Restudy of pyroxene-pyroxene equilibration temperatures for ordinary chrondrite meteorites

Abstract

Electron microprobe analyses of orthopyroxene and Ca-clinopyroxene in 21 ordinary petrographic type 6 chondrites (7 H-, 8 L-, and 6 LL-group chondrites) result in differentK D (distribution coefficient) values for H-, L-, and LL-group chondrites, which suggest different equilibration temperatures for each group. If we consider the Blander model (Blander 1972), the differences in Fe-Mg distributions for these groups reflect only their different Wo (CaSiO3) contents, and are not due to differences in equalibration temperature. Ca-clinopyroxene and orthopyroxene in contact with each other give a lower analytical spread and more meaningful results than those analyzed at random. Olivine and orthopyroxene in nearly all analyzed chondrites are homogenous within and between grains; Ca-clinopyroxene shows heterogenous compositions in petrographic types 4 and 5, but is homogenous in type 6. Ca-clinopyroxene in disequilibrated chondrites contains unusual amounts of minor elements Al, Ti, and Cr, a feature that can be used to denote meteorites of this type. Equilibration temperatures for ordinary chondrites, based on the pyroxene-pyroxene geothermometer are not known, although a range of 750–950° C appears to be a fair estimate. The exact equilibration temperatures can be determined only after this geothermometer is carefully calibrated experimentally or empirically, with due consideration given to Wo content.