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Sample records for anfengshan basalts subducted

  1. Melting of subducted basalt at the core-mantle boundary.

    PubMed

    Andrault, Denis; Pesce, Giacomo; Bouhifd, Mohamed Ali; Bolfan-Casanova, Nathalie; Hénot, Jean-Marc; Mezouar, Mohamed

    2014-05-23

    The geological materials in Earth's lowermost mantle control the characteristics and interpretation of seismic ultra-low velocity zones at the base of the core-mantle boundary. Partial melting of the bulk lower mantle is often advocated as the cause, but this does not explain the nonubiquitous character of these regional seismic features. We explored the melting properties of mid-oceanic ridge basalt (MORB), which can reach the lowermost mantle after subduction of oceanic crust. At a pressure representative of the core-mantle boundary (135 gigapascals), the onset of melting occurs at ~3800 kelvin, which is ~350 kelvin below the mantle solidus. The SiO2-rich liquid generated either remains trapped in the MORB material or solidifies after reacting with the surrounding MgO-rich mantle, remixing subducted MORB with the lowermost mantle. PMID:24855266

  2. H 2O in basalt and basaltic andesite glass inclusions from four subduction-related volcanoes

    NASA Astrophysics Data System (ADS)

    Sisson, T. W.; Layne, G. D.

    1993-06-01

    Total dissolved H 2O and major element abundances were measured in basalt and basaltic andesite glass inclusions in olivine phenocrysts from Quaternary eruptions of four subduction-related volcanoes to test the hypothesis that low-MgO high-alumina basalts contain high H 2O at depth [1] and to reveal any petrogenetically significant correlations between arc basalt compositions and H 2O contents. Total dissolved H 2O (combined molecular H 2O and OH groups) measured by ion microprobe in mafic glass inclusions from the 1974 eruption of Fuego, Guatemala, reaches 6.2 wt.%. Dissolved H 2O contents decrease in more evolved Fuego glasses. Correlations of H 2O with MgO, Na 2O, K 2O, S and Cl indicate that aqueous fluid exsolution during magma ascent forced crystallization and differentiation of residual liquids. Low-K 2O magnesian high-alumina basalt glass inclusions from the 3 ka eruption of Black Crater (Medicine Lake volcano, California) have low H 2O contents, near 0.2 wt.%, which are consistent with the MORB-like character of these and other primitive lavas of the Medicine Lake region. Basalt and basaltic andesite glass inclusions from Copco Cone and Goosenest volcano on the Cascade volcanic front north of Mt. Shasta have H 2O contents of up to 3.3 wt.%. The range of H 2O contents in Cascade mafic magmas is too large to have resulted solely from enrichment by crystallization and indicates the participation of an H 2O-rich component in magma generation or crustal-level modification. Whereas fluid-absent melting of amphibole-bearing peridotite can account for the H 2O in most mafic arc liquids, the very high H 2O/alkali ratios of the 1974 Fuego eruptives suggest that an aqueous fluid was involved in the generation of Fuego basalts.

  3. The influence of the basalt-eclogite transition on incipient subduction dynamics

    NASA Astrophysics Data System (ADS)

    Hall, C.; Gurnis, M.

    2004-12-01

    Subduction becomes self-sustaining when the negative buoyancy of the slab is larger than the sum of elastic bending, fault friction, and viscous drag. The oceanic lithosphere, consisting of basaltic crust overlying mantle which is depleted in iron, is compositionally buoyant relative to the asthenosphere and will resist subduction. This is not a critical impediment for mature subduction zones because the basaltic crust should metamorphose into denser eclogite, with the result that the slab has nearly neutral compositional buoyancy at depths greater than this phase transition. During subduction initiation, however, compositional buoyancy may offer significant resistance to subduction and initially limit subduction to plates which have gained sufficient negative buoyancy from cooling. We have shown that while including a 6 km thick basaltic crust does not greatly affect the maximum force required to bend the lithosphere, it substantially increases the net work required to reach a self-sustaining state. These earlier models did not consider the basalt-eclogite transition. We use a visco-elastoplastic numerical method to model the evolution of the force balance during incipient subduction occurring at a fixed convergence rate. Phases are tracked using a particle advection scheme. In some cases, the buoyant basaltic crust buoyantly detaches from the mantle lithosphere and rises to the base of the over-riding lithosphere. Buoyant detachment removes most of the compositional impediment to subduction initiation, but will deny the slab of the additional driving force that would occur after the transformation of basalt to eclogite. Crustal detachment is encouraged by mechanical thickening during convergence, and therefore may depend on the degree of coupling and fault strength at the plate interface.

  4. Effect of subduction components on production of basalts from Tateshina volcano, central Japan: geochemical calculation of dehydration of subducting oceanic crust and partial melting of overlying sediments, and subsequent fluid-mantle interaction

    NASA Astrophysics Data System (ADS)

    Katoh, Masayasu; Shuto, Kenji

    Effect of subduction components on production of basalts from Tateshina volcano, central Japan: geochemical calculation of dehydration of subducting oceanic crust and partial melting of overlying sediments, and subsequent fluid-mantle interaction

  5. Shock-Induced Melting of Martian Basalts: Insights on Subducting Oceanic Crust Melting Processes

    NASA Astrophysics Data System (ADS)

    Beck, P.; Gillet, P.; Barrat, J.-A.; Gautron, L.; Daniel, I.; El Goresy, A.

    2003-04-01

    Experiments carried out on rocks at upper and lower mantle P-T conditions have produced series of candidate minerals for the Earth mantle mineralogical model. Basaltic compositions can also suffer ultra high-pressure and temperature when subducting in the mantle. The phase diagram of basalts has been studied to characterize potential chemical and mineralogical heterogeneities produced by partial melting and phase transformations of the oceanic crust. Shergottites that represent the most important sub-class of Martian meteorite have compositions close to terrestrial basalts and gabbros. During their extraction from Mars, they were severely shocked with pressures up to 50 GPa. These shocks induced partial melting. These melt pocket are an opportunity to study melting phenomena of basaltic compositions (i.e. oceanic crust) under high-pressure. We have performed a Raman spectroscopy investigation to determine the mineralogy of the melt pockets. Four shergottites were studied, NWA 480, NWA 856, NWA 1068 and Zagami. In each meteorite, abundant "large" minerals in melt pockets are hollandite (both Ca-Na and K-Na hollandite), stishovite, amorphous pyroxene and high-pressure phosphate. Meltpocket matrix seems to have a similar mineralogy as "megacrysts". In NWA 856 we observed at a melt pocket rim that maskelynite successively transforms into hollandite, and a polycrystalline aggregate. This aggregate was identified as a mixture of stishovite and a calcium aluminosilicate phase (CAS), a phase previously described in high-pressure experiments, but never observed in natural samples. The Raman spectra identifies unambiguously this silicate of composition CaAl_4Si_2O11 and of Ba-ferrite type structure. Such a phase is supposed to be present in basalt subsolidus melting experiments for pressures above 25 GPa and temperatures between 2500 and 2700 K. Its discovery reinforces the proposition that this CAS phase is a valuable candidate for hosting Al in subducting oceanic crust.

  6. Heterogeneous source components of intraplate basalts from NE China induced by the ongoing Pacific slab subduction

    NASA Astrophysics Data System (ADS)

    Chen, Huan; Xia, Qun-Ke; Ingrin, Jannick; Deloule, Etienne

    2016-04-01

    In recent few years, the recycled oceanic slab has been increasingly suggested to be the enriched component in the mantle source of widespread intra-plate small-volume basaltic magmatism in eastern China. The recycled oceanic slab is a mixture of sediment, upper oceanic crust and lower gabbro oceanic crust, and will undergo alteration and dehydration during the recycling progress. The influence of these different components on the mantle source needs to be further constrained. The Chaihe-aershan volcanic field in Northeast China is located close to the surface position of the front edge of the subducted Pacific slab and includes more than 35 small-volume Quaternary basaltic volcanoes, which provides an opportunity to study the evolution of mantle source in detail and the small-scale geochemical heterogeneity of the mantle source. We measured the oxygen isotopes and water content of clinopyroxene (cpx) phenocrysts by secondary ion mass spectrometry (SIMS) and Fourier transform infrared spectrometry (FTIR), respectively. The water content of magma was then estimated based on the partition coefficient of H2O between cpx and basaltic melt. The measured δ18O of cpx phenocrysts (4.27 to 8.57) and the calculated H2O content of magmas (0.23-2.70 wt.%) show large variations, reflecting the compositional heterogeneity of the mantle source. The δ18O values within individual samples also display a considerable variation, from 1.28 to 2.31‰ suggesting mixing of magmas or the sustained injection of magmas with different δ18O values during the crystallization. The relationship between the averaged δ18O values of cpx phenocrysts and the H2O/Ce, Ba/Th, Nb/La ratios and Eu anomaly of whole-rocks demonstrates the contribution to three components in the mantle source (hydrothermally altered upper oceanic crust or marine sediments, altered lower gabbroic oceanic crust, ambient mantle). The proportions of these three components varied strongly within a limited period (˜1.27 Ma to

  7. Niobium-enriched basalts from the Wabigoon subprovince, Canada: evidence for adakitic metasomatism above an Archean subduction zone

    NASA Astrophysics Data System (ADS)

    Wyman, D. A.; Ayer, J. A.; Devaney, J. R.

    2000-06-01

    Late Archean niobium-enriched basalts from the Central Sturgeon Lake assemblage and Neepawa group of the western Wabigoon subprovince have mantle-normalized Nb/La between 0.8 and 1.3 and Zr/Y between 4 and 7. They are compositionally similar to basalts attributed to adakite metasomatism of mantle wedge regions in Cenozoic subduction zones [Sajona et al., J. Petrol. 37 (1996) 693-726]. In detail, their Sc-REE systematics suggest the Archean basalts were generated above the garnet stability field. An association with adakite-like volcanic rocks, an absence of komatiites and the arc-like attributes of their host sequences suggest a subduction-related origin for the basalts. If current models of adakite and Niobium-enriched basalt genesis are valid, then additional examples of these rocks should be relatively common in other Archean greenstone belts.

  8. Sulfur isotope systematics of basaltic lavas from Indonesia: implications for the sulfur cycle in subduction zones

    NASA Astrophysics Data System (ADS)

    de Hoog, J. C. M.; Taylor, B. E.; van Bergen, M. J.

    2001-07-01

    We report sulfur isotope compositions of basaltic and basaltic andesite lavas from selected volcanoes in the Indonesian arc system covering the spectrum from low-K tholeiitic to high-K calc-alkaline compositions. The results of 25 samples from seven volcanoes, which are associated with different subduction regimes, show a range in δ34S values of +2.0-+7.8‰ (VCDT) with an average of +4.7±1.4‰ (1σ). Averages and within-suite variations of two larger sets of samples from Batur and Soputan volcanoes (+4.2±1.3‰ with n=9 and +5.7±1.4‰ with n=7, respectively) are comparable to those of the entire sample set. Sulfur concentrations are low (mostly between 2 and 74 ppm, average=19 ppm) and do not show correlations with sulfur isotope composition and whole-rock chemistry, or systematic changes with time in any of the lava suites. From model calculations we infer that basaltic magmas will undergo sulfur isotope fractionation during degassing, most commonly towards lower δ34S values, but that the extent is limited at P-T conditions and oxidation states of interest. Hence, δ34S signatures of basaltic lavas will generally be within a few permil from primary magmatic values, even in cases of extensive sulfur loss. Consequently, magmas in the Indonesian arc system originate from mantle sources that are enriched in 34S relative to MORB and OIB sources and are likely to have δ34S values of about +5-+7‰. The enrichment in 34S is considered to reflect addition of slab-derived material, presumably from sediments rather than altered oceanic crust, with fluids being the most likely transport medium. Absence of correlation between δ34S values of Indonesian basalts and chemical proxies for source components or processes at the slab-wedge interface suggests that sulfur isotopes are relatively insensitive to variations in subduction setting and dynamics. This is supported by the modest range in δ34S of the Indonesian volcanoes studied despite significant variations in the

  9. Geochemistry of basaltic lavas from the southern Lau Basin suggests input of variable subduction component

    NASA Astrophysics Data System (ADS)

    Yan, Q.; Castillo, P. R.; Shi, X.

    2011-12-01

    New major element, trace element and Sr-Nd-Pb isotope data are presented for 18 volcanic lavas and 6 basaltic glasses collected in situ from the Eastern Lau Spreading center (ELSC) and Valu Fa ridge (VFR) in the southern Lau Basin. All lava samples are aphanitic with rare microlites and microphenocrysts of plagioclase and clinopyroxene. The samples belong to sub-alkaline series and range from basalt and basaltic andesite to more differentiated andesite. In terms of trace element composition, the samples are transitional between typical normal mid-ocean ridge basalt (MORB) and island arc basalt (IAB) although VFR samples have higher large ion lithophile element/high field strength element (LILE/HFSE) ratios (e.g., Ba/Nb) than ELSC samples. Their Sr-Nd isotopic compositions plot between Indian MORB and Tonga arc lavas, but VFR samples have higher 87Sr/86Sr ratios for given 143Nd/144Nd ratios than ELSC samples. Their Pb isotopic compositions indicate a gradual transition from Pacific Ocean-type to Indian Ocean-type mantle source from VFR to ELSC. Our new data, combined with literature data for CLSC, ELSC and VFR, show that the mantle beneath the southern Lau Basin was originally a Pacific Ocean-type both in trace element and isotopic compositions and was displaced by an Indian Ocean-type mantle. The mantle beneath the spreading axies in the southern Lau Basin also receives variable influences of subducted components, mainly fluids released from the altered oceanic crust (AOC) and the influence of the subducted componensts is best seen in the VFR lavas. We suggest that from CLSC via ELSC to VFR, partial melting is the direct, first order controlling factor on the differences in axis depths, and both aqueous fluids and mantle source compositions appear to be the indirect, first order controlling factors. Large degree of partial melting (induced by the addition of aqueous fluids) of depleted mantle source produced shallow spreading axes at VFR and part of ELSC (close to

  10. Compositional diversity of Late Cenozoic basalts in a transect across the southern Washington Cascades: Implications for subduction zone magmatism

    NASA Astrophysics Data System (ADS)

    Leeman, William P.; Smith, Diane R.; Hildreth, Wes; Palacz, Zen; Rogers, Nick

    1990-11-01

    magmas had little residence time in the crust. A majority of the SWC basaltic samples studies are indistinguishable from oceanic island basalts (OIB) in terms of trace element and isotopic compositions, and more importantly, most do not display the typical high field strength element (HFSE) depletion seen in subduction-related magmas in volcanic arcs elsewhere. LILE enrichment and HRSE depletion characteristics of most arc magmas are generally attributed to the role of fluids released by dehydration of subducted oceanic lithosphere and to the effects of sediment subduction. Because most SWC basalts lack these compositional features, we conclude that subducted fluids and sediments do not play an essential role in producing these magmas. Rather, we infer that they formed by variable degree melting of a mixed mantle source consisting mainly of heterogeneously distributed OIB and mid-ocean ridge basalt source domains. Relatively minor occurrences of HFSE-depleted arclike basalts may reflect the presence of a small proportion of slab-metasomatized subarc mantle. The juxtaposition of such different mantle domains within the lithospheric mantle is viewed as a consequence of (1) tectonic mixing associated with accretion of oceanic and island arc terranes along the Pacific margin of North America prior to Neogene time, and possibly (2) a seaward jump in the locus of subduction at about 40 Ma. The Cascades arc is unusual in that the subducting oceanic plate is very young and hot. We suggest that slab dehydration outboard of the volcanic front resulted in a diminished role of aqueous fluids in generating or subsequently modifying SWC magmas compared to the situation at most convergent margins. Furthermore, with low fluid flux conditions, basalt generation is presumably triggered by other processes that increase the temperature of the mantle wedge (e.g., convective mantle flow, shear heating, etc.).

  11. Subduction of hydrated basalt of the oceanic crust: Implications for recycling of water into the upper mantle and continental growth

    NASA Technical Reports Server (NTRS)

    Rapp, R. P.

    1994-01-01

    Subduction zones are presently the dominant sites on Earth for recycling and mass transfer between the crust and mantle; they feed hydrated basaltic oceanic crust into the upper mantle, where dehydration reactions release aqueous fluids and/or hydrous melts. The loci for fluid and/or melt generation will be determined by the intersection of dehydration reaction boundaries of primary hydrous minerals within the subducted lithosphere with slab geotherms. For metabasalt of the oceanic crust, amphibole is the dominant hydrous mineral. The dehydration melting solidus, vapor-absent melting phase relationships; and amphibole-out phase boundary for a number of natural metabasalts have been determined experimentally, and the pressure-temperature conditions of each of these appear to be dependent on bulk composition. Whether or not the dehydration of amphibole is a fluid-generating or partial melting reaction depends on a number of factors specific to a given subduction zone, such as age and thickness of the subducting oceanic lithosphere, the rate of convergence, and the maturity of the subduction zone. In general, subduction of young, hot oceanic lithosphere will result in partial melting of metabasalt of the oceanic crust within the garnet stability field; these melts are characteristically high-Al2O3 trondhjemites, tonalites and dacites. The presence of residual garnet during partial melting imparts a distinctive trace element signature (e.g., high La/Yb, high Sr/Y and Cr/Y combined with low Cr and Y contents relative to demonstrably mantle-derived arc magmas). Water in eclogitized, subducted basalt of the oceanic crust is therefore strongly partitioned into melts generated below about 3.5 GPa in 'hot' subduction zones. Although phase equilibria experiments relevant to 'cold' subduction of hydrated natural basalts are underway in a number of high-pressure laboratories, little is known with respect to the stability of more exotic hydrous minerals (e.g., ellenbergite) and

  12. The halogen cycle in subduction zones: insight from back-arc basin basalts

    NASA Astrophysics Data System (ADS)

    Chavrit, Deborah; Ruzie, Lorraine; Burgess, Ray; Hilton, David; Sumino, Hirochika; Sinton, John; Ballentine, Chris

    2014-05-01

    The extent to which the subduction process preserves the volatile elements signature of the downgoing slab and the mechanisms by which these elements are transferred into the mantle wedge are not well understood. Halogens (Cl, Br, I) are good candidates to trace these processes, due to their incompatibility and their relatively high concentrations in seawater and marine sediments. A technique developed at the University of Manchester allows the high precision measurements of these elements on neutron-irradiated samples using noble gas mass spectrometry. To better constrain the cycle of halogens in subduction zones, we analyzed the halogens in 15 volcanic glasses (BABB) from three back-arc basins which are known to contain slab-derived components viz Manus basin, Lau basin and Mariana trough. The three back-arc basins have relatively constant Br/Cl weight ratios (4.0±0.4×10-3) which are 2x higher than the mid-ocean ridge basalts (MORB) value. The I/Cl weight ratios (0.9 to 7.1×10-5) range from values close to seawater to MORB values. These results suggest that the halogen composition of the BABB mantle source is affected by a slab-derived component. However, the I/Cl ratios positively correlate with Ba/Nb ratios that are between 5-33 (weight), which reflect the extent of the slab contribution. Thus, it indicates the presence of an unknown end member with a MORB-like Ba/Nb ratio and with low I/Cl and high Br/Cl ratios. It is notable that the halogen ratios of this component are similar to that of the fluid phases trapped in altered oceanic crust. Another component with higher Ba/Nb, higher I/Cl and lower Br/Cl ratios, is consistent with the presence of a sedimentary-derived component. The possible origins of the signature of the halogen BABB mantle source will be discussed by comparing with the different components characterizing the subducted oceanic crust.

  13. Compositional diversity of Late Cenozoic basalts in a transect across the southern Washington Cascades: Implications for subduction zone magmatism

    SciTech Connect

    Leeman, W.P. ); Smith, D.R. ); Hildreth, W. ); Palacz, Z.; Rogers, N. )

    1990-11-10

    Major volcanoes of the Southern Washington Cascades (SWC) include the large quaternary stratovolcanoes of Mount St. Helens (MSH) and Mount Adams (MA) and the Indian Heaven (IH) and Simcoe Mountain (SIM) volcanic fields. There are significant differences among these volcanic centers in terms of their composition and evolutionary history. The authors conclude that subducted fluids and sediments do not play an essential role in producing these magmas. Rather, they infer that they formed by variable degree melting of a mixed mantle source consisting mainly of heterogeneously distributed OIB and mid-ocean ridge basalt source domains. Relatively minor occurrences of high field strength element (HFSE) depleted arclike basalts may reflect the presence of a small proportion of slab-metasomatized subarc mantle. The juxtaposition of such different mantle domains within the lithospheric mantle is viewed as a consequence of (1) tectonic mixing associated with accretion of oceanic and island arc terranes along the Pacific margin of North America prior to Neogene time, and possibly (2) a seaward jump in the locus of subduction at about 40 Ma. The Cascades arc is unusual in that the subducting oceanic plate is very young and hot. They suggest that slab dehydration outboard of the volcanic front resulted in a diminished role of aqueous fluids in generating or subsequently modifying SWC magmas compared to the situation at most convergent margins. Furthermore, with low fluid flux conditions, basalt generation is presumably triggered by other processes that increase the temperature of the mantle wedge (e.g., convective mantle flow, shear heating, etc.).

  14. Subduction influence of Philippine Sea plate on the mantle beneath northern Kyushu, SW Japan: An examination of boron contents in basaltic rocks

    NASA Astrophysics Data System (ADS)

    Miyoshi, Masaya; Fukuoka, Takaaki; Sano, Takashi; Hasenaka, Toshiaki

    2008-03-01

    Northern Kyushu, characterized by the subduction of two oceanic slabs (a hot Shikoku basin and a cold Philippine Sea plate) beneath the Eurasian plate, forms a complex portion of Southwestern Japan arc. In order to evaluate the effect of slab-derived fluids from these two contrasting oceanic plates, we determined the boron (B) contents in basaltic rocks from ten volcanoes and three old volcanic fields which erupted since 11 Ma. Since B is distinctly concentrated into slab-derived fluids among the earth's materials, we attempted to estimate the influences of subduction on the sub-arc mantle composition from the interpretation of the B data in basaltic rocks. Old (11-6 Ma) basaltic rocks contain low ratios of B/Sm (0.5-1.3), B/Zr (0.02-0.05) and B/Nb (0.2-0.5), suggesting little influence of subduction. Similarly, backarc basaltic rocks occurring throughout the observed period show little influence of subduction. In contrast, volcanic products from young Aso volcano, located at the volcanic front, show a strong influence of subduction, as indicated by the high B/Sm (1.6-4.3), B/Zr (0.07-0.16) and B/Nb (1.4-3.7) ratios. After 6 Ma, the volcanic arc segment containing Aso volcano is associated with the subduction of the cold Philippine Sea plate; hence a B-rich fluid was probably added to the sub-arc mantle. However, Yufu, Tsurumi and Kuju volcanoes, which are located at the same volcanic front, show small B/Sm (0.9-1.9), B/Zr (0.04-0.07) and B/Nb (0.5-0.9) ratios. This implies that the subduction of the hot Shikoku basin released fluids from the slab at shallow depths; therefore it is depleted of B and other subduction components by the time it reaches the volcanic front.

  15. Olivine and melt inclusion chemical constraints on the source of intracontinental basalts from the eastern North China Craton: Discrimination of contributions from the subducted Pacific slab

    NASA Astrophysics Data System (ADS)

    Li, Hong-Yan; Xu, Yi-Gang; Ryan, Jeffrey G.; Huang, Xiao-Long; Ren, Zhong-Yuan; Guo, Hua; Ning, Zhen-Guo

    2016-04-01

    Contributions from fluid and melt inputs from the subducting Pacific slab to the chemical makeup of intraplate basalts erupted on the eastern Eurasian continent have long been suggested but have not thus far been geochemically constrained. To attempt to address this question, we have investigated Cenozoic basaltic rocks from the western Shandong and Bohai Bay Basin, eastern North China Craton (NCC), which preserve coherent relationships among the chemistries of their melt inclusions, their hosting olivines and their bulk rock compositions. Three groups of samples are distinguished: (1) high-Si and (2) moderate-Si basalts (tholeiites, alkali basalts and basanites) which were erupted at ∼23-20 Ma, and (3) low-Si basalts (nephelinites) which were erupted at <9 Ma. The high-Si basalts have lower alkalies, CaO and FeOT contents, lower trace element concentrations, lower La/Yb, Sm/Yb and Ce/Pb but higher Ba/Th ratios, and lower εNd and εHf values than the low-Si basalts. The olivines in the high-Si basalts have higher Ni and lower Mn and Ca at a given Fo value than those crystallizing from peridotite melts, and their corresponding melt inclusions have lower CaO contents than peridotite melts, suggesting a garnet pyroxenitic source. The magmatic olivines from low-Si basalts have lower Ni but higher Mn at a given Fo value than that of the high-Si basalts, suggesting more olivine in its source. The olivine-hosted melt inclusions of the low-Si basalts have major elemental signatures different from melts of normal peridotitic or garnet pyroxenitic mantle sources, pointing to their derivation from a carbonated mantle source consisting of peridotite and garnet pyroxenite. We propose a model involving the differential melting of a subduction-modified mantle source to account for the generation of these three suites of basalts. Asthenospheric mantle beneath the eastern NCC, which entrains garnet pyroxenite with an EM1 isotopic signature, was metasomatized by carbonatitic

  16. Recycling of Volatiles in Subduction Zones: Evidence from H-O-S Isotopic Signatures of Indonesian Arc Basalts

    NASA Astrophysics Data System (ADS)

    de Hoog, J. C.; Taylor, B. E.; van Bergen, M. J.

    2001-05-01

    Subaerial lavas from eight volcanoes, situated in different sectors of the Indonesian arc system, have been analyzed for hydrogen, oxygen and sulfur isotopic compositions. The lavas are mostly basaltic and cover a range from low-K tholeiitic to high-K calc-alkaline affinities. The following whole-rock compositions were observed: δ D = -109 to -57‰ V-SMOW (avg = -89‰ ), δ 18O = +5.3 to +6.7‰ V-SMOW (avg = +5.9‰ ), δ 34S = +2.0 to +7.8‰ V-CDT (avg = +4.7‰ ). All samples are strongly degassed, as average water contents are ~0.15 wt.% and average sulfur concentrations are ~14 ppm. Because degassing-induced shifts in sulfur isotopes are limited at magmatic temperatures and oxidation states typical of basalts, the data suggest that, in all cases, the magma source in the sub-arc mantle is significantly enriched in 34S compared to MORB-source mantle. Hydrogen isotopes fractionate strongly, and our data are most consistent with multi-stage degassing of magmatic water. Based on primary H-O-S isotopic signatures inferred for the Indonesian arc basalts and on assumed compositions of the pre-subduction mantle, we performed mass-balance calculations to estimate volatile concentrations in the slab component. The results indicate that addition of about 1-1.5 wt.% aqueous fluid containing 1.5% sulfur will produce the H-O-S isotopic compositions of primary basaltic melts of the volcanoes studied. Alternatively, the slab-derived material may consist of a mix of melts and fluids, in which melts can be more voluminous, but fluids dominate the budgets of sulfur, hydrogen, and other hydrophile elements. Irrespective of the preferred model, S and H2O contents must have increased considerably in the sub-arc mantle (to ~350 ppm and ~1.3 wt.%, respectively). The S value is consistent with estimates based on primary sulfur contents in arc basalts ( ~2000 ppm), but the H2O concentration is higher than the mantle can accommodate, as it will induce melting. We argue, however, that

  17. Miocene to Late Quaternary Patagonian basalts (46 47°S): Geochronometric and geochemical evidence for slab tearing due to active spreading ridge subduction

    NASA Astrophysics Data System (ADS)

    Guivel, Christèle; Morata, Diego; Pelleter, Ewan; Espinoza, Felipe; Maury, René C.; Lagabrielle, Yves; Polvé, Mireille; Bellon, Hervé; Cotten, Joseph; Benoit, Mathieu; Suárez, Manuel; de la Cruz, Rita

    2006-01-01

    Miocene to Quaternary large basaltic plateaus occur in the back-arc domain of the Andean chain in Patagonia. They are thought to result from the ascent of subslab asthenospheric magmas through slab windows generated from subducted segments of the South Chile Ridge (SCR). We have investigated three volcanic centres from the Lago General Carrera-Buenos Aires area (46-47°S) located above the inferred position of the slab window corresponding to a segment subducted 6 Ma ago. (1) The Quaternary Río Murta transitional basalts display major, trace elements, and Sr and Nd isotopic features similar to those of oceanic basalts from the SCR and from the Chile Triple Junction near Taitao Peninsula (e.g., ( 87Sr/ 86Sr) o = 0.70396-0.70346 and ɛNd = + 5.5 - + 3.0). We consider them as derived from the melting of a Chile Ridge asthenospheric mantle source containing a weak subduction component. (2) The Plio-Quaternary (< 3.3 Ma) post-plateau basanites from Meseta del Lago Buenos Aires (MLBA), Argentina, likely derive from small degrees of melting of OIB-type mantle sources involving the subslab asthenosphere and the enriched subcontinental lithospheric mantle. (3) The main plateau basaltic volcanism in this region is represented by the 12.4-3.3-Ma-old MLBA basalts and the 8.2-4.4-Ma-old basalts from Meseta Chile Chico (MCC), Chile. Two groups can be distinguished among these main plateau basalts. The first group includes alkali basalts and trachybasalts displaying typical OIB signatures and thought to derive from predominantly asthenospheric mantle sources similar to those of the post-plateau MLBA basalts, but through slightly larger degrees of melting. The second one, although still dominantly alkalic, displays incompatible element signatures intermediate between those of OIB and arc magmas (e.g., La/Nb > 1 and TiO 2 < 2 wt.%). These intermediate basalts differ from their strictly alkalic equivalents by having lower High Field Strength Element (HFSE) and higher ɛNd (up to

  18. Along-strike trace element and isotopic variation in Aleutian Island arc basalt: Subduction melts sediments and dehydrates serpentine

    NASA Astrophysics Data System (ADS)

    Singer, Brad S.; Jicha, Brian R.; Leeman, William P.; Rogers, Nick W.; Thirlwall, Matthew F.; Ryan, Jeff; Nicolaysen, Kirsten E.

    2007-06-01

    Trace element and Sr-Nd-Pb isotope compositions of basaltic lavas from 11 volcanoes spanning 1300 km of the Aleutian Island arc provide new constraints on the recycling of elements in melts and fluids derived from subducted oceanic crust and sediment. Despite a nearly twofold variation in the flux of sediment subducted along the Aleutians, proxies indicating the presence of sediment melt in the magma source, including Th/La and Th/Nd, do not vary systematically along strike. In contrast, ratios including B/La, B/Nb, B/Be, Cs/La, Pb/Ce, and Li/Y suggest that the quantity or composition of fluid transferred from the slab into the mantle wedge varies an order of magnitude along strike and is apparently correlated with sediment flux. However, the most distinctive fluid addition corresponds spatially with subduction of the Amlia Fracture Zone (AFZ), a likely repository for H2O-rich serpentinite. Sr, Nd, and Pb isotope ratios, together with Th/Nd and B/La ratios, show that the majority of these basalts reflect a common baseline metasomatism of the mantle that accumulated, perhaps over millions of years, via small additions of both slab fluids and partially melted sediment. The paradox of requiring slab surface temperatures high enough to melt a layer of sediment, while lower-temperature dehydration reactions that supply water occur sufficiently deep to flux melting >80 km beneath the volcanoes is reconciled in a four-stage model: (1) as sediment and altered ocean crust is carried to ˜60 km depth and temperatures increase to ˜650°C, metamorphic dehydration reactions release most of the fluid and B to the shallow mantle wedge beneath the fore arc, but some of this mantle is metasomatized and flows downward; (2) the uppermost layer of sediment begins to melt at ˜750°C and >60 km depth; this small volume of melt physically mingles with the overlying metasomatized mantle wedge as it flows further downdip; (3) below the sediment veneer, the uppermost 1 km of ocean crust

  19. Effects of Compositional Variation of Basalt on Subducting Slabs over Time

    NASA Astrophysics Data System (ADS)

    Ko, B.; Shim, S. H.

    2015-12-01

    In much of Earth history, basaltic crust has been injected into the mantle due to its high density. In Archean, downwelling basaltic crust is thought to be much more mafic (Mg/Si=0.72) than that of the modern mid-ocean ridge basalt (MORB) (Mg/Si=0.25). This difference in composition may result in different behaviors of basaltic crust in the mantle with time. We have studied the two compositions of ancient basalt: x=0.8 (Mg/Si=0.72) and x=0.4 (Mg/Si=0.54). The former represents a composition at 3.5 Ga. The glass starting materials were mixed with gold (10 wt%) for internal pressure scale and loaded into the laser-heated diamond-anvil cell (LHDAC) together with a Ne or Ar medium. We have conducted in-situ X-ray diffraction at APS up to 34 GPa and 2200 K. The recovered samples have been analyzed using energy-dispersive X-ray spectroscopy (EDS) in aberration-corrected electron microscopy (ACEM) at ASU. The previous study of the modern MORB reported the post-garnet transition occurs at 26-27 GPa and 1800 K. The same transition was observed at a similar pressure range within 2 GPa for both compositions. The post-spinel transition was observed in x=0.8 at 24±2 GPa, while it does not exist in x=0.4 and modern MORB due to the absence of ringwoodite. At 34 GPa and 2200 K, mineralogy of x=0.8 consists of 82% of bridgmanite (bm), 16% of Ca perovskite (CaPv), and 2% of stishovite (stv), while the modern MORB consists of 30% of stv, 26% of CaPv, 23% of bm, and 22% of aluminous phases (in mole fraction). The mineralogy of x=0.4 is similar to x=0.8 with different proportions, but still different from that of modern MORB. The unit-cell volumes of quenched bm at 1 bar and 300 K for x=0.8 and 0.4 were 166.05(5) and 165.69(6) Å3, respectively, which are smaller than bm in modern MORB (170.0(1) Å3). At high pressures, the unit-cell volume of CaPv is greater in the x=0.4 and 0.8 compositions. The compositions of bm and CaPv in x=0.8 show less Al, but more Mg than those found in

  20. Nd-isotope systematics of ˜2.7 Ga adakites, magnesian andesites, and arc basalts, Superior Province: evidence for shallow crustal recycling at Archean subduction zones

    NASA Astrophysics Data System (ADS)

    Polat, Ali; Kerrich, Robert

    2002-09-01

    An association of adakite, magnesian andesite (MA), and Nb-enriched basalt (NEB) volcanic flows, which erupted within 'normal' intra-oceanic arc tholeiitic to calc-alkaline basalts, has recently been documented in ˜2.7 Ga Wawa greenstone belts. Large, positive initial ɛNd values (+1.95 to +2.45) of the adakites signify that their basaltic precursors, with a short crustal residence, were derived from a long-term depleted mantle source. It is likely that the adakites represent the melts of subducted late Archean oceanic crust. Initial ɛNd values in the MA (+0.14 to +1.68), Nb-enriched basalts and andesites (NEBA) (+1.11 to +2.05), and 'normal' intra-oceanic arc tholeiitic to calc-alkaline basalts and andesites (+1.44 to +2.44) overlap with, but extend to lower values than, the adakites. Large, tightly clustered ɛNd values of the adakites, together with Th/Ce and Ce/Yb systematics of the arc basalts that rule out sediment melting, place the enriched source in the sub-arc mantle. Accordingly, isotopic data for the MA, NEBA, and 'normal' arc basalts can be explained by melting of an isotopically heterogeneous sub-arc mantle that had been variably enriched by recycling of continental material into the shallow mantle in late Archean subduction zones up to 200 Ma prior to the 2.7 Ga arc. If the late Archean Wawa adakites, MA, and basalts were generated by similar geodynamic processes as their counterparts in Cenozoic arcs, involving subduction of young and/or hot ocean lithosphere, then it is likely that late Archean oceanic crust, and arc crust, were also created and destroyed by modern plate tectonic-like geodynamic processes. This study suggests that crustal recycling through subduction zone processes played an important role for the generation of heterogeneity in the Archean upper mantle. In addition, the results of this study indicate that the Nd-isotope compositions of Archean arc- and plume-derived volcanic rocks are not very distinct, whereas Phanerozoic plumes

  1. A Strongly Calc-alkaline Suite in the Midst of the Tholeiitic Columbia River Basalt Province: Implications for Generating the Calc-alkaline Trend Without Subduction Processes

    NASA Astrophysics Data System (ADS)

    Steiner, A. R.; Streck, M. J.

    2012-12-01

    The mid-Miocene lavas of the Strawberry Volcanics (SV), distributed over 3,400 km2 in NE Oregon, comprise a diverse volcanic suite, which span the range of compositions from basalt to rhyolite. The volcanic suite is mainly composed of calc-alkaline (CA) basaltic andesite and andesite, yet tholeiitic (TH) lavas of basalt to andesite occur as well. The SV lies in the heart of nearly coeval flood basalts of the Columbia River province of the Pacific Northwest. The unique combination of strongly CA rocks of the SV in a non-subduction setting provide an excellent opportunity to study controls on inducing CA evolution in the midst of a TH province and independent of processes taking places at an active subduction zone. New 40Ar/39Ar ages indicate CA basaltic andesites to andesites of the SV erupted at least from 14.78±0.13 Ma to 12.44±0.12 Ma demonstrating that CA magmatism of the SV was ongoing during the eruptions of the tholeiitic Wanapum Basalt member of the Columbia River Basalt Group (CRBG). This range will likely be extended to even older ages in the future because existent age dates did not include samples from near the base of the SV. Thickness of intermediate lavas flows of the SV range from 15 m to as thin as 2 m and lavas are characterized by mostly phenocryst poor lithologies. When phenocrysts are abundant they are very small suggesting growth late during eruption. Single lava flow sections can include on the order of 30 conformable flows, testifying to a vigorous eruption history. The thickest andesitic sections are located in the glacially carved mountains of the Strawberry Mountain Wilderness (i.e. Strawberry Mountain, High Lake, and Slide Lake) where several vent complexes are exposed, which are delineated by dikes and plugs with finely interlocking plutonic textures, cross-cutting SV lava flows. Dikes generally strike NW-SE. Subtle variations in major and trace element compositions exist between TH and CA lavas of the SV. The CA lavas of the SV are

  2. The effect of anhydrite saturation on the fate of sulfur during fluid-present melting of subducting basaltic crust

    NASA Astrophysics Data System (ADS)

    Jego, S.; Dasgupta, R.

    2012-12-01

    The apparent sulfur enrichment of sub-arc mantle is thought to derive from an oxidized downgoing slab, and it has been suggested that the slab-derived sulfate species is responsible for oxidizing the mantle wedge [1]. However, the conditions and extent of sulfur transfer from the subducting slab to the mantle wedge are poorly understood. In particular, the relative mobility of sulfur as a function of oxygen fugacity (fO2) is unconstrained at sub-arc depths. To add to our recent study on sulfur mobility during fluid-present melting of a sulfide-bearing basaltic crust [2], here we constrain the fate of sulfur during similar melting at relatively oxidizing conditions, i.e., at sulfate saturation. Experiments were performed using a piston cylinder device at P = 2-3 GPa, T = 950-1050 °C. A synthetic MORB + 6.8 wt.% H2O doped with 1 wt% S (added as pyrite) was contained in AuPd inner capsules and hematite-magnetite (HM: ~FMQ+3.9 to +4.6) mixture used as fO2 buffer was housed in Pt outer capsules, following the recently proposed design of ref. [3]. Sulfur concentration in quenched silicate glasses, the major element phase compositions, and fO2 of the experiments based on dissolved Fe contents in AuPd and added Pt sensor [4, 5], were determined using EPMA. All experiments contain silicate melt, cpx, garnet, anhydrite, rutile and/or Ti-magnetite, and are fluid saturated. The partial melt compositions are rhyolitic to rhyodacitic with increasing T and melting degree. Sulfur contents in the melt range from ~700 to 3000 ppm, and increase with increasing P and T, in agreement with published SCAS models [6, 7]. Mass balance calculations show that the proportion of sulfur dissolved in silicate melt can be >13% of the bulk sulfur at 1050 °C. However, at slab surface (<900 °C), the major part of the bulk sulfur present in the slab is dissolved in the aqueous fluid phase, the rest being stored as anhydrite crystals. Moreover, our results suggest that sulfur partition coefficient

  3. High-Mg# andesites and basalts from the Kamchatka-Kurile subduction system: Implications for primitive arc magma genesis and mantle wedge processes

    NASA Astrophysics Data System (ADS)

    Bryant, J. A.; Yogodzinski, G. M.; Churikova, T. G.; Volynets, O. N.

    2007-12-01

    Primitive arc magmatism and mantle wedge processes are investigated through a petrologic and geochemical study of high Mg# (Mg/Mg+Fe>0.65) basalts and andesites from the Kurile-Kamchatka subduction system. The primitive andesites are from the Shisheisky complex (Portnyagin et al., AGU Monograph 172, 2007), a field of Quaternary-age, monogenetic cones located in the Aleutian-Kamchatka junction, north of Shiveluch Volcano, the northernmost active composite cone in Kamchatka. The Shisheisky lavas are similar to primitive andesites from Mt. Shasta, Piip Volcano, and Setouchi, Japan. They have Mg# of 0.66-0.73 at intermediate SiO2 (54-58 wt%), low CaO/Al2O3 (<0.54), and high Ni (184-243 ppm) and Cr (418-880 ppm). Olivine phenocryst core compositions of ~FO90 appear to be in equilibrium with whole-rock `melts', consistent with the aphyric to sparsely phyric nature of these lavas. Compared to the Shishiesky andesites, primitive basalts from the region (Alaid, Tolbachik, Kharchinsky) have higher CaO/Al2O3 (0.69-0.86), and lower whole-rock Ni (105-182 ppm), Cr (395-531 ppm), and Ni/MgO (10-17) at similar Mg# (0.66-.70). Olivine phenocrysts in the basalts have similarly higher CaO, lower Ni, and lower Ni/MgO at ~FO88 compared to the andesites. The absence of plagioclase phenocrysts from the primitive andesites strongly contrasts petrographic observations of the plagioclase-phyric basalts, indicating relatively high pre-eruptive water contents for the andesites compared to the basalts. Petrographic and mineral composition data suggest that the Shisheisky primitive andesites were liquids in equilibrium with mantle peridotite, and were not produced by mixing between primitive basalts and evolved felsic magmas or from contamination by xenocrystic olivine. The key features of the Shisheisky primitive andesites (e.g., low CaO/Al2O3 and high Ni/MgO at high Mg#) appear to have been acquired at sub-moho depths, by processes and under physical conditions in the mantle wedge (lower

  4. 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

  5. Hf-Nd Isotopes in West Philippine Basin Basalts: Results from International Ocean Discovery Program (IODP) Site U1438 and Implications for the Early History of the Izu-Bonin-Mariana (IBM) Subduction System

    NASA Astrophysics Data System (ADS)

    Yogodzinski, G. M.; Hocking, B.; Bizimis, M.; Hickey-Vargas, R.; Ishizuka, O.; Bogus, K.; Arculus, R. J.

    2015-12-01

    Drilling at IODP Site U1438, located immediately west of Kyushu-Palau Ridge (KPR), the site of IBM subduction initiation, penetrated 1460 m of volcaniclastic sedimentary rock and 150 m of underlying basement. Biostratigraphic controls indicate a probable age for the oldest sedimentary rocks at around 55 Ma (51-64 Ma - Arculus et al., Nat Geosci in-press). This is close to the 48-52 Ma time period of IBM subduction initiation, based on studies in the forearc. There, the first products of volcanism are tholeiitic basalts termed FAB (forearc basalt), which are more depleted than average MORB and show subtle indicators of subduction geochemical enrichment (Reagan et al., 2010 - Geochem Geophy Geosy). Shipboard data indicate that Site U1438 basement basalts share many characteristics with FABs, including primitive major elements (high MgO/FeO*) and strongly depleted incompatible element patterns (Ti, Zr, Ti/V and Zr/Y below those of average MORB). Initial results thus indicate that FAB geochemistry may have been produced not only in the forearc, but also in backarc locations (west of the KPR) at the time of subduction initiation. Hf-Nd isotopes for Site 1438 basement basalts show a significant range of compositions from ɛNd of 7.0 to 9.5 and ɛHf of 14.5 to 19.8 (present-day values). The data define a well-correlated and steep array in Hf-Nd isotope space. Relatively radiogenic Hf compared to Nd indicates an Indian Ocean-type MORB source, but the dominant signature, with ɛHf >16.5, is more radiogenic than most Indian MORB. The pattern through time is from more-to-less radiogenic and more variable Hf-Nd isotopes within the basement section. This pattern culminates in basaltic andesite sills, which intrude the lower parts of the sedimentary section. The sills have the least radiogenic compositions measured so far (ɛNd ~6.6, ɛHf ~13.8), and are similar to those of boninites of the IBM forearc and modern IBM arc and reararc rocks. The pattern within the basement

  6. 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.

  7. Subduction-modified oceanic crust mixed with a depleted mantle reservoir in the sources of the Karoo continental flood basalt province

    NASA Astrophysics Data System (ADS)

    Heinonen, Jussi S.; Carlson, Richard W.; Riley, Teal R.; Luttinen, Arto V.; Horan, Mary F.

    2014-05-01

    The great majority of continental flood basalts (CFBs) have a marked lithospheric geochemical signature, suggesting derivation from the continental lithosphere, or contamination by it. Here we present new Pb and Os isotopic data and review previously published major element, trace element, mineral chemical, and Sr and Nd isotopic data for geochemically unusual mafic and ultramafic dikes located in the Antarctic segment (Ahlmannryggen, western Dronning Maud Land) of the Karoo CFB province. Some of the dikes show evidence of minor contamination with continental crust, but the least contaminated dikes exhibit depleted mantle - like initial ɛNd (+9) and 187Os/188Os (0.1244-0.1251) at 180 Ma. In contrast, their initial Sr and Pb isotopic compositions (87Sr/86Sr = 0.7035-0.7062, 206Pb/204Pb = 18.2-18.4, 207Pb/204Pb = 15.49-15.52, 208Pb/204Pb = 37.7-37.9 at 180 Ma) are more enriched than expected for depleted mantle, and the major element and mineral chemical evidence indicate contribution from (recycled) pyroxenite sources. Our Sr, Nd, Pb, and Os isotopic and trace element modeling indicate mixed peridotite-pyroxenite sources that contain ˜10-30% of seawater-altered and subduction-modified MORB with a recycling age of less than 1.0 Ga entrained in a depleted Os-rich peridotite matrix. Such a source would explain the unusual combination of elevated initial 87Sr/86Sr and Pb isotopic ratios and relative depletion in LILE, U, Th, Pb and LREE, high initial ɛNd, and low initial 187Os/188Os. Although the sources of the dikes probably did not play a major part in the generation of the Karoo CFBs in general, different kind of recycled source components (e.g., sediment-influenced) would be more difficult to distinguish from lithospheric CFB geochemical signatures. In addition to underlying continental lithosphere, the involvement of recycled sources in causing the apparent lithospheric geochemical affinity of CFBs should thus be carefully assessed in every case.

  8. Devonian Nb-enriched basalts and andesites of north-central Tibet: Evidence for the early subduction of the Paleo-Tethyan oceanic crust beneath the North Qiangtang Block

    NASA Astrophysics Data System (ADS)

    Zhang, Hongrui; Yang, Tiannan; Hou, Zengqian; Bian, Yeke

    2016-07-01

    The early evolution of the Tethyan Ocean in north-central Tibet is currently poorly constrained. A sequence of volcanic rocks ranging from basic to intermediate in composition has been identified in the Zaduo area of the North Qiangtang Block. SHRIMP U-Pb dating of zircons from a sample of Zaduo andesite suggests an eruption age of Late Devonian (~ 380 Ma). The Zaduo volcanic rocks exhibit geochemical characteristics similar to those of typical Nb-enriched basalts, with relatively high Nb, Ta, and Zr contents, resulting in high Nb/La ratios (0.70-1.08) and Nb/U ratios (10.57-34.37). The relative enrichment in high field strength elements, together with positive εNd(t) values of + 4.6 to + 5.8 and low (87Sr/86Sr)i ratios of 0.70367-0.70532, indicates the Zaduo volcanic rocks were derived from a depleted mantle source metasomatized by silicate melts of a subducted oceanic slab. The occurrence of Nb-enriched volcanic rocks in the North Qiangtang Block suggests that the subduction of Paleo-Tethyan oceanic crust was initiated in the Late Devonian. Available geochronological data from ophiolites surrounding the North Qiangtang Block suggest that the subducted slab is most likely the Longmucuo-Shuanghu Paleo-Tethyan oceanic crust.

  9. Magmatic relationships between depleted mantle harzburgites, boninitic cumulate gabbros and subduction-related tholeiitic basalts in the Puerto Plata ophiolitic complex, Dominican Republic: Implications for the birth of the Caribbean island-arc

    NASA Astrophysics Data System (ADS)

    Escuder-Viruete, Javier; Castillo-Carrión, Mercedes; Pérez-Estaún, Andrés

    2014-05-01

    The Lower Cretaceous Puerto Plata ophiolitic complex (PPC) occurs west of the main collisional suture between the Caribbean and North American plates in the northern Dominican Republic, and imposes important constraints on the geochemical and tectonic processes associated with the birth of the Caribbean island-arc. The PPC exposes a tectonically dismembered 3.0-km-thick section of upper mantle harzburgites, lower crustal cumulate gabbroic rocks and upper crustal basaltic pillow lavas, volcanic breccias and pelagic sediments. The harzburgites exhibit a highly depleted signature in terms of their modal compositions, mineral chemistry and whole rock major and trace element contents, suggesting that they are residues after high-degrees of partial melting. Melt modeling suggests that they were similar in trace element characteristics to a boninite. In the crustal sequence, three magmatic episodes have been recognized based on field, mineral and geochemical data. The first phase is composed of the lower layered gabbronorites, which are variably deformed and recrystallized at high-temperature conditions. Trace element modeling suggests that the gabbronorites crystallized from LREE-depleted island-arc tholeiitic (IAT) melts. The second phase is composed of the intermediate layered troctolites (126 Ma), which are undeformed and preserve igneous cumulate textures. Modeling indicates that the troctolites crystallized from boninitic melts. The gabronorite-troctolite substrate was intruded by a third, supra-subduction zone tholeiitic magmatic phase at < 126 Ma, which formed the upper olivine gabbros and gabbronorites. These gabbroic rocks formed from melts similar in composition to the IAT basalts and basaltic andesites of the overlying Los Caños Fm. Contemporaneous Aptian to lower Albian mafic volcanic rocks of the Los Ranchos Fm and Cacheal complex have comparable IAT geochemical and isotopic signatures, suggesting that all of them may have erupted over a single piece of the

  10. Fore-arc mantle peridotites and back-arc basin basalts from the Izu-Bonin-Mariana subduction factory (ODP LEGs 125 and 195): a modern analogue for Mediterranean ophiolites

    NASA Astrophysics Data System (ADS)

    Zanetti, A.; D'Antonio, M.; Vannucci, R.; Raffone, N.; Spadea, P.

    2009-04-01

    occurring in all IBM forearc peridotites (e.g. crystallisation of late cpx, embayment of opx porphyroclasts), and likely marks the accretion of the mantle sequence to the thermal boundary layer. It was accompanied by the devolopment of transient geochemical gradients in the migrating liquids mainly governed by chromatographic-type chemical exchange with the peridotite. The West Philippine Basin (WPB) is a back-arc basin that opened in the Philippine Sea Plate (PSP) between the current position of the Palau-Kyushu Ridge (PKR) and the margin of East Asia. Spreading occurred at the Central Basin Fault (CBF) from 54 to 30 Ma. The PKR was active since ~48 to 35 Ma constituting a single volcanic arc with the Izu-Bonin-Mariana Arc. ODP Leg 195 Site 1201 is located in the WPB, ~100 km west of the PKR, on 49 Ma basaltic crust formed by NE-SW spreading at the CBF. From ~35 to 30 Ma, pelagic sedimentation at Site 1201 was followed by turbidite sedimentation, fed mostly by early Mariana Arc (PKR)-derived volcanic clasts. These volcanics are calc-alkaline, whereas PKR rocks from literature have mostly boninitic and arc tholeiitic affinity; the WPB basement basalts have MORB to arc-like affinity, as expected for a back-arc basin. Sr, Nd, Pb and Hf isotope data highlight the Indian Ocean MORB-like character of WPB basement basalts, suggesting an upper mantle domain distinct from that underlying the Pacific Plate. The geochemical and isotopic features of PKR volcanics reflect higher amounts of subduction-derived components, added mostly as siliceous melts, in the source of arc magmas relative to that of basement basalts. In that respect, Site 1201 PKR volcanics resemble calc-alkaline volcanics of the currently active Mariana Arc. In addition, their calc-alkaline affinity, unradiogenic neodymium, and inferred Middle Oligocene age, suggest they might represent an evolved stage of arc volcanism at Palau-Kyushu Ridge, perhaps shortly before the end of its activity.

  11. Partial melting of subducting oceanic crust

    NASA Astrophysics Data System (ADS)

    Peacock, Simon M.; Rushmer, Tracy; Thompson, Alan Bruce

    1994-01-01

    The conditions under which partial melting of subducting oceanic crust occurs can be determined by combining a partial melting model for basaltic compositions with two-dimensional thermal models of subduction zones. For porosities of approximately 1% containing H2O the amount of partial melt generated at the wet basaltic solidus is limited to less than 5 vol%. At higher temperatures (approximately 1000 C at 1.5 GPa) large amounts of partial melt, up to 50 vol%, form by the breakdown of amphibole and the release of structurally bound H2O. In most subduction zones, substantial partial melting of subducting oceanic crust will only occur if high shear stresses (greater than approximately 100 MPa) can be maintained by rocks close to, or above, their melting temperatures. In the absence of high shear stresses, substantial melting of the oceanic crust will only occur during subduction of very young (less than 5 Ma) oceanic lithosphere. Partial melting of hydrated basalt (amphibolites) derived from the mid-ocean ridge has been proposed as being responsible for the generation of certain recent high-Al andesitic to dacitic volcanic rocks (adakites). Three of these volcanic suites (Mount St. Helens, southern Chile, and Panama) occur in volcanic arcs where oceanic crust less than 25 Ma is being subducted at rates of 1 - 3 cm/yr and the calculated thermal regime is several hundreds of degrees hotter than more typical subduction zone environments. However, oceanic lithosphere is not currently being subducted beneath Baja and New Guinea, where recent adakites are also present, suggesting that some adakite magmas may form by water-undersaturated partial melting of underplated mafic lower crust or previously subducted oceanic crust. Further experimental work on compositions representative of oceanic crust is required to define the depth of possible adakite source regions more accurately.

  12. Links between fluid circulation, temperature, and metamorphism in subducting slabs

    USGS Publications Warehouse

    Spinelli, G.A.; Wang, K.

    2009-01-01

    The location and timing of metamorphic reactions in subducting lithosph??re are influenced by thermal effects of fluid circulation in the ocean crust aquifer. Fluid circulation in subducting crust extracts heat from the Nankai subduction zone, causing the crust to pass through cooler metamorphic faci??s than if no fluid circulation occurs. This fluid circulation shifts the basalt-to-eclogite transition and the associated slab dehydration 14 km deeper (35 km farther landward) than would be predicted with no fluid flow. For most subduction zones, hydrothermal cooling of the subducting slab will delay eclogitization relative to estimates made without considering fluid circulation. Copyright 2009 by the American Geophysical Union.

  13. Seismic coupling and uncoupling at subduction zones

    NASA Technical Reports Server (NTRS)

    Ruff, L.; Kanamori, H.

    1983-01-01

    Some of the correlations concerning the properties of subduction zones are reviewed. A quantitative global comparison of many subduction zones reveals that the largest earthquakes occur in zones with young lithosphere and fast convergence rates. Maximum earthquake size is directly related to the asperity distribution on the fault plane. This observation can be translated into a simple model of seismic coupling where the horizontal compressive stress between two plates is proportional to the ratio of the summed asperity area to the total area of the contact surface. Plate age and rate can control asperity distribution directly through the horizontal compressive stress associated with the vertical and horizontal velocities of subducting slabs. The basalt to eclogite phase change in the down-going oceanic crust may be largely responsible for the uncoupling of subduction zones below a depth of about 40 km.

  14. A late-Carboniferous to early early-Permian subduction-accretion complex in Daqing pasture, southeastern Inner Mongolia: Evidence of northward subduction beneath the Siberian paleoplate southern margin

    NASA Astrophysics Data System (ADS)

    Liu, Jianfeng; Li, Jinyi; Chi, Xiaoguo; Qu, Junfeng; Hu, Zhaochu; Fang, Shu; Zhang, Zhong

    2013-09-01

    A subduction-accretion complex is identified from previously defined late-Carboniferous and early-Permian strata in Daqing pasture, southern Xiwuqi, Inner Mongolia. The subduction-accretion complex is composed of a matrix of siltstone and exotic blocks of bioclastic limestone, pillow basalt, foliated basalt, chert and asbestos. The pillow basalt possesses the geochemical characteristics of mid-ocean ridge basalt (N-MORB), whereas the foliated basalt displays the geochemical characteristics of island arc basalt (IAB), indicating that these basalts are of different origins. U-Pb (zircon) dating indicates that the foliated basalt formed in the late-Carboniferous (314.5 to 318.4 Ma) and the bioclastic limestone formed in the early early-Permian. Combined with regional geological data, the subduction-accretion complex and coeval calc-alkaline granitic belt to the north constitute the essential elements of the late-Carboniferous to early early-Permian subduction zone on the southern margin of the Siberian paleoplate. The zircon εHf(t) values of the foliated basalt are positive (+ 14.4 to + 23.9), suggesting that this basalt originated directly from depleted mantle. The temporal-spatial distribution of the subduction-accretion complex and ophiolite belts in southeastern Inner Mongolia indicates that there was significant lateral crustal growth on the southern margin of the Siberian paleoplate in the late Paleozoic.

  15. The thermal effect of fluid circulation in the subducting crust on slab melting in the Chile subduction zone

    NASA Astrophysics Data System (ADS)

    Spinelli, Glenn A.; Wada, Ikuko; He, Jiangheng; Perry, Matthew

    2016-01-01

    Fluids released from subducting slabs affect geochemical recycling and melt generation in the mantle wedge. The distribution of slab dehydration and the potential for slab melting are controlled by the composition/hydration of the slab entering a subduction zone and the pressure-temperature path that the slab follows. We examine the potential for along-strike changes in temperatures, fluid release, and slab melting for the subduction zone beneath the southern portion of the Southern Volcanic Zone (SVZ) in south central Chile. Because the age of the Nazca Plate entering the subduction zone decreases from ∼14 Ma north of the Guafo Fracture Zone to ∼6 Ma to the south, a southward warming of the subduction zone has been hypothesized. However, both north and south of Guafo Fracture Zone the geochemical signatures of southern SVZ arc lavas are similar, indicating 3-5 wt.% sediment melt and little to no contribution from melt of subducted basalt or aqueous fluids from subducted crust. We model temperatures in the system, use results of the thermal models and the thermodynamic calculation code Perple_X to estimate the pattern of dehydration-derived fluid release, and examine the potential locations for the onset of melting of the subducting slab. Surface heat flux observations in the region are most consistent with fluid circulation in the high permeability upper oceanic crust redistributing heat. This hydrothermal circulation preferentially cools the hottest parts of the system (i.e. those with the youngest subducting lithosphere). Models including the thermal effects of fluid circulation in the oceanic crust predict melting of the subducting sediment but not the basalt, consistent with the geochemical observations. In contrast, models that do not account for fluid circulation predict melting of both subducting sediment and basalt below the volcanic arc south of Guafo Fracture Zone. In our simulations with the effects of fluid circulation, the onset of sediment

  16. Metamorphic Perspectives of Subduction Zone Volatiles Cycling

    NASA Astrophysics Data System (ADS)

    Bebout, G. E.

    2008-12-01

    Field study of HP/UHP metamorphic rocks provides "ground-truthing" for experimental and theoretical petrologic studies estimating extents of deep volatiles subduction, and provides information regarding devolatilization and deep subduction-zone fluid flow that can be used to reconcile estimates of subduction inputs and arc volcanic outputs for volatiles such as H2O, N, and C. Considerable attention has been paid to H2O subduction in various bulk compositions, and, based on calculated phase assemblages, it is thought that a large fraction of the initially structurally bound H2O is subducted to, and beyond, subarc regions in most modern subduction zones (Hacker, 2008, G-cubed). Field studies of HP/UHP mafic and sedimentary rocks demonstrate the impressive retention of volatiles (and fluid-mobile elements) to depths approaching those beneath arcs. At the slab-mantle interface, high-variance lithologies containing hydrous phases such as mica, amphibole, talc, and chlorite could further stabilize H2O to great depth. Trench hydration in sub-crustal parts of oceanic lithosphere could profoundly increase subduction inputs of particularly H2O, and massive flux of H2O-rich fluids from these regions into the slab-mantle interface could lead to extensive metasomatism. Consideration of sedimentary N concentrations and δ15N at ODP Site 1039 (Li and Bebout, 2005, JGR), together with estimates of the N concentration of subducting altered oceanic crust (AOC), indicates that ~42% of the N subducting beneath Nicaragua is returned in the corresponding volcanic arc (Elkins et al., 2006, GCA). Study of N in HP/UHP sedimentary and basaltic rocks indicates that much of the N initially subducted in these lithologies would be retained to depths approaching 100 km and thus available for addition to arcs. The more altered upper part of subducting oceanic crust most likely to contribute to arcs has sediment-like δ15NAir (0 to +10 per mil; Li et al., 2007, GCA), and study of HP/UHP eclogites

  17. Continental Flood Basalts

    NASA Astrophysics Data System (ADS)

    Continental flood basalts have been receiving considerable scientific attention lately. Recent publications have focused on several particular flood-basalt provinces (Brito-Arctic, Karoo, Parana', Deccan, and Columbia Plateau), and much attention has been given to the proposed connection between flood-basalt volcanism, bolide impacts, and mass extinctions. The editor of Continental Flood Basalts, J. D. Macdougall, conceived the book to assemble in a single volume, from a vast and scattered literature, an overview of each major post-Cambrian flood-basalt province.Continental Flood Basalts has 10 chapters; nine treat individual flood-basalt provinces, and a summary chapter compares and contrasts continental flood-basalts and mid-oceanic ridge basalts. Specifically, the chapters address the Columbia River basalt, the northwest United States including the Columbia River basalt, the Ethiopian Province, the North Atlantic Tertiary Province, the Deccan Traps, the Parana' Basin, the Karoo Province, the Siberian Platform, and Cenozoic basaltic rocks in eastern China. Each chapter is written by one or more individuals with an extensive background in the province.

  18. A new source of water in seismogenic subduction zones

    NASA Astrophysics Data System (ADS)

    Kameda, Jun; Yamaguchi, Asuka; Saito, Saneatsu; Sakuma, Hiroshi; Kawamura, Katsuyuki; Kimura, Gaku

    2011-11-01

    Seismogenic plate-boundary faults at accretionary margins (e.g., the Nankai margin, southwest Japan) may occur where the uppermost part of subducting oceanic crust, composed of basaltic rocks, is in contact with the overriding plate of a lithified accretionary prism. The plate-boundary faults in ancient accretionary complexes typically record high-velocity slip under fluid-rich conditions. Although previous studies have emphasized the mechanical significance of fluids in terms of dynamic slip-weakening, the source of fluid in seismogenic subduction zones remains poorly constrained. In this work, we focus on the hydrous smectite in the uppermost oceanic crust, an alteration product of intact basalt before arrival at the trench axis. A comparison between (1) new mineralogical data on basalt drillcore recovered by Integrated Ocean Drilling Program (IODP) Expedition 322 at site C0012, a reference site for subduction input to the Nankai Trough, and (2) mineralogical data on basalt within ancient oceanic crust embedded in a fossil accretionary complex of the Shimanto Belt, southwest Japan, suggests that progressive smectite-chlorite conversion would liberate bound fluids at a rate of 0.34 to 0.65 × 10-14 s-1 along the plate interface. This rate of fluid production appears to be more than an order of magnitude greater than that from other possible sources, including from overlying sediments via smectite-illite conversion and the expulsion of pore fluids, and may facilitate seismic slip along plate-boundary faults.

  19. Three-dimensional dynamic laboratory models of subduction with an overriding plate and variable interplate rheology

    NASA Astrophysics Data System (ADS)

    Duarte, João C.; Schellart, Wouter P.; Cruden, Alexander R.

    2013-10-01

    Subduction zones are complex 3-D features in which one tectonic plate sinks underneath another into the deep mantle. During subduction the overriding plate (OP) remains in physical contact with the subducting plate and stresses generated at the subduction zone interface and by mantle flow force the OP to deform. We present results of 3-D dynamic laboratory models of subduction that include an OP. We introduce new interplate materials comprising homogeneous mixtures of petrolatum and paraffin oil to achieve progressive subduction. The rheology of these mixtures is characterized by measurements using a strain rate controlled rheometer. The results show that the strength of the mixture increases with petrolatum content, which can be used as a proxy for the degree of mechanical coupling along the subduction interface. Results of subduction experiments are presented with different degrees of mechanical coupling and the influence this has on the dynamics and kinematics of subduction. The modelling results show that variations in the degree of mechanical coupling between the plates have a major impact on subduction velocities, slab geometry and the rate of OP deformation. In all experiments the OP is displaced following trench migration and experiences overall extension localized in the plate interior. This suggests that OP deformation is driven primarily by the toroidal component of subduction-related mantle return flow. The subduction rate is always very slow in experiments with medium mechanical coupling, and subduction stops prematurely in experiments with very high coupling. This implies that the shear forces along the plate interface in natural subduction zone systems must be relatively low and do not vary significantly. Otherwise a higher variability in natural subduction velocities should be observed for mature, non-perturbed subduction zones. The required low shear force is likely controlled by the rheology of highly hydrated sedimentary and basaltic rocks.

  20. Evolution of the Archaean crust by delamination and shallow subduction.

    PubMed

    Foley, Stephen F; Buhre, Stephan; Jacob, Dorrit E

    2003-01-16

    The Archaean oceanic crust was probably thicker than present-day oceanic crust owing to higher heat flow and thus higher degrees of melting at mid-ocean ridges. These conditions would also have led to a different bulk composition of oceanic crust in the early Archaean, that would probably have consisted of magnesium-rich picrite (with variably differentiated portions made up of basalt, gabbro, ultramafic cumulates and picrite). It is unclear whether these differences would have influenced crustal subduction and recycling processes, as experiments that have investigated the metamorphic reactions that take place during subduction have to date considered only modern mid-ocean-ridge basalts. Here we present data from high-pressure experiments that show that metamorphism of ultramafic cumulates and picrites produces pyroxenites, which we infer would have delaminated and melted to produce basaltic rocks, rather than continental crust as has previously been thought. Instead, the formation of continental crust requires subduction and melting of garnet-amphibolite--formed only in the upper regions of oceanic crust--which is thought to have first occurred on a large scale during subduction in the late Archaean. We deduce from this that shallow subduction and recycling of oceanic crust took place in the early Archaean, and that this would have resulted in strong depletion of only a thin layer of the uppermost mantle. The misfit between geochemical depletion models and geophysical models for mantle convection (which include deep subduction) might therefore be explained by continuous deepening of this depleted layer through geological time. PMID:12529633

  1. Central Cascadia subduction zone creep

    NASA Astrophysics Data System (ADS)

    Schmalzle, Gina M.; McCaffrey, Robert; Creager, Kenneth C.

    2014-04-01

    Cascadia between 43°N and 46°N has reduced interseismic uplift observed in geodetic data and coseismic subsidence seen in multiple thrust earthquakes, suggesting elevated persistent fault creep in this section of the subduction zone. We estimate subduction thrust "decade-scale" locking and crustal block rotations from three-component continuous Global Positioning System (GPS) time series from 1997 to 2013, as well as 80 year tide gauge and leveling-derived uplift rates. Geodetic observations indicate coastal central Oregon is rising at a slower rate than coastal Washington, southern Oregon and northern California. Modeled locking distributions suggest a wide locking transition zone that extends inland under central Oregon. Paleoseismic records of multiple great earthquakes along Cascadia indicate less subsidence in central Oregon. The Cascade thrust under central Oregon may be partially creeping for at least 6500 years (the length of the paleoseismic record) reducing interseismic uplift and resulting in reduced coseismic subsidence. Large accretions of Eocene age basalt (Siletzia terrane) between 43°N and 46°N may be less permeable compared to surrounding terranes, potentially increasing pore fluid pressures along the fault interface resulting in a wide zone of persistent fault creep. In a separate inversion, three-component GPS time series from 1 July 2005 to 1 January 2011 are used to estimate upper plate deformation, locking between slow-slip events (SSEs), slip from 16 SSEs and an earthquake mechanism. Cumulative SSEs and tectonic tremor are weakest between 43°N and 46°N where partial fault creep is increased and Siletzia terrane is thick, suggesting that surrounding rock properties may influence the mode of slip.

  2. Geochemical constraints on possible subduction components in lavas of Mayon and Taal Volcanoes, Southern Luzon, Philippines

    USGS Publications Warehouse

    Castillo, P.R.; Newhall, C.G.

    2004-01-01

    Mayon is the most active volcano along the east margin of southern Luzon, Philippines. Petrographic and major element data indicate that Mayon has produced a basaltic to andesitic lava series by fractional crystallization and magma mixing. Trace element data indicate that the parental basalts came from a heterogeneous mantle source. The unmodified composition of the mantle wedge is similar to that beneath the Indian Ocean. To this mantle was added a subduction component consisting of melt from subducted pelagic sediment and aqueous fluid dehydrated from the subducted basaltic crust. Lavas from the highly active Taal Volcano on the west margin of southern Luzon are compositionally more variable than Mayon lavas. Taal lavas also originated from a mantle wedge metasomatized by aqueous fluid dehydrated from the subducted basaltic crust and melt plus fluid derived from the subducted terrigenous sediment. More sediment is involved in the generation of Taal lavas. Lead isotopes argue against crustal contamination. Some heterogeneity of the unmodified mantle wedge and differences in whether the sediment signature is transferred into the lava source through an aqueous fluid or melt phase are needed to explain the regional compositional variation of Philippine arc lavas. ?? Oxford University Press 2004; all rights reserved.

  3. Second critical endpoints and their bearing on subduction zone magmatism

    NASA Astrophysics Data System (ADS)

    Mibe, K.

    2011-12-01

    Understanding the phase relations in silicate-H2O systems is fundamental for clarifying the physical and chemical evolution of the Earth, because H2O affects melting temperature of rocks, composition of magmas generated, and rheology of rocks. Under high pressure and high temperature conditions, it is known that the solubility of both water in silicate melt and silicate in aqueous fluid increases with increasing pressure. As a result, silicate melt and aqueous fluid in the Earth's interior is expected to become supercritical fluid and the hydrous solidus of the system can no longer be defined beyond a certain critical condition. This condition is called the second critical endpoint and is the point of intersection between the critical curve and hydrous solidus. In recent years, the second critical endpoints in the systems peridotite-H2O and basalt-H2O have been determined using high-pressure and high-temperature X-ray radiography technique [Mibe et al., 2007, JGR; 2011, PNAS]. In these studies, it was concluded that the second critical endpoints in the systems peridotite-H2O and basalt-H2O occurred at around 3.8 and 3.4 GPa, respectively. These results suggest that the aqueous fluid and silicate melt becomes indistinguishable at the depths deeper than ~120 km in the mantle wedge peridotite and ~100 km in the subducting basaltic oceanic crust in subduction zones. The melting temperature of the subducting oceanic crust can no longer be defined beyond this critical condition. The fluid released from subducting oceanic crust at depths deeper than 100 km under volcanic arcs are supercritical fluid rather than aqueous fluid and/or hydrous melts. It is suggested that the position of the second critical endpoint explains why there is a limitation of slab depth (~90 km) where Adakitic magmas are produced and also explains the origin of across-arc geochemical variations of trace elements in volcanic rocks in subduction zones.

  4. Lead transport in intra-oceanic subduction zones: 2D geochemical-thermo-mechanical modeling of isotopic signatures

    NASA Astrophysics Data System (ADS)

    Baitsch-Ghirardello, Bettina; Stracke, Andreas; Connolly, James A. D.; Nikolaeva, Ksenia M.; Gerya, Taras V.

    2014-11-01

    Understanding the physical-chemical mechanisms and pathways of geochemical transport in subduction zones remains a long-standing goal of subduction-related research. In this study, we perform fully coupled geochemical-thermo-mechanical (GcTM) numerical simulations to investigate Pb isotopic signatures of the two key "outputs" of subduction zones: (A) serpentinite mélanges and (B) arc basalts. With this approach we analyze three different geodynamic regimes of intra-oceanic subduction systems: (1) retreating subduction with backarc spreading, (2) stable subduction with high fluid-related weakening, and (3) stable subduction with low fluid-related weakening. Numerical results suggest a three-stage Pb geochemical transport in subduction zones: (I) from subducting sediments and oceanic crust to serpentinite mélanges, (II) from subducting serpentinite mélanges to subarc asthenospheric wedge and (III) from the mantle wedge to arc volcanics. Mechanical mixing and fluid-assisted geochemical transport above slabs result in spatially and temporarily variable Pb concentrations in the serpentinized forearc mantle as well as in arc basalts. The Pb isotopic ratios are strongly heterogeneous and show five types of geochemical mixing trends: (i) binary mantle-MORB, (i) binary MORB-sediments, (iii) double binary MORB-mantle and MORB-sediments, (iv) double binary MORB-mantle and mantle-sediments and (v) triple MORB-sediment-mantle. Double binary and triple mixing trends are transient and characterize relatively early stages of subduction. In contrast, steady-state binary mantle-MORB and MORB-sediments trends are typical for mature subduction zones with respectively low and high intensity of sedimentary melange subduction. Predictions from our GcTM models are in agreement with Pb isotopic data from some natural subduction zones.

  5. Metamorphic density controls on early-stage subduction dynamics

    NASA Astrophysics Data System (ADS)

    Duesterhoeft, Erik; Oberhänsli, Roland; Bousquet, Romain

    2013-04-01

    contribution to the slab pull, where eclogitization does not occur. Thus, the lithospheric mantle acts as additional ballast below the sinking slab shortly after the initiation of subduction. Our calculation shows that the dogma of eclogitized basaltic, oceanic crust as the driving force of slab pull is overestimated during the early stage of subduction. These results improve our understanding of the force budget for slab pull during the intial and early stage of subduction. Therefore, the complex metamorphic structure of a slab and mantle wedge has an important impact on the development and dynamics of subduction zones. Further Reading: Duesterhoeft, Oberhänsli & Bousquet (2013), submitted to Earth and Planetary Science Letters

  6. Melt Inclusions as Windows on Subduction Zone Processes - A Retrospective

    NASA Astrophysics Data System (ADS)

    Sisson, T. W.

    2002-12-01

    A.T. (Fred) Anderson, in a series of papers in the interval 1972-1984, presented evidence from melt inclusions for high dissolved water and Cl concentrations in many subduction zone basalts through andesites. His observations, subsequently shown to be correct, were not widely accepted because (1) phase equilibrium experiments on Paricutin and Mount Hood andesites indicated moderate water concentrations, and some workers reasoned that potentially parental basalts would have been drier still, (2) common basalts lack hydrous phenocrysts, and (3) water content estimates were indirect (water-by-difference) or involved difficult, unfamiliar measurements (single inclusion manometry) and thus were discounted. Subsequent development of techniques for the direct and precise measurement of water and CO2 in melt inclusions (SIMS, FTIR), new hydrous phase-equilibrium studies on arc basalts through rhyolites, and wider appreciation of the diversity of arc magmatic suites changed this situation. Melt inclusion evidence shows that subduction zone basalts can have pre-eruptive dissolved water concentrations as high as ~6 wt% (Sisson and Layne 1993 EPSL; Roggensack et al. 1997 Science), confirming predictions from phase-equilibrium experiments (Sisson and Grove 1993a,b CMP), and supporting the now standard model of water-fluxed melting to drive arc magmatism. An important discovery, presaged in the original Anderson data, is that there is a wide range of pre-eruptive water contents in arc basalts, with some as dry as MORB (Sisson and Bronto 1998 Nature). Nearly dry arc basalts can erupt at the volcanic front (Galunggung, Java) and sporadically along the arc axis over distances of hundreds of km (Cascades, USA), in some cases in proximity to demonstrably water-rich magmatic centers (Mt. Shasta, Crater Lake). To produce dry primitive basalts requires upwelling and pressure-release melting of peridotite in the mantle wedge at temperatures (~1300° C) well above those predicted by

  7. Continental crustal formation and recycling: Evidence from oceanic basalts

    NASA Technical Reports Server (NTRS)

    Saunders, A. D.; Tarney, J.; Norry, M. J.

    1988-01-01

    Despite the wealth of geochemical data for subduction-related magma types, and the clear importance of such magmas in the creation of continental crust, there is still no concensus about the relative magnitudes of crustal creation versus crustal destruction (i.e., recycling of crust into the mantle). The role of subducted sediment in the formation of the arc magmas is now well documented; but what proportion of sediment is taken into the deeper mantle? Integrated isotopic and trace element studies of magmas erupted far from presently active subduction zones, in particular basaltic rocks erupted in the ocean basins, are providing important information about the role of crustal recycling. By identifying potential chemical tracers, it is impossible to monitor the effects of crustal recycling, and produce models predicting the mass of material recycled into the mantle throughout long periods of geological time.

  8. 3D Numerical simulations of oblique subduction

    NASA Astrophysics Data System (ADS)

    Malatesta, C.; Gerya, T.; Scambelluri, M.; Crispini, L.; Federico, L.; Capponi, G.

    2012-04-01

    In the past 2D numerical studies (e.g. Gerya et al., 2002; Gorczyk et al., 2007; Malatesta et al., 2012) provided evidence that during intraoceanic subduction a serpentinite channel forms above the downgoing plate. This channel forms as a result of hydration of the mantle wedge by uprising slab-fluids. Rocks buried at high depths are finally exhumed within this buoyant low-viscosity medium. Convergence rate in these 2D models was described by a trench-normal component of velocity. Several present and past subduction zones worldwide are however driven by oblique convergence between the plates, where trench-normal motion of the subducting slab is coupled with trench-parallel displacement of the plates. Can the exhumation mechanism and the exhumation rates of high-pressure rocks be affected by the shear component of subduction? And how uprise of these rocks can vary along the plate margin? We tried to address these questions performing 3D numerical models that simulate an intraoceanic oblique subduction. The models are based on thermo-mechanical equations that are solved with finite differences method and marker-in-cell techniques combined with multigrid approach (Gerya, 2010). In most of the models a narrow oceanic basin (500 km-wide) surrounded by continental margins is depicted. The basin is floored by either layered or heterogeneous oceanic lithosphere with gabbro as discrete bodies in serpentinized peridotite and a basaltic layer on the top. A weak zone in the mantle is prescribed to control the location of subduction initiation and therefore the plate margins geometry. Finally, addition of a third dimension in the simulations allowed us to test the role of different plate margin geometries on oblique subduction dynamics. In particular in each model we modified the dip angle of the weak zone and its "lateral" geometry (e.g. continuous, segmented). We consider "continuous" weak zones either parallel or increasingly moving away from the continental margins

  9. Diagenesis and dehydration of subducting oceanic crust within seismogenic subduction zones

    NASA Astrophysics Data System (ADS)

    Kameda, J.; Yamaguchi, A.; Hamada, Y.; Hashimoto, Y.; Kimura, G.

    2012-12-01

    Diagenesis and dehydration of subducting oceanic crust is thought to have strong influence on mechanical and hydrologic properties of seismogenic plate interfaces beneath the accretionary wedges (Kameda et al., 2011). In this work, we analyzed five representative pillow basalts exposed in the ancient accretionary complex, the Shimanto belt in southwest Japan, in order to derive details on a suite of mineral reactions within the subducting oceanic crust. Based on the vitrinite reflectance measurement of terrigenous sediments accompanied by these rocks, they are estimated to have been subjected to burial diagenenesis at 150-300 C. Whole rock and clay-fraction X-ray diffraction (XRD) analyses indicate that sequential saponite to chlorite transformation through mixed-layer phases proceeds under the relatively constant bulk rock composition. Such clay mineral reaction may persist to deep crustal level (~290 C) and contribute to bulk dehydration as a dominant fluid supplier to the plate-boundary fault system. The dehydration may cause abnormal fluid pressurization around the plate-boundary fault zone with a maximum at a certain horizon below the fault (within the intact oceanic crust), resulting in underplating of the upper basement rock into the overriding accretionary prism. Such dehydration-induced weakening process well explains the thickness distribution of the accreted basaltic crust fragments as observed in the onland exposures (Kimura and Ludden, 1995). The breakage of the oceanic crust potentially nucleates seismic slip to propagate along the seismogenic plate interface.

  10. Diverse magmatic effects of subducting a hot slab in SW Japan: Results from forward modeling

    NASA Astrophysics Data System (ADS)

    Kimura, Jun-Ichi; Gill, James B.; Kunikiyo, Tomoyuki; Osaka, Isaku; Shimoshioiri, Yusuke; Katakuse, Maiko; Kakubuchi, Susumu; Nagao, Takashi; Furuyama, Katsuhiko; Kamei, Atsushi; Kawabata, Hiroshi; Nakajima, Junichi; van Keken, Peter E.; Stern, Robert J.

    2014-03-01

    response to the subduction of the young Shikoku Basin of the Philippine Sea Plate, arc magmas erupted in SW Japan throughout the late Cenozoic. Many magma types are present including ocean island basalt (OIB), shoshonite (SHO), arc-type alkali basalt (AB), typical subalkalic arc basalt (SAB), high-Mg andesite (HMA), and adakite (ADK). OIB erupted since the Japan Sea back-arc basin opened, whereas subsequent arc magmas accompanied subduction of the Shikoku Basin. However, there the origin of the magmas in relation to hot subduction is debated. Using new major and trace element and Sr-Nd-Pb-Hf isotope analyses of 324 lava samples from seven Quaternary volcanoes, we investigated the genetic conditions of the magma suites using a geochemical mass balance model, Arc Basalt Simulator version 4 (ABS4), that uses these data to solve for the parameters such as pressure/temperature of slab dehydration/melting and slab flux fraction, pressure, and temperature of mantle melting. The calculations suggest that those magmas originated from slab melts that induced flux melting of mantle peridotite. The suites differ mostly in the mass fraction of slab-melt flux, increasing from SHO through AB, SAB, HMA, to ADK. The pressure and temperature of mantle melting decreases in the same order. The suites differ secondarily in the ratio of altered oceanic crust to sediment in the source of the slab melt. The atypical suites associated with hot subduction result from unusually large mass fractions of slab melt and unusually cool mantle temperatures.

  11. New Field, Geochronological, and Geochemical Data on the Miocene Lovejoy Basalt and its Source, Northern California

    NASA Astrophysics Data System (ADS)

    Garrison, N. J.; Busby, C. B.; Gans, P. B.; Wagner, D. L.

    2003-12-01

    The Lovejoy Basalt is a distinctive, aphanitic olivine basalt that flowed through a paleovalley across the northern end of the Sierra Nevada from the Diamond Mountains near Honey Lake to the Sacramento Valley in northern California. Approximately 150 km3 of basalt was erupted in a tectonic setting transitional between subduction, Basin and Range extension, and hot spot volcanism, but the eruptive source and timing of the Lovejoy Basalt have remained controversial. Recent field mapping and geochemical analyses suggest that Thompson Peak, located south of Susanville, California, is the source vent for the Lovejoy Basalt. At Thompson Peak, Lovejoy Basalt forms an elongate, NNW-SSE trending ridge that is capped in its central section by the Basalt of Thompson Peak. Scoria deposits, volcanic bombs, agglutinate and other near vent deposits indicate Thompson Peak as the source for the Lovejoy Basalt. Previous and new mapping indicate that the basalt was channelized within granitic basement, and flowed south from the vent 30 km to Red Clover Creek before bending to the southwest and flowing 65 km to the Sacramento Valley. The Lovejoy Basalt consists of up to 13 flows, all aphyric except for an uppermost plagioclase-phyric flow present between Thompson Peak and Red Clover Creek that locally contains xenocrysts of olivine and garnet. At Table Mountain, the basalt appears to have ponded or inflated, forming very thick flows with prominent N-S trending pressure ridges. The Lovejoy Basalt is a quartz-normative tholeiite that is geochemically similar to the Columbia River Basalt. While a conclusive statement cannot yet be made about the source magma for the Lovejoy Basalt, initial analyses show that it does not appear to be related to the main phase of Cascade arc volcanism, which is predominantly calc-alkaline. Recently published age data for the Lovejoy Basalt suggests that it is coeval with the main phase of the Columbia River Basalt Group; however, our further attempts to

  12. Petrogenesis of pillow basalts from Baolai in southwestern Taiwan

    NASA Astrophysics Data System (ADS)

    Liu, Chih-Chun; Yang, Huai-Jen

    2016-04-01

    .9-21.0 versus 26.0-33.5), and Dy/Yb (~2.7 versus 2.97-3.62) with higher Lu/Hf (~0.056 versus ~0.045). Based on model calculations, the eastern Guangdong basalts represent mixtures containing large proportions (> 90%) of melt generated by < 2% melting from a source with residual garnet and small proportions (< 10%) of low degree melts (< 1%) from spinel lherzolite. The Baolai basalts are explained as involving higher proportions (10-20%) of melt from spinel lherzolite by higher degrees (2-3%) of partial melting. The unusually high Nb/La ratio of > 1.6 in the Baolai basalts is best explained as reflecting a component in the recycled dehydrated residues, indicating derivation from asthenospheric mantle source that involves subduction components. It is inferred that the subduction components are associated with the subduction of paleo-Pacific Ocean. If this is the case, a relatively high mantle circulation rate (i.e., 1 cm/yr; Wang et al., 2013) is required. Smith and Lewis (2007), International Geology Review 49, 1-13. Wang et al. (2013), Earth and Planetary Science Letters 377-378, 248-259.

  13. Pliocene Basaltic Volcanism in The East Anatolia Region (EAR), Turkey

    NASA Astrophysics Data System (ADS)

    Oyan, Vural; Özdemir, Yavuz; Keskin, Mehmet

    2016-04-01

    East Anatolia Region (EAR) is one of the high Plateau which is occurred with north-south compressional regime formed depending on continent-continent collision between Eurasia and Arabia plates (Şengör and Kidd, 1979). Recent studies have revealed that last oceanic lithosphere in the EAR have completely depleted to 20 million years ago based on fission track ages (Okay et al. 2010). Our initial studies suggest that extensively volcanic activity in the EAR peaked in the Pliocene and continued in the same productivity throughout Quaternary. Voluminous basaltic lava plateaus and basaltic lavas from local eruption centers occurred as a result of high production level of volcanism during the Pliocene time interval. In order to better understand the spatial and temporal variations in Pliocene basaltic volcanism and to reveal isotopic composition, age and petrologic evolution of the basaltic volcanism, we have started to study basaltic volcanism in the East Anatolia within the framework of a TUBITAK project (project number:113Y406). Petrologic and geochemical studies carried out on the Pliocene basaltic lavas indicate the presence of subduction component in the mantle source, changing the character of basaltic volcanism from alkaline to subalkaline and increasing the amount of spinel peridotitic melts (contributions of lithospheric mantle?) in the mantle source between 5.5-3.5 Ma. FC, AFC and EC-AFC modelings reveal that the while basaltic lavas were no or slightly influenced by crustal contamination and fractional crystallization, to more evolved lavas such as bazaltictrachyandesite, basalticandesite, trachybasalt might have been important processes. Results of our melting models and isotopic analysis data (Sr, Nd, Pb, Hf, 18O) indicate that the Pliocene basaltic rocks were derived from both shallow and deep mantle sources with different melting degrees ranging between 0.1 - 4 %. The percentage of spinel seems to have increased in the mantle source of the basaltic

  14. Basaltic island sand provenance

    SciTech Connect

    Marsaglia, K.M. . Dept. of Geological Sciences)

    1992-01-01

    The Hawaiian Islands are an ideal location to study basaltic sand provenance in that they are a series of progressively older basaltic shield volcanoes with arid to humid microclimates. Sixty-two sand samples were collected from beaches on the islands of Hawaii, Maui, Oahu and Kauai and petrographically analyzed. The major sand components are calcareous bioclasts, volcanic lithic fragments, and monomineralic grains of dense minerals and plagioclase. Proportions of these components vary from island to island, with bioclastic end members being more prevalent on older islands exhibiting well-developed fringing reef systems and volcanic end members more prevalent on younger, volcanically active islands. Climatic variations across the island of Hawaii are reflected in the percentage of weathered detritus, which is greater on the wetter, northern side of the island. The groundmass of glassy, basaltic lithics is predominantly black tachylite, with lesser brown sideromelane; microlitic and lathwork textures are more common than holohyaline vitric textures. Other common basaltic volcanic lithic fragments are holocrystalline aggregates of silt-sized pyroxene or olivine, opaque minerals and plagioclase. Sands derived from alkalic lavas are texturally and compositionally indistinguishable from sands derived from tholeiitic lavas. Although Hawaiian basaltic sands overlap in composition with magmatic arc-derived sands in terms of their relative QFL, QmPK and LmLvLs percentages, they are dissimilar in that they lack felsic components and are more enriched in lathwork volcanic lithic fragments, holocrystalline volcanic lithic fragments, and dense minerals.

  15. Temporal control of subduction magmatism in the eastern Trans-Mexican Volcanic Belt: Mantle sources, slab contributions, and crustal contamination

    NASA Astrophysics Data System (ADS)

    Gómez-Tuena, Arturo; Lagatta, Alexandra B.; Langmuir, Charles H.; Goldstein, Steven L.; Ortega-GutiéRrez, Fernando; Carrasco-NúñEz, Gerardo

    2003-08-01

    The magmatic record of the easternmost part of the Trans-Mexican Volcanic Belt elucidates how temporal changes in subduction parameters influence convergent margin volcanism. In the Palma Sola massif, three phases of magmatic rocks with distinct chemical characteristics were emplaced in a relatively short time span (˜17 Ma): Miocene calc-alkaline plutons, latest Miocene-Pleistocene alkaline plateau basalts, and Quaternary calc-alkaline cinder cones. Plutons have arc-like trace element patterns (Ba/Nb = 16-101), and their Sr, Nd, and Pb isotopic compositions become more "depleted" with increasing SiO2 contents. Their Pb isotopes are bracketed by the subducted sediments and Pacific mid-ocean ridge basalts (MORB), requiring the participation of an unradiogenic component that mixes with a sediment contribution. High Sr/Y and Gd/Yb ratios in the least radiogenic pluton might indicate a melt coming from the subducted MORB. Trace element patterns of the plateau basalts show moderate or negligible subduction contributions (Ba/Nb = 6-31). Rocks without subduction signatures are similar to ocean island basalts, indicating melting of an enriched mantle wedge. The plateau basalts form an array in 206Pb/204Pb-207Pb/204Pb space that trends toward the composition of the subducted sediment. The sediment component is also indicated by the inverse correlations between Pb isotopes and subduction signals. This component has high Th/Nd coupled with low 143Nd/144Nd, but lower Pb/Nd and Sr/Nd ratios than the bulk sediment. These suggest melting of a sediment that has lost fluid mobile elements prior to melting. The Quaternary cinder cones have moderate subduction signals (Ba/Nb = 16-41), and their isotopic compositions correlate with differentiation indices. Contamination with the local Paleozoic basement can explain the petrogenesis of the youngest rock suite. The geochemical differences among the suites indicate temporal modifications in the chemical characteristics of the slab input

  16. Experimental Insights into the Subduction Filter

    NASA Astrophysics Data System (ADS)

    Till, C. B.; Grove, T. L.

    2010-12-01

    We present an experimental investigation of H2O-saturated undepleted peridotite at pressures and temperatures relevant to subduction zone melting processes. Piston cylinder and multi-anvil experiments were conducted from 740-1200°C at 3.2 to 6 GPa to locate the H2O-saturated peridotite solidus and quantify changes in mineral and melt compositions with increased degrees of melting. Abrupt changes in texture, persistent changes in mineral composition (e.g., Mg# and Cr2O3) with increasing temperature, and analyses of both quenched melt and quenched solute, demonstrate that the transition we observe at ˜810°C at 3.2 to 6 GPa is the H2O-saturated solidus rather than a change in the solute composition or content of the vapor phase. Our experiments indicate that melts of H2O-saturated peridotite produced in the base of the mantle wedge evolve from low-alkali quartz tholeiites at low degrees of melting (5 wt.%) to olivine tholeiites at higher degrees of melting (≤30 wt.%). Between 860°C and 1100°C at 3.2 GPa, the H2O-saturated peridotite melting reaction consumes orthopyroxene, clinopyroxene and garnet to produce olivine and melt, while above 1100°C, only melt is produced by the eutectic melting reaction involving olivine and orthopyroxene as the only residues of mantle melting. Our mineral and melt compositional data plays a crucial role in characterizing the chemical composition of the component added to the continental crust as the result of mantle wedge melting, as well as the residual component returned to the mantle. Furthermore, the temperature difference between the H2O-saturated peridotite solidus and other solidi relevant to subduction zone melting, such as those for subducted sediment and oceanic crust, decreases from 100°C to ˜0°C from 2 to 4.5 GPa (or ˜60-140 km depth), likely depths for melting at subduction zones. Thus there is a critical range where melts of both sediment and basalt may form at typical mantle wedge conditions, ascend into the

  17. A record of spontaneous subduction initiation in the Izu-Bonin-Mariana arc

    NASA Astrophysics Data System (ADS)

    Arculus, Richard J.; Ishizuka, Osamu; Bogus, Kara A.; Gurnis, Michael; Hickey-Vargas, Rosemary; Aljahdali, Mohammed H.; Bandini-Maeder, Alexandre N.; Barth, Andrew P.; Brandl, Philipp A.; Drab, Laureen; Do Monte Guerra, Rodrigo; Hamada, Morihisa; Jiang, Fuqing; Kanayama, Kyoko; Kender, Sev; Kusano, Yuki; Li, He; Loudin, Lorne C.; Maffione, Marco; Marsaglia, Kathleen M.; McCarthy, Anders; Meffre, Sebastién; Morris, Antony; Neuhaus, Martin; Savov, Ivan P.; Sena, Clara; Tepley, Frank J., III; van der Land, Cees; Yogodzinski, Gene M.; Zhang, Zhaohui

    2015-09-01

    The initiation of tectonic plate subduction into the mantle is poorly understood. If subduction is induced by the push of a distant mid-ocean ridge or subducted slab pull, we expect compression and uplift of the overriding plate. In contrast, spontaneous subduction initiation, driven by subsidence of dense lithosphere along faults adjacent to buoyant lithosphere, would result in extension and magmatism. The rock record of subduction initiation is typically obscured by younger deposits, so evaluating these possibilities has proved elusive. Here we analyse the geochemical characteristics of igneous basement rocks and overlying sediments, sampled from the Amami Sankaku Basin in the northwest Philippine Sea. The uppermost basement rocks are areally widespread and supplied via dykes. They are similar in composition and age--as constrained by the biostratigraphy of the overlying sediments--to the 52-48-million-year-old basalts in the adjacent Izu-Bonin-Mariana fore-arc. The geochemical characteristics of the basement lavas indicate that a component of subducted lithosphere was involved in their genesis, and the lavas were derived from mantle source rocks that were more melt-depleted than those tapped at mid-ocean ridges. We propose that the basement lavas formed during the inception of Izu-Bonin-Mariana subduction in a mode consistent with the spontaneous initiation of subduction.

  18. Flexural Mechanics of Subduction

    NASA Astrophysics Data System (ADS)

    Ribe, N. M.; Li, Z. H.

    2014-12-01

    Deformation of subducting lithosphere is controlled by a balance of four forces: the negative buoyancy of the slab, its internal viscous resistance to stretching and bending, and the drag of the ambient mantle. To elucidate the complex dynamics of this system, we study a model in which a 3-D sheet of viscous fluid with thickness hh and viscosity γη1gammaeta_1 subducts in an ambient fluid with viscosity η1eta_1 and depth DD. Numerical solutions for the sheet's evolution are obtained using a boundary element method, starting from an initial configuration comprising a short protoslab attached to a longer horizontal plate that is free to move laterally. The dynamics of the sheet are controlled by its dimensionless `stiffness' S≡γ(h/ℓb)3Sequiv gamma (h/ell_b)^3, where the `bending length' ℓbell_b is the sum of the lengths of the slab and of the flexural bulge. The slab's sinking speed is controlled by its own viscosity if S≫1Sgg 1, and by that of the ambient fluid if S≤1Sleq 1. Time-dependent solutions with passive tracers demonstrate a partial return flow around the leading edge of a retreating slab and return flow around its sides. A systematic investigation of the slab's interaction with the bottom boundary as a function of η2/η1eta_2/eta_1 and D/hD/h delineates a rich regime diagram of subduction modes (trench retreating, slab folding, trench advancing, etc.) that agrees well with laboratory observations. The solutions show that mode selection is controlled by the dip of the slab's leading edge at the time when it first encounters the bottom boundary. We will discuss several geophysical applications of the model, including seismic evidence for slab folding, the radius-of-curvature constraint on the slab/mantle viscosity ratio η2/η1eta_2/eta_1, and the distribution of seismic anisotropy around subducting slabs.

  19. Geochemistry of rare high-Nb basalt lavas: Are they derived from a mantle wedge metasomatised by slab melts?

    NASA Astrophysics Data System (ADS)

    Hastie, Alan R.; Mitchell, Simon F.; Kerr, Andrew C.; Minifie, Matthew J.; Millar, Ian L.

    2011-09-01

    Compositionally, high-Nb basalts are similar to HIMU (high U/Pb) ocean island basalts, continental alkaline basalts and alkaline lavas formed above slab windows. Tertiary alkaline basaltic lavas from eastern Jamaica, West Indies, known as the Halberstadt Volcanic Formation have compositions similar to high-Nb basalts (Nb > 20 ppm). The Halberstadt high-Nb basalts are divided into two compositional sub-groups where Group 1 lavas have more enriched incompatible element concentrations relative to Group 2. Both groups are derived from isotopically different spinel peridotite mantle source regions, which both require garnet and amphibole as metasomatic residual phases. The Halberstadt geochemistry demonstrates that the lavas cannot be derived by partial melting of lower crustal ultramafic complexes, metasomatised mantle lithosphere, subducting slabs, continental crust, mantle plume source regions or an upper mantle source region composed of enriched and depleted components. Instead, their composition, particularly the negative Ce anomalies, the high Th/Nb ratios and the similar isotopic ratios to nearby adakite lavas, suggests that the Halberstadt magmas are derived from a compositionally variable spinel peridotite source region(s) metasomatised by slab melts that precipitated garnet, amphibole, apatite and zircon. It is suggested that high-Nb basalts may be classified as a distinct rock type with Nb > 20 ppm, intraplate alkaline basalt compositions, but that are generated in subduction zones by magmatic processes distinct from those that generate other intraplate lavas.

  20. Subduction Initiation in Eastern Indonesia

    NASA Astrophysics Data System (ADS)

    Hall, R.

    2014-12-01

    Subduction is often reported to be difficult to initiate, yet in the West Pacific and Eastern Indonesia there are many young subduction zones. Few theoretical or modelling studies consider such settings in which subduction commonly began close to boundaries between ocean crust and thickened crust of arc or continental origin. In Eastern Indonesia there are subduction zones at different stages of development. Some young examples such as the Banda Arc developed by propagation of an existing trench into a new area by tearing, probably along an ocean-continent boundary. This 'solves' the problem since the older subducted slab provides the driving force to drag down unsubducted ocean lithosphere. However, similar explanations cannot account for other subduction zones, such as North Sulawesi, nearby examples in which the subducted slab is not yet at 100 km depth, or troughs where subduction appears to be beginning. These examples show that subduction initiated at a point, such as a corner in an ocean basin, where there were very great differences in elevation between land and adjacent ocean floor. Depression of ocean crust by flow of arc/continent crust is associated with granitic magmatism and detachments within the upper crust. Once the oceanic corner reaches depths of c.100 km, eclogite formation may lead to slab pull that causes the new subduction zone to grow in both directions along strike; arc magmatism may or may not begin. The close relationship between subduction and extension in Eastern Indonesia links dramatic elevation of land, exhumation of deep crust, and spectacular subsidence of basins imaged by oil exploration seismic and multibeam data. Exhumed granites and high-grade metamorphic rocks at elevations up to 3 km, separated by Neogene alluvial sediments from carbonate reefs now at depths of 2 kilometres, imply vertical movements of several kilometres in a few million years. These observations raise the question of whether subduction is driving extension

  1. A possible source of water in seismogenic subduction zones

    NASA Astrophysics Data System (ADS)

    Kameda, J.; Yamaguchi, A.; Kimura, G.; Iodp Exp. 322 Scientists

    2010-12-01

    Recent works on the subduction megathrusts have emphasized the mechanical function of fluids contributing dynamic slip-weakening. Basalt-hosting fault zones in on-land accretionary complexes present several textures of seismic slip under fluid-assisted condition such as implosion breccia with carbonate matrix and decrepitation of fluid inclusion. In order to clarify initiation and evolution processes of such fault zones as well as possible source of fluid in the seismogenic subduction zone, we examined a mineralogical/geochemical feature of basaltic basement recovered by IODP Exp. 322 at C0012, that is a reference site for subduction input in the Nankai Trough. A total of 10 samples (about 4 m depth interval from the basement top) were analyzed in this study. XRD analyses indicate that all of the samples contain considerable amount of smectite. The smectite does not appear as a form of interstratified phase with illite or chlorite. Preliminary chemical analyses by EDS in TEM suggest that the smectite is trioctahedral saponite with Ca as a dominant interlayer cation. To determine the saponite content quantitatively, cation exchange capacity (CEC) of bulk samples was measured. The samples show almost similar CEC of around 30 meq/100g, implying that bulk rock contains about 30 wt% of saponite, considering a general CEC of 100 meq/100g for monomineralic saponite. Such abundance of saponite might be a result from intense alteration of oceanic crust due to sea water circulation at low temperature. Previous experimental work suggests that saponite might be highly hydrated (two to three water layer hydration form) at the seismogenic P-T condition. Hence, altered upper oceanic crust is a possible water sink in the seismogenic zone. The water stored in the smectite interlayer region will be expelled via smectite to chlorite transition reaction, that might contribute to the dynamic weakening of the seimogenic plate boundary between the basement basalt and overlying

  2. Slab melting versus slab dehydration in subduction-zone magmatism.

    PubMed

    Mibe, Kenji; Kawamoto, Tatsuhiko; Matsukage, Kyoko N; Fei, Yingwei; Ono, Shigeaki

    2011-05-17

    The second critical endpoint in the basalt-H(2)O system was directly determined by a high-pressure and high-temperature X-ray radiography technique. We found that the second critical endpoint occurs at around 3.4 GPa and 770 °C (corresponding to a depth of approximately 100 km in a subducting slab), which is much shallower than the previously estimated conditions. Our results indicate that the melting temperature of the subducting oceanic crust can no longer be defined beyond this critical condition and that the fluid released from subducting oceanic crust at depths greater than 100 km under volcanic arcs is supercritical fluid rather than aqueous fluid and/or hydrous melts. The position of the second critical endpoint explains why there is a limitation to the slab depth at which adakitic magmas are produced, as well as the origin of across-arc geochemical variations of trace elements in volcanic rocks in subduction zones. PMID:21536910

  3. Slab melting versus slab dehydration in subduction-zone magmatism

    PubMed Central

    Mibe, Kenji; Kawamoto, Tatsuhiko; Matsukage, Kyoko N.; Fei, Yingwei; Ono, Shigeaki

    2011-01-01

    The second critical endpoint in the basalt-H2O system was directly determined by a high-pressure and high-temperature X-ray radiography technique. We found that the second critical endpoint occurs at around 3.4 GPa and 770 °C (corresponding to a depth of approximately 100 km in a subducting slab), which is much shallower than the previously estimated conditions. Our results indicate that the melting temperature of the subducting oceanic crust can no longer be defined beyond this critical condition and that the fluid released from subducting oceanic crust at depths greater than 100 km under volcanic arcs is supercritical fluid rather than aqueous fluid and/or hydrous melts. The position of the second critical endpoint explains why there is a limitation to the slab depth at which adakitic magmas are produced, as well as the origin of across-arc geochemical variations of trace elements in volcanic rocks in subduction zones. PMID:21536910

  4. Why subduction zones are curved

    NASA Astrophysics Data System (ADS)

    Mahadevan, L.; Bendick, R.; Liang, Haiyi

    2010-12-01

    We give an explanation for the polarity, localization, shape, size, and initiation of subduction zones on Earth. By considering a soft, thin, curved lithospheric cap with either elastic or viscous rheology supported by a thick, nearly incompressible mantle, we find two different characteristic subduction geometries arise depending on boundary conditions: (1) plate boundaries where subduction results primarily from the gravitational body force (free subduction) have characteristic plate lengths and form arc-shaped dimpled segments resulting from the competition between bending and stretching in edge buckling modes of thin spherical shells, and (2) subduction zones due to localized applied loads that push one slab of thin, positively buoyant lithosphere beneath an overriding plate (forced subduction) form localized straight segments, consistent with the deformation of indented spherical shells. Both types of subduction are nonlinear subcritical instabilities, so small perturbations in the mechanical properties of the lithosphere have pronounced effects on subduction initiation and evolution. Yet in both cases, geometric relationships determined by the shape of the Earth itself play the most critical role in controlling the basic morphology and characteristic length scales of subduction zones.

  5. Does subduction zone magmatism produce average continental crust

    NASA Technical Reports Server (NTRS)

    Ellam, R. M.; Hawkesworth, C. J.

    1988-01-01

    The question of whether present day subduction zone magmatism produces material of average continental crust composition, which perhaps most would agree is andesitic, is addressed. It was argued that modern andesitic to dacitic rocks in Andean-type settings are produced by plagioclase fractionation of mantle derived basalts, leaving a complementary residue with low Rb/Sr and a positive Eu anomaly. This residue must be removed, for example by delamination, if the average crust produced in these settings is andesitic. The author argued against this, pointing out the absence of evidence for such a signature in the mantle. Either the average crust is not andesitic, a conclusion the author was not entirely comfortable with, or other crust forming processes must be sought. One possibility is that during the Archean, direct slab melting of basaltic or eclogitic oceanic crust produced felsic melts, which together with about 65 percent mafic material, yielded an average crust of andesitic composition.

  6. The fate of the downgoing oceanic plate: Insight from the Northern Cascadia subduction zone

    NASA Astrophysics Data System (ADS)

    Piana Agostinetti, Nicola; Miller, Meghan S.

    2014-12-01

    In this study, we use teleseismic receiver function analysis to image the seismic structure of the Juan de Fuca oceanic plate during its subduction beneath the North American plate. Seismic data have been recorded at 58 seismic stations deployed along the northern Cascadia subduction zone. Harmonic decomposition of the receiver function data-set along a trench-normal profile allows us to image both the isotropic and the anisotropic structure of the plate (slab). Our images highlight the presence of a highly anisotropic region at 40-70 km depths across the Cascadia subduction zone. The detected seismic anisotropy is interpreted to be related to both metamorphic facies (e.g. blueschists) and fluid released during the dehydration of the subducting mantle. The processes of dehydration and metamorphism produce the variations of the seismic properties within each lithologic unit that constitutes the subducted slab, i.e. basalts, gabbro layer and upper mantle, as the oceanic plate sinks in the upper mantle. Such variations make it almost impossible to recognize the “plate boundary” as a characteristic “velocity-jump” at depth (neither positive nor negative) along the Cascadia subduction zone. Based on the comparative interpretation of both the isotropic and the anisotropic structures retrieved, we propose a 4-stage model of the evolution of the Juan de Fuca oceanic plate during its subduction beneath the North American plate.

  7. Hf-Nd input flux in the Izu-Mariana subduction zone and recycling of subducted material in the mantle

    NASA Astrophysics Data System (ADS)

    Chauvel, Catherine; Marini, Jean-Christophe; Plank, Terry; Ludden, John N.

    2009-01-01

    In subduction zones, two major mass fluxes compete: the input flux of altered oceanic crust and sediments subducted into the mantle and the output flux of magma that forms the volcanic arc. While the composition and the amount of material erupted along volcanic arcs are relatively well known, the chemical and isotopic composition of the subducted material (altered oceanic crust and sediments) is poorly constrained and is an important factor in the mass balance calculation. Ocean Drilling Program Leg 185 in the Western Pacific used systematic sampling of the altered basaltic basement and sediment pile and the creation of composite mixtures to quantify the total chemical flux subducted at the Izu-Mariana margin. Here, we report Hf and Nd isotopic compositions of materials recovered from this Leg. The Hf and Nd isotopic compositions of altered basalts from Hole 801C are indistinguishable from those of recent unaltered Pacific mid-ocean ridge basalt, suggesting that hydrothermal alteration had no effect on either isotopic systems. The complete Site 1149 sedimentary pile has a weighted average ɛNd of -5.9 and ɛHf of +4.4, values similar to those of Fe-Mn crusts and nodules. Therefore, the Hf and Nd isotopic compositions of the sediments collected at Site 1149 indicate minimal contributions from continental detrital material to the rare earth elements and high field strength elements. However, the Hf isotopic budget of the oldest sediments is more influenced by continental material than the younger sediments, despite the large distances to continental masses 130 Ma ago. In the Izu subduction zone, we calculate a sedimentary input of less than about 2% in the volcanic lava source. In contrast, at least 85% of the sedimentary Nd and Hf are recycled into the mantle to affect its general composition. Assuming that sediments have been recycled in a similar manner into the mantle for millions of years, large chemical heterogeneities must be produced in the mantle. In

  8. Elemental responses to subduction-zone metamorphism: Constraints from the North Qilian Mountain, NW China

    NASA Astrophysics Data System (ADS)

    Xiao, Yuanyuan; Niu, Yaoling; Song, Shuguang; Davidson, Jon; Liu, Xiaoming

    2013-02-01

    Subduction zone metamorphism (SZM) and behaviors of chemical elements in response to this process are important for both arc magmatism and mantle compositional heterogeneity. In this paper, we report the results of our petrographic and geochemical studies on blueschist and eclogite facies rocks of sedimentary and basaltic protoliths from two metamorphic sub-belts with different metamorphic histories in the North Qilian Mountain, Northwest China. The protolith of low-grade blueschists is basaltic in composition and is most likely produced in a back-arc setting, while the protoliths of high-grade blueschists/eclogites geochemically resemble the present-day normal and enriched mid-oceanic ridge basalts plus some volcanic arc rocks. The meta-sedimentary rocks, including meta-graywacke, meta-pelite, meta-chert and marble, show geochemical similarity to global oceanic (subducted) sediments. Assuming that high field strength elements (HFSEs) are relatively immobile, the correlated variations of rare earth elements (REEs) and Th with HFSEs suggest that all these elements are probably also immobile, whereas Pb and Sr are mobile in rocks of both basaltic and sedimentary protoliths during SZM. Ba, Cs and Rb are immobile in rocks of sedimentary protoliths and mobile in rocks of basaltic protolith. The apparent mobility of U in rocks of basaltic protolith may be inherited from seafloor alterations rather than caused by SZM. On the basis of in situ mineral compositional analysis (both major and trace elements), the most significant trace element storage minerals in these subduction-zone metamorphic rocks are: lawsonite, pumpellyite, apatite, garnet and epidote group minerals for REEs, white micas (both phengite and paragonite) for large ion lithophile elements, rutile and titanite for HFSEs. The presence and stability of these minerals exert the primary controls on the geochemical behaviors of most of these elements during SZM. The immobility of REEs, Th and U owing to their

  9. Timing of subduction and exhumation in a subduction channel: Evidence from slab melts from La Corea Mélange (eastern Cuba)

    NASA Astrophysics Data System (ADS)

    Blanco-Quintero, I. F.; Rojas-Agramonte, Y.; García-Casco, A.; Kröner, A.; Mertz, D. F.; Lázaro, C.; Blanco-Moreno, J.; Renne, P. R.

    2011-11-01

    High pressure igneous rocks (tonalites), generated by partial melting of subducted basaltic rocks accreted to the mantle wedge, are present in the La Corea serpentinite-matrix mélange (eastern Cuba) as centimeter- to meter-sized blocks and as concordant to crosscutting veins within high-pressure parent amphibolite blocks. The slab melts have adakitic signatures, in agreement with formation after partial melting of metabasite. Thermobarometric calculations indicate 620-680 °C and 13-15 kbar during crystallization of tonalites and down to 250-300 °C, 6 kbar during retrogression, indicating counter-clockwise P-T paths (hot subduction-cool exhumation). Free water required for melting of amphibolite at moderate temperature (700-750 °C) and moderate pressure (13-16 kbar) close to the wet basaltic solidus is inferred to have been provided after dehydration of sediments, altered basaltic crust and serpentinite of the subducting Proto-Caribbean lithosphere. Single zircon (SHRIMP) and phengite 40Ar/39Ar age data constrain the P-T-t evolution of the mélange from the timing of crystallization of melts at ~ 110-105 Ma to cooling at ~ 87-84 Ma, ca. 350 °C, ca. 9 kbar. These figures are consistent with subduction of an oblique ridge, shortly before 115 Ma. Furthermore, our data indicate very slow exhumation (ca. 1 mm/yr) in the subduction channel during the oceanic convergence stage (120-70 Ma) until final fast exhumation to the surface occurred at 70-65 Ma during a regional arc-platform collision event.

  10. Supertoxic Flood Basalts: The CAMP - Siberian Trap Connection

    NASA Astrophysics Data System (ADS)

    Puffer, J. H.

    2007-12-01

    Several diverse magma types are represented throughout the CAMP and Siberian Trap LIPs, however, the main extrusive phase of each province is highly unusual among continental flood basalts. The most widespread extrusions were intermediate titanium (ITi-type) CAMP basalt and the lower portion of the Upper Sequence of Siberian Trap. New and recently published data indicate that the geochemistry and petrology of these basalt suites closely resemble each other and infer similar origins. The basalts are characterized by strong negative Nb- Ta anomalies and unusual island arc-like depletion in high field strength elements, particularly Ti, plotted on spider diagrams. The geochemical data is consistent with significant contributions from subducted slabs into the magma source regions. If contaminated, volatile enriched mantle wedges were trapped beneath thick continental plates during the assembly of Pangea, fertile magma sources would have remained dormant until decompression melting was triggered during failed rift, then early rift stages of continental plate disassembly. The combination of volatile enriched sources and highly extensional tectonism would create rare perfect storms of toxicity. Calculated low viscosities assuming negligible carbon dioxide are consistent with rapid crustal penetration. Resulting aphyric melts extruded at enormous effusive rates as thick sub-parallel flows across wide subareal terrains through fissures extending several hundred km in length. High fountain heights would afford ample opportunity for efficient degassing, perhaps into the stratosphere. When the supply of volatile flux was exhausted magmatism ceased. The mass extinctions that coincide with CAMP and Siberian volcanism contrast with some large plume and superplume events that correlate with expansions of biodiversity. This may be due in part to contrasting magma access to sources of toxic volatiles, particularly sulfur concentrations in anoxic subducted sediments.

  11. Teaching about Subduction Zone Magmagenesis using MARGINS Subduction Factory Focus Site Geochemical Compilations and ABS3 (Invited)

    NASA Astrophysics Data System (ADS)

    Stern, R. J.; Jordan, E.; Raye, U.; Carr, M. J.; Feigenson, M.; Gill, J. B.; Hacker, B. R.; Kimura, J.; Lehnert, K. A.; Tamura, Y.; van Keken, P. E.

    2010-12-01

    Processes and inputs involved in generating arc magmas are reasonably well known but quantitative modeling is often overlooked when teaching about subduction zone magmagenesis. In order to appreciate these complexities, students need to be able to explore subduction zone magmagenetic processes using trace element compositions of igneous rocks. The MARGINS Subduction Factory experiment selected two endmember convergent margins, the Izu-Bonin-Mariana (IBM) arc, which subducts old, cold, dense seafloor, and the Central American (CentAm) arc, which subducts young, hot, buoyant seafloor. We have compiled high-quality trace element and isotopic data for young, fresh lavas from along the magmatic fronts of these endmember arcs, using the EarthChem database. Comparing data for primitive magmas from the two arc systems allows for first-order distinctions, including the greater relative abundances of fluid-mobile elements (e.g., K, Sr, U) in IBM lavas and greater relative abundances of elements requiring sediment melting (e.g., Th, LREE, Zr) in CentAm lavas. These differences can be explored quantitatively using the Arc Basalt Simulator version 3 (ABS3). ABS3 is a free Excel-based spreadsheet forward model that allows the user to control compositions of subducted sediment and altered oceanic crust in tandem with realistic thermal models to predict metamorphic conditions in the subducted slab, using simplified results from Perple_X, and to understand when sediment- and slab-melting is likely (See J.-I. Kimura et al, this meeting "V15 The Subduction Filter" session for more information about ABS3). Prograde metamorphism along with experimentally-determined partition coefficients are used to predict hydrous fluid compositions; experimental results along with mineral-melt distribution coefficients are used to predict slab melt compositions. Hydrous fluid or melt is allowed to rise into and metasomatize overlying mantle, and the modified fluid allowed to trigger mantle melting

  12. Subduction cycling of U, Th, and Pb [rapid communication

    NASA Astrophysics Data System (ADS)

    Kelley, Katherine A.; Plank, Terry; Farr, Linda; Ludden, John; Staudigel, Hubert

    2005-06-01

    Many studies argue, based partly on Pb isotopic evidence, that recycled, subducted slabs reside in the mantle source of ocean island basalts (OIB) [1-3] [A.W. Hofmann, W.M. White, Mantle plumes from ancient oceanic crust. Earth Planet. Sci. Lett. 57 (1982) 421-436; B.L. Weaver, The origin of ocean island basalt end-member compositions: trace element and isotopic constraints. Earth Planet. Sci. Lett. 104 (1991) 381-397; J.C. Lassiter, E.H. Hauri, Osmium-isotope variations in Hawaiian lavas: evidence for recycled oceanic lithosphere in the Hawaiian plume, Earth Planet. Sci. Lett. 164 (1998) 483-496]. Such models, however, have remained largely untested against actual subduction zone inputs, due to the scarcity of comprehensive measurements of both radioactive parents (Th and U) and radiogenic daughter (Pb) in altered oceanic crust (AOC). Here, we discuss new, comprehensive measurements of U, Th, and Pb concentrations in the oldest AOC, ODP Site 801, and consider the effect of subducting this crust on the long-term Pb isotope evolution of the mantle. The upper 500 m of AOC at Site 801 shows > 4-fold enrichment in U over pristine glass during seafloor alteration, but no net change to Pb or Th. Without subduction zone processing, ancient AOC would evolve to low 208Pb/ 206Pb compositions unobserved in the modern mantle [4] [S.R. Hart, H. Staudigel, Isotopic characterization and identification of recycled components, in: Crust/Mantle Recycling at Convergence Zones, Eds. S.R. Hart, L. Gülen, NATO ASI Series. Series C: Mathematical and Physical Sciences 258, pp. 15-28, D. Reidel Publishing Company, Dordrecht-Boston, 1989]. Subduction, however, drives U-Th-Pb fractionation as AOC dehydrates in the earth's interior. Pacific arcs define mixing trends requiring 8-fold enrichment in Pb over U in AOC-derived fluid. A mass balance across the Mariana subduction zone shows that 44-75% of Pb but < 10% of U is lost from AOC to the arc, and a further 10-23% of Pb and 19-40% of U is

  13. Constraining the Conditions Required for the Delamination of Subducting Crust

    NASA Astrophysics Data System (ADS)

    Maunder, B. L.; Van Hunen, J.; Magni, V.; Bouilhol, P.

    2014-12-01

    It is commonly accepted that the building of the continental crust is linked to subduction zone processes, but the refining mechanism isolating the felsic product from its basaltic counterpart, leading to a stratified crust, remains poorly understood. Delamination of subducting material and its subsequent melting and segregation, with the felsic part being underplated and added to the crust from below has been suggested to be a viable scenario.In this study we use thermo-mechanical numerical models of subduction to explore the possibility of delamination of the igneous slab crust and determine the conditions that are required by varying key parameters, such as subduction speed and angle, slab age, crustal thickness and density, overriding plate thickness, mantle temperature, depth of eclogitisation and the rheological properties for crustal and mantle material. We also quantify the extent of the resultant crustal melting, and its composition.Our preliminary models demonstrate that for present day mantle potential temperatures and average slab crustal thickness, the slab crust may only delaminate for extreme rheologies (i.e very weak crust), making slab mafic crust delamination unlikely. Contrastingly, in an early earth setting (High mantle temperature potential and thicker mafic slab crust) we find that the whole crustal scale delamination of the subducting mafic crust is a dynamically viable mechanism for a reasonable rheology when slabs are younger than ~20Ma. The resulting delamination leads to buoyant upwelling and ponding of mafic crustal material beneath the overriding lithosphere. After only ~5 Myrs from the onset of delamination, delaminated mafic crust would sit in the hot mantle wedge, where it would likely cross its solidus. These melts would be readily segregated from the migmatitic mafic source and contribute to the formation of felsic crust with little interaction with the mantle wedge, explaining part of the spectrum of TTG forming the earliest

  14. Evidence for pressure-release melting beneath magmatic arcs from basalt at Galunggung, Indonesia

    USGS Publications Warehouse

    Sisson, T.W.; Bronto, S.

    1998-01-01

    The melting of peridotite in the mantle wedge above subduction zones is generally believed to involve hydrous fluids derived from the subducting slab. But if mantle peridotite is upwelling within the wedge, melting due to pressure release could also contribute to magma production. Here we present measurements of the volatile content of primitive magmas from Galunggung volcano in the Indonesian are which indicate that these magmas were derived from the pressure-release melting of hot mantle peridotite. The samples that we have analysed consist of mafic glass inclusions in high-magnesium basalts. The inclusions contain uniformly low H2O concentrations (0.21-0.38 wt%), yet relatively high levels of CO2 (up to 750 p.p.m.) indicating that the low H2O concentrations are primary and not due to degassing of the magma. Results from previous anhydrous melting experiments on a chemically similar Aleutian basalts indicate that the Galunggung high-magnesium basalts were last in equilibrium with peridotite at ~1,320 ??C and 1.2 GPa. These high temperatures at shallow sub-crustal levels (about 300-600 ??C hotter than predicted by geodynamic models), combined with the production of nearly H2O- free basaltic melts, provide strong evidence that pressure-release melting due to upwelling in the sub-are mantle has taken place. Regional low- potassium and low-H2O (ref. 5) basalts found in the Cascade are indicate that such upwelling-induced melting can be widespread.

  15. The Karoo igneous province — A problem area for inferring tectonic setting from basalt geochemistry

    NASA Astrophysics Data System (ADS)

    Duncan, Andrew R.

    1987-06-01

    Tholeiitic basalts and associated intrusives are the major component of the Karoo igneous province. They are of Mesozoic age and constitute one of the world's classic continental flood basalt (CFB) provinces. It has been argued that most Karoo basalts have not undergone significant contamination with continental crust and that their lithospheric mantle source areas were enriched in incompatible minor and trace elements during the Proterozoic. The only exceptions to this are late-stage MORB-like dolerites near the present-day continental margins which are considered to be of asthenospheric origin. When data for the "southern" Karoo basalts are plotted on many of the geochemical discriminant diagrams which have been used to infer tectonic setting, essentially all of them would be classified as calc-alkali basalts (CAB's) or low-K tholeiites. Virtually none of them plot in the compositional fields designated as characteristic of "within-plate" basalts. There is little likelihood that the compositions of the Karoo basalts can be controlled by active subduction at the time of their eruption and no convincing evidence that a "subduction component" has been added to the subcontinental lithospheric mantle under the entire area in which the basalts crop out. It must be concluded that the mantle source areas for CAB's and the southern Karoo basalts have marked similarities. In contrast, the data for "northern" Karoo basalts largely plot in the "within-plate" field on geochemical discriminant diagrams. Available data suggest that the source composition and/or the restite mineralogy and degree of partial melting are different for southern and northern Karoo basalts. There is no evidence for any difference in tectonic setting between the southern and northern Karoo basalts at the time they were erupted. This appears to be clear evidence that specific mantle source characteristics and/or magmatic processes can vary within a single CFB province to an extent that renders at least

  16. Fluid processes in subduction zones.

    PubMed

    Peacock, S A

    1990-04-20

    Fluids play a critical role in subduction zones and arc magmatism. At shallow levels in subduction zones (<40 kilometers depth), expulsion of large volumes of pore waters and CH(4)-H(2)O fluids produced by diagenetic and low-grade metamorphic reactions affect the thermal and rheological evolution of the accretionary prism and provide nutrients for deep-sea biological communities. At greater depths, H(2)O and CO(2) released by metamorphic reactions in the subducting oceanic crust may alter the bulk composition in the overlying mantle wedge and trigger partial melting reactions. The location and conse-quences of fluid production in subduction zones can be constrained by consideration of phase diagrams for relevant bulk compositions in conjunction with fluid and rock pressure-temperature-time paths predicted by numerical heat-transfer models. Partial melting of subducting, amphibole-bearing oceanic crust is predicted only within several tens of million years of the initiation of subduction in young oceanic lithosphere. In cooler subduction zones, partial melting appears to occur primarily in the overlying mantle wedge as a result of fluid infiltration. PMID:17784486

  17. Lithium isotope fractionation in the southern Cascadia subduction zone

    NASA Astrophysics Data System (ADS)

    Magna, Tomáš; Wiechert, Uwe; Grove, Timothy L.; Halliday, Alex N.

    2006-10-01

    We present lithium (Li) abundances and isotope compositions for a suite of anhydrous olivine tholeiites (HAOTs) and hydrous basalt-andesitic (BA) lavas from the Mt. Shasta and Medicine Lake regions, California. The values of δ7Li vary from + 0.9‰ to + 6.4‰ and correlate inversely with distance from the trench. These data are consistent with continuous isotope fractionation of Li during dehydration of the subducted oceanic lithosphere, an interpretation corroborated by uniformly high pre-eruptive H 2O contents in basaltic andesites accompanied by high Li, Rb, Sr, Ba and Pb abundances. The subduction-derived component that was added to these hydrous magmas is shown to be very similar beneath both Mt. Shasta and Medicine Lake volcanoes despite characteristically distinct Li isotope compositions in the magmas themselves. More evolved andesites and dacites from Mt. Shasta have δ7Li from + 2.8 to + 6.9‰ which is identical with the range obtained for HAOTs and BA lavas from Mt. Shasta. Therefore, Li isotopes do not provide evidence for any other crustal component admixed to Mt. Shasta andesites or dacites during magmatic differentiation and magma mixing in the crust.

  18. Very high potassium (VHK) basalt - Complications in mare basalt petrogenesis

    NASA Technical Reports Server (NTRS)

    Shervais, J. W.; Taylor, L. A.; Laul, J. C.; Shih, C.-Y.; Nyquist, L. E.

    1985-01-01

    The first comprehensive report on the petrology and geochemistry of Apollo 14 VHK (Very High Potassium) basalts and their implications for lunar evolution is presented. The reported data are most consistent with the hypothesis that VHK basalts formed through the partial assimilation of granite by a normal low-Ti, high-Al mare basalt magma. Assimilation was preceded by the diffusion-controlled exchange of alkalis and Ba between basalt magma and the low-temperature melt fraction of the granite. Hypotheses involving volatile/nonvolatile fractionations or long-term enrichment of the source regions in K are inconsistent with the suprachondritic Ba/La ratios and low initial Sr-87/Sr-86 ratios of VHK basalt. An important implication of this conclusion is that granite should be a significant component of the lunar crust at the Apollo 14 site.

  19. Decarbonation reaction of magnesite in subducting slabs at the lower mantle

    NASA Astrophysics Data System (ADS)

    Takafuji, N.; Fujino, K.; Nagai, T.; Seto, Y.; Hamane, D.

    2006-12-01

    High-pressure and temperature experiments (28 62 GPa, and 1,490 2,000 K, corresponding to approximately 770 1,500 km depth in the mantle) have been conducted on a MgCO3 + SiO2 mixture using a laser-heated diamond anvil cell combined with analytical transmission electron microscope observation of the product phases to constrain the fate of carbonates carried on the subducting basalt into the lower mantle. At these conditions, the decarbonation reaction MgCO3 (magnesite) + SiO2 (stishovite) → MgSiO3 (perovskite) + CO2 (solid) has been recognized. This indicates that above reaction takes place as a candidate for decarbonation of the carbonated subducting mid ocean ridge basalts in the Earth’s lower mantle.

  20. Evolution of a Subduction Zone

    NASA Astrophysics Data System (ADS)

    Noack, Lena; Van Hoolst, Tim; Dehant, Veronique

    2014-05-01

    The purpose of this study is to understand how Earth's surface might have evolved with time and to examine in a more general way the initiation and continuance of subduction zones and the possible formation of continents on an Earth-like planet. Plate tectonics and continents seem to influence the likelihood of a planet to harbour life, and both are strongly influenced by the planetary interior (e.g. mantle temperature and rheology) and surface conditions (e.g. stabilizing effect of continents, atmospheric temperature), but may also depend on the biosphere. Employing the Fortran convection code CHIC (developed at the Royal Observatory of Belgium), we simulate a subduction zone with a pre-defined weak zone (between oceanic and continental crust) and a fixed plate velocity for the subducting oceanic plate (Quinquis et al. in preparation). In our study we first investigate the main factors that influence the subduction process. We simulate the subduction of an oceanic plate beneath a continental plate (Noack et al., 2013). The crust is separated into an upper crust and a lower crust. We apply mixed Newtonian/non-Newtonian rheology and vary the parameters that are most likely to influence the subduction of the ocanic plate, as for example density of the crust/mantle, surface temperature, plate velocity and subduction angle. The second part of our study concentrates on the long-term evolution of a subduction zone. Even though we model only the upper mantle (until a depth of 670km), the subducted crust is allowed to flow into the lower mantle, where it is no longer subject to our investigation. This way we can model the subduction zone over long time spans, for which we assume a continuous inflow of the oceanic plate into the investigated domain. We include variations in mantle temperatures (via secular cooling and decay of radioactive heat sources) and dehydration of silicates (leading to stiffening of the material). We investigate how the mantle environment influences

  1. The Sulfur Cycle at Subduction Zones

    NASA Astrophysics Data System (ADS)

    de Moor, M. J.; Fischer, T. P.; Sharp, Z. D.

    2013-12-01

    We present sulfur (S) isotope data for magmatic gases emitted along the Central American (CA) Arc (oxidizing conditions ΔQFM ~+ 1.5) and at the East African Rift (reduced conditions ΔQFM ~0). The results are interpreted through mass balance calculations to characterize the S cycle through subduction zones with implications for the redox conditions of arc magmas. Voluminous gas emissions from Masaya, an open vent basaltic volcano in Nicaragua, represent >20% of the SO2 flux from the CA arc [1]. Samples from the Masaya plume have S isotope compositions of + 4.8 × 0.4 ‰ [2]. Degassing fractionation modeling and assessment of differentiation processes in this oxidized volcano suggest that this value is close to that of the source composition. High T gas samples from other CA volcanoes (Momotombo, Cerro Negro, Poas, Turrialba) range from + 3 ‰ (Cerro Negro) to + 7 ‰ (Poas; [3]). The high δ34S values are attributed to recycling of subducted oxidized sulfur (sulfate ~ + 20 ‰) through the CA arc. The δ34S values of the more reduced samples from East African Rift volcanoes, Erta Ale - 0.5 × 0.6 ‰ [3] and Oldoinyo Lengai -0.7 ‰ to + 1.2 ‰) are far lower and are probably sourced directly from ambient mantle. The subduction of oxidized material at arcs presents a likely explanation for the oxidized nature of arc magmas relative to magmas from spreading centers. We observe no distinguishable change in melt fO2 with S degassing and attribute these differences to tectonic setting. Monte Carlo modeling suggests that subducted crust (sediments, altered oceanic crust, and serpentinized lithospheric mantle) delivers ~7.7 × 2.2 x 1010 mols of S with δ34S of -1.5 × 2.3‰ per year into the subduction zone. The total S output from the arc is estimated to be 3.4 × 1.1 x 1010 mols/yr with a δ34S value similar to that of Masaya gas (+5 × 0.5 ‰). Considering δ34S values for ambient upper mantle (0 ‰ [4]) and slab-derived fluids (+14 ‰ [5]) allows calculation

  2. Near-Primary Oxidized Basalts from the Submarine Vanuatu Arc

    NASA Astrophysics Data System (ADS)

    Gentes, Z.; Kelley, K. A.; Cottrell, E.; Arculus, R. J.

    2014-12-01

    Near-primary melt compositions (i.e., in equilibrium with >Fo89 olivine) are rare in arc systems. Yet, such melts provide essential views of mantle-derived melts, without further modification by fractional crystallization or other crustal processes, and reveal the diversity of melt compositions that exist in the arc mantle wedge. Here, we present new measurements of naturally glassy, near-primary olivine-hosted melt inclusions from one dredge of Evita seamount (SS07/2008 NLD-02) in the southern Vanuatu arc system. Two distinct basalt types were identified in hand sample upon collection, based on contrasting phenocryst assemblage (Type 1: 1% phenocrysts; Type 2: 15% phenocrysts). We selected melt inclusions from each type and determined major elements, S, and Cl by EMP, H2O and CO2 by FTIR, trace elements by LA-ICP-MS, and Fe3+/∑Fe ratios by XANES. Melt inclusions from both lava types show equilibrium with ≥Fo90 olivine, consistent with host olivine compositions, and thus are near-primary melt compositions that have escaped major modification since departing the mantle wedge. Both have similar maximum dissolved H2O (~2.3 wt.%), high Mg# (48-75), and are basalt to basaltic andesite (SiO2 49-55 wt.%). However, the two lava types have very different major and trace element compositions. Inclusions from Type 1 show relatively flat REE patterns and classic negative anomalies in Nb and Ta, and positive anomalies in Pb and Sr typical of normal arc basalts, and have Fe3+/∑Fe ratios similar to global arc basalts (~0.24). In contrast, melt inclusions from Type 2 exhibit steeply sloped REE patterns with strong depletions in the HREE that suggest garnet in the source lithology for these magmas, either in the subducting slab or the mantle wedge. Moreover, the Type 2 inclusions have high La/Yb (29.5-43) and Sr/Y (50-58), which are classically attributed to partial melting of the basaltic slab, although these inclusions are basaltic, not andesitic. Type 2 inclusions also

  3. Continental flood basalts derived from the hydrous mantle transition zone.

    PubMed

    Wang, Xuan-Ce; Wilde, Simon A; Li, Qiu-Li; Yang, Ya-Nan

    2015-01-01

    It has previously been postulated that the Earth's hydrous mantle transition zone may play a key role in intraplate magmatism, but no confirmatory evidence has been reported. Here we demonstrate that hydrothermally altered subducted oceanic crust was involved in generating the late Cenozoic Chifeng continental flood basalts of East Asia. This study combines oxygen isotopes with conventional geochemistry to provide evidence for an origin in the hydrous mantle transition zone. These observations lead us to propose an alternative thermochemical model, whereby slab-triggered wet upwelling produces large volumes of melt that may rise from the hydrous mantle transition zone. This model explains the lack of pre-magmatic lithospheric extension or a hotspot track and also the arc-like signatures observed in some large-scale intracontinental magmas. Deep-Earth water cycling, linked to cold subduction, slab stagnation, wet mantle upwelling and assembly/breakup of supercontinents, can potentially account for the chemical diversity of many continental flood basalts. PMID:26169260

  4. Sulfide Stability of Planetary Basalts

    NASA Technical Reports Server (NTRS)

    Caiazza, C. M.; Righter, K.; Gibson, E. K., Jr.; Chesley, J. T.; Ruiz, J.

    2004-01-01

    The isotopic system, 187Re 187Os, can be used to determine the role of crust and mantle in magma genesis. In order to apply the system to natural samples, we must understand variations in Re/Os concentrations. It is thought that low [Os] and [Re] in basalts can be attributed to sulfide (FeS) saturation, as Re behaves incompatibly to high degrees of evolution until sulfide saturation occurs [1]. Previous work has shown that lunar basalts are sulfide under-saturated, and mid-ocean ridge, ocean-island and Martian (shergottites) basalts are saturated [2,3]. However, little is known about arc basalts. In this study, basaltic rocks were analyzed across the Trans-Mexican Volcanic Belt.

  5. Subduction initiation at relic arcs

    NASA Astrophysics Data System (ADS)

    Leng, Wei; Gurnis, Michael

    2015-09-01

    Although plate tectonics is well established, how a new subduction zone initiates remains controversial. Based on plate reconstruction and recent ocean drilling within the Izu-Bonin-Mariana, we advance a new geodynamic model of subduction initiation (SI). We argue that the close juxtaposition of the nascent plate boundary with relic oceanic arcs is a key factor localizing initiation of this new subduction zone. The combination of thermal and compositional density contrasts between the overriding relic arc, and the adjacent old Pacific oceanic plate promoted spontaneous SI. We suggest that thermal rejuvenation of the overriding plate just before 50 Ma caused a reduction in overriding plate strength and an increase in the age contrast (hence buoyancy) between the two plates, leading to SI. The computational models map out a framework in which rejuvenated relic arcs are a favorable tectonic environment for promoting subduction initiation, while transform faults and passive margins are not.

  6. Water and the Oxidation State of Subduction Zone Magmas

    SciTech Connect

    Kelley, K.; Cottrell, E

    2009-01-01

    Mantle oxygen fugacity exerts a primary control on mass exchange between Earth's surface and interior at subduction zones, but the major factors controlling mantle oxygen fugacity (such as volatiles and phase assemblages) and how tectonic cycles drive its secular evolution are still debated. We present integrated measurements of redox-sensitive ratios of oxidized iron to total iron (Fe{sup 3+}/{Sigma}Fe), determined with Fe K-edge micro-x-ray absorption near-edge structure spectroscopy, and pre-eruptive magmatic H{sub 2}O contents of a global sampling of primitive undegassed basaltic glasses and melt inclusions covering a range of plate tectonic settings. Magmatic Fe{sup 3+}/{Sigma}Fe ratios increase toward subduction zones (at ridges, 0.13 to 0.17; at back arcs, 0.15 to 0.19; and at arcs, 0.18 to 0.32) and correlate linearly with H{sub 2}O content and element tracers of slab-derived fluids. These observations indicate a direct link between mass transfer from the subducted plate and oxidation of the mantle wedge.

  7. Global overview of subduction seismicity

    NASA Astrophysics Data System (ADS)

    Funiciello, F.; Presti, D.; Heuret, A.; Piromallo, C.

    2013-12-01

    In the framework of the EURYI Project ';Convergent margins and seismogenesis: defining the risk of great earthquakes by using statistical data and modelling', we propose the first global overview of subduction seismicity. Previous studies have been focused on interplate seismicity, intraslab seismicity, upper plate deformation, or relation between interplate and intraslab seismicity, but the three components of subduction seismicity have been never approached in an systematic and exhaustive study. To allow such a study, nodal planes and seismic moments of worldwide subduction-related earthquakes heve been extracted by EHB hypocenter and CMT Harvard catalogues for the period 1976 - 2007. Data were collected for centroid depths between sea level and 700 km and for magnitude Mw 5.5. For each subduction zone, a set of trench-normal transects were constructed choosing a 120km width of the cross-section on each side of a vertical plane and a spacing of 1 degree along the trench. For each of the 505 resulting transects, the whole subduction seismogenic zone was mapped as focal mechanisms projected on to a vertical plane after their faulting type classification according to the Aki-Richards convention. Transect by transect, fist the seismicity that can be considered not related to the subduction process under investigation was removed, then was selected the upper plate seismicity (i.e. earthquakes generated within the upper plate as a result of the subduction process). After deletion from the so obtained event subset of the interplate seismicity as identified in the framework of this project by Heuret et al. (2011), we can be reasonably confident that the remaining seismicity can be related to the subducting plate. Among these earthquakes we then selected the shallow (0-70 km), intermediate (70-300 km) and deep (300-660 km) depth seismicity. Following Heuret et al. (2011), the 505 transects were merged into 62 larger segments that were ideally homogeneous in terms of their

  8. Thrusting-related Fluid Flow in Subduction Zone: Geologic and Isotopic Evidence From Ancient Subduction Complex

    NASA Astrophysics Data System (ADS)

    Yamaguchi, A.; Ujiie, K.; Kimura, G.

    2004-12-01

    Role of fluid in a seismogenic plate boundary of subduction zone remains poorly understood. To reveal the origin and behavior of syn-tectonic fluid in the ancient subduction complex, we examined occurrences of veins and carbon and oxygen isotope composition of vein calcite of the Mugi Melange in the Shimanto Belt. The melange preserves a relationship between rock deformation and fluid flow near the up-dip limit of seismogenic zone. The Mugi Melange is a late Cretaceous to Paleocene tectonic melange, and is characterized by blocks of sandstone and basalt in shale matrix. The Mugi Melange is divided into five units based on the repetition of basaltic layer of N-MORB, representing duplex-underplated assemblage. Paleo-temperature determined by vitrinite reflectance is 120-150 \\deg C in the studied section, and fluid inclusion thermo-barometry indicates 125-245 \\deg C and 92-149 MPa (Matsumura et al., 2003). This P-T condition is in the vicinity of up-dip limit of the seismogenic zone. Major deformational stage of the Mugi Melange is underthrusting (D1), underplating (D2), and uplifting (D3). There are various occurrences of quartz and calcite veins in melange formed during various stages. We classified three occurrences of veins named intra-boudin veins (IBV), network veins (NWV), and fault-parallel veins (FPV). IBV are observed only around the neck of boudined sandstone block in the black shale matrix, so they were precipitated when block-in-matrix structure was formed due to layer-parallel extension during D1. NWV are distributed in the damage zone of duplex ramp thrust, and are mainly perpendicular to shear surface of the thrust. Such occurrences suggest that NWV formed inter-faulting period of D2. FPV show implosion breccia-like texture (Sibson, 1986), and they are observed just below the basalt / shale boundary. Vein minerals are dominantly calcite. These features suggest that FPV would be precipitated as a result of co-slip fluid migration during oceanic crust

  9. Origin of low δ26Mg Cenozoic basalts from South China Block and their geodynamic implications

    NASA Astrophysics Data System (ADS)

    Huang, Jian; Li, Shu-Guang; Xiao, Yilin; Ke, Shan; Li, Wang-Ye; Tian, Ye

    2015-09-01

    Origin of low δ26Mg basalts is a controversial subject and has been attributed to interaction of isotopically light carbonatitic melts derived from a subducted oceanic slab with the mantle (Yang et al., 2012), or alternatively, to accumulation of isotopically light ilmenite (FeTiO3) in their mantle source (Sedaghatpour et al., 2013). To study the origin of low δ26Mg basalts and evaluate whether Mg isotope ratios of basalts can be used to trace deeply recycled carbon, high-precision major and trace element and Mg isotopic analyses on the Cenozoic alkaline and tholeiitic basalts from the South China Block (SCB), eastern China have been carried out in this study. The basalts show light Mg isotopic compositions, with δ26Mg ranging from -0.60 to -0.35. The relatively low TiO2 contents (<2.7 wt.%) of our basalts, roughly positive correlations between δ26Mg and Ti/Ti∗ and their constant Nb/Ta ratios (16.4-20) irrespective of variable TiO2 contents indicate no significant amounts of isotopically light ilmenite accumulation in their mantle source. Notably, the basalts display negative correlations between δ26Mg and the amounts of total alkalis (i.e., Na2O + K2O) and incompatible trace elements (e.g., Ti, La, Nd, Nb, Th) and trace element abundance ratios (e.g., Sm/Yb, Nb/Y). Generally, with decrease of δ26Mg values, their Hf/Hf∗ and Ti/Ti∗ ratios decrease, whereas Ca/Al and Zr/Hf ratios increase. These features are consistent with incongruent partial melting of an isotopically light carbonated mantle, suggesting that large variations in Mg isotope ratios occurred during partial melting of such carbonated mantle under high temperatures. The isotopically light carbonated mantle were probably formed by interaction of the mantle with low δ26Mg carbonatitic melts derived from the deeply subducted low δ26Mg carbonated eclogite transformed from carbonate-bearing oceanic crust during plate subduction. As only the Pacific slab has an influence on both the North China

  10. Why Hexagonal Basalt Columns?

    PubMed

    Hofmann, Martin; Anderssohn, Robert; Bahr, Hans-Achim; Weiß, Hans-Jürgen; Nellesen, Jens

    2015-10-01

    Basalt columns with their preferably hexagonal cross sections are a fascinating example of pattern formation by crack propagation. Junctions of three propagating crack faces rearrange such that the initial right angles between them tend to approach 120°, which enables the cracks to form a pattern of regular hexagons. To promote understanding of the path on which the ideal configuration can be reached, two periodically repeatable models are presented here involving linear elastic fracture mechanics and applying the principle of maximum energy release rate. They describe the evolution of the crack pattern as a transition from rectangular start configuration to the hexagonal pattern. This is done analytically and by means of three-dimensional finite element simulation. The latter technique reproduces the curved crack path involved in this transition. PMID:26550724

  11. Late Cenozoic volcanism, subduction, and extension in the Lassen region of California, Southern Cascade Range

    SciTech Connect

    Guffanti, M. ); Clynne, M.A.; Smith, J.G.; Muffler, L.J.P.; Bullen, T.D. )

    1990-11-10

    The authors identify 537 volcanic vents younger than 7 Ma, and they classify these into five age intervals and five compositional categories based on SiO{sub 2} content. Maps of vents by age and composition illustrate regionally representative volcanic trends. Most mafic volcanism is calcalkaline basalt and basaltic andesite. However, lesser volume of low-potassium olivine tholeiite (LKOT), a geochemically distinctive basalt type found in the northern Basin and Range province, also has erupted throughout the Lassen segment of the Cascade arc since the Pliocene. Normal faults and linear groups of vents are evidence of widespread crustal extension throughout most of the Lassen region. NNW alignments of these features indicate NNW orientation of maximum horizontal stress (ENE extension), which is similar to the stress regime in the adjacent northwestern Basin and Range and northern Sierra Nevada provinces. They interpret the western limit of the zone of NNW trending normal faults as the western boundary of the Basin and Range province where it overlaps the Lassen segment of the Cascade arc. The Lassen volcanic region occurs above the subducting Gorda North plate but also lies within a broad zone of distributed extension that occurs in the North American lithosphere east and southeast of the present Cascadia subduction zone. The scarcity of volcanic rocks older than 7 Ma suggests that a more compressive lithospheric stress regime prior to the late Miocene extensional episode may have suppressed volcanism, even though subduction probably was occurring beneath the Lassen region.

  12. Project EMSLAB examines subduction

    NASA Astrophysics Data System (ADS)

    Hermance, John F.

    As part of the International ELAS (Electrical Studies of the Asthenosphere) Project (Eos, July 31, 1979, p. 551), which has been designated as a “key project” by the International Council of Scientific Unions (ICSU) Interunion Commission on the Lithosphere, large land and sea electromagnetic arrays have been deployed by workers from Canada, the United States, Mexico, and other countries during the summer and early fall of 1985. Soviet workers are expected to participate in the data analysis. This array, known as Project EMSLAB (for “electromagnetic study of the lithosphere and asthenosphere beneath (the Juan de Fuca Plate and adjacent continents)”), will use geomagnetic variation and magnetotelluric techniques to study the electrical conductivity of the crust, lithosphere, and asthenosphere in and above the actively subducting Juan de Fuca Plate. The area to be studied includes the region from the Juan de Fuca Ridge to the North American continent, across the Cascade Range of southern Canada, Washington and Oregon states, and into the Basin and Range Province of the western United States. Complementing the magnetic variometer array will be a profile of magnetotelluric and short-period magnetic variation sites along a swath through central Oregon.

  13. Basalt Weathering Rates Across Scales

    NASA Astrophysics Data System (ADS)

    Navarresitchler, A.; Brantley, S.

    2006-12-01

    Weathering of silicate minerals is a known sink for atmospheric CO2. An estimated 30%-35% of the consumption of CO2 from continental silicate weathering can be attributed to basalt weathering (Dessert et al., 2003). To assess basalt weathering rates we examine weathering advance rates of basalt (w, mm/yr) reported at four scales: denudation rates from basalt watersheds (tens of kilometers), rates of soil formation from soil profiles developed on basaltic parent material of known age (meters), rates of weathering rind formation on basalt clasts (centimeters), and laboratory dissolution rates (millimeters). Basalt weathering advance rates calculated for watersheds range between 0.36 and 9.8x10-3 mm/yr. The weathering advance rate for a basalt soil profile in Hawaii is 8.0x10-3 mm/yr while advance rates for clasts range from 5.6x10-6 to 2.4x10-4 mm/yr. Batch and mixed flow laboratory experiments performed at circum- neutral pH yield advance rates of 2.5x10^{-5} to 3.4x10-7 mm/yr when normalized to BET surface area. These results show increasing advance rates with both increasing scale (from laboratory to watersheds) and increasing temperature. If we assume that basalt weathers at an intrinsic rate that applies to all scales then we conclude that variations in weathering advance rates arise from variations in surface area measurement at different scales (D); therefore, basalt weathering is a fractal system. We measure a fractal dimension (dr) of basalt weathering of 2.2. For Euclidean geometries, measured surface area does not vary with the scale at which it is measured and dr equals 2. For natural surfaces, surface area is related to the scale at which it is measured. As scale increases, the minimum size of the surface irregularities that are measurable also increases. The ratio between BET and geometric normalized laboratory dissolution rates has been defined as a roughness parameter, λ, which ranges from ~10-100. We extend the definition of this roughness parameter

  14. An inverted double seismic zone in chile: evidence of phase transformation in the subducted slab.

    PubMed

    Comte, D; Suárez, G

    1994-01-14

    Data from two microseismic field experiments in northern Chile revealed an elongated cluster of earthquakes in the subducted Nazca plate at a depth of about 100 kilometers in which down-dip tensional events were consistently shallower than a family of compressional earthquakes. This double seismic zone shows a distribution of stresses of opposite polarity relative to that observed in other double seismic zones in the world. The distribution of stresses in northern Chile supports the notion that at depths of between 90 to 150 kilometers, the basalt to eclogite transformation of the subducting oceanic crust induces tensional deformation in the upper part of the subducted slab and compressional deformation in the underlying mantle. PMID:17839180

  15. Low H2O/Ce in Icelandic basalts as evidence for crustal recycling

    NASA Astrophysics Data System (ADS)

    Neave, David; Shorttle, Oliver; Hartley, Margaret; Maclennan, John

    2016-04-01

    The generation of new crust at mid-ocean ridges is balanced by the subduction of partially hydrothermally altered basaltic material back into the mantle. This subducted material may then be recycled and returned via mantle plumes to the Earth's surface at hot spots. Long-identified isotopic and trace element signatures of oceanic crust recycling in ocean island basalts (OIBs) have been recently supplemented by evidence of major element, i.e. lithological, heterogeneity in the melting region. For example, combined major and trace element systematics from Iceland suggest that the mantle source contains at least 5% recycled basalt. Observations of high water (H2O) contents in subglacially quenched basalts from Iceland have previously been attributed to the incorporation of wet recycled material into the mantle source. However, when combined with trace element analyses, recent volatile analyses from the Laki-Grímsvötn and Bárðarbunga-Veiðivötn systems in the Eastern Volcanic Zone (EVZ) of Iceland suggest that the underlying mantle is comparatively depleted in H2O for its degree of major and trace element enrichment. Correlations between H2O and cerium (Ce) within individual mid-ocean ridge basalt (MORB) suites reveal that these elements partition similarly prior to H2O degassing at low pressures; H2O/Ce remains constant during melting and fractionation, and hence reflects the average H2O/Ce of the melting region. MORBs from the Mid-Atlantic Ridge south of Iceland have a mean H2O/Ce value of 304±48 at a mean La/Yb of 2.1±1.5. In contrast, basalts from the EVZ have a lower mean H2O/Ce of 180±20 at a higher mean La/Yb of 3.1±0.5. Thus, despite coming from an enriched section of the Mid-Atlantic ridge in terms of trace element content, basalts from the EVZ have the lowest H2O/Ce values known from the ridge, and are hence comparatively depleted in H2O. Given that H2O/Ce from un-degassed basalts is considered to represent mantle source values, we suggest that low H

  16. Geochemistry of basalts from small eruptive centers near Villarrica stratovolcano, Chile: Evidence for lithospheric mantle components in continental arc magmas

    NASA Astrophysics Data System (ADS)

    Hickey-Vargas, R.; Sun, M.; Holbik, S.

    2016-07-01

    In the Central Southern Volcanic Zone (CSVZ) of the Andes, the location of stratovolcanoes and monogenetic small eruptive centers (SEC) is controlled by the Liquiñe-Ofqui Fault Zone (LOFZ), a trench-parallel strike-slip feature of over 1000 km length. The geochemistry of basalts from SEC is different from those of stratovolcanoes, and are termed Type 2 and Type 1 basalts, respectively. In the region of Villarrica stratovolcano, contemporaneous SEC are more MgO-rich, and have greater light rare earth element (LREE) enrichment, lower 87Sr/86Sr and 143Nd/144Nd, and lower ratios of large ion lithophile elements (LILE) to LREE and high field strength elements (HFSE). A unique finding in this region is that basalts from one SEC, San Jorge, has Type 1 character, similar to basalts from Villarrica stratovolcano. Type 1 basalts from Villarrica and San Jorge SEC have strong signals from time-sensitive tracers of subduction input, such as high 10Be/9Be and high (238U/230Th), while Type 2 SEC have low 10Be/9Be and (238U/230Th) near secular equilibrium. Based on new trace element, radiogenic isotope and mineral analyses, we propose that Type 1 basaltic magma erupted at San Jorge SEC and Villarrica stratovolcano forms by melting of the ambient actively subduction-modified asthenosphere, while Type 2 SEC incorporate melts of pyroxenite residing in the supra-subduction zone mantle lithosphere. This scenario is consistent with the close proximity of the volcanic features and their inferred depths of magma separation. The pyroxenite forms from arc magma produced during earlier episodes of subduction modification and magmatism, which extend back >300 Ma along this segment of the western South American margin. Type 2 basaltic magmas may reach the surface during LOFZ-related decompression events, and they may also be a normal but episodic part of the magma supply to large stratovolcanoes, resulting in cryptic geochemical variations over time. The presence and mobilization of stored

  17. Accretion, subduction, and underplating along the southern Alaska continental margin

    SciTech Connect

    Plafker, G.; Ambos, E.L.; Fuis, G.S.; Mooney, W.D.; Nokleberg, W.J.; Campbell, D.L.

    1985-01-01

    In 1984-1985 the Trans Alaska Crustal Transect (TACT) program completed geologic, seismic refraction, gravity, and magnetic studies along a 350-km-long corridor that extends northward from the Gulf of Alaska coast near Cordova to the Denali fault at the Richardson Highway. From south to north, this segment of the transect traverses: 1) part of the Prince William terrance (PWT), composed of an accreted Paleocene and Eocene deep-sea fan complex, oceanic volcanic rocks, and pelagic sediments; 2) the Chugach terrane (CGT) composed of a) accreted Late Cretaceous flysch and oceanic basaltic rocks, b) accreted and subducted (.) Late Jurassic to Early Cretaceous sheared melange, and c) subducted Early (.) Jurassic or older blueschist/greenschist; and 3) Wrangellia-Peninsular terranes (WRT/PET) consisting primarily of late Paleozoic intraoceanic andesitic arc rocks with associated mafic and ultramafic plutonic rocks, an overlying distinctive Triassic sedimentary and volcanic sequence, and superposed intrusive and extrusive magmatic rocks of the Jurassic Talkeetna arc. At the southern margin of both the CGT and WRT/PET, shallow high-velocity zones characterized by positive gravity and magnetic anomalies reflect uplift of mafic and ultramafic basement along these thrusts. The Contact and Border Ranges fault systems appear to merge into a subhorizontal low-velocity zone of uncertain origin that underlies the CGT and southern WRT/PET at 5-9 km depth. A few kilometers beneath the shallow low-velocity zone in a 30-km-thick stack of eight northward-dipping layers of alternating high and low velocity, interpreted as subducted and underplated mantle and oceanic crust rocks. Distribution of earthquake hypocenters suggests that active subduction involves at least the lowest two and possibly the lower four layers.

  18. Potential Fields Illuminate Earthquake Sources in Subduction-Margin Settings

    NASA Astrophysics Data System (ADS)

    Blakely, R. J.; Wells, R. E.

    2008-05-01

    Potential-field anomalies can be used to map the geologic structure of earthquake source regions in subduction zones and thus provide promise for assessing future earthquake hazards. Satellite free-air gravity anomalies over subduction zones consist of an offshore trench-parallel gravity low and a subparallel coastal gravity high that reflect the topography and structure of the inner trench slope and coast ranges, respectively. Large coseismic slip in shallow megathrust earthquakes correlates with gravity lows centered on large forearc sedimentary basins along the deep sea terrace offshore. The trench-parallel gravity low, the basins, and the earthquakes are all thought to be related to the resistance to slip along the plate boundary, and basin-centered gravity lows in similar settings may be the likely source of asperities in future earthquakes. Landward of the shallow megathrust, magnetic anomalies provide clues to processes occurring within the subducting slab and overlying mantle wedge. In some subduction zones, water released from the transformation of basalt to eclogite hydrates overlying sub-continental mantle, producing serpentinite, and embrittles the downgoing slab, promoting intraslab earthquakes. The 1970 central Peru earthquake (MW 7.5 to 8.0) and the 1949 Olympia, Washington, earthquake (MW 7.1) are recent examples. Thermal models indicate that the hydrated mantle wedge in most subduction zones is below the Curie temperature of magnetite. If serpentinite is sufficiently abundant, hydrated mantle wedges will produce long-wavelength magnetic anomalies observable at the earth's surface. A crust-mantle model of the Cascadia subduction margin based on magnetic, gravity, and seismic data, is consistent with the presence of significant volumes of hydrated mantle. The advent of new global magnetic databases may allow us to map hydrated mantle worldwide. The World Digital Magnetic Anomaly Map and the CHAMP satellite magnetic field, processed to emphasize

  19. A discussion of numerical subduction initiation

    NASA Astrophysics Data System (ADS)

    Buiter, Susanne; Ellis, Susan

    2016-04-01

    In nature, subduction can initiate in various ways: Shortening can localise at oceanic transform faults, extinct spreading centres, or inherited passive margin faults; or, alternatively, subduction can be triggered from existing subduction systems by along-strike trench propagation, polarity reversals, or trench jumps. Numerical studies that specifically address subduction initiation have highlighted the roles of sediment loading, rheological strength contrasts, strain softening, and continental topographic gradients, among others. Usually, however, numerical models that aim to investigate subduction dynamics prefer to bypass the subduction initiation phase and its complexities, and focus instead on the stages during which the slab is descending into the mantle. However, even in these models, subduction still needs to begin. It is disturbingly easy to define initial model geometries that do not result in subduction. The specific combination of initial model geometries and values for rheological parameters that successfully initiates subduction has even been referred to as 'the sweet spot' in model space. One cause of subduction initiation failure is when the subducting and overriding plates lock, resulting in either indentation or severe dragging downwards of the overriding plate. This may point to a difficulty in maintaining a weak subduction interface during model evolution. A second factor that may cause difficulties is that initial model geometry and stresses need to balance, as otherwise the first model stages may show spurious deformation associated with reaching equilibrium. A third requirement that may cause problems is that the surface needs to have sufficient displacement freedom to allow the overriding plate to overthrust the subducting plate. That also implies an exclusion of sharp corners in the subduction interface near the surface. It is the interplay of subduction interface geometry, interface strength and subducting plate rheology that determines

  20. Geodynamic models of deep subduction

    NASA Astrophysics Data System (ADS)

    Christensen, Ulrich

    2001-12-01

    Numerical and laboratory models that highlight the mechanisms leading to a complex morphology of subducted lithospheric slabs in the mantle transition zone are reviewed. An increase of intrinsic density with depth, an increase of viscosity, or phase transitions with negative Clapeyron slope have an inhibiting influence on deep subduction. The impingement of slabs on a viscosity and density interface has been studied in laboratory tanks using corn syrup. Slab interaction with equilibrium and non-equilibrium phase transitions has been modelled numerically in two dimensions. Both the laboratory and the numerical experiments can reproduce the variety of slab behaviour that is found in tomographic images of subduction zones, including cases of straight penetration into the lower mantle, flattening at the 660-km discontinuity, folding and thickening of slabs, and sinking of slabs into the lower mantle at the endpoint of a flat-lying segment. Aside from the material and phase transition properties, the tectonic conditions play an important role. In particular, the retrograde motion of the point of subduction (trench-rollback) has an influence on slab penetration into the lower mantle. A question that still needs to be clarified is the mutual interaction between plate kinematics and the subduction process through the transition zone.

  1. Petrochemistry and tectonic significance of Lower Cretaceous Barros Arana Formation basalts, southernmost Chilean Andes

    NASA Astrophysics Data System (ADS)

    Stern, C. R.; Mohseni, P. P.; Fuenzalida, P. R.

    The Lower Cretaceous Barros Arana Formation (Albian, hornblende KAr age of 104 Ma), in the Magallanes region of Chile, consists of a sequence of spilitized clinopyroxene- and amphibole-bearing mafic dikes and lavas, and volcaniclastic breccias, occurring within the sedimentary infill of the Rocas Verdes marginal basin and its eastward extension onto the Cretaceous continental platform. Although the original alkali and alkaline earth element concentrations of the basaltic lavas and dikes have been altered by spilitization, the presence of relict pargasitic amphibole phenocrysts, the absence of orthopyroxene, and high LREE contents and LREE/HREE ratios imply mildly alkaline affinities for these basalts. Their low TiO 2 and HFSE (Zr, Nb, Ta, and Hf) contents and high LREE/HFSE ratios suggest affinities with convergent plate boundary arc magmas. The Barros Arana basalts are interpreted as mafic members of the mildly alkaline shoshonitic rock suite of subduction-related arcs. They may have formed as subduction geometry began to undergo the changes (flattening) that ultimately led to the initiation of the closure, deformation, and uplift of the Rocas Verdes basin by the late or post-Albian. The low initial 87Sr/ 86Sr (0.7031) and high initial 143Nd/ 144Nd (0.51277) of the basalts indicate that a generally extensional tectonic regime east of the main calc-alkaline arc allowed eruption of these mafic shoshonites without interaction with continental crust (in contrast to the contemporaneous plutons of the Patagonian batholith).

  2. Hanford basalt flow mineralogy

    SciTech Connect

    Ames, L.L.

    1980-09-01

    Mineralogy of the core samples from five core wells was examined in some detail. The primary mineralogy study included an optical examination of polished mounts, photomicrographs, chemical analyses of feldspars, pyroxenes, metallic oxides and microcrystalline groundmasses and determination from the chemical analyses of the varieties of feldspars, pyroxenes and metallic oxides. From the primary mineralogy data, a firm understanding of the average Hanford basalt flow primary mineralogy emerged. The average primary feldspar was a laboradorite, the average pyroxene was an augite and the average metallic oxide was a solid solution of ilmenite and magnetite. Secondary mineralization consisted of vug filling and joint coating, chiefly with a nontronite-beidellite clay, several zeolites, quartz, calcite, and opal. Specific flow units also were examined to determine the possibility of using the mineralogy to trace flows between core wells. These included units of the Pomona, the Umatilla and a high chromium flow just below the Huntzinger. In the Umatilla, or high barium flow, the compositional variation of the feldspars was unique in range. The pyroxenes in the Pomona were relatively highly zoned and accumulated chromium. The high chromium flow contained chromium spinels that graded in chromium content into simple magnetites very low in chromium content. A study of the statistical relationships of flow unit chemical constituents showed that flow unit constituents could be roughly correlated between wells. The probable cause of the correlation was on-going physical-chemical changes in the source magma.

  3. Geochemistry characteristics of Seamounts in the Tonga arc : Influence of subduction component

    NASA Astrophysics Data System (ADS)

    Myeong, B.; Kim, J. H.; Woo, H.; Jang, Y. D.

    2015-12-01

    Located in the southwest Pacific ocean, The seamounts, from TA07 seamount to TA26, in the Tonga arc are located from 20 °S to 25 °S. At 25 °S, the Tonga trench is intersected by the Louisville ridge, a ~4,300-km-long chain of seamounts and the Osbourn trough, a paleo-spreading center. For this reason, magma which created these seamounts may have various origin. Based on this, the seamounts which forming the Tonga arc are divided into three groups; including group 1(region that the earliest Louisville ridge subduction arised, correspond to TA07-12), group 2(region that the second Louisville ridge subduction arised, correspond to TA14-24); and group 3(region that the Louisville ridge and the Osbourn trough are subducting, correspond to TA25-26). These seamounts are mostly stratovolcanoes with caldera. Rocks recovered by dredging have been identified as pumice, dacite, andesite, basaltic andesites and basalts(most abundant). Major element concentrations are constant, trace element concentrations are enriched LILE, depleted HFSE compared with MORB. The Tonga arc is affected by subduction components divided into the shallow and deep subduction components. Related to subduction components, variables include mantle source, AOC(altered oceanic crust), PS(pelagic sediment), LSC(Louisville seamount chain) and OS(Osbourn trough). In the case of shallow subduction component, it tends to have higher contents in group 1, 3 and lower contents in group 2. Thus, comparatively speaking, it seems that group 1, 3 have been heavily influenced by the fluid. However, origin of the fluid seems to be different, since its locations are not continuous. In the case of deep subduction component, it shows similar range in the group 1, 2, and shows a significantly lower ratio in the group 3. The reason why its values are similar is that it is effected by the melt during the Louisville ridge was subducting and the reason why the ratio in the group 3 shown lower is because of the fluid effect

  4. The Cambrian initiation of intra-oceanic subduction in the southern Paleo-Asian Ocean: Further evidence from the Barleik subduction-related metamorphic complex in the West Junggar region, NW China

    NASA Astrophysics Data System (ADS)

    Liu, Bo; Han, Bao-Fu; Xu, Zhao; Ren, Rong; Zhang, Jin-Rui; Zhou, Jing; Su, Li; Li, Qiu-Li

    2016-06-01

    In this study, we present new evidence from the Barleik subduction-related metamorphic complex in the southern West Junggar region, northwestern China, for the Cambrian initiation of intra-oceanic subduction in the southern Paleo-Asian Ocean. The Barleik metamorphic complex is mainly composed of blueschist and amphibolite blocks within an ophiolitic mélange and their protoliths are calc-alkaline andesite and alkali and tholeiitic basalts. The calc-alkaline andesite has a zircon U-Pb age of 502 ± 2 Ma, obtained from magmatic cores of zircon grains, and shares geochemical features similar to the 515-485 Ma intra-oceanic arc magmatic rocks in the West Junggar region. By contrast, the alkali and tholeiitic basalts have trace element features similar to ocean island and enriched mid-ocean ridge basalts, respectively. Rutile and sodic-calcic amphibole from the amphibolite have a U-Pb age of 502 ± 25 Ma and a 40Ar/39Ar age of ∼504 Ma, respectively, which are in good agreement within errors with a 40Ar/39Ar age of 492 ± 4 Ma for phengite from the blueschist. These metamorphic ages of ∼500 Ma are interpreted to represent the timing of Pacific-type subduction-related metamorphism and are also compatible with ages of the oldest supra-subduction zone ophiolites (531-512 Ma) and intra-oceanic arc plutons (515-485 Ma) in the southern West Junggar region. Being one of the oldest subduction-related metamorphic complexes (509-490 Ma) in the southern Central Asian Orogenic Belt, the Barleik metamorphic complex, together with the oldest arc plutons, definitely indicate the initial intra-oceanic subduction in the southern Paleo-Asian Ocean at least in the Early Cambrian.

  5. Flood basalts and mass extinctions

    NASA Technical Reports Server (NTRS)

    Morgan, W. Jason

    1988-01-01

    There appears to be a correlation between the times of flood basalts and mass-extinction events. There is a correlation of flood basalts and hotspot tracks--flood basalts appear to mark the beginning of a new hotspot. Perhaps there is an initial instability in the mantle that bursts forth as a flood basalt but then becomes a steady trickle that persists for many tens of millions of years. Suppose that flood basalts and not impacts cause the environmental changes that lead to mass-extinctions. This is a very testable hypothesis: it predicts that the ages of the flows should agree exactly with the times of extinctions. The Deccan and K-T ages agree with this hypothesis; An iridium anomaly at extinction boundaries apparently can be explained by a scaled-up eruption of the Hawaiian type; the occurrence of shocked-quartz is more of a problem. However if the flood basalts are all well dated and their ages indeed agree with extinction times, then surely some mechanism to appropriately produce shocked-quartz will be found.

  6. Thickness of western mare basalts

    NASA Technical Reports Server (NTRS)

    Dehon, R. A.

    1979-01-01

    An isopach map of the basalt thickness in the western mare basins is constructed from measurements of the exposed external rim height of partially buried craters. The data, although numerically sparse, is sufficiently distributed to yield gross thickness variations. The average basalt thickness in Oceanus Procellarum and adjacent regions is 400 m with local lenses in excess of 1500 m in the circular maria. The total volume of basalt in the western maria is estimated to be in the range of 1.5 x 10 to the 6th power cu km. The chief distinction between the eastern and western maria appears to be one of basalt volumes erupted to the surface. Maximum volumes of basalt are deposited west of the central highlands and flood subjacent terrain to a greater extent than on the east. The surface structures of the western maria reflect the probability of a greater degree of isostatic response to a larger surface loading by the greater accumulation of mare basalt.

  7. Incorporating Cutting Edge Scientific Results from the Margins-Geoprisms Program into the Undergraduate Curriculum: The Subduction Factory

    NASA Astrophysics Data System (ADS)

    Penniston-Dorland, S.; Stern, R. J.; Edwards, B. R.; Kincaid, C. R.

    2014-12-01

    The NSF-MARGINS Program funded a decade of research on continental margin processes. The NSF-GeoPRISMS Mini-lesson Project, funded by NSF-TUES, is designed to integrate fundamental results from the MARGINS program into open-source college-level curriculum. Three Subduction Factory (SubFac) mini-lessons were developed as part of this project. These include hands-on examinations of data sets representing 3 key components of the subduction zone system: 1) Heat transfer in the subducted slab; 2) Metamorphic processes happening at the plate interface; and 3) Typical magmatic products of arc systems above subduction zones. Module 1: "Slab Temperatures Control Melting in Subduction Zones, What Controls Slab Temperature?" allows students to work in groups using beads rolling down slopes as an analog for the mathematics of heat flow. Using this hands-on, exploration-based approach, students develop an intuition for the mathematics of heatflow and learn about heat conduction and advection in the subduction zone environment. Module 2: "Subduction zone metamorphism" introduces students to the metamorphic rocks that form as the subducted slab descends and the mineral reactions that characterize subduction-related metamorphism. This module includes a suite of metamorphic rocks available for instructors to use in a lab, and exercises in which students compare pressure-temperature estimates obtained from metamorphic rocks to predictions from thermal models. Module 3: "Central American Arc Volcanoes, Petrology and Geochemistry" introduces students to basic concepts in igneous petrology using the Central American volcanic arc, a MARGINS Subduction Factory focus site, as an example. The module relates data from two different volcanoes - basaltic Cerro Negro (Nicaragua) and andesitic Ilopango (El Salvador) including hand sample observations and major element geochemistry - to explore processes of mantle and crustal melting and differentiation in arc volcanism.

  8. Changing recycled oceanic components in the mantle source of the Shuangliao Cenozoic basalts, NE China: New constraints from water content

    NASA Astrophysics Data System (ADS)

    Chen, Huan; Xia, Qun-Ke; Ingrin, Jannick; Jia, Zu-Bing; Feng, Min

    2015-05-01

    The mantle source of the Cenozoic intraplate basalts from the Shuangliao volcanoes in the Songliao Basin in northeastern China (Xu et al., 2012) has been suggested to contain young subducted recycled oceanic crust (ROC), whereas the mantle source of the Cenozoic basalts from the nearby Changbaishan volcanoes contain recent and ancient recycled marine sediments (Kuritani et al., 2011). To determine the possible contributions of recycled marine sediments to the mantle source of the Shuangliao Cenozoic basalts, we measured the H2O content in clinopyroxene (cpx) phenocrysts by Fourier transform infrared spectrometry (FTIR) and calculated H2O contents of equilibrated melts using the partition coefficient of H2O between cpx and basaltic melt. Major and trace element compositions measured in the Shuangliao whole rocks are consistent with a previous study by Xu et al. (2012) and display typical HIMU characteristics. The calculated H2O content of the "primary" magmas varies from 0.90 to 3.06 wt.%, falling in the range of back-arc basin basalts (BABBs) and island arc basalts (IABs) and it is higher than that of mid-ocean ridge basalts (MORBs) and ocean island basalts (OIBs). The relationships between H2O/Ce and (Ba/Th)n (n denotes normalization to primitive mantle composition), (Nb/La)n and Ce/Pb of the Shuangliao basalts indicate that in addition to ROC with low H2O/Ce and high (Nb/La)n, recycled marine sediments with high H2O/Ce, (Ba/Th)n and low Ce/Pb and a normal depleted mantle (DMM) component should also be involved in the magma source. In addition, the mantle source of the later (~ 43 Ma) basalts should contain more recycled marine sediments than that of the earlier (~ 51 Ma) basalts.

  9. Fine-scale isotopic variation in Mariana Trough basalts: evidence for heterogeneity and a recycled component in backarc basin mantle

    NASA Astrophysics Data System (ADS)

    Volpe, Alan M.; Douglas Macdougall, J.; Lugmair, Gunter W.; Hawkins, James W.; Lonsdale, Peter

    1990-10-01

    Fine-scale sampling with ALVIN and by dredging of the axial ridge in the Mariana Trough between 17°40'N and 18°30°N recovered basalts with isotopic compositions that span the range between N-type MORB and Mariana island arc basalts. There is a local tectonic-morphological control on basalt compositions; MORB-like basalts are found on the deeper ridge segment bounded by the Pagan transform and the ridge offset at 17°56'N, while basalts from the shallower ridge to the north are typical Mariana Trough basalts (MTB) having compositions intermediate between the two endmember rock types. Arc-like basalts were recovered from one site on the axial ridge. The discovery of basalts with such diverse isotopic characteristics from a short (100 km) section of this backarc spreading center constrains the chemical characteristics and distribution of mantle source variability in the Mariana Trough. Sr sbnd Nd sbnd Pb isotopic variability suggests that the MTB source is heterogeneous on the scale of individual melt batches. The principal component in the MTB mantle source region is depleted peridotite similar to the source of MORB. The enriched component, most evident in the arc-like basalts and intimately mixed in MTB, has isotopic characteristics similar to those observed in the Mariana arc basalts. The isotopic data suggest that source variability for Mariana axial ridge basalts can be explained by mixed arc-like and MORB-like mantle. We hypothesize that there are fragments of old oceanic lithosphere in the backarc source region. This lithospheric component may reflect remnants of subducted seafloor or forearc-volcanic arc mantle that predate rifting in the backarc basin.

  10. Nickel in high-alumina basalts

    USGS Publications Warehouse

    Hedge, C.E.

    1971-01-01

    New analyses of high-alumina basalts reveal an average nickel content higher than previously indicated. Ni in high-alumina basalts correlates with magnesium in the same way as it does in other basalt types. There is therefore no reason, based on Ni contents, to hypothesize a special origin for high-alumina basalts and it is permissible (based on Ni contents) to form andesites by fractional crystallization from high-alumina basalts. ?? 1971.

  11. Separation of supercritical slab-fluids to form aqueous fluid and melt components in subduction zone magmatism

    PubMed Central

    Kawamoto, Tatsuhiko; Kanzaki, Masami; Mibe, Kenji; Ono, Shigeaki

    2012-01-01

    Subduction-zone magmatism is triggered by the addition of H2O-rich slab-derived components: aqueous fluid, hydrous partial melts, or supercritical fluids from the subducting slab. Geochemical analyses of island arc basalts suggest two slab-derived signatures of a melt and a fluid. These two liquids unite to a supercritical fluid under pressure and temperature conditions beyond a critical endpoint. We ascertain critical endpoints between aqueous fluids and sediment or high-Mg andesite (HMA) melts located, respectively, at 83-km and 92-km depths by using an in situ observation technique. These depths are within the mantle wedge underlying volcanic fronts, which are formed 90 to 200 km above subducting slabs. These data suggest that sediment-derived supercritical fluids, which are fed to the mantle wedge from the subducting slab, react with mantle peridotite to form HMA supercritical fluids. Such HMA supercritical fluids separate into aqueous fluids and HMA melts at 92 km depth during ascent. The aqueous fluids are fluxed into the asthenospheric mantle to form arc basalts, which are locally associated with HMAs in hot subduction zones. The separated HMA melts retain their composition in limited equilibrium with the surrounding mantle. Alternatively, they equilibrate with the surrounding mantle and change the major element chemistry to basaltic composition. However, trace element signatures of sediment-derived supercritical fluids remain more in the melt-derived magma than in the fluid-induced magma, which inherits only fluid-mobile elements from the sediment-derived supercritical fluids. Separation of slab-derived supercritical fluids into melts and aqueous fluids can elucidate the two slab-derived components observed in subduction zone magma chemistry. PMID:23112158

  12. Separation of supercritical slab-fluids to form aqueous fluid and melt components in subduction zone magmatism.

    PubMed

    Kawamoto, Tatsuhiko; Kanzaki, Masami; Mibe, Kenji; Matsukage, Kyoko N; Ono, Shigeaki

    2012-11-13

    Subduction-zone magmatism is triggered by the addition of H(2)O-rich slab-derived components: aqueous fluid, hydrous partial melts, or supercritical fluids from the subducting slab. Geochemical analyses of island arc basalts suggest two slab-derived signatures of a melt and a fluid. These two liquids unite to a supercritical fluid under pressure and temperature conditions beyond a critical endpoint. We ascertain critical endpoints between aqueous fluids and sediment or high-Mg andesite (HMA) melts located, respectively, at 83-km and 92-km depths by using an in situ observation technique. These depths are within the mantle wedge underlying volcanic fronts, which are formed 90 to 200 km above subducting slabs. These data suggest that sediment-derived supercritical fluids, which are fed to the mantle wedge from the subducting slab, react with mantle peridotite to form HMA supercritical fluids. Such HMA supercritical fluids separate into aqueous fluids and HMA melts at 92 km depth during ascent. The aqueous fluids are fluxed into the asthenospheric mantle to form arc basalts, which are locally associated with HMAs in hot subduction zones. The separated HMA melts retain their composition in limited equilibrium with the surrounding mantle. Alternatively, they equilibrate with the surrounding mantle and change the major element chemistry to basaltic composition. However, trace element signatures of sediment-derived supercritical fluids remain more in the melt-derived magma than in the fluid-induced magma, which inherits only fluid-mobile elements from the sediment-derived supercritical fluids. Separation of slab-derived supercritical fluids into melts and aqueous fluids can elucidate the two slab-derived components observed in subduction zone magma chemistry. PMID:23112158

  13. Osmium Recycling in Subduction Zones

    PubMed

    Brandon; Creaser; Shirey; Carlson

    1996-05-10

    Peridotite xenoliths from the Cascade arc in the United States and in the Japan arc have neodymium and osmium isotopic compositions that are consistent with addition of 5 to 15 percent of subducted material to the present-day depleted mantle. These observations suggest that osmium can be partitioned into oxidized and chlorine-rich slab-derived fluids or melts. These results place new constraints on the behavior of osmium (and possibly other platinum group elements) during subduction of oceanic crust by showing that osmium can be transported into the mantle wedge. PMID:8662577

  14. Basaltic Lava Channels

    NASA Astrophysics Data System (ADS)

    Cashman, K. V.; Griffiths, R. W.; Kerr, R. C.

    2004-12-01

    or channel bends that exposes more core lava to cooling than simply that of the shear zones. Thus the channel geometry plays a major role in the thermal history of a flow. As lava flows rarely flow through pre-existing channels of prescribed geometry, we have performed an additional set of analog laboratory experiments to determine the relationship between flow rate, slope, and channel formation in solidifying flows. All flows develop stable uniform channels within solidified levees except when the flow rate is sufficiently low to permit flow front solidification, inflation, and tube formation. On constant slopes, increasing flow rates result in increases in both the rate of flow advance rate and the channel width, and a decrease in levee width. At constant flow rates, both channel width and levee width decrease with increasing slope while flow advance rate increases. Limited data on the geometry of basaltic lava channels indicate that experimental data are consistent with field observations, however, both additional field data and scaling relationships are required to fully utilize the laboratory experiments to predict channel development in basaltic lava flows.

  15. Subduction & orogeny: Introduction to the special volume

    NASA Astrophysics Data System (ADS)

    Rolland, Y.; Bosch, D.; Guillot, S.; de Sigoyer, J.; Martinod, J.; Agard, P.; Yamato, P.

    2016-05-01

    Subduction processes play a major role in plate tectonics and the subsequent geological evolution of Earth. This special issue focuses on ongoing research in subduction dynamics to a large extent (oceanic subduction, continental subduction, obduction…) for both past and active subduction zones and into mountain building processes and the early evolution of orogens. It puts together various approaches combining geophysics (imaging of subduction zones), petrology/geochemistry (metamorphic analysis of HP-UHP rocks, fluid geochemistry and magmatic signal, geochronology), seismology and geodesy (present-day evolution of subduction zones, active tectonics), structural geology (structure and evolution of mountain belts), and numerical modelling to provide a full spectrum of tools that can be used to constrain the nature and evolution of subduction processes and orogeny. Studies presented in this special issue range from the long-term (orogenic cycle) to short-term (seismic cycle).

  16. Origin of arc-like continental basalts: Implications for deep-Earth fluid cycling and tectonic discrimination

    NASA Astrophysics Data System (ADS)

    Wang, Xuan-Ce; Wilde, Simon A.; Xu, Bei; Pang, Chong-Jin

    2016-09-01

    Continental basalts generally display enrichment of fluid-mobile elements and depletion of high-field-strength elements, similar to those that evolved in the subduction environment, but different from oceanic basalts. Based on the continental flood basalt database for six large igneous provinces, together with rift-related basalt data from the Basin and Range Province, this study aimed to test the validity of geochemical tectonic discrimination diagrams in distinguishing arc-like intra-continental basalts from arc basalts and to further investigate the role of deep-Earth water cycling in producing arc-like signatures in large-scale intra-continental basalts. Our evaluation shows that arc-like intra-continental basalts can be distinguished from arc basalts by integrating the following factors: (1) the FeO, MgO, and Al2O3 concentrations of the primary melt; (2) Tisbnd V, Zrsbnd Zr/Y, Zrsbnd Ti, and Ti/Vsbnd Zr/Smsbnd Sr/Nd discrimination diagrams; (3) the coexistence of arc-like and OIB-like subtype basalts within the same province; (4) primitive mantle-normalized trace element distribution patterns. The similarity of enrichment in fluid-mobile elements (Ba, Rb, Sr, U, and K) between arc-like and true arc basalts suggests the importance of water flux melting in producing arc-like signatures in continental basalts. Experimentally determined liquid lines of descent (LLD) imply high magma water concentrations for continental flood basalts (CFBs) and the Basin and Range basalts. Furthermore, estimates based on the Al2O3-LLD method indicates 4.0-5.0 wt% pre-eruptive magma H2O concentration for CFBs and the Basin and Range basalts. The tight relationships between H2O/Ce and Ba/La, Ba/Nb and Rb/Nb based on global arc basalt data were further used to estimate the primary H2O concentrations. With the exception of the Emeishan CFBs (mainly containing 4.0-5.6 wt% H2O), all other CFBs investigated have similar estimated primary H2O contents, with values ranging from 1.0 to 2

  17. Thrust-type subduction-zone earthquakes and seamount asperites: A physical model for seismic rupture

    SciTech Connect

    Cloos, M. )

    1992-07-01

    A thrust-type subduction-zone earthquake of M{sub W} 7.6 ruptures an area of {approximately}6,000 km{sup 2}, has a seismic slip of {approximately}1 m, and is nucleated by the rupture of an asperity {approximately}25km across. A model for thrust-type subduction-zone seismicity is proposed in which basaltic seamounts jammed against the base of the overriding plate act as strong asperities that rupture by stick-slip faulting. A M{sub W} 7.6 event would correspond to the near-basal rupture of a {approximately}2-km-tall seamount. The base of the seamount is surrounded by a low shear-strength layer composed of subducting sediment that also deforms between seismic events by distributed strain (viscous flow). Planar faults form in this layer as the seismic rupture propagates out of the seamount at speeds of kilometers per second. The faults in the shear zone are disrupted after the event by aseismic, slow viscous flow of the subducting sediment layer. Consequently, the extent of fault rupture varies for different earthquakes nucleated at the same seamount asperity because new fault surfaces form in the surrounding subducting sediment layer during each fast seismic rupture.

  18. The deep water cycle and origin of cratonic flood basalts: two examples from the Siberian craton

    NASA Astrophysics Data System (ADS)

    Ivanov, A. V.

    2014-12-01

    Cratonic flood basalt volcanism is the most puzzling phenomenon compared to all other types of intraplate volcanism. Cratons are thick and cold; the two parameters which suppress melting of either sublithospheric or lithospheric mantle in dry conditions. Fusible eclogites at hot plume geotherm start to melt in sublithospheric depth (~ 230 km), but geochemical arguments require that, in addition to eclogitic component, significant volume of flood basalts are from peridotitic mantle. Dry peridotitic mantle cannot be melted unless the lithospheric thickness reduced to about 60 km. That is why modern plume models incorporate lithospheric delamination and/or initially thinned lithosphere to explain cratonic flood basalts. However, if lithosphere remained thick, which was the case for the Siberian craton by the time of formation of its Devonian (Vilyui) and Permo-Triassic (Siberian) flood basalt provinces, then plume model is unable to explain the flood basalt volcanism. If mantle is wet, the peridotitic solidus lowered such as it can start to melt at sublithospheric depth (for example, 2 wt. % H2O-bearing peridotite starts to melt at ~320 km depth even at a normal mantle geotherm). In this presentation I will show that fluxing of mantle via the deep water cycle process may explain the Siberian craton flood basalts and many other continental flood basalts. According to the deep water cycle model, water is carried to the mantle transition zone by fast subducting slabs (may be in form of solid ice VII), then water is released from the slabs due to warming to the ambient mantle temperature, then localized hydration creates buoyant wet diapirs (or melt-bearing diapirs), the diapirs raise up to the sublithosheric depth were melt accumulates for the following tectonically triggered flood basalt eruptions.

  19. Geochronology and geochemistry of late Cenozoic basalts from the Leiqiong area, southern China

    NASA Astrophysics Data System (ADS)

    Ho, Kung-suan; Chen, Ju-chin; Juang, Wen-shing

    2000-06-01

    The Leiqiong area, which includes the Leizhou Peninsula and the northern part of the Hainan Island, is the largest province of exposed basalts in southern China. Ar-Ar and K-Ar dating indicates that incipient volcanism in the Leiqiong area may have taken place in late Oligocene time and gradually increased in tempo toward the Miocene and Pliocene Epoch. Volcanic activities were most extensive during Pleistocene, and declined and ended in Holocene. Based on radiometric age dating and geographic distribution, Pliocene and Quaternary volcanism in Hainan Island can be grouped into two stages and six eruptive regions. The early volcanism is dominated by flood type fissure eruption of quartz tholeiites and olivine tholeiites whereas the later phase is dominated by central type eruption of alkali olivine basalts and olivine tholeiites. The systematic decrease of MgO, ΣFeO and TiO 2 with increasing SiO 2 content for basalts from Hainan Island indicates that fractional crystallization of olivine, clinopyroxene and Ti-bearing opaques may have occurred during magmatic evolution. From coexisting Fe-Ti oxide minerals, it is estimated that the equilibrium temperatures range from 895-986°C and oxygen fugacities range from 10 -13.4 to 10 -10.7 atmospheres in the basaltic magmas. The incompatible element ratios and the chondrite-normalized REE patterns of basalts from the Leiqiong area are generally similar to OIB. The Nb/U ratios (less than 37) in most of the tholeiitic rocks and the negative Nb anomaly observed in the spidergram of some basalts indicated that the influence of a paleo-subduction zone derived component can not be excluded in considering the genesis of the basalts from the Leiqiong area. The tholeiites in the Leiqiong area may have mixed with a more enriched lithospheric mantle component as well as undergone relatively larger percentages of partial melting than the alkali basalts.

  20. Snowball Earth and basaltic traps

    NASA Astrophysics Data System (ADS)

    Dupre, B.; Godderis, Y.; Nedelec, A.; Donnadieu, Y.; Dessert, C.; Francois, L. M.; Grard, A.

    2003-04-01

    The causes of the Neo-Proterozoic glaciations is still a matter of debate. One potential trigger for those glaciations is a major perturbation of the global carbon cycle, leading to the consumption of atmospheric CO_2, and finally to the cooling of the global climate. The first glacial episode is characterized by intense rift formations. The Proto-Pacific ocean starts to open within the Sturtian stage (800-750 Ma). The onset of rifts cutting through continental surfaces might have been coeval with the spreading of continental flood basalts. As demonstrated by Dessert et al (2001) for the K-T boundary, such events might severely impacts the long term evolution of the global climate, through intense consumption of atmospheric CO_2 by fresh basaltic surfaces, leading to non negligible global cooling at the million year timescale. Based on weathering laws for basaltic and granitic surfaces, we estimate that the onset of continental flood basalts over 6 million km^2 along the equator (crossed by the Proto-Pacific rift) will drive the Earth into global glaciation 1.5 My after the event, assuming a pre-perturbation level of 280 ppmv of CO_2 and a solar luminosity reduced by 6%. The δ13C of carbonates accumulating between the start of the continental plume and the onset of the global glaciation is expected to fall by about 3 ppm in response to the degassing of large amount of mantle carbon into the atmosphere, in agreement with data. This hypothesis raises the question of the cyclicity of the glaciations. Once the glaciation ends, the basaltic surface starts again to weather, and plunge the Earth into a new deep glaciation. Within 30 My, the basaltic trap, originally located at the equator, might have migrated 3500 km southward, within the dryer tropical area. Such migration reduces the consumption of CO_2 by the basaltic surface, preventing the Earth from a new global glaciation.

  1. Building a Subduction Zone Observatory

    USGS Publications Warehouse

    Gomberg, Joan S.; Bodin, Paul; Bourgeois, Jody; Cashman, Susan; Cowan, Darrel; Creager, Kenneth C.; Crowell, Brendan; Duvall, Alison; Frankel, Arthur; Gonzalez, Frank; Houston, Heidi; Johnson, Paul; Kelsey, Harvey; Miller, Una; Roland, Emily C.; Schmidt, David; Staisch, Lydia; Vidale, John; Wilcock, William; Wirth, Erin

    2016-01-01

    Subduction zones contain many of Earth’s most remarkable geologic structures, from the deepest oceanic trenches to glacier-covered mountains and steaming volcanoes. These environments formed through spectacular events: Nature’s largest earthquakes, tsunamis, and volcanic eruptions are born here.

  2. Bubble Growth in Lunar Basalts

    NASA Astrophysics Data System (ADS)

    Zhang, Y.

    2009-05-01

    Although Moon is usually said to be volatile-"free", lunar basalts are often vesicular with mm-size bubbles. The vesicular nature of the lunar basalts suggests that they contained some initial gas concentration. A recent publication estimated volatile concentrations in lunar basalts (Saal et al. 2008). This report investigates bubble growth on Moon and compares with that on Earth. Under conditions relevant to lunar basalts, bubble growth in a finite melt shell (i.e., growth of multiple regularly-spaced bubbles) is calculated following Proussevitch and Sahagian (1998) and Liu and Zhang (2000). Initial H2O content of 700 ppm (Saal et al. 2008) or lower is used and the effect of other volatiles (such as carbon dioxide, halogens, and sulfur) is ignored. H2O solubility at low pressures (Liu et al. 2005), concentration-dependent diffusivity in basalt (Zhang and Stolper 1991), and lunar basalt viscosity (Murase and McBirney 1970) are used. Because lunar atmospheric pressure is essentially zero, the confining pressure on bubbles is completely supplied by the overlying magma. Due to low H2O content in lunar basaltic melt (700 ppm H2O corresponds to a saturation pressure of 75 kPa), H2O bubbles only grow in the upper 16 m of a basalt flow or lake. A depth of 20 mm corresponds to a confining pressure of 100 Pa. Hence, vesicular lunar rocks come from very shallow depth. Some findings from the modeling are as follows. (a) Due to low confining pressure as well as low viscosity, even though volatile concentration is very low, bubble growth rate is extremely high, much higher than typical bubble growth rates in terrestrial melts. Hence, mm-size bubbles in lunar basalts are not strange. (b) Because the pertinent pressures are so low, bubble pressure due to surface tension plays a main role in lunar bubble growth, contrary to terrestrial cases. (c) Time scale to reach equilibrium bubble size increases as the confining pressure increases. References: (1) Liu Y, Zhang YX (2000) Earth

  3. Striking Local Distinctions in Basaltic Melts within Nicaraguan Cross-arc Lineaments

    NASA Astrophysics Data System (ADS)

    Her, X.; Walker, J. A.; Roggensack, K.

    2015-12-01

    The Nejapa-Miraflores (NM) and Granada (G) lineaments which cut across the Central American volcanic front (CAVF) host numerous monogenetic vents which have erupted diverse basaltic magmas (e.g., Walker, 1984). As previously shown by Walker (1984), the basaltic magmas loosely fall into two groups: a high Ti, low K group which are reminiscent of MORB or BABB; and a low Ti, high K group which are more typical of subduction zones worldwide. Major element data obtained from over 200 olivine-hosted melt inclusions found within NM and G tephras from six separate monogenetic vents confirm this unusual compositional dichotomy. Melt inclusions from four of the six monogenetic vents are exclusively high- or low-Ti, while two of the volcanoes have both high- and low-Ti melt inclusions. New volatile and trace element data on over 40 of the NM and G melt inclusions has yielded additional compositional distinctions between the high- and low-Ti groups. Least degassed high-Ti melts tend to have lower water contents than their low-Ti counterparts. The high-Ti Inclusions also have lower concentrations of U, Th, Pb, Ba and Cs and lower La/Yb ratios. In addition, there are subtle HFSE variations between the two types of basalts. The overall geochemical differences between the high- and low-Ti groups suggest that the mantle wedge source of the latter contains a greater slab-derived (hemipelagic) sediment melt component than the former linked to a larger flux of hydrous fluids from deeper in the subducting Cocos plate. What is particularly significant is that the contrasting mafic emanations from these monogenetic volcano lineaments demonstrate that transport of fluids, volatiles and basaltic melts in subduction zones can be quite variable and complex on a very localized scale.

  4. Mantle upwelling and trench-parallel mantle flow in the northern Cascade arc indicated by basalt geochemistry

    NASA Astrophysics Data System (ADS)

    Mullen, E.; Weis, D.

    2013-12-01

    Cascadia offers a unique perspective on arc magma genesis as an end-member ';hot' subduction zone in which relatively little water may be available to promote mantle melting. The youngest and hottest subducting crust (~5 Myr at the trench) occurs in the Garibaldi Volcanic Belt, at the northern edge of the subducting Juan de Fuca plate [1]. Geochemical data from GVB primitive basalts provide insights on mantle melting where a slab edge coincides with high slab temperatures. In subduction zones worldwide, including the Cascades, basalts are typically calc-alkaline and produced from a depleted mantle wedge modified by slab input. However, basalts from volcanic centers overlying the northern slab edge (Salal Glacier and Bridge River Cones) are alkalic [2] and lack a trace element subduction signature [3]. The mantle source of the alkalic basalts is significantly more enriched in incompatible elements than the slab-modified depleted mantle wedge that produces calc-alkaline basalts in the southern GVB (Mt. Baker and Glacier Peak) [3]. The alkalic basalts are also generated at temperatures and pressures of up to 175°C and 1.5 GPa higher than those of the calc-alkaline basalts [3], consistent with decompression melting of fertile, hot mantle ascending through a gap in the Nootka fault, the boundary between the subducting Juan de Fuca plate and the nearly stagnant Explorer microplate. Mantle upwelling may be related to toroidal mantle flow around the slab edge, which has been identified in southern Cascadia [4]. In the GVB, the upwelling fertile mantle is not confined to the immediate area around the slab edge but has spread southward along the arc axis, its extent gradually diminishing as the slab-modified depleted mantle wedge becomes dominant. Between Salal Glacier/Bridge River and Glacier Peak ~350 km to the south, there are increases in isotopic ratios (ɛHf = 8.3 to13.0, ɛNd = 7.3 to 8.5, and 208Pb*/206*Pb* = 0.914 to 0.928) and trace element indicators of slab

  5. Saline Fluids in Subduction Channels and Mantle Wedge

    NASA Astrophysics Data System (ADS)

    Kawamoto, T.; Hertwig, A.; Schertl, H. P.; Maresch, W. V.; Shigeno, M.; Mori, Y.; Nishiyama, T.

    2015-12-01

    Saline fluids can transport large-ion-lithophile elements and carbonate. Subduction-zone fluids contain salts with various amounts of NaCl equivalent similar to that of the present and/or Phanerozoic seawater (about 3.5 wt% NaCl). The salinity of aqueous fluids in the mantle wedge decreases from trench side to back-arc side, although available data have been limited. Such saline fluids from mantle peridotite underneath Pinatubo, a frontal volcano of the Luzon arc, contain 5.1 wt% NaCl equivalent and CO2 [Kawamoto et al., 2013 Proc Natl Acad Sci USA] and in Ichinomegeta, a rear-arc volcano of the Northeast Japan arc, contain 3.7 wt% NaCl equivalent and CO2 [Kumagai et al., Contrib Mineral Petrol 2014]. Abundances of chlorine and H2O in olivine-hosted melt inclusions also suggest that aqueous fluids to produce frontal basalts have higher salinity than rear-arc basalts in Guatemala arc [Walker et al., Contrib Mineral Petrol 2003]. In addition to these data, quartz-free jadeitites contain fluid inclusions composed of aqueous fluids with 7 wt% NaCl equivalent and quartz-bearing jadeitite with 4.6 wt% NaCl equivalent in supra-subduction zones in Southwest Japan [Mori et al., 2015, International Eclogite Conference] and quartz-bearing jadeitite and jadeite-rich rocks contain fluid inclusions composed of aqueous fluids with 4.2 wt% NaCl equivalent in Rio San Juan Complex, Dominica Republic [Kawamoto et al., 2015, Goldschmidt Conference]. Aqueous fluids generated at pressures lower than conditions for albite=jadeite+quartz occurring at 1.5 GPa, 500 °C may contain aqueous fluids with higher salinity than at higher pressures.

  6. Influence of subducted components on back-arc melting dynamics in the Manus Basin

    NASA Astrophysics Data System (ADS)

    Beier, Christoph; Turner, Simon P.; Sinton, John M.; Gill, James B.

    2010-06-01

    Lavas erupted in back-arc basins afford the opportunity to explore the extent to which decompression and subduction-related components influence partial melting in this setting. We present U-Th-Ra disequilibria data from 24 well-characterized lavas from the Manus Basin behind the New Britain volcanic arc, supplemented by some additional trace element and Sr-Nd-Pb isotope data. The lavas range in composition from 49.6 to 57.7 wt % SiO2 and can be subdivided into those that are broadly like mid-ocean ridge basalts (MORB) with Ba/Nb < 16 and back-arc basin basalts (BABB) that are variably influenced by subduction components and have Ba/Nb > 16. Rifts closest to the arc are dominated by BABB, whereas both lavas types erupt further away at the Manus Spreading Center. The MORB have small 230Th excesses (up to 5%) and are displaced below the global correlation of (230Th/238U) with ridge depth. In most respects the BABB closely resemble lavas erupted along the New Britain arc front, including 238U excesses that reach 26%. The Pb isotope data can be explained by mixing of a subduction component into an Indian MORB mantle source. The Pb in the subduction component is derived from both the subducted sediment (5%) and fluids from the subducting altered Solomon Sea oceanic crust (95%), and these were mixed prior to addition to the mantle wedge. U/Th ratios, Fe3+/ΣFe, and H2O contents all increase with increasing 206Pb/204Pb. A model in which addition of the subduction component to the mantle wedge is followed by 230Th in-growth during decompression and dynamic melting all less than 140 kyr prior to eruption can simulate the data. However, our preferred model is one of dynamic decompression melting in which subduction-modified, more oxidized mantle had DU ≪ DTh leading to 238U excesses in contrast to unmodified mantle that yields 230Th excess. Large 226Ra excesses in some southern rift samples require addition of a fluid <8 kyr ago but elsewhere reflect melting under low

  7. Lithospheric-folding-based understanding on the origin of the back-arc basaltic magmatism beneath Jeju volcanic island, Korea

    NASA Astrophysics Data System (ADS)

    Yun, S.; Shin, Y.; CHOI, K.; Koh, J.; Nakamura, E.; Na, S.

    2012-12-01

    Jeju Island is an intraplate volcanic island located at the eastern margin on the East Asia behind the Ryukyu Trench, the collisional/subduction boundary between the Eurasian plate and Philippine Sea plate. It is a symmetrical shield volcano, having numerous monogenetic cinder cones, over 365, on the Mt. Halla volcanic edifice. The basement rock mainly consists of Precambrian gneiss, Mesozoic granite and volcanic rocks. Unconsolidated sedimentary rock is found between basement rock and surface lava. The lava plateau is composed of voluminous basaltic lava flows, which extend to the coast region with a gentle slope. Based on the evidence obtained from volcanic stratigraphy, paleontology, and geochronology, the age of the Jeju basalts ranges from the early Pleistocene to Holocene(Historic). The alkaline and tholeiitic basalts exhibits OIB composition from intraplate volcanism which is not associated with plate subduction, while the basement xenolith contained in the volcanic rock indicates that there were volcanic activities associated with the Mesozoic plate subduction. The Geochemical characteristics have been explained with the plume model, lithospheric mantle origin, and melting of shallow asthenosphere by the rapid change of stress regimes between the collision of the India-Eurasia plates and subduction of the Pacific plate, while there has not been any geophysical investigation to disclose it. Compression near collisional plate boundaries causes lithospheric folding which results in the decrease of pressure beneath the ridge of the fold while the pressure increases beneath trough. The decompression beneath lithosphere is likely to accelerate basaltic magmatism along and below the ridge. We investigate the subsurface structure beneath Jeju volcanic island, South Korea and its vicinity and propose an alternative hypothesis that the basaltic magma beneath the island could be caused by episodic lithospheric folding. Unlike the prevailing hypothesis of the

  8. Thermal buoyancy on Venus - Underthrusting vs subduction

    NASA Technical Reports Server (NTRS)

    Burt, Jeffrey D.; Head, James W.

    1992-01-01

    The thermal and buoyancy consequences of the subduction endmember are modeled in an attempt to evaluate the conditions distinguishing underthrusting and subduction. Thermal changes in slabs subducting into the Venusian mantle with a range of initial geotherms are used to predict density changes and, thus, slab buoyancy. Based on a model for subduction-induced mantle flow, it is then argued that the angle of the slab dip helps differentiate between underthrusting and subduction. Mantle flow applies torques to the slab which, in combination with torques due to slab buoyancy, act to change the angle of slab dip.

  9. The Southern Mariana Forearc: An Active Subduction Initiation (SI) Analogue

    NASA Astrophysics Data System (ADS)

    Stern, R. J.; Bloomer, S. H.; Brounce, M. N.; Ishii, T.; Ishizuka, O.; Kelley, K. A.; Martinez, F.; Ohara, Y.; Pujana, I.; Reagan, M. K.; Ribeiro, J.

    2014-12-01

    It is important to understand how new subduction zones form. Some subduction zones begin spontaneously, with sinking of dense oceanic lithosphere adjacent to a lithospheric weakness. The Eocene evolution of the Izu-Bonin-Mariana convergent margin is the type example of this process, with an increasingly well-documented evolution including results from IODP 352 drilling. A lack of any active examples of spontaneous SI hinders our understanding, but our studies of the evolution of the southernmost Mariana convergent margin provides important insights. Here the Mariana Trough backarc basin terminates against the Challenger Deep trench segment, where it has opened ~250 km in the past ~4 Ma. This corresponds to GPS opening rate of ~4.5cm/y at the latitude of Guam (Kato et al., 2003). This newly formed and rapidy widening margin faces the NW-converging Pacific plate and causes it to contort and tear. Pacific plate continues to move NW but the upper plate response is illustrative of a newly formed subduction zone. Slab-related earthquakes can be identified to ~200 km deep beneath this margin; with convergence rate of 3cm/yr, this may reflect no more than 7 Ma of subduction. The usual well-defined magmatic arc is missing; its position ~100 km above the subducted slab is occupied by the magma-rich (inflated) Malaguana-Gadao Ridge (MGR), and hydrous MORB-like basalts with ~2 wt. % H2O have erupted unusually close to the trench where they overly mantle peridotites ~6 km water depth. HMR-1 sonar backscatter mapping reveals a chaotic fabric that is at a high angle to the trend of the MGR to the east but is concordant to the west. This unusual spreading fabric may have formed by chaotic upper plate extension in response to rapid rollback of the short, narrow Pacific slab in a manner similar to that thought to occur during SI. Further interdisciplinary studies are needed to understand this rapidly-evolving tectono-magmatic province and what it can teach us about SI.

  10. Distribution of Hydrous Minerals in the Subduction System beneath Mexico

    NASA Astrophysics Data System (ADS)

    Kim, Y.; Clayton, R. W.; Jackson, J. M.; Asimow, P. D.

    2011-12-01

    Teleseismic converted phases are used to estimate the mineralogy of subducting material as a function of depth along the Cocos slab in central and southern Mexico. Modeling of the receiver function (RF) conversion amplitudes of the horizontal slab, south of the Trans-Mexican Volcanic Belt (TMVB) in central Mexico, reveals a thin low-velocity zone between the Cocos plate and the continental crust that appears to absorb nearly all of the strain between the upper plate and the Cocos slab. Using Vp/Vs versus Vs in a range of likely pressures and temperatures for candidate mineral phases, this thin (~4 km thick) layer in the flat slab region of central Mexico is determined to be enriched with hydrous minerals such as talc over normal oceanic crustal compositions such as MORB-like gabbro. On the other hand, the mineralogy of the oceanic crust downdip in the steep part of the slab beneath the TMVB is enriched with lawsonite and zoisite, which then transitions into gabbroic and eclogitic assemblages at 100 km. This supports arc volcanism in the TMVB directly above the slab as well as the slab rollback. In contrast, the dominant mineral phase in the upper oceanic crust of southern Mexico beneath the Isthmus of Tehauntepec is amphibole on top of unaltered gabbroic oceanic crust. Based on the P-T curves for equilibria involving talc derived from available thermodynamic data, the generation of talc from the basaltic lithology of the oceanic crust subducting from the trench side is nearly impossible. We therefore propose that the talc-rich layer on top of the subducting plate is generated from the mantle wedge side during the slab flattening process coupled with trench rollback. The talc-rich rocks at the slab interface can be formed in the mantle by the addition of silica transported by rising fluids via the dehydration reaction from the subducting oceanic crust and by mechanical mixing of mantle and siliceous rocks. Thus, it appears that the thin low-strength layer, which

  11. The basalts of Mare Frigoris

    NASA Astrophysics Data System (ADS)

    Kramer, G. Y.; Jaiswal, B.; Hawke, B. R.; Öhman, T.; Giguere, T. A.; Johnson, K.

    2015-10-01

    This paper discusses the methodology and results of a detailed investigation of Mare Frigoris using remote sensing data from Clementine, Lunar Prospector, and Lunar Reconnaissance Orbiter, with the objective of mapping and characterizing the compositions and eruptive history of its volcanic units. With the exception of two units in the west, Mare Frigoris and Lacus Mortis are filled with basalts having low-TiO2 to very low TiO2, low-FeO, and high-Al2O3 abundances. These compositions indicate that most of the basalts in Frigoris are high-Al basalts—a potentially undersampled, yet important group in the lunar sample collection for its clues about the heterogeneity of the lunar mantle. Thorium abundances of most of the mare basalts in Frigoris are also low, although much of the mare surface appears elevated due to contamination from impact gardening with the surrounding high-Th Imbrium ejecta. There are, however, a few regional thorium anomalies that are coincident with cryptomare units in the east, the two youngest mare basalt units, and some of the scattered pyroclastic deposits and volcanic constructs. In addition, Mare Frigoris lies directly over the northern extent of the major conduit for a magma plumbing system that fed many of the basalts that filled Oceanus Procellarum, as interpreted by Andrews-Hanna et al. (2014) using data from the Gravity Recovery and Interior Laboratory mission. The relationship between this deep-reaching magma conduit and the largest extent of high-Al basalts on the Moon makes Mare Frigoris an intriguing location for further investigation of the lunar mantle.

  12. Earthquake hazards on the cascadia subduction zone

    SciTech Connect

    Heaton, T.H.; Hartzell, S.H.

    1987-04-10

    Large subduction earthquakes on the Cascadia subduction zone pose a potential seismic hazard. Very young oceanic lithosphere (10 million years old) is being subducted beneath North America at a rate of approximately 4 centimeters per year. The Cascadia subduction zone shares many characteristics with subduction zones in southern Chile, southwestern Japan, and Colombia, where comparably young oceanic lithosphere is also subducting. Very large subduction earthquakes, ranging in energy magnitude (M/sub w/) between 8 and 9.5, have occurred along these other subduction zones. If the Cascadia subduction zone is also storing elastic energy, a sequence of several great earthquakes (M/sub w/ 8) or a giant earthquake (M/sub w/ 9) would be necessary to fill this 1200-kilometer gap. The nature of strong ground motions recorded during subduction earthquakes of M/sub w/ less than 8.2 is discussed. Strong ground motions from even larger earthquakes (M/sub w/ up to 9.5) are estimated by simple simulations. If large subduction earthquakes occur in the Pacific Northwest, relatively strong shaking can be expected over a large region. Such earthquakes may also be accompanied by large local tsunamis. 35 references, 6 figures.

  13. Earthquake hazards on the cascadia subduction zone.

    PubMed

    Heaton, T H; Hartzell, S H

    1987-04-10

    Large subduction earthquakes on the Cascadia subduction zone pose a potential seismic hazard. Very young oceanic lithosphere (10 million years old) is being subducted beneath North America at a rate of approximately 4 centimeters per year. The Cascadia subduction zone shares many characteristics with subduction zones in southern Chile, southwestern Japan, and Colombia, where comparably young oceanic lithosphere is also subducting. Very large subduction earthquakes, ranging in energy magnitude (M(w)) between 8 and 9.5, have occurred along these other subduction zones. If the Cascadia subduction zone is also storing elastic energy, a sequence of several great earthquakes (M(w) 8) or a giant earthquake (M(w) 9) would be necessary to fill this 1200-kilometer gap. The nature of strong ground motions recorded during subduction earthquakes of M(w) less than 8.2 is discussed. Strong ground motions from even larger earthquakes (M(w) up to 9.5) are estimated by simple simulations. If large subduction earthquakes occur in the Pacific Northwest, relatively strong shaking can be expected over a large region. Such earthquakes may also be accompanied by large local tsunamis. PMID:17789780

  14. Subduction Controls of Hf and Nd Isotopes in Lavas of the Aleutian Island Arc

    SciTech Connect

    Yogodzinski, Gene; Vervoort, Jeffery; Brown, Shaun Tyler; Gerseny, Megan

    2010-08-29

    The Hf and Nd isotopic compositions of 71 Quaternary lavas collected from locations along the full length of the Aleutian island arc are used to constrain the sources of Aleutian magmas and to provide insight into the geochemical behavior of Nd and Hf and related elements in the Aleutian subduction-magmatic system. Isotopic compositions of Aleutian lavas fall approximately at the center of, and form a trend parallel to, the terrestrial Hf-Nd isotopic array with {var_epsilon}{sub Hf} of +12.0 to +15.5 and {var_epsilon}{sub Nd} of +6.5 to +10.5. Basalts, andesites, and dacites within volcanic centers or in nearby volcanoes generally all have similar isotopic compositions, indicating that there is little measurable effect of crustal or other lithospheric assimilation within the volcanic plumbing systems of Aleutian volcanoes. Hafnium isotopic compositions have a clear pattern of along-arc increase that is continuous from the eastern-most locations near Cold Bay to Piip Seamount in the western-most part of the arc. This pattern is interpreted to reflect a westward decrease in the subducted sediment component present in Aleutian lavas, reflecting progressively lower rates of subduction westward as well as decreasing availability of trench sediment. Binary bulk mixing models (sediment + peridotite) demonstrate that 1-2% of the Hf in Aleutian lavas is derived from subducted sediment, indicating that Hf is mobilized out of the subducted sediment with an efficiency that is similar to that of Sr, Pb and Nd. Low published solubility for Hf and Nd in aqueous subduction fluids lead us to conclude that these elements are mobilized out of the subducted component and transferred to the mantle wedge as bulk sediment or as a silicate melt. Neodymium isotopes also generally increase from east to west, but the pattern is absent in the eastern third of the arc, where the sediment flux is high and increases from east to west, due to the presence of abundant terrigenous sediment in the

  15. Reconciling the shadow of a subduction signature with rift geochemistry and tectonic environment in Eastern Marie Byrd Land, Antarctica

    NASA Astrophysics Data System (ADS)

    LeMasurier, Wesley E.; Choi, Sung Hi; Hart, Stanley R.; Mukasa, Sam; Rogers, Nick

    2016-09-01

    Basalt-trachyte volcanoes in the Marie Byrd Land (MBL) Cenozoic province lie along the Amundsen Sea coast on the north flank of the West Antarctic rift. Basalts here are characterized by OIB-like geochemistry, restricted ranges of 87Sr/86Sr (0.702535-0.703284) and 143Nd/144Nd (0.512839-0.513008) and a wide range of 206Pb/204Pb (19.357-20.934). Basalts at three MBL volcanoes display two anomalies compared with the above and with all other basalts in West Antarctica. They include 143Nd/144Nd (0.512778-0.512789) values at Mt. Takahe and Mt. Siple that are 2σ lower than other West Antarctic basalts, and Ba/Nb, Ba/La, and Ba/Th values at Mt. Murphy and Mt. Takahe that are 3-8 times higher than normal OIB. Isotope and trace element data do not support crustal and lithospheric mantle contamination, or the presence of residual mantle amphibole or phlogopite as explanations of these anomalies. The apparent coincidence of these anomalies with the site of a pre-Cenozoic convergence zone along the Gondwanaland margin suggests a subduction influence. Major episodes of subduction and granitic plutonism took place in MBL during the Devonian, Permian, and Late Cretaceous. Relicts in the source region, of components from these subducted slabs, provide a credible explanation for the uncoupling of Ba from other large ion lithophile elements (LILE), for its erratic distribution, and for the anomalously low 143Nd/144Nd at Mt. Takahe. The last episode of subduction ended ~ 85 Ma, and was followed by continental break-up, rifting and lithospheric attenuation that produced the West Antarctic rift as we know it today. Thus, the enigmatic geochemical signatures in these three volcanoes seem to have been preserved roughly 61-85 m.y. after subduction ended. New calculations of source melting depth and a new determination of lithospheric thickness suggest that the source of the anomalies resides in a fossil mélange diapir that rose from the Cretaceous subducting slab, became attached to the

  16. Beginning the Modern Regime of Subduction Tectonics in Neoproterozoic time: Inferences from Ophiolites of the Arabian-Nubian Shield

    NASA Astrophysics Data System (ADS)

    Stern, R.

    2003-04-01

    . Boninites are unusual melts of harzburgite that result from asthenospheric upwelling interactng with slab-derived water. This environment is only common during subduction initiation events. Boninites associated with ophiolites have been reported from Egypt, Ethiopia and Eritrea, but most of the geochemical studies of ANS ophiolitic basalts are based on studies that are a decade or more old. The abundance of ANS ophiolites implies an episode of subduction initiation occurred in Neoproterozoic time.

  17. Early Neoproterozoic multiple arc-back-arc system formation during subduction-accretion processes between the Yangtze and Cathaysia blocks: New constraints from the supra-subduction zone NE Jiangxi ophiolite (South China)

    NASA Astrophysics Data System (ADS)

    Wang, Xin-Shui; Gao, Jun; Klemd, Reiner; Jiang, Tuo; Zhai, Qing-Guo; Xiao, Xu-Chang; Liang, Xin-Quan

    2015-11-01

    The NE Jiangxi ophiolite in the eastern Jiangnan Orogen is a tectonic mélange that mainly consists of individual tectonic blocks comprising pyroxenite, gabbro, basalt, diorite, granite and chert in a matrix of serpentinite or tuffaceous greywacke. A combined geochemical and geochronological study of the NE Jiangxi ophiolite was undertaken to constrain the timing and tectonic setting of its formation. The basalts were geochemically subdivided into three groups with different FeOt and TiO2 contents. Group 1 basalts have the lowest FeOt (12.17-13.07 wt.%) and TiO2 (1.48-1.62 wt.%) contents and the lowest Nb/Yb (0.80-0.88) and Th/Nb (0.02-0.03) ratios. Furthermore, they have normal mid-ocean ridge basalt (N-MORB)-like trace element patterns, suggesting derivation from an N-MORB-type mantle source without subduction input. Group 2 Fe-Ti basalts have the highest FeOt (15.52-16.30 wt.%) and TiO2 (3.06-3.23 wt.%) contents, Nb/Yb and Th/Nb ratios from 1.75 to 1.89 and from 0.11 to 0.15, respectively, and trace element patterns similar to those of back-arc basin basalts. The geochemical characteristics suggest that Group 2 basalts were derived from a slightly enriched MORB-type mantle source with a minor subduction contribution. In contrast, Group 3 Fe-Ti basalts have moderate FeOt (12.98-13.40 wt.%) and TiO2 (2.37-2.71 wt.%) contents, and Nb/Yb and Th/Nb ratios from 1.28 to 1.45 and from 0.27 to 0.30, respectively. These basalts further display markedly negative Nb-Ta anomalies and show a geochemical affinity to island-arc basalts (IAB), indicating a slightly enriched MORB-type mantle source that was significantly influenced by subduction-derived fluids and/or melts. SIMS zircon U-Pb dating on gabbros gave ages of 995 ± 22 Ma and 993 ± 12 Ma, which are interpreted as the formation age of the NE Jiangxi ophiolite. Positive zircon εHf(t) (+ 8.8 to + 13.8) values for the gabbros and whole-rock εNd(t) (+ 5.5 to + 6.6) values for the basalts indicate that the NE Jiangxi

  18. Subducted upper and lower continental crust contributes to magmatism in the collision sector of the Sunda-Banda arc, Indonesia

    NASA Astrophysics Data System (ADS)

    Elburg, M. A.; van Bergen, M. J.; Foden, J. D.

    2004-01-01

    Pb isotopes in igneous rocks from the Banda-Sunda arc show extreme along-arc variations, which correspond to major lithologic changes in crustal components entering the subduction system. An increase in 206Pb/204Pb ratios toward the zone of collision with the Australian continent reflects input of subducted upper-crustal material; maximum values coincide with anomalously radiogenic 3He/4He ratios that have been earlier attributed to the involvement of the continental margin. The collision zone is further characterized by 208Pb/204Pb ratios that are higher for a given 207Pb/204Pb value than observed in the noncollisional sectors, and in the central part of the collision zone, the 206Pb/204Pb ratios are lower than the most radiogenic values in the adjacent areas. We propose that these Pb isotope signatures reflect input of subducted lower crust, mobilized as a result of slab-window formation during arc-continent collision. Variations in Pb isotopes in the collision zone are solely determined by variations in the nature and proportions of different subducted components. The Pb isotope arrays in the noncollision area may be dominated by slab components as well and could reflect mixing between subducted oceanic crust and entrained sediments, rather than between subarc mantle and subducted sediments. Our new interpretation of the Indonesian Pb isotope data does not call for involvement of ocean-island basalt (OIB) type mantle or Australian subcontinental lithospheric mantle, as has been suggested previously.

  19. Drainage Asperities on Subduction Megathrusts

    NASA Astrophysics Data System (ADS)

    Sibson, R. H.

    2012-12-01

    Geophysical observations coupled with force-balance analyses suggest that the seismogenic shear zone interface of subduction megathrusts is generally fluid-overpressured to near-lithostatic values (λv = Pf/σv > 0.9) below the forearc hanging-wall, strongly modulating the profile of frictional shear resistance. Fluid sources include the accretionary prism at shallow levels and, with increasing depth, metamorphic dehydration of material entrained within the subduction shear zone together with progressive metamorphism of oceanic crust in the downgoing slab. Solution transfer in fine-grained material contained within the deeper subduction shear zone (150 < T < 350°C) likely contributes to hydrothermal sealing of fractures. A dramatic difference may therefore exist between low prefailure permeability surrounding the megathrust and high postfailure fracture permeability along the rupture zone and adjacent areas of aftershock activity. Observed postseismic changes in the velocity structure of the fore-arc hanging-wall led Husen and Kissling (2001) to propose massive fluid loss across the subduction interface following the 1995 Antofagasta, Chile, Mw8.0 megathrust rupture. Such trans-megathrust discharges represent a variant of 'fault-valve' action in which the subduction interface itself acts as a seal trapping overpressured fluids derived from metamorphic dehydration beneath. In low-permeability assemblages the maximum sustainable overpressure is limited by the activation or reactivation of brittle faults and fractures under the prevailing stress state. Highest overpressures tend to occur at low differential stress in compressional stress regimes. Loci for fluid discharge are likely determined by stress heterogeneities along the megathrust (e.g. the hangingwall of the rupture at its downdip termination). Discharge sites may be defined by swarm aftershocks defining activated fault-fracture meshes. However, fluid loss across a subduction interface will be enhanced when

  20. Serpentine in active subduction zones

    NASA Astrophysics Data System (ADS)

    Reynard, Bruno

    2013-09-01

    Serpentinization is a key phenomenon for understanding the geodynamics of subduction zones in the 10-200 km depth range. Serpentines are a major water carrier, and their rheological properties have a strong influence on deformation partitioning and seismicity at depths. I review experimental investigations that have been conducted on serpentines, with emphasis on the large body of data acquired over the past decade. Determinations of physical properties at the pressure and temperature conditions of subductions allow interpreting geophysical data in active subduction in terms of mineralogy and petrology, and to link the presence of serpentinites with deformation and fluid circulation. The fluid budget can be partially constrained from geophysical data. Elasticity data provide a quantitative basis for mapping serpentinization in the mantle wedge and slab from seismic tomography. Anisotropy suggests the existence of thin serpentinite channels above the plate interface, that account for mechanical decoupling inferred from down-dip limit of the seismogenic zone and heat flow. Strain-rate dependent rheology of antigorite serpentine is consistent with stable deformation of this thin layer or channel over timescales ranging from those of the seismic cycle to those of thermal equilibration and exhumation of high-pressure rocks, and with the geological record of subduction-related deformation. Circulation of serpentinizing fluids depends on the permeability structure, and is imaged by electrical conductivity tomography. It could be controlled by fracturing in the undeformed cold nose of the mantle wedge, and by plastic deformation along the plate interface. Fluid migration mechanisms are similar to those inferred from petrological and geochemical data on exhumed serpentinites. Estimation of the fluid budget associated with serpentine formation will rely on numerical simulations for which coupling of kinetics of hydration and dehydration at scales ranging from grain size up

  1. Temperature dependence of basalt weathering

    NASA Astrophysics Data System (ADS)

    Li, Gaojun; Hartmann, Jens; Derry, Louis A.; West, A. Joshua; You, Chen-Feng; Long, Xiaoyong; Zhan, Tao; Li, Laifeng; Li, Gen; Qiu, Wenhong; Li, Tao; Liu, Lianwen; Chen, Yang; Ji, Junfeng; Zhao, Liang; Chen, Jun

    2016-06-01

    The homeostatic balance of Earth's long-term carbon cycle and the equable state of Earth's climate are maintained by negative feedbacks between the levels of atmospheric CO2 and the chemical weathering rate of silicate rocks. Though clearly demonstrated by well-controlled laboratory dissolution experiments, the temperature dependence of silicate weathering rates, hypothesized to play a central role in these weathering feedbacks, has been difficult to quantify clearly in natural settings at landscape scale. By compiling data from basaltic catchments worldwide and considering only inactive volcanic fields (IVFs), here we show that the rate of CO2 consumption associated with the weathering of basaltic rocks is strongly correlated with mean annual temperature (MAT) as predicted by chemical kinetics. Relations between temperature and CO2 consumption rate for active volcanic fields (AVFs) are complicated by other factors such as eruption age, hydrothermal activity, and hydrological complexities. On the basis of this updated data compilation we are not able to distinguish whether or not there is a significant runoff control on basalt weathering rates. Nonetheless, the simple temperature control as observed in this global dataset implies that basalt weathering could be an effective mechanism for Earth to modulate long-term carbon cycle perturbations.

  2. Geochemistry and genesis of behind-arc basaltic lavas from eastern Nicaragua

    NASA Astrophysics Data System (ADS)

    Janoušek, V.; Erban, V.; Holub, F. V.; Magna, T.; Bellon, H.; Mlčoch, B.; Wiechert, U.; Rapprich, V.

    2010-05-01

    The petrology and chemistry of the Behind the Volcanic Front (BVF) lavas from eastern mainland Nicaragua and the adjacent Great Corn Island in the Caribbean Sea illustrate the complex nature of sources and processes operating in such a tectonic setting. The older, Early Miocene (˜ 17 Ma) group of low-Ti (< 1 wt.%) basalts-andesites is characterized by a strong LILE/HFSE depletion. The low-Ti lavas from El Rama and El Bluff areas are interpreted as relics of Early Miocene volcanic arc, largely analogous to the nowadays extinct Coyol arc further west. However, these rocks differ in some parameters from the modern volcanic front lavas, most notably in having lower δ7Li values, Ba/Yb ratios and lower U contents. The younger high-Ti (Ti > 1.5%) lavas, rich in other HFSE as well, are represented both by alkaline (Quaternary trachybasalts: Volcán Azul and Kukra Hill) and subalkaline (basalts-basaltic andesites: Late Miocene, ˜ 11 Ma Great Corn Island and Quaternary, Pearl Lagoon) volcanic rocks. The Late Miocene and Quaternary high-Ti BVF lavas probably represent small-volume decompression melts of a source similar to that of the OIB-like magmas, most likely upwelling asthenosphere having a strong Galápagos mantle imprint. The positive Sr-Nd isotopic correlation indicates an interaction between this OIB component and a depleted lithospheric mantle modified by a subduction-related influx of Sr and, to a lesser extent, other hydrous fluid-mobile elements. However, the rocks show no recognizable influence of the modern subduction. The feeble trace-element (e.g., slightly elevated Ba, K, and Sr at some localities) and a more pronounced Sr-Li isotopic subduction-related signal stems most likely from the Miocene convergence episode. Subduction of the Galápagos hot-spot tracks in Costa Rica produces magmas that can be readily recognized by their elevated Sr isotopic ratios due to seafloor alteration; the Nd isotopic signature remains unaffected. Such a component with

  3. The Lithium Isotopic Signature of Hawaiian Basalts

    NASA Astrophysics Data System (ADS)

    Harrison, L.; Weis, D.; Hanano, D. W.

    2013-12-01

    shield stage volcanoes or amongst individual volcanoes; 2) differences in degree of mantle melt and spatial structure of the mantle heterogeneity melting in the case of pre-shield, shield, and post-shield differences. In Hawaiian basalts, lithium isotopes help distinguish between 'enriched' Loa source components: Ko';olau Makapu';u shield stage lavas may have between 4-10% of a carbonate input and Hualalai post-shield and shield lavas may reflect incorporation of subduction eroded lower continental crust. Comparison of this dataset with worldwide OIB published lithium isotopic data indicates that the lithium isotopic system behaves systematically on a mantle-wide scale. Hawai'i is generally characterized as EM-I like, with Hualalai post-shield lavas exhibiting both the lightest lithium isotopic signature and the most extreme EM-I characteristics. Lithium isotopes thus provide an additional insight into the nature of EM-I type mantle. [1] Weis, D. et al. (2011) Nature Geoscience 4, doi:10.1038/NGEO1328. [2] Nobre Silva, I.G. et al. (2013) Geochem. Geophy. Geosys. 14(3), doi: doi:10.1002/ggge.20047. [3] Chan, L.H., and Frey, F.A. (2003) Geochem. Geophy. Geosys. 4(3), doi: 10.1029/2002GC000365.

  4. Overriding Plate Controls on Subduction Zone Evolution

    NASA Astrophysics Data System (ADS)

    Sharples, W. K.; Jadamec, M. A.; Moresi, L. N.; Capitanio, F. A.

    2014-12-01

    Seismic data, rock deformation experiments, and geochemical studies indicate variability in the thickness, buoyancy, and strength of the lithosphere at plate boundaries. However, geodynamic models of subduction commonly either omit an overriding plate or do not investigate role of the variation in overriding plate properties on the subduction evolution. We present time-dependent numerical models of subduction that vary the overriding plate thickness, strength, and density and allow for a plate interface that evolves with time via an anisotropic brittle failure rheology. We examine the emergence of (a) asymmetric versus symmetric subduction, (b) trench retreat versus advance, (c) subduction zone geometry, (d) slab stagnation versus penetration into the lower mantle, and (e) flat slab subduction. The majority of the models result in sustained asymmetric subduction. The models demonstrate that trench retreat is correlated with a thin overriding plate, whereas, trench advance is correlated with a thick and/or strong overriding plate. Slab dip, measured at a depth below the plate boundary interface, has a negative correlation with an increase in overriding plate thickness. Overriding plate thickness exerts a first order control over slab penetration into the lower mantle, with penetration most commonly occurring in models with a thick overriding plate. Periods of flat slab subduction occur with thick, strong overriding plates producing strong plate boundary interface coupling. The results provide insight into how the overriding plate plays a role in establishing advancing and retreating subduction, as well as providing an explanation for the variation of slab geometry observed in subduction zones on Earth.

  5. Modeled Temperatures and Fluid Source Distributions for the Mexico Subduction Zone: Effects of Hydrothermal Cooling and Implications for Plate Boundary Seismic Processes

    NASA Astrophysics Data System (ADS)

    Perry, M. R.; Spinelli, G. A.; Wada, I.

    2014-12-01

    In subduction zones, spatial variations in pore fluid pressure are hypothesized to control the distribution and nature of slip behavior (e.g., "normal" earthquakes, slow slip events, non-volcanic tremor, very low frequency earthquakes) on the plate boundary fault. A primary control on the pore fluid pressure distribution in subduction zones is the distribution of fluid release from hydrous minerals in the subducting sediment and rock. The distributions of these diagenetic and metamorphic fluid sources are controlled by the pressure-temperature paths that the subducting material follows. Thus, constraining subduction zone thermal structure is required to inform conceptual models of seismic behavior. Here, we present results of thermal models for the Mexico subduction zone, a system that has received recent attention due to observations of slow-slip events and non-volcanic tremor. We model temperatures in five margin-perpendicular transects from 96 ˚W to 104 ˚W. In each transect, we examine the potential thermal effects of vigorous fluid circulation in a high permeability aquifer within the basaltic basement of the oceanic crust. In the transect at 100˚W, hydrothermal circulation cools the subducting material by up to 140 ˚C, shifting peak slab dehydration landward by ~100 km relative to previous estimates from models that do not include the effects of fluid circulation. The age of the subducting plate in the trench increases from ~3 Ma at 104 ˚W to ~18 Ma at 96 ˚W; hydrothermal circulation redistributes the most heat (and cools the system the most) where the subducting plate is youngest. For systems with <20 Ma subducting lithosphere, hydrothermal circulation in oceanic crust should be considered in estimating subduction zone temperatures and fluid source distributions.

  6. Evidence for shallow dehydration of the subducting plate beneath the Mariana forearc: New insights into the water cycle at subduction zones

    NASA Astrophysics Data System (ADS)

    Ribeiro, J.; Stern, R. J.; Kelley, K. A.; Shaw, A. M.; Martinez, F.; Ohara, Y.

    2014-12-01

    Water is efficiently recycled at subduction zones. It is fluxed from the surface into the mantle by the subducted plate and back to the surface or crust through explosive arc volcanism and degassing. Fluids released from dehydrating the subducting plate are transfer agents of water. Geophysical modeling [1] and the geochemistry of arc glasses [2] suggest that at cold-slab subduction zones, such as the Mariana convergent margin, the downgoing plate mostly dehydrates beneath the volcanic arc front (≥ ~ 80 -100 km depth to slab) to trigger volcanism. However, there is a gap in our understanding of the water fluxes released beneath forearcs, as examples of forearc magmatism are extremely rare. Here, we investigate the Southernmost Mariana Forearc Rift (SEMFR), where MORB-like spreading occurred unusually close to the trench, sampling slab-derived aqueous fluids released at ~ 30 to 100 km depth from the subducted plate. Examining the trace element and water contents of olivine-hosted melt inclusions and glassy rinds from the young (2 - 4 Ma) and fresh SEMFR pillowed basalts provide new insights into the global water cycle. SEMFR lavas contain ~2 wt % H2O, and the olivine-hosted melt inclusions have the highest subduction-related H2O/Ce ratios (H2O/Ce = 6000 - 19000) ever recorded in arc magmas (H2O/Ce < 10600 and global averaged H2O/Ce < 3000). Our findings show that (i) slab-derived fluids released beneath forearcs are water-rich compared to the deeper fluids released beneath the arc system; and (ii) cold downgoing plates lose most of their water at shallow depths (~ 70 - 80 km slab depth), suggesting that water is efficiently recycled beneath the forearc (≥ 90%). 1. Van Keken, P.E., et al., Subduction factory: 4. Depth-dependent flux of H2O from subducting slabs worldwide. Journal of Geophysical Research: Solid Earth, 2011. 116(B1): p. B01401, DOI: 10.1029/2010jb007922. 2. Ruscitto, D.M., et al., Global variations in H2O/Ce: 2. Relationships to arc magma

  7. The geochemistry and petrogenesis of basalts from the Taupo Volcanic Zone and Kermadec Island Arc, S.W. Pacific

    NASA Astrophysics Data System (ADS)

    Gamble, J. A.; Smith, I. E. M.; McCulloch, M. T.; Graham, I. J.; Kokelaar, B. P.

    1993-01-01

    Basalts from the Taupo Volcanic Zone (TVZ), New Zealand, the Kermadec Island Arc (KA) and its back-arc basin, the Havre Trough show systematic variations in trace-element and isotope geochemistry which are attributed to differences in tectonic setting and source heterogeneity along a more or less continuous plate boundary. Basalts from the Kermadec Arc are characterised by low abundances of high field strength elements (HFSE) such as Ti, Zr, Nb, Ta and Hf and have high ratios of Ti/Zr and low ratios of Ti/Sc and Ti/V relative to typical MORB. Basalts from TVZ also show low abundances of the HFS elements relative to MORB but show lower Ti/Zr, higher Ti/V and Ti/Sc ratios and generally higher Zr abundances than KA most basalts. The Havre Trough basalt is mildly alkaline (< 1% normative nepheline) like many back-arc basin basalts from the Pacific rim, contrasting with the hypersthene normative TVZ and KA rocks. It has higher Zr than most TVZ basalts and all KA basalts. Ratios such as Ti/V, Ti/Sc and Ti/Zr are within the range of TVZ and MORB basalts but distinct from KA basalts. The depleted (relative to MORB) HFSE characteristics of the KA and TVZ basalts are complemented by high abundances of large ion lithophile elements (LIL), such as Ba, Rb and K, when compared to MORB, yielding the distinctive LIL-enriched pattern of subduction related rocks on a normalised multi-element plot. In contrast, the Havre Trough basalt is MORB-like. Chondrite-normalised Rare Earth Element (REE) patterns for the TVZ basalts show a field overlapping with that defined by the southern KA (Rumble Sea Mounts), with light REE enriched patterns (Ce/Yb n = ˜ 1.8-3) and flat heavy REE (Tb-Lu). Basalts from the northern KA are typically light REE depleted (Ce/Yb n = 0.5) or slightly enriched (Ce/Yb n = 1.5). The REE pattern of the Havre Trough basalt is distinctive from both the KA and TVZ fields, being richer in the heavy REE, yet similar to many basalts from back-arc basins. Sr and Nd

  8. Archean Subduction or Not? The Archean Volcanic Record Re-assessed.

    NASA Astrophysics Data System (ADS)

    Pearce, Julian; Peate, David; Smithies, Hugh

    2013-04-01

    Methods of identification of volcanic arc lavas may utilize: (1) the selective enrichment of the mantle wedge by 'subduction-mobile' elements; (2) the distinctive preconditioning of mantle along its flow path to the arc front; (3) the distinctive combination of fluid-flux and decompression melting; and (4) the effects of fluids on crystallization of the resulting magma. It should then be a simple matter uniquely to recognise volcanic arc lavas in the Geological Record and so document past subduction zones. Essentially, this is generally true in the oceans, but generally not on the continents. Even in recent, fresh lavas and with a full battery of element and isotope tools at our disposal, there can be debate over whether an arc-like geochemical signature results from active subduction, an older, inherited subduction component in the lithosphere, or crustal contamination. In the Archean, metamorphism, deformation, a different thermal regime and potential non-uniformitarian tectonic scenarios make the fingerprinting of arc lavas particularly problematic. Not least, the complicating factor of crustal contamination is likely to be much greater given the higher magma and crustal temperatures and higher magma fluxes prevailing. Here, we apply new, high-resolution immobile element fingerprinting methods, based primarily on Th-Nb fractionation, to Archean lavas. In the Pilbara, for example, where there is a volcanic record extending for over >500 m.y., we note that lavas with high Th/Nb (negative Nb anomalies) are common throughout the lava sequence. Many older formations also follow a basalt-andesite-dacite-rhyolite (BADR) sequence resembling present-day arcs. However, back-extrapolation of their compositions to their primitive magmas demonstrates that these were almost certainly crustally-contaminated plume-derived lavas. By contrast, this is not the case in the uppermst part of the sequence where even the most primitive magmas have significant Nb anomalies. The

  9. Lithological nature of the subduction channel: Insights from the Karabakh suture zone (Lesser Caucasus) and general comparisons

    NASA Astrophysics Data System (ADS)

    Hässig, Marc; Rolland, Yann; Sosson, Marc; Avagyan, Ara

    2016-05-01

    The lithological nature of major interplate boundaries is estimated by a field analysis of a well preserved exhumed subduction channel in the Caucasus Karabakh region. From this field example the subduction channel is a narrow geological object of about 500 m width formed at approximate depth of 10 km along an Andean-type subduction zone. It is comprised by an upper 'sedimentary' channel formed by an upper section of detrital and volcanic rocks thrusted on top of pelagic sediments scrapped off the oceanic floor. This sedimentary mélange is thrusted on top of an intensely deformed tectonic mélange. The tectonic mélange comprises blocks of basalt from the oceanic floor and a focussed deformation zone 50-100 m in width. This zone is mainly formed by mud-supported conglomerates exhibiting a chlorite + carbonate matrix with blocks of basalt, cross-cut by numerous chlorite-carbonate-epidote-albite veins. It overlies an undeformed ocean floor section. Superposed chlorite- and calcite-bearing veins in the mélange evidence high fluid:rock ratios of 0.3-2.3, with varied δ18O and δ13C isotopic ratios (+17 < δ18O < +25‰; -7 < δ13C < +4‰), which agrees with fluid mixing between pelagic sediments and a hydrothermal component at temperatures ranging from 120 to 400 °C, and thus mixing between deep and shallow reservoirs along the subduction interface. These data show that the several fluid reservoirs situated along the interplate boundary could have been connected by high-magnitude co-seismic displacements along the subduction zone. These subduction channel features are confronted to other similar fossil examples and current settings, such as the Andes accretionary prism to propose a reconstructed geometry of the interplate contact zone from the surface to the base of the crust.

  10. Mantle source variations beneath the Eastern Lau Spreading Center and the nature of subduction components in the Lau basin-Tonga arc system

    NASA Astrophysics Data System (ADS)

    Escrig, S.; BéZos, A.; Goldstein, S. L.; Langmuir, C. H.; Michael, P. J.

    2009-04-01

    New high-density sampling of the Eastern Lau Spreading Center provides constraints on the processes that affect the mantle wedge beneath a back-arc environment, including the effect of the subduction input on basalt petrogenesis and the change in subduction input with distance from the Tonga arc. We obtained trace element and Pb-Sr-Nd isotopic compositions of 64 samples distributed between 20.2°S and 22.3°S with an average spacing of ˜3.6 km. The trace element and isotope variations do not vary simply with distance from the arc and reflect variations in the mantle wedge composition and the presence of multiple components in the subduction input. The mantle wedge composition varies form north to south, owing to the southward migration of Indian-like mantle, progressively replacing the initially Pacific-like mantle wedge. The mantle wedge compositions also require an enriched mid-ocean ridge basalt-like trace element enrichment that has little effect on isotope ratios, suggesting recent low-degree melt enrichment events. The composition of the subduction input added to the mantle wedge is geographically variable and mirrors the changes observed in the Tonga arc island lavas. The combination of the back-arc and arc data allows identification of several components contributing to the subduction input. These are a fluid derived from the altered oceanic crust with a possible sedimentary contribution, a pelagic sediment partial melt, and, in the southern Lau basin, a volcaniclastic sediment partial melt. While on a regional scale, there is a rough decrease in subduction influence with the distance from the arc, on smaller scales, the distribution of the subduction input reflects different mechanisms of the addition of the subduction input to a variable mantle wedge.

  11. Genetic interpretation of lead-isotopic data from the Columbia River basalt group, Oregon, Washington, and Idaho.

    USGS Publications Warehouse

    Church, S.E.

    1985-01-01

    Lead-isotopic data for the high-alumina olivine plateau basalts and most of the Colombia River basalt group plot within the Cascade Range mixing array. The data for several of the formations form small, tight clusters and the Nd and Sr isotopic data show discrete variation between these basalt groups. The observed isotopic and trace-element data from most of the Columbia River basalt group can be accounted for by a model which calls for partial melting of the convecting oceanic-type mantle and contamination by fluids derived from continental sediments which were subducted along the trench. These sediments were transported in the low-velocity zone at least 400 km behind the active arc into a back-arc environment represented by the Columbia Plateau province. With time, the zone of melting moved up, resulting in the formation of the Saddle Mt basalt by partial melting of a 2600 m.y.-old sub-continental lithosphere characterized by high Th/U, Th/Pb, Rb/Sr and Nd/Sm ratios and LREE enrichment. Partial melting of old sub-continental lithosphere beneath the continental crust may be an important process in the formation of continental tholeiite flood basalt sequences world-wide. -L.di H.

  12. Middle Miocene near trench volcanism in northern Colombia: A record of slab tearing due to the simultaneous subduction of the Caribbean Plate under South and Central America?

    NASA Astrophysics Data System (ADS)

    Lara, M.; Cardona, A.; Monsalve, G.; Yarce, J.; Montes, C.; Valencia, V.; Weber, M.; De La Parra, F.; Espitia, D.; López-Martínez, M.

    2013-08-01

    Field, geochemical, geochronological, biostratigraphical and sedimentary provenance results of basaltic and associated sediments northern Colombia reveal the existence of Middle Miocene (13-14 Ma) mafic volcanism within a continental margin setting usually considered as amagmatic. This basaltic volcanism is characterized by relatively high Al2O3 and Na2O values (>15%), a High-K calc-alkaline affinity, large ion lithophile enrichment and associated Nb, Ta and Ti negative anomalies which resemble High Al basalts formed by low degree of asthenospheric melting at shallow depths mixed with some additional slab input. The presence of pre-Cretaceous detrital zircons, tourmaline and rutile as well as biostratigraphic results suggest that the host sedimentary rocks were deposited in a platform setting within the South American margin. New results of P-wave residuals from northern Colombia reinforce the view of a Caribbean slab subducting under the South American margin. The absence of a mantle wedge, the upper plate setting, and proximity of this magmatism to the trench, together with geodynamic constraints suggest that the subducted Caribbean oceanic plate was fractured and a slab tear was formed within the oceanic plate. Oceanic plate fracturing is related to the splitting of the subducting Caribbean Plate due to simultaneous subduction under the Panama-Choco block and northwestern South America, and the fast overthrusting of the later onto the Caribbean oceanic plate.

  13. Melt Inclusions in Mariana Arc Lavas: Volatiles, Trace Elements and Linkages to Subducted Components

    NASA Astrophysics Data System (ADS)

    Kelley, K. A.; Newman, S.; Plank, T.; Grove, T. L.; Parman, S.

    2001-12-01

    Melt inclusions (MIs) are particularly useful to study in volcanic arcs because they may provide samples of primitive liquids prior to extensive magma fractionation, assimilation, or crystal accumulation. Although volatile contents can also be inferred by a number of indirect petrological techniques, MIs also provide the only direct means of measuring volatile (H2O, CO2, etc.) concentrations in arc magmas, as most all other eruptive products are degassed. Volatiles are critical to the operation of the subduction factory since they are both the transport medium of material from the slab and the driver of melting in the mantle wedge. The presence of ``fluid components'' originating from subducted sediment or basaltic crust in arc magmas, however, is typically deduced from trace element compositions more than it is directly measured. We studied MI populations within four basaltic scoria samples from Guguan, Pagan and Agrigan islands of the Mariana arc. All MIs selected for study are hosted by olivine (Fo 68-82), are 50-200 †m in size, and are brown glass with no visible evidence of devitrification. We have analyzed these MIs for H2O and CO2 by FTIR, major elements by EMP and trace elements by laser ablation ICP-MS. H2O data reveal a range in water content of ~1-4 wt%, and a tighter grouping of 2-4 wt% for basaltic inclusions (<52% SiO2, Fo 72-82), higher than the highest water content reported for the Mariana back-arc trough. The single inclusion with detectable dissolved CO2 (630 ppm) also had the highest H2O content, which may indicate that lower H2O contents in the other inclusions could be due to degassing. The MIs are broadly similar in both major and trace elements to lavas from the same islands, and appear to define liquid lines of descent consistent with plagioclase suppression increasing with water concentrations. These Mariana MIs do not include high-Ca compositions like those reported by Schiano, et al. (2000), rather they are normal tholeiitic arc

  14. Resistivity logging of fractured basalt

    SciTech Connect

    Stefansson, V.; Axelsson, G.; Sigurdsson, O.

    1982-01-01

    A lumped double porosity model was studied in order to estimate the effect of fractures on resistivity - porosity relations. It is found that the relationship between resistivity and porosity for fractured rock is in general not simple and depends both on the amounts of matrix porosity as well as the fracture orientation. However, when fractures dominate over matrix porosity the exponent is close to 1.0. Resistivity-porosity relations have been determined for large amounts of basaltic formations in Iceland. An exponent close to 1.0 is found in all cases investigated. This is interpreted as fractures constitute a considerable part of the porosity of the basalts. In the IRDP-hole in Eastern Iceland it is found that the ratio of fracture porosity to total porosity decreases with depth.

  15. Permeability within basaltic oceanic crust

    NASA Astrophysics Data System (ADS)

    Fisher, Andrew T.

    1998-05-01

    Water-rock interactions within the seafloor are responsible for significant energy and solute fluxes between basaltic oceanic crust and the overlying ocean. Permeability is the primary hydrologic property controlling the form, intensity, and duration of seafloor fluid circulation, but after several decades of characterizing shallow oceanic basement, we are still learning how permeability is created and distributed and how it changes as the crust ages. Core-scale measurements of basaltic oceanic crust yield permeabilities that are quite low (generally 10-22 to 10-17 m²), while in situ measurements in boreholes suggest an overlapping range of values extending several orders of magnitude higher (10-18 to 10-13 m²). Additional indirect estimates include calculations made from borehole temperature and flow meter logs (10-16 to 10-11 m²), numerical models of coupled heat and fluid flow at the ridge crest and within ridge flanks (10-16 to 10-9 m²), and several other methods. Qualitative indications of permeability within the basaltic oceanic crust come from an improved understanding of crustal stratigraphy and patterns of alteration and tectonic modification seen in ophiolites, seafloor samples and boreholes. Difficulties in reconciling the wide range of estimated permeabilities arise from differences in experimental scale and critical assumptions regarding the nature and distribution of fluid flow. Many observations and experimental and modeling results are consistent with permeability varying with depth into basement and with primary basement lithology. Permeability also seems to be highly heterogeneous and anisotropic throughout much of the basaltic crust, as within crystalline rocks in general. A series of focused experiments is required to resolve permeability in shallow oceanic basement and to directly couple upper crustal hydrogeology to magmatic, tectonic, and geochemical crustal evolution.

  16. Paleotethyan subduction process revealed from Triassic blueschists in the Lancang tectonic belt of Southwest China

    NASA Astrophysics Data System (ADS)

    Fan, Weiming; Wang, Yuejun; Zhang, Yanhua; Zhang, Yuzhi; Jourdan, Fred; Zi, Jianwei; Liu, Huichuan

    2015-11-01

    The subduction of the Paleotethyan Ocean and subsequent continental collision along the Lancang tectonic belt of the southeastern Paleotethyan belt is a major tectonic event in Southwest China, but the event of the subduction preceding the final collision is still not well-constrained. The mafic blueschists exposed in the Lancang accretionary complex provide crucial records of the Paleotethyan subduction process. In this paper, we present a set of new petrologic, geochronological and geochemical data for the Suyi mafic blueschists in the Lancang metamorphic zone. The mineral assemblage of these blueschists consists of zoned sodic amphibole (25-30%), albite (15-20%), epidote (25-30%), phengite (5-10%), chlorite (~ 5-10%), and minor amounts of actinolite, apatite, sphene, zircon, ilmenite, quartz and secondary limonite. This suggests a prograde metamorphism from ~ 0.5 to ~ 0.9 GPa and retrograde metamorphic overprinting (back to ~ 0.6 GPa) within the temperature range of 300-450 °C. The Suyi blueschists give a zircon U-Pb age of 260 ± 4 Ma and glaucophane minerals formed during prograde metamorphism yield a 40Ar/39Ar plateau age of 242 ± 5 Ma (MSWD = 0.77; P = 0.54). The blueschists have geochemical compositions of subalkaline basalt and show typical OIB-type REE and multi-elemental patterns and εNd(t) values ranging from + 3.35 to + 4.85. Based on available data, it is inferred that the protolith formed at 260 Ma and originated from a basaltic seamount. The basaltic rocks subducted down to 30-35 km depths beneath the Lincang arc to form the epidote blueschists at ~ 242 Ma. The blueschists were subsequently transported to shallower crustal levels in response to the continuous underthrust of the subducted slab and the continent-continent collision in the middle-late Triassic. These results provide a systematic constraint on the tectonic evolution and temporal framework of the southeastern Paleotethyan belt in Southwest China.

  17. Crustal recycling by subduction erosion in the central Mexican Volcanic Belt

    NASA Astrophysics Data System (ADS)

    Straub, Susanne M.; Gómez-Tuena, Arturo; Bindeman, Ilya N.; Bolge, Louise L.; Brandl, Philipp A.; Espinasa-Perena, Ramón; Solari, Luigi; Stuart, Finlay M.; Vannucchi, Paola; Zellmer, Georg F.

    2015-10-01

    Recycling of upper plate crust in subduction zones, or 'subduction erosion', is a major mechanism of crustal destruction at convergent margins. However, assessing the impact of eroded crust on arc magmas is difficult owing to the compositional similarity between the eroded crust, trench sediment and arc crustal basement that may all contribute to arc magma formation. Here we compare Sr-Nd-Pb-Hf and trace element data of crustal input material to Sr-Nd-Pb-Hf-He-O isotope chemistry of a well-characterized series of olivine-phyric, high-Mg# basalts to dacites in the central Mexican Volcanic Belt (MVB). Basaltic to andesitic magmas crystallize high-Ni olivines that have high mantle-like 3He/4He = 7-8 Ra and high crustal δ18Omelt = +6.3-8.5‰ implying their host magmas to be near-primary melts from a mantle infiltrated by slab-derived crustal components. Remarkably, their Hf-Nd isotope and Nd/Hf trace element systematics rule out the trench sediment as the recycled crust end member, and imply that the coastal and offshore granodiorites are the dominant recycled crust component. Sr-Nd-Pb-Hf isotope modeling shows that the granodiorites control the highly to moderately incompatible elements in the calc-alkaline arc magmas, together with lesser additions of Pb- and Sr-rich fluids from subducted mid-oceanic ridge basalt (MORB)-type altered oceanic crust (AOC). Nd-Hf mass balance suggests that the granodiorite exceeds the flux of the trench sediment by at least 9-10 times, corresponding to a flux of ⩾79-88 km3/km/Myr into the subduction zone. At an estimated thickness of 1500-1700 m, the granodiorite may buoyantly rise as bulk 'slab diapirs' into the mantle melt region and impose its trace element signature (e.g., Th/La, Nb/Ta) on the prevalent calc-alkaline arc magmas. Deep slab melting and local recycling of other slab components such as oceanic seamounts further diversify the MVB magmas by producing rare, strongly fractionated high-La magmas and a minor population of

  18. Thermoluminescence dating of Hawaiian basalt

    USGS Publications Warehouse

    May, Rodd James

    1979-01-01

    The thermoluminescence (TL) properties of plagioclase separates from 11 independently dated alkalic basalts 4,500 years to 3.3 million years old and 17 tholeiitic basalts 16 years to 450,000 years old from the Hawaiian Islands were investigated for the purpose of developing a TL dating method for young volcanic rocks. Ratios of natural to artificial TL intensity, when normalized for natural radiation dose rates, were used to quantify the thermoluminescence response of individual samples for age-determination purposes. The TL ratios for the alkalic basalt plagioclase were found to increase with age at a predictable exponential rate that permits the use of the equation for the best-fit line through a plot of the TL ratios relative to known age as a TL age equation. The equation is applicable to rocks ranging in composition from basaltic andesite to trachyte over the age range from about 2,000 to at least 250,000 years before present (B.P.). The TL ages for samples older than 50,000 years have a calculated precision of less than :t 10 percent and a potential estimated accuracy relative to potassium-argon ages of approximately :t 10 percent. An attempt to develop a similar dating curve for the tholeiitic basalts was not as successful, primarily because the dose rates are on the average lower than those for the alkalic basalts by a factor of 6, resulting in lower TL intensities in the tholeiitic basalts for samples of equivalent age, and also because the age distribution of dated material is inadequate. The basic TL properties of the plagioclase from the two rock types are similar, however, and TL dating of tholeiitic basalts should eventually be feasible over the age range 10,000 to at least 200,000 years B.P. The average composition of the plagioclase separates from the alkalic basalts ranges from oligoclase to andesine; compositional variations within this range have no apparent effect on the TL ratios. The average composition of the plagioclase from the tholeiitic

  19. Mars Crust: Made of Basalt

    NASA Astrophysics Data System (ADS)

    Taylor, G. J.

    2009-05-01

    By combining data from several sources, Harry Y. (Hap) McSween (University of Tennessee), G. Jeffrey Taylor (University of Hawaii) and Michael B. Wyatt (Brown University) show that the surface of Mars is composed mostly of basalt not unlike those that make up the Earth's oceanic crust. McSween and his colleagues used data from Martian meteorites, analyses of soils and rocks at robotic landing sites, and chemical and mineralogical information from orbiting spacecraft. The data show that Mars is composed mostly of rocks similar to terrestrial basalts called tholeiites, which make up most oceanic islands, mid-ocean ridges, and the seafloor beneath sediments. The Martian samples differ in some respects that reflect differences in the compositions of the Martian and terrestrial interiors, but in general are a lot like Earth basalts. Cosmochemistst have used the compositions of Martian meteorites to discriminate bulk properties of Mars and Earth, but McSween and coworkers' synthesis shows that the meteorites differ from most of the Martian crust (the meteorites have lower aluminum, for example), calling into question how diagnostic the meteorites are for understanding the Martian interior.

  20. Flood basalts and extinction events

    NASA Technical Reports Server (NTRS)

    Stothers, Richard B.

    1993-01-01

    The largest known effusive eruptions during the Cenozoic and Mesozoic Eras, the voluminous flood basalts, have long been suspected as being associated with major extinctions of biotic species. Despite the possible errors attached to the dates in both time series of events, the significance level of the suspected correlation is found here to be 1 percent to 4 percent. Statistically, extinctions lag eruptions by a mean time interval that is indistinguishable from zero, being much less than the average residual derived from the correlation analysis. Oceanic flood basalts, however, must have had a different biological impact, which is still uncertain owing to the small number of known examples and differing physical factors. Although not all continental flood basalts can have produced major extinction events, the noncorrelating eruptions may have led to smaller marine extinction events that terminated at least some of the less catastrophically ending geologic stages. Consequently, the 26 Myr quasi-periodicity seen in major marine extinctions may be only a sampling effect, rather than a manifestation of underlying periodicity.

  1. Geochemistry of metamorphosed pillow basalts of the Chara Zone, NE Kazakhstan

    NASA Astrophysics Data System (ADS)

    Volkova, N. I.; Khlestov, V. V.; Sukhorukov, V. P.; Khlestov, M. V.

    2016-04-01

    Geochemical study of the metamorphosed pillow lavas of the Chara Zone revealed that their protoliths have N-MORB compositions. Elevated concentrations of K, Rb, Cs, and lower Ca may be related not to blueschist metamorphism, but rather they may be inherited from underwater alteration of parental basalts. Comparative analysis of the compositions of massive glaucophanites and vein rocks demonstrated the relative mobility of Sr, U, P, Ba, Rb, K, Cs, Ca, and LREEs, at least on the local scale. Their mobility has been provided by fluids circulating in the subduction zone, where the fluids have been generated through slab dehydratation processes. These studies of metamorphosed pillow lavas of the Chara Zone displayed only local alterations in the bulk composition of their protoliths, which evidences a low intensity of fluid flows: this allowed preservation of the geochemistry of oceanic basalts.

  2. Recycling of geochemically heterogenous oceanic crust: Significance for the origin of ocean island basalts

    NASA Astrophysics Data System (ADS)

    Duggen, S.; Hoernle, K.; Hauff, F.; Park, S.-H.; Geldmacher, J.

    2009-04-01

    Explaining the isotopic signature and origin of ocean island basalts (OIBs) is a challenge in Earth sciences. There is general agreement that lithospheric material, recycled into the Earth's mantle, is involved in the mantle sources of OIBs. The relative roles, however, of 1) subducted marine sediments, 2) altered oceanic basaltic crust (AOC), 3) oceanic lithospheric mantle and/or 4) delaminated metasomatised subcontinental lithosphere and continental lower crust, however, are much debated. We present results from geochemical modeling in the Sr-Nd-Pb-isotope space following a new approach that takes into account the trace element and isotope heterogeneity of subducted oceanic crust (sediments + AOC). By means of backward and forward modeling, we examine how a geochemically heterogeneous package of oceanic crust may evolve in terms of Sr-Nd-Pb-isotopic composition through time and compare the results with present day radiogenic isotope ratios of OIBs. Our study suggests that recycling of AOC, modified during the subduction process, and stored in the Earth's mantle for several hundreds of millions of years can explain the Sr-Nd-Pb-isotopic composition of OIBs with relatively high Nd-isotope ratios that form elongated fields along or below the Northern Hemisphere Reference Line (NHRL) in the Pb-isotopic diagrams (e.g. Canaries, Galapagos, Iceland, Madeira). Explaining the origin of OIBs with relatively low Nd-isotope ratios and Pb-isotopic composition above the NHRL, and thus geochemical affinity to enriched mantle (EM) components (e.g. Pitcairn, Tristan, Samoa), however, seems to also require recycling of other lithospheric material such as subducted sediments, lower continental crust and/or subcontinental lithosphere.

  3. Tectono-magmatic response to major convergence changes in the North Patagonian suprasubduction system; the Paleogene subduction-transcurrent plate margin transition

    NASA Astrophysics Data System (ADS)

    Aragón, Eugenio; D'Eramo, Fernando; Castro, Antonio; Pinotti, Lucio; Brunelli, Daniele; Rabbia, Osvaldo; Rivalenti, Giorgio; Varela, Ricardo; Spakman, Wim; Demartis, Manuel; Cavarozzi, Claudia E.; Aguilera, Yolanda E.; Mazzucchelli, Maurizio; Ribot, Alejandro

    2011-08-01

    The southern and central Andes reflect significant along-strike differences of tectonic activity, including shortening, alternating flat-to-normal subduction styles and magmatism. In northern Patagonia, the subduction/supra-subduction system, fore arc, arc and back arc basins developed in an extensional setting during the Paleogene. This was accompanied by landward migration of calc-alkalic magmatism which changed to synextensional bimodal volcanism of rhyolitic ignimbrites and interbedded tholeiitic and alkalic basalts. These Paleogene events occurred during a time when the Farallon-Aluk active ridge reached the South American plate, and the Farallon plate subduction was interrupted. They represent a new tectonic regime, characterized by a transcurrent plate margin. The presence in the back arc of a rigid lithospheric block of 100,000 km 2 represented by the North Patagonian Massif focused the rotation of the coastal blocks. This resulted in the development of two Paleogene extensional regions to the north and south, respectively, of the Massif and replaced the former back arc. Plate rearrangement caused by the inauguration of the Nazca plate and its regime of orthogonal subduction at the beginning of the Miocene, re-established typical calc-alkaline arc magmatism at the former upper Cretaceous arc locus. Present seismic activity in the subducted plate and tomographic modeling of p-wave velocity anomalies in the upper mantle also suggest the presence of a subduction gap that lasted for most of the Paleogene in northern Patagonia.

  4. Flood Basalts and Neoproterozoic Glaciation

    NASA Astrophysics Data System (ADS)

    Halverson, G. P.; Cox, G. M.; Kunzmann, M.; Strauss, J. V.; Macdonald, F. A.

    2014-12-01

    Large igneous provinces (LIPs), which are commonly associated with supercontinental break-up, are the product of the emplacement of >106 km3 of mafic rocks in less than a few million years. LIP magmatism, in particular continental flood basalt (CFB) volcanism, perturbs global climate on shorter time scales through the radiative effects of degassed SO2 and CO2. On longer time scales, CFBs alter climate through the effect of the high weatherabilty of mafic rocks (5-10 times greater than average continental crust) on global silicate weathering. A link between flood basalt weathering, Rodinia break-up, and Neoproterozoic snowball glaciation has been postulated. Here we present a new compilation of Nd isotope data on Neoproterozoic mudstones from Laurentia, Australia, and South China along with a new seawater strontium isotope record from well preserved carbonates that support this hypothesis. These datasets are consistent with an outsized role of basalt weathering on the global silicate weathering budget during the second half of the Tonian period (~850 to 725 Ma). Along with Os isotope data, they also suggest that an additional pulse of basalt weathering at the end of the Tonian may have initiated the Sturtian snowball glaciation. CFBs have relatively high concentrations of phosphorous. Hence, the drawdown in atmospheric CO2 required to trigger the Sturtian snowball Earth was likely accomplished through a combination of increased silicate weathering rates and enhanced biological productivity driven by greater nutrient supply to the oceans. CFBs were also the likely source of the iron in Neoproterozoic iron formation (IF), all significant occurrences of which are restricted to Sturtian-aged glacial successions. Dramatic declines in ɛNd following the Cryogenian snowball glaciations are mirrored by stepwise increases in 87Sr/86Sr, reflecting the scouring of the continents by global ice sheets. This continental resurfacing removed the extensive basalt carapace as well as

  5. Filamentous microbial fossil from low-grade metamorphosed basalt in northern Chichibu belt, central Shikoku, Japan

    NASA Astrophysics Data System (ADS)

    Sakakibara, M.; Sugawara, H.; Tsuji, T.; Ikehara, M.

    2014-05-01

    The past two decades have seen the reporting of microbial fossils within ancient oceanic basalts that could be identical to microbes within modern basalts. Here, we present new petrographic, mineralogical, and stable isotopic data for metabasalts containing filamentous structures in a Jurassic accretionary complex within the northern Chichibu Belt of the Yanadani area of central Shikoku, Japan. Mineralized filaments within these rocks are present in interstitial domains filled with calcite, pumpellyite, or quartz, and consist of iron oxide, phengite, and pumpellyite. δ13CPDB values for filament-bearing calcite within these metabasalts vary from -2.49‰ to 0.67‰. A biogenic origin for these filamentous structures is indicated by (1) the geological context of the Yanadani metabasalt, (2) the morphology of the filaments, (3) the carbon isotope composition of carbonates that host the filaments, and (4) the timing of formation of these filaments relative to the timing of low-grade metamorphism in a subduction zone. The putative microorganisms that formed these filaments thrived between eruption (Late Paleozoic) and accretion (Early Jurassic) of the basalt. The data presented here indicate that cryptoendolithic life was present within water-filled vesicles in pre-Jurassic intraplate basalts. The mineralogy of the filaments reflects the low-grade metamorphic recrystallization of authigenic microbial clays similar to those formed by the encrustation of prokaryotes in modern iron-rich environments. These findings suggest that a previously unusual niche for life is present within intraplate volcanic rocks in accretionary complexes.

  6. Variability of South Pacific Tropical Water Subduction

    NASA Astrophysics Data System (ADS)

    Lu, X.; Fine, R. A.; Qu, T.

    2014-12-01

    Collection of Argo data provides an opportunity to carefully examine South Pacific Tropical Water (SPTW) subduction rate variability. SPTW is characterized by a vertical salinity maximum exceeding 36.2 psu centered at 20°S and 120°W and lying in the upper thermocline between 24.0 and 25.0 σθ. Subduction rates for SPTW for two different periods are calculated using two methods. Monthly one degree by one degree Argo data covering the South Pacific are used to calculate subduction rates from September 2005 to August 2013, also lateral induction and vertical pumping are calculated. There are two spatial subduction maxima, and the lateral induction process dominates in both maxima. Subduction rates from Argo data vary from 15 to 26 m/yr +/- 7.5% during the 8 year period. Subduction rates are shown to be positively and highly correlated with Southern Oscillation Index. Additionally, using CFC-12 data from the 1990s World Ocean Circulation Experiment, average subduction rate is calculated to be 35 +/- 16.5 m/yr. Some of the difference between Argo and tracer rates is due to a difference in the methods, and some difference may be due to decadal variability. Thus, SPTW subduction rates are shown to vary on interannual and possible decadal time scales.

  7. Obduction of western Anatolian ophiolites: from birth to steady state of a subduction zone

    NASA Astrophysics Data System (ADS)

    Plunder, Alexis; Agard, Philippe; Chopin, Christian; Whitechurch, Hubert; Okay, Aral

    2015-04-01

    During Cretaceous times, the convergence between the Anatolide Tauride block (following the movement of Africa) and Eurasia lead to the closure of a branch of the Neotethyan ocean and to ophiolite obduction. Obducted ophiolite and their sub-ophiolitic units can be found along a 400 kilometre-long north to south transect in western Anatolia. The aim of this contribution is twofold: (1) (re)-appraise the metamorphic pressure-temperature (PT) conditions and evolution of the sub-ophiolitic units of western Anatolia, by constraining the formation of the metamorphic sole during the first stages of subduction and the unusual accretion of ocean-derived units along a subduction interface in an evolving, cooling thermal regime, and (2) understand the dynamics of a large-scale and long-lived obduction. Directly below the ophiolite (mostly made of mantle-derived rocks) lies a metamorphic sole. The upper part is this sole is made of garnet and garnet clinopyroxene amphibolites, the lower part consisting in amphibolite or green-schist facies metapelites and metabasite suggestive of discrete accretion steps. In the northern part of the section the metamorphic sole is characterised by an important blueschist-facies overprint destabilizing the amphibolite paragenesis. This high-pressure overprint is lacking in the southern area. Using field and petrological observations, three units (namely and from top to bottom, OC1, OC2 and OC3) were distinguished in the accretionary complex with PT conditions ranging from incipient metamorphism to blueschist facies conditions. OC1 represents most of the outcropping unit, is found all along the section and shows only low-grade metamorphism. Metamorphic conditions remains hard to establish in this unit made of a stack of hm-thick tectonic slices showing subtle differences in their metamorphic grade (from pristine pillow basalts and hydrothermalized lavas to lawsonite pumpellyite-lawsonite bearing basalts). In OC2, Fe-Mg carpholite-bearing layers

  8. What lies below the Columbia River Basalt?

    NASA Astrophysics Data System (ADS)

    Reidel, S.; Kauffman, J.; Garwood, D.; Bush, J.

    2006-12-01

    More than 200,000 sq km of the Pacific Northwest are covered by the Miocene Columbia River Basalt Group (CRB). The lavas were erupted onto a complex structural setting dominated by cratonic rocks, and accreted terranes at a convergent plate margin. Few boreholes penetrate the basalt so the sub-basalt structure must be deduced from geophysical data, the surrounding area and structures within the basalt. In Oregon (OR) and Idaho (ID) the eastern edge of the basalt follows the boundary between the craton and accreted terranes but the suture zone becomes lost beneath the basalt in eastern WA. In northern OR and Washington (WA), a thick basalt sequence in the western part of the province overlies an early Tertiary basin with kms of sediment fill which, in turn, overlies accreted terranes. In eastern WA and western ID, a much thinner basalt sequence overlies cratonic and accreted terrane rocks without thick intervening Tertiary sediments. This basin began in the Eocene and continued into the present; the sediment now controls the location of the Yakima fold belt (YFB). Prior to basalt eruptions, a rugged mountainous terrane existed in eastern WA and ID that probably extended to the west. NW faults and folds (e.g. the Orofino fault zone ID, and Chiwaukum graben and White River-Naches River fault zone, Cascade Range) dominate the prebasalt rocks and must extend under the basalt. Remanents of this NW trend are present in YFB (e.g. Rattlesnake-Wallula fault zone) but these are less prominent than the large basalt anticlinal folds that are decoupled from the basement. CRB dikes have a NW to N trend and are thought to reflect a basement structural weakness. In the basalt province many folds and faults follow this dike trend. Major NE trending faults in the basalts do not have major counterparts beyond the basalt. One fault, the Hite Fault, must form a significant sub-basalt boundary. Dikes to the east of the Hite fault trend N-N20W whereas dikes to the west trend N40-50W

  9. The Use of Basalt, Basalt Fibers and Modified Graphite for Nuclear Waste Repository - 12150

    SciTech Connect

    Gulik, V.I.; Biland, A.B.

    2012-07-01

    New materials enhancing the isolation of radioactive waste and spent nuclear fuel are continuously being developed.. Our research suggests that basalt-based materials, including basalt roving chopped basalt fiber strands, basalt composite rebar and materials based on modified graphite, could be used for enhancing radioactive waste isolation during the storage and disposal phases and maintaining it during a significant portion of the post-closure phase. The basalt vitrification process of nuclear waste is a viable alternative to glass vitrification. Basalt roving, chopped basalt fiber strands and basalt composite rebars can significantly increase the strength and safety characteristics of nuclear waste and spent nuclear fuel storages. Materials based on MG are optimal waterproofing materials for nuclear waste containers. (authors)

  10. Flat Subduction and Dynamic Topography

    NASA Astrophysics Data System (ADS)

    Lithgow-Bertelloni, C. R.; Dávila, F. M.; Eakin, C. M.; Crameri, F.

    2014-12-01

    Mantle dynamics manifests at the surface via the horizontal motions of plates and the vertical deflections that influence topography and the non-hydrostatic geoid. The pioneering work of Mitrovica et al. (1989) and Gurnis (1990) on this dynamic topography revolutionized our understanding of sedimentary basin formation, sea level changes and continental flooding. The temporal evolution of subduction can explain the migration of basins and even the drainage reversal of the Amazon (Shephard et al., 2012; Eakin et al., 2014). Until recently, flat subduction has been seen as enhancing downward deflection of the overriding plate and increasing flooding. However, this interpretation depends crucially on the details of the morphology and density structure of the slab, which controls the loci and amplitude of the deflection. We tend to ignore morphological details in mantle dynamics because flow can smooth out short wavelength variations. We have shown instead that details matter! Using South America as a natural laboratory because of the large changes in morphology of the Nazca slab along strike, we show that downward deflection of the overriding plate and hence basin formation, do not occur over flat segments but at the leading edge, where slabs plunge back into the mantle. This is true in both Argentina and Peru. The temporal evolution from a 'normally' dipplng slab to a flat slab leads to uplift over flat segments rather than enhanced subsidence. Critical for this result is the use of a detailed morphological model of the present-day Nazca slab with a spatial resolution of 50-100 km and based on relocated seismicity and magnetotelluric results. The density structure of the slab, due to age and the presence of overthickened crust from aseismic ridge subduction is essential. Overthickened crust leads to buoyant slabs. We reproduce formation and deposition of the Acres-Solimoes basin and the evolution of the Amazon drainage basin in Peru as well as the Mar Chiquita

  11. Mare basalt magma source region and mare basalt magma genesis

    SciTech Connect

    Binder, A.B.

    1982-11-15

    Given the available data, we find that the wide range of mare basaltic material characteristics can be explained by a model in which: (1) The mare basalt magma source region lies between the crust-mantle boundary and a maximum depth of 200 km and consists of a relatively uniform peridotite containing 73--80% olivine, 11--14% pyroxene, 4--8% plagioclase, 0.2--9% ilmenite and 1--1.5% chromite. (2) The source region consists of two or more density-graded rhythmic bands, whose compositions grade from that of the very low TiO/sub 2/ magma source regions (0.2% ilmenite) to that of the very high TiO/sub 2/ magma source regions (9% ilmenite). These density-graded bands are proposed to have formed as co-crystallizing olivine, pyroxene, plagioclase, ilmenite, and chromite settled out of a convecting magma (which was also parental to the crust) in which these crystals were suspended. Since the settling rates of the different minerals were governed by Stoke's law, the heavier minerals settled out more rapidly and therefore earlier than the lighter minerals. Thus the crystal assemblages deposited nearest the descending side of each convection cell were enriched in heavy ilmenite and chromite with respect to lighter olivine and pyroxene and very much lighter plagioclase. The reverse being the case for those units deposited near the ascending sides of the convection cells.

  12. Subseafloor basalts as fungal habitats

    NASA Astrophysics Data System (ADS)

    Ivarsson, M.; Bengtson, S.

    2013-12-01

    The oceanic crust makes up the largest potential habitat for life on Earth, yet next to nothing is known about the abundance, diversity and ecology of its biosphere. Our understanding of the deep biosphere of subseafloor crust is, with a few exceptions, based on a fossil record. Surprisingly, a majority of the fossilized microorganisms have been interpreted or recently re-interpreted as remnants of fungi rather than prokaryotes. Even though this might be due to a bias in fossilization the presence of fungi in these settings can not be neglected. We have examined fossilized microorganisms in drilled basalt samples collected at the Emperor Seamounts in the Pacific Ocean. Synchrotron-radiation X-ray tomography microscopy (SRXTM) studies has revealed a complex morphology and internal structure that corresponds to characteristic fungal morphology. Chitin was detected in the fossilized hyphae, which is another strong argument in favour of a fungal interpretation. Chitin is absent in prokaryotes but a substantial constituent in fungal cell walls. The fungal colonies consist of both hyphae and yeast-like growth states as well as resting structures and possible fruit bodies, thus, the fungi exist in vital colonies in subseafloor basalts. The fungi have also been involved in extensive weathering of secondary mineralisations. In terrestrial environments fungi are known as an important geobiological agent that promotes mineral weathering and decomposition of organic matter, and they occur in vital symbiosis with other microorganisms. It is probable to assume that fungi would play a similar role in subseafloor basalts and have great impact on the ecology and on biogeochemical cycles in such environments.

  13. Mantle Heterogeneities and Crustal Processes of the Cascade Arc Represented by Basalts of the Poison Lake Chain, Lassen Volcanic Center, California

    NASA Astrophysics Data System (ADS)

    Wenner, J. M.; Teasdale, R.; Hiebing, M. S.; Lenz, Q. A.; Kroeninger, K.

    2013-12-01

    Basalts in the Poison Lake chain (PLC) include eight chemically distinct groups of primitive calc-alkaline basalts (defined by major element geochemistry and mineralogy). Located east of the Lassen Volcanic Center, PLC primitive basalts span the range of basalt compositions exposed throughout the entire Cascade arc (e.g. Ba: 100-1000 ppm; (Sr/P)n: 1.3 - 3.8; La/Yb: 4-26). PLC groups have trace-element and isotope ratios that show little evidence of direct genetic relationships among groups or a common source. Major, trace element and isotope ratios show evidence of contributions from multiple mantle sources including MORB, fluid rich subduction component and subduction-related sediment. Some groups record compositional variations from multiple mantle sources with minimal crustal processing. Similarly, preliminary probe data for olivine-spinel pairs suggest that some PLC groups are derived from heterogeneous mantle sources. Geochemical evidence indicates that other groups have petrogenetic histories that include crustal processes such as fractional crystallization, mixing or crustal contamination. Isotope ratios, major and trace element compositions and crystal compositions provide insights into the extent of source heterogeneities versus the degree of crustal processing. The broad range of compositional variations in basalts of PLC provides the opportunity to examine the extent of mantle heterogeneities and crustal processing in a small geographic area (50km2) for rocks that are nearly the same age (100-110 ka). The diverse primitive compositions erupted in the constrained time and space of the Poison Lake chain and the lack of genetic relationship among groups make it the ideal place to investigate the small scale nature of mantle domains and the roles of subduction and modification processes in the generation of basaltic compositions in arcs such as the Cascades, Mexico, Japan.

  14. The geochemical behaviour of W in subduction zones: constraints from high precision isotope dilution measurements

    NASA Astrophysics Data System (ADS)

    Konig, S.; Munker, C.; Schuth, S.

    2007-12-01

    Assessing the behaviour of W during silicate Earth's differentiation is hampered by low abundances of W in terrestrial reservoirs, making sufficiently precise and accurate measurements difficult. Previous results (e.g., Newsom et al. 1996) indicate a lower W/Th of the mantle (ca. 0.19) compared to the Earth's crust, (ca. 0.26), suggesting that W appears to be more incompatible than Th. New data for MORB (Munker et al. 2007), however, demonstrate that W/Th is not significantly fractionated during dry peridotite melting, tentatively suggesting a fractionation of the two elements during crust formation by subduction related processes. We present high precision W and Nb-Ta, Zr-Hf data obtained by isotope dilution, using a mixed 183W-180Ta- 94Zr-180Hf-176Lu tracer and multiple collector inductively coupled plasma mass spectrometry (MC-ICPMS). This enables the direct determination of W and HFSE from one sample digestion. For some samples, a "nugget effect" as previously reported for PGE was observed, reflecting sample heterogeneity. Measured Ta and W abundances determined in basaltic glasses and whole rock powders from various island arc settings yield Ta/W ratios of 0.6 to 1.7, significantly lower than the values reported for MORB (4-6). In contrast, Nb/Ta for the samples overlap with MORB values, suggesting that Nb and Ta were not mobile in the magma sources. These systematic differences indicate that W does not behave as other HFSE (Nb-Ta, Zr-Hf) in subduction zones but rather displays a higher mobility in slab components. Intra-oceanic arc suites involving subducted pelagic sediment in their sources generally display higher W/Th compared to magmas without sediment-derived components in their sources, reflecting the higher initial W abundances in subducted pelagic sediments. A fractionation of W/Th during crust formation could consequently be explained by a selective W enrichment relative to Th during subduction processes.

  15. The Contribution from Hot, Subducted Lithosphere to Mantle Wedge: Melt or Fluid?

    NASA Astrophysics Data System (ADS)

    Grove, T. L.

    2005-12-01

    In the Mt. Shasta region, N. Calif., USA, primitive basaltic andesites and andesites (similar to adakites) preserve a remarkable record of subducted lithosphere and mantle wedge elemental contributions. Estimates of pre-eruptive water contents allow the development of models of magma generation. When combined with a mantle melting model, one can characterize the chemical composition of the subducted slab contribution. Mt. Shasta lies above the young Juan de Fuca plate, where a hot slab environment has been proposed for the origin of the lavas. Melting is modeled as a process where an initial melt is formed in the mantle wedge above the slab by vapor-saturated melting of peridotite, metasomatized and enriched by the slab-derived melt or fluid. Vapor-saturated melting leads to the production of a water-rich melt (25-30 wt. % H2O) that ascends into the overlying mantle and continuously reacts as it encounters hotter, shallower mantle. The melt fraction increases, the water content decreases and the slab contribution is modified and diluted. The result is a flux melt whose major elements are dominantly derived by mantle melting and whose trace elements and isotopic characteristics reflect the subducted oceanic lithosphere. Two distinct sources are indicated by Sr, Nd and Pb isotopic evidence: a MORB and a sediment source. When the major element signature of the mantle wedge is removed, the slab contribution more closely resembles a low degree melt (2 to 5 wt. %) of a garnet + clinopyroxene source. The subducted component is less similar to experimental fluids equilibrated with eclogite. Although the LIL elements are a good match with a fluid, the model abundances of rare earth elements (Ce, Sm and Yb) are low by several orders of magnitude. Thus, a melt seems a better fit at Mt. Shasta based on our current state of understanding of melt vs. fluid equilibrium in the deep subduction environment.

  16. Fluids escape in subduction zones: new constraints from 3-D microtomography data

    NASA Astrophysics Data System (ADS)

    Le Roux, V.; Gaetani, G. A.; Slaugenwhite, J.; Miller, K.

    2013-12-01

    Large amounts of H2O are carried into trenches via subduction of the sediments, basaltic crust and uppermost mantle that make up the oceanic lithosphere. A major question is how much of this subducted H2O is released into the overlying mantle wedge, promoting melting, and how much is carried deeper into the mantle. This depends, at least in part, on whether H2O is able to form an interconnected network among the mineral grains that make up the rock down to very low fluid fractions. In order to achieve connectivity and allow the fluid phase to escape, a minimum amount of fluid (critical porosity) is required when dihedral angles are more than 60 degrees. We investigated the distribution of seawater in simplified sediment analogs (i.e. quartz for siliceous sediments; calcite for carbonate sediments), in natural clays (kaolinite and montmorillonite) and in bulk eclogite. Experiments were performed in a piston-cylinder apparatus at 2 GPa and 650°C. Fluid fractions ranged from ~10% to ~1% to determine the porosity at which connectivity of the seawater network is lost for each rock type. We used synchrotron X-ray microtomographic techniques (at Argonne National Laboratory, IL) to obtain 3-D images of the pore space network in order to constrain the grain scale distribution of fluids in a subducted slab. This nondestructive 3-D imaging technique has a spatial resolution of 0.7 μm and provides quantitative information on geometrical parameters of fluid topology, such as porosity, dihedral angle distribution, fluid channel sizes and connectivity. The geometrical parameters were extracted using the VSG Avizo software. This study lays the groundwork for determining the 3-D grain scale distribution of fluids in a range of subducted lithologies. Results from this study provide important new insights into the amount of fluid that can be transported into the deep mantle by subduction.

  17. Generation of hydrous-carbonated plumes in the mantle transition zone linked to tectonic erosion and subduction

    NASA Astrophysics Data System (ADS)

    Safonova, Inna; Maruyama, Shigenori; Litasov, Konstantin

    2015-11-01

    This paper presents a model for the generation of hydrous-carbonated plumes (HCPs) in the mantle transition zone (MTZ) linking (i) the Pacific-type convergent margins; (ii) melt generation in the MTZ under the influence of volatiles (water, carbon dioxide) and subducted granitic material and oceanic slabs and (iii) the Meso-Cenozoic intra-plate magmatism in Central Asia. The model is based on four groups of evidences obtained from geology, petrology, seismic tomography and numerical simulations. The double-sided subduction at the Pacific-type margins around post-Miocene Asia supplies hydrated-carbonated oceanic crust and continental crust materials down to the deep mantle, which accumulate in the MTZ at 410-660 km. The delivery of crustal material to the MTZ is provided by the direct subduction of intra-oceanic arcs in the Western Pacific and by the tectonic erosion of convergent margin hanging walls. The U-Th-K-enriched continental material accumulated in the MTZ can serve an additional source of heat. Evidence for the subduction of continental crust materials comes from seismic tomography and numerical modelling data. The subducting oceanic slab consisting of serpentinites, hydrated sediments, carbonates and carbonatized basalts can supply water and carbon dioxide to the deep mantle and metasomatize it. The presence of volatiles, which can reduce melting temperature, and the presence of the subducted crustal material, which may serve an additional heater, can synergistically trigger the generation of HCPs. Those HCPs can induce mantle upwelling, melting of the metasomatized mantle and subducted MORB slabs, ascent of melts, surface rifting and formation of mafic and bimodal volcanic series. In addition, they can contribute to the supercontinent cycle. The HCPs generated in the MTZ beneath Central and East Asia resulted in a shift of the tectonic regime from transpression to transtension and in the formation of numerous Meso-Cenozoic intra-plate volcanic fields.

  18. Generation of Talc from the Mantle Wedge and its Role on the Subduction Dynamics in Central Mexico

    NASA Astrophysics Data System (ADS)

    Kim, Y.; Clayton, R. W.; Asimow, P. D.; Jackson, J. M.

    2013-05-01

    Geophysical evidence shows the existence of low-seismic velocity material at the surface of subducting slabs in subduction zones although its termination point may vary depending on various parameters such as its thermal state and slab dip angle. In central Mexican subduction zone, modeling of the teleseismic converted amplitudes of the horizontal oceanic crust previously revealed a thin (~4 km) low-velocity zone at the upper oceanic crust between the Cocos plate and the continental crust that appears to absorb nearly all of the strain between two plates. Using Vp/Vs as a function of S wave velocity (Vs) in a range of likely pressures (P) and temperatures (T) for candidate mineral phases, this thin layer in the flat slab region is determined to be enriched with hydrous (low-strength) minerals such as talc over the normal oceanic crustal compositions such as MORB-like gabbro. Based on the P-T curves for equilibria involving talc derived from available thermodynamic data, the generation of talc from the basaltic lithology of the oceanic crust subducting at the trench side is nearly impossible. Also, there is no clear evidence to explain the origin of the flat subduction based on the offshore anomalies, which may lead to the current configuration. We therefore propose that the talc-rich layer on top of the subducting plate is generated from the mantle wedge side during the slab flattening process coupled with trench rollback. The evolution of this low-strength zone has important implications for the dynamics of the subduction system including the flattening process of the slab as well as the geochemistry of the mantle wedge and arc in central Mexico.

  19. Origin and Role of Recycled Crust in Flood Basalt Magmatism: Case Study of the Central East Greenland Rifted Margin

    NASA Astrophysics Data System (ADS)

    Brown, E.; Lesher, C. E.

    2015-12-01

    Continental flood basalts (CFB) are extreme manifestations of mantle melting derived from chemically/isotopically heterogeneous mantle. Much of this heterogeneity comes from lithospheric material recycled into the convecting mantle by a range of mechanisms (e.g. subduction, delamination). The abundance and petrogenetic origins of these lithologies thus provide important constraints on the geodynamical origins of CFB magmatism, and the timescales of lithospheric recycling in the mantle. Basalt geochemistry has long been used to constrain the compositions and mean ages of recycled lithologies in the mantle. Typically, this work assumes the isotopic compositions of the basalts are the same as their mantle source(s). However, because basalts are mixtures of melts derived from different sources (having different fusibilities) generated over ranges of P and T, their isotopic compositions only indirectly represent the isotopic compositions of their mantle sources[1]. Thus, relating basalts compositions to mantle source compositions requires information about the melting process itself. To investigate the nature of lithologic source heterogeneity while accounting for the effects of melting during CFB magmatism, we utilize the REEBOX PRO forward melting model[2], which simulates adiabatic decompression melting in lithologically heterogeneous mantle. We apply the model to constrain the origins and abundance of mantle heterogeneity associated with Paleogene flood basalts erupted during the rift-to-drift transition of Pangea breakup along the Central East Greenland rifted margin of the North Atlantic igneous province. We show that these basalts were derived by melting of a hot, lithologically heterogeneous source containing depleted, subduction-modified lithospheric mantle, and <10% recycled oceanic crust. The Paleozoic mean age we calculate for this recycled crust is consistent with an origin in the region's prior subduction history, and with estimates for the mean age of

  20. Petrology of the Basalt of Summit Creek: A [Slab] Window into Pacific Northwest Tectonics during the Eocene

    NASA Astrophysics Data System (ADS)

    Kant, L. B.; Tepper, J. H.; Nelson, B. K.

    2012-12-01

    Variation in composition of basalts within the Cascade arc reflects the regional effects of subducting slab windows. The earliest preserved Tertiary manifestation of this process is the 55-44 Ma Basalt of Summit Creek (BSC), located southeast of Mount Rainier. At the base of this steeply dipping 2000 m section of subaerial lavas are basalts / diabases with arc traits (e.g., HFSE depletions, 1.0-1.2 wt. % K2O) and isotopic compositions (207Pb/204Pb > 15.58; ɛNd = +5.8 to +6.7) that overlap those of modern Cascade arc rocks. Conformably overlying these arc rocks (and separated by ~35m of shale, sandstone and conglomerate) are tholeiitic basalts with OIB affinities (<0.4 wt. % K2O, Y/Nb = 1.1-2.3, concave spidergram profiles) and isotopic signatures of a more depleted mantle source (207Pb/204Pb < 15.56; ɛNd = +7.1 to +7.8). In major element, trace element, and isotopic composition the upper BSC lavas are broadly similar to the voluminous Crescent Formation basalts on the Olympic Peninsula, which are coeval with the BSC but located ~100 km farther west. Compositional diversity within the upper BSC section (Mg# 66-30) appears to reflect both fractional crystallization and source heterogeneity. Modeling with MELTS (Ghiroso and Sack, 1995) indicates that differentiation dominated by removal of clinopyroxene and plagioclase took place at mid crustal depths (P = 5 kbar) and that the parent magma had <0.2 wt. % water. However, this process cannot account for all incompatible element data, which indicate the existence of two distinct magma series that differ most notably in Sr, Zr, and K2O contents. Arc basalts of the lower BSC may represent the southernmost extension of the Cretaceous-Tertiary North Cascades arc (Miller et al., 2009); however, basalts higher in the section have OIB traits and reflect a different tectonic setting. We propose that the transition from arc to OIB magmatism in the BSC records the arrival beneath the arc of a slab window produced by subduction

  1. Continental subduction induced tremor activity?

    NASA Astrophysics Data System (ADS)

    Tai, H. J.; Chen, K. H.; Ide, S.; Mouyen, M.; Byrne, T. B.

    2015-12-01

    Southern Central Range of Taiwan, a place where deep-seated tectonic tremors (a proxy of slow slip) and earthquake swarms are closely located in space and highly correlated in time, provides rare opportunity towards the understanding of physical mechanisms governing different style of slip. To identify tremor events, we used the identification scheme similar to Ide et al. (2015) but applied slightly different techniques: (1) Higher waveform cross-correlation coefficient (>0.6) (2) careful visual inspection for excluding local earthquakes and short-lasted event (duration < 60 s) (3) Signal to noise ratio higher than 1.2 and lower than 30 (4) No spatio-temporal clustering technique used. During the study period of 2007-2012, we identified 2320 tremor events with duration ranging from 60 s to 1550 s. They are located underneath southern Central Range, forming a NS-striking and SE-dipping pipe-like structure at a depth of 20-40 km. The up-dip extension of this tremor structure reaches an aseismic zone under the western flank of Central Range at shallow depths, where is an area characterized by high heat flow, low Vp and Vs anomaly. Such seismic gap was explained by the buoyancy induced crust detachment during continental subduction of Eurasian Plate. This detachment may open a new channel for hot and ductile material ascending to shallow depth, producing high temperatures along the way. This provides a common mechanism for down-dip tremor and up-dip shallow seismic gap along the same eastern dipping channel. In addition, the tremor events are found to be mostly occurred in high tides and exhibit higher correlation with tide data from west coast of Taiwan. This may again imply the association between tremor activity and subduction of Eurasian Plate.

  2. Modeling Central American basalts using the Arc Basalt Simulator

    NASA Astrophysics Data System (ADS)

    Feigenson, M.; Carr, M. J.

    2011-12-01

    We have used the Arc Basalt Simulator (ABS), developed by JI Kimura, to explore the conditions and components of melting beneath the Central American volcanic front. ABS is a comprehensive forward model that incorporates slab dehydration and melting and mantle wedge fluxing and melting using realistic P-T conditions and experimentally determined phase relations. We have applied ABS versions 3 and 4 to model representative magma types in Nicaragua, which span a broad geochemical range including proximal high- and low-Ti lavas in Nicaragua. Sr-Nd-Pb data require appropriate selection of previously identified sources, including: separate carbonate and hemipelagic sediments, DMM, an enriched mantle isotopically similar to the alkaline basalts of Yojoa, a Himu-influenced mantle derived from Galapagos material and altered oceanic crust (AOC) derived from both MORB and Galapagos seamounts. Following the dry solidus, the dominant arc basalts, exemplified by Cerro Negro lavas, can be generated at about 80-90 km where lawsonite and zoisite break down, releasing LILEs into a hydrous fluid that travels into the wedge. The fluid-triggered melting occurs just above the garnet stability field in the wedge to fit the HREEs. Below 90 Km, slab melting begins and the AOC component dominates, generating a fluid with little or no HFSE depletions, consistent with the unusual high-Ti lavas found in Nicaragua. However, the isotopic data require a much lower sediment input for the high-Ti lavas (consistent with 10Be results on the high-Ti lavas) and an enriched component for the AOC and/or mantle wedge. Following the wet solidus, fits to the Cerro Negro magma only occur in the absence of phengite in the AOC and with the presence of HFSE attracting minerals, rutile, zircon and allanite. The depth of the best fit is 135 km, consistent with current best estimates of the depth to the seismic zone beneath Cerro Negro. Below 150 km, the high-Ti lavas can be generated if the HFSE retaining

  3. Reconciling the Shadow of a Subduction Signature with Rift Geochemistry and Tectonic Environment in Eastern Marie Byrd Land, Antarctica

    NASA Astrophysics Data System (ADS)

    LeMasurier, W. E.; Choi, S.

    2013-12-01

    Basalt-trachyte volcanoes in the Marie Byrd Land (MBL) Cenozoic province lie along the Amundsen Sea coast on the north flank of the West Antarctic rift. In the province as a whole, the basalts are characterized by OIB-like geochemistry, restricted ranges of 87Sr/86Sr (0.70254 - 0.70368) and 143Nd/144Nd (0.51286 - 0.51368) and a wide range of 206Pb/204Pb (19.50 - 20.69). Basalts at three volcanoes in central and eastern MBL, of Miocene and Quaternary age, display a variety of geochemical anomalies compared with the above. These include low 143Nd/144Nd (0.51276 - 0.51281), very high Ba (e.g. 1398ppm) associated with low K and low Th, slightly depressed Nb and Ta, and elevated EM2 signatures. These are only erratically displayed, from one volcano to another, and even from one sample locality to another in the same volcano. In some cases, anomalous sample localities lie above or below sample localities with relatively 'normal' characteristics. Furthermore, the whole complement of anomalies is rarely displayed in a single sample. These characteristics suggest a subduction influence, but one that seems to have been filtered, or partly masked. Major episodes of subduction and granite plutonism in MBL took place in the late Devonian, Permian, and late Cretaceous. The last of these ended ~90 Ma, and was followed by continental break-up, rifting and lithospheric attenuation that produced the West Antarctic rift as we know it today. Thus, the enigmatic geochemical signatures in these three volcanoes may have been acquired 80-90 m.y. after subduction ended, and following the subsequent tectonic reorganization to a rift environment. We suspect that the sublithospheric source was heterogeneously and incompletely metasomatized by fluids that originated with slab dewatering during the subduction episodes. Interestingly, pelagic rocks, probably similar to those that were subducted, have geochemical characteristics that seem to be reflected in the geochemical anomalies of the

  4. Seismic anisotropy above a subducting plate

    SciTech Connect

    Shih, X.R.; Meyer, R.P. ); Schneider, J.F. )

    1991-08-01

    Shear-wave splitting observed in northeastern Colombia has provided evidence of seismic anisotropy in a shear zone immediately above a subducting plate. In an upper mantle composed mainly of olivine (57%) and orthopyroxene (17%), the splitting can be interpreted by wave propagation in an anisotropic medium of orthorhombic symmetry that results from alignment of these intrinsically anisotropic minerals. The mechanism of alignment is most likely the shearing associated with the subduction, aided by fluids migrating from the subducting plate when the plate exceeds 100 km in depth.

  5. The systematics of chlorine, fluorine, and water in Izu arc front volcanic rocks: Implications for volatile recycling in subduction zones

    NASA Astrophysics Data System (ADS)

    Straub, Susanne M.; Layne, Graham D.

    2003-11-01

    We studied the systematics of Cl, F and H 2O in Izu arc front volcanic rocks using basaltic through rhyolitic glass shards and melt inclusions (Izu glasses) from Oligocene to Quaternary distal fallout tephra. These glasses are low-K basalts to rhyolites that are equivalent to the Quaternary lavas of the Izu arc front (Izu VF). Most of the Izu glasses have Cl ˜400-4000 ppm and F ˜70-400 ppm (normal-group glasses). Rare andesitic melt inclusions (halogen-rich andesites; HRA) have very high abundances of Cl (˜6600-8600 ppm) and F (˜780-910 ppm), but their contents of incompatible large ion lithophile elements (LILE) are similar to the normal-group glasses. The preeruptive H 2O of basalt to andesite melt inclusions in plagioclase is estimated to range from ˜2 to ˜10 wt% H 2O. The Izu magmas should be undersaturated in H 2O and the halogens at their preferred levels of crystallization in the middle to lower crust (˜3 to ˜11 kbar, ˜820° to ˜1200°C). A substantial portion of the original H 2O is lost due to degassing during the final ascent to surface. By contrast, halogen loss is minor, except for loss of Cl from siliceous dacitic and rhyolitic compositions. The behavior of Cl, F and H 2O in undegassed melts resembles the fluid mobile LILE (e.g.; K, Rb, Cs, Ba, U, Pb, Li). Most of the Cl (>99%), H 2O (>95%) and F (>53%) in the Izu VF melts appear to originate from the subducting slab. At arc front depths, the slab fluid contains Cl = 0.94 ± 0.25 wt%, F = 990 ± 270 ppm and H 2O = 25 ± 7 wt%. If the subducting sediment and the altered basaltic crust were the only slab sources, then the subducted Cl appears to be almost entirely recycled at the Izu arc (˜77-129%). Conversely, H 2O (˜13-22% recycled at arc) and F (˜4-6% recycled) must be either lost during shallow subduction or retained in the slab to greater depths. If a seawater-impregnated serpentinite layer below the basaltic crust were an additional source of Cl and H 2O, the calculated percentage of

  6. Magmatic response to early aseismic ridge subduction: the Ecuadorian margin case (South America)

    NASA Astrophysics Data System (ADS)

    Bourdon, Erwan; Eissen, Jean-Philippe; Gutscher, Marc-André; Monzier, Michel; Hall, Minard L.; Cotten, Joseph

    2003-01-01

    A geochemical and isotopic study of lavas from Pichincha, Antisana and Sumaco volcanoes in the Northern Volcanic Zone (NVZ) in Ecuador shows their magma genesis to be strongly influenced by slab melts. Pichincha lavas (in fore arc position) display all the characteristics of adakites (or slab melts) and were found in association with magnesian andesites. In the main arc, adakite-like lavas from Antisana volcano could be produced by the destabilization of pargasite in a garnet-rich mantle. In the back arc, high-niobium basalts found at Sumaco volcano could be produced in a phlogopite-rich mantle. The strikingly homogeneous isotopic signatures of all the lavas suggest that continental crust assimilation is limited and confirm that magmas from the three volcanic centers are closely related. The following magma genesis model is proposed in the NVZ in Ecuador: in fore arc position beneath Pichincha volcano, oceanic crust is able to melt and produces adakites. En route to the surface, part of these magmas metasomatize the mantle wedge inducing the crystallization of pargasite, phlogopite and garnet. In counterpart, they are enriched in magnesium and are placed at the surface as magnesian andesites. Dragged down by convection, the modified mantle undergoes a first partial melting event by the destabilization of pargasite and produces the adakite-like lavas from Antisana volcano. Lastly, dragged down deeper beneath the Sumaco volcano, the mantle melts a second time by the destabilization of phlogopite and produces high-niobium basalts. The obvious variation in spatial distribution (and geochemical characteristics) of the volcanism in the NVZ between Colombia and Ecuador clearly indicates that the subduction of the Carnegie Ridge beneath the Ecuadorian margin strongly influences the subduction-related volcanism. It is proposed that the flattening of the subducted slab induced by the recent subduction (<5 Ma?) of the Carnegie Ridge has permitted the progressive warming of

  7. Carbonate Mineralization of Volcanic Province Basalts

    SciTech Connect

    Schaef, Herbert T.; McGrail, B. Peter; Owen, Antionette T.

    2010-03-31

    Flood basalts are receiving increasing attention as possible host formations for geologic sequestration of anthropogenic CO2, with studies underway in the United States, India, Iceland, and Canada. As an extension of our previous experiments with Columbia River basalt, basalts from the eastern United States, India, and South Africa were reacted with aqueous dissolved CO2 and aqueous dissolved CO2-H2S mixtures under supercritical CO2 (scCO2) conditions to study the geochemical reactions resulting from injection of CO2 in such formations. The results of these studies are consistent with cation release behavior measured in our previous experiments (in press) for basalt samples tested in single pass flow through dissolution experiments under dilute solution and mildly acidic conditions. Despite the basalt samples having similar bulk chemistry, mineralogy and apparent dissolution kinetics, long-term static experiments show significant differences in rates of mineralization as well as compositions and morphologies of precipitates that form when the basalts are reacted with CO2-saturated water. For example, basalt from the Newark Basin in the United States was by far the most reactive of any basalt tested to date. Carbonate reaction products for the Newark Basin basalt were globular in form and contained significantly more Fe than the secondary carbonates that precipitated on the other basalt samples. In comparison, the post-reacted samples associated with the Columbia River basalts from the United States contained calcite grains with classic dogtooth spar morphology and trace cation substitution (Mg and Mn). Carbonation of the other basalts produced precipitates with compositions that varied chemically throughout the entire testing period. Examination of polished cross sections of the reacted grains by scanning electron microscopy and energy dispersive x-ray spectroscopy show precipitate overgrowths with varying chemical compositions. Compositional differences in the

  8. Fractionation products of basaltic komatiite magmas at lower crustal pressures: implications for genesis of silicic magmas in the Archean

    NASA Astrophysics Data System (ADS)

    Mandler, B. E.; Grove, T. L.

    2015-12-01

    Hypotheses for the origin of crustal silicic magmas include both partial melting of basalts and fractional crystallization of mantle-derived melts[1]. Both are recognized as important processes in modern environments. When it comes to Archean rocks, however, partial melting hypotheses dominate the literature. Tonalite-trondhjemite-granodiorite (TTG)-type silicic magmas, ubiquitous in the Archean, are widely thought to be produced by partial melting of subducted, delaminated or otherwise deeply buried hydrated basalts[2]. The potential for a fractional crystallization origin for TTG-type magmas remains largely unexplored. To rectify this asymmetry in approaches to modern vs. ancient rocks, we have performed experiments at high pressures and temperatures to closely simulate fractional crystallization of a basaltic komatiite magma in the lowermost crust. These represent the first experimental determinations of the fractionation products of komatiite-type magmas at elevated pressures. The aim is to test the possibility of a genetic link between basaltic komatiites and TTGs, which are both magmas found predominantly in Archean terranes and less so in modern environments. We will present the 12-kbar fractionation paths of both Al-depleted and Al-undepleted basaltic komatiite magmas, and discuss their implications for the relative importance of magmatic fractionation vs. partial melting in producing more evolved, silicic magmas in the Archean. [1] Annen et al., J. Petrol., 47, 505-539, 2006. [2] Moyen J-F. & Martin H., Lithos, 148, 312-336, 2012.

  9. Primitive, high-Mg basaltic andesites: direct melts of the shallow, hot, wet mantle

    NASA Astrophysics Data System (ADS)

    Andrews, A.; Grove, T. L.

    2013-12-01

    Direct mantle melts are rare in subduction zone settings. Such melts are identified by Mg #s (Mg # = Mg / (Mg+Fe)) greater than ~0.73, indicating chemical equilibrium with Fo90 mantle olivine. Most of these primitive arc melts are basaltic, characterized by SiO2 contents of ~48-50 wt % and MgO contents ranging from 8-10 wt %. However, primitive basaltic andesites with mantle-equilibrated Mg #s have also been found at subduction zones worldwide. These basaltic andesites have higher SiO2 contents (53-58 wt %) than typical primitive basalts as well as high MgO (8-10 wt %). Because these rocks have high SiO2 contents and yet retain evidence for chemical equilibrium with the mantle (Mg #s), their petrogenesis has sparked intense debate as researchers have tried to discern how these samples fit into the paradigm of mantle melting at subduction zones. Through an understanding of the conditions and processes that produce the SiO2 enrichment in these rocks, we also aim to understand the role of these melts in producing the observed andesitic compositional characteristics of the continental crust. To understand the petrogenesis of primitive, high-Mg basaltic andesites, this study investigates the experimental melts of undepleted mantle peridotite plus a slab component (Na-2O + K2O) from 1,205-1,470°C at 1.0-2.0 GPa under water-undersaturated conditions (0-5 wt % H2O). At 1.0 and 1.2 GPa, the experimental melts reproduce the compositions of natural primitive, high-Mg basaltic andesites in all major elements (SiO2, TiO2, Al2O3, FeO, MnO, MgO, and Na2O+K2O) except CaO. CaO contents are higher than the range of the natural samples by ~2 wt % at the highest silica contents of the experiments (54-56 wt% SiO2). This suggests that at 1.0-1.2 GPa, a higher percent of melting (30-35 %) with 3-5 wt % H2O is required to drive the chemical compositions of the experiments toward the representative compositions of the natural rocks. The experimental melts also show that mantle-wall rock

  10. Shock metamorphism of lunar and terrestrial basalts

    NASA Technical Reports Server (NTRS)

    Schaal, R. B.; Hoerz, F.

    1977-01-01

    Lonar Crater (India) basalt and lunar basalt 75035 were shock loaded under controlled laboratory conditions up to 1000 kbar, generally in a CO/CO2 (1:1) environment evacuated to 10 to the minus seventh power torr. The Kieffer et al. (1976) classification scheme of progressive shock metamorphism is found to apply to lunar basalts. The major shock features of the five classes that span the range 0 to 1000 kbar are described. Only three out of 152 basalt specimens show shock effects in their natural state as severe as Class 2 features. The scarcity of shocked basalt hand samples in contrast to the abundance of shock-produced agglutinates and homogeneous glass spheres in the lunar regolith indicates the dominant role of micrometeorite impact in the evolution of the lunar regolith. The overall glass content in asteroidal and Mercurian regoliths is considered.

  11. Postcollisional mafic igneous rocks record recycling of noble gases by deep subduction of the continental crust

    NASA Astrophysics Data System (ADS)

    Dai, Li-Qun; Zheng, Yong-Fei; He, Huai-Yu; Zhao, Zi-Fu

    2016-05-01

    Recycling of noble gases from crustal rocks into the mantle is indicated not only by oceanic basalts and mantle xenoliths, but also by ultrahigh-pressure metamorphic rocks in collisional orogens. It is intriguing whether noble gases in continental crust were recycled into the mantle by deep subduction of the continental crust to mantle depths. Here we firstly report the He, Ne and Ar isotopic compositions of pyroxene from postcollisional mafic igneous rocks in the Dabie orogen, China. The results show that the pyroxene separates from the mafic rocks have low 3He/4He ratios of 0.002 to 1.8 Ra and air-like Ne isotope compositions. Furthermore, the pyroxene exhibits low 40Ar/36Ar ratios of 393.6 to 1599.8, close to those of the air. In combination with whole-rock geochemistry it is found that pyroxene 3He/4He ratios are correlated with whole-rock (La/Yb)N and Sr/Y ratios, εNd(t) values and MgO contents. These observations demonstrate the mass transfer from the deeply subducted continental crust to the overlying mantle wedge, recording the source mixing between the crust-derived melt and the mantle peridotite in the continental subduction zone. A direct addition of the crustal He via crust-derived melt to the mantle leads to the extremely low 3He/4He ratios in the orogenic lithospheric mantle, and the dissolved atmospheric Ar and Ne in the subducted supracrustal rocks results in the air-like Ar and Ne isotope ratios. Therefore, the noble gas isotopic signatures of supracrustal rocks were carried into the mantle by the continental deep subduction to subarc depths and then transferred to the postcollisional mafic igneous rocks via the melt-peridotite reaction at the slab-mantle interface in a continental subduction channel. Our finding firstly establishes the slab-mantle interaction model for recycling of supracrustal noble gases in the continental subduction zone.

  12. Separate zones of sulfate and sulfide release from subducted mafic oceanic crust

    NASA Astrophysics Data System (ADS)

    Tomkins, Andrew G.; Evans, Katy A.

    2015-10-01

    Liberation of fluids during subduction of oceanic crust is thought to transfer sulfur into the overlying sub-arc mantle. However, despite the importance of sulfur cycling through magmatic arcs to climate change, magma oxidation and ore formation, there has been little investigation of the metamorphic reactions responsible for sulfur release from subducting slabs. Here, we investigate the relative stability of anhydrite (CaSO4) and pyrite (FeS2) in subducted basaltic oceanic crust, the largest contributor to the subducted sulfur budget, to place constraints on the processes controlling sulfur release. Our analysis of anhydrite stability at high pressures suggests that this mineral should dominantly dissolve into metamorphic fluids released across the transition from blueschist to eclogite facies (∼450-650 °C), disappearing at lower temperatures on colder geothermal trajectories. In contrast, we suggest that sulfur release via conversion of pyrite to pyrrhotite occurs at temperatures above 750 °C. This higher temperature stability is indicated by the preservation of pyrite-bornite inclusions in coesite-bearing eclogites from the Sulu Belt in China, which reached temperatures of at least 750 °C. Thus, sulfur may be released from subducting slabs in two separate pulses; (1) varying proportions of SO2, HSO4- and H2S are released via anhydrite breakdown at the blueschist-eclogite transition, promoting oxidation of remaining silicates in some domains, and (2) H2S is released via pyrite breakdown well into the eclogite facies, which may in some circumstances coincide with slab melting or supercritical liquid generation driven by influx of serpentinite-derived fluids. These results imply that the metallogenic potential in the sub-arc mantle above the subducting slab varies as a function of subduction depth, having the greatest potential above the blueschist-eclogite transition given the association between oxidised magmas and porphyry Cu(-Au-Mo) deposits. We speculate

  13. Pacific slab subduction-induced carbonatite mantle metasomatism in the eastern North China Craton

    NASA Astrophysics Data System (ADS)

    Deng, L. L.; Liu, Y.; Gao, S.

    2014-12-01

    Trace element and Sr-Li isotopic compositions of pyroxene and olivine in mantle peridotite xenoliths entrained by the Cenozoic Changle basalt from the eastern block of the North China Craton (NCC) were analyzed by LA-(MC)-ICPMS. Calcite and carbonatitic melt inclusions occur in clinopyroxene and olivine. Clinopyroxene in these xenoliths generally have a relatively higher Mg# (92.3-93.0) than that of olivine (Mg# = 90.8). Clinopyroxene is characterized by enrichments in LREE and zonations of REE, Li and Sr increasing from the cores to the rims. Furthermore, they have high Ca/Al (8.4-8.7), Zr/Hf (69.9-78.4) and (La/Yb)N (21-36) ratios and low Ti/Eu (618-755) ratios, which are features of mantle peridotite metasomatized by carbonatitic melt. These observations indicate that the peridotitic mantle beneath the Changle area could have suffered a carbonatitic metasomatism. Clinopyroxene, othopyroxene and olivine in these peridotites exhibit variably low δ7Li (+3.9 to -27.2‰) and slightly high 87Sr/86Sr (0.7030-0.7036), suggesting involvement of recycled crustal material. It has been previously suggested that the Mesozoic destruction of the eastern NCC was closely related to the hydration and mantle upwelling associated with Pacific plate subduction. The Cenozoic Changle basalt locates at the eastern margin of the NCC. Thus, we suggest that the carbonatitic metasomatism of the lithopheric mantle beneath the Changle area could have been induced by the westward subduction of the Pacific plate. The subduction-related carbonatitic metasomatism can transform the depleted harzburgite/dunite mantle to clinopyroxne/orthopyroxene-rich mantle, which could have played an important role in the lithospheric thinning of the NCC.

  14. The earthquake cycle in subduction zones

    NASA Technical Reports Server (NTRS)

    Melosh, H. J.; Fleitout, L.

    1982-01-01

    A simplified model of a subduction zone is presented, which incorporates the mechanical asymmetry induced by the subducted slab to anchor the subducting plate during post-seismic rebound and thus throw most of the coseismic stream release into the overthrust plate. The model predicts that the trench moves with respect to the deep mantle toward the subducting plate at a velocity equal to one-half of the convergence rate. A strong extensional pulse is propagated into the overthrust plate shortly after the earthquake, and although this extension changes into compression before the next earthquake in the cycle, the period of strong extension following the earthquake may be responsible for extensional tectonic features in the back-arc region.

  15. Density model of the Cascadia subduction zone

    USGS Publications Warehouse

    Romanyuk, T.V.; Mooney, W.D.; Blakely, R.J.

    2001-01-01

    The main goal of this work is to construct self-consistent density models along two profiles crossing the northern and central Cascadia subduction zone that have been comprehensively studied on the basis of geological, geophysical, etc. data.

  16. Th isotope and U-series studies of subduction-related volcanic rocks

    SciTech Connect

    Gill, J.B.; Williams, R.W. )

    1990-05-01

    The activities of 6-7 radionuclides measured by alpha spectrometry are reported for 35 samples of recent volcanic rocks from 24 volcanoes in 9 arcs. These include continental margins (Cascades, Alaska, Costa Rica), island arcs (the Sunda, Banda, and Sangihe arcs of Indonesia, Marianas, Japan, Aleutians, Antilles, Tonga), and a backarc basin (Lau Basin). Enrichment of {sup 238}U over {sup 230}Th is more common and greater in these subduction-related volcanics than in those from other tectonic environments. {sup 230}Th/{sup 232}Th ratios also extend to higher values, both in absolute numbers and relative to other isotope ratios. Enrichment of {sup 210}Po and {sup 226}Ra over {sup 230}Th is widespread and is more common and greater in island arcs than continental margins. The level of Po or Ra enrichment is similar to that in ocean island and ridge basalts and deceases with differentiation. The differences in Th-U systematics between subduction-related and other volcanic rocks are attributed to variations in the process of melt extraction, changes in bulk partition coefficients within the mantle wedge, or preferential addition of U from subducted lithosphere. Ra enrichments are attributed to partial melting processes which are similar to those at ocean ridges. Smaller excesses at continental margins are attributed either to slower ascent or to differences in the process of melt formation beneath continents.

  17. Andean flat subduction maintained by slab tunneling

    NASA Astrophysics Data System (ADS)

    Schepers, Gerben; van Hinsbergen, Douwe; Kosters, Martha; Boschman, Lydian; McQuarrie, Nadine; Spakman, Wim

    2016-04-01

    In two segments below the Andean mountain belt, the Nazca Plate is currently subducting sub-horizontally below South America over a distance of 200-300 km before the plate bends into the mantle. Such flat slab segments have pronounced effects on orogenesis and magmatism and are widely believed to be caused by the downgoing plate resisting subduction due to its local positive buoyancy. In contrast, here we show that flat slabs primarily result from a local resistance against rollback rather than against subduction. From a kinematic reconstruction of the Andean fold-thrust belt we determine up to ~390 km of shortening since ~50 Ma. During this time the South American Plate moved ~1400 km westward relative to the mantle, thus forcing ~1000 km of trench retreat. Importantly, since the 11-12 Ma onset of flat slab formation, ~1000 km of Nazca Plate subduction occurred, much more than the flat slab lengths, which leads to our main finding that the flat slabs, while being initiated by arrival of buoyant material at the trench, are primarily maintained by locally impeded rollback. We suggest that dynamic support of flat subduction comes from the formation of slab tunnels below segments with the most buoyant material. These tunnels trap mantle material until tearing of the tunnel wall provides an escape route. Fast subduction of this tear is followed by a continuous slab and the process can recur during ongoing rollback of the 7000 km wide Nazca slab at segments with the most buoyant subducting material, explaining the regional and transient character of flat slabs. Our study highlights the importance of studying subduction dynamics in absolute plate motion context.

  18. Heterogeneity in titaniferous lunar basalts

    NASA Technical Reports Server (NTRS)

    Walker, D.; Longhi, J.; Hays, J. F.

    1976-01-01

    Small but real chemical differences exist between subsamples of fine-grained quench-textured titaniferous lunar basalts. The existence of different textural domains with different chemistries is thought to account for most of this variation. In addition to the textural domains, lunar sample 74275 has a population of olivine 'megacrysts' as well as dunite fragments. These materials are thought to be extraneous and to compromise the primary nature of 74275. Recognition of the small chemical variations present may aid in understanding some discrepancies in the experimental-petrology literature. However, these small variations have a distressing petrogenetic significance since they severely limit resolution in recognizing the number and depth of origin of primary magmas.

  19. Subseafloor basalts as fungal habitats

    NASA Astrophysics Data System (ADS)

    Ivarsson, M.

    2012-02-01

    The oceanic crust is believed to host the largest potential habitat for microbial life on Earth, yet, next to nothing is known about this deep, concealed biosphere. Here fossilised fungal colonies in subseafloor basalts are reported from three different seamounts in the Pacific Ocean. The fungal colonies consist of various characteristic structures interpreted as fungal hyphae, fruit bodies and spores. The fungal hyphae are well preserved with morphological characteristics such as hyphal walls, septa, thallic conidiogenesis, and hyphal tips with hyphal vesicles within. The fruit bodies consist of large (~50-200 μm in diameter) body-like structures with a defined outer membrane and an interior filled with calcite. The fruit bodies have at some stage been emptied of their contents of spores and filled by carbonate forming fluids. A few fruit bodies not filled by calcite and with spores still within support this interpretation. Spore-like structures (ranging from a few μm:s to ∼20 μm in diameter) are also observed outside of the fruit bodies and in some cases concentrated to openings in the membrane of the fruit bodies. The hyphae, fruit bodies and spores are all closely associated with a crust lining the vein walls that probably represent a mineralized biofilm. The results support a fungal presence in deep subseafloor basalts and indicate that such habitats were vital between ∼81 and 48 Ma, and probably still is. It is suggested that near future ocean drilling programs prioritize sampling of live species to better understand this concealed biosphere.

  20. Subseafloor basalts as fungal habitats

    NASA Astrophysics Data System (ADS)

    Ivarsson, M.

    2012-09-01

    The oceanic crust is believed to host the largest potential habitat for microbial life on Earth, yet, still we lack substantial information about the abundance, diversity, and consequence of its biosphere. The last two decades have involved major research accomplishments within this field and a change in view of the ocean crust and its potential to harbour life. Here fossilised fungal colonies in subseafloor basalts are reported from three different seamounts in the Pacific Ocean. The fungal colonies consist of various characteristic structures interpreted as fungal hyphae, fruit bodies and spores. The fungal hyphae are well preserved with morphological characteristics such as hyphal walls, septa, thallic conidiogenesis, and hyphal tips with hyphal vesicles within. The fruit bodies consist of large (∼50-200 µm in diameter) body-like structures with a defined outer membrane and an interior filled with calcite. The fruit bodies have at some stage been emptied of their contents of spores and filled by carbonate-forming fluids. A few fruit bodies not filled by calcite and with spores still within support this interpretation. Spore-like structures (ranging from a few µm to ∼20 µm in diameter) are also observed outside of the fruit bodies and in some cases concentrated to openings in the membrane of the fruit bodies. The hyphae, fruit bodies and spores are all closely associated with a crust lining the vein walls that probably represent a mineralized biofilm. The results support a fungal presence in deep subseafloor basalts and indicate that such habitats were vital between ∼81 and 48 Ma.

  1. Interplate coupling at oblique subduction zones: influence on upper plate erosion.

    NASA Astrophysics Data System (ADS)

    Malatesta, Cristina; Gerya, Taras; Crispini, Laura; Federico, Laura; Scambelluri, Marco; Capponi, Giovanni

    2014-05-01

    In active subduction zones, when the converging plates cannot slip freely past each other, "plate coupling" occurs. The moving subducting slab and therefore the coupling/decoupling relationship between plates control both short- and long-term deformation of the upper plate. Short-term deformation is dominantly elastic, occurs at human timescales and can be directly associated with earthquakes. Long-term deformation is cumulative, permanent and prevails at the geological timescale (Hoffman-Rothe et al., 2006, Springer Berlin Heidelberg). Here we used 3D numerical simulations to test oblique subduction zones and to investigate: 1) how long-term deformation and coupling relationship vary along the trench-axis; 2) how this relationship influences erosion and down-drag of upper plate material. Our models are based on thermo-mechanical equations solved with finite differences method and marker-in-cell techniques combined with a multigrid approach (Gerya, 2010, Cambridge Univ. Press). The reference model simulates an intraoceanic subduction close to the continental margin (Malatesta et al., 2013, Nature Communications, 4:2456 DOI:10.1038/ncomms3456). The oceanic crust is layered with a 5-km-thick layer of gabbro overlain by a 3-km-thick layer of basalt. The ocean floor is covered by 1-km-thick sediments. Plates move with a total velocity of 3.15 cm/yr; the oblique convergence is obtained using velocity vectors that form an angle of 45° with the initial starting point of subduction (weak zone in the lithosphere). After initiation of plate convergence, part of sediments on top of the incoming plate enters the subduction zone and is buried; another part is suddenly transferred along strike at shallow depths and along the subducting slab according to the direction of the along-trench velocity component of subduction. The lateral migration of sediment causes the evolution of the trench along its strike from sediment-poor to sediment-rich. As soon as subduction starts, where

  2. From Subduction to Continental Breakup: Neogene and Quaternary Magmatic Evolution of Baja California Peninsula and the Adjacent Gulf of California, Mexico

    NASA Astrophysics Data System (ADS)

    Calmus, T.; Maury, R.; Pallares, C.; Aguillón-Robles, A.; Benoit, M.; Bellon, H.; Royer, J.; Cotten, J.; Bourgois, J.

    2007-05-01

    After the subduction stopped at ca. 12.5 Ma off Baja California Peninsula, calc-alkaline volcanic activity vanished progressively, and the Comondú volcanic arc was replaced by a post-subduction diversified volcanism which derived from at least five distinct magmatic sources located beneath the Baja California Peninsula and the Gulf of California. New field data, 40K-40Ar datings and geochemical and petrological studies of some volcanic fields and isolated volcanic centers, including San Esteban island, allow us to define the following volcanic suites from Late Miocene to Quaternary: 1) magnesian andesites, or "bajaites", 2) adakites, 3) tholeiitic basalts, 4) niobium- rich basalts (NEB), 5) calc-alkaline andesites, and 6) alkali basalts, in order of decreasing volumetric abundance. In the northern part of the Peninsula until the latitude of 29°N, where the oceanic ridge subduction has been documented, volcanism is mainly represented by magnesian andesites and alkali basalts. To the south, alkali basalts have not been reported, but, from Vizcaino peninsula to La Purísima volcanic field, we observe a close association of Late Miocene magnesian andesites, adakites, niobium-rich basalts and tholeiitic basalts. These rocks together indicate the presence of an asthenospheric window, related to the slab tear which developed within the downgoing oceanic plate. Tholeiites are considered as a product of partial melting of the lower plate asthenosphere, whereas adakites likely derive from partial melting of the edges of the slab window, and NEB and magnesian andesites from the melting of the supraslab mantle following its interaction with adakitic liquids. We propose that, after the oceanic accretion ceased off southern Baja California between 12 and 8 Ma, the slab tearing was due to the combination of the capture of the unsubducted remnant of the Magdalena plate by the Pacific plate and of the progressive sinking of the deep subducted oceanic plate. The slab tearing was

  3. Investigating Mechanisms of South American Flat Subduction

    NASA Astrophysics Data System (ADS)

    Hu, J.; Hermosillo, A.; Liu, L.

    2014-12-01

    Flat-slab subduction is a pronounced tectonic phenomenon occurring at 10% of the convergence plate boundaries today. Causes of flat-slab formation remain debated, where proposed mechanisms include subduction of buoyancy anomalies such as oceanic plateaus and aseismic ridges, dynamic suction from thickened overriding plate, and enhanced subduction speed and reduced seafloor ages. South America represents an ideal place to test these hypotheses, with ongoing flat subduction as well as possible flat-slab scenarios during the geological past. Here, we use geodynamic models with plate kinematics and seafloor ages as boundary conditions to reproduce the history of South American subduction since the Late Cretaceous, during which we attempt to investigate the dynamic causes and impacts of flat subduction. The modeling results will be compared to present-day upper mantle slab geometry through slab 1.0 [Hayes et al, 2012] and lower mantle structures in several tomography models including GyPSuM [Simmons et al, 2010] and S20RTS [Ritsema et al. 1999].

  4. An oceanic flood basalt province within the Caribbean plate

    NASA Astrophysics Data System (ADS)

    Sinton, C. W.; Duncan, R. A.; Storey, M.; Lewis, J.; Estrada, J. J.

    1998-02-01

    The thick oceanic crust of the Caribbean plate appears to be the tectonized remnant of an eastern Pacific oceanic plateau that has been inserted between North and South America. The emplacement of the plateau into its present position has resulted in the obduction and exposure of its margins, providing an opportunity to study the age relations, internal structure and compositional features of the plateau. We present the results of 40Ar- 39Ar radiometric dating, major-, trace-element, and isotopic compositions of basalts from some of the exposed sections as well as drill core basalt samples from Leg 15 of the Deep Sea Drilling Project. Five widely spaced, margin sections yielded ages ranging from 91 to 88 Ma. Less well-constrained radiometric ages from the drill cores, combined with the biostratigraphic age of surrounding sediments indicate a minimum crystallization age of ˜90 Ma in the Venezuelan Basin. The synchroneity of ages across the region is consistent with a flood basalt origin for the bulk of the Caribbean plateau (i.e., large volume, rapidly erupted, regionally extensive volcanism). The ages and compositions are also consistent with plate reconstructions that place the Caribbean plateau in the vicinity of the Galápagos hotspot at its inception. The trace-element and isotopic compositions of the ˜90 Ma rocks indicate a depleted mantle and an enriched, plume-like mantle were involved in melting to varying degrees across the plateau. Within the same region, a volumetrically secondary, but widespread magmatic event occurred at 76 Ma, as is evident in Curaçao, western Colombia, Haiti, and at DSDP Site 152/ODP Site 1001 near the Hess Escarpment. Limited trace-element data indicate that this phase of magmatism was generally more depleted than the first. We speculate that magmatism may have resulted from upwelling of mantle, still hot from the 90 Ma event, during lithospheric extension attending gravitational collapse of the plateau, and/or tectonic

  5. An oceanic flood basalt province within the Caribbean plate

    NASA Astrophysics Data System (ADS)

    Estrada, J. J.; Lewis, J.; Storey, M.; Duncan, R. A.; Sinton, C. W.

    1998-02-01

    The thick oceanic crust of the Caribbean plate appears to be the tectonized remnant of an eastern Pacific oceanic plateau that has been inserted between North and South America. The emplacement of the plateau into its present position has resulted in the obduction and exposure of its margins, providing an opportunity to study the age relations, internal structure and compositional features of the plateau. We present the results of 40Ar-39Ar radiometric dating, major-, trace-element, and isotopic compositions of basalts from some of the exposed sections as well as drill core basalt samples from Leg 15 of the Deep Sea Drilling Project. Five widely spaced, margin sections yielded ages ranging from 91 to 88 Ma. Less well-constrained radiometric ages from the drill cores, combined with the biostratigraphic age of surrounding sediments indicate a minimum crystallization age of ~90 Ma in the Venezuelan Basin. The synchroneity of ages across the region is consistent with a flood basalt origin for the bulk of the Caribbean plateau (i.e., large volume, rapidly erupted, regionally extensive volcanism). The ages and compositions are also consistent with plate reconstructions that place the Caribbean plateau in the vicinity of the Galápagos hotspot at its inception. The trace-element and isotopic compositions of the ~90 Ma rocks indicate a depleted mantle and an enriched, plume-like mantle were involved in melting to varying degrees across the plateau. Within the same region, a volumetrically secondary, but widespread magmatic event occurred at 76 Ma, as is evident in Curaçao, western Colombia, Haiti, and at DSDP Site 152/ODP Site 1001 near the Hess Escarpment. Limited trace-element data indicate that this phase of magmatism was generally more depleted than the first. We speculate that magmatism may have resulted from upwelling of mantle, still hot from the 90 Ma event, during lithospheric extension attending gravitational collapse of the plateau, and/or tectonic emplacement

  6. Origin of magmas in subduction zones: a review of experimental studies

    PubMed Central

    Kushiro, Ikuo

    2007-01-01

    Studies of the origin of magmas in subduction zones, particularly in the Japanese island arc, have been significantly advanced by petrological, geochemical, geophysical and experimental studies during last 50 years. Kuno’s original model1) for magma generation in the Japanese island arc, that tholeiite magmas are formed at relatively shallow levels in the mantle on the Pacific Ocean side whereas alkali basalt magmas are formed in deeper levels on the Japan Sea side, stimulated subsequent studies, particularly high-pressure experimental studies in which the author participated. Recent seismic tomographic studies of regions beneath the Japanese island arc demonstrate that seismic low-velocity zones where primary magmas are formed have finger-like shapes and rise obliquely from the Japan Sea side toward the Pacific Ocean side. Based on experimental studies, it is suggested that the compositions of primary magmas depend mainly on the H2O content and degree of melting in the melting zones, and that primary tholeiite magmas are formed by 10–25% melting of the source mantle containing less than 0.2 wt.% H2O. High-alumina basalt and alkali basalt magmas are formed by smaller degrees of melting of similar mantle, whereas primary boninite magmas are formed by more than 20% melting of the source mantle with more than 0.2 wt.% H2O, and finally, high-magnesia andesite magmas are formed by smaller degrees of melting of similar mantle. PMID:24019580

  7. Origin of magmas in subduction zones: a review of experimental studies.

    PubMed

    Kushiro, Ikuo

    2007-02-01

    Studies of the origin of magmas in subduction zones, particularly in the Japanese island arc, have been significantly advanced by petrological, geochemical, geophysical and experimental studies during last 50 years. Kuno's original model(1)) for magma generation in the Japanese island arc, that tholeiite magmas are formed at relatively shallow levels in the mantle on the Pacific Ocean side whereas alkali basalt magmas are formed in deeper levels on the Japan Sea side, stimulated subsequent studies, particularly high-pressure experimental studies in which the author participated. Recent seismic tomographic studies of regions beneath the Japanese island arc demonstrate that seismic low-velocity zones where primary magmas are formed have finger-like shapes and rise obliquely from the Japan Sea side toward the Pacific Ocean side. Based on experimental studies, it is suggested that the compositions of primary magmas depend mainly on the H2O content and degree of melting in the melting zones, and that primary tholeiite magmas are formed by 10-25% melting of the source mantle containing less than 0.2 wt.% H2O. High-alumina basalt and alkali basalt magmas are formed by smaller degrees of melting of similar mantle, whereas primary boninite magmas are formed by more than 20% melting of the source mantle with more than 0.2 wt.% H2O, and finally, high-magnesia andesite magmas are formed by smaller degrees of melting of similar mantle. PMID:24019580

  8. Petrochemistry and genesis of olivine basalts from small monogenetic parasitic cones of Bazman stratovolcano, Makran arc, southeastern Iran

    NASA Astrophysics Data System (ADS)

    Saadat, Saeed; Stern, Charles R.

    2011-07-01

    Small monogenetic Quaternary parasitic cones around Bazman stratovolcano, located at the western edge of the Makran arc, southeastern Iran, erupted low-Ti subalkaline olivine basalts with MgO (3.8-8.6 wt.%) and Al2O3 (16.5-18.6 wt.%). Positive correlation of decreasing MgO, Ni and Cr indicates that formation of low MgO basalts involved limited crystal-liquid fractionation of olivine and clinopyroxene, the common phenocrysts. The basalts have variable 87Sr/86Sr (0.704177-0.705139) and 143Nd/144Nd (0.512689-0.512830) ratios, within the range of OIB-like intra-plate alkaline basalts erupted in eastern Iran north of the Makran arc. This, and the lack of correlation between Sr content and Sr-isotopic ratio, suggest that upper crustal contamination was not significant in their formation, consistent with the relatively thin crust (≤ 40 km) in the area. Enrichment of large-ion-lithophile elements (LILE) relative to light rare-earth-elements (LREE; Ba/La = 9-25), and depletions in Nb relatively to LILE (Ba/Nb = 12-35; La/Nb = 0.8-2.1), are similar in most cases to other convergent plate boundary arc basalts, suggesting that the Bazman basalts formed by melting of subcontinental mantle modified by dehydration of subducted Oman Sea oceanic lithosphere. Pb isotopic ratios of the basalts define a linear trend above the Northern Hemisphere Line, consistent with their derivation from mantle contaminated by Pb derived from subducted sediment. Trace element contents and ratios (LaN = 10-25; YbN = 3-6; (La/Yb)N = 3-8) suggest that these basalts formed as a result of low (~ 10%) degrees of partial melting of subarc mantle modified only moderately by subducted components. Relatively low Ba/Nb < 15, La/Nb < 1.5 and Ba/La < 15 ratios for some basalts confirm only limited contamination of the source of these samples, consistent with observations in other arcs that parasitic cones tap sources less affected by slab-derived fluids than the larger stratovolcanoes they surround. Comparison

  9. Subduction erosion processes with application to southern Mexico

    NASA Astrophysics Data System (ADS)

    Keppie, Duncan Fraser

    Finite-element numerical models of ocean-continent subduction are used to investigate the roles of crustal frictional strength, subduction angle, and convergence rate in subduction erosion processes. These models exhibit two distinct modes of subduction erosion: (1) slow and steady, removing small blocks of material continually, and (2) fast and non-steady, removing a large forearc block in a single event. The slow mode, called edge-weakening subduction erosion, is enhanced by steeper subduction angles but acts to shallow the subduction angle at crustal depths. The fast mode, called internal-weakening subduction erosion, is enhanced by shallow subduction angles but acts to steepen the subduction angle at crustal depths. The two modes may alternate cyclically in nature and may account, in part, for the variation in subduction angle observed at the modern western American subduction zones. The slow, edge-weakening subduction erosion mode correlates well to subduction erosion processes widely reported for natural subduction zones. The fast, internal-weakening subduction erosion mode has previously been described only for subduction zones involving continental lithosphere on the lower plate. The removal of a 150--250 km wide forearc block from southern Mexico between 27--25 Ma and 21--19 Ma may be a first type example of internal-weakening subduction erosion at an ocean-continent subduction zone. The numerical models showing internal-weakening subduction erosion and the geological record of southern Mexico share the following geological features synchronous with forearc removal: (1) rapid trench migration rates approaching orthogonal plate convergence rates, (2) a step-wise shift in the locus of arc magmatism towards the upper plate, (3) forearc subsidence at the new margin of the upper plate, (4) a zone of crustal unroofing within the upper plate's new forearc region, and (5) a zone of subduction-antithetic thrust-sense shearing inboard of the crustal unroofing. These

  10. Mass-dependent molybdenum isotopes in mid-ocean ridge basalts: A new mantle reference

    NASA Astrophysics Data System (ADS)

    Hibbert, K.; Freymuth, H.; Willbold, M.; Elliott, T.

    2013-12-01

    Molybdenum isotopes have been proposed as a novel tracer for subduction components in arc magmas as well as for recycled crustal components in the source of ocean island basalts. In order to investigate these hypotheses, it is important to establish a reference value for the molybdenum isotope composition of the mantle, which has so far been only poorly constrained and based on continental material and volcanic rocks with large analytical errors [1]. Analysis of samples of basalts from the Mariana arc shows that samples are enriched in Mo relative to Pr, an element with a similar degree of incompatibility during mantle melting. This enrichment correlates with δ98Mo such that the heaviest samples (~+0.16‰ relative to NIST SRM 3134) also have the highest Mo/Pr. The resulting array is interpreted as a result of fluid enrichment of a presumed mantle composition, with isotopically heavy fluids derived from fluid-solid fractionation during slab dehydration [2]. This scenario further implies that the deep recycled, subduction zone processed crust should be isotopically light. There is evidence for this in the Mo isotopic composition of some ocean island basalts (OIBs). Notably, basalts from La Palma, which have radiogenic Pb and Os isotopic compositions proposed to result from a component of recycled mafic oceanic crust, have isotopically light Mo (δ98Mo -0.24 to -0.49‰). Implicit in this apparently self-consistent model is that MORB has a δ98Mo intermediate between the isotopically heavy, fluid-rich arc lavas of the Marianas and the isotopically light basalts of La Palma. The low [Mo] of MORB, coupled with the potential for Fe-Mn coatings to perturb δ98Mo to lower values, has made this a more challenging quest. Here we present new data to constrain this datum for the convecting upper mantle. We have processed up to ~1g of carefully handpicked glasses and obtain values of ~-0.15 to -0.25‰, entirely in keeping with the scenario outlined above. Analysis of

  11. Subducting Seamounts and the Rupturing Process of Great Subduction Zone Earthquakes

    NASA Astrophysics Data System (ADS)

    Das, S.

    2009-05-01

    It was suggested in the 1970's that subducting ocean floor features may delimit the along-strike rupture lengths of large subduction zone earthquakes. With the dramatic improvement in data quality, both for seismic and ocean floor bathmetry data, we can now see how the actual rupturing process of great earthquakes is also influenced by such subducting features. Here we present three great (Mw > 8) subduction zone earthquakes, in very different parts of the world, for which a relation between the ocean floor and the earthquake source process is seen. These include the 1986 Andreanof Islands, Alaska and the 1996 Biak, Indonesia earthquakes, in which the regions of large slip concentrate in patches, reminiscent of the "asperity model" of earthquakes, and appear to be related to subducted seamounts. For the 2001 Peru earthquake, a subducting fracture zone, with its associated bathymetric peak and trough, seems to have been the cause of the rupture being stalled for ~30s, before producing an earthquake of Mw 8.4, the third largest earthquake worldwide since 1965. Similarities and differences in the earthquake rupturing properties for these two different types of subducting features will be discussed. An outstanding question is what controls whether a seamount obducts or subducts.

  12. The Mineralogy of the Youngest Lunar Basalts

    NASA Astrophysics Data System (ADS)

    Staid, M. I.; Pieters, C. M.

    1999-01-01

    The last stage of lunar volcanism produced spectrally distinct basalts on the western nearside of the Moon, which remain unsampled by landing missions. The spectral properties of these late-stage basalts are examined using high-spatial-resolution Clementine images to constrain their mineralogic composition. The young high-Ti basalts in the western Procellarum and Imbrium Basins display a significantly stronger ferrous absorption than earlier mare basalts, suggesting that they may be the most Fe-rich deposits on the Moon. The distinct long-wavelength shape of this ferrous absorption is found to be similar for surface soils and materials excavated from depth. The pervasive character of this absorption feature supports the interpretation of abundant olivine within these late-stage lunar deposits. Important distinctions exist between the early-stage eastern maria and the late-stage western basalts, even though both appear to be Ti-rich. For example, the western maria are more radiogenic than eastern deposits. Telescopic spectra of the high-Ti western maria also exhibit a unique combination of a strong 1 micron feature and a relatively weak or attenuated 2-micron absorption. Pieters et al. concluded that the unusual strength and shape of the 1-micron absorption in western basalts results from an additional absorption from abundant olivine and/or Fe-bearing glass. Either mineralogy could produce the strong long wavelength 1-micron band, but a glassy Fe-rich surface could only form by rapid cooling along the exterior surfaces of flows. Clementine UV-VIS data of late-stage basalts are examined for regions in Oceanus Procellarum and Mare Imbrium. The spectral properties of western regions are compared to the sampled Apollo 11 basalts in Mare Tranquillitatis, which contain similar albedos and UV-VIS spectral properties. For reference, the western basalts are also compared to the low-Ti and Fe-rich basalts in Mare Serenitatis (mISP). Serenitatis basalts have the strongest

  13. Mare basalts - Crystal chemistry, mineralogy, and petrology

    NASA Technical Reports Server (NTRS)

    Papike, J. J.; Hodges, F. N.; Bence, A. E.; Cameron, M.; Rhodes, J. M.

    1976-01-01

    The paper attempts a synthesis of the major-element chemistry, petrography, mineral chemistry, and crystal chemistry of the mare basalts returned by Apollo and Luna missions. A classification of the mare basalts based on major-element chemistry is given, and textural sequences within each major-element group are identified. The mineral chemistry and crystal chemistry of each mineral group are considered within the framework of the major-element groups and the textural sequences. The various classes of models for the origin of the mare basalts and the nature of their source regions are discussed in the context of the major- and trace-element chemistries and experimental investigations.

  14. Petrogenesis and evolution of Quaternary basaltic rocks from the Wulanhada area, North China

    NASA Astrophysics Data System (ADS)

    Fan, Qi-Cheng; Chen, Sheng-Sheng; Zhao, Yong-Wei; Zou, Hai-Bo; Li, Ni; Sui, Jian-Li

    2014-10-01

    The origin of alkali basalts in eastern China has been the subject of considerable debate. Here we focus on the Wulanhada basalts located in the western block of North China Craton to provide new insights into recent deep mantle dynamics. The Wulanhada volcanic group has 30 volcanic cones with variable volumes, consisting of scoria cone (cinder cone + spatter cone) and lava. The Wulanhada volcanoes exhibit Strombolian eruption activities during late Pleistocene epoch and Holocene. The Wulanhada basalts are strongly alkaline rocks (tephrite). According to the characteristics of trace elements and Sr-Nd-Pb-Hf isotopic compositions, the Wulanhada magmas were mainly derived from garnet-bearing peridotite within the asthenosphere and underwent fractional crystallization of olivine and clinopyroxene without significant crustal contamination. Their elevated values of Na, Al, Sr/Sm, Sm/Hf, Zr/Hf, and Nb/Ta, positive Ba, K, Pb, and Sr anomalies and negative Zr, Hf anomalies, combined with a negative correlation between 176Hf/177Hf and 143Nd/144Nd and relatively low 87Sr/86Sr, suggest that the magma source may be a mixture of garnet peridotites and carbonated melts. The presence of carbonated melts is likely associated with the sediments or fluids carried by the subducted or stagnant Pacific Plate.

  15. Oceanic asthenosphere subduction and its geological implications

    NASA Astrophysics Data System (ADS)

    Zhou, Q.; Liu, L.

    2014-12-01

    We investigate the evolution of oceanic asthenosphere during subduction by exploring various scenarios including plate kinematics and plausible values of asthenosphere viscosity and density. We find that the oceanic asthenosphere will always subduct with the down-going slab as long as its average viscosity value is no smaller than 1×1018 Pa s. In order to allow slabs to subduct into the deep upper mantle, a maximum oceanic asthenosphere density reduction relative to the underlying mantle should be no larger than 0.7%, assuming a 200-km-thick asthenosphere channel. We find that a significant portion of the asthenosphere buoyancy should result from its excess temperature from the long-term thermal evolution of mantle convection. Our results are in contrast to an earlier suggestion that negligible amount (<30 km thick) of asthenosphere could get subducted, which is likely due to over-simplicity of subduction geometry and model boundary conditions. The recycling of a weak and hot asthenosphere provides a novel mechanism for the formation of slow seismic anomalies within the deep mantle. This, in turn, questions the commonly believed deep mantle plume origin of intra-plate volcanism, with a typical example being the Yellowstone volcanic system. Our current results suggest that a buoyant asthenosphere can be dragged down into the lower mantle and then moves upward due to its buoyancy when the overlying slab barrier is removed. To further test our hypothesis, we construct a 4D subduction model for western North America during the Cenozoic. We use data assimilation techniques to incorporate plate kinematics and sea floor ages as boundary conditions, and seismic anomalies converted density structure as internal buoyancy source. The subduction history is calibrated through a hybrid of forward and adjoint simulations satisfying multiple observational constraints. Some preliminary results will be presented.

  16. Anaglyph: Basalt Cliffs, Patagonia, Argentina

    NASA Technical Reports Server (NTRS)

    2000-01-01

    Basalt cliffs along the northwest edge of the Meseta de Somuncura plateau near Sierra Colorada, Argentina show an unusual and striking pattern of erosion. Stereoscopic observation helps to clarify the landform changing processes active here. Many of the cliffs appear to be rock staircases that have the same color as the plateau's basaltic cap rock. Are these the edges of lower layers in the basalt or are they a train of slivers that are breaking off from, then sliding downslope and away from, the cap rock. They appear to be the latter. Close inspection shows that each stair step is too laterally irregular to be a continuous sheet of bedrock like the cap rock. Also, the steps are not flat but instead are little ridges, as one might expect from broken, tilted, and sliding slices of the cap rock. Stream erosion has cut some gullies into the cliffs and vegetation (appears bright in this infrared image) shows that water springs from and flows down some channels, but land sliding is clearly a major agent of erosion here.

    This anaglyph was generated by first draping a Landsat Thematic Mapper image over a topographic map from the Shuttle Radar Topography Mission, then producing the two differing perspectives, one for each eye. When viewed through special glasses, the result is a vertically exaggerated view of the Earth's surface in its full three dimensions. Anaglyph glasses cover the left eye with a red filter and the right eye with a blue filter.

    Landsat satellites have provided visible light and infrared images of the Earth continuously since 1972. SRTM topographic data match the 30-meter (99-foot) spatial resolution of most Landsat images and provide a valuable complement for studying the historic and growing Landsat data archive. The Landsat 7 Thematic Mapper image used here was provided to the SRTM project by the United States Geological Survey, Earth Resources Observation Systems (EROS) Data Center,Sioux Falls, South Dakota.

    Elevation data used in this

  17. Trace element signature of subduction-zone fluids, melts and supercritical liquids at 120-180 km depth.

    PubMed

    Kessel, Ronit; Schmidt, Max W; Ulmer, Peter; Pettke, Thomas

    2005-09-29

    Fluids and melts liberated from subducting oceanic crust recycle lithophile elements back into the mantle wedge, facilitate melting and ultimately lead to prolific subduction-zone arc volcanism. The nature and composition of the mobile phases generated in the subducting slab at high pressures have, however, remained largely unknown. Here we report direct LA-ICPMS measurements of the composition of fluids and melts equilibrated with a basaltic eclogite at pressures equivalent to depths in the Earth of 120-180 km and temperatures of 700-1,200 degrees C. The resultant liquid/mineral partition coefficients constrain the recycling rates of key elements. The dichotomy of dehydration versus melting at 120 km depth is expressed through contrasting behaviour of many trace elements (U/Th, Sr, Ba, Be and the light rare-earth elements). At pressures equivalent to 180 km depth, however, a supercritical liquid with melt-like solubilities for the investigated trace elements is observed, even at low temperatures. This mobilizes most of the key trace elements (except the heavy rare-earth elements, Y and Sc) and thus limits fluid-phase transfer of geochemical signatures in subduction zones to pressures less than 6 GPa. PMID:16193050

  18. Effect of Fluorine on Near-Liquidus Phase Equilibria of Basalts

    NASA Technical Reports Server (NTRS)

    Filiberto, Justin; Wood, Justin; Loan, Le; Dasgupta, Rajdeep; Shimizu, Nobumichi; Treiman, Allan H.

    2010-01-01

    Volatile species such as H2O, CO2, F, and Cl have significant impact in generation and differentiation of basaltic melts. Thus far experimental work has primarily focused on the effect of water and carbon dioxide on basalt crystallization, liquid-line of descent, and mantle melting [e.g., 1, 2] and the effects of halogens have received far less attention [3-4]. However, melts in the planetary interiors can have non-negligible chlorine and fluorine concentrations. Here, we explore the effects of fluorine on near-liquidus phase equilibria of basalt. We have conducted nominally anhydrous piston cylinder experiments using graphite capsules at 0.6 - 1.5 GPa on an Fe-rich model basalt composition. 1.75 wt% fluorine was added to the starting mix in the form of AgF2. Fluorine in the experimental glass was measured by SIMS and major elements of glass and minerals were analyzed by EPMA. Nominally volatile free experiments yield a liquidus temperature from 1330 C at 0.8GPa to 1400 at 1.6GPa and an olivine(Fo72)-pyroxene(En68)-liquid multiple saturation point at 1.25 GPa and 1375 C. The F-bearing experiments yield a liquiudus temperature from 1260 C at 0.6GPa to 1305 at 1.5GPa and an ol(Fo66)-pyx(En64)-MSP at 1 GPa and 1260 C. This shows that F depresses the basalt liquidus, extends the pyroxene stability field to lower pressure, and forces the liquidus phases to be more Fe-rich. KD(Fe-Mg/mineral-melt) calculated for both pyroxenes and olivines show an increase with increasing F content of the melt. Therefore, we infer that F complexes with Mg in the melt and thus increases the melt s silica activity, depressing the liquidus and changing the composition of the crystallizing minerals. Our study demonstrates that on a weight percent basis, the effect of fluorine is similar to the effect of H2O [1] and Cl [3] on freezing point depression of basalts. But on an atomic fraction basis, the effect of F on liquidus depression of basalts is xxxx compared to the effect of H. Future

  19. Three-dimensional numerical modeling of temperature and mantle flow fields associated with subduction of the Philippine Sea plate, southwest Japan

    NASA Astrophysics Data System (ADS)

    Ji, Yingfeng; Yoshioka, Shoichi; Matsumoto, Takumi

    2016-06-01

    We investigated temperature and mantle flow distributions associated with subduction of the Philippine Sea (PHS) plate beneath southwest Japan, by constructing a three-dimensional parallelepiped model incorporating a past clockwise rotation, the bathymetry of the Philippine Sea plate, and distribution of the subducting velocity within its slab. The geometry of the subducting plate was inferred from contemporary seismic studies and was used as a slab guide integrated with historical plate rotation into the 3-D simulation. Using the model, we estimated a realistic and high-resolution temperature field on the subduction plate interface, which was constrained by a large number of heat flow data, and attempted to clarify its relationship with occurrences of megathrust earthquakes, long-term slow slip events (L-SSEs), and nonvolcanic low-frequency earthquakes (LFEs). Results showed that the oblique subduction coupled with the 3-D geometry of subducting PHS plate was a key factor affecting the interplate and intraplate temperature distributions, leading to a cold anomaly in the plate interface beneath western Shikoku, the Bungo Channel, and the Kii Peninsula. Temperatures in the slab core in these regions at a depth near the continental Moho were nearly 200°C lower than that in eastern Shikoku, indicating a high thermal lateral heterogeneity within the subducting plate. The geothermal control of the LFEs beneath western Shikoku was estimated to be within a range from 400 to 700°C, and the interplate temperature for the L-SSEs with a slip larger than 15 cm beneath the Bungo Channel was estimated to be approximately 350-500°C. A large horizontal temperature gradient of 2.5 ~ °C/km was present where the LFEs occurred repeatedly. The steep temperature change was likely to be related to the metamorphic phase transformation from lawsonite or blueschist to amphibolite of hydrous minerals of the mid-ocean ridge basalt of the subducting PHS plate.

  20. Stability of Basalt plus Anhydrite plus Calcite at HP-HT: Implications for Venus, the Earth and Mars

    NASA Technical Reports Server (NTRS)

    Martin, A. M.; Righter, K.; Treiman, A. H.

    2010-01-01

    "Canali" observed at Venus surface by Magellan are evidence for very long melt flows, but their composition and origin remain uncertain. The hypothesis of water-rich flow is not reasonable regarding the temperature at Venus surface. The length of these channels could not be explained by a silicate melt composition but more likely, by a carbonate-sulfate melt which has a much lower viscosity (Kargel et al 1994). One hypothesis is that calcite CaCO3 and anhydrite CaSO4 which are alteration products of basalts melted during meteorite impacts. A famous example recorded on the Earth (Chicxulub) produced melt and gas rich in carbon and sulfur. Calcite and sulfate evaporites are also present on Mars surface, associated with basalts. An impact on these materials might release C- and S-rich melt or fluid. Another type of planetary phenomenon (affecting only the Earth) might provoke a high pressure destabilization of basalt+anhydrite+calcite. Very high contents of C and S are measured in some Earth s magmas, either dissolved or in the form of crystals (Luhr 2008). As shown by the high H content and high fO2 of primary igneous anhydrite-bearing lavas, the high S content in their source may be explained by subduction of an anhydrite-bearing oceanic crust, either directly (by melting followed by eruption) or indirectly (by release of S-rich melt or fluid that metasomatize the mantle) . Calcite is a major product of oceanic sedimentation and alteration of the crust. Therefore, sulfate- and calcite-rich material may be subducted to high pressures and high temperatures (HP-HT) and release S- and C-rich melts or fluids which could influence the composition of subduction zone lavas or gases. Both phenomena - meteorite impact and subduction - imply HP-HT conditions - although the P-T-time paths are different. Some HP experimental/theoretical studies have been performed on basalt/eclogite, calcite and anhydrite separately or on a combination of two. In this study we performed piston

  1. Basaltic Crater in Color IR

    NASA Technical Reports Server (NTRS)

    2004-01-01

    [figure removed for brevity, see original site]

    Released August 6, 2004 This image shows two representations of the same infra-red image near Nili Fosse in the the Isidis region of Mars. On the left is a grayscale image showing surface temperature, and on the right is a false-color composite made from 3 individual THEMIS bands. The false-color image is colorized using a technique called decorrelation stretch (DCS), which emphasizes the spectral differences between the bands to highlight compositional variations. In many cases craters trap sand in their topographic depressions, interrupting the sand's migration across the Martian surface. This image is particularly interesting because there appears to be more than 1 type of sand in the bottom of this crater and in the hummocky terrain near the bottom of the image. The pink/magenta areas are characteristic of a basaltic composition, but there are also orange areas that are likely caused by the presence of andesite. These two compositions, basalt and andesite, are some of the most common found on Mars.

    Image information: IR instrument. Latitude 24, Longitude 80.7 East (297.3 West). 100 meter/pixel resolution.

    Note: this THEMIS visual image has not been radiometrically nor geometrically calibrated for this preliminary release. An empirical correction has been performed to remove instrumental effects. A linear shift has been applied in the cross-track and down-track direction to approximate spacecraft and planetary motion. Fully calibrated and geometrically projected images will be released through the Planetary Data System in accordance with Project policies at a later time.

    NASA's Jet Propulsion Laboratory manages the 2001 Mars Odyssey mission for NASA's Office of Space Science, Washington, D.C. The Thermal Emission Imaging System (THEMIS) was developed by Arizona State University, Tempe, in collaboration with Raytheon Santa Barbara Remote Sensing. The THEMIS investigation is led by Dr. Philip

  2. Apollo 17 KREEPy basalt - A rock type intermediate between mare and KREEP basalts

    NASA Technical Reports Server (NTRS)

    Ryder, G.; Stoeser, D. B.; Wood, J. A.

    1977-01-01

    The Apollo 17 KREEPy basalt is a unique lunar volcanic rock, observed only as clasts in the light friable breccia matrix (72275) of Boulder 1, Station 2 at Taurus-Littrow. Its status as a volcanic rock is confirmed by the absence of any meteoritic contamination, a lack of cognate inclusions or xenocrystal material, and low Ni contents in metal grains. The basalt was extruded 4.01 + or - 0.04 b.y. ago, approximately contemporaneously with the high-alumina mare basalts at Fra Mauro; shortly afterwards it was disrupted, probably by the Serenitatis impact, and its fragments emplaced in the South Massif. The basalt, which is quartz-normative and aluminous, is chemically and mineralogically intermediate between the Apollo 15 KREEP basalts and the high-alumina mare basalts in most respects. It consists mainly of plagioclase and pigeonitic pyroxene in approximately equal amounts, and 10-30% of mesostatis.

  3. Experimental research on continuous basalt fiber and basalt-fibers-reinforced polymers

    NASA Astrophysics Data System (ADS)

    Zhang, Xueyi; Zou, Guangping; Shen, Zhiqiang

    2008-11-01

    The interest for continuous basalt fibers and reinforced polymers has recently grown because of its low price and rich natural resource. Basalt fiber was one type of high performance inorganic fibers which were made from natural basalt by the method of melt extraction. This paper discusses basic mechanical properties of basalt fiber. The other work in this paper was to conduct tensile testing of continuous basalt fiber-reinforced polymer rod. Tensile strength and stress-strain curve were obtained in this testing. The strength of rod was fairly equal to rod of E-glass fibers and weaker than rod of carbon fibers. Surface of crack of rod was studied. An investigation of fracture mechanism between matrix and fiber was analyzed by SEM (Scanning electron microscopy) method. A poor adhesion between the matrix and fibers was also shown for composites analyzing SEM photos. The promising tensile properties of the presented basalt fibers composites have shown their great potential as alternative classical composites.

  4. Carbonate-Iron Interaction: Kinetic Approach for Carbonate Subduction Down to the Mantle Transition Zone

    NASA Astrophysics Data System (ADS)

    Martirosyan, N.; Yoshino, T.; Shatskiy, A.; Litasov, K. D.; Chanyshev, A.

    2015-12-01

    One of the principal redox exchange reactions at the slab-mantle interface during subduction is CaCO3-Fe0 interaction. Using uniaxial press multianvil apparatus, we investigate this reaction at temperatures from 650 to 1400 °C, and pressures from 4 to 16 GPa. These conditions are representative for the PT path of the slab from the surface to the mantle transition zone. Based on our results, redox reaction can be written as: 3 CaCO3 (aragonite) + 13 Fe0 (metal) = Fe7C3 (carbide) + 3 CaFe2O3 (Ca-wüstite). It is a diffusion-controlled process with the reaction rate constant (k) being log-linear in 1/T. The experimental results allow to calculate the length scale over which the reaction kinetics between aragonite and metallic iron is likely effective. Using relatively simple relationship between the characteristic distance of diffusion as a function of k and duration of the process, we estimate the length scale of the reaction for time scales 4-16 Myr, which corresponds to subduction rates of 2-8 cm/year from 250 km (metal-saturation boundary) to 470 km, for different slab PT-profiles. Assuming that carbonates are in a direct contact with iron, the maximum degree of carbonate reduction can be evaluated using the results of our study and the data on CO2 distribution in altered oceanic basalts. The estimates suggest that up to 0.5, 12, and 20 vol. % of carbonates can be reduced in such case during subduction down to the mantle transition zone at the conditions of cold, medium and hot geotherms, respectively. The sluggish kinetics of established CaCO3-Fe0 interaction suggests that even over the entire history of the Earth, carbonates could survive during subduction from metal saturation boundary near 250 km depth, down to the transition zone, and presumably to the lower mantle.

  5. Melt Inclusion Evidence for Subduction-modified Mantle Beneath the Woodlark Spreading Center, Solomon Islands

    NASA Astrophysics Data System (ADS)

    Chadwick, J.; Turner, A.; Collins, E.

    2015-12-01

    The Woodlark Spreading Center (WSC) to the east of Papua New Guinea separates the Indo-Australian plate and Solomon Sea microplate. At its eastern terminus, the WSC is being subducted at the New Britain trench, forming a triple junction near the New Georgia Group arc in the Solomon Islands. Previous studies have shown that lavas recovered from greater than 100 km from the trench on the WSC are N-MORB, but closer to the trench they have arc-like Sr-Nd-Pb isotopic ratios, enrichments in LILE, and depletions in HFSE. In the complex triple junction area of the WSC on the Simbo and Ghizo Ridges, island arc tholeiites to medium-K calc-alkaline andesites and dacites have been recovered, many with trace element and isotopic characteristics that are similar to the true arc lavas in the New Georgia Group on the other side of the trench. We suggest that subduction-modified arc mantle migrates through slab windows created by the subduction of the WSC as the plates continue to diverge after subduction. This transfer of mantle across the plate boundary leads to variable mixing between arc and N-MORB end-members, forming the hybrid to arc-like lavas recovered on the WSC. To test this hypothesis and to characterize the end-member compositions, we have analyzed melt inclusions in olivine, pyroxene, and plagioclase phenocrysts in Simbo and Ghizo Ridge lava samples. Major elements were analyzed using the electron microprobe facility at Fayetteville State University and volatiles were analyzed on the ion probe facility at Woods Hole Oceanographic Institution. The melt inclusions show a wide diversity of magmas from basalts to dacites, and mixing modeling shows that most Woodlark Spreading Center lava compositions are explained by mixing between the most extreme mafic (MORB) and felsic (arc) inclusion compositions.

  6. Episodic Entrainment of Subducted Oceanic Crust into Primordial Reservoirs of the Lower Mantle

    NASA Astrophysics Data System (ADS)

    Li, M.; McNamara, A. K.; Garnero, E. J.

    2012-12-01

    The origin and dynamic nature of proposed large-scale compositional heterogeneity in Earth's lowermost mantle is not well understood. The preservation of primordial dense materials in the lowermost mantle has long been proposed by both geochemical and geophysical studies. As is shown in geodynamic models, primordial reservoirs can be passively swept to upwelling regions by mantle flow and form stable piles. In addition, oceanic crust can be subducted into the lowermost mantle, providing an additional source of compositional heterogeneity. We performed high resolution 2D calculations to study the interaction between primordial reservoirs and subducted oceanic crust that is compositionally different from the reservoirs, and its implications for deep mantle structure, dynamics, and chemical evolution. We find that oceanic crust subducted to the lowermost mantle is viscously dragged toward upwelling regions, where primordial reservoirs are hypothesized to exist. While most oceanic crust avoids interaction with a reservoir itself and is entrained into thermal plumes that form along the tops of reservoirs, some crust accumulates at the base of the plumes, along reservoir tops. Eventually the volume of accumulated crust at the base of a plume gains enough negative buoyancy such that it is flushed into the reservoir below, through the reservoir's top margin. Crust that is flushed into the reservoirs may explain some of the internal heterogeneity within LLSVPs observed by seismic studies. Experiments and theoretical calculations have suggested a reduction in viscosity within cold regions due to the post-perovskite phase transition. We find that this viscosity drop may act to increase the amount of crust that is entrained into primordial reservoirs. Furthermore, plumes that originate from reservoir tops will exhibit a time-dependent variation in chemistry, containing all three components (background mantle, primordial reservoir, subducted oceanic crust), perhaps explaining

  7. Subduction obliquity as a prime indicator for geotherm in subduction zone

    NASA Astrophysics Data System (ADS)

    Plunder, Alexis; Thieulot, Cédric; van Hinsbergen, Douwe

    2016-04-01

    The geotherm of a subduction zone is thought to vary as a function of subduction rate and the age of the subducting lithosphere. Along a single subduction zone the rate of subduction can strongly vary due to changes in the angle between the trench and the plate convergence vector, namely the subduction obliquity. This phenomenon is observed all around the Pacific (i.e., Marianna, South America, Aleutian…). However due to observed differences in subducting lithosphere age or lateral convergence rate in nature, the quantification of temperature variation due to obliquity is not obvious. In order to investigate this effect, 3D generic numerical models were carried out using the finite element code ELEFANT. We designed a simplified setup to avoid interaction with other parameters. An ocean/ocean subduction setting was chosen and the domain is represented by a 800 × 300 × 200 km Cartesian box. The trench geometry is prescribed by means of a simple arc-tangent function. Velocity of the subducting lithosphere is prescribed using the analytical solution for corner flow and only the energy conservation equation is solved in the domain. Results are analysed after steady state is reached. First results show that the effect of the trench curvature on the geotherm with respect to the convergence direction is not negligible. A small obliquity yields isotherms which are very slightly deflected upwards where the obliquity is maximum. With an angle of ˜30°, the isotherms are deflected upwards of about 10 kilometres. Strong obliquity (i.e., angles from 60° to almost 90°) reveal extreme effects of the position of the isotherms. Further model will include other parameter as the dip of the slab and convergence rate to highlight their relative influence on the geotherm of subduction zone.

  8. Gravity anomalies, crustal structure, and seismicity at subduction zones: 1. Seafloor roughness and subducting relief

    NASA Astrophysics Data System (ADS)

    Bassett, Dan; Watts, Anthony B.

    2015-05-01

    An ensemble averaging technique is used to remove the long-wavelength topography and gravity field from subduction zones. >200 residual bathymetric and gravimetric anomalies are interpreted within fore arcs, many of which are attributed to the tectonic structure of the subducting plate. The residual-gravimetric expression of subducting fracture zones extends >200 km landward of the trench axis. The bathymetric expression of subducting seamounts with height ≥1 km and area ≥500 km2 (N=36), and aseismic ridges (N>10), is largest near the trench (within 70 km) and above shallow subducting slab depths (SLAB1.0 <17 km). Subducting seamounts are similar in wavelength, amplitude, and morphology to unsubducted seamounts. Morphology, spatial distributions, and reduced levels of seismicity are considered inconsistent with mechanical models proposing wholesale decapitation, and the association of subducting seamounts with large-earthquakes. Subducting aseismic ridges are associated with uplift and steepening of the outer fore arc, a gradual reduction in residual bathymetric expression across the inner fore arc, and a local increase in the width and elevation of the volcanic-arc/orogen. These contrasting expressions reflect the influence of margin-normal variations in rigidity on where and how the upper plate deforms, both to accommodate subducting relief and in response to stresses transmitted across the plate interface. The outer fore arc and arc have lower rigidity due to fracturing and thermal weakening, respectively. Similar associations with complex earthquakes and fault creep suggest aseismic ridge subduction may also be accommodated by the development and evolution of a broad fracture network, the geometrical strength of which may exceed the locking strength of a smooth fault.

  9. 3D numerical modeling of subduction dynamics: plate stagnation and segmentation, and crustal advection in the mantle transition zone

    NASA Astrophysics Data System (ADS)

    Yoshida, M.; Tajima, F.

    2012-04-01

    in the behaviors of subducting plate and the trace of crustal materials between the models under dry and wet conditions. Under wet conditions, the weaker and denser crustal materials are fed into the WFZs and the subducting plate tends to stagnate with a maximum lateral extent of over 1000 km, and is segmented by crustal materials. Under wet condition the crustal layer of garnetite is advected faster than the peridotite body due to the rheology and density contrasts in the MTZ, resulting in its separation from the main body subducting plate. The viscosity contrast between the crustal material and the peridotite permits the separation of the crustal layer from the bulk peridotite and then, crustal materials gradually sink into the lower mantle along with the penetration of the subducting slab main body. As a deep mantle cycling of basaltic oceanic crust is suggested by mineralogy recently [Walter et al., 2011], this numerical result may suggest to re-examine the fate of subducted oceanic crust in the deep mantle.

  10. In-situ formation of Indian Mantle in global subduction zones

    NASA Astrophysics Data System (ADS)

    Nebel, Oliver; Arculus, Richard; Davies, Rhodri

    2014-05-01

    The isotopic signatures of Sr-Nd-Pb-Hf-Os in mid-ocean ridge basalts (MORB) in the Indian Ocean are clearly distinct compared with their Atlantic/Pacific (A/P) counterparts. The origin of this isotopic distinction has been a matter of debate since its discovery by Dupré and Allègre (1983). Current models advocate: (i) delamination of ancient, negatively buoyant lower crust/lithosphere from a supercontinent; (ii) contamination of A/P-style mantle with plumes (the original association with the DUPAL anomaly); or (iii) long-term overprint by a subduction component (SC) surrounding a former supercontinent. The sum of various stable and radiogenic isotope proxies appears to support a delamination scenario, but alternatives, or the combination of the aforementioned scenarios, are possible. Irrespective of the origin of the Indian mantle domain, isotopic signatures similar to those of Indian MORB and hot-spots are observed in arc/back-arc systems associated with western Pacific subduction zones. These isotope signatures have been regarded as unequivocally derived from Indian-type mantle, and accordingly used to trace eastward flow of that type of mantle. Here we show the majority of igneous rocks associated with subduction zone systems mimic Indian-type mantle in Pb isotope space, but are distinct in Hf-Nd isotope co-variations. We suggest isotopic signatures believed to be derived from Indian mantle in subduction zones are the result of medium-term subduction overprint of evolving A/P-type mantle wedges. This feature results from the relative mobility of U-Pb>Sm-Nd>Lu-Hf in subducted slab-derived components and Th/U (k) fractionation in the mantle wedge. Elevation of k in the wedge from 2.6 (MORB) to about 6-12 can account for the shift in Pb isotope space over a duration of ca. 100-200 Myrs; "decoupling" of Hf-Nd isotopes reflect the subduction component vs mantle wedge contribution. More generally, "Pseudo-Indian mantle" is noted as common in subduction zones

  11. Conditions of basaltic magma generation at Mount Baker Volcanic Field, North Cascades

    NASA Astrophysics Data System (ADS)

    McCallum, I. S.; Mullen, E. K.

    2011-12-01

    Significant unresolved questions remain on the processes of mantle melting throughout the wide range of thermal conditions encompassed by subduction zones. For example, subducting slabs in "hot" arc settings are thought to dehydrate at relatively shallow depths, yet volcanoes develop in locations indistinguishable from those in "cold" arcs. The northern Cascade arc is considered a classic end-member example of a "hot" subduction zone because the subducting crust is extremely young, 6-10 Ma at the trench [1], with a thick layer of insulating sediment and a relatively low convergence rate [2]. The most magmatically productive volcanic center of the northern Cascades is the Mt. Baker volcanic field (MBVF) [3], and here we glean information from the most primitive MBVF lavas to develop a petrogenetic model for basalt generation in a "hot" arc setting. Whole-rock geochemical data and the compositions of coexisting minerals are used to establish the initial water contents and redox states of the magmas, and the temperatures and pressures of segregation from the mantle. Melt silica activities indicate the MBVF magmas segregated from their residual mantle source assemblages at depths ranging from 60 to 40 km, corresponding to a few km shallower than the hot core of the mantle wedge [4] to the base of the crust. Plagioclase core compositions indicate that the initial water contents of the magmas ranged from 1.7 to 2.3 wt. % H2O, and show a good inverse correlation with segregation depths. Fe-Ti oxide pairs and spinel inclusions in olivine phenocrysts indicate redox states slightly more oxidizing than the quartz-fayalite-magnetite buffer. Segregation depths are also strongly correlated with temperatures calculated from olivine-liquid equilibria, which range from 1286°C to 1350°C. Coupled with the most recent thermal model for the subducting slab in northern Cascadia [4], we use petrologic phase equilibria for the P-T stability of mineral assemblages in the mantle and

  12. Crustal Accretion at Subduction Initiation Along Izu-Bonin-Mariana Arc and the Link to SSZ Ophiolites

    NASA Astrophysics Data System (ADS)

    Ishizuka, O.; Tani, K.; Reagan, M. K.; Kanayama, K.; Umino, S.; Harigane, Y.; Sakamoto, I.

    2014-12-01

    The Izu-Bonin-Mariana (IBM) forearc preserves the earliest arc magmatic history from subduction initiation to the establishment of the arc. Recent investigations have established a bottom to top igneous stratigraphy of: 1) mantle peridotite, 2) gabbroic rocks, 3) a sheeted dyke complex, 4) basaltic pillow lavas (forearc basalts: FAB), 5) boninites and magnesian andesites, 6) tholeiites and calcalkaline arc lavas. This stratigraphy has many similarities to supra-subduction zone (SSZ) ophiolites. One of the most important common characteristics between the SSZ ophiolites and the forearc crust is the occurrence of MORB-like basaltic lavas underlying or accompanying boninites and early arc volcanic suites. A key observation from the IBM forearc is that FAB differs from nearby back-arc lavas in chemical characteristics, including a depletion in moderately incompatible elements. This indicates that FAB is not a pre-existing oceanic basement of the arc, but the first magmatic product after subduction initiation. Sheeted dikes of FAB composition imply that this magmatism was associated with seafloor spreading, possibly triggered by onset of slab sinking. Recognition of lavas with transitional geochemical characteristics between the FAB and the boninites strongly implies genetic linkage between these two magma types. The close similarity of the igneous stratigraphy of SSZ ophiolites to the IBM forearc section strongly implies a common magmatic evolutionary path, i.e., decompressional melting of a depleted MORB-type mantle is followed by melting of an even more depleted mantle with the addition of slab-derived fluid/melt to produce boninite magma. Similarity of magmatic process between IBM forearc and Tethyan ophiolites appears to be reflected on common characteristics of upper mantle section. Peridotite from both sections show more depleted characteristics compared to upper mantle rocks from mid-ocean ridges. Age determinations reveal that first magmatism at the IBM arc

  13. The Origin of Noble Gas Isotopic Heterogeneity in Icelandic Basalts

    NASA Technical Reports Server (NTRS)

    Dixon, E. T.; Honda, M.; McDougall, I.

    2001-01-01

    Two models for generation of heterogeneous He, Ne and Ar isotopic ratios in Icelandic basalts are evaluated using a mixing model and the observed noble gas elemental ratios in Icelandic basalts,Ocean island Basalt (OIBs) and Mid-Ocean Ridge Basalt (MORBs). Additional information is contained in the original extended abstract.

  14. Shock metamorphism of granulated lunar basalt

    NASA Technical Reports Server (NTRS)

    Schaal, R. B.; Thompson, T. D.; Hoerz, F.; Bauer, J. F.

    1979-01-01

    The paper deals with an extensive series of shock-recovery experiments performed on both nonporous crystalline basalt and its granulated and sieved counterpart to study the role of porosity and grain size in shock motomorphic effects under otherwise identical conditions. Shocked samples are compared with unshocked starting material in terms of textural and mineralogical modifications attributable to shock. A comparative petrographic and chemical characterization is presented of pulverized and sieved lunar basalt 75035 shocked between 6 and 75 GPa in comparison with holocrystalline disks of the same basalts shocked in 10 earlier experiments. Specifically, a petrographic classification of shock features is given, along with an estimation of relative amounts of shock glasses and a chemical characterization of shock glasses in each shocked granular basalt.

  15. Reduction of mare basalts by sulfur loss

    USGS Publications Warehouse

    Brett, R.

    1976-01-01

    Metallic Fe content and S abundance are inversely correlated in mare basalts. Either S volatilization from the melt results in reduction of Fe2+ to Fe0 or else high S content decreases Fe0 activity in the melt, thus explaining the correlation. All considerations favor the model that metallic iron in mare basalts is due to sulfur loss. The Apollo 11 and 17 mare basalt melts were probably saturated with S at the time of eruption; the Apollo 12 and 15 basalts were probably not saturated. Non-mare rocks show a positive correlation of S abundance with metallic Fe content; it is proposed that this is due to the addition of meteoritic material having a fairly constant Fe0/S ratio. If true, metallic Fe content or S abundance in non-mare rocks provides a measure of degree of meteoritic contamination. ?? 1976.

  16. Basalts Dredged from the Northeastern Pacific Ocean.

    PubMed

    Engel, C G; Engel, A E

    1963-06-21

    Volcanic rocks dredged from seamounts, fault ridges, and other major geological features of the northeast Pacific Ocean include a wide variety of basalts. Most of these are vesicular, porphyritic types with near analogues in the Hawaiian and other oceanic islands. In addition, aluminous basalts and diabasic theoleiites impoverished in potassium also occur. There is no simple correlation of composition, degree of oxidation, vesiculation, or hydration of these basalts with texture, or depth of dredge site. Most samples appear to have been extruded at much shallower depths than those now pertaining at the dredge site. The distribution of these basalts suggests that the andesite line coincides with or lies on the continent side of the foot of the continental slope. PMID:17802173

  17. Basalts dredged from the northeastern Pacific Ocean

    USGS Publications Warehouse

    Engel, C.G.; Engel, A.E.J.

    1963-01-01

    Volcanic rocks dredged from seamounts, fault ridges, and other major geological features of the northeast Pacific Ocean include a wide variety of basalts. Most of these are vesicular, porphyritic types with near analogues in the Hawaiian and other oceanic islands. in addition, aluminous basalts and diabasic tholeiites impoverished in potassium also occur. There is no simple correlation of composition, degree of oxidation, vesiculation, or hydration of these basalts with texture, or depth of dredge site. Most samples appear to have been extruded at much shallower depths than those now pertaining at the dredge site. the distribution of these basalts suggests that the andesite line coincides with or lies on the continent side of the foot of the continental slope.

  18. Experimental Hydrous Partial Melting of Natural Pristine and Altered MORB Beneath Subduction Zones

    NASA Astrophysics Data System (ADS)

    Carter, L. B.; Skora, S. E.; Blundy, J.

    2012-12-01

    Experimental determination of the hydrous phase relations and trace element partitioning behaviour of Mid-Ocean Ridge Basalt (MORB) constrains the conditions for melting of subducted oceanic crust. This study utilises natural pristine MORB (from the Kolbeinsey ridge, north of Iceland) and altered MORB from the altered oceanic crust (AOC, from the DSDP leg 46, Mid Atlantic, ~20°N), hydrated with about 15wt% water, in experiments using piston-cylinder apparatus to simulate pressures and temperatures equivalent to about 100 km depth (3.0 GPa, 800-1000°C). Our motivation of (re-)studying the melting behaviour of undoped subducted basalt is two-fold. First, previous studies that focused on the trace element behaviour 'doped' their starting materials with trace elements to facilitate analysis and positive identification of accessory phases. Only by applying the 'doping' method has it been found that allanite may exert a key control on the light rare earth element (approx. La-Sm) budget in subducted basalt. However, it is still a matter of debate whether the ubiquitous presence of allanite in these experimental studies is solely due to doping of La-Sm. In addition, bulk fluid-solid partition coefficients so obtained may not be uncritically applied to model the trace element transfer in nature because of the enhanced proportions of accessory phases in the experimental solid residue. Second, there appears to be a clear difference in the melting behaviour of K-free and K-bearing MORB (and sediment), even if K2O only appears in minor concentrations. These differences are difficult to quantify, as the various studies were carried out at differing P-T-XH2O conditions. In agreement with earlier studies, we find that the vapour-saturated solidus is shifted toward higher temperatures at 3 GPa in pristine MORB (800°C < Tsolidus < 850°C), because it is virtually K-free (approx. 0.03 wt% K2O), in contrast to altered MORB where melting starts at T < 800°C (approx. 0.26 wt% K2O

  19. Molybdenum isotope systematics in subduction zones

    NASA Astrophysics Data System (ADS)

    König, Stephan; Wille, Martin; Voegelin, Andrea; Schoenberg, Ronny

    2016-08-01

    This study presents Mo isotope data for arc lavas from different subduction zones that range between δ 98 / 95 Mo = - 0.72 and + 0.07 ‰. Heaviest isotope values are observed for the most slab fluid dominated samples. Isotopically lighter signatures are related to increasing relevance of terrigenous sediment subduction and sediment melt components. Our observation complements previous conclusions that an isotopically heavy Mo fluid flux likely mirrors selective incorporation of isotopically light Mo in secondary minerals within the subducting slab. Analogue to this interpretation, low δ 98 / 95 Mo flux that coincides with terrigenous sediment subduction and sediment melting cannot be simply related to a recycled input signature. Instead, breakdown of the controlling secondary minerals during sediment melting may release the light component and lead to decreasing δ 98 / 95 Mo influx into subarc mantle sources. The natural range between slab dehydration and hydrous sediment melting may thus cause a large spread of δ 98 / 95 Mo in global subduction zone magmas.

  20. Basaltic cannibalism at Thrihnukagigur volcano, Iceland

    NASA Astrophysics Data System (ADS)

    Hudak, M. R.; Feineman, M. D.; La Femina, P. C.; Geirsson, H.

    2014-12-01

    Magmatic assimilation of felsic continental crust is a well-documented, relatively common phenomenon. The extent to which basaltic crust is assimilated by magmas, on the other hand, is not well known. Basaltic cannibalism, or the wholesale incorporation of basaltic crustal material into a basaltic magma, is thought to be uncommon because basalt requires more energy than higher silica rocks to melt. Basaltic materials that are unconsolidated, poorly crystalline, or palagonitized may be more easily ingested than fully crystallized massive basalt, thus allowing basaltic cannibalism to occur. Thrihnukagigur volcano, SW Iceland, offers a unique exposure of a buried cinder cone within its evacuated conduit, 100 m below the main vent. The unconsolidated tephra is cross-cut by a NNE-trending dike, which runs across the ceiling of this cave to a vent that produced lava and tephra during the ~4 Ka fissure eruption. Preliminary petrographic and laser ablation inductively coupled mass spectrometry (LA-ICP-MS) analyses indicate that there are two populations of plagioclase present in the system - Population One is stubby (aspect ratio < 1.7) with disequilibrium textures and low Ba/Sr ratios while Population Two is elongate (aspect ratio > 2.1), subhedral to euhedral, and has much higher Ba/Sr ratios. Population One crystals are observed in the cinder cone, dike, and surface lavas, whereas Population Two crystals are observed only in the dike and surface lavas. This suggests that a magma crystallizing a single elongate population of plagioclase intruded the cinder cone and rapidly assimilated the tephra, incorporating the stubbier population of phenocrysts. This conceptual model for basaltic cannibalism is supported by field observations of large-scale erosion upward into the tephra, which is coated by magma flow-back indicating that magma was involved in the thermal etching. While the unique exposure at Thrihnukagigur makes it an exceptional place to investigate basaltic

  1. Basaltic Soil of Gale Crater: Crystalline Component Compared to Martian Basalts and Meteorites

    NASA Technical Reports Server (NTRS)

    Treiman, A. H.; Bish, D. L.; Ming, D. W.; Morris, R. V.; Schmidt, M.; Downs, R. T.; Stolper, E. M.; Blake, D. F.; Vaniman, D. T.; Achilles, C. N.; Chipera, S. J.; Bristow, T. F.; Crisp, J. A.; Farmer, J. A.; Morookian, J. M.; Morrison, S. M.; Rampe, E. B.; Sarrazin, P.; Yen, A. S.; Anderosn, R. C.; DesMarais, D. J.; Spanovich, N.

    2013-01-01

    A significant portion of the soil of the Rocknest dune is crystalline and is consistent with derivation from unweathered basalt. Minerals and their compositions are identified by X-ray diffraction (XRD) data from the CheMin instrument on MSL Curiosity. Basalt minerals in the soil include plagioclase, olivine, low- and high-calcium pyroxenes, magnetite, ilmenite, and quartz. The only minerals unlikely to have formed in an unaltered basalt are hematite and anhydrite. The mineral proportions and compositions of the Rocknest soil are nearly identical to those of the Adirondack-class basalts of Gusev Crater, Mars, inferred from their bulk composition as analyzed by the MER Spirit rover.

  2. Demise of Flat-slab Subduction at the end of the Laramide Orogeny (Invited)

    NASA Astrophysics Data System (ADS)

    Humphreys, E.

    2013-12-01

    absence of major change in California subduction presumably indicates that while the flat slab fell off of North America east of California, it remained at the base of California. More recently, the arrival of Yellowstone beneath S. Oregon triggered a delamination of the flat slab remaining beneath N. Oregon, drawing flood basalt activity north.

  3. Geochemical and Sr-Nd isotope variations within Cretaceous continental flood-basalt suites of the Canadian High Arctic, with a focus on the Hassel Formation basalts of northeast Ellesmere Island

    NASA Astrophysics Data System (ADS)

    Estrada, Solveig

    2015-11-01

    Early- to mid-Cretaceous flood-basalt suites of the northeast Canadian High Arctic assigned to a High Arctic Large Igneous Province (HALIP) were studied for their whole-rock geochemistry and Sr-Nd isotopes. Data from basalt flows within the upper Albian to lower Cenomanian Hassel Formation of northeast Ellesmere Island are compared with former published data and new inductively coupled plasma mass spectrometry data of the stratigraphic equivalent Strand Fiord basalts and the older, late Hauterivian to Aptian Isachsen basalts from Axel Heiberg Island. The transitional to mildly alkaline aphyric Hassel basalts, with ocean island basalt (OIB)-like geochemical signatures in parts, have an Ar-Ar whole-rock age of on average 96.4 ± 1.6 Ma. They represent two geochemically different flow units without a fractional crystallization relationship: the high-phosphorous (HP) and low-phosphorous (LP) basalts. The Hassel HP basalts differ from the LP basalts by additionally higher Ba, K, Rb, Th and LREE contents, a pronounced positive Eu anomaly (Eu/Eu* = 1.74-1.76), as well as lower Ta, Nb, Zr and Hf concentrations. The Nd and Sr isotope ratios of the Hassel HP basalts [ ɛ Nd( t) of -1.3 to -1.4, 87Sr/86Sr( t) of 0.70706-0.70707] and the LP basalts [ ɛ Nd( t) of 4.5-4.9, 87Sr/86Sr( t) of 0.7038-0.7040] indicate an origin from different mantle sources. The geochemically similar tholeiitic Isachsen (ca. 130-113 Ma) and Strand Fiord basalts (ca. 105-95 Ma) are also incompatible element enriched relative to the primitive mantle, however, with negative Sr-P anomalies as well as partially negative K, Ta and Nb anomalies. In terms of incompatible element ratios (Zr/Nb, Nb/Th), several mantle components are involved in the formation of the flood-basalt suites: a component with primitive mantle composition, an OIB-like component (probably subducted and recycled oceanic crust) and an enriched lithospheric component. The latter component, probably metasomatized subcontinental

  4. Origins of felsic magmas in Japanese subduction zone: Geochemical characterizations of tephra from caldera-forming eruptions <5 Ma

    NASA Astrophysics Data System (ADS)

    Kimura, Jun-Ichi; Nagahashi, Yoshitaka; Satoguchi, Yasufumi; Chang, Qing

    2015-07-01

    Dacitic to rhyolitic glass shards from 80 widespread tephras erupted during the past 5 Mys from calderas in Kyushu, and SW, central, and NE Japan were analyzed. Laser ablation inductively coupled plasma mass spectrometry was used to determine 10 major and 33 trace elements and 207Pb/206Pb-208Pb/206Pb isotope ratios. The tephras were classified into three major geochemical types and their source rocks were identified as plutonic, sedimentary, and intermediate amphibolite rocks in the upper crust. A few tephras from SW Japan were identified as adakite and alkali rhyolite and were regarded to have originated from slab melt and mantle melt, respectively. The Pb isotope ratios of the tephras are comparable to those of the intermediate lavas in the source areas but are different from the basalts in these areas. The crustal assimilants for the intermediate lavas were largely from crustal melts and are represented by the rhyolitic tephras. A large heat source is required for forming large volumes of felsic crustal melts and is usually supplied by the mantle via basalt. Hydrous arc basalt formed by cold slab subduction is voluminous, and its heat transfer with high water content may have melted crustal rocks leading to effective felsic magma production. Coincidence of basalt and felsic magma activities shown by this study suggests caldera-forming eruptions are ultimately the effect of a mantle-driven cause.

  5. Asthenosphere-lithosphere interaction triggered by a slab window during ridge subduction: Trace element and Sr-Nd-Hf-Os isotopic evidence from Late Carboniferous tholeiites in the western Junggar area (NW China)

    NASA Astrophysics Data System (ADS)

    Tang, Gong-Jian; Wyman, Derek A.; Wang, Qiang; Li, Jie; Li, Zheng-Xiang; Zhao, Zhen-Hua; Sun, Wei-Dong

    2012-05-01

    Tholeiites occur in a variety of geological settings, e.g., mid-ocean ridge, back-arc basin, ocean island, island arc and intra-continent, and their geochemical and isotopic characteristics vary according to the corresponding geodynamic environments. Here we investigated the Hatu tholeiitic basalts and basaltic andesites of the western Junggar region, Central Asian Orogenic Belt (CAOB). LA-ICPMS zircon U-Pb analyses indicate that the Hatu tholeiites were generated in the Late Carboniferous (~ 315 Ma). All the studied rock samples are characterized by flat rare earth elements pattern on chondrite-normalized plot, and negligible Nb, Ta and Ti anomalies on mid-ocean-ridge basalt normalized plots. They are also characterized by moderate positive ɛNd(t) (+ 5.25 to + 5.94), ɛHf(t) (+ 13.24 to + 14.89), highly radiogenic Os isotope compositions (187Os/188Os315Ma = 0.1338-0.3547), and relatively low (87Sr/86Sr)i ratios (0.7044 to 0.7048). Taking into account their geological characteristics, the occurrence of nearby ophiolites and the types of contemporaneous magmatic rocks found in the western Junggar region, we propose that the Hatu basalts were generated by slab window-related processes following a spreading ridge subduction beneath the Keramay intra-oceanic island arc. During this process, deep and enriched asthenospheric mantle rose to the edge of the subducted oceanic lithosphere, its melts infiltrating the subducted oceanic lithosphere and reacting with peridotites. Therefore, the Hatu tholeiites are interpreted as a result of melting of a mixed mantle source consisting of subducted depleted oceanic lithosphere and a deep, enriched upwelling asthenospheric mantle. Incongruent dynamic melting modeling of trace element compositions indicates that the Hatu basalts could have been derived from large degrees of melting (~ 10%) of such a mixed mantle source. This newly recognized mechanism is a natural consequence of the diversity of contemporaneous potential mantle

  6. Subduction, collision and initiation of hominin dispersal

    NASA Astrophysics Data System (ADS)

    Schattner, Uri; Lazar, Michael

    2009-09-01

    Subduction is the main driving force of plate tectonics controlling the physiography of the Earth. The northward subduction of the Sinai plate was interrupted during the Early Pleistocene when the Eratosthenes Seamount began to collide with the Cyprian arc. A series of synchronous structural deformations was triggered across the entire eastern Mediterranean, and local topography was drastically accentuation along the Levantine corridor - one of the main pathways of hominin dispersal out of Africa. However, the choice of this preferred pathway and timing of dispersal has not been resolved. Though causes for dispersal out of Africa are in debate, we show that the transition from subduction to collision in the eastern Mediterranean set the route.

  7. Predicting the Isotopic Composition of Subduction-Filtered Subducted Oceanic Crust and Sediment

    NASA Astrophysics Data System (ADS)

    White, W. M.

    2010-12-01

    The chemical and isotopic character of mantle plumes, which produce oceanic island volcanoes, are widely thought to reflect the presence of recycled oceanic crust and sediment. Isotopic systematics suggest the “cycle time” for this process is 1 Ga or longer, but it should be possible to use a simple mass balance approach to discern how the presently operating subduction zone filter affects the ratios of radioactive parent to radiogenic daughter isotopes. Simple uniformitarian assumptions can then be used to predict the present isotopic composition of anciently subducted lithosphere. Our underlying assumption in deciphering the subduction zone filter is that the flux of an element into the deep mantle is simply equal to the flux of element into the subduction zone less the flux of that element into subduction zone magmas. The former is readily calculated from published data. The latter can be calculated by estimating parental magma compositions, arc accretion rates, and the assumption that arc magma compositions differ from MORB only because of material derived from subducting crust and sediment. Using this approach for 8 intra-oceanic subduction zones, we find 73% of Th and Pb, 79% of U, 80% of Rb and Sr, 93% of Nd and 98% of Sm survive the subduction zone filter. The subduction zone filter systematically increases Sm/Nd ratios in all subduction zones, but the effect is small, with a weighted mean increase of 1.5%. The effect of subduction is to decrease the Sm/Nd of the mantle, but only slightly. The effect of subduction is to increase the Rb/Sr of the mantle, but the subduction zone filter does not have a systematic effect on Rb/Sr ratios: it significantly increases in Rb/Sr in 3 subduction zones and significantly decreases it in one; the weighted mean shows no significant change. The effect of the subduction zone filter on U/Pb is also not systematic. U/Pb ratios in the mantle fluxes are bimodal, with values equal to or lower than the bulk Earth value in 4

  8. Tomographic search for missing link between the ancient Farallon subduction and the present Cocos subduction

    NASA Astrophysics Data System (ADS)

    Gorbatov, Alexei; Fukao, Yoshio

    2005-03-01

    A striking feature of the tomographic images of the Earth's mid-mantle is the long, high-velocity belt extending in a north-south direction under the North and South American continents, which is believed to be the remnant subduction of the Farallon Plate. In the Oligocene epoch the North Farallon Plate subduction terminated off Baja California and the South Farallon Plate broke into the Cocos and Nazca plates. This important period of the Farallon subduction history is not clearly understood, due in part to the lack of high-resolution tomographic images. Our P-wave tomographic image of the mantle below Mexico indicates that the currently subducting slab of the Cocos Plate is torn apart from the already subducted slab of the ancient Farallon Plate in a region behind the slab window or slab gap off Baja California. We suggest that the southeastward advance of this slab tearing was synchronous with the counter-clockwise rotation of the Cocos Plate against the eastward to northeastward subduction of the ancient Farallon Plate. The Cocos slab torn apart from the deeper Farallon slab dips to the north to northeast with its strike oblique to the trench axis. This slab configuration delineates well with the intermediate-depth earthquake activity and the volcanic activity known as the Trans Mexican Volcanic Belt. Further to the south of the slab tearing of the subducted slab continues from the deeper Farallon part to the shallower Cocos part but with considerable distortion in the shallower part.

  9. Subduction of Organic Carbon into the Earth

    NASA Astrophysics Data System (ADS)

    Plank, T. A.; Malinverno, A.

    2015-12-01

    Seafloor sediments approach active subduction zones with small amounts (generally < 1 dry weight %) of organic carbon, but this small concentration nonetheless constitutes a significant flux over geological time with respect to the size, isotopic composition and electron balance of the carbosphere. In order to quantify the flux of organic carbon into subduction zones, we have examined carbon concentration and density measurements provided by successive drilling programs (DSDP, ODP, IODP) for cores that sample complete sediment sections to basement near deep sea trenches. An interpolation scheme compensates for sparse or non-uniform analyses, and a weighted bulk concentration is calculated for organic carbon (OC) and inorganic carbonate (IC) for each site. When multiplied by the sediment thickness, the trench length and the convergence velocity, a subducted mass flux can be obtained. Sites with the greatest concentration of OC include those that pass beneath regions of high biological productivity (such as the Eastern Equatorial Pacific) and those that receive terrigenous turbidites (e.g., Indus and Begal Fans, Gulf of Alaska, Cascadia, etc). Together, terrigenous turbidites make up about 60% of the global subducted sediment (GLOSS), and thus have a strong control on the concentration of OC in GLOSS. Sites dominated by terrigenous turbidites have 0.4 wt% OC on average (1sd = 0.1 wt%), and GLOSS is very similar, yielding about 6 MtC/yr OC subducted globally. This flux is enough to subduct the entire surface pool of OC every 2.6 Ga, which if not returned, or returned in a more oxidized form, could contribute to a significant rise in oxygen at the surface of the Earth. Seemingly fortuitously, the OC/total carbon fraction in GLOSS is 20%, very near the long term average required to maintain the isotopic composition of marine carbonates at ~ 0 per mil d13C over much of Earth history.

  10. Origin of ophiolite complexes related to intra-oceanic subduction initiation: implications of IODP Expedition 352 (Izu-Bonin fore arc)

    NASA Astrophysics Data System (ADS)

    Robertson, Alastair; Avery, Aaron; Carvallo, Claire; Christeson, Gail; Ferré, Eric; Kurz, Walter; Kutterolf, Steffen; Morgan, Sally; Pearce, Julian; Reagan, Mark; Sager, William; Shervais, John; Whattam, Scott; International Ocean Discovery Program Expedition 352 (Izu-Bonin-Mariana Fore Arc), the Scientific Party of

    2015-04-01

    Ophiolites, representing oceanic crust exposed on land (by whatever means), are central to the interpretation of many orogenic belts (e.g. E Mediterranean). Based mostly on geochemical evidence, ophiolites are widely interpreted, in many but by no means all cases, as having formed within intra-oceanic settings above subduction zones (e.g. Troodos ophiolite, Cyprus). Following land geological, dredging and submersible studies, fore arcs of the SW Pacific region became recognised as likely settings of supra-subduction zone ophiolite genesis. This hypothesis was tested by recent drilling of the Izu-Bonin fore arc. Four sites were drilled, two on the outer fore arc and two on the upper trench slope. Site survey seismic data, combined with borehole data, indicate that three of the sites are located in fault-controlled sediment ponds that formed in response to dominantly down-to the-west extensional faulting (with hints of preceding top-to-the-east compressional thrusting). The sediments overlying the igneous basement, of maximum Late Eocene to Recent age, document ash and aeolian input, together with mass wasting of the fault-bounded sediment ponds. At the two more trenchward sites (U1440 and U1441), mostly tholeiitic basalts were drilled, including massive and pillowed lavas and hyaloclastite. Geochemically, these extrusives are of near mid-oceanic ridge basalt composition (fore arc basalts). Subtle chemical deviation from normal MORB can be explained by weakly fluid-influenced melting during decompression melting in the earliest stages of supra-subduction zone spreading (not as 'trapped' older MORB). The remaining two sites, c. 6 km to the west (U1439 and U1442), penetrated dominantly high-magnesian andesites, known as boninites, largely as fragmental material. Their formation implies the extraction of highly depleted magmas from previously depleted, refractory upper mantle in a supra-subduction zone setting. Following supra-subduction zone spreading, the active

  11. How was the Iapetus infected with subduction

    NASA Astrophysics Data System (ADS)

    Waldron, John; Schofield, David; Brendan Murphy, J.; Thomas, Chris

    2015-04-01

    The history of the Iapetus Ocean is the archetype for the "Wilson cycle". The most poorly understood part of the Wilson cycle is the transition between ocean opening and ocean closing. It is often assumed that subduction is initiated by subsidence of old, cold ocean floor at passive margins. However, in the best modern analogue, Atlantic margins formed at ~180 Ma are still passive, suggesting that some other mechanism is required to initiate subduction. In most tectonic reconstructions of the Appalachian-Caledonide orogen, the continental blocks (Laurentia, Baltica, and Amazonia - West Africa), which separated to form the Iapetus during the breakup of Rodinia, are the same three continents that subsequently collided during closure, making the Iapetus a test case for models of subduction initiation. The margin of Laurentia underwent protracted rifting from ~615 Ma to at least 550 Ma, and perhaps later. The earliest "drift" successions on the Newfoundland margin are as young as ~515 Ma. Subduction, recorded by arc volcanics preserved in the orogen, began relatively early in the history of the new ocean at ~515-505 Ma, and the earliest collisional events are recorded almost simultaneously in peri-Laurentian and peri-Gondwanan microcontinents around 490-480 Ma. However, the stable passive margin of Laurentia survived until after 470 Ma before being converted to an active margin. Closure of the ocean between Avalonia and Laurentia was complete by ~425 Ma. These relationships are difficult to reconcile with a classic Wilson cycle in which subduction is initiated by inversion of an extensional margin. It is much more likely that closure was initiated at a subduction zone migrating westward into the Iapetus, analogous to the eastward Mesozoic-Cenozoic entry of the Caribbean and Scotia plates into the Atlantic realm. This process was probably initiated at a transform boundary between the "internal" ocean formed during the breakup of Rodinia, and "external" Panthalassan

  12. Segmentation of the Cascade Arc as indicated by Sr and Nd isotopic variation among diverse primitive basalts

    NASA Astrophysics Data System (ADS)

    Schmidt, Mariek E.; Grunder, Anita L.; Rowe, Michael C.

    2008-02-01

    In the central Oregon Cascades, extension of the arc has promoted eruption of primitive basalts that are of three types, calcalkaline (CAB), low K tholeiitic (LKT) and rare absarokitic (ABS) in the forearc. Based on a comparison with the distribution of primitive magma types and their 87Sr/ 86Sr and 143Nd/ 144Nd isotopic signature in the Cascades, we divide the arc into four segments that correspond to distinct tectonic settings and reflect mantle domains and melting regimes at depth. The segments are: 1) the North Segment from Mt. Meager to Glacier Peak; 2) the Columbia Segment from Mt. Rainier to Mt. Jefferson; 3) the Central Segment from the Three Sisters to Medicine Lake, and 4) the South Segment from Mt. Shasta to Lassen Peak. Calcalkaline basalts (CABs) are found all along the arc axis and are produced by fluxing of variable mantle domains by subduction-derived fluid. In the South Segment, the degree of fluxing and melting is greatest as indicated by high 87Sr/ 86Sr and Ba/Ce of CABs relative to other types of ambient basalt and is consistent with the greater abundance of high-Mg basaltic andesite, relative to other segments. High flux and abundant melt is enhanced by the presence of a slab window and subduction of the altered and deformed Gorda Plate. In the northern part of the arc, small degrees of flux melting are coupled with the presence of an enriched mantle component to yield abundant high-field strength element-enriched (HFSE-rich) basalts. Extension and higher heat flow favors the production of abundant low potassium tholeiites LKT in the Central Segment. A distinct shift in 87Sr/ 86Sr of low LKTs occurs between the Columbia and Central Segments (0.7028 vs. 0.7034, respectively), which we interpret as juxtaposition of mantle of accreted oceanic terranes, including the enriched large igneous province Siletz Terrane, with encroaching mantle related to the adjacent Basin and Range Province. The latter, although depleted, carries an enrichment signature

  13. Rapid time scales of basalt to andesite differentiation at Anatahan volcano, Mariana Islands

    NASA Astrophysics Data System (ADS)

    Reagan, Mark; Tepley, Frank J.; Gill, James B.; Wortel, Matthew; Hartman, Brian

    2005-08-01

    We present comprehensive U-series data ( 238U- 234U- 230Th- 226Ra- 210Pb- 210Po and ( 230Th)/( 232Th)) for an andesite from an oceanic arc. The juvenile Anatahan andesite has U-Th systematics colinear with other historical Mariana volcanic rocks, and is most similar to those of the other volcano in the Mariana arc with a significant proportion of silicic andesite: Uracas. Like Uracas, the parental basalt for the Anatahan andesite was generated by relatively low degrees of flux melting from a source previously enriched in a sediment component from the subducting slab. However, the Anatahan andesite is much more strongly enriched in 226Ra over 230Th than Uracas lavas, and has one of the highest ( 226Ra)/( 232Th) ratios of siliceous andesites globally. The long-lived disequilibria between 238U- 230Th- 226Ra in the Anatahan andesite are inherited from basalt genesis, not created during differentiation or eruption. Thus, the time between genesis of the parental basalt and eruption of andesite at Anatahan is shorter than for Uracas. Moreover, the near-equilibrium ( 210Pb)/( 226Ra) value indicates that the magma body did not persistently lose or gain 222Rn for more than 2 years before eruption. This permits differentiation of the parental basalt to form andesite within this 2-year time period, although a differentiation time period between 100 and a few thousand years also is possible. The relative activities between 210Po and 210Pb suggest erupted scoria degassed Po less than most lavas despite eruption plume heights of ˜10 km, which further suggests an unusually rapid ascent before eruption. These data also show that juvenile material was ejected from the first day of the eruption. Phreatomagmatic ejecta overlying the main Anatahan scoria is strongly enriched in 210Po over 210Pb, indicating that a significant proportion of the Po degassed from rising magmas sublimes in its shallow fumarolic conduit system.

  14. Tomographically-imaged subducted slabs and magmatic history of Caribbean and Pacific subduction beneath Colombia

    NASA Astrophysics Data System (ADS)

    Bernal-Olaya, R.; Mann, P.; Vargas, C. A.; Koulakov, I.

    2013-12-01

    We define the length and geometry of eastward and southeastward-subducting slabs beneath northwestern South America in Colombia using ~100,000 earthquake events recorded by the Colombian National Seismic Network from 1993 to 2012. Methods include: hypocenter relocation, compilation of focal mechanisms, and P and S wave tomographic calculations performed using LOTOS and Seisan. The margins of Colombia include four distinct subduction zones based on slab dip: 1) in northern Colombia, 12-16-km-thick oceanic crust subducts at a modern GPS rate of 20 mm/yr in a direction of 110 degrees at a shallow angle of 8 degrees; as a result of its low dip, Pliocene-Pleistocene volcanic rocks are present 400 km from the frontal thrust; magmatic arc migration to the east records 800 km of subduction since 58 Ma ago (Paleocene) with shallow subduction of the Caribbean oceanic plateau starting ~24-33 Ma (Miocene); at depths of 90-150 km, the slab exhibits a negative velocity anomaly we associate with pervasive fracturing; 2) in the central Colombia-Panama area, we define an area of 30-km-thick crust of the Panama arc colliding/subducting at a modern 30/mm in a direction of 95 degrees; the length of this slab shows subduction/collision initiated after 20 Ma (Middle Miocene); we call this feature the Panama indenter since it has produced a V-shaped indentation of the Colombian margin and responsible for widespread crustal deformation and topographic uplift in Colombia; an incipient subduction area is forming near the Panama border with intermediate earthquakes at an eastward dip of 70 degrees to depths of ~150 km; this zone is not visible on tomographic images; 3) a 250-km-wide zone of Miocene oceanic crust of the Nazca plate flanking the Panama indenter subducts at a rate of 25 mm/yr in a direction of 55 degrees and at a normal dip of 40 degrees; the length of this slab suggests subduction began at ~5 Ma; 4) the Caldas tear defines a major dip change to the south where a 35 degrees

  15. Rare-earth element geochemistry and the origin of andesites and basalts of the Taupo Volcanic Zone, New Zealand

    USGS Publications Warehouse

    Cole, J.W.; Cashman, K.V.; Rankin, P.C.

    1983-01-01

    Two types of basalt (a high-Al basalt associated with the rhyolitic centres north of Taupo and a "low-Al" basalt erupted from Red Crater, Tongariro Volcanic Centre) and five types of andesite (labradorite andesite, labradorite-pyroxene andesite, hornblende andesite, pyroxene low-Si andesite and olivine andesite/low-Si andesite) occur in the Taupo Volcanic Zone (TVZ), North Island, New Zealand. Rare-earth abundances for both basalts and andesites are particularly enriched in light rare-earth elements. High-Al basalts are more enriched than the "low-Al" basalt and have values comparable to the andesites. Labradorite and labradorite-pyroxene andesites all have negative Eu anomalies and hornblende andesites all have negative Ce anomalies. The former is probably due to changing plagioclase composition during fractionation and the latter to late-stage hydration of the magma. Least-squares mixing models indicate that neither high-Al nor "low-Al" basalts are likely sources for labradorite/labradorite-pyroxene andesites. High-Al basalts are considered to result from fractionation of olivine and clinopyroxene from a garnet-free peridotite at the top of the mantle wedge. Labradorite/labradorite-pyroxene andesites are mainly associated with an older NW-trending arc. The source is likely to be garnet-free but it is not certain whether the andesites result from partial melting of the top of the subducting plate or a hydrated lower portion of the mantle wedge. Pyroxene low-Si andesites probably result from cumulation of pyroxene and calcic plagioclase within labradorite-pyroxene andesites, and hornblende andesites by late-stage hydration of labradorite-pyroxene andesite magma. Olivine andesites, low-Si andesites and "low-Al" basalts are related to the NNE-trending Taupo-Hikurangi arc structure. Although the initial source material is different for these lavas they have probably undergone a similar history to the labradorite/labradorite-pyroxene andesites. All lavas show evidence

  16. The redox budget of subduction zones

    NASA Astrophysics Data System (ADS)

    Evans, K. A.

    2012-06-01

    Elements that can occur in more than one valence state, such as Fe, C and S, play an important role in Earth's systems at all levels, and can drive planetary evolution as they cycle through the various geochemical reservoirs. Subduction introduces oxidised Fe, C and S in sediments, altered ocean crust, and partially serpentinised lithospheric mantle to the relatively reduced mantle, with short- and long-term consequences for the redox state of the mantle. The distribution of redox-sensitive elements in the mantle controls the redox state of mantle-derived material added to the lithosphere and atmosphere, such as arc volcanic gases and the magmas that form arc-related ore deposits. The extent of mantle oxidation induced by subduction zone cycling can be assessed, albeit with large uncertainties, with redox budget calculations that quantify the inputs and outputs to subduction zones. Literature data are augmented by new measurements of the chemical composition of partially serpentinised lithospheric mantle from New Caledonia and ODP 209. Results indicate that there is a net addition of Fe (55 ± 13 × 1012 mol year- 1), C (4.6 ± 4.0 × 1012 mol year- 1), S (2.4 ± 0.9 × 1012 mol year- 1), and redox budget (5-89 × 1012 mol year- 1) at subduction zones. Monte Carlo calculations of redox budget fluxes indicate that fluxes are 46 ± 12 × 1012 mol year- 1 entering subduction zones, if input and output parameters are assumed to be normally distributed, and 46-58 × 1012 mol year- 1 if input and output parameters are assumed to be log-normally distributed. Thus, inputs into subduction zones for Fe, C, S and redox budget are in excess of subduction zone outputs. If MORB and plume-related fluxes are taken into account then Fe, C and S fluxes balance, within error. However, the redox budget does not balance, unless the very lowest estimates for the extent of slab oxidation are taken. Thus it is likely that subduction continuously increases the redox budget of the mantle

  17. Nickel and Cobalt Partitioning Between Spinel and Basaltic Melt: Applications to Planetary Basalt Suites

    NASA Technical Reports Server (NTRS)

    Righter, K.

    2002-01-01

    New experimental spinel/melt partition coefficients for Ni and Co have been measured in basalt samples with natural levels of Ni and Co, are lower than previous high doping experiments, and are applied to several planetary basalt suites. Additional information is contained in the original extended abstract.

  18. Generating Single-sided Subduction with Parameterized Mantle Wedge

    NASA Astrophysics Data System (ADS)

    Lin, C. J.; Tan, E.; Ma, K. F.

    2015-12-01

    Subduction on Earth is one-sided, where one oceanic plate sinks beneath the overriding plate. However, subduction zones in most numerical models tends to develop two-sided subduction, where both plates sink to the mantle. In this study, we use numerical model to find out how the existence of low viscosity wedge (LVW) can enable single-sided subduction and affects the flow in the subduction system.At the mantle wedge, water released from dehydrated oceanic crust serpentinized the mantle, which forms the LVW. LVW is an important part of the subduction system and provides efficient lubricant between the subducting slab and overriding lithosphere. Single-sided subduction can be generated in numerical models by different techniques, including prescribed plate velocity, non-Newtonian rheology, and free surface. These techniques either requires kinematic boundary condition, which produce mantle flow inconsistent with the buoyancy, or costs great amount of computational resources when solving nonlinear equations. In this study, we tried to generating single-sided subduction with Newtonian viscosity and free slip surface. A set of tracers representing hydrated oceanic crust are placed near the surface. As the tracers subducted with the lithosphere, we assume that the oceanic crust becomes dehydrated and serpentinizes the mantle wedge above. A parameterized LVW is placed above the subducted tracers in the models. We test with different upper/lower depth limits of the LVW and the viscosity of the LVW. Both overriding plate and subducting plate's surface velocity relative to the trench is calculated in order to determine whether the subduction is one-sided.Results of our numerical models show that not only the low viscosity wedge above the slab is essential for the formation of one-side subduction, a low viscosity layer in between two tectonic plates is also needed to provide the slab efficient lubricant after the subduction started. On the other hand, the plate's age, which

  19. Subduction and exhumation of continental crust: insights from laboratory models

    NASA Astrophysics Data System (ADS)

    Bialas, Robert W.; Funiciello, Francesca; Faccenna, Claudio

    2011-01-01

    When slivers of continental crust and sediment overlying oceanic lithosphere enter a subduction zone, they may be scraped off at shallow levels, subducted to depths of up to 100-200 km and then exhumed as high pressure (HP) and ultra-high pressure (UHP) rocks, or subducted and recycled in the mantle. To investigate the factors that influence the behaviour of subducting slivers of continental material, we use 3-D dynamically consistent laboratory models. A laboratory analogue of a slab-upper mantle system is set up with two linearly viscous layers of silicone putty and glucose syrup in a tank. A sliver of continental material, also composed of silicone putty, overlies the subducting lithosphere, separated by a syrup detachment. The density of the sliver, viscosity of the detachment, geometry of the subducting system (attached plate versus free ridge) and dimensions of the sliver are varied in 34 experiments. By varying the density of the sliver and viscosity of the detachment, we can reproduce a range of sliver behaviour, including subduction, subduction and exhumation from various depths and offscraping. Sliver subduction and exhumation requires sufficient sliver buoyancy and a detachment that is strong enough to hold the sliver during initial subduction, but weak enough to allow adequate sliver displacement or detachment for exhumation. Changes to the system geometry alter the slab dip, subduction velocity, pattern of mantle flow and amount of rollback. Shallower slab dips with more trench rollback produce a mantle flow pattern that aids exhumation. Steeper slab dips allow more buoyancy force to be directed in the up-dip direction of the plane of the plate, and aide exhumation of subducted slivers. Slower subduction can also aide exhumation, but if slab dip is too steep or subduction too slow, the sliver will subduct to only shallow levels and not exhume. Smaller slivers are most easily subducted and exhumed and influenced by the mantle flow.

  20. Comparing the composition of the earliest basalts erupted by the Iceland and Afar mantle plumes.

    NASA Astrophysics Data System (ADS)

    Stuart, Finlay M.

    2013-04-01

    The first basalts erupted by mantle plumes are typically generated by mantle melting at temperatures 200-300°C higher than average ambient mantle. This is consistent with the derivation of from a thermal boundary layer at the core-mantle boundary. Mantle plume temperatures decrease with time, likely as large plume heads give way to thin plume conduits. Consequently the early, hot plume basalts are a window into the deep mantle. At it's simplest they provide a test of whether the discrete plume source regions are primordial mantle that have been isolated since soon after Earth accretion, or have substantial contributions from subducted slabs. Here I present new isotopic and trace element determinations of the earliest picritic basalts from the ~30 Ma Afar plume in Ethiopia. They will be compared with similar material from the ~60 Ma proto-Iceland plume (PIP) in an effort to test prevailing models regarding the source of mantle plumes. The extremely primordial nature of the helium in the PIP picrites (3He/4He ~ 50 Ra) contrasts with much lower values of the Ethiopian flood basalt province (~21 Ra). The Iceland plume 3He/4He has decreased (linearly) with time, mirroring the secular cooling of the Iceland mantle plume identified by decreasing MgO and FeO in primary melts. In 60 million years the Iceland plume 3He/4He is still higher than the maximum Afar plume value. The Sr-Nd-Pb isotopic composition of the high 3He/4He Ethiopian flood basalt province picrites are remarkably homogenous (e.g. 87Sr/86Sr = 0.70396-0.70412; 206Pb/204Pb = 18.82-19.01). In comparison the PIP picrites have ranges that span nearly the global range of E-MORB and N-MORB. The Afar and proto-Iceland mantle plumes are clearly not initiated in a single deep mantle domain with the same depletion/enrichment and degassing histories, and the same scale of heterogeneity. This implies that there is more than one plume source region/mechanism that is capable of generating comparable volumes of basalt melt

  1. Volatiles in Submarine HIMU Basalts from the Austral Islands, South Pacific

    NASA Astrophysics Data System (ADS)

    Nichols, A. R.; Hanyu, T.; Shimizu, K.; Dosso, L.

    2014-12-01

    Submarine basalts have been collected from the slopes of Rurutu and Tubuai in the Austral Islands, South Pacific with the manned submersible Shinkai 6500. Previous work on the bulk radiogenic isotope and trace element chemistry of these samples suggests that the basalts were generated from a HIMU reservoir derived from an ancient subducted slab that was entrained and mixed with the depleted asthenospheric mantle. Olivines and glasses from the submarine basalts show lower 3He/4He than MORB, similar to subaerial basalts from these islands. Sixteen glass chips from the same submarine samples have now undergone in-situ analysis for major elements (including S and Cl) by EPMA, trace elements by LA-ICP-MS, H2O and CO2 by FTIR, and bulk volatile analysis (S, Cl, F) by ion chromatography combined with pyrohydrolysis. H2O ranges from 0.62-2.44 wt%, while CO2 is below detection (<20 ppm). S measured by EPMA ranges from 612-1889 ppm and by bulk analysis from 582-1301 ppm and, with the exception of one sample, concentrations agree well. Cl measured by EPMA ranges from 151-538 ppm, and by bulk analysis from 188-980 ppm. The higher values suggest that the bulk samples may be contaminated by seawater; otherwise Cl correlates strongly with incompatible elements. F measured in the bulk samples ranges from 221-1243 ppm. S correlates positively with FeO and Cu, but not with incompatible elements, suggesting sulfide saturation. While the highest H2O contents may reflect late-stage hydration and are oversaturated at the depth of collection, the low H2O contents (11 samples with 0.62-0.96 wt%) are undersaturated, and there is a positive correlation between the H2O contents of all chips and their incompatible element concentrations. This suggests that H2O/Ce and Cl/Ce filtered for shallow level processes may reflect source compositions, providing constraints on volatiles in the sources of Rurutu and Tubuai, and indications about the efficiency of subduction-related volatile-loss in the

  2. Stress orientations in subduction zones and the strength of subduction megathrust faults

    USGS Publications Warehouse

    Hardebeck, Jeanne L.

    2015-01-01

    Subduction zone megathrust faults produce most of the world’s largest earthquakes. Although the physical properties of these faults are difficult to observe directly, their frictional strength can be estimated indirectly by constraining the orientations of the stresses that act on them. A global investigation of stress orientations in subduction zones finds that the maximum compressive stress axis plunges systematically trenchward, consistently making a 45°-60° angle to the subduction megathrust fault. These angles indicate that the megathrust fault is not substantially weaker than its surroundings. Together with several other lines of evidence, this implies that subduction zone megathrusts are weak faults in a low-stress environment. The deforming outer accretionary wedge may decouple the stress state along the megathrust from the constraints of the free surface.

  3. Subduction and volatile recycling in Earth's mantle

    NASA Technical Reports Server (NTRS)

    King, S. D.; Ita, J. J.; Staudigel, H.

    1994-01-01

    The subduction of water and other volatiles into the mantle from oceanic sediments and altered oceanic crust is the major source of volatile recycling in the mantle. Until now, the geotherms that have been used to estimate the amount of volatiles that are recycled at subduction zones have been produced using the hypothesis that the slab is rigid and undergoes no internal deformation. On the other hand, most fluid dynamical mantle flow calculations assume that the slab has no greater strength than the surrounding mantle. Both of these views are inconsistent with laboratory work on the deformation of mantle minerals at high pressures. We consider the effects of the strength of the slab using two-dimensional calculations of a slab-like thermal downwelling with an endothermic phase change. Because the rheology and composition of subducting slabs are uncertain, we consider a range of Clapeyron slopes which bound current laboratory estimates of the spinel to perovskite plus magnesiowustite phase transition and simple temperature-dependent rheologies based on an Arrhenius law diffusion mechanism. In uniform viscosity convection models, subducted material piles up above the phase change until the pile becomes gravitationally unstable and sinks into the lower mantle (the avalanche). Strong slabs moderate the 'catastrophic' effects of the instabilities seen in many constant-viscosity convection calculations; however, even in the strongest slabs we consider, there is some retardation of the slab descent due to the presence of the phase change.

  4. Earth's oldest mantle fabrics indicate Eoarchaean subduction

    PubMed Central

    Kaczmarek, Mary-Alix; Reddy, Steven M.; Nutman, Allen P.; Friend, Clark R. L.; Bennett, Vickie C.

    2016-01-01

    The extension of subduction processes into the Eoarchaean era (4.0–3.6 Ga) is controversial. The oldest reported terrestrial olivine, from two dunite lenses within the ∼3,720 Ma Isua supracrustal belt in Greenland, record a shape-preferred orientation of olivine crystals defining a weak foliation and a well-defined lattice-preferred orientation (LPO). [001] parallel to the maximum finite elongation direction and (010) perpendicular to the foliation plane define a B-type LPO. In the modern Earth such fabrics are associated with deformation of mantle rocks in the hanging wall of subduction systems; an interpretation supported by experiments. Here we show that the presence of B-type fabrics in the studied Isua dunites is consistent with a mantle origin and a supra-subduction mantle wedge setting, the latter supported by compositional data from nearby mafic rocks. Our results provide independent microstructural data consistent with the operation of Eoarchaean subduction and indicate that microstructural analyses of ancient ultramafic rocks provide a valuable record of Archaean geodynamics. PMID:26879892

  5. The State of Subduction in Southern Peru

    NASA Astrophysics Data System (ADS)

    Clayton, R. W.

    2014-12-01

    The crustal thickness in southern Peru has doubled to 70-75 km in the last 40Ma. Various causes are thought to contribute the additional volume including subduction related volcanism and compression and the curvature of the Arica Bend, but the sum of these appears to fall short of accounting for the needed volume. In a recent detailed seismic survey in southern Peru (PeruSE), we have been able to image the suduction zone, including the overriding continent and the slab in the regions of both flat and normal subduction in southern Peru. One surprising aspect of the image is the presence of a mid-crustal layer boundary that is pervasive in the back-arc of both the normal and flat subduction regimes. Our interpretation of this feature is an extension of an older model in which the Brazilian Shield is underthrusting the Andes, with the difference that this process (according to our image) extends under the Western Cordillera, almost to the current subduction arc. This would explain the missing crustal thickness.

  6. Earth's oldest mantle fabrics indicate Eoarchaean subduction.

    PubMed

    Kaczmarek, Mary-Alix; Reddy, Steven M; Nutman, Allen P; Friend, Clark R L; Bennett, Vickie C

    2016-01-01

    The extension of subduction processes into the Eoarchaean era (4.0-3.6 Ga) is controversial. The oldest reported terrestrial olivine, from two dunite lenses within the ∼3,720 Ma Isua supracrustal belt in Greenland, record a shape-preferred orientation of olivine crystals defining a weak foliation and a well-defined lattice-preferred orientation (LPO). [001] parallel to the maximum finite elongation direction and (010) perpendicular to the foliation plane define a B-type LPO. In the modern Earth such fabrics are associated with deformation of mantle rocks in the hanging wall of subduction systems; an interpretation supported by experiments. Here we show that the presence of B-type fabrics in the studied Isua dunites is consistent with a mantle origin and a supra-subduction mantle wedge setting, the latter supported by compositional data from nearby mafic rocks. Our results provide independent microstructural data consistent with the operation of Eoarchaean subduction and indicate that microstructural analyses of ancient ultramafic rocks provide a valuable record of Archaean geodynamics. PMID:26879892

  7. Earth's oldest mantle fabrics indicate Eoarchaean subduction

    NASA Astrophysics Data System (ADS)

    Kaczmarek, Mary-Alix; Reddy, Steven M.; Nutman, Allen P.; Friend, Clark R. L.; Bennett, Vickie C.

    2016-02-01

    The extension of subduction processes into the Eoarchaean era (4.0-3.6 Ga) is controversial. The oldest reported terrestrial olivine, from two dunite lenses within the ~3,720 Ma Isua supracrustal belt in Greenland, record a shape-preferred orientation of olivine crystals defining a weak foliation and a well-defined lattice-preferred orientation (LPO). [001] parallel to the maximum finite elongation direction and (010) perpendicular to the foliation plane define a B-type LPO. In the modern Earth such fabrics are associated with deformation of mantle rocks in the hanging wall of subduction systems; an interpretation supported by experiments. Here we show that the presence of B-type fabrics in the studied Isua dunites is consistent with a mantle origin and a supra-subduction mantle wedge setting, the latter supported by compositional data from nearby mafic rocks. Our results provide independent microstructural data consistent with the operation of Eoarchaean subduction and indicate that microstructural analyses of ancient ultramafic rocks provide a valuable record of Archaean geodynamics.

  8. Naming Lunar Mare Basalts: Quo Vadimus Redux

    NASA Astrophysics Data System (ADS)

    Ryder, G.

    1999-01-01

    Nearly a decade ago, I noted that the nomenclature of lunar mare basalts was inconsistent, complicated, and arcane. I suggested that this reflected both the limitations of our understanding of the basalts, and the piecemeal progression made in lunar science by the nature of the Apollo missions. Although the word "classification" is commonly attached to various schemes of mare basalt nomenclature, there is still no classification of mare basalts that has any fundamental grounding. We remain basically at a classification of the first kind in the terms of Shand; that is, things have names. Quoting John Stuart Mill, Shand discussed classification of the second kind: "The ends of scientific classification are best answered when the objects are formed into groups respecting which a greater number of propositions can be made, and those propositions more important than could be made respecting any other groups into which the same things could be distributed." Here I repeat some of the main contents of my discussion from a decade ago, and add a further discussion based on events of the last decade. A necessary first step of sample studies that aims to understand lunar mare basalt processes is to associate samples with one another as members of the same igneous event, such as a single eruption lava flow, or differentiation event. This has been fairly successful, and discrete suites have been identified at all mare sites, members that are eruptively related to each other but not to members of other suites. These eruptive members have been given site-specific labels, e.g., Luna24 VLT, Apollo 11 hi-K, A12 olivine basalts, and Apollo 15 Green Glass C. This is classification of the first kind, but is not a useful classification of any other kind. At a minimum, a classification is inclusive (all objects have a place) and exclusive (all objects have only one place). The answer to "How should rocks be classified?" is far from trivial, for it demands a fundamental choice about nature

  9. Oxygen consumption in subseafloor basaltic crust

    NASA Astrophysics Data System (ADS)

    Orcutt, B. N.; Wheat, C. G.; Hulme, S.; Edwards, K. J.; Bach, W.

    2012-12-01

    Oceanic crust is the largest potential habitat for life on Earth and may contain a significant fraction of Earth's total microbial biomass, yet little is known about the form and function of life in this vast subseafloor realm that covers nearly two-thirds of the Earth's surface. A deep biosphere hosted in subseafloor basalts has been suggested from several lines of evidence; yet, empirical analysis of metabolic reaction rates in basaltic crust is lacking. Here we report the first measure of oxygen consumption in young (~ 8 Ma) and cool (<25 degrees C) basaltic crust, calculated from modeling oxygen and strontium profiles in basal sediments collected during Integrated Ocean Drilling Program (IODP) Expedition 336 to 'North Pond', a sediment 'pond' on the western flank of the Mid-Atlantic Ridge (MAR), where vigorous fluid circulation within basaltic crust occurs. Dissolved oxygen concentrations increased towards the sediment-basement interface, indicating an upward diffusional supply from oxic fluids circulating within the crust. A parametric reaction-transport model suggests oxygen consumption rates on the order of 0.5-500 nmol per cubic centimeter fluid per day in young and cool basaltic crust, providing sufficient energy to support a subsurface crustal biosphere.

  10. Can we identify source lithology of basalt?

    PubMed

    Yang, Zong-Feng; Zhou, Jun-Hong

    2013-01-01

    The nature of source rocks of basaltic magmas plays a fundamental role in understanding the composition, structure and evolution of the solid earth. However, identification of source lithology of basalts remains uncertainty. Using a parameterization of multi-decadal melting experiments on a variety of peridotite and pyroxenite, we show here that a parameter called FC3MS value (FeO/CaO-3*MgO/SiO2, all in wt%) can identify most pyroxenite-derived basalts. The continental oceanic island basalt-like volcanic rocks (MgO>7.5%) (C-OIB) in eastern China and Mongolia are too high in the FC3MS value to be derived from peridotite source. The majority of the C-OIB in phase diagrams are equilibrium with garnet and clinopyroxene, indicating that garnet pyroxenite is the dominant source lithology. Our results demonstrate that many reputed evolved low magnesian C-OIBs in fact represent primary pyroxenite melts, suggesting that many previous geological and petrological interpretations of basalts based on the single peridotite model need to be reconsidered. PMID:23676779

  11. The global systematics of subduction zones

    NASA Astrophysics Data System (ADS)

    Syracuse, Ellen M.

    To better understand the global systematics of subduction zones, a series of studies investigates their variability taking advantage of comprehensive and accurate seismicity catalogs and advances in computing for wave propagation and geodynamic modeling. Every subduction zone with sufficient intermediate-depth earthquakes (IDE) from a global teleseismic catalog is analyzed and the top of the IDE, presumably the top of the slab, is digitized. Subduction zones are separated into a total of 52 segments 500-km-long. Parameters such as dip, age, convergence velocity, and slab depth beneath arc volcanoes (H) are compiled, resulting in a comprehensive, complete suite of subduction zone descriptions. Surprisingly, H ranges from 70-190 km globally and varies smoothly between arc segments, even though most models and textbooks assume constant H for all arcs, placing new constraints on magma generation models at arcs. To assess regional biases in earthquake location due to large-scale structure, IDEs in each arc segment are relocated in a three-dimensional global velocity model. Although the absolute position of slab surfaces shifts 0-25 km regionally, global variations in H persist. These variations in geometry, as well as slab age, convergence velocity, sediments, the overlying plate, and the location of the transition from localized slip and distributed flow create a large range in the thermal states of subduction zones. Two dimensional thermal kinematic-flow models using these slab geometries for each 500-km segment indicate that slab crust and sediments dehydrate before reaching beneath the arc, whereas slab mantle may still be hydrated, for all slabs and a variety of assumptions. To test these inferences, a temporary array of seismographs was deployed in Central America, sampling the slab and sub-arc mantle. P and S arrival times were inverted for earthquake locations and high-resolution regional velocity structure. Hypocenters confirm high H here. The velocity models

  12. Subduction: The Gatekeeper for Mantle Melting.

    NASA Astrophysics Data System (ADS)

    Kincaid, C. R.; Druken, K. A.; Griffiths, R. W.

    2011-12-01

    Geodynamic models are used to show the importance of subduction in controlling vertical thermal and chemical fluxes from Earth's interior to surface. In our models subduction-induced circulation produces conditions favorable to both steady-state and episodic melt production and also plays the role of gatekeeper in thwarting large scale melt production from rising plumes. We use laboratory experiments to characterize three-dimensional (3D) flow fields in convergent margins in response to a range of subduction and back-arc deformation styles, and how these flows interact with upwellings. Models utilize a glucose working fluid with a temperature dependent viscosity to represent the upper 2000 km of the mantle. Subducting lithosphere is modeled with a descending Phenolic plate and back-arc extension is produced by moving Mylar sheets. Thermal plumes are generated from a pressurized, temperature controlled source. Our results show that naturally occurring transitions from downdip- to rollback-dominated subduction produce conditions that favor both widespread decompression melting in the mantle wedge and short-lived pulses of extensive slab melting. For cases of plume-subduction interaction, 3D slab-induced flow quickly converts the active upwelling to a passive thermal anomaly that bears little to no resemblance to traditional models for plume surface expressions. Instead of rising to make LIPs with age-progressive chains, the bulk of the original plume material is trapped below depths of melt production before being re-subducted by the slab. A limited volume of this passive, former plume material is capable of surfacing. Interestingly, this is seen to occur through a range of morphologies that are consistently offset from the original rise location (e.g., conduit). Surface expressions include anything from small circular patches to long, linear features with complex age trends (e.g., progressive or regressive) resulting from the competition between plate motions and

  13. Numerical comparison of different convergent plate contacts: subduction channel and subduction fault

    NASA Astrophysics Data System (ADS)

    De Franco, Roberta; Govers, Rob; Wortel, Rinus

    2007-10-01

    At convergent plate boundaries, the properties of the actual plate contact are important for the overall dynamics. Convergent plate boundaries both mechanically decouple and link tectonic plates and accommodate large amounts of strain. We investigate two fundamental physical states of the subduction contact: one based on a fault and the other based on a subduction channel. Using a finite element method, we determine the specific signatures of both states of the subduction contact. We pay particular attention to the overriding plate. In a tectonic setting of converging plates, where the subducting plate is freely moving, the subduction channel reduces compression relative to the fault model. In a land-locked basin setting, where the relative motion between the far field of the plates is zero, the subduction channel model produces tensile stress regime in the overriding plate, even though the amount of slab roll-back is small. The fault model shows a stronger development of slab roll-back and a compressive stress regime in the upper plate. Based on a consistent comparison of fault and channel numerical models, we find that the nature of the plate contact is one of the controlling factors in developing or not of backarc extension. We conclude that, the type of plate contact plays a decisive role in controlling the backarc state of stress. To obtain backarc extension, roll-back is required as an underling geodynamic process, but it is not always a sufficient condition.

  14. Geochemistry of the Early Miocene volcanic succession of Northland, New Zealand, and implications for the evolution of subduction in the Southwest Pacific

    NASA Astrophysics Data System (ADS)

    Booden, Mathijs A.; Smith, Ian E. M.; Black, Philippa M.; Mauk, Jeffrey L.

    2011-01-01

    Latest Oligocene and Early Miocene volcanic rocks occur on the Northland Peninsula, New Zealand, and record the inception of Cenozoic subduction-related volcanism in the North Island that eventually evolved to its present manifestation in the Taupo Volcanic Zone. This NW-striking Northland Arc is continuous with the Reinga Ridge and comprises two parallel belts of volcanic centres ca. 60 km apart. A plethora of tectonic models have been proposed for its origins. We acquired new trace element and Sr-Nd isotope data to better constrain such models. All Northland Arc rocks carry an arc-type trace element signature, however distinct differences exist between rocks of the eastern and western belt. Eastern belt rocks are typically andesites and dacites and have relatively evolved isotope ratios indicating assimilated crustal material, and commonly contain hornblende. Additionally some eastern belt rocks with highly evolved isotope compositions show fractionated REE compositions consistent with residual garnet, and some contain garnetiferous inclusions in addition to schistose crustal fragments. In contrast, western belt rocks are mostly basalts or basaltic andesites with relatively primitive Sr-Nd isotope compositions, do not contain hornblende and show no rare earth element evidence for cryptic amphibole fractionation. Eastern and western belt rocks contain comparable slab-derived fractions of fluid-mobile trace elements and invariably possess an arc signature. Therefore the difference between the belts may be best explained as due to variation in crustal thickness across the Northland Peninsula, where western belt centres erupted onto a thinner crustal section than eastern belt rocks. The consistent arc signature throughout the Northland arc favours an origin in response to an actual, if short-lived subduction event, rather than slab detachment as proposed in some models. No Northland Arc rocks possess a convincing adakite-like composition that might reflect the

  15. Lithoautotrophic microbial ecosystems in deep basalt aquifers

    SciTech Connect

    Stevens, T.O.; McKinley, J.P.

    1995-10-20

    Bacterial communities were detected in deep crystalline rock aquifers within the Columbia River Basalt Group (CRB). CRB ground waters contained up to 60 {mu}M dissolved H{sub 2} and autotrophic microorganisms outnumbered heterotrophs. Stable carbon isotope measurements implied that autotrophic methanogenesis dominated this ecosystem and was coupled to the depletion of dissolved inorganic carbon. In laboratory experiments, H{sub 2} a potential energy source for bacteria, was produced by reactions between crushed basalt and anaerobic water. Microcosms containing only crushed basalt and ground water supported microbial growth. These results suggest that the CRB contains a lithoautotrophic microbial ecosystem that is independent of photosynthetic primary production. 38 refs., 4 figs., 3 tabs.

  16. Sintering of lunar glass and basalt

    NASA Technical Reports Server (NTRS)

    Allen, Carlton C.; Hines, Joy A.; Mckay, David S.; Morris, Richard V.

    1992-01-01

    Experiments were conducted to study the sintering behavior of glass and basalt lunar soil simulants. The degree of sintering was assessed by compressive strength testing and microanalysis. Both crushed glass and basalt sinter significantly at 1000 C, with the basalt attaining its maximum strength at 1100 C. Initial sintering occurs in less than 15 min, and the degree of sintering does not increase significantly with time after about 30 min. Glass sinters more readily than crystalline material. Sintering and devitrification both occur on a time scale of minutes in the heated glass, but sintering is apparently more rapid. The processes of sintering and oxygen release by hydrogen reduction of lunar soil are synergistic, and could be combined to produce two extremely useful products at a lunar base.

  17. Hf-Nd Isotopic and Trace-Element Geochemistry of Global Subducting Sediments

    NASA Astrophysics Data System (ADS)

    Vervoort, J. D.; Plank, T.; Patchett, P. J.

    2001-12-01

    Ferromanganese nodules, crusts, and associated metalliferous clays have long been known to have anomalously high Lu/Hf ratios and highly radiogenic Hf relative to Nd (Patchett et al., 1984; White et al., 1986). These oceanic sediments are some of the few terrestrial materials where Hf and Nd isotopes deviate from the crust-mantle array. This distinctive isotopic signature, therefore, has the potential to trace the fate of oceanic sediments through the subduction zone and into the mantle. It has recently been suggested, for example, that pelagic sediments can be detected in some Hawaiian basalts (Blichert-Toft et al., 1999) and in volcanic rocks from the Luzon arc (Marini et al., 2000) based on their Hf-Nd isotopic compositions. The weak link in this approach, however, is that we do not know, in any quantitative way, how widespread this anomalous signature is in oceanic sediments, what compositions are responsible for this signature, or how volumetrically important these compositions are in terms of the total sediment flux into subduction zones and the mantle. Most marine sediments analyzed thus far have been collected on or near the ocean floor and constitute an incomplete and unrepresentative inventory of the sediment column bound for the subduction zone. There is some reason to suspect that much of the sediment flux is not particularly anomalous, either in terms of Lu/Hf ratios or Hf and Nd isotopic compositions. The most dominant sediment types entering many subduction zones (terrigenous and other continentally derived sediments), have normal Lu/Hf ratios and Hf-Nd isotopic compositions that are indistinguishable from the crust-mantle array. An examination is needed of the Hf-Nd isotopic composition of oceanic sediments, the major and trace-element geochemistry of global sediment flux, how such compositions may relate to Hf-Nd isotopic behavior, and potential Lu/Hf and Nd/Hf fractionation in subduction zones. In addition, the origin of the high Lu/Hf and

  18. Garnet growth as a proxy for progressive dehydration in subduction zones

    NASA Astrophysics Data System (ADS)

    Caddick, M. J.; Baxter, E. F.

    2012-12-01

    The release of volatiles from subducting lithologies is a crucial triggering process for arc magmatism, seismicity, the growth and maturation of continents, and the global geological H2O-CO2 cycle. While numerous models have been developed to predict slab volatile release, it has proven challenging to reconstruct and test these fluid fluxes released from specific lithologies in the rock record. Here we show that the growth of garnet may be used as a proxy for progressive devolatilization at blueschist to eclogite facies conditions in subduction zones. Generally, as garnet grows in a subducting rock, fluid is produced due to metamorphic dehydration reactions. Using rigorous thermodynamic analysis, which includes the crucial effects of phase fractionation, we model the proportional relationship between garnet and water production in common lithologies (pelitic sediment and hydrated MORB) along three representative subduction geotherms. The results show that several dehydration reactions contribute to garnet growth especially within a crucial span of the subduction zone (~1.5 to 2.5 GPa) within which slab-mantle decoupling has been predicted to occur in some models and volatile fluxes may be focused. The water:garnet production ratio varies during garnet growth, constrained by the specific hydrous reactant phases that are breaking down, but the average water:garnet production ratios are surprisingly consistent regardless of composition and geotherm. Over the garnet growth interval ~400 to 700 C (and corresponding depths for each geotherm) the average production ratio for altered MORB compositions is 0.52 (wt % water per vol % garnet) in cooler geotherms (Honshu and Nicaragua) and 0.27 in hotter (Cascadia) geotherms, with predictably lower ratios if the input basalt previously experienced less hydrous alteration. Over the same interval the water production ratios are approximately 50 % lower for pelite (0.24 and 0.13, respectively). Lower temperature water release is

  19. 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

  20. CO2 sequestration in basalts: laboratory measurements

    NASA Astrophysics Data System (ADS)

    Otheim, L. T.; Adam, L.; van Wijk, K.; McLing, T. L.; Podgorney, R. K.

    2010-12-01

    Geologic sequestration of CO2 is proposed as the only promising large-scale method to help reduce CO2 gas emission by its capture at large point sources and subsequent long-term storage in deep geologic formations. Reliable and cost-effective monitoring will be important aspect of ensuring geological sequestration is a safe, effective, and acceptable method for CO2 emissions mitigation. Once CO2 injection starts, seismic methods can be used to monitor the migration of the carbon dioxide plume. To calibrate changes in rock properties from field observations, we propose to first analyze changes in elastic properties on basalt cores. Carbon dioxide sequestration in basalt rocks results in fluid substitution and mixing of CO2 with water and rock mineralizations. Carbon dioxide sequestration in mafic rocks creates reactions such as Mg2SiO 4 + CaMgSi2O 6 + 4CO2 = Mg 3Ca(CO 3) 4 + 3SiO2 whereby primary silicate minerals within the basalt react with carbonic acid laden water to creating secondary carbonate minerals and silicates. Using time-lapse laboratory scale experiments, such as laser generated ultrasonic wave propagation; it is possible to observe small changes in the physical properties of a rock. We will show velocity and modulus measurements on three basalt core samples for different saturation. The ultimate goal of the project is to track seismic changes due to fluid substitution and mineralization. The porosity of our basalts ranges from 8% to 12%, and the P-wave velocity increases by 20% to 40% from dry to water saturated conditions. Petrographic analysis (CT-scans, thin sections, XRF, XRf) will aid in the characterization of the mineral structure in these basalts and its correlation to seismic properties changes resulting from fluid substitution and mineralization.

  1. Bimodal volcanism in northeast Puerto Rico and the Virgin Islands (Greater Antilles Island Arc): Genetic links with Cretaceous subduction of the mid-Atlantic ridge Caribbean spur

    NASA Astrophysics Data System (ADS)

    Jolly, Wayne T.; Lidiak, Edward G.; Dickin, Alan P.

    2008-07-01

    Bimodal extrusive volcanic rocks in the northeast Greater Antilles Arc consist of two interlayered suites, including (1) a predominantly basaltic suite, dominated by island arc basalts with small proportions of andesite, and (2) a silicic suite, similar in composition to small volume intrusive veins of oceanic plagiogranite commonly recognized in oceanic crustal sequences. The basaltic suite is geochemically characterized by variable enrichment in the more incompatible elements and negative chondrite-normalized HFSE anomalies. Trace element melting and mixing models indicate the magnitude of the subducted sediment component in Antilles arc basalts is highly variable and decreases dramatically from east to west along the arc. In the Virgin Islands, the sediment component ranges between< 0.5 to ˜ 1% in Albian rocks, and between ˜ 1 and 2% in succeeding Cenomanian to Campanian strata. In comparison, sediment proportions in central Puerto Rico range between 0.5 to 1.5% in the Albian to 2 to > 4% during the Cenomanian-Campanian interval. The silicic suite, consisting predominantly of rhyolites, is characterized by depleted Al 2O 3 (average < 16%), low Mg-number (molar Mg/Mg + Fe < 0.5), TiO 2 (< 1.0%), and Sr/Y (< 10), oceanic or arc-like Sr, Nd, and Pb isotope signatures, and by the presence of plagioclase. All of these features are consistent with an anatexic origin in gabbroic sources, of both oceanic and arc-related origin, within the sub-arc basement. The abundance of silicic lavas varies widely along the length of the arc platform. In the Virgin Islands on the east, rhyolites comprise up to 80% of Lower Albian strata (112 to 105 Ma), and about 20% in post-Albian strata (105 to 100 Ma). Farther west, in Puerto Rico, more limited proportions (< 20%) of silicic lavas were erupted. The systematic variation of both sediment flux and abundance of crustally derived silicic lavas are consistent with current tectonic models of Caribbean evolution involving approximately

  2. Basaltic Volcanism and Ancient Planetary Crusts

    NASA Technical Reports Server (NTRS)

    Shervais, John W.

    1993-01-01

    The purpose of this project is to decipher the origin of rocks which form the ancient lunar crust. Our goal is to better understand how the moon evolved chemically and, more generally, the processes involved in the chemical fractionation of terrestrial planetoids. This research has implications for other planetary bodies besides the Moon, especially smaller planetoids which evolved early in the history of the solar system and are now thermally stable. The three main areas focused on in our work (lunar mare basalts, KREEP basalts, and plutonic rocks of the lunar highlands) provide complementary information on the lunar interior and the processes that formed it.

  3. Vapor deposition in basaltic stalactites, Kilauea, Hawaii

    NASA Astrophysics Data System (ADS)

    Baird, A. K.; Mohrig, D. C.; Welday, E. E.

    Basaltic stalacties suspended from the ceiling of a large lava tube at Kilauea, Hawaii, have totally enclosed vesicles whose walls are covered with euhedral FeTi oxide and silicate crystals. The walls of the vesicles and the exterior surfaces of stalactites are Fe and Ti enriched and Si depleted compared to common basalt. Minerals in vesicles have surface ornamentations on crystal faces which include alkali-enriched, aluminosilicate glass(?) hemispheres. No sulfide-, chloride-, fluoride-, phosphate- or carbonate-bearing minerals are present. Minerals in the stalactites must have formed by deposition from an iron oxide-rich vapor phase produced by the partial melting and vaporization of wall rocks in the tube.

  4. Evidence for Sulfur Degassing in Oceanic Basalts

    NASA Astrophysics Data System (ADS)

    Wetzel, D. T.; Saal, A. E.; Rutherford, M. J.; Hauri, E. H.

    2010-12-01

    Sulfur concentration in basaltic magmas is highly dependent on pressure, temperature, fO2, fS2, and bulk composition and therefore undergoes a complex history during melting, melt transport, degassing, and eruption. It was first recognized as a volatile in submarine basalts by Moore and Fabbi (1971) when the S content in glassy pillow rims was higher than the amount of S measured in degassed lavas at the surface. Subsequent studies concluded that degassing of S is not expected to occur in basaltic magmas erupting deeper than 500m below sea level (~50 bars). Therefore, once sulfide saturation (Liu et al, 2007) and fractionation of sulfide melts have been considered, pre-eruptive sulfur concentrations in basaltic magmas have been used to estimate the primitive S content in the melts and their mantle sources. Based on a large compilation of submarine glasses, we will show that basaltic magmas have lost not only CO2 and H2O but also S during degassing at pressures greater than 50 bars. Melt inclusion and glass compositions from submarine oceanic basalts were compiled to examine sulfur variations. Data was filtered for sulfide undersaturated samples using Liu et al.’s (2007) calculation since H2O content was available. A positive correlation between S and Dy was expected as seen by previous studies, which considered S for sulfide undersaturated basalts to behave similar to HREE (Dy) during melting and crystal fractionation (Morgan, 1986; Saal et al, 2002). A S/Dy ratio of 240±20 ppm was determined for the data compilation of the basaltic glasses. A subgroup of samples, dominantly glasses enriched in incompatible trace elements, showed lower S/Dy than the ratio determined above. This lower S/Dy would be controlled either by a change in S or Dy. A simple melting model was used to confirm that the change in S/Dy was not controlled by Dy variation due to the effect of garnet during mantle melting, but by the decrease in S content. The most likely explanation for this

  5. Slab and sediment melting during subduction initiation: granitoid dykes from the mantle section of the Oman ophiolite

    NASA Astrophysics Data System (ADS)

    Rollinson, Hugh

    2015-09-01

    New geochemical data are presented for a suite of tonalites, granodiorites, trondhjemites and granites intrusive into depleted mantle harzburgites of the Oman-UAE ophiolite. A detailed field, petrological and geochemical examination suggests that these `mantle granitoids' are the product of three processes: (a) the mixing of melts derived from both mafic and metasedimentary sources, (b) interaction with the mantle harzburgite host and (c) the fractional crystallisation of plagioclase, hornblende ± accessory phases. Geochemical data are used to characterise the identity of the protolith(s) by first screening the data for those samples which have experienced fractional crystallisation during emplacement. The resultant `reduced' data set has moderately fractionated REE, with small negative Eu anomalies and fractionated primitive mantle-normalised trace element patterns with high concentrations of fluid mobile elements and lower concentrations of HFS elements and with positive peaks for Rb and Pb and negative troughs for Ba, Nb, Sr and Ti. The character of the protolith was quantified using a melting model based upon a MORB-type basalt similar in composition to the Oman Geotimes lavas and a model using the MUQ (MUd from Queensland) global sediment composition (Kamber et al. Geochim Cosmochim Acta 69:1041-1058, 2005) both with an amphibolite/granulite facies mineralogy. The two compositions bracket the mantle granitoid data set with partial melts of the MORB source yielding trace element compositions lower than the granitoids, whereas melts of the MUQ source yield melts with compositions higher than the granitoids. Mixing of the calculated melt compositions indicates that the measured granitoid compositions represent between 10 and 30 % mixing of a metasedimentary melt into the melt of a mafic source. Current petrological, structural and geochronological data suggest a model for the origin of the Oman ophiolite in which it is formed by spreading above a subduction zone

  6. Decarbonation of subducting slabs: insight from thermomechanical-petrological numerical modelling

    NASA Astrophysics Data System (ADS)

    Gonzalez, Christopher M.; Gorczyk, Weronika; Gerya, Taras

    2015-04-01

    This work extends a numerical geodynamic modelling code (I2VIS) to simulate subduction of carbonated lithologies (altered basalts and carbonated sediments) into the mantle. Code modifications now consider devolatilisation of H2O-CO2 fluids, a CO2-melt solubility parameterisation for molten sediments, and allows for carbonation of mantle peridotites. The purpose is to better understand slab generated CO2 fluxes and consequent subduction of carbonates into the deep mantle via numerical simulation. Specifically, we vary two key model parameters: 1) slab convergence rate (1,2,3,4,5 cm y-1) and 2) converging oceanic slab age (20,40,60,80 Ma) based on a half-space cooling model. The aim is to elucidate the role subduction dynamics has (i.e., spontaneous sedimentary diapirism, slab roll-back, and shear heating) with respect to slab decarbonation trends not entirely captured in previous experimental and thermodynamic investigations. This is accomplished within a fully coupled petrological-thermomechanical modelling framework utilising a characteristics-based marker-in-cell technique capable of solving visco-plastic rheologies. The thermodynamic database is modified from its original state to reflect the addition of carbonate as CO2 added to the rock's overall bulk composition. Modifications to original lithological units and volatile bulk compositions are as follows: GLOSS average sediments (H2O: 7.29 wt% & CO2: 3.01 wt%), altered basalts (H2O: 2.63 wt% & CO2: 2.90 wt%), and metasomatised peridotite (H2O: 1.98 wt% & CO2: 1.5 wt%). We resolve stable mineralogy and extract rock properties via PerpleX at a resolution of 5K and 25 MPa. Devolatilisation/consumption and stability of H2O-CO2 fluid is determined by accessing the thermodynamic database. When fluid is released due to unstable conditions, it is tracked via markers that freely advect within the velocity field until consumed. 56 numerical models were completed and our results show excellent agreement in dynamics with

  7. Behavior of K in Eclogite from Subducted Slab at Pressures from 4 to 10 GPa

    NASA Astrophysics Data System (ADS)

    Han, L.; Fei, Y.; Zhang, L.

    2014-12-01

    The behavior of potassium (K) in eclogite from deep subducted slab is rarely studied, even though there have been many experimental studies in basalt or clinopyroxene systems at high pressure and temperature. We have performed high P-T sub-solidus experiments in the eclogite system with excess K up to 10 GPa, using the multi-anvil press. From 4 to 10 GPa, there is a systematic increase of K2O content in omphacite from ~0.4 wt% to ~4.4 wt%. Therefore, eclogitic omphacite could be an important carrier to transfer K into the deep mantle. The partitioning coefficient of K (Dk(Grt/Omp)) between garnet and clinopyroxene has a negative trend with increasing pressure, indicating that K is preferred to be incorporated into omphacite when the slab is subducted to deep interior. As the amount of AlVI in the stoichiometric formula of omphacite is limited, the content of the end-member NaAlVISi2O6 should be decreased with increasing pressure, because K substitutes for AlVI to produce the end-member KAlVISi2O6. The end-member substitution of diopside (CaMgSi2O6) by K-jadetie (KAlSi2O6) in omphacite results in the increases of Ca, Mg in garnet and of K, Al in omphacite. The K content in clinopyroxene is strongly depended on the mineral composition and pressure.

  8. Geochemical and petrologic investigation of the Ola Plateau-basalts from the Okhotsk-Chukotka Volcanic Belt (NE Russia)

    NASA Astrophysics Data System (ADS)

    Leitner, Jürgen; Ntaflos, Theodoros; Akinin, Vyacheslav; Tschegg, Cornelius

    2010-05-01

    The Okhotsk-Chukotka volcanic belt to a large degree consists of coeval Cretaceous and Early Tertiary volcanic and plutonic rocks that occur along the continental margin in northeast Russia. These igneous-arc related rocks build up an Andean-style magmatic arc sequence that occurs for about 3.500 km along the entire length of the Eurasian continent, from Chukotka Peninsula in the north down to north-east China. The rocks of the Okhotsk-Chukotka Volcanic Belt (OCVB) comprise Late Cretaceous, andesitic basalts, andesites, dacites, rhyolites, tuffs, rare beds of nonmarine clastic rocks with conglomerates and sandstones in the base and locally Paleocene gently dipping basalts. The duration of the magmatic activity in the Okhotsk-Chukotka volcanic belt is still in debate but generally it has been estimated from middle of Albian to Campanian. The studied area, the Ola Plateau Basalts (OPB) and the Hypotetica Basalts (HB), comprise basaltic andesites, trachy- basalts, basaltic trachy- andesite and rhyolitic dykes, belongs to the Okhotsk-Cukotka volcanic belt and represents the last volcanic activity related to the subduction of the palaeo-Pacific plate in this region. The exposed lavas have a thickness of 0.5 km and the estimated volume is about 222 km³. Fine grained 4 m thick rhyolitic dykes represent the very last event of the studied sequence. According to Ar/Ar and U/Pb dating (Hourigan, Akinin, 2004;), the average age of the OPB/ HB is 78.8 to 74 Ma. The basaltic rocks that build up the Ola Plateau are mainly fine grained calc- alkaline basalts with clinopyroxene, plagioclase and strongly to moderately altered olivine phenocrysts with spinel inclusions. The Mg# of the calc- alkaline basalts vary from 0.35 to 0.57 and the TiO2 from 1.2 to 2.2 wt% whereas CaO correlates positive with MgO contents. The OPB and HB lavas, according to their primitive mantle normalized trace elements, can be divided into three groups: Group (I) is characterized by positive Sr anomaly with

  9. The Seismic Cycle on Spontaneously Evolving Subduction Faults in Geodynamic Simulations

    NASA Astrophysics Data System (ADS)

    Mai, P. M.; van Dinther, Y.; Gerya, T.; Dalguer, L. A.; Morra, G.

    2012-12-01

    vertical displacements of several meters. The spatio-temporal evolution of geodetic displacements identifies these events, but has minor impact on long-term deformation as these elastic events almost fully rebound to their original displacements. We find that the downdip limit of the seismogenic zone is located at temperatures around 400 degrees Celsius (depth ~ 45 km), where ductile flow starts to dominate and plastic strengths are high. Our results reveal also interaction between the main thrust, dominating the seismic response and located within the partially locked basaltic crust, and occasionally activated faults within the outer-rise (trench-ward dipping normal faults related to slab bending) and sedimentary wedge (both splay faults and back-thrusts). Spatio-temporal energy dissipation between different event clusters vary with different subduction regimes, which are most likely distinguished by thrust coupling. Our results also demonstrate that hydrational weakening (1-10% of its original strength) is needed to sustain a shallow dipping thrust interface along the subducting plate.

  10. Long-distance fluid and heat transport in the oceanic crust entering the Nankai subduction zone, NanTroSEIZE transect

    NASA Astrophysics Data System (ADS)

    Spinelli, Glenn A.

    2014-03-01

    I examine the potential causes of anomalous seafloor heat flux on the oceanic plate in the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) transect offshore southern Japan. The most prominent anomaly is a ˜50 mW m change in heat flux between Integrated Ocean Drilling Program Sites C0011 and C0012 over a distance of <10 km. I develop thermal models to investigate the effects of hydrothermal circulation in the basaltic basement of the oceanic crust and variations in heat input from the mantle. The <10 km wide transition in surface heat flux reflects a process in the shallow subsurface; variations in heat input from ⩾5 km depth would generate a >30 km wide transition at the seafloor. The observed surface heat flux pattern is indicative of hydrothermal circulation in the basement aquifer and advection of heat from the subducted crust into the aquifer on the incoming plate. For a 600 m thick aquifer, the permeability is likely ⩾7×10-11 m, and hydrothermal circulation transports at least 300 times more heat than conduction alone. The heat flux from the subduction zone seaward to the incoming plate is consistent with hydrothermal circulation in the subducting crust persisting to ˜100 km landward of the deformation front. Vigorous fluid circulation in the basaltic basement is consistent with both the seafloor thermal anomalies and geochemical anomalies near the sediment-basement interface.

  11. Subduction-related metamorphism beneath ophiolites (Oman) and during early stages of continental collision (Himalaya)

    NASA Astrophysics Data System (ADS)

    Searle, Mike; Waters, David; Cowan, Robert; Cherry, Alan; Cooper, Charles

    2014-05-01

    Subduction-related metamorphism occurs beneath ophiolites (Oman), beneath island arcs (Kohistan) and during the early stages of continental collision (Kaghan, Tso Morari; Himalaya). Ophiolite obduction necessarily involves subduction of first oceanic, then continental crust to mantle depths beneath the ophiolite. In Oman an inverted pressure and temperature profile is exposed beneath the Semail ophiolite from garnet+clinopyroxene-bearing granulite to hornblende+plagioclase amphibolite down through epidote amphibolite and a variety of greenschist facies meta-sediments, dominantly cherts, marbles and quartzites. Thermobarometry on Grt+Cpx-bearing amphibolites immediately beneath the contact with mantle sequence harzburgites shows that the upper sole rocks formed at PT conditions of 770-900°C and 11-13 kbar, equivalent to depths of 30-40 km in oceanic lithosphere. Heat for metamorphism can only have been derived from the overlying mantle peridotites. Pressures are higher than can be accounted for by the thickness of the preserved ophiolite (15-20 km). Timing of peak metamorphism was synchronous with formation of the ophiolite gabbroic - trondhjemite crustal sequence and eruption of the pillow lavas (Cenomanian; 96-95 Ma). During the later stages of obduction the continental margin was dragged down to depths of nearly 100 km and basaltic sills within calc-schists were converted to eclogites (20-25 kbar; 500-560oC; 79.1 Ma), then exhumed back up the same subduction channel. Apparent 'extensional' fabrics throughout the HP units are related to upward flow of deeply buried rocks in a wholly compressional environment. Eclogites in a similar structural position occur along the Himalaya in the northernmost exposures of Indian plate rocks. These eclogites formed either during the latest stage of ophiolite obduction or the earliest stage of continental collision.

  12. Hydrogeological responses to incoming materials at the erosional subduction margin, offshore Osa Peninsula, Costa Rica

    NASA Astrophysics Data System (ADS)

    Kameda, Jun; Harris, Robert N.; Shimizu, Mayuko; Ujiie, Kohtaro; Tsutsumi, Akito; Ikehara, Minoru; Uno, Masaoki; Yamaguchi, Asuka; Hamada, Yohei; Namiki, Yuka; Kimura, Gaku

    2015-09-01

    Bulk mineral assemblages of sediments and igneous basement rocks on the incoming Cocos Plate at the Costa Rica subduction zone are examined by X-ray diffraction analyses on core samples. These samples are from Integrated Ocean Drilling Program Expedition 334 reference Site U1381, ˜ 5 km seaward of the trench. Drilling recovered approximately 100 m of sediment and 70 m of igneous oceanic basement. The sediment includes two lithologic units: hemipelagic clayey mud and siliceous to calcareous pelagic ooze. The hemipelagic unit is composed of clay minerals (˜50 wt.%), quartz (˜5 wt.%), plagioclase (˜5 wt.%), calcite (˜15 wt.%) and ˜30 wt.% of amorphous materials, while the pelagic unit is mostly made up of biogenic amorphous silica (˜50 wt.%) and calcite (˜50 wt.%). The igneous basement rock consists of plagioclase (˜50-60 wt.%), clinopyroxene (˜>25 wt.%), and saponite (˜15-40 wt.%). Saponite is more abundant in pillow basalt than in the massive section, reflecting the variable intensity of alteration. We estimate the total water influx of the sedimentary package is 6.9 m3/yr per m of trench length. Fluid expulsion models indicate that sediment compaction during shallow subduction causes the release of pore water while peak mineral dehydration occurs at temperatures of approximately ˜100°C, 40-30 km landward of the trench. This region is landward of the observed updip extent of seismicity. We posit that in this region the presence of subducting bathymetric relief capped by velocity weakening nannofossil chalk is more important in influencing the updip extent of seismicity than the thermal regime.

  13. The dynamic history of the Trans-Mexican Volcanic Belt and the Mexico subduction zone

    NASA Astrophysics Data System (ADS)

    Ferrari, Luca; Orozco-Esquivel, Teresa; Manea, Vlad; Manea, Marina

    2012-02-01

    The Trans-Mexican Volcanic Belt (TMVB) is a 1000 km long Neogene continental arc showing a large variation in composition and volcanic style, and an intra-arc extensional tectonics. It overlies the Rivera and Cocos slabs, which display marked changes in geometry. Geophysical studies indicate that lithospheric mantle is very thin or absent beneath the forearc and arc, the fluids from the slab are released in a 40 to 100 km wide belt beneath the frontal part of the arc, and the lower crust beneath the arc is partially molten. East of 101°W the TMVB is built on a Precambrian to Paleozoic crust with thickness of 50-55 km. West of 101°W the TMVB is underlain by Jurassic to Cenozoic marine and continental arcs with a 35-40 km thick crust. The evolution of the TMVB occurred in four stages: 1) from ~ 20 to 10 Ma the initial andesitic arc moved inland showing progressively drier melting and, eventually, slab melting, suggesting flattening of the subducted slab; 2) since ~ 11 Ma a pulse of mafic volcanism migrated from west to east reaching the Gulf of Mexico by 7 Ma. This mafic lavas marks the lateral propagation of a slab tear, triggered by cessation of subduction beneath Baja California; 3) thereafter, the volcanic front started moving trenchward, with a marked phase of silicic volcanism between 7.5 and 3 Ma, local emplacement of small volume intraplate-like basalts since 5 Ma, and development of extensional faulting. These features are related to slab rollback, enhancing asthenophere flux into the mantle wedge and promoting partial melting of the crust; 4) the modern arc consists of a frontal belt dominated by flux and slab melting, and a rear belt characterized by more differentiated rocks or by mafic lavas with little or no evidence of subduction fluids but higher asthenosphere fingerprint.

  14. Petrogenesis of dacite in an oceanic subduction environment: Raoul Island, Kermadec arc

    NASA Astrophysics Data System (ADS)

    Smith, Ian E. M.; Worthington, Timothy J.; Price, Richard C.; Stewart, Robert B.; Maas, R.

    2006-09-01

    Raoul Volcano in the northern Kermadec arc is typical of volcanoes in oceanic subduction systems in that it is composed mainly of low-K high-Al basalts and basaltic andesite. However, during the last 4 ka Raoul Volcano has produced mainly dacite magma in pyroclastic eruptions associated with caldera formation. The rocks produced in these episodes are almost aphyric containing only sparse crystals of plagioclase, clinopyroxene, orthopyroxene and magnetite. These apparent phenocrysts have chemical compositions that suggest that they did not crystallise from melts with the chemical composition of their host rocks. Rather they are xenocrysts and only their rims show evidence for crystallisation from their host melt. Chemical compositions of samples of the dacites show that each eruption has tapped a distinct magma batch. Compositional variations through the analysed suite cannot be accommodated in any reasonable model of fractional crystallisation from likely parental magma compositions. The hypothesis that best fits the petrology of Raoul Island dacites is one of crustal anatexis. This model requires heating of the lower crust by a magma flux to the point where dehydration melting associated with amphibole breakdown produces magma from a preconditioned source. It is suggested that Raoul is passing through an adolescent stage of development in which siliceous melts are part of an open system in which felsic and mafic magmas coexist.

  15. Modeling the effects of geological heterogeneity and metamorphic dehydration on slow slip and shallow deformation in subduction zones

    NASA Astrophysics Data System (ADS)

    Skarbek, Robert M.

    Slow slip and tectonic tremor in subduction zones take place at depths (˜20 - 50 km) where there is abundant evidence for distributed shear over broad zones (˜10 - 103 m) composed of rocks with marked differences in mechanical properties and for near lithostatic pore pressures along the plate interface where the main source of fluids must be attributed to chemical dehydration reactions. In Chapter II, I model quasi-dynamic rupture along faults composed of material mixtures characterized by different rate-and-state-dependent frictional properties to determine the parameter regime capable of producing slow slip in an idealized subduction zone setting. Keeping other parameters fixed, the relative proportions of velocity-weakening (VW) and velocity-strengthening (VS) materials control the sliding character (stable, slow, or dynamic) along the fault. The stability boundary between slow and dynamic is accurately described by linear analysis of a double spring-slider system with VW and VS blocks. In Chapter III, I model viscoelastic compaction of material subducting through the slow slip and tremor zone in the presence of pressure and temperature-dependent dehydration reactions. A dehydration fluid source is included using 1) a generalized basalt dehydration reaction in subducting oceanic crust or 2) a general nonlinear kinetic reaction rate law parameterized for an antigorite dehydration reaction. Pore pressures in excess of lithostatic values are a robust feature of simulations that employ parameters consistent with the geometry of the Cascadia subduction margin. Simulations that include viscous deformation uniformly generate traveling porosity waves that transport increased fluid pressures within the slow slip region. Slow slip and tremor also occur in shallow (< 10 km depth) accretionary prism sections of subduction zones. In Chapter IV, I examine how geologic heterogeneities affect the mechanics of accretionary prisms in subduction zones by showing how spatial

  16. Late Cretaceous-Early Eocene Climate Change Linked to Tectonic Eevolution of Neo-Tethyan Subduction Systems

    NASA Astrophysics Data System (ADS)

    Jagoutz, O. E.; Royden, L.; Macdonald, F. A.

    2015-12-01

    In this presentation we demonstrate that the two tectonic events in the late Cretaceous-Early Tertiary triggered the two distinct cooling events that followed the Cretaceous Thermal Maximum (CTM). During much of the Cretaceous time, the northern Neo Tethyan ocean was dominated by two east-west striking subduction system. Subduction underneath Eurasia formed a continental arc on the southern margin of Eurasia and intra oceanic subduction in the equatorial region of the Neo Tethys formed and intra oceanic arc. Beginning at ~85-90 Ma the western part of the TTSS collided southward with the Afro-Arabian continental margin, terminating subduction. This resulted in southward obduction of the peri-Arabian ophiolite belt, which extends for ~4000 km along strike and includes the Cypus, Semail and Zagros ophiolites. At the same time also the eastern part of the TTS collided northwards wit Eurasia. After this collisional event, only the central part of the subduction system remained active until it collided with the northern margin of the Indian continent at ~50-55 Ma. The collision of the arc with the Indian margin, over a length of ~3000 km, also resulted in the obduction of arc material and ophiolitic rocks. Remnants of these rocks are preserved today as the Kohistan-Ladakh arc and ophiolites of the Indus-Tsangpo suture zone of the Himalayas. Both of these collision events occurred in the equatorial region, near or within the ITCZ, where chemical weathering rates are high and are contemporaneous with the onset of the global cooling events that mark the end of the CTM and the EECO. The tectonic collision events resulted in a shut down of subduction zone magmatism, a major CO2 source and emplacement of highly weatherable basaltic rocks within the ITCZ (CO2 sink). In order to explore the effect of the events in the TTSS on atmospheric CO2, we model the potential contribution of subduction zone volcanism (source) and ophiolite obduction (sink) to the global atmospheric CO2

  17. Primitive basalts and andesites from the Mt. Shasta region, N. California: products of varying melt fraction and water content

    NASA Astrophysics Data System (ADS)

    Baker, Michael B.; Grove, Timothy L.; Price, Richard

    1994-06-01

    Quaternary volcanism in the Mt. Shasta region has produced primitive magmas [Mg/(Mg+Fe*)>0.7, MgO>8 wt% and Ni>150 ppm] ranging in composition from high-alumina basalt to andesite and these record variable extents ofmelting in their mantle source. Trace and major element chemical variations, petrologic evidence and the results of phase equilibrium studies are consistent with variations in H2O content in the mantle source as the primary control on the differences in extent of melting. High-SiO2, high-MgO (SiO2=52% and MgO=11 wt%) basaltic andesites resemble hydrous melts (H2O=3 to 5 wt%) in equilibrium with a depleted harzburgite residue. These magmas represent depletion of the mantle source by 20 to 30 wt% melting. High-SiO2, high-MgO (SiO2=58% and MgO=9 wt%) andesites are produced by higher degrees of melting and contain evidence for higher H2O contents (H2O=6 wt%). High-alumina basalts (SiO2=48.5% and Al2O3=17 wt%) represent nearly anhydrous low degree partial melts (from 6 to 10% depletion) of a mantle source that has been only slightly enriched by a fluid component derived from the subducted slab. The temperatures and pressures of last equilibration with upper mantle are 1200°C and 1300°C for the basaltic andesite and basaltic magmas, respectively. A model is developed that satisfies the petrologic temperature constraints and involves magma generation whereby a heterogeneous distribution of H2O in the mantle results in the production of a spectrum of mantle melts ranging from wet (calc-alkaline) to dry (tholeiitic).

  18. Ubiquitous radiogenic Os in Miocene to recent basalts from diverse mantle domains beneath the Colorado Plateau, USA

    NASA Astrophysics Data System (ADS)

    Schlieder, T.; Reid, M. R.; Widom, E.; Blichert-Toft, J.

    2015-12-01

    The source of magmatism and mechanisms responsible for the observed geochemical signatures in Miocene to Recent Colorado Plateau (CP) basalts has been a renewed focus of investigation in light of Earthscope results. We report new Os and Nd isotopic data for magnesian basalts (Mg#=62-72) and interpret them in light of previously reported Hf isotope data to help constrain contributions from olivine-poor source lithologies and subduction-derived metasomatism in the genesis of recent CP volcanism. The basalts studied span a large range in Hf isotope compositions and represent melts last equilibrated at a variety of depths beneath the Colorado Plateau and its transition zones. We distinguished at least three mantle domains on the basis of paired Hf-Nd isotope, other isotopic, and geochemical characteristics of CP lavas. Domain 1 likely represents a depleted, variably metasomatized, lithospheric source, with relatively radiogenic ɛHf (+5.2 to +11.8) and highly variable ɛNd (-6.2 to +6.2). Domain 2 could represent either ancient or Farallon subduction-modified mantle and is displaced above the Hf-Nd mantle array (ɛHf=+1.0 to +7.3; ɛNd=-6.1 to -3.5). Domain 3 may be melts of pyroxenite/mica-rich veins or layers within lithospheric mantle and is characterized by unradiogenic Hf and Nd (ɛHf=-12.9 to +0.6; ɛNd=-10.0 to -2.9). The isotopic variability in CP-related lavas can largely be attributed to contributions from these mantle domains. Preliminary Os isotope data show no correlation with proxies for differentiation or crustal contamination. Osmium and Hf isotope compositions are negatively correlated between domains 1 and 2 (187Os/188Os=0.31 to 0.59), whereas the Os isotope ratios in two domain 3 basalts have both lower and higher values (187Os/188Os=0.25 and 0.68). Significantly, Os isotope signatures are highly radiogenic (vs. values of <0.12 for SW US peridotite xenoliths [1]), overlapping and extending the range for inferred melts of pyroxene- and mica-rich veins

  19. A new P wave velocity model beneath East Asia: insights on the relationship between intraplate volcanism and Pacific subduction

    NASA Astrophysics Data System (ADS)

    Huang, T.; Niu, F.; Obayashi, M.

    2013-12-01

    The Pacific plate subducted beneath the East China since the Middle Mesozoic and it went through several different stages of subduction. Knowing its detailed configuration in the mantle can help better understand the geological events happened in the East Asia. Here we combine P-wave traveltime data from the EHB (Engdahl, van der Hilst, and Buland 1998) catalog of 1964-2007, and manually picks from the regional networks of the China Earthquake Administration (CEArray) consisting of more than one thousand stations from 2007 to 2010 as well as the NorthEast China Extended Array (NECESSArray) with 127 broadband stations from 2009 to 2011 in order to produce a global P-wave velocity model with a focus on the velocity structure beneath the East Asia. The mantle is parameterized into irregular blocks and the size of each block depends on the number of the rays penetrating the block. The minimum block size is 1.25°x1.25°. The large dataset and the adaptive block size yield a high-resolution 3D P-wave velocity model beneath Asia. Our tomographic model shows high velocity roots under Archean cratons, such as the Ordos and the Sichuan basin. Our model also displays low velocity anomalies at a depth from 50km to 350km beneath the Changbaishan and Datong volcanoes. Massive basaltic volcanism occurred in these places in the Cenozoic. There is also a pronounced slow anomaly that extends to the transition zone beneath the South China block, but it is only limited in the upper mantle. The Cenozoic basaltic magmatism in Hainan Island might be related with this slow anomaly. Our model also shows clear segments of slabs inside the transition zone, which could be imaged as flat slabs with insufficient data sampling. Different segments also appear to have different intensity and may correspond to different episodes of the Pacific subduction. The fragmented nature of the subducted Pacific plate also implies that several slab detachment events may have occurred during the subduction

  20. Equilibration of Leachants with Basalt Rock for Repository Simulation Tests

    SciTech Connect

    Jantzen, C.M.

    2001-07-02

    In a nuclear waste repository in basalt, the groundwater will have a low redox potential (Eh) which may affect the leach rate of SRP waste glass. Accurate laboratory simulations of conditions in a basalt reposition must maintain low Eh values throughout the course of the experiment. In this report, important parameters affecting the ability of basalt to maintain appropriate Eh-pH conditions are examined, in particular basalt type and groundwater simulation.

  1. A new view into the Cascadia subduction zone and volcanic arc: Implications for earthquake hazards along the Washington margin

    USGS Publications Warehouse

    Parsons, T.; Trehu, A.M.; Luetgert, J.H.; Miller, K.; Kilbride, F.; Wells, R.E.; Fisher, M.A.; Flueh, E.; ten Brink, U.S.; Christensen, N.I.

    1998-01-01

    In light of suggestions that the Cascadia subduction margin may pose a significant seismic hazard for the highly populated Pacific Northwest region of the United States, the U.S. Geological Survey (USGS), the Research Center for Marine Geosciences (GEOMAR), and university collaborators collected and interpreted a 530-km-long wide-angle onshore-offshore seismic transect across the subduction zone and volcanic arc to study the major structures that contribute to seismogenic deformation. We observed (1) an increase in the dip of the Juan de Fuca slab from 2??-7?? to 12?? where it encounters a 20-km-thick block of the Siletz terrane or other accreted oceanic crust, (2) a distinct transition from Siletz crust into Cascade arc crust that coincides with the Mount St. Helens seismic zone, supporting the idea that the mafic Siletz block focuses seismic deformation at its edges, and (3) a crustal root (35-45 km deep) beneath the Cascade Range, with thinner crust (30-35 km) east of the volcanic arc beneath the Columbia Plateau flood basalt province. From the measured crustal structure and subduction geometry, we identify two zones that may concentrate future seismic activity: (1) a broad (because of the shallow dip), possibly locked part of the interplate contact that extends from ???25 km depth beneath the coastline to perhaps as far west as the deformation front ???120 km offshore and (2) a crustal zone at the eastern boundary between the Siletz terrane and the Cascade Range.

  2. Evaluation of crustal recycling during the evolution of Archean-age Matachewan basaltic magmas

    NASA Technical Reports Server (NTRS)

    Nelson, Dennis O.

    1989-01-01

    The simplest model for the Matachewan-Hearst Dike (MHD) magmas is assimilation-fractional crystallization (AFC), presumably occurring at the base of the crust during underplating. Subduction zone enriched mantle sources are not required. Trace elements suggest that the mantle sources for the MHD were depleted, but possessed a degree of heterogeneity. Rates of assimilation were approximately 0.5 (= Ma/Mc); the contaminant mass was less than 20 percent. The contaminant was dominated by tonalites-randodiorites, similar to xenoliths and rocks in the Kapuskasing Structural Zone (KSZ). Assimilation of partial melts of light-rare earth and garnet-bearing basaltic precursors may have produced some the MHD magmas. Apparently, previous underplating-AFC processes had already produced a thick crust. The silicic granitoid assimilant for the MHD magmas was probably produced by earlier processing of underplated mafic crust (4, 5, 10, 21 and 30). Calculations suggest that the derived silicic rocks possess negative Ta and Ti anomalies even though they were not the product of subduction.

  3. Thermal models for basaltic volcanism on Io

    USGS Publications Warehouse

    Keszthelyil, L.; McEwen, A.

    1997-01-01

    We present a new model for the thermal emissions from active basaltic eruptions on Io. While our methodology shares many similarities with previous work, it is significantly different in that (1) it uses a field tested cooling model and (2) the model is more applicable to pahoehoe flows and lava lakes than fountain-fed, channelized, 'a'a flows. This model demonstrates the large effect lava porosity has on the surface cooling rate (with denser flows cooling more slowly) and provides a preliminary tool for examining some of the hot spots on Io. The model infrared signature of a basaltic eruption is largely controlled by a single parameter, ??, the average survival time for a lava surface. During an active eruption surfaces are quickly covered or otherwise destroyed and typical values of ?? for a basaltic eruption are expected to be on the order of 10 seconds to 10 minutes. Our model suggests that the Galileo SSI eclipse data are consistent with moderately active to quiescent basaltic lava lakes but are not diagnostic of such activity. Copyright 1997 by the American Geophysical Union.

  4. Utilization of lunar ilmenite: Basalt or regolith?

    NASA Technical Reports Server (NTRS)

    Kawatra, S. K.; Delao, K. L.

    1991-01-01

    A critical discussion of whether lunar basalt or regolith should be used as a resource for mineral processing schemes on the lunar surface, with pros and cons for each argument is presented. A literature review has shown that the majority of authors feel that mining the lunar basalt, crushing it, and then processing to remove the desired minerals, would be the route to take. The argument that this method would not be a sound mineral processing practice is presented. Mining and crushing are difficult propositions even on Earth; to attempt such processes in the hostile lunar environment would be a phenomenal task. It would be better to start with a simpler scheme, such as processing the regolith, which can be adapted to the multitude of unknowns facing the first lunar production plant. If, however, the lunar mining trend is followed, it must be kept in mind that mining and processing technology which is radically different from what is currently available and used on Earth will have to be developed. Podnieks and Roepke (1987) and Lindroth and Podnieks (1987) have summarized the new technology that may be applicable, but this technology is very similar to the current, 99 percent inefficient technology used on Earth. One such possible technique is sodium vapor fragmentation of basalt. Initial testwork was conducted at Michigan Technological University on terrestrial basalt with extremely promising results, though much time and effort will be needed to fully develop this process.

  5. The biological consequences of flood basalt volcanism

    NASA Astrophysics Data System (ADS)

    Clapham, M.

    2012-12-01

    Flood basalt eruptions are among the largest environmental perturbations of the Phanerozoic. The rapid release of CO2 from a large igneous province would have triggered a chain of events that can include climate warming, ocean acidification, reduced seawater carbonate saturation, and expanded oceanic anoxia. Those stressors have widely negative impacts on marine organisms, especially on calcified taxa, by affecting their respiratory physiology and reducing energy available for growth and reproduction. Many Phanerozoic extinctions, most notably the end-Permian and end-Triassic mass extinctions, coincided with flood basalt eruptions and shared distinctive patterns of taxonomic and ecological selectivity. In these extinctions, highly active organisms were more likely to survive because they possess physiological adaptations for maintaining internal pH during activity, which also proves useful when buffering pH against ocean acidification. In contrast, species that did not move and had low metabolic rates, such as brachiopods and sponges, suffered considerable losses during these extinctions. Heavily-calcified organisms, especially corals, were particularly vulnerable; as a result, ocean acidification and saturation state changes from flood basalt eruptions often triggered crises in reef ecosystems. This characteristic pattern of selectivity during "physiological" extinctions that closely coincided with flood basalts provides a template for assessing the causes of other extinction events. Because these crises also provide deep time analogues for the ongoing anthropogenic crisis of warming, ocean acidification, and expanded anoxia, the selectivity patterns can also help constrain "winners" and "losers" over upcoming decades.

  6. Basalt-Block Heat-Storage Plant

    NASA Technical Reports Server (NTRS)

    Sullivan, Thomas A.

    1992-01-01

    Concept for storage of solar heat for later use based on use of basalt, cast into blocks and stacked in inflatable gas-tight enclosure serving as heat-storage chamber. Heat flows to blocks from solar collector during day and from blocks to heat engine at night.

  7. Pressure grouting of fractured basalt flows

    SciTech Connect

    Shaw, P.; Weidner, J.; Phillips, S.; Alexander, J.

    1996-04-01

    This report describes a field trial of pressure grouting in basalt and the results of subsequent coring and permeability measurement activities. The objective was to show that the hydraulic conductivity of fractured basalt bedrock can be significantly reduced by pressure injection of cementitious materials. The effectiveness of the pressure grout procedure was evaluated by measuring the change in the hydraulic conductivity of the bedrock. The extent of grout penetration was established by analyzing postgrout injection drilling chips for the presence of a tracer in the grout and also by examining cores of the treated basalt. Downhole radar mapping was used to establish major lava flow patterns and follow water movement during a surface infiltration test. A site called Box Canyon, which is located northwest of the INEL, was chosen for this study due to the similarity of this surface outcrop geology to that of the underlying bedrock fracture system found at the Radioactive Waste Management Complex. This study showed that hydraulic conductivity of basalt can be reduced through pressure grouting of cementitious material.

  8. Basaltic injections into floored silicic magma chambers

    NASA Astrophysics Data System (ADS)

    Wiebe, R. A.

    Recent studies have provided compelling evidence that many large accumulations of silicic volcanic rocks erupted from long-lasting, floored chambers of silicic magma that were repeatedly injected by basaltic magma. These basaltic infusions are commonly thought to play an important role in the evolution of the silicic systems: they have been proposed as a cause for explosive silicic eruptions [Sparks and Sigurdsson, 1977], compositional variation in ash-flow sheets [Smith, 1979], mafic magmatic inclusions in silicic volcanic rocks [Bacon, 1986], and mixing of mafic and silicic magmas [Anderson, 1976; Eichelberger, 1978]. If, as seems likely, floored silicic magma chambers have frequently been invaded by basalt, then plutonic bodies should provide records of these events. Although plutonic evidence for mixing and commingling of mafic and silicic magmas has been recognized for many years, it has been established only recently that some intrusive complex originated through multiple basaltic injections into floored chambers of silicic magma [e.g., Wiebe, 1974; Michael, 1991; Chapman and Rhodes, 1992].

  9. Subduction zone earthquakes and stress in slabs

    NASA Technical Reports Server (NTRS)

    Vassiliou, M. S.; Hager, B. H.

    1988-01-01

    Simple viscous fluid models of subducting slabs are used to explain observations of the distribution of earthquakes as a function of depth and the orientation of stress axes of deep (greater than 300 km) and intermediate (70-300 km) earthquakes. Results suggest the following features in the distribution of earthquakes with depth: (1) an exponential decrease from shallow depths down to 250 to 300 km, (2) a minimum near 250 to 300 km, and (3) a deep peak below 300 km. Many shallow subducting slabs show only the first characteristic, while deeper extending regions tend to show all three features, with the deep peak varying in position and intensity. These data, combined with the results on the stress orientations of various-depth earthquakes, are consistent with the existence of a barrier of some sort at 670-km depth and a uniform viscosity mantle above this barrier.

  10. On the dip angle of subducting plates

    NASA Technical Reports Server (NTRS)

    Hsui, Albert T.; Tang, Xiao-Ming; Toksoz, M. Nafi

    1990-01-01

    A new approximate analytic model is developed for the thermal structure of a subducting plate with a finite length. This model provides the capability of easily examining the thermal and mechanical structure of a subducting plate with different lengths and at different angles. Also, the torque balance of a descending plate can be examined, and effects such as the leading edge effect, the adiabatic compression effect, and the phase change effect can be incorporated. A comparison with observed data indicates that short slabs are likely under torque equilibrium at present, while long slabs are probably dominated by their gravitational torques such that their dip angles are transient, moving toward a steeper dip angle similar to that of the Mariana slab.

  11. Global Oceanic Basalt Geochemistry From EarthChem Databases

    NASA Astrophysics Data System (ADS)

    Hofmann, A. W.; Sarbas, B.; Jochum, K. P.; Stracke, A.

    2004-12-01

    For the past 21 years, global trace element systematics in oceanic basalts have been systematically developed, using mass spectrometry, by the MPI geochemistry department in Mainz, starting with Ba-Rb-Cs, and continuing with K-U-Th, Nb-U-Th, Pb-Ce, Pr-Mo, Nb-Ta, Sn-REE, Sb-REE, and Y-Ho relations. These were complemented by other groups, contributing e.g. Sr-REE relationships and more refined Nb-Ta systematics. Goal of these investigations was to establish relative trace element compatibilities during mantle melting, the corresponding enrichment and depletion patterns in MORB, OIB, subduction-related volcanics and the continental crust, and the relationships to Bulk Silicate Earth abundances through comparisons with element abundances in meteorites and in the continental crust (see e.g. [1]). Most of these studies were initially based on extremely limited data sets, often fewer than 100 analyses, because routine analytical techniques such as XRF and INAA were inadequate, either in sensitivity or accuracy or both, for many of the elements of interest. The advent of ICPMS technology has increased the volume of available, useable data enormously. The most recent development of laser source ICPMS is accelerating the acquisition of comprehensive trace element data even more dramatically. Although the general quality of recent trace element analyses has improved significantly, there remain large differences in reliability between published data because of varying analytical uncertainties and sample alteration. Thus, quality assurance remains an enormous task. In any case, now and in the foreseeable future, it will be impossible to assess global geochemical data without the use of comprehensive databases. Such databases are now available under http://www.earthchem.org/, comprising http://beta.petdb.ciesin.columbia.edu/; http://georoc.mpch-mainz.gwdg.de/, and http://navdat.geo.ku.edu/ . Unfortunately, the use of such databases is not without pitfalls. Often, appropriate

  12. Major and trace element and Sr-Nd isotope signatures of lavas from the Central Lau Basin: Implications for the nature and influence of subduction components in the back-arc mantle

    NASA Astrophysics Data System (ADS)

    Tian, Liyan; Castillo, Paterno R.; Hawkins, James W.; Hilton, David R.; Hanan, Barry B.; Pietruszka, Aaron J.

    2008-12-01

    New major and trace element and Sr-Nd isotope data are presented for basaltic glasses from active spreading centers (Central Lau Spreading Center (CLSC), Relay Zone (RZ) and Eastern Lau Spreading Center (ELSC)) in the Central Lau Basin, SW Pacific. Basaltic lavas from the Central Lau Basin are mainly tholeiitic and are broadly similar in composition to mid-ocean ridge basalts (MORB). Their generally high 87Sr/ 86Sr ratios, combined with relatively low 143Nd/ 144Nd ratios are more akin to MORB from the Indian rather than Pacific Ocean. In detail, the CLSC, RZ and ELSC lavas are generally more enriched in large ion lithophile elements (Rb, Ba, Sr, and K) than average normal-MORB, which suggests that the mantle beneath the Central Lau Basin was modified by subducted slab-derived components. Fluid mobile/immobile trace element and Sr - Nd isotope ratios suggest that the subduction components were essentially transferred into the mantle via hydrous fluids derived from the subducted oceanic crust; contributions coming from the subducted sediments are minor. Compared to CLSC lavas, ELSC and RZ lavas show greater enrichment in fluid mobile elements and depletion in high field strength elements, especially Nb. Thus, with increasing distance away from the arc, the influence of subduction components in the mantle source of Lau Basin lavas diminishes. The amount of hydrous fluids also influences the degree of partial melting of the mantle beneath the Central Lau Basin, and hence the degree of melting also decreases with increasing distance from the arc.

  13. Geochemical evolution of Monowai volcanic center: New insights into the northern Kermadec arc subduction system, SW Pacific

    NASA Astrophysics Data System (ADS)

    Timm, Christian; Graham, Ian J.; de Ronde, Cornel E. J.; Leybourne, Matthew I.; Woodhead, Jon

    2011-08-01

    We present trace element and Sr-Nd-Pb isotope data on volcanic rocks recovered from the submarine Monowai volcanic center, which marks the midpoint of the ˜2500 km long Tonga-Kermadec arc. The center consists of a large (12 × 9 km) partly hydrothermally active caldera and a 12 km diameter ˜1500 m high volcanically and hydrothermally active stratovolcano. The stratovolcano lavas are tholeiitic basalts, which show variable evidence for plagioclase (±pyroxene) accumulation. The caldera lavas range from basalt to andesite, with the compositional variation being consistent with fractional crystallization as the dominant process. The mafic Monowai magmas were generated by relatively high degrees (12%-20%) of partial melting of a previously depleted MORB-type spinel-peridotitic mantle, metasomatized by slab-derived fluids. Strongly fluid mobile 87Sr/86Sr and 207Pb/204Pb ratios of the Monowai basaltic lavas are similar to those from the Putoto, Raoul, and Macauley volcanic centers 200-400 km to the south, suggesting derivation largely from subducted sediment. In contrast, variably fluid immobile 143Nd/144Nd ratios suggest an isotopically heterogeneous mantle along this segment of the arc. Higher 206Pb/204Pb in Monowai lavas imply some influence from the nearby subducting Louisville seamounts in melt generation. The formation of one of the Earth's largest submarine mafic calderas can best be explained through drainage of a single magma reservoir and subsequent collapse caused by trench-perpendicular extension, probably via southward progressive rifting of the northern Havre Trough.

  14. Hydrogen isotope systematics of submarine basalts

    USGS Publications Warehouse

    Kyser, T.K.; O'Neil, J.R.

    1984-01-01

    The D/H ratios and water contents in fresh submarine basalts from the Mid-Atlantic Ridge, the East Pacific Rise, and Hawaii indicate that the primary D/H ratios of many submarine lavas have been altered by processes including (1) outgassing, (2) addition of seawater at magmatic temperature, and (3) low-temperature hydration of glass. Decreases in ??D and H2O+ from exteriors to interiors of pillows are explained by outgassing of water whereas inverse relations between ??D and H2O+ in basalts from the Galapagos Rise and the FAMOUS Area are attributed to outgassing of CH4 and H2. A good correlation between ??D values and H2O is observed in a suite of submarine tholeiites dredged from the Kilauea East Rift Zone where seawater (added directly to the magma), affected only the isotopic compositions of hydrogen and argon. Analyses of some glassy rims indicate that the outer millimeter of the glass can undergo lowtemperature hydration by hydroxyl groups having ??D values as low as -100. ??D values vary with H2O contents of subaerial transitional basalts from Molokai, Hawaii, and subaerial alkali basalts from the Society Islands, indicating that the primary ??D values were similar to those of submarine lavas. Extrapolations to possible unaltered ??D values and H2O contents indicate that the primary ??D values of most thoteiite and alkali basalts are near -80 ?? 5: the weight percentages of water are variable, 0.15-0.35 for MOR tholeiites, about 0.25 for Hawaiian tholeiites, and up to 1.1 for alkali basalts. The primary ??D values of -80 for most basalts are comparable to those measured for deep-seated phlogopites. These results indicate that hydrogen, in marked contrast to other elements such as Sr, Nd, Pb, and O, has a uniform isotopic composition in the mantle. This uniformity is best explained by the presence of a homogeneous reservoir of hydrogen that has existed in the mantle since the very early history of the Earth. ?? 1984.

  15. Diamond Growth in the Subduction Factory

    NASA Astrophysics Data System (ADS)

    Bureau, H.; Frost, D. J.; Bolfan-Casanova, N.; Leroy, C.; Estève, I.

    2014-12-01

    Natural diamonds are fabulous probes of the deep Earth Interior. They are the evidence of the deep storage of volatile elements, carbon at first, but also hydrogen and chlorine trapped as hydrous fluids in inclusions. The study of diamond growth processes in the lithosphere and mantle helps for our understanding of volatile elements cycling between deep reservoirs. We know now that inclusion-bearing diamonds similar to diamonds found in nature (i.e. polycrystalline, fibrous and coated diamonds) can grow in hydrous fluids or melts (Bureau et al., GCA 77, 202-214, 2012). Therefore, we propose that the best environment to promote such diamonds is the subduction factory, where highly hydrous fluids or melts are present. When oceanic plates are subducted in the lithosphere, they carry an oceanic crust soaked with seawater. While the slabs are traveling en route to the mantle, dehydration processes generate saline fluids highly concentrated in NaCl. In the present study we have experimentally shown that diamonds can grow from the saline fluids (up to 30 g/l NaCl in water) generated in subducted slabs. We have performed multi-anvil press experiments at 6-7 GPa and from 1300 to 1400°C during 6:00 hours to 30:00 hours. We observed large areas of new diamond grown in epitaxy on pure diamond seeds in salty hydrous carbonated melts, forming coated gems. The new rims are containing multi-component primary inclusions. Detailed characterizations of the diamonds and their inclusions have been performed and will be presented. These experimental results suggest that multi-component salty fluids of supercritical nature migrate with the slabs, down to the deep mantle. Such fluids may insure the first stage of the deep Earth's volatiles cycling (C, H, halogen elements) en route to the transition zone and the lower mantle. We suggest that the subduction factory may also be a diamond factory.

  16. Subducting Slabs: Jellyfishes in the Earth's Mantle

    NASA Astrophysics Data System (ADS)

    Loiselet, C.; Braun, J.; Husson, L.; Le Carlier de Veslud, C.; Thieulot, C.; Yamato, P.; Grujic, D.

    2010-12-01

    The constantly improving resolution of geophysical data, seismic tomography and seismicity in particular, shows that the lithosphere does not subduct as a slab of uniform thickness but is rather thinned in the upper mantle and thickened around the transition zone between the upper and lower mantle. This observation has traditionally been interpreted as evidence for the buckling and piling of slabs at the boundary between the upper and lower mantle, where a strong contrast in viscosity may exist and cause resistance to the penetration of slabs into the lower mantle. The distribution and character of seismicity reveal, however, that slabs undergo vertical extension in the upper mantle and compression near the transition zone. In this paper, we demonstrate that during the subduction process, the shape of low viscosity slabs (1 to 100 times more viscous than the surrounding mantle) evolves toward an inverted plume shape that we coin jellyfish. Results of a 3D numerical model show that the leading tip of slabs deform toward a rounded head skirted by lateral tentacles that emerge from the sides of the jellyfish head. The head is linked to the body of the subducting slab by a thin tail. A complete parametric study reveals that subducting slabs may achieve a variety of shapes, in good agreement with the diversity of natural slab shapes evidenced by seismic tomography. Our work also suggests that the slab to mantle viscosity ratio in the Earth is most likely to be lower than 100. However, the sensitivity of slab shapes to upper and lower mantle viscosities and densities, which remain poorly constrained by independent evidence, precludes any systematic deciphering of the observations.

  17. Subducting slabs: Jellyfishes in the Earth's mantle

    NASA Astrophysics Data System (ADS)

    Loiselet, Christelle; Braun, Jean; Husson, Laurent; Le Carlier de Veslud, Christian; Thieulot, Cedric; Yamato, Philippe; Grujic, Djordje

    2010-08-01

    The constantly improving resolution of geophysical data, seismic tomography and seismicity in particular, shows that the lithosphere does not subduct as a slab of uniform thickness but is rather thinned in the upper mantle and thickened around the transition zone between the upper and lower mantle. This observation has traditionally been interpreted as evidence for the buckling and piling of slabs at the boundary between the upper and lower mantle, where a strong contrast in viscosity may exist and cause resistance to the penetration of slabs into the lower mantle. The distribution and character of seismicity reveal, however, that slabs undergo vertical extension in the upper mantle and compression near the transition zone. In this paper, we demonstrate that during the subduction process, the shape of low viscosity slabs (1 to 100 times more viscous than the surrounding mantle) evolves toward an inverted plume shape that we coin jellyfish. Results of a 3D numerical model show that the leading tip of slabs deform toward a rounded head skirted by lateral tentacles that emerge from the sides of the jellyfish head. The head is linked to the body of the subducting slab by a thin tail. A complete parametric study reveals that subducting slabs may achieve a variety of shapes, in good agreement with the diversity of natural slab shapes evidenced by seismic tomography. Our work also suggests that the slab to mantle viscosity ratio in the Earth is most likely to be lower than 100. However, the sensitivity of slab shapes to upper and lower mantle viscosities and densities, which remain poorly constrained by independent evidence, precludes any systematic deciphering of the observations.

  18. Nonvolcanic tremors in the Mexican subduction zone

    NASA Astrophysics Data System (ADS)

    Payero, J. S.; Kostoglodov, V.; Mikumo, T.; Perez-Campos, X.; Iglesias, A.; Clayton, R.

    2007-05-01

    Nonvolcanic low frequency tremors (NVT) have been discovered and studied recently in Japan and Cascadia subduction zones and deep beneath the San Andreas Fault. The tremors activity is increasing during so-called silent earthquakes (SQ) in Japan and Cascadia. NVT clusters also migrate following the propagation of the SQ. The origin of the NVT is still unclear. The studies of NVT and SQ in different subduction zones are required to understand the cause for these phenomena. We discovered a number of NVT from daily spectrograms of continuous broad band records at seismic stations of Servicio Seismológico Nacional (SSN) an MASE project. The analyzed data cover a period of 2001-2004 (SSN) when in 2002 a large SQ has occurred in the Guerrero- Oaxaca region, and a steady-state interseismic epoch of 2005 and a new large SQ in 2006 (MASE). NVT occurred in the central part of the Mexican subduction zone (Guerrero) at approximately 200 km from the coast. We can not accurately localize the tremors because of sparse station coverage in 2001-2004. The MASE data of 2005-2006 show that NVT records in Mexico are very similar to those obtained in Cascadia subduction zone. The tremors duration is of 10-60 min, and they appear to travel at S-wave velocities. More than 100 strong NVT were recorded by most of the MASE stations with the epicenters clustered in the narrow band of ~40x150 km to the south of Iguala city and parallel to the coast line. NVT depths are poorly constrained but seem to be less than 40 km deep. We noticed a some increase of NVT activity during the 2001-2002 and 2006 SQs compared with an NVT activity for the "SQ quiet" period of 2003-2004 nevertheless. A lack of NVT for the period of 2-3 months after the SQ is apparent in 2002 and 2006.

  19. Structural Responses to the Chile Ridge Subduction, Southern South America

    NASA Astrophysics Data System (ADS)

    Rodriguez, E. E.; Russo, R. M.; Mocanu, V. I.; Gallego, A.; Murdie, R.; Comte, D.

    2015-12-01

    The Nazca-Antarctic plate boundary, the Chile spreading ridge, subducts beneath South America, forming the northward-migrating Chile Triple Junction (CTJ), now at ~46.5°S, where an actively spreading segment is currently in the Nazca trench. Ridge subduction is associated with diachronously developed variable structure and magmatism of overriding South America. To assess the effects of ridge subduction, we deployed a network of 39 broadband seismometers in southern Chile between 43 - 49°S and 71 - 76°W from Dec. 2004 - Feb. 2007, recording 102 earthquakes suitable for receiver function analyses, i.e., M > 5.9, of various backazimuths, and at epicentral distances of 30 - 90°. The network encompassed onland portions of the current triple junction and ridge subduction, areas to the south of the CTJ where ridge segments subducted during the last 6 m.y., and regions north of the CTJ not yet affected by ridge subduction, allowing the assessment of the effects of ridge subduction on crustal structure of overriding South America. We constructed 551 teleseismic receiver functions to estimate crustal thicknesses, H, and average compressional to shear wave velocity ratios, Vp/Vs = k, using the iterative time deconvolution method of Ligorria and Ammon (1999). H and k were calculated using the grid search method of Zhu and Kanamori (2000). Beneath stations closest to the trench, where the Nazca plate subducts, we found Moho depths between 28 and 55 km, thickening northward. At the locus of current ridge subduction, in the Taitao Pennisula, thinner crust ranges from 27 - 36 km. H is 36-38 km where the Antarctic plate subducts and the Chile ridge recently subducted. The direct effect of the subducting ridge on South America can be seen in H differences between forearc regions that have sustained ridge subduction versus those that have not. South American forearc crust above the subducted Nazca plate is as much as 28 km thicker than forearc crust recently affected by ridge

  20. Coatings on Atacama Desert Basalt: A Possible Analog for Coatings on Gusev Plains Basalt

    NASA Technical Reports Server (NTRS)

    Sutter, B.; Golden, D. C.; Amundson, R.; Chong-Diaz, G.; Ming, D. W.

    2007-01-01

    Surface coatings on Gusev Plains basalt have been observed and may contain hematite and nanophase Fe-oxides along with enrichments in P, S, Cl, and K relative to the underlying rock. The Gusev coatings may be derived from the dissolution of adhering soil and/or parent rock along with the addition of S and Cl from outside sources. Transient water for dissolution could be sourced from melting snow during periods of high obliquity, acid fog, and/or ground water (Haskin et al., 2005). Coatings on basalt in the hyper-arid (less than 2mm y(sup -1)) Atacama Desert may assist in understanding the chemistry, mineralogy and formation mechanisms of the Gusev basalt coatings. The Atacama Desert climate is proposed to be analogous to a paleo-Mars climate that was characterized by limited aqueous activity when the Gusev coatings could have formed. The objectives of this work are to (i) determine the chemical nature and extent of surface coatings on Atacama Desert basalt, and (ii) assess coating formation mechanisms in the Atacama Desert. Preliminary backscattered electron imaging of Atacama basalt thin-sections indicated that the coatings are as thick as 20 m. The boundary between the coating and the basalt labradorite, ilmenite, and augite grains was abrupt indicating that the basalt minerals underwent no chemical dissolution. The Atacama coatings have been added to the basalt instead of being derived from basalt chemical weathering. Semi-quantitative energy dispersive spectroscopy shows the coatings to be chemically homogeneous. The coating is depleted in Ca (0.9 wt% CaO) and enriched in K (1.3 wt.% K2O) and Si (69.1 wt.% SiO2) relative to the augite and labradorite grains. A dust source enriched in Si (e.g., poorly crystalline silica) and K and depleted in Ca appears to have been added to the basalt surface. Unlike the Gusev coatings, no P, S, and Cl enrichment was observed. However, Fe (3.2 wt.% FeO) was present in the Atacama coatings suggesting the present of Fe

  1. Barren Island Volcano (NE Indian Ocean): Island-arc high-alumina basalts produced by troctolite contamination

    NASA Astrophysics Data System (ADS)

    Luhr, James F.; Haldar, Dhanapati

    2006-01-01

    Barren Island (BI) is a subduction-related volcanic island lying in the northeastern Indian Ocean, about 750 km north of the northern tip of Sumatra. Rising from a depth of ˜2300 m on the Andaman Sea floor, BI has a submarine volume estimated at ˜400 km 3, but the island is just 3 km across, reaches a maximum elevation of 355 m, and has a subaerial volume of only ˜1.3 km 3. The first historical eruption began in 1787 when a cinder cone grew in the center of a pre-historical caldera 2-km in diameter and sent lava flows westward to reach the sea; activity continued intermittently until 1832. Two subsequent eruptions modified the central cone and also sent lava flows westward to reach the sea in 1991 and 1994-1995. A suite of 28 lava, scoria, and ash samples were investigated from various stages of the subaerial eruptive history of BI. Most are basalts (including all 10 samples from the 1994-1995 eruption) and basaltic andesites (including 7 of 8 samples from the 1991 eruption), but 2 pre-1787 andesites were also studied. On multi-element spider diagrams the BI suite shows subparallel trends for most elements that reflect an important role for fractional crystallization, along with the characteristic depletions of Nb-Ta and enrichments of K-Rb-Pb found in other subduction-related island-arc suites. The typical relative enrichment of Ba is not present, likely because the subducted sediments in the Andaman arc are not Ba-rich. Wide compositional ranges for Cs, Th, Rb, U, and Pb may trace different degrees of scavenging from the underlying volcanic pile. BI basalts and basaltic andesites have variable abundances of phenocrystic-microphenocrystic olivine plus Cr-Al-Mg spinel inclusions, plagioclase, and clinopyroxene, embedded in a matrix of glass, the same minerals, and titanomagnetite (mostly exsolved). The most remarkable mineralogical feature of certain BI basalts and basaltic andesites is the presence of abundant (to 40 vol.%) and large (to 5 mm) crystals of

  2. Adakitic-like volcanism in Southern Mexico and subduction of the Tehuantepec Ridge

    NASA Astrophysics Data System (ADS)

    Manea, M.; Manea, V. C.

    2007-05-01

    The origin of El Chichón volcano is poorly understood, and our attempt in this study is to demonstrate that Tehuantepec Ridge, a major tectonic discontinuity on the Cocos plate, plays a key role in the slab dehydration budget and therefore in partial melting of the mantle beneath El Chichón. Using marine magnetic anomalies we show that the upper mantle beneath TR undergo partial serpentinization, a 5-7 km thick serpentinized root extending along TR and below the oceanic crust. Another key aspect of the magnetic anomaly over southern México is a long-wavelength (~150 km) high amplitude (~500 nT) magnetic anomaly located between the trench and the coast. Using a 2D joint magnetic-gravity forward model, constrained by the subduction P-T structure, slab geometry and seismicity, we find a highly magnetic and low-density source located at 40-130 km depth. We interpret this result as a serpentinized mantle wedge by fluids expelled from the subducting Cocos plate beneath southern Mexico. Such a deep hydrated mantle requires a low temperature wedge (T<600° C) because serpentine is stable below this temperature and also the magnetic properties are preserved for temperature less than the Currie point for magnetite (~580° C). This result explains the lack of volcanism in southern México where the slab depth is ~ 100 km. Using phase diagrams for sediments, basalt and peridotite, and the subduction P-T structure beneath El Chichón we find that sediments and basalt dehydrate ~ 50% at depths corresponding with the location of serpentinized mantle wedge, whereas the serpentinized root beneath TR strongly dehydrates (60-80%) at higher depths (170-180 km) comparable with the slab depth beneath El Chichón. We conclude that this strong deserpentinization pulse of mantle lithosphere beneath TR at great depths triggers arc melting, explaining the unusual location and probably the adakitic signature of El Chichón.

  3. Intra-slab Fluid Flow and Eclogite-facies Metasomatism in Subducted Oceanic Lithosphere

    NASA Astrophysics Data System (ADS)

    Angiboust, Samuel; Pettke, Thomas; Agard, Philippe; De Hoog, Cees-Jan; Collins, Nathan; Oncken, Onno

    2013-04-01

    The Monviso ophiolite Lago Superiore Unit (LSU) constitutes a well-preserved, almost continuous fragment of upper oceanic lithosphere subducted down to ca. 80 km (between 50 and 40 Ma) and later exhumed along the subduction interface. The LSU is made of (i) a variably thick (50-500 m) section of eclogitized mafic crust (associated with minor calcschist lenses) overlying a 100-400 m thick metagabbroic body, and of (ii) a serpentinite sole (ca. 1000 m thick). This section is cut by two 10 to 100m thick eclogite-facies shear zones, found at the boundary between basalts and gabbros (Intermediate Shear Zone), and between gabbros and serpentinites (Lower Shear Zone: LSZ). Fragments of mylonitic basaltic eclogites and calcschists were dragged and dismembered within serpentinite schists along the LSZ during eclogite-facies deformation. Metasomatic rinds formed on these fragments at the contact with the surrounding antigorite schists during lawsonite-eclogite facies metamorphism, testifying to prominent fluid-rock interaction along with deformation. We present new petrological and geochemical data on four types of metasomatically altered eclogites (talc-, chlorite-, lawsonite- and phengite-bearing eclogites) and on a (serpentinite-derived) magnesite-bearing talc schist from the block rind. Bulk-rock compositions, in situ LA-ICP-MS analysis and X-ray maps of garnet demonstrate that (i) these samples underwent significant Cr, Mg, Ni enrichment and Fe, Al, V depletion during eclogitic metasomatism and (ii) garnet composition show strong variation from core to rim. These compositional patterns point to a massive, fluid-mediated element transfer along with deformation, originating from the surrounding serpentinite (locally, with possible contributions from metasediments-equilibrated fluids). Antigorite breakdown, occurring ca. 15 km deeper than the maximum depth reached by these eclogites, could have provided significant amounts of fluid promoting extensive fluid

  4. Continental aggregation, subduction initiation, and plume generation

    NASA Astrophysics Data System (ADS)

    Heron, P. J.; Lowman, J. P.

    2013-12-01

    Several processes unfold during the supercontinent cycle, more than one of which might result in an elevation in subcontinental mantle temperatures through the generation of mantle plumes. Paleogeographic plate reconstructions have indicated that sub-continental mantle upwellings appear below large continents that are extensively ringed by subduction zones. Moreover, several numerical simulations of supercontinent formation and dispersal attribute the genesis of sub-continental plumes to the generation of subduction zones on the edges of the supercontinent, rather than resulting from continental insulation. However, the role of the location of downwellings in producing a return-flow upwelling, and on increasing sub-continental mantle temperatures, is not fully understood. In this mantle convection study, we examine the evolution of mantle dynamics after supercontinent accretion over a subduction zone (analogous to the formation of Pangea) for a range of continental coverage. We present 2D and 3D Cartesian geometry mantle convection simulations, featuring geotherm- and pressure-dependent viscosity with thermally and mechanically distinct oceanic and continental plates. Through changing the size of the continent we are able to analyze the factors involved in the generation of mantle plumes in purely thermal convection. Furthermore, we change the upper and lower mantle viscosity to determine their relation to plume formation in vigorous mantle convection simulations. Elevated sub-continental temperatures are analyzed in relation to continental coverage to further understand the influence of continental tectonics on the thermal evolution of the mantle.

  5. Production of mildly alkaline basalts at complex ocean ridge settings: Perspectives from basalts emitted during the 2010 eruption at the Eyjafjallajökull volcano, Iceland

    NASA Astrophysics Data System (ADS)

    Viccaro, Marco; Nicotra, Eugenio; Urso, Salvatore

    2015-11-01

    The early phase of the 2010 eruption at the Eyjafjallajökull volcano (Iceland) produced poorly evolved mildly alkaline basalts that have a signature more enriched with respect to the typically depleted basalts emitted at ocean ridges. The whole rock geochemistry of these basaltic magmas offers a great opportunity to investigate the mantle source characteristics and reasons leading to this enriched fingerprint in proximity of the ocean ridge system. Some basaltic products of Katla volcano, ∼25 km east of Eyjafjallajökull, have been chosen from the literature, as they display a similar mildly alkaline signature and can be therefore useful to explore the same target. Major and trace element variations of the whole rock suggest a very limited evolutionary degree for the 2010 Eyjafjallajökull products and the selected Katla magmas, highlighting the minor role played by differentiation processes such as fractional crystallization. Nevertheless, effects of the limited fractionation have been erased through re-equilibration of the major and trace element abundances at primary conditions. Concentrations of Th after re-equilibration have been assumed as indexes of the partial melting degree, given the high incompatibility of the element, and enrichment ratios calculated for each trace element. Especially for LILE (Rb, Ba, K, Sr), the pattern of resulting enrichment ratios well matches that obtained from fractional melting of peridotite bearing hydrous phases (amphibole/phlogopite). This put forward the idea that magmas have been generated through partial melting of enriched mantle domains where hydrous minerals have been stabilized as a consequence of metasomatic processes. Refertilization of the mantle has been attributed to intrusion of hydrous silicate melts and fractional crystallization of hydrous cumulates. These refertilizing melts, inherited from an ancient subducted oceanic crust, intruded into a depleted oceanic lithosphere that remained stored for a long time

  6. Searching for Non-Vestoid Basaltic Asteroids

    NASA Astrophysics Data System (ADS)

    Moskovitz, Nicholas; Jedicke, R.; Willman, M.; Gaidos, E.

    2006-12-01

    We present an observational campaign designed to target Main Belt asteroids that: (1) have a photometric signature similar to that of 4 Vesta and the Vestoid family, i.e. a basaltic surface indicative of past mineral differentiation, and (2) are dynamically distinct from this family so that they lie outside of the domain of orbital element phase space occupied by the V-type asteroids. The only known non-vestoid asteroid with a basaltic surface is 1459 Magnya (Lazzaro et al. 2000). This is a paradox; collected meteorites include representatives from tens or even hundreds of past or present differentiated parent bodies in the Main Belt, a number that is at least an order of magnitude larger than the two known occurrences of Main Belt differentiation (Scott 2002). Asteroids with potentially basaltic surfaces are chosen based on their Sloan Digital Sky Survey (SDSS) ugriz photometric magnitudes. Ten different ugriz color combinations, including two principal component colors (Nesvorny et al. 2005), are used to select asteroids that show similarity to Vestoid colors. Objects with colors that meet these criteria are then prioritized based on their dynamical dissimilarity to Vesta. Low-resolution spectroscopy of the highest priority objects is performed using the Echellette Spectrograph and Imager (ESI) on Keck II in order to unambiguously determine whether or not our candidates have basaltic surfaces. The 0.4-1.0 micron wavelength coverage of this instrument is well suited to resolving both the 0.9 micron olivine/pyroxene absorption feature and the 0.5-0.7 micron slope that are indicative of a basaltic surface and thus that a given asteroid derives from a parent body that has experienced either partial melting or differentiation. Preliminary results from two observing runs in August and September of this year are presented here. This research is supported in part by NSF Planetary Astronomy grant AST04-07134, P.I. R. Jedicke.

  7. Sulfur isotope homogeneity of lunar mare basalts

    NASA Astrophysics Data System (ADS)

    Wing, Boswell A.; Farquhar, James

    2015-12-01

    We present a new set of high precision measurements of relative 33S/32S, 34S/32S, and 36S/32S values in lunar mare basalts. The measurements are referenced to the Vienna-Canyon Diablo Troilite (V-CDT) scale, on which the international reference material, IAEA-S-1, is characterized by δ33S = -0.061‰, δ34S ≡ -0.3‰ and δ36S = -1.27‰. The present dataset confirms that lunar mare basalts are characterized by a remarkable degree of sulfur isotopic homogeneity, with most new and published SF6-based sulfur isotope measurements consistent with a single mass-dependent mean isotopic composition of δ34S = 0.58 ± 0.05‰, Δ33S = 0.008 ± 0.006‰, and Δ36S = 0.2 ± 0.2‰, relative to V-CDT, where the uncertainties are quoted as 99% confidence intervals on the mean. This homogeneity allows identification of a single sample (12022, 281) with an apparent 33S enrichment, possibly reflecting cosmic-ray-induced spallation reactions. It also reveals that some mare basalts have slightly lower δ34S values than the population mean, which is consistent with sulfur loss from a reduced basaltic melt prior to eruption at the lunar surface. Both the sulfur isotope homogeneity of the lunar mare basalts and the predicted sensitivity of sulfur isotopes to vaporization-driven fractionation suggest that less than ≈1-10% of lunar sulfur was lost after a potential moon-forming impact event.

  8. 3-D numerical modeling of plume-induced subduction initiation

    NASA Astrophysics Data System (ADS)

    Baes, Marzieh; Gerya, taras; Sobolev, Stephan

    2016-04-01

    Investigation of mechanisms involved in formation of a new subduction zone can help us to better understand plate tectonics. Despite numerous previous studies, it is still unclear how and where an old oceanic plate starts to subduct beneath the other plate. One of the proposed scenarios for nucleation of subduction is plume-induced subduction initiation, which was investigated in detail, using 2-D models, by Ueda et al. (2008). Recently. Gerya et al. (2015), using 3D numerical models, proposed that plume-lithosphere interaction in the Archean led to the subduction initiation and onset of plate tectonic. In this study, we aim to pursue work of Ueda et al. (2008) by incorporation of 3-D thermo-mechanical models to investigate conditions leading to oceanic subduction initiation as a result of thermal-chemical mantle plume-lithosphere interaction in the modern earth. Results of our experiments show four different deformation regimes in response to plume-lithosphere interaction, that are a) self-sustaining subduction initiation where subduction becomes self-sustained, b) freezing subduction initiation where subduction stops at shallow depths, c) slab break-off where subducting circular slab breaks off soon after formation and d) plume underplating where plume does not pass through the lithosphere but spreads beneath it (failed subduction initiation). These different regimes depend on several parameters such as plume's size, composition and temperature, lithospheric brittle/plastic strength, age of the oceanic lithosphere and presence/absence of lithospheric heterogeneities. Results show that subduction initiates and becomes self-sustained when lithosphere is older than 10 Myr and non-dimensional ratio of the plume buoyancy force and lithospheric strength above the plume is higher than 2.

  9. A new integrated tectonic model for the Mesozoic-Early Cenozoic subduction, spreading, accretion and collision history of Tethys adjacent to the southern margin of Eurasia (NE Turkey)

    NASA Astrophysics Data System (ADS)

    Robertson, Alastair; Parlak, Osman; Ustaömer, Timur; Taslı, Kemal; İnan, Nurdan; Dumitrica, Paulian; Karaoǧlan, Fatih

    2014-05-01

    A major Tethyan suture zone (İzmir-Ankara-Erzincan-Kars Suture Zone) borders the southern margin of Eurasia throughout the Pontides. In eastern Turkey the suture zone includes a range of redeposited terrigenous and volcanogenic sedimentary rocks, pelagic sedimentary rocks and also igneous/metamorphic rocks. The igneous rocks are mostly basaltic blocks and thrust sheets within melange, plus relatively intact, to dismembered, ophiolitic rocks (oceanic crust). Two alternative hypotheses have been developed and tested during this work: 1. The suture zone preserves a single Andean-type active continental margin associated with northward subduction, accretion and arc magmatism during Mesozoic-early Cenozoic time; 2. The suture zone preserves the remnants of two different subduction zones, namely a continental margin subduction zone (as above) and an intra-ocean subduction zone (preferred model). To determine the age of the oceanic crust, relevant to both hypotheses, zircons were extracted from basic ophiolitic rocks (both intact and dismembered) and dated by the U/Pb method (U238/U236) using an ion probe at Edinburgh University. This yielded the following results for the intact ophiolites (Ma): plagiogranite cutting sheeted dykes of the Refahiye ophiolite (east of Erzincan), 183.6±1.7 (2σ); isotropic gabbro from the Karadaǧ ophiolite (northeast of Erzurum), 179.4±1.7 (2σ). In addition, dismembered ophiolites gave the following ages: gabbro cumulate (Bayburt area), 186.2±1.4 (2σ), gabbro cumulate (N of Horasan), 178.1±1.8 (2σ). Furthermore, two samples from a kilometre-sized (arc-related) tonalite body, mapped as cutting a thrust sheet of ophiolitic isotropic gabbro in the Kırdaǧ area, yielded ages of 182.1±3.2 (2σ) and 185.1±3.0 (2σ) Ma. We infer that the ophiolitic and related magmatic arc rocks formed by spreading in a supra-subduction zone setting during the late Early Jurassic (Pliensbachian-Toarcian). This amends former assumptions of a Late

  10. High-alumina basalts from the Bogda Mountains suggest an arc setting for Chinese Northern Tianshan during the Late Carboniferous

    NASA Astrophysics Data System (ADS)

    Xie, Wei; Xu, Yi-Gang; Chen, Yi-Bing; Luo, Zhen-Yu; Hong, Lu-Bing; Ma, Liang; Liu, Hai-Quan

    2016-07-01

    Considerable debate persists as to the tectonic setting of the Tianshan Orogen during the Late Paleozoic, with active subduction system and intraplate large igneous provinces as two dominant schools. With aims of providing constraints on this issue, geochronological and geochemical analyses have been carried out on the Late Carboniferous high-Al basaltic lava (HAB) from the Bogda Mountains. These lavas, in conformable contact with the felsic rocks, belong to the Upper Carboniferous Liushugou Group. Zircon SHRIMP U-Pb dating of two felsic ignimbrites further suggest that they were mainly erupted during 315-319 Ma. The Bogda basaltic lava is classified as HAB given their high Al contents > 16% and their chemical resemblance to those from modern arcs such as Aleutian and Kamchatka. They are characterized by strong enrichment in large ion lithophile elements (LILE), strong negative Nb-Ta and Ti anomalies, and distinct positive Pb anomalies. Hence, they are significantly different from the mantle plume-related basalts, as exemplified by those from Siberian, Emeishan, and Tarim large igneous provinces. Instead, their MORB-like Nd-Hf-Pb isotopes and arc-like trace elements indicate that the Bogda HABs may have been generated from a mantle wedge metasomatized by sediment-derived melts. The sector and oscillatory zoning in clinopyroxene phenocrysts in the Bogda HABs is attributable to rapid dynamic crystallization during magma ascent. High Al content is due to delayed plagioclase nucleation likely by the high crystallization pressure rather than water content. Collectively, our data lend support to an island arc environment during the Late Paleozoic, probably related to southward subduction of the Paleo-Tianshan Ocean.

  11. Subduction recycling of continental sediments and the origin of geochemically enriched reservoirs in the deep mantle

    SciTech Connect

    Rapp, R.P.; Irifune, T.; Shimizu, N.; Nishiyama, N.; Norman, M.D.; Inoue, T.

    2008-10-08

    Isotopic and trace element geochemical studies of ocean island basalts (OIBs) have for many years been used to infer the presence of long-lived ({approx} 1-2 Ga old) compositional heterogeneities in the deep mantle related to recycling of crustal lithologies and marine and terrigenous sediments via subduction [e.g., Zindler, A., Hart, S.R., 1986. Chemical geodynamics. Annu. Rev. Earth Planet. Sci. 14, 493-571; Weaver, B.L., 1991. The origin of ocean island basalt end-member compositions: trace element and isotopic constraints. Earth Planet. Sci. Lett. 104, 381-397; Chauvel, C., Hofmann, A.W., Vidal, P., 1992. HIMU-EM: the French Polynesian connection. Earth Planet. Sci. Lett. 110, 99-119; Hofmann, A.W., 1997. Mantle geochemistry: the message from oceanic volcanism. Nature 385, 219-229; Willbold, M., Stracke, A., 2006. Trace element composition of mantle end-members: Implications for recycling of oceanic and upper and lower continental crust. Geochem. Geophys. Geosyst. Q04004. 7, doi:10.1029/2005GC001005]. In particular, models for the EM-1 type ('enriched mantle') OIB reservoir have invoked the presence of subducted, continental-derived sediment to explain high {sup 87}Sr/{sup 86}Sr ratios, low {sup 143}Nd/{sup 144}Nd and {sup 206}Pb/{sup 204}Pb ratios, and extreme enrichments in incompatible elements observed in OIB lavas from, for example, the Pitcairn Island group in the South Pacific [Woodhead, J.D., McCulloch, M.T., 1989; Woodhead, J.D., Devey, C.W., 1993. Geochemistry of the Pitcairn seamounts, I: source character and temporal trends. Earth Planet. Sci. Lett. 116, 81-99; Eisele, J., Sharma, M., Galer, S.J.G., Blichert-Toft, J., Devey, C.W., Hofmann, A.W., 2002. The role of sediment recycling in EM-1 inferred from Os, Pb, Hf, Nd, Sr isotope and trace element systematics of the Pitcairn hotspot. Earth Planet. Sci. Lett. 196, 197-212]. More recently, ultrapotassic, mantle-derived lavas (lamproites) from Gaussberg, Antarctica have been interpreted as the product

  12. Magnesium Isotopic Composition of Subducting Marine Sediments

    NASA Astrophysics Data System (ADS)

    Hu, Y.; Teng, F. Z.; Plank, T. A.; Huang, K. J.

    2015-12-01

    Subducted marine sediments have recently been called upon to explain the heterogeneous Mg isotopic composition (δ26Mg, ‰) found in mantle wehrlites (-0.39 to +0.09 [1]) in the context of a homogeneous mantle (-0.25 ± 0.07 [2]). However, no systematic measurements of δ26Mg on marine sediments are currently available to provide direct support to this model. To characterize the Mg inputs to global subduction zones, we measured δ26Mg data for a total of 90 marine sediments collected from 12 drill sites outboard of the world's major subduction zones. These sediments span a 1.73‰ range in δ26Mg. The detritus-dominated sediments have δ26Mg (-0.59 to +0.53) comparable to those of weathered materials on continents (e.g. -0.52 to +0.92 [3]), while the calcareous oozes yield δ26Mg (as light as -1.20) more similar to the seawater value (-0.83 [4]). The negative correlation between δ26Mg and CaO/Al2O3 in these sediments indicates the primary control of mineralogy over the Mg isotopic distribution among different sediment types, as carbonates are enriched in light Mg isotopes (-5.10 to -0.40 [5]) whereas clay-rich weathering residues generally have heavier δ26Mg (e.g. up to +0.65 in saprolite [6]). In addition, chemical weathering and grain-size sorting drive sediments to a heavier δ26Mg, as indicated by the broad positive trends between δ26Mg with CIA (Chemical Index of Alteration [7]) and Al2O3/SiO2, respectively. Collectively, the arc systems sampled in this study represent ~30% of global arc length and the extrapolated global Mg flux of subducting marine sediments accounts for ~9% of the yearly Mg riverine input with a flux-weighted average δ26Mg at -0.26. Subduction of these heterogeneous sediments may not cause significant mantle heterogeneity on a global scale, but the highly variable Mg fluxes and δ26Mg of sediments delivered to different trenches are capable of producing local mantle variations. Volcanic rocks sourced from these mantle domains are thus

  13. Subduction zone guided waves in Northern Chile

    NASA Astrophysics Data System (ADS)

    Garth, Thomas; Rietbrock, Andreas

    2016-04-01

    Guided wave dispersion is observed in subduction zones as high frequency energy is retained and delayed by low velocity structure in the subducting slab, while lower frequency energy is able to travel at the faster velocities associated with the surrounding mantle material. As subduction zone guided waves spend longer interacting with the low velocity structure of the slab than any other seismic phase, they have a unique capability to resolve these low velocity structures. In Northern Chile, guided wave arrivals are clearly observed on two stations in the Chilean fore-arc on permanent stations of the IPOC network. High frequency (> 5 Hz) P-wave arrivals are delayed by approximately 2 seconds compared to the low frequency (< 2 Hz) P-wave arrivals. Full waveform finite difference modelling is used to test the low velocity slab structure that cause this P-wave dispersion. The synthetic waveforms produced by these models are compared to the recorded waveforms. Spectrograms are used to compare the relative arrival times of different frequencies, while the velocity spectra is used to constrain the relative amplitude of the arrivals. Constraining the waveform in these two ways means that the full waveform is also matched, and the low pass filtered observed and synthetic waveforms can be compared. A combined misfit between synthetic and observed waveforms is then calculated following Garth & Rietbrock (2014). Based on this misfit criterion we constrain the velocity model by using a grid search approach. Modelling the guided wave arrivals suggest that the observed dispersion cannot be solely accounted for by a single low velocity layer as suggested by previous guided wave studies. Including dipping low velocity normal fault structures in the synthetic model not only accounts for the observed strong P-wave coda, but also produces a clear first motion dispersion. We therefore propose that the lithospheric mantle of the subducting Nazca plate is highly hydrated at intermediate

  14. Seismicity of the eastern Hellenic Subduction Zone

    NASA Astrophysics Data System (ADS)

    Bruestle, A.; Kueperkoch, L.; Rische, M.; Meier, T.; Friederich, W.; Egelados Working Group

    2012-04-01

    The Hellenic Subduction Zone (HSZ) is the seismically most active region of Europe. The African plate is subducting beneath the Aegean lithosphere with a relative velocity of 4 cm per year. A detailed picture of the microseismicity of the eastern HSZ was obtained by the recordings of the temporary networks CYCNET (September 2002 - September 2005) and EGELADOS (October 2005 - March 2007). In total, nearly 7000 earthquakes were located with a location uncertainty of less than 20 km. The SE Aegean is dominated by (1) shallow intraplate seismicity within the Aegean plate, by (2) interplate seismicity at the plate contact and by (3) intermediate deep seismicity along the subducting African slab. Strong shallow seismicity in the upper plate is observed along the Ptolemy graben south of Crete extending towards the Karpathos Basin, indicating intense recent deformation of the forearc. In contrary, low shallow seismicity around Rhodes indicates only minor seismic crustal deformation of the upper plate. An almost NS-striking zone of microseismicity has been located, running from the Karpathos basin via the Nisyros volcanic complex towards the EW striking Gökova graben. In the SE Aegean the geometry of the Wadati-Benioff-Zone (WBZ) within the subducting African plate is revealed in detail by the observed microseismicity. Between about 50 to 100 km depth a continuous band of intermediate deep seismicity describes the strongly curved geometry of the slab. From the central to the eastern margin of the HSZ, the dip direction of the WBZ changes from N to NW with a strong increase of the dip angle beneath the eastern Cretan Sea. The margin of the dipping African slab is marked by an abrupt end of the observed WBZ beneath SW Anatolia. Below 100 km depth, the WBZ of the eastern HSZ is dominated by an isolated cluster of intense intermediate deep seismicity (at 100-180 km depth) beneath the Nisyros volcanic complex. It has an extension of about 100x80 km and is build up of 3 parallel

  15. Modeling the effects of 3-D slab geometry and oblique subduction on subduction zone thermal structure

    NASA Astrophysics Data System (ADS)

    Wada, I.; Wang, K.; He, J.

    2013-12-01

    In this study, we revisit the effects of along-strike variation in slab geometry and oblique subduction on subduction zone thermal structures. Along-strike variations in slab dip cause changes in the descending rate of the slab and generate trench-parallel pressure gradients that drive trench-parallel mantle flow (e.g., Kneller and van Keken, 2007). Oblique subduction also drives trench-parallel mantle flow. In this study, we use a finite element code PGCtherm3D and examine a range of generic subduction geometries and parameters to investigate the effects of the above two factors. This exercise is part of foundational work towards developing detailed 3-D thermal models for NE Japan, Nankai, and Cascadia to better constrain their 3-D thermal structures and to understand the role of temperature in controlling metamorphic, seismogenic, and volcanic processes. The 3-D geometry of the subducting slabs in the forearc and arc regions are well delineated at these three subduction zones. Further, relatively large compilations of surface heat flow data at these subduction zones make them excellent candidates for this study. At NE Japan, a megathrust earthquake occurred on March 11, 2011; at Nankai and Cascadia, there has been a great effort to constrain the scale of the next subduction thrust earthquake for the purpose of disaster prevention. Temperature influences the slip behavior of subduction faults by (1) affecting the rheology of the interface material and (2) controlling dehydration reactions, which can lead to elevated pore fluid pressure. Beyond the depths of subduction thrust earthquakes, the thermal structure is affected strongly by the pattern of mantle wedge flow. This flow is driven by viscous coupling between the subducting slab and the overriding mantle, and it brings in hot flowing mantle into the wedge. The trench-ward (up-dip) extent of the slab-mantle coupling is thus a key factor that controls the thermal structure. Slab-mantle decoupling at shallow

  16. Oligocene and Miocene arc volcanism in northeastern California: evidence for post-Eocene segmentation of the subducting Farallon plate

    USGS Publications Warehouse

    Colgan, J.P.; Egger, A.E.; John, D.A.; Cousens, B.; Fleck, R.J.; Henry, C.D.

    2011-01-01

    The Warner Range in northeastern California exposes a section of Tertiary rocks over 3 km thick, offering a unique opportunity to study the long-term history of Cascade arc volcanism in an area otherwise covered by younger volcanic rocks. The oldest locally sourced volcanic rocks in the Warner Range are Oligocene (28–24 Ma) and include a sequence of basalt and basaltic andesite lava flows overlain by hornblende and pyroxene andesite pyroclastic flows and minor lava flows. Both sequences vary in thickness (0–2 km) along strike and are inferred to be the erosional remnants of one or more large, partly overlapping composite volcanoes. No volcanic rocks were erupted in the Warner Range between ca. 24 and 16 Ma, although minor distally sourced silicic tuffs were deposited during this time. Arc volcanism resumed ca. 16 Ma with eruption of basalt and basaltic andesite lavas sourced from eruptive centers 5–10 km south of the relict Oligocene centers. Post–16 Ma arc volcanism continued until ca. 8 Ma, forming numerous eroded but well-preserved shield volcanoes to the south of the Warner Range. Oligocene to Late Miocene volcanic rocks in and around the Warner Range are calc-alkaline basalts to andesites (48%–61% SiO2) that display negative Ti, Nb, and Ta anomalies in trace element spider diagrams, consistent with an arc setting. Middle Miocene lavas in the Warner Range are distinctly different in age, composition, and eruptive style from the nearby Steens Basalt, with which they were previously correlated. Middle to Late Miocene shield volcanoes south of the Warner Range consist of homogeneous basaltic andesites (53%–57% SiO2) that are compositionally similar to Oligocene rocks in the Warner Range. They are distinctly different from younger (Late Miocene to Pliocene) high-Al, low-K olivine tholeiites, which are more mafic (46%–49% SiO2), did not build large edifices, and are thought to be related to backarc extension. The Warner Range is ∼100 km east of the

  17. Origin of Columbia River flood basalt controlled by propagating rupture of the Farallon slab.

    PubMed

    Liu, Lijun; Stegman, Dave R

    2012-02-16

    The origin of the Steens-Columbia River (SCR) flood basalts, which is presumed to be the onset of Yellowstone volcanism, has remained controversial, with the proposed conceptual models involving either a mantle plume or back-arc processes. Recent tomographic inversions based on the USArray data reveal unprecedented detail of upper-mantle structures of the western USA and tightly constrain geodynamic models simulating Farallon subduction, which has been proposed to influence the Yellowstone volcanism. Here we show that the best-fitting geodynamic model depicts an episode of slab tearing about 17 million years ago under eastern Oregon, where an associated sub-slab asthenospheric upwelling thermally erodes the Farallon slab, leading to formation of a slab gap at shallow depth. Driven by a gradient of dynamic pressure, the tear ruptured quickly north and south and within about two million years covering a distance of around 900 kilometres along all of eastern Oregon and northern Nevada. This tear would be consistent with the occurrence of major volcanic dikes during the SCR-Northern Nevada Rift flood basalt event both in space and time. The model predicts a petrogenetic sequence for the flood basalt with sources of melt starting from the base of the slab, at first remelting oceanic lithosphere and then evolving upwards, ending with remelting of oceanic crust. Such a progression helps to reconcile the existing controversies on the interpretation of SCR geochemistry and the involvement of the putative Yellowstone plume. Our study suggests a new mechanism for the formation of large igneous provinces. PMID:22337059

  18. What's happening inside the subduction factory?

    NASA Astrophysics Data System (ADS)

    Penniston-Dorland, S. C.; Bebout, G. E.; Gorman, J. K.; Piccoli, P. M.; Walker, R. J.

    2012-12-01

    Much research has focused on the inputs and outputs of the 'subduction factory,' however a variety of metamorphic processes occur within the subducting slab and at its interface with the mantle wedge that contribute to creating the mixed signals observed in arc magmas. Subduction-related metamorphic rocks from the Catalina Schist represent a range of metamorphic grades and provide a natural laboratory to investigate these processes. Hybrid rock types such as reaction zones or 'rinds' between mafic (crustal) and ultramafic (mantle) rocks have attracted recent interest since they have a different bulk chemistry and mineralogy compared to the original inputs to the subduction factory. Here we explore the mineralogical and geochemical differences between the metamorphic rocks, their reaction zones, and endmember subduction input lithologies over a range of metamorphic grades including lawsonite albite, lawsonite blueschist, and amphibolite facies (with peak T ranging from ~ 275 to ~ 750°C and peak P ranging up to ~1.1 GPa). The results shed light on chemical changes occurring within the subduction zone and the processes happening inside the 'subduction factory', including mass transfer of elements by both fluid infiltration and mechanical mixing. Elements commonly enriched in arc magmatic rocks, such as the LILE (e.g. Ba, K), are enriched in metamafic rocks at all metamorphic grades relative to likely MORB protoliths. These enrichments are interpreted as the product of metamorphic fluid infiltration. Many major- and trace-element concentrations in reaction rinds fall between those of metamafic blocks and surrounding ultramafic-rich mélange matrix (including TiO2, MgO, FeO, Al2O3, Zr, Ni and Cr). Spatial distributions of these elements within the rinds suggest that the intermediate concentrations may be due to mechanical mixing of crustal and mantle materials. Rind concentrations of the highly siderophile elements (HSE: including Os, Ir, Ru, Pt, Pd, Re) as well as

  19. Velocities of Subducted Sediments and Continents

    NASA Astrophysics Data System (ADS)

    Hacker, B. R.; van Keken, P. E.; Abers, G. A.; Seward, G.

    2009-12-01

    The growing capability to measure seismic velocities in subduction zones has led to unusual observations. For example, although most minerals have VP/ VS ratios around 1.77, ratios <1.7 and >1.8 have been observed. Here we explore the velocities of subducted sediments and continental crust from trench to sub-arc depths using two methods. (1) Mineralogy was calculated as a function of P & T for a range of subducted sediment compositions using Perple_X, and rock velocities were calculated using the methodology of Hacker & Abers [2004]. Calculated slab-top temperatures have 3 distinct depth intervals with different dP/dT gradients that are determined by how coupling between the slab and mantle wedge is modeled. These three depth intervals show concomitant changes in VP and VS: velocities initially increase with depth, then decrease beyond the modeled decoupling depth where induced flow in the wedge causes rapid heating, and increase again at depth. Subducted limestones, composed chiefly of aragonite, show monotonic increases in VP/ VS from 1.63 to 1.72. Cherts show large jumps in VP/ VS from 1.55-1.65 to 1.75 associated with the quartz-coesite transition. Terrigenous sediments dominated by quartz and mica show similar, but more-subdued, transitions from ~1.67 to 1.78. Pelagic sediments dominated by mica and clinopyroxene show near-monotonic increases in VP/ VS from 1.74 to 1.80. Subducted continental crust that is too dry to transform to high-pressure minerals has a VP/ VS ratio of 1.68-1.70. (2) Velocity anisotropy calculations were made for the same P-T dependent mineralogies using the Christoffel equation and crystal preferred orientations measured via electron-backscatter diffraction for typical constituent phases. The calculated velocity anisotropies range from 5-30%. For quartz-rich rocks, the calculated velocities show a distinct depth dependence because crystal slip systems and CPOs change with temperature. In such rocks, the fast VP direction varies from slab

  20. Modeled temperatures and fluid source distributions for the Mexican subduction zone: Effects of hydrothermal circulation and implications for plate boundary seismic processes

    NASA Astrophysics Data System (ADS)

    Perry, Matthew; Spinelli, Glenn A.; Wada, Ikuko; He, Jiangheng

    2016-02-01

    In subduction zones, spatial variations in pore fluid pressure are hypothesized to control the sliding behavior of the plate boundary fault. The pressure-temperature paths for subducting material control the distributions of dehydration reactions, a primary control on the pore fluid pressure distribution. Thus, constraining subduction zone temperatures are required to understand the seismic processes along the plate interface. We present thermal models for three margin-perpendicular transects in the Mexican subduction zone. We examine the potential thermal effects of vigorous fluid circulation in a high-permeability aquifer within the basaltic basement of the oceanic crust and compare the results with models that invoke extremely high pore fluid pressures to reduce frictional heating along the megathrust. We combine thermal model results with petrological models to determine the spatial distribution of fluid release from the subducting slab and compare dewatering locations with the locations of seismicity, nonvolcanic tremor, slow-slip events, and low-frequency earthquakes. Simulations including hydrothermal circulation are most consistent with surface heat flux measurements. Hydrothermal circulation has a maximum cooling effect of 180°C. Hydrothermally cooled crust carries water deeper into the subduction zone; fluid release distributions in these models are most consistent with existing geophysical data. Our models predict focused fluid release, which could generate overpressures, coincident with an observed ultraslow layer (USL) and a region of nonvolcanic tremor. Landward of USLs, a downdip decrease in fluid source magnitude could result in the dissipation in overpressure in the oceanic crust without requiring a downdip increase in fault zone permeability, as posited in previous studies.

  1. Subduction Zone Science - Examples of Seismic Images of the Central Andes and Subducting Nazca Slab

    NASA Astrophysics Data System (ADS)

    Beck, S. L.; Zandt, G.; Scire, A. C.; Ward, K. M.; Portner, D. E.; Bishop, B.; Ryan, J. C.; Wagner, L. S.; Long, M. D.

    2015-12-01

    Subduction has shaped large regions of the Earth and constitute over 55,000 km of convergent plate margin today. The subducting slabs descend from the surface into the lower mantle and impacts earthquake occurrence, surface uplift, arc volcanism and mantle convection as well as many other processes. The subduction of the Nazca plate beneath the South America plate is one example and constitutes the largest present day ocean-continent convergent margin system and has built the Andes, one of the largest actively growing mountain ranges on Earth. This active margin is characterized by along-strike variations in arc magmatism, upper crustal shortening, crustal thickness, and slab geometry that make it an ideal region to study the relationship between the subducting slab, the mantle wedge, and the overriding plate. After 20 years of portable seismic deployments in the Central Andes seismologists have combined data sets and used multiple techniques to generate seismic images spanning ~3000 km of the South American subduction zone to ~800 km depth with unprecedented resolution. For example, using teleseismic P- waves we have imaged the Nazca slab penetrating through the mantle transition zone (MTZ) and into the uppermost lower mantle. Our tomographic images show that there is significant along-strike variation in the morphology of the Nazca slab in the upper mantle, MTZ, and the lower mantle, including possible tears, folding, and internal deformation. Receiver function studies and surface wave tomography have revealed major changes in lithospheric properties in the Andes. Improved seismic images allow us to more completely evaluate tectonic processes in the formation and uplift of the Andes including: (1) overthickened continental crust driven by crustal shortening, (2) changes in slab dip and coupling with the overlying plate (3) localized lithospheric foundering, and (4) large-scale mantle and crustal melting leading to magmatic addition and/or crustal flow. Although

  2. Additive Construction using Basalt Regolith Fines

    NASA Technical Reports Server (NTRS)

    Mueller, Robert P.; Sibille, Laurent; Hintze, Paul E.; Lippitt, Thomas C.; Mantovani, James G.; Nugent, Matthew W.; Townsend, Ivan I.

    2014-01-01

    Planetary surfaces are often covered in regolith (crushed rock), whose geologic origin is largely basalt. The lunar surface is made of small-particulate regolith and areas of boulders located in the vicinity of craters. Regolith composition also varies with location, reflecting the local bedrock geology and the nature and efficiency of the micrometeorite-impact processes. In the lowland mare areas (suitable for habitation), the regolith is composed of small granules (20 - 100 microns average size) of mare basalt and volcanic glass. Impacting micrometeorites may cause local melting, and the formation of larger glassy particles, and this regolith may contain 10-80% glass. Studies of lunar regolith are traditionally conducted with lunar regolith simulant (reconstructed soil with compositions patterned after the lunar samples returned by Apollo). The NASA Kennedy Space Center (KSC) Granular Mechanics & Regolith Operations (GMRO) lab has identified a low fidelity but economical geo-technical simulant designated as Black Point-1 (BP-1). It was found at the site of the Arizona Desert Research and Technology Studies (RATS) analog field test site at the Black Point lava flow in adjacent basalt quarry spoil mounds. This paper summarizes activities at KSC regarding the utilization of BP-1 basalt regolith and comparative work with lunar basalt simulant JSC-1A as a building material for robotic additive construction of large structures. In an effort to reduce the import or in-situ fabrication of binder additives, we focused this work on in-situ processing of regolith for construction in a single-step process after its excavation. High-temperature melting of regolith involves techniques used in glassmaking and casting (with melts of lower density and higher viscosity than those of metals), producing basaltic glass with high durability and low abrasive wear. Most Lunar simulants melt at temperatures above 1100 C, although melt processing of terrestrial regolith at 1500 C is not

  3. Geochemistry of apollo 15 basalt 15555 and soil 15531.

    PubMed

    Schnetzler, C C; Philpotts, J A; Nava, D F; Schuhmann, S; Thomas, H H

    1972-01-28

    Major and trace element concentrations have been determined by atomic absorption spectrophotometry, colorimetry, and isotope dilution in Apollo 15 mare basalt 15555 from the Hadley Rille area; trace element concentrations have also been determined in plagioclase and pyroxene separates from basalt 15555 and in soil 15531 from the same area. Basalt 15555 most closely resembles in composition the Apollo 12 olivine-rich basalts. The concentrations of lithium, potassium, rubidium, barium, rare-earth elements, and zirconium in basalt 15555 are the lowest, and the negative europium anomaly is the smallest, reported for lunar basalts; this basalt might be the least differentiated material yet returned from the moon. Crystallization and removal of about 6 percent of plagioclase similar to that contained in the basalt would account for the observed europium anomaly; if plagioclase is not on the liquidus of this basalt, a multistage origin is indicated. Mineral data indicate that plagioclase and pyroxene approached quasi-equilibrium. Most of the chemical differences between basalt 15555 and soil 15531 would be accounted for if the soil were a mixture of 88 percent basalt, 6 percent KREEP (a component, identified in other Apollo soils, rich in potassium, rare-earth elements, and phosphorus) and 6 percent plagioclase (anorthosite?). PMID:17731364

  4. Molybdenum mobility and isotopic fractionation during subduction at the Mariana arc

    NASA Astrophysics Data System (ADS)

    Freymuth, Heye; Vils, Flurin; Willbold, Matthias; Taylor, Rex N.; Elliott, Tim

    2015-12-01

    The fate of crustal material recycled into the convecting mantle by plate tectonics is important for understanding the chemical and physical evolution of the planet. Marked isotopic variability of Mo at the Earth's surface offers the promise of providing distinctive signatures of such recycled material. However, characterisation of the behaviour of Mo during subduction is needed to assess the potential of Mo isotope ratios as tracers for global geochemical cycles. Here we present Mo isotope data for input and output components of the archetypical Mariana arc: Mariana arc lavas, sediments from ODP Sites 800, 801 and 802 near the Mariana trench and the altered mafic, oceanic crust (AOC), from ODP Site 801, together with samples of the deeper oceanic crust from ODP Site 1256. We also report new high precision Pb isotope data for the Mariana arc lavas and a dataset of Pb isotope ratios from sediments from ODP Sites 800, 801 and 802. The Mariana arc lavas are enriched in Mo compared to elements of similar incompatibility during upper mantle melting, and have distinct, isotopically heavy Mo (high 98Mo/95Mo) relative to the upper mantle, by up to 0.3 parts per thousand. In contrast, the various subducting sediment lithologies dominantly host isotopically light Mo. Coupled Pb and Mo enrichment in the Mariana arc lavas suggests a common source for these elements and we further use Pb isotopes to identify the origin of the isotopically heavy Mo. We infer that an aqueous fluid component with elevated [Mo], [Pb], high 98Mo/95Mo and unradiogenic Pb is derived from the subducting, mafic oceanic crust. Although the top few hundred metres of the subducting, mafic crust have a high 98Mo/95Mo, as a result of seawater alteration, tightly defined Pb isotope arrays of the Mariana arc lavas extrapolate to a fluid component akin to fresh Pacific mid-ocean ridge basalts. This argues against a flux dominantly derived from the highly altered, uppermost mafic crust or indeed from an Indian

  5. Mission Immiscible: Distinct subduction components generate two primary magmas of Pagan Volcano, Mariana arc

    NASA Astrophysics Data System (ADS)

    Tamura, Y.; Ishizuka, O.; Stern, R. J.; Nunokawa, A.; Shukuno, H.; Kawabata, H.; Hirahara, Y.; Chang, Q.; Miyazaki, T.; Kimura, J.; Embley, R. W.; Bloomer, S. H.; Tatsumi, Y.

    2012-12-01

    Pagan is one of the largest (2,160 km3; Bloomer et al., 1989) volcanoes along the Mariana arc magmatic front. Pagan has a maximum elevation of 570 m (Mt. Pagan), but its submarine flanks descend to 2,000-3,000 m, and most of the volcano is submarine and unexplored. Bathymetric mapping and ROV Hyper-Dolphin (HPD1147) dive on the NE submarine flank of Pagan were carried out during NT10-12 (R/V Natsushima) in July 2010. There are no systematic differences between subaerial and submarine lavas with > 52 wt % SiO2, suggesting derivation from the same magmatic system. Twenty least-fractionated basalts (48.5-50 wt % SiO2) extend to higher MgO (10-11 wt %) and Mg# (66-70) than subaerial lavas. Compositions of olivine (up to Fo94) and spinels (Cr# up to 0.8) suggest that Pagan primitive magmas formed from high degrees of mantle melting. Two geochemical groups of basalts can be distinguished at similar 10-11 wt % MgO; these erupted about the same time, 500 m apart. Both contain clinopyroxene and olivine phenocrysts, thus, these two groups are referred to as COB1 and COB2. Lower TiO2, FeO, Na2O, K2O, incompatible trace element abundances, and Nb/Yb suggest that COB1 formed from higher degrees of mantle melting. In addition, LREE-enrichment and higher Th/Nb in COB2 contrast with LREE-depletion and lower Th/Nb in COB1. Higher Ba/Th and Ba/Nb and lower Th/Nb indicate that main subduction addition in COB1 was dominated by hydrous fluid, whereas that in COB2 was dominated by sediment melt. Sr-Nd-Pb-Hf isotopes are also consistent with this scenario. Importantly, the subduction addition, that caused more melting of the COB1 source was mostly hydrous fluid. In contrast to Pagan, we observed two primary magmas (COB and POB) in the NW Rota-1 volcano (NWR1), ~40 km behind the volcanic front. NWR1 COB has a greater subduction component, both hydrous fluid and sediment melt, than POB, perhaps reflecting that the subducting slab below NWR1 is > 100 km deeper than that beneath Pagan. At

  6. Fe-XANES analyses of Reykjanes Ridge basalts: Implications for oceanic crust's role in the solid Earth oxygen cycle

    NASA Astrophysics Data System (ADS)

    Shorttle, Oliver; Moussallam, Yves; Hartley, Margaret E.; Maclennan, John; Edmonds, Marie; Murton, Bramley J.

    2015-10-01

    The cycling of material from Earth's surface environment into its interior can couple mantle oxidation state to the evolution of the oceans and atmosphere. A major uncertainty in this exchange is whether altered oceanic crust entering subduction zones can carry the oxidised signal it inherits during alteration at the ridge into the deep mantle for long-term storage. Recycled oceanic crust may be entrained into mantle upwellings and melt under ocean islands, creating the potential for basalt chemistry to constrain solid Earth-hydrosphere redox coupling. Numerous independent observations suggest that Iceland contains a significant recycled oceanic crustal component, making it an ideal locality to investigate links between redox proxies and geochemical indices of enrichment. We have interrogated the elemental, isotope and redox geochemistry of basalts from the Reykjanes Ridge, which forms a 700 km transect of the Iceland plume. Over this distance, geophysical and geochemical tracers of plume influence vary dramatically, with the basalts recording both long- and short-wavelength heterogeneity in the Iceland plume. We present new high-precision Fe-XANES measurements of Fe3+ / ∑ Fe on a suite of 64 basalt glasses from the Reykjanes Ridge. These basalts exhibit positive correlations between Fe3+ / ∑ Fe and trace element and isotopic signals of enrichment, and become progressively oxidised towards Iceland: fractionation-corrected Fe3+ / ∑ Fe increases by ∼0.015 and ΔQFM by ∼0.2 log units. We rule out a role for sulfur degassing in creating this trend, and by considering various redox melting processes and metasomatic source enrichment mechanisms, conclude that an intrinsically oxidised component within the Icelandic mantle is required. Given the previous evidence for entrained oceanic crustal material within the Iceland plume, we consider this the most plausible carrier of the oxidised signal. To determine the ferric iron content of the recycled component ([Fe2O

  7. Source Evolution After Subduction Initiation as Recorded in the Izu-Bonin-Mariana Fore-arc Crust

    NASA Astrophysics Data System (ADS)

    Shervais, J. W.; Reagan, M. K.; Pearce, J. A.; Shimizu, K.

    2015-12-01

    Drilling in the Izu-Bonin-Mariana (IBM) fore-arc during IODP Expedition 352 and DSDP Leg 60 recovered consistent stratigraphic sequences of volcanic rocks reminiscent of those found in many ophiolites. The oldest lavas in these sections are "fore-arc basalts" (FAB) with ~51.5 Ma ages. Boninites began eruption approximately 2-3 m.y. later (Ishizuka et al., 2011, EPSL; Reagan et al., 2013, EPSL) and further from the trench. First results from IODP Expedition 352 and preliminary post-cruise data suggest that FAB at Sites U1440 and U1441 were generated by decompression melting during near-trench sea-floor spreading, and that fluids from the subducting slab were not involved in their genesis. Temperatures appear to have been unusually high and pressures of melting appear to have been unusually low compared to mid-ocean ridges. Spreading rates at this time appear to have been robust enough to maintain a stable melt lens. Incompatible trace element abundances are low in FAB compared to even depleted MORB. Nd and Hf Isotopic compositions published before the expedition suggest that FAB were derived from typical MORB source mantle. Thus, their extreme deletion resulted from unusually high degrees of melting immediately after subduction initiation. The oldest boninites from DSDP Site 458 and IODP Sites U1439 and U1442 have relatively high concentrations of fluid-soluble elements, low concentrations of REE, and light depleted REE patterns. Younger boninites, have even lower REE concentrations, but have U-shaped REE patterns. Our first major and trace element compositions for the FAB through boninite sequence suggests that melting pressures and temperatures decreased through time, mantle became more depleted though time, and spreading rates waned during boninite genesis. Subduction zone fluids involved in boninite genesis appear to have been derived from progressively higher temperatures and pressures over time as the subducting slab thermally matured.

  8. Fluid-induced petrophysical changes of blueschist-facies overprinted eclogites: consequences for exhumation processes along the subduction plate interface

    NASA Astrophysics Data System (ADS)

    John, Timm; van der Straaten, François; Schenk, Volker; Li, Ji-Lei; Gao, Jun

    2014-05-01

    The bulk chemical data of the Tianshan eclogite-blueschist sequence points to a back-arc derived oceanic basalt. The sample describes a counter clockwise P-T path, at distinct higher temperatures and a retrograde contemporaneous cooling and decompression, which typically results from uplift and exhumation in the subduction channel. During retrograde metasomatic fluid infiltration and associated metamorphism Mg, transition metals, and LILE were gained in the replacing blueschist compared to the precursor eclogite, while HFSE and Al behaved immobile. Additionally Si, Ca and REE are mobilised and removed during this conversion process. These chemical changes together with the nearly total LREE lost in the sample indicate that the fluid prior to infiltration had been in equilibrium with a rock of peridotitic composition, most likely with the partially serpentinised mantle wedge. Fluid-induced element loss and slight volume increase result in a decreased density of the affected blueschist and eclogite. Compared to the assumed serpentinite matrix in the subduction channel the calculated density contrast between the HP-rocks of the Tianshan is high enough that the serpentinite is able to carry these HP-rocks to shallower depths. Serpentinite with incorporated low-density rocks in a matrix of unaltered and thus denser mantle peridotite is able to rise up, particularly by buoyancy-driven forces. Although the changed chemistry of the eclogites and blueschists points to infiltration of an external fluid that was in equilibrium with a peridotitic source (most likely the serpentinite in the subduction channel), occurrences of outcropping serpentinite in the Tianshan are very rare. Alternatively the meta-sedimentary units in the Tianshan are suitable to overtake the role of the serpentinite as carrier matrix in the subduction channel.

  9. Diapiric flow at subduction zones: a recipe for rapid transport.

    PubMed

    Hall, P S; Kincaid, C

    2001-06-29

    Recent geochemical studies of uranium-thorium series disequilibrium in rocks from subduction zones require magmas to be transported through the mantle from just above the subducting slab to the surface in as little as approximately 30,000 years. We present a series of laboratory experiments that investigate the characteristic time scales and flow patterns of the diapiric upwelling model of subduction zone magmatism. Results indicate that the interaction between buoyantly upwelling diapirs and subduction-induced flow in the mantle creates a network of low-density, low-viscosity conduits through which buoyant flow is rapid, yielding transport times commensurate with those indicated by uranium-thorium studies. PMID:11431563

  10. Towards the Dynamics of Subduction Initiation within an Historical Context

    NASA Astrophysics Data System (ADS)

    Gurnis, M.; Lavier, L.; House, M. A.

    2001-12-01

    Little progress was made on understanding the dynamics of subduction initiation following the establishment of plate tectonics. Modelers quickly established that subduction initiation would be difficult if a homogeneous, unbroken lithosphere underwent convective instability. Modelers focused on attempting to establish the means by which an Atlantic-type margin could founder, creating a theory for an idealized 'Wilson-cycle'. On the other hand, considerable progress was made at finding places where subduction nucleated in the past. Subduction zones often tend to form in close proximity to other subduction zones and/or near preexisting zones of weakness within the lithosphere, such as old plate margins. Only recently have we attempted to understand the dynamics of subduction inititation with realistic initial conditions. Associated with major plate reorganizations, there may be two types of subduction initiation: spontaneous nucleation and forced; both may occur on old plate margins. The Eocene reorganization of the Pacific plate provides the context for this theory. As shown by Uyeda and Hilde the Izu-Bonin-Mariana subduction zone may have formed along the Palau Kyushu Ridge, potentially a transform margin; Stern has assembled evidence suggesting that the IBM formed by spontaneous nucleation. Much further to the south, the Tonga-Kermadec subduction zone may have simultaneously formed by thrusting leading to the preserved New Caledonia ophiolite. The IBM subduction may have caused the change in Pacific plate motion at 43 Ma, while other subduction zones, such as Tonga-Kermadec, may have resulted from the change in plate motion. Unfortunately, the geological record within subducting plate boundaries becomes deformed and buried so that the ability to test dynamic models is limited. Seeking a better preserved record, recent work has focused on the Macquarie Ridge complex, the Australia-Pacific plate boundary south of New Zealand where subduction has been nucleating

  11. Biogenic Mn-Oxides in Subseafloor Basalts

    PubMed Central

    Ivarsson, Magnus; Broman, Curt; Gustafsson, Håkan; Holm, Nils G.

    2015-01-01

    The deep biosphere of the subseafloor basalts is recognized as a major scientific frontier in disciplines like biology, geology, and oceanography. Recently, the presence of fungi in these environments has involved a change of view regarding diversity and ecology. Here, we describe fossilized fungal communities in vugs in subseafloor basalts from a depth of 936.65 metres below seafloor at the Detroit Seamount, Pacific Ocean. These fungal communities are closely associated with botryoidal Mn oxides composed of todorokite. Analyses of the Mn oxides by Electron Paramagnetic Resonance spectroscopy (EPR) indicate a biogenic signature. We suggest, based on mineralogical, morphological and EPR data, a biological origin of the botryoidal Mn oxides. Our results show that fungi are involved in Mn cycling at great depths in the seafloor and we introduce EPR as a means to easily identify biogenic Mn oxides in these environments. PMID:26107948

  12. Northwest Africa 5298: A Basaltic Shergottite

    NASA Technical Reports Server (NTRS)

    Hui, Hejiu; Peslier, Anne; Lapen, Thomas J.; Brandon, Alan; Shafer, John

    2009-01-01

    NWA 5298 is a single 445 g meteorite found near Bir Gandouz, Morocco in March 2008 [1]. This rock has a brown exterior weathered surface instead of a fusion crust and the interior is composed of green mineral grains with interstitial dark patches containing small vesicles and shock melts [1]. This meteorite is classified as a basaltic shergottite [2]. A petrologic study of this Martian meteorite is being carried out with electron microprobe analysis and soon trace element analyses by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Oxygen fugacity is calculated from Fe-Ti oxides pairs in the sample. The data from this study constrains the petrogenesis of basaltic shergottites.

  13. Lead isotope studies of mare basalt 70017

    NASA Technical Reports Server (NTRS)

    Mattinson, J. M.; Tilton, G. R.; Todt, W.; Chen, J. H.

    1977-01-01

    Uranium, thorium, and isotopic lead data for components of basalt 70017 are reported, and it is found that the whole rock, pyroxene, and ilmenite points in a concordia diagram plot along a chord intersecting the curve at 3.7 and 4.33 eons. The plagioclase data do not seem to lie on this line. The data for 70017 appear to plot along a distinctly different chord in a concordia diagram than do the data for 75055 and 75035, two other Apollo 17 mare basalts. The lead data are in accord with Sm-Nd results. A 3.7 eon crystallization age for 70017 would be consistent with the same kind of parentless lead that is indicated by previous studies of soils and soil breccias from stations at Taurus-Littrow. The Th/U ratio in ilmenite is 2.2, and the concentrations of these two elements are approximately twice those in pyroxene.

  14. Biogenic Mn-Oxides in Subseafloor Basalts.

    PubMed

    Ivarsson, Magnus; Broman, Curt; Gustafsson, Håkan; Holm, Nils G

    2015-01-01

    The deep biosphere of the subseafloor basalts is recognized as a major scientific frontier in disciplines like biology, geology, and oceanography. Recently, the presence of fungi in these environments has involved a change of view regarding diversity and ecology. Here, we describe fossilized fungal communities in vugs in subseafloor basalts from a depth of 936.65 metres below seafloor at the Detroit Seamount, Pacific Ocean. These fungal communities are closely associated with botryoidal Mn oxides composed of todorokite. Analyses of the Mn oxides by Electron Paramagnetic Resonance spectroscopy (EPR) indicate a biogenic signature. We suggest, based on mineralogical, morphological and EPR data, a biological origin of the botryoidal Mn oxides. Our results show that fungi are involved in Mn cycling at great depths in the seafloor and we introduce EPR as a means to easily identify biogenic Mn oxides in these environments. PMID:26107948

  15. Pb isotopic heterogeneity in basaltic phenocrysts

    SciTech Connect

    Bryce, Julia G.; DePaolo, Donald J.

    2002-06-01

    The Pb isotopic compositions of phenocrystic phases in young basaltic lavas have been investigated using the Getty-DePaolo method (Getty S. J. and DePaolo D. J. [1995] Quaternary geochronology by the U-Th-Pb method. Geochim. Cosmochim. Acta 59, 3267 3272), which allows for the resolution of small isotopic differences. Phenocryst, matrix, and whole rock analyses were made on samples from the 17 Myr-old Imnaha basalts of the Columbia River Group, a zero-age MORB from the Mid-Atlantic Ridge, and a ca. 260 kyr-old tholeiite from Mount Etna. Plagioclase feldspar phenocrysts have low-(U, Th)/Pb, and in each sample the plagioclase has significantly lower 206Pb/207Pb and 208Pb/207Pb values than whole rock, matrix, and magnetite-rich separates. The Pb isotopic contrast between plagioclase and matrix/whole rock is found in three samples with varying grain sizes (0.5 2 cm for the Imnaha basalt and MORB and <1 mm for the Etna sample) from different tectonic settings, suggesting that these results are not unique. The isotopic contrasts are only slightly smaller in magnitude than the variations exhibited by whole rock samples from the region. The Imnaha basalts also have Sr isotopic heterogeneity evident only in plagioclase phenocrysts, but the MORB and Etna lavas do not. The isotopic heterogeneities reflect magma mixing, and indicate that isotopically diverse magmas were mixed together just prior to eruption. The results reinforce indications from melt inclusion studies that magma source region isotopic heterogeneities have large amplitudes at short length scales, and that the isotopic variations imparted to the magmas are not entirely homogenized during segregation and transport processes.

  16. Continental basaltic volcanoes — Processes and problems

    NASA Astrophysics Data System (ADS)

    Valentine, G. A.; Gregg, T. K. P.

    2008-11-01

    Monogenetic basaltic volcanoes are the most common volcanic landforms on the continents. They encompass a range of morphologies from small pyroclastic constructs to larger shields and reflect a wide range of eruptive processes. This paper reviews physical volcanological aspects of continental basaltic eruptions that are driven primarily by magmatic volatiles. Explosive eruption styles include Hawaiian and Strombolian ( sensu stricto) and violent Strombolian end members, and a full spectrum of styles that are transitional between these end members. The end-member explosive styles generate characteristic facies within the resulting pyroclastic constructs (proximal) and beyond in tephra fall deposits (medial to distal). Explosive and effusive behavior can be simultaneous from the same conduit system and is a complex function of composition, ascent rate, degassing, and multiphase processes. Lavas are produced by direct effusion from central vents and fissures or from breakouts (boccas, located along cone slopes or at the base of a cone or rampart) that are controlled by varying combinations of cone structure, feeder dike processes, local effusion rate and topography. Clastogenic lavas are also produced by rapid accumulation of hot material from a pyroclastic column, or by more gradual welding and collapse of a pyroclastic edifice shortly after eruptions. Lava flows interact with — and counteract — cone building through the process of rafting. Eruption processes are closely coupled to shallow magma ascent dynamics, which in turn are variably controlled by pre-existing structures and interaction of the rising magmatic mixture with wall rocks. Locations and length scales of shallow intrusive features can be related to deeper length scales within the magma source zone in the mantle. Coupling between tectonic forces, magma mass flux, and heat flow range from weak (low magma flux basaltic fields) to sufficiently strong that some basaltic fields produce polygenetic

  17. Nanoparticulate mineral matter from basalt dust wastes.

    PubMed

    Dalmora, Adilson C; Ramos, Claudete G; Querol, Xavier; Kautzmann, Rubens M; Oliveira, Marcos L S; Taffarel, Silvio R; Moreno, Teresa; Silva, Luis F O

    2016-02-01

    Ultra-fine and nano-particles derived from basalt dust wastes (BDW) during "stonemeal" soil fertilizer application have been the subject of some concern recently around the world for their possible adverse effects on human health and environmental pollution. Samples of BDW utilized were obtained from companies in the mining district of Nova Prata in southern Brazil for chemical characterization and nano-mineralogy investigation, using an integrated application of advanced characterization techniques such as X-ray diffraction (XRD), High Resolution-Transmission Electron microscopy (HR-TEM)/(Energy Dispersive Spectroscopy) EDS/(selected-area diffraction pattern) SAED, Field Emission-Scanning Electron Microscopy (FE-SEM)/EDS and granulometric distribution analysis. The investigation has revealed that BDW materials are dominated by SiO2, Al2O3 and Fe2O3, with a complex micromineralogy including alkali feldspar, augite, barite, labradorite, hematite, heulandrite, gypsum, kaolinite, quartz, and smectite. In addition we have identified a number of trace metals such as Cd, Cu, Cr, Zn that are preferentially concentrated into the finer, inhalable, dust fraction and could so present a health hazard in the urban areas around the basalt mining zone. The implication of this observation is that use of these nanometric-sized particulates as soil fertilizer may present different health challenges to those of conventional fertilizers, inviting future work regarding the relative toxicities of these materials. Our investigation on the particle size distribution, nano-particle mineralogy and chemical composition in typical BDW samples highlights the need to develop cleaning procedures to minimise exposure to these natural fertilizing basalt dust wastes and is thus of direct relevance to both the industrial sector of basalt mining and to agriculture in the region. PMID:26551199

  18. Metamorphic probing of subduction dynamics and rheology

    NASA Astrophysics Data System (ADS)

    Agard, Philippe

    2015-04-01

    Understanding subduction dynamics and rheology, and particularly the role of fluids and deformation, strongly relies on integrated tectonic, petrological and geochemical studies able to retrieve from our most direct and reliable natural probes (i.e., preserved metamorphic assemblages) their pressure-temperature-time (P-T-t) evolution. I first provide two examples of such integrated studies that allow tracking rock trajectories and exhumation dynamics in subduction zones -- thanks to the considerable progress made over the last ten years on estimating P-T-t conditions. The Oman example shows how EPMA mapping and the detailed study of local, low-temperature equilibria help constrain the behaviour and dynamics of upper crustal units during continental subduction, demonstrating the importance of slicing, accretion at depths of ~30 km and short-lived tectonic expulsion. In the Western Alps, the extensive coverage of field exposures by means of the Raman Spectrometry of carbonaceous matter and by dedicated pseudosection modelling allows to identify the existence of tens of km long, fairly continuous slices of downgoing slab exhumed from similar eclogitic depths (~80 km), and to assess the role of the overall fluid content in enabling their exhumation/preservation. I then illustrate how metamorphic rocks can provide ideal probes (though still partly to be improved) to address key, large-scale tectonic processes and not 'simply' histories, and do stress the importance of adequate field-based data acquisition. Three examples (and present-day limitations) are reviewed here: (1) The regional-scale exhumation of blueschists from the downdip end of the seismogenic zone across thousands of kilometers along the Neotethys (at ~1-1.5 GPa, 350°C) is a major geodynamic event providing insights into changes in interplate mechanical coupling and subduction dynamics. (2) Eclogite breccias recently reported in the Monviso area (W. Alps) allow constraining short-term processes involving

  19. The origin of Cenozoic basalts from central Inner Mongolia, East China: The consequence of recent mantle metasomatism genetically associated with seismically observed paleo-Pacific slab in the mantle transition zone

    NASA Astrophysics Data System (ADS)

    Guo, Pengyuan; Niu, Yaoling; Sun, Pu; Ye, Lei; Liu, Jinju; Zhang, Yu; Feng, Yue-xing; Zhao, Jian-xin

    2016-01-01

    We present new major element, trace element and Sr-Nd-Hf isotope data on Cenozoic basalts from central Inner Mongolia (CIM) in eastern China to study the origin of the incompatible-element enriched component in these basalts by testing whether or not the paleo-Pacific plate lying in the mantle transition zone beneath eastern China is the immediate cause. The Cenozoic CIM basalts have a large variation in major element, trace element and isotope compositions. Fractional crystallization of olivine and clinopyroxene can readily explain much of the major element compositional variation, while trace element and isotope ratio variation largely reflect source heterogeneities and source histories. The variably low 87Sr/86Sr, high εNd, high εHf and elevated ratios of high field strength element over large ion lithophile element (HFSE/LILE, e.g., Nb/U, Nb/La) indicate that the CIM basalts are of asthenospheric origin, which is characterized by mixing between DMM and EM1. However, the CIM basalts are enriched in incompatible elements and enriched in the progressively more incompatible elements (e.g., variably high [La/Sm]N = 1.66-3.38), suggesting that the magma source(s) must have been enriched prior to the major episode of the magmatism. Participation of subducted ocean crust in the mantle source region of these basalts is recognized, but cannot be the major source material because the subducted ocean crust is expectedly too depleted in incompatible elements (e.g., [La/Sm]N ≪ 1) to produce magmas highly enriched in incompatible elements with [La/Sm]N ≫ 1. With the new data, we consider that low mass fraction (low-F) melt metasomatism in the seismic low velocity zone (LVZ) beneath eastern China as the most likely process to generate incompatible-element enriched source(s) for mantle melts parental to the Cenozoic CIM basalts. The low-F metasomatic agent most likely resulted from dehydration melting of the transition-zone paleo-Pacific slab, which has been taking place

  20. Subduction zone tectonic studies to develop concepts for the occurrence of sediment subduction (Phase I). Final technical report

    SciTech Connect

    Hilde, T.W.C.

    1984-08-01

    The objective was to determine the fate of sediments at convergent lithospheric plate boundaries. The study focuses on the structures of the Circum-Pacific trenches and shallow portions of the associated subduction zones. Sediment distribution and the nature of sediment deformation was defined through the various stages of plate convergence to determine if the sediments are subducted or accreted. The controlling factors for sediment subduction and/or accretion were determined. 50 figs. (ACR)

  1. Subduction-related halogens (Cl, Br and I) and H2O in magmatic glasses from Southwest Pacific Backarc Basins

    NASA Astrophysics Data System (ADS)

    Kendrick, Mark A.; Arculus, Richard J.; Danyushevsky, Leonid V.; Kamenetsky, Vadim S.; Woodhead, Jon D.; Honda, Masahiko

    2014-08-01

    Submarine magmatic glasses from the Manus, Woodlark, North Fiji and Lau backarc basins in the Southwest Pacific, and Volcano A on the volcanic front of the Tonga Arc adjacent to the Lau Basin, were investigated to characterise the Cl, Br and I elemental budgets in subduction systems. In particular we seek to determine the extent of variability in the Br/Cl and I/Cl ratios of backarc basin basalts (BABB) and evaluate if these ratios could improve constraints on the source of subducted volatile components in backarc basins worldwide. The selected glasses represent variably evolved melts of boninite, basalt, basaltic-andesite, dacite and rhyolite composition and were selected from spreading centres and seamounts located at varying distances from the associated arcs. In general the strongest subduction signatures (e.g. Ba/Nb of 100-370) occur in the samples closest to the arcs and lower more MORB-like Ba/Nb of <16 are found in the more distal samples. The glasses investigated have extremely variable halogen concentrations (e.g. 3-4200 ppm Cl), with the highest concentrations in enriched glasses with the most evolved compositions. As observed in previous studies, the K/Cl, Br/Cl and I/Cl ratios of glasses from individual settings do not vary as a function of MgO and are considered representative of the magma sources because these ratios are not easily altered by partial melting or fractional crystallisation. Systematic variations in these ratios between basins can therefore be related to mixing of halogens from different sources including: (i) the mantle wedge which has MORB-like Br/Cl and I/Cl; (ii) a subduction-derived slab fluid with estimated salinity of ∼4-10 wt% salts and variable I/Cl; and (iii) brines characterised by salinities of 55±15 wt% salts and Br/Cl slightly higher than seawater, that are sometimes assimilated in crustal magma chambers. The slab fluids enriching the Woodlark Basin, North Fiji Basin and the Fonualei Spreading Centre of the Lau Basin

  2. Imaging segmentation along the Cascadia subduction zone

    NASA Astrophysics Data System (ADS)

    Allen, R. M.; Hawley, W. B.; Martin-Short, R.

    2015-12-01

    As we learn more about the Cascadia subduction zone, there is clear evidence for segmentation in the character of the many physical processes along its 1000 km length. There is segmentation in the arc magmas, in the seismicity, episodic tremor and slip, crustal structure and mantle structure all the way down to ~400 km depth. What is striking is the fact that the segment boundaries for these processes at depths of a few kilometers to hundreds of kilometers align. We must determine if this is coincidence, or if not, what the causative process is. The seismic deployments of the Cascadia Initiative onshore and offshore allow us to image the structure of the subduction zone, including the incoming Juan de Fuca plate, with unprecedented resolution. We use data from three one-year deployments of 70 ocean bottom seismometers across the Juan de Fuca plate, along with hundreds of onshore stations from the Pacific Northwest Seismic Network, the Berkeley Digital Seismic Network, the Earthscope Transportable Array, and smaller temporary seismic deployments. Our 3D tomographic models show significant variation in the structure of the subducting slab along its length. It extends deepest in the south (the Gorda section) where the plate is youngest, and shallows to the north across southern Oregon. There is a gap in the slab beneath northern Oregon, which appears to correlate with the geochemistry of the arc magmas. The slab is then visible again beneath Washington. We also constrain mantle flow paths using shear-wave splitting measurements at the offshore and onshore seismic stations. Beneath the Juan de Fuca plate the flow is sub-parallel to the motion of the plate. However, beneath the Gorda section of the Juan de Fuca place the flow is sub-parallel to the motion of the Pacific plate, not the Juan de Fuca plate. We are thus beginning to image a complex mantle flow pattern that may also play a role in the observed segmentation.

  3. Iron isotopic systematics of oceanic basalts

    NASA Astrophysics Data System (ADS)

    Teng, Fang-Zhen; Dauphas, Nicolas; Huang, Shichun; Marty, Bernard

    2013-04-01

    The iron isotopic compositions of 93 well-characterized basalts from geochemically and geologically diverse mid-ocean ridge segments, oceanic islands and back arc basins were measured. Forty-three MORBs have homogeneous Fe isotopic composition, with δ56Fe ranging from +0.07‰ to +0.14‰ and an average of +0.105 ± 0.006‰ (2SD/√n, n = 43, MSWD = 1.9). Three back arc basin basalts have similar δ56Fe to MORBs. By contrast, OIBs are slightly heterogeneous with δ56Fe ranging from +0.05‰ to +0.14‰ in samples from Koolau and Loihi, Hawaii, and from +0.09‰ to +0.18‰ in samples from the Society Islands and Cook-Austral chain, French Polynesia. Overall, oceanic basalts are isotopically heavier than mantle peridotite and pyroxenite xenoliths, reflecting Fe isotope fractionation during partial melting of the mantle. Iron isotopic variations in OIBs mainly reflect Fe isotope fractionation during fractional crystallization of olivine and pyroxene, enhanced by source heterogeneity in Koolau samples.

  4. How thick are lunar mare basalts

    NASA Technical Reports Server (NTRS)

    Hoerz, F.

    1978-01-01

    It is argued that De Hon's estimates of the thickness of lunar mare basalts, made by analyzing 'ghost' craters on mare surfaces, were inflated as the result of the crater morphometric data of Pike (1977) to reconstruct rim heights of degraded craters. Crater rim heights of 82 randomly selected highland craters of various states of degradation were determined, and median rim height was compared to that of corresponding fresh impact structures. Results indicate that the thickness estimates of De Hon may be reduced by a factor of 2, and that the total volume of mare basalt produced throughout lunar history could be as little as 1-2 million cubic kilometers. A survey of geochemical and petrographic evidence indicates that lateral transport of regolith components over distances of much greater than 10 km is relatively inefficient; it is suggested that vertical mixing of a highland substrate underlying the basaltic fill may have had a primordial role in generating the observed mare width distributions and high concentrations of exotic components in intrabasin regoliths.

  5. Lunar sample studies. [breccias basalts, and anorthosites

    NASA Technical Reports Server (NTRS)

    1977-01-01

    Lunar samples discussed and the nature of their analyses are: (1) an Apollo 15 breccia which is thoroughly analyzed as to the nature of the mature regolith from which it derived and the time and nature of the lithification process, (2) two Apollo 11 and one Apollo 12 basalts analyzed in terms of chemistry, Cross-Iddings-Pirsson-Washington norms, mineralogy, and petrography, (3) eight Apollo 17 mare basalts, also analyzed in terms of chemistry, Cross-Iddings-Pirsson-Washington norms, mineralogy, and petrography. The first seven are shown to be chemically similar although of two main textural groups; the eighth is seen to be distinct in both chemistry and mineralogy, (4) a troctolitic clast from a Fra Mauro breccia, analyzed and contrasted with other high-temperature lunar mineral assemblages. Two basaltic clasts from the same breccia are shown to have affinities with rock 14053, and (5) the uranium-thorium-lead systematics of three Apollo 16 samples are determined; serious terrestrial-lead contamination of the first two samples is attributed to bandsaw cutting in the lunar curatorial facility.

  6. Old Shield Basalts from Savai'i Island, Samoa: Validation of A Plume-Hotspot Model for the Samoan Chain

    NASA Astrophysics Data System (ADS)

    Jackson, M.; Hart, S.; Koppers, A.; Staudigel, H.

    2006-12-01

    The standard model for the generation of an oceanic hotspot track maintains that a deep-seated mantle plume impinges on, and generates melts that escape through, the overriding tectonic plate, thereby generating an age-progressive series of volcanic islands and seamounts. The Samoan hotspot track exhibits several characteristics that are consistent with this model, including an active submarine volcano—Vailulu'u—that anchors the eastern extremity of the chain. The older, submarine portions of the 1700 km chain display a reasonable fit to Pacific plate velocity. However, Savai'i, the westernmost island, has been almost completely re-surfaced with recent, rejuvenated volcanism and there are no subaerial shield basalts with ages (~5 my) predicted by the plume-hotspot model. This lack of a clear age-progression has been taken as evidence that the Samoan hotspot chain is not plume-driven, but is simply a tectono-magmatic manifestation of the nearby Tonga subduction zone. We report geochemical analyses of old basalts recovered from the deep flanks of Savai'i during the 2005 Samoan ALIA cruise. In Samoa, Pb-isotopes provide a good discriminant between shield and rejuvenated basalts, as basalts mapped as rejuvenated in the field have an isotopic signature that is distinct from basalts mapped as shield. Basalts from four separate dredges on the submarine flanks Savai'i exhibit Pb-isotope pedigrees previously observed only in the young, shield-building province of the eastern Samoan islands and seamounts: Dredge 114 has two samples similar in chemistry to Ta'u island, dredge 128 samples are similar to lavas from Tutuila Island, dredge 115 lavas plot either within or very near the Samoan shield basalt field, and dredge 118 reveals surprising heterogeneity, with samples that lie well outside the field for published Samoan basalt data. A fifth dredge (116) on the west rift zone of Savai'i yielded a basalt with clear post-erosional affinities. Dredges 115, 118 and 128 also

  7. Phenocryst He-Ar isotopic and whole-rock geochemical constraints on the origin of crustal components in the mantle source of Cenozoic continental basalt in eastern China

    NASA Astrophysics Data System (ADS)

    Xu, Zheng; Zheng, Yong-Fei; He, Huai-Yu; Zhao, Zi-Fu

    2014-02-01

    -mantle interaction in oceanic subduction channel. Therefore, the noble gas isotopic compositions of mantle sources for intraplate basalts are primarily determined by the budget between the mantle and crustal components during the melt-peridotite in oceanic subduction factory.

  8. Depths and Temperatures of Mantle Melt Extraction in the Southern Cascadia Subduction Zone (Invited)

    NASA Astrophysics Data System (ADS)

    Till, C.; Grove, T. L.; Donnelly-Nolan, J. M.; Carlson, R. W.

    2013-12-01

    Plagioclase and spinel lherzolite thermometry and barometry applied to an extensive suite of <10.5 Ma primitive basaltic lavas (most Mg#>0.70) containing variable H2O contents (<<1 to ~4 wt%) suggests these melts were extracted from the mantle at 40-58 km below Oregon's High Lava Plains, 41-51 km below California's Modoc Plateau, and 37-60 km below the central-southern Cascades volcanic arc. Of the 155 basalt samples investigated, 33 are calc-alkaline basalts (CAB) and the remainder are high alumina olivine tholeiites (HAOT) or mildly alkaline basalts (MAB). All 33 of the CAB are from the subduction-influenced volcanic centers of Lassen, Mt. Shasta, Three Sisters, Medicine Lake, and Newberry in the present-day Cascades arc or rear arc. All of these volcanic centers also erupted HAOT or MAB. Olivine-plagioclase hygrometry for a representative subset of the 20 CAB from Newberry indicates they contained ~4 wt% H2O prior to eruption. Water contents for the remaining CAB were approximated using the H2O-melt composition scaling relationship developed by Ruscitto et al. [2010, EPSL 298(1-2), 153-161] yielding ≤1-3 wt% H2O. The calculated pressures and temperatures of last equilibration with mantle lherzolite for all 33 CAB were adjusted for the effects of H2O following Till et al. [2012, JGR 117(B06206)] and are on average 50×15°C (1s) cooler and 1.65×0.27 km deeper than their calculated temperatures and depths for anhydrous conditions. The minimum depths of melt extraction calculated for all basalts considered (including the CAB) are close to the Moho, as determined by regional geophysical studies. Thus, our results suggest that the geophysical Moho and lithosphere-asthenosphere boundary are located in close proximity to one another (within 5-10 km). The basalts originated at 1185-1383°C and the presence of both wet and dry basalts that were generated at such different temperatures at similar times, depths, and geographic locations in the Cascades arc and rear arc

  9. Degassing of carbon dioxide from basaltic magma at spreading centers: I. Afar transitional basalts

    NASA Astrophysics Data System (ADS)

    Gerlach, Terrence M.

    1989-11-01

    This study investigates the hypothesis that a significant fraction of the CO 2 in basalt supplied to axial volcanic ranges of spreading centers in the Afar depression, escapes by degassing during residence in crustal magma reservoirs. The investigation employs volcanic gas data to test the degassing hypothesis. Volcanic gases emitted from source vents at Erta'Ale lava lake are used to represent volatiles present in the basalt supplied to magma reservoirs underlying Afar spreading centers. For comparison, volcanic gases from a large fissure eruption at Ardoukoba are used to represent volatiles in basalt after a period of storage in the Afar magma reservoirs. The results confirm the hypothesis. Gases from the lava lake and fissure eruption are the same except for CO 2. They lie along a common CO 2 control line. The fissure eruption gases are six-fold depleted in CO 2 compared to gases from the continuously supplied lava lake. This difference corresponds to a loss of approximately 85% of the initial CO 2. Moreover, gases from the fissure eruption are nearly identical to those emitted by Kilauea basalts that have lost CO 2 by magma reservoir degassing. Mass balance modeling indicates an initial CO 2 content for Afar basalt of 0.12 wt.% compared to CO 2 concentrations as low as 0.02 wt.% after degassing at depth.

  10. Improving Seismic Constraints on Subduction Zone Geometry

    NASA Astrophysics Data System (ADS)

    Syracuse, E. M.; Abers, G. A.; Fischer, K. M.; van Keken, P. E.; Kneller, E. A.; Rychert, C. A.

    2007-12-01

    Accurate slab geometries are necessary for 3D flow modeling, and for understanding the variations in temperature and melting geometry between different subduction zones. Recent studies have shown that the depth to slab beneath arc volcanoes varies by as much as a factor of two between subduction zones, but these results are based on teleseismic earthquake catalogs with potentially large errors. When available, local seismic arrays provide better constraints. The TUCAN array (Tomography Under Costa Rica and Nicaragua) deployed 48 three component broadband PASSCAL instruments for 18 months with station spacing of 10-50 km across the Central America arc. This dataset provides some of the best control anywhere for ground-truth comparison of teleseismic catalogs in steeply dipping subduction zones. Joint inversion of TUCAN arrival times for velocity and hypocenters illuminate a 10-15 km thick Wadati-Benioff zone (WBZ), with absolute hypocenter uncertainties of 1-5 km. Besides providing accurate hypocenters, the tomographic images provide independent constraints on melting and temperature, through the imaging of low Vp (7.5-7.8 km/s) and highly attenuating (40subducting slab is assumed to lie at the top of the WBZ seismicity, for example if double seismic zones are expected. Because of the large scatter, the TWBZ is biased too shallow compared to the TUCAN data, vertically by as much as 50 km for the steeply-dipping Nicaragua slab. Relative

  11. Imaging of the subducted Kyushu-Palau Ridge in the Hyuga-nada region, western Nankai Trough subduction zone

    NASA Astrophysics Data System (ADS)

    Yamamoto, Yojiro; Obana, Koichiro; Takahashi, Tsutomu; Nakanishi, Ayako; Kodaira, Shuichi; Kaneda, Yoshiyuki

    2013-03-01

    We performed 3D seismic tomography of the Hyuga-nada region, western Nankai subduction zone, to investigate the relationship of the subducted part of Kyushu-Palau Ridge (KPR) to coseismic rupture propagation, seismicity, and shallow very low frequency earthquakes. Combining active-source and passive-source data recorded both onshore and offshore, we imaged the deep slab from near the trough axis to the coastal area. Our results show the subducted KPR as a low-velocity belt oriented NW-SE extending down the plate boundary to around 30 km depth. At this depth, we suggest that the subducted KPR detaches from the slab and becomes underplated on the overriding continental plate. As the coseismic slip areas of past large earthquakes do not extend into the subducted KPR, we suggest that it may inhibit rupture propagation. The interior of the subducted KPR shows active intraslab seismicity with a wide depth distribution. Shallow very low frequency earthquakes are continuously active above the location of the subducted KPR, whereas they are intermittent to the northeast of the subducted KPR. Thus, the subducted KPR appears to be an important factor in coseismic rupture propagation and seismic phenomena in this region.

  12. Isotopic variations within upper oceanic crust at IODP Site 1256: Implications for crustal recycling and the formation of ocean island basalts

    NASA Astrophysics Data System (ADS)

    Duggen, S.; Hoernle, K.; Geldmacher, J.; Hauff, F.

    2007-12-01

    The origin of ocean island basalts (OIBs) is a fundamental question facing Earth scientists. It is commonly agreed that lithospheric material recycled in the mantle is involved in the magma source of OIBs. The relative importance of 1) subducted altered oceanic basaltic crust (AOC), 2) subducted marine sediments and/or 3) delaminated metasomatised subcontinental lithosphere and continental lower crust remains to be resolved. We examine the geochemical composition of a complete in situ section of oceanic crust drilled at Site 1256 during IODP Expeditions 309 and 312. It includes the extrusive layer, sheeted dikes and gabbros of ca. 15 Ma old oceanic crust of the Cocos Plate formed during a period of superfast spreading at the East Pacific Rise. Modeling in the Sr-Nd-Pb-isotope space and comparison with present day radiogenic isotope ratios of OIBs provides constraints on the significance of recycled oceanic crust in the OIB mantle source(s). Our study shows that the generation of sulphides during low- and high-temperature alteration of oceanic crust has a strong influence on U/Pb and Th/Pb ratios and whether an AOC domain evolves relatively low or high Pb-isotope ratios over geological timescales. The model suggests that AOC as the sole precursor material, modified during the subduction process, and after relatively low to moderate recycling ages of ca. 300-800 Ma, is sufficient to explain the Sr-Nd-Pb-isotopic composition of OIBs with Pb-isotopic compositions along or below the Northern Hemisphere Reference Line (NHRL) and relatively high Nd-isotope ratios (e.g. Canaries, Galapagos, Iceland, Madeira). This indicates that additional EM-components, potentially associated with recycled lithospheric material such as subducted sediments, lower continental crust or subcontinental lithosphere, are not required for an array of OIBs, but are only necessary to explain OIBs with Pb-isotope ratios above the NHRL and relatively low Nd- isotope ratios (e.g. Pitcairn, Tristan

  13. Anatomy of rhyolite production at the Tonga-Taupo subduction system: Okataina volcano, New Zealand

    NASA Astrophysics Data System (ADS)

    Shane, P. A.; Smith, V.; Storm, S.; Schmitt, A. K.

    2013-12-01

    Okataina is an exceptionally active rhyolite volcano located within the continental part of the Tonga-Taupo subduction system, in New Zealand. The present tectonic regime is one of active extension and exceptionally high heatflow, resulting in a thin continental crust (about 16 km) and vigorous rhyolite magma production. The volcano experienced caldera collapse (100 km3 magma) at 46 ka, followed by 25 rhyolitic eruptions, each involving magma volumes of 1-20 km3. The most recent rhyolite eruption occurred in AD1314, followed by a basaltic plinian eruption in AD1886. The magmatic system is still present. Published geophysical data indicate a region of low resistivity associated with slower S-wave velocities 8-15 km beneath the volcano, interpreted as interconnected melt. This is consistent petrological evidence that points to magma storage prior to eruption at about 200 MPa (= 8 km depth), including: (1) rhyolite melt projection on the Qtz-Ab-Or ternary phase diagram; (2) H2O/CO2 contents in quartz-hosted melt inclusions; and (3) the presence of cummingtonite. The brittle-ductile boundary is seismically located at about 7 km and may influence the depth of magma ponding by arresting dykes and promoting sill formation. Zircon U-series data provide evidence for a long-lived crystal-rich root zone beneath Okataina. Most of the crystals are antecrysts that variously grew continuous or intermittently over periods up to 100 ka, unaffected by magma-extraction events including the last caldera-forming episode. Intra-caldera rhyolite eruptions have occurred along linear vent zones (up to 20 km long), influenced by local tectonic fabric. Each eruption comprised several magma batches, distinguished by fast equilibrating melts and Fe-Ti oxides, erupted simultaneously along the vent zones. This suggests that the magmas occur as short-lived semi-isolated lenses at the top of the crystal mush. There is amble evidence for basaltic magma involvement in the rhyolite eruptions

  14. Does eddy subduction matter in the northeast Atlantic Ocean?

    NASA Astrophysics Data System (ADS)

    Gebbie, Geoffrey

    2007-06-01

    Mesoscale eddies are an important contributor to subduction in the Gulf Stream region and the Antarctic Circumpolar Current, but is eddy subduction also important in the relatively quiescent interior of the world's subtropical gyres? Observations from the Subduction Experiment of the northeast Atlantic do not have the spatial resolution necessary to calculate eddy subduction and answer this question. Regional numerical models can diagnose subduction, but their representativeness is unknown. Furthermore, water mass budgets in an open-ocean domain show that the simulated properties of subducted water directly depend upon uncertain open-boundary conditions and surface fluxes. To remedy these problems, a state estimate of the ocean circulation is formed by constraining an eddy-permitting general circulation model to observations by adjusting the model parameters within their uncertainty. The resulting estimate is self-consistent with the equations of motion and has the necessary resolution for diagnosing subduction. In the northeast Atlantic during 1991-1993, the time-variable circulation contributes less than 1 Sv of net subduction, while the total subduction is 4 Sv. Eddy volume fluxes of 40 m/yr in the North Equatorial Current and the Azores Current, however, are significant and rival the subduction by Ekman pumping locally. Furthermore, a state estimate at 1/6° resolution has 2-3 Sv more subduction in the density bands centered around σ = 24.0 kg/m3 and σ = 26.0 kg/m3 than a 2° state estimate. This result implies that the inability to accurately simulate mesoscale phenomena and surface fluxes in climate models would lead to an accumulation of errors in water mass properties over 10-20 years, even in the interior of the subtropical gyre.

  15. Deep view of the Subduction-Transform Edge Propagator (STEP) fault in the Calabrian Subduction Zone

    NASA Astrophysics Data System (ADS)

    Emanuele Maesano, Francesco; Tiberti, Mara Monica; Basili, Roberto

    2016-04-01

    The Calabrian Subduction Zone plays a key role in the evolution of the central Mediterranean in the framework of the convergence between Africa and Europe. Here, the remnants of the World's oldest oceanic crust form a narrow NW-dipping slab passively subducting beneath the Calabrian Arc. Recently published high-resolution seismic profiles and bathymetric data of the western Ionian Sea highlight the presence of a NNW-SSE faulting system connected with a series of Plio-Pleistocene syn-tectonic basins. These features are correlated with the recent activity of a major NNW-SSE deformation zone confining the active subduction to the SW and interpreted as a Subduction-Transform Edge Propagator (STEP) fault. The goal of this work is to jointly reconstruct the geometry of the STEP fault and the subduction interface in its surroundings. We use multichannel seismic profiles acquired in the southwestern part of the Calabrian accretionary wedge to focus on the STEP fault geometry at depth and to analyse its relationships with shallow deformation features. The quantitative analysis and enhancement of seismic data provided an accurate image of the internal structure of the accretionary wedge at various depths, showing growth strata in the Plio-Pleistocene succession and major discontinuities in the lower crust. Our results depict a main subvertical, slightly east-dipping, lithospheric fault cutting the oceanic crust down to the Moho, and a rich set of associated secondary synthetic and antithetic faults. This picture also provides new insights on the STEP fault propagation mechanism. In addition, the tridimensional correlation of the STEP fault occurrences in various seismic profiles provides a preliminary scheme of its segmentation and highlights the relationships of this master fault with other main structural elements of the Calabrian Arc and Eastern Sicily, including some of the faults deemed to be responsible for major historical earthquakes in the area.

  16. Making rhyolite in a basalt crucible

    NASA Astrophysics Data System (ADS)

    Eichelberger, John

    2016-04-01

    Iceland has long attracted the attention of those concerned with the origin of rhyolitic magmas and indeed of granitic continental crust, because it presents no alternative for such magmas other than deriving them from a basaltic source. Hydrothermally altered basalt has been identified as the progenitor. The fact that rhyolite erupts as pure liquid requires a process of melt-crustal separation that is highly efficient despite the high viscosity of rhyolite melt. Volcanoes in Iceland are foci of basaltic magma injection along the divergent plate boundary. Repeated injection produces remelting, digestion, and sometimes expulsion or lateral withdrawal of material resulting in a caldera, a "crucible" holding down-dropped and interlayered lava flows, tephras, and injected sills. Once melting of this charge begins, a great deal of heat is absorbed in the phase change. Just 1% change in crystallinity per degree gives a melt-present body an effective heat capacity >5 times the subsolidus case. Temperature is thus buffered at the solidus and melt composition at rhyolite. Basalt inputs are episodic ("fires") so likely the resulting generation of rhyolite by melting is too. If frequent enough to offset cooling between events, rhyolite melt extractions will accumulate as a rhyolite magma reservoir rather than as discrete crystallized sills. Evidently, such magma bodies can survive multiple firings without themselves erupting, as the 1875 eruption of Askja Caldera of 0.3 km3 of rhyolite equilibrated at 2-km depth without previous leakage over a ten-millennium period and the surprise discovery of rhyolite magma at 2-km depth in Krafla suggest. Water is required for melting; otherwise melting cannot begin at a temperature lower than that of the heat source. Because the solubility of water in melt is pressure-dependent and almost zero at surface pressure, there must be a minimum depth at which basalt-induced melting can occur and a rhyolite reservoir sustained. In practice, the

  17. Channelized Fluid Flow and Metasomatism in Subducted Oceanic Lithosphere recorded in an Eclogite-facies Shear Zone (Monviso Ophiolite, Italy)

    NASA Astrophysics Data System (ADS)

    Angiboust, S.; Agard, P.; Pettke, T.

    2012-04-01

    The Monviso ophiolite Lago Superiore Unit (LSU) constitutes a well-preserved, almost continuous fragment of upper oceanic lithosphere subducted down to ca. 80 km (between 50 and 40 Ma) and later exhumed along the subduction interface. The LSU is made of (i) a variably thick (50-500 m) section of eclogitized mafic crust (associated with minor calcschist lenses) overlying a 100-400 m thick metagabbroic body, and of (ii) a serpentinite sole (ca. 1000 m thick). This section is cut by two 10 to 100m thick eclogite-facies shear zones, found at the boundary between basalts and gabbros (Intermediate Shear Zone: ISZ), and between gabbros and serpentinites (Lower Shear Zone: LSZ). Fragments of mylonitic basaltic eclogites and marbles were dragged and dismembered within serpentinite schists along the LSZ during eclogite-facies deformation [Angiboust et al., Lithos, 2011]. Metasomatic rinds formed on these fragments at the contact with the surrounding antigorite schists during lawsonite-eclogite facies metamorphism, testifying to prominent fluid-rock interaction along with deformation. We present new petrological and geochemical data on four types of metasomatically altered eclogites (talc-, chlorite-, lawsonite- and phengite-bearing eclogites) and on a (serpentinite-derived) talc schist from the block rind. Bulk-rock compositions, in situ LA-ICP-MS analysis and X-ray Cr/Mg maps of garnet demonstrate that (i) these samples underwent significant B, Cr, Mg, Ni and Co enrichment and Fe, V and As depletion during eclogite-facies metamorphism (while Li and Pb behaved inconsistently) and (ii) garnet composition and chemistry of inclusions show extreme variation from core to rim. These compositional patterns point to a massive, pulse-like, fluid-mediated element transfer along with deformation, originating from the surrounding serpentinite (locally, with contributions from metasediments-equilibrated fluids). Antigorite breakdown, occurring ca. 10 km deeper than the maximum depth

  18. Acceleration spectra for subduction zone earthquakes

    USGS Publications Warehouse

    Boatwright, J.; Choy, G.L.

    1989-01-01

    We estimate the source spectra of shallow earthquakes from digital recordings of teleseismic P wave groups, that is, P+pP+sP, by making frequency dependent corrections for the attenuation and for the interference of the free surface. The correction for the interference of the free surface assumes that the earthquake radiates energy from a range of depths. We apply this spectral analysis to a set of 12 subduction zone earthquakes which range in size from Ms = 6.2 to 8.1, obtaining corrected P wave acceleration spectra on the frequency band from 0.01 to 2.0 Hz. Seismic moment estimates from surface waves and normal modes are used to extend these P wave spectra to the frequency band from 0.001 to 0.01 Hz. The acceleration spectra of large subduction zone earthquakes, that is, earthquakes whose seismic moments are greater than 1027 dyn cm, exhibit intermediate slopes where u(w)???w5/4 for frequencies from 0.005 to 0.05 Hz. For these earthquakes, spectral shape appears to be a discontinuous function of seismic moment. Using reasonable assumptions for the phase characteristics, we transform the spectral shape observed for large earthquakes into the time domain to fit Ekstrom's (1987) moment rate functions for the Ms=8.1 Michoacan earthquake of September 19, 1985, and the Ms=7.6 Michoacan aftershock of September 21, 1985. -from Authors

  19. Driving forces: Slab subduction and mantle convection

    NASA Technical Reports Server (NTRS)

    Hager, Bradford H.

    1988-01-01

    Mantle convection is the mechanism ultimately responsible for most geological activity at Earth's surface. To zeroth order, the lithosphere is the cold outer thermal boundary layer of the convecting mantle. Subduction of cold dense lithosphere provides tha major source of negative buoyancy driving mantle convection and, hence, surface tectonics. There are, however, importnat differences between plate tectonics and the more familiar convecting systems observed in the laboratory. Most important, the temperature dependence of the effective viscosity of mantle rocks makes the thermal boundary layer mechanically strong, leading to nearly rigid plates. This strength stabilizes the cold boundary layer against small amplitude perturbations and allows it to store substantial gravitational potential energy. Paradoxically, through going faults at subduction zones make the lithosphere there locally weak, allowing rapid convergence, unlike what is observed in laboratory experiments using fluids with temperature dependent viscosities. This bimodal strength distribution of the lithosphere distinguishes plate tectonics from simple convection experiments. In addition, Earth has a buoyant, relatively weak layer (the crust) occupying the upper part of the thermal boundary layer. Phase changes lead to extra sources of heat and bouyancy. These phenomena lead to observed richness of behavior of the plate tectonic style of mantle convection.

  20. Viscous Dissipation and Criticality of Subducting Slabs

    NASA Astrophysics Data System (ADS)

    Riedel, Mike; Karato, Shun; Yuen, Dave

    2016-04-01

    Rheology of subducting lithosphere appears to be complicated. In the shallow part, deformation is largely accomodated by brittle failure, whereas at greater depth, at higher confining pressures, ductile creep is expected to control slab strength. The amount of viscous dissipation ΔQ during subduction at greater depth, as constrained by experimental rock mechanics, can be estimated on the basis of a simple bending moment equation [1,2] 2ɛ˙0(z) ∫ +h/2 2 M (z) = h ṡ ‑h/2 4μ(y,z)y dy , (1) for a complex multi-phase rheology in the mantle transition zone, including the effects of a metastable phase transition as well as the pressure, temperature, grain-size and stress dependency of the relevant creep mechanisms; μ is here the effective viscosity and ɛ˙0(z) is a (reference) strain rate. Numerical analysis shows that the maximum bending moment, Mcrit, that can be sustained by a slab is of the order of 1019 Nm per m according to Mcrit˜=σp ∗h2/4, where σp is the Peierl's stress limit of slab materials and h is the slab thickness. Near Mcrit, the amount of viscous dissipation grows strongly as a consequence of a lattice instability of mantle minerals (dislocation glide in olivine), suggesting that thermo-mechanical instabilities become prone to occur at places where a critical shear-heating rate is exceeded, see figure. This implies that the lithosphere behaves in such cases like a perfectly plastic solid [3]. Recently available detailed data related to deep seismicity [4,5] seems to provide support to our conclusion. It shows, e.g., that thermal shear instabilities, and not transformational faulting, is likely the dominating mechanism for deep-focus earthquakes at the bottom of the transition zone, in accordance with this suggested "deep criticality" model. These new findings are therefore briefly outlined and possible implications are discussed. References [1] Riedel, M. R., Karato, S., Yuen, D. A. Criticality of Subducting Slabs. University of Minnesota

  1. GPS Monitoring of Subduction Zone Deformation in Costa Rica

    NASA Technical Reports Server (NTRS)

    Lundgren, Paul

    1997-01-01

    The subduction of the Cocos plate beneath Costa Rica is among the highest convergence rates in the world. The high subduction rate and nearness of the Nicoya Peninsula, Costa Rica to the Middle America Trench (MAT) provide a unique opportunity to map variations in interseismic strain of the crust above the seismogenic zone in response to variations in seismic coupling.

  2. Nature of the plate contact and subduction zones diversity

    NASA Astrophysics Data System (ADS)

    De Franco, Roberta; Govers, Rob; Wortel, Rinus

    2008-07-01

    In recent studies we showed that the nature of the plate contact in subduction zones is an important physical feature in both oceanic lithospheric subduction and continental collision. We investigated two fundamental states of the plate contact: one based on a fault and the other based on a subduction channel. Using geodynamic modeling, we determined the specific signatures of both states of the subduction contact. We established that the nature of the plate contact influences the dynamic response of the overriding and subducting plate, and is one of the controlling factors whether back-arc extension develops or not. In the present study, we combine results of our previous numerical experiments with a re-analysis of published observations. Overall, our synthesis connects seismic moment release with back-arc deformation and tectonic processes at the margin. It leads us to identify four classes of subduction zones. The first two classes result directly from our numerical experiments. In class 1, subduction zones are characterized by a plate contact that is largely fault-like with an accretionary margin. In class 2, the plate contacts are largely channel-type and have an erosive margin. Class 3, where the plate contact is entirely channel-like, consists of accretionary margins with a high sediment supply. Subduction zones of class 4, mostly characterized by an erosive convergent margin (northern Chile, Peru, Honshu and Kuril), are more complicated. They can be explained by incorporating regional observations.

  3. Seamount, ridge, and transform subduction in southern Central America

    NASA Astrophysics Data System (ADS)

    Morell, Kristin D.

    2016-02-01

    Understanding the factors that control subduction zone processes is a first-order goal in the study of convergent margins. In southern Central America, a growing body of research reveals strong links between the character of the subducting slab and the mechanics of important processes that include subduction erosion, fluid flow, deformation, and seismogenesis. In this paper, I evaluate the role that seamount, ridge, and transform subduction have in the development of upper plate deformation and volcanism by summarizing previous work across a >500 km long region of Central America where each of these three scenarios are present along strike. The data show that the subduction of short-wavelength bathymetry (e.g., seamounts and faults on the seafloor) produces short-wavelength deformation that persists for relatively short timescales (104-105 years), whereas the subduction of longer-wavelength bathymetry (e.g., the aseismic Cocos Ridge) results in longer-wavelength deformation that endures over a longer time scale (106 years). The timing and distribution of upper plate deformation are consistent with subhorizontal Cocos Ridge subduction driving upper plate deformation, and the increased crustal thickness (>20 km) of the subducting Cocos Ridge is likely one of the most important factors in the production of upper plate contraction and crustal thickening. The data illustrate a fundamental connection between lower plate properties and upper plate deformation and highlight the profound influence that bathymetry and crustal thickness have in the localization and kinematics of upper plate strain and volcanism in Middle America.

  4. Geochemical Variation of Alkaline Basalts from Petit-spot Volcanoes in the Northwestern Pacific

    NASA Astrophysics Data System (ADS)

    Machida, S.; Hirano, N.; Kato, Y.; Abe, N.

    2009-12-01

    Petit-spot, new type young volcanoes discovered on the old Pacific plate, originate from melting in the uppermost mantle asthenosphere, and that the magma exudes where the plate flexes and fractures before subducting (Hirano et al., 2006, Science). Our recent studies defined that alkaline basalt lava from petit-spot have (1) high concentrations of incompatible trace elements indicating extreme enrichment in highly incompatible elements, (e.g., Rb, Ba, U, Th, and Nb) and light rare earth elements (REE), and depletion in heavy REE (Hirano et al., 2006), and (2) Dupal, or extreme EM-1-like, Sr-Nd-Pb isotopic compositions (Machida et al., 2009, GCA). Machida et al. (2009) proposed that melting of small-scale recycled plate material produces petit-spot magma. Thus, detailed geochemical investigation of the upper mantle using these petit-spot volcanoes will provide key constraints on understanding the nature of heterogeneity of the northwestern Pacific upper mantle. Petit-spot volcanoes distribute at the site in the Japan Trench (Site A) and the site approximately 600 km ESE of the Japan Trench (Site B) (Hirano et al., 2006). Furthermore, another petit-spot lava field was discovered on the Cretaceous Pacific plate, but approximately 200 km S of the Site A (Site C; Abe et al., 2007, AGU abstract). Petit-spot basalts from these sites are geochemically classified into two distinct groups. Basalts from the Site A and B are enriched in large ion lithophile elements (LILEs), such as Ba, and light rare earth elements (LREEs). Then, they have large negative U, Th, Nb, and Ta anomalies relative to primitive mantle. In contrast, basalts from the Site C do not have negative anomalies. Isotopic compositions (Machida et al., 2009), especially in 143Nd/144Nd, correlate to such compositional difference. These observations suggest that petit-spot magma at the Site C is derived by melting of different source material from that of other sites. Although we consider that LILEs

  5. Primitive Submarine Basalts and Magmatic Variation of Pagan and Daon, Mariana Arc

    NASA Astrophysics Data System (ADS)

    Tamura, Y.; Ishizuka, O.; Stern, R. J.; Nunokawa, A.; Shukuno, H.; Kawabata, H.; Embley, R. W.; Bloomer, S. H.; Nichols, A. R.; Tatsumi, Y.

    2011-12-01

    Pagan is an active volcano located in the central island province of the Mariana arc (18°07'N) and is one of the largest volcanoes in the Mariana arc; its main edifice rises from a base ~3,000 m below sea level (b.s.l.) and has a volume of 2,160 km3 (Bloomer et al., 1989). Daon is a small reararc seamount 25 km SW of Pagan (17°58'N). We visited the submarine portions of the two volcanoes in 2010 (NT10-12), using ROV Hyper-Dolphin and RV Natsushima. Rocks were collected from the northeastern and southwestern flanks of the Pagan volcano at 1,500-2,000 m b.s.l. (dive HPD1147) and at 2,020-2,330 m b.s.l. (HPD1148), respectively, and from the southern flank of Daon at 2,360-2,580 m b.s.l. (HPD1149). Fresh pillow lavas dominate in all three dives, but the rocks recovered from HPD1147 seem to be the youngest based on very light sediment cover and no Mn coating. Sediment cover is considerably more extensive at HPD1148, and all rocks from Daon (HPD1149) had 0-10 mm thick Mn coating. Submarine Pagan lavas show major element compositions typical of subaerial Pagan basalts (Marske et al., 2011; Elliott et al., 1997; Woodhead, 1989), although the least fractionated compositions recovered from HPD1147 extend to much higher MgO (7-11 wt %) and Mg# (60-70), than the subaerial lavas. We recognize two types of primitive basalts from Pagan and Daon. Daon has plagioclase-olivine basalt (POB) and clinopyroxene-olivine basalt (COB), petrographic types that are similar to those reported from NW Rota-1 volcano (Tamura et al., 2011). Pagan has two types of COB, both having 10-11 wt % MgO; COB-1 has higher Ba/Zr and Sr/Zr and lower Zr/Y than COB-2 at the same MgO content, indicating that COB-1 has a greater subduction component and formed from higher degrees of mantle melting than COB-2. Similar distinct primitive magmas like those recognized from NW Rota-1 also coexist at Pagan and Daon.

  6. How the Ferric Iron Proportion in Basalts Changes Towards the Iceland Plume

    NASA Astrophysics Data System (ADS)

    Shorttle, O.; Moussallam, Y.; Hartley, M. E.; Edmonds, M.; Maclennan, J.; Murton, B. J.

    2014-12-01

    Planetary differentiation has been driven by the Earth's giant convective system, which has been redistributing heat, volatile elements and myriad other chemical species for 4.5 billion years. A key exchange in this transport process is between the mantle and the atmosphere through the volcanic degassing of sulfur, carbon and hydrogen from silicate melts forming in the deep Earth. The speciation and mobility of volatile elements during silicate melting is modulated by the oceanic mantle's oxygen fugacity (fO2), which away from subduction zones has long been considered uniform. However, a recent study has challenged this paradigm with new measurements of ferric iron proportions (Fe3+/Fe) in glasses from mid-ocean ridge basalts (Cottrell & Kelley, 2013). These new results suggest mantle domains containing material recycled from the Earth's surface are more reducing than ambient mantle and contain high concentrations of carbon. The pervasive mantle heterogeneity well documented in other geochemical indices may therefore be systematically associated with changes in oxidation state In this study we have produced a dataset of combined XANES, volatile element (C, S, F, Cl, H, B) and boron isotope analyses of 65 basalts from the Mid-Atlantic Ridge south of Iceland. These samples form a transect from 1000 km south of the Iceland plume to within 300 km of the plume centre, crossing into the zone experiencing the greatest geophysical and geochemical influence from the plume. Accordingly there are major changes in the isotopic and trace element composition of the basalts in this sample set, driven by both an increase in the proportion of recycled oceanic crustal components towards Iceland and a shift to a plume driven flow field. This suite of basalts therefore form an excellent test of the global correlations observed by Cottrell & Kelley (2013), where ferric iron contents anti-correlated with isotopic enrichment, with a high resolution regional dataset. By combining major

  7. Constraints on the Timing and Geometry of Kula-Farallon Ridge Subduction and Implications for the Eocene Magmatic and Tectonic History of the Pacific Northwest

    NASA Astrophysics Data System (ADS)

    Tepper, J. H.; Clark, K. P.; Wolfe, M. R.

    2005-12-01

    Dates on basalts in the WA-OR Coast Ranges and on scattered centers of adakite and near-trench magmatism in WA and Vancouver Island constrain the timing and geometry of subduction of the Kula-Farallon Ridge (KFR) beneath western North America between ~60 and 35 Ma. These data indicate that: (1) Coast Range basalts from widely separated areas show extensive age overlap with no apparent geographic trend over time, (2) 43 Ma adakites in WA are coeval with the Tillamook volcanics in Oregon, and (3) between 48 and 35 Ma the focus of adakite / near trench magmatism migrated northward from the southern Olympic Peninsula to central Vancouver Island. Points (1) and (2) require left-stepping offsets of the KFR, which would have led to multiple ridge-trench intersections and the development of a series of fraternal slab windows (Thorkelson, 1996). The exact geometry of this fraternal window system (FWS) cannot be reconstructed as there are no constraints on the geometry of subducted ridge offsets, but it would have been larger in area than a window resulting from a single ridge intersection (e.g., Breitsbacher et al., 2004). Preliminary calculations of the maximum extent of the KFR FWS indicate it could have extended from SE Oregon to southern BC and as far inland as Montana. Development of this FWS provides an explanation for widespread Eocene volcanism of the Challis event. The arc geochemical signature of many Challis rocks may reflect: (1) the presence of the subducting Farallon plate under some portions of the region, and/or (2) contamination of the lithosphere during earlier subduction episodes (Hooper et al., 1995). Subsequent establishment of the modern Cascade arc at ~39 Ma corresponds in space and time with the reappearance of a subducting slab beneath OR/WA as the KFR migrated northward. This model for the transition from Challis to Cascade magmatism eliminates the need for a steepening or foundering of the subducting Farallon slab, accounts for the width and

  8. The basaltic volcanism of the Dumisseau Formation in the Sierra de Bahoruco, SW Dominican Republic: A record of the mantle plume-related magmatism of the Caribbean Large Igneous Province

    NASA Astrophysics Data System (ADS)

    Escuder-Viruete, Javier; Joubert, Marc; Abad, Manuel; Pérez-Valera, Fernando; Gabites, Janet

    2016-06-01

    The basaltic volcanism of the Dumisseau Fm in the Sierra de Bahoruco, SW Dominican Republic, offers the opportunity to study, on land, the volcanism of the Caribbean Large Igneous Province (CLIP). It consists of an at least 1.5 km-thick sequence of submarine basaltic flows and pyroclastic deposits, intruded by doleritic dykes and sills. Three geochemical groups have been identified: low-Ti tholeiites (group I); high-Ti transitional basalts (group II); and high-Ti and LREE-enriched alkaline basalts (group III). These geochemical signatures indicate a plume source for all groups of basalts, which are compositionally similar to the volcanic rocks that make up various CLIP fragments in the northern region of the Caribbean Plate. Trace element modelling indicates that group I magmas are products of 8-20% melting of spinel lherzolite, group II magmas result 4-10% melting of a mixture of spinel and garnet lherzolite, and group III basalts are derived by low degrees (0.05-4%) of melting of garnet lherzolite. Dynamic melting models suggest that basalts represent aggregate melts produced by progressive decompression melting in a mantle plume. There is no compositional evidence for the involvement of a Caribbean supra-subduction zone mantle or crust in the generation of the basalts. Two 40Ar/39Ar whole-rock ages reflect the crystallisation of group II magmas at least in the late Campanian (~ 74 Ma) and the lower Eocene (~ 53 Ma). All data suggest that the Dumisseau Fm is an emerged fragment of the CLIP, which continues southward through the Beata Ridge

  9. Global correlation of lower mantle structure and past subduction

    NASA Astrophysics Data System (ADS)

    Domeier, Mathew; Doubrovine, Pavel V.; Torsvik, Trond H.; Spakman, Wim; Bull, Abigail L.

    2016-05-01

    Advances in global seismic tomography have increasingly motivated identification of subducted lithosphere in Earth's deep mantle, creating novel opportunities to link plate tectonics and mantle evolution. Chief among those is the quest for a robust subduction reference frame, wherein the mantle assemblage of subducted lithosphere is used to reconstruct past surface tectonics in an absolute framework anchored in the deep Earth. However, the associations heretofore drawn between lower mantle structure and past subduction have been qualitative and conflicting, so the very assumption of a correlation has yet to be quantitatively corroborated. Here we show that a significant, time-depth progressive correlation can be drawn between reconstructed subduction zones of the last 130 Myr and positive S wave velocity anomalies at 600-2300 km depth, but that further correlation between greater times and depths is not presently demonstrable. This correlation suggests that lower mantle slab sinking rates average between 1.1 and 1.9 cm yr-1.

  10. Hotspots, basalts, and the evolution of the mantle.

    PubMed

    Anderson, D L

    1981-07-01

    The trace element concentration patterns of continental and ocean island basalts and of mid-ocean ridge basalts are complementary. The relative sizes of the source regions for these fundamentally different basalt types can be estimated from the trace element enrichment-depletion patterns. Their combined volume occupies most of the mantle above the 670 kilometer discontinuity. The source regions separated as a result of early mantle differentiation and crystal fractionation from the resulting melt. The mid-ocean ridge basalts source evolved from an eclogite cumulate that lost its late-stage enriched fluids at various times to the shallower mantle and continental crust. The mid-ocean ridge basalts source is rich in garnet and clinopyroxene, whereas the continental and ocean island basalt source is a garnet peridotite that has experienced secondary enrichment. These relationships are consistent with the evolution of a terrestrial magma ocean. PMID:17741173

  11. 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

  12. Numerical Modeling of Continental Subduction In Apennines Style Context

    NASA Astrophysics Data System (ADS)

    Toussaint, G.; Burov, E.; Jolivet, L.

    We performed numerical experiments of early stages of continental subduction and slab detachment at the mantle scale, using a 2D thermo-mechanical model which ac- counts for brittle-elasto-ductile behaviour, phase changes and surface processes. Our goal is to study the influence of various parameters on subduction-related orogens (for instance the Apennines) such as density contrasts, convergence rate, thermal state, eclogitisation and intensity of surface processes. The model reproduces the subduction cycle from early stages to penetration of the slab to the 660 km boundary. Depending of the parameters' combination, the evolution of the initiated slab varies from rapid detachment, stagnation of the subduction to stable, oceanic type subduction. An im- portant feature of our experiments is the almost systematic occurrence of underplating of the lower plate crust beneath the upper plate, leading to formation of double-Moho like structures. It appears that the subducted upper crust and sediments may play a role of lubrification layer, like the low friction coefficients used in some previous models, The results show that the main controlling parameter of slab evolution is the density contrast between the slab, subducted crust, and surrounding asthenosphere : for in- stance, when the contrast between the slab and the asthenosphere is larger than 0.02, the sinking rate of the slab is high, and slab detachment can occur within the first mil- lion years after onset of subduction. For smaller contrasts, stable subduction occurs only in case of full crustal metamorphism. The rate of convergence appears to be a second-order parameter, a high rate being able to prevent detachment. The intensity of surface processes predominantly controls the surface topography, and also seems to be able to influence deep processes such as the sinking rate. Thus, the same topography can be maintained for completely different subduction scenarios.

  13. Terminal Stage Subduction and Slab Detachment: From Structure to Dynamics

    NASA Astrophysics Data System (ADS)

    Wortel, M. J.; Spakman, W.; Govers, R.

    2007-12-01

    In this contribution we focus on terminal stage subduction, often triggered by continent-continent or arc-continent collision. Rupturing of subducted lithosphere, including slab detachment, is a key process in this stage. Seismic tomography information on the 3D structure of subduction zones has been key towards developing a better understanding of subduction processes. We show how structural information on lithosphere and mantle structure, obtained from seismic tomography, can be used to formulate hypotheses concerning the evolution and dynamics of subduction zones which (1) can be subjected to numerical modeling to investigate the physical soundness of the hypotheses, and (2) can subsequently be tested against geological, geophysical, and geodetical data to assess their general or regional validity. In combination, this approach increases our understanding of the relation between deep processes and the geological processes at the surface. The landlocked basin setting of the Mediterranean region provides unique opportunities to study terminal stage subduction and its consequences. Subducted lithosphere is subject to various types of rupturing: Slab detachment, vertical slab tearing and the formation of STEP (tear) faults have been identified and analysed. The different Mediterranean arc systems being in slightly different stages of evolution provides additional clues towards unraveling the dynamics of the process. Using examples from the western-central and eastern Mediterranean, we show how temporal and spatial variations in slab structure affect the dynamics and stress field of convergent plate boundaries, with arc migration and back-arc extension as a prominent expression. In addition, terminal stage subduction appears to be have played a crucial role in Italian magmatism. Terminal stage subduction involving slab detachment is a strongly transient stage with great impact on geological processes, on a wide variety of scales. The information obtained from

  14. Subduction-Driven Recycling of Continental Margin Lithosphere

    NASA Astrophysics Data System (ADS)

    Levander, A.; Bezada, M. J.; Niu, F.; Palomeras, I.; Thurner, S.; Humphreys, E.; Miller, M. S.; Carbonell, R.; Gallart, J.; Schmitz, M.

    2014-12-01

    While subduction recycling of oceanic lithosphere is one of the central themes of plate tectonics, recycling continental lithosphere appears far more complicated and is less well understood. Delamination and convective downwelling are two widely recognized processes invoked to explain the removal of lithospheric mantle under or adjacent to orogenic belts. Here we describe another process that can lead to the loss of continental lithosphere adjacent to a subduction zone: Subducting oceanic plates can entrain and recycle lithospheric mantle from an adjacent continent and disrupt the continental lithosphere far inland from the subduction zone. Seismic images from recent dense broadband arrays on opposite sides of the Atlantic show higher than expected volumes of positive anomalies identified as the subducted Atlantic (ATL) slab under northeastern South America (SA), and the Alboran slab beneath the Gibraltar arc region (GA). The positive anomalies lie under and are aligned with the continental margins at depths greater than 200 km. Closer to the surface we find that the continental margin lithospheric mantle is significantly thinner than expected beneath the orogens adjacent to the subduction zones. Thinner than expected lithosphere extends inland as far as the edges of nearby cratonic cores. These observations suggest that subducting oceanic plates viscously entrain and remove continental mantle lithosphere from beneath adjacent continental margins, modulating the surface tectonics and pre-conditioning the margins for further deformation. The latter can include delamination of the entire lithospheric mantle, as around GA, inferred by results from active and passive seismic experiments. Secondary downwellings develop under the continental interior inland from the subduction zone: We image one under SA and one or more in the past were likely under GA. The process of subduction-driven continental margin lithosphere removal reconciles numerous, sometimes mutually

  15. 4-D Subduction Models Incorporating an Upper Plate

    NASA Astrophysics Data System (ADS)

    Stegman, D.; Capitanio, F. A.; Moresi, L.; Mueller, D.; Clark, S.

    2007-12-01

    Thus far, relatively simplistic models of free subduction have been employed in which the trench and plate kinematics are emergent features completely driven by the negative buoyancy of the slab. This has allowed us to build a fundamental understanding of subduction processes such as the kinematics of subduction zones, the strength of slabs, and mantle flow-plate coupling. Additionaly, these efforts have helped to develop appreciable insight into subduction processes when considering the energetics of subduction, in particular how energy is dissipated in various parts of the system such as generating mantle flow and bending the plate. We are now in a position to build upon this knowledge and shift our focus towards the dynamic controls of deformation in the upper plate (vertical motions, extension, shortening, and dynamic topography). Here, the state of stress in the overriding plate is the product of the delicate balance of large tectonic forces in a highly-coupled system, and must therefore include all components of the system: the subducting plate, the overriding plate, and the underlying mantle flow which couples everything together. We will present some initial results of the fully dynamic 3-D models of free subduction which incorporate an overriding plate and systematically investigate how variations in the style and strength of subduction are expressed by the tectonics of the overriding plate. Deformation is driven in the overriding plate by the forces generated from the subducting plate and the type of boundary condition on the non-subducting side of the overriding plate (either fixed or free). Ultimately, these new models will help to address a range of issues: how the overriding plate influences the plate and trench kinematics; the formation and evolution of back-arc basins; the variation of tractions on the base of the overriding plate; the nature of forces which drive plates; and the dynamics controls on seismic coupling at the plate boundary.

  16. Arc Evolution in Response to the Subduction of Buoyant Features

    NASA Astrophysics Data System (ADS)

    Jenkins, Luke; Fourel, Loic; Goes, Saskia; Morra, Gabriele

    2015-04-01

    The subduction of buoyant features such as aseismic ridges or oceanic plateaux has been invoked to explain arc deformation, flat subduction and increase in seismic coupling. Other studies have challenged these ideas, attributing a larger role to the overriding plate. However, many open questions remain about the dynamics of the relative simple case of a single freely subducting plate. How big does a plateau need to be to change the arc shape? What is the control of plate's strength on the impact of buoyant features? How do the velocities adapt to the subduction of less dense material? In the present study, we propose a systematic approach in order to tackle these questions. We use a new 3-D coupled fluid-solid subduction model where the interaction between the slab and the isoviscous mantle is only calculated on the slab surface, significantly increasing computational efficiency. The oceanic plate rheology is visco-elasto-plastic and its top surface is free. We find that arc shape is significantly altered by the subduction of buoyant plateaux. Along the subduction plane through the plateau and depending on its size, the dip angle and the retreat velocity significantly decrease. Flat subduction is obtained in the case of large and strongly buoyant plateau/ridge. An interesting feature is that retreat velocity increases right after the plateau or ridge has finished subducting in order to catch up with the rest of the plate. The gradient in retreat velocity obtained along the trench may cause the slab to have a heterogeneous response to ridge push, eventually leading to slab advance where buoyant material is present. We apply our models to the Izu-Bonin-Marianas (IBM) trench and propose that subduction of the buoyant Caroline Island Ridge at the southern edge of the Mariana trench can explain both trench motion history and the current morphology of the IBM slab as imaged by seismic tomography.

  17. Subduction of the South-Chile active spreading ridge: a 17 Ma to 3 Ma magmatic record in central Patagonia (western edge of Meseta del Lago Buenos Aires, Argentina)

    NASA Astrophysics Data System (ADS)

    Boutonnet, Emmanuelle; Arnaud, Nicolas; Guivel, Christèle; Lagabrielle, Yves; Scalabrino, Bruno; Espinoza, Felipe

    2010-05-01

    The Chile Triple Junction is a natural laboratory to study the interactions between magmatism and tectonics during the subduction of an active spreading ridge beneath a continent. The MLBA plateau (Meseta del Lago Buenos Aires) is one of the Neogene alkali basaltic plateaus located in the back-arc region of the Andean Cordillera at the latitude of the current Chile Triple Junction. The genesis of MLBA can be related with successive opening of slabs windows beneath Patagonia: within the subducting Nazca Plate itself and between the Nazca and Antarctic plates. Detailed 40Ar/39Ar dating and geochemical analysis of bimodal magmatism from the western flank of the MLBA show major changes in the back-arc magmatism which occurred between 14.5 Ma and 12.5 Ma with the transition from calc-alkaline lavas (Cerro Plomo) to alkaline lavas (MLBA) in relation with slab window opening. In a second step, at 4- 3 Ma, alkaline felsic intrusions were emplaced in the western flank of the MLBA coevally with the MLBA basalts with which they are genetically related. These late OIB-like alkaline to transitional basalts were generated by partial melting of the subslab asthenosphere of the subducting Nazca plate during the opening of the South Chile spreading ridge-related slab window. These basalts differentiated with small amounts of assimilation in shallow magma chambers emplaced along transtensional to extensional zones. The close association of bimodal magmatism with extensional tectonic features in the western MLBA is a strong support to the model of Patagonian collapse event proposed to have taken place between 5 and 3 Ma as a consequence of the presence of the asthenospheric window (SCR-1 segment of South Chile Ridge) below the MLBA area.

  18. Thermal modeling of the southern Alaska subduction zone: Insight into the petrology of the subducting slab and overlying mantle wedge

    SciTech Connect

    Ponko, S.C.; Peacock, S.M.

    1995-11-10

    This report discusses a two-dimensional thermal model of the southern Alaska subduction zone. This model allows specfic predictions to be made about the pressure-temperature conditions and mineralogy of the subducting oceanic crust and the mantle wedge and assess different petrologic models for the generation of Alaskan arc magmas.

  19. Inside the subduction factory: Modeling fluid mobile element enrichment in the mantle wedge above a subduction zone

    NASA Astrophysics Data System (ADS)

    Shervais, John W.; Jean, Marlon M.

    2012-10-01

    Enrichment of the mantle wedge above subduction zones with fluid mobile elements is thought to represent a fundamental process in the origin of arc magmas. This "subduction factory" is typically modeled as a mass balance of inputs (from the subducted slab) and outputs (arc volcanics). We present here a new method to model fluid mobile elements, based on the composition of peridotites associated with supra-subduction ophiolites, which form by melt extraction and fluid enrichment in the mantle wedge above nascent subduction zones. The Coast Range ophiolite (CRO), California, is a Jurassic supra-subduction zone ophiolite that preserves mantle lithologies formed in response to hydrous melting. We use high-precision laser ablation ICP-MS analyses of relic pyroxenes from these peridotites to document fluid-mobile element (FME) concentrations, along with a suite of non-fluid mobile elements that includes rare earth and high-field strength elements. In the CRO, fluid-mobile elements are enriched by factors of up to 100× DMM, whereas fluid immobile elements are progressively depleted by melt extraction. The high concentrations of fluid mobile elements in supra-subduction peridotite pyroxene can be attributed to a flux of aqueous fluid or fluid-rich melt phase derived from the subducting slab. To model this enrichment, we derive a new algorithm that calculates the concentration of fluid mobile elements added to the source: C=[C/[[D/(D-PF)]∗[1-(PF/D)

  20. The mean composition of ocean ridge basalts

    NASA Astrophysics Data System (ADS)

    Gale, Allison; Dalton, Colleen A.; Langmuir, Charles H.; Su, Yongjun; Schilling, Jean-Guy

    2013-03-01

    mean composition of mid-ocean ridge basalts (MORB) is determined using a global data set of major elements, trace elements, and isotopes compiled from new and previously published data. A global catalog of 771 ridge segments, including their mean depth, length, and spreading rate enables calculation of average compositions for each segment. Segment averages allow weighting by segment length and spreading rate and reduce the bias introduced by uneven sampling. A bootstrapping statistical technique provides rigorous error estimates. Based on the characteristics of the data, we suggest a revised nomenclature for MORB. "ALL MORB" is the total composition of the crust apart from back-arc basins, N-MORB the most likely basalt composition encountered along the ridge >500 km from hot spots, and D-MORB the depleted end-member. ALL MORB and N-MORB are substantially more enriched than early estimates of normal ridge basalts. The mean composition of back-arc spreading centers requires higher extents of melting and greater concentrations of fluid-mobile elements, reflecting the influence of water on back-arc petrogenesis. The average data permit a re-evaluation of several problems of global geochemistry. The K/U ratio reported here (12,340 ± 840) is in accord with previous estimates, much lower than the estimate of Arevalo et al. (2009). The low Sm/Nd and 143Nd/144Nd ratio of ALL MORB and N-MORB provide constraints on the hypothesis that Earth has a non-chondritic primitive mantle. Either Earth is chondritic in Sm/Nd and the hypothesis is incorrect or MORB preferentially sample an enriched reservoir, requiring a large depleted reservoir in the deep mantle.

  1. Trace Element Diffusion in Basaltic Melt

    NASA Astrophysics Data System (ADS)

    Holycross, M.; Watson, E. B.

    2015-12-01

    We conducted high pressure, high temperature experiments to determine simultaneously the diffusivities of 24 trace elements (Sc, V, Rb, Y, Zr, Nb, Ba, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, Lu, Hf, Ta, Th, U) in liquids of basaltic composition. Pre-synthesis runs were conducted in graphite capsules in a piston-cylinder apparatus to create two glasses having relatively high and low trace element contents. These glasses were then powdered and paired in diffusion couples by repacking in graphite capsules. All diffusion experiments were executed in a piston cylinder apparatus at 1 GPa pressure and temperatures ranging from 1250-1500º C. Concentration gradients that developed in the glasses were characterized using a laser ablation inductively coupled plasma mass spectrometer (LA-ICP-MS). Diffusion coefficients were determined from concentration profiles and show Arrhenian behavior within experimental error. Errors were assigned based on the linear fit of five time series experiments conducted over 500-9000 s to accurately represent the total experimental reproducibility of our results. Data show the highest activation energies are obtained for high field strength elements. Values for the pre-exponential factor, D0, also peak for the high field strength elements. We suggest that trace element diffusion in basaltic melts follows the compensation law (Winchell, 1969), with log D0 exhibiting linear dependence on activation energy. Calculated diffusivities indicate that transport through basaltic melt could be an effective mechanism for fractionating high field strength elements over geologically relevant time scales. Winchell (1969) High Temp. Sci. 1: 200-215

  2. Twilight of a Volcanic Field: 11 Million Years of Basaltic Volcanism in the Southwestern Nevada Volcanic Field, USA

    NASA Astrophysics Data System (ADS)

    Perry, F. V.; Valintine, G. A.

    2007-12-01

    Following the end of major caldera-forming silicic volcanism in the Southwestern Nevada Volcanic Field (SNVF), at least 10 episodes of alkalic basaltic volcanism have occurred over the last ~11 Ma. An understanding of the past behavior of the volcanic field provides insight for forecasting future eruptive behavior for use in hazard assessment for the high-level radioactive waste repository at Yucca Mountain. A program of geophysics, drilling, Ar-Ar dating and geochemistry conducted since 2004 by Los Alamos National Laboratory and the U.S. Geological Survey, combined with previous and ongoing petrogenetic and physical volcanology studies, sheds more light on the early and middle evolution of the volcanic field, much of which has been buried in alluvial basins. Volumes of erupted basalt have drastically declined over the history of the field, from as much as 50 km3 in the Miocene to about 0.5 km3 in the Pleistocene. The volume decrease is accompanied by a drastic decrease in extension rate, suggesting a close link between magmatism and tectonism. Neodymium and strontium isotopic analyses indicate that enriched lithospheric mantle has been the source of basalt throughout the history of the field. Decreasing eruption volumes are accompanied by an approximate doubling of Ce/Yb ratios, indicating that the volume decrease reflects a decrease in degree of partial melting of the lithospheric source. Eruption style has also changed with time, reflecting an increase in magma volatile content, consistent with decreased amounts of partial melting of a volatile-bearing source. These observations are consistent with a model in which the lithospheric mantle source was hottest during the period of major silicic volcanism and the presence of an active subduction system. After the breakdown of subduction, continued thermal input into the lithosphere ceased, and the lithosphere began to conductively cool. Melt accumulation in non-convecting, static lithosphere is probably related to

  3. Mare basalt genesis - Modeling trace elements and isotopic ratios

    NASA Astrophysics Data System (ADS)

    Binder, A. B.

    1985-11-01

    Various types of mare basalt data have been synthesized, leading to the production of an internally consistent model of the mare basalt source region and mare basalt genesis. The model accounts for the mineralogical, major oxide, compatible siderophile trace element, incompatible trace element, and isotopic characteristics of most of the mare basalt units and of all the pyroclastic glass units for which reliable data are available. Initial tests of the model show that it also reproduces the mineralogy and incompatible trace element characteristics of the complementary highland anorthosite suite of rocks and, in a general way, those of the lunar granite suite of rocks.

  4. Variations in chemical composition of Apollo 15 mare basalts

    NASA Technical Reports Server (NTRS)

    Butler, J. C.

    1976-01-01

    Chemical analyses of 30 different Apollo 15 mare basalts were examined to evaluate the effects of closure on the pearson moment correlation coefficient. It is shown possible to describe the Apollo 15 mare basalts in terms of an opaque, an olivine/pyroxene, an anorthite, and a KREEP component, if significant correlations are identified using the expected correlations as null values. Using Q-mode cluster analysis and nonlinear mapping, it is possible to recognize three groups of the mare basalts, groups 1 and 2 belonging to the olivine normative basalt cluster and group 3 to the quartz normative cluster.

  5. An estimate of the juvenile sulfur content of basalt

    USGS Publications Warehouse

    Moore, J.G.; Fabbi, Brent P.

    1971-01-01

    Sulfur analyses by X-ray fluorescence give an average content of 107 ppm for 9 samples of fresh subaerially-erupted oceanic basalt and 680 ppm for 38 samples of submarine erupted basalt. This difference is the result of retention of sulfur in basalt quenched on the sea floor and loss of sulfur in basalt by degassing at the surface. The outer glassy part of submarine erupted basalt contains 800??150 ppm sulfur, and this amount is regarded as an estimate of the juvenile sulfur content of the basalt melt from the mantle. The slower cooled interiors of basalt pillows are depleted relative to the rims owing to degassing and escape through surface fractures. Available samples of deep-sea basalts do not indicate a difference in original sulfur content between low-K tholeiite, Hawaiian tholeiite, and alkali basalt. The H2O/S ratio of analyzed volcanic gases is generally lower than the H2O/S ratio of gases presumed lost from surface lavas as determined by chemical differences between pillow rims and surface lavas. This enrichment of volcanic gases in sulfur relative to water may result from a greater degassing of sulfur relative to water from shallow intrusive bodies beneath the volcano. ?? 1971 Springer-Verlag.

  6. Quantifying glassy and crystalline basalt partitioning in the oceanic crust

    NASA Astrophysics Data System (ADS)

    Moore, Rachael; Ménez, Bénédicte

    2016-04-01

    The upper layers of the oceanic crust are predominately basaltic rock, some of which hosts microbial life. Current studies of microbial life within the ocean crust mainly focus on the sedimentary rock fraction, or those organisms found within glassy basalts while the potential habitability of crystalline basalts are poorly explored. Recently, there has been recognition that microbial life develops within fractures and grain boundaries of crystalline basalts, therefore estimations of total biomass within the oceanic crust may be largely under evaluated. A deeper understanding of the bulk composition and fractionation of rocks within the oceanic crust is required before more accurate estimations of biomass can be made. To augment our understanding of glassy and crystalline basalts within the oceanic crust we created two end-member models describing basalt fractionation: a pillow basalt with massive, or sheet, flows crust and a pillow basalt with sheeted dike crust. Using known measurements of massive flow thickness, dike thickness, chilled margin thickness, pillow lava size, and pillow lava glass thickness, we have calculated the percentage of glassy versus crystalline basalts within the oceanic crust for each model. These models aid our understanding of textural fractionation within the oceanic crust, and can be applied with bioenergetics models to better constrain deep biomass estimates.

  7. Study on basalt fiber parameters affecting fiber-reinforced mortar

    NASA Astrophysics Data System (ADS)

    Orlov, A. A.; Chernykh, T. N.; Sashina, A. V.; Bogusevich, D. V.

    2015-01-01

    This article considers the effect of different dosages and diameters of basalt fibers on tensile strength increase during bending of fiberboard-reinforced mortar samples. The optimal dosages of fiber, providing maximum strength in bending are revealed. The durability of basalt fiber in an environment of cement, by means of microscopic analysis of samples of fibers and fiberboard-reinforced mortar long-term tests is examined. The article also compares the behavior of basalt fiber in the cement stone environment to a glass one and reveals that the basalt fiber is not subject to destruction.

  8. Mapping Yakutat Subduction with Tectonic Tremor

    NASA Astrophysics Data System (ADS)

    Wech, A.

    2015-12-01

    Subduction of the Yakutat microplate (YAK) in south-central Alaska may be responsible for regional high topography, large slip during the 1964 earthquake, and the anomalous gap in arc volcanism, but the exact geodynamics and its relationship with the underlying Pacific Plate (PP) are not fully understood. Refraction data support distinct subducting layers, and both GPS and body wave tomography suggest the YAK extends from the Cook Inlet volcanoes in the west to the Wrangell volcanic field in the east. Earthquakes, however, are limited to normal faulting within the PP with an abrupt eastern boundary 80 km west of the inferred YAK edge, and more recent active source seismic data suggest subduction of one homogenous thickened oceanic plateau. Here, I perform a search for tectonic tremor to investigate the role of tremor and slow slip in the system. I scan continuous waveforms from 2007-2015 using all available data from permanent and campaign seismic stations in south-central Alaska. Using envelope cross-correlation, I detect and locate ~9,000 tectonic tremor epicenters, providing a map of the transition zone downdip of the 1964 earthquake. Tremor epicenters occur downdip of discrete slow slip events, and tremor rates do not correlate temporally with slow slip behavior. Depth resolution is poor, but horizontal locations are well constrained and spatially correlate with the velocity images of the YAK. Likewise, tremor extends 80 km further east than intraslab seismicity. Tremor swarms occur intermittently and manifest as ambient tremor. I interpret tremor to mark slow, semi-continuous slip occurring at the boundary between the YAK and North American plates, whose interface continues beyond the eastern edge of the PP. In this model, the YAK is welded to the underlying PP in the west, but extends past the eastern terminus of the PP. This geometry explains the correlation between tremor and the YAK, the discrepancy between deep seismicity and tremor, and the paucity of

  9. Microbial colonization and alteration of basaltic glass

    NASA Astrophysics Data System (ADS)

    Einen, J.; Kruber, C.; Øvreås, L.; Thorseth, I. H.; Torsvik, T.

    2006-03-01

    Microorganisms have been reported to be associated with the alteration of the glassy margin of seafloor pillow basalts (Thorseth et al., 2001, 2003; Lysnes et al., 2004). The amount of iron and other biological important elements present in basalts and the vast abundance of basaltic glass in the earth's crust, make glass alteration an important process in global element cycling. To gain further insight into microbial communities associated with glass alteration, five microcosm experiments mimicking seafloor conditions were inoculated with seafloor basalt and incubated for one year. Mineral precipitations, microbial attachment to the glass and glass alteration were visualized by scanning electron microscopy (SEM), and the bacterial community composition was fingerprinted by PCR and denaturing gradient gel electrophoresis (DGGE) in combination with sequencing. SEM analysis revealed a microbial community with low morphological diversity of mainly biofilm associated and prosthecate microorganisms. Approximately 30 nm thick alteration rims developed on the glass in all microcosms after one year of incubation; this however was also seen in non inoculated controls. Calcium carbonate precipitates showed parallel, columnar and filamentous crystallization habits in the microcosms as well as in the sterile controls. DGGE analysis showed an alteration in bacterial community profiles in the five different microcosms, as a response to the different energy and redox regimes and time. In all microcosms a reduction in number of DGGE bands, in combination with an increase in cell abundance were recorded during the experiment. Sequence analysis showed that the microcosms were dominated by four groups of organisms with phylogenetic affiliation to four taxa: The Rhodospirillaceae, a family containing phototrophic marine organisms, in which some members are capable of heterotrophic growth in darkness and N2 fixation; the family Hyphomicrobiaceae, a group of prosthecate oligotrophic

  10. Plagioclase mineralogy of olivine alkaline basalt

    NASA Technical Reports Server (NTRS)

    Hoffer, J. M.

    1973-01-01

    A geological and mineralogical study of the Potrillo volcanics is reported. The investigation consisted first of field mapping to establish and identify the different rock types and volcanic features in order to determine the geological history. Next, samples were collected and analyzed petrographically to determine suitable rocks from the various stratigraphic units for study of plagioclase. Samples selected for further study were crushed and the plagioclase extracted for the determination of composition and structural state. These results were then related to the petrology and crystallization of the basalt.

  11. History and evolution of Subduction in the Precambrium

    NASA Astrophysics Data System (ADS)

    Fischer, R.; Gerya, T.

    2013-12-01

    Plate tectonics is a global self-organising process driven by negative buoyancy at thermal boundary layers. Phanerozoic plate tectonics with its typical subduction and orogeny is relatively well understood and can be traced back in the geological records of the continents. Interpretations of geological, petrological and geochemical observations from Proterozoic and Archean orogenic belts however (e.g. Brown, 2006), suggest a different tectonic regime in the Precambrian. Due to higher radioactive heat production the Precambrian lithosphere shows lower internal strength and is strongly weakened by percolating melts. The fundamental difference between Precambrian and Phanerozoic subduction is therefore the upper-mantle temperature, which determines the strength of the upper mantle (Brun, 2002) and the further subduction history. 3D petrological-thermomechanical numerical modelling experiments of oceanic subduction at an active plate at different upper-mantle temperatures show these different subduction regimes. For upper-mantle temperatures < 175 K above the present day value a subduction style appears which is close to present day subduction but with more frequent slab break-off. At upper-mantle temperatures 175 - 250 K above present day values steep subduction changes to shallow underplating and buckling. For upper-mantle temperatures > 250 K above the present day value no subduction occurs any more. The whole lithosphere starts to delaminate and drip-off. But the subduction style is not only a function of upper-mantle temperature but also strongly depends on the thickness of the subducting plate. If thinner present day oceanic plates are used in the Precambrian models, no shallow underplating is observed but steep subduction can be found up to an upper-mantle temperature of 200 K above present day values. Increasing oceanic plate thickness introduces a transition from steep to flat subduction at lower temperatures of around 150 K. Thicker oceanic plates in the

  12. East Mariana Basin tholeiites: Cretaceous intraplate basalts or rift basalts related to the Ontong Java plume?

    USGS Publications Warehouse

    Castillo, P.R.; Pringle, M.S.; Carlson, R.W.

    1994-01-01

    Studies of seafloor magnetic anomaly patterns suggest the presence of Jurassic oceanic crust in a large area in the western Pacific that includes the East Mariana, Nauru and Pigafetta Basins. Sampling of the igneous crust in this area by the Deep Sea Drilling Program (DSDP) and the Ocean Drilling Program (ODP) allows direct evaluation of the age and petrogenesis of this crust. ODP Leg 129 drilled a 51 m sequence of basalt pillows and massive flows in the central East Mariana Basin. 40Ar 39Ar ages determined in this study for two Leg 129 basalts average 114.6 ?? 3.2 Ma. This age is in agreement with the Albian-late Aptian paleontologic age of the overlying sediments, but is distinctively younger than the Jurassic age predicted by magnetic anomaly patterns in the basin. Compositionally, the East Mariana Basin basalts are uniformly low-K tholeiites that are depleted in highly incompatible elements compared to moderately incompatible ones, which is typical of mid-ocean ridge basalts (MORB) erupted near hotspots. The Sr, Nd and Pb isotopic compositions of the tholeiites ( 87Sr 86Srinit = 0.70360-0.70374; 143Nd 144Ndinit = 0.512769-0.512790; 206Pb 204Pbmeas = 18.355-18.386) also overlap with some Indian Ocean Ridge MORB, although they are distinct from the isotopic compositions of Jurassic basalts drilled in the Pigafetta Basin, the oldest Pacific MORB. The isotopic compositions of the East Mariana Basin tholeiites are also similar to those of intraplate basalts, and in particular, to the isotopic signature of basalts from the nearby Ontong Java and Manihiki Plateaus. The East Mariana Basin tholeiites also share many petrologic and isotopic characteristics with the oceanic basement drilled in the Nauru Basin at DSDP Site 462. In addition, the new 110.8 ?? 1.0 Ma 40Ar 39Ar age for two flows from the bottom of Site 462 in the Nauru Basin is indistinguishable from the age of the East Mariana Basin flows. Thus, while magnetic anomaly patterns predict that the igneous

  13. Possible coseismic laminar and non-laminar flow along subduction megathrusts

    NASA Astrophysics Data System (ADS)

    Ujiie, K.; Noguchi, K.; Saito, T.; Tsutsumi, A.

    2014-12-01

    Coseismic deformation mechanisms during subduction earthquakes remained unclear other than frictional melting recorded in pseudotachylytes. However, the recent mineralogical studies in the shallow plate-boundary thrust in the Nankai subduction zone and the underplating-related duplex-fault zone in the Shimanto accretionary complex exhumed from 4-6 km depth have identified increased heating along the 2 mm-thick, clay-rich fault gouge and the few-centimeters-thick, basalt-derived ultracataclasite, respectively. The microstructures of the fault gouge are characterized by strong preferred orientation of clay particles along the gouge, while those of the ultracataclasite show the random fabric. High-velocity friction experiments were conducted on the disaggregated fault rocks under wet (water-saturated) conditions at different normal stresses, using the rotary shear frictional testing apparatus. The results show the rapid slip weakening with low peak and steady-state shear stress, and a very small slip weakening distance and fracture energy, suggesting the ease of earthquake rupture propagation through the fault materials. The steady-state shear stress is almost independent of normal stress, indicating that the gouge behaved like a fluid during high-velocity shearing. The microstructures after the experiments are marked by the development of foliated zone in the gouge layer, but the random fabric develops in the outermost region of the circular gouge layer. Given the nearly independence of steady-state shear stress on normal stress and the increase in the rotation velocity from the center of the rotation axis during the rotary shear, the change from foliated zone to non-foliated, random fabric in the circular gouge layer could represent the change from laminar to non-laminar (or turbulent) flow associated with the increase in the Reynolds number. The implications for the fault rocks are that the development of foliated and non-foliated zones may represent coseismic

  14. Fluid budgets along the northern Hikurangi subduction margin, New Zealand: the effect of a subducting seamount on fluid pressure

    NASA Astrophysics Data System (ADS)

    Ellis, Susan; Fagereng, Åke; Barker, Dan; Henrys, Stuart; Saffer, Demian; Wallace, Laura; Williams, Charles; Harris, Rob

    2015-07-01

    We estimate fluid sources around a subducted seamount along the northern Hikurangi subduction margin of New Zealand, using thermomechanical numerical modelling informed by wedge structure and porosities from multichannel seismic data. Calculated fluid sources are input into an independent fluid-flow model to explore the key controls on overpressure generation to depths of 12 km. In the thermomechanical models, sediment transport through and beneath the wedge is calculated assuming a pressure-sensitive frictional rheology. The change in porosity, pressure and temperature with calculated rock advection is used to compute fluid release from compaction and dehydration. Our calculations yield more precise information about source locations in time and space than previous averaged estimates for the Hikurangi margin. The volume of fluid release in the wedge is smaller than previously estimated from margin-averaged calculations (˜14 m3 yr-1 m-1), and is exceeded by fluid release from underlying (subducting) sediment (˜16 m3 yr-1 m-1). Clay dehydration contributes only a small quantity of fluid by volume (˜2 m3 yr-1 m-1 from subducted sediment), but the integrated effect is still significant landward of the seamount. Fluid source terms are used to estimate fluid pressures around a subducting seamount in the fluid-flow models, using subducted sediment permeability derived from porosity, and testing two end-members for décollement permeability. Models in which the décollement acts as a fluid conduit predict only moderate fluid overpressure in the wedge and subducting sediment. However, if the subduction interface becomes impermeable with depth, significant fluid overpressure develops in subducting sediment landward of the seamount. The location of predicted fluid overpressure and associated dehydration reactions is consistent with the idea that short duration, shallow, slow slip events (SSEs) landward of the seamount are caused by anomalous fluid pressures; alternatively

  15. Strain accumulation along the Cascadia Subduction Zone

    NASA Astrophysics Data System (ADS)

    Murray, Mark H.; Lisowski, Michael

    2000-11-01

    We combine triangulation, trilateration, and GPS observations to determine horizontal strain rates along the Cascadia subduction zone from Cape Mendocino to the Strait of Juan de Fuca. Shear-strain rates are significantly greater than zero (95% confidence) in all forearc regions (26-167 nanoradians/yr), and are not significant in the arc and backarc regions. The deformation is primarily uniaxial contraction nearly parallel to Juan de Fuca-North America plate convergence (N55°-80°E). The strain rates are consistent with an elastic dislocation model for interseismic slip with a shallow 100-km wide locked zone and a deeper 75-km transition zone along the entire megathrust, except along the central Oregon coast where relatively lower strain rates are consistent with 30-40 km wide locked and transition zones.

  16. Crystal Stratigraphy of Two Basalts from Apollo 16: Unique Crystallization of Picritic Basalt 606063,10-16 and Very-Low-Titanium Basalt 65703,9-13

    NASA Technical Reports Server (NTRS)

    Donohue, P. H.; Neal, C. R.; Stevens, R. E.; Zeigler, R. A.

    2014-01-01

    A geochemical survey of Apollo 16 regolith fragments found five basaltic samples from among hundreds of 2-4 mm regolith fragments of the Apollo 16 site. These included a high-Ti vitrophyric basalt (60603,10-16) and one very-low-titanium (VLT) crystalline basalt (65703,9-13). Apollo 16 was the only highlands sample return mission distant from the maria (approx. 200 km). Identification of basaltic samples at the site not from the ancient regolith breccia indicates input of material via lateral transport by post-basin impacts. The presence of basaltic rocklets and glass at the site is not unprecedented and is required to satisfy mass-balance constraints of regolith compositions. However, preliminary characterization of olivine and plagioclase crystal size distributions indicated the sample textures were distinct from other known mare basalts, and instead had affinities to impact melt textures. Impact melt textures can appear qualitatively similar to pristine basalts, and quantitative analysis is required to distinguish between the two in thin section. The crystal stratigraphy method is a powerful tool in studying of igneous systems, utilizing geochemical analyses across minerals and textural analyses of phases. In particular, trace element signatures can aid in determining the ultimate origin of these samples and variations document subtle changes occurring during their petrogenesis.

  17. Gravity and Flexure Modelling of Subducting Plates

    NASA Astrophysics Data System (ADS)

    Hunter, J. A.; Watts, A. B.; SO 215 Shipboard Scientific Party

    2012-04-01

    The long-term strength of the lithosphere is determined by its flexural rigidity, which is commonly expressed through the effective elastic thickness, Te. Flexure studies have revealed a dependence of Te on thermal age. In the oceans, loads formed on young (70 Ma) seafloor. In the continents, loads on young (1000 Ma) lithosphere. Recent studies have questioned the relationship of Te with age, especially at subduction zones, where oceanic and continental lithosphere are flexed downwards by up to ~6 km over horizontal distances of up to ~350 km. We have therefore used free-air gravity anomaly and topography profile data, combined with forward and inverse modelling techniques, to re-assess Te in these settings. Preliminary inverse modelling results from the Tonga-Kermadec Trench - Outer Rise system, where the Pacific plate is subducting beneath the Indo-Australian plate, show large spatial variations in Te that are unrelated to age. In contrast to the southern end of the system, where Te is determined by the depth to the 600° C and 900° C isotherms, the northern end of the system shows a reduction in strength. Results also suggest a reduction in Te trenchward of the outer rise that is coincident with a region of pervasive extensional faulting visible in swath bathymetry data. In a continental setting, the Ganges foreland basin has formed by flexure of the Indo-Australian plate in front of the migrating loads of the Himalaya. Preliminary forward modelling results, using the Himalaya as a known surface topographic load, suggest that Te is high - consistent with the great age of Indian cratonic lithosphere. However, results from inverse modelling that solves for unknown loads (vertical shear force and bending moment) show significant scatter and display trade-offs between Te and these driving loads.

  18. Petrogenesis of the Eocene and Mio Pliocene alkaline basaltic magmatism in Meseta Chile Chico, southern Patagonia, Chile: Evidence for the participation of two slab windows

    NASA Astrophysics Data System (ADS)

    Espinoza, Felipe; Morata, Diego; Pelleter, Ewan; Maury, René C.; Suárez, Manuel; Lagabrielle, Yves; Polvé, Mireille; Bellon, Hervé; Cotten, Joseph; De la Cruz, Rita; Guivel, Christelle

    2005-06-01

    The Meseta Chile Chico (MCC, 46.4°S) is the westernmost exposure of Eocene (lower basaltic sequence, LBS; 55-40 Ma, K-Ar ages) and Mio-Pliocene (upper basaltic sequence, UBS; 16-4 Ma, K-Ar ages) flood basalt volcanism in Patagonia. The MCC is located south of the Lago General Carrera-Buenos Aires (LGCBA), southeast from the present day Chile Triple Junction (CTJ), east of the actual volcanic gap between Southern South Volcanic Zone and Austral Volcanic Zone (SSVZ and AVZ, respectively) and just above the inferred location of the South Chile Ridge segment subducted at ˜6 Ma (SCR-1). Erupted products consist of mainly ne-normative olivine basalt with minor hy-normative tholeiites basalt, trachybasalt and basanite. MCC lavas are alkaline (42.7-53.1 wt.% SiO 2, 3-8 wt.% Na 2O+K 2O) and relatively primitive (Ni: 133-360 ppm, Cr: 161-193 ppm, Co: 35-72 ppm, 4-16.5 MgO wt.%). They have a marked OIB-like signature, as shown by their isotopic compositions ( 87Sr/ 86Sr o=0.70311-0.70414 and ɛNd=+4.7-+5.1) and their incompatible trace elements ratios (Ba/La=10-20, La/Nb=0.46-1.09, Ce/Pb=15.52-27.5, Sr/La<25), reflecting deep mantle origin. UBS-primitive lavas have characteristics similar to those of the Eocene LBS basalts, while UBS-intermediate lavas show geochemical imprints (La/Nb>1, Sr/La>25, low Ce/Pb, Nb/U) compatible with contamination by arc/slab-derived and/or crustal components. We propose that the genesis and extrusion of magmas is related to the opening of two slab windows due to the subduction of two active ridge segments beneath Patagonia during Eocene and Mio-Pliocene.

  19. 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

  20. Southern Cordilleran basaltic andesite suite, southern Chihuahua, Mexico: A link between Tertiary continental arc and flood basalt magmatism in North America

    NASA Astrophysics Data System (ADS)

    Cameron, K. L.; Nimz, G. J.; Kuentz, D.; Niemeyer, S.; Gunn, S.

    1989-06-01

    Mid-Cenozoic orogenic andesites and ignimbrites of western Mexico, southwestern New Mexico, and Arizona are commonly capped by basaltic andesites, most from 29-20 Ma. We refer to these mafic lavas as the Southern Cordilleran Basaltic Andesite (SCORBA) suite, and they may constitute the most extensive Cenozoic basaltic suite in North America. The SCORBA suite has trace element and isotopic characteristics of orogenic (arc) rocks (e.g., Ba/Nb>40), and silica content (53-56% SiO2) like the Grande Ronde Basalt, which represents about 80% of the volume of the Columbia River Group. Geochemical and isotopic data are presented on SCORBA lavas and rare mafic lavas (PRE-SCORBA) interlayered with older ignimbrites from a 700-km-long NE-SW transect of southern Chihuahua, Mexico. SCORBA and PRE-SCORBA lavas with relatively low K/P (<7) and differing Ba/Nb (50 versus 18) have similar isotopic compositions, arguing against their isotopic signatures being controlled by crustal assimilation. Along the entire length of the transect, the basaltic rocks have ɛNd and 87Sr/86Sr near bulk Earth and 206Pb/204Pb and 207Pb/204Pb ratios that lie along a 1.7 Ga pseudoisochron. The Pb isotopic variation is geographically controlled, becoming more radiogenic from east to west, reflecting mixing in mantle source regions. The eastern mantle source has low 206Pb/204Pb and is a mixture of an enriched, enriched-mantle-like (EMI) component with one or more depleted components, which could include an intraplate component with relatively high Nb/Y (>0.8). The western mantle source contains less of the intraplate component and is more oceanic in character. Overprinting both the eastern and western sources is a Cenozoic subduction component that is responsible for the western radiogenic Pb, and this component fades out inland to the east. This transect crosses the inferred position of the Mojave-Sonora megashear, previously proposed to be a major lithospheric boundary, separating Proterozoic basement to