Quantification of water in hydrous ringwoodite

Abstract

Ringwoodite, γ-(Mg,Fe) 2 SiO 4 , in the lower 150 km of Earth's mantle transition zone (410-660 km depth) can incorporate up to 1.5-2 wt% H 2 O 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 spectrometry (SIMS) and proton-proton (pp)-scattering on a suite of synthetic Mg-and Fe-bearing hydrous ringwoodites. H 2 O concentrations in the crystals studied here range from 0.46 to 1.7 wt% H 2 O (absolute methods), with the maximum H 2 O 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 78,180 to 158,880 Lmol −1 cm −2 , depending upon frequency of the OH absorption. We further report a linear wavenumber IR calibration for H 2 O quantification in hydrous ringwoodite across the Mg 2 SiO 4-Fe 2 SiO 4 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% H 2 O, thus confirming near-maximum amounts of H 2 O for this sample from the transition zone.

Examination of Water Quantification and Incorporation in Transition Zone Minerals: Wadsleyite, Ringwoodite and Phase D Using ERDA (Elastic Recoil Detection Analysis)

“…Based on the data presented here we propose an absorptivity of 118,000 ± 5,000 for Fe-free ringwoodite and 135,000 ± 9,000 for Fe-bearing (Fo90) ringwoodite. Our value of ε for Fe-free ringwoodite is 15% higher than that extrapolated by Koch-Muller and Rhede (2010) and 10% higher than the more recent value determined using proton-proton scattering from Thomas et al (2015). Indeed, Thomas et al (2015) did not take into account the infrared feature at 2,500 cm −1 , which we did.…”

Comparative compressibility of hydrous wadsleyite and ringwoodite: Effect of H2O and implications for detecting water in the transition zone

“…[] calibration. Given the importance of hydrous wadsleyite to modeling the mineralogy and geochemistry of Earth's transition zone, future work on calibrating both spectroscopic and SIMS H 2 O contents for wadsleyite against absolute methods is warranted [e.g., Thomas et al ., ].…”

“…We present an interlaboratory comparison made by measuring the H 2 O content of wadsleyite Z0570 crystals on two different instruments: first on the Cameca IMS-3F ion mass microanalyzer at Hokkaido University and subsequently on the Cameca NanoSIMS 50L at the Carnegie Institution of Washington, Department of Terrestrial Magnetism (DTM). Thomas et al, 2015].…”

A New Look at Gaia's Relationship with Water

“…Three world oceans' worth of water were recently discovered in the Ringwoodite layer between Earth's upper and lower mantles (Thomas et al 2015), its life juices pumping up through deep ocean vents and volcanic eruption. Apparently water pervades all of space.…”

“…Apparently water pervades all of space. Three world oceans' worth of water were recently discovered in the Ringwoodite layer between Earth's upper and lower mantles (Thomas et al 2015), its life juices pumping up through deep ocean vents and volcanic eruption. Water "in-forms" all life within Earth's governing membrane, from folded rock to Earth's "crown of creation," man.…”