Syneruptive magma ascent revealed by crystallization processes Recent progresses with decompression experiments

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1 /+ (,**0) ,**0,,2,**0 3,+ 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 liquidus temperature 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 and have 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 ( T e#). 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 with respect to the other parameters. The experiments confirmed that the rates of crystal growth and nucleation show classical bell-shaped curves with T e#. Growth rate reaches a peak at a lesser T e# than for nucleation. Variation of T e# 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 constant speed, 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 is thus influenced by errors in quantitative analysis of natural ejecta and the correlation of the decompression rate with textural parameters. Such experimental approaches improve our interpretation of eruption mechanisms by enabling 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 of groundmass microlite, experimental replication of crystals 32* 2/12 Department of Mineralogy, Petrology and Economic Geology, Graduate School of Science, Tohoku University, Aoba-ku, Sendai, 32* 2/12, Japan suzukiyk@ganko.tohoku.ac.jp + Cashman,,**. ; Cashman et al.,,*** ; Jaupart, +332 ; Rutherford and Gardner,,***, +33*

2 374 Fig. +. Procedures investigating magma ascent condition using crystallization. Italic letter indicates result in each procedure. Procedure and result at the starting point of arrow control those at the end point. (Fig. +a),*** Cashman,,**. -., Rutherford and Hill, +33- (Jaupart, +332 ; Rutherford and Gardner,,***),, + Mastin and Ghiorso,*** Conflow (Fig.,a) Fig. -

3 375 Fig.,. Phase diagrams for isothermally decompressed felsic melt. a) A diagram showing increase of liquidus temperature with decrease of pressure for water saturated conditions. Modified from Eichelberger (+33/).,**,,*, + MPa correspond to reservoir, conduit and near-surface, respectively. L, C and V represent liquid, crystal and vapor, respectively. Boundaries of L vs. L C and L V vs. L C V denote liquidus. When reservoir magma is water saturated and free from vapor, its melt part locates at Melt. As magma ascends to surface and the melt is transformed to groundmass ("GMS ) of ejecta, the liquidus increases from T,** to T +. b) H,O-saturated phase relations. Modified from Cashman (,**.). Each phase is stable over lowpressure side of phase boundary, except hornblende. Orhopyroxene (Opx), augite (Aug), plagioclase (Pl), hornblende (Hb) and quartz(qtz). ; Fig. - Fig.,b Rutherford and Hill, +33- Fig. -,,, -,. (Cashman,,**.), -,.,, Blundy and Cashman (,**+) (Qz-Ab-Or) Fig. -. BSE image of groundmass (micropumice in Usu,*** eruption). Microlites are magnetite (Mt), pigeonite (Pig) and plagioclase in order of decreasing brightness. Skeletal plagioclase microlite appears to show a swallow tail form (S.T.) or to have a hollow filled with glass (H. L.) depending on the orientation of the section of each crystal. An,* (Qz, Ab, Or, An) Qz, Ab, Or

4 376 Fig... Crystallization path for water-saturated isothermal decompression (2** ) plotted in Qz- Ab-Or haplogranitic system. Crystallizing phases are feldspar (fs) and quartz (qz). Cotectic lines, composition of H,O-saturated minima (filled circle) and eutectics are also shown as a function of pressure. Redrafted from Figs + and,b of Blundy and Cashman (,**+). + (Ebadi and Johannes, +33+ ; Luth et al., +30. ; Tuttle and Bowen, +3/2 ; Fig..) Blundy and Cashman (,**+),** MPa (2** ) Tuttle and Bowen (+3/2) Holtz et al. (+33,a) (Fig..) Qz /* MPa Qz /* MPa Qz Qz-Or -* MPa 2** Fig.. a Qz b Fig..,* MPa (Tuttle and Bowen, +3/2) (,**/),**. Qz (Qz /*) MPa, - + Swanson et al. (+323) (Cashman, +33, ; Gardner et al., +332 ; Hammer et al., +333,,*** ; Nakada and Motomura, +333) - Dowty, +32* ; Kirkpatrick, +32+ ; Swanson, +311 : -, Swanson, +311 ; Fig. / (Lasaga, +32+ ; Lofgren, +32*) Cashman and Blundy,,*** Hammer et al. (,***) (Fig. 0a) Hammer et al. (+333) +33+

5 377 Fig. /. Relationship between e#ective undercooling ( T e#) and crystallization rates (nucleation and growth). Modified from Couch et al. (,**-). Arrows indicate increases in T e# and rate. Nucleation requires T e# exceeds threshold. With increase in T e#, rates increase over small T e# and decrease over large T e#. This results from interplay between thermodynamic driving force and kinetic barrier. (Fig. 0b) 3* Geschwind and Rutherford (+33/) Cashman (+33,) Gardner et al. (+332) Hammer et al. (+333,***) Nakada and Motomura (+333),., (,. +) (,.,),. + (,**+),*** (Fig. 1) Noguchi et al. (,**0) 2-2 -/ / MPa,., CSD CSD (Marsh, ) + CSD CSD (CSD) + mm - (mm) (mm - ) (mm. ) (mm. ) (Fig. 2) CSD Marsh (+322) CSD CSD CSD CSD L

6 378 Fig. 0. Correlations between eruptive parameters and microlite textures. a) E#usive flux and number density (modified from Hammer et al.,,***), b) Repose time and crystallinity (modified from Hammer et al., +333). n n * exp L Gt (+) n * : L: G : t : n Ln n L Gt + n * J J n * G (,) CSD G t t Cashman, +33, ; Hammer, et al., Marsh ( ) (batch crystallization) CSD CSD (Fig. 2a) CSD, CSD Fig. 2b + CSD Fig. 2b, CSD (Fig. 2c),. + CSD CSD Fig. 2a, b (Fig. 2c), CSD CSD Armienti et al., +33. ; Cashman, +33, ; Castro et al.,,**- ; Hammer et al., +333 ; Noguchi et al.,,**0 ; Sato, +33/ ; Suzuki et al.,,**0 ;,**0

7 379 Fig. 1. Magma plumbing system beneath Usu volcano, and ascent process (a) and textural evolution (b) of Usu,*** magma supplied from shallow reservoir. From Suzuki et al. (,**0). For reservoir at +* km depth, Tomiya and Miyagi (,**,) was referred. indicates experimental result in Suzuki et al. (,**0). Fig. 2a, Fig. 2c + CSD (Cashman, +33, ; Castro et al.,,**- ; Hammer et al., +333 ; Noguchi et al.,,**0 ; Sato, +33/ ; Suzuki et al.,,**0) Marsh ( ) CSD CSD CSD (Eberl et al.,,**, ; Maal e et al., +323) Suzuki et al. (,**0),*** CSD Fig. 2b + (Fig. 1) CSD Fig. 2b CSD Armienti et al. (+33.) (,**0),**. - + Geschwind and Rutherford (+33/) +32* (Couch,,**- ; Couch et al.,,**- ; Hammer and Rutherford,,**, ; Martel and Schmidt,,**-) Fig. / (Fig. /) Martel and Schmidt (,**-)

8 380 Fig. 2. Representative CSDs from groundmass of volcanic ejecta. For interpretation, see text. a) Linear CSD, b) Kinked CSDs, c) CSD with negative slope at large sizes and positive slope at small sizes. ( 0,*** MPa h) - + PH,O P total PH,O P total (Couch,,**-) (SiO, 1* 2* wt. ; Table +) Martel and Schmidt,**- LP; Table + (Ps ; Table +) (Pf ; Table +) T (Fig. 3, Table +) Fig. 3 Fig. 3 T SSD MSD (Hammer and Rutherford,,**, ) SSD (single-step decompression) (Table +, Fig. +*) MSD (multistep decompression) (Fig. +*) SSD MSD (Hammer and Rutherford,,**,) SSD MSD Hammer and Rutherford (,**,) SSD 01/ -,,/ MPa h. ; Table + MSD +., MPa h (Table +) MSD SSD MSD (Fig. +*) -, T eff + T eff T Couch et al.,**- T eff T eff ( P) ( P t) MSD (Fig. +*)

9 381 T eff T SSD, Table + ; Fig. +* Couch (,**-) Couch et al. (,**-) Hammer and Rutherford (,**,) SSD (Fig. ++) Fig. ++a Hammer and Rutherford,**, (Fig ++b Hammer and Rutherford (,**,) MSD Ps P t ; Table + Fig. 3. Undercooling simply defined by di#erence between temperature (12* ) and plagioclase liquidus ( T in text). Modified from Hammer and Rutherford (,**,). Fig. +*. Styles of decompression. Modified from Couch et al. (,**-). Table +. Conditions of experiments for decompression-induced crystallization in felsic system. For detail of references, see text. Decompression rate (m/s) in MSD experiments in Couch (,**-) was calculated assuming +** MPa is equivalent to,./ km. Martel and Schmidt (,**-) tested the most variable decompression rate in MSD experiments. When final pressure was set at /* MPa, experimental time varied from 0 second to +/ days.

10 382 Pf +** MPa, /* MPa -**.** mm,,,/ MPa +,*** mm, / MPa.*,*** mm, ; Table., Couch et al. (,**-) MSD (Fig. +,) Martel and Schmidt (,**-) Fe-Ti -, SSD +, Fig. /, - -,, Couch (,**-) Couch et al. (,**-) Hammer and Rutherford (,**,) SSD ( P) P (Fig. +-) (Fig. +-b) - Couch et al. (,**-) MSD (Fig. +.) Couch (,**-) SSD Fig. /, Fig. ++. Relationship between T (simply defined by P) and plagioclase nucleation rate (I) in SSD. a) and b) are modified from Hammer and Rutherford (,**,) and Couch et al. (,**-), respectively. In SSD, T is equal to T e# (refer text). There exists time lag between occurrence of undercooling and nucleation start. In addition, nucleation stops when system approaches equilibrium and undercooling becomes small (e.g. decrease in nucleation rate, as shown in b)). To evaluate e#ect of P, we thus need to compare samples held at final pressure for the same duration. a) shows data from samples which were held for +02 hours at final pressure, while b) shows incremental rates for various durations (. /*. hours) SSD (Hammer and Rutherford,,**,) Table, ; Fig. +/

11 383 Fig. +,. Relationships among T (simply defined by P), decompression rate and plagioclase nucleation rate (I) in MSD. Modified from Couch et al. (,**-). Relative decompression rate is known from the legend showing duration of experiment. Lofgren, +32* -. Hammer and Rutherford (,**,) Table + (Table,) SSD, MSD SSD 01/ -,,/ MPa h MSD +., MPa h (Table +) MSD MSD (Table,) Table, ( T eff) a T eff Pf +** /* MPa MSD; Table, : b T eff : c T eff : d T eff Pf / MPa SSD ; Table, : Fig. +-. Relationship between T (simply defined by P) and plagioclase growth rate (Y and G i) in SSD. In SSD, T is equal to T e# (refer text). a) is modified from Hammer and Rutherford (,**,) where average rates for +02 hours are shown. b) is modified from Couch (,**-) which includes incremental growth rates for various durations (2 /*. hours). For growth rate estimation, ten largest microlites were used (both a) and b)). T eff Fig. / Fig. ++a Fig. +-a a d Fig. +/ Suzuki et al. (,**0) MSD T eff P P t T eff P P t T eff -, P P t

12 384 +** MPa Fig. +.. Relationship among T (simply defined by P), decompression rate and plagioclase growth rate (G m) in MSD. Modified from Couch et al. (,**-). Legend indicates duration of experiment. Relative decompression rate is known from the legend showing duration of experiment. P t P T eff P P (Fig. 3) (Suzuki,,**+ ; Hammer,,**0) Blundy and Cashman (,**/) +32* Suzuki (,**+) 0 (.** MPa) +** MPa Fig. +0 An ; Fig. +0 +** MPa (Fig. 3) ( T) (Suzuki,,**+).,,*** (Suzuki et al.,,**0) (Couch et al.,,**- ; Geschwind and Rutherford, +33/). + a (SSD, MSD) ; Fig. +a-,,. b c (Fig. +a-+) c - (Fokin et al., +333),*** - -+,**, ; Oshima and Ui,,**-,.,,,**,

13 Table,. 385 Textual change of run product with ﬁnal pressure and decompression rate. Excerpt of Table. of Hammer and Rutherford (,**,). Results are from +02 hour experiments for both SSD and MSD. Te# in table corresponds to T (simply deﬁned by P) in text. Fig. +/. BSE images of decompression products in Suzuki et al. (,**0) where samples were decompressed from +,/ MPa to /* MPa. MSD and SSD show style of decompression (see Fig. +*). Numbers after MSD and SSD indicate experimental time (hour). Phases are Fe-Ti Oxides (Ox), clinopyroxene (Cpx), plagioclase (Pl), glass and bubble in order of decreasing brightness. Most crystals are microlite. Crystals which existed before decompression are labeled with pe (abbreviation of pre-existing). Plagioclase pe in MSD-+./ is fragmented. Plagioclase microlites in SSD-,. are skeletal (for reason, see caption of Fig. -), while those in MSD products are tabular and euhedral. Note systematic change in microlite number density with time in MSD run (increases from MSD-0* to MSD-0 and decreases to MSD-+./ ; also see text). Among all run products in Suzuki et al. (,**0), SSD-,. best resembles ejecta (Fig. -) in terms of microlite. Actual magma was decompressed to +*ῌ/ MPa after crystallization was completed (Stage-, ; Fig. 1), so there exists textural di#erence of bubbles between ejecta and SSD-,..

14 386 Fig. +0. BSE image for plagioclase phenocryst (Pl-Ph) and groundmass (GMS) of a pumice sample from Futatsudake eruption at the middle of 0th century. Low-An rim records syneruptive magma decompression from reservoir (.** MPa) to +** MPa (Suzuki,,**+). Also note no microlite in groundmass. Fr-Ph ; fragmented phenocryst. Fig. 1 + ; Fig. 1,**+,. +, + + Fig. +* SSD MSD. Fig. +1 F (3** PH,O +,/ MPa) (/* MPa) SSD MSD SSD +, +.., MSD +./ +.. MSD /* MPa /* MPa SSD MSD /* MPa., /* MPa An SSD (An./.0.3.+) SSD (An..., /-.*) MSD /* MPa /* MPa Fig. +/ MSD ; Fig. / +./ +./ (Fig. 1) SSD /* MPa (vol. ) (/* MPa) +, Fig. 1 ; Fig. +* SSD (Fig. -) MSD SSD Fig. +/ CSD Fig. 2c SSD Fig. +/ SSD-,. BSE. -, km,**, /* MPa +./, km

15 387 Fig. +1. Seismic activity and ground deformation before and just after the -+ March eruption in Usu volcano (modified from Oshima and Ui,,**-). (B) temporal change in base line distance (for location, see (C)), (E) hypocenter distribution in EW cross section, (F) temporal variation in hypocenter depth. Three phases in the precursory seismicity (I-III), proposed in Oshima and Ui (,**-), are shown with the vertical dotted lines in (B) and (F)., +* ; -./ ; Fig. +1 F, Fig. +1F -,3 ; Oshima and Ui,,**-,, +* km ;,**,,**+ Fig. +1 B Suzuki et al. (,**0) / / + / + +,*** ;,**, (,**,)

16 388,*** ;,**, Fig. 1,***,**+,**, ; Suzuki et al.,,**0 Noguchi et al. (,**0) (Gardner and Denis,,**. ; Hurwitz and Navon, +33.), / +,. (Fig. +a) - (Fig. +b vs a-+) - (Couch,,**- ; Couch et al.,,**- ; Hammer and Rutherford,,**, ; Martel and Schmidt,,**-) CSD CSD CSD (Marsh, ) CSD;,., Fig. +* MSD - +,**- Mourtada-Bonnefoi and Laporte,**. (Table +) +* MPa h /*** MPa h,.,***. + P ( T eff) -. T eff P P t -, P P t., - (Blundy and Cashman,,**+) (Fig. +*) Hammer and Rutherford (,**,) SSD MSD P., (Suzuki et al.,,**0) P P t - /,. ;. - Hammer

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19 391 zation for a binary eutectic system with application to igneous textures. J. Geophys. Res., +*0,.*-1.*0*.,**0,**. / Tuttle, O. F. and Bowen, N. L. (+3/2) Origin of granite in the light of experimental studies in the system NaAlSi -O 2 - KAlSi -O 2-SiO,-H,O. Geol. Soc. Amer. Mem., 1., +/- p.