Oxygen and hydrogen isotope geochemistry of thermal springs of the Western Cape, South Africa: Recharge at high altitude?

A number of thermal springs with temperature up to 64°C are found in the Western Cape Province of South Africa. The average δ¹³C value of gas (CO₂+CH₄) released at three springs is −22, which is consistent with an entirely biogenic origin for the C and supports previous investigations which showed that the springs are not associated with recent or nascent volcanic activity. Most springs issue from rocks of the Table Mountain Group, where faulted and highly jointed quartzites and sandstones of the Cape Fold Belt act as the main deep aquifer. The δD and δ¹⁸O values of the springs range from −46 to −18 and from −7.3 to −3.9, respectively. Although the thermal springs have isotope compositions that plot close to the local meteoric water line, their δD and δ¹⁸O values are significantly lower than ambient meteoric water or groundwater. It is, therefore, suggested that the recharge of most of the thermal springs is at a significantly higher altitude than the spring itself. The isotope ratios decrease wuth increasing distance from the west coast of South Africa, which is in part related to the continental effect. However, a negative correlation between the spring water temperature and the δ¹⁸O value in the thermal springs closest to the west coast indicates a progressive in increase in the average altitude of recharge away from the coast.     

Oxygen and carbon isotope composition of structural carbonate in weathering apatites from laterites, southern Brazil and western Senegal

The oxygen (δ¹⁸O_c) and carbon (δ¹³C_c) isotope compositions of the structural carbonate group (CO₃) in apatites from lateritic profiles were investigated. The weathering profiles, located in southern Brazil and in western Senegal, are developed on three different types of apatite-rich parent rock: carbonatite, metamorphosed marine phosphorite and sedimentary marine phosphorite. The parent rock apatites are of magmatic, hydrothermal, metamorphic and sedimentary origins. The in situ formation of apatite of weathering origin in the profiles is well documented petrographically and geochemically.The overall range of measured δ¹⁸O_c and δ¹³C_c values of apatites of weathering origin (22 to 27 SMOW for δ¹⁸O_c and −15 to −10 PDB for δ¹³C_c) is much smaller than the range of measured and/or published isotope compositions of parent rock apatites (4–35 for δ¹⁸O_c and −11 to +1 for δ¹³C_c). In any profile, the apatites of weathering origin can exhibit lower, similar or higher δ¹⁸O_c values than parent rock apatites. In contrast, their δ¹³C_c values are systematically and significantly lower than those of the parent rock apatites. Apatites formed as a result of weathering in laterites can therefore be readily distinguished from apatites of other origin on the basis of their isotope composition.Assuming that apatite CO₃ fractionates O in a way similar to calcite CO₃, the structural carbonate group of the apatites of weathering origin appears to form in approximate isotopic equilibrium with the weathering solutions. The very low δ¹³C_c values exhibited by these apatites indicate that the dominant sources of dissolved CO₂ in the soil water are organic. The isotope composition of structural carbonate in apatite of weathering origin in lateritic profiles may provide useful information for paleoenvironmental studies.     

Do Stable Isotope Data from Calcrete Record Late Pleistocene Monsoonal Climate Variation in the Thar Desert of India?

Late Pleistocene terrestrial climate records in India may be preserved in oxygen and carbon stable isotopes in pedogenic calcrete. Petrography shows that calcrete nodules in Quaternary sediments of the Thar Desert in Rajasthan are pedogenic, with little evidence for postpedogenic alteration. The calcrete occurs in four laterally persistent and one nonpersistent eolian units, separated by colluvial gravel. Thermoluminescence and infrared- and green-light-stimulated luminescence of host quartz and feldspar grains gave age brackets for persistent eolian units I–IV of ca. 70,000–60,000, ca. 60,000–55,000, ca. 55,000–43,000, and ca. 43,000–25,000 yr, respectively. The youngest eolian unit (V) is <10,000 yr old and contains no calcrete. Stable oxygen isotope compositions of calcretes in most of eolian unit I, in the upper part of eolian unit IV, and in the nonpersistent eolian unit, range between −4.6 and −2.1‰ PDB. These values, up to 4.4‰ greater than values from eolian units II and III, are interpreted as representing nonmonsoonal¹⁸O-enriched “normal continental” waters during climatic phases when the monsoon weakened or failed. Conversely, 25,000–60,000-yr-old calcretes (eolian units II and III) probably formed under monsoonal conditions. The two periods of weakened monsoon are consistent with other paleoclimatic data from India and may represent widespread aridity on the Indian subcontinent during isotope stages 2 and 4. The total variation in δ¹³C is 1.7‰ (0.0–1.7‰), and δ¹³C covaries positively and linearly with δ¹⁸O. δ¹³C values are highest when δ¹⁸O values indicate the most arid climatic conditions. This is best explained by expansion of C₄grasses at the expense of C₃plants at low latitudes during glacial periods when atmosphericpCO₂was lowered. C₄dominance was overridingly influenced by global change in atmosphericpCO₂despite the lowered summer rainfall.     

Stable carbon and oxygen isotope investigation in historical lime mortar and plaster – Results from field and experimental study

Lime mortar and plaster were sampled from Roman, medieval and early modern buildings in Styria. The historical lime mortar and plaster consist of calcite formed in the matrix during setting and various aggregates. The stable C and O isotopic composition of the calcite matrix was analyzed to get knowledge about the environmental conditions during calcite formation. The δ¹³C_matrix and δ¹⁸O_matrix values range from −31 to 0‰ and −26 to −3‰(VPDB), respectively. Obviously, such a range of isotope values does not represent the local natural limestone assumed to be used for producing the mortar and plaster. In an ideal case, the calcite matrix in lime mortar and plaster is isotopically lighter in the exterior vs. the interior mortar layer according to the relationship δ¹⁸O_matrix = 0.61 · δ¹³C_matrix − 3.3 (VPDB). Calcite precipitation by uptake of gaseous CO₂ into alkaline Ca(OH)₂ solutions shows a similar relationship, δ¹⁸O_calcite = 0.67 · δ¹³C_calcite − 6.4 (VPDB). Both relationships indicate that the ¹³C/¹²C and ¹⁸O/¹⁶O values of the calcite reflect the setting behaviour of the lime mortar and plaster. Initially, CO₂ from the atmosphere is fixed as calcite, which is accompanied by kinetic isotope fractionation mostly due to the hydroxylation of CO₂ (δ¹³C_matrix ≈  −25‰ and δ¹⁸O_matrix ≈ −20‰). As calcite formation continued the remaining gaseous CO₂ is subsequently enriched in ¹³C and ¹⁸O causing later formed calcite to be isotopically heavier along the setting path in the matrix. Deviations from such an ideal isotopic behaviour may be due to the evolution of H₂O, e.g. evaporation, the source of CO₂, e.g. from biogenic origin, relicts of the natural limestone, and secondary effects, such as recrystallization of calcite. The results of the field and experimental study suggest that isotope values can be used as overall proxies to decipher the origin of carbonate and the formation conditions of calcite in the matrix of ancient and recent lime mortar and plaster. Moreover, these proxies can be used to select calcite matrix from historical lime mortar and plaster for ¹⁴C dating.     

Investigation of the stable isotope fractionation in speleothems with laboratory experiments

Many speleothems show evidence for calcite precipitation under disequilibrium conditions. To improve the understanding of these kinetic processes, several laboratory experiments were performed to study the fractionation of stable oxygen and carbon isotopes during the precipitation of calcite. Carbonate was precipitated under controlled conditions from both a body of standing water (beaker experiments) and a solution flowing along a channel (channel experiments) at a relative humidity of 100%. Slow degassing of CO₂, simulated by the beaker experiments, results in δ¹⁸O values in equilibrium with the solution. In contrast, the δ¹³C values show a significant enrichment, inversely proportional to the height of the solution in the beakers. Fast degassing of CO₂, simulated by the channel experiments, showed an enrichment of both δ¹³C and δ¹⁸O and a slope of Δδ¹³C/Δδ¹⁸O of 1.4±0.6. These results represent experimental evidence for the Hendy effect, which is manifested in (i) a progressive increase in δ¹⁸O and δ¹³C away from the growth axis and (ii) a positive correlation between δ¹⁸O and δ¹³C along a single growth layer of a stalagmite.     

Stable carbon isotope compositions and source rock geochemistry of the giant gas accumulations in the Ordos Basin, China

Ordos Basin, the second largest sedimentary basin in China, contains enormous natural gas resources. Each of the four giant gas fields discovered so far in this basin (i.e., Sulige, Yulin, Wushenqi and Jingbian) has over 100 billion cubic meters (bcm) or 3.53 trillion cubic feet (tcf) of proven gas reserves. This study examines the stable carbon isotope data of 125 gas samples collected from the four giant gas fields in the Ordos Basin. Source rocks in the Upper Paleozoic coal measures are suggested by the generally high δ¹³C values of C₁–C₄ gaseous hydrocarbons in the gases from the Sulige, Yulin and Wushenqi gas fields. While the δ¹³C_iC4 value is higher than that of the δ¹³C_nC4, the dominant ranges for the δ¹³C₁, δ¹³C₂, and δ¹³C₃ values in these Upper Paleozoic reservoired gases are −34 to −32‰, −27 to −23‰, and −25 to −24‰, respectively. The δ¹³C values of methane, benzene and toluene in gases from the Lower Paleozoic reservoirs of the Jingbian field indicate a significant contribution from humic source rocks, as they are similar to those in the Upper Paleozoic reservoirs of the Sulige, Yulin and Wushenqi gas fields. However, the wide variation and reversal in the δ¹³C₁, δ¹³C₂ and δ¹³C₃ values in the Jinbian gases cannot be explained using a single source scenario, thus the gases were likely derived dominantly from the Carboniferous-Permian coal measures with some contribution from the carbonates in the Lower Permian Taiyuan Formation. The gas isotope data and extremely low total organic carbon contents (<0.2% TOC) suggest that the Ordovician Majiagou Formation carbonates are unlikely to be a significant gas source rock, thus almost all of the economic gas accumulations in the Ordos Basin were derived from Upper Paleozoic source rocks.     

Cryogenic carbonates in cave environments: A review

Cryogenic cave carbonate (CCC) represents a specific type of speleothem. Its precipitation proceeds at the freezing point and is triggered by freezing-induced concentration of solutes. Compared to classical speleothems (stalagmites, flowstones), CCC occurs as accumulations of loose uncemented aggregates. The grain sizes range from less than 1 μm to over 1 cm in diameter. Karst groundwater chemistry and its freezing rate upon entering the cave are responsible for highly variable grain morphology. Rapid freezing of water results in the formation of CCC powders with grain size typically below 50 μm. Slow freezing of water in caves (usually in systems where the CO₂ escape is partly restricted; e.g., ice covered water pools) results in the formation of large mineral grains, with sizes from less than 1 mm to about 20 mm. The range of carbon and oxygen stable isotope compositions of CCC is larger than for a typical carbonate speleothem. Rapid freezing of water accompanied by a quick kinetic CO₂ degassing results in large ranges of δ¹³C of the CCC powders (between –10‰ and +18‰ PDB). Slow freezing of water, with a restricted CO₂ escape results in gradual increase of δ¹³C values (from −9‰ to +6‰ PDB; data ranges in individual caves are usually much more restricted), accompanied by a δ¹⁸O decrease of the precipitated carbonate (overall range from −10‰ to −24‰ PDB). These unusual trends of the carbonate δ¹⁸O evolution reflect incorporation of the heavier ¹⁸O isotope into the formed ice. New isotope data on CCC from three Romanian ice caves allow better understanding of the carbon and oxygen isotope fingerprint in carbonates precipitated from freezing of bulk water. CCCs are proposed as a new genetic group of speleothems.     

A 37-Meter Record of Paleoclimatological Events from Stable Isotope Data on Continental Molluscs in Valle di Castiglione, Near Rome, Italy

New stable isotope analyses on molluscan shells from a long core drilled in the crater lake of Valle di Castiglione, near Rome, extended the investigated portion of the core to 37 m. The succession of δ¹⁸O‰ values in the core interval 37–2.3 m ranges from −2.8 to +6.9‰ with only six samples below 0‰ _(PDB). These results point to arid climatic phases coupled with the high measured δ¹⁸O values of the biogenic carbonate. In contrast, depleted ¹⁸O samples correspond to wet climatic periods, in agreement with a strong evaporative control on the lake water isotopic composition. The ¹³C content of the shells shows sharp changes controlled by the dissolved inorganic carbon isotope budget. Isotopic data suggest that the whole body of water behaved as a closed system, thus resembling lacustrine systems located in arid and semiarid regions where hydrological control dominates the geochemical parameters.     

Geochemistry and occurrence of inorganic gas accumulations in Chinese sedimentary basins

Inorganic gases are commonly seen in eastern China and occasionally in southern China from the shallow water columns above hot and cold springs. The gases contain 68% to nearly 100% CO₂, with δ¹³C_CO2 and δ¹³C₁ values in the range of −1.18‰ to −6.00‰ and −19.48‰ to −24.94‰, respectively. All of the 34 large inorganic CO₂ and one inorganic methane accumulations discovered in China are distributed in eastern parts of the country, from both onshore and continental shelf basins. No commercial inorganic gas accumulation has been found in central and western China. This is a review of the occurrence and geochemical characteristics of inorganic gas accumulations in Chinese sedimentary basins. A detailed study of gas samples collected from four representative inorganic CO₂ pools and one possible inorganic methane pool indicates that inorganic alkane gases typically show δ¹³C₁ values greater than −10‰ versus PDB (mostly −30‰), with a positive stable carbon isotope sequence of δ¹³C₁ < δ ¹³C₂ < δ¹³C₃ < δ ¹³C₄. In contrast, the δ¹³C₁ values of biogenic alkane gases are lighter than −30‰, with a negative isotope sequence (i.e. δ¹³C₁ > δ¹³C₂ > δ ¹³C₃ > δ¹³C₄). Inorganic gases also tend to show less negative δ¹³C_CO2 values (−10‰) than biogenic gases (<−10‰).     

Isotopic and molecular composition of coal-bed gas in the Amasra region (Zonguldak basin—western Black Sea)

Previous studies on the coal-bed methane potential of the Zonguldak basin have indicated that the gases are thermogenic and sourced by the coal-bearing Carboniferous units. In this earlier work, the origin of coal-bed gas was only defined according to the molecular composition of gases and to organic geochemical properties of the respective source rocks, since data on isotopic composition of gases were not available. Furthermore, in the western Black Sea region there also exist other source rocks, which may have contributed to the coal-bed gas accumulations. The aim of this study is to determine the origin of coal-bed gas and to try a gas-source rock correlation. For this purpose, the molecular and isotopic compositions of 13 headspace gases from coals and adjacent sediments of two wells in the Amasra region have been analyzed. Total organic carbon (TOC) measurements and Rock-Eval pyrolysis were performed in order to characterize the respective source rocks. Coals and sediments are bearing humic type organic matter, which have hydrogen indices (HI) of up to 300 mgHC/gTOC, indicating a certain content of liptinitic material. The stable carbon isotope ratios (δ¹³C) of the kerogen vary from −23.1 to −27.7‰. Air-free calculated gases contain hydrocarbons up to C₅, carbon dioxide (<1%) and a considerable amount of nitrogen (up to 38%). The gaseous hydrocarbons are dominated by methane (>98%). The stable carbon isotope ratios of methane, ethane and propane are defined as δ¹³C₁: −51.1 to −48.3‰, δ¹³C₂: −37.9 to −25.3‰, δ¹³C₃: −26.0 to −19.2 ‰, respectively. The δD₁ values of methane range from −190 to −178‰. According to its isotopic composition, methane is a mixture, partly generated bacterially, partly thermogenic. Molecular and isotopic composition of the gases and organic geochemical properties of possible source rocks indicate that the thermogenic gas generation took place in coals and organic rich shales of the Westphalian-A Kozlu formation. The bacterial input can be related to a primary bacterial methane generation during Carboniferous and/or to a recent secondary bacterial methane generation. However, some peculiarities of respective isotope values of headspace gases can also be related to the desorption process, which took place by sampling.     

Hydrochemistry and isotope compositions of groundwater from the Shihongtan sandstone-hosted uranium deposit, Xinjiang, NW China

Groundwaters and surface water in the Shihongtan sandstone-hosted U ore district, Xinjiang, NW China, were sampled and analyzed for their major-, and trace element concentrations and oxygen, hydrogen, boron and strontium isotope compositions in order to assess the possible origins of the waters and water–rock interactions that occurred in the deep aquifer system. The waters in the study district have been grouped into three hydrochemical facies: Facies 1, potable spring-water, is a pH neutral (7.0), Na–Ca–HCO₃ type water with low total dissolved solids (TDS; 0.2 g/l, fresh) and has δ¹⁸O of − 8.3‰, δD of − 48.2‰,δ¹¹B of 1.5‰, and ⁸⁷Sr/⁸⁶Sr of 0.70627. Facies 2 groundwaters are mildly acidic to mildly alkaline (pH of 6.5–8.0, mean 7.3), Na–Ca–Mg–Cl–SO₄ type waters with moderate TDS (8.2 g/l–17.2 g/l, mean 9.3 g/l, brackish) and haveδ¹⁸O values in the − 5.8‰ to − 9.3‰ range (mean − 8.1‰), δD values in the − 20.8‰ to − 85.5‰ range (mean − 47.0‰),δ¹¹B values in the + 9.5‰ to + 39.1‰ range (mean + 17.1‰), and ⁸⁷Sr/⁸⁶Sr values in the 0.70595 to 0.70975 range (mean 0.70826). Facies 3, Aiting Lake water, is a mildly alkaline (pH = 7.4), Na–Ca–Mg–Cl–SO₄ type water with the highest TDS (249.1 g/l, brine) and has δ¹⁸O of − 2.8‰, δD of − 45.8‰,δ¹¹B of 21.2‰, and ⁸⁷Sr/⁸⁶Sr of 0.70840. The waters from the study district show a systematic increase in major, trace element and TDS concentrations and δ¹¹B values along the pathway of groundwater migration which can only be interpreted in terms of water–rock interaction at depth and strong surface evaporation. The hydrochemical and isotopic data presented here confirm that the groundwaters in the Shihongtan ore district are the combined result of migration, water–rock interaction and mixing of meteoric water with connate waters contained in sediments.     

Isotopic evidence for temporal variation in proportion of seasonal precipitation since the last glacial time in the inland Pacific Northwest of the USA

Large-scale atmospheric circulation patterns determine the quantity and seasonality of precipitation, the major source of water in most terrestrial ecosystems. Oxygen isotope (δ¹⁸O) dynamics of the present-day hydrologic system in the Palouse region of the northwestern U.S.A. indicate a seasonal correlation between the δ¹⁸O values of precipitation and temperature, but no seasonal trends of δ¹⁸O records in soil water and shallow groundwater. Their isotope values are close to those of winter precipitation because the Palouse receives  75% of its precipitation during winter. Palouse Loess deposits contain late Pleistocene pedogenic carbonate having ca. 2 to 3‰ higher δ¹⁸O values and up to 5‰ higher carbon isotope (δ¹³C) values than Holocene and modern carbonates. The late Pleistocene δ¹⁸O values are best explained by a decrease in isotopically light winter precipitation relative to the modern winter-dominated infiltration. The δ¹³C values are attributed to a proportional increase of atmospheric CO₂ in soil CO₂ due to a decrease in soil respiration rate and ¹³C discrimination in plants under much drier paleoclimate conditions than today. The regional climate difference was likely related to anticyclonic circulation over the Pleistocene Laurentide and Ice Sheet.     

Carbon isotopes of Middle–Lower Jurassic coal-derived alkane gases from the major basins of northwestern China

Coal-derived hydrocarbons from Middle–Lower Jurassic coal-bearing strata in northwestern China are distributed in the Tarim, Junggar, Qaidam, and Turpan-Harmi basins. The former three basins are dominated by coal-derived gas fields, distributed in Cretaceous and Tertiary strata. Turpan-Harmi basin is characterized by coal-derived oil fields which occur in the coal measures. Based on analysis of gas components and carbon isotopic compositions from these basins, three conclusions are drawn in this contribution: 1) Alkane gases with reservoirs of coal measures have no carbon isotopic reversal, whereas alkane gases with reservoirs not of coal measures the extent of carbon isotopic reversal increases with increasing maturity; 2) Coal-derived alkane gases with high δ¹³C values are found in the Tarim and Qaidam basins (δ¹³C₁: − 19.0 to − 29.9‰; δ¹³C₂: − 18.8 to − 27.1‰), and those with lowest δ¹³C values occur in the Turpan-Harmi and Junggar basins (δ¹³C₁: − 40.1 to − 44.0‰; δ¹³C₂: − 24.7 to − 27.9‰); and 3) Individual specific carbon isotopic compositions of light hydrocarbons (C_5–8) in the coal-derived gases are lower than those in the oil-associated gases. The discovered carbon isotopic reversal of coal-derived gases is caused by isotopic fractionation during migration and secondary alteration. The high and low carbon isotopic values of coal-derived gases in China may have some significance on global natural gas research, especially the low carbon isotope value of methane may provide some information for early thermogenic gases. Coal-derived methane typically has much heavier δ¹³C than that of oil-associated methane, and this can be used for gas–source rock correlation. The heavy carbon isotope of coal-derived ethane is a common phenomenon in China and it shed lights on the discrimination of gas origin. Since most giant gas fields are of coal-derived origin, comparative studies on coal-derived and oil-associated gases have great significance on future natural gas exploration in the world.     

Pleistocene paleoclimate of the arid region of Israel as recorded in calcite deposits along regional transverse faults and in veins

The δ¹⁸O and δ¹³C values of the calcites associated with E-W and NE-SW transverse faults in the Negev, Israel, indicate that calcite was deposited from meteoric water. A regional change in the δ¹⁸O and δ¹³C values was observed. The ¹⁸O content in the calcite increases, from the southwestern (δ¹⁸O = −17.8‰) to the northeastern (δ¹⁸O = −2.9‰) part of the region. The δ¹³C values show the opposite trend of the ¹³C content decrease: from +2‰ in the south to −10‰ in the northeast. These trends had to reflect changes in regional paleoclimate, suggesting a change in the isotopic composition of the solution from which the calcite was deposited in different periods. The variations in the δ¹⁸O values reflect shifts in the δ¹⁸O values of precipitation and are associated with a change in the source of moist air masses which came from the equatorial Atlantic in the early Pleistocene and from the Mediterranean during a later period. Variations in δ¹³C values reflect changes from humid to arid conditions. Two modes of calcite deposition are suggested: (1) precipitation of calcite minerals in the unsaturated zone following the dissolution in the soil or (2) calcite deposition that occurred as CO₂ was lost during emergence of paleogroundwater from Lower Cretaceous and Jurassic aquifers.     

Carbon and oxygen isotope zoning around Carlin-type gold deposits: a reconnaissance survey at Twin Creeks, Nevada

This study was undertaken to determine whether wallrocks around the Twin Creeks Carlin-type gold deposits exhibit oxygen isotope haloes similar to those found around other types of hydrothermal deposits. Mineralization at Twin Creeks is hosted by Ordovician Sequence shales containing some carbonate minerals and by Pennsylvanian–Permian Etchart Formation limestone. Analysis of orthophosphate-soluble carbonate from these rocks shows that oxygen isotope haloes are detectable in Ordovician Sequence shales but not in Etchart Formation limestone. The soluble fraction of Ordovician Sequence shales at Twin Creeks has δ¹⁸O values of 12 to 24‰ and δ¹³C values of 0 to −10‰. Most samples fall along a poorly defined trend that extends from δ¹⁸O of about 24‰ and δ¹³C values of about 0, which are typical of unaltered limestones, toward lower values for both isotope systems, which are typical of rocks that have undergone alteration by hydrothermal fluids. Plots of these values along two sections through the ore body show that δ¹⁸O values of wallrocks are lowest in the ore zone and increase outward, forming a halo several hundred meters in size. In the same plots, δ¹³C values of the wallrocks do not show systematic spatial variations. The soluble fraction of Etchart Formation limestones at Twin Creeks have δ¹⁸O values of 25 to 5‰ and δ¹³C values of 4 to −10‰, but do not show any systematic spatial variation relative to mineralization at the scale of our samples. Failure of the Etchart Formation samples to show detectable haloes is probably related to deposition of post-ore carbonate minerals or lower ore fluid : rock ratios. Material balance calculations used to model the isotopic composition of average Ordovician Sequence shales indicate that changes in temperature and water : rock ratio were probably not sufficient to account for the wide range of isotope compositions observed in these rocks. The most likely additional factor contributing to this range of values was a change in the composition of the altering fluid, probably by mixing of the ore fluid with surrounding meteoric water. These results suggest that Carlin-type gold deposits are surrounded by haloes of low δ¹⁸O values, but that detection of these haloes could be complicated by local compositional variations and post-ore modification of the wallrocks.     

Temperature dependence of the compressional wave velocity in an anisotropic dunite: Measurements to 500°C at 10 kbar

Measurements of compressional wave velocity V_p were made in a gas apparatus to 500°C at 10 kbar in three cores of an anisotropic dunite specimen from Twin Sisters Mountain. The axial directions of the three chosen cores coincide with the preferred directions and concentration of olivine crystallographic axes (a [100], b [010], andc [001]).Measured (δV_p/δT)p values at 10 kbar in the three cores (−6.7, −5.4 and −6.2 · 10⁻⁴ km/sec · deg, respectively), and the mean value for the dunite (−6.1 · 10⁻⁴ km/sec · deg) are larger than the Voigt-Reuss-Hill values calculated from single-crystal data. This discrepancy is explained by the presence of internal thermal stresses, due to anisotropic expansion of olivine grains, causing grain boundary cracks to widen.It is concluded that high negative values of (δV_p/δT)p for rocks reported in the literature should be carefully evaluated in terms of the formation of new cracks or widening of cracks already present under high pressure-temperature environments.     

Fluid geochemistry in the Ivigtut cryolite deposit, South Greenland

The 1.27 Ga old Ivigtut (Ivittuut) intrusion in South Greenland is world-famous for its hydrothermal cryolite deposit [Na₃AlF₆] situated within a strongly metasomatised A-type granite stock. This detailed fluid inclusion study characterises the fluid present during the formation of the cryolite deposit and thermodynamic modelling allows to constrain its formation conditions.Microthermometry revealed three different types of inclusions: (1) pure CO₂, (2) aqueous-carbonic and (3) saline-aqueous inclusions. Melting temperatures range between − 23 and − 15 °C for type 2 and from − 15 to − 10 °C for type 3 inclusions. Most inclusions homogenise between 110 and 150 °C into the liquid.Stable isotope compositions of CO₂ and H₂O were measured from crushed inclusions in quartz, cryolite, fluorite and siderite. The δ¹³C values of about − 5‰ PDB are typical of mantle-derived magmas. The differences between δ¹⁸O of CO₂ (+ 21 to + 42‰ VSMOW) and δ¹⁸O of H₂O (− 1 to − 21.7‰ VSMOW) suggest low-temperature isotope exchange. δD (H₂O) ranges from − 19 to − 144‰ VSMOW. The isotopic composition of inclusion water closely follows the meteoric water line and is comparable to Canadian Shield brines. Ion chromatography revealed the fluid's predominance in Na, Cl and F. Cl/Br ratios range between 56 and 110 and may imply intensive fluid–rock interaction with the host granite.Isochores deduced from microthermometry in conjunction with estimates for the solidification of the Ivigtut granite suggest a formation pressure of approximately 1–1.5 kbar for the fluid inclusions. Formation temperatures of different types of fluid inclusions vary between 100 and 400 °C. Thermodynamic modelling of phase assemblages and the extraordinary high concentration in F (and Na) may indicate that the cryolite body and its associated fluid inclusions could have formed during the continuous transition from a volatile-rich melt to a solute-rich fluid.     

Carbon and Oxygen Isotope Geochemistry of the Amba Dongar Carbonatite Complex, Gujarat, India

A carbon and oxygen isotope survey based on 42 samples from the Amba Dongar carbonatite complex of Gujarat, India, indicates that the magmatic differentiation series sövite → alvikite → ankeritic carbonatite is beset with a distinct isotope trend characterized by a moderate rise in ¹³C coupled with a sizeable increase in ¹⁸O. From an average of −4.6 ± 0.4 ‰ [PDB] for the least differentiated (coarse) sövite member, δ¹³C values slowly increase in the alvikite (−3.7 ± 0.6 ‰) and ankeritic fractions (−3.0 ± 1.1 ‰), whereas δ¹⁸O rises from 10.3 ± 1.7 ‰ [SMOW] to 17.5 ± 5.8 ‰ over the same sequence, reaching extremes between 20 and 28 ‰ in the latest generation of ankeritic carbonatite. While an apparent correlation between δ¹³C and δ¹⁸O over the δ¹⁸O range of 7–13 ‰ conforms with similar findings from other carbonatite complexes and probably reflects a Rayleigh fractionation process, the observed upsurge of ¹⁸O notably in the ankeritic member is demonstrably related to a late phase of low-temperature hydrothermal activity involving large-scale participation of ¹⁸O-depleted groundwaters. As a whole, the Amba Dongar carbonatite province displays the characteristic ¹³C/¹²C label of deep-seated (primordial) carbon, reflecting the carbon isotope composition of the subcontinental upper mantle below the Narmada Rift Zone of the Indian subcontinent.     

Hydrochemical and isotopic characteristics of groundwater in the Souss Upstream Basin, southwestern Morocco

Heterogeneous shallow Plio-Quaternary formations of the Souss Plain represent the most important aquifer in southern High Atlas Mountains in Morocco. The present work was conducted in the Souss Upstream Basin to identify the chemical characteristics and the origin of groundwater in an aquifer under semi-arid climate. Isotopic and hydrochemical compositions combined with geological and hydrogeological data were used for this purpose. The total dissolved solids vary from 239 to 997 mg l⁻¹, and the following groundwater types are recognized: Ca²⁺–Mg²⁺–HCO₃⁻, Ca²⁺–Mg²⁺–SO₄²⁻ and Ca²⁺–Mg²⁺–Cl⁻. The groundwater is saturated and slightly supersaturated with respect to carbonate minerals and undersaturated with respect to evaporite minerals, which means that the groundwater composition is largely controlled by the dissolution of carbonate rocks known in the basin. The isotopic contents of groundwaters ranged from −8‰ to −5.2‰ for δ¹⁸O, from −52‰ to −34‰ for δD, and from 0 to 5.5 TU for tritium. The hydrogen (δD) and oxygen (δ¹⁸O) isotope signatures reveal a significant infiltration before evaporation takes place, indicating a major recharge directly from fractures in the crystalline and limestone formations of Atlas Mountains (above 800 m a.s.l.) and infiltration of surface water in the alluvial cones at the border of the Atlas basins. The very low tritium values suggest that the groundwater recharge follows a long flow path and a mixing between old and modern water is shown. However, a slight evaporation effect is noted in the southern part of the basin close to the Anti-Atlas Mountains.     

Calibration of sclerosponge oxygen isotope records to temperature using high-resolution δO data

A revised calibration is presented relating the oxygen isotope composition of the aragonite-secreting sclerosponge Ceratoporella nicholsoni, oxygen isotope composition of seawater, and ambient water temperature. This new relationship has been obtained using high-resolution δ¹⁸O data measured in sclerosponges from the Bahamas and Jamaica compared to ambient temperature measurements and δ¹⁸O values of seawater from the two locations, both measured and published. New data improve an existing calibration which was determined using measurements of salinity rather than directly measured δ¹⁸O values of the seawater and was composed of measurements from different species of sclerosponge and other aragonite-secreting organisms. The updated calibration (n = 12, r² = 0.95) is:

T(°C)=16.1(±3.1)-[6.5(±1.1)](δ_arag-δ_sw),