Thermal Constraints from Siderophile Trace Elements in Acapulcoite- Lodranite Metals

Introduction: A fundamental process in the formation of differentiated bodies is the segregation of metal-sulfide and silicate phases, leading to the formation of a metallic core. The only known direct record of this process is preserved in some primitive achondrites, such as the acapulcoite-lodranites. Meteorites of this clan are the products of thermal metamorphism of a chondritic parent. Most acapulcoites have experienced significant partial melting of the metal-sulfide system but not of silicates, while lodranites have experienced partial melting and melt extraction of both. The clan has experienced a continuum of temperatures relevant to the onset of metal mobility in asteroidal bodies and thus could yield insight into the earliest stages of core formation. Acapulcoite GRA 98028 contains relict chondrules, high modal sulfide/metal, has the lowest 2-pyroxene closure temperature, and represents the least metamorphosed state of the parent body among the samples examined. Comparison of the metal-sulfide component of other clan members to GRA 98028 can give an idea of the effects of metamorphism. Methods: We have analyzed trace elements in metallic phases in eight acapulcoite-lodranites from the U.S. Antarctic Meteorite Collection by LA-ICP-MS at the ICP-MS facility of the National High Magnetic Field Laboratory/Florida State University for major and trace siderophile elements using the methodology of [1] and obtaining similar analytical precision. Metal, sulfide, and silicate phases in these samples had been characterized previously by EPMA at NASA Johnson Space Center. Results: Siderophile element concentrations in acapulcoite-lodranite metals are generally higher than in bulk chondrite or H chondrite metals. Variability in the concentration of incompatible siderophile elements (ISE), those that partition most readily into the melt phase (e.g. As, Au, Pd), relative to compatible siderophile elements (CSE), those that partition most readily into the solid phase (e.g. Pt, Ir, Re, Os), is observed among metals of different samples. Samples with ISE/CSE both higher and lower than H chondrite were observed (Fig 1). Metal in all samples is predominantly kamacite but Ni-rich phases are also present, chiefly plessite, with average Ni content of metals from different samples ranging 6-11 wt%, and 14.5 wt% for sample MAC 88177. The representative metal composition of each sample was taken to be the average of several analyses. Discussion: In general, the observed ISE/CSE of metals of different samples demonstrate relationships with textural and mineralogic indications of metamorphic grade, a relationship that suggests progressive melt extraction with increased temperature. The fact that some samples contain little or no sulfide phase (e.g. EET 84302) provides further evidence that metalsulfide melt extraction was an efficient process accompanying metamorphism. We modeled the evolution of siderophile trace elements in metals during 0.1 wt% incremental metal-sulfide batch melt extraction using average GRA 98028 metal as a starting composition. Since the S content of the melt phase exhibits primary control over the partitioning behavior of many elements [2], predicted D values for a range of melt S contents were employed. By comparing the resultant solid phase compositions with those measured in actual metals, the extent of melt extraction and the S content of equilibrium melt were estimated (Fig 2).