Hygrothermobarometry for granites using melt inclusions in zircon

Abstract

Granites (sensu lato) represent unerupted products of felsic magmas in the crust. In this study, we estimate water contents of melts and pressure–temperature conditions of zircon crystallization in granitic magma using melt inclusions in zircon, a ubiquitous accessory mineral in granites. Homogenization experiments of polymineralic inclusions hosted in zircon have been conducted for granitoid samples from the Miocene Kaikomagatake and Miuchi plutons in Japan, which were emplaced at different crustal levels. The homogenized melt inclusions have high SiO₂ contents (77–79 wt% anhydrous basis), implying that they represent fractionated interstitial melts trapped in growing zircon crystals. The absence of older inherited UPb ages precludes the possibility that the zircon and its melt inclusions were derived from the surrounding metasedimentary rocks. Higher water contents for the Kaikomagatake pluton (4.8–9.0 wt% H₂O) than the Miuchi pluton (2.4–6.0 wt% H₂O) are revealed, which can be attributed to the difference in emplacement depths between the plutons. We examine the results of five melt geobarometers and find that a recently proposed machine learning-based melt–phase assemblage geobarometer (MagMaTaB) yields reasonable pressures for both the Kaikomagatake and Miuchi plutons of 303–185 MPa and 235–92 MPa, respectively, interpreted as zircon crystallization pressures. The melt inclusion data plot along the H₂O solubility curve in the pressure–H₂O diagram, suggesting the high water activity of the fractionated interstitial melts within the magmas during zircon crystallization. The new thermobarometric estimates suggest that temperature increased with decreasing pressure for both plutons, which probably reflects the reheating of reservoirs during magma ascent.