Sulfur is a key element in terrestrial magmatic processes yet its geochemical behavior remains one of the most difficult to
model due to its heterovalent chemistry. The maximum amount of sulfur a silicate melt can dissolve before saturating with
sulfide (e.g., pyrrhotite) or sulfate (e.g., anhydrite) changes with the redox state of the system and has important implications
for the sulfur budget of a magmatic system. Several empirical models have been developed to predict the sulfur content of a
silicate melt at either sulfide (under reducing conditions) or sulfate (under oxidizing conditions) saturation, but only one model
existed that systematically assessed how the sulfur content of a basaltic melt changes as a function of oxygen fugacity (fO2)
across the transition from sulfide- to sulfate-dominated conditions. The applicability of that model to intermediate and felsic
melts rests on the assumption that changes in melt composition do not affect how sulfide or sulfate dissolves in the melt. Here,
we report new experimental data that constrain the sulfur concentration at sulfide saturation (SCSS) and the sulfur concentration
at anhydrite saturation (SCAS) in a dacitic melt as a function of fO2. The experiments were conducted using a H2Osaturated
natural dacitic melt at 1000 C, 300 MPa, and at log fO2 varying over four orders of magnitude encompassing the
sulfide-sulfate transition (log fO2 = DFMQ 0.7, DFMQ+0, DFMQ+0.5, DFMQ+1, DFMQ+1.48, DFMQ+1.54, DFMQ
+1.75, DFMQ+2.08 and DFMQ+3.3). New SCSS and SCAS data and modeling for dacitic melts reveals that the sulfidesulfate
transition occurs at DFMQ+1.81 ± 0.56, defined by the following equations to predict the sulfur content of intermediate
to evolved silicate melts as a function of fO2:
SCSSdacitic = [S2 ] (1 + 10(2.00DFMQ – 3.05))
SCASdacitic = [S6+] (1 + e(1.26 – 2.00DFMQ))
The results presented here demonstrate that the basaltic-derived SCSS-SCAS model is not appropriate for dacitic melts and that the
sulfide-sulfate transition is shifted to higher fO2 in more evolved silicate melts. Implications include the stability of sulfides to higher
fO2 in more evolved silicate melts and the potential for a narrower transition from a sulfide- to a sulfate-dominated melt than that
predicted by thermodynamics.
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Shock experiments on basalt—Ferric sulfate mixes and their possible relevance to the sulfide bleb clusters in large impact melts in shergottites
Large impact‐melt pockets in shergottites contain both Martian regolith components and sulfide/sulfite bleb clusters that yield high sulfur concentrations locally compared to bulk shergottites. The regolith may be the source of excess sulfur in the shergottite melt pockets. To explore whether shock and release of secondary Fe‐sulfates trapped in host rock voids is a plausible mechanism to generate the shergottite sulfur bleb clusters, we carried out shock recovery experiments on an analog mixture of ferric sulfate and Columbia River basalt at peak pressures of 21 and 31 GPa. The recovered products from the 31 GPa experiment show mixtures of Fe‐sulfide and Fe‐sulfite blebs similar to the sulfur‐rich bleb clusters found in shergottite impact melts. The 21 GPa experiment did not yield such blebs. The collapse of porosity and local high‐strain shear heating in the 31 GPa experiment presumably created high‐temperature hotspots (~2000 °C) sufficient to reduce Fe3+to Fe2+and to decompose sulfate to sulfite, followed by concomitant reduction to sulfide during pressure release. Our results suggest that similar processes might have transpired during shock production of sulfur‐rich bleb clusters in shergottite impact melts. It is possible that very small CO presence in our experiments could have catalyzed the reduction process. We plan to repeat the experiments without CO.
more » « less- NSF-PAR ID:
- 10361154
- Publisher / Repository:
- Wiley-Blackwell
- Date Published:
- Journal Name:
- Meteoritics & Planetary Science
- Volume:
- 56
- Issue:
- 12
- ISSN:
- 1086-9379
- Page Range / eLocation ID:
- p. 2250-2264
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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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.more » « less
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