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

Abstract

Hydrous wadsleyite, ringwoodite, and phase D have been synthesized in the MgO-SiO 2-H 2 O and MgO-FeO-SiO 2-H 2 O systems at mantle transition zone conditions (15-21 GPa and 1,100-1,700 • C) to investigate their water incorporation using Elastic Recoil Detection Analysis (ERDA), which is an absolute quantitative method. Wadsleyite and ringwoodite water contents vary from 0.1 to 3.2 wt% H 2 O and for ringwoodite containing up to 1.16 wt% H 2 O we observe that the (Mg+Fe)/Si ratio vs. water content follows the same trend as for wadsleyite, indicating that H is substituting for Mg as for wadsleyite. We also measured, for the first time, the water content of phase D and observed that it varies from 6.7 to 11.2 wt% H 2 O, up to twice less than estimated from electron microprobe analysis totals. Using these experiments, we were able to determine the absorptivity coefficient for OH infrared absorption bands in four wadsleyite and five ringwoodite samples. The average for the two iron-free wadsleyite samples leads to an absorptivity of 69,000 ± 7,000 L/moles H 2 O/cm 2 , in very good agreement with previous determinations. The wadsleyite with 8 mole% Fe displays an absorptivity of 67,000 ± 5,000 L/moles H 2 O/cm 2. The absorptivity values vary from 118,500 ± 5,000 for Fefree ringwoodite to ±135,000 ± 9,000 for Fe-bearing ringwoodite (10% mole Fe). Our results show that absorptivity coefficient for OH infrared absorption of ringwoodite do not drastically change with Fe content and that the frequency-based calibration of Paterson (1982) underestimates its water determinations by 50%. This is very important to know when comparing data from different studies where different extinction coefficients have been used.

Water Solubility in Fe‐Bearing Wadsleyite at Mantle Transition Zone Temperatures

“…The C H2O in Fe‐bearing wadsleyite measured in this study is higher than the values reported by Bolfan‐Casanova et al. (2018). Note that the H 2 O content in the starting material in Bolfan‐Casanova et al.…”

“…Note that the Paterson (1982) calibration was used in Sun et al (2018), which may underestimate C H2O . By adjusting their results to a newer calibration using a correction factor of 100/55 given by Bolfan-Casanova et al (2018), the C H2O in Sun et al (2018) becomes 1.7 ∼ 1.8 wt.%, which is slightly higher than in this study (∼1.4 wt.% at 1720 K) (Figure 3). However, this difference could be caused by experimental uncertainties, including the uncertainty of the correction factor between different infrared calibrations, experimental temperature, and FTIR spectra baseline subtraction.…”

“…(2018) reported the H 2 O content ( C H2O ) of approximately 1.0 wt.% in Fe‐bearing wadsleyite synthesized at 1720 K under H 2 O‐saturated conditions, which is more than twice that reported by Bolfan‐Casanova et al. (2018) under comparable conditions. Kohlstedt et al.…”

“…Note that the H 2 O content in the starting material in Bolfan‐Casanova et al. (2018) was relatively small (∼5%). Since only two or three phases (wadsleyite, melt, enstatite/superhydrous phaseB) coexist in the Fe‐bearing system which has four (MgO + SiO 2 + H 2 O + FeO) or five (+Fe 2 O 3 ) components, the number of phases is smaller than the number of components.…”

Constraining the Volume of Earth's Early Oceans With a Temperature‐Dependent Mantle Water Storage Capacity Model

“…To ensure the compiled water storage capacity measurements are internally consistent, we have corrected all the FTIR measurements for olivine to the Withers et al (2012) calibration. Though no independent infrared absorption calibration has been established specifically for OH in either wadsleyite or ringwoodite, it has been shown that the Paterson (1982) calibration underestimates water concentrations in wadsleyite and ringwoodite by ∼55% relative to the ERDA technique (Bolfan-Casanova et al, 2018). Here, we corrected those water concentrations in wadsleyite and ringwoodite determined by FTIR based on the Paterson (1982) calibration by this amount.…”