HR: 08:30h
AN: V51B-01 INVITED     [Abstracts]
TI: Element Transfer from Slab to Wedge; the Volcanic Arc Perspective
AU: * Pearce, J A
EM: PearceJA@cf.ac.uk
AF: Cardiff University, Park Place, Cardiff, CF10 3YE United Kingdom
AB: Volcanic arc magmas are characterised, as is well known, by geochemical patterns which feature the selective enrichment of some elements (the subduction-mobile or non-conservative elements) relative to others (the subduction-immobile or conservative elements). More detailed evaluation of these patterns highlights three groups of subduction-mobile elements: a low-temperature group (e.g., As, Ba, Pb, Rb) comprising elements released at temperatures of diagenesis and above; an intermediate-temperature group (e.g. Th, LREE) comprising elements released mainly at temperatures of amphibolite facies and above; and a high-temperature group (e.g., (e.g., Nb, Ta, Hf, Zr) comprising elements released mainly at melting temperatures and above. Typically, the fluxes of these three groups of elements vary according not only to input flux but also to the thermal and geodynamic state of the subduction system. Using element ratios such as Ba/Th and Nb/Th as proxies for these three groups, it is possible to demonstrate that arc segments characterised by shallow convecting mantle exhibit the greatest selective addition of the low-temperature group, while segments characterised by collisions, ridge subduction, rifting and edge flow exhibit the greatest addition of the high-temperature group. The HREE and the relatively compatible elements appear to be effectively conservative in all cases. A particular outcome of the arc-based mobility studies is the relationship between slab temperature and lava chemistry. Leeman and co-workers have established that B provides a potential proxy for slab temperature because of the temperature-controlled release of B at shallow depth. This work demonstrates that Hf and Zr also provide a proxy for slab temperature based on the temperature-controlled dissolution of zircon in fluids and melts at greater depths. By quantifying the chemistry-temperature link using Hf isotope and element-based ratios, it is possible to confirm the inference from geochemical patterns that the mobility of this element is largely restricted to unusual subduction events such as the beginning and end of the subduction cycle and episodes of ridge subduction and rifting. Thus, the transfer of elements from slab to wedge may vary considerably within the subduction cycle and this may explain, in part, why studies of arc lavas and subduction complexes do not always produce the same conclusions.
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