Ruthenium in chromite as indicator for magmatic sulfide liquid equilibration in mafic-ultramafic systems

Abstract

The platinum-group element ruthenium (Ru) is an important petrogenetic tracer of Earth’s accretion history, core-mantle interaction, mantle evolution and the exploration for magmatic sulfide deposits. However, its geochemical behavior in mafic-ultramafic systems is still not fully understood, which limits its usefulness in the predictive modelling of geochemical systems.To further develop the use of Ru as a petrogenetic tracer, we analyzed the Ru contents of chromites from a global sample set of komatiites, komatiitic basalts, and ferropicrites by laser ablation ICP-MS and Carius tube isotope dilution ICP-MS analysis. The Ru data are combined with full major and minor element microprobe analyses. The data show that two groups of chromite can be distinguished on the basis of their Ru contents. This bimodal distribution occurs across komatiites, komatiitic basalts and ferropicrites and corresponds to the sulfide saturation state of the magma during chromite crystallization: chromites from sulfide-undersaturated magmas contain between ∼150 and 600 ppb Ru, whereas chromites that crystallized from sulfide-bearing magmas mostly contain less than ∼150 ppb Ru. The Ru contents are independent of elements that typically document a modification of chromite, suggesting that the determined Ru concentrations reflect the primary magmatic chromite composition. The Ru contents are furthermore independent of magma type (i.e. komatiites, komatiitic basalts, ferropicrites), the magma source regions (i.e. different cratons, belt and localities), the geochemical affinity (i.e. Munro-type and Karasjok-type), and age (i.e. 2.7 Ga and 2.0 Ga) and neither local fluctuations in fO2, nor emplacement styles (i.e. intrusive vs. extrusive) can account for the bimodal Ru distribution in chromite observed during this study.As a consequence, it is argued that the Ru contents of chromites from mafic and ultramafic systems are indicative of the presence or absence of a sulfide liquid during chromite crystallization. In sulfide-saturated systems, the chalcophile Ru will dominantly partition into sulfides, whereas in the absence of sulfides, Ru preferentially partitions into chromite over all other available phases. Because (i) high Ru contents in chromites are exclusively associated with sulfide-undersaturated systems, and (ii) the Ru contents of chromites can be measured via fast and cost-effective laser ablation ICP-MS, Ru variability patterns in chromites allow the identification of magmas that have equilibrated with magmatic sulfide liquids prior to or during chromite crystallization and hence have potential to host metal sulfide orebodies.