5:15 PM - 7:15 PM
[SGC37-P07] Oxybarometry and hygrothermobarometry for granites using zircon and melt inclusions: examples of the Kaikomagatake, Miuchi, Daito, and Gamano granitoids in Japan
Keywords:granite, melt inclusions in zircon, hygrothermobarometry, oxybarometry
Granites (sensu lato) represent unerupted products of felsic magmas in the crust. Redox state, melt water contents, and pressure-temperature conditions collectively control the fundamental properties of the magma by influencing the stability and crystallization of mineral phases as well as the viscosity and density of melts, all of which affect magma rheology. The processes that occur during the segregation of felsic melts in the deep crust and transport of granitic magma with dissolved volatiles from the source to the emplacement level are strongly dependent on the rheological properties of the melt and of the magma, leading to mass transfer and ultimately contributing to chemical differentiation of the continental crust. Therefore, estimating redox state, melt water contents, and pressure-temperature conditions of felsic melts can help to unravel the evolution of granitic magmas and the tectonic history of orogens. In this study, we estimate these conditions of granitic magma using zircon and melt inclusions, a ubiquitous accessory mineral in granites.
Homogenization experiments of polymineralic inclusions hosted in zircon have been conducted for granitoid samples from magnetite-bearing Neogene Kaikomagatake pluton, magnetite-free Neogene Miuchi pluton, magnetite-bearing Paleogene Daito pluton, and magnetite-free late Cretaceous Gamano granodiorite. SEM–EDS analysis has revealed that the homogenized melt inclusions have high SiO2 contents (76–79 wt% anhydrous basis), implying that they represent fractionated interstitial melts trapped in growing zircon crystals. A recently proposed machine learning-based melt–phase assemblage geothermobarometer (Weber and Blundy, 2024, Jour. Petrol.) yields 303–185 MPa and 731–702 °C from the Kaikomagatake pluton (Taniwaki et al., under revision, submitted to Lithos), 235–92 MPa and 785–733 °C from the Miuchi pluton (Taniwaki et al., under revision), 265–161 MPa and 763–705 °C from the Daito pluton (this study), and 563–266 MPa and 708–731 °C from the Gamano granodirite (Kawashima et al., 2024, Jour. Mineral. Petrol. Sci.), interpreted as zircon crystallization pressure-temperature conditions. We also estimated water contents of melt inclusions from SEM-EDS analysis following the method described in Geshi et al. (2017, Bull. Volcanol. Soc. Japan) which resulted in 4.8–9.0 wt% and 2.4–6.0 wt% for the Kaikomagatake and Miuchi plutons, respectively (Taniwaki et al., under revision), 4.1–8.1 wt% for the Daito pluton (this study), and 6.4–11.3 wt% for the Gamano granodiorite (Kawashima et al., 2024, Jour. Mineral. Petrol. Sci.). The estimated pressures and water contents are plotted 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.
U-Pb zircon dating using LA-ICP-MS yields 12.67 ± 0.07 Ma and 14.47 ± 0.12 Ma for the Kaikomagatake and Miuchi plutons, respectively (Taniwaki et al., under revision), 55.50 ± 0.32 Ma for the Daito pluton, and 92.41 ± 0.37 Ma for the Gamano granodiorite (this study). The absence of older inherited U-Pb ages in each sample precludes the possibility that the zircons were derived from the surrounding metasedimentary rocks. The zircon oxybarometer (Loucks et al., 2020, Jour. Petrol.) using LA-ICP-MS analyzed trace element compositions together with the U-Pb age yields delta FMQ values of -1.6 – +2.8 and -2.6 – -0.5 for the Kaikomagatake and Miuchi plutons, +0.3 – +2.0 for the Daito granodiorite, and -2.8 – +0.1 for the Gamano granodirite. The delta FMQ values of the Kaikomagatake and Daito plutons are comparable to the magnetite-series granitoid, whereas those of the Miuchi pluton and the Gamano diorite are comparable to the ilmenite-series granitoid (Wones, 1981, Mining Geol.), consistent with the presence or absence of the magnetite in the studied samples (Ishihara, 1977, Mining Geol.). The oxybarometric and hygrothermobarometric approach using zircons and melt inclusions presented here would be applicable to most granitoids, which could provide fundamental data to better understand fates of volatiles in the crust, granite petrogenesis, and the tectonic evolution of orogenic belts.
Homogenization experiments of polymineralic inclusions hosted in zircon have been conducted for granitoid samples from magnetite-bearing Neogene Kaikomagatake pluton, magnetite-free Neogene Miuchi pluton, magnetite-bearing Paleogene Daito pluton, and magnetite-free late Cretaceous Gamano granodiorite. SEM–EDS analysis has revealed that the homogenized melt inclusions have high SiO2 contents (76–79 wt% anhydrous basis), implying that they represent fractionated interstitial melts trapped in growing zircon crystals. A recently proposed machine learning-based melt–phase assemblage geothermobarometer (Weber and Blundy, 2024, Jour. Petrol.) yields 303–185 MPa and 731–702 °C from the Kaikomagatake pluton (Taniwaki et al., under revision, submitted to Lithos), 235–92 MPa and 785–733 °C from the Miuchi pluton (Taniwaki et al., under revision), 265–161 MPa and 763–705 °C from the Daito pluton (this study), and 563–266 MPa and 708–731 °C from the Gamano granodirite (Kawashima et al., 2024, Jour. Mineral. Petrol. Sci.), interpreted as zircon crystallization pressure-temperature conditions. We also estimated water contents of melt inclusions from SEM-EDS analysis following the method described in Geshi et al. (2017, Bull. Volcanol. Soc. Japan) which resulted in 4.8–9.0 wt% and 2.4–6.0 wt% for the Kaikomagatake and Miuchi plutons, respectively (Taniwaki et al., under revision), 4.1–8.1 wt% for the Daito pluton (this study), and 6.4–11.3 wt% for the Gamano granodiorite (Kawashima et al., 2024, Jour. Mineral. Petrol. Sci.). The estimated pressures and water contents are plotted 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.
U-Pb zircon dating using LA-ICP-MS yields 12.67 ± 0.07 Ma and 14.47 ± 0.12 Ma for the Kaikomagatake and Miuchi plutons, respectively (Taniwaki et al., under revision), 55.50 ± 0.32 Ma for the Daito pluton, and 92.41 ± 0.37 Ma for the Gamano granodiorite (this study). The absence of older inherited U-Pb ages in each sample precludes the possibility that the zircons were derived from the surrounding metasedimentary rocks. The zircon oxybarometer (Loucks et al., 2020, Jour. Petrol.) using LA-ICP-MS analyzed trace element compositions together with the U-Pb age yields delta FMQ values of -1.6 – +2.8 and -2.6 – -0.5 for the Kaikomagatake and Miuchi plutons, +0.3 – +2.0 for the Daito granodiorite, and -2.8 – +0.1 for the Gamano granodirite. The delta FMQ values of the Kaikomagatake and Daito plutons are comparable to the magnetite-series granitoid, whereas those of the Miuchi pluton and the Gamano diorite are comparable to the ilmenite-series granitoid (Wones, 1981, Mining Geol.), consistent with the presence or absence of the magnetite in the studied samples (Ishihara, 1977, Mining Geol.). The oxybarometric and hygrothermobarometric approach using zircons and melt inclusions presented here would be applicable to most granitoids, which could provide fundamental data to better understand fates of volatiles in the crust, granite petrogenesis, and the tectonic evolution of orogenic belts.