Long-term preservation of Hadean protocrust in Earth’s mantle

With plate tectonics operating on Earth, the preservation potential for mantle reservoirs from the Hadean Eon (>4.0 Ga) has been regarded as very small. The quest for such early remnants has been spurred by the observation that many Archean rocks exhibit excesses of (182)W, the decay product of short-lived (182)Hf. However, it remains speculative whether Archean (182)W anomalies and also (182)W deficits found in many young ocean island basalts (OIBs) mirror primordial Hadean mantle differentiation or merely variable contributions from older meteorite building blocks delivered to the growing Earth. Here, we present a high-precision (182)W isotope dataset for 3.22- to 3.55-Ga-old rocks from the Kaapvaal Craton, southern Africa. In expanding previous work, our study reveals widespread (182)W deficits in different rock units from the Kaapvaal Craton and also the discovery of a negative covariation between short-lived (182)W and long-lived (176)Hf–(143)Nd–(138)Ce patterns, a trend of global significance. Among different models, these distinct patterns can be best explained by the presence of recycled mafic restites from Hadean protocrust in the ancient mantle beneath the Kaapvaal Craton. Further, the data provide unambiguous evidence for the operation of silicate differentiation processes on Earth during the lifetime of (182)Hf, that is, the first 60 million y after solar system formation. The striking isotopic similarity between recycled protocrust and the low-(182)W endmember of modern OIBs might also constitute the missing link bridging (182)W isotope systematics in Archean and young mantle-derived rocks.