ELSEVIER Earth and Planetary Science Letters 159 (1998) 13–23 Decomposition of phase D in the lower mantle and the fate of dense hydrous silicates in subducting slabs Sean R. Shieh a,b,L , Ho-kwang Mao a , Russell J. Hemley a , Li Chung Ming b a Geophysical Laboratory and Center forHigh Pressure Research, Carnegie Institution of Washington, 5251 Broad Branch Rd. N.W., Washington, D.C. 20015, USA b Department of Geology and Geophysics, Hawaii Institute of Geophysics and Planetology, University of Hawaii at Manoa, 2525 Correa Rd., Honolulu, HI 96822, USA Received 14 July 1997; revised version received 27 February 1998; accepted 25 March 1998 Abstract High pressure, high temperature quench-type experiments were carried out on serpentine to pressures of 53 GPa and temperatures between 800–1800ºC. X-ray analyses show that recovered phase assemblages varied considerably for the different high pressure and high temperature studies. The dense hydrous magnesium silicate phases decompose sequentially as the pressure increases and appear to serve as reservoirs for H 2 O in the mantle, eventually releasing it at the highest pressure. Superhydrous phase B is stable up to pressure equivalent to the boundary between the transition zone and lower mantle. Phase D decomposes at pressures of 44 GPa, equivalent to about 1250 km depth. This may define the lower depth limit for the presence of dense hydrous magnesium silicates. Beyond this pressure, only nominally anhydrous phases have been recovered in high-pressure experiments. This may indicate a lack of stoichiometric-hydrogen bearing silicate phases at higher pressure.  1998 Elsevier Science B.V. All rights reserved. Keywords: X-ray diffraction analysis; lower mantle; serpentine; high pressure 1. Introduction Water and hydrogen play an important role in the dynamics and composition of subducting material. The water released by dehydration of a subducting oceanic slab has long been considered the cause of arc magmatism, the trigger of partial melting [1–5], and possibly deep-focus earthquakes [6]. Potentially, it could have an even broader role in global crustal and upper mantle processes, for its presence can change the physical and chemical properties of min- erals and rocks and, for example, enhance the mi- L Corresponding author. E-mail: shieh@gl.ciw.edu gration of magma [1]. H 2 O has also been proposed as a mechanism to broaden seismic discontinuities [7]. Its presence would also increase the electrical conductivity [8] in the mantle. The exact role of H 2 O in the transition zone and lower mantle, how it is stored, and the total mantle H 2 O budget remain unclear. From laboratory experiments, it has been argued that hydrogen in the Earth’s mantle could be present as OH in many nominally anhydrous mantle minerals [9,10] or as dense hydrous magnesium silicates (DHMS-A, B, Superhydrous phase B, D, E) [11–19]. If DHMS phases are, indeed, the mantle reservoir for hydro- gen in cold region, such as subducting slabs, total 0012-821X/98/$19.00  1998 Elsevier Science B.V. All rights reserved. PII S0012-821X(98)00062-4