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Here we present the first thermodynamic models that predict the full range of possible S-liberating reactions during the subduction of mafic oceanic crust. Models for MORB and AOC were created in Perple_X utilizing the combined thermodynamic databases of [1] and [2]. Transitions from pyrrhotite to pyrite and pyrite to anhydrite are observed with increasing P-T. The pressure of the pyrite-anhydrite transition depends on initial Fe3+/6Fe: 3.2 GPa for MORB (Fe3+/6Fe = 0.16) and 2.3 GPa for AOC (Fe3+/6Fe = 0.28) at 650 °C. Reactions were monitored along slab-top geotherms for Honshu and Cascadia (D80, [3]). Above the pyrite-anhydrite transition HSO4-, SO42-, and HSO3- are the dominant fluid species (<0.9 mol/kg), whereas HS- is dominant in the sulfide fields (<0.1 mol/kg). Along the Honshu path, oxidized Sspecies increase from 0.05 to 0.4 mol/kg over 550 to 625 °C (82-84 km depth), concurrent with an increase of 70 % in the total fluid volume due to reactions such as lawsonite-out. Sulfur oxidation is balanced by the reduction of Fe3+, with a 48 % decrease in Fe3+/6Fe. In contrast, oxidized S-species increase from 0.0 to 0.4 mol/kg over 600 to 675 °C (68-77 km depth) along the Cascadia path, concurrent with a 15 % increase in total fluid volume. Nearly 85 % of the total fluid released along the Cascadia path occurs where HS- dominates and S concentrations in fluid are low. Our data suggest that slab-derived S-bearing fluids are a viable mechanism for oxidation of arc magmas. Coeval sulfur and water loss along cold P-T paths are expected to result in high fluxes of oxidized sulfur to volcanic arcs, whereas significant dehydration prior to sulfur oxidation will result in low sulfur fluxes along hot P-T paths. This discrepancy is expected to be accentuated by the less oxidized nature of younger oceanic crust at hot subduction zones. [1] Holland & Powell (2011) JMG [2] Servjensky et al. (2014) GCA [3] Syracuse et al. (2010) PEPI 

Examination of a global suite of eclogite-facies metabasites and metasediments suggests that eclogites tend to exhibit reduced mineral assemblages relative to their protoliths. High-pressure rocks tend to lack sulfides and Fe3+-bearing oxides in the eclogite facies. We suggest that eclogite-facies mineral assemblages are consistent with prograde reactions that balance the oxidation of S2- or S- to S6+ by reducing Fe3+in silicates or oxides: (1)8Fe3+Si O (OH) +S2-=8Fe2+Si O +SO 2-+(H O) abc de42f The oxidation of one mole of S2-or S-is balanced by the reduction of 7 to 8 moles of Fe3+, and typical S concentrations in the oceanic crust are capable of fully reducing the entire Fe3+ budget of metabasites. As most eclogite facies rocks do not preserve peak metamorphic sulfides, petrographic evidence for prograde S oxidation reactions are cryptic; however, textures associated with sulfate reduction in response to influx of external fluids are common (reaction 1 in reverse). These reactions produce Fe3+-rich phases and are observed in both metasedimentary and metabasic rocks across a range of retrograde P-T paths (blueschist to granulite facies). For example, high-P calc- schists exhibit reaction textures that suggest the breakdown of garnet and white mica to produce pyrite + chalcopyrite + epidote + biotite + magnetite. Our thermodynamic models of aS2 and aO2 at subduction zone P-T conditions suggest assemblages of this type are indicative of aO2 0.7 to 4.5 log units above the quartz-fayalite-magnetite buffer. In rehydrated eclogites, pyrite is commonly associated with the breakdown of garnet + omphacite to amphibole + pyrite. Additionally, direct precipitation of sulfide from sulfate is observed in two samples: 1) The retrograde assemblage pyrite + ilmenite + gypsum occurs in one retrogressed metagabbroic eclogite, and 2) Coronas of secondary pyrite + barite + gypsum enclose early retrograde pyrite in a retrogressed garnet blueschist. In many eclogites, S- is reduced to S2- as pyrite is replaced by pyrrhotite, chalcopyrite, and mixed valence Co-Ni sulfides. These reactions are balanced by oxidation of divalent to trivalent Fe-Co-Ni. Reactions of this type are consistent with increasing aS2 during retrograde metamorphism. Thus, ample evidence exists for oxidized S-bearing fluids released from subducting slabs. 

Sulfide breakdown during subduction releases oxidizing fluids that transport chalcophile and siderophile elements (CSE) such as Ni ,Co, and As. These fluids are reincorporated into high-pressure rocks such as eclogites during exhumation and rehydration along the slab-mantle interface. Evidence for these rehydration reactions takes the form of large sulfide (pyrite, pyrrhotite, chalcopyrite) grains (up to 5 mm) associated with hydrous Fe3+-bearing minerals. Here we present results of trace element determination by LA-ICP-MS coupled with mass balance calculations for sulfide-silicate reactions in rehydrated eclogites from the Mariánské Lázně Complex and Moldanubian Zone, Bohemian Massif, Czech Republic. One key texture observed in these rocks is the breakdown of garnet + omphacite in the presence of fluid to produce hornblende + diopside + plagioclase + pyrite. This rehydration reaction involves the oxidation of Fe2+ in garnet to Fe3+ in hornblende. In order to oxidize the iron from the garnet, we propose that sulfate is brought into the rock by an infiltrating fluid, where it is reduced to form pyrite, consistent with the observed textures. Trace element analyses reveal the Co distribution within rehydrated eclogite: Co is measurable in garnet (~50 μg/g), omphacite (~26 μg/g), hornblende (~80 μg/g), and pyrite (~5000 μg/g). Mass balance calculations suggest that of the total amount of Co present in the rehydration products, only ~35 % can be supplied by the breakdown of garnet and omphacite, leaving ~65 % of the Co to be supplied by another source. Average concentrations of Ni are: in garnet (1–4 μg/g), omphacite (~57 μg/g), hornblende (~90 μg/g), and pyrite (~2500 μg/g). Mass balance calculations suggest that of the total amount of Ni present in the rehydration products, ~70 % comes from the breakdown of garnet + omphacite, with the other 30 % supplied external to this reaction. Arsenic is not present in the silicate minerals, but is in the 10s of μg/g range in pyrite, and must be supplied externally to the rock, likely from a fluid. We conclude that the fluids released from subducting slabs carry sulfate and CSEs, which infiltrate the slab-mantle interface and eventually make their way into the sub-arc mantle, where they can be incorporated into the arc magmatic system. 

The large range in oxidation states of sulfur (-II to +VI) provides it with a large oxidation potential in rocks, even at relatively low concentrations. Most importantly, the transition from sulfide to sulfate species in rocks and silicate melts occurs in the same approximate fO2 region (for a given temperature) as the transition from ferrous to ferric iron, and reduced S species can coexist with oxidized Fe and vice versa. The result is a large potential for reactions involving sulfur to oxidize or reduce Fe in silicate minerals, since Fe only occurs in two oxidation states (+II and +III). In order for sulfur to be released during slab dehydration, sulfur in sulfide must be converted into an easily dissolved species, such as SO42− or H2S, through either oxidation or reduction. We propose that oxidation of sulfur in sulfide follows the generalized reaction: 8Fe3+SiaOb(OH)c +S2− = 8Fe2+SidOe +SO42− +(H2O)f (1) In this type of reaction, sulfur participates in the dehydration of greenschist- or blueschist-facies hydrous silicates during transition to the eclogite facies: ferric Fe in Fe-bearing silicates (chlorite, amphibole, epidote) is reduced to ferrous Fe in anhydrous ferromagnesian silicates (pyroxene, garnet). At the same time, the reaction consumes sulfide by oxidation of S2− to produce SO42−, which is readily dissolved in the fluid produced during dehydration. Additionally, a similar redox reaction could oxidize sulfur by reducing ferric Fe in oxides. It is important to note that one mole of S has the same redox potential as 8 moles of Fe. The molar ratio of 8 moles of Fe per 1 mole of S translates to a mass ratio of approximately 14; therefore, small concentrations of sulfur can have a large impact on reduction/oxidation of the silicate assemblage. Our observations show that sulfide minerals that can be identified as primary or related to the peak metamorphic stage are rare in eclogites and restricted to inclusions in garnet, consistent with reaction (1). Thermodynamic modeling is currently underway to assess the influence of sulfur on the phase equilibria of silicate phases during high pressure metamorphism. 

Despite their geochemical and economic importance, very little work has focused on the behavior of subducted chalcophile and siderophile elements (CSE). Here we present the first survey of these elements in metasediments, metabasites, and hybrid mélange rocks from exhumed terranes worldwide. These samples represent greenschist- to eclogite-facies conditions. EPMA X-ray maps display significant Co, Ni, and As zoning in pyrite; however, no zoning was observed in pyrrhotite or chalcopyrite. In situ LA- ICP-MS analyses of sulfides reveal Co, Ni, Cu, Zn, As, Pb, and Cr at concentrations above 10 μg/g, whereas Ga, Ge, Mo, Ag, Cd, In, Sn, Sb, Tl, and Bi are typically below 1 μg/g. Pyrite is enriched in Co, As, Zn, and Cr relative to pyrrhotite and chalcopyrite, whereas pyrrhotite contains abundant Ni. Pyrite is also enriched in Cu relative to pyrrhotite. Amphiboles and phyllosilicates were found to contain up to hunderds of μg/g of Ni, Cr, and Zn, and tens of μg/g of Co and Ga. In eclogites, Co in silicates mainly occurs in garnet, whereas Ni, Ga, and Zn occur in pyroxene. Both phases contain similar concentrations of Cr and Ge. Most silicates were found to have less than 1 μg/g of Cu; Cu in garnet was below detection, and As was below detection in all silicates. Contrasting behavior of Co and Ni is displayed in hybrid mélange samples. Transects of pyrite in chlorite schists show no correlation between these two elements, consistent with the hightened fluid mobility of Co over Ni observed in hydrothermal ore deposits. In one glacophane-omphacite rock, Co and Ni are anti-correlated. This behavior may be explained by alternation between fluid-buffered conditions, in which cobalt is mobile, and rock-buffered conditions, in which reactions with silicates release Ni. Matrix sulfides are absent in most eclogite-facies samples. Sulfide breakdown during subduction likely drives the release of As, Pb, and Cu into fluids that flux the overlying mantle, whereas both silicates and sulfides may contribute Co to these fluids. The elements Cr, Zn, Ga, and Ge likely persist into the eclogite facies, but may also be released during silicate dehydration. 

The sulfur isotope composition of volcanic rocks in arcs can be difficult to constrain because significant fractionation can occur during degassing. Mafic and ultramafic cumulates represent the least degassed part of the magmatic arc system, thereby offering an opportunity to investigate undegassed sulfur in arcs. Recent work on high pressure metamorphic rocks has suggested that subducted materials can retain their original isotopic composition to sub-arc depths. In particular, extreme negative δ34S values can be retained in subducted sediments. The purpose of this project is to investigate to what extent these deep subduction zone processes are reflected in the sulfur isotope signature of arc magmas. In the Lesser Antilles arc, there is a gradual decrease in terrigenous sediment being subducted from south to north. An estimated ~15% subducted sediment in the south and ~2% in the north is reflected in the chemical and isotopic composition of the Lesser Antilles arc magmas. Sulfides in these magma- derived cumulates record the earliest stages of magma evolution and are a more faithful monitor of the sulfur isotopic composition of the magma source region in the mantle than erupting lavas. We hypothesize that the decrease in terrigenous sediment being subducted from the south to north will be reflected in the S isotopes in cumulate samples. Samples of mafic and ultramafic cumulates have been collected from fourteen islands across the Lesser Antilles arc. Primary rock types are olivine gabbro, amphibole gabbro, plagioclase gabbro, and olivine gabbronorite. Sulfide minerals include pyrite, chalcopyrite, and pyrrhotite, and typically occur as spherical blebs. Sulfides are found primarily as inclusions in clinopyroxene, amphibole, olivine, and plagioclase. Sulfides occur less frequently as inclusions in magnetite and within the matrix. Analyses of sulfur isotopes in cumulate sulfides are currently underway. The decrease in the amount sediment being subducted from south to north in the Lesser Antilles arc should result in δ34S values that increase from south to north (more sediment subducted = more negative δ34S values).