Quantification of water in hydrous ringwoodite

Sylvia-Monique Thomas,Steven D Jacobsen,Craig R Bina,Monika Koch-Mã¼Ller,Erik H Hauri,Patrick Reichart,Marcus Moser,Gã¼Nther Dollinger,Joseph R Smyth

doi:10.3389/feart.2014.00038

Abstract

Ringwoodite, γ-(Mg,Fe)2SiO4, in the lower 150 km of Earth’s mantle transition zone (410-660 km depth) can incorporate up to 1.5-2 wt% H2O as hydroxyl defects. We present a mineral-specific IR calibration for the absolute water content in hydrous ringwoodite by combining results from Raman spectroscopy, secondary ion mass spectrometery (SIMS) and proton-proton (pp)-scattering on a suite of synthetic Mg- and Fe-bearing hydrous ringwoodites. H2O concentrations in the crystals studied here range from 0.46 to 1.7 wt% H2O (absolute methods), with the maximum H2O in the same sample giving 2.5 wt% by SIMS calibration. Anchoring our spectroscopic results to absolute H-atom concentrations from pp-scattering measurements, we report frequency-dependent integrated IR-absorption coefficients for water in ringwoodite ranging from 78180 to 158880 L mol-1cm-2, depending upon frequency of the OH absorption. We further report a linear wavenumber IR calibration for H2O quantification in hydrous ringwoodite across the Mg2SiO4-Fe2SiO4 solid solution, which will lead to more accurate estimations of the water content in both laboratory-grown and naturally occurring ringwoodites. Re-evaluation of the IR spectrum for a natural hydrous ringwoodite inclusion in diamond from the study of Pearson et al. (2014) indicates the crystal contains 1.43 ± 0.27 wt% H2O, thus confirming near-maximum amounts of H2O for this sample from the transition zone.

Highlights

Ringwoodite was first described in the Tenham meteorite found in Queensland, Australia, by Binns et al (1969) and named after the Australian mineral physicist Ted Ringwood
The spectrum of pure Mg-ringwoodite (Figure 2A) displays expected Raman modes at ∼322, 444, 665, 794, and 829 cm−1, which are due to antisymmetric (T2g) and symmetric (A1g) stretching vibrations of the isolated SiO4 tetrahedra (Chopelas et al, 1994), whereas an association of the Raman bands at ∼300 cm−1 and 400 cm−1 with octahedral cation vibrations is under discussion (McMillan and Akaogi, 1987; Chopelas et al, 1994)
Low-frequency bands with slightly varying intensities and positions at ∼200 cm−1 are present in the Fe-bearing samples (Figure 2B). These signals might be associated with localized modes generated by Fe substitution (Fe2+, Fe3+) (Kleppe et al, 2002; Kleppe and Jephcoat, 2006), which explains their absence in pure Mg-ringwoodite (Figures 2A,B)

Summary

Introduction

Ringwoodite was first described in the Tenham meteorite found in Queensland, Australia, by Binns et al (1969) and named after the Australian mineral physicist Ted Ringwood. Ringwoodite is one of the major components of the Earth’s mantle transition zone (e.g., Bernal, 1936; Akaogi and Akimoto, 1977; Anderson and Bass, 1986; Bina and Wood, 1986; Irifune, 1987; Ringwood and Major, 1967) and nominally anhydrous, ringwoodite is wellknown to incorporate up to 1.5–2 wt% of water as (OH)− point defects in both laboratory-grown and naturally occurring samples (e.g., Kohlstedt et al, 1996; Bolfan-Casanova et al, 2000; Smyth et al, 2003; Pearson et al, 2014). Because of its likely abundance (>50% of a pyrolite model) in the lower 150 km of the Earth’s mantle transition zone (410–660 km depth) and for its ability to incorporate significant amounts of H2O, we have undertaken this study on determining the absolute water content in ringwoodite. The absolute water content in minerals was conventionally determined by quantitative dehydration weight analyses (e.g., Aines and Rossman, 1984), but more suitable for the typically very small amounts (μg) of material from high-pressure and high-temperature syntheses, two additional methods have been developed: elastic recoil detection analysis (e.g., Aubaud et al, 2009; Bureau et al, 2009; Withers et al, 2012) and protonproton (pp) scattering (e.g., Gose et al, 2008; Thomas et al, 2008; Reichart and Dollinger, 2009)

Methods

Results

Conclusion