Calculation of sulfur isotope fractionation in sulfides

The increment method has been successfully applied to calculate thermodynamic isotope fractionation factors of oxygen in silicates, oxides, carbonates, and sulfates. In this paper, we modified the increment method to calculate thermodynamic isotope fractionation factors of sulfur in sulfides, based on chemical features of sulfur-metal bonds and crystal features of sulfide minerals. To approximate the bond strength of sulfides, a new constant, known as the Madelung constant, was introduced. The increment method was then extended to calculate the reduced partition function ratios of sphalerite, chalcopyrite, galena, pyrrhotite, greenockite, bornite, cubanite, sulvanite, and violarite, as well as the isotope fractionation factors between them over the temperature range from 0 to 1000 °C. The order of ³⁴S enrichment in these nine minerals is pyrrhotite > greenockite > sphalerite > chalcopyrite > cubanite > sulvanite > bornite > violarite > galena. Our improved method constitutes another model for calculating the thermodynamic isotope fractionation factors of sulfur in sulfides of geochemical interest.     

New U-Pb titanite age data on the rocks from the Karelian craton and the Belomorian mobile belt, Fennoscandian Shield

We test the possibility of applying titanite as indicator of the boundaries between the Belomorian mobile belt and the Karelian craton of the Fennoscandian Shield. U-Pb isotope dating established wide variations of titanite ages in the Belomorian mobile belt and the Karelian craton. The titanites from the Karelian craton are mainly Archean in age (2.52–2.86 Ga), whereas the Belomorian mobile belt contains, with few exceptions, Paleoproterozoic titanites (1.74–1.95 Ga). In the Karelian craton, the age of titanite, in general, records the cratonization of the Earth’s crust (2.8 Ga and 2.6–2.7 Ga, respectively). In the Belomorian mobile belt, it presumably reflects the timing of the exhumation of tectonic nappes from the mid-crustal depths during the collisional stage of the evolution of the Lapland-Kola orogen.     

Carbon and sulfur isotopes in Precambrian sediments from the Canadian Shield

Carbon and sulfur isotope ratios have been determined for more than 200 samples of Precambrian graphitic and sulfidic metasediments from the Superior and Churchill Provinces of Canada. The sediments were deposited in small sedimentary basins related to various Canadian greenstone belts. The age of the Archean samples is approximately 2.7 Ga., the Proterozoic samples about 1.8 Ga.,

The Archean organic material shows C isotope values between −47 and −15%. vs. PDB. The Proterozoic metasediments show a smaller range, with δ¹³C between −30 and −17%.,

A few carbonate samples yielded C isotopic compositions between −5.9 and −4.7%.,

Associated iron sulfides have δ³⁴S-values ranging from −6 to +8%., which are consistent with isotopic fractionation effects either by inorganic or bacterial reactions in the sedimentary sulfur cycle.

The great variability of organic C isotopic composition, especially in the late Archean, although affected by extensive postdepositional alteration, might indicate differences in the environmental conditions and/or the organic content of these sedimentary basins. Some extremely ¹³C-depleted organic carbon values support a global occurrence of environments likely dominated by methanotrophic organisms at the end of the Archean.     

Multiple sulfur isotopes of sulfides from sediments in the aftermath of Paleoproterozoic glaciations

Geochemical evidence reported from Paleoproterozoic sediments has long been used to evaluate the transition from the anoxic Archean atmosphere to an oxygenated atmosphere. Sulfur isotopes (³²S, ³³S, ³⁴S and ³⁶S) in sedimentary sulfides and sulfates are an especially sensitive means to monitor this transition, such that the timing of the Paleoproterozoic “Great Oxidation Event” can be investigated using mass-independently fractionated (MIF) sulfur isotope systematics expressed as Δ³³S. Here we report data from 83 individual analyses of pyrite, pyrrhotite and chalcopyrite on a new suite of 30 different samples from Finland, South Africa, Wyoming and Ontario that span ∼600 My and follow one or several “Snowball Earth” events in the Paleoproterozoic. The samples were measured using a high-resolution secondary ion mass spectrometry technique in multicollection mode that investigates multiple sulfur isotopes in microdomains (<30 μm) within individual sulfide grains while preserving petrographic context. We focused on sediments deposited in the aftermath of the Paleoproterozoic glaciations (between 1.9 and 2.2 Ga) to trace fluctuations in atmospheric O₂ concentrations that were likely affected by an interplay of O₂ sinks in the atmosphere and the upper ocean and continental crust, and by the emergence and diversification of aerobic organisms. Our results demonstrate that MIF sulfur isotopes are absent in sediments deposited after the period of protracted global cooling in the Paleoproterozoic and independently confirm observations that MIF ceased during this time. We interpret our results by integrating Δ³³S and δ³⁴S data in sulfides, δ¹³C data in carbonates and the estimated timing of glaciation events in the Paleoproterozoic. Data strongly hint at the presence of microbial sulfate reduction and fluctuations in the concentration of dissolved seawater sulfate and/or in δ³⁴S_sulfate in the aftermath of glaciations and likely were affected by changing erosion rates and nutrient delivery to the oceans. These changes modulated the population of primary producers, especially oxygenic photosynthesizers, and led to fluctuations in the abundance of atmospheric O₂, CO₂ and CH₄. Our results support the interpretation that the world-wide δ¹³C_carb excursion observed between ∼2.25 and 2.05 Ga (Karhu and Holland, 1996) was a period of significant accumulation of O₂ in the atmosphere.     

Early Proterozoic layered intrusions in the northeastern part of the Fennoscandian Shield

Summary Early Proterozoic layered intrusions, about 2440 Ma in age, are widespread over a large area of the northeastern Fennoscandian Shield in Finland, Sweden and the Soviet Union. Only one intrusion, the Kukkola intrusion, is encountered in Sweden whereas in Finland, their number exceeds twenty. These are concentrated principally in two areas, the dicontinuous Tornio-Närdnkävaara intrusion belt which crosses northern Finland and the Koitelainen intrusion with its satellites located in central Finnish Lapland. The intrusions in the Soviet Union are concentrated in three areas: (i) on the Kola Peninsula, (ii) in the Paanajärvi area close to the Finnish border and (iii) northeast of Lake Onega.Examples of all the ore types characteristic of layered intrusions have been found in these intrusions. Chromitite layers are encountered in the Kukkola/Tornio, Kemi, Penikat, Koitelainen and Burakovsky intrusions, but only one, the Kemi chromitite, has so far been mined. The Portimo, Koillismaa, Monchegorsk and Fedorova intrusions are characterized by PGE-bearing Cu-Ni-deposits in their marginal series. Mineralized zones enriched in PGE are also encountered in the layered series. Those in the Penikat intrusion and in the Portimo intrusions are the most remarkable and the best known to date. Vanadium-bearing Fe-Ti-oxide layers are encountered in several intrusions, but only one, the Mustavaara deposit, is presently being exploited.Two types of parental magma have tentatively been proposed for these intrusions. The first type is represented by a magma which was relatively rich in magnesium and chromium and was as a whole boninitic in composition, whereas the plagioclase-rich intrusions and megacyclic units are interpreted as having crystallized from a magma which was greatly depleted in these elements, especially Cr, and had melted crustal material incorporated in it.The emplacement of the early Proterozoic layered intrusions in Fennoscandia was part of the world-wide igneous activity indicated by other layered intrusions and mafic dyke swarms of similar age in other ancient cratons, i.e. the Jimberlana intrusion in Australia, the Great Dyke in Zimbabwe, the Scouric picrite suite in Scotland, the Hearst-Matachewan dyke swarm, Copper Cliff Formation and East Bull Lake intrusion in Ontario, Canada, and the Vestfold Hills and Napier Complex dyke swarms in Antarctica. This almost contemporaneous occurrence in different parts of the world would suggest a more intimate relationship between the Fennoscandian Shield, northwest Scotland, Canadian Shield, Yilgarn Block, Zimbabwe Craton and East Antarctic Shield at the beginning of the Proterozoic than at present.

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Früh-Proterozoische geschichtete Intrusionen im nordöstlichen Teil des Fennoskandischen Schildes

Zusammenfassung Im Nordost-Teil des Fennoskandischen Schildes in Finnland, Schweden und der Sowjetunion kommen fast vierzig frühproterozoische geschichtete Intrusionen, die ungefähr 2440 Mio J. alt sind, vor. Nur eine davon, die Kukkola Intrusion, liegt in Schweden, während in Finnland mehr als zwanzig Intrusionen vorkommen. Diese sind hauptsächlich in zwei Gebieten konzentriert, nämlich in dem nicht-zusammenhängenden Tornio-Näränkävaara Gürtel, der das nördliche Finnland durchzieht, und die Koitelainen-Intrusion mit ihren Satelliten im zentralen Finnischen Lapland. Die Intrusionen in der Sowjetunion sind in drei Gebieten konzentriert: (i) auf der Kola Halbinsel (ii) im Paanajärvi Gebiet nahe der Finnischen Grenze und (iii) östlich vom Onega-See.Beispiele aller für geschichtete Intrusionen charakteristischen Erztypen kommen vor. Chromititlagen sind in den Intrusionen von Kukkola/Tornio, Kemi, Penikat, Koitelainen und Burakovsky zu finden, aber nur eine davon, der Kemi Chromitit, ist bisher in Abbau genommen worden.Die Portimo-, Koillismaa-, Monchegorsk- und Fedorova-Intrusionen werden durch PGE-führende Kupfer-Nickel-Lagerstätten in ihren randlichen Bereichen charakterisiert. Mineralisierte Zonen die an PGE angereichert sind kommen auch in den geschich teten Serien vor. Die bemerkenswertesten sind die PGE-Vererzungen der Penikat- und der Portimo-Intrusionen. Vanadium-führende Fe-Ti-Oxidlagen kommen in verschiedenen Intrusionen vor, aber nur eine davon, die Mustavaara-Lagerstätte, ist bisher abgebaut worden.Diese Intrusionen werden auf zwei verschiedene Magmentypen zurückgeführt. Ersteres ist ein Magma das relativ reich an Magnesium und Chrom war und eine boninitische Zusammensetzung hatte, während die Plagioklas-reichen Intrusionen, und die megazyklischen Einheiten auf ein Magma das an diesen Elementen (besonders Cr) verarmt war, und das Krustenmaterial aufgeschmolzen hat, zurückgehen.Die Platznahme der frühproterozoischen geschichteten Intrusionen in Fennoskandien stellt einen Teil weltweiter magmatischer Aktivität dar, die durch andere geschichtete Intrusionen und mafische Gänge von fast identischem Alter in anderen alten Kratonen repräsentiert wird. Hier ist die Jimberlana-Intrusion in Australien, der Great Dyke in Zimbabwe, die Pikrit-Suite von Scourie in Schottland, die Gänge von Hearst-Matachewan, die Copper Cliff Formation und die East Bull Lake Intrusion in Ontario, Kanada ebenso wie die Gangsysteme der Vesthold Hills und des Napier Komplexes in Antarctica zu nennen. Diese fast gleichaltrigen Vorkommen in verschiedenen Teilen der Welt weisen auf eine engere Beziehung zwischen dem Fennoskandischen Schild, Nordwest-Schottland, dem Kanadischen Schild, dem Yilgarn Block, dem Zimbabwe-Craton und dem Ostantarktischen Schild zum Beginn des Proterozoikums hin.

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With 7 Figures     

硫化物中硫同位素分馏理论计算研究进展

硫化物是重要的矿物类,通常是一个或多个金属元素与硫结合而形成硫化物.硫化物作为大多数金属的主要来源具有重大的经济利用价值,硫化物中硫同位素分馏的研究一直是同位素地球化学研究的热点.研究不同金属硫化物之间的硫同位素分馏效应,对于利用硫同位素对成矿作用过程和成矿物质来源开展地球化学示踪,具有非常重要的意义.本文结合笔者近期的工作概述了硫化物中硫同位素分馏的理论计算研究,认为虽然半经验半理论的增量方法在同位素分馏计算中存在一定的局限性,但在没有其他实用的理论计算方法时,改进的增量方法可以作为硫化物中硫同位素分馏计算的一种理论估算方法.     

Paleoproterozoic mafic dyke swarms of the Belomorian Province,eastern Fennoscandian Shield

Paleoproterozoic mafic igneous rocks (2450–1970 Ma) are exposed in the form of layered intrusions, dykes, and volcanic rocks in the Karelian, Kola and Murmansk provinces and in the form of dykes and small intrusions in the Belomorian Province, Eastern Fennoscandian Shield. The age and sequence of mafic dyke emplacement during the Paleoproterozoic are very similar in these regions. Further comparisons of geochemical characteristics of mafic dyke swarms in the Belomorian Province and neighboring cratons show considerable similarities.     

Evidence for the Mesozoic endogenous activity in the northeastern part of the Fennoscandian Shield

Paleomagnetic study of dykes and intrusions remanent in the central part of the Kola Peninsula has been carried out; the Devonian age of these objects has been confirmed by isotopic-geochronological studies. The component analysis of the magnetization vector in the samples has shown that there are two magnetization components in most samples. The paleomagnetic pole corresponding to the direction of a more stable component is located in the close vicinity of the Middle Devonian segment of the apparent polar wander path (APWP) for the East European Craton, so this enables us to estimate its age to be as old as the Devonian. The second magnetization component was found in Devonian dykes of both northern and southern parts of the Kola Peninsula; the paleomagnetic pole corresponding to this component is located close to the Mesozoic (Early Jurassic) part of the APWP for the East European Craton. It is suggested that the extensive remagnetization of Devonian intrusions in the Kola Peninsula was caused by the thermal effect of the Barents-Amerasian superplume and by the appearance of an extensive area with trap magmatism within the modern Arctic Basin region. Discovery of a significant thermal event that covered the Fennoscandian northeast allows us to explain the geochronological problem concerning the Mesozoic ages of particular singular zircon grains from Precambrian rocks of the shield derived via the SHRIMP method.     

胶东地块东部变质岩锆石U-Pb定年和氧同位素研究

对胶东地块东部高压-超高压变质岩作了系统的锆石U-Ph定年和矿物氧同位素分析,结果对这些变质岩的原岩性质提供了制约。研究得到:(1)区域花岗片麻岩及岩浆锆石普遍不同程度地亏损~(18)O,3个片庥岩样品中岩浆核锆石U-Pb年龄分别为723±36Ma、738±17Ma和744±63Ma,低δ~(18)O值(-0.42～4.14‰)岩浆核锆石说明其原岩为新元古代低δ~(18)O值岩浆岩,石英-石榴石氧同位素温度及少量印支期变质锆石的出现,指示片麻岩与榴辉岩曾经共同经历了印支期超高压变质作用;(2)花岗片麻岩中的榴辉岩原岩年龄有两种,一种是新元古代,其U-Ph年龄为806±79Ma。另一种是古元古代晚期,其 U-Pb年龄为1838±41Ma。这2个榴辉岩的超高压变质年龄分别为229±3Ma和242±21Ma。多数榴辉岩中的变质增生或变质重结晶锆石也具有低δ~(18)O值特征(0.22～3.4‰),指示在榴辉宕相变质作用之前,这些榴辉岩原岩为低δ~(18)O值蚀变岩或低δ~(18)O值基性岩浆岩;(3)大理岩中榴辉岩变质增生锆石δ~(18)O值高达15.9‰,U-Pb年龄为229±4Ma,指示在榴辉岩相变质作用之前,榴辉岩原岩与大理岩一样具有高δ~(18)O值;(4)斜长角闪岩的原岩U-Pb年龄为1719±18Ma,与同时期榴辉岩原岩一起构成扬子板块北缘新元古代岩浆侵位时的裂谷肩部围岩,在三叠纪大陆碰撞时同样受到变质改造。     

Lithological interpretation of crustal composition in the Fennoscandian Shield with seismic velocity data

In this study, we report the results of an investigation of lithological interpretation of the crust in the central Fennoscandian Shield (in Finland) using seismic wide-angle velocity models and laboratory measurements on P- and S-wave velocities of different rock types. The velocities adopted from wide-angle velocity models were compared with laboratory velocities of different rock types corrected for the crustal PT conditions in the study area. The wide-angle velocity models indicate that the P-wave velocity does not only increase step-wise at boundaries of major crustal layers, but there is also gradual increase of velocity within the layers. On the other hand, the laboratory measurements of velocities indicate that no single rock type is able to provide the gradual downward increasing trends. Thus, there must be gradual vertical changes in rock composition. The downward increase of velocities indicates that the composition of the crust becomes gradually more mafic with increasing depth. We have calculated vertical velocity profiles for a range of possible crustal lithological compositions. The Finnish crustal velocity profiles require a more mafic composition than an average global continental model would suggest. For instance, on the SVEKA'81 transect, the calculated models suggest that the crustal velocity profiles can be simulated with rock type mixtures where the upper crust consists of felsic gneisses and granitic–granodioritic rocks with a minor contribution of amphibolite and diabase. In the middle crust, the amphibolite proportion increases. The lower crust consists of tonalitic gneiss, mafic garnet granulite, hornblendite, pyroxenite and minor mafic eclogite. Assuming that these rock types are present in sufficiently extensive and thick layers, they would also have sufficiently high acoustic reflection coefficients for generating the generally well-developed reflectivity in the crust in the central part of the shield. Density profiles calculated from the lithological models suggest that there is practically no density contrast at Moho in areas of the high-velocity lower crust. Comparison of reflectors from FIRE-1 and FIRE-3 transects and the velocity model from SVEKA'81 wide-angle transect indicated that the reflectors correlate with velocity layering, but the three-dimensional structures of the crust complicate such comparisons.     

Sr isotopic composition of Paleoproterozoic C-rich carbonate rocks: The Tulomozero Formation,SE Fennoscandian Shield

Rb–Sr systematics has been studied in ¹³C-rich carbonate rocks of the Paleoproterozoic (2.09 ± 0.07 Ga) Tulomozero Formation in the northern Onega Lake area, the SE Fennoscandian Shield. The formation is divided into eight members (A–F) consisting of greenschist-facies-grade, variegated sandstones, siltstones, mudstones, stromatolitic dolostones and subordinate crystalline limestones. Samples of carbonate rocks were obtained from two overlapping drillholes intersecting the entire thickness of the Tulomozero Formation. Prior to isotope analysis, the rocks powders were treated with 1N ammonium acetate for partial removal of the late epigenetic carbonate phases. Major resetting of the Rb–Sr systems in the Tulomozero carbonate rocks appears to take place during the Svecofennian regional metamorphic event, and it was screened by using Mn/Sr, Fe/Sr, Mg/Ca, and ¹⁸O/¹⁶O ratios. High Sr content (up to 2080 μg/g in limestones, and 530 μg/g in dolostones) coupled with low Fe/Mn (<0.40) ratios in the Tulomozero carbonate rocks of Members A, B (the lower part), D, F, and E are consistent with accumulation of original carbonate sediments in evaporitic lacustrine, playa, and sabkha environments. A decrease in the Sr content with concurrent increase in the Fe/Mn ratio (>0.40) in dolostones of the upper part of Member B, and of Members G and H is indicative of seawater influxes (sea transgression) into the Tulomozero basin. The ⁸⁷Sr/⁸⁶Sr values in the least altered (Mn/Sr < 2.0) marine dolostones are 0.70418–0.70442 and 0.70343–0.70409 for the earlier and late phases of the marine transgression, respectively. The decrease in the ⁸⁷Sr/⁸⁶Sr ratio in ca. 2.1 Ga seawater is attributable to an increase in hydrothermal flux Sr into the Palaeoproterozoic ocean.     

Metallogeny of gold in the Fennoscandian Shield

Gold occurs in a number of different ore types in the Fennoscandian Shield ranging in age from Late Archean to Late Proterozoic. Until recently, the metal was exploited primarily as a byproduct in volcanogenic massive sulphide deposits but during the 1980s more gold mines have been opened than during any other episode in the mining history of northern Europe. The occurrence of gold in the Fennoscandian Shield is reviewed in the context of the major tectonostratigraphic units:

The sulfur isotopic composition of sulfides from ores and host rocks of the Upper Kolyma region,Magadan Oblast

More than 200 analyses of the sulfur isotopic composition of sulfides from various terrigenous and intrusive host rocks, metasomatically altered wall rocks, and gold lodes of the Upper Kolyma region are presented. In accessory pyrite of the metaterrigenous rocks, δ³⁴S varies from ?23.1 to +5.7‰ δ³⁴S of pyrite and arsenopyrite from gold-quartz mineralization is within the range ?10.6 to ?0.4‰ and is close to the average δ³⁴S of pyrite from the metaterrigenous rocks (?4.4‰). In the intrusive rocks, δ³⁴S of pyrite varies from ?3.8 to +2.6‰ (+0.7‰, on average) and drastically differs from δ³⁴S of arsenopyrite from postmagmatic gold-rare-metal mineralization (?7.9 to ?2.7‰; ?5.2‰, on average). The comparison of the δ³⁴S of accessory sulfides from the host rocks with δ³⁴S of sulfides from the gold deposits suggests that sulfur mobilized from the terrigenous sequences participated in the hydrothermal process. The results obtained are consistent with the metamorphic model of the formation of gold-quartz deposits in the Upper Kolyma region.     

Petrogenesis of siliceous high-Mg series:Evidence from Early Paleoproterozoic mafic volcanic rocks of the Vodlozero Domain,Fennoscandian Shield

Compositional peculiarities of the siliceous high-Mg series(SHMS)rocks formed at the Archean-Paleoproterozoic boundary as a function of plume activity are discussed using example of Early Paleoproterozoic mafic volcanic rocks of the Vodlozero Domain,Fennoscandian Shield.These rocks are characterized by wide variations in Mg#(33-67)and Cr contents(25-1123 ppm),LREE enrichment,and weakly negative_(εNd)(from-0.7 to-2.9).The high Gd/Yb ratio in the primitive high-Mg rocks of the Vodlozero Domain suggests their generation from a garnet-bearing source.At the same time,their negative _(εNd)in combination with LREE enrichment points to the crustal contamination.A new model was proposed to explain the remarkable global-scale similarity of SHMS.Such rocks can be generated by the contamination of a high-degree(30%)partial melt derived from a depleted mantle.The lower crustal sanukitoid-type rocks can be considered as a universal crustal contaminant.Modeling showed that such mixing can provide the observed narrow_(εNd)variations in Early Paleoproterozoic volcanics.The Neoarchean sanukitoid suites,which are widespread on all cratons,presumably composed the lower crust at the beginning of the Paleoproterozoic.Therefore,this mechanism can be considered universal for the genesis of the SHMS rocks.The high-to low-Cr rock series can be produced by the fractionation of the mafic melt coupled with an insignificant crustal assimilation of felsic end members of the sanukitoid suite of the Vodlozero Domain en route to the surface,as suggested by the positive correlation of_(εNd)with Cr and Mg#,negative correlation with Th,and slight decrease of_(εNd)in the more evolved varieties.     

Distinguishing Cambrian from Upper Ordovician source rocks: Evidence from sulfur isotopes and biomarkers in the Tarim Basin

The reported source rocks for the abundant petroleum in the Tarim Basin, China range from Cambrian to Lower Ordovician and/or Upper Ordovician in age. However, the difference between the two groups of source rocks is not well characterized. In this study, pyrite was removed from eleven mature to over mature kerogen samples from source rocks using the method of CrCl₂ reduction and grinding. The kerogen and coexisting pyrite samples were then analyzed for δ³⁴S values. Results show that the kerogen samples from the Cambrian have δ³⁴S values between +10.4‰ and +19.4‰. The values are significantly higher than those from the Lower Ordovician kerogen (δ³⁴S of between +6.7‰ and +8.7‰), which in turn are generally higher than from the Upper Ordovician kerogen samples (δ³⁴S of between ?15.3 and +6.8‰). The associated pyrite shows a similar trend but with much lower δ³⁴S values. This stratigraphically controlled sulfur isotope variation parallels the evolving contemporary marine sulfate and dated oil δ³⁴S values from other basins, suggesting that seawater sulfate and source rock age have an important influence on kerogen and pyrite δ³⁴S values. The relatively high δ³⁴S values in the Cambrian to Lower Ordovician source rocks are associated with abundant aryl isoprenoids, gammacerane and C₃₅ homohopanes in the extractable organic matter, indicating that these source rocks were deposited in a bottom water euxinic environment with water stratification. Compared with the Upper Ordovician, the Cambrian to Lower Ordovician source rocks show abundance in C₂₈ 20R sterane, C₂₃ tricyclic terpanes, 4,23,24-trimethyl triaromatic dinosteroids and depletion in C₂₄ tetracyclic terpane, C₂₉ hopane. Thus, δ³⁴S values and biomarkers of source rock organic matter can be used for distinguishing the Cambrian and Upper Ordovician source rocks in the Tarim Basin.     

The origin of Zn isotope fractionation in sulfides

Isotope fractionation of Zn between aqueous sulfide, chloride, and carbonate species (Zn²⁺, Zn(HS)₂, , , ZnS(HS)⁻, ZnCl⁺, ZnCl₂, , and ZnCO₃) was investigated using ab initio methods. Only little fractionation is found between the sulfide species, whereas carbonates are up to 1‰ heavier than the parent solution. At pH > 3 and under atmospheric-like CO₂ pressures, isotope fractionation of Zn sulfides precipitated from sulfidic solutions is affected by aqueous sulfide species and the δ⁶⁶Zn of sulfides reflect these in the parent solutions. Under high P_CO2 conditions, carbonate species become abundant. In high P_CO2 conditions of hydrothermal solutions, Zn precipitated as sulfides is isotopically nearly unfractionated with respect to a low-pH parent fluid. In contrast, negative δ⁶⁶Zn down to at least −0.6‰ can be expected in sulfides precipitated from solutions with pH > 9. Zinc isotopes in sulfides and rocks therefore represent a potential indicator of mid to high pH in ancient hydrothermal fluids.     

Geochemistry and origin of saline groundwaters in the Fennoscandian Shield

Chemical and isotopic analyses of water from drill holes and mines throughout the Fennoscandian Shield show that distinct layers of groundwater are present. An upper layer of fresh groundwater is underlain by several sharply differentiated saline layers, which may differ in salinity, relative abundance of solutes, and O, H, Sr and S isotope signature. Saline groundwater can be classified into four major groups based on geochemistry and presumed origin. Brackish and saline waters from 50–200 m depth in coastal areas around the Baltic Sea exhibit distinct marine chemical and isotopic fingerprints, modified by reactions with host rocks. These waters represent relict Holocene seawater. Inland, three types of saline groundwater are observed: an uppermost layer of brackish and saline water from 300–900 m depth; saline water and brines from 1000–2000 m depth; and superdeep brines which have been observed to a depth of at least 11 km in the drill hole on the Kola Peninsula, U.S.S.R. Electrical and seismic studies in shield areas suggest that such brines are commonly present at even greater depths. The salinity of all inland groundwaters is attributed predominantly to water-rock interaction. The main solutes are Cl, Ca, Na and Mg in varying proportions, depending on the host rock lithology. The abundance of dissolved gases increases with depth but varies from site to site. The main gas components are N₂, CH₄ (up to 87 vol.%) and locally H₂. The δ¹³C value for methane is highly variable (−25 to −46%), and it is suggested that hydrothermal or metamorphic gases trapped within the surrounding rocks are the most obvious source of CH₄. The uppermost saline water has meteoric oxygen-hydrogen isotopic compositions, whereas values from deeper water plot above the meteoric water line, indicating considerably longer mean residence time and effective low temperature equilibration with host rocks. Geochemical and isotopic results from some localities demonstrate that the upper saline water cannot have been formed through simple mixing between fresh water and deep brines but rather is of independent origin. The source of water itself has not been satisfactorily verified although superdeep brines at least may contain a significant proportion of relict Precambrian hydrothermal or metamorphic fluids.     

An ultraviolet laser microprobe for the in situ analysis of multisulfur isotopes and its use in measuring Archean sulfur isotope mass-independent anomalies

A laser fluorination microprobe system has been constructed for high-accuracy, high-precision multisulfur isotope analysis with improved spatial resolution. The system uses two lasers: (a) a KrF excimer laser for in situ spot analysis by ultraviolet (UV) photoablation with λ = 248 nm and (b) a CO₂ laser for whole-grain analysis of powdered samples by infrared heating at λ = 10.6 μm. A CO₂ laser is necessary for the analysis of interlaboratory isotope reference materials because they are supplied as powders. The δ³⁴S and δ³³S compositions of reference materials measured with a CO₂ laser fluorination system agree (±0.2‰, 1σ) with the recommended values by the Sulfur Isotope Working Group of the International Atomic Energy Agency [Ding et al 2001] and [Taylor]. The precision of replicate analyses of powdered sulfide minerals with the CO₂ laser is typically ±0.2‰ (1σ) for δ³⁴S.The in situ fluorination of sulfides with a KrF excimer laser (λ = 248 nm) was validated by comparison of measurements of side-by-side laser craters and powders excavated from drill holes. Powders from drill holes were analyzed with the CO₂ laser. In situ laser craters and drill hole powders give the same δ³⁴S_V-CDT and δ³³S_V-CDT values within 0.2‰. The δ³⁴S_V-CDT and δ³³S_V-CDT values of both powders and in situ analyses are independent of F₂ gas pressure over a range of 15 to 65 torr. No dependence of δ³⁴S_V-CDT and δ³³S_V-CDT values on UV laser energy fluence has been observed. Mineral-specific fractionation of sulfur isotopes in analyzing pyrite, sphalerite, galena, troilite, and chalcopyrite has not been observed with a KrF excimer laser (λ = 248 nm). Test analyses with an ArF excimer laser (λ = 193 nm), however, gave fractionated sulfur isotope ratios.A range of Δ³³S anomalies of from - 1.5 to +3.0‰ in Archean samples from the North Pole district, Pilbara Craton, Australia, and from black shale of the Lokamonna Formation, South Africa, were verified by in situ analysis of individual pyrite grains with a KrF excimer laser. These results show that a combination of high-accuracy, high-precision analyses with improved spatial resolution permits locating and analyzing host minerals of non-mass-dependent sulfur isotope anomalies.