Multi-stage water enrichment processes in the wake of the Farralon slab (UNESCO IGCP 557)

In this study we supply petrological evidence for a hydrous upwelling beneath the Colorado Plateau in the Western United States. We trace a hydrous upwelling by its signature of multi-stage water enrichment processes in garnet- and Cr-spinel bearing lherzolithes collected from the sample volcano "the Thumb". Multi-stage hydration is witnessed from a depth of 150 km to just below Moho depth by various constraints. We use high-resolution synchrotron based FTIR, to resolve the hydration at 150 km depth (T= 1200°C) by mapping the water content in homogenous olivine crystals, around spinel inclusions and fully embedded cracks from lherzolites samples. We interpret these lherzolites to originate from the lower part of the Farallon slab. These xenoliths are presumably released during break off of the slab. In these samples background hydration in the homogenous olivine is 140 ppm H2O wt but around 2-D defects the water content can rise up to 800 ppm H20 wt. A second set of samples of harzburgitic composition is interpreted to be derived from the subcontinental Mantle (T= 600°C). In these samples hydration is visible through cracks that are serpentinized. From this observation we conclude that hydration causes multi-stage enrichment of the mantle wedge through a process that is dominated by the growth of ubiquitous crack. Our results do not confirm the hypothesis of pervasive hydration through point defect diffusion. Cracks in the mantle are a key element in the subduction zone water cycle.