2.1 Definition of “Water”
Although water is primarily in the form of H2 O molecules in the hydrosphere (atmosphere
and oceans), deeper in the Earth, what is generally referred to as “water” is actually H
incorporated as various species in minerals, melts and fluids (Fig. 1). In the latter, i.e. sil-
icate/carbonate melts or volatile-rich fluids, H can occur as water H 2 O, hydroxyl OH, hy-
drogen gas H 2 , methane CH4 , hydrogen sulfide H 2 S, and possibly more complex H-bearing
molecules at high pressures (e.g., Stolper 1982; Dixon et al. 1988; Carroll and Webster 1994;
Kohn 2000; Luth 2003; Mookherjee et al. 2008; Song et al. 2009; Zhang and Duan 2009;
Hirschmann et al. 2012; Bali et al. 2013; Armstrong et al. 2015). Melts and aqueous fluids
are generally thought to become miscible at high pressure (> 6 GPa) in the upper mantle
and the resulting super-critical fluids may contain water as both H 2 O and OH (e.g., Kennedy
et al. 1962; Bureau and Keppler 1999; Mibe et al. 2007).