HiDe: A Highly Heterogeneous Depleted Upper Mantle?

PI在过去的一年中产生的证据提供了一个强有力的建议，大洋中脊玄武岩（MORB）显着低估了其地幔源的成分多样性，并认为耗尽的上地幔实际上可能是高度异质性的。现在关键是要确定这个诱人的假设是否可靠，因为地幔在全球地球化学循环中起着核心作用。它还充当地球动力学历史的磁带记录器，记录早期地核形成和大陆地壳形成期间的亏损，以及随后通过板块构造进行的地壳岩石再循环。一个有影响力的范例已经出现了从所有这些数据的洋中脊玄武岩的主要耗尽地幔源，连同大规模的富集域作为地幔柱。这些富集区域反映了板块构造过程中地壳物质的再循环。然而，有一个主要的问题，使用MORB作为一个代理组成的亏损上地幔：MORB已被均匀的岩浆混合之前，它的爆发，因此不忠实地捕捉完整的异质性熔体产生的上地幔源。因此，真正的水平的不均匀性的耗尽上地幔仍然uncertaint.Our新的观测是基于Nd同位素组成的个别矿物从下大洋地壳部分的中大西洋海岭，测试的假设，这些矿物的结晶熔融侵位后，在地壳早于岩浆混合。结果表明，在下地壳的Nd同位素比值的多样性超过了相关的MORB的一个因素的7，和Nd同位素比值在这个“耗尽”的脊段包括成分，如冰岛和亚速尔群岛的大西洋地幔柱富集。这需要从根本上重新评估我们对耗尽上地幔的理解，以及其中回收的地壳物质的规模和组成。这些数据表明，为了努力建立耗尽上地幔的真实非均匀性水平，我们必须利用下大洋地壳记录的潜力。Hide项目将这样做。它将采用一种新的、专门设计的钕同位素分析方法，对现有的两套最全面的下地壳样品进行分析，包括来自（超）慢速和快速扩张海脊的样品。这些数据将对耗尽的上地幔的不均匀性进行严格的定量评估，从而能够确定其中存在的再循环地壳物质的最低比例。此外，我们将确定MORB是地幔不均匀性的忠实记录者的程度。因此，这些结果将决定全球MORB数据库的重要性，并提供前所未有的洞察地球耗尽上地幔的异质性水平。这将是了解地球地球动力学历史的及时和关键的一步。

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.