Investigations on modification of relict olivine composition during serpentinization of ultrabasic rocks; An example from peridotites from North west of Piranshahr, NW Iran

Olivine is one of the major crystals in ultrabasic rocks of Piranshahr Ophiolitic Complex. This mineral occurs as anhedral with cracks and fractures. Commonly, olivine is altered to serpentine along the fractures and rims. In some cases, serpentine crosscut the olivine. According to BSE images and mineral chemistry studies of olivine, the secondary iron- rich olivines occur at the rim of primary olivines. The forsterite content of the secondary olivines is lower than the primary olivines. XFo in the secondary olivines varies 85.6 to 87.7. Textural studies suggest that iron- rich olivines formed during serpentinization. Occurrence of Mg- rich serpentine and lack of magnetite during serpentinization of olivine caused formation of Fe- rich fluids. The iron- rich fluids may infiltrated through the olivine crystal and formed iron- rich rims.