Diamond exploration: novel techniques for the interpretation of diamond indicator minerals

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Most diamonds crystallize at great depths (>140 km) in ancient, stable regions of the earth's upper mantle called cratonic lithosphere, and they only get to the earth's surface after accidental entrainment in deeply sourced, rapidly ascending magma (kimberlite or more rarely lamproite). Kimberlite and lamproite also transport non-diamond fragments of the cratonic lithosphere ( xenoliths ) other minerals ( xenocrysts ) and because these minerals are present in much greater abundances than diamonds they are more easily detected and can be used in novel ways to act as diamond indicator minerals (DIMs).

Once emplaced into the near surface environment by kimberlite volcanism, DIMs may be geographically dispersed by surface processes. Diamond explorers often recover them from heavy mineral concentrates (HMC), obtained from regolith and modern or paleo-drainage systems. Often, their geographical dispersion from the suspected volcanic source can be back-tracked upstream to determine the source's location.

Xenoliths and DIMs also contain important additional information enabling assessment of the likely diamond grade of lithosphere sampled by their source volcanic rocks. DIMs' chemical compositions record the temperature at which the grains equilibrated with olivine in the mantle. In garnet peridotite xenoliths, their pressure and temperature of origin in the mantle can be calculated using a number of well-calibrated thermometers and barometers. Garnet compositions can reveal how oxidising or reducing the lithosphere was and this is important for diamond stability.

We are currently undertaking research aimed at calibrating new thermometers for application to kimberlite-bourne minerals and rock fragments. In addition, we are developing a new synchrotron-based technique for measuring Fe 3+ /?Fe in garnet, which will allow determination of lithopsheric oxygen fugacity. This information gives diamond explorers important clues to assess whether or not the lithosphere sampled by kimberlites was likely to be rich in diamonds or relatively barren.

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