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>