HR: 0800h
AN: U41A-0725    [Abstracts]
TI: Solubility of Helium in Olivine at 1 Atmosphere
AU: * Parman, S W
EM: parman@mit.edu
AF: Mass. Inst. of Technology, MIT 54-1212, Cambridge, MA 02139 United States
AU: Kurz, M D
EM: mkurz@whoi.edu
AF: Woods Hole Oceanographic Inst., WHOI, Woods Hole, MA 02543 United States
AU: Hart, S R
EM: shart@whoi.edu
AF: Woods Hole Oceanographic Inst., WHOI, Woods Hole, MA 02543 United States
AU: Grove, T L
EM: tlgrove@mit.edu
AF: Mass. Inst. of Technology, MIT 54-1212, Cambridge, MA 02139 United States
AB: We have measured the solubility of He in olivine at 1 atm. Previous measurements of noble gas solubility in mantle minerals have found mineral/melt partition coefficients (D) that are higher [1;2] or close to [3] the D values of U and Th in the mantle. In contrast, geochemical systematics suggest that D$^{He}$ is lower than D$^{U}$ and D$^{Th}$. Our experiments were specifically designed to avoid gas trapped in melt pockets or between sintered grains, which may have affected previous studies [1;2]. The starting materials are gem quality San Carlos olivine and synthetic pure forsterite. These materials were examined before and after the experiments for inclusions and bubbles using optical, scanning electron and scanning transmission electron microscopic techniques. No bubbles were found. The primary experiments were performed on cm size grains to avoid trapping of gas in sintered powders. The experiments were run for 17-21 days at 1350$\deg$C, in crucibles made from large San Carlos olivine crystals, in an atmosphere of 50% He and 50% a mix of CO$_{2}$ and H$_{2}$ (to control fO$_{2}$ at NNO and QFM). At no time was the olivine in contact with a melt phase. To examine the effects of powder sintering, experiments that span a range of smaller grain sizes (100-1000 microns) were also performed. He concentrations in the olivines were measured by noble gas mass spectrometry using sequential in vacuo crushing followed by melting of the powders. The experimental results are consistent: 8.3(+/-2.6) e$^{-7}$ cc STP He/g are released by crushing and 6.2(+/-1.3) e$^{-7}$ cc He STP/g are released by melting. Over 50% of the total gas is released by crushing. Powdered samples release unusually high amounts of gas on the first crush step (interpreted to be trapped between grains), but subsequently follow the same release pattern as the unpowdered samples and yield the same solubility values (excluding the first step). The consistency of the results over a range of compositions, grain sizes and fO$_{2}$ conditions, and our careful examination of the experimental materials indicate that the gas released during crushing is not trapped in bubbles or inclusions, but is loosely bound within the crystal lattice. Where it resides in the olivine crystal structure is not clear. Using only the gas released by melting, along with published solubilities of He in basaltic melts [4], the ol/melt D$^{He}$ is 0.003(+/-0.001) at 1 atm. This is most likely an upper limit because the crushing experiments may not have completely eliminated the loosely bound helium. If the gas from both the crushing and melting steps are combined, the apparent D$^{He}$ is 0.006(+/-0.001). We caution against extrapolating these preliminary values to mantle melting conditions until the location of He in olivine is identified and the effects of pressure and temperature are quantified. [1] Broadhurst et al. (1990) GCA 54: 299-309 [2] Hiyagon and Ozima (1986) GCA 50: 2045-2057 [3] Brooker et al. (2003) Nature 423: 738-741 [4] Jambon et al. (1986) GCA 50: 401-408
DE: 3630 Experimental mineralogy and petrology
DE: 3670 Minor and trace element composition
DE: 1040 Isotopic composition/chemistry
DE: 1065 Trace elements (3670)
DE: 1000 GEOCHEMISTRY (New field, replaces Rock Chemistry)
SC: Union [U]
MN: 2004 AGU Fall Meeting