MSA PRESIDENTIAL ADDRESS: DIAMOND — PREMIER MINERAL FOR UNDERSTANDING THE GEOLOGY OF THE DEEP EARTH

Diamond is the only mineral that samples the mantle at depths up to 800 km and completely preserves included minerals. As such, diamond provides extraordinary information about Earth’s interior. Diamond crystallizes from mobile C-O-H-bearing fluids or melts and can display remarkable zonation -- giving it the ability to track carbon sources and mobility in the deep mantle. Through the fortuitous co-crystallization or entrapment of silicate, sulfide, metal, and carbide inclusions, information about fluid sources, host lithologies, diamond ages, mantle mineralogy, and mantle redox state can be obtained. Information on diamond age, C composition, and inclusion mineralogy gleaned from many single diamonds can be combined into large-scale patterns that can be related to continental geologic structure and/or the geodynamics of mantle convection. Since diamonds from the continental lithospheric mantle have great antiquity (up to 3.5 Ga; Ricahrdson et al. 1984; Westerlund et al. 2006), diamond has great potential to examine these processes in the geologic past. Direct studies on diamond or studies considering its role in mantle mineralogy have pinpointed the initiation of subduction (Shirey and Richardson 2011), traced recycling (including water) into the mantle transition zone (Stachel et al. 2005; Walter et al. 2011; Pearson et al. 2014), recorded the passage of fluids into the continental lithosphere (Richardson et al. 1984; Pearson et al. 1998; Shirey et al. 2004), preserved the signature of carbonatitic fluids that trigger deep mantle melting (Walter et al, 2008), and revealed the change in mantle redox with depth (Stagno et al., 2013). In the future, better knowledge of fluid compositions, diamond forming reactions, C and N fractionation factors, and new suites of sublithospheric (superdeep) diamonds hold the promise of changing our understanding of the geology of the deep Earth.

Pearson et al., EPSL 297 1998; Pearson et al., Nature 507 2014; Richardson et al., Nature 310 1984; Shirey et al., Lithos 288 2004; Shirey and Richardson, Science 333 2011; Stachel et al., Elements 1 2005; Stagno et al., Nature 493 2013; Walter et al., Nature 454 2008; Walter et al., Science 334 2011; Westerlund et al., CMP 342 2006.