Nucleation Environment of Diamonds from Yakutian Kimberlites

G. P. Bulanova, W. L. Griffin and C. G. Ryan
TSNIGRI, Varshavsky sh. 129B, 113545 Moscow, Russia
GEMOC Key Centre, School of Earth Sciences, Macquarie University, Sydney 2109, Australia
CSIRO Exploration and Mining, Box 136, North Ryde 2113, Australia

Abstract: The micro-inclusions located in the genetic centre of Yakutian diamond monocrystals have been studied using optical (anomalous birefringence, photoluminescence, cathodoluminescence) and microanalytical (electron-microprobe, proton-microprobe, scanning electron microscope) methods. Most diamonds nucleated heterogeneously on mineral seeds, that lowered the energy barrier to nucleation. Nucleation of peridotitic diamonds occurred on a matrix of graphite+iron+wüstite, in an environment dominated by forsteritic olivine and Fe-Ni sulfide. Nucleation of eclogitic diamonds occurred on a matrix of sulfide ± iron in an environment dominated by Fe-sulfide and omphacite (±K-Na-Al-Si-melt). The mineral assemblages recorded in the central inclusions of Yakutian diamonds indicate that they grew in a reduced environment, with oxygen fugacity controlled by the iron-wüstite equilibrium. Nucleation of diamond occurred in the presence of a fluid, possibly a volatile-rich silicate melt, highly enriched in LIL (K, Ba, Rb, Sr) and HFSE (Nb, Ti, Zr) elements. This fluid also carried immiscible Fe-Ni-sulfide melts, and possibly a carbonatitic component; the introduction of this fluid into a reduced refractory environment may have been accompanied by a thermal pulse, and may have created the conditions necessary for the nucleation and growth of diamond.

Keywords: diamond • inclusions • nucleation • growth • Yakutia • kimberlite

Mineralogical Magazine; June 1998 v. 62; no. 3; p. 409-419; DOI: 10.1180/002646198547675
© 1998, The Mineralogical Society
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