HR: 09:45h
AN: V11B-07    [Abstracts]
TI: Controls on helium solubility in olivine at low pressures
AU: * Parman, S W
EM: stephen.parman@durham.ac.uk
AF: University of Durham, Science Labs, Durham, DH1 3LE United Kingdom
AU: Kurz, M D
EM: mkurz@whoi.edu
AF: Woods Hole Oceanographic Inst., Clark Building, Woods Hole, MA 02543 United States
AU: Hart, S R
EM: shart@whoi.edu
AF: Woods Hole Oceanographic Inst., Clark Building, Woods Hole, MA 02543 United States
AU: Grove, T L
EM: tlgrove@mit.edu
AF: Massachusetts Inst. of Technology, 54-1228, Cambridge, MA 02139 United States
AB: Experiments on the solubility of helium in olivine have been conducted over a range of conditions to evaluate the thermodynamic controls on helium partitioning during melting. The experiments were performed at 1 to 150 atm. total pressure at temperatures between 1350 and 1550° C. Helium partial pressures ranged from 0.5 to 150 atm. The effect of oxygen fugacity was examined by running experiments at both the QFM and Ni-NiO buffer, and by using both Fe-bearing and Fe-free olivines. Temperature: Experiments at 1350, 1450 and 1550° C (Ni-NiO buffer) show a clear decreasing solubility of helium with temperature, as expected for a gas dissolving in a solid. In contrast, previous experiments on basaltic melts show little change of helium solubility with temperature. Therefore the olivine-melt D value decreases steadily with temperature. Linear regression yields the equation D^He=-8.8e-6T+ 0.014 (with R² > 0.999), where T is in degrees C. Using the van't Hoff equation, this implies an enthalpy of solution of -2.0 kJ/mole and an entropy of solution of -1.97 J/mole K. Pressure: Experiments on powdered samples at helium partial pressures of 0.5 and 150 atmospheres suggest a Henry's law constant between 1e-6 and 3e-6 STP cc/g/atm. This value is around 2 orders of magnitude lower than the Henry's law constant for basaltic melts, consistent with the higher density of olivine. Note that these values should be taken as preliminary as only two helium partial pressures have been studied so far and there are large uncertainties in analyzing powdered samples. Oxygen Fugacity: Experiments on Fe-bearing and Fe-free olivine as well as at varying oxygen fugacity indicate that at least some He is located in vacancies associated with Fe³⁺ in the olivine crystal lattice. A single experiment at the QFM buffer suggests a D^He at 1 atm. of ~0.001, a factor of 2-3 lower than at Ni-NiO. Assuming that the measured Henry's law constant is valid at high pressure, that mantle melting takes place between temperatures of 1200 and 1450° C and that fO₂ conditions are near the QFM buffer, our experiments suggest D values during mantle melting of 0.001 to 0.002.
DE: 1037 Magma genesis and partial melting (3619)
DE: 1038 Mantle processes (3621)
DE: 1042 Mineral and crystal chemistry (3620)
DE: 1065 Major and trace element geochemistry
DE: 3630 Experimental mineralogy and petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005