Generation of mantle-derived basaltic andesites in volcanic arcs.

• High-MgO basaltic andesites from Klyuchevskoy volcano are products of mantle melting. • Pressures of melting correspond to near-Moho depths (23–36 km). • Melting temperatures are 1220 to 1240 °C; melt fraction is 8 to 11 wt%. • Melting is driven by rifting-induced decompression of amphibole-bearing lherzolite. • Primary arc magma chemistry is controlled by wedge thermal structure and H 2 O activity. Primary magmas in volcanic arcs exhibit wide compositional diversity on both local and global scales. Processes responsible for this diversity are generally ascribed to some combination of mantle melting or crustal differentiation processes. One widespread view is that arc magmagenesis is driven by combination of H 2 O-fluxed and decompression melting of peridotitic mantle wedge, and that primary, mantle-derived melts are high-MgO basalt. However, a variety of other mantle-derived primitive arc magmas, ranging in composition from high-Mg andesite to picrite, has been recognised and it remains unclear to what extent this diversity can be generated by mantle melting processes modulated, for example, by changes in the thermal state of the mantle wedge or the supply of fluid from the slab. Here we use high pressure and temperature experiments to constrain magma generation conditions of a primitive magnesian (8.8 wt% MgO) basaltic andesite from Klyuchevskoy volcano, Kamchatka arc, Russia. We use an inverse experimental approach to define a multiple saturation point on the liquidus surface of the basaltic andesite. The experimental multiple saturation point defines the pressure and temperature at which an erupted melt could have last been in equilibrium with a polymineralic source rock, such as mantle peridotite, and hence provides a robust estimate of magma source conditions. Equilibrium piston-cylinder experiments were carried out between 0.5 and 1.0 GPa under hydrous conditions (3 to 6 wt% added H 2 O) at f O 2 = ΔNNO+1. We show that Klyuchevskoy basaltic andesite is multiply saturated with the lherzolite assemblage olivine (Fo 90) + clinopyroxene + orthopyroxene + Cr-spinel close to its liquidus (≥ 95% melt) in the pressure range of 0.6 to 1 GPa (23 to 36 km depth) and 1220–1240 °C. Amphibole is present at temperatures just below the multiple saturation point (≤ 1200 °C). Our results show that basaltic andesite was produced by 8 to 11 wt% partial melting of amphibole-lherzolite source and therefore represents a primary, undifferentiated magma extracted from its source at near-Moho depths. These findings are in a good agreement with geophysical studies of Klyuchevskoy volcano that show magmas are supplied directly from a reservoir at near-Moho depths (25–30 km) through a sub-vertical, pipe-like feeder system. Coeval high-MgO basalts from the volcano may correspond to higher degree mantle melts extracted at slightly greater depths. Our results provide a tight constraint on the thermal structure of the mantle wedge beneath Klyuchevskoy. Experimental temperatures are higher than those calculated from steady-state thermal models suggesting that upwelling asthenosphere might directly impinge the Moho in a similar fashion to mid-ocean ridges. Intra-arc rifting promotes asthenospheric decompression beneath the Central Kamchatka Depression. The presence of amphibole in our experiments at temperatures up to 1200 °C indicates that dehydration melting of amphibole peridotite formed by metasomatism of mantle wedge by slab-derived fluids is the primary magma generating process. Amphibole stability exercises an important control on melting conditions. Integrating our results with published multiple-saturation experiments we show that Klyuchevskoy basaltic andesite is one of a family of primary arc magmas whose compositions depend on mantle wedge thermal structure and H 2 O activity. [ABSTRACT FROM AUTHOR]