The influence of chromite on osmium, iridium, ruthenium and rhodium distribution during early magmatic processes

Chromite-rich plutonic rocks are enriched in Os, Ir, Ru (IPGE) and Rh which is usually attributed to the presence of micron-size inclusions of platinum-group minerals. It is also known that volcanic rocks show positive correlations between Cr and IPGE and Rh with the most primitive lavas (olivine + chromite phyric) being enriched in these elements compared to the more fractionated lavas suggesting that chromite phenocrysts somehow influence and concentrate the IPGE and Rh. Previous results from in situ analysis of chromite phenocrysts from komatiites and from oxidized arc lavas showed that they contain IPGE and Rh in their structure. Our new results confirm these previous observations and show that the enrichment is a common phenomenon observed in volcanic rocks from several geological settings including MORB, boninite, komatiite, picrite and Hawaiian tholeiite. In contrast, in situ analysis of plutonic chromites show that they contain low concentrations of IPGE and Rh. Mass balance calculations indicate that chromite phenocrysts from volcanic samples are not the major hosts of Os, Ir and Rh accounting for < 40%, < 25% and < 30% of the whole rock budgets, respectively, but they account for ≥ 30% of the whole rock Ru budget. As in the case of volcanic chromites, plutonic chromites do not have a great influence on Os, Ir and Rh whole rock budget in accounting for < 25%, in contrast with volcanic chromites, plutonic chromites account for ≤ 10% of the whole rock Ru budget. These new results show that chromite from rapidly cooled environments can act as the main Ru-carrier phase but has a minor role in hosting Os, Ir and Rh. Overall, plutonic chromite has a minor role in hosting IPGE and Rh. Chromites from Bushveld ultramafic sills have also been analyzed. They show IPGE and Rh enrichments comparable to chromites from plume related magmas (komatiite and picrite) whereas chromites from Bushveld chromitites show low IPGE and Rh concentrations like other chromitite samples despite the fact that they crystallized from magmas of similar composition. This clearly indicates a change in Os, Ir, Ru and Rh behavior between rapidly cooled and slowly cooled environments.