Geochemical types of kimberlites and their mantle sources
A.V. Lapin * , A.V. Tolstov **
*The Institute of Mineralogy, Geochemistry and Crystal Chemistry of Rare Elements. Moscow, Russia. **Botuoba Geoexploration Expedition ,ALROSA, Mirnyi , Yakutia, Russia.
Revealing of geochemical heterogeneity of kimberlites became, in recent years, one of the main achievements in study of the abyssal mantle magmatism. It was firstly demonstrated in connection with discovery of Arkhangelsk diamond bearing province (Arkhangelsk Diamondiferous Province, 2000).Geochemical parameters of kimberlites in this province proved to be rather different from rock parameters of traditional diamond bearing districts of Yakutia, that can be recognized as a formation standard of diamond bearing kimberlites. Later, after discovery of diamond bearing kimberlites in Middle-Markha district (Nakynsk field) , the geochemical heterogeneity of kimberlites was defined within Yakutsk province itself (Lapin, Tolstov, 2007). Similar, geochemically unusual kimberlites were also recently revealed in Slave Province, Canada (Pokhilenko et al., 2003; Lapin, Tolstov, 2005).
Relatively late discovery of a number of unusual manifestations of kimberlites is explained by abnormal properties of these rocks, which are hardly detected by conventional exploration methods. They are characterized by extremely low ilmenite contents and, consequently by low magnetization and low degree of contrast in the magnetic field. Pyrope content in rocks (the main indicator element for kimberlites) is often not much higher than diamond content, which makes it difficult to use mineralogical research methods. However, the most radical difference between newly revealed occurrences of kimberlites and rocks of the formation petrotype concerns geochemical properties of rocks. The received representative parameters of two kimberlite types show the high degree of their geochemical contrast, which concerns, first of all, a group of high charge rare and radioactive elements and has deep genetic reasons.
Taking into account the available data for kimberlites of the formation petrotype, which has moderately high concentration of titanium and HFS elements, we can recommend the name ôkimberlites of geochemical type Nö; the rocks with very low content of HFS elements and relatively poor in titanium (kimberlites of Zolotitskoe field in Arkhangelsk Province and Nakynsk field, Yakutia) can be called ôkimberlites of geochemical type Dö. Ce/Y and Nb/Zr ratios, alongside with Ce, Nb, Zr, U, and Th concentrations, that are higher in kimberlites of N type, are important indicators for determination of a geochemical type of kimberlites. Correlation relations of elements (Nb ľ Ti, Ce ľ Y, Ce ľ P and oth.) are individual in the rocks of two geochemical types (Fig. 1, 2).
Fig. 1. Correlations between the contents of (a) Zr and Nb,(b) Th and U in the kimberlites of the Nakyn and Zolotitsa fields and traditional diamond-bearing regions of Yakutia. (1) Kimberlites from the traditional diamond-bearing regions of Yakutia (Mir, International,Udachnaya, Aikhal, Yubileinaya, Komsomolĺskaya, 23rd Party Congress, Taezhnaya, and Dachnaya pipes); (2) kimberlites from the Nakyn field of Yakutia (Botuobiya and Nyurbinskaya pipes); (3) V. Grib pipe in the Verkhotina field of the Arkhangelsk province; and (4) kimberlites of the Zolotitsa field of the Arkhangelsk province (Lomonosovskaya, Pionerskaya, and Karpinskii pipes).
Fig. 2. Correlations between the contents of (a) Ce and Y and (b) Ce and P2O5 in the kimberlites of the Middle Markha region and traditional diamond-bearing regions of Yakutia. Kimberlite pipes: (1) Yubileinaya, (2) Komsomolĺskaya, (3) Dachnaya, (4) Udachnaya, (5) Aikhal, (6) International, (7) Mir, (8) 23rd Party Congress, (9) Taezhnaya, (10) Botuobiya, and (11) Nyurbinskaya.
Contrast differences in HFS elements distribution in petrochemically rather homogeneous types of rocks result from the particular geochemical history of these elements in mantle rocks, which differs from geochemical history of main components and coherent admixture elements (Cr, Ni, V and oth.). According to the analysis, the main factor, determining distribution of non-coherent HFS elements in the mantle kimberlite source, is abyssal metasomatism, caused by juvenile fluids flows, which is always accompanied by significant introduction of alkalies, titanium, high charge rare and radioactive elements. Thus, there is a good reason to consider the lithosphere substrate, metasomatized under the influence of juvenile abyssal fluids, as a mantle source of kimberlites, belonging to the geochemical type N. It is also clear, that generation of kimberlites type D takes place under different conditions. Geochemical properties of this type of kimberlites and their confinement to collision margins of ancient cratons and terrains allows to think, that rocks with negative HFS elements and titanium anomaly are generated from the lithosphere substrate that was metasomatized under the influence of volatiles introduced from the subducted oceanic crust.
The obtained data essentially change the idea of kimberlites as a geochemical object. The outlined concept has a number of petrologic, mineralogical and exploration results. For petrology, it is important to get the reliable criteria for participation of subduction processes in the mantle metasomatism and magmatism. For diamond minerageny it is important , that the degree of kimberlite diamond content does not depend on their geochemical type and, hence, the diamond rather falls into the category of protomineral of the mantle substrate, the subsequent metasomatism and fusion of which, as well as transportation of the abyssal matter with kimberlite magma, can be accompanied by crushing, growth and solution of diamond. In addition, the adequate geochemical image of kimberlites, which essentially changes in the light of new data, is important for prediction of diamond bearing magmatites and diamond exploration geochemistry.
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