Hotspot volcanism and highly siderophile elements

Application of the Re-Os isotope system to hotspot volcanism over the past 20 years has expanded our understanding of chemical geodynamics. The utility of the 187Re-187Os isotope system (t1/2 = ~42 Ga) lies in the contrasting compatibilities of Re (less compatible) and Os (more compatible) during basaltic melt generation, and their siderophile and chalcophile behaviours, which offer unique and complementary information to other long-lived parent-daughter isotope systems (e.g., Rb-Sr, Sm-Nd, Lu-Hf, U-Th-Pb). Present-day mantle-derived hotspot lavas, which typically derive from sources with distinct long-term time-integrated parent-daughter fractionations for Sr-Nd-Hf-Pb isotopes, preserve a large range in 187Os/188Os ratios (~0.12-0.18), translating to variable long-term time-integrated Re/Os ratios. This observation supports the standard view of mantle geochemistry that isotopic heterogeneity observed in hotspot volcanism derives from subduction of sediments, crust and lithosphere into a heterogeneous convecting upper mantle. More recently, understanding of mantle processes has been enhanced by combined Re-Os isotope and highly siderophile element (HSE: Os, Ir, Ru, Ru, Pt, Pd, Re, Au) abundance measurements, which allow assessment of both long-term parent-daughter fractionation, and inter-element fractionation of the HSE that are strongly controlled by S in the mantle, and hence to S fugacity and melting behaviour. Additionally, high-precision measurement of the long-lived 190Pt-186Os system (t1/2 = ~470 Ga) enables assessment of long-term Os-Pt-Re fractionation within and between large-scale planetary reservoirs. Here I provide examples from continental flood basalt and ocean island basalt localities of how Os isotopes and the relative and absolute abundances of the HSE may be utilized to trace the character of their source mantle, and the relative proportions of recycled oceanic/continental crust and/or lithosphere that may be preserved within their mantle sources. For some hotspot volcanoes, melting and emplacement processes, such as concomitant assimilation and fractional crystallization, may also have profound influence on HSE fractionation behaviour, and may impact Os isotopic ratios of parental melts. Furthermore, while there remains potential to trace core-mantle interaction using Pt-Os isotope systematics, there are other reservoirs in the crust and mantle which have HSE concentrations and long-term Pt/Os ratios sufficient to generate 186Os/188Os anomalies, and further work is required to explore this isotope system in hotspot lavas. This work is supported by grants from the NSF (EAR 1116089) and The San Diego Foundation (C-2011-00204).