Dynamic crystallization experiments on the Angra dos Reis achondritic meteorite

Abstract

With the recent discovery of angrite meteorites in the Antarctic collection which clearly have a melt origin the hypothesis that Angra dos Reis (ADOR) itself is a cumulate is being reconsidered. A recent model suggests that ADOR is a porphyry, not a cumulate. Dynamic crystallization experiments have been conducted to determine whether a melt of ADOR composition can crystallize with porphyritic textures at reasonable cooling rates under appropriate heterogeneous nucleation conditions. The nucleation conditions were varied by melting the starting material at different degrees of superheat. Porphyritic textures were produced if nuclei are absent from the melt and embryos are few in number. If some degree of supercooling is developed while the embryos are growing to critical size, the phenocrysts grow with skeletal shapes. Fassaitic pyroxene grows over a large temperature interval before the appearance of kirschsteinite, spinel, and a pyroxferroite-like phase. Alternatively, if the nucleation density is high and irregularity distributed at the beginning of cooling, a granular texture very similar to ADOR is produced. In this case the large fassaites would grow later and poikilitic enclose the early formed granular fassaite. The phases present and their composition depend on the oxygen fugacity, and a complex redox history for ADOR can be inferred. Olivine is not present in quantities compatible with ADOR and kirschsteinite appears to have been stabilized kinetically in the experiments. While a volcanic or hypabyssal history for ADOR is possible based on the experiments, the presence of olivine, particularly as a phenocryst, remains unexplained.