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Minerals in Volcanic Ash 1:
Primary Minerals and Volcanic Glass
MITSUHIRO NAKAGAWA1 and TSUKASA OHBA2
1Division of Earth and Planetary Sciences, Graduate School of Science, Hokkaido University
Kita-10 Nishi-8, Kita-ku, Sapporo, 060-0810 Japan
e-mail: nakagawa@ep.sci.hokudai.ac.jp
2Department of Mineralogy, Petrology and Economic Geology
Graduate School of Science, Tohoku University
Aramaki Aza Aoba, Aoba-ku, Sendai, 980-8578 Japan
e-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 chemical
compositions of magma usually show distinct features in each volcano, and because assemblages and
compositions of minerals reflect their host melt. Using assemblages and chemical compositions of
minerals 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 and
eruption 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 portions
of these materials are called volcanic ash, and are dis-
tributed over a wide area far from the source volcano
according to the dominant wind direction above the
volcano. 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 sus-
pended in the melt. Many petrological and geoche-
mical studies of volcanoes all over the world have
revealed that chemical compositions of magma can be
distinguished among volcanoes. This has been used
in classifying regional magma types (e.g. LeBas,
1988). In addition, comparing magmas of adjacent
volcanoes, the magma of each volcano could be
distinguished from that of the neighboring volcano
(e.g., Sakuyama, 1983). Thus, volcanic ash can be
found 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 in
volcanic ash, and review their scientific applications.
We mention variations of magma types and represen-
tative phenocrystic minerals in magma. We also
introduce methods of analyzing minerals and glass in
the ash. We present several examples of correlation
of volcanic ash over a wide area. In conclusion, we
show several examples of research using minerals and
glass 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 a
mixture of melt and mineral crystal. In many cases,
the minerals crystallize in the magma during the
ascent to the surface due to cooling of the magma.
Chemical compositions of magma vary according to
both chemical compositions and mineral assemblage
of source materials. Also, magma varies in terms of
conditions of melting, such as degree of melting,
pressure, volatile components, etc. Integrated geo-
chemical and petrological studies have revealed that
there exist variations in magma types corresponding to
regional 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 compo-
sitional variations, there exist two types of mecha-
nisms for compositional variations of magmas; varia-
tions in primary magmas and variations due to
differentiation of a primary magma. In Figure 1, the
former is variations in alkali and K2O content at
similar SiO2 content. These variations are mainly
derived from diversity of primary magmas. On the
other hand, the latter variations are produced by
differentiation 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 differen-
tiation processes. These processes usually modify
the primary magma toward felsic compositions, as in
Fig. 1.
Many explosive eruptions have been caused by
felsic magma, and have produced widespread volcanic
ash. Thus, in the case of non-alkaline rocks, magma
types from explosive eruptions are usually dacite and
rhyolite. Explosive eruptions by more mafic magma
often produce tephra. In the case of Japan (Fig. 2),
for example, the Fuji, Iwate and Izu-Oshima vol-
canoes have produced basaltic tephra. However,
these basaltic tephras are not as widely distributed
compared with those of the same scale (magmatic
volume) 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 (pheno-
crysts). Thus, in order to determine melt composi-
tions during eruption, we have to subtract the com-
positions of minerals from the whole-rock
compositions. Table 1 shows examples of chemical
compositions of magmatic melt (volcanic glass) found
in the Tyatya volcano (Fig. 2). In many cases, melt
Fig. 1 (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 Glass 43
compositions are less silicic than those of the
whole-rock composition. For example, although the
eruptive products of historic eruptions of the
Hokkaido-Komagatake volcano (Fig. 2) are andesitic
(SiO2=60%), their melt compositions are rhyolitic
(Table 1) (Kozu, 1934).
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 of
transition 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.
Sample Ty-136 Ty-134 Ty-42 Ty-133 Ty-138 Ty-170
Analysis No. 136-1 134-9 42-5 133-6 138-1 170-6
Tephra - Ko-c1 Ko-c2 Ta-a Ma-b B-Tm
Source Tyatya Koma. Koma. Tarumai Mashu Baito.
Age ? 1856 AD 1669 AD 1739 AD ca. 1 ka ca. 969 AD
(wt%)
SiO2 53.07 75.16 77.08 76.88 73.91 72.15
TiO2 1.55 0.44 0.33 0.23 0.66 0.21
Al2O3 13.40 11.85 11.90 12.03 12.72 8.92
total FeO 12.68 1.94 1.75 1.77 3.03 3.73
MnO 0.23 0.08 0.01 0.08 0.08 0.00
MgO 4.53 0.36 0.38 0.29 0.64 0.01
CaO 9.07 2.16 1.89 1.83 3.33 0.23
Na2O 2.79 3.25 3.44 3.57 4.27 5.00
K2O 0.89 1.79 1.75 2.32 0.72 4.70
Total 98.19 97.02 98.52 98.98 99.36 94.95
(Abbreviations: Koma., Hokkaido-Komagatake; Baito., Baitoushan)
44 M. NAKAGAWA and T. OHBA
the colorless minerals do not contain transition metal
ions as a major constituent. Besides the silicates
minerals, Fe-Ti oxide minerals are also contained in
almost all magmas. Table 2 summarizes major
rock-forming minerals occurring in volcanic ash.
Major colored minerals in widespread tephra are
Table 2 Representative rock-forming minerals in volcanic ash.
Group minerals major
end-member
Chemical formula* Solid solution Remarks
Feldspar group
Plagioclase between An and Ab
Anorthite (An) CaAl2Si2O8
Albite (Ab) NaAlSi3O8
Alkali feldpar between pottasium
feldspar and Ab
Sanidine is
common
potasium feldspar KAlSi3O8
Silica minerals
Quartz SiO2
Olivine group
Olivine between Fo and Fa
Forstelite (Fo) Mg2SiO4
Fayalite (Fa) Fe2+
2SiO4
Pyroxene group
Orthopyroxene between En and Fs
Enstatite (En) MgSiO3
Ferrosilite (Fs) Fe2+SiO3
Clinopyroxene among En, Fs and
Wo (Wo<50 mole%)
Augite is
common
Wollastonite CaSiO3
Amphibole
group
Hornblende among Act, Ed, Prg
and Ts
AlIV>0.8
Actinolite (Act) Ca2(Mg, Fe2+)5Si8O22(OH)2
Edenite (Ed) NaCa2(Mg, Fe2+)5AlSi7O22(OH)2
Pargasite (Prg) NaCa2(Mg, Fe2+)4Al3Si6O22(OH)2
Tschermakite (Ts) Ca2(Mg, Fe2+)3Al4Si6O22(OH)2
Cummingtonite (Mg, Fe2+)7Si8O22(OH)2
Mica group
Biotite among Ann, Sdp, Phl
and Est
Relatively
Mg-poor
Annite (Ann) K2Fe2+
6Al2Si6O20(OH)4
Siderophyllite
(Sdp)
K2Fe2+
5Al4Si5O20(OH)4
Phlogopite (Phl) K2Mg6Al2Si6O20(OH)4
Eastonite (Est) K2Mg2+
5Al4Si5O20(OH)4
Spinel group
Ti-magnetite between Mag and
Ulv
Magnetite (Mag) Fe2+Fe3+
2O4
Ulvospinel (Ulv) Fe2+
2TiO4
Ilmenite Fe2+TiO3 include Fe3+
2O3
component
* major elements, and trace elements are not included.
Minerals in Volcanic Ash 1: Primary Minerals and Volcanic Glass 45
pyroxene, amphibole and mica groups. In addition,
olivine is often recognized in tephra. Major colorless
ones 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-
postions of these minerals vary. This is because
many rock-forming minerals are a solid solution,
which can exchange ions in the crystal structure of
various 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 other
in 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
So urc e Toya Toya Toy a Tarum a i Toy a Toy a
Mineral Clinopyroxene Orthopyroxene Hornblende Olivine Plagioclase Plagioclase
(wt%)
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 13.86 21.78 14.45 37.05 0.00 0.01
CaO 20.93 0.97 10.99 0.15 2.32 16.44
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 99.32 100.64 100.36
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 0.019 0.038 0.076 0.009 ---- ----
Mg 0.775 1.210 3.138 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 3.999 3.993 15.393 3.002 4.988 4.996
Number of O 6 6 23 4 8 8
Fo 73.4
Mg/(Mg+Fe) 72.1 63.9 65.5
Wo 43.9 2.0
En 40.5 62.6
Fs 15.7 35.4
Or 3.0 0.1
An 10.9 80.4
Ab 86.1 19.5
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 % in
olivine (Table 3).
3. Analytical Methods
Constituents of volcanic ash are small in size, and
can 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 and
minerals. Thus, it has been widely accepted that
direct measurement of glass and mineral compositions
is essential for research of volcanic ashes. Chemical
compositions 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 an
electron-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 trace
elements (~several hundreds ppm), because it utilizes
a strong electron beam. However, such a strong
beam may damage the sample, especially glass and
hydrous minerals. In contrast, the latter type can be
applied to glass and hydrous minerals without serious
damage, because it does the analysis with a weak
electron beam. However, detection limits of
analyzed 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 case
of 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 ele-
ments, such as rare earth elements. In this case, sev-
eral 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 be
essential.
4. Application of Glass Chemistry in
Volcanic Ash Studies
In this section, we overview several examples of
research using chemical compositions of volcanic
glass in ash. The primary purpose of using volcanic
glass is to correlate volcanic ash, and to identify the
source volcano of the volcanic ash. Another
example shows that volcanic ash also contains a
record 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 of
giant 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 the
index 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 of
correlation by the refractive index of volcanic glass
and 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 been
correlated. In addition, the source volcanoes of these
layers have also been identified by
tephrochronological and petrological studies of these
layers. These results are reviewed by Machida and
Arai (1992).
Since the 1980s, in addition to measurement of
refractive index, chemical compositions of volcanic
glass determined by EPMA have been adopted for
correlation 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 from
different volcanoes. In addition, Tokui (1984) and
Okumura (1980) summarized many tephras in eastern
Hokkaido, some of which were derived from large
eruptions of volcanoes in southwestern Hokkaido dur-
ing 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 volcanic
ash, 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) summar-
ized the eruption history of the Hokkaido-Komagatake
volcano. However, on the basis of field observations
at proximal sites, Hayakawa (1991) suggested that the
so-called AD 1694 ash of the volcano was the product
not of a distinct eruption but of a later phase of the
1640 eruption, and pointed out that the occurrence of
the 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 of
which have volcanic glass with chemical composi-
tions derived from Hokkaido-Komagatake. The
Minerals in Volcanic Ash 1: Primary Minerals and Volcanic Glass 47
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 a
later phase of the 1640 eruption.
If the source volcano of ash and its age can be
identified, the ash becomes a possible time marker to
construct a stratigraphy for the area. Nakagawa et al.
(2002) investigated geological studies of the Tyatya
volcano, southern Kuril Islands, and constructed a
stratigraphic relationship among the eruptive products
of the volcano. They also found several widespread
ash layers among these eruptive products. Using
chemical compositions of the volcanic glass of these
ashes, 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 eruptions of
Hokkaido-Komagatake, the 1739 of Tarumai, and
eruption around 1 ka of the Mashu and Baitoushan (at
the 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 erup-
tion of the Tyatya volcano.
4.2 Magma variations and eruption processes
In the case of correlation of volcanic ash, average
chemical compositions of volcanic glasses have been
used in many cases. However, there exist possible
compositional variations in glass chemistry beyond
analytical error. In this section, we introduce our on-
going research of the Baitoushan volcano to show that
glass chemistry of ash can provide important informa-
tion 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 erup-
tion occurred around the 10th century and produced
widespread tephra covering northern Japan (B-Tm
tephra; Machida and Arai, 1984). Chemical composi-
tions of glasses of the B-Tm tephra have been
determined by many authors. According to these
data, the glass of the tephra shows wide compositional
variations, for example in the SiO2-K2O diagram.
However, these variations have not been emphasized
yet.
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 erup-
tion 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 also
collect 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 for
the ash are the same as previously reported variations
in B-Tm tephra (Machida and Arai, 1992). In
addition, compositional differences in the ash also
exist among the three parts. Taking the mineral
effects in the proximal deposits into consideration,
compositional variations in proximal deposits are
similar to those in distal deposits. Moreover, the
compositional 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 compositional
variations in glass from B-Tm tephra are not due to
analytical error but possible variations in melt
compositions. The variations in the 10th century
eruptive products may reflect variations of magma
types in the magma chamber(s) just before the
eruption. Although detailed magmatic processes
during the 10th century eruption will be submitted to
another journal, we would like to emphasize that
compositional 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 of
albite (Na-rich plagioclase: An~10), eulite (Fe-rich
orthopyroxene: En~25) and cummingtonite (mineral
of the amphibole group). These minerals and their
chemical compositions are not common in other
Fig. 4 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 49
volcanic ashes in Japan during the late Quaternary
(Machida and Arai, 1992). B-Tm ash from the
Baitoushan volcano is also characterized by the pre-
sence K-feldspar. K-feldspar is not present in many
ashes of Japan during the late Quaternary. This is
because the magma which produced B-Tm ash was
strongly alkaline. Such alkaline-rock series are char-
acteristic of volcanism on the continent, and have not
occurred 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 of
minerals strongly depend on melt compositions and/or
magmatic temperatures. For example, the Fe/Mg
ratios of olivine and orthopyroxene reflect the ratios in
the magma. Thus, if melts of two volcanoes have
similar Fe/Mg ratios, and if the melts can be distin-
guished by other elements, the Fe/Mg ratios of these
minerals must be similar. Thus, although mineral
chemistry 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.
Using a Mn/Mg – Al diagram, Tomiya (1995) sug-
gested that the Ti-magnetite of each historical eruption
of 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 composi-
tional 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 caused
by new magma, which was distinct from any other
historic magmas.
5.2 Implications for study of magma genesis
Besides identification of ash, mineral chemistry
could provide important information on magma gene-
sis, because the assemblage and chemistry of minerals
depend on magma compositions and crystallization
conditions (temperature, pressure, etc.). As mention-
ed before, Toya tephras contain minerals with unique
compositions. For example, although albite plagio-
clase 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 plagio-
clase 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 calcu-
lated using mineral chemistry. In this section, we
show an example from the 1739 Tarumai eruption
(Fig. 2). The eruptive materials contained both
T-magnetite and ilmenite. Compositional variations
of Mn/Mg ratios are wide in Ti-magnetite, whereas
they are narrow in ilmenite (Fig. 6). Equilibrium
relationships between both minerals can be checked
by 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
Tarumai eruptive materials. The Ti-magnetite and
ilmenite in equilibrium indicate a magmatic tempera-
ture of ~900°C whereas the other Ti-magnetites with
higher Mn/Mg ratios must be derived from another
magma having a much higher magmatic temperature.
This temperature cannot be calculated, because there
is no ilmenite coexisting the Ti-magnetite in equilib-
rium. Thus, on the basis of the compositional relation-
ship between Ti-magnetite and ilmenite, it can be con-
cluded that the eruptive materials of the 1739 Tarumai
eruption are products of mixing, and that the mag-
matic temperature of one of end-member magma with
a lower temperature was about 900°C.
6. Summary
Volcanic ash is fragments of magma, and consists
of glass and minerals. Thus, both glass and minerals
can provide information on the magma. Chemical
compositions 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
Fig. 6 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 histogram
of ilmenite includes minerals in four pumices.
Minerals in Volcanic Ash 1: Primary Minerals and Volcanic Glass 51
just before and/or during eruption. Thus it should
be noted that petrological and geochemical studies of
glass and minerals in volcanic ash must be developed,
in addition to tephrochronological study.
Acknowledgements
The authors would like to express their apprecia-
tion Prof. M. Taniguchi for inviting us to contribute to
this 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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Fig. 7 Mg/Mn ratios between Ti-magnetite and ilmenite. If both minerals are in equilibrium, the ratios of the
minerals are plotted near the solid line in the figure.
