Experimental constraints on mercury solubility both in Fe-Ni(-S) metal and volatile-bearing silicate melts at high pressure and temperature

Magmas generated by partial melting of mantle rocks are the main carriers of volatile species (e.g., CO₂, H₂O, SO₂) and trace elements (rare earth elements, Hg, Co, etc.) to the Earth’s surface. Mercury (Hg) is of particular interest because it has been widely used over the last decade as a marker of large-scale volcanic eruptions in sedimentary records, owing to its relatively long atmospheric residence time (0.5–2 years; Bagnato et al. 2007), and its association with mass extinction events (Percival et al. 2018). Although Hg is present at low abundance in the silicate Earth (10 ppb; McDonough and Sun 1995), isotopic studies on sediments point to a predominantly volcanic origin (Grasby et al. 2019). However, no experimental work has yet constrained the mechanisms by which Hg is mobilized from mantle sources to the atmosphere, and only a few geochemical studies on meteorites and peridotite xenoliths suggest that sulfide minerals are the main Hg host at depth (Canil et al. 2015).

In this study, Hg solubility in Fe–Ni–S alloy was investigated at 6 GPa and 700–1400 °C using a rotating multi-anvil apparatus (MavoPress LPT 500-400/50 with a Walker-type module) at the Department of Earth Sciences, Sapienza University of Rome. The starting materials consisted of a mixture of pure Fe and Ni powders doped with 5 wt.% natural cinnabar (HgS) as the Hg source, allowing quantitative analysis by electron microprobe. In addition, Hg solubility in synthetic 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). Two starting compositions were employed, a synthetic picritic glass and a carbonate–silicate glass, each mixed with ~5 wt.% natural HgS.

The results show that Hg increasingly partitions into the Fe-Ni alloy with rising temperature. In the presence of silicate melts, Hg concentrations of up to ~1700 ppm under sulfur-saturated conditions are observed, with similar contents in both carbonate-silicate and picritic melts. Additionally, Hg abundance is primarily controlled by the concentration of dissolved sulfur. These experimental constraints are finally compared with the limited available data on Hg concentrations in natural volcanic rocks to better quantify the deep Hg cycle.

References

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Canil et al. (2015). Chem. Geol., 396, 134-142

Grasby S.E. et al. (2019). ESR, 196, 102880.

McDonough W.F., Sun S.S. (1995). Chem. Geol., 120 (3-4), 223-253

Percival L. M. et al. (2018). AJS, 318(8), 799-860

Wu P. et al. (2024). Matter Radiat. Extremes 9, 027402

Xu Y. et al. (2025). Matter Radiat. Extremes 11, 017803