Age of the Archean Talga-Talga Subgroup, Pilbara Block, Western Australia, and early evolution of the mantle: new Sm [sbnd]Nd isotopic evidence

Abstract

Archean komatiites, high-Mg basalts and tholeiites from the North Star Basalt and the Mount Ada Basalt formations of the Talga-Talga Subgroup, Warrawoona Group, Pilbara Block, Western Australia, define a linear correlation on the normal 143Nd/ 144Nd vs. 147Sm/ 144Nd isochron plot. The data give an age of 3712 ± 98 Ma and initial ε Nd(T) of +1.64 ± 0.40. The 3712 ± 98 Ma date is consistent with the regional stratigraphic sequence and available age data and the Sm Nd linear array may be interpreted as an isochron giving the eruption age of the Talga-Talga Subgroup. An alternative interpretation is that the isochron represents a mixing line giving a pre-volcanism age for the Subgroup. Consideration of geochemical and isotopic data indicates that the true eruptive age of the Talga-Talga Subgroup is possibly closer to about 3500 Ma. Regardless of the age interpretation, the new Nd isotopic data support an existence of ancient LREE-depleted reservoirs in the early Archean mantle, and further suggest that source regions for the Pilbara volcanic rocks were isotopically heterogeneous, with ε Nd(T) values ranging from at least 0 to +4.0.