Equilibrium and disequilibrium trace element partitioning in hydrous eclogites (Trescolmen, Central Alps)

Abstract

Despite the widespread presence of hydrous phases in subduction-related systems, experimental DMin/Fluid trace element values for many hydrous phases are lacking. To fill this gap, we present a set of DMin/Clinopyroxene values (where Min indicates amphibole, zoisite, phengite, paragonite or apatite) derived from equilibrium parageneses 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 circumventing experimental problems with the direct determination of such values. We analysed Li, Be, B, Sr, Y, Zr, Nb, Ba, Ce, Nd, Sm, Pb, Th and U in coexisting phases by laser ablation microprobe inductively coupled plasma mass spectrometry. Many of the values are extremely low; for example, Nb, Ba, Ce, The and U are in the lower ppb range in clinopyroxene. Attainment of equilibrium was evaluated by textural, and major and trace element characteristics. Nonequilibrated assemblages are common in most eclogite localities, 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 clinopyroxene compositions and preferred orientation of high-pressure phases yielded consistent DMin/Clinopyroxene values in all four samples (where Min indicates amphibole, phengite, paragonite, apatite), and hence were studied in detail. The low abundances in some phases result from strong preferential incorporation of trace elements into other minor phases. From the investigated hydrous phases (amphibole, zoisite, clinozoisite, phengite, paragonite, apatite and talc), zoisite was found to be the most important carrier of Sr, light rare earth elements, Pb, Th and U, whereas, phengite hosts Ba and is, along with clinopyroxene and paragonite, an important phase for B. However, because of their low modal abundance in ecologite-facies rocks, phengite and paragonite do not control the B whole-rock budget. We infer that estimated DMin/Clinopyroxene values from equilibrium assemblages can be used as a good approximation for partition coefficients under the given PT conditions (∼650°C and 2.0 GPa). Deformation-induced dynamic recrystallization seems to be the driving process necessary for a close approach towards trace element equilibrium. This process should work efficiently during subduction close to the slab-mantle interface, affecting sediments and low-T altered mid-ocean ridge basalt, but is of less importance inside the slab. Knowledge of D values from equilibrium assemblages determined in this study is useful not only for large-scale subduction-zone modelling, but also as an aid in the selection of equilibrium subassemblages in imperfectly equilibrated samples, so that a crystallization sequence can be derived.