Ba-Mg isotopes in high Ba-Sr granites: implications for mantle mass transfer and crustal growth

High Ba-Sr granites have continental Hf-Nd-Sr-O isotope characteristics but evidence for mantle derivation. Consequently, crustal evolution models that rely on Hf-O isotopes incorrectly exclude them from the juvenile budget. It is therefore critical to understand the “enriched” crustal signature and constrain its volumetric proportion. Plutons from the Caledonian-type area show a range of enrichment severity ideal for novel isotopic investigations. Mg isotopes (δ26Mg value of −0.30 to −0.25‰ in a “depleted” example vs. −0.42 to −0.23‰ in an enriched comparator) lie largely within the normal mantle range. Ba isotopes (δ138/134Ba values of -0.05 to +0.23‰ and -0.19 to +0.03‰ for depleted and enriched, respectively) overlap normal mantle values but extend to lighter values, consistent with the addition of 2-5% pelagic sediment. Elemental relationships suggest sediment melting as the admixture mechanism. First-order estimates from the Caledonian orogen and global geochemical data suggest that ca. 10% of granites have high Ba-Sr character, thus representing significant unrecognized crustal growth.