Petrology and chemistry of the Miles IIE iron. I: Description and petrology of twenty new silicate inclusions

A petrologic study of twenty new silicate inclusions (15 gabbroic, 5 cryptocrystalline) in the Miles IIE iron was undertaken in order to help clarify the petrogenesis of this meteorite and its relationship to other IIE irons with silicates, as well as H-group chondrites. Nine of the inclusions were analyzed by the INAA method, which is presented in a companion paper. The results indicate that Miles was formed by shock events on an H-group chondrite parent body. It formed silicate melt and Fe-Ni-S-P melt by about 25% partially melting, and left behind olivine-orthopyroxene residues. The silicate melt was a crystal mush which consisted of phenocrysts of high-Ca pyroxene, orthopyroxene and plagioclase, and residual melt. The phenocryst-enriched mush formed irregular-shaped gabbroic inclusions, which also contain minor olivine, inverted pigeonite, antiperthite, K-feldspar, tridymite, glass, chromite, rutile, ilmenite, armalcolite, chlorapatite, whitlockite, kamacite, taenite, schreibersite, troilite and pentlandite. The residual melt formed round- to ellipsoidal-shaped cryptocrystalline inclusions, which contain microphenocrysts of alkali feldspar or phenocrysts of pyroxene. The crystal mush was mixed with abundant Fe-Ni-S-P melt, from which most of the sulfur escaped, and from which some of the phosphorus reacted with the silicate melts, resulting in their reduction. This reduction produced Ca-poor magnesian orthopyroxene and Ca-poor plagioclase in the gabbroic inclusions. The degree of reduction was, however, more intense in the cryptocrystalline inclusions than in the gabbroic inclusions. The remaining phosphorus in the solid host metal exsolved as schreibersite around the silicate inclusions.