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 201

thus the chloride liquid was supercooled. On the other hand, it was close to the point of
solid solution unmixing in the system 75% NaCl - 25% KCl (543 o C at 1 atm), and in this case
unmixing of liquids rather than solids is more likely.
Crystallisation from a homogeneous chloride-carbonate liquid (i.e., prior to immiscibility) is
possible, and very unusual Na-Mg carbonates containing a NaCl molecule (northupite
NaCl*Na 2 Mg(CO 3 ) 2 , Fig. 10e), is an example. Disruption of the melt structure caused by
chloride-carbonate immiscibility and followed by reduction in solubility of the phosphate
and Fe-Mg aluminosilicate components, prompted rapid crystallisation of zoned and often
skeletal micro-crystals of apatite and phlogopite - tetraferriphlogopite. Fibrous aggregates of
phlogopite in carbonates and sylvite are common and suggestive of incomplete extraction
of the phlogopite component from carbonate and chloride melts by post-immiscibility
crystallisation. After chloride-carbonate liquid unmixing the sulphate component of the
original melt was largely accommodated within the carbonate melt. It was partially released
as an aphthitalite melt at the chloride-carbonate interfaces (Fig. 11d), leaving porous K- and
S-free carbonate behind (Fig. 11b), and it was also partially exsolved and re-distributed
within the carbonate at subsolidus temperatures.
7.4 Rheological properties of kimberlite magmas
Kimberlites, especially those with preserved diamonds (Haggerty, 1999) are undoubtedly
fast ascending magmas (>4 m/s; see review in Sparks et al., 2006). Support to this contention
also comes from experimentally studied rates of dissolution of garnet in H 2 O-bearing
kimberlite melt (Canil & Fedortchouk, 1999) and Ar diffusive loss profiles of phlogopite in
mantle xenoliths (Kelley & Wartho, 2000). Other indirect evidence includes inferred low
viscosity of the kimberlite magma and its low density, contributing to high buoyancy
(Spence & Turcotte, 1990). The unique physical properties of the kimberlite magma are
governed by high abundances of chemical components that reduce melt polymerization (e.g.
volatiles). The kimberlite magmas are assigned significant H 2 O contents in controlling
transport and eruption, and only a few studies cast doubts on magmatic origin of H 2 O in
kimberlites (e.g., Marshintsev, 1986; Sheppard & Dawson, 1975; Sparks et al., 2006).
Rapid transport and emplacement of the Udachnaya-East kimberlite is supported by the fact
that this pipe is one of the most diamond-enriched in the world. However, our study denies
the control from H2O on rheological properties of the Udachnaya-East kimberlite magma as
the measured H2O abundances are particular low (<0.5 wt%). Instead, we are in position to
draw analogy with the Oldoinyo Lengai natrocarbonatite magma, given the observed
similarities in temperature (Kamenetsky et al., 2004) and composition. At low eruption
temperature (< 600 o C) the natrocarbonatite magma has exceptionally low density (2170
kg/m 3 ; Dawson et al. (1996), viscosity (0.1-5 Pa s ; Dawson et al., 1996; Keller & Krafft, 1990;
Norton & Pinkerton, 1997) and fast flow velocities (1-5 m/s ; Keller & Krafft, 1990). The
effect of halogens on reducing apparent viscosity of the carbonatite magma (three orders of
magnitude for a three-fold increase in halogen content; Norton & Pinkerton, 1997) makes us
confident that enrichment of the Udachnaya-East kimberlite in chlorine (at least 3 wt%) is a
key chemical factor responsible for unique rheological properties of kimberlite magmas.
7.5 Implications from kimberlites and carbonatites worldwide
The enrichment of the Udachnaya-East kimberlite in alkali carbonates and chlorides, if a
primary mantle-derived signature, could have been present in other group-I kimberlites