CLASSIFICATION OF MAFIC CLASTS FROM MESOSIDERITES - IMPLICATIONS FOR ENDOGENOUS IGNEOUS PROCESSES

We have analyzed thirteen igneous pebbles from the Vaca Muerta, EET87500, and Bondoc mesosiderites by electron microprobe and instrumental neutron activation and combined these data with literature data for forty-three analyzed mesosiderite clasts. We classify these well-characterized clasts into the following five principal groups: 1) Polygenic and monogenic cumulates (39%) are coarse-grained gabbros that are highly depleted in incompatible elements (relative to H chondrites); they formed at moderate depth either as residues of low-degree partial melting of pre-existing cumulate eucrites or as cumulates from parent melts similar to cumulate eucrites. 2) Polygenic basalts (30%) are finer-grained rocks with positive europium anomalies, La/Lu ratios < 1, and lower rare earth element abundances than basaltic eucrites. It seems likely that these rocks were formed near their parent body surface by remelting mixtures of major amounts of basaltic eucrites and lesser amounts of cumulate eucrites. 3) Quench-textured rocks comprise two compositional groups, (a) those which resemble basaltic eucrites (5%), and (b) those which resemble cumulate eucrites (2%). The quench-textured rocks are probably monogenic; they formed most likely when small-scale impacts at their parent body surface totally melted small amounts of basaltic or cumulate eucrite material. 4) Monogenic basalts (11 %) resemble basaltic eucrites and formed by endogenous igneous processes on the mesosiderite parent body (MPB). The monogenic basalts and those quench-textured rocks that resemble basaltic eucrites in composition plot subparallel to the Nuevo Laredo trend eucrites on diagrams of incompatible element concentration vs. mg*. Previous work has shown that the Nuevo-Laredo trend eucrites were probably produced by fractional crystallization; it seems likely that the monogenic basalts from the MPB had a similar origin. 5) Ultramafic rocks are cumulates consisting mainly of large crystals of orthopyroxene (9%) or olivine (4%). Orthopyroxenite clasts closely resemble diogenites and were formed most likely by endogenous igneous processes. Olivine crystals vary significantly in major element composition; some may be endogenous, but others may have been derived from the disrupted mantle of a different parent body.