Melting the hydrous, subarc mantle: the origin of primitive andesites
This experimental study is the first comprehensive investigation of the melting behavior of an olivine orthopyroxene spinel bearing fertile mantle (FM) composition as a function of variable pressure and water content. The fertile composition was enriched with a metasomatic slab component of =0.5 alkalis and investigated from 1135 to 1470 degC at 1.0 2.0 GPa. A depleted lherzolite with 0.4 alkali addition was also studied from 1225 to 1240 degC at 1.2 GPa. Melts of both compositions were water undersaturated: fertile lherzolite melts contained 0 6.4 wt% H2O, and depleted lherzolite melts contained ~2.5 wt% H2O. H2O contents of experimental glasses are measured using electron microprobe, secondary ion mass spectrometry, and synchrotron source reflection Fourier transform infrared spectroscopy, a novel technique for analyzing H2O in petrologic experiments. Using this new dataset in conjunction with results from previous hydrous experimental studies, a thermobarometer and a hygrometer thermometer are presented to determine the conditions under which primitive lavas were last in equilibration with the mantle. These predictive models are functions of H2O content and pressure, respectively. A predictive melting model is also presented that calculates melt compositions in equilibrium with an olivine orthopyroxene spinel residual assemblage (harzburgite) This model quantitatively predicts the following influences of H2O on mantle lherzolite melting: (1) As melting pressure increases, melt compositions become more olivine normative, (2) as melting extent increases, melt compositions become depleted in the normative plagioclase component, and (3) as melt H2O content increases, melts become more quartz normative. Natural high Mg# [molar Mg/(Mg Fe2+ high MgO basaltic andesite and andesite lavas or primitive andesites (PAs) contain high SiO2 contents at mantle equilibrated Mg#s. Their compositional characteristics cannot be readily explained by melting of mantle lherzolite under anhydrous conditions. This study shows that experimental melts of a FM peridotite plus the addition of alkalis reproduce the compositions of natural PAs in SiO2, Al2O3, TiO2, Cr2O3, MgO, and Na2O at 1.0 1.2 GPa and H2O contents of 0 7 wt% Our results also suggest that PAs form under a maximum range of extents of melting from F 0.2 0.3. The CaO contents of the melts produced are 1 5 wt% higher than the natural samples. This is not a result of a depleted source composition or of extremely high extents of melt but is potentially caused by a very low CaO content contribution from deeper in the mantle wedge.
Contributions to Mineralogy and Petrology