Timescales of convection in magma chambers below the Mid-Atlantic ridge from melt inclusions investigations

Abstract

Closed hopper and complex swallowtail morphologies of olivine microcrysts have been described in the past in both mid-oceanic ridge basalts and subaerial tholeitic volcanoes and indicate fluctuations in magma undercooling. We describe similar morphologies in a Mid-Atlantic ridge pillow basalt (sample RD87DR10), and in addition we estimate the duration of temperature fluctuations required to produce these textures as follows: (1) Pairs of melt inclusions are arranged symmetrically around the centre of hopper crystals and each pair represents a heating–cooling cycle. Using the literature olivine growth rates relevant to the observed morphologies, and measuring the distance between two successive inclusions, we estimate the minimum time elapsed during one convection cycle. (2) The major element composition of melt inclusions (analysed by electron microprobe) was found to be in the range of the boundary layer measured in the glass surrounding the olivines, irrespective of their size. Several major elements demonstrate that this boundary layer results from rapid quenching on the seafloor, and not from crystal growth at depth, implying the inclusions had the same composition as the surrounding magma when they were sealed. Using diffusivity of slow diffusing elements such as Al₂O₃, we estimate the minimum time required for inclusion formation. These two independent approaches give concordant results: each cooling–heating cycle lasted between a few minutes and 1 h minimum. Thus, these crystals probably recorded thermal convection in small magmatic bodies (a dyke or shallow magma chamber) during the last hour or hours before eruption.