Abstract :
[en] Knowing the extent of mantle wedge melting below volcanic arcs is critical to improve magma genesis models. During the last decades, experimental petrology provided significant data and ready to be used models to retrieve the melting conditions producing primary magmas (Mg# >0.7, Ni > 150ppm, Cr > 1000 ppm after Baker et al. (1994) and Grove et al. (2012)). The Central Southern Volcanic zone (CSVZ, southern Chile) of the Andean arc lies on a thin continental crust (50-30 km : Tassara and Echaurren, 2012) and is crosscut by a major transcrustal fault (LOFZ : Cembrano and Lara, 2009) which speed up magma ascent. As a consequence, (near-) primary magmas have been sampled in the area avoiding the blurring of its original characteristics by deep (MASH) (Hildreth and Moorbath, 1988) to shallower (differentiation) magmatic processes. Osorno is one of the CSVZ volcano that possess the most primitive recorded rocks (Mg# =0.72, MgO: 10.23-10.53 wt%, Cr: 584-745 ppm, Ni: 171-179 ppm, Fo# of olivines up to Fo89) in the area. Using lherzolite melting experiments (Hirose and Kushiro, 1993) as well as numerical (Lee et al., 2009), empirical (Wood, 2004) and chemical models (Kelley et al., 2006) together with modal batch melting equations (Hickey-Vargas et al., 2016a, 2016b), we retrieved temperature, pressure, mantle water content and mantle melting rate (F) below Osorno. Temperatures range from 1303 to 1327 °C, pressures from 10.5 to 13.6 kbar (around MOHO depth, ca. 35-44 km), mantle water content from 0.08 to 0.33 wt% and F from 0.12 to 0.22. The uncertainty on F values reflects the difficulty to precisely estimate this parameter. However, this range allows better constraining geophysical models. Values estimated using a global arc numerical model (Turner et al., 2016; F ≈ 0.11 at ca. 60 km and T≈1100°C, mantle H2O =0.6 wt%) or using Arc basalt simulator, isotopes and trace elements (Jacques et al., 2014; F = 1.6-5.5, P=19 kbar, T = 1240°C) vary significantly from our data emphasizing the remaining gap of knowledge of the mantle melting conditions. Our results nonetheless agree with those calculated at La Picada (Vander Auwera et al., 2019) or southwards (Watt et al., 2013; Weller and Stern, 2018) in the southern SVZ.
Commentary :
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