Pergamon Geochimica et Cosmochimica Acra, Vol. 59, No. 14, pp. 3071-3084. 1995 Copyright 0 1995 Elsevier Science Lid Printed in the USA. All rights reserved zyxwvutsrqpo 0016-7037/95-$9.50 + .I0 0016-7037(95)00196-4 Diogenites as asteroidal cumulates: Insights from orthopyroxene trace element chemistry G. W. FOWLER, C. K. SHEARER, J. J. PAPIKE, and G. D. LAYNE Institute of Meteoritics, Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87 13 1 -I 126, USA (Received September 12, 1994; accepted in revised form April 6, 1995) Abstract-Eucrite, howardite, and diogenite members of the achondrite meteorites are considered by many to be genetically related. Therefore, each provides a piece of the puzzle for reconstructing magmatic processes on the eucrite parent body zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA (EPB ). The interpretation of the magmatic history of the diogenites (orthopyroxenites) is compromised to a great extent because the magmatic major element signature of orthopyroxene has been reset and some minor elements such as Al have been compromised by coupled substitution mechanisms. As a further test of the models for the origin of diogenites, we have analyzed a suite of twenty-one diogenites (= 160 individual analyses) for minor and trace elements using ion micro- probe techniques. The concentrations of incompatible elements are low in the orthopyroxenes analyzed, while their vari- ability in the orthopyroxenes is both extensive and consistent. The range of averages in Yb varies by a factor of 16 from Ellemeet to LEW 88679. Over this suite of diogenites, Zr varies by a factor of 117 and Y varies by a factor of 151. This variability exceeds the range noted by previous INAA studies of ortho- pyroxene separates. These incompatible trace elements exhibit a strong positive correlation with Ti. The consistent incompatible element variability among diogenites, limited textural evidence for subsolidus exsolution modification, and the expected slower diffusion rates of the REE, Ti, and Y relative to Fe-Mg indicate that the trace elements in the diogenitic orthopyroxene may reliably preserve the magmatic history of the diogenites. Based on the incompatible trace element systematics of Y and Yb, over 90% crystallization is necessary to explain the variation in concentrations from Peckelsheim (most depleted) to LEW 88679 (most enriched) assuming constant D’s. Over 70% crystallization of orthopyroxene is required if Dy and Dyb increase by a factor of three over the same suite of diogenites. Based on terrestrial analogs, it appears highly unlikely that a single basaltic magma will produce such a mono-mineralic orthopyroxene cumulate horizon with 70-908 crystallization of the parental melt. Two models that potentially explain this extensive incompat- ible element variability are: ( 1) the melts from which the diogenites formed are normative orthopyroxene enriched and normative plagioclase depleted or; (2) the suite of diogenites represent multiple basaltic melts with distinctly different incompatible element enrichments. Melt compositions that were back-calculated from the orthopyroxene data indicate that the diogenites crystallized from melts that had a wider range in incompatible elements than that exhibited by the main group eucrites. If the assumptions made in the calculation of these melts compositions are correct, this may be interpreted to mean that either many of the diogenites are not fractional crystallization products of eucritic melts or that the eucritic melts that were parental to the incompatible element enriched diogenites have not yet been sampled. 1. Ih’TRODUCTION The howardite, eucrite, and diogenite (HED) lithologies are believed to have originated on asteroid 4 Vesta during an extensive melting event at -4.6 b.y. (Consolmagno and Drake, 1977; Drake, 1979; Binzel and Xu, 1993). Eucrites are pigeonite/plagioclase basalts while diogenites are ortho- pyroxenites, with minor to trace amounts of olivine, clino- pyroxene, plagioclase, and chrornite. As a first approximation, howardites are two-component brecciated mixtures of eucrites and diogenites. There are two distinctly different models that have been proposed to explain the relationship between the eucrites and diogenites: ( 1) fractional crystallization and (2) partial melting. In fractional crystallization models (Mason, 1967; Warren, 1985; Warren and Jerde, 1987; Bartels and Grove, 1991; Grove and Bartels, 1992), eucrites and dioge- nites represent a complementary continuum of planetary frac- tional crystallization products in which diogenites are ortho- pyroxene cumulates that formed during the crystallization of primary magmas and eucrites formed from residual melts after the crystallization of most of this magma. Other models suggest that the eucrites represent partial melts from a prim- itive, chondritic mantle source (Stolper, 1977; Consolmagno and Drake, 1977). Within this type of a model, diogenites are still generally considered to be cumulates, but their pet- rogenetic relationship to the eucrites is less clear. Stolper ( 1977) and Consolmagno and Drake ( 1977) suggested di- ogenites are cumulates produced by crystallization of mag- mas produced by higher degrees of partial melting than eu- critic magmas thus far represented in the present meteorite collection. Sack et al. ( 1991) proposed that the olivine di- ogenites represent residua from the partial melting events that produced eucritic liquids. However, the interpretation of Sack et al. (1991) has been challenged by Papike et al. (1993), Shearer et al. (1993), Mittlefehldt (1994), and Fowler et al. ( 1994a). Clearly, the nature of magmatism in the parent planetary body for eucrites and diogenites is still not resolved. 3071