Different oxygen buffers of magmas, such as FMQ and NNO, have been used in the papers. How to transfer the FMQ to NNO if we want to make comparision?
Hi Dr. Huang,
Normally, if you know the magma's temperature, pressure, and oxygen fugacity relative to a buffer (e.g., ΔFMQ, ΔIW, ΔNNO), you can calculate the magma logfO2 directly, and then convert the magma fO2 from one buffer to another. Here is a paper that you can get expressions and related constants (Table 1):
B. Ronald Frost; Introduction to oxygen fugacity and its petrologic importance. Reviews in Mineralogy and Geochemistry 1991; 25 (1): 1–9.
Thanks for your good question,
Dian
Dian Ji ：
Thanks for your recommended paper. But I think the convertion is not convenient to use the equation, just because the equation is metioned by different people in various conditions, there are even different formulas for calculating the same oxygen buffer. For example, to calculate the logfO2(FMQ), Myers and Eugster(1983) and O'Neill(1987) proposed different equations. So I want to know if exist some experimental results about the convertion in different oxygen buffers.
[1]Myers J, Eugster H P. The system Fe-Si-O: Oxygen buffer calibrations to 1,500 K[J]. Contributions to Mineralogy and Petrology, 1983, 82: 75-90.
[2]O'Neill H S C. Quartz-fayalite-iron and quartz-fayalite-magnetite equilibria and the free energy of formation of fayalite (Fe 2 SiO 4) and magnetite (Fe 3 O 4)[J]. American Mineralogist, 1987, 72(1-2): 67-75.
Hi Haoqian Zhang :
Thank you for your further question. You are right, there are different formulas for calculating the same buffer, as each study has their own calibrated dataset. For example, the formula of FMQ buffer in Myers and Eugster (1983) has been recalibrated at 1 atm pressure, so there is not pressure term in their equation. In addition, the T range in Myers and Eugster (1983) is also different from that in Frost (1991). We really need to read the experimental conditions of calibrated data before we applied a specific formula.
Based on my experience, if I choose calibrated datasets and corresponding formulas that overlap with my target magma temperature and pressure range, even if the formulas of the same buffer look different, the results should be similar. In this case, converting between different buffers should not be a problem.
Hope this helps,
Dian
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