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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Variable Sulfur Release Predicted for Hot and Cold Subducting Slabs
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
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- Award ID(s):
- 1725301
- NSF-PAR ID:
- 10111065
- Date Published:
- Journal Name:
- Goldschmidt Abstracts
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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