712 ISSN 1075-7015, Geology of Ore Deposits, 2009, Vol. 51, No. 8, pp. 712–722. © Pleiades Publishing, Ltd., 2009. Original Russian Text © M.S. Babushkina, L.P. Nikitina, A.G. Goncharov, N. I. Ponomareva, 2009, published in Zapiski RMO (Proceedings of the Russian Mineralogical Society), 2009, Pt. CXXXVII, No. 1, pp. 3–19. INTRODUCTION Fluids and dissolved water affect many physical and chemical properties of the upper mantle. A minor amount of water in minerals of peridotites decreases their mechanical strength (Makswell et al., 1985; Chen et al., 1998), facilitates ion diffusion in the structure of a mineral (Goldsmith, 1987), changes the electric con- ductivity of the mantle (Karato, 1990), increases seis- mic wave velocity (Karato and Jung, 1998), and changes phase relations and melt composition (Kushiro, 1990; Gaetani et al., 1993; Inoue, 1994). The first information on occurrence of water in structures of rock-forming minerals from mantle rocks appeared in the 1980s, and it was suggested that precisely nomi- nally anhydrous minerals (NAM) rather than hydroxyl- bearing minerals with stoichiometric OH are the main water reservoir in the mantle (Bell and Rossman, 1992a). The study of structural water in NAM has and continues to develop in three directions. (1) Elaboration of the techniques for determining hydrous species and their structural position in min- erals (Fourier spectroscopy, nuclear methods of hydro- Corresponding author: L.P. Nikitina. E-mail: LPN@LN10839.spb.edu gen determination, etc.) and techniques for quantita- tively assessing water dissolved in minerals (Patterson, 1982; Yesinovski et al., 1988; Cho and Rossman, 1993; Bell et al., 1995, 2004; Kohn, 1996). Fourier spectroscopy is the most sensitive method to quantitatively determine water in NAM, which does not damage minerals and has a high degree of analytical accuracy. The instrumental combination of a Fourier spectrometer and infrared microscope makes it possible to distinguish structural water from the water in fluid inclusions, fractures, and products of replacement. The spectra obtained in polarized radiation allow determi- nation of the type and orientation of the bond in the mineral structure. This method is, however, relative and requires calibration data obtained by independent methods. Calibration for many minerals had until recently been lacking. The amount of H in structures of minerals is determined also by secondary-ion mass spectroscopy (SIMS) (Kurosawa et al., 1997; Koga et al., 2003); elastic recoil detection analysis (ERDA) (Sweeney et al., 1997); nuclear magnetic resonance ( 1 H magic-angle spinning nuclear magnetic resonance (MAS NMR)), (Yesinovski et al., 1988; Cho and Ross- man, 1993, Kohn, 1996)], where the area of spectrum is Water in the Structure of Minerals from Mantle Peridotites As Controlled byThermal and Redox Conditions in the Upper Mantle M. S. Babushkina a , L. P. Nikitina a , A. G. Goncharov a , and N. I. Ponomareva b a Institute of Precambrian Geology and Geochronology, Russian Academy of Sciences, nab. Makarova 2, St. Petersburg, 199034 Russia b Department of Mineralogy, Faculty of Geology, St. Petersburg State University, Universitetskaya nab. 7/9, St. Petersburg, 199034 Russia Received May 4, 2008 Abstract—Hydrous species and the amount of water (OH – ions and crystal hydrate H 2 O) in structures of nom- inally anhydrous rock-forming minerals (olivine, ortho- and clinopyroxenes) were studied with Fourier spec- troscopy in peridotite nodules (19 samples) from Cenozoic alkali basalts of the Baikal–Mongolia region (Dari- ganga Plateau, Taryat Depression, and Vitim Plateau). Single-crystal samples oriented relative to the crystallo- graphic axes of minerals were examined with an FTIR spectrometer equipped with an IR microscope at the points of platelets free from fluid inclusions. FTIR spectra were measured in regions of stretching vibrations of OH – and H 2 O (3800–3000 cm –1 ) and deformation vibrations of H 2 O (1850–1450 cm –1 ). The water content in mineral structures was determined from integral intensities. To estimate the conditions of entrapment and loss of structural water in minerals, their chemical composition, including Fe 2+ and Fe 3+ contents, was determined with an electron microprobe analysis and Mössbauer spectroscopy. The bulk chemical composition of some nodules was determined with XRF and ICP MS. The total water content (OH – + H 2 O) varies from 150 to 1140 ppm in olivines, from 45 to 870 ppm in clinopyroxenes, and from 40 to 1100 ppm in orthopyroxenes. Both water species in the mineral structures are retained down to a depth of 150–160 km in wide temperature and pressure ranges (1100–1500°C, 32–47 kbar) at the oxygen fugacity of –1.4 to –0.1 log units relative to that of the quartz–fayalite–magnetite buffer. DOI: 10.1134/S1075701509080042