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  • 1
    Language: English
    In: Earth and Planetary Science Letters, 2011, Vol.308(1), pp.65-76
    Description: We investigated the halogen (Cl, F, Br, and I) chemistry of serpentinites that record progressive dehydration during subduction from shallow oceanic environments via increased pressure and temperature conditions to complete breakdown of antigorite. The aim is to evaluate the relevance of serpentinites for halogen recycling in subduction zones and for deep mantle recharge of these elements. The halogen compositions of the analyzed samples indicate input from seawater and sedimentary sources during initial serpentinization of either subducting lithospheric mantle during slab bending or forearc mantle by uprising slab fluids. During the first dehydration stage (antigorite + brucite → olivine + H O), fluids with high Br/Cl and I/Cl ratios are released resulting in residual serpentinites with lower Br/Cl and I/Cl ratios. Veins associated with this event and with the final antigorite breakdown (antigorite → olivine + orthopyroxene + H O) show higher halogen ratios compared to their adjacent wall rocks, and they are similar to those found in arc volcanoes (F/Cl and I/Cl between ca. 0.083–1.5, and ca. 0.00038–0.0013, respectively). All measured deserpentinization samples show a narrow range in δ Cl values (between − 0.42‰ and + 0.92‰) overlapping the δ Cl values of seafloor serpentinites and confirming that no significant Cl isotope fractionation occurs during subduction dehydration of serpentinites. Our findings document the conservative behavior of halogens during subduction. Mass balance constraints reveal that serpentinites strongly control the halogen chemistry of deep subduction zone fluids and that descent of rock residues after deserpentinization strongly affects the halogen budget of the mantle. ► Halogen concentrations and ratios and Cl isotope data indicate halogen input from sedimentary reservoirs during shallow serpentinization, either at the outer rise during slab bending or in suprasubduction shallow environments. ► During deserpentinization, fluids with high Br/Cl and I/Cl ratios are released which drives the residual serpentinites to lower Br/Cl and Cl/I and during ongoing dehydration. F/Cl ratios increase strongest with increasing degree of dehydration throughout the whole deserpentinization process. The high-pressure slab fluids liberated during the antigorite to olivine reaction have F/Cl and I/Cl ratios that are similar to those of arc volcanoes. ► The overall δ Cl values of the deserpentinization fluids are close to 0‰, which are within the range of known arc volcanoes. ► The concentrations and ratios found in the serpentinization–deserpentinization sequence indicate the conservative behavior of halogens during subduction and fluid release. ► Mass balance calculations show that serpentinites seem to strongly control the halogen budget, and likely also the Cl isotope signature, of the deeper subduction zone fluids.
    Keywords: Subduction Zone ; Serpentinites ; Dehydration ; Halogens ; Chlorine Isotopes ; Deep Recycling ; Geology ; Physics
    ISSN: 0012-821X
    E-ISSN: 1385-013X
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  • 2
    Language: English
    In: Earth and Planetary Science Letters, 2010, Vol.298(1), pp.175-182
    Description: Subduction of oceanic lithosphere is a key feature of terrestrial plate tectonics. However, the effect of this recycled crustal material on mantle composition is debated. Ocean island basalts (OIB) provide direct insights into the composition of Earth's mantle. The distinct composition of the HIMU (high U/ Pb)- and EM (enriched mantle)-type OIB mantle sources may be due to either recycling of oceanic crust and sediment into the mantle or metasomatic processes within the mantle. Chlorine derived from seawater or crustal fluids potentially provides a tracer for recycled material. Previously reported δ Cl values for mid-ocean ridge basalts (MORB) range from ca. − 3.0 to near 0‰. In contrast to MORB, we find a larger variation in OIB glasses representing HIMU- and EM-type mantle sources based on replicate SIMS analyses with δ Cl values ranging from − 1.6 to + 1.1‰ for HIMU-type and − 0.4 to + 2.9‰ for EM-type lavas. These δ Cl values correlate positively with Sr/ Sr ratios for both the HIMU- and EM-type samples. The negative δ Cl values of some HIMU-type lavas overlap with those of altered oceanic lithosphere, which is assumed to be present in the HIMU source. The EM lavas have high Sr/ Sr and primarily positive δ Cl values. We hypothesize that subducting sediments may have developed high δ Cl values by expelling Cl-depleted pore fluids, thus accounting for the positive δ Cl values recorded in the EM-type lavas.
    Keywords: Chlorine Isotopes ; Crustal Recycling ; Sims ; Subduction Zone ; Ocean Island Basalt ; Mantle Chemistry ; Geology ; Physics
    ISSN: 0012-821X
    E-ISSN: 1385-013X
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  • 3
    Language: English
    In: Geochimica et Cosmochimica Acta, 01 December 2015, Vol.170, pp.225-246
    Description: Apatite (Ca (PO ) (OH, F, Cl)) is one of the main host of halogens in magmatic and metamorphic rocks and plays a unique role during fluid–rock interaction as it incorporates halogens (i.e. F, Cl, Br, I) and OH from hydrothermal fluids to form a ternary solid solution of the endmembers F-apatite, Cl-apatite and OH-apatite. Here, we present an experimental study to investigate the processes during interaction of Cl-apatite with different aqueous solutions (KOH, NaCl, NaF of different concentration also doped with NaBr, NaI) at crustal conditions (400–700 °C and 0.2 GPa) leading to the formation of new apatite. We use the experimental results to calculate partition coefficients of halogens between apatite and fluid. Due to a coupled dissolution–reprecipitation mechanism new apatite is always formed as a pseudomorphic replacement of Cl-apatite. Additionally, some experiments produce new apatite also as an epitaxial overgrowth. The composition of new apatite is mainly governed by complex characteristics of the fluid phase from which it is precipitating and depends on composition of the fluid, temperature and fluid to mineral ratio. Furthermore, replaced apatite shows a compositional zonation, which is attributed to a compositional evolution of the coexisting fluid in local equilibrium with the newly formed apatite. Apatite/fluid partition coefficients for F depend on the concentration of F in the fluid and increase from 75 at high concentrations (460 μg/g F) to 300 at low concentrations (46 μg/g F) indicating a high compatibility of F in apatite. A correlation of Cl-concentration in apatite with Cl concentration of fluid is not observed for experiments with highly saline solutions, composition of new apatite is rather governed by OH concentration of the hydrothermal fluid. Low partition coefficients were measured for the larger halogens Br and I and vary between 0.7 * 10 –152 * 10 for Br and 0.3 * 10 –17 * 10 for I, respectively. Br seems to have values of about one order of magnitude higher than I. These data allow an estimation of the values for the other halogens based on a lattice strain model which displays a sequence with of ∼120, of ∼100, of ∼2.3 ∼0.045, and ∼0.0025. Results from this experimental study help to better understand fluid–rock interaction of an evolving fluid, as it enables the composition of hydrothermally derived apatite to be used as a fluid probe for halogens at crustal conditions. It further shows the importance of mineral replacement as one of the key reactions to generate apatite of different composition.
    Keywords: Geology
    ISSN: 0016-7037
    E-ISSN: 1872-9533
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  • 4
    Language: English
    In: Earth and Planetary Science Letters, 2011, Vol.311(3), pp.411-419
    Description: The rheological properties of subducting rocks represent a fundamental parameter in the dynamics of subduction zones. Making robust predictions about these properties and the general strength evolution of subducting plates is hampered by a relatively restricted understanding of the mechanisms by which rocks deform at high- and ultrahigh pressure. This uncertainty is relates to the discrepancy between experimental and field-based observations of eclogite deformation and, in particular, the role of garnet. To further investigate this important aspect, we performed a textural and micro-structural investigation, applying optical microscopy, element mapping, and electron backscatter diffraction on deformed garnet polycrystals from an eclogite mylonite. The results were compared to those from a study of undeformed polycrystals that formed from a supercooled frictional melt at HP conditions. The mylonites' polycrystals are flattened parallel to the main high-pressure foliation in the rock and individual grains were shortened by an average 15%. Although dislocation creep is commonly presumed to dominate garnet straining in eclogites, no record of this mechanism was found. Instead, the garnet grains have dissolution surfaces indicative of deformation by intergranular pressure solution. The observations provide compelling evidence for the role of fluids and syn-tectonic porosity in the weakening of garnet: a supposedly rigid eclogite component. Such weakening represents a crucial step in the fundamental feedback loop between fluid ingress, metamorphism and near-instantaneous competence loss in rocks undergoing deep subduction. ► Garnet in eclogite mylonites underwent extensive intergranular pressure solution. ► Straining of garnet occurred at temperatures where garnet is expected to be rigid. ► Fluids trigger high-kinetic diffusion creep processes in subducting mafic rocks. ► These processes control the rates at which garnet and eclogite deforms. ► Competence contrasts in slabs reflect contrasts in syn-tectonic permeability.
    Keywords: Garnet ; Eclogite ; Rheology ; Deformation ; Subduction ; Creep ; Geology ; Physics
    ISSN: 0012-821X
    E-ISSN: 1385-013X
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  • 5
    Language: English
    In: Earth and Planetary Science Letters, Jan 15, 2014, Vol.386, p.64(11)
    Description: To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.epsl.2013.10.050 Byline: Laura Jonas, Timm John, Helen E. King, Thorsten Geisler, Andrew Putnis Abstract: The pseudomorphic replacement of Carrara marble by calcium phosphates was used as a model system in order to study the influence of different fluid pathways for reaction front propagation induced by fluid-rock interaction. In this model, grain boundaries present in the rock as well as the transient porosity structures developing throughout the replacement reaction enable the reaction front to progress further into the rock as well as to the center of each single grain until transformation is complete. Hydrothermal treatment of the marble using phosphate bearing solutions led to the formation of hydroxylapatite and [beta]-TCP; the formation of the latter phase was probably promoted by the presence of [approximately equal to]0.6 wt.% Mg in the parent carbonate phase. Completely transformed single grains show a distinctive zoning, both in composition and texture. Whereas areas next to the grain boundary consist of nearly pure hydroxylapatite and show a coarse porosity, areas close to the center of the single grains show a high amount of [beta]-TCP and a very fine porous microstructure. If fluorine was added as an additional solution component, up to 3 wt.% of F were incorporated into the product apatite and the formation of [beta]-TCP was avoided. The use of the isotope.sup.18O as a chronometer for the replacement reaction makes it possible to reconstruct the chronological development of the calcium phosphate reaction front. Raman analysis revealed that the incorporation of.sup.18O in the PO.sub.4 tetrahedron of hydroxylapatite results in the development of distinct profiles in the calcium phosphate reaction front perpendicular to the grain boundaries of the marble. Through the use of the.sup.18O chronometer, it is possible to estimate and compare the time effectiveness of the different fluid pathways in this model system. The results demonstrate that the grain boundaries are an effective pathway enabling the fluid to penetrate the rock more than one order of magnitude faster compared to the newly developing channel-like porosity structures, which act as pathways towards the center of single mineral grains. Thus, after only short reaction durations, it may be possible for the fluid to progress relatively large distances along the grain boundaries without developing broad reaction fronts along the path. Author Affiliation: (a) Institut fur Mineralogie, Westfalische Wilhelms-Universitat Munster, Corrensstra[sz]e 24, D-48149 Munster, Germany (b) Steinmann-Institut fur Geologie, Mineralogie und Palaontologie, Rheinische Friedrich-Wilhelms-Universitat Bonn, Poppelsdorfer Schloss, D-53115 Bonn, Germany Article History: Received 28 March 2013; Revised 14 September 2013; Accepted 23 October 2013 Article Note: (miscellaneous) Editor: T. Elliott
    Keywords: Marble -- Analysis ; Apatites -- Analysis ; Phosphate Minerals -- Analysis ; Porosity -- Analysis ; Carbonates -- Analysis
    ISSN: 0012-821X
    Source: Cengage Learning, Inc.
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  • 6
    Language: English
    In: Geology, July, 2006, Vol.34(7), p.557(4)
    Description: Field evidence preserved in ancient subducted oceanic crust documents that eclogite facies frictional failure with melting (pseudotachylyte formation during intermediate-depth earthquakes) was accompanied and followed, not preceded, by infiltration of external fluids and progressive vein formation in the eclogites. Eclogitization began during seismic failure and fluid passage through the shear zone. Subsequent fluid flow produced hydraulic fracturing and continuous vein formation during ongoing burial. We suggest that this kind of shear zone may allow channelized fluid flow within and out of slabs. Keywords: pseudotachylyte, intermediate-depth earthquake, eclogite facies, oceanic crust, subduction zone, fluid flow.
    Keywords: Earthquakes -- Analysis ; Oceans
    ISSN: 0091-7613
    E-ISSN: 19432682
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  • 7
    Language: English
    In: Geochimica et Cosmochimica Acta, Nov 1, 2013, Vol.120, p.326(37)
    Description: To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.gca.2013.06.023 Byline: Ji-Lei Li, Jun Gao, Timm John, Reiner Klemd, Wen Su Abstract: High-pressure (HP) veins in eclogites provide insight into element mobility during fluid-rock interaction in subduction zones. Here, we present a petrological-geochemical study of a sulfide-bearing HP vein and its massive lawsonite eclogite host rock from the Chinese Tianshan (ultra-)high-pressure/low-temperature metamorphic belt. The omphacite-dominated vein is enveloped by a garnet-poor, sulfide-bearing eclogite-facies reaction selvage. Lawsonite, garnet, omphacite, glaucophane and other HP minerals occur as inclusions in pyrite porphyroblasts of the selvage rock, indicating that the selvage formed prograde under eclogite-facies conditions. Relicts of wall-rock garnet (Grt_I) cores in recrystallized selvage garnet (Grt_II) close to the wall rock and the Ca distribution in Grt_II, which images the overgrown selvage matrix, indicate that the selvage formed due to dissolution-precipitation processes as a consequence of fluid-rock interaction of the wall rock eclogite with the vein-forming fluid. The peak metamorphic P-T conditions of the wall-rock eclogite are estimated at ca. 590[degrees]C and 23kbar. Mass-balance calculations indicate that the reaction selvage experienced: (1) a depletion of the large-ion lithophile elements (K-Rb-Ba) of up to 100% relative to their concentrations in the wall-rock eclogite; (2) a moderate depletion of the HREE and some transition metal elements including Fe, Cu, Ni, Zn, Co, Cr, and Mn (10-40%); (3) a significant enrichment of CaO (up to 50-80%), Sr (up to [approximately equal to]200%), Pb (up to [approximately equal to]170%) and S (up to [approximately equal to]210%); (4) a slight to moderate enrichment of the LREE (10-20%) and MREE (0-40%); whereas (5) the HFSE show no significant variations. The chemical changes in the selvage suggest that the fluid, which caused the dissolution of the wall-rock and the precipitation of the selvage assemblage while the vein formed, was probably a mixture of an "internal" fluid derived from the prograde dehydration (e.g., lawsonite breakdown) of the wall rock and an "external" fluid most likely derived from dehydrating altered oceanic crust located in stratigraphically lower units of the subducting slab. The external fluid introduced Ca, S, Sr, Pb and at least parts of the LREE and the MREE into the selvage, whereas some elements, such as the remaining LREE and MREE, may have been derived from the wall rock eclogite via diffusional transport into the selvage. The enrichment of Ca and L-MREE is displayed by the abundant growth of selvage epidote. In contrast, the dissolution of garnet and phengite released significant amounts of HREE and LILE (K-Rb-Ba) into the passing fluid, because the chemical changes within the selvage prevented the formation of a mineral assemblage with sufficiently high bulk-fluid partition coefficients for these elements. Significant amounts of transitional metal elements were released into the fluid during the dissolution of white mica and the dissolution-precipitation behavior of garnet, omphacite, dolomite and sulfides. Thus the LILE and HREE along with some transition metal elements (e.g., Fe, Cu, Ni and Zn) were mobilized during the dissolution-precipitation processes that led to the selvage formation. Accordingly the slab fluids are not only strongly enriched in LILE and depleted in HFSE, but also carry significant amounts of transition metals. It is most likely that slab fluids strongly contribute to the metal flux into the arc magma systems finally resulting in giant arc-related ore deposits. Article History: Received 14 January 2013; Accepted 20 June 2013 Article Note: (miscellaneous) Associate editor: Edward M. Ripley
    Keywords: Ore Deposits ; Sulfides ; Subduction Zones ; Computer Industry
    ISSN: 0016-7037
    Source: Cengage Learning, Inc.
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  • 8
    Language: English
    In: Geochimica et Cosmochimica Acta, 15 November 2018, Vol.241, pp.38-55
    Description: An important question regarding the formation of the solar system is how planetary bodies developed from dust and ice into the planets and planetary bodies. A particularly interesting topic is the thermal evolution of carbonaceous chondrites and volatile-rich clasts that could have originated from CM- and CI-like parent bodies. Two types of these volatile-rich clasts, which are a particular type of dark clasts, can be found. These clasts are mineralogically very similar to CM and CI chondrites and can occasionally be found in achondritic meteorites. Mineral assemblages suggest that both CM and CI chondrites as well as volatile-rich clasts experienced low peak temperatures. However, these mineral assemblages only offer large estimated temperature ranges to describe the thermal history of CM and CI chondrites, and the thermal history of volatile-rich clasts has not been previously described. In this study, to gain a better understanding of the thermal history of both CM and CI chondrites and volatile-rich clasts, we estimated peak temperatures of 30 volatile-rich clasts (16 CI-, and 14 CM-like) in 10 different host meteorites (4 polymict ureilites, 5 polymict eucrites and 1 howardite) by Raman carbon thermometry. An automated method was developed in order to describe over 4000 collected Raman spectra using four pseudovoigt functions. The full width half maximum (FWHM) of the D1-band was then used to calculate peak temperatures. Results were then compared to Raman data of 8 different well-studied carbonaceous chondrites (including CI and CM chondrites) to evaluate the suggestion that volatile-rich clasts are composed of similar material to the equivalent CI and CM chondrites. Our results show that the peak temperatures experienced by CI-like clasts range between 30–110 °C with an average of about 65 ± 25 °C; the peak temperatures experienced by CM-like clasts range from 50 to 110 °C with an average of about 70 ± 25 °C. Six of the 8 studied carbonaceous chondrites (CM, CI, CR or C2 ) also plot in the same low-temperature range between 50 °C and 75 °C and can thus be considered to have formed under similar temperature conditions as the volatile-rich clasts. This is in agreement with previous suggestions, based on their mineral compositions that volatile-rich clasts and CI and CM carbonaceous chondrites are composed of similar materials. The peak temperatures for carbonaceous chondrites determined in this study considerably reduce the range of temperature estimates proposed previously for these chondrites by different methods. By highlighting the ability of our methodology to evaluate data in an automated way, this study shows that Raman carbon thermometry is a good analytical technique for obtaining information about peak temperatures in small and delicate samples.
    Keywords: Raman ; Carbon Thermometry ; Organic Matter ; Volatile-Rich Clasts ; CM Chondrite ; Ci Chondrite ; Dark Clasts ; Peak Temperatures ; Geology
    ISSN: 0016-7037
    E-ISSN: 1872-9533
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  • 9
    Language: English
    In: Geochimica et Cosmochimica Acta, 2010, Vol.74(6), pp.1892-1922
    Description: In the subduction complex of the Tianshan mountains, western China, massive blueschist is cross-cut by an eclogite-facies major fluid conduit surrounded by a reaction zone which is mainly composed of omphacite and garnet. Petrological as well as geochemical evidence suggest that formation of the vein and the eclogitic selvage around the vein was caused by fluid infiltration under peak metamorphic conditions of 21 ± 1.5 kbar and 510 ± 30 °C. The combination of whole-rock with mineral trace-element data as well as mass-balance calculations indicate that substantial differences exist between the unaltered host rock and the part of the system which was altered by fluid–rock interaction. These differences include: (1) depletion of mainly large-ion lithophile elements (LILE) and Li of up to 60% relative to their concentrations in the unaltered host rock; (2) an extreme enrichment of CaO (∼115%), Sr and Pb (〉300%) in the altered parts of the vein-wall-rock system; (3) redistribution of heavy rare earth elements (HREE) from partly replaced rutile and recrystallized titanite in the blueschist–eclogite transition zone into newly grown garnet rims in the eclogitic selvage around the vein; (4) transformation of high Nb/Ta rutile into low Nb/Ta titanite which is associated with preferred mobilization of Nb over Ta; and (5) decoupling of Zr and Hf from Nb and Ta; the latter are depleted by ∼30% relative to the unaltered blueschist host rock whereas the former are depleted by only ∼10%. The prerequisite for the transformation of Ca-poor blueschist (6–7 wt.% CaO) into Ca-rich eclogite (up to 13 wt.% CaO) was the infiltration of a Ca-rich fluid. The release of trace elements can be attributed to partitioning of these elements into the passing fluid phase during dissolution–reprecipitation processes in the course of eclogitization. The reactivity of the precursor mineral assemblage and the chemical gradients between the reacting and passing fluid of the conduit are mainly responsible for trace-element mobilization in the studied samples. The suite of trace elements released upon fluid-induced eclogitization of the reactive wall-rock resembles that in island arc magmas showing strong enrichment of LIL elements, strong depletions in HFS elements and intermediate concentrations of REE.
    Keywords: Geology
    ISSN: 0016-7037
    E-ISSN: 1872-9533
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  • 10
    Language: English
    In: Contributions to Mineralogy and Petrology, 2011, Vol.161(1), pp.35-45
    Description: During the evolution of the Earth, distinct geochemical reservoirs with different Nb/Ta ratios have developed. Archean granitoids of the tonalite–trondhjemite–granodiorite (TTG) suite, which represent the Earth’s early continental crust, show larger Nb/Ta variations than any other Earth reservoir. This implies that significant Nb–Ta fractionation must have occurred during early crust formation, while the underlying mechanism behind is still unclear. Here, we present a new model on how Nb may be fractionated from Ta during partial melting of subducted oceanic crust. Our data show that Nb/Ta ratios in melts derived from rutile- and titanite-bearing eclogite are largely controlled by the modal relative abundances of rutile and titanite in the source. High modal ratios of titanite over rutile generate melts with very high Nb/Ta (〉60), whereas low modal titanite/rutile produces melts with much lower Nb/Ta (≤30). Very low Nb/Ta (〈16) occur when all Ti-phases are consumed at very high degrees of melting. As the modal ratio of titanite to rutile is a function of pressure, the Nb/Ta of melts is a function of melting depth. Our new model helps to explain the extreme variation of Nb/Ta observed in many TTGs and thus how Nb and Ta were fractionated during the early evolution of the Earth. Furthermore, the model also indicates that simple one-stage melting models for mafic crust are not sufficient to explain the formation of TTGs.
    Keywords: Crustal evolution ; Nb–Ta ; Partial melting ; Rutile ; Titanite ; Subduction zone ; Eclogite
    ISSN: 0010-7999
    E-ISSN: 1432-0967
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