Mg–O isotopes trace the origin of Mg-rich fluids in the deeply subducted continental crust of Western Alps

Abstract

Fluids are important for mass transfer at the slab–mantle interface in subduction zones. However, it is usually difficult to trace fluids from specific sources in a subducting slab, especially those derived from dehydration of serpentinite. Coesite-bearing whiteschist at Dora-Maira in the Western Alps is characterized by strong Mg enrichment relative to the country rocks, which requires infiltration of Mg-rich fluids into the supracrustal rock. In order to constrain the origin of such Mg-rich fluids, we have performed an integrated study of whole-rock Mg and O isotopes, zircon U–Pb ages and O isotopes for the whiteschist and related rocks. Zircons in the whiteschist show two groups of U–Pb ages at ∼262 Ma and ∼34 Ma, respectively, for relict and newly grown domains. The Permian U–Pb ages of relict magmatic domains are consistent with the protolith age of host metagranite, suggesting that their common protolith is the Permian granite. The Tertiary U–Pb ages occur in coesite-bearing metamorphic domains, consistent with the known age for ultrahigh-pressure metamorphism. The metamorphic domains have δ18O values of 5.8–6.8‰, whereas the relict magmatic domains have high δ18O values of ∼10‰. Such high δ18O values are also characteristic of the metagranite, indicating that the whiteschist protolith underwent metasomatism by metamorphic fluids with low δ18O value of ∼2–4‰. The whiteschist mostly has whole-rock δ26Mg values of −0.07 to 0.72‰, considerably higher than country-rock δ26Mg values of −0.54 to −0.11‰. Thus, the metamorphic fluids are not only rich in Mg but also heavy in Mg isotopes. They were probably derived from the breakdown of Mg-rich hydrous minerals such as talc and antigorite in serpentinite at the slab–mantle interface in the subduction channel. Therefore, the dehydration of mantle wedge serpentinite during the subduction and exhumation of continental crust can provide the Mg-rich fluids responsible for the metasomatism of crustal rocks at subarc depths.