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EVIDENCE FROM OXYGEN ISOTOPES AND METAL CHEMISTRY FOR A COMMON PARENT BODY FOR H CHONDRITES AND IIE IRON METEORITES

Introduction: The IIE iron meteorites and H chondrites have been linked by some authors [e.g. 1, 2], whereas others suggest that IIE irons are derived from an ordinary chondrite (OC) parent body that is further reduced and enriched in metallic iron compared to the H chondrite parent body [e.g. 3, 4]. Mo and Ru isotopic compositions for IIE irons overlap with OC values, which is interpreted to indicate formation of the IIE irons through impact melting of an OC parent body [5, 6]. IIE iron meteorites have a δGemetal range that encompasses all OC values including H chondrites [7 and ref. within]. Here we present geochemical evidence that IIE irons formed on the H chondrite parent body under high-temperature, low-pressure, low-oxygen fugacity conditions. Petrogenetic context: Peak temperatures vary amongst the IIE iron meteorite group; Miles cooled rapidly from >1160°C [8], whereas Netschaëvo reached temperatures of 920 1080°C [9]. Pressure during crystallization was <0.7 GPa [8]. IIE irons formed under highly reducing conditions (IW -1.43 to -2.09) [8]. Metal composition: Siderophile elements will mostly remain in the metal portion of meteorites during planetary processes such as differentiation and density separation, impact melting etc. Therefore, to examine relationships between meteorite groups we only compared analyses the metal portions and not bulk analyses. When the concentrations of Ni, Ga, Ge, Ir, Co, and Au in the metal portion of OCs are plotted on the iron meteorite classification diagram, the H chondrites overlap in composition with the IIE irons and Mont Dieu (ungrouped iron), whereas the other OCs form distinct clusters (Fig. 1). The Ge vs Ni and Ir vs Ni contents show a complete overlap of IIE irons and H chondrite metal, with Co vs Au showing close agreement. These elements are classified as moderately siderophile to highly siderophile elements [11], thus melting a H chondrite source under high-T, low-P, lowfO2 conditions is expected to preserve the concentrations of these elements in the metal (IIE). As a moderately siderophile element with a (GaBSE/GaCI)/(FeBSE/FeCI)>1 (where XBSE = Bulk Silicate Earth abundance and XCI = CI chondrite abundance) [11], Ga is found in both the metal and silicate of H chondrites [12]. However under high-T, low-P, low-fO2 conditions the silicate:metal partition