Tracking Hadean processes in modern basalts with 142-Neodymium
Résumé
The short-lived 146 Sm® 142 Nd isotope system (t1/2 = 103 Ma) can constrain terrestrial silicate fractionation during the early Hadean. Although some Archean terranes preserve variability in 142 Nd/ 144 Nd, no anomalies have been previously resolved in young rocks. This study provides high precision 142 Nd/ 144 Nd data on a suite of ocean island basalts from Samoa and Hawaii previously shown to have variable depletions in 182 W/ 184 W that are inversely correlated with 3 He/ 4 He ratios. Improved analytical techniques and multiple replicate analyses of Nd show a variation in µ 142 Nd values between-1.3 and +2.7 in the suite, relative to the JNdi standard. Given the reproducibility of the standard (±2.9 ppm, 2 SD), two Samoan samples exhibit resolved variability in their 142 Nd/ 144 Nd ratios outside of their 95% confidence intervals, suggesting minor variability in the Samoan hotspot. One sample from Samoa has a higher µ 142 Nd of +2.7, outside the 95% confidence interval (±1.0 ppm) of the average of the standard. Limited, but resolved, variation in 142 Nd/ 144 Nd within the suite suggests the preservation of early Hadean silicate differentiation in the sources of at least some basalts from Samoa. Larger variations of 182 W/ 184 W and 3 He/ 4 He ratios in the same samples suggest that metal-silicate separation and mantle outgassing left a more persistent imprint on the accessible mantle compared to 142 Nd/ 144 Nd ratios which are impacted by early silicate differentiation.
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