HR: 11:05h
AN: V42A-04    [Abstracts]
TI: Relationship between Diamond Dissolution Features, Oxygen Fugacity and Temperatures of Lac de Gras Kimberlite Magmas
AU: * Fedortchouk, Y
EM: yana@uvic.ca
AF: School of Earth and Ocean Sciences, University of Victoria, Canada, P.O.Box 3055,STN CSC,, Victoria, BC V8W 3P6 Canada
AU: Canil, D
EM: dcanil@uvic.ca
AF: School of Earth and Ocean Sciences, University of Victoria, Canada, P.O.Box 3055,STN CSC,, Victoria, BC V8W 3P6 Canada
AU: Carlson, J A
EM: Jon.A.Carlson@BHPBilliton.com
AF: BHPBilliton Diamonds Inc., Kelowna, Canada, #8-2604 Enterprise Way, Kelowna, BC V1X 7Y5 Canada
AB: Natural diamonds recovered from kimberlites usually show a variety of morphological forms and surface features, reflecting the complex history of diamond growth, dissolution and transformation during its residence in the mantle and ascent in kimberlite melts. High-temperature kimberlite magma is a reactive media for diamonds. The reaction with kimberlite is an important mechanism of diamond destruction that may greatly influence the grade and value of diamonds in a kimberlite pipe. Crystallization temperatures (T) and oxygen fugacity (fO2) of kimberlite melts may notably influence the kinetics of diamond dissolution process and the character of the resorption features developed on the diamond surface. In order to see how notable is the impact of the parameters of the host kimberlite melt on the characteristics of the diamond population we compare the T and fO2 of kimberlite melt crystallization with the diamond contents and the characters of resorption in diamonds from several kimberlite pipes in the Lac de Gras region (NWT, Canada). Exceptionally fresh kimberlites from the Lac de Gras region allowed us to apply olivine-spinel (Ol-Sp) thermometry and oxygen barometry to chromite inclusions in olivine phenocrysts in these kimberlites. We extended our previous estimates to a larger number of kimberlite pipes from different clusters within the Lac de Gras area. A detailed description of the morphology and surface features of individual stones was compiled for five kimberlite pipes from the both clusters. The T and fO2 values calculated at an assumed pressure of 1 GPa are in the range 970 - 1140 \pm 50 \deg C and 2.8 to 4.4 log fO2 units below the nickel-nickel oxide (NNO) buffer. Kimberlites from one cluster show very similar T and fO2 values, whereas variations between the clusters are within 150 \deg C and 1 log unit respectively. These differences correlate well with variations in the characteristics of the diamonds. An increase in diamond resorption in the kimberlites corresponds to increase in T. The development of various surface dissolution pits and structures is more extensive in the more oxidized kimberlites. The two processes of diamond dissolution - volume resorption and surface etching, do not show a strong correlation with each other. The diamond grade correlates with decrease in kimberlite fO2, while the quality of diamonds decreases with T. The established correlations between T- fO2 values of kimberlites and diamond resorption features suggest that conditions of the host kimberlite may notably influence the entrained diamonds. Our data shows that the development of the surface etching mainly occurs in kimberlite melt and therefore is controlled by the conditions of the melt, whereas the volume resorption depends on the conditions in both the kimberlite and in the mantle source and its relationship with the kimberlite conditions are obscure. The small but distinct variation in the fO2 between the kimberlites from the different clusters may have notably influenced their diamond populations.
DE: 3600 MINERALOGY AND PETROLOGY (replaces
DE: 3640 Igneous petrology
DE: 1025 Composition of the mantle
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting