Convective isolation of Hadean mantle reservoirs through Archean time

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.

Among the terrestrial planets, Earth is unique in that plate tectonic processes efficiently mix and homogenize its silicate mantle. Surprisingly, however, geochemical studies have revealed that both Archean and Phanerozoic mantle reservoirs still carry primordial geochemical signatures, thus escaping efficient convective homogenization as also predicted by geodynamic models (1, 2). The main evidence for such ancient geochemical heterogeneities stems from noble gas systematics (3) and from short-lived nuclide decay series that became extinct after the Hadean eon (>4.0 Ga) (4, 5). For instance, the relative abundance of daughter isotopes from short-lived nuclide series such as ¹⁴²Nd and ¹⁸²W shows significant variations in ancient rocks when compared to Earth’s modern mantle composition (4, 5). From these short-lived isotope systems, the ¹⁸²Hf–¹⁸²W decay system has proven particularly useful in constraining the timing of planetary core formation (6), timescales of late accretion, and silicate differentiation (7, 8).

There are two competing explanations for the origin of ¹⁸²W isotope anomalies found in the terrestrial rock record, arising from the markedly different geochemical behavior of Hf and W during both core formation and silicate differentiation in planetary bodies. As primitive meteorites exhibit strong ¹⁸²W isotope deficits (µ¹⁸²W = −190) (6), the observation of positive ¹⁸²W anomalies in Eoarchean rocks was interpreted as evidence that these rocks lacked a late veneer component (5). Conversely, the presence of some late accreted material is required to explain the elevated abundances of highly siderophile elements (HSEs) in Earth’s modern silicate mantle (9). Notably, some Archean rocks with apparent pre-late veneer like ¹⁸²W isotope excesses were shown to display HSE concentrations that are indistinguishable from modern mantle abundances (10, 11), which is difficult to reconcile with the missing late veneer hypothesis. An alternative suggestion is therefore that early silicate differentiation during the lifetime of ¹⁸²Hf might have caused the formation of mantle reservoirs with anomalous ¹⁸²W signatures (10, 12). In addition, recent studies have revealed variable ¹⁸²W isotope deficits in the mantle plume sources of ocean island basalts (13). In line with noble gas systematics and seismic properties of such deep-rooted mantle plumes, these ¹⁸²W deficits have been taken as evidence for the presence of primordial domains that have been convectively isolated since Earth’s earliest history (13). The origin of this modern igneous reservoir remains ambiguous (14–16), and its presence in pre-Phanerozoic time is highly speculative. However, the contribution of modern mantle plume sources with ¹⁸²W deficits to the convecting mantle offers an additional explanation for the decrease of ¹⁸²W isotope excesses since the Archean as a consequence of increasing mantle homogenization (SI Appendix).

Surprisingly few studies have directly assessed the ¹⁸²W record of mantle-derived rock assemblages that span a relatively long time frame of Archean geodynamic evolution. Arguably, there are already comprehensive studies available from the Slave Craton (17), southwest Greenland (12, 18), and southern Africa (19) that investigated lithostratigraphic successions covering time series of several hundred million years. However, these studies only covered crustal reservoirs (Tonalite-throndhjemite-granodiorite rocks [TTGs] and diamictites) but not mafic rocks that allow direct tracing of mantle-derived magmatic pulses. Notably, fluid mobility of W already may have caused significant disturbance of primary ¹⁸²W isotope systematics in hydrated mafic precursors of such previously investigated TTG suites (17, 20, 21). Hence, W/Th ratios of TTGs may not necessarily reflect the primary W/Th ratios of their mafic precursors. In fact, subcanonical W/Th ratios are often observed in TTGs (e.g., ref. 22), that may reflect 1) preferential loss of W during dehydration at amphibolite facies conditions prior to partial anataxis or 2) preferential retention of W in residual rutile (23). It is therefore more than plausible that apparently invariant ¹⁸²W isotope excesses in felsic crustal assemblages (17) may simply mirror fluid-mediated W redistribution within their hydrated mafic precursors (20, 21), obscuring primary magmatic trends. Moreover, ¹⁸²W studies on sediments like glacial diamictites only provide average crustal compositions and protracted information on convective processes in the mantle. An exceptional case in this regard are studies from southwest Greenland (5, 12, 18). These 3.8- to 3.4-Ga-old mantle-derived rocks from southwest Greenland show uniform ¹⁸²W excesses but decreasing ¹⁴²Nd anomalies through time in the same lithostratigraphic successions (12, 18). This relationship suggests that an original decrease in ¹⁸²W was obscured by secondary W redistribution during metamorphism (18), as also suggested for Archean rocks from the Acasta Gneiss Complex (24), from the North Atlantic Craton (25), and from the Superior Province (26). Collectively, these examples illustrate that most studies have only provided snapshots of the ¹⁸²W isotope evolution of the mantle beneath individual Archean cratons (SI Appendix) and that robust information on the ¹⁸²W isotope evolution of the Archean mantle with time is scarce and often ambiguous, due to secondary disturbance. In fact, only 15% of all available samples previously analyzed for ¹⁸²W display W abundances that are pristine with respect to an unmodified mantle source (Fig. 3 and SI Appendix). In order to allow for a more comprehensive understanding of Archean geodynamic evolution, studies are required to assess the ¹⁸²W isotope evolution of a particular region over a long time period, with a focus on mantle-derived rocks.

Fig. 3.

Compilation illustrating the secular ¹⁸²W isotope evolution of the terrestrial mantle. This dataset includes all available ¹⁸²W isotope literature data for mantle-derived rocks. For Nuvvuagittuq, we assume a minimum emplacement age of 3.75 Ga (50), being well aware that it might be older (51). The literature data are subdivided into samples with overprinted elemental W budgets (grey) and samples with canonical W/Th (red). Symbols with no color fill refer to samples with unknown W/Th ratios. Data for Pilbara rocks presented in this study are squares with thick black frames. References and information on the data compilation are provided in Dataset S4. Error bars are omitted for visual clarity, but uncertainties are given in Dataset S4.

Here we present a nearly continuous ¹⁸²W isotope record for an Archean craton that spans an age range of 800 Ma, from the Paleoarchean to the Neoarchean, and covers both mafic and felsic compositions. We investigated the ¹⁸²W isotope evolution of the exceptionally well preserved 3.58- to 2.76-Ga-old Pilbara Craton (Western Australia). Since much of this craton is only slightly affected by metamorphism (27), many units within the lithostratigraphic successions are more likely to have preserved their primary W systematics than other more deformed cratons of similar age. Therefore, our data may allow a much more robust assessment of ¹⁸²W isotope evolution through the Archean. A detailed outline of the geologic evolution of the Pilbara Craton is provided in SI Appendix. In short, mantle-derived (mafic and ultramafic) magmatic sequences in the east (East Pilbara Terrane [EPT]; Warrawoona, Kelly, and Sulphur Springs groups) and west (West Pilbara Superterrane [WPS]; Ruth Well Formation) of the craton cover an age range from 3.52 Ga to 3.23 Ga and tapped rather depleted mantle sources that were emplaced during distinct mantle plume events, similar to modern plateau basalts (28, 29). Burial of these mafic sequences, together with older protocrust, caused melting to form four supersuites of sodic (tonalites, trondhjemites, and granodiorites) and potassic granitoids (granites) that can be regarded as probes of early mafic crust. The younger evolution of the EPT involved plume-initiated rifting (Soanesville Group) at 3.18 Ga and subsequent accretion (3.07 Ga) of the younger WPS that also includes ∼3.1-Ga subduction-related mafic lithologies (Whundo Group) (30, 31). After amalgamation, postorogenic, lithosphere-derived magmatism included mafic rocks from the Bookingarra Group (Opaline Well Intrusion, Louden Volcanics, and Mount Negri Volcanics) and crust-derived posttectonic granites (e.g., Split Rock Supersuite) (27).

We employed two strategies in selecting our samples. First, we analyzed mafic volcanic rocks that tapped the ambient asthenospheric mantle of the EPT (plume derived) and the WPS (subduction related). Secondly, to understand the evolution of the lithosphere and to obtain average crustal compositions, we analyzed mafic dikes, sediments, and granitoids of different ages. We studied a total of 56 samples from more than 10 major stratigraphic units of the two different terranes (SI Appendix, Fig. S1), also covering different tectonic regimes (vertical tectonics in the EPT and horizontal tectonics in the WPS). For all these samples, selected trace element abundances (Zr, Nb, Ta, W, Th, and U) were obtained by isotope dilution measurements (details in SI Appendix). Ten ultramafic (komatiite) samples were also previously analyzed for HSE (32). High-precision W isotope measurements were performed on a subset of 30 samples. We followed a slightly modified analytical protocol of reference (18) as outlined in SI Appendix. Results of high-precision W isotope analyses are reported in the µ-notation (equivalent to parts per million) relative to National Institute of Standards and Technology Standard Reference Material 3163 and given in Dataset S1 (session mean values) and Dataset S2 (single measurements). The µ¹⁸²W isotope compositions addressed in this manuscript always refer to the measured ¹⁸²W/¹⁸⁴W ratio that has been corrected for mass bias by using ¹⁸⁶W/¹⁸⁴W = 0.92767 (denoted “6/4”) (33). Major and trace element compositions and isotope dilution data are reported in Dataset S3.

High-precision concentration measurements are required to identify samples with primary W abundances, because W is strongly fluid mobile when compared to elements of similar incompatibility (e.g., Th, U, and Ta) (34). Thus, W mobility during fluid alteration results in noncanonical W/Th ratios (34), whereas, in undisturbed magmatic systems, the similarly high incompatibility of W and Th results in canonical W/Th ratios between 0.09 and 0.24 over a wide range of mafic to felsic melts (34). Our Archean rock samples display primary magmatic trends for Th versus Zr (Fig. 1A), but not for W versus Zr, as W was enriched in many samples (Fig. 1B). Samples with canonical W/Th ratios, however, define a pristine magmatic trend (Fig. 1B), allowing us to restrict further consideration primarily to samples with ¹⁸²W isotope composition unmodified by secondary processes.

Fig. 1.

Trace element variation diagrams showing Th and W vs. Zr contents in mafic−ultramafic rocks from the Pilbara Craton (A and B) and high-precision µ¹⁸²W data for rocks from the Pilbara Craton versus W/Th ratios (C). All concentration measurements were conducted by isotope dilution. Also included are Pilbara samples that were previously analyzed for their ¹⁸²W isotope composition (15, 36). (A) Nearly all Pilbara samples have preserved robust magmatic differentiation trends for Th vs. Zr, indicating that Th was not significantly affected by secondary processes. Only the postorogenic samples from the Bookingarra Group were contaminated by crustal components, in line with previous studies (35). (B) In contrast to Th, the scattered concentrations of W mirror postmagmatic W enrichment by secondary processes. Primary magmatic differentiation trends are preserved in samples with canonical W/Th ratios. Notably, all samples from ref. 36 display strongly elevated W concentrations that indicate postmagmatic W enrichment. (C) Samples with undisturbed elemental W budgets (red squares, W/Th lower than 0.30; SI Appendix) display variable ¹⁸²W isotope excesses that illustrate a decrease of ¹⁸²W excesses with time (Fig. 2). Samples with elevated W/Th ratios (grey squares) were affected by secondary W enrichment through metasomatic agents that integrate the ¹⁸²W isotope composition of the whole Pilbara Craton. Accordingly, these samples display intermediate ¹⁸²W isotope excesses yielding the same average as lithosphere derived samples (grey and blue bars, respectively).

¹⁸²W Systematics of the Pilbara Craton

The oldest plume-related mafic volcanic samples from the EPT (Warrawoona Group) all display resolvable ¹⁸²W excesses. However, Warrawoona Group rocks, and a slightly older gabbroic enclave from the Shaw Granitic Complex (Mount Webber Gabbro; Pil17-07), with supracanonical W/Th ratios, display markedly lower µ¹⁸²W compositions when compared to samples with undisturbed elemental W budgets. Samples with primary W concentrations (canonical W/Th) display an average of µ¹⁸²W = +12.6 ± 1.4 ppm [95% CI, n = 8, also including previously published data for two samples from the Apex basalt (15); red bar in Fig. 2], whereas rocks affected by secondary W enrichment display a resolvably lower excess of ∼+8.1 ± 1.4 ppm (95% CI, n = 8; gray bar in Fig. 2). Interestingly, the average ¹⁸²W excess in the metasomatized samples is indistinguishable from the long-term lithospheric average (Fig. 2 and below). Samples from the younger, plume-derived ∼3.35-Ga Kelly Group (Euro Basalt) exhibit a contrasting pattern. The altered sample Pil16-39a displays a µ¹⁸²W value of +8.5 ± 2.1 ppm, again indistinguishable from the long-term lithospheric average. However, the unaltered sample Pil16-53a shows a resolvably lower excess of µ¹⁸²W = +5.2 ± 2.6 ppm when compared to unaltered samples from the Warrawoona Group. The youngest sample from the EPT, a 3.18-Ga Honeyeater basalt, displays no ¹⁸²W isotope excess, overlapping the modern upper mantle value (Pil16-51, µ¹⁸²W = +1.1 ± 3.9 ppm). The eruption of the Honeyeater Basalt marks extension at the onset of clear plate tectonic style processes (the earliest recorded Wilson Cycle) in the Pilbara Craton (31). Thus, decreasing µ¹⁸²W values in the unaltered samples indicate a diminishing ¹⁸²W isotope excess with time during a change in the tectonic regime. Additional evidence for change in geodynamic pattern is manifested in rocks from the 3.13- to 3.11-Ga Whundo Group (WPS), a sequence with geochemical characteristics that are consistent with derivation from a depleted mantle source that was variably affected by subduction components in an arc-like setting (30). Samples from the Whundo Group exhibit consistently lower µ¹⁸²W (Pil16-67: µ¹⁸²W = +5.3 ± 4.8; Pil16-74: µ¹⁸²W = +5.9 ± 3.2) than Warrawoona Group samples with canonical W/Th ratios. Beside data for 3.48-Ga komatiites from the Komati Formation of the Kapvaal Craton (10), our data provide the oldest evidence for modern upper mantle-like W isotope compositions in the early Archean rock record.

Fig. 2.

High-precision µ¹⁸²W analyses for rocks from the Pilbara Craton presented in stratigraphic order. Associated uncertainties refer to the corresponding 95% CIs of multiple digestions. Mantle-derived samples with undisturbed elemental W budgets (having canonical W/Th ratios) are represented by red squares; mantle-derived samples that were affected by secondary W enrichment (supracanonical W/Th ratios) are shown as grey squares. Undisturbed Warrawoona Group samples display an average of µ¹⁸²W = +12.6 ± 1.4 ppm (95% CI, n = 8; red bar), whereas rocks that were affected by secondary W enrichment display resolvably lower excesses of ∼+8.1 ± 1.4 ppm (95% CI, n = 8; gray bar). Younger mantle-derived rocks with undisturbed W budgets display significantly lower ¹⁸²W excesses. Lithosphere-derived rocks are shown as blue symbols. Granitoid rocks (blue squares) include nongneissic tonalites and more fractionated and postorogenic granitoids. Shales (blue circles) provide information about the upper continental crust, whereas lithospheric mafic magmatism is recorded in samples from the Bookingarra and Fortescue groups. The lithospheric samples provide an average µ¹⁸²W of +8.3 ± 1.0 ppm (blue bar) which is significantly lower than the average µ¹⁸²W of +12.6 ± 1.4 ppm for Warrawoona Group samples with canonical W/Th (red bar). Notably, samples that were affected by secondary W enrichment (gray bar) display the same average as the lithosphere-derived rocks (blue bar). R, previously published data for two samples from the Apex basalt (15); K, Kelly Group; SS, Sulphur Springs Group; S, Soanesville Group.

The ¹⁸²W isotope excesses also slightly decrease in granitoids: Sodic and potassic granitoids (Pil16-34 and Pil16-35, 3.47-Ga North Shaw Tonalite and Pil16-41, 3.47-Ga Homeward Bound Granite) display µ¹⁸²W values that are indistinguishable from the Warrawoona Group average (only considering samples with canonical W/Th ratios). However, highly fractionated postorogenic monzogranites (Pil16-36A, 2.85-Ga Spearhill Monzogranite and Pil16-12, 3.45-Ga North Pole Monzogranite; blue squares in Fig. 2) display significantly lower µ¹⁸²W values than this Warrawoona Group average. Hence, the granitoids in the EPT mimic the ¹⁸²W isotopic evolution of the ambient mantle, although with some delay. Given that the oldest and less evolved sodic granites are directly derived from mafic protocrust, this protocrust must have had an average ¹⁸²W excess similar to that of the Warrawoona Group samples. Lower ¹⁸²W isotope excesses in younger and more evolved granitoids mirror the temporal decrease in ¹⁸²W as also observed for mantle-derived mafic rocks. Prevalent ¹⁸²W excesses that are lower than the pristine Warrawoona Group average (canonical (W/Th) are also recorded in Mesoarchean to Neoarchean shales (Pil16-50B, 3.19-Ga Paddy Market Formation and Pil16-38b, 2.76-Ga Hardey Formation, µ¹⁸²W between +7.1 and +7.6 ppm), a rift-related lithosphere-derived dolerite from the Fortescue Group (µ¹⁸²W = +7.6 ± 3.1 ppm Pil16-31, 2.78-Ga Black Range dolerite) and mafic rocks from the Bookingarra Group (ca. 2.95 Ga; Pil16-63, Pil16-65, and Pil16-75), which are derived from a metasomatized, lithospheric mantle source that was refertilized by subducted sediments in a subduction zone-like setting (35) (µ¹⁸²W between +8 and +11 ppm; Fig. 2). The shales provide an average of the ¹⁸²W isotope composition of the upper crust, and the lithosphere-derived mafic intrusion probes the composition of the subcontinental lithospheric keel. Unlike the shales, a Paleoarchean Algoma-type banded iron formation from the EPT (Pil16-62B, 3.47-Ga Duffer Formation) has negligible detrital components (0.4% Al₂O₃), and carries a ¹⁸²W excess of +9.0 ± 3.3 ppm. We interpret this signature as being seawater-derived and reflecting the average composition of the ambient hydrothermal input provided by local volcanism with similar W isotope composition.

Altogether, the integrated ¹⁸²W isotope compositions recorded in granitoids, shales, and lithosphere-derived rocks provide a lithospheric average µ¹⁸²W of +8.3 ± 1.0 ppm (n = 11; blue bar in Fig. 2), which is significantly lower than the average µ¹⁸²W of +12.6 ± 1.4 ppm for Warrawoona Group samples with canonical W/Th (n = 8; red bar in Fig. 2) but higher than the modern upper mantle-like µ¹⁸²W of +1.1 ± 3.9 ppm for the Honeyeater Basalt. Thus, the lithospheric average seems to record the integrated ¹⁸²W isotope composition of different mantle reservoirs. Samples from the EPT that were affected by secondary W enrichment (grey bar in Fig. 2) show an average similar to the lithosphere-derived rocks, which is evidence for a craton-wide homogenization of ambient ¹⁸²W isotope compositions and the partial homogenization of primary ¹⁸²W isotope variability. Hence, using elemental W-Th systematics is a key tool, as this can unambiguously clarify the origin of ¹⁸²W isotope variability. In contrast to a previous study (36), where this tool has not been applied and altered samples were studied, we can now distinguish between diminishing ¹⁸²W isotope anomalies in the ambient mantle and secondary processes that redistribute ¹⁸²W isotope anomalies (Fig. 1C).

Our study demonstrates that the oldest mantle-derived rocks from the Pilbara Craton with canonical W/Th display ¹⁸²W isotope excesses of a magnitude similar to other Archean cratons. As outlined above, however, there is no uniformly accepted explanation for the origin of these ¹⁸²W excesses. For the Pilbara Craton, the missing late veneer hypothesis provides a plausible explanation, as decreasing depletions of platinum group elements (PGE) in komatiites with decreasing age are consistent with a progressive in-mixing of late veneer material (32). Among the 10 komatiite samples from the EPT and WPS that were previously analyzed for HSE (32), we found only one sample (179738) that preserved its primary W budget. All other komatiites from this sample selection were metasomatically overprinted and exhibit strongly elevated W/Th ratios up to 51 (sample 160245). This confirms previous evidence that ultramafic rocks are extremely susceptible to (fluid mediated) second-stage enrichment of W(17), which causes decoupling of HSE and ¹⁸²W systematics. The komatiite sample 179738 (3.46-Ga Apex Basalt of the Warrawoona Group) with a canonical W/Th ratio is strongly depleted in PGE (37) and displays an elevated ¹⁸²W of +11.9 ± 2.9 ppm, consistent with a model assuming an incomplete late veneer contribution to the mantle source of this sample. Such an interpretation is also in line with the absence of ¹⁴²Nd anomalies in Pilbara rock samples that show significant PGE depletions (36), thereby making unlikely early silicate crystal−liquid differentiation as an additional explanation for the occurrence of ¹⁸²W isotope excesses in the Pilbara samples. Collectively, our data indicate that Hadean, pre-late veneer mantle was preserved in the mantle sources of Pilbara igneous rocks until ca. 3.3 Ga to 3.1 Ga, much longer than evident from other cratons (12, 26). The long-standing apparent mismatch between virtually constant ¹⁸²W isotope excesses and progressively vanishing ¹⁴²Nd anomalies through Archean time (12) is therefore well explained by the larger mobility of W compared to Nd, and its redistribution during secondary processes. By only considering samples with strictly canonical W/Th ratios, we show that the prevailing ¹⁸²W isotope excesses even in the younger Archean rock record (Fig. 3) may be a vestige of older rocks, from which W was progressively redistributed. Hence, we rather argue that mantle convection homogenized primordial ¹⁸²W isotope anomalies during the Paleoarchean and Mesoarchean (Fig. 3) at the same timescales as ¹⁴²Nd systematics (38).

Geodynamic Implications

Our ¹⁸²W results for the Pilbara Craton have important implications for understanding timescales of geodynamic processes on early Earth and provide a future reference parameter for geodynamic models addressing mantle mixing in early Earth. Geodynamic models addressing the Pilbara data have to account for the observations that 1) a Hadean pre-late veneer signature remains isolated for more than 1 billion years and 2) such an isotopically isolated mantle domain continuously triggered prolonged plume-driven magmatism with a distinct isotopic signature, most likely in a stagnant lid regime (39). In contrast to present-day mantle plumes with negative ¹⁸²W signatures (13), Archean mantle plumes beneath the Pilbara Craton appear to have carried rather uniform ¹⁸²W isotope excesses that were similar to the shallower mantle, thus arguing that compositionally distinct deep mantle sources such as large low-shear velocity provinces or ultra-low-velocity zones may not have been involved or, alternatively, even may not have formed (39, 40).

The marked geodynamic transition at 3.3 Ga to 3.1 Ga in the Pilbara Craton with influx of modern-like, upper mantle material carrying smaller ¹⁸²W isotope excesses coincides with models claiming a global transition from stagnant lid-type regimes toward modern plate tectonics. This transition is, for instance, postulated from crustal growth curves based on the zircon record (41), from the increased occurrence of subduction-like magmatism in the Archean (42), from the change of composition in the subcontinental lithospheric mantle (43), and from increasingly larger volumes of exposed felsic crust (44). It is therefore tempting to postulate that the initiation of plate tectonics caused a more efficient homogenization of Earth’s mantle than vertical, plume-driven dynamics that prevailed in a stagnant lid regime as proposed for the older units of the Pilbara Craton. Indeed, geodynamic models for a stagnant lid regime show that, contrary to expectation, the timescales of mantle mixing in the Archean were not accelerated due to a hotter thermal state of the mantle but were rather prolonged (38). The Archean regime was possibly characterized by small, isolated convection cells and episodic mantle overturn events that vertically homogenized discrete regions of the mantle (39, 45) but maintained lateral heterogeneities for billions of years (2). Most likely, the initiation of modern plate tectonics was not a sudden event but rather occurred gradually (46). These circumstances might explain why 1) other Archean lithostratigraphic successions of similar age to the Pilbara Craton display invariant ¹⁸²W patterns (17, 24) or already show modern upper mantle-like µ¹⁸²W isotope compositions (10) and, 2) in some cases, ¹⁸²W anomalies survived at least until the Late Archean (47). In fact, our interpretation is in line with a recent study interpreting heterogeneous ¹⁸²W isotope compositions in Archean rocks as expressing localized subduction events (as in the Pilbara Craton) during a transition period (∼3.6 Ga to 2.7 Ga) from plume tectonics to modern plate tectonics (40).

In summary, our study demonstrates that the Pilbara Craton preserves a natural laboratory for studying Archean geodynamics that may serve as a future test case for interdisciplinary studies assessing timescales of mantle dynamics through deep time. Complementary to attempts trying to infer timescale estimates for mantle stirring times from global modeling (48), our approach adds to a more detailed information pattern providing individual homogenization histories of isotopically anomalous Archean mantle domains. Therefore, the rock record of the Pilbara Craton provides important observational constraints on convective processes within the Archean mantle which are indispensable for numerical simulations, as such input parameters define the evolutionary pathway of computational convection models (49).

Data Availability.

All study data are included in the article and supporting information.