International Workshop on the mechanism of Phreatic Eruption

Petrological and geochemical analysis on non-juvenile lithics fromphreatic/hydrothermal eruptions
Tsukasa Ohba1, Takumi Imura1, Syahreza Angkasa2
1 Akita University, 2 Pertamina University
E-mail: t-ohba(at)gipc.akita-u.ac.jp

Terminology
Suijoki-bakuhatsu 水蒸気爆発 and Suijoki-Funka 水蒸気噴火 are the terms used for nonjuvenile eruption in the community of Japanese volcanologists, and the term phreatic eruption
is preferred as the translated word, although Suijoki-bakuhatsu 水 蒸 気 爆 発 includes
phreatic, hydrothermal, and even ultravulcanian (or gas) eruptions. Here, steam-blast
eruption is used as the translation for Suijoki-bakuhatsu because of the similarity to theoriginal word (Suijoki is steam, bakuhatsu is blast, and Funka is eruption).
Eruption derived from hydrothermal system beneath Akita-Yakeyama and Ontake, Japan
Non-juvenile ash from steam-blast eruptions are often intensely altered. Alteration types ofthe ash from the Akita Yakeyama 1997 eruption and the Ontake 2014 eruption were
petrologically determined to investigate chemical characteristics of volcanic fluid under thecraters (Ohba et al. 2007; Minami et al., 2016). Various types of alteration types are identified
for each ash deposit, and these assemblages are similar to the hydrothermal alteration zones
in geothermal fields and around epithermal-porphyry ores. Well established geologic models
under volcanoes proposed by economic geologists (e.g., Silitoe, 2010) can be applied tothese active volcanoes. Boiling of the hydrothermal fluid in the porphyry-system-like volcanohydrothermal system is the origin of these eruptions (hydrothermal eruptions).
Brief acid alteration recorded in ash from Tokachi, Japan
The petrological analysis was carried out for volcanic ash from the steam-blast eruptions
at Tokachi volcano, Hokkaido. The non-juvenile lithic ash consists of unaltered, partiallyaltered, and completely altered rocks. Alteration mineralogy is monotonous, consisting ofsilica mineral, alunite, and minor kaolinite. Partial alteration with thin alteration film (<100micrometer) is the most dominant in the ash, implying brief reaction with a flush of acid fluid(volcanic gas/fluid). The volcano is characterized by high frequency of magma intrusion(intervals of 20-40 years); this probably results in an unstable magmatic-gas-dominantsystem under the crater.

International Workshop on the mechanism of Phreatic Eruption

Magma intrusion into acid hydrothermal system beneath the crater at Tangkuban Parahu,
Indonesia
Geological study and petrological analysis was carried out for volcanic ash from the steamblast eruptions at Tangkuban Parahu volcano, Indonesia (Angkasa et al.,2019). Ten units ofsteam-blast eruptions were identified as Holocene volcanic ash deposits. All the samples areabundant in hydrothermally altered lithics and the alteration mineralogy exclusively showssiliceous alteration. Samples from two units contain abundant fresh juvenile fragments. Thisimplies the origin of steam-blast eruptions, or hydrothermal eruption, is basically shallow
acidic hydrothermal system, and intruding magma occasionally interacts with thehydrothermal system to result in magmatic-hydrothermal eruptions.
Preliminary report on sulfur isotopic study
We have measured sulfur isotopic compositions of the volcanic ash samples. Expected
δ34S values of ash are (1) bimodal for hydrothermal eruption (as high as +20 in sulfate andlow in sulfide) and (2) ca. zero (up to +10) for ultravulcanian. The volcanic ash from Ontake
shows the bimodal values, indicating isotopic fractionation between sulfate and sulfide inhydrothermal system. On the other hand, the samples from Tokachi and Tagkuban Parahu
show lower δ34S values from sulfate (0 - +10), implying magmatic sulfur is dominant.
Stratigraphic variation in δ34S at Tanguban Parahu indicates that after a big magmatic
contribution to eruption, the δ34S dropped from +10 to +3. Assumed the magmatic gas has
the value around +3, the values around +10 of early hydrothermal eruptions may be themixing resultant of steam-heated water with high δ34S (ca. +20) and the later eruptions areless influenced by the isotopically fractionated hydrothermal fluid. Alternatively, the isotopic
composition of magmatic gas might have changed from high to the low value. δ34S values ofmagma (or fresh volcanic rock) from the volcanoes must be measured to compare the values
from these steam-blast eruptions.
Main References
Ohba T et al. (2007) Mud plumbing system of an isolated phreatic eruption at Akita Yakeyama
volcano, northern Honshu, Japan, JVGR, 146, 35-46
Minami Y et al. (2016) Mineralogical study on volcanic ash of the eruption on September 27,
2014 at Ontake volcano, central Japan: correlation with porphyry copper systems, EPS,
161 35-46.
Angkasa S et al. (2019) Tephra-stratigraphy and Ash Componentry Studies of Proximal
Volcanic Products at Mount Tangkuban Parahu, Indonesia: An Insight to Holocene
Volcanic Activity IJOG, in press