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The role of sulphur in upper mantle processes

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Odling, Nicholas W A(Nicholas William A) (1989) The role of sulphur in upper mantle processes. PhD thesis, University of Tasmania.

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Abstract

This thesis consists of a review of experimental studies of sulphur solubility in mafic
magmas and the stability of magmatic sulphides, a theoretical examination of the
thermodynamics of C-O-S-H fluids to high temperature and pressure and three
experimental studies examining the role of sulphur under both volatile absent and
volatile saturated conditions.
Data concerning the sulphur solubility in mafic melts comes from experimental
studies on natural basalts and from metallurgical sources concerned with the
equilibrium between silicate slags and liquid metals. All of the metallurgical studies and
most of those involving natural compositions have been conducted at one atmosphere
and have identified the iron activity and the ambient oxygen and sulphur fugacities as
being the major factors controlling the sulphur solubility in a silicate melt. Studies at
pressures greater than one atmosphere are few and have examined only a few
compositions under relatively unconstrained sulphur and oxygen fugacities. Therefore
there is a lack of sulphur solubility data at high pressures and the partitioning of
sulphur between solid, melt and fluid phases remains an unknown of potential
significance.
Recent models for the genesis of MORB have emphasized the non-primary
nature of the majority of MORB glass compositions. The compositions of a range of
sulphide saturated potential MORB parent picritic liquids in equilibrium with either
lherzolitic or harzburgitic assemblages have been determined. The liquids produced are
similar to those generated from a similar but sulphur free source composition (pyrolite)
indicating that the presence of sulphur does not have the same fluxing effect as either
CO2 or H20.
Whereas volatile-poor conditions is appropriate for the modelling of MORB,
xenolithic material from the upper mantle frequently shows evidence of fluid
activity. A thermodynamic model for super-critical C-O-S-H fluids at high
temperature and pressure has been derived. Available P-V-T data for sulphur
bearing species has been combined with data for C-O-H species to calculate
fugacity coefficients by means of a modified Redlich-Kwong equation of state.
Using fugacity coefficients for multicomponent fluids the species distribution\as I
been determined as a function of the intensive variables P, T, f02 and fS2. The
results show that reduced fluids (f02≤1W) may contain a significant sulphur
fraction as H2S at moderate fS2 (IT+1) whereas more oxidized fluids (f02≥GC0)
by comparison contain little sulphur even at high fS2. SO2 is not a significant fluid
component in the range of f02's thought to characterize the upper mantle redox
range (IW<f02<NNO).
Two series of experiments have been carried out to examine the role of
sulphur in fluid saturated systems. Both employ simultaneous buffering of both
sulphur and oxygen by a tungsten based buffer assemblage ( f02~1W+1;
fS2~IT+1). This assemblage when combined with carbon saturation results in
invariant fluid compositions.
The first examines the system olivine-C-O-S-H. A technique is developed
by which fluid inclusions are induced to form in single crystals of natural olivine
(Fo90) from a tungsten assemblage buffered C-0-S-H fluid (H20>CH4). By
analysis of the quenched fluid from the capsule, the relationship between the
parent fluid and the daughter fluid inclusions may be examined. Laser Raman micro
analysis of inclusions formed from a variety of pressure/temperature combinations
show compositional variation between individual inclusions and between inclusions
and the parent fluid showing that the assumption of isochemical entrapment may not
be valid for fluid inclusions formed under mantle conditions. Inclusions formed
also show development of daughter crystals indicating appreciable cation solubility
in the fluid phase at the run conditions. Infra-red spectrometry of the host olivine
shows development of structural O-H during the experiments revealing a possible
'sink' for inclusion hydrogen; a model for post entrapment chemical change in the
fluid phase is devised which has implications for the origin of CO2 rich inclusions
in mantle xenoliths.'
In the second series of experiments the same oxygen/sulphur fluid buffer
technique has been used to explore the fluid saturated phase relations for the system
pyrolite-C-0-S-H. The condition of melting (solidus) appears to coincide with the
disappearance of a hydrous phase (amphibole and/or phlogopite) and occurs midway
between the reduced C-0-H solidus as determined by Taylor and Green(1988)
and the water saturated solidus. Fractured natural olivine crystals were incorporated
into the capsule assembly and the inclusions produced show fluids dominated by
CH4-H20 mixtures in agreement with the analysis of the capsule gasses by capsule
piercing/mass spectrometry. In addition to these volatile phases substantial amounts
of daughter crystals are observed in the fluid inclusions from conditions below the
'solidus' indicating significant cation solubility in supercritical CH4-H20 mixtures.
At higher temperatures this solid component increases continuously in abundance
until true melt inclusions are obtained at conditions above the solidus. These
sulphide/silicate compositional relationships are examined and a model derived
which at a given pressure and temperature relates the compositions of olivine,
orthopyroxene and sulphide to the ambient sulphur and oxygen fugacities. Such a
reaction may be used as an oxygen/sulphur fugacity sensor if an independent means
of estimating the temperature and pressure is available.

Item Type: Thesis (PhD)
Keywords: Sulfur compounds
Copyright Holders: The Author
Additional Information:

Thesis (Ph.D.)--University of Tasmania, 1990. Includes bibliography

Date Deposited: 03 Feb 2015 03:08
Last Modified: 11 Mar 2016 05:55
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