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

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