Gas Leakage From Shallow Ponding Magma and Trapdoor Faulting at Sierra Negra Volcano (Isabela Island, Galápagos)

Abstract

Magmatic gas delivered by intraplate, hot-spot related volcanism offers important insight into the abundance and distribution of volatiles in the Earth's upper mantle (Aiuppa et al., 2021) and hence into the rates and mechanisms of volatile exchange in and out our planet (Dasgupta & Hirschmann, 2010). Hot-spot magmatic gases have long been recognized (Gerlach, 1982;Symonds et al., 1994) to exbibit CO 2 -richer (and H 2 O-poorer) compositions relative to arc magmatic gases (Fischer, 2008;Fischer & Chiodini, 2015;Oppenheimer et al., 2014;Taran & Zelenski, 2015), attesting for the presence of a carbon-rich mantle reservoir (Aiuppa et al., 2021, and references therein) at depths higher than the shallow (<50 km) Depleted Mantle (DM) sampled by MORBs (Mid-Ocean Ridge Basalts;Hauri et al., 2019). Combined with the specific trace element and radiogenic isotope signatures of hot-spot volcanic rocks (Hoffman, 2003;Zindler & Hart, 1986), the composition of intraplate magmatic gases may thus provide unique information on volatiles' heterogeneities in mantle plumes. Unfortunately, however, the existing data set for hot-spot magmatic gases is still limited (Aiuppa et al., 2021), detailed information being available for only the

Craters of habit: Patterns of deformation in the western Galápagos

“…Uplift immediately resumed after the 2005 eruption, and was sustained for 13 years (with > 4 sub-phases of deformation) [Geist et al 2008;Bell et al 2021b]. In total, the caldera inflated by > 6.5 m as magma accumulated at two kilometres depth [Bell [Geist et al 2008], and agrees with that suggested by gas composition analysis, as well as magma equilibration depths [Bell et al 2021b;Aiuppa et al 2022]. The sills that fed the distal fissure are spectacularly observed in the InSAR displacement maps of the eruption (e.g.…”

“…Jónsson et al 2005;Chadwick et al 2006;Geist et al 2008;Jónsson 2009;Bell et al 2021b]. Sierra Negra, like Alcedo, has an active hydrothermal system, on the western segment of the caldera fault [Reynolds et al 1995;Goff et al 2000;Aiuppa et al 2022]. Fumaroles here produce magma-derived gases that have exsolved from the shallow sill, and been channeled to the surface by the trapdoor fault [Aiuppa et al 2022].…”

“…Sierra Negra, like Alcedo, has an active hydrothermal system, on the western segment of the caldera fault [Reynolds et al 1995;Goff et al 2000;Aiuppa et al 2022]. Fumaroles here produce magma-derived gases that have exsolved from the shallow sill, and been channeled to the surface by the trapdoor fault [Aiuppa et al 2022]. The trapdoor fault dominates the expression of volcanism at Sierra Negra: it actively promotes fumarolic activity in the west of the caldera [Aiuppa et al 2022], slip along it can trigger eruptions , the location of which is controlled by the accompanying stress field , and interplay between the fault and the shallow sill produces asymmetric deformation patterns [Amelung et al 2000].…”

“…Fumaroles here produce magma-derived gases that have exsolved from the shallow sill, and been channeled to the surface by the trapdoor fault [Aiuppa et al 2022]. The trapdoor fault dominates the expression of volcanism at Sierra Negra: it actively promotes fumarolic activity in the west of the caldera [Aiuppa et al 2022], slip along it can trigger eruptions , the location of which is controlled by the accompanying stress field , and interplay between the fault and the shallow sill produces asymmetric deformation patterns [Amelung et al 2000]. The 2005 eruption (October 22 nd ) initiated explosively, with a 13 km plume, followed by three stages of variable lava fountaining (total erupted volume of 1.5 × 10 8 m 3 , from a sill located at 2.1 km depth [Geist et al 2008], with a source volume loss of 1.2 × 10 8 m 3 [Manconi and Casu 2012]).…”

“…At Sierra Negra, the intra-caldera fault is located in the southwestern portion of the caldera, and facilitates "trapdoor faulting," where the caldera floor hinges upwards like a trapdoor during inter-eruptive inflation [Amelung et al 2000;Jónsson et al 2005]. Eruptions here in 2005 and 2018 were accompanied by slip along this fault [Jónsson 2009;Bell et al 2021b], while it also acts as a conduit for magmatic fluids to the shallow surface fumarole field [Aiuppa et al 2022]. Similar magmatic-tectonic interactions at Alcedo modify displacement patterns during periods of uplift [Galetto et al 2019].…”