HR: 09:05h
AN: V51F-05 INVITED     [Abstracts]
TI: Halogens in diamonds and the origin of their variability
AU: * Burgess, R
EM: Ray.Burgess@manchester.ac.uk
AF: School od Earth, Atmospheric and Environmental Sciences, University of Manchester, Oxford Rd, Manchester, M13 9PL United Kingdom
AU: Cartigny, P
EM: j.harris@ges.gla.ac.uk
AF: Laboratoire de G‚ochimie des Isotopes Stables, IPG-Paris, T54-64 1er etage, 4 place Jussieu 75252, Paris, Cedex 05 France
AU: Harris, J W
EM: j.harris@ges.gla.ac.uk
AF: Department of Geographical and Earth Sciences, University of Glasgow, Glasgow, G12 8QQ United Kingdom
AB: Micro-inclusions in fibrous diamonds provide unique samples of deep fluids from the sub-continental lithospheric mantle. Investigation of the isotopic and chemical composition of these fluids in diamonds that have formed at different epochs and from different geographical regions has the potential to trace the time-resolved, global evolution of volatiles in the mantle. Previous studies have shown that the isotopic composition of volatiles such as C and N and noble gases in micro-inclusion-bearing diamonds from Africa, Siberia and Canada record a clear upper mantle signature. However the fluids show strong chemical fractionations that include the halogens, a group of elements not normally considered to be fractionated by mantle processes, and yet showing Br/Cl and I/Cl that exceed crustal values. The extreme enrichment of halogens in the fluids, up to four orders of magnitude higher than present -day upper mantle values suggests that the fluids have accumulated from large volumes of the mantle and therefore unlikely to represent local heterogeneities. The formation of carbonatitic-hydrous silicic fluid mixtures is associated with major K/Cl fractionation and minor Br/Cl and I/Cl fractionation from the MORB ratios typified by diamonds from Africa, Siberia and a few Canadian diamonds. In contrast, carbonatitic-brine mixtures are typified by relatively constant K/Cl with major Br/Cl and I/Cl variations and are predominant in Canadian diamonds. The variations in K/Br/I/Cl between diamonds from Canada, Africa and Siberia can not be explained by crystallisation of a single Cl-bearing mineral phase, and the upper mantle He, Ar, C and N isotope ratios appear to rule-out the presence of recycled seawater or crustal halogens in the mantle fluids. In Siberian and African diamonds minor halogen fractionation may occur during partitioning controlled by halide ion radius between a hydrous silicic and carbonatitic melts. Much larger halogen fractionation is present in most Canadian diamonds associated with unmixing of immiscible brine fluids.
DE: 1025 Composition of the mantle
DE: 1065 Major and trace element geochemistry
DE: 3621 Mantle processes (1038)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005