The zonal distribution of
trace elements in garnets
deformed lherzolites in the Udachnaya pipe (Yakutia)
Kalashnikova T. V.*,
Solov’eva L. V.**, Kostrovitsky*
* - Vinogradov Institute of
geochemistry, Siberian Branch of RAS, Irkutsk, Russia
** - Institute of Earth
crust, Siberian Branch of RAS, Irkutsk, Russia Solv777@crust.irk.ru
The development of
Middle-Paleozoic kimberlite cycle in Siberian craton is supposed to
connect with Yakutian plume [Ernst and Buchan, 1997; Solov’eva, 2008].
The sources of basic melts evolved in this period beneath lithosphere of
Siberian craton. The melts percolated through substance of upper
asthenoshere and lower lithosphere and effected metasomatic modification
of rocks on the boundary lithosphere-asthenosphere. The exchange of
trace elements occurred between melts and solid matter. As a result the
low-Cr megacrysts’ series and deformed peridotites were generated. The
latter are original mantle blastomylonites [Solov’eva et. al, 2008].
Two petrographic types
(coarse-porphyroclastic and fine-porphyroclastic) were distinguished
among deformed peridotites. We investigated the trace element
distribution in garnet grains from two samples of deformed lherzolites
related to these different types.
The distribution of trace
element concentrations normalized to chondrite in two garnet grains from
coarse-porphyroclastic Grt lherzolite 00-69 are showed on plot (Fig. 1
a, b). The REE normal distribution with consistent increase from La to
Yb is fixed in both grains. The distribution of all trace elements is
similar to that in garnet from low-Cr Grt megacrysts and coarse-porphyroclastic
lherzolites from Udachnaya pipe [Solov’eva et. al, 2008]. The Sr - La
minimum and Ta – Hf maximum are evidently seen on all lines (Fig. 1 a,
b). The weak increase of normalized concentrations in succession La - Gd
and for Ba, U, Ta (line 3, Fig. 1a; line P1, Fig. 1b) is evident. Nearby
kelyphytic rims on garnet grains (line 4, Fig. 1a; lines P5-P6, Fig. 1b)
the normalized concentrations La - Yb grow with the most increasing for
elements Zr - Y. The maximums Zr, Hf and Ti are prominent and the lines
get essentially flat from Gd to Yb.
The distribution of
incompatible trace elements is different in principle for garnet in
fine-porphyroclastic lherzolite 4-06 (Fig. 1c). In central part of grain
(lines 1, 2, 2a) sinusoid form of REE distribution is observed. The such
form is usual for low-temperature coarse-grained peridotites
representing lithosphere mantle [Solov’eva et. al, 2008]. The
distinctive minimuma for Ba, Sr, Zr, Hf, Dy, Y and maximuma for U are
seen in these lines. In centre of grain (line 1, fig. 1c) Ti
demonstrated weak maximum, replacing for noticeable maximum toward
kelyphytic rim (lines 2, 2a). Then lines for majority of elements move
up in the direction of grain rim. The line of near rim border (4) are
similar to line of kelyphytic rim (5). These lines are demonsrated sharp
maximum of Ba, Ta, Zr, Hf, Ti, minimum of Ti and weaken maximum of U. In
central part of Grt grain from 4-06 the incompatible trace element
distribution is similar to that of central part of Grt-grains in fine-porphyroclastic
deformed peridotites and low-temperature coarse-grained peridotites from
Udachnaya pipe. The type of incompatible trace element distribution get
similar to that in Grt from coarse-porphyroclastic lherzolite toward
Thus, in coarse-porphyroclastic
type the normal distribution of REE is observed with various
concentration level in different zones. The REE distribution zonality
indicate disequilibrated crystallization process and its development
shortly before trapping by kimberlite melts. In fine-porphyroclastic
lherzolite 4-06 the central part of Grt grain preserved trace elements
distribution corresponding to that in Grt from low-temperature
coarse-grained lherzolites from lithosphere mantle.
It may suppose that coarse-porphyroclastic
lherzolite present the asthenosphere matter, whereas fine-porphyroclastic
lherzolite is lithosphere substance getting into percolating asthosphere
melt. The such trace elements distribution may be explained the rising
of thermo-chemical Yakutian plume to Siberian craton lithosphere base.
The plume effected mechanical and chemical erosion of lithosphere plate
and involved lithosphere substance in the form of slaces and xenoliths
to convective plastic asthenosphere.
The distinction of trace
elements distribution are determined between garnets and clinopyroxenes
in different zones from these minerals. This fact may be evidence of
these xenoliths staying in different sources of asthenosphere melts.
As a whole, considered
peculiarities of incompatible trace elements behavior in zonal Grt
grains don’t contradict the model of percolating melt with synchronous
crystallization (“percolative fractional crystallization”- [Harte et
al., 1993]). It may assume that in xenolith 4-06 the change of
composition in garnet occurred due to elements diffusion from
intergranular melt in early existing grain rather than melt
Fig. 1. COMP photos of
garnet grains - on the left.
a) – Sample 00-69 – the
part of more large Grt grain, in centre – oval Cpx inclusion;
b) – Sample 00-69 – the
part of another Grt grain with former melt inclusion;
c) – Sample 4-06 – the part
of Grt grain.
On the right – C1 chondrite-normalized
[McDonough and Sun, 1995] trace elements concentration spidergrams in
Grt grains (00-69 and 4-06). Line numbers are corresponding to analyses
points on left photos.
The work was supported by
grant RFFI – 07-05-00589-a.
Solov’eva, L.V., Lavrent’ev,
Yu.G., Egorov, K.N., Kostrovitsky, S.I., Korolyuk, V.N., & Suvorova, L.F.
(2008) The genetic relationship of the deformed peridotites and garnet
megacrysts from kimberlites with asthenospheric melts. Russian Geology
and Geophysics, v.49(4), pp.207-224.
Ernst R.E., Buchan K.L.
Giant radiating dyke swarms: Their use in indetifying pre-Mesozoic large
igneous provinces and mantle plumes. In: Large igneous provinces:
continental, oceanic and planetary volcanism. Am. Geophys. Union. Monogr.
100. P. 297 – 333.
Harte B., Hunter R.H.,
Kinny P.D. Melt geometry, movement and crystallization, in relation to
mantle dykes, veins and metasomatism. In: Philosophical Transaction of
the Royal Society of London. Series A. 1993. V. 342. P. 1–21.
McDonough W.F., Sun S.-S.
The composition of the Earth. Chem. Geol. - 1995. V. 120. - P. 223-253.