Convective isolation of Hadean mantle reservoirs through Archean time Jonas Tuscha,1, Carsten Münkera, Eric Hasenstaba, Mike Jansena, Chris S. Mariena, Florian Kurzweila, Martin J. Van Kranendonkb,c, Hugh Smithiesd, Wolfgang Maiere, and Dieter Garbe-Schönbergf 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; and fInstitut für Geowissenschaften, Universität zu Kiel, 24118 Kiel, Germany Edited by Richard W. Carlson, Carnegie Institution for Science, Washington, DC, and approved November 18, 2020 (received for review June 19, 2020) Although Earth has a convecting mantle, ancient mantle reservoirs anomalies in Eoarchean rocks was interpreted as evidence that that formed within the first 100 Ma of Earth’s history (Hadean these rocks lacked a late veneer component (5). Conversely, the Eon) appear to have been preserved through geologic time. Evi- presence of some late accreted material is required to explain the dence for this is based on small anomalies of isotopes such as elevated abundances of highly siderophile elements (HSEs) in 182W, 142Nd, and 129Xe that are decay products of short-lived nu- Earth’s modern silicate mantle (9). Notably, some Archean rocks clide systems. Studies of such short-lived isotopes have typically with apparent pre-late veneer like 182W isotope excesses were focused on geological units with a limited age range and therefore shown to display HSE concentrations that are indistinguishable only provide snapshots of regional mantle heterogeneities. Here from modern mantle abundances (10, 11), which is difficult to 182 182 we present a dataset for short-lived Hf− W (half-life 9 Ma) in reconcile with the missing late veneer hypothesis. An alternative a comprehensive rock suite from the Pilbara Craton, Western Aus- suggestion is therefore that early silicate differentiation during tralia. The samples analyzed preserve a unique geological archive 182 182 the lifetime of Hf might have caused the formation of mantle covering 800 Ma of Archean history. Pristine W signatures that reservoirs with anomalous 182W signatures (10, 12). In addition, directly reflect the W isotopic composition of parental sources are recent studies have revealed variable 182W isotope deficits in the only preserved in unaltered mafic samples with near canonical mantle plume sources of ocean island basalts (13). In line with W/Th (0.07 to 0.26). Early Paleoarchean, mafic igneous rocks from EARTH, ATMOSPHERIC, AND PLANETARY SCIENCES noble gas systematics and seismic properties of such deep-rooted the East Pilbara Terrane display a uniform pristine μ182W excess of mantle plumes, these 182W deficits have been taken as evidence 12.6 ± 1.4 ppm. From ca. 3.3Ga onward, the pristine 182W signa- tures progressively vanish and are only preserved in younger rocks for the presence of primordial domains that have been con- vectively isolated since Earth’s earliest history (13). The origin of of the craton that tap stabilized ancient lithosphere. Given that the – anomalous 182W signature must have formed by ca. 4.5 Ga, the this modern igneous reservoir remains ambiguous (14 16), and mantle domain that was tapped by magmatism in the Pilbara Cra- its presence in pre-Phanerozoic time is highly speculative. ton must have been convectively isolated for nearly 1.2 Ga. This However, the contribution of modern mantle plume sources with 182 finding puts lower bounds on timescale estimates for localized W deficits to the convecting mantle offers an additional ex- ENVIRONMENTAL SCIENCES 182 convective homogenization in early Earth’s interior and on the planation for the decrease of W isotope excesses since the widespread emergence of plate tectonics that are both important Archean as a consequence of increasing mantle homogenization input parameters in many physical models. (SI Appendix). late veneer | Pilbara Craton | tungsten isotopes | early Earth | mantle Significance convection Geological processes like mantle convection or plate tectonics mong the terrestrial planets, Earth is unique in that plate are an essential factor controlling Earth’s habitability. Our Atectonic processes efficiently mix and homogenize its silicate study provides insights into timescales of convective homo- mantle. Surprisingly, however, geochemical studies have revealed genization of Earth’s early mantle, employing the novel tool of that both Archean and Phanerozoic mantle reservoirs still carry high-precision 182W isotope measurements to rocks from the primordial geochemical signatures, thus escaping efficient con- Pilbara Craton in Australia, that span an age range from 3.5 182 vective homogenization as also predicted by geodynamic models billion years to 2.7 billion years. Previous W studies mostly (1, 2). The main evidence for such ancient geochemical hetero- covered snapshots through geologic time, so the long-term 182 geneities stems from noble gas systematics (3) and from short- W evolution of the mantle has been ambiguous. Together lived nuclide decay series that became extinct after the Hadean with sophisticated trace element approaches, we can now eon (>4.0 Ga) (4, 5). For instance, the relative abundance of provide an improved insight into such timescales, arguing for daughter isotopes from short-lived nuclide series such as 142Nd local preservation of primordial geochemical heterogeneities within Earth’s mantle as late as around 3.0 billion years, the and 182W shows significant variations in ancient rocks when putative onset of widespread plate tectonics on Earth. compared to Earth’s modern mantle composition (4, 5). From 182 –182 these short-lived isotope systems, the Hf W decay system Author contributions: J.T. and C.M. designed research; J.T. performed research; J.T., M.J., has proven particularly useful in constraining the timing of C.S.M., W.M., and D.G.-S. contributed new reagents/analytic tools; J.T. analyzed data; and planetary core formation (6), timescales of late accretion, and J.T., C.M., E.H., F.K., M.J.V.K., and H.S. wrote the paper. silicate differentiation (7, 8). The authors declare no competing interest. 182 There are two competing explanations for the origin of W This article is a PNAS Direct Submission. isotope anomalies found in the terrestrial rock record, arising Published under the PNAS license. from the markedly different geochemical behavior of Hf and W 1To whom correspondence may be addressed. Email: [email protected]. during both core formation and silicate differentiation in plan- This article contains supporting information online at https://www.pnas.org/lookup/suppl/ 182 etary bodies. As primitive meteorites exhibit strong W isotope doi:10.1073/pnas.2012626118/-/DCSupplemental. 182 182 deficits (μ W = −190) (6), the observation of positive W Published December 21, 2020. PNAS 2021 Vol. 118 No. 2 e2012626118 https://doi.org/10.1073/pnas.2012626118 | 1of6 Downloaded by guest on September 28, 2021 Surprisingly few studies have directly assessed the 182W record supersuites of sodic (tonalites, trondhjemites, and granodiorites) of mantle-derived rock assemblages that span a relatively long and potassic granitoids (granites) that can be regarded as probes time frame of Archean geodynamic evolution. Arguably, there of early mafic crust. The younger evolution of the EPT involved are already comprehensive studies available from the Slave plume-initiated rifting (Soanesville Group) at 3.18 Ga and sub- Craton (17), southwest Greenland (12, 18), and southern Africa sequent accretion (3.07 Ga) of the younger WPS that also in- (19) that investigated lithostratigraphic successions covering time cludes ∼3.1-Ga subduction-related mafic lithologies (Whundo series of several hundred million years. However, these studies Group) (30, 31). After amalgamation, postorogenic, lithosphere- only covered crustal reservoirs (Tonalite-throndhjemite-grano- derived magmatism included mafic rocks from the Bookingarra diorite rocks [TTGs] and diamictites) but not mafic rocks that Group (Opaline Well Intrusion, Louden Volcanics, and Mount allow direct tracing of mantle-derived magmatic pulses. Notably, Negri Volcanics) and crust-derived posttectonic granites (e.g., fluid mobility of W already may have caused significant distur- Split Rock Supersuite) (27). bance of primary 182W isotope systematics in hydrated mafic We employed two strategies in selecting our samples. First, we precursors of such previously investigated TTG suites (17, 20, analyzed mafic volcanic rocks that tapped the ambient as- 21). Hence, W/Th ratios of TTGs may not necessarily reflect the thenospheric mantle of the EPT (plume derived) and the WPS primary W/Th ratios of their mafic precursors. In fact, sub- (subduction related). Secondly, to understand the evolution of canonical W/Th ratios are often observed in TTGs (e.g., ref. 22), the lithosphere and to obtain average crustal compositions, we that may reflect 1) preferential loss of W during dehydration at analyzed mafic dikes, sediments, and granitoids of different ages. amphibolite facies conditions prior to partial anataxis or 2) We studied a total of 56 samples from more