doi:10.1016/j.gca.2004.07.011
Experimental data on the speciation of sulfur as a function
of oxygen fugacity in basaltic melts
PEDRO J. JUGO,* ROBERT W. LUTH, and JEREMY P. RICHARDS
Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, Canada, T6G 2E3
(Received March 11, 2004; accepted in revised form July 9, 2004)
Abstract—The speciation of sulfur as a function of oxygen fugacity was calculated in glasses of basaltic
composition saturated experimentally with either sulfide or sulfate phases. The experiments were conducted
on mixtures of synthetic and natural materials equilibrated at 1300 °C and 1 GPa in a piston-cylinder
apparatus. Sulfur speciation was calculated by measuring the peak shift of the sulfur K radiation relative to
a sulfide standard, whereas oxygen fugacity was calculated from the composition of olivine and spinel present
in the assemblages. The results are consistent with sulfur being present as sulfite (S
4+
) in addition to sulfate
(S
6+
) in oxidized melts. Therefore, sulfur speciation derived from SK peak shifts should be seen as ”sulfate
mole fraction equivalents“ (X(S
6+
)
eq.
). Using the data available, an empiric function:
X(S
6+
)
eq.
= 0.86 / (1 + exp(2.89 - 2.23FMQ))
relating sulfate mole fraction equivalents and oxygen fugacity was derived. This function can be approx-
imated by a linear function:
FMQ = 2.28X(S
6+
)
eq.
+ 0.32
which could be used as an independent oxybarometer over the range in which most of the change of speciation
occurs (+0.8 FMQ +1.9). Copyright © 2005 Elsevier Ltd
1. INTRODUCTION
Sulfur can be present in silicate melts as various species,
most notably as sulfide (S
2-
) and sulfate (SO
4
)
2-
, and under-
standing the relationship between sulfur speciation and oxygen
fugacity is of interest for various reasons. For example, sulfur
is a significant component in gases released by volcanic activity
and sulfur-rich volcanic eruptions, some of them associated
with the presence of anhydrite in the erupted magmas (e.g.,
Pinatubo in 1991), have been linked to short-term, high-impact
variations in global climate (Rampino and Ambrose, 2000).
Saturation or elimination of sulfide phases (crystalline or as
immiscible liquids) during magmatic evolution or partial melt-
ing dictates the behavior of chalcophile elements (e.g., Cu, Ni)
and highly siderophile elements (HSE = Ru, Rh, Pd, Re, Os, Ir,
Pt, Au). These elements are of interest because of their eco-
nomic value as base- or precious metals (e.g., Cu, Ni, Pd, Pt,
Au) or because they are used as tracers of mantle processes
(e.g., Re and Os).
In relatively reduced magmas, such as those generated in
midocean ridge settings, sulfur is dominantly present as sulfide.
Because of this, sulfides were considered to be the dominant
sulfur species in magmatic systems. However, some magmas
are sufficiently oxidized for sulfur to be present as sulfate.
Carroll and Rutherford (1988) used the peak shift in the sulfur
K radiation, determined by electron probe microanalysis
(EPMA), to estimate the change in the speciation of sulfur as a
function of oxygen fugacity and showed that the transition from
sulfide-dominance to sulfate-dominance occurred between
FMQ and FMQ =+ 2 (FMQ: fayalite-magnetite-quartz
reference buffer). Wallace and Carmichael (1994) combined
the data from Carroll and Rutherford (1988) with data on
natural basaltic glasses from spreading centers, seamounts, and
back-arc basins to derive an expression relating the ratio be-
tween the sulfate and the sulfide mole fractions (X(SO
4
)
2-
/
X(S)
2-
) and fO
2
. This relationship has been used to infer the
oxidation state of natural samples from calculated sulfur spe-
ciation (e.g., Métrich and Clocchiatti, 1996; Gurenko and
Schmincke, 1998, 2000). Matthews et al. (1999) measured
sulfur speciation and oxidation state in basaltic andesites and
andesites from the Lascar volcano (Chile) and compiled the
available data to generate a polynomial fit relating fO
2
as a
function of the sulfate mole fraction (X(S
6+
)). In this paper, we
present new data on the speciation of sulfur in basaltic glasses
from experiments with independently constrained values of
oxygen fugacity. These new data are compared with existing
data on sulfur speciation as a function of oxygen fugacity. The
different models for the relationship between sulfur speciation
and oxygen fugacity are discussed and a new empiric function
relating the apparent sulfate mole fraction and fO
2
is derived.
This function provides a better data fit than the function of
Wallace and Carmichael (1994), has a simpler form than that of
Matthews et al. (1999), and is consistent with the presence of
sulfite species (S
4+
) in silicate melts as shown by Métrich et al.
(2002). This function could be used as an independent oxyba-
rometer in oxidized basaltic melts.
2. EXPERIMENTAL AND ANALYTICAL METHODS
A summary of experimental conditions and the method for estimat-
ing sulfur speciation are described in this section. Further details of the
* Author to whom correspondence should be addressed, at Institut für
Mineralogie, J.W. Goethe Universität, D-60054 Frankfurt am Main,
Germany (Pedro.Jugo@em.uni-frankfurt.de).
Geochimica et Cosmochimica Acta, Vol. 69, No. 2, pp. 497–503, 2005
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