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[SVC28-10] How much fluid released during the formation of the silicious magma in the upper crust?
Keywords:Water budget , Rhyolite-MELTS simulation, Geothermal fluid, Magma degassing
Formation of any shallow magma chamber related to the crystallization of mantle-derived less evolved magma. The extensive crystallization of basalt may supply excessive magmatic volatile components to an overlying felsic magma chamber. But water dissolved in the parental mantle-derived magmas cannot be completely dissolved in the felsic endmember magmas dislocated in shallow conditions due to the limited solubility of volatiles. Thus, fluid excess exists. That undissolved water migrated upward and formed potential geothermal reservoirs and ore deposits. Here we consider how much water released in two different petrological cases: (1) Naruko caldera rhyolite (Miyagi Pref. Northeast Japan), supposed to be formed by the crystallization of primitive basalts; (2) Mendeleev caldera (Kunashir Island, Kurils) dacite, that formed through partial melting of amphibole bearing lower crust. Using known data on the petrology of magmas, storage conditions, and volatile content, we conduct simulations in Rhyolite-Melts software to estimate the degree of crystallization of the parental melt to form the final shallow reservoir and the mass of water that is (1) dissolved in the final reservoir and (2) released water fluid. Therefore, the amount of separated fluid can be estimated.
We applied Rhyolite-Melts modelling for the equilibrium batch crystallization of the most primitive known sample from modern postcaldera Naruko volcano. The best fit scenario suggests producing Naruko rhyolites through 70% crystallization of the starting basalt under 150 MPa and H2O=4.2 wt. %, CO2=0.001 wt. % in parental magma. The calculated mass of the required parental melt at 70% crystallization is 92 gigatons (1Gt=1012 kg), equivalent to approximately 40 km3 of magma. That parental melt contains 3.86 Gt of water. Using the average concentration of water in the rhyolite melt inclusions (4.9 wt.%) and the estimated mass of the Naruko rhyolites (28 Gt), we estimate the mass of dissolved water to be 1.36 Gt, which is equivalent to 35 wt.% of the original bulk water in the parental magma (as 1.36/3.86). Thus, it is possible to estimate that approximately 2.5 Gt of water has not been dissolved in the rhyolite endmember but exceeds due to the limited water solubility.
Another considered case is formation of voluminous dacites through dehydration partial melting in the lower crust on the example of Mendeleev volcano (Kotov et al., 2023). Dehydration melting does not lead to the release of free fluid by itself, however, the later migration of magmas from the source region to the upper crust leads to degassing. To estimate water excess, we model this process through Rhyolite-Melts. We take the less-evolved composition of rhyolitic melt inclusion observed in the restitic mafic clots for the starting composition. The results show that final dacitic assemblage formed after 14-36% crystallization of the initial melt. The mass of the rhyolite melts in erupted Mendeleev magma estimated to be 94-101 Gt. The mass of water in the final shallow reservoir varies within 3.8-5.7 Gt. The water excess is 1.03-5.34 Gt (18-53% of initial mass).
We recalculated the obtained values of excess free fluid into the value t/yr/m to understand how much fluid per unit time can be released in the magma generation region. Using a wide range of magma input into the crust from 0.001 to 0.0001 km3/yr, we estimated water flux from 1 to 11 t/y/m. This result is slightly lower than total water input into the NE Honshu crust (13 t/y/m), which confirms the correctness of the calculations. But future additional refinements to the rate of magma supply into the upper crust should help clarify this value.
Kotov A., Smirnov S, Nizametdinov I, Uno M, Tsuchiya N, Maksimovich I, Partial Melting under Shallow-Crustal Conditions: A Study of the Pleistocene Caldera Eruption of Mendeleev Volcano, Southern Kuril Island Arc, Journal of Petrology, V. 64, Iss. 6, 2023, egad033, https://doi.org/10.1093/petrology/egad033
We applied Rhyolite-Melts modelling for the equilibrium batch crystallization of the most primitive known sample from modern postcaldera Naruko volcano. The best fit scenario suggests producing Naruko rhyolites through 70% crystallization of the starting basalt under 150 MPa and H2O=4.2 wt. %, CO2=0.001 wt. % in parental magma. The calculated mass of the required parental melt at 70% crystallization is 92 gigatons (1Gt=1012 kg), equivalent to approximately 40 km3 of magma. That parental melt contains 3.86 Gt of water. Using the average concentration of water in the rhyolite melt inclusions (4.9 wt.%) and the estimated mass of the Naruko rhyolites (28 Gt), we estimate the mass of dissolved water to be 1.36 Gt, which is equivalent to 35 wt.% of the original bulk water in the parental magma (as 1.36/3.86). Thus, it is possible to estimate that approximately 2.5 Gt of water has not been dissolved in the rhyolite endmember but exceeds due to the limited water solubility.
Another considered case is formation of voluminous dacites through dehydration partial melting in the lower crust on the example of Mendeleev volcano (Kotov et al., 2023). Dehydration melting does not lead to the release of free fluid by itself, however, the later migration of magmas from the source region to the upper crust leads to degassing. To estimate water excess, we model this process through Rhyolite-Melts. We take the less-evolved composition of rhyolitic melt inclusion observed in the restitic mafic clots for the starting composition. The results show that final dacitic assemblage formed after 14-36% crystallization of the initial melt. The mass of the rhyolite melts in erupted Mendeleev magma estimated to be 94-101 Gt. The mass of water in the final shallow reservoir varies within 3.8-5.7 Gt. The water excess is 1.03-5.34 Gt (18-53% of initial mass).
We recalculated the obtained values of excess free fluid into the value t/yr/m to understand how much fluid per unit time can be released in the magma generation region. Using a wide range of magma input into the crust from 0.001 to 0.0001 km3/yr, we estimated water flux from 1 to 11 t/y/m. This result is slightly lower than total water input into the NE Honshu crust (13 t/y/m), which confirms the correctness of the calculations. But future additional refinements to the rate of magma supply into the upper crust should help clarify this value.
Kotov A., Smirnov S, Nizametdinov I, Uno M, Tsuchiya N, Maksimovich I, Partial Melting under Shallow-Crustal Conditions: A Study of the Pleistocene Caldera Eruption of Mendeleev Volcano, Southern Kuril Island Arc, Journal of Petrology, V. 64, Iss. 6, 2023, egad033, https://doi.org/10.1093/petrology/egad033