Fluid effect on zircon O and U-Pb isotopes during ultrahigh-pressure metamorphism: Insights from the Dora-Maira Massif of the Western Alps

Zircon geochemistry such as U-Pb and O isotopes have been widely used in dating and tracing complex geological processes. However, it still remains unclear how fluid action affects zircon geochemistry during metamorphic and metasomatic processes in subduction zones. Here a systematic study on zircon U-Pb dating, O isotopes and trace elements as well as whole-rock O isotopes was carried out for the coesite-bearing whiteschists, jadeite quartzites and granitic gneisses from the Dora-Maira Massif, Western Alps. Whole-rock and zircon geochemistry supports a common protolith, i.e., Permian S-type granites, for the above three types of rocks and an intense fluid metasomatism during the Alpine orogeny to form whiteschists and jadeite quartzites. Zircon cores in all samples have nearly identical δ¹⁸O values (9‰–11‰), whereas their apparent ²⁰⁶Pb/²³⁸U ages show a greater variability due to Pb loss during metamorphism. Zircon rims formed in the late Eocene to early Oligocene can be categorized into two types. Type-Ⅰ rims occur in granitic gneisses and jadeite quartzites. They have high δ¹⁸O values consistent with zircon cores, but much lower contents of P and Y as well as lower Th/U ratios than the cores. Their growth can be attributed to internal metamorphic fluid action at the UHP metamorphic stage. Type-Ⅱ rims occur in whiteschists and jadeite quartzites. They have remarkably lower δ¹⁸O values (5‰–8‰) and Th/U ratios (< 0.01), compared with zircon cores and Type-Ⅰ rims. Their growth can be ascribed to external fluids during the metasomatic process. Some zircon domains in whiteschists and jadeite quartzites show a positive correlation between δ¹⁸O values and apparent ²⁰⁶Pb/²³⁸U ages, which suggest the simultaneous impacts on U-Pb-O isotopes during external fluid metasomatism. This process can be attributed to the fluid-assisted dissolution and recrystallization of protolith zircons. Especially, coesite inclusions that would have been expected to occur only in young zircon rims formed during UHP metamorphism are also observed in the relict magmatic zircon cores, indicating that the fluid-related metasomatism at the UHP metamorphic conditions also affected these pre-existing protolith-related cores. Therefore, fluid action in subduction zones reveals significant impacts on both the U-Pb and O isotope systems of zircon, especially when external metasomatic fluids are involved. Therefore, a detailed study on zircon, including microstructure, mineral inclusion and geochemical data of different growth and recrystallization domains, is needed in order to unravel continental crustal evolution based on zircon U-Pb ages and O isotope compositions.