TY - JOUR
T1 - Trace and minor elements in minerals of nakhlites and Chassigny
T2 - Clues to their petrogenesis
AU - Wadhwa, Meenakshi
AU - Crozaz, Ghislaine
N1 - Funding Information:
Acknowledgments--This work was funded by NASA grant NAG 9-55 to GC. We would like to acknowledge the Smithsonian Institution and the University of New Mexico for the loan of the samples. Thanks are due to Dan Kremser for helping with the electron microprobe measurements and to Pat Swan for his assistance with the SEM analyses. We are grateful to Glenn MacPherson and Brad Jolliff for useful discussions. Also, we thank Ralph Harvey, Allan Treiman, and Charles Shearer for their reviews and constructive comments.
PY - 1995/9
Y1 - 1995/9
N2 - Ion microprobe measurements of REE (rare earth element) and other selected trace and minor element concentrations were made in clinopyroxene, olivine, feldspar, and chlorapatite in Chassigny and the three nakhlites (Nakhla, Governador Valadares, and Lafayette). In Chassigny, we have found pigeonite occurring in a poikilitic habit; it is the first reported occurrence of this mineral in this meteorite. The most significant implication of this finding is that estimates of the major element composition of the Chassigny parent melt will have to accommodate pigeonite as a primary mineral crystallizing before plagioclase. Chlorapatite contains the highest REE abundances in all meteorites studied here. The REE patterns of chlorapatite, plagioclase, and augite in nakhlites and Chassigny are similar (although the downward trends of REE patterns are somewhat steeper for these minerals in Chassigny than in the nakhlites). Pyroxenes in nakhlites and Chassigny are extensively zoned in REEs and other trace and minor elements, such as Y, Zr, Ti, and Al, which appear to be "resistant" to diffusive reequilibration. Thus, primary igneous zoning of these elements has been preserved in the pyroxenes of these meteorites and can be used to infer their crystallization histories. The calculated REE patterns of the parent melts of nakhlites and Chassigny are LREE enriched and parallel to their whole-rock REE patterns, indicating that, once crystal accumulation occurred, the intercumulus trapped melt fractionally crystallized in an essentially closed-system and that infiltration (i.e., metasomatism) by LREE-enriched magmas need not be invoked to explain the LREE enrichment in bulk samples of these meteorites. Fractional crystallization model calculations for Nakhla also confirm these conclusions. The distributions of trace and minor elements in augites in nakhlites and Chassigny suggest that all three nakhlites could be samples from different horizons of the same lithologic unit but that Chassigny was not comagmatic with the nakhlites.
AB - Ion microprobe measurements of REE (rare earth element) and other selected trace and minor element concentrations were made in clinopyroxene, olivine, feldspar, and chlorapatite in Chassigny and the three nakhlites (Nakhla, Governador Valadares, and Lafayette). In Chassigny, we have found pigeonite occurring in a poikilitic habit; it is the first reported occurrence of this mineral in this meteorite. The most significant implication of this finding is that estimates of the major element composition of the Chassigny parent melt will have to accommodate pigeonite as a primary mineral crystallizing before plagioclase. Chlorapatite contains the highest REE abundances in all meteorites studied here. The REE patterns of chlorapatite, plagioclase, and augite in nakhlites and Chassigny are similar (although the downward trends of REE patterns are somewhat steeper for these minerals in Chassigny than in the nakhlites). Pyroxenes in nakhlites and Chassigny are extensively zoned in REEs and other trace and minor elements, such as Y, Zr, Ti, and Al, which appear to be "resistant" to diffusive reequilibration. Thus, primary igneous zoning of these elements has been preserved in the pyroxenes of these meteorites and can be used to infer their crystallization histories. The calculated REE patterns of the parent melts of nakhlites and Chassigny are LREE enriched and parallel to their whole-rock REE patterns, indicating that, once crystal accumulation occurred, the intercumulus trapped melt fractionally crystallized in an essentially closed-system and that infiltration (i.e., metasomatism) by LREE-enriched magmas need not be invoked to explain the LREE enrichment in bulk samples of these meteorites. Fractional crystallization model calculations for Nakhla also confirm these conclusions. The distributions of trace and minor elements in augites in nakhlites and Chassigny suggest that all three nakhlites could be samples from different horizons of the same lithologic unit but that Chassigny was not comagmatic with the nakhlites.
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U2 - 10.1016/0016-7037(95)00228-R
DO - 10.1016/0016-7037(95)00228-R
M3 - Article
AN - SCOPUS:0029473431
SN - 0016-7037
VL - 59
SP - 3629
EP - 3645
JO - Geochimica et Cosmochimica Acta
JF - Geochimica et Cosmochimica Acta
IS - 17
ER -