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Syneruptive magma ascent revealed by crystallization processes
Recent progresses with decompression experiments
Yuki SUZUKI῍This paper reviews the principles and the methods used to investigate the ascent process of water-saturated
magmas based on crystal texture and composition, which are especially relevant to recent progress made in
decompression experiments. The primary cause of syneruptive crystallization is an increase in the liquidustemperature and resultant undercooling due to a decrease in dissolved H,O in the melt. Analyses of ejecta crystal
texture provide time-resolvable information.
Firstly, recent decompression experiments have improved knowledge regarding crystallization kinetics andhave conﬁrmed correlations made between ascent conditions, texture and crystal composition. The number
density of groundmass microlite, crystal size distribution, and crystal form all reﬂect e#ective undercooling (Te#). This increases with an increased pressure drop (P), and also with an increased decompression rate (P/t). If P or P/t match in an experiment series, we can compare the texture of the run products withrespect to the other parameters. The experiments conﬁrmed that the rates of crystal growth and nucleation showclassical bell-shaped curves with Te# . Growth rate reaches a peak at a lesser Te# than for nucleation.
Variation of Te# causes a shift in crystallization style.
Secondly, decompression experiments help in estimation of the rate and style of magma ascent during a
speciﬁc eruption through replication of crystals found in ejecta. To best reproduce natural ascent conditions one
needs to know a) the number of syneruptive ascent stages, each of which can be assigned an approximate constantspeed, b) temperature and pressure of magma at the start and end of each ascent stage, c) how texture evolved
during the syneruptive ascent. Ascent conditions are estimated for each stage based on the experimental
reproduction of the texture and composition of the natural ejecta. Accuracy in the estimation of ascent rate isthus inﬂuenced by errors in quantitative analysis of natural ejecta and the correlation of the decompression ratewith textural parameters. Such experimental approaches improve our interpretation of eruption mechanisms byenabling us to combine observation made upon the eruptive product and time-resolved geophysical data. The
application of this approach to the ,*** A.D. eruption of Usu Volcano in Japan is presented. With further
improvement in the experimental techniques both laboratory and observational studies on ejecta will play a more
important role in linking various volcanology research ﬁelds.
Key words : rate and style of magma ascent, e#ective undercooling, nucleation and growth rates, quantiﬁcation ofgroundmass microlite, experimental replication of crystals+ῌ ῐ ῎ ῑ ῏ !" #$%&V32*ῌ2/12 WXYZ[\]^Z[_`aNaNbNPQMcNdecf5LMNM
Department of Mineralogy, Petrology and EconomicGeology, Graduate School of Science, Tohoku University, Aoba-ku, Sendai, 32*ῌ2/12, Japan !" 3456789:;<'e-mail : suzukiyk@ganko.tohoku.ac.jp

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