Arc crust formation and differentiation constrained by experimental petrology

Abstract

We review igneous experimental data designed to understand fractionation processes in the roots of magmatic arcs, focusing on fractional crystallization experiments in dry and hydrous basalts to andesites to simulate the liquid and cumulate lines of descent of primary mantle derived magmas. An evaluation of the major element composition indicates that the cumulate line of descent (CLD) of hydrous systems is fundamentally different from dry systems. Cumulates derived from hydrous experiments display elevated Al₂O₃ and CaO contents at low SiO₂, producing voluminous andesitic to rhyolitic liquids that closely overlap with compositions of natural systems. The mineralogical and chemical composition of cumulates converges for very different hydrous primary mantle magmas, indicating that fundamental phase equilibria under the conditions prevailing in the roots of magmatic arcs exert a strong control on the compositions of derivative andesitic to rhyolitic liquids. Melting experiments on amphibolite or basaltic to andesitic compositions equally produce granitic – rhyolitic liquids of similar composition yet their restites do not present the same variability of the CLD of hydrous fractional crystallization experiments.

The comparison of our experimentally produced cumulates with natural rock suites from the Jurassic Talkeetna arc (South Central Alaska) and the Cretaceous Kohistan arc (Pakistan) show that fractionation experiments closely reproduce the natural rock compositions and in particular the cumulate sequences exposed in the roots of island arcs. Both dry and hydrous fractionation series are preserved yet only the hydrous fractionation series produces voluminous ultramafic cumulates (50–70%) that are negatively buoyant with respect to mantle peridotite. Delamination of dense ultramafic roots can explain the andesitic composition of continental crust. Our fractionation experiments show that generation of granites is a natural consequence of fractional crystallization of primary basaltic magmas. Discrepancies of liquid compositions produced in the lower crust and the arc volcanic record can readily be explained by further differentiation, partial assimilation and mingling and mixing processes during ascent and decompression in transcrustal magmatic systems. We therefore favor the interpretation that Earth-like continental crust is primarily produced by fractional crystallization in the lower crust and upper mantle, and that magma mixing, assimilation, metamorphism and crustal melting are important secondary processes that refine and amplify the difference and stratification between mafic lower crust and a granitic upper crust.