14:15 〜 14:30
[MIS35-15] 沈み込みチャネルとマントルウェッジの炭素を含む塩水
キーワード：水流体, 流体包有物, 島弧マグマ
We find C-bearing saline fluids in the subduction channel and mantle wedge. Saline fluids are found with or without methane in jadeitites of serpentinite melanges located in Southwest Japan [Mori, Shigeno, Kawamoto, Nishiyama, in progress]. Carbon dioxide-bearing saline fluid inclusions are also reported from sub-arc mantle peridotite xenoliths: 3.7 wt% NaCl in Ichinomegata lherzolites, Northeast Japan arc [Kumagai et al., 2014] and 5.1 wt% NaCl in Pinatubo harzburgites, Luzon arc [Kawamoto et al., 2013]. These findings indicate that aqueous fluids in the subduction channel and mantle wedge can contain certain amounts of C and Cl.
We suggested that separation of slab-derived supercritical fluids into aqueous fluids and melts plays an important role in elemental transfer from subducting slab to the mantle wedge [Kawamoto et al., 2012]. It is, therefore, important to determine the effect of Cl on the trace element partitioning between aqueous fluids and melts. Synchrotron radiation X-ray fluorescence (XRF) analysis is conducted to know Rb, Sr, and Pb partitioning between aqueous fluids and melts simultaneously at high-temperature and high-pressure conditions. There is a positive correlation between partition coefficients and pressure, as well as salinity [Kawamoto et al., 2014]. Two slab-derived components, melt and fluid components, are suggested to explain trace element characteristics of arc-basalts in the Mariana arc [Pearce et al., 2005]. The fluid component is characterized by enrichment of alkali, alkali earth elements, and Pb. Such features can be explained if the fluid component is a saline fluid, because alkali earth elements and Pb are much less mobile with Cl-free fluids than Cl-rich fluids [Kawamoto et al., 2014].
We suggest that slab-derived components have compositional features consistent with a saline fluid and a melt, which can be formed through a separation of a slab-derived supercritical fluid [Kawamoto et al., 2012, 2014]. Slab-derived supercritical fluids contain Cl, and separated aqueous fluids inherit much of the Cl and some of the large-ion lithophile elements. Dissolution of carbon materials into aqueous fluids is enhanced by the salinity [Newton and Manning 2002] and their species can be controlled by oxygen fugacity.
Reference
Kawamoto T., Kanzaki M., Mibe K., Matsukage K. N., Ono S., 2012, Separation of supercritical slab-fluids to form aqueous fluid and melt components in subduction zone magmatism. Proceedings of the National Academy of Sciences, U. S. A., 109, 18695-18700.
Kawamoto T., Yoshikawa M., Kumagai Y., Mirabueno M. H. T., Okuno M., Kobayashi T., 2013, Mantle wedge infiltrated with saline fluids from dehydration and decarbonation of subducting slab. Proceedings of the National Academy of Sciences, U. S. A., 110, 9663-9668.
Kawamoto T., Mibe K., Bureau H., Reguer S., Mocuta C., Kubsky S., Thiaudiere D., Ono S., Kogiso T., 2014, Large ion lithophile elements delivered by saline fluids to the sub-arc mantle, Earth, Planets and Space, 66, 61.
Kumagai Y., Kawamoto T., Yamamoto J., 2014, Evolution of carbon dioxide bearing saline fluids in the mantle wedge beneath the Northeast Japan arc, Contributions to Mineralogy and Petrology, 168, 1056.
Newton, R.C. and Manning, C.E., 2002, Experimental determination of calcite solubility in H2O-NaCl solutions at deep crust/upper mantle pressures and temperatures: implications for metasomatic processes in shear zones. American Mineralogist, 87, 1401-1409.
Pearce J. A., Stern R. J., Bloomer S. H., Fryer P., 2005, Geochemical mapping of the Mariana arc-basin system: Implications for the nature and distribution of subduction components. Geochemistry, Geophysics, Geosystems, 6, Q07006.
We suggested that separation of slab-derived supercritical fluids into aqueous fluids and melts plays an important role in elemental transfer from subducting slab to the mantle wedge [Kawamoto et al., 2012]. It is, therefore, important to determine the effect of Cl on the trace element partitioning between aqueous fluids and melts. Synchrotron radiation X-ray fluorescence (XRF) analysis is conducted to know Rb, Sr, and Pb partitioning between aqueous fluids and melts simultaneously at high-temperature and high-pressure conditions. There is a positive correlation between partition coefficients and pressure, as well as salinity [Kawamoto et al., 2014]. Two slab-derived components, melt and fluid components, are suggested to explain trace element characteristics of arc-basalts in the Mariana arc [Pearce et al., 2005]. The fluid component is characterized by enrichment of alkali, alkali earth elements, and Pb. Such features can be explained if the fluid component is a saline fluid, because alkali earth elements and Pb are much less mobile with Cl-free fluids than Cl-rich fluids [Kawamoto et al., 2014].
We suggest that slab-derived components have compositional features consistent with a saline fluid and a melt, which can be formed through a separation of a slab-derived supercritical fluid [Kawamoto et al., 2012, 2014]. Slab-derived supercritical fluids contain Cl, and separated aqueous fluids inherit much of the Cl and some of the large-ion lithophile elements. Dissolution of carbon materials into aqueous fluids is enhanced by the salinity [Newton and Manning 2002] and their species can be controlled by oxygen fugacity.
Reference
Kawamoto T., Kanzaki M., Mibe K., Matsukage K. N., Ono S., 2012, Separation of supercritical slab-fluids to form aqueous fluid and melt components in subduction zone magmatism. Proceedings of the National Academy of Sciences, U. S. A., 109, 18695-18700.
Kawamoto T., Yoshikawa M., Kumagai Y., Mirabueno M. H. T., Okuno M., Kobayashi T., 2013, Mantle wedge infiltrated with saline fluids from dehydration and decarbonation of subducting slab. Proceedings of the National Academy of Sciences, U. S. A., 110, 9663-9668.
Kawamoto T., Mibe K., Bureau H., Reguer S., Mocuta C., Kubsky S., Thiaudiere D., Ono S., Kogiso T., 2014, Large ion lithophile elements delivered by saline fluids to the sub-arc mantle, Earth, Planets and Space, 66, 61.
Kumagai Y., Kawamoto T., Yamamoto J., 2014, Evolution of carbon dioxide bearing saline fluids in the mantle wedge beneath the Northeast Japan arc, Contributions to Mineralogy and Petrology, 168, 1056.
Newton, R.C. and Manning, C.E., 2002, Experimental determination of calcite solubility in H2O-NaCl solutions at deep crust/upper mantle pressures and temperatures: implications for metasomatic processes in shear zones. American Mineralogist, 87, 1401-1409.
Pearce J. A., Stern R. J., Bloomer S. H., Fryer P., 2005, Geochemical mapping of the Mariana arc-basin system: Implications for the nature and distribution of subduction components. Geochemistry, Geophysics, Geosystems, 6, Q07006.