Other
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.v34b..08t&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #V34B-08
Other
1012 Reactions And Phase Equilibria (3612, 8412), 1031 Subduction Zone Processes (3060, 3613, 8170, 8413), 1034 Hydrothermal Systems (0450, 3017, 3616, 4832, 8135, 8424), 1036 Magma Chamber Processes (3618), 1038 Mantle Processes (3621)
Scientific paper
Since the discovery of the second critical endpoint (CP2) in the albite-water system, numerous attempts have been made to determine the pressure and temperature of this CP2 and the mutual solubilities within more complex systems. The P-T position of the CP2 has been estimated for many systems: SiO2 (<10 kb/900 °C, Newton and Manning, 2008); NaAlSi3O8 (15 kb/800 °C, Burnham and Davis, 1974; Shen and Keppler, 1997); Pelite (50 kb/1,000 °C, Schmidt et al., 2004), basalt (50 kb/ 1000 °C, Kessel et al., 2004), Peridotite (38 kb/1000 °C, Mibe et al., 2007). A number of experimental techniques have been used to determine phase relations and H2O solubility in experiments. These include in-situ experimental techniques (e.g., HYDAC; Shen and Keppler, 1997), fluid trap techniques (e.g., diamond traps; Stalder et al., 2000), and single crystal weight-loss techniques (e.g., SiO2-H2O techniques employed by Newton and Manning, 2008). None of these techniques is without difficulties, as H2O rich experiments need to overcome huge retrograde fluid solubilities upon quench in order to determine mutual solubilities at experimental conditions. We have developed a new technique to determine "rock"-H2O relationships at high-P conditions, with particular focus on the shape and locus of solvi in pressure temperature space. In this series of experiments, an oxygen fugacity buffer (Re-ReO2) and a sliding H-fugacity sensor (NiO-Ni-Pd mixture) are combined to monitor H2O activity over the entire range of pressure and temperature. Unlike other techniques, the use of sensor capsules does not require textural interpretation of experiments. H2O activity is related to oxygen and hydrogen fugacity by the reaction: H2O = H2 + ½O2 NiO-Ni-Pd mixtures were placed within a ZrO2 jacket and sealed within a welded 2.3 mm Pt capsule. This 2.3 mm Pt sensor capsule was then encased within a larger, thick walled 6 mm diameter Ag capsule. Pelite-H2O mixtures and oxygen buffers were held within this larger Ag capsule and sealed by capsule swaging. Changes in H2O activity (measured by shifts in the Ni content of the alloy) that occur with changes in pelite- water ratios at isothermal and isobaric conditions, can be used to determine the shape and position of the melt-H2O solvus. Pelite-H2O experiments at 25 kbar show solvus behaviour at 900 °C (temperatures >200 °C) above the wet solidus), indicating that the wet solidus and critical curve do not intersect at these conditions. In contrast, experiments at 35 kbar and 800-900 °C, show a gradual change in H2O activity with increasing water:pelite ratios, indicating that the critical curve and wet solidus become closer with increasing pressure. Solvus topology in the pelite-H2O system, as determined by this series of experiments at 25 and 35 kbar, can also be used to estimate the shape and orientation of solubility isopleths (in terms of P-T). While the seminal work of Burnham and Davis (1974) demonstrated the importance of pressure on inducing H2O saturation in silicate melts, experiments presented here indicate that temperature may also play an important role. Burnham C. W. and Davis N. F. 1974, American Journal of Science. 274; 8, 902-940. Kessel, R., et al 2005, Earth and Planetary Science Letters, 237, 873-892. Mibe, K et al. 2007, Geochimica et Cosmochimica Acta. 68, 24, 5189-5195. Newton, R. C., and Manning, C. E., 2008, Earth and Planetary Science Letters. Schmidt, M.W.et al, 2004, Earth and Planetary Science Letters, 228, 65-84. Shen, A., and Keppler, H. 1997 Nature, 385, 710-712. Stadler, R., et al, 2000, American Mineralogist, 85, 68-77.
class="ab'>
Hermann Jérg
Mavrogenes John A.
O'Neill H. S.
Tailby N.
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