Systematics of noble gas, carbon and nitrogen isotope compositions in Australian diamonds from the Merlin and Argyle Mines

Rong Shi

Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia

Diamonds from Merlin and Argyle mines show different characteristics both on visual appearances and isotopic compositions, although the two mines are 1350 km apart from each other in opposite directions of south Darwin, Northern Territory. The Merlin diamonds (kimberlitic) are predominantly characterized by yellow colour, cube and dodecahedral form, whereas Argyle diamonds (lamproitic) are mainly octahedral in pale brown colour.

Previous carbon isotopic studies on diamonds from both locations indicated the diamonds sharing δ13C range from -5‰ to -16.66‰ (Lee et al 1997; Hall & Smith 1984; Jaques et al 1989). Peridotitic diamonds from Argyle mine exhibit a restricted δ¹³C range of -4.4‰ to -9.1‰, compared to a wider δ¹³C from -5‰ to -16‰ for eclogitic Argyle diamonds (Jaques et al 1989). Limited noble gas studies on Argyle diamonds yielded a higher ³He/⁴He value 1.57±0.75X10^-4 and solar Ne (²⁰Ne/²²Ne=12.6±1.8, ²¹Ne/²²Ne=0.0304±0.0048) for colourless diamonds, and a lower ³He/⁴He value (<4.5X10^-8) for coloured composite diamonds (Honda et al 1987).

A further systematically integrated study of noble gas, nitrogen and carbon isotopic compositions on Australian diamonds will help to understand regional time-integrated mantle history. Furthermore carbon and nitrogen isotopic compositions, combined with noble gases, in diamonds can elucidate the evolution and outgassing history of the subcontinental lithosphere.

 

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