Earth's evolving geodynamic regime recorded by titanium isotopes

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

doi:10.1038/s41586-023-06304-0

Earth's evolving geodynamic regime recorded by titanium isotopes

Nature. 2023 Sep;621(7977):100-104. doi: 10.1038/s41586-023-06304-0. Epub 2023 Jul 26.

Authors

Zhengbin Deng^{1

2}, Martin Schiller³, Matthew G Jackson⁴, Marc-Alban Millet⁵, Lu Pan^{3

6}, Katrine Nikolajsen³, Nikitha S Saji³, Dongyang Huang⁷, Martin Bizzarro^{3

8}

Affiliations

¹ Centre for Star and Planet Formation, Globe Institute, University of Copenhagen, Copenhagen, Denmark. zhengbindeng@ustc.edu.cn.
² Deep Space Exploration Laboratory/CAS Key Laboratory of Crust-Mantle Materials and Environments, School of Earth and Space Sciences, University of Science and Technology of China, Hefei, China. zhengbindeng@ustc.edu.cn.
³ Centre for Star and Planet Formation, Globe Institute, University of Copenhagen, Copenhagen, Denmark.
⁴ Department of Earth Science, University of California, Santa Barbara, Santa Barbara, CA, USA.
⁵ School of Earth and Environmental Sciences, Cardiff University, Cardiff, UK.
⁶ Deep Space Exploration Laboratory/Laboratory of Seismology and Physics of Earth's Interior, School of Earth and Space Sciences, University of Science and Technology of China, Hefei, China.
⁷ Institute of Geochemistry and Petrology, ETH Zürich, Zürich, Switzerland.
⁸ Institut de Physique du Globe de Paris, Université Paris Cité, Paris, France.

Abstract

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. ^1,2). 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^3-7. 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.