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.
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CSEs in high pressure rocks from exhumed terranes
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.
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- Award ID(s):
- 1725301
- NSF-PAR ID:
- 10108910
- Date Published:
- Journal Name:
- Goldschmidt Abstracts
- Page Range / eLocation ID:
- 2671
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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