Other
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27r.302w&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 302
Other
1
Scientific paper
The nakhlites (Nakhla, Lafayette, and Governador Valadares) are pyroxenites that belong to the SNC group of meteorites, for which a martian origin is currently favored. These meteorites are considered to be cumulate rocks, and therefore their whole rock chemistries most probably do not reflect parent melt compositions. One way to determine their parent melt compositions is to use appropriate distribution coefficient values to invert the elemental concentrations in the cores of the cumulus augite grains; this method, however, assumes that the cores of the cumulus grains have retained their original compositions, and have not reequilibrated since their crystallization. Recently Harvey and McSween (1992) studied major element zoning in augite and olivine in all three nakhlites, and concluded that the nakhlites had undergone varying degrees of late magmatic and subsolidus diffusion. According to these authors, Nakhla was least affected, while Lafayette was the most reequilibrated. In order to determine whether trace elements in augites in the nakhlites have been affected by diffusive reequilibration, we made ion microprobe analyses on augites in thin sections of Nakhla, Lafayette, and Governador Valadares. Preliminary work (Wadhwa and Crozaz, 1992) indicated that trace element zoning is preserved not only in Nakhla, but also in Lafayette. This result, however, was based on few analyses of augites in Lafayette. The present study is a continuation of the work presented at the 23rd LPS conference (Wadhwa and Crozaz, 1992); additional data on Lafayette as well as analyses of augites from Governador Valadares are presented. Trace element concentrations of 14 augites in Nakhla, 16 in Lafayette, and 21 in Governador Valadares were determined. Extensive zoning of many trace and minor elements is preserved in augites in all three nakhlites (e.g., the ratios of extreme Y concentrations in augites in Nakhla, Lafayette, and Governador Valadares are, respectively, 2.3, 3.7, and 4.1) and indicates that the augite cores preserved their original REE abundances. Data for the three nakhlites follow the same trends (as shown for Y and Ti in Fig. 1, but also observed for a number of other elements); therefore, the nakhlites originated from magmas of similar compositions. Calculated REE patterns for melts in equilibrium with augite, plagioclase, and apatite are remarkably parallel to their whole rock REE patterns and indicate that the LREE-rich whole rock patterns for the nakhlites are not due to late-stage infiltration metasomatism (Berkley et al., 1980). The order of crystallization, inferred from REE concentrations in equilibrium melts, is consistent with the observed textural relationships. References: Berkley J.L., Keil K., and Prinz M. (1980) Proc. Lunar Planet. Sci. Conf. 11th, 1089-1102. Harvey R.P. and McSween H.Y., Jr. (1992) Geochim. Cosmochim. Acta 56, 1655-1663. Wadhwa M. and Crozaz G. (1992) Lunar Planet. Sci. (abstract) 23, 1483- 1484. Figure 1, which in the hard copy appears here, shows Y vs. Ti in augites in the nakhlites.
Crozaz Ghislaine
Wadhwa Meenakshi
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