The abundances of F, Cl, and H2O in eucrites: Implications for the origin of volatile depletion in the asteroid 4 Vesta

We conducted a petrologic study of apatite within eight unbrecciated, non-cumulate eucrites and two monomict, non-cumulate eucrites. These data were combined with previously published data to quantify the abundances of F, Cl, and H2O in the bulk silicate portion of asteroid 4 Vesta (BSV). Using a combination of apatite-based melt hygrometry/chlorometry and appropriately paired volatile/refractory element ratios, we determined that BSV has 3.0-7.2 ppm F, 0.39-1.8 ppm Cl, and 3.6-22 ppm H2O. The abundances of F and H2O are depleted in BSV relative to CI chondrites to a similar degree as F and H2O in the bulk silicate portion of the Moon. This degree of volatile depletion in BSV is similar to what has been determined previously for many moderately volatile elements in 4 Vesta (e.g., Na, K, Zn, Rb, Cs, and Pb). In contrast, Cl is depleted in 4 Vesta by a greater degree than what is recorded in samples from Earth or the Moon. Based on the Clisotopic compositions of eucrites and the bulk rock Cl-/F ratios determined in this study, the eucrites likely formed through serial magmatism of a mantle with heterogeneous delta Cl-37 and Cl/F, not as extracts from a partially crystallized global magma ocean. Furthermore, the volatile depletion and Cl-isotopic heterogeneity recorded in eucrites is likely inherited, at least in part, from the precursor materials that accreted to form 4 Vesta and is unlikely to have resulted solely from degassing of a global magma ocean, magmatic degassing of eucrite melts, and/or volatile loss during thermal metamorphism. Although our results can be reconciled with the past presence of wide-scale melting on 4 Vesta (i.e., a partial magma ocean), any future models for eucrite petrogenesis involving a global magma ocean would need to account for the preservation of a heterogeneous eucrite source with respect to Cl/F ratios and Cl isotopes. Published by Elsevier Ltd.

Automated summary

Through petrologic study and data analysis of apatite in eucrites, this research quantified the abundances of F, Cl, and H2O in the bulk silicate portion of asteroid 4 Vesta. The study revealed a depletion of F and H2O in BSV compared to CI chondrites, while Cl depletion in 4 Vesta was found to be greater than in samples from Earth or the Moon. Eucrites are likely formed through serial magmatism from a mantle with heterogeneous delta Cl-37 and Cl/F, and the volatile depletion and Cl-isotopic heterogeneity recorded in eucrites may be inherited from precursor materials that accreted to form 4 Vesta.