Evidence for subduction-related components in the subcontinental mantle from low 3He/ 4He and 40Ar/ 36Ar ratio in mantle xenoliths from Far Eastern Russia

Abstract

By applying both vacuum crushing and stepwise heating methods for the extraction of the noble gases, we have discovered 3He/ 4He ratios much lower than the atmospheric ratio (∼0.3 R A; R A is the atmospheric 3He/ 4He ratio of 1.4×10 −6) and relatively low 40Ar/ 36Ar ratios (<1000) in olivine separates from some subcontinental mantle-derived xenoliths from Far Eastern Russia. The low 3He/ 4He ratios cannot be explained by the addition of radiogenic 4He generated in-situ after the eruption of magma entraining the xenoliths. Furthermore, petrographic evidence suggests that incorporation of crustal fluids is not likely. Hence, it must reflect a feature of the Far Eastern Russian upper mantle. Spectroscopic and petrographic observations confirm that there are at least two compositionally distinct fluids in these xenoliths; liquid CO 2 inclusions and melt inclusions with shrinkage bubbles. Based on the crushing experiments, it is inferred that the inclusions of liquid CO 2 have a 3He/ 4He ratio similar to that of MORB, and the component with the low 3He/ 4He ratio is derived from the shrinkage bubbles in the melt inclusions. For the present samples, the 40Ar/ 36Ar ratios obtained by crushing were less than 1000, suggesting incorporation of atmospheric components in the source materials. Since low 40Ar/ 36Ar ratios were observed irrespective of the occurrence of the liquid CO 2 inclusions, the atmospheric component exists in the melt inclusions. Ne and Xe isotopes are also consistent with incorporation of atmospheric components. Since the Far Eastern Russia area was located at a subduction zone in the Jurassic–early Cretaceous Period, it is most likely that the melt inclusions displaying atmospheric noble gas characteristics together with low 3He/ 4He ratios have been derived from the Jurassic–Cretaceous subducted slab. Although we cannot exclude the possibility that low 3He/ 4He ratios are due to the existence of minor U-bearing minerals in the lithospheric continental mantle caused by metasomatism, we have no petrographical evidence for such minerals in this area. On the other hand, 3He/ 4He ratios observed in the liquid CO 2 inclusions, which are similar to the MORB-like value, might reflect the general character of the upper mantle. The Far Eastern Russian mantle may therefore be a MORB-like source that has been partly infiltrated by subduction-related fluids.