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Deng Z, Schiller M, Jackson MG, Millet MA, Pan L, Nikolajsen K, Saji NS, Huang D, Bizzarro M. Earth's evolving geodynamic regime recorded by titanium isotopes. Nature 2023; 621:100-104. [PMID: 37495699 PMCID: PMC10482698 DOI: 10.1038/s41586-023-06304-0] [Citation(s) in RCA: 1] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/24/2022] [Accepted: 06/09/2023] [Indexed: 07/28/2023]
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 residues3-7. 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.
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Affiliation(s)
- Zhengbin Deng
- Centre for Star and Planet Formation, Globe Institute, University of Copenhagen, Copenhagen, Denmark.
- 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.
| | - Martin Schiller
- Centre for Star and Planet Formation, Globe Institute, University of Copenhagen, Copenhagen, Denmark
| | - Matthew G Jackson
- Department of Earth Science, University of California, Santa Barbara, Santa Barbara, CA, USA
| | - Marc-Alban Millet
- School of Earth and Environmental Sciences, Cardiff University, Cardiff, UK
| | - Lu Pan
- Centre for Star and Planet Formation, Globe Institute, University of Copenhagen, Copenhagen, Denmark
- 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
| | - Katrine Nikolajsen
- Centre for Star and Planet Formation, Globe Institute, University of Copenhagen, Copenhagen, Denmark
| | - Nikitha S Saji
- Centre for Star and Planet Formation, Globe Institute, University of Copenhagen, Copenhagen, Denmark
| | - Dongyang Huang
- Institute of Geochemistry and Petrology, ETH Zürich, Zürich, Switzerland
| | - Martin Bizzarro
- Centre for Star and Planet Formation, Globe Institute, University of Copenhagen, Copenhagen, Denmark
- Institut de Physique du Globe de Paris, Université Paris Cité, Paris, France
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Long-term core-mantle interaction explains W-He isotope heterogeneities. Proc Natl Acad Sci U S A 2023; 120:e2215903120. [PMID: 36649424 PMCID: PMC9942857 DOI: 10.1073/pnas.2215903120] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/19/2023] Open
Abstract
The isotopic characteristics of ocean island basalts have long been used to infer the nature of their source and the long-term evolution of the Earth's mantle. Anticorrelation between tungsten and helium isotopic signatures is a particularly puzzling feature in those basalts, which no single process appears to explain. Traditionally, the high 3He/4He signature has been attributed to an undegassed reservoir in the deep mantle. Additional processes needed to obtain low 182W/184W often entail unobserved ancillary geochemical effects. It has been suggested, however, that the core feeds the lower mantle with primordial helium, obviating the need for an undegassed mantle reservoir. Independently, the tungsten-rich core has been suggested to impart the plume source with anomalous tungsten isotope signatures. We advance the idea that isotopic diffusion may simultaneously transport both tungsten and helium across the core-mantle boundary, with the striking implication that diffusion can naturally account for the observed isotopic trend. By modeling the long-term isotopic evolution of mantle domains, we demonstrate that this mechanism can account for more than sufficient isotopic ratios in plume-source material, which, after dynamical transport to the Earth's surface, are consistent with the present-day mantle W-He isotopic heterogeneities. No undegassed mantle reservoir is required, bearing significance on early Earth conditions such as the extent of magma oceans.
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