Presentation Information
[T2-O-9]Estimation of melt water contents, pressure-temperature conditions, and redox state of granitic magma using zircon and melt inclusions
*Satoshi Saito1, Chiho Ihira1, Taichi Kawashima1, Yuka Taniwaki1, Mayuko Fukuyama2 (1. Ehime University, 2. Akita University)
Keywords:
Granitoid,Zircon,Melt inclusion,Melt water contents,Pressure-temperature conditions,Redox state
Granites (sensu lato) represent unerupted products of felsic magmas in the crust. Melt water contents, pressure-temperature conditions, and redox state 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 melt water contents, pressure-temperature conditions, and redox state 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 the 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) yields 303–185 MPa and 731–702 °C from the Kaikomagatake pluton (Taniwaki et al., 2025), 235–92 MPa and 785–733 °C from the Miuchi pluton (Taniwaki et al., 2025), 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; Taniwaki et al., 2025), 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) which resulted in 4.8–9.0 wt% and 2.4–6.0 wt% for the Kaikomagatake and Miuchi plutons, respectively (Taniwaki et al., 2025), 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). 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., 2025), 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) using LA-ICP-MS analyzed trace element compositions together with the U-Pb age yields ΔFMQ values of–2.8 — –0.3 for the Miuchi plutons, +0.1 — +1.7 for the Daito granodiorite, and –2.2 — –0.2 for the Gamano granodirite. The ΔFMQ values of the 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), consistent with the presence or absence of the magnetite in the studied samples (Ishihara, 1977). The hygrothermobarometric and oxybarometric approach using zircons and melt inclusions presented here would be applicable to most granitoids, which could provide fundamental data to better understand granite petrogenesis and the tectonic evolution of orogenic belts.
REFERENCES
Geshi et al., 2017. Bulletin of the Volcanological Society of Japan 62, 13–22; Ishihara, S., 1977. Mining Geology 27, 293-305; Kawashima et al., 2024, Journal of Mineralogical and Petrological Sciences 119, 018; Loucks et al., 2020. Journal of Petrology 61, egaa034; Taniwaki et al., 2025. Lithos 504-505, 108029; Weber and Blundy, 2024. Journal of Petrology 65, egae020; Wones, 1981. Mining Geology 31, 191-212.
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 the 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) yields 303–185 MPa and 731–702 °C from the Kaikomagatake pluton (Taniwaki et al., 2025), 235–92 MPa and 785–733 °C from the Miuchi pluton (Taniwaki et al., 2025), 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; Taniwaki et al., 2025), 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) which resulted in 4.8–9.0 wt% and 2.4–6.0 wt% for the Kaikomagatake and Miuchi plutons, respectively (Taniwaki et al., 2025), 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). 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., 2025), 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) using LA-ICP-MS analyzed trace element compositions together with the U-Pb age yields ΔFMQ values of–2.8 — –0.3 for the Miuchi plutons, +0.1 — +1.7 for the Daito granodiorite, and –2.2 — –0.2 for the Gamano granodirite. The ΔFMQ values of the 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), consistent with the presence or absence of the magnetite in the studied samples (Ishihara, 1977). The hygrothermobarometric and oxybarometric approach using zircons and melt inclusions presented here would be applicable to most granitoids, which could provide fundamental data to better understand granite petrogenesis and the tectonic evolution of orogenic belts.
REFERENCES
Geshi et al., 2017. Bulletin of the Volcanological Society of Japan 62, 13–22; Ishihara, S., 1977. Mining Geology 27, 293-305; Kawashima et al., 2024, Journal of Mineralogical and Petrological Sciences 119, 018; Loucks et al., 2020. Journal of Petrology 61, egaa034; Taniwaki et al., 2025. Lithos 504-505, 108029; Weber and Blundy, 2024. Journal of Petrology 65, egae020; Wones, 1981. Mining Geology 31, 191-212.