Computer Science
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27r.263m&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 263
Computer Science
Scientific paper
MAC88177 is a coarse-grained ultramafic rock and is related to lodranite (Takeda et al., 1991), although it was initially described as a carbon-free ureilite. Modal abundances of minerals in the MAC88177,55 thin section are 39 vol% olivine (Ol), 44% orthopyroxene (Opx), 6% augite (Aug), 5% Fe-Ni metal and troilite. We measured chemical zoning profiles of Opx, Aug, and Ol with an electron microprobe and discussed the origin of this meteorite in the context of a planetesimal-scale collision (PSC) model proposed for ureilites (Takeda et al., 1991). The core compositions are Ol Fa(sub)13.2, Opx Ca(sub)3.6Mg(sub)84.3Fe(sub)12.1, and Aug (bulk) Ca(sub)39.7Mg(sub)53.8Fe(sub)6.5. The mg# (=100xMg/(Mg+Fe)) of Ol 87, Opx 87, and Aug 89 are in equilibrium and uniform within and between the crystals. The CaO content of Ol is 0.01 to 0.04 wt%, which is less than that of ureilites (0.2 to 0.4 wt%). The chemical zonings of Opx are unique. The CaO content of Opx decreases towards the rim, although mg# is uniform within the crystal (Fig. 1). This trend is the same as that in Lodran. The Al, Cr, and Na contents also decrease towards the rim. There are no trends in the Ti and Mn contents. Aug shows no clear chemical zonings, although the Ca content scatters partly due to the fine exsolution lamellae. The Ca content of Ol appears to decrease towards the rim, although it is very low. The Ca and Al contents increase at the extreme edge of Opx and Ol grains that do not satisfy the stoichiometry and probably corresponds to interstitial materials as were found in ureilites (Ogata et al., 1991). These results imply the introduction of partial melts after crystallization. The chemical zonings of Opx may be explained as follows: Heating by decay of ^26Al and planetesimal-scale collision produced partial melts rich in Ca, Al, Cr, and Na and are partially removed. Mafic silicates grow from such partial melts under a temperature gradient together with the removal of the residual liquids and volatiles through grain boundaries during and after the crystal growth. The enrichment of Ca and Al in the interstitial materials among the grain boundaries found in MAC88177 may be traces of such partial melts in the final stage. We thank the Antarctic Meteorite Working Group in the USA for the sample. References Ogata H., Mori H., and Takeda H. (1991) Meteoritics 26, 195-201. Takeda H., Saito J., and Miyamoto M. (1991) Lunar Planet. Sci. (abstract) 22, 1375. Figure 1, which in the hard copy appears here, shows chemical zonings of an Opx in MAC88177.
Miyamoto Manabu
Takeda Hidenori
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