Melting phase relations in the systems Mg2SiO4–H2O and MgSiO3–H2O and the formation of hydrous melts in the upper mantle. (1st May 2017)

• Record Type: Journal Article
• Authors: Novella, Davide; Dolejš, David; Myhill, Robert; Pamato, Martha G.; Manthilake, Geeth; Frost, Daniel J.
• Is Part Of: Geochimica et cosmochimica acta. Volume 204(2017)
• Page Start: 68
• Page End: 82
• Publication Date: 2017-05-01
• Subjects: Olivine -- Enstatite -- Phase equilibria -- Hydrous melting -- Cryoscopic equation -- Water speciation; Geochemistry -- Periodicals; Meteorites -- Periodicals; Géochimie -- Périodiques; Météorites -- Périodiques; Geochemie; Astrochemie; Electronic journals; 551.905
• DOI: 10.1016/j.gca.2016.12.042
• Languages: English
• ISSNs: 0016-7037

Abstract

Abstract: High-pressure and high-temperature melting experiments were conducted in the systems Mg2 SiO4 –H2 O and MgSiO3 –H2 O at 6 and 13 GPa and between 1150 and 1900 °C in order to investigate the effect of H2 O on melting relations of forsterite and enstatite. The liquidus curves in both binary systems were constrained and the experimental results were interpreted using a thermodynamic model based on the homogeneous melt speciation equilibrium, H2 O + O 2− = 2OH −, where water in the melt is present as both molecular H2 O and OH − groups bonded to silicate polyhedra. The liquidus depression as a function of melt H2 O concentration is predicted using a cryoscopic equation with the experimental data being reproduced by adjusting the water speciation equilibrium constant. Application of this model reveals that in hydrous MgSiO3 melts at 6 and 13 GPa and in hydrous Mg2 SiO4 melts at 6 GPa, water mainly dissociates into OH − groups in the melt structure. A temperature dependent equilibrium constant is necessary to reproduce the data, however, implying that molecular H2 O becomes more important in the melt with decreasing temperature. The data for hydrous forsterite melting at 13 GPa are inconclusive due to uncertainties in the anhydrous melting temperature at these conditions. When applied to results on natural peridotite melt systems at similar conditions, the same model infers the presence mainly of molecular H2 O, implying a significant difference in physicochemical behaviour between simple and complex hydrous melt systems. As pressures increase along a typical adiabat towards the base of the upper mantle, both simple and complex melting results imply that a hydrous melt fraction would decrease, given a fixed mantle H2 O content. Consequently, the effect of pressure on the depression of melting due to H2 O could not cause an increase in the proportion, and hence seismic visibility, of melts towards the base of the upper mantle.