Rocks powdered through different approaches aided in testing possible Hg loss during sample preparation and the efficiency of DMA-80 and CV-AFS in analysing this element in crystalline rocks. Hg was not lost during sample preparation because temperatures measured soon after powdering were < 40 °C, lower than the temperature at which Hg began to volatilise from solid matrix (~ 100 °C). Samples were hence analysed with DMA-80 and CV-AFS. We found that DMA-80 is less accurate and precise in analysing Hg in crystalline rocks when these have Hg concentrations <10 ng/g. By reviewing 717 Hg analyses on crystalline rocks, we also find that more than 80 % have Hg concentrations ≤10 ng/g with a large overlap between all lithologies, and of this 80 % of analyses, more than 80 % are determined with DMA-80 and similar facilities for which accuracy limit is suggested to be at 10 ng/g. Conversely, results from acid digestion-CV-AFS analyses show more accurate and precise results. Also, two DMA-combusted residual powders from granite, gabbro and peridotite showing the highest and lowest Hg concentrations at DMA-80 were recovered and analysed with CV-AFS. Results display that in peridotite powders, there is still Hg as opposed to granitic powders, suggesting that DMA-80 does not efficiently release all Hg from some lithologies. Overall, these first results indicate that uncertainties are mostly related to the analytical method most often employed (e.g. DMA-80).
In the second part of the action, we use CV-AFS to analyse Hg in the granites of the Valle Mosso pluton of the SMS exposing to surface features representing magma accumulation, extraction, and degassing. Hence, by applying our new methodological approach to this exposed sub-volcanic system, we can investigate the behaviour of Hg prior to and during a volcanic event. Also, some of these granites were used for µ-XRF investigation of Hg distribution in minerals. Collectively, the STECALMY project offers the most complete geochemical dataset for a magmatic system with the possibility to provide new constraints on the cycle of Hg within a continental magmatic system and, importantly, understand the partitioning of this and other elements between magmas, minerals, and volatile phases, with a direct application to volcano forecasting.
