New identity of the kimberlite melt: Constraints from unaltered diamondiferous Udachnaya-East pipe kimberlite, Russia

Kimberlite magmas are in many aspects unusual compared to other terrestrial magmatic liquids. They are very rare and occur in small volumes, but their intimate relationships with diamonds make them invaluable to the scientific and exploration communities. The association of kimberlite rocks with diamonds and deep-seated mantle xenoliths links the origin of parental kimberlite magmas to the highest known depths (> 150 km) of magma derivation (e.g. Dawson, 1980; Eggler, 1989; Girnis & Ryabchikov, 2005; Mitchell, 1986; Mitchell, 1995; Pasteris, 1984). Kimberlite magmas would have one of the lowest viscosities and highest buoyancies that enable exceptionally rapid transport from the source region (Canil & Fedortchouk, 1999; Eggler, 1989; Haggerty, 1999; Kelley & Wartho, 2000; Sparks et al., 2006) and preservation of diamonds. Despite significant research efforts, there is still uncertainty about the true chemical identity of kimberlite parental melts and their derivates. Kimberlite magmas are always contaminated by large quantities of lithic fragments and crystals, unrelated to the evolution of the parental melt. In most cases kimberlites are severely modified by syn- and post-magmatic changes that have altered the original alkali and volatile element abundances. These problems are reflected in the definition of the kimberlite rock as both a contaminated and altered sample of its parent melt‚ÄövÑvp (Pasteris, 1984). Numerous other definitions of the kimberlite commonly reflect on ultramafic compositions and enrichment in volatiles (CO2 and H2O; Clement et al., 1984; Kjarsgaard et al., 2009; Kopylova et al., 2007; Mitchell, 1986; Mitchell, 2008; Patterson et al., 2009; Skinner & Clement, 1979) which are supposedly inherited from parental magmas. The physical properties of a kimberlite magma directly, and occurrence of diamonds indirectly, relate to the enrichment in carbonate components which are represented in common kimberlites by calcite and dolomite. The abundant carbonate component in kimberlite rocks is counter-balanced by a more abundant olivine (ultramafic) component, represented by olivine fragments and crystals that are commonly affected by serpentinisation. The ultramafic silicate compositions of kimberlites are ascribed to abundant olivine macrocrysts and phenocrysts, whereas significant CO2 and H2O contents are attributed respectively to carbonate minerals (calcite and dolomite) and serpentine (+ other H2O-bearing magnesian silicates). Unfortunately, the masking effects of deuteric and post-magmatic alteration do not permit routine recognition of olivine generations, and so the olivine component originally dissolved in the kimberlite parental melt remains controversial (Brett et al., 2009; Francis & Patterson, 2009; Mitchell & Tappe, 2010; Patterson et al., 2009). Similarly, the original magmatic abundances of volatile and fluid-mobile alkali elements are disturbed by syn- and post-emplacement modifications, thus complicating complicating quantification of the parental melt composition if inferred from bulk kimberlite analyses. The existing dogma about correspondence between compositions of whole rock kimberlites and their parental melt has been recently challenged by the newcomers to the kimberlite scientific community (e.g., Kamenetsky et al., 2004; Kamenetsky et al., 2007a; Kamenetsky et al., 2007b; Kamenetsky et al., 2008; Kamenetsky et al., 2009a; Kamenetsky et al., 2009b; Kamenetsky et al., 2009c; Maas et al., 2005). A breakthrough into understanding of the kimberlite magma chemical and physical characteristics was made possible by detailed studies of the diamondiferous Udachnaya-East kimberlite pipe in Siberia. Unlike other kimberlites worldwide, severely modified by syn- and post-magmatic changes, the Udachnaya-East kimberlite is the only known fresh rock of this type, and thus it is invaluable source of information on the composition and temperature of primary melt, its mantle source, rheological properties of ascending kimberlite magma. This kimberlite preserved unequivocal evidence for olivine populations, olivine paragenetic assemblages and olivine-hosted melt inclusions, and the role of mantle-derived alkali carbonate and alkali chloride components in the parental melt.