Publication Date:
2021-06-21
Description:
Sulphur behaviour and variations in redox conditions during magma differentiation and degassing
in the Mt Etna (Italy) volcanic system have been explored by integrating the study of olivine-hosted
melt inclusions (MIs) with an experimental survey of sulphur solubility in hydrous basaltic magmas.
Sulphur solubility experiments were performed at conditions relevant to the Etnean plumbing
system (1200 C, 200MPa and oxygen fugacity between NNOþ0 2 and NNOþ1 7, with NNO being
the nickel–nickel oxide buffer), and their results confirm the important control of oxygen fugacity
(fO2) on S abundance in mafic magmas and on S partitioning between fluid and melt phases
(DSfluid/melt). The observed DSfluid/melt value increases from 5164 to 14666 when fO2 decreases
from NNOþ1 760 5 to NNOþ0 3. Based on the calculated DSfluid/melt and a careful selection of
previously published data, an empirical model is proposed for basaltic magmas to predict the variation
of DSfluid/melt values with variations in P (25–300 MPa), T (1030–1200 C) and fO2 (between
NNO– 0 8 and NNOþ2 4). Olivine-hosted melt inclusions (Fo89-91) from tephra of the prehistoric
(4 ka BP) sub-plinian picritic eruption, named FS (‘Fall Stratified’), have been investigated for their
major element compositions, volatile contents and iron speciation (expressed as Fe3þ/PFe ratio).
These primitive MIs present S content from 235677 to 34456168 ppm, and oxygen fugacity
values, estimated from Fe3þ/PFe ratios, range from NNOþ0 760 2 to NNOþ1 660 2. Iron speciation
has also been investigated in more evolved and volatile-poorer Etnean MIs. The only primitive
melt inclusion from the Mt Spagnolo eruption (4–15 ka BP) presents a S content of
1515649ppm and an estimated fO2 of NNOþ1 460 1. The more evolved MIs (from 2002–2003,
2006, 2008–2009 and 2013 eruptions) have S content lower than 500 ppm, and their Fe3þ/RFe ratios
result in fO2 between NNO– 0 960 1 and NNOþ0 460 1. Redox conditions and S behaviour in
Etnean magmas during degassing and fractional crystallization were modelled coupling MELTS
code with our empirical DSfluid/melt model. Starting from an FS-type magma composition and upon
decrease of T and P, fractional crystallization of olivine, clinopyroxene, spinel and plagioclase
causes a significant fO2 decrease. The fO2 reduction, in turn, causes a decrease in sulphur solubility
and an increase in DSfluid/melt, promoting S exsolution during magma ascent, which further
enhances the reduction of fO2. For the evolved MIs of 2002–2013 eruptions, magma differentiation
may therefore have played a crucial role in decreasing redox conditions and favouring efficient S
degassing. Differently, during the unusual FS eruption, only limited melt evolution is observed and
S exsolution seems to have been triggered by a major pressure decrease accompanied by H2O and
CO2 exsolution during fast magmatic ascent.
Description:
Published
Description:
egaa095
Description:
4V. Processi pre-eruttivi
Description:
JCR Journal
Keywords:
melt inclusions
;
sulphur solubility experiments
;
XANES
;
Mt. Etna
;
04.08. Volcanology
Repository Name:
Istituto Nazionale di Geofisica e Vulcanologia (INGV)
Type:
article
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