Petrology of the Asuka 881931 ureilite with special reference to crystallization of interstitial silicate melt

Abstract

The Asuka 881931 meteorite is an unbrecciated ferroan ureilite and consists mainly of equigranular olivine and pigeonite grains, a metal-sulfide network, interstitial silicates, and glass. Peripheral portions of equigranular olivine grains are often replaced by fine-grained forsterite-metal aggregates and sometimes by fine-grained enstatite-metal aggregates. These aggregates may have been produced from the equigranular olivine by reduction. Peripheral portions of equigranular pigeonite grains also are sometimes replaced by fine-grained orthopyroxene aggregates with tiny patches of Si-rich glass and may have been produced from the pigeonite by reduction reaction with silicate melt. Interstitial silicates are mainly orthopyroxene, magnesian pigeonite, high-Ca pyroxene (diopside/fassaite), and CaO-poor enstatite; and they crystallized from interstitial silicate melt. Interstitial glass is classified into two types-Si-poor and Si-rich. The Si-poor glass is always in contact with equigranular olivine, but the Si-rich glass never contacts equigranular olivine and is in contact with pyroxene and the metal-sulfide network. Both types of glass were produced from an original interstitial silicate melt, but the Si-poor glass formed mainly by fractional crystallization of pyroxenes, and the Si-rich glass may have formed by addition of Si mainly from nearby metal-sulfide melt, as well as crystallization of pyroxenes. The Si-poor and Si-rich melts were finally quenched as interstitial glasses under rapid cooling conditions.