Mantle-like Hf-Nd isotope signatures in-3.5 Ga greenstones: No evidence for Hadean crust beneath the East Pilbara Craton

The geological setting of extensive -3.5 Ga greenstone sequences in the Pilbara Craton (Australia) has been controversial for many years. Models range from a purely oceanic origin to emplacement over a pre-existing, possibly Hadean, sialic basement. Here we present new isotopic (Nd-Hf) and trace element data for a geographically extensive suite of komatiitic-tholeiitic metavolcanic rocks which comprise much of the Pilbara Supergroup. Most of the samples have flat to mildly enriched normalised trace element patterns, often associated with a slight Nb-Ta depletion. A small subset of samples, represented throughout the stratigraphy, show strong LREE enrichment and pronounced HFSE anomalies (low Nb/Th, high La/Nb and Zr/Y) either requiring a different magma source or the addition of an enriched component (possibly pre-existing continental crust). Despite this compositional diversity, the Sm-Nd and Lu-Hf data for all samples define -3.5 Ga isochrons and yield homogeneous initial ratios (epsilon Nd = 1.15 +/- 0.19; epsilon Hf = 2.29 +/- 0.65) suggesting a magma source - or sources - slightly more depleted than the putative primitive (chondritic) mantle but consistent with recent observations of other primordial, deep mantle reservoirs. Published Sm-Nd data for the Dresser Formation obtained during the Pilbara Drilling Project appear to be affected by alteration processes and are considered unreliable for the estimation of an accurate initial isotopic composition. The data for the enriched samples are inconsistent with assimilation of known examples of locally exposed continental crust as a mechanism responsible for their trace element characteristics. If an alternate crustal contaminant is invoked it cannot be much older than the basaltic magmatism itself. As a consequence, we see no evidence for an extensive Hadean (>4.0 Ga) sialic basement in the region.

Automated summary

The research on the 3.5 billion-year-old greenstone sequences in the Pilbara Craton of Australia revealed a diverse composition of rocks with potential magma sources from further distances. Despite the complexity of origins, the isotopic data for all samples point to a 3.5 billion-year isochron, ruling out the possibility of extensive Hadean sialic basement in the region.