Chlorine isotope geochemistry of Icelandic thermal fluids: Implications for geothermal system behavior at divergent plate boundaries

Stefánsson, Andri; Barnes, Jaime D.

doi:10.1016/j.epsl.2016.05.041

Cited by 22 publications

(18 citation statements)

References 42 publications

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“…Low concentrations of Cl in experimental fluids were caused by precipitation of minor amounts of salts (e.g., halite) in the very late stage of the boiling process. High concentrations of Cl and other volatile elements such as C and S in the IDDP-1 fluids could be attributed to minor magmatic degassing [48][49][50]. The experimental results thus support previous findings that supercritical IDDP-1 [51], and Watanabe et al, [52] using an average fluid mass flux of 10 -5 kg m -2 s -1 [54].…”

Section: Comparison Of Experimental and Modeling Results Withsupporting

confidence: 87%

“…Similar concentration trends were observed for the IDDP-1 supercritical and subcritical geothermal fluids at Krafla, Iceland, and have been predicted from geochemical modeling [31]. The experimental results further support findings that the supercritical IDDP-1 fluids likely form by conductive heating of subcritical geothermal fluids of meteoric origin, with minor input of magmatic gases [31,48,50]. Such fluids may be suitable for power production.…”

Section: Discussionsupporting

confidence: 82%

“…fluids likely form by conductive heating of subcritical geothermal reservoir fluids of meteoric water origin with minor input of magmatic gases [31,48,50]. Our experiments, in combination with geochemical modeling [31], revealed that varying the initial concentrations of volatile elements (C, S, and B) in the reservoir geothermal fluid does not affect the chemical composition of the supercritical fluid nor the alteration mineralogy, due to early partitioning of volatile elements into the vapor (Figure 8).…”

Section: Geofluidsmentioning

confidence: 74%

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Supercritical Fluid Geochemistry in Geothermal Systems

2019

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Supercritical fluids exist in the roots of many active high-temperature geothermal systems. Utilization of such supercritical resources may multiply energy production from geothermal systems; yet, their occurrence, formation mechanism, and chemical properties are poorly constrained. Flow-through experiments at 260°C and 400-420°C were performed to study the chemical and mineralogical changes associated with supercritical fluid formation near shallow magmatic intrusions by conductive heating and boiling of conventional subcritical geothermal fluids. Supercritical fluids formed by isobaric heating of liquid geothermal water had similar volatile element concentrations (B, C, and S) as the subcritical water. In contrast, mineral-forming element concentrations (Si, Na, K, Ca, Mg, and Cl) in the supercritical fluid were much lower. The results are consistent with the observed mineral deposition of quartz, aluminum silicates, and minor amount of salts during boiling. Similar concentration patterns have been predicted from geochemical modeling and were observed at Krafla, Iceland, for the IDDP-1 supercritical fluid discharge. The experimental results confirm previous findings that supercritical fluids may originate from conductive heating of subcritical geothermal reservoir fluids characterized by similar or lower elemental concentrations with minor input of volcanic gas.

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Section: Comparison Of Experimental and Modeling Results Withsupporting

confidence: 87%

Section: Discussionsupporting

confidence: 82%

Section: Geofluidsmentioning

confidence: 74%

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Supercritical Fluid Geochemistry in Geothermal Systems

2019

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“…Chlorine isotope results for several wells in Krafla, show δ 37 Cl ranging from 0.2 to 2.1‰, with an average of 0.9‰ [8]. No chlorine isotopes have been reported for well fluids from the Bjarnarflag field.…”

Section: Figmentioning

confidence: 97%

The origin of the warm groundwater near Lake Mývatn, NE Iceland, traced by stable isotopes

Óskarsson

2019

E3S Web Conf.

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The origin of the warm groundwater which feeds Lake Mývatn is unknown, but it has been affected by volcanic episodes as well as geothermal activity and utilisation. In this contribution stable isotopes of hydrogen (2H), oxygen (18O), sulphur (34S), chlorine (37Cl) and strontium (86Sr and 87Sr) in 20 groundwater and effluent samples from the Lake Mývatn area are used to constrain the origin of the warm groundwater. The results suggest that the warm groundwater is partly formed by mixing with geothermal effluent water and partly by mixing with geothermal steam.

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“…Previous work on the chlorine isotope composition of thermal waters (Yellowstone, western United States; Iceland; Indonesia; Cascadia, North American Pacific coast; Taupo Volcanic Zone, New Zealand) reports values ranging from ~−1‰ to ~+2‰, interpreted to reflect magmatic input and/or leaching of Cl from the host rock during water-rock interaction (Bernal et al, 2014;Cullen et al, 2015;Eggenkamp, 1994;Li et al, 2015;Stefánsson and Barnes, 2016;Zhang et al, 2004). Given the low temperature of the cold springs from the Hikurangi margin compared to these previous studies, it is likely that leaching of Cl from host rock is limited.…”

Section: Volatile Sourcementioning

confidence: 99%

The role of the upper plate in controlling fluid-mobile element (Cl, Li, B) cycling through subduction zones: Hikurangi forearc, New Zealand

et al. 2019

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In order to trace the cycling of fluid-mobile elements (FMEs) through subduction zone forearcs, we collected water samples from two warm and 16 cold springs along the subaerially exposed forearc of the Hikurangi subduction zone in New Zealand. Water samples were analyzed for their cation and anion concentrations, as well as their B, Li, Cl, and O stable isotope compositions. Fluids discharging through the prism have high concentrations of Cl (2400-16,000 mg/L), Br (6-70 mg/L), I (0.4-72 mg/L), Sr (0.1-200 mg/L), B (3-130 mg/L), Li (0.1-13 mg/L), and Na (33-6600 mg/L), consistent with data from previous studies. Most of these elements decrease overall in concentration from north to south, have a concentration peak in the central part of the margin, and have had limited concentration variability during the last three decades. Because Li, Cl, and B are all fluid-mobile elements, their incompatibility potentially limits modification by fluid-rock interaction, making them reliable tracers of fluid source. δ 37 Cl, δ 11 B, and δ 7 Li values range from −1.3‰ to +0.4‰ (n = 36), +11.8‰ to +41.9‰ (n = 25), and −3.1‰ to +29.0‰ (n = 29), respectively. Despite the change in concentrations along the margin, there is no corresponding trend in isotopic composition. Chlorine and boron isotope compositions are consistent with fluids dominated by seawater (δ 37 Cl = 0‰; δ 11 B = 40‰) and sedimentary pore fluids (δ 37 Cl ≈ −8‰ to 0‰; δ 11 B > ~17‰). Br/Cl (0.0025-0.005) and I/Cl (0.00005-0.007) weight ratios also support a dominant seawater and pore-fluid source. Lithium isotope data also suggest fluids sourced from seawater (δ 7 Li = +31‰) as well as dehydrating sediments and/or modified by interaction with local sediment. The fluid geochemical data cannot be explained by a change in the fluid source along the margin, but rather by a change in the upper-plate structural permeability. In the north, extension likely results in a highly permeable forearc, whereas transpression in the south traps fluids within the upper plate and dilutes saline fluids with groundwater. Such changes in upper-plate structural permeability may influence fluid pressure conditions within the forearc, which in turn may influence the observed change in slip behavior on the interface from north Hikurangi (aseismic creep) to south Hikurangi (deep locking). This work highlights the role the upper plate may play in the geochemical modification and transport of slab-derived fluids, and supports the long-standing but poorly documented assumption that seawater and pore fluids are expelled at shallow levels in the subduction zone (<15 km) and therefore play a limited role in the transport of FMEs to great depths in subduction zones. ■ INTRODUCTION Lithium, chlorine, and boron are all highly fluid-mobile elements (FMEs). Their incompatibility in rocks limits modification by fluid-rock interaction, thereby making them potentially excellent tracers of fluid source. Isotopic systems of all three elements have been used as a fluid tracer in subduction zones or to trace re...

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Chlorine isotope geochemistry of Icelandic thermal fluids: Implications for geothermal system behavior at divergent plate boundaries

Cited by 22 publications

References 42 publications

Supercritical Fluid Geochemistry in Geothermal Systems

Supercritical Fluid Geochemistry in Geothermal Systems

The origin of the warm groundwater near Lake Mývatn, NE Iceland, traced by stable isotopes

The role of the upper plate in controlling fluid-mobile element (Cl, Li, B) cycling through subduction zones: Hikurangi forearc, New Zealand

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