Trace element disequilibria and magnesium isotope heterogeneity in 3655A: Evidence for a complex multi-stage evolution of a typical Allende Type B1 CAI

Abstract

We used the Panurge ion microprobe to measure concentrations of the rare earth elements (REEs), Ba, Hf, and Sr in melilite, clinopyroxene, plagioclase, and perovskite and Mg isotopes in plagioclase, spinel, melilite, fassaite, hibonite, grossular, and monticellite from the Allende Type B1 calcium-, aluminum-rich inclusion (CAI), USNM 3655A. The distribution and concentration of Ba and the REE in melilite from the melilite-rich mantle of 3655A are unlike those predicted from melilite-melt REE partitioning experiments for closed system crystal fractionation. REE concentrations are lower than expected in the first crystallized gehlenitic melilite, increase rapidly to higher than expected concentrations in melilite with intermediate åkermanite contents (Ak30–Ak40), and decrease as expected only during the late stage of mantle crystallization. Barium concentrations in melilite are 10–50 times those expected, and the LREE/HREE ratio increases continuously rather than remaining constant. The unexpected distribution of trace elements in melilite reflects a progressive enrichment of trace elements in the melt during the early stages of crystallization. A partial explanation for this observation is the dissolution of precursor perovskite that contained half or more of the total REE budget of the inclusion. In addition, there are large trace element enrichments adjacent to included spinel in melilite and similar but smaller enrichments adjacent to spinet in clinopyroxene. These enrichments are consistent with the existence of trace element enriched boundary layers at the mineral/melt interfaces. The fact that kinetic processes partially control trace element abundances and distributions suggests rapid cooling during crystallization of the melilite-rich mantle. Similar trace element signatures are ubiquitous in Type B1 CAI, suggesting that each experienced a similar thermal history. The Mg isotope record of 3655A is distinguished by four salient features: (1) large ^(26)Mg excesses correlated with the respective AI/Mg ratios in plagioclase, melilite, and hibonite, (2) F_(Mg), the mass-dependent fractionation of Mg, is positive, with enrichment of the heavier Mg isotopes in all primary phases, (3) a heterogeneous distribution of F_(Mg) values, with F_(Mg) in melilite systematically greater than in either spinel or fassaite, and (4) isotopically normal Mg in the secondary alteration phases, grossular and monticellite. The occurrence of ^(26)Mg^∗, the decay product of ^(26)Al, in anorthite implies early formation of 3655A, while ^(26)Al was extant at nearly the canonical solar system value of ∼5 × 10^(−5). The trace element and Mg isotope heterogeneities suggest a formation scenario for 3655A which includes (1) flash heating to partially melt a solid precursor, (2) rapid cooling to allow survival of relict phases, (3) diffusive exchange of Mg between melilite and a nebular reservoir, and (4) alteration at low temperature. Although this model explains most of the trace element and isotopic characteristics of 3655A and other Type BI CAIs, it does not provide an explanation for the rapid change of LREE/HREE ratios of melilite during crystallization.

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