Differentiation history of the mesosiderite parent body: Constraints from trace elements and manganese-chromium isotope systematics in Vaca Muerta silicate clasts

We report here the results of a study of trace element microdistributions and Mn-53-Cr-53 systematics in several basaltic and orthopyroxenitic clasts from the Vaca Muerta mesosiderite. Ion microprobe analyses of selected trace and minor element abundances in minerals of the silicate clasts indicate that, following igneous crystallization, these clasts underwent extensive metamorphic equilibration that resulted in intra- and inter-grain redistribution of elements. There is also evidence in the elemental microdistributions that these clasts were subsequently affected to varying degrees by alteration resulting from redox reactions involving the indigenous silicates and externally derived reducing agents (such as phosphorus, derived from the mesosiderite metal) at the time of metal-silicate mixing. Furthermore, our results suggest that the varying degrees of alteration by redox reactions recorded in the different clasts were most likely facilitated by different degrees of remelting induced by heating during the metal-silicate mixing event. After taking into account the effects of these postmagmatic secondary processes, comparison of the trace and minor element concentrations and distributions in minerals of basaltic and orthopyroxenitic clasts with those of noncumulate eucrites and diogenites, respectively, suggests that the primary igneous petrogenesis, including parent magma and source compositions, of Vaca Muerta silicates were similar to those of achondritic meteorites of the Howardite-Eucrite-Diogenite (HED) association. Internal Mn-53-Cr-53 isochrons obtained for two basaltic (pebble 16 and 4679) and two orthopyroxenitic (4659 and 4670) clasts show that chromium isotopes are equilibrated within each clast. Nevertheless, just as for noncumulate eucrites and diogenites, Cr-53 excesses in whole-rock samples of the basaltic clasts (similar to1.01 epsilon in pebble 16; similar to1.07 epsilon in 4679) are significantly higher than in the orthopyroxene-rich clasts (similar to0.62 epsilon in 4659; similar to0.53 epsilon in 4670). As in the case of the HED parent body, this suggests that Mn/Cr fractionation in the parent body of the Vaca Muerta silicate clasts occurred very early in the history of the solar system, when Mn-53 was still extant. However, the slope of the Mn-53-Cr-53 isochron 3 155 defined by the whole-rock samples of Vaca Muerta clasts (corresponding to a Mn-53/Mn-55 ratio of 3.3 +/- 0.6 X 10(-6)) is distinctly lower than that defined by the HED whole-rock samples (corresponding to a Mn-53/Mn-55 ratio of 4.7 +/- 0.5 X 10(-6)), indicating that the global Mn/Cr fractionation event that established mantle source reservoirs on the parent body of the Vaca Muerta silicate clasts occurred similar to2 Ma after a similar event on the HED parent body. Copyright (C) 2003 Elsevier Ltd.