Decoding degassing modes of magma chamber of arc volcanoes: Insights from CO2/Cl and Cl/H2O ratios of magmatic fluids in groundwater

The H₂O-CO₂-Cl composition of the fluids released from the magma chambers for arc volcanoes was calculated based on the solubilities of H₂O and CO₂ in the silicate melt and the partition coefficient of Cl between the aqueous fluid and melt for the stages defined by a simple evolution model of the magma chamber. The degassing modes consisted of mafic bubbling (MB: first boiling) and felsic bubbling (FB: second boiling) releasing fluids as separate bubbles due to the mafic magma supply and felsic magma formation by differentiation, respectively, and felsic solidification (FS) releasing fluid due to felsic pluton formation. The variations in the CO₂/Cl and Cl/H₂O ratios of the fluids were examined for each degassing mode at pressures ranging from 150 to 400 MPa and were found to have very large variations at 0.00024–845 for CO₂/Cl and 0.00043–0.041 for Cl/H₂O. The CO₂/Cl is concluded to be a good indicator of magma chamber conditions because each mode has a specific ratio distinguished from that of the other modes; i.e., CO₂/Cl > 43 for MB, 0.027–23 for FB, and < 0.62 for FS.

The alteration of magmatic composition of fluid during ascent was discussed considering the phase separation of fluid, CO₂ bubbling in groundwater, dissolution, and deposition of minerals; furthermore, the limitation of the method when applied to natural groundwater systems were outlined. This method was tested for a natural volcanic system, the Kuju volcano in central Kyusyu, Japan, which is an active volcano complex composed of various magma types from rhyolitic to basaltic, replacing the eruption centers from west to east. Groundwater samples from springs and boreholes were collected and analyzed for chemical and isotopic compositions to determine the magmatic H₂O, dissolved inorganic carbon (DIC), and Cl concentrations. The new approach to determine the magmatic CO₂ concentration in groundwater using ³He concentration proposed herein cancelled the effect of carbonate deposition/dissolution, addition of DIC from organic matter, and bubbling of CO₂. The estimated magmatic CO₂/Cl ratios were 0.0009–14, which overlapped consistently with the modeled variation in the CO₂/Cl ratio. The spatial variation of magmatic CO₂/Cl in groundwater showed that higher-CO₂/Cl are found near the central part of the volcanic complex which composed of felsic to mafic volcanoes, and low-CO₂/Cl are only found around old felsic volcanoes. The CO₂ bubbling springs were found to be associated with younger mafic volcanoes. The condition of the magma chambers, as shown by the spatial variation in CO₂/Cl, were consistent with the record of the eruptive activities of the Kuju volcano. The CO₂/Cl method proposed in this study using groundwater is useful for estimating magmatic conditions in chambers.