This programme of research builds on very recently published work and aims to examine several suits of diamonds and their inclusions to illuminate the nature of Earth’s deep He-C-N cycles (Mikhail et al., 2019a,b). This student will apply major and trace element geochemistry alongside stable and radiogenic isotopic tracers (13C/12C, 15N/14N, 3He/4He, 143Nd/144Nd) to establish the origin of and temporal development of diamond-forming media in Earth’s mantle. The samples in this study represent all three mantle diamond types; monocrystalline, fibrous and polycrystalline. The PhD will use different samples to answer different questions. The chapter breakdown is envisaged as described below:
1. Using 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, 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., 2014; 2019a,b).
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 diamondites 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 diamondite-formation to known tectono-thermal events such as craton stabilisation, craton break-up, and plume-lithosphere interaction (see methods Timmerman et al. 2017)
4. 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.