­GROWTH CONDITIONS OF MIXED-HABIT DIAMONDS FROM MARANGE, ZIMBABWE

Diamonds plates from the Marange placer deposits in eastern Zimbabwe, are known for both cuboid and octahedral growth sectors with distinctive centre cross patterns. Previous studies suggest mixed-habit diamonds may only precipitate from N-and H-rich source fluids (Rondeau et al., 2004; Howell et al., 2012). Similarly, Marange mixed-habit diamonds have large FTIR peaks at 3107 cm^-1 related to the VN₃H defect (Goss et al., 2014), indicating they crystallised from reduced N-and H-bearing fluids. Accordingly, they provide an opportunity to evaluate the cycling of reduced ammonium-bearing species into the deep mantle.

Growth sectors in these diamonds are visible in plane light due to grey ‘clouds’ in the cuboid sectors, revealed as disordered graphite by Raman analyses (broad ‘D’ and ‘G’ bands associated with amorphous sp² bonded graphite; e.g. Ferrari and Roberston, 2000). Graphite could have formed due to heating and deformation in the mantle (Evans, 1979, Howell et al., 2013) causing graphitisation along discoid-crack-like defects in cuboid sectors (e.g., Welbourn et al., 1989). Alternatively, it could represent metastable graphite formed as a precursor to diamond at low temperatures (~600 °C); where disordered sp² carbon would precipitate due to the sluggish kinetics of the sp² to sp³ conversion (Bundy and Kasper, 1967; Frezzotti et al., 2014).

Nitrogen aggregation characteristics in these diamonds do not yield useful mantle storage information due to two competing effects: 1) Cuboid sectors contain Ni defects that may enhance nitrogen aggregation (Kiflawi et al., 1998), with some diamonds having cuboid sectors with 30 % more aggregated nitrogen than octahedral sectors and 2) nitrogen contained in H-related defects (as VN₃H at 3017 cm^-1) that would not easily migrate to aggregated nitrogen centres. Therefore, low levels of nitrogen aggregation could be preserved in H-rich Marange diamonds even if they were not stored at low temperatures in the shallow Zimbabwe lithosphere.

SIMS analyses of δ¹³C and δ¹⁵N values are currently underway to evaluate diamond source speciation and any involvement of recycled crustal components in the lithosphere below the eastern Zimbabwe craton. Additionally, any isotopic fractionation between octahedral and cuboid sectors of the same growth horizon will be investigated.