The role of H2O during crystallization of primitive arc magmas under uppermost mantle conditions and genesis of igneous pyroxenites: an experimental study

Abstract

Exposed, subduction-related magmatic arcs commonly include sections of ultrama®c plutonic rocks that are composed of dunite, wehrlite, and pyroxenite. In this experimental study we examined the eects of variable H 2 O concentration on the phase proportions and compositions of igneous pyroxenites and related ultrama®c plutonic rocks. Igneous crystallization experiments simulated natural, arc magma compositions at 1.2 GPa, corresponding to conditions of the arc lower crust. Increasing H 2 O concentration in the liquid changes the crystallization sequence. Low H 2 O concentration in the liquid stabilizes plagioclase earlier than garnet and amphibole while derivative liquids remain quartz normative. Higher H 2 O contents (>3%) suppress plagioclase and lead to crystallization of amphibole and garnet thereby producing derivative corundum normative andesite liquids. The experiments show that alumina in the liquid correlates positively with Al in pyroxene, as long as no major aluminous phase crystallizes. Extrapolation of this correlation to natural pyroxenites in the Talkeetna and Kohistan arc sections indicates that clinopyroxenes with low Ca-Tschermaks component represent near-liquidus phases of primitive, Si-rich hydrous magmas.Density calculations on the residual solid assemblages indicate that ultrama®c plutonic rocks are always denser than upper mantle rocks in the order of 0.05 to 0.20 g/cm 3 . The combination of high pressure and high H 2 O concentration in the liquid suppresses plagioclase crystallization, so that ultrama®c plutonic rocks form over a signi®cant proportion of the crystallization interval (up to 50% crystallization of ultrama®c rocks from initial, mantle-derived liquids). This suggests that in subductionrelated magmatic arcs the seismic Moho might be shallower than the petrologic crust/mantle transition. It is therefore possible that calculations based on seismic data have overestimated the normative plagioclase content (e.g., SiO 2 , Al 2 O 3 ) of igneous crust in arcs.

TTG-type plutonic rocks formed in a modern arc batholith by hydrous fractionation in the lower arc crust

“…9). The fact that garnetites are only observed in the lowermost part of the arc indicates a strong pressure control on their formation in agreement with the results from early-phase Trace element concentrations and field occurrence (normalized to primitive mantle Mcdonough and Sun, 1995) of hornblendite (left, black) and garnetite (right, red) cumulates as preserved in the lowermost part of the arc (data after Dhuime et al 2007;Jagoutz et al 2011) Contrib Mineral Petrol (2013) 166:1099-11181113 diagram determinations (e.g., Lambert and Wyllie 1972) and from hydrous fractionation experiments (Alonso-Perez et al 2009;Müntener et al 2001). In these experiments garnet can be the sole or the dominant liquidus phase at high pressure (C1.2 GPa) in andesitic to basaltic andesite liquids.…”

“…In Fig. 11 we calculated viscosities employing the model of Giordano et al (2008) for melt compositions from appropriate hydrous and anhydrous fractionation experiments (Alonso-Perez et al 2009;Müntener et al 2001;Villiger et al 2004). Based on these calculations, the viscosity of the derivative melts starts to differ significantly at *52-55 wt% SiO 2 .…”

“…Consequently, the low viscosities of derivative liquids from the hydrous sequence facilitate the separation of these liquids from their source region. In contrast, gabbronorites of the less hydrous fractionation line retain more easily their residual tonalitic to granodioritic liquids and thus correspond commonly to Müntener et al 2001;Villiger et al 2004) against liquidus temperature. Stars denote viscosities calculated for a hydrous LLD modified after the model of Jagoutz (2010).…”

Pyroxenite xenoliths from Marsabit (Northern Kenya): evidence for different magmatic events in the lithospheric mantle and interaction between peridotite and pyroxenite

“…The opx/cpx ratio during high-P fractionation of opx-saturated liquids (websterite crystallisation) decreases with decreasing temperature and/or increasing H 2 O in the melt (Müntener et al 2001). In their experiments, Müntener et al (2001) found grt pyroxenite (with 4.9 wt% opx) to crystallise at 1,110°C and amph-bearing grt pyroxenite (8.8 wt% opx; 8.9 wt% amph) at 1,070°C (in both cases at 1.2 GPa with *5 wt% H 2 O in the melt). Similar phase proportions and thermal conditions (but higher P; see Fig.…”

Volatile and C-H-O isotopic compositions of giant Fe-Ti-V oxide deposits in the Panxi region and their implications for the sources of volatiles and the origin of Fe-Ti oxide ores

“…Experimental results indicate that at 200 MPa and H 2 O-saturated conditions, involvement of H 2 O could lower crystallization temperature of silicates but affect little on Fe-Ti oxides, resulting in early crystallization of magnetite [45]. However, the control of H 2 O on the relative timing of magnetite crystallization depends on how much H 2 O in magmas [22,46]. In general, solubility of H 2 O in basaltic melt is 3.1 wt% at 100 MPa, and increases with pressure [39].…”