In situ observations of bubble growth in basaltic

Authors: M Masotta H Ni H Keppler

Publish Date: 2014/02/15

Volume: 167, Issue: 2, Pages: 976-

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Abstract

Bubble growth strongly affects the physical properties of degassing magmas and their eruption dynamics Natural samples and products from quench experiments provide only a snapshot of the final state of volatile exsolution leaving the processes occurring during its early stages unconstrained In order to fill this gap we present in situ hightemperature observations of bubble growth in magmas of different compositions basalt andesite and rhyodacite at 1100 to 1240 °C and 01 MPa 1 bar obtained using a moissanite cell apparatus The data show that nucleation occurs at very small degrees of supersaturaturation 60 MPa in basalt and andesite 200 MPa in rhyodacite probably due to heterogeneous nucleation of bubbles occurring simultaneously with the nucleation of crystals During the early stages of exsolution melt degassing is the driving mechanism of bubble growth with coalescence becoming increasingly important as exsolution progresses Ostwald ripening occurs only at the end of the process and only in basaltic melt The average bubble growth rate G R ranges from 34 × 10−6 to 52 × 10−7 mm/s with basalt and andesite showing faster growth rates than rhyodacite The bubble number density N B at nucleation ranges from 79 × 104 mm−3 to 18 × 105 mm−3 and decreases exponentially over time While the rhyodacite melt maintained a wellsorted bubble size distribution BSD through time the BSDs of basalt and andesite are much more inhomogeneous Our experimental observations demonstrate that bubble growth cannot be ascribed to a single mechanism but is rather a combination of many processes which depend on the physical properties of the melt Depending on coalescence rate annealing of bubbles following a single nucleation event can produce complex bubble size distributions In natural samples such BSDs may be misinterpreted as resulting from several separate nucleation events Incipient crystallization upon cooling of a magma may allow bubble nucleation already at very small degrees of supersaturation and could therefore be an important trigger for volatile release and explosive eruptionsWe are grateful to Hubert Schulze and Raphael Njul for sample preparation Sven Linhardt for technical assistance Detlef Krauße Ulrike Trenz and Florian Heidelbach for assistance during EMPA and SEM analyses Insightful comments from the Editor and two anonymous reviewers greatly improved the manuscript This work was supported by Humboldt fellowship to MM

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