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

限制高 3He/4He 地幔的起源：使用多同位素方法进行假设检验。

• 批准号: NE/E015069/1
• 负责人: Robert Ellam
• 金额: $ 42.01万
• 依托单位: Scottish Universities Environmental Research Centre
• 依托单位国家: 英国
• 项目类别: Research Grant
• 财政年份: 2007
• 资助国家: 英国
• 起止时间: 2007 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=NE%2FE015069%2F1
• 关键词: Constraining; origin; 3He; 4He; mantle

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.

在新洋壳形成的洋中脊喷发的玄武岩具有非常均匀的氦（He）同位素组成。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变的He同位素组成，并且通常富含3 He。He的两种同位素有着非常不同的起源。3他主要是“原始的”，也就是说，它是在地球形成时从外层空间合并而来的。4 He主要是放射性的，是地球内U和Th放射性衰变时释放的α粒子和电子的产物。因此，高的3 He/4 He被认为是洋岛玄武岩中原始物质贡献的证据。一个被广泛接受的海洋岛屿火山起源模型将它们置于起源于地球深处的热岩上升羽状物之上。高3 He/4 He与这些“地幔柱”的关联鼓励了含有原始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。从地震资料中观察到的650 km深度的显著不连续面上方的上地幔约占地幔的1/3。因此，出现了一个标准的模型，其中上地幔是亏损的地壳形成的元素和脱气的He，而上地幔是未耗尽和未脱气的原始特征。这一标准模型所做的预测没有得到过去10-15年收集的数据的支持，而关于地球具有原始下地幔的分层证据也越来越受到挑战。已经提出了多种高3 He/4 He的替代来源，包括：1.地球的外核，由起源于地幔底部的地幔柱取样。2.与地幔对流隔绝了10 - 20亿年或更长时间的古老贫化地幔。3.一个非常早期的地球分化事件，由现在已经灭绝的放射性同位素的子产物记录。4.一个次要的组成部分，丰富的氦，可能是原始的，地幔，当被岩浆挖掘占主导地位的He同位素组成的混合物，但只是一个次要的组成部分，为其他同位素示踪剂。利用各种同位素技术，其中许多只是在过去几年中开发的，我们能够提出一个同位素分析方案，将测试这些不同的建议。这些假设检验测量将限制高3 He/4 He的起源，从而澄清最神秘的地球化学示踪剂之一的意义。我们的研究结果将显著影响对地球深部、岩浆生成和地球演化的理解。

项目成果

142Nd/144Nd inferences on the nature and origin of the source of high 3He/4He magmas

142Nd/144Nd 高 3He/4He 岩浆来源的性质和起源推断

• DOI: 10.1016/j.epsl.2017.05.005
• 发表时间: 2017
• 期刊: Earth and Planetary Science Letters
• 影响因子: 5.3
• 作者: De Leeuw G