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摘要
The buoyancy of continental crust opposes its subduction to mantle depths, except where mineral reactions substantially increase rock density. Sluggish kinetics limit such densification, especially in dry rocks, unless deformation and hydrous fluids intervene. Here we document how hydrous fluids in the subduction channel invaded lower crustal granulites at 50-60 km depth through a dense network of probably seismically induced fractures. We combine analyses of textures and mineral composition with thermodynamic modeling to reconstruct repeated stages of interaction, with pulses of high-pressure (HP) fluid at 650-670 degrees C, rehydrating the initially dry rocks to micaschists. SIMS oxygen isotopic data of quartz indicate fluids of crustal composition. HP growth rims in allanite and zircon show uniform U-Th-Pb ages of similar to 65 Ma and indicate that hydration occurred during subduction, at eclogite facies conditions. Based on this case study in the Sesia Zone (Western Italian Alps), we conclude that continental crust, and in particular deep basement fragments, during subduction can behave as substantial fluid sinks, not sources. Density modeling indicates a bifurcation in continental recycling: Chiefly mafic crust, once it is eclogitized to >60%, are prone to end up in a subduction graveyard, such as is tomographically evident beneath the Alps at similar to 550 km depth. By contrast, dominantly felsic HP fragments and mafic granulites remain positively buoyant and tend be incorporated into an orogen and be exhumed with it. Felsic and intermediate lithotypes remain positively buoyant even where deformation and fluid percolation allowed them to equilibrate at HP.
Plain Language Summary Processes inside subduction zones are fundamental for continental accretion and the recycling of the Earth's crust into the mantle, but many aspects remain poorly understood. As tectonic plates converge, continental rocks resist subduction into the denser mantle, except where they are transformed to high-pressure mineral assemblages. This densification process is known to be particularly sluggish for dry rocks. We document that tectonic bodies of dry continental crust now in the Western Alps were extensively (re) hydrated during subduction. The effects of brittle deformation, providing access to hydrous fluids at >60 km depth, is investigated by detailed petrology, U-Th-Pb geochronology, oxygen isotopes, and modeling of phase equilibria. These results - combined with density modeling and then scaled up suggest that, unless hydration occurs, continental crust may undergo density separation beneath the overlying plate. A dense layer at similar to 550 km depth, visible by tomography beneath the Western Alps, is interpreted as a subduction graveyard that separated from the hydrated rocks documented in this study.
- 出版日期2018-3
