Diamond: Depth of Crystallization Inferred from Compressed Included Garnet

Data on the compressed state of garnet included in diamond are used to place restrictions on the pressure (P)-temperature (T) conditions of inclusion. To do this we first locate a P-T point or region within which the relative compression between host and inclusion is eliminated. Then we use comparison dilatometry of the two minerals to 7 kbar to determine a set of P-T curves of constant difference in natural strain. From that set we then select a subset encompassing the P-T condition(s) found in the first step. Extrapolation of such curves to plausible geotherms shows that the probable source of many diamonds associated with garnets of the type found in eclogites lies within the stability field of diamond somewhere between 145 km and 300 km beneath the surface, consistent with independent inferences from chemical petrology.