Quantitative Model to Predict Melts on the Ol-Opx Saturation Boundary during Mantle Melting: The Role of H2O

Abstract:

Two quantitative, empirical models are presented that predict mantle melt compositions in equilibrium with olivine (ol) + orthopyroxene (opx) ± spinel (sp) as a function of variable pressure and H₂O content. The models consist of multiple linear regressions calibrated using new data from H₂O-undersaturated primitive and depleted mantle lherzolite melting experiments as well as experimental literature data. The models investigate the roles of H₂O, Pressure, 1-Mg# (1-[X_Mg/(X_Mg+X_Fe)]), NaK# ((Na₂O+K₂O)/(Na₂O+K₂O+CaO)), TiO₂, and Cr₂O₃ on mantle melt compositions. Melts are represented by the pseudoternary endmembers Clinopyroxene (Cpx), Olivine (Ol), Plagioclase (Plag), and Quartz (Qz) of Tormey et al. (1987). Model A returns predictive equations for the four endmembers with identical predictor variables, whereas Model B chooses predictor variables for the four compositional endmember equations and temperature independently. We employ the use of Akaike Information Criteria (Akaike, 1974) to determine the best predictor variables from initial variables chosen through thermodynamic reasoning and by previous models.

In both Models A and B, the coefficients for H₂O show that increasing H₂O drives the melt to more Qz normative space, as the Qz component increases by +0.012(3) per 1 wt.% H₂O. The other endmember components decrease and are all three times less affected by H₂O (Ol: -0.004(2); Cpx: -0.004(2); Plag: -0.004(3)). Consistent with previous models and experimental data, increasing pressure moves melt compositions to more Ol normative space at the expense of the Qz component. The models presented quantitatively determine the influence of H₂O, Pressure, 1-Mg#, NaK#, TiO₂, and Cr₂O₃ on mantle melts in equilibrium with ol+opx±sp; the equations presented can be used to predict melts of known mantle source compositions saturated in ol+opx±sp.

References

Tormey, Grove, & Bryan (1987), doi: 10.1007/BF00375227.

Akaike (1974), doi: 10.1109/TAC.1974.1100705.