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

Abstract

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 182W, the decay product of short-lived 182Hf. However, it remains speculative whether Archean 182W anomalies and also 182W 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 182W 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 182W deficits in different rock units from the Kaapvaal Craton and also the discovery of a negative covariation between short-lived 182W and longlived 176Hf-143Nd-138Ce 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 182Hf, that is, the first 60 million y after solar system formation. The striking isotopic similarity between recycled protocrust and the low-182W endmember of modern OIBs might also constitute the missing link bridging 182Wisotope systematics in Archean and young mantle-derived rocks.