Serpentine mud volcanism in the Mariana forearc region provides an opportunity to investigate slab-derived pore fluids and metamorphism of the subducted plate and overlying suprasubduction zone mantle and to determine the pressure and temperature conditions of the subduction zone at depths (up to ~30 km) that are impossible to reach by drilling. Previous drilling during Leg 125 on Conical Seamount, another serpentine mud volcano, showed that these features are bringing materials and pore fluids from the region of the décollement between the subducting Pacific Ocean lithosphere and the overlying wedge of the forearc plate (Fryer, 1992b; Mottl, 1992).
Site 1200 is located at the summit of one of the Mariana forearc serpentine mud volcanoes. The target site is a 200-m-high tumescent knoll on South Chamorro Seamount and lies at 13°47´N, 146°00´E, at a water depth of ~2930 meters below sea level (mbsl), ~125 km east of Guam in the western Pacific Ocean (Figs. F1, F2). It is situated ~85 km from the trench, where the depth to the slab is ~26.5 km, based on Isacks and Barazangi (1977). Pore fluids collected in gravity cores from this seamount exhibit a strong slab signal (Fryer et al., 1999). It is the only known site of active blueschist mud volcanism in the world and hosts the only documented megafaunal assemblages associated with serpentine/blueschist mud volcanism (Fryer and Mottl, 1997).
The objectives of drilling at South Chamorro Seamount were to (1) examine the processes of mass transport and geochemical cycling in the subduction zones and forearcs of nonaccretionary convergent margins; (2) ascertain the spatial variability of slab-related fluids within the forearc environment as a means of tracing dehydration, decarbonation, and water-rock reactions in subduction and suprasubduction zone environments; (3) study the metamorphic and tectonic history of nonaccretionary forearc regions; (4) investigate the physical properties of the subduction zone as controls over dehydration reactions and seismicity; and (5) investigate biological activity associated with subduction zone material from great depths.
To achieve these objectives, drilling at Site 1200 was designed to recover sufficient material to permit petrologic and mineralogic characterization of the serpentine mud flow units, characterize their pore fluid compositions, collect any biological material contained therein, and establish a cased reentry hole for deploying downhole instrumentation for a long-term observatory at South Chamorro Seamount.
Downhole instrumentation for deployment in Hole 1200C included a downhole thermistor string, pressure sensor, and osmotic fluid sampler sealed with a circulation obviation retrofit kit (CORK). This installation will provide a long-term record of (1) the rebound of temperatures toward formation conditions after the emplacement of the seal; (2) possible temporal variations in temperatures and pressures due to lateral flow in discrete zones, regional and/or local seismicity, and short-term pressure effects; and (3) composition of the circulating fluids obtained with the osmotic fluid sampler. Data from the downhole instruments will be collected during a Jason/DSL 120 cruise that is tentatively scheduled for 18 months after completion of Leg 195.