Phosphates in pallasite meteorites as probes of mantle processes in small planetary bodies

Abstract

THE stony-iron pallasite meteorites, which are generally believed to come from the core–mantle boundaries of asteroids, contain small quantities of phosphate minerals. Here we use trace element analyses of these phosphates to investigate the magmatic history of the silicate portions of pallasites. In Eagle Station and seven other pallasites, the phosphates have relatively low concentrations of rare earth elements (REE) and are strongly enriched in heavy relative to light REE. These patterns are consistent with formation of phosphate by subsolidus reactions between metal and silicate, in which phosphate inherits the REE pattern of olivine. In Spring-water and Santa Rosalia, calcium-rich phosphates have higher concentrations of REE, are enriched in light relative to heavy REE and have negative europium anomalies. These patterns are consistent with crystallization of phosphate from a europium-depleted chondritic liquid. Tihis is unlikely to have happened near the base of the differentiating parent-body mantle, because phosphates are late-crystallizing phases; this suggests that some pallasites may come from regions of their parent bodies much nearer the surface than the core–mantle boundary.