The phosphates of IIIAB iron meteorites

Abstract

Thirteen phosphate minerals are found in IIIAB iron meteorites. Four of these (sarcopside, graftonite, johnsomervilleite, and galileiite) constitute the majority of occurrences. The IIIB iron meteorites are confined to occurrences of only these four phosphates. The IIIA iron meteorites may contain one or more of these four phases; they may also contain other rarer phosphates, and silica (in two instances) and a silicate rock (in one instance). Thus, the IIIA lithophile chemistry is more varied than that of the IIIB meteorites. Based on petrographic relations, sarcopside appears to be the first phosphate to form. Graftonite is probably formed by recrystallization of sarcopside. Johnsomervilleite and galileiite exsolved as enclaves in sarcopside or graftonite at lower temperatures, although some of these also nucleated as separate crystals. The IIIAB phosphates are carriers of a group of incompatible lithophile elements: Fe, Mn, Na, Ca, and K, and, rarely, Mg as well as Pb. These elements (and O) were concentrated in a residual, S-rich liquid during igneous fractional crystallization of the IIIAB core mass. The phosphates formed by oxidation of P as the core solidified and excluded O, which increased its partial pressure in the residual liquid. The trace siderophile trends in bulk IIIAB metal are paralleled by a mineralogical trend of the phosphate minerals that formed. For IIIAB meteorites with low-Ir contents in the metal, the phosphates are mainly Fe-Mn phases; at intermediate 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 elements in the core by the overlying mantle.