Effect of clinopyroxene composition on Fe–Mg distribution coefficient between garnet and clinopyroxene

Abstract

This study examines the effect of jadeite content (Xjd) in omphacite on Fe–Mg distribution coefficient between garnet and clinopyroxene (K_D) by using natural data obtained from ultrahigh–pressure eclogites of the Sulu region, China. A previous study has already pointed out that a negative correlation between lnK_D and Xjd and a positive correlation between Fe# [= Fe²⁺/(Fe²⁺ + Mg)] of omphacite and Xjd are present. On the other hand, this study newly recognized that K_D decreases with decreasing Ca + Na + K contents in M2 site of omphacite, indicating that excess Fe²⁺ may be incorporated into M2 site of omphacite with decreasing Ca + Na + K contents and hence total Fe# values of omphacite may become excessively high. Thus, application of garnet–clinopyroxene geothermometer for omphacite with significant amount of jadeite and/or ferrosilite (and enstatite) components may overestimate the peak metamorphic temperatures in some cases. As a first approximation, it is better to avoid the use of omphacite with Ca + Na + K contents significantly lower than 1.0 a.p.f.u. for the application of garnet–clinopyroxene geothermometer.