Nanoscale Diopside and Spinel Exsolution in Olivine from Dunite of the Tethyan Ophiolites, Southwestern Turkey: Implications for the Multi-Stage Process

Diopside and Chromian spinel exsolution lamellae occur as oriented intergrowths within olivine of dunite from the Tethyan ophiolite, Turkey. Fresh samples of the dunite consist of olivine, chromian spinel and diopside. Olivine compositions are Fo_89.3–89.8, which are lower than those of olivine in harzburgite containing high-Cr chromian spinel (Fo_90.1–95.3) that lacks exsolution lamellae. Equilibrium temperatures between the olivine and exsolved Chromian spinel are 810–860 °C, indicating that these occurred at relatively low temperatures. The low-Cr#s (49.7–51.3) of the chromian spinel indicate metasomatism by rich Cr melts, produced by ~22% partial melting of mantle peridotite. However, olivine in the harzburgite with high-Cr chromian spinel (Cr# = 55.5–78.2), which lacks exsolution lamellae, is formed by reaction with boninitic melts produced by 25–45% partial melting. On the basis of these findings, we suggest that the Tethyan ophiolite was generated in two stages. In stage 1, abyssal peridotites were formed in a mid-ocean ridge environment. During the stage 2, hydrous, high-Cr boninitic melts produced by high degrees of partial melting in a suprasubduction zone mantle wedge reacted with the peridotites to form dunite pods. These melts were relatively rich in Ti⁴⁺, Cr³⁺, Ca²⁺ and Fe³⁺, which were added to the olivine grains by solid solution. Upon cooling, these elements were exsolved as lamellae of chromian spinel and clinopyroxene. Thus, as the oceanic crustal slabs were trapped in the Mid Ocean Ridge, they were modified by tholeiitic magmas or partial melting, which caused interaction or metasomatism and subsequent reaction with boninitic magma in suprasubduction zones (SSZ).