Zonal and sectorial diamonds from some of Yakutian kimberlite pipes

Skuzovatov S.Yu.

V.S. Sobolev Institute of Geology and Mineralogy sb ras, Novosibirsk, Russia

justsquall@gmail.com

According to literature data, natural diamonds have diverse morphology and physical properties [3, 4]. This diversity must reflect the widely varying conditions of diamond formation in the mantle. There are cases of changing morphology from cubic to octahedral one [5, 7] and in reverse way (from octahedral to cuboid) [1, 3]. Beside of these cases there are sectorial diamonds with simultaneous growth of octahedral faces and cuboid surfaces observed [2]. Here data on impurities content and distribution, carbon isotope composition and also the composition of microinclusions for diamonds from some of Yakutian kimberlite pipes are reported.

Data on studied diamonds reveal few factors that may control a change of diamond habit. It is obvious that for coated diamonds and diamonds with cloudy microinclusions growth mechanism changed at the moment, that is consistent with “octahedral-cuboid” boundary. For coated diamonds this may be due to: a) increase in carbon oversaturation in growth medium, b) different growth conditions for cores and coats (growth of cubic coats in the diffusion regime with octahedra faces stability lost), c) influence of impurity content in crystallization medium. Character of nitrogen impurity distribution, δ¹³C values for coats and cores of different samples, and internal structures on such diamonds allows considering a weak role of impurity factor and possible change of carbon source – this can be a new portion of fluid/melt with relatively homogenous nitrogen and carbon isotope composition. Similar scheme is supposed for diamonds with cloudy microinclusions. Cuboid-octahedra transition here is supposed to proceed during continuous fractionation of fluid/melt with decreasing carbon oversaturation. Composition of cloudy microinclusions and microinclusions in coats of IV variety diamonds hardly allow us to consider some direct dependence of fibrous cores and coats growth on crystallization medium composition. Other regularities are seen for sectorial diamonds. It is quite clear that there is no considerable difference between {111} and {100} sectors growth rates, and eventually in growth mechanisms, as it supposed for octahedral and cuboid crystals [6].

References:

1. Boyd, S.R., Pineau, F., Javoy, M. (1994) Modeling the growth of natural diamonds. Chemical Geology 116, 29-42.

2. Lang, A. R. (1979) Internal structure. The Properties of Diamond (Ed. by J. E. Field). Academic Press, New York. 425-469.

3. Orlov, Yu.L. (1984). Diamond Mineralogy [in Russian]. Nauka, Moscow.

4. Shatsky, V.S., Rylov, G.M., Efimova, E.S., de Corte, K., Sobolev, N.V. (1998) .Morphology and real structure of microdiamonds from metamorphic rocks (Kokchetav Massif), kimberlites, and alluvial placers. Russian Geology and Geophysics 39 (7), 949–961.

5. Skuzovatov, S.Yu., Zedgenizov, D.A., Shatsky, V.S., Ragozin, A.L., Kuper, K.E. (2011) Composition of cloudy microinclusions in octahedral diamonds from Internatsional’naya kimberlite pipe. Russian Geology and Geophysics 52, 85–96.

6. Sunagawa, I. (1990) Growth and morphology of diamond crystals under stable and metastable conditions. Journal of Crystal Growth 99, 1156-1161.

7. Zedgenizov, D.A., Harte, B., Shatsky, V.S., Politov, A.A., Rylov, G.M., Sobolev, N.V., 2006. Directional chemical variations in diamonds showing octahedral following cuboid growth. Contrib. Mineral. Petrol. 151 (1), 45–57.