Other
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28q.428s&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 428
Other
Cumulates, Equilibrium, Eucrites, Fractional Crystallization, Medanitos, Pyroxenes
Scientific paper
Cumulate eucrites have bulk compositions between diogenites and ordinary (noncumulate) eucrites (OE). The study of polymict eucrite Y791439 shows that ordinary to cumulate eucrites are located adjacent to each other in the parent body crust, and shows that there are four stages in the crystallization sequence of pyroxene, including two types of cumulate eucrite [1]. In this study we calculate the activities of some elements in the liquid equilibrated with pyroxenes in Y791439 and some cumulate eucrites by single element partition coefficients for the better understanding of the liquid evolution on the HED parent body's crust. Polished thin sections (PTS) of Y791439,51 and Y791195,92-1 were supplied from the National Institute of Polar Research (NIPR) in Japan. Medanitos was supplied from the British Museum of Natural History. A PTS of ALH85001 was made from a chip supplied from Meteorite Working Group (MWG) in the U.S. These PTSs were investigated by SEM (JEOL840A) equipped with a chemical mapping system (PXQUAD system) originally constructed by us and the chemical compositions of pyroxenes were obtained by EPMA (JEOL 733 mark II). Y791439 is composed of four types of pyroxene; JV (Juvinas), MC (Moore County), BD (Binda), and D (Diogenite)-type in order from Fe-rich to Mg-rich. The activities of some network-modifier elements (e.g., Fe, Mg, and Ca) in the liquid equilibrated with the pyroxenes in Y791439 are calculated and plotted in the Ca/Fe vs. Mg/Fe (atomic ratio) diagram. For the calculation we adopted the modified Bottinga-Weill two-lattice melt model [2] and single "M"-site pyroxene model and applied Nielsen-Drake partition coefficients [3]. The bulk chemical composition of Y7308 is taken from Ikeda and Takeda (1985) and those of the other meteorites are taken from BVSP (1981). The bulk of Y7308 is one of the probable candidates for the parental liquid that crystallized diogenites and eucrites and the bulks of OE are candidates for the most evolved liquid compositions. The tendency of liquid evolution is roughly estimated as Path 1 in Fig. 1. The liquid equilibrated with the D-type pyroxene corresponds to OEs. There is a high possibility that the D-type pyroxenes crystallized from the liquid, which would solidify as scarf of an OE such as the JV type. The liquids equilibrated with Fe-rich pyroxenes (JV and MC type of Y791439 and Y791195) are more evolved magmas than the OE bulk. Such Fe-rich materials, however, have not been found. We propose that some cumulate eucrites especially Fe-rich ones may not be cumulus, but slightly evolved liquid, themselves solidified slowly under an OE scarf. Based on its major element chemistries (mg# = 45) and non-preferred orientation texture, Y791195 is a good candidate for such a kind of noncumulate eucrite. Medanitos may be cumulus from an Fe-rich liquid like Nuevo Laredo or may be slightly evolved liquid itself. Y791195 and Medanitos show finer exsolution textures than Moore County. This work is supported by Fellowships of the Japan Society for the Promotion of Science for Japanese Junior Scientists. References: [1] Saiki K. et al (1992) Meteoritics, 27, 284. [2] Bottinga Y. and Weill D. F. (1972) AJS, 272, 438-475. [3] Nielsen R. L. and Drake M. J. (1979) GCA, 43, 1259-1272. Figure 1, which appears in the hard copy, shows hypothetical liquids on the HED parent body.
Saiki Koichiro
Takeda Hidenori
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