Abstract
Enigmatic dunitic veins and veinlets crosscutting a podiform chromitite ore body discovered in Wadi Rajmi, northern Oman ophiolite, display a peculiar characteristic of being almost or completely spinel-free. Olivines show an evolution trend from the spinel-free dunites (Fo (sub 93-94) and 0.4 wt% NiO) to the spinel-bearing dunites (Fo (sub 91-93) and 0.2-0.3 wt% NiO). The Cr/(Cr+Al) of chromian spinel grains increases from 0.6 to 0.8, accompanied by a slight increase in Fe (super 3+) ratio, and the Fo content of olivine decreases in the spinel-bearing dunites. A high-Mg magma, initially undersaturated with chromian spinel, precipitated the spinel-free dunites. Once the spinel saturation of the melt was attained by fractionation, the spinel-bearing dunites precipitated. The initial magma was possibly komatiitic in nature, which was produced at an early stage of detachment of the oceanic lithosphere that formed the Oman ophiolite.