V.B. Vasilenko

Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, Novosibirsk, 630090 Russia

The Yakutian Kimberlite Province includes 24 volcanic rock fields. Their compositions vary from deep diamondiferous kimberlites in the south to ultrabasic–alkalic rocks with carbonatites in the north. The decrease of depths of melt generation tend to correspond to the decrease of the depth of the lithosphere bottom (see figure).

Location of kimberlite fields in the Yakutian Diamondiferous Province.

1, Mirny; 2, Nakyn; 3, Alakit-Markha; 4, Daldyn; 5, Verkhne-Muna; 6, Chomurdakh; 7, Ogoner-Yuryakh; 8, West Ukukit; 9, East Ukukit; 10, Merchimden; 11, Molodo; 12, Toluop; 13, Kuoik; 14, Khorbusuon; 15, Tomtor; 16, Ebelyakh; 17, Yargin; 18, Starorechenskoe; 19, Ary-Mastakh; 20, Dyuken; 21, Luchakan; 22, Birigindin; 23, Kuranakh; 24, Anabar.

Kimberlites. Melting of peridotites saturated with carbon dioxide occurs by transfer to the liquidus of the clinopyroxene–olivine cotectic. The composition of the first drops of the selective melt corresponded to calcite, whose amount could be as large as 80% of the parental melt volume. Higher contents of calcite in the parental melt resulted in metasomatism of syngenetic olivine and restite peridotite. During further melting, the proportion of olivine in kimberlites increased up to the formation of high-olivine varieties. Olivine accumulation occurred step-by-step; therefore, 12 varieties can be recognized in kimberlites according to the CaO : MgO ratio. The volume ratio between carbonate and olivine varieties can be different, depending on the composition of the magma-producing matter. Predominance of calcite-rich rocks results in formation of carbonate kimberlites. They constitute no less than 15% in major diamond fields of Yakutia. As a rule, they are prepipe or postpipe carbonate-rich rocks. The typochemical feature of these rocks is the inverse correlation between MgO and CaO. This phenomenon reflects the evolution of thermal conditions of the melting of kimberlite parental melts at temperatures within 900–1500œ. Melts of diamondiferous kimberlites form at depths within approximately 100–200 km or, possibly, even deeper. Part of diamonds may be of xenogenous origin, but the majority of them formed concurrently with kimberlite parental melts, whose composition determined the amount of idiogenous diamonds.

Above the aforementioned depths, the lithosphere section is geochemically heterogeneous even in the model with uniform peridotite composition. Under increasing pressure, admixing elements are redistributed over peridotite minerals. In peridotite clinopyroxene, the titanium admixture is redistributed to other minerals, whereas potassium is accumulated. Each range of the lithosphere section is characterized by a certain TiO₂ : K₂O ratio in clinopyroxenes. Hence, the contents of titanium and potassium in kimberlite parental melts depend on the depth ranges where they formed.

Thousands of chemical analyses of Yakutian kimberlites were considered and subdivided into seven populations, the first of them being the deepest, and the seventh, the shallowest. They differed in the contents of TiO₂ and K₂O. Ranking of the mean contents of these oxides in the populations shows that TiO₂ contents form a sequence within  3.0–0.3%. Correspondingly, the mean K₂O contents form an inverse sequence. The population with the lowest TiO₂ content contains 0.85% K₂O. Higher  K₂O contents in kimberlites are produced by subducted oceanic crust fragments entering the magma formation zone. The thermal factor, namely, the inverse correlation between CaO and MgO, is clearly observable in each population. The deepest populations have the highest diamond potential.

Mean TiO₂ contents over all kimberlite bodies within each kimberlite field form sequences from the deepest to shallowest populations. This phenomenon results from vertical migration of magmatic chambers and sequential formation of kimberlite bodies. The deepest (1 and 2) kimberlite populations are absent from some kimberlite fields (Daldyn and Verkhne-Muna) because of the rise of the lithosphere bottom and reduction of its thickness. Thus, the geochemical zoning of the lithosphere is responsible for type geochemical indices of the P–T conditions of kimberlite formation and diamond potential.

Picrites. This statement is confirmed by the occurrence of picrites, rocks different from kimberlites, in northern fields of the province, where they are often present in the same bodies as kimberlites. The maximum contents of picrites are recorded in the Ary-Mastakh  and Dyuken fields of the Anabar group of fields and in the  West Ukukit and  Ogoner-Yuryakh fields of the Olenek group. The predominance of picrites in northern fields of the province indicates that the lithosphere is thinner there. The depths of the formation of the picrite parental melts vary approximately from  –100 to –70 km. Picrites show the same features as kimberlites except for higher TiO₂ contents and absence of diamonds and their associates. Carbonate varieties are typical for picrites, as well  as for kimberlites. However, their  proportion is larger there.

Alkalic–ultrabasic rocks with carbonatites occur most often in the Tomtor field of the province. The depth of the formation of their parental melts is estimated to be 60–70 km. They are characterized, first of all, by separate bodies of carbonate rocks (carbonatites) enriched in rare earths. The silicate and silicate–carbonate rocks of the Tomtor field can  be subdivided into two groups: rich in CaO and rich in Al₂O₃. The former can be assigned to the carbonatite association, and the other, to ultrabasic foidolites (see table). In our opinion, these rocks form in a way similar to that of kimberlites. In both cases lime is leached from silicate minerals of basaltoid rocks in the course of carbonate metasomatism.

The variation of rock compositions from diamondiferous kimberlites in the south to carbonatites of the Tomtor field in the north is characterized by higher contents of TiO₂ and SFe₂O₃ in kimberlites of northern fields, dramatic increase of TiO₂ and SFe₂O₃ contents in picrites of the same fields. The typochemical feature of the carbonate rocks, kimberlites and picrites, is the inverse correlation between CaO and MgO, whereas in the carbonate rocks of the Tomtor field the typochemical feature is the inverse correlation between CaO и Al₂O₃.

A common feature of rocks of all assemblages is the presence of carbonate matter and predominance of potassium. These features may have arisen from widespread subduction of the oceanic crust from the Upper Yana fold–thrust belt. This subduction favored the emergence of mantle plumes and their penetration to the lithospheric mantle beneath the Yakutian Diamondiferous Province.

Table. Composition of rocks of various assemblages in the Yakutian Diamondiferous Province