Distribution and concentrations of magmatic mercury from the earliest submarine activity of Mt. Etna: the case of Aci Castello pillow lavas

Volcanic systems release a variety of volatiles (e.g., CO2, SO2, H2O) during magma ascent and degassing, along with potentially toxic trace metals such as mercury (Hg). Due to its high volatility, Hg is emitted from mantle-derived melts in widely variable amounts, reaching up to 340 t·yr-1 [Yin et al. 2024]. In the atmosphere, Hg occurs mainly as gaseous elemental Hg (GEM), which has a long residence time (~1-2 years) [Bagnato et al. 2007] and can be transported globally, eventually accumulating in sediments. Over the last decades, Hg has emerged as a valuable geochemical tracer in sedimentary rocks for large-scale magmatic events, such as Large Igneous Provinces (LIPs) [Grasby et al. 2019]. However, available data provide limited constraints on primary Hg distribution and concentrations in mantle-derived melts. Even less is known about the effect of magmatic fractionation and crystallization on Hg partitioning. Available data on volcanic rocks are relative to MORB and indicate Hg concentrations of 1.4 ± 0.6 ng/g [Yin et al. 2024]. In this study, we investigated the Hg concentration of the earliest submarine volcanic products of Mt. Etna outcropped at Aci Castello (south-east flank of the volcano). These ~500 kyr-old tholeiitic pillow lavas represent the initial stages of Etna volcanism and were emplaced at water depths of ~800-400 m onto unconsolidated Lower Pleistocene marine clays [Corsaro and Cristofolini, 2000]. Their submarine emplacement provides a unique opportunity to evaluate Hg contents in magmas that were minimally affected by shallow magmatic differentiation and conduit-related processes. A total of 40 samples were collected from 13 pillow lavas consisting of tholeiitic basalts with low porphyricity index (6-10%), with olivine and subordinate plagioclase phenocrysts set in a groundmass with intersertal texture made of plagioclase and pyroxene [Corsaro and Mazzoleni, 2002]. Sampling was carried out across individual pillow lavas, from the massive core to the glassy rim, including the clay-rich inter-pillow material. Accurate analyses of Hg were performed using a Milestone Direct Mercury Analyzer (DMA-80) Tricell available at the Earth Sciences Department of Sapienza University of Rome, employing thermal decomposition at 750 °C, amalgamation, and atomic absorption spectrophotometry. Preliminary results indicate Hg concentrations of ~1-2 ng/g in both the massive cores and the glassy rims of the pillow lavas; while intermediate portions of the pillows show lower Hg concentrations, generally <1 ng/g. In contrast, the clay-rich inter-pillow material displays higher Hg contents and this explain the observed Hg variability measured within each pillow. These preliminary data, supported by recent experimental models [Boulliung et al., 2025] confirm the high volatility of Hg in basaltic magmas and allow a first estimate of Hg outflux compared with present-day volcanic Hg emissions.