A transparent IHPV for the in-situ geochemical characterization of magmatic volatile phases and melts

Abstract

Chemical exchanges between magmas and volatile-rich fluids and gases are fundamental processes of magmatic and volcanic activity, but also play a critical role in the formation of various ore deposits. Yet, the composition and properties of the so-called magmatic volatile phases (MVPs) remain elusive, due to difficulties in their sampling, both in natural systems and in experimental laboratories.

Here, we present a novel ‘transparent’ internally-heated high-pressure vessel (T-IHPV) that enables the geochemical characterization of coexisting hydrous melts and MVPs in-situ, under typical shallow magmatic conditions. The experimental design is validated through the observation of haplogranite and rhyodacite melting to 900 °C and 130 MPa and the in-situ X-ray absorption (XAS) study of bromine and strontium distribution and speciation in the haplogranite-H₂O system to 800 °C and 100 MPa. These preliminary experiments confirm the efficient partitioning of Br in MVPs in differentiated systems ($D_{\mathrm{Br}}^{\mathrm{MVP}/\mathrm{melt}}$ around 41 for haplogranite) and reveal the stability of hydrated Br species instead of HBr in the MVPs. Coupled to other spectroscopic methods (Raman, SAXS/WAXS, XRD), we expect the T-IHPV to shed a new light on the thermodynamics and kinetics of chemical reactions at stake in shallow magmatic and hydrothermal reservoirs.