Trace Elements in Mafic and Ultramafic Xenoliths from Kimberlites of Yakutia

Zdislav Spetsius Institute of Diamond Industry, Mirny, Yakutia, Russia

More than 60 xenoliths from Udachnaya and Obnazhonnaya kimberlite pipes were investigated for trace elements abundance in their silicate, oxide and sulfide minerals on the HIAF proton microprobe (PMP) and partly by the sensitive, high resolution microprobe (SHRIMP). Investigated samples have been chosen from different varieties of xenoliths, including diamondiferous.

Results and Discussion

PMP and SHRIMP data of trace elements displayed a high their variation, especially for eclogites from Udachnaya. Collation of our and literature data has shown some differences and inconsistency in content and distribution of trace elements of mantle xenoliths from this pipe in comparison with those from Mir or Obnazhonnaya and analogues rocks from kimberlites of South Africa. Salient peculiarities can be summarized as follows: 1) The subcalcic garnets in megacrystalline peridotites from Udachnaya are more depleted in Zr and Y and more enriched in Sr than those in either Siberian or Kaapvaal lherzolites (Pearson et al., 1994); 2) Garnets from eclogite xenoliths of Udachnaya show similar trace element concentrations to those from eclogites of South Africa(Griffin et al., 1988;), but in detail, they range to more less Ni values and depleted in Y, Zr
and Sr; 3) Eclogitic clinopyroxenes show a wide range in Ni (45-1500 ppm). By comparison with clinopyroxenes from eclogites of South Africa and those of inclusions in diamonds of eclogitic paragenesis from Monastery and Argyle (Griffin et al., 1988; Moore et al., 1989) they range to higher Sr values and are slightly depleted in Y and Zr concentrations and enriched in Ga and Ni (Spetsius and Griffin, 1995);
4) Udachnaya clinopyroxenes are more LREE-depleted than from pipes Mir or Obnazhonnaya or those from South Africa and less radiogenic in Sr isotopic composition (Jacob et al., 1994; Snyder et al., 1995).

The singular nature of the Udachnaya eclogites were explained by Snyder et al.(1995) as a result of more deeper conditions of their formation and long time of mantle equilibration. It is a reasonable explanation of some signatures
of these rocks but others provide evidence of the later metasomatism and partial melting of xenoliths. The following features are particularly noteworthy: (a) complex metasomatic history of peridotitic xenoliths confirmed their
Nd-Sr and Re-Os systematics (Pearson et al., 1994);
(b) chemical zoning of garnets in peridotites (Shimizu et al., 1994); (c) enhanced abundances of the HFSE (Nb, Zr and Hf) in the host garnet in comparison with inclusion in diamond of eclogitic xenolith due to metasomatic enrichment by passing melt (Ireland et al., 1994); (d) evidence that some sulfides and partly rutile in eclogites are related to metasomatic and/or partial melting processes.


Our data suggest that the most part of xenoliths from Udachnaya have undergone metasomatism and/or partial melting, which define partly the trace elements signatures
of these rocks. Eclogitic mantle substance under Siberian platform has experienced more stronger enrichment by some LIL and HFSE than under South African or Australian cratons and is slightly depleted in some other trace elements. Abundances of trace elements in eclogites and some peridotites are determined by the conditions of formation in the mantle and essentially depend on the subsequent processes of metasomatism and partial melting. Estimation of the contribution of each component to the content of trace elements in mantle xenoliths will give an insight into
the geochemistry and evolution of the mantle. It must be admitted that such data cannot be obtained by simple determination of trace elements in rocks and minerals but may be done only by careful petrographic study of being analyzed samples and especially by investigation of trace elements in diamond inclusions.


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