The History of Water and Other Associated Volatiles in Howardite-Eucrite-Diogenite Meteorites

Abstract

Recent in situ measurements of volatiles (H2O, Cl and F), particularly in lunar and martian samples, have changed our understanding of the volatile inventory of the inner Solar System. Whilst this has provided a wealth of data for these planetary bodies, currently little is known about the history of volatiles in other bodies in the inner Solar System. The howardite, eucrite, diogenite (HED) meteorites form the largest suite of rocks representing a differentiated basaltic asteroid (4 Vesta) and are some of the oldest igneous rocks in the Solar System. Understanding their volatile histories, therefore, can help us to constrain some of the processes that operated on Vesta early in Solar System history. Using the NanoSIMS 50L, six eucrites were analysed for their apatite H2O abundance and hydrogen isotopes, whilst seven eucrites (five of which were also investigated for H) were analysed for their apatite Cl content and isotopic composition. Apatite H2O abundances range from ~ 30 to ~ 3500 ppm and are associated with a weighted average δD value of - 34 ± 67 ‰. No systematic variations or correlations are observed in H2O abundance or δD value with eucrite geochemical trend or metamorphic grade. These results are comparable to published literature and confirm the striking homogeneity in the H-isotopic composition of water in eucrites, and are consistent with a common source of water in the inner Solar System. Chlorine abundance in apatite ranges from ~ 25 to 4900 ppm and the δ37Cl values range from – 3.98 to + 35.6 ‰. Samples with lower H2O content typically were enriched in δ37Cl, however, no obvious correlation between δ37Cl and δD values is seen. Cl isotope fractionation is likely to have occurred for some samples via magmatic degassing of ZnCl2. The observed variation in Cl isotopes, particularly the residual eucrites, however, remains enigmatic.