Links and Gaps between
Volcanic Gas and Geothermal Fluids
SHINOHARA Hiroshi(Geological Survey of Japan, AIST)

Hedenquist and Lowenstern (1996)

Origin of various hot spring types:
Acid HCl-SO4 type to Neutral NaCl type
Volcanic gas: Acid, High S/Cl
Condensation
HCl-H2SO4-type: Acid, High S/Cl
Neutralization+ S-mineral deposition
NaCl-type: Neutral, Low S/Cl

Giggenbach (1996), Ohba et al. (1995)

Origin of various hot spring types:
Acid HCl-SO4 type to Neutral NaCl type
Volcanic gas: Acid, High S/ClCondensation
HCl-H2SO4-type: Acid, High S/Cl
Neutralization+ S-mineral deposition
NaCl-type: Neutral, Low S/Cl

Giggenbach (1996), Ohba et al. (1995)

Volcanic Hydrothermal SystemHedenquist and Lowenstern (1996)

HCl/KCl decrease with T-decrease:
Neutralization with cooling
Giggenbach (1996)

HCl/NaCl increase with P-decrease

Shinohara and Fujimoto (1994)

Low P: HCl-rich Fluid
High P: HCl-poor Fluid

Magma degassing: HCl/NaCl VS Pressure
Andesitic melt, 1250°C

Botcharnikov et al. (2015)

Low P: HCl-rich Acid Fluid
High P: HCl-poor Neutral (?) Fluid

Acid Volcanic Gas vsNeutral Hydrothermal Fluid

Shallow Fluids
Acid

Deep Fluids
Not Acid
Neutral?
Weak Acid?

Hedenquist and Lowenstern (1996)

Origin of various hot spring types:
Acid HCl-SO4 type to Neutral NaCl type
Volcanic gas: Acid, High S/Cl
Condensation
HCl-H2SO4-type: Acid, High S/Cl
Neutralization+ S-mineral deposition
NaCl-type: Neutral, Low S/Cl

Giggenbach (1996), Ohba et al. (1995)

Average volcanic gas compositionof volcanoes in Japan
High flux High-T gas
Low flux Hight-T gas
Average
JT: This study
JF: Fischer, 2008
Shinohara (2013)

SU: Suwanosejima, MI: Miyake,
AM:Asama, SI:Satsuma-Iwojima,
AS:Aso, TK: Tokachi,
SK:Sakurajima, UN: Unzen,
US:Usu,SS: Showa-Shinzan

S/Cl mol ratio
High flux High-T gas= 2〜20
Japan Average = 6Asama

SakurajimaGlobal variation of volcanic gas composition
VU: Vulcano, ST: Stromboli, GA: Galeras, WI: White Island,
MM: Momotombo, MR: Merapi, MS: Masaya, YS: Yasur,
GO: Gorely, VI: Villarrica, KU: Kudryavy
Global Average
GT: This study
GF: Fischer, 2008
GH: Hilton et al., 2003
GW: Wallace, 2005

Shinohara (2013)S/Cl mol ratio
Global Average = 3
Japan Average = 6Etna, Italy

Villarrica, Chile

Cl-S content in Arc Magma Melt Inclusions
Shinohara (2008)

S/ClVolcanic Gas = 3-6
Basalt Magma = 2Dacite Magma = 0.2
Rhyolite Magma = 0.1

S/Cl ratio in magma <S/Cl ratio in volcanic gas
Volcanic Gas: Japan Average
Volcanic Gas: Global AverageCl-rich Magma vs S-rich Volcanic Gas
Low S solubility:
Effective degassing
High Cl solubility:
Cl remaining in magmas
Lowsolubility

S/Cl ratio in magma <S/Cl ratio in volcanic gasHighVolcanic Gas: Japan Average
Volcanic Gas: Global AverageMagma degassing:Volcanic VS Hydrothermal
Volcanic Gas
Decompression driven degassing (Magma ascent)
H2O, CO2, S :Low solubility
effective degassing
Cl: High solubility: remaining in melt
Deep hydrothermal fluid
Crystallization driven degassing
All volatiles expelled from magma(Little volatiles in magmatic minerals)
Crystallization

Chelle-Michou et al. (2017)

Volatiles in Igneous Rocks
Cl remains in lavas by low-P degassing

Dacite Magma

Glassy Lavas

Plutonic Rocks

H2O ~5wt.%
CO2 1000ppm
S500ppm
Cl 3000ppmH2O 0.5wt.%
0ppm
CO2S50ppm
Cl 2800ppm

H2O ~1wt.%
CO20ppm
S300ppm
Cl300ppm

Shallow degassing
Volcanic Gas

Deep degassing
Hydrothermal Fluids

H2O

CO2S

Cl

S/Cl

Volcanic Gas

98.3

0.9

0.6

0.2

2.4

Hydrothermal Fluid

95.4

1.0

0.3

3.4

0.1

(mol%)

S-rich Volcanic gas vs
Cl-rich Hydrothermal fluids
S/Cl=3
Volcanic Gas
S/Cl=0.5
Crater Lake

S/Cl<0.01
Neutral NaCl

S/Cl= 0.1??
DeepHydrothermal
Fluid?

S/Cl=0.1
Porphyry Mo
Fluid Inclusion(Audétat, 2015)

S/Cl=0.1~2
Deep Magma
Hedenquist and Lowenstern (1996)

Summary
Volcanic Gas
VS Geothermal Fluid
Acid & S-rich
VS Neutral & Cl-richClassic concept
Neutralization + S-mineral deposition with T-decrease
Other factors
P-dependence
Low P:Acid Gas
VS High-P:Neutral Fluid
Magma degassing
Low P: S-rich degassing VS Cl-rich degassing byCrystallization

Summary: Research Target
Review of S and Cl origins in various fluids
in a volcanic hydrothermal system(isotopes, S/Cl, Cl/Br/I ratios)

Fluid flow modelling based on deep
resistivity structure survey(Deep structure of Acid Cl-SO4 Spring)Characterization of deep hydrothermal fluid (pH, S/Cl ratios)
Sampling: Fluid inclusions or magma drilling
Magma degassing experiment:
pH of S-containing fluid?
Hedenquist and Lowenstern (1996)