The 142Nd/144Nd Variations in Mantle-Derived Rocks Provide Constraints on the Stirring Rate of the Mantle from the Hadean to the Present

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The 142Nd/144Nd variations in mantle-derived rocks provide constraints on the stirring rate of the mantle from the Hadean to the present

Eugenia Hyunga,1,2 and Stein B. Jacobsena

aDepartment of Earth and Planetary Sciences, Harvard University, Cambridge, MA 02138

Edited by Donald E. Canfield, Institute of Biology and Nordic Center for Earth Evolution, University of Southern Denmark, Odense M., Denmark, and approved May 1, 2020 (received for review April 15, 2020) Early silicate differentiation events for the terrestrial planets can variations can be used as a tracer for geodynamic mixing of enriched be traced with the short-lived 146Sm-142Nd system (∼100-My half- and depleted reservoirs (1). Measurements of μ142Nd by thermal life). Resulting early Earth-produced 142Nd/144Nd variations are an ionization mass spectrometry (TIMS) of roughly present day rocks excellent tracer of the rate of mantle mixing and thus a potential (7, 11), including mantle peridotites, mid-ocean ridge basalts tracer of plate tectonics through time. Evidence for early silicate (MORBs), ocean island basalts (OIBs), kimberlites, and continental differentiation in the Hadean (4.6 to 4.0 Ga) has been provided by flood basalts, have not revealed clearly resolved variations at the 142 144 Nd/ Nd measurements of rocks that show both higher and external reproducibility level of ±5 to 8 ppm (2σ)exceptforsome ± lower ( 20 ppm) values than the present-day mantle, demonstrat- recent results from Réunion and Samoa (12, 13), which have been ing major silicate Earth differentiation within the first 100 My of σ reported to be statistically resolved. solar system formation. We have obtained an external 2 uncer- The typical level of external 2σ reproducibility for published tainty at 1.7 ppm for 142Nd/144Nd measurements to constrain its 142Nd/144Nd measurements range from 2σ =±3 to 8 ppm. There homogeneity/heterogeneity in the mantle for the last 2 Ga. We re- are many published 142Nd/144Nd measurements at this level of port that most modern-day mid-ocean ridge basalt and ocean island basalt samples as well as continental crustal rocks going back to 2 precision, including data for relatively young rocks (e.g., ref. 7). Ga are within 1.7 ppm of the average Earth 142Nd/144Nd value. Con- Data of this quality could be used to argue for relatively large

142 144 EARTH, ATMOSPHERIC, AND PLANETARY SCIENCES sidering mafic and ultramafic compositions, we use a mantle-mixing Nd/ Nd heterogeneities, even in the present mantle (13). To model to show that this trend is consistent with a mantle stirring test whether large heterogeneities are real and common in the time of about 400 My since the early Hadean. Such a fast mantle modern mantle, it is necessary to test for with high-precision stirring rate supports the notion that Earth’s thermal and chemical measurements, as this is important for understanding the rate evolution is likely to have been largely regulated by plate tectonics of mixing in the mantle through time. A recent study has achieved for most of its history. Some young rocks have 142Nd/144Nd signa- a high degree of external reproducibility (2σ <1 to 2.6 ppm) tures marginally resolved (∼3 ppm), suggesting that the entire man- using multicollector inductively coupled plasma mass spectrom- tle is not equally well homogenized and that some silicate mantle etry and reported new 142Nd/144Nd measurements of this quality signatures from an early differentiated mantle (>4.1 Ga ago) are for early Archean samples (14). However, except for ref. 2 there preserved in the modern mantle. are no data with this quality for post-Archean samples, particularly between 500 Ma and 2.0 Ga. Such data are needed for a igh-precision measurements of the now-extinct 146Sm-142Nd proper interpretation of the 142Nd/144Nd record for all of Earth’s Hsystem are an excellent tool for tracing early silicate differ- history. entiation processes that occurred within the first 500 My of solar system history and subsequent mixing through convective processes Significance (1–6). Sm and Nd are concentrated to different degrees in the melt with respect to the solid during partial melting or fractional crys- We have obtained ultrahigh-precision 142Nd/144Nd measurements tallization, and thus these processes produce variations in Sm/Nd 142 144 for post-Archean terrestrial samples by thermal ionization ratios. Variations in Nd/ Nd can only be developed in early- mass spectrometry. The post-Archean mantle is homogeneous in formed Sm/Nd fractionated reservoirs (less than 500 My after Earth 142 144 147 143 Nd/ Nd, in contrast to the large variability in the Archean. formation). In contrast, the long-lived Sm- Nd system records Using the data as constraints, we arrive at an average mantle Sm/Nd fractionation throughout Earth’shistoryas143Nd/144Nd 142 144 stirring rate throughout time. Our results are consistent with the variations. Variations in Nd/ Nd are now usually reported as ’ μ142 = 142 144 142 144 − × perspective that Earth s chemical evolution has been largely Nd [( Nd/ Ndsample/ Nd/ Ndstandard) 1] 1,000,000, regulated by plate tectonics for most of its history. units of parts per million (ppm), with respect to the modern terrestrial value as a reference point (2, 6, 7). The Nd isotope stan- Author contributions: E.H. and S.B.J. designed the study. E.H. processed the samples and 142 144 dard JNdi-1 (8) is thought to give the average Nd/ Nd value of performed the measurements. E.H. and S.B.J. performed calculations and wrote the modern mantle (7). The first evidence for early silicate dif- the paper. ferentiation in the Hadean was discovered in the 3.8-Ga-old The authors declare no competing interest. 142 Greenland Isua supracrustal rocks as positive μ Nd values (>0), This article is a PNAS Direct Submission. preserving a record of the early depleted (high Sm/Nd ratio) Published under the PNAS license. mantle (6). A complementary, enriched (low Sm/Nd) Hadean Data deposition: Data for this paper have been deposited at EarthChem (https://ecl. 142 mantle reservoir, with negative μ Nd values is the source of the earthchem.org/view.php?id=1545). 3.4-Ga-old mafic Greenland Ameralik dykes (9). The presence of 1To whom correspondence may be addressed. Email: [email protected]. μ142 such variations of Nd in rocks formed well after the extinction 2Present address: Division of Geological and Planetary Sciences, California Institute of 146 of Sm is a measure of the process of mantle mixing and may Technology, Pasadena, CA 91125. reflect binary mixing to varying degrees (10). Subsequent mixing of This article contains supporting information online at https://www.pnas.org/lookup/suppl/ mantle reservoirs can only result in smaller μ142Nd variations doi:10.1073/pnas.2006950117/-/DCSupplemental. as a function of time, reflecting the extent of mixing. Thus, μ142Nd

www.pnas.org/cgi/doi/10.1073/pnas.2006950117 PNAS Latest Articles | 1of7 Downloaded by guest on October 1, 2021 external 2σ reproducibility of ∼±1.7 ppm. Measurements of the long-lived 147Sm-143Nd system are also reported (SI Appendix, 143 143 144 143 Table S7) and use the « Nd value (= [( Nd/ Ndsample/ Nd/ 144 NdCHUR) − 1] × 10,000) to determine whether the mantle source of the samples had long-term enrichment («143Nd < 0) or depletion («143Nd > 0) of a light rare earth-enriched component (typical for melts). MORB samples include HIMU, enriched mantle 1 (EM1), and the depleted MORB mantle whose «143Nd values are >10, with the exception of an EM1 MORB with a low «143Nd value of +0.8. A depleted N-MORB is resolved to be ∼4 ppm lower (2σ =±1.0) in μ142Nd than an enriched MORB, determined at a long-term reproducibility of better than 1.7 ppm. In contrast, all our other modern MORB and OIB measurements agree to within ±1.7 ppm of the JNdi-1 standard. The enriched 142Nd/144Nd signature (μ142Nd < 0) in a depleted MORB sample indicates the decoupling of the long-lived 147Sm-143Nd and short- lived 146Sm-142Nd systems and reflects the complex history of the mantle, where an initially enriched mantle reservoir is recycled to later become depleted by melt extraction. A number of continental crustal samples were selected to cover the age range from 300 Ma to 2 Ga; they are all from the Baltic Shield. These measurements exhibit μ142Nd-values within ±1.7 ppm (2σ)oftheJNdi- Fig. 1. (A) 142Nd/144Nd plotted as deviations in ppm from the JNdi-1 stan- 1 standard (Fig. 2). We deduce from our new data that the mantle dard, using standard multidynamic data reduction with an exponential law has developed a μ142Nd-value that is limited to within ±1.7 ppm of (RED1). (B) The same data with our reduction/measurement method (RED2) μ142Nd = 0, suggested to have persisted for the past 2 Ga, while 142 144 shows a lot less scatter in the calculated Nd/ Nd values and demon- some slightly larger heterogeneities may exist. The 142Nd/144Nd σ ≤ ± strates a 2 1.7 reproducibility. This data reduction method is used for uniformity for the past 2 Ga (Fig. 3, green squares) is in strong the results presented in this paper. (C) This plot demonstrates the relation- contrast to pre-2.5 Ga data (Fig. 3, blue circles), where 142Nd/144Nd ship between reduction methods RED1 and RED2. The ratio of the cup fac- – – variations are the rule rather than the exception. tors (fH2/fAx) for the two Faraday detectors, raised to the power of ( 1 p), is a 142 simple function of the ppm difference of 142Nd/144Nd from the two methods. The persistence of Nd heterogeneity into the Archean, and Based on this plot, we find this reduction method to be the main contrib- as “late” as 2.7 Ga, particularly for felsic compositions (SI Ap- uting factor to the improvement of 142Nd/144Nd reproducibility. The slope of pendix, Fig. S9), has been suggested to be due to the sluggish the data is roughly consistent with SI Appendix, Eq. S15 for this relationship. mixing behavior of the mantle in the Hadean and Archean, due The error bars are 2σ. The averages for JNdi-1 for RED1 vs. RED2 are to stagnant lid tectonics or sluggish mantle dynamics that would 1.1418343 and 1.1418395, respectively. The JNdi-1 standard measurement have slowed the mixing rate of the mantle (3, 15). The data in ∼ for RED1 is lower than that of RED2 by 4 ppm. Fig. 3 demonstrate this rapid changeover of μ142Nd from ±20 ppm in the Archean–Hadean to ±2 ppm in the post-Archean Here, we report ultrahigh-precision 142Nd/144Nd measurements Earth. In contrast to the suggestion that this is due to the sluggish behavior of the mantle, we show here that the persistence of by TIMS spanning from the present day to 2 Ga ago to investigate – the extent of homogenization of 142Nd/144Nd ratios throughout the such heterogeneities in the Archean Hadean can be attributed to a “fast” mantle mixing (∼400 My stirring rate; range: 250 to Proterozoic and the modern day. These data will be used to assess the extent of mantle mixing, providing a perspective from Nd isotopes to infer the dominant mode of tectonics throughout a large extent of Earth’s history (3). Our newly developed multidynamic method for measuring 142Nd/144Nd ratios (RED2) was aimed at canceling out differences in detector efficiencies when using the exponential law for fractionation corrections (Fig. 1 and SI Appendix, section 6). Measurements consist of just 142Nd and 144Nd measured in two Faraday cups and then 144Nd and 146Nd measured in the same two cups. Time-dependent drift corrections are then used to both cancel out differences in cup efficiencies and to obtain the correct fractionation in multidynamic runs. The standard multidynamic reduction that is commonly used (RED1 in Fig. 1A) yields a much larger scatter in the data than using the RED2 method because it cannot completely eliminate the cup factor dependence of the results (for further details see SI Ap- pendix,section6). With this measurement/reduction scheme, we obtained a 2σ external reproducibility of ±1.7 ppm. The results by two reduction methods show a tight linear relation for standards (Fig. 1C), providing a means to understand some of the scatter shown in TIMS data (SI Appendix, section 6). Fig. 2. μ142Nd measurements from this study. Modern-day MORBs (blue), Our ultrahigh-precision 142Nd/144Nd measurements of terrestrial OIBs (red), and continental crustal samples (green) whose ages range from μ142 samples constrain the extent of mantle 142Nd/144Nd heterogeneity 288 Ma to 1.792 Ga are shown. The Nd values are plotted relative to the μ142 JNdi-1 standard, which is assumed to be representative of the average through the past 2 Ga. The Nd values of terrestrial samples are mantle. Dashed lines indicate our measured 2σ uncertainty of ±1.7 ppm for plotted in Fig. 2 relative to the JNdi-1 standard, which is assumed to this standard. All of these samples are within the ±1.7 ppm 2σ error of the 142 144 be representative of average mantle. The Nd/ Nd measurements JNdi-1 standard, except for one Atlantic N-MORB sample which is slightly (Figs. 1 and 2 and SI Appendix, Table S6) were determined with an lower and outside the 2σ uncertainty of JNdi-1.

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Fig. 3. (Upper) The μ142Nd data (green squares) reported here for samples ranging in age from 0 to 2 Ga are compared to published μ142Nd data for Archean rocks, particularly mafic to ultramafic in composition, of 2.4 to 4.0 Ga age (blue circles; refs. 2, 3, 9, 10, 11, 15, and 40–54). Modern-day measured samples of this study (green squares), which are clustered around T = 0, are slightly offset from 0 to better exhibit the distribution of the data in comparison to model results. The data are compared to results from a model for mantle mixing (18). The rapid changeover happens when the layers in the convectively stirred mantle are sufficiently thin that sampling of the mantle by basalt melt extraction will always only give the average value. At early times one will expect to see the full range of μ142Nd for a stirring rate that is similar to what is calculated for the modern day (16). (Lower) Synthetic data generated for various stirring rates.

750 My) throughout the history of the Earth (16, 17). We present abundances of the enriched and depleted signatures (Fig. 4B). calculations within a quantitative framework using a stochastic The model scenario should not be considered analogous to an mixing model (16, 18) to test this idea. We explore a simplistic “absence of mixing” as the reservoirs are still too large at this scenario where the upper and lower mantle volumes are set up to stage in this idealized model compared to the length scale of represent the early (Hadean) enriched (EEM) and early de- sampling (modeled after a typical magma column) and need time pleted mantle reservoirs (EDM), respectively, and compare the to be shaped, as further discussed later. Initially heterogeneous results with the distribution of data throughout time. We also reservoirs may generate a histogram distribution that better re- explore various crustal growth models to which we compare the sembles that of the data. Notably, the presence of 142Nd/144Nd extant data. In the stochastic model, the enriched and depleted distributions that exhibit μ142Nd = 0 but are not chondritic reservoirs are assumed to be generated through a magma ocean («143Nd ≠ 0) at 3.8 Ga (Fig. 4A) strongly suggests that the mantle spanning more or less the whole mantle, with a stirring rate τ, has already started to efficiently homogenize early on subsequent resulting in stretching and thinning of layers with initial hetero- to large-scale mantle differentiation, due to mantle processes geneity length scales of l0-upper and l0-lower. Tests involving smaller that persisted before this time. In turn, ultrahigh-precision data reservoirs tend to homogenize the Archean 142Nd/144Nd varia- may better clarify the distribution of μ142Nd values, and partic- tions much more rapidly and are unable to reproduce the data ularly the true distribution around μ142Nd = 0, as lower precision distribution in the Archean (SI Appendix, Fig. S6), even when levels may introduce ambiguity as to the extent of homogeniza- assuming a slower mixing rate that span the history of the Earth. tion. A μ142Nd value of ∼10 ppm at 3.8 Ga, instead of 20 as The short length scales of the mantle reservoirs are stretched suggested for the Isua Eoarchean crust (14), would not affect and thinned such that they decay exponentially as a function of how quickly μ142Nd is homogenized, as such a rate is not pri- −t/τ 142 time t, lt = l0e , due to toroidal flow. The extent of the Nd/ marily affected by the magnitude of Nd anomalies but by the 144Nd signatures used in our model is chosen to cover the range stirring rate (SI Appendix, Fig. S8). of Archean 142Nd/144Nd signatures. In assuming two initially In contrast, an overall slower mantle stirring rate closer to 800 homogeneous reservoirs, synthetic isotopic signatures demon- My, as in the stagnant lid scenario, would be unable to reproduce strate a bimodal distribution of heterogeneities in the first few the range of μ142Nd signatures detected throughout the Pro- hundred million years of this model, as shown in Fig. 3, where terozoic. In this case, large-scale μ142Nd variations detectable at the relative sizes of the reservoirs may be reflected in the relative the 2σ =±5 ppm level would be predicted to persist and have a

Hyung and Jacobsen PNAS Latest Articles | 3of7 Downloaded by guest on October 1, 2021 However, this is a rather marginal effect that could also be due to nonideal behavior of this sample during mass spectrometry, as suggested by the deviations from the linear trend between the cup factor and the two dynamic reduction methods for the two measurements, expressed in Fig. 1C. The μ142Nd variations that deviate from the idealized model suggest that some domains of the modern mantle are not as well mixed as the dominant part of the mantle producing magmas. Such “islands” are suggested to survive in mantle convection models due to localized regions of high viscosity or non-Newtonian rheology (19). In testing various stirring rates that can roughly reproduce this data distribution, a stirring rate in the mantle of about 400 My explains the overall pattern of 142Nd/144Nd through Earth’s history for mafic to ultramafic compositions (Fig. 3). When also considering all 142Nd/ 144Nd signatures throughout time, a stirring rate of about 500 My explains the negative μ142Nd anomalies at 2.7 Ga (SI Appendix, Fig. S9). This is within the ranges that were derived from the long-lived decay systems 147Sm-143Nd and 87Rb-87Sr, based on mantle mixing in the presence of plate tectonics through time (16). As the early enriched and early depleted reservoirs are gradually stretched and thinned due to convection, the mantle is gradually homogenized, and the μ142Nd changeover from an early heter- ogenous distribution to a late (essentially gone by 2 Ga; see Figs. 3 and 5) homogenization of μ142Nd happens rapidly. We also explore mixing scenarios with more than one mixing rate throughout the history of the Earth, paying attention to the relative distributions of enriched and depleted μ142Nd signatures. 142 142 Fig. 4. Comparison of μ Nd histograms (A and C) with μ Nd histogrames In the case of a singular stirring rate, a value of ∼400 My best ∼ predicted from the model with a mantle stirring rate of 300 My until 3.7 reflects the data distribution for mafic/ultramafic compositions. Ga, after which a stirring rate of 600 My is applied (B and D). The green histograms indicate mafic/ultramafic compositions, whereas the yellow his- Since the stirring rate might be expected to be faster during the togram bars indicate intermediate/felsic compositions and metasediments. Hadean, we also tested models with a dual stirring rate. We The histogram occurences for each composition are not stacked on top of found that starting with a 300-My stirring rate that switches to 142 each other. The yellow is shown in front of the green. The model exhibits a 600 My at 3.7 Ga, the synthetic histogram distribution of μ Nd bimodal distribution of reservoirs when assuming two initial homogeneous resembles the distribution of μ142Nd data (Fig. 4 C and D) even reservoirs, as long as the length scale of heterogeneity is twice as long as the better than for a single stirring rate of 400 My. Generally, the 142 length scale of sampling (18). The μ Nd data could be argued to be con- shorter stirring rates ranging from 400 to 600 My fit the Archean ’ ’ sistent with Earth s thermal and chemical evolution s being largely regulated data well. We also show data for intermediate/felsic rocks in by plate tectonics for most of Earth’s history. Fig. 4D. While these rocks are not derived directly from the mantle, they are in the Archean known to largely reflect the high likelihood of being sampled as recent as 1.0 Ga (SI Ap- isotopic evolution of the mantle due to their typical short (50 to pendix, Fig. S7). Instead, the broad ranges of μ142Nd values are 100 My) crustal prehistories shown by Rb-Sr systematics (20). In observed to converge into a limited distribution of values at contrast, we find that a “slow” mantle mixing scenario through- around 2 Ga, predicting a relatively quick change from a wide out the Hadean (τ = 800 My), would still predict a significant range of μ142Nd values (40 ppm maximum range at 3.5 Ga) into a much smaller range of ±1.8 ppm (2σ), as early as 2.4 Ga (Fig. 3). In our model, this results when the layers in the mantle, carrying the Hadean 142Nd/144Nd signatures, are sufficiently thin that sampling of the mantle by basalt melt extraction always gives the average value. Once the average 142Nd/144Nd of a sampling box is no longer distinguishable from one another after repeat sampling on the scale of the sampling box (melting column), the terrestrial mantle can be deduced to be homogeneous on the length scale of melting (SI Appendix, section 2 and Fig. S5C). The data show an abundance in depleted signatures at 3.8 Ga and an abundance in the enriched signatures at 3.4 Ga and as recent as 2.7 Ga when considering felsic compositions, as op- posed to a relative abundance of depleted signatures observed at 3.8 Ga. Ultrahigh-precision measurements of Phanerozoic and Proterozoic samples are compared with model results in Fig. 5. Most samples that are measured in this study are shown to Fig. 5. μ142Nd modern MORB and OIB sample measurements of this study cluster around μ142Nd = 0. Our results support using JNdi-1 as (green squares), compared to the last 2 Ga of the evolution diagram in Fig. 3. μ142 The modern-day data (T = 0) of this study are offset from zero for clarity. The representative of Nd in the average modern mantle. While σ =± the Proterozoic μ142Nd data all are within ±1.7 ppm, most of the 2 1.7 ppm error band around 0 is indicated with broken lines. The model results from Fig. 3 are plotted in purple for comparison. Previously published data plot slightly above the average modern mantle, whereas one data refs. 5, 7, 11–13, and 25–27 are indicated in gray, where modern-day 1.08 Ga sample (GN 12-03) plots significantly below these val- samples are offset from “0” to the left for clarity and to avoid overlap with ues. This may suggest that the 1.08-Ga-old sample records a dated samples. The lowest-most data point near 1.0 Ga is offset from the mantle that is different from the main trend of the samples. empirical trend in Fig. 1C and may reflect artifacts from fractionation.

4of7 | www.pnas.org/cgi/doi/10.1073/pnas.2006950117 Hyung and Jacobsen Downloaded by guest on October 1, 2021 abundance in depleted signatures at 3.0 Ga, with more than half the synthetic data points at that point still having a high likelihood of being depleted. In accounting for the abundance of enriched signatures between 3.8 and 3.4 Ga for all sample types, the stirring rate of the Hadean was likely to have been faster (300 My) rather than slower (>800 My), followed by a slightly slower (600 My) stirring rate. A “slow” (800 My) scenario starting from the Hadean followed by an abrupt “fast” mantle mixing scenario (300 My) would incur a scenario where there was simply not enough time to adequately mix the mantle to reflect the change in the distribution of extant μ142Nd data that occurs between 3.8 Ga to 2.7 Ga (SI Appendix,section3). The timing of the changeover of the wide 142Nd/144Nd data distribution apparent in the Archean to a narrow, limited band in the Proterozoic is particularly important, as it reflects the rate of mixing of the mantle pertaining to early Earth’s history. The sensitivity of the isotopic pattern to average mantle stirring rates over Earth’s history is shown in Fig. 3. Recently, it was proposed by Rosas and Korenaga (21) (R&K18 hereafter) that the Sm-Nd isotope evolution of the early Earth may be best interpreted as the result of extremely rapid early continental crustal growth combined with extremely high early continental crustal recycling rates akin to the Armstrong model (22). In revisiting Jacobsen and Harper (1) (J&H96 hereafter), we note that suppressing recycling in the first 500 My of Earth evolution is equally effective in generating depleted Archean 142Nd signatures. This is shown in Fig. 6 by comparing the μ142Nd depleted mantle evolutions for the continental crustal growth and recycling curves of R&K18 (21) with those of J&H96 (1, 23),

calculated using the exact analytical solutions for this problem EARTH, ATMOSPHERIC, AND PLANETARY SCIENCES (24). Additional calculated parameters are shown in SI Appendix, Figs. S1–S3. It is the fractionation of Sm and Nd in the mantle (quantified by the fSm/Nd value) that is required to generate a positive μ142Nd-value in the depleted mantle. The J&H96 (1, 23) model without recycling during the Hadean makes half the continental crust in the first 40 My and results in fSm/Nd growing to +0.12 during this time. In contrast, the R&K18 model makes the full continental volume in the first ∼100 My and results in fSm/Nd growing to +0.23 in the first 100 My. These two extremely different models result in very similar μ142Nd evolution during the Hadean. The reason is that without recycling, an fSm/Nd value of 0.12 is sufficient to produce the observed μ142Nd evolution, while when there is major recycling this value needs to be doubled to Fig. 6. The effect of crustal growth (A) and recycling (B)ontheμ142Nd 142 make up for recycling working to reduce the μ Nd value. During isotope evolution of the depleted mantle. The red crustal growth and 142 the post-Hadean, the μ Nd evolution is no longer sensitive to recycling functions correspond to the red curves in figure 3 in ref. 21, the Sm/Nd fractionation and is only affected (reduced) by recycling green curves to a two-stage recycling model from figure 17 of ref. 1, and the and remixing of reservoirs with positive and negative μ142Nd val- blue curves to the recycling model of ref. 1. The resulting μ142Nd isotope ues. In comparing models with a constant rate of recycling (0.7 × evolutions of the depleted mantle are shown in C. The green curve was 22 · −1 obtained by using the crustal growth and recycling functions of ref. 1 but 10 kg Gy ) and a second with exponentially decaying recycling = × 22 · −1 × 22 · −1 modified to use the same starting point (ts 4.51Ga) and D-values for Sm from an initial value of 2.0 10 kg Gy to 0.50 10 kg Gy , = = = = 142 and Nd as those of ref. 21 [DNd(ts) 35, DNd(tp) 45, DSm(ts) 20, DSm(ts) both yield very similar post-Hadean μ Nd evolutions for the “ ” SI Appendix, section 1 25] and is therefore labeled modified. The corresponding evolution of Hadean mantle ( ). Finally, we also compare «143Nd as well as other parameters are given in SI Appendix, Figs. S1–S3. The 142 with the μ Nd evolution calculated based on constraints only results show clearly that very similar μ142Nd can be generated both without 143 Sm/Nd from the « Nd and f evolution of the mantle and crust. recycling in the Hadean (1) and with extremely high early recycling rates It is clear from these considerations that there is no requirement (21). The μ142Nd evolution of the depleted mantle therefore cannot be used for continental growth to have occurred through all of the Hadean as a unique interpretation in favor of the Armstrong model. or have large recycling rates during that time (SI Appendix, section 1). In further exploring the enriched 142Nd signatures of the crustal development models, R&K18 tends to deviate from the average depleted mantle reservoirs, and how they are sampled in com- crustal «143Nd at the modern day (SI Appendix,Fig.S1). parison to a typical magma column. μ142 The extent of possible scenarios for exploring crustal growth in The combination of a wide range of Nd variations prior to the Hadean through 142Nd isotopes is limited, as they must occur about 2.5 Ga as well as their duration throughout the Archean when the 146Sm-142Nd system is still live. The scenarios of R&K18 must have involved one or more massive initial chemical dif- and the modified J&H96 bracket two extreme scenarios of μ142Nd ferentiation events, during the first 100 My of solar system his- 142 evolution due to crustal growth, one where there is fast recycling tory, as this is the only time when Nd heterogeneities can form in the Hadean, and the other where there is none. The stochastic relatively rapidly. This may have involved both the formation of mixing model presented in this study shows a mechanism through an early magma ocean following an event such as the Moon- which the switchover (Fig. 3) in data distribution of μ142Nd may be forming impact, as well as the earliest formation of a crust and a reflection of the relative sizes of the early formed enriched and its recycling (SI Appendix, section 1) to result in the EEM and

Hyung and Jacobsen PNAS Latest Articles | 5of7 Downloaded by guest on October 1, 2021 EDM forming within the first of 100 My. While the earliest crust scenario would push the initiation of plate tectonics as early as 4.2 likely developed sometime in the Hadean, it would likely not Ga. Such differences in perspective reflect various lines of evidence have been similar to today’s continental crust and probably was that may serve as strong indications of the initiation of plate tec- more mafic in nature, where the extent of Sm/Nd fractionation tonics. Our perspective based on inferences from mantle mixing due to crustal growth would likely have been limited. maybemoreinlinewiththelatterscenario. Most previous 142Nd/144Nd measurements of modern-day sam- Here we have demonstrated the homogenenization of 142Nd ples were revealed to be homogeneous at the 2σ =±2 to 8 ppm heterogeneity throughout Earth’s history, with a fast type of mantle level (2, 5, 7, 11, 25–27). In contrast, two resolved low and high stirring rate. Nd isotope heterogeneity itself cannot clearly dem- μ142Nd values of –7.9 and 7.3 (2σ =±4.4) ppm have been onstrate the early processes that created the early enriched and reported for Réunion Island lavas and were interpreted as due to depleted reservoirs or their starting compositions. However, it is 142Nd/144Nd heterogeneities in the modern convecting mantle required that they were generated early on, within the first ∼100 My (13). While these values could result from high-viscosity regions of solar system formation, and subsequently erased. The mechanism of the mantle, there is in this case another possible interpreta- through which they were generated may not be as important as the tion. Recently, Mauritius, a young island associated with the fact that they were generated then homogenized. Réunion hotspot, has revealed Archean zircons (28), attributed The evolution of the enriched and depleted reservoirs of sili- to Madagascar to the east of Mauritius and Réunion. Thus, cate Earth can be traced through the extinct Sm-Nd system and traces of Archean continental crust that are present in the oce- provides implications for the mode of convection that may have anic crust along this region could be the cause of the anomalous governed the mantle mixing process, to the suggested timing of μ142Nd values in Réunion. These continental crustal fragments when plate tectonics was initiated to how long it has persisted. are proposed to have formed from the ancient nucleus of Disagreements in the literature suggest that the matter is not a Madagascar and India, which rifted apart at about 100 Ma. The settled issue, and more investigation may be needed. closeness of the Sr-Nd-Pb isotope ratios demonstrated in the Mauritius lavas when compared to those of Réunion (29), shows Methods that Archean crustal contamination should not be ruled out and All sample preparation and mass spectrometric measurements were made at that additional studies are required to settle this issue. the Department of Earth and Planetary Sciences, Harvard University. Heterogeneities in 142Nd/144Nd appear to be rare in modern- day igneous rocks (2, 5, 7, 11, 13, 25–27), to the degree they occur Analytical Technique. Samples (∼100 mg) were dissolved in a mixture of con- for 182W/184W (30, 31), which sometimes occur in the samples with centrated HF and HNO3 (1:4 ratio) using a CEM microwave system. The tem- 142 144 perature was ramped to 180 °C in a span of 20 min, and held at 1 h, then lack of Nd/ Nd variability (32). Although the short-lived Sm-Nd ramped down to room temperature. The solution was dried down and then and Hf-W systems are affected during silicate–mantle differentia- redissolved in a mixture of concentrated HCl, HNO3, and MilliQ water (1:4:1) tion, the lack of isotopic variability in one system over the other, mixture. Samples that formed precipitates were repeatedly dissolved using the

suggests the decoupling of the two systems owing to different pro- second step in different ratios of HCl to HNO3 to ensure complete dissolution. cesses involved in the generation of heterogeneity. One viable ex- The dissolved samples were processed through ion exchange chemistry using planation is the leakage of the core, which has been proposed in BioRad AG 50W-X8 Resin (100 to 200 mesh), and ion exchange columns with a explaining modern-day 182W/184W heterogeneities (30). 1-cm internal diameter, 30 cm in length. The separated rare earth elements 142 144 The distribution of Nd/ Nd anomalies in the Archean (REEs) were then treated with 30% H2O2 and further dried down and passed subsequent to one or more early massive differentiation events, through REE columns twice with BioRad AG 50W-X8 Resin (200 to 400 mesh) whose internal diameters were 2 mm, to separate Nd from other REEs with and the contrasted uniformity persisting in the past 2 Ga reflect α-HIBA (0.2 M) calibrated at a pH of 4.62 and air-pressurized. Samples were homogenization due to efficiency in mantle mixing, a process passed twice through the columns. Nd blanks typically did not exceed 20 pg. To that was ongoing long before it was reflected in the decrease of make 142Nd/144Nd measurements, Nd aliquots that were purified through col- 142 144 142 144 Nd/ Nd anomalies. The Archean Nd/ Nd anomalies in umn chromatography (∼1,000 to 1,500 ng) were loaded on 99.999% triple Re the context of a well-mixed component eliminate the need for zone-refined filament (H Cross) at ∼0.6 A, where Nd isotope measurements were the currently proposed stagnant lid tectonics (3) with regard to carried with an Isoprobe T (HCT010) thermal ionization mass spectrometer the 142Nd/144Nd record in rocks. The isotopic evidence from equipped with new Xact amplifiers with 1011 Ω resistors (SI Appendix,section6). 142Nd/144Nd is consistent with plate tectonics and related geology Stochastic Mixing Model Parameters and Calculations of the Short-Lived and experimental evidence (33, 34) back to at least 3.8 Ga and 146 142 requires the mixing process to start shortly after the very early Sm- Nd System. The terrestrial mantle is assumed to differentiate into two homogeneous layers 100 My after solar system formation, corresponding chemical differentiation event that resulted in the variability of roughly to the mass of the upper and lower mantles to represent the EEM and 142Nd heterogeneities in the early mantle, and their subsequent “ ” EDM reservoirs, respectively. Here the length scale of heterogeneity is de- homogenization. The persistence of a fast mixing rate in the early termined by the short length of the reservoir, corresponding to the thickness development of the Earth supports the notion of the persistence of of the layer, which decreases exponentially as a function of time. A melting plate tectonics in some form that may have developed sometime event is represented by a sampling box that is 100 km in each dimension, during the Hadean. Such a connection may be considered consis- which is placed in a mixture of cubes of reservoirs (SI Appendix, Fig. S5). This tent with various geophysical models proposed (e.g., ref. 17), al- process is repeated 1,000 times for each time step for every 100 My from the though the style of plate tectonics may not have necessarily been timing of differentiation (T4.47Ga) to the present day (T0). The resulting iso- modern, where the plate and mantle are closely coupled (35). topic composition from the sampling box is calculated from the averaged isotopic compositions of the EEM and EDM and their corresponding volu- The timing of the initiation to the persistence of plate tec- metric proportions present in the box (SI Appendix, section 2). tonics on Earth is a debated issue. In a brief literature survey, the initiation of the Wilson cycle was proposed to have been around Data Availability. Data tables, in addition to the mathematical principles and 3.0 Ga based on diamond inclusions (36), while the initiation of corresponding MATLAB scripts for processing data, are available in the SI plate tectonics was similarly proposed to have been relatively Appendix. In addition, the data will be made available through EarthChem recent (∼3.0 Ga) based on inferences about the evolution of (https://ecl.earthchem.org/view.php?id=1545). composition of the upper crust (37). Meanwhile, the investigation of titanium isotopes of shales (38) suggests that plate tec- ACKNOWLEDGMENTS. This work was partially supported by NASA Emerging tonics must have been initiated more than 3.5 Ga. Another study Worlds Grants NNX15AH66G and 80NSSC20K0346. We thank the four anonymous reviewers whose comments and suggestions greatly improved (39) has proposed high silica content in the early crust based on the manuscript. Some of the samples were provided by Charles Langmuir Rb/Sr ratios, suggesting that the early Earth was capable of forming (KN207-2, CH59-2, A12DR44, and HLY102-096), Bjørn Larsen (K1714), Rita continental crust within <350 My of solar system formation. Such a Parai (DICE), and Jonathan Tucker (RC2806 2D-1).

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