An initial evaluation of olivine diffusion timescales from monogenetic-style volcanoes in the Oregon High Cascades

Abstract:

Until recently, monogenetic-style volcanoes were thought to represent small-volume, “simple, single-eruption” volcanoes whose chemical compositions provided a window into melt generation and extraction processes in the upper mantle. However, recent work has highlighted chemical diversity of magmas within, and between individual volcanoes in a volcanic field, considered remarkable considering their small volume, as well as between volcanic fields worldwide.

Diffusion chronometry of elemental exchange across zoned minerals, in conjunction with other petrological information, can be used to constrain and quantify the timescales of magmatic processes occurring pre-, syn- and post eruption, including magma ascent. This technique has been implemented within a variety of tectonic settings, more commonly at polygenetic volcanoes, to infer the dynamics of magma plumbing systems. Recent diffusion studies of crystals from monogenetic volcanoes, also show evidence for complex magmatic processes, including magma mixing, magma recharge, xenocryst incorporation, deep seated magma storage and the possible presence of a shallow transient reservoir - further evidence that these monogenetic volcanic systems may not be as simple as was first thought.

Despite this new work, monogenetic volcanism associated with subduction zone processes is under-represented in existing petrological and geochemical studies and thus under-represented in the global dataset. We present diffusion timescales for zoned olivine crystals from several monogenetic centres near the Santiam-McKenzie Pass in the Oregon High Cascades. Whilst it has been demonstrated that the small shield volcanoes in this region have more complex, shallow magmatic plumbing systems than those of the cinder cones, magma residence and transport timescales within these contrasting systems were until now, unknown.