Subduction Zone Chemical Fluxing
The subduction zone provides an idiosyncratic chemical "filter", between surface and deep Earth reservoirs, which preferentially allows or disallows deep subduction of various volatile and non-volatile components. Estimates exist of the masses (and compositions) or downgoing materials (subducting sediment, altered oceanic crust, and pore fluids), affording some at least first-pass evaluation of convergent margin fluxing on both margin-by-margin and global bases. In this section, we organize (and solicit) attempts to quantify the flux of material through subduction zones. In doing so, we assume a simplified, generic/uniform physical model for subduction zones that we first briefly explain.
Here we consider three "stages" across an Idealized Subduction Zone [click here to view cartoon] recognizing that dominant processes are likely to vary among individual modern convergent margins. This approach doubtless simplifies what is probably a continuum.
Subduction Zone Input
Pore Fluids Seafloor Sediment.
Altered Oceanic Crust.
Stage I (Forearc Processes)
Compaction-related dewatering of sediment and forearc prograde mineral devolatilization reactions in sediment and oceanic crust. These processes favor the liberation of aqueous fluids which can then be delivered to the seafloor along structural heterogeneities or into the forearc mantle wedge, the latter potentially resulting in hydration and production of serpentinite seamounts. Forearc accretion and erosion processes will also be considered.
State II (Subarc Processes)
Magma production at the main volcanic arc, resulting from delivery of slab+ sediment chemical components to mantle wedge, in addition to AFC processes in the arc crust. Contributions to crustal growth and atmosphere (the latter through volcanic degassing).
Stage III (Backarc Processes)
Magma production during back-arc rifting.
Deep Subduction Zone Output
Deeply Recycled Material.
Many studies have shown that the composition of arc lava's (Stage II) strongly depend on the composition of subducting (input) material. Furthermore, the physical characteristics of individual arc-trench systems (age of subducting oceanic lithosphere, convergence rate) dictate such factors as thermal structure critical to the history of deep recycling in subduction zones. Thus, rather than initially attempting a global average of Stages I-III, we elect to first focus on assessment of chemical flux in a subset of Earth's modern arc-trench systems.
These systems are shown on the map of the Earth's Subduction Zones [click here to view map] and we include here a summary of the Physical and Chemical Characteristics of the focus arc-trench systems. Global flux estimates for individual elements/components will in turn eventually be gleaned from the information on individual systems. In response to discussions at the recent La Jolla GERM Workshop (March, 1998) and other recent meetings (MARGINS Subduction Factory Initiative Meeting, June, 1998), we focus on the acquisition, organization, and interpretation of data for eight modern arc-trench systems that span a wide range in physical and chemical behaviors. Data where available will be organized and provided and additional contributions are welcomed. Please send contributions to the Editors for this section Gray Bebout and Tim Elliott. We will also help organize the development, curation, and distribution of reference sample suites of arc volcanic sample suites for the eight systems. Accordingly, a contact person who has such samples will be indicated as they are identified.
The eight arc-trench systems we will initially consider are:
The eight arc-trench systems identified for special attention result from a diverse range of criteria, but ones that hopefully provide a representative range of subduction zone processes. To some extent, areas have been selected as a result of existing sample and data coverage, that results in at least a first-pass understanding of their behavior, and should maximize the effect of future detailed work. Age of subducting plate ranges from 8Ma beneath Cascadia to 155 Ma beneath the Marianas, and rates of subducting drop from 10cm/a at Tonga to 2cm/a at Lesser Antilles (see summary of the Physical and Chemical Characteristics).
The Aleutians provide an example of oblique convergence and oceanic-continent boundary. Subducting sediment assemblages include a range of lithologies from the thick pelagic sequences from productive tropical water off Central America, an important contribution of Cretaceous volcanics out-board the Marianas, to dominant Archean cratonic material at Banda. Ocean island arcs, where geochemical signatures from deep are best preserved, and continental arcs, that clearly play an important role in continental growth, are both well represented. Arcs with (e.g., Tonga) and without (e.g., Aleutians) active back-arc spreading are included. There are clearly many other arcs that may also be of great interest, but in an attempt to focus concerted activity we are trying to highlight the eight listed.
Ultimately, tables of averaged compositions of outputs for Stages I-III will be made where possible, affording mass-balancing with (or, at least, tracing) known compositions of subduction inputs. Compilation of results for individual arc-trench systems will ideally allow estimates of global subduction-zone flux.