Chemical differences between mid-ocean ridge basalts (MORBs) and ocean island basalts (OIBs) provide critical evidence that the Earth's mantle is compositionally heterogeneous. MORBs generally exhibit a relatively low and narrow range of 3He/4He ratios on a global scale, whereas OIBs display larger variability in both time and space. The primordial origin of 3He in OIBs has motivated hypotheses that high 3He/4He ratios are the product of mantle plumes sampling chemically distinct material, but do not account for lower MORB-like 3He/4He ratios in OIBs, nor their observed spatial and temporal variability. Here we perform thermochemical convection calculations which show the variable 3He/4He signature of OIBs can be reproduced by deep isolated mantle reservoirs of primordial material that are viscously entrained by thermal plumes. Entrainment is highly time-dependent, producing a wide range of 3He/4He ratios similar to that observed in OIBs worldwide and indicate MORB-like 3He/4He ratios in OIBs cannot be used to preclude deep mantle-sourced hotspots.
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机译：大洋中脊玄武岩（MORB）与大洋洲玄武岩（OIBs）之间的化学差异提供了重要的证据，证明地球的地幔在成分上是异质的。 MORB通常在全球范围内呈现出 3 He / 4 He比值相对较低且较窄的范围，而OIBs在时间和空间上均表现出较大的可变性。 OIB中 3 He的原始起源提出了这样的假设：高 3 He / 4 He比率是地幔柱中化学成分不同的样品的产物，但不能解释OIB中类似MORB的 3 He / 4 He比值较低，也不能说明其观察到的时空变异性。在这里，我们进行热化学对流计算，结果显示了OIBs的变量 3 He / 4 He的特征可以通过原始羽状深层孤立的地幔储层来再现，这些储层被热羽状粘性夹带。夹带是高度时间依赖性的，产生广泛的 3 He / 4 He比率，与世界范围内的OIBs中观察到的比率相似，表明类似MORB的 3 He / 4 在OIB中的He比不能用来排除深部幔源热区。
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