Alkali-feldspathic material entrained in Fe,S-rich veins in a monomict ureilite

Abstract

The Elephant Moraine (EET) 96001 ureilite contains a remarkable diversity of feldspars, which occur as tiny (no more than 60 micrometers maximum dimension) grains within a few Fe,S-rich (now weathered to mostly Fe oxide) veins. Molar S:Fe ratio in the veins averages 0.08 0.02. The veins meander and feature large fluctuations in apparent width; they appear to have entered this monomict breccia by a gentle, percolative process, not by violent impact injection. The feldspars are accompanied by a diverse suite of K-rich (and generally also Ti-rich) feldspathic glasses, and also major proportions of silica and pyroxene, which is largely fassaitic. A rhnite-like phase is also found, and, as inclusions in one of the fassaites, a Cr-poor spinel-like phase. The feldspars mostly feature remarkably high K/Na compared to feldspars of comparable An from polymict ureilites. The EET 96001 feldspathic component was probably once part of a thin basaltic crust on a ureilite asteroid. The spinel included in one of the fassaites formed at remarkably high fO2 (apparent oxidation state of iron: 41 atom% Fe3+), suggesting that the parent magma possibly assimilated near-surface water (however, the Fe3+was not directly measured, and has conceivably been affected by terrestrial weathering; also, there is no assurance that this fassaite originated together with the typical feldspar). We speculate that the feldspathic component was mixed into the dense, Fe,S-rich vein material, and very soon thereafter the Fe,S-rich vein material was emplaced adjacent to the EET 96001 host ureilite, at an advanced stage in a chaotic catastrophic disruption and partial reassembly process that affected all ureilites. The high-K nature of the EET 96001 feldspathic component, including the feldspathic glasses, suggests that fractional fusion may not have been as common during ureilite anatexis as has been inferred from recent studies of clast assemblages in polymict ureilites.