SOURCES AND REACTIONS OF VOLATILES IN ICELANDIC THERMAL FLUID

Andri Stefánsson, Sæmundur A. Halldórsson, David Hilton, Árný E. Sveinbjörnsdóttir, Jaime D. Barnes, Shuhei Ono, Jan

Heinemeier, Jens Fiebig, Finnbogi Óskarsson, Peter Torssander, Stefán Arnórsson

Volatiles

• Volatiles are a group of elements and compounds that typically have low boiling points and are commonly found within planetary crusts and the atmosphere. • In thermal fluids, volatile elements include for example: H,

O, C, S, Cl, F, N, B, He and other noble gases• Use isotopes to understand sources and reactions• Sampling• Analysis• Isotope geochemical modeling

Sampling

Sampling

Analysis

Hekla 1980• Cl <10 to >20,000

ppm• 5-450°C• Hot springs, fumaroles

and wells

Thermal fluids

Hekla 1980• 5-450°C• Hot springs, fumaroles

and wells• S <1 to >1500 ppm

Thermal fluids

Hekla 1980• 5-450°C• Hot springs, fumaroles

and wells• S <1 to >1500 ppm

• CO2 <10 to >10,000 ppm

Thermal fluids

Hekla 1980• 5-450°C• Hot springs, fumaroles

and wells• S <1 to >1500 ppm

• CO2 <10 to >10,000 ppm

• Meteoric and seawater water source

• Dominated by H2O (>95 mol%)

Thermal fluids

• Trace sources and reactions of volatiles• Approach• Conceptual geological and

geochemical models• Data collection and

analysis• Simulations of chemical

and isotope behavior of the conceptual models• Comparison of the two

Isotopes of volatile elements

Sulfur isotopes and low-temperature SO4

• Multiple oxidation states (S-II to S+VI)

• Four stable isotopes (32S, 33S, 34S and 36S)

• SO4 in low-T water on the Southern Lowlands

• Three trends:• Meteoric water SO4

source• Water-rock interaction

and SO4 in rocks• Seawater mixing

Sulfur isotopes and low-temperature SO4

• Multiple oxidation states (S-II to S+VI)

• Four stable isotopes (32S, 33S, 34S and 36S)

• SO4 in low-T water on the Southern Lowlands

• Three trends:• Meteoric water SO4

source• Water-rock interaction

and SO4 in rocks• Seawater mixing

Hekla 1980

Sulfur isotopes and low-temperature SO4

Chlorine δ34S (sulfate)

Carbon in low- and high-temperature fluid

• Multiple oxidation states (C-IV to C+IV)

• Two stable isotopes (12C, 13C)

• Both as gas phase and liquid phase

• Fluid-rock interaction a major source of CO2

• Melt (mantle) CO2 input on- and off-rift

Carbon in low- and high-temperature fluid

• Multiple oxidation states (C-IV to C+IV)

• Two stable isotopes (12C, 13C)

• Both as gas phase and liquid phase

• Fluid-rock interaction a major source of CO2

• Melt (mantle) CO2 input on- and off-rift

Hekla 1980

Carbon in low- and high-temperature fluid

Helium isotopes

• 3He dominates the atmosphere where 4He is produced by decay of heavy elements in Earth

• He is non-reactive (as well as other noble gases) – can be used to look at source(s) and source(s) heterogeneity

• Values given relative to air, RA

• MORB is ~8• Higher values indicate

primordial mantle (early earth) input

RA

0 5 10 15 20 25 30

Count

0

20

40

60

80

100

Hekla 1980

Helium in low- and high-temperature fluid

Sæmi A. Halldórsson

Andri Stefánsson

Shuhei Ono

David R.Hilton

Árný E.Sveinbjörnsdóttir

Jaime D. Barnes

Jan Heinemeier Jens Fiebig Finnbogi Óskarsson

Peter Torssander

Stefán Arnórsson