Partitioning of mercury between picritic melts, silicate minerals and Fe-Ni (-S) metals under high pressure and temperature conditions

Magmas generated by partial melting of mantle rocks are the primary carriers of volatile elements (e.g., CO2, H2O, SO2) and trace elements (e.g., REE, Hg, Co) to the Earth’s surface. Among these, mercury (Hg) is a key geochemical marker for volcanic events at a global scale [Percival et al. 2018] Despite the predicted low Hg abundance in the Bulk Silicate Earth (~10 ppb) [McDonough and Sun, 1995], isotopic signatures in the sedimentary record provide clear evidence of its predominantly deep (magmatic) origin [Grasby et al. 2019]. However, the deep Hg cycle remains poorly understood, as no experimental work has yet constrained the mechanisms of Hg mobilization and speciation during mantle melting. Currently, the hypothesis that sulphide minerals are the primary Hg host at depth [Canil et al., 2015] relies solely on limited geochemical data from meteorites and peridotite xenoliths, highlighting a significant need for experimental validation. In this study, Hg solubility in synthetic picritic melts was examined at 3-6 GPa, 1300-1550 °C, and oxygen fugacity buffered near the graphite-CO2 redox equilibrium, using six-anvils cubic presses at the Center for High Pressure Science & Technology Advanced Research (HPSTAR), Beijing [Wu et al. 2024; Xu et al. 2025]. The starting material consisted of a synthetic glass prepared by mixing high-purity reagents by proper amounts and then, heated at 1500 °C for 1 h before quench. The homogeneous glass was powdered and mixed with ~5 wt.% natural HgS (Almaden, Spain). Graphite capsules were used as containers filled with the starting material, compressed to the target P and the temperature monitored with a C-type thermocouple. In addition, Hg solubility in Fe-Ni-S alloy was investigated at 6 GPa and 700-1400 °C using a rotating multi-anvil apparatus with a Walker-type module available at the Department of Earth Sciences, Sapienza University of Rome. The quenched runs were embedded in epoxy resin, polished and analysed by scanning electron microscopy and electron microprobe. Experimental results show Hg concentrations up to ~1700 ppm in picritic melts likely controlled by the amount of dissolved sulphur. The Hg partitioning into the FeNiS alloy increases significantly with rising temperature up to 3-5 wt%. These experimental constraints, compared with the limited data available for natural volcanic rocks, allow to better quantify the deep Hg cycle and its mobilization processes. In particular, when a batch melting model is applied to our data, we show that 20% of picritic melt fraction can mobilize up to 5% of the primordial Hg stored in the deep Earth, making such melts an efficient carrier from the interior to the atmosphere.