Geochemical evidence of volcanic plumbing system processes from fumarolic gases and diffuse CO2 degassing of Taal volcano, Philippines, prior to the January 2020 eruption&#160;
Authors: Hernández Pedro A. , Melian Gladys , Asensio-Ramos María , Padron Eleazar , Sumino Hirochicka , Perez Nemesio M. , Padilla German , Barrancos Jose , Baldago Mª Criselda , Rodriguez Fatima , Alonso Mar , Amonte Cecilia , Arcilla Carlo , Lagmay Mahar 
Published: March 2021

&lt;p&gt;Significant temporal variations in the chemical and isotopic composition of Taal fumarolic gas as well as in diffuse CO&lt;sub&gt;2&lt;/sub&gt; emission from Taal Main Crater Lake (TMLC) have been observed across the ~12 years of geochemical monitoring (Arpa et al., 2013; Hern&amp;#225;ndez et a., 2017), with significant high CO&lt;sub&gt;2 &lt;/sub&gt;degassing rates, typical of plume degassing volcanoes, measured in 2011 and 2017. In addition to these CO&lt;sub&gt;2&lt;/sub&gt; surveys at the TCML, soil CO&lt;sub&gt;2&lt;/sub&gt; efflux continuous monitoring was implemented at Taal volcano since 2016 and a clear increasing trend of the soil CO&lt;sub&gt;2&lt;/sub&gt; efflux in 2017 was also observed. Increasing trends on the fumarolic CO&lt;sub&gt;2&lt;/sub&gt;/St, He/CO&lt;sub&gt;2&lt;/sub&gt;, CO/CO&lt;sub&gt;2&lt;/sub&gt; and CO&lt;sub&gt;2&lt;/sub&gt;/CH&lt;sub&gt;4&lt;/sub&gt; ratios were recorded during the period 2010-2011 whereas increasing SO&lt;sub&gt;2&lt;/sub&gt;/H&lt;sub&gt;2&lt;/sub&gt;S, H&lt;sub&gt;2&lt;/sub&gt;/CO&lt;sub&gt;2&lt;/sub&gt; ratios were recorded during the period 2017-2018. A decreasing on the CO&lt;sub&gt;2&lt;/sub&gt;/CH&lt;sub&gt;4&lt;/sub&gt; and CO&lt;sub&gt;2&lt;/sub&gt;/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&lt;sub&gt;2&lt;/sub&gt; and CO contents suggest increases in temperature and pressure in the upper parts of the hydrothermal system of Taal volcano. The &lt;sup&gt;3&lt;/sup&gt;He/&lt;sup&gt;4&lt;/sup&gt;He ratios corrected (Rc/Ra), and &amp;#948;&lt;sup&gt;13&lt;/sup&gt;C of fumarolic gases also increased during the periods 2010-2011 and 2017-2018 before the eruption onset. During this study, diffuse CO&lt;sub&gt;2&lt;/sub&gt; emission values measured at TMCL showed a wide range of values from &gt;0.5&amp;#8201;g&amp;#8201;m&lt;sup&gt;&amp;#8722;2&lt;/sup&gt; d&lt;sup&gt;&amp;#8722;1&lt;/sup&gt; up to 84,902 g&amp;#8201;m&lt;sup&gt;&amp;#8722;2&lt;/sup&gt; d&lt;sup&gt;&amp;#8722;1&lt;/sup&gt;. The observed relatively high and anomalous diffuse CO&lt;sub&gt;2&lt;/sub&gt; emission rate across the ~12 years reached values of 4,670 &amp;#177; 159 t d&lt;sup&gt;-1 &lt;/sup&gt;on March 24, 2011, and 3,858 &amp;#177; 584 t d&lt;sup&gt;-1&lt;/sup&gt; on November 11, 2017. The average value of the soil CO&lt;sub&gt;2&lt;/sub&gt; 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&lt;sub&gt;2&lt;/sub&gt; efflux from ~0.1 up to 1.1 kg m&lt;sup&gt;-2&lt;/sup&gt; d&lt;sup&gt;-1&lt;/sup&gt; was measured in 9 hours and continued to show a sustained increase in time up to 2.9 kg m&lt;sup&gt;-2&lt;/sup&gt; d&lt;sup&gt;-1&lt;/sup&gt; in 2 November, that represents the main long-term variation of the soil CO&lt;sub&gt;2&lt;/sub&gt; 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.&lt;/p&gt;&lt;p&gt;Arpa, M.C., et al., 2013. Bull. Volcanol. 75, 747. https://doi.org/10.1007/s00445-013-0747-9.&lt;/p&gt;&lt;p&gt;Hern&amp;#225;ndez, P.A., et al.,&amp;#160; 2017. Geol. Soc. Lond. Spec. Publ. 437:131&amp;#8211;152. https://doi.org/10.1144/SP437.17.&lt;/p&gt;