Petrology and geochemistry of the Elephant Moraine A79002 diogenite: A genomict breccia containing a magnesian harzburgite component

The Elephant Moraine A79002 (EETA79002) diogenite is a fragmental breccia w ith a subtle light-dark structure. It is composed of orthopyroxene, with mi nor olivine, chromite, and ubiquitous, inhomogeneously distributed, approxi mately 5-500 mu m sized troilite and metal grains. These latter are present in the matrix, and as inclusions in and as symplectic intergrowths with or thopyroxene and olivine. Trace amounts of silica and diopside are also pres ent. Most orthopyroxene compositions (typical orthopyroxenes) are in the na rrow range Wo(2.1-2.7)En(74.1-75.6)Fs(22.2-23.8) like those of most diogeni tes. A few magnesian orthopyroxenes are present with compositions of Wo(1.7 -2.5)En(77.5-80.2) Fs(18.2-20.3). These are among the most magnesian orthop yroxenes known from diogenites. A few ferroan orthopyroxenes have compositi ons of Wo(2.1-2.9)En(71.7-73,7)Fs(24.2-25.5). Differences in Al2O3, TiO2, a nd Cr2O3 between the different orthopyroxene groups are inconsistent with a simple igneous fractionation relationship between them. Olivine compositio ns are Fo(75.0-76.9). The olivines could be in equilibrium with the magnesi an orthopyroxenes, but not with the typical or ferroan orthopyroxenes that form the bulk of EETA79002. Metal grains exhibit a range of Ni and Co conte nts and Ni/Co ratios; their compositions indicate that they are primary ign eous metal. Metal and troilite grains are more prevalent in the dark sample s. The trace incompatible lithophile element contents of 16 samples are remark ably uniform. Their Yb concentrations are all within their 2 sigma analytic al uncertainties of the mean. The uniformity and low content of light rare earth elements in EETA79002 indicate that negligible amounts of a trapped l iquid component, or foreign material mixed in the breccia, could be present . The siderophile and chalcophile element data show that the light-dark str ucture is due to the distribution of metal and troilite grains; dark sample s contain higher Ni, Co, and Se compared to light samples. Meteorite EETA79002 appears to contain material from three or more related plutons, a magnesian harzburgite, and two orthopyroxenites, and is a genomi ct breccia.