The in vacuo release of Ar from minerals: 1. Hydrous minerals

Abstract

The systematics of Ar release during in vacuo stepwise heating of hydrous minerals is examined. By means of common-denominator three-isotope correlation diagrams, in use for 60 years, it is possible to recognize the presence of polymineralic mixtures in a step-heated mineral separate and characterize their components. These correlation diagrams, when applied to literature studies proposing Fick's Law diffusion during in vacuo stepwise heating, show that in actual fact these studies were based on partly retrogressed micas that were intergrown with their alteration products, probably at a scale < 20 μm so as to escape recognition during handpicking. The degassing of hydrous minerals in vacuo does not follow Fick's Law in an inert, homogeneous matrix, and instead proceeds from the structural collapse during dehydration. An alternative presentation discussed here quantifies the differential release of the Ar isotopes. Their release follows the structural collapse of their host phases and gives rise to remarkably well resolved patterns even in polymineralic samples.