Other
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27..212d&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 212
Other
1
Scientific paper
Most studies of refractory inclusions in C2 chondrites have focused on the presumably more primitive and refractory hibonite-rich inclusions, but the most abundant type of inclusion in Mighei and other C2 chondrites are spinel-rich ones. A preliminary report on Mg isotopes in 6 Mighei inclusions and trace elements in 3 of them was made by MacPherson and Davis (1991). We have now measured trace element and Mg isotopic compositions in 22 spinel-rich inclusions from 3 thin sections of Mighei in order to constrain models for their origin. Mighei spinel-rich inclusions range from nodular, with a compact interior of spinel with subordinate pyroxene and perovskite rimmed by pyroxene, to chainlike, in which several pyroxene-rimmed spinel-rich areas are close to one another but separated by 10-20 micrometers of meteorite matrix. The spinel-rich inclusions are small, so that 2 to 6 ion microprobe analyses of spinel-rich areas using a 25-micrometer-diameter beam spot sample a significant fraction of each inclusion. Of the 22 inclusions analyzed, 7 have REE patterns that are unfractionated or have negative Eu anomalies (group I), 7 have volatility-fractionated group II patterns, 2 chainlike inclusion have both group I and group II patterns in different spinel-rich areas, 2 have group III patterns with depletions in Eu and Yb, 3 have irregular patterns that are different from group II but appear to be volatility-fractionated and one (Davis, 1991) has an ultrarefractory REE pattern. The enrichment factors for LREE in all but the latter inclusion range from ~5 to ~100 x C1 chondrites. The dominant carrier of REE and most other refractory lithophile elements is perovskite, as nearly all spinel-rich inclusions contain some perovskite and there is a good correlation between TiO2 and refractory element contents among the spot analyses of each inclusion. Delta^25Mg values in these inclusions are low, -2.4 to +7.5 o\oo, and appear to correlate with trace element classification: group I and group II inclusions have average delta^25Mg values of 3.2+- 1.3 and 0.6+-1.2 o\oo (+-2 sigma), respectively. Similarly, in the larger CAIs in the C3V chondrites, group I inclusions contain isotopically heavy Mg and Si and group II inclusions generally contain isotopically light Mg and Si. These fractionations have been attributed to kinetic isotope effects caused by evaporation and condensation. Although the spinel-rich inclusions do not usually contain a primary Si-bearing phase, it appears that the same processes operated on C3V inclusions and the spinel-rich inclusions in Mighei. The origin of the spinel-rich inclusions remains unclear, but some models can be excluded. Kornacki and Fegley (1984) suggested that spinel-rich inclusions formed by partial melting and loss of the liquid by evaporation or splashing during collisions, leaving a spinel-rich residue. Evaporation would be expected to produce significant enrichments in the heavy isotopes of Mg (Davis et al., 1990), but the effects seen in spinel are small. Partial melting followed by mechanical loss of liquid would be expected to produce igneous fractionation effects. Group I and group III inclusions studied here do not show the depletions or enrichments in light compared to heavy REE expected from mineral/melt or mineral/mineral partitioning. We favor a condensation origin for the spinel-rich inclusions, although it is not clear that they are primary condensates. References: Davis A. M. (1991) Meteoritics 26, 330. Davis A. M., Hashimoto A., Clayton R. N., and Mayeda T. K. (1990) Nature 347, 655-658. Kornacki A. S. and Fegley B. (1984) Proc. Lunar Planet. Sci. Conf. 14th, B588-B596. MacPherson G. J. and Davis A. M. (1991) Lunar and Planetary Science 22, 841-842.
Davis Aileen M.
MacPherson Glenn J.
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