Volatile element (B, Cl, F) behaviour in the roof of an axial magma chamber from the East Pacific Rise

Abstract

Understanding the behaviour of volatile elements at mid-ocean ridges is important for reasons ranging from their influence on mantle viscosity through to their role as a food source for the deep biosphere. With the aim of constraining what processes control the distribution of volatiles in the ocean crust at fast-spreading ridges, we present a detailed study of the compositional variability in magmatic amphibole formed in the upper part of the plutonic sequence at the East Pacific Rise (EPR). These amphiboles are massively enriched in chlorine (by more than an order of magnitude), and moderately enriched in boron, with respect to magmatic amphiboles in cumulates from the Mid-Atlantic Ridge (MAR). Similar enrichments have been reported for basaltic glasses from the EPR and are interpreted as indicative of assimilation. The greater enrichments observed in the plutonic section suggest both that assimilation occurs at the roof of the axial magma chamber (AMC) and that lava compositions may record minimum amounts of exogenic contamination. Amphiboles with compositions indicative of crystallisation from a contaminated magma occur to depths of 800 m beneath the sheeted dyke complex. This is interpreted to indicate that at least this upper portion of the plutonic section forms via crystallisation within the AMC followed by subsidence of a crystal mush. Amphibole boron isotope compositions show that assimilation of altered sheeted dykes plus hydrothermal fluids drives AMC magmas to heavier δ11B values (up to +5.8‰). Subsequent degassing within a solidifying crystal mush leads to a negative trend in δ11B–B with the most degassed magma having δ11B as low as −21.2‰. This degassing was associated with hydrofracturing of the partially molten crystal mush and could have facilitated a temporal link with the overlying hydrothermal system.