By applying both vacuum crushing and stepwise heating methods for the extraction of the noble gases, we have discovered 3 He/4He ratios much lower than the atmospheric ratio (∼0.3 RA; RA 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 CO2 inclusions and melt inclusions with shrinkage bubbles. Based on the crushing experiments, it is inferred that the inclusions of liquid CO2 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 CO2 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 CO2 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.
All Science Journal Classification (ASJC) codes
- Geochemistry and Petrology