Articles | Volume 9, issue 2
Research article
16 Apr 2018
Research article |  | 16 Apr 2018

Integrating field, textural, and geochemical monitoring to track eruption triggers and dynamics: a case study from Piton de la Fournaise

Lucia Gurioli, Andrea Di Muro, Ivan Vlastélic, Séverine Moune, Simon Thivet, Marina Valer, Nicolas Villeneuve, Guillaume Boudoire, Aline Peltier, Patrick Bachèlery, Valérie Ferrazzini, Nicole Métrich, Mhammed Benbakkar, Nicolas Cluzel, Christophe Constantin, Jean-Luc Devidal, Claire Fonquernie, and Jean-Marc Hénot

Abstract. The 2014 eruption at Piton de la Fournaise (PdF), La Réunion, which occurred after 41 months of quiescence, began with surprisingly little precursory activity and was one of the smallest so far observed at PdF in terms of duration (less than 2 days) and volume (less than 0.4  ×  106 m3). The pyroclastic material was composed of golden basaltic pumice along with fluidal, spiny iridescent and spiny opaque basaltic scoria. Density analyses performed on 200 lapilli reveal that while the spiny opaque clasts are the densest (1600 kg m−3) and most crystalline (55 vol. %), the golden pumices are the least dense (400 kg m−3) and crystalline (8 vol. %). The connectivity data indicate that the fluidal and golden (Hawaiian-like) clasts have more isolated vesicles (up to 40 vol. %) than the spiny (Strombolian-like) clasts (0–5 vol. %). These textural variations are linked to primary pre-eruptive magma storage conditions. The golden and fluidal fragments track the hotter portion of the melt, in contrast to the spiny fragments and lava that mirror the cooler portion of the shallow reservoir. Exponential decay of the magma ascent and output rates through time revealed depressurization of the source during which a stratified storage system was progressively tapped. Increasing syn-eruptive degassing and melt–gas decoupling led to a decrease in the explosive intensity from early fountaining to Strombolian activity. The geochemical results confirm the absence of new input of hot magma into the 2014 reservoir and confirm the emission of a single shallow, differentiated magma source, possibly related to residual magma from the November 2009 eruption. Fast volatile exsolution and crystal–melt separation (second boiling) were triggered by deep pre-eruptive magma transfer and stress field change. Our study highlights the possibility that shallow magma pockets can be quickly reactivated by deep processes without mass or energy (heat) transfer and produce hazardous eruptions with only short-term elusive precursors.

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Short summary
We prove here that macroscopic and microscopic studies of emitted pyroclastic and effusive products provide valuable information to track and understand small explosive eruptions for hazard and risk assessment. This is especially true for Piton de La Fournaise, La Réunion, whose activity has recently been characterized by effusive and mild explosive activity in highly visited areas. We confirm that petrological monitoring is essential to forecast changes in the magmatic system.