New Identity of the Kimberlite Melt: Constraints from Unaltered Diamondiferous Udachnaya – East Pipe Kimberlite, Russia - Advances in Data, Methods, Models and Their Applications in Geoscience - page 205

olivine from this melt implies saturation in the olivine component, which makes this melt
different from the alkali carbonate melt experimentally produced at mantle P-T conditions
and low melting extents (Sweeney et al., 1995; Wallace & Green, 1988). How and where is
the saturation in olivine acquired?
Study of the olivine populations and complex zoning of the groundmass olivine in the
Udachnaya-East and other kimberlites (Kamenetsky et al., 2008; Kamenetsky et al., 2009b)
provides evidence that olivine crystals were first entrapped by the melt at depth, then partly
abraded, dissolved and recrystallised on ascent, and finally regenerated during
emplacement. We suggest that the history of kimberlitic olivine is owed to the extraordinary
melt composition, as well as conditions during melt generation and emplacement. In our
scenario, a key role is played by the chloride-carbonate (presumably protokimberlite) melt,
which forces strong mechanical abrasion and dissolution of the silicate minerals from
country rocks in the mantle and lithosphere. Such a melt is capable of accumulating Si and
Mg, but only to a certain limit, above which an immiscible Cl-bearing carbonate-silicate
liquid appears (Safonov et al., 2007). The amount of forsterite that can be dissolved in the
sodium carbonate liquid at 10 kbar and 1300 o C is found to be 16 wt% (Hammouda &
Laporte, 2000). Dissolution of olivine and other silicate phases at high pressure does not
proceed beyond the saturation, and is closely followed by precipitation of olivine
(Hammouda & Laporte, 2000). Therefore, ascending kimberlite magma, although being
more Si-rich than its parental melt and loaded with xenocrysts and xenoliths, remains
buoyant enough to continue rapid ascent. At emplacement, the magma releases the
dissolved silicate component in the form of groundmass olivine rims and minor silicate
minerals, thus driving the residual melt towards original chloride-carbonate compositions
(Kamenetsky et al., 2007a).
8. Concluding remarks
Dry, chlorine-bearing alkali minerals in the Udachnaya-East kimberlite are products of
crystallisation of the mantle-derived, uncontaminated melt. We suggest that a composition
rich in alkalies, CO 2 and Cl may be a viable alternative to the currently favoured ultramafic
kimberlite magma. A “salty” kimberlite composition can explain trace element signatures
consistent with low degrees of partial melting, low temperatures of crystallisation and
exceptional rheological properties responsible for fast ascent and the magma's ability to
carry abundant high-density mantle nodules and crystals. Evidence for these components,
notably Cl and alkalies, is only preserved in an ultrafresh kimberlite such as Udachnaya-
East. Nevertheless, Cl-bearing minerals of the type reported here have also been found in
the groundmass and melt inclusions in kimberlites from Canada and Greenland
(Kamenetsky et al., 2009b). The possible existence of chloride-carbonate liquids within the
diamond stability field can be inferred from experiments in the model silicate system with
addition of Na-Ca carbonate and K-chloride (Safonov et al., 2010; Safonov et al., 2007;
Safonov et al., 2009). These experiments also show that Cl-bearing carbonate-silicate and Si-
bearing chloride-carbonate melts evolve towards Cl-rich carbonatitic liquids with
decreasing temperature, providing a possible explanation for chlorine- and alkali-enriched
microinclusions in some diamonds from Udachnaya-East (Zedgenizov & Ragozin, 2007) and
other kimberlites in South Africa and Canada (Izraeli et al., 2001; Klein-BenDavid et al.,
2007; Tomlinson et al., 2006). Brine inclusions in diamonds from various kimberlites, and the
inferred role of chlorides in diamond nucleation and growth (Palyanov et al., 2007;