TY - JOUR
T1 - Isotope systematics of Icelandic thermal fluids
AU - Stefansson, Andri
AU - Hilton, David R.
AU - Sveinbjornsdottir, Arny E.
AU - Torssander, Peter
AU - Heinemeier, Jan
AU - Barnes, Jaime D.
AU - Ono, Shuhei
AU - Halldorsson, Saemundur Ari
AU - Fiebig, Jens
AU - Arnorsson, Stefan
PY - 2017/5/1
Y1 - 2017/5/1
N2 - Thermal fluids in Iceland range in temperature from < 10 °C to > 440 °C and are dominated by water (> 97 mol%) with a chloride concentration from < 10 ppm to > 20,000 ppm. The isotope systematics of the fluids reveal many important features of the source(s) and transport properties of volatiles at this divergent plate boundary. Studies spanning over four decades have revealed a large range of values for δD (− 131 to + 3.3‰), tritium (− 0.4 to + 13.8 TU), δ
18O (− 20.8 to + 2.3‰),
3He/
4He (3.1 to 30.4 R
A), δ
11B (− 6.7 to + 25.0‰), δ
13C
∑ CO
2
(− 27.4 to + 4.6‰),
14C
∑ CO
2
(+ 0.6 to + 118 pMC), δ
13C
CH
4
(− 52.3 to − 17.8‰), δ
15N (− 10.5 to + 3.0‰), δ
34S
∑ S
− II
(− 10.9 to + 3.4‰), δ
34S
SO
4
(− 2.0 to + 21.2‰) and δ
37Cl (− 1.0 to + 2.1‰) in both liquid and vapor phases. Based on this isotopic dataset, the thermal waters originate from meteoric inputs and/or seawater. For other volatiles, degassing of mantle-derived melts contributes to He, CO
2 and possibly also to Cl in the fluids. Water-basalt interaction also contributes to CO
2 and is the major source of H
2S, SO
4, Cl and B in the fluids. Redox reactions additionally influence the composition of the fluids, for example, oxidation of H
2S to SO
4 and reduction of CO
2 to CH
4. Air-water interaction mainly controls N
2, Ar and Ne concentrations. The large range of many non-reactive volatile isotope ratios, such as δ
37Cl and
3He/
4He, indicate heterogeneity of the mantle and mantle-derived melts beneath Iceland. In contrast, the large range of many reactive isotopes, such as δ
13C
∑ CO
2
and δ
34S
∑ S
− II
, are heavily affected by processes occurring within the geothermal systems, including fluid-rock interaction, depressurization boiling, and isotopic fractionation between secondary minerals and the aqueous and vapor species. Variations due to these geothermal processes may exceed differences observed among various crust and mantle sources, highlighting the importance and effects of chemical reactions on the isotope systematics of reactive elements.
AB - Thermal fluids in Iceland range in temperature from < 10 °C to > 440 °C and are dominated by water (> 97 mol%) with a chloride concentration from < 10 ppm to > 20,000 ppm. The isotope systematics of the fluids reveal many important features of the source(s) and transport properties of volatiles at this divergent plate boundary. Studies spanning over four decades have revealed a large range of values for δD (− 131 to + 3.3‰), tritium (− 0.4 to + 13.8 TU), δ
18O (− 20.8 to + 2.3‰),
3He/
4He (3.1 to 30.4 R
A), δ
11B (− 6.7 to + 25.0‰), δ
13C
∑ CO
2
(− 27.4 to + 4.6‰),
14C
∑ CO
2
(+ 0.6 to + 118 pMC), δ
13C
CH
4
(− 52.3 to − 17.8‰), δ
15N (− 10.5 to + 3.0‰), δ
34S
∑ S
− II
(− 10.9 to + 3.4‰), δ
34S
SO
4
(− 2.0 to + 21.2‰) and δ
37Cl (− 1.0 to + 2.1‰) in both liquid and vapor phases. Based on this isotopic dataset, the thermal waters originate from meteoric inputs and/or seawater. For other volatiles, degassing of mantle-derived melts contributes to He, CO
2 and possibly also to Cl in the fluids. Water-basalt interaction also contributes to CO
2 and is the major source of H
2S, SO
4, Cl and B in the fluids. Redox reactions additionally influence the composition of the fluids, for example, oxidation of H
2S to SO
4 and reduction of CO
2 to CH
4. Air-water interaction mainly controls N
2, Ar and Ne concentrations. The large range of many non-reactive volatile isotope ratios, such as δ
37Cl and
3He/
4He, indicate heterogeneity of the mantle and mantle-derived melts beneath Iceland. In contrast, the large range of many reactive isotopes, such as δ
13C
∑ CO
2
and δ
34S
∑ S
− II
, are heavily affected by processes occurring within the geothermal systems, including fluid-rock interaction, depressurization boiling, and isotopic fractionation between secondary minerals and the aqueous and vapor species. Variations due to these geothermal processes may exceed differences observed among various crust and mantle sources, highlighting the importance and effects of chemical reactions on the isotope systematics of reactive elements.
KW - Iceland
KW - Isotopes
KW - Thermal fluids
KW - Volatiles
KW - SUBMARINE HYDROTHERMAL VENTS
KW - DIVERGENT PLATE BOUNDARIES
KW - GEOTHERMAL SYSTEMS
KW - CARBON-ISOTOPE
KW - MANTLE PLUME
KW - SUBGLACIAL BASALTS
KW - LAKAGIGAR ERUPTION
KW - HELIUM-ISOTOPES
KW - REDOX REACTIONS
KW - TRACE-ELEMENTS
UR - http://www.scopus.com/inward/record.url?scp=85016733612&partnerID=8YFLogxK
U2 - 10.1016/j.jvolgeores.2017.02.006
DO - 10.1016/j.jvolgeores.2017.02.006
M3 - Journal article
SN - 0377-0273
VL - 337
SP - 146
EP - 164
JO - Journal of Volcanology and Geothermal Research
JF - Journal of Volcanology and Geothermal Research
ER -