Other
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28q.366h&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 366
Other
Chervony Kut, Eucrites, Ibitira, Ion Probe, Pigeonite, Plagioclase, Rare-Earth Elements, Stannern
Scientific paper
Most trace element studies of eucrites have been limited to the analysis of bulk samples. However, the use of trace element zoning in single grains may provide more information about the evolution of the eucritic melts. We report here ion microprobe data for individual mineral grains and evaluate, in this preliminary study, whether pyroxene and/or plagioclase, the first two minerals to crystallize from a eucritic melt, can be used to derive information about its nature. Although many eucrites have igneous textures that reflect fast cooling, some of their minerals seem to have experienced reequilibration. Of the two major minerals, pigeonite and plagioclase, the former was the most affected. Except in a few eucrites classified as unequilibrated, pigeonite shows both a narrow range of Fe/Mg as well as a range of Ca concentrations due to the presence of microscopic exsolution lamellae. In contrast, plagioclase seems to have retained some of its original major element zoning and, therefore, may be a better candidate to gain insights into the nature of eucrite parent melts and to reevaluate the relationships between various eucrites. We studied three eucrites: Chervony Kut (CK: unbrecciated but severely shocked, medium-grained, non-cumulate), Ibitira (IB: unbrecciated, fine-grained, vesicular), and Stannern (ST: brecciated, monomict). In plagioclase, typically, there is a range of REE concentrations in a given grain. Chondrite- normalized abundances all show a strong positive Eu anomaly and, for CK and IB, a decrease from La to Lu. In ST, the decrease of chondrite-normalized abundances from La to Sm is followed by an increase from Gd to Lu, in agreement with the data of [1] who recently analyzed single ST plagioclase grains by INAA. Lower REE concentrations are observed in CK plagioclase than in the other two meteorites. In pyroxene, REE concentrations are constant within a given crystal and the REE patterns show an increase from La to Lu with, however, a strong negative Eu anomaly. The HREE pattern is more flat in IB than in CK and ST pyroxenes. Concentrations in CK pyroxene are lower than in the two other eucrites by about a factor of 2. The clouding of plagioclase and pyroxene [2] has altered some of the REE patterns in ST and CK. In particular, the increase of chondrite-normalized abundances from Gd to Lu in ST plagioclase is due to the inclusion of small pyroxene crystals and, similarly, the relatively flat LREE pattern observed in one CK pyroxene grain can be traced to the presence of minute plagioclase grains. None of these effects is observed in IB. If it is assumed that plagioclase retained its original REE zoning, one can use the lowest REE abundances measured in the cores of plagioclase grains to calculate the REE composition of the eucritic parent melts. Using the same partition coefficients as [1], the estimated REE patterns are essentially flat for all three eucrites (e.g., Fig. 1 for CK) and the absolute REE abundances are consistent with whole rock values previously reported for these meteorites. We conclude that plagioclase seems to be a promising mineral to study the petrogenesis of eucrites. References: [1] Phinney W. C. et al. (1993) LPSC XXIV, 1137-1138. [2] Harlow G. E. and Klimentidis R. (1980) LPS XI, 1131-1143.
Crozaz Ghislaine
Hsu Weibiao
MacPherson Glenn J.
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