Evidence for subduction-related components in the subcontinental mantle from low 3He/4He and 40Ar/36Ar ratio in mantle xenoliths from Far Eastern Russia
Evidence for subduction-related components in the subcontinental mantle from low 3He/4He and 40Ar/36Ar ratio in mantle xenoliths from Far Eastern Russia
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DOI：
10.1016/j.chemgeo.2004.03.007
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发表时间：
2004-07
期刊：
影响因子：
3.9
通讯作者：
J. Yamamoto;I. Kaneoka;S. Nakai;H. Kagi;V. Prikhod'ko;S. Arai
中科院分区：
文献类型：
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作者：
J. Yamamoto;I. Kaneoka;S. Nakai;H. Kagi;V. Prikhod'ko;S. Arai
采用真空破碎和分步加热提取稀有气体的方法，我们在俄罗斯远东地区的一些地幔包体中发现了远低于大气的3 He/4 He比值（~ 3 He/4 He = 1.4×10−6）和较低的40 Ar/36 Ar比值（<1000）。低的3 He/4 He比值不能解释为附加的放射性4 He产生后，岩浆喷发夹带捕虏体原位。此外，岩相学证据表明，地壳流体的合并是不可能的。因此，它必须反映远东俄罗斯上地幔的一个特征。光谱和岩相学观察证实，至少有两种成分不同的流体在这些捕虏体，液体CO2包裹体和熔融包裹体收缩气泡。通过破碎实验，推测液态CO2包裹体中的3 He/4 He比值与MORB包裹体中的3 He/4 He比值相似，低3 He/4 He比值的组分来源于熔融包裹体中的收缩气泡。对于目前的样品，通过破碎获得的40 Ar/36 Ar比值小于1000，这表明在源物质中掺入了大气组分。由于低的40 Ar/36 Ar比值观察到的液态CO2包裹体的出现无关，大气成分存在于熔融包裹体。Ne和Xe同位素也与大气成分的结合一致。由于俄罗斯远东地区位于侏罗纪-早白垩世的俯冲带，具有大气惰性气体特征的熔融包裹体和低的3 He/4 He比值很可能来源于侏罗纪-白垩纪俯冲板片。虽然我们不能排除低的3 He/4 He比值是由于岩石圈大陆地幔中存在交代作用引起的少量含U矿物的可能性，但我们没有这类矿物在该地区的岩相学证据。另一方面，CO2包裹体中的3 He/4 He比值与类MORB值相似，可能反映了上地幔的一般特征。因此，远东俄罗斯地幔可能是一个MORB样的来源，已部分渗入俯冲相关的流体。
By applying both vacuum crushing and stepwise heating methods for the extraction of the noble gases, we have discovered3He/4He ratios much lower than the atmospheric ratio (∼0.3 RA; RAis the atmospheric3He/4He ratio of 1.4×10−6) and relatively low40Ar/36Ar ratios (<1000) in olivine separates from some subcontinental mantle-derived xenoliths from Far Eastern Russia. The low3He/4He ratios cannot be explained by the addition of radiogenic4He 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 CO2inclusions and melt inclusions with shrinkage bubbles. Based on the crushing experiments, it is inferred that the inclusions of liquid CO2have a3He/4He ratio similar to that of MORB, and the component with the low3He/4He ratio is derived from the shrinkage bubbles in the melt inclusions. For the present samples, the40Ar/36Ar ratios obtained by crushing were less than 1000, suggesting incorporation of atmospheric components in the source materials. Since low40Ar/36Ar ratios were observed irrespective of the occurrence of the liquid CO2inclusions, 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 low3He/4He ratios have been derived from the Jurassic–Cretaceous subducted slab. Although we cannot exclude the possibility that low3He/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 CO2inclusions, 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.