Icosahedral quasicrystals (i-phases) in the Al–Cu–Fe system are of great interest because of their perfect quasicrystalline structure and natural occurrences in the Khatyrka meteorite. The natural quasicrystal of composition Al 62 Cu 31 Fe 7 , referred to as i-phase II, is unique because it deviates significantly from the stability field of i-phase and has not been synthesized in a laboratory setting to date. Synthetic i-phases formed in shock-recovery experiments present a novel strategy for exploring the stability of new quasicrystal compositions and prove the impact origin of natural quasicrystals. In this study, an Al–Cu–W graded density impactor (GDI, originally manufactured as a ramp-generating impactor but here used as a target) disk was shocked to sample a full range of Al/Cu starting ratios in an Fe-bearing 304 stainless-steel target chamber. In a strongly deformed region of the recovered sample, reactions between the GDI and the steel produced an assemblage of co-existing Al 61.5 Cu 30.3 Fe 6.8 Cr 1.4 i-phase II + stolperite (β, AlCu) + khatyrkite (θ, Al 2 Cu), an exact match to the natural i-phase II assemblage in the meteorite. In a second experiment, the continuous interface between the GDI and steel formed another more Fe-rich quinary i-phase (Al 68.6 Fe 14.5 Cu 11.2 Cr 4 Ni 1.8 ), together with stolperite and hollisterite (λ, Al 13 Fe 4 ), which is the expected assemblage at phase equilibrium. This study is the first laboratory reproduction of i-phase II with its natural assemblage. It suggests that the field of thermodynamically stable icosahedrite (Al 63 Cu 24 Fe 13 ) could separate into two disconnected fields under shock pressure above 20 GPa, leading to the co-existence of Fe-rich and Fe-poor i-phases like the case in Khatyrka. In light of this, shock-recovery experiments do indeed offer an efficient method of constraining the impact conditions recorded by quasicrystal-bearing meteorite, and exploring formation conditions and mechanisms leading to quasicrystals.
more »
« less
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
More Like this
-
-
more » « less
Abstract The sulfur isotope budget of Martian regolith breccia (NWA 7533) has been addressed from conventional fluorination bulk rock analyses and ion microprobe in situ analyses. The bulk rock analyses yield 865 ± 50 ppm S in agreement with LA‐ICP‐MS analyses. These new data support previous estimates of 80% S loss resulting from terrestrial weathering of NWA 7533 pyrite. Pyrite is by far the major S host. Apatite and Fe oxyhydroxides are negligible S carriers, as are the few tiny igneous pyrrhotite–pentlandite sulfide grains included in lithic clasts so far identified. A small nonzero Δ33S (−0.029 ± 0.010‰) signal is clearly resolved at the 2σ level in the bulk rock analyses, coupled with negative CDT‐normalized δ34S (−2.54 ± 0.10‰), and near‐zero Δ36S (0.002 ± 0.09‰). In situ analyses also yield negative Δ33S values (−0.05 to −0.30‰) with only a few positive Δ33S up to +0.38‰. The slight discrepancy compared to the bulk rock results is attributed to a possible sampling bias. The occurrence of mass‐independent fractionation (MIF) supports a model of NWA 7533 pyrite formation from surface sulfur that experienced photochemical reaction(s). The driving force that recycled crustal S in NWA 7533 lithologies—magmatic intrusions or impact‐induced heat—is presently unclear. However, in situ analyses also gave negative δ34S values (+1 to −5.8‰). Such negative values in the hydrothermal setting of NWA 7533 are reflective of hydrothermal sulfides precipitated from H2S/HS‐aqueous fluid produced via open‐system thermochemical reduction of sulfates at high temperatures (>300 °C).
-
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
-
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.more » « less
-
more » « less
Summary Chemotrophic microorganisms gain energy for cellular functions by catalyzing oxidation–reduction (redox) reactions that are out of equilibrium. Calculations of the Gibbs energy (
ΔG r ΔG r + 2H+ ⇌ 4S0+ 4H2O). We show that at elevated concentrations of sulfide and sulfate in acidic environments over a broad temperature range, this putative metabolism can be exergonic (ΔG r ⇌ + H2O) and to sulfite (H2S + 3 ⇌ 4+ 2H+), are only moderately exergonic or endergonic even at ideal geochemical conditions. Natural and impacted environments, including sulfidic karst systems, shallow‐sea hydrothermal vents, sites of acid mine drainage, and acid–sulfate crater lakes, may be ideal hunting grounds for finding microbial sulfur comproportionators.
