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. However, many Archean rocks exhibit excesses of
182 W, the decay product of short-lived 182 Hf. The exact causes for
these 182 W excesses, however, have remained ambiguous and it remains
speculative, if the Archean 182 W anomalies and also 182 W 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 182 W isotope dataset for 3.22-3.55 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 very first discovery of a
negative co-variation between short-lived 182 W and long-lived 176
Hf-143 Nd-138 Ce 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 182 Hf, i.e., the first 60
million years after solar system formation, thereby also providing lower
bounds on the age of the Earth-Moon system. The striking isotopic
similarity between recycled protocrust and the low 182 W endmember of
modern OIBs might also be the missing link bridging 182 W isotope
systematics in Archean and young mantle-derived rocks.