HiDe: A Highly Heterogeneous Depleted Upper Mantle?

HiDe：高度异质的贫化上地幔？

• 批准号: NE/T000317/1
• 负责人: Cornelis Lissenberg
• 金额: $ 52.49万
• 依托单位: CARDIFF UNIVERSITY
• 依托单位国家: 英国
• 项目类别: Research Grant
• 财政年份: 2020
• 资助国家: 英国
• 起止时间: 2020 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=NE%2FT000317%2F1
• 关键词: HiDe; Highly; Heterogeneous; Depleted; Upper

Evidence generated by the PI over the last year provides a strong suggestion that mid-ocean ridge basalt (MORB) significantly underestimates the compositional diversity of its mantle source, and that the depleted upper mantle may in fact be highly heterogeneous. It is now critical to establish if this tantalising hypothesis is robust, as the mantle plays a central role in global geochemical cycles. It also acts as the tape recorder of Earth's dynamic history, recording depletion during early core formation and continental crust formation, and the subsequent recycling of crustal rocks through plate tectonics.For more than 50 years, the isotopic compositions of oceanic lavas have been used to map the geochemical diversity of the upper mantle. An influential paradigm has emerged from all this data of a predominantly depleted mantle source for mid-ocean ridge basalts, together with large-scale enriched domains present as mantle plumes. These enriched domains reflect crustal material recycled during plate tectonics. However, there is a major problem with using MORB as a proxy for the composition of the depleted upper mantle: MORB has been homogenised by magma mixing prior to its eruption, and therefore does not faithfully capture the full heterogeneity of melts generated in its upper mantle source. Hence, the true level of heterogeneity of the depleted upper mantle remains uncertain.Our new observations are based on the Nd isotopic composition of individual minerals from a lower oceanic crustal section on the Mid-Atlantic Ridge, testing the hypothesis that crystallisation of these minerals upon melt emplacement in the crust predates magma mixing. The results show that the diversity in Nd isotopic ratios in the lower crust exceeds that of the associated MORB by a factor of seven, and that the Nd isotopic ratios in this 'depleted' ridge segment include compositions that are as enriched as Atlantic mantle plumes such as Iceland and the Azores. This requires a fundamental re-assessment of our understanding of the depleted upper mantle, and the scale and compositions of recycled crustal material within itThese data demonstrate that in order to work towards establishing the true level of heterogeneity of the depleted upper mantle, we must harness the potential of the lower oceanic crustal record. The HiDe project will do so. It will apply a new, purpose-designed analytical approach for Nd isotopic analysis to the two most comprehensive lower crustal sample suites currently available, including samples from both an (ultra)slow- and a fast-spreading ridge. These data will provide a rigorous, quantitative assessment of the heterogeneity of the depleted upper mantle, enabling the minimum proportion of recycled crustal material present within it to be determined. Furthermore, we will establish the extent to which MORB are faithful recorders of mantle heterogeneity. Hence, the results will determine the significance of the global MORB database, and provide unprecedented insight into the level of heterogeneity of the Earth's depleted upper mantle. This will represent a timely and critical step towards understanding the geodynamic history of the Earth.

PI在过去一年中产生的证据提供了强烈的暗示，即中海脊玄武岩（MORB）显着低估了其地幔源的组成多样性，并且耗尽的上层地幔实际上可能是高度异质的。现在，确定这种诱人的假设是否强大，这是至关重要的，因为地幔在全球地球化学循环中起着核心作用。它还充当地球动态历史的磁带记录器，在早期核心形成期间记录耗竭和大陆地壳形成，以及随后通过板块构造的地壳岩石的回收。绘制上地幔的地球化学多样性。来自中洋脊玄武岩的所有这些数据的所有这些数据都从所有这些数据的数据中出现了有影响力的范式，以及作为地幔羽的大规模富集域。这些富集的结构域反映了板块构造过程中回收的地壳材料。但是，将MORB用作耗尽的上套的组成的代理存在一个主要问题：MORB在喷发前通过Magma混合而均质，因此并未忠实地捕获其上层层中产生的完整异质性的异质性来源。因此，耗尽的上地幔的异质性的真实水平仍然不确定。我们的新观察结果基于来自中大西洋山脊的下海洋甲壳层的单个矿物质的ND同位素组成，证实了这些矿物质在这些矿物质上的假设地壳中的熔体扩展早于岩浆混合。结果表明，下壳中ND同位素比的多样性超过了相关的病态的倍数，而该“耗尽”的山脊段中的ND同位素比包括与大西洋羽毛一样丰富的成分作为冰岛和亚速尔群岛。这需要对我们对耗尽的上地幔的理解进行基本的重新评估，以及在ITCH中的回收地壳材料的规模和组成，这些数据表明，为了建立耗尽的上层底层异质性的真实水平，我们必须努力。下海壳记录的潜力。隐藏项目将这样做。它将对当前可用的两个最全面的下地壳样品套件应用新的，专门设计的分析方法，以进行ND同位素分析，包括来自（Ultra）慢速和快速播放的山脊的样品。这些数据将对耗尽的上地幔的异质性进行严格的定量评估，从而确定其内部存在的可回收地壳材料的最小比例。此外，我们将确定MORB是地幔异质性的忠实录音机的程度。因此，结果将决定全球MORB数据库的重要性，并为地球上层枯竭的异质性水平提供前所未有的见解。这将代表理解地球地球动力学历史的及时和批判步骤。