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Abstracts of International conference

Ore potential of alkaline, kimberlite

and carbonatite magmatism

The zonal distribution of trace elements in garnets

from 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 Kalashnikova@igc.irk.ru

** - 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 grain rim.

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 crystallization.

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.

References:

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. 1997. Ò. 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.