Equilibrium and Disequilibrium Trace Element Partitioning in Hydrous Eclogites (Trescolmen, Central Alps)

Abstract

elements, Pb, Th and U, whereas phengite hosts Ba and is, along Despite the widespread presence of hydrous phases in subductionwith clinopyroxene and paragonite, an important phase for B. related systems, experimental D trace element values for many However, because of their low modal abundance in eclogite-facies hydrous phases are lacking. To fill this gap, we present a set of rocks, phengite and paragonite do not control the B whole-rock budget. D values (where Min indicates amphibole, zoisite, We infer that estimated D values from equilibrium phengite, paragonite or apatite) derived from equilibrium parageneses assemblages can be used as a good approximation for partition of eclogites from Trescolmen (Central Alps, Switzerland). These coefficients under the given PT conditions (>650°C and 2·0 data can be combined with experimental data for D, GPa). Deformation-induced dynamic recrystallization seems to be to estimate D values for the hydrous phases, thus circumventing the driving process necessary for a close approach towards trace experimental problems with the direct determination of such values. element equilibrium. This process should work efficiently during We analysed Li, Be, B, Sr, Y, Zr, Nb, Ba, Ce, Nd, Sm, Pb, Th subduction close to the slab–mantle interface, affecting sediments and U in coexisting phases by laser ablation microprobe inductively and low-T altered mid-ocean ridge basalt, but is of less importance coupled plasma mass spectrometry. Many of the values are extremely inside the slab. Knowledge of D values from equilibrium assemblages low; for example, Nb, Ba, Ce, Th and U are in the lower ppb determined in this study is useful not only for large-scale subductionrange in clinopyroxene. Attainment of equilibrium was evaluated zone modelling, but also as an aid in the selection of equilibrium by textural, and major and trace element characteristics. Nonsubassemblages in imperfectly equilibrated samples, so that a equilibrated assemblages are common in most eclogite localities, crystallization sequence can be derived. including Trescolmen, and using such samples would lead to the derivation of erroneous values for equilibrium partitioning. However, four of the 10 studied eclogites from Trescolmen having homogeneous