Lithium and boron isotopic fractionation in sedimentary rocks during metamorphism - The role of rock composition and protolith mineralogy

The variation in the isotopic composition of Li and B during metamorphism is smaller (Li ∼10 δ-units, B ∼8 δ-units) than the isotopic variation in Palaeozoic shales from the former Gondwana shelf, where individual units have a relatively small variation (∼4-6 δ-units) in δ7Li and δ11B, whereas the entire section - depending on weathering history and deposition environment - encompasses a range of more than ∼20 δ-units in δ7Li and δ11B. Thus, inferences on the behavior of Li and B and their isotopic composition during metamorphism that are derived from the comparison of metamorphic rocks with an “average protolith“ may be misleading.