High- ³ He Plume Origin and Temporal-Spatial Evolution of the Siberian Flood Basalts

Abstract

An olivine nephelinite from the lower part of a thick alkalic ultrabasic and mafic sequence of volcanic rocks of the northeastern part of the Siberian flood basalt province (SFBP) yielded a (40)Ar/(39)Ar plateau age of 253.3 +/- 2.6 million years, distinctly older than the main tholeiitic pulse of the SFBP at 250.0 million years. Olivine phenocrysts of this rock showed (3)He/(4)He ratios up to 12.7 times the atmospheric ratio; these values suggest a lower mantle plume origin. The neodymium and strontium isotopes, rare earth element concentration patterns, and cerium/lead ratios of the associated rocks were also consistent with their derivation from a near-chondritic, primitive plume. Geochemical data from the 250-million-year-old volcanic rocks higher up in the sequence indicate interaction of this high-(3)He SFBP plume with a suboceanic-type upper mantle beneath Siberia.

On the ages of flood basalt events

“…There was no documented subsequent successful rifting (apart from possible continental rifting in the Kazakhstan basin [37]) which may in part be responsible for the absence of an 'age tail', as found for instance in the Paraná. Basu et al [12] determined an 40 Ar/ 39 Ar age of 253.0 ± 2.6 Ma in alkalic-ultramafic lavas infilling a pre-existing Late Permian graben in the Northeast; this might be an indication of a slightly older onset of volcanism, though we find little convincing evidence, for this (Wignall's figure 5 [190], comparing the timing of the various events, is misleading, because it ignores these uncertainties). The basal parts of the Siberian traps contain a high proportion of fragmental basaltic lavas.…”

Evaluation of different models for the origin of the Siberian Traps

“…3). Basu et al (1995) (Renne and Basu, 1991). However, Kamo et al (2003) reported the UPb perovskite age of 251.7 ± 0.4 Ma for the Arydjansky suite, which is indistinguishable from the Permo-Triassic U-Pb zircon and baddeleyite age 251.2 ± 0.3 Ma for the Noril'sk-I orebearing intrusion at Noril'sk-Kharaelakh subprovince (Kamo et al, 1996).…”

“…3. Relative probability distribution of 40 Ar/ 39 Ar ages (after Ivanov et al, 2006) and original ages (from Baksi and Farrar, 1991;Basu et al, 1995;Dalrymple et al, 1995;Reichow et al, 2002;Renne and Basu, 1991;Venkatesan et al, 1997;Walderhaug et al, 2005). The Permo-Triassic boundary is from Renne et al (1995).…”

The Siberian Traps and the End-Permian mass extinction: a critical review

“…Published 40 Ar/ 39 Ar determinations (whole-rock and plagioclase separates) exist for basalts from Noril'sk [14,22,23] , Putorana [14] , Maymecha-Kotuy [24] and the West Siberian Basin [25] (Figure 3 [16] at Meishan, China (see below). Zircons and baddeleyites from intrusions at Noril'sk, together with perovskite, zircon and baddeleyite from Maymecha-Kotuy, give U/Pb concordia ages that are slightly but significantly older than the 40 Ar/ 39 Ar ages.…”

“…Error propagation was established by including the uncertainty of each age as the square root of the sum of the squares of the errors. Sources of data: green symbols: Reichow et al [1] ; red symbols: Noril'sk [14,22,23] , Putorana [14] , Maymecha-Kotuy [24] , and West Siberian Basin [25] . Data for Meishan Bed 28 are from Reichow et al [1] .…”