PETROGENETIC MODELS FOR MAGMATISM ON THE EUCRITE PARENT BODY - EVIDENCE FROM ORTHO-PYROXENE IN DIOGENITES

Diogenites are recognized as a major constituent of the howardite, euc rite and diogenite (HED) meteorite group. Recently, several papers (Mi ttlefehldt, 1994; Fowler et al., 1994, 1995) have identified trace-ele ment systematics in diogenites that appeared to mimic simple magmatic processes that involved large degrees of crystallization (up to 95% or thopyroxene) of basalt with extremely high normative hypersthene. Such a crystallization scenario linking all the diogenites is highly unlik ely. The purpose of this study is to explore other possible models rel ating the diogenites. Computational major-element melting models of a variety of different potential bulk compositions for the eucrite paren t body (EPB) mantle indicate that these compositions show a similar se quence in residuum mineral assemblage with increasing degrees of parti al melting. Numerous bulk compositions would produce melts with Mg# ap propriate for diogenitic parent magmas at low to moderate degrees of p artial melting (15% to 30%). These calculations also show that melts w ith similar Mg# and variable incompatible element concentrations may b e produced during small to moderate degrees of EPB mantle melting. The trace-element characteristic of the orthopyroxene in diogenites does not support a model for large amounts of fractional crystallization of a single ''hypersthene normative'' basaltic magma following either sm all-scale or large-scale EPB mantle melting. Small degrees of fraction al crystallization of a series of distinct basaltic magmas are much mo re likely. Only two melting models that we considered hold any promise for producing different batches of ''diogenitic magmas.'' The first m odel involves the fractional melting of a homogeneous source that prod uces parental magmas to diogenites with an extensive range of incompat ible elements and limited variations in Mg#. There are several require ments for this model to work. The first requirement of this model is t hat the D-orthopyroxene/melt must change during melting or crystalliza tion to compress the range of incompatible elements in the calculated diogenitic magmas. The second prerequisite is that either some of the calculated diogenitic magmas are parental to eucrites or the Mg# in di ogenitic magmas are influenced by slight changes in oxygen fugacity du ring partial melting. The second model involves batch melting of a sou rce that reflects accretional heterogeneities capable of generating di ogenitic magmas with the calculated Mg# and incompatible element conte nts. Both of these models require small to moderate degrees of partial melting that may limit the efficiency of core separation.