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154 Sm/147 Sm). Because of the high precision of the measurements, the error on Nd and Sm concentrations is only carried by the error on the composition of the spike, which is better than 0.5% (2σ). Results are presented in Supplementary Table 1. The unspiked Nd cuts for high-precision

142 Nd/144 Nd measurements were performed on the 9-faraday cup Triton thermal ionization mass spectrometer at the Johnson Space Center (Supplementary Table 2). The analytical protocol described by Caro et al.27 was employed, where Nd isotopes are measured in multidynamic mode with rotating amplifiers between blocks. The Nd isotopes were measured as positive metal ions. All measured data are normalized to

146 Nd/144 Nd = 0.7219 to correct for instrument mass fractionation during the runs. The interference of

142 Ce on

142 Nd was monitored with

140 Ce. Corrections for this interference to

142 Nd/144 Nd ranged from 0.2 to 9.5 ppm, and for all but one AMES run and

4 Iceland picrite runs, ≤

4 ppm and within 2σ internal and established 2σ external precisions on

142 Nd/144 Nd (Supplementary Table 2). The external precision is expressed as

2 standard deviations (2σ): 2σ =

2 * x ? x ( )

2 ∑ n ?

1 ( ) Samarium interferences on the Nd isotopes were monitored with

147 Sm, which was never detectable on the faraday cups above background noise. Scans on the electron multiplier showed that

147 Sm was never above

1000 counts/sec, and therefore negligible. The amount of Nd extracted in the SNC samples measured ranged from approximately

300 to

600 ng. In order to constrain the accuracy and external precisions for the measured SNC data, standards were run with similar concentrations. A series of runs for two standards, an AMES in-house internal standard and LaJolla, are shown in supplementary Figure 1. Internal precisions of ≤ ±3 ppm were achieved for

600 ng standard aliquots for measuring times of

4 to

6 hours (Standard Set 1, Supplementary Table 2). The signal intensities were

4 to

6 V on the

142 Nd beam. The average for these standards was

142 Nd/144 Nd = 1.1418402±0.0000034 (n=14, 2σ) and is used as the reference value of ε142 Nd =

0 for calculating ε142 Nd of the standards and unknowns. The 2σ external precision for these

14 standard runs for ε142 Nd is ±0.03 (±3 ppm). The average

143 Nd/144 Nd values for Ames standards (n=10) and La Jolla standards (n=4) give 0.5121359±0.0000026 (2σ) and 0.5118491±0.0000007 (2σ) respectively (Supplementary Table 2). A set of

142 Nd-enriched standards were measured in order to check for accuracy over a range of

142 Nd/144 Nd. The enriched standards were prepared by Chi-Yu Shih at the Johnson Space Center by adding >

99% pure

142 Nd spike to the in-house AMES Nd standard in measured quantities. All the data are presented in Supplementary Table 2. 1.141820 1.141840 1.141860 1.141880 1.141900 1.141920

142 Nd/144 Nd AMES La Jolla Std. Avg. 0.218 AMES 0.430 AMES 0.638 AMES ±3 ppm 2σ 0.218 AMES 0.430 AMES 0.638 AMES ±3 ppm 2σ 1:1 ε142 Nd - Gravimetric

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 ε142 Nd - Measured Supplementary Figure 1. High-precision measurements of standards for

142 Nd/144 Nd. Error bars are ±2σ. As shown in Supplementary Fig.

1 and Supplementary Table 2, for gravimetrically prepared

142 Nd-enriched standards with ε142 Nd at 0.218, 0.430, and 0.638, the measured ratios for these enriched standards fall on a 1:1 line with the values obtained gravimetrically (Supplementary Fig. 1). This demonstrates that the accuracy of these measurements are reproducible within this external precision of ±3 ppm (2σ) for materials that vary over a range of >