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

- The Mg-rich whiteschist in Western Alps has a protolith of Permian granite.
- Low δ18O metamorphic zircon indicates metasomatism by low δ18O fluids.
- High δ26Mg values for whiteschist suggest metasomatism by high δ26Mg fluids.
- The metasomatic fluid was derived from dehydration of talc and antigorite in serpentinite.
- The supracrustal rock was infiltrated by the Mg-rich fluid at the slab-mantle interface.

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.