Convective isolation of Hadean mantle reservoirs through Archean time
- aInstitut für Geologie und Mineralogie, Universität zu Köln, 50674 Köln, Germany;
- bSchool of Biological, Earth and Environmental Sciences, The University of New South Wales, Kensington, NSW 2052, Australia;
- cAustralian Center for Astrobiology, The University of New South Wales, Kensington, NSW 2052, Australia;
- dDepartment of Mines, Industry Regulations and Safety, Geological Survey of Western Australia, East Perth, WA 6004, Australia;
- eSchool of Earth and Ocean Sciences, Cardiff University, Cardiff CF10 3AT, United Kingdom;
- fInstitut für Geowissenschaften, Universität zu Kiel, 24118 Kiel, Germany
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Edited by Richard W. Carlson, Carnegie Institution for Science, Washington, DC, and approved November 18, 2020 (received for review June 19, 2020)

Significance
Geological processes like mantle convection or plate tectonics are an essential factor controlling Earth’s habitability. Our study provides insights into timescales of convective homogenization of Earth’s early mantle, employing the novel tool of high-precision 182W isotope measurements to rocks from the Pilbara Craton in Australia, that span an age range from 3.5 billion years to 2.7 billion years. Previous 182W studies mostly covered snapshots through geologic time, so the long-term 182W evolution of the mantle has been ambiguous. Together with sophisticated trace element approaches, we can now provide an improved insight into such timescales, arguing for local preservation of primordial geochemical heterogeneities within Earth’s mantle as late as around 3.0 billion years, the putative onset of widespread plate tectonics on Earth.
Abstract
Although Earth has a convecting mantle, ancient mantle reservoirs that formed within the first 100 Ma of Earth’s history (Hadean Eon) appear to have been preserved through geologic time. Evidence for this is based on small anomalies of isotopes such as 182W, 142Nd, and 129Xe that are decay products of short-lived nuclide systems. Studies of such short-lived isotopes have typically focused on geological units with a limited age range and therefore only provide snapshots of regional mantle heterogeneities. Here we present a dataset for short-lived 182Hf−182W (half-life 9 Ma) in a comprehensive rock suite from the Pilbara Craton, Western Australia. The samples analyzed preserve a unique geological archive covering 800 Ma of Archean history. Pristine 182W signatures that directly reflect the W isotopic composition of parental sources are only preserved in unaltered mafic samples with near canonical W/Th (0.07 to 0.26). Early Paleoarchean, mafic igneous rocks from the East Pilbara Terrane display a uniform pristine µ182W excess of 12.6 ± 1.4 ppm. From ca. 3.3Ga onward, the pristine 182W signatures progressively vanish and are only preserved in younger rocks of the craton that tap stabilized ancient lithosphere. Given that the anomalous 182W signature must have formed by ca. 4.5 Ga, the mantle domain that was tapped by magmatism in the Pilbara Craton must have been convectively isolated for nearly 1.2 Ga. This finding puts lower bounds on timescale estimates for localized convective homogenization in early Earth’s interior and on the widespread emergence of plate tectonics that are both important input parameters in many physical models.
Footnotes
- ↵1To whom correspondence may be addressed. Email: j.tusch{at}uni-koeln.de.
Author contributions: J.T. and C.M. designed research; J.T. performed research; J.T., M.J., C.S.M., W.M., and D.G.-S. contributed new reagents/analytic tools; J.T. analyzed data; and J.T., C.M., E.H., F.K., M.J.V.K., and H.S. wrote the paper.
The authors declare no competing interest.
This article is a PNAS Direct Submission.
This article contains supporting information online at https://www.pnas.org/lookup/suppl/doi:10.1073/pnas.2012626118/-/DCSupplemental.
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