41

Minerals in Volcanic Ash 1:
Primary Minerals and Volcanic Glass
MITSUHIRO NAKAGAWA1 and TSUKASA OHBA2
1

Division of Earth and Planetary Sciences, Graduate School of Science, Hokkaido University
Kita-10 Nishi-8, Kita-ku, Sapporo, 060-0810 Japane-mail: nakagawa@ep.sci.hokudai.ac.jp
2
Department of Mineralogy, Petrology and Economic Geology
Graduate School of Science, Tohoku University
Aramaki Aza Aoba, Aoba-ku, Sendai, 980-8578 Japane-mail: ohbatu@mail.cc.tohoku.ac.jp

Abstract
Volcanic ash is fragments of magma, and consists of minerals and volcanic glass. These materials
in the ash can provide important information on the nature of the magma, because chemicalcompositions of magma usually show distinct features in each volcano, and because assemblages andcompositions of minerals reflect their host melt. Using assemblages and chemical compositions ofminerals and glass chemistry, it may be possible to correlate volcanic ash distributed over a wide area.
In addition, we may identify the source volcano of the ash. We show several examples of correlating
volcanic ash and determining source volcanoes. Moreover, it is possible to discuss magmatic anderuption processes recorded in volcanic ash. We also show several examples of discussing eruption
sequences and magma mixing processes using analysis of volcanic ash.
Key words: eruption process, magma, minerals, volcanic ash, volcanic glass

1. Introduction
In explosive eruptions, magma is fragmented by
rapid exsolution of dissolved volatile components to
produce pyroclastic materials. Fine-grained portionsof these materials are called volcanic ash, and are distributed over a wide area far from the source volcano
according to the dominant wind direction above thevolcano. Magma is usually composed of silicate
melt and crystallized minerals (called phenocryst).
Thus, volcanic ash consists of minerals and volcanic
glass. The glass is quenched melt of magma during
an eruption, and the minerals are phenocrysts suspended in the melt. Many petrological and geochemical studies of volcanoes all over the world haverevealed that chemical compositions of magma can be
distinguished among volcanoes. This has been usedin classifying regional magma types (e.g. LeBas,
1988). In addition, comparing magmas of adjacentvolcanoes, the magma of each volcano could be
distinguished from that of the neighboring volcano(e.g., Sakuyama, 1983). Thus, volcanic ash can befound to correspond to magma of a source volcano,and analysis of minerals and volcanic glass in the ash
can provide information on magma from the source
volcano.

In this article, we describe minerals and glass involcanic ash, and review their scientific applications.
We mention variations of magma types and representative phenocrystic minerals in magma. We also
introduce methods of analyzing minerals and glass inthe ash. We present several examples of correlationof volcanic ash over a wide area. In conclusion, weshow several examples of research using minerals andglass in volcanic ash.

2. Magma Types and Volcanic Ash
2.1 Magma types and melt chemistry
Magma is produced by partial melting of materials
from the earth’s crust and mantle, and is usually amixture of melt and mineral crystal. In many cases,
the minerals crystallize in the magma during theascent to the surface due to cooling of the magma.
Chemical compositions of magma vary according toboth chemical compositions and mineral assemblageof source materials. Also, magma varies in terms ofconditions of melting, such as degree of melting,pressure, volatile components, etc. Integrated geochemical and petrological studies have revealed that
there exist variations in magma types corresponding toregional tectonic settings. These variations have42

M. NAKAGAWA and T. OHBA

been classified into rock types in terms of SiO2 – total
alkali and K2O diagrams (Fig. 1).
Considering magmatic processes forming compositional variations, there exist two types of mechanisms for compositional variations of magmas; variations in primary magmas and variations due todifferentiation of a primary magma. In Figure 1, theformer is variations in alkali and K2O content atsimilar SiO2 content. These variations are mainly
derived from diversity of primary magmas. On theother hand, the latter variations are produced bydifferentiation processes for a primary magma during
ascent. The latter processes are complex, including
crystallization-differentiation, contamination of crustal
materials, magma mixing processes, etc. Once a
primary magma is segregated from its source, its
chemical compositions will change due to differentiation processes. These processes usually modify
the primary magma toward felsic compositions, as inFig. 1.

Fig. 1

Many explosive eruptions have been caused byfelsic magma, and have produced widespread volcanicash. Thus, in the case of non-alkaline rocks, magma
types from explosive eruptions are usually dacite andrhyolite. Explosive eruptions by more mafic magma
often produce tephra. In the case of Japan (Fig. 2),for example, the Fuji, Iwate and Izu-Oshima volcanoes have produced basaltic tephra. However,these basaltic tephras are not as widely distributed
compared with those of the same scale (magmaticvolume) of explosive eruption of felsic magma.
The above description of magmatic compositions
is based on whole-rock chemistry of eruptive products(rocks), which include melt plus minerals (phenocrysts). Thus, in order to determine melt compositions during eruption, we have to subtract the compositions of minerals from the whole-rockcompositions. Table 1 shows examples of chemicalcompositions of magmatic melt (volcanic glass) foundin the Tyatya volcano (Fig. 2). In many cases, melt(a) Chemical classification and nomenclature of volcanic rocks using the total alkali versus silica diagram(Le Maitre, 1989).
(b) Division of basalts, basaltic andesites, andesites, dacites and rhyolites into low-K, medium-K and high-K
types (Le Maitre, 1989).

Minerals in Volcanic Ash 1: Primary Minerals and Volcanic Glasscompositions are less silicic than those of thewhole-rock composition. For example, although theeruptive products of historic eruptions of the
Hokkaido-Komagatake volcano (Fig. 2) are andesitic(SiO2=60%), their melt compositions are rhyolitic(Table 1) (Kozu, 1934).

43

2.2 Minerals in magma
Rock-forming minerals crystallizing from magma
are mainly silicate minerals. These silicate minerals
can be divided into colored and colorless minerals.
The colored minerals contain considerable amounts oftransition metal ions, such as iron. On the other hand,Fig. 2 Index map showing volcanoes in the text.
Table 1 Chemical compositions of volcanic glass of tephras found in the Tyatya volcano.
SampleAnalysis No.
TephraSource
Age
(wt%)
SiO2
TiO2
Al2O3
total FeO
MnO
MgO
CaO
Na2O
K2O

Ty-136
136-1
Tyatya?

Ty-134
134-9
Ko-c1
Koma.
1856 AD

Ty-42
42-5
Ko-c2
Koma.
1669 AD

Ty-133133-6Ta-a
Tarumai
1739 AD

Ty-138Ty-170
138-1
170-6
Ma-b
B-Tm
Mashu
Baito.
ca. 1 ka ca. 969 AD

53.07
1.55
13.40
12.68
0.23
4.53
9.07
2.79
0.89

75.16
0.44
11.85
1.94
0.08
0.36
2.16
3.25
1.79

77.08
0.33
11.90
1.75
0.01
0.38
1.89
3.44
1.75

76.88
0.23
12.03
1.77
0.08
0.29
1.83
3.57
2.32

73.91
0.66
12.72
3.03
0.08
0.64
3.33
4.27
0.72

72.15
0.21
8.92
3.73
0.00
0.01
0.23
5.00
4.70

Total
98.19
97.02
98.52
98.98
(Abbreviations: Koma., Hokkaido-Komagatake; Baito., Baitoushan)

99.36

94.95

44

M. NAKAGAWA and T. OHBA

the colorless minerals do not contain transition metal
ions as a major constituent. Besides the silicatesminerals, Fe-Ti oxide minerals are also contained inalmost all magmas. Table 2 summarizes majorrock-forming minerals occurring in volcanic ash.
Major colored minerals in widespread tephra are

Table 2 Representative rock-forming minerals in volcanic ash.
Group

mineralsmajor
end-member

Chemical formula*

Anorthite (An)
Albite (Ab)CaAl2Si2O8
NaAlSi3O8

Solid solution

Remarks

Feldspar group
Plagioclase

between An and Ab

Alkali feldpar

between pottasium
feldspar and Ab

Sanidine iscommonpotasium feldspar KAlSi3O8
Silica minerals
QuartzSiO2

Olivine group
Olivinebetween Fo and Fa
Forstelite (Fo)

Mg2SiO4

Fayalite (Fa)Fe2+2SiO4

Enstatite (En)
Ferrosilite (Fs)MgSiO3Fe2+SiO3

Pyroxene group
Orthopyroxene

between En and Fs

Clinopyroxene

among En, Fs andAugite is
Wo (Wo<50 mole%) common
Wollastonite

CaSiO3

Amphibolegroup
HornblendeActinolite (Act)Edenite (Ed)

Ca2(Mg, Fe2+)5Si8O22(OH)2
NaCa2(Mg, Fe2+)5AlSi7O22(OH)2

Pargasite (Prg)

NaCa2(Mg, Fe2+)4Al3Si6O22(OH)2

among Act, Ed, Prg
and Ts

AlIV>0.8

Tschermakite (Ts) Ca2(Mg, Fe2+)3Al4Si6O22(OH)2(Mg, Fe2+)7Si8O22(OH)2

Cummingtonite
Mica group
BiotiteAnnite (Ann)K2Fe2+6Al2Si6O20(OH)4Siderophyllite(Sdp)
Phlogopite (Phl)

K2Fe2+5Al4Si5O20(OH)4

Eastonite (Est)

K2Mg2+5Al4Si5O20(OH)4

among Ann, Sdp, Phl Relatively
and EstMg-poorK2Mg6Al2Si6O20(OH)4

Spinel group
Ti-magnetite
Magnetite (Mag) Fe2+Fe3+2O4

between Mag andUlv

Ulvospinel (Ulv) Fe2+2TiO4Ilmenite

Fe2+TiO3

include Fe3+2O3component* major elements, and trace elements are not included.

Minerals in Volcanic Ash 1: Primary Minerals and Volcanic Glasspyroxene, amphibole and mica groups. In addition,
olivine is often recognized in tephra. Major colorlessones are the feldspar group and quartz. In alkaline
magma, alkali feldspar is often present. Major Fe-Ti
oxide minerals are Ti-magnetite and ilmenite (Table 2).
Examples of mineral compositions are also shown in
Table 3.
Even if minerals have the same crystal structure
and are named as the same mineral, chemical com-45

postions of these minerals vary. This is because
many rock-forming minerals are a solid solution,
which can exchange ions in the crystal structure ofvarious ratios (Table 3). For example, olivine has
two end-member components, forsterite (Mg2SiO4)
and fayalite (Fe2SiO4). Between both end-members,
Fe2+ and Mg2+ ions can be exchanged with each otherin various proportions. In the case of a solid solution,minerals are usually described by the mole ratio of an

Table 3 Mineral compositions and numbers of ions of representative silicate rock-forming minerals
Source

Toya

Toya

Toya

Tarumai

Toya

Toya

MineralClinopyroxene

Orthopyroxene

HornblendeOlivinePlagioclase

Plagioclase

SiO2

52.74

53.32

48.21

37.77

66.78

48.83

TiO2

0.20

0.16

1.60

0.03

nd

nd

Al2O3

1.29

0.70

7.11

0.02

20.81

32.29

Cr2O3

0.00

0.01

0.00

0.00

nd

nd

FeO

9.56

21.94

13.54

23.87

0.10

0.57

MnO

0.60

1.19

0.62

0.41

nd

nd

MgO
CaO

13.86
20.93

21.78
0.97

14.45
10.99

37.05
0.15

0.00
2.32

0.01
16.44

(wt%)

NiO

nd

nd

nd

0.02

nd

nd

Na2O

0.32

0.00

1.36

0.02

10.09

2.21

K2O

nd

nd

0.08

nd

0.54

0.01

total

99.50

100.07

97.95

99.32

100.64100.36

Si

1.978

1.987

7.020

0.998

2.919

2.232

Ti

0.006

0.004

0.175

0.001

----

----

Al

0.057

0.031

1.220

0.001

1.072

1.740

Cr

0.000

0.000

0.000

0.000

----

----

Fe

0.300

0.684

1.651

0.528

0.004

0.022

Mn
Mg

0.019
0.775

0.038
1.210

0.076
3.138

0.009
1.460

---0.000---0.001

Ca

0.841

0.039

1.715

0.004

0.108

0.805

Ni

----

----

----

0.000

----

----

Na

0.023

0.000

0.383

0.001

0.855

0.196

K

----

----

0.014

----

0.030

0.001

total
Number of O

3.999
6

3.993
6

15.393
23

3.002
4

4.988
8

4.996
8

Fo
Mg/(Mg+Fe)

72.1

63.9

65.5

Or

3.0

0.1

An

10.9

80.4

Ab

86.1

19.5

73.4

Wo

43.9

2.0

En

40.5

62.6

Fs

15.7

35.4

Abbreviations; Fo, forstelite mole%; Wo, wollastonite mole%; En, enstatite mole%; Fs, ferrosilite mole%;
Or, orthoclase (KAlSi3O8); An, anorthite; Ab, albite. See table 2

46

M. NAKAGAWA and T. OHBA

end-member; e.g., the forsterite (Fo) mole % inolivine (Table 3).

3. Analytical Methods
Constituents of volcanic ash are small in size, andcan be identified by using a microscope or polarized
microscope. To identify volcanic glass and minerals,
refraction indexes have been widely used (e.g.,
Machida and Arai, 1990). However, the index may
also be affected by trace elements in the glass andminerals. Thus, it has been widely accepted that
direct measurement of glass and mineral compositions
is essential for research of volcanic ashes. Chemicalcompositions of volcanic glass and minerals are
determined by EPMA (electron probe micro analyzer),
which determine chemical compositions of small
areas or spots (~several µm in diameter) using anelectron-beam. On the basis of the X-ray dispersion
method, the EPMA is divided into two types, wave
length (WDS) and energy dispersive spectrometry(EDS).
The former type can determine traceelements (~several hundreds ppm), because it utilizes
a strong electron beam. However, such a strong
beam may damage the sample, especially glass andhydrous minerals. In contrast, the latter type can beapplied to glass and hydrous minerals without serious
damage, because it does the analysis with a weakelectron beam.
However, detection limits ofanalyzed elements by EDS are higher than those of
WDS. Considering its ability to get high detection
limits, the WDS type is used widely now. In the caseof analysis of glass and hydrous minerals by the WDS
system, defocused and/or scanned electron beams
have been applied to avoid sample damage.
Analytical methods have also been developed
especially for volcanic glass focusing on trace elements, such as rare earth elements. In this case, several hundred mg of concentrated glass is treated for
ICP-AES. Although major element analysis of glass
by EPMA must be enough to correlate ashes, if we try
to discuss magma genesis by using volcanic ash,further analysis of glass and minerals, such as by
ICP-Mass with a laser ablation system, may beessential.

4. Application of Glass Chemistry in
Volcanic Ash Studies
In this section, we overview several examples ofresearch using chemical compositions of volcanic
glass in ash. The primary purpose of using volcanic
glass is to correlate volcanic ash, and to identify thesource volcano of the volcanic ash.
Anotherexample shows that volcanic ash also contains arecord of magmatic processes during eruption.

4.1 Correlation and source volcano of volcanic ash
The pioneering and most successful research of
volcanic glass is correlation of widespread tephra ofgiant eruptions forming calderas in southern Kyushu.
Machida and Arai (1976) identified Aira-Tn tephra(AT) over a wide area of the Japanese islands. They
correlated these tephras using a refractive index of
volcanic glass and minerals. Moreover, by using theindex in addition to stratigraphic investigation, they
revealed that the ash is derived from a 20-25 ka
eruption of the Aira caldera (Fig. 2). In addition,
Machida and Arai (1978) also recognized
Kikai-Akahoya tephra (K-Ah) from the Kikai caldera
in southern Kyushu (Fig. 2), on the basis ofcorrelation by the refractive index of volcanic glassand minerals. This tephra has been dated as 6.3 ka(Machida & Arai, 1990). Since then, many major
volcanic ash layers in the Japanese islands have beencorrelated. In addition, the source volcanoes of theselayers
have
also
been
identifiedbytephrochronological and petrological studies of theselayers. These results are reviewed by Machida andArai (1992).
Since the 1980s, in addition to measurement ofrefractive index, chemical compositions of volcanic
glass determined by EPMA have been adopted forcorrelation of volcanic ash. For example, Tokui(1984) correlated volcanic ash distributed over eastern
Hokkaido to AD 1739 ash of the Tarumai volcano(Ta-a) in southwestern Hokkaido. These ashes had
been correlated as distinct ashes originating fromdifferent volcanoes. In addition, Tokui (1984) andOkumura (1980) summarized many tephras in eastern
Hokkaido, some of which were derived from large
eruptions of volcanoes in southwestern Hokkaido during the 17-19 centuries. In southwestern Hokkaido,
it has been widely accepted that three volcanoes,
Hokkaido-Komagatake, Usu and Tarumai volcanoes,
erupted sequentially from AD 1640 to 1667. Thus, it
might be hard to identify each ash in distal areas,
because there exists thin soil between each ash.
However, using chemical compositions of volcanicash, distal ash from the three volcanoes can be
identified, for example, in the TiO2-K2O diagram (Fig.
3).
Distal volcanic ash can also reveal the eruption
history of a volcano. Katsui et al. (1989) summarized the eruption history of the Hokkaido-Komagatake
volcano. However, on the basis of field observationsat proximal sites, Hayakawa (1991) suggested that theso-called AD 1694 ash of the volcano was the productnot of a distinct eruption but of a later phase of the1640 eruption, and pointed out that the occurrence ofthe 1694 eruption might be doubtful. Furukawa et al.
(1995) also investigated volcanic ash at distal areas,
and indicated that there exists distinct ash of the AD
1667 Tarumai ash between two ash layers, both ofwhich have volcanic glass with chemical compositions derived from Hokkaido-Komagatake.
TheMinerals in Volcanic Ash 1: Primary Minerals and Volcanic Glass

upper layer is covered by another ash of the 1739
Tarumai eruption. Thus, according to stratigraphy of
distal ash layers, it can be concluded that the
Hokkaido-Komagatake volcano erupted just before
AD 1667, and between 1667 and 1739. Thus, it has
been proven that the Hokkaido-Komagatake volcano
did erupt in 1694, and that the so-called 1694 ash of
Hokkaido-Komagatake volcano is not a product of alater phase of the 1640 eruption.
If the source volcano of ash and its age can be
identified, the ash becomes a possible time marker toconstruct a stratigraphy for the area. Nakagawa et al.
(2002) investigated geological studies of the Tyatya
volcano, southern Kuril Islands, and constructed astratigraphic relationship among the eruptive productsof the volcano. They also found several widespread
ash layers among these eruptive products. Usingchemical compositions of the volcanic glass of theseashes, the source volcano and eruption age were
determined (Fig. 3). They identified five widespread
ash layers of volcanoes far from the Tyatya volcano,
the
1694
and
1856
eruptionsof
Hokkaido-Komagatake, the 1739 of Tarumai, anderuption around 1 ka of the Mashu and Baitoushan (atthe boundary of North Korea and China) volcanoes(Fig. 2). Based on the presence of these ash layers,
they discussed the eruption history and mode of eruption of the Tyatya volcano.

47

4.2 Magma variations and eruption processes
In the case of correlation of volcanic ash, averagechemical compositions of volcanic glasses have beenused in many cases. However, there exist possible
compositional variations in glass chemistry beyond
analytical error. In this section, we introduce our ongoing research of the Baitoushan volcano to show thatglass chemistry of ash can provide important information for understanding of eruption processes.
Baitoushan volcano is located at boundary between
China and North Korea (Fig. 2), and is a large shield
volcano with a summit caldera. Recent eruptions
have occurred at the summit caldera, and discharged
large amounts of felsic magma. The last large eruption occurred around the 10th century and produced
widespread tephra covering northern Japan (B-Tm
tephra; Machida and Arai, 1984). Chemical compositions of glasses of the B-Tm tephra have been
determined by many authors. According to these
data, the glass of the tephra shows wide compositionalvariations, for example in the SiO2-K2O diagram.
However, these variations have not been emphasizedyet.
We investigated the 10th century eruption of the
Baitoushan volcano both in distal and proximal areas.
In the proximal area, the eruption can be divided into
phase 1 and phase 2. In phase 1, a large plinian eruption occurred to produce pumice fall deposits, which
are trachytic. Although there might exist a consider-Fig. 3 TiO2-K2O diagram for identification of volcanic ash layers in Hokkaido (shaded area). Shaded areas are after
Furukawa et al. (1997) and Nakagawa et al. (2002). Chemical compositions of volcanic glass found at the
Tyatya volcano are also shown.

48

M. NAKAGAWA and T. OHBA

able break between the two phases, subsequent large
eruptions (phase 2) occurred to produce a plinian fall
and pyroclastic flow, which are more mafic. We alsocollect distal ash fall in Hokkaido 1,000 km east of the
volcano, where the thickness of the ash is about
1-2 cm. We divided the ash into three layers, lower,
middle and upper, and collected each part carefully.
Chemical compositions of both proximal and distal
ashes are plotted in Fig. 4. Our analytical results forthe ash are the same as previously reported variationsin B-Tm tephra (Machida and Arai, 1992). In
addition, compositional differences in the ash also
exist among the three parts. Taking the mineraleffects in the proximal deposits into consideration,
compositional variations in proximal deposits are
similar to those in distal deposits. Moreover, thecompositional difference of the proximal deposits
between the two phases can be also recognized in
distal ash from lower to upper parts. Thus, we could
conclude that the previously reported compositionalvariations in glass from B-Tm tephra are not due toanalytical error but possible variations in melt

Fig. 4compositions. The variations in the 10th century
eruptive products may reflect variations of magma
types in the magma chamber(s) just before theeruption. Although detailed magmatic processes
during the 10th century eruption will be submitted toanother journal, we would like to emphasize thatcompositional variations in the glass of volcanic ash
can provide an important basis for discussion of
eruption processes.

5. Application of Mineral Chemistry in
Volcanic ash
5.1 Correlation of tephra
The presence of rare minerals in volcanic ash may
be useful for correlating and identifying the ash. For
example, volcanic ash from the Toya volcano (Fig. 2),
northern Japan, is characterized by the presence ofalbite (Na-rich plagioclase: An~10), eulite (Fe-richorthopyroxene: En~25) and cummingtonite (mineralof the amphibole group). These minerals and theirchemical compositions are not common in other

SiO2 versus FeO diagrams for volcanic glass of B-Tm tephra and whole-rock compositions of pumice clasts
from the Baitoushan volcano. Symbols are shown in the text.

Minerals in Volcanic Ash 1: Primary Minerals and Volcanic Glass

volcanic ashes in Japan during the late Quaternary(Machida and Arai, 1992). B-Tm ash from the
Baitoushan volcano is also characterized by the presence K-feldspar. K-feldspar is not present in manyashes of Japan during the late Quaternary. This isbecause the magma which produced B-Tm ash wasstrongly alkaline. Such alkaline-rock series are characteristic of volcanism on the continent, and have notoccurred in many volcanoes of the Japanese Islands.
Therefore, the presence of such minerals must be
strong evidence for identifying Toya and B-Tm ash.
Even if the same minerals are contained in the ash,mineral chemistry might be also useful for identifying
each ash. This is because chemical compositions ofminerals strongly depend on melt compositions and/or
magmatic temperatures. For example, the Fe/Mg
ratios of olivine and orthopyroxene reflect the ratios inthe magma. Thus, if melts of two volcanoes have
similar Fe/Mg ratios, and if the melts can be distinguished by other elements, the Fe/Mg ratios of theseminerals must be similar. Thus, although mineralchemistry might be useful for identifying ash, it would
not be easier than using glass chemistry.
Even if major elements in minerals are similar,
trace elements are often available to identify the ash.

49

Using a Mn/Mg – Al diagram, Tomiya (1995) suggested that the Ti-magnetite of each historical eruptionof the Usu volcano from 1640 to 1977 shows distinct
compositional variations in the diagram, and that
eruptive materials can identify eruption age by using
the diagram. On the basis of systematic compositional variations in T-magnetite, Tomiya et al. (2002)
indicated that the initial eruption of a series of
eruptions of the Usu volcano during 2000 was causedby new magma, which was distinct from any otherhistoric magmas.
5.2 Implications for study of magma genesis
Besides identification of ash, mineral chemistry
could provide important information on magma genesis, because the assemblage and chemistry of minerals
depend on magma compositions and crystallization
conditions (temperature, pressure, etc.). As mentioned before, Toya tephras contain minerals with uniquecompositions. For example, although albite plagioclase is most dominant in Toya ash, more calcic
plagioclase is often recognized in the ash. These
calcic and albitic plagioclases could not coexist in
equilibrium. Figure 5 shows histograms of plagioclase from three pumices in the same pyroclastic flow.

Fig. 5 Histograms for plagioclase phenocrysts from three pumice clasts of a Toya pyroclastic flow.
An mole %; 100*Ca/(Ca+Na+K)

50

M. NAKAGAWA and T. OHBA

According to whole-rock chemistry of these pumices,
the compositional distributions of the plagioclase
differ. More felsic pumice contains albite plagioclase
only, whereas chemical compositions of plagioclase in
more mafic pumices range from An (100*Ca/(Ca+Na+K)) ~90 to ~9. Such wide variations could not be
achieved by crystallization from a single magma.
This strongly suggests that mixing between two
magmas with different chemistries and temperatures
occurred during the Toya eruptions.
Equilibrium magmatic temperature can be calculated using mineral chemistry. In this section, weshow an example from the 1739 Tarumai eruption(Fig. 2).
The eruptive materials contained bothT-magnetite and ilmenite. Compositional variationsof Mn/Mg ratios are wide in Ti-magnetite, whereas
they are narrow in ilmenite (Fig. 6). Equilibrium
relationships between both minerals can be checkedby the relationship of the Mn/Mg ratio of the minerals(Bacon and Harshmann, 1994). Ilmenite can coexist
with type 1 Ti-magnetite at low Mn/Mg ratios,
whereas other types of Ti-magnetite cannot coexist
with these ilmenites (Fig. 7). In summary, there exist
more than two types of Ti-magnetite in the 1739

Fig. 6

Tarumai eruptive materials. The Ti-magnetite andilmenite in equilibrium indicate a magmatic temperature of ~900°C whereas the other Ti-magnetites withhigher Mn/Mg ratios must be derived from anothermagma having a much higher magmatic temperature.
This temperature cannot be calculated, because there
is no ilmenite coexisting the Ti-magnetite in equilibrium. Thus, on the basis of the compositional relationship between Ti-magnetite and ilmenite, it can be concluded that the eruptive materials of the 1739 Tarumai
eruption are products of mixing, and that the magmatic temperature of one of end-member magma witha lower temperature was about 900°C.

6. Summary
Volcanic ash is fragments of magma, and consistsof glass and minerals. Thus, both glass and minerals
can provide information on the magma. Chemicalcompositions of these glasses and minerals have been
determined not only to correlate ashes from different
areas but also to identify their source volcanoes. In
addition to these purposes, melt and minerals can also
provide a record of magmatic and eruption processes

Histograms of Mg/Mn ratio of Ti-magnetite and ilmenite phenocrysts of pumice of the AD 1739 eruption of
the Tarumai volcano. In the case of Ti-magnetite, each histogram corresponds to each pumice. The histogramof ilmenite includes minerals in four pumices.

Minerals in Volcanic Ash 1: Primary Minerals and Volcanic Glass

Fig. 751

Mg/Mn ratios between Ti-magnetite and ilmenite. If both minerals are in equilibrium, the ratios of theminerals are plotted near the solid line in the figure.

just before and/or during eruption. Thus it should
be noted that petrological and geochemical studies ofglass and minerals in volcanic ash must be developed,
in addition to tephrochronological study.

Acknowledgements
The authors would like to express their appreciation Prof. M. Taniguchi for inviting us to contribute tothis special issue. We also thank many persons who
have studied volcanic ash with us for a long time: Y.
Katsui, S. Aramaki, T. Ui, K. Wada, N. Miyaji, C. A.
Feebrey, M. Yoshida, M. Okuno, S. Miyamoto, R.
Furukawa, Y. Ishizuka, M. Yoshimoto, M.
Miyasaka-Amma, R. Takahashi, E. Ishii and A.
Matsumoto.
Without their valuable comments,
collaboration, encouragements and continuous
discussion, our studies of volcanic ash could not be
performed.
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