In situ determination of Os, Ir, and Ru in chromites formed from komatiite, tholeiite and boninite magmas: Implications for chromite control of Os, Ir and Ru during partial melting and crystal fractionation

Osmium, Ir and Ru behave as compatible elements during partial melting and crystal fractionation, whilst Pt, Pd and Rh behave as incompatible elements. This produces a fractionation of the platinum-group elements (PGE). There is a debate as to which mineral or minerals control Os, Ir and Ru (referred collectively as the Ir platinum-group elements, IPGE). Chromite appears to have an important role in concentrating these elements, because chromite-rich cumulates from mantle and crustal settings tend to be enriched in IPGE and in basalts and komatiites there is a positive correlation between Cr and IPGE. Broadly speaking there are two mechanisms that have been suggested whereby chromite could concentrate IPGE: i) these elements could partition into the chromite and be present in solid solution; ii) these elements could also have a very low solubility in mafic magmas and crystallise as IPGE-rich minerals together with chromite.In order to investigate whether IPGE are present in solid solution in chromite and whether minerals other than chromite are important in controlling the IPGE we have determined the IPGE content of chromites from a range of geological settings. As representatives of chromites formed from primary magmas, chromites from two boninites, a komatiite and an ocean island tholeiite were analysed. As representatives of cumulate chromites, chromites from the Stillwater Complex and as representative of chromites formed under mantle conditions, chromites from the Thetford Mines Ophiolite were analysed. These analyses were combined with whole-rock and microprobe analyses to consider the control of chromite on the IPGE.In the komatiite, IPGE spectra are homogeneous across the chromite grains and IPGE appear to have partitioned into the chromite. Mass balance calculations show that all of the Ru and 15–18% of the Os and Ir are present in the chromite. The calculated partition coefficients for the IPGE into the chromite from the komatiitic melt are DOsChr/kom = 8.3, DIrChr/kom = 9.5, and DRuChr/kom = 79. For all of the other chromites, IPGE concentrations were less than the detection limits and mass balance calculations indicate that chromite controls less than 25% of the whole-rock Os budget, less than 10% of the whole-rock Ir budget, and less than 20% of the whole-rock Ru budget. Small IPGE mineral inclusions were observed in chromites from the ophiolite and the layered intrusion and presumably these control the IPGE in most of the rock types we analysed. Thus, in our study both mechanisms that have been proposed for the association of chromite and IPGE appear to be viable. However, except for Ru in the komatiite, chromite is not the dominant mineral controlling these elements and IPGE minerals are probably the main controlling phases.

► Chromites from plutonic and volcanic rocks were analysed by LA-ICP-MS to characterise their Os, Ir and Ru content.
► Komatiitic chromite contains Ru (~ 450 ppb), Os (19 ppb) and Ir (20 ppb).
► Komatiitic chromite controls ~ 100% of Ru but < 20% of Os and Ir whole-rock budgets.
► For all other samples, chromite is not the phase that controls Os, Ir and Ru.
► IPGE-rich phases must account for the whole-rock IPGE budgets except for Ru in komatiitic chromite.