Generation of mantle-derived basaltic andesites in volcanic arcs

• Published in: Earth and planetary science letters 2024-09, Vol.641, p.118791, Article 118791
• Main Authors: Melekhova, Elena, Blundy, Jon
• Format: Article
• Language: English
• Subjects: Basaltic andesite; Experimental petrology; Kamchatka; Mantle melting; Multiple saturation point; Primary arc magmas
• ISSN: 0012-821X; 1385-013X
• DOI: 10.1016/j.epsl.2024.118791

•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 H2O 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 H2O-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 H2O) at fO2 = ΔNNO+1. We show that Klyuchevskoy basaltic andesite is multiply saturated with the lherzolite assemblage olivine (Fo90) + 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-Mo