A comparison of Siberian meimechites and kimberlites: Implications for the source of high‐Mg alkalic magmas and flood basalts

Abstract

Chemical and radiogenic isotope (Sr, Nd, Hf, Os, and Pb) data are presented for a variety of mafic‐alkalic rocks from the Maymecha‐Kotuy section of the Siberian flood‐volcanic province. These data are compared to a similar data set for Siberian kimberlites that were emplaced both before and after the flood‐volcanic event in order to examine the spatial‐temporal evolution of Paleozoic magma sources in the mantle beneath this site of voluminous magmatic activity. As shown in previous studies, the high‐Mg, meimechitic composition rocks extend the range in Sr and Nd isotopic composition seen in the flood basalts in the direction of more “depleted” compositions, i.e., higher 143Nd/144Nd and lower 87Sr/86Sr, overlapping values typically observed in intraplate ocean‐island basalts. Sr, Nd, Hf, and Pb isotopic compositions show little correlation with major‐ and trace‐element compositions in the Maymecha‐Kotuy rocks. Os isotopic compositions, on the other hand, show rough correlations with a number of major‐element characteristics of the magmas. The Os data suggest that the magma sources range from peridotite for the meimechitic magmas to a mixture of peridotite and pyroxenite for the nephelinitic, melilititic, and trachybasaltic compositions. The isotopic overlap of both old and young kimberlites with the Maymecha‐Kotuy rocks is consistent with all these magmas being derived from mantle sources that were present beneath Siberia long before, and long after, the flood‐volcanic event. The isotopic characteristics of the mantle source of these magmas best match the FOZO component observed in ocean‐island basalts, which suggests that this mantle composition may be prevalent in the upper mantle outside of ocean basins. The long‐lived presence of this source beneath Siberia makes it unnecessary to appeal to a mechanism, such as a plume, to bring this type of mantle into play only during the flood‐volcanic episode.