Mixing and crystal scavenging in the Main Ethiopian Rift revealed by trace element systematics in feldspars and glasses
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Abstract For many magmatic systems, crystal compositions preserve a complex and protracted history which may be largely decoupled from their carrier melts. The crystal cargo may hold clues to the physical distribution of melt and crystals in a magma reservoir and how magmas are assembled prior to eruptions. Here we present a geochemical study of a suite of samples from three peralkaline volcanoes in the Main Ethiopian Rift. Whilst whole-rock data shows strong fractional crystallisation signatures, the trace element systematics of feldspars, and their relationship to their host glasses, reveals complexity. Alkali feldspars, particularly those erupted during caldera-forming episodes, have variable Ba concentrations, extending to high values that are not in equilibrium with the carrier liquids. Some of the feldspars are antecrysts, which we suggest are scavenged from a crystal-rich mush. The antecrysts crystallised from a Ba-enriched (more primitive) melt, before later entrainment into a Ba-depleted residual liquid. Crystal-melt segregation can occur on fast timescales in these magma reservoirs, owing to the low viscosity nature of peralkaline liquids. The separation of enough residual melt to feed a crystal-poor post-caldera rhyolitic eruption may take as little as months to tens of years (much shorter than typical repose periods of 300-400 years). Our observations are consistent with these magmatic systems spending significant portions of their life cycle dominated by crystalline mushes containing ephemeral, small (<1 km3) segregations of melt. This interpretation helps to reconcile observations of high crustal electrical resistivity beneath Aluto, despite seismicity and ground deformation consistent with a magma body.
Iddon , F , Jackson , C , Hutchison , W , Fontijn , K , Pyle , D M , Mather , T A , Yirgu , G & Edmonds , M 2019 , ' Mixing and crystal scavenging in the Main Ethiopian Rift revealed by trace element systematics in feldspars and glasses ' , Geochemistry, Geophysics, Geosystems , vol. Early View . https://doi.org/10.1029/2018GC007836
Geochemistry, Geophysics, Geosystems
Copyright © 2018. American Geophysical Union. All Rights Reserved. This work is made available online in accordance with the publisher’s policies. This is the final published version of the work, which was originally published at: https://doi.org/10.1029/2018GC007836
DescriptionThis project is funded by the Natural Environment Research Council grant NE/L013932/1 (RiftVolc).
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