These articles will help you.

Article Fe-Mg Partitioning between Olivine and High-magnesian Melts ...

and

Article Olivine-liquid equilibria and the chemical activities of FeO...

Dear Tamalkanti Mukherjee,

The determination for the presence of FeO can be realized by two approaches. One is in the presence of a real experiment in which we have a reaction with Fe and O. The reading of this phase can be realized by X-ray structural analysis (X-RAY). Another approach is by simulating the process, using research data or literature for input data in the software. In this variant, a mathematical model can be created for the optimized variant, which can be verified experimentally. When differences arise, new adjustments are made to the mathematical model. This is repeated until a similarity is found between the experiment and the mathematical model.

Olivine structure contains only 2-valent iron and can be written in sub-lattice form of (Fe,Mg)2(Si)(O)4. Thus, the molar fraction of (Fe+Mg) to Si is always constant and the fact can be used for estimating the total concentration of olivine in a system.

Pekka's suggestion would work well if you need a rough estimate of FeO concentration. Natural olivines would also have small amount of NiO, MnO, CaO, TiO2, Al2O3, you'll get more accurate result if these minor elements concentrations are known.

Thank You all for your suggestions.