Calculated phase equilibria for a morb composition in a P-T range, 450-650 °C and 18-28 kbar: the stability of eclogite

Abstract

Lower temperature eclogite (with T = 600 °C) represents a significant part of the occurrences of eclogite in orogenic belts. 'True' eclogite, with, for example, garnet + omphacite >70%, is well represented in such an occurrence. Calculated phase equilibria in Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2-O (NCKFMASHTO), for just one rock composition - that of a representative mid-ocean ridge basalt, morb- are used to see under what circumstances 'true' eclogite is predicted to occur. The variables considered are not only pressure (P) and temperature (T) but also water content and oxidation state. The latter two variables are known to exert a significant control on mineral assemblage but are diffi..