The Mariana system contains numerous large mud volcanoes composed principally of unconsolidated flows of serpentinite mud and serpentinized mantle peridotites, along with minor amounts of clasts of metabasic rock clasts (Johnson, 1992; Maekawa et al., 1992, 1993; Fryer et al., 1999; Guggino et al., 2002). The serpentinite muds were likely formed as fault gouge during tectonic deformation of the underlying forearc mantle, resulting in unconsolidated silt- to clay-sized serpentinite sediments that migrate as a result of gravitational instability to the surface through faults and fractures in the forearc crust (Fryer, Pearce, Stokking, et al., 1992; Fryer et al., 1995). Based on earthquake locations, the estimated depth to the subducting Pacific plate under the South Chamorro and Conical seamounts is ~30 km, and the metamorphic schists recovered from the sampled forearc sites likely originated from the décollement zone as entrained clasts within the upwelling serpentinite muds (Fryer et al., 1999). We will explore the geochemical influences that metabasic rocks from the subducting slab, through dehydration and low-temperature metamorphism, may have on the trace element and fluid-mobile element systematics of the extensively studied serpentinite muds and pore fluids as the muds and fluids rise through the mantle wedge and the forearc crust.
The discovery of metamorphic schists containing Na-rich amphiboles (indicating pressures = 5 kbar) in the grit-sized fraction of the serpentinite mud from Site 1200 (Salisbury, Shinohara, Richter, et al., 2002; Gharib et al., 2002) provides strong evidence for the deep (slab) origin of the muds that are actively protruding at the summit knoll of South Chamorro Seamount (for a more detailed discussion see Fryer et al., submitted [N1]). Unlike Conical Seamount, where only two metamorphic fragments were found in six holes drilled, South Chamorro Seamount provides a significant proportion of metabasites that allow the investigation of the paragenesis of the slab-derived lithologic fraction.