Bitumens in the Mir Kimberlite Pipe (Yakutia)

Abstract

Solid bitumens in the Mir kimberlite pipe form vein-like segregations several centimeters in size. They are distributed irregularly in the pipe body, regardless of the kimberlite breccia varieties distribution. The bitumen content in the kimberlites ranges from 0.001 to 0.12 wt %. Bitumen-rich areas are confined to the pipe contacts and faults. The question of the origin of bitumens in kimberlites is hotly debated. Therefore, this work is aimed at determining their origin in the Mir pipe. The studied bitumen-bearing kimberlite samples were collected in the Mir pipe, from depths of 100 and 130 m. The chloroform bitumoid was extracted from bitumen. The carbon isotope compositions of bitumoids were determined, and biomarker analysis was carried out by gas chromatography mass spectrometry. Biomarker hydrocarbons were detected in the bitumoid, indicating a biogenic origin of the organic matter. The following biomarkers were identified: n-alkanes, isoprenoids Pr and Ph, tri- and pentacyclic terpanes–hopanes, and steranes. The pristane to phytane ratio Pr/Ph = 0.8 indicates reducing conditions of formation corresponding to marine environment. The oddness ratio of n-alkanes, sterane and hopane maturity indices show that the thermal maturity of organic matter (OM) corresponds to the initial stage of the oil formation. The low content of long-chain alkanes and the predominance of C29 over C27 among regular steranes (C29/C27 = 2.2) allow us to assume that the initial biota for the OM of the bitumen sample under study could be phytoplankton. The δ¹³C_VPDB values of the studied bitumoid (from –29.4 to –31.6‰) correspond to the isotopic compositions of bitumoids in the Paleozoic and Mesozoic sediments of the Siberian Platform (from –25.8 to –33.8‰) and differ significantly from the isotopic composition of endogenous carbon (from –2 to –10‰). According to these features, the investigated bitumen from the Mir pipe is of marine origin.