Trace Elements in High-ca Pyroxene & Plagioclase in the Bilanga Diogenite: Implications for the Magmatic Evolution of Diogenites

Introduction: High-Ca pyroxene and plagioclase are typically present as minor phases in diogenites [1]. However, although the trace element content of diogenite orthopyroxene has been measured in a number of studies e.g., [2, 3]; almost no trace element data is available for the high-Ca pyroxene and plagioclase with which it routinely coexists in these meteorites. These data could provide insights into the nature and evolution of the melts from which diogenites crystallized in the HED parent body. In this study we have obtained initial measurements of several REEs in highCa pyroxene, plagioclase, and orthopyroxene in the Bilanga. Measurement of additional incompatible trace element concentrations in these phases is currently in progress. Bilanga Diogenite: The Bilanga diogenite contains numerous aggregates of high-Ca pyroxene and in some cases plagioclase that are 10s 100s of μm in size. Some of these appear to be remnants of trapped intercumulus melt that has been relatively undisturbed by later shock melting (Type 1 assemblages of Kolar et al. [4]). Others appear to have been partly re-melted by later shock events (Type 3 assemblages). Heterogeneously exsolved blebs of high-Ca pyroxene and chromite, more fractionated zones of silica-rich mesostasis associated with small REE-rich phosphate minerals, and exotic clasts consisting of high-Ca pyroxene, plagioclase, silica with minor K-feldspar and troilite are also observed. The average compositions of orthopyroxene and high-Ca pyroxene in all of these occurrences are mg# 80; En78, Fs21, Wo1 and mg# 89; En47, Fs6, Wo47 respectively. The Al content of orthopyroxene in Bilanga is 0.03 cations per 6 oxygens and the avg. Ti content is 0.055 wt%. Based on the observations of Fowler et al. [3, 5] this suggests Bilanga falls approximately in the mid-range of diogenites with respect to the degree of fractionation it has experienced. Plagioclase compositions are An88 in Type 1 assemblages, An79 in exotic clasts, and variable (An92 – An46) in Type 3 assemblages. Data obtained for high-Ca pyroxene, plagioclase, and orthopyroxene in Type 1 assemblages, high-Ca pyroxene and plagioclase in exotic clasts, and high-Ca pyroxene and orthopyroxene in the Type 3 assemblages, are presented in the current study. Analytical Methods: Analyses were made using the Cameca 4f ion microprobe at the Institute of Fig. 1: Chondrite normalized REE contents of high-Ca pyroxene, plagioclase, and orthopyroxene for different petrographic phase assemblages in the Bilanga diogenite. La, Pr, Gd, Tb, Ho, Tm, Lu were not measured.