Constraints on the formation of rims on CAIs

Details Constraints on the formation of rims on CAIs Constraints on the formation of rims on CAIs

Jul 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994metic..29..458d&link_type=abstract

Meteoritics (ISSN 0026-1114), vol. 29, no. 4, p. 458-459

Other

2

Abundance, Chondrites, Meteoritic Composition, Rims, Trace Elements, Anomalies, Ion Probes, Perovskites, Spinel

Scientific paper

Several years ago we examined trace-element abundances and Mg isotopic compositions of Wark-Lovering rims (W-L rims) in the Vigarano type A Ca-Al rich inclusion (CAI) 477-5 and concluded that enrichments in refractory elements in the rim relative to the interior of this Ca-Al rich inclusion (CAI) were probably not due to evaporation of the more volatile major elements. We have reexamined 477-5 and begun study of two other Vigarano CAIs, 1623-2 (type A) and 1623-11 (type B), as well as Leoville 3536-3b (type A). All these CAIs have experienced less secondary alteration than the widely studied Allende inclusions, so that primary rim features are better preserved. The ion microprobe was used to profile trace-element contents from the interior of each CAI through the rim into the matrix, attempting to analyze each individual rim layer in turn. All four CAIs studied have the same sequence of outer rim layers. The rim layers in these CAIs share common chemical features. (1) In all three type A CAIs, the spinel-rich layer contains perovskite and is enriched in Rare Earth Elements (REE) and other refractory lithophile elements by approximately 100x CI chondrites. (2) The clinopyroxene layer of the W-L rim, analyzed in two CAIs, has a REE pattern typical of fassaitic pyroxene. (3) Rims and cores of all four CAIs have either no Yb anomaly or small Yb anomalies of the same size. The lack of Ce anomalies and absence of isotopically heavy Mg in rims compared to cores of CAIs indicate that W-L rims are not evaporation residues. W-L rim formation on CAIs appears to involve local melting and reaction between the CAI and surrounding gas.

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