Limited lithium isotopic fractionation during progressive metamorphic dehydration in metapelites: A case study from the Onawa contact aureole, Maine

The major, trace element, and Li isotopic compositions of granodiorite and metapelites from the Onawa pluton and surrounding contact aureole have been measured to document the behavior of Li and its isotopes during progressive metamorphic dehydration. Major and trace element concentrations in metapelites of the contact aureole change little, while loss on ignition (LOI) decreases with increasing metamorphic grade, which ranges from regional chlorite-zone metamorphism far removed from the pluton to partially melted rocks adjacent to the pluton. Lithium concentrations in metapelites from all zones correlate with LOI and abundances of the main Li-bearing minerals, chlorite + biotite + muscovite, decreasing by a factor of two (from 130 ppm far removed from the pluton to 64 ppm adjacent to the pluton). In contrast, Li isotopic compositions remain relatively unchanged across the aureole (δ7Li = − 3.5 to + 1.0), and are comparable to the range observed in schists and unmetamorphosed shales. Compared to the surrounding metasediments, the granodiorite has a lower Li content (45 ppm) and a comparable δ7Li value of − 0.2, similar to those of other granites and the average upper crust (0 ± 2). observations are consistent with the removal of Li from metapelites via Rayleigh distillation during progressive metamorphic dehydration, with isotopic fractionation factors between fluids and rocks ranging between 1.001 and 1.004. These values are similar to those of a recent experimental study and produce isotopic fractionation that is barely beyond analytical uncertainty (± 1‰) for the observed degree of Li depletion. The good correlations between Li concentrations and LOI with mineralogical abundances reflect the importance of mineralogy in controlling Li concentrations and isotopic compositions of metamorphic rocks. Comparison of these results with those from a study of a contact halo around the Tin Mountain pegmatite suggests that the nature of the intrusion plays a critical role in controlling the behavior of Li in country rocks. Large, diffusive-driven isotopic fractionation over a small scale is expected to occur when rocks are infiltrated by Li-rich magmatic fluids, while metamorphic devolatilization results in minor isotopic fractionation on the aureole scale.