Carbon in howardite, eucrite and diogenite basaltic achondrites

Abstract

Abstract—The C contents and isotopic compositions of four eucrites, four diogenites and two howardites have been determined. Stepped heating in an O atmosphere was employed to convert selectively different carbonaceous materials to CO2gas at various temperatures. This technique successfully distinguishes between terrestrial contaminants and indigenous C. With the exception of the Kapoeta howardite, the howardite, eucrite, and diogenite (HED) meteorites contain ∼10–30 ppm indigenous C with δ13C between −29% and −19%. Kapoeta (a regolith breccia) has an elevated C content and δ13C, due to the presence of13C‐enriched carbonate minerals (δ13C ∼ +20%) in CM2‐ or CR2‐like clasts. The range in δ13C displayed by HED samples is similar to that of other solar system basalts, such as lunar rocks and Martian meteorites but distinctly different from that of the terrestrial mantle.The diogenites have a slightly lower total C yield and higher δ13C than the eucrites, which is a result of degassing of trapped CO/CC2/CO2–3from the silicate lattice during metamorphism or annealing. However, three out of the four diogenites studied appear to contain a discrete component, possibly of graphitic C coating silicate grains, that is seemingly unaffected by the extended annealing period experienced by the diogenites. It is possible that this component might host the indigenous primitive Xe recently identified in diogenites.