We report the finding of rare eclogite-facies vugs forming millimeter- to centimeter-sized pockets in meta-ophiolites from the western European Alps. Euhedral garnet crystals covering the vug walls display oscillatory chemical zoning for a wide range of major and trace elements, including Cr, Mn, and rare earth elements. Thermodynamic modeling revealed that closed-system fluid production through the breakdown of prograde glaucophane, lawsonite, and chlorite between 505 °C and 525 °C can successfully explain porosity creation of ~4% and the mineralogical properties of the vugs. Available geologic and geochronologic constraints indicate that the eclogitization of the downgoing mafic crust spanned a window of at least 1 m.y. These observations can only be explained by the presence of extremely low permeability values (<10–22 m2) to keep the fluid confined at the meter scale within vugs on such time scales. Our field-based report of eclogite porosity provides the first in situ confirmation of previous experimental data and geophysical estimates on active margins. A substantial amount of fluid trapped in this porosity may be carried deeper than expected into Earth’s mantle, with implications for volatile recycling budgets.

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