MULTICOMPONENT DIFFUSION IN BASALTIC MELTS

Abstract

Experimental results are presented for eighteen experiments exploring multicomponent chemical-diffusion in basaltic liquids. Experiments were performed in Columbia River Basalt (CRB) composition doped with about 5 wt% SiO2, TiO2, Al2O3, FeO, MgO, and CaO, under reducing conditions at 1 atm., at 1473 K, 1573 K, and 1723 K. Results indicate that diffusion behavior in CRB compositions is consistent with a simple Fick's law formulation. This Fickian behavior in CRB compositions contrasts with more complicated diffusion behavior observed in MgO-Al2O3-SiO2 and CaO-MgO-Al2O3-SiO2 melts. Results of CRB experiments are combined to calibrate a diffusion matrix (D) in CRB liquids at 1473 K, 1573 K, and 1723 K. Our D estimates indicate negative coupling between CaO and both FeO and Al2O3 components, though diagonal elements still dominate. These general features persist across the temperature range considered.Self- and tracer-diffusion data from the literature are used with the predictive model of Richter (1993) to estimate a D matrix at 1573 K. The resulting matrix does not compare well with our measured 1573 K diffusion matrix. It cannot be established if this discrepancy indicates a failure of the Richter (1993) model, or merely reflects deficiencies in the available tracer- and self-diffusion data, or limitations in the melt activity model.Effective Binary Diffusion Coefficients (EBDC) were also estimated for TiO2, Al2O3, FeO, MgO, and CaO at 1473 K, 1573 K, and 1723 K. These EBDC estimates are used to constrain a polythermal Arrhenian model for the prediction of EBDC values at super-liquidus temperatures. Results of experiments on alkali diffusion in CRB compositions are included to add Na2O and K2O to the polythermal EBDC model.