Dear Professor:

I hope this email finds you well. I am currently facing a question regarding the calculation of oxygen fugacity, particularly about the specific minerals chosen for these calculations.

For instance, in the calculation of FMQ, only olivine and spinel are considered. However, I wonder if this approach is comprehensive enough, especially in systems where olivine, spinel, garnet, and pyroxenes (both monoclinic and orthopyroxenes) are present. Would you agree that this might limit the accuracy of the oxygen fugacity calculations?

To address this concern, I have been considering a different approach. Suppose I introduce Ir into the system; this would create a balance as follows:

2Fe+O2=2FeO

In my assumption, all the iron in the system originates from the Fe-Ir alloy. Assuming that the iron in the minerals is primarily composed of FeO (even though high-pressure phases of olivine, such as wadsleyite, contain about 2 wt.% Fe³⁺and garnets typically have around 3 wt.% Fe³⁺), would it be more accurate to calculate the activity of each FeO in the minerals? Then, based on the molar percentage of each mineral in the rock, we could determine the average activity of FeO in the rock.

I would greatly appreciate your insights on this method and whether you think it is a suitable approach for more accurately calculating oxygen fugacity.

Thank you for your time, and I look forward to your feedback.