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conditions of olivine formation are closely similar; 3) olivine cores and rims originate in
different conditions; and 4) variable Fo compositions of cores reflect varying sources or
changing conditions, whereas similar Fo values of rims reflect major buffering event.
Composition and zoning of the Udachnaya-East olivine-II are not unique; similar principle
compositional characteristics of groundmass olivine phenocrysts (variable and constant Fo
of cores and rims, respectively, and variable trace elements at a given Fo of the olivine rims;
Fig. 6b) are described in a number of kimberlite suites (e.g., Boyd & Clement, 1977; Emeleus
& Andrews, 1975; Fedortchouk & Canil, 2004; Hunter & Taylor, 1984; Kirkley et al., 1989;
Mitchell, 1978; Mitchell, 1986; Moore, 1988; Nielsen & Jensen, 2005; Skinner, 1989).
Compared to the ambiguous origin of the olivine cores, the rims of olivine-II most certainly
crystallised from a melt transporting these crystals to the surface. This is best supported by
the cases where several cores of different size, shape and composition are enclosed within a
single olivine-II grain (Fig. 7 g, h). As indicated by mineral inclusions, the olivine-II rims
formed together with phlogopite, perovskite, minerals of spinel group, rutile and
orthopyroxene, i.e. common groundmass minerals (except orthopyroxene) from a melt that
is present as melt inclusions in the olivine rims and healed fractures in the olivine-II cores
and olivine-I (Fig. 3, 8).
Numerous studies indicate that most common xenoliths in kimberlites are garnet lherzolites,
but surprisingly low abundance of orthopyroxene among xenocrysts and macrocrysts has
been intriguing (Mitchell, 1973; Mitchell, 2008; Patterson et al., 2009; Skinner, 1989). Low
silica activity in the kimberlite magma was offered as an explanation for instability of
orthopyroxene, especially at sub-surface pressures (Mitchell, 1973). On the other hand,
crystallising groundmass olivine rims and the presence of orthopyroxene inclusions in this
olivine (Kamenetsky et al., 2008; Kamenetsky et al., 2009a) seem to be inconsistent with each
other. One explanation is that orthopyroxene inclusions (often in groups and always
associated with CO
2
bubbles) can result from the local reaction of olivine with CO
2
fluid
(2SiO
4
-4
+ 2CO
2
Si
2
O
6
-4
+ 2CO
3
-2
).
A limited range of Fo content in the olivine-II rims, but variable trace element abundances
(Fig. 6b) suggest crystallisation over a small temperature range or/and buffering of the
magma at a constant Fe/Mg with fractionating Ni, Mn and Ca. In many instances, where the
cores are seemingly affected by diffusion (Fig. 7 b, c, f, h) and have a surrounding layer of
distinct composition (Fig. 7 a, e, h), the uniform Fo in the rims can reflect attempts by the
crystals to equilibrate with a final hybrid magma (Mitchell, 1986). We also propose that the
buffering of Fe/Mg can occur if the Mg-Fe distribution coefficient (Kd) between olivine and
a carbonate-rich kimberlite melt is significantly higher than for common basaltic systems
(i.e. 0.3±0.03). This reflects significantly smaller Mg-Fe fractionation between silicates and
carbonate melt, possibly as a result of complexing between carbonate and Mg
2+
ions (Green
& Wallace, 1988; Moore, 1988). The implied higher Kd for carbonatitic liquids, and
especially Ca-rich carbonate, has been supported by experimental evidence (Dalton &
Wood, 1993; Girnis et al., 2005). Probably an increase in Kd is even more pronounced for
alkali-rich carbonatitic liquids.
The melt crystallising the rims of the Udachnaya-East groundmass olivine is represented by
the carbonate-chloride matrix of the rocks (Kamenetsky et al., 2004; Kamenetsky et al.,
2007a), and by the melt inclusions in olivine (Fig. 3, 8). The composition of this melt is
unusually enriched in alkali carbonates and chlorides, but low in aluminosilicate
components (Kamenetsky et al., 2004; Kamenetsky et al., 2007a). The crystallisation of
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