This student will apply major and trace element geochemistry alongside stable and radiogenic isotopic tracers (He-C-N-O-Nd) to establish the origin of and temporal development of diamond-forming media in Earth’s mantle. The samples in this study represent all three types of mantle diamond; monocrystalline, fibrous and polycrystalline (Fig.1). This programme of research builds on very recently published work and aims to examine several suites of diamonds and their inclusions to illuminate the nature of Earth’s deep volatile cycles (Mikhail et al., 2019a,b; 2021).
The PhD will use different samples to answer different questions. The chapter breakdown is envisaged as described below:
1. Using samples from Orapa (Botswana) as a case study, the major and trace element geochemistry of fluids and silicates alongside stable and radiogenic isotope data from the diamonds and their fluid inclusions (13C/12C, 15N/14N, 18O/16O, 3He/4He) will be used to explain the similarities and differences for the source(s) of carbon for the three main diamond types (see methods in Mikhail et al., 2019a,b; 2014)
2. Fluid-rich fibrous diamonds and near gem-quality diamonds from the Congo, Canada (Ekati, Diavik, Snap Lake, Victor), and South Africa (Finsch, Newlands) will be used to constrain the degree of helium isotope homogeneity in the sub-continental lithospheric mantle as a function of mantle lithology (e.g. eclogitic vs peridotitic; see methods in Mikhail et al., 2019a)
3. Garnets extracted from southern African samples will be used to date silicate and fluid-rich polycrystalline diamond-formation (using 143Nd/144Nd) into the wider context of diamond-formation. E.g., relating polycrystalline diamond-formation to known tectono-thermal events such as craton stabilisation, craton break-up, and plume-lithosphere interaction (see methods Timmerman et al. 2017)
Collectively, these thesis chapters/papers will allow the student to chart, describe, and model the evolution of helium, carbon, and nitrogen isotope systems in the sub-continental lithospheric mantle and to place these cycles in the context of diamond-formation and potentially, diamond grade.