Periodic Reporting for period 1 - STECALMY (Source and mobiliTy of mErCury within a continentAL Magmatic sYstem)
Reporting period: 2022-07-01 to 2024-06-30
Mercury (Hg) is a highly volatile, neurotoxic trace metal mainly emitted naturally into the environment by volcanic degassing, whose concentrations have increased due to several industrial activities. However, its cycle within the Earth and its source and mobility within magmatic systems are still unknown. This is due to the analytical challenges in measuring Hg at very low concentrations (ng/g, parts per billion) because of its high geochemical complexity and the lack of available studies investigating the Hg distribution within the different parts of an individual magmatic plumbing system. To better understand the cycle of Hg in magmatic systems, project STECALMY (Source and mobiliTy of mErcury in ContinentAL Magmatic sYstems) designed two objectives: i) to develop a robust, time- and cost-effective methodological protocol for measuring Hg concentration in crystalline rocks and ii) to place constraints on the source and mobility of Hg in a continental magmatic system using as a study case the Permian (~ 280 – 285 Ma, millions of years) Sesia Magmatic System (SMS, Western Italian Alps). This is a transcrustal igneous section traceable from its deepest roots that belongs to the Ivrea-Verbano zone and Serie dei Laghi units, exposing an almost complete section of the continental crust tilted by ~ 90° during the Alpine orogenesis. The actions will improve the knowledge about Hg in magmatic environments. The outcomes will also provide geochemical backgrounds on Hg that can be used for environmental purposes.
In the first stage, the literature review about the SMS favoured a rapid understanding of the geology of this area and its lithologies. This allowed correlating field observations and outcomes of these publications with the samples and their thin sections available at the Department of Mathematics, Informatics and Geosciences (MIGe) of the University of Trieste (UNITS), Italy. It helped in revising the plans for the field campaigns planned in the project, minimising any waste of time and resources, and raising the probability of the achievement of most of the designed goals. The literature review also focused on the general geochemistry of Hg in magmatic systems and the analytical methods currently used to analyse this metal. Literature review was also devoted to sulfur (S), gold (Au), copper (Cu), and arsenic (As) because they are chalcophile (i.e. sulfur-loving) elements like Hg and capable of giving other information on the behaviour of Hg in the deep crust.
The samples selected to develop a cost-effective methodological and analytical protocol for Hg analyses were crushed and powdered through different methods. Mercury was successfully analysed for these samples with the Direct Mercury Analyser (DMA-80) via thermal decomposition, gold amalgamation and atomic absorption spectrometry. The same samples were dissolved through a mixing of different acid reagents, and Hg was successfully analysed with cold-vapor atomic fluorescence spectroscopy (CV-AFS) and gold amalgamation. The results allowed us to constrain how Hg has to be analysed and also how to pursue the goal of the second objective. For this purpose, sixteen carefully selected granite samples from the Valle Mosso pluton were analysed via CV-AFS. The Valle Mosso pluton is a well-characterized igneous body representing the uppermost part of the large SMS, feeding a large caldera-forming volcanic event.
Another important achievement during this action was the investigation of the distribution of Hg and other trace metals and halogens in minerals of five Valle Mosso granites with micro-X-ray fluorescence (µ-XRF) at the Elettra Synchrotron of Trieste, Italy. The results are of high scientific impact and are expected to strengthen our understanding of the Hg cycle in magmatic systems.
The samples selected to develop a cost-effective methodological and analytical protocol for Hg analyses were crushed and powdered through different methods. Mercury was successfully analysed for these samples with the Direct Mercury Analyser (DMA-80) via thermal decomposition, gold amalgamation and atomic absorption spectrometry. The same samples were dissolved through a mixing of different acid reagents, and Hg was successfully analysed with cold-vapor atomic fluorescence spectroscopy (CV-AFS) and gold amalgamation. The results allowed us to constrain how Hg has to be analysed and also how to pursue the goal of the second objective. For this purpose, sixteen carefully selected granite samples from the Valle Mosso pluton were analysed via CV-AFS. The Valle Mosso pluton is a well-characterized igneous body representing the uppermost part of the large SMS, feeding a large caldera-forming volcanic event.
Another important achievement during this action was the investigation of the distribution of Hg and other trace metals and halogens in minerals of five Valle Mosso granites with micro-X-ray fluorescence (µ-XRF) at the Elettra Synchrotron of Trieste, Italy. The results are of high scientific impact and are expected to strengthen our understanding of the Hg cycle in magmatic systems.
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.
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.