Chromium-silicates, Feldspars and Highly Silicic Glasses Formed from Felsic Melts in Post-depressurization Ferroan Ureilites, Especially

Depressurization is the dominant process that produces mantle melts on Earth. Depressurization had equally profound but quite different effects in the evolution of ureilite meteorites. The second most abundant type of achondrite, ureilites formed as anatectic restites (olivine + pyroxene + minor, 0.2-6 wt%, carbon) in the mantle(s) of carbonaceous asteroid(s). In ureilites, de-pressurization was probably initially caused by impact disruption of the asteroid(s), and involved only a few tens of bars. Despite a simultaneous abrupt transition from the anatectic thermal regime into one of rapid cooling, depressurization triggered a series of chemical transformations, starting with oxidation of the carbon, the oxygen coming from reduction of olivine (rims only) and minor melt components [1]. In some of the most FeO-rich ureilites, post-depressurization redox (DR) had consequences beyond the usual olivine-rim effects. Warren [2] described evidence for reduction of Cr 2 O 3 from Cr-spinel (cm) into CrO-rich silicates in NWA 776. In this work, we describe the corresponding but more diverse suite of DR phases in LEW 88774, and discuss the origin of these materials, including the highly silicic (typically ~70 wt% SiO 2) glasses that are found in contact with almost all reduced ureilitic Cr-spinels. LEW 88774 is by far the most cm-rich ureilite, 3-6 vol% [3-5]. Early work [3,4] documented the presence of Cr-rich sulfides and carbides, (Fe,Cr) 2 C with Fe/Cr wt. ratio generally ~2, clustered near the cm's. The long, thin, arcs of CrO-rich (up to 40 wt%) silicates that occur shortly beyond the rims of LEW 88774 cm's are very similar to the corresponding materials in NWA 776; and like the NWA 776 Cr-silicate arcs [6], these LEW 88774 arc materials were in 2001 misinterpreted as uvarovitic (Cr 3+-rich) garnet [7]. These Cr-silicate arcs occur as growths " in " toward cm from semi-adjacent olivine; not fully adjacent because there is a layer of silicic glass between arc and cm. It now appears unlikely that the Cr-silicate arc phase was ever a garnet of any kind. The hypothesis that this material is (or formerly was) garnet fails the test of stoichiomet-ric charge balance, i.e., the Cr cannot be mainly Cr 3+. A small minority of the Cr-silicate arcs in LEW 88774, occurring as CrO-rich (up to 28 wt%) growths " in " toward cm from semi-adjacent low-Ca pyroxene, have much higher Si/O ratios. We concur with the interpretation [7] that these high-Si/O Cr-silicates originated as …