Diffusion-controlled growth of albite and pyroxene reaction rims

The growth rates of albite and pyroxene (enstatite + diopside + spinel) reaction rims were measured at 1000°C and ˜700 MPa and found to be parabolic indicating diffusion-controlled growth. The parabolic rate constants for the pyroxene (+ spinel) rims in samples with 0.5 wt% H₂O added or initially vacuum dried at 25°C and 250°C are 1.68 ± 0.09, 0.54 ± 0.05 and 0.25 ± 0.06 μm²/h, respectively. The values for albite rim growth in samples initially dried at 60°C and with 0.1 wt% H₂O added are 0.25 ± 0.04 and 0.33 ± 0.03 μm²/h, respectively. The latter values were used to derive the product of the grain boundary diffusion coefficient D′_A, where A = SiO₂, NaAlO₂, or NaAlSi₋₁, and the grain boundary thickness δ in albite. The calculated D′_SIO2δ in the albite aggregate for the situations of two different water contents are about 9.9 × 10⁻²³ and 1.4 × 10⁻²² m³ s⁻¹, respectively. Both the rate constants and the calculated D′_Aδ demonstrate that the effect of water content on the grain boundary diffusion rate in monomineralic albite and polymineralic pyroxene (+ spinel) aggregates is small, consistent with recent studies of monomineralic enstatite and forsterite rims. Received: 1 July 1995 / Accepted: 1 August 1996     

Volume and grain boundary diffusion of calcium in natural and hot-pressed calcite aggregates

 Calcium self-diffusion rates in natural calcite single crystals were experimentally determined at 700 to 900° C and 0.1 MPa in a stream of CO₂. Diffusion coefficients (D) were determined from ⁴²Ca concentration profiles measured with an ion microprobe. The Arrhenius parameters yield an activation energy (Q)=382±37 kJ/mol and pre-exponential factor (D₀)=0.13 m²/s, and there is no measurable anisotropy. Calcium grain boundary diffusion rates were experimentally determined in natural (Solnhofen) limestone and hot-pressed calcite aggregates at 650° to 850° C and 0.1 to 100 MPa pressure. The Solnhofen limestone was first pre-annealed for 24 h at 700° C and 100 MPa confining pressure under anhydrous conditions to produce an equilibrium microstructure for the diffusion experiments. Values for the product of the grain boundary diffusion coefficient (D′) and the effective grain boundary diffusion width (δ) were determined from ⁴²Ca concentration profiles measured with an ion microprobe. The results show that there is no measurable difference between D′δ values obtained for pre-annealed Solnhofen samples at 0.1 and 100 MPa or between hot-pressed calcite aggregates and pre-annealed Solnhofen samples. The temperature dependence for calcium grain boundary diffusion in Solnhofen samples annealed at 0.1 MPa is described by the Arrhenius parameters D^′ ₀δ=1.5×10⁻⁹ m³/s and Q=267±47 kJ/mol. Comparison of the results of this study with previously published data show that calcium is the slowest volume diffusing species in calcite. The calcium diffusivities measured in this study place constraints on several geological processes that involve diffusive mass transfer including diffusion-accommodated mechanisms in the deformation of calcite rocks. Received: 19 December 1994/Accepted: 30 June 1995     

Organic matter in Silurian rocks from the Chernov uplift

This study presents data on the composition of organic matter from the Late Silurian sediments of the Chernov uplift. These sediments are characterized by low C_org contents, which may reach 1–3% in individual layers. A relatively high thermal maturity of organic matter is confirmed by polycyclic biomarker distributions and Rock-Eval pyrolyisis data. Despite its higher thermal maturity level (T _max = 456°C), kerogen in carbonaceous shales from the Padymeityvis River exhibits good preservation of long-chain n-alkyl structures, which are readily identified in the ¹³C NMR spectra and by the molecular analysis of the kerogen pyrolysis products.     

Gold ore-forming fluids of the Tanami region, Northern Australia

Fluid inclusion studies have been carried out on major gold deposits and prospects in the Tanami region to determine the compositions of the associated fluids and the processes responsible for gold mineralization. Pre-ore, milky quartz veins contain only two-phase aqueous inclusions with salinities ≤19 wt% NaCl eq. and homogenization temperatures that range from 110 to 410°C. In contrast, the ore-bearing veins typically contain low to moderate salinity (<14 wt% NaCl eq.), H₂O + CO₂ ± CH₄ ± N₂-bearing fluids. The CO₂-bearing inclusions coexist with two-phase aqueous inclusions that exhibit a wider range of salinities (≤21 wt% NaCl eq.). Post-ore quartz and carbonate veins contain mainly two-phase aqueous inclusions, with a last generation of aqueous inclusions being very CaCl₂-rich. Salinities range from 7 to 33 wt% NaCl eq. and homogenization temperatures vary from 62 to 312°C. Gold deposits in the Tanami region are hosted by carbonaceous or iron-rich sedimentary rocks and/or mafic rocks. They formed over a range of depths at temperatures from 200 to 430°C. The Groundrush deposit formed at the greatest temperatures and depths (260–430°C and ≤11 km), whereas deposits in the Tanami goldfield formed at the lowest temperatures (≥200°C) and at the shallowest depths (1.5–5.6 km). There is also evidence in the Tanami goldfield for late-stage isothermal mixing with higher salinity (≤21 wt% NaCl eq.) fluids at temperatures between 100 and 200°C. Other deposits (e.g., The Granites, Callie, and Coyote) formed at intermediate depths and at temperatures ranging from 240 to 360°C. All ore fluids contained CO₂ ± N₂ ± CH₄, with the more deeply formed deposits being enriched in CH₄ and higher level deposits being enriched in CO₂. Fluids from deposits hosted mainly by sedimentary rocks generally contained appreciable quantities of N₂. The one exception is the Tanami goldfield, where the quartz veins were dominated by aqueous inclusions with rare CO₂-bearing inclusions. Calculated δ ¹⁸O values for the ore fluids range from 3.8 to 8.5‰ and the corresponding δD values range from −89 to −37‰. Measured δ ¹³C values from CO₂ extracted from fluid inclusions ranged from −5.1 to −8.4‰. These data indicate a magmatic or mixed magmatic/metamorphic source for the ore fluids in the Tanami region. Interpretation of the fluid inclusion, alteration, and structural data suggests that mineralization may have occurred via a number of processes. Gold occurs in veins associated with brittle fracturing and other dilational structures, but in the larger deposits, there is also an association with iron-rich rocks or carbonaceous sediments, suggesting that both structural and chemical controls are important. The major mineralization process appears to be boiling/effervescence of a gas-rich fluid, which leads to partitioning of H₂S into the vapor phase resulting in gold precipitation. However, some deposits also show evidence of desulfidation by fluid–rock interaction and/or reduction of the ore-fluid by fluid mixing. These latter processes are generally more prevalent in the higher crustal-level deposits.     

Genesis of organic and carbonate carbon in sediments of the North and Middle Caspian basins inferred from the isotope data

Isotopic compositions of organic (δ¹³C-C_org) and carbonate (δ¹³C-C_carb) carbon were analyzed in the particulate matter (hereafter, particulates) and sediments from the North and Middle Caspian basins. Isotopic composition of C_org was used for assessing proportions of the allochthonous and autochthonous organic matter in the particulates. Difference between the δ¹³C-C_org values in surface sediments and particulates is explained by the aerobic and anaerobic diagenetic transformations. Isotopic composition of C_org in sediments may be used as a tool for reconstructing the Quaternary transgressive-regressive history of the Caspian Sea.     

A fluid inclusion and stable isotope study of 200 Ma of fluid evolution in the Galway Granite, Connemara, Ireland

Fluid inclusions in granite quartz and three generations of veins indicate that three fluids have affected the Caledonian Galway Granite. These fluids were examined by petrography, microthermometry, chlorite thermometry, fluid chemistry and stable isotope studies. The earliest fluid was a H₂O-CO₂-NaCl fluid of moderate salinity (4–10 wt% NaCl eq.) that deposited late-magmatic molybdenite mineralised quartz veins (V₁) and formed the earliest secondary inclusions in granite quartz. This fluid is more abundant in the west of the batholith, corresponding to a decrease in emplacement depth. Within veins, and to the east, this fluid was trapped homogeneously, but in granite quartz in the west it unmixed at 305–390 °C and 0.7–1.8 kbar. Homogeneous quartz δ¹⁸O across the batholith (9.5 ± 0.4‰n = 12) suggests V₁ precipitation at high temperatures (perhaps 600 °C) and pressures (1–3 kbar) from magmatic fluids. Microthermometric data for V₁ indicate lower temperatures, suggesting inclusion volumes re-equilibrated during cooling. The second fluid was a H₂O-NaCl-KCl, low-moderate salinity (0–10 wt% NaCl eq.), moderate temperature (270–340 °C), high δD (−18 ± 2‰), low δ¹⁸O (0.5–2.0‰) fluid of meteoric origin. This fluid penetrated the batholith via quartz veins (V₂) which infill faults active during post-consolidation uplift of the batholith. It forms the most common inclusion type in granite quartz throughout the batholith and is responsible for widespread retrograde alteration involving chloritization of biotite and hornblende, sericitization and saussuritization of plagioclase, and reddening of K-feldspar. The salinity was generated by fluid-rock interactions within the granite. Within granite quartz this fluid was trapped at 0.5–2.3 kbar, having become overpressured. This fluid probably infiltrated the Granite in a meteoric-convection system during cooling after intrusion, but a later age cannot be ruled out. The final fluid to enter the Granite and its host rocks was a H₂O-NaCl-CaCl₂-KCl fluid with variable salinity (8–28 wt% NaCl eq.), temperature (125–205 °C), δD (−17 to −45‰), δ¹⁸O (−3 to + 1.2‰), δ¹³C_CO2 (−19 to 0‰) and δ³⁴S_sulphate (13–23‰) that deposited veins containing quartz, fluorite, calcite, barite, galena, chalcopyrite sphalerite and pyrite (V₃). Correlations of salinity, temperature, δD and δ¹⁸O are interpreted as the result of mixing of two fluid end-members, one a high-δD (−17 to −8‰), moderate-δ¹⁸O (1.2–2.5‰), high-δ¹³C_CO2 (> −4‰), low-δ³⁴S_sulphate (13‰), high-temperature (205–230 °C), moderate-salinity (8–12 wt% NaCl eq.) fluid, the other a low-δD (−61 to −45‰), low-δ¹⁸O (−5.4 to −3‰), low-δ¹³C (<−10‰), high-δ³⁴S_sulphate (20–23‰) low-temperature (80–125 °C), high-salinity (21–28 wt% NaCl eq.) fluid. Geochronological evidence suggests V₃ veins are late Triassic; the high-δD end-member is interpreted as a contemporaneous surface fluid, probably mixed meteoric water and evaporated seawater and/or dissolved evaporites, whereas the low-δD end-member is interpreted as a basinal brine derived from the adjacent Carboniferous sequence. This study demonstrates that the Galway Granite was a locus for repeated fluid events for a variety of reasons; from expulsion of magmatic fluids during the final stages of crystallisation, through a meteoric convection system, probably driven by waning magmatic heat, to much later mineralisation, concentrated in its vicinity due to thermal, tectonic and compositional properties of granite batholiths which encourage mineralisation long after magmatic heat has abated. Received: 3 April 1996 / Accepted: 5 May 1997     

Fluid evolution at the Variscan front in the vicinity of the Aachen thrust

Quartz–carbonate–chlorite veins were studied in borehole samples of the RWTH-1 well in Aachen. Veins formed in Devonian rocks in the footwall of the Aachen thrust during Variscan deformation and associated fluid flow. Primary fluid inclusions indicate subsolvus unmixing of a homogenous H₂O–CO₂–CH₄–(N₂)–Na–(K)–Cl fluid into a H₂O–Na–(K)–Cl solution and a vapour-rich CO₂–(H₂O, CH₄, N₂) fluid. The aqueous end-member composition resembles that of metamorphic fluids of the Variscan front zone with salinities ranging from 4 to 7% NaCl equiv. and maximum homogenisation temperatures of close to 400°C. Pressure estimates indicate a burial depth between 4,500 and 8,000 m at geothermal gradients between 50 and 75°C/26 MPa, but pressure decrease to sublithostatic conditions is also indicated, probably as a consequence of fracture opening during episodic seismic activity. A second fluid system, mainly preserved in pseudo-secondary and secondary fluid inclusions, is characterised by fluid temperatures between 200 and 250°C and salinities of <5% NaCl equiv. Bulk stable isotope analyses of fluids released from vein quartz, calcite, and dolomite by decrepitation yielded δD_H2O values from −89 to −113 ‰, δ¹³C_CH4 from −26.9 to −28.9‰ (VPDB) and δ¹³C_CO2 from −12.8 to −23.3‰ (VPDB). The low δD and δ¹³C range of the fluids is considered to be due to interaction with cracked hydrocarbons. The second fluid influx caused partial isotope exchange and disequilibrium. It is envisaged that an initial short lived flux of hot metamorphic fluids expelled from the epizonal metamorphic domains of the Stavelot–Venn massif. The metamorphic fluid was focused along major thrust faults of the Variscan front zone such as the Aachen thrust. A second fluid influx was introduced from formation waters in the footwall of the Aachen thrust as a consequence of progressive deformation. Mixing of the cooler and lower salinity formation water with the hot metamorphic fluid during episodic fluid trapping resulted in an evolving range of physicochemical fluid inclusion characteristics.     

The effects of metamorphism on O and Fe isotope compositions in the Biwabik Iron Formation, northern Minnesota

The Biwabik Iron Formation of Minnesota (1.9 Ga) underwent contact metamorphism by intrusion of the Duluth Complex (1.1 Ga). Apparent quartz–magnetite oxygen isotope temperatures decrease from ∼700°C at the contact to ∼375°C at 2.6 km distance (normal to the contact in 3D). Metamorphic pigeonite at the contact, however, indicates that peak temperatures were greater than 825°C. The apparent O isotope temperatures, therefore, reflect cooling, and not peak metamorphic conditions. Magnetite was reset in δ¹⁸O as a function of grain size, indicating that isotopic exchange was controlled by diffusion of oxygen in magnetite for samples from above the grunerite isograd. Apparent quartz–magnetite O isotope temperatures are similar to calculated closure temperatures for oxygen diffusion in magnetite at a cooling rate of ∼5.6°C/kyr, which suggests that the Biwabik Iron Formation cooled from ∼825 to 400°C in ∼75 kyr at the contact with the Duluth Complex. Isotopic exchange during metamorphism also occurred for Fe, where magnetite–Fe silicate fractionations decrease with increasing metamorphic grade. Correlations between quartz–magnetite O isotope fractionations and magnetite–iron silicate Fe isotope fractionations suggest that both reflect cooling, where the closure temperature for Fe was higher than for O. The net effect of metamorphism on δ¹⁸O–δ⁵⁶Fe variations in magnetite is a strong increase in δ¹⁸O_Mt and a mild decrease in δ⁵⁶Fe with increasing metamorphic grade, relative to the isotopic compositions that are expected at the low temperatures of initial magnetite formation. If metamorphism of Iron Formations occurs in a closed system, bulk O and Fe isotope compositions may be preserved, although re-equilibration among the minerals may occur for both O and Fe isotopes. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

The significance of fixed ammonium in Palaeozoic sediments for the generation of nitrogen-rich natural gases in the North German Basin

In contrast to predominantly hydrocarbon-rich natural gases in the western part of the Central European Basin (CEB), accumulations of natural gases from the eastern part of the North German Basin (NGB) are nitrogen-rich with up to 90% N₂. This study is focused on the behaviour of fixed ammonium in clay minerals of organic-rich Palaeozoic sediments in the eastern part of the NGB as a major source of nitrogen-rich natural gases. Carboniferous shales have been investigated for a better understanding of nitrogen fixing during diagenesis, storage during burial and release during devolatilization processes or fluid–rock interactions. The total nitrogen contents in the studied Carboniferous shales of the NGB reach up to 2700 ppm with an inorganic fixed portion (in the form of NH₄ ⁺–N) of more than 60%. The results of this study indicate an increasing proportion of the mineralogically fixed ammonium with increasing thermal maturity and storage up to catagenetic conditions. The isotopic composition of fixed-NH₄ is relatively homogeneous in the majority of the shales and ranges from +1 to +3.5‰. In contrast, samples from the basin centre show a significant decrease in ammonium contents down to 460 ppm coupled with a shift in δ¹⁵N up to +5.6‰ suggesting a release of nitrogen on a large scale. Calculation of nitrogen loss and isotopic fractionation indicate that more than 30% of nitrogen was released as ammonium probably as a consequence of fluid-rock interaction with highly saline brines.     

Nitrogen isotopic studies in the suboxic Arabian Sea

Measurements of¹⁵N/¹⁴N in dissolved molecular nitrogen (N₂), nitrate (NO ₃ ⁻ ) and nitrous oxide (N₂O) and¹⁸O/¹⁶O in N₂O [expressed as δ¹⁵N and δ¹⁸O, relative to atmospheric N₂ and oxygen (O₂), respectively] have been made in water column at several locations in the Arabian Sea, a region with one of the thickest and most intense O₂ minima observed in the open ocean. Microbially-mediated reduction of NO ₃ ⁻ to N₂ (denitrification) in the oxygen minimum zone (OMZ) appears to greatly affect the natural isotopic abundances. The δ¹⁵N of NO ₃ ⁻ increases from 6‰ in deep waters (2500 m) to 15‰ within the core of the denitrifying layer (250–350 m); the δ¹⁵N of N₂ concurrently decreases from 0.6‰ to 0.20‰ Values of the isotopic fractionation factor (ε) during denitrification estimated using simple advection-reaction and diffusion-reaction models are 22‰ and 25‰, respectively. A strong decrease in δ¹⁵N of NO ₃ ⁻ is observed from ∼ 200m (> 11‰) to 80m (∼ 6‰); this is attributed to the input of isotopically light nitrogen through nitrogen fixation. Isotopic analysis of N₂O reveals extremely large enrichments of both¹⁵N and¹⁸O within the OMZ, presumably due to the preferential reduction of lighter N₂O to N₂. However, isotopically light N₂O is observed to accumulate in high concentrations above the OMZ indicating that the N₂O emitted to the atmosphere from this region cannot be very heavy. The isotope data from the intense upwelling zone off the southwest coast of India, where some of the highest concentrations of N₂O ever found at the sea surface are observed, show moderate depletion of¹⁵N, but slight enrichment of¹⁸O relative to air. These results suggest that the ocean-atmosphere exchange cannot counter inputs of heavier isotopes (particularly¹⁸O) associated with the stratospheric back flux, as proposed by previous workers. This calls for additional sources and/or sinks of N₂O in the atmosphere. Also, the N₂O isotope data cannot be explained by production through either nitrification or denitrification, suggesting a possible coupling between the two processes as an important mechanism of N₂O production.     

Contact metamorphism in the Los Santos W skarn (NW Spain)

Summary The intrusion of the Lower Permian Los Santos-Valdelacasa granitoids in the Los Santos area caused contact metamorphism of Later Vendian-Lower Cambrian metasediments. High grade mineral assemblages are confined to a 7 km wide contact aureole. Contact metamorphism was accompanied by intense metasomatism and development of skarns, and it generated the following mineral assemblages: diopside, forsterite, phlogopite (±clintonite) and humites and spinel-bearing assemblages or diopside, grossular, vesuvianite ± wollastonite in the marbles, depending on the bulk rock composition. Cordierite, K-feldspar, andalusite and, locally, sillimanite appear in the metapelitic rocks. Mineral assemblages of marbles and hornfelses indicate pressure conditions ranging from 0.2 to 0.25 GPa and maximum temperatures between 630 and 640 °C. ¹³C and ¹⁸O depletions in calcite marbles are consistent with hydrothermal fluid–rock interaction during metamorphism. Calcites are depleted in both ¹⁸O (δ¹⁸O = 12.74‰) and ¹³C (δ¹³C = −5.47‰) relative to dolomite of unmetamorphosed dolostone (δ¹⁸O = 20.79‰ and δ¹³C = −1.52‰). The δ¹³C variation can be interpreted in terms of Rayleigh distillation during continuous CO₂ fluid removal from the carbonates. The δ¹⁸O values reflect hydrothermal exchange with an externally derived fluid. Microthermometric analyses of fluid inclusions from vesuvianite indicate that the fluid was water dominated with minor contents of CO₂ (±CH₄ ± N₂) suggesting a metamorphic origin. Fluorine-bearing minerals such as chondrodite, norbergite and F-rich phlogopite indicate that contact metamorphism was accompanied by fluorine metasomatism. Metasomatism was more intense in the inner-central portion of the contact aureole, where access to fluids was extensive. The irregular geometry of the contact with small aplitic intrusives between the metasediments and the Variscan granitoids probably served as pathways for fluid circulation.     

Stable isotope variations in a coral (Favia speciosa) from the Gulf of Kutch during 1948–1989 A.D.: Environmental implications

The stable isotopic analyses (δ¹⁸O and δ¹³C) of a coralFavia speciosa spanning forty two years (1948–89 A.D.), collected from the Pirotan island (22.6°N, 70°E) in the Gulf of Kutch have been carried out to assess its potential for retrieving past environmental changes in this region. It is seen that the summer (minima) δ¹⁸O variations in the coral CaCO₃ are negatively correlated with seasonal (summer) monsoon rainfall in the adjoining region of Kutch and Saurashtra and a qualitative reconstruction of historical rainfall variations in this region can be obtained by analyzing the δ¹⁸O in this species of coral. The observed mean seasonal range of δ¹⁸O variations is 0.34 ±0.17‰ (n = 42), whereas the expected range calculated (from available SST and measured δ¹⁸O of sea water) is ∼ 1.1 ±0.15‰ The difference is due to the coarse resolution of sampling, which can be corrected. The seasonal range in δ¹³C is ∼ l‰ and is explained by changes in: a) the light intensity related to the cloudiness during monsoons and b) phytoplankton productivity.     

Isotopic Geochemistry of Non—hydrocarbons in Natural Gases from the Sanshui Basin,Guangdong Province,China

This study is focused on geothermal heat flow and the origin of non-hydrocarbons in natural gases in terms of the isotope geochemical characteristics of Ar, He, CO₂ and N₂ in natural gases from the Sanshui Basin, Guangdong Province. China.³He/⁴He ratios are of (1.60-6.39) × 10^-6, and⁴⁰Ar/³⁶Ar ratios of 450–841. The carbon isotopic composition (δ_l3C PDB) of carbon dioxide ranges from -20‰ to -2‰. δ^l5N(air) ratios have a wider range of-57 ‰- +95 ‰. The isotope geochemical characteristics of non-hydrocarbons indicate that He, Ar and N₂ in the gas reservoirs enriched in non-hydrocarbons were derived largely from the upper mantle. Non-hydrocarbons in gaseous hydrocarbon reservoirs consist mainly of crustal radiogenic He and⁴⁰Ar and some mantle-derived He and Ar, as well as of¹³C-depleted carbon dioxide and nitrogen generated as a result of thermal decomposition of organic matter in strata. Carbon dioxide enriched in¹³C was derived largely from carbonate rocks and partially from the lower crust and upper mantle. Based on the relationship between geothermal heat flow (Q) and³He/⁴ He ratio in natural gases, the Q values for the area studied have been calculated. Similar Q values are reported from the upper mantle uplift area (77 mWm^-2) in Huabei and the Tancheng-Lujiang Rift Zone (88 mWm^-2). More than 60 percent of geothermal heat flow in the Sanshui Basin may have been derived from the upper mantle. The project is financially supported by the National Natural Science Foundation of China.     

Sealing of fluid pathways in overpressure cells: a case study from the Buntsandstein in the Lower Saxony Basin (NW Germany)

We studied veins in the Triassic Buntsandstein of the Lower Saxony Basin (NW Germany) with the aim of quantifying the evolution of in-situ stress, fluids and material transport. Different generations of veins are observed. The first generation formed in weakly consolidated rocks without a significant increase in fracture permeability and was filled syntectonically with fibrous calcite and blocky to elongate-blocky quartz. The stable isotopic signature (δ¹⁸O and δ¹³C) indicates that the calcite veins precipitated from connate water at temperatures of 55–122°C. The second vein generation was syntectonically filled with blocky anhydrite, which grew in open fractures. Fluid inclusions indicate that the anhydrite veins precipitated at a minimum temperature of 150°C from hypersaline brines. Based on δ³⁴S measurements, the source of the sulphate was found in the underlying Zechstein evaporites. The macro- and microstructures indicate that all veins were formed during subsidence and that the anhydrite veins were formed under conditions of overpressure, generated by inflation rather than non-equilibrium compaction. The large amount of fluids which are formed by the dehydrating gypsum in the underlying Zechstein and are released into the Buntsandstein during progressive burial form a likely source of overpressures and the anhydrite forming fluids.     

Hydrogeological investigations of thermal waters in the Sfax Basin (Tunisia)

The Sfax Basin in eastern Tunisia is bounded to the east by the Mediterranean Sea. Thermal waters of the Sfax area have measured temperatures of 23–36°C, and electrical conductivities of 3,200 and 14,980 μS/cm. Most of the thermal waters are characterized as Na–Cl type although there are a few Na–SO₄–Cl waters. They issue from Miocene units which are made up sands and sandstones interbedded with clay. The Quaternary sediments cap the system. The heat source is high geothermal gradient which are determined downhole temperature measurements caused by graben tectonics of the area. The results of mineral equilibrium modeling indicate that the thermal waters of the Sfax Basin are undersaturated with respect to gypsum, anhydrite and fluorite, oversaturated with respect to kaolinite, dolomite, calcite, microcline, quartz, chalcedony, and muscovite. Assessments from various chemical geothermometers, Na–K–Mg ternary and mineral equilibrium diagrams suggest that the reservoir temperature of the Sfax area can reach up to 120°C. According to δ¹⁸O and δ²H values, all thermal and cold groundwater is of meteoric origin.     

Isotope geochemistry and fluid inclusion study of skarns from Vesuvius

Summary We present new mineral chemistry, fluid inclusion, stable carbon and oxygen, as well as Pb, Sr, and Nd isotope data of Ca-Mg-silicate-rich ejecta (skarns) and associated cognate and xenolithic nodules from the Mt. Somma-Vesuvius volcanic complex, Italy. The typically zoned skarn ejecta consist mainly of diopsidic and hedenbergitic, sometimes “fassaitic” clinopyroxene, Mg-rich and Ti-poor phlogopite, F-bearing vesuvianite, wollastonite, gehlenite, meionite, forsterite, clinohumite, anorthite and Mg-poor calcite with accessory apatite, spinell, magnetite, perovskite, baddeleyite, and various REE-, U-, Th-, Zr- and Ti-rich minerals. Four major types of fluid inclusions were observed in wollastonite, vesuvianite, gehlenite, clinopyroxene and calcite: a) primary silicate melt inclusions (T_HOM = 1000–1050 °C), b) CO₂ ± H₂S-rich fluid inclusions (T_HOM = 20–31.3 °C into the vapor phase), c) multiphase aqueous brine inclusions (T_HOM = 720–820 °C) with mainly sylvite and halite daughter minerals, and d) complex chloride-carbonate-sulfate-fluoride-silicate-bearing saline-melt inclusions (T_HOM = 870–890 °C). The last inclusion type shows evidence for immiscibility between several fluids (silicate melt – aqueous chloride-rich liquid – carbonate/sulfate melt?) during heating and cooling below 870 °C. There is no evidence for fluid circulation below 700 °C and participation of externally derived meteoric fluids in skarn formation. Skarns have considerably variable ²⁰⁶Pb/²⁰⁴Pb (19.047–19.202), ²⁰⁷Pb/²⁰⁴Pb (15.655–15.670), and ²⁰⁸Pb/²⁰⁴Pb (38.915–39.069) and relatively low ¹⁴³Nd/¹⁴⁴Nd (0.51211–0.51244) ratios. The carbon and oxygen isotope compositions of skarn calcites (δ¹³C_V-PDB = −5.4 to −1.1‰; δ18O_V-SMOW = 11.7 to 16.4‰) indicate formation from a ¹⁸O- and ¹³C-enriched fluid. The isotope composition of skarns and the presence of silicate melt inclusion-bearing wollastonite nodules suggests assimilation of carbonate wall rocks by the alkaline magma at moderate depths (< 5 km) and consequent exsolution of CO₂-rich vapor and complex saline melts from the contaminated magma that reacted with the carbonate rocks to form skarns. Received March 1, 2000; revised version accepted November 2, 2000     

Isotope geochemistry of ore fluids for the Dongsheng sandstone-type uranium deposit, China

The Dongsheng sandstone-type uranium deposit is one of the large-sized sandstone-type uranium deposits discovered in the northern part of the Ordos Basin of China in recent years. Geochemical characteristics of the Dongsheng uranium deposit are significantly different from those of the typical interlayered oxidized sandstone-type uranium ore deposits in the region of Middle Asia. Fluid inclusion studies of the uranium deposit showed that the uranium ore-forming temperatures are within the range of 150–160℃. Their 3He/4He ratios are within the range of 0.02–1.00 R/Ra, about 5–40 times those of the crust. Their 40Ar/36Ar ratios vary from 584 to 1243, much higher than the values of atmospheric argon. The δ18OH2O and δD values of fluid inclusions from the uranium deposit are -3.0‰– -8.75‰ and -55.8‰– -71.3‰, respectively, reflecting the characteristics of mixed fluid of meteoric water and magmatic water. The δ18OH2O and δD values of kaolinite layer at the bottom of the uranium ore deposit are 6.1‰ and -77‰, respectively, showing the characteristics of magmatic water. The δ13CV-PDB and δ18OH2O values of calcite veins in uranium ores are -8.0‰ and 5.76‰, respectively, showing the characteristics of mantle source. Geochemical characteristics of fluid inclusions indicated that the ore-formation fluid for the Dongsheng uranium deposit was a mixed fluid of meteoric water and deep-source fluid from the crust. It was proposed that the Jurassic-Cretaceous U-rich metamorphic rocks and granites widespread in the northern uplift area of the Ordos Basin had been weathered and denudated and the ore-forming elements, mainly uranium, were transported by meteoric waters to the Dongsheng region, where uranium ores were formed. Tectonothermal events and magmatic activities in the Ordos Basin during the Mesozoic made fluids in the deep interior and oil/gas at shallow levels upwarp along the fault zone and activated fractures, filling into U-bearing clastic sandstones, thus providing necessary energy for the formation of uranium ores.     

Laboratory thermal conversion of sedimentary lipids to kerogen-like matter

A laboratory heating experiment was conducted in an attempt to evaluate the possible role of lipids as precursors for petroleum hydrocarbons. Lipids were extracted from a Recent lake sediment (Lake Haruna, Japan), and heated under N₂ atmosphere, at 125–370°C, for 1–7 days. A significant amount of lipids was polymerized to kerogen-like matter (lipid-derived kerogen) at the low temperature of 175°C for 1 day. The polymerization follows first-order kinetics, and the half life of lipids is calculated to be 10⁴–10⁵ yr at 0–30°C. The lipid-derived kerogen generated a significant amount (62 mg/g) of n-alkanes (C₁₄–C₃₆) on heating at 350°C for 1 day.The results indicate a possible occurrence of lower temperature thermal polymerization of lipids in a relatively early stage of diagenesis as one of the formation pathways of kerogen with high hydrocarbon producing potential.     

Sources and transformations of N in reclaimed coastal tidelands: evidence from soil δ<Superscript>15</Superscript>N data

Electrical conductivity of saturated soil extracts (EC_e) in three reclaimed tideland (RTL) soils on the west coast of Korea decreased with time since reclamation, indicating natural desalinization through leaching of salts by precipitation water. Soil N concentration increased with decreasing EC_e. With the increase in soil N concentration, the δ¹⁵N decreased, likely caused by the input of ¹⁵N-depleted N sources. As N₂-fixing plant species were found in the oldest RTL, atmospheric N₂ fixation likely contributed to the increase in soil N concentration in the oldest RTL. Negative δ¹⁵N (−7.1 to −2.0‰) of total inorganic N (NH₄ ⁺+NO₃ ⁻) and published data on N deposition near the study area indicate that atmospheric N deposition might be another source of N in the RTLs. Meanwhile, the consistently negative δ¹⁵N of soil NO₃ ⁻ excluded N input from chemical fertilizer through groundwater flow as a potential N source, since NO₃ ⁻ in groundwater generally have a positive δ¹⁵N. The patterns of δ¹⁵N of NH₄ ⁺ (+2.3 to +5.1‰) and NO₃ ⁻ (−9.2 to −5.0‰) suggested that nitrification was an active process that caused ¹⁵N enrichment in NH₄ ⁺ but denitrification was probably minimal which would otherwise have caused ¹⁵N enrichment in NO₃ ⁻. A quantitative approach on N budget would provide a better understanding of soil N dynamics in the studied RTLs.     

Pyrolysis-GC characterization of whole rock and kerogen-concentrate samples of immature Jurassic source rocks from NW-Germany

Nine rock samples from three Jurassic stratigraphic units of a shallow core from NW Germany were analyzed by pyrolysis-gas chromatography. The units contain a mixed Type-II/III kerogen (Dogger-α), a hydrogen-rich Type-II kerogen (Lias-), and a hydrogen-poor Type-III kerogen (Lias-δ). All of the kerogen was immature (R_o = 0.5%). Two sets of kerogen concentrates (“AD”: HCl/HF followed by a density separation, and “A”: only acid treatment) prepared from the rock samples were also analyzed to make a detailed comparison of the pyrolysates of rock and corresponding kerogen-concentrates.Hydrogen-index (HI) values of the kerogen concentrates prepared from organic-carbon poor rock were nearly 200% higher than HI values of the rock samples. Changes in HI were minimal for the samples containing Type-II kerogen. The A and AD samples from the C_org-poor rock yielded pyrolysates with n-alkane series of very different molecular lengths. Pyrograms of the rock samples had n-alkane series extending to n-C₁₄; the chromatograms of the A samples reached the n-C₁₄-nC₂₀ range. The AD samples from C_org-poor rock and all three sample types from the C_org-rich rock had n-alkane series up to n-C₂₉. The benzene/hexane and toluene/heptane ratios for the C_org-poor rock and A samples were far higher than for the AD samples, which had ratios similar to those of all three sample types from the C_org-rich rocks. These results indicate that choice of kerogen preparation method is critical when C_org-poor samples are analyzed.