Impact of crystallinity evolution on volatile phase transport in hydrous magmatic intrusions during the magmatic-hydrothermal transition

Abstract

Exploiting high-enthalpy geothermal systems is an important component of expanding low-carbon electricity generation in volcanically active regions. One possibility to enhance power production is harnessing supercritical geothermal resources that underlay conventional high-enthalpy systems, but utilizing them requires a deeper understanding of the physical and chemical processes taking place at the magmatic-hydrothermal transition immediately below the supercritical reservoir.          In water-rich, silicic magma bodies, the exsolution of a magmatic volatile phase (MVP) can result from magma crystallization. In a certain crystal fraction range, the MVP can be abundant enough to percolate through permeable pathways within the magma mush and coalesce at the intrusion’s apex, where fluid pressure build-up can lead to hydro-fracturing.    For the case of a prolate, shallow-seated (situated at 4 to 7 km, 0.94 km width), water-rich silicic magma body, we conducted 2D numerical fluid flow simulations, exploring the transient patterns of MVP exsolution and flow inside and out of the cooling intrusion, and hydrothermal fluid flow outside of it. We examined the impact of varying temperature-crystallinity trends on the timing of MVP production, the formation of permeable pathways, overpressure build-up and hydro-fracturing.        Our findings show that for silicic magmas, where crystallization primarily takes place at low magmatic temperature (near the haplogranitic solidus), permeable pathways (“channels”) form within the mush, but fluid pressures at the intrusion apex are not sufficient to expel MVPs forcefully. In contrast, for chemically less evolved melts, with crystallization shifted to higher temperatures, overpressure build-up is large enough to periodically release MVPs via hydro-fractures into the ductile carapace around the intrusion and beyond.        This periodic MVP release from the magma contributes a small fraction of the total water in the geothermal system above the cooling intrusion, where it is mixed with meteoric water. At typical borehole depths for conventional, high-enthalpy geothermal systems (e.g. 2 km), the contribution of magmatic water is usually ≤ 5%, and its thermal effect is negligible.  The observed degassing patterns could elucidate differences in degassing style between hydrous magmatic systems of different chemical compositions, e.g. the silicic intrusions of the Central Taupō Volcanic Zone versus the more mafic Whakaari White Island volcano (both in New Zealand). Further, these insights into the timing and extent of hydro-fracturing and the related advection of metals and ligands are key to understand the conditions relevant to the formation of magmatic-hydrothermal ore deposit such as granite-related tin-tungsten deposits and porphyry-copper deposits.