HiDe: A Highly Heterogeneous Depleted Upper Mantle?

点击翻译按钮获取中文摘要

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.