182W and 142Nd anomalies in 2.7 Ga old basalts indicate long-term preservation of distinct mantle heterogeneities

The combined short-lived ¹⁴⁶Sm–¹⁴²Nd and ¹⁸²Hf–¹⁸²W radiogenic isotope systems provide critical insights into early Earth differentiation and long-term preservation of mantle heterogeneities. Although Archean rocks often display resolvable ¹⁴²Nd and ¹⁸²W anomalies, these signatures diminish with time due to convective homogenization of the mantle. Proposed origins for these anomalies include early silicate-silicate or metal-silicate differentiation, and heterogeneous incorporation of late-accreted material [e.g., 1-4]. To further investigate these processes, we obtained high-precision ¹⁸²W isotope data for the 2.7 Ga old Theo’s Flow tholeiites. We show that, in addition to the previously reported ¹⁴²Nd excess [5], the Theo's Flow tholeiites also have a positive ¹⁸²W anomaly, indicating the survival of an early-formed mantle domain for nearly 1.8 Ga. The ¹⁴²Nd anomaly is best attributed to silicate differentiation at ~4.4 Ga, whereas the ¹⁸²W signature likely reflects incomplete mixing of late-accreted materials, consistent with a ‘grainy’ accretion scenario. Comparison with coeval komatiites from Boston Creek, which were derived from the same mantle plume [6], highlight spatial heterogeneity in the incorporation of late-accreted material at the single plume scale. These findings support inefficient mantle mixing during the Archean and are more consistent with a stagnant-lid tectonic regime than with modern-style plate tectonics.

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