Geoscience Reference
In-Depth Information
abundant olivine macrocrysts and phenocrysts, whereas significant CO
2
and H
2
O contents
are attributed respectively to carbonate minerals (calcite and dolomite) and serpentine (+
other H
2
O-bearing magnesian silicates). Unfortunately, the masking effects of deuteric and
post-magmatic alteration do not permit routine recognition of olivine generations, and so
the olivine component originally dissolved in the kimberlite parental melt remains
controversial (Brett et al., 2009; Francis & Patterson, 2009; Mitchell & Tappe, 2010; Patterson
et al., 2009). Similarly, the original magmatic abundances of volatile and fluid-mobile alkali
elements are disturbed by syn- and post-emplacement modifications, thus complicating
complicating quantification of the parental melt composition if inferred from bulk
kimberlite analyses.
The existing dogma about correspondence between compositions of whole rock kimberlites
and their parental melt has been recently challenged by the newcomers to the kimberlite
scientific community (e.g., Kamenetsky et al., 2004; Kamenetsky et al., 2007a; Kamenetsky et
al., 2007b; Kamenetsky et al., 2008; Kamenetsky et al., 2009a; Kamenetsky et al., 2009b;
Kamenetsky et al., 2009c; Maas et al., 2005). A breakthrough into understanding of the
kimberlite magma chemical and physical characteristics was made possible by detailed
studies of the diamondiferous Udachnaya-East kimberlite pipe in Siberia. Unlike other
kimberlites worldwide, severely modified by syn- and post-magmatic changes, the
Udachnaya-East kimberlite is the only known fresh rock of this type, and thus it is
invaluable source of information on the composition and temperature of primary melt, its
mantle source, rheological properties of ascending kimberlite magma. This kimberlite
preserved unequivocal evidence for olivine populations, olivine paragenetic assemblages
and olivine-hosted melt inclusions, and the role of mantle-derived alkali carbonate and
alkali chloride components in the parental melt.
2. Udachnaya-East kimberlite: Location and samples
The Udachnaya diamondiferous kimberlite pipe is located in the Daldyn-Alakit region of
the Siberian diamondiferous kimberlite province (Fig. 1). Most Siberian pipes are tuff-
breccias essentially devoid of unaltered olivine, but some contain large blocks of massive
fresh kimberlite. A remarkable characteristic of this region is that it contains more pipes
with fresh, unaltered olivine than any other kimberlite region within the Siberian province.
About 10% of the intrusions exhibit either two adjacent channelways or repeated intrusion
of magma through the same chimney. The Udachnaya pipe, the best known example of
these twin diatremes, is located in the northwest part of the Daldyn field (Fig. 1). At the
surface it consists of two adjacent bodies (East and West) that are separated at depth >250-
270 m. Based on stratigraphic relationships both intrusions formed at the Devonian-
Carboniferous boundary (~350 Ma), and the age estimates vary from 389 to 335 Ma (Burgess
et al., 1992; Kamenetsky et al., 2009c; Kinny et al., 1997; Maas et al., 2005; Maslovskaja et al.,
1983). The eastern and western bodies of the Udachnaya kimberlite pipe are different from
each other in terms of mineralogy, petrography, composition, and degree of alteration. As
the alteration of the western pipe can be considered typical of this rock type, the rocks of the
Udachnaya-East are unique in having lesser alteration, and in some places they are
completely unaltered.
At depths greater than 350 m a particularly fresh porphyritic kimberlite has been found.
These rocks are described as dark-grey massive kimberlite, characterized by unaltered
euhedral-subhedral olivine phenocrysts set in a dominantly carbonate matrix (Marshintsev
Search WWH ::
Custom Search