and olivine (ol) in mantle xenoliths to investigate Ca isotopic fractionation in the upper mantle. δ44/40Ca (δ44/40Ca (‰) = (44Ca/40Ca)SAMPLE/(44Ca/40Ca)SRM915a - 1) in opx varies from 0.95 ± 0.05‰ to 1.82 ± 0.01‰ and cpx from 0.71 ± 0.06‰ to 1.03 ± 0.12‰ (2se). δ44/40Ca in ol (P-15) is 1.16 ± 0.08‰, identical to δ44/40Ca of the co-existing opx (1.12 ± 0.09‰, 2se). The Δ44/40Caopx-cpx (Δ44/40Caopx-cpx = δ44/40Caopx-δ44/40Cacpx) shows a large variation ranging from -0.01‰ to 1.11‰ and it dramatically increases with decreasing of Ca/Mg (atomic ratio) in opx. These observations may reflect the effect of opx composition on the inter-mineral equilibrium fractionation of Ca isotopes, consistent with the theoretical prediction by first-principles theory calculations (Feng et al., 2014). Furthermore, Δ44/40Caopx-cpx decreases when temperature slightly increases from 1196 to 1267 K. However, the magnitude of such inter-mineral isotopic fractionation (1.12‰) is not consistent with the value calculated by the well-known correlation between inter-mineral isotope fractionation factors and 1/T2 (Urey, 1947). Instead, it may reflect the temperature control on crystal chemistry of opx (i.e., Ca content), which further affects Δ44/40Caopx-cpx. The calculated δ44/40Ca of bulk peridotites and pyroxenites range from 0.76 ± 0.06‰ to 1.04 ± 0.12‰ (2se). Notably, δ44/40Ca of bulk peridotites are positively correlated with CaO and negatively with MgO content. Such correlations can be explained by mixing between a fertile mantle end-member and a depleted one with low δ44/40Ca, indicating that Ca isotopes could be a useful tool in studying mantle evolution.

  9. First data for deep seated xenoliths and mantle geotherms of Zarnitsa kimberlite pipe, Daldyn, Yakutia.

    NASA Astrophysics Data System (ADS)

    Ashchepkov, Igor; Pokhilenko, Nikolai; Vladykin, Nikolai; Spetsius, Zdislav; Logvinova, Alla; Palessky, Stanislav; Khmelnikova, Olga; Shmarov, Gleb

    2014-05-01

    First discovered in Yakutia and the largest in Daldyn region kimberlite pipe is composed from several phases including breccias and porphyric kimberlites. Commonly mantle xenolith from this pipe especially included in the prevailing grey breccia are nearly completely altered. Only relatively fresh material from the brownish - grey breccia from the drilling core and porphyric kimberlites includes material which could be used for the mineral thermobarometry. The picroilmenites from the Zarnitsa pipe are forming three clusters according to the Cr- content: 0.5; 1.0 and 2.5 % Cr2O3 (Ashchepkov,Amshinsky, Pokhilenko, 1980; Amshinsky, Pokhilenko,1984; Alymova et al., 2003) due to the different contamination degree of protokimberlites in mantle peridotites. The ilmenites from porphyric kimberlites are forming stepped trend consisting from three groups of different pressure intervals from 6.5 to 4.0 GPa but more continuous than those determined for the ilmenites from breccia (Ashchepkov et al ., 2010). The relatively low Cr diopsides are corresponding to the deeper part while those containing to 2 -3 of Cr2O3 are from the middle part of the mantle section and are in association with the phlogopites contain the reflecting processes of the protokimbelite differentiation and contamination. Peridotites from the lithosphere base are of Hi temperature type and slightly Fe - enriched and are referred to the porphyroclustic types where garnets contain up to 10% Cr2O3 are they are relatively low in TiO2. But there are alos varieties of reduced Cr and the Fe-enriched which are closer to the deformed type (Agashev et al., 2013). The cold clot in the 60-5.5 GPa (34 mwm-2) are represented by Fe- low peridotites with the garnets of sub-Ca types. The Cr- low and LT eclogites are correspondent to the low 4.5-6.0 GPa interval similar to those from Udachnaya pipe. But near the pyroxenites lens the varieties enriched in Fe and sometimes hybrid pyroxenites appear like in most pf mantle

  10. Garnet lherzolite xenoliths in the kimberlites of northern Lesotho: revised P-T equilibration conditions and upper mantle Palaeogeotherm

    NASA Astrophysics Data System (ADS)

    Carswell, D. A.; Gibb, F. G. F.

    1987-12-01

    Evidence is presented that the inflected palaeogeotherm for northern Lesotho, previously highlighted by Boyd (1973), Boyd and Nixon (1973, 1975), Finnerty and Boyd (1984, 1987), is essentially an artifact of the unsatisfactory, over-simplified barometer formulation (based on MacGregor 1974) employed. The absence of an inflection in the palaeogeotherm for Udachnaya, Siberia based on P-T estimates for garnet lherzolite xenoliths calculated with the same barometer, does not prove the reality of an inflected palaeogeotherm for northern Lesotho. Rather, it reflects, at least in part, chemical differences between the equivalent deformed, high- T xenoliths in these two areas — most importantly expressed in the respective contents of Jadeite relative to ureyite in the constituent orthopyroxenes. Accurate estimation of P-T equilibration conditions for garnet lherzolite xenoliths requires both complete and precise mineral analyses and adequate consideration of the influence of minor elements, such as Cr and Na, on the element exchange reaction thermometers and barometers employed. The barometer formulation of Nickel and Green (1985) is judged to be the best currently available. As no single thermometer is entirely satisfactory and dependable throughout the P-T range of interest, equilibration temperatures are currently best assessed as a mean value obtained from application of the most accurate formulations for both the two-pyroxene solvus thermometer (Bertrand and Mercier 1985) and Fe2+-Mg2+ exchange reactions between garnet-clinopyroxene (Powell 1985), garnet-orthopyroxene (Harley 1984a) and garnet-olivine (O'Neill and Wood 1979) mineral pairs. Such ‘best’ P-T estimates for xenoliths in the kimberlites of northern Lesotho indicate a somewhat elevated, non-inflected, upper mantle palaeogeotherm, compatible with a 120 145 km thick thermally conductive lithosphere above a convecting asthenosphere. The common coarse textured, chemically depleted, garnet lherzolite

  11. Water in the lithospheric mantle beneath a Phanerozoic continental belt: FTIR analyses of Alligator Lake Xenoliths (Yukon, Canada)

    NASA Astrophysics Data System (ADS)

    Gelber, M.; Peslier, A. H.; Brandon, A. D.

    2015-12-01

    Water in the mantle influences melting, metasomatism, viscosity and electrical conductivity. The Alligator Lake mantle xenolith suite is one of three bimodal peridotite suites from the northern Canadian Cordillera brought to the surface by alkali basalts, i.e., it consists of chemically distinct lherzolites and harzburgites [1-2]. The lherzolites have equilibration temperatures about 50 °C lower than the harzburgites and are thought to represent the fertile upper mantle of the region. The harzburgites might have come from slightly deeper in the mantle and/or be the result of a melting event above an asthenospheric upwelling detected as a seismic anomaly at 400-500 km depth [3]. Major and trace element data are best interpreted as the lherzolite mantle having simultaneously experienced 20-25% partial melting and a metasomatic event to create the harzburgites [3]. Well-characterized xenoliths are being analyzed for water by FTIR. Harzburgites contain 29-52 ppm H2O in orthopyroxene (opx) and ~140 ppm H2O in clinopyroxene (cpx). The lherzolites have H2O contents of 27-150 ppm in opx and 46-361 ppm in cpx. Despite correlating with enrichments in LREE, the water contents of the harzburgite pyroxenes are low relative to those of typical peridotite xenoliths [4], suggesting that the metasomatic agents were water-poor, contrarily to what has been suggested before [3]. The water content of cpx is about double that of opx indicating equilibrium. Olivine water contents are low (< 5 ppm H2O) and out of equilibrium with those of opx and cpx, which may be due to H loss during xenolith ascent. This is consistent with olivines containing more water in their cores than their rims. Olivines exclusively exhibit water bands in the 3400-3000 cm-1 range, which may be indicative of a reduced environment [5]. [1] Francis. 1987 JP 28, 569-97. [2] Eiche et al. 1987 CMP 95, 191-201. [3] Shi et al. 1997 CMP 131, 39-53. [4] Peslier et al. 2015 GGG 154, 98-117. [5] Bai et al. 1993 PCM 19, 460-71.

  12. Mesozoic-Cenozoic thermal evolution of lithospheric mantle beneath the North China Craton: evidence from REE-in-two-pyroxene temperatures of mantle xenoliths

    NASA Astrophysics Data System (ADS)

    Wang, C.; Xu, W.; Liang, Y.

    2014-12-01

    Thermal state is an important property for us to understand the nature of the lithospheric mantle beneath the North China Craton (NCC). Traditionally, it was obtained by calculating equilibrium temperatures for the mantle xenoliths using thermometers based on major element compositions of coexisting minerals. A REE-in-two-pyroxene thermometer developed by Liang et al. (2013) is able to extract near-solidus temperatures,which can deduce the thermal histories of mantle rocks in combination with major-element-in-two-pyroxene temperatures (Tmaj). We calculated REE temperatures (TREE) for mantle samples from the NCC including ancient refractory peridotites entrained by Early Cretaceous high-Mg diorites from the central NCC (Fushan), Mantle pyroxenites entrained by Early Cretaceous basalts from the eastern NCC (Feixian and Fangcheng), and fertile/moderately depleted peridotites entrained by <100 Ma basalts from the central and eastern NCC. The Fushan peridotites have low Tmaj (<880°C) and mismatched high TREE (780-1150°C), indicating that the ancient mantle was subjected to melt-rock reactions. The Feixian and Fangcheng pyroxenites have both high Tmaj (>890°C) and high TREE - Tmaj values (80-220°C), suggestive of a hot and fast-cooled mantle in Early Cretaceous. The peridotites in <100 Ma basalts have similar TREE and Tmaj, characteristics of well-equilibrated mantle. Based on the thermometric data, we suggest that the transition of nature of the NCC lithospheric mantle is marked by the interaction between ancient lithospheric mantle and hot melt derived from recycling lower crust in Early Cretaceous. After that a fertile mantle was accreted from upwelling asthenosphere, which accomplished the replacement of the NCC lithsospheric mantle. Reference: Liang et al., 2013, A REE-in-two-pyroxene thermometer for mafic and ultramafic rocks. Geochimica et Cosmochimia Acta 102, 246-260.

  13. Petrological features of anhydrous and hydrous mantle xenoliths from Harrow Peaks, Antarctica

    NASA Astrophysics Data System (ADS)

    Pelorosso, Beatrice; Bonadiman, Costanza; Coltorti, Massimo; Giacomoni, Pier Paolo; Ntaflos, Theodoros

    2014-05-01

    A preliminary study on the petrological features of a new xenoliths population, collected in the area of Harrow Peaks (HP) Northern Victoria Land, Antarctica is presented. The presence of modal hydrous phases (amphibole and rare phlogopite) allows to explore the water circulation and volatile reservoirs of this mantle domain. HP samples are medium to coarse grain peridotites, protogranular to phorphyroclastic in texture. They vary in composition from fertile lherzolite to harzburgite. Both anhydrous and hydrous peridotites show matrix/melt interaction forming secondary minerals and resorbed rims in primary olivines and spongy textures or cloudy rims of the other peridotite minerals. Opx occur as large crystals (opx1) with thin and resorbed spongy rims or texturally well equilibrated small, elongated grains (opx2). Primary unmetasomatized cpx are rare, the majority is spongy, resorbed grains or newly formed small crystals. Spinel (sp) always occur as small anhedral crystals, or larger, often dendritic primary grains. Amphibole occurs both as disseminated and in veins; the latter frequently associated with newly formed, secondary cpx crystals (Coltorti et al.,2004). Glassy patches are rare, not associated with amphibole, but occur related to resorbed/spongy cpx and spinel(sp).Mineral and glass major element analyses evidence that HP peridotites are following a residual trend, but are characterized by low mg#[=100*Mg/Mg+Fetot]values. Fo in primary unmetasomatized olivine range between 87.49 and 89.07 reflecting an anomalous fertile character respect to the lithological type (PM ol: Fo= 89.5). CaO(< 0.1 wt%)and NiO(0.28 to 0.41 wt%) contents are in the range of variably residual mantle values. In term of mg# values (87.24 - 89.56),opx1 and opx2 are coherent with primary ol. Both types show a narrow range of variation in terms of Al2O3(2.11-3.32 wt%), TiO2(0.05-0.14 wt%) and CaO(0.36-0.96 wt%). Spongy rims and resorbed crystals in both olivine and opx record a sensible

  14. Peridotite xenoliths from western Grand Canyon and The Thumb: A probe into the subcontinental mantle of the Colorado Plateau

    NASA Astrophysics Data System (ADS)

    Alibert, Chantal

    1994-11-01

    Rare earth elements (REE) and Sr and Nd isotopic compositions are presented for spinel peridotite xenoliths from western Grand Canyon, on the margin of the Colorado Plateau (CP), and for garnet peridotite inclusions from The Thumb, central CP. The Grand Canyon spinel peridotites show original textures and geochemical characteristics which do not have counterparts in the xenoliths from the nearby Basin and Range. The restricted range of mineral compositions in these 1herzolites suggests that they represent a limited series of residues of partial melting. Pyroxene equilibration temperatures are in the range 800-1000 deg C. The Nd-143/Nd-144 ratios in clinopyroxene (cpx) are extremely variable. Sample TRU 2, with Epsilon(sub Nd) = +147, records an ancient depletion event around 1.2 Ga. More than half of the cpx separates show concave downward REE profiles between Gd and Yb, indicating that partial melting occurred in the garnet stability field before final recrystallization of the residues in the spinel peridotite field, probably during decompressional upwelling. The strong light REE (LREE) depletion in some of the cpx cannot be modeled by batch melting of a LREE-depleted source but requires 5-10% melting with segregation in approximately 1% increments. A second group of xenoliths, characterized by cpx with strong LREE enrichment, includes amphibole-bearing peridotites and has a more limited range of Nd-143/Nd-144 ratios. Interactions with melts of variable composition are inferred on the basis of mineralogical and geochemical evidence. Contrasting with the extreme isotopic heterogeneity of Nd, a high, uniform Sr-87/Sr-86 ratio around 0.7045 is associated with high Sr/Nd ratios. Aqueous fluids derived from a subducted slab could account for this preferential Sr enrichment. Six porphyroclastic garnet 1herzolites from The Thumb sample a deeper part of the subcontinental mantle. One xenolith has a refractory chemistry, low temperature of equilibration and Sr-Nd isotopic

  15. Deformation, annealing, reactive melt percolation, and seismic anisotropy in the lithospheric mantle beneath the southeastern Ethiopian rift: Constraints from mantle xenoliths from Mega

    NASA Astrophysics Data System (ADS)

    Tommasi, Andréa; Baptiste, Virginie; Vauchez, Alain; Holtzman, Benjamin

    2016-07-01

    We explore the relations between deformation, annealing, and melt percolation during rifting and the effect of these processes on seismic anisotropy by analyzing the microstructures and crystal preferred orientations (CPO) in a suite of mantle xenoliths from Mega, in the southern end of the Ethiopian rift. Previous geochemical studies on these xenoliths showed evidence for interactions with variable melt types and volumes during the rifting process. The peridotites have dominantly coarse-porphyroclastic microstructures, but coarse granular or partially recrystallized microstructures also occur. The olivine CPO, characterized by orthorhombic to fiber-[100] patterns and moderate intensities, the common occurrence of (100) tilt walls, and the predominance of <0vw> rotation axes accommodating low angle misorientations in olivine support deformation by dislocation creep with dominant activation of the [100](010) system. Annealing (static recrystallization) of variable intensity followed this deformation. Modal enrichment in pyroxenes in > 60% of the studied peridotites corroborates extensive, but spatially heterogeneous reactive melt percolation leading to refertilization of the lithospheric mantle beneath the southern Ethiopian rift. The common interstitial shapes of the pyroxenes and lack of correlation between the pyroxenes and the olivine CPOs in many samples suggest that part of the refertilization is post-kinematic. However, there is no simple relation between reactive melt percolation and annealing of the olivine deformation microstructure. Comparison with data from other xenolith localities points to changes in the metasomatic imprint in the lithospheric mantle along the East African rift system correlated with the evolution in the rift maturity. Seismic properties averaged over all samples show typical lithospheric mantle patterns with fast propagation of P- and polarization of the fast S-waves parallel to the lineation. The anisotropy is moderate (< 6% for P

  16. Cobalt and precious metals in sulphides of peridotite xenoliths and inferences concerning their distribution according to geodynamic environment: A case study from the Scottish lithospheric mantle

    NASA Astrophysics Data System (ADS)

    Hughes, Hannah S. R.; McDonald, Iain; Faithfull, John W.; Upton, Brian G. J.; Loocke, Matthew

    2016-01-01

    Abundances of precious metals and cobalt in the lithospheric mantle are typically obtained by bulk geochemical analyses of mantle xenoliths. These elements are strongly chalcophile and the mineralogy, texture and trace element composition of sulphide phases in such samples must be considered. In this study we assess the mineralogy, textures and trace element compositions of sulphides in spinel lherzolites from four Scottish lithospheric terranes, which provide an ideal testing ground to examine the variability of sulphides and their precious metal endowments according to terrane age and geodynamic environment. Specifically we test differences in sulphide composition from Archaean-Palaeoproterozoic cratonic sub-continental lithospheric mantle (SCLM) in northern terranes vs. Palaeozoic lithospheric mantle in southern terranes, as divided by the Great Glen Fault (GGF). Cobalt is consistently elevated in sulphides from Palaeozoic terranes (south of the GGF) with Co concentrations > 2.9 wt.% and Co/Ni ratios > 0.048 (chondrite). In contrast, sulphides from Archaean cratonic terranes (north of the GGF) have low abundances of Co (< 3600 ppm) and low Co/Ni ratios (< 0.030). The causes for Co enrichment remain unclear, but we highlight that globally significant Co mineralisation is associated with ophiolites (e.g., Bou Azzer, Morocco and Outokumpu, Finland) or in oceanic peridotite-floored settings at slow-spreading ridges. Thus we suggest an oceanic affinity for the Co enrichment in the southern terranes of Scotland, likely directly related to the subduction of Co-enriched oceanic crust during the Caledonian Orogeny. Further, we identify a distinction between Pt/Pd ratio across the GGF, such that sulphides in the cratonic SCLM have Pt/Pd ≥ chondrite whilst Palaeozoic sulphides have Pt/Pd < chondrite. We observe that Pt-rich sulphides with discrete Pt-minerals (e.g., PtS) are associated with carbonate and phosphates in two xenolith suites north of the GGF. This three

  17. Osmium isotope evidence for Early to Middle Proterozoic mantle lithosphere stabilization and concomitant production of juvenile crust in Dish Hill, CA peridotite xenoliths

    NASA Astrophysics Data System (ADS)

    Armytage, Rosalind M. G.; Brandon, Alan D.; Peslier, Anne H.; Lapen, Thomas J.

    2014-07-01

    The 187Os/188Os compositions in peridotite samples from the sub-continental lithospheric mantle (SCLM) can be used to constrain the timing of melt extraction and potentially test the link between large-scale mantle melting and juvenile crust production. The SCLM has often experienced a complex history such that some lithophile elements such as REEs (rare earth elements) in these rocks typically record overprinting during metasomatism. New 187Os/188Os, major and trace element compositional data were obtained on sixteen Dish Hill peridotite xenoliths (California, USA) and are used to examine these issues. The samples show strong correlations between 187Os/188Os and indicators of melt depletion such as Lu abundance in clinopyroxene, modal abundance of clinopyroxene, bulk rock Al2O3 and the Cr# (Cr/(Cr + Al) in spinel. These relationships indicate that metasomatism did not compromise the 187Os/188Os systematics. The data appear to form two melt depletion trends consistent with Re depletion model ages (TRD) obtained from the two Al2O3 versus 187Os/188Os trends are 2.1 ± 0.5 Ga and 1.3 ± 0.3 Ga (±95% conf.). It has been suggested that the SCLM under Dish Hill may be fragments of oceanic lithosphere emplaced as the result of Farallon plate subduction during the Late Cretaceous (Luffi et al., 2009). However, the strong melt depletion trends, major element compositions and Re-depletion ages are not consistent with the interpretation of this suite of xenoliths having an oceanic lithospheric origin. Rather, the 2.1 Ga age coincides with Nd model ages of 2-2.3 Ga (Bennett and DePaolo, 1987; Rämö and Calzia, 1998) for the overlying Mojavia crustal province. The 1.3 Ga age is consistent with large-scale A-type magmatism in the nearby region at this time that is purported to be the result of mantle plume melting processes. Therefore, data from this study point to the SCLM under Dish Hill being formed by two ancient mantle-melting events, which could be the result of

  18. In-situ assimilation of mantle minerals by kimberlitic magmas - Direct evidence from a garnet wehrlite xenolith entrained in the Bultfontein kimberlite (Kimberley, South Africa)

    NASA Astrophysics Data System (ADS)

    Soltys, Ashton; Giuliani, Andrea; Phillips, David; Kamenetsky, Vadim S.; Maas, Roland; Woodhead, Jon; Rodemann, Thomas

    2016-07-01

    The lack of consensus on the possible range of initial kimberlite melt compositions and their evolution as they ascend through and interact with mantle and crustal wall rocks, hampers a complete understanding of kimberlite petrogenesis. Attempts to resolve these issues are complicated by the fact that kimberlite rocks are mixtures of magmatic, xenocrystic and antecrystic components and, hence, are not directly representative of their parental melt composition. Furthermore, there is a lack of direct evidence of the assimilation processes that may characterise kimberlitic melts during ascent, which makes understanding their melt evolution difficult. In this contribution we provide novel constraints on the interaction between precursor kimberlite melts and lithospheric mantle wall rocks. We present detailed textural and geochemical data for a carbonate-rich vein assemblage that traverses a garnet wehrlite xenolith [equilibrated at ~ 1060 °C and 43 kbar (~ 140-145 km)] from the Bultfontein kimberlite (Kimberley, South Africa). This vein assemblage is dominated by Ca-Mg carbonates, with subordinate oxide minerals, olivine, sulphides, and apatite. Vein phases have highly variable compositions indicating formation under disequilibrium conditions. Primary inclusions in the vein minerals and secondary inclusion trails in host wehrlite minerals contain abundant alkali-bearing phases (e.g., Na-K bearing carbonates, Mg-freudenbergite, Na-bearing apatite and phlogopite). The Sr-isotope composition of vein carbonates overlaps those of groundmass calcite from the Bultfontein kimberlite, as well as perovskite from the other kimberlites in the Kimberley area. Clinopyroxene and garnet in the host wehrlite are resorbed and have Si-rich reaction mantles where in contact with the carbonate-rich veins. Within some veins, the carbonates occur as droplet-like, globular segregations, separated from a similarly shaped Si-rich phase by a thin meniscus of Mg-magnetite. These textures are

  19. Petrology and geochemistry of mantle xenoliths from the Kapsiki Plateau (Cameroon Volcanic Line): Implications for lithospheric upwelling

    NASA Astrophysics Data System (ADS)

    Tamen, Jules; Nkoumbou, Charles; Reusser, Eric; Tchoua, Felix

    2015-01-01

    Mantle xenoliths hosted by Oligocene alkaline basalts of the Kapsiki Plateau, northern end of the Cameroon Volcanic Line consist of group I spinel and plagioclase peridotites, mainly protogranular and accessorily porphyroclastic. The sub-continental lithospheric mantle here is heterogeneous and encloses both depleted and fertile components. Minerals exhibit wide range major element compositions compared to Nyos and Kumba grabens equivalent rocks. Spinel occurs as homogeneous brown crystals or as composite (brown-core-dark-rim) crystals when in contact with diopside or swatted in melt pools. Clinopyroxene crystals are either spinel exsolution-bearing or exsolution-free, the latter being often skeletal or frameworked and riddled with intracrystalline melt pools. Intraxenolith melt pockets and veinlets are always associated to plagioclase-bearing samples. Feldspars depict two distinctive compositions (An37-66Ab57-32Or6-2 and An3-7Ab52-62Or31-48) partly attributed to host xenolith type and to the involvement in the spinel and/or diopside melting reaction of an infiltrating alkali and carbonate-rich liquid. Petrographic and geochemical data discriminate melt pockets from their host basalts, excluding thus infiltration of basaltic melt as prospective origin. Thermo-barometric estimates reveal that prior to their entrainment the Kapsiki mantle xenoliths experienced two P-T equilibrium stages resulting in subsolidus re-equilibration from spinel- to plagioclase-facies conditions. Furthermore mineral textural relations show that the occurrence of plagioclase and melts inclusions is linked to spinel and/or diopside breakdown, likely subsequent to decompression and/or metasomatic induced melting events predating Oligo-Miocene volcanism.

  20. Petrological processes in mantle plume heads: Evidence from study of mantle xenoliths in the late Cenozoic alkali Fe-Ti basalts in Western Syria

    NASA Astrophysics Data System (ADS)

    Sharkov, Evgenii

    2015-04-01

    It is consensus now that within-plate magmatism is considered with ascending of mantle plumes and adiabatic melting of their head. At the same time composition of the plumes' matter and conditions of its adiabatic melting are unclear yet. The major source of objective information about it can be mantle xenoliths in alkali basalts and basanites which represent fragments of material of the plume heads above magma-generation zone. They are not represent material in melting zone, however, carry important information about material of modern mantle plumes, its phase composition and components, involved in melting. Populations of mantle xenoliths in basalts are characterized by surprising sameness in the world and represented by two major types: (1) dominated rocks of ``green'' series, and (2) more rare rocks of ``black'' series, which formed veins in the ``green'' series matrix. It can evidence about common composition of plume material in global scale. In other words, the both series of xenoliths represent two types of material of thermochemical mantle plumes, ascended from core-mantle boundary (Maruyama, 1994; Dobretsov et al., 2001). The same types of xenoliths are found in basalts and basanites of Western Syria (Sharkov et al., 1996). Rocks of ``green'' series are represented by Sp peridotites with cataclastic and protogranular structures and vary in composition from dominated spinel lherzolites to spinel harzburgites and rare spinel pyroxenites (websterites). It is probably evidence about incomplete homogenizing of the plume head matter, where material, underwent by partial melting, adjoins with more fertile material. Such heterogeneity was survived due to quick cooling of upper rim of the plume head in contact with relatively cold lithosphere. Essential role among xenoliths of the ``black'' series play Al-Ti-augite and water-bearing phases like hornblende (kaersutute) and Ti-phlogopite. Rocks of this series are represented by wehrlite, clinopyroxenite, amphibole

  1. Os, Sr, Nd, and Pb isotope systematics of southern African peridotite xenoliths - Implications for the chemical evolution of subcontinental mantle

    NASA Technical Reports Server (NTRS)

    Walker, R. J.; Carlson, R. W.; Shirey, S. B.; Boyd, F. R.

    1989-01-01

    Isotope analyses of Os, Sr, Nd, and Pb elements were caried out on twelve peridotite xenoliths from the Jagersfontein, Letseng-la-terae, Thaba Patsoa, Mothae, and Premier kimberlites of southern Africa, to investigate the timing and the nature of melt extraction from the continental lithosphere and its relation to the continent formation and stabilization. The distinct Os and Pb isotopic characteristics found in these samples suggested that both the low- and the high-temperature peridotites reside in an ancient stable lithospheric 'keel' to the craton that has been isolated from chemical exchange with the sublithospheric mantle for time periods in excess of 2 Ga.

  2. Mantle Recycling of Crustal Materials through Study of Ultrahigh-Pressure Minerals in Collisional Orogens, Ophiolites, and Xenoliths

    NASA Astrophysics Data System (ADS)

    Liou, J. G.; Tsujimori, T.; Yang, J.; Zhang, R. Y.; Ernst, W. G.

    2014-12-01

    Newly recognized ultrahigh-pressure (UHP) mineral occurrences including diamonds in ultrahigh-temperature (UHT) felsic granulites of orogenic belts, in chromitites associated with ophiolitic complexes, and in mafic/ultramafic xenoliths suggest the recycling of crustal materials through profound subduction, mantle upwelling, and return to the Earth's surface. Recycling is supported by unambiguously crust-derived mineral inclusions in deep-seated zircons, chromites, and diamonds from collision-type orogens, from eclogitic xenoliths, and from ultramafic bodies of several Alpine-Himalayan and Polar Ural ophiolites; some such phases contain low-atomic number elements typified by crustal isotopic signatures. Ophiolite-type diamonds in placer deposits and as inclusions in chromitites together with numerous highly reduced minerals and alloys appear to have formed near the mantle transition zone. In addition to ringwoodite and stishovite, a wide variety of nanometric minerals have been identified as inclusions employing state-of-the-art analysis. Reconstitution of now-exsolved precursor UHP phases and recognition of subtle decompression microstructures produced during exhumation reflect earlier UHP conditions. Some podiform chromitites and associated peridotites contain rare minerals of undoubted crustal origin, including Zrn, corundum, Fls, Grt, Ky, Sil, Qtz, and Rtl; the zircons possess much older U-Pb ages than the formation age of the host ophiolites. These UHP mineral-bearing chromitites had a deep-seated evolution prior to extensional mantle upwelling and its partial melting at shallow depths to form the overlying ophiolite complexes. These new findings plus stable isotopic and inclusion characteristics of diamonds provide compelling evidence for profound underflow of both oceanic and continental lithosphere, recycling of biogenic carbon into the lower mantle, and ascent to the Earth's surface through deep mantle ascent.

  3. The Diamondiferous Lithospheric Mantle Underlying the Eastern Superior Craton: Evidence From Mantle Xenoliths From the Renard Kimberlites, Quebec

    NASA Astrophysics Data System (ADS)

    Hunt, L.; Stachel, T.; Armstrong, J. P.; Simonetti, A.

    2009-05-01

    The Renard kimberlite cluster consists of nine pipes located within a 2km2 area in the northern Otish Mountains of Quebec. The pipes are named Renards 1 to 10, with subsequent investigation revealing Renards 5 and 6 to join at depth (now Renard 65). The pipes are located within the eastern portion of the Superior craton, emplaced into Archean granitic and gneissic host rocks of the Opinica Subprovince (Percival, 2007). Amphibolite grade metamorphism, locally passing into the granulite facies (Percival et al., 1994) occurred in late Archean time (Moorhead et al., 2003). Radiometric dating of the hypabyssal Renard 1 kimberlite indicates Neoproterozoic emplacement, with a 206Pb/238U model age of 631.6±3.5 Ma (2σ) (Birkett et al., 2004). A later study on the main phases in Renard 2 and 3 gave a similar emplacement, with a 206Pb/238U model age of 640.5±2.8Ma (Fitzgerald et al., 2008). This makes this kimberlite district one of the oldest in Canada, similar in eruption age to the Wemindji kimberlites (629±29Ma: Letendre et al., 2003). These events are broadly coeval with the conversion from subduction magmatism to rifting in northern Laurentia (Birkett et al., 2004). The bodies are part of a late Neoproterozoic to Cambrian kimberlite field in eastern Canada (Girard, 2001; Moorhead et al, 2002; Letendre et al., 2003) and fit into the north-east of the Eocambrian/Cambrian Labrador Sea Province of Heaman et al. (2004). To better understand the diamondiferous lithospheric mantle beneath the Renard kimberlites, 116 microxenoliths and xenocrysts were analysed. The samples were dominantly peridotitic, composed primarily of purple garnet, emerald green clinopyroxene and olivine, with a few pink and red garnets. A minor eclogitic component comprises predominantly orange garnets and lesser amounts of clinopyroxene. A detailed study on the major, minor and trace element composition of xenolith minerals is currently underway. All but three of the clinopyroxenes analysed to date

  4. Deep-seated xenoliths and xenocrysts from Sytykanskaya pipe: evidence for the evolution of the mantle beneath Alakit, Yakutia.

    NASA Astrophysics Data System (ADS)

    Ashchepkov, Igor; Vladykin, Nikolai; Ntaflos, Theodoros; Logvinova, Alla; Yudin, Denis; Karpenko, Mikhail; Palessky, Stanislav; Khmelnikova, Olga; Travin, Alexey; Salikhov, Ravil

    2014-05-01

    The concentrate from two phases of the kimberlite (breccia and porphyritic kimberlite) and about 130 xenoliths from the Sytykanskaya pipe of the Alakit field (Yakutia) were studied by EPMA and LAM ICP methods. Reconstructions of the PTXfO2 mantle sections were made separately for the two phases. The porphyritic kimberlites and breccia show differences in the minerals although the layering and pressure interval remains the same. For the porphyritic kimberlite the trends P- Fe# - CaO in garnet, fO2 are sub-vertical while the xenocrysts from the breccia show stepped and curved trends possibly due to interaction with fluids. Minerals within xenoliths show the widest variation in all pressure intervals. PT points for the ilmenites which trace the magmatic system show splitting of the magmatic source into two levels at the pyroxenite lens (4GPa) accompanied by peridotite contamination and an increase in Cr in ilmenites. Two groups of metasomatites with Fe#Ol ~ 10-12% and 13-15% were created by the melts derived from protokimberlites and trace the mantle columns from the lithosphere base (Ilm - Gar - Cr diopside) to Moho becoming essentially pyroxenitic (Cr-diopside with Phl). The first Opx-Gar-based mantle geotherm from the Alakit field has been constructed from15 associations and is close to 35 mw/m2 in the lower part of mantle section but deviates to high temperatures in the upper part of the mantle section. The oxidation state for the protokimberlite melts determined from ilmenites is higher than for the other pipes in the Yakutian kimberlite province which probably accounts for the decrease in the diamond grade of this pipe. The geochemistry of the minerals (garnets and clinopyroxenes) from breccias, metasomatic peridotite xenoliths and pyroxenites systematically differ. Xenocrysts from the breccia were produced by the most differentiated melts and enriched protokimberlite or carbonatite; they show highly inclined nearly linear REE patterns and deep troughs of HFSE

  5. Petrological study of Greene Point mantle xenoliths, Northern Victoria Land, Antarctica.

    NASA Astrophysics Data System (ADS)

    Pelorosso, Beatrice; Bonadiman, Costanza; Faccini, Barbara; Coltorti, Massimo; Ntaflos, Theodoros; Grégoire, Michel

    2015-04-01

    negative correlation between cr# (Cr/(Cr+Al) *100mol) (17.5-50.5) and mg# (67.3-81.6). Glasses are silica-rich (SiO2=59.16-68.51 wt%) with K2O and Na2O contents varying from 5.89 to 6.12 and from 5.76 to 9.72wt%, respectively. Trace elements are characterized by positive fractioned REE patterns at low HREE (YbN 2.10-2.72). Based on major and trace element models this mantle domain underwent a degree of partial melting variable between 10 and 18%. The Fe/Mg distribution between ol and sp evidences equilibrium for the majority of ol-sp pairs, leading to choose the ol-sp geothermometer of Ballhaus et al. (1991) to evaluate the GP thermal condition. Assuming a P of 15 Kbar, the majority of the samples has T close to 950°C; fO2 ranges from Δlog fO2 (QFM) -1.70 to -0.38 (Ballhaus et al., 1991). On the whole these new data confirm the tendency for anhydrous GP xenolith population to have higher equilibration T and comparable redox condition with respect to the nearby hydrous Baker Rocks peridotites (Bonadiman et al., 2014). Ballhaus et al. (1991) CMP 106, 27-40 Bonadiman et al., (2014) CMP 167:984

  6. Recycling of crustal materials through study of ultrahigh-pressure minerals in collisional orogens, ophiolites, and mantle xenoliths: A review

    NASA Astrophysics Data System (ADS)

    Liou, Juhn G.; Tsujimori, Tatsuki; Yang, Jingsui; Zhang, R. Y.; Ernst, W. G.

    2014-12-01

    Newly recognized occurrences of ultrahigh-pressure (UHP) minerals including diamonds in ultrahigh-temperature (UHT) felsic granulites of orogenic belts, in chromitites associated with ophiolitic complexes, and in mantle xenoliths suggest the recycling of crustal materials through deep subduction, mantle upwelling, and return to the Earth's surface. This circulation process is supported by crust-derived mineral inclusions in deep-seated zircons, chromites, and diamonds from collision-type orogens, from eclogitic xenoliths in kimberlites, and from chromitities of several Alpine-Himalayan and Polar Ural ophiolites; some of these minerals contain low-atomic number elements typified by crustal isotopic signatures. Ophiolite-type diamonds in placer deposits and as inclusions in chromitites together with numerous highly reduced minerals and alloys appear to have formed near the mantle transition zone. In addition to ringwoodite and inferred stishovite, a number of nanometric minerals have been identified as inclusions employing state-of-the-art analytical tools. Reconstitution of now-exsolved precursor UHP phases and recognition of subtle decompression microstructures produced during exhumation reflect earlier UHP conditions. For example, Tibetan chromites containing exsolution lamellae of coesite + diopside suggest that the original chromitites formed at P > 9-10 GPa at depths of >250-300 km. The precursor phase most likely had a Ca-ferrite or a Ca-titanite structure; both are polymorphs of chromite and (at 2000 °C) would have formed at minimum pressures of P > 12.5 or 20 GPa respectively. Some podiform chromitites and host peridotites contain rare minerals of undoubted crustal origin, including zircon, feldspars, garnet, kyanite, andalusite, quartz, and rutile; the zircons possess much older U-Pb ages than the time of ophiolite formation. These UHP mineral-bearing chromitite hosts evidently had a deep-seated evolution prior to extensional mantle upwelling and partial

  7. Mantle oddities: A sulphate fluid preserved in a MARID xenolith from the Bultfontein kimberlite (Kimberley, South Africa)

    NASA Astrophysics Data System (ADS)

    Giuliani, A.; Phillips, D.; Fiorentini, M. L.; Kendrick, M. A.; Maas, R.; Wing, B. A.; Woodhead, J. D.; Bui, T. H.; Kamenetsky, V. S.

    2013-08-01

    Sulphur in the lithospheric mantle is concentrated in sulphide minerals, with limited evidence for the occurrence of sulphate phases. Here we describe an unusual assemblage of celestine (SrSO4), clinopyroxene and minor phlogopite, pectolite, sphene, apatite, barite (BaSO4) and Ca-Sr carbonates in a MARID mantle xenolith sampled by the Bultfontein kimberlite (Kimberley, South Africa). This assemblage occurs in veins that pervasively traverse the xenolith, indicating that celestine and the other vein minerals crystallised from a fluid. In the MARID host rock, K-richterite is resorbed where in contact with celestine and is overgrown by clinopyroxene. Celestine hosts the other metasomatic vein phases, but also occurs as inclusions in euhedral clinopyroxene, suggesting co-precipitation of these minerals. Celestine was partly replaced by serpentine during alteration by hydrous fluids after kimberlite emplacement in the upper crust. Celestine has relatively radiogenic Sr isotopes (87Sr/86Sr = 0.70677), which overlap those of K-richterite in the MARID host rock and fall within the range of other MARID and phlogopite-K-richterite peridotites sampled by southern African kimberlites. Celestine displays S isotopes (δ34S=+5.9‰) that are slightly heavier than typical mantle values (δ34S∼0‰) and there is no evidence of mass-independent fractionation (Δ33S=-0.01‰). The texture and chemical composition of the metasomatic phases indicate that the MARID rock was infiltrated by a sulphate fluid enriched in Sr, Ba, Na and Ca, with lesser P, Ti, LREE, CO2 and F. The similar Sr/Ba ratios and Sr isotopic compositions of celestine and K-richterite suggest that K-richterite breakdown contributed to the alkali enrichment of the sulphate fluid. A mantle origin for the sulphate fluid is supported by (i) comparisons between the Sr-S isotopic compositions of celestine and the host kimberlite, crustal and mantle lithologies from the area, and (ii) alteration of celestine by late

  8. Mantle water contents beneath the Rio Grande Rift (NM, USA): FTIR analysis of Rio Puerco and Kilbourne Hole peridotite xenoliths

    NASA Astrophysics Data System (ADS)

    Schaffer, L. A.; Peslier, A. H.; Brandon, A. D.; Selverstone, J.

    2015-12-01

    Peridotite xenoliths from the Rio Grande Rift (RGR) are being analyzed for H2O contents by FTIR as well as for major and trace element compositions. Nine samples are from the Rio Puerco Volcanic Field (RP) which overlaps the central RGR and southeastern Colorado Plateau; seventeen samples are from Kilbourne Hole (KH) in the southern RGR. Spinel Cr# (Cr/(Cr+Al) = 0.08-0.46) and olivine Mg# (Mg/(Mg+Fe) = 0.883-0.911) of samples fall within the olivine-spinel mantle array from [1], an indicator that these are residues of partial melting. Pyroxene H2O contents in KH correlate with bulk rock and pyroxene Al2O3 contents. The KH clinopyroxene rare earth element (REE) variations fit models of 0-13% fractional melting of a primitive upper mantle. Most KH peridotites have bulk-rock light REE depleted patterns, but five are enriched in light REEs consistent with metasomatism. Variation in H2O content seems unrelated to REE enrichment. Metasomatism is seen in RP pyroxenite xenoliths [2] and will be examined in the peridotites studied here. Olivine H2O contents are low (≤20 ppm), and decrease from core to rim within grains. This is likely due to H loss during xenolith transport by the host magma [3]. Diffusion models of H suggest that mantle H2O contents are still preserved in cores of KH olivine, but not those of RP olivine. The average H2O content of Colorado Plateau clinopyroxene (670 ppm) [4] is ~300 ppm higher than RGR clinopyroxene (350 ppm). This upholds the hypothesis that hydration-induced lithospheric melting occurred during flat-slab subduction of the Farallon plate [5]. Numerical models indicate hydration via slab fluids is possible beneath the plateau, ~600 km from the paleo-trench, but less likely ~850 km away beneath the rift [6]. [1]Arai, 1994 CG 113, 191-204.[2]Porreca et al., 2006 Geosp 2, 333-351.[3]Peslier and Luhr, 2006 EPSL 242, 302-319.[4]Li et al., 2008 JGR 113, 1978-2012.[5]Humphreys et al., 2003 Int Geol Rev 45, 575-595.[6]English et al., 2003 EPSL

  9. Mantle Water Contents Beneath the Rio Grande Rift (NM, USA): FTIR Analysis of Rio Puerco and Kilbourne Hole Peridotite Xenoliths

    NASA Technical Reports Server (NTRS)

    Schaffer, L. A.; Peslier, A. H.; Brandon, A.; Selverstone, J.

    2015-01-01

    Peridotite xenoliths from the Rio Grande Rift (RGR) are being analyzed for H (sub 2) O contents by FTIR (Fourier Transform Infrared) as well as for major and trace element compositions. Nine samples are from the Rio Puerco Volcanic Field (RP) which overlaps the central RGR and southeastern Colorado Plateau; seventeen samples are from Kilbourne Hole (KH) in the southern RGR. Spinel Cr# (Cr/(Cr+Al)) (0.08-0.46) and olivine Mg# (Mg/(Mg plus Fe)) (0.883-0.911) of all RGR samples fall within the olivine-spinel mantle array from [1], an indicator that peridotites are residues of partial melting. Pyroxene H (sub 2) O in KH correlate with bulk rock and pyroxene Al (sub 2) O (sub 3).The KH clinopyroxene rare earth element (REE) variations fit models of 0-13 percent fractional melting of a primitive upper mantle. Most KH peridotites have bulk-rock light REE depleted patterns, but five are enriched in light REEs consistent with metasomatism. Variation in H (sub 2) O content is unrelated to REE enrichment. Metasomatism is seen in RP pyroxenite xenoliths [2] and will be examined in the peridotites studied here. Olivine H (sub 2) O contents are low (less than or equal to 15 parts per million), and decrease from core to rim within grains. This is likely due to H loss during xenolith transport by the host magma [3]. Diffusion models of H suggest that mantle H (sub 2) O contents are still preserved in cores of KH olivine, but not RP olivine. The average H (sub 2) O content of Colorado Plateau clinopyroxene (670 parts per million) [4] is approximately 300 parts per million higher than RGR clinopyroxene (350 parts per million). This upholds the hypothesis that hydration-induced lithospheric melting occurred during flat-slab subduction of the Farallon plate [5]. Numerical models indicate hydration via slab fluids is possible beneath the plateau, approximately 600 kilometers from the paleo-trench, but less likely approximately 850 kilometers away beneath the rift [6].

  10. Anisotropic Structure of the Upper Mantle in the Carpathian-Pannonian Region: From SKS Splitting data and Xenolith Constraints

    NASA Astrophysics Data System (ADS)

    Bokelmann, G.; Qorbani, E.; Kovacs, I.; Falus, G.

    2014-12-01

    The Carpathian-Pannonian region (CPR) is the northeastern end of the Alpine mountain belt. This study is aimed at evaluating the seismic anisotropy of the upper mantle beneath the CPR concentrating on the eastward elongation of the fast anisotropic pattern of the Eastern Alps. We use data recorded by the temporary stations, set up for the Carpathian Basin Project (CBP) extending from the Vienna basin through Hungary into Serbia, together with the petrologic indicator of deformation in basalt-hosted upper mantle xenoliths from Szentbékkála. While some of the results agree with another recently published anisotropy analysis of the Eastern Alps and earlier results for the Pannonian basin, some don't. We reprocess data from 45 temporary CBP stations consisting recorded waveform of teleseismic events with magnitude greater than 6.0 Mw that have occurred in the distance range from 90° to 130° between 2005 and 2007. The dominant fast polarization orientation is NW-SE for the Hungarian part of the stations as well as at the stations located in the easternmost of the Pannonian Basin. This orientation matches well with the anisotropy structure under the Eastern Alps, which has been attributed to asthenospheric flow. The NW-SE trend turns more into WNW-ESE at the stations situated in Austria, mostly NW of the Vienna basin. Apart from predominately NW-SE fast orientation, there are a few measurements showing NE-SW and almost E-W trend. The SKS measurements most likely originate mainly from the asthenosphere and consequently past (Miocene) asthenospheric flow in the upper mantle may still have a significant contribution to the fast azimuth directions, however, the effect of other factors cannot be excluded. Xenoliths from the deeper lithospheric mantle have LPO structures of minerals as A-type of olivine in which the fast axis azimuth may correspond to the shear direction, possibly present-day asthenospheric flow in the upper mantle. Large delay times (>1.0 s) might also

  11. Water in the Cratonic Mantle: Insights from FTIR Data on Lac De Gras Xenoliths (Slave Craton, Canada)

    NASA Technical Reports Server (NTRS)

    Peslier, Anne H.; Brandon, Alan D.; Schaffer, Lillian Aurora; O'Reilly, Suzanne Yvette; Griffin, William L.; Morris, Richard V.; Graff, Trevor G.; Agresti, David G.

    2014-01-01

    The mantle lithosphere beneath the cratonic part of continents is the deepest (> 200 km) and oldest (>2-3 Ga) on Earth, remaining a conundrum as to how these cratonic roots could have resisted delamination by asthenospheric convection over time. Water, or trace H incorporated in mineral defects, could be a key player in the evolution of continental lithosphere because it influences melting and rheology of the mantle. Mantle xenoliths from the Lac de Gras kimberlite in the Slave craton were analyzed by FTIR. The cratonic mantle beneath Lac de Gras is stratified with shallow (<145 km) oxidized ultradepleted peridotites and pyroxenites with evidence for carbonatitic metasomatism, underlain by reduced and less depleted peridotites metasomatized by kimberlite melts. Peridotites analyzed so far have H O contents in ppm weight of 7-100 in their olivines, 58 to 255 in their orthopyroxenes (opx), 11 to 84 in their garnet, and 139 in one clinopyroxene. A pyroxenite contains 58 ppm H2O in opx and 5 ppm H2O in its olivine and garnet. Olivine and garnet from the deep peridotites have a range of water contents extending to higher values than those from the shallow ones. The FTIR spectra of olivines from the shallow samples have more prominent Group II OH bands compared to the olivines from the deep samples, consistent with a more oxidized mantle environment. The range of olivine water content is similar to that observed in Kaapvaal craton peridotites at the same depths (129-184 km) but does not extend to as high values as those from Udachnaya (Siberian craton). The Slave, Kaapvaal and Siberian cratons will be compared in terms of water content distribution, controls and role in cratonic root longevity.

  12. Ultramafic xenoliths from the Bearpaw Mountains, Montana, USA: Evidence for multiple metasomatic events in the lithospheric mantle beneath the Wyoming craton

    USGS Publications Warehouse

    Downes, H.; Macdonald, R.; Upton, B.G.J.; Cox, K.G.; Bodinier, J.-L.; Mason, P.R.D.; James, D.; Hill, P.G.; Hearn, B.C.

    2004-01-01

    Ultramafic xenoliths in Eocene minettes of the Bearpaw Mountains volcanic field (Montana, USA), derived from the lower lithosphere of the Wyoming craton, can be divided based on textural criteria into tectonite and cumulate groups. The tectonites consist of strongly depleted spinel lherzolites, harzbugites and dunites. Although their mineralogical compositions are generally similar to those of spinel peridotites in off-craton settings, some contain pyroxenes and spinels that have unusually low Al2O3 contents more akin to those found in cratonic spinel peridotites. Furthermore, the tectonite peridotites have whole-rock major element compositions that tend to be significantly more depleted than non-cratonic mantle spinel peridotites (high MgO, low CaO, Al2O3 and TiO2) and resemble those of cratonic mantle. These compositions could have been generated by up to 30% partial melting of an undepleted mantle source. Petrographic evidence suggests that the mantle beneath the Wyoming craton was re-enriched in three ways: (1) by silicate melts that formed mica websterite and clinopyroxenite veins; (2) by growth of phlogopite from K-rich hydrous fluids; (3) by interaction with aqueous fluids to form orthopyroxene porphyroblasts and orthopyroxenite veins. In contrast to their depleted major element compositions, the tectonite peridotites are mostly light rare earth element (LREE)-enriched and show enrichment in fluid-mobile elements such as Cs, Rb, U and Pb on mantle-normalized diagrams. Lack of enrichment in high field strength elements (HFSE; e.g. Nb, Ta, Zr and Hf) suggests that the tectonite peridotites have been metasomatized by a subduction-related fluid. Clinopyroxenes from the tectonite peridotites have distinct U-shaped REE patterns with strong LREE enrichment. They have 143Nd/144Nd values that range from 0??5121 (close to the host minette values) to 0??5107, similar to those of xenoliths from the nearby Highwood Mountains. Foliated mica websterites also have low 143Nd

  13. Depleted subcontinental lithospheric mantle and its tholeiitic melt metasomatism beneath NE termination of the Eger Rift (Europe): the case study of the Steinberg (Upper Lusatia, SE Germany) xenoliths

    NASA Astrophysics Data System (ADS)

    Kukuła, Anna; Puziewicz, Jacek; Matusiak-Małek, Magdalena; Ntaflos, Theodoros; Büchner, Jörg; Tietz, Olaf

    2015-12-01

    The ca. 30 Ma Steinberg basanite occurs at the NE termination of the Eger (Ohře) Rift in the NW Bohemian Massif, Central Europe, and belongs to the Cenozoic alkaline Central European Volcanic Province. The basanite hosts a suite of mantle xenoliths, most of which are harzburgites containing relatively magnesian olivine (Fo 90.5-91.6) and Al-poor (0.04-0.13 a pfu) orthopyroxene (mg# 0.90-0.92). Some of these harzburgites also contain volumetrically minor clinopyroxene (mg# 0.92-0.95, Al 0.03-0.13 a pfu) and have U-shaped LREE-enriched REE patterns. The Steinberg harzburgites are typical for the Lower Silesian - Upper Lusatian domain of the European subcontinental lithospheric mantle. They represent residual mantle that has undergone extensive partial melting and was subsequently affected by mantle metasomatism by mixed carbonatite-silicate melts. The Steinberg xenolith suite comprises also dunitic xenoliths affected by metasomatism by melt similar to the host basanite, which lowered the Fo content in olivine to 87.6 %. This metasomatism happened shortly before xenolith entrainment in the erupting lava. One of the xenoliths is a wehrlite (olivine Fo 73 %, clinopyroxene mg# 0.83-0.85, subordinate orthopyroxene mg# 0.76-0.77). Its clinopyroxene REE pattern is flat and slightly LREE-depleted. This wehrlite is considered to be a tholeiitic cumulate. One of the studied harzburgites contains clinopyroxene with similar trace element contents to those in wehrlite. This type of clinopyroxene records percolation of tholeiitic melt through harzburgite. The tholeiitic melt might be similar to Cenozoic continental tholeiites occurring in the Central European Volcanic Province (e.g., Vogelsberg, Germany).

  14. Frozen melt-rock reaction in a peridotite xenolith from sub-arc mantle recorded by diffusion of trace elements and water in olivine

    NASA Astrophysics Data System (ADS)

    Tollan, P. M. E.; O'Neill, H. St. C.; Hermann, J.; Benedictus, A.; Arculus, R. J.

    2015-07-01

    Inferring the ambient state of the lithospheric mantle from xenoliths may be misleading if the magmatic activity responsible for the exhumation has modified the xenolith. Changes due to melt-xenolith interactions during exhumation are usually identifiable because of the short timescales involved, but changes due to earlier pulses of magma passing through the lithosphere may be more insidious because the longer timescales allow extensive overprinting of the ambient petrography. Here we describe an intermediate stage of melt-rock reaction recorded in a porphyroclastic peridotite xenolith from the upper mantle wedge adjacent to the West Bismarck Island Arc. The texture and chemistry, frozen during rapid exhumation, reflects the progressive change from harzburgite to clinopyroxene-bearing dunite caused by the infiltration of hydrous basaltic melt into the xenolith. The unusually low equilibration temperature of the xenolith (∼650 °C) prior to this event is reflected in very low concentrations of incompatible elements in the olivine, enhancing the diffusion profiles of these elements into the olivine from the interaction with the melt. The Ca concentration profiles correspond to a timescale of approximately three months, which we interpret as the time between the first infiltration of melt into the lithospheric mantle at this locality and the subsequent exhumation of the xenolith by a second pulse of magma. The diffusion profiles of other trace elements (Li, Na, Sc, Ti, V, Cr, Mn, Ni, Cu, Y) and major elements (Fe-Mg) confirm that all these elements diffuse through olivine at rates that differ from each other by less than one order of magnitude. Infrared spectroscopy reveals OH- ("water") contents and incorporation mechanisms typical of other arc mantle peridotites in the olivine cores. Water contents increase towards the crystal rims, similarly to the profiles of other measured trace elements. The increase in water concentration is accompanied by subtle changes in

  15. Anisotropic Structure of the Upper Mantle in the Pannonian Basin: From SKS Splitting data and Xenolith Constraints

    NASA Astrophysics Data System (ADS)

    Qorbani, Ehsan; Bokelmann, Götz; Kovács, Istvan; Falus, György

    2015-04-01

    The Carpathian-Pannonian region (CPR) is the northeastern end of the Alpine mountain belt. In the Western Alps, available results of anisotropy investigations (SKS splitting) show clear belt-parallel anisotropy. In the Eastern Alps this pattern not only is broken but also does not follow the strike of Carpathians and Dinarides. This study is aimed at evaluating the seismic anisotropy of the upper mantle beneath the Pannonian Basin concentrating on the eastward elongation of the fast anisotropic pattern of the Eastern Alps. We use data recorded by the temporary stations, set up for the Carpathian Basin Project (CBP) extending from the Vienna basin through Hungary into Serbia, together with the petrological indicator of deformation in basalt-hosted upper mantle xenoliths from Szentbékkála analyzed with the EBSD system at Geosciences Montpellier, France (Kovács et al., 2012). We reprocess data from 45 temporary CBP stations consisting of recorded waveform of teleseismic events. The minimum energy method is applied to measure the splitting delay time and fast axis orientation. The dominant fast polarization orientation is NW-SE for the Hungarian part of the stations as well as at the stations located in the easternmost of the Pannonian Basin. This orientation matches well with the anisotropy structure under the Eastern Alps, which has already been attributed to the asthenospheric flow (Qorbani et al., 2015). The NW-SE trend turns more into WNW-ESE at the stations situated in Austria, mostly NW of the Vienna basin. Apart from predominately NW-SE fast orientation, there are a few measurements showing NE-SW and almost E-W trend. The lithospheric thickness in the CPR region is estimated about 60 km, the SKS measurements thus most likely originate mainly from the asthenosphere. Lattice preferred orientation of xenoliths from the deeper lithospheric mantle show A-type fabric of olivine, in which the fast axis azimuth may correspond to the shear direction, possibly present

  16. A Dynamic study of Mantle processes applying In-situ Methods to Compound Xenoliths: implications for small to intermediate scale heterogeneity

    NASA Astrophysics Data System (ADS)

    Baziotis, Ioannis; Asimow, Paul; Koroneos, Antonios; Ntaflos, Theodoros; Poli, Giampero

    2013-04-01

    The mantle is the major geochemical reservoir of most rock-forming elements in the Earth. Convection and plate-tectonic driven processes act to generate local and regional heterogeneity within the mantle, which in turn through thermal and chemical interactions modulates ongoing geophysical processes; this feedback shapes the dynamics of the deep interior. Consequently, these processes contribute to the evolution of the earth throughout its geological history. Up to now, the heterogeneity of the mantle has been extensively studied in terms of conventional methods using basalt chemistry, bulk rock and mineral major and trace element analysis of isolated xenolith specimens of varying lithology, and massif exposures. The milestone of the present study, part of an ongoing research project, is the application of in-situ analytical methods such as microprobe, LA-ICP-MS and high resolution SEM in order to provide high quality major and trace element analyses as well as elemental distribution of the coexisting phases in the preserved intra-mantle lithologies, Particularly, in the context of the current study we used selected compound xenoliths from San Carlos (Arizona, USA), Kilbourne Hole (New Mexico, USA), Cima Dome and Dish Hill suites (California, USA), San Quintin (Baja California, Mexico) and Chino Valley (Arizona, USA), from the Howard Wilshire collection archived at the Smithsonian Institution. The selection of these compound xenoliths was based upon freshness and integrity of specimens, maximum distance on both sides of lithologic contacts, and rock types thought most likely to represent subsolidus juxtaposition of different lithologies that later partially melted in contact. The San Carlos samples comprise composite xenoliths with websterite, lherzolite and clinopyroxenite layers or clinopyroxenite veins surrounded by lherzolite or orthopyroxenite-rich rims. The Kilbourne Hole suite comprises spinel-(olivine) clinopyroxenite and orthopyroxenite dikes cutting

  17. A negative Ce anomaly in a peridotite xenolith: Evidence for crustal recycling into the mantle or mantle metasomatism

    SciTech Connect

    Neal, C.R.; Taylor, L.A. )

    1989-05-01

    The presence of negative cerium anomalies in island arc lavas has been considered enigmatic. Such negative Ce anomalies must be inherent in the source region and can be produced by subducting pelagic sediments + seawater-altered basalts (SWAB) into the mantle. A mantle peridotite from the Malaitan alnoeite also contains a negative Ce anomaly, which can be produced by sediment recycling into the upper mantle. However, in spite of the poorly defined effects of cryptic metasomatism and associated fO{sub 2} conditions, such a process also seems plausible for the generation of negative Ce anomalies. In order to define the petrogenesis of this peridotite within the constraints of the present experimental data, the authors have attempted various mixing models with the end-members: mantle peridotite, Pacific sediment and seawater-altered basalt. In their model, it was assumed that negative Ce anomalies cannot be produced by magmatic or metasomatic processes. Best results were obtained from mixing a maximum of 3 to 5% seawater-altered basalt and 2 to 3% pelagic sediment, with a LREE-enriched mantle precursor. The authors' model stresses the importance of the contribution that recycled crustal materials can have on the composition of the upper mantle, in particular the recognition of a crustal signature in a mantle regime.

  18. Rapid growth of phosphorus-rich olivine in mantle xenolith from Middle Atlas Mountains (Morocco, Africa)

    NASA Astrophysics Data System (ADS)

    Baziotis, Ioannis; Mavrogonatos, Konstantinos; Flemetakis, Stamatios; Papoutsa, Angeliki; Klemme, Stephan; Berndt, Jasper; Asimow, Paul

    2016-04-01

    Phosphorus(P)-rich zones in olivine may reflect incorporation of P in excess of equilibrium partitioning during rapid growth (e.g. Milman-Barris et al. 2008). We investigated a mantle xenolith from Middle Atlas Mountains (Morocco) by optical microscopy and electron microprobe. It contains spinel-bearing lherzolite and orthopyroxenite layers, cross-cut by veins dominated by glass and secondary phases including P-rich olivines. The host lava, presumed to be alkali basalt (El Messbahi et al. 2015), is present on the margins of the hand sample but not included in our thin section. The studied melt veins (MV) generally contain Ol+Gl+Cpx+Pl+Spl±Ap. Olivines in the MV have (Fo72.1‑83.4) with 0.02-0.3 wt.% P2O5; olivines with P2O5 >0.1 wt.% are Fo75.3 ‑82.8. Some olivine grains are inclusion-free; others contain rounded glass inclusions or subhedral spinel or ilmenite inclusions. Olivines is generally found in contact with plagioclase and glass. Glass (5-15 vol%) has variable composition with P2O5 up to 1.52 wt.%, K2O 1.65-2.37 wt%, CaO 6.39-9.55 wt%, Na2O 0.78-6.70 wt% and SiO2 45.2-49.6 wt%. Where glass is in contact with matrix olivine, Fe-rich outer rims on olivine indicate mineral-melt reaction. In MgO variation diagrams, glass compositions display a coherent single trend for all oxides, with the exception of a discrete low-Na group. Clinopyroxene is present both as isolated subhedral to euhedral crystals within the MV and as replacive rims on matrix minerals. Very fine-grained dendritic clinopyroxene quench crystals up to 10 μm long are also present. Plagioclase occurs as prismatic, flow-oriented crystals parallel or sub-parallel to the layering. Spinel shows anhedral and euhedral shapes and occurs both as inclusions in olivine and as discrete grains associated with plagioclase and glass. Spinel in contact with glass shows a spongy outer rim and normal zonation towards Fe-rich rim compositions. Apatite is found mostly as very small crystals embedded in glass

  19. Rapid growth of phosphorus-rich olivine in mantle xenolith from Middle Atlas Mountains (Morocco, Africa)

    NASA Astrophysics Data System (ADS)

    Baziotis, Ioannis; Mavrogonatos, Konstantinos; Flemetakis, Stamatios; Papoutsa, Angeliki; Klemme, Stephan; Berndt, Jasper; Asimow, Paul

    2016-04-01

    Phosphorus(P)-rich zones in olivine may reflect incorporation of P in excess of equilibrium partitioning during rapid growth (e.g. Milman-Barris et al. 2008). We investigated a mantle xenolith from Middle Atlas Mountains (Morocco) by optical microscopy and electron microprobe. It contains spinel-bearing lherzolite and orthopyroxenite layers, cross-cut by veins dominated by glass and secondary phases including P-rich olivines. The host lava, presumed to be alkali basalt (El Messbahi et al. 2015), is present on the margins of the hand sample but not included in our thin section. The studied melt veins (MV) generally contain Ol+Gl+Cpx+Pl+Spl±Ap. Olivines in the MV have (Fo72.1-83.4) with 0.02-0.3 wt.% P2O5; olivines with P2O5 >0.1 wt.% are Fo75.3 -82.8. Some olivine grains are inclusion-free; others contain rounded glass inclusions or subhedral spinel or ilmenite inclusions. Olivines is generally found in contact with plagioclase and glass. Glass (5-15 vol%) has variable composition with P2O5 up to 1.52 wt.%, K2O 1.65-2.37 wt%, CaO 6.39-9.55 wt%, Na2O 0.78-6.70 wt% and SiO2 45.2-49.6 wt%. Where glass is in contact with matrix olivine, Fe-rich outer rims on olivine indicate mineral-melt reaction. In MgO variation diagrams, glass compositions display a coherent single trend for all oxides, with the exception of a discrete low-Na group. Clinopyroxene is present both as isolated subhedral to euhedral crystals within the MV and as replacive rims on matrix minerals. Very fine-grained dendritic clinopyroxene quench crystals up to 10 μm long are also present. Plagioclase occurs as prismatic, flow-oriented crystals parallel or sub-parallel to the layering. Spinel shows anhedral and euhedral shapes and occurs both as inclusions in olivine and as discrete grains associated with plagioclase and glass. Spinel in contact with glass shows a spongy outer rim and normal zonation towards Fe-rich rim compositions. Apatite is found mostly as very small crystals embedded in glass. High

  20. Crystal preferred orientations of minerals from mantle xenoliths in alkali basaltic rocks form the Catalan Volcanic Zone (NE Spain)

    NASA Astrophysics Data System (ADS)

    Fernández-Roig, Mercè; Galán, Gumer; Mariani, Elisabetta

    2015-04-01

    Mantle xenoliths in alkali basaltic rocks from the Catalan Volcanic Zone, associated with the Neogene-Quaternary rift system in NE Spain, are formed of anhydrous spinel lherzolites and harzburgites with minor olivine websterites. Both peridotites are considered residues of variable degrees of partial melting, later affected by metasomatism, especially the harzburgites. These and the websterites display protogranular microstructures, whereas lherzolites show continuous variation between protogranular, porphyroclastic and equigranular forms. Thermometric data of new xenoliths indicate that protogranular harzburgites, lherzolites and websterites were equilibrated at higher temperatures than porphyroclastic and equigranular lherzolites. Mineral chemistry also indicates lower equilibrium pressure for porphyroclastic and equigranular lherzolites than for the protogranular ones. Crystal preferred orientations (CPOs) of olivine and pyroxenes from these new xenoliths were determined with the EBSD-SEM technique to identify the deformation stages affecting the lithospheric mantle in this zone and to assess the relationships between the deformation fabrics, processes and microstructures. Olivine CPOs in protogranular harzburgites, lherzolites and a pyroxenite display [010]-fiber patterns characterized by a strong point concentration of the [010] axis normal to the foliation and girdle distribution of [100] and [001] axes within the foliation plane. Olivine CPO symmetry in porphyroclastic and equigranular lherzolites varies continuously from [010]-fiber to orthorhombic and [100]-fiber types. The orthorhombic patterns are characterized by scattered maxima of the three axes, which are normal between them. The rare [100]-fiber patterns display strong point concentration of [100] axis, with normal girdle distribution of the other two axes, which are aligned with each other. The patterns of pyroxene CPOs are more dispersed than those of olivine, especially for clinopyroxene, but

  1. Petrological constraints on evolution of continental lithospheric mantle beneath the northwestern Ethiopian plateau: Insight from mantle xenoliths from the Gundeweyn area, East Gojam, Ethiopia

    NASA Astrophysics Data System (ADS)

    Alemayehu, Melesse; Zhang, Hong-Fu; Zhu, Bin; Fentie, Birhanu; Abraham, Samuel; Haji, Muhammed

    2016-01-01

    Detailed petrographical observations and in-situ major- and trace-element data for minerals from ten spinel peridotite xenoliths from a new locality in Gundeweyn area, East Gojam, have been examined in order to understand the composition, equilibrium temperature and pressure conditions as well as depletion and enrichment processes of continental lithospheric mantle beneath the Ethiopian plateau. The peridotite samples are very fresh and, with the exception of one spinel harzburgite, are all spinel lherzolites. Texturally, the xenoliths can be divided into two groups as primary and secondary textures. Primary textures are protogranular and porphyroclastic while secondary ones include reaction, spongy and lamellae textures. The Fo content of olivine and Cr# of spinel ranges from 86.5 to 90.5 and 7.7 to 14.1 in the lherzolites, respectively and are 89.8 and 49.8, respectively, in the harzburgite. All of the lherzolites fall into the lower Cr# and Fo region in the olivine-spinel mantle array than the harzburgite, which indicates that they are fertile peridotites that experienced low degrees of partial melting and melt extraction. Orthopyroxene and clinopyroxene show variable Cr2O3 and Al2O3 contents regardless of their lithology. The Mg# of orthopyroxene and clinopyroxene are 87.3 to 90.1 and 85.8 to 90.5 for lherzolite and 90.4 and 91.2 for harzburgite, respectively. The peridotites have been equilibrated at a temperature and pressure ranging from 850 to 1100 °C and 10.2 to 30 kbar, respectively, with the highest pressure record from the harzburgite. They record high mantle heat flow between 60 and 150 mW/m2, which is not typical for continental environments (40 mW/m2). Such a high geotherm in continental area shows the presence of active mantle upwelling beneath the Ethiopian plateau, which is consistent with the tectonic setting of nearby area of the Afar plume. Clinopyroxene of five lherzolites and one harzburgite samples have a LREE enriched pattern and the rest

  2. Sub-continental lithospheric mantle structure beneath the Adamawa plateau inferred from the petrology of ultramafic xenoliths from Ngaoundéré (Adamawa plateau, Cameroon, Central Africa)

    NASA Astrophysics Data System (ADS)

    Nkouandou, Oumarou F.; Bardintzeff, Jacques-Marie; Fagny, Aminatou M.

    2015-11-01

    Ultramafic xenoliths (lherzolite, harzburgite and olivine websterite) have been discovered in basanites close to Ngaoundéré in Adamawa plateau. Xenoliths exhibit protogranular texture (lherzolite and olivine websterite) or porphyroclastic texture (harzburgite). They are composed of olivine Fo89-90, orthopyroxene, clinopyroxene and spinel. According to geothermometers, lherzolites have been equilibrated at 880-1060 °C; equilibrium temperatures of harzburgite are rather higher (880-1160 °C), while those of olivine websterite are bracketed between 820 and 1010 °C. The corresponding pressures are 1.8-1.9 GPa, 0.8-1.0 GPa and 1.9-2.5 GPa, respectively, which suggests that xenoliths have been sampled respectively at depths of 59-63 km, 26-33 km and 63-83 km. Texture and chemical compositional variations of xenoliths with temperature, pressure and depth on regional scale may be ascribed to the complex history undergone by the sub-continental mantle beneath the Adamawa plateau during its evolution. This may involve a limited asthenosphere uprise, concomitantly with plastic deformation and partial melting due to adiabatic decompression processes. Chemical compositional heterogeneities are also proposed in the sub-continental lithospheric mantle under the Adamawa plateau, as previously suggested for the whole Cameroon Volcanic Line.

  3. A Lower Mantle Origin for Megacryst Suite Pyroxene-Ilmenite Xenoliths in Kimberlites: High-Pressure Experimental Constraints and Geodynamic Significance

    NASA Astrophysics Data System (ADS)

    Collerson, K. D.; Terasaki, H.; Ohtani, E.; Suzuki, A.; Kondo, T.

    2004-12-01

    Megacryst suite xenoliths (MSX's) in kimberlites, alnoites and alkali basalts are an important and poorly understood association. MSX's comprise medium- to coarse-grained monomineralic, or rare, multi-grain aggregates of low Cr, high Ti-Na pyrope, Mg ilmenite, sub-calcic pyroxene, Fo85±3 olivine, orthopyroxene and zircon. Some MSX's exceed 30 cm in diameter. They are interpreted to form by fractional crystallization from their host magma, near the base of the lithosphere [1-2]. However, majorite, and other high-pressure phases in some garnetite MSX's, indicates a mantle transition zone (TZ) origin [3]. A sub-lithospheric, deeply subducted slab source is also supported by Hf isotopic data [4]. A common member of the MSX suite, are graphic intergrowths of pyroxene and Mg-ilmenite interpreted to reflect cotectic, or non-equilibrium crystallization [5-6] from the kimberlite magma. However, Pb isotopic data for Monastery [7], and Namibian [8] megacrysts shows that MSX's and their host magmas are unrelated. Thus the mineralogy of the Ti-rich px-ilm MSX's needs to be determined at TZ and higher P. We have conducted multi-anvil (MA) and diamond anvil (DA) experiments on natural px-ilm xenoliths from Monastery and Malaita with different TiO2 contents (17% and 12%), in an attempt to synthesize the pre-exsolution phase. MA experiments were carried out on both starting compositions at 18 and 21 GPa, at 1800° C and 2100° C. None of the experiments yielded a single phase. Phases identified (EPMA, Raman & XRD) include: majorite, Si-rich ilmenite and Ca-Si-Ti Pv. At 21 GPa and 2100° C wadsleyite formed part of the assemblage, and melt was locally developed. Majorite is the most abundant phase in all experiments. Maximum majorite TiO2 occurs at 18 GPa (i.e., 5.4% - Malaita and 6.2% - Monastery). In the lower Ti Malaita composition, at 25 GPa and 1800° C, the assemblage is dominated by almost equal amounts of majorite (TiO2 1.3% to 2.1%) and Ca-Si-Ti Pv, with a small amount of

  4. Distribution and behaviour during metasomatism of PGE-Re and Os isotopes in off-craton mantle xenoliths from Namibia

    NASA Astrophysics Data System (ADS)

    Aulbach, S.; Luchs, T.; Brey, G. P.

    2014-01-01

    Platinum-group element (PGE) and Re-Os isotopes were measured by isotope dilution ICPMS in five well-characterised peridotite xenoliths from the Gibeon kimberlite field in the Proterozoic Rehoboth Terrane, Namibia. The distribution of these elements and Os isotopes between different size and mineral fractions within the samples indicates that incompatible Pd and Pt likely reside in minerals typically < 1 mm size, such as interstitial sulphides of magmatic-metasomatic origin, whereas compatible Ir and Os may be controlled by primary-residual sulphide inclusions in large silicates. The lowest Pd/Ir, which decreases during partial melting and increases during melt metasomatism, and the most depleted (least radiogenic) 187Os/188Os (0.1112-0.1190) are consistently obtained for opx-rich fractions in cpx-poor samples, which therefore represent the most promising targets to date melt extraction from the mantle. Significant Os isotope disequilibrium between fractions containing opx and secondary cpx in the same sample implies that non-modal partial melting of such rocks could generate melts with 187Os/188Os different from their average source mantle. Like Lu-Hf, but unlike Sm-Nd, PGE concentrations and Os isotopes appear to be transparent to cryptic metasomatism by carbonate melts or fluids, as evidenced by preservation of the least radiogenic Os and low Pd/Ir in the sample with the most unradiogenic (enriched) Nd and radiogenic (depleted) Hf. In contrast, both modal metasomatism (clinopyroxene addition) at 0.45 Ga and silicate melt metasomatism at 1.85 Ga led to higher Pd/Ir and direct addition of radiogenic Os and/or Re addition followed by ingrowth of radiogenic Os, with 187Os/188Os up to 0.179. Although the mantle beneath the Rehoboth Terrane, which adjoins the Kaapvaal craton, has features resembling Archaean lithospheric mantle (e.g., cool conductive geothermal gradient, occasional opx enrichment), only one aliquot in one of the fractions measured in this study

  5. Quantifying the effects of metasomatism in mantle xenoliths: Constraints from secondary chemistry and mineralogy in Udachnaya edlogites, Yakutia

    SciTech Connect

    Sobolev, V.N.; Taylor, L.A.; Snyder, G.A.; Jerde, E.A.; Neal, C.R.; Sobolev, N.V.

    1999-05-01

    In mantle xenoliths, metasomatism is recorded by compositional variations within and between minerals, and by the introduction of secondary minerals. However, metasomatism has not been quantitatively evaluated as a process with respect to the fluid composition involved. Diamondiferous eclogites from the Udachnaya kimberlite provide a unique suite of samples that allow a semi-quantitative estimation of metasomatic fluid composition. The basis of this analysis involves comparison of reconstructed whole-rock compositions with measured whole-rock analyses. Primary minerals in these samples are relatively homogeneous, and permit the use of modal analyses and mineral chemistry for reconstruction of pristine whole-rock compositions. The metasomatic overprint, which is similar in all samples studied, has produced depletions in SiO{sub 2}, Na{sub 2}O, and FeO and enrichments in TiO{sub 2}, K{sub 2}O, MgO, and LREE. Secondary minerals from the samples are interpreted as the direct result of metasomatism (i.e., typical metasomatic minerals such as phlogopite, amphibole, djerfisherite, and sodalite are present in these xenoliths). Enrichment/depletion signatures demonstrate that the major metasomatic source for Udachnaya eclogites was not derived from the host kimberlite. These metasomatic agents appear to have been more enriched in TiO{sub 2}, K{sub 2}O, Cl, FeO, and LREE than are kimberlites, and may have contained significant amounts of F, CO{sub 2}, and H{sub 2}O. The high Ca contents of two samples are interpreted to be the product of metasomatism by a carbonatite-like fluid.

  6. 40Ar/ 39Ar-ages of phlogopite in mantle xenoliths from South African kimberlites: Evidence for metasomatic mantle impregnation during the Kibaran orogenic cycle

    NASA Astrophysics Data System (ADS)

    Hopp, Jens; Trieloff, M.; Brey, G. P.; Woodland, A. B.; Simon, N. S. C.; Wijbrans, J. R.; Siebel, W.; Reitter, E.

    2008-12-01

    We applied the 40Ar/ 39Ar dating method to an extensive suite of phlogopites from kimberlite-hosted mantle xenoliths (dominantly garnet bearing) from the mines of Bultfontein (South Africa), Letseng-la-Terae and Liqhobong (Lesotho). Argon extraction was performed by conventional high resolution stepwise heating technique, laser incremental heating technique and laser spot analysis. All age spectra obtained by conventional analysis indicate various degrees of 40Ar loss during kimberlite emplacement, but never resulted in a total reset of the argon system. Most intriguingly, the sample-specific maximum apparent ages cluster between 1.0 and 1.22 Ga for the phlogopites with the least disturbed age spectra. A maximum apparent age of 1.02 Ga was observed during laser heating analysis. Individual grains tend to yield older ages in their cores, with successively younger ages at their rims. The range in age obtained via the laser fusion technique and with conventional stepwise heating technique agrees with each other, as well as with literature data. The often inferred presence of excess 40Ar in those phlogopites cannot explain the coherent age pattern in the large suite of samples. Hence, the age constraint of 1.0-1.25 Ga is regarded as geologically meaningful and assigned to metasomatism of the local cratonic mantle during the advent of Kibaran orogenesis (1.00-1.25 Ga). The major consequences of our findings are: (i) The argon system of phlogopite can remain closed for long time scales, even at ambient temperatures of 800-1200 °C within the mantle, most likely because the solid/solid partitioning behaviour of Ar between phlogopite and other major phases in the mantle strongly favours phlogopite, or because conventionally inferred diffusivity of argon in phlogopite is seriously overestimated. Thus, the 40Ar/ 39Ar phlogopite system appears to be a valuable tool for deciphering ancient metasomatic events affecting the lithospheric mantle. (ii) The cratonic lithospheric

  7. Xenoliths From Isla Isabel, Nayarit, Mexico: The Nature of the Upper Mantle Underneath the Western Part of the Mexican Volcanic Belt

    NASA Astrophysics Data System (ADS)

    Housh, T. B.; Aranda-Gomez, J. J.; Luhr, J. F.

    2007-12-01

    Isla Isabel is located ~65 km NW of San Blas (Nayarit), off the Pacific coast of central Mexico. The island is a Quaternary (Ar/Ar < 0.7 Ma) volcanic complex built atop attenuated continental crust. Isabel lies on the east side of the mouth of the Gulf of California, near the area previously occupied (early Pliocene) by Los Cabos Block. Southeast of Isabel, on the mainland, is the NW-trending Tepic-Zacoalco rift, a major volcano-tectonic structure in the western part of the Mexican Volcanic Belt. On land, the rift is the boundary between the Jalisco and Sierra Madre Occidental blocks, and Isabel lies along its projection. Immediately S of Isabel is the San Blas Trough, a swale that trends NW-SE, co-linear with a gravity lineation parallel to the Tamayo and San Blas fault zones, which are the transform boundaries between the northern Rivera and North American plates. Plio-Quaternary alkaline and calc-alkaline lavas have erupted contemporaneously in the Tepic-Zacoalco rift, but so far no mantle xenoliths have been reported in them. Isabel's rocks are intra-plate type alkaline basalts to trachybasalts, with 5-6%\\ normative Ne. Primary paragenesis in the lavas is: Ol + Pl + Cpx + TMt. Small (< 5 cm) peridotite xenoliths, and xenocrysts derived from them, are ubiquitous in the rocks. Eleven xenoliths were studied comprising 3 dunites, 7 harzburgites (one Pl-bearing), and 1 gabbro. Compared to other Mexican xenolith localities N of the MVB, they are refractory as they are depleted in, or lack, Cpx. Ol crystals in xenoliths are homogenous and their Mg#\\ s range as follows: peridotites (92-88), gabbro (84), and Pl-harzburgite (80). Cores of primary Ol phenocrysts (90.5-86.5) in Isabel's volcanic rocks are significantly higher in Mg#\\ s than in Ol from other Mexican xenolith localities (max. 86) and overlap with the associated peridotite xenoliths. Such overlap has not been reported for other Mexican xenolith localities. Xenolith equilibration temperatures for 5

  8. Flow in the shallow mantle in the westernmost Mediterranean: insights from xenoliths in Plio-Pleistocene alkali basalts from the eastern Betic Cordillera (SE Spain)

    NASA Astrophysics Data System (ADS)

    Konc, Zoltán; Hidas, Károly; Garrido, Carlos J.; Tommasi, Andréa; Vauchez, Alain; Padrón Navarta, José Alberto; Marchesi, Claudio; Acosta-Vigil, Antonio; Szabó, Csaba; Varas-Reus, Maria Isabel

    2016-04-01

    Peridotite mantle xenoliths in Plio-Pleistocene alkali basalts of the eastern Betic Cordillera (Cartagena area, Murcia, SE Spain) provide a snapshot of the structure and composition of the lithospheric mantle at the northern limb of the Alpine Betic-Rif arched belt in the westernmost Mediterranean. The xenoliths are spinel and plagioclase lherzolite with minor harzburgite and wehrlite, displaying porphyroclastic to equigranular textures. Regardless of composition and texture, the Crystal Preferred Orientation (CPO) of olivine shows an axial-[100] pattern characterized by a strong alignment of [100]-axes near or parallel to the peridotite lineation and a girdle distribution of [010]-axes with a maximum normal to the peridotite foliation. This CPO pattern is consistent with ductile deformation accommodated by dislocation creep with dominant activation of the high temperature {0kl}[100] olivine slip system, indicative of deformation by simple shear or combinations of simple shear and pure shear with a transtensional component. Calculated seismic properties are characterized by fast propagation of P-waves and polarization of fast S-waves parallel to olivine [100]-axis, indicating the flow direction. SKS and Pn anisotropy in the eastern Betics can be explained by a lithospheric mantle peridotite with similar fabric to the one displayed by the studied mantle xenoliths. Considering the limited thickness of the mantle lithosphere in the Betics (40-80 km), the measured azimuths and delays of SKS waves in the eastern Betics are consistent with a steeply dipping mantle foliation and a subhorizontal lineation with ENE strike. This geometry of the lithospheric fabrics implies active or frozen mantle flow with a dominantly strike-slip component subparallel to the paleo-Iberian margin. Synkinematic overprinting of mineral assemblages from the garnet-spinel to the plagioclase facies demonstrates 36-40 km uplift continuously accommodated by ductile shear thinning of the

  9. Geochemistry and Sr-Nd isotopic compositions of mantle xenoliths from the Monte Vulture carbonatite-melilitite volcano, central southern Italy

    NASA Astrophysics Data System (ADS)

    Downes, H.; Kostoula, T.; Jones, A. P.; Beard, A. D.; Thirlwall, M. F.; Bodinier, J.-L.

    2002-08-01

    Spinel peridotite xenoliths found in the Monte Vulture carbonatite-melilitite volcano have been derived from the subcontinental lithospheric mantle beneath central southern Italy. Clinopyroxene-poor lherzolites and harzburgites are the most common rock types, with subordinate wehrlites and dunites. Small quantities of phlogopite and carbonate are present in a few samples. The peridotites record a large degree of partial melting and have experienced subsequent enrichment which has increased their LILE and LREE contents, but in most cases their HFSE contents are low. Despite being carried to the surface by a carbonatite-melilitite host, the whole-rock and clinopyroxene compositions of the xenoliths have a trace-element signature more closely resembling that of silicate-melt metasomatised mantle rather than carbonatite-metasomatised peridotites. 87Sr/86Sr and 143Nd/144Nd isotopic ratios for clinopyroxene from the Vulture peridotites are 0.7042-0.7058 and 0.51260-0.5131 respectively. They form a trend away from the depleted mantle to the composition of the host magmas, and show a significant enrichment in 87Sr/86Sr compared with most European mantle samples. The mantle beneath Monte Vulture has had a complex evolution - we propose that the lithosphere had already undergone extensive partial melting before being affected by metasomatism from a silicate melt which may have been subduction-related.

  10. Olivine-mica pyroxenite xenoliths from northern Tanzania: metasomatic products of upper-mantle peridotite

    NASA Astrophysics Data System (ADS)

    Dawson, J. B.; Smith, J. V.

    1992-04-01

    Olivine-mica-pyroxene blocks in Neogene pyroclastics from Oldoinyo Lengai and Loluni, Tanzania, result from K, Ca, Fe, Ti, Al, REE, Cl, F and OH metasomatism of upper-mantle peridotite. Deformed olivine relicts and high Cr and Ni in bulk-rock analyses indicate a peridotite precursor.

  11. Diamond-bearing Rocks among Mantle Xenoliths in Kimberlites as Indicatory for the Chambers of Diamond-parental Carbonatite Magma

    NASA Astrophysics Data System (ADS)

    Litvin, Yuriy; Kuzyura, Anastasia

    2014-05-01

    may originate and evolve by: (1) metasomatic-magmatic stage resulted in partial carbonatization of mantle peridotite under attack of K-CO2-bearing metasomatic agents and generation of carbonate melts; (2) dissolving-magmatic stage when major and accessory minerals of peridotite host-rock, volatiles and carbon dissolve in carbonate melt whereas insoluble sulphide phases penetrate into melts; eventually, completely miscible peridotite-carbonatite-carbon magma parental for diamond and paragenetic minerals (hosting xenogenetic sulphide minerals and melts) are formed; (3) fraction-crystallization stage (in chamber consolidated into a self-dependent body) during natural cooling of parental magma up to solidus temperature; the cooling activates physicochemical control that is created by PT-phase relations for the parental magma composition, i.e., syngenesis phase diagram on a representative polythermal section of peridotite-eclogite-carbonatite-diamond system at 7 GPa under conditions of fractional crystallization (Litvin, 2013). Parental carbonatite melts, while compositionally evolve under fractional crystallization, are physicochemically capable to form diamond and sequentially minerals of peridotitic and eclogitic parageneses (presented as primary inclusions in diamonds). Paragenetic peridotite-eclogite transition in the course of ultrabasic-basic fractional evolution of parental melts is revealed in physicochemical experiments as the effect of 'peridotite-to-eclogite' tonnel (Litvin, 2013). Diamond-bearing peridotite and eclogite rocks and intimate mineral intergrowths with diamond are also formed in the chambers of diamond-parental carbonatite magmas under these physicochemical conditions. Diamond-free rocks among mantle xenoliths in kimberlites represent samples of the enclosing host-rocks for the chambers of diamond-parental carbonatite magma. Support: grant of the RF President #MK-1386.2013.5, RFBR grants 12-05-33044, 13-05-00835 and 14-05-00537.

  12. Eclogite xenoliths from the Lace kimberlite, Kaapvaal craton: From convecting mantle source to palaeo-ocean floor and back

    NASA Astrophysics Data System (ADS)

    Aulbach, S.; Viljoen, K. S.

    2015-12-01

    Major- and trace-element compositions of eclogite and pyroxenite xenoliths of ≥2.5 Ga age (in situ Pb-Pb data on clinopyroxene) from the Lace kimberlite on the Kaapvaal craton were investigated in order to constrain: (1) the nature and evolution of their protoliths; (2) the extent to which they preserve information on the state of the asthenospheric mantle source that gave rise to their low-pressure protoliths; and (3) the effect of their deep recycling on the radiogenic isotope evolution of the convecting mantle. Their elemental relationships are consistent with low-pressure fractionation of olivine ± plagioclase and clinopyroxene during oceanic crust formation, whereby the residual melt was enriched in rare-earth elements (REE), high field-strength elements and Y, producing inverse correlations of ΣREE with the size of Eu- and Sr-anomalies. LREE-depletion may indicate loss of on average 20% of a partial melt upon subduction and metamorphism (eclogitisation) of oceanic crust, which did not, however, contribute to juvenile growth of continental crust. The eclogites have median Sm/Nd (0.40) and Lu/Hf (0.27) similar to Depleted Mantle, and lower U/Pb (0.02) and Th/Pb (0.02). If deeply subducted, these rocks cannot explain unradiogenic Nd and Hf, and radiogenic Pb isotope compositions in the sources of some modern ocean island basalts. Low incompatible trace-element contents similar to picrites, and Yb concentrations at a given TiO2 content similar to modern MORB, indicate derivation of the protoliths by average melt fractions of ∼ 0.20- 0.25 that left a spinel peridotite residue at pressures ≤2.5 to 3.0 GPa. This shallow intersection of the peridotite solidus suggests moderate Archaean ambient mantle potential temperatures of ≤1420 to 1470 °C. Samples filtered for clinopyroxene fractionation and metasomatism have V/Sc (4.7 ± 1.2; n = 11) indicating lower fO2 (-1.9 relative to the fayalite-magnetite-quartz buffer = ΔFMQ) than modern MORB. This is in part

  13. Combining kimberlite (U-Th)/He dating with the mantle xenolith record to decipher elevation change in continental interiors: an example from the southern African Plateau

    NASA Astrophysics Data System (ADS)

    Stanley, J. R.; Flowers, R. M.; Bell, D. R.

    2013-12-01

    Deciphering the patterns and causes of erosion and elevation change histories in continental interiors is commonly not straightforward. Many continental shield regions are repeatedly intruded by small volume kimberlite magmas, which often contain rich xenolith records of the state of the lithosphere and the sedimentary cover at the time of eruption. Here we show that dating kimberlites with apatite (U-Th)/He thermochronometry (AHe), a tool used to constrain thermal histories within the upper 1-3 km of the crust, can tightly bracket the timing of erosion through comparison of cooling dates and eruption ages. Mantle xenoliths from kimberlites erupted at different times also record perturbations to the lithospheric mantle, indicators of changes to the mantle below. The coeval deep and shallow records of kimberlite pipes thus allow the potential to link deep earth processes with the surface response. The southern African Plateau was elevated from sea level to >1000 m elevation in post-Paleozoic time while distal from convergent plate boundaries and with little surface deformation. The timing and mechanisms of surface uplift are debated. AHe data for four kimberlites off the southwestern corner of the Kaapvaal Craton indicate a substantial Mesozoic unroofing episode that was largely completed by 90 Ma. This erosion phase is contemporaneous with significant warming, metasomatism, and thinning of the lithospheric mantle revealed in the peridotite xenoliths and garnet xenocrysts in these same pipes. We suggest that this surface signal is the erosional response to regional, mantle-induced surface uplift. These data also detect a lesser Cenozoic erosion signature in some pipes, focused around a proposed Tertiary paleo-tributary to the Orange River, suggesting that the Cenozoic signal is associated with drainage network evolution rather than long-wavelength surface uplift. Preliminary data from an E-W transect of kimberlites across the Kaapvaal Craton from Kimberley to the

  14. Sulfur isotope composition of metasomatised mantle xenoliths from the Bultfontein kimberlite (Kimberley, South Africa): Contribution from subducted sediments and the effect of sulfide alteration on S isotope systematics

    NASA Astrophysics Data System (ADS)

    Giuliani, Andrea; Fiorentini, Marco L.; Martin, Laure A. J.; Farquhar, James; Phillips, David; Griffin, William L.; LaFlamme, Crystal

    2016-07-01

    Sulfur isotopes are a powerful geochemical tracer in high-temperature processes, but have rarely been applied to the study of mantle metasomatism. In addition, there are very limited S isotope data on sub-continental lithospheric mantle (SCLM) material. For cratonic regions, these data are restricted to sulfide inclusions in diamonds. To provide new constraints on the S isotope composition of the SCLM and on the source(s) of mantle metasomatic fluids beneath the diamondiferous Kimberley region (South Africa), we investigated the S isotope systematics of five metasomatised mantle xenoliths from the Bultfontein kimberlite. Pentlandite and chalcopyrite in these xenoliths were analysed by in situ secondary-ion mass spectrometry (SIMS), with bulk-rock material measured by gas source isotope ratio mass spectrometry techniques. Based on previous studies, the xenoliths experienced different types of metasomatism to one another at distinct times (∼180 and ∼90-80 Ma). Contained pentlandite grains show variable alteration to heazlewoodite (i.e. Ni sulfide) + magnetite. The in situ S isotope analyses of pentlandite exhibit a relatively restricted range between -5.9 and - 1.4 ‰δ34 S (compared to VCDT), with no statistically meaningful differences between samples. Chalcopyrite only occurs in one sample and shows δ34 S values between -5.4 and - 1.0 ‰. The bulk-rock Ssulfide isotope analyses vary between -3.4 and + 0.8 ‰δ34 S. Importantly, the only sample hosting dominantly fresh sulfides shows a bulk-rock δ34 S value consistent with the mean value for the sulfides, whereas the other samples exhibit higher bulk 34S/32S ratios. The differences between bulk-rock and average in situδ34 S values are directly correlated with the degree of sulfide alteration. This evidence indicates that the elevated 34S/32S ratios in the bulk samples are not due to the introduction of heavy S (commonly as sulfates) and are best explained by isotopic fractionation coupled with the removal

  15. Chalcophile and Siderophile Element Abundances in Kilbourne Hole Lherzolites: Distinguishing the Signature of Melt Depleted Primitive Mantle from Metasomatic Overprints

    NASA Astrophysics Data System (ADS)

    Harvey, J.; König, S.; Luguet, A.

    2013-12-01

    Selenium, tellurium and the highly siderophile elements in peridotites have the potential to illustrate planetary scale processes that are opaque to lithophile elements. However, the interpretation of chalcophile and siderophile element abundances relies heavily on the selection of representative mantle material and the determination of what processes have affected these elements since melt depletion. Whole rock and in-situ sulfide data demonstrate that chalcophile and HSE systematics of the upper mantle could be significantly modified through sulfide-metasomatism, particularly by C-O-H-S × Cl fluids[1] or sulfide melts[2] i.e., chalcophile and siderophile element abundances result from a complex interplay between sulfide addition and alteration of pre-existing sulfide. Here we present new bulk-rock S-Se-Te-PGE abundances on a suite (n = 17) of lherzolite and harzburgite xenoliths from Kilbourne Hole, USA[3, 4]. Mineral modal abundances, major element contents and LREE/HREE ratios for 10 of these xenoliths are consistent with varying degrees of melt depletion (≤ 20 %) whereas the remainder appear to have been affected by cryptic metasomatism, refertilization, or melt-rock interaction which affected lithophile element abundances [4]. While sulfur, Se and PGE budgets are primarily controlled by sulfides, 50 × 30% of Te in peridotite may be accounted for by Pt-Pd tellurides[5]. Although most Kilbourne Hole peridotite xenoliths have PGE characteristics consistent with varying degrees of melt depletion and somewhat scattered Se/Te ratios, KH96-24 has Pt-Pd-Te abundances consistent with Pt-Pd-telluride precipitation, in addition to petrographic evidence for alteration by secondary processes[4]. S/Se are well correlated within the suite. However, lherzolites that retain a strong melt-depletion signature have distinctly lower abundances of both S and Se (<65 ppm and <31 ppm respectively) compared to peridotites that have had their lithophile element budgets perturbed

  16. Relationships between oxygen fugacity and metasomatism in the Kaapvaal subcratonic mantle, represented by garnet peridotite xenoliths in the Wesselton kimberlite, South Africa

    NASA Astrophysics Data System (ADS)

    Hanger, Brendan J.; Yaxley, Gregory M.; Berry, Andrew J.; Kamenetsky, Vadim S.

    2015-01-01

    A suite of 12 peridotite xenoliths from the Wesselton kimberlite was studied and found to sample the subcratonic lithospheric mantle over a pressure range from 3.6 to 4.7 GPa and a temperature range of 880 to 1120 °C. Major, minor and trace element compositions indicate that both metasomatised and un-metasomatised samples are present over this pressure range. Fe3 +/∑ Fe in garnet from four xenoliths was determined using Fe K-edge XANES spectroscopy, enabling the redox state of the sampled subcratonic mantle to be determined for three garnet bearing samples. ΔlogfO2[FMQ] varied from 0 to - 3.3 over the sampled pressure interval, with the un-metasomatised samples falling within the global trend of decreasing ΔlogfO2[FMQ] with increasing depth. Superimposed on this was an oxidation trend, at higher pressures (≥ 4.5 GPa), with ΔlogfO2 increasing by 1.5 to 2 units in the metasomatically enriched samples, indicating a clear link between metasomatism and oxidation. One potential source of this oxidation is a carbonated silicate melt, which will increase in carbonate content as ΔlogfO2 increases. Mantle minerals affected by such a melt have the potential to shift from the field of diamond stability into that of carbonate, threatening the stability of diamond.

  17. Highly Siderophile Elements as Tracers for the Subcontinental Mantle Evolution Beneath the Southwestern USA: The San Carlos and Kilbourne Hole Peridotite Xenoliths Revisited

    NASA Technical Reports Server (NTRS)

    vanAcken, D.; Brandon, A. D.; Peslier, A. H.; Lee, C.-T. A.

    2010-01-01

    Peridotite xenoliths from San Carlos, Arizona, and Kilbourne Hole, New Mexico, have been studied since the 1970 s to give insights into melting and metasomatism in the subcontinental mantle beneath the southwestern USA. More recently, the highly siderophile elements (HSE; Os, Ir, Ru, Rh, Pt, Pd, and Re) and the included Re-Os isotope system have been established as powerful tools for the study of mantle processes because of their range in compatibility during mantle melting and their siderophile and chalcophile geochemical behavior. Model aluminachron Re-Os ages for San Carlos and Kilbourne Hole, as well as for the nearby Dish Hill and Vulcan's Throne sites, give consistent depletion ages of around 2.2 Ga. This age can be interpreted as a single large scale mantle melting event linked to crustal formation and continental growth under the southwestern USA. Highly siderophile elements, however, may be added to depleted peridotites via melt-rock interaction, especially the more incompatible and hence mobile Pt, Pd, and Re. This may result in overprinting of the signature of melt extraction, thus abating the usefulness of Re-Os mantle extraction model ages. A comprehensive characterization of the suite of mantle xenoliths from the SW USA in terms of HSE concentrations is thus necessary to re-assess the Re-Os system for dating purposes. San Carlos peridotites are depleted to moderately fertile, as indicated by their bulk Al2O3 contents between 0.66 wt% and 3.13 wt%. Bulk Os-187/Os-188 in San Carlos peridotites range from 0.1206 to 0.1357. In contrast, Kilbourne Hole peridotites tend to be more fertile with Al2O3 between 2.11 and 3.78 wt%, excluding one extremely depleted sample with 0.30 wt% Al2O3, and have Os-187/Os-188 between 0.1156 and 0.1272, typical for mantle peridotites. No large fractionation between the more compatible HSE Os, Ir, and Ru are observed. The more incompatible HSE Re, Pd, and to a minor extent, Pt, however, are depleted in a number of samples by

  18. Diversity of minor elements in olivines from mantle xenoliths (Wołek Hill, SW Poland) - PIXE measurements

    NASA Astrophysics Data System (ADS)

    Nowak, Monika; Munnik, Frans; Michalak, Przemysław P.; Renno, Axel

    2016-04-01

    Wołek Hill is one of the best examined exposures of Cenozoic volcanic rocks from SW Poland (Nowak, 2012). This is related with two facts: a great amount of mantle xenoliths were collected from that outcrop and this is one of two occurrences in Poland were modal metasomatism (related with amphibole crystals presence) was recognized. Wołek Hill is a relatively small exposure and belongs to the Złotoryja Volcanic Field, which is one of the volcanic concentrations in the Polish part of the Central European Volcanic Province (Ladenberger et al. 2006). Based on previous observations olivine crystals from the inside part of xenoliths occasionally display internal inhomogeneity visible on a BSE image. Such inhomogeneity has been related to olivine "sub-grains" with slightly shifted crystal axis. Those "sub-grains" are visible in optical microscope as transitional lamellae (Nowak, Stawikowski 2009). Besides the mentioned visible inhomogeneity olivine crystals also show diversity in Ca content inside single crystals (sometimes even more than 200 ppm). EPMA standard measurements (15 kV, 20 nA, time: 40 seconds) were limited to beam size and detection limits of the microprobe (most of the obtained results oscillate close to the detection limits - or even below it). Special conditions EPMA analyses (15 kV, 100nA, time: 100s) confirmed the differences in Ca content in the studied olivines, but did not provide any idea on how to interpret the results (Nowak, 2012). In this short summary we present preliminary data of olivine minor element composition (Ca, Zn, Cr, Ti, Co, K and also Mn, Ni, Fe) obtained with Particle Induced X-ray Emission(PIXE) measurements performed at the HZDR in Dresden using a 3 MeV proton beam, 1-1.5 nA current and an acquisition time of 3 hrs for each scan. The size of individual scans varied from ca. 30 μm up to 60-65 μm, with 8 x 8 measurement points - min. 4 μm in diameter. The measurements have been analysed with the GeoPIXE software (Ryan, 2001

  19. Preliminary data on sulfides from mantle xenoliths from Wilcza Góra and Krzeniów basanites (SW Poland)

    NASA Astrophysics Data System (ADS)

    Bukała, Michał; Puziewicz, Jacek; Ntaflos, Theodoros; Wojtulek, Piotr

    2015-04-01

    The basanites from Krzeniów (19.57 Ma) and Wilcza Góra (20.07 Ma; K-Ar ages by Birkenmajer et al. 2007 Ann. Soc. Geol. Polon.) in SW Poland belong to the Lower Silesian part of the Cenozoic Central European Volcanic Province (CEVP). Both basanites are rich in mantle xenoliths, predominantly of harzburgitic composition. The Krzeniów harzburgites are anhydrous, whereas those from Wilcza Góra contain small amounts of amphibole. Sulfides occur in those harzburgites as: (1) rare, small (<15 μm in diameter) singular grains, enclosed in silicate phases. They have mostly composition of pentlandite (Ni 25.2 - 32.5, Fe 20.3 - 27.0 mole %, atomic metal/sulfur ratio = 1,11). Locally, pentlandite is associated with Ni-pyrrhotite (up to 6,76 mole% of Ni) within the same grain. The grains of pentlandite/Ni-pyrrhotite are anhedral, rounded and elongated. Anhedral millerite grains (Fe ~ 2.39 mole %, metal/sulfur ratio = 0.98) are subordinate. (2) small (< 5 μm in diameter) grains enclosed in silicates, forming sulfide inclusion trails located close to grain margins. These sulfides have the composition of pentlandite (21.5 mole% Fe, 30.8 mole% Ni). Grains are anhedral, oval. Only two analyses were not contaminated due to small size of this kind of sulfides. (3) blebs associated with fine-grained intergranular aggregates of silicate minerals. The aggregates carry majority of sulfides. The sulfide blebs are relatively large (up to 300 μm across) and occur as single grains or in aggregates. Their grains are heterogeneous and consist of various phases. In Krzeniów aggregates consist of clinopyroxene+ olivine± spinel± glass± feldspar± sulfides (pentlandite and Ni-pyrrhotite). Pentlandite contains up to 1.25 mole% of Cu. Sulfides occur in the marginal parts of fine-grained aggregates. One of the pentlandite grains exhibits broad variation of Fe (4.6 to 20.3) and Ni (32.5 to 46.0 mole %). In xenoliths from Wilcza Góra,3 kinds of fine-grained intergranular aggregates occur: (A

  20. He-Ne-Ar isotope studies of mafic volcanic rocks and mantle xenoliths from the East African Rift System - contrasting isotope signals in different rift branches

    NASA Astrophysics Data System (ADS)

    Halldorsson, S. A.; Hilton, D. R.; Scarsi, P.; Abebe, T.; Massi, K. M.; Barry, P. H.; Fischer, T. P.; de Moor, J.; Rudnick, R. L.

    2010-12-01

    Helium isotope studies of the East African Rift System (EARS) suggest the involvement of a deep mantle plume(s) beneath the northern (Ethiopian) segment [1-3]. The highest 3He/4He (RA) signatures found to date show a close association with the greatest magma volumes erupted since the Early Cenozoic in the region. While the helium isotope characteristics are well established in the Ethiopia-Afar region, Ne and Ar systematics remain poorly constrained. Using a combined He-Ne-Ar isotope approach, our aim is to determine the regional extent of the influence of the Afar plume and to distinguish between subcontinental lithospheric mantle (SCLM) and/or a possible second mantle plume sources located to the south of the Turkana Depression. Xenoliths and mafic lavas from N-Tanzania display a limited range in He isotopes (5-7 RA) with exceptions at Arusha (7.8RA) and Labait (8.7RA), through 7.1-8.7 RA in N-Kenya and S-Ethiopia, to 14.3 RA in the Main Ethiopian Rift and Afar, spanning nearly the entire range of previously reported values. The mean 3He/4He ratio from of lavas and xenoliths from N-Tanzania is remarkably close to the global average of 6.1±0.9 (RA) for continental xenoliths and basalts, thought to represent the SCLM [4]. Thus far, only MORB-like values of 7.3-8.3 RA have been found in volcanics of the Western rift. Initial Ne isotope data reveal the presence of a solar-like Ne component in xenoliths from the Ethiopia-Afar region, with extrapolated 21Ne/22Neex ratios of 0.0365 (assuming Ne-B = 12.5). This trend overlaps that of the Loihi-Kilauea line (L-K). Interestingly, a xenolith from N-Tanzania has a 21Ne/22Neex ratio of 0.0415, falling on a trajectory intermediate between MORB and L-K. The highest 40Ar/36Ar ratio obtained on phenocrysts/xenoliths to date is 1510. The generally low 3He/4He ratios of N-Tanzania likely result from different mixing proportions of asthenospheric sources with lithospheric material, the latter having developed lower 3He/4He ratios

  1. The age and history of the lithospheric mantle of the Siberian craton: Re-Os and PGE study of peridotite xenoliths from the Obnazhennaya kimberlite

    NASA Astrophysics Data System (ADS)

    Ionov, Dmitri A.; Carlson, Richard W.; Doucet, Luc S.; Golovin, Alexander V.; Oleinikov, Oleg B.

    2015-10-01

    The formation age of the lithospheric mantle of the Siberian craton (one of the largest on Earth) is not well established; nearly all published whole-rock Re-Os data are for mantle xenoliths from a single kimberlite in the center of the craton (Udachnaya). We report Re-Os isotope and PGE concentration data for 19 spinel and garnet peridotite xenoliths from the Obnazhennaya kimberlite in the northeastern portion of the craton. Most samples in this study, and many Obnazhennaya peridotites in general, show a combination of relatively low Al2O3 (0.1-2%) with high CaO (1.4-4%) concentrations. Only four dunites and harzburgites in our sample suite have low contents of both Al2O3 and CaO (0.1-0.8%), but their relatively low Mg# (0.888-0.919) and highly variable Os concentrations (0.6-35 ppb) suggest they may have formed in melt migration channels rather than as residues of partial melt extraction. A group of six Ca-rich (2.0-3.2% CaO) peridotites yields the highest Re-Os model ages (mean TRD = 2.8 Ga, mean TMA = 3.5 Ga). Eight peridotites with low to moderate Al2O3 (<2%) and Mg# ≥0.91, including three low-Ca harzburgites, yield lower Re-Os model ages (mean TRD = 1.9 Ga, mean TMA = 2.2 Ga). The remainder of the samples may not yield meaningful TRD ages because they are not refractory (Al2O3 >2.6% and/or Mg# ≤0.90). We interpret these results as evidence for a two-stage formation of the lithospheric mantle. The peridotites formed at the two stages show very similar chemical compositions. The enrichment in Ca, which we attribute to widespread post-melting metasomatism by carbonate-rich melts, may have taken place either at the end of the Archean melting event, when at least one Ca-Al-rich peridotite was formed, or later. The combined Re-Os age data on xenoliths from Obnazhennaya and Udachnaya suggest that the lithospheric mantle beneath the Siberian craton was not formed in a single event, but grew in at least two events, one in the late Archean and the other in the

  2. Isla Isabel (Nayarit, México): Quaternary alkalic basalts with mantle xenoliths erupted in the mouth of the Gulf of California

    NASA Astrophysics Data System (ADS)

    Housh, Todd B.; Aranda-Gómez, José Jorge; Luhr, James F.

    2010-11-01

    Isla Isabel is a small island (~ 1.06 km 2) off the coast of Nayarit where intraplate-type alkali-basaltic volcanic rocks erupted during the Quaternary, and one of two Mexican islands (along with Isla Guadalupe) where it is known that the alkali basalts have transported peridotite xenoliths to the surface. The volume of the island is estimated at approximately 4.2 km 3; the volume above sea level is estimated to be approximately 0.03 km 3. The dominant volcanic rock type exposed above sea level is analcime-cemented basaltic tuff-breccia, the rapidly lithified products of hydrovolcanic eruptions; however, subaerial scoria-fall deposits and lava flows are also present. Ten analyzed Isla Isabel volcanic rocks are relatively homogeneous alkali basalts with 4.5-6.4 wt.% nepheline in the CIPW norm. Compared to other analyzed Mexican intraplate-type volcanic rocks from oceanic and continental localities, at the same MgO content, the Isabel samples are depleted in TiO 2, FeO total, and Na 2O, and moderately incompatible trace elements (e.g., Sm, Eu, Tb, and Zr). On the other hand, they are enriched in CaO and Al 2O 3, and relatively compatible trace elements (e.g., Lu and Sc). It is suggested that relative to other Mexican intraplate alkaline lavas the Isabel lavas were derived by higher degrees of melting at lower pressures of mantle that is characterized by ancient HIMU-like isotope systematics. Isabel olivine phenocrysts and their spinel inclusions range to Mg#s that overlap with the peridotite minerals in the accompanying mantle xenoliths, which has not been reported at other Mexican xenolith localities. Olivine-hosted glass inclusions in rapidly quenched scoria yielded the first estimates for volatile contents in Mexican intraplate-type mafic melts; maximum values are: 1.3 wt.% H 2O, 2241 ppm CO 2, 1500 ppm F, 1300 ppm S, and 1200 ppm Cl. Isabel peridotites (Ol ± Hy ± Di + Sp ± Pl) include dunites, harzburgites, and rare plagioclase-harzburgite, and are depleted in

  3. Post-Archean formation of the lithospheric mantle in the central Siberian craton: Re-Os and PGE study of peridotite xenoliths from the Udachnaya kimberlite

    NASA Astrophysics Data System (ADS)

    Ionov, Dmitri A.; Doucet, Luc S.; Carlson, Richard W.; Golovin, Alexander V.; Korsakov, Andrey V.

    2015-09-01

    The formation age of the Siberian cratonic mantle is not well established. Re-Os data on various mantle-derived materials brought up by kimberlite magmas have shown that it contains Archean components, but the reported ages range broadly (3.4 to <1 Ga). We report Re-Os isotope and PGE concentration data for a suite of 29 fresh, well-characterized xenoliths from the Udachnaya-East kimberlite representing all major peridotite rock types and a large part of the cratonic mantle profile. Several xenoliths with very low Os contents (<0.3 ppb) and/or high Re/Os ratios are not suitable for age estimates. The Os (and Ir) depletions are common in cpx-bearing spinel harzburgites and coarse garnet harzburgites, but are not found in deformed, high-T peridotites. Twenty refractory (Al2O3 0.1-1.6%) peridotites yield TRD ages from 0.9 to 2.2 Ga. TRD for a subset of six high-Mg# (0.92-0.93), low-T (⩽930 °C) spinel harzburgites and a single garnet harzburgite yield a narrow range from 2.0 to 2.2 Ga with an average of 2.1 ± 0.1 Ga, which we consider the best estimate for the age of the melting event that initially formed the lithospheric mantle beneath Udachnaya. The TRD estimates for less refractory (Mg# 0.907-0.919) deformed garnet peridotites show a greater range and are generally lower (0.9-2.0 Ga; average 1.54 ± 0.28 Ga) apparently due to the effects of melt metasomatism on the initial melting residues. The predominant part of the mantle in the central Siberian craton formed in the Paleoproterozoic and not in the Archean, unlike cratons in southern Africa and North America. Minor older components reported earlier from Udachnaya may be fragments of pre-existing lithosphere trapped during stacking of melting residues formed about 2 Ga ago. We argue that the formation of cratonic lithospheric mantle, with common high-Mg# (⩾0.92) and opx-enriched peridotites, was not limited to the Archean as previously thought, but continued in the Paleoproterozoic, i.e. that asthenospheric

  4. Peridotite xenoliths from the Polynesian Austral and Samoa hotspots: Implications for the destruction of ancient 187Os and 142Nd isotopic domains and the preservation of Hadean 129Xe in the modern convecting mantle

    NASA Astrophysics Data System (ADS)

    Jackson, M. G.; Shirey, S. B.; Hauri, E. H.; Kurz, M. D.; Rizo, H.

    2016-07-01

    The Re-Os systematics in 13 peridotite xenoliths hosted in young (<0.39 myr) rejuvenated lavas from the Samoan island of Savai'i and 8 peridotite xenoliths from 6 to 10 myr old lavas from the Austral island of Tubuai have been examined to evaluate the history of the oceanic mantle in this region. Modal mineralogy, trace element compositions and 187Os/188Os ratios suggest that these peridotites are not cognate or residual to mantle plumes but rather samples of Pacific oceanic lithosphere created at the ridge. Savai'i and Tubuai islands lie along a flow line in the Pacific plate, and provide two snapshots (separated by over 40 Ma in time) of Pacific mantle that originated in the same region of the East Pacific rise. Tubuai xenoliths exhibit 187Os/188Os from 0.1163 to 0.1304, and Savai'i (Samoa) xenoliths span a smaller range from 0.1173 to 0.1284. The 187Os/188Os ratios measured in Tubuai xenoliths are lower than (and show no overlap with) basalts from Tubuai. The 187Os/188Os of the Savai'i xenoliths overlap the isotopic compositions of lavas from the island of Savai'i, but also extend to lower 187Os/188Os than the lavas. 3He/4He measurements of a subset of the xenoliths range from 2.5 to 6.4 Ra for Tubuai and 10.8 to 12.4 Ra for Savai'i. Like abyssal peridotites and xenoliths from oceanic hotspots that sample the convecting mantle, Os isotopes from the Savai'i and Tubuai xenolith suites are relatively unradiogenic, but do not preserve a record of depleted early-formed (Hadean and Archean) mantle domains expected from earlier cycles of ridge-related depletion, continent extraction, or subcontinental lithospheric mantle erosion. The lack of preservation of early-formed, geochemically-depleted Os-isotopic and 142Nd/144Nd domains in the modern convecting mantle contrasts with the preservation of early-formed (early-Hadean) 129Xe/130Xe isotopic heterogeneities in the convecting mantle. This can be explained if the initial isotopic signatures in Re-Os and Sm-Nd systems

  5. Refertilization of deep continental arc lithosphere: constraints from major element and trace element systematics in mantle xenoliths from the Sierra Nevada, California

    NASA Astrophysics Data System (ADS)

    Chin, E. J.; Lee, C.; Luffi, P. I.

    2010-12-01

    Thickening of continental arc lithosphere, either by underplating, magmatic inflation, or other tectonic processes, is an important stage in the maturation of primitive island arcs into Phanerozoic continental crust. The Sierra Nevada continental arc is one example of a mature arc that has experienced significant thickening and recent orogenic collapse. Previous studies of mantle xenoliths from the Sierras point to delamination of a dense mafic root that represented the residua of the evolved crust distilled out of a >90 km thickened lithospheric column. What has not received much attention, however, is how this lithospheric column has been compositionally modified over the lifespan of the Sierra Nevada arc. As the interface between the crust and mantle, the base of the thickened lithospheric column (upper mantle and lowermost crust) is potentially a zone of refertilization, wherein previously depleted mantle is replenished with fertile components from ascending melts and/or fluids. Refertilization of deep lithosphere not only influences the composition of arc magmas in their early stages, but also has implications for the stability of highly melt-depleted, refractory mantle that may become “rejuvenated” by such refertilization. In this study, we evaluate the role of refertilization in continental arc evolution by investigating upper mantle xenoliths from the Sierra Nevada. Cr# in spinel cores is chosen as a baseline index of melt depletion, and is compared to whole-rock major- and trace-element concentrations (Yb, Ti, Mg, Al) to assess the degree of refertilization. Whole-rock trace element patterns indicate depletions in high field strength elements and complementary enrichments in fluid mobile elements, suggesting that the deep Sierran lithosphere was refertilized by a melt containing a large component of slab-derived fluid. Although high degrees of melting (>15%) are recorded by Cr# in spinel, many Sierran peridotites contain diffuse bands comprised of

  6. Composition of garnet and clinopyroxene in peridotite xenoliths from the Grib kimberlite pipe, Arkhangelsk diamond province, Russia: Evidence for mantle metasomatism associated with kimberlite melts

    NASA Astrophysics Data System (ADS)

    Kargin, A. V.; Sazonova, L. V.; Nosova, A. A.; Tretyachenko, V. V.

    2016-10-01

    Here we present major and trace element data for garnet and clinopyroxene from mantle-derived peridotite xenoliths of the Grib kimberlite, the Arkhangelsk diamond province, Russia, and provide new insights into the metasomatic processes that occur within the subcontinental lithospheric mantle (SCLM) during the kimberlite generation and ascent. The mantle xenoliths examined in this study are both coarse and sheared garnet peridotites and consist of olivine, orthopyroxene, clinopyroxene, garnet with minor ilmenite, magnetite, and Cr-spinel. Based on garnet and clinopyroxene composition, two groups of peridotite are recognized. One group contains high-Ti, light rare earth elements (LREE) enriched garnets and low-Mg# clinopyroxenes with low (La/Sm)n (C1 chondrite-normalized) values. This mineral assemblage was in equilibrium with a high-temperature carbonate-silicate metasomatic agent, presumably, a protokimberlite melt. Pressure-temperature (P-T) estimates (T = 1220 °C and P = 70 kbar) suggest that this metasomatic event occurred at the base of the SCLM. Another group contains low-Ti garnet with normal to sinusoidal rare earth elements (REE) distribution patterns and high-Mg# clinopyroxenes with wide range of (La/Sm)n values. The geochemical equilibrium between garnet and clinopyroxene coupled with their REE composition indicates that peridotite mantle experienced metasomatic transformation by injection of a low-Ti (after crystallizations of the ilmenite megacrysts) kimberlite melt that subsequently percolated through a refractory mantle column. Peridotites of this group show a wide range of P-T estimates (T = 730-1070 °C and P = 22-44 kbar). It is suggested that evolution of a kimberlite magma from REE-enriched carbonate-bearing to carbonate-rich ultramafic silicate compositions with lower REE occurs during the ascent and interaction with a surrounding lithospheric mantle, and this process leads to metasomatic modification of the SCLM with formation of both high

  7. Mantle metasomatism did not modify the initial H2O content in peridotite xenoliths from the Tianchang basalts of eastern China

    NASA Astrophysics Data System (ADS)

    Hao, Yan-Tao; Xia, Qun-Ke; Tian, Zhen-Zhen; Liu, Jia

    2016-09-01

    Metasomatism induced by melts/fluids is ubiquitous in the lithospheric mantle and can potentially modify the initial water content of the mantle. However, the preservation of correlations between H2O content and partial melting indices (e.g., Yb content in clinopyroxene, Cr / (Cr + Al) in spinel) and the lack of correlations between H2O content and metasomatic indices (e.g., La/Yb in clinopyroxene) in peridotite xenoliths from several localities suggest that variations in the initial H2O content were controlled by partial melting processes rather than by subsequent metasomatic event(s) (Hao et al., 2014; Denis et al., 2015). However, the applied partition coefficients of H2O between peridotite and melt (Dperidotite/melt = 0.1-0.3) in the partial melting models cast doubts on the reasonability of such explanations. Whether metasomatism always modifies the initial H2O content of the lithospheric mantle remained a topic of debate. In this paper, we measure major and trace element concentrations and H2O contents of minerals in the peridotite xenoliths hosted by the Tianchang Cenozoic basalts in eastern China by using electron microprobe, laser-ablation ICP-MS and Fourier transform infrared spectroscopy, respectively. The H2O contents (weight in ppm) of clinopyroxene, orthopyroxene and olivine are 70-280 ppm, 35-140 ppm and below detection limit (< 2 ppm H2O), respectively. Although water diffusion during xenolith ascent cannot be excluded for olivine, pyroxenes largely retain the initial H2O content of the mantle source, as supported by (1) the correlation between H2O content and major element content of pyroxene, and (2) the equilibrium H2O partitioning between clinopyroxene and orthopyroxene. The calculated whole-rock H2O contents range from 14 to 93 ppm (average 52 ± 25 ppm) assuming 0.1 for the H2O partition coefficient between olivine and clinopyroxene. Although no hydrous minerals are found, the enrichment in light rare earth elements and large ion lithophile

  8. Xenoliths in ultrapotassic volcanic rocks in the Lhasa block: direct evidence for crust-mantle mixing and metamorphism in the deep crust

    NASA Astrophysics Data System (ADS)

    Wang, Rui; Collins, William J.; Weinberg, Roberto F.; Li, Jin-xiang; Li, Qiu-yun; He, Wen-yan; Richards, Jeremy P.; Hou, Zengqian; Zhou, Li-min; Stern, Richard A.

    2016-07-01

    Felsic granulite xenoliths entrained in Miocene (~13 Ma) isotopically evolved, mantle-derived ultrapotassic volcanic (UPV) dykes in southern Tibet are refractory meta-granitoids with garnet and rutile in a near-anhydrous quartzo-feldspathic assemblage. High F-Ti (~4 wt.% TiO2 and ~3 wt.% F) phlogopite occurs as small inclusions in garnet, except for one sample where it occurs as flakes in a quartz-plagioclase-rich rock. High Si (~3.45) phengite is found as flakes in another xenolith sample. The refractory mineralogy suggests that the xenoliths underwent high-T and high-P metamorphism (800-850 °C, >15 kbar). Zircons show four main age groupings: 1.0-0.5 Ga, 50-45, 35-20, and 16-13 Ma. The oldest group is similar to common inherited zircons in the Gangdese belt, whereas the 50-45 Ma zircons match the crystallization age and juvenile character (ɛHf i +0.5 to +6.5) of Eocene Gangdese arc magmas. Together these two age groups indicate that a component of the xenolith was sourced from Gangdese arc rocks. The 35-20 Ma Miocene ages are derived from zircons with similar Hf-O isotopic composition as the Eocene Gangdese magmatic zircons. They also have similar steep REE curves, suggesting they grew in the absence of garnet. These zircons mark a period of early Miocene remelting of the Eocene Gangdese arc. By contrast, the youngest zircons (13.0 ± 4.9 Ma, MSWD = 1.3) are not zoned, have much lower HREE contents than the previous group, and flat HREE patterns. They also have distinctive high Th/U ratios, high zircon δ18O (+8.73-8.97 ‰) values, and extremely low ɛHf i (-12.7 to -9.4) values. Such evolved Hf-O isotopic compositions are similar to values of zircons from the UPV lavas that host the xenolith, and the flat REE pattern suggests that the 13 Ma zircons formed in equilibrium with garnet. Garnets from a strongly peraluminous meta-tonalite xenolith are weakly zoned or unzoned and fall into four groups, three of which are almandine-pyrope solid solutions and have low

  9. P-T Equilibrium Conditions of Xenoliths from the Udachnaya Kimberlite Pipe: Thermal Perturbations in the Lithospheric Mantle

    NASA Astrophysics Data System (ADS)

    Tychkov, Nikolay; Agashev, Alexey; Malygina, Elena; Pokhilenko, Nikolay

    2014-05-01

    Integrated study of 250 peridotite xenoliths from Udachnaya -East pipe show difference in mineral paragenesises and textural-structural peculiarities in the different level of cratonic lithosphere mantle (CLM). The compositions of minerals were determined using EPMA. Thermobarometric parameters (Brey, Kohller, 1990) were determined for all rocks occupying different fields on geothermal curve. The deepest layer (the pressure interval of 5.0-7.0 GPa) contains mostly pophyroclastic lherzolites. Anyway, some rocks of this layer have an idiomorphic texture being also enriched in incompatible components. Higher in the CLM sequence, the interval (4.2-6.3 GPa) is composed of the most depleted rocks: megacristalline ultradepleted harzburgite-dunites and depleted granular harzburgite-dunites, as well as lherzolites in a subordinate amount. They correspond strate to 35 mW/m2 and partly overlap the deeper layer in dapth. It is likely that rocks of this layer are in equilibrium and were not subject to significant secondary changes due to kimberlite magma intrusion. Thus, this interval of the CLM sequence reflects the true (relic) geotherm for the area of the Udachnaya kimberlite pipe. Moreover, it is obvious that this interval was a major supplier of diamonds into kimberlites of the Udachnaya pipe. The interval of 4.2-2.0 GPa in the CLM sequence is also composed of coarse depleted lherzolites and harzburgites. Rocks of this interval are slightly more enriched than those of the underlying interval. This is confirmed by the distinct predominance of lherzolites over harzburgite-dunites. The heat flow in this layer varies in the range of 38-45 mW/m2 and shows a general tendency to increase with decreasing depth. According to occurrence of nonequilibrium mineral assemblages and increased heat flow relative to the major heat flow of 35 mW/m2, this interval is similar to the deepest interval of secondary enriched rocks. Interval of less than 2.0 GPa composed of spinel lherzolites and

  10. The first finding of graphite inclusion in diamond from mantle rocks: The result of the study of eclogite xenolith from Udachnaya pipe (Siberian craton)

    NASA Astrophysics Data System (ADS)

    Mikhailenko, D. S.; Korsakov, A. V.; Golovin, A. V.; Zelenovskiy, P. S.; Pohilenko, N. P.

    2016-08-01

    A xenolith of eclogite from the kimberlite pipe Udachnaya-East, Yakutia Grt+Cpx+Ky + S + Coe/Qtz + Dia + Gr has been studied. Graphite inclusions in diamond have been studied in detail by Confocal Raman (CR) mapping. The graphite inclusion in diamond has a highly ordered structure and is characterized by a substantial shift in the band (about 1580 cm-1) by 7 cm-1, indicating a significant residual strain in the inclusion. According to the results of FTIR spectroscopic studies of diamond crystals, a high degree of nitrogen aggregation has been detected: it is present mainly in form A, which means an "ancient" age of the diamonds. In the xenolith studied, the diamond formation occurred about 1 Byr, long before their transport by the kimberlite melt, and the conditions of the final equilibrium were temperatures of 1020 ± 40°C at 4.7 GPa. Thus, these graphite inclusions found in a diamond are the first evidence of crystallization of metastable graphite in a diamond stability field. They were formed in rocks of the upper mantle significantly below (≥20 km) the graphite-diamond equilibrium line.

  11. Complex metasomatism of lithospheric mantle by asthenosphere-derived melts: Evidence from peridotite xenoliths in Weichang at the northern margin of the North China Craton

    NASA Astrophysics Data System (ADS)

    Zou, Dongya; Zhang, Hongfu; Hu, Zhaochu; Santosh, M.

    2016-11-01

    The petrology, in situ analyses of major and trace elements as well as Sr isotopic compositions of spinel-facies lherzolite and harzburgite xenoliths from Weichang within the northern margin of the North China Craton (NCC) are reported for the first time in this study to evaluate the nature and evolution of the lithospheric mantle. These peridotite xenoliths display porphyroclastic texture and can be subdivided into two groups. Group I peridotites have slightly higher forsterite contents (Fo) (90.6-91.2) and 87Sr/86Sr ratios (0.7025-0.7043) in the cores than those in the rims (89.8-90.8; 0.7025-0.7038) of olivines and clinopyroxenes, respectively. The clinopyroxenes in these rocks exhibit uniform LREE-depleted patterns. These geochemical features suggest that the Group I peridotites were weakly metasomatized by recent asthenospheric melts. In contrast, Group II peridotites show sieve-texture and clear compositional zoning in minerals. The olivines and clinopyroxenes from these rocks have higher Fo (86.9-91.3) and 87Sr/86Sr ratios (0.7035-0.7049) in the cores than those in the rims (76.9-90.6; 0.7021-0.7046). The clinopyroxenes show three types of REE patterns: LREE-enriched, convex-upward and spoon-shaped. The LREE-enriched clinopyroxenes have the highest (La/Yb)N and lowest Ti/Eu and those with spoon-shaped REE patterns show an increase in LREE, Ba, Th and U contents from the cores to the rims. These features indicate that the Group II peridotites witnessed a high degree of refertilization by recent asthenosphere-derived silicate and carbonatite melts or their mixture. Compared with the data of the Mesozoic NCC lithospheric mantle, the heterogeneous and low 87Sr/86Sr ratios (0.7025-0.7049) in the LREE-depleted clinopyroxenes reveal that the ancient lithospheric mantle could have been modified by asthenospheric melts before the recent metasomatism event. We conclude that the lithospheric mantle beneath Weichang underwent multiple modifications through asthenosphere

  12. Nd, Sr and Pb isotopic composition of metasomatised xenoliths from the backarc Patagonian Mantle Wedge: Insights into the origin of the uprising melts

    NASA Astrophysics Data System (ADS)

    Zanetti, Alberto; Mazzucchelli, Maurizio; Hemond, Christope; Cipriani, Anna; Bertotto, Gustavo W.; Cingolani, Carlos; Vannucci, Riccardo

    2010-05-01

    Information about the geochemical composition of metasomatic melts migrating through the Patagonian mantle wedge is provided by the ultramafic xenoliths occurrence of Tres Lagos (TL; lat. 49.13°S, long. 71.18°W), Argentina. Such a locality is placed at the eastern border of the Meseta de la Muerte backarc basaltic plateau, where a post-plateau volcanic diatreme contains mantle xenoliths in both pyroclastites and lavas. Its latitude corresponds with the Northern limit of the Austral Volcanic Arc (AVZ), which is separated from the Southern Volcanic Zone (SVZ) by a gap in the arc magmatism ranging between 49° and 46°30' latitude S. The analysed xenoliths have been distinguished into two groups (Group 1 & 2). Group 1 consists of lherzolites and harzburgites, whereas Group 2 is formed by harzburgites. The texture of the Group 1 lherzolites varies from protogranular to granoblastic to porphyroblastic, whereas Group 1 harzburgites have always granoblastic texture. Group 2 harzburgites have granular texture, which may change to porphyroblastic owing to the random concentration of large olivine and orthopyroxene crystals. The clinopyroxenes (Cpx) from Group 1 lherzolites have PM-normalised REE patterns ranging from LREE-depleted (LaN/SmN= 0.24-0.37), to LREE-enriched (LaN/YbN up to 4.08) and spoon-shaped: the latter have minimum at Pr and Pr-Yb concentrations similar to those shown by the LREE-depleted Cpx. The Cpx from Group 1 harzburgites have lower REE concentrations with respect to the lherzolite ones and their REE patterns vary from HREE-enriched, steadily fractionated, (LaN/YbN = 0.21-0.35, Ybn ~ 1-2) to spoon-shaped (LaN/SmN = 2.81; SmN/YbN = 0.89; YbN ~ 3. The Cpx from the Group 2 harzburgites have convex-upward (LaN/SmN = 0.31; SmN/YbN = 1.50) to LREE-enriched (LaN/YbN = 2.94) patterns. The Sr, Nd and Pb isotopic compositions of the Group 1 clinopyroxenes form arrays spanning from DM to the field delimited by the TL basaltic lavas, pointing to EMI end

  13. Peridotite xenoliths from the Shiribeshi Seamount, Japan Sea: insights into mantle processes in a back-arc basin

    NASA Astrophysics Data System (ADS)

    Ichiyama, Yuji; Morishita, Tomoaki; Tamura, Akihiro; Arai, Shoji

    2016-10-01

    Orthopyroxene-rich and orthopyroxene-poor peridotite xenoliths were sampled from quaternary basaltic to andesitic lava flows of the Shiribeshi seamount, Japan Sea. These xenoliths were affected by reactions with the host magma during transportation to the surface, which caused partial orthopyroxene dissolution and intergrowth with vermicular spinel. Chromian spinel and clinopyroxene major element compositions in the Shiribeshi peridotite are similar to those in abyssal peridotites. REE modeling indicates that the Opx-rich peridotite experienced decompression partial melting from the garnet to the spinel peridotite stability field. Rare earth element (REE) patterns of clinopyroxene in the Opx-rich peridotite show various degrees of enrichment in light REE, which resulted from melt percolation through the reaction with host magma. Comparison with peridotite xenoliths from two other localities (Seifu and Oshima-Ōshima) in the Japan Sea suggests that the Oshima-Ōshima peridotite record higher degree of partial melting than the Shiribeshi and Seifu peridotites. Oxygen fugacities calculated from chromian spinel in the Japan Sea peridotites are comparable to those of arc peridotites. The high degree of partial melting of the Oshima-Ōshima peridotite was possibly caused by the infiltration of a H2O-bearing flux released from the subducted slab. The Shiribeshi peridotite is interpreted as the residue formed after the extraction of depleted back-arc basalts during a later stage of the Japan Sea opening in the Middle Miocene, whereas the Oshima-Ōshima peridotite is residual after the extraction of enriched basalts during an earlier stage of the opening of the Japan Sea.

  14. Paleo-Asian oceanic subduction-related modification of the lithospheric mantle under the North China Craton: Evidence from peridotite xenoliths in the Datong basalts

    NASA Astrophysics Data System (ADS)

    Wang, Chengyuan; Liu, Yongsheng; Min, Ning; Zong, Keqing; Hu, Zhaochu; Gao, Shan

    2016-09-01

    In-situ major and trace elements and Sr isotopic compositions of peridotite xenoliths of the Datong Quaternary alkaline basalt were analyzed to evaluate the influences of the southward subduction of the Paleo-Asian oceanic plate (PAOP) on the lithospheric mantle transformation of the North China Craton (NCC). These peridotite xenoliths including spinel harzburgites and lherzolites were classified into three groups. The type 1 peridotites have the lowest temperatures (961-1007 °C). Clinopyroxenes in these peridotites exhibit LREE-depleted REE patterns and have the lowest 87Sr/86Sr ratios of 0.70243-0.70411. The type 2 and 3 peridotites show higher temperatures (1017-1022 °C). Clinopyroxenes in the type 2 peridotite have V-shaped REE patterns and relatively higher 87Sr/86Sr ratios of 0.70418-0.70465. Clinopyroxenes in the type 3 peridotite have concave-downward REE patterns and unusually high 87Sr/86Sr ratios of 0.70769-0.70929. Carbonatitic veinlets are found in the type 1 peridotites. They show steep LREE-enriched REE patterns with enrichment in LILE and depletion in HFSE, and have the highest 87Sr/86Sr ratios of 0.71145-0.71285. The mineral chemistries and modal calculations suggest that the protolith of these peridotites experienced a variable degree of partial melting. The type 2 and 3 peridotites sampled from deeper depth experienced latter cryptic carbonatitic metasomatism. The carbonatitic veinlets have generally consistent trace element patterns and Sr isotopic ratios with the calculated melts equilibrated with clinopyroxenes in the type 3 peridotite, which may represent the percolated carbonatitic melt quickly solidified in the relatively cold and shallow mantle. The remarkable negative Eu anomalies (0.37-0.61) and highly radiogenic Sr isotopic compositions of the calculated metasomatic agents preclude indicate melt derived from carbonated peridotite or carbonated eclogite but point to a crustal sedimentary origin. Considering the tectonic setting and

  15. Mantle metasomatism in the Kaapvaal Craton lithosphere: constraints on the composition of the metasomatic agent from fluid inclusions in MARID-type xenoliths

    NASA Astrophysics Data System (ADS)

    Konzett, J.; Krenn, K.; Hauzenberger, Ch.

    2012-04-01

    The emplacement of both group I and group II kimberlites in the Kaapvaal Craton of the Kimberley region in South Africa is associated with an intense metasomatic alteration of the country rocks as evidenced by a diverse suite of xenoliths sampled by the kimberlites mainly comprising metasomatized peridotites and minor MARID-type xenoliths. These are characterized by hydrous potassic silicates and LILE-HFSE-rich titanates. Because the metasomatic agent is not preserved in these rocks its composition has to be inferred from that of the metasomatic assemblages. Here we present for the first time data on fluid inclusions from two MARID-xenoliths sampled by group-I kimberlites of the Kimberley cluster. They provide direct evidence for the nature of the metasomatic fluids involved in kimberlite-related metsomatism. The xenoliths contain phlogopite+K-richterite+diopside+ilmenite±rutile±apatite±zircon. Fluid inclusions with 4-10 µm in size were found in diopside, K-richterite and zircon and contain L+V+one-to-several daughter phases. Investigations with the freezing and heating stage indicate two different chemical systems for the fluids: (1) H2O-NaCl dominant fluids found as L+V+S inclusions in zircon together with abundant needle-like apatite, rutile and phlogopite solid inclusions. The fluid inclusions in part occur along zircon host-rutile/apatite inclusion grain boundaries which indicates that the fluids were trapped during zircon growth. They contain 30-32 mass% NaCl and show a density of 0.87-0.94 g/cm3. Halos of tiny fluid inclusions, however, indicate that most if not all zircon inclusions are decrepitated during ascent from depth and/or superheating during entrainment of the xenoliths into the kimberlite. Using EMPA, enstatite and a SiO2 polymorph were identified in opened fluid inclusions exposed at the surface of polished thin sections. Because these phases were exclusively found in the fluid inclusions, they are considered daughter crystals. The enstatite

  16. Xenoliths from Late Cretaceous seamounts in the Central Pacific: Cumulates of fractionating alkalic basalt magma chambers

    SciTech Connect

    Davis, A.S.; Friesen, W.B.; Pickthorn, L.; Pringle, M.S.; Clague, D.A. )

    1990-06-01

    Abundant xenoliths in alkalic basalt were recovered from two Late Cretaceous seamounts in the Central Pacific. One seamount, located in the Phoenix archipelago (lat 0{degree}22'5, long 176{degree}05'W), is dated by {sup 40}Ar/{sup 39}Ar techniques as 65 Ma. The other seamount, located in the northern Line Islands (lat 15{degree}39'N, long 170{degree}23'W), is dated as 70 Ma. Host lavas are basanite and differentiated alkalic basalt. Mafic xenolith assemblages consist of clinopyroxene with variable amounts of amphibole and mica. Intermediate assemblages have abundant feldspar in addition to the mafic minerals. Rare felsic xenoliths consist of two or more feldspars. Variable amounts of apatite, titanite, and magnetite are poikilitically enclosed in mafic phases, and minor feldspathoids are present in some xenoliths. Most xenoliths are holocrystalline with fine- to medium-grained, equigranular cumulus texture, but two xenoliths have a seriate, interlocking crystal framework in a small amount of glassy to microcrystalline matrix. Clinopyroxene in the holocrystalline samples is partially replaced by amphibole. In a few samples, extensive replacement of clinopyroxene by rounded amphibole grains results in a nearly granoblastic texture. Clinopyroxene compositions range from diopside to ferrosalite and are essentially Cr-free but generally have high Ti and Al contents. Cr-rich diopside and Al-augite, characteristic of mantle clinopyroxene, are absent. Feldspars include plagioclase, anorthoclase, and sanidine. Mineral compositions of xenoliths are similar to those of phenocrysts in the host lavas, indicating that these xenoliths are not metasomatized mantle material, but rather are cumulates from fractionating alkalic basalt magma chambers.

  17. Orthopyroxene-enrichment in the lherzolite-websterite xenolith suite from Paleogene alkali basalts of the Poiana Ruscă Mountains (Romania)

    NASA Astrophysics Data System (ADS)

    Nédli, Zsuzsanna; Szabó, Csaba; Dégi, Júlia

    2015-12-01

    In this paper we present the petrography and geochemistry of a recently collected lherzolite-websterite xenolith series and of clinopyroxene xenocrysts, hosted in Upper Cretaceous-Paleogene basanites of Poiana Ruscă (Romania), whose xenoliths show notable orthopyroxene-enrichment. In the series a slightly deformed porphyroclastic-equigranular textured series could represent the early mantle characteristics, and in many cases notable orthopyroxene growth and poikilitic texture formation was observed. The most abundant mantle lithology, Type A xenoliths have high Al and Na-contents but low mg# of the pyroxenes and low cr# of spinel suggesting a low degree (< 10 %) of mafic melt removal. They are also generally poor in overall REE-s (rare earth elements) and have flat REY (rare earth elements+ Y) patterns with slight LREE-depletion. The geochemistry of the Type A xenoliths and calculated melt composition in equilibrium with the xenolith clinopyroxenes suggests that the percolating melt causing the poikilitization can be linked to a mafic, Al-Na-rich, volatile-poor melt and show similarity with the Late Cretaceous-Paleogene (66-72 Ma) subduction-related andesitic magmatism of Poiana Ruscă. Type B xenoliths, with their slightly different chemistry, suggest that, after the ancient depletion, the mantle went through a slight metasomatic event. A subsequent passage of mafic melts in the mantle, with similar compositions to the older andesitic magmatism of Poiana Ruscă, is recorded in the pyroxenites (Fe-rich xenoliths), whereas the megacrysts seem to be cogenetic with the host basanite. The Poiana Ruscă xenoliths differ from the orthopyroxene-enriched mantle xenoliths described previously from the Carpathian-Pannonian Region and from the Dacia block.

  18. Iron geochemistry of the mantle

    NASA Astrophysics Data System (ADS)

    Humayun, M.; Campbell, T. J.; Brandon, A. D.; Davis, F. A.; Hirschmann, M. M.

    2011-12-01

    The Fe/Mg ratio is an important constraint on the compositionally controlled density of the mantle. However, this ratio cannot be inferred from erupted lavas from OIB or MORB sources, but must be determined directly from mantle peridotites. Recently, the Fe/Mn ratio of erupted lavas has been used as an indicator of potential Fe variability in the mantle driven by core-mantle interaction, recycled oceanic crust, or even variations in the temperature of mantle melting. The classic compilation of McDonough & Sun (1995) provided the currently accepted Fe/Mn ratio of the upper mantle, 60±10. The uncertainty on this ratio allows for 15-30% variability in mantle iron abundances, which is equivalent to a density variation larger than observed by seismic tomography in the mantle. To better understand the relationship between mantle peridotites and erupted lavas, and to search for real variability in the Fe/Mn ratio of mantle peridotites, we report precise new ICP-MS measurements of the transition element geochemistry of suites of mantle xenoliths that have known Fe/Mg ratios. For 12 Kilbourne Hole xenoliths, we observe a clear correlation between Fe/Mn and MgO (or Fe/Mg) over an Fe/Mn range of 59-72. Extrapolation of this trend to a Primitive Mantle (PM) MgO content of 37.8 yields an Fe/Mn of 59±1 for the PM. Our new analyses of KLB-1 powder and fused glass beads yield an Fe/Mn of 61.4 for both samples, which plots on the Kilbourne Hole Fe/Mn vs. MgO trend. A set of ten xenoliths from San Carlos yield a wide range of Fe/Mn (56-65) not correlated with MgO content. The San Carlos xenoliths may have experienced a metasomatic effect that imprinted variable Fe/Mn. A clinopyroxene-rich lithology from San Carlos yields an Fe/Mn of 38, which plots on an extension of the Kilbourne Hole Fe/Mn vs. MgO trend. These new results, and those from other xenolith localities being measured in our lab, provide new constraints on the compositional variability of the Earth's upper mantle. Mc

  19. Nature and evolution of the lithospheric mantle beneath the eastern Central Asian Orogenic Belt: Constraints from peridotite xenoliths in the central part of the Great Xing'an Range, NE China

    NASA Astrophysics Data System (ADS)

    Pan, Shaokui; Zheng, Jianping; Griffin, W. L.; Xu, Yixian; Li, Xiyao

    2015-12-01

    Our knowledge of the lithospheric mantle beneath the Central Asian Orogenic Belt is still sparse. Petrologic, major- and trace-element studies on the peridotite xenoliths from the Cenozoic volcanic fields in the Aershan area, the central part of the Great Xing'an Range, NE China, provide insights into the nature and evolution of the lithospheric mantle beneath the eastern part of the belt. According to the REE patterns of clinopyroxene, these peridotites can be divided into three groups which show clear differences in microstructure, geochemistry and equilibration temperature. Group 1 xenoliths (LREE-depleted patterns of Cpx) are lherzolites, with protogranular microstructure and high modal Cpx (8-13 wt.%), low Cr# in spinel (< 21.1), high whole-rock CaO and Al2O3 contents and estimated temperatures of 834-849 °C. Group 2 xenoliths (flattened REE patterns of Cpx) are harzburgites, with microstructures transitional between mosaic and tabular and low Cpx content (2-3 wt.%); they have high Cr# in spinel (41.1-49.6), low whole-rock CaO and Al2O3 levels and equilibration temperatures of 1183-1244 °C. Group 1 peridotites represent the newly accreted fertile mantle which was not significantly affected by post-melting enrichment; while Group 2 xenoliths may be older relics of moderately refractory mantle that underwent H2O bearing silicate-melt metasomatism (Ti/Eu > 3300, (La/Yb)N < 1 and occurrence of amphibole). Group 3 peridotites (convex-up REE patterns of Cpx) comprise both lherzolite and harzburgite; they have porphyroclastic microstructures, and show a broad range of Cpx modes (0-7 wt.%), spinel-Cr# (26.5-71.1), bulk rock CaO and Al2O3 contents and temperatures (941-1239 °C). The high TiO2 contents (up to 1.42 wt.%) in spinels of Group 3 imply the involvement of melt/rock reactions. We suggest that the upwelling of asthenospheric material played a key role in modifying the lithospheric mantle underneath the eastern Central Asian Orogenic Belt, and resulted in

  20. Characterization of hydration in the mantle lithosphere: Peridotite xenoliths from the Ontong Java Plateau as an example

    NASA Astrophysics Data System (ADS)

    Demouchy, Sylvie; Ishikawa, Akira; Tommasi, Andréa; Alard, Olivier; Keshav, Shantanu

    2015-01-01

    We report concentrations of hydrogen (H) in upper mantle minerals of peridotites (olivine and pyroxenes) transported by alnöitic lavas, which erupted on the southwestern border of the Ontong Java Plateau (Malaita, Solomon Islands, West Pacific). Unpolarized FTIR analyses show that olivine, orthopyroxene, and diopside contain 2-32 ppm, 162-362 ppm and 159-459 ppm wt H2O, respectively. In the studied lherzolites, garnets are anhydrous. The concentration of hydrogen within individual olivine and pyroxene grains is almost homogeneous, indicating no evidence of dehydration or hydration by ionic diffusion. In the lherzolite, the concentration of hydrogen in olivine tends to increase weakly with depth (based on geothermobarometry), consistent with the increase of water solubility with increasing water fugacity as a function of pressure, but concentrations remain well below water-saturation values determined experimentally. The highest concentration of H in olivine (32 ppm wt H2O) is, however, found in refractory spinel harzburgites, which equilibrated at depths of 85 km., while deeper specimens as the high-temperature spinel harzburgites, and some of the garnet lherzolites, contain less hydrogen in olivine. Olivines from pyroxene- or pargasite-rich peridotites have also lower hydrogen concentrations. We interpret the high hydrogen concentrations in olivine from the refractory spinel harzburgites as due to (1) simultaneous hydration and metasomatism of the lithospheric mantle by a water-rich silicate melt/fluid, during which hydrogen follows MREE and where spinel harzburgite have experienced 'stealth' metasomatism, and/or (2) to a late 'fleeting' hydrogen metasomatism, which would hydrate the rock after this first 'stealth' metasomatism event. In the second case, the composition of the 'fleeting' percolating fluid (small volume fraction of very evolved fluids, with high volatiles concentration and transient properties) is likely to be linked to the decrease of the plume

  1. The comparative characteristic of PGE distribution in the mantle xenoliths of the Udachnaya pipe (Siberian craton) from the deformed and granular peridotites and eclogites.

    NASA Astrophysics Data System (ADS)

    Ilyina, Olga; Pokhilenko, Lyudmila; Agashev, Aleksey; Tychkov, Nikolay; Surgutanova, Evgenia

    2016-04-01

    We report the first data of PGE distribution in the unusually fresh deformed peridotites, granular peridotites and eclogites from the Udachnaya pipe. Mantle xenoliths from the Udachnaya pipe have different origin, structure and chemical composition and represent comprehensive depths in the Siberian craton. Equilibration temperatures and pressures for Udachnaya deformed and granular peridotites are 1250-1400° C and 5,7 - 7 GPa and 750 - 1250 ° C i 3,5 - 6 GPa, respectively [1]. Equilibration temperatures and pressure estimated for eclogites ranged from 1245 to 1320 ° C and 6 - 6,5 GPa [2]. We assume that the silphides are the main host mineral of PGE in our rocks [3]. The sulphides from eclogites have a narrow range and little concentration of compatible elements (Ir+Ru from 0,002 to 0,144 ppb) in contrast to incompatible PGE (Pt+Pd from 0,001 to 23,24 ppb). Ir show good positive correlation with major elements (CaO+Al2O3). Pt and Pd have no correlation with these elements. There is a good positive correlation between PGE and Fe2O3. Thus, PGE in eclogites are not controlled by silica components and belong to iron phase enrichment. The same situation is observed in granular peridotites. The sulphides in these rocks are not identified, but there is a good correlation of Fe2O3 with PGE in the whole-rock. And PGE show negative correlation with major elements (CaO+ Al2O3). The sulphides were identified in two samples of the deformed peridotite. Chondrite normalized PGE concentrations in the sulphides are three orders higher than that in the deformed peridotites whole-rock. But the pattern shapes is similar. The distribution of PGE in the deformed peridotites generally corresponds to that in granular peridotites of the Udachnaya pipe and xenoliths from Lesotho [4]. However, in contrast with broad range concentrations PGE in the granular peridotites of the Udachnaya pipe ((0,0003 - 0,02) × chondritic), the deformed peridotites show nearly flat pattern from Os to Pt (~0

  2. Cumulate xenoliths from St. Vincent, Lesser Antilles Island Arc: a window into upper crustal differentiation of mantle-derived basalts

    NASA Astrophysics Data System (ADS)

    Tollan, P. M. E.; Bindeman, I.; Blundy, J. D.

    2012-02-01

    In order to shed light on upper crustal differentiation of mantle-derived basaltic magmas in a subduction zone setting, we have determined the mineral chemistry and oxygen and hydrogen isotope composition of individual cumulus minerals in plutonic blocks from St. Vincent, Lesser Antilles. Plutonic rock types display great variation in mineralogy, from olivine-gabbros to troctolites and hornblendites, with a corresponding variety of cumulate textures. Mineral compositions differ from those in erupted basaltic lavas from St. Vincent and in published high-pressure (4-10 kb) experimental run products of a St. Vincent high-Mg basalt in having higher An plagioclase coexisting with lower Fo olivine. The oxygen isotope compositions (δ18O) of cumulus olivine (4.89-5.18‰), plagioclase (5.84-6.28‰), clinopyroxene (5.17-5.47‰) and hornblende (5.48-5.61‰) and hydrogen isotope composition of hornblende (δD = -35.5 to -49.9‰) are all consistent with closed system magmatic differentiation of a mantle-derived basaltic melt. We employed a number of modelling exercises to constrain the origin of the chemical and isotopic compositions reported. δ18OOlivine is up to 0.2‰ higher than modelled values for closed system fractional crystallisation of a primary melt. We attribute this to isotopic disequilibria between cumulus minerals crystallising at different temperatures, with equilibration retarded by slow oxygen diffusion in olivine during prolonged crustal storage. We used melt inclusion and plagioclase compositions to determine parental magmatic water contents (water saturated, 4.6 ± 0.5 wt% H2O) and crystallisation pressures (173 ± 50 MPa). Applying these values to previously reported basaltic and basaltic andesite lava compositions, we can reproduce the cumulus plagioclase and olivine compositions and their associated trend. We conclude that differentiation of primitive hydrous basalts on St. Vincent involves crystallisation of olivine and Cr-rich spinel at depth

  3. Oxo-amphiboles in mantle xenoliths: evidence for H2O-rich melt interacting with the lithospheric mantle of Harrow Peaks (Northern Victoria Land, Antarctica)

    NASA Astrophysics Data System (ADS)

    Gentili, S.; Bonadiman, C.; Biagioni, C.; Comodi, P.; Coltorti, M.; Zucchini, A.; Ottolini, L.

    2015-12-01

    Amphiboles are the most widespread hydrous metasomatic phases in spinel-bearing mantle peridotites from Harrow Peaks (HP), Northern Victoria Land (Antarctica). They occur both in veinlets and disseminated in the peridotite matrix (preferentially associated with clinopyroxene and spinel grains). Four amphibole crystals were investigated by single-crystal X-ray diffraction (SC-XRD), electron microprobe analysis (EMPA), secondary ion mass spectrometry (SIMS) and micro-Mössbauer spectroscopy; these crystal-chemical data allow to constrain upper mantle conditions during growth of these amphiboles and the role of volatile circulation during metasomatic processes in the Antarctic region. The HP amphiboles have low Mg# values (69.3-84.1), high TiO2 (2.74-5.30 wt%) and FeOtot contents (3.40 to 6.90 wt%). The Fe3+/Fetot ratios are significantly high (0.53-0.66). The W-site is mainly occupied by O2- (0.984-1.187 apfu) plus OH (H2O: 0.70-1.01 wt%) and minor F (0.04-0.24 wt%) and Cl (0.03-0.08 wt%). Consequently, HP amphiboles are actually characterized by a significant oxo component. The aH2O values were calculated at 1.5 GPa by dehydration equilibrium equations written as H2O-buffering equilibria among end-member components of amphibole and coexisting peridotite phases. Three out of four HP amphibole-bearing peridotites have values of aH2O ranging from 0.122 to 0.335; whereas one sample has aH2O remarkably higher (0.782) approaching an ideal H2O basalt solubility. The HP fO2 values, determined by the olivine-spinel-orthopyroxene oxygeobarometer (ΔQFM = -1.77 : +0.01), are remarkably different from those calculated on the basis of the amphibole dehydration equilibrium and the application of the dissociation reaction (ΔQFM = -2.60 : +6.8). The high aH2O and the extremely high fO2 values, determined by the oxy-amphibole equilibrium with respect to the redox conditions recorded by the co-existing anhydrous minerals (close to QFM buffer), revealed that: i) the amphibole

  4. Insight into the upper mantle beneath an active extensional zone: the spinel-peridotite xenoliths from San Quintin (Baja California, Mexico)

    NASA Astrophysics Data System (ADS)

    Cabanes, N.; Mercier, J.-C. C.

    1988-11-01

    Many of the peridotite xenoliths included in the San Quintin (Baja California Norte, Mexico) quaternary alkali-basalts have undergone a very intense shear deformation (deviatoric stresses up to 0.1 GPa), hence a first-order classification into coarse-grained lherzolites and deformed peridotites (porphyroclastic and mosaic textures) has been applied. All of these rocks show a very limited compositional variability in the Mg/(Mg+Fe2+) ratios (olivine: 0.894 0.905±0.005; orthopyroxene: 0.899 0.9105±0.005), and the observed trends in the Cr/(Cr+Al) spinel ratios (from 0.1 to 0.6) can be interpreted as resulting from gradual partial melting followed by homogenization of the bulk phases. A later and less accentuated melting event is also evidenced by internal core-rim variations in the spinels from a few samples and ascribed to the thermal effect of the host lava. Simultaneous application of exchange geothermometers which give the latest equilibrium temperatures (i.e. at the time of eruption: Fe-Mg exchange between olivine and spinel) and of pyroxene transfer thermobarometers yields two distinct behaviours: the porphyroclastic and mosaic peridotites record an event of deformation and recrystallization and were equilibrated at 800° 950° C and P≲-1 GPa at the time of eruption, but have also retained evidence of higher temperatures (1000° 1050° C) and pressures; the coarsegrained lherzolites, which yield conditions of 1000° 1050° C and P<-2 GPa at the time of eruption, were originally equilibrated at higher temperature and pressure conditions and were subsequently re-equilibrated to 1000° 1050° C by solid-state bulk diffusion, without exsolution. Clinopyroxenite veins provide evidence of magma injection into the host-peridotite, before deformation but after the major melting event. To explain the simultaneous sampling of both groups of peridotites by the San Quintin alkali basalts, we suggest that the ascending magma reached the critical limit for hydraulic

  5. Wehrlitisation in the upper mantle beneath the Nógrád-Gömör Volcanic Field (Northern Pannonian Basin)

    NASA Astrophysics Data System (ADS)

    Patkó, Levente; Előd Aradi, László; Liptai, Nóra; Szabó, Csaba

    2013-04-01

    The Nógrád-Gömör Volcanic Field is situated in the northern part of the Pannonian Basin, where Plio-Pleistocene alkaline basalts brought upper mantle xenoliths to the surface. We collected great number of ultramafic xenoliths from the central part of the region, Medves-plateau (Eresztvény, Magyarbánya) and Baby hill (Ratka, Filakovske Kovace, Terbelovce), and detailed petrographic studies were carried out. As a result, beside the dominating lherzolite xenoliths, large number of wehrlite xenoliths also appeared, in which the modal proportion of clinopyroxene was increased in contrast to the descending amount of ortopyroxene. These wehrlite xenoliths show very unique texture, which is characterized by irregularly shaped olivine grains hosted in clinopyroxene and vermicular spinel inclusions in clinopyroxenes. According to petrographic features, ten wehrlite xenoliths have been selected for a detailed study. Based on the major elements of rock forming minerals, Fe and Mn enrichment in olivines, Ti, Al and Fe enrichment in clinopyroxenes, and Fe and Ti enrichment in spinels can be observed compared to those of lherzolite xenoliths. In the studied wehrlite xenoliths silicate, fluid and sulfide inclusions are also abundant. We focused on the latter ones in this thesis. The mineralogy of these multi-phase sulfides is in agreement with those usually found in the upper mantle with domination of pyrrhotite, pentlandite and chalcopyrite. However, bulk composition of the sulfides slightly differs from the lherzolite xenoliths. Sulfides in wehrlite xenoliths show higher Fe and lower Cu concentrations. Based on our detailed petrography and geochemistry of rock forming constituents and sulfide minerals, wehrlite xenoliths are products of a process called stealth mantle metasomatism where new minerals, in our case clinopyroxene is introduced to the system that is mineralogically indistinguishable from common upper mantle peridotites. This metasomatism is supposed to be

  6. Comparing measured OH concentrations from olivine, orthopyroxene and clinopyroxene (NAMs) with aH2O estimated from amphibole equilibria in mantle xenoliths: a test of NAM OH retention

    NASA Astrophysics Data System (ADS)

    Hunt, L. E.; Holyoke, C. W.; Lamb, W. M.; Popp, R. K.

    2013-12-01

    Determining values of H2O activity (aH2O) for mantle rocks will yield a better understanding of mantle processes that are controlled, in part, by the availability of H2O (e.g., melting and deformation). Values of mantle aH2O may, in some cases, be inferred from the OH contents of nominally anhydrous minerals (NAMs) contained in mantle xenoliths. However, mantle NAMs may suffer H2O loss during emplacement on the Earth's surface. H2O-buffering amphibole equilibria may also be used to determine values of mantle aH2O1. If NAMs reflect mantle H2O concentrations, then there should be a positive correlation between the OH contents of NAMs and values of aH2O estimated from amphibole equilibria. The chemistries of co-existing phases were characterized using the electron microprobe to estimate values of T and aH2O in nine amphibole-bearing xenoliths. We compared these values of aH2O with the OH concentrations (COH) of NAMs as measured using FTIR spectroscopy. In addition to amphibole, all samples contain olivine, two-pyroxenes, and spinel. Temperatures (T) estimated from the compositions of co-existing pyroxenes range from approximately 810 to 960oC. Minimum pressures (P), estimated based on the depth of the Moho at each location, are ≈10 - 12 kbar. Amphibole equilibria, in conjunction with these P-T estimates, yield low values of aH2O for all samples (≈ 0.02 - 0.14). The COH of olivine in all nine samples ranged from <2 - 13 wt. ppm. The relatively low values of aH2O estimated from amphibole equilibria yield predicted olivine OH concentrations that range from 1 and 20 ppm wt. ppm, as determined using experimentally derived relations between aH2O and olivine COH 2,3. The COH in olivine, in combination with DOH opx/ol ≈ 6.7 4, yield orthopyroxene (opx) OH concentrations of <13 to 87 ppm. This is within the measured opx COH of 12 - 150 ppm for eight of nine samples. COH was determined in clinopyroxene (cpx) for 5 of the 9 samples and these values ranged from 52 to 217

  7. Investigating Mantle Sources of Basaltic Melts Using Olivine LA-ICPMS Analysis, Mount Taylor Volcanic Field, New Mexico

    NASA Astrophysics Data System (ADS)

    Schrader, C. M.; Schmidt, M. E.; Thomas, A.; Bryce, J. G.; Fahnestock, M. F.

    2015-12-01

    The Mount Taylor Volcanic Field (MTVF), New Mexico, is located along the Jemez Lineament, a major crustal feature and a focus of post-Laramide magmatism. The MTVF comprises at least three regions containing contemporaneous (~3.7 to 1.26 Ma) Ne-normative volcanic rocks. The intermediate Mount Taylor (MT) strata-volcano contains early central and flanking mantle xenolith-bearing alkali basalts and hawaiites. The Rio Puerco (RP) volcanic necks contain mantle xenolith-bearing basanites and alkali basalts and no evolved lavas. Mesa Chivato (MC) contains an alkaline mafic to felsic suite with geochemical similarities to RP and MT lavas but no known mantle-xenoliths. The MTVF xenoliths are diverse (e.g., Thomas et al., 2012, AGU Fall Meeting, V43A-2825) and suggest varying degrees of melt enrichment/fertilization. By LA-ICPMS, we are characterizing olivine trace element chemistry from the mantle xenoliths and basaltic (sensu lato) phenocrysts to test how much can be determined about likely source rocks by phenocryst olivine alone. This is part of a continuing project to investigate spatial trends in Laramide mantle melt metasomatism and its relation to post-Laramide magma compositions. We have analyzed samples from a RP neck (lherzolite in alkali basalt); flows from the MT flank (websterite in alkali basalt) and from the MT amphitheater (wehrlite in hawaiite). Additionally, we analyzed olivine phenocrysts from three xenolith-free lavas: a MT basanite and MC alkali basalt and hawaiite. (1) As diverse as the xenoliths are, their olivine clusters together with regards to most trace elements, though the xenoliths can be discriminated from each other by Co abundances alone or in Cr-Al and Cr-Zn space. (2) Phenocrysts from xenolith-bearing alkali basalts cluster with the xenolith olivine, suggesting the melt was in equilibrium with a lithology of a similar trace element budget. (3) Phenocrysts from the xenolith-bearing MT hawaiite and most MC phenocrysts are depleted in

  8. The first allanite-bearing eclogite xenolith in kimberlite

    NASA Astrophysics Data System (ADS)

    Trojman-Nichols, S.; Heaman, L.

    2015-12-01

    Here we report the first allanite-bearing mantle eclogite xenolith, entrained in the 173 Ma Jericho kimberlite pipe, located in the Slave craton, northwestern Canada. This eclogite is unique among the other Jericho eclogites by an extreme LREE enrichment in all phases, and garnet alteration rims that are more calcic than the garnet cores. Allanite is an abundant accessory phase, present as dull orange, subhedral crystals. Other minerals in the paragenesis are garnet, clinopyroxene, apatite and sulfides; two compositionally and texturally distinct generations of phlogopite constitute a secondary paragenesis where allanite is no longer stable. Allanite in this sample is La-, Ce- and Th- rich, with concentrations at the weight % level, while Y is only present at the relatively low concentration of ~100 ppm. Electron backscatter imaging reveals complex zonation within the allanite crystals that is off-centre, non-symmetric, and patchy. It is often asserted that eclogite xenoliths represent subducted oceanic lithosphere, despite significant differences in the composition and mineralogy between mantle-derived eclogite xenoliths and eclogite massif material. Both types of eclogite occurrences can contain quartz/coesite; massif eclogites often have small, sparse allanite inclusions, but allanite has never been reported in eclogite xenoliths in kimberlite. Allanite in massif eclogite is thought to form during subduction by the break-down of lawsonite and the incorporation of LREE into zoisite. Lawsonite breaks down into grossular and H20 at high pressures, which may explain the anomalous high-Ca rims measured in some garnets in this sample. This allanite-bearing eclogite may provide an unprecedented window for exploring a crucial stage of eclogite metamorphism and fluid mobilization in subduction zones. In addition, the U-Pb systematics currently under investigation may constrain the age of eclogitization.

  9. HSE Abundances in Angrites and HEDs: Core-Mantle Equilibration or Late Accretion Addition of a Chondritic Component

    NASA Astrophysics Data System (ADS)

    Rai, N.; Downes, H.; Smith, C. L.

    2016-08-01

    Using metal-silicate partitioning of HSEs together with their mantle abundances in Vesta and the APB respectively, we test whether formation of a metallic core could have led to the observed abundances of the HSEs, in the mantles of these bodies.

  10. Germanium abundances in lunar basalts: Evidence of mantle metasomatism

    SciTech Connect

    Dickinson, T.; Taylor, G.J.; Keil, T.K.; Bild, R.W.

    1988-01-01

    To fill in gaps in the present Ge data base, mare basalts were analyzed for Ge and other elements by RNAA and INAA. Mare basalts from Apollo 11, 12, 15, 17 landing sites are rather uniform in Ge abundance, but Apollo 14 aluminous mare basalts and KREEP are enriched in Ge by factors of up to 300 compared to typical mare basalts. These Ge enrichments are not associated with other siderophile element enrichments and, thus, are not due to differences in the amount of metal segregated during core formation. Based on crystal-chemical and inter-element variations, it does not appear that the observed Ge enrichments are due to silicate liquid immiscibility. Elemental ratios in Apollo 14 aluminous mare basalts, green and orange glass, average basalts and KREEP suggest that incorporation of late accreting material into the source regions or interaction of the magmas with primitive undifferentiated material is not a likely cause for the observed Ge enrichments. We speculate that the most plausible explanation for these Ge enrichments is complexing and concentration of Ge by F, Cl or S in volatile phases. In this manner, the KREEP basalt source regions may have been metasomatized and Apollo 14 aluminous mare basalt magmas may have become enriched in Ge by interacting with these metasomatized areas. The presence of volatile- and Ge-rich regions in the Moon suggests that the Moon was never totally molten. 71 refs., 1 fig., 6 tabs.

  11. Geochemistry and radiogenic isotope characteristics of xenoliths in Archean diamondiferous lamprophyres: Implications for the Superior Province cratonic keel

    NASA Astrophysics Data System (ADS)

    Wyman, D. A.; Hollings, P.; Conceição, R. V.

    2015-09-01

    Xenoliths retrieved from lamprophyric hosts in the Michipicoten belt fall into four groups defined by Al-Mg contents but do not include mantle peridotite. Based on immobile trace element abundances, the xenoliths are derived from magmas associated with the main phase of arc volcanism between 2.75 and 2.70 Ga or are co-genetic with the orogenic shoshonite suite. Trace elements distinguish two styles of metasomatism characterized either by LILE enrichment or both LILE and Zr-Hf (± Nb-Ta). The first is likely associated with a hydrous fluid while the second is related to melts that permeated underplated shoshonitic mafic magmas and cumulates or the older sub-arc mantle. The Sm-Nd isotopic compositions of the xenoliths indicate that an aged, highly depleted, source was tapped during the orogenic event. The formation depths of the lamprophyric magmas, and the xenoliths they contain, contrast with the calculated depths to the base of the depleted lithosphere based on xenoliths retrieved from post-Archean kimberlites. The differences imply a late docking of the ~ 150-160 km deep Archean keel beneath the Abitibi-Wawa terrane following the emplacement of major orogenic gold deposits.

  12. Group II Xenoliths from Lunar Crater Volcanic Field, Central Nevada: Evidence for a Kinked Geotherm

    NASA Astrophysics Data System (ADS)

    Roden, M.; Mosely, J.; Norris, J.

    2015-12-01

    Group II xenoliths associated with the 140 Ka Easy Chair Crater, Lunar Crater volcanic field, NV, consist of amphibole rich-inclusions including amphibolites, pyroxenites, and gabbros. Abundant minerals in these inclusions are kaersutite, aluminous (7.3-9.7 wt% Al2O3), calcic clinopyroxene, primarily diopside, and olivine (Mg# 69-73) with accessory spinel, sulfide and apatite. Although most apatites are fluor-hydroxyapatite solid solutions, one xenolith contains Cl- and OH-rich apatite suggesting that Cl may have been an important constituent in the parent magma(s) . The xenoliths show abundant evidence for equilibration at relatively low temperatures including amphibole and orthopyroxene exsolution in clinopyroxene, and granules of magnetite in hercynite hosts. If latter texture is due to exsolution, then this particular Group II xenolith equilibrated at temperatures near or below 500oC or at a depth of about 15 km along a conductive geotherm. It may be that all the Group II xenoliths equilibrated at low temperatures given the abundant exsolution textures although Fe-Mg exchange relations suggest equilibration at temperatures in excess of 800oC. Low equilibration temperatures are in conflict with the unusually high equilibration temperatures, >1200oC (Smith, 2000) displayed by Group I xenoliths from this same volcanic field. Taken at face value, the geothermometric results indicate unusually high temperatures in the upper mantle, normal temperatures in the crust and the possibility of a kinked geotherm in the region. Curiously the LCVF lies in an area of "normal" heat flow, south of the Battle Mountain area of high heat flow but the number of heat flow measurements in the Lunar Crater area is very low (Humphreys et al., 2003; Sass, 2005). References: Humphreys et al., 2003, Int. Geol. Rev. 45: 575; Sass et al., 2005, http://pubs.usgs.gov/of/2005/1207/; Smith, 2000, JGR 105: 16769.

  13. Mantle metasomatism

    SciTech Connect

    Menzies, M.; Hawkesworth, C.

    1986-01-01

    The concept of metasomatism and its role in the geochemical enrichment and depletion processes in upper mantle rocks remains contentious. This volume makes a comprehensive contribution to the study of metasomatic and enrichment processes: origin and importance in determining trace element and isotopic heterogeneity in the lithospheric mantle. It begins with a theoretical thermodynamic and experimental justification for metasomatism and proceeds to present evidence for this process from the study of mantle xenoliths. Finally the importance of metasomatism in relation to basaltic volcanism is assessed. The contents are as follows: Dynamics of Translithospheric Migration of Metasomatic Fluid and Alkaline Magma. Solubility of Major and Trace Elements in Mantle Metasomatic Fluids: Experimental Constraints. Mineralogic and Geochemical Evidence for Differing Styles of Metasomatism in Spinel Lherzolite Xenoliths: Enriched Mantle Source Regions of Basalts. Characterization of Mantle Metasomatic Fluids in Spinel Lherzolites and Alkali Clinophyroyxenites from the West Eifel and South-West Uganda. Metasomatised Harzburgites in Kimberlite and Alkaline Magmas: Enriched Resites and ''Flushed'' Lherzolites. Metasomatic and Enrichment Phenomena in Garnet-Peridotite Facies Mantle Xenoliths from the Matsoku Kimberlite Pipe Lesotho. Evidence for Mantle Metasomatism in Periodite Nodules from the Kimberley Pipes South Africa. Metasomatic and Enrichment Processes in Lithospheric Peridotites, an Effective of Asthenosphere-Lithosphere Interaction. Isotope Variations in Recent Volcanics: A Trace Element Perspective. Source Regions of Mid-Ocean Ridge Basalts: Evidence for Enrichment Processes. The Mantle Source for the Hawaiian Islands: Constraints from the Lavas and Ultramafic Inclusions.

  14. Ilmenite Exsolution in Xenolithic Garnets From the Hawaiian Hot Spot: Evidence for the Existance of High-titanium Garnets in the Earth's Mantle

    NASA Astrophysics Data System (ADS)

    Keshav, S.; Sowerby, J. R.; Sen, G.

    2001-12-01

    Oxide inclusions in a pyrope host were discovered in a single garnet clinopyroxenite xenolith (sample 115954-20 B; Jackson Collection, Smithsonian) from Salt Lake Crater, Oahu, Hawaii. These inclusions, ~ 5-6 μ m thick and 10 μ m long, appear to radiate from a point, rather than align along the <111> direction, as found previously by other authors (e.g., Haggerty, 1991b). In the same section there are other garnet grains that have ilmenite of the same morphology but along <111>. Electron microprobe analysis and Raman spectroscopy show that both types of inclusions are ilmenite (FeTiO3) with a large geikelite (MgTiO3) component, and minor amounts of Al3+, Cr3+, and Fe3+. Note that ilmenite does not occur as a discrete phase in the xenolith. Inclusions of ilmenite and rutile, have previously been found in host garnets from eclogitic and lherzolitic xenoliths in kimberlites and ultra-high pressure terrains. This is the first report of such occurrence from an oceanic hot spot source. Based on texture, we suggest that the ilmenite inclusions in garnet in the Hawaiian xenolith are of exsolution origin as opposed to an origin by epitaxal precipitation (as proposed by Wang et al, 1999). At the present time, because of the lack of appropriate experimental study it is difficult to speculate on the P,T conditions under which ilmenite exsolved from the garnet host. However, Van Roermund et al (2000) have hypothesized on crystal chemical grounds that such inclusions are the result of the break down of a high P,T `super-titanic' garnet to a lower P,T pyrope with exsolved ilmenite, in a similar fashion to the breakdown of `super-silicic' or majoritic garnet to pyrope with exsolved pyroxene.

  15. Neodymium Isotope Variability at the Grain Scale in the Sub-Continental Lithospheric Mantle: NdO+ Analyses of Individual Clinopyroxene Grains (<5 ng Nd aliquots) from a Kilbourne Hole Harzburgitic Xenolith.

    NASA Astrophysics Data System (ADS)

    Harvey, J.; Honn, D.; Baxter, E. F.; Warren, J. M.; Hammond, S.; Walshaw, R.

    2014-12-01

    It is evident that at scales of 102 to 10-2 m there is significant isotopic heterogeneity in the mantle that is not always reflected in primitive melts. The "Os isotopic gap"[1] is one such manifestation of this phenomenon but a similar offset exists between the Nd isotope composition of abyssal peridotites and the mid-ocean ridge basalts that they are inferred to have produced[2]. This study takes advantage of recent advances in the analysis of Nd isotopes as NdO+[3,4] which permit the precise analysis of single clinopyroxene grains (<1 mg mass; <5 ng Nd) from a continental harzburgitic xenolith from Kilbourne Hole, NM. Analyses of aggregates of clinopyroxenes from 5 Kilbourne Hole xenoliths reveal a wide range of 143Nd/144Nd (0.513011 ± 28 to 0.513615 ±19)[5]. This study demonstrates significant grain-to-grain isotopic heterogeneity at a scale of 10-2 m (143Nd/144Nd = 0.513089 ± 78 to 0.513364 ± 74) which (i) is equivalent to the range of values for Pacific MORB[6] and (ii) is more primitive than local basalts with an asthenospheric signature[7]. This suggests that small-scale refractory domains exist within the mantle which are either not sampled during partial melting or whose presence is obscured by the melting of higher volumes of more fusible material. Ref:[1]Alard et al. (2005) Nature 436, 1005-1008 [2]Warren et al. (2009) JGR 114, B12203, doi:10.1029/2008JB006186 [3]Harvey and Baxter (2009) Chem. Geol. 258, 251-257 [4]Honn et al. (2013) AGU Fall abstr. V33-2722 [5]Harvey et al. (2012) J. Petrol. 53, 1709-1742 [6]Hofmann (1997) Nature 385, 219-229 [7]Thompson et al. (2005) J. Petrol. 46, 1603-1643

  16. The effects of melt depletion and metasomatism on highly siderophile and strongly chalcophile elements: S-Se-Te-Re-PGE systematics of peridotite xenoliths from Kilbourne Hole, New Mexico

    NASA Astrophysics Data System (ADS)

    Harvey, Jason; König, Stephan; Luguet, Ambre

    2015-10-01

    The composition of the Earth's upper mantle is a function of melt depletion and subsequent metasomatism; the latter obscuring many of the key characteristics of the former, and potentially making predictions of Primitive Upper Mantle (PUM) composition problematic. To date, estimates of PUM abundances of highly siderophile element (HSE = platinum group elements (PGE) and Re) and the strongly chalcophile elements Se and Te, have been the subject of less scrutiny than the lithophile elements. Critically, estimates of HSE and strongly chalcophile element abundances in PUM may have been derived by including a large number of metasomatized and refertilized samples whose HSE and chalcophile element abundances may not be representative of melt depletion alone. Unravelling the effects of metasomatism on the S-Se-Te-HSE abundances in peridotite xenoliths from Kilbourne Hole, New Mexico, USA, potentially provides valuable insights into the abundances of HSE and strongly chalcophile element abundances in PUM. Superimposed upon the effects of melt depletion is the addition of metasomatic sulfide in approximately half of the xenoliths from this study, while the remaining half have lost sulfide to a late S-undersaturated melt. Despite these observations, the Kilbourne Hole peridotite xenoliths have HSE systematics that are, in general, indistinguishable from orogenic peridotites and peridotite xenoliths used for determination of PUM HSE abundances. This study represents the first instance where Se-Te-HSE systematics in peridotite xenoliths are scrutinized in detail in order to test their usefulness for PUM estimates. Despite earlier studies attesting to the relative immobility of Se during supergene weathering, low S, Se, Os and Se/Te in peridotite xenoliths suggests that Se may be more mobile than originally thought, and for this reason, peridotite xenoliths may not be suitable for making predictions of the abundance of these elements in PUM. Removal of Se, in turn, lowers the

  17. Thinning of Refertilized Sub-Continental Lithospheric Mantle (SLCM) beneath the Main Ethiopian Rift During Tertiary Rifting: Petrologic and Thermal Constraints from (Garnet)-Spinel Peridotite Xenoliths (Mega, Ethiopia).

    NASA Astrophysics Data System (ADS)

    Casagli, A.; Frezzotti, M. L.; Peccerillo, A.; Tiepolo, M.; De Astis, G.

    2014-12-01

    The East African Rift System (EARS) represents a key locality for the knowledge of the nature and evolution of SCLM during continental rifting processes, traditionally ascribed to ascending mantle plumes. We report petrological and geothermobarometric data from mantle xenoliths in Quaternary alkali-basalt lava flows and scoria cones at Mega (Sidamo Region; EARS) in the southern Main Ethiopian Rift (MER), that give evidence for refertilization of SCLM and for thinning during Tertiary rifting. Studied samples consist of seven lherzolites, five harzburgites and one olivine-websterite that contain spinel-pyroxene symplectites, interpreted as products of garnet breakdown reactions. These rocks were analyzed for major (whole rock and minerals) and trace elements (pyroxenes). Major element data have been used to reconstruct original garnet composition (pyrope). Equilibration temperatures range from 985 ± 40°C in the garnet facies (2.9-2.2 GPa) to 960 ± 55°C in the spinel facies (1.3 GPa). Xenoliths consist of depleted and fertile peridotites. Five lherzolites have up to 4 wt% of CaO, high CaO/Al2O3 (1.42-4.46), and the most fertile are more enriched than primitive mantle. Variations of major oxides in bulk rocks and minerals are consistent with variable degrees of melt extraction. Evidence for modal and cryptic metasomatism is given by addition of clinopyroxene ± phlogopite, and by LILE and LREE enrichment in clinopyroxene. Refertilization process appears to have been induced by sub-lithospheric volatile-rich melts at high melt/rock ratio, and were followed by cooling. To account for the geodynamic evolution of SCLM beneath the southern MER, which implies a temperature gradient from 50-60 to ˜ 90 mW/m2, we propose that thinning of the base of fertile SCLM from 90-95 to ˜45km depth and associated magmatism occurred along a normal-mantle adiabat above an upwelling asthenosphere (i.e., decompression melting) without the need for significant heat sources.

  18. Os and HSE of the hot upper mantle beneath southern Tibet: Indian mantle affinity?

    NASA Astrophysics Data System (ADS)

    Zhao, Z.; Dale, C. W.; Pearson, D. G.; Niu, Y.; Zhu, D.; Mo, X.

    2011-12-01

    The subduction of the Indian plate (including cratonic continental crust and/or upper mantle) beneath southern Tibet is widely accepted from both geological and geophysical studies. Mantle-derived xenoliths from this region provide a means of directly investigating the mantle underlying the southern part of the plateau. Studies of xenoliths hosted in the Sailipu ultrapotassic volcanic rocks, erupted at ~17 Ma, have indicated that the subcontinental mantle of southern Tibetan Plateau is hot and strongly influenced by metasomatism (Zhao et al., 2008a, b; Liu et al., 2011). Here we report comprehensive EPMA and LA-ICP-MS major and trace element data for the Sailipu xenoliths and also whole rock Os isotope and HSE data in order to constrain the depletion history of the mantle and to identify the presence of any potential Indian cratonic mantle. The xenoliths, ranging in size from 0.5cm to 1.5cm in diameter, are mostly peridotites. The calculated temperatures are 1010-1230°C at the given pressures of ~1.6-2.0 GPa (n=47). These P-T conditions are similar to rift-related upper mantle regimes (e.g. Kenya), indicating the influence of regional extension beneath southern Tibet in the Miocene. A series of compositional discriminations for minerals (Cpx, Opx, Ol, and Phl), e.g. Fo<90, suggest that the xenoliths are non-cratonic spinel-peridotite (cratonic peridotite olivine Fo> ~91), with a clear metasomatic signature We obtained Os isotope data and abundances of highly siderophile elements (HSE, including Os, Ir, Ru, Pt, Pd and Re) on a set of six olivine-dominated peridotite samples from Sailipu volcanics, less than 1 cm in dimension. They allow us to further constrain the nature and state of the upper mantle beneath the southern Tibet. Sailipu samples display low total HSE abundances (Os+Ir+Ru+Pt+Pd+Re) ranging from 8.7 to 25 ppb, with nearly constant Os, Ir , and Ru, but rather varied Pt (2-13), Pd (0.4-5.2), and Re (0.01-0.5). Chondrite-normalised Pd/Ir ratios range from

  19. Water Content of the Oceanic Lithosphere at Hawaii from FTIR Analysis of Peridotite Xenoliths

    NASA Technical Reports Server (NTRS)

    Peslier, Anne H.; Bizmis, Michael

    2013-01-01

    Although water in the mantle is mostly present as trace H dissolved in minerals, it has a large influence on its melting and rheological properties. The water content of the mantle lithosphere beneath continents is better constrained by abundant mantle xenolith data than beneath oceans where it is mainly inferred from MORB glass analysis. Using Fourier transform infrared (FTIR) spectrometry, we determined the water content of olivine (Ol), clinopyroxene (Cpx) and orthopyroxene (Opx) in spinel peridotite xenoliths from Salt Lake Crater, Oahu, Hawaii, which are thought to represent fragments of the Pacific oceanic lithosphere that was refertilized by alkalic Hawaiian melts. Only Ol exhibits H diffusion profiles, evidence of limited H loss during xenolith transport to the surface. Water concentrations (Ol: 9-28 ppm H2O, Cpx: 246-566 ppm H2O, Opx: 116-224 ppm H2O) are within the range of those from continental settings but higher than those from Gakkel ridge abyssal peridotites. The Opx H2O contents are similar to those of abyssal peridotites from Atlantic ridge Leg 153 (170-230 ppm) but higher than those from Leg 209 (10- 14 ppm). The calculated bulk peridotite water contents (94 to 144 ppm H2O) are in agreement with MORB mantle source water estimates and lower than estimates for the source of Hawaiian rejuvenated volcanism (approx 540 ppm H2O) . The water content of Cpx and most Opx correlates negatively with spinel Cr#, and positively with pyroxene Al and HREE contents. This is qualitatively consistent with the partitioning of H into the melt during partial melting, but the water contents are too high for the degree of melting these peridotites experienced. Melts in equilibrium with xenolith minerals have H2O/Ce ratios similar to those of OIB

  20. Mantle rare gas relative abundances in a steady-state mass transport model

    NASA Technical Reports Server (NTRS)

    Porcelli, D.; Wasserburg, G. J.

    1994-01-01

    A model for He and Xe was presented previously which incorporates mass transfer of rare gases from an undegassed lower mantle (P) and the atmosphere into a degassed upper mantle (D). We extend the model to include Ne and Ar. Model constraints on rare gas relative abundances within P are derived. Discussions of terrestrial volatile acquisition have focused on the rare gas abundance pattern of the atmosphere relative to meteoritic components, and the pattern of rare gases still trapped in the Ear,th is important in identifying volatile capture and loss processes operating during Earth formation. The assumptions and principles of the model are discussed in Wasserburg and Porcelli (this volume). For P, the concentrations in P of the decay/nuclear products 4 He, 21 Ne, 40 Ar, and 136 Xe can be calculated from the concentrations of the parent elements U, Th, K, and Pu. The total concentration of the daughter element in P is proportional to the isotopic shifts in P. For Ar, ((40)Ar/(36)Ar)p - ((40)Ar/(36)Ar)o =Delta (exp 40) p= 40 Cp/(exp 36)C where(i)C(sub j) the concentration of isotope i in j. In D, isotope compositions are the result of mixing rare gases from P, decay/nuclear products generated in the upper mantle, and subducted rare gases (for Ar and Xe).

  1. Ge abundances in the lunar mantle and implications for the origin of the Moon

    NASA Technical Reports Server (NTRS)

    Dickinson, T.; Newsom, H.

    1984-01-01

    Regardless of the origin of the Moon, metal segregation must have occurred within the moon in order to account for its low siderophile element abundances relative to the Earth or chondrites. Germanium is a strongly siderophile element whose bulk distribution coefficient indicates that it is not fractionated during igneous processes on the Moon. The variability in absolute Ge abundances in mare basalts and pristine highland rocks, rather than elemental ratios, can be used to infer lunar mantle abundances and processes. Literature data have been compiled for Ge abundances in mare basalts and pristine highland rocks. For some landing sites, samples with 12 ppb Ge were considered to be extreme outliers and are not included. The Apollo 15 samples are enriched in Ge by a factor of 2.5 over the Apollo 12, 16 and 17 samples. Other siderophile element variations have been found in the Moon. Based on this data, best estimate of the average Ge abundance in the silicate portion of the Moon is 3.52 ppb. The Moon is depleted, relative to chondritic abundances, by a factor of 38,000 normalized to Si. Two possible explanations for the observed variations in Ge abundance in the Moon are: (1) more metal may have segregated from some regions of the Moon than from others; or Ge-bearing material may been been added later in the evolution of the Moon.

  2. Mantle metasomatism: the REE story.

    USGS Publications Warehouse

    Wilshire, H.G.

    1984-01-01

    Refractory rocks with light REE/heavy REE ratios > chondrites are common as xenoliths in basalts and kimberlites and are found in some oceanic peridotite massifs. Structural and major-element geochemical evidence from these rocks suggest that the metasomatic effects resulting in addition of light REE are local and are related to emplacement of partial melts. The melts are represented by dykes of pyroxenites, hydrous minerals and gabbro that were emplaced in mantle peridotites of various origins. Metasomatic interaction between dykes and peridotite wall rock results in light REE enrichment in peridotite and depletion in dykes relative to the original liquid. Differentiation of the intrusions and separation of residual liquids may further enhance the REE exchange and extend the volume of metasomatized peridotite. Differences in the relative abundances of altered peridotite in xenoliths and massifs are seen as a sampling problem rather than a difference in process.-L.diH.

  3. The formation of saline mantle fluids by open-system crystallization of hydrous silicate-rich vein assemblages - Evidence from fluid inclusions and their host phases in MARID xenoliths from the central Kaapvaal Craton, South Africa

    NASA Astrophysics Data System (ADS)

    Konzett, Jürgen; Krenn, Kurt; Rubatto, Daniela; Hauzenberger, Christoph; Stalder, Roland

    2014-12-01

    The composition of texturally primary fluid inclusions and their host phases clinopyroxene, K-richterite, and zircon were investigated in two MARID-type (mica-amphibole-rutile-ilmenite-diopside) mantle xenoliths sampled by the Kimberley cluster of Cretaceous kimberlites erupted in the central Kaapvaal Craton, South Africa. P-T conditions of crystallization for the MARID assemblages of 4.2 GPa and 960 °C were estimated based on Ca-in-opx thermometry and the assumption of a 40 mW/m2 geotherm applied to two orthopyroxene-bearing MARIDs collected from the same locality. Cooling/heating stage measurements and Raman spectroscopy indicate a fluid system dominated by H2O-NaCl-MgCl2 with variable total salinities in the range ⩽6.4-32.4 mass% and minor amounts of MgCl2, the latter inferred from the crystallization of MgCl2 × 12H2O during cooling of the inclusions. In addition to liquid and vapour, enstatite, baddeleyite, barite, calcite and a K-Ba-Fe-Cr-titanate were identified as solid phases in opened fluid inclusions, indicating high LIL-(HFS) element concentrations in the saline hydrous fluids prior to solid phase precipitation. The Cl contents of apatite (⩽0.35 wt.%), phlogopite (⩽0.09 wt.%) and K-richterite (⩽0.025 wt.%) follow the enrichment pattern Clap ≫ Clphl > ClKr which is typical for upper mantle rocks. Fluid inclusion-bearing clinopyroxenes show very low H2O contents of ∼45 μg/g which is consistent with a reduced aH2O of the fluids due to the presence of NaCl-MgCl2-SiO2-LILE combined with high fO2 and very low Al3+ contents of the clinopyoxenes. The zircons show a complex compositional zoning with variable and positively correlated Y (⩽1260 μg/g), P (⩽1870 μg/g) and Sc (⩽1373 μg/g) contents, indicating a pretulite-type substitution Si4+ + Zr4+ = P5+ + (Sc, Y, REE)3+. The Sc contents of the zircons are amongst the highest Sc concentrations reported so far for upper mantle silicates. Oxygen isotope analyses of selected zircons yields δ18O

  4. A new and simple approach to determine the abundance of hydrogen molecules on interstellar ice mantles

    NASA Astrophysics Data System (ADS)

    Hincelin, U.; Chang, Q.; Herbst, E.

    2015-02-01

    Context. Water is usually the main component of ice mantles, which cover the cores of dust grains in cold portions of dense interstellar clouds. When molecular hydrogen is adsorbed onto an icy mantle through physisorption, a common assumption in gas-grain rate-equation models is to use an adsorption energy for molecular hydrogen on a pure water substrate. However, at high density and low temperature, when H2 is efficiently adsorbed onto the mantle, its surface abundance can be strongly overestimated if this assumption is still used. Unfortunately, the more detailed microscopic Monte Carlo treatment cannot be used to study the abundance of H2 in ice mantles if a full gas-grain network is utilized. Aims: We present a numerical method adapted for rate-equation models that takes into account the possibility that an H2 molecule can, while diffusing on the surface, find itself bound to another hydrogen molecule, with a far weaker bond than the H2-water bond, which can lead to more efficient desorption. We label the ensuing desorption "encounter desorption". Methods: The method is implemented first in a simple system consisting only of hydrogen molecules at steady state between gas and dust using the rate-equation approach and comparing the results with the results of a microscopic Monte Carlo calculation. We then discuss the use of the rate-equation approach with encounter desorption embedded in a complete gas-grain chemical network. Results: For the simple system, the rate-equation model with encounter desorption reproduces the H2 granular coverage computed by the microscopic Monte Carlo model at 10 K for a gas density from 104 to 1012 cm-3, and yields up to a factor 4 difference above 1012 cm-3. The H2 granular coverage is also reproduced by a complete gas-grain network. We use the rate-equation approach to study the gas-grain chemistry of cold dense regions with and without the encounter desorption mechanism. We find that the grain surface and gas phase species can be

  5. Textural relationship and compositions of ilmenite-corundum exsolutions in rutile from kimberlitic kyanite eclogite xenoliths: microstructural evidence using EBSD

    NASA Astrophysics Data System (ADS)

    Sobolev, N. V.; Schertl, H.; Neuser, R. D.; Lavrentiev, Y. G.; Logvinova, A. M.; Usova, L. V.

    2007-12-01

    Rutile is one of the most common accessory minerals in high pressure (HP) and ultrahigh-pressure (UHP) rocks of crustal and mantle origin. Among those rocks eclogites are very abundant in metamorphic belts and as xenoliths in kimberlite pipes. Some of these xenoliths contain coesite or diamond. Eclogite xenolith rutiles contain high abundance of minor elements reaching up to 0.8 wt.% Al2O3, 2.5 wt.% Fe2O3, 1.4 wt.% Nb2O5, 0.45 wt.% ZrO2 and are mostly heterogeneous with widely varying Al, Fe and Mg contents. These heterogeneities are caused by the presence of closely associated sigmoidal oriented lamellae of ilmenite and corundum which have been detected for the first time in rutile from Roberts Victor mine eclogite xenolith (Sobolev, Yefimova, 2000, Intern. Geol. Rev., v. 42, p. 758-767). We report here on the wide occurrence of such lamellae in rutiles both from diamondiferous kyanite eclogites of Udachnaya mine, Siberia and more samples from Roberts Victor mine confirmed by EMPA. Electron backscatter diffraction (EBSD) measurements on two rutile grains from Roberts Victor mine kyanite eclogite confirm the presence of ilmenite plus corundum exsolutions. We found evidence for respective different crystallographic orientations related to the surrounded rutile host which will be demonstrated in detail in the present study.

  6. Breakdown of orthopyroxene contributing to melt pockets in mantle peridotite xenoliths from the Western Qinling, central China: constraints from in situ LA-ICP-MS mineral analyses

    NASA Astrophysics Data System (ADS)

    Su, Ben-Xun; Zhang, Hong-Fu; Yang, Yue-Heng; Sakyi, Patrick Asamoah; Ying, Ji-Feng; Tang, Yan-Jie

    2012-03-01

    Major and trace element compositions of constituent minerals, partly decomposed rims of orthopyroxenes (DRO), `closed' melt pockets (CMP) and open melt pockets (OMP) in some Western Qinling peridotite xenoliths were obtained by LA-ICP-MS. Systematic core-to-rim compositional variations of garnet, clinopyroxene and orthopyroxene demonstrate that these minerals underwent variable degrees of subsolidus breakdown or partial melting. Both DROs and CMPs consist of similar mineral assemblages and are characterized by high TiO2, CaO + Na2O and low MgO contents; they are enriched in LREE and LILE compositions, have positive anomalies in Pb, Sr and particularly Ti, negative Th and U, and variable Zr and Hf anomalies. These chemical features are distinct and reflect reactions involving the orthopyroxenes. Compared to the CMPs, the OMPs, which are composed of a complex assemblage of minerals, display lower FeO and MgO contents, larger ranges in SiO2 and Na2O, higher TiO2, Al2O3, CaO and trace element concentrations, slightly negative Zr and Hf anomalies, and apparently negative Ti anomalies. Modeling calculations of partial fusion of orthopyroxenes and clinopyroxenes suggest that the CMPs most likely originated from the breakdown of orthopyroxenes with variably minor contribution of external melts from the melting of clinopyroxenes, whereas the OMPs were probably formed from the modification of the CMPs through the interaction with large amount of external melts.

  7. Fabrics and Rheology of the Mojave Lower Crust and Lithospheric Mantle

    NASA Astrophysics Data System (ADS)

    Bernard, R. E.; Behr, W. M.

    2014-12-01

    We use xenoliths from young (3 Ma to present) cinder cones in the tectonically active Mojave desert region of southern California to characterize the rheological properties of the lower crust and upper mantle. The xenoliths were collected from two localities ~90 km apart: the Cima and Dish Hill volcanic fields. The xenolith suites represent a depth range of ~25-60 km and include spinel and plagioclase facies peridotites and lower crustal gabbros. We document how stress, temperature, water content, deformation mechanism, lattice preferred orientation, and style of localization vary with increasing depth in both xenolith suites. Key findings thus far include the following: (1) Both xenolith suites exhibit a wide range of deformation textures, ranging from granular, to protogranular, to porphyroclastic and mylonitic. The higher strain fabrics show no evidence for static annealing, thus are likely reflecting youthful deformation and strain gradients at depth. (2) Both xenolith suites show abundant dynamic recrystallization and other evidence for dislocation creep as the dominant deformation mechanism. This is consistent with recent models of upper mantle post-seismic relaxation following the Landers and Hector Mine earthquakes, which require a component of power-law creep in order to fit the post-seismic surface response. (3) A- and E-type olivine LPOs occur in both xenolith suites. Further work will determine whether these fabrics are related to changes in water content as inferred from experimental studies. (4) Deformation in most lower crustal gabbros is weak, but some show strong fabrics associated with plagioclase-rich zones. (5) Measurements of olivine subgrain sizes in Dish Hill samples are similar to previously published measurements from Cima, suggesting similar stress magnitudes at depth in both locations. Paleopiezometers for olivine and plagioclase indicate stress magnitudes of 11-20 MPa for the uppermost mantle, and 0.1 MPa for the lowermost crust.

  8. Ultramafic xenoliths and megacrysts from a melilitite tuff cone, Deeti, northern Tanzania

    NASA Astrophysics Data System (ADS)

    Johnson, L. H.; Jones, A. P.; Church, A. A.; Taylor, W. R.

    1997-07-01

    Deeti, a ˜50 m high 'ubehebe' tuff ring is situated in a small field of similar tuff cones of Quaternary age, next to the two nephelinite-carbonatite volcanoes of Kerimasi and Oldoinyo Lengai, south of Lake Natron. The tuff cones, lying on small parallel faults, may have been closely associated and possibly triggered by the same events that have given rise to the volcanic activity at Oldoinyo Lengai and Kerimasi. The host extrusive at Deeti is a melilitite (SiO 2 37.20%, MgO 15.0%, TiO 2 3.93%, CaO 9.26%, K 2O 2.76% and Mg66.4); it is mica porphyritic and contains ijolitic xenolith fragments. The upper part of the cone is formed of spectacular carbonate-cemented, coarse bedded deposits of cored, golf ball size lapilli. A distinctive megacryst suite is composed of phlogopitic mica (FeO 0.5%, TiO 2 4.8%, Mg80), pargasitic amphibole ( Mg71) and diopsidic clinopyroxene ( Mg80). Numerous ultramafic xenoliths dominated by amphibole-mica peridotites and pyroxenites, form the cores of larger lapilli and exist as bombs up to 30 cm across. This amphibole is chromian-pargasite, with very rare relics of richterite ( Mg88). The xenoliths show abundant evidence of multiple veining, overgrowths and substantial fabric modification. On the basis of electron microprobe data, we show that these petrographic textures probably developed as a result of metasomatism by alkaline silicate, and possibly carbonatite melts. The original protoliths include more primitive spinel peridotites (Fo >88) that have been significantly Fe-enriched. The lava has sampled upper mantle wall rock to depths of origin of the melilitite (> 60 km) and these xenoliths may constrain possible mantle source compositions for the adjacent larger carbonatite volcanoes.

  9. A Disequilibrium Melting Spectrum: Partially Melted Crustal Xenoliths from the Wudalianchi Volcanic Field, NE China.

    NASA Astrophysics Data System (ADS)

    McLeod, C. L.; McGee, L. E.

    2015-12-01

    Disequilibrium melting has been established as a common process occurring during crustal anatexis and thus demonstrates that crustal assimilation by ascending mantle-derived magmas is likley not a closed system. Observations of extreme compositional heterogeneity within partial melts derived from crustal xenoliths have been documented in several recent examples, however, the retention or transfer of elements to and from residues and glasses, and their relative contributions to potential crustal contaminants warrants further investigation. Sampled lavas from the Huoshaoshan volcano in the Holocene Wudalianchi volcanic field of Northeast China contain crustal xenoliths which preserve a spectrum of partial melting both petrographically and geochemically, thus providing an excellent, natural example of crustal anatexis. Correlations exist between the volume of silicic glass preserved within the xenoliths and bulk rock SiO2 (70-83 wt%), Al2O3 (16-8 wt%), glass 87Sr/86Sr (0.715-0.908), abundances of elements common in feldspars and micas (Sr, Ba, Rb) and elements common in accessory minerals (Y, Zr, Nb). These correlations are likely associated with the consumption of feldspars and micas and the varying retention of accessory phases during partial melting. The xenoliths which contain the greater volumes of silicic glass and residual quartz (interpreted as being the most melted) were found within pahoehoe lava, whilst the least melted xenoliths were found within scoria of the summit cone of Huoshaoshan; thus it is interpreted that the extent of melting is linked to the immersion time in the lava. Small-scale (mm) mingling and transfer of material from the enclosing lava to the xenolith is observed, however, modelling of potential contaminant compositions is inconsistent with crustal contamination during lava petrogenesis. It is inferred that crustal contamination in sampled lavas is localized within the open magmatic system and most likely occurs at the contact zone

  10. X-Ray Tomography of the Most Diamondiferous Peridotite: A Unique Xenolith from Udachnaya, Siberia

    NASA Astrophysics Data System (ADS)

    Taylor, L. A.; Howarth, G. H.; Logvinova, A. M.; Reustsy, V.; Fedorova, E.; Ketcham, R. A.; Wirth, R.; Sobolev, N. V.

    2014-12-01

    The Udachnaya kimberlite pipe is dominated by ultramafic xenoliths; however, it is the diamondiferous eclogites that are most recovered, with precious few diamondiferous peridotites. A truly unique diamondiferous garnet-peridotite xenolith was recovered, consisting of equal amounts of enstatite (Mg# 92.7) and lherzolitic garnet (Mg# 83.8), with lesser unaltered olivine (Mg# 92-95). High-Resolution X-ray Computerized Tomography (HRXCT) of this strange xenolith produced numerous 2-D and 3-D images, revealing detailed relationship between the diamonds and the host minerals. The modal abundances of the minerals are: diamonds (9.5 vol%); enstatite (38 vol%); pyrope (35 vol%); sulfides (4 vol%; pyrrhotite, pentlandite, and chalcopyrite); and the remainder being mainly alteration products, consisting of "spinach" - probably serpentine - mostly in the interior of the peridotite, along with the majority of the diamonds. This 10.5 g peridotite contains >30,000 colorless, all euhedral, octahedral micro-diamonds, with sizes of 100 to 700 mm, many occurring in clusters. This is the absolute highest-yield of diamonds ever in a mantle xenolith - 106 cts/tonne - but tiny Ds. The δ13C isotopic composition in these crystals has a value of -22 ‰, not typical for diamonds of ultramafic paragenesis, which are usually in the narrow range in δ13C (-2 to -8 ‰), characteristic of the mantle (Cartigny, Elements, 2005). Diamonds with such light-isotopic carbon, as low as -44 ‰ typical for eclogitic diamonds, are considered as indicative of subduction of oceanic crust (Sobolev & Sobolev, Russ. G & G, 1980). The majority of diamonds are Type IIa; others are Type IaB. Nitrogen impurity is of the B form indicating a high-aggregation state, considered evidence for long mantle residence times at high temperatures. In some Ds, nano-size inclusions were discovered using TEM techniques and composed of Mg-Al silicate-rich phases, a Ca-carbonate phase, graphite, and fluid. These micro

  11. Gold in the mantle: The role of pyroxenites

    NASA Astrophysics Data System (ADS)

    Saunders, J. Edward; Pearson, Norman J.; O'Reilly, Suzanne Y.; Griffin, William L.

    2016-02-01

    Mantle pyroxenites are the crystallised products of mafic silicate melts, which are commonly invoked as metasomatic agents in the upper mantle. This study has analysed the trace elements of sulfides, with a specific focus on gold, hosted in a suite of mantle pyroxenite xenoliths from Qilin in the Cathaysia Block, southeast China. These are compared with sulfides hosted in peridotite xenoliths from the same locality to assess the difference in the abundances of Au, and a suite of siderophile and chalcophile elements between the sulfides hosted in mobile melts in the upper mantle and their host "wall" rocks. Both the peridotite- and pyroxenite-hosted sulfides show a wide spectrum of trace element contents. The pyroxenite-hosted sulfides typically have PGE and Au concentrations that are an order of magnitude or more below those measured in the peridotite-hosted sulfides (lherzolite-hosted sulfides: total PGE = 95 ± 118 ppm, Au = 1.4 ± 2.6 ppm; pyroxenite-hosted sulfides: total PGE = 0.25 ± 0.70 ppm, Au = 0.14 ± 0.39 ppm). Furthermore, the Ir group PGE (Ir, Os and Ru) are present in lower concentrations than the Pd-group PGE (Pd, Pt and Rh). This may lead to a distinct signature if the melts from which these sulfides crystallise interact with lherzolitic sulfides. The overall low abundances of these elements within the pyroxenites suggests that the parent melts are an inefficient medium for enriching any of these elements in the upper mantle.

  12. Simultaneous Quantification of Temperature, Pyroxenite Abundance, and Upwelling Rates in the Iceland Mantle Source

    NASA Astrophysics Data System (ADS)

    Brown, E.; Lesher, C. E.

    2014-12-01

    The compositions and volumes of basalts erupted at the earth's surface are a function of mantle temperature, mantle composition, and the rate at which the mantle upwells through the melting zone. Thus, basaltic magmatism has long been used to probe the thermal and physiochemical state of the earth's mantle. Great insight has been gained into the mantle beneath the global spreading ridge system, where the mantle source is assumed to be homogeneous peridotite that upwells passively [1]. However, it is now recognized that many basalt source regions are lithologically heterogeneous (i.e. containing recycled lithospheric material ranging from harzburgite to pyroxenite) and upwell at rates in excess of those governed by plate separation. To account for these complexities, we have developed a forward melting model for lithologically heterogeneous mantle that incorporates thermodynamically and experimentally constrained melting functions for a range of peridotite and pyroxenite lithologies. The model is unique because it quantifies mantle upwelling rates based on the net buoyancy of the source, thus providing a means for linking basalt compositions/volumes to mantle flow while accounting for source heterogeneity. We apply the model to investigate the mantle properties governing magmatism along different rift segments in Iceland, where lithologic heterogeneity and variable upwelling rates have been inferred through geochemical means [2,3]. Using constraints from seismically determined crustal thicknesses and recent estimates of the proportion of pyroxenite-derived melt contributing to Icelandic basalt compositions [4,5], we show that mantle sources beneath Iceland have excess potential temperatures >85 °C, contain <7% pyroxenite, and maximum upwelling rates ~14 times the passive rate. Our modeling highlights the dominant role of elevated mantle temperature and enhanced upwelling for high productivity magmatism in Iceland, and a subordinate role for mantle heterogeneity

  13. Lithology and evolution of the crust-mantle boundary region in the southwestern Basin and Range province

    SciTech Connect

    Wilshire, H.G. )

    1990-01-10

    Seismic transects in this area show a strongly reflective Moho of generally low relief, which, in the area of modern transects, consists of a thin zone (< 2 km thick) of short reflectors. The upper mantle is transparent and has a P{sub n} of 7.8-8.0 km/s similar to much of the western US. A lower crustal zone, 2-13 km thick, has variable internal reflectivity and a relatively low velocity of 6.6-6.8 km/s. Upper mantle peridotite xenoliths show both ductile and brittle deformational features and have structures and composition affected by magmatic intrusion; intrusions form complex dike systems and extensive zones of grain boundary infiltration in peridotite xenoliths. Whereas melt infiltration preceded and followed ductile deformation, brittle deformation, represented by closely spaced joint systems and faults, followed ductile deformation and is related to the youngest magmatic episodes. Lower crustal xenoliths are dominantly igneous-textured pyroxenites and mafic to intermediate gabbros identical to the dikes in peridotite xenoliths. The crustal xenoliths also commonly are jointed, and in addition many show partial melting and have abundant cavities that probably were filled with CO{sub 2}-rich fluids. These rocks are interpreted as products of underplated magmas that were fed through the mantle dike systems and may represent the lowest crustal unit identified in the seismic records. The mafic compositions and high densities of the crustal xenoliths indicate that the low velocity of the lower crust mat be caused in part by fracture systems, partial melts, and high temperatures. The preferred model for the evolution of the lower lithosphere is one in which extension affects the upper mantle as well as the crust and is overlapped in time by multiple magmatic episodes. The earliest magmatic events preceded extension, and later events accompanied and followed extension.

  14. Trace Element and Os-Hf-Nd-Sr Isotope Systematics of Pervasively Metasomatised Ancient Lithospheric Mantle at the Southeastern rim of the Siberian Craton

    NASA Astrophysics Data System (ADS)

    Ionov, D.; Weis, D.; Shirey, S.; Prikhodko, V.; Chazot, G.

    2001-12-01

    Spinel peridotite xenoliths in Late Cenozoic basalts from the Aldan-Stanovoi shield show effects of Meso-Cenozoic tectonic re-activation and magmatism on the ancient lithospheric mantle. Most of the xenoliths are harzburgites and cpx-poor lherzolites; less common are fertile lherzolites and olivine-rich cumulates. Petrographic and chemical data indicate profound metasomatic alteration of the refractory peridotites, possibly due to interaction with evolved magmatic liquids: precipitation of secondary clinopyroxene and gabbroic interstitial material, low Mg-numbers of olivine and whole-rocks in combination with high Cr in spinel; high whole-rock Ca/Al, enrichments in highly incompatible elements and/or inversely U-shaped REE patterns. Re abundances in all xenoliths are <0.06 ppb; Os abundances range from 0.1 to 4 ppb. Re and Os (0.9-3 ppb) in a subset of samples (including all cpx-rich lherzolites) that show no or limited metasomatism are positively correlated with modal clinopyroxene or whole-rock Al. 187/188Os in those xenoliths show linear correlations with Al or modal cpx consistent with a depletion age about 2 Ga and the formation of the lithosphere in the Precambrian. By contrast, the metasomatised refractory (2-7% cpx) xenoliths show a broad range in Os abundances and 187/188Os values (0.116-0.127), possibly due to disturbance of the Re-Os system during metasomatism. 176/177Hf is above the N-MORB average in one clinopyroxene separate and range between BSE and MORB values in the few other samples analysed. The 176/177Hf variations could be explained by mixing of ancient depleted mantle with an OIB-type metasomatic agent. We conclude that the xenoliths represent cratonic mantle strongly modified by metasomatism in hot-spot or subduction-related environments, possibly following removal of the cratonic keel and involving underplating of basaltic melts and their cumulates.

  15. Re-Os and PGE Systematics of Neoarchean Websterite Xenoliths and Diamondiferous Lamprophyres of the Wawa Area, Superior Province, Canada

    NASA Astrophysics Data System (ADS)

    Shirey, S. B.; Ayer, J. A.; Wyman, D. A.

    2009-12-01

    The Michipicoten greenstone belt (MGB) is situated in the Wawa-Abitibi subprovince, a largely juvenile crustal addition to the southern Superior Province. The MGB comprises three mafic to felsic volcanic successions. The Mesoarchean basal succession also contains komatiites and granitoids unconformably overlain by 2 Neoarchean units. Numerous 2.68 Gyr old lamprophyre dikes and their diatreme facies equivalents occur in the youngest succession and are notable for their diamonds and ultramafic xenoliths; they are, in fact, the world’s oldest igneous diamond hosts. This occurrence is ideal for understanding deep-seated petrogenetic processes beneath greenstone belts, the possible subduction origin of diamond, and how juvenile oceanic crust became cratonized to form the continents. The lamprophyres are calc-alkaline or shoshonitic with original minerals mostly replaced by greenschist grade metamorphic assemblages of actinolite+chlorite+albite+epidote+titanite+chromite. They contain xenocrysts of chromite and diamond, and abundant crustal and ultramafic xenoliths. The ultramafic xenoliths are compositionally websteritic and recrystallized to assemblages of actinolite +/- talc. Both ultramafic xenoliths and lamprophyres were studied previously for their major and trace elements and Nd and Pb isotopes [1]. Re-Os and the platinum group elements (PGE) were analyzed in these xenoliths and their lamprophyre hosts to examine their depletion/enrichment history. As expected, lamprophyres are mildly enriched in the incompatible PGE (Pt, Pd) and depleted in the compatible PGE (Os, Ir, Ru) whereas websterites are the reverse. All samples have low Re content that leads to low 187Re/188Os (lamprophyres 0.14 - 0.30; websterites 0.03 - 0.06) and hence yields accurate initial 187Os/188Os. The lamprophyres have initial 187Os/188Os ranging from 0.114 to 0.122 which is enriched by 4-10% over Neoarchean chondritic mantle. Similar relative enrichments are seen in modern lamprophyres such

  16. Water in the Cratonic Mantle Lithosphere

    NASA Technical Reports Server (NTRS)

    Peslier, A. H.

    2016-01-01

    The fact that Archean and Proterozoic cratons are underlain by the thickest (>200 km) lithosphere on Earth has always puzzled scientists because the dynamic convection of the surrounding asthenosphere would be expected to delaminate and erode these mantle lithospheric "keels" over time. Although density and temperature of the cratonic lithosphere certainly play a role in its strength and longevity, the role of water has only been recently addressed with data on actual mantle samples. Water in mantle lithologies (primarily peridotites and pyroxenites) is mainly stored in nominally anhydrous minerals (olivine, pyroxene, garnet) where it is incorporated as hydrogen bonded to structural oxygen in lattice defects. The property of hydrolytic weakening of olivine [4] has generated the hypothesis that olivine, the main mineral of the upper mantle, may be dehydrated in cratonic mantle lithospheres, contributing to its strength. This presentation will review the distribution of water concentrations in four cratonic lithospheres. The distribution of water contents in olivine from peridotite xenoliths found in kimberlites is different in each craton (Figure 1). The range of water contents of olivine, pyroxene and garnet at each xenolith location appears linked to local metasomatic events, some of which occurred later then the Archean and Proterozoic when these peridotites initially formed via melting. Although the low olivine water contents (<10 ppm wt H2O) at > 6 GPa at the base of the Kaapvaal cratonic lithosphere may contribute to its strength, and prevent its delamination, the wide range of those from Siberian xenoliths is not compatible with providing a high enough viscosity contrast with the asthenophere. The water content in olivine inclusions from Siberian diamonds, on the other hand, have systematically low water contents (<20 ppm wt H2O). The xenoliths may represent a biased sample of the cratonic lithosphere with an over-­abundance of metasomatized peridotites with

  17. Thermobarometry for spinel lherzolite xenoliths in alkali basalts

    NASA Astrophysics Data System (ADS)

    Ozawa, Kazuhito; Youbi, Nasrrddine; Boumehdi, Moulay Ahmed; Nagahara, Hiroko

    2016-04-01

    Application of geothermobarometers to peridotite xenoliths has been providing very useful information on thermal and chemical structure of lithospheric or asthenospheric mantle at the time of almost instantaneous sampling by the host magmas, based on which various thermal (e.g., McKenzie et al., 2005), chemical (e.g., Griffin et al., 2003), and rheological (e.g., Ave Lallemant et al., 1980) models of lithosphere have been constructed. Geothermobarometry for garnet or plagioclase-bearing lithologies provide accurate pressure estimation, but this is not the case for the spinel peridotites, which are frequently sampled from Phanerozoic provinces in various tectonic environments (Nixon and Davies, 1987). There are several geobarometers proposed for spinel lherzolite, such as single pyroxene geothermobarometer (Mercier, 1980) and geothermobarometer based on Ca exchange between olivine and clinopyroxene (Köhler and Brey, 1990), but they have essential problems and it is usually believed that appropriated barometers do not exist for spinel lherzolites (O'Reilly et al., 1997; Medaris et al., 1999). It is thus imperative to develop reliable barometry for spinel peridotite xenoliths. We have developed barometry for spinel peridotite xenoliths by exploiting small differences in pressure dependence in relevant reactions, whose calibration was made through careful evaluation of volume changes of the reactions. This is augmented with higher levels of care in application of barometer by choosing mineral domains and their chemical components that are in equilibrium as close as possible. This is necessary because such barometry is very sensitive to changes in chemical composition induced by transient state of the system possibly owing to pressure and temperature changes as well as chemical modification, forming chemical heterogeneity or zoning frequently reported from various mantle xenoliths (Smith, 1999). Thus very carful treatment of heterogeneity, which might be trivial for

  18. Crustal evolution and the eclogite to granulite phase transition in xenoliths from the West African Craton

    NASA Technical Reports Server (NTRS)

    Haggerty, S. E.; Hills, D. V.; Toft, P. B.

    1988-01-01

    A suite of eclogite and granulite facies xenoliths from kimberlite pipes in the Archean Man Shield of West Africa is described. The xenoliths include lithologies ranging in composition from komatiite to anorthosite and appear to be geochemically, petrologically, and geophysically related. The suite may represent fractionation of felsic material separated from ancient mantle and added to early Archean crust. The samples can be used to define a xenolith geotherm, which may represent an ancient episode of high heat flow. The samples also imply that the crust-mantle boundary is a gradational and possibly interlayered geochemical, mineralogical, and seismic transition. It is speculated that the depleted subcontinental mantle required by diamond bearing coalescence of smaller depletion cells formed by extraction of ancient crustal components. These depleted zones are surrounded by fertile asthenospheric mantle, which may have given rise to later flood basalts such as the Karroo and Parana Provinces.

  19. To the origin of Icelandic rhyolites: insights from partially melted leucocratic xenoliths

    NASA Astrophysics Data System (ADS)

    Gurenko, Andrey A.; Bindeman, Ilya N.; Sigurdsson, Ingvar A.

    2015-05-01

    We have studied glass-bearing leucocratic (granitic to Qz-monzonitic) crustal xenoliths from the Tindfjöll Pleistocene volcanic complex, SW Iceland. The xenoliths consist of strongly resorbed relicts of anorthitic plagioclase, K-rich feldspar and rounded quartz in colorless through pale to dark-brown interstitial glass. Spongy clinopyroxene and/or rounded or elongated crystals of orthopyroxene are in subordinate amount. Magnetite, ilmenite, zircon, apatite, allanite and/or chevkinite are accessory minerals. The xenoliths more likely are relicts of earlier-formed, partially melted Si-rich rocks or quartz-feldspar-rich crystal segregations, which suffered latter interaction with hotter and more primitive magma(s). Icelandic lavas are typically low in δ 18O compared to mantle-derived, "MORB"-like rocks (~5.6 ± 0.2 ‰), likely due to their interaction with, or contamination by, the upper-crustal rocks affected by rain and glacial melt waters. Surprisingly, many quartz and feldspar crystals and associated colorless to light-colored interstitial glasses of the studied xenoliths are not low but high in δ 18O (5.1-7.2 ‰, excluding three dark-brown glasses of 4-5 ‰). The xenoliths contain abundant, low- to high- δ 18O (2.4-6.3 ‰) young zircons (U-Pb age 0.2-0.27 ± 0.03 Ma; U-Th age 0.16 ± 0.07 Ma), most of them in oxygen isotope equilibrium with interstitial glasses. The δ 18O values >5.6 ‰ recorded in the coexisting zircon, quartz, feldspar and colorless interstitial glass suggest crystallization from melts produced by fusion of crustal rocks altered by seawater, also reflecting multiple melting and crystallization events. This suggests that "normal"- δ 18O silicic magmas may not be ultimately produced by crystallization of mafic, basaltic magmas. Instead, our new single-crystal laser fluorination and ion microprobe O-isotope data suggest addition of diverse partial crustal melts, probably originated from variously altered and preconditioned crust.

  20. Crustal make-up of the northern Andes: evidence based on deep crustal xenolith suites, Mercaderes, SW Colombia

    NASA Astrophysics Data System (ADS)

    Weber, Marion B. I.; Tarney, John; Kempton, Pamela D.; Kent, Raymond W.

    2002-02-01

    Samples of the deep crust and upper mantle in the Northern Andes occur as abundant xenoliths in the Granatı´fera Tuff, a late Cenozoic vent in the Mercaderes area of SW Colombia. The lower crustal assemblage includes granulites, hornblendites, pyribolites, pyroxenites and gneisses; mafic rocks predominate, but felsic material is also common. P-T conditions for the pyribolite assemblages (i.e. Hbl+Fs/Scp+Grt+Cpx+Qtz±Bt), which are the best constrained, are 720-850 °C and 10-14 kbar, consistent with a deep-to-lower crustal origin. A notable feature of this xenolith suite is that it is dominated by hornblende. However, mineral reactions within the suite show that there is a transition from amphibolite to granulite facies, and there is a probable restite-melt relationship represented within the suite. However, the latter appears to be dominated by hornblende and garnet. The mafic rocks mostly lack the high Cr and Ni that would be expected of cumulates. Neither do they possess the positive Sr and Eu anomalies that would be consistent with resite or cumulate models for the lower crust. They bear greatest similarity to oceanic basalts (s.l.). The Rb contents of the xenoliths, whether mafic or silicic, are very low, and the more silicic members of the suite tend to have small positive Sr and Eu anomalies, which are transitional to adakitic compositions. The Sr isotopic compositions of the xenoliths lie between 0.704 and 0.705; however, the Nd isotopic compositions are much more variable, indicating considerable long-term heterogeneity. Few of the xenoliths can be compositionally recognised as metasedimentary; however, a sedimentary component is evident in the Pb isotopic compositions. Within these constraints, our favoured model is a deep crust formed by basaltic components (subduction-accretion?), and minor sediment, which is subject to an increase in thermal gradient to produce the granulites, any melting being dominated by hornblende-out reactions involving garnet

  1. Ultra-depleted isotopic compositions in fertile asthenosphere-derived peridotites: constraints on the composition of the upper mantle

    NASA Astrophysics Data System (ADS)

    Byerly, B. L.; Lassiter, J. C.

    2012-12-01

    Recent studies of abyssal peridotites (AP) and OIB xenoliths have reported refractory, isotopically ultra-depleted domains within the convecting upper mantle with Nd- and Hf-isotope compositions that extend far beyond the MORB field. These results have important implications regarding the average composition of the depleted upper mantle and the genetic relationship between MORB and AP. However, the abundance of ultra-depleted domains in the mantle is unclear. In addition, recent melt extraction processes at mid-ocean ridges make it difficult to evaluate the compositions of ultra-depleted domains prior to exhumation and thus evaluate their role in melt generation. To better constrain the abundance and composition of typical convecting upper mantle, we examined a suite of spinel peridotite xenoliths from the central Rio Grande Rift (RGR) where most of the preexisting lithosphere has been convectively removed and replaced with depleted upper mantle. Seismic tomography indicates that the lithosphere beneath the RGR has been substantially removed (Gao, 2004), and geochemical evidence supports this. Two distinct populations of xenoliths are observed from Elephant Butte, central RGR. One population, interpreted to derive from residual Proterozoic lithospheric mantle, is refractory (bulk Al2O3 <2.3 wt.%), LREE- and LILE-enriched, has enriched Sr, Nd, and Pb isotopic compositions and along with xenoliths from the Eastern Colorado Plateau define a strong Lu/Hf-176Hf/177Hf "pseudo-isochron" with an apparent age of ~1.6 Ga. In contrast, the majority of the RGR xenoliths have fertile major element compositions (bulk Al2O3 ~ 4.0 wt %), low spinel Cr# (~10), and LREE-depleted trace element patterns, and overlap with composition estimates for the depleted mantle (Workman & Hart, 2005). We interpret these xenoliths to reflect recent replacement of the pre-existing lithosphere with material from the convecting upper mantle. The fertile xenoliths have cpx Sr-, Nd-, and Hf

  2. Platinum group elements in mantle melts and mantle samples

    NASA Astrophysics Data System (ADS)

    Barnes, Stephen J.; Mungall, James E.; Maier, Wolfgang D.

    2015-09-01

    A large data compilation has been assembled of platinum group element (PGE) analyses in mantle melts and mantle rocks, the latter including an assortment of xenoliths and obducted mantle massifs. The degree of correlation has been investigated among the PGEs and with other major element variables such as Al2O3, TiO2 and Mg number, and the results are considered in the context of the current paradigm for the behaviour of highly siderophile elements in the silicate Earth. Primitive mantle melts have a wide range of PGE contents. Komatiites have the highest abundances of all the PGEs, show the strongest correlations between Pt and Rh, Pt and Pd and between the iridium-group PGEs Ir, Ru and Os (IPGEs). Most basalts of all affinities have lower levels of Pt and Pd and much lower levels of Ir, Ru and Os than komatiites. Within the basalt grouping Rh has stronger affinities with the IPGEs. Picrites and Archaean basalts are intermediate between these two groups. MORBs and a small proportion of continental LIP basalts show strong depletions in all PGEs attributable to retention of sulfide in their mantle source rocks, or sulfide liquid fractionation on ascent. The degree of PGE depletion in other basalts is probably attributable to equilibration with sulfide, but is less than would be expected under conventional models of sulfide extraction, and is instead attributed to mixing of magmas generated at variable depths incorporating both sulfide-saturated and undersaturated components. Basalts with Pt and Pd contents higher than typical komatiites are rare, a notable example being B1-type parent magmas to the Bushveld Complex, which have komatiite-like relative PGE abundances and Pt, Pd and Rh abundances up to a factor of two higher than komatiites for comparable Ti contents. The mantle composition array as a whole is characterized by variable degrees of depletion of Pt, Pd and Rh in Al-poor, melt-depleted harzburgite/dunite lithologies; lack of depletion in these elements in

  3. Craton vs. rift uppermost mantle contributions to magnetic anomalies in the United States interior

    NASA Astrophysics Data System (ADS)

    Friedman, S. A.; Feinberg, J. M.; Ferré, E. C.; Demory, F.; Martín-Hernández, F.; Conder, J. A.; Rochette, P.

    2014-06-01

    The interpretation of satellite magnetic information (Magsat, Oersted, CHAMP, Swarm) requires the understanding of the mineralogy of crustal and mantle sources. Also, spectral analysis of magnetic data over forearcs and cratons calls for upper mantle contribution. The prospect of such a contribution contradicts the view that the mantle is too hot and its magnetism is too weak to influence magnetic anomalies. Here we examine the rock magnetic properties of fresh mantle xenoliths from four settings across the United States: phlogopite-spinel dunites from the Bearpaw Mountains, Montana, and lherzolites/harzburgites from San Carlos, Arizona; Kilbourne Hole, New Mexico; and Knippa, Texas. Paleomagnetic results show single-component natural remanent magnetizations (NRMs), which, combined with optical and secondary electron microscopy support the lack of post-eruption alteration and absence of host-rock contamination. The NRM carriers include magnetite at Bearpaw Mountain and San Carlos, and pyrrhotite at Kilbourne Hole and Knippa. These four areas show continental crust of distinct thicknesses and various geotherms. The potential mantle contribution to magnetic anomalies is forward modeled using crustal thickness, current geotherm and average magnetic properties of xenoliths. The San Carlos and Kilbourne Hole mantle, situated near the Rio Grande Rift is too hot and its magnetism is too weak to contribute to anomalies. The sulfide-dominated assemblage at Knippa does not support magnetization at mantle depths. In contrast, the Bearpaw Mountains combine a relatively cold geotherm (craton) and abundance of magnetite formed at mantle depth. This cratonic mantle, metasomatized by fluids from the Farallon plate, may contribute to long wavelength magnetic anomalies.

  4. New constraints on the textural and geochemical evolution of the upper mantle beneath the Styrian basin

    NASA Astrophysics Data System (ADS)

    Aradi, Laszlo; Hidas, Károly; Zanetti, Alberto; János Kovács, István; Patkó, Levente; Szabó, Csaba

    2016-04-01

    Plio-Pleistocene alkali basaltic volcanism sampled sporadically the upper mantle beneath the Carpathian-Pannonian Region (CPR, e.g. [1]). Lavas and pyroclasts often contain mantle derived xenoliths, and the majority of them have been extensively studied [1], except the westernmost Styrian Basin Volcanic Field (SBVF, Eastern Austria and Slovenia). In the SBVF only a few volcanic centers have been studied in details (e.g. Kapfenstein & Tobaj). Based on these studies, the upper mantle beneath the SBVF is consists of dominantly high temperature, texturally and geochemically homogeneous protogranular spinel lherzolite. New major and trace element data from rock-forming minerals of ultramafic xenoliths, coupled with texture and deformation analysis from 12 volcanic outcrops across the SBVF, suggest that the lithospheric roots of the region are more heterogeneous than described previously. The studied xenoliths are predominantly lherzolite, amphibole is a common phase that replaces pyroxenes and spinels and proves modal metasomatism. Phlogopite coupled with apatite is also present in amphibole-rich samples. The texture of the xenoliths is usually coarse-grained and annealed with low abundance of subgrain boundaries in both olivine and pyroxenes. Olivine crystal preferred orientation (CPO) varies between the three most abundant one: [010]-fiber, orthogonal and [100]-fiber symmetry [2]. The CPO of pyroxenes is usually coherent with coeval deformation with olivine, however the CPO of amphibole is suggesting postkinematic epitaxial overgrowth on the precursor pyroxenes. According to equilibrium temperatures, the studied xenolith suite samples a broader temperature range (850-1100 °C) than the literature data, corresponding to mantle depths between 30 and 60 km, which indicates that the xenolith suite only represents the shallower part of the recent 100 km thick lithospheric mantle beneath the SBVF. The equilibrium temperatures show correlation with the varying CPO symmetries

  5. Lower crustal xenoliths, Chinese Peak lava flow, central Sierra Nevada.

    USGS Publications Warehouse

    Dodge, F.C.W.; Calk, L.C.; Kistler, R.W.

    1986-01-01

    This assemblage of pyroxenite, peridotite and mafic granulite xenoliths in the toe of a 10 m.y. trachybasalt flow remnant overlying late Cretaceous granitic rocks, indicates the presence of a mafic-ultramafic complex beneath this part of central California; orthopyroxenites, websterites and clinopyroxenites are dominant. A few of the xenoliths contain ovoid opaque patches that are apparently pseudomorphs after garnet and have pyralspite garnet compositions; using a garnet-orthopyroxene geobarometer, they indicate a lower crustal depth of approx 40 km. Abundant mafic granulites can be subdivided into those with Al2O3 = or 15% and showing considerable scatter on oxide variation diagrams. The high-alumina granulite xenoliths have relatively low 87Rb/86Sr but high 87Sr/86Sr, whereas the low-alumina and ultramafic xenoliths have a wide range of 87Rb/86Sr, but lower 87Sr/86Sr; the isotopic data indicate roughly the same age as that of overlying granitic plutons (approx 100 m.y.). However, the granitic rocks have initial 87Sr/86Sr ratios intermediate between those of the high-alumina and ultramafic xenoliths, suggesting that they result from the mixing of basaltic magma (represented by the ultramafic rocks) and crustal materials, with subsequent crystal fractionation.-R.A.H.

  6. Complex evolution of the lower crust beneath the southeastern North China Craton: the Junan xenoliths and xenocrysts

    NASA Astrophysics Data System (ADS)

    Tang, Huayun; Zheng, Jianping; Griffin, William L.; O‧Reilly, Suzanne Y.; Yu, Chunmei; Pearson, Norman J.; Ping, Xianquan; Xia, Bing; Yang, Huaben

    2014-10-01

    Knowledge of the lower crust beneath the southeastern parts of North China Craton (NCC) is still sparse. The Junan basalts (67 Ma) in the southeastern NCC contain abundant xenoliths of lower crustal granulites, pyroxenites and mantle peridotites. We present integrated in-situ U-Pb ages and Hf isotopes of zircons from the Junan basalts and granulite xenoliths, to investigate accretion and modification processes in the lower crust. The granulite xenoliths define three distinct U-Pb age populations of ca 2.3 Ga, ca 2.0 Ga and 114-126 Ma. The ca 2.3 Ga zircons have widely variable εHf(t) and Paleo-Neoarchean model ages (Tcrust = 2.6-4.0 Ga), whereas the ca 2.0 Ga structureless grains give negative εHf(t) and Tcrust of 2.7-3.3 Ga. In addition to a few discordant Early Paleoproterozoic xenocrysts, zircons from the basalts are dominantly Early Cretaceous (115-125 Ma), with some Neoproterozoic (550-800 Ma) and Early Paleozoic (437-493 Ma), as well as minor Late Triassic and Late Jurassic grains. These results, combined with previous petrological and geochemical studies and P-T estimates, suggest that the upper part of the Junan lower crust consists of mafic-intermediate granulites, mainly formed at ca 2.3 Ga by crystallization of depleted-mantle-derived magmas that assimilated ancient crust and then fractionated. More significantly, zircon ages and Hf isotopes imply that this lower crust had a complex history of accretion and modification, including initial growth at 3.0-4.0 Ga and 2.5-2.7 Ga, conversion to the dominant granulitic assemblages in the Early Paleoproterozoic (ca 2.3 Ga), modification or metamorphism in the Late Paleoproterozoic (1.8-2.0 Ga) and possibly slight heating in Neoproterozoic time. Episodic thermal events during the Early Paleozoic, Late Triassic and Late Jurassic may also have reworked this Precambrian lower crust. Most of the Early Cretaceous zircons and xenocrysts have uniform εHf(t) values similar to those of the nearby coeval magmatic rocks

  7. Boron Isotopic Variation in the Subcontinental Lithospheric Mantle

    NASA Astrophysics Data System (ADS)

    Guild, M. R.; Bell, D. R.; Hervig, R. L.

    2013-12-01

    Boron contents and isotopic compositions (δ11B) of phlogopite mica, amphibole, and selected coexisting anhydrous phases were measured by secondary ion mass spectrometry in mantle xenolith samples from the Kaapvaal Craton of South Africa, in order to better understand processes of volatile element transfer in the mantle. We have documented a wide range of δ11B (>40‰) and B contents (<10ppb to 10's of ppm) in mica from three broad groups identified based on petrographic and compositional criteria, and B geochemistry. The first group, characterized by light δ11B values (-17‰ to -30‰) and low B contents (a few ppb to 100's ppb), consists of mica megacrysts in kimberlite and mica in garnet harzburgites (gt hz) and lherzolites (gt lz) containing variably abundant metasomatic mica, orthopyroxene (opx) and, in some cases, clinopyroxene (cpx). Boron contents and δ11B show a broad positive correlation with modal intensity of metasomatism from gt hz to mica-rich websteritic gt lz. Metasomatic fluids, parental to this group, are proposed to originate in partially-dehydrated subducting oceanic lithosphere, consistent with high LILE/HFSE mineral chemistry. The second group is characterized by relatively B-rich (~1ppm) micas and amphiboles from MARID xenoliths, cpx and gt in cpx-rich peridotite, and (B-poor) subcalcic cpx megacrysts, which all have δ11B of ~-10‰, indistinguishable from primitive mantle estimates. The fluids associated with the second group of samples may have originated in mantle plumes. The third group is heterogeneous showing δ11B values from ~-5 to +15‰ with B contents from 0.5-10 ppm. These samples (all micas) exhibit secondary textures that appear to result from fluid processes associated with kimberlite emplacement. Other analyzed samples (hydrous and anhydrous) may record contact with multiple fluids. The current dataset shows that boron is a useful tracer of fluids in the mantle and can contribute to the understanding of global geochemical

  8. Origin of ultramafic xenoliths containing exsolved pyroxenes from Hualalai Volcano, Hawaii

    USGS Publications Warehouse

    Bohrson, W.A.; Clague, D.A.

    1988-01-01

    Hualalai Volcano, Hawaii, is best known for the abundant and varied xenoliths included in the historic 1800 Kaupulehu alkalic basalt flow. Xenoliths, which range in composition from dunite to anorthosite, are concentrated at 915-m elevation in the flow. Rare cumulate ultramafic xenoliths, which include websterite, olivine websterite, wehrlite, and clinopyroxenite, display complex pyroxene exsolution textures that indicate slow cooling. Websterite, olivine websterite, and one wehrlite are spinel-bearing orthopyroxene +olivine cumulates with intercumulus clinopyroxene +plagioclase. Two wehrlite samples and clinopyroxenite are spinel-bearing olivine cumulates with intercumulus clinopyroxene+orthopyroxene + plagioclase. Two-pyroxene geothermometry calculations, based on reconstructed pyroxene compositions, indicate that crystallization temperatures range from 1225?? to 1350?? C. Migration or unmixing of clinopyroxene and orthopyroxene stopped between 1045?? and 1090?? C. Comparisons of the abundance of K2O in plagioclase and the abundances of TiO2 and Fe2O3in spinel of xenoliths and mid-ocean ridge basalt, and a single 87Sr/ 86Sr determination, indicate that these Hualalai xenoliths are unrelated to mid-ocean ridge basalt. Similarity between the crystallization sequence of these xenoliths and the experimental crystallization sequence of a Hawaiian olivine tholeiite suggest that the parental magma of the xenoliths is Hualalai tholeiitic basalt. Xenoliths probably crystallized between about 4.5 and 9 kb. The 155??-230?? C of cooling which took place over about 120 ka - the age of the youngest Hualalai tholeiitic basalt - yield maximum cooling rates of 1.3??10-3-1.91??10-3 ??C/yr. Hualalai ultramafic xenoliths with exsolved pyroxenes crystallized from Hualalai tholeiitic basalt and accumulated in a magma reservoir located between 13 and 28 km below sealevel. We suspect that this reservoir occurs just below the base of the oceanic crust at about 19 km below sealevel

  9. Anomalous isotopes and trace element zoning in plagioclase peridotite xenoliths of Oahu (Hawaii): implications for the Hawaiian plume

    NASA Astrophysics Data System (ADS)

    Sen, Gautam; Yang, Huai-Jen; Ducea, Mihai

    2003-02-01

    Survival of plagioclase in the residual melting column during melting can have a significant impact on the melting process beneath a mid-oceanic ridge [Asimow et al., Phil. Trans. R. Soc. London Ser. A 355 (1997) 255-281]. Here we investigate the origin of plagioclase that occurs in some rare mantle xenoliths from Oahu, Hawaii. The xenoliths are harzburgitic with less than 2 modal% clinopyroxene and are characterized by strong foliation and porphyroclastic texture. Olivine and orthopyroxene are common porphyroclasts; and only one xenolith (77PAII-9) contains a single large clinopyroxene porphyroclast with thick exsolved orthopyroxene lamellae. The strongly foliated groundmass shows well-developed triple-point junctions and is dominantly composed of olivine (ol 85-90 opx 7-14 cpx <1-2 plag 3-5 spinel trace). Spinel grains are small and dispersed through the groundmass and show extreme variation in Cr/Al ratio within individual thin sections, indicating that they are out of equilibrium with the other phases in these xenoliths. A highly anorthitic plagioclase (An 92-96) occurs only in the groundmass and its modal abundance (˜3-5%) is too high relative to the abundance of clinopyroxene (commonly <1%) for it to be a residual phase, implying that plagioclase may have an exotic origin. The porphyroclasts show strong compositional zoning near the rims and appear to be relict phases (as are all the spinel grains) that had once equilibrated with melts within the stability field of spinel peridotite (pressure ˜1-3 GPa). Clinopyroxene neoblasts and the single porphyroclast in PAII-9 are all characterized by lithosphere-like strongly depleted light rare earth element (chondrite-normalized) patterns. The clinopyroxene porphyroclast in PAII-9 is zoned in Al, Eu, Cr, and Na. The porphyroclastic ortho- and clinopyroxenes give a homogenized (host+exsolution) temperature of 1300°C, which is inferred to be the temperature at which the porphyroclast cores equilibrated with a MORB

  10. Widespread refertilization of cratonic and circum-cratonic lithospheric mantle

    NASA Astrophysics Data System (ADS)

    Tang, Yan-Jie; Zhang, Hong-Fu; Ying, Ji-Feng; Su, Ben-Xun

    2013-03-01

    Studies of mantle xenoliths have confirmed that Archean subcontinental lithospheric mantle (SCLM) is highly depleted in basaltic components (such as Al, Ca and Na) due to high-degree extraction of mafic and ultramafic melts and thus is refractory and buoyant, which made it chronically stable as tectonically independent units. However, increasing studies show that ancient SCLM can be refertilized by episodic rejuvenation events like infiltration of upwelling fertile material. The North China Craton is one of the most typical cases for relatively complete destruction of its Archean keel since the eruption of Paleozoic kimberlites, as is evidenced by a dramatic change in the compositions of mantle xenoliths sampled by Paleozoic to Cenozoic magmas, reflecting significant lithospheric thinning and the change in the character of the SCLM. The compositional change has been interpreted as the result of refertilization of Archean SCLM via multiple-stage peridotite-melt reactions, suggested by linear correlations between MgO and indices of fertility, covariations of Al2O3 with CaO, La/Yb, 87Sr/86Sr, 143Nd/144Nd, 187Os/188Os and Re-depletion ages (TRD), high Re abundances, scatter in Re-Os isotopic plot, variable in situ TRD ages of sulfides, and correlation between TRD ages and olivine Fo of peridotite xenoliths in Paleozoic kimberlites and Cenozoic basalts on the craton. By integrating major and trace element, Sr, Nd and Os isotopic compositions of peridotite xenoliths and orogenic massif peridotites from the continents of Europe, Asia, America, Africa and Australia, together with previous studies of petrology and geochemistry of global peridotites, we suggest that (1) refertilization of cratonic and circum-cratonic lithospheric mantle is widespread; (2) Archean SCLM worldwide has experienced a multi-stage history of melt depletion and refertilization since segregation from the convecting mantle; (3) cratonic SCLM may be more susceptible to compositional change caused by

  11. The continental lithospheric mantle: characteristics and significance as a mantle reservoir.

    PubMed

    Pearson, D G; Nowell, G M

    2002-11-15

    The continental lithospheric mantle (CLM) is a small-volumed (ca. 2.5% of the total mantle), chemically distinct mantle reservoir that has been suggested to play a role in the source of continental and oceanic magmatism. It is our most easily identifiable reservoir for preserving chemical heterogeneity in the mantle. Petrological and geophysical constraints indicate that the maximum depth of the CLM is ca. 250 km. There is a clear secular variation of CLM composition, such that CLM formed in the last 2 Gyr is less depleted and therefore less dynamically stable than ancient CLM formed in the Archean. We present new trace-element data for kimberlite-hosted lithospheric peridotites and metasomites. These data, combined with other data for spinel peridotites from non-cratonic regions, show that neither hydrous nor anhydrous lithospheric mantle xenoliths make suitable sources for continental or oceanic basalts. Addition of a hydrous phase, either amphibole or phlogopite, to depleted peridotite results in positive Nb and Ti anomalies that are the opposite of those predicted for some flood-basalt sources on the basis of their trace-element abundances. Overall, the Sr and Nd isotopic composition of cratonic and non-cratonic CLM is close to bulk Earth, with cratonic CLM showing small numbers of extreme compositions. Thus, while the CLM is certainly ancient in many locations, its average composition is not significantly 'enriched' over primitive upper mantle, in terms of either radiogenic isotopes or trace elements. These characteristics, plus a change in lithospheric chemistry with depth, indicate that the elemental and isotopic composition of lithospheric mantle likely to be re-incorporated into convecting mantle via delamination/thermal erosion processes is probably not very distinct from that of the convecting mantle. These observations lead us to question the requirement for CLM participation in the source of oceanic magmas and to promote consideration of a mantle that

  12. The continental lithospheric mantle: characteristics and significance as a mantle reservoir.

    PubMed

    Pearson, D G; Nowell, G M

    2002-11-15

    The continental lithospheric mantle (CLM) is a small-volumed (ca. 2.5% of the total mantle), chemically distinct mantle reservoir that has been suggested to play a role in the source of continental and oceanic magmatism. It is our most easily identifiable reservoir for preserving chemical heterogeneity in the mantle. Petrological and geophysical constraints indicate that the maximum depth of the CLM is ca. 250 km. There is a clear secular variation of CLM composition, such that CLM formed in the last 2 Gyr is less depleted and therefore less dynamically stable than ancient CLM formed in the Archean. We present new trace-element data for kimberlite-hosted lithospheric peridotites and metasomites. These data, combined with other data for spinel peridotites from non-cratonic regions, show that neither hydrous nor anhydrous lithospheric mantle xenoliths make suitable sources for continental or oceanic basalts. Addition of a hydrous phase, either amphibole or phlogopite, to depleted peridotite results in positive Nb and Ti anomalies that are the opposite of those predicted for some flood-basalt sources on the basis of their trace-element abundances. Overall, the Sr and Nd isotopic composition of cratonic and non-cratonic CLM is close to bulk Earth, with cratonic CLM showing small numbers of extreme compositions. Thus, while the CLM is certainly ancient in many locations, its average composition is not significantly 'enriched' over primitive upper mantle, in terms of either radiogenic isotopes or trace elements. These characteristics, plus a change in lithospheric chemistry with depth, indicate that the elemental and isotopic composition of lithospheric mantle likely to be re-incorporated into convecting mantle via delamination/thermal erosion processes is probably not very distinct from that of the convecting mantle. These observations lead us to question the requirement for CLM participation in the source of oceanic magmas and to promote consideration of a mantle that

  13. Origin of water and mantle-crust interactions on Mars inferred from hydrogen isotopes and volatile element abundances of olivine-hosted melt inclusions of primitive shergottites

    NASA Astrophysics Data System (ADS)

    Usui, Tomohiro; Alexander, Conel M. O'D.; Wang, Jianhua; Simon, Justin I.; Jones, John H.

    2012-12-01

    Volatile elements have influenced the differentiation and eruptive behavior of Martian magmas and played an important role in the evolution of Martian climate and near-surface environments. However, the abundances of volatiles, and in particular the amount of water in the Martian interior, are disputed. A record of volatile reservoirs is contained in primitive Martian basalts (shergottites). Olivine-hosted melt inclusions from a geochemically depleted shergottite (Yamato 980459, representing a very primitive Martian melt) possess undegassed water with a chondritic and Earth-like D/H ratio (δD≤275‰). Based on volatile measurements in these inclusions, the water content of the depleted shergottite mantle is calculated to be 15-47 ppm, which is consistent with the dry mantle hypothesis. In contrast to D/H in the depleted shergottite, melt from an enriched shergottite (Larkman Nunatak 06319), which either formed by melting of an enriched mantle or by assimilation of crust, exhibits an extreme δD of ˜5000‰, indicative of a surface reservoir (e.g., the Martian atmosphere or crustal hydrosphere). These data provide strong evidence that the Martian mantle had retained the primordial low-δD component until at least the time of shergottite formation, and that young Martian basalts assimilated old Martian crust.

  14. Petrology, geochemistry, and petrogenesis of ultramafic xenoliths from 1800-1801 Kaupulehu flow, Hualalai Volcano, Hawaii

    SciTech Connect

    Chen, C.H.

    1986-01-01

    The 1800-1801 Kaupulehu alkalic flow on Hualalai Volcano, Hawaii, contains abundant xenoliths of dunite, wehrlite, and olivine-clinopyroxenite with minor gabbro, troctolite, anorthosite, and websterite. The petrography and mineral chemistry of forty-six dunite, wehrlite, and olivine-clinopyroxenite xenoliths have been studied; eight were selected for determination of trace element concentrations and isotopic ratios of separated clinopyroxenes. Temperatures of equilibrium obtained from both olivine-spinel and pyroxene geo-thermometers range from 1000 C to 1200 C for these ultramafic xenoliths. A depth of 8-25 km is suggested for the formation of these ultramafic xenoliths. The rarity of othopyroxene, presence of clinopyroxene, Fe-rich olivine and clinopyroxene compositions, and high TiO content in spinel and clinopyroxene indicate that these xenoliths have a cumulate origin and are not residues from partial melting. Sr and Nd isotopic ratios from clinopyroxene are different from those of most Mid-Ocean Ridge Basalts. Rare earth element (REE) concentrations in liquid that equilibrated with xenolith clinopyroxenes have light rare earth element (LREE) enriched patterns with (Ce/Yb)n between 4 and 10. Similar olivine, spinel, and clinopyroxene compositions in xenoliths and Hawaiian basalts as well as good agreement of their Sr and Nd isotopic ratios suggests a genetic relationship between Hualalai ultramafic xenoliths and Hawaiian basalts. Some xenoliths possibly are cumulates from alkalic or tholeiitic basalts. However, Hualalai tholeiitic basalts are excluded due to their different /sup 3/He//sup 4/He values and REE patterns. The magmas that crystallized the Mg-rich (>Fo/sub 87/) dunites with high REE contents are similar in Sr and Nd isotopic values to Hualalai 1800-1801 alkalic basalts but have higher REE and Sr contents.

  15. Gabbroic xenoliths from the northern Gorda Ridge: implications for magma chamber processes under slow spreading centers

    USGS Publications Warehouse

    Davis, A.S.; Clague, D.A.

    1990-01-01

    Abundant gabbroic xenoliths in porphyritic pillow basalt were dredged from the northern Gorda Ridge. The host lava is a moderately fractionated, normal mid-ocean ridge basalt with a heterogeneous glass rind (Mg numbers 56-60). Other lavas in the vicinity range from near primary (Mg number 69) to fractionated (Mg number 56). On the basis of textures and mineral compositions, the xenoliths are divided into five types. The xenoliths are not cognate to the host lava, but they are genetically related. Chemistry of mineral phases in conjunction with textural features suggests that the xenoliths formed in different parts of a convecting magma chamber that underwent a period of closed system fractionation. The chamber was filled with a large proportion of crystalline mush when new, more primitive, and less dense magma was injected and mixed incompletely with the contents in the chamber, forming the hybrid host lava. -from Authors

  16. Midcrustal xenoliths from Beistein, Austria

    NASA Astrophysics Data System (ADS)

    Németh, B.; Badenszki, E.; Koller, F.; Török, K.; Mogassie, A.; Szabó, Cs.

    2009-04-01

    The main aim of this work is to study crustal granulite xenoliths from Beistein (Styrian Basin, Austria). Styrian-basin is situated at the westernmost part the Pannonian-basin-system. We collected 17 felsic granulite xenoliths, from basaltic pyroclastics near Beistein. We made petrography, geothermo-barometry and fluid inclusion studies to find out their evolution and fluid content and to make a comparison with other crustal xenoliths found in the Pannonian Basin. The petrography shows, that these xenoliths experienced 3 evolutional stages. At the beginning, a felsic magmatic body intruded into the middle crust. It contained feldspar, pyroxenes, quartz, biotite, opaque minerals, apatite and zircon. This intrusion experienced cooling. This cooling generated the stable mineral assemblage of the granulite facies (feldspar + orthopyroxene + quartz ± garnet) at p=5.5-6.7 kbar and T= 800-850oC and resulted in antiperthitic exsolution of feldspars. The third step was the uplift of the xenoliths in the basalt to the surface which generated temperature increase, and pressure decrease. Considering the petrology of the fluid inclusions (FI) in these xenoliths, there are primary and secondary FIs in the samples. This shows us, that there were minimum of two fluid events (fluid-rock interaction). The very first fluid event coeval with the emplacement and crystallization of the magmatic body in the deep crust, witnessed by primary inclusions in the relict magmatic minerals like apatite and zircon. There are other primary FIs also in the main rock-forming minerals, such as plagioclase, quartz and pyroxenes. The second fluid event happened still in the lower crust. This produced the secondary FI rows along healed fractures mostly in the felsic minerals. The microthermometry shows, that CO2 was the dominant fluid phase in the crust, but genetically the different FIs have different compositions. The melting temperatures (Tm) of primary FIs were between -58,0 and -65,6 °C, and that

  17. Mantle hydrocarbons: abiotic or biotic?

    PubMed

    Sugisaki, R; Mimura, K

    1994-06-01

    Analyses of 227 rocks from fifty localities throughout the world showed that mantle derived rocks such as tectonized peridotites in ophiolite sequences (tectonites) arid peridotite xenoliths in alkali basalts contain heavier hydrocarbons (n-alkanes), whereas igneous rocks produced by magmas such as gabbro arid granite lack them. The occurrence of hydrocarbons indicates that they were not derived either from laboratory contamination or from held contamination; these compounds found in the mantle-derived rocks are called here "mantle hydrocarbons." The existence of hydrocarbons correlates with petrogenesis. For example, peridotite cumulates produced by magmatic differentiation lack hydrocarbons whereas peridotite xenoliths derived from the mantle contain them. Gas chromatographic-mass spectrometric records of the mantle hydrocarbons resemble those of aliphatics in meteorites and in petroleum. Features of the hydrocarbons are that (a) the mantle hydrocarbons reside mainly along grain boundaries and in fluid inclusions of minerals; (b) heavier isoprenoids such as pristane and phytane are present; and (c) delta 13C of the mantle hydrocarbons is uniform (about -27%). Possible origins for the mantle hydrocarbons are as follows. (1) They were in organically synthesized by Fischer-Tropsch type reaction in the mantle. (2) They were delivered by meteorites and comets to the early Earth. (3) They were recycled by subduction. The mantle hydrocarbons in the cases of (1) and (2) are abiogenic and those in (3) are mainly biogenic. It appears that hydrocarbons may survive high pressures and temperatures in the mantle, but they are decomposed into lighter hydrocarbon gases such as CH4 at lower pressures when magmas intrude into the crust; consequently, peridotite cumulates do not contain heavier hydrocarbons but possess hydrocarbon gases up to C4H10. PMID:11541663

  18. Mantle hydrocarbons: Abiotic or biotic?

    SciTech Connect

    Sugisaki, Ryuichi; Mimura, Koichi

    1994-06-01

    Analyses of 227 rocks from fifty localities throughout the world showed that mantle derived rocks such as tectonized peridotites in ophiolite sequences (tectonites) and peridotite xenoliths in alkali basalts contain heavier hydrocarbons (n-alkanes), whereas igneous rocks produced by magmas such as gabbro and granite lack them. The occurrence of hydrocarbons indicates that they were not derived either from laboratory contamination or from field contamination; these compounds found in the mantle-derived rocks are called here {open_quotes}mantle hydrocarbons.{close_quotes} The existence of hydrocarbons correlates with petrogenesis. For example, peridotite cumulates produced by magmatic differentiation lack hydrocarbons whereas peridotite xenoliths derived from the mantle contain them. Gas chromatographic-mass spectrometric records of the mantle hydrocarbons resemble those of aliphatics in meteorites and in petroleum. Features of the hydrocarbons are that (a) the mantle hydrocarbons reside mainly along grain boundaries and in fluid inclusions of minerals; (b) heavier isoprenoids such as pristane and phytane are present; and (c) {delta}{sup 13}C of the mantle hydrocarbons is uniform (about {minus}27{per_thousand}). Possible origins for the mantle hydrocarbons are as follows. (1) They were inorganically synthesized by Fischer-Tropsch type reaction in the mantle. (2) They were delivered by meteorites and comets to the early Earth. (3) They were recycled by subduction. The mantle hydrocarbons in the cases of (1) and (2) are abiogenic and those in (3) are mainly biogenic. It appears that hydrocarbons may survive high pressures and temperatures in the mantle, but they are decomposed into lighter hydrocarbon gases such as CH{sub 4} at lower pressures when magmas intrude into the crust; consequently, peridotite cumulates do not contain heavier hydrocarbons but possess hydrocarbon gases up to C{sub 4}H{sub 10}. 76 refs., 5 figs., 3 tabs.

  19. Mantle hydrocarbons: abiotic or biotic?

    PubMed

    Sugisaki, R; Mimura, K

    1994-06-01

    Analyses of 227 rocks from fifty localities throughout the world showed that mantle derived rocks such as tectonized peridotites in ophiolite sequences (tectonites) arid peridotite xenoliths in alkali basalts contain heavier hydrocarbons (n-alkanes), whereas igneous rocks produced by magmas such as gabbro arid granite lack them. The occurrence of hydrocarbons indicates that they were not derived either from laboratory contamination or from held contamination; these compounds found in the mantle-derived rocks are called here "mantle hydrocarbons." The existence of hydrocarbons correlates with petrogenesis. For example, peridotite cumulates produced by magmatic differentiation lack hydrocarbons whereas peridotite xenoliths derived from the mantle contain them. Gas chromatographic-mass spectrometric records of the mantle hydrocarbons resemble those of aliphatics in meteorites and in petroleum. Features of the hydrocarbons are that (a) the mantle hydrocarbons reside mainly along grain boundaries and in fluid inclusions of minerals; (b) heavier isoprenoids such as pristane and phytane are present; and (c) delta 13C of the mantle hydrocarbons is uniform (about -27%). Possible origins for the mantle hydrocarbons are as follows. (1) They were in organically synthesized by Fischer-Tropsch type reaction in the mantle. (2) They were delivered by meteorites and comets to the early Earth. (3) They were recycled by subduction. The mantle hydrocarbons in the cases of (1) and (2) are abiogenic and those in (3) are mainly biogenic. It appears that hydrocarbons may survive high pressures and temperatures in the mantle, but they are decomposed into lighter hydrocarbon gases such as CH4 at lower pressures when magmas intrude into the crust; consequently, peridotite cumulates do not contain heavier hydrocarbons but possess hydrocarbon gases up to C4H10.

  20. Isotopic characterisation of the sub-continental lithospheric mantle beneath Zealandia, a rifted fragment of Gondwana

    NASA Astrophysics Data System (ADS)

    Waight, Tod E.; Scott, James M.; van der Meer, Quinten H. A.

    2013-04-01

    The greater New Zealand region, known as Zealandia, represents an amalgamation of crustal fragments accreted to the paleo-Pacific Gondwana margin and which underwent significant thinning during the subsequent split from Australia and Antarctica in the mid-Cretaceous following opening of the Tasman Sea and the Southern Ocean. We present Sr, Nd and Pb isotopes and laser ablation trace element data for a comprehensive suite of clinopyroxene separates from spinel peridotite xenoliths (lherzolite to harzburgite) from the sub-continental lithospheric mantle across southern New Zealand. These xenoliths were transported to the surface in intra-plate alkaline volcanics that erupted across the region in the Eocene and Miocene (33-10 m.y.a.). Most of the volcanic suites have similar geochemical and isotopic properties that indicate melting of an OIB-like mantle source in the garnet stability zone and that contained a HIMU component. The volcanics have tapped two adjacent but chemically contrasting upper mantle domains: a fertile eastern domain and an extremely depleted western domain. Both domains underlie Mesozoic metasedimentary crust. Radiogenic isotope compositions of the clinopyroxene have 87Sr/86Sr between 0.7023 to 0.7035, 143Nd/144Nd between 0.5128 and 0.5132 (corresponding to ?Nd between +3 and +13) with a few samples extending to even more depleted compositions, 206Pb/204 Pb between ca. 19.5 to 21.5 and 208Pb/204 Pb between ca. 38.5 to 40.5. No correlations are observed between isotopic composition, age or geographical separation. These isotopic compositions indicate that the sub-continental lithospheric mantle under southern New Zealand has a regionally distinct and pervasive FOZO to HIMU - like signature. The isotopic signatures are also similar to those of the alkaline magmas that transported the xenoliths and suggest that most of the HIMU signature observed in the volcanics could be derived from a major source component in the sub-continental lithospheric mantle

  1. Petrochemical and petrophysical characterization of the lower crust and the Moho beneath the West African Craton, based on Xenoliths from Kimberlites

    NASA Technical Reports Server (NTRS)

    Haggerty, Stephen E.; Toft, Paul B.

    1988-01-01

    Additional evidence to the composition of the lower crust and uppermost mantle was presented in the form of xenolith data. Xenoliths from the 2.7-Ga West African Craton indicate that the Moho beneath this shield is a chemically and physically gradational boundary, with intercalations of garnet granulite and garnet eclogite. Inclusions in diamonds indicate a depleted upper mantle source, and zenolith barometry and thermometry data suggest a high mantle geotherm with a kink near the Moho. Metallic iron in the xenoliths indicates that the uppermost mantle has a significant magnetization, and that the depth to the Curie isotherm, which is usually considered to be at or above the Moho, may be deeper than the Moho.

  2. Carbon isotopes in xenoliths from the Hualalai Volcano, Hawaii, and the generation of isotopic variability

    SciTech Connect

    Pineau, F. ); Mathez, E.A. )

    1990-01-01

    The isotopic composition of carbon has been determined in a suite of xenoliths from lava of the 1800-1801 Kaupulehu eruption of Hualalai Volcano, Hawaii. Several lithologies are represented in the suite, including websterite, dunite, wehrlite, pyroxenite, and gabbro. In addition, there are composite xenoliths in which contacts between lithologies are preserved. Most of the xenoliths represent deformed cumulates. The contact relations in the composite samples indicate that the lithologies originated from the same source region, which, based on pressures determined from fluid inclusions, is estimated to be at a depth of {approx}20 km, or near the crust-mantle boundary. The observations and isotopic results demonstrate that isotopic variability can be generated by multistage fractionation processes such as degassing of CO{sub 2} from magma and precipitation of CO{sub 2}-rich fluids to form graphitic compounds. Such processes operated over regions the scales of which were determined by style and intensity of deformation and by lithology.

  3. Millennia of magmatism recorded in crustal xenoliths from Southwest Greenland

    NASA Astrophysics Data System (ADS)

    Smit, Matthijs; Waight, Tod; Nielsen, Troels

    2016-04-01

    Eruption of CO2-rich ultramafic magma involves rapid ascent of mantle-derived magmas loaded with mantle xenoliths and xenocrysts (>30 vol%). The dynamics and duration of such eruptions are increasingly well constrained; the causes are nevertheless largely unclear. To address this issue, we performed a petrological and speedometric analysis of well-preserved crustal xenoliths from aillikite dikes at Sisimiut and Sarfartôq alkaline provinces, W Greenland. The xenoliths represent mafic granulites, scavenged from c. 25-36 km depth within the mid-to-lower crust. The rocks are infiltrated by various types of melt in grain boundaries, cracks and veins. Zirconium-in rutile thermometry and Fe-in-rutile speedometry indicate melt temperature of c. 1,015 °C and melt exposure time of a few hours for the host aillikite, implying an average ascent rate of c. 2 m/s. This is slower than average ascent rates of mantle cargo (4-40 m/s [1]), suggesting a slowing-down of transport at shallow levels. Local diffusive zoning in garnet indicates up to several millenia of melt-assisted mass transport. This demonstrates a two-stage magmatic process of rapid melt ascent preceded by a previously unrecognized long magmatic episode. Melt infiltration at Sismiut lasted 10 times longer than at Sarfartôq, and unlike at Sarfartôq was initially associated with carbonate- and sulfide-rich melt. This contrast reflects a fundamental difference in the devolatilization efficiency of parental carbonatite magma. The rapid development of the Sarfartôq system is ascribed to the local lithospheric mantle being highly depleted [2] and rich in the decarbonation reactant orthopyroxene [3]. A link is also proposed between this feature, and the occurrence of REE-carbonatite and diamond-bearing mantle cargo at that particular location. References [1] Sparks, R.S.J. et al., 2006. J. Volcanol. Geotherm. Res. 155, 8-48. [2] Bizzarro, M. and Stevenson, R.K., 2003. Contrib. Mineral. Petrol. 146, 223-240. [3] Russell

  4. Carbonate assimilation during magma evolution at Nisyros (Greece), South Aegean Arc: Evidence from clinopyroxenite xenoliths

    NASA Astrophysics Data System (ADS)

    Spandler, Carl; Martin, Lukas H. J.; Pettke, Thomas

    2012-08-01

    To contribute to the understanding of magma evolution in arc settings we investigate the oldest volcanic unit (Kanafià Synthem) of Nisyros volcano, located in the eastern Aegean Sea (Greece). The unit consists of porphyritic pillow lavas of basaltic andesite composition with trace element signatures that are characteristic of island-arc magmas. Two lava types are distinguished on the basis of geochemistry and the presence or absence of xenoliths, with the xenolith-bearing lavas having distinctly elevated Sr, MREE/HREE and MgO/Fe2O3 compared to the xenolith-free lavas. Xenoliths include relatively rare quartzo-feldspathic fragments that represent continental-type material, and coarse clinopyroxenite xenoliths that consist largely of aluminous and calcic clinopyroxene, and accessory aluminous spinel. Anorthite-diopside reaction selvages preserved around the clinopyroxenite xenoliths demonstrate disequilibrium between the xenoliths and the host magma. The xenolith clinopyroxene is distinctly enriched in most lithophile trace elements compared to clinopyroxene phenocrysts in the host magmas. A notable exception is the Sr concentration, which is similar in both clinopyroxene types. The high Al and low Na contents of the clinopyroxenites preclude a cumulate, deep metamorphic, or mantle origin for these xenoliths. Instead, their composition and mineralogy are diagnostic of skarn rocks formed by magma-carbonate interaction in the mid/upper crust. The Kanafià lavas are interpreted to have undergone crystal fractionation, magma mixing/mingling and crustal assimilation while resident in the upper crust. We show that magma-carbonate reaction and associated skarn formation does not necessarily result in easily recognised modification of the melt composition, with the exception of increasing Sr contents. Carbonate assimilation also releases significant CO2, which will likely form a free vapour phase due to the low CO2 solubility of arc magmas. In the broader context, we stress

  5. Abnormal lithium isotope composition from the ancient lithospheric mantle beneath the North China Craton.

    PubMed

    Tang, Yan-Jie; Zhang, Hong-Fu; Deloule, Etienne; Su, Ben-Xun; Ying, Ji-Feng; Santosh, M; Xiao, Yan

    2014-01-01

    Lithium elemental and isotopic compositions of olivines in peridotite xenoliths from Hebi in the North China Craton provide direct evidence for the highly variable δ(7)Li in Archean lithospheric mantle. The δ(7)Li in the cores of olivines from the Hebi high-Mg# peridotites (Fo > 91) show extreme variation from -27 to +21, in marked deviation from the δ(7)Li range of fresh MORB (+1.6 to +5.6) although the Li abundances of the olivines are within the range of normal mantle (1-2 ppm). The Li abundances and δ(7)Li characteristics of the Hebi olivines could not have been produced by recent diffusive-driven isotopic fractionation of Li and therefore the δ(7)Li in the cores of these olivines record the isotopic signature of the subcontinental lithospheric mantle. Our data demonstrate that abnormal δ(7)Li may be preserved in the ancient lithospheric mantle as observed in our study from the central North China Craton, which suggest that the subcontinental lithospheric mantle has experienced modification of fluid/melt derived from recycled oceanic crust. PMID:24589693

  6. Abnormal lithium isotope composition from the ancient lithospheric mantle beneath the North China Craton

    PubMed Central

    Tang, Yan-Jie; Zhang, Hong-Fu; Deloule, Etienne; Su, Ben-Xun; Ying, Ji-Feng; Santosh, M.; Xiao, Yan

    2014-01-01

    Lithium elemental and isotopic compositions of olivines in peridotite xenoliths from Hebi in the North China Craton provide direct evidence for the highly variable δ7Li in Archean lithospheric mantle. The δ7Li in the cores of olivines from the Hebi high-Mg# peridotites (Fo > 91) show extreme variation from −27 to +21, in marked deviation from the δ7Li range of fresh MORB (+1.6 to +5.6) although the Li abundances of the olivines are within the range of normal mantle (1–2 ppm). The Li abundances and δ7Li characteristics of the Hebi olivines could not have been produced by recent diffusive-driven isotopic fractionation of Li and therefore the δ7Li in the cores of these olivines record the isotopic signature of the subcontinental lithospheric mantle. Our data demonstrate that abnormal δ7Li may be preserved in the ancient lithospheric mantle as observed in our study from the central North China Craton, which suggest that the subcontinental lithospheric mantle has experienced modification of fluid/melt derived from recycled oceanic crust. PMID:24589693

  7. Abnormal lithium isotope composition from the ancient lithospheric mantle beneath the North China Craton.

    PubMed

    Tang, Yan-Jie; Zhang, Hong-Fu; Deloule, Etienne; Su, Ben-Xun; Ying, Ji-Feng; Santosh, M; Xiao, Yan

    2014-03-04

    Lithium elemental and isotopic compositions of olivines in peridotite xenoliths from Hebi in the North China Craton provide direct evidence for the highly variable δ(7)Li in Archean lithospheric mantle. The δ(7)Li in the cores of olivines from the Hebi high-Mg# peridotites (Fo > 91) show extreme variation from -27 to +21, in marked deviation from the δ(7)Li range of fresh MORB (+1.6 to +5.6) although the Li abundances of the olivines are within the range of normal mantle (1-2 ppm). The Li abundances and δ(7)Li characteristics of the Hebi olivines could not have been produced by recent diffusive-driven isotopic fractionation of Li and therefore the δ(7)Li in the cores of these olivines record the isotopic signature of the subcontinental lithospheric mantle. Our data demonstrate that abnormal δ(7)Li may be preserved in the ancient lithospheric mantle as observed in our study from the central North China Craton, which suggest that the subcontinental lithospheric mantle has experienced modification of fluid/melt derived from recycled oceanic crust.

  8. Nickeliferous sulfides in xenoliths, olivine megacrysts and basaltic glass

    NASA Astrophysics Data System (ADS)

    Fleet, Michael E.; Stone, William E.

    1990-11-01

    The composition of olivine and nickeliferous sulfide inclusions from a selection of mafic and ultramafre rocks, xenoliths and megacrysts, including picritic basalts from Kilauea Volcano, Hawaii, kimberlite from Fayette County, Pennsylvania, and megacrysts from Mount Shasta, California are compared with the mean experimental value of the distribution coefficient for Ni/Fe exchange (KD3=32). Only nine of the forty five olivipe/bulk-sulfide pairs investigated have compositions consistent with equilibration at high temperature, yielding calculated KD3 values in the range 22 to 41. The remaining pairs have calculated KD3 values which range from 0 to 19. Bulk-sulfides in disequilibrated assem-blages are consistently depleted in nickel and within both indivudual associations and individual petrographic sections they exhibit a wide variation in NiS content. The bulk copper contents of olivine-and groundmass-hosted sulfides from Kilauea Volcano range from 0.5 to 43 at%, and samples from the Kilauea Iki lava lake are more Fe-and Cu-rich and generally have lower KD3 values than those from the eruption itself. As with magmatic Ni-Cu sulfide deposits, most nickeliferous sulfide inclusions in mantle-related rocks and xenoliths and in volcanic rocks do not have pristine early-magmatic bulk compositions, and it would seem to be premature to attribute these sulfides solely to either a mantle or an early-magnatic origin.

  9. Cognate xenoliths in Mt. Etna lavas: witnesses of the high-velocity body beneath the volcano

    NASA Astrophysics Data System (ADS)

    Corsaro, Rosa Anna; Rotolo, Silvio Giuseppe; Cocina, Ornella; Tumbarello, Gianvito

    2014-01-01

    Various xenoliths have been found in lavas of the 1763 ("La Montagnola"), 2001, and 2002-03 eruptions at Mt. Etna whose petrographic evidence and mineral chemistry exclude a mantle origin and clearly point to a cognate nature. Consequently, cognate xenoliths might represent a proxy to infer the nature of the high-velocity body (HVB) imaged beneath the volcano by seismic tomography. Petrography allows us to group the cognate xenoliths as follows: i) gabbros with amphibole and amphibole-bearing mela-gabbros, ii) olivine-bearing leuco-gabbros, iii) leuco-gabbros with amphibole, and iv) Plg-rich leuco gabbros. Geobarometry estimates the crystallization pressure of the cognate xenoliths between 1.9 and 4.1 kbar. The bulk density of the cognate xenoliths varies from 2.6 to 3.0 g/cm3. P wave velocities (V P ), calculated in relation to xenolith density, range from 4.9 to 6.1 km/s. The integration of mineralogical, compositional, geobarometric data, and density-dependent V P with recent literature data on 3D V P seismic tomography enabled us to formulate the first hypothesis about the nature of the HVB which, in the depth range of 3-13 km b.s.l., is likely made of intrusive gabbroic rocks. These are believed to have formed at the "solidification front", a marginal zone that encompasses a deep region (>5 km b.s.l.) of Mt. Etna's plumbing system, within which magma crystallization takes place. The intrusive rocks were afterwards fragmented and transported as cognate xenoliths by the volatile-rich and fast-ascending magmas of the 1763 "La Montagnola", 2001 and 2002-03 eruptions.

  10. Heterogeneous distribution of H2O in the Martian interior: Implications for the abundance of H2O in depleted and enriched mantle sources

    NASA Astrophysics Data System (ADS)

    McCubbin, Francis M.; Boyce, Jeremy W.; Srinivasan, Poorna; Santos, Alison R.; Elardo, Stephen M.; Filiberto, Justin; Steele, Andrew; Shearer, Charles K.

    2016-04-01

    We conducted a petrologic study of apatite within 12 Martian meteorites, including 11 shergottites and one basaltic regolith breccia. These data were combined with previously published data to gain a better understanding of the abundance and distribution of volatiles in the Martian interior. Apatites in individual Martian meteorites span a wide range of compositions, indicating they did not form by equilibrium crystallization. In fact, the intrasample variation in apatite is best described by either fractional crystallization or crustal contamination with a Cl-rich crustal component. We determined that most Martian meteorites investigated here have been affected by crustal contamination and hence cannot be used to estimate volatile abundances of the Martian mantle. Using the subset of samples that did not exhibit crustal contamination, we determined that the enriched shergottite source has 36-73 ppm H2O and the depleted source has 14-23 ppm H2O. This result is consistent with other observed geochemical differences between enriched and depleted shergottites and supports the idea that there are at least two geochemically distinct reservoirs in the Martian mantle. We also estimated the H2O, Cl, and F content of the Martian crust using known crust-mantle distributions for incompatible lithophile elements. We determined that the bulk Martian crust has ~1410 ppm H2O, 450 ppm Cl, and 106 ppm F, and Cl and H2O are preferentially distributed toward the Martian surface. The estimate of crustal H2O results in a global equivalent surface layer (GEL) of ~229 m, which can account for at least some of the surface features on Mars attributed to flowing water and may be sufficient to support the past presence of a shallow sea on Mars' surface.

  11. Efremovka E49: A compact type-A CAI containing a partially molten spinel-melilite-diopside xenolith

    NASA Astrophysics Data System (ADS)

    El Goresy, A.; Zinner, E. K.

    1994-07-01

    Eframovka E49 is a triangular 2-mm segment from a Compact Type A (CTA) inclusion with large portions of intact core and rim sequence. It is probably a fragment from an originally round approximately equal to 4-mm Ca-Al rich Inclusion (CAI). The core consists of two lithologically different assemblages: (1) The major portion of the CAI contains melilite sprinkled with rare spinel, perovskite, and the new Ca-Ti silicate. It is covered by a six-layer rim consisting of (from the interior outward): two layers of Zr- and Y-rich perovskite, spinel, Al-diopside, diopside, and forsteritic olivine. (2) A 650-micron wide complex xenolith contains coarse spinel, melilite, perovskite, and metal in its interior, surrounded by a broad shell of Al-diopside, diopside, and minor fassaite and anorthite, and in the rim fassaite yields Al-diopside yields diopside. Coarse spinels abundantly display resorbtion outlines and some of the grains have been broken down to several amoeboid fragments floating in the eutectic assemblage. All these textures are evidence of local melting of the xenolith followed by fast cooling. No such features are observed in the host CAI. Since melting is confined to the xenolith, the melt event must have predated its capture into the core of E49. Ion microprobe trace-element studies reveal distinct differences between Rare Earth Element (REE) abundances in perovskites in the xenolith and the host CAI. Perovskites in the xenolith display REE patterns with prominent Yb and small Ce excesses and large Eu depletions. Perovskites in the xenolith show higher abundances of Nb, Zr, and V. Magnesium in xenolith and the host is almost unfractionated. Excesses of Mg-26 are found both in the xenolith and the host with data points plotting along a line with a slope of 4 x 105. This is in accord with the petrographic interpretation and indicates that the melting of the xenolith and its capture in E49 took place early.

  12. Water Distribution in the Continental and Oceanic Upper Mantle

    NASA Technical Reports Server (NTRS)

    Peslier, Anne H.

    2015-01-01

    Nominally anhydrous minerals such as olivine, pyroxene and garnet can accommodate tens to hundreds of ppm H2O in the form of hydrogen bonded to structural oxygen in lattice defects. Although in seemingly small amounts, this water can significantly alter chemical and physical properties of the minerals and rocks. Water in particular can modify their rheological properties and its distribution in the mantle derives from melting and metasomatic processes and lithology repartition (pyroxenite vs peridotite). These effects will be examined here using Fourier transform infrared spectrometry (FTIR) water analyses on minerals from mantle xenoliths from cratons, plume-influenced cratons and oceanic settings. In particular, our results on xenoliths from three different cratons will be compared. Each craton has a different water distribution and only the mantle root of Kaapvaal has evidence for dry olivine at its base. This challenges the link between olivine water content and survival of Archean cratonic mantle, and questions whether xenoliths are representative of the whole cratonic mantle. We will also present our latest data on Hawaii and Tanzanian craton xenoliths which both suggest the intriguing result that mantle lithosphere is not enriched in water when it interacts with melts from deep mantle upwellings (plumes).

  13. Hf sbnd Nd sbnd Sr isotopes and incompatible element abundances in island arcs: implications for magma origins and crust-mantle evolution

    NASA Astrophysics Data System (ADS)

    White, William M.; Patchett, Jonathan

    1984-02-01

    We present Hf, Nd and Sr isotopic data and abundances of K, Rb, Cs, Ba, Sr, Hf and REE for 32 samples from seven intra-oceanic island arcs. Samples from the Marianas, Izu, Aleutian and New Britain arcs have tightly grouped 176Hf/ 177Hf˜ 0.28320, 143Nd/ 144Nd˜ 0.51303 and 87Sr/ 86Sr˜ 0.7035 close to, but distinct from, mid-ocean ridge basalts (MORB) for 143Nd/ 144Nd and 87Sr/ 86Sr . In contrast, samples from the Sunda, Banda and Lesser Antilles arcs are much more variable towards lower 176Hf/ 177Hf and 143Nd/ 144Nd , and higher 87Sr/ 86Sr . Isotopically, island arcs on the whole are closely similar to ocean islands. Some commonly-occurring features of the trace element geochemistry of island arcs are apparent in our data: alkali and alkaline-earth elements, particularly Cs, have high abundance relative to LREE compared to oceanic basalts; negative Ce anomalies occur in six out of seven arcs. However, Hf does not appear underabundant relative to REE. The isotopic data require a continental component in all island arcs, in addition to probable mantle and oceanic crust contributions, even for the arcs with isotope ratios close to MORB. In the absence of continental crust, we can best explain this component by subducted pelagic sediment in the arc magma source region. The involvement of sediments in all arcs implies that there is an inherent recycling of older continent to island arcs, and potentially to new continent, of at least 1%. Conservative calculations show that the upper subducted slab (basalt + sediment) passes beyond the arc magma genesis zone and enters the deep mantle with a minimum of 500-1000 ppm K, and corresponding amounts of other incompatible elements. If this material is not completely homogenized with the mantle and later becomes part of the source of ocean island magmas, then the ocean island—island arc isotopic similarity is a result of their similar mix of source materials—mantle peridotite with trace element signatures from oceanic crust

  14. High-K andesite petrogenesis and crustal evolution: Evidence from mafic and ultramafic xenoliths, Egmont Volcano (Mt. Taranaki) and comparisons with Ruapehu Volcano, North Island, New Zealand

    NASA Astrophysics Data System (ADS)

    Price, Richard C.; Smith, Ian E. M.; Stewart, Robert B.; Gamble, John A.; Gruender, Kerstin; Maas, Roland

    2016-07-01

    compositions approximating the solid material extracted during these processes. Some Group 2 xenoliths have relatively unevolved Sr and Nd isotopic compositions and they are interpreted to have derived as crystal cumulates from a more primitive parental basalt or through metasomatic alteration of other xenolith types by isotopically less evolved fluids or melts. Some Group 3 xenoliths could have originally been pyroxene cumulates but the granoblastic textures of others are more consistent with an origin as restites generated during anatexis of amphibolite. Group 4 xenoliths have textures similar to those of mantle-derived peridotite xenoliths found in intraplate basalts. The geochemical variation is consistent with a system fed by mantle-derived magmas that underplated and intruded the lower crust. At this level AFC and FC and crustal anatexis generated cumulates and pyroxene restite represented by the mafic and ultramafic xenoliths. The magmas segregating at these deep levels moved higher into the crust where a complex dispersed magma storage and plumbing system formed. Here magmas evolved further through AFC and FC with the formation of cumulates and crystal mushes that are represented by some Group 1 and Group 2 xenoliths. Xenoliths were further modified by interaction with host magmas or by alteration at the side walls of magma storages and conduits.

  15. Metasomatically altered peridotite xenoliths from the Hessian Depression (Northwest Germany)

    SciTech Connect

    Hartmann, G.; Wedepohl, K.H. )

    1990-01-01

    A set of 36 spinel lherzolite and spinel harzburgite xenoliths from 7 occurrences of Tertiary alkali olivine basalt and olivine nephelinite have been investigated for modal composition, major elements, and 39 minor elements. The periodotites were depleted during former partial melting events. The proportion of clinopyroxene, elements indicating depletion, and compatible trace elements are on average close to the means of the worldwide subcontinental lithosphere. The metasomatic alteration of the sampled mantle varies in a complicated three-dimensional pattern without vertical layering. Mobilization of elements in mantle fluids and accumulation in certain peridotites is a complex open-system process. The exceptional behavior of high field-strength elements might be controlled by the formation of Ti minerals. The metasomatic transport can be explained by extraction of the incompatible elements either from mantle minerals or from subducted crustal sources into water-rich fluids. Chlorine in phlogopite has equilibrated with fluids containing chloride concentrations close to seawater. Values of {delta}{sup 18}O in bulk peridotites and in phlogopite exceeding typical mantle compositions probably reflect a crustal contribution to the metasomatic fluids. Nd and Sr isotopic ratios of both peridotites and alkali basalts received a crustal signature from an environment with time-integrated LREE accumulation and Rb depletion.

  16. Distribution and transport of hydrogen in the lithospheric mantle: A review

    NASA Astrophysics Data System (ADS)

    Demouchy, Sylvie; Bolfan-Casanova, Nathalie

    2016-01-01

    The minerals constituting the Earth's upper mantle are nominally anhydrous silicates (NAMs). However they do contain hydrogen as a trace element, decorating point defects in their crystalline structure. Experimental petrology and mineralogy have quantified the maximum concentration under several compositional and thermodynamic conditions, but systematic studies on the hydrogen concentration in minerals from mantle-derived rocks have only recently been carried out. Here, we have compiled the distribution of hydrogen in upper mantle peridotite xenoliths, from which several conclusions can be drawn. NAMs from peridotite xenoliths contain a few ppm wt. H2O in their structure. From the current database, the hydrogen concentrations in olivine regularly increase with increasing depth. The amount of hydrogen in NAMs from peridotite xenoliths from subduction contexts is not higher than in other geological context for similar temperature and pressure conditions. The highest hydrogen concentrations is found in peridotitic olivines from cratonic mantle, and are likely due to the depth of origin. The increasing hydrogen concentration in olivine with increasing depth is likely controlled by the increase of H partitioning into olivine at the expense of orthopyroxene as imposed by a decrease in Al content in opx with depth. However, the sparse data could also indicate that the bulk hydrogen concentration slightly increases with depth > 150 km. In this case, it would suggest, locally (Udachnaya for example), a possible increase in water fugacity due to fluid saturation. Even if the most abundant mineral in mantle rocks is olivine, the bulk hydrogen concentration in peridotites is controlled by the amount of hydrogen stored in pyroxenes. However, hydrogen concentration in olivine remains crucial for consequences on physical properties such as rheology and electrical conductivity. Kinetics of hydrogen transport is reviewed and hydrous melt/fluid percolation appears necessary to

  17. Highly siderophile element (HSE) abundances in the mantle of Mars are due to core formation at high pressure and temperature

    NASA Astrophysics Data System (ADS)

    Righter, K.; Danielson, L. R.; Pando, K. M.; Williams, J.; Humayun, M.; Hervig, R. L.; Sharp, T. G.

    2015-04-01

    Highly siderophile elements (HSEs) can be used to understand accretion and core formation in differentiated bodies, due to their strong affinity for FeNi metal and sulfides. Coupling experimental studies of metal-silicate partitioning with analyses of HSE contents of Martian meteorites can thus offer important constraints on the early history of Mars. Here, we report new metal-silicate partitioning data for the PGEs and Au and Re across a wide range of pressure and temperature space, with three series designed to complement existing experimental data sets for HSE. The first series examines temperature effects for D(HSE) in two metallic liquid compositions—C-bearing and C-free. The second series examines temperature effects for D(Re) in FeO-bearing silicate melts and FeNi-rich alloys. The third series presents the first systematic study of high pressure and temperature effects for D(Au). We then combine our data with previously published partitioning data to derive predictive expressions for metal-silicate partitioning of the HSE, which are subsequently used to calculate HSE concentrations of the Martian mantle during continuous accretion of Mars. Our results show that at midmantle depths in an early magma ocean (equivalent to approximately 14 GPa, 2100 °C), the HSE contents of the silicate fraction are similar to those observed in the Martian meteorite suite. This is in concert with previous studies on moderately siderophile elements. We then consider model calculations that examine the role of melting, fractional crystallization, and sulfide saturation/undersaturation in establishing the range of HSE contents in Martian meteorites derived from melting of the postcore formation mantle. The core formation modeling indicates that the HSE contents can be established by metal-silicate equilibrium early in the history of Mars, thus obviating the need for a late veneer for HSE, and by extension volatile siderophile elements, or volatiles in general.

  18. Application of Markov Chain Monte Carlo Method to Mantle Melting: An Example from REE Abundances in Abyssal Peridotites

    NASA Astrophysics Data System (ADS)

    LIU, B.; Liang, Y.

    2015-12-01

    Markov chain Monte Carlo (MCMC) simulation is a powerful statistical method in solving inverse problems that arise from a wide range of applications, such as nuclear physics, computational biology, financial engineering, among others. In Earth sciences applications of MCMC are primarily in the field of geophysics [1]. The purpose of this study is to introduce MCMC to geochemical inverse problems related to trace element fractionation during concurrent melting, melt transport and melt-rock reaction in the mantle. MCMC method has several advantages over linearized least squares methods in inverting trace element patterns in basalts and mantle rocks. First, MCMC can handle equations that have no explicit analytical solutions which are required by linearized least squares methods for gradient calculation. Second, MCMC converges to global minimum while linearized least squares methods may be stuck at a local minimum or converge slowly due to nonlinearity. Furthermore, MCMC can provide insight into uncertainties of model parameters with non-normal trade-off. We use MCMC to invert for extent of melting, amount of trapped melt, and extent of chemical disequilibrium between the melt and residual solid from REE data in abyssal peridotites from Central Indian Ridge and Mid-Atlantic Ridge. In the first step, we conduct forward calculation of REE evolution with melting models in a reasonable model space. We then build up a chain of melting models according to Metropolis-Hastings algorithm to represent the probability of specific model. We show that chemical disequilibrium is likely to play an important role in fractionating LREE in residual peridotites. In the future, MCMC will be applied to more realistic but also more complicated melting models in which partition coefficients, diffusion coefficients, as well as melting and melt suction rates vary as functions of temperature, pressure and mineral compositions. [1]. Sambridge & Mosegarrd [2002] Rev. Geophys.

  19. Thorium abundances of basalt ponds in South Pole-Aitken basin: Insights into the composition and evolution of the far side lunar mantle

    USGS Publications Warehouse

    Hagerty, J.J.; Lawrence, D.J.; Hawke, B.R.

    2011-01-01

    Imbrian-aged basalt ponds, located on the floor of South Pole-Aitken (SPA) basin, are used to provide constraints on the composition and evolution of the far side lunar mantle. We use forward modeling of the Lunar Prospector Gamma Ray Spectrometer thorium data, to suggest that at least five different and distinct portions of the far side lunar mantle contain little or no thorium as of the Imbrian Period. We also use spatial correlations between local thorium enhancements and nonmare material on top of the basalt ponds to support previous assertions that lower crustal materials exposed in SPA basin have elevated thorium abundances, consistent with noritic to gabbronoritic lithologies. We suggest that the lower crust on the far side of the Moon experienced multiple intrusions of thorium-rich basaltic magmas, prior to the formation of SPA basin. The fact that many of the ponds on the lunar far side have elevated titanium abundances indicates that the far side of the Moon experienced extensive fractional crystallization that likely led to the formation of a KREEP-like component. However, because the Imbrian-aged basalts contain no signs of elevated thorium, we propose that the SPA impact event triggered the transport of a KREEP-like component from the lunar far side and concentrated it on the nearside of the Moon. Because of the correlation between basaltic ponds and basins within SPA, we suggest that Imbrian-aged basaltic volcanism on the far side of the Moon was driven by basin-induced decompressional melting. Copyright ?? 2011 by the American Geophysical Union.

  20. Stable isotopic (O, H) evidence for hydration of the central Colorado Plateau lithospheric mantle by slab-derived fluids

    NASA Astrophysics Data System (ADS)

    Marshall, E. W.; Barnes, J.; Lassiter, J. C.

    2013-12-01

    The Colorado Plateau is a tectonically stable, relatively undeformed Proterozoic lithospheric province in the North America Cordillera. Although the stability of the Colorado Plateau suggests that it is rheologically strong, evidence from xenoliths show that the lithospheric mantle is extensively hydrated (e.g., presence of hydrous minerals, 'high' water contents in nominally anhydrous minerals), and therefore weakened. In addition, LREE enrichments in clinopyroxene (cpx) imply that the lithospheric mantle has been metasomatized ([1],[2]). Here we analyze mineral separates from spinel and garnet peridotite xenoliths from the Navajo Volcanic Field (NVF), located in the center of the Plateau, for their oxygen and hydrogen isotope compositions. These compositions are compared to those of xenoliths at the margins of the Plateau: spinel peridotites from the Grand Canyon Volcanic Field (GCVF) in the west and Zuni-Bandera Volcanic Field (ZBVF) in the east. NVF xenoliths are significantly more hydrous than the xenoliths on the margins of the Colorado Plateau based on modal abundances of hydrous minerals and structural water in olivine (e.g. [3]). All hydrous phases have high δD values (antigorite = -71 to -46‰ (n = 6 xenoliths); chlorite = -49 to -31‰ (n=3); amphibole = -47‰ (n=1)) compared to normal mantle (~-80‰), suggesting the addition of a fluid that is enriched in D compared to typical mantle. δ18O values for the same hydrous minerals range from 6.0 to 6.6‰ (n=6). δ18O values of olivine from NVF spinel peridotites have a narrow range, 5.0 to 5.4‰ (n = 4), near mantle olivine values (~5.2‰). Olivines from spinel peridotites from the GCVF and ZBVF also have mantle-like δ18O values (5.1 to 5.2‰ (n=3) and 5.1 to 5.4‰ (n=7), respectively). However, olivines and orthopyroxenes (opx) from NVF garnet peridotites have a slightly larger range and some record 18O enrichment (olivine = 5.1 to 5.6‰ (n = 3); opx = 5.9‰ (n=1)). The high δ18O values of

  1. Rapid kimberlite ascent and the significance of Ar-Ar ages in xenolith phlogopites

    PubMed

    Kelley; Wartho

    2000-07-28

    Kimberlite eruptions bring exotic rock fragments and minerals, including diamonds, from deep within the mantle up to the surface. Such fragments are rapidly absorbed into the kimberlite magma so their appearance at the surface implies rapid transport from depth. High spatial resolution Ar-Ar age data on phlogopite grains in xenoliths from Malaita in the Solomon Islands, southwest Pacific, and Elovy Island in the Kola Peninsula, Russia, indicate transport times of hours to days depending upon the magma temperature. In addition, the data show that the phlogopite grains preserve Ar-Ar ages recorded at high temperature in the mantle, 700 degrees C above the conventional closure temperature.

  2. Rapid kimberlite ascent and the significance of Ar-Ar ages in xenolith phlogopites

    PubMed

    Kelley; Wartho

    2000-07-28

    Kimberlite eruptions bring exotic rock fragments and minerals, including diamonds, from deep within the mantle up to the surface. Such fragments are rapidly absorbed into the kimberlite magma so their appearance at the surface implies rapid transport from depth. High spatial resolution Ar-Ar age data on phlogopite grains in xenoliths from Malaita in the Solomon Islands, southwest Pacific, and Elovy Island in the Kola Peninsula, Russia, indicate transport times of hours to days depending upon the magma temperature. In addition, the data show that the phlogopite grains preserve Ar-Ar ages recorded at high temperature in the mantle, 700 degrees C above the conventional closure temperature. PMID:10915621

  3. Mantle metasomatism beneath western Victoria, Australia: I. Metasomatic processes in Cr-diopside lherzolites

    NASA Astrophysics Data System (ADS)

    O'Reilly, Suzanne Y.; Griffin, W. L.

    1988-02-01

    Most Cr-diopside spinel lherzolite xenoliths from Bullenmerri and Gnotuk Maars, western Victoria, show modal metasomatism involving the growth of amphibole ± mica ± apatite at the expense of primary pyroxenes + spinel. The metasomatism is attributed to CO 2-rich fluids, observed in fluid inclusions. Median values of Ni, Cr, V, Sc, Y, Ti, K and Na are similar to those in depleted mantle xenoliths and the source regions of N-type MORB. Median concentrations of Ba, Th, U, Ta, Nb, Sr and REE range from 1-10 times "primordial" values. REE patterns of anhydrous lherzolites range from LREE-depleted ( (La/Yb) n ≈ 0.3 ) to LREE-enriched ( (La/Yb) n = 30-60 ), and show an inverse correlation of Nd/Sm with CaO. Amphibole-rich peridotites are enriched in LREE ( (La/Yb) n = 10-30 ), Zr and Ta, with high K/Rb. Mica-rich rocks are enriched in K, Rb, Ba, Ta and Ti, with low K/ Rb. Introduction of apatite leads to high σREE (with (La/Yb) n = 40-100 ), Sr, U and Th contents. The distribution of trace and minor elements in the Iherzolites is thus controlled by the crystal chemistry of the primary and metasomatic phases. Micaceous xenoliths may be derived from thin selvedges on pyroxenite veins. Abundant amphibole lherzolites may form a matrix enclosing relict volumes of anhydrous lherzolites showing varying degrees of cryptic metasomatism. The overall pattern of trace-element enrichment in the mantle beneath Victoria will depend on the volumetric proportions of these rock types.

  4. H Diffusion in Olivine and Pyroxene from Peridotite Xenoliths and a Hawaiian Magma Speedometer

    NASA Technical Reports Server (NTRS)

    Peslier, A. H.; Bizimis, M.

    2014-01-01

    Hydrogen is present as a trace element in olivine and pyroxene and its content distribution in the mantle results from melting and metasomatic processes. Here we examine how these H contents can be disturbed during decompression. Hydrogen was analyzed by FTIR in olivine and pyroxene of spinel peridotite xenoliths from Salt Lake Crater (SLC) nephelinites which are part of the rejuvenated volcanism at Oahu (Hawaii) [1,2]. H mobility in pyroxene resulting from spinel exsolution during mantle upwelling Most pyroxenes in SLC peridotites exhibit exsolutions, characterized by spinel inclusions. Pyroxene edges where no exsolution are present have less H then their core near the spinel. Given that H does not enter spinel [3], subsolidus requilibration may have concentrated H in the pyroxene adjacent to the spinel exsolution during mantle upwelling. H diffusion in olivine during xenolith transport by its host magma and host magma ascent rates Olivines have lower water contents at the edge and near fractures compared to at their core, while the concentrations of all other chemical elements appear homogeneous. This suggests that some of the initial water has diffused out of the olivine. Water loss from the olivine is thought to occur during host-magma ascent and xenolith transport to the surface [4-6]. Diffusion modeling matches best the data when the initial water content used is that measured at the core of the olivines, implying that mantle water contents are preserved at the core of the olivines. The 3225 cm(sup -1) OH band at times varies independantly of other OH bands, suggesting uneven H distribution in olivine defects likely acquired during mantle metasomatism just prior to eruption and unequilibrated. Diffusion times (1-48 hrs) combined with depths of peridotite equilibration or of magma start of degassing allow to calculate ascent rates for the host nephelinite of 0.1 to 27 m/s.

  5. The Paradox of a Wet (High H2O) and Dry (Low H2O/Ce) Mantle: High Water Concentrations in Mantle Garnet Pyroxenites from Hawaii

    NASA Technical Reports Server (NTRS)

    Peslier, Anne H.; Bizimis, Michael

    2013-01-01

    Water dissolved as trace amounts in anhydrous minerals has a large influence on the melting behavior and physical properties of the mantle. The water concentration of the oceanic mantle is inferred from the analyses of Mid-Ocean Ridge Basalt (MORB) and Oceanic Island Basalt (OIB). but there is little data from actual mantle samples. Moreover, enriched mineralogies (pyroxenites, eclogites) are thought as important sources of heterogeneity in the mantle, but their water concentrations and their effect on the water budget and cycling in the mantle are virtually unknown. Here, we analyzed by FTIR water in garnet clinopyroxenite xenoliths from Salt Lake Crater, Oahu, Hawaii. These pyroxenites are high-pressure (>20kb) crystal fractionates from alkalic melts. The clinopyroxenes (cpx) have 260 to 576 ppm wt H2O, with the least differentiated samples (Mg#>0.8) in the 400-500 ppm range. Orthopyroxene (opx) contain 117-265 ppm H2O, about half of that of cpx, consistent with other natural sample studies, but lower than cpx/opx equilibrium from experimental data. The pyroxenite cpx and opx H2O concentrations are at the high-end of on-and off-craton peridotite xenolith concentrations and those of Hawaiian spinel peridotites. In contrast, garnet has extremely low water contents (<5ppm H2O). There is no correlation between H2O in cpx and lithophile element concentrations. Phlogopite is present in some samples, and its modal abundance shows a positive correlation in Mg# with cpx, implying equilibrium. However, there is no correlation between H2O concentrations and or the presence of phlogopite. These data imply that cpx and opx may be at water saturation, far lower than experimental data suggest. Reconstructed bulk rock pyroxenite H2O ranges from 200-460 ppm (average 331 +/- 75 ppm), 2 to 8 times higher than H2O estimates for the MORB source (50-200 ppm), but in the range of E-MORB, OIB and the source of rejuvenated Hawaiian magmas. The average bulk rock pyroxenite H2O/Ce is 69

  6. Nd-isotopes in selected mantle-derived rocks and minerals and their implications for mantle evolution

    USGS Publications Warehouse

    Basu, A.R.; Tatsumoto, M.

    1980-01-01

    The Sm-Nd systematics in a variety of mantle-derived samples including kimberlites, alnoite, carbonatite, pyroxene and amphibole inclusions in alkali basalts and xenolithic eclogites, granulites and a pyroxene megacryst in kimberlites are reported. The additional data on kimberlites strengthen our earlier conclusion that kimberlites are derived from a relatively undifferentiated chondritic mantle source. This conclusion is based on the observation that the e{open}Nd values of most of the kimberlites are near zero. In contrast with the kimberlites, their garnet lherzolite inclusions show both time-averaged Nd enrichment and depletion with respect to Sm. Separated clinopyroxenes in eclogite xenoliths from the Roberts Victor kimberlite pipe show both positive and negative e{open}Nd values suggesting different genetic history. A whole rock lower crustal scapolite granulite xenolith from the Matsoku kimberlite pipe shows a negative e{open}Nd value of -4.2, possibly representative of the base of the crust in Lesotho. It appears that all inclusions, mafic and ultramafic, in kimberlites are unrelated to their kimberlite host. The above data and additional Sm-Nd data on xenoliths in alkali basalts, alpine peridotite and alnoite-carbonatites are used to construct a model for the upper 200 km of the earth's mantle - both oceanic and continental. The essential feature of this model is the increasing degree of fertility of the mantle with depth. The kimberlite's source at depths below 200 km in the subcontinental mantle is the most primitive in this model, and this primitive layer is also extended to the suboceanic mantle. However, it is clear from the Nd-isotopic data in the xenoliths of the continental kimberlites that above 200 km the continental mantle is distinctly different from their suboceanic counterpart. ?? 1980 Springer-Verlag.

  7. Magnetic mineralogy of pyroxenite xenoliths from Hannuoba basalts, northern North China Craton: Implications for magnetism in the continental lower crust

    NASA Astrophysics Data System (ADS)

    Li, Zhiyong; Zheng, Jianping; Zeng, Qingli; Liu, Qingsheng; Griffin, W. L.

    2014-02-01

    Studies of the petrology, mineral chemistry, and rock magnetic properties of nine pyroxenite xenoliths from Hannuoba basalts, northern North China Craton, have been made to determine the magnetization signature of the continental lower crust. These pyroxenites are weakly magnetic with low average susceptibility (χ) and saturation isothermal remanent magnetization (Mrs) of 39.59 × 10-8 m3 kg-1 and 12.05 × 10-3 Am2 kg-1, respectively. The magnetic minerals are mainly magnetite, pyrrhotite, and Fe-rich spinel, which significantly contribute to χ and natural remanent magnetization. Magnetite occurs as interstitial microcrystals together with zeolite aggregates, indicating a secondary origin in a supergene environment. In contrast, pyrrhotite and Fe-rich spinel were formed prior to the xenoliths' ascent to the surface, as evidenced by their dominant occurrence as tiny inclusions and thin exsolution lamellae in pyroxene. The Fe-rich spinel has ~ 50% mole fraction of Fe3O4 and corresponds to the strongest magnetization, and its coexistence with Mg-rich spinel implies a reheating event due to the underplating of basaltic magma. Besides, armalcolite and ilmenite were found in the reaction rims between xenoliths and the basalt, but they contribute little to the whole rock magnetization. However, these pyroxenite xenoliths would be nonmagnetic at in situ depths, as well as peridotite and mafic granulite xenoliths derived from the crust-mantle transition zone (~ 32-42 km). Therefore, we suggest the limiting depth of magnetization at the boundary between weakly magnetic deep-seated (lower crust and upper mantle) xenoliths and strongly magnetic Archean granulite facies rocks (~ 32 km) in Hannuoba, northern North China Craton.

  8. Thermal history of Colorado plateau lithoshere from Sm-Nd mineral geochronology of xenoliths

    SciTech Connect

    Wendlandt, E.; DePaolo, D.J.; Baldridge, W.S.

    1996-07-01

    The thermal history of the lower crust and upper mantle of the Colorado Plateau region is reconstructed on the basis of Nd and Sr isotopes in minerals and whole rock xenoliths hosted by Tertiary minette and kimberlite. Whole rock data indicate that the crustal rocks were extracted from the mantle at ca. 1900 Ma. The mineral ages, which are 30-100 m.y. younger than crystallization ages of Proterozoic `anorogenic` granitoids from regions bordering the Colorado Plateau, are interpreted as cooling ages set following the crustal thermal maximum at 1380-1440 Ma. The eclogite mineral ages are probably the ages of the host Garnet Ridge and Moses Rock diatremes, and require that Nd isotopes were maintained in equilibrium right up to the time of entrainment. The isotopic data and the mineral textures suggest that the eclogites were undergoing active recrystallization at 21 Ma. The contrast in mineral ages between granulite and eclogite xenoliths indicates that the equilibration temperatures of the two rock types reflect different times of equilibration, and therefore cannot be considered as evidence for a negative thermal gradient at depth. The Rb-Sr mineral data from the xenoliths give variable early Paleozoic and Proterozoic ages that cannot easily be assigned to geologic events. 55 refs., 9 figs., 2 tabs.

  9. A study of REE and Pb, Sr and Nd isotopes in garnet-lherzolite xenoliths from Mingxi, Fujian Province

    USGS Publications Warehouse

    Wankang, H.; Junwen, W.; Basu, A.R.; Tatsumoto, M.

    1993-01-01

    The REE and Pb, Sr, Nd isotopes in three xenoliths from limburgite and scoria-breccias, including spinel-lherzolite, spinel-garnet-lherzolite and phlogopite-gamet-lherzolite, were analysed. The REE contents of the xenoliths are 1.3 to 3.3 times those of the chondrites with their REE patterns characterized by weak LREE depletion. The143Nd/144Nd values of whole rocks and minerals range from 0.51306 to 0.51345 with ??Nd=+ 8.2- +15.8,206Pb/204 Pb < 18.673, and207Pb/204Pb < 15.574. All this goes to show that the upper mantle in Mingxi at the depth of 67-82 km is a depleted mantle of MORB type, with87Sr/86 Sr ratios 0.70237-0.70390. In Nd-Sr diagram the data points of whole rocks are all out of the mantle array, implying that the xenoliths from Mingxi have more radiogenic Sr isotopes than those of the mantle array. ?? 1993 Institute of Geochemistry, Chinese Academy of Sciences.

  10. Unusual δ56Fe values in Samoan rejuvenated lavas generated in the mantle

    NASA Astrophysics Data System (ADS)

    Konter, Jasper G.; Pietruszka, Aaron J.; Hanan, Barry B.; Finlayson, Valerie A.; Craddock, Paul R.; Jackson, Matthew G.; Dauphas, Nicolas

    2016-09-01

    Several magmatic processes contribute to the Fe isotope composition of igneous rocks. Most basalts fall within a limited range of δ56Fe (+ 0.10 ± 0.05 ‰), although more differentiated lavas trend towards slightly elevated values (up to + 0.3 ‰). New data for basalts and olivine crystals from the Samoan Islands show higher δ56Fe values than have previously been reported for basalts worldwide. Common magmatic processes - from partial melting of average mantle to subsequent differentiation of melts - cannot sufficiently fractionate the Fe isotopes to explain the elevated δ56Fe values (∼ + 0.3 ‰) in rejuvenated Samoan lavas. Instead, a mantle source with an elevated δ56Fe value - in conjunction with effects due to common magmatic processes - is required. The Samoan mantle source is known to be unique in its radiogenic isotope composition and indications that melting of the Samoan mantle source can generate elevated δ56Fe values in lavas comes from: (1) High fO2 values of Samoan lavas and their likely sources affecting Fe isotope fractionation during melting; (2) Metasomatism that caused elevated δ56Fe in the Samoan mantle, as observed in xenoliths; and (3) Involvement of a pyroxenite source lithology, based on the Zn/Fe ratios and TiO2 (and other high field-strength element) abundances of the lavas, that can generate melts with elevated δ56Fe values. Two models are presented to explain the elevated δ56Fe values in Samoan lavas: a metasomatized source (∼ + 0.07 ‰) or the presence of a pyroxenite source component (∼ + 0.12 ‰). Both models subsequently elevate δ56Fe values with both partial melting (∼ + 0.14 ‰) and fractional crystallization (∼ + 0.1 ‰). These processes may be related to an upwelling mantle plume with a pyroxenite component, or melting of previously metasomatized upper mantle.

  11. Surface and Mantle Expression of the Early Permian Tarim Mantle Plume

    NASA Astrophysics Data System (ADS)

    Chen, Mimi; Tian, Wei

    2015-04-01

    The mantle process during the Early Permian Tarim plume event is revealed by flood basalt and mantle xenoliths. Permian Tarim flood basalts have typical two pulses' eruption. The first pulse of the Tarim flood basalt was erupted at 291-290Ma, characterized by OIB-like Zr/Nb (~5.83), Nb/La and Ce/Pb ratios, and PUM-like initial 187Os/188Os ratios (0.1308-0.1329). They're plotted along a 290±11Ma isochron, implying a pristine "plume mantle" source. The second pulse of the Tarim flood basalt was erupted at 283-281 Ma, with Zr/Nb (~13.6), Nb/La and Ce/Pb ratios similar or close to the lower crust and initial 187Os/188Os ratios (0.1743~19.6740) that deviated from the ~290 Ma isochron line, indicative of significant crustal assimilation. Mantle-derived peridotite and pyroxenite xenoliths hosted in Cenozoic alkali basalts (~20 Ma) are found in the Xikeer, western Tarim Block. Based on their petrographic and geochemical characteristics, peridotite xenoliths can be divided into three groups. Group 1 peridotites, with the presence of the high Mg-number of olivines (91-93) and spinel-pyroxenes clusters, experienced high-degree melt extraction (~17% fractional melting) from garnet- to spinel-stable field. Groups 2 and 3 peridotites, characterized by the clinopyroxenes with spoon-shaped and highly fractionated REE patterns respectively, underwent extensive silicate melt metasomatism at low melt/rock ratios (15) and that the host basanite is incapable of being the metasomatic agent. The Re-Os isotopic systematics of the Xikeer peridotites and pyroxenites yield an isochron of 290±11 Ma, virtually identical to the age of Tarim flood basalts. Their PUM-like Os initial ratios and convecting mantle-like ɛNd(t=290 Ma) strongly suggest that the Xikeer mantle xenoliths derive from the plume mantle. We propose that the Xikeer xenolith suite recorded mantle 'auto-refertilization' process, i.e., they may have been initially formed by melt extraction from the convecting mantle and

  12. Osmium isotopic evidence for ancient subcontinental lithospheric mantle beneath the kerguelen islands, southern indian ocean

    PubMed

    Hassler; Shimizu

    1998-04-17

    Upper mantle xenoliths found in ocean island basalts are an important window through which the oceanic mantle lithosphere may be viewed directly. Osmium isotopic data on peridotite xenoliths from the Kerguelen Islands, an archipelago that is located on the northern Kerguelen Plateau in the southern Indian Ocean, demonstrate that pieces of mantle of diverse provenance are present beneath the Islands. In particular, peridotites with unradiogenic osmium and ancient rhenium-depletion ages (to 1.36 x 10(9) years old) may be pieces of the Gondwanaland subcontinental lithosphere that were incorporated into the Indian Ocean lithosphere as a result of the rifting process. PMID:9545216

  13. Osmium isotopic evidence for ancient subcontinental lithospheric mantle beneath the kerguelen islands, southern indian ocean

    PubMed

    Hassler; Shimizu

    1998-04-17

    Upper mantle xenoliths found in ocean island basalts are an important window through which the oceanic mantle lithosphere may be viewed directly. Osmium isotopic data on peridotite xenoliths from the Kerguelen Islands, an archipelago that is located on the northern Kerguelen Plateau in the southern Indian Ocean, demonstrate that pieces of mantle of diverse provenance are present beneath the Islands. In particular, peridotites with unradiogenic osmium and ancient rhenium-depletion ages (to 1.36 x 10(9) years old) may be pieces of the Gondwanaland subcontinental lithosphere that were incorporated into the Indian Ocean lithosphere as a result of the rifting process.

  14. Deep crust vs shallow mantle: sources of long wavelength magnetic anomalies

    NASA Astrophysics Data System (ADS)

    Ferre, E. C.; Friedman, S. A.; El Atrassi, F.; Conder, J. A.; Demory, F.; Feinberg, J. M.; Filiberto, J.; Khakhalova, E.; Knafelc, J.; Martín-Hernández, F.; Neal, C. R.; Rochette, P.; Till, J. L.; Walsh, K. B., Jr.

    2015-12-01

    Recent petromagnetic results on shallow mantle xenoliths suggest that the uppermost mantle is significantly more magnetic than previously thought, particularly in metasomatised or cold region. This magnetic mantle bears minute amounts of magnetite, a mineral indicative of higher oxygen fugacities. While the exact origin of this magnetite remains uncertain, its contribution to long wavelength magnetic anomalies adds to the potential contributions of crustal rocks. Here we present a compilation of rock magnetic and paleomagnetic data enabling the quantitification of the respective contributions in distinct tectonic settings including subduction zones and stable cratons. The magnetic properties of over 400 specimens of mantle xenoliths are compared to published data from hundreds of deep crustal xenoliths. The picture emerging from this comparison supports a layered distribution of oxidation state in the lithosphere reflecting both the tectonic setting and the degree of melt extraction.

  15. The role of mantle CO2 in volcanism

    USGS Publications Warehouse

    Barnes, I.; Evans, William C.; White, L.D.

    1988-01-01

    Carbon dioxide is the propellant gas in volcanic eruptions and is also found in mantle xenoliths. It is speculated that CO2 occurs as a free gas phase in the mantle because there is no reason to expect CO2 to be so universally associated with volcanic rocks unless the CO2 comes from the same source as the volcanic rocks and their xenoliths. If correct, the presence of a free gas in the mantle would lead to physical instability, with excess gas pressure providing the cause of both buoyancy of volcanic melts and seismicity in volcanic regions. Convection in the mantle and episodic volcanic eruptions are likely necessary consequences. This suggestion has considerable implications for those responsible for providing warnings of impending disasters resulting from volcanic eruptions and earthquakes in volcanic regions. ?? 1988.

  16. Millennia of magmatism recorded in crustal xenoliths from alkaline provinces in Southwest Greenland

    NASA Astrophysics Data System (ADS)

    Smit, Matthijs A.; Waight, Tod E.; Nielsen, Troels F. D.

    2016-10-01

    Mantle-derived CO2-rich magma ascends rapidly through the lithospheric column, supporting upward transport of large mantle-xenoliths and xenocryst (>30 vol%) loads to the (sub-)surface within days. The regional magmatism during which such pulses occur is typically well characterized in terms of general duration and regional compositional trends. In contrast, the time-resolved evolution of individual ultramafic dyke and pipe systems is largely unknown. To investigate this evolution, we performed a geochemical and speedometric analysis of xenoliths from ultramafic (aillikite) dykes in two Neoproterozoic alkaline provinces in West Greenland: 1) Sarfartôq, which overlies Archean ultra-depleted SCLM and yielded ultra-deep mineral indicators, and 2) Sisimiut, where the SCLM is refertilized and deep xenoliths (>120 km) are lacking. We focused on the rare and understudied crustal xenoliths, which preserve a rich record of melt injection. The xenoliths are derived from 25-36 km depth and were transported to the sub-surface within 4 ± 1h (Fe-in-rutile speedometry), during which they were exposed to the magmatic temperature of 1 , 015 ± 50°C (Zr-in-rutile thermometry). Garnet major-element speedometry shows that before the xenolith-ascent stage the lower crust had already been exposed to a variety of magmas for 700 (Sarfartôq) and 7,100 (Sisimiut) years. The Sisimiut samples contain exotic carbonate- and sulfide-rich assemblages, which occurred during the early stages of melt infiltration. Absence of such exotic assemblages and the faster magmatic development at Sarfartôq are tentatively linked to higher decarbonation kinetics in the more depleted SCLM at this location. The data reveal the so far unrecognized pre-eruptive development of ultramafic systems. This stage involves non-steady state melt-silicate interaction between ascending magmas and the immediate SCLM wall-rock, during which the composition of both is modified. The progress and duration of this interaction

  17. Multi-Isotopic evidence from West Eifel Xenoliths

    NASA Astrophysics Data System (ADS)

    Thiemens, M. M.; Sprung, P.

    2015-12-01

    Mantle Xenoliths from the West Eifel intraplate volcanic field of Germany provide insights into the nature and evolution of the regional continental lithospheric mantle. Previous isotope studies have suggested a primary Paleoproterozoic depletion age, a second partial melting event in the early Cambrian, and a Variscan metasomatic overprint. Textural and Sr-Nd isotopic observations further suggest two episodes of melt infiltration of early Cretaceous and Quaternary age. We have investigated anhydrous, vein-free lherzolites from this region, focusing on the Dreiser Weiher and Meerfelder Maar localities. Hand separated spinel, olivine, ortho- and clinopryoxene, along with host and bulk rocks were dissolved and purified for Rb-Sr, Sm-Nd, and Lu-Hf analysis on the Cologne/Bonn Neptune MC-ICP-MS. We find an unexpected discontinuity between mineral separates and whole rocks. While the latter have significantly more radiogenic ɛNd and ɛHf, mineral separates imply close-to chondritic compositions. Our Lu-Hf data imply resetting of the Lu-Hf systematic after 200 Ma. Given the vein-free nature of the lherzolites, this appears to date to the second youngest metasomatic episode. We suggest that markedly radiogenic Nd and Hf were introduced during the Quarternary metasomatic episode and most likely reside on grain boundaries.

  18. The modal mantle metasomatism in the NE part of the Cenozoic Central European Volcanic Province: the example of the Wołek basanite (SW Poland)

    NASA Astrophysics Data System (ADS)

    Nowak, M.; Puziewicz, J.; Grégoire, M.; Muszyński, A.

    2012-04-01

    The Cenozoic lava occurrences in SW Poland belong to the Central European Volcanic Province (CEVP). In some of them (ca.10) mantle xenoliths were recognized. Most of xenoliths show no signs of modal metasomatism (Blusztajn and Shimizu 1994, Matusiak-Małek et al. 2010, Puziewicz et al. 2011). One of the rare exposures with amphibole-bearing peridotite xenoliths in SW Poland is the Wołek Hill basanite. It is located about 100 km south-west of Wrocław (Nowak et al. 2010) in the Złotoryja volcanic field (Matusiak-Małek et al. 2010). Peridotite xenoliths from Wołek

  19. Hydrous metasomatism and melt percolation in the lithospsheric mantle wedge underneath Comallo, Rio Negro Province, Argentina

    NASA Astrophysics Data System (ADS)

    Papadopoulou, Martha; Ntaflos, Theodoros; Bjerg, Ernesto; Gregoire, Michel; Hauzenberger, Christoph

    2015-04-01

    Xenoliths from Comallo, N. Patagonia, are sp-lherzolites, sp-harzburgites, dunites, wehrlites and clinopyroxenites. The rock-forming minerals are olivine, ortho- and clinopyroxene and spinel. Amphibole and phlogopite are present as relicts, suggesting that the region was affected by modal metasomatism. The majority of xenoliths show a dominant well-equilibrated equigranular texture. Small rounded spinels and sulfides enclosed within olivine as well as amphiboles enclosed in clinopyroxenes indicate that these xenoliths are recrystallized. The recrystallized samples show secondary protogranular textures. The amphibole inclusions in clinopyroxenes indicate that the peridotite has experienced a dehydration reaction during the recrystallization process. Amphibole and phlogopite, where present, have been destabilized and show breakdown reactions at the margin, forming secondary ol, cpx and sp. The clinopyroxene REE patterns display a concave-up shape in LREE and MREE whereas the HREE abundances are low. Depending on the presence or not of amphibole and/or phlogopite the cpx REE patterns can be divided into two different groups, both of which show absence of Sr- and weak Zr, Hf and Ti-negative anomalies. These features combined with the REE patterns highlight a cryptic metasomatism due to melt infiltration of alkali basaltic composition. The differences occuring between the two groups may indicate a differentiation at distance from the percolation front. A third group with steep patterns, negative slope and slightly positive Eu anomaly shows a progression from LREE enrichments to depleted HREE. A carbonatitic metasomatism is evidenced by the LREE enrichment as well as a positive Eu-anomaly combined with a negative Ti-anomaly. Calculated equilibrium temperatures at 1.5GPa using the cores of crystals range between 790 and 950°C, whereas the estimated temperatures using rims are ~70°C higher. Such temperatures are relatively low for the lithospheric mantle below Comallo

  20. Multi-stage metasomatism revealed by trace element and Li isotope distributions in minerals of peridotite xenoliths from Allègre volcano (French Massif Central)

    NASA Astrophysics Data System (ADS)

    Gu, Xiaoyan; Deloule, Etienne; France, Lydéric; Ingrin, Jannick

    2016-11-01

    The modal, chemical, and isotopic compositions of mantle peridotite are largely modified by metasomatic processes, which may affect them repeatedly. Xenoliths are commonly used to characterize those metasomatic processes along with the structure, and chemical and isotopic compositions of mantle domains. Nevertheless, the original mantle signatures born by mantle xenoliths are potentially obscured by the interactions occurring between the host magma and the xenolith itself. Here we attempt to identify to which degree the original Li content and isotopic composition, as well as other trace element contents of mantle xenoliths, can be modified by interaction with the host magma. Peridotite xenoliths that have suffered extensive exchange with the entraining magma were sampled in the solidified lava lake of Allègre, Southern French Massif Central, in order to decipher the signature related to peridotite-melt interaction, and to further unravel the evolution of the sub-continental lithospheric mantle. In-situ trace element analyses of clinopyroxene (Cpx) were performed via LA-ICP-MS, and the Li content and isotopic composition of pyroxene and olivine (Ol) via SIMS. Negative HFSE anomalies (Ti/Eu ratios as low as 437) and markedly high LREE/HREE ratios ((La/Yb)N as high as 79) are characteristic of mantle metasomatism at depth. Lithium isotope systematics indicates that at least two different metasomatic events affected the peridotite. Exceptionally high Li contents in Cpx (up to 50 ppm) and slight Li enrichment of Ol rims are ascribed to diffusive Li influx with a positive δ7Li value (+ 3.2‰) from the host magma after entrainment. Conversely, Ol cores preserve extremely light Li isotopic compositions (δ7Li as low as - 25‰) with high Li contents (up to 4.4 ppm) compared to normal mantle, indicating a metasomatic event that occurred before xenolith entrainment. The negative δ7Li signature of this early metasomatism may be related to subduction-related fluids

  1. Mantle metasomatism beneath western Victoria, Australia. I. Metasomatic processes in Cr-diopside lherzolites

    SciTech Connect

    O'Reilly, S.Y.; Griffin, W.L.

    1988-02-01

    Most Cr-diopside spinel lherzolite xenoliths from Bullenmerri and Gnotuk Maars, western Victoria, show modal metasomatism involving the growth of amphibole +- mica +- apatite at the expense of primary pyroxenes + spinel. The metasomatism is attributed to CO/sub 2/-rich fluids, observed in fluid inclusions. REE patterns of anhydrous lherolites range from LREE-depleted ((LaYb)/sub n/ approx. = 0.3) to LREE-enriched ((LaYb)/sub n/ = 30-60), and show an inverse correlation of NdSm with CaO. Amphibole-rich peridotites are enriched in LREE ((LaYb)/sub n/ = 10-30), Ar and Ta, with high KRb. Mica-rich rocks are enriched in K, Rb, Ba, Ta and Ti, with low KRb. Introduction of apatite leads to high ..sigma..REE (with (LaYb)/sub n/ = 40-100), Sr, U and Th contents. The distribution of trace and minor elements in the ilherzolites is thus controlled by the crystal chemistry of the primary and metasomatic phases. Micaceous xenoliths may be derived from thin selvedges on pyroxenite veins. Abundant amphibole lherzolites may form a matrix enclosing relic volumes of anhydrous lherzolites showing varying degrees of cryptic metasomatism. The overall pattern of trace-element enrichment in the mantle beneath Victoria will depend on the volumetric proportions of these rock types.

  2. Thorium Abundances of Basalt Ponds in South Pole-Aitken Basin: Insights into the Composition and Evolution of the Far Side Lunar Mantle

    NASA Astrophysics Data System (ADS)

    Hagerty, J. J.; Lawrence, D. J.; Hawke, B. R.

    2011-03-01

    We used forward modeling of Lunar Prospector thorium (Th) data derived from basalts in South Pole-Aitken basin to demonstrate that large expanses of basalt could be reliably used to obtain compositional information about the far side lunar mantle.

  3. First finding of burkeite in melt inclusions in olivine from sheared lherzolite xenoliths.

    PubMed

    Korsakov, Andrey V; Golovin, Alexander V; De Gussem, Kris; Sharygin, Igor S; Vandenabeele, Peter

    2009-08-01

    For the first time burkeite was observed as a daughter phase in the melt inclusions in olivine by Raman spectroscopy. The olivine comes from sheared lherzolite xenoliths from the Udachnaya-East kimberlite pipe (Yakutia, Russia). This anhydrous sulfate-carbonate mineral (Na(6)(CO(3))(SO(4))(2)) is generally considered to be a characteristic mineral in saline soils or in continental lacustrine evaporite deposits. Recently, however, this mineral was identified in hydrothermal fluids. Our observations indicate that burkeite can also be formed from a mantle-derived melt.

  4. First finding of burkeite in melt inclusions in olivine from sheared lherzolite xenoliths.

    PubMed

    Korsakov, Andrey V; Golovin, Alexander V; De Gussem, Kris; Sharygin, Igor S; Vandenabeele, Peter

    2009-08-01

    For the first time burkeite was observed as a daughter phase in the melt inclusions in olivine by Raman spectroscopy. The olivine comes from sheared lherzolite xenoliths from the Udachnaya-East kimberlite pipe (Yakutia, Russia). This anhydrous sulfate-carbonate mineral (Na(6)(CO(3))(SO(4))(2)) is generally considered to be a characteristic mineral in saline soils or in continental lacustrine evaporite deposits. Recently, however, this mineral was identified in hydrothermal fluids. Our observations indicate that burkeite can also be formed from a mantle-derived melt. PMID:19058996

  5. Structure and evolution of the lithospheric mantle beneath Siberian craton, thermobarometric study

    NASA Astrophysics Data System (ADS)

    Ashchepkov, Igor V.; Pokhilenko, Nikolai P.; Vladykin, Nikolai V.; Logvinova, Alla M.; Afanasiev, Valentin P.; Pokhilenko, Lyudmila N.; Kuligin, Sergei S.; Malygina, Elena V.; Alymova, Natalia A.; Kostrovitsky, Sergey I.; Rotman, Anatolii Y.; Mityukhin, Sergey I.; Karpenko, Mikhail A.; Stegnitsky, Yuri B.; Khemelnikova, Olga S.

    2010-04-01

    Monomineral thermobarometry (MTB) data derived from EMP analyses for heavy monomineral separates from > 20 Yakutian kimberlite pipes were used to compile a SSE-NNW traverse of the mantle beneath the Siberian craton. Orthopyroxene (Opx) MBT for the mantle section beneath Udachnaya gives three PT paths: a low temperature (LT) conductive branch (Boyd et al., 1997), estimated with thermometers of Krogh (1988) or/and O'Neill and Wood (1979), and two other HT paths. The three paths correspond to different values of Fe# in Ol (0.075, 0.085 and 0.11). They are reproduced by the modified MTB equations for clinopyroxene, garnet, chromite and ilmenites (Ashchepkov et al., 2008a), a mono-version of the O'Neill and Wood (1979) thermometer with corrections to Cr and Ca/Mg ratios which mark conductive geotherm. PT estimates for garnets and pyroxenes reflect mantle layering, whereas those for ilmenite reflect varying conditions of polybaric mantle protokimberlite systems and metasomatism. MTB for xenoliths from the Udachnaya, Mir, Dalnyaya, and Komsomolskaya kimberlites show the colder branches if PT path using heavy mineral analyses them according to PT for xenoliths according to TB (O'Neill and Wood, 1979; Finnerty and Boyd, 1987; McGregor, 1974; Brey and Kohler, 1990) producing smoother geotherms. MTB gives a wider range PT points reflecting heating near magmatic channels. Regularities of mantle sections and layering in the Daldyn field are recognized on the PT and P- X diagrams. The lower part of the mantle sections were heated by protokimberlite melts which created the megacrystalline associations. PT values of sub-calcic garnets correlate with those of picroilmenites. Mantle columns beneath the large pipes reveal stepped 7-12 layering for Udachnaya pipe units correlating with the peaks of Re/Os ages (Griffin et al., 2002b) is marked by periodic increase in Fe# in minerals. Pyroxenites and 'hot' eclogites (Pokhilenko et al., 1999) are found in layers at ˜ ˜ 40, 50-65, and

  6. Controls on volatile content and distribution in the continental upper mantle of Southern Gondwana (Patagonia & W. Antarctica)

    NASA Astrophysics Data System (ADS)

    Rooks, Eve; Gibson, Sally; Petrone, Chiara; Leat, Phillip

    2015-04-01

    Water content is known to affect many physical and chemical properties of the upper mantle, including melting temperature and viscosity. Water is hosted by hydrous phases, such as amphibole and phlogopite, and also by more dominant, nominally-anhydrous mantle minerals (e.g. olivine and pyroxene). The latter have the potential to incorporate hundreds of ppm of water in point defects, and may explain geophysical observations such as seismic and conductivity anomalies in the upper mantle [1]. However, the significance of the reported concentrations of H2O in nominally anhydrous minerals in mantle xenoliths is still a subject of debate primarily due to the effects of post-entrainment loss [1,2]. Unlike H2O and Li, F is less susceptible to post entrainment loss and can potentially be used to constrain the source of volatiles. We present high-precision SIMS analyses of H2O, Li and F in mantle xenoliths hosted by recently-erupted (5-10 Ka) alkali basalts from south Patagonia and the Antarctic Peninsula. These two regions formed part of the southern margin of the Gondwana supercontinent, prior to break-up, and were located above long-lived subduction zones for at least 200 M.yr., making them highly-appropriate to investigating long term evolution of the sub-continental lithospheric mantle in this setting. The xenoliths are well characterised peridotites, sourced from the off-craton spinel- and spinel-garnet facies lithospheric mantle (40-80 km). Samples are relatively dry: H2O contents of olivine span 0-49 ppm, orthopyroxene 150-235 ppm and clinopyroxene 100-395 ppm. West Antarctic samples are more hydrated than Patagonian samples, on average. These H2O concentrations fall within the global measured range for off-craton mantle minerals [4]. We attribute low H2O concentrations in olivine to diffusive loss, either by exchange with the host magma, shallow level degassing or during cooling [2]. F shows less variability than H2O and is most highly concentrated in clinopyroxenes

  7. Plutonic xenoliths from Martinique, Lesser Antilles: evidence for open system processes and reactive melt flow in island arc crust

    NASA Astrophysics Data System (ADS)

    Cooper, George F.; Davidson, Jon P.; Blundy, Jon D.

    2016-10-01

    The Lesser Antilles Volcanic Arc is remarkable for the abundance and variety of erupted plutonic xenoliths. These samples provide a window into the deeper crust and record a more protracted crystallisation history than is observed from lavas alone. We present a detailed petrological and in situ geochemical study of xenoliths from Martinique in order to establish their petrogenesis, pre-eruptive storage conditions and their contribution to construction of the sub-volcanic arc crust. The lavas from Martinique are controlled by crystal-liquid differentiation. Amphibole is rarely present in the erupted lavas, but it is a very common component in plutonic xenoliths, allowing us to directly test the involvement of amphibole in the petrogenesis of arc magmas. The plutonic xenoliths provide both textural and geochemical evidence of open system processes and crystal `cargos'. All xenoliths are plagioclase-bearing, with variable proportions of olivine, spinel, clinopyroxene, orthopyroxene and amphibole, commonly with interstitial melt. In Martinique, the sequence of crystallisation varies in sample type and differs from other islands of the Lesser Antilles arc. The compositional offset between plagioclase (~An90) and olivine (~Fo75), suggests crystallisation under high water contents and low pressures from an already fractionated liquid. Texturally, amphibole is either equant (crystallising early in the sequence) or interstitial (crystallising late). Interstitial amphibole is enriched in Ba and LREE compared with early crystallised amphibole and does not follow typical fractionation trends. Modelling of melt compositions indicates that a water-rich, plagioclase-undersaturated reactive melt or fluid percolated through a crystal mush, accompanied by the breakdown of clinopyroxene, and the crystallisation of amphibole. Geothermobarometry estimates and comparisons with experimental studies imply the majority of xenoliths formed in the mid-crust. Martinique cumulate xenoliths are

  8. Petrology of ultramafic, mafic, and felsic xenoliths from Ruddon's Point basanite, Fife, Scotland, UK - preliminary results.

    NASA Astrophysics Data System (ADS)

    Sobczak, Paweł; Matusiak-Malek, Magdalena; Puziewicz, Jacek; Upton, Brian

    2016-04-01

    Numerous dykes of Carboniferous alkaline volcanic rocks occur in the county of Fife, Scotland, United Kingdom. Basanitic dyke from Ruddon's Point encloses mafic, ultramafic, and felsic xenoliths as well as megacryts of alkali feldspar and xenoliths of felsic rocks. The studied set of rocks comprises wehrlite, clinopyroxenites, gabbro, anorthosite, and anorthoclasite. Wehrlite contains pseudomorphs after biotite, the Mg# of clinopyroxene varies from 0.78 to 0.81, the Fo content in olivine is 0.68-0.71. Clinopyroxenites have cumulative textures and are typically olivine± sulfides bearing. Most of them contained biotite which is now replaced by brownish aggregates formed of chlorite with scarce biotite intergrowths. The Mg# of clinopyroxene (Al, Ti - augite) varies from 0.77 to 0.84. The Fo content in olivine is 0.81-0.85 in plagioclase-free clinopyroxenites, but in xenolith where minor amounts of plagioclase (Ab48-51An47-48) occur, the Fo content is 0.70 - 0.72. Biotite's Mg# is ~70%. Gabbro is titanite-bearing and contains trace amounts of amphibole. Diopside forming the gabbro is characterized by Mg#=0.56-0.64, plagioclase is potassium-free (Ab14-22An77-86). Anorthosite also encloses brownish post-biotitic aggregates. Plagioclase has composition of Ab35-43An54-64. Anorthoclasite (Or65-72 Ab65-72) is characterized by unusual mineral composition - it contains corundum, zircon, apatite, and niobates. Previous study on the felsic xenoliths from Scotland showed their lower crustal origin, but with possible ultramafic affinity (e.g. Upton et al., 2009, Min.Mag., 73, 943-956). Crystallization from met- and peraluminous melts was also suggested. Mantle-derived xenoliths from Scotland are from almost primitive to strongly depleted (Upton et al.; 2010, J. Geol. Soc. London, 168, 873-886), but more data from individual localities are necessary for precise description and interpretation of mantle and lower crustal processes beneath Scotland. This study was possible thanks to

  9. Petrology of gabbroic xenoliths in 1960 Kilauea basalt: crystalline remnants of prior (1955) magmatism

    USGS Publications Warehouse

    Fodor, R.V.; Moore, R.B.

    1994-01-01

    The 1960 Kapoho lavas of Kilauea's east rift zone contain 1-10 cm xenoliths of olivine gabbro, olivine gabbro-norite, and gabbro norite. Textures are poikilitic (ol+sp+cpx in pl) and intergranular (cpx+pl??ol??opx). Poikilitic xenoliths, which we interpret as cumulates, have the most primitive mineral compositions, Fo82.5, cpx Mg# 86.5, and An80.5. Many granular xenoliths (ol and noritic gabbro) contain abundant vesicular glass that gives them intersertal, hyaloophitic, and overall 'open' textures to suggest that they represent 'mush' and 'crust' of a magma crystallization environment. Their phase compositions are more evolved (Fo80-70, cpx Mg# 82-75, and An73-63) than those of the poikilitic xenoliths. Associated glass is basaltic, but evolved (MgO 5 wt%; TiO2 3.7-5.8 wt%). The gabbroic xenolith mineral compositions fit existing fractional crystallization models that relate the origins of various Kilauea lavas to one another. FeO/MgO crystal-liquid partitioning is consistent with the poikilitic ol-gabbro assemblage forming as a crystallization product from Kilauea summit magma with ???8 wt% MgO that was parental to evolved lavas on the east rift zone. For example, least squares calculations link summit magmas to early 1955 rift-zone lavas (???5 wt% MgO) through ???28-34% crystallization of the ol+sp+cpx+pl that comprise the poikilitic ol-gabbros. The other ol-gabbro assemblages and the olivine gabbro-norite assemblages crystallized from evolved liquids, such as represented by the early 1955 and late 1955 lavas (???6.5 wt% MgO) of the east rift zone. The eruption of 1960 Kapoho magmas, then, scoured the rift-zone reservoir system to entrain portions of cumulate and solidification zones that had coated reservoir margins during crystallization of prior east rift-zone magmas. ?? 1994 Springer-Verlag.

  10. Carbonaceous xenoliths from the Erevan howardite

    NASA Technical Reports Server (NTRS)

    Nazarov, M. A.; Brandstaetter, F.; Kurat, G.

    1993-01-01

    Preliminary studies of the Erevan howardite showed that the meteorite is a polymict breccia. We report on our study of CM-type carbonaceous xenoliths. All of these clasts are enriched in tochilinite and carbonate inclusion as compared to CM chondrites. They also contain a new, P-rich sulphide beside pentlandite. The P-rich sulphide represents a new type of P-bearing phases. It indicates a chalcophile behavior of P under certain nebular conditions.

  11. Xenoliths of mafic/ultramafic igneous rocks as carriers of information on lower crust beneath Złotoryja - Jawor volcanic complex (SW Poland).

    NASA Astrophysics Data System (ADS)

    Dajek, Michał; Matusiak-Malek, Magdalena; Puziewicz, Jacek; Lipa, Danuta; Ntaflos, Theodors

    2016-04-01

    The Cenozoic alkaline volcanic rocks in Lower Silesia (SW Poland) are known for their mantle peridotite xenoliths. However, the mafic and ultramafic xenoliths with cumulative textures and of composition of olivine- or hornblende clinopyroxenite, clinopyroxenite, websterite, norite and gabbro occur in some of the lavas (6 sites) of the Złotoryja-Jawor volcanic complex. The xenoliths are anhydrous, only in Wilcza Góra minor amount of amphibole occurs. The Mg# of clinopyroxene varies from 0.54 (Ostrzyca Proboszczowicka clinopyroxenite) to 0.89 (Góra Świątek clinopyroxenite). Forsterite content in olivine varies from 64% (Winna Góra gabbro) to 86% (Wilcza Góra hornblende clinopyroxenite). Anortite content in plagioclase in nortite and gabbros is 33-56%. The Mg# in amphibole is 0.43 to 0.76. Clinopyroxene trace element composition is typically LREE enriched, but in Wilcza Góra norite and Mnisza Góra clinopyroxenite it is LREE-depleted. The calculated pressures of clinopyroxene crystallization (calculated by the algorithm of Nimis and Ulmer, 1998, CMP, 1998, 122-135, assuming all Fe to be 2+) is from 0.45 to 0.96 GPa pointing to crystallization of the pyroxenitic rocks in lower crust or at crust/mantle boundary. Theoretical melts in equilibrium with clinopyroxene enriched in LREE resemble the alkaline lavas from the area and we suggest they are cognate with host magmas. We explain variations in composition of mafic xenoliths from Wilcza Góra, Winna Góra and Grodziec to be a result of magma fractionation. Xenoliths containing clinopyroxene impoverished in LREE may represent lithologies inherited from Variscan oceanic crust. Megacrysts of clinopyroxene present in some of the localities cannot result from disintegration of mafic xenoliths This study was possible thanks to project NCN UMO-2014/15/B/ST10/00095 of Polish National Centre for Science.

  12. Geochemical constraints on the origin of serpentinization of oceanic mantle

    NASA Astrophysics Data System (ADS)

    Li, Z.; Lee, C. A.

    2004-12-01

    The lower seismic zone of double seismic zones in subducting oceanic lithosphere is suggested to be a result of serpentine or chlorite dehydration in the lithospheric mantle (Hacker et al., 2003). However, the mechanism by which oceanic lithospheric mantle is serpentinized is unclear. One way is through hydrothermal circulation where the lithospheric mantle represents part of the circuit through which seawater passes and then returns to the ocean. Another way is to inject seawater into the lithospheric mantle through fractures in the overlying crust without having a return path of water to the ocean. The two mechanisms differ in that the former is an open system process whereas the latter is a closed system process in which the mantle serves as a ¡°sponge¡± for water. Identifying the dominant process is important. For example, if the mantle is part of a hydrothermal circulation cell, the interaction of seawater with the mantle will influence the composition of seawater. This also has important implications for the heat flow out of seafloor. On the other hand, if serpentinization occurs by a closed system process, there will be no influence on seawater composition. Previous studies have suggested that serpentinization of ophiolite bodies was an isochemical process, hence closed system, but it was not clear in these studies whether serpentinization occurred in situ in the oceanic lithosphere. To better understand serpentinization processes in the oceanic lithosphere, we investigated a continuous transition zone of relatively unaltered harzburgite to completely serpentinized harzburgite in the Feather River Ophiolite in northern California. These samples are highly enriched in Na, K, Rb, Cs, U, and Sr, which strongly suggests that serpentinization occurred while the oceanic lithosphere was beneath the ocean. All samples (n=19) have Al2O3 contents ranging from 0.6 to 2.5 wt.% and have extremely depleted light rare-earth element abundances, indicating that these

  13. Eclogites, pyroxene geotherm, and layered mantle convection.

    PubMed

    Basu, A R; Ongley, J S; Macgregor, I D

    1986-09-19

    Temperatures of equilibration for the majority (81 percent) of the eclogite xenoliths of the Roberts Victor kimberlite pipe in South Africa range between 1000 degrees and 1250 degrees C, falling essentially on the gap of the lower limb of the subcontinental inflected geotherm derived from garnet peridotite xenoliths. In view of the Archean age (>2.6 x 10(9) years) of these eclogites and their stratigraphic position on the geotherm, it is proposed that the inflected part of the geotherm represents the convective boundary layer beneath the conductive lid of the lithospheric plate. The gradient of 8 Celsius degrees per kilometer for the inflection is characteristic of a double thermal boundary layer and suggests layered convection rather than whole mantle convection for the earth. PMID:17843357

  14. Eclogites, pyroxene geotherm, and layered mantle convection.

    PubMed

    Basu, A R; Ongley, J S; Macgregor, I D

    1986-09-19

    Temperatures of equilibration for the majority (81 percent) of the eclogite xenoliths of the Roberts Victor kimberlite pipe in South Africa range between 1000 degrees and 1250 degrees C, falling essentially on the gap of the lower limb of the subcontinental inflected geotherm derived from garnet peridotite xenoliths. In view of the Archean age (>2.6 x 10(9) years) of these eclogites and their stratigraphic position on the geotherm, it is proposed that the inflected part of the geotherm represents the convective boundary layer beneath the conductive lid of the lithospheric plate. The gradient of 8 Celsius degrees per kilometer for the inflection is characteristic of a double thermal boundary layer and suggests layered convection rather than whole mantle convection for the earth.

  15. Carbon isotope and abundance systematics of Icelandic geothermal gases, fluids and subglacial basalts with implications for mantle plume-related CO2 fluxes

    NASA Astrophysics Data System (ADS)

    Barry, P. H.; Hilton, D. R.; Füri, E.; Halldórsson, S. A.; Grönvold, K.

    2014-06-01

    We report new carbon dioxide (CO2) abundance and isotope data for 71 geothermal gases and fluids from both high-temperature (HT > 150 °C at 1 km depth) and low-temperature (LT < 150 °C at 1 km depth) geothermal systems located within neovolcanic zones and older segments of the Icelandic crust, respectively. These data are supplemented by CO2 data obtained by stepped heating of 47 subglacial basaltic glasses collected from the neovolcanic zones. The sample suite has been characterized previously for He-Ne (geothermal) and He-Ne-Ar (basalt) systematics (Füri et al., 2010), allowing elemental ratios to be calculated for individual samples. Geothermal fluids are characterized by a wide range in carbon isotope ratios (δ13C), from -18.8‰ to +4.6‰ (vs. VPDB), and CO2/3He values that span eight orders of magnitude, from 1 × 104 to 2 × 1012. Extreme geothermal values suggest that original source compositions have been extensively modified by hydrothermal processes such as degassing and/or calcite precipitation. Basaltic glasses are also characterized by a wide range in δ13C values, from -27.2‰ to -3.6‰, whereas CO2/3He values span a narrower range, from 1 × 108 to 1 × 1012. The combination of both low δ13C values and low CO2 contents in basalts indicates that magmas are extensively and variably degassed. Using an equilibrium degassing model, we estimate that pre-eruptive basaltic melts beneath Iceland contain ∼531 ± 64 ppm CO2 with δ13C values of -2.5 ± 1.1‰, in good agreement with estimates from olivine-hosted melt inclusions (Metrich et al., 1991) and depleted MORB mantle (DMM) CO2 source estimates (Marty, 2012). In addition, pre-eruptive CO2 compositions are estimated for individual segments of the Icelandic axial rift zones, and show a marked decrease from north to south (Northern Rift Zone = 550 ± 66 ppm; Eastern Rift Zone = 371 ± 45 ppm; Western Rift Zone = 206 ± 24 ppm). Notably, these results are model dependent, and selection of a lower

  16. Contrasting styles of Pre-Cenozoic and mid-Tertiary crustal evolution in northern Mexico: Evidence from deep crustal xenoliths from La Olivina

    SciTech Connect

    Cameron, K.L.; Robinson, J.V.; Kuentz, D.C.; Collerson, K.D.; Niemeyer, S.; Harmon, R.S.; Bohlen, S.R.

    1992-11-10

    The principal deep crustal rock types found at the La Olivina xenolith locality in southeastern Chihuahua, Mexico, are mafic granulites, paragneisses, and intermediate- to silicic-composition orthogneisses. These granulite facies xenoliths are interpreted in terms of two age groups, pre-Cenozoic and mid-Tertiary, based on previous ion probe dating of zircons from the xenoliths and on isotopic comparisons of the xenoliths to rocks of known age. The mafic granulites have Pb, Nd, and Sr isotopic compositions identical to those of Oligocene volcanic rocks from the La Olivina region. Compositionally, they are olivine-normative gabbroic cumulates, and they precipitated from two or more mid-Tertiary basalt to dacite or rhyolite assimilation/fractional crystallization series. Mineral assemblages in the xenoliths record pressures of {le} 7.2 kbar or depths of < 25 km. If these are the maximum pressures the rocks experienced and if the crust was > 35 km thick in Oligocene time as inferred from regional tectonic considerations, then the mafic granulites cannot be samples of basaltic magmas underplated near the crust-mantle boundary. The cumulate protoliths for the mafic granulites probably formed in magma chambers well above the Moho. The mafic granulites are plausibly ignimbrite fields of Mexico. Pre-Cenozoic and mid-Tertiary crustal evolution followed very different paths in northern Mexico. For example, Nd isotopic evidence for crustal recycling is much more evident in rocks associated with the Paleozoic convergence than in rocks produced during mid-Tertiary magmatism. Furthermore, mafic rocks are very rare in the pre-Cenozoic xenolith suite, but they dominate the mid-Tertiary one. The rarity of pre-Cenozoic mafic xenoliths suggests that Proterozoic and Paleozoic lower crust may have delaminated in response to crustal overthickening associated with the Ouachita collision event. 97 refs., 14 figs., 5 tabs.

  17. U-Pb Thermochronology of lower crustal xenoliths: creating a temporal record of lithosphere thermal evolution

    NASA Astrophysics Data System (ADS)

    Blackburn, T.; Bowring, S. A.; Schoene, B.; Dudas, F. O.; Mahan, K. H.

    2009-12-01

    Lower crustal xenoliths often contain U-bearing accessory minerals (titanite, apatite, rutile) with a range of closure temperatures for Pb-diffusion (~400-700 °C). These minerals record a time-temperature path which can be then be related to the thermal history of the underlying lithosphere mantle from igneous crystallization through multiple periods of metamorphism and thermal relaxation. In this study, we constrain the age and evolution of lithospheric thermal structure of Medicine Hat Block beneath Montana using geochronology, thermochronology, and thermal/diffusion modeling of lower crustal xenoliths. The cooling history of individual xenoliths is recorded by the Pb diffusion profiles within xenolithic rutile and titanite grains. Pb diffusion profiles can be inferred by making multiple measurements of grains with varying diffusion domain sizes (grain size). These analyses yield a dispersion in measured U-Pb dates that when plotted on a U-Pb Concordia diagram define a linear array. Xenoliths derived from depths between 20 and 30km have Proterozoic upper intercept dates and lower intercepts 700-900 Ma younger. This extreme discordance indicates a loss of equilibrium within the U-Pb system that could potentially be interpreted as the result of two different t-T paths: 1) reheating and 2) slow-cooling. To test the validity of each model, finite-difference approximations to the diffusion equation allow us to determine Pb diffusion profiles, and thus modeled U-Pb dates for any pre-assumed t-T path. Forward modeling of slow-cooling and reheating t-T paths reveals that each t-T path has a very unique data topology that allows us to distinguish between extreme examples of each case. This technique has been applied to accessory minerals from xenoliths from the Sweet Grass Hills, Montana where an Eocene minette that has exhumed lower crustal xenoliths from the Archean Medicine Hat Block. Rutile U-Pb thermochronologic analyses from three xenoliths that record metamorphic

  18. Geochemistry of Eclogite Xenoliths from Kimberlite Pipe Udachnaya

    NASA Astrophysics Data System (ADS)

    Agashev, Aleksey; Pokhilenko, Ludmila; Pokhilenko, Nikolai

    2016-04-01

    A suite of 17 unique big (1 to 20 kg) and fresh ecligite xenoliths from Udachnaya kimberlite pipe have been studied for their whole-rock and minerals major and trace elements composition.Whole rock major elements composition of the Udachnaya eclogite xenoliths suite have a great variability in their MgO contents (9-19Wt%). Based on major elements composition Udachnaya eclogites can be subdivided in two subsets, high magnesian (Mg# 68.8-81.9) and low magnesian (Mg# 56.8-59). High variations also shown by Al2O3 and Na2O concentrations and high Mg# samples tend to contain less of those oxides then low Mg# samples with some exceptions. Two eclogitic groups are clearly different in style of inter-elements correlations. FeO and CaO contents are positively correlate with MgO in low Mg# group of eclogites but negatively in high Mg# group. The same relations present between Al2O3 contents of eclogite group with their Mg#. Compared to present day MORB composition eclogite samples have similar contents of most of elements with some depletion in TiO2 and P2O5 and enrichment in MgO and K2O. The variability of these elements concentrations can be related to melt extraction while elevated K2O can indicate late metasomatic enrichment. In terms of trace elements composition Udachnaya eclogites are enriched over PM but comparable to that of MORB composition, except significant enrichment in LILE elements (Rb, Ba, K, Sr). The records of both subduction related processes and mantle metasomatism could be find in geochemical features of these rocks. Most of the eclogites show positive Eu anomaly which is direct evidence of plagioclase accumulation in eglogites protolith. Variation of La/Yb ratio (1-11), in majority of samples are the range 2-4 indicates different degrees of samples metasomatic enrichment in LREE. Udachnaya eclogites have range of Sm/Nd ratio from 0.25 to 0.5 (MORB is 0.32) which positive covariates with Nd content. This trend could not be a result of melt extraction nor

  19. Xenoliths of Phl- Ol breccia and lamproite- like dykes in Udachanaya pipe

    NASA Astrophysics Data System (ADS)

    Ashchepkov, Igor; Ntaflos, Theodoros; Samsonov, Gerasim; Mityukhin, Sergei; Khmelnikova, Olga

    2013-04-01

    Several xenoliths of the unusual fine grained essentially Phl-Ol rocks there were found in the Udachnaya Vostochnaya kimberlite pipe. There two varieties. The fist type ids composed mainly from Phl, and Ol and Chromites with Phl-Ol, Chromite while the other contain more ilmenites and phlogopites and rare. Among the xenocrysts the rounding Ol xenocrysts and mantle xenoliths including dunite, sheared peridotites, Mg- ilmenites, prevail. In the second type Ol -pyroxenites and xenocrysts of low Cr garnets occur (surrounded by thick kelyphite). The first type contain Ol (Fe#~15), Na -Ti-rich kersutite and low. In intergranular space Sr -apitite, Ba selestine as well as barites. Sulfides are pyrotites . chalcopyrite, pentlandite. Such a mineralogy are typical of the lamproite but the bulk rock composition gives relatively low Na (1.3%) and K (2.8%) and Al2O3 contents with ~32% of MgO and 11 % Fe due to abundant Ol xenocrysts. This allow to interpreted rock Ol breccia cemented by the lamprorite magma. the Ol xenocryst are zonal with Mg - rich cores 8-10% and Fe(14%) rims. The Chromite xenocrysts and phenocrsts show 50 -22 % Cr2O3 compositional range. and pressure range 5.0 -2.0 GPa The second type represent the zonal rocks with the Cpx outer xone the essentally Phl zone and then fine grained material of Phl. Ol, Cr-low Cpx, and ilmenites. Variation o Cpx from the contact to the inner part show an essential increase of Fe (4 - 7 %) TiO2 And Al2O3 as well as CaO suggesting the crystallization differentiation of magma. Starting parameters of the xenocrysts crystallization of the Ilm-Phl-Ol rock refer to the basement of the lithosphere beneath Udachnaya ~6.0 -5.5 GPa. But the final stage of crystallization was near Moho 1.2 -1.5 GPa according to Cpx barometry Ashchepkov et al ., 2011. and T 1100-1250 oC. The Ol-Chr sample possibly was crystallizing upper. The trace elements were measured by the LAM ICP MS method in IGM SD RAS using Finnigan ELEMENT with the YAG Nd 266 Laser

  20. Water in the Oceanic Lithosphere: Salt Lake Crater Xenoliths, Oahu, Hawaii

    NASA Technical Reports Server (NTRS)

    Peslier, Anne H.; Bizimis, Michael

    2010-01-01

    Water can be present in nominally anhydrous minerals of peridotites in the form of hydrogen bonded to structural oxygen. Such water in the oceanic upper mantle could have a significant effect on its physical and chemical properties. However, the water content of the MORB source has been inferred indirectly from the compositions of basalts. Direct determinations on abyssal peridotites are scarce because they have been heavily hydrothermally altered. Here we present the first water analyses of minerals from spinel peridotite xenoliths of Salt Lake Crater, Oahu, Hawaii, which are exceptionally fresh. These peridotites are thought to represent fragments of the Pacific oceanic lithosphere that was refertilized by alkalic Hawaiian melts. A few have unradiogenic Os and radiogenic Hf isotopes and may be fragments of an ancient (2 Ga) depleted and recycled lithosphere. Water contents in olivine (Ol), orthopyroxene (Opx), and clinopyroxene (Cpx) were determined by FTIR spectrometry. Preliminary H_{2}O contents show ranges of 8-10 ppm for Ol, 151-277 ppm for Opx, and 337-603 ppm for Cpx. Reconstructed bulk rock H_{2}O contents range from 88-131 ppm overlapping estimates for the MORB source. Water contents between Ol minerals of the same xenolith are heterogeneous and individual OH infrared bands vary within a mineral with lower 3230 cm^{-1} and higher 3650-3400 cm^{-1} band heights from core to edge. This observation suggests disturbance of the hydrogen in Ol likely occurring during xenolith entrainment to the surface. Pyroxene water contents are higher than most water contents in pyroxenes from continental peridotite xenoliths and higher than those of abyssal peridotites. Cpx water contents decrease with increasing degree of depletion (e.g. increasing Fo in Ol and Cr# in spinel) consistent with an incompatible behavior of water. However Cpx water contents also show a positive correlation with LREE/HREE ratios and LREE concentrations consistent with refertilization. Opx water

  1. The Role of the Mantle Lithosphere in Continent Stability

    NASA Astrophysics Data System (ADS)

    Carlson, R. W.; Ancuta, L. D.; Fouch, M. J.; Idleman, B. D.; Ionov, D. A.; James, D. E.; Meltzer, A.; Pearson, G.; Shirey, S. B.; Zeitler, P. K.

    2012-12-01

    Most Archean cratons are underlain by up to 200 km thick sections of mantle characterized by high seismic velocities. Xenoliths from cratonic mantle lithosphere show them to consist of refractory peridotites that are the residues of very high degrees of partial melt removal leaving the majority with less than 2% Al2O3. The partial melt removal leaves the lithospheric mantle compositionally buoyant, strong, and with very little internal radioactive heat generating capacity so that even after cooling it contributes to the strength, longevity, and relative geologic inactivity of the overlying crust. Re-Os studies, particularly in the Kaapvaal Craton of southern Africa, show a strong correspondence between the ages of melt depletion of the cratonic mantle and significant crust building events. The main age peak in the Kaapvaal lithospheric mantle is 2.9 Ga, coincident with assembly of the western and eastern blocks of the craton. The only significant disruption to this age pattern is seen below the 2 Ga Bushveld intrusion where the mantle lithosphere is characterized by slower seismic velocities and xenolith ages closer to 2 than 3 Ga. The surrounding Proterozoic mobile belts have even slower seismic velocities and xenolith ages generally less than 1.5 Ga. An interesting contrast to this picture of cold, old, stable cratonic lithosphere is that displayed by central Mongolia. This area, more or less in the middle of the huge Asian continental plate, is far removed from plate boundary processes yet in the Hangay Mountains shows elevations approaching 4 km along with extensive late Cenozoic basaltic volcanism. In contrast to cratonic lithosphere, mantle xenoliths from the Hangay region are dominantly fertile peridotite. Fifty-six percent of a large collection of peridotites from 4 Mongolian localities have more than 3.5% Al2O3 and only 4% have Al2O3 contents of less than 2%. Cenozoic basalts from the region have subchondritic 143Nd/144Nd and MORB-like He isotopic

  2. Birch's Mantle

    NASA Astrophysics Data System (ADS)

    Anderson, D. L.

    2002-12-01

    Francis Birch's 1952 paper started the sciences of mineral physics and physics of the Earth's interior. Birch stressed the importance of pressure, compressive strain and volume in mantle physics. Although this may seem to be an obvious lesson many modern paradoxes in the internal constitution of the Earth and mantle dynamics can be traced to a lack of appreciation for the role of compression. The effect of pressure on thermal properties such as expansivity can gravitational stratify the Earth irreversibly during accretion and can keep it chemically stratified. The widespread use of the Boussinesq approximation in mantle geodynamics is the antithesis of Birchian physics. Birch pointed out that eclogite was likely to be an important component of the upper mantle. Plate tectonic recycling and the bouyancy of oceanic crust at midmantle depths gives credence to this suggestion. Although peridotite dominates the upper mantle, variations in eclogite-content may be responsible for melting- or fertility-spots. Birch called attention to the Repetti Discontinuity near 900 km depth as an important geodynamic boundary. This may be the chemical interface between the upper and lower mantles. Recent work in geodynamics and seismology has confirmed the importance of this region of the mantle as a possible barrier. Birch regarded the transition region (TR ; 400 to 1000 km ) as the key to many problems in Earth sciences. The TR contains two major discontinuities ( near 410 and 650 km ) and their depths are a good mantle thermometer which is now being exploited to suggest that much of plate tectonics is confined to the upper mantle ( in Birch's terminology, the mantle above 1000 km depth ). The lower mantle is homogeneous and different from the upper mantle. Density and seismic velocity are very insensitive to temperature there, consistent with tomography. A final key to the operation of the mantle is Birch's suggestion that radioactivities were stripped out of the deeper parts of

  3. A radiogenic Os component in the oceanic lithosphere? Constraints from Hawaiian pyroxenite xenoliths

    NASA Astrophysics Data System (ADS)

    Sen, Indra Sekhar; Bizimis, Michael; Sen, Gautam; Huang, Shichun

    2011-09-01

    Platinum Group Element (PGE) concentrations in garnet pyroxenite xenoliths from Oahu, Hawaii, are significantly lower than those in mantle peridotites and show fractionated patterns (e.g. Pd N/Os N = 2-10, Pd N/Ir N = 4-24; N = chondrite normalized) and very high Re N/Os N ratios (˜9-248). Mass balance calculations show that the bulk rock pyroxenite PGE inventory is controlled by the presence of sulfide phases. The 187Os/ 188Os ratios of these pyroxenites vary from subchondritic to suprachondritic (0.123-0.164); and the 187Os/ 188Os ratios show good correlations with bulk rock and clinopyroxene major and trace element compositions, and bulk rock PGE and sulfur abundances. These observations suggest that the Os isotope compositions in these pyroxenites largely reflect primary processes in the oceanic mantle and Pacific lithosphere. In contrast, bulk rock 187Os/ 188Os ratios do not correlate with other lithophile isotopic tracers (e.g. Rb-Sr, Sm-Nd, Lu-Hf) which show limited isotopic variability ( Bizimis et al., 2005). This and the lack of 187Os/ 188Os vs. Re/Os correlations suggest that the range in Os isotope ratios is not likely the result of mixing between long-lived depleted and enriched components or aging of these pyroxenites within the Pacific lithosphere after its formation at a mid-oceanic ridge setting some 80-100 million years ago. We interpret the Os isotopes, PGE and lithophile element systematics as the result of melt-lithosphere interaction at the base of the Pacific lithosphere. The major and trace element systematics of the clinopyroxenes and bulk rock pyroxenites and the relatively constant lithophile element isotope systematics are best explained by fractional crystallization of a rather homogenous parental magma. We suggest that during melt crystallization and percolation within the lithosphere, the parental pyroxenite melt assimilated radiogenic Os from the grain boundaries of the peridotitic lithosphere. This radiogenic Os component may

  4. Fractionation of zirconium and hafnium during processes of mantle metasomatism

    NASA Astrophysics Data System (ADS)

    Kogarko, L. N.

    2016-06-01

    For the first time, fractionation of zirconium and hafnium in carbonatized mantle xenoliths from the eastern Antarctic has been studied. An elevation relative to the chondrite values of Zr/Hf in the metasomatized xenoliths has been revealed. The main reactions of the carbonate metasomatism lead to replacement of primary orthopyroxene by secondary clinopyroxene. A substantial broadening of the clinopyroxene crystallization field results in an increase of Zr/Hf in an equilibrated melt due to a higher partition coefficient of Hf in clinopyroxene, relative to that of Zr. Migration of reaction-active carbonate and carbonate-silicate melts, equilibrated to metasomatic wehrlite, causes an increase in the Zr/Hf value in the carbonatized mantle substrate.

  5. Chemical Exchange Between the Core and the Convecting Mantle of the Earth: Evidence from Highly Siderophile Elements (HSE)

    NASA Astrophysics Data System (ADS)

    Schmidt, G.; Palme, H.; Kratz, K. L.

    1995-09-01

    Core formation is a major physical and chemical event in the evolution of a differentiated planet. The core is the dominant repository of HSE in the Earth. Element ratios of HSE in peridotites provide insights into the accretion processes of the Earth and the effect of core formation. Depletion of HSE in the Earth's mantle results from core formation. Refractory siderophile elements are about a factor of > 100 depleted in the Earth's mantle compared to CI carbonaceous chondrites. Nevertheless, the concentrations of PGE, Re and Au (7.1 +/- 0.8 x 10^-3 CI chondrite abundances) are higher than would be expected from metal-silicate partitioning during core formation [1]. Several different explanations have been suggested to explain the low absolute abundances of these elements. (1) Os, Re, Ir, Ru, Rh, Pd, Pt, and Au were added with a late chondritic veneer containing less than 1% of a CI component [2-9]. (2) Insufficient core formation, i.e. some metallic Fe-Ni was retained in the upper mantle during core formation [10]. (3) Disequilibrium during core formation; Segregation of metal from the upper mantle in later stages of accretion was so rapid that equilibrium was not attained [4,11,12]. (4) There was continuous formation of the core during accretion; Equilibrium between sinking metal grains and a molten magma ocean at high temperatures (3000-3500 K) [13]. (5) Increase in silicate/metal partition coefficients by pressure, temperature, or high f(O2) [5,14]; Solution of FeO in the core raises the f(O2) conditions at the core-mantle interface sufficiently to increase the equilibrium concentrations of the siderophile elements in the mantle [15]. Studies of mantle-derived samples such as massif peridotites and peridotite xenoliths provide direct information on the nature and composition of the upper mantle. Massive peridotitic rocks from Zabargad island (Red Sea), Lanzo (Italy), Ronda (Spain) and peridotitic xenoliths from Mongolia were analysed for Os, Re, Ir, Ru, Rh, Pd

  6. Water content and hydrogen behaviour during metasomatism in the uppermost mantle beneath Ray Pic volcano (Massif Central, France)

    NASA Astrophysics Data System (ADS)

    Denis, Carole M. M.; Alard, Olivier; Demouchy, Sylvie

    2015-11-01

    To understand the deep cycle of water, upper mantle water content and distribution between nominally anhydrous minerals (NAMs) and hydrous minerals (e.g., amphibole) must be constrained. We need then to understand H behaviour during mantle melting and metasomatism. Major, minor and trace element compositions including water contents were obtained on ten xenoliths of spinel-bearing peridotites from the Ray Pic locality, in the Southern part of the Massif Central (France). The sample suite investigated here is composed of rather fertile lherzolites (89.4 ≤ Fo ≤ 90.8%; 11.3 ≤ cr# in spinel ≤ 21.1%; 0.942 ≤ [Yb]cpx ≤ 1.90 ppm; cpx: clinopyroxene), which can be best explained by batch melting, with degree of melting between 3 and 10%. These xenoliths contain up to 8% modal amphibole. Three groups are defined as a function of the amphibole modal abundance (above or below 1%) and equilibrium temperature (above or below 1000 °C). Results show no clear positive correlation between modal metasomatism (amphibole) and incompatible element enrichment in cpx. Trace element compositions in cpx show strong enrichments of the most incompatible elements (e.g., (La/Sm)PM as high as 15.7; PM: normalised to primitive mantle values), but strong negative anomalies of the high field strength elements (e.g., (Th/Nb)PM as high as ~ 680). Such trace element fractionations are usually ascribed to the so-called carbonatitic metasomatism involving the percolation of small volume melts which are enriched in volatiles. The hydrogen concentrations in cpx range from 203 to 330 ppm wt. H2O, in orthopyroxene from 66 to 160 ppm wt. H2O and in olivine from 2 to 6 ppm wt. H2O. These values are within the common concentration range of other spinel-bearing peridotites. Amphiboles contain 1.9 ± 0.5 wt.% of H2O. The effect of metasomatism on water abundances in NAMs is not straightforward. Hydrous metasomatism (i.e., leading to the crystallisation of OH-bearing amphibole) has no effect on the

  7. Silica- and LREE-enriched spinel peridotite xenoliths from the Quaternary intraplate alkali basalt, Jeju Island, South Korea: Old subarc fragments?

    NASA Astrophysics Data System (ADS)

    Woo, Yonghoon; Yang, Kyounghee; Kil, Youngwoo; Yun, Sung-Hyo; Arai, Shoji

    2014-11-01

    Spinel harzburgite to lherzolite xenoliths are entrapped in Quaternary intraplate alkali basalts on Jeju Island, South Korea. These xenoliths are unusual in containing late-stage secondary orthopyroxene, free of deformation and exsolution that is replacing olivine as the main pervasive metasomatic mineral. These xenoliths are characterized by high Mg# in olivine, orthopyroxene, and clinopyroxene (89-93) and variable Cr# of spinel (9-53), representing residues left after variable degrees of melt extraction (~ 25%). In contrast to their depleted major-element compositions, clinopyroxenes in the xenoliths are enriched in most incompatible trace elements. Clinopyroxenes display enrichment in light rare earth elements (LREE) or spoon-shaped REE with a general enrichment in La over Ce, and depletion in high field strength elements (HFSE; e.g., Nb-Ta, Zr-Hf, Ti). Orthopyroxenes (either primary or secondary) are characterized by low TiO2, high Al2O3, and moderate CaO contents, and resemble those of sub-continental arc peridotites from the eastern Pacific. The geochemical evidence, in addition to the formation of secondary orthopyroxene, indicates that Jeju peridotite xenoliths have been subjected to different degrees of metasomatism by subduction-related silica- and LREE-enriched fluids (or melts). However, chemical equilibrium is evident between the primary and secondary orthopyroxene, implying that the duration of post-metasomatic high temperatures enabled complete resetting/reequilibration of the mineral compositions. The metasomatic enrichment pre-dates the host Jeju Quaternary magmatism, and a genetic relationship with the host magmas is considered unlikely. We therefore propose that the Jeju peridotite xenoliths went through a two-stage evolution, with their composition primarily controlled by early fractional melt extraction, which was subsequently modified by residual slab-derived fluids (or melts). Following enrichment in the peridotite protolith in the mantle

  8. Long-term cycling of mantle Pb: A trace element study of the major mantle mineral phases in abyssal peridotites

    NASA Astrophysics Data System (ADS)

    D'Errico, M. E.; Warren, J. M.; Godard, M.; Ildefonse, B.

    2012-12-01

    Peridotites from ultraslow-spreading ridges preserve signatures of the depleted mantle, while also reflecting the fine scale compositional variability present in the mantle. Traditional analyses of these depleted rocks have focused on clinopyroxene, the main trace element host in spinel peridotites. However, key isotopic systems, such as lead and osmium, are hosted in other phases at low but significant concentration levels. The amount of lead contained within mantle mineral phases is of critical importance to understanding the long-term evolution of the Earth, because the radiogenic isotopes of lead are sensitive to past material cycling and melt-rock interaction. Sulfides have long been suggested as the main host for lead (Pb) in the mantle, but recent studies have demonstrated that Pb is not exclusively hosted in this trace phase. Therefore, the Pb contents of the major peridotite mineral phases (olivine, orthopyroxene, and clinopyroxene) need to be reassessed. Lead concentration data is available for orogenic and xenolith peridotite samples, which are typically more enriched than abyssal peridotites, but these do not provide direct information on the oceanic upper mantle. Direct measurement of Pb in abyssal peridotites has so far been limited because of its extremely low concentration (often <1 ppm). We report Pb and other trace element concentration data for peridotite phases determined by in-situ laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). The LA-ICP-MS technique achieves high spatial resolution combined with detection of low elemental abundances. External precision varied from 6% to 17%, with a precision of 6% for Pb, based on 14 repeat analyses of BIR-1G standard basalt glass. Laser spot size varied from 102-163 microns, which produced a detection limit of 0.42-0.81 ppb for Pb. This study focused on abyssal peridotites from the ultra-slow spreading Gakkel and Southwest Indian Ridges (SWIR), with samples coming from segments with

  9. An experimental study of Li partitioning between olivine and diopside at mantle conditions

    NASA Astrophysics Data System (ADS)

    Yakob, J. L.; Feineman, M. D.; Penniston-Dorland, S. C.; Eggler, D. H.

    2010-12-01

    . 139, 356-371. Brenan, J.M., Ryerson, F.J., Shaw, H.F. (1998b) The role of aqueous fluids in the slab-to-mantle transfer of boron, beryllium, and lithium during subduction: Experiments and models. Geochim. Cosmochim. Acta 62, 3337-3347. Caciagli-Warman, N. (2010) Experimental constraints on lithium exchange between clinopyroxene, olivine and aqueous fluids at high pressures and temperatures. Unpublished PhD thesis, University of Toronto. Ionov, D.A. and Seitz, H.M. (2008) Lithium abundances and isotopic compositions in mantle xenoliths from subduction and intra-plate settings: Mantle sources vs. eruption histories. Earth Planet. Sci. Lett. 266, 316-331. Jeffcoate, A.B., Elliot, T., Kasemann, S.A., Ionov, D., Cooper, K., Brooker, R. (2007) Li isotope fractionation in peridotites and mafic melts. Geochim. Cosmochim. Acta 71, 202-218. Rudnick, R.L. and Ionov, D.A. (2007) Lithium elemental and isotopic disequilibrium in minerals from peridotite xenoliths from far-east Russia: Product of recent melt/fluid-rock reaction, Earth Planet. Sci. Lett. 256, 278-293.

  10. The paradox of a wet (high H2O) and dry (low H2O/Ce) mantle: High water concentrations in mantle garnet pyroxenites from Hawaii

    NASA Astrophysics Data System (ADS)

    Bizimis, M.; Peslier, A. H.

    2013-12-01

    Water dissolved as trace amounts in anhydrous minerals has a large influence on the melting behavior and physical properties of the mantle. The water concentration of the oceanic mantle is inferred from the analyses of MORB and OIB [1], but there is little data from actual mantle samples. Moreover, enriched mineralogies (pyroxenites, eclogites) are thought as important sources of heterogeneity in the mantle, but their water concentrations and their effect on the water budget and cycling in the mantle are virtually unknown. We analyzed by FTIR water concentrations in garnet clinopyroxenite xenoliths from Salt Lake Crater, Oahu, Hawaii. These pyroxenites are high-pressure (>20kb) crystal fractionates from alkalic melts. The clinopyroxenes (cpx) have 260 to 576 ppm wt. H2O, with the least differentiated samples (Mg#>0.8) in the 400-500 ppm range. Orthopyroxene (opx) contain 117-265 ppm H2O, about half of that of cpx, consistent with other natural sample studies, but lower than experimental cpx/opx equilibrium data. These pyroxenite cpx and opx water concentrations are at the high-end of on-and off-craton peridotite xenolith concentrations and megacrysts from kimberites [2] and those of Hawaiian spinel peridotites. In contrast, garnet has extremely low water contents (<5ppm H2O). There is no correlation between water in cpx and lithophile element concentrations. Phlogopite is present in some samples, and its modal abundance shows a positive correlation in Mg# with cpx, implying equilibrium. However, there is no correlation between water concentrations and the presence of phlogopite. These data imply that cpx and opx water concentrations may be buffered by phlogopite crystallization. Reconstructed bulk rock pyroxenite water concentrations (not including phlogopite, i.e. minimum) range from 200-460 ppm (average 331× 75 ppm), significantly higher than water estimates for the MORB source (50-200 ppm), but in the range of E-MORB, OIB and the source of rejuvenated Hawaiian

  11. Large-scale pattern of mantle evolution through rifting in hyper-extended margins

    NASA Astrophysics Data System (ADS)

    Picazo, Suzanne; Müntener, Othmar; Manatschal, Gianreto; Bauville, Arthur

    2016-04-01

    New ideas and concepts have been developed to understand and be able to give a simplified large-scale view of the evolution of the mantle lithosphere in hyper-extended magma-poor rifted margins based on the ancient Alpine Tethys rifted margin. In contrast to the classical assumption assuming a simple, isotropic mantle lithosphere, these new models integrate observations from exposed and drilled mantle rocks and propose that the mantle lithosphere evolved and was modified during an extensional cycle from post-orogenic collapse through several periods of rifting to embryonic oceanic (ultra-) slow seafloor spreading. But it is, at present, unclear how far these ideas can be generalized at Atlantic type rifted margins. In our presentation, we review the available mantle data from dredged samples in the North Atlantic and from ophiolite massifs and xenoliths in preserved and reactivated passive margins i.e. the Alpine Tethys, the Pyrenean domain, and the Dinarides and Hellenides. We revisit the available terminology concerning mantle massifs and xenoliths and compile the available data to identify different mantle domains. We define chemical and petrological characteristics of mantle domains based on clinopyroxene and spinel compositions and compile them on present-day and paleo-geographic maps of Western Europe. Finally we link the observed distribution of mantle domains to the post-Variscan extensional cycle and link domains to processes related to the late post-Variscan extension, the rift evolution and refertilization associated to hyper-extension and the development of embryonic oceanic domains.

  12. Metasomatism in the lithospheric mantle beneath southern Patagonia, Argentina

    NASA Astrophysics Data System (ADS)

    Kolosova-Satlberger, Olesya; Ntaflos, Theodoros; Bjerg, Ernesto

    2015-04-01

    Mantle xenoliths from Gobernador Gregores, southern Patagonia are spinel- lherzolites, harzburgites and wehrlites. Composite xenoliths consisting of websterites, olivine-websterites and spinel- lherzolites or harzburgites are present as well. The lithospheric mantle beneath Gobernador Gregores region was affected by multiple modal metasomatic events as can be inferred by the presence of amphibole, phlogopite and apatite. The existence of amphibole as inclusion in clinopyroxene suggests dehydration reaction of peridotites, which previously experienced modal metasomatism. This textural evidence records the earliest detectable metasomatic event. A second distinct modal metasomatic event consists of disseminated up to 6 mm in diameter coarse grained amphiboles (100*mg# =89.9) which show breakdown reactions and pseudomorphic replacement by glass and fine grained second generation of olivine, clinopyroxene and spinel. The intensity of the breakdown reaction is variable. In most cases amphibole occurs as a relict within these pseudomorphs. However, melt pockets of up to 10 mm in diameter are abundant, irregular in shape and having the same minerals such as in the pseudomorphs, indicate clearly amphibole breakdown because remnants of it were found enclosed by second generation clinopyroxene. Similar breakdown reactions experienced the phlogopite in the samples where is present. The Phlogopite (100*mg# =88.6) breakdown produces the same mineral phases as the amphibole. The second generation minerals formed after breakdown of amphibole and phlogopite show minor differences in their composition. However, the chemical composition of glass varies considerably. The glasses formed after breakdown of amphibole and phlogopite have trachyandesitic and tephriphonolitic composition, respectively. Some harzburgites and composite xenoliths reveal another metasomatic event: peridotite, enriched in orthopyroxene (mainly orthopyroxenite veinlets, rare websterite), suggests interaction with

  13. Shear zones in the upper mantle - relation between geochemical enrichment and deformation in mantle peridotites

    SciTech Connect

    Downes, H. )

    1990-04-01

    Textural variations in mantle-derived spinel peridotites have previously been interpreted as evidence of the existence of asthenospheric mantle diapirs, indicating deformational heterogeneity on a large lateral scale (kilometers to tens of kilometers). However, many volcanic vents entrain both deformed and undeformed xenoliths, and field relations in peridotite massifs show the scale of alternation between deformed and undeformed peridotite to be small (centimeters to meters) because of the presence of numerous lithospheric shear zones. Some rare xenoliths contain both deformed and undeformed peridotite. These is also an apparent relation between deformation and the growth of metasomatic minerals; amphibole is often concentrated in strongly deformed zones in peridotite massifs and deformed xenoliths, although it is also found in crosscutting veins and aureoles around such veins. A relation can also be seen between deformation and indicators of geochemical enrichment. Clinopyroxenes from many deformed spinel peridotites show light rate earth element (REE) enrichment, whereas clinopyroxenes from undeformed spinel peridotites commonly have mid-ocean ridge basalt (MORB)-type light REE-depleted patterns. Sr and Nd radiogenic isotopic compositions of undeformed peridotites are generally MORB-like, and have low {epsilon}Sr and high {epsilon}Nd. In contrast, deformed peridotites, with or without amphibole, often have higher {epsilon}Sr and low {epsilon}Nd values, indicating geochemical enrichment by large ion lithophile (LIL)- and light REE-enriched fluids or melts. These observations can be used to infer that the shallow mantle contains lithospheric ductile shear zones in which metasomatic fluids precipitated amphibole and clinopyroxene.

  14. Extraction and carbon isotope analysis of CO 2 from scapolite in deep crustal granulites and xenoliths

    NASA Astrophysics Data System (ADS)

    Moecher, David P.; Valley, John W.; Essene, Eric J.

    1994-01-01

    Carbon isotope compositions of scapolite from granulite facies gneisses and lower crustal xenoliths document the composition and constrain the source of carbon in scapolite from the lower crust. CO 2 is extracted from scapolite without fractionating carbon isotopes by reaction with phosphoric acid at 25 or 75°C. Thus, partial yields of CO 2 from scapolite are sufficient for accurate carbon isotopic analysis. Isotopic compositions of coexisting scapolite and calcite in high-grade calc-silicate gneisses and marbles, and consideration of the crystal chemical environment of CO 3 in the scapolite structure, indicate little fractionation of 13C /12C between scapolite and calcite (0.1 ± 1.2%.) at equilibrium conditions of 650-800°C. The carbon isotope composition of CO 2 extracted from scapolite in twenty-nine samples of regional granulite facies gneisses, amphibolites, calc-silicate gneisses, and crustal xenoliths yield values of δ 13C that range from -10 to -1%. (PDB). High-grade marbles and graphitic paragneisses are precluded as major sources of carbon for scapolite in the high grade rocks analyzed in this study, as the former are isotopically enriched, and the latter isotopically depleted in 13C /12C relative to the range of isotopic compositions determined here. The δ 13C values for mafic granulites and amphibolites in granulite terranes composed of supracrustal sequences (-10.1 to -4.0%.) may reflect the isotopic composition of diagenetic carbonate present in their basaltic protoliths. The values of δ 13C for scapolite in mafic xenoliths and some granulite facies orthogneisses (-8.2 to -1.2%.) are consistent with crystallization of the scapolite from a mafic melt or derivation of CO 2 from mafic melts emplaced in the lower crust or upper mantle. The values of δ 13C for scapolites from calc-silicate gneisses and calc-silicate xenoliths (-10.0 to -2.9%.) may result from depletion of 13C /12C as a result of decarbonation of calcite-bearing protoliths during

  15. The Evolution of Floreana Island, Galapagos: Mantle Metasomatism as a Control of Structural and Geochemical Variations

    NASA Astrophysics Data System (ADS)

    Koleszar, A. M.; Rollins, N. A.; Harpp, K. S.; Geist, D. J.

    2004-05-01

    Floreana, the 6th largest island in the Galapagos Archipelago, is situated ESE of the current proposed location of the hotspot, believed to be near Fernandina Island. Floreana is the most distant Galapagos volcano from the Galapagos Spreading Center and is located on 12 Ma lithosphere. Both normally- and reversely-polarized flows are present on Floreana, which emerged more than 1 Ma. The emergent shield is constructed of lava flows and >80 cinder cones. In the final stage of island building, approximately 0.3 Ma, the eruptive activity on Floreana became more explosive and produced the largest cinder cones on the island. Spatter ramparts, cinder cones, vents, and pit craters are arranged in at least 3 major parallel to sub-parallel alignments oriented N40E. The basalts of Floreana are notably alkalic, primitive, and highly enriched in incompatible trace elements (ITE). MgO concentrations in the lavas range from <8 wt% to >13 wt%, and many of the magmas are likely related by fractional crystallization of olivine and clinopyroxene. The volcano has erupted ultramafic xenoliths, which are observed predominantly in the older, reversely-polarized flows and cones. Floreana lavas have the greatest light REE enrichment observed in the archipelago and the most radiogenic Sr- and Pb- isotopic ratios, indicative of an ITE-enriched source. Elevated ratios of alkali and alkaline earth contents to those of high-field strength elements indicate contributions from metasomatic fluids to Floreana melts. Although the effects of metasomatism are apparent in most Floreana basalts, normally-polarized lavas may have been affected to a greater extent by the metasomatism than the older flows. Temporal-compositional trends in trace element concentrations also suggest that the depth of melt generation may have decreased slightly over the course of the island's formation. Floreana is distinct from the rest of the Galapagos Archipelago in its explosive history, abundant mantle xenoliths

  16. Composition of the earth's upper mantle-I. Siderophile trace elements in ultramafic nodules

    USGS Publications Warehouse

    Morgan, J.W.; Wandless, G.A.; Petrie, R.K.; Irving, A.J.

    1981-01-01

    Seven siderophile elements (Au, Ge, Ir, Ni, Pd, Os, Re) were determined by radiochemical neutron activation analysis in 19 ultramafic rocks, which are spinel lherzollites-xenoliths from North and Central America, Hawaii and Australia, and garnet Iherzolitexenoliths from Lesotho. Abundances of the platinum metals are very uniform in spinel lherzolites averaging 3.4 ?? 1.2 ppb Os, 3.7 ?? 1.1 ppb Ir, and 4.6 ?? 2.0 ppb Pd. Sheared garnet lherzolite PHN 1611 has similar abundances of these elements, but in 4 granulated garnet lherzolites, abundances are more variable. In all samples, the Pt metals retain cosmic ( Cl-chondrite) ratios. Abundances of Au and Re vary more than those of Pt metals, but the Au/Re ratio remains close to the cosmic value. The fact that higher values of Au and Re approach cosmic proportions with respect to the Pt metals, suggests that Au and Re have been depleted in some ultramafic rocks from an initially chondrite-like pattern equivalent to about 0.01 of Cl chondrite abundances. The relative enrichment of Au and Re in crustal rocks is apparently the result of crust-mantle fractionation and does not require a special circumstance of core-mantle partitioning. Abundances of moderately volatile elements Ni, Co and Ge are very uniform in all rocks, and are much higher than those of the highly siderophile elements Au, Ir, Pd, Os and Re. When normalized to Cl chondrites, abundances of Ni and Co are nearly identical, averaging 0.20 ?? 0.02 and 0.22 ?? 0.02, respectively; but Ge is only 0.027 ?? 0.004. The low abundance of Ge relative to Ni and Co is apparently a reflection of the general depletion of volatile elements in the Earth. The moderately siderophile elements cannot be derived from the same source as the highly siderophile elements because of the marked difference in Cl chondrite-normalized abundances and patterns. We suggest that most of the Ni, Co and Ge were enriched in the silicate by the partial oxidation of pre-existing volatile-poor Fe

  17. Constraints on plateau architecture and assembly from deep crustal xenoliths, northern Altiplano (SE Peru)

    NASA Astrophysics Data System (ADS)

    Chapman, Alan; Ducea, Mihai; McQuarrie, Nadine; Coble, Matthew; Petrescu, Lucian; Hoffman, Derek

    2016-04-01

    Newly discovered xenoliths within Pliocene and Quaternary intermediate volcanic rocks from southern Peru permit examination of lithospheric processes by which thick crust (60-70 km) and high average elevations (3-4 km) resulted within the Altiplano, the second most extensive orogenic plateau on Earth. The most common petrographic groups of xenoliths studied here are igneous or meta-igneous rocks with radiogenic isotopic ratios consistent with recent derivation from asthenospheric mantle (87Sr/ 86Sr = 0.704-0.709, 143Nd/144Nd = 0.5126-0.5129). A second group, consisting of felsic granulite xenoliths exhibiting more radiogenic compositions (87Sr/86Sr = 0.711-0.782, 143Nd/144Nd = 0.5121-0.5126), is interpreted as supracrustal rocks that underwent metamorphism at ~9 kbar (~30-35 km paleodepth, assuming a mean crustal density of 2.8 g/cm3) and ~750 °C. These rocks are correlated with nonmetamorphosed rocks of the Mitu Group and assigned a Mesozoic (Upper Triassic or younger) age based on detrital zircon U-Pb ages. A felsic granulite Sm-Nd garnet whole-rock isochron of 42 ± 2 Ma demonstrates that garnet growth took place in Eocene time. Monazite grains associated with quenched anatectic melt networks in the same rocks yield ion microprobe U-Pb ages ranging from 3.2 ± 0.2 to 4.4 ± 0.3 Ma (2σ). These disparate geochronologic data sets are reconciled by a model wherein Mesozoic cover rocks were transferred to >30 km depth beneath the plateau in the Eocene and progressively heated until at least Pliocene time. Isothermal decompression and partial melting ensued as these rocks were entrained as xenoliths in volcanic host magmas and transported toward the surface. Mafic granulites and peridotites from the same xenolith suite comprise the basement of the metasedimentary sequence, exhibiting isotopic characteristics of Central Andean crust. Calculated equilibrium pressures for these basement rocks are >11 kbar, suggesting that the basement-cover interface lies beneath the

  18. The Moho as a magnetic boundary. [Earth crust-mantle boundary

    NASA Technical Reports Server (NTRS)

    Wasilewski, P. J.; Thomas, H. H.; Mayhew, M. A.

    1979-01-01

    Magnetism in the crust and the upper mantle and magnetic results indicating that the seismic Moho is a magnetic boundary are considered. Mantle derived rocks - peridotites from St. Pauls rocks, dunite xenoliths from the Kaupulehu flow, and peridotite, dunite, and eclogite xenoliths from Roberts Victor and San Carlos diatremes - are weakly magnetic with saturation magnetization values from 0.013 emu/gm to less than 0.001 emu/gm which is equivalent to 0.01 to 0.001 wt% Fe304. Literature on the minerals in mantle xenoliths shows that metals and primary Fe304 are absent, and that complex Cr, Mg, Al, and Fe spinels are dominant. These spinels are non-magnetic at mantle temperatures, and the crust/mantle boundary can be specified as a magnetic mineralogy discontinuity. The new magnetic results indicate that the seismic Moho is a magnetic boundary, the source of magnetization is in the crust, and the maximum Curie isotherm depends on magnetic mineralogy and is located at depths which vary with the regional geothermal gradient.

  19. Kaersutite-bearing xenoliths and megacrysts in volcanic rocks from the Funk Seamount in the souhtwest Indian Ocean

    NASA Technical Reports Server (NTRS)

    Reid, Arch M.; Le Roex, Anton P.

    1988-01-01

    The petrography, mineral chemistry, and whole-rock compositions of volcanic rocks dredged from the Funk Seamount, located 60 km NW of Marion Island in the southwestern Indian Ocean, are presented together with the mineral chemistry of their inclusions. On the basis of these characteristics, the possible relationships between the Funk Seamount's volcanic rocks and the megacrysts and xenoliths in these rocks are discussed. It is argued that the Funk Seamount lavas derive from a similar mantle source region as that of the Marion Island and Prince Edward Island hotspot lavas. The geochemical signature of these lavas implies derivation from a source that is enriched (e.g., in Ti, K, P, and Nb) over the depleted mantle source regions for the adjacent mid-ocean ridge basalts.

  20. Contrasting water contents of Hawaiian peridotite and pyroxenite: Implications for the origin of EM-mantle reservoirs and the electrical conductivity of the oceanic mantle.

    NASA Astrophysics Data System (ADS)

    Bizimis, Michael; Peslier, Anne

    2014-05-01

    Water (as hydrogen) dissolved in nominally anhydrous minerals exerts a strong influence on the physical properties of the mantle, but most water concentration estimates of oceanic mantle reservoirs are inferred from erupted lavas. Mantle xenoliths brought to the surface by volcanism in oceanic plates provide direct access to the convective mantle and can place unique constrains on the water distribution in mantle reservoirs. Here we report FTIR water concentration data on spinel peridotite (residual lithosphere) and garnet pyroxenite xenoliths from Salt Lake Crater, Oahu, Hawaii. We use these data to test whether enriched mineralogies (pyroxenites) can generate the water variability inferred for different mantle reservoirs. Reconstructed bulk peridotite concentrations (70-114 ppm H2O) fall at the low side of the MORB source estimates (50-200 ppm H2O)[1] and consistent with an oceanic lithosphere (i.e. residual) origin. In turn, the garnet pyroxenites have higher bulk water concentrations (200-460 ppm H2O) that mostly reflect the high modal abundance and high water concentration in cpx (up to 570 ppm H2O). The pyroxenite bulk H2O concentrations are significantly higher than the MORB source, but also have low H2O/Ce ratios (<100), both close to estimates of EM-type OIB sources [1,2]. Even accounting for the presence of trace phlogopite (up to 0.5% modal) with ~5 wt% H2O the bulk pyroxenites do not have the high water concentrations inferred for FOZO-type OIB sources (>700ppm, [1]). We suggest that EM-type OIB reservoirs with high H2O concentrations and low H2O/Ce ratios may be sourced from recycled oceanic lithosphere that is mineralogically enriched with pyroxenite veins as high pressure cumulates produced from previously unerupted melts. This model is consistent with other trace element and isotopic models for EM-type sources [2,3], and relaxes the requirements for the presence of dehydrated recycled slab in the source of EM basalts [1]. Finally, pyroxene

  1. The Mantle and Basalt-Crust Interaction Below the Mount Taylor Volcanic Field, New Mexico

    NASA Technical Reports Server (NTRS)

    Schrader, Christian M.; Crumpler, Larry S.; Schmidt, Marick E.

    2010-01-01

    The Mount Taylor Volcanic Field (MTVF) lies on the Jemez Lineament on the southeastern margin of the Colorado Plateau. The field is centered on the Mt. Taylor composite volcano and includes Mesa Chivato to the NE and Grants Ridge to the WSW. MTVF magmatism spans approximately 3.8-1.5 Ma (K-Ar). Magmas are dominantly alkaline with mafic compositions ranging from basanite to hy-basalt and felsic compositions ranging from ne-trachyte to rhyolite. We are investigating the state of the mantle and the spatial and temporal variation in basalt-crustal interaction below the MTVF by examining mantle xenoliths and basalts in the context of new mapping and future Ar-Ar dating. The earliest dated magmatism in the field is a basanite flow south of Mt. Taylor. Mantle xenolith-bearing alkali basalts and basanites occur on Mesa Chivato and in the region of Mt. Taylor, though most basalts are peripheral to the main cone. Xenolith-bearing magmatism persists at least into the early stages of conebuilding. Preliminary examination of the mantle xenolith suite suggests it is dominantly lherzolitic but contains likely examples of both melt-depleted (harzburgitic) and melt-enriched (clinopyroxenitic) mantle. There are aphyric and crystal-poor hawaiites, some of which are hy-normative, on and near Mt. Taylor, but many of the more evolved MTVF basalts show evidence of complex histories. Mt. Taylor basalts higher in the cone-building sequence contain >40% zoned plagioclase pheno- and megacrysts. Other basalts peripheral to Mt. Taylor and at Grants Ridge contain clinopyroxene and plagioclase megacrysts and cumulate-textured xenoliths, suggesting they interacted with lower crustal cumulates. Among the questions we are addressing: What was the chemical and thermal state of the mantle recorded by the basaltic suites and xenoliths and how did it change with time? Are multiple parental basalts (Si-saturated vs. undersaturated) represented and, if so, what changes in the mantle or in the tectonic

  2. Petrology and Geochemistry of a Mg- and Al-Rich Orthopyroxenite Xenolith in the EETA79001 Shergottite: Implications for Mars Crustal Evolution

    NASA Technical Reports Server (NTRS)

    Berkley, John L.

    1999-01-01

    EETA79001 is a Mars meteorite (SNC) consisting of multiple rock types, including two basalt types, olivine and pyroxene xenocrysts, and ultramafic xenoliths. This study is focused on the petrology and geochemistry of one orthopyroxenite xenolith in PTS 68, designated X-1. It consists of chemically homogeneous orthopyroxene cores with exceptionally high Mg/Fe (mg#=85) and Al. Cores are permeated by minute high-Si+Al glassy inclusions, some with augite microlites. Magnesian core areas are mantled by more Fe-rich orthopyroxene rims grading to pigeonite away from cores. The xenolith is transected by cross-cutting shear planes, some of pre-incorporation origin. Major and minor element composition and variation suggest that core areas are primarily igneous, crystallized from a high temperature mafic melt. However, nearly constant mg# across cores suggest metamorphic equilibration. Si+Al inclusions may result from, among other processes, exsolution of feldspathic material during subsolidus cooling, or may be solid materials (alkali feldspar) poikilitically enclosed by growing igneous orthopyroxene crystals. Late reaction with more fractionated melts produced Fe-rich mantles, the whole assemblage later cut by tectonic micro-shear planes. Raw electron microprobe data produced during this study are available on request from the author.

  3. Xenoliths of Cerro las Mercedes, Costa Rica: a Geochemical Record of Arc History?

    NASA Astrophysics Data System (ADS)

    Lindsay, F. N.; Gazel, E.; Herzberg, C. T.; Carr, M. J.; Feigenson, M. D.

    2006-12-01

    Over 70 ultramafic xenoliths, many with diameters of at least 3cm, open a window into the mantle beneath Cerro las Mercedes, Costa Rica. This quaternary vent is 70km behind the active volcanic front and near the northern boundary of the Caribbean Plateau. Both xenoliths and host lava remain well preserved in spite of substantial soil development in a rain forest environment. We analyzed the host lava and a subset of 30 rocks for bulk and mineral chemistry, which include both peridotite and pyroxenite populations. The host rock is Plio-Quaternary potassic alkaline basalt; depleted in SiO2 and Al2O3 and enriched in MgO and P2O5 relative to both present day and Miocene volcanics. In terms of trace elements, the host basalt has enriched LREEs and relative depletion of HFSEs, typical of island-arc basalt, both present and past. The coarse-grained pyroxenites have trace element chemistry akin to the volcanic front lavas of Costa Rica, although they are closer to chondritic values. Truly, they are more similar to basalts from Sarapiquí Miocene arc than to present day volcanic front basalts. Although there is enrichment in incompatible elements such as Ba and U and depletions in HFSEs, common in island arc basalts, the pyroxenites have a strong positive Pb anomaly that is characteristic only of the Miocene arc, not present day lavas. We interpret the pyroxenites as cumulates. The peridotite population includes dunites, spinel lherzolites and lherzolites. These rocks have Mg-numbers ranging from 87 to 92 and Cr-numbers ranging from 6 to 61. Whole rock geochemistry indicates that the peridotite xenoliths are fragments of mantle associated with the western Caribbean Plateau. SiO2, FeO, Al2O3 and MgO contents of several Cerro Mercedes peridotites are similar to those expected of hot residues that would form in a plume environment. Trace elements reveal some samples with enrichment of Ba and U, and depletion in Nb and Ta indicating variable extents of melt-rock reaction and

  4. Origin and Distribution of Water Contents in Continental and Oceanic Lithospheric Mantle

    NASA Technical Reports Server (NTRS)

    Peslier, Anne H.

    2013-01-01

    The water content distribution of the upper mantle will be reviewed as based on the peridotite record. The amount of water in cratonic xenoliths appears controlled by metasomatism while that of the oceanic mantle retains in part the signature of melting events. In both cases, the water distribution is heterogeneous both with depth and laterally, depending on localized water re-enrichments next to melt/fluid channels. The consequence of the water distribution on the rheology of the upper mantle and the location of the lithosphere-asthenosphere boundary will also be discussed.

  5. Mesozoic thermal evolution of the southern African mantle lithosphere

    NASA Astrophysics Data System (ADS)

    Bell, David R.; Schmitz, Mark D.; Janney, Philip E.

    2003-12-01

    The thermal structure of Archean and Proterozoic lithospheric terranes in southern Africa during the Mesozoic was evaluated by thermobarometry of mantle peridotite xenoliths erupted in alkaline magmas between 180 and 60 Ma. For cratonic xenoliths, the presence of a 150-200 °C isobaric temperature range at 5-6 GPa confirms original interpretations of a conductive geotherm, which is perturbed at depth, and therefore does not record steady state lithospheric mantle structure. Xenoliths from both Archean and Proterozoic terranes record conductive limb temperatures characteristic of a "cratonic" geotherm (˜40 mW m -2), indicating cooling of Proterozoic mantle following the last major tectonothermal event in the region at ˜1 Ga and the probability of thick off-craton lithosphere capable of hosting diamond. This inference is supported by U-Pb thermochronology of lower crustal xenoliths [Schmitz and Bowring, 2003. Contrib. Mineral. Petrol. 144, 592-618]. The entire region then suffered a protracted regional heating event in the Mesozoic, affecting both mantle and lower crust. In the mantle, the event is recorded at ˜150 Ma to the southeast of the craton, propagating to the west by 108-74 Ma, the craton interior by 85-90 Ma and the far southwest and northwest by 65-70 Ma. The heating penetrated to shallower levels in the off-craton areas than on the craton, and is more apparent on the southern margin of the craton than in its western interior. The focus and spatial progression mimic inferred patterns of plume activity and supercontinent breakup 30-100 Ma earlier and are probably connected. Contrasting thermal profiles from Archean and Proterozoic mantle result from penetration to shallower levels of the Proterozoic lithosphere by heat transporting magmas. Extent of penetration is related not to original lithospheric thickness, but to its more fertile character and the presence of structurally weak zones of old tectonism. The present day distribution of surface heat flow

  6. Deep Mantle Fluids Bottled Up in Diamonds

    NASA Astrophysics Data System (ADS)

    Weiss, Y.; Pearson, D. G.

    2015-12-01

    Many mantle xenoliths and mineral inclusions in diamonds reflect refertilisation and enrichment by mantle metasomatism, a key mechanism for controlling abrupt changes in the chemical and physical properties of the continental lithospheric mantle (CLM) globally. However, the nature of the fluids involved can normally only be constrained indirectly from geochemical proxies or calculated using mineral/melt partition coefficients. Direct samples of mantle metasomatic fluids, shielded from any late stage alteration, are encased as microinclusions in fast-growing diamonds - "fibrous diamonds". These trapped high-density fluids (HDFs) provide a unique chemical and physical record for tracing the sources of deep mantle fluids and constraining the processes that shape their nature.Diamond HDFs vary between four major compositional types: saline, silicic and high-Mg plus low-Mg carbonatitic. A strong connection has been established between high-Mg carbonatitic HDFs and a carbonated peridotite source. In addition, the silicic and low-Mg carbonatitic HDFs have been related to hydrous eclogite (±carbonate). However, the compositionally extreme saline fluid endmember remained enigmatic and its source in the deep lithosphere has remained ambiguous. Our new data on fluid-rich diamonds show the geochemical fingerprints of a subducting slab as the source of deep mantle fluids of saline composition. In addition, for the first time, we show that these deep saline fluids are parental, via fluid rock interaction, to in-situ forming carbonatitic and silicic melts in the lithosphere. This model provides a strong platform for resolving the effects of the compositional spectrum of mantle fluids, which alter the deep lithosphere globally and play key roles in diamond formation.

  7. Metasomatic enrichment of Proterozoic mantle south of the Kaapvaal Craton, South Africa: origin of sinusoidal REE patterns in clinopyroxene and garnet

    NASA Astrophysics Data System (ADS)

    le Roex, Anton; Class, Cornelia

    2016-02-01

    Xenoliths of mantle peridotite have been sampled from four kimberlite intrusions, Melton Wold, Hebron, Uintjiesberg and Markt, emplaced through the Mesoproterozoic Namaqua-Natal Belt, along the southern border of the Kaapvaal Craton. Although many of the xenoliths are heavily altered, constituent clinopyroxene, garnet and phlogopite are fresh and have been analysed by electron microprobe for major elements and by laser ablation ICP-MS for trace elements. Primitive mantle-normalised REE abundances in clinopyroxene are all strongly LREE enriched and show a range of patterns including uniformly MREE-HREE sloped (referred to here as `normal'), sinusoidal and humped sinusoidal patterns. HREE abundances are extremely low (Yb = 0.3-0.06 × PM). REEN patterns in coexisting garnets show a similar range of patterns. When normalised to primitive mantle values, trace element patterns in some clinopyroxenes show strong relative depletion in Rb-Ba, Ta-Nb and Ti, with some samples also being relatively depleted in Zr-Hf. These trace element characteristics are indistinguishable from those found in clinopyroxene and garnet from peridotites from the adjacent cratonic mantle. Numerical modelling of reactive porous flow of an enriched metasomatic melt through a geochemically depleted peridotite matrix can account for the full range in observed REEN patterns. The relative depletion in Rb-Ba, Ta-Nb and Ti can be accounted for by an early crystallisation of phlogopite from the percolating melt. The relative depletion in Zr-Hf in some clinopyroxenes requires either zircon to crystallise in the proximal metasomatic assemblage, or metasomatism by a carbonatitic melt. Modelling results, together with the absence of clinopyroxene with depleted or even partially enriched REEN patterns, suggest that all clinopyroxene has been modally introduced through metasomatism into an initially highly depleted harzburgitic protolith. The range in Sr and Pb isotopic composition of the clinopyroxenes

  8. Retrograded eclogite xenoliths from mid-Tertiary potassic lavas along the southwest margin of the Colorado Plateau

    NASA Astrophysics Data System (ADS)

    Schroeder, T. J.; Riggs, N.; Ort, M. H.

    2010-12-01

    A suite of xenolith-bearing, alkalic volcanic rocks erupted in small domes and plugs along the southwest margin of the Colorado Plateau (current Transition Zone) in Arizona between 30 Ma and 22 Ma. The onset of volcanic activity slightly precedes the earliest phase of mid-Tertiary extension and metamorphic core complex denudation in western/southern Arizona (27-25 Ma), which occurred along detachment faults that dip gently beneath the CP margin. Lavas are highly potassic (up to 6% Wt. K2O) and rich in incompatible trace elements (Ba up to 1900 ppm and Sr up to 1250 ppm), suggesting an enriched mantle melting source. Xenoliths are dominantly eclogite and pyroxenite that have been slightly to completely retrograded to amphibolite assemblages. The most common xenolith mineral assemblage consists of: 1) omphacite that has been completely replaced by an irregular symplectite of cpx and low-Ca plagioclase, 2) garnet that is partially replaced and rimmed by a fine, lamellar symplectite of high-Ca plagioclase, pargasitic amphibole, and Fe-Ti oxides, 3) coarse pargasitic amphibole that has overgrown or partially replaced the cpx-plag symplectite, 4) rutile that has been partially replaced by ilmenite and magnetite, and 4) apatite, zircon, and other accessory minerals. Xenoliths contain evidence for melt interaction prior to entrainment, including plagioclase veins containing hercynite and hogbomite that are cut by sharp contacts with the host lava. Results of this and other studies suggest that these veins formed by reactions between the eclogite assemblage (likely including spinel) and a locally derived melt. However, textural relations and geochemical differences suggest that this melt was not the host melt that entrained the xenoliths. Pyroxene and garnet thermobarometry from this and other studies on the primary eclogitic assemblages indicate temperatures from 500 to 900C at 11 to 20 kb. Mineral chemistry analyses of amphibole, plagioclase, hornblende, and garnet in the

  9. Plutonic and metamorphic xenoliths from the Cascada Tuff, Chihuahua, Mexico, as evidence indicating the composition of the basement rocks beneath the Sierra Madre Occidental

    SciTech Connect

    Duex, T.W.

    1985-01-01

    The Sierra Madre Occidental of western Mexico is composed dominantly of Mid-Tertiary felsic and subordinate mafic volcanic rocks with only sparse outcrops of non-volcanic rocks. There are widely scattered but small exposure of plutonic rocks but regionally metamorphosed rocks are not known to occur in the Sierra. To this date the only known area where plutonic and metamorphic xenoliths have been found is near the village of Basaseachic in western Chihuahua where thick outcrops of the Cascada Tuff occur. The xenoliths are the only known occurrence of regionally metamorphosed rocks for a distance of about 400 km between exposures of Precambrian rocks to the west in Sonora and the east in central Chihuahua. Non-volcanic xenoliths from a few cm to about one meter in diameter occur most abundantly in the upper portions of the Cascada Tuff. They can be divided into four main groups in decreasing order of abundance as follows: (1) coarse-grained phaneritic felsic igneous rocks; (2) cataclastically deformed plutonic rocks; (3) fine-grained phaneritic, mafic to intermediate igneous rocks; and (4) low-grade schistose, gneissic, and non-foliated metamorphic rocks. The lithological composition of the xenoliths is grossly similar to that described for Precambrian metamorphic and plutonic rocks from northern Mexico and the southwestern US.

  10. Geochemical evolution of lithospheric mantle underlying Intrasudetic Fault (SW Poland).

    NASA Astrophysics Data System (ADS)

    Ćwiek, Mateusz; Matusiak-Małek, Magdalena; Puziewicz, Jacek; Ntaflos, Theodoros

    2015-04-01

    alkaline silicate melt. Xenolith where clinopyroxene shows constant enrichment in LREE may represent a part of mantle affected by pure carbonatitic metasomatism. Xenoliths with the lowest Fo content in olivine are probably cumulates of mafic silicate melt. Intergranular aggregates originated during reaction between primary phases and infiltrating, possibly mafic melt. The "glassy patch" might be a product of complete breakdown of amphibole in upper mantle conditions (Shaw, 2009, Lithos). However, presence of rhönite in host basanite suggests that some amphibole might have been broken also in lower pressures. Chemical composition of peridotites from Pilchowice resembles that recorded by group A mantle xenoliths from Krzeniów (Matusiak-Małek et al., 2014, JoP). In Pilchowice we have described the first xenolith affected by purely carbonatitic metasomatism. We also suggest that hydrous phases might have been present in upper mantle beneath this Pilchowice, which is believed to be nominally anhydrous (Puziewicz et al., 2015, IJES, DOI 10.1007/s00531-014-1134-2). This study was a part of MSc thesis of the first author and was possible thanks to the project NCN 2011/03/B/ST10/06248 of Polish National Centre for Science.

  11. Origin of carbonate xenoliths in Siddanpalli kimberlites, Southern India

    NASA Astrophysics Data System (ADS)

    Anand, M.; Osborne, I.; Gilmour, M. A.; Chalapath Rao, N. V.

    2009-04-01

    A number of carbonate xenoliths have been recently discovered from the three kimberlites (designated as SK1, SK2, SK3) of the Siddanpalli kimberlite cluster (SKC; Sridhar et al., 2004) of Southern India. These kimberlites intrude the Precambrian granite-greenstone terrain of Gadwal schist belt of Eastern Dharwar Craton (e.g., Dongre et al., 2008). Rb-Sr phlogopite/whole-rock dating of one of these bodies, SK1, has yielded an age of 1093 ± Ma (Kumar et al. 2007). Previously, Dongre et al. (2008) reported an occurrence of a limestone xenolith from the SK2 kimberlite. Based on petrographic, geochemical, and C and O isotope data these authors suggested a sedimentary origin for the limestone xenolith. However, now we have documented a larger variation in the nature of carbonate material occurring as xenoliths in these kimberlites, ranging from micro-crystalline segregations to well-formed carbonate crystals. Thus, it is likely that a number of sources and/or processes have been involved in their formation. We are in the process of measuring carbon and oxygen isotopic compositions of individual carbonate xenoliths in order to further constrain their origin.

  12. Application of Fe K-edge XANES determinations of Fe3+/totalFe in garnet to peridotite xenoliths from the Udachnaya Kimberlite

    NASA Astrophysics Data System (ADS)

    Yaxley, G. M.; Berry, A. J.; Woodland, A. B.; Kamenetsky, V. S.; Paterson, D.; de Jonge, M. D.; Howard, D. L.

    2012-04-01

    The garnet structure can accommodate both Fe2+ and Fe3+. Garnet Fe3+/∑Fe in kimberlite-bourne garnet peridotite xenoliths can be used to determine the oxygen fugacity (fO2) of the cratonic lithosphere. This is important as an indicator of diamond (versus carbonate) stability. In cratonic lithosphere the ƒO2 of peridotite is expected to broadly decrease with increasing depth, and is consistent with graphite or diamond stability. However metasomatic events may locally perturb this trend, possibly leading to oxidation that could result in diamond breakdown or resorption. Such events will usually be recorded by the coexisting garnet. Fe3+/∑Fe of garnets has traditionally been determined by Mössbauer Spectroscopy of powdered samples. This lacks spatial resolution and the data for each measurement take several days to acquire. X-ray Absorption Near Edge Structure (XANES) spectroscopy is now commonly being used to determine Fe3+/∑Fe in minerals, is capable of micron spatial resolution and spectra can be recorded in ~15 minutes. We have recently reported a new method for quantifying Fe3+/∑Fe from the XANES spectra of mantle garnets with an accuracy and precision comparable to Mössbauer Spectroscopy. We applied the XANES technique to investigate the ƒO2-depth variation in the Siberian Craton using a suite of fresh garnet lherzolites from the Udachnaya East kimberlite. Garnet Fe3+/∑Fe was detemined using XANES spectroscopy on the X-ray Fluorescence Microscopy beamline of the Australian Synchrotron. XANES spectra were recorded in fluorescence mode from garnets prepared as either polished thin sections or electron probe mounts. A calibration curve relating the spectra to Fe3+/∑Fe of mantle garnets previously analysed by Mössbauer spectroscopy allowed garnet unknowns to be quantified. Thermobarometry established that the samples range in pressure from 3.9-7.1 GPa and lie along a typical cratonic geotherm. Several samples exhibit elevated abundances of Ti, Zr

  13. Evolution of LILE-enriched small melt fractions in the lithospheric mantle: a case study from the East African Rift

    NASA Astrophysics Data System (ADS)

    Bedini, R. M.; Bodinier, J.-L.; Dautria, J.-M.; Morten, L.

    1997-12-01

    Spinel-peridotite xenoliths from Mega (East African Rift, Sidamo region, SE Ethiopia) show variable degrees of recrystallization coupled with trace-element variations. The less recrystallized samples (deformed xenoliths) consist of apatite-bearing porphyroclastic peridotites. They are strongly enriched in LILE (Ba, Th, U, Sr and LREE), with negative anomalies of the HFSE (Nb, Ta, Zr, Hf and Ti). The most recrystallized samples (granular xenoliths) consist of apatite-free peridotites with coarse-grained, granular textures. These samples are depleted or only slightly enriched in LILE and display no significant HFSE anomaly. We suggest that the inverse relationship between recrystallization and trace-element enrichment results from km-scale variation in volume and composition of melts pervasively infiltrated in the lithosphere. The deformed xenoliths record interaction with LILE-enriched small melt fractions, at low melt/rock ratio, while the granular xenoliths were extensively re-equilibrated with a higher fraction of basaltic melt, at higher melt/rock ratio. With a numerical simulation of reactive porous flow at the transition between adiabatic and conductive geotherms in the mantle, it is shown that these two processes were possibly coeval and associated with thermo-mechanical erosion of the lower lithosphere above a mantle plume.

  14. Cadomian (˜560 Ma) crust buried beneath the northern Arabian Peninsula: Mineral, chemical, geochronological, and isotopic constraints from NE Jordan xenoliths

    NASA Astrophysics Data System (ADS)

    Stern, Robert J.; Ali, Kamal A.; Ren, Minghua; Jarrar, Ghaleb H.; Romer, Rolf L.; Leybourne, Matthew I.; Whitehouse, Martin J.; Ibrahim, Khalil M.

    2016-02-01

    In order to better understand the nature and formation of the lower continental crust beneath northern Arabia, we studied lower crustal xenoliths brought up by Neogene basalts in NE Jordan. Most of these xenoliths are comprised of primary phases plagioclase + two-pyroxenes with magnetite and ilmenite. Most clinopyroxene are augite whereas orthopyroxene mostly are hypersthene (Mg# = 50-80). Plagioclase feldspar is dominantly andesine-labradorite; pyrope-rich garnet and Fe-rich olivine (Fo75 to Fo62) are rare. These xenoliths represent cumulates formed from intermediate magmas that pooled in the lower crust. Many xenoliths also contain small, fine-grained K-rich zones interpreted as melt pockets reflecting late magmatic infiltration of the lower crust. The xenoliths display a wide range in major element compositions (37-51 wt.% SiO2, 4-15 wt.% MgO and 0.1-6.3 wt.% TiO2), enrichment in Ba, K, Sr, Pb and Eu, and some trace element ratios atypical of bulk continental crust (e.g., K/Rb = 1265 ± 565, K/U = 63 000 ± 60 080 and Th/U = 0.96 ± 0.56); these extreme ratios reflect widespread K-metasomatism associated with melt pockets. The magmas from which these cumulates formed may have been generated at a reararc convergent margin setting. Four U-Pb zircon populations yield indistinguishable ages of 554 ± 4 Ma; 559 ± 5 Ma; 559 ± 6 Ma, and 563 ± 5 Ma. Initial 87Sr/86Sr values (0.70260-0.70352) and positive εNd(560) (with the exception of a single, more radiogenic sample (+9.6), range = + 1.3 to +4.8) indicate that the lower crust sampled by the xenoliths originated in the asthenospheric mantle, with little or no interaction with older crust, although Pb isotopic compositions allow for some interaction with older or subducted crustal materials. We interpret the geochemistry and mineralogy of these xenoliths to indicate that the lower crust beneath NE Jordan is mafic and comprised of plagioclase-rich 2-pyroxene igneous rocks. The lower crust of this area formed by

  15. Asthenosphere-lithosphere interactions in Western Saudi Arabia: Inferences from 3He/4He in xenoliths and lava flows from Harrat Hutaymah

    NASA Astrophysics Data System (ADS)

    Konrad, Kevin; Graham, David W.; Thornber, Carl R.; Duncan, Robert A.; Kent, Adam J. R.; Al-Amri, Abdullah M.

    2016-04-01

    volcanic field situated above thinned lithosphere beneath the Makkah-Medinah-Nafud volcanic lineament. Previous work established that spinel lherzolites at Hutaymah are sourced near the lithosphere-asthenosphere boundary (LAB), while other xenolith types there are derived from shallower depths within the lithosphere itself (Thornber, 1992). Helium isotopes are consistent with melts originating near the LAB beneath many of the Arabian harrats, and any magma derived from the Afar mantle plume currently appears to be of minor importance.

  16. Water contents in pyroxenes of intraplate lithospheric mantle

    NASA Astrophysics Data System (ADS)

    Bonadiman, C.; Hao, Y.-T.; Coltorti, M.; Dallai, L.; Faccini, B.; Hu, H.; Qunke, X.

    2009-04-01

    Water contents of clinopyroxene and orthopyroxene in mantle peridotites from various xenolith occurrences in intraplate settings (both oceanic and continental) were determined by Fourier Transform Infrared Spectroscopy (FTIR). The localities are as follow: Sal Island (Cape Verde Archipelago); Baker Rocks and Greene Point (Northern Victoria Land, Antarctica); Panshishan and Lianshan (Subei Basin, Eastern China). They represent well-known localities where detailed petrographical and geochemical studies have already been carried out or areas which are currently under investigation. The water incorporated in these pyroxenes is low (cpx, 37-399ppm; opx: 9-166ppm)(or very low as in Greene Point, Antarctica; cpx, 5-16ppm; opx: 9-16ppm) and, among each population, no clear correlation with melting parameters (MgO contents) in single mineral is evident. Results are compared with the available literature data on water contents in mantle pyroxene which includes peridotites from on-craton (hosted by kimberlitic-type magmas) and off-craton (hosted by alkaline basic magmas), as well as subarc mantle settings. The "relatively dry" (cpx: 140-528 ppm; opx: 38-280 ppm) sub-arc mantle xenoliths (Peslier et al., 2002) are shown to be wetter than the intraplate (off-craton) xenoliths. Cratonic mantle pyroxenes are only represented by a few determinations on garnet peridotites and eclogite from Kaapvaal and Colorado Plateau. They record the highest water contents (cpx: 342-1012 ppm; opx: 180-491 ppm) so far measured in mantle pyroxenes from various tectonic settings. Despite the limited data set, the indication that the cratonic mantle is strongly hydrated is compelling. Rehydration for the Colorado Plateau craton may be due to the Farallon plate subduction (Li et al., 2008), while for Kaapvaal Craton it might be related to young (<100Ma) metasomatic enrichments (Griffin et al., 2003a; Kobussen et al., 2008). If this is the case then the Archean mantle water content needs to be

  17. First direct evidence of sedimentary carbonate recycling in subduction-related xenoliths

    NASA Astrophysics Data System (ADS)

    Liu, Yongsheng; He, Detao; Gao, Changgui; Foley, Stephen; Gao, Shan; Hu, Zhaochu; Zong, Keqing; Chen, Haihong

    2015-06-01

    Carbon in rocks and its rate of exchange with the exosphere is the least understood part of the carbon cycle. The amount of carbonate subducted as sediments and ocean crust is poorly known, but essential to mass balance the cycle. We describe carbonatite melt pockets in mantle peridotite xenoliths from Dalihu (northern China), which provide firsthand evidence for the recycling of carbonate sediments within the subduction system. These pockets retain the low trace element contents and δ18OSMOW = 21.1 ± 0.3 of argillaceous carbonate sediments, representing wholesale melting of carbonates instead of filtered recycling of carbon by redox freezing and melting. They also contain microscopic diamonds, partly transformed to graphite, indicating that depths >120 km were reached, as well as a bizarre mixture of carbides and metal alloys indicative of extremely reducing conditions. Subducted carbonates form diapirs that move rapidly upwards through the mantle wedge, reacting with peridotite, assimilating silicate minerals and releasing CO2, thus promoting their rapid emplacement. The assimilation process produces very local disequilibrium and divergent redox conditions that result in carbides and metal alloys, which help to interpret other occurrences of rock exhumed from ultra-deep conditions.

  18. First direct evidence of sedimentary carbonate recycling in subduction-related xenoliths.

    PubMed

    Liu, Yongsheng; He, Detao; Gao, Changgui; Foley, Stephen; Gao, Shan; Hu, Zhaochu; Zong, Keqing; Chen, Haihong

    2015-01-01

    Carbon in rocks and its rate of exchange with the exosphere is the least understood part of the carbon cycle. The amount of carbonate subducted as sediments and ocean crust is poorly known, but essential to mass balance the cycle. We describe carbonatite melt pockets in mantle peridotite xenoliths from Dalihu (northern China), which provide firsthand evidence for the recycling of carbonate sediments within the subduction system. These pockets retain the low trace element contents and δ(18)OSMOW = 21.1 ± 0.3 of argillaceous carbonate sediments, representing wholesale melting of carbonates instead of filtered recycling of carbon by redox freezing and melting. They also contain microscopic diamonds, partly transformed to graphite, indicating that depths >120 km were reached, as well as a bizarre mixture of carbides and metal alloys indicative of extremely reducing conditions. Subducted carbonates form diapirs that move rapidly upwards through the mantle wedge, reacting with peridotite, assimilating silicate minerals and releasing CO2, thus promoting their rapid emplacement. The assimilation process produces very local disequilibrium and divergent redox conditions that result in carbides and metal alloys, which help to interpret other occurrences of rock exhumed from ultra-deep conditions. PMID:26100577

  19. First direct evidence of sedimentary carbonate recycling in subduction-related xenoliths.

    PubMed

    Liu, Yongsheng; He, Detao; Gao, Changgui; Foley, Stephen; Gao, Shan; Hu, Zhaochu; Zong, Keqing; Chen, Haihong

    2015-06-23

    Carbon in rocks and its rate of exchange with the exosphere is the least understood part of the carbon cycle. The amount of carbonate subducted as sediments and ocean crust is poorly known, but essential to mass balance the cycle. We describe carbonatite melt pockets in mantle peridotite xenoliths from Dalihu (northern China), which provide firsthand evidence for the recycling of carbonate sediments within the subduction system. These pockets retain the low trace element contents and δ(18)OSMOW = 21.1 ± 0.3 of argillaceous carbonate sediments, representing wholesale melting of carbonates instead of filtered recycling of carbon by redox freezing and melting. They also contain microscopic diamonds, partly transformed to graphite, indicating that depths >120 km were reached, as well as a bizarre mixture of carbides and metal alloys indicative of extremely reducing conditions. Subducted carbonates form diapirs that move rapidly upwards through the mantle wedge, reacting with peridotite, assimilating silicate minerals and releasing CO2, thus promoting their rapid emplacement. The assimilation process produces very local disequilibrium and divergent redox conditions that result in carbides and metal alloys, which help to interpret other occurrences of rock exhumed from ultra-deep conditions.

  20. First direct evidence of sedimentary carbonate recycling in subduction-related xenoliths

    PubMed Central

    Liu, Yongsheng; He, Detao; Gao, Changgui; Foley, Stephen; Gao, Shan; Hu, Zhaochu; Zong, Keqing; Chen, Haihong

    2015-01-01

    Carbon in rocks and its rate of exchange with the exosphere is the least understood part of the carbon cycle. The amount of carbonate subducted as sediments and ocean crust is poorly known, but essential to mass balance the cycle. We describe carbonatite melt pockets in mantle peridotite xenoliths from Dalihu (northern China), which provide firsthand evidence for the recycling of carbonate sediments within the subduction system. These pockets retain the low trace element contents and δ18OSMOW = 21.1 ± 0.3 of argillaceous carbonate sediments, representing wholesale melting of carbonates instead of filtered recycling of carbon by redox freezing and melting. They also contain microscopic diamonds, partly transformed to graphite, indicating that depths >120 km were reached, as well as a bizarre mixture of carbides and metal alloys indicative of extremely reducing conditions. Subducted carbonates form diapirs that move rapidly upwards through the mantle wedge, reacting with peridotite, assimilating silicate minerals and releasing CO2, thus promoting their rapid emplacement. The assimilation process produces very local disequilibrium and divergent redox conditions that result in carbides and metal alloys, which help to interpret other occurrences of rock exhumed from ultra-deep conditions. PMID:26100577

  1. Mesozoic Thermal Evolution of the Kaapvaal Craton Mantle Root.

    NASA Astrophysics Data System (ADS)

    Bell, D. R.

    2002-12-01

    Recent thermobarometry1 and accessory-mineral thermochronology2 studies on mantle and lower crustal xenoliths proposed a thermal pulse that affected the southern African mantle lithosphere during the Mesozoic and may relate to supercontinent breakup and associated mantle upwelling. Effects are concentrated on the margins of the Archean craton and in the surrounding Proterozoic terrains, but are not obvious within the core of the craton. In order to examine more closely the evidence for a thermal effect within the craton itself, we performed thermobarometry studies of a large suite of peridotite xenoliths from the 114 Ma Newlands orangeite (group II kimberlite). A variety of mineral thermobarometers were applied to analyses of coexisting minerals separated from fifty xenoliths. The resultant P-T arrays lie sub-parallel to those defined by xenoliths from the 84 Ma Kimberley group I kimberlites, some 50 km to the SE, but at temperatures from 60 to 120 C lower (depending on thermobarometer) at a given depth. This result, which is consistent with seismic images of a 250 - 300 km thick mantle root, confirms previous reports3,4 of unusually low temperatures, and inferences from garnet xenocryst studies5 that proposed a change from <37 to 40 mW/m2 model geotherms over this 30 Ma time interval. The corresponding depth difference for Kimberley and Newlands isotherms ranges from 12 - 20 km and exceeds expectations for mid-Cretaceous uplift5. It thus appears that the mantle root of the Kaapvaal craton has been heated, at least in the local regions sampled by Group I kimberlites, and that both the inflected and un-inflected limbs of Mesozoic group-I kimberlite-hosted geotherms on the Kaapvaal Craton may represent a perturbed thermal state of limited lateral extent. Petrologic studies on Kimberley xenoliths have documented the common presence of metasomatic diopside, phlogopite, and amphibole that are not common in the Newlands peridotites. Modal, trace element and isotope data

  2. Emplacement of Xenolith Nodules in the Kaupulehu Lava Flow, Hualalai Volcano, Hawaii

    NASA Technical Reports Server (NTRS)

    Guest, J. E.; Spudis, P. D.; Greeley, R.; Taylor, G. J.; Baloga, S. M.

    1995-01-01

    The basaltic Kaupulehu 1800-1801 lava flow of Hualalai Volcano, Hawaii contains abundant ultramafic xenoliths. Many of these xenoliths occur as bedded layers of semi-rounded nodules, each thinly coated with a veneer (typically 1 mm thick) of lava. The nodule beds are analogous to cobble deposits of fluvial sedimentary systems. Although several mechanisms have been proposed for the formation of the nodule beds, it was found that, at more than one locality, the nodule beds are overbank levee deposits. The geological occurrence of the nodules, certain diagnostic aspects of the flow morphology and consideration of the inferred emplacement process indicate that the Kaupulehu flow had an exceptionally low viscosity on eruption and that the flow of the lava stream was extremely rapid, with flow velocities of at least 10 m/s (more than 40 km/h. This flow is the youngest on Hualalai Volcano and future eruptions of a similar type would pose considerable hazard to life as well as property.

  3. Recognizing Mantle Domains Related to an Extensional Cycle: the Record from Western Europe

    NASA Astrophysics Data System (ADS)

    Picazo, S.; Muntener, O.; Manatschal, G.

    2015-12-01

    Most of the studies on rifted margins have shown that the classical predictions of models assuming a "homogeneous" mantle lithosphere without some inheritance are unable to capture the observed large variety of magmatic budgets as a function of extension. More recently, new ideas and concepts have been developed to understand the evolution of the mantle lithosphere in hyper-extended magma-poor rifted margins that are mainly based on observations from the present-day Iberia-Newfoundland and ancient Alpine Tethys rifted margins and the Pyrenean systems. In contrast to the classical assumption assuming a simple, isotropic mantle lithosphere, these new models integrate observations from exposed and drilled mantle rocks and propose that the mantle lithosphere evolved and was modified during an extensional cycle from post-orogenic collapse through several periods of rifting to embryonic oceanic (ultra-) slow seafloor spreading. But it is, at present, unclear how far these ideas can be generalized and if they can explain the nature of mantle rocks observed across Western Europe and, in a more general way, at Atlantic type rifted margins. We review the available mantle data from Western Europe, i.e. ophiolite massifs, xenoliths and dredged samples, revisit the available terminology concerning mantle massifs and xenoliths and compile the available data to identify different mantle domains. We define chemical and petrological characteristics of mantle domains based on clinopyroxene and spinel compositions and compile them on present-day and paleo-geographic maps of Western Europe. Finally we link the observed distribution of mantle domains to the post-Variscan extensional cycle and link domains to processes related to the late post-Variscan extension, the rift evolution and refertilization associated to hyper-extension and the development of embryonic oceanic domains.

  4. Alkali basalts and enclosed ultramafic xenoliths near Ushuaia, Tierra Del Fuego, Argentina.

    PubMed

    Acevedo, Rogelio Daniel

    2016-01-01

    At the southernmost part of Tierra del Fuego a few outcrops and erratic boulders of alkali basaltic rocks with ultramafic enclaves have been studied. Alkali basalt plugs or pipes hitherto identified are scarce, and host rocks are constituted by slates that belong to Mesozoic deposition. The petrography, texture and composition of the basalt and xenoliths were investigated by petrographic microscope and electron microprobe analysis. Xenocrysts of amphibole and alkali feldspar, phenocrysts of nepheline, olivine, spinel, phlogopite and Fe-Ti minerals (10 %) and a diversity of xenoliths, mainly lherzolitic, pyroxenite and wehrlitic nodules (15 %), but also from metamorphic rocks provenance, are contained in the basalt groundmass (75 %). This finer-grained material is made up of laths or needles of plagioclase, pyroxene, opaque minerals, apatite and glass, with intersertal, hyalopilitic and pilotaxitic. Locally, rock has an even granoblastic texture. Former amygdules are filled by analcite, zeolites, sodalite and calcite. The normative classification, based on nepheline content, conclude that this rock is an alkali basalt. The chemical classification, considering immobile elements as Zr/TiO2 versus Nb/Y indicate an alkali basalt too and plots over the TAS diagram fall in the foidite (Na-rich or nephelinite) and basanite fields. The REE patterns are fractionated (La/Yb primitive mantle normalized is approximately 30). The K-Ar isotopic technique on individual macrocrysts gave ages of 146 ± 5 Ma (amphibole) and 127 ± 4 Ma (alkali feldspar); and K-Ar whole rock datum reported 8.3 ± 0.3 Ma. Nevertheless, fertile samples show geochemical features typical of deep derived material thus, based on the position in the actual tectonic setting, indicate that the basalt is older than its isotopic age. PMID:27610313

  5. Alkali basalts and enclosed ultramafic xenoliths near Ushuaia, Tierra Del Fuego, Argentina.

    PubMed

    Acevedo, Rogelio Daniel

    2016-01-01

    At the southernmost part of Tierra del Fuego a few outcrops and erratic boulders of alkali basaltic rocks with ultramafic enclaves have been studied. Alkali basalt plugs or pipes hitherto identified are scarce, and host rocks are constituted by slates that belong to Mesozoic deposition. The petrography, texture and composition of the basalt and xenoliths were investigated by petrographic microscope and electron microprobe analysis. Xenocrysts of amphibole and alkali feldspar, phenocrysts of nepheline, olivine, spinel, phlogopite and Fe-Ti minerals (10 %) and a diversity of xenoliths, mainly lherzolitic, pyroxenite and wehrlitic nodules (15 %), but also from metamorphic rocks provenance, are contained in the basalt groundmass (75 %). This finer-grained material is made up of laths or needles of plagioclase, pyroxene, opaque minerals, apatite and glass, with intersertal, hyalopilitic and pilotaxitic. Locally, rock has an even granoblastic texture. Former amygdules are filled by analcite, zeolites, sodalite and calcite. The normative classification, based on nepheline content, conclude that this rock is an alkali basalt. The chemical classification, considering immobile elements as Zr/TiO2 versus Nb/Y indicate an alkali basalt too and plots over the TAS diagram fall in the foidite (Na-rich or nephelinite) and basanite fields. The REE patterns are fractionated (La/Yb primitive mantle normalized is approximately 30). The K-Ar isotopic technique on individual macrocrysts gave ages of 146 ± 5 Ma (amphibole) and 127 ± 4 Ma (alkali feldspar); and K-Ar whole rock datum reported 8.3 ± 0.3 Ma. Nevertheless, fertile samples show geochemical features typical of deep derived material thus, based on the position in the actual tectonic setting, indicate that the basalt is older than its isotopic age.

  6. Geochronology and Geochemistry of Lower Crustal Xenoliths: Exploring the Formation of the Lower Crust Beneath Central Mongolia

    NASA Astrophysics Data System (ADS)

    Ancuta, L. D.; Carlson, R.; Ionov, D. A.

    2015-12-01

    Central Mongolia is far removed from any active margin yet it is marked by the anomalously high Hangay Mountains. Near Tariat on the flanks of the Hangay range we recovered lower crustal xenoliths from the Shavaryn-Tsaram Quaternary basaltic breccia pipe. Two-pyroxene Fe-Mg exchange thermometry indicates the xenoliths equilibrated at 840 ± 30ºC. Previous studies indicated pressure ranges between 12.5 and 15.5 kbar for samples with similar equilibration temperatures from the same locality (Stosch et al., 1995). Abundant zircon in the samples dated by U-Pb laser ablation ICP-MS show a wide range of ages from 50 to 300 Ma with a broad peak around 200 Ma. Each of the seven dated samples shows a similar spread of ages with a maximum age range of 200 Ma for individual xenoliths. The new data suggest that the lower crust in the Tariat area formed in the Late Paleozoic to early Mesozoic during the accretion of the Central Asian Orogenic Belt (CAOB), which is consistent with arc like geochemical signatures of whole-rocks and with common Permo-Triassic Sm-Nd and U-Pb isotope ages for magmatic rocks exposed on the surface in the region. Whether the younger Mesozoic zircon ages reflect later metamorphic events or result from diffusive Pb loss is unclear. Based on elevated Th/U ratios the latter may be more likely, though more work is needed to fully resolve the origin of the zircons. Early Mesozoic whole-rock Sm-Nd model ages for the suite of xenoliths are broadly consistent with the zircon U-Pb data. Stachnik et al. (2014) indicated that the high elevations of the Hangay region could be isostatically supported by a thick crust. Our new data suggests the lower crust, and by inference the high topography, formed in the Late Paleozoic to Early Mesozoic during the formation of the CAOB.

  7. Mantle cryptology

    SciTech Connect

    Zindler, A.; Jagoutz, E.

    1988-02-01

    A group of anhydrous peridotites from Peridot Mesa, Arizona, document isotopic and trace element heterogeneity in the source mantle. LREE enrichments in two spinel periodotites may have occurred immediately prior to entrainment through interaction with a melt similar to the hose basanite. Detailed characterization of inclusion-free peridotite phases, and washed and unwahsed whole-rock samples, verifies the presence of a ubiquitous secondary contaminant which derives from interaction of the peridotites with local ground waters and host magma. Once the veil of this contamination is removed, coexisting phases are found to be in isotopic equilibrium. Further, a comparison of washed whole rocks and calculated clean-bulk compositions documents the occurrence of an important intragranular fluid-hosted trace element component. For the very incompatible elements (K, Rb, Cs, and Ba, and probably U, Th, Pb and gaseous components as well) this component dominates the nodule budget for two of the three samples studied in detail. Production of basaltic magmas from fertile but incompatible-element-depleted peridotite requires the action of melting processes such as those recently proposed by McKenzie (1985) and O'Hara (1985). The distinctive feature of these models is that they call on effectively larger source volumes for more incompatible elements. In this context, depletions of incompatible trace elements in MORB source mantle will be more extreme than has heretofore been suspected. This would essentially preclude the long-term total isolation of a MORB source mantle above the 670 km seismic discontinuity.

  8. Peridotite xenoliths from the Jagersfontein kimberlite pipe: I. Primary and primary-metasomatic mineralogy

    SciTech Connect

    Harte, B. ); Gurney, J.J. ); Winterburn, P.A. Isotope Geochemistry Facility, Pretoria )

    1990-02-01

    The geochemistry and textures of peridotite xenoliths from the Jagersfontein kimberlite pipe are reported. The xenoliths have a primary mineralogy of olivine {plus minus} orthopyroxene {plus minus} clinopyroxene {plus minus} garnet {plus minus} spinel. They are subdivided into coarse and deformed xenoliths corresponding to high- and low-temperature estimates, respectively. Coarse-grained xenoliths are further subdivided into low- and medium-temperature groups. Mineral chemistry of these two groups is distinct, e.g., clinopyroxene 100 Al/(Al + Cr) 24 to 60 and 60 to 70 in the medium- and low-temperature groups, respectively. Low-temperature xenoliths have undergone exsolution of pyroxene, spinel, and garnet in their pyroxenes. Primary modal metasomatism has occurred in the coarse xenoliths with the replacement of orthopyroxene by edenitic amphibole in the low-temperature xenoliths and of clinopyroxene by low-Ti phlogopite in the medium-temperature xenoliths. The amphibole stability limit confines it to shallower depths. Metasomatized xenoliths have been enriched in K, Na, Al, and Ca, and trace incompatible elements. Metasomatism is considered to have occurred at round 1 Ga by the infiltration of, and reaction with, ascending H{sub 2}O-rich fluids with Sr and Nd isotopic characteristics similar to group II kimberlites. The widespread chemical equilibrium seen in metasomatized xenoliths suggests that the particularly distinctive features of the low-temperature Jagersfontein xenoliths, namely exsolution and very low equilibration temperatures, may also be a result of the primary metasomatism.

  9. Ultramafic xenoliths from Damaping (Hannuoba region, NE-China): Petrogenetic implications from crystal chemistry of pyroxenes, olivine and Cr-spinel and trace element content of clinopyroxene

    NASA Astrophysics Data System (ADS)

    Princivalle, F.; De Min, A.; Lenaz, D.; Scarbolo, M.; Zanetti, A.

    2014-02-01

    Olivine, orthopyroxene, clinopyroxene and spinel from 16 different lherzolite mantle xenoliths from Damaping (Hannuoba region, NE-China) have been analyzed by X-ray single crystal diffraction and electron microprobe to define their structure, chemistry and cation distribution. LA-ICP-MS analysis has been carried out on clinopyroxenes to determine their LILE (Large-Ion Lithophile Elements), HFSE (High Field Strength Elements) and REE (Rare Earth Elements) contents. These latter define Depleted, Enriched, U-shaped and Transitional CI-normalized patterns suggesting that the xenolith suite here studied experienced at least two metasomatic events related to the porous flow percolation of melts with tholeiitic and alkaline affinity, respectively. The cation distribution of the mineral phases allowed determination of their intra- and inter-crystalline temperatures of equilibration. The closure temperature of intra-crystalline reactions for the constituent phases is in the range 600-800 °C, suggesting a relatively slow cooling rate, while the inter-crystalline temperature interval found out is in the range 866-1048 °C. The pressure conditions estimated for Hannuoba xenoliths through structural considerations correspond to a depth of about 50-60 km, consistent with the Moho depth beneath the Hannuoba area, which is estimated at about 42 km. The pressures of equilibration, as estimated from the cell and T site volumes of the clinopyroxenes, have been compared with the trace element and REE signatures; they demonstrate that Enriched, Depleted, U-shaped and Transitional clinopyroxenes formed at different pressures and are not restricted to a specific region of the mantle, testifying that the metasomatism took place at different depths in the mantle.

  10. Mafic and ultramafic xenoliths in San Bartolo lava field: New insights on the ascent and storage of Stromboli magmas

    NASA Astrophysics Data System (ADS)

    Laiolo, Marco; Cigolini, Corrado

    2006-06-01

    Mafic and ultramafic xenoliths are well represented within a large basaltic lava field of Stromboli. These basalts, known as San Bartolo lavas, show a high-K calc-alkaline (HKCA) affinity and were erupted <5 ka BP. Xenoliths consist of olivin-gabbro, gabbronorite, anorthosite, dunite, wehrlite and clinopyroxenite. Thermobarometric estimates for the crystallization of gabbroic materials show minima equilibration pressures of 0.17 0.24 GPa, at temperatures ranging from 940 to 1,030°C. These materials interacted with hydrous ascending HKCA basaltic magmas (with temperatures of 1,050 1,100°C) at pressures of about 0.2 0.4 GPa. These pressure regimes are nearly identical to those found for the crystallization of phenocrystic phases within HKCA basaltic lavas. Gabbroic inclusions are regarded as cumulates and represent crystallized portions of earlier HKCA Strombolian basalts. Dunite and wehrlite show porphyroclastic-heterogranular textures, whereas the clinopyroxenite exhibit a mosaic-equigranular texture typical of mantle peridotites. These ultramafic materials are in equilibrium with more primitive basaltic magmas (under moderately hydrous and anhydrous conditions) at pressures of 0.8 1.2 GPa, which is below the crust-mantle transition, located at about 20 km depth under Stromboli. Major and trace element distributions indicate comagmatism between the host basaltic lava and the mafic and ultramafic inclusions. REE patterns for mafic nodules are relatively regular and overlap the field of basaltic lavas (HKCA). They show moderate to high LREE enrichments and moderate enrichments in HREE relative to chonrites. Spider diagrams also show significant similarities between the lavas and the mafic-ultramafic xenoliths as well. During their ascent, primitive Strombolian magmas may be stored in upper-mantle regions where they interact with peridotitic materials and partly differentiate (to give dunite and wehrlite) before migrating to upper crustal levels. In this region

  11. Integrative Analysis of Mantle Lithosphere Rheology

    NASA Astrophysics Data System (ADS)

    Hirth, G.; Collins, J. A.; Molnar, P. H.; Kelemen, P. B.

    2014-12-01

    We will present an analysis of the rheology of mantle lithosphere based on extrapolation of lab-based flow laws, microstructural characterization of mantle shear zones and xenoliths, and the spatial distribution of mantle earthquakes and seismic anisotropy. As a starting point, we illustrate the similarity in the evolution of olivine lattice preferred orientation (LPO) for cm-scale lab samples (e.g., Zhang et al., 2000) and 100 meter-scale shear zones (e.g., Warren et al., 2008; Skemer et al., 2010). This correlation provides strong support for the extrapolation of lab data in both time and scale. The extrapolation of these results to plate-scale processes is supported by the analysis of shear wave splitting across the Alpine Fault on the South Island of New Zealand and its surrounding ocean basins (Zietlow et al., 2014). For the same region, the similarity in the fast Pn azimuth with the fast shear wave polarization directions indicates high strain deformation of relatively cold (~500-700oC) mantle lithosphere across a region 100-200 km wide (Collins and Molnar, 2014). This latter observation suggests that the lithosphere is significantly weaker than predicted by the extrapolation of dislocation creep or Peierls creep flow laws. Weakening via promotion of grain size sensitive creep mechanisms (diffusion creep and DisGBS) is likely at these conditions; however, studies of exhumed mantle shear zones generally indicate that the activation of these processes leads to strain localization at scales <<200 km. These observations motivate us to consider rheological constraints derived from geodetic studies and earthquake depths in regions where deformation of the lithosphere occurs at similar conditions. At face value, these data provide additional support for the extrapolation of lab data; the depth extent of earthquakes is consistent with estimates for the conditions where a transition from stable to unstable frictional sliding occurs (e.g., Boettcher et al., 2007) - and

  12. Osmium isotope variations in the Pacific mantle: implications for the distribution of heterogeneity in the convecting mantle

    NASA Astrophysics Data System (ADS)

    Ishikawa, A.; Senda, R.; Suzuki, K.; Tani, K.; Ishii, T.

    2015-12-01

    Recent accumulation of Os isotope data obtained either from abyssal peridotites or from ocean island peridotite xenoliths has clearly demonstrated that the modern convecting mantle is substantially heterogeneous in Os-isotope composition. Unlike other radiogenic isotope heterogeneities observed in oceanic basalts, largely controlled by incorporation of recycled crustal materials, it seems likely that the observed range of Os-isotope compositions in oceanic peridotites directly reflect varying degrees of ancient melt extraction from peridotitic mantle. Hence, global variations of Os-isotope compositions in oceanic peridotites may provide an important piece of information in unraveling the geochemical and geodynamic evolution of the convecting mantle. Here we present the Os-isotope variations in peridotite-serpentinite recovered from the Pacific area because the number of data available is yet scarce when compared with data from other oceans (Atlantic, Arctic and Indian Ocean). Our primary purpose is to test whether mantle domains underlying four major oceans are distinct in terms of Os isotope variations, reflecting the pattern of mantle convection or mixing efficiency. We examined 187Os/188Os ratios and highly siderophile element concentrations in serpentinized harzburgite recovered from Hess Deep in the East Pacific Rise, a mantle section in the Taitao ophiolite, Chile (Schulte et al., 2009), serpentinized harzburgite bodies in the Izu-Ogasawara and Tonga forearc (Parkinson et al., 1998), peridotite xenoliths from the Pali-Kaau vent in O'ahu island, Hawaii (Bizimis et al., 2007), and low-temperature type peridotite xenoliths from Malaita, Solomon Islands (Ishikawa et al., 2011). The results demonstrate that samples from each area display very similar Os-isotope variations with a pronounced peak in 187Os/188Os = 0.125-0.128. Moreover, the relatively larger datasets obtained from Hess Deep, Taitao and Malaita clearly exhibit the presence of secondary peak in 187Os

  13. Copper systematics during mantle melting and crustal differentiation in arcs: implications for S and Pb budgets of the continental crust

    NASA Astrophysics Data System (ADS)

    Lee, C.; Chin, E. J.; Dasgupta, R.; Luffi, P. I.; Le Roux, V.

    2010-12-01

    During mid-ocean ridge melting, Cu behaves like Sc and is therefore moderately incompatible, as evidenced by the twofold increase in Cu content in MORBs compared to the mantle. However, Cu content in the continental crust is comparable to that of the mantle, implying that during continental crust formation, Cu becomes effectively compatible. Cu is one of the only elements that exhibits Jekyll and Hyde behavior. This switch to being compatible is consistent with the observation that for the majority of arc magmas (as well as MORBs), Cu decreases with increasing SiO2 and decreasing MgO. Using natural samples, we infer new partition coefficients that indicate Cu is incompatible in olivine, pyroxenes, amphiboles, and biotite. The only mineral to exert significant control on Cu partitioning is sulfide. Cu behaves incompatibly during mantle melting because the modal abundance of sulfides relative to silicate minerals is extremely low. The monotonic decrease in Cu in most differentiating arc magmas requires sulfide saturation. In addition, the similar abundances of Cu in many primitive arc magmas compared to MORBs suggests that mantle melting in both environments occurs not only at sulfide saturation but without the need for excess Cu (or S). In a few cases, however, primitive arc magmas begin with high Cu or show increases in Cu with differentiation, which most likely requires unusually high oxygen fugacities in the source or magmatic evolution towards high oxygen fugacity. Such cases may be important for the origin of Cu porphyry deposits, but are generally rare. Because of the close link between Cu and sulfide during magma differentiation, Cu can be used as a proxy for the pre-degassed S content of arc magmas. The S content of continental crust, like many volatile elements, is basically unconstrained, but it can be inferred from Cu, which is much better constrained. Finally, the fact that the continental crust is highly depleted in Cu implies that there is a missing

  14. Noble gases, their carrier phases, and argon chronology of upper mantle rocks from Zabargad Island, Red Sea

    NASA Astrophysics Data System (ADS)

    Trieloff, M.; Weber, H. W.; Kurat, G.; Jessberger, E. K.; Janicke, J.

    1997-12-01

    Three ultramafic bodies on Zabargad Island contain fresh peridotites with mostly unfractionated primitive bulk major and trace element abundances and mostly monomineralic vein rocks (pyroxenites, olivinites, homblendites, etc.). We analyzed a set of coarse grained vein rocks with the 40Ar- 39Ar technique applying high resolution stepheating. Neutron induced argon isotopes derived from Ca, K, and Cl, and the specific degassing behaviour of major and accessory minerals enabled us to separate and identify different trapped and radiogenic argon components and their hosts. Within two clinopyroxenites trapped argon is present in (1) low temperature, low 40Ar/36Ar phases (serpentine and/or fluid inclusions), (2) pyroxene-related Cl-rich carriers (pyroxene and/or associated microinclusions) and (3) amphiboles which are intimately and nonseparably intergrown with pyroxene. The amphiboles, which can texturally, chemically, and isotopically be divided into different generations, formed by interaction of spinels and pyroxenes with mantle fluids during different stages of diapiric uplift (Agrinier et al., 1993). Formation of these amphiboles and microinclusions in pyroxenes, along with incorporation of isotopically distinct Ar with 40Ar/36Ar ratios up to 8000, can be related to recent mantle metasomatism also evident in Arabian xenoliths (Henjes-Kunst et al., 1990) and must have been induced by a variety of mantle fluids. For a homblendite, in situ radiogenic and excess argon components could be separated: the plateau age of 18.7 ± 1.3 Ma is in perfect agreement with a zircon Pb/Pb age of 18.4 ± 1.0 Ma (Oberli et al., 1987) interpreted as the age of crustal intrusion. Obviously, the formation of the hornblendite occurred during the final stage of uplift, most probably by interaction with seawater, as suggested by strontium, oxygen, and hydrogen isotopic data (Agrinier et al., 1993) and the low 40Ar/36Ar ratio (305) of the trapped argon. 4He, 20Ne, 40Ar, and 36Ar were measured

  15. Superplume Under the Siberian Craton: Evidence from Metasomatic Signatures in Kimberlitic Xenoliths

    NASA Astrophysics Data System (ADS)

    Pokhilenko, N.; Taylor, L. A.; Agashev, A. M.; Pokhilenko, L.; Baziotis, I. P.; Carmody, L.; Smith, G. B.

    2012-12-01

    A superplume is proposed to have existed under the Siberian Craton from the mid-Paleozoic to the late Mesozoic. The most dramatic result of this was the eruption of 17 million km3 of the Siberian trap magma at 250 ±2 Ma. However, the effects of this plume were recorded earlier in Devonian (~360 Ma) kimberlitic peridotites, as minor "basaltic" metasomatic overprinting. Indeed, peridotites from later kimberlites (~145 Ma) show evidence for similar, yet extensive, metasomatism, thereby bracketing the major plume climax with possible effects both prior and after this extensive activity. Indeed, a comparative analysis of petrological, mineralogical, and geochemical results from these early and late, Devonian versus Jurassic, peridotites have shown that the thickness of lithosphere of North-Eastern part of Siberian Platform was around 230 km at Middle Paleozoic time and decreased to 140-150 km by Late Mesozoic time. The Devonian Udachnaya kimberlite (central craton), one of the largest diamondiferous pipes in the world, provides a suite of garnet peridotites (>90%), many of which possess a 'sheared' texture. These peridotites possess evidence for a late-stage cryptic metasomatism, particularly in the garnets, exemplified by a notable increase in CaO, with a near constant Cr2O3 content, as the garnet compositions move from the harzburgitic field into that of lherzolites. Conversely, the xenolith suite from the Mesozoic Obnazhennaya kimberlite (north-east craton) is comprised of more pyroxenite xenoliths, with rare peridotite lithologies. The metasomatic over-print on these xenoliths has produced a more fertile chemistry, with the garnets becoming less Cr-rich, following a lherzolitic trend. These two trends in the garnets suggest that a long-lasting metasomatic ultra-event, the same metasomatic processes (and possibly similar sources), operated during both time periods. Changes from harzburgitic in the older Udachnaya peridotite rims to lherzolitic core-rim compositions

  16. Asthenosphere–lithosphere interactions in Western Saudi Arabia: Inferences from 3He/4He in xenoliths and lava flows from Harrat Hutaymah

    USGS Publications Warehouse

    Konrad, Kevin;; Graham, David W; Thornber, Carl; Duncan, Robert A; Kent, Adam J.R.; Al-Amri, Abdulla

    2016-01-01

    Elevated 3He/4He in the western harrats has been observed only at Rahat (up to 11.8 RA; Murcia et al., 2013), a volcanic field situated above thinned lithosphere beneath the Makkah-Medinah-Nafud volcanic lineament. Previous work established that spinel lherzolites at Hutaymah are sourced near the lithosphere-asthenosphere boundary (LAB), while other xenolith types there are derived from shallower depths within the lithosphere itself (Thornber, 1992). Helium isotopes are consistent with melts originating near the LAB beneath many of the Arabian harrats, and any magma derived from the Afar mantle plume currently appears to be of minor importance.

  17. Metasomatizing effects of serpentinization-related hydrothermal fluids in abyssal peridotites: new contributions from Hyblean peridotite xenoliths (southeastern Sicily)

    NASA Astrophysics Data System (ADS)

    Manuella, Fabio Carmelo; Ottolini, Luisa; Carbone, Serafina; Scavo, Lidia

    2016-11-01

    We studied a partially serpentinized peridotite xenolith, found in the diatreme tuff-breccia deposit at Valle Guffari (Hyblean Plateau, southeastern Sicily, Italy), which is representative of the Hyblean peridotite xenolith suite. We also considered all published (21) whole-rock analyses of Hyblean peridotites, to investigate the metasomatizing effects of seawater-related hydrothermal fluids in the Hyblean basement, an in-situ remnant of the ultraslow-spreading Permian Tethys. In detail, we analyzed the serpentine veins by different techniques (scanning electron microscopy-SEM, electron-probe microanalysis-EPMA, micro-Raman spectroscopy, X-ray powder diffraction-XRPD) to determine the crystal-chemical composition and the structure of the veins. In addition, secondary ion mass spectrometry (SIMS) was applied to measure the abundance of trace elements. Serpentine veins are made up of two Fe-rich polytypes, chrysotile 2Mc1 and lizardite 1T. The chondrite-normalized rare earth element compositions of both serpentine polytypes are lower than 1, except for a modest light rare earth element (LREE) enrichment, and also in some fluid-mobile elements (FME: B, Rb, Sr, U). Conversely, the whole-rock composition of the studied peridotite xenolith is enriched with LREE and other trace elements (B, Sr, P, Th, U, Pb), like most Hyblean peridotites. The REE and multi-element patterns of Hyblean peridotites are akin to those of hydrothermal sediments from the Mid-Atlantic Ridge and St. Demetrio hill (northern Hyblean Plateau), and abyssal peridotites (serpentinites) whose trace element abundance is generally ascribed to melt-rock interaction. The integrated interpretation of the data and the documentation of hydrothermal minerals [(Na,S)-rich apatite, carbonates] in serpentine veins indicate that serpentinization-related hydrothermal fluids do have a primary role in metasomatism (mainly for the abundance of LREE and high field strength elements-HFSE) of ancient (Permian Tethys) and

  18. Diamonds in an upper mantle peridotite nodule from kimberlite in southern wyoming.

    PubMed

    McCallum, M E; Eggler, D H

    1976-04-16

    Diamonds in a serpentinized garnet peridotite nodule from a diatreme in southern Wyoming are the first known occurrence in an upper mantle peridotite xenolith from a kimberlite intrusion in North America as well as the second authenticated occurrence of diamonds from kimberlite pipes in North America. The nodule is believed to have come from a section of depleted (partially melted) lherzolite at a depth of 130 to 180 kilometers. PMID:17831161

  19. Diamonds in an upper mantle peridotite nodule from kimberlite in southern wyoming

    USGS Publications Warehouse

    McCallum, M.E.; Eggler, D.H.

    1976-01-01

    Diamonds in a serpentinized garnet peridotite nodule from a diatreme in southern Wyoming are the first known occurrence in an upper mantle peridotite xenolith from a kimberlite intrusion in North America as well as the second authenticated occurrence of diamonds from kimberlite pipes in North America. The nodule is believed to have come from a section of depleted (partially melted) Iherzolite at a depth of 130 to 180 kilometers.

  20. Diamonds in an upper mantle peridotite nodule from kimberlite in southern wyoming.

    PubMed

    McCallum, M E; Eggler, D H

    1976-04-16

    Diamonds in a serpentinized garnet peridotite nodule from a diatreme in southern Wyoming are the first known occurrence in an upper mantle peridotite xenolith from a kimberlite intrusion in North America as well as the second authenticated occurrence of diamonds from kimberlite pipes in North America. The nodule is believed to have come from a section of depleted (partially melted) lherzolite at a depth of 130 to 180 kilometers.

  1. Activation volume for creep in the upper mantle.

    PubMed

    Ross, J V; Ave'lallemant, H G; Carter, N L

    1979-01-19

    The activation volume for creep, V*, of olivine-rich rocks has been determined in pressure-differential creep experiments on dunite at temperatures from 1100 degrees to 1350 degrees C and confining pressures from 5 to 15 kilobars. Values of V* range from 10.6 to 15.4 cubic centimeters per mole with a mean value of 13.4 cubic centimeters per mole, near that expected for oxygen ion self-diffusion. The quantity V* is incorporated into existing flow equations; in combination with observations on naturally deformed mantle xenoliths, estimates are given of the variation with depth of stress, strain rate, and viscosity. PMID:17738997

  2. Sr, Nd, and Pb isotopes of ultramafic xenoliths in volcanic rocks of Eastern China: enriched components EMI and EMII in subcontinental lithosphere

    USGS Publications Warehouse

    Tatsumoto, M.; Basu, A.R.; Wankang, H.; Junwen, W.; Guanghong, X.

    1992-01-01

    The UThPb, SmNd, and RbSr isotopic systematics of mafic and ultramafic xenolithic rocks and associated megacrystic inclusions of aluminous augite and garnet, that occur in three alkalic volcanic suites: Kuandian in eastern Liaoning Province, Hanluoba in Hebei Province, and Minxi in western Fujian Province, China are described. In various isotopic data plots, the inclusion data invariably fall outside the isotopic ranges displayed by the host volcanic rocks, testifying to the true xenolithic nature of the inclusions. The major element partitioning data on Ca, Mg, Fe, and Al among the coexisting silicate minerals of the xenoliths establish their growth at ambient mantle temperatures of 1000-1100??C and possible depths of 70-80 km in the subcontinental lithosphere. Although the partitioning of these elements reflects equilibrium between coexisting minerals, equilibria of the Pb, Nd, and Sr isotopic systems among the minerals were not preserved. The disequilibria are most notable with respect to the 206Pb 204Pb ratios of the minerals. On a NdSr isotopic diagram, the inclusion data plot in a wider area than that for oceanic basalts from a distinctly more depleted component than MORB with higher 143Nd 144Nd and a much broader range of 87Sr 86Sr values, paralleling the theoretical trajectory of a sea-water altered lithosphere in NdSr space. The garnets consistently show lower ?? and ?? values than the pyroxenes and pyroxenites, whereas a phlogopite shows the highest ?? and ?? values among all the minerals and rocks studied. In a plot of ??207 and ??208, the host basalts for all three areas show lower ??207 and higher ??208 values than do the xenoliths, indicating derivation of basalts from Th-rich (relative to U) sources and xenoliths from U-rich sources. The xenolith data trends toward the enriched mantle components, EMI and EMII-like, characterized by high 87Sr 86Sr and ??207 values but with slightly higher 143Nd 144Nd. The EMI trend is shown more distinctly by the host

  3. Iron isotope tracing of mantle heterogeneity within the source regions of oceanic basalts

    NASA Astrophysics Data System (ADS)

    Williams, Helen M.; Bizimis, Michael

    2014-10-01

    Mineralogical variations in the Earth's mantle and the relative proportions of peridotitic versus enriched and potentially crustally-derived pyroxenitic domains within the mantle have important implications for mantle dynamics, magma generation, and the recycling of surface material back into the mantle. Here we present iron (Fe) stable isotope data (δ57Fe, deviation in 57Fe/54Fe from the IRMM-014 standard in parts per thousand) for peridotite and garnet-pyroxenite xenoliths from Oahu, Hawaii and explore Fe isotopes as tracer of both peridotitic and pyroxenitic components in the source regions of oceanic basalts. The pyroxenites have δ57Fe values that are heavy (0.10 to 0.27‰) relative to values for mid-ocean ridge and ocean island basalts (MORB; OIB; δFe57∼0.16‰) and the primitive mantle (PM; δFe57∼0.04‰). Pyroxenite δ57Fe values are positively correlated with bulk pyroxenite titanium and heavy rare earth element (REE) abundances, which can be interpreted in terms of stable isotope fractionation during magmatic differentiation and pyroxene cumulate formation. In contrast, the peridotites have light δ57Fe values (-0.34 to 0.14‰) that correlate negatively with degree of melt depletion and radiogenic hafnium isotopes, with the most depleted samples possessing the most radiogenic Hf isotope compositions and lightest δ57Fe values. While these correlations are broadly consistent with a scenario of Fe isotope fractionation during partial melting, where isotopically heavy Fe is extracted into the melt phase, leaving behind low-δ57Fe peridotite residues, the extent of isotopic variation is far greater than predicted by partial melting models. One possibility is derivation of the samples from a heterogeneous source containing both light-δ57Fe (relative to PM) and heavy-δ57Fe components. While pyroxenite is a viable explanation for the heavy-δ57Fe component, the origin of the depleted light-δ57Fe component is more difficult to explain, as melting

  4. Constraints on Mantle Water from Peridotite Pyroxenes

    NASA Astrophysics Data System (ADS)

    Warren, J. M.; Hauri, E. H.

    2014-12-01

    The occurrence of volatiles in the Earth's mantle influences properties such as melting temperature, conductivity, and viscosity. To constrain upper mantle water content, secondary ion mass spectrometry was used to measure H2O in olivine, orthopyroxene and clinopyroxene in xenolith, orogenic, and abyssal peridotites. The comparison of fresh and altered peridotites demonstrates that low to moderate levels of alteration do not affect H2O concentrations, in agreement with mineral diffusion data. In all samples, olivine diffusively lost water during emplacement, as indicated by disequilibrium between olivine and co-existing pyroxenes. In contrast, clinopyroxene and orthopyroxene preserve their high temperature water contents, based on partitioning that agrees with published values from experiments and xenoliths. However, preservation of water in pyroxenes suggests that existing experimental diffusion data overestimates the diffusion coefficient for water in pyroxene. Among samples in this study, clinopyroxenes have 60-670 ppm H2O and orthopyroxenes have 10-300 ppm. Using the experimentally determined value of 0.11 for the olivine/orthopyroxene partition coefficient, olivine is calculated to have contained 8-34 ppm H2O. The highest olivine water concentration translates to an effective viscosity of 6×1019 Pa s at 1250°C and ~15 km depth, compared to a dry effective viscosity of 2.5×1021 Pa s. Bulk rock water concentrations, calculated using mineral modes, are 20-220 ppm and correlate with peridotite indices of melt depletion. However, trace element melt modeling indicates that peridotites have too much water relative to their rare earth element concentrations, which may be explained by late-stage melt addition, during which only hydrogen diffuses fast enough for re-equilibration.

  5. Water concentrations in mantle peridotite minerals

    NASA Astrophysics Data System (ADS)

    Warren, J. M.; Hauri, E. H.

    2010-12-01

    The concentration and distribution of volatiles in the mantle is important for constraining many key properties, including melting systematics at ridges and subduction zones. We present measurements of water concentrations in nominally anhydrous minerals from abyssal, orogenic and xenolith peridotites. Analyses of fresh and altered samples from a variety of locations are used to assess the extent to which mineral water concentrations reflect primary mantle compositions, versus diffusive loss and/or hydration due to secondary processes. Water concentrations were measured in olivine (Ol), orthopyroxene (Opx) and clinopyroxene (Cpx) by ion microprobe, using mineral specific standards and monitoring background concentrations by analysis of synthetic forsterite. Analytical reproducibility, based on 11 repeat analyses of an Ol grain, is 10%, while background H2O levels varied from 7-19 ppm. Samples include xenoliths from Pali Aike, Samoa and Spitsbergen, along with unusually fresh oceanic peridotites from the Gakkel Ridge and the Tonga Trench. In addition, samples were analyzed from the Southwest Indian Ridge (SWIR) and the Josephine Peridotite, both of which have moderate degrees of alteration. In olivine, water concentrations are <11 ppm, with the exception of Pali Aike xenoliths, which have water concentrations of 16-33 ppm. On average, peridotite Opx have 187 ppm and Cpx have 474 ppm. Pyroxenite veins from the Southwest Indian Ridge have systematically lower concentrations, with an average of 12 ppm in Opx and 55 ppm in Cpx. Water partition coefficients for Opx/Ol have an average value of 28 and Cpx/Ol of 57, significantly higher than previous estimates (e.g., Hirth and Kohlstedt, 1996). Excluding the pyroxenites, the average Cpx/Opx partition coefficient is 2, in agreement with published estimates. This suggests that Cpx and Opx preserve mantle water concentrations, whereas Ol has undergone hydrogen loss. Mineral rims have water concentrations that are within error

  6. Density heterogeneity of the cratonic mantle and dynamic topography in southern Africa

    NASA Astrophysics Data System (ADS)

    Artemieva, Irina; Vinnik, Lev

    2016-04-01

    An unusually high topography in southern Africa may be caused by the dynamic support of the mantle below the lithosphere base and/or by a low density (high depletion) of the cratonic lithospheric mantle. We use free-board constraints to examine the relative contributions of the both factors to surface topography and present the model of density structure of the lithospheric mantle in southern Africa. The results indicate that 0.5-1.0 km of topography requires contribution from the sublithospheric mantle because it cannot be explained by the lithosphere structure within the petrologically permitted range of mantle densities. We propose that this additional topography may be associated with the low-density region below the depth of isostatic compensation (LAB). A likely candidate is the low velocity layer between the lithospheric base and the mantle transition zone, where a temperature anomaly of 100-200 deg may produce the required extra contribution to regional topographic uplift. The calculated lithospheric mantle density values are in an overall agreement with xenolith-based data for lithospheric terranes of different ages and show an overall trend in mantle density increase from Archean to younger lithospheric terranes. A significant anomaly in mantle depletion beneath the Limpopo belt and the Bushveld Complex may result from regional melt-metasomatism. Density anomalies in the lithospheric mantle show an overall inverse correlation with seismic Vp, Vs velocities at 100-150 km depth; however, density-velocity relationship is strongly non-unique. Manuscripts in revision, Gondwana Research (2016)

  7. Pervasive, tholeiitic refertilisation and heterogeneous metasomatism in Northern Victoria Land lithospheric mantle (Antarctica)

    NASA Astrophysics Data System (ADS)

    Pelorosso, Beatrice; Bonadiman, Costanza; Coltorti, Massimo; Faccini, Barbara; Melchiorre, Massimiliano; Ntaflos, Theodoros; Gregoire, Michel

    2016-04-01

    The petrology of peridotite xenoliths in the Cenozoic volcanics from Greene Point (Northern Victoria Land, Antarctica) provides new constraints on the characterisation of the lithospheric mantle beneath the West Antarctic Rift. Based on mineral major and trace element models, this mantle domain is proposed to represent a residuum after 10% and 20% partial melting. Moreover, melting models and isotopic results for Sr and Nd systematics highlight the substantial contribution of tholeiitic melts percolating through peridotites. Close correlation with trace element contents in clinopyroxene phenocrysts from Ferrar and Karoo tholeiites allows us to ascribe this refertilisation event to the Jurassic. This asthenospheric melt was also able to transfer a garnet signature to the Northern Victoria Land mantle segment. The rare presence of glass and secondary phases indicate that Greene Point xenoliths were heterogeneously affected by alkaline metasomatism, probably related to the West Antarctic Rift System opening; this has also been widely observed in other Northern Victoria Land localities (i.e., Baker Rocks). Temperature and fO2 were calculated (950 °C; Δlog fO2 (QFM), - 1.70 to - 0.39) at a fixed pressure of 15 kbar, confirming the tendency of the anhydrous Greene Point xenolith population to have higher equilibration temperatures and comparable redox conditions, compared to the nearby amphibole-bearing peridotites from Baker Rocks.

  8. Did diamond-bearing orangeites originate from MARID-veined peridotites in the lithospheric mantle?

    PubMed

    Giuliani, Andrea; Phillips, David; Woodhead, Jon D; Kamenetsky, Vadim S; Fiorentini, Marco L; Maas, Roland; Soltys, Ashton; Armstrong, Richard A

    2015-04-17

    Kimberlites and orangeites (previously named Group-II kimberlites) are small-volume igneous rocks occurring in diatremes, sills and dykes. They are the main hosts for diamonds and are of scientific importance because they contain fragments of entrained mantle and crustal rocks, thus providing key information about the subcontinental lithosphere. Orangeites are ultrapotassic, H2O and CO2-rich rocks hosting minerals such as phlogopite, olivine, calcite and apatite. The major, trace element and isotopic compositions of orangeites resemble those of intensely metasomatized mantle of the type represented by MARID (mica-amphibole-rutile-ilmenite-diopside) xenoliths. Here we report new data for two MARID xenoliths from the Bultfontein kimberlite (Kimberley, South Africa) and we show that MARID-veined mantle has mineralogical (carbonate-apatite) and geochemical (Sr-Nd-Hf-O isotopes) characteristics compatible with orangeite melt generation from a MARID-rich source. This interpretation is supported by U-Pb zircon ages in MARID xenoliths from the Kimberley kimberlites, which confirm MARID rock formation before orangeite magmatism in the area.

  9. Did diamond-bearing orangeites originate from MARID-veined peridotites in the lithospheric mantle?

    PubMed

    Giuliani, Andrea; Phillips, David; Woodhead, Jon D; Kamenetsky, Vadim S; Fiorentini, Marco L; Maas, Roland; Soltys, Ashton; Armstrong, Richard A

    2015-01-01

    Kimberlites and orangeites (previously named Group-II kimberlites) are small-volume igneous rocks occurring in diatremes, sills and dykes. They are the main hosts for diamonds and are of scientific importance because they contain fragments of entrained mantle and crustal rocks, thus providing key information about the subcontinental lithosphere. Orangeites are ultrapotassic, H2O and CO2-rich rocks hosting minerals such as phlogopite, olivine, calcite and apatite. The major, trace element and isotopic compositions of orangeites resemble those of intensely metasomatized mantle of the type represented by MARID (mica-amphibole-rutile-ilmenite-diopside) xenoliths. Here we report new data for two MARID xenoliths from the Bultfontein kimberlite (Kimberley, South Africa) and we show that MARID-veined mantle has mineralogical (carbonate-apatite) and geochemical (Sr-Nd-Hf-O isotopes) characteristics compatible with orangeite melt generation from a MARID-rich source. This interpretation is supported by U-Pb zircon ages in MARID xenoliths from the Kimberley kimberlites, which confirm MARID rock formation before orangeite magmatism in the area. PMID:25882074

  10. Calcio-carbonatite melts and metasomatism in the mantle beneath Mt. Vulture (Southern Italy)

    NASA Astrophysics Data System (ADS)

    Rosatelli, Gianluigi; Wall, Frances; Stoppa, Francesco

    2007-12-01

    At Mt. Vulture volcano (Basilicata, Italy) calcite globules (5-150 μm) are hosted by silicate glass pools or veins cross-cutting amphibole-bearing, or more common spinel-bearing mantle xenoliths and xenocrysts. The carbonate globules are rounded or elongated and are composed of a mosaic of 2-20 μm crystals, with varying optical orientation. These features are consistent with formation from a quenched calciocarbonatite melt. Where in contact with carbonate amphibole has reacted to form fassaitic pyroxene. Some of these globules contain liquid/gaseous CO 2 bubbles and sulphide inclusions, and are pierced by quench microphenocrysts of silicate phases. The carbonate composition varies from calcite to Mg-calcite (3.8-5.0 wt.% MgO) both within the carbonate globules and from globule to globule. Trace element contents of the carbonate, determined by LAICPMS, are similar to those of carbonatites worldwide including ΣREE up to 123 ppm. The Sr-Nd isotope ratios of the xenolith carbonate are similar to the extrusive carbonatite and silicate rocks of Mt. Vulture testifying to derivation from the same mantle source. Formation of immiscibile silicate-carbonatite liquids within mantle xenoliths occurred via disequilibrium immiscibility during their exhumation.

  11. Did diamond-bearing orangeites originate from MARID-veined peridotites in the lithospheric mantle?

    NASA Astrophysics Data System (ADS)

    Giuliani, Andrea; Phillips, David; Woodhead, Jon D.; Kamenetsky, Vadim S.; Fiorentini, Marco L.; Maas, Roland; Soltys, Ashton; Armstrong, Richard A.

    2015-04-01

    Kimberlites and orangeites (previously named Group-II kimberlites) are small-volume igneous rocks occurring in diatremes, sills and dykes. They are the main hosts for diamonds and are of scientific importance because they contain fragments of entrained mantle and crustal rocks, thus providing key information about the subcontinental lithosphere. Orangeites are ultrapotassic, H2O and CO2-rich rocks hosting minerals such as phlogopite, olivine, calcite and apatite. The major, trace element and isotopic compositions of orangeites resemble those of intensely metasomatized mantle of the type represented by MARID (mica-amphibole-rutile-ilmenite-diopside) xenoliths. Here we report new data for two MARID xenoliths from the Bultfontein kimberlite (Kimberley, South Africa) and we show that MARID-veined mantle has mineralogical (carbonate-apatite) and geochemical (Sr-Nd-Hf-O isotopes) characteristics compatible with orangeite melt generation from a MARID-rich source. This interpretation is supported by U-Pb zircon ages in MARID xenoliths from the Kimberley kimberlites, which confirm MARID rock formation before orangeite magmatism in the area.

  12. Apatite, SiO2, rutile and orthopyroxene precipitates in minerals of eclogite xenoliths from Yakutian kimberlites, Russia

    NASA Astrophysics Data System (ADS)

    Alifirova, T. A.; Pokhilenko, L. N.; Korsakov, A. V.

    2015-06-01

    Eclogite mantle xenoliths from the central part of Siberian craton (Udachnaya and Zarnitsa kimberlite pipes) as well as from the northeastern edge of the craton (Obnazhennaya kimberlite) were studied in detail. Garnet and clinopyroxene show evident exsolution textures. Garnet comprises rutile, ilmenite, apatite, and quartz/coesite oriented inclusions. Clinopyroxene contains rutile (± ilmenite) and apatite precipitates. Granular inclusions of quartz in kyanite and garnet usually retain features of their high-pressure origin. According to thermobarometric calculations, the studied eclogitic suite was equilibrated within lithospheric mantle at 3.2-4.9 GPa and 813-1080 °C. The precursor composition of garnets from Udachnaya and Zarnitsa eclogites suggests their stability at depths 210-260 km. Apatite precipitation in clinopyroxenes of Udachnaya and Zarnitsa allows us to declare that original pyroxenes could have been indicative of their high P-T stability. Raman spectroscopic study of quartz and coesite precipitates in garnet porphyroblasts confirms our hypothesis on the origin of the exsolution textures during pressure-temperature decrease. With respect to mineralogical data, we suppose the rocks to be subjected to stepwise decompression and cooling within mantle reservoir.

  13. Serpentinisation of the Mantle Wedge Controls the Chemistry of Arc Magmas

    NASA Astrophysics Data System (ADS)

    Yardley, B. W.

    2008-12-01

    Arc magmas are derived from mantle wedge source regions that are more highly oxidised than other parts of the mantle but the cause of the oxidation is not well understood. Conventionally it has been attributed to influx of water derived from dehydration of the underlying slab, but this possibility has been comprehensively debunked by Frost et al. (1998). A direct link to the introduction of the fluids that flux melting fails to explain the ubiquitous high oxygen fugacity of xenoliths from this region, which point to extensive oxidation of the wedge. If it is not possible to introduce excess oxygen to the mantle wedge, it is likely that the oxidation arises from residual enrichment in oxygen due to dissociation of water and subsequent loss of hydrogen. Serpentinisation is the only geological process that generates sufficiently reducing conditions for water to dissociate, and it allows hydrogen fugacities to rise to sufficiently high levels for hydrogen to be lost as a migrating vapour phase. Because it creates magnetite, serpentinisation results in an increase in the Fe3+ content of the serpentinised rocks, which means that if subsequent heating regenerates less hydrous peridotite assemblages, the magnetite content of their spinel phase will be higher than in the precursor peridotite. For these regenerated peridotites, the greater magnetite content results in a higher oxygen fugacity than for the precursor peridotite. Hence the high Fe3+ signature generated during serpentine growth is carried down by flow in the wedge beyond the temperatures of serpentine breakdown and results in generation of high Fe3+:Fe2+ magmas. The main introduction of slab volatiles to the mantle wedge takes place at low temperatures, probably above 50km depth, where fluids are more abundant than at greater depths. This interpretation is consistent with geological and geophysical evidence for the existence of a serpentine body immediately above the slab in both ancient and modern subduction

  14. Petrology and geochemistry of mafic and ultramafic cumulates occurring as xenoliths in volcanic rocks from Polish part of Central European Volcanic Province.

    NASA Astrophysics Data System (ADS)

    Dajek, Michał; Matusiak-Małek, Magdalena; Puziewicz, Jacek; Ntaflos, Theodoros

    2015-04-01

    of diopside, but its mg# is lower than in other rocks (0.68-0.88). It is always LREE enriched and shows negative Eu, Ti and Sr anomalies. Orthopyroxene occurs only in norite from Wilcza Góra. It composition is strongly heterogenous and the mg# vary from 0.60 to 0.75. Fo- and NiO-poor (62-69% and 0.04-0.10 wt.%, respectively) olivine is present in gabbro from Winna Góra. Feldspar is typically plagioclase (An30-60), but in rims the composition grades toward ternary feldspar (Or up to 45%). Plagioclase is LREE enriched and shows strong positive Eu anomaly. It is enriched in Sr and depleted in Zr-Hf. Composition of opaques is similar to that in plagioclase-free rocks. Modelling based on the trace element composition of clinopyroxene suggest that all the studied xenoliths are precipitates from alkaline silicate magmas, usually similar to the host volcanic-rock. Relatively high content of iron in silicates suggest crystallization at crustal depths or at crust/mantle boundary, but ratios between AIV and AlVI in clinopyroxene suggest that plagioclase¬-bearing rocks crystalized in different conditions than pyroxenites (Aoki and Shiba, 1976). Origin of xenoliths from Złotoryja-Jawor Volcanic Complex is different than that of clinopyroxene-rich mafic rocks from Lutynia basanite (Lądek Zdrój Volcanic Complex (Ackerman et al., 2012; J. of Geosciences). This study was possible thanks to the project NCN 2011/03/B/ST10/06248 of Polish National Centre for Science.

  15. Ancient recycled crust beneath the Ontong Java Plateau: Isotopic evidence from the garnet clinopyroxenite xenoliths, Malaita, Solomon Islands

    NASA Astrophysics Data System (ADS)

    Ishikawa, Akira; Kuritani, Takeshi; Makishima, Akio; Nakamura, Eizo

    2007-07-01

    We present a Sr, Nd, Hf and Pb isotope investigation of a set of garnet clinopyroxenite xenoliths from Malaita, Solomon Islands in order to constrain crustal recycling in the Pacific mantle. Geological, thermobarometric and petrochemical evidence from previous studies strongly support an origin as a series of high-pressure (> 3 GPa) melting residues of basaltic material incorporated in peridotite, which was derived from Pacific convective mantle related to the Ontong Java Plateau magmatism. The present study reveals isotopic variations in the pyroxenites that are best explained by different extents of chemical reaction with ambient peridotite in the context of a melting of composite source mantle. Isotopic compositions of bimineralic garnet clinopyroxenites affected by ambient peridotite fall within the oceanic basalt array, similar to those of Ontong Java Plateau lavas. In contrast, a quartz-garnet clinopyroxenite, whose major element compositions remain intact, has lower 206Pb/ 204Pb- 143Nd/ 144Nd and higher 87Sr/ 86Sr- 207Pb/ 204Pb ratios than most oceanic basalts. These isotopic signatures show some affinity with proposed recycled sources such as the so-called EM-1 or DUPAL types. Constraints from major and trace element characteristics of the quartz-garnet clinopyroxenite, the large extent of Hf-Nd isotopic decoupling and the good coincidence of Pb isotopes to the Stacey-Kramers curve, all indicate that pollution of southern Pacific mantle occurred by the subduction or delamination of Neoproterozoic granulitic lower crust (0.5-1 Ga). This crustal recycling could have taken place around the suture of Rodinia supercontinent, a part of which resurfaced during mantle upwelling responsible for creating the Cretaceous Ontong Java Plateau.

  16. Composition and structure of mantle lithosphere in the Russian Far East according to xenolths study.

    NASA Astrophysics Data System (ADS)

    Prikhodko, V.; Ashchepkov, I.; Ntaflos, T.; Barkar, A.; Vysotsky, S.; Esin, S.; Kutolin, V.; Prussevich, A.

    2012-04-01

    Lherzolitic mantle xenoliths from the Pliocene - basalts of Russian Primorie referred to the different volcanic regions (plateaus) show spatial -temporal variations of thee mineral chemistry determined for 550 xenoliths and TRE in IGM Novosibirsk but rather similar bulk rock compositions. In the N Eastern volcanic zone in Sovgavan plateau the xenoliths bearing basalts occur in late stages of the Miocen - Pleocene basalt plateau (Tuttochi), in the late extrusions (necks) and dykes and the post erosion enclosed valley flows (Sunku and Kamky) scoria cones (MountKurgan) where amphiboles occurred in hybrid websterites. In Southern Sikhote Alin in Shkotkov plteau Fe- lherzolites with amphiboles and mica dominate in the basement lavas. The Pliocene Pogelbanochny neck and lava flow contain y large xenoliths (to 1 m) (Scheka , 1981) sapphires and some other gems (Vysotsky et al ., 2009). The xenolith in the western volcanic zones - Lesozovoskaya, Medvezhy contains kelyphites after garnets and Phl veins The Cr- diopsides in Tuttochi are more (Na, Al , Ti) depleted and dispersed, in Kamky flow Fe-rich trends is found similar to relation for CPx in Sunku flow and Mount Kurgan. The early stage Nelma and Shkotov palateu Cr-Di show high dispersion and Fe-metasomatism. Mesozoic Anyui Cpx are less Na-Ti-Al riched. The Sp refer to most Al rich OSMA part with are Cr-picotites equilibrated with garnets (16-24% Cr2O3). Calculated PT geotherms ~90 mWm-2 everywhere starts near Gar stability at18kabrs. The Western fields show lower mantle thermal gradients. In basaltic plateau P-Fe# trends show percolation trends increasing P-Fe# with Cpx pressure lower then Opx. Those from latest scoria cones demonstrates sub adiabatic PT trajectories (MountKurgan) or Fe# rising to bottom (Medvezhy) formed by melt interaction. The basement plateau Shkotov xenolith reveal first thermal plum impact and subvertical magma channel trend TRE determined by LAMICP IGM for Sovgavan Cr- diopsides (Sanky

  17. Compositionally Controlled Volatile Content of Nominally Volatile-Free Minerals in the Continental Upper Mantle of Southern Gondwana (Patagonia & W. Antarctica)

    NASA Astrophysics Data System (ADS)

    Rooks, E. E.; Gibson, S. A.; Leat, P. T.; Petrone, C. M.

    2015-12-01

    H2O and F contents affect many physical and chemical properties of the upper mantle, including melting temperature and viscosity. These elements are hosted by hydrous and F-rich phases, and by modally abundant, nominally-anhydrous/halogen-free mantle minerals, which can potentially accommodate the entire volatile budget of the upper mantle. We present high-precision SIMS analyses of H2O, and F in mantle xenoliths hosted by recently-erupted (5-10 Ka) alkali basalts from south Patagonia (Pali Aike) and older (c. 25 Ma) alkali basalts from localities along the Antarctic Peninsula. Samples are well characterised peridotites and pyroxenites, from a range of depths in the off-craton lithospheric mantle. Minerals are relatively dry: H2O contents of olivine span 0-49 ppm, orthopyroxene 150-235 ppm and clinopyroxene 100-395 ppm, with highest concentrations found in spinel-garnet lherzolites from Pali Aike. These H2O concentrations fall within the global measured range for off-craton mantle minerals. H2O and F are correlated, and the relative compatibility of F in mantle phases is clinopyroxene>orthopyroxene>olivine. However, elevated F concentrations of 100-210 ppm are found in pyroxenites from two Antarctic localities. This elevated F content is not correlated with high H2O, suggesting that these rocks interacted with a F-rich melt. In clinopyroxenes, F concentration is correlated with Ti, and the ratio of M1Ti to M1Al + M1Cr, suggesting a charge balanced substitution. Consistency between samples (excepting high-F pyroxenites) suggests a constant F-budget, and that concentrations in clinopyroxenes are controlled by mineral chemistry. In orthopyroxene, F correlates with CaO, but no other major or minor elements. Large variability of H2O concentrations within samples is attributed to diffusive loss during ascent. Cl is negligible in all samples, indicating little or no influence of slab fluids from this long-lived subduction zone.

  18. Thermo-chemical heterogeneity of continental lithospheric mantle: examples from Europe, Siberia, and North America

    NASA Astrophysics Data System (ADS)

    Artemieva, I. M.

    2015-12-01

    I present models of lithosphere density and the non-thermal part of upper mantle Vs anomalies in different tectonic provinces of Eurasia and North America. The focus is on compositional heterogeneity of the lithospheric mantle, and therefore the effect of regional temperature variations on density and Vs is removed by applying regional temperature corrections, which are constrained by heat flow data. Significant parts of Precambrian cratons of Laurasia are characterized by extremely low surface heat flow values (<25-30 mW/m2), which imply the depth extent of the lithospheric keels down to 300-350 km, at least locally. These values are in apparent contradiction with a worldwide compilation of cratonic xenolith P-T arrays, which are usually consistent with surface heat flow of around 40 mW/m2 and the lithosphere thickness of 200-250 km depth. Models of lithosphere density and seismic velocity structure indicate that xenoliths do not sample mantle with the lowest density and the highest velocity. Density structure of continental lithosphere mantle correlates with crustal structure and surface tectonics. This observation is illustrated by examples from the East European and the Siberian cratons, where lateral variations in density structure of the lithospheric mantle are compared with petrological studies of mantle-derived xenoliths from the Fennoscandian and Siberian kimberlite provinces. The results indicate that in the Siberian craton isopycnicity is satisfied only in major kimberlite provinces. High lithosphere density in major sedimentary basins suggests the presence of eclogitic material. Since the depth distribution of density anomalies is unknown, the analysis is complemented by seismic data in order to understand better geodynamic causes of mantle density heterogeneity. Temperature-corrected seismic velocity structure based on published high-resolution tomography models indicates a pronounced stratification of lithospheric mantle in many Precambrian terranes

  19. Lower-crustal xenoliths from Jurassic kimberlite diatremes, upper Michigan (USA): Evidence for Proterozoic orogenesis and plume magmatism in the lower crust of the southern Superior Province

    USGS Publications Warehouse

    Zartman, Robert E.; Kempton, Pamela D.; Paces, James B.; Downes, Hilary; Williams, Ian S.; Dobosi, Gábor; Futa, Kiyoto

    2013-01-01

    unique peraluminous composition. It has the lowest εNd and εHf values of the suite. Its isotopic compositions indicate that it is significantly older than the other granulites. Broken zircon cores encased by younger overgrowths suggest that this granulite includes a large component of pre-existing sedimentary rocks. Two distinct populations of zircons from S69-5 were dated by sensitive high-resolution ion microprobe. Abundant rounded zircons yield ages of 1104 ± 42 (2σ) Ma, which coincide with the Mid-Continent Rift flood basalt eruptions. Their morphology is similar to those found in lower-crustal rocks that have undergone granulite-facies metamorphism and thus they are considered to represent the age of Group 2 granulites. Also present are less abundant elongate zircon grains that yield a mean age of 1387 ± 32 (2σ) Ma. Their elongate shapes indicate growth from a melt or fluid, possibly associated with 1·3–1·5 Ga anorogenic granite magmatism exposed in the shallow crust to the south in Wisconsin, or related to an initial encroachment of the Keweenawan plume upon the lower crust. Older ages recognized in zircon cores are less well constrained but may be related to tectono-magmatic events in the southern Superior craton. Within the studied suite only S69-5 was recognized as a remnant of the Late Archean lower crust into which the Group 1 and 2 mafic granulite precursor basalts were intruded. Collectively, the data show that the lower crust beneath northern Michigan formed in Archean times and underwent a variety of tectono-magmatic processes throughout the Proterozoic, including orogenesis, partial melting and mafic magmatic underplating in response to upwelling mantle plumes.

  20. Subduction signature in backarc mantle?

    NASA Astrophysics Data System (ADS)

    Nelson, W. R.; Snow, J. E.; Brandon, A. D.; Ohara, Y.

    2013-12-01

    Abyssal peridotites exposed during seafloor extension provide a rare glimpse into the processes occurring within the oceanic mantle. Whole rock and mineral-scale major element data from abyssal peridotites record processes intimately associated with melt-depletion and melt-rock interaction occurring just prior to exposure of the mantle at the surface. Isotopic data, however, can provide insight into the long-term evolution of the oceanic mantle. A number of studies of mantle material exposed along mid-ocean ridges have demonstrated that abyssal peridotites from Mid-Atlantic Ridge, Gakkel Ridge, and Southwest Indian Ridge commonly display a range of whole rock Os isotopic ratios (187Os/188Os = 0.118- 0.130; Brandon et al., 2000; Standish et al., 2002; Alard et al., 2005; Harvey et al., 2006; Liu et al., 2008). The range of isotopic values in each region demonstrates that the oceanic mantle does not melt uniformly over time. Instead, anciently depleted regions (187Os/188Os ≈ 0.118) are juxtaposed against relatively fertile regions (187Os/188Os ≈ 0.130) that are isotopically similar to established primitive mantle values (187Os/188Os = 0.1296; Meisel et al. 2001). Abyssal peridotites from the Godzilla Megamullion and Chaotic Terrain in the backarc Parece Vela Basin (Philippine Sea) display a range of Os isotopic values extending to similar unradiogenic values. However, some of the backarc basin abyssal peridotites record more radiogenic 187Os/188Os values (0.135-0.170) than mid-ocean ridge peridotites. Comparable radiogenic signatures are reported only in highly weathered abyssal peridotites (187Os/188Os ≤ 0.17, Standish et al., 2002) and subduction-related volcanic arc peridotites (187Os/188Os ≤ 0.16, Brandon et al., 1996; Widom et al., 2003). In both the weathered peridotites and arc peridotites, the 187Os/188Os value is negatively correlated with Os abundance: the most radiogenic value has the lowest Os abundance (< 1 ppb) making them highly susceptible to

  1. Mantle density beneath the Siberian craton based on free board constrains

    NASA Astrophysics Data System (ADS)

    Cherepanova, Yulia; Artemieva, Irina

    2014-05-01

    We present the mantle density model of the Archean-Proterozoic Siberian Craton (SC). The density model is constrained by free-board (buoyancy) modeling (Lachenbruch and Morgan, 1990). The approach assumes isostatic compensation of the region, and is justified by the near-zero free-air gravity for most of the region, except for the flanking orogenic belts with high topography. Despite a relatively uniform topography of the SC (ca. 400 m for most of the region and reaching 700 m in the shields), the craton has a strongly heterogeneous crustal structure with large regional variations in Moho and average crustal Vp (Cherepanova et al., 2013) which reflects its complex tectonic evolution. Formed by amalgamation of several Archean terranes, the craton has been significantly affected by Proterozoic collisional and extensional events, the late- Proterozoic rifting at its margins, the Devonian rifting of the Vilyui rift, several pulses of kimberlite magmatism, and the Permo-Triassic trap basalt magmatism. The strong lateral and vertical heterogeneity of the lithospheric mantle has been documented so far in the studied of the mantle xenoliths from kimberlite pipes and in a limited number of geophysical studies. Here we extend geophysical analysis of mantle compositional heterogeneity by evaluating mantle density structure and interpreting its regional variations in terms of mantle mg#. We link regional large-amplitude variations in mantle depletion to the tectonic evolution of the craton and compare these results with geophysical models and petrologic data. We speculate on the origin of compositional heterogeneity of the lithospheric mantle, which is in overall agreement with results of a joint analysis of seismic and thermal data (Artemieva, 2009) and mantle xenolith studies which provide information on metasomatic enrichment of the depleted lithospheric mantle by the tectonic events. The results indicate the heterogeneous structure of mantle density, with the average

  2. Petrology and geochronology of crustal xenoliths from the Bering Strait region: Linking deep and shallow processes in extending continental crust

    USGS Publications Warehouse

    Akinin, V.V.; Miller, E.L.; Wooden, J.L.

    2009-01-01

    Petrologic, geochemical, and metamorphic data on gneissic xenoliths derived from the middle and lower crust in the Neogene Bering Sea basalt province, coupled with U-Pb geochronology of their zircons using sensitive high-resolution ion microprobe-reverse geometry (SHRIMP-RG), yield a detailed comparison between the P-T-t and magmatic history of the lower crust and magmatic, metamorphic, and deformational history of the upper crust. Our results provide unique insights into the nature of lithospheric processes that accompany the extension of continental crust. The gneissic, mostly maficxenoliths (constituting less than two percent of the total xenolith population) from lavas in the Enmelen, RU, St. Lawrence, Nunivak, and Seward Peninsula fields most likely originated through magmatic fractionation processes with continued residence at granulite-facies conditions. Zircon single-grain ages (n ??? 125) are interpreted as both magmatic and metamorphic and are entirely Cretaceous to Paleocene in age (ca. 138-60 Ma). Their age distributions correspond to the main ages of magmatism in two belts of supracrustal volcanic and plutonic rocks in the Bering Sea region. Oscillatory-zoned igneous zircons, Late Cretaceous to Paleocene metamorphic zircons and overgrowths, and lack of any older inheritance in zircons from the xenoliths provide strong evidence for juvenile addition of material to the crust at this time. Surface exposures of Precambrian and Paleozoic rocks locally reached upper amphibolite-facies (sillimanite grade) to granulite-facies conditions within a series of extension-related metamorphic culminations or gneiss domes, which developed within the Cretaceous magmatic belt. Metamorphic gradients and inferred geotherms (??30-50 ??C/km) from both the gneiss domes and xenoliths aretoo high to be explained by crustal thickening alone. Magmatic heat input from the mantle is necessary to explain both the petrology of the magmas and elevated metamorphic temperatures. Deep

  3. Mantle wedge infiltrated with saline fluids from dehydration and decarbonation of subducting slab.

    PubMed

    Kawamoto, Tatsuhiko; Yoshikawa, Masako; Kumagai, Yoshitaka; Mirabueno, Ma Hannah T; Okuno, Mitsuru; Kobayashi, Tetsuo

    2013-06-11

    Slab-derived fluids play an important role in heat and material transfer in subduction zones. Dehydration and decarbonation reactions of minerals in the subducting slab have been investigated using phase equilibria and modeling of fluid flow. Nevertheless, direct observations of the fluid chemistry and pressure-temperature conditions of fluids are few. This report describes CO2-bearing saline fluid inclusions in spinel-harzburgite xenoliths collected from the 1991 Pinatubo pumice deposits. The fluid inclusions are filled with saline solutions with 5.1 ± 1.0% (wt) NaCl-equivalent magnesite crystals, CO2-bearing vapor bubbles, and a talc and/or chrysotile layer on the walls. The xenoliths contain tremolite amphibole, which is stable in temperatures lower than 830 °C at the uppermost mantle. The Pinatubo volcano is located at the volcanic front of the Luzon arc associated with subduction of warm oceanic plate. The present observation suggests hydration of forearc mantle and the uppermost mantle by slab-derived CO2-bearing saline fluids. Dehydration and decarbonation take place, and seawater-like saline fluids migrate from the subducting slab to the mantle wedge. The presence of saline fluids is important because they can dissolve more metals than pure H2O and affect the chemical evolution of the mantle wedge.

  4. Mantle wedge infiltrated with saline fluids from dehydration and decarbonation of subducting slab

    PubMed Central

    Kawamoto, Tatsuhiko; Yoshikawa, Masako; Kumagai, Yoshitaka; Mirabueno, Ma. Hannah T.; Okuno, Mitsuru; Kobayashi, Tetsuo

    2013-01-01

    Slab-derived fluids play an important role in heat and material transfer in subduction zones. Dehydration and decarbonation reactions of minerals in the subducting slab have been investigated using phase equilibria and modeling of fluid flow. Nevertheless, direct observations of the fluid chemistry and pressure–temperature conditions of fluids are few. This report describes CO2-bearing saline fluid inclusions in spinel-harzburgite xenoliths collected from the 1991 Pinatubo pumice deposits. The fluid inclusions are filled with saline solutions with 5.1 ± 1.0% (wt) NaCl-equivalent magnesite crystals, CO2-bearing vapor bubbles, and a talc and/or chrysotile layer on the walls. The xenoliths contain tremolite amphibole, which is stable in temperatures lower than 830 °C at the uppermost mantle. The Pinatubo volcano is located at the volcanic front of the Luzon arc associated with subduction of warm oceanic plate. The present observation suggests hydration of forearc mantle and the uppermost mantle by slab-derived CO2-bearing saline fluids. Dehydration and decarbonation take place, and seawater-like saline fluids migrate from the subducting slab to the mantle wedge. The presence of saline fluids is important because they can dissolve more metals than pure H2O and affect the chemical evolution of the mantle wedge. PMID:23716664

  5. Mantle wedge infiltrated with saline fluids from dehydration and decarbonation of subducting slab.

    PubMed

    Kawamoto, Tatsuhiko; Yoshikawa, Masako; Kumagai, Yoshitaka; Mirabueno, Ma Hannah T; Okuno, Mitsuru; Kobayashi, Tetsuo

    2013-06-11

    Slab-derived fluids play an important role in heat and material transfer in subduction zones. Dehydration and decarbonation reactions of minerals in the subducting slab have been investigated using phase equilibria and modeling of fluid flow. Nevertheless, direct observations of the fluid chemistry and pressure-temperature conditions of fluids are few. This report describes CO2-bearing saline fluid inclusions in spinel-harzburgite xenoliths collected from the 1991 Pinatubo pumice deposits. The fluid inclusions are filled with saline solutions with 5.1 ± 1.0% (wt) NaCl-equivalent magnesite crystals, CO2-bearing vapor bubbles, and a talc and/or chrysotile layer on the walls. The xenoliths contain tremolite amphibole, which is stable in temperatures lower than 830 °C at the uppermost mantle. The Pinatubo volcano is located at the volcanic front of the Luzon arc associated with subduction of warm oceanic plate. The present observation suggests hydration of forearc mantle and the uppermost mantle by slab-derived CO2-bearing saline fluids. Dehydration and decarbonation take place, and seawater-like saline fluids migrate from the subducting slab to the mantle wedge. The presence of saline fluids is important because they can dissolve more metals than pure H2O and affect the chemical evolution of the mantle wedge. PMID:23716664

  6. Oxidation state of the lithospheric mantle beneath the Massif Central,France

    NASA Astrophysics Data System (ADS)

    Uenver-Thiele, L.; Woodland, A. B.; Downes, H.; Altherr, R.

    2012-04-01

    The Tertiary and Quaternary volcanism of the French Massif Central sampled the underlying subcontinental lithospheric mantle (SCLM) in the form of xenoliths over a wide geographic area of ~20.000km2. Such an extensive distribution of xenoliths provides an unique opportunity to investigate regional variations in mantle structure and composition. On the basis of textural and geochemical differences, Lenoir et al. (2000) and later Downes et al. (2003) identified two distinct domains in the SCLM lying north and south of latitude 45° 30' N, respectively. The northern domain is relatively refractory, but has experienced pervasive enrichment of LREE. The southern domain is generally more fertile, exhibiting depletion in LREE. A metasomatic overprint has developed to variable extents in many xenolith suites. The different histories of these two juxtaposed blocks of SCLM should also be reflected in their oxidation state, with local variations also to be expected due to metasomatic interactions. For example, if carbonate-melt metasomatism played a role in the LREE enrichment of the northern domain (Lenoir et al. 2000; Downes et al. 2003), then such mantle should be relatively oxidised. Since surprisingly little redox data are currently available, we are undertaking a study to determine the oxidation state of the SCLM beneath the Massif Central over the largest geographical area possible. All xenoliths investigated are spinel peridotites, mostly with protogranular textures (although some samples are porphyroclastic or equigranular). Most samples are nominally anhydrous although minor amphibole is present in some xenolith suites. Major element compositions of the individual minerals were determined by microprobe. Two-pyroxene temperatures (BKN) range from 750° to ~1200° C. Ferric iron contents of spinel were determined by Mössbauer spectroscopy and gave a range of Fe3+/ Fetot from 0.191 to 0.418, with a conservative uncertainty of ±0.02. These data were used to calculate

  7. Mantle geodynamics and implications for Earth's mantle

    NASA Astrophysics Data System (ADS)

    Lassak, Teresa Mae

    Seismic evidence suggests the presence of two large, low shear wave velocity provinces in Earth's lowermost mantle beneath Africa and the central Pacific. The origin and evolution of these provinces is not constrained but is thought to be linked to Earth's large-scale mantle dynamics. The focus of this work is to determine if seismic models of core-mantle boundary (CMB) topography can be useful in characterizing Earth's mantle dynamics. The dynamic motions in Earth's mantle exert stress on the boundaries of Earth's mantle, which results in topography at the Earth's CMB. Better understanding topography on Earth's CMB could provide important constraints on mantle dynamics and on lower mantle heterogeneity. The work in this dissertation investigates two proposed dynamical hypotheses for Earth's mantle: thermal upwellings (plume clusters) and large intrinsically dense yet thermally buoyant piles of primitive mantle material (thermochemical piles). CMB topography is calculated for each model in order to identify topography patterns that are unique to each model. CMB topographic relief beneath thermochemical piles is relatively flat and upwarped compared to downwarping CMB topography beneath downwelling regions. In plume cluster models, there is a direct correlation between upwarping relief on the CMB and upwellings while downwarping relief occurs exclusively beneath downwelling regions. The results show that both thermochemical pile and plume cluster models produce unique CMB topography signatures; thus as seismic studies better resolve the global topography of Earth's CMB, there is potential to constrain the chemical and dynamic nature of Earth's lower mantle.

  8. Dikes, joints, and faults in the upper mantle

    USGS Publications Warehouse

    Wilshire, H.G.; Kirby, S.H.

    1989-01-01

    Three different types of macroscopic fractures are recognized in upper-mantle and lower-crustal xenoliths in volcanic rocks from around the world: 1. (1) joints that are tensile fractures not occupied by crystallized magma products 2. (2) dikes that are tensile fractures occupied by mafic magmas crystallized to pyroxenites, gabbros or hydrous-mineral-rich rocks, 3. (3) faults that are unfilled shear fractures with surface markings indicative of shear displacement. In addition to intra-xenolith fractures, xenoliths commonly have polygonal or faceted shapes that represent fractures exploited during incorporation of the xenoliths into the host magma that brought them to the surface. The various types of fractures are considered to have formed in response to the pressures associated with magmatic fluids and to the ambient tectonic stress field. The presence of fracture sets and crosscutting relations indicate that both magma-filled and unfilled fractures can be contemporaneous and that the local stress field can change with time, leading to repeated episodes of fracture. These observations give insight into the nature of deep fracture processes and the importance of fluid-peridotite interactions in the mantle. We suggest that unfilled fractures were opened by volatile fluids exsolved from ascending magmas to the tops of growing dikes. These volatile fluids are important because they are of low viscosity and can rapidly transmit fluid pressure to dike and fault tips and because they lower the energy and tectonic stresses required to extend macroscopic cracks and to allow sliding on pre-existing fractures. Mantle seismicity at depths of 20-65 km beneath active volcanic centers in Hawaii corresponds to the depth interval where CO2-rich fluids are expected to be liberated from ascending basaltic magmas, suggesting that such fluids play an important role in facilitating earthquake instabilities in the presence of tectonic stresses. Other phenomena related to the fractures

  9. Evolution of the middle crust beneath the western Pannonian Basin: a xenolith study

    NASA Astrophysics Data System (ADS)

    Török, Kálmán; Németh, Bianka; Koller, Friedrich; Dégi, Júlia; Badenszki, Eszter; Szabó, Csaba; Mogessie, Aberra

    2014-02-01

    Felsic to mafic granulite xenoliths from late Neogene basalt pyroclastics in four localities of the western Pannonian Basin (Beistein, Kapfenstein, Szigliget and Káptalantóti (Sabar-hegy) were studied to find out their metamorphic and fluid history. The characteristic mineral assemblage of the granulites consists of Pl + Opx + Qtz ± Cpx ± Bt ± Grt ± Kfs. Based on abundant magmatic relic microstructural domains occurring in these rocks, the potential precursors might have been predominantly felsic igneous or high to ultrahigh temperature rocks. Ternary feldspar thermometry provides a rough estimate of temperatures of about 920-1070 °C. The first fluid invasion event, which is linked with this early high to ultrahigh temperature stage is characterised by primary pure CO2 inclusions in apatite and zircon. The densest primary CO2 inclusions indicate 0.52-0.64 GPa pressure at the estimated temperature range of crystallization. According to mineral equilibria and geothermobarometry, the high to ultrahigh temperature rock cooled and crystallized to granulite of predominantly felsic composition at about 750-870 °C and 0.50-0.75 GPa in the middle crust, between 20 and 29 km depths. The second fluid invasion event is recorded by primary CO2-rich fluid inclusions hosted in the granulitic mineral assemblage (plagioclase, quartz and orthopyroxene). In addition to CO2, Raman spectroscopy revealed the presence of minor N2, H2S, CO and H2O in these inclusions. Partial melting of biotite-bearing assemblages could be connected to the next fluid invasion shown by secondary CO2-rich fluids recorded along with healed fractures in plagioclase, clinopyroxene and orthopyroxene. This event could have happened at depths similar to the previous ones. The final step in the granulite evolution was the sampling in the middle crust and transportation to the surface in form of xenoliths by mafic melt. This event generated temperature increase and pressure decrease and thus, limited

  10. Evolution of the middle crust beneath the western Pannonian Basin: a xenolith study

    NASA Astrophysics Data System (ADS)

    Török, Kálmán; Németh, Bianka; Koller, Friedrich; Dégi, Júlia; Badenszki, Eszter; Szabó, Csaba; Mogessie, Aberra

    2013-05-01

    Felsic to mafic granulite xenoliths from late Neogene basalt pyroclastics in four localities of the western Pannonian Basin (Beistein, Kapfenstein, Szigliget and Káptalantóti (Sabar-hegy) were studied to find out their metamorphic and fluid history. The characteristic mineral assemblage of the granulites consists of Pl + Opx + Qtz ± Cpx ± Bt ± Grt ± Kfs. Based on abundant magmatic relic microstructural domains occurring in these rocks, the potential precursors might have been predominantly felsic igneous or high to ultrahigh temperature rocks. Ternary feldspar thermometry provides a rough estimate of temperatures of about 920-1070 °C. The first fluid invasion event, which is linked with this early high to ultrahigh temperature stage is characterised by primary pure CO2 inclusions in apatite and zircon. The densest primary CO2 inclusions indicate 0.52-0.64 GPa pressure at the estimated temperature range of crystallization. According to mineral equilibria and geothermobarometry, the high to ultrahigh temperature rock cooled and crystallized to granulite of predominantly felsic composition at about 750-870 °C and 0.50-0.75 GPa in the middle crust, between 20 and 29 km depths. The second fluid invasion event is recorded by primary CO2-rich fluid inclusions hosted in the granulitic mineral assemblage (plagioclase, quartz and orthopyroxene). In addition to CO2, Raman spectroscopy revealed the presence of minor N2, H2S, CO and H2O in these inclusions. Partial melting of biotite-bearing assemblages could be connected to the next fluid invasion shown by secondary CO2-rich fluids recorded along with healed fractures in plagioclase, clinopyroxene and orthopyroxene. This event could have happened at depths similar to the previous ones. The final step in the granulite evolution was the sampling in the middle crust and transportation to the surface in form of xenoliths by mafic melt. This event generated temperature increase and pressure decrease and thus, limited

  11. A ‘hidden’ 18O-enriched reservoir in the sub-arc mantle

    PubMed Central

    Liu, Chuan-Zhou; Wu, Fu-Yuan; Chung, Sun-Lin; Li, Qiu-Li; Sun, Wei-Dong; Ji, Wei-Qiang

    2014-01-01

    Plate subduction continuously transports crustal materials with high-δ18O values down to the mantle wedge, where mantle peridotites are expected to achieve the high-δ18O features. Elevated δ18O values relative to the upper mantle value have been reported for magmas from some subduction zones. However, peridotites with δ18O values significantly higher than the well-defined upper mantle values have never been observed from modern subduction zones. Here we present in-situ oxygen isotope data of olivine crystals in Sailipu mantle xenoliths from South Tibet, which have been subjected to a long history of Tethyan subduction before the India-Asia collision. Our data identify for the first time a metasomatized mantle that, interpreted as the sub-arc lithospheric mantle, shows anomalously enriched oxygen isotopes (δ18O = +8.03 ± 0.28 ‰). Such a high-δ18O mantle commonly does not contribute significantly to typical island arc basalts. However, partial melting or contamination of such a high-δ18O mantle is feasible to account for the high-δ18O signatures in arc basalts. PMID:24577190

  12. Origin of xenoliths in the trachyte at Puu Waawaa, Hualalai Volcano, Hawaii

    USGS Publications Warehouse

    Clague, D.A.; Bohrson, W.A.

    1991-01-01

    Rare dunite and 2-pyroxene gabbro xenoliths occur in banded trachyte at Puu Waawaa on Hualalai Volcano, Hawaii. Mineral compositions suggest that these xenoliths formed as cumulates of tholeiitic basalt at shallow depth in a subcaldera magma reservoir. Subsequently, the minerals in the xenoliths underwent subsolidus reequilibration that particularly affected chromite compositions by decreasing their Mg numbers. In addition, olivine lost CaO and plagioclase lost MgO and Fe2O3 during subsolidus reequilibration. The xenoliths also reacted with the host trachyte to form secondary mica, amphibole, and orthopyroxene, and to further modify the compositions of some olivine, clinopyroxene, and spinel grains. The reaction products indicate that the host trachyte melt was hydrous. Clinopyroxene in one dunite sample and olivine in most dunite samples have undergone partial melting, apparently in response to addition of water to the xenolith. These xenoliths do not contain CO2 fluid inclusions, so common in xenoliths from other localities on Hualalai, which suggests that CO2 was introduced from alkalic basalt magma between the time CO2-inclusion-free xenoliths erupted at 106??6 ka and the time CO2-inclusion-rich xenoliths erupted within the last 15 ka. ?? 1991 Springer-Verlag.

  13. Constraints on the timing and tectonic setting of mantle metasomatism beneath the Colorado Plateau and Rio Grande rift

    NASA Astrophysics Data System (ADS)

    Byerly, B. L.; Lassiter, J. C.

    2011-12-01

    Cryptic metasomatism of mantle xenoliths from the Jemez lineament, a transition zone between the Rio Grande rift (RGR) and Colorado Plateau (CP), is manifested by LREE-enrichment and heterogeneous isotopic compositions. The timing and source of metasomatism is unclear. Dehydration of the subducted Farallon plate during flat-slab subduction (~75-40Ma) has been proposed to explain Tertiary volcanism in western North America as well as metasomatic signatures in peridotites and eclogite xenoliths from the Colorado Plateau (Lipman 1992, Smith et al. 2004). Alternatively, metasomatism could be related to the formation of the Yavapai province crust and lithosphere ~1.7Ga (Bennett and DePaolo, 1987). We examined major element, trace element, and isotopic compositions of a suite of xenoliths from the Jemez lineament in an attempt to understand nature and timing of metasomatism of the lithospheric mantle beneath the CP and RGR. We sampled a suite of xenoliths hosted in an alkalic basalt cinder cone, Cerro Chato, in the Chain of Craters area SW of Grants, NM, USA. The xenoliths include type-I spinel peridotites (olivine + orthopyroxene + clinopyroxene + spinel ± Fe-sulfides). The type-I xenoliths are LREE-enriched and display HFSE depletion in CPX. The HFSE depletion is not seen in whole-rock analyses. Strong positive correlations of Sr/Nd with 87Sr/86Sr and La/Sm with 87Sr/86Sr are also present. Type-II peridotites (wehrlites), which do not display LREE-enrichment or strong HFSE depletion, of which some contain trace amounts of amphibole, are also present at Cerro Chato. The isotopic composition of the two xenolith types overlap, with 87Sr/86Sr 0.7040-.7043, ɛNd ranging from +4.7 to +6.1 and 187Os/188Os 0.118-0.122. The TRD age for the most refractory xenolith from Cerro Chato yields an age of 1.66 Ga, consistent with the age of the local continental crust. Depleted mantle model extraction ages range from 152-219 Ma. This is consistent with a pseudo-isochron age for the

  14. Hydrogen storage in Earth's mantle and core

    NASA Technical Reports Server (NTRS)

    Prewitt, Charles T.

    1994-01-01

    Two different approaches to explaining how hydrogen might be stored in the mantle are illustrated by a number of papers published over the past 25-30 years, but there has been little attempt to provide objective comparisons of the two. One approach invokes the presence in the mantle of dense hydrous magnesium silicates (DHMS) stable at elevated pressures and temperatures. The other involves nominally anhydrous minerals (NAM) that contain hydrogen as a minor constituent on the ppm level. Experimental studies on DHMS indicate these phases may be stable to pressures and temperatures as high at 16 GPa and 1200 C. This temperature is lower than that indicated by a mantle geotherm at 16 GPa, but may be reasonable for a subducting slab. It is possible that other DHMS could be stable to even higher pressures, but little is known about maximum temperature limits. For NAM, small amounts of hydrogen (up to several hundred ppm) have been detected in olivine, orthopyroxene, clinopyroxene, and garnet recovered from xenoliths in kimberlites, eclogites, and alkali basalts; it has been demonstrated that synthetic wadsleyite and perovskite can accommodate significant amounts of hydrogen. A number of problems are associated with each possibility. For NAM originating in the mantle, one would like to assume that the hydrogen measured in samples recovered on Earth's surface was incorporated when the phase-crystallized at high temperatures and pressures, but it could have been introduced during transport to the surface. Major problems for the DHMS proponents are that none of these phases have been found as minerals and little is yet known about their stabilities in systems containing other cations such as Fe, Al, and Ca.

  15. Mantle oxidation state and its relationship to tectonic environment and fluid speciation.

    PubMed

    Wood, B J; Bryndzia, L T; Johnson, K E

    1990-04-20

    The earth's mantle is degassed along mid-ocean ridges, while rehydration and possibly recarbonaton occurs at subduction zones. These processes and the speciation of C-H-O fluids in the mantle are related to the oxidation state of mantle peridotite. Peridotite xenoliths from continental localities exhibit an oxygen fugacity (fo(2)) range from -1.5 to +1.5 log units relative to the FMQ (fayalite-magnetite-quartz) buffer. The lowest values are from zones of continental extension. Highly oxidized xenoliths (fo(2) greater than FMQ) come from regions of recent or acive subduction (for example, Ichinomegata, Japan), are commonly amphibole-bearing, and show trace element and isotopic evidence of fluid-rock interaction. Peridotites from ocean ridges are reduced and have an averae fo(2) of about -0.9 log units relative to FMQ, virtually coincident with values obtained from mid-ocean ridge basalt (MORB) glasses. These data are further evidence of the genetic link between MORB liquids and residual peridotite and indicate that the asthenosphere, although reducing, has CO(2) and H(2)O as its major fluid species. Incorporation of oxidized material from subduction zones into the continental lithosphere produces xenoliths that have both asthenospheric and subduction signatures. Fluids in the lithosphere are also dominated by CO(2) and H(2)O, and native C is generally unstable. Although the occurrence of native C (diamond) in deep-seated garnetiferous xenoliths and kimberlites does not require reducing conditions, calculations indicate that high Fe(3+) contents are stabilized in the garnet structure and that fo(2) deareases with increasing depth. PMID:17784487

  16. Mantle oxidation state and its relationship to tectonic environment and fluid speciation.

    PubMed

    Wood, B J; Bryndzia, L T; Johnson, K E

    1990-04-20

    The earth's mantle is degassed along mid-ocean ridges, while rehydration and possibly recarbonaton occurs at subduction zones. These processes and the speciation of C-H-O fluids in the mantle are related to the oxidation state of mantle peridotite. Peridotite xenoliths from continental localities exhibit an oxygen fugacity (fo(2)) range from -1.5 to +1.5 log units relative to the FMQ (fayalite-magnetite-quartz) buffer. The lowest values are from zones of continental extension. Highly oxidized xenoliths (fo(2) greater than FMQ) come from regions of recent or acive subduction (for example, Ichinomegata, Japan), are commonly amphibole-bearing, and show trace element and isotopic evidence of fluid-rock interaction. Peridotites from ocean ridges are reduced and have an averae fo(2) of about -0.9 log units relative to FMQ, virtually coincident with values obtained from mid-ocean ridge basalt (MORB) glasses. These data are further evidence of the genetic link between MORB liquids and residual peridotite and indicate that the asthenosphere, although reducing, has CO(2) and H(2)O as its major fluid species. Incorporation of oxidized material from subduction zones into the continental lithosphere produces xenoliths that have both asthenospheric and subduction signatures. Fluids in the lithosphere are also dominated by CO(2) and H(2)O, and native C is generally unstable. Although the occurrence of native C (diamond) in deep-seated garnetiferous xenoliths and kimberlites does not require reducing conditions, calculations indicate that high Fe(3+) contents are stabilized in the garnet structure and that fo(2) deareases with increasing depth.

  17. Processes accompanying of mantle plume emplacement into continental lithosphere: Evidence from NW Arabian plate, Western Syria

    NASA Astrophysics Data System (ADS)

    Sharkov, E. V.

    2015-12-01

    Lower crustal xenoliths occurred in the Middle Cretaceous lamprophyre diatremes in Jabel Ansaria (Western Syria) (Sharkov et al., 1992). They are represented mainly garnet granulites and eclogite-like rocks, which underwent by deformations and retrograde metamorphism, and younger fresh pegmatoid garnet-kaersutite-clinopyroxene (Al-Ti augite) rocks; mantle peridotites are absent in these populations. According to mineralogical geothermobarometers, forming of garnet-granulite suite rocks occurred under pressure 13.5-15.4 kbar (depths 45-54 kn) and temperature 965-1115oC. At the same time, among populations of mantle xenoliths in the Late Cenozoic platobasalts of the region, quite the contrary, lower crustal xenoliths are absent, however, predominated spinel lherzolites (fragments of upper cooled rim of a plume head), derived from the close depths (30-40 km: Sharkov, Bogatikov, 2015). From this follows that ancient continental crust was existed here even in the Middle Cretaceous, but in the Late Cenozoic was removed by extended mantle plume head; at that upper sialic crust was not involved in geomechanic processes, because Precambrian metamorphic rocks survived as a basement for Cambrian to Cenozoic sedimentary cover of Arabian platform. In other words, though cardinal rebuilding of deep-seated structure of the region occurred in the Late Cenozoic but it did not affect on the upper shell of the ancient lithosphere. Because composition of mantle xenolithis in basalts is practically similar worldwide, we suggest that deep-seated processes are analogous also. As emplacement of the mantle plume heads accompanied by powerful basaltic magmatism, very likely that range of lower (mafic) continental crust existence is very convenient for extension of plume heads and their adiabatic melting. If such level, because of whatever reasons, was not reached, melting was limited but appeared excess of volatile matters which led to forming of lamprophyre or even kimberlite.

  18. Seismic Structure of the Mantle Beneath the Pacific Hemisphere

    NASA Astrophysics Data System (ADS)

    Bagley, Brian C.

    Aside from xenoliths, the Earth's mantle is a region that is inaccessible directly, leaving us with limited tools to investigate its characteristics indirectly. Seismology is a tool well-suited for this purpose, and has provided valuable insight regarding many fundamental processes occurring within the mantle. It is fortuitous that the mantle is layered, and that these layers are often punctuated by distinct changes in density and/or velocity that are seismically detectable. By investigating the seismic structure of the mantle we are able to infer properties such as composition, temperature, anisotropy, and water content. Seismic tomography has informed our understanding of subduction and the fate of slabs, and we are beginning to realize that the lower mantle might also be rich with heterogeneity. Our picture of the Earth's mantle is becoming clearer, however, there is much that we do not understand. Receiver function studies of the oceans are fewer and suffer the common malady of looking beneath oceanic islands, not generic oceanic crust. Most of the detailed information regarding the seismic discontinuity structure of open ocean mantle comes from bottom-side reflections that are precursors to SS phases (a shear wave that has traveled from source to receiver with one bottom-side surface bounce in between). SS and PP (a compressional wave with a path analogous to SS) precursors offer extensive geographic coverage and good sensitivity to small velocity contrasts and reasonable localization. They do not perform well for shallow reflectors, or reflectors near the larger transition zone discontinuities. In our studies we use multiple ScS reverberations to gain better resolution of these features. The primary goals of this research are to study mantle discontinuities, and fill in some of the missing detail regarding mantle heterogeneity. We do this by examining the Pacific ocean, beginning with the open ocean mantle, then moving to the subduction zones in the west

  19. Hotspots and the evolution of the mantle

    NASA Technical Reports Server (NTRS)

    Anderson, D. L.

    1979-01-01

    Trace element patterns show that continental and ocean island basalts are complementary to mid-ocean ridge basalts (MORB). The relative sizes of the two source regions can be estimated from enrichment/depletion patterns. Their combined volume, computed from estimates of whole mantle abundances, occupies the entire upper mantle. The source regions appear to be the result of an early differentiation of the mantle. The MORB source evolved from the melt fraction which lost its late stage enriched fluids to the overlying plume source. The MORB source is primarily garnet and clinopyroxene, consistent with it being an eclogite cumulate.

  20. Ancient mantle metasomatism recorded in subcalcic garnet xenocrysts: Temporal links between mantle metasomatism, diamond growth and crustal tectonomagmatism

    NASA Astrophysics Data System (ADS)

    Shu, Qiao; Brey, Gerhard P.

    2015-05-01

    We have identified carbonatitic melts as the main agent for metasomatism in cpx-free garnet harzburgites from the Kaapvaal craton. Substantial overlap of the Ti/Eu and Zr/Hf ratios of subcalcic garnets from xenoliths and garnet inclusions in diamonds corroborate previous findings that mantle metasomatism and the growth of diamonds are connected processes. The key process involves the interaction of a carbonatitic melt with depleted garnet harzburgite leading to dissolution and regrowth of the constituent minerals and the growth of diamonds by redox reactions. Model calculations show that only small amounts of a carbonatite melt, between 0.3 to 3%, are needed to generate the range of sinusoidal Rare Earth Element patterns in the garnets from harzburgites and the inclusions in diamonds. The εHf values of the xenolith garnets range from extremely positive (∼+1000) to extremely negative values (-65) with accompanying εNd values varying from +38 to -41. It can be shown that the negative εHf and εNd values correspond to the initial ratios of the metasomatizing agent. They can only stem from a very old (early Archean or Hadean) crustal component. Very negative εNd values have been found previously in subcalcic garnets from xenoliths and inclusions in diamonds. They yield early Archean model ages for the growth of diamonds. In these studies, very high, unsupported 87Sr/86Sr ratios were found as well. The combined evidence suggests that the metasomatic agents are derived from an old source with a high Rb/Sr ratio, typical for pelitic crustal rocks. Metasomatism in the mantle, the formation of diamonds and tectonomagmatic events in the crust occurred contemporaneously at least within the time span between the early Archean and the middle Proterozoic as can be seen from the comparison of crustal ages, Lu-Hf isochron ages from subcalcic garnets and Sm-Nd and Re-Os ages of inclusions in diamonds.

  1. Concentrates and mantle xenocrysts from the Lao river Guinea and reconstructions of the mantle structure

    NASA Astrophysics Data System (ADS)

    Afanasiev, Valentin; Ashchepkov, Igor; Nikolenko, Eeugeny

    2015-04-01

    The discovery of the kimberlite dykes in Quinea in the basin of the Lao river means finding of the new diamondiferous kimberlite field in the West Africa. It is locating 100 km SW from Bubudu and contains large dykes and placers in Lao river. The kimberlite concentrate and diamondiferous placers are containing pyropes, chromites, Ilmenites and Cr diosides and low Cr- pyroxenes. Two river placers in Lao and Bobeko and newly discovered dyke slightly differ in compositions of minerals. mainly in representatives of the minerals though their compositional trends are in general similar. The concentrates from and Druzhba pipe (Bunudu) contains mainly ilmenite and more are pyropes. Garnets from all localities are close and belong mainly to the lherzolite field to 10 wt%Cr2O3. But the dyke contains essential amount of harzburgitic garnets starting from 2 %wt Cr2O3 and they became prevailing from 6 wt %Cr2O3. There are also megacrystic low - Cr pyropes in dyke concentrate. Dyke is rich in peridotites and coarse grained garnet pyroxenite xenoliths which are ranging from the low Cr -to Cr -diopside type Chromites from Dyke are Ti - low but are often Al rich. While chromites from Bobeko and especially Lao placers define the Cr- rich trend from 60 to 40 wt%Cr2O3 and demonstrate the deviation to ulvospinel trends with increasing of Al2O3 . Cr - Diopsides clinopyroxenites trace the Fe- Ti-Na-Al enrichment trend. Ilmenites from three localities - define close trends splitting to the two intervals 60-40 wt% TiO2 and 33-27 TiO2 which are enriched in Cr2O3 to 5wt% reflecting the crystallization of megacrystalline association at the lithosphere base and Ilm metasomatic vein stockwark near the Moho in pre-eruption feeding system. The Dyke ilmenites are Mg rich and mainly are captured from the deep part of the mantle section. Babeko and Druzhba localities are similar variations of ilmenite trends. Some ilmenites from Lao and Druzhba are Mn - rich and are less in Cr possibly reflecting

  2. Constraints on mantle evolution from 187Os/ 188Os isotopic compositions of Archean ultramafic rocks from southern West Greenland (3.8 Ga) and Western Australia (3.46 Ga)

    NASA Astrophysics Data System (ADS)

    Bennett, Victoria C.; Nutman, Allen P.; Esat, Tezer M.

    2002-07-01

    Initial 187Os/ 188Os isotopic compositions for geochronologically and geologically well -constrained 3.8-Ga spinel peridotites from the Itsaq Gneiss Complex of southern West Greenland and chromite separates from 3.46-Ga komatiites from the Pilbara region of Western Australia have been determined to investigate the osmium isotopic evolution of the early terrestrial mantle. The measured compositions of 187Os/ 188Os(0) = 0.10262 ± 2, from an olivine separate, and 0.10329 ± 3, for a spinel separate from ˜3.8-Ga peridotite G93/42, are the lowest yet reported from any terrestrial sample. The corrections for in situ decay over 3.8 Ga for these low Re/Os phases are minimal and change the isotopic compositions by only 0.5 and 2.2% for the spinel and the olivine, respectively, resulting in 187Os/ 188Os (3.8 Ga) = 0.1021 ± 0.0002 and 0.1009 ± 0.0002, respectively. These data extend direct measurement of Os isotopic compositions to much earlier periods of Earth history than previously documented and provide the best constraints on the Os isotopic composition of the early Archean terrestrial mantle. Analyses of Pilbara chromites yield 3.46-Ga mantle compositions of 0.1042 ± 0.0002 and 0.1051 ± 0.0002. These new data, combined with published initial Os isotopic compositions from late Archean and early Proterozoic samples, are compatible with the mantle, or at least portions of it, evolving from a solar system initially defined by meteorites to a modern composition of 187Os/ 188Os(0) = 0.1296 ± 0.0008 as previously suggested from peridotite xenolith data ( Meisel et al., 2001); the associated 187Re/ 188Os(0) = 0.435 ± 0.005. Thus, chondritic 187Os/ 188Os compositions were a feature of the upper mantle for at least 3.8 billion years, requiring chondritic Re/Os ratios to have been a characteristic of the very early terrestrial mantle. In contrast, nonchondritic initial compositions of some Archean komatiites demonstrate that Os isotopic heterogeneity is an ancient feature

  3. Lithosphere Structure in Southern Africa: Mantle Density, Dynamic Topography, Moho Sharpness, and Kimberlite Magmatism

    NASA Astrophysics Data System (ADS)

    Artemieva, I. M.; Vinnik, L. P.

    2015-12-01

    In southern Africa, both the Archean and Proterozoic blocks have the topography 500-700 m higher than in any other craton worldwide, except for the Tanzanian craton. An unusually high topography may be caused by a low density of the cratonic lithospheric mantle and/or by the dynamic support of the mantle with origin below the depth of isostatic compensation (assumed to be at the lithosphere base). We use free-board constraints to examine the relative contributions of the both factors to surface topography in the cratons of southern Africa and present regional model of density structure of the lithospheric mantle. The results indicate that 0.5-1.0 km of topography requires the dynamic contribution from the sublithospheric mantle because it cannot be explained by the lithosphere structure within the petrologically permitted range of mantle densities. The calculated lithospheric mantle density values are in an overall agreement with xenolith-based data and show an overall trend in mantle density increase from Archean to younger lithospheric terranes. Notable exceptions are the Limpopo belt and the Bushveld Intrusion Complex, which have an increased mantle density, probably as a result of melt-metasomatism. The Western Cape Fold Belt has a moderately depleted mantle with density within the range expected for Phanerozoic mantle, while mantle densities beneath the Eastern Cape Fold Belt require the presence of a significant amount of eclogite in the mantle. Mantle density structure correlates with distribution of kimberlites and with seismic velocity contrast across the Moho: kimberlite-rich regions have sharp Moho and low-density (3.32-3.33 g/cc) mantle, while kimberlite-poor regions have transient Moho and denser mantle (3.34-3.35 g/cc). We explain this pattern by melt-metasomatism which affects both mantle depletion and the Moho sharpness. We also find that regions with high mantle density host non-diamondiferous kimberlites, while diamondiferous kimberlites are

  4. Constraints on the seismic properties of the mantle beneath the Nógrád-Gömör Volcanic Field (Northern Pannonian Basin)

    NASA Astrophysics Data System (ADS)

    Klebesz, Rita; Liptai, Nora; Kovacs, Istvan; Patko, Levente; Pinter, Zsanett; Falus, Gyorgy; Graczer, Zoltan; Szanyi, Gyongyver; Wesztergom, Viktor; Szabo, Csaba

    2014-05-01

    In the Carpathian Pannonian Region (CPR) Plio-Pleistocene alkali basalts have sampled the upper mantle at five known occurrences, bringing upper mantle xenoliths to the surface. One of these is the Nógrád-Gömör Volcanic Field (NGVF), which is located in the northern part of the Pannonian Basin. For this study, 25 representative lherzolite and wehrlite xenoliths were selected from the central and southern parts of the NGVF. These xenoliths sample a small volume (~4000 km3) of the upper mantle from a depth of about 35-50 km. Xenoliths collected from the southern part of NGVF originate from shallower depth (35-40 km) than those from the central part (40-50 km) [1]. Crystal preferred orientations (CPO) of the minerals were measured by electron backscatter diffraction (EBSD). Two distinct orientation types (A-Type, D-Type) based on the distribution and alignment of crystallographic axes were recognized, which show some correlation not only with the macroscopic texture, but also with olivine J-factors that indicate the strength of the xenolith fabric [2]. The seismic properties, i.e. seismic anisotropy and velocities, of these 25 mantle xenoliths were calculated based on the CPO and volume fractions of olivine, ortho- and clinopyroxene. It was found that P wave and fast split shear wave polarization direction is always close to the density maximum of the a-axis of olivine. Seismic anisotropy is higher for stronger CPO. Maximum P wave azimuthal anisotropy ranges are 4.5%-6.9% and 5.3%-11.9%, for the southern and the central area respectively. Maximum S wave polarization anisotropy ranges are 2.92%-5.31% and 3.97%-7.46%for the southern area and the central area respectively. The anisotropy that would be measured by SKS, Rayleigh and Love waves for end-member orientations of the lineation and foliation could be predicted based on the already calculated seismic properties of the xenoliths [3]. The calculated anisotropy is compared to the results of S receiver function

  5. Petrochemistry of ultrapotassic tephrites and associated cognate plutonic xenoliths with carbonatite affinities from the late Quaternary Qa’le Hasan Ali maars, central Iran

    NASA Astrophysics Data System (ADS)

    Saadat, S.; Stern, C. R.; Moradian, A.

    2014-08-01

    The Quaternary Qa’le Hasan Ali (QHA) maars in central Iran occur at the intersection of the north-south Nayband fault, which defines the western boundary of the Lut micro-continental block, and a system of northwest-southeast faults, subparallel to the Zagros suture zone, that formed during the Arabian-Eurasian collision. These post-collisional maars intrude Eocene volcanic rocks of the Urumieh-Dokhtar magmatic belt, which was generated by the subduction of Neotethys oceanic lithosphere below Iran. The highly potassic, Ti-phlogopite + Mg-rich (Fo89-92) olivine + diopside-augite + aegirine-augite basanite tephrites forming the tuff rims of the QHA maars contain tephrite-coated plutonic xenoliths, some of which are interpreted as co-genetic with the tephrites based on their similar mineralogy and Sr isotopic composition (87Sr/86Sr = 0.70590). Cognate plutonic xenoliths have up to ∼20 vol% calcite, considered to be magmatic calcite because of (1) its grain size, which is similar to feldspars and aegirine-augite pyroxenes in these rocks, (2) the occurrence of fine-grained inclusions of pyroxene and apatite within these calcite grains, and (3) the similarity of the Sr-isotopic composition of this calcite with the other minerals in these rocks. The fact that the magmatic calcite has remained intact and did not volatilize during the transport of these xenoliths to the surface in the hot tephrite magma implies a short transit time, indicating that they are samples of a shallow plutonic complex, as does the presence of anorthoclase in these plutonic xenoliths. Their high modal proportion of magmatic calcite suggests that this shallow plutonic complex has affinities with carbonatites. The magmatic calcite-bearing plutonic xenoliths have high LREE/HREE ratios and contain REE-rich allanite (with up to ∼20 wt% LREE) and britholite (∼60 wt% LREE) that make up ∼3 modal percent of the most calcite-rich samples. Similar to many post-collisional highly potassic rocks

  6. Zircon megacrysts from kimberlite: oxygen isotope variability among mantle melts

    NASA Astrophysics Data System (ADS)

    Valley, John W.; Kinny, Peter D.; Schulze, Daniel J.; Spicuzza, Michael J.

    The oxygen isotope ratios of Phanerozoic zircons from kimberlite pipes in the Kaapvaal Craton of southern Africa and the Siberian Platform vary from 4.7 to 5.9‰ VSMOW. High precision, accurate analyses by laser reveal subtle pipe-to-pipe differences not previously suspected. These zircons have distinctive chemical and physical characteristics identifying them as mantle-derived megacrysts similar to zircons found associated with diamond, coesite, MARID xenoliths, Cr-diopside, K-richterite, or Mg-rich ilmenite. Several lines of evidence indicate that these 18O values are unaltered by kimberlite magmas during eruption and represent compositions preserved since crystallization in the mantle, including: U/Pb age, large crystal size, and the slow rate of oxygen exchange in non-metamict zircon. The average 18O of mantle zircons is 5.3‰, 0.1 higher and in equilibrium with values for olivine in peridotite xenoliths and oceanic basalts. Zircon megacrysts from within 250 km of Kimberley, South Africa have average 18O=5.32+/-0.17 (n=28). Small, but significant, differences among other kimberlite pipes or groups of pipes may indicate isotopically distinct reservoirs in the sub-continental lithosphere or asthenosphere, some of which are anomalous with respect to normal mantle values of 5.3+/-0.3. Precambrian zircons (2.1-2.7 Ga) from Jwaneng, Botswana have the lowest values yet measured in a mantle zircon, 18O=3.4 to 4.7‰. These zircon megacrysts originally crystallized in mafic or ultramafic rocks either through melting and metasomatism associated with kimberlite magmatism or during metamorphism. The low 18O zircons are best explained by subduction of late Archean ocean crust that exchanged with heated seawater prior to underplating as eclogite and to associated metasomatism of the mantle wedge. Smaller differences among other pipes and districts may result from variable temperatures of equilibration, mafic versus ultramafic hosts, or variable underplating. The narrow

  7. Noble gas composition of subcontinental lithospheric mantle: An extensively degassed reservoir beneath Southern Patagonia

    NASA Astrophysics Data System (ADS)

    Jalowitzki, Tiago; Sumino, Hirochika; Conceição, Rommulo V.; Orihashi, Yuji; Nagao, Keisuke; Bertotto, Gustavo W.; Balbinot, Eduardo; Schilling, Manuel E.; Gervasoni, Fernanda

    2016-09-01

    Patagonia, in the Southern Andes, is one of the few locations where interactions between the oceanic and continental lithosphere can be studied due to subduction of an active spreading ridge beneath the continent. In order to characterize the noble gas composition of Patagonian subcontinental lithospheric mantle (SCLM), we present the first noble gas data alongside new lithophile (Sr-Nd-Pb) isotopic data for mantle xenoliths from Pali-Aike Volcanic Field and Gobernador Gregores, Southern Patagonia. Based on noble gas isotopic compositions, Pali-Aike mantle xenoliths represent intrinsic SCLM with higher (U + Th + K)/(3He, 22Ne, 36Ar) ratios than the mid-ocean ridge basalt (MORB) source. This reservoir shows slightly radiogenic helium (3He/4He = 6.84-6.90 RA), coupled with a strongly nucleogenic neon signature (mantle source 21Ne/22Ne = 0.085-0.094). The 40Ar/36Ar ratios vary from a near-atmospheric ratio of 510 up to 17700, with mantle source 40Ar/36Ar between 31100-6800+9400 and 54000-9600+14200. In addition, the 3He/22Ne ratios for the local SCLM endmember, at 12.03 ± 0.15 to 13.66 ± 0.37, are higher than depleted MORBs, at 3He/22Ne = 8.31-9.75. Although asthenospheric mantle upwelling through the Patagonian slab window would result in a MORB-like metasomatism after collision of the South Chile Ridge with the Chile trench ca. 14 Ma, this mantle reservoir could have remained unhomogenized after rapid passage and northward migration of the Chile Triple Junction. The mantle endmember xenon isotopic ratios of Pali-Aike mantle xenoliths, which is first defined for any SCLM-derived samples, show values indistinguishable from the MORB source (129Xe/132Xe =1.0833-0.0053+0.0216 and 136Xe/132Xe =0.3761-0.0034+0.0246). The noble gas component observed in Gobernador Gregores mantle xenoliths is characterized by isotopic compositions in the MORB range in terms of helium (3He/4He = 7.17-7.37 RA), but with slightly nucleogenic neon (mantle source 21Ne/22Ne = 0.065-0.079). We

  8. Recycling of crustal minerals into the upper mantle: evidence from ophiolites

    NASA Astrophysics Data System (ADS)

    Robinson, P. T.; Yang, J.; Trumbull, R.

    2009-05-01

    A wide variety of ultrahigh pressure and crustal minerals has been recovered from podiform chromitites of the Donqiao and Luobusa ophiolites of Tibet, the Ray-Iz ophiolite of the Polar Urals and the Semail ophiolite of Oman. Microdiamonds are abundant in the Luobusa, Donqiao and Ray-Iz ophiolites, coesite and kyanite occur in Luobusa and moissanite is present in all four ophiolites. Numerous crustal minerals, including zircon, corundum, quartz, almandine garnet, rutile, and feldspar are also present. The diamonds are mostly euhedral grains,100-200 µm across, commonly containing metallic and Mg-Fe silicate inclusions. One small grain occurs as an inclusion in an Os-Ir alloy. Coesite and kyanite are intergrown with each other on the rim of a grain of Ti-Fe alloy, and the coesite has a prismatic form suggesting it may be pseudomorphic after stishovite. Moissanite is common in all four ophiolites and occurs as small colorless, green or blue, vitreous fragments. Zircon grains range from 20 to 300 µm, and are mostly well rounded with very complex internal structures. They commonly contain low-pressure inclusions of quartz, rutile, orthoclase, mica, ilmenite and apatite. 206Pb/238U SIMS and SHRIMP dates for the zircons are mostly Paleozoic and Precambrian, far older than the ophiolites. The grains of quartz, almandine garnet, corundum and feldspar range up to about 0.5 mm and are moderately to well rounded. Smaller, angular fragments of such grains are also present. The rounded morphology of these grains, as well as the zircons, strongly suggests derivation from sedimentary material, presumably transported into the mantle by subduction. The microdiamonds and moissanite could also have been derived from crustal materials recycled into the mantle. We suggest that the various minerals were picked up by melts from which the chromitites precipitated and carried to shallow crustal levels. The preservation of such minerals, particularly quartz and coesite, in the mantle

  9. Using chalcophile elements to constrain crustal contamination and xenolith-magma interaction in Cenozoic basalts of eastern China

    NASA Astrophysics Data System (ADS)

    Zeng, Gang; Huang, Xiao-Wen; Zhou, Mei-Fu; Chen, Li-Hui; Xu, Xi-Sheng

    2016-08-01

    Continental basalts have complicated petrogenetic processes, and their chemical compositions can be affected by multi-staged geological evolution. Compared to lithophile elements, chalcophile elements including Ni, platinum-group elements (PGEs) and Cu are sensitive to sulfide segregation and fractional crystallization during the evolution of mantle-derived magmas and can provide constraints on the genesis of continental basalts. Cenozoic intra-continental alkaline basalts in the Nanjing basaltic field, eastern China, include high-Ca and low-Ca varieties. All these basalts have poor PGE contents with Ir ranging from 0.016 ppb to 0.288 ppb and high Cu/Pd ratios from 0.7 × 105 to 4.7 × 105 (5.7 × 103 for DMM), indicating that they were derived from sulfide-saturated mantle sources with variable amounts of residual sulfide during melting or might undergo an early-sulfide segregation in the mantle. Relatively high Cu/Pd ratios along with high Pd concentrations for the high-Ca alkaline basalts indicate an additional removal of sulfide during magma ascent. Because these basalts have high, variable Pd/Ir ratios (2.8-16.8) with low Ce/Pb (9.9-19.7) ratios and εNd values (+ 3.6-+6.4), crustal contamination is proposed to be a potential process to induce the sulfide saturation and removal. Significantly increased Pd/Ir ratios for few high-Ca basalts can be explained by the fractionation of laurite or Ru-Os-Ir alloys with olivine or chromite. For low-Ca alkaline basalts, their PGE contents are well correlated with the MgO, Sc contents, incompatible element ratios (Lu/Hf, Na/Ti and Ca/Al) and Hf isotopes. Good correlations are also observed between Pd/Ir (or Rh/Ir) and Na/Ti (or Ca/Al) ratios. Variations of these elemental ratios and Hf isotopes is previously documented to be induced by the mixing of peridotite xenolith-released melts during ascent. Therefore, we suggest that such xenolith-magma interaction are also responsible for the variable PGE compositions of low

  10. Metasomatism in the lithospheric mantle beneath southern Patagonia, Argentina

    NASA Astrophysics Data System (ADS)

    Kolosova-Satlberger, Olesya; Ntaflos, Theodoros; Bjerg, Ernesto

    2014-05-01

    Mantle xenoliths from Gobernador Gregores, southern Patagonia are spinel- lherzolites, harzburgites and wherlites. A large number of the studied xenoliths have experienced cryptic and modal metasomatism. The xenoliths are mainly coarse-grained with prevalent protogranular texture but equigranular tabular and mosaic textures are present as well. Xenoliths that have undergone modal metasomatism bear hydrous phases such as amphibole, phlogopite ± apatite and melt pockets. The latter are of particular interest because of their unusually large size (up to 1 cm in diameter) and freshness. They consist of second generation olivine, clinopyroxene and spinel ± relict amphibole ± sulfides that are surrounded by a yellowish vesicular glass matrix. The melt pockets are found in amphibole- and/or phlogopite-bearing wehrlites and harzburgites as well as anhydrous lherzolites. Subhedral primary olivines enclosed by melt pockets show in the BSE images a dark grey margin up to 80 microns thick attributed to the reaction of the primatry olivine with melt. Fine grained spinel inclusions are always associated with the dark grey margin, indicating that they belong to the secong generation assemblage. There are considerable differences between first and second generation minerals found in melt pockets. While primary olivine has Fo-contents that range from 88.0 to 93.3, second generation olivines in melt pockets vary from Fo89.3 to Fo94.4. Both primary and second generation cpx are diopsides with the latter systematically enriched in TiO2. The glasses that occur in melt pockets or propagate intergranular have compositions varying from trachyandesite to phonolite. The variable composition of the glass could be attributed to host basalt infiltration and decompressional melting of amphiboles. Some of the studied xenoliths show melt propagation of two compositional different glasses crosscutting primary generation minerals and finally mixing with each other. Microprobe analyses suggest

  11. Plume-cratonic lithosphere interaction recorded by water and other trace elements in peridotite xenoliths from the Labait volcano, Tanzania

    NASA Astrophysics Data System (ADS)

    Hui, Hejiu; Peslier, Anne H.; Rudnick, Roberta L.; Simonetti, Antonio; Neal, Clive R.

    2015-06-01

    Water and other trace element concentrations in olivine (1-39 ppm H2O), orthopyroxene (10-150 ppm H2O), and clinopyroxene (16-340 ppm H2O) of mantle xenoliths from the Labait volcano, located on the edge of the Tanzanian craton along the eastern branch of the East African Rift, record melting and subsequent refertilization by plume magmas in a stratified lithosphere. These water contents are at the lower end of the range observed in other cratonic mantle lithospheres. Despite correlations between water content and indices of melting in orthopyroxene from the shallow peridotites, and in both olivine and orthopyroxene from the deep peridotites, water concentrations are too high for the peridotites to be simple residues. Instead, the Labait water contents are best explained as reflecting interaction between residual peridotite with a melt having relatively low water content (<1 wt.% H2O). Plume-derived melts are the likely source of water and other trace element enrichments in the Labait peridotites. Only garnet may have undergone addition of water from the host magma as evidenced by water content increasing toward the kelyphite rim in one otherwise homogeneous garnet. Based on modeling of the diffusion profile, magma ascent occurred at 4-28 m/s. In summary, plume-craton interaction appears to result in only moderate water enrichment of the lithosphere.

  12. On the recent enrichment of subcontinental lithosphere: A detailed UPb study of spinel lherzolite xenoliths, Yukon, Canada

    NASA Astrophysics Data System (ADS)

    Carignan, Jean; Ludden, John; Francis, Don

    1996-11-01

    lead relative to coexisting cpx and, depending on the K d values used for the calculations, is either enriched or depleted in uranium compared to cpx. Mantle cpx from the subcontinental lithosphere that has suffered metasomatism is U- and Pb-rich when compared to "unmetasomatised" cpx suggesting that cpx scavanges these elements from metasomatic fluids. Fractionation of uranium and lead between fluids and cpx is not easy to constrain, but the data for Alligator Lake suggest a possible uranium enrichment over lead in metasomatised cpx. Three whole rock samples have variable 187Os /188Os corresponding to γOs of -0.3, +0.4, and +3.0. In contrast to the lead and strontium data, the 187Os /188Os and 187Re /188Os ratios are positively correlated with calcium and aluminum contents of the xenoliths. Their 187Re /188Os ratios are slightly higher than the mean chondritic values, and an age of 650 Ma is obtained when the data are regressed in a ReOs isochron diagram. This age may date partial melting of the lherzolite and/or re-fertilization with rhenium enrichment of the mantle lithosphere. The protolithic lithosphere would have had a γOs typical of depleted MORB (ca. -1.4) at 650 Ma.

  13. Comparison of the mantle modification of the mantle column between two phases of kimberlite intrusion in Dalnyaya pipe, Yakutia

    NASA Astrophysics Data System (ADS)

    Ashchepkov, Igor; Spetsius, Zdislav; Salikhov, Ravil; Khmelnikova, Olga

    2013-04-01

    Dalnaya pipe is one of the largest in Daldyn field, Yakutia is composed of autolite breccia (AKB) and porphyric kimberlite (PK). Minerals from concentrates of both phases were compared and with the peridotite xenoliths minerals. Cpx from Dalnyaya are showing common tendencies Fe--Ti rise and Cr, Al, Na decrease. Garnets belong to lherzolite field with more deviation to harzbuirgitic one for (PK) . The chromites show two subtrends for Cr Fe, Ni vs TiO2. In general the variations of the AKB minerals and dispersion are higher but amount of depleted varieties is higher in PK. We used >50 xenoliths and ~1200 concentrate minerals for PT reconstructions. Combine PTX diagram show deep SCLM root beneath Dalnyaya with the main heating ~7 GPa. The HT 45mwm-2 branch is traced by some xenoliths from base to 2GPa. Essential inflection and heating detected by PT for OPx ~3GPa referring to Ca- enriched pyroxenitic garnets . Small Fe enrichment for Cpx and Gar found near 6 GPa referring to heated porhyroclastic varieties. Continuous and irregular growth of Fe# for Gar and low Fe Cpx Fe# 6 to 12# suggest that primary mantle layering beneath this pipe was smoothed by the high scale interaction with melts. The refertilization trend with Fe#9-15% rising upward in two branches refer to the Ilm and Cpx parental melt evolutions produced the intergrowth sometimes with garnets. In the PFO2 diagrams garnets and Cpx show continuous reduction to the lithosphere base to 4QMF higher for Cpx. Ilm - garnet trend is rising upward between -2 -0 QMF. The PT diagram for the AKB minerals from Dalnyaya pipe is nearly the same with the high dispersion to Fe rich varieties and smaller amount o f Mg rich minerals. Since the diamond grade is often determined by the amount of depleted varieties it is higher for the PK. Trace elements determined for Gar and Cpx from 13 xenoliths from the middle part of mantle section reveal very similar patterns in general. Supported by RBRF grant 11-05-00060.

  14. Constraining the Protolith of Large, Macroscopically Layered Kyanite-bearing Eclogite Xenoliths from the Kaapvaal Craton, South Africa

    NASA Astrophysics Data System (ADS)

    Rebelo, C. C.; Gurney, J. J.; Richardson, S. H.; Shaw-Kahle, B.

    2015-12-01

    We describe the geochemistry of a suite of ten layered, kyanite-bearing eclogites from the Roberts Victor kimberlite, Kaapvaal Craton, South Africa. All samples are characterized by clear zonation corresponding to the presence or absence of kyanite and the state of preservation of clinopyroxene. The sampled zones are defined as KF (free of kyanite, with well-preserved garnet and clinopyroxene), KZ (preserved clinopyroxene is absent; garnet occurs with kyanite); and TZ (partially preserved clinopyroxene and more altered garnet; kyanite may or may not be present). We report on results of petrographic observations, mineral chemistry, and trace element and oxygen isotope analyses for the different zones. Results from adjacent zones are presented as geochemical transects perpendicular to the layering for each xenolith. We use the results to infer the geochemical evolution of likely protoliths for the various samples. Mineral chemistry of garnets across the different zones shows decreasing FeO*, Cr2O3, MgO and MnO and increasing in CaO from KF into KZ. Clinopyroxene shows increasing in Al2O3 and Na2O from KF into KZ. Clinopyroxenes approach a more jadeitic composition towards the KZ and are more diopsidic in KF. Trace element analyses were conducted with laser ablation ICP-MS on hand-picked mineral separates. Garnets show subchondritic LREE depletion and superchondritic HREE enrichment in both KF and KZ. Positive europium anomalies in garnet are present in all zones, with relatively larger anomalies in garnet grains from KZ. Preserved clinopyroxenes show complementary REE patterns to garnet grains, with superchondritic LREEs and subchondritic HREEs. For the suite of xenoliths, results from mass spectrometry on oxygen extracted by laser fluorination on mineral separates show that δ18O values are above the accepted mantle value and fall within the range of hydrothermally altered oceanic crustal material. At this stage of the research, the bulk protolith is interpreted to

  15. Storage and recycling of water in the Earth's mantle

    NASA Astrophysics Data System (ADS)

    Bolfan-Casanova, N.

    2015-12-01

    Most natural samples originating from the mantle contain traces of water. It can be observed that water content varies laterally as a function of the geodynamic context, but also with depth in cratons. Basalts from mid-ocean ridges, which sample the convecting upper mantle, contain generally below 0.6 wt% H2O leading to 50-330 parts per million by weight in the source. Oceanic Islands Basalts are more hydrated with contents ranging from 0.6 to 1.1 wt%, leading to 350-1100 ppm wt H2O in the source. Arc basalts are even more hydrated with water contents ranging from 0.2 to 5-6 wt% H2O testifying of the recycling of water by subduction. Kimberlite magmas are also the proof that local saturation in volatiles is possible. Among xenoliths, the samples from cratons are very interesting because they may provide a depth profile of water. However, the variation of water content in olivine with depth differs from craton to craton, and is the result of a complex geological history. Also, olivine inclusions in diamond and olivine from peridotite xenoliths do not give the same message regarding to water activity. The water storage capacity of the mantle is defined as the maximum water or hydroxyl that can be incorporated in its constitutive minerals before a free fluid phase appears. It can be determined experimentally and confronted to geophysical observations, such as low seismic velocities, and electrical conductivity. In this talk we will review our current knowledge of water incorporation in NAMs as determined experimentally and compare it with available observations. New data concerning clinopyroxenes will be shown. The aim being to understand the deep water cycle.

  16. Osmium Isotope Constraints on the Timing of Production and Destruction of Mantle Lithosphere in the Southwest United States

    NASA Astrophysics Data System (ADS)

    Brandon, A. D.

    2013-12-01

    When convecting mantle melts, the residual peridotite becomes less dense and may be become stabilized as lithosphere. The Re-Os isotope chronometer has been successfully applied to determining the timing of melt extraction in mantle peridotite. In continental regions where multiple mantle xenolith locales are present, the Re-Os chronometer can be applied to assessing the timing of mantle melting in relation to juvenile continental crust production, stabilization, and destruction of mantle lithosphere. This is evaluated here for the off-craton mantle lithosphere in the Southwest United States by examining 5 mantle xenolith suites from locales spanning a region hundreds of kilometers north to south and east to west - Dish Hill, California; Lunar Crater Nevada; Grand Canyon and San Carlos, Arizona; and Kilbourne Hole, New Mexico. Because Re is mobile in mantle peridotites at surface conditions, direct Re-Os isochrons representing mantle melting ages are typically absent. Instead melting proxies for Re such as Al2O3 can be used to obtain ';aluminachron' ages or to assess disturbances of the mantle lithosphere following partial melting. The Dish Hill, Grand Canyon, and Kilbourne Hole suites display lithophile element evidence for post-melting, multiple modal and cryptic metasomatic events in combination with positive and well correlated Os isotope versus Al2O3 trends. For example, each of these xenolith suites has samples with light rare earth element (LREE) depleted to LREE-enriched bulk rock and clinopyroxene compositions. However, no correlation exists between LREE differences and their Os isotope, bulk rock Al compositions, or other indices of melt-rock interaction. The Os-aluminachron age obtained for Dish Hill is 2.15 Ga, for Grand Canyon is 2.31 Ga, and for Kilbourne Hole is 1.96 Ga. These ages overlap TDM ages for the overlying crustal provinces confirming a link between melting that creates mantle lithosphere and production of juvenile continental crust. A

  17. An ancient depleted mantle sample from a 42-Ma dike in Montana: Constraints on persistence of the lithosphere during Eocene Magmatism

    USGS Publications Warehouse

    Dudas, F.O.; Harlan, S.S.

    1999-01-01

    Recent models for the Cenozoic tectonic evolution of the western margin of North America propose that delamination of ancient lithosphere accompanied asthenospheric upwelling, magmatism, and uplift subsequent to Laramide deformation. On the basis of the age of an alkaline dike in south-central Montana, thermometry of mantle xenoliths from the dike, and Sr, Nd, and Pb isotopic compositions of the dike and a xenocryst, we show that refractory lithosphere, derived from ancient, depleted mantle, remained in place under the Wyoming Craton as late as 42 Ma. The Haymond School Dike, a camptonite, yields a 40Ar/39Ar plateau date of 41.97 ?? 0.19 Ma (2??). Paleomagnetic data are consistent with this date and indicate intrusion during chron C19r. The dike has Sr, Nd, and Pb isotopic compositions similar to those of other Eocene alkaline rocks from central Montana. A clinopyroxene megacryst from the dike has ??42 = 17, and 87Sr/86Sr = 0.70288, indicating that it derives from ancient, depleted mantle isotopically distinct from the source of the host camptonite. Thermometry of xenoliths from the dike shows pyroxene populations that formed at 880?? and 1200??C. Combining thermometry with previous estimates of the regional Eocene geotherm inferred from xenoliths in kimberlites, and with the Al-in-orthopyroxene barometer, we infer that lithospheric mantle remained intact to depths of 110-150 km as late as 42 Ma. Eocene magmatism was not accompanied by complete removal of ancient lithosphere.

  18. Osmium isotopic evidence for mesozoic removal of lithospheric mantle beneath the sierra nevada, california

    PubMed

    Lee; Yin; Rudnick; Chesley; Jacobsen

    2000-09-15

    Thermobarometric and Os isotopic data for peridotite xenoliths from late Miocene and younger lavas in the Sierra Nevada reveal that the lithospheric mantle is vertically stratified: the shallowest portions (<45 to 60 kilometers) are cold (670 degrees to 740 degrees C) and show evidence for heating and yield Proterozoic Os model ages, whereas the deeper portions (45 to 100 kilometers) yield Phanerozoic Os model ages and show evidence for extensive cooling from temperatures >1100 degrees C to 750 degrees C. Because a variety of isotopic evidence suggests that the Sierran batholith formed on preexisting Proterozoic lithosphere, most of the original lithospheric mantle appears to have been removed before the late Miocene, leaving only a sliver of ancient mantle beneath the crust. PMID:10988067

  19. Osmium isotopic evidence for mesozoic removal of lithospheric mantle beneath the sierra nevada, california

    PubMed

    Lee; Yin; Rudnick; Chesley; Jacobsen

    2000-09-15

    Thermobarometric and Os isotopic data for peridotite xenoliths from late Miocene and younger lavas in the Sierra Nevada reveal that the lithospheric mantle is vertically stratified: the shallowest portions (<45 to 60 kilometers) are cold (670 degrees to 740 degrees C) and show evidence for heating and yield Proterozoic Os model ages, whereas the deeper portions (45 to 100 kilometers) yield Phanerozoic Os model ages and show evidence for extensive cooling from temperatures >1100 degrees C to 750 degrees C. Because a variety of isotopic evidence suggests that the Sierran batholith formed on preexisting Proterozoic lithosphere, most of the original lithospheric mantle appears to have been removed before the late Miocene, leaving only a sliver of ancient mantle beneath the crust.

  20. Rapid Cenozoic ingrowth of isotopic signatures simulating "HIMU" in ancient lithospheric mantle: Distinguishing source from process

    NASA Astrophysics Data System (ADS)

    McCo