Boron and strontium isotopic signature in rainwaters from Mt. Etna, Italy

The isotopic compositions of boron (δ11B‰) and strontium (87Sr/86Sr) were measured, for the first time, in 10 rainwater samples from Mt. Etna, Italy. The samples were collected during the paroxysmal sequence of 2021-2022. The chemical composition was determined using an ICP-MS, while the isotopic ratios of B and Sr were measured using a mass spectrometer MC-ICP-MS (Neptune Plus™), after specific pre-concentration procedures in clean rooms (class 100 - 1000). In the analysed rainwater samples, the concentrations of B and Sr were between 4.6 µg L-1 and 42.2 µg L-1, and between 9.7 µg L-1 and 541 µg L-1, respectively. Overall, the isotopic composition of B ranging from +1.29‰ ± 0.30‰ to +42.9‰ ± 0.20‰, with a median value of +22.0‰ ± 0.19‰. The lowest δ11B‰ values were measured in the site closest to the main active craters (3.6 km), with a median value of +11.4‰ ± 0.21‰; the highest was measured in the site close to the Ionian Sea (Zafferana Etnea), with a median value of +38.1‰ ± 0.21‰. Strontium (87Sr/86Sr) ratios were between 0.703728 ± 0.000008 and 0.710363 ± 0.000006, with a median of 0.707328 ± 0.000007. The highest and the lowest 87Sr/86Sr ratios were measured in the sites most and less affected by the contribution of the volcanic emissions, with median values of 0.704339 ± 0.000007 and 0.709583 ± 0.000007, respectively. Although exists few data of isotopic ratios of boron and strontium on fluids in volcanic systems (δ11B‰ between -9.3 to 21.4‰), there are no studies on rainwater influenced by volcanic emissions. Nevertheless, the data available in the literature are sufficient to attribute Etna's volcanic source as the major contributor to B and Sr emissions in the atmospheres of this area. The rainwater chemistry of the Zafferana Etnea site was partly influenced by the volcanic source, but the measured isotopic ratios at this site showed a strong contribution from the marine source. The results provide the first comprehensive study of B and Sr isotopes in Mt. Etna rainwater. Two main sources of atmospheric emissions of B and Sr were recognised: sea-salt aerosols and volcanic gases. This research also adds to the potential for the use of B and Sr isotopes as volcanic emissions contribution tracers.