MANTLE SOURCES OF THE KIMBERLITE VOLCANISM OF THE SIBERIAN PLATFORM

Abstract

The paper presents results of the investigation of the major-, trace-, and rare-earth element compositions of kimberlites from various regions of the Yakutian diamond-bearing province with application to determining mantle sources of their melts. The kimberlites of the Middle Markha, Daldyn-Alakit, and Upper Muna regions (data on the Malaya Botuobiya region were taken from the literature) were analyzed by the methods of X-ray fluorescence and mass spectrometry with inductively coupled plasma. These regions are controlled by two main northeast-trending fault zones (mineragenic zones): Vilyui-Markha and Daldyn-Olenek. Essentially all the kimberlites studied are similar to the group I kimberlites of southern Africa with respect to trace element ratios: Zr/Nb < 2, Nb/La = 1.33-2.29, and Ce/Sr = 0.04-0.29. An exception is the kimberlite of the diamond-rich Nyurbinskaya pipe (Middle Markha region), which shows Zr/Nb = 2.2 (2.6 for the autolith of kimberlite breccia) typical of the group II kimberlite of southern Africa. The kimberlite of this pipe is also conspicuous in (1) high Ba/Nb, Rb/Nb, Ba/Th, Ba/La, Pb/U, and Th/U; (2) low La/Yb, Th/Pb, and Rb/Sr; (3) elevated contents of large ion lithophile (LIL) elements (Rb, Cs, Pb, Sr, and Li) and Ta; and (4) minima at high-field strength (HFS) elements (Ti, U, and Th). These features suggest that the mantle source of the kimberlite melt was depleted in HFS and slightly enriched in LIL elements by the process of mantle metasomatism. These and some other characteristics of the Middle Markha kimberlites suggest the existence of a slightly LILE enriched mantle source of the EMI type. The geochemical and isotopic characteristics of kimberlites from two main diamond-bearing regions of the Yakutian province (Malaya Botuobiya and Daldyn-Alakit) and the Lower Olenek region suggest that they were formed from a BSE-type mantle reservoir (areas of the most primitive isotopically homogeneous mantle). The kimberlites of the Upper Muna region show high U/Pb = 0.72-1.11 and Th/Pb = 4.15-5.77 in contrast to low Rb/Sr = 0.00-0.11, which indicates a mantle source of the HIMU type enriched in U and Th relative to Pb. On the basis of these data, it is believed that there is a northeast-trending lateral zoning with respect to the type of mantle reservoirs within the two main mineragenic zones of the Yakutian diamond-bearing province.