Xenopumice erupted on 15 October 2011 offshore of El Hierro (Canary Islands): a subvolcanic snapshot of magmatic, hydrothermal and pyrometamorphic processes

Abstract

On 15 October 2011, a submarine eruption offshore of El Hierro Island gave rise to floating volcanic products, known as xenopumices, i.e., pumiceous xenoliths partly mingled and coated with the juvenile basanitic magma. Over the last few years, no consensus in the scientific community in explaining the origin of these products has been reached. In order to better understand the formation of xenopumice, we present a textural, mineralogical, and geochemical study of the possible magmatic, hydrothermal, and pyrometamorphic processes, which usually operate in the plumbing systems of active volcanoes. We carried out a comprehensive SEM investigation and Sr-Nd-Pb isotope analyses on some samples representative of three different xenopumice facies. All the data were compared with previous studies, new data for El Hierro extrusives and a literature dataset of Canary Islands igneous and sedimentary rocks. In the investigated xenopumices, we emphasize the presence of restitic magmatic phases as well as crystallization of minerals (mainly olivine + pyroxene + magnetite aggregates) as pseudomorphs after pre-existing mafic phenocrysts, providing evidence of pyrometamorphism induced by the high-T juvenile basanitic magma. In addition, we identify veins consisting of zircon+REE-oxides+mullite associated with Si-rich glass and hydrothermal quartz, which indicate the fundamental role played by hydrothermal fluid circulation in the xenopumice protolith. The petrological data agree with a pre-syneruptive formation of the xenopumice, when El Hierro basanite magma intruded hydrothermally altered trachyandesite to trachyte rocks and triggered local partial melting. Therefore, the El Hierro xenopumice represents a snapshot of the transient processes at the magma-wall rock interface, which normally occurs in the feeding system of active volcanoes.

New insight into the 2011-2012 unrest and eruption of El Hierro Island (Canary Islands) based on integrated geophysical, geodetical and petrological data

“…We suggest that the seismic silence and seismic activity at shallow depth might occur when the ascending magma found a ductile zone represented by a stagnant magma pocket/s and hydrothermally altered volcanic rocks [Meletlidis et al 2012, Sigmarsson et al 2012, Del Moro et al 2015. The interaction (heating/pyrometamorphism) between the basanitic magma feeding the eruption, and the rocks/magma of the duc-tile zone would have initially resulted in an almost aseismic, horizontal expansion of the magma and later in the shallow seismicity.…”

“…The subaerial part of El Hierro is formed by two main volcanic edifices, the Tiñor volcano on the north-east flank of the island, built approximately between 0.88-1.12 Ma and El Golfo volcano in the north-west flank, built between 550-130 ka. Products belonging to Tiñor volcano [Troll et al 2011, Meletlidis et al 2012, Sigmarsson et al 2012, Del Moro et al 2015]; (c) example of the juvenile ash and fine lapilli sampled in water column on November 5, 2011, andFebruary 12, 2012; floating lava block erupted during the first days of the eruption; (e) elliptical-shaped "lava balloon". mainly consist of picritic to hawaiitic-tephritic massive lavas whereas El Golfo volcano is characterized by nephelinitic to trachytic composition [Carracedo et al 2001].…”

“…sition [Troll et al 2011, Meletlidis et al 2012, Sigmarsson et al 2012, Del Moro et al 2015. This type of products was emitted exclusively during the first days of the eruption and the main volume was produced on October 15, 2011.…”

Understanding volcanic systems and their dynamics combining field and physical volcanology with petrology studies

“…Classical methods embrace compositional investigations on bulk rocks or on single phases, using information derived from radiogenic isotopes and trace elements. Some examples include recent eruptions or eruptive periods of well-known volcanoes (Pompilio et al, 2011;Meletlidis et al, 2012 ;Troll et al, 2012;Marti et al, 2013;Longpré et al, 2014;Del Moro et al, 2015, Meletlidis et al, 2015, Di Renzo et al, 2018. Textural analyses including both descriptive (e.g., habitus, shape) and quantitative information derived from crystal or vesicle size distributions have been also largely employed, in most of active volcanoes, thanks to the tremendous recent development of image analysis techniques.…”

Repeated magmatic intrusions at El Hierro Island following the 2011–2012 submarine eruption

“…Tremor amplitude continually increased until 12 October 2011 when this abruptly ceased and green-colored water and the first rock samples were observed floating on the sea . These rocks were called xenopumices due to their glassy basanitic crusts of decimeter-scale and cores of pumice-like texture and glassy matrix (Del Moro et al, 2015;Meletlidis et al, 2012;Sigmarsson et al, 2013;Troll et al, 2012).…”