Bull. Volcanol. Soc. Japan
Vol. 58 (2013) No. 1, pp. 213-226

ArticleVariations of Color and Leachate Contents of Volcanic Ashes
from Sakurajima Volcano, Japan
Isoji MIYAGI＊, Hiroshi SHINOHARA＊ and Junʼichi ITOH＊(Received September 20, 2011 ; Accepted June 20, 2012)
To understand magma degassing processes near the top of volcanic conduit, we investigated a series of volcanic
ashes from the Sakurajima volcano, Japan. We describe temporal changes in the color and the amount of leachates (Cl,F, S) of ash erupted from 1981 to 2011. Based on the amount of leachates present, ash samples are classified into two
major groups: one is with a molar S/Cl ratio of 〜10, and another with S/Cl of 〜1 and that is relatively depleted in S.
Ash samples that were erupted during 1981-1991 from the Minamidake summit crater belong to the latter group.
Ashes erupted from the Showa crater in early 2008 belong the former group, of which the S content was found to
decrease systematically through time, although in 2011, ashes of the latter group erupted for the first time from thiscrater. Based on coloration, the ash samples in this study are classified into two groups: one with a yellowish color,
another with a less yellowish color. The coloration of the former group can be explained by the existence of yellowish
native sulfur as well as other hydrothermally altered minerals. We observed positive correlations between the intervalof successive eruptions and both the yellowness and the amount of ash leachates. Our interpretation is that the observed
temporal changes in volcanic ash result from a transition in the amount of fumarolic sulfur accumulation in partly
solidified magma near the top of volcanic conduit, which we interpret to reflect the mean residence time of the magma.
The magma probably periodically renewed in response to ash eruption and/or magma convection near the top of the
volcanic conduit.
Key words : Sakurajima; volcanic ash, color, volcanic gas, magma degassing, S, F, Cl

1．Introduction
Developing a sound understanding of magma degassing
processes can ultimately yield clues to the mechanismsthat cause volcanic eruptions (Eichelberger et al., 1986;
Eichelberger and Westrich, 1981; Jaupart, 1998; Kazahaya
et al., 1994; Melnik et al., 2005; Papale et al., 1998; Wilson
et al., 1980; Woods and Koyaguchi, 1994). The Sakurajima volcano is one of the best places for conducting
petrological investigations of magmatic degassing processes, especially because fresh ash samples are frequently
discharged by eruptions at relatively short time intervals.
The volcanic activitiy of Sakurajima has been extensively
monitored in recent years, including for example, thetiming of eruptions, the height of volcanic clouds, thermal
infrared imaging of the vent, SO2 gas fluxes, volcanic
earthquakes, and crustal deformation (e.g., Iguchi et al.,2008; Yokoo, 2009, Kazahaya et al., this volume, Japan
Metrological Agency (JMA)). Here we describe temporalchanges in the color and amount of leachates (Cl, F, S)
absorbed on the ash. The analysis of ash leachates (Witham
et al., 2005) has been used in the past to estimate the composition of volcanic gases, and also in monitoring thestages of eruption of a volcano (e.g., Nogami et al., 2002).
＊

Geological Survey of Japan, Tsukuba Central 7, 1-1-1
Higashi, Tsukuba, Ibaraki 305-8567, Japan.

In this study, the ashes were collected shortly after each
eruption took place, without having exposed to rain. The
analysis of the color of volcanic ash can provide information about the temperature and oxidation state of the post
eruptive magma (Miyagi et al., 1998; Miyagi and Tomiya,
2002; Moriizumi et al., 2008; Yamanoi et al., 2008).
2．Recent activity of Sakurajima
Over the past three decades, eruptions of the Sakurajima
volcano occurred at the Showa crater and the Minamidake summit crater. The current center of volcanic activity is the Showa crater (N31° 3443, E130° 3945) located about 750 m above sea level on the eastern hill of thesummit of Minami-dake. Volcanic activity within the Showa
crater commenced on 26 October 1939, and subsequently
ceased by 1948. Since 1948, the Minami-dake summit
crater has been the only active volcanic center of the
Sakurajima volcano, until eventually recommencement of
volcanic activity in the Showa crater took place in June
2006. On 3 February 2008, explosive eruptions began
within the Showa crater. Since mid-April 2008, red-hot
ejecta have been observed at night by network cameras
installed around the volcano. Seemingly in response to theCorresponding author: Isoji Miyagi
e-mail: miyagi.iso14000@aist.go.jp

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Isoji MIYAGI, Hiroshi SHINOHARA and Junʼichi ITOH
Table 1.

List of samples.

vigorous activity of the Showa crater, the frequency oferuptions at Minami-dake has since declined.
3．Samples
Ash samples collected from the Showa and Minamidake summit crater from 1981 to 2009 were documented
previously by Miyagi et al. (2010), in addition to thatyounger samples collected from 2009 to 2011 (Table 1) are

the subject of the present work. Ash samples in this study
were collected either by the JMA or by the Geological
Survey of Japan (GSJ) shortly after extrusion without
having been exposed to rain, with exception of sample
Skr2008 0203 (A) (Miyagi et al., 2010).
4．Analysis
The chemical analyses of ash leachates (i.e., for Cl, F,Variations of Color and Leachate Contents of Volcanic Ashes from Sakurajima Volcano, Japan
Table 1.

and SO4 contents) was performed by an ion chromatograph on aqueous solutions prepared as follows. The magmatic volatiles adsorbed on the surfaces of ash samples
were first dissolved with deionized water. For eachanalysis, about 0.5 g of ash sample was sealed in a glass
tube with about a fifty-fold weight of water, and then215

continued.

activated for 30 minutes using an ultrasonic cleaner. The
supernatant fluid was subsequently recovered about 2hours after the ultrasonic cleaning, and was then filteredusing a 0. 45μm membrane filter. The error associatedwith each analysis was estimated based on replicate
analyses of individual samples, which yielded errors of±7

216

Isoji MIYAGI, Hiroshi SHINOHARA and Junʼichi ITOH
Table 1.

% (1σ),±5 % (1σ) and±4 % (1σ) for F, Cl, and SO4,
respectively.
The color analyses were performed using a colorimeter(Minolta SPAD-503) on bulk ash samples and also on finerparticles extracted via the method outlined below, in whichcolor data are reported in a Commission internationale de lʼéclairage (CIE) L*a*b* color space proposed by the CIE in
1976 (Wyszecki and Stiles, 1982). For each color analysis, bulk ash or finer particles were brought into directcontact with the window (10 mm in diameter) of thecolorimeter while taking care not to expose the sensor tooutside light. Each color analysis consisted of seven setsof measurements, of which the first and last were done on astandard (for quality control), and then the five unknown(sample) values were averaged and considered to represent
the empirically determined color of the sample.
The finer particles of volcanic ash were extracted fromthe samples as follows. Each ash sample (comprisingcontinued.

1-30 g of material) was washed with deionized water(〜300 g) in a beaker for 30 minutes in an ultrasoniccleaner. About 1 minute after the ultrasonic cleaning, each
mixture was then separated by sedimentation (grain
settling) through the water column into a thick bottom
deposit and a thin upper part, comprising clay- to silt-sizedparticles suspended in water. Taking special care not toinclude any material from the thick bottom deposit, theupper part was decanted into an evaporating dish. The
finer particles were then recovered from the suspension by
overnight drying at 90℃ to evaporate water from thesuspension, leaving only the fine ash particles behind in
the evaporating dish.
The aftermentioned thick bottom deposit was further
washed, dried, sieved, and weighed to obtain the grain size
distribution of the ash particles (Table 2), and these particles were then observed under a binocular microscope.

Variations of Color and Leachate Contents of Volcanic Ashes from Sakurajima Volcano, Japan
Table 2.

217

Cumulative grain size distribution.

5．Results
5-1 Particle size and composition
The results of grain size analysis of volcanic ash samples from this study are shown in Table 2 along with thepreviously characterized samples reported by Miyagi et al.
(2010). The finer particles recovered from the liquidsuspension are noted as “silt” in Table 2, and those frac-

tions generally comprised at least a few tens of wt.% of thebulk ash sample in total. All of the ash samples examinedin this study are dominated by fine particles smaller than 2
φ (i.e., less than 0.25 mm in diameter), except for thosesamples originating from earlier eruptions in 2008 (i.e.,
Skr2008 0203(A) and Skr2008 0408).
The coarser ash particles tend to be non-vesicular, round-218

Isoji MIYAGI, Hiroshi SHINOHARA and Junʼichi ITOH
Table 2.

ed, and are interpreted to represent recycled ash particles
that were crushed long before eruption. Relative to thecoarser particles, the moderate grain-size fractions exhibit
surface features consistent with crushing upon eruption; e.
g., freshly fractured surfaces and angular grains. Fresh ash

continued.

particles are found throughout the ash deposits originatingfrom activity of the Showa volcanic vent since February
2008. These fresh particles consist of both crystal-rich andcrystal-poor varieties, with or without tiny bubbles.
Despite having freshly fractured surfaces and angularVariations of Color and Leachate Contents of Volcanic Ashes from Sakurajima Volcano, Japan
Table 3. Amount of ash leachates (F, Cl, SO4) and the
Cl/S ratios. Data marked with asterisks are from
Miyagi et al. (2010).

shapes associated with post-vitrification crushing during
eruption, the ash particles show various degrees of alteration, the intensity of which was estimated on the basis ofobserved variations in brightness of BEI/SEM images of
the ash particles (Miyagi et al., 2010). The ratio of alteredto unaltered ash particles is high in cases where ash samples were erupted a long time after the previous eruption,and this ratio was observed to drop rapidly for samples thatwere erupted at short intervals (e.g., figure 6a in Miyagi etal., 2010). In addition, the proportion of glassy (crystalpoor) to blocky (crystal-rich) particles varied systemat-

Table 3.

219

continued.

ically with time, similar to the observed variation in the
proportion of altered to unaltered particles. This variationin glassy/blocky constituents of ash sample is considered
to reflect the faster cooling rate of magma in cases where
eruption occurred through a less-heated volcanic conduit
after longer repose (Miyagi et al., 2010).
5-2 Water-soluble magmatic volatiles
The amount of water-soluble ions of chlorine and sulphate adsorbed onto volcanic ash particles from Sakura-220

Isoji MIYAGI, Hiroshi SHINOHARA and Junʼichi ITOH

Group

A

B

C

D

2006

2008

2010
Year

Fig. 2. Temporal changes in ash leachate compositionalgroups. See Fig. 1 for the grouping criteria.
Fig. 1. Amount of water-soluble chlorine and sulphateions (in mmol/kg ash or in mg/kg ash) adsorbed onto
ash surfaces for deposits erupted during 1981-2010.
Data are from Table 3 and Miyagi et al. (2010). Solidcircles represent data collected from the Minami-dakesummit vent during 1981 to 1992. Open circles represent data from the Showa volcanic vent, with the
exception of one sample (Skr2008 0428) that is a
mixture of ashes from the Showa and Minami-dakesummit vents. Two reference lines are also shown on
the plot as guide lines for examining the data ̶the
S/Cl molar ratio of the upper reference line is 10, andfor the lower reference line this value is 1.

jima eruptions during the time span of 1981-2010 ranges
across three orders of magnitude (Table 3, Fig. 1). Whilethe data for ashes from the Minami-dake summit vent forthe period 1981-1992 (open circles) fall along a lower
reference line (with S/Cl of 〜1), those from the Showa
volcanic vent from 2006-2010 (solid circles) have higher
sulphur concentrations that plot along an upper referenceline with S/Cl of 〜10 (Fig. 1). Thus, in terms of theamount of water-soluble chlorine and sulfur, the volcanic
ash samples are classified into two major groups: one witha molar S/Cl ratio of 〜10, and another with a molar S/Cl
ratio of 〜1 and that contains notably less sulfur (group D).
The former group is subdivided into groups A, B, and C,
which are ordered according to their decreasing amountsof sulfur.
There is also a documented temporal association whereby the timing of eruptions correlates with these different
geochemical groupings of ash leachates (Fig. 2). For
example, all ashes from the Minami-dake summit venterupted from 1981 to 1991 belong to group D, with one
outlier plotting in the group C region (Fig. 1; 5 January1992). Ash samples from the Showa vent extruded on 6
June 2006 and before 2008 belong to group C. Samples
from explosive eruptions at the Showa vent initially belong
to groups A and B; i.e., in the early stages of eruption in2008. On 9 July 2008, however, ashes of group C reappeared. With time, the frequency of group A ashes seems
to have decreased, while the frequency of group B and C
ash accumulations was observed to increase with time, andfinally on 9 March 2010, ashes of group D were erupted
for the first time from the Showa volcanic vent. Thus, theobserved temporal change in ash types from the Showa
vent is C→A→B→C→D (Figs. 1 and 2).
To summarize, ash samples from the Showa volcanic
vent show a temporal change in geochemical groupings
defined by adsorbed water-soluble chlorine and sulfate,from group C→(longer interval)→A→B→C→D. Thechange in groups defined by ash leachate compositions
from A→B→C reflects a decrease in the measured amountof sulphur and chlorine at a relatively constant rate, while
the change from C→D is simply the result of a decrease insulphur.
5-3 Color of the finer fraction
The colors of bulk volcanic ashes erupted from 3 May
2009 to 3 June 2011 range from 0.4<a*<2.1 and 0.5<b*<5.6, where a* and b* values are notations drawn fromthe CIE L*a*b* color space. The color of the finerparticles (recovered from suspension noted as “silt” inTable 2) range from 1.5<a*<4.2 and 4.7<b*<11.2(Table 4). These observations are consistent with previous
reports that the variation in color of finer particles is larger
than that of the bulk ash (Miyagi et al., 2010). The color
data determined for finer ash particles in this study areplotted on an a*b* plane (Fig. 3) to allow for directcomparisons with previously determined color data fromashes of the Sakurajima volcano (i. e., from eruptions
between 3 February 2008 and October 2009); also plotted
are relevant data from Miyagi et al. (2010) (Fig. 3). Since
the finer ash particles show a remarkably large variation incolor ̶and because they are the major constituent of theash in terms of surface area̶ we will focus hereafter on
the color of these finer ash particles in our discussions of
the color data.
On the basis of color alone, the ash samples from theVariations of Color and Leachate Contents of Volcanic Ashes from Sakurajima Volcano, Japan
Table 4.

Color of volcanic ash.

Sakurajima volcano erupted from 3 February 2008 to 3
June 2011 are classified into two groups: one that plots
along the line b* = 2a* (group R), and another defined byhaving b*>2a* (group Y). A typical example of group Y
is those ashes erupted in the early days of re-activation of
the Showa volcanic vent, from February to May 2008
(open circles in Fig. 3). A key feature of the group Y
ashes is their yellowish color relative to the group R ashes.
In order to quantify the deviation of the group Y color
from the reference line defined by the color of group R
ashes, we introduce a new parameter “d”, defined as the
distance between the color data and the reference line b*=
2a* on the a*-b* plane, where d=|b*−2a*|/ 5 .
6．Discussion
6-1 Abundance and composition of ash leachates
The measured amount of water-soluble sulfur fromashes erupted during the early stages of re-activation of the
Showa volcanic vent in 2008 (i.e., 44,600 mg SO4/kg ash,Miyagi et al., 2010) ̶along with most of the data presented in this study (table 3)̶ is higher than the concentration of sulfate dissolved in rhyolitic melt inclusions inphenocryst fragments within ash from the Sakurajima
volcano (〜900 mg SO4 /kg melt; Miyagi, unpublisheddata). This observation requires the existence of some
additional source for such excess sulfur adsorbed onto theash particles, and this might include CO2 as the dominantmagmatic gas carrier (Wallace and Gerlach, 1994), anhy-

Table 4.

221

continued.

drite decomposition during eruption (Devine et al., 1984),
a separate C-O-S vapor phase occurring as bubbles (Luhret al., 1984), exsolved sulfur originating from a rapid dropin sulfide solubility within the silicate melt due to magma
mixing (Kress, 1997), and native sulfur deposited aroundvolcanic fumaroles. In contrast the amount of watersoluble chlorine (Fig. 1) in these ashes is comparable to Cl
concentrations observed in melt inclusions (〜1000 mg
Cl/kg melt; Miyagi, unpublished data), with the exceptionof those ashes erupted in the early stages of re-activation of
the Showa volcanic vent in 2008. The observed change inash group form CD outlined above, corresponds to theobserved change in the amount of water-soluble sulfur
from very S-rich ashes (i.e., far above S solubility values
from the parent melt) to those ashes with adsorbed SO4
values comparable to the observed SO4 concentration inmelt inclusions.
6-2 Relationships between color and ash leachatecompositions
We observed a positive correlation between the d valueof ash samples and the amount of ash leachates present
within the ashes collected during 2008-2011 (Fig. 4).
Accordingly, the samples with higher d values meet thegroup A ash (i.e., have a higher amount of ash leachates).
Since air-oxidized andesitic volcanic ash shows a change
in color along the line b* = 2a* (Miyagi and Tomiya,2002), the parameter d reflects the color change due to222

Isoji MIYAGI, Hiroshi SHINOHARA and Junʼichi ITOH

Fig. 3. Color of volcanic ashes from the Sakurajima
volcano plotted on the a*-b* system of CIE L*-a*-b*
color space. The values of a* and b* correspond to theintensities of red (+a*) and green (-a*), and yellow(+b*) and blue (-b*) color, respectively. Open circles
represent the colors of ash particles (samples erupted
from eruptions that took place from February to May
2008). Crosses represent samples of volcanic ashes
erupted from May 2008 to December 2009. Solidcircles represent samples of volcanic ashes erupted
from January 2010 to June 2011. For reference, blackstars indicate the color of coarse volcanic ash particles(but ground individually via agate mortar and pestle
before each color measurement) for ash samples
erupted from April to September 2010. The black line
represents the color variation (b* = 2a*), whichindicates the general trend of colors observed in theheating experiments (of volcanic ash in air) performedby Miyagi and Tomiya (2002). This line is also nearly
equal to the color variation observed in bulk-ash
samples erupted from the Minami-dake summit ventduring 1981-1992, and from the Showa volcanic ventfrom May 2008 to March 2009 (Miyagi et al., 2010).

processes other than oxidation. For example, the additionof native sulfur, which has a strong yellow color (a*=3,
b*=23), could be responsible for the color change. In
fact, we observed native sulfur within the group Y ashes,
the identity of which we subsequently confirmed unequivocally using X-ray diffractometry (Miyagi et al., 2010).
The group A ashes are characterized by minerals indicativeof hydrothermal alteration, such as native sulfur, pyrite,kaolinite, gypsum, basanite, opal, and Na-alunite. These
hydrothermal minerals are considered to be accumulate via
fumarolic activity immediately preceding the eruption

Fig. 4. Relationships between color (d value) and ash
leachate compositions. The parameter d is defined as
the distance on the a*-b* plane between the color data
and for a given ash sample and the reference line b*=
2a* on the a*-b* plane (d=|b*−2a*|/ 5 ).

(Miyagi et al., 2010), which suggests a possible connection
between the value of d and the amount of ash leachates
accumulated through pre-eruptive hydrothermal alteration.
The group A ashes were erupted only during the earlystages of reactivation of the Showa volcanic vent in 2008
(Miyagi et al., 2010), which suggests a possible control on
the extent of hydrothermal alteration by the length of time
between eruptions.
6-3 Links with duration of eruption intervals
The time intervals between eruptions of the Sakurajima
volcano were examined in order to test whether the duration of pre-eruptive fumarolic activity affects the extent towhich alteration takes place near the top of the volcanic
conduit, and whether, as a consequence, this affects the ash

Variations of Color and Leachate Contents of Volcanic Ashes from Sakurajima Volcano, Japan

Fig. 5. Frequency plot of the durations of intervals
between successive eruptions of Sakurajima volcano.
Horizontal axis represents the duration of the timeinterval between successive eruptions, and the verticalaxis represents the number of neighboring data points
for a given interval duration (within±3 minutes, i.e., awindow size of 6 minutes). The duration of the interval
between successive eruptions for the group A ash
samples (Fig. 1) is highlighted with arrows.

color and the amount of ash leachates extracted. Timing
data for each eruption at Sakurajima ̶e.g., “2011/02/07
08: 05”̶ are drawn from the JMA website (http : //www.
seisvol. kishou. go. jp). Our eruption-interval data range
from February 2008 to April 2011, with the exception of ashort (3-month) gap in available data from January to
March 2010. The total number of timing data on eruption
activity is 1670 data points, and the intervals of time
between successive eruptions ranged from 0 to 88, 624
minutes. The frequency of eruption intervals of the samelength of time determined from these data is shown in Fig.
5, and we observe a notable peak in eruption interval
duration at about 45 minutes. The arrows shown in Fig. 5
indicate the eruption intervals for group A samples (Fig.
1). Individual arrows are located on the right side of theeach peak from which it can be seen that the yellowish ash
samples with higher sulfur contents are characterized byhaving resided throughout longer time intervals between
eruptions. We also observed a positive correlation between the measured d values of ashes and the interval
between successive eruptions (Fig. 6). In other words,
fluctuations in d values are small when the interval
between successive eruptions is short, and vice versa. We
also observed similar correlations between the duration of
eruptive intervals and the amount of leachates extracted
from the ashes (Fig. 7). This result suggests that theamount of leachates extracted from ash sample is a reflection of the extent/duration of the pre-eruptive hydrothermal activity that affected the deposits from which they
were collected.

223

Fig. 6. Relationships between the color (d values) of volcanic ashes and the interval between successive eruptions. Some analytical results of ash samples originating from multiple eruptive events (e. g., Skr2009
0503 is a composite sample comprised of ashes erupted
at 09 : 23 and 09 : 50 3 May 2009) are shown as pluralspots.

6-4 Pre-eruptive hydrothermal alteration
The most important aspects of all of the above observations can be summarized as follows: (1) the amount ofleachates extracted from ash sample varies by nearly threeorders of magnitude over the entire range of sampled
eruptions through time; (2) the amount of adsorbed ash
leachates present, especially sulfur, exceeds the range of
those same chemical compounds dissolved within theoriginal melt; (3) the shorter the interval between successive eruptions, the lower the total amount of ash leachatespresent and the lower the d value of the ash; (4) the longer
the interval between successive eruptions, the greater therange of the amount of ash leachates present and the larger
the d value; and (5) the ratio of altered to the unaltered ash
particles is higher in those cases where the ash deposits
resided for a longer repose time before the eruption, andthis ratio drops correspondingly (and rapidly) with a dropin the repose time (e.g., Fig. 6a; Miyagi et al., 2010).
Fig. 8 is a schematic illustration of the cross section near
the top of volcanic vent of the Sakurajima volcano. Given
that a large amount of SO2 was emitted from the Showa
volcanic vent even during non-eruptive periods (i. e.,
150-1880 ton/day; Kazahaya et al., this volume), convective magma degassing (Kazahaya et al., 1994; Shinohara
et al., 2003) probably taking place beneath the volcanic
conduit during these times. The lower half of Fig. 8 shows
an upwelling current of bubbly magma (white) and a descending current of degassed magma (black), which isessentially identical to previously proposed models of convective magma degassing (Kazahaya et al., 1994; Shinohara
et al., 2003).
Since effective magma degassing probably only takes

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Isoji MIYAGI, Hiroshi SHINOHARA and Junʼichi ITOH

Fig. 7. Relationships between the interval between successive eruptions and the amount of ash leachatespresent. Some analytical results of ash samples originating from multiple eruptive events (e. g., Skr2009
0503 is a composite sample comprised of ashes erupted
at 09 : 23 and 09 : 50 3 May 2009) are shown as pluralspots.

place after an initially impermeable porous magma suddenly turns highly permeable when the bubble fraction in
the magma reaches about 80 vol.% (Takeuchi et al., 2009),
the productive center of magma degassing likely comprises the upper half of the diagram (Fig. 8) only (i.e., in
the lower half degassing is inefficient). The upper half of
this illustration depicts a complex mixture of foamy
degassing magma (white) and dense degassed magma(black). It is likely that lava within the uppermost part ofthe diagram especially, is partly solidified and hydro-

Fig. 8. A schematic cross-section near the top of volcanic
vent of the Sakurajima volcano showing the distribution of pre-eruptive hydrothermal alteration. See themain text for details.

thermally altered (shadowed area surrounded by dashedcurve) (Fig. 8). The bubbly degassing magma originating
at depth, likely becomes quite foamy through a decompression process as it reaches shallower levels, and when it
ultimately collapses (degasses) while losing its form, ittransforms into the dense magma that is depicted (black) in
Fig. 8. As a result, the upper half of this magmatic/volcanic complex may suffer hydrothermal alteration due toextensive volcanic out-gassing, ultimately causing theexcess amount of sulfur present in ash leachates. It is
likely that this hydrothermally altered area is distributed
heterogeneously within in the upper volcanic complex.
According to our proposed model, the observed excess
sulfur in ash leachates is derived from hydrothermal or
fumarolic repositories in the upper part of the volcanic

Variations of Color and Leachate Contents of Volcanic Ashes from Sakurajima Volcano, Japan

conduit, where native sulfur and/or sulfur-bearing hydrothermal minerals are deposited in and around the volcanic
fumaroles. We observe that, relative to chlorine, native
sulfur can become concentrated efficiently in the magmatic/
volcanic complex, although we suggest that it probably
then takes a substantially longer time period to replenish
the sulfur repository. The longer the residence time of themagma within this upper half of the volcanic complex (i.e.,
the longer the eruptive interval), the greater the extent ofaccumulation of native sulfur in the magmatic/volcanic
complex, and vice versa. Upon eruption, the upper part ofthe volcanic complex becomes replenished with fresh
magma originating from deeper levels. The observed decrease in S/Cl (i.e., the change from group C→D ashes)
probably reflects renewal of the hydrothermally altered
upper part of the volcanic complex with higher S/Cl magmas relative to the S/Cl of the high-temperature volcanic
gases being emitted.
7．Concluding remarks
We examined temporal changes in the color and amountof ash leachates (Cl, F, S) present within ashes erupted
from the Sakurajima volcano during 1981-2011. The
amount of leachates extracted from these ashes exceeded
the concentration of those same chemical components
dissolved within the actual melt, and varied by nearly threeorders of magnitude over the eruption history studied.
This variation is positively correlated with the length oftime between successive eruptions, and variations in color(d value) of the ashes correlate with the interval between
eruptions. We propose a model for this volcanic ventwhereby heterogeneously distributed, eruption-renewed
pre-eruptive hydrothermal alteration takes place, which
ultimately explains the observed temporal changes in ash
colors, and variations in abundances of adsorbed chemical
species on the exterior surfaces of the ash particles.
Acknowledgments
Ash samples were collected by the JMA (Kagoshima
Observatory) and by the Geological Survey of Japan (Drs.
K. Nishiki and N. Geshi). Dr. R. Oikawa, Miss. T.
Yamaguchi and Mrs. K. Asaoka helped with sample
preparation. Constructive comments and encouragement
provided by an anonymous referee, and by Drs. K. Notsu,
and T. Kobayashi, improved the overall quality of themanuscript.
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(Editorial handling Tetsuo Kobayashi)

桜島から放出された火山灰の水溶性付着成分と色の時間変化
宮城磯治・篠原宏志・伊藤順一

火山灰の噴出を繰り返す火道上部におけるマグマ脱ガス過程を理解するため，1981〜2011 年の桜島の火
山灰の水溶性付着成分 (Cl, F, S) と色の時間変化を調べた．水溶性付着成分量により火山灰は，S/Cl 比が約10 のグループと S/Cl 比が約 1 のグループに大別できる．1981〜1991 年に南岳山頂火口の活発な噴火で放出された火山灰は，S/Cl 比が約 1 のグループに属する．2008 年に活動を再開した昭和火口の火山灰は，ほとんどが S/Cl 比が約 10 のグループに属するが，時間とともに付着成分量が低下し，2010 年には S/Cl 比が
約 1 のグループも出現した．2008 年に昭和火口から放出された火山灰の色調は，時間とともに黄色みが強いものから弱いものへと推移した．黄色みの強い色調は，自然硫黄（黄色）や熱水変質鉱物の存在を反映していると考えられる．火山灰の水溶性付着成分量および黄色味の強さは，その火山灰をもたらした個々の噴
火前の休止時間の長さと正の相関が有ることが判明した．この相関は，火道頂部に滞留しているマグマが，火山灰となって噴火する前に，滞留時間に応じ，噴気ガスの一部を蓄積したためだと思われる．1981〜2011年にかけて観察された火山灰の水溶性付着成分量と色の変化は，火道内マグマ対流と噴火によるマグマの入れ替わりに対応した，火道頂部におけるマグマ平均滞留時間の変遷を反映していると解釈される．