Activation-energy parameter search. Low- and midlatitude model–data comparison metrics for the Gaussian effective activation-energy distribution. (a) RMSE (Equation (4)) between model and M³ inferred surface abundance. (b) Mean bias (Equation (5)) model minus M³ abundance. The star marks the preferred grid case, which has the lowest RMSE and a mean bias close to zero. The shallow minimum near E_c = 0.520 eV and E_w = 0.090 eV indicates a preferred effective activation-energy response region for the M³-sensitive reservoir, not a unique mineralogical activation energy.