HR: 10:50h
AN: V52A-03    [Abstracts]
TI: The Decisive but Quite Limited Role of Hydrous and Carbonaceous Minerals During Subduction - an Opinion
AU: * Schmidt, M W
EM: max.schmidt@erdw.ethz.ch
AF: ETH Zürich, Inst.Min.Pet., Zürich, 8092 Switzerland
AB: Subduction is interesting because it enables geochemical exchange between the subducting crust and the Earth's mantle. Such exchange occurs through two processes: element transport in a mobile phase which moves between the reservoirs or mechanical mixture of the reminder of devolatilization with the mantle. After initial expulsion of pore fluids and decomposition of clay minerals, hydrous and carbonaceous phases are fundamental for the production of a fluid, melt or supercritical liquid within the subducting lithosphere. The role and contribution of the different hydrous minerals can be quantified when superimposing thermo-mechanical models and phase diagrams (e.g. Connolly 2005, EPSL). Chlorite, amphibole, epidote/zoisite, lawsonite and chloritoid are the most important hydrous minerals in mafic and pelagic lithologies, this list is appended by the role of phengite in defining melting conditions in any lithology with significant K₂O. In some subduction zones, carbonates may be omnipresent, nevertheless, dolomite and magnesite are extremely refractory and X_CO2 in the fluid is generally low, thus making it difficult to mobilize CO₂ at typical subduction zone conditions. With regard to the mantle, the contributions and conditions for serpentine, clinohumite, chondrodite, phase A and phase E are reviewed. Whereas the major element geochemistry as well as fluid and melt production are clearly dominated by hydrous phases, this is only valid to a much diminished extent for trace elements. Many of the major hydrous phases are quite limited as residual phases, in particular, because trace element budgets are established in an essentially diffusion-free (low temperature) environment: distribution coefficients between hydrous and mobile phases are important, but fluids equilibrate only with the (mostly unhydrous) phases formed anew during hydrous phase breakdown, an aspect that is generally ignored in trace element modeling. Furthermore, fluids originating from lower parts will flush upper parts of the subducting lithosphere and, depending on their pathway, may play a major role in trace element geochemistry (due to leaching) and melt production. Finally, further limitations to the role of volatile bearing phases in geodynamics arise from the fact, that hitherto a direct correlation between the location of fluid or melt producing reactions in the slab and the position of volcanic arcs has not been made plausible. The straightforward relation between the result of thermo-mechanical models and devolatilization reactions is only of limited help, as thermo-mechanical models based on steady state and actual kinematic subduction parameters fail to describe major subduction phenomena (i.e. back arc basins, slab rollback), which in turn requires more dynamic models in which the correlation between actual subduction geometry and the thermal field (and thus reactions) is more complex.
DE: 1012 Reactions and phase equilibria (3612, 8412)
DE: 1030 Geochemical cycles (0330)
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 1065 Major and trace element geochemistry
DE: 8104 Continental margins: convergent
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005