METAMORPHISM OF ORDINARY CHONDRITES - A REVIEW

Textural variations among the ordinary chondrites are paralleled by mineralogioal and chemical trends, most of which are consistent with the view that these chondrites have experienced various degrees of alteration in the solid state. On the basis of mineralogical and textural data, it is inferred that this alteration took place at temperatures of roughly 400 to greater than 820°C, under load pressures on the order of 1-2 kbar, in the absence of penetrative stress and under relatively dry, reducing conditions. The mineralogical data do not indicate whether alteration took place during reheating of cold material (progressive metamorphism) or during cooling of a hot agglomerate (autometamorphism). The former mechanism seems more likely, for the latter requires extremely rapid and deep burial, which would be difficult to achieve within existing models for the origin of chondritic material. Alternative models which attribute the textural, mineralogical and chemical variations among ordinary chondrites to crystal-liquid-vapor interactions prior to accumulation find little support in the chondrites and are regarded as less satisfactory than the metamorphic hypothesis. © 1969.