A Subsolidus Olivine Water Solubility Equation for the Earth's Upper Mantle. Issue 12 (26th December 2017)

• Record Type: Journal Article
• Authors: Padrón‐Navarta, J. A.; Hermann, J.
• Is Part Of: Journal of geophysical research. Volume 122:Issue 12(2017)
• Page Start: 9862
• Page End: 9880
• Publication Date: 2017-12-26
• Subjects: subduction zones -- hydrous defects -- olivine -- deepwater cycle -- rheology -- titanium; Geomagnetism -- Periodicals; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 551.1
• DOI: 10.1002/2017JB014510
• Languages: English
• ISSNs: 2169-9313

Abstract

Abstract: The pressure and temperature sensitivity of the two most important point hydrous defects in mantle olivine involving Si vacancies (associated to trace amounts of titanium [TiChu‐PD] or exclusively to Si vacancies [Si]) was investigated at subsolidus conditions in a fluid‐saturated natural peridotite from 0.5 to 6 GPa (approximately 20–200 km depth) at 750 to 1050°C. Water contents in olivine were monitored in sandwich experiments with a fertile serpentine layer in the middle and olivine and pyroxene sensor layers at the border. Textures and mineral compositions provide evidence that olivine completely recrystallized during the weeklong experiments, whereas pyroxenes displayed only partial equilibration. A site‐specific water solubility law for olivine has been formulated based on the experiments reconciling previous contradictory results from low‐ and high‐pressure experiments. The site‐specific solubility laws permit to constrain water incorporation into olivine in the subducting slab and the mantle wedge, as these are rare locations on Earth where fluid‐present conditions exist. Chlorite dehydration in the hydrated slab is roughly parallel to the isopleth of 50 ± 20 ppm wt H2 O in olivine, a value which is independent of the pressure and temperature trajectory followed by the slab. Hydrous defects are dominated by [Si] under the relevant conditions for the mantle wedge affected by fluids coming from the slab dehydration (slab‐adjacent low viscosity/seismic low‐velocity channel, P > 3 GPa). In cold subduction zones at 5.5 km from the slab surface the storage capacity of the mantle wedge at depths of 100–250 km ranges from 400 to 2, 000 ppm wt H2 O. Key Points: Site‐specific equation for the two most important hydrous defects in olivine Hydrous Ti is the most relevant defect in shallow upper mantle, whereas Si defects dominate at higher pressure The new equation constrains water incorporation in the subducting slab and the mantle wedge