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

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.