DIOGENITES AS ASTEROIDAL CUMULATES - INSIGHTS FROM ORTHO-PYROXENE TRACE-ELEMENT CHEMISTRY

Eucrite, howardite, and diogenite members of the achondrite meteorites are considered by many to be genetically related. Therefore, each pro vides a piece of the puzzle for reconstructing magmatic processes on t he eucrite parent body (EPB). The interpretation of the magmatic histo ry of the diogenites (orthopyroxenites) is compromised to a great exte nt because the magmatic major element signature of orthopyroxene has b een reset and some minor elements such as Al have been compromised by coupled substitution mechanisms. As a further test of the models for t he origin of diogenites, we have analyzed a suite of twenty-one diogen ites (approximate to 160 individual analyses) for minor and trace elem ents using ion microprobe techniques. The concentrations of incompatib le elements are low in the orthopyroxenes analyzed, while their variab ility in the orthopyroxenes is both extensive and consistent. The rang e of averages in Yb varies by a factor of 16 from Ellemeet to LEW 8879 . Over this suite of diogenites, Zr varies by a factor of 117 and Y va ries by a factor of 151. This variability exceeds the range noted by p revious INAA studies of orthopyroxene separates. These incompatible tr ace elements exhibit a strong positive correlation with Ti. The consis tent incompatible element variability among diogenites, limited textur al evidence for subsolidus exsolution modification, and the expected s lower 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 incompat ible trace element systematics of Y and Yb, over 90% crystallization i s necessary to explain the variation in concentrations from Peckelshei m(most depleted) to LEW 88679 (most enriched) assuming constant D's. O ver 70% crystallization of orthopyroxene is required if D-y and D-yb i ncrease by a factor of three over the same suite of diogenites. Based on terrestrial analogs, it appears highly unlikely that a single basal tic magma will produce such a mono-mineralic orthopyroxene cumulate ho rizon with 70-90% crystallization of the parental melt. Two models tha t potentially explain this extensive incompatible element variability are: (1) the melts from which the diogenites formed are normative orth opyroxene enriched and normative plagioclase depleted or; (2) the suit e of diogenites represent multiple basaltic melts with distinctly diff erent incompatible element enrichments. Melt compositions that were ba ck-calculated from the orthopyroxene data indicate that the diogenites crystallized from melts that had a wider range in incompatible elemen ts 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 eucr itic melts that were parental to the incompatible element enriched dio genites have not yet been sampled.