A unique omphacite, amphibole, and graphite-bearing clast in Queen Alexandra Range (QUE) 99177: A metamorphosed xenolith in a pristine CR3 chondrite

A metamorphosed, unshocked, chondritic clast, containing approximately equal volumes of matrix and chondrules, has been identified in the primitive CR3 chondrite QUE 99177. The most striking features of this clast are the presence of ferroan omphacitic pyroxenes, amphiboles (pargasite and pargasitic hornblende) and coarse-grained (up to 70 μm) graphite laths. The clast also contains relatively equilibrated FeO-rich olivine, low-Ca pyroxene, and sulfides (pentlandite and troilite). The clast's matrix contains coarse, interlocking crystals. The chondrules are extensively recrystallized, contain no mesostasis or metal, and present poorly-defined boundaries. Based on its compositional and crystallographic features, omphacite appears to have formed at high pressures (>6 GPa). Amphibole formation probably postdated graphite formation and occurred by reaction of olivine, clinopyroxene, and omphacite in the presence of an oxidizing, Na-rich fluid at high temperatures (>700 °C). The absence of metal suggests that this fluid was also rich in S.