The Subduction Factory: Rationale for a Workshop
At convergent margins, raw materials (sediments, crust and mantle of the subducting and overlying plates) are fed into a "factory" where numerous processes (compaction dewatering, diagenesis, dehydration, metamorphism, melting and melt migration, mantle convection) shape the final products: fluids, mud and serpentinite diapirs and volcanic, plutonic and metamorphic rocks. The operation of this Subduction Factory has a number of consequences. It causes hazardous seismicity, explosive volcanism, and associated climate modification. Ore deposits and new continental crust are formed by the factory. Subduction of pore fluids and hydrous minerals in oceanic sediments and altered basalt may have a profound effect on the global budgets of selected volatiles such as H2O. The return of subducted fluids to the surface above a slab 0-50 km deep can support chemosynthetic communities, affect seawater chemistry for selected elements, partially control deformation, and extensively serpentinize shallow lithospheric mantle. Subducted altered ocean crust and sediments contribute to the chemistry of arc, rear-arc and some back-arc volcanoes, thereby providing a probe of physical and chemical processes operating deeper than can be drilled or imaged seismically. Subducted materials not consumed at the arc or recycled at shallower depths are carried into the deep mantle, where they alter its chemistry and rheology. And despite the central role for subduction in the evolution of the Earth, initiation of subduction remains a mystery.
Understanding these processes represents a major objective of the MARGINS program. Therefore, we propose to hold a small (ca. 50 participants) international workshop on the Subduction Factory in June of 1998 in San Diego, CA in order to prioritize scientific objectives and geographical foci for studies of the Subduction Factory. The workshop will be multidisciplinary, encompassing geological, geophysical, experimental, theoretical, petrological and geochemical approaches. The workshop will build upon the report prepared at a JOI/USSAC workshop held at Avalon, California in June of 1994. At Avalon, participants discussed the important science to be addressed at subduction zones, evaluated geophysical and geochemical methods for studying them and developed a series of general criteria for choosing margins suitable for drilling.
Our goal for this next workshop is to prioritize the important scientific themes and to further refine the general criteria so that the community may endorse one or two specific convergent margins for a focused, integrated study. The behavior of fluids entering and leaving the subduction zone will be included along with the study of solids and melts. We will develop a plan for integrating seismic reflection and tomographic studies; experimental geochemistry, petrology and rheology; heat flow and thermal structure of the slab; mantle geodynamics; mantle melting and melt migration with physical and chemical studies of materials recovered by drilling, ROV and seafloor observatories, and field work. Studies of solids and fluids on the incoming plate link closely with the plan developed at the Seismogenic Zone workshop held in Hawaii in June of 1997, and will build on those results wherever possible.
Ocean drilling will be an essential component of this plan. We need good recovery of the oxidative and reducing alteration zones of the incoming basaltic crust (ca. 500m penetration), as well as complete sediment recovery outboard of the trench. We need deeper drilling into fore-arc serpentine diapirs, and into arc and back-arc hydrothermal systems, along with CORKed holes. We need drilling into partly subducted oceanic plates and their sediments, as well as into the older and deeper parts of volcanic arcs and volcaniclastic sedimentary basins, to evaluate changes through time and assess assumptions of steady state used in all flux calculations. Drilling into juvenile fore-arc to explore subduction initiation may allow us to understand how and why the subduction factory starts up. Temperature and heat flow measurements made in drill holes will increase our understanding of the thermal structure of the convergent margin. While much of this can be done with existing technology, the deeper penetration of riser drilling would enable us to look at more than just the top few hundred meters of serpentinite diapirs and volcanic or volcanosedimentary piles. Riser drilling will also provide better hole stability and recovery, essential for many of the geochemical aspects of this plan.
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