HR: 1340h
AN: V13C-0569    [Abstracts]
TI: Cosmogenic Helium In Alluvial Diamonds From Namaqualand, South Africa
AU: * Burgess, R
EM: Ray.Burgess@manchester.ac.uk
AF: School of Earth, Atmospheric and Environmanela Sciences, University of Manchester Oxford Rd, Manchester, M13 9PL United Kingdom
AU: Harris, J W
EM: j.harris@ges.gla.ac.uk
AF: Department of Geographical and Earth Sciences, University of Glasgow, Gregory Bldg , Glasgow, G12 8QQ United Kingdom
AB: The interpretation of He in diamonds is not straightforward, potential sources include trapped mantle-derived He, radiogenic 4He, and 3He produced in situ by cosmic-ray spallation. The presence of cosmogenic 3He is manifested by high 3He/4He values and ratios of >200 Ra have been measured. 3He contents can be used to distinguish pipe from alluvial diamonds and the time interval that they have been involved in the sedimentary cycle, i.e. their surface exposure history. This is important information for locating the source of alluvial diamonds, understanding their transport histories and may provide a useful tool for diamond exploration. In this study we are analysing the He composition of ten alluvial diamonds from the Koignass-Namaqualand area along the south-west coast of South Africa and Namibia. Diamonds are currently mined at a depth of 100m below the surface from deposits of gravel beaches and river channels. Helium is extracted from the diamonds using a newly developed high temperature filament furnace having a blank approximately 200x lower than conventional resistance furnaces. The low blank of this system means it is possible to extract He by stepwise heating of diamonds weighing 0.05 g. Helium isotopes were analysed using a mass spectrometer with a 3He detection limit of 2,000,000 atoms equivalent to about 20 ka of surface exposure. Results from one diamond from Namaqualand illustrate the range of data obtained so far. This diamond was analysed using eight temperature steps and shows an overall increase in 3He/4He value with temperature from 8.8 x 10-6 to 2.6 x 10-2, the latter being only 10x lower than the pure spallogenic ratio. The cosmogenic 3He content is estimated at 73.5 x 10-12 cm3/g which is at the upper end of the range determined previously in alluvial diamonds from west African sources. Assuming a sea level 3He production rate at 30°S, then the 3He content of this diamond is equivalent to 16 Ma of surface exposure. Most of this exposure is likely to have occurred since the mid to late Tertiary when diamonds were no longer being released from primary sources, but were being reworked from older terrace deposits.
DE: 1150 Cosmogenic-nuclide exposure dating (4918)
DE: 1194 Instruments and techniques
DE: 9305 Africa
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005