Other
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30..481a&link_type=abstract
Meteoritics, vol. 30, no. 5, page 481
Other
Basalts, Mare, Crystallization, Meteorites, Asuka 881757, Lunar, Pyroxene, Spinel
Scientific paper
Lunar meteorite Asuka (A)881757 is a distinct new type of low-Ti (LT) mare basalt [e.g. 1,2], considerably older than nearly all Apollo and Luna mare basalt samples [3], and may be an product of the earliest mare volcanism. It shows an extraordinarily coarse-grained gabbroic texture, and is mainly composed of Fe-rich pyroxene, plagioclase, olivine, ilmenite, Ti-rich spinel, troilite and Fe-Ni [4]. Extensive chemical zonings, which are recognized in pyroxenes and spinels, were studied by means of elemental distribution maps in order to deduce crystallization condition of pyroxene and spinel, and their mutual relationship during crystallization. We studied polished thin sections (PTS) A881757, 51-4, A881757, 53D-2, and A881757, 53E-2 supplied by the National Institute of Polar Research (NIPR). Mineral chemistries and textures were examined by EPMA and SEM. Elemental distribution maps of pyroxenes and spinels for Fe, Mg, Ti, Cr, and Al were obtained by EPMA. Pyroxenes are generally zoned from Mg-rich core Fe(sub)37Mg(sub)40Ca(sub)23 to Fe-rich rim Fe(sub)65Mg(sub)10Ca(sub)25 in three PTSs. Spinels are distributed quite heterogeneously through three PTS and modal abundance of spinel reachs 11% only in one PTS. Spinels in three PTS can be divided in two groups: spinels in mesostases and interstitial ones between pyroxenes. Mesostasis spinels, which are homogeneous and have similar compositions from grain to grain, crystallized with very Fe-rich pyroxene. Interstitial spinels which are zoned for TiO2 and Cr2O3 within crystals, and show different composition between crystals, cocrystallized with relatively Mg- rich pyroxenes. Interstitial spinels (TiO2= 23-28wt%, Cr2O3= 8-17 wt%) are more Ti-rich and less Cr-rich than mesostasis ones (TiO2= 30-31wt%, Cr2O3= 3-4 wt%). It is noted that more Ti-rich spinels tend to be surrounded by more Fe-rich pyroxene. Spinels in mare basalts generally change their compositions from Fe2TiO4 (ulvospinel) to FeCr2O4 (chromite) during fractional crystallization. Spinels in A881757 are Cr-bearing ulvospinel, which suggests that the spinels crystallized at a relatively late stage. Differences of TiO2 and Cr2O3 among spinels probably reflect different stages in a series of crystallization. More Fe-rich pyroxene crystallized adjacent to more Ti-rich interstitial spinels, or spinels within mesostases, crystallized with greatly Fe-rich pyroxene. These spinels and pyroxenes probably crystallized simultaneously from compositionally evolving liquid[5]. In other words, fractional crystallization of pyroxene and spinel occurred even in PTS scale, because of rapid cooling rate. A881757 was cooled only slightly more slowly than typical mare basalts despite its old age [3] and unusually coarse grain sizes. Together, the inferred rapid cooling and the coarse grain sizes suggest origin in the interior of the relatively thick lava flow. We thank NIPR for samples, Profs. I. Kushiro for discussions, and Mr. H. Yoshida and O. Tachikawa for technical assistances. References: [1] Yanai K.et al. (1993) LPS XXIV, 1555-1556. [2] Warren P. H. and Lindstrom M. M. (1993) LPS XXIV, 1483-1484. [3] Misawa K.et al. (1992) in Proc. 17th Symp. Antarc. Meteorites, 119-121. [4] Yanai K (1991) Proc. LPS, Vol. 21, 317-324. [5] Arai T. et al. (1995) in Proc. 20th Symp. Antarc. Meteorites, 4-7.
Arai Toshiaki
Takeda Hidenori
Warren Harry P.
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