Geoscience Reference
In-Depth Information
dissolution of alkali chlorides in crustal environments (Zaitsev & Keller, 2006) can be
responsible for depriving kimberlites (carbonatites) of their original sodium and potassium.
7.2 Alkali carbonate-chloride parental melt
The source and origin of alkali carbonates and chlorides in the groundmass of the
Udachnaya-East kimberlites is still controversial, given the fact that other group-I
kimberlites are devoid of these minerals, but have serpentine. Three possible scenarios of
the alkali carbonate-chloride enrichment of the Udachnaya-East rocks can be considered:
postmagmatic alteration, contamination of the magma in the crust en route to the surface
and derivation from melting of the respectable mantle source. A possibility of post-
emplacement ingress of chloride- and carbonate-bearing fluids can be confidently rejected
on the basis of petrographic evidence. Any alteration features, typical of kimberlite rocks,
are absent in this case; macrocrysts and phenocrysts of olivine bear no serpentine, and the
olivine- and phlogopite hosted melt inclusions, trapped at magmatic temperatures (>
660oC) are compositionally similar to the groundmass (Golovin et al., 2007; Golovin et al.,
2003; Kamenetsky et al., 2004; Kamenetsky et al., 2007a; Kamenetsky et al., 2009c). Moreover,
water-soluble carbonate and chloride minerals in the groundmass were an important factor
in preventing ingress of external fluids.
A choice between crustal and mantle origin of the Udachnaya-East unique compositional
features is utterly important in deciding whether the Udachnaya-East kimberlite is a “black
sheep” in the kimberlite clan or a bearer of the true identity of the primary kimberlite melt,
and by inference, the composition of the mantle source and mantle melting process. A
potential Na- and Cl-rich contaminant in the form of carbonate-evaporate sedimentary
sequence is present in the south and southwest of the Siberian platform, however, it is not
confidently recorded in the north, beneath the Daldyn kimberlite field (Brasier & Sukhov,
1998). Moreover, such contaminant is not pronounced in the composition of kimberlites
from upper levels of the Udachnaya-East pipe (< 450 m), Udachnaya-West and other pipes
from the same field and other kimberlite fields in Siberia. In addition to indirect evidence
against likelihood of contamination of the kimberlite magma by evaporites reported in
(Kamenetsky et al., 2007a), the deep mantle origin of the carbonate-chloride enrichment of
the Udachnaya-East melt is well supported by the isotope composition of Sr in groundmass
carbonates and perovskite (Kamenetsky et al., 2009c).
The non-silicate residual kimberlite magma has low temperatures (<650-750oC), as shown
by the study of the Udachnaya-East melt inclusions (Kamenetsky et al., 2004), experimental
data on the fluorine-bearing Na2CO3-CaCO3 system (Jago & Gittins, 1991) and direct
temperature measurements in the halogen-rich (up to 15 wt% F+Cl, Jago & Gittins, 1991)
natrocarbonatite lava lakes and flows of the Oldoinyo Lengai volcano (Dawson et al., 1990;
Keller & Krafft, 1990; Krafft & Keller, 1989). However, even at these temperatures it is highly
fluid. Thus, we envisage that droplets of residual melt separate from a solid aluminosilicate
framework of the magma, percolate into weaker, less solidified zones, and finally coalesce,
forming melt pockets. The latter are now seen in the kimberlite as chloride-carbonate
nodules.
We emphasise that in the Udachnaya-East kimberlite the combination of such features such
as extraordinary freshness, high abundances of Na, K and Cl, depletion in H2O, and
preservation of water-soluble minerals and chloride-carbonate melt pockets cannot be
coincidental. From the analogy with dry carbonatite magmas of Oldoinyo Lengai (Keller &
Search WWH ::
Custom Search