Groundmass Composition for EET 79001A Using a Novel Microprobe Technique for Estimating Bulk Compositions. Lithology A as an Impact Melt

摘要

Petrologic investigation of the shergottites has been hampered by the fact that most of these meteorites are partial cumulates. Two lines of inquiry have been used to evaluate the compositions of parental liquids: (i) perform melting experiments at different pressures and temperatures until the compositions of cumulate crystal cores are reproduced (e.g., 1); and (ii) use point-counting techniques to reconstruct the compositions of intercumulus liquids (e.g., 2). The second of these methods is hampered by the approximate nature of the technique. In effect, element maps are used to construct mineral modes; and average mineral compositions are then converted into bulk compositions. This method works well when the mineral phases are homogeneous (3). However, when minerals are zoned, with narrow rims contributing disproportionately to the mineral volume, this method becomes problematic. Decisions need to be made about the average composition of the various zones within crystals. And, further, the proportions of those zones also need to be defined. We have developed a new microprobe technique to see whether the point-count method of determining intercumulus liquid composition is realistic. In our technique, the approximating decisions of earlier methods are unnecessary because each pixel of our x-ray maps is turned into a complete eleven-element quantitative analysis. The success or failure of our technique can then be determined by experimentation. As discussed earlier, experiments on our point-count composition can then be used to see whether experimental liquidus phases successfully reproduce natural mineral compositions. Regardless of our ultimate outcome in retrieving shergottite parent liquids, we believe our pixel-bypixel analysis technique represents a giant step forward in documenting thin-section modes and compositions. For a third time, we have analyzed the groundmass composition of EET 79001, 68 (Eg). The first estimate of Eg was made by (4) and later modified by (5), to take phase diagram considerations into account. The Eg composition of (4) was too olivine normative to be the true Eg composition, because the ,68 groundmass contains no forsteritic olivine. A later mapping by (2) basically reconfirmed the modifications of (5). However, even the modified composition of (5) has olivine on the liquidus for approximately 50 C before low-Ca pyroxene appears (6).