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rounded grains (olivine-I of disputed origin), whereas another type of olivine is typically
smaller but better shaped crystals (olivine-II or groundmass phenocrysts). It has been
advocated in the literature that olivine may provide valuable clues to processes of kimberlite
formation, transport and emplacement (e.g. Boyd & Clement, 1977; Mitchell, 1973; Mitchell,
1986; Moore, 1988; Skinner, 1989).
Mitchell & Tappe (2010), Mitchell (1973; 1986), and Moore (1988) considered olivine from
both populations to be phenocrysts (cognate phenocrysts of olivine-I from high-pressure
crystallisation of the kimberlite melt, and groundmass olivine-II), although up to 40 % of
olivine was assigned to xenocrystic origin from various mantle and lithospheric sources. A
similar conclusion can be endorsed by the extreme diversity of peridotite xenoliths within
the Udachnaya-East kimberlite (Shimizu et al., 1997; Sobolev, 1977; Sobolev et al., 2009). The
absence of primary melt inclusions and presence of Cr-diopside inclusions in olivine-I also
argue against their phenocrystic origin.
Xenocrystic origin of some or all grains of olivine-I does not preclude this olivine being
overgrown by the “phenocrystic” olivine. Both types of olivine are transported together, and
thus all changes related to chemical and mechanical resorption should be equally imposed
on them, making a morphological distinction subjective. Both olivine populations in the
studied Udachnaya-East samples demonstrate striking compositional similarity in their Fo
values (Fig. 6) and oxygen isotope values (Kamenetsky et al., 2008). Trace elements
abundances are also indistinguishable for the olivine-I and core sections of the groundmass
olivine (Fig. 6). Moreover, in many cases the olivine-II cores have original crystal faces
ground away (Fig. 7), and thus their shapes are similar to those of round olivine-I. It is most
likely that crystals that now show as relics in the olivine-II cores were formed at depth and
transported upwards in a crystal mush.
Morphological and chemical resemblance between olivine-I and cores of olivine-II can be
related to similar chemical and physical conditions exerted during olivine growth (or re-
crystallisation) and transport to the surface. If both olivine populations are related, their
common origin might be tracked down to the earliest and deepest stages of the kimberlite
evolutionary story, i.e. when and where primary (protokimberlite) magma derived and
started ascent.
7.7 Evolutionary storyline of the kimberlite parental melt
The Udachnaya-East groundmass olivine has a clear compositional structure, where the
cores with variable Fo values can be distinguished from the rims with limited range in Fo
values (Fig. 6b). It should be emphasised again that the olivine-II rims are essentially
uniform with respect to major elements, but minor elements fluctuate strongly, especially Ni
abundances which reach maximum near the core-rim boundary, then decrease rapidly
towards the outer rims (Fig. 6b). Broadly similar compositional features, namely two groups
of olivine with normal and reversed core to rim zonation and similar ranges in Fo and trace
element contents, have been previously described in the groundmass olivine in other
kimberlites, diamondiferous and barren (Fedortchouk & Canil, 2004; Moore, 1988; Skinner,
1989).
Although the origin of olivine cores (cognate vs exotic) is still debatable, the overall
compositional analogy between groundmass olivine from different pipes and different
kimberlite provinces argue for that 1) origin of cores and rims of groundmass olivine are
intimately linked to kimberlite genesis and evolution; 2) in each case physical and chemical
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