Fluids in the Earth's Lower Mantle - Phase Relations in the System MgO-SiO2-H2O

Water plays an important role in the mantle affecting both mineral properties and mantle dynamics. Water is held in the mantle in hydrous minerals, nominally anhydrous minerals (NAMs) and fluids or melts.1 The effect of water on phase relations and the stability of hydrous fluids/melts have been investigated at upper mantle conditions within the MgO-SiO2-H2O (MSH) system.2,3,4 However this system has not been investigated systematically at lower mantle conditions. New Laser Heated Diamond Anvil Cell (LH-DAC) experiments were conducted at lower mantle temperature and pressure conditions to understand the phase relations within the MSH system. We investigated a wide range of compositions produced by mixing various proportions of enstatite glass, silica glass, and brucite powder. Samples were synthesised at 25-60 GPa and 1600-2400 K in a DAC using a double-sided laser-heating geometry. Phases identified by synchrotron micro-X-ray diffraction include perovskite, periclase, stishovite, and phase D, with subordinate brucite and/or magnesite. Some of these phases are thought to be precipitated from a fluid during cooling, and the presence of magnesite indicates some reaction with the diamond. However, a combination of reversal and in-situ experiments and variation in modal abundances as a function of bulk composition allow the primary phases to be identified. Phase relations change systematically as a function of water content and pressure. For a model mantle Si/Mg ratio our results indicate that Phase D can be stable along a whole-mantle convective geotherm up to about 45 GPa for a range of H2O contents. Further, chemographic relations suggest that in hydrous model mantle solid phases will coexist with fluids having very high MgO-contents. Two implications of these results which we explore are (1) fluids originating in the lower mantle, either primordially or expelled from subducted slabs, may transport MgO into the upper mantle, and (2) a reservoir for water residing in Phase D may exist in the upper part of the lower mantle to a depth of ~ 1300 km. References (1) Hirschmann, M. M. 2006 Ann. Rev. Earth Planet. Sci. 34, 629-653 (2) Stalder, R. et al. 2001 Contrib. Mineral. Petrol. 140, 607-618 (3) Mibe, K. et al. 2002 Geochim. Cosmochim. Acta 66, 2273-2285 (4) Melekhova, E. et al. 2007 Geochim. Cosmochim. Acta 71, 3348-3360