丹麦哥本哈根大学Deng, Zhengbin及其团队成员的一项最新研究提出了钛同位素记录的地球演化动力学机制。2023年7月26日出版的《自然》杂志发表了这项成果。

研究报道了超高精度的球粒陨石、38~20 亿年(Ga)前的古地幔熔岩和现代洋岛玄武岩(OIBs)的 ⁴⁹Ti/⁴⁷Ti 比值。研究基于球粒陨石的新 Ti 块状硅酸盐陆地(BSE)数据，估计比正常大洋中脊玄武岩(N-MORBs)取样的现代上地幔重 0.052±0.006 ‰。地球上地幔的 ⁴⁹Ti/⁴⁷Ti 比值在 3.5 Ga之前为球粒陨石型，并在大约 3.5-2.7 Ga之间演化为类似正常大洋中脊玄武岩的成分，表明在此时期有更多的大陆地壳被提取出来。

块状硅酸盐陆地和正常大洋中脊玄武岩之间的 +0.052±0.006 ‰ 的偏移要求需小于 30 % 的地幔，与再循环地壳物质保持平衡，这意味着上地幔和下地幔之间的物质交换有限，因此，在地球地质历史的大部分时间里，原始下地幔储层得以保存。现代洋岛玄武岩记录了可变的 ⁴⁹Ti/⁴⁷Ti 比率，从球粒陨石到正常大洋中脊玄武岩成分，表明地球原始地幔的持续破裂。因此，上地幔和下地幔之间的高质量转移的现代形式的板块构造，仅仅代表了地球历史的最近特征。

据介绍，地球的地幔有两层结构，包括由660公里的地震分裂带分割形成的上地幔和下地幔。然而，人们对地球历史上这些地幔区域之间的物质转移程度知之甚少。大陆地壳提取的结果导致钛稳定同位素分馏，产生同位素轻熔残留物。这些成分的地幔再循环性，可以赋予 Ti 同位素在深部时间的可追踪性。

附：英文原文

Title: Earth’s evolving geodynamic regime recorded by titanium isotopes

Author: Deng, Zhengbin, Schiller, Martin, Jackson, Matthew G., Millet, Marc-Alban, Pan, Lu, Nikolajsen, Katrine, Saji, Nikitha S., Huang, Dongyang, Bizzarro, Martin

Issue&Volume: 2023-07-26

Abstract: Earth’s mantle has a two-layered structure, with the upper and lower mantle domains separated by a seismic discontinuity at about 660km . The extent of mass transfer between these mantle domains throughout Earth’s history is, however, poorly understood. Continental crust extraction results in Ti-stable isotopic fractionation, producing isotopically light melting residues. Mantle recycling of these components can impart Ti isotope variability that is trackable in deep time. We report ultrahigh-precision ⁴⁹Ti/⁴⁷Ti ratios for chondrites, ancient terrestrial mantle-derived lavas ranging from 3.8 to 2.0 billion years ago (Ga) and modern ocean island basalts (OIBs). Our new Ti bulk silicate Earth (BSE) estimate based on chondrites is 0.052±0.006‰ heavier than the modern upper mantle sampled by normal mid-ocean ridge basalts (N-MORBs). The ⁴⁹Ti/⁴⁷Ti  ratio of Earth’s upper mantle was chondritic before 3.5 Ga and evolved to a N-MORB-like composition between approximately 3.5 and 2.7Ga, establishing that more continental crust was extracted during this epoch. The +0.052±0.006‰ offset between BSE and N-MORBs requires that <30% of Earth’s mantle equilibrated with recycled crustal material, implying limited mass exchange between the upper and lower mantle and, therefore, preservation of a primordial lower-mantle reservoir for most of Earth’s geologic history. Modern OIBs record variable ⁴⁹Ti/⁴⁷Ti  ratios ranging from chondritic to N-MORBs compositions, indicating continuing disruption of Earth’s primordial mantle. Thus, modern-style plate tectonics with high mass transfer between the upper and lower mantle only represents a recent feature of Earth’s history.

DOI: 10.1038/s41586-023-06304-0

Source: https://www.nature.com/articles/s41586-023-06304-0