The water and fluorine content of 4 Vesta

Adam R. Sarafian, Sune G. Nielsen, Horst R. Marschall, Glenn A. Gaetani, Kevin Righter, Eve L. Berger

Geochimica et Cosmochimica Acta


“The processes that controlled accretion of water and volatiles to the inner solar system remain enigmatic, because it is difficult to determine the absolute concentrations of volatile elements in planetary bodies. In this contribution we study rare unequilibrated eucrites derived from the asteroid 4 Vesta, to determine the water and fluorine content of this asteroid by measuring the volatile content of pyroxene. Common thermal metamorphism in most equilibrated eucrites would have diffusively reset magmatic volatile contents. The unequilibrated eucrites are, therefore, the most suitable samples to determine primary magmatic volatile contents of 4 Vesta. We find H2O and F contents in pyroxenes of 4–11 µg/g and 0.12–0.23 µg/g. We also determine a H2O partition coefficient for clinopyroxene and melt equilibrated at 0.1 MPa of DH2O = 0.1, which is higher than values previously reported for higher pressures. The higher compatibility of H2O in this experiment could partially be due to high OH/H2O ratio at the low total water contents in this experimental charge, but only further more detailed experiments will fully delineate the reasons for the more compatible behavior for water at lower pressures. However, given the lack of H2O partitioning data at low pressures we conclude that our 0.1 MPa experiment is the most appropriate to calculate magmatic water contents for melt in equilibrium with eucrite pyroxene. After using appropriate partition coefficients we calculate melt concentrations of 50–70 µg/g H2O and 1.5–2.4 µg/g F. In turn, these are converted into bulk 4 Vesta water and F contents of 10–70 µg/g H2O and 0.3–2 µg/g F by assuming eucrite formation via either mantle partial melting or extraction from a magma ocean. We also measure the D/H of the clinopyroxenes and show that these are identical to the results of previous studies that reported D/H in eucrite apatite. These values match those found in carbonaceous chondrites suggesting that water in 4 Vesta accreted from carbonaceous chondrites and not from cometary material.”