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Carbon and Hydrogen in Melts and Fluids in Planetary Interiors

James Tuff 1 , David H. Green 1 , Michael Shelley 1

1 Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia

Important characteristics of the Earth's mantle, such as the temperature of it's solidus at a given depth and the compositions of mantle-derived magmas (e.g. silica-saturated vs. silica-undersaturated), are dependent on the prevailing oxidation state and the solubilities of volatile species. In particular, C-H-O species are known to have large effects on mantle solidus temperatures and magma compositions. Magmas on the Earth's surface are generally oxidized (ƒO 2 ~ FMQ) and CO 2 -bearing. However, the source region of these magmas - the convecting upper mantle - is likely to be significantly reduced at depth, probably ~ FMQ-2 to FMQ-3 log units for basalt source regions and < FMQ-3 log units for kimberlite source regions (e.g. Green et al., 1987; Taylor & Green, 1989). The stable C-H-O volatile phase under these ƒO 2 conditions will be a CH 4 -H 2 ±H 2 O fluid, rather than CO 2 -H 2 O. Calculations suggest that upper mantle ƒO 2 may decrease by as much as 0.6 log ƒO 2 units per GPa (relative to FMQ). The question arises as to how oxidized magmas can be extracted from a reduced source.

We are using high pressure, high temperature experimental techniques in order to constrain the role of carbon and hydrogen volatile species within the Earth's upper mantle. Our experimental work has focussed on the development of new methods that enable us to analyse the fluid phases produced from experimental run products. An innovative system we have built consists of a gas chromatograph that is capable of measuring trace quantities of gases within experimental run charges; at present, we are able to measure N 2 , O 2 , CO, CO 2 , CH 4 , C 2 H 6 and H 2 O, but slight modifications will allow us to analyse a variety of other gases. Experiments are based on the design of Taylor & Foley (1989) and consist of a W-C-WO buffer in order to produce the required reducing environment (FMQ-3 log units at 2.8 GPa). Early results indicate that a CH 4 - and H 2 O - rich fluid is produced under such conditions and agree with results from experiements by Matveev et al. (1997) and Taylor & Foley (1989). Our development of a reliable method of gas analysis provides a versatile system that will have many useful applications for future experimental studies.

References: Green D.H., Falloon T.J., and Taylor W.R. (1987) Mantle-derived magmas: Roles of variable source peridotite and variable C-H-O fluid compositions. Geochemical Soiety Special Pubication. 1 , 139-154.

Taylor W.R., and Green D.H. (1989) The role of reduced C-O-H fluids in mantle partial melting. Geological Society of Ausralia Special. Pubication. 14 , 592-602.

Taylor, W.R., and Foley, S.F. (1989) Improved Oxygen-buffering techniques for C-O-H fluid-saturated eperiments at high pressure. Journal of Geophysical Research 94 , 4146-4158.

Matveev, S., Ballhaus, C., Fricke, K., Truckenbrodt, J. and Ziegenbein, D. (1997) Volatiles in the Earth's mantle: I. Synthesis of CHO fluids at 1273 K and 2.4 GPa. Geochimica et Cosmochimica Acta 61 , 3081-3088.