Japan Geoscience Union Meeting 2014

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International Session (Oral)

Symbol S (Solid Earth Sciences) » S-IT Science of the Earth's Interior & Techtonophysics

[S-IT02_29PM2] Aqueous fluids and melts in subduction zones: Experiment, modeling, and geophysical observations

Tue. Apr 29, 2014 3:45 PM - 5:00 PM 211 (2F)

Convener:*Bjorn Mysen(Geophysical Laboratory, Carnegie Inst. Washington), Junichi Nakajima(Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan), Eiji OHTANI(Graduate School of Science Tohoku University 6-3 Aoba, Aramaki, Aoba-ku Sendai 980-8578), Chair:Bjorn Mysen(Geophysical Laboratory, Carnegie Inst. Washington)

4:45 PM - 5:00 PM

★ [SIT02-P03_PG] Solution mechanism of water in depolymerized silicate melts

3-min talk in an oral session

*Nadezda CHERTKOVA1, Shigeru YAMASHITA1 (1.Okayama University, ISEI)

Keywords:water speciation, hydrothermal diamond anvil cell, near-infrared spectroscopy, Raman spectroscopy

It is known that the effect of dissolved water on the viscosity of silicate melts is larger for polymerized melts than for depolymerized melts [e.g., 1, 2]. Direct spectroscopic measurements of melt structure and water speciation at high temperature provide information about the mechanism of water dissolution and its influence on the physical properties of the melts. While in situ measurements of water speciation were widely conducted for rhyolitic melts and their analogues [e.g., 3, 4, 5], only limited data are available for depolymerized silicate melts.We performed high-temperature near-infrared and Raman spectroscopic measurements of hydrous Na2Si2O5 melts (2.3-8.1wt% H2O) using externally heated diamond anvil cell (HDAC). Na2Si2O5 composition was chosen as a structural analogue of basaltic melt (anhydrous NBO/T = 1). Experimental pressure was monitored with the pressure- and temperature-dependent Raman shift of 13C diamond [6]. Near-infrared spectra of the homogeneous liquid phase, observed above 820 degree C, 1.7GPa in the Na2Si2O5+2.3wt%H2O system and above 700 degree C, 1.6GPa in the Na2Si2O5+8.1wt%H2O system, contain absorption peaks corresponding to molecular H2O (at ~5200 cm-1) and structurally bound OH groups (at ~4500 cm-1). At 900 degree C and 1.6-1.9GPa the ratio of these peaks height remains approximately constant (2.6-2.2), implying a constant (structurally bound OH)/(molecular H2O) ratio for this range of water contents. This observation differs from the regularities reported for more polymerized melts (rapid decrease of OH/H2O with total water content) [e.g., 4, 7]. At the same time no pressure effect on the ratio of 4500 cm-1 peak height to 5200 cm-1 peak was observed below 2.4 GPa. References[1] Whittington A., Richet P., Holtz F. (2000) Geochim Cosmochim Acta 64, 3725-3736.[2] Giordano D., Russell J.K., Dingwell D.B. (2008) Earth. Planet. Sci. Lett. 271, 123-134.[3] Sowerby J.R., Keppler H. (1999) Am. Mineral. 84, 1843-1849.[4] Nowak M., Behrens H. (2001) Earth. Planet. Sci. Lett. 184, 515-522.[5] Shen A.H., Keppler H. (1995) Am. Mineral. 80, 1335-1338.[6] Mysen B.O., Yamashita S. (2010) Geochim. Cosmochim. Acta 74, 4577-4588.[6] Mysen B.O. (2010) Geochim. Cosmochim. Acta 74, 4123-4139.[7] Botcharnikov R.E., Behrens H., Holtz F. (2006) Chem. Geol. 229, 125-143.