Showing papers by "Nicole Métrich published in 2017"

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

Abstract: . The 2014 eruption at Piton de la Fournaise (PdF), La Reunion, 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.

New Insights into Magma Differentiation and Storage in Holocene Crustal Reservoirs of the Lesser Sunda Arc: the Rinjani–Samalas Volcanic Complex (Lombok, Indonesia)

Abstract: Large explosive eruptions of dacitic magmas, their relationship with their basaltic parent and their conditions of transfer and crystallization are widely debated. Here we report new timing constraints and a detailed study of the mineralogy and chemistry of magmas erupted over the last 12 kyr at the Rinjani–Samalas volcanic complex on Lombok, in the Lesser Sunda arc. Rinjani–Samalas products define a calc-alkaline series, moderately rich in K2O. High-alumina basalts (HABs) evolved towards trachydacites (50 to 68 wt % SiO2) mainly by fractional crystallization, and required the peritectic formation of (cryptic) amphibole. The pre-caldera stage is characterized by chemical bimodality (basalt–trachydacite) of the erupted magmas. Conversely, the post-caldera magmatism produced basaltic andesites. These present-day magmas possibly result from the mixing between basalt and trachydacite melts, in main proportions 0 7:0 3, before crystallization. The AD 1257 caldera-forming eruption delivered a large volume of chemically homogeneous trachydacitic magma. Its mineral paragenesis typically consists of plagioclase showing a bimodal distribution with patchy zoned cores (An82–75) surrounded by bands of An50 to An43, in association with amphibole (magnesio-hastingsite), orthopyroxene (Mg# 0 66–0 73), titanomagnetite, iron sulfide and apatite. Rare extremely calcic plagioclase (An91–92) records the early stage of crystallization. Extensive Mg diffusion demonstrates an overall re-equilibration of plagioclase compositions with a trachydacitic melt in equilibrium with plagioclase An5061 (312 6 42 ppm Mg). Discrepancies between the crystallization temperature of plagioclase ( An50) measured on melt inclusions (989 6 10 C) and those provided by geothermometers (in the range between 895 and 980 C) bring evidence of heterogeneity in both the temperature and the water content of the trachydacitic magma batches. Trace element geochemistry and mineralogy of bulk-rocks and volatile contents of melt inclusions suggest that the HABs initially crystallized in the lower crust; trachydacite magma batches were then extracted, transferred to shallow depths, and crystallized under conditions of watersaturation. Extensive stratigraphic data and new radiocarbon ages reveal that explosive and effusive emissions of trachydacitic magmas and strongly explosive eruptions of HABs occurred contemporaneously before the caldera-forming eruption. This implies a transfer of basaltic magma through dykes independently of the central system, a feature ascribed to the interplay between edifice mass loading, magma buoyancy and possibly the regional tectonic stress field. VC The Author(s) 2018. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@oup.com 2257 J O U R N A L O F P E T R O L O G Y Journal of Petrology, 2017, Vol. 58, No. 11, 2257–2284 doi: 10.1093/petrology/egy006 Advance Access Publication Date: 3 February 2018

From magma ascent to ash generation: Investigating volcanic conduit processes by integrating experiments, numerical modeling, and observations

Abstract: Processes occurring in volcanic conduits, the pathways through which magma travels from its storage region to the surface, have a fundamental control on the nature of eruptions and associated phenomena. It has been well established that magma flows, crystallizes, degasses, and fragments in conduits, that fluids migrate in and out of conduits, and that seismic and acoustic waves are generated and travel within conduits. A better understanding of volcanic conduits and related processes is of paramount importance for improving eruption forecasting, volcanic hazard assessment and risk mitigation. However, despite escalating advances in the characterization of individual conduit processes, our understanding of their mutual interactions and the consequent control on volcanic activity is still limited. With the purpose of addressing this topic, a multidisciplinary workshop led by a group of international scientists was hosted from 25 to 27 October 2014 by the Pisa branch of the Istituto Nazionale di Geofisica e Vulcanologia under the sponsorship of the MeMoVolc Research Networking Programme of the European Science Foundation. The workshop brought together the experimental, theoretical, and observational communities devoted to volcanological research. After 3 days of oral and poster presentations, breakout sessions, and plenary discussions, the participants identified three main outstanding issues common to experimental, analytical, numerical, and observational volcanology: unsteadiness (or transience), disequilibrium, and uncertainty. A key outcome of the workshop was to identify the specific knowledge areas in which exchange of information among the sub-disciplines would lead to efficient progress in addressing these three main outstanding issues. It was clear that multidisciplinary collaboration of this sort is essential for progressing the state of the art in understanding of conduit magma dynamics and eruption behavior. This holistic approach has the ultimate aim to deliver fundamental improvements in understanding the underlying processes generating and controlling volcanic activity.

Degassing vs. eruptive styles at Mt. Etna volcano (Sicily, Italy): Volatile stocking, gas fluxing, and the shift from low-energy to highly-explosive basaltic eruptions

Abstract: Abstract Basaltic magmas can transport and release large amounts of volatiles into the atmosphere, especially in subduction zones, where slab-derived fluids enrich the mantle wedge. Depending on magma volatile content, basaltic volcanoes thus display a wide spectrum of eruptive styles, from common Strombolian-type activity to Plinian events. Mt. Etna, in Sicily, is a typical basaltic volcano where the volatile control on such a variable activity can be investigated. Based on a melt inclusion study in products from Strombolian or lava-fountain activity to Plinian eruptions, here we show that for the same initial volatile content, different eruptive styles reflect variable degassing paths throughout the composite Etnean plumbing system. The combined influence of i) crystallization, ii) deep degassing and iii) CO2 gas fluxing can explain the evolution of H2O, CO2, S and Cl in products from such a spectrum of activity. Deep crystallization produces the CO2-rich gas fluxing the upward magma portions, which will become buoyant and easily mobilized in small gas-rich batches stored within the plumbing system. When reaching gas-dominated conditions (i.e., a gas/melt mass ratio of ~ 0.3 and CO2,gas/H2Ogas molar ratio ~ 5), magma batches will erupt effusively or mildly explosively. In case of the 122 BC Plinian eruption, open-system degassing conditions took place within the plumbing system, such that earlier CO2-fluxing determined gas accumulation on top of the magmatic system, likely followed by H2O-fluxing further hydrating the shallow magma. The emission of such a cap in the early eruptive phase triggered the arrival of deep H2O-rich magma whose fast decompression and bubble nucleation led to the highly explosive character, enhanced by abundant microlite crystallization and consequent increase of magma effective viscosity. This could explain why open system basaltic systems like Etna may experience highly explosive or even Plinian episodes during eruptions that start with effusive to mildly explosive phases. The proposed mechanism also determines a depression of chlorine contents in CO2-fluxed (and less explosive) magmas with respect to those feeding Plinian events like 122 BC. The opposite is seen for sulfur: low to mild-explosive fluxed magmas are S-enriched, whereas the 122 BC Plinian products are relatively S-poor, likely because of early sulfide separation accompanying magma crystallization. The proposed mechanism involving CO2 separation and fluxing may suggest a subordinate role for variable mixing of different sources having different degrees of K-enrichment. However, such a mechanism requires further experimental studies about the effects on S and Cl dissolution and does not exclude self-mixing between degassed and undegassed parcels within the Etna plumbing system. Finally, our findings may represent a new interpretative tool for the geochemical and petrologic monitoring of plume gas discharges and melt inclusions, and allow tracking the switch from mild-explosive to highly explosive or even Plinian events at Etna.