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South Chamorro Seamount, Mariana Forearc

Geologic processes at convergent plate margins control geochemical cycling, seismicity, and deep biosphere activity within subduction zones. The study of input into a convergent plate margin by sampling the downgoing plate provides the geochemical reference necessary to learn what geochemical factors influence the production of suprasubduction zone crust and mantle in these environments. The study of the output in terms of magma and volatiles in volcanic arcs and backarc basin settings constrains processes at work deep in the subduction zone, but these studies are incomplete without an understanding of the throughput, the nature of geochemical cycling that takes place between the time the subducting plate enters the trench and the time it reaches the zone of magma genesis beneath the arc. Tectonically induced circulation of fluids at convergent margins is a critical element in the understanding of chemical transport and cycling within convergent plate margins and, ultimately, in understanding global mass balance (e.g., COSOD II, 1987; Langseth et al., 1988; Kulm and Suess, 1990; Langseth and Moore, 1990; Martin et al., 1991). In the shallow to intermediate suprasubduction zone region, dehydration reactions release pore fluids from bound volatiles in oceanic sediments and basalts of the downgoing plate (Fryer and Fryer, 1987; Peacock, 1987, 1990; Mottl, 1992; Liu et al., 1996). Fluid production and transport affect the thermal regime of the convergent margin, metamorphism in the suprasubduction zone region, diagenesis in forearc sediments, biological activity in the region, and, ultimately, the composition of arc and backarc magmas. Furthermore, these fluids, their metamorphic effects, and the temperature and pressure conditions in the contact region between the plates (the dècollement) affect the physical properties of the subduction zone, where most major earthquakes occur.

The discovery of Earth's deep biosphere is recognized as one of the most outstanding breakthroughs in the biological sciences. The extent of this biosphere is currently unknown, but we are becoming increasingly aware that life has persisted in environments ranging from active hydrothermal systems on mid-ocean ridges to deep ocean sediments, but so far no detailed investigations have been made of the potential for interaction of the deep biosphere with processes active in convergent plate margins.

Determining unequivocally the composition of slab-derived fluids and their influences over the physical properties of the subduction zone, biological activity, or geochemical cycling in convergent margins requires direct sampling of the dècollement region. To date, studies of dècollement materials, mass fluxes, and geochemical interchanges have been based almost exclusively on data from drill cores taken in accretionary convergent margins (e.g., Kastner et al., 1992; Carson and Westbrook, 1995; Maltman et al., 1997). Large wedges of accreted sediment bury the underlying crystalline basement, making it inaccessible to drilling, and the wedges interact with slab-derived fluids, altering the original slab signal. The dehydration reactions and metamorphic interchanges in intermediate and deeper parts of the dècollements have not been studied in these margins. By contrast, nonaccretionary convergent margins permit direct access to the crystalline basement and produce a more pristine slab-fluid signature for two reasons: the fluids do not suffer interaction with a thick accretionary sediment wedge, and they pass through fault zones that have already experienced water-rock interactions, thus minimizing interaction with subsequently escaping fluids. Regardless of the type of margin studied, the deeper dècollement region is directly inaccessible with current or even foreseeable ocean drilling technologies. A locality is needed where some natural process brings materials from great depths directly to the surface. The Mariana convergent margin provides precisely the sort of environment needed.

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