fO₂ calculation in CHNOSZ

[ZongwenLi]

Dear Dr. Jeffrey M. Dick,

I hope this email finds you well. My name is Zongwen Li, a graduate student from China University of Geosciences , studying skarn deposit. I am writing to seek your expertise on oxygen fugacity (fO₂) calculation using EPMA data of garnet, pyroxene, and magnetite within the CHNOSZ framework.I have EPMA oxide wt% data (FeO, MnO, MgO, etc.). However, I am unsure how to programmatically link these reactions to fO₂ calculation in CHNOSZ.
Thank you very much for your time and guidance. I greatly appreciate your contribution to thermodynamic modeling and would be grateful for any suggestions.

Best regards,

Zongwen Li,

China University of Geosciences

[jedick]

Hi Zongwen, thanks for your question! As far as I can tell, the use of EPMA data for fO₂ calculations is usually based on compositional variation of minerals. For instance, how much Ti or V substitutes for Fe in the magnetite structure depends on fO₂ and other variables.

The minerals in the OBIGT database in CHNOSZ can be used for relatively simple calculations of fO₂. For example, the magnetite-hematite reaction defines a value of fO₂ at any temperature. Because this calculation, in a first approximation, is based on the assumption of unit activity of the pure endmembers, it doesn't take into account compositional data from EPMA for the minerals in the reaction. The compositional variation of magnetite could probably be modeled as a solid solution, which would correspond to variable activities of the endmembers. If you were able to obtain the activities of endmembers from such a model, then they could be used in CHNOSZ to calculate values of fO₂ that would differ from those calculated using the assumption of unit activity.

I'm sorry that I can't suggest a complete solution using CHNOSZ. You might have some luck searching for packages that can perform oxybarometry. Please reach out if you know of other ways that your data could be used to formulate reactions between minerals that CHNOSZ has available in the database.