HR: 0800h
AN: V41E-1517    [Abstracts]
TI: Influence of H2O on Liquidus Temperatures of Primitive Basalts and Olivine-Liquid Thermometry.
AU: * Medard, E
EM: emedard@mit.edu
AF: Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, 77 Mass. Ave, Cambridge, MA 02139 United States
AU: Grove, T L
EM: tlgrove@mit.edu
AF: Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, 77 Mass. Ave, Cambridge, MA 02139 United States
AB: We have undertaken a systematic experimental study aimed at understanding the effect of water on olivine liquidus temperatures of primitive basalts. Experiments have been performed on a primitive tholeiitic basalt from Medicine Lake Volcano, California (sample 82-72f, Bartels et al. 1991). The dry liquidus has been characterized from 0.1 MPa to 1.2 GPa. The wet liquidus has been constrained to within 15 °C using water-saturated experiments performed in a MHC externally heated pressure vessel. Preliminary results show that the olivine-liquidus depression (i.e., the difference between dry and water-bearing liquidus) is essentially a linear function of the water content of the melt: ΔT = 560  X(HO0.5), where X(HO0.5) is the mole fraction of water, calculated on a single-cation oxide basis. For 82-72f, this roughly translates into a 30 °C / wt% H2O depression. Simple systems (e.g., diopside/H2O, albite/H2O) suggest that melt structure / composition may have an influence on H2O liquidus depression, and this potential influence is currently under investigation. Experimental phase equilibria and thermobarometry of primitive basalts provide the primary evidence for estimating melting conditions and thermal structures in the Earth's mantle. Assessing the influence of H2O is critical, because it is the dominant volatile component involved in igneous processes, and it has been shown to cause a significant reduction in liquidus temperatures. However, recent model parameterizations vary from very large effects at low H2O contents (about 75 °C at 1 wt% H2O, Falloon and Danyushevsky 2000) to linear effect of H2O vs liquidus temperature (about 25 °C at 1 wt% H2O, Sugawara 2000). Our experimental determination more closely approximates the latter model. A key consequence is that the presence of small amounts of water in MORB magmas (< 1wt%) will only have a very small effect (< 30 °C) on liquidus temperature determination for mid-ocean-ridges. For magmas that are more water-rich, as observed in subduction zones, ocean island and continental magmatism, this parameterization of H2O liquidus depression can be included in existing thermodynamic models to retrieve magmatic temperatures from petrology of primitive basalts.
DE: 3611 Thermodynamics (0766, 1011, 8411)
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
DE: 3630 Experimental mineralogy and petrology
DE: 3640 Igneous petrology
DE: 3651 Thermobarometry
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005