Trace element and petrologic clues to the formation of forsterite-bearing Ca-Al-rich inclusions in the Allende meteorite

This work presents new trace element and petrographic data for three forsterite-bearing, Ca-Alrich inclusions from the Allende meteorite: TE, 818a, and 110-A. Such inclusions form a continuum with Type B1 and B2 Ca-Al-rich inclusions (CAIs), and we refer to them as "Type B3" CAIs. Textures, mineral chemistries, crystal-chemically fractionated REE patterns, and other properties suggest that Type B3 crystallized from partly molten evaporative residues. The concentrations of refractory lithophile elements are lower than in Type B1 and Type B2, in approximately inverse proportion to the higher concentrations of Mg and Si in the Type B3's. The refractory trace element abundances of the forsterite-bearing, isotopically anomalous FUN CAIs TE and CG14 suggest that they formed at higher temperatures and under more oxidizing conditions than other Type B CAIs, thus strengthening the previously observed link between relatively oxidized CAI compositions and FUN properties. We also present evidence that 818a was strongly re-heated and modified in the nebula after its initial crystallization: it consists of a core of coarse-grained Ti-Al-pyroxene (Tpx), forsterite, spinel and metal grains and a thick, surrounding mantle of melilite that has been almost totally converted to fine-grained alteration products. In the core, the mean concentrations of refractory lithophiles and siderophiles are similar (both ~ 14 × CI), but in the mantle, the refractory siderophiles are a factor of 2 lower (~ 9 × CI) than the refractory lithophiles (~18 × CI). Because the core and mantle display similar, mineralogically-fractionated REE patterns (both sloping up from La to Lu), the pre-alteration mantle could not have formed during fractional crystallization of the primary CAI nor as a later condensate over the core. A 3-stage formation process is required for 818a: (1) crystallization of the primary CAI rich in Tpx throughout; (2) re-heating and partial volatilization of Mg and Si from the outer portion of the CAI, causing an increase in the concentration of refractory lithophiles, a loss of siderophiles, and converting Tpx to melilite; (3) metasomatic alteration of the melilite-rich mantle.