Subducted Farallon Plate Carries Water for Hydration Above the Flat Slab and Deep into the Mantle: Evidence from the Navajo Volcanic Field HP and UHP Xenolith Suite

Abstract:

Xenoliths in the Navajo Volcanic Field diatremes include HP-LT and UHP metamorphic eclogites (with lawsonite, phengite, coesite, zircon), exotic hydrous Cr-omphacitites (with guyanaite - CrOOH, carmichaelite, eskolaite, tawmawite, redledgeite) and hydrous and anhydrous peridotite, pyroxenite and lower crustal rocks. The eclogites (primarily dated at 30-80 Ma), omphacitites (30 Ma) and some of the (serpentinized) peridotites were derived from the subducted Farallon Plate (and possibly near-trench mantle wedge material accompanying the slab) from the flat slab by the diatremes at 30 Ma, approximately 700 km from the trench. Dehydration reactions in this assemblage (primarily prograde metamorphism of serpentinite yielding peridotite but also breakdown of guyanaite to eskolaite + water) provided water that hydrated overlying mantle materials (garnet and spinel peridotite to antigorite/chlorite serpentinite, garnet pyroxenite to chlorite-bearing eclogite, garnet and spinel pyroxenite to pargasite + chlorite). The accompanying volume expansion contributed to uplift of the Colorado Plateau, as originally suggested by Hess (1955). Phengite and possibly lawsonite remained stable and continued to carry water to greater depths. Guyanaite is stable to over 13.5 GPa and can transport water at least into the transition zone and thus may be a vehicle for hydrating ringwoodite in the transition zone (the present location of the Farallon slab). It is not known to how deep other hydrous minerals in these assemblages are stable (e.g., carmichaelite, redledgeite, tawmawite) but they, too, have potential for carrying water deep into the mantle.