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

Degassing vs. eruptive styles at Mt. Etna volcano (Sicily, Italy). Part I: Volatile stocking, gas... 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 122BC 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 122BC. The opposite is seen for sulfur: low to mild-explosive fluxed magmas are S-enriched, whereas the 122BC 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. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Chemical Geology Elsevier

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

Loading next page...
 
/lp/elsevier/degassing-vs-eruptive-styles-at-mt-etna-volcano-sicily-italy-part-i-qQ1XCRHVJk
Publisher
Elsevier
Copyright
Copyright © 2017 Elsevier Ltd
ISSN
0009-2541
eISSN
1872-6836
D.O.I.
10.1016/j.chemgeo.2017.09.017
Publisher site
See Article on Publisher Site

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 122BC 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 122BC. The opposite is seen for sulfur: low to mild-explosive fluxed magmas are S-enriched, whereas the 122BC 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.

Journal

Chemical GeologyElsevier

Published: Apr 5, 2018

References

You’re reading a free preview. Subscribe to read the entire article.


DeepDyve is your
personal research library

It’s your single place to instantly
discover and read the research
that matters to you.

Enjoy affordable access to
over 18 million articles from more than
15,000 peer-reviewed journals.

All for just $49/month

Explore the DeepDyve Library

Search

Query the DeepDyve database, plus search all of PubMed and Google Scholar seamlessly

Organize

Save any article or search result from DeepDyve, PubMed, and Google Scholar... all in one place.

Access

Get unlimited, online access to over 18 million full-text articles from more than 15,000 scientific journals.

Your journals are on DeepDyve

Read from thousands of the leading scholarly journals from SpringerNature, Elsevier, Wiley-Blackwell, Oxford University Press and more.

All the latest content is available, no embargo periods.

See the journals in your area

DeepDyve

Freelancer

DeepDyve

Pro

Price

FREE

$49/month
$360/year

Save searches from
Google Scholar,
PubMed

Create lists to
organize your research

Export lists, citations

Read DeepDyve articles

Abstract access only

Unlimited access to over
18 million full-text articles

Print

20 pages / month

PDF Discount

20% off