Geochemistry of Chromium in the Earth’s Mantle

A huge amount of data has been accumulated in the field of high-pressure mineralogy to date (Agee 1998; Stachel 2001; Akaogi 2007; Irifune and Tsuchiya 2007; Kaminsky 2012; and others). Direct study of the substance of the Earth’s mantle using data on the minerals of mantle xenolith and inclusions in natural diamonds is significantly restricted. According to the geothermobarometric estimates, most of such minerals are formed at a depth of 150–200 km, i.e., their associations characterize the P–T conditions of the upper mantle (Sobolev 1977; Sobolev et al. 1997; Taylor and Anand 2004). At the same time, there is a continuous replenishment of the database on mineral inclusions in diamonds related to the depths of the Earth’s transition zone (410–660 km) (Davies et al. 2004; Stachel et al. 2000a) and lower mantle (>660 km) (Harte et al. 1999; Harte and Harris 1994; Kaminsky et al. 2001; Hayman et al. 2005; Stachel et al. 2000b). Along with the mineralogical data, the geophysical data and experimental results obtained at high pressures and temperatures are important sources for ideas on the deep structure of the Earth. Deep analysis of mineralogical, geophysical, and experimental information provided evidence for the most important phase transitions in the Earth’s mantle, showed their relationships to the jumps in seismic wave velocities, and defined the major phase assemblages typical of the different parts of the upper mantle, transition zone, and lower mantle, which ultimately allowed to clarify the existing models of the structure of the deep geospheres (Harte 2010; Pushcharovsky and Pushcharovsky 2012).