ORIGINAL PAPER Yana Fedortchouk Æ Dante Canil Æ Jon A. Carlson Dissolution forms in Lac de Gras diamonds and their relationship to the temperature and redox state of kimberlite magma Received: 1 November 2004 / Accepted: 14 April 2005 / Published online: 20 July 2005 Ó Springer-Verlag 2005 Abstract The degree and character of diamond dissolu- tion were compared to crystallization temperatures (T) and oxygen fugacities (fO 2 ) estimated from chromite inclusions in olivine phenocrysts in several kimberlites from Lac de Gras, Northwest Territories, Canada. The T and fO 2 values calculated at an assumed pressure of 1 GPa are in the range of 970–1,140±50°C and 2.8–4.4 log fO 2 units below the nickel–nickel oxide (NNO) buf- fer. The T and fO 2 vary between kimberlites from northwest and southeastern clusters within 150°C and 1 log unit, respectively. A detailed description of mor- phological forms and surface dissolution features for diamond parcels from the Panda, Beartooth, Grizzly, Misery and Jay kimberlites (>7000 stones) show that an increase in diamond resorption in the kimberlites cor- responds to increase in T. The development of various surface dissolution pits and structures correlates with higher fO 2 of kimberlites and therefore mainly happens in the magma. The two processes of diamond dissolu- tion, volume resorption and surface etching, do not show a strong correlation with each other, since some of the resorption occurs in the mantle. We suggest that the fO 2 of the kimberlite magma plays an important role in both the processes. The proportion of plastically de- formed brown diamonds does not correlate with the degree of volume resorption, but does correlate with the development of surface forms. The diamond grade is higher in kimberlites with lower fO 2 , confirming that conditions of kimberlite crystallization can have notable effect on diamond dissolution. Introduction Natural diamonds recovered from kimberlites usually show a variety of morphological forms and surface features (Sunagawa 1984), reflecting the complex history of diamond growth, dissolution and transformation during its residence in the mantle and ascent in kim- berlite melts. Diamond dissolution (resorption) can be a complex multi-stage process (Gurney et al. 2004) resulting in a variety of dissolution forms. Etch pits, ruts, corrosion sculptures, frosting and other surface features are commonly accepted to be the result of resorption and may lead to a very irregular form, and decrease in quality, of a diamond. For the formation of dodecahedra and tetrahexahedron (THH) diamond forms, two mechanisms, primary growth (Mendelssohn and Milledge 1995, and references therein) and resorp- tion of octahedron stones (Robinson et al. 1989) were proposed. The latter is a commonly accepted mechanism supported by recent experimental studies of diamond dissolution that showed significant volume loss (Kozai and Arima 2003). Therefore, diamond resorption pro- cesses greatly influence the grade and value of diamonds in a kimberlite pipe. High-temperature kimberlite magma is a reactive media for diamonds. Experimental studies on diamond dissolution in alkaline melts (Khokhryakov and Palya- nov 1990; Kozai and Arima 2003; Sonin et al. 2002) have produced many of the resorption features observed in natural diamonds. Their results support the sugges- tion (Robinson et al. 1989) that the reaction with kim- berlite is the main mechanism of diamond destruction. Resorption in the mantle was also documented (Gurney et al. 2004 and references therein). Electronic Supplementary Material Supplementary material is available for this article at http://dx.doi.org/10.1007/s00410-005- 0003-1 Communicated by T.L. Grove Y. Fedortchouk (&) Æ D. Canil School of Earth and Ocean Sciences, University of Victoria, Victoria, 3800 Finnerty Rd, BC V8W 3P6, Canada E-mail: yana@uvic.ca Tel.: +1-250-4724181 Fax: +1-250-7216200 J. A. Carlson BHP Billiton Diamonds Inc, Kelowna, BC, Canada Contrib Mineral Petrol (2005) 150: 54–69 DOI 10.1007/s00410-005-0003-1