Mercury in some arc crustal rocks and mantle peridotites and relevance to the moderately volatile element budget of the Earth

Abstract

We measured Hg concentrations in 37 igneous rocks from an arc crustal section and in 30 mantle peridotites from ophiolite, orogenic massif and xenolith settings. Mercury is heterogeneously distributed in the igneous rocks and shows a ‘nugget effect’, suggesting it is concentrated in a trace phase, likely sulfide. The abundance of Hg in the crustal samples varies from 0.9 to 8ppb and correlates with S and Cu but no other element indicative of differentiation. The average of our data produces 2.9±2.6 Hg for the bulk crust, a factor of 10 lower than previous estimates. The mantle peridotites contained 0.2–5ppb Hg and a correlation of Hg with Al, Cu, S or loss on ignition (LOI) depending on sample type. Secondary uptake of Hg due to low-temperature alteration or mantle metasomatism is evident in the ophiolite and orogenic massif samples, respectively. The primitive upper mantle (PUM) contains 0.4–0.6ppb Hg based on the depletion/enrichment trends in the fresh xenolith samples that demonstrably retained primary Cu/S during emplacement. During mantle melting to produce the crust, Hg behaves as a mildly incompatible element (DHg residue/melt~0.1), not unlike Cu. For a chondritic abundance of 310ppb Hg, our estimate for Hg in the mantle requires this element has a similar depletion to Se, Te or S in the bulk silicate Earth.