Guatemala jadeitites and albitites were formed by deuterium-rich serpentinizing fluids deep within a subduction zone

Jadeitites and albitites from the Motagua Valley, Guatemala, are high-pressure–low-temperature metasomatic rocks that occur as tectonic inclusions in serpentinite-matrix melange. Metasomatism was driven by a fluid with a δ¹⁸O_H2O value of 6‰, and a δD_H2O value that is high in comparison with metamorphic fluids at other high-pressure–low-temperature localities of similar grade. We infer that the fluid was originally seawater that was entrained during subduction either as mineral-bound H₂O or as free pore waters. The fluid drove serpentinization reactions in ultramafic rocks, possibly leading to deuterium enrichment of H₂O, prior to forming the jadeitites and albitites at a depth of 29 ± 11 km. There are isotopic and fluid-inclusion similarities to rodingites, which are Ca-rich metasomatites found at other serpentinite localities. Our results suggest that the serpentinization process, whether it occurs within subduction zones or on the flanks of oceanic spreading ridges, may produce residual fluids that are H₂O rich, have 1–8 wt% equivalent NaCl, and have high, perhaps seawater-like, δD values.