Constraining the origin of high 3He/4He mantle: hypothesis testing using a multi-isotope approach.

在新洋壳形成的洋中脊喷发的玄武岩具有非常均匀的氦元素（He）同位素组成。相比之下，在海洋岛屿（如夏威夷和冰岛）喷发的玄武岩的He同位素组成变化更大，往往富含3He。氦的两种同位素有非常不同的起源。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。氦主要是放射性的，是U和Th在地球内部放射性衰变过程中释放的α粒子和电子的产物。因此，高3He/4He被认为是洋岛玄武岩中原始物质贡献的证据。一个被广泛接受的关于海洋岛屿火山起源的模型将它们置于源自地球深处的热岩上升柱之上。高3He/4He与这些“地幔柱”的联系鼓励了含有原始He的深层储层的建议。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。在地震资料观测到的深度650 km处的一个突出的不连续之上的上地幔约占地幔的1/3。由此形成了上地幔贫壳、脱气、未贫壳、未脱气、具有原始特征的标准模型。这一标准模型做出的预测没有得到过去10-15年收集的数据的支持，而且地球是分层的，有原始下地幔的证据日益受到挑战。已经提出了多种高3He/4He的替代来源，包括：1。地球的外核是由源自地幔底部的地幔柱取样的。2. 古老的枯竭的地幔与地幔对流隔离了10 - 20亿年或更长时间。3. 一个非常早期的地球分化事件，由一种现已灭绝的放射性同位素的子产物记录。4. 富氦地幔的一种小成分，可能是原始的，当被岩浆开采时，在混合物的He同位素组成中占主导地位，但在其他同位素示踪剂中只是一个小成分。利用各种同位素技术，其中许多是最近几年才发展起来的，我们能够提出一个同位素分析计划，来检验这些不同的建议。这些测试假设的测量将限制高3He/4He的起源，从而澄清最神秘的地球化学示踪剂之一的意义。我们的结果将显著影响对地球深部、岩浆的产生和地球演化的理解。

Basalts erupted at the mid ocean ridges, where new ocean crust is forming, have a very uniform isotopic composition of the element 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 and electrons released during radioactive decay of U and Th within 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 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. Using a variety of isotope techniques, many of which have only been developed in the last few years, we are able to propose a programme of isotopic analyses that will test these various suggestions. These hypothesis-testing measurements will constrain the origin of high 3He/4He and thereby clarify the significance of one of the most enigmatic geochemical tracers. Our results will significantly influence understanding of the deep Earth, the generation of magmas and the evolution of the planet.