MINERALOGY OF ANTARCTIC UREILITES AND A WORKING HYPOTHESIS FOR THEIR ORIGIN AND EVOLUTION

Abstract

Mineralogical and crystallographic data and electron microprobe analyses of silicate minerals for Antarctic ureilites Y74123, Y74130, Y74659, Y790981, ALHA77257, ALHA78019 and ALH82106 extend the chemical and textural variations among ureilites beyond those known for non-Antarctic ureilites. Low-Ca pyroxenes, possibly orthopyroxene, poikilitically enclosed in olivine and coexisting with Mg-rich pigeonites, have been identified in Y74659 and ALHA77257. Coexisting low-Ca pyroxene, pigeonite, and augite, together with the very Fe-rich olivine in ureilites were observed in Y74130. Almost Fe-free pigeonite and diopside coexist in ALH82106. No systematic differentiation trend has been reconstructed from these data, but the pyroxene pairs yield formation temperatures above 1200°C. A weak anticorrelation between MnO and FeO delineated by these pyroxenes suggests that a reduction reaction is responsible for the chemical variations. A working hypothesis for ureilite formation involves high-temperature episoded and a planetesimal-scale collision, which might have facilitated compaction and recrystallization of mafic silicates. During the episodes, FeNiS eutectic melt and a partial melt rich in Ca, Al and Fe might have been lost from a carbonaceous-chondrite-like source material.