Guiana Shield diamonds, the sub-cratonic lithosphere, and kimberlite emplacement in the upper crust.
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Bassoo, Roy Raymond, Jr., 1985-
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The origin of diamonds from Guyana have remained an enigma despite being mined for nearly 150 years within the Guiana Shield. These gemstones are an undescribed diamond suite within an understudied portion of the Amazonian Craton. Our study confirms they are likely eroding as paleo-placers from Roraima Supergroup rocks, but may have been primarily derived from older ultramafic rocks (≥ 2 Ga) with a high H2O/CO2 volatile budget. These ancient diamonds also contain inclusions that preserve high Cr, Al, Mg, and low Ca, suggesting a harzburgitic paragenesis. Forsterite with elevated Mn, and chromite with low Fe3+/Fe2+ ratios suggest metasomatic alteration within low oxygen fugacity conditions (log ƒO2 (ΔFMQ) -1.6±1.1). Raman thermobarometry suggests entrapment at 5 – 7 GPa within cratonic roots. Low OH active infrared absorbances indicate low ppm H2O contents of forsterite inclusions, from which we were able to estimate high viscosities of 1023.7±2.1 Pa∙s within the cratonic root which incidentally resists delamination through geologic time. We applied similar mineralogical and spectroscopic techniques to diamonds from the Prairie Creek lamproite in Arkansas, USA. Coesite and forsterite inclusions record entrapment conditions of 4.8±0.5 GPa, typical of cratonic settings. However, Arkansas diamonds preserve brown body colors and one example of purple luminescence suggesting vacancy clusters and a distorted cubic lattice. In a highly viscous and turbulent upper mantle of an edge-driven convection cell, as inferred for Arkansas lamproite magma genesis, diamonds may be subjected to extreme forces which distort the atomic lattice. Finally we conducted high temperature (300 – 900 °C) and cold seal experiments at 100-200 MPa on hypabyssal kimberlite from the Jericho kimberlite (Northwest Territories, Canada) to examine sub-solidus reactions that occur syn- to post- emplacement. We observe that olivine in the presence of calcite is unstable and monticellite is precipitated, within the span of hours, suggesting at least rapid syn-emplacement This decarbonation reaction is directly proportional to temperature and indirectly proportional to available fluid CO2, where +12 wt.% CO2 increases calcite stability by 100 °C. Additionally, calcite preservation in hypabyssal kimberlite provides an observational constraint that diamond grade has not been diminished by post-emplacement conditions.