Changes in carbon and oxygen isotope composition during limestone diagenesis

The calcite fossils of the Derbyhaven Beds, Isle of Man, have δ¹³C values (+ 1·8 PDB) similar to modern, shallow-water marine skeletons, but the δ¹⁸O values (?6·1 PDB) are much lighter than modern skeletons. The light oxygen values indicate either re-equilibration with isotopically light water before cementation started, or Carboniferous sea water with δ¹⁸O of ?6‰. Aragonite dissolution was followed by precipitation of zoned calcite cement. In this cement, up to six intracrystalline zones, recognized in stained thin sections, show isotopic variation. Carbon varies from + 3-8 to + 1-2‰. and oxygen from ? 2-6 to ? 12-4‰. with decreasing age of the cement. This trend is attributed to increasing temperature and to isotopic evolution of the pore waters during burial. The zoned calcite is sequentially followed by dolomite and kaolinite cements which continue the trend towards light isotopic values. This trend is continued with younger, fault-controlled dolomite, and is terminated by vein-filling calcite and dolomite. The younger calcite, interpreted as a near-surface precipitate from meteoric waters, is unrelated to the older sequence of carbonates and has distinctly different carbon isotope ratios: δ¹³C ? 6-8‰.     

Pedogenic-phreatic carbonates on a Middle Devonian (Givetian) terrigenous alluvial-deltaic plain, Gilwood Member (Watt Mountain Formation), northcentral Alberta, Canada

In the Muskeg Trough of northcentral Alberta the Gilwood Member contains widespread carbonate deposits that formed within terrigenous mudstone and sandstone hosts. Stratigraphic, depositional and petrographic relationships indicate that these carbonates represent calcretes and dolocretes. Calcretes, observed best with cathodoluminescence, display microcrystalline alpha fabrics, circumgranular cracks, root networks, displacive growth fabrics, elongate channel voids and rare coloform growths with flower spar. Similarly, dolocretes have microcrystalline alpha fabrics, brecciation, gradational contacts with host mudstones, extensive layered nodular horizons and are associated with anhydrite and pyrite. δ¹³C values range between ?7‰ to +1‰ and –6‰ to +3‰ for calcretes and dolocretes, respectively. Oxygen isotopes are more variable and differ with host lithologies. δ¹⁸O of calcretes ranges between ?11‰ to ?8‰ for sandstones and ?8‰ to ?3‰ for mudstones, whereas δ¹⁸O of dolocretes ranges between ?3‰ to 1‰ for marine mudstones and ?6‰ to ?2‰ for pedogenic mudstones. Regional mapping indicates that calcretes thicken towards the deepest parts of the Muskeg Trough. Widespread dolocretes extend beyond the eastern and western limits of Muskeg Trough and are useful marker intervals for regional correlations. Dolocretes of restricted lateral extent are found within gleyed palaeosol mudstones next to calcretized channel sandstones. Calcrete isotopic values are interpreted as indicative of carbonate precipitation from waters with meteoric water input. However, the higher δ¹⁸O values in dolocretes are indicative of a contribution from an isotopically heavier source such as seawater. Stratigraphically, calcretes are most common along the western and northern edges of Muskeg Trough; thus, calcrete accumulation was further controlled by meteoric water in-flow from the highland to the west and sluggish groundwater flow in Muskeg Trough. In contrast, regionally widespread dolocrete horizons appear to have formed from mixing of fresh waters derived from the highland to the west and seawaters introduced from the east. Regionally restricted dolocretes which are found next to channel sandstones formed from groundwater out-flow from the permeable channel sandstones which resulted in calcretization in channel proximal mudstones and dolomitization in channel distal mudstones.     

The recharge of glacial meltwater and its influence on the geochemical evolution of groundwater in the Ordovician-Cambrian aquifer system,northern part of the Baltic Artesian Basin

The geochemical evolution of groundwater in the Ordovician-Cambrian aquifer system in the northern part of the Baltic Artesian Basin (BAB) illustrates how continental glaciations have influenced groundwater systems in proglacial areas. The aquifer system contains water that has originated from various end-members: recent meteoric water, glacial meltwater and relict Na-Cl brine. The saline formation water that occupied the aquifer system prior to the glacial meltwater intrusion has been diluted by meltwaters of advancing-retreating ice sheets. The diversity in the origin of groundwater in the aquifer system is illustrated by a wide variety in δ¹⁸O values that range from −11‰ to −22.5‰. These values are mostly depleted with respect to values found in modern precipitation in the area. The chemical and isotopic composition of groundwater has been influenced by mixing between waters originating from different end-members. In addition, the freshening of a previously saline water aquifer due to glacial meltwater intrusion has initiated various types of water-rock interaction (e.g. ion exchange, carbonate mineral dissolution).     

Investigation on the hydrodynamics of Ganga Alluvial Plain using environmental isotopes: a case study of the Gomati River Basin, northern India

An investigation using environmental isotopes (δ¹⁸O and δD) was conducted to gain insight into the hydrological processes of the Ganga Alluvial Plain, northern India. River-water, shallow-groundwater and lake-water samples from the Gomati River Basin were analyzed. During the winter season, the δ¹⁸O and δD compositions of the Gomati River water ranged from ?1.67 to ?7.62 ‰ and ?25.08 to ?61.50 ‰, respectively. Deuterium excess values in the river water (+0.3 to ?13 ‰) and the lake water (?20 ‰) indicate the significance of evaporation processes. Monthly variation of δ¹⁸O and δD values of the Gomati River water and the shallow groundwater follows a similar trend, with isotope-depleted peaks for δ¹⁸O and δD synchronized during the monsoon season. The isotopically depleted peak values of the river water (δ¹⁸O?=??8.30 ‰ and δD?=??57.10 ‰) can be used as a proxy record for the isotopic signature of the monsoon precipitation in the Ganga Alluvial Plain.     

The diagenesis of the late Dinantian Derbyshire-East Midland carbonate shelf, central England

Carbonate cements in late Dinantian (Asbian and Brigantian) limestones of the Derbyshire carbonate platform record a diagenetic history starting with early vadose meteoric cementation and finishing with burial and localized mineral and oil emplacement. The sequence is documented using cement petrography, cathodoluminescence, trace element geochemistry and C and O isotopes. The earliest cements (Pre-Zone 1) are locally developed non-luminescent brown sparry calcite below intrastratal palaeokarsts and calcretes. They contain negligible Fe, Mn and Sr but up to 1000 ppm Mg. Their isotopic compositions centre around δ¹⁸O =?8.5‰, δ¹³C=?5.0‰. Calcretes contain less ¹³C. Subsequent cements are widespread as inclusion-free, low-Mg, low-Fe crinoid overgrowths and are described as having a‘dead-bright-dull’cathodoluminescence. The‘dead’cements (Zone 1) are mostly non-luminescent but contain dissolution hiatuses overlain by finely detailed bright subzones that correlate over several kilometres. Across‘dead'/bright subzones there is a clear trend in Mg (500–900 ppm), Mn (100–450 ppm) and Fe (80-230 ppm). Zone 1 cements have isotopic compositions centred around δ¹⁸O =?8.0‰ and δ¹³C=?2.5‰. Zone 2 cement is bright, thin and complexly subzoned. It is geochemically similar to bright subzones of Zone 1 cements. Dull Zone 3 cement pre-dates pressure dissolution and fills 70% or more of the pore space. It generally contains little Mn, Fe and Sr but can have more than 1000 ppm Mg, increasing stratigraphically upwards. The δ¹⁸O compositions range from ?5.5 to ?15‰ and the δ¹³C range is ?1 to + 3.2^0/00. Zone 4 fills veins and stylolite seams in addition to pores. It is synchronous with Pb, Ba, F ore mineralization and oil migration. Zone 4 is ferroan with around 500 ppm Fe, up to 2500 ppm Mg and up to 1500 ppm Mn. Isotopic compositions range widely; δ¹⁵O =?2.7 to ?9‰ and δ¹³C=?3.8 to+2.50‰. Unaltered marine brachiopods suggest a Dinantian seawater composition around δ¹⁵O = 0‰ (SMOW), but vital isotopic effects probably mask the original δ¹³C (PDB) value. Pre-Zone 1 calcites are meteoric vadose cements with light soil-derived δ¹³C and light meteoric δ¹⁸O. An unusually fractionated‘pluvial’δ¹⁵O(SMOW) value of around — 6‰ is indicated for local Dinantian meteoric water. Calcrete δ¹⁸O values are heavier through evaporation. Zone 1 textures and geochemistry indicate a meteoric phreatic environment. Fe and Mn trends in the bright subzones indicate stagnation, and precipitation occurred in increments from widespread cyclically developed shallow meteoric water bodies. Meteoric alteration of the rock body was pervasive by the end of Zone 1 with a general resetting of isotopic values. Zone 3 is volumetrically important and external sources of water and carbonate are required. Emplacement was during the Namurian-early Westphalian by meteoric water sourced at a karst landscape on the uplifted eastern edge of the Derbyshire-East Midland shelf. The light δ¹⁸O values mainly reflect burial temperatures and an unusually high local heat flow, but an input of highly fractionated hinterland-derived meteoric water at the unconformity is also likely. Relatively heavy δ¹³C values reflect the less-altered state of the source carbonate and aquifer. Zone 4 is partly vein fed and spans burial down to 2000 m and the onset of tectonism. Light organic-matter-derived δ¹³C and heavy δ¹⁸O values suggest basin-derived formation water. Combined with textural evidence of geopressures, this relates to local high-temperature ore mineralization and oil migration. Low water-to-rock ratios with host-rock buffering probably affected the final isotopic compositions of Zone 4, masking extremes both of temperature and organic-matter-derived CO₂.     

Spatio-temporal variation of stable isotopes of river waters, water source identification and water security in the Heishui Valley (China) during the dry-season

Spatial variations of δD and δ¹⁸O among seven tributaries and their water sources were investigated in the Heishui Valley of the Yangtze River, China during the dry-season in 2004. A one-way ANOVA (analysis of variation) test showed that both δD (p?18O (p?=?0.045) spatially varied among the seven tributaries. The plot of δ¹⁸O versus δD for the river water collected at different locations showed that isotopic fractionation occurred during the snow and glacial melting process. The depleted δ¹⁸O and δD in the tributary waters distributed above the local meteoric water line (LMWL) suggested that the glacial and early snowpack meltwater largely recharged these streams during the early spring. The meltwater was isotopically distinguishable from the precipitation and river water, which had been evaporated during warmer and drier times. If glaciers and snow accumulation diminish with future climate warming, the recharge of these tributaries’ baseflow will decline and the security of the water resource in this watershed will be threatened.     

Hydrogen and oxygen isotopic compositions of sedimentary kaolin deposits,Egypt: Paleoclimatic implications

The clay fractions of sedimentary kaolin deposits representing different ages (Carboniferous and Cretaceous), types (pisolitic flint and plastic), and localities (Sinai and Aswan) from Egypt were analyzed for their H and O isotopic compositions to examine the paleoclimate conditions during their formation. The δD values of the Carboniferous deposits in Sinai range between −67‰ and −88‰, while the values for the Cretaceous deposits in Sinai range between −59‰ and −75‰. The δ¹⁸O values of the Carboniferous deposits range from 17.9‰ to 19.4‰ and the values for the Cretaceous deposits range between 19.2‰ and 20.4‰. The relatively low δD and δ¹⁸O values of the Carboniferous deposit at the Abu Natash area (−67‰ and 17.9‰, respectively) compared to other Carboniferous deposits (averages of −83.3‰, and 18.8‰ for δD and δ¹⁸O, respectively) could be due to isotopic exchange between this deposit and the adjacent dolomite and/or the enclosed hydrothermally-formed Mn ores of the Carboniferous Um Bogma Formation. The δD and δ¹⁸O values of the Cretaceous pisolitic flint kaolin deposit from Aswan (averages of −65‰ and 20.3‰, respectively) and plastic kaolin from the same area (averages of −66‰ and 19.5‰, respectively) are almost identical. The differences in the δ¹⁸O values between the clay fractions of the pisolitic flint kaolin (20.3‰) and the previously analyzed bulk kaolin of the same deposit (average of 17.5‰) suggest a significant effect of non-clay minerals on the isotopic compositions of the kaolin deposits.The H and O isotopic compositions plot close to the kaolinite line that marks the isotopic composition of kaolinite in equilibrium with meteoric water at 20 °C. This indicates that the kaolinite from both the Carboniferous and Cretaceous deposits in Egypt formed by meteoric water weathering of the source rock(s). The δD and δ¹⁸O values also suggest that kaolinite of these deposits formed under warm-temperate to tropical conditions. The slight deviations of some samples from the kaolinite line suggest post-depositional modifications of the isotopic compositions of studied deposits probably due to the interaction between earlier-formed kaolinite and downward percolating meteoric water.The δD and δ¹⁸O values of the Cretaceous and Carboniferous deposits from all localities suggest that both deposits formed under similar climatic conditions due to the location of Egypt at almost the same distance from the equator either to the south during the Carboniferous or to the north during the Cretaceous.     

Isotopic constraints on growth conditions of multiphase calcite–pyrite–barite concretions in Carboniferous mudstones

Carbonate concretions in the Lower Carboniferous Caton Shale Formation contain diagenetic pyrite, calcite and barite in the concretion matrix or in different generations of septarian fissures. Pyrite was formed by sulphate reduction throughout the sediment before concretionary growth, then continued to form mainly in the concretion centres. The septarian calcites show a continuous isotopic trend from δ¹³C=?28·7‰ PDB and δ¹⁸O=?1·6‰ PDB through to δ¹³C=?6·9‰ PDB and δ¹⁸O=?14·6‰ PDB. This trend arises from (1) a carbonate source initially from sulphate reduction, to which was added increasing contributions of methanogenic carbonate; and (2) burial/temperature effects or the addition of isotopically light oxygen from meteoric water. The concretionary matrix carbonates must have at least partially predated the earliest septarian cements, and thus used the same carbonate sources. Consequently, their isotopic composition (δ¹³C=?12·0 to ?10·1‰ PDB and δ¹⁸O=?5·7 to ?5·6‰ PDB) can only result from mixing a carbonate cement derived from sulphate reduction with cements containing increasing proportions of carbonate from methanogenesis and, directly or indirectly, also from skeletal carbonate. Concretionary growth was therefore pervasive, with cements being added progressively throughout the concretion body during growth. The concretions contain barite in the concretion matrix and in septarian fissures. Barite in the earlier matrix phase has an isotopic composition (δ³⁴S=+24·8‰ CDT and δ¹⁸O=+16·4‰ SMOW), indicating formation from near‐surface, sulphate‐depleted porewaters. Barites in the later septarian phase have unusual isotopic compositions (δ³⁴S=+6 to +11‰ CDT and δ¹⁸O=+8 to +11‰ SMOW), which require the late addition of isotopically light sulphate to the porewaters, either from anoxic sulphide oxidation (using ferric iron) or from sulphate dissolved in meteoric water. Carbon isotope and biomarker data indicate that oil trapped within septarian fissures was derived from the maturation of kerogen in the enclosing sediments.     

Isotope geochemistry of fluid inclusions in Permian halite with implications for the isotopic history of ocean water and the origin of saline formation waters

δD and δ¹⁸O values have been determined for fluid inclusions in 45 samples of Permian halite. The inclusions are enriched in ¹⁸O relative to the meteoric water line but are depleted in D relative to ocean water. Inclusions with the more positive δ-values coincide with the isotopic composition expected for evaporating sea water which follows a hooked trajectory on a δD-δ¹⁸O diagram. Inclusions with more negative δ-values may represent more highly evaporated sea water but probably reflect synsedimentary or diagenetic mixing of meteoric water with evaporite brines. The isotope systematics in these inclusions are sufficiently similar to those of a modern evaporite pan to indicate that Permian sea water was isotopically similar to modern sea water.Connate evaporite brines can have negative δ-values because of the probable hooked isotope trajectory of evaporating sea water and/or synsedimentary mixing of evaporite brines with meteoric waters. Subsurface formation waters composed of mixtures of remnant primary evaporite brines and later meteoric waters may be more common than previous isotopic evidence has suggested.     

Geochemistry of stable oxygen and hydrogen isotopes in minerals and corundum-bearing rocks in northern Karelia as an indicator of their unusual genesis

The paper presents newly obtained data on the oxygen and hydrogen isotopic composition of minerals in corundum occurrences and their host rocks in northern Karelia. Minerals in the Khitoostrov and Varaka corundum-bearing zones have extremely low δ¹⁸O (lower than ?26‰) and δ D (lower than ?215‰), which suggest that the mineral-forming process involved glacial waters and that the minerals preserve the isotopic ratios of their protolith. Aluminous corundum plagioclasites were produced by high-pressure Svecofennian (1.9-1.8 Ga) metamorphism of Paleoproterozoic rocks that had been metasomatized with the involvement of meteoric waters during the Guronian glaciation epoch.     

Multiple isotope composition (S, Pb, H, O, He, and Ar) and genetic implications for gold deposits in the Jiapigou gold belt, Northeast China

The Jiapigou gold belt (>150 t Au), one of the most important gold-producing districts in China, is located at the northeastern margin of the North China Craton. It is composed of 17 gold deposits with an average grade around 10 g/t Au. The deposits are hosted in Archean gneiss and TTG rocks, and are all in shear zones or fractures of varying orientations and magnitudes. The δ³⁴S values of sulfide from ores are mainly between 2.7?‰ and 10?‰. The Pb isotope characteristics of ore sulfides are different from those of the Archean metamorphic rocks and Mesozoic granites and dikes, and indicate that they have different lead sources. The sulfur and lead isotope compositions imply that the ore-forming materials might originate from multiple, mainly deep sources. Fluid inclusions in pyrite have ³He/⁴He ratios of 0.6 to 2.5 Ra, whereas their ⁴⁰Ar/³⁶Ar ratios range from 1,444 to 9,805, indicating a dominantly mantle fluid with a negligible crustal component. δ¹⁸O values calculated from hydrothermal quartz are between ?0.2?‰ and +5.9?‰, and δD values of the fluids in the fluid inclusions in quartz are from ?70?‰ to ?96?‰. These ranges suggest dominantly magmatic water with a minor meteoric component. The noble gas isotopic data, along with the stable isotopic data, suggest that the ore-forming fluids have a dominantly mantle source with minor crustal addition.     

Chemical Dynamics and Evaluation of Biogeochemical Processes in Alpine River Kamniška Bistrica, North Slovenia

Biogeochemical processes were investigated in alpine river—Kamni?ka Bistrica River (North Slovenia), which represents an ideal natural laboratory for studying anthropogenic impacts in catchments with high weathering capacity. The Kamni?ka Bistrica River water chemistry is dominated by HCO₃ ^?, Ca²⁺ and Mg²⁺, and Ca²⁺/Mg²⁺ molar ratios indicate that calcite weathering is the major source of solutes to the river system. The Kamni?ka Bistrica River and its tributaries are oversaturated with respect to calcite and dolomite. pCO₂ concentrations were on average up to 25 times over atmospheric values. δ¹³C_DIC values ranged from ?12.7 to ?2.7 ‰, controlled by biogeochemical processes in the catchment and within the stream; carbonate dissolution is the most important biogeochemical process affecting carbon isotopes in the upstream portions of the catchment, while carbonate dissolution and organic matter degradation control carbon isotope signatures downstream. Contributions of DIC from various biogeochemical processes were determined using steady state equations for different sampling seasons at the mouth of the Kamni?ka Bistrica River; results indicate that: (1) 1.9–2.2 % of DIC came from exchange with atmospheric CO₂, (2) 0–27.5 % of DIC came from degradation of organic matter, (3) 25.4–41.5 % of DIC came from dissolution of carbonates and (4) 33–85 % of DIC came from tributaries. δ¹⁵N values of nitrate ranged from ?5.2 ‰ at the headwater spring to 9.8 ‰ in the lower reaches. Higher δ¹⁵N values in the lower reaches of the river suggest anthropogenic pollution from agricultural activity. Based on seasonal and longitudinal changes of chemical and isotopic indicators of carbon and nitrogen in Kamni?ka Bistrica River, it can be concluded that seasonal changes are observed (higher concentrations are detected at low discharge conditions) and it turns from pristine alpine river to anthropogenic influenced river in central flow.     

USGS46 Greenland Ice Core Water – A New Isotopic Reference Material for δ2H and δ18O Measurements of Water

Ice core from Greenland was melted, filtered, homogenised, loaded into glass ampoules, sealed, autoclaved to eliminate biological activity, and calibrated by dual‐inlet isotope‐ratio mass spectrometry. This isotopic reference material (RM), USGS46, is intended as one of two secondary isotopic reference waters for daily normalisation of stable hydrogen (δ²H) and stable oxygen (δ¹⁸O) isotopic analysis of water with a mass spectrometer or a laser absorption spectrometer. The measured δ²H and δ¹⁸O values of this reference water were ?235.8 ± 0.7‰ and ?29.80 ± 0.03‰, respectively, relative to VSMOW on scales normalised such that the δ²H and δ¹⁸O values of SLAP reference water are, respectively, ?428 and ?55.5‰. Each uncertainty is an estimated expanded uncertainty (U = 2u_c) about the reference value that provides an interval that has about a 95‐percent probability of encompassing the true value. This reference water is available in cases containing 144 glass ampoules that are filled with either 4 ml or 5 ml of water per ampoule.     

Oxygen and Hydrogen Isotopes of Waters in the Ordos Basin,China: Implications for Recharge of Groundwater in the North of Cretaceous Groundwater Basin

Hundreds of precipitation samples collected from meteorological stations in the Ordos Basin from January 1988 to December 2005 were used to set up a local meteoric water line and to calculate weighted average isotopic compositions of modern precipitation. Oxygen and hydrogen isotopes, with averages of ?7.8‰ and ?53.0‰ for δ18O and δD, respectively, are depleted in winter and rich in spring, and gradually decrease in summer and fall, illustrating that the seasonal effect is considerable. They also show that the isotopic difference between south portion and north portion of the Ordos Basin are not obvious, and the isotope in the middle portion is normally depleted. The isotope compositions of 32 samples collected from shallow groundwater (less than a depth of 150 m) in desert plateau range from ?10.6‰ to ?6.0‰ with an average of ?8.4‰ for δ18O and from ?85‰ to ?46‰ with an average of ?63‰ for δD. Most of them are identical with modern precipitation. The isotope compositions of 22 middle and deep groundwaters (greater than a depth of 275 m) fall in ranges from ?11.6‰ to ?8.8‰ with an average of ?10.2‰ for δ18O and from ?89‰ to ?63‰ with an average of ?76‰ for δD. The average values are significantly less than those of modern precipitation, illustrating that the middle and deep groundwaters were recharged at comparatively lower air temperatures. Primary analysis of 14C shows that the recharge of the middle and deep groundwaters started at late Pleistocene. The isotopes of 13 lake water samples collected from eight lakes define a local evaporation trend, with a relatively flat slope of 3.77, and show that the lake waters were mainly fed by modern precipitation and shallow groundwater.     

Isotopic composition of thermal waters in Chukotka

Twenty three groups of thermomineral springs in eastern Chukotka with the discharge temperature of 2 to 97°C and mineralization of 1.47 to 37.14 g/l are studied and compared with surface freshwater from their localities. The δD and δ¹⁸O values in surface waters vary from ?121.4 to ?89.5‰ and from ?16.4 to ?11.1‰, respectively, while respective values in thermomineral waters range from ?134.2 to ?92.5‰ and from ?17.6 to ?10.5‰. The δD value in surface waters decreases from the east to west, i.e., toward interior areas of the peninsula. Hydrothermal springs most depleted in deuterium (δD < ?120‰) are localized in the geodynamically active Kolyuchinskaya-Mechigmen Depression. According to the proposed formation model of Chukotka thermomineral waters, their observed chemical and isotopic characteristics could result from the mixing (in different proportions) of surface waters with the deep-sourced isotopically light mineralized component (δD ≈ ?138‰, δ¹⁸O ≈ ?19‰, M = 9.5?14.7 g/l). The latter originates most likely from subpermafrost waters subjected to slight cryogenic metamorphism.     

Stable isotope evidence for the origin of diagenetic carbonate minerals from the Lower Jurassic Inmar Formation, southern Israel

The oxygen isotope compositions of diagenetic carbonate minerals from the Lower Jurassic Inmar Formation, southern Israel, have been used to identify porewater types during diagenesis. Changes in porewater composition can be related to major geological events within southern Israel. In particular, saline brines played an important role in late (Pliocene-Pleistocene) dolomitization of these rocks. Diagenetic carbonates included early siderite (δ¹⁸O_SMOW=+24.4 to +26.5‰δ¹³C_PDB=?1.1 to +0.8‰), late dolomite, ferroan dolomite and ankerite (δ¹⁸O_SMOW=+18.4 to +25.8‰; δ¹³C_PDB=?2.1 to +0.2‰), and calcite (δ¹⁸O_SMOW=+21.3 to +32.6‰; δ¹³C_PDB=?4.2 to + 3.2‰). The petrographic and isotopic results suggest that siderite formed early in the diagenetic history at shallow depths. The dolomitic phases formed at greater depths late in diagenesis. Crystallization of secondary calcite spans early to late diagenesis, consistent with its large range in isotopic values. A strong negative correlation exists between burial depth (temperature) and the oxygen isotopic compositions of the dolomitic cements. In addition, the δ¹⁸O values of the dolomitic phases in the northern Negev and Judea Mountains are in isotopic equilibrium with present formation waters. This behaviour suggests that formation of secondary dolomite post-dates the tectonic activity responsible for the present relief of southern Israel (Upper Miocene to Pliocene) and that the dolomite crystallized from present formation waters. Such is not the case in the Central Negev. In that locality, present formation waters have much lower salinities and δ¹⁸O values, indicating invasion of freshwater, and are out of isotopic equilibrium with secondary dolomite. Recharge of the Inmar Formation by meteoric water in the Central Negev occurred in the Pleistocene, and halted formation of dolomite.     

Carbon and Oxygen Isotope Compositions of Calcite and Rhodochrosite from Geothermal Exploratory Drills TH‐4 and TH‐6 near the Toyoha Deposit,Hokkaido, Japan

We studied calcite and rhodochrosite from exploratory drill cores (TH‐4 and TH‐6) near the Toyoha deposit, southwestern Hokkaido, Japan, from the aspect of stable isotope geochemistry, together with measuring the homogenization temperatures of fluid inclusions. The alteration observed in the drill cores is classified into four zones: ore mineralized zone, mixed‐layer minerals zone, kaolin minerals zone, and propylitic zone. Calcite is widespread in all the zones except for the kaolin minerals zone. The occurrence of rhodochrosite is restricted in the ore mineralized zone associated with Fe, Mn‐rich chlorite and sulfides, the mineral assemblage of which is basically equivalent to that in the Toyoha veins. The measured δ¹⁸O_SMOW and δ¹³C_PDB values of calcite scatter in the relatively narrow ranges from ?2 to 5‰ and from ?9 to ?5‰, respectively; those of rhodochrosite from 3 to 9‰ and from ?9 to ?5‰, excluding some data with large deviations. The variation of the isotopic compositions with temperature and depth could be explained by a mixing process between a heated surface meteoric water (100°C δ¹⁸O =?12‰, δ¹³C =?10‰) and a deep high temperature water (300°C, δ¹⁸O =?5‰, δ¹³C =?4‰). Boiling was less effective in isotopic fractionation than that of mixing. The plots of δ¹⁸O and δ¹³C indicate that the carbonates precipitated from H₂CO₃‐dominated fluids under the conditions of pH = 6–7 and T = 200–300°C. The sequential precipitation from calcite to rhodochrosite in a vein brought about the disequilibrium isotopic fractionation between the two minerals. The hydrothermal fluids circulated during the precipitation of carbonates in TH‐4 and TH‐6 are similar in origin to the ore‐forming fluids pertaining to the formation of veins in the Toyoha deposit.     

Hydrothermal flow systems in the Midcontinent Rift: oxygen and hydrogen isotopic studies of the North Shore Volcanic Group and related hypabyssal sills,Minnesota

Rift-related lavas of the North Shore Volcanic Group (NSVG) are intruded by plutonic rocks of the Duluth Complex along the unconformity between the NSVG and the underlying Proterozoic metasedimentary rocks (Animikie Group) and Archean volcano-sedimentary and plutonic rocks. Heat associated with the emplacement of the mafic intrusions generated fluid flow in the overlying plateau lavas. δ¹⁸O values for whole rocks from the NSVG and hypabyssal sills range from 5.5 to 17.7‰ and 5.3 to 11.5‰, respectively, and most values are higher than those considered “normal” for basaltic rocks (5.4 to 6.0‰). In general, there is a positive correlation between whole rock δ¹⁸O and water content, which suggests that elevated δ¹⁸O values are related primarily to secondary mineral growth and isotopic exchange during hydrothermal alteration and metamorphism. δ¹⁸O_H2O values computed from amygdule-filling minerals such as smectite, chlorite, and epidote found in low- to high-temperature metamorphic zones range from ∼−1 to 6‰ with an average value of ∼3‰. Smectite in the lower-grade zones gives computed δD_H2O values between −26 and −83‰, whereas epidote in the higher-grade zones gives δD_H2O values of −15 to 6‰. Fluid isotopic compositions computed from epidote and smectite values are suggestive of the involvement of at least two fluids during the early stages of amygdule filling. Fluid δD and δ¹⁸O values determined from epidote at the higher metamorphic grades indicate that seawater dominated the deeper portions of the system where greenschist facies assemblages and elevated δ¹⁸O values were produced in flow interiors, as well as margins. Smectite isotopic compositions suggest that meteoric water was predominant in the shallower portions of the system. The increase in δ¹⁸O values of massive flow interiors with depth is interpreted as a result of rock interaction with a fluid of constant oxygen isotopic composition with increasing temperature. The stable isotopic data are supportive of previous suggestions that seawater was involved in the hydrothermal system associated with the Midcontinent Rift. Although the origin of the seawater remains problematic, it appears that marine incursions may have occurred during the late stages of Portage Lake volcanism, and periodically thereafter.     

Hydrogeochemical and isotopic characteristics of the Ilica geothermal system (Erzurum,Turkey)

In this paper, the hydrochemical isotopic characteristics of samples collected from geothermal springs in the Ilica geothermal field, Eastern Anatolia of Turkey, are examined and described. Low-temperature geothermal system of Ilica (Erzurum, Turkey) located along the Eastern Anatolian fault zone was investigated for hydrogeochemical and isotopic characteristics. The study of ionic and isotopic contents shows that the thermal water of Ilica is mainly, locally fed by groundwater, which changes chemically and isotopically during its circulation within the major fault zone reaching depths. The thermal spring has a temperature of 29–39 °C, with electrical conductivity ranging from 4,000 to 7,510 µS/cm and the thermal water is of Na–HCO₃–Cl water type. The chemical geothermometers applied in the Ilica geothermal waters yielded a maximum reservoir temperature of 142 °C according to the silica geothermometers. The thermal waters are undersaturated with respect to gypsum, anhydrite and halite, and oversaturated with respect to dolomite. The dolomite mineral possibly caused scaling when obtaining the thermal waters in the study area. According to the enthalpy chloride-mixing model, cold water to the thermal water-mixing ratio is changing between 69.8 and 75 %. The δ¹⁸O–δ²H compositions obviously indicate meteoric origin of the waters. Thermal water springs derived from continental precipitation falling on to higher elevations in the study area. The δ¹³C ratio for dissolved inorganic carbonate in the waters lies between 4.63 and 6.48 ‰. In low-temperature waters carbon is considered as originating from volcanic (mantle) CO₂.     

A stable isotope study of serpentinization and metamorphism in the Highland Border Suite,Scotland, U.K.

$\text{D}\text{H}$ and ${}^{18}\text{O}{}^{16}\text{O}$ ratios have been measured for whole-rock samples and mineral separates from the mafic and ultramatic rocks of the Cambro-Ordovician Highland Border Suite. The H- and O- isotopic compositions of these rocks record individual stages in a relatively complex 500 Myr old hydrothermal/metamorphic history. Lizardite serpentinites (δD ～ ? 105‰; δ¹⁸O ～ + 6.2‰) record a premetamorphic history and indicate that parent harzburgites, dunites, and pyroxenites were serpentinized through low-temperature interaction with meteoric waters during cooling. The other rocks of the Highland Border Suite record subsequent interaction with metamorphic fluids. Amphibolite facies hornblende schists were produced through thrust-related (dynamothermal) metamorphism of spilitic pillow lavas. During dehydration, D-enriched fluids were driven off from the spilites thus leaving the hornblende schists to equilibrate with a relatively D-depleted internal fluid reservoir (δD ～ ? 45‰). The expelled D-enriched fluids may have mixed with more typical Dalradian metamorphic waters which then exchanged with the remaining mafic rocks and lizardite serpentinites during greenschist facies regional metamorphism to produce antigorite serpentinites (δD ～ ? 62‰; δ¹⁸O ～ + 8‰) and greenschist metaspilites (δD ～ ? 57‰; δ¹⁸O ～ + 7.3‰) with similar H- and O-isotopic compositions. Serpentinites which have been only partially metamorphosed show intermediate H-isotopic compositions between that of metamorphic antigorite (δD ～ ? 62‰) and non-metamorphic lizardite δD ～ ? 105‰) end members.