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

Borisov, A.; Veksler, I.V.

Авторы: Borisov Alexander, Veksler Ilya V.
Журнал: Contributions to Mineralogy and Petrology
Том: 176
Номер: 6
Год издания: 2021
Издательство: Springer Verlag
Местоположение издательства: Germany
Номер статьи: 47
DOI: 10.1007/s00410-021-01798-1
Аннотация: 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 modelling 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:logKdK/Fe = 3.796ΔXSiO2sf + 4.85ΔXAl2O3sf + 7.235ΔXP2O5sf - 0.108where KdK/Fe is a ratio of K and Fe mole fractions in the silica-rich (s) and Fe-rich (f) immiscible liquids: KdK/Fe = (XKs/XKf)/ (XFes/XFef) and ΔXisf is a difference in mole fractions of a network-forming oxide i between the liquids (s) and (f): ΔXisf = Xis - Xif. 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.
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