Investigating the distribution of mercury in the transcrustal Sesia Magmatic System (Western Alps, Italy)

The mercury (Hg) budget outgassed in the atmosphere from active volcanoes is genetically related to the underlying magmatic plumbing system. However, the source, distribution, and transport of Hg in magmas remain uncertain. Differentiation and/or crustal assimilation would increase Hg in igneous and volcanic rocks, while low concentrations (< 10 ng/g) would reflect degassing. Nonetheless, these hypotheses remain ambiguous because i) they have been proposed using rocks that are not from the same magmatic system and ii) the results could be biased by unintentional under- or overestimation of Hg caused by analytical challenges related to its high volatility, extremely low concentration (< 10 ng/g) in crystalline rocks, sample heterogeneity at the microscale, and laboratory contamination. To investigate the source, distribution, and transport of Hg in continental magmatic systems, we designed the STECALMY project to present novel data showcasing Hg distribution in crustal lithologies using the Sesia Magmatic System (SMS) (Western Alps, Italy) as a case study. The SMS is an archetype of a quasi-continuous transcrustal igneous section exposing the deep-level of mantle-derived lithologies and gabbros in the lower crust to granites and the remnants of a volcanic caldera in the upper crust. Rocks representing these different lithologies are firstly powdered through different approaches and then analyzed through a Direct Mercury Analyzer and a Cold-Vapour-Atomic-Fluorescence spectrometer. These are used to set up a novel approach for sample preparation and analytical procedure to optimize Hg analyses in crystalline rocks. The first outcomes show that Hg is highly variable within any rock (peridotite, gabbro, paragneiss, granite, rhyolite), varying from ~ 2 to ~ 300 ng/g. Since Hg might be preferentially partitioned into sulfides, these results could reflect the heterogeneous distribution of sulfides within each lithology and possibly among different aliquots of the same sample. We also show new data on total Hg concentration in the upper crustal Valle Mosso pluton (VMP) of the SMS representing the uppermost part of the plumbing system feeding the large (≥15-km-diameter) caldera-forming system of Sesia volcano erupting in the Lower Permian (ca. 289 to 280 Ma). Hg analyses on the lower, central, and upper VMP granites exhibiting cumulitic, storage, and degassing textures (i.e., miarolitic cavities), respectively, will provide more robust constraints on the source and mobility of Hg before and during magma degassing.