Computer Science
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27..284s&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 284
Computer Science
Scientific paper
Cumulate eucrites are located compositionally between diogenites and eucrites. In order to obtain a better understanding of the relationships between diogenites and eucrites in the pyroxene differentiation trend of the HED parent body, we have investigated three meteorites Y791439, Y791201, and ALH85001. We developed techniques to obtain quantitative information on the chemistry and texture, including a new chemical mapping utility "Elemental Distribution Map for Pyroxene Quadrilateral (EDMPQ) System" (1). The system also provides us with a modal analysis tool for pyroxenes. Y791439 and Y791201 contain Mg-rich cumulate eucrites and Fe-rich diogenite components (2), and show brecciated textures with glassy matrix, which are characteristics of the Y75032-type HED achondrites. Y791439 contains rare Fe-rich ordinary eucrite pyroxenes. ALH85001 is a cumulate eucrite similar to Binda (3). Accumulated data of Y791439 suggest that its pyroxene chemical compositions are divided into four types: D(Diogenite)-type, BD(Binda)-type, MC(Moore County)-type, JV(Juvinas)-type in the order from Mg-rich to Fe-rich. A diagram plotting 100 x Fe / (Fe+Mg) atomic ratio (Fe#) versus Lamellae/Host areal ratio (L/H) of pyroxenes has been constructed from the data of the L/H of Y791439, Y791201, and ALH85001 obtained by the EDMPQ system and their Fe#s computed from the chemical compositions of the host pyroxenes obtained by EPMA (Fig.1). Modal abundances of four types of pyroxene in Y791439 obtained by the EDMPQ system are shown in Fig.1. On the Fe# vs. L/H diagram (Fig.1), the Y791439 pyroxenes define the trend having the ascending slope. The L/H ratios depend on not only the bulk Ca contents of pyroxenes, but also on cooling history, and some more complex factors. The Y791201 trend plots on almost the same points on the Y791439 trend. This similarity strongly suggests that there was a particular liquid path and the crystallization steps of the HED magma or that these two meteorites are paired or both. Even from the same magma chemical composition, the Fe#s of crystallizing pyroxenes may change according to the crystallization condition such as cooling rate, supersaturation rate, nucleation density, and so on. There is a high possibility that these two samples are paired. The modal data (Fig.1) suggest the possibility that there are large amounts of the BD-type cumulate eucrites existing deep in the HED parent body. Assuming that these types constitute a layered crust, we suggest that large amount of cumulate eucrites are located at a region between diogenites and ordinary eucrites. This interpretation means that cumulate eucrites are link materials between diogenites and ordinary eucrites. Previously it was thought that the BD-type cumulate eucrite was a rare component of the HED parent body, but considering the modal data and the fact that ALH85001 has the BD-type pyroxenes, the BD-type cumulate eucrites may be common components on the HED parent body. We propose the significance of cumulate eucrites for the study of genetic links between diogenites and eucrites, and potential usefulness of the L/H ratio vs. Fe# diagram for expressing differentiation trend of pyroxenes of the HED parent body, and identifying the pairing of polymict eucrites. We thank NIPR and MWG for the samples. References (1) Saiki, K., Yamaguchi, A., and Takeda, H. (1992) Lunar Planet. Sci. (abstract). pp. 1201-1202. (2) Takeda, T. and Hidaka, O. (1986) Meteoritics. 24. (Abstract). p. 331. (3) Mason, B.(1986) MWG Newsletter. p. 16. Figure 1, which in the hard copy appears here, shows the plot of Fe# (Fe/(Fe+Mg)x100) atomic ratio vs. Lamellae/Host areal ratio of the pyroxene in Y791439, Y791201, and ALH85001. The included table shows modal abundance *(areal%) of each type pyroxenes in Y791439. *These data are normalized to 100% for the entire surface of pyroxenes.
Saiki Koichiro
Tagai Tokuhei
Takeda Hidenori
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