Geochemical processes and petrogenetic evolution of rodingite dykes in the ophiolite complex of Othrys (Central Greece)

Rodingite dykes occur in serpentinised harzburgites in the Othrys ophiolite. Hydrogrossular and diopside along with chlorite appeared first and mark the initiation of rodingitisation of the dolerite protolith. Subsequently calcite formed after diopside and consequently reacted with chlorite to enrich the rodingites in diopside and garnet. Apatite also forms at the late stages of alteration. Several geochemical exchanges were triggered by variably alkaline and continuously evolving fluids passing through the serpentinising peridotites. A possible T-XCO2 path along with a reaction series are proposed for the development of the rodingitic assemblages that entail a fluid phase. Leaching of the peridotite was triggered by fluids very rich in CO2 and carbonate complexes are thought to be responsible for mobilization of Cr and Ni. Under such conditions, REE also were mobile and the presence of CO32− and PO43− ions that complexed them further assisted their transportation. Zr mobilised at increased pH conditions with the aid of the OH− and PO43− during the late stage of alteration, and due to the incorporation of more saline fluids. After formation of calcite and apatite in the two metasomatic stages, respectively, the REE complexes were destabilised due to reduced activity of CO32− and PO43− and free REEs were able to precipitate onto garnet. Cr and Ni hydroxides were precipitated after formation of calcite in the rodingite. The saline fluids were also reducing, hence Eu remained in its divalent state and was highly absorbed on garnet surfaces. The rodingitisation at Othrys ophiolite occurred in a subduction regime as it is implicated by the very rich in CO2 fluid phase which is readily explained by involvement of subducted carbonate sediments and is favoured by the development of structural zones that acted as conduits to the metasomatising fluid phase.