Spinel-free dunites as a proxy to komatiitic melt activity in the mantle

Abstract

High-Mg dunites, which are spinel-free or spinel-poor, occur as veins that cut across a pod of chromitite within the mantle section deep below the Moho transition zone of the northern Oman ophiolite (Wadi Rajmi). Only thick (>50cm) veins were considered in order to avoid the effects of subsolidus Mg-Fe2+ exchanges with the chromitite wall rock on the compositions of olivine. The spinel-free dunites contain especially high-Mg (Fo>93) olivines, whereas olivines in the spinel-poor dunites have lower Fo contents (Fo91-93). In addition, chromian spinels in the spinel-poor dunites display high values of Cr# up to 0.8 [Cr#=Cr/(Cr+Al) atomic ratio]. We found that these komatiitic dunites formed via an olivine fractional crystallization process, and not by replacement, and that the chromitites served as isolating capsules where melts were preserved from melt-rock reactions with peridotite minerals. The olivines show a continuous trend in their chemistry from the spinel-free dunites, through the spinel-bearing dunites, to the high-Ca boninites described from the northern Oman ophiolite (Fizh Block). This strongly suggests a genetic linkage between the high-Mg spinel-free or spinel-poor dunites and the high-Ca boninites, which respectively represent cumulates and the resultant fractionated melts of a primary komatiitic melt. In this paper, we challenge the prevalent models that rule out the genesis of komatiites in supra-subduction zones and shed light on the conditions of the mantle at an early stage of arc evolution.