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Structure and equilibria among silicate species in aqueous fluids in the upper mantle: Experimental SiO 2 –H 2 O and MgO–SiO 2 –H 2 O data recorded in situ to 900°C and 5.4 GPa
Author(s) -
Mysen B. O.,
Mibe K.,
Chou I.M.,
Bassett W. A.
Publication year - 2013
Publication title -
journal of geophysical research: solid earth
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.983
H-Index - 232
eISSN - 2169-9356
pISSN - 2169-9313
DOI - 10.1002/2013jb010537
Subject(s) - enstatite , forsterite , coesite , silicate , stishovite , analytical chemistry (journal) , isothermal process , isochoric process , equilibrium constant , quartz , aqueous solution , mineralogy , chemistry , materials science , thermodynamics , geology , inorganic chemistry , chondrite , physics , meteorite , composite material , tectonics , paleontology , chromatography , organic chemistry , astronomy , eclogite , subduction
Abstract Experiments to determine silicate structural species in silicate‐saturated aqueous fluids in equilibrium with silica polymorphs (quartz and coesite), enstatite, and enstatite + forsterite in the SiO 2 –H 2 O and MgO–SiO 2 –H 2 O systems have been carried out in situ in the 0.4–5.4 GPa and 700–900°C pressure and temperature ranges, respectively. MicroRaman spectroscopy was the structural probe. In the SiO 2 –H 2 O system (1.6–5.4 GPa/700–900°C), the detected silicate species are Q 0 (SiO 4 4− ), Q 1 (0.5 Si 2 O 7 6− ), and Q 2 (SiO 3 2− ). The expression 2 Q 1 ⇌ Q 0 + Q 2 describes the equilibrium among these species with ∆ H and ∆ V values from the isochoric temperature and isothermal pressure dependence of its equilibrium constant, K = X Q 0 · X Q 2 /( X Q 1 ) 2 , range from −23 to −69 kJ/mol and −1 to −2 cm 3 /mol, respectively. In the system MgO–SiO 2 –H 2 O the calculated silica solubility, using literature algorithms, is approximately 50% of that in the SiO 2 –H 2 O system at similar temperature and pressure. Only Q 1 and Q 0 species were detected in the MgO–SiO 2 –H 2 O fluids, whether in equilibrium with enstatite + forsterite ( P < 3 GPa) or enstatite only ( P > 3 GPa). The temperature and pressure dependence of the equilibrium constant, K = X Q 1 / X Q 0 , for this system yields average values of ∆ H = 40 ± 5 kJ/mol and ∆ V = −2.3 ± 0.4 cm 3 /mol. The speciation of silicate in aqueous fluids resembles that in hydrous melts as a function of temperature and pressure at deep crustal and upper mantle temperature and pressure conditions, and they become increasingly similar with depth. As the silicate speciation and solubility in the aqueous fluid depend on silicate composition, the pressure and temperature at which complete miscibility occurs will also vary with silicate composition. The structural similarity between fluids and melts will also lead to fluid/melt element partition coefficients trending toward 1 and mineral/fluid partition coefficients trending toward mineral/melt values in the upper mantle as the silicate‐H 2 O systems approach complete miscibility with increasing temperature and pressure.