Water contents and pressures of melts in unerupted felsic magma constrained by SEM-EDS analysis of homogenized melt inclusions in zircon

抄録

Granitic rocks (sensu lato) represent unerupted felsic magmas crystallized in the crust. In this study, we estimate water contents of melts and crystallization pressures of zircons in granitoid magma using melt inclusions in zircon, a ubiquitous accessory mineral in granitoids. Homogenization experiments of polymineralic inclusions hosted in zircon have been conducted for a granitoid sample from the Cretaceous Gamano granodiorite in Yashiro–jima Island, southwest Japan, using a piston-cylinder high-pressure–high-temperature apparatus. SEM-EDS analysis reveals that the homogenized melt inclusions have high water contents (6.4–11.3 wt%) and high SiO₂ contents (76–78 wt% anhydrous basis) implying that they represent fractionated interstitial hydrous melts trapped in growing zircon crystals. A recently proposed machine learning-based melt–phase assemblage geobarometer yields pressures ranging from 563 to 266 MPa interpreted as crystallization pressures of the zircons. The results of this study suggest high water activity of the interstitial melts within the Gamano granodiorite magma at the time of zircon crystallization. The melt inclusions in zircons record a wide range of pressures, from intrusion of the magma into the deeper crustal levels (∼563–500 MPa) to final solidification at shallower levels (∼266 MPa).