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Abstract

Evidence from natural magmas and mantle xenoliths argues that fluids
in the system C-0-H exercise important influences on petrogenetic
processes in the Earth's upper mantle. In this thesis, experimental,
spectroscopic and theoretical constraints are applied to provide the
necessary basis for understanding the nature of mantle melting as a
function of oxygen fugacity (f02) in the model system "peridotite"-C-0-H.
Until very recently, most petrogenetic models were based on the
assumption that f02 -conditions in magma source-regions of the upper mantle
were relatively oxidised, lying near the f02 defined by the synthetic
assemblage fay a li te-magneti te-quartz (FMQ). This was cons is tent with the
petrogenetic role inferred for oxidised co2-H20 volatiles and carbonated
peridotite. However, if magma generation involving volatile components
takes place in a reduced environment, for example at f02's near the
synthetic iron-wustite (IW) oxygen buffer - as suggested by intrinsic f02
measurements on mantle-derived minerals - then in the model system
"peridotite"-C-0-H, volatiles will be dominantly CH4 > H20 > H2
mixtures and crystalline carbonates will not be stable relative to diamond or
graphite. The nature of mantle melts produced under reduced conditions is
expected to be very different from melts originating in an oxidised
environment.
To investigate the role of reduced volatiles in upper mantle
processes, a two-fold approach has been pursued. Firstly, a thermodynamic
model for supercritical C-0-H fluids under elevated P,T conditions was
derived. Available volumetric data for the important species: H20, C02,
CO, H2, CH4 and C2H6 were used to calibrate an MRK-type equation of state.
Using fugacity coefficients derived from the MRK-equation, the distribution
of C-0-H species as a function of P,T and f02 has been determined.
Application of this model to fluids incorporated in natural diamond
(perhaps the only unequivocal example of upper mantle fluids) suggests that
the majority of natural diamonds and coexisting C-0-H fluids were at
equilibrium in the mantle under redox conditions near or slightly above
f02 = IW. A genetic link between methane-bearing fluids and diamond is
indicated.
The second approach taken in this study was directed at understanding
the nature of mantle melting in the presence of reduced volatile species.
Because little is known of the mechanism of reduced volatile interaction
with silicate systems, an investigation of the CH4-H2-C2H6 fluid solubility
mechanism in aluminosilicate melts was undertaken. Jadeite and sodamelilite composition melts were saturated with a C-H fluid at P = 30 kbar and Fourier transform infrared (FTIR) spectra of the resultant high-P
quenched glasses were recorded. FTIR spectra distinguish both oxidised and
reduced components dissolved in the melt as predicted from theoretical
considerations. Low total carbon concentrations (<2000 ppm) in the
quenched glasses coupled with FTIR identification of 0-H species and a
shift in Si-0 spectral bands, favour a dissolution mechanism for C-H
volatiles that involves formation of 0-H groups together with a reduced
silicate network component ("silicon monoxide" unit). Dissolution of the
latter component results in reduction in overall O:Si stoichiometry of the
melt such that O/Si < 2.
The effects of C-0-H fluids on aluminosilicate melt structure are
revealed indirectly by movement of liquidus field boundaries, particularly
those between enstatite and forsterite. Liquidus phase relations in the
system Ne-Fo-Q under conditions of C-H volatile saturation indicate that
C-H volatile dissolution increases the activity of non-network aluminium
and results in silicate network depolymerisation. This is in agreement
with the proposed C-H volatile dissolution mechanism.
Recognising that reduced volatiles such as CH4
are important primordial components in fluids degassed at mid-ocean ridges and other
environments, an hypothesis for redox-reaction induced partial melting of
the upper mantle is presented. This hypothesis relates oxidation of
methane-rich fluids, precipitation of diamond/graphite and production of
incipient H20-rich melts as the combined effects of interaction of deepseated
reduced volatiles with oxidised lithosphere.

Item Type:	Thesis - PhD
Authors/Creators:	Taylor, WR
Keywords:	Petrogenesis, Peridotite
Copyright Holders:	The Author
Copyright Information:	Copyright 1985 the Author - The University is continuing to endeavour to trace the copyright owner(s) and in the meantime this item has been reproduced here in good faith. We would be pleased to hear from the copyright owner(s).
Additional Information:	Thesis (Ph.D.)--University of Tasmania, 1986. Includes bibliography
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