High hydrogen solubility in Al-rich stishovite and water transport in the lower mantle

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Abstract

Stishovite is an important phase in subducting oceanic crust. The post-garnet assemblage from a precursor eclogite lithology contains up to 25% stishovite at pressures above 25 GPa. This stishovite may contain up to 5 wt.% Al2O3. We measured the hydrogen contents of stishovite samples synthesized at 20–25 GPa and 1200–1800 °C from several starting materials containing up to 10 wt.% Al2O3. FTIR spectra of stishovite show major bands at 3111–3134 cm 1, with the frequencies increasing as H2O and Al2O3 contents increase, and several minor bands at 2659–2667, 3240, 3261, 3312–3334, and 3351 cm 1. The H2O content of Al-free stishovite is in the range of 16–30 wt. ppm. The maximum H2O content of Al-bearing stishovite (4.4 wt.% Al2O3) synthesized at 20 GPa and 1400 °C is 3010 ± 300 wt. ppm. Most hydrogen in stishovite is associated with Al3+ substitutional defects on the octahedral (Si4+) site. The hydrogen can occupy 40% of vacancies created by incorporation of Al3+ at 20 GPa. This observation along with some anomalies in the FTIR spectra may indicate an alternative mechanism of Al3+ incorporation into stishovite via the formation of oxygen vacancies or interstitial Aliradical dotradical dotradical dot defects. We report the highest H2O concentrations in Al-stishovite to date, and argue that it is the most important carrier of water into the lower mantle post-garnet eclogitic assemblages.

Introduction

Recent experimental studies show that water can be transported to the deep lower mantle by hydrous phases (Frost, 1999, Ohtani et al., 2001, Litasov and Ohtani, 2003, Ohtani et al., 2004, Frost, 2006), nominally anhydrous minerals (Bolfan-Casanova et al., 2000, Skogby, 2006, Beran and Libowitzky, 2006, Litasov and Ohtani, in press), and as a fluid captured in disconnected interstitial patches in mantle rocks (Mibe et al., 2003). The role of nominally anhydrous minerals might be most important among these, due to their ability to retain water even at elevated temperatures. Many studies have shown that nominally anhydrous olivine (and its high-pressure modifications wadsleyite and ringwoodite), garnet, and pyroxenes may contain significant amount of structurally bound water, typically as hydroxyl ions (Skogby, 2006, Beran and Libowitzky, 2006, Bell and Rossman, 1992, Kohlstedt et al., 1996, Smyth, 2006).

Most hydrous phases, of which are serpentine, chlorite, clinohumite, chondrodite, and class of so-called dense hydrous magnesium silicates (phases A, E, D, and superhydrous phase B), are stable at low temperatures, corresponding to the cold subducting slabs, and are not stable at normal or ambient mantle temperatures (Litasov and Ohtani, 2003, Litasov and Ohtani, in press, Kawamoto, 2004). Moreover, these phases are stable in a peridotite composition and are not stable in other mantle constituents, such as the eclogite that forms from former oceanic crust. While peridotite comprises the major part of subduction slabs descending into the deep mantle, its degree of hydration is equivocal (e.g. Kerrick, 2002, Rüpke et al., 2004). The part of a subducted slab which is substantially hydrated, at least in ‘cold subduction’ environments, is oceanic basaltic crust. No hydrous minerals are stable in a typical basaltic composition (eclogite) above 10 GPa (the stability limit of phengite and lawsonite) (e.g. Poli and Schmidt, 1998, Okamoto and Maruyama, 2004, Litasov and Ohtani, 2005). Minor K-amphibole can be stable to 15 GPa (Inoue et al., 1998). Therefore, water can be transported only by the nominally anhydrous minerals of eclogite or by trapped fluids (e.g. Mibe et al., 2003, Ono et al., 2002a, Ono et al., 2002b, Ono et al., 2002c). The major minerals in eclogite are garnet, clinopyroxene, and the SiO2-polymorphs coesite and stishovite. At pressures of 23–28 GPa the garnet-bearing assemblage transforms to a post-garnet assemblage consisting of Al- and Fe-rich Mg-perovskite, Al-rich NAL or CF phases, Ca-perovskite, and stishovite (e.g. Litasov and Ohtani, 2005, Hirose and Fei, 2002, Litasov et al., 2004). Stishovite is one of the most important phases in this lower-mantle post-eclogite assemblage (20–25 modal%).

Pawley et al. (1993) first showed that stishovite, in particular Al-bearing stishovite, can contain significant amounts of water at levels up to 82 wt. ppm H2O. Thereafter, Chung and Kagi (2002) found up to 844 wt. ppm H2O in Al-bearing stishovite from eclogite assemblages at 10–15 GPa. The mechanisms of hydrogen incorporation into Al-bearing stishovite were investigated experimentally by Bromiley et al. (2006) and theoretically by Gibbs et al. (2004), leading to the conclusion that H+ is coupled with Al3+ substitutional defects on adjacent octahedral (Si4+) sites. Recently, Lakshtanov et al. (2007b) reported a drastic shift of the post-stishovite transition to CaCl2-structured SiO2 to lower pressures in Al- and H-bearing system, and argued that Al- and H-bearing stishovite may be responsible for intermittent seismic reflectors observed in the mid-lower mantle (e.g. Le Stunff et al., 1995, Kaneshima and Helffrich, 1999).

In present paper we have determined high water concentrations in stishovite with various Al2O3 contents, and argue that the role of stishovite as possible water carrier to the deep mantle might have previously been underestimated.

Section snippets

Experimental procedures

We used several starting compositions to synthesize Al-rich stishovites. The starting materials for the synthesis experiments were pure SiO2 and Al2O3 mixtures with different proportions of the oxides. H2O was added as Al(OH)3 and as pure distilled H2O with a total content appropriate for the desired stoichiometry (Table 1). In addition, we used Al-free stishovite crystals from experiments on Mg-perovskite syntheses in the (Mg,Fe)SiO3–KHCO3–Mg(OH)2 systems (Shatskiy et al., in press). Some data

Results

It should be noted that hydrogen is incorporated into stishovite as a hydroxyl ion (OH), which is observed in FTIR spectra. However, for simplicity and consistency with previous studies we express hydrogen/hydroxyl contents of stishovite in ppm H2O by weight, which can be simply recalculated to wt. ppm hydroxyl or H/106Si contents.

The experimental PT conditions, compositions, and H2O contents of the stishovite samples are listed in Table 2. The sizes of the stishovite crystals were typically

Solubility of hydrogen and aluminium in stishovite

H2O contents of pure stishovite measured at 20 GPa and 1600 °C and 24 GPa and 1750 °C are 16 and 31 wt. ppm, respectively. This is slightly higher relative to values obtained previously (7 wt. ppm at 10 GPa and 1200 °C (Pawley et al., 1993); 3 wt. ppm at 15 GPa and 1500 °C (Bromiley et al., 2006), and is similar to those reported by Bolfan-Casanova et al. (2000) (15–72 wt. ppm in stishovite synthesized at 15–21 GPa and 1200–1500 °C). High H2O concentrations (72 wt. ppm) in pure stishovite

Conclusions

  • 1.

    We measured hydrogen contents in stishovite synthesized at 20–25 GPa and 1200–1800 °C from several starting materials containing up to 10 wt.% Al2O3. FTIR spectra of stishovite show major bands at 3111–3134 cm 1, with the frequencies increasing as H2O and Al2O3 content increases (to a maximum observed Al2O3 content of 7.62 wt.%), and several minor bands at 2659–2667, 3240, 3261, 3312–3334, and 3351 cm 1.

  • 2.

    The H2O contents of Al-free stishovite were 16–30 wt. ppm. The maximum H2O content of

Acknowledgements

We thank two anonymous reviewers for helpful comments. This work was conducted as a part of the 21st Century Center-of-Excellence program at Tohoku and Okayama Universities and supported by the grants in Aid for Scientific Researches from the Ministry of Education, Culture, Sports, Science and Technology, Japan (Nos 14102009 and 16075202) to EO and a grant in Aid for young scientists from Japanese Society for Promotion of Science (No 17740344) to KDL. Support was also provided by the US

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