EQUILIBRATION TEMPERATURES OF EL CHONDRITES - A MAJOR DOWNWARD REVISION IN THE FERROSILITE CONTENTS OF ENSTATITE

A study of enstatite grains separated from EL6 chondrites shows ferros ilite contents to be approximately 0.02-0.04 mol%, about an order of m agnitude lower than reported in the literature. The higher values repo rted earlier can probably be attributed to the excitation of Fe atoms in Fe-Ni and FeS, in part from mum-sized blebs of FeS and Fe-Ni common ly present in enstatite grains but mainly from large neighboring opaqu e grains excited by bremsstrahlung or by doubly backscattered electron s. Our EL6 ferrosilite contents are similar to those observed in coars e enstatite grains in some aubrites. The equilibrium between ferrosili te and Si-bearing kamacite is highly temperature dependent. At constan t Si concentration in kamacite, the equilibrium ferrosilite mole fract ion drops by a factor of roughly 2.5 for each 100 K decrease in the eq uilibrium temperature. As a result, the compositions of these two most common EL6 phases can be used to estimate metamorphic equilibration t emperatures. The inferred temperatures are near 1200 K.