HR: 0800h
AN: V31C-0624    [Abstracts]
TI: Internally consistent mineral solubility and hydrous melting relations for simple silicate-H2O systems and their significance
AU: * Hack, A C
EM: alistair.hack@erdw.ethz.ch
AF: Research School of Earth Sciences, Australian National University, Canberra, 0200 Australia
AU: * Hack, A C
EM: alistair.hack@erdw.ethz.ch
AF: Institute for Mineralogy and Petrology, ETH, Zrich, 8092 Switzerland
AU: Hermann, J
EM: joerg.hermann@anu.edu.au
AF: Research School of Earth Sciences, Australian National University, Canberra, 0200 Australia
AU: Mavrogenes, J A
EM: john.mavrogenes@anu.edu.au
AF: Research School of Earth Sciences, Australian National University, Canberra, 0200 Australia
AB: Mineral solubility and melting relations in the systems SiO2-H2O, NaAlSi3O8(albite)-H2O and MgO-SiO2-H2O have been the subject of many experimental studies. This is a direct reflection of their importance as analogues for crust and mantle rock types, and as guides to the behavior of more complex silicate-H2O systems. A particularly interesting feature of all of these systems is that the wet solidus is known to terminate with increasing pressure (the so-called second critical point). The nature of fluids that occur above the wet solidus is a source of considerable confusion but is of key importance if certain ultra-high pressure localities and subduction processes are to be understood. Although none of these simple systems has been completely explored experimentally we have critically evaluated and assimilated the information available from each to construct a general and internally consistent PTX description of phase relations and solubility in silicate-H2O systems. It is presented as a series of PTX projections. We identify two possible end members in a continuum of phase relation topologies for silicate-H2O, and show that these result from seemingly minor but important differences in the compositional behavior of silicate melt coexisting with H2O-rich fluid. The analysis bears directly on the nature of high pressure magma genesis and degassing histories. We also consider mineral-buffered supercritical fluid, elucidate the PT region in which this phenomenon occurs for different rock-types, its relation to subsolidus mineral solubility isopleths, aqueous vapor-saturated and the dry liquidus curves and general compositional behavior. We argue that mineral-saturated fluids at pressures above the second critical point are compositionally well defined and that fluid compositions and physical properties are strongly temperature dependent. Fluids occurring in the continental crust should remain in the dilute supercritical aqueous fluid regime and likewise subsolidus peridotite-buffered fluids will also be dilute. In contrast fluids buffered by less mafic rock types (e.g., pelite) at subduction zone and upper mantle conditions are also supercritical but have solute-rich, hydrous melt-like compositions.
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
DE: 3616 Hydrothermal systems (0450, 1034, 3017, 4832, 8135, 8424)
DE: 3619 Magma genesis and partial melting (1037)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005