Challenges, pitfalls, and advances in understanding the source and mobility of mercury (Hg) in a continental magmatic system

Mercury (Hg) is a volatile chalcophile element primarily outgassed by volcanoes. However, due to its complex behavior and analytical challenges, Hg's source, distribution, and transport in magmas before degassing and within any magmatic plumbing system are largely unknown. Here we present novel data showcasing Hg distribution in crustal lithologies of the Lower Permian (~289 to 280 Ma) Sesia Magmatic System (SMS) (Western Alps, Italy), a quasi-continuous transcrustal igneous section traceable from its deepest roots to the exposed caldera. Peridotites, gabbros, diorites, orthogneisses, and granites of the SMS are powdered through different methods and analyzed with Direct Mercury Analyzer (DMA) and Cold Vapour Atomic Fluorescence spectrometer (CV-AFS) to set up a novel sample preparation and analytical procedure for Hg analyses in crystalline rocks. DMA shows that Hg ranges from ~2 to 30 ng/g among different lithologies. A possibility is that this content variability may reflect a heterogeneous distribution of Hg-bearing sulfides in each lithology and among separate aliquots of the same sample. Alternatively, it may indicate that the DMA combustion temperature (650 °C) is not high enough to desorb all Hg from rocks with high solidus temperatures (>650 °C). Here the release of Hg from minerals in the rock powders (15-30 µm diameter) could occur only by diffusion, which has to be fast enough to allow all Hg release during any heating step (~1 min). Ongoing petrographic and CV-AFS analyses, the latter also on the same powders analyzed by DMA, will provide crucial constraints on these uncertainties. We also provide first insights into the mobility of Hg in the upper crust by analyzing its concentration on the Valle Mosso pluton (VMP) representing the upper crustal segment of SMS feeding the large (≥15-km-diameter) caldera-forming Sesia volcano1. From the lower, central, and upper VMP granites exhibiting, respectively, cumulitic, storage, and degassing textures (i.e., miarolitic cavities), Hg content increases from ~7 to 30 ng/g, as seen for SiO2 (~66 to 77 wt.%) and Rb (90 to 103 µg/g) for instance1. Further analyses will better constrain the source and mobility of Hg before and during magma degassing and compare its behavior with that of other volatile metals.