HR: 16:15h
AN: V34A-02 INVITED     [Abstracts]
TI: Evidence for mobility of fluids and trace elements in high pressure rocks
AU: * Zack, T
EM: tzack@min.uni-heidelberg.de
AF: Mineralogisches Institut Universitaet Heidelberg, INF 236, Heidelberg, 69120 Germany
AB: Eclogite-facies rocks show widespread evidence for fluid mobility under high pressure conditions. Occurrence of high-pressure veins (qtz+omp+/-ky+/-phe+/-rt), documenting fracture-controlled fluid movement, are visible in most eclogite localities. Percolative fluid movement is also indicated by near-equilibrium trace element partitioning between minerals, at least on a thin section scale, in a number of eclogite-facies rocks. Trace element mobility on a meter to km scale is more difficult to prove in eclogite-facies rocks. The crucial issue is that in environments with low fluid fluxes most trace elements are not mobile enough to be significantly depleted. Significant depletion can be defined as the depletion of a residuum remarkable beyond the expected scatter associated with the rock protolith composition. With the exception of accretionary wedge environments, it should be recognized that fluid fluxes in high pressure rocks are mostly low (e.g. blueschist-facies mafic rocks have only stored a few wt percent H2O). However, "trace element mobility" should not only be observed from a rock viewpoint, but also as a trace-element component in fluids. Simple mass balance calculations involving protolith concentrations, D(Min/Fluid) values and modal abundance of relevant minerals clearly demonstrate that trace element concentration of moderately mobile elements (e.g. Rb, Sr, Cs, Ba, Pb) can be 100x enriched in a fluid compared to primitive mantle values, while the residual rock has lost less than 10 percent of these elements.
DE: 1000 GEOCHEMISTRY
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 3600 MINERALOGY AND PETROLOGY
DE: 3653 Fluid flow
DE: 3660 Metamorphic petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005