| Popis: |
Yasur and Ambrym volcanoes, which are located within the Vanuatu Arc, are amongst the most regularly active in the world, with both experiencing ongoing eruption throughout their historical record. They offer a natural laboratory for understanding magmatic processes that sustain persistent volcanic activity. Furthermore, both volcanoes have experienced higher magnitide eruptions within their history, albeit with different frequency-magnitude relationships.Over a dozen eruptions of VEI 2-3 have been experienced at Ambrym throughout the past century, while at Yasur eruptions of VEI 5 or more have ocurred at intervals of ~10-25 kyr. These allow the relationship between low-magnitude, persistent activity and higher magnitude,explosive and effusive eruptions to be explored. Both of these motivating themes are primarily investigated by petrological and geochemical means, including mineral, tephra and whole rock major element analysis and whole rock trace element and Sr, Nd, Hf and U-series isotope analysis. This geochemical data is reconciled with historical and geological records of eruptive behvaiour to link changes in magmatic processes with variations in eruptive activity. At Yasur, persistent volcanic activity, which takes the form of regular Strombolian eruptions, has been maintained for at least 600 years. Throughout this period basaltic trachyandesitic magmas (SiO2 ~ 56 wt.%) have been erupted at a constant output rate of 410-480m3 day-1. The invariance of magma composition and output rate through this time suggest that processes of magma replenishment, fractionation and tapping are in equilibrium. Magma replenishment events are estimated to occur at intervals of years, on the basis of thermal and gas output models. Elevated gas fluxes show that an open conduit connects the shallow plumbing system with the surface. Gas fluxes at both volcanoes cannot be reconciled with magma output rates, suggesting that large volumes of magma remain un-erupted. These are likley stored within the magmatic system as a resident magma or a crystal mush. Uranium-series isotopes reveal otherwise cryptic interaction between this component and ascending magmas during persistent activity. Less frequent, high magnitude eruptions show greater evidence for interaction between an ascending magma and a resident magma/mush component. Volcanic rocks from these eruptions display diverse petrology, including mineral disequilbrium textures, such as resorbed and skeletal phenocrysts. At Ambrym, effusive eruptions from the central vents show evidence for mixing between replenishment and residual magmas, while at Yasur, such is less frequent,but involves wholesale rejuvenation of a stored crystal mush leading to catastrophic, ignimbrite producing, caldera-forming eruptions. Fissure-fed lavas erupted from Ambrym’s flanks show no evidence for interaction with stored magmas, suggesting they have by-passed the shallow plumbing system, avoiding this component. As such, stored magmas resulting from persistent activity likely reside at depths of ~1-2 km. MELTS models suggest a similar scenario for Yasur, Sr, Nd and Hf isotopes show no evidence for assimilation with other crustal components at either volcano. Combined, these observations demonstrate the role of persistent activity within the magmatic system as a whole. They raise the possibility that persistent volcanic activity is responsible for the generation of crystal mushes beneath these volcanoes, and that such a component plays a significant role in higher magnitude eruptions. |
