Integrated Petrological and Geophysical Constraints on Magma System Architecture in the Western Galápagos Archipelago: Insights From Wolf Volcano

Abstract

The 2015 eruption of Wolf volcano was one of the largest eruptions in the Galápagos Islands since the onset of routine satellite‐based volcano monitoring. It therefore provides an excellent opportunity to combine geophysical and petrological data, to place detailed constraints on the architecture and dynamics of subvolcanic systems in the western archipelago. We present new geodetic models that show that pre‐eruptive inflation at Wolf was caused by magma accumulation in a shallow flat‐topped reservoir at ~1.1 km, whereas edifice‐scale deformation during the eruption was related to a deflationary source at 6.1–8.8 km. Petrological observations suggest that the erupted material was derived from both a subvolcanic mush and a liquid‐rich magma body. Using a combination of olivine‐plagioclase‐augite‐melt (OPAM) and clinopyroxene‐melt barometry, we show that the majority of magma equilibration, crystallization, and mush entrainment occurred at a depth equal to or greater than the deep geodetic source, with little petrological evidence of material sourced from shallower levels. Hence, our multidisciplinary study does not support a fully transcrustal magmatic system beneath Wolf volcano before the 2015 eruption but instead indicates two discrete storage regions, with a small magma lens at shallow levels and the major zone of magma storage in the lower crust, from which most of the erupted material was sourced. A predominance of lower crustal magma storage has previously been thought typical of subvolcanic systems in the eastern Galápagos Archipelago, but our new data suggest that this may also occur beneath the more active volcanoes of the western archipelago.

Noneruptive Unrest at the Caldera of Alcedo Volcano (Galápagos Islands) Revealed by InSAR Data and Geodetic Modeling

“…The 2‐ to 3‐km depth of the sills centroids in our study is also consistent with the depth of the source that caused the subsidence of the southern part of the caldera from 1997 to 2001 (Hooper et al, ). Furthermore, this depth agrees with that of the top of the shallow magmatic systems of the nearby calderas at Fernandina, Sierra Negra, and Wolf (from 1 to 3 km depth below the caldera; Bagnardi et al, ; Jónsson, ; Stock et al, ). Such depths are more consistent with those of magmatic sources than with that of the shallow hydrothermal system of Alcedo.…”

“…Successively, between 1997 and 2001, uplift switched to subsidence (<11 cm), which has been interpreted as deflation of an ellipsoid‐like body at ~2.2 km below sea level (Hooper et al, ). Conversely to Cerro Azul, Fernandina, and Wolf (Amelung et al, ; Bagnardi et al, ; Stock et al, ; Xu et al, ), no deformation is recorded outside the caldera of Alcedo in the last 30 years.…”

Cryptic evolved melts beneath monotonous basaltic shield volcanoes in the Galápagos Archipelago

“…2; ref. 19 ; Supplementary Dataset 1). These do not show any significant compositional variability outside of uncertainty, despite sampling lava flows that erupted at different times through the circumferential fissure phase of the eruption.…”

“…Recent Galápagos eruptions have afforded geophysical constraints on magma storage depths 19,21,32,33 , making them good targets for understanding the processes responsible for 71 . The sampling locations of the lavas and tephra analysed in this study are shown as green circles and blue diamonds, respectively.…”

Sector-zoned clinopyroxene as a recorder of magma history, eruption triggers, and ascent rates

“…Mollo and Hammer, 2017) is always necessary for the crystals to grow to the size of phenocrysts. In natural volcanic rocks, augite crystals often deviate from the frequently assumed 'treering' model and develop sector zoning (Hollister and Gancarz, 1971;Arculus, 1973;Ferguson, 1973;Downes, 1974;Leung, 1974;Dowty, 1976;Duncan and Preston, 1980;Shimizu, 1981;Watson and Liang, 1995;Brophy et al, 1999;Hammer et al, 2016;Welsch et al, 2016;Neave and Putirka, 2017;Mollo and Hammer, 2017;Stock et al, 2018), generating an hourglass form ( Fig. 1).…”