Immiscible silicate liquids: K and Fe distribution as a test for chemical equilibrium and insight into the kinetics of magma unmixing

Abstract

Silicate liquid immiscibility leading to formation of mixtures of distinct iron-rich and silica-rich liquids is common in basaltic and andesitic magmas at advanced stages of magma evolution. Experimental modeling of the immiscibility has been hampered by kinetic problems and attainment of chemical equilibrium between immiscible liquids in some experimental studies has been questioned. On the basis of symmetric regular solutions model and regression analysis of experimental data on compositions of immiscible liquid pairs, we show that liquid–liquid distribution of network-modifying elements K and Fe is linked to the distribution of network-forming oxides SiO2, Al2O3 and P2O5 by equation: log K_{{text{d}}}^{{text{K/Fe}}} = , 3.796Delta X_{{{text{SiO}}_{2} }}^{{{text{sf}}}} + , 4.85Delta X_{{{text{Al}}_{2} {text{O}}_{3} }}^{{{text{sf}}}} + , 7.235Delta X_{{{text{P}}_{2} {text{O}}_{5} }}^{{{text{sf}}}} - , 0.108,where K_{{text{d}}}^{{text{K/Fe}}} is a ratio of K and Fe mole fractions in the silica-rich (s) and Fe-rich (f) immiscible liquids: K_{d}^{{text{K/Fe}}} = , left( {X_{{text{K}}}^{s} /X_{{text{K}}}^{f} } right)/ , left( {X_{{{text{Fe}}}}^{s} /X_{{{text{Fe}}}}^{f} } right) and Delta X_{{text{i}}}^{sf} is a difference in mole fractions of a network-forming oxide i between the liquids (s) and (f): Delta X_{i}^{sf} = X_{i}^{s} - X_{i}^{f}. We use the equation for testing chemical equilibrium in experiments not included in the regression analysis and compositions of natural immiscible melts found as glasses in volcanic rocks. Departures from equilibrium that the test revealed in crystal-rich multiphase experimental products and in natural volcanic rocks imply kinetic competition between liquid–liquid and crystal–liquid element partitioning. Immiscible liquid droplets in volcanic rocks appear to evolve along a metastable trend due to rapid crystallization. Immiscible liquids may be closer to chemical equilibrium in large intrusions where cooling rates are lower and crystals may be spatially separated from liquids.