Rapid magmatic processes drive persistently active volcanism

Abstract

Rapid transitions between long periods of relatively benign low magnitude activity and short episodes of higher magnitude eruption, pose significant risk to local populations. Here we investigate the relationship between persistent, low magnitude activity and upsurges to more sever eruptions at four persistently to intermittently active volcanoes in the Vanuatu Arc (Yasur, Ambrym, Ambae and Lopevi) using uranium-series isotopes. This approach offers the unique ability to examine both magmatic processes and the timescales over which they operate. Significant 226Ra excesses at Ambrym, Ambae and Lopevi, as high as 6.56, indicate fast transit of magma through the crust, suggesting that persistent activity is driven by continuous magma supply. At Yasur, equilibrium between 210Pb and 226Ra in erupted magmas reveals rapid magma replenishment and degassing. Rapid ascent is facilitated by crustal weakness that favours the formation of open conduits, either as a result of regional tectonic regimes or localised caldera formation. At Yasur (230Th/232Th) in lavas and tephras varies between 1.449 and 1.482 and (226Ra/230Th) ranges between 1.165 and 2.069, while at Ambrym (226Ra/230Th) extends from 2.796 to 6.557. This observed variation in ThRa isotopes cannot be explained by simple models of magma replenishment and mixing, and instead requires cryptic assimilation with older, stored crystal mushes. Evidence for this cryptic assimilation differs between the examined volcanoes, suggesting differences in the location of mush zones. At Ambrym, Ambae and Lopevi the tectonic regime favours rifting, and mushes accumulate in the volcano's flanks, while at Yasur a ‘typical’ subduction regime leads to development of a large crystal mush body directly beneath the volcano. These differences in mush storage have significant impact on the frequency and magnitude of upsurges in eruption intensity at these volcanoes, with mush bodies stored beneath the volcano remobilised to produce lower frequency/higher magnitude eruptions.