Fractional crystallization of primitive, hydrous arc magmas: an experimental study at 0.7 GPa

Differentiation of mantle-derived, hydrous, basaltic magmas is a fundamental process to produce evolved intermediate to SiO2-rich magmas that form the bulk of the middle to shallow continental and island arc crust. This study reports the results of fractional crystallization experiments conducted in a piston cylinder apparatus at 0.7 GPa for hydrous, calc-alkaline to arc tholeiitic magmas. Fractional crystallization was approached by synthesis of starting materials representing the liquid composition of the previous, higher temperature experiment. Temperatures ranged from near-liquidus at 1,170 degrees C to near-solidus conditions at 700 degrees C. H2O contents varied from 3.0 to more than 10 wt%. The liquid line of descent covers the entire compositional range from olivine-tholeiite (1,170 degrees C) to high-silica rhyolite (700 degrees C) and evolves from metaluminous to peraluminous compositions. The following crystallization sequence has been established: olivine -> clinopyroxene -> plagioclase, spinel -> orthopyroxene, amphibole, titanomagnetite -> apatite -> quartz, biotite. Anorthite-rich plagioclase and spinel are responsible for a marked increase in SiO2-content (from 51 to 53 wt%) at 1,040 degrees C. At lower temperatures, fractionation of amphibole, plagioclase and Fe-Ti oxide over a temperature interval of 280 degrees C drives the SiO2 content continuously from 53 to 78 wt%. Largest crystallization steps were recorded around 1,040 degrees C and at 700 degrees C. About 40 % of ultramafic plutonic rocks have to crystallize to generate basaltic-andesitic liquids, and an additional 40 % of amphibole-gabbroic cumulate to produce granitic melts. Andesitic liquids with a liquidus temperature of 1,010 degrees C only crystallize 50 % over an 280 degrees C wide range to 730 degrees C implying that such liquids form mobile crystal mushes (<50 % crystals) in long-lived magmatic systems in the middle crust, allowing for extensive fractionation, assimilation and hybridization with periodic replenishment of more mafic magmas from deeper magma reservoirs.