Reykjanes Geothermal System Research Articles

The behaviour of metals in deep fluids of NE Iceland

In this contribution, we present some of the first data on the elemental signature of deep crustal fluids in a basalt-hosted, low-chloride magmatic-hydrothermal system. Down-hole fluid samples (850–1600 m) from wells in the Theistareykir and Krafla geothermal fields in the Northern Volcanic Zone of Iceland were combined with well-head samples of condensed vapor, cuttings of altered rock, and fresh basalt (being some of the first concentration data for volatile and semi-volatile elements (Sb, Tl, Bi, Cd and As) for this area of Iceland). Results show that the deep fluids are relatively enriched in base metals and (semi)-volatile metals (in particular Te, Hg, Re and Tl) compared to local basalt. We interpret this enrichment in volatile metals to reflect a significant element input from magma degassing. Boiling of this deep fluid results in a well-head fluid composition that is significantly depleted in most elements. This well-head fluid has a distinct elemental signature, including a depletion in Sb that is mirrored in the altered rocks, and a depletion in the base metals that shows their selective sequestration in scale minerals, likely sulphides. As expected, the element content and patterns in surface fluids can thus not be interpreted to directly reflect that of the deep reservoir fluid. The behaviour of elements in Theistareykir and Krafla fluids is consistent, and largely agrees with similar data obtained for the Reykjanes geothermal system in SW Iceland. We therefore posit that our results are representative for this geological setting and indicate a significant magmatic degassing cation input to deep fluids, variably modified by water–rock interaction.

Short‐ and Long‐Term Variations in the Reykjanes Geothermal Reservoir From Seismic Noise Interferometry

AbstractThe Reykjanes Geothermal System (RGS) is a high‐temperature geothermal system located on the Reykjanes peninsula, a transtensional plate‐boundary zone located on the southwestern tip of Iceland. The area is characterized by high seismicity, recent volcanism, and high‐temperature geothermal fields. We use seismic noise records from April 2014 to August 2015 to study stress changes and potential deformation of the subsurface caused by injection and production operations at RGS through seismic interferometry. We retrieve continuous time series of waveform similarity values and seismic velocity changes during this period. The S‐transform of the similarity values allows us to clearly identify three variations in the mechanical properties of the Reykjanes peninsula related to rapid changes of RGS production. In addition, we observe a slow seismic velocity decrease of 0.36%/year in the reservoir due to the water deficit and seasonal variations associated with the energy production demand.

Analysis of 2014–2015 on- and off-shore passive seismic data on the Reykjanes Peninsula, SW Iceland

Passive seismic techniques are a valuable tool to map active faults, follow changes in the local stress field caused by drilling and exploitation processes in geothermal areas and estimate seismic velocity ratios. They have, however, not been used extensively in basaltic crust as we find it in Iceland due to the data processing challenges that high impedance contrasts between layers of igneous rocks pose. To improve conventional passive seismic techniques and to test new ones, a dense network of seismometers recorded seismic activity on- and off-shore the Reykjanes peninsula in southwest Iceland from spring 2014 until August 2015. In addition to the existing long-term and permanent networks, a temporary network consisting of 30 on-land stations and 24 ocean bottom seismometers (OBS) was deployed. The network was laid out as concentric circles around the Reykjanes geothermal area at the tip of the peninsula and was about 100 km in diameter. The data of the on-land stations were collected every 70 days and data recorded by the OBSs were read out after one year when they were recovered. We used the SeisComp3 software to automatically detect earthquakes and then manually revised the P and S arrival times. A total of 2066 earthquakes could be located along the mid-ocean ridge and within and around the Reykjanes geothermal system. Earthquake hypocentres associated with the geothermal area occur in the uppermost 2 km while events along the spreading axis occur at 4–6 km depth on-land and appear to deepen with increased distance from the shore. We see a cluster of about 200 events, induced by the drilling and onset of operation of a local injection borehole in the Reykjanes geothermal area. We find a vp/vs ratio from all picks of 1.78 consistent with earlier studies in the area. Focal mechanisms calculated for selected events reveal a regime dominated by strike-slip and normal faulting.

Rare earth element systematics in boiled fluids from basalt-hosted geothermal systems

Hydrothermal processes that lead to REE fractionation and redistribution are important for understanding water-rock interactions in geothermal energy resources and mineral deposits, and for determining how submarine hydrothermal activity affects the composition of oceanic crust. Much previous work on REE transport and deposition has focused on submarine hydrothermal vents. We report REE concentrations in boiled fluids sampled from five subaerial, basalt-hosted geothermal fields, and explore controls on aqueous REE concentrations by ligand complexation and mineral supersaturation. Samples that boiled at pressures between 0.8 and 2.83 MPa were obtained from the Reykjanes, Svartsengi, Hellisheidi, and Nesjavellir geothermal systems in Iceland, and the Puna geothermal system in Hawaii. For comparison, we also report REE concentrations in hydrothermal fluids from the sediment hosted submarine Middle Valley hydrothermal system, which boiled at >250 MPa. The pH(25°C) values of the sampled subaerial geothermal fluids range from 3.94 to 6.77, and Cl concentrations range from near seawater (502 mmol/kg) to dilute (1.9 mmol/kg). La, Ce and Eu are the only REE present at levels above 5 picomole/kg (pmol/kg) in the boiled geothermal fluids; and there are notable CI chondrite normalized La and Eu anomalies in the saline fluids. REE concentrations in Middle Valley hydrothermal fluids fall within the typical range reported for submarine hydrothermal fluids and have around two orders of magnitude higher REE than the boiled subaerial geothermal fluids. Bulk samples of precipitates in pipes from the Reykjanes geothermal system have detectable REE, confirming that downhole fluids have lost REE during boiling and production of fluids for geothermal energy. Isenthalpic boiling models show that the proportions of La and Eu chloride complexes increase relative to other aqueous species as boiling progresses, attenuating the incorporation of La and Eu into precipitated well scale solids. Fluorapatite is calculated to precipitate on boiling of low pH and saline fluids and calcite is calculated to precipitate from dilute and near-neutral pH fluids, and these minerals likely sequester REE in boiled subaerial fluids. Submarine hydrothermal fluids are constrained to boiling at higher temperatures than subaerial geothermal fluids owing to pressure from overlying cold seawater, therefore secondary minerals and solids that incorporate REE are not extensively precipitated and REE concentrations in the fluids are higher.

Elemental changes and alteration recorded by basaltic drill core samples recovered from in situ temperatures up to 345°C in the active, seawater‐recharged Reykjanes geothermal system, Iceland

AbstractHydrothermal activity results in element exchanges between seawater and oceanic crust that contribute to many aspects of ocean chemistry; therefore, improving knowledge of the associated chemical processes is of global significance. Hydrothermally altered basaltic drill core samples from the seawater‐recharged Reykjanes geothermal system in Iceland record elemental gains and losses similar to those observed in samples of hydrothermally altered oceanic crust. At Reykjanes, rocks originally emplaced on the seafloor were buried by continued volcanism and subsided to the current depths (>2250 m below surface). These rocks integrate temperature‐dependent elemental gains and losses from multiple stages of hydrothermal alteration that correspond to chemical exchanges observed in rocks from different crustal levels of submarine hydrothermal systems. Specifically, these lithologies have gained U, Mg, Zn, and Pb and have lost K, Rb, Ba, Cu, and light rare earth elements (La through Eu). Alteration and elemental gains and losses in lithologies emplaced on the seafloor can only be explained by a complex multistage hydrothermal alteration history. Reykjanes dolerite intrusions record alteration similar to that reported for the sheeted dike section of several examples of oceanic crust. Specifically, Reykjanes dolerites have lost K, Rb, Ba, and Pb, and gained Cu. The Reykjanes drill core samples provide a unique analog for seawater‐oceanic crust reactions actively occurring at high temperatures (275–345°C) beneath a seafloor hydrothermal system.

Lithogeochemical approaches in geothermal system characterization: An application to the Reykjanes geothermal field, Iceland

This article presents the results of a comprehensive lithogeochemical study that was conducted using downhole rock samples from the Reykjanes geothermal system in Iceland. Magmatic fractionation trends established for least-altered rocks were used to correct mass change estimates for pseudo-enrichments/-depletions due to fluid-rock interaction. Net mass changes of altered rocks in the Reykjanes geothermal system range from −26 to +22g/100gp, and are dominated by changes in SiO2>CaO>MgO>Na2O,K2O, S>C. Trace elements undergoing the greatest net mass changes are Ba>Zn>Cu, Ni>Sr>Rb>As>Cd. The lithogeochemical data are interpreted with petrographic data and data from parallel studies of mineral chemistry (detailed in separate manuscripts). Trends in the distribution of reactive-mobile elements correspond closely to the large-scale hydrology of the Reykjanes geothermal system, as determined by measured isotherms and hydrothermal mineral zonation. A hypothesis of deep inflow to the system from the southeast is supported by lithogeochemical and mineralogical evidence and downhole temperature measurements. Whole-rock mass changes of specific components counter their documented enrichments and depletions in the Reykjanes geothermal fluid relative to the initial seawater input. Volume estimates for the Reykjanes geothermal system are made based on the redistribution of select reactive-mobile elements.

Compositions of hydrothermal silicates and carbonates as indicators of physicochemical conditions in the Reykjanes geothermal system, Iceland

This paper presents results of a quantitative investigation of mineral chemistry in the Reykjanes geothermal system, Iceland, designed to assess the relative influence of various hydrothermal minerals on the observed bulk-rock mass changes of elements. Detailed petrography and electron microprobe analyses (n=∼1400) were performed on samples of rock cuttings from eleven wells that intersect different parts of the hydrothermal system. An important finding was the strong partitioning of barium into adularia, which becomes incorporated through “capture” substitution into the 12-fold coordination site, coupled with Al3+ substitution into the tetrahedral site. Adularia has average and maximum Ba contents of 0.8 and 3.5wt.%, respectively. In addition, there is no relationship between Ba concentration in adularia and crystal size, occurrence type (replacement versus vein/vug), or rock composition. There is also no correlation between the Ba content of replacement adularia with that of the igneous plagioclase being altered, demonstrating that Ba concentration in the altered rocks of the Reykjanes geothermal system is being controlled by large-scale hydrothermal redistribution processes and not primary chemostratigraphic variability. Calcite exerts a major control on strontium proportions in the bulk rock, and contains up to 1wt.% Sr. Concentrations of many minor and trace elements and some major elements in hydrothermal minerals at Reykjanes correlate positively with measured downhole temperature (to varying degrees), e.g.: Ba and Rb in adularia; Al(iv) in chlorite; Fe, Mg, Mn, and Sr in calcite; and the andradite component in garnet. The Fe/(Fe+Mg) of chlorite is unrelated to the precursor host rock ratio. Chlorite geothermometry, based on Al(iv) substitution and Fe/(Fe+Mg) content, predicts temperatures of 94–313°C (average=236°C). These temperatures are similar to or less than the present-day temperatures measured for corresponding sample depths in all wells (with the exception of a shallow interval from RN10). This may indicate a prolonged and ongoing heating trend in the deep liquid-phase region of the Reykjanes reservoir.

Geochemical bias in drill cutting samples versus drill core samples returned from the Reykjanes Geothermal System, Iceland

The wholerock major and trace element composition of drill cutting samples are compared to drill core samples from adjacent depths in the seawater recharged Reykjanes geothermal system in Iceland. The first appearance of alteration minerals and lithologies in drill cutting samples is a useful tool for interpreting broad subsurface characteristics. However, use of drill cutting samples for determining igneous affinity and elemental exchanges during hydrothermal alteration is problematic. Samples recovered from immediately above and below the cored intervals in wells RN-17B and RN-30 demonstrate that drill-cutting samples are biased towards preservation of least altered primary igneous minerals and more resistant alteration minerals, including albite, quartz, and epidote, with preferential loss of finer-grained and less resistant minerals including chlorite and actinolite. This selective recovery obscures elemental exchanges resulting from hydrothermal alteration processes. For some elements, compositional variations (enrichments and depletions) measured from 9.5m of core exceeds that observed in ∼3000m of cutting analyses. Concentration ratios of hydrothermally immobile elements including Zr, Nb, V, Y, HREE, Hf, Ta and Th in deep (>2245m) spot drill core samples record bimodal, trace element-enriched and trace element-depleted precursor compositions similar to subaerial Reykjanes Peninsula basalts. The same elements in nearly 3000m of drill cutting samples from well RN-17 overwhelmingly reflect the more common trace element-enriched igneous precursor, demonstrating that mixing of drill cutting samples obscures details of their igneous affinity. A new and different drill rig was used to deepen well RN-17 below 2266m in a sidetrack hole (RN-17ST), which resulted in a change in drilling conditions, accompanied with an increased well deviation angle from ∼0° to ∼4°. Wholerock geochemical results for drill cutting samples from RN-17ST are homogenous for virtually every element; suggesting the change in drilling conditions resulted in extreme mixing of the drill cuttings. Anomalously high concentrations of Cu, Ni, Cr and Ta in some drill cutting samples likely reflects contamination of drill cutting samples by metal alloys used in drill bits and drill collars or more resistant spinel and sulfide phases.

Relating sulfide mineral zonation and trace element chemistry to subsurface processes in the Reykjanes geothermal system, Iceland

The nature and distribution of sulfide minerals and their trace element chemistry in the seawater-dominated Reykjanes geothermal system was determined through the study of cuttings and core from wells that intersect different regions of the hydrothermal cell, from the near surface to depths of >3000m. The observed sulfide mineral zonation and trace element enrichment correlate well with the present-day thermal structure of the system. Isocubanite and pyrrhotite are confined to the deep, low permeability regions, whereas an assemblage of chalcopyrite and pyrite predominates in the main convective upflow path. The presence of marcasite in the uppermost regions of the system reflects weakly acidic conditions (pH<5) marginal to the upflow, where outflow and downward percolating fluids have dissolved deeply exsolved CO2. The presence of “chalcopyrite disease” in sphalerite may be an indication that the system is experiencing a heating trend, following the logic of “zone-refining” in volcanogenic massive sulfide systems. Sulfide sulfur at all analyzed depths in the Reykjanes geothermal system was derived from a mixture of basaltic and reduced seawater sources. Petrographic evidence suggests that seawater-derived hydrothermal fluids have altered primary igneous sulfides in the host rocks, a process that has been proposed as a major control of aqueous sulfide production in mid-ocean ridge environments. Calculations show that igneous sulfides in the host basalts likely account for less than 5% of the total available ore metal budget in the system, however, their contribution to fluid metal budgets is probably significant because of their relatively high solubility. The processes documented by this study are likely analogous to those operating in the feeder and deep reaction zones of mid-ocean ridge seafloor hydrothermal systems. The results show that sulfide mineral zonation and trace element chemistry vary as a function of physicochemical parameters that are relevant to the characterization and exploration of geothermal energy resources.

Strontium and oxygen isotopic profiles through 3 km of hydrothermally altered oceanic crust in the Reykjanes Geothermal System, Iceland

The Iceland Deep Drilling Program well RN-17 was drilled 3km into a section of hydrothermally altered basaltic crust in the Reykjanes geothermal system in Iceland. The system is located on the landward extension of the Mid-Atlantic Ridge, and the circulating hydrothermal fluid is modified seawater, making Reykjanes a useful analog for mid-oceanic ridge hydrothermal systems. We have determined whole-rock Sr and O isotope compositions, and Sr isotope compositions of epidote grains from the RN-17 cuttings and RN-17B core. Whole rock oxygen isotope ratios range from −0.13 to 3.61‰ V-SMOW, and are isotopically lighter than fresh MORB (5.8±0.2‰). The concentrations of Sr in the altered basalt range from well below to well above concentrations in fresh rock, and appear to be strongly correlated with the dominant alteration mineralogy. Whole rock Sr isotope ratios ranged from 0.70329 in the least altered crystalline basalt, to 0.70609 in the most altered hyaloclastite samples; there is no correlation with depth. Sr isotope ratios in epidote grains measured by laser ablation MC-ICP-MS ranged from 0.70360 to 0.70731. Three depth intervals, at 1000m, 1350m, and 1650m depth, have distinctive isotopic signatures, where 87Sr/86Sr ratios are elevated (mean value>0.7050) relative to background levels (mean altered basalt value ~0.7042). These areas are proximal to geothermal feed zones, and the 1350m interval directly overlies the transition from dominantly extrusive to dominantly intrusive lithologies. Oxygen isotope measurements yield integrated water/rock ratios of 0.4 to 4.3, and suggest that hydrothermal fluids must have formerly had a component of meteoric water. Strontium isotopic measurements provide a more sensitive indication of seawater interaction and require significant exchange with seawater strontium. Both isotopic systems indicate that the greenschist-altered basalts were in equilibrium with hydrothermal fluids at a relatively high mean water/rock (Wt.) ratio ranging from about 0.5 to 4. These ratios are higher than estimates from ODP Hole 504B and IODP Hole 1256D, but are consistent with values inferred from vent fluids from 21° and 13°N on the East Pacific Rise (Albarède et al., 1981; Michard et al., 1984; Alt et al., 1996; Harris et al., 2015).

Evolution of fluid–rock interaction in the Reykjanes geothermal system, Iceland: Evidence from Iceland Deep Drilling Project core RN-17B

We describe the lithology and present spatially resolved geochemical analyses of samples from the hydrothermally altered Iceland Deep Drilling Project (IDDP) drill core RN-17B. The 9.3m long RN-17B core was collected from the seawater-dominated Reykjanes geothermal system, located on the Reykjanes Peninsula, Iceland. The nature of fluids and the location of the Reykjanes geothermal system make it a useful analog for seafloor hydrothermal processes, although there are important differences. The recovery of drill core from the Reykjanes geothermal system, as opposed to drill cuttings, has provided the opportunity to investigate evolving geothermal conditions by utilizing in-situ geochemical techniques in the context of observed paragenetic and spatial relationships of alteration minerals. The RN-17B core was returned from a vertical depth of ~2560m and an in-situ temperature of ~345°C. The primary lithologies are basaltic in composition and include hyaloclastite breccia, fine-grained volcanic sandstone, lithic breccia, and crystalline basalt. Primary igneous phases have been entirely pseudomorphed by calcic plagioclase+magnesium hornblende+chlorite+titanite+albitized plagioclase+vein epidote and sulfides. Despite the extensive hydrothermal metasomatism, original textures including hyaloclastite glass shards, lithic clasts, chilled margins, and shell-fragment molds are superbly preserved. Multi-collector LA-ICP-MS strontium isotope ratio (87Sr/86Sr) measurements of vein epidote from the core are consistent with seawater as the dominant recharge fluid. Epidote-hosted fluid inclusion homogenization temperature and freezing point depression measurements suggest that the RN-17B core records cooling through the two-phase boundary for seawater over time to current in-situ measured temperatures. Electron microprobe analyses of hydrothermal hornblende and hydrothermal plagioclase confirm that while alteration is of amphibolite-grade, it is in disequilibrium and the extent of alteration is dependent upon protolith type and water/rock ratio. Alteration in the RN-17B core bares many similarities to that of Type II basalts observed in Mid-Atlantic Ridge samples.

A geochemical investigation of trace elements in well RN-17 at Reykjanes geothermal system, SW-Iceland

Here we present the results of an investigation of well RN-17 at Reykjanes geothermal system (Reykjanes Peninsula, SW-Iceland). We show that the adoption of micro-spot secondary ion mass spectrometry (SIMS) together with bulk-rock techniques, such as X-ray fluorescence (XRF), inductively coupled plasma mass spectrometry (ICP-MS), and thermal ionisation mass spectrometry (TIMS) carried out on 10 basaltic drill-cuttings over a depth of 3000 m gives us insights into the geochemical characteristics of the well. Whole rock and mineral phases, i.e., plagioclase, clinopyroxene, epidote and amphibole, indicate that the rocks pertaining to the cuttings were affected by hydrothermal alteration that has been able to modify significantly the Sr isotope ratios and the concentration of K, Rb, Cs, Ba and B. Seawater/rock ratio, evaluated for 87Sr/86Sr, achieved its maximum for the shallow cutting RN-17-650, decreasing although not systematically with depth, with the minimum value for the deep cutting RN-17-2800. Moreover, the occurrence of Na-rich plagioclase, epidote and amphibole down to the well bottom are evidence of the persistence of fluid/rock interaction at depth.

High-grade contact metamorphism in the Reykjanes geothermal system: Implications for fluid-rock interactions at mid-oceanic ridge spreading centers

Granoblastic hornfels identified in cuttings from the Reykjanes seawater-dominated hydrothermal system contains secondary pyroxene, anorthite, and hornblendic amphibole in locally equilibrated assemblages. Granoblastic assemblages containing secondary orthopyroxene, olivine, and, locally, cordierite and spinel occur within groups of cuttings that show dominantly greenschist facies hydrothermal alteration. Granoblastic plagioclase ranges continuously in composition from An54 to An96, in contrast with relict igneous plagioclase that ranges from An42 to An80. Typical hydrothermal clinopyroxene compositions range from Wo49En3Fs48 to Wo53En30Fo17; clinopyroxene from the granoblastic grains is less calcic with an average composition of Wo48En27Fs25. The hornfels is interpreted to form during contact metamorphism in response to dike emplacement, resulting in local recrystallization of previously hydrothermally altered basalts. Temperatures of granoblastic recrystallization estimated from the 2-pyroxene geothermometer range from 927°C to 967°C. Redox estimates based on the 2-oxide oxybarometer range from log fO2 of −13.4 to −15.9. Granoblastic hornfels comprised of clinopyroxene, orthopyroxene, and calcic plagioclase have been described in a number of ancient hydrothermal systems from the conductive boundary layer between the hydrothermal system and the underlying magma source, most notably in Integrated Ocean Drilling Program Hole 1256D, Ocean Drilling Program Hole 504B, and in the Troodos and Oman ophiolites. To our knowledge, this is the first evidence of high-grade contact metamorphism from an active geothermal system and the first description of equilibrated amphibole-absent pyroxene hornfels facies contact metamorphism in any mid-ocean ridge (MOR) hydrothermal system. This contribution describes how these assemblages develop through metamorphic reactions and allows us to predict that higher-temperature assemblages may also be present in MOR systems.

Cu-Rich Scales in the Reykjanes Geothermal System, Iceland

Seawater-dominated fluids discharge from the subaerial Reykjanes geothermal system, Iceland. There is a sharp pressure decrease in surface pipes at an orifice (throttle point), and Cu-rich scales deposit at this orifice that consist largely of bornite and digenite, along with sphalerite and other sulfides. The bornite and digenite form complex intergrowths with sphalerite and galena, accompanied by high concentrations of gold and silver (up to 590 ppm and 2.3 wt %, respectively). The precipitates form in response to rapid boiling (flashing) of the hydrothermal fluids due to the sharp decrease in pressure (~37–22 bars) at the orifice, downstream of the well-head. The boiling, and concomitant separation of the vapor phase, results in a temperature decrease from 252° to 220°C across this throttle over distances of centimeters. There is a nearly quantitative deposition of metals from solution compared to concentrations in reservoir liquids, resulting in formation of the metastable Cu-FeS solid solution, rich in silver and gold, plus sphalerite and galena. A high degree of supersaturation of metals in the quenched liquid is indicated by the very fine grain size of the sulfides and dendritic textures. Postdepositional cooling likely caused exsolution of bornite and digenite from the high-temperature solid solution; in addition, silver was expelled from the Cu-Fe-S solid solution and remobilized into late fractures in the scales, whereas the gold appears to have remained in solid solution or as submicroscopic inclusions in the bornite and digenite. The observed mineral assemblage at Reykjanes contrasts with that of sea-floor black smokers, despite similar fluid compositions. The sea-floor hydrothermal vents characteristically have Cu-rich linings composed predominantly of chalcopyrite, which is precipitated by a combination of conductive cooling of high-temperature (~350°C) hydrothermal fluids and mixing with seawater. The difference in sulfide mineralogy between the Reykjanes orifice scales, with Cu sulfides dominated by bornite and digenite, and many sea-floor vents may provide an indication of the subsea-floor mineralization where fluids boil before discharging at sea-floor vents.

An investigation of trace and isotope light elements in mineral phases from well RN-17 (Reykjanes Peninsula, SW Iceland)

The light lithophile (Li, Be and B) and halogen (F, Cl) elements are powerful tracers of fluid transfer due to their mobility during high temperature hydrothermal processes and metamorphic devolatilisation. Moreover, although a great deal of studies have been carried out on these elements in whole rock and minerals of altered rocks from divergent and convergent plate margins, an inventory for mineral phases from the altered Icelandic oceanic crust is still incomplete.In the present paper we report the results of in situ EPMA and SIMS investigations on variously altered magmatic (plagioclase and clinopyroxene) and hydrothermal phases (amphibole and epidote) from selected cuttings drilled at different depths (400 – 3000 m) of the well RN-17, Reykjanes geothermal system (SW Iceland). Our study has benefited from the use of high-magnification SEM investigations; from ICP-MS on Li, P-TIMS determinations of boron isotope composition (δ11B) and ID analyses of B contents on the whole rock.Particularly, SIMS data on epidote have shown that alteration beneath Reykjanes has been more efficient in the shallow and intermediate cuttings, while whole rock data on boron isotope composition have revealed that the alteration has been caused firstly by δ11B-poor fluids and successively by δ11B-rich seawater-hydrothermal fluids.

Metals in deep liquid of the Reykjanes geothermal system, southwest Iceland: Implications for the composition of seafloor black smoker fluids

Seafloor hydrothermal systems precipitate Cu, Zn, and Fe sulfides at and below black smoker vents on the seafloor; as a result, the metal concentrations in the vent fluids are minimum values. We sampled deep, unboiled liquids from the Reykjanes geothermal reservoir, Iceland, and measured the metal concentrations. This active, seawater-dominated system, situated on the Mid-Atlantic Ridge, is the subaerial equivalent to mid-ocean-ridge hydro thermal systems. The liquids, collected at 1350–1500 m depth and 284–295 °C, contain 154–2431 μ M Fe (9–140 ppm), 207–261 μ M Cu (14–17 ppm), 79–393 μ M Zn (5–27 ppm), 0.6–1.4 μ M Pb (120–290 ppb), 6–31 n M Au (1–6 ppb), and 250–960 n M Ag (28–107 ppb). Fluids discharged at surface from the same wells have orders of magnitude lower metal concentrations due to precipitation caused by boiling and vapor loss during depressurization. The concentrations of Cu, Zn, and Pb in the high-temperature reservoir liquids at Reykjanes are similar to those in the highest-temperature black smoker discharges, whereas Au and Ag concentrations are one to two orders of magnitude higher at Reykjanes; lower-temperature seafloor fluids have lower metal contents, suggesting subseafloor deposition before discharge. The Reykjanes heat flux of 130 MW requires a liquid flux of ~100 kg/s; over 104 yr, the minimum life of the system, 0.5 Mt each of Cu and Zn may have precipitated at depth.

Hydrothermal alteration in the Reykjanes geothermal system: Insights from Iceland deep drilling program well RN-17

The Reykjanes geothermal system is a seawater-recharged hydrothermal system that appears to be analogous to seafloor hydrothermal systems in terms of host rock type and low water/rock alteration. The similarities make the Reykjanes system a useful proxy for seafloor vents. At some time during the Pleistocene, the system was dominated by meteoric water recharge, and fluid composition at Reykjanes has evolved through time as a result of changing proportions of meteoric water influx as well as differing pressure and temperature conditions. The purpose of this study is to characterize secondary mineralization, degree of metasomatic alteration, and bulk composition of cuttings from well RN-17 from the Reykjanes geothermal system. The basaltic host rock includes hyaloclastite, breccia, tuff, extrusive basalt, diabase, as well as a marine sedimentary sequence. The progressive hydrothermal alteration sequence observed with increasing depth results from reaction of geothermal fluids with the basaltic host rock. An assemblage of greenschist facies alteration minerals, including actinolite, prehnite, epidote and garnet, occurs at depths as shallow as 350 m; these minerals are commonly found in Icelandic geothermal systems at temperatures above 250 °C (Bird and Spieler, 2004). This requires hydrostatic pressures that exceed the present-day depth to boiling point curve, and therefore must record alteration at higher fluid pressures, perhaps as a result of Pleistocene glaciation. Major, minor, and trace element profiles of the cuttings indicate transitional MORB to OIB composition with limited metasomatic shifts in easily mobilized elements. Changes in MgO, K 2O and loss on ignition indicate that metasomatism is strongly correlated with protolith properties. The textures of alteration minerals reveal alteration style to be strongly dependent on protolith as well. Hyaloclastites are intensely altered with calc-silicate alteration assemblages comprising calcic hydrothermal plagioclase, grandite garnet, prehnite, epidote, hydrothermal clinopyroxene, and titanite. In contrast, crystalline basalts and intrusive rocks display a range in alteration intensity from essentially unaltered to pervasive and nearly complete albitization of igneous feldspar and uralitization of clinopyroxene. Hydrothermal anorthite (An92–An98) occurs in veins in the most altered basalt cuttings and is significantly more calcic than igneous feldspar (An48–An79). Amphibole compositions change from actinolite to hornblende at depth. Hydrothermal clinopyroxene, which occurs in veins, has greater variation in Fe content and is systematically more calcic than igneous pyroxene and also lacks uralitic textures. Solid solutions of prehnite, epidote, and garnet indicate evolving equilibria with respect to aluminum and ferric iron.

Hydrothermal minerals record CO2 partial pressures in the Reykjanes geothermal system, Iceland

The Reykjanes Peninsula in southwest Iceland is the landward extension of the Mid-Atlantic Ridge spreading center. At present seawater penetrates the coastal Reykjanes geothermal system at depth, mixes with magmatic volatiles and reacts with the basaltic host rock to form secondary geothermal minerals in progressively higher-grade mineral alteration zones with depth where highest recorded temperatures are ~320°C. Within the epidote-chlorite and portions of the epidote-actinolite zones of alteration, epidote-prehnite-calcite-quartz-fluid constitutes a quadravariant assemblage that, under conditions of specified temperature, pressure, and activity of H2O allows prediction of geothermal fluid PCO2 as a function of the composition of the solid solution minerals epidote or prehnite. This assemblage is most common at temperatures >250°C and 600m between the reference temperatures of 275°C and 310°C range from ~1 to ~6.5 bars, when only epidote, prehnite and quartz are observed. When all members of the assemblage are present, including calcite, the calculated range in PCO2 is from ~0.5 to 6 bars. If calcite is absent from the assemblage, the computation of PCO2 using equilibria for reaction 1 provides the maximum value of PCO2 at which calcite will not be present. Actual PCO2 values of geothermal fluids from the Reykjanes system were derived from analytical data on liquid and vapor samples collected at the surface using both the WATCH and SOLVEQ speciation programs. The geothermal fluids at reference temperature between 275°C and 310°C have PCO2 concentrations ranging from 1.3 bars to 4.0 bars. The calculated PCO2 values based on epidote compositions are in close agreement with present-day measured CO2 in the Reykjanes geothermal system fluids. 74% of the calculated PCO2 values based on epidote compositions where the complete epidote-prehnite-calcite-quartz assemblage is observed fall within the range of measured present-day fluids, while 62% of the calculated PCO2 values fall within the range when calcite is not present in the assemblage. Therefore, our method for calculating fluid PCO2 is proven quite reliable when all four index minerals are present. Additionally, if only epidote, prehnite and quartz are observed, our method still serves as a moderately accurate predictive proxy for fluid PCO2 composition in the Reykjanes geothermal system. Ultimately, the correlation between measured and predicted fluid compositions, provides insight into future abilities to characterize spatial and temporal concentrations of CO2 in active and fossil hydrothermal and low-grade metamorphic environments in mafic lithologies based on compositional variations and paragenesis of hydrothermal minerals. In addition, this study will aid in understanding the nature of reactions that involve natural sequestration of CO2 derived from magmatic degassing, and injection of industrial CO2-rich fluids within hydrothermal environments in basaltic rocks.

Isotopic constraints on ice age fluids in active geothermal systems: Reykjanes, Iceland

The Reykjanes geothermal system is located on the landward extension of the Mid-Atlantic Ridge in southwest Iceland, and provides an on-land proxy to high-temperature hydrothermal systems of oceanic spreading centers. Previous studies of elemental composition and salinity have shown that Reykjanes geothermal fluids are likely hydrothermally modified seawater. However, δD values of these fluids are as low as −23‰, which is indicative of a meteoric water component. Here we constrain the origin of Reykjanes hydrothermal solutions by analysis of hydrogen and oxygen isotope compositions of hydrothermal epidote from geothermal drillholes at depths between 1 and 3 km. δD EPIDOTE values from wells RN-8, -9, -10 and -17 collectively range from −60 to −78‰, and δ 18 O EPIDOTE in these wells are between −3.0 and 2.3‰. The δD values of epidote generally increase along a NE trend through the geothermal field, whereas δ 18 O values generally decrease, suggesting a southwest to northeast migration of the geothermal upflow zone with time that is consistent with present-day temperatures and observed hydrothermal mineral zones. For comparative analysis, the meteoric-water dominated Nesjavellir and Krafla geothermal systems, which have a δD FLUID of ∼ −79‰ and −89‰, respectively, show δD EPIDOTE values of −115‰ and −125‰. In contrast, δD EPIDOTE from the mixed meteoric-seawater Svartsengi geothermal system is −68‰; comparable to δD EPIDOTE from well RN-10 at Reykjanes. Stable isotope compositions of geothermal fluids in isotopic equilibrium with the epidotes at Reykjanes are computed using published temperature dependent hydrogen and oxygen isotope fractionation curves for epidote–water, measured isotope composition of the epidotes and temperatures approximated from the boiling point curve with depth. Calculated δD and δ 18 O of geothermal fluids are less than 0‰, suggesting that fluids of meteoric or glacial origin are a significant component of the geothermal solutions. Additionally, δD FLUID values in equilibrium with geothermal epidote are lower than those of modern-day fluids, whereas calculated δ 18 O FLUID values are within range of the observed fluid isotope composition. We propose that modern δD EPIDOTE and δD FLUID values are the result of diffusional exchange between hydrous alteration minerals that precipitated from glacially-derived fluids early in the evolution of the Reykjanes system and modern seawater-derived geothermal fluids. A simplified model of isotope exchange in the Reykjanes geothermal system, in which the average starting δD ROCK value is −125‰ and the water to rock mass ratio is 0.25, predicts a δD FLUID composition within 1‰ of average measured values. This model resolves the discrepancy between fluid salinity and isotope composition of Reykjanes geothermal fluids, explains the observed disequilibrium between modern fluids and hydrothermal epidote, and suggests that rock–fluid interaction is the dominant control over the evolution of fluid isotope composition in the hydrothermal system.