CHOS gas/fluid‐induced reduction in ureilites

Abstract

Ureilite meteorites contain regions of localized olivine reduction to Fe metal widely accepted to have formed by redox reactions involving oxidation of graphite, a process known as secondary smelting. However, the possibility that other reductants might be responsible for this process has largely been ignored. Here, 17 ureilite samples are investigated to assess whether, instead of smelting involving only solid reactants, a CHOS gas/fluid could have caused much of the smelting. Features consistent with gas‐ or supercritical fluid‐driven reduction were found to be abundant in all ureilites, such as fracture‐focused smelting, plume‐like reaction fronts, and addition of sulfur. Many of these are developed away from graphite. In some ureilites, it is clear that the redox process coincided with annealing, and we suggest that this was caused by enhanced diffusion facilitated by a higher density gas or fluid, rather than slow cooling, which requires elevated pressure. The C‐CO and CH4‐C‐H2O buffers were modeled to examine their relative potential to drive reduction. This modeling showed that a CH4‐rich fluid is able to produce the observed mineral compositions at elevated pressures. This result, coupled with the observed textures, is used to develop a likely series of reactions. We suggest that at higher pressures, a H2‐CH4‐H2S‐S2‐bearing fluid‐like phase, and at lower pressures, an equivalent gas, were able to infiltrate grain boundaries and fine fractures. Sulfidation to form troilite may have acted to maintain highly reduced gas/fluid conditions. The presence of hydrocarbons in ureilites supports a role for reduction driven by CHOS gas/fluid.

Sequential Lonsdaleite to Diamond Formation in Ureilite Meteorites via In Situ Chemical Fluid/Vapor Deposition

“…S10) that straddles the H-CO mixing line (45), so mixtures of H 2 -CH 4 -CO are ideal. This process is similar to a series of reactions we suggested for gas/fluid-driven smelting in ureilites, where infiltrating H 2 , CH 4 , S 2 , and H 2 S drove reduction of olivine and pyroxene to Fe metal and FeS (2). Smelting thus created a spatially variable mixture of CH 4 , H 2 , H 2 S, CO, and H 2 O, produced by redox reactions either side of the fO 2 required for carbon deposition.…”

“…diamond j lonsdaleite j ureilite j meteorite j chemical vapor deposition Ureilites are primitive achondrite meteorites that are residues of fractional melt extraction from deep within the ureilite parent body (UPB) (1). Because the UPB underwent extensive melting and had a diameter that may have been >530 km (2), it was likely a dwarf planet (massive enough to form a spheroid under hydrostatic equilibrium; for comparison, the diameter of the impact-modified spheroid Vesta is 525 km), and these meteorites are therefore our only large suite of samples from the mantle of such a body.…”

“…Are the ureilites really from such a large body? If not, and many assume the UPB was closer to 500 km diameter (2,28,30), there is currently no satisfactory explanation for diamond and lonsdaleite formation in ureilites.…”

“…These textures developed around graphite (39) and along fractures and grain boundaries. The predominant development of these textures along fracture arrays and grain boundaries (2,33,40), and the extensive addition of sulfur, are clear indications that gas/fluid addition drove much of the reduction (2). We recently showed that the gas/fluid responsible contained H 2 , CH 4 , S 2 , and H 2 S, with O derived from oxidation of silicates during smelting, creating a locally varying mix of gas/ fluid molecules in the system C-H-O-S (2).…”

“…Smelting thus created a spatially variable mixture of CH 4 , H 2 , H 2 S, CO, and H 2 O, produced by redox reactions either side of the fO 2 required for carbon deposition. Maintenance of low XH 2 O conditions during cooling in the UPB is implied by the lack of hydrous silicates in ureilites and the presence of aromatic hydrocarbons among graphite and diamond (16,46,47), which likely evolved via reaction between the C-H-O-S fluid/gas and graphite (2). Within that spatially varying mix, there would be domains that fall within the diamond stability window of SI Appendix, Fig.…”