The compositions of chromites from volcanic rocks is of interest for two reasons. First, most chromite-rich rocks from intrusions and the mantle are enriched in Os, Ir, Ru (Ir-platinum-group elements, IPGE), and Rh relative to volcanic rocks and in some cases are also enriched in Pd and Pt. There is a small data base showing that chromites from volcanic rocks contain IPGE and Rh, but that chromites from intrusions generally do not contain any of the PGE. Thus, the role that chromite plays in enriching chromite-rich rocks in PGE is not clear. Secondly, the major ore deposits are found within three layered intrusions and the compositions of the magmas which formed the intrusions and conditions under which the deposits formed are a subject of debate. All three intrusions contain massive chromite layers and because chromite is one of the first minerals to crystallize it is possible that chromite compositions could indicate the compositions of the magmas. However, the composition of chromite in an intrusion may change after crystallization due to a number of processes, including re-equilibration during slow cooling or injection of new magma and alteration during metamorphism. In order to evaluate what the composition of chromite can tell us a baseline of the initial chromite composition is required.

A comparison of the variations in the compositions of chromites from three komatiite flows shows that the chromite compositions reflect the composition of the evolving liquids. Furthermore, the Cr# records higher equilibration temperatures than the Fe#. The 3+ and 4+ ions (Al, Ti, Sc, Ga, V, and Zr) correlate with Cr#, whereas 2+ ions (Mn, Co, Ni, and Zn) correlate with Fe#, indicating that 3+ and 4+ ions are more reliable indicators of initial magma composition. Compared with compositions of volcanic chromites from the literature, the 2+ ions concentrations are not distinctive. In contrast, the 3+ and 4+ ions combined with Cr# distinguish chromites from MORB, boninite, arc picrites, high-Ti picrites, and komatiites.

The Alexo komatiite chromites contain IPGE and Rh, but only Ru is controlled by chromite. Another mineral, either olivine or an Ir-Rh platinum-group mineral, controls Ir and Rh. Compared with chromites from volcanic rocks of island arcs, the komatiite chromites and chromites from large igneous provinces are richer in Ru and poorer in Rh. This difference is attributed to the higher fO2 of arc magmas, a point reinforced by the lower V content and higher Fe3+/FeT ratios of chromites from volcanic rocks of island arcs.

You do not have access to this content, please speak to your institutional administrator if you feel you should have access.