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Scientific Objectives

The overall objectives of drilling at South Chamorro Seamount were to (1) study geochemical cycling and mass transport in the subduction zones and forearcs of nonaccretionary convergent margins; (2) determine the spatial variability of slab-related fluids within the forearc environment as a means of tracing dehydration, decarbonation, and water-rock reactions both in the subduction zone and the overlying suprasubduction zone environments; (3) study the metamorphic and tectonic history of nonaccretionary forearc regions; (4) investigate the physical properties of the subduction zone and their influence on dehydration reactions and seismicity; and (5) investigate biological activity associated with subduction zone material from great depth.

To achieve these scientific objectives, operations during Leg 195 were designed to recover sufficient materials to permit petrologic and mineralogic characterization of the serpentine mud flow units, to analyze their pore fluid compositions, to collect any biological material contained in the muds, and to deploy a long-term geochemical observatory at South Chamorro Seamount.

Establishment of a Seafloor Geochemical Observatory

The primary objective at South Chamorro Seamount was to deploy a long-term geochemical observatory in a cased reentry hole in the central conduit of the serpentine mud volcano. The reentry hole for the installation of a downhole thermistor string, a pressure sensor, and osmotic-fluid samplers (Osmo-Samplers) was designed to be sealed with a circulation obviation retrofit kit (CORK). The techniques that were used to install the CORK were similar to those used during successful installations during Legs 139, 164, 168, and 174B (Davis et al., 1992). The hole at Site 1200 was CORKed with a thermistor cable to obtain a long-term record of the temperature variations in the sealed hole as the natural hydrologic system reestablishes itself after drilling with a pressure sensor package and two Osmo-Samplers. 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 temperature and pressure due to lateral flow in discrete zones, regional and/or local seismicity, and short-term pressure effects; and (3) the composition of deep circulating fluids obtained with the Osmo-Samplers. Data from the downhole instruments will be collected during an NSF-funded Jason/DSL 120 cruise that is tentatively scheduled to be conducted 18 months after Leg 195.

Fluid Transport

The drill site on South Chamorro Seamount was designed to help assess the variability of fluid transport and composition within the forearc. Previous field studies indicate that most of the fluid flow in the Mariana forearc is channeled along forearc faults and fault-controlled conduits in mud volcanoes. The pore fluid compositions are expected to vary depending on the nature of the channeling structures (diffuse network of small faults, major faults, and mud volcano conduits). In particular, fluids ascending through mud-volcano conduits along well-established paths in contact with previously metamorphosed wall rock should carry the most pristine slab signature. This was certainly the case at Conical Seamount, drilled during Leg 125. The summit Site 780 produced by far the purest deep slab-derived fluids, based on their much lower chlorinity and higher K, Rb, B, H2S, and sulfate, whereas the flank Sites 778 and 779 produced combinations of slab-derived fluid and seawater that had reacted with peridotite and basalt at shallower crustal levels (Mottl, 1992).

Fluid Budgets

Although total fluid budgets are difficult to ascertain in any convergent margin, they are likely to be more readily determined at nonaccretionary active margins because the hydrologic flow systems operate on longer timescales than do those at accretionary margins. Attempts to determine the total fluid budgets at accretionary active margins have been hindered by the presence of lateral heterogeneity and transient flow processes. Lateral heterogeneity results in different flow rates and compositions along the strike of the margin. Transient flow apparently results largely from the valvelike influence of the accretionary complexes themselves.

Sediment properties vary with fluid pressure, and fluid pressure varies as a function of fluid production rate and transient hydrologic properties. Thus, the accretionary system acts as both a seal and a relief valve on the fluid flow system. The absence of such a short timescale, fluid pressure, and formation-properties modulator at nonaccretionary systems should allow fluids to escape more steadily. To test this hypothesis, the physical nature of fluid flow at nonaccretionary settings must be determined. Then fluid budgets can be constructed to determine whether the expected long-term flow is consistent with observations or if the flow must occur in transient pulses. The CORK experiment planned for the South Chamorro Seamount site will address this problem.

Along-Strike Variability

The composition of slab-derived fluids and deep-derived rock materials may differ along the strike of the forearc, reflecting regional variations in composition within the slab and suprasubduction zone lithosphere. The pore fluids from several of the forearc mud volcanoes already sampled are chemically distinct, and it was anticipated that the pore waters from South Chamorro Seamount would also be chemically distinct. These differences are probably associated not only with the depth to the slab but also with the physical conditions under which water-rock reactions occur and the variations in the regional composition of the plate and overriding forearc wedge.

The geochemistry of the fluids from Conical Seamount is described in detail in several publications (Fryer et al., 1990; Haggerty, 1991; Haggerty and Chaudhuri, 1992; Haggerty and Fisher, 1992; Mottl, 1992; Mottl and Alt, 1992). These investigators have shown the origin of the Conical Seamount fluids to be from dehydration of oceanic crustal basalt and sediment at the top of the subducting lithospheric slab. The compositions of the fluids from PACMANUS hydrothermal field and seamounts farther south are reported in Fryer et al. (1999). Pore fluids from these indicate a slab source, as shown by their lower chlorinity and higher K and Rb, similar to that observed at Conical Seamount by Mottl (1992).

Pressure and Temperature Indicators from Fluids

The composition of slab-derived and deep-derived metamorphosed rock is useful in defining geochemical processes and estimates of the thermal and pressure regime at depth, and, thus, for determining the physical properties of the dècollement region. It was hoped to constrain some of the pressure and temperature conditions under which certain dehydration reactions take place in the subducted slab. Pore fluids from Ocean Drilling Program (ODP) Site 780 at the summit of Conical Seamount are unusual because of geochemical and physical processes at depth. The observed enrichments in alkali elements and B in fluids from Site 780 are unambiguous indicators of a source temperature in excess of 150°C, yet the fact that these elements are depleted at Sites 778 and 779 on the flanks of Conical Seamount, relative to their concentrations in seawater, indicates that the deep slab signal can be readily overprinted by local peridotite-seawater reactions at lower temperatures. Not all chemical species are affected by this overprinting, however (i.e., sulfur isotopic composition of dissolved sulfate) (Mottl and Alt, 1992). Thus, to avoid potential reactions between sediment and slab-derived fluids, we planned to collect fluids from mud volcano conduits, where continued focused flow provides a pathway for slab-derived "basement" fluids to reach the seafloor.

Metamorphic Parageneses

Studies of deep-derived minerals and metamorphic rock fragments brought to the surface in mud flows in serpentine seamounts can be used to constrain the pressure and temperature regimes under which the metamorphism that formed them took place. It is known, for instance, that the minimum pressures of formation for incipient blueschist materials from Conical Seamount are 6-7 kbar (Maekawa et al., 1995). Similarly, from the paragenesis of crossite schist recovered in cores from South Chamorro Seamount, it can be shown that pressures >7 kbar are consistent with their metamorphism. Examination of a more extensive collection of the muds and clasts from South Chamorro Seamount should make it possible to quantify the assemblages of muds and xenoliths present in the flows and constrain the ranges of pressure and temperature that exist in the source regions for these materials.

Biological Activity Associated with Deep-Derived Subduction Zone Material

Interest in the deep subsurface biosphere has grown dramatically as a result of recent studies linking extreme environments to the first living organisms that inhabited the Earth. The search for the last common ancestor in the geologic record is moving toward high-temperature environments, such as those at spreading centers and hotspots both on the ocean floor and on land. Microbes and microbial products are abundant in oceanic hydrothermal environments and are presumed to be representative of a community of thermophilic and hyperthermophilic organisms that originated beneath the seafloor (Fisk et al., 1998). Microbes are also involved in the transformation of minerals in the oceanic crust and in the cycling of elements in the crust; however, the origin of these microbes is much more controversial.

Drilling at Chamorro Seamount provides a unique opportunity to determine the nature of microbiological activity in a very different kind of extreme environment, the high-pH, low-temperature environment associated with serpentine/blueschist mud volcanism (Fryer and Mottl, 1997; Fryer et al., 1999), and to reexamine the hypothesis that microbes are capable of using alternative energy sources that would support a heterotrophic subsurface ecosystem. In addition, because the pore fluids are more pristine in nonaccretionary convergent margins, it should be easier to assess from the chemistry of both the muds and the fluids whether organic syntheses capable of supporting life are active in these settings.

Understanding the origin of the deep biosphere is a fundamental ODP objective and will further address the compelling question of whether life arose in extreme environments rather than on the surface of the early Earth. Although several experimental studies indicate that a thermophilic origin of life is possible, definitive proof must await an assessment of the full range of conditions in which life exists and the nature of life in these environments.

Mechanics and Rheology

The mechanics and rheology of serpentine muds in the Mariana forearc seamounts control the processes that formed the seamounts and their morphology. It was thus planned to conduct a rheological study of the serpentine muds to place realistic constraints on the mechanisms governing the ascent of the muds to the surface, the maintenance of the conduits, and the construction of the seamounts.

Shipboard torsion-vane testing during Leg 125 at Conical Seamount in the Mariana forearc and at Torishima Forearc Seamount in the Bonin forearc showed that the serpentine muds are plastic solids with a rheology that bears many similarities to the idealized Cam clay model and is well described by critical-state soil mechanics (Phipps and Ballotti, 1992). These muds are thus orders of magnitude weaker than salt and are, in fact, comparable in strength to common deep-sea pelagic clays. The rate at which the muds rise relative to the fluids will likely influence the water-rock reactions and the character of the slab signal in fluids from these mud volcanoes. Better constraints on the nature of the fluids will permit a more accurate determination of the physical conditions of the dècollement, where the fluids originate.

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