Siberian Traps Geochemistry

All of the analysed basalts from Noril'sk, Putorana and the West Siberian Basin have evolved compositions, indicating extensive fractionation of the melts following their segregation from a peridotite mantle. Many suites are also evidence of contamination of the original magmas. This is shown by the high La/Sm (Figure 2, opposite), Th/Ta, ⁸⁷Sr/⁸⁶Sr and low ¹⁴³Nd/¹⁴⁴Nd of, for example, the Ivakinsky, Syverminsky and Nadezhdinsky suites (Fedorenko et al., 1996). The constant Th/Ta ratios of many of these suites also indicates that the contamination occurred early in the evolution of the basalts. The contaminant may be continental crust (e.g., Arndt et al., 1992).
The depth of segregation of the magmas appears to have changed through time. Thus, the lower suites have significantly higher Sm/Yb ratios (Figure 1), consistent with deeper, and more limited extents of melting in equilibrium with garnet. With time, the Sm/Yb ratio has decreased. This could be explained by shallower melting without garnet being present, or more extensive melting which has resulted in garnet being exhausted from the source.

Sm/Yb (Fig. 1) and La/Sm (Fig. 2) versus nominal depth for Siberian Trap basalts from the Noril'sk region. These plots show the large compositional range - related to age - in the Trap basalts, and also indicate some of the processes that may have occurred. Qualitatively, the Sm/Yb values indicate depth or extent of melting in the mantle. High Sm/Yb ratios in the older basalts are generated by small degrees of melting at greater depths (~100 km or deeper) whereas Sm/Yb decreases during more shallow melting. This is due to the preferential incorporation of Yb over Sm into garnet. The decreasing Sm/Yb values suggest that the average depth of melting has shallowed, and/or become more extensive, during emplacement of younger Noril'sk lavas. La/Sm provides a useful index for the amount of crustal contamination of the magmas, with high La/Sm indicating more contamination. This applies in particular to the Nadezhdinsky, Ivakinsky, and Syverminsky Suites (Figure 2). Data sources: Lightfoot et al. (1990, 1993); Wooden et al. (1993); Hawkesworth et al. (1995).

Determining the nature of the original source material (depleted upper mantle, a more deeply sourced 'enriched plume' mantle , or a primordial mantle) is fraught with difficulty because of the pervasive contamination (see discussion by Sharma, 1997). This has effectively masked the composition of the mantle-derived liquids for most of the suites.
The fractionated character of the basalts raises some intriguing possibilities, however. On the one hand, if they were derived from a peridotite source, this implies that the primary magmas were substantially more magnesian than those sampled to date. Were these dense, high-Mg magmas trapped deep in the crust, where they maintained large magma bodies (such large bodies are required for the production of the voluminous flows)? On the other hand, were the primary magmas produced by melting of an eclogite- or pyroxenite-rich source, as has been proposed for some Hawaiian basalts (Sobolev et al., 2005), in which case the amount of fractionation may have been substantially less?