Spinel cataclasites in 15445 and 72435: Petrology and criteria for equilibrium

Abstract

Petrologic methods widely used by metamorphic petrologists offer one approach to this problem by enabling depth determinations to be made on certain lunar highland clasts (Herzberg and Baker, 1980). The two fundamental elements of such methods are: establishing the existence of equilibrium among the coexisting phases in the rock, and applying experimentally determined phase equilibria in temperature-pressure-composition space to the natural assemblage. For the spinel cataclasites (plagioclase, olivine, orthopyroxene, pleonaste spinel ± cordierite) phase equilibria and resulting temperatures and pressures of formation are discussed in detail by Herzberg and Baker (1980), along with inferences on the structure of the lunar crust. In this paper we concentrate on the first of the above two elements by presenting petrographic and mineral chemistry data on a new spinel cataclasite from 15445 (clast H), and more extensive data than has previously been available on two clasts in 72435.