显生宙全球海水化学成分演化及其对蒸发岩沉积的约束

通过搜集显生宙以来不同地质时期内海相碳酸盐岩鲕粒及胶结物矿物成分、钾盐矿床矿物种类及组合特征、蒸发岩盆地中石盐流体包裹体成分,并利用这些资料与人工海水模拟实验得到的石盐中Br分配特征的对比,得出海水成分在5.5亿年以来的显生宙期间,经历了五个阶段:其中晚元古代至寒武纪早期、二叠纪早期至中生代早期、新生代早期至现今,这些时期的原始海水组成特征系数m(SO_4~(2-))+m(HCO_3~-)/2m(Ca~(2+)),为Na-Mg-K-SO_4-Cl型海水,此期间沉积的钾盐矿床的钾镁盐矿物主要为钾盐镁矾、无水钾镁矾、杂卤石、硫酸镁石等含MgSO_4矿物,海相鲕粒和碳酸盐胶结物矿物成分为文石;而寒武纪早期至石炭纪、中生代早期至新生代早期,原始海水组成特征系数m(Ca~(2+))m(SO_4~(2-))+m(HCO_3~-)/2,为Na-Mg-KCa-Cl型海水,此期间沉积的钾镁盐矿物主要为光卤石和钾石盐,甚至含有溢晶石,海相鲕粒和碳酸盐胶结物矿物成分为方解石。根据石盐流体包裹体成分计算得出:显生宙期间,海水K+含量大部分时间变化幅度较小,为9.3～11.5mmol/kg H_2O(除了石炭纪和晚元古代),平均为10.55mmol/kg H_2O。Mg~(2+)含量在早寒武世≥67mmol/kg H_2O、晚志留世至中泥盆世31～41mmol/kg H_2O、晚古生代≥48mmol/kg H22O、晚白垩世34mmol/kg H_2O和现代55.1mmol/kg H_2O。Ca~+含量在晚元古代至古生代早期≤11mmol/kg H_2O、古生代早期至石炭纪22～35mmol/kg H_2O、石炭纪至中生代早期≤17mmol/kg H_2O、中生代早期至新生代早期19～39mmol/kg H_2O及新生代早期至今7～21mmol/kg H_2O。SO_4~(2-)含量在晚元古代至古生代早期≥23mmol/kg H_2O、古生代早期至石炭纪5～17mmol/kg H_2O、石炭纪至中生代早期13～22mmol/kg H_2O、中生代早期至新生代早期5～19mmol/kg H_2O及新生代早期至今12～29.2mmol/kg H_2O。海水Ca~(2+)与SO_4~(2-)含量的相对变化是控制海相钾盐矿床钾镁盐矿物类型的基本因素。同时,利用以上数据计算得到的显生宙各时期海水[m(Mg~(2+))+m(SO_4~(2-))]/[m(K~+)+m(Ca~(2+))]的变化与各时期海相蒸发岩系石盐层底部的Br含量变化具有同步性,进一步验证了显生宙期间海水成分是不断变化的,是约束海相蒸发岩钾盐矿物类型的主要因素。海水成分变化的控制因素为洋中脊热液和陆地水,其中洋中脊热液起主要作用,而控制这些因素变化的根本原因为板块构造运动。     

The major-ion composition of Permian seawater

The major-ion (Mg²⁺, Ca²⁺, Na⁺, K⁺, SO₄²⁻, and Cl⁻) composition of Permian seawater was determined from chemical analyses of fluid inclusions in marine halites. New data from the Upper Permian San Andres Formation of Texas (274-272 Ma) and Salado Formation of New Mexico (251 Ma), analyzed by the environmental scanning electron microscopy (ESEM) X-ray energy-dispersive spectrometry (EDS) method, along with published chemical compositions of fluid inclusions in Permian marine halites from North America (two formations of different ages) and the Central and Eastern European basins (eight formations of four different ages) show that Permian seawater shares chemical characteristics with modern seawater, including SO₄²⁻ > Ca²⁺ at the point of gypsum precipitation, evolution into Mg²⁺-Na⁺-K⁺-SO₄²⁻-Cl⁻ brines, and Mg²⁺/K⁺ ratios ∼5. Permian seawater, however, is slightly depleted in SO₄²⁻ and enriched in Ca²⁺, although modeling results do not rule out Ca²⁺ concentrations close to those in present-day seawater. Na⁺ and Mg²⁺ in Permian seawater are close to (slightly below) their concentrations in modern seawater. Permian and modern seawater are both classified as aragonite seas, with Mg²⁺/Ca²⁺ ratios >2, conditions favorable for precipitation of aragonite and magnesian calcite as ooids and cements.The chemistry of Permian seawater was modeled using the chemical composition of brine inclusions for three periods: Lower Permian Asselian-Sakmarian (296-283 Ma), Lower Permian Artinskian-Kungurian (283-274 Ma), and Upper Permian Tatarian (258-251 Ma). Parallel changes in the chemistry of brine inclusions from equivalent age evaporites in North America, Central Europe, and Eastern Europe show that seawater underwent secular variations in chemistry over the 50 million years of the Permian. Modeled SO₄²⁻ concentrations are 20 mmol per kg H₂O (mmolal) and 19 mmolal in the Asselian-Sakmarian and Artinskian-Kungurian, with higher concentrations in the Upper Permian Tatarian (23 mmolal). Modeled Ca²⁺ is at or above its concentration in modern seawater throughout the Permian. Mg²⁺ is close to (slightly below) its concentration in modern seawater (55 mmolal) in the Asselian-Sakmarian (52 mmolal), and Tatarian (52 mmolal), but slightly higher than modern seawater in the Artinskian-Kungurian (60 mmolal). Mg²⁺/Ca²⁺ ratios are 3.5 (total range = 2.7 to 5.5) in the Lower Permian and rose slightly to 3.7 (total range = 3.1 to 5.8) in the Upper Permian, primarily due to decreases in Ca²⁺. These results are consistent with models that predict oscillations in the major-ion composition of Phanerozoic seawater on the basis of changes in the midocean ridge/river water flux ratio driven by changes in the rate of midocean ridge crust production.The Permian was characterized by low sea levels, icehouse conditions, and southern hemisphere glaciation. Such conditions, analogous to the present ice age, and the similarities between Permian seawater and modern seawater, all suggest that general Phanerozoic supercycles, driven by mantle convection and global volcanicity, also control the major-ion chemistry of seawater.     

The major-ion composition of silurian seawater

One-hundred fluid inclusions in Silurian marine halite were analyzed in order to determine the major-ion composition of Silurian seawater. The samples analyzed were from three formations in the Late Silurian Michigan Basin, the A-1, A-2, and B Evaporites of the Salina Group, and one formation in the Early Silurian Canning Basin (Australia), the Mallowa Salt of the Carribuddy Group. The results indicate that the major-ion composition of Silurian seawater was not the same as present-day seawater. The Silurian ocean had lower concentrations of Mg²⁺, Na⁺, and SO₄²⁻, and much higher concentrations of Ca²⁺ relative to the ocean’s present-day composition. Furthermore, Silurian seawater had Ca²⁺ in excess of SO₄²⁻. Evaporation of Silurian seawater of the composition determined in this study produces KCl-type potash minerals that lack the MgSO₄-type late stage salts formed during the evaporation of present-day seawater. The relatively low Na⁺ concentrations in Silurian seawater support the hypothesis that oscillations in the major-ion composition of the oceans are primarily controlled by changes in the flux of mid-ocean ridge brine and riverine inputs and not global or basin-scale, seawater-driven dolomitization. The Mg²⁺/Ca²⁺ ratio of Silurian seawater was ∼1.4, and the K⁺/Ca²⁺ ratio was ∼0.3, both of which differ from the present-day counterparts of 5 and 1, respectively. Seawaters with Mg²⁺/Ca²⁺ <2 facilitate the precipitation of low-magnesian calcite (mol % Mg < 4) marine ooids and submarine carbonate cements whereas seawaters with Mg²⁺/Ca²⁺ >2 (e.g., modern seawater) facilitate the precipitation of aragonite and high-magnesian calcite. Therefore, the early Paleozoic calcite seas were likely due to the low Mg²⁺/Ca²⁺ ratio of seawater, not the pCO₂ of the Silurian atmosphere.     

Secular variation in the major-ion chemistry of seawater: Evidence from fluid inclusions in Cretaceous halites

The major-ion (Mg²⁺, Ca²⁺, Na⁺, K⁺, , and Cl⁻) chemistry of Cretaceous seawater was determined from analyses of seawater-derived brines preserved as fluid inclusions in marine halites. Fluid inclusions in primary halite from three evaporite deposits were analyzed by the environmental scanning electron microscopy (ESEM) X-ray energy dispersive spectrometry (EDS) technique: the Early Cretaceous (Aptian, 121.0-112.2 Ma) of the Sergipe basin, Brazil and the Congo basin, Republic of the Congo, and the Early to Late Cretaceous (Albian to Cenomanian, 112.2-93.5 Ma) of the Khorat Plateau, Laos, and Thailand. The fluid inclusions in halite indicate that Cretaceous seawater was enriched several fold in Ca²⁺, depleted in , Na⁺, and Mg²⁺, and had lower Na⁺/Cl⁻, Mg²⁺/Ca²⁺, and Mg²⁺/K⁺ ratios compared to modern seawater. Elevated Ca²⁺ concentrations, with Ca²⁺ >  at the point of gypsum saturation, allowed Cretaceous seawater to evolve into Mg²⁺-Ca²⁺-Na⁺-K⁺-Cl⁻ brines lacking measurable .The major-ion composition of Cretaceous seawater was modeled from fluid inclusion chemistries for the Aptian and the Albian-Cenomanian. Aptian seawater was extreme in its Ca²⁺ enrichment, more than three times higher than present day seawater, with a Mg²⁺/Ca²⁺ ratio of 1.1-1.3. Younger, Albian-Cenomanian seawater had lower Ca²⁺ concentrations, and a higher Mg²⁺/Ca²⁺ ratio of 1.2-1.7. Cretaceous (Aptian) seawater has the lowest Mg²⁺/Ca²⁺ ratios so far documented in Phanerozoic seawater from fluid inclusions in halite, and within the range chemically favorable for precipitation of low-Mg calcite ooids and cements. Results from halite fluid inclusions, together with Mg²⁺/Ca²⁺ ratios measured from echinoderm and rudist calcite, all indicate that Early Cretaceous seawater (Hauterivian, Barremian, Aptian, and Albian) had lower Mg²⁺/Ca²⁺ ratios than Late Cretaceous seawater (Coniacian, Santonian, and Campanian). Low Aptian-Albian Mg²⁺/Ca²⁺ seawater ratios coincide with negative excursions of ⁸⁷Sr/⁸⁶Sr ratios and δ³⁴S_SO4, and peak Cretaceous ocean crust production rates, all of which suggests a link between seawater chemistry and midocean ridge hydrothermal brine flux.     

Comparison of two potential strategies of planktonic foraminifera for house building: Mg or H removal?

Marine organisms must possess strategies enabling them to initiate calcite precipitation despite the unfavorable conditions for inorganic precipitation in surface seawater. These strategies are poorly understood. Here we compare two potential strategies of marine calcifyers to manipulate seawater chemistry in order to initiate calcite precipitation: Removal of Mg²⁺ and H⁺ ions from seawater solutions. An experimental setup was used to monitor the onset of inorganic precipitation on seed crystals as a function of the Mg²⁺ concentration and pH in artificial seawater. We focused on precipitation rates typical for biogenic calcification in planktonic foraminifera (∼10⁻³ mol m⁻² h⁻¹) and time scales typical for the initiation of calcification in these organisms (minutes to hours). We find that the carbonate ion concentration has to increase by a factor of ∼13 when [Mg²⁺] increases from 0 to 53 mmol kg⁻¹ in order to maintain a typical biogenic precipitation rate. Model calculations for the energy requirement for various scenarios of Mg²⁺ and H⁺ removal including Ca²⁺ exchange and CO₂ diffusion are presented. We conclude that the more cost-effective strategy to initiate calcite precipitation in foraminifera is H⁺ removal, rather than Mg²⁺ removal.     

Chemical composition of seawater in Neoproterozoic: Results of fluid inclusion study of halite from Salt Range (Pakistan) and Amadeus Basin (Australia)

Data on chemical composition of brines in primary inclusions of marine halites and on mineralogy of marine evaporites and carbonates lead to the conclusion that during the Phanerozoic two long-term cycles of chemical composition of seawater existed. During each of those cycles, seawater dominantly a Na-K-Mg-Ca-Cl (Ca-rich) type changed to a Na-K-Mg-Cl-SO₄ (SO₄-rich) type. Recrystallised halite from the uppermost Neoproterozoic Salt Range Formation (ca. 545 Ma) in Pakistan, contains solitary inclusions indicating SO₄-rich brines. This supports the concept derived from the study on primary fluid inclusions from the Neoproterozoic Ara Formation of Oman; SO₄-rich seawater existed during latest Neoproterozoic time (ca. 545 Ma). In contrast, samples of recrystallised halite from the Bitter Springs Formation (840–830 Ma) in Australia contain inclusion brines that are entirely Ca-rich, indicating that basin brines and seawater were Ca-rich during deposition of central Australian evaporites. These combined data supported by the timing of aragonite and calcite seas suggest that during the Proterozoic, significant oscillations of the chemical composition of marine brines, and seawater, occurred, which are similar to those known to exist during the Phanerozoic. It is suggested that Ca-rich seawater dominated for a substantial period of time (more than 200 Ma), at 650 Ma, this was replaced by SO₄-rich seawater, finally returning to Ca-rich seawater at 530 Ma.     

四川盆地三叠纪古盐湖已达钾石盐析出阶段——来自石盐流体包裹体化学组成的约束

石盐的流体包裹体成分可提供古流体组成的物理化学信息,用以探查卤水组成变化及环境演化规律等。四川盆地位于上扬子地台,其中的早-中三叠纪沉积建造是中国海相找钾的有利层位之一。获取石盐沉积时期的卤水成分信息,是深刻认识四川盆地古海水蒸发浓缩程度的重要途径。文章利用激光剥蚀电感耦合等离子体质谱法,对采自川东地区长平3井嘉陵江组的石盐流体包裹体开展了化学组成分析,结果显示古卤水化学类型为Mg_SO4型;流体包裹体中的ρ(K~+)与现代海水浓缩到钾石盐析出阶段的ρ(K~+)基本一致,可能揭示了盆地三叠纪时期古卤水已达到钾石盐析出阶段,对四川盆地沉积环境演化及钾盐成矿规律研究等具有重要的理论意义。     

Formation of tonalite in island arcs by seawater-induced anatexis of mafic rocks; evidence from the Lyngen Magmatic Complex, North Norwegian Caledonides

The Lyngen Magmatic Complex (LMC) of North Norway, consists of a western suite of layered gabbros of normal-mid oceanic ridge basalt (N-MORB) affinity and an eastern suite of layered gabbronorites, quartz-bearing gabbros and diorites/quartz-diorites of IAT (island-arc tholeiitte) to boninitic affinity. The boundary between the suites is defined by a large-scale ductile shear zone, the Rypdalen shear zone (RSZ). In this shear zone anatectic tonalites were generated by partial melting of the gabbro in the presence of an H₂O bearing fluid phase.Quartz from the tonalites contains early secondary and secondary liquid-dominated inclusions (88-99 wt.% H₂O), with an average salinity of 18 wt.% (calculated as NaCl_eq). Combined gas and ion chromatography shows that the major ions in the fluid are Cl⁻, Ca²⁺, Na⁺ with smaller amounts of K⁺, Mg²⁺, Sr²⁺, Br⁻ and NO₃⁻. The dominant non-H₂O volatile species is N₂ (0.5-10%), and small amounts of CO₂, CH₄ and other hydrocarbons are also present.The cation concentrations in the fluid are variable, due to element exchange during interaction of the fluids with the tonalites, amphibolites and metagabbros of the RSZ. The fluid contributed Na⁺ and K⁺ to the melt and gained Ca²⁺ in exchange, explaining the variable Na⁺/Ca²⁺ ratio of the fluid. The Br⁻ and Cl⁻ contents of the fluid inclusions plot on the same line as evaporating sea water, which strongly suggests a seawater origin for the fluid phase, and a seawater source fits well with other geochemical signatures and the tectonic setting of the LMC.It is suggested that seawater escaped from a subducting slab and was channelled along the Rypdalen shear zone. This caused anatexis of the gabbro, generating tonalitic melts at 0.5-0.9 GPa and 680-800°C.     

The Behavior of Mixed Ca–Mn Carbonates in Water and Seawater: Controls of Manganese Concentrations in Marine Porewaters

The dissolution behavior of natural, ordered kutnahorite (Mn_1.14Ca_0.82Mg_0.04Fe_0.012(CO₃)₂) and a disordered, calcian rhodochrosite (Mn_1.16Ca_0.78Mg_0.06(CO₃)₂) precipitated in the laboratory was investigated in deionized distilled water and artificial seawater in both open and closed systems at 25 °C, one atmosphere total pressure, and various pCO₂s. Both solids dissolved congruently in distilled water in an open system and yielded identical long-term equilibration or extrapolated ion activity products, IAP_pkt = a_Ca ²⁺a_Mn ²⁺(a_{CO 3} ^2?)² = 1.7 (±0.12)× 10^?21 or pIAP_pkt = 20.77 (±0.03). This value is believed to be the thermodynamic solubility product of pseudokutnahorite. In contrast, the steady state ion concentration products, ICP_pkt = [Ca²⁺][Mn²⁺][CO₃ ^2?]², measured following the dissolution of both minerals in artificial seawater increase as the CO₂ partial pressure decreases and the [Mn²⁺]:[Ca²⁺] ratio increases. These observations are interpreted as resulting from the formation of phases of different stoichiometry in response to large variations of the [Mn²⁺]:[Ca²⁺] ratio in solution. These data and results of calcite-seawater equilibration experiments in the presence of various dissolved Mn(II) concentrations define the fields of stability of manganoan calcites and calcian rhodochrosites in seawater within Lippmann phase diagrams for the CaCO₃–MnCO₃–H₂O system. Results of this study reveal that the nature (i.e., mineralogy) and composition of manganese-rich carbonate phases that may form under suboxic/anoxic conditions in marine sediments are dictated by the porewater [Mn²⁺]:[Ca²⁺] ratio, the abundance of calcite surfaces and reaction kinetics.     

The Dolomite Problem Revisited1

A reassessment of the abundance of dolomite in carbonate sediments has confirmed that carbonates deposited during the past 150 Ma contain, on average, less dolomite than Proterozoic and Paleozoic carbonates. The lower dolomite content of the more recent carbonate sediments results from the increase in the deposition of CaCO₃ in deep-sea sediments, and to the difficulty of dolomitizing deep-sea CaCO₃ by reaction with cold, unevaporated seawater. The decrease in the rate of dolomite formation during the past 150 Ma has led to an increase in the output of oceanic Mg+² by the reaction of seawater with clay minerals and with ocean-floor basalts. The increase in the output of marine Mg+² into these reservoirs has been brought about by an increase in the Mg+² concentration of seawater. During the past 40 Ma, the concentration of Mg+² in seawater has probably increased by ～18 mmol/kg, and probably has been accompanied by an equimolar increase in the concentration of SO₄?².     

The estimation of the pK1HA of acids in seawater using the Pitzer equations

The equations of Pitzer have been used to calculate the stoichiometric ionization constants, pK¹_HA, for acids in NaCl media at 25°C. The calculated results for the ionization of HAc, H₂O, B(OH)₃, H₂CO^?₃, H₃PO₄, H₂PO^?₄, HPO^2?₄, H₃AsO₄, H₂AsO^?₄ and HAsO^2?₄ are in good agreement with the measured values, providing higher order interaction terms (θ and ψ) are used. The pK¹_HA measurements of these acids in NaCl media containing Mg²⁺ and Ca²⁺ were used to determine Pitzer specific interaction parameters at I = 0.7. With these Pitzer coefficients, it was possible to make reliable estimates for the activity coefficients of anions in seawater (S = 35) that form strong interactions with Mg²⁺ and Ca²⁺. The calculated activity coefficients yield reliable estimates for the pK¹_HA of acids in seawater.     

Heterogeneous Mg isotopic composition of the early Carboniferous limestone: implications for carbonate as a seawater archive

Carbonate precipitation and hydrothermal reaction are the two major processes that remove Mg from seawater. Mg isotopes are significantly (up to 5‰) fractionated during carbonate precipitation by preferential incorporation of ²⁴Mg, while hydrothermal reactions are associated with negligible Mg isotope fractionation by preferential sequestration of ²⁶Mg. Thus, the marine Mg cycle could be reflected by seawater Mg isotopic composition (δ²⁶Mg_sw), which might be recorded in marine carbonate. However, carbonates are both texturally and compositionally heterogeneous, and it is unclear which carbonate component is the most reliable for reconstructing δ²⁶Mg_sw. In this study, we measured Mg isotopic compositions of limestone samples collected from the early Carboniferous Huangjin Formation in South China. Based on petrographic studies, four carbonate components were recognized: micrite, marine cement, brachiopod shell, and mixture. The four components had distinct δ²⁶Mg: (1) micrite samples ranged from ?2.86‰ to ?2.97‰; (2) pure marine cements varied from ?3.40‰ to ?3.54‰, while impure cement samples containing small amount of Rugosa coral skeletons showed a wider range (?3.27‰ to ?3.75‰); (3) values for the mixture component were ?3.17‰ and ?3.49‰; and (4) brachiopod shells ranged from ?2.20‰ to ?3.07‰, with the thickened hinge area enriched in ²⁴Mg. Due to having multiple carbonate sources, neither the micrite nor the mixture component could be used to reconstruct δ²⁶Mg_sw. In addition, the marine cement was homogenous in Mg isotopes, but lacking the fractionation by inorganic carbonate precipitation that is prerequisite for the accurate determination of δ²⁶Mg_sw. Furthermore, brachiopod shells had heterogeneous C and Mg isotopes, suggesting a significant vital effect during growth. Overall, the heterogeneous δ²⁶Mg of the Huangjin limestone makes it difficult to reconstruct δ²⁶Mg_sw using bulk carbonate/calcareous sediments. Finally, δ²⁶Mg_sw was only slightly affected by the faunal composition of carbonate-secreting organisms, even though biogenic carbonate accounts for more than 90% of marine carbonate production in Phanerozoic oceans and there is a wide range (0.2‰–4.8‰) of fractionation during biogenic carbonate formation.     

Geochemical processes and solute transport at the seawater/freshwater interface of a sandy aquifer

Geochemical processes occurring at a seawater/freshwater interface were studied in a shallow coastal siliclastic aquifer containing minor amounts of calcite. Data were collected from 106 piezometers in a 120-m transect from the coastline and landward. In the first 40 m from the coastline, a wedge of saltwater is intruding below the freshwater aquifer. The aquifer is strongly reduced with mineralization of organic matter by methanogenesis in the freshwater aquifer, and sulfate reduction dominating in the most seaward part of the saline aquifer. The spatial separation of cations in the aquifer indicated a slow freshening process where Ca²⁺ from freshwater displaced the marine cations Na⁺ and Mg²⁺ from the exchanger complex. The resulting loss of Ca²⁺ from solution decreases the saturation state for calcite and possibly causes calcite dissolution. A storm-flooding event was recorded where pulses of dense seawater sank through the fresh aquifer. As a result, the terminal electron accepting process switched from methanogenesis to sulfate reduction. The pulses of sinking seawater also triggered cation exchange reactions where Ca²⁺ was expelled from the exchanger by seawater Na⁺ and Mg²⁺. The released Ca²⁺ is being flushed from the aquifer by groundwater flow, and this export of Ca²⁺ will, in the long term, cause decalcification of the sediment. The water composition in the aquifer is in a transient state as the result of various processes that operate on different timescales. Oxidation of organic matter occurs continuously but at a rate decreasing on a geological time scale. The freshening of the aquifer operates on the timescale of a few years. The episodic flooding and sinking of seawater through the aquifer proceeds in the course of days to weeks, but occurs irregularly with years in between.     

Hydrogeochemical characterization of major factors affecting the quality of shallow groundwater in the coastal area at Kimje in South Korea

This study has evaluated the main hydrogeochemical characteristics that affect the quality of shallow groundwater in the coastal area at Kimje in South Korea. In this area, the chemical composition of groundwater is mostly classified into Na⁺?Cl^? and Ca²⁺+Mg²⁺?NO₃^-+Cl^? types. These types are affected by three major factors: seawater intrusion, fertilizers and redox processes. The Na⁺?Cl^? type, which is generally localized in the coastal area, shows typical characteristics of groundwater affected not only by modern seawater but also by old seawater residing in the reclaimed land. In contrast, the Ca²⁺+Mg²⁺?NO₃^-+Cl^? type, which is usually found inland, is very contaminated by fertilizers. As groundwater flows from the inland area to the coast, the redox condition changes from an oxic condition to a suboxic/anoxic condition. In addition, the reclamation has caused a large amount of underlying organic matter. These effects provide optimum conditions for the occurrence of redox processes in the coastal area.     

Dating and Fluid Geochemistry of the Sarkobu Gold Deposit in Altay, Xinjiang, China

The dating of fluid inclusions of quartz yields an Ar-Ar isochrone age of 320.4±6 Ma. Three types of fluid inclusions have been identified with the homogenization temperature ranging from 157℃ to 362℃. The homogenization temperature consists of two groups. The first group varies from 157℃ to 166℃, and the second from 232℃ to 362℃. Their chemical composition is dominated by Na+-Ca2+-Mg2+ and Cl-. The relative concentration of ions is characteristic by Na+>Ca2+>K+>Mg2+ and C1->SO42-> F-. The δD and δ18O values indicate that the ore-forming fluid originates from mixing of multi-source water. The Sarkobu gold deposit has experienced two mineralization stages: gold was enriched during the volcanic-exhalative-sedimentary process in the early stage, while the gold deposit was finally formed under compression-shearing during the orogenic period.     

Fluid inclusions in quartz from deep-seated granitic intrusions,south Norway

H₂O, CO₂, and H₂OCO₂ inclusions were observed in quatz from deep-seated granitic intrusions belonging to the Precambrian Farsund plutonic complex, south Norway. These inclusions represent solidus and/or sub-solidus fluids that were present in these rocks at some period between the initial melt and the present. Early CO₂ and H₂OCO₂ inclusions with about 20 mole% CO₂ contain up to 10 mole% CH₄ in the CO₂ phase and have densities from 0.96 to 0.85 g/cc. These inclusions are considered to most nearly approximate solidus vapour phases and suggest conditions of final solidification of the magma at 5 to 6 Kb and 700°C to 800°C. The H₂O inclusions have salinities between 2 and 60 wt%; the majority contain 5 to 20 equivalent wt.% NaCl and have densities from 1.05 to 0.85 g/cc. Microthermometry indicates that other cations such as K⁺, Ca²⁺ and / or Mg²⁺ are present in these aqueous fluids. The H₂O inclusions primarily represent fluids present at a post-magmatic stage of fracturing and healing of these rocks during uplift.     

Origin and mechanisms of K–Si-metasomatism of ca. 3.4–3.3 Ga volcaniclastic deposits and implications for Archean seawater evolution: Examples from cherts of Kittys Gap (Pilbara craton, Australia) and Msauli (Barberton Greenstone Belt, South Africa)

The effects of K–Si-metasomatism during the formation of Early Archean replacement cherts have been quantified in this study by the investigation of two well-known stratigraphic sections: the Msauli chert (MC, Barberton greenstone belt, South Africa) and the Kittys Gap chert (KGC, Pilbara craton, Western Australia). The KGCs have a dacitic precursor similar to Duffer Formation dacites (Pilbara craton), while the MCs are derived from Al-depleted komatiites similar to those from the Weltevreden Formation (Barberton greenstone belt). Mass balance calculations reveal that the volcaniclastic deposits had initial porosities of up to 85 vol.% for the KGC and of 65 vol.% for the MC. Secondary porosities (27 vol.%: MC, 8 vol.%: KGC) produced during K-metasomatism are proportional to the dissolution of Fe, Ca, Mg-rich glass and precursor minerals. Komatiites have a higher chemical exchange potential than dacites, each gram releasing 1.2 mmol Fe²⁺, 2.8 mmol Mg²⁺, 1.4 mmol Ca²⁺ and 1.1 mmol Na⁺ to seawater, together with 4.4 mmol O²⁻. K-metasomatism of 1 g of komatiite further implies an uptake of 0.67 mmol of K⁺ and 2.7 mmol of H⁺. The highest silica uptake is achieved for the KGC (82 mmol/g of precursor). This silica enrichment most likely operated in the water column and at the sediment–water interface by sorption mechanisms on the surface of detrital particles and particulate organic matter, as a result of seawater silica-saturation. Acidic conditions (pH 5.5–6.5) and hot temperatures (>70 °C) favored the formation of K-rich phyllosilicates by interaction with seawater during the early diagenetic alteration of the volcaniclastic particles. The widespread occurrence of K–Si-metasomatism in volcanic and sedimentary rocks can be regarded as a general alteration process of the Early Archean seafloor, with a major influence on seawater composition. The highly K-selective metasomatism confirms previous studies suggesting that the Archean ocean was acidic and probably in equilibrium with a CO₂-rich atmosphere.     

Origin and distribution of saline groundwaters in the upper Miocene aquifer system, coastal Rhodope area, northeastern Greece

 This paper describes the origins and distribution of saline groundwaters in the coastal area of Rhodope, Greece. The aquifer system includes two aquifers within coarse-grained alluvial sediments in the coastal part of the study area. Two major water-quality groups occur in the study area, namely Ca²⁺-rich saline groundwater and Ca²⁺-poor, almost fresh groundwater. The main process controlling the groundwater chemistry is the exchange of calcium and sodium between the aquifer matrix and intruding seawater. The natural salt water in the study area is probably residual water that infiltrated the aquifer system during repeated marine transgressions in late Pleistocene time. Seawater intrusion into the coastal aquifer system occurs as a result of overpumping in two seawater wedges separated vertically by a low-permeability layer. The rate of intrusion averages 0.8 m/d and is less than expected due to a decline of the aquifer's permeability at the interface with the seawater. The application of several hydrochemical techniques (Piper and Durov diagrams; Na⁺/Cl^–, Ca²⁺/Cl^–, Mg²⁺/Cl^–, and Br^–/Cl^– molar ratios; Ca²⁺/Mg²⁺ weight ratio; and chloride concentrations), combined with field observations, may lead to a better explanation of the origin of the saline groundwater. Received, May 1997 / Revised, May 1998, December 1998 / Accepted, February 1999     

Diagenetic and other highly mineralized waters in the Polish Carpathians

Highly mineralized waters of different chemical types and origin occur in the flysch formations and their bedrocks in the western part of the Polish Carpathians. The marine sedimentation water of the flysch formations is not preserved, as the most mineralized and the heaviest isotopic values of flysch waters are characterized by δ¹⁸O and δ²H values in the ranges of 5–7‰ and −(20–30)‰, respectively. Their origin is related to the dehydration of clay minerals during burial diagenesis, with molecules of marine water completely removed by molecules of released bound water. They are relatively enriched in Na⁺ in respect to the marine water, supposedly due to the release of Na⁺ during the illitization of smectites and preferable incorporation of other cations from the primary brine into newly formed minerals. In some parts of younger formations, i.e. in the Badenian sediments, brines occur with isotopic composition close to SMOW and Cl⁻ contents greatly exceeding the typical marine value of about 19.6 g/L, supposedly due to ultrafiltration. Most probably, the marine water of the flysch formations was similarly enriched chemically in its initial burial stages. Final Cl⁻ contents in diagenetic waters depend on different Cl⁻ contents in the primary brines and on relationships between diagenetic and further ultrafiltration processes. In some areas, diagenetic waters migrate to the surface along fault zones and mix with young local meteoric waters becoming diluted, with the isotope composition scattering along typical mixing lines. In areas with independent CO₂ flow from great depths, they form chloride CO₂-rich waters. Common CO₂-rich waters are formed in areas without near-surface occurrences of diagenetic waters. They change from the HCO₃–Ca type for modern waters to HCO₃–Mg–Ca, HCO₃–Na–Ca and other types with elevated TDS, Mg²⁺ and/or Na⁻ contents for old waters reaching even those of glacial age. Bedrocks of the flysch are represented by Mesozoic and Paleozoic mudstones, sandstones and carbonates, and in some areas by Badenian sediments. Brines of the Mesozoic and Paleozoic bedrocks are usually significantly enriched in Ca²⁺ and Mg²⁺ in comparison with the Badenian brines. By analogy to the deepest brines in the adjacent Upper Silesian Coal Basin, they are supposed to originate from paleometeoric waters of a hot climate.     

Strontium,SO42−/Cl− and Mg2+/Ca2+ ratios as tracers for the evolution of seawater into coastal aquifers: the example of Castell de Ferro aquifer (SE Spain)

The strontium content and the SO₄^2?/Cl^? and Mg²⁺/Ca²⁺ ratios were used as natural tracers of the residence time of seawater intrusion into the Castell de Ferro aquifer. Analysis of these parameters indicated the existence of two principal flowpaths in the aquifer. The first flows through the eastern part of the aquifer, through the karstified Castell de Ferro massif; it accommodates a larger and more rapid flow, so that the residence time is shorter, leading to lower SO₄²⁺/Cl^? ratios, lower Sr²⁺ content and higher Mg²⁺/Ca²⁺ ratios. The second flowpath is in the western sector, and flows exclusively through alluvial deposits; the flow here is slower, particularly that flowing towards the sea. Thus the residence time of the water here will be longer and there is scant flushing of the intruded seawater; this is manifested in the high Sr²⁺ content, high SO₄²⁺/Cl^? and low Mg²⁺/Ca²⁺ ratios. To cite this article: P. Pulido-Leboeuf et al., C. R. Geoscience 335 (2003).