Studies of Brazilian meteorites: III. Origin and history of the Angra dos Reis achondrite

Abstract

The Angra dos Reis meteorite fell in 1869 and is a unique achondrite. It is an ultramafic igneous rock, pyroxenite, with 93% fassaite pyroxene which has 15.7% Ca-Tschermak's molecule, plus calcic olivine (Fo 53.1; 1.3% CaO), green hercynitic spinel, whitlockite (merrillite), metallic Ni-Fe, troilite, as well as magnesian kirschsteinite (Ks 62.3Mo 37.7), within olivine grains, and celsian (Cs 90.2An 7.7Ab 1.7Or 0.4) which are phases reported in a meteorite for the first time, and plagioclase (An 86.0), baddeleyite, titanian magnetite (TiO 2, 21.9%), and terrestrial hydrous iron oxide which are phases reported for the first time in this meteorite. Petrofabric analysis shows that fassaite has a preferred orientation and lineation which is interpreted as being due to cumulus processes, possibly the effect of post-depositional magmatic current flow or laminar flow of a crystalline mush. The mineral chemistry indicates crystallization from a highly silica-undersaturated melt at low pressure. Since the meteorite formed as a cumulate, pyroxene crystals may have gravitationally settled from a melt which crystallized melilite first. Plagioclase would be unstable in such a highly undersaturated melt, and feldspathoids would be rare or absent due to the very low alkali contents of the melt. The presence of rare grains of plagioclase and celsian may be the result of late-stage crystallization of residual liquids in local segregations. Thus, the Eu anomaly in Angra dos Reis may be the result of pyroxene separation from a melt which crystallized melilite earlier, rather than plagioclase as previously suggested.