The role of C-O-H fluids in upper mantle processes : a theoretical, experimental and spectroscopic study

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 PT 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 PT 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."