Science.gov

Sample records for abundant mantle xenoliths

  1. Evolution of the lithosphere beneath Oahu, Hawaii: rare earth element abundances in mantle xenoliths

    NASA Astrophysics Data System (ADS)

    Sen, Gautam; Frey, Frederick A.; Shimizu, Nobumichi; Leeman, William P.

    1993-08-01

    Rare earth element contents of clinopyroxenes in Hawaiian mantle xenoliths from Oahu were determined with an ion microprobe. The analyzed xenoliths are from four vents of the alkali Honolulu Volcanics (HV). Three (Kaau, Pali and Kalihi—KPK) are located close to the caldera of the extinct Koolau shield volcano, and the fourth, Salt Lake Crater (SLC), is on the periphery of the shield volcano. Systematic differences exist in REE contents between clinopyroxenes of the KPK and SLC xenoliths: (1) KPK pyroxenes are typically zoned in REE contents whereas SLC pyroxenes are homogeneous, (2) the LREE-depleted (chondrite-normalized) patterns that characterize many of the KPK xenoliths are not found in SLC xenoliths, and (3) the convex-upward REE patterns that are characteristic of SLC xenoliths are not found in KPK xenoliths. Relative to abyssal peridotites, the LREE-depleted Hawaiian lherzolite pyroxenes (interpreted to be residual oceanic lithosphere) have higher contents of REE, Na 2O, TiO 2 and FeO, and more modal clinopyroxene. These LREE-depleted Hawaiian xenoliths represent deeper, less-depleted parts of the melting column, whereas the abyssal peridotites represent the uppermost, more strongly depleted part of the mantle. The spoon-shaped, LREE-enriched and convex-upward REE patterns in the xenoliths have resulted from metasomatic enrichment of the lithosphere caused by reaction with magmas that formed the Honolulu Volcanics. A model for the evolution of the oceanic lithosphere is presented in which fractures were the main mode of transport of the Honolulu Volcanics. Metasomatic enrichment resulted from interaction between percolating Honolulu Volcanics magmas and wallrock. The differences between SLC and KPK xenoliths are attributed to chromatographic fractionation effects: SLC xenoliths are postulated to have come from a greater depth where they equilibrated to a larger extent with the percolating magmas than the KPK rocks.

  2. Superplume metasomatism: Evidence from Siberian mantle xenoliths

    NASA Astrophysics Data System (ADS)

    Howarth, Geoffrey H.; Barry, Peter H.; Pernet-Fisher, John F.; Baziotis, Ioannis P.; Pokhilenko, Nikolay P.; Pokhilenko, Lyudmila N.; Bodnar, Robert J.; Taylor, Lawrence A.; Agashev, Aleksey M.

    2014-01-01

    The Siberian craton contains > 1000 kimberlite intrusions of various ages (Silurian to Jurassic), making it an ideal locality for a craton-wide study on the evolution of the sub-continental lithospheric mantle (SCLM). The primary objective of this study is to characterize the temporal and spatial metasomatic effects on the Siberian SCLM, focusing on the metasomatic imprint rendered by the Siberian superplume. We report new major- and trace-element mineral data for mantle peridotite xenoliths, obtained from the Late Devonian Udachnaya and the Late Jurassic Obnazhennaya kimberlites, which bracket the thermal climax of the Siberian superplume.

  3. 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

  4. Evidence from mantle xenoliths for lithosphere removal beneath the central Rio Grande Rift

    NASA Astrophysics Data System (ADS)

    Byerly, Benjamin L.; Lassiter, John C.

    2012-11-01

    Seismic tomography beneath the Central Rio Grande Rift (RGR) at ˜34°N shows a low P and S wave velocity zone in the mantle that extends up the base of the Moho. This low-velocity region has been interpreted by (Gao et al., 2004) to be the result of convective removal of a portion of the once >100 km thick Proterozoic lithosphere. The amount of extension in the central RGR is thought to be low (˜25%) and thus cannot account for the amount of lithosphere thinning suggested by seismic tomography. We measured whole rock and mineral major element, trace element, and isotopic compositions of spinel-peridotite xenoliths erupted along the central axis of the rift (Elephant Butte) and the eastern margin of the Colorado Plateau (Cerro Chato) to determine their depth of origin and mantle provenance and to test the delamination hypothesis. If lithosphere removal has not occurred and the low P and S wave velocities are instead the result of hydration or melt infiltration in the lithosphere, then xenoliths erupted on the rift axis should have geochemical compositions similar to Proterozoic sub-continental lithospheric mantle (SCLM). At Cerro Chato, on the margin of the Colorado Plateau, xenoliths were derived from ˜60 km depth and have geochemical signatures similar to Proterozoic sub-continental lithospheric mantle (e.g. refractory major element compositions, LREE-enrichment, enriched Sr and Nd isotopes, unradiogenic Os isotopes). At Elephant Butte, along the central rift axis, two distinct groups of xenoliths are present. The majority of xenoliths from Elephant Butte are LREE-depleted and have fertile major element compositions. Additionally, these xenoliths have isotopic signatures similar to the range for DMM (e.g. 87Sr/86Sr ranging from 0.7018 to 0.7023, ɛNd ranging from 7 to 21, and 187Os/188Os ranging from 0.122 to 0.130). We interpret this group of xenoliths to be derived from asthenospheric mantle. A less-abundant group of xenoliths at Elephant Butte are LREE

  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. PMID:17745405

  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. Composition and structure of the lithospheric mantle beneath NE Iran: Constraints from mantle xenoliths

    NASA Astrophysics Data System (ADS)

    Su, Ben-Xun; Chung, Sun-Lin; Zarrinkoub, Mohammad Hossein; Pang, Kwan-Nang; Chen, Ling; Ji, Wei-Qiang; Brewer, Aaron; Ying, Ji-Feng; Khatib, Mohammad Mahdi

    2014-08-01

    A detailed study on petrology and mineral chemistry of 32 mantle xenoliths has been conducted to decipher the physical and chemical characteristics of the lithosphere beneath NE Iran. Spinel lherzolite, the most abundant xenolith type, is made up of olivine, orthopyroxene, clinopyroxene, and spinel. Clinopyroxenes in the spinel lherzolites display a primitive mantle-like composition, typical of non-cratonic peridotites. Pyroxenite, another major xenolith type, shows equilibrated textures and highly variable compositions including olivine websterite, websterite and clinopyroxenite. These pyroxenites, together with an equigranular dunite, delineate a clear metasomatic trend, characterized by systematic Mg#, Cr#, Al2O3, and TiO2 variations in the constituent minerals, coupled with light rare earth element enrichment and high field strength element depletion in clinopyroxene. The pyroxenites are therefore suggested to have formed by the interaction between garnet-bearing peridotites within the lithospheric mantle and melts from a stagnant slab within the asthenosphere. The lithospheric mantle may have undergone multiple stages of partial melting. The earliest stage, evidenced by the equigranular dunite, resulted in significant NiO depletion in olivine, low Al2O3 and TiO2 coupled with high Mg# and Cr# in clinopyroxene, and high Cr# in spinel. The second stage occurred more widely and gave rise to the large ion lithophile element depletion in clinopyroxenes of all rock types. The extent of melting is lower in the spinel lherzolites than that in the pyroxenites, implying that the partial melting was not caused by decompression and thus most likely related to Tethyan subduction. A third and more recent melting stage, responsible for the spongy texture in some clinopyroxenes, is attributed to the extensional tectonic regime that started in the middle Miocene in the region. Temperature estimates show that both the spinel lherzolites and pyroxenites equilibrated at ~ 900

  8. Geochemical heterogeneity in mantle xenoliths of the Western Grand Canyon

    NASA Astrophysics Data System (ADS)

    Schultz, L.; Low, P. C.

    2011-12-01

    The degree of heterogeneity in the rocks of the upper mantle is not entirely understood, though xenoliths can provide important insight into compositional and other characteristic variances in the deep Earth. Upper mantle xenoliths were sampled from three basalt flows along the Western Grand Canyon in the vicinity of Vulcan's Throne (179.4-184.7 river miles downstream from Lee's Ferry). Host basalts from the three flows are similar in composition and texture, and most samples are porphyritic with olivine phenocrysts of 1 mm or less in diameter, though basalts from two flows also feature samples with vesicular textures. Whole rock geochemical data is consistent with calc-alkaline basalts within plate boundaries, though some tholeiitic iron enrichment is also apparent. Mg/(Mg+Fe) in olivine phenocrysts including texturally identified xenocrysts ranges from 0.78 to 0.90, and a negative sloping REE diagram (chondrite normalized La/Lu ~30) suggests that the basalts are the product of partial melting from a previously depleted mantle source. Mantle xenoliths from these flows include spinel-bearing lherzolites, harzburgites, and dunites with high modal olivine at 74-94% and low modal clinopyroxene at 0-5%, withholding one clinopyroxenite xenolith. In contrast to the basalts, the xenoliths are significantly heterogeneous in modal composition, and single flows contain both crustal and mantle xenoliths, as well as diverse mineral strain histories and varying levels of iddingsitization among mantle xenoliths, even within a single rock sample. A limited compositional range in highly magnesian olivines (Mg/(Mg+Fe) =.895-.912) is consistent with their formation as residues of partial melting, and clinopyroxenes and orthopyroxenes also exhibit a limited compositional range. A variety of geothermometers suggest equilibration temperatures between 730 to 1000°C, while thermodynamic modeling of partial melting processes fits best with observed mineral and bulk rock compositional

  9. 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

  10. Magnetite Nucleation in Mantle Xenoliths During Quasi-Adiabatic Ascent

    NASA Astrophysics Data System (ADS)

    Walsh, K. B., Jr.; Filiberto, J.; Friedman, S. A.; Knafelc, J.; Conder, J. A.; Ferre, E. C.; Khakhalova, E.; Feinberg, J. M.; Neal, C. R.; Ionov, D. A.; Hernandez, F. M.

    2014-12-01

    Can magnetite be a stable phase in the lithospheric mantle? Equilibrium-based thermodynamic calculations and petrologic models predict that it should not be stable. Studies of mantle xenoliths during the 1980s concluded that even though there were rare exceptions, mantle rocks do not host sufficient concentrations of ferromagnetic minerals and are too hot to allow any magnetic remanence. Thus, conventional wisdom dictates that the Moho constitutes a fundamental magnetic boundary. Yet, growing evidence from a more complete global mantle xenolith survey indicates the presence of ferromagnetic minerals in mantle materials. Examination of mantle xenoliths devoid of serpentinization and meteoric alteration show the presence of ferromagnetic minerals within primary silicate mineral phases, including olivine, pyroxene, and spinel. Nucleation of these magnetic minerals could occur at three different stages: in-situ in the mantle, upon ascent, and at the surface. This study reports the results of laboratory-based quasi-adiabatic decompression experiments that aim to simulate the ascent of mantle xenoliths through the lithosphere and test if magnetite growth is promoted during the process. The starting material for these experiments is San Carlos olivine, which holds a magnetic remanence of less than ~10-10 A/m2-1kg2 (the detection limit of the vibrating sample magnetometer). This low starting remanence will allow us to identify whether new magnetic minerals are formed during the decompression experiments using either vibrating sample magnetometry or SQUID-based rock magnetometers. All olivine grains in these experiments were hand-picked under a light microscope in an effort to avoid the inclusion of grains with spurious magnetic minerals. Olivine powders from these carefully selected grains will be used to represent average mantle olivine compositions (Fo90-Fo92). Experiments will start at 1 GPa and be decompressed to 0.3 GPa over 60 hrs at constant temperature (1200° C

  11. Li-Be-B Systematics of mantle Xenoliths from Harrat Uwayrid (Saudi Arabia)

    NASA Astrophysics Data System (ADS)

    Kaliwoda, M.; Altherr, R.

    2003-04-01

    The Harrat Uwayrid is a Late Miocene to Quaternary volcanic field located in the northwestern part of the Arabian plate and related to the opening of the Red Sea. Numerous cinder cones contain abundant mantle xenoliths. Based on mineralogy and textures, these xenoliths can be subdivided into 3 different groups. Group IA1 are "anhydrous" spinel lherzolites and harzburgites consisting of olivine (ol), orthopyroxene (opx), clinopyroxene (cpx) and spinel (spl). Group IA2 xenoliths represent group IA1 materials that were moderately metasomatized. In addition to cpx, opx, ol and spl these xenoliths contain subordinate amounts of Cr-pargasite (par). Group IB xenoliths were strongly metasomatized by fluids and fluid-rich melts resulting in newly formed Ba-phlogopite (phl), Ba-bearing pargasite, Ba-feldspar (celsian) and Ba-rich phonolitic glasses. Li-Be-B systematics of the different xenoliths were studied by secondary ion mass spectrometry (SIMS). The results show that the partitioning of Li, Be and B among the various minerals is virtually independent of pressure and temperature. The absolute abundances, however, depend on the degree of initial depletion and later metasomatic overprint. Group IA1 spinel lherzolites are characterized by Li(cpx) = 0.65-1.19 µg/g, Li(opx) = 1.29-1.63 µg/g and Li(ol) = 2.15-2.43 µg/g, while group IB xenoliths show much higher abundances of Li: Li(cpx) = 1.89-2.16 µg/g, Li(opx) = 1.88-2.83 µg/g, Li(ol) = 3.06-4.12 µg/g. Similar differences are observed for the abundances of B (IA1: B(cpx) = 0.46-0.83 µg/g, B(opx) = 0.047-0.189 µg/g, B(ol) = 0.16-1.15 µg/g; IB: B(cpx) = 0.16-1.15 µg/g, B(opx)= 1.01-1.47 µg/g, B(ol) = 0.44-0.55 µg/g). In marked contrast to Li and B, the abundances of Be were not changed during metasomatism. Group IA1 xenoliths are characterized by Be(cpx) = 0.09-0.13 µg/g, Be(opx) = 0.023-0.027 µg/g and Be(ol) = 0.002 µg/g and group IB xenoliths show Be(cpx) = 0.05-0.09 µg/g, Be(opx) = 0.03-0.08 µg/g and Be

  12. 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.

  13. 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.

  14. 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

  15. 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.

  16. 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

  17. Mantle Xenoliths from Central and South Vietnam: Petrology and Geochemistry

    NASA Astrophysics Data System (ADS)

    Hauzenberger, Christoph; Konzett, Juergen; Nguyen, Hoang; Nguyen, Khoi

    2014-05-01

    Mantle xenoliths, spinel lherzolites and subordinate amounts of spinel harzburgites and pyroxenites, are commonly found in alkali basalts from south-central Vietnam. The basalts are part of widespread Neogene volcanism found in southern China and Indochina regions. Samples from different localities between the cities of Ban Me Thuot and Saigon were recovered. In addition one xenolith sample from an off-shore volcano SE of Ho Chi Minh City in the South China Sea was investigated. The mineral assemblage in most samples consists of the simple lherzolitic mineral assemblage Ol-Opx-Cpx-Sp. The Ol, Cpx and Opx crystals are equigranular while Sp occurs usually as smaller sized intersertal phase or as partly oriented inclusions in Cpx. Cpx II occurs in some samples as recrystallized "spongy rim" around Cpx I. Cpx I has a a very uniform composition between different samples with a typical XMg (=Mg/(Mg+Fe2+) of 0.92 to 0.98, a XNa (=Na/(Na+Ca) of 0.10 to 0.16, a Cr2O3 content of 0.6-0.9 wt. .% and Al2O3 values of c. 6 to 8 wt.%. Cpx II has a lower XNa and Al content as well as higher XMg and Cr content compared to Cpx I. Orthopyroxene typically has a XMg of c. 0.90 to 0.93. The XMg values for Ol differ slightly between different samples but are within 0.84 to 0.94. Spinel grains have a variable composition with XMg from 0.65 to 0.92 and XCr (Cr/Cr+Al+Fe3+) of 0.08 to 0.25. The use of the Cpx-Opx thermometer (BREY & KOEHLER, 1990) and the Al and Cr in Ol thermometer (DE HOOG et al., 2010) allowed to constrain the temperature with 800 to 1100 °C. Trace and rare earth element composition of Cpx was determined by LA-ICPMS. While most Cpx compositions are slightly depleted in LREE, typical for average depleted mantle compositions, some samples are strongly enriched in LREE indicating mantle metasomatic processes. The sample displaying the highest level of LREE enrichment in Cpx has the lowest calculated temperature (T = c. 800°C) and the highest Ni content in olivine (3000 ppm

  18. Lithospheric mantle evolution above a subducting plate: Direct constraints from Antarctic Peninsula spinel peridotite xenoliths

    NASA Astrophysics Data System (ADS)

    Gibson, Lydia; Gibson, Sally; Leat, Phil

    2010-05-01

    Our understanding of the tectono-magmatic processes in subduction zones generally relies on interpretations of the bulk-rock compositions of associated volcanic rocks. These, however, have typically undergone extensive modification in the crust (fractionation and/or contamination) and interpreting the mantle processes that have contributed to their genesis is complex. Direct evidence of the composition of the mantle beneath subduction-related volcanics is rare as mantle xenoliths are seldom brought to the surface. An exception is the Antarctic Peninsula, which consists of a series of suspect arc terranes accreted to the margin of Gondwana. Subduction occurred along a trench, off the west coast, and lasted over 200 Ma. It finally ceased after a series of ridge-trench collisions, which began at ~50 Ma in the south and ended at ca. 4 Ma in the north. This was followed by extensive alkaline volcanism along the length of the Antarctic Peninsula. At several localities these post-subduction volcanics contain abundant, fresh spinel-bearing lherzolites, harzburgites and pyroxenites. The widest variety of xenoliths were collected from basanites and tephrites emplaced on Alexander Island and Rothschild Island in the accreted Western Domain. The mineral chemistry of the xenolith suite as a whole is highly varied, e.g. olivine ranges in composition from Fo77 to Fo91, but within individual xenoliths typically only limited variation is apparent. Xenolith textures and plots of mineral chemistry suggest that the constituent mineral phases are in equilibrium and can be used to determine pressures and temperatures. PT estimates based on pyroxene compositions indicate that the lithosphere beneath the Antarctic Peninsula has a normal, unperturbed mantle geotherm and a thickness of ~90 km; the base of the mechanical boundary layer is at ~70 km and the xenoliths appear to have been entrained from within this region. Preliminary modelling of incompatible-trace-element ratios of diopsides

  19. Progressive enrichment of island arc mantle by melt-peridotite interaction inferred from Kamchatka xenoliths

    NASA Astrophysics Data System (ADS)

    Kepezhinskas, Pavel; Defant, Marc J.; Drummond, Mark S.

    1996-04-01

    The Pliocene (7 Ma) Nb-enriched arc basalts of the Valovayam Volcanic Field (VVF) in the northern segment of Kamchatka arc (Russia) host abundant xenoliths of spinel peridotites and pyroxenites. Textural and microstructural evidence for the high-temperature, multistage creep-related deformations in spinel peridotites supports a sub-arc mantle derivation. Pyroxenites show re-equilibrated mosaic textures, indicating recrystallization during cooling under the ambient thermal conditions. Three textural groups of clinopyroxenes exhibit progressive enrichment in Na, Al, Sr, La, and Ce accompanied by increase in Sr/Y, La/Yb, and Zr/Sm. Trace elements in various mineral phases and from felsic veins obtained through ion microprobe analysis suggest that the xenoliths have interacted with a siliceous (dacitic) melt completely unlike the host basalt. The suite of xenoliths grade from examples that display little evidence of metasomatic reaction to those containing an assemblage of minerals that have been reproduced experimentally from the reaction of a felsic melt with ultramafic rock, e.g., pargasitic amphibole, albite-rich plagioclase, Al-rich augite, and garnet. The dacitic veins within spinel lherzolite display a strong enrichment in Sr and depletion in Y and the heavy rare earth elements (e.g., Yb). The dacites are comparable to adakites (melts derived from subducted metabasalt), and not typical arc melts. We believe that these potential slab melts were introduced into the mantle beneath this portion of Kamchatka subsequent to partial melting of a relatively young (and hot) subducted crust. Island arc metasomatism by peridotite-slab melt interaction is an important mantle hybridization process responsible for arc-related alkaline magma generation from a veined sub-arc mantle.

  20. 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.

  1. 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

  2. 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

  3. 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.

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

    USGS Publications Warehouse

    Carter, Hearn B., Jr.

    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

  5. 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

  6. 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

  7. Chemical and petrological heterogenity of lithospheric mantle beneath N Patagonia (Argentina) - case study of Cerro Chenque xenoliths.

    NASA Astrophysics Data System (ADS)

    Kozdrowska, Dominika; Matusiak-Małek, Magdalena; Ntaflos, Theodoros; Puziewicz, Jacek; Bjerg, Ernesto

    2015-04-01

    Mantle-xenoliths-bearing, back-arc Pliocene - Quaternary alkali basalts occur in N Patagonia, Argentina (Bjerg et al., 2005, J. of S. Am. Sci.). The Cerro Chenque (Rio Negro province) trachybasaltic lavas carry small (up to 10 cm in diameter) xenoliths of anhydrous, spinel bearing harzburgites, dunites and less abundant clino- , orthopyroxenites, websterites. The xenolith suite comprises also gabbros and norites, which are not discussed in this study. All the phases forming xenoliths are rich in Mg (Fo=90.5-93.5%; mg#Opx=0.90-0.94; mg#Cpx=0.91-0.95). Composition of spinel is extremely variable (mg#=0.65-0.85; cr#=0.00-0.70). Three types (A, B, C) of REE patterns occur in clinopyroxene and orthopyroxene: (1) type A (harzburgites and orthopyroxenites) is U-shaped in both the pyroxenes, REE contents vary significantly (e.g. La=~0.5 primitive mantle values (PM), ~5 PM, and ~50PM); (2) type B (dunites and harzburgites) clinopyroxene has flat HREE and is continuously enriched in LREE up to 10x PM, orthopyroxene is U-shaped; (3) type C (harzburgites) clinopyroxene is convex upward, La=5-7PM), orthopyroxene is continuously depleted in LREE. Clinopyroxene of all the types is poor in Ti, while normalized content of other trace elements is strongly variable. Trace element compositions of Cerro Chenque xenolithc clinopyroxene cover whole compositional range of mantle-derived clinopyroxene from Rio Negro province presented by Bjerg et al., 2005 (op.cit.). In xenoliths where clino- and orthopyroxene are in equilibrium, the calculated temperatures are always around 1000°C (Brey and Köhler,1990, JoP). No spinel-clinopyroxene symplectites suggesting peridotite provenance from garnet stability field were observed. Strong variations in rock-type and chemical composition of minerals forming Cerro Chenque xenoliths suggest complicated structure of upper mantle beneath N Patagonia. At present stage of study we suggest that Earth's lithospheric mantle in this region: - suffered from

  8. Platinum-group element abundance patterns in different mantle environments

    SciTech Connect

    Rehkaemper, M.; Halliday, A.N.; Barfod, D.; Fitton, J.G.; Dawson, J.B.

    1997-11-28

    Mantle-derived xenoliths from the Cameroon Line and northern Tanzania display differences in their platinum-group element (PGE) abundance patterns. The Cameroon Line lherzolites have uniform PGE patterns indicating a homogeneous upper mantle over several hundreds of kilometers, with approximately chondritic PGE ratios. The PGE patterns of the Tanzanian peridotites are similar to the PGE systematics of ultramafic rocks from ophiolites. The differences can be explained if the northern Tanzanian lithosphere developed in a fluid-rich suprasubduction zone environment, whereas the Cameroon Line lithosphere only experienced melt extraction from anhydrous periodotites. 32 refs., 2 figs., 1 tab.

  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. 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.

  12. 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

  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. 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

  15. Heterogeneous lithospheric mantle metasomatism in the eastern North China Craton: He-Ar isotopes in peridotite xenoliths from Cenozoic basalts

    NASA Astrophysics Data System (ADS)

    Tang, Huayun; Matsumoto, Takuya; Zheng, Jianping; Czuppon, György; Yu, Chunmei; Miyakawa, Chie; Ping, Xianquan

    2014-02-01

    The abundances and isotopic compositions of Helium and Argon have been analyzed in a suite of fresh spinel peridotite xenoliths in Cenozoic basalts from the eastern North China Craton (NCC) by step-wise heating experiments, to investigate the nature of noble gas reservoirs in the subcontinental lithospheric mantle beneath this region. The xenoliths include one harzburgite collected from Hebi in the interior of the NCC, two lherzolites from Hannuoba at the northern margin of the craton, and three lherzolites from Shanwang and Nushan on the eastern margin. 3He/4He ratios in most of the xenoliths are similar to those of mid-ocean ridge basalts (MORB) or slightly lower (2-10.5 Ra, where Ra is the 3He/4He ratio of the atmosphere), suggesting mixing of MORB-like and radiogenic components. One olivine separate from Nushan has a helium value of 25.3 Ra, probably suggesting cosmogenic 3He addition. The 40Ar/36Ar ratios vary from atmospheric value (296) to 1625, significantly lower than the MORB value. Available data of the peridotite xenoliths indicate the He and Ar isotopic systematics of the mantle reservoirs beneath the NCC can be interpreted as mixtures of at least three end-members including MORB-like, radiogenic and atmospheric components. We suggest that the MORB-like noble gases were derived from the underlying asthenosphere during mantle upwelling, whereas the radiogenic and recycled components probably were incorporated into the lithospheric mantle during circum-craton subduction of oceanic crust. Available data suggest that the MORB-like fluids are better preserved in the interior of the NCC, whereas the radiogenic ones are more prevalent at the margins. The Paleo-Asian ocean subduction system probably was responsible for the enriched and recycled noble gas signatures on the northern margin of the craton, while the Pacific subduction system could account for the observed He-Ar isotopic signatures beneath the eastern part. Therefore, integration of helium and

  16. 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

  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. Melt-Rock Reactions in the Uppermost Sub-Arc Mantle Beneath Kamchatka: Evidence from Peridotite Xenoliths from Shiveluch Volcano

    NASA Astrophysics Data System (ADS)

    Bryant, J.; Yogodzinski, G. M.; Churikova, T. G.

    2005-12-01

    Ultramafic xenoliths from Shiveluch Volcano are predominantly spinel harzburgites with 55-90 modal percent olivine and 7-45 percent orthopyroxene (OPX). Abundant kink-banded olivine and textures that vary from protogranular to porphyroclastic, and granuloblastic, are consistent with a history of plastic deformation under conditions of mantle flow. Metasomatic OPX, phlogopite, clinopyroxene and amphibole, crosscut the xenoliths in mm-scale veins and form irregular patches that appear to replace the olivine-dominant primary mineralogy. Textural features suggest that high modal OPX in the harzburgites was produced by melt-rock reactions involving the replacement of olivine by OPX (e.g., OPX occurs mostly along grain boundaries between coarse olivine crystals). Primary mineral compositions are refractory, with olivine from FO89-94 and Cr# (Cr*100/Al+Cr) in spinel from 40-80. Equilibration temperatures and pressures, calculated using two-pyroxene thermometry and Ca-in-olivine barometry, are between 800-1000°C and 10-25kb. Our best estimate for the temperature and pressure of equilibration of the xenoliths, based on results from samples that have well developed two-pyroxene + olivine mineral assemblages, is approximately 900°C and 12-14kb. Oxygen barometry shows that the xenoliths are strongly oxidized (log (fO2)FMQ from 2.5-4.5) compared to abyssal and continental peridotites. This high fO2 may also reflect a history of melt-rock interaction beneath Shiveluch (e.g., Parkinson and Arculus, Chem. Geol., 1999). These results suggest that melt-rock reactions may play a strong role in creating and modifying the uppermost mantle and deepest crust beneath active subduction-related volcanic arcs.

  19. In-situ geochemistry of sulfides in highly metasomatized mantle xenoliths from Kerguelen, southern Indian Ocean

    NASA Astrophysics Data System (ADS)

    Delpech, Guillaume; Lorand, Jean-Pierre; Grégoire, Michel; Cottin, Jean-Yves; O'Reilly, Suzanne Y.

    2012-12-01

    Mantle xenoliths from the Kerguelen Archipelago record a complex multistage history involving a high degree (15 to 25%) partial melting that created a harzburgitic mantle completely stripped of Base Metal Sulfides (BMS), followed by pervasive melt-rock reaction with alkaline melts above the Kerguelen mantle plume. Subsequent reaction of the highly refractory protolith with small volumes of carbonate-rich silicate melts led to a re-enrichment in BMS (up to 0.05 wt.%). Two BMS precipitation mechanisms are suggested: immiscibility from the silicate-carbonate melt and sulfidation reactions from a CO2-rich supercritical fluid. In-situ analyses of chalcophile and siderophile elements (major and trace levels) in the BMS shed new light on their origin. The BMS phases that precipitated via immiscibility are metal-rich sulfide melts which progressively evolved toward Ni and Cu-rich end-members by cumulate fractionation of monosulfide solid solution (mss) during percolation inside the peridotites. Some cumulate mss have elevated and fractionated IPGE contents (200-900 × C1-Chondrite abundances), indicating random digestion of preexisting Os, Ir, Ru-rich PGM by the percolating sulfide melt. The BMS that precipitated by sulfidation reactions from a CO2-rich vapour phase are subsolidus exsolution products from Cu-bearing but Ni-poorer mss. They have the highest concentrations of PGEs and show selective enrichment in S, Pd, Pt and Os over Cu, S, Ir, Ru and Rh. Their PGE compositions confirm experimental data, which demonstrate that S, Pd, Pt and Os can be efficiently transported in a CO2-rich supercritical fluid. Superchondritic (S/Se), (Os/Ir) and (Pd/Pt) in both bulk-rocks and individual sulfides are inferred to be the geochemical fingerprints of sulfide crystallisation from a CO2-rich vapour exsolved from a highly evolved carbonate-rich metasomatic melt.

  20. Compositional Variability of the Mantle beneath West Antarctica and its Relationship to Terrane Tectonics: Evidence from Mantle Xenoliths

    NASA Astrophysics Data System (ADS)

    Ross, A. J.; Gibson, S. A.; Leat, P. T.; Vaughan, A. P. M.

    2009-04-01

    This work examines the petrography and mineral chemistry of sixteen previously undescribed mantle xenoliths suites from the West Antarctic Peninsula. The xenoliths are from the Jones Mountains (a Palaeozoic volcanic arc terrane on the margin of Gondwana), Adelaide Island (a Mesozoic volcanic arc terrane) and Alexander Island (an accretionary complex). They were entrained by subduction and rift-related magmatism, including 50 Ma calc-alkaline lamprophyres (Adelaide Island) and 10-5 Ma alkali basalts (Alexander Island and Jones Mountains). The xenoliths range in composition from pyroxenites (Adelaide Island) to spinel peridotites (Alexander Island and Jones Mountains). At Alexander Island, the spinel peridotites occur as both ‘fertile' lherzolites and ‘depleted' harzburgites. The xenoliths show a wide variation in mineral chemistry, for example olivine varies from Fo91.5 in the harzburgites to Fo71 in the pyroxenites. Significant variations have also been observed in the major-element chemistry of the pyroxenes. Those in the spinel peridotites are Cr-diopsides whereas those in the pyroxenites are Al-augites. Rare-earth element patterns of clinopyroxenes and Cr/(Cr+Al) ratios of spinels provide information on the origin of the xenoliths. We conclude that the harzburgites experienced a complex evolution involving extraction of up to 20% melt, perhaps in the mantle wedge, followed by accretion on to the base of the lithosphere and enrichment in Cr by large degree hydrous melts (boninites). There is also evidence of enrichment in strongly incompatible trace elements by carbonate melts and fluids from the subducted Phoenix plate. However, some of the spinel lherzolites from Alexander Island, and also those from the Jones Mountains, have compositions that are similar to fertile mantle and have not been subjected to large scale melting. The pyroxenites from Adelaide Island are believed to represent samples of veined lithospheric mantle caused by percolation and reaction

  1. 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.

  2. 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

  3. 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

  4. 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.

  5. 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

  6. 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

  7. 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

  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. Contrasting lithospheric mantle domains beneath the Massif Central (France) revealed by geochemistry of peridotite xenoliths

    NASA Astrophysics Data System (ADS)

    Lenoir, Xavier; Garrido, Carlos J.; Bodinier, Jean-Louis; Dautria, Jean-Marie

    2000-09-01

    We report major and trace element analyses for 82 coarse-grained peridotite xenoliths from 25 Cenozoic volcanic centres throughout the Massif Central (France). These data cover a region of about 150×150 km, allowing an investigation of large scale compositional variations in the subcontinental lithospheric mantle (SCLM). In agreement with textural variations, geochemical data define two contrasting lithospheric domains, situated north and south of latitude 45°30'. Peridotites of the northern domain show protogranular textures, characterised by clustered pyroxene-spinel distributions. They are rather refractory and depleted in MREE relative to HREE, but pervasively enriched in LREE and other highly incompatible elements. The samples show mantle-normalised patterns with negative anomalies of Nb, Ta, Zr and Hf, similar to enriched mantle xenoliths ascribed to carbonatitic metasomatism. In contrast, the peridotites of the southern domain are devoid of pyroxene-spinel clusters and are therefore referred to as coarse-granular. They are distinguished from the northern suite by more fertile compositions and relatively flat MREE-HREE patterns. In addition, only the harzburgites and a few lherzolites are enriched in LREE. Most southern domain lherzolites are depleted in these elements and the average composition of the southern suite is comparable to that of depleted MORB-source mantle (DMM). The main compositional differences between the two domains cannot be accounted for by a secular evolution of the Massif Central SCLM caused by Cenozoic plume upwelling. Instead, these differences record the existence of distinct lithospheric blocks assembled during the Variscan orogeny. To some degree, the northern and southern domains are reminiscent of cratonic and circumcratonic SCLM domains. Being relatively refractory and pervasively enriched in LREE, the northern domain displays similarities with cratonic SCLM. It is interpreted as a relatively ancient (pre

  10. 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

  11. 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

  12. 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).

  13. 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.

  14. Petrology and Geochemistry of mantle xenoliths from Margasan, Baikal Lake region (Russia) Preliminary results

    NASA Astrophysics Data System (ADS)

    Dippenaar, Elijah; Ntaflos, Theodoros; Ashchepkov, Igor

    2014-05-01

    Off-cratonic mantle xenoliths collected from the Margasan volcanic field in eastern Siberia (Russia) were analysed to characterize their mineralogical composition, their petrological and geochemical properties. The xenoliths are spinel bearing lherzolithe and have been transported to the surface by alkali basalts. The studied samples are fertile lherzolithes with modal compositions as follows: Forsterite 56% - 78%, orthopyroxene 35% - 16%, clinopyroxene 6% - 20% and spinel 0% - 4.5%. The majority of the samples have textures ranging from protogranular to equigranular. Some of the samples with equigranular texture show characteristic foliation suggesting deformation in the depths. Frequently, olivine grains show kink-bands. The clinopyroxenes show characteristic spongy rims with variable width as a consequence of interaction with interganulate circulating melts/fluids. Alternatively they could be formed en route to the surface due to pressure decrease. Microprobe analyses have shown that Forstertite (Fo) contents in olivine vary from 89 to 90. The mg#*100 in orthopyroxene and clinopyroxene ranges from 90 to 90.5 and 89 to 91 respectively. Spinels are Al2O3-rich with 100*cr# ranging from 9 to 17. The TiO2, Cr2O3 and Na2O contents in clinopyroxene vary from 0.4% to 0.7%, 0.6% to 1.1% and 0.6 to 2.1 respectively. The clinopyroxene spongy rims consist of glass and second generation clinopyroxene, which differs from the primary clinopyroxenes in that the TiO2, Al2O3 and Na2O contents are lower than in the primary clinopyroxenes. According to the model of batch melting the studied xenoliths indicate that they are the residues after 1.5% to 7% partial melting. The calculated equilibrium temperature at a pressure of 15kbar ranges from 940°C to 1120°C, indicating that xenoliths have been sampled from different depths.

  15. 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

  16. 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.

  17. 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

  18. Hydrogen Content Of The Mantle From Combined Magnetotelluric, Mineral Physics And Xenolith Data

    NASA Astrophysics Data System (ADS)

    Selway, K.; Karato, S.

    2012-12-01

    Determining the composition of the mantle is a significant challenge. The only direct measurements can be made from mantle xenoliths, which sample only a small fraction of the lithsopheric mantle and are unlikely to have strictly representative compositions. Therefore it is vital that geophysical data that image the mantle are also analysed and that their relationship to compositional factors is well understood. The geophysical techniques with the best resolution at mantle depths are seismic tomography and magnetotellurics (MT). MT is a passive electromagnetic method that relies on the fact that the Earth's time-varying magnetic field induces electrical currents in conductive bodies within the Earth and thus images the electrical resistivity structure of the Earth. Mineral physics experiments on the nominally anhydrous silicates that dominate the continental mantle have shown that, at a given temperature, their electrical conductivities are very sensitive to small amounts of hydrogen while being relatively insensitive to major element composition. The combination of MT data and experimental mineral physics data is therefore a powerful tool to constrain the hydration state of the mantle. However, in order to obtain reliable estimates of water content, the influence of other factors such as temperature and oxygen fugacity needs to be analysed with a great care. With this in mind, an MT survey was carried out in north-east Tanzania, crossing from the Tanzanian Craton over the East Africa Rift. The Tanzanian Craton has remained largely stable since the Archean whereas the adjacent lithosphere has been reworked during several major orogenic events including the Paleoproterozoic Usagaran Orogen, the Neoproterozoic East African Orogen and the current East Africa Rifting. The very different responses of the Tanzanian Craton and the adjacent lithosphere to these tectonothermal events suggests that there are significant compositional differences between them. Much of the

  19. 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

  20. The Xinchang peridotite xenoliths reveal mantle replacement and accretion in southeastern China

    NASA Astrophysics Data System (ADS)

    Liu, Chuan-Zhou; Wu, Fu-Yuan; Sun, Jing; Chu, Zhu-Yin; Qiu, Zhi-Li

    2012-10-01

    The Xinchang mantle xenoliths, including garnet lherzolites, spinel lherzolites and harzburgites, have been studied for the nature and age of the lithospheric mantle beneath southeastern (SE) China. The spinel harzburgites have refractory compositions, with 0.95-1.73 wt.% Al2O3, 0.65-1.47 wt.% CaO, 0.03-0.16 wt.% TiO2 and 2134-2422 ppm Ni. The lherzolites (both spinel- and garnet-) have more fertile compositions, containing 2.4-5.43 wt.% Al2O3, 2.17-3.22 wt.% CaO, 0.1-0.38 wt.% TiO2 and 1733-2055 ppm Ni. Olivines in the harzburgites have Fo contents of 88.7-91.4, which is 88.4-90.2 for the lherzolites. Both clinopyroxene and orthopyroxene have lower Al2O3 but higher Cr2O3 contents than those in the lherzolites. Modeling of the Y and Yb contents in clinopyroxenes indicates that the spinel harzburgites have been subjected to ca. 20% degrees of partial melting. Reaction textures, in particular the sieve-texture in clinopyroxene, is widely developed in the Xinchang mantle xenoliths, which resulted from reaction with the host lavas. The sieve-textured clinopyroxenes show compositional zonation; the sieve-textured rims have lower Na2O and Al2O3 but higher Cr2O3 and CaO than the inclusion-free cores. The inclusion-free cores of clinopyroxene in the harzburgites show clearly enriched characteristics in trace elements, reflecting they have been metasomatized before the entrainment in the host lavas. Clinopyroxenes have high Ti/Eu but low La/Ya ratios, suggesting that the Xinchang mantle xenoliths were metasomatized by silicate melts. The spinel harzburgites have 187Os/188Os of 0.11999-0.12258, giving TRD age of 0.99-1.35 Ga relative to the primitive upper mantle (PUM). In comparison, both spinel and garnet lherzolites have more radiogenic 187Os/188Os ratios (0.12424-0.12801) and younger model ages (0.22-0.75 Ga). We explain that the spinel harzburgites represent the ancient lithosphere relic beneath the Xinchang region, whereas both spinel and garnet lherzolites represent

  1. 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

  2. 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

  3. 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.

  4. Toward a general view of mantle peridotite beneath the volcanic front: petrology of peridotite xenoliths from Bezymyanny volcano (central Kamchatka)

    NASA Astrophysics Data System (ADS)

    Ishimaru, S.; Arai, S.; Tamura, A.; Okrugin, V. M.; Shcherbakov, V.; Plechov, P.

    2012-04-01

    We have a large amount of data about petrological and geochemical features of upper mantle peridotites based on researches of mantle xenoliths, ophiolites or solid intrusions. But the nature of sub-arc mantle, especially beneath a volcanic front, has not been fully understood due to the scarcity of occurrences of mantle-derived materials there. Kamchatka Peninsula is one of the active volcanic arcs, having 29 active volcanoes, and 13 volcanoes of them contain cognate or mantle peridotite xenoliths (Erlich et al., 1979). Peridotite xenoliths derived from the upper mantle beneath the volcanic front are expected from 9 of them (Erlich et al., 1979). Avachinsky (Avacha) volcano is the most famous of them because of its easy accessibility and high xenolith production. Peridotite xenoliths from Avacha record high degree of melting and multiple stages of metasomatism (e.g., Ishimaru et al., 2007; Ionov, 2010). Formation of secondary orthopyroxenes replacing olivine is one of characteristics of arc-derived peridotite xenoliths (e.g., Arai & Kida, 2000; McInnes et al., 2001). In addition, we found peculiar metasomatisms, e.g., Ni enrichment (e.g., Ishimaru and Arai, 2008), in the Avacha peridotite xenolith suite. Here, we show petrological and geochemical features of ultramafic xenoliths from Bezymyanny volcano, central Kamchatka, to obtain a more generalized view of the sub-front mantle. We examined 2 harzburgite xenoliths from Bezymyanny. They are composed of fine-grained minerals (cf. Arai and Kida, 2000), and occasionally contain hornblende and/or phlogopite. Almost all orthopyroxenes show irregular shapes and replace olivine, indicating a secondary origin. At the boundary between the harzburgite and host andesite, we observed hornblende and secondary orthopyroxenes. At the xenoliths' interior, Fo content of olivine and Cr# (= Cr/(Cr + Al) atomic ratio) of chromian spinel are high, 91-92 and 0.43-0.69, respectively, and the Fo content decreases to 76 at the boundary

  5. Mineral chemistry of a zircon-bearing, composite, veined and metasomatised upper-mantle peridotite xenolith from kimberlite

    NASA Astrophysics Data System (ADS)

    Dawson, J. B.; Hill, P. G.; Kinny, P. D.

    2001-02-01

    Zircon-bearing veins in a harzburgite xenolith from kimberlite have imposed Ca-metasomatism on the harzburgite wall rock, in addition to adding K, Fe, Ti and OH. The zircon, previously dated to have an age similar to that of the xenolith-hosting kimberlite, shows higher Y, Nb, Ba, REE, Th and U contents than other mantle-derived zircons. Peripheral alteration of the zircon to baddeleyite and zirconolite, and alteration of vein ilmenite to perovskite suggest reaction with an evolving carbonatitic kimberlite melt. The high Cr2O3 content (0.77 wt%) of the zirconolite extends the compositional range of terrestrial zirconolite.

  6. 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.

  7. 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.

  8. 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.

  9. Melting and Metasomatism in the Mantle Lithosphere beneath the Pribilof Islands: Petrology and ICP-MS Analyses of Spinel Peridotite Xenoliths from St. George Island, Bering Sea, Alaska

    NASA Astrophysics Data System (ADS)

    Feeley, T. C.; Ulianov, A.; Underwood, S. J.

    2006-12-01

    A suite of protogranular spinel lherzolite xenoliths from an alkali basalt lava flow on St. George Island, Alaska, have been analyzed by for their bulk compositions by XRF and ICP-MS and for mineral compositions by electron microprobe and laser ablation ICP-MS. Bulk compositions of the peridotites range from relatively fertile (15-19% modal diopside) to depleted (10-8% modal diopside) and are consistent with variable degrees of melt extraction (7-18%) from a primitive mantle source. No hydrous minerals (e.g., mica or amphibole) were observed in any of the studied xenoliths. However, all xenoliths show melt-reaction textures characterized by glass-bearing, sieved textured rims on clinopyroxene and spinel. In several samples, complete clinopyroxene grains show sieve texture and are associated with melt pockets composed of coexisting silicic glass (Na- ± K-rich) and quench-textured olivine, spinel, clinopyroxene, and feldspar. In general, clinopyroxene and whole-rock chondrite-normalized REE patterns are similar, indicating that the whole-rock compositions reliably record the pre-entrainment REE patterns of the xenoliths. Whole-rock and clinopyroxene REE patterns show a considerable range from LREE-depleted to LREE-enriched, with most samples having flat to MREE-enriched patterns. The REE systematics suggest that most St. George lherzolites experienced little metasomatic overprinting of highly incompatible trace elements following melt extraction. In this regard, only the most refractory sample studied shows clear compositional evidence for metasomatism, as reflected in high whole-rock and clinopyroxene LREE/HREE ratios and abundances of other highly incompatible trace elements (e.g., Th, Ta, Nb, Rb, Sr). Based on the pervasive homogenization of this metasomatic signature, it is likely that it does not reflect a recent event related to the magmatism that brought the xenoliths to the surface. In spite of this evidence, the melt-reaction textures clearly demonstrate

  10. 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

  11. 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

  12. Petrological characterization of mantle xenoliths from Handler Ridge, Northern Victoria Land (Antarctica)

    NASA Astrophysics Data System (ADS)

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

    2014-05-01

    A study of ultramafic xenoliths in Cenozoic alkali basalts from Handler Ridge has been carried out in order to characterize the petrological features of the lithospheric mantle beneath Handler Ridge, Northern Victoria Land (NVL) and to extend the petrological knowledge of the mantle over a large portion of the Western Antarctic Rift, from M.te Melbourne (74°21'S 164°42'E) to Handler Ridge (72°31'S 167°18'E). Xenoliths are anhydrous spinel(sp) bearing lherzolites. Two textural types were recognized: i) medium to coarse grain protogranular and ii) fine grain equigranular. Some xenoliths present evidences of peridotite-melt interactions, such as spongy textures, cloudy rims and glassy patches. Large primary sp are often lobated. Newly formed, secondary crystals are referred to cpx only. Regardless textural types, olivine (ol) is forsteritic in composition with Fo varying from 87.5 to 91.0, with a more fertile group ranging between 87.6 and 88.6. NiO contents between 0.31 and 0.44 wt% match the typical mantle values. As observed in ol, orthopyroxene (opx) can be distinguished in two clusters: the more fertile group with mg# [=Mg/(Mg+Fe)*100 mol] ranging between 87.6 and 88.3 and the more residual one with mg# from 88.9 to 91.0. In cpx mg# varies between 87.06 and 91.7. Cpx in depleted lherzolite and secondary cpx are characterized by high values of Na2O (1.3-1.63 wt%) and Al2O3 (4.62-6.62 wt%). Glasses are SiO2 (≥55.50 wt%) oversaturated with TiO2 content between 0.6 and 1wt% and Na2O/K2O in the range of 0.80-2.02. Sp are characterized by mg# values between 62 and 78, and cr# [=Cr/(Cr+Al)*100 mol] from 14 to 42 with the usual negative correlation between the two parameters. The relationship between ol, opx and sp suggest that partial melting is the main process controlling the chemical variation of these three phases, while cpx were affected by metasomatic event/s. On the basis of the formalism of Brey & Kohler (1990), Liermann & Ganguly (2003) and Ballhaus (1991

  13. 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.

  14. A Morphological and Chemical Study of Carbonate Globules Contained Within Mantle Xenoliths of the Sverrefjell Volcano Spitsbergen -- Implications for ALH84001

    NASA Astrophysics Data System (ADS)

    Steele, A.; Amundsen, H. E. F.; Fries, M.; Vicenzi, E. P.; Benning, L.; Maule, J.; Mysen, B.; Toporski, J.; Schweizer, M.; Fogel, M.

    2005-03-01

    We describe raman and elemental composition of globules similar to those found in ALH84001. We appear to see direct evidence of zonation of carbonate, carbon and silicon phases within terrestrial olivine mantle xenoliths.

  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. CO2 fluid inclusion barometry in mantle xenoliths from central Mexico: A detailed record of magma ascent

    NASA Astrophysics Data System (ADS)

    Levresse, Gilles; Cervantes-de la Cruz, Karina Elizabeth; Aranda-Gómez, José Jorge; Dávalos-Elizondo, María Guadalupe; Jiménez-Sandoval, Sergio; Rodríguez-Melgarejo, Francisco; Alba-Aldave, Leticia Araceli

    2016-01-01

    Chemical equilibrium and CO2 fluid inclusion barometry are complementary techniques, as they register different geological processes. Chemical equilibrium barometry records the pressure at the site where the xenoliths were incorporated into the transporting magma, but it is not a sensible technique to document magma ascent. CO2 fluid inclusion pore pressure in xenoliths does not register the P-T conditions in the source area, but it allows illustrating a fairly detailed record of different geological processes that occurred during the magma transport to the surface and as the eruption proceeded. Mantle xenoliths from Ventura-Espíritu Santo and Santo Domingo volcanic fields contain dominant CO2 pseudosecondary and secondary fluid inclusions trapped in cpx and ol. Cpx chemical equilibrium pressures indicate a maximum pressure of 10 kbar for the source area. Pore pressures obtained in CO2 pseudosecondary and secondary fluid inclusions show a distribution with three maximum peaks at ca. 8, 5-7, and less than 3 kbar. A comparison with geophysical models for the area where the xenoliths-bearing volcanoes are located shows that the three peaks in the pore pressures correspond to three physico-chemical transitions within the continental crust. Likewise, the pore pressure suggests that rapid magma ascent is momentarily interrupted by these discontinuities, a fact that allows the formation of new fluid inclusions and the re-equilibration of some of the inclusions already present in the primary minerals of the xenoliths.

  18. 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

  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. 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

  2. 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

  3. Upper mantle can in-situ fracture: an implication from a cataclastic peridotite xenolith from Megata, Northeast Japan arc

    NASA Astrophysics Data System (ADS)

    Takeuchi, Miyuki; Arai, Shoji

    2015-04-01

    A peculiar peridotite xenolith with cataclastic texture was found from Ichinomegata crater, Megata volcano, the Northeast Japan arc. This peridotite xenolith is the same in mineral assemblage and mineral chemistry (olivine, Fo90; spinel, Cr/(Cr + Al) atomic ratio, 0.2) to some fertile mantle lherzolites, but quite different in texture from all the documented mantle peridotite xenoliths from Ichinomegata and other localities on Earth. The peridotite is a mixture of coarse and fine grains of olivine, orthopyroxene, clinopyroxene, chromian spinel. The coarse mineral grains are angular but never elongated like those in mylonite. All minerals have been fragmented, and the fragmented pyroxenes and chromian spinel form thin streaks in fine-grained olivine-rich matrix. These features indicate that brittle fracturing does occur even in the upper mantle possibly along pre-existing deep-seated faults that have been frequent in the Northeast Japan arc. Some of the fine grains (<100 μm across) of olivine and pyroxenes display strong intra-grain and inter-grain chemical variations; some are more refractory and the others are more evolved in chemistry than the coarse grains. This suggests a possibility of very small degree frictional melting of peridotite upon cataclastic fracturing.

  4. 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

  5. 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

  6. Flow in the uppermost mantle during back-arc spreading revealed by Ichinomegata peridotite xenoliths, NE Japan

    NASA Astrophysics Data System (ADS)

    Satsukawa, Takako; Michibayashi, Katsuyoshi

    2014-02-01

    Spinel peridotite xenoliths from the Ichinomegata Volcano (NE Japan) have distinct foliations defined by compositional layering between olivine-rich and pyroxene-rich layers as well as lineations defined by elongated spinel grains. Crystallographic preferred orientations (CPOs) of olivine are consistent with slip on (010)[100] and {0kl}[100]. The angles between the foliation and the olivine slip planes decrease with increasing values of the J-index (i.e. CPO strength). Such composite planar relationships within the peridotite xenoliths could result from shearing in the uppermost mantle, so that shear strains can be estimated by the angles between the foliation and the olivine slip plane in terms of simple shear strain (0.31-4.26). From these observations, we argue that a suite of the peridotite xenoliths recorded a rare snapshot of uppermost-mantle flow related to back-arc spreading during the opening of the Japan Sea. The peridotite xenoliths with higher J-indices (i.e. higher shear strain) tend to have slightly lower minimum temperatures, possibly defining a vertical strain gradient in the uppermost mantle section at the time of the volcano's eruption. The CPO data have been used to calculate the seismic properties of the xenoliths at PT conditions obtained from geothermobarometry, and are compared to field geophysical data from the literature. Our results are consistent with a roughly EW-oriented fastest P-wave propagation direction in the uppermost mantle beneath the northeast part of the Japan arc. Average samples are calculated based on three different structural reference frames; horizontal plane parallel to 1) foliation, 2) the plane containing the maximum concentration of olivine [100], and 3) P-wave maximum direction. S-wave anisotropy deduced from CPOs requires a reasonable thickness of the anisotropic layer (24.1-26.6 km), and the structural reference frame does not have significant effect on the estimation of thickness. Consequently, Ichinomegata

  7. 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.

  8. Fabric Characterization of Mantle beneath South Central North America: Constraints from Peridotite Xenoliths from Knippa and Kilbourne Hole

    NASA Astrophysics Data System (ADS)

    Satsukawa, T.; Michibayashi, K.; Raye, U.; Stern, R. J.; Anthony, E. Y.

    2009-12-01

    Knippa quarry and Kilbourne Hole are sites where we can study samples of the upper mantle beneath Texas and New Mexico. Knippa peridotite xenoliths are hosted by ~86 Ma nepehlinites of the Balcones Igneous Province in central Texas. Kilbourne Hole maar, Potrillo volcanic field (PVF) near El Paso, erupted basanite approximately 10 Ka ago and thus the xenoliths represent present-day thermal and compositional character of this segment of the continental mantle. Both provide rare snapshots of upper mantle processes and compositions beneath south-central Laurentia. The xenolith suites are dominated by lherzolite, harzburgite and dunite, although pyroxenites are also common at Kilbourne Hole. Temperatures determined using the Ca in opx thermometer (Brey and Kohler, 1990) range between 900 and 1000C. Kilbourne Hole peridotites are several types. Fine-grained lherzolite has the lowest temperatures, porphyroclastic lherzolite intermediate temperature, and harzburgite and dunite the highest temperature. Large olivine grains commonly contain subgrain boundaries. The dominant slip system in olivine was determined from the orientations of the axes of subgrain rotation and CPO data. Olivine CPO data show strong concentration in [100] and [100]{0kl} patterns in Knippa peridotites, and [100](010) and [010]-fiber patterns in Kilbourne peridotites. We interpret the development of [010]-fiber patterns to: (i) axial shortening or 3D transpressive deformation, (ii) activation of multiple glide directions, (iii) deformation in the presence of melt. Presence of melt is also indicated by xenolith elemental compositions, which indicate melt infiltration and reaction. CPO strength is characterized by the dimensionless texture index “J”. J-index of Knippa peridotites varies from 4.6 to 11.4. Kilbourne Hole peridotite J-indices positively correlate with temperature, varying from 5 to 10 for the low temperature lherzolite to an unusually high value of 20 for the harzburgite and dunite

  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. A Physical Description of the Lithosphere and Seismic Low-Velocity Zone beneath Oahu: Perspectives from Hawaiian Mantle Xenoliths

    NASA Astrophysics Data System (ADS)

    Sen, G.; Bizimis, M.; Keshav, S.

    2003-12-01

    Mantle xenoliths provide a random sampling of a significant portion of the uppermost mantle beneath Oahu, including all of the lithosphere and the lithosphere-asthenosphere transition (LAT). We combine all the petrological, geochemical, and isotopic information on these xenoliths and evaluate existing models of the lithosphere and LAT beneath Oahu. The xenoliths are principally of two kinds - spinel peridotites and garenet clinopyroxenites; and they occur within the post-erosional Honolulu Volcanics (HV). Plagioclase peridotites are rare, and plagioclases in them are exotic, having crystallized from Koolau-like (enriched) magma. Trace element and Nd-Hf isotope ratios indicate recent metasomatic enrichment of the depleted peridotitic lithosphere by HV-like fluids. The pyroxenite suite xenoliths are dominantly composed of clinopyroxene and garnet with variable proportions of olivine, minor spinel and orthopyroxene, and rare phlogopite, carbonate, amphibole, and melt pockets. Phase equilibrium considerations indicate that these rocks form accumulates at the LAT and along the walls of fracture-conduits within the deep lithosphere. Nd-Sr isotope data on whole rocks and garnet and cpx separates show them to be depleted as well, partially overlapping with the host HV lavas and lithospheric spinel peridotite xenoliths. We view the Oahu lithosphere (below the crustal part) as follows: 20-30 km - depleted harzburgite with veins of plagioclase; 30 - 90 km - spinel peridotite layer (we model the vertical modal variation), 90 -130 km - cumulate garnet clinopyroxenites and "piclogites" with small amounts of (5 percent) intergranular H2O+CO2-rich fluids that may be of kimberlitic-carbonatite affinity. Our inferred compsition of the lithosphere and LAT are generally similar to geophysical models but predict a 200-250oC lower temperature within the LVZ beneath Oahu than the latter. In our model, Hawaiian plume-derived shield-stage magmas have little interaction with the lithosphere

  11. 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

  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. Carbonatite melt infiltration in mantle xenoliths from the Eurasian plate - North American modern plate collision zone (Ruditch, Yakutia)

    NASA Astrophysics Data System (ADS)

    Tschegg, Cornelius; Ntaflos, Theodoros; Akinin, Viacheslav; Hauzenberger, Christoph

    2010-05-01

    Within the seismic active Chersky belt, the modern border between North American and Eurasian plates (Indigirka River area, Sakha-Yakutia Republic), mantle xenoliths were found in eroded alkaline basalt dike remnants.The peridotite xenoliths are represented by mainly anhydrous spinel lherzolites that appear together with subordinate orthpyroxene, clinopyroxene and feldspar megacrysts. Spinel lherzolites have protogranular textures and are well equilibrated, lacking any mineral zonation. The constituent minerals have minor compositional variations whithin and between different samples. Olivine compositions range from Fo 89-90.5, with CaO contents between 0.04 and 0.06 wt.%. Orthopyroxenes indicate a very narrow composititional variance (Wo1En63Fs36, Mg# 90-91 and Al2O3 from 4 to 4.7 wt.%), just like clinopyroxene phases that are represented by Wo38En40Fs22, with Mg#s from 90 to 91 and Al2O3 between 6.8 and 7.6 wt.%. Spinels also show a fertile composition with Cr#s ranging between 26 and 29 and Mg#s between 77 and 78 respectively. Equlibration temperature estimations gives approx. 1000 °C at 15 kbar pressure for all studied samples. In one xenolith, a round melt pocket with 200 microns in diameter consisting of well crystallized dolomite (25 wt.% CaO, 31 wt.% MgO) in perfect contact with homogeneous glass (16 wt.% Na2O, 51 wt.% SiO2, 20 wt.% Al2O3), apparently an immiscibility of carbonatite and silicate melt, was found at the triple point of olivine, orthopyroxene and clinopyroxene. Mineral chemistries show that the lithospheric mantle underneath the study area is a fertile lherzolith. Clinopyroxene LA-ICP-MS trace element analyses confirm the fertile nature of the xenoliths. The primitive mantle normalized REE patterns show a slight depletion of LREE with respect to HREE. The majority of the analyzed cpx have (La/Yb)N that vary between 0.1 and 0.5 and (Tb/Yb)N from 1.0 to 1.1 indicating the overal absense and metasomatic processes and low degree of melt

  14. 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.

  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. Insights into the evolution of the uppermost continental mantle from xenolith localities on and near the Colorado Plateau and regional comparisons

    NASA Astrophysics Data System (ADS)

    Smith, D.

    2000-07-01

    The interplay between the evolution of mantle and crust has been investigated by textural and electron probe analysis of Cr-diopside spinel peridotite xenoliths from six localities and by comparisons to a database of mineral compositions. The six localities are on a 300-km transect from the Colorado Plateau into the adjoining Rio Grande rift. Typical xenolith populations from the Colorado Plateau province have magnesian olivine (average Fo90.5) and Ti- and Na-poor clinopyroxene. In contrast, typical xenolith populations from the Rio Grande rift and the Basin and Range are characterized by more Fe-rich olivine (average Fo89.6) and clinopyroxene with a range of Ti and Na (average 0.44wt%TiO2). The contrast between the xenoliths from these adjacent provinces documents that the Colorado Plateau has a depleted mantle root that helps to stabilize it. At Mount Taylor, however, a locality high on a composite volcano within the eastern Colorado Plateau, most xenoliths are not depleted, and the Fe-rich olivine (Fo86-Fo88) in them may characterize mantle affected by silicate melt metasomatism at high melt/rock ratios. Low-Al orthopyroxene occurs at two of the six transect localities, and part of a large porphyroblast contains only 0.13wt%Al2O3 because of the unusual compositions and textures, water-rock reactions are inferred to have been important in parts of the refractory continental mantle. Zonation of Ca and Al in orthopyroxene provides constraints on the temperature histories of the unusual pyroxenes. Equilibration temperatures for xenoliths from Lunar Crater, Nevada, average 1270°C, much higher than the average of 1012°C for others from the Basin and Range; Lunar Crater xenoliths may be samples of a plume or asthenosphere emplaced recently into the uppermost mantle.

  20. Seismic properties and deformation record of the shallow upper mantle beneath SE Iberia: insights from mantle xenoliths in Neogene alkaline basalts

    NASA Astrophysics Data System (ADS)

    Konc, Z.; Garrido, C.; Tommasi, A.; Padrón-Navarta, J.; Hidas, K.; Szabo, C.

    2011-12-01

    The volcanic activity in the SE Iberian Volcanic Province 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 snapshot of the structure and composition 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 olivine crystal preferred orientation (CPO) that is similar in the two investigated volcanic centers. The dominant olivine CPO is [100]-fiber pattern 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 and suggests simple shear or combinations of simple shear and pure shear with a transtensional component were the dominant deformation regimes. These olivine CPO contrast with those of Ronda peridotite -sampling the lithospheric mantle of the Alboran domain at 23 Ma- which are consistent with a transpressional deformation regime, indicating a temporal evolution of the deformation regime to transpressive to a transtensive in the Miocene. The seismic anisotropy calculated from xenolith's CPO and modal compositions is characterized by fast propagation of P-waves and polarization of fast S-waves parallel to the lineation. Highest delay times are observed for S-waves propagating within the foliation, but at high angle to the lineation, whereas S-waves propagating along the lineation

  1. 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.

  2. 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

  3. 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.

  4. Water-rock interactions, orthopyroxene growth, and Si-enrichment in the mantle: evidence in xenoliths from the Colorado Plateau, southwestern United States

    NASA Astrophysics Data System (ADS)

    Smith, Douglas; Alexis Riter, J. C.; Mertzman, Stanley A.

    1999-01-01

    Water-rock interactions and consequent orthopyroxene growth are documented by mantle xenoliths from opposite margins of the Colorado Plateau province. The interactions are inferred from a distinctive texture plus composition of orthopyroxene in spinel peridotite, in which porphyroblasts of orthopyroxene with inclusions of resorbed olivine are zoned to interiors exceptionally low in Al 2O 3 (<0.20 wt%). This texture plus composition has been found in xenoliths from Bandera Crater, New Mexico, in the southeastern margin of the Colorado Plateau, extending the known distribution from the northwestern margin (Grand Canyon field). Evidence that Si-enrichment locally accompanied movement of aqueous fluid is provided by an orthopyroxenite xenolith that is about 95% enstatite. The enstatite occurs in curved laths to 25 mm long, and the texture and composition (Al 2O 3 1 to 2 wt%, Mg/(Mg + Fe) 0.92) are attributed to growth during subsolidus interaction between peridotite and hydrous fluid. Modal orthopyroxene calculated from 4 bulk rock analyses of peridotite xenoliths from the Grand Canyon field ranges from 26 to 29%, more than in comparably depleted oceanic mantle. The mantle root of the Colorado Plateau may have formed from accreted ocean lithosphere and subsequently been enriched in Si by aqueous metasomatism at widely distributed sites. Similar fluid-rock interaction may have contributed to the orthopyroxene-enrichment characteristic of some mantle xenoliths from roots of Archaean cratons.

  5. 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.

  6. Sulfide mineralogy and chalcophile and siderophile element abundances in the Ivrea-Verbano mantle peridotites (Western Italian Alps)

    NASA Astrophysics Data System (ADS)

    Garuti, G.; Gorgoni, C.; Sighinolfi, G. P.

    1984-09-01

    The mineralogy of sulfide assemblages and the abundances of chalcophile and siderophile elements are determined in samples of peridotite massifs (supposed to be mantle fragments partially melted beneath a plate-collision area) from Baldissero, Balmuccia, and Finero in the Western Italian Alps. The S content is measured by pyrolysis; sulfide phases are subjected to microscopic examination and microprobe analysis; and trace and ultratrace elements are determined by flame and nonflame AAS analyses and vapor hydride-forming techniques. The results are presented in tables and graphs and characterized in detail, and the implications for the composition of the early mantle are explored. The sulfide component is found to determine the abundances of both chalcophile and strongly siderophile elements and to vary closely with the degree of melting. It is inferred from the nonprimary nature of sulfide components in ultramalfic xenoliths that these materials are not representative of the primitive mantle.

  7. 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

  8. 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

  9. Pyroxenite and granulite xenoliths from beneath the Scottish Northern Highlands Terrane: evidence for lower-crust/upper-mantle relationships

    NASA Astrophysics Data System (ADS)

    Upton, B. G. J.; Aspen, P.; Hinton, R. W.

    2001-08-01

    , Ti and OH and ascending from the asthenosphere, interact with the ultramafic cumulates, modifying them texturally and modally to produce a complex veined assemblage of clinopyroxene- and pargasite-rich rocks. The metasomatism involved an increase in LREE, HFSE and LILE contents. Some modal and cryptic metasomatism may also have affected the granulites, accounting for the presence of amphibole and relatively high LREE/HREE values (La/Lu 38-206). Since closely comparable xenolith assemblages also occur in Mull at the southwestern extremity of the Northern Highland Terrane, such metasomatised olivine- and orthopyroxene-deficient ultramafic rocks may characterise the shallowest part of the mantle beneath the entire terrane. The strongly bimodal character of the xenolith populations (either ultramafic or mafic grading to felsic) is taken to reflect the sharpness of the petrological Moho in this region.

  10. 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

  11. Petrogenesis of Cenozoic, alkalic volcanic lineages at Mount Morning, West Antarctica and their entrained lithospheric mantle xenoliths: Lithospheric versus asthenospheric mantle sources

    NASA Astrophysics Data System (ADS)

    Martin, Adam P.; Cooper, Alan F.; Price, Richard C.

    2013-12-01

    Two volcanic lineages are identified at Mount Morning, a Cenozoic to recent, eruptive centre in the Ross Sea, West Antarctica, which is part of the McMurdo Volcanic Group. Both the older (at least 18.7-11.4 Ma), mildly alkalic, nepheline- or quartz-normative Mason Spur Lineage, and the younger (at least 6-0.02 Ma), nepheline normative, strongly alkalic Riviera Ridge Lineage evolved by fractional crystallization from nominally anhydrous (<0.5 wt% H2O) parental magmas. Both lineages are analogous to other, relatively anhydrous lineages in the McMurdo Volcanic Group and distinctly different from those in which kaersutite is present on the liquid line of descent. Sub-continental lithospheric mantle (SCLM) xenoliths entrained in Riviera Ridge Lineage rocks show trace element and isotopic Sr-Nd-Pb variation that is consistent with four-component mixing whereby depleted mantle has been refertilised by enriched, HIMU-like and Nb-enriched (carbonatite) components. Refertilization may have occurred c. 530-490 Ma ago when fluids derived from subduction associated with Gondwanaland amalgamation infiltrated the SCLM. Similar trace element and isotope variation (Sr-Nd-Pb) in Mount Morning basaltic rocks and entrained xenoliths suggests that the source for the basaltic magmas lies (at least in part) in the lithospheric mantle. It has long been recognized that Cenozoic volcanic rocks in Antarctica (Victoria Land - including Mount Morning - and Marie Byrd Land), Zealandia and eastern Australia share common chemical and isotopic source characteristics and they have been argued to collectively constitute a single diffuse alkaline magmatic province (DAMP). Source characteristic similarities suggest DAMP volcanic rocks inherit at least some of their trace element and isotopic characteristics from the lithospheric mantle. Super-chondritic Nb/Ta values measured in some SCLM xenoliths and volcanic rocks at Mount Morning, and in volcanic rocks across the DAMP, can be explained by addition

  12. 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

  13. 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].

  14. Comparing Mantle Xenoliths from Mount Taylor and Rio Puerco Necks, New Mexico: Evidence for Metasomatism

    NASA Astrophysics Data System (ADS)

    Thomas, A. E.; Schmidt, M. E.; Schrader, C. M.; Crumpler, L. S.

    2012-12-01

    The Mount Taylor Volcanic Field (MTVF) is located along the Jemez Lineament at the south eastern margin of the Colorado Plateau in north western New Mexico. To learn about its underlying lithospheric mantle, we conducted a survey of xenoliths from basaltic vents peripheral to the Mount Taylor edifice and the Rio Puerco Volcanic Necks. A total of 89 thin sections (32 from Mount Taylor and 57 from Rio Puerco) were examined. The population of thin sections from Mount Taylor and Rio Puerco listed respectively is: 18 and 20 lherzolites; 8 and 24 pyroxenites; 4 and 3 wehrlites; 1 and 6 dunites; and 1 and 4 harzburgites. Pyroxenite grain size ranged from 1 to 9 mm and lherzolite grains were typically 0.5 to 2 mm. Spinels ranged in colour from dark green, brown to black and they were generally <1 mm and interstitially concentrated. Spinel concentrations between the two suites were comparable, with an average of 2.6% for Mount Taylor and 2.0% for Rio Puerco. The largest concentration of spinels was in a pyroxenite at 12.5% from Mount Taylor. Up to 5% primary calcite is present in the Rio Puerco suite; in contrast calcite has not been identified in the Mount Taylor suite. Calcite grains were <0.5 mm in size and located at grain boundaries and as inclusions in clinopyroxene and orthopyroxene. Equilibrium textures include triple junction grain boundaries between olivines and clinopyroxenes in some lherzolites. Disequilibrium textures include rounded, optically continuous olivine and orthopyroxene in clinopyroxene, complex intergrowths between clinopyroxene and orthopyroxene, and sheared olivine. Electron microprobe analyses were performed on 9 representative thin sections with 5 pyroxenites, 4 lherzolites and 1 wehrlite; samples included green, brown, red and black spinels. Pyroxenes for the two suites decrease in Cr2O3 and increase in Al2O3 with decreasing Mg numbers, increase in CaO with decreasing MgO and increase in Na2O with increasing Al2O3. The presence of optically

  15. 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

  16. 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.

  17. Direct evidence for upper mantle structure in the NW Pacific Plate: Microstructural analysis of a petit-spot peridotite xenolith

    NASA Astrophysics Data System (ADS)

    Harigane, Yumiko; Mizukami, Tomoyuki; Morishita, Tomoaki; Michibayashi, Katsuyoshi; Abe, Natsue; Hirano, Naoto

    2011-02-01

    Petit-spots, the late Miocene alkali basaltic volcanoes on the Early Cretaceous NW Pacific Plate, originate at the base of the lithosphere. The petit-spot volcanic rocks enclose fragments of tholeiitic basalt, dolerite, gabbro, and mantle peridotite, providing a unique window into the entire section of subducting oceanic lithosphere. We provide here the first direct observations on the deep structure of the Pacific lithosphere using microstructural analyses of a petit-spot peridotite xenolith. The xenolith is a lherzolite that consists mainly of coarse- and medium-grained olivine, orthopyroxene, and clinopyroxene, as well as fine-grained aggregates of spinel and orthopyroxene that probably represent replaced pyrope-rich garnet. A strong deformational fabric is marked by a parallel alignment of millimeter-sized elongate minerals and their crystallographic preferred orientation. The olivine displays a [010] fiber pattern with a girdle of [100] axes and a maximum of [010] perpendicular to the foliation, a pattern which is consistent with a transpressional deformation in high temperature conditions at the base of oceanic lithosphere. Our microstructural observations and seismic data indicate that the lower part of the NW Pacific lithosphere possess an early stage structure of mantle flow at the asthenosphere. This interpretation is compatible with a conventional model in which oceanic lithosphere is thickened during cooling and plate convection. A discrepancy between the weak anisotropy in the petit-spot peridotite and the strong azimuthal anisotropy from the seismic data in the NW Pacific plate implies the existence of a highly anisotropic component in the deep oceanic lithosphere.

  18. 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).

  19. 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

  20. 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

  1. Methane-related diamond crystallization in the Earth's mantle: Stable isotope evidences from a single diamond-bearing xenolith

    NASA Astrophysics Data System (ADS)

    Thomassot, E.; Cartigny, P.; Harris, J. W.; (Fanus) Viljoen, K. S.

    2007-05-01

    Mineralogical studies of deep-seated xenoliths and mineral inclusions in diamonds indicate that there is significant variability in oxygen fugacity within the Earth's upper mantle. This variability is consistent both with the occurrence of reduced (methane-bearing) or oxidized (CO 2/carbonate-bearing) fluids. Invariably, direct sampling of reduced deep fluids is not possible as they are unquenchable and re-equilibrate with either the surrounding mantle or are affected by degassing. Key information about the nature of such fluids might be found in diamond if it were possible to study a population related to a single source. Usually, diamonds within a kimberlite pipe have different parageneses and can be shown to have formed at different times and depths. We studied 59 diamonds extracted from a single diamondiferous peridotite xenolith (with a volume of only 27 cm 3), from the Cullinan mine (formerly called the Premier mine) in South Africa. Diamond sizes range from 0.0005 to 0.169 carats (0.1 to 33.8 mg). A correlation between the nitrogen contents of the diamonds (range 40 to 1430 ppm) and their nitrogen aggregation state (varying from 10 to 85% of IaB defects) is compatible with a single growth event. δ 13C-values range from - 4.2‰ to - 0.1‰, with slight internal variability measured in the largest diamonds. Nitrogen isotope measurements show δ 15N ranging from - 1.2‰ to + 7.2‰. On the centimeter scale of this upper mantle rock, the variations for nitrogen content, nitrogen aggregation state, carbon and nitrogen isotopic compositions, respectively, cover 64%, 75%, 15% and 23% of the ranges known for peridotitic diamonds. In spite of such large ranges, N-content, δ 13C and δ 15N within this diamond population are distinctly coupled. These relationships do not support a mixing of carbon sources, but are best explained by a Rayleigh distillation within the sub-continental mantle at depths > 150 km and T > 1200 °C, which precipitates diamonds from methane

  2. 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

  3. 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.

  4. 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

  5. Mantle sources of quaternary volcanism on Zhokhov Island (De Long Islands, East Arctic): Isotope-geochemical features of the basalts and spinel lherzolite xenoliths

    NASA Astrophysics Data System (ADS)

    Sakhno, V. G.; Krymsky, R. Sh.; Belyatsky, B. V.; Shevchenko, S. S.; Sergeev, S. A.

    2015-02-01

    This paper reports the results of isotope-geochemical studies and distribution patterns for trace elements and rare earth elements in a collection of specimens of Cenozoic alkaline olivine basalts and spinel lherzolite xenoliths sampled at Zhokhov Island, De Long Archipelago (New Siberian Islands), East Arctic. In spite of various contributions of xenogenic minerals of mantle lherzolite xenoliths (olivine, pyroxene and spinel), the bulk-rock isotope composition of the studied specimens deviates within insignificant limits. This evidences the isotopic and geochemical homogeneity of the mantle source and the absence of a significant effect of processes of contamination by crustal host rocks, which frequently occur within the basalts as xenoliths. On the other hand this reflects the short lifetime of existence and evolution of the mantle melt source. The studied basalts by their isotope and geochemical characteristics resemble volcanics of oceanic islands, intraplate continental volcanics, and those of ocean rises, relate to the activity of mantle plumes. The lack of elevated U/Pb in the studied specimens permits us to correlate the occurrence of the Cenozoic volcanism at De Long Islands of East Arctic with intraplate continental plume volcanism of the rifting stage (analogous to plume magmatism of the East African Rift, Mesozoic alkaline ultramafic volcanism of Eastern India, and others).

  6. 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

  7. The evolution of spinel lherzolite xenoliths and the nature of the mantle at Kilbourne Hole, New Mexico

    NASA Astrophysics Data System (ADS)

    Perkins, Dexter; Anthony, Elizabeth Y.

    2011-12-01

    In peridotites, olivine, clinopyroxene, and orthopyroxene are complex solid solutions with wide stability fields. Depending mostly on bulk composition and pressure, these minerals may be accompanied by plagioclase (low pressure), spinel (moderate pressure), or garnet (high pressure), resulting in 4-phase and rarer 5-phase assemblages. Although a particular mineral assemblage is stable over a range of P-T, the compositions of the individual minerals vary with changing P-T conditions. Application of standard geothermobarometers to olivine-clinopyroxene-orthopyroxene-spinel peridotites is problematic. An alternative approach is to use a bulk rock composition to calculate equilibrium phase diagrams to determine the conditions under which a particular assemblage is stable. This requires consideration of the 7-component system SiO2-Al2O3-Cr2O3-FeO-MgO-CaO-Na2O, internally consistent thermodynamic data for end members, and reliable mixing models for all mineral solutions. Experimental studies in simpler systems, and solution models from the literature, permit derivation of multicomponent thermodynamic mixing models for the key minerals. The models, when applied to xenoliths from Kilbourne Hole, constrain P and T of equilibration and are less sensitive to mineral compositional variations, or uncertainty in activity models, than standard thermobarometry. Our modeling provides the first tightly constrained pressure estimates for Kilbourne Hole, placing the xenoliths in the spinel stability field at depths (30-45 km) that correspond to the uppermost mantle beneath the Rio Grande Rift. The fine-grained equigranular lherzolite, porphyroclastic lherzolite, and some harzburgite-dunite specimens equilibrated at average conditions of 11.5 Kbar-930°C, 12 Kbar-990°C, and 13 Kbar-1,080°C, respectively. The mantle beneath the Rio Grande Rift is layered; the fine-grained equigranular lherzolite derives from relatively shallow depth (35 km average), and the porphyroclastic lherzolite

  8. 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

  9. 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

  10. 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.

  11. 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.

  12. 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

  13. Water contents in mantle xenoliths from the Colorado Plateau and vicinity: Implications for the mantle rheology and hydration-induced thinning of continental lithosphere

    NASA Astrophysics Data System (ADS)

    Li, Zheng-Xue Anser; Lee, Cin-Ty A.; Peslier, Anne H.; Lenardic, Adrian; Mackwell, Stephen J.

    2008-09-01

    Nominally anhydrous minerals (e.g., olivine, clinopyroxene, and orthopyroxene) in peridotite xenoliths collected from the Colorado Plateau and southern Basin and Range in western North America were systematically analyzed by Fourier transform infrared spectroscopy for water contents. Measured water contents range from 2 to 45 ppm for olivine, from 53 to 402 ppm for orthopyroxene, and from 171 to 957 ppm for clinopyroxene. The Colorado Plateau has the highest water contents (up to 45 ppm H2O in olivine, 402 ppm H2O in orthopyroxene, and 957 ppm H2O in clinopyroxene), while San Carlos in the southern Basin and Range has the lowest water contents (up to 4 ppm H2O in olivine, 82 ppm H2O in orthopyroxene, and 178 ppm H2O in clinopyroxene). With the exception of San Carlos, the olivine and pyroxenes from all other localities (Dish Hill, Grand Canyon, and Navajo) have water contents close to or higher than that inferred for the fertile asthenospheric mantle. We interpret the high water contents measured here to have been introduced into the base of the lithospheric mantle by rehydration associated with the subduction of the Farallon plate beneath North America during the early Cenozoic. Application of an updated flow law for dislocation creep of wet olivine to lithospheric mantle conditions beneath the Colorado Plateau predicts that for a given background shear stress, hydration alone can result in approximately 1 order of magnitude drop in the effective viscosity at the base of the lithosphere. If viscosity alone is used to distinguish the lithosphere from underlying asthenosphere, this suggests that hydration could have resulted in more than 10 km of lithospheric thinning. Viscosity reduction and lithospheric thinning of even larger extents (up to ˜100 km) are predicted when thicker lithosphere (such as Archean cratons) and larger water contents (up to water-saturated conditions) are considered. If our interpretations are correct, the implications of our study go

  14. K-rich glass-bearing wehrlite xenoliths from Yitong, Northeastern China: petrological and chemical evidence for mantle metasomatism

    NASA Astrophysics Data System (ADS)

    Xu, Y.; Mercier, J.-C. C.; Lin, Chuanyong; Shi, Lanbin; Menzies, M. A.; Ross, J. V.; Harte, B.

    1996-11-01

    possibility of percolation of host-basalt related melts for the origin of these glasses. Thus the glasses must have resulted from local penetration of mantle metasomatic melts which may have been produced by partial melting of peridotites with involvement of deep-seated fluids. Such melts may have been significantly modified by subsequent fractional crystallization of ol, cpx and sp, extensive reaction with the mantle conduit and the xenolith transport process.

  15. 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

  16. 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.

  17. Probing the structure of the sub-Salinia mantle lithosphere using spinel lherzolite xenoliths from Crystal Knob, Santa Lucia Range, California

    NASA Astrophysics Data System (ADS)

    Quinn, D. P.; Saleeby, J.; Ducea, M. N.; Luffi, P. I.

    2013-12-01

    We present the first petrogenetic analysis of a suite of peridotite xenoliths from the Crystal Knob volcanic neck in the Santa Lucia Range, California. The neck was erupted during the Plio-Pleistocene through the Salinia terrane, a fragment of the Late Cretaceous southern Sierra-northwest Mojave supra-subduction core complex that was displaced ~310 km in the late Cenozoic along the dextral San Andreas fault. The marginal tectonic setting makes these xenoliths ideal for testing different models of upper-mantle evolution along the western North American plate boundary. Possible scenarios include the early Cenozoic underplating of Farallon-plate mantle lithosphere nappes (Luffi et al., 2009), Neogene slab window opening (Atwater and Stock, 1998), and the partial subduction and stalling of the Monterey microplate (Pisker et al., 2012). The xenoliths from Crystal Knob are spinel lherzolites, which sample the mantle lithosphere underlying Salinia, and dunite cumulates apparently related to the olivine-basalt host. Initial study is focused on the spinel lherzolites: these display an allotriomorphic granular texture with anisotropy largely absent. However, several samples exhibit a weak shape-preferred orientation in elongate spinels. Within each xenolith, the silicate phases are in Fe-Mg equilibrium; between samples, Mg# [molar Mg/(Mg+Fe)*100] ranges from 87 to 91. Spinels have Cr# [molar Cr/(Cr+Al)*100] ranging from 10 to 27. Clinopyroxene Rb-Sr and Sm-Nd radiogenic isotope data show that the lherzolites are depleted in large-ion lithophile (LIL) elements, with uniform enrichment in 143Nd (ɛNd from +10.3 to +11.0) and depletion in 87Sr (87/86Sr of .702). This data rules out origin in the continental lithosphere, such as that observed in xenoliths from above the relict subduction interface found at at Dish Hill and Cima Dome in the Mojave (Luffi et al., 2009). The Mesozoic mantle wedge, which is sampled by xenoliths from beneath the southern Sierra Nevada batholith

  18. 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.

  19. Mantle peridotite xenoliths in andesite lava at El Peñon, central Mexican Volcanic Belt: Isotopic and trace element evidence for melting and metasomatism in the mantle wedge beneath an active arc

    NASA Astrophysics Data System (ADS)

    Mukasa, Samuel B.; Blatter, Dawnika L.; Andronikov, Alexandre V.

    2007-08-01

    Peridotites in the mantle wedge and components added to them from the subducting slab are thought to be the source of most arc magmas. However, direct sampling of these materials, which provides a glimpse into the upper mantle beneath an active margin, is exceedingly rare. In the few arc localities where found, peridotite xenoliths are usually brought to the surface by basaltic magmas. Remarkably, the hornblende-bearing ultramafic xenoliths and clinopyroxene megaxenocrysts from El Peñon in the central Mexican Volcanic Belt were brought to the surface by a Quaternary high-Mg siliceous andesite, a rock type usually considered too evolved to be a direct product of mantle melting. The xenoliths and megaxenocrysts from El Peñon represent lithospheric mantle affected by significant subduction of oceanic lithosphere since as early as the Permian. Trace element and radiogenic isotope data we report here on these materials suggest a history of depletion by melt extraction, metasomatism involving a fluid phase, and finally, limited reaction between the ultramafic materials and the host andesite, probably during transport. They also show that high-Mg siliceous andesite can be a direct product of 1-5% melting of H 2O-bearing spinel lherzolite.

  20. Heterogeneous distribution of phosphorus in olivine from otherwise well-equilibrated spinel peridotite xenoliths and its implications for the mantle geochemistry of lithium

    NASA Astrophysics Data System (ADS)

    Mallmann, Guilherme; O'Neill, Hugh St. C.; Klemme, Stephan

    2009-10-01

    The major- and trace-element abundances of the coexisting phases of four metasomatized spinel peridotite xenoliths from the Anakies locality (SE Australia) were determined by electron microprobe and laser-ablation ICP-MS. The compositions of all phases are remarkably homogeneous, with the exception of phosphorus (P), lithium (Li) and sodium (Na) in olivine. These three elements are enriched in large parts of most olivine crystals due to a second metasomatic episode. Apart from these elements, all phases are in mutual equilibrium with respect to both their major- and trace-element compositions. Li and Na show a strong correlation with P in olivine, although molar Li + Na are an order of magnitude less than molar P, indicating that the substitution mechanism of these elements is more complex than the simple charge-balanced coupled exchange IVSi4+ + VI(FeMg)2+ = IVP5+ + VI(LiNa)+. We suggest that Li and Na are decorating octahedral-site cation vacancies formed by the original incorporation of P. Elemental maps revealed that the P zoning patterns are concentric in a few large olivine porphyroblasts, but form irregular patches in most crystals. This distribution of P is proposed to be the result of a two-stage process, whereby the initial concentric zoning, caused by its exceptionally sluggish diffusion after metasomatic influx, is broken up by extensive sub-solidus deformation and recrystallization, attesting to large grain-scale strains even within the lithosphere. Such strains must be an efficient means of ensuring trace-element equilibrium during partial melting. The association of Li with P in olivine may help to explain the variability of Li abundances in mantle minerals and to interpret Li diffusion experiments and Li isotopic fractionation.

  1. Short-scale variability of the SCLM beneath the extra-Andean back-arc (Paso de Indios, Argentina): Evidence from spinel-facies mantle xenoliths

    NASA Astrophysics Data System (ADS)

    Ponce, Alexis D.; Bertotto, Gustavo W.; Zanetti, Alberto; Brunelli, Daniele; Giovanardi, Tommaso; Aragón, Eugenio; Bernardi, Mauro I.; Hémond, Christophe; Mazzucchelli, Maurizio

    2015-10-01

    Cenozoic basalts carrying ultramafic mantle xenoliths occur in the Matilde, León and Chenque hills in the Paso de Indios region, Argentina. The mantle xenoliths from the Chenque and León hills mainly present porphyroclastic textures, whereas the Matilde hill xenoliths have coarse-grained to porphyroclastic textures. The equilibrium temperatures are in the range of 780 to 940ºC, indicating a provenance from shallow sectors of the lithospheric mantle column that were subjected to a relatively low heat ffiux at Cenozoic Era. According to the modal compositions of xenoliths, the mantle beneath Matilde and León hills was affected by greater than 22% partial melting, while less depleted peridotites occur in the Chenque suite (starting from 10% partial melting). Such an observation is confirmed by the partial melting estimates based on Cr#Sp, which vary from 8 to 14% for the selected Chenque samples and from 14 to 18% for the Matilde ones. The common melting trend is overlapped by small-scale cross cutting local trends that may have been generated by open-system processes, such as open-system partial melting and/or post partial-melting metasomatic migration of exotic Na-Cr-rich melts. The two main mineralogical reaction schemes are: i) the dissolution of pyroxenes and the segregation of new olivine in olivine-rich peridotites, and ii) the replacement of primary olivine by orthopyroxene±clinopyroxene in orthopyroxene-rich peridotites. These were produced by channelled and/or pervasive melt extraction/ migration. Enhanced pyroxene dissolution is attributed to channelling of silica- undersaturated melts, whereas the replacement of primary olivine by orthopyroxene±clinopyroxene points to reaction with silica-saturated melts. Late disequilibrium reactions identified in the xenoliths comprise: the breakdown of orthopyroxene in contact with the host basalt, and (rarely) reaction coronae on orthopyroxene, clinopyroxene and spinel linked to glassy veins. Such features are

  2. 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

  3. 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.

  4. Metasomatised lower crustal and upper mantle xenoliths from north Queensland: Chemical and isotopic evidence bearing on the composition and source of the fluid phase

    NASA Astrophysics Data System (ADS)

    Stolz, A. J.; Davies, G. R.

    1989-03-01

    A diverse range of ultramafic and mafic granulite xenoliths from cone #32, north Queensland display mineralogical and chemical evidence for enhanced fluid activities in the lowermost crust and uppermost mantle. Metasomatism of spinel lherzolite, garnet and spinel websterite and garnet granulite produced variable amounts of amphibole ± mica, locally resulting in development of hornblendite as the end-product of reaction between spinel websterite and C-O-H fluids. In contrast, associated felsic and 2-pyroxene granulite xenoliths have remained relatively unaffected. The garnet granulites and spinel websterites are interpreted as products of underplating of the lower crust by Cenozoic magmatic activity, whereas Nd-Sr isotope systematics for the 2-pyroxene and felsic granulite xenoliths suggest that they may be of Proterozoic age. The development of amphibole and mica in the mafic and ultramafic xenoliths was accompanied by increases in TiO 2, Na 2O, K 2O, A1 2O 2, Sr, Ba, Zr, Nb, LREE and Fe 2O 3/FeO, and decreased MgO, SiO 2, Cr and Ni. Virtually anhydrous spinel websterite xenoliths have 87Sr /86Sr and 143Nd/144Nd almost identical to amphibole-rich variants and hornblendite, and very similar values to the host nepheline mugearite and an amphibole megacryst. The chemical and isotopic data suggest that the metasomatism is a relatively young feature caused by oxidized fluids exsolved from alkaline magmas similar to the host as they ascended through the lithosphere. This style of metasomatism is considered a relatively localized phenomenon which is unlikely to provide an important contribution to the source regions of alkali basaltic magmas.

  5. Carbonatite melt-CO 2 fluid inclusions in mantle xenoliths from Tenerife, Canary Islands: a story of trapping, immiscibility and fluid-rock interaction in the upper mantle

    NASA Astrophysics Data System (ADS)

    Frezzotti, Maria Luce; Andersen, Tom; Neumann, Else-Ragnhild; Simonsen, Siri Lene

    2002-10-01

    Three types of fluid inclusions have been identified in olivine porphyroclasts in the spinel harzburgite and lherzolite xenoliths from Tenerife: pure CO 2 (Type A); carbonate-rich CO 2-SO 2 mixtures (Type B); and polyphase inclusions dominated by silicate glass±fluid±sp±silicate±sulfide±carbonate (Type C). Type A inclusions commonly exhibit a "coating" (a few microns thick) consisting of an aggregate of a platy, hydrous Mg-Fe-Si phase, most likely talc, together with very small amounts of halite, dolomite and other phases. Larger crystals (e.g. (Na,K)Cl, dolomite, spinel, sulfide and phlogopite) may be found on either side of the "coating", towards the wall of the host mineral or towards the inclusion center. These different fluids were formed through the immiscible separations and fluid-wall-rock reactions from a common, volatile-rich, siliceous, alkaline carbonatite melt infiltrating the upper mantle beneath the Tenerife. First, the original siliceous carbonatite melt is separated from a mixed CO 2-H 2O-NaCl fluid and a silicate/silicocarbonatite melt (preserved in Type A inclusions). The reaction of the carbonaceous silicate melt with the wall-rock minerals gave rise to large poikilitic orthopyroxene and clinopyroxene grains, and smaller neoblasts. During the metasomatic processes, the consumption of the silicate part of the melt produced carbonate-enriched Type B CO 2-SO 2 fluids which were trapped in exsolved orthopyroxene porphyroclasts. At the later stages, the interstitial silicate/silicocarbonatite fluids were trapped as Type C inclusions. At a temperature above 650 °C, the mixed CO 2-H 2O-NaCl fluid inside the Type A inclusions were separated into CO 2-rich fluid and H 2O-NaCl brine. At T<650 °C, the residual silicate melt reacted with the host olivine, forming a reaction rim or "coating" along the inclusion walls consisting of talc (or possibly serpentine) together with minute crystals of NaCl, KCl, carbonates and sulfides, leaving a residual CO 2

  6. Fabric characterization associated with asthenospheric upwelling in the uppermost mantle, back-arc region of the southwest Japan arc: Evidence from peridotite xenoliths, Oki-Dogo Island

    NASA Astrophysics Data System (ADS)

    Satsukawa, Takako; Michibayashi, Katsuyoshi

    2010-05-01

    Oki-Dogo Island is an important site in terms of xenoliths because it makes the most continent ward occurrence of mantle peridotite xenoliths in the back-arc region of the southwest Japan arc. We describe the microstructure of peridotite xenoliths obtained from Oki-Dogo Island with aim of understanding the evolution of the uppermost mantle beneath the back-arc side of the Japan arc. Basement on the island consists of gneissic metamorphic complex. The alkaline basalts that contain the xenoliths were erupted during the Pliocene-Pleistocene after the opening of the Japan Sea which is thought to have occurred during the Oligocene-Miocene as a consequence of back-arc spreading. Peridotite xenoliths found on Oki-Dogo Island are up to 10 cm in size, show granular texture, and are mainly spinel lherzolites, with some harzburgites. Large xenoliths (> 3 cm) contain a foliation defined by compositional banding and aligned spinel grains, and a lineation defined by the long axes of spinel grains. All spinel lherzolites contain spinel with a low Cr# (=Cr/Fr+Al) (< 0.45); this feature, combined with their mineral assemblages and high NiO content in olivine, suggests that they are of residual origin. The Mg# (=Mg/Mg+Fe) of silicate minerals in some spinel lherzolites is lower (e.g. down to Fo86) than that in typical residual peridotites of the upper mantle, indicating that the observed Fe enrichment occurred in mantle rocks during metasomatism. We have measured crystallographic preferred orientations (CPOs) of olivine grains from highly polished thin sections using a scanning electron microscope equipped with an electron backscatter diffraction system. The dominant slip system in olivine, as determined from kink bands and CPO data, was {0kl}[100] slip. Moreover, peridotites with low olivine Mg# tend to show a AG-type ([010]-fiber) CPO pattern. This finding suggests that the peridotites were deformed in the presence of melt and represent various degrees of rock-melt interaction as

  7. 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

  8. 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

  9. 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.

  10. Geochemical composition of subcontinental lithospheric mantle in the westernmost Mediterranean: constrains from peridotite xenoliths in Plio-Pleistocene alkali basalts (eastern Betic Cordillera, SE Spain)

    NASA Astrophysics Data System (ADS)

    Marchesi, Claudio; Konc, Zoltán; Bosch, Delphine; Garrido, Carlos J.; Hidas, Károly; Varas-Reus, María Isabel

    2016-04-01

    Peridotite xenoliths in Plio-Pleistocene alkali basalts from the eastern Betic Cordillera (Murcia, SE Spain) provide key information on Alpine tectono-magmatic processes that affected the subcontinental lithospheric mantle beneath the westernmost Mediterranean. Here we present a detailed geochemical study comprising whole-rock and mineral major- and trace-element, as well as Sr-Nd-Pb isotopic compositional data of spinel ± plagioclase lherzolite, spinel ± plagioclase harzburgite and spinel wehrlite xenoliths from Tallante and Los Perez volcanic centers. The whole-rock major element compositions and mineral chemistry of the studied xenoliths reflect increasing fertility from clinopyroxene-poor peridotites (Group I; Mg# up to 91.5), to common lherzolites (Group II; Mg# up to 90.6), fertile lherzolites (Group III; Mg# = 86.8-88.9) and wehrlites (Mg# = 86.7-87.4). The mineral major element chemistry records the geochemical imprint of maximum 10-12 % partial melting in the most depleted Group I peridotites. However, trace element and isotopic data attest for various degrees of metasomatic enrichment that overprinted the previously depleted lithospheric mantle. Interaction with melts produced enrichment of LREE in Group II and Group III peridotites, as well as in wehrlites. In contrast to major and trace elements, Sr-Nd-Pb radiogenic isotope systematic is unrelated to compositional groups and shows isotopic variations between DMM and EM2 end-members and contribution of an Atlantic sediment-like component. Different whole-rock trace element compositions coupled to similar isotopic signatures indicate that metasomatism was caused by external melt(s) issued from a common source not before the Tertiary. These geochemical evidences attest for the percolation of slab-derived, SiO2-undersaturated melts (and hydrous fluids) with carbonate sediment affinity in the pre-Miocene supra-subduction continental lithospheric mantle beneath the Alboran Basin.

  11. 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

  12. 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

  13. 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

  14. 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

  15. 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

  16. Nd, Sr and Os isotope systematics in young, fertile spinel peridotite xenoliths from northern Queensland, Australia: A unique view of depleted MORB mantle?

    NASA Astrophysics Data System (ADS)

    Handler, M. R.; Bennett, V. C.; Carlson, R. W.

    2005-12-01

    Northeastern Queensland, a part of the Phanerozoic composite Tasman Fold Belt of eastern Australia, has a Paleozoic to Mesozoic history dominated by subduction zone processes. A suite of 13 peridotite xenoliths from the <3 Ma Atherton Tablelands Volcanic Province, predominantly from Mount Quincan, comprise fertile (1.8-3.4 wt.% Al 2O 3 and 38.7-41.9 wt.% MgO) spinel lherzolites free from secondary volatile-bearing phases and with only weak metasomatic enrichment of incompatible trace elements (Sm N/Yb N = 0.23-1.1; La N/Yb N = 0.11-4.9). The suite is isotopically heterogeneous, with measured Sr ( 87Sr/ 86Sr = 0.7027-07047), Nd ( 143Nd/ 144Nd = 0.51249-0.51362), and to a lesser extent, Os ( 187Os/ 188Os = 0.1228-0.1292) compositions broadly overlapping MORB source mantle (DMM) and extending to more depleted compositions, reflecting evolution in a time-integrated depleted reservoir. Major and rare earth element systematics are consistent with mantle that is residual after low to moderate degrees of melt extraction predominantly in the spinel facies, but with a few samples requiring partial melting at greater pressures in the garnet field or near the garnet-spinel transition. In contrast to most previously studied suites of continental lithospheric mantle samples, the incompatible trace element contents and Sr and Nd isotopic systematics of these samples suggest only minimal modification of the sampled lithosphere by metasomatic processes. Five of six Mount Quincan xenoliths preserving depleted middle to heavy REE patterns form a whole rock Sm-Nd isochron with an age of ˜275 Ma (ɛ Ndi = +9), coincident with widespread granitoid emplacement in the overlying region. This isochron is interpreted to indicate the timing of partial melting of a DMM-like source. Xenoliths from other Atherton localities scatter about the isochron, suggesting that the sampled mantle represents addition of DMM mantle to the lithosphere in the Permian, when the region may have broadly been

  17. 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

  18. 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

  19. Thermal state, oxygen fugacity and COH fluid speciation in cratonic lithospheric mantle: New data on peridotite xenoliths from the Udachnaya kimberlite, Siberia

    NASA Astrophysics Data System (ADS)

    Goncharov, A. G.; Ionov, D. A.; Doucet, L. S.; Pokhilenko, L. N.

    2012-12-01

    Oxygen fugacity (fO2) and temperature variations in a complete lithospheric mantle section (70-220 km) of the central Siberian craton are estimated based on 42 peridotite xenoliths in the Udachnaya kimberlite. Pressure and temperature (P-T) estimates for the 70-140 km depth range closely follow the 40 mW/m2 model conductive geotherm but show a bimodal distribution at greater depths. A subset of coarse garnet peridotites at 145-180 km plots near the "cold" 35 mW/m2 geotherm whereas the majority of coarse and sheared rocks at ≥145 km scatter between the 40 and 45 mW/m2 geotherms. This P-T profile may reflect a perturbation of an initially "cold" lithospheric mantle through a combination of (1) magmatic under-plating close to the crust-mantle boundary and (2) intrusion of melts/fluids in the lower lithosphere accompanied by shearing. fO2 values estimated from Fe3+/∑Fe in spinel and/or garnet obtained by Mössbauer spectroscopy decrease from +1 to -4 Δlog fO2 (FMQ) from the top to the bottom of the lithospheric mantle (˜0.25 log units per 10 km) due to pressure effects on Fe2+-Fe3+ equilibria in garnet. Garnet peridotites from Udachnaya appear to be more oxidized than those from the Kaapvaal craton but show fO2 distribution with depth similar to those in the Slave craton. Published fO2 estimates for Udachnaya xenoliths based on C-O-H fluid speciation in inclusions in minerals from gas chromatography are similar to our results at ≤120 km, but are 1-2 orders of magnitude higher for the deeper mantle, possibly due to uncertainties of fO2 estimates based on experimental calibrations at ≤3.5 GPa. Sheared peridotites containing garnets with u-shaped, sinusoidal and humped REE patterns are usually more oxidized than Yb, Lu-rich, melt-equilibrated garnets, which show a continuous decrease from heavy to light REE. This further indicates that mantle redox state may be related to sources and modes of metasomatism.

  20. 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

  1. 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

  2. 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

  3. Water contents and electrical conductivity of peridotite xenoliths from the North China Craton: Implications for water distribution in the upper mantle

    NASA Astrophysics Data System (ADS)

    Wang, Qin; Bagdassarov, Nikolai; Xia, Qun-Ke; Zhu, Beibei

    2014-02-01

    The eastern North China Craton (NCC) experienced significant lithospheric thinning and widespread magmatism in the Mesozoic, and is characterized by high surface heat flow and high conductivity layers (HCL) in the upper mantle. An integrated study of petrology and petrophysics will improve our understanding of the relationships between the electrical structure, thermal structure and chemical compositions (iron content and water content) of the upper mantle. Nine spinel peridotite xenoliths were collected from four Cenozoic basalts (Yangyuan, Hannuoba, Hebi and Nushan) in the Eastern Block and the Taihang Mountains of the NCC. These samples show compositional variations from depleted harzburgites to fertile lherzolites, representing the relic Archean lithosphere, modified Proterozoic lithosphere and newly accreted lithosphere after the lithospheric thinning event. The water contents of the peridotite samples were analyzed using Fourier transform infrared spectrometry. The water contents in olivine are very low (2-13 ppm H2O), hence the whole-rock water concentration is controlled by orthopyroxene (Opx) and clinopyroxene (Cpx). Different hydration states of peridotites are distinguished according to the water contents in Opx: saturated (350 ppm), water-rich (> 120 ppm), water-poor (40-90 ppm) and dry (~ 1 ppm). Using a piston cylinder press and Solartron 1260 phase-gain analyzer, the electrical conductivity of sintered peridotites was measured at pressures of 1-2 GPa and temperatures of 350-1150 °C. The electrical conductivity (σ) follows an Arrhenius equation: σ = σ0 · exp(- ΔH / kT), where T is in Kelvin and k is the Boltzmann constant. The pre-exponential factor (σ0) and activation enthalpy of electric conductivity (ΔH) of spinel peridotites vary in the range of 100.65-102.38 S/m and 1.03-1.45 eV, respectively. Based on electrical conductivity of mantle minerals, we proposed a new equation to model the effect of iron content and water content on the

  4. Megacrysts and xenoliths in kimberlite, Elliott County, Kentucky: A mantle sample from beneath the Permian Appalachian Plateau

    NASA Astrophysics Data System (ADS)

    Garrison, James R.; Taylor, Lawrence A.

    1980-12-01

    Two kimberlite pipes in Elliott County contain rare ultramafic xenoliths and abundant megacrysts of olivine (Fo85 93), garnet (0.21 9.07% Cr2O3), picroilmenite, phlogopite, Cr-poor clinopyroxene (0.56 0.88% Cr2O3), and Cr-poor orthopyroxene (<0.03 0.34% Cr2O3) in a matrix of olivine (Fo88 92), picroilmenite, Cr-spinel, magnetite, perovskite, pyrrhotite, calcite, and hydrous silicates. Rare clinopyroxene-ilmenite intergrowths also occur. Garnets show correlation of mg (0.79 0.86) and CaO (4.54 7.10%) with Cr2O3 content; the more Mg-rich garnets have more uvarovite in solution. Clinopyroxene megacrysts show a general decrease in Cr2O3 and increase in TiO2 (0.38 0.56%) with decreasing mg (0.87 0.91). Clinopyroxene megacrysts are more Cr-rich than clinopyroxene in clinopyroxene-ilmenite intergrowths (0.06 0.38% Cr2O3) and less Cr-rich than peridotite clinopyroxenes (1.39 1.46% Cr2O3). Orthopyroxene megacrysts and orthopyroxene inclusions in olivine megacrysts form two populations: high-Ca, high-Al (1.09 1.16% CaO and 1.16 1.18% Al2O3) and low-Ca, low-Al (0.35 0.46% CaO and 0.67 0.74% Al2O3). Three orthopyroxenes belonging to a low-Ca subgroup of the high-Ca, high-Al group were also identified (0.86 0.98% CaO and 0.95 1.01% Al2O3). The high-Ca, high-Al group (Group I) has lower mg (0.88 0.90) than low-Ca, low-Al group (Group II) with mg=0.92 0.93; low mg orthopyroxenes (Group Ia) have lower Cr2O3 and higher TiO2 than high mg orthopyroxenes (Group II). The orthopyroxene megacrysts have lower Cr2O3 than peridotite orthopyroxenes (0.46 0.57% Cr2O3). Diopside solvus temperatures indicate equilibration of clinopyroxene megacrysts at 1,165° 1,390° C and 1,295° 1,335° C for clinopyroxene in clinopyroxene-ilmenite intergrowths. P-T estimates for orthopyroxene megacrysts are bimodal: high-Ca, high-Al (Group I) orthopyroxenes equilibrated at 1,165° 1,255° C and 51 53 kb (± 5kb) and the low-Ca, low-Al (Group II) orthopyroxenes equilibrated at 970° 1,020°C and 46 56 kb (

  5. 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

  6. 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

  7. Evolution of Mojavian mantle lithosphere influenced by Farallon plate subduction: Evidence from Hf and Nd isotopes in peridotite xenoliths from Dish Hill, CA

    NASA Astrophysics Data System (ADS)

    Armytage, Rosalind M. G.; Brandon, Alan D.; Andreasen, Rasmus; Lapen, Thomas J.

    2015-06-01

    A major issue in the assembly of continents is the role of subduction in building and reworking the continental mantle lithosphere. Spinel lherzolite xenoliths from Dish Hill, CA represent Mojavian sub-continental lithospheric mantle (SCLM) that existed along an off-craton continental edge during late Cretaceous Farallon plate subduction. The Dish Hill locale is well situated for recording any Farallon plate influence, be it as oceanic lithosphere accretion or for its role in providing metasomatic agents to the Mojavian SCLM. The 176Hf/177Hf and 143Nd/144Nd isotopic compositions of clinopyroxenes from these xenoliths are radiogenic with εHf from +12.9 to +134.4 and εNd from +2.2 to +26.1, indicative of ancient Proterozoic melt depletion. Four out of the sixteen samples lie on a 2.1 Ga reference line for melt extraction from primitive mantle for both 176Hf/177Hf and 143Nd/144Nd, confirming their position on the 2.1 Ga 187Os/188Os aluminachron from previous work on these peridotites (Armytage et al., 2014). A second Re-depletion age obtained from an 187Os/188Os aluminachron of 1.3 Ga is also observed in the 176Lu-176Hf and 147Sm-143Nd systematics. The 176Hf/177Hf-143Nd/144Nd data from Dish Hill do not provide strong evidence for the existence of a duplex of oceanic lithosphere and SCLM, or for these peridotites being sourced from modern asthenospheric mantle upwelling after lithospheric removal. However, subchondritic 176Lu/177Hf and 147Sm/144Nd ratios and trace element compositions in some of the peridotites point to the influence of metasomatic processes. In seven of the peridotites 176Hf/177Hf ratios are not complemented by similarly radiogenic 143Nd/144Nd ratios. Such decoupling, relative to the mantle array, indicates that the 176Hf/177Hf record in these peridotites is more robust to resetting by these local metasomatic processes than 143Nd/144Nd. The 87Sr/86Sr ratios measured in these samples fall into two distinct groups based on (Ce/Yb)PM, with the less

  8. 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.

  9. 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.

  10. Murchison xenoliths

    NASA Technical Reports Server (NTRS)

    Olsen, Edward J.; Davis, Andrew M.; Clayton, Robert N.; Mayeda, Toshiko K.; Grossman, Lawrence

    1988-01-01

    C3 xenoliths in a C2 host (Murchison) are unique among known meteoritic xenolith-host occurrences. They offer an opportunity to determine possible effects on the xenoliths by the hydrated host. Eleven xenoliths were found ranging from 2 to 13 mm. Four of these Murchison Xenoliths (MX1, MX2, MX3 and MX4) have been studied in detail. MX1 and MX2 were large enough for trace element, oxygen isotope, carbon isotope, bulk carbon and bulk nitrogen determinations. All four were studied petrographically and by analytical SEM. The xenoliths cannot be unequivocally identified as C3V or C3O subtypes. MX1 contains some matrix phyllosilicate, indicating reaction with water. MX1, MX2 and MX3 all show extensive alteration by an FeO-rich medium, and some minerals in them contain ferric iron. MX4, however, exhibits very minor alteration by FeO only. Oxygen isotopic and chemical data show that the alteration of these xenoliths did not take place in the Murchison host. The alterations occurred in one or more parent bodies, which were later disrupted to release these xenoliths that ultimately accreted onto the Murchison parent body.

  11. Stable isotope (O, H, S) relationships in Tertiary basalts and their mantle xenoliths from the Northern Hessian Depression, W.-Germany

    NASA Astrophysics Data System (ADS)

    Harmon, R. S.; Hoefs, J.; Wedepohl, K. H.

    1987-03-01

    18O/16O, 34S/32S, and D/H ratios as well as vacuum-fusion H2O+ contents were measured for late Tertiary volcanic basaltic rocks ranging in composition from quartz tholeiites and alkali olivine basalts to melilite-bearing olivine nephelinites and for peridotite xenoliths from the Northern Hessian Depression of W.-Germany. Measured Oisotope ratios in both basalts and peridotites were corrected for variable degree of post-eruption, secondary alteration. The ranges and means of corrected δ 18O values (‰ SMOW) for the North Hessian lavas and peridotites are: (i) 8 tholeiites: ca. +6.1 to +7.3 ( ¯x=+6.6), (ii) 21 alkali olivine basalts: ca. +5.4 to +7.6 ( ¯x=+6.5), (iii) 19 nepheline basanites, limburgites, and olivine nephelinites: ca. +5.3 to +8.0 ( ¯x=+6.6), and (iv) 23 peridotites: +5.1 to 7.0 ( ¯x+6.0). The δ 34S values (‰ CDT) for the tholeiites range from -0.6 to +1.4 ( ¯x=-0.03) and for the alkali basalts range from +0.9 to +8.6 ( ¯x=+2.5). The approximate δD value (‰ SMOW) of the pristine basalts and peridotites is estimated to have been ca. -90‰ The quartz tholeiites appear to have had a different genetic history than the alkali basalts. Supported by chemical evidence, the 18O and 87Sr enrichment observed in the tholeiites suggests low crustal contamination of parental olivine tholeiite melts, derived from a depleted mantle source. The contamination by crustal partial melts may have occurred in granulitic lower crust during differentiation. By contrast the high δ 18O and δ 34S values observed for the alkali basalts and peridotites are best explained in terms of metasomatic alteration of the mantle source region by fluids enriched in 18O, K, and incompatible trace elements prior to partial melting. The δ 18O-K relationships for the peridotites indicate that the mantle beneath the Northern Hessian Depression has had a complex stable isotope history involving at least two distinct metasomatic events. The earlier event involved a CO2-rich fluid

  12. 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, C.; Liu, Y.; Min, N.; Zong, K.; Hu, Z.; Gao, S.

    2015-12-01

    In-situ major and trace elements and Sr isotopic compositions of peridotite xenoliths carried by the Datong Quaternary alkaline basalt were analyzed. These peridotite xenoliths were classified into three groups. The type 1 peridotites preserve depleted trace element and Sr isotopic signatures and record the lowest temperature (930 - 980 °C). Clinopyroxenes in these peridotites exhibit LREE-depleted REE patterns, and have the lowest 87Sr/86Sr ratios of 0.70243 - 0.70411. The types 2 and 3 peridotites are featured by enriched trace element and Sr isotopic signatures and record a higher temperature (1003 - 1032 °C). Clinopyroxenes in the type 2 peridotite have U-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 enrichments in LILE and depletions in HFSE, and have the highest 87Sr/86Sr ratios of 0.71147 - 0.71285. The types 2 and 3 peridotites suffered latter cryptic carbonatitic metasomatism, as indicated by the decreased Ti/Eu and increased Zr/Hf and CaO/Al2O3 ratios of clinopyroxenes. The carbonatitic veinlets have generally consistent trace element patterns and Sr isotopic ratios with the calculated melts being equilibrated with the clinopyroxenes in the type 3 peridotite, and may represent the metasoamtic agent solidified in the relatively cold and shallow mantle. The negative Eu anomalies (0.37 - 0.61) and high 87Sr/86Sr ratios of the calculated melts indicate a crustal sedimentary origin. It is speculated that the REE-rich and high-87Sr/86Sr metasoamtic agent should be carbonatitic melt derived from the carbonated pelite carried by the subducted PAOP, which could have contributed to the transformation of the lithospheric mantle beneath the NCC.

  13. Low Abundances of Highly Siderophile Elements in the Lunar Mantle: Evidence for Prolonged Late Accretion

    NASA Technical Reports Server (NTRS)

    Walker, R. J.; Horan, M. F.; Shearer, C. K.; Papike, J. J.

    2004-01-01

    The highly siderophile elements (HSE: including Re, Au, Ir, Os, Ru, Pt, Pd, Rh) are strongly partitioned into metal relative to silicates. In the terrestrial planets these elements are concentrated in metallic cores. Earth s mantle has sufficiently high abundances of the HSE (0.008 times CI abundances) that it has been hypothesized approximately 0.1-0.5% of the mass of the Earth was added following the last major interaction between the core and mantle [e.g. 1]. The additional material added to the Earth and Moon has been termed a late veneer , and the process has often been termed late accretion [2]. The timing of the dominant late accretionary period of the Earth and Moon is still poorly known. The abundances of HSE in the lunar mantle could provide important constraints on when the late veneer was added. The material that ultimately became the silicate portion of the Moon was likely stripped of most of its HSE prior to and during coalescence of the Moon. Consequently the initial lunar mantle likely had very low concentrations of the HSE. Unlike Earth, the generation of permanent lunar crust by 4.4 Ga prevented subsequent additions of HSE to the lunar mantle via continued accretion. Thus, if a substantial portion of the late veneer was added after 4.4 Ga, the lunar mantle should have retained very low HSE concentrations. Conversely, if the late veneer was mostly added prior to 4.4 Ga, HSE abundances in the lunar mantle may be roughly similar to abundances in the terrestrial mantle.

  14. 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

  15. 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

  16. 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

  17. Lattice-preferred Orientation and Volatile Content of Olivine in Arc Mantle Lithosphere, Based on Xenoliths from the Sierra Nevada Batholith, California

    NASA Astrophysics Data System (ADS)

    Chin, E. J.; Hirth, G.; Saal, A. E.; Eiler, J. M.

    2014-12-01

    Peridotite xenoliths from the Mesozoic Sierra Nevada Batholith, California, USA record the origin and growth of arc mantle lithosphere. The peridotites vary from coarse-grained, clinopyroxene-poor spinel peridotite to fine-grained, recrystallized, clinopyroxene-rich garnet peridotite. Garnet coronas around spinel and garnet exsolution indicate cooling and/or increasing pressure. Previous thermobarometry (Chin et al., 2012) interpreted these peridotites as originally shallow (<3 GPa), melt-depleted residues that were subsequently refertilized by basaltic melt, transported to final pressures ~3 GPa, and cooled to 650 - 845 C. Here, we investigate deformation microstructures and volatile content of Sierran peridotites. Owing to the dependence of olivine lattice preferred orientation (LPO) on stress, temperature, and water content, we can gain new constraints on the relative chronology of deformation and introduction of volatiles, which may be related to the refertilization event. Olivine LPOs were determined using the electron backscatter diffraction (EBSD) technique with the HKL Channel 5 software. Measurements were done manually on a grain-by-grain basis along 3 mm-spacing profiles parallel to the long axis of each thin section. We analyzed three garnet peridotites and one mylonitic spinel peridotite. Of the garnet peridotites, one sample shows alignment of [001] parallel to lineation with [010] aligned normal to the foliation (B-type fabric), while the other samples display a girdle distribution of [100] and [001] parallel to lineation (possibly suggesting axial compression). The mylonitic spinel peridotite is also characterized by a B-type fabric. The B-type fabric is associated with low temperatures, high stress, and hydrous conditions, and could be the dominant fabric type in the cold part of the mantle wedge. The presence of B-type fabric in the Sierran peridotites is consistent with their equilibration at cold temperatures and high pressures, conditions that

  18. Quartz diorite veins in a peridotite xenolith from Tallante, Spain: implications for reaction and survival of slab-derived SiO2-oversaturated melt in the upper mantle

    NASA Astrophysics Data System (ADS)

    Arai, S.; Shimizu, Y.; Gervilla, F.

    We found quartz diorite veins (up to 5 mm thick), composed mainly of plagioclase and quartz, in a plagioclase-bearing spinel lherzolite xenolith in alkali basalt from Tallante, Southern Spain. The quartz diorite veins are coarse-grained, the average grain size being 0.5 mm, and have thin orthopyroxenite rim along olivine wall. Thinner veins free of quartz and composed solely of plagioclase with orthopyroxene selvage are much more common in other xenoliths from Tallante. The involved melt was strongly reactive with olivine to form orthopyroxene, which can protect against further reaction. This suggests how the silica-oversaturated melts, after supplied from downgoing slabs, can move through peridotite and reach the shallow mantle with preserving the silica-oversaturated character. The armor of orthopyroxenite is of vital importance for the melt to keep its silica-oversaturated character within peridotite. Precipitation of orthopyroxene combined with olivine consumption somewhat controls the general chemical trend of adakite. Orthopyroxenite vein network at the expense of olivine is expected to be common as fossil conduits within the mantle wedge. This kind of orthopyroxenite has contributed to Si-enrichment of the mantle wedge.

  19. 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

  20. 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

  1. 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.

  2. Peraluminous websterite and granulite xenoliths from the Chyulu Hills volcanic field, Kenya: Plagioclase-rich cumulates re-equilibrated at uppermost mantle and crustal conditions?

    NASA Astrophysics Data System (ADS)

    Ulianov, A.; Kalt, A.; Pettke, T.

    2004-12-01

    Basanites of the Chyulu Hills volcanic field, Kenya, contain a suite of meta-igneous peraluminous spinel-garnet olivine websterite, Mg-Al sapphirine-bearing and Ca-Al hibonite-bearing granulite xenoliths. The websterites are the most mafic and magnesian members of this sequence. The Mg-Al sapphirine-bearing granulites are more Si- and Al-rich and less magnesian. They consist of ortho- and clinopyroxene, corundum, spinel, sapphirine, sillimanite, plagioclase and garnet. The Ca-Al hibonite-bearing granulites are most enriched in Si and Al. They are dominated by clinopyroxene and plagioclase and may contain hibonite closely associated with spinel, mullite, sapphirine and sillimanite. Hibonite, which is very rare in terrestrial rocks, is the earliest mineral in the crystallization sequence. All rocks are poor in REE, HFSE and Cr and enriched in LILE and Ni. They follow a magmatic fractionation trend and form a cumulate sequence finally equilibrated in the range of uppermost mantle (websterites) and crustal (granulites) depths. The websterites could have formed by high-pressure metamorphism of low-pressure troctolite-like cumulates and appear similar to some websteritic lithologies from exhumed high-pressure ultramafic complexes (Kornprobst, 1990; Morishita et al., 2003). The final P-T conditions for most websterites correspond to 920-1000 C / 17-22 kbar, whereas the final equilibration in granulites occurred at ca. 600-740 C / <8 kbar, the pressure limit being defined by the stability field of sillimanite. The geodynamic interpretation of spatially close granulite terranes (Moeller et al., 1998) coupled with the petrological evidence suggest that the studied rocks were metamorphosed and may have formed in the environment of a Pan-African active continental margin. Moeller, A., Mezger, K., Schenk, V. (1998). Journal of Petrology 39, 749-783. Kornprobst, J., Piboule, M., Roden, M., Tabit, A. (1990). Journal of Petrology 31, 717-745. Morishita, T., Arai, S., Gervilla, F

  3. 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

  4. Tungsten Abundances in Hawaiian Picrites: Implications for the Mantle Sources of Hawaiian Volcanoes

    NASA Astrophysics Data System (ADS)

    Ireland, T. J.; Arevalo, R. D.; Walker, R. J.; McDonough, W. F.

    2008-12-01

    Tungsten abundances have been measured in a suite of Hawaiian picrites (MgO >13 wt.%) from nine Hawaiian shield volcanoes (Mauna Kea, Mauna Loa, Hualalai, Loihi, Koolau, Kilauea, Kohala, Lanai and Molokai). Tungsten concentrations in the parental melts for these volcanoes have been estimated via the intersection of linear W-MgO trends with the putative MgO content of the parental melt (~16 wt.%). Tungsten behaves as a highly incompatible trace element in mafic to ultramafic systems; thus, given an independent assessment of the degree of partial melting for each volcanic center, the W abundances in their mantle sources can be determined. The mantle sources for Hualalai, Kilauea, Kohala and Loihi have non- uniform estimated W abundances of 11, 13, 16 and 27 ng/g, respectively, giving an average source abundance of 17±5 ng/g. This average source abundance is nearly six times more enriched than Depleted MORB Mantle (DMM: 3.0±2.3 ng/g) and slightly elevated relative to the Bulk Silicate Earth (BSE: 13±10 ng/g). The relatively high abundances of W in the Hawaiian sources relative to the DMM can potentially be explained as a consequence of crustal recycling. For example, incorporation of 30% oceanic crust (30 ng/g W), including 3% sediment (1500 ng/g W), into a DMM source could create the W enrichment observed in the Loihi source, consistent with estimates from earlier models based on other trace elements and isotope systems. The Hualalai source, however, has also been suggested to contain a substantial recycled component, as implied by similarly radiogenic 187Os/188Os, yet this source has the lowest estimated W abundance among the volcanic centers studied. The conflict between these results may: 1) reflect chemical differences among recycled components, 2) indicate a more complex history for Hualalai samples, e.g. involvement of a melt percolation component, or 3) implicate other sources of W.

  5. 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.

  6. 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.

  7. 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.

  8. 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.

  9. Correction for volatile fractionation in ascending magmas: noble gas abundances in primary mantle melts

    NASA Astrophysics Data System (ADS)

    Burnard, Pete

    2001-09-01

    Accurate relative noble gas abundances of mantle-derived melts are required in order to further understand the distribution of noble gases in the mantle and fractionation of noble gases during the melting process. Noble gas relative abundances in the majority of oceanic basalts are highly fractionated, at least in part due to late stage, solubility controlled fractionation. Noble gas concentrations in the volatile phase (≡ noble gas:CO 2 ratio) will vary systematically during solubility controlled degassing of a magma. This contribution models the noble gas concentrations in the volatile phase during degassing at different pressures and vesicularities in order to develop a method for correcting fractionation resulting from magmatic degassing, and thereby estimate the "initial" (pre-degassing) noble gas compositions. Correcting for fractionation during magmatic degassing requires: a) a method for determining the volatile fractionation trajectory during degassing; and b) one well constrained mantle volatile composition with which to "fix" the extrapolation. The trajectory of volatile fractionation can be estimated by sequential crushing of basaltic glasses. Vesicles grow during ascent, therefore large vesicles trap early (less fractionated) volatiles while small vesicles trap late (fractionated) volatiles. Sequential crushing of basaltic glasses releases volatiles from progressively smaller vesicles, thereby allowing the fractionation trajectory resulting from degassing to be determined on individual samples. The production rate of both 21Ne and 4He in the mantle is a function of U concentration only, resulting in a constant 21Ne/ 4He production ratio in the mantle which can be used to "fix" the degassing fractionation trajectory determined by sequential crushing. This correction then allows fractionation of 4He from 40Ar prior to degassing to be assessed. This method is illustrated using multiple crushes of a single basaltic glass from the mid-Atlantic Ridge that

  10. 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

  11. 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

  12. Lead Abundance In The Martian Mantle Deduced From The Isotopic Data In Snc Meteorites

    NASA Astrophysics Data System (ADS)

    Dreibus, G.; Jagoutz, E.

    Isotopic data are a powerful tool for the study of planetary evolution. Assuming that the SNC meteorites are rocks from Mars their Sm-Nd-, Rb-Sr- and Pb-Pb-isotope systematics reveal the time scale for the chemical evolution of the Martian mantle. From the Rb -Sr isotopic systematic the existence of 3 isotopically distinct reservoirs on Mars was postulated, which remained isolated for a period of 4.3 +/- 0.2 Ga. The basaltic shergottites Shergotty, Zagami and Los Angeles have relatively high radiogenic Sr, which might come from a planetary crust. A second group, characterized by non radiogenic Sr, consists of the two mafic cumulates Nakhla and Chassigny, the olivine rich basaltic shergottites DaG 476, SaU 005, Dhofar 019and the basaltic shergottite QUE 94201, which may represent the depleted mantle. The depletion of this reservoir must have taken place during a very early process. as derived from the primitive Sr isotopes and the existence of Nd-142, the daughter product of the extinct Sm-146, found in Chassigny, the Nakhlites, SaU 005, and DaG476. A third group, with intermediate Sr isotopic composition, represented by the lherzolitic shergottites, could be derived from a primitive, unfractionated mantle. Our observed correlation of Sr-isotopes with Pb-isotopes in SNC's permits to estimate the Pb abundance for the Martian mantle. The Pb isotopes of all measured SNCs show a similar pattern as Sr isotopes. The initial Pb data of Los Angeles, Shergotty, and Zagami from the enriched crustal reservoir and of Nakhla and SaU 005 from the depleted mantle reservoir plot close to the 4.5 Ga Pb -Pb isochron.. We used this correlation to estimate the µ value (238U/204Pb) of 3.1 for the Martian mantle. This corresponds to 366 ppb Pb. Compared to the Earth with a µ = 8.8, Pb is enriched on Mars by at least a fact or of 2.5. The same enrichment was found for all other moderately volatile and volatile elements on Mars. From the high abundance of Pb in the sulfide phases of iron

  13. 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

  14. Pétrologie des xénolites ultramafiques du puy Beaunit (Massif central français) : un gisement atypique du manteau sous-continentalPetrology of ultramafic xenoliths from the Puy Beaunit (French Massif Central): an unusual occurrence for the sub-continental mantle

    NASA Astrophysics Data System (ADS)

    Féménias, Olivier; Mercier, Jean-Claude C.; Demaiffe, Daniel

    2001-05-01

    The Puy Beaunit maar presents a large variety of mantle xenoliths (spinel peridotites, pyroxenites and layered rocks). A detailed study of the textures and mineral equilibria shows the unusual character of this occurrence and the local complexity of the upper mantle beneath the French Massif Central. Ultramafic nodules have a metamorphic, magmatic or pyrometamorphic origin; they display different stages of deformation, metasomatism, partial melting and fractional crystallisation. The upper mantle appears stratified (as in other regional occurrences of the area); it has been intruded by a differentiated magmatic complex.

  15. Depletion, cryptic metasomatism, and modal metasomatism of central European lithospheric mantle: evidence from elemental and Li isotope compositions of spinel peridotite xenoliths, Kozákov volcano, Czech Republic

    NASA Astrophysics Data System (ADS)

    Medaris, L. Gordon; Ackerman, Lukáš; Jelínek, Emil; Magna, Tomáš

    2015-11-01

    Spinel peridotite xenoliths in 4.1 Ma basanite lava at Kozákov volcano vary in equilibration temperature from 675 to 1,135 °C and provide a continuous sample of lithospheric mantle from the Moho to a depth of ~82 km. The sub-Kozákov mantle is layered, consisting of an upper equigranular layer (UEL) from 32 to 45 km, an intermediate protogranular layer (PGL) from 45 to 66 km, and a lower equigranular layer (LEL) below 66 km. Relative to primitive mantle, all three layers are depleted in major incompatible elements and heavy rare earth elements, with the UEL being most depleted among the three layers, consisting of harzburgite and having experienced >15 % fractional melting. In contrast, the PGL and LEL experienced <10-15 % melting and consist of lherzolite; the PGL and LEL overlap in major element composition, with the PGL displaying a decreasing degree of depletion with depth. Subsequent metasomatism by silicate melt led to cryptic enrichments in large-ion lithophile elements, light REE, and high field strength elements over all the layers and, locally, modal enrichment in orthopyroxene. Metasomatism is accompanied by elevated whole-rock Li contents (1.2-3.6 ppm) and isotopically light δ7Li (-0.8 to -5.8 ‰). Lithium contents and δ7Li show no strong correlation with rock type or depth, although values of δ7Li are <-3.0 ‰ in the PGL and >-3.5 ‰ in the UEL and LEL. The layered structure and geochemical characteristics of sub-Kozákov lithospheric mantle are the product of Variscan or pre-Variscan melting, Variscan tectonics, and Neogene volcanism and metasomatism.

  16. 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.

  17. Xenoliths from the sub-volcanic lithosphere of Mt Taranaki, New Zealand

    NASA Astrophysics Data System (ADS)

    Gruender, Kerstin; Stewart, Robert B.; Foley, Stephen

    2010-02-01

    Mount Taranaki is located 140 km west of the Taupo Volcanic Zone and represents the most westerly expression of subduction-related volcanism on the North Island of New Zealand. Taranaki is a predominantly high-K arc volcano but compositions range from basaltic andesite to andesite with minor dacite and basalt. The sub-volcanic basement under Taranaki is thought to comprise calc-alkaline plutonic and metamorphic rocks of the Median Batholith, overlain by a sequence of Cretaceous and Tertiary sediments. Taranaki lavas contain abundant xenoliths that represent samples of the upper to lower crust beneath the volcano. The xenolith suite has been initially organised into six groups based on petrography, geochemistry and inferred genetic relationships: supracrustal sedimentary rocks (1), mafic hornfels (2), garnet gneiss (3), granite and granodiorite (4), finely banded amphibolitic gneiss (5) and gabbros and ultramafic rocks (6). Groups 1, 3 and 4 are derived from the Median Batholith basement and Cretaceous-Tertiary sediments of the Taranaki Basin while Groups 2, 5 and some fine grained gabbros from Group 6 could either be derived from the Median Batholith or be cognate xenoliths. Group 6 gabbros and ultramafic rocks are dominated by clinopyroxene, amphibole and plagioclase and are predominantly cumulate in origin. The Egmont xenoliths can also be classified into the Type I and Type II xenoliths defined by Frey and Prinz (1978). Type I dunite and wehrlite xenoliths are only present in basaltic andesite host rocks and are sourced from depleted upper mantle whereas Type II xenoliths predominate in the more siliceous andesites and are sourced from the lower crust. The separate source depths for the two rock types can be explained by the "hot zone" model where the andesites have much greater interaction with the lower crust than the basaltic andesites. Some xenoliths contain glass of rhyolitic to trachyitic compositions with up to 6% K 2O that represent partial melts of the

  18. Mafic granulite xenoliths in the Chilka Lake suite, Eastern Ghats Belt, India: evidence of deep-subduction of residual oceanic crust

    NASA Astrophysics Data System (ADS)

    Bhattacharya, S.; Chaudhary, A. K.; Saw, A. K.; Das, P.; Chatterjee, D.

    2012-11-01

    Granulite xenoliths preserve key geochemical and isotopic signatures of their mantle source regions. Mafic granulite and pyroxinite xenoliths within massif-type charnockitic rocks from the Eastern Ghats Belt have recently been reported by us. The mafic granulite xenoliths from the Chilka Lake granulite suite with abundant prograde biotite are geochemically akin to Oceanic Island Basalt (OIB). They can be distinguished from the hornblende-mafic granulite xenoliths with signatures of Arc-derived basalt occurring in the other suites of the Eastern Ghats Belt. These two groups of xenoliths in the Paleoproterozoic Eastern Ghats Province have quite distinct Nd-model ages- 1.9 Ga and 2.5 Ga respectively, which may be interpreted as their crustal residence ages. Strong positive Nb anomalies, indicating subducted oceanic crust in the source, LREE enrichment and strongly fractionated REE pattern are key geochemical signatures attesting to their origin as OIB-type magma. Also low Yb and Sc contents and high (La / Yb)N ratios can be attributed to melting in the presence of residual garnet and hence at great depths (> 80 km). The variable enrichment in radiogenic 87Sr, between 0.70052 and 0.71092 at 1.9 Ga and less radiogenic 143Nd between ɛ-1.54 and 7.46 are similar to those of the OIBs compared to MORBs. As OIBs commonly contain some recycled oceanic crust in their sources, we suggest that the residue of the oceanic crust from a previous melting event (~ 2.5 Ga) that produced the Arc-derived basalts (protoliths of hornblende-mafic granulite xenoliths) could have subducted to great depths and mechanically mixed with the mantle peridotite. A subsequent re-melting event of this mixed source might have occurred at ca. 1.9 Ga as testified by the crustal residence ages of the biotite-mafic granulite xenoliths of the Chilka Lake granulite suite.

  19. 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

  20. 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.

  1. 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.

  2. Selenium and Tellurium abundances in residual mantle peridotites from Baldissero and Balmuccia (Ivrea Zone, Northern Italy)

    NASA Astrophysics Data System (ADS)

    Wang, Z.; Becker, H.; Gawronski, T.

    2011-12-01

    Precise and accurate abundances of S, Se and Te in terrestrial samples may help to constrain the behavior and processes that control highly siderophile (HSE) and chalcophile elements during planetary and mantle processes. On the basis of higher S/Se and Se/Te in basaltic magma relative to peridotites, mantle-basalt bulk partition coefficients of DSabundances in spinel lherzolites (Al2O3=2.0-3.1%) and a harzburgite (Al2O3=1.2%) from the Balmuccia and Baldissero peridotite massifs, Ivrea Zone, Italy [5] have been studied by the isotope dilution-MS method. Due to their freshness and absence of late alteration (loss on ignition <1 %), these samples are particularly useful for such a study. Se and Te contents in variably fertile lherzolites and a harzburgite display a positive linear relationship, and become more depleted with decreasing Al2O3, from 108.0 to 6.5 ng/g, from 20.5 to 1.0 ng/g, respectively. Except for one sample, the data roughly match predicted results of partial melting controlled by sulfide-silicate equilibrium, assuming similar DSe and DTe (1770) [6], implying that both elements have similar partition coefficients during magmatic processes in the mantle. This conclusion is further corroborated by relatively constant Se/Te at 5.3-9.0 (7.0±1.1, 1s), even for depleted harzburgite (6.5). Similar values have been obtained on lherzolites from Lherz (Pyrenees), which are believed to the results of melt

  3. Petrology of ultramafic xenoliths in Cenozoic alkaline rocks of northern Madagascar (Nosy Be Archipelago)

    NASA Astrophysics Data System (ADS)

    Rocco, Ivana; Lustrino, Michele; Zanetti, Alberto; Morra, Vincenzo; Melluso, Leone

    2013-01-01

    Late Miocene basanites of Nosy Be and Nosy Sakatia islands (Nosy Be Archipelago, northern Madagascar) carry spinel-facies anhydrous ultramafic xenoliths (lherzolites, harzburgites and wehrlites). Geothermobarometric estimates indicate that these xenoliths derive from shallow mantle depths of 35-40 km, with those from Nosy Be island showing equilibration T (averages in the range of 945-985 °C) lower than the Nosy Sakatia analogues (averages ranging from 1010 to 1110 °C). One Nosy Sakatia mantle xenolith exhibits relatively fertile lherzolite composition, with trace and major element mineral chemistry consistent with a residual character after low degrees (1-2%) of mafic melt extraction. We interpret this composition as that resembling a depleted mantle (DM)-like lithospheric composition before metasomatic overprints. The other lherzolites and harzburgites display petrochemical characters consistent with variable extent of partial melting (up to 18%), associated with pronounced metasomatic overprints caused by migrating melts, as highlighted by enrichments in highly incompatible trace elements (e.g. light rare earth elements, LREE and Sr), together with the abundant occurrence of wehrlitic lithologies. The variability of petrochemical features points to different styles of metasomatism and metasomatic agents. The estimated composition of the parental melts of wehrlites matches that of host basanites. The combination of this evidence with the petrographic features, characterized by coarse-granular to porphyroclastic textures and by the presence of olivine without kink-banding, suggests that wehrlites are veins or pockets of high pressure cumulates within the mantle peridotite. The same melts also metasomatized via porous-flow percolation some lherzolites and harzburgites. Distinctly, a group of lherzolites and harzburgites was metasomatized by a different alkaline melt having markedly lower incompatible trace element contents. Late infiltration of metasomatic fluids

  4. The Hyblean xenolith suite (Sicily): an unexpected legacy of the Ionian-Tethys realm

    NASA Astrophysics Data System (ADS)

    Manuella, Fabio Carmelo; Scribano, Vittorio; Carbone, Serafina; Brancato, Alfonso

    2015-07-01

    The extensive study of a great number of deep-seated xenoliths from Tortonian tuff-breccia pipes in the Hyblean area (Sicily) revealed the following fundamental evidence: (1) typical continental crust rocks are completely absent in the entire xenolith suite; (2) mantle ultramafics are more abundant than gabbroids; (3) sheared oxide-gabbros, closely resembling those from oceanic fracture zones, are relatively common; (4) secondary mineral assemblages, compatible with alteration processes in serpentinite-hosted hydrothermal systems, occur both in peridotites and gabbros. Among the products of this hydrothermal activity, organic compounds, having abiotic origin via Fischer-Tropsch synthesis, occur in some hydrothermally altered gabbro and ultramafic xenoliths, as well as in hydrothermal clays. Moreover, the U-Pb dating of hydrothermal zircon grains, hosted in a xenolith of metasomatized tectonic breccia, indicated an Early-Middle Triassic age of the fossil hydrothermal system. Another line of evidence for the oceanic nature of the Hyblean-Pelagian basement is the complete absence of continental crust lithologies (granites, felsic metaigneous, and metasedimentary rocks) in outcrops and in boreholes, and the oceanic affinity of the Tertiary volcanic rocks from the Hyblean Plateau and the Sicily Channel (Pantelleria and Linosa Islands), which lack of any geochemical signature for continental crust contamination. A reappraisal of existing geophysical data pointed out that serpentinites form the dominant lithologies in the lithospheric basement of the Hyblean-Pelagian area down to a mean depth of 19 km, which represents the regional Moho considered as the serpentinization front, marking the transition from serpentinites to unaltered peridotites. On these grounds, we confirm that Hyblean xenoliths contain mineralogical, compositional, and textural evidence for tectonic, magmatic, and hydrothermal processes indicating the existence of fossil oceanic core complexes, in the

  5. 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.

  6. 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

  7. 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

  8. 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.

  9. 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

  10. Phosphorus and other trace elements from secondary olivine in composite mantle xenoliths (CMX) from Cima Volcanic Field (CVF; California, USA): implications for crystal growth kinetics

    NASA Astrophysics Data System (ADS)

    Baziotis, Ioannis; Asimow, Paul; Ntaflos, Theodoros; Boyce, Jeremy; Koroneos, Antonios; Perugini, Diego; Liu, Yongsheng; Klemme, Stephan; Berndt, Jasper

    2015-04-01

    embedded in Gl and as large (~100 μm) idiomorphic to hypidiomorphic crystals with Gl and Spl inclusions; Mg# ranges from Fo74.5 (rim in contact with Gl) to Fo90.3; P2O5 reaches 3.5 wt% (in a ~Fo84 rim); Li varies from 2.80 (core) to 6.35 ppm (rim). Clinopyroxene (Wo41-43En50-54Fs5-8; P2O5 0.04-0.08 wt%; Li 3.33 ppm) is found both within the ML and as a reaction product between melt and matrix Opx. Trace element geochemistry shows possible equilibrium with ML glass for some elements, but clear disequilibrium for others. Apatite occurs as large (~100 μm) crystals in contact with Ol or Gl, as near-rim inclusions in P-rich Fo84 and as tiny prismatic crystals in Gl; REEs show slight negative Eu anomalies (Eu/Eu*=0.79-0.86) due to prior crystallization of plagioclase. High-resolution X-ray mapping of P in Ol from Ci-1-196 reveals 3-7 μm wide P-rich bands parallel to facets. P2O5 correlates negatively with Si and Mg+Fe+Ca, suggesting a substitution Mg2SiO4 + 1 /2 P2O5 →Mg1.5[]0.5PO4 + 1 /2MgO+SiO2. P-Al-rich areas may grow in minutes, whereas P-Al-poor over few weeks (Jambon et al., 1992). At such rates, dendritic growth (Welsch et al., 2014) implies that core to rim zoning may not be simple growth stratigraphy. A slight correlation between P and Al in our data implies either diffusive relaxation of Al gradients or, judging by dynamic experiments (Grant & Kohn, 2013), cooling rates >10° C/h that generate disequilibrium solute trapping of P but near-equilibrium incorporation of Al. The petrogenetic history following melt intrusion requires rapid cooling and reaction with matrix minerals and crystallization sequence Ol→Cpx→Pl→Ap→Fe-Ox→quench of Gl. P and Li concentrations set upper and lower limits on growth rates after intrusion of melt into CVF xenoliths. Early-crystallized olivine grew rapidly enough that sluggish P became over-enriched but not so fast as to over-enrich other elements. Cpx formed later either as neoblasts or reaction rims in which P was

  11. Melt inclusions in scoria and associated mantle xenoliths of Puy Beaunit Volcano, Chaîne des Puys, Massif Central, France

    NASA Astrophysics Data System (ADS)

    Jannot, Séverine; Schiano, Pierre; Boivin, Pierre

    2005-07-01

    In order to characterize the composition of the parental melts of intracontinental alkali-basalts, we have undertaken a study of melt and fluid inclusions in olivine crystals in basaltic scoria and associated upper mantle nodules from Puy Beaunit, a volcano from the Chaîne des Puys volcanic province of the French Massif Central (West-European Rift system). Certain melt inclusions were experimentally homogenised by heating-stage experiments and analysed to obtain major- and trace-element compositions. In basaltic scoria, olivine-hosted melt inclusions occur as primary isolated inclusions formed during growth of the host phase. Some melt inclusions contain both glass and daughter minerals that formed during closed-system crystallisation of the inclusion and consist mainly of clinopyroxene, plagioclase and rhönite crystals. Experimentally rehomogenised and naturally quenched, glassy inclusions have alkali-basalt compositions (with SiO2 content as low as 42 wt%, MgO>6 wt%, Na2O+K2O>5 wt%, Cl~1,000 3,000 ppm and S~400 2,000 ppm), which are consistent with those expected for the parental magmas of the Chaîne des Puys magmatic suites. Their trace-element signature is characterized by high concentration(s) of LILE and high LREE/HREE ratios, implying an enriched source likely to have incorporated small amounts of recycled sediments. In olivine porphyroclasts of the spinel peridotite nodules, silicate melt inclusions are secondary in nature and form trails along fracture planes. They are generally associated with secondary CO2 fluid inclusions containing coexisting vapour and liquid phases in the same trail. This observation and the existence of multiphase inclusions consisting of silicate glass and CO2-rich fluid suggest the former existence of a CO2-rich silicate melt phase. Unheated glass inclusions have silicic major-element compositions, with normative nepheline and olivine components, ~58 wt% SiO2, ~9 wt% total alkali oxides, <3 wt% FeO and MgO. They also have high

  12. 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.

  13. The Heldburg Phonolite, Central Germany: Reactions between phonolite and xenocrysts from the upper mantle and lower crust

    NASA Astrophysics Data System (ADS)

    Grant, Thomas B.; Milke, Ralf; Pandey, Sanjay; Jahnke, Hannes

    2013-12-01

    The Heldburg Phonolite, Central Germany contains abundant fragments of xenolithic material most commonly seen as single xenocrysts of olivine and orthopyroxene as well as larger poly-mineralic micro-xenoliths. The xenocryst and micro-xenolith compositions indicate two cumulate source rocks; a spinel-bearing lherzolite and a pyroxene rich gabbro-norite. Disequilibrium between the host melt and xenocrysts lead to the formation of phlogopite-diopside double rims on olivine and either amphibole-phlogopite or amphibole-diopside double rims on orthopyroxene. The rim assemblages and infiltration of melt into some micro-xenoliths suggest that the xenolithic material was sampled by the phonolite directly from their source rocks. The idea that the phonolite originated from the upper mantle is supported by thermobarometry of amphibole and clinopyroxene phenocrysts. The reaction rims therefore provide small-scale analogues of a metasomatic event involving an evolved alkali enriched melt and upper mantle and lower crust wall rocks. Chemical zoning within the rims and the inheritance of compositional features from their hosts indicates rapid rim growth rates with slow diffusion rates of components through the rims. Residence times for the xenocrysts in the melt are in the order of several months to a year.

  14. 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

  15. 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

  16. 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.

  17. 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.

  18. 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

  19. 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

  20. Redox state of subcontinental lithospheric mantle and relationships with metasomatism: insights from spinel peridotites from northern Victoria Land (Antarctica)

    NASA Astrophysics Data System (ADS)

    Perinelli, Cristina; Andreozzi, Giovanni B.; Conte, Aida M.; Oberti, Roberta; Armienti, Pietro

    2012-12-01

    Rift-related Cenozoic alkaline mafic lavas from northern Victoria Land (Antarctica) carry abundant mantle xenoliths whose oxygen fugacities ( fO2) were determined to assess how the metasomatism, related to Cenozoic magmatism, affected the state of oxidation of the lithospheric mantle. The xenoliths used for this study are anhydrous spinel peridotites sampled in two localities, Greene Point and Baker Rocks, that show different extents of metasomatism: these are limited to incompatible element enrichments in Greene Point and to enrichments in major, minor and trace elements at Baker Rocks. The data set includes a composite xenolith from Baker Rocks, formed by a depleted lherzolite crosscut by an amphibole-bearing vein. Mössbauer spectroscopy was used to accurately determine the Fe3+/Fetot ratios in spinel and amphibole minerals. Amphiboles were also characterized by Single-Crystal X-ray Diffraction, and the crystallographic data were used to calculate the dehydrogenation. The oxidation state recorded by the xenoliths ranges from 0.2 to 1.5 log-bar units below the fayalite-magnetite-quartz (FMQ) buffer (Δlog fO2) with the highest values observed in the metasomatized samples from Greene Point. For the vein of composite Baker Rocks xenolith, Δlog fO2 was estimated on the basis of the amphibole in -1.7 log-bar units, a value close to those calculated for all Baker Rocks xenoliths (Δlog fO2 = -1.5 to -1.1 log-bar units). These results indicate a similar oxidation state for lithospheric mantle prior to the metasomatic event at Greene Point and Baker Rocks (Δlog fO2 ~ -1.3 log-bar units). Metasomatism produced different effects in the shallow mantle at the two sites. At Greene Point, an oxidizing metasomatic melt caused the rise of fO2 in peridotite portions close to melt conduits up to FMQ. In contrast, at Baker Rocks, a metasomatizing melt with fO2 similar to that of the peridotite matrix produced chemical changes in the surrounding mantle rocks and amphibole

  1. A Chemistry-Based Classification for Peridotite Xenoliths

    NASA Astrophysics Data System (ADS)

    Block, K. A.; Ducea, M.; Raye, U.; Stern, R. J.; Anthony, E. Y.; Lehnert, K. A.

    2007-12-01

    The development of a petrological and geochemical database for mantle xenoliths is important for interpreting EarthScope geophysical results. Interpretation of compositional characteristics of xenoliths requires a sound basis for comparing geochemical results, even when no petrographic modes are available. Peridotite xenoliths are generally classified on the basis of mineralogy (Streckeisen, 1973) derived from point-counting methods. Modal estimates, particularly on heterogeneous samples, are conducted using various methodologies and are therefore subject to large statistical error. Also, many studies simply do not report the modes. Other classifications for peridotite xenoliths based on host matrix or tectonic setting (cratonic vs. non-cratonic) are poorly defined and provide little information on where samples from transitional settings fit within a classification scheme (e.g., xenoliths from circum-cratonic locations). We present here a classification for peridotite xenoliths based on bulk rock major element chemistry, which is one of the most common types of data reported in the literature. A chemical dataset of over 1150 peridotite xenoliths is compiled from two online geochemistry databases, the EarthChem Deep Lithosphere Dataset and from GEOROC (http://www.earthchem.org), and is downloaded with the rock names reported in the original publications. Ternary plots of combinations of the SiO2- CaO-Al2O3-MgO (SCAM) components display sharp boundaries that define the dunite, harzburgite, lherzolite, or wehrlite-pyroxenite fields and provide a graphical basis for classification. In addition, for the CaO-Al2O3-MgO (CAM) diagram, a boundary between harzburgite and lherzolite at approximately 19% CaO is defined by a plot of over 160 abyssal peridotite compositions calculated from observed modes using the methods of Asimow (1999) and Baker and Beckett (1999). We anticipate that our SCAM classification is a first step in the development of a uniform basis for

  2. Thermal history of the deepest parts of orogens through U-Pb thermochronology of Tanzanian deep crustal xenoliths

    NASA Astrophysics Data System (ADS)

    Blondes, M. S.; Rudnick, R. L.; Bowring, S. A.; Piccoli, P. M.; Ramezani, J.

    2009-12-01

    information from a comparison of rutile, apatite, and titanite derived from different depths. The two lowest closure temperature phases, apatite and rutile, contain no appreciable radiogenic Pb in all but one xenolith rock type, consistent with pre-eruptive temperatures above 450 - 550 °C in the middle and lower crust. The presence of abundant radiogenic Pb in coexisting titanite, as well as the diffusion kinetics of Pb in titanite, apatite, and rutile, suggest that the Pb-loss is not due to magmatic entrainment, but rather reflects a widespread lithospheric thermal anomaly. This could be associated with modern rifting and/or reflect increased heat flow since the pan-African due to a thin lithospheric mantle relative to the craton. Closure of Pb in lower crustal titanite and Pb loss in middle crustal apatite is consistent with a measured surface heat flow value of 47 mW/m 2 and a conductive geotherm only if crustal radiogenic heat production is very low (< 0.5 μWm -3 ). As the MB is thought to represent the base of the Pan-African Orogen, these samples from Northern Tanzania offer a unique window into the thermal evolution of deep portions of a large continental orogen.

  3. 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

  4. Nature and origin of eclogite xenoliths from kimberlites

    NASA Astrophysics Data System (ADS)

    Jacob, D. E.

    2004-09-01

    Eclogites from the Earth's mantle found in kimberlites provide important information on craton formation and ancient geodynamic processes because such eclogites are mostly Archean in age. They have equilibrated over a range of temperatures and pressures throughout the subcratonic mantle and some are diamond-bearing. Most mantle eclogites are bimineralic (omphacite and garnet) rarely with accessory rutiles. Contrary to their overall mineralogical simplicity, their broadly basaltic-picritic bulk compositions cover a large range and overlap with (but are not identical to) much younger lower grade eclogites from orogenic massifs. The majority of mantle eclogites have trace element geochemical features that require an origin from plagioclase-bearing protoliths and oxygen isotopic characteristics consistent with seawater alteration of oceanic crust. Therefore, most suites of eclogite xenoliths from kimberlites can be satisfactorily explained as samples of subducted oceanic crust. In contrast, eclogite xenoliths from Kuruman, South Africa and Koidu, Sierra Leone stem from protoliths that were picritic cumulates from intermediate pressures (1-2 Ga) and were subsequently transposed to higher pressures within the subcratonic mantle, consistent with craton growth via island arc collisions. None of the eclogite suites can be satisfactorily explained by an origin as high pressure cumulates from primary melts from garnet peridotite.

  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. Ultradeep (>300 kilometers) ultramafic xenoliths: petrological evidence from the transition zone.

    PubMed

    Sautter, V; Haggerty, S E; Field, S

    1991-05-10

    The seismologically delineated transition zone, at depths between 400 and 670 kilometers, is a fundamental discontinuity in the earth that separates the upper mantle from the lower mantle. Xenoliths from within or close to the transition zone are dominated by pyropic garnet and associated pyroxene or mineralogically heterogeneous garnet lherzolite. These xenoliths show evidence for the high-pressure (90 to 120 kilobars) transformation of pyroxene to a solid solution of pyroxene in garnet (majorite) and silicon in octahedral coordination; low-pressure (less than 80 kilobars) exsolution of clinopyroxene or orthopyroxene from the original majorite is preserved. Although mineral modes and rock proportions below the transition zone and the relative amount of eclogite present cannot be accurately assessed from the xenoliths, it is likely that both majorite and beta-spinel help produce the observed seismic gradient of the transition zone. PMID:17744263

  7. 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.

  8. 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

  9. 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

  10. 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

  11. Mantle in the Manihiki Plateau source with ultra-depleted incompatible element abundances but FOZO-like isotopic signature

    NASA Astrophysics Data System (ADS)

    Golowin, R.; Hoernle, K.; Portnyagin, M.; Hauff, F.; Gurenko, A.; Garbe-Schoenberg, C. D.; Werner, R.

    2014-12-01

    The ~120Ma Manihiki Plateau basement consists of high-Ti tholeiitic basalts with EM-I type isotopic signatures, similar to the Singgalo basalts at Ontong Java, and low-Ti tholeiitic basalts with FOZO (Kwaimbaita/Kroenke) to HIMU-type isotopic compositions, similar to late stage volcanism on Hikurangi and Manihiki Plateaus (Hoernle et al. 2010; Timm et al. 2011). The low-Ti basalts have affinities to boninites and have been interpreted to be derived from residual mantle wedge mantle (Ingle et al. 2007). New major, volatile and trace element and radiogenic isotope data have been generated from fresh low-Ti glass samples recovered during R/V Sonne cruises SO193 and SO225. The low-Ti samples have distinctly lower Ti/V ratios compared to lavas from Ontong Java Plateau (Kwaimbaita-Kroenke and Singgalo), but similar to boninitic rocks. Glasses and melt inclusions in olivine have low volatile contents (0.12-0.25 wt% H2O). Olivine chemistry points to derivation from peridotite source. Therefore we interpret the low-Ti lavas to have formed through melting of dry and depleted peridotite at high temperatures, consistent with Timm et al (2011). The low-Ti group is characterized by U-shaped trace element patterns. The glass samples form linear mixing arrays on radiogenic isotope diagrams, pointing to the involvement of two components: 1) a component ultra-depleted in highly incompatible elements (UDC) but with intermediate Pb, Sr and Nd isotopic compositions, being similar to Kwaimbaita/Kroenke lavas from Ontong Java, and 2) an enriched component with HIMU-type incompatible element and isotopic characteristics, similar to late-stage volcanism on Manihiki, Hikurangi and Ontong Java (e.g. Hoernle et al. 2010). The ultra-depleted, FOZO-like mantle component could represent second stage melting of FOZO type mantle or re-melting of young recycled oceanic lithosphere within the plume head. Enrichment with HIMU type melts is required to explain the enrichment in the most incompatible

  12. 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

  13. 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.

  14. 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

  15. Mantle hydrocarbons: Abiotic or biotic?

    NASA Astrophysics Data System (ADS)

    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 "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 (c) δ13C of the mantle hydrocarbons is uniform (about -27%.). 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 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 4H 10.

  16. 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.

  17. 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.

  18. Are Colorado Plateau Eclogite Xenoliths Franciscan?: Oxygen Isotope Evidence From Zoned Garnet

    NASA Astrophysics Data System (ADS)

    Hoover, W. F.; Page, F. Z.; Schulze, D. J.; Kitajima, K.; Valley, J. W.

    2014-12-01

    Eclogite xenoliths from the Moses Rock diatreme, UT, USA are of controversial (Proterozoic or Phanerozoic) age. In this study, seven garnets from four Moses Rock eclogite xenoliths were analyzed for δ18O by ion microprobe. Garnet core δ18O values are 7.8-10.3‰ VSMOW. All samples have a sharp change between cores and rim values of 5.8-6.9‰. These garnets have the first reported oxygen isotope zoning from mantle xenoliths. The core values are well outside the range of garnets equilibrated with the mantle, suggesting that they began growth during subduction from an altered oceanic crustal protolith. Most rim values reach the mantle range. This decrease in δ18O from core to rim is consistent with continued subduction of the eclogites into the mantle. The failure of some garnet rims to reach mantle δ18O values may indicate that they did not equilibrate fully with the mantle, or were exposed to a mixed mantle-slab fluid. Zoning in the samples from this study record a stepped shift from an altered upper oceanic crust protolith, to a mantle-influenced environment. The preservation of zoning in some of the samples from this study suggests that these eclogites were protected within the cool subducting slab and experienced a short mantle residence time. The preservation of cation and oxygen isotope zoning is more consistent with an origin during Franciscan subduction than Proterozoic subduction, unless the zoning is a late feature that formed just prior to volcanic emplacement. This is further supported by the similar patterns of increased pyrope content and decreasing δ18O found in some Franciscan eclogite garnets (e.g., Errico et al., 2013, CMP).

  19. 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

  20. 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

  1. 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

  2. Oxygen Fugacity Recorded by Xenoliths from Pacific Oceanic Islands

    NASA Astrophysics Data System (ADS)

    Wall, K.; Davis, F. A.; Cottrell, E.

    2014-12-01

    Oxygen fugacity (fO2) plays a vital role in determining mineral stability and depth of melting in the mantle. Several studies have used the spinel peridotite oxybarometer to estimate fO2; yet few data exist from ocean islands, despite the importance of fO2 to understanding ocean island basalt petrogenesis (Herzberg and Asimow, 2008). We report fO2 recorded by peridotite xenoliths from three ocean islands: Savai'i (average fO2 = QFM -1.4 to +0.9), Tahiti (QFM +0.6 to +0.7) and Tubuai (QFM -1.1 to +0.2). We calculate fO2 using methods and standards from Wood and Virgo (1989) and Wood (RiMG, 1990). Oxygen fugacities span a similar range to those reported for El Hierro, Oahu, and Tahiti by Ballhaus (1993): more reduced than arc peridotites, but more oxidized than abyssal peridotites. Spinels in several of the xenoliths are heterogeneous and record a range of apparent fO2 at the mm scale. We propose two distinct mechanisms for introducing fO2 heterogeneity: melt refertilization (Tubuai) and diffusive reequilibration (Savai'i and Tubuai). Spinels in one Tubuai sample record increasing fO2 from QFM-0.6 in the xenolith interior to +1.1 at the basalt interface. Apparent fO2 recorded by these spinels correlate with TiO2, an indicator of melt refertilization (Pearce et al., 2000). We suggest that spinels from the xenolith interior record the relatively low fO2 conditions of the lithospheric mantle, while host basalt has oxidized near-interface spinels. Uniformly high TiO2, fO2, and low olivine Mg# in Tahitian xenoliths from this study may indicate that refertilization has reset the fO2 recorded by these rocks. Closed-system diffusive reequilibration, caused by changes in temperature, can also change the fO2 recorded by a peridotite. In samples from Savai'i and Tubuai with multiple spinel habits, fine intergrowth spinels and the rims of large, equant spinels record higher apparent fO2 and lower Al2O3 than cores of large grains. Canil and O'Neill (1996) suggest that the MgAl2O4

  3. The provenance of sub-cratonic mantle beneath the Limpopo Mobile Belt (South Africa)

    NASA Astrophysics Data System (ADS)

    van der Meer, Quinten H. A.; Klaver, Martijn; Waight, Tod E.; Davies, Gareth R.

    2013-06-01

    Petrological, whole rock major element and mineral chemical analysis of mantle xenoliths from the Venetia kimberlite pipes (533 Ma) in South Africa reveals an apparently stratified cratonic mantle beneath the Central Zone of the Limpopo Mobile Belt (LMB) that separates the Kaapvaal and Zimbabwe Cratons. Combined pressure-temperature (P-T) data and petrographic observations indicate that the mantle consists of an upper layer of Low-T coarse-equant garnet + spinel lherzolite (~ 50 to ~ 130 km depth). This layer is underlain by a region of mixed garnet harzburgites and garnet lherzolites that are variably deformed (~ 130 to ~ 235 km depth). An equilibrated geotherm did not exist at the time of kimberlite eruption (533 Ma) and a localised heating event involving the introduction of asthenospheric material to the High-T lithosphere below 130 km is inferred. Low-T garnet-spinel lherzolites are highly melt depleted (40% on average). In contrast, the High-T lithosphere (mostly at diamond stable conditions) consists of a mixed zone of variably sheared and melt depleted (30% on average) garnet harzburgite and mildly melt depleted (20% on average) garnet lherzolite. The chemistry of the High-T xenoliths contrasts with that of minerals included in diamond originating from the same depth. Inclusions suggest diamond crystallisation in a more melt depleted lithosphere than represented by either Low- or High-T xenoliths. High-T xenoliths are proposed to represent formerly melt depleted lithosphere, refertilised by asthenosphere-derived melts during the diapiric rise of a proto-kimberlitic melt pocket. This process is coupled to the positive temperature perturbation observed in the High-T xenoliths and may represent a common process in the lower lithosphere related to localised but intense tectono-magmatic events immediately preceding kimberlite eruption. The presence of clinopyroxene, garnet and abundant orthopyroxene in the Low-T lherzolite implies a history of melt depletion

  4. Xenolith incorporation, distribution, and dissemination in a mid-crustal granodiorite, Vega pluton, central Norway

    NASA Astrophysics Data System (ADS)

    Marko, W.; Barnes, M.; Vietti, L.; McCulloch, L.; Anderson, H.; Barnes, C.; Yoshinobu, A.

    2005-12-01

    A wide compositional variety of enclaves have been observed in intrusions of various tectonic settings and emplacement histories. Some enclaves may have been incorporated as the result of magma mingling (e.g. mafic magmatic enclaves) while others (xenoliths or residuum) are the product of processes acting to disaggregate solid host or source rocks. These processes may include stoping, diking, partial melting and dissolution. The Vega intrusion in central Norway contains numerous xenoliths ranging in size from sub-mm to sub-km in map extent. These xenoliths occur in a heterogeneous and in some cases strongly peraluminous granodiorite host and appear to have been distributed throughout most of the intrusion. Xenolith compositions include marble, utramafic, gneiss, biotite garnet schist, calc-silicate, quartzite, and meta- and diatexite. Additionally a number of xenocrystic minerals in the host granodiorite may include feldspar, quartz, cordierite, and biotite. Calc-silicate, gneissic and schistose xenoliths are the most abundant. Calc-silicate xenoliths tend to exhibit an angular geometry and may be partially surrounded by leucogranitic magmas. Gneissic and schistose xenoliths are commonly less angular and may exhibit reactions rims in the form of grain coarsening and/or changes in mineralogy. Migmatitic xenoliths have lobate or circular 2-D contacts. In some cases xenoliths appear composite in composition with calc-silicate apparently folded around diatexitic cores that are enclosed by host granodiorite. Xenoliths >= 5 cm2 (short axis) have average population densities on the order of 2/m2 to 4/m2 of host and some show a statistical alignment of long axis at the outcrop scale in measured areas up to 100 m2. Assuming elliptical 2-D area from maximum measured axial ratios the xenolith areas range from 0.1 m2 to 0.3 m2 per m2 of host. Area-frequency (size) distributions for enclaves >= 5 cm2 are positively skewed for both composite calc-silicate xenolith populations

  5. 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.

  6. 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.

  7. 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

  8. Granulite xenoliths from Cenozoic Basalts in SE China provide geochemical fingerprints to distinguish lower crust terranes from the North and South China tectonic blocks

    NASA Astrophysics Data System (ADS)

    Yu, Jin-Hai; Xu, Xisheng; O'Reilly, Suzanne Y.; Griffin, W. L.; Zhang, Ming

    2003-03-01

    A large suite of lower crustal xenoliths from Cenozoic basaltic rocks from three widely spaced localities in the eastern part of the South China Block (SCB; Xilong, Qilin and Leizhou) and two localities (Nushan and Guizishan) close to the suture between the North and South China Blocks in Southeast China has been studied in detail. They are used to define the nature of the lower crust beneath these regions and to define two distinct types of lithospheric domains resulting from different tectonic episodes. Lower crustal xenoliths from the Nushan area have distinctive petrologic and geochemical (including isotopic) characteristics indicating formation by underplating of continental basaltic magmas around the crust-mantle boundary. Geochemistry of these granulites demonstrates that an isotopically enriched old source component had played a significant role in the formation of the Nushan xenoliths. The lower crustal xenoliths from the South China Block reveal different origins and source compositions from those from the Nushan area. The South China Block xenoliths are considered to be derived from the underplating of continental arc-type basaltic magmas, but also show evidence of significant assimilation and fractional crystallisation (AFC) that produced abundant cumulates and fractionated liquids with strong crustal contamination signatures. They are geochemically and isotopically similar to the outcropping Late Mesozoic gabbros and basalts in the same region and are inferred to be their underplated high-pressure analogues and products of AFC with the lower crustal sources parental to the more silicic Mesozoic outcropping magmatic rocks. These Mesozoic basaltic magmas from the South China Block are geochemically distinct from the Cenozoic basaltic volcanics in this region, indicating a secular source change from a continental arc setting to an intraplate extensional regime. This significant change from Mesozoic to Cenozoic may indicate eastward movement of the

  9. 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

  10. Petrology of two diamondiferous eclogite xenoliths from the Lahtojoki kimberlite pipe, eastern Finland

    NASA Astrophysics Data System (ADS)

    Peltonen, P.; Kinnunen, K. A.; Huhma, H.

    2002-08-01

    Diamondiferous Group A eclogites constitute a minor portion of the mantle-derived xenoliths in the eastern Finland kimberlites. They have been derived from the depth interval 150-230 km where they are inferred to occur as thin layers or small pods within coarse-grained garnet peridotites. The chemical and isotopic composition of minerals suggest that they represent (Proterozoic?) mantle-derived melts or cumulates rather than subducted oceanic lithosphere. During magma ascent and emplacement of the kimberlites, the eclogite xenoliths were mechanically and chemically rounded judging from the types of surface markings. In addition, those octahedral crystal faces of diamonds that were partially exposed from the rounded eclogite xenolith became covered by trigons and overlain by microlamination due to their reaction with the kimberlite magma. The diamonds bear evidence of pervasive plastic deformation which is not, however, evident in the eclogite host. This suggests that annealing at ambient lithospheric temperatures has effectively recrystallised the silicates while the diamond has retained its lattice imperfections and thus still has the potential to yield information about ancient mantle deformation. One of our samples is estimated to contain approximately 90,000 ct/ton diamond implying that some diamonds occur within very high-grade pods or thin seams in the lithospheric mantle. To our knowledge, this is one of the most diamondiferous samples described.

  11. Helium, neon, and argon systematics of the European subcontinental mantle: Implications for its geochemical evolution

    NASA Astrophysics Data System (ADS)

    Dunai, T. J.; Baur, H.

    1995-07-01

    In this study we present a comprehensive noble gas study of mantle xenoliths from various European Cenozoic volcanic provinces. The main body of samples is from the Massif Central, France, and the Eifel, Germany. Smaller subsets of samples are from Spitsbergen and the Graz Basin, Austria. In all the helium, neon, and argon isotopic abundances of a total of forty-five mantle xenoliths, phenocrysts, and xenocrysts were determined. The 3He/4He-ratios within each volcanic province are very uniform, irrespective of the diverse lithologies and P-T conditions which are represented by our sample suite. Mean 3He/4He ratios of the Massif Central, Eifel, Spitsbergen, and Kapfenstein are 6.53 ± .25, 6.03 ± .14, 6.65 ± .25, and 6.1 ± .7 (1σ) times the atmospheric ratio (Ra.), respectively. The strontium and neodymium isotopic composition of some of the cpx-separates are highly variable and therefore in contrast to the uniform He signature. We thus conclude that He and probably also the other noble gases in the xenoliths are effectively decoupled from the non-volatile elements. Therefore, the He signature that is preserved in the xenoliths is actually that of their host magmas. Published strontium, neodymium, and lead isotope data of the unevolved host magmas of the xenoliths correlate well with our xenolith He data. The position of the fields of the investigated volcanic provinces in Hesbnd Sr, Hesbnd Nd, and Hesbnd Pb variation diagrams depict ternary mixtures between DMM-EM-HIMU endmembers as the source the host magmas and their volatiles. The neon isotopic composition of the gases released from the xenoliths is in most cases atmospheric and probably reflects atmospheric contamination; only a few samples reveal indications for MORB-type Ne or evidence of mass-fractionation. The 40Ar/36Ar-ratios of the xenoliths are mostly radiogenic, with the highest ratio being 17,000. However, all samples have suffered a certain degree of atmospheric contamination. We calculate that

  12. 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.

  13. 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.

  14. 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.

  15. 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).

  16. 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.

  17. Crust Formation and Stabilization of the Western Archean Kaapvaal Craton: Evidence from U-Pb Geochronology of Basement Blocks and Deep Crustal Xenoliths from the Kimberley Region, South Africa

    NASA Astrophysics Data System (ADS)

    Schmitz, M. D.; Bowring, S. A.

    2001-05-01

    The kimberlites of the Kimberley region of South Africa have yielded one of the most abundantly sampled and studied suites of lithospheric mantle xenoliths in the world, providing a detailed picture of the composition and thermal evolution of the continental mantle beneath the western Kaapvaal craton. Surprisingly however, little published data exist regarding the nature of the basement and deeper crustal rocks in the western craton, with which to contrast the evolution of the crustal and mantle portions of this Archean cratonic region. Crustal xenoliths collected in the various mine dumps around Kimberley are predominantly large blocks of near-surface basement lithologies, including deformed granitic to tonalitic gneisses and amphibolites, weakly deformed pegmatoids, and non-deformed biotite granite. U-Pb zircon geochronological data for a number of xenoliths have been used to develop a preliminary framework for the age and evolution of the Archean crust of the Kimberley region. The youngest component of the Kimberley basement is a non-deformed sample of biotite granite with an age of 2724+/-2 Ma. A major episode of metamorphism and crustal anatexis is recorded by 2928+/-2 Ga metamorphic zircon growth in amphibolitic and tonalitic components of banded gneisses, and igneous zircons of identical age in weakly deformed cross-cutting pegmatoids. Zircons from these same pegmatoids also have inherited cores which yield 207Pb/206Pb dates as old as 3265 Ma. These inherited zircons, as well as cores of zircons from a foliated granodioritic xenolith with 207Pb/206Pb dates as old as 3184 Ma, indicate the antiquity of the oldest crustal components of the Kimberley basement. These data are consistent with cursory SHRIMP U-Pb zircon geochronological information reported for lithologies collected in situ in the diamond mine walls of Kimberley. Two important implications of this data are considered: first, we interpret the major metamorphism and crustal anatexis at 2.93 Ga as

  18. 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

  19. 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.

  20. 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.

  1. 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

  2. Lithium isotopic composition of the lower continental crust: A xenolith perspective

    NASA Astrophysics Data System (ADS)

    Teng, F.; McDonough, W. F.; Rudnick, R. L.; Tomascak, P. B.; Saal, A. E.

    2003-12-01

    Study of high-grade metamorphic rocks is important for understanding the lithium isotopic composition of the lower crust and determining the degree to which lithium isotopes fractionate during metamorphism. Here we present Li isotopic data for two well-characterized suites of lower crustal xenoliths from North Queesland, Australia [1, 2]. These xenoliths have average major-and trace-element compositions similar to estimates of the bulk lower continental crust, and can therefore be used to place broad constraints on its Li isotopic composition. Furthermore, the Chudleigh xenoliths are a suite of cogenetic cumulates that formed through AFC of Cenozoic basaltic magma that intruded Proterozoic lower crust, and thus may provide insight into the regional average δ 7Li of the lower crust. The samples display a large range of Li concentrations and isotopic compositions with the average, concentration-weighted δ 7Li value = 0. McBride xenoliths are enriched in Li (7 +/- 4 ppm versus 3 +/- 2 ppm) and have a larger range of δ 7Li values (-13 to +7 versus -9 to +6) than Chudleigh xenoliths, consistent with differences in lithology and genetic diversity: McBride xenoliths range from paragneisses to mafic and felsic orthogneisses whereas the Chudleigh xenoliths are mafic cumulates. Mg#, 87Sr/86Sr and 143Nd/144Nd in the Chudeigh suite correlate with δ 7Li, reflecting assimilation of isotopically light Proterozoic lower crust (δ 7Li < -9) by heavier (δ 7Li ˜ +6) mantle-derived basalt. The δ 7Li values of the lower continental crust are heterogeneous, and its average composition is similar to the upper continental crust [3] or lighter; this suggests that the continents are isotopically lighter than the mantle [4]. The isotopically light continental crust is probably derived from two processes involving fluid-rock interactions: surface weathering and metamorphic dehydration. Both processes drive the hydrosphere to heavier (+30) and bulk continental crust to lighter (-1 to 0

  3. 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

  4. Eclogite xenoliths from Wajrakarur kimberlites, southern India

    NASA Astrophysics Data System (ADS)

    Patel, S. C.; Ravi, S.; Thakur, S. S.; Rao, T. K.; Subbarao, K. V.

    2006-09-01

    Mineralogical characteristics of eclogite xenoliths from three kimberlite pipes (KL2, P2 and P10) of the Proterozoic Wajrakarur kimberlite field of southern India have been studied. In a rare sample of enstatite eclogite from the KL2 pipe garnet contains microscopic triangular arrays of needles or blebs of omphacite, enstatite and rutile consistent with an origin by exsolution parallel to the isometric form {111}. Discrete omphacite grains in the sample contain exsolved needles or blebs of enstatite and garnet. Kyanite eclogites are abundant in the KL2 pipe which occasionally show a secondary ring of pure celsian around kyanite grains. Omphacite Na2O contents in the eclogites of the KL2 and P2 pipes are typically between 3 and 6 wt%, and garnet has widely variable composition with end member ranges of Prp22-81Grs0-47Alm10-30Sps0-1Adr0-5Uv0-3. Eclogites of the P10 pipe comprise chromian omphacite and garnet. Phase relations in the ACF projection exhibit systematic increase of the Ca-Tschermak’s component in omphacite from enstatite eclogite through biminerallic eclogite to kyanite eclogite. Garnet-clinopyroxene Fe-Mg geothermometry yields temperatures mostly in the range of 900-1100 °C. A formerly supersilicic nature of garnet in enstatite eclogite as inferred from exsolution mineralogy indicates minimum peak pressure of 5 GPa.

  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. 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.

  7. Lower crustal xenoliths from Junan, Shandong province and their bearing on the nature of the lower crust beneath the North China Craton

    NASA Astrophysics Data System (ADS)

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

    2010-10-01

    Geochronological, petrological and geochemical studies were performed on the granulite xenoliths from a Late Cretaceous basaltic breccia dike in Junan, Shandong province, eastern China. These xenoliths show close similarities to the Nushan granulite xenoliths from the southern margin of the North China Craton (NCC) and the Archean granulite terrains in terms of mineralogy and bulk rock compositions, but are quite different from the Hanuoba mafic granulite xenoliths from the northern NCC. In-situ zircon U-Pb age and Hf isotopic analyses, together with geochemical data reveal that the protolith of these xenoliths was formed around 2.3 Ga ago, through assimilation-fractional crystallization of a mafic magma. P-T conditions of these xenoliths suggest that the lower crust beneath the Junan region reaches to a depth of 35 km, which agree well with the result deduced from various geophysical methods. The consistent petrological and seismic Moho depths, the observed velocity structure and calculated velocity of these xenoliths imply the absence of underplating induced crust-mantle transition zone, which was well formed in the northern NCC. Compared to 40-50 km depth of the lower crust in Early Jurassic, the lower crust beneath Junan extended to a depth of 30 km in Late Cretaceous, suggesting that the lower crust of NCC was significantly thinned during Late Mesozoic.

  8. Sources and fate of xenoliths in the Wooley Creek batholith-a geochemical perspective

    NASA Astrophysics Data System (ADS)

    Barnes, C. G.; Coint, N.; Rämö, O. T.; Barnes, M. A.

    2011-12-01

    xenoliths of diverse lithologic types underwent partial melting, resulting in pyroxene-rich assemblages. Among mafic xenoliths derived from the RCt, lower Zr and higher Sr contents than the protolith suggest that plagioclase was residual during partial melting. In contrast, metasedimentary xenoliths from the eHt have somewhat higher Zr contents compared to their protoliths. These xenoliths contain detrital zircons plus zircons whose U-Pb age is the same as the emplacement age, an indication that zircon was residual in the xenoliths during partial melting. Clearly, partial melts from xenoliths could not uniformly contaminate the host magma. The Nd isotope data suggest that the source terranes of xenoliths in the WCb can be clearly identified. If so, the data indicate that a faint, yet discernable ghost stratigraphy exists in the pluton and that stoping was an important mechanism of magma emplacement. Moreover, the lithologic diversity of the host terranes is NOT preserved in the xenolith suite, which is biased against Sr-rich rocks of the wHt and the abundant ultramafic rocks of the RCt.

  9. 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.

  10. Moho vs crust-mantle boundary: Evolution of an idea

    NASA Astrophysics Data System (ADS)

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

    2013-12-01

    The concept that the Mohorovicic Discontinuity (Moho) does not necessarily coincide with the base of the continental crust as defined by rock-type compositions was introduced in the early 1980s. This had an important impact on understanding the nature of the crust-mantle boundary using information from seismology and from deep-seated samples brought to the surface as xenoliths in magmas, or as tectonic terranes. The use of empirically-constrained P-T estimates to plot the locus of temperature vs depth for xenoliths defined a variety of geotherms depending on tectonic environment. The xenolith geotherms provided a framework for constructing lithological sections through the deep lithosphere, and revealed that the crust-mantle boundary in off-craton regions commonly is transitional over a depth range of about 5-20 km. Early seismic-reflection data showed common layering near the Moho, correlating with the petrological observation of multiple episodes of basaltic intrusion around the crust-mantle boundary. Developments in seismology, petrophysics and experimental petrology have refined interpretation of lithospheric domains. The expansion of in situ geochronology (especially zircon U-Pb ages and Hf-isotopes; Os isotopes of mantle sulfides) has defined tectonic events that affected whole crust-mantle sections, and revealed that the crust-mantle boundary can change in depth through time. However, the nature of the crust-mantle boundary in cratonic regions remains enigmatic, mainly due to lack of key xenoliths or exposed sections. The observation that the Moho may lie significantly deeper than the crust-mantle boundary has important implications for modeling the volume of the crust. Mapping the crust using seismic techniques alone, without consideration of the petrological problems, may lead to an overestimation of crustal thickness by 15-30%. This will propagate to large uncertainties in the calculation of elemental mass balances relevant to crust-formation processes

  11. Distribution of incompatible trace elements between the constituents of spinel peridotite xenoliths: ICP-MS data from the East African rift

    NASA Astrophysics Data System (ADS)

    Bedini, R. M.; Bodinier, J.-L.

    1999-11-01

    To constrain the geochemical models of the lithospheric mantle, we have carried out a detailed study of the distribution of incompatible trace elements between the various constituents of spinel peridotites. Predominant and accessory minerals were separated in 12 mantle xenoliths from Mega (East African Rift, Sidamo region, SE Ethiopia). The samples range in composition from cpx-rich lherzolites to refractory harzburgites and are devoid of modal metasomatism, except for minor amount of apatite in some of them. Their trace element concentration encompasses almost the whole range reported in the literature for basalt-born xenoliths. Mineral separates (ol, opx, cpx, spinel and apatite) and their leachates were analyzed by ICP-MS, for rare earth elements (REE) and several incompatible trace elements (Rb, Sr, Ba, Zr, Hf, Nb, Ta, Th, U, and Ti). Spinel surfaces were investigated by SEM and EPMA to determine the composition of the attached micro-phases. Mass-balance inversion shows that the trace element composition of whole rocks is controlled by five distinct components: 1). the silicate minerals which account for the total HREE abundance, and 50-90% of LREE, Sr, and Zr-Hf, in the apatite-free peridotites; 2). the mineral-hosted fluid inclusions which play a significant role for Rb (20-25%), and to a lesser degree for the other LILEs; 3). a pervasive grain-boundary component selectively enriched in highly incompatible elements, which contributes 25-90% of the whole-rock budget for Ba, Th and U, and 10-50% for Nb and LREE, in the apatite-free samples; 4). thin reaction layers (<10 μm thick) coating the surfaces of spinel grains and mainly composed of Ti-oxides and phlogopite. They are the predominant repository of Nb-Ta (45-60%) and Rb-Ba (30-80%) in all the studied xenoliths; 5). apatite which largely predominates the budget of Th, U, Sr and LREE (25-75%) in the samples containing this mineral. Compared to the other peridotite constituents, fluid-derived inclusions in

  12. 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.

  13. 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.

  14. 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

  15. Source of Mesozoic intermediate-felsic igneous rocks in the North China craton: Granulite xenolith evidence

    NASA Astrophysics Data System (ADS)

    Jiang, Neng; Carlson, Richard W.; Guo, Jinhui

    2011-07-01

    Four intermediate to felsic igneous rocks from the Zhangjiakou region, along the northern margin of the North China craton, have magmatic zircon U-Pb ages from 122 to 144 Ma. Two of these samples have inherited zircon U-Pb ages of ~ 2.5 Ga, similar to the zircon ages of rocks from the surrounding granulite terrain. Zircons from two intermediate composition granulite xenoliths (JN0811 and JN0919) in the nearby Cenozoic Hannuoba basalts yield two groups of ages. The rims have concordant Mesozoic ages mostly between 120 and 145 Ma, coeval with the Mesozoic intermediate-felsic magmatism in the region, while the cores have discordant U-Pb ages with upper-intercepts of ~ 2.5 Ga, overlapping the zircon ages of granulite terrain rocks, and lower-intercept ages of ~ 130 Ma, approximating the ages of the Mesozoic intermediate-felsic magmatism. The Sr-Nd isotopic compositions of the Mesozoic intermediate-felsic igneous rocks are completely different from those expected for basaltic melts from either the lithospheric mantle or the asthenospheric mantle, precluding a derivation by extensive fractional crystallization of mantle-derived magmas. The lack of correlation between (86Sr/87Sr)i, εNd(t) and SiO2 for the Mesozoic igneous rocks, the very narrow range of zircon εHf(t) for individual intermediate-felsic igneous rocks, and simple binary mixing calculations argue against them being formed by mixing between mantle-derived magma and preexisting crust that has extremely evolved Sr-Nd isotopic compositions like granulite xenoliths JN0811 and JN0919. Hf isotopic compositions of the Mesozoic zircons and whole-rock geochemistry show that the granulite xenoliths with extremely evolved Sr-Nd isotopic compositions have not undergone partial melting during the Mesozoic and thus do not contribute to the Mesozoic intermediate-felsic magmas. Further comparisons show that the source rocks for the Mesozoic intermediate-felsic magmas likely were late Archean lower crustal rocks similar in

  16. 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.

  17. Coupling, decoupling and metasomatism: Evolution of crust-mantle relationships beneath NW Spitsbergen

    NASA Astrophysics Data System (ADS)

    Griffin, W. L.; Nikolic, N.; O'Reilly, Suzanne Y.; Pearson, N. J.

    2012-09-01

    The Bockfjord area of NW Spitsbergen (Norwegian Arctic) exposes a long history of crustal evolution, culminating in the Caledonian (400-500 Ma) orogeny; prior to the opening of the N. Atlantic Ocean, this area was part of the Laurentian (Greenland) side of the orogen. The N-striking Breibogen-Bockfjorden (BB) fault marks the western margin of a large graben filled with Devonian redbeds. West of the fault the basement consists of gneisses, schists and granites of the Hekla Hoek formation, inferred to represent a Caldeonian thrust sheet. U-Pb and Hf-isotope data for detrital zircons from this area show that the Hekla Hoek protoliths formed at ca 1.8 Ga, but were heavily reworked ca 800-1000 Ma ago, and again during the Caledonian orogeny. Quaternary alkali-basalt volcanism has provided abundant xenoliths of mantle and crustal rocks from both sides of the BB fault. Lower-crustal xenoliths are mainly mafic to intermediate granulites. Whole-rock Sr-, Nd- and Hf-isotope data for the granulites from both sides of the BB fault show significant disequilibrium, implying the removal of melts late in the evolution of the lower crust. Most zircons from eight xenoliths have Neoarchean/Paleoproterozoic and Paleozoic U-Pb ages; several also contain zircons with ages and/or Hf model ages > 3.2 Ga. The peridotite xenoliths are dominantly spinel lherzolites, metasomatized to varying extents. Xenoliths from basalts east of the BB fault commonly contain metasomatic amphibole, phlogopite and apatite; peridotites from west of the fault rarely display these phases. West of the fault, there is no clear correlation between cpx REE patterns and whole-rock Al contents; east of the fault there is a clear negative correlation between LREE enrichment and whole-rock Al2O3. In-situ Re-Os isotope analysis of sulfides in the peridotites shows another dichotomy. Xenoliths from west of the fault contain sulfides with Re depletion (TRD) model ages extending back to 3.3 Ga, with major populations at 2

  18. Shallow plumbing systems for small-volume basaltic volcanoes, 2: Evidence from crustal xenoliths at scoria cones and maars

    NASA Astrophysics Data System (ADS)

    Valentine, Greg A.

    2012-04-01

    Monogenetic basaltic volcanoes record complex eruption processes and the relationships between those processes and shallow plumbing are poorly understood. This paper explores these relationships, building upon earlier studies of exposed shallow plumbing (in the upper hundreds of meters of crust) beneath volcanoes produced by magmatic eruption processes and those produced by phreatomagmatic processes, such as scoria cones and maars, respectively. Eruptive facies and xenolith abundances are described at three scoria cones and at tephra rings around two maars in the San Francisco Volcanic Field (Arizona, USA). Well-constrained subvolcanic sedimentary stratigraphy provides constraints on the depths of origin of xenoliths of different types. Sedimentary xenolith contents at scoria cones are < 10- 3 and commonly < 10- 4 (volume fraction of xenoliths) and are composed almost entirely of fragments from the uppermost sedimentary formation (Kaibab Formation, depth interval ~ 50-200 m). These xenolith contents are consistent with conduits or dikes that widen mainly in the uppermost tens of meters of the crust, as observed at exposed plumbing systems of eroded scoria cones. Sedimentary xenolith contents in tephra ring deposits at one of the studied maar volcanoes also are typically < 10- 3 (volume fraction) and the remaining fraction is dominated by clasts of pre-maar volcanic rocks that formed an ~ 50 thick surface layer over the sedimentary formations; the second studied maar has much higher xenolith contents but this also appears to be dominated by the shallowest unit. The maars' tephra ring deposits contain xenoliths from all of the major sedimentary units beneath the volcanoes (to depths of ~ 1200 m) but the abundances and proportions of xenoliths are not consistent with the volumes of sub-volcanic units that would be disrupted assuming dimensions that are commonly observed in exposed maar plumbing systems (diatremes). These differences illustrate the different mechanisms

  19. 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.

  20. Effects of melt percolation on highly siderophile elements and Os isotopes in subcontinental lithospheric mantle: A study of the upper mantle profile beneath Central Europe

    NASA Astrophysics Data System (ADS)

    Ackerman, Lukáš; Walker, Richard J.; Puchtel, Igor S.; Pitcher, Lynnette; Jelínek, Emil; Strnad, Ladislav

    2009-04-01

    The effects of melt percolation on highly siderophile element (HSE) concentrations and Re-Os isotopic systematics of subcontinental lithospheric mantle are examined for a suite of spinel peridotite xenoliths from the 4 Ma Kozákov volcano, Bohemian Massif, Czech Republic. The xenoliths have previously been estimated to originate from depths ranging from ˜32 to 70 km and represent a layered upper mantle profile. Prior petrographic and lithophile trace element data for the xenoliths indicate that they were variably modified via metasomatism resulting from the percolation of basaltic melt derived from the asthenosphere. Chemical and isotopic data suggest that lower sections of the upper mantle profile interacted with melt characterized by a primitive, S-undersaturated composition at high melt/rock ratios. The middle and upper layers of the profile were modified by more evolved melt at moderate to low melt/rock ratios. This profile permits an unusual opportunity to examine the effects of variable melt percolation on HSE abundances and Os isotopes. Most HSE concentrations in the studied rocks are significantly depleted compared to estimates for the primitive upper mantle. The depletions, which are most pronounced for Os, Ir and Ru in the lower sections of the mantle profile, are coupled with strong HSE fractionations (e.g., Os N/Ir N ratios ranging from 0.3 to 2.4). Platinum appears to have been removed from some rocks, and enriched in others. This enrichment is coupled with lithophile element evidence for the degree of percolating melt fractionation (i.e., Ce/Tb ratio). Osmium isotopic compositions vary considerably from subchondritic to approximately chondritic ( γOs at 5 Ma from -6.9 to +2.1). The absence of correlations between 187Os/ 188Os and indicators of fertility, as is common in many lithospheric mantle suites, may suggest significant perturbation of the Os isotopic compositions of some of these rocks, but more likely reflect the normal range of isotopic

  1. Les xénolites ultramafiques du volcanisme alcalin quaternaire d'Oranie (Tell, Algérie occidentale), témoins d'une lithosphère cisaillée et enrichieUltramafic xenoliths from Quaternary alkali volcanism from Oranie (Tell, western Algeria): witnesses of a sheared and enriched lithosphere

    NASA Astrophysics Data System (ADS)

    Zerka, Mohamed; Cottin, Jean-Yves; Grégoire, Michel; Lorand, Jean-Pierre; Megartsi, M'Hamed; Midoun, Mohamed

    Numerous ultramafic xenoliths occur within the Aı̈n-Temouchent volcanic complex (Northwestern Oranie, Algeria). Most of them are type I mantle tectonites (lherzolites and harzburgites) and composite xenoliths (harzburgite/clinopyroxenite) are rare. Only a few samples of spinel lherzolites display relatively fertile compositions when the major part of type I xenoliths have refractory major element compositions but enriched LREE contents showing that they have been affected by mantle metasomatism. The composite xenoliths are witnesses of reactions of alkaline magmas with the upper mantle. An asthenospheric rising, in relation with the large strike slip fault affecting the North African plate margin at Trias time is proposed as a possible geodynamical setting. To cite this article: M. Zerka et al., C. R. Geoscience 334 (2002) 387-394.

  2. 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

  3. Kimberlite and related rocks from Garnet Lake, West Greenland, including their mantle constituents, diamond occurrence, age and provenance

    NASA Astrophysics Data System (ADS)

    Hutchison, Mark T.; Frei, Dirk

    2009-11-01

    Observations of thickness, orientation and morphology and mineral chemistry of the principal diamondiferous intrusive sheet and associated bodies in the vicinity of Garnet Lake, Sarfartoq, West Greenland are reported. The principal body dips to the east on a NE/SW (true) trend and reaches a maximum thickness of 4.25 m. Multiple intrusive events are identified within the main sheet including sub-parallel bands occasionally exhibiting grain size sorting, cross-cutting layers and late-stage carbonate-rich emplacement, particularly at the contacts with country rock. Phenocrystic mineral assemblages and compositional measurements reveal two principal petrological types. The dominant type is an aillikite and the second rock type is a kimberlite. The kimberlite exhibits thin Ba-rich rims (towards kinoshitalite) on Al-rich phlogopite crysts, and an abundance of perovskite. Compositional zonation in groundmass spinels suggest a later transition towards an aillikite component. The aillikite is characterised by abundant phlogopite, heavily zoned with tetraferriphlogopite rims, transitional Type 1-Type 2 spinel compositions, rare Al,Ti-rich groundmass clinopyroxene and occasional exotic Sr-carbonate phases such as olekminskite. The Garnet Lake main sheet is characterised by mantle phases occurring as individual grains, most strikingly as garnet xenocrysts up to 5 mm and disaggregated mantle olivine crysts. Xenoliths occur rarely and are typically garnet dunites and garnet lherzolites. Heavy mineral separation reveals an abundance of G10D garnets and, whilst peridotitic garnets dominate, eclogitic G3D and G4D garnets also occur. Trace element compositions of garnet crysts reveal sinusoidal REE patterns in harzburgitic garnets however a component of flat and REE-enriched G11 garnets is apparent, reflecting significant mantle refertilisation. Thermorbarometric calculations on assemblages in Garnet Lake main sheet garnet lherzolites reveal equilibrium conditions clustering closely

  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. Composition of the lithospheric mantle in the Siberian craton : New constraints from fresh peridotites from the Udachnaya-East Kimberlite

    NASA Astrophysics Data System (ADS)

    Doucet, Luc-Serge; Ionov, Dmitri A.; Ashchepkov, Igor

    2010-05-01

    contents. The broad range of heavy REE appears to be controlled by the presence and the abundance of garnet and is also related to microstructures such that granular spinel harzburgites have lower HREE contents than "fertile" porphyroclastic garnet lherzolites. Trace elements in cpx and garnet have equilibrated patterns in porphyroclastic peridotites and complex sinusoidal shapes in granular peridotites. Bulk-rock major element compositions show important variations in Mg# (0.89 - 0.93), SiO2 (41.5 - 46.6%), Al2O3 (0.3 - 4%) and CaO (0.3 - 4%). As for compatible trace elements, the major element compositions appear to be related to microstructures. Calculated modal compositions show highly variable opx contents (4.5 - 24%), which are generally lower than in Kaapvaal peridotites but are similar to those from the North Atlantic craton [3]. Overall, modal compositions and the contents of low-mobility elements, are consistent with an origin by variable degrees of partial melting of fertile mantle [1-3]. The range in FeO contents (6-8.5%) may indicate either variable melting depths [2] or post-melting enrichments. Enrichments in SiO2 show some similarities to those in supra-subduction xenoliths [4]; enrichments in highly incompatible elements can be explained by metasomatism with possible involvement of subduction-related fluids. Strong correlations between chemical compositions and microstructures indicate the involvement of tectonic processes in melt percolation and metasomatism. We suggest that the cratonic lithosphere in Siberia was formed in three stages: (1) formation of proto-cratonic mantle by high-degree melting at variable depth, (2) accretion of the proto-craton domains in subduction-related settings, (3) metasomatism commonly accompanied by deformation. [1] Boyd et al (1997) Contrib. Mineral. Petrol. 128, 228-246. [2] Herzberg (2004) J. Petrol. 45, 2507-2530. [3] Wittig et al (2008) Lithos 71, 289-322. [4] Ionov (2009) J. Petrol. In press

  6. Eclogite xenoliths in west African kimberlites as residues from Archaean granitoid crust formation

    NASA Astrophysics Data System (ADS)

    Rollinson, Hugh

    1997-09-01

    Eclogites are a comparatively rare but petrologically important member of kimberlite xenolith suites. Their broadly basaltic chemistry has led many authors to propose that they represent ancient, subducted ocean crust. Recent studies, however, have suggested an alternative origin and propose that kimberlitic eclogites are residues from the process of Archaean granitoid crust formation. Geochemical arguments in support of this new model were previously based on the trace-element chemistry of eclogitic minerals. Here I report that the major-element chemistry of eclogite xenoliths also supports a crustal residue model. I examine eclogite xenoliths from kimberlite pipes at Koidu, Sierra Leone, which sample the lithospheric mantle underlying the Archaean (2.8Gyr) granitoid crust of the West African craton. Geochemical plots of major elements measured in unaltered, whole-rock samples of low-silica eclogite demonstrate that they are complementary to the granitoids of the West African craton and have compositions which indicate that both were derived from a common basaltic parent rock.

  7. Mantle Heterogeneity Beneath Europe Resolved from GOCE Satellite Gravity

    NASA Astrophysics Data System (ADS)

    Herceg, M.; Artemieva, I. M.; Thybo, H.

    2014-12-01

    The objectives of this study is to construct a continent-scale model of density variations in the European upper mantle. The residual mantle gravity anomalies are derived from the GOCE satellite gravity data, from which gravitational effects of the deep mantle and the crust are removed. We take advantage of a recently released seismic model EUNAseis for the crustal structure of the European continent and the adjacent regions. We examine the propagation of crustal model uncertainties into determinations of lithospheric mantle density. Given a relatively small range of expected density variations in the lithospheric mantle, knowledge on the uncertainties associated with incomplete knowledge of density structure of the crust is of utmost importance for further progress in density heterogeneity studies. Our model of mantle density structure has lateral resolution of ca. 100 km, which allows to distinguish small-scale mantle anomalies and to link them to regional geodynamic processes. To understand better geodynamic causes of mantle density heterogeneity, we compare mantle residual gravity anomalies for the European upper mantle with regional upper mantle velocity structure constrained by high-resolution seismic tomography studies, and compare our regional upper mantle density model with petrological studies of mantle-derived xenoliths from the Baltic shield, the Arkhangelsk region, and worldwide.

  8. Determining the geochemical structure of the mantle from surface isotope distribution patterns? Insights from Ne and He isotopes and abundance ratios

    NASA Astrophysics Data System (ADS)

    Stroncik, N.; Niedermann, S.; Schnabel, E.; Erzinger, J.

    2011-12-01

    It is a common procedure among geochemists to use surface distribution patterns of e.g. Sr, Nd, Pb or He isotopes of lavas erupted at oceanic islands to map the geochemical structure of the Earth's mantle. Advances in noble gas mass spectrometry within the last decade resulting in an increasing availability of Ne isotope data sets allow us to test the strength of this approach. 4He and 21Ne are coupled through the same parent nuclides and therefore should show analogous isotope distribution patterns. Here we present He and Ne fusion data of fresh olivines derived from Big Island, Hawaii, together with He and Ne fusion data of fresh glasses from the Easter Seamount Chain (ESC), indicating that the observed isotope distribution patterns are mainly controlled by melting and shallow mixing processes. He isotopic ratios of the investigated olivines vary from MORB-like (8 ± 1 RA) to ratios more typical for a primitive mantle source (up to 20 and 26 RA for Hawaii and the ESC, respectively; RA = atmospheric 3He/4He ratio of 1.39 x 10-6). In contrast, all Ne isotope data plot within error limits along the Loihi-Kilauea line in a Ne three-isotope diagram. The Loihi-Kilauea line is regarded to be typical for a primitive mantle source. Thus, the Ne isotope data are inconsistent with any kind of zoned plume model or even a heterogeneous mantle source. The combined He and Ne data show that these He and Ne isotope systematics are produced by a pre-degassing fractionation process and subsequent melt mixing. Basically, this process causes a He deficit in melts generated by the plume, as shown by 3He/22NeS below current estimates of solar or planetary composition and 4He/21Ne* lower than the production ratio, making the He isotopic composition more susceptible to changes than the Ne isotopic composition. This can best be explained by a model in which He is fractionated from Ne during formation of melts from a plume (or enriched parts of a plume) at low melting degrees, which

  9. 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

  10. 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

  11. 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.

  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. Origin and U-Pb dating of zircon-bearing nepheline syenite xenoliths preserved in basaltic tephra (Massif Central, France)

    NASA Astrophysics Data System (ADS)

    Paquette, Jean-Louis; Mergoil-Daniel, Juliette

    2009-08-01

    Zircon-bearing xenoliths in continental basalts are often interpreted as witnesses of the continental basement uplifted during volcanic eruptions. Nevertheless, their origin is still debated. The Devès basaltic plateau belongs to the alkaline volcanic province of the French Massif Central. In few outcrops, zircon-bearing nepheline syenite xenoliths were preserved. U-Pb dating of the zircon crystals define an age of 956 ± 11 kyr constraining the crystallisation time of the zircons and consequently of the host xenoliths. This age, together with mineral chemistry arguments lead us to conclude that these minerals do not derive from a continental protolith. Rather, they likely result from the crystallisation of a liquid characterised by a nepheline-felspar composition and produced by the differentiation of a basaltic magma or, alternatively, by the low degree partial melting of a metasomatised lithospheric mantle. Such alkaline sialic rock and xenoliths may occur in large volumes at depth and generate the large amounts of zircon megacrysts discovered worldwide in secondary deposits within continental basaltic provinces.

  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. PMID:17843357

  15. Heterogeneous magnesium isotopic composition of the lower continental crust: A xenolith perspective

    NASA Astrophysics Data System (ADS)

    Teng, Fang-Zhen; Yang, Wei; Rudnick, Roberta L.; Hu, Yan

    2013-09-01

    We report 26 high-precision whole-rock Mg isotopic analyses for two suites of well-characterized granulite xenoliths from Chudleigh and McBride, North Queensland, Australia, in order to constrain the behavior of Mg isotopes during deep crustal processes and the Mg isotopic composition of the lower continental crust. Previous studies suggest that the Chudleigh granulites are a suite of cogenetic cumulates crystallized from mafic magmas that intruded into and assimilated the preexisting lower crust via combined assimilation and fractional crystallization (AFC). The δ26Mg values of the xenoliths range from -0.31 to -0.21‰ and correlate with radiogenic isotopes, reflecting mixing of mantle-derived mafic magma (δ26Mg = -0.31‰) with preexisting isotopically heavy crustal materials (δ26Mg = ˜ +0.5‰) through the AFC process. The McBride granulites range compositionally from mafic to felsic, and originated as cumulates, solidified mafic/felsic melts, and restites that formed during basaltic underplating and reworking of preexisting lower crust. Their δ26Mg values vary widely from -0.72 to +0.19‰. The large Mg isotopic variation in the McBride xenoliths reflects both distinct source compositions and metamorphic enrichment of garnet, which is isotopically light. Based on these results, the lower continental crust has a heterogeneous Mg isotopic composition, with a weighted average δ26Mg of -0.18‰. The bulk continental crust, based on available data, has an average Mg isotopic composition of -0.19‰, and is slightly heavier than the mantle. The highly heterogeneous Mg isotopic distribution in the crust indicates that chemical weathering not only modifies the upper crust compositions but also significantly influences lower crust compositions through emplacement of upper crustal materials into the deep crust.

  16. 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.

  17. Thermal history and origin of the Tanzanian Craton from Pb isotope thermochronology of feldspars from lower crustal xenoliths

    NASA Astrophysics Data System (ADS)

    Bellucci, Jeremy J.; McDonough, William F.; Rudnick, Roberta L.

    2011-01-01

    Common and radiogenic Pb isotopic compositions of plagioclase and anti-perthitic feldspars from granulite-facies lower crustal xenoliths from the Labait Volcano on the eastern margin of the Tanzanian Craton have been measured via laser ablation MC-ICP-MS. Common Pb in plagioclase and a single stage Pb evolution model indicate that the lower crust of the Tanzanian Craton was extracted from mantle having a 238U/ 204Pb of 8.1 ± 0.3 and a 232Th/ 238U of 4.3 ± 0.1 at 2.71 ± 0.09 Ga (all uncertainties are 2σ). Since 2.4 Ga, some orthoclase domains within anti-perthites have evolved with a maximum 238U/ 204Pb of 6 and 232Th/ 238U of 4.3. The spread in Pb isotopic composition in the anti-perthitic feldspars yields single crystal Pb-Pb isochrons of ˜ 2.4 Ga, within uncertainty of U-Pb zircon ages from the same sample suite. The Pb isotopic heterogeneities imply that these granulites resided at temperatures < 600 °C in the lower crust of the Tanzanian Craton from ca. 2.4 Ga to the present. In concert with the chemistry of surface samples, mantle xenoliths, and lower crustal xenoliths, our data imply that the cratonic lithosphere in Tanzania formed ca. ˜ 2.7 Ga, in a convergent margin setting, and has remained undisturbed since 2.7 Ga.

  18. 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.

  19. 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.

  20. Mineral chemistry and thermobarometry of peridotite xenoliths from Central Tien Shan basalts

    NASA Astrophysics Data System (ADS)

    Egorova, V.; Batalev, V.; Simonov, V.; Bagdassarov, N.; Litasov, Yu.

    2009-04-01

    Meso-Cenozoic basaltoids were revealed in the Tien Shan on a vast area (>285.000 km2), from the mountainous framing of the Fergana basin in the west to the Dzhungar Alatau spurs in the east. They occur as dikes and stocks among the Paleozoic Tien Shan complexes. Basaltic flows and sills were found among the deposits of the Suluterek Formation localized in the basement of the section of continental sediments filling the Tien Shan neotectonic depressions. In the Toyun basin in China and in the Fergana valley, basaltic flows and sills occur among Cretaceous-Paleogene marine deposits. Meso-Cenozoic effusive bodies are mainly olivine and plagioclase basalts. The trace- and rare-earth-element compositions of rocks show that most of the studied basaltic series in the Tien Shan formed in within-plate magmatic systems related to mantle plume sources. Spinel lherzolite xenoliths were found in basalts from Ortosuu sites located in Kyrgyzstan. New basalt sites with ultramafic xenoliths have high significance for reconstruction of the composition, structure and evolution of the upper mantle of Tien Shan and geodynamic processes in Central Asia. Spinel lherzolite xenoliths are characterized by an anhydrous four-phase mineral assemblage: olivine, clinopyroxene, orthopyroxene, and brown Cr-spinel. Peridotite textures are largely protogranular. Rock forming minerals have high Mg# = 0.87-0.91 in Ol, 0.87-0.91 in Opx, 0.88-0.91 in Cpx, and 0.75-0.77 in Sp. NiO content in olivine reaches 0.4 wt.%. Clinopyroxenes are Cr-diopside and characterized by high Al2O3 (4.6-7.5 wt%), Cr2O3 (0.7 - 1.11 wt%), Na2O (1.4-1.7 wt%) contents and 0.44-0.5 Ca/(Ca+Mg) ratio. Cr/(Cr+Al) ratio in spinel is equal to 0.09-0.18. Temperatures of equilibration for spinel lherzolites range from 920 to 1150C for Opx-Cpx thermometer and from 920 to 1070C for Ca-in-opx thermometer of Brey, Kohler (1990) and 912-1080C for Opx-Cpx thermometer of Wells (1977). Pressures calculated from Cpx barometer of Nimis (1999

  1. An Ultra-Depleted Mantle Component in the Ontong Java Plateau Revealed by Major, Trace and Volatile Element Abundances in Olivine-Hosted Melt Inclusions

    NASA Astrophysics Data System (ADS)

    Jackson, M. G.; Cabral, R. A.; Rose-Koga, E. F.; Koga, K. T.; Price, A. A.; Hauri, E. H.; Michael, P. J.

    2014-12-01

    The Ontong Java Plateau (OJP) represents the most voluminous large igneous province (LIP) preserved in the geologic record. The most voluminous volcanic stages of the OJP—the Kroenke and Kwaimbaita stages, which dominate the accessible portions of the plateau—have relatively flat primitive mantle normalized rare earth element (REE) patterns, or spidergrams. With the exception of relatively small volumes of late-stage melts—referred to as the Singgalo stage—that are characterized by slightly enriched REE spidergrams, the volcanic stages that dominated the eruptive history of the OJP exhibit remarkably homogeneous, flat REE patterns. Here we isolate, for the first time, olivine-hosted melt inclusions from OJP. We show that the melt inclusions have two clear populations defined by having distinct trace element characteristics. The first population has relatively flat trace element patterns that are similar to that observed in whole rock lavas from the most voluminous volcanic stages (Kroenke and Kwaimbaita stages) recorded in the OJP. In contrast, a second group of melt inclusions, referred to as UDM (ultra-depleted melt) inclusions, exhibit strikingly depleted REE spidergrams; these trace element patterns are far more depleted than any previously reported lava from OJP. The UDM have unique trace element signatures that preclude an origin by assimilation of hydrothermally-altered oceanic crust or re-melting the depleted mantle source left over after melt extraction during construction of the OJP. We interpret the new UDM compositions to be the result of melting of a previously unrecognized ultra-depleted component hosted in the OJP mantle source.

  2. The really "stealth" mantle metasomatism

    NASA Astrophysics Data System (ADS)

    Puziewicz, Jacek; Matusiak-Małek, Magdalena; Ntaflos, Theodoros; Grégoire, Michel; Kukuła, Anna; Wojtulek, Piotr

    2015-04-01

    The Lower Silesian/Upper Lusatian domain of European subcontinental lithospheric mantle is dominated by two kinds of harzburgites: A - not affected or slightly affected by silicate melt metasomatism related to migration of lavas during formation of Cenozoic Central European Volcanic Province, and B - strongly overprinted by those lavas (Puziewicz et al. 2015, IJES, DOI 10.1007/s00531-014-1134-2). The study of Matusiak-Małek et al. (2014, J Petrol 55, 1799-1828) shows that the A harzburgites untouched by metasomatic events contain no clinopyroxene. Part of the A harzburgites contains clinopyroxene which has "primary" appearance but was added to the host during metasomatic event(s) overprinting the primary mineral assemblage. The metasomatic nature of this clinopyroxene can be recognized by its major and trace element chemical composition, and the mineral is a good example of the "stealth" metasomatic phase (O'Reilly & Griffin 2013, Springer). One of the typical features of this kind of clinopyroxene are LREE enriched REE patterns. We have discovered single xenoliths containing clinopyroxene with LREE depleted patterns in Steinberg near Görlitz (Lower Silesian/Upper Lusatian Region) and in Feldstein near Suhl (Heldburger Gangschar in Thuringia). Usually these kind of REE patterns is considered to be a relic of primary mineral assemblage subjected to strong melt-depletion. However, clinopyroxene from Steinberg is texturally late phase. Its major element chemical composition suggests that it is not a residue after partial melting, but a late silicate-melt metasomatic addition to the host rock which preceded the xenolith entrainment in the erupting lava. Thus, the metasomatising melt must have had characteristics enabling the precipitation of LREE depleted clinopyroxene. The existence of such the melts is clearly shown by the clinopyroxene from websterite cumulate from Dobkovičky in Eger Rift (Ackerman et al. 2012, J Geosci 58, 199-219), which has LREE depleted

  3. Carbonate-rich melt infiltration in peridotite xenoliths from the Eurasian-North American modern plate boundary (Chersky Range, Yakutia)

    NASA Astrophysics Data System (ADS)

    Tschegg, Cornelius; Ntaflos, Theodoros; Akinin, Vyacheslav V.; Hauzenberger, Christoph

    2012-09-01

    A suite of mainly spinel peridotite and subordinate pyroxenite xenoliths and megacrysts were studied in detail, enabling us to characterize upper mantle conditions and processes beneath the modern North American-Eurasian continental plate boundary. The samples were collected from 37-Ma-old basanites cropping out in the Main Collision Belt of the Chersky Range, Yakutia Republic (Russian Far East). The spinel lherzolites reflect a mantle sequence, equilibrated at temperatures of 890-1,025 °C at pressures of 1.1-2 GPa, with melt extraction estimated to be around 2-6 %. The spinel harzburgites are characterized by lower P-T equilibration conditions and estimated melt extraction up to 12 %. Minor cryptic metasomatic processes are recorded in the clinopyroxene trace elements, revealing that percolating hydrous fluid-rich melts and basaltic melts affected the peridotites. One of the lherzolites preserves a unique melt droplet with primary dolomite in perfect phase contact with Na-rich aluminosilicate glass and sodalite. On the basis of the well-constrained P-T frame of the xenolith suite, as well as the rigorously documented melt extraction and metasomatic history of this upper mantle section, we discuss how a carbonated silicate melt infiltrated the lherzolite at depth and differentiated into an immiscible carbonate and silicate liquid shortly before the xenolith was transported to the surface by the host basalt. Decreasing temperatures triggered crystallization of primary dolomite from the carbonate melt fraction and sodalite as well as quenched glass from the Na-rich aluminosilicate melt fraction. Rapid entrainment and transport to the Earth's surface prevented decarbonatization processes as well as reaction phenomena with the host lherzolite, preserving this exceptional snapshot of upper mantle carbonatization and liquid immiscibility.

  4. 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

  5. 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

  6. 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

  7. The Pb isotopic compositions of lower crustal xenoliths and the evolution of lower crustal Pb

    NASA Astrophysics Data System (ADS)

    Rudnick, Roberta L.; Goldstein, Steven L.

    1990-05-01

    Pb isotopic compositions for three suites of well-characterized granulite facies xenoliths from a diversity of crustal settings (the Chudleigh and McBride volcanic provinces, Queensland, Australia and the Eifel volcanics, West Germany) are presented here. All three suites plot to the right of the 4.57 Ga geochron, similar to the published Pb results of other mafic granulite xenoliths. Correlations between Sr, Nd and Pb isotopes in the three suites measured here point to an origin by mixing of mantle-derived basaltic magmas with lower crust at the time of basaltic underplating (i.e., < 100 Ma for Chudleigh, ˜ 300 Ma for McBride, ˜ 450 Ma for Eifel). Because the Pb concentration of the continental crust is much greater than that of mantle-derived basaltic magmas, the Pb isotopic compositions of the magmas are shifted dramatically by the mixing, allowing delineation of the isotopic characteristics of the lower crust. In all three cases, this lower crust had radiogenic Pb and Sr isotopic compositions and unradiogenic Nd isotopic compositions, yielding Proterozoic Nd model ages. Such radiogenic lower crust contrasts markedly with the Pb isotopic characteristics of most Precambrian granulite facies terrains. Whereas the Nd isotopes reflect the average age of crust formation, the Pb isotopic characteristics of the lower crust appear to be a function of the tectonothermal age of the crust: unradiogenic Pb can only develop in regions which have remained stable for long time periods (e.g., cratons), whereas in areas where orogenies have occurred subsequent to crustal formation, the Pb isotopic composition of the lower crust is "rejuvenated" through mixing with radiogenic Pb from upper crust and mantle-derived magmas. Thus, after orogeny, the Pb isotopic composition of the lower crust resembles that of the upper crust. On the basis of this proposed orogenic age-Pb isotope correlation, we estimate the Pb isotopic composition of the lower crust using the data for granulite

  8. 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

  9. 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

  10. 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

  11. 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

  12. 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

  13. Cumulate Origins of the High MgO Eclogite Xenoliths from Koidu, West Africa

    NASA Astrophysics Data System (ADS)

    Barth, M. G.; Rudnick, R. L.; Horn, I.; McDonough, W. F.; Spicuzza, M. J.; Valley, J. W.; Haggerty, S. E.

    2001-05-01

    Xenolithic eclogites (bi-mineralic rocks composed of garnet and omphacite) from kimberlite pipes equilibrated at upper mantle pressures and temperatures, but the origin of the protoliths of these rocks and their subsequent metamorphic evolution are less certain. Xenolithic eclogites have been variously interpreted as crystallized high-pressure magmas or their cumulates, or as fragments of recycled oceanic crust. Eclogites from the Koidu kimberlite pipe, Sierra Leone, fall into two groups based on their major element chemistry. The low MgO group (6-13 wt% MgO) shows evidence for being derived from recycled oceanic crust that has been through a melting episode associated with subduction (Barth et al., 2001, GCA, in press). In contrast, the high MgO eclogites (>16 wt% MgO) have mantle-like δ 18O. Analyses of multiple mineral generations suggest that their variable light rare earth element (REE) patterns reflect variable degrees of metasomatic enrichment. High MgO and Al2O3 contents of these eclogites are suggestive of a cumulate origin, either as high-pressure (2-3 GPa) garnet-pyroxene cumulates or low-pressure (<1 GPa) plagioclase-pyroxene-olivine cumulates. The absence of modal olivine and orthopyroxene coupled with liquid lines of descent of mantle-derived magmas suggest that these eclogites formed at pressures lower than their current equilibrium pressures (>4 GPa). The high proportion of modal garnet (up to 65%) is difficult to explain as a cumulate phase in light of the relatively flat heavy to middle REE patterns. This, coupled with positive Sr anomalies, and low to moderate transition element contents is consistent with a low-pressure cumulate origin of the high MgO eclogites as metamorphosed olivine gabbros and troctolites. If this interpretation is correct, the high MgO eclogites could represent either the basal section of subducted oceanic crust or foundered mafic lower continental crust.

  14. Sr, Nd, Pb and Hf Isotopic Compositions of Late Cenozoic Alkali Basalts in South Korea: Evidence for Mixing Between the Two Dominant Asthenospheric Mantle Domains beneath East Asia

    NASA Astrophysics Data System (ADS)

    Choi, S.; Mukasa, S. B.; Kwon, S.; Andronikov, A. V.

    2004-12-01

    We determined the Sr, Nd, Pb and Hf isotopic compositions of late Cenozoic basaltic rocks from six lava-field provinces in South Korea, including Baengnyeong Island, Jogokni, Ganseong area, Jeju Island, Ulleung Island and Dog Island, in order to understand the nature of the mantle source. The basalts have OIB-like trace element abundance patterns, and also contain mantle-derived xenoliths. Available isotope data of late Cenozoic basalts from East Asia, along with ours, show that the mantle source has a DMM-EM1 array for northeast China and a DMM-EM2 array for Southeast Asia. We note that the basalts falling on an array between DMM and an intermediate end member between EM1 and EM2, are located between the two large-scale isotopic provinces, i.e., around the eastern part of South Korea. The most intriguing observation on the isotopic correlation diagrams is spatial variation from predominantly EM2 signatures in the basaltic lavas toward increasingly important addition of EM1, starting from Jeju Island to Ulleung and Dog Islands to Ganseong area, and to Baengnyeong Island. This is without any corresponding changes in the basement and the lithospheric mantle beneath the region. These observations suggest that the asthenospheric mantle source is dominant for the Cenozoic intraplate volcanism in East Asia, which is characterized by two distinct, large-scale domains. Previous studies on East Asian Cenozoic volcanic rocks have invoked origins by either plume activity or decompressional melting in a rift environment. On the basis of our new trace element and isotopic compositions which have OIB-like characteristics, we prefer a plume origin for these lavas. However, because tomographic images do not show distinct thermal anomaly that would be interpreted as a plume, we suggest that the magmatism might be the product of small, difficult to image multiple plumes that tapped the shallow part of the asthenosphere (probably the transition zone in the upper mantle).

  15. 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

  16. The geochemical signature of fluid-saturated magma determined from silicate melt inclusions in ascension island granite xenoliths

    NASA Astrophysics Data System (ADS)

    Webster, J. D.; Rebbert, C. R.

    2001-01-01

    Silicate glasses in crystal-free to crystal-poor melt inclusions from two coarse-grained granite xenoliths in alkaline volcanic rocks of Ascension Island were analyzed for 29 major, minor, and trace elements. For most constituents, the glass compositions are similar to those of the volcanic whole rocks and the xenoliths; we interpret the glasses to be chemically representative of granite melt. The melt inclusions are silicic; alkaline; contain low S and P abundances; and are enriched in H 2O, Cl, F, and Na relative to K. Inclusions from one xenolith contain 1.3 wt.% F, on average, whereas those from the other xenolith contain half that amount. The melt inclusion compositions allow investigation of the means and extent of granite magma evolution. The presence of magmatic fluid inclusions in close proximity to melt inclusions in the phenocrysts indicates that the granite melt was saturated in one or more volatile phases (Roedder and Coombs, 1967). The Cl and H 2O contents of the melt inclusions are consistent with the exsolution of volatile phase(s) at pressures of 3000 to 2000 bars. The glasses also show trends involving the (Cl/H 2O) ratio and the incompatible trace elements in melt that are indicative of melt evolution via crystal fractionation of volatile phase-saturated melt. These trends should be useful for recognizing volatile phase saturation in other Cl-enriched melts.

  17. 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.

  18. 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

  19. Subduction zone Hf-anomalies: Mantle messenger, melting artefact or crustal process?

    NASA Astrophysics Data System (ADS)

    Woodhead, Jon; Hergt, Janet; Greig, Alan; Edwards, Louise

    2011-04-01

    The origin of Hf elemental depletions in subduction zone magmas is investigated using new major- and trace-element data for cumulate xenoliths from the Mariana arc, and deep sea sediments recovered by the DSDP and ODP drilling programmes. Results indicate that most of the rare earth element (REE) and Hf inventory in the xenoliths is contained within two minerals—clinopyroxene and titanomagnetite—and that removal of a typical gabbroic fractionating assemblage reduces the depletion in Hf relative to neighbouring REE on a mantle normalised trace element diagram (commonly denoted Hf/Hf*) in the evolving magmas. Confirmation of this observation is provided by a variety of literature data from different subduction zones in which bulk-rock samples also define a positive correlation between Hf/Hf* and the silica content of the magmas. In agreement with experimental studies on REE-HFSE partitioning, we observe that the ability of clinopyroxene to influence the Hf/Hf* of fractionating magmas is associated with its aluminium content. This decoupling of Hf from the REE in differentiating arc magmas suggests that bulk rock Hf/Hf* values, when used in isolation, are unlikely to provide a robust measure of source REE-Hf characteristics, even when suites are filtered to exclude all but the most mafic samples. It may be possible to normalise data to a constant degree of fractionation, and in this way distinguish subtle changes in source Hf/Hf* but most existing datasets are of neither the size nor quality to attempt such calculations. Modification of Hf/Hf* is also seen when modelling mantle melting processes and there are strong suggestions that source variations are influenced by not only subducted sediment, which exhibits a remarkably wide range in Hf/Hf*, but also subduction zone fluids. These observations remove some of the constraints imposed on recent models that attempt to reconcile Hf isotope data with Hf-REE abundance data in some arc suites. Although a case may be

  20. 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.

  1. Back-arc Mantle Evolution inferred from Peridotite Xenotlishs from the Japan Sea

    NASA Astrophysics Data System (ADS)

    Morishita, T.; Ichiyama, Y.; Tamura, A.; Arai, S.

    2014-12-01

    Peridotite xenoliths are recovered in basaltic to andesitic lavas from several localities in the Japan Sea, a Miocene back-arc basin of the Western Pacific region. These peridotites are classified into two groups: two-pyroxene peridotitess and dunite-wehrlite groups. Although slight chemical modifications are observed in these peridotite samples, two-pyroxene peridotite group has retained their original residual mantle geochemical signatures left after partial melting. The dunite-wehrlite group is, on the other hand, probably formed by extensive interaction of the two-pyroxene peridotite group. We examined trace element characteristics of clinopyroxene in these xenoliths. Light REE-depleted clinopyroxenes that are usually interpreted as a simple residual mantle after anhydrous partial melting are similar to those of abyssal peridotite recovered from mid-ocean ridges and back-arc basins. Other samples show LREE-enriched patterns that are residues after influx melting caused by hydrous melt/supercritical fluids released from the subducted slab at high pressure conditions. The geotectonic and geochemical variations of the peridotite xenoliths from the Japan Sea (Shirabeshi Seamount, Seifu Seamount and Oshima-Oshima Island) suggest that the mantle beneath the Japan Sea are suffered from hydrous to anhydrous melting as the Japan Sea forms. This is consistent with the geochemical and isotopic results from Miocene basaltic rocks formed during opening of the Japan Sea (Sato et al., Jour. Petrol., 2013). The Japan Sea peridotite xenoliths also shed lights on the origin of ophiolites.

  2. 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.

  3. An application of GOCE satellite gravity to resolve mantle heterogeneity in Europe

    NASA Astrophysics Data System (ADS)

    Herceg, Matija; Artemieva, Irina M.; Thybo, Hans

    2015-04-01

    The aim of this study is to obtain new information on the density structure of the European upper mantle by incorporating the state-of-the-art global gravity data derived from the GOCE satellite gravity mission and recently released seismic model for the crustal structure, EUNAseis. The residual mantle gravity anomalies are derived from the GOCE data, from which gravitational effects of the deep mantle and the crust are removed. Our model of mantle density structure has lateral resolution of ca. 100 km, which allows to distinguish small-scale mantle anomalies and to link them to regional geodynamic processes. Given a relatively small range of expected density variations in the lithospheric mantle, knowledge on the uncertainties associated with incomplete knowledge of density structure of the crust is of utmost importance for further progress in density heterogeneity studies. Therefore, we examine the propagation of crustal model uncertainties into determinations of lithospheric mantle density. To understand better geodynamic causes of mantle density heterogeneity, we compare mantle residual gravity anomalies for the European upper mantle with upper mantle velocity structure constrained by seismic tomography. Furthermore, we compare our regional upper mantle density model with petrological studies of mantle-derived xenoliths from the Baltic shield and the Arkhangelsk region.

  4. The mantle and basalt-crust interaction below the Mount Taylor Volcanic Field, New Mexico

    NASA Astrophysics Data System (ADS)

    Schrader, C. M.; Crumpler, L. S.; Schmidt, M. E.

    2010-12-01

    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 ~3.8-1.5 Ma (K-Ar, Perry et al., 1990). 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 (Perry et al., 1990). Mantle xenolith-bearing alkali basalts and basanites occur on Mesa Chivato (Crumpler, 1980) 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 cone-building. 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 (Perry et al., 1990), 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

  5. 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

  6. Lithium partitioning between olivine and diopside at upper mantle conditions: An experimental study

    NASA Astrophysics Data System (ADS)

    Yakob, Jessica L.; Feineman, Maureen D.; Deane, James A.; Eggler, David H.; Penniston-Dorland, Sarah C.

    2012-05-01

    Experiments were conducted at 1.5 GPa and temperatures between 700 °C and 1100 °C in order to assess the equilibrium distribution of lithium between olivine and diopside in the upper mantle. Lithium in olivine and diopside from natural mantle xenoliths displays a broad array of apparent partition coefficients ranging from ~ 0.2 to 10. In addition, a strikingly large range of lithium isotope ratios is observed in olivine and diopside from mantle xenoliths, with ∆7Liol-di (= δ7Liol - δ7Lidi) ranging from nearly zero to greater than 20‰. Both of these observations might be explained if the distribution of Li between olivine and diopside is strongly temperature dependent at mantle conditions such that a change in temperature, i.e. cooling upon exhumation, initiates diffusive re-equilibration of Li between phases in the xenolith. Accompanying dynamic fractionation of 6Li from 7Li due to differing diffusion rates of the two isotopes could then be permanently recorded in the xenolith if its temperature drops below the closure temperature before a new equilibrium is reached. The results of this study indicate a partition coefficient for Li between olivine and diopside (DLiol/di) of 2.0 ± 0.2 that is independent of temperature (within the error of our analyses) over the range 700 °C to 1100 °C. This lack of temperature dependence holds true when data from previous experiments at temperatures as high as 1375 °C are considered. Thus it appears that closed-system diffusion of Li between olivine and diopside in response to changing temperature is not an appropriate explanation for the observed range of elemental and isotopic distributions in natural xenoliths. Other possible explanations include Li redistribution in response to changing oxygen fugacity in the system, or diffusive addition or subtraction of Li during open-system interaction with an infiltrating melt or fluid.

  7. 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

  8. 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.

  9. Pallasites as a proxy for understanding core-mantle partitioning

    NASA Astrophysics Data System (ADS)

    Mikhail, S.; Crispin, K. L.; Shahar, A.

    2012-12-01

    During planetary differentiation in the core-mantle system the elements that are iron-loving (siderophile) are partitioned into the core, conversely, the rock-loving (lithophile) elements are partitioned into the mantle. However, empirical data from the studies of upper mantle xenoliths and chondritic meteorites are at odds with experimentally calibrated partition coefficients under conditions akin to the early Earth during core-formation. In short, the concentration of moderate to highly siderophile elements in the mantle is too high. This has led to the late-veneer hypothesis, which increases the mantles siderophile element budget post core-formation to explain the observed siderophile element-enrichment. To investigate the need for such a late-veneer, and to also test the experimentally calibrated partition coefficients, we have analyzed pallasite meteorites, thought to come from the core-mantle boundary of planetary bodies. Pallasites are stony-iron meteorites consisting chiefly of olivine within a Fe-Ni metal matrix. We will present the results from the determinations of a variety of siderophile and lithophile trace elements in the olivines ('mantle') and the Fe-Ni alloy ('core') from 9 pallasite meteorites and 1 aubrite meteorite. We discuss the data in light of two notions; (i) siderophile element-enrichment in the mantle requires a late-veneer and (ii) the pallasites share a common parent body.

  10. 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

  11. 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.

  12. 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.

  13. 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.

  14. 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

  15. Mapping Mantle Mixing and the Extent of Superplume Influence Using He-Ne-Ar-CO2-N2 Isotopes: The Case of the East Africa Rift System

    NASA Astrophysics Data System (ADS)

    Hilton, D. R.; Halldorsson, S. A.; Scarsi, P.; Castillo, P.; Abebe, T.; Kulongoski, J. T.

    2014-12-01

    Earth's mantle possesses distinct and variable volatile characteristics as sampled by magmatic activity in different tectonic environments. In general, trace element depleted mid-ocean ridge basalts, with low Sr and Pb isotope values (but high ɛNd and ɛHf), release mantle-derived noble gases characterised by 3He/4He ~8 ± 1RA, (21Ne/22Ne)ex ~0.06 and 40Ar/36Ar ≥ 10,000 with CO2 and N2 having δ13C~-5‰ and δ15N ~-5‰, respectively. In contrast, enriched intraplate lavas possess higher 3He/4He (up to 50RA), lower (21Ne/22Ne)ex ~0.035 and 40Ar/36Ar ≤ 10,000 with generally higher but variable δ13C and δ15N. These isotopic attributes of mantle-derived volatiles can be exploited to map the extent, and mixing characteristics, of enriched (plume) mantle with depleted asthenospheric mantle ± the effects of over-riding lithosphere and/or crust. The East African Rift System (EARS) is superimposed upon two massive plateaux - the Ethiopia and Kenya domes - regarded as geophysical manifestations of a superplume source, a huge thermochemical anomaly originated at the core-mantle boundary and providing dynamic support for the plateaux. We present new volatile isotopic and relative abundance data (on the same samples) for geothermal fluids (He-CO2-N2), lavas (He-Ne-Ar) and xenoliths (He-Ne-Ar-CO2-N2) which provide an unprecedented overview of the distribution of mantle volatiles of the Ethiopia Dome, from the Red Sea via the Afar region and Main Ethiopian Rift (MER) to the Turkana Depression. Notably, peaks in geothermal fluid 3He/4He (16RA) and δ15N (+6.5‰) are coincident within the MER but the maximum δ13C (-0.78‰) lies ~100 km to the south. Highs in 3He/4He (14RA), δ13C (~-1‰) and δ15N (+3.4‰) for mafic crystals occur in the Afar region ~ 500km to the north. We assess the significance of the off-set in these volatile isotope signals, for sampling volatile heterogeneity in the plume source and/or the relative sensitivity of different volatiles to

  16. 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.

  17. On-, off-cratonic, orogenic and oceanic mantle roots: lithosphere unite

    NASA Astrophysics Data System (ADS)

    Wittig, Nadine; Pearson, D. Graham

    2010-05-01

    Associated with Earth's crust is the underlying lithospheric mantle, which is often categorized according its surface availability for sampling. For example, we frequently describe sub-continental lithospheric peridotite xenoliths as on- or off-cratonic to highlight the approximate lithosphere stabilization age as being Archean or post-Archean and contrast them against the lithospheric peridotites recovered from present-day oceanic basins. Orogenic peridotites and ophiolites are obducted in subduction zones and mark paleo-suture zones in amalgamated continents. Regardless of its present-day occurrence, lithospheric mantle is buoyant as a result of substantial extraction or mafic to ultramafic silicate melts. This depletion and buoyancy results in a thermal and mechanical boundary layer (lithosphere-asthenosphere boundary layer, LAB) and isolates Earth's lithosphere from the convecting mantle. The mineralogical and geochemical similarities of some of the oldest cratonic sub-continental lithosphere and highly-depleted young oceanic peridotites require common geochemical mechanisms that are capable of introducing well-correlated major element variability - until metasomatic enrichment masks the depletion signature. Efficient extraction of basaltic melts from the convecting mantle occurs at mid-ocean ridges and removes clinopyroxene and to some extent garnet leaving the olivine-rich buoyant residual lithospheric mantle. We will examine the oceanic heritage of garnet and spinel-facies sub-continental lithospheric mantle in general, but focus on xenoliths (n = 62) sampled across the North Atlantic Craton (~700km, NAC), West Greenland at c. 600 and 200 Ma. These Greenlandic samples are strongly serpentinized, harzburgitic to dunitic peridotites and generally comprise less than 15% orthopyroxene and very little clinopyroxene (<< 5%). If garnet is present it occurs in variable amounts (12 to 0.3%). Major element systematics of these peridotites are highly refractory with Al

  18. 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.

  19. 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

  20. 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

  1. Large magnesium isotope fractionation in peridotite xenoliths from eastern North China craton: Product of melt-rock interaction

    NASA Astrophysics Data System (ADS)

    Xiao, Yan; Teng, Fang-Zhen; Zhang, Hong-Fu; Yang, Wei

    2013-08-01

    To investigate the effects of melt-rock interaction on Mg isotope fractionation and mantle Mg isotopic heterogeneity, we report high-precision Mg isotopic data of olivine (Ol), orthopyroxene (Opx), clinopyroxene (Cpx) and spinel (Spl) for 25 peridotite xenoliths from the eastern North China craton. These xenoliths range from lherzolites, Cpx-rich lherzolites to wehrlites, and are variably metasomatised. The lherzolites have Ol with Fo contents from 89 to 90 and have a homogeneous Mg isotopic composition (δ26Mg = -0.26 to -0.20), similar to the typical mantle value. By contrast, Cpx-rich lherzolites and wehrlites have lower Ol with Fo contents (78-88) and exhibit larger Mg isotopic variations, with δ26Mg ranging from -0.39 to +0.09. The δ26Mg values of minerals in these xenoliths are also variable and range from -0.45 to -0.03 in Ol, from -0.26 to -0.01 in Opx, from -0.34 to +0.22 in Cpx and from -0.16 to +0.25 in Spl. Inter-mineral fractionations between coexisting pyroxene and Ol in lherzolites and most isotopically light wehrlites (Δ26MgOpx-Ol = -0.04 to +0.09‰; Δ26MgCpx-Ol = + 0.02 to +0.25‰) vary as a function of temperature and are consistent with equilibrium inter-mineral isotope fractionations. By contrast, large disequilibrium Mg isotope fractionation occurs between coexisting pyroxene and Ol in the majority of Cpx-rich lherzolites (Δ26MgOpx-Ol = +0.16 to +0.32‰; Δ26MgCpx-Ol = +0.04 to +0.34‰). Both types of isotope fractionations also occur between Spl and Ol, with Spl being consistently heavier than Ol (Δ26MgSpl-Ol = +0.15 to +0.55‰). Overall, the isotopically equilibrated but light wehrlites may result from mantle metasomatism by isotopically light melt, whereas large disequilibrium isotope fractionations in Cpx-rich peridotites likely reflect kinetic isotope fractionation during melt-peridotite interaction. Both processes result in the heterogeneous Mg isotopic composition of the lithospheric mantle beneath the eastern North China

  2. Fertile Lithospheric Mantle beneath Far East Russia; evidence for Lithospheric delamination

    NASA Astrophysics Data System (ADS)

    Ntaflos, T.; Koutsovitis, P.; Aschchepkov, I.; Hauzenberger, C. A.; Prikhodko, V.; Barkar, A.

    2012-12-01

    In the back-arc environment of Far East Russia, mantle xenoliths from Sikhoti-Alin( Komku area, KO) and Primorie (Sviyaginsky area, SV), Far East Russia are fertile spinel lherzolites with traces of amphibole, phlogopite and hyalophane in some of the studied samples. Though samples from both localities are fertile there is a systematic difference in their fertility. The KO samples have mg# varying from 0.891 to 0.899 and are slightly more fertile than the SV samples that have mg# ranging from 0.898 to 0.904. LA-ICP-MS analyses on clinopyroxenes confirm this trend as the (La/Yb)N in KO samples range from 1.49 to 5.4 and in SV samples from 0.15 to 1.73. The estimated equilibration temperatures for the KO suite range from 940 °C to 1035 °C and for the SV suite from 770 to 945. The differences in the estimated equilibrium temperatures between the KO and SV suites suggest that the less fertile SV suite originated in shallower depths than the more fertile KO suite. Kaersutite, and extremely Ti-rich phlogopite, up to 14 wt% TiO2, are associated with intergranular glass indicating clearly metasomatism of undersaturated alkaline melts. Pargasitic amphibole occurs as inclusion in clinopyroxene. Incompatible element abundances, besides Ba, Sr and Ti that are slightly enriched in the amphibole, are similar in both phases suggesting minor metasomatism due to percolation of small amounts of water-rich fluids. The lithospheric mantle beneath the studied area represents the residue after partial melting of up to 2 % of a primitive mantle and is comparable to that of Mongolia. Despite the fact that the studied area experienced several subducting episodes, the lithospheric mantle appears to be unaffected from the upwelling fluids/melts of the subducted slab(s). Since there is no indication for plume activity, and/or evidence for refertilization, it is likely that the lithospheric mantle has been delaminated as the result of tectonic events (lithospheric attenuation, inverse

  3. 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

  4. 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.

  5. 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 ɛNd(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

  6. Peridotite xenoliths from the Chersky belt (Yakutia): Infiltrated carbonate-rich melts leaving no metasomatic record

    NASA Astrophysics Data System (ADS)

    Tschegg, C.; Ntaflos, Th.

    2012-04-01

    The Chersky seismic belt (NE-Russia) forms the modern plate boundary of the Eurasian-North American continental plate. The geodynamic evolution of this continent-continent setting is highly complex and it remains a matter of debate, how the extent of the Mid-Arctic Ocean spreading influenced the North Asian continent in this region since the Eocene. We constrained a model (Tschegg et al. 2011, Lithos) showing that volcanism in the Chersky area was triggered by extension and thinning of the lithosphere combined with adiabatic upwelling of the underlying mantle at 37 Ma. This implicates that the rift tectonics of the Mid-Arctic Ocean, at that time, affected the North Asian continent causing volcanic activity. Luckily, the basanites that were studied for these purposes host a representative number of peridotite xenoliths, which allow further constraints on the evolution of this area. The suite of spinel peridotites (lherzolites and harzburgites), pyroxenites and mega-crysts enable to characterize upper mantle conditions as well as to observe different processes within the lithospheric mantle beneath the Chersky belt. Equilibration temperatures of the spinel lherzolites reveal approx. 900-1000 °C at pressures of 1-2 GPa, with melt extraction volumes around 4 %. The analyzed spinel harzburgites reflect equilibration at lower P-T conditions and around 8 % higher melt extraction rates. We were able to find a completely preserved interstitial melt droplet in a lherzolite, in which a primary dolomite is in perfect phase contact with Na-rich alumosilicate glass and sodalite. Based on detailed and integrated investigations, we reconstructed origin and evolution of this spectacular carbonatic liquid that at depth differentiated from a carbonated silicate melt to an immiscible carbonate and silicate liquid, entered the lherzolite and quenched shortly before it was transported in the xenolith to the earth surface. To our surprise, the carbonate-rich melt infiltration did not

  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. 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

  9. 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.

  10. 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

  11. Regional heterogeneity in the water content of the Cenozoic lithospheric mantle of Eastern China

    NASA Astrophysics Data System (ADS)

    Hao, Yan-Tao; Xia, Qun-Ke; Jia, Zu-Bing; Zhao, Qi-Chao; Li, Pei; Feng, Min; Liu, Shao-Chen

    2016-02-01

    The major and trace elements and H2O contents of minerals in peridotite xenoliths hosted by the Cenozoic basalts in Northeast China (NEC) were evaluated using electron microprobe, laser-ablation inductively coupled plasma-mass spectrometry and Fourier transform infrared spectroscopy, respectively. Although a potential loss of H during the xenoliths' ascent cannot be excluded for olivine, orthopyroxene (opx) and clinopyroxene (cpx) largely preserved the H2O contents of their mantle source in all of the samples, as inferred from (1) the homogenous H2O contents within single pyroxene grains and (2) the equilibrium H2O partitioning between cpx and opx. No OH was detected for pyroxenes of peridotite xenoliths from the north part of NEC (NNEC). Combined with previously published data from the North China Craton (NCC) and the South China Block (SCB), the regional heterogeneity in the water contents in the Cenozoic lithospheric mantle beneath the whole Eastern China has been revealed. The lithospheric mantle beneath the NNEC is completely dry. The "bulk" water contents of the lithospheric mantle of the south part of NEC and the NCC have similar ranges and average values, whereas those of the SCB are much higher (12-195 ppm, average 90 ± 45 ppm for whole rock). The regional variations in the H2O content of the Cenozoic lithospheric mantle of Eastern China cannot be caused by partial melting, mantle metasomatism, or variations in redox state. We propose that the lithospheric mantle beneath the different regions of Eastern China may have distinct origins and may have undergone distinct geodynamic processes.

  12. 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.

  13. Lithium isotopic composition of xenolithic eclogites: implications for subduction zone processes

    NASA Astrophysics Data System (ADS)

    Rudnick, R. L.; McDonough, W. F.; Tomascak, P. B.; Baker, E.

    2002-12-01

    Lithium isotopes are strongly fractionated at the Earth's surface. Seawater Li is very heavy (δ7Li = +32 ‰ ) compared to that in mantle-derived magmas (MORB = +2 to +6 ‰ ). Because heavy seawater Li is incorporated into altered oceanic crust (Chan et al., 1992, EPSL), Li isotopes may provide a new tool for tracing recycled oceanic crust in the Earth's mantle. However, Li systematics of island arc basalts are not straightforward. In most arc lavas no correlations exist between d7Li and other indicators of subduction (e.g., δ11B) and most arc lavas' δ7Li values overlap with those of MORB (Tomascak et al., 2002, EPSL). What happens to the heavy Li that is subducted? Recent investigation of Alpine eclogites, which are analogs of subducted, hydrothermally altered oceanic crust (Zack et al., EPSL, submitted), show them to have light δ7Li, ranging from -11 to +3 ‰ . This is significantly lighter than fresh or altered MORB, suggesting that Li is fractionated during dehydration of the slab, with heavy seawater Li being lost early in the subduction process, perhaps to the forearc mantle. We have measured the δ7Li of clean omphacites separated from xenolithic eclogites from the Mesozoic Koidu kimberlites, Sierra Leone. Previous studies have shown that the low MgO suite of eclogites from Koidu has geochemical properties of hydrothermally altered Archean oceanic crust (Barth et al., 2001, GCA; 2003, Prec. Res., in press). Omphacites from these eclogites have δ7Li ranging from -2 to +5 ‰ . Eclogites with δ18O heavier than normal mantle range are the only samples to show δ7Li that deviates from the MORB range (to lighter values), whereas elcogites with δ18O lighter than normal mantle have δ7Li that falls within the MORB range. We interpret the elcogites with heavy δ18O and light Li to represent former oceanic crust that was hydrothermally altered at low temperatures, which increased both δ18O and δ7Li. Like the Alpine eclogites, this crust lost much of its

  14. Stable isotopic constraints on formation of continental lithospheric mantle: a case study from the Colorado Plateau

    NASA Astrophysics Data System (ADS)

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

    2014-12-01

    Mantle melt depletion is fundamental to stabilization of thick lithospheric mantle and to craton formation and survival. Three potential mechanisms to grow lithospheric mantle are: partial melting of upwelling mantle plumes; imbrication and accretion of oceanic lithosphere at subduction zones; and flux melting of the mantle wedge at volcanic arcs. Stable isotopes can distinguish these models because stable isotopes fractionate substantially at low temperature, making the mantle sensitive to the incorporation of subducted material. Correlations between stable isotopes and trace elements can then distinguish seafloor weathering processes from flux melting processes, allowing subducted oceanic lithosphere to be discerned from flux-melted lithosphere. We find that indices of melt depletion (e.g. cpx Cr#, Yb concentrations and Mg#) in xenoliths from the Colorado Plateau correlate with δ18O values of olivine. Xenoliths from The Thumb volcanic neck have δ18Ool values ranging from +5.05 to +5.50‰ (n=12) that correlate positively with indices of melt depletion, and xenoliths from the Green Knobs diatreme have δ18Ool values that range from +4.85 to +5.36‰ (n=9) and correlate negatively with indices of melt depletion. These trends may reflect coupled fluid input and melt depletion, typical of flux melting. The different trends at each locality may be due to fluxing of different fluids- either high δ18O fluids derived from altered oceanic crust or low δ18O fluids derived from recycled serpentinite. Canil and Lee (2009) interpreted correlations between whole rock δ18O values and MgO/SiO2 in xenoliths from The Thumb to be due to seafloor weathering and Mg loss of abyssal peridotites followed by later accretion of the rocks to the Colorado Plateau lithosphere. However, chrome and alumina are immobile during seafloor weathering, so correlations between δ18O values and Cr# also present in The Thumb xenoliths are inconsistent with seafloor weathering. Evidence for flux

  15. 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.

  16. 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

  17. 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

  18. 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

  19. Ancient mantle trapped in the Mariana arc-basin system: Insights from the platinum group elements and Os isotopes

    NASA Astrophysics Data System (ADS)

    Savov, I. P.; Shirey, S. B.; Horan, M. F.; Mock, T. D.

    2006-12-01

    Serpentinized harzburgites recently drilled during ODP Leg 195 at South Chamorro Seamount in the Mariana forearc region have been studied for their platinum group element (PGE) concentrations and Os isotopic compositions. The samples allow a look at the slab fluid-modified subarc mantle immediately overlaying the actively subducting Pacific slab at depths of ~ 30 km. The average PGE (Os 2.3 ppb, Ir 1.5 ppb, Ru 5.4 ppb, Pd 1.6 ppb and Pt 16.3 ppb) and Re (60 ppt) abundances are comparable to those measured in other subarc mantle xenolith suites. The PGE and Re abundances are depleted up to 3 orders of magnitude relative to chondrites, with relative order of depletions Ru > Os > Ir> Pt> Re> Pd. The variable Pd contents (0.01-4.5 ppb) and the low Pd/Os (<2) in the Mariana forearc samples differ significantly from that of altered oceanic crust (Pd/Os~ 22), suggesting Os remained relatively unchanged during low temperature subduction-fluid additions. Serpentinitization of the harzburgites occurred in equilibrium with fluids that were both reducing and highly alkaline. Os in its original phases may be stabilized in such an environment, therefore can preserve evidence for ancient melting despite later slab interactions. Finally, the high Pt/Pd (ave. ~ 25) and low Re contents (ave.~ 60 ppt) in the Leg 195 forearc samples are complementary to those measured in boninites from the Izu-Bonin-Mariana arc-basin system, showing a genetic relationship. Our samples reveal an average 187Os/188Os of 0.123 [range = 0.119- 0.127], making them similar to abyssal peridotites from elsewhere. Radiogenic 187Os/188Os ratios would have been imparted to the serpentinites by slab fluids, so the subchondritic Os isotopic compositions implies that peridotite-slab fluid interactions did not alter the Os isotope systematics of the mantle protoliths. Low alumina abundances (<1 %) and trace element signatures (low HFSE; REE with U-shaped chondrite-normalized patterns and 2-3 times lower than

  20. 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

  1. 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.

  2. Timing of Magmatism and Metamorphism in the Lower Crust Beneath the Colorado Plateau From Coupled U-Pb and Lu-Hf Isotopic Analysis of Zircon From Xenoliths

    NASA Astrophysics Data System (ADS)

    Crowley, J. L.; Schmitz, M. D.; Bowring, S. A.; Williams, M. L.; Karlstrom, K. E.

    2003-12-01

    have oscillatory-zoned grains with U-Pb dates of 1710 and 1640-1620 Ma, and unradiogenic Hf isotope compositions with depleted mantle model ages of 1.8-1.6 Ga; this concordance of crystallization and source extraction ages supports an igneous protolith interpretation. Three mafic xenoliths have sector- and oscillatory-zoned grains that crystallized at ca. 1435 Ma, however their unradiogenic Hf isotope compositions and model ages of 1.7-1.6 Ga complicate interpretation of the ca. 1435 Ma dates as representing mafic magmatic crystallization, unless these melts were substantially contaminated by older crust. Further petrologic and geochemical studies of the mafic xenoliths are necessary to evaluate this hypothesis. The results suggest a complex lower crust beneath the Colorado Plateau with ca. 1435 Ma mafic magmatism in ca. 1.7-1.6 Ga felsic rocks followed by episodic metamorphism that coincided with regional granitic magmatism and metamorphism in exposed basement rocks. This coincidence suggests a link between ca. 1.4 Ga mafic magmatism and metamorphism in the lower crust and tectonothermal events in the middle crust. The 1.7-1.6 Ga Hf model ages from many metamorphic zircons and all igneous zircons in the mafic xenoliths indicate that they are crustally contaminated ca. 1.4 Ga mafic magmas or restites from ca. 1.4 Ga granite extraction.

  3. 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.

  4. 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

  5. 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.

  6. 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

  7. 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.

  8. 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

  9. 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.

  10. 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

  11. 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.

  12. 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.

  13. 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

  14. 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.

  15. 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

  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. Chemical equilibration of the Earth's core and upper mantle

    USGS Publications Warehouse

    Brett, R.

    1984-01-01

    The oxygen fugacity (fO2) of the Earth's upper mantle appears to lie somewhat above that of the iron-wu??stite buffer, its fO2 is assumed to have been similar to the present value at the time of core formation. In the upper mantle, the Fe-rich liquid protocore that would form under such conditions of fO2 at elevated temperatures would lie predominantly in the system Fe-S-O. Distribution coefficients for Co, Cu, Ni, Ir, Au, Ir, W, Re, Mo, Ag and Ga between such liquids and basalt are known and minimum values are known for Ge. From these coefficients, upper mantle abundances for the above elements can be calculated by assuming cosmic abundances for the whole Earth and equilibrium between the Fe-S-O protocore and upper mantle. These calculated abundances are surprisingly close to presently known upper mantle abundances; agreements are within a factor of 5, except for Cu, W, and Mo. Therefore, siderophile element abundances in the upper mantle based on known distribution coefficients do not demand a late-stage meteoritic bombardment, and a protocore formed from the upper mantle containing S and O seems likely. As upper mantle abundances fit a local equilibrium model, then either the upper mantle has not been mixed with the rest of the mantle since core formation, or else partition coefficients between protocore and mantle were similar for the whole mantle regardless of P, T, and fO2. The latter possibility seems unlikely over such a P-T range. ?? 1984.

  18. 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.

  19. 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

  20. 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.

  1. 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.

  2. Measured and calculated elastic wave speeds in partially equilibrated mafic granulite xenoliths: Implications for the properties of an underplated lower continental crust

    NASA Astrophysics Data System (ADS)

    Rudnick, Roberta L.; Jackson, Ian

    1995-06-01

    Ultrasonic compressional wave velocities measured at 1.0 GPa and room temperature are compared with calculated velocities (based on single-crystal data and modal mineralogy) for a suite of mafic granulite xenoliths from the Chudleigh volcanic province, north Queensland, Australia. The xenoliths have nearly constant major element compositions but widely variable modal mineralogy, reflecting recrystallization under variable pressure-temperature conditions at depth in the continental crust (20-45 km). They thus provide an excellent opportunity to investigate velocity variation with depth in a mafic lower crust. Measured P wave velocities, corrected for the decompression-induced breakdown of garnet, range from 6.9 to 7.6 km/sec and correlate with derivation depth. These velocities are 5-12% lower than the calculated velocities (7.5-8.0 km/sec), apparently as a result of grain boundary alteration as well as irreversible changes that occurred in the xenoliths during rapid decompression. Calculated P wave velocities are similar to those estimated by Furlong and Fountain (1986) and Sobolev and Babeyko (1989) for mafic granulites formed through basaltic underplating of the continental crust. Depending upon in situ temperature, P wave velocities in the deepest samples may be interpreted as crustal (e.g., 7.3-7.6 km/sec, if heat flow is high) or mantle (7.7-7.8 km/sec, in areas of low heat flow). The range of velocities in the xenolith suite is larger than predicted for a fully equilibrated underplated basaltic layer, highlighting the importance of kinetic effects in determining the ultimate velocity profile of magmatically underplated crust. Comparison of our results with seismic profiles illustrates that the lower crust rarely reaches such high velocities, suggesting quartz-bearing rocks (country rocks?) are present within magmatically underplated layers of the deep crust.

  3. 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

  4. 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

  5. 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.

  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. 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.

  8. 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

  9. A 'hidden' 18O-enriched reservoir in the sub-arc mantle.

    PubMed

    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-δ(18)O values down to the mantle wedge, where mantle peridotites are expected to achieve the high-δ(18)O features. Elevated δ(18)O values relative to the upper mantle value have been reported for magmas from some subduction zones. However, peridotites with δ(18)O 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 (δ(18)O = +8.03 ± 0.28 ‰). Such a high-δ(18)O mantle commonly does not contribute significantly to typical island arc basalts. However, partial melting or contamination of such a high-δ(18)O mantle is feasible to account for the high-δ(18)O signatures in arc basalts. PMID:24577190

  10. Metasomatic control of water contents in the Kaapvaal cratonic mantle

    NASA Astrophysics Data System (ADS)

    Peslier, A. H.; Woodland, A. B.; Bell, D. R.; Lazarov, M.; Lapen, T. J.

    2012-11-01

    Water and trace element contents were measured by FTIR and laser ablation-ICPMS on minerals from peridotite xenoliths in kimberlites of the Kaapvaal craton from Finsch, Kimberley, Jagersfontein (South Africa), Letseng-La-Terae, and Liqhobong (Lesotho) mines. The peridotites record a wide range of pressure, temperature, oxygen fugacity, and metasomatic events. Correlations between water content or OH vibration bands with major, minor and trace elements in pyroxene and garnet precludes disturbance during xenolith entrainment by the host kimberlite magma and indicate preservation of mantle water contents. Clinopyroxene water contents (150-400 ppm H2O, by weight) correlate with those of orthopyroxene (40-250 ppm). Olivines (Peslier et al., 2008, 2010) and garnets have 0-86 and 0-20 ppm H2O, respectively. Relations in individual xenolith suites between the amount of water and that of incompatible elements Ti, Na, Fe3+ and rare earths in minerals suggests that metasomatism by oxidizing melts controls the water content of olivine, pyroxene and garnet. At pressures ⩽5.5 GPa, hydrous, alkaline, siliceous fluids or melts metasomatized Liqhobong and Kimberley peridotites, producing high water contents in their olivine, pyroxenes and garnet. At higher pressures, the percolation of ultramafic melts reacting with peridotite resulted in co-variation of Ca, Ti and water at the edge of garnets at Jagersfontein, and the overall crystallization of garnet with lower water contents than those in the original peridotites. The upward migration of these ultramafic melts through the lithospheric mantle also increased the water content of olivines with decreasing pressure at Finsch Mine. H2O/Ce ratios of melts in equilibrium with Kaapvaal peridotites range from 100 to 20,000 and the larger values may indicate metasomatism in subduction zone settings. Metasomatic events in Kaapvaal peridotites are thought to have occurred from the Archean to the Mesozoic. However, circumstantial evidence

  11. 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

  12. Significance of ancient sulfide PGE and Re-Os signatures in the mantle beneath Calatrava, Central Spain

    NASA Astrophysics Data System (ADS)

    González-Jiménez, José María; Villaseca, Carlos; Griffin, William L.; O'Reilly, Suzanne Y.; Belousova, Elena; Ancochea, Eumenio; Pearson, Norman J.

    2014-08-01

    Spinel lherzolite and wehrlite xenoliths from the Cenozoic Calatrava volcanic field carry the geochemical imprint of metasomatic agents that have affected the subcontinental lithospheric mantle beneath Central Iberia. Some xenoliths (mainly wehrlites) were enriched in REE, Sr, P, and CO2 by silicic-carbonate-rich metasomatic melts/fluids, while others record the effects of subduction-related hydrous silicate fluids that have precipitated amphibole and induced high Ti/Eu in primary clinopyroxene. The petrographic observations and geochemical data suggest that interstitial glass in the xenoliths represent the quenched products of Si-rich melts that infiltrated the mantle peridotite shortly before the entrainment of the xenoliths in the host magmas that erupted ca 2 million years ago. During their infiltration, the metasomatic melts reacted with peridotite, resulting in silica enrichment, while remobilizing grains of iron-rich monosulfide solid solution (Fe-rich Mss) initially enclosed in, or intergranular to, primary olivine and pyroxenes. In situ laser ablation inductively coupled plasma-mass spectrometry analysis of single sulfide grains reveals that the Fe-rich Mss in glass shows platinum-group element (PGE) patterns and 187Os/188Os compositions identical to the Fe-rich Mss occurring as inclusions in, or at grain boundaries of primary silicates. Moreover, independent of its microstru