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

Hongzhan Fei,Tomoo Katsura

doi:10.1029/2021gl092836

Abstract

AbstractWadsleyite can store significant amounts of H2O in its crystal structure as hydroxyl. However, H2O solubility in Fe‐bearing wadsleyite remains poorly constrained at mantle transition zone temperatures. Previous studies (e.g., Demouchy et al., 2005 [https://doi.org/10.2138/am.2005.1751]; Litasov et al., 2011 [https://doi.org/10.1007/s00269-010-0382-3]) focused primarily on Fe‐free systems, which do not represent the Earth's interior because Fe may affect the H2O solubility. Here, we investigated the temperature dependence of H2O solubility in Fe‐bearing and Fe‐free wadsleyite at 1500–2100 K. The results indicate that H2O solubility in Fe‐bearing wadsleyite is higher than in Fe‐free samples at 1800–1900 K, corresponding to transition zone geotherm, but there is no clear Fe content dependence in the Fe‐bearing samples. Wadsleyite can contain approximately 1.0 wt.% H2O at transition zone temperatures. The H2O solubility in wadsleyite is lower than ringwoodite along a plume geotherm, which may result in dehydration melting at the 520‐km discontinuity by upwelling flow in plumes.

Highlights

Wadsleyite, with a chemical formula of (Mg,Fe)2SiO4, is the dominant mineral in the upper part of the mantle transition zone from 410 to 520 km depth
The presence of H2O can significantly affect the physical and chemical properties of wadsleyite, including atomic diffusivity, electrical conductivity, phase transition, elasticity, and rheology (Buchen et al, 2018; Chen et al, 2002; Dai & Karato, 2009; Demouchy et al, 2011; Manthilake et al, 2009; Shimojuku et al, 2010; Yoshino et al, 2008; Yoshino & Katsura, 2013; Zhang et al, 2021; Zhang & Xia, 2021), information regarding H2O incorporation in wadsleyite is essential for understanding mantle dynamics
Because H2O-rich fluid and silicate melt are miscible at high pressure, the H2O solubility in high-pressure minerals is defined as the maximum amount of H2O that can be dissolved in a crystal coexisting with hydrous silicate melt under a given pressure and temperature conditions (Demouchy et al, 2005; Hirschmann et al, 2005)

Summary

Introduction

Wadsleyite, with a chemical formula of (Mg,Fe)2SiO4, is the dominant mineral in the upper part of the mantle transition zone from 410 to 520 km depth. H2O solubility has been extensively studied in the Fe-free wadsleyite endmember (Demouchy et al, 2005; Druzhbin et al, 2021; Litasov et al, 2011; Zhu et al, 2019), that in the Fe-bearing system remains poorly constrained. By considering the temperature dependence of H2O solubility in minerals (Demouchy et al, 2005; Litasov et al, 2011) and possible Fe effect on H2O incorporation, a systematic study regarding H2O solubility in Fe-bearing wadsleyite is required to understand the H2O incorporation and storage capacity in the mantle transition zone. As we investigated recently (Druzhbin et al, 2021), H2O solubility in Fe-free wadsleyite is independent from fO2 It should be the case for Fe-bearing wadsleyite because dissociation of H2O is negligible under transition zone conditions (Druzhbin et al, 2021). The fO2 in this study is self-buffered by the samples without additional buffering materials

Starting Material and High-Pressure Experiments

Sample Analysis

Sample Textures

FTIR Spectra

Implications for H2O Storage Capacity in the Mantle Transition Zone