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-SiO2-H2 O and MgO-FeO-SiO2-H2O 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% H2 O and for ringwoodite containing up to 1.16 wt% H2O 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% H2O, 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 H2O/cm2, in very good agreement with previous determinations. The wadsleyite with 8 mole% Fe displays an absorptivity of 67,000 ± 5,000 L/moles H2 O/cm2. The absorptivity values vary from 118,500 ± 5,000 for Fe-free 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) under-estimates its water determinations by 50%. This is very important to know when comparing data from different studies where different extinction coefficients have been used.