Chemical studies of enstatite chondrites

Abstract

The unique mineral and chemical composition of enstatite chondrites has been difficult to explain. Contrary to the conclusions of other workers, there is no evidence for a significant chemical difference between EH and EL chondrites, except in bulk S. Analyzed splits of the enstatite chondrites vary widely in major element composition, largely as a result of analyzing inadequate sample sizes. The phases in enstatite chondrites most likely to reflect conditions in the solar nebula include enstatite, forsterite, diopside, silica polymorph, albite, metal, troilite, oldhamite, niningerite, schreibersite, minerals A and B, caswellsilverite, and djerfisherite. There is evidence that the enstatite chondrites did not form in complete equilibrium. The Na-Cr sulfides known as minerals A and B contain oxygen, but contain little hydrogen, indicating that these sulfides did not necessarily form under hydrous conditions. The mineralogy and modal composition of type 3 enstatite chondrites are best matched by thermodynamic models that involve two chemical fractionations from a gas of solar or cosmic composition. The first fractionation entailed the removal from the enstatite chondrite formation location of solids in equilibrium with a solar composition gas at ∼1270K. The second fractionation involved the subsequent removal of water vapor from the enstatite chondrite formation location. These fractionations can be understood if the enstatite chondrites formed in a water-depleted gas from which solids at 1270K had been previously removed, sunward of a water condensation front in the solar nebula. Enstatite chondrites largely equilibrated in the nebula at ∼925K, although sulfidation of metal occurred to temperatures as low as ∼675K.