HR: 0800h
AN: V41C-1411    [Abstracts]
TI: Effect of H2O on Ca-Na Partitioning Between Plagioclase and Melt: Implications for the Origin of An-rich Plagioclase in Low-Alkali Arc Tholeiite
AU: TAKAGI, D
EM: plagio@s.vodafone.ne.jp
AF: Graduate School of Science and Technology, Kobe Univ., Rokko-dai 1-1, Nada-ku, Kobe, 657-8501 Japan
AU: * SATO, H
EM: hsato@kobe-u.ac.jp
AF: Dept. of Earth & Plan. Sci., Kobe Univ., Rokko-dai 1-1, Nada-ku, Kobe, 657-8501 Japan
AU: NAKAGAWA, M
EM: nakagawa@epms.hokudai.ac.jp
AF: Dept. of Earth & Plan. Sci., Hokkaido Univ., Kita 10jyo, Nishi8 chome, Kita-ku, Sapporo, 060-0810 Japan
AB: We conducted high pressure (100-500 MPa) melting experiments on a low-alkali tholeiite (SiO2=53wt% MgO=6.5 wt%, CaO/Na2O=4.4, Al2O3/SiO2=0.33) at both H2O-undersaturated and H2O-saturated conditions to investigate the effect of H2O content on Ca-Na partitioning between plagioclase and melt. The experiments were carried out using IHPV at 100 and 200 MPa, and a solid media 0.5-inch piston cylinder apparatus at 500 MPa in the temperature ranges of 1000-1300­‘, and varying H2O contents of 0-12wt%. Redox condition was about 0-2 log unit above NNO (nickel-nickel oxide) buffer. In this study, we focused on near liquidus plagioclases-melt partitioning, where melt composition remained constant and the effect of water/temperature on the partitioning was elucidated. Our experimental results show that, at each experimental pressure, An content of the near liquidus plagioclase and the Kd(Ca-Na) almost linearly increases as H2O content in melt increases. Each of the An content and the Kd(Ca-Na) variations in a low-alkali tholeiitic system (CaO/Na2O=4.0-4.5, Al2O3/SiO2=0.27-0.33) can be described by one equation using temperature, pressure(kb), and melt H2O content as parameters (lnAn=927.91/T-0.86298"C0.02693*P/T+0.01674*H2O, R=0.97; ln Kd(Ca-Na)=10695/T-6.7781-0.1009*P/T-0.00860*H2O, R=0.97). An content and the Kd(Ca-Na) of liquidus plagioclase increases with increasing melt H2O and decreasing pressure, indicating that the condition of nearly H2O-saturated at 200-300 MPa is preferable for the crystallization of most An-rich plagioclase ($>$An90) at nearly constant melt composition. We suggest this pressure condition of 200-300 MPa, corresponding to a depth of 7-10 km plays an important role for the crysatllization of An-rich plagioclase in H2O-rich arc tholeiite.
DE: 8439 Physics and chemistry of magma bodies
DE: 3600 MINERALOGY AND PETROLOGY (replaces
DE: 3630 Experimental mineralogy and petrology
DE: 3640 Igneous petrology
DE: 1000 GEOCHEMISTRY (New field, replaces Rock Chemistry)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting