THE PETROGRAPHY, MINERALOGY AND ORIGINS OF CALCIUM-SULFATE WITHIN THE COLD BOKKEVELD CM CARBONACEOUS CHONDRITE

A detailed scanning and transmission electron microscopy study of Cold Bokkeveld has shown that calcium sulphate is widespread, occluding approximately 30 mum wide matrix-cutting fractures in addition to mum-sized veins and irregular dissolution pores within calcitized chondrules, matrix calcite grains and Ca- and Al-rich inclusions (CAI). The majority of calcium sulphate crystals have fibrous habits, especially those which have grown within straight-sided fractures and veins. Mineralogically, the calcium sulphate is composed of a fine-scale mixture of hemihydrate and anhydrite which have probably formed by the dehydration of primary gypsum during sample preparation. In all contexts, calcium sulphate precipitation was the last identifiable diagenetic event in the pre-terrestrial history of Cold Bokkeveld and followed pervasive aqueous alteration of the meteorite matrix to phyllosilicates. The fibrous fracture- and vein-filling calcium sulphates are morphologically similar to a terrestrial form of gypsum termed ''satinspar'' and so may have formed in a similar manner by precipitation from supersaturated aqueous solutions during fracture and vein dilation. The aqueous solutions were most probably generated by melting of water ice due to internal heating and/or post-accretional impact heating of the parent body. Although impact-produced fractures and veins may have provided conduits for fluid advection, the dissolution of calcite, alteration of metal sulphides and precipitation of gypsum probably took place when aqueous solutions were more-or-less static. Despite their similarity to late-stage mineralized fractures in CI meteorites, the calcium sulphate-filled fractures in Cold Bokkeveld probably do not have any significance regarding a shared CM-CI parent body.