Visualizing the Distribution of Water in Nominally Anhydrous Minerals at the Atomic Scale: Insights From Atom Probe Tomography on Fayalite

Abstract

While the chemical formulas of mantle-derived minerals such as olivine ((Mg,Fe) 2 SiO 4 ) indicate they are anhydrous, they are capable of structurally incorporating small amounts of hydrogen (H) as hydroxyl (OH) (colloquially referred to as water) (Demouchy & Bolfan-Casanova, 2016). At the planetary scale, these nominally anhydrous minerals (NAM) may, in turn, be significant repositories of mantle water. Thus, understanding the occurrence of water in NAM and its incorporation mechanisms has broad implications, from understanding planetary evolution to the determination of Earth's water budget. For instance, the presence of water in NAM influences mantle rheology, lubricating and facilitating the movement of faults via hydrolytic weakening (Sommer et al., 2008), as well as its electrical conductivity (Karato, 1990). Additionally, the insight gained from NAMs on Earth can be utilized to understand the evolution of planetary systems like that of Mars (Peslier, 2010).Quantification of water in NAM has been extensively studied for the last three decades (Bell et al., 2003), though understanding its mode of incorporation is largely ambiguous due to challenges in directly observing the distribution of water at the atomic level. That is, the amount of H in NAM is readily obtained using infrared (IR) spectroscopic, elastic recoil detection analysis (ERDA; Bolfan-Casanova et al., 2018;Withers et al., 2012), and