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Equilibrium and Disequilibrium Trace Element Partitioning in Hydrous Eclogites (Trescolmen, Central Alps) |
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| Authors: | ZACK T; FOLEY S F; RIVERS T |
| Institution: | 1MINERALOGISCHES INSTITUT, UNIVERSITÄT HEIDELBERG, IM NEUENHEIMER FELD 236, 69120 HEIDELBERG, GERMANY 2INSTITUT FÜR GEOLOGISCHE WISSENSCHAFTEN, UNIVERSITÄT GREIFSWALD, JAHNSTR. 17A, 17487 GREIFSWALD, GERMANY 3DEPARTMENT OF EARTH SCIENCES, MEMORIAL UNIVERSITY OF NEWFOUNDLAND, ST. JOHN’S, NEWFOUNDLAND, A1B 3X5, CANADA |
| Abstract: | Despite the widespread presence of hydrous phases in subduction-related systems, experimental DMin/Fluid trace element valuesfor many hydrous phases are lacking. To fill this gap, we presenta set of DMin/Clinopyroxene values (where Min indicates amphibole,zoisite, phengite, paragonite or apatite) derived from equilibriumparageneses of eclogites from Trescolmen (Central Alps, Switzerland).These data can be combined with experimental data for DClinopyroxene/Fluid,to estimate DMin/Fluid values for the hydrous phases, thus circumventingexperimental problems with the direct determination of suchvalues. We analysed Li, Be, B, Sr, Y, Zr, Nb, Ba, Ce, Nd, Sm,Pb, Th and U in coexisting phases by laser ablation microprobeinductively coupled plasma mass spectrometry. Many of the valuesare extremely low; for example, Nb, Ba, Ce, Th and U are inthe lower ppb range in clinopyroxene. Attainment of equilibriumwas evaluated by textural, and major and trace element characteristics.Non-equilibrated assemblages are common in most eclogite localities,including Trescolmen, and using such samples would lead to thederivation of erroneous values for equilibrium partitioning.However, four of the 10 studied eclogites from Trescolmen havinghomogeneous clinopyroxene compositions and preferred orientationof high-pressure phases yielded consistent DMin/Clinopyroxenevalues in all four samples (where Min indicates amphibole, phengite,paragonite, apatite), and hence were studied in detail. Thelow abundances in some phases result from strong preferentialincorporation of trace elements into other minor phases. Fromthe investigated hydrous phases (amphibole, zoisite, clinozoisite,phengite, paragonite, apatite and talc), zoisite was found tobe the most important carrier of Sr, light rare earth elements,Pb, Th and U, whereas phengite hosts Ba and is, along with clinopyroxeneand paragonite, an important phase for B. However, because oftheir low modal abundance in eclogite-facies rocks, phengiteand paragonite do not control the B whole-rock budget. We inferthat estimated DMin/Clinopyroxene values from equilibrium assemblagescan be used as a good approximation for partition coefficientsunder the given PT conditions ( |
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