Other
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28..395m&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 395
Other
4
Minor Elements, Nakhla, Pyroxenes
Scientific paper
Nakhla is one of the SNC meteorites, which are generally believed to be of martian origin. It is a medium-grained augite-olivine cumulate with a variolitic ground mass of sodic plagioclase, alkali feldspar, and Fe-rich pyroxenes and olivine [e.g., 1]. One ofthe major tasks in deciphering Nakhla's petrogenesis is determining the composition of its parent melt. Gaining an understanding of the composition and petrogenesis of this parent melt may help unravel Nakhla's relationship to the other SNCs, and provide clues to martian petrogenesis in general. Several compositions have been proposed for this parent melt [e.g., 2-4]. A major difference among these compositions is the Al content. Longhi and Pan [2] proposed a low-Al parent melt based on inversion of the observed composition of cumulus pyroxene cores, using experimental Al partition coefficients. McKay et al. [5] applied this approach to their experiments on synthetic analogs to several proposed parent melt compositions, and noted that the most aluminous experiments in their experiments overlapped the least-aluminous observed Nakhla cores. However, they observed a range of Al contents in Nakhla pyroxenes, and it is unclear whether it is the high-Al or low-Al pyroxenes that should be matched. To obtain a better understanding of these Al variations, we have begun a study of the variation of minor elements in Nakhla pyroxenes. Results from over 500 analyses may be summarized as follows: (1) Despite being very homogeneous in Fe/Mg, cores vary in Al2O3 from 0 4 to 1.7 wt%, while rims show a positive correlation between Fe/Mg and Al, with Al2O3 ranging up to nearly 3% (Fig. 1). (2) In pyroxene cores, molar Al is strongly correlated with Ti along a line with 1/4 slope, but intercepting the Ti axis at positive Al; rims deviate toward lower Ti/AI. (3) Cr7O3 in cores varies from 0.3 to 0.5, and shows no correlation with Al or Fe/Mg; Cr2O3 in rims ranges from 0.5% to below detection limits, and is negatively correlated with Fe/Mg. (4) Na2O in both cores and rims varies from 0.2 to 0.3% (except for a few outliers), and shows no correlation with Fe/Mg, but may be slightly correlated with Al The cause of the Al variations is not clear from the above data. The rim trend in Fig. 1 suggests that the rims evolved from the low-Al portion of the cores, further suggesting that the high-Al portion of the cores may have been the first to crystallize. However, it is also possible that Al is controlled by some crystallographic effect, such as sector zoning. We have begun to map spatial variations in minor elements to explore this possibility, but results on three grains are inconclusive, and we are planning to analyze more grains. Thus, to date, it is impossible to say whether it is the high- or low-Al pyroxenes that should be inverted to give information on the composition of the Nakhla parent melt. References: [1] Treiman (1986) GCA, 50, 1061. [2] Longhi and Pan (1989) Proc. LPSC 19th, 451. [3] Harvey and McSween (1991) EPSL, 111, 467. [4] Treiman (1993) GCA, submitted. [5] McKay et al. (1993) LPS XXlV, 966. Fig. 1 appears here in the hard copy.
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