REVEALING THE CONTRASTING HISTORIES OF MAFIC MAGMAS IN THE CENTRAL OREGON CASCADES USING DIFFUSION CHRONOMETRY AND MINERAL CHEMISTRY

Basaltic volcanoes are common in the Central Oregon Cascades and include small shields, flank eruptions and thousands of cinder cones. It has been suggested that small shields can have a shallow plumbing system where magmas are stored prior to eruption, which may enable complex magma interaction (e.g. magma mixing) to take place. In contrast, monogenetic, small-volume “simple, single-eruption” volcanoes have been suggested to have less complex magma interaction prior to eruption. This view of monogenetic-style centres has been challenged in recent years.

Detailed eruption timescales for the basaltic volcanoes in the Cascades are rare in the regional dataset. We present petrological data and diffusion modelling timescales from Belknap (a small shield volcano) and Four in One (a small, monogenetic-style fissure eruption) located west of Sisters, OR.

Both volcanic centres contain olivine and plagioclase crystals that are compositionally zoned. Belknap olivine are equant and normally zoned with core compositions of Fo_84-80 and rims of Fo₇₂. Four in One olivines are both normally zoned (core compositions of Fo_84-81, rim compositions of Fo_82-70) and reversely zoned (core compositions of Fo_{74 and 82} and rim compositions of Fo_{78 and 83}) with dendritic overgrowths. Plagioclase from Four in One have two core populations -An₄₅ and An₈₀. An₄₅ cores are surrounded by a fritted inner rim, followed by a thin An₆₅ outer rim. An₈₀ cores are only surrounded by the An₆₅ rim. Plagioclase from Belknap have core compositions of ~An₈₁ with rims ~An_70-65 and they do not display any fritted textures. Mg-Fe diffusion timescales are on the order of weeks to months for the Belknap olivine, and days to weeks for the Four in One olivine. Mg diffusion timescales in Four in One plagioclase cores are ~1 month. These plagioclase timescales and textures indicate short storage before magma mixing, with ascent from depth prior to eruption recorded by the olivines. Diffusion timescales for Belknap olivines indicate longer storage timescales prior to eruption.

These data will be important to shape responses and for mitigation during future geophysical unrest in areas where basaltic volcanism has been prevalent. The results also support the notion that complex magmatic interaction also occurs beneath both shield and monogenetic-style centres.