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

Abstract

Differentiation of mantle-derived, hydrous, basaltic magmas is a fundamental process to produce evolved intermediate to SiO 2-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°C to near-solidus conditions at 700°C. H 2 O 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°C) to high-silica rhyolite (700°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 SiO 2-content (from 51 to 53 wt%) at 1,040°C. At lower temperatures, fractionation of amphibole, plagioclase and Fe-Ti oxide over a temperature interval of 280°C drives the SiO 2 content continuously from 53 to 78 wt%. Largest crystallization steps were recorded around 1,040°C and at 700°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°C only crystallize 50 % over an 280°C wide range to 730°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. Keywords Liquid line of descent Á Fractional crystallization Á Calc-alkaline magmas Á Mid-crustal magma reservoirs Communicated by T. L. Grove.

High-Mg adakitic rocks and their complementary cumulates formed by crystal fractionation of hydrous mafic magmas in a continental crustal magma chamber

“…Increasing producing pyroxenite-dominated cumulate assemblages. As mentioned before, the differentiation of the Ningcheng melts followed a path of websterites-(Ol −/Hbl-)pyroxenites-gabbros in terms of mineral assemblage, comparable with the solid assemblages left by fractional crystallization of hydrous magmas (Blatter et al, 2013;Müntener et al, 2001;Nandedkar et al, 2014) and cumulates from hydrous arc magmas (Jagoutz et al, 2011) at lower-crustal levels. Plagioclase is absent from the cumulus assemblage of the high-Mg websterites (early fractionation stage), low-Mg websterites and Hbl-pyroxenites (intermediate fractionation stage), and is present in higher proportions during the late differentiation of the magma.…”

The temporal evolution of chemical and physical properties of magmatic systems

“…A key feature is that melts, crystals and gases can move independently of each other through the system. phase relations of Nandedkar et al (2014). The description of the model, for the analogous case of upper crustal intrusion, is reported in Caricchi et al (2014); in the present simulations the initial temperature of the rock surrounding the intrusion was increased to 9008C to better match the lower crust.…”

“…The effect of phase equilibria on the distribution of residual melt composition can be illustrated by considering the evolution of residual melt fraction (F) and silica content of an H 2 O-bearing basalt undergoing fractional crystallization (Nandedkar et al 2014;Fig. 3).…”

“…The lower SiO 2 end of this compositional gap corresponds to the silica content of the residual melt at the onset of hornblende crystallization ( Fig. 3; Nandedkar et al 2014). These simulations demonstrate how magma fluxes can play an important role in controlling the chemical composition of residual melt present within magmatic systems.…”

“…Histograms showing the distribution in time of the relative volume of residual melt present in magmatic reservoirs assembled at the base of the crust (30 km) by the continuous injection of basalt from the mantle. The calculations were made using the approach to thermal modelling presented in Caricchi et al (2014) combined with the basaltic phase equilibria of Nandedkar et al (2014). The histograms are shaded for time from the beginning of magma intrusion and show the relative volumes of residual melt of different compositions present within the magmatic reservoir.…”