As noted by Bebout (1995), Ryan et al. (1995, 1996), and Bebout et al. (1999), metamorphism at the slab/mantle interface may prevent the deep subduction of highly volatile components such as H2O, N, B, As, Cs, and I (Snyder et al., this volume). The large but selective enrichments of fluid-mobile elements in the serpentinite muds (this study) and clasts (Savov, 2004), their distinct REE distributions, and the fractionation of B, Cs, and As from Rb, Li, Ba, Pb, and U reveal a distinctive elemental signature produced through the depletion of the subducted Mariana slab assemblage (Plank and Langmuir, 1998; Kelley et al., 2003). The mechanical mixing of mineralogically diverse metabasalt fragments with serpentinized peridotites in the forearc mantle wedge may help explain the large B and Li isotope fractionations required to produce the isotopic oddities of various forearc mantle and slab-derived rocks (Benton et al., 2001, 2004; Savov et al., 2002; Marschall et al., 2003). The extreme enrichments of fluid-mobile elements in serpentinite muds could not be explained by simple mechanical mixing with slab-derived metamorphic schists. Because the fluid-mobile element enrichment patterns in the serpentinized muds resemble those of serpentinized peridotites, we suggest that they both were created by interaction with deeply derived slab fluids having very high B, As, and Cs contents.