Late Archaean basalts from the Yilgarn craton record evidence of thin lithosphere prior to cratonisation

The dynamics of Earth’s early mantle and the timing of the onset of plate tectonics remain a topic of debate. Proposed hypotheses for the Archaean eon range from a stagnant-lid Earth all the way to modern-style plate tectonics. Here, we estimate temperatures and depths of melt generation in the late Archaean mantle using a new geochemical data compilation of mafic igneous rocks from the Yilgarn craton, Australia. We combine these results with stratigraphic and geodynamic constraints to resolve the tectonic regime and upper mantle dynamics at the time.

Primitive volcanic rocks can preserve signatures of the melting processes in the mantle: depth and temperature of melting are recorded in magma major and trace element chemistry. We have collated a data compilation of mafic volcanic samples from the Archaean Yilgarn craton in Western Australia. In order to identify those samples most representative of melting conditions in the convecting mantle, the data were screened to minimise the effects of crystal fractionation and assimilation of crustal or cumulate material (9 wt% < MgO < 15 wt%; no Eu anomalies, no positive Pb anomalies; Nb/U > 30). We further correct these screened compositions for olivine fractionation. This screened dataset predominantly comprises tholeiitic basalts in the Kalgoorlie terrane that erupted prior to the main komatiite sequence and the felsic magmas that make up the bulk of the Yilgarn cratonic crust. The mafic compositions investigated here therefore represent melting conditions immediately before the onset of cratonisation.

The screened data display depleted, MORB-like rare earth element patterns with no evidence of a garnet signature. Forward and inverse geochemical modelling of these compositions, assuming a primitive mantle source, predicts melting at depths as shallow as ~40 km and mantle potential temperatures elevated by ~200 °C compared to present-day ambient mantle. These results are consistent with melting of a rising plume head combined with moderate extension of the pre-existing lithospheric lid.