HiDe: A Highly Heterogeneous Depleted Upper Mantle?

过去一年由PI产生的证据提供了一个强有力的证据，表明大洋中脊玄武岩(MORB)大大低估了其地幔来源的成分多样性，而且贫化的上地幔实际上可能是高度不均匀的。现在关键是要确定这一诱人的假说是否站得住脚，因为地幔在全球地球化学循环中扮演着核心角色。它也是地球动力学历史的磁带记录器，记录了早期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重新循环的地壳岩石。50多年来，大洋熔岩的同位素组成一直被用来绘制上地幔的地球化学多样性图。从所有这些数据中出现了一个有影响力的范例，即大洋中脊玄武岩的主要亏损地幔来源，以及以地幔羽流形式存在的大规模富集域。这些富集域反映了在板块构造过程中回收的地壳物质。然而，使用MORB作为贫化上地幔成分的替代存在一个主要问题：MORB在喷发前被岩浆混合而均一，因此不能忠实地捕捉到其上地幔来源产生的熔体的全部不均质性。因此，贫化的上地幔的不均匀程度的真实水平仍然不确定。我们的新观察基于大西洋中脊下部大洋地壳剖面中单个矿物的Nd同位素组成，验证了这些矿物在熔体侵位时在地壳中结晶早于岩浆混合的假设。结果表明，下地壳中的Nd同位素分异程度比伴生的MORB高出7倍，而且这个“亏损”的海脊段中的Nd同位素比包括了与大西洋地幔热柱一样富集的成分，如冰岛和亚速尔群岛。这需要从根本上重新评估我们对贫化上地幔及其内部再循环地壳物质的规模和组成的理解。这些数据表明，为了努力确定贫化上地幔的真正不均质性水平，我们必须利用下部大洋地壳记录的潜力。Hide项目将做到这一点。它将对现有的两套最全面的下地壳样品套，包括来自(超)慢速扩张海脊和快速扩张海脊的样品，应用一种新的、专门设计的分析方法进行钕同位素分析。这些数据将对枯竭的上地幔的不均一性进行严格、定量的评估，从而能够确定其内部存在的回收地壳物质的最低比例。此外，我们将确定MORB在多大程度上是地幔非均质性的忠实记录者。因此，这一结果将决定全球MORB数据库的重要性，并为了解地球枯竭的上地幔的不均一性程度提供前所未有的见解。这将是向理解地球动力学历史迈出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.