Water speciation and Sr isotopic exchange during water–melt interaction: a combined NMR–TIMS study on the Cretaio Tephra (Ischia Island, south Italy)

Abstract

In order to improve the microscopic understanding of the water–magma interaction process during explosive volcanism, volcanic glasses representative of deposits with sedimentological characteristics suggesting different water/melt ratios were studied by a combination of the nuclear magnetic resonance (NMR) and TIMS methods. The glasses were separated from pumices of two surge layers and one fallout bed of the Cretaio Tephra (Ischia Island, Italy), which is the product of an explosive eruption that occurred at Ischia in the second century BC. The 29Si CP–MAS NMR experiments indicate the occurrence of 1H– 29Si dipolar couplings in glasses from the phreatomagmatic activity, suggesting the presence of hydrogen atoms in proximity of silicon atoms. This feature is not detected in the glass from the deposit of the magmatic explosion. 1H MAS NMR spectra reveal different peaks attributed to different hydrous species characterized by different motional properties. These include ‘rigid’ H 2O groups isolated in the glass structure, more mobile water species and possibly structural hydroxyl groups. 1H MAS NMR spectra recorded after deuteration experiments of the glass at a temperature up to 300°C revealed that the exchange reactions of the D 2O vapor with hydrogen were limited to the most mobile water species, possibly on vesicle surfaces or in channels. The hydrogen concentration linearly correlates with the 87Sr/ 86Sr isotope ratio in glasses, suggesting isotopic tracer exchanges between the Sr dissolved in the water vapor and the Sr in the silicon–oxygen network during hydration. It is proposed that the uprising melt interacted with a hydrothermal system of seawater-derived fluids, characterized by relatively high Sr isotopic composition.