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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

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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 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, particu- larly 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 ter- restrial 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, de- termined 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 conti- nental crustal samples were selected to cover the age range from 300 Ma to 2 Ga; they are all from the Baltic Shield. These mea- surements 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.
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