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Volatile composition of microinclusions in diamonds from the Panda kimberlite, Canada: Implications for chemical and isotopic heterogeneity in the mantle |
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| Authors: | Ray Burgess Pierre Cartigny Emily Hobson |
| Institution: | a School of Earth, Atmospheric and Environmental Sciences, University of Manchester, Oxford Rd, Manchester M13 9PL, UK b IPGP et Universite Paris 7, Laboratoire de Géochimie des Isotopes Stables, Institut de Physique du Globe, UMR CNRS 7047, 4 Place Jussieu, T54-64 E1, 75251 Paris cedex 05, France c Department of Geographical and Earth Sciences, University of Glasgow, Gregory Building, Glasgow G12 8QQ, Scotland, UK |
| Abstract: | In order to better investigate the compositions and the origins of fluids associated with diamond growth, we have carried-out combined noble gas (He and Ar), C and N isotope, K, Ca and halogen (Cl, Br, I) determinations on fragments of individual microinclusion-bearing diamonds from the Panda kimberlite, North West Territories, Canada. The fluid concentrations of halogens and noble gases in Panda diamonds are enriched by several orders of magnitude over typical upper mantle abundances. However, noble gas, C and N isotopic ratios (3He/4He = 4-6 Ra, 40Ar/36Ar = 20,000-30,000, δ13C = −4.5‰ to −6.9‰ and δ15N = −1.2‰ to −8.8‰) are within the worldwide range determined for fibrous diamonds and similar to the mid ocean ridge basalt (MORB) source value. The high 36Ar content of the diamonds (>1 × 10−9 cm3/g) is at least an order of magnitude higher than any previously reported mantle sample and enables the 36Ar content of the subcontinental lithospheric mantle to be estimated at ∼0.6 × 10−12 cm3/g, again similar to estimates for the MORB source. Three fluid types distinguished on the basis of Ca-K-Cl compositions are consistent with carbonatitic, silicic and saline end-members identified in previous studies of diamonds from worldwide sources. These fluid end-members also have distinct halogen ratios (Br/Cl and I/Cl). The role of subducted seawater-derived halogens, originally invoked to explain some of the halogen ratio variations in diamonds, is not considered an essential component in the formation of the fluids. In contrast, it is considered that large halogen fractionation of a primitive mantle ratio occurs during fluid-melt partitioning in forming silicic fluids, and during separation of an immiscible saline fluid. |
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