Lower crustal heterogeneity and fractional crystallisation control evolution of small volume magma batches at ocean island volcanoes (Ascension Island, South Atlantic)

Abstract

Ocean island volcanoes erupt a wide range of magmatic compositions via a diverse range of eruptive styles. Understanding where and how these melts evolve is thus an essential component in the anticipation of future volcanic activity. Here we examine the role of crustal structure and magmatic flux in controlling the location, evolution and ultimately composition of melts at Ascension Island. Ascension Island, in the south Atlantic, is an ocean island volcano which has produced a continuum of eruptive compositions from basalt to rhyolite in its 1-million-year subaerial eruptive history. Volcanic rocks broadly follow a silica undersaturated subalkaline evolutionary trend and new data presented here show a continuous compositional trend from basalt through trachyte to rhyolite. Detailed petrographic observations are combined with in-situ geochemical analyses of crystals and glass, and new whole rock major and trace element data from mafic and felsic pyroclastic and effusive deposits that span the entire range in eruptive ages and compositions found on Ascension Island. These data show that extensive fractional crystallisation is the main driver for the production of felsic melt for Ascension Island; a volcano built on thin, young, oceanic crust. Strong spatial variations in the compositions of erupted magmas reveals the role of a heterogeneous lower crust: differing degrees of interaction with a zone of plutonic rocks are responsible for the range in mafic lava composition, and for the formation of the central and eastern felsic complexes. A central core of nested small-scale plutonic, or mush-like, bodies inhibits the ascent of mafic magmas, allowing sequential fractional crystallisation within the lower crust, and generating felsic magmas in the core of the island. There is no evidence for magma mixing preserved in any of the studied eruptions, suggesting that magma storage regions are transient, and material is not recycled between eruptions.