HR: 14:15h
AN: V33F-02    [Abstracts]
TI: Brine Rich Diamond-Forming Fluids
AU: * Klein-BenDavid, O
EM: ofrak@vms.huji.ac.il
AF: Institute of Earth Sciences, The Hebrew University of Jerusalem, Givat Ram, Jerusalem, 91904 Israel
AU: Wirth, R
EM: wirth@gfz-potsdam.de
AF: GeoForschungsZentrum, Telegrafenberg, Potsdam, 14473 Germany
AU: Izraeli, E S
EM: eladi@earth.es.huji.ac.il
AF: Institute of Earth Sciences, The Hebrew University of Jerusalem, Givat Ram, Jerusalem, 91904 Israel
AU: Navon, O
EM: oded.navon@huji.ac.il
AF: Institute of Earth Sciences, The Hebrew University of Jerusalem, Givat Ram, Jerusalem, 91904 Israel
AB: Micro-inclusions in diamonds provide pristine information on the composition of mantle fluids. We explored the composition of sub-micrometer inclusions in 12 fibrous diamonds from Diavik, Slave Craton, Canada. TEM investigation of the inclusions revealed a multi-phase halide-carbonate assemblage with minor apatite. Fluid is also present indicating crystallization during cooling from a primary fluid, trapped during the diamond growth. Potassium is concentrated in halide and fluid; no K-bearing carbonates were found. Ten diamonds carry brine-rich fluid with an average composition of K$_{6}$ Na$_{4}$CaMgFeBa(Si,Al)O$_{2}$Cl$_{9.4}$(CO$_{3}$)$_{4.3}$(H$_{2}$O)$_{10}$ (determined using EPMA and FTIR). In one zoned diamond carbonatitic melt inclusions populates the rim (K$_{13}$Na$_{24}$Ca$_{11}$Mg$_{22}$ Fe$_{5}$Ba$_{2}$Si$_{7}$AlP$_{2}$Cl$_{11}$); the mantle carries brine (K$_{18}$Na$_{24}$Ca$_{6}$Mg$_{10}$Fe$_{4}$Ba$_{3}$Si$_{5}$PCl$_{26}$). One diamond carries composition intermediate between hydro-silicic and carbonatitic melt. The brine in the Canadian diamonds is similar to that found by Izraeli et al. (2001) in cloudy eclogitic and peridotitic diamonds from Koffiefontein, but is richer in Na, Fe and Ba. Micro-inclusions of peridotitic minerals were found in two of the Canadian diamonds. Integrating diamond fluid data from Africa, Brazil, Siberia, and Canada, we found a narrow ranges of fluid composition varying between four end members: hydrous melts rich in silica and alkalis, carbonatitic melts rich in Mg, Fe and Ca, brine rich in Cl, K and Na and sulfide melts rich in Fe and Ni. Carbonatitic melts were found together with all other fluids. The other three components were never detected together in any single diamond and no mixing lines were observed between them. Brine may be generated from parental carbonatitic melts by carbonate crystallization and separation of the residual melt into two immiscible fluids: brine and hydrous-silicic melt. Diamonds can grow from all these fluids. The trace element chemistry of the diamond-forming fluids is similar to that of kimberlites. It is possible that kimberlitic magmas at depth are closer in composition to the trapped fluids and to carbonate and halide-rich fluids recently found in olivine phenocrysts in an Udachnayan kimberlite. The volatile content of erupting kimberlites represents magma that degassed during most of its ascent.
DE: 9350 North America
DE: 1025 Composition of the mantle
DE: 1010 Chemical evolution
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting