6b. Fe³⁺/Fe_T to f_O2 dependence

To add a new parameterisation for Fe³⁺/Fe_T to f_O2 dependence involves creating a new “terms” function, which details how any parameters depend on pressure, temperature, melt composition, etc.

Here is an example for Borisov et al. (2018) in model_dependent_variables.py:

def FefO2_Borisov18_terms(
    PT, melt_wf, models=default_models
    ):  # Eq. (4) from Borisov et al. (2018) CMP 173:98 doi:10.1007/s00410-018-1524-8
    T_K = PT["T"] + 273.15
    # Borisov et al. (2018) CMP 173
    a = 0.207
    # melt mole fraction with no volatiles and all Fe as FeOT
    melt_comp = mg.melt_mole_fraction(
        melt_wf, models, "no", "no", molmass="ONeill22", majors="ONeill22"
    )
    B = (
        4633.3 / T_K
        - 0.445 * melt_comp["SiO2"]
        - 0.900 * melt_comp["TiO2"]
        + 1.532 * melt_comp["MgO"]
        + 0.314 * melt_comp["CaO"]
        + 2.030 * melt_comp["Na2O"]
        + 3.355 * melt_comp["K2O"]
        - 4.851 * melt_comp["P2O5"]
        - 3.081 * melt_comp["SiO2"] * melt_comp["Al2O3"]
        - 4.370 * melt_comp["SiO2"] * melt_comp["MgO"]
        - 1.852
    )
    return a, B

Then, the new parameterisation must be added to the functions fO22Fe3FeT() and f_O2() in model_dependent_variables.py, which is shown here for Borisov et al. (2018):

def fO22Fe3FeT(fO2, PT, melt_wf, models=default_models):  # converting fO2 to Fe3/FeT

    model = models.loc["fO2", "option"]

    # Eq. (4) from Borisov et al. (2018) CMP 173:98 doi:10.1007/s00410-018-1524-8
    if model == "Borisov18":
        a, B = FefO2_Borisov18_terms(PT, melt_wf, models)
        Fe3Fe2 = 10.0 ** (a * math.log10(fO2) + B)
        return Fe3Fe2 / (Fe3Fe2 + 1.0)

def f_O2(PT, melt_wf, models=default_models):

    model = models.loc["fO2", "option"]

    # Eq. (4) from Borisov et al. (2018) CMP 173:98 doi:10.1007/s00410-018-1524-8
    elif model == "Borisov18":
        F = mg.Fe3Fe2(melt_wf)
        a, B = FefO2_Borisov18_terms(PT, melt_wf, models)
        fO2 = 10.0 ** ((math.log10(F) - B) / a)
        return fO2

To use it, the “fO2” option in “models” would be set to “Borisov18”.