Origins of Majoritic Inclusions in Diamond

Abstract:

Mineral inclusions in diamonds are the only available samples from the transition zone and the lower mantle. The dominant type of inclusions for the range of transition zone (410-660 km) pressures is majoritic garnet, a high-pressure Si-rich tetragonal garnet which crystallises when pyroxene breaks down and dissolves in the garnet structure. Two majoritic garnet endmembers: Na₂MgSi₅O₁₂ and Mg₄Si₄O₁₂ can be distinguished for eclogitic and peridotitic parageneses, respectively. In our recent study [1] we used these idealised substitutions to show that the majority of majoritic garnets reported in the literature belong to neither eclogitic nor peridotitic lithologies and rather crystallised from a wide range of intermediate compositions, referred to as pyroxenites [1].

Here we elaborate on the origin and composition of the pyroxenite lithology. According to Si geobarometry most majoritic garnet inclusions formed at mantle transition zone pressures, predominantly within the stability field of clinopyroxene + garnet. Using experimental partition coefficients for Na, Al, Mg and Fe between garnet and clinopyroxene, we have calculated the compositions of clinopyroxene in equilibrium with majoritic garnet, which allows us to estimate the locus of possible bulk rock compositions. These lie in the field of upper mantle pyroxenites as defined by Hirschmann and Stolper [2]. We also find that occasional clinopyroxene inclusions in diamond actually coexisting with majoritic garnet inclusions are all close in composition to those predicted by our study. Additionally, we show experimentally that the pyroxenite-diamond association is probably a consequence of the interaction between basaltic and peridotitic compositions in the presence of carbonate melt and that layering of pyroxenites is a natural consequence of this interaction. Reduction of carbonate to carbon at high pressures is responsible for the genetic connection between pyroxenite and diamond and the abundance of pyroxenitic inclusions reflects this connection rather than a high abundance of this rock type in the mantle.

[1] Kiseeva E.S et al (2013) Geology doi:10.1130/G34311.1

[2] Hirschmann, M.M., Stolper, E.M. (1996) Contrib Mineral Petrol 124:185-208