A Subsolidus Olivine Water Solubility Equation for the Earth's Upper Mantle

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 H2O 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 H2O.

Pressure Dependence of Proton Incorporation and Water Solubility in Olivine

“…A comparison of FTIR spectra at various pressures ( Figure 6, as well as Bali et al, 2008;Withers et al, 2011) shows that the contribution of Mgsite protons (absorption at 3,150-3,250 cm −1 ) is significant under only relatively low pressure ≤2.5 GPa) and high silica activity conditions (buffered with pyroxene). With increasing pressure, it becomes much less significant and finally negligible at 12 GPa (e.g., Bali et al, 2008;Berry et al, 2005;Lemaire et al, 2004;Padron-Navarta & Hermann, 2017;Smyth et al, 2006). In contrast, with excess MgO, the incorporation of protons in the Mg sites is always negligible.…”

“…To examine this hypothesis, we plot previously reported experimental data of water solubility in forsterite/olivine as a function of pressure in Figure 7 (This study; Bai & Kohlstedt, 1992;Bali et al, 2008;Ferot & Bolfan-Casanova, 2012;Lemaire et al, 2004;Litasov et al, 2007;Matveev et al, 2001;Mosenfelder et al, 2006;Smyth et al, 2006;Withers and Hirschmann, 2008;Withers et al, 2011;Zhao et al, 2004). Because temperature has a significant effect on water solubility (Bali et al, 2008;Ferot & Bolfan-Casanova, 2012), only the literature data at temperatures of 1,200 to 1,300°C are plotted alongside our data in Figure 7 by considering a ± 50°C temperature uncertainty in large-volume high-pressure experiments even though data exit over a wider temperature range (e.g., Kohlstedt et al, 1996;Padron-Navarta & Hermann, 2017;Withers & Hirschmann, 2008). Additionally, because different infrared calibrations (Bell et al, 2003;Paterson, 1982;Withers et al, 2012) were adopted in these studies, all data are adjusted to the Withers et al (2012) calibration by simply using…”

Halogens and noble gases in serpentinites and secondary peridotites: Implications for seawater subduction and the origin of mantle neon

“…The long-term retention of noble gases and halogens in subducting slabs is also made more likely by the increasing solubility of H 2 O in nominally anhydrous minerals, including olivine polymorphs, with depth (Kohlstedt et al, 1996). A recent experimental study has shown that in addition to fluid inclusions (Fig 2f; Scambelluri et al, 2015), about 50 ppm H 2 O is incorporated into the lattice of olivine after the breakdown of chlorite (Padrón-Navarta and Hermann, 2017). An even greater amount of H 2 O would be expected in the lattice of olivine formed at greater depths by the eventual breakdown of Phase A on cold slab geotherms (cf.…”

In Situ Infrared Spectra for Hydrous Forsterite up to 1243 K: Hydration Effect on Thermodynamic Properties

“…Many experimental and theoretical studies have been reported exploring the hydrogen substitution mechanisms: (1) Mg 2+ /Fe 2+ vacancies [1,2,6,[13][14][15][16][17][18][19][20][21][22][23][24][25][26], which is the most common substitution in many hydrous silicate minerals; (2) Si 4+ vacancies [27][28][29][30][31][32][33][34][35]; (3) combinations of Mg 2+ , Si 4+ , Ti 4+ , with M 3+ vacancies [28,29,31,[36][37][38][39][40][41][42][43]. In addition, another hydration mechanism with F-substitution is also proposed in both Fe-free and Fe-bearing olivine [44].…”