Precipitation of Minerals from Water-Rich Fumarolic Gas Using a Flow-through Benchtop Installation

Volcanic fumaroles are openings in the earth's surface, where volcanic gases discharge to the atmosphere. Metallic and non-metallic elements contained in gases form specific mineral precipitates upon cooling. Although the presence of metals in fumarolic gases has long been known, their concentrations are generally low and difficult to measure directly. A laboratory model of a fumarole may resolve the situation if the complex gas composition could be accurately reproduced. Here we describe a new experimental approach that allows accurately simulating fumarolic gases in terms of their main components (H2O, CO2, S, HCl), as well as adding volatile metal compounds. Gas is generated inside a special flow-through reactor, at the outlet of which the elements contained in the gas form temperature-dependent mineral sequence inside the attached silica-glass tube. Using this installation, we obtained laboratory sublimates from reducing (H2S-rich) gases similar to natural ones in terms of mineral composition and mineral habits. Twenty-one phases have been identified in sublimates, among which are simple and complex chlorides, simple sulfides and six sulfosalts. Comparison of the sublimate deposition from H2O-rich gas at 1 bar with similar works performed in evacuated ampoules at low pressure showed that fumarolic gases behave like an ideal gas, in which molecules do not interact with each other, and reactive compounds in the gas serve in fact as an inert carrier of volatile metals species. Changing the composition of the gas at the outlet of the installation, its flow rate and temperature, we can observe the corresponding changes in mineral precipitates and in such a way study the factors affecting mineral formation on natural fumarolic fields.

Behavior of magmatic components in fumarolic gases related to the 2018 phreatic eruption at Ebinokogen Ioyama volcano, Kirishima Volcanic Group, Kyushu, Japan

Direct sampling and analysis of fumarolic gas was conducted at Ebinokogen Ioyama volcano, Japan, between December 2015 and July 2020. Notable changes in the chemical composition of gases related to volcanic activity included a sharp increase in SO2 and H2 concentrations in May 2017 and March 2018. The analyses in March 2018 immediately preceded the April 2018 eruption at Ioyama volcano. The isotopic ratios of H2O in fumarolic gas revealed the process of formation. Up to 49% high-enthalpy magmatic vapor mixed with 51% of cold local meteoric water to generate coexisting vapor and liquid phases at 100–160 °C. Portions of the vapor and liquid phases were discharged as fumarolic gases and hot spring water, respectively. The CO2/SO2 ratio of the fumarolic gas was higher than that estimated for magmatic vapor due to SO2 hydrolysis during the formation of the vapor phase. When the flux of the magmatic vapor was high, effects of hydrolysis were small resulting in low CO2/SO2 ratios in fumarolic gases. The high apparent equilibrium temperature defined for reactions involving SO2, H2S, H2 and H2O, together with low CO2/SO2 and H2S /SO2 ratios were regarded to be precursor signals to the phreatic eruption at Ioyama volcano. The apparent equilibrium temperature increased rapidly in May 2017 and March 2018 suggesting an increased flux of magmatic vapor. Between September 2017 and January 2018, the apparent equilibrium temperature was low suggesting the suppression of magmatic vapor flux. During this period, magmatic eruptions took place at Shinmoedake volcano 5 km away from Ioyama volcano. We conclude that magma sealing and transport to Shinmoedake volcano occurred simultaneously in the magma chamber beneath Ioyama volcano.

Geochemical evidence of volcanic plumbing system processes from fumarolic gases and diffuse CO2 degassing of Taal volcano, Philippines, prior to the January 2020 eruption

<p>Significant temporal variations in the chemical and isotopic composition of Taal fumarolic gas as well as in diffuse CO<sub>2</sub> emission from Taal Main Crater Lake (TMLC) have been observed across the ~12 years of geochemical monitoring (Arpa et al., 2013; Hernández et a., 2017), with significant high CO<sub>2 </sub>degassing rates, typical of plume degassing volcanoes, measured in 2011 and 2017. In addition to these CO<sub>2</sub> surveys at the TCML, soil CO<sub>2</sub> efflux continuous monitoring was implemented at Taal volcano since 2016 and a clear increasing trend of the soil CO<sub>2</sub> efflux in 2017 was also observed. Increasing trends on the fumarolic CO<sub>2</sub>/St, He/CO<sub>2</sub>, CO/CO<sub>2</sub> and CO<sub>2</sub>/CH<sub>4</sub> ratios were recorded during the period 2010-2011 whereas increasing SO<sub>2</sub>/H<sub>2</sub>S, H<sub>2</sub>/CO<sub>2</sub> ratios were recorded during the period 2017-2018. A decreasing on the CO<sub>2</sub>/CH<sub>4</sub> and CO<sub>2</sub>/St ratios was observed for 2017-2018. These changes are attributed to an increased contribution of magmatic fluids to the hydrothermal system in both periods. Observed changes in H<sub>2</sub> and CO contents suggest increases in temperature and pressure in the upper parts of the hydrothermal system of Taal volcano. The <sup>3</sup>He/<sup>4</sup>He ratios corrected (Rc/Ra), and δ<sup>13</sup>C of fumarolic gases also increased during the periods 2010-2011 and 2017-2018 before the eruption onset. During this study, diffuse CO<sub>2</sub> emission values measured at TMCL showed a wide range of values from >0.5 g m<sup>−2</sup> d<sup>−1</sup> up to 84,902 g m<sup>−2</sup> d<sup>−1</sup>. The observed relatively high and anomalous diffuse CO<sub>2</sub> emission rate across the ~12 years reached values of 4,670 ± 159 t d<sup>-1 </sup>on March 24, 2011, and 3,858 ± 584 t d<sup>-1</sup> on November 11, 2017. The average value of the soil CO<sub>2</sub> efflux data measured by the geochemical station showed oscillations around background values until 14 March, 2017. Since then at 22:00 hours, a sharp increase of soil CO<sub>2</sub> efflux from ~0.1 up to 1.1 kg m<sup>-2</sup> d<sup>-1</sup> was measured in 9 hours and continued to show a sustained increase in time up to 2.9 kg m<sup>-2</sup> d<sup>-1</sup> in 2 November, that represents the main long-term variation of the soil CO<sub>2</sub> emission time series. All the above variations might be produced by two episodes of magmatic intrusion which favored degassing of a gas-rich magma at depth. During the 2010-2011 the magmatic intrusion of volatile-rich magma might have occurred from the mid-crustal storage region at shallower depths producing important changes in pressure and temperature conditions, whereas a new injection of more degassed magma into the deepest zone of the hydrothermal system occurring in 2017-2018 might have favored the accumulation of gases in the subsurface, promoting conditions leading to a phreatic eruption. These geochemical observations are most simply explained by magma recharge to the system, and represent the earliest warning precursor signals to the January 2020 eruptive activity.</p><p>Arpa, M.C., et al., 2013. Bull. Volcanol. 75, 747. https://doi.org/10.1007/s00445-013-0747-9.</p><p>Hernández, P.A., et al.,  2017. Geol. Soc. Lond. Spec. Publ. 437:131–152. https://doi.org/10.1144/SP437.17.</p>