The phosphates of IIIAB iron meteorites

Thirteen phosphate minerals are found in IIIAB iron meteorites. Four of the se (sarcopside, graftonite, johnsomervilleite, and galileiite) constitute t he majority of occurrences. The IIIB iron meteorites are confined to occurr ences of only these four phosphates. The IIIA iron meteorites may contain o ne or more of these four phases; they may also contain other rarer phosphat es, and silica tin two instances) and a silicate rock tin one instance). Th us, the IIIA lithophile chemistry is more varied than that of the IIIB mete orites. Based on petrographic relations, sarcopside appears to be the first phosphate to form. Graftonite is probably formed by recrystallization of s arcopside. Johnsomervilleite and galileiite exsolved as enclaves in sarcops ide or graftonite at lower temperatures, although some of these also nuclea ted as separate crystals. The IIIAB phosphates are carriers of a group of i ncompatible lithophile elements: Fe, Mn, Na, Ca, and K, and, rarely, Mg as well as Pb. These elements land O) were concentrated in a residual, S-rich liquid during igneous fractional crystallization of the IIIAB core mass. Th e phosphates formed by oxidation of P as the core solidified and excluded O , which increased its partial pressure in the residual liquid. The trace si derophile trends in bulk IIIAB metal are paralleled by a mineralogical tren d of the phosphate minerals that formed. For IIIAB meteorites with low-Ir c ontents in the metal, the phosphates are mainly Fe-Mn phases; at intermedia te Ir values, more Na-bearing phosphates appear; at the highest Ir values, the rarer Na-, K-, Mg-, Cr-, and Pb-bearing phosphates appear. The absence of significant amounts of Mg, Si, Al, and Ti suggest depletion of these ele ments in the core by the overlying mantle.