Parent body thermal metamorphism of enstatite chondrites: Disentangling the effects of shock melting

Abstract

Enstatite chondrites (ECs) formed on at least two parent bodies, EH and EL. After the accretion of the EC parent bodies, EC material was subjected to varying degrees of parent body thermal metamorphism (measured by petrologic types 3–6), due to heat released by radioactive isotope decay. Current schemes to determine the petrologic type of the ECs are qualitative and ambiguous, and many studies have included known or misclassified shock-melted ECs, which altogether have led to inconsistent classifications. In this study, we attempt to distinguish shock-melted ECs from other ECs so that we can assess the effects of thermal metamorphism alone. We identified a suite of geochemical parameters that allow us to classify rapidly cooled, quenched shock-melt ECs, including high-Fe (Mg,Mn,Fe)S monosulfide, high-Cr troilite, and high-Ni kamacite. We then screened out shock-melted samples. This then allowed us to establish a quantitative scheme to determine the petrologic type of an EC. This classification scheme is based on the petrography and geochemistry of glass, silicate minerals, sulfides, and metal. Specifically, for EH chondrites (which are similar but distinct from the EL group), among other parameters, the size and abundance of feldspar progressively increase from EH3 to EH6 (<13 μm, <8.5% modal% to >13 μm, 11.5 modal%), while the FeO content of enstatite changes from types 3–4 to types 5–6 (<0.45 wt% to >0.45 wt%). Additionally, we build on the work of others to propose a scheme that subdivides the EH3. Using the average Cr2O3 content of olivine, we divide the EH3 and EH4 chondrites into EH3Low (mean Cr2O3 > 0.25 wt%) and EH3High-EH4 subtypes (Cr2O3 < 0.25 wt%).