The magmatic-hydrothermal transition: a perspective from fluid & melt inclusions and mineral chemistry

The exsolution of volatile phases from a silicate melt constitutes the magmatic-hydrothermal transition, which plays a vital role in determining volcanic eruption styles or in generating a sus- tained and focused fluid flux to form magmatic-hydrothermal ore deposits. In particular, metal and sulfur transport by magmatic volatiles is an important step in porphyry Cu ore genesis, but the details and relative importance of processes that shape large deposits are still debated. A major impediment in quantitative studies of volatiles is the intricacy of their preservation in the rock record. Here, the composition of magmatic and hydrothermal minerals was investigated as a potential proxy for the volatile record: (1) Plagioclase phenocrysts were analyzed to test if they could inform about the water content of the melt they crystallized from, and (2) the composition of quartz in a crystallization regime changing from magmatic to lower-temperature hydrothermal was examined, analogous to previous studies that focused on the evolution of min- eralized pegmatites. Furthermore, pristine fluid and melt inclusions from natural samples may provide small aliquots of the magmatic-hydrothermal system. Previous studies largely focused on inclusions hosted in quartz veins from ore deposits or in quartz lining miarolitic cavities. These samples, however, likely suffered from re-equilibration to changing conditions, resulting in changes from the modification of water concentrations and small, monovalent cations like Cu to the point of complete replacement by later fluids. Inclusions in rapidly-quenched volcanic ejecta, on the other hand, are more likely to preserve original compositions. Water concentrations in silicate melts are commonly estimated through phase equilibria, associated with an appreciable uncertainty, or by analyzing undegassed glass, which is not al- ways available. The incorporation of more Al than stoichiometrically allowed in plagioclase, a ubiquitous magmatic mineral, was recently hypothesized to scale with the ...