Origins of the terrestrial Hf-Nd mantle array: Evidence from a combined geodynamical-geochemical approach
Origins of the terrestrial Hf-Nd mantle array: Evidence from a combined geodynamical-geochemical approach
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DOI：
10.1016/j.epsl.2019.04.015
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发表时间：
2019-07
影响因子：
5.3
通讯作者：
Rosemary E. Jones;P. V. van Keken;E. Hauri;J. Tucker;J. Vervoort;C. Ballentine
中科院分区：
文献类型：
--
作者：
Rosemary E. Jones;P. V. van Keken;E. Hauri;J. Tucker;J. Vervoort;C. Ballentine
洋壳和陆壳与地幔的形成和分离，以及洋壳和陆壳通过俯冲和（或）拆沉作用返回地幔，导致了陆地地幔中独特的地球化学储集层的发育。这些储层的位置、性质和停留时间，以及洋壳和陆壳在地幔地球化学端元发育过程中各自所起的作用等基本问题仍然存在。Lu-Hf和Sm-Nd同位素系统在岩浆系统中表现相似，并共同形成陆相地幔Hf-Nd同位素阵列。本文将地幔对流地球动力学模型与同位素和微量元素（TE）地球化学相结合，研究了Hf-Nd地幔阵列的演化。本研究考察了海洋地壳形成过程中对TE分割系数的敏感性；俯冲洋壳和地幔之间的密度对比；大陆地壳的形成和再循环。我们表明，父（Lu和Sm）和子（Hf和Nd）之间的分馏需要比从当今熔融环境确定的分馏系数所表明的要高。这与地球历史早期更深层次的地幔融化和残留石榴石的作用增加的说法是一致的。致密洋壳的俯冲和聚集在深部地幔中产生了大量不相容的TE富集物质。在洋中脊地幔源Hf、Nd同位素和TE组成的发育过程中，这种深部地幔富集作用似乎比大陆地壳的提取和再循环作用更重要。这一结果的推论是，与目前公认的范式相反，大陆地壳的形成起着次要的作用。然而，将大陆地壳的形成和再循环考虑在内，产生了一个更广泛的模型地幔阵列，更好地再现了自然数据集中的分布。该模型还生成了上地幔和大陆地壳的Hf、Nd同位素和TE组成，以及类似于羽状供养的洋岛玄武岩的深部地幔组成。我们的模型与基于锆石Lu-Hf同位素组成的大陆生长模型一致，表明现今大陆地壳质量的50-70%是在3ga之前产生的，并且在此之后大陆地壳的再循环更加普遍。
The formation and segregation of oceanic and continental crust from the mantle, and its return to the mantle via subduction and/or delamination, leads to the development of distinct geochemical reservoirs in the terrestrial mantle. Fundamental questions remain regarding the location, nature, and residence time of these reservoirs, as well as the respective roles of oceanic and continental crust in the development of the mantle's geochemical endmembers. The Lu-Hf and Sm-Nd isotope systems behave similarly in magmatic systems and together form the terrestrial mantle Hf-Nd isotopic array. Here we combine a geodynamic model of mantle convection with isotope and trace element (TE) geochemistry to investigate the evolution of the Hf-Nd mantle array. This study examines the sensitivity to: TE partition coefficients used in the formation of oceanic crust; density contrasts between subducting oceanic crust and the mantle; and the formation and recycling of continental crust. We show that the fractionation between the parent (Lu and Sm) and daughter (Hf and Nd) species needs to be higher than is indicated by partition coefficients determined from the present-day melting environment. This is consistent with the suggestion of deeper mantle melting earlier in Earth history and an increased role for residual garnet. Subduction and accumulation of dense oceanic crust produces a large mass of incompatible TE enriched material in the deep mantle. This deep mantle enrichment appears to play a more significant role than the extraction and recycling of continental crust in developing the Hf and Nd isotope and TE compositions of the mid-ocean ridge mantle source. The corollary of this result is that the formation of the continental crust plays a secondary role, contrary to the currently accepted paradigm. Nevertheless, the inclusion of continental crust formation and recycling produces a broader model mantle array, which better reproduces the spread in the natural data set. This model also produces the Hf and Nd isotope and TE compositions of the upper mantle and continental crust, as well as deep mantle compositions similar to those of plume-fed ocean island basalts. Our model is consistent with continental growth models based on the Lu-Hf isotopic composition of zircon, which suggest that 50–70% of the present-day mass of the continental crust is produced prior to 3 Ga, and that the recycling of continental crust becomes more prevalent after this time.