Source composition or melting effect: New evidence from Archean komatiites concerning the origin of low highly siderophile element abundances in Earth’s mantle

Abstract

Highly siderophile element (HSE) contents of komatiites have been widely used to estimate the HSE composition of Earth’s mantle. However, the interpretation of existing komatiite data is controversial, with some authors arguing that the Archean deep mantle komatiite source was impoverished in HSE due to slow admixture of a late accretion component, while others invoke a melting process that would allow observed komatiite abundances to be obtained from a mantle source with present-day HSE abundances. To obtain insight into this issue, we present new HSE abundance data for komatiites from the Gorumahishani greenstone belt of the Singhbhum Craton, eastern India. Our Sm-Nd and Re-Os isotope data indicate a ∼3.5 Ga age for these little-studied rocks, which provide extreme examples of Al-depleted and Ti-depleted komatiite varieties, juxtaposed over a short-length scale. The calculated parental melt compositions for the Al-depleted komatiites have 2.7 ± 0.2 ppb Ru, 3.4 ± 0.2 ppb Pt, and 3.2 ± 0.6 ppb Pd, whereas, for the Ti-depleted type these values are 4.4 ± 0.3 ppb Ru, 3.2 ± 0.6 ppb Pt, 3.0 ± 0.5 ppb Pd. These concentrations are similar to those found in most Archean komatiites at >3.4 Ga. For the Al-depleted samples, these values would correspond to mantle abundances equivalent to ∼38 % of modern Bulk Silicate Earth (BSE) Ru contents and ∼24 and ∼21 % of BSE Pt and Pd contents, respectively, if it is assumed that simple extrapolation of the measured values to the MgO content of fertile peridotite provides an adequate approximation of the HSE composition of the BSE. To examine the alternative model that the low contents of Ru, Pd and Pt in Gorumahishani komatiites could be obtained from a mantle source with BSE-like HSE contents, we apply a simple two-stage critical melting model using current experimental HSE partitioning coefficients. The Ru abundances of the Gorumahishani Al-depleted komatiitic magmas can be produced from the pooled melts of a fertile source with BSE-like Ru and S contents during the first melting stage. The Ru abundances of the Ti-depleted komatiitic magmas can then be produced from remelting the residue left by this first melting stage. On the other hand, Pt and Pd abundances cannot be successfully modelled for either the Al-depleted or the Ti-depleted komatiites using available partition coefficients, though our current understanding of Pt and Pd partitioning after sulfide exhaustion is limited. The use of komatiites to characterize the abundance and distribution of HSE in the early mantle critically depends on developing a better understanding of the partitioning behaviors of these elements between mantle sources and komatiitic magmas.