Thermodynamics of multicomponent olivines and the solution properties of (Ni,Mg,Fe)2SiO4 and (Ca,Mg,Fe)2SiO4olivine—Reply

Abstract

Fleet (2000) criticizes several aspects of the thermodynamic model for (Ni,Mg,Fe)2SiO4-bearing olivine from Hirschmann (1991) and its subsequent recalibration in my Ph.D. thesis (Hirschmann 1992). He also justifies the thermodynamic methods used by Fleet (1989) to extract activity coefficients of NiSi0.5O2 in olivine. I am pleased that somebody has taken the time to read parts of my Ph.D. thesis and will take this opportunity to reply to Fleet’s comments on some of my early work and elaborate on the problems with Fleet (1989). Fleet (2000) finds fault with three aspects of the thermodynamic model for (Ni,Mg,Fe)2SiO4 olivine from Hirschmann (1991 Hirschmann (1992). He criticizes the formulation of the model, but his contentions reflect misapprehensions regarding thermodynamic models that account for long-range ordering. He questions the quality of the ordering data used to calibrate the model, but does not acknowledge available data that corroborate the site occupancy measurements. He also disparages the accuracy of the predicted activity-composition relations, but fails to account for experimental uncertainties and discounts considerable evidence for agreement between the model and experimental constraints on NiSi0.5O2 olivine activities. Fleet (2000) also defends the activity coefficients for NiSi0.5O2 extracted in the thermodynamic analysis of Fleet (1989). He suggests that there is a posteriori evidence that silica activity was nearly constant in a suite of experiments that were not designed to buffer silica but, as detailed below, there is good reason to suspect significant silica activity variation in these experiments. He reiterates evidence presented in Fleet (1989) suggesting that the oxysulfide liquids in the experiments of interest are reasonable approximations of FeS-NiS binary solutions. However, these liquids are unlikely to be close to monosulfide composition because their reported compositions are critically …