Primitive melt recharge, and magma-mush mixing in the weeks and months preceding the 2005-06 eruption, EPR, 9˚46’N-9˚56’N




Moore, Aerona

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At fast spreading ridges such as the East Pacific Rise (EPR) volcanic eruptions are predicted to occur on a decadal timescale. Due to the limited ability to observe submarine eruptions, little is known about the magmatic processes occurring in the underlying magma chamber leading up to a volcanic event, including differentiation and magma mixing. The recent 2005-06 eruption at 9˚46’N-9˚56’N along the EPR provides a unique opportunity to gain a better understanding of rates of magma transport and magma replenishment associated with a typical eruption. This study examines the geochemistry of phenocrysts from the 2005-06 eruption in order to determine if they are in equilibrium with their host melt, or if magma mixing occurred prior to eruption. A diffusion model is used to model those crystals which are out of equilibrium with their erupted host to determine timescales of magma mixing. The major and trace element contents of plagioclase and olivine phenocrysts provides evidence for melts both more evolved (> 3.5 wt % MgO) and more primitive (< 8.8 wt % MgO) than those found within the host lava (7.7-8.3 wt % MgO; Goss et al., 2010). Glomerocrysts and resorbed crystals in equilibrium with evolved melts (3.5-6.5 wt % MgO) suggests an origin in a roof mush zone, and were disrupted and entrained into their host melt within days of eruption. Modelling of the zoning profiles of phenocrysts suggest the 2005-06 eruption was likely triggered by an influx of hotter, more primitive melt (~ 9.0 wt % MgO) which was injected into the melt lens a few weeks to months prior to the eruption. With decreasing time before eruption, there is an overall increase in the number of crystals with modelled timescales representing mixing events in the magma chamber. This increase in modelled timescales appears to correlate with the increase in seismic activity recorded prior to the eruption (Tolstoy et al., 2006). This suggests magma mixing events within the underlying magma chamber may be linked to seismic activity at fast spreading ridges.



mid-ocean ridge, basalt, diffusion modelling, East Pacific Rise