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2 years ago in Geochemistry , Igneous Petrology By Lukas

How to calculate FeO from Fe2O3 (whole rock analysis)‑ Can this FeO be used for altered metavolcanics?

My lab only provides total iron as Fe2O3(T), but I need FeO for Jensen or AFM diagrams to classify these proteozoic metavolcanics. The standard molar mass conversion is simple, but my concern is that alteration and metamorphism have likely changed the Fe2+/Fe3+ ratio. Can I trust a calculated FeO value in this context, or is it misleading?

 

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By Prajwal Sharma Answered 1 year ago

I have seen this practice lead to significant misclassification, especially in oxidized terranes. The conversion assumes all iron is ferrous, which is rarely true after seafloor alteration or weathering. For fresh basalts, it's a fair estimate, but for altered metavolcanics, it can place your samples incorrectly in the tholeiitic vs. calc-alkaline fields. I would recommend this calculated value only for a first-pass look. For robust classification, you need a direct measure, like wet-chemical titration for FeO, or to use immobile element diagrams (e.g., Zr/Ti) instead, which are unaffected by the oxidation state changes.

 

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