Trace and REE content of clinopyroxenes from supra-subduction zone peridotites. Implications for melting and enrichment processes in island arcs

Abstract

We measured trace element and Rare Earth Element (REE) contents of clinopyroxenes (cpx) in peridotites from ophiolite complexes from the Hellenic Peninsula: Vourinos, Pindos, Othris (Greece), and Bulqiza (Albania). Compared to abyssal peridotites the Hellenic peridotites have a highly depleted mineralogy (<1% modal cpx) and the cpxs have extremely low concentrations in Ti (30–150 ppm) and heavy REE. The light and middle REE, Zr and Sr contents of the cpxs show enrichments compared to cpx from abyssal peridotites. Our cpx data falls in the field of previous studies on modern arc peridotites ([Parkinson, I.J., Pearce, J.A., Thirwall, M.F., Johnson, K.T.M., Ingram, G., 1992. Trace element geochemistry of peridotites from the Izu–Bonin–Mariana forearc, Leg 125. In: Fryer, P., Pearce, J.A., Stokking, L.B. (Eds.), Proc. Ocean Drilling Program. pp. 487–506; Bonatti, unpubl.]) suggesting that these complexes originated above a subduction zone. Dry melting of an upper mantle source similar to a mid-ocean ridge basalt (MORB) source (e.g., [Johnson, K.T.M., Dick, H.J.B., Shimizu, N., 1990. Melting in the oceanic upper mantle: an ion microprobe study of diopsides in abyssal peridotites. J Geophys. Res. 95, 2661–2678.]) cannot generate the extreme depletions in Ti and heavy rare earth element (HREE) seen in the cpxs from these supra-subduction zone (SSZ) peridotites. We propose that these high degrees of depletion can be achieved during hydrous melting of a MORB-depleted peridotite above a subduction zone. The melting rate of cpx during hydrous melting is expected to be less than in dry melting, while the rate of orthopyroxene (opx) consumption increases. These changes in melting modes allow for larger degrees of total melting and larger depletions in Ti and HREE in the cpx before cpx is exhausted from the residual peridotite. The enrichments in light rare earth element (LREE), MREE, Zr and Sr are best modeled by a constant flux of a slab-derived fluid component to the mantle wedge during melting. Ti and the HREE are not mobile in this fluid and are not enriched in the mantle peridotite. The continuous flux is necessary to sustain hydrous melting in the sub-arc mantle during the complete melting range. The calculated fluid composition matches well previous estimates based on the arc basalt chemistry ([McCulloch, M.T., Gamble, J.A., 1991. Geochemical and geodynamical constraints on subduction zone magmatism. Earth Planet. Sci. Lett. 102, 358–374; Stolper, E., Newman, S., 1994. The role of water in the petrogenesis of Mariana trough magmas. Earth Planet. Sci. Lett. 121, 293–325.]) and experiments on fluids ([Ayers, J.C., Dittmer, S.K., Graham, D.L., 1997. Partitioning of elements between peridotite and H 2O at 2.0–3.0 GPa and 900–1100°C, and application to models of subduction zone processes. Earth Planet. Sci. Lett. 150, 381–398; Ayers, J.C., 1998. Trace element modeling of aqueous fluid–peridotite interaction in the mantle wedge of subduction zones. Contrib. Mineral. Petrol. 132, 390–404.]). The resulting melts from our model closely resemble melts from an arc environment (boninites) further arguing for the plausibility of this model.