Are He isotopes decoupled from other geochemical tracers in the deep Earth?

在大洋中脊喷发的玄武岩，那里正在形成新的洋壳，其气体氦(He)的同位素组成非常均匀。相比之下，在大洋岛屿(如夏威夷和冰岛)喷发的玄武岩具有更多可变的He同位素组成，并且经常富含3He。他的两种同位素有着非常不同的来源。3他大部分是“原始的”，也就是说，它是在地球形成时从外太空合并而来的。4He主要是放射性成因，是地球中U和Th放射性衰变过程中释放出的阿尔法粒子的产物。因此，高3He/4He被认为是洋岛玄武岩中原始物质贡献的证据。一种被广泛接受的海洋岛屿火山起源模型将它们置于源自地球深处的上升的热岩羽流之上。高~3He/~4He与这些“地幔热柱”的联系鼓励了含有原始氦的深层储集层的推测。所有稀有气体(包括Ar和Xe)的质量平衡模型表明，需要1/2到1/3的地幔来平衡大陆地壳和大气。从地震资料观测到的650公里深度的显著不连续面上方的上地幔，约占地幔的1/3。因此，出现了一种标准模式，即上地幔亏损造壳元素并脱气He，而上地幔未亏损且未脱气，具有原始特征。这一标准模型作出的预测没有得到过去10-15年收集的数据的支持，关于地球具有原始下地幔的分层的证据越来越受到质疑。高~3He/~4He的各种替代来源已被提出，包括：1.来自地幔底部的地幔热柱采样的地球外核。2.与地幔对流隔离了10-20亿年或更长时间的古代亏损地幔。3.由一种现已灭绝的放射性同位素的子产物记录的非常早期的地球分化事件。4.富氦地幔的一种次要成分，可能是原始地幔，当被岩浆敲击时，它主导着混合体的氦同位素组成，但对其他同位素示踪剂来说只是一种次要成分。来自加拿大北极和格陵兰岛的玄武岩的最新结果表明，同位素可能根本不能追踪地幔来源，而且地球的地幔可能确实几乎完全脱气。利用各种同位素技术，其中许多是在最近几年才发展起来的，以及在埃塞俄比亚北部人迹罕至的地区收集的一些壮观的玄武岩样本，我们能够提出一个将检验这一建议的同位素分析方案。我们的结果将显著影响对地球深处的理解、岩浆的产生和地球的演化。

Basalts erupted at the mid-ocean ridges, where new ocean crust is forming, have a very uniform isotopic composition of the gas helium (He). By contrast, basalts erupted at oceanic islands (e.g. Hawaii and Iceland) have much more variable He isotope composition and are often enriched in 3He. The two isotopes of He have very different origins. 3He is mostly 'primordial' i.e. it was incorporated from outer space when the Earth formed. 4He is mostly radiogenic, the product of alpha-particles released during radioactive decay of U and Th in the Earth. Thus, high 3He/4He has been viewed as evidence for a contribution from primordial material in the ocean island basalts. A widely accepted model for the origin of ocean island volcanoes places them above rising plumes of hot rock that originate deep in the Earth. The association of high 3He/4He with these 'mantle plumes' encouraged the suggestion of a deep reservoir containing primordial He. Mass balance models of all the noble gases (including Ar and Xe) suggest that 1/2 to 1/3 of the mantle is required to balance the continental crust and atmosphere. The upper mantle, above a prominent discontinuity at 650 km depth observed from earthquake seismic data, represents about 1/3 of the mantle. Thus a standard model emerged in which the upper mantle is depleted in crust-forming elements and degassed of He while the upper mantle is undepleted and undegassed with primordial characteristics. This standard model makes predictions that have not been supported by data collected over the past 10-15 years and the evidence for a layered Earth with a primordial lower mantle has increasingly been challenged. A variety of alternative sources of high 3He/4He have been suggested, including: 1. The Earth's outer core sampled by mantle plumes originating at the bottom of the mantle. 2. Ancient depleted mantle that has remained isolated from mantle convection for 1-2 billion years or more. 3. A very early Earth differentiation event recorded by the daughter products of a radioactive isotope that is now extinct. 4. A minor component of He-rich, possibly primordial, mantle that when tapped by magmas dominates the He isotope composition of mixtures but is only a minor component for the other isotopic tracers. Recent results from basalts from the Canadian Arctic and Greenland suggest that He isotopes may not trace the mantle source at all, and that the Earth's mantle may indeed be nearly completely degassed. Using a variety of isotope techniques, many of which have only been developed in the last few years, and some spectacular basalt samples collected inaccessible parts of northern Ethiopia we are able to propose a programme of isotopic analyses that will test this suggestion. Our results will significantly influence understanding of the deep Earth, the generation of magmas and the evolution of the planet.