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

[1] 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.