Multiple sulfur isotope compositions of thermal fluids from Iceland were measured in order to evaluate the sources and reactions of sulfur and sulfur isotope fractionation in geothermal systems at Icelandic divergent plate boundaries, characterized by MORB-like basalts. The geothermal systems studied had a wide range of reservoir temperatures of 56–296 °C and Cl concentrations of 18–21,000 ppm. Dissolved sulfide (∑ S− II) and SO4 concentrations in liquid water measured < 0.01–165 ppm and 1.3–300 ppm, respectively, and H2S(g) concentrations in the vapor 4.9–2000 ppm. The δ34S and Δ33S values for different phases and oxidation states were highly variable: δ34S∑ S− II = − 11.6 to 10.5‰ (n = 99), ∆33S∑ S− II = − 0.12 to 0.00‰ (n = 45), δ34SSO4 = − 1.0 to 24.9‰ (n = 125), ∆33SSO4 = − 0.04 to 0.02‰ (n = 50), δ34SH2S(g) = − 2.6 to 5.9‰ (n = 112) and ∆33SH2S(g) = − 0.03 to 0.00‰ (n = 56). The multiple sulfur isotope values of the thermal fluids are interpreted to reflect various sources of sulfur in the fluids, as well as isotope fractionation occurring within the geothermal systems associated with fluid-rock interaction, boiling and oxidation and reduction reactions. The results of isotope geochemical modeling demonstrate that the sources of S− II in the thermal fluid are leaching of basalt (MORB) and seawater SO4 reduction for saline systems with insignificant magma gas input, and that the observed ranges of δ34S and Δ33S for ∑ S− II and H2S(g) reflect isotope fractionation between minerals and aqueous and gaseous species upon fluid-rock interaction and boiling. The sources of SO4 are taken to be multiple, including oxidation of S− II originating from basalt, leaching of SVI from the basalts and the seawater itself in the case of saline systems. In low-temperature fluids, the δ34S and Δ33S values reflect the various sources of sulfur. For high-temperature fluids, fluid-rock interaction, ∑ S− II oxidation and SO4 reduction and sulfide and sulfate mineral formation result in a large range of δ34S and Δ33S values for ∑ S− II and SO4 in the fluids, highlighting the importance and effects of chemical reactions on the isotope systematics of reactive elements like sulfur. Such effects needed to be quantified in order to reveal the various sources of an element.

Thermal fluids in Iceland range in temperature from <10°C to >440°C and are dominated by water (>97mol%) with a chloride concentration from <10ppm to >20,000ppm. 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), δ18O (−20.8 to +2.3‰), 3He/4He (3.1 to 30.4 RA), δ11B (−6.7 to +25.0‰), δ13C∑CO2 (−27.4 to +4.6‰), 14C∑CO2 (+0.6 to +118 pMC), δ13CCH4 (−52.3 to −17.8‰), δ15N (−10.5 to +3.0‰), δ34S∑S−II (−10.9 to +3.4‰), δ34SSO4 (−2.0 to +21.2‰) and δ37Cl (−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, CO2 and possibly also to Cl in the fluids. Water-basalt interaction also contributes to CO2 and is the major source of H2S, SO4, Cl and B in the fluids. Redox reactions additionally influence the composition of the fluids, for example, oxidation of H2S to SO4 and reduction of CO2 to CH4. Air-water interaction mainly controls N2, Ar and Ne concentrations. The large range of many non-reactive volatile isotope ratios, such as δ37Cl 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 δ13C∑CO2 and δ34S∑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.

The Ngualla area of the Lupa terrane in the Ubendian Belt of SW Tanzania is underlain by well-preserved greenschist facies calc-alkaline volcanic rocks which have been intruded by low-Sr, peraluminous K-rich granites. The Ngualla volcanic rocks are subdivided into four groups: the basaltic andesitestrachyandesites, Group I porphyritic dacites-trachytes, Group II porphyritic dacites-trachytes, and rhyolites. The basaltic andesites-trachyandesites were emplaced at similar to 1943 Ma and their most primitive members are characterized by MgO contents of up to 7.43 wt % at a SiO2 content of 54.0 wt %, Ni contents of up to 170 ppm and Mg# of up to 62. They show fractionated REE patterns (La/YbcN = 11.5-36.5), subchondritic Zr/Hf (37.8-43.9) and Nb/Ta (7.14-20.0) ratios, epsilon Nd (1943 Ma) values of 3.33 to 6.24 and TOM ages of 2521-2883 Ma. These geochemical features are consistent with derivation of the basaltic andesites-trachyandesites by at least 0.1% partial melting of garnet peridotite mantle, followed by 50 -80% fractional crystallization involving the removal of 61% clinopyroxene, 20% hornblende, 4% plagioclase and 15% orthopyroxene. The magmas were contaminated by Neoarchaean crustal materials incorporated in the Ubendian Belt during their ascent to the surface. The evolution of these magmas led to the formation of Group I porphyritic dacites-trachytes and rhyolites in a Palaeoproterozoic continental arc setting.The Group II porphyritic dacites-trachytes (1871 5 Ma) were coevally emplaced with the low-Sr, peraluminous K-granites (1878 15 Ma) forming a felsic plutonic-volcanic suite. Despite some differences, these rocks share geochemical features including negative Eu anomalies (mean Eu/Eu* = 0.19 for the dacites and 0.5 for the granites) and eNd (1871 Ma) values of -5.72 for the dacites and eNd (1878 Ma) values of -6.00 to -11.2 for the granites. These geochemical features are consistent with the generation of the similar to 1.87 Ga Ngualla felsic plutonic -volcanic suite by partial melting of heterogeneous Neoarchaean crust at low pressure conditions in the stability field of plagioclase in an intracontinental setting along a sutured margin.The 1.89-1.87 Ga felsic plutonic -volcanic suite also occurs in the Ndembera area of the Usagaran belt to the SW. Our results corroborate previous evidence that the Palaeoproterozoic regional K-granitic magmatism that affected parts of the Ubendian and Usagaran belts was caused by a regional thermal anomaly that was induced by large scale magmatic underplating that precipitated widespread crustal anatexis. (C) 2017 Elsevier Ltd. All rights reserved.

This study describes the chemical composition of dissolved, degased and suspended fluxes of the 2002 Skaftá glacial flood, which emerged from one of the Skaftá subglacial lake due to geothermal activity beneath the Icelandic Vatnajökull glacier. The dissolved and suspended fluxes during the flood are compared with those normally observed in the Skaftá river to determine the effect of such floods on the annual fluxes of material delivered to the coastal waters. Concentrations of most dissolved elements during the flood were significantly higher than those normally observed in the Skaftá river. In addition, dissolved concentrations of nutrients such as SiO2, Fe, and V, increased more than an order of magnitude during the flood. These will affect biological processes on a local scale. The δ34S composition in the flood water suggests that the dissolved SO4 was derived from the oxidation of H2S and the geothermal fluid. The total suspended particulate load measured in the Skaftá river during the 8-day 2002 flood was approximately half of the non-flood total annual Skaftá suspended load. As particles carry the bulk of limiting nutrients to the oceans, this demonstrates the importance of glacial floods for primary production of coastal waters. The composition of the flood water and the Skaftá subglacial lake, together with reaction path modelling suggest that substantial degasing of CO2 and H2S occurred at the glacial outlet during the flood. This degasing may have released as much as 262,000 and 7,980tonnes of CO2 and H2S, respectively, to the atmosphere having a considerable impact on the local carbon and sulphur cycles during the flood event.

Samples of precipitation have been collected and analysed from four sampling stations in southern Iceland. Sea-salt in the precipitation has the same elemental ratios as in seawater and its concentration decreases with distance from the shore. The sampling station at Mjóanes, at Thingvallavatn Lake, is located around 100km west of Eyjafjallajökull and during and after its eruption in 2010, the concentration of fluoride in precipitation increased from 10-20μg/l to 90μg/l and the annual average concentration elevated from 11μg/l to 28μg/l. From 1980 to 1998, pH of rainwater at Írafoss increased due to emissions reductions of anthropogenic sulphur. Since 1998, pH of the precipitation has decreased again at Írafoss. Similar pH changes are seen in Mjóanes, which is close to Írafoss, but not in other sampling stations further away. River water samples collected in the vicinity of the rainwater samplers, from the spring fed river Sog have elevated non-SO4 sulphur concentration from 2005-2010 and decreased δ34S from 2005-2007. Írafoss and Mjóanes are located approximately 12-20km away from two geothermal power plants, Nesjavellir- and Hellisheidi power plants that emit large quantities of CO2 and H2S, among other gases, to the atmosphere. The data presented here suggest that geothermal power production can cause local environmental effects, like rainwater acidification and increases in dissolved sulphur in rivers in the vicinity of the power plants.

We have analysed the chemical and stable isotope compositions of four spring waters situated just northwest of the Hekla volcano, where cold water emerges from the base of the lava flows. The stable isotope ratios of water (H, O), dissolved inorganic carbon (C) and sulphate (S) were used to determine whether magmatic gases are mixing with the groundwater. The waters can be characterised as Na-HCO3 type. The results show that deep-seated gases mix with groundwater, substantially affecting the concentration of solutes and the isotopic composition of dissolved carbon and sulphate.

Fresh pinewood blocks were submerged in sulfate and iron(II) containing media, inoculated with bacterial consortia isolated from seawater, aiming to simulate the seabed conditions of the Vasa shipwreck (1628). The consortia contained erosion (EB) and sulfate-reducing bacteria (SRB). Sulfur K-edge X-ray absorption near edge structure (XANES) spectroscopy and scanning X-ray spectromicroscopy images showed that organic sulfur, mainly thiols (R-SH), had accumulated in the lignin-rich middle lamella in EB-degraded parts of the wood. The sulfur content in the wood increased more than 10 times in 2 years. In another series with active inoculums from marine archaeological wood, the sulfur XANES spectra showed, after 4 years of anaerobic treatment, considerable amounts also of inorganic iron sulfides, Fe1−xS, which oxidized at atmospheric exposure. A sediment sample from the Vasa's seabed was also rich in iron sulfides, including pyrite, FeS2. X-ray fluorescence mappings of sulfur and phosphorous distributions indicate that scavenging SRB penetration, producing hydrogen sulfide in situ, is restricted to EB-degraded parts of the wood structure. The sulfur isotope depletion of 34S from δ34S=21‰ in marine sulfate to δ34S=6‰ and 1.8‰ for fractions of reduced sulfur and sulfate separated from a Vasa wood sample, respectively, suggests bacterial transformation. A fuller understanding of the routes of sulfur accumulation, as reactive iron sulfides and as organic sulfur, has important implications for improving conservation methods of marine archaeological wood. Moreover, the biogenic accumulation of organically bound sulfur, specifically in lignin-rich parts of waterlogged wood, has wider geochemical significance for fossil fuels of marine origin, as lignin-rich humic matter is important for the diagenetic formation of kerogens from anoxic marine sediments.

Episodic hydrological events, such as snowmelt during spring, have a marked effect on stream water chemistry. Here we investigated how spring snowmelt affected δ34S values of sulfate in six streams situated in northern Sweden. Four streams had high δ34SSO4 values during base flow with values ranging from +11.9 to +8.6‰. During snowmelt the δ34SSO4 decreased to around +6‰. In one of the streams and in the forested upper reaches of a second stream, δ34SSO4 values were close to +5‰ during base flow and decreased to about +3.8‰ during the spring snowmelt. One stream, which drained cultivated postglacial sediments dominated by acid sulfuric soils, was differentiated from the other streams by low δ34SSO4 values (−5.0‰ to −0.5‰). We could identify two stream water SO4 sources: sedimentary sulfides and anthropogenic S. Bacterial dissimilatory sulfate reduction was identified as an important process affecting stream water δ34SSO4 values and suggests that in this boreal landscape, peatlands and possibly riparian zones have a large influence on the biogeochemistry of SO42− during base flow conditions. Our results suggest that during the spring snowmelt, snow S and desorbing SO4 of mainly anthropogenic origin are the two major S sources in four of the investigated streams. Two streams in forested areas also indicate that reoxidation of reduced S may be released during the spring flood. The stream in the cultivated area was found to be strongly influenced by the acid sulfuric soils independent of stream flow conditions.

Long-term climate moderation is commonly attributed to chemical weathering; the greater the temperature and precipitation the faster the weathering rate. To test this widely-held hypothesis, we performed a field study and determined the weathering rates of eight nearly pristine north-east Iceland river catchments with varying glacial cover over 44 y. Statistically significant linear positive correlations were found between mean annual temperature and chemical weathering in all eight catchments and between mean annual temperature and mechanical weathering and runoff in seven of the eight catchments. The runoff, mechanical weathering flux, and chemical weathering fluxes in these catchments are found to increase from 6 to 16%, 8 to 30%, and 4 to 14%, respectively, depending on the catchment for each degree of temperature increase. Positive correlations were found between time and mechanical and chemical weathering for all catchments. In summary, these results demonstrate a significant feedback between climate and Earth surface weathering, and suggest that this weathering rate is currently increasing with time due to global warming.

Long-term climate moderation is commonly attributed to chemical weathering; the higher the temperature and precipitation the faster the weathering rate. Weathering releases divalent cations to the ocean via riverine transport where they promote the drawdown of CO2 from the atmosphere by the precipitation and subsequent burial of carbonate minerals. To test this widely-held hypothesis, we performed a field study determining the weathering rates of 8 nearly pristine north-eastern Iceland river catchments with varying glacial cover over 44 years. The mean annual temperature and annual precipitation of these catchments varied by 3.2 to 4.5 degrees C and 80 to 530%, respectively during the study period. Statistically significant linear positive correlations were found between mean annual temperature and chemical weathering in all 8 catchments and between mean annual temperature and both mechanical weathering and runoff in 7 of the 8 catchments. For each degree of temperature increase, the runoff, mechanical weathering flux, and chemical weathering fluxes in these catchments are found to increase from 6 to 16%, 8 to 30%, and 4 to 14% respectively, depending on the catchment. In contrast, annual precipitation is less related to the measured fluxes; statistically significant correlations between annual precipitation and runoff, mechanical weathering, and chemical weathering were found for 3 of the least glaciated catchments. Mechanical and chemical weathering increased with time in all catchments over the 44 year period. These correlations were statistically significant for only 2 of the 8 catchments due to scatter in corresponding annual runoff and average annual temperature versus time plots. Taken together, these results 1) demonstrate a significant feedback between climate and Earth surface weathering, and 2) suggest that weathering rates are currently increasing with time due to global warming. (c) 2008 Elsevier B.V. All rights reserved.

The Björkdal gold deposit, bound to a quartz vein system which is mainly hosted by a quartz-monzodioritic intrusion, is situated at the easternmost part of the 1.9 Ga Skellefte base metal district in the Fennoscandian shield. Three fluid stages may be distinguished, referred to as a “barren” stage, a main gold stage, and a remobilization stage, respectively. From oxygen and hydrogen isotope evidence, it is argued that fluids of different origins (magmatic and surface waters) penetrated the ore zone at the inferred stages, but regional metamorphic fluids appear essentially only to have redistributed elements. Early quartz veining took place during a pre-metamorphic stage at ca. 1.88 Ga, as evidenced by unradiogenic galena data and an Sm–Nd scheelite errorchron of 1,915 ± 32 Ma (MSWD = 0.25). Temporarily, the main ore-forming stage was closely related to the first barren stage and took place during a major uplift event close to 1.88 Ga. Although other source rocks cannot be totally ruled out, available isotope data (O, S, Sr and Pb) are seemingly consistent with the view that these elements, and by inference other ore elements, were derived from the host intrusion.

A high-resolution carbon isotope profile through the uppermost Neoproterozoic–Lower Cambrian part of the Sukharikha section at the northwestern margin of the Siberian platform shows prominent secular oscillations of δ13C with peak-to-peak range of 6–10 ‰. There are six minima, 1n–6n, and seven maxima 1p–7p, in the Sukharikha Formation and a rising trend of δ13C from the minimum 1n of − 8.6 ‰ to maximum 6p of + 6.4 ‰. The trough 1n probably coincides with the isotopic minimum at the Precambrian–Cambrian boundary worldwide. Highly positive δ13C values of peaks 5p and 6p are typical of the upper portion of the Precambrian–Cambrian transitional beds just beneath the Tommotian Stage in Siberia. A second rising trend of δ13C is observed through the Krasnoporog and lower Shumny formations. It consists of four excursions with four major maxima that can be correlated with Tommotian–Botomian peaks II, IV, V, and VII of the reference profile from the southeastern Siberian platform. According to the chemostratigraphic correlation, the first appearances of the index forms of archaeocyaths are earlier in the Sukharikha section than in the Lena–Aldan region.

Granitoids intruding the late Archaean sequences of the Sukumaland Greenstone Belt of northern Tanzania belong to two distinct geochemical suites. Suite 1 is characterised by Na2O/K2O > 1 (1.04 – 4.67), high Sr/Y (56 – 204) and Ba/Rb ratios (6.1 – 27.1) and low Rb/Sr ratios (0.08 - 0.25). The rocks are enriched in Sr (405 – 1264 ppm) and depleted in Yb (0.17 – 0.93 ppm) and Rb (56 – 132 ppm). On chondrite-normalised REE diagrams, the rocks display highly fractionated patterns characterised by relative LREE enrichment ((La/Yb)N = 23 – 128 and (Gd/Yb)N = 3.10 – 8.54) and lower concentrations of the HREE (YbN = 0.80 – 4.45). On primitive mantle-normalised spidergrams, Nb and Ti, together with P and Y are depleted relative to adjacent elements. The major and trace element characteristics of Suite 1 are comparable to those of typical Archaean TTG suites and High Silica Adakites (HSA). Suite 2 granitoids are characterised by Na2O/K2O < 1, low Sr/Y (2.80 – 41.7) and Ba/Rb (0.40 – 8.91) ratios and high Rb/Sr (0.30 – 6.27) ratios. Suite 2 is also characterised by low Sr (53 - 326 ppm) and high Rb (40 - 365 ppm) and Yb (0.44 – 1.36 ppm) contents. Compared to Suite 1, Suite 2 rocks display less fractionated REE patterns ((La/Yb)N = 15 – 86 and (Gd/Yb)N = 1.73 – 6.74) and are characterised by higher concentrations of the HREE (YbN = 2.1 – 6.5). On primitive mantle-normalised spidergrams, Suite 2 samples, like those of Suite 1, show relative depletion in Th, Nb and Ti, together with P and Y relative to adjacent elements. Sm-Nd mean crustal residence ages for both suites are indistinguishable and range between 2470 and 2720 Ma with a mean of 2610  35 Ma (2 SE), similar to the emplacement age of 2620  40 Ma. The granitoids are interpreted to have formed by partial melting at the base of a late Archaean thickened sub-arc basaltic crust. Melting to form the Suite 1 granitoids occurred in the eclogite stability field whereas Suite 2 formed by melting at shallower depth in the garnet amphibolite stability field.