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Book Chapter

Volatiles in magmatic-hydrothermal systems and volcanic degassing

Published:
January 01, 2015

Abstract

Pleistocene basalts from Daisen and Mengameyama in the SW Japan volcanic arc of western Honshu are characterized by an abundance of olivine crystals with Fe-rich rims. At Daisen, these have previously been interpreted to have formed from their host melt by equilibrium crystal fractionation and by disequilibrium fractionation during supercooling. Here we use combined electron probe microanalysis, isotopography, transmission electron microscopy and selected area electron diffraction to show that crystal rims are significantly enriched in aluminium (up to c. 1 wt%) and hydrogen (up to c. 10 000 ppm) hosted in oriented low-density amorphous domains. These domains are interpreted to have formed by melting of deuteric and/or post-deuteric metasomatic alteration minerals upon uptake of older olivine crystals into fresh, initially aphyric host melts up to a few hours prior to eruption. It is argued that uptake of variably altered crystals into initially aphyric or sparsely phyric melts may be a common process at subduction zones, and can account for typical disequilibrium textures displayed by arc magmas erupted in SW Japan and elsewhere. Analyses of the altered crystal cargo in arc volcanic rocks therefore provides an important tool for understanding subvolcanic hydrothermal systems and the interaction of ascending melts with such systems.

Supplementary material:

Olivine mineral chemistry data from two typical Daisen basalts and one typical Mengameyama basalt, and a figure showing the locations of all focussed ion beam (FIB) sections studied here, are available at http://www.geolsoc.org.uk/SUP18760.

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Contents

Geological Society, London, Special Publications

The Role of Volatiles in the Genesis, Evolution and Eruption of Arc Magmas

G. F. Zellmer
G. F. Zellmer
Massey University, New Zealand
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M. Edmonds
M. Edmonds
University of Cambridge, UK
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S. M. Straub
S. M. Straub
Columbia University, USA
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Geological Society of London
Volume
410
ISBN electronic:
9781862396982
Publication date:
January 01, 2015

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