Exsolution Lamellae in Olivine Grains of Dunite Units from Different Types of Mafic-Ultramafic Complexes

Abstract

Exsolution microstructures in olivine grains from dunite units in a few selected tectonic environments are reported here. They include lamellae of clinopyroxene and clinopyroxene‐magnetite intergrowth in the Gaositai and Yellow Hill Alaskan‐type complexes, clinopyroxene‐magnetite intergrowth in the Kızıldağ ophiolite, and chromite lamellae in the Hongshishan mafic‐ultramafic intrusive complex. These lamellae commonly occur as needle‐ or rod‐like features and are oriented in olivine grains. The host olivine grains have Fo contents of 92.5–92.6 in the Gaositai complex, 86.5–90.1 in the Yellow Hill complex, 93.2–93.4 in the Kızıldağ ophiolite and 86.9–88.3 in the Hongshishan complex. Clinopyroxene in the rod‐like intergrowth exsolved in olivine grains in the Gaositai and Yellow Hill is diopside with similar major element compositions of CaO (23.6–24.3wt%), SiO2 (52.2–54.0wt%), Al2O3 (0.67–2.15wt%), Cr2O3 (0.10–0.42wt%) and Na2O (0.14–0.26wt%). It falls into the compositional field of hydrothermal clinopyroxene and its origin is thus probably related to reaction between dunite and fluids. The enrichment of the fluids in Ca²⁺, Fe³⁺, Cr³⁺ and Na⁺, resulted in elevated concentrations of these cations in olivine solid solutions via the reaction. With decreasing temperature, the olivine solid solutions altered to an intergrowth of magnetite and clinopyroxene. The Fe³⁺ and Cr³⁺ preferentially partitioned into magnetite, while Ca²⁺ and Na⁺ entered clinopyroxene. Since the studied Alaskan‐type complexes and ophiolite formed in a subduction environment, the fluids were probably released from the subducted slab. In contrast, the exsolved chromite in olivine grains from the Hongshishan complex that formed in post‐orogenic extension setting can be related to olivine equilibrated with Cr‐bearing liquid. Similarly, these lamellae have all been observed in serpentine surrounding olivine grains, indicating genetic relations with serpentinization.