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, if Archean 182W anomalies and also 182W deficits found in
many young ocean island basalts (OIBs) mirror primordial Hadean mantle
differentiation or just variable contributions from older meteorite
building blocks delivered to the growing Earth. Here, we present a
high-precision 182W isotope dataset for 3.22-3.55 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 very first discovery of a negative
co-variation between short-lived 182W and long-lived 176Hf-143Nd-138Ce
patterns, a trend of global significance. Amongst 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, i.e., the first 60 million years
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 182W isotope systematics in Archean
and young mantle-derived rocks.