HR: 08:45h
AN: V51B-02 INVITED     [Abstracts]
TI: Element Transfer from Slab to Wedge: the Subducted Plate Perspective
AU: * Pearce, J A
EM: PearceJA@cf.ac.uk
AF: Cardiff University, Park Place, Cardiff, CF10 3YE United Kingdom
AB: Element release from subducted materials can be investigated using thermodynamics, experiment and observation. Thermodynamic calculations quantify free energy changes for reactions in which an element is converted from a chosen solid state to a chosen dissolved state. Such calculations illustrate the dependency of element mobility on, in particular, ionic potential and complexing agents, but have restricted application because of our limited knowledge of thermodynamic parameters for subduction pressures and temperatures. The experimental approach takes real subducted materials and subjects them to subduction pressures and temperatures in the laboratory. This provides data on element mobility under the conditions of the experiment, but is limited by the fact that experiments cannot easily simulate the real world of open systems, complex fluid compositions, and progressive chemical change during subduction. Analysis of subducted materials that have experienced known metamorphic conditions give a good indication of element addition provided the protolith composition can be estimated. However, whereas it is straightforward to use immobile elements to fingerprint the composition of the fresh protolith and then to estimate chemical gains and losses relative to that composition, it is not always clear whether these gains and losses took place on the ocean floor before subduction, during the early stages of subduction, at the pressure and temperature of the peak metamorphic event, or during exhumation. Despite these caveats and gaps in our knowledge, the existing database essentially supports the general concept of selected element enrichment prior to subduction followed by progressive release of these and other elements during subduction. Of the three main types of subducted material, hydrated mantle is enriched in H, B, U and other elements and releases an important proportion of its H (as water) - and perhaps other elements - below 100km. Metabasic rocks become selectively enriched in elements of low ionic potential, predominantly in brownstone and zeolite facies prior to subduction. They then release these during progressive subduction, while elements of higher ionic potential are probably released only if eclogite undergoes melting or if fluid-rock ratios are particularly high. Subducted sediments often provide the greatest budget of elements of high ionic potential (such as Th and LREE) and probably release these at depth in supercritical fluids and melts. Behaviour of the high field strength elements remains ambiguous, with most experiments indicating transport in slab-derived melts, but with few eclogites (even those recycled through the deep Earth) exhibiting HFSE depletion.
DE: 1020 Composition of the crust
DE: 1025 Composition of the mantle
DE: 1040 Isotopic composition/chemistry
DE: 1065 Trace elements (3670)
DE: 8124 Earth's interior--composition and state (old 8105)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2005 Joint Assembly