Earth's evolving geodynamic regime recorded by titanium isotopes.
Earth's evolving geodynamic regime recorded by titanium isotopes.
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DOI：
10.1038/s41586-023-06304-0
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发表时间：
2023-09
期刊：
影响因子：
64.8
通讯作者：
Bizzarro, Martin
中科院分区：
文献类型：
--
作者：
Deng, Zhengbin;Schiller, Martin;Jackson, Matthew G.;Millet, Marc-Alban;Pan, Lu;Nikolajsen, Katrine;Saji, Nikitha S.;Huang, Dongyang;Bizzarro, Martin
地幔具有两层结构，上地幔域和下地幔域被约 660 km 处的地震间断面分开（参考文献 ）。然而，在整个地球历史上，这些地幔区域之间的质量传递程度却知之甚少。大陆地壳提取导致钛稳定同位素分馏，产生同位素轻熔化残留物。这些成分的地幔再循环可以赋予钛同位素的变化性，这种变化性是可在深时间追踪的。我们报告了球粒陨石、距今 3.8 至 20 亿年前 (Ga) 的古代陆地幔熔岩和现代洋岛玄武岩 (OIB) 的超高精度 49Ti/47Ti 比率。我们基于球粒陨石的新钛块状硅酸盐地球（BSE）估计比正常洋中脊玄武岩（N-MORB）采样的现代上地幔重 0.052±0.006‰。地球上地幔的 49Ti/47Ti 比率在 3.5Ga 之前是球粒状的，并在大约 3.5Ga 和 2.7Ga 之间演化为类似 N-MORB 的成分，这表明在这个时期有更多的大陆地壳被提取出来。 BSE 和 N-MORB 之间的 +0.052±0.006‰ 偏移量要求 <30% 的地球地幔与回收的地壳物质达到平衡，这意味着上地幔和下地幔之间的质量交换有限，因此在地球的大部分地质历史中都保留了原始的下地幔储层。现代 OIB 记录了从球粒陨石到 N-MORB 成分的各种 49Ti/47Ti 比率，表明地球原始地幔的持续破坏。因此，上地幔和下地幔之间具有高质量传递的现代板块构造仅代表了地球历史的一个新特征。对球粒陨石、古代陆地幔熔岩和现代海洋岛屿玄武岩的钛同位素测量意味着，在地球的大部分地质历史中，原始下地幔储层都得到了保存。
Earth’s mantle has a two-layered structure, with the upper and lower mantle domains separated by a seismic discontinuity at about 660 km (refs. ). 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 2.7 Ga, 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. Titanium isotope measurements for chondrites, ancient terrestrial mantle-derived lavas and modern ocean island basalts imply the preservation of a primordial lower-mantle reservoir for most of Earth’s geologic history.
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