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

Abstract

The metamorphic nappes of the Erzgebirge all include the same sequence of volcanosedimentary protoliths, but were metamorphosed to different extents during the Variscan orogeny. Metamorphic conditions range from very-low grade to eclogite facies. Geochemical and Sr-isotope data identify distinct protoliths, which can be linked to their unmetamorphosed equivalents from the former Gondwana shelf. Comparison of the Li and B contents and isotope ratios in metamorphic rocks from two distinctive metasedimentary units and their unmetamorphosed protoliths show that there is an overall loss of Li and B and and a decrease of δ7Li and δ11B during progressive metamorphism, but Li and B do not behave coherently. Additions from late fluids affect the B isotopic composition (shifting δ11B to higher values) more than the Li isotopic composition. The mobility of Li and B is dependent on the metamorphic history and pre-metamorphic composition of the rocks.In the sedimentary protoliths, exchangeable Li is mainly bound to illite and sericitic muscovite and structurally-bound Li is hosted in chlorite and illite/sericite, whereby chlorite-rich sedimentary rocks have distinctly higher Li contents. Structurally-bound Li has a narrow δ7Li range (−3.4 to +0.4), whereas samples with significant contributions of exchangeable Li have a broad range of δ7Li (−6 to +14.5) that depends on the depositional environment. In contrast, B contents do not vary with the relative contribution of chlorite and illite/sericite, but seem to have distinctly higher δ11B values in chlorite-rich samples. Loss of exchangeable Li starts during very low-grade metamorphism. Hence, δ7Li in sedimentary rocks with a significant relative amount of exchangeable Li is changed significantly, whereas δ7Li remains largely unaffected by very low-grade metamorphism in sedimentary rocks where most of the Li is structurally bound. At increasing metamorphic grade, the mobility of Li and B is largely controlled by the stability of muscovite/phengite and chlorite and whether metamorphically formed minerals sequester Li and B. However, overall loss of B and reduction of δ11B demonstrates that metamorphic tourmaline does no sequester released B quantitatively.The variation in the isotopic composition of Li and B during metamorphism is smaller (Li ∼10δ-units, B ∼8δ-units) than the isotopic variation in Palaeozoic shales from the former Gondwana shelf, where individual units have a relatively small variation (∼4–6δ-units) in δ7Li and δ11B, whereas the entire section – depending on weathering history and deposition environment – encompasses a range of more than ∼20δ-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.