Nucleation and growth of bubbles on plagioclase crystals during experimental decompression degassing of andesitic melts

摘要

The nucleation, growth and attachment/detachment of gas bubbles on crystals in silicate melts is one of the key drivers of volcanic eruptions and can greatly influence their explosivity. Formerly, oxides were considered the best candidates for heterogeneous bubble nucleation, but recent studies showed that silicate crystals can also be bubble nucleation sites at 1 atmosphere (atm). This study examines whether bubbles can nucleate on plagioclase crystal surfaces during decompression, which is a more common natural situation than 1 atm degassing. Experimental samples were synthesised from andesitic rock powder, H2O and plagioclase seed crystals, and melted and decompressed in a piston-cylinder apparatus. The products were imaged with 3D X-ray computed tomography. Due to partial melting, the plagioclase crystals developed sieve rims and rough crystal-melt interfaces. All plagioclase crystals in all samples were covered with bubbles. Bubble size distribution comparisons between bubbles in contact with plagioclase and bubbles in the melt, show that bubbles belong to two populations, generated by two different events, one of which is heterogeneous nucleation on plagioclase crystal surfaces and the other is homogeneous nucleation within the melt, the latter potentially accompanied by heterogeneous nucleation on oxide microlite surfaces. The calculated attachment force between the bubbles and the plagioclase surface is stronger than the detachment force; hence, such bubbles would remain attached during crystal movements. In our experiments, the net buoyancy of the bubble-crystal aggregates indicates that they could sink in an andesitic melt. Our findings highlight the need to reconsider the role of plagioclase crystals in magmatic degassing and shows interesting possibilities for magma mixing-triggered degassing. (C) 2019 Elsevier B.V. All rights reserved.