Isotopic Composition Of Calcite Research Articles

Stable isotope geochemistry evidences from fossil carbonate and sulfur minerals on the origin of geothermal water, Kızıldere Geothermal Field, Western Turkey

The Kızıldere geothermal field, located at the eastern part of the Büyük Menderes graben in Western Turkey, is the most important geothermal reservoir suitable for electricity generation. Current and fossil fumaroles and alteration zones are directly related to the tectonic zones influenced by N-S directional extension since Miocene period. Associated to fossil geothermal activities carbonate (calcite, dolomite) and sulfate (gypsum, anhydrite) minerals were occurred in the form of void/crack fill and bands/lenses parallel to bedding of Neogene clastic and carbonate rocks. The carbon (δ13CPDB ‰) and oxygen (δ18OPDB ‰) isotope compositions of hydrothermal calcites and dolomites and sulfur (δ34SCDT) and oxygen (δ18OSMOW) isotope compositions of gypsum and anhydrites are analyzed first time and correlated current geothermal water composition. The carbon and oxygen isotope data of calcites and dolomites have similar carbon but different oxygen isotope composition which increases in the direction of surface calcite – drill cuttings calcite – drill cuttings dolomite – surface dolomite. The isotope compositions of calcite and dolomite minerals range between limestone and marble host rock compositions and indicate the carbonate mineral-forming fluids originated from dissolution of carbonate rocks during the circulation of hot geothermal waters. According to the calcite-CO2 and dolomite-CO2 isotopic fractionation data for the 0–300 °C temperature range, the fossil isotope composition is higher than the composition of current CO2 and reflects relatively lower temperature conditions. The isotope compositions of gypsum and anhydrite minerals indicate that hot thermal waters dissolved terrestrial evaporites and formed a sulfur-rich geothermal solution, and hydrothermal gypsum and anhydrite precipitated from this solution. The δ34SCDT compositions of hydrothermal gypsum and anhydrites are similar to the current geothermal water compositions. Stable isotope geochemistry data of hydrothermal carbonate and sulfate minerals in the Kızıldere geothermal field have shown that the fluids forming these minerals were originated from host rocks instead of magmatic volatiles.

Distribution Patterns of Calcite Cement in the Turbidite Sandstones of the Shahejie Formation in the Dongying Depression, Eastern China

AbstractCalcite cements are volumetrically dominant among the most diagenetic constituents in turbidite sandstones of the Paleogene Shahejie Formation in the Dongying Depression. The results show carbonate cements mainly occur in three phases: Calcite I (pre‐compaction), Calcite II (post‐compaction) and ankerite in the turbidite sandstone. The isotopic composition of Calcite I and Calcite II range from –4.3‰ to 4.4‰, –3.5‰ to 4.3‰ PDB, respectively, suggesting that the calcite cements are mainly derived from the interbedded shales. The early calcite cement precipitated from the mixing of lacustrine and meteoric waters at 38–69°C. The late‐phase calcite precipitated from pore waters modified by water‐rock interactions at 64–126°C. Two distribution patterns of calcite cements can be identified, depending on their position within the sandstone. The peripheral cementation of the sandstones is near the sand‐shale contact. The development of cement is controlled by sandstone thickness and the properties of the interbedded shale. For scattered cementation, the distribution of the calcite cement is not controlled by the sand‐shale contact. Calcite cement was preferentially distributed in coarsegrained sandstones, indicating that sandstones with high original permeability are the preferred pathway for the migration of early diagenetic fluids.

Sources of Hydrothermal Fluids Inferred from Oxygen and Carbon Isotope Composition of Calcite, Keweenaw Peninsula Native Copper District, Michigan, USA

The Mesoproterozoic North American Midcontinent Rift hosts the world’s largest accumulation of native copper in Michigan’s Keweenaw Peninsula. During a regional metamorphogenic-hydrothermal event, native copper was deposited along with spatially zoned main-stage minerals in a thermal high. This was followed by deposition of late-stage minerals including minor copper sulfide. Inferences from the oxygen and carbon isotopic composition of main-stage hydrothermal fluids, as calculated from 296 new and compiled isotopic measurements on calcite, are consistent with existing models that low-sulfur saline native copper ore-forming fluids were dominantly derived by burial metamorphic processes from the very low sulfur basalt-dominated rift fill at depth below the native copper deposits. Co-variation of oxygen and carbon isotopic compositions are consistent with mixing of metamorphic-derived fluids with two additional isotopically different fluids. One of these is proposed to be evolved seawater that provided an outside source of salinity. This fluid mixed at depth and participated in the formation of a well-mixed hybrid metamorphic-dominated ore-forming fluid. Secondary Ion Mass Spectrometry in-situ isotopic analyses of calcite demonstrate a high degree of variability within samples that is attributed to variable degrees of shallow mixing of the hybrid ore-forming fluid with sulfur-poor, reduced evolved meteoric water in the zone of precipitation. The oxygen and carbon isotopic compositions of 100 new and compiled measurements on late-stage calcite are mostly isotopically different than the main-stage hydrothermal fluids. The late-stage hydrothermal fluids are interpreted as various proportions of mixing of evolved meteoric water, main-stage hybrid ore-forming fluid, and shallow, evolved seawater in the relatively shallow zone of precipitation.

Evolution of the magmatic–hydrothermal system and formation of the giant Zhuxi W–Cu deposit in South China

The Zhuxi deposit is a recently discovered W–Cu deposit located in the Jiangnan porphyry–skarn W belt in South China. The deposit has a resource of 3.44 million tonnes of WO3, making it the largest on Earth, however its origin and the evolution of its magmatic–hydrothermal system remain unclear, largely because alteration–mineralization types in this giant deposit have been less well-studied, apart from a study of the calcic skarn orebodies. The different types of mineralization can be classified into magnesian skarn, calcic skarn, and scheelite–quartz–muscovite (SQM) vein types. Field investigations and mineralogical analyses show that the magnesian skarn hosted by dolomitic limestone is characterized by garnet of the grossular–pyralspite (pyrope, almandine, and spessartine) series, diopside, serpentine, and Mg-rich chlorite. The calcic skarn hosted by limestone is characterized by garnet of the grossular–andradite series, hedenbergite, wollastonite, epidote, and Fe-rich chlorite. The SQM veins host high-grade W–Cu mineralization and have overprinted the magnesian and calcic skarn orebodies. Scheelite is intergrown with hydrous silicates in the retrograde skarn, or occurs with quartz, chalcopyrite, sulfide minerals, fluorite, and muscovite in the SQM veins.Fluid inclusion investigations of the gangue and ore minerals revealed the evolution of the ore-forming fluids, which involved: (1) melt and coexisting high–moderate-salinity, high-temperature, high-pressure (>450 °C and >1.68 kbar), methane-bearing aqueous fluids that were trapped in prograde skarn minerals; (2) moderate–low-salinity, moderate-temperature, moderate-pressure (~210–300 °C and ~0.64 kbar), methane-rich aqueous fluids that formed the retrograde skarn-type W orebodies; (3) low-salinity, moderate–low-temperature, moderate-pressure (~150–240 °C and ~0.56 kbar), methane-rich aqueous fluids that formed the quartz–sulfide Cu(–W) orebodies in skarn; (4) moderate–low-salinity, moderate-temperature, low-pressure (~150–250 °C and ~0.34 kbar) alkanes-dominated aqueous fluids in the SQM vein stage, which led to the formation of high-grade W–Cu orebodies. The S–Pb isotopic compositions of the sulfides suggest that the ore-forming materials were mainly derived from magma generated by crustal anatexis, with minor addition of a mantle component. The H–O isotopic compositions of quartz and scheelite indicate that the ore-forming fluids originated mainly from magmatic water with later addition of meteoric water. The C–O isotopic compositions of calcite indicate that the ore-forming fluid was originally derived from granitic magma, and then mixed with reduced fluid exsolved from local carbonate strata. Depressurization and resultant fluid boiling were key to precipitation of W in the retrograde skarn stage. Mixing of residual fluid with meteoric water led to a decrease in fluid salinity and Cu(–W) mineralization in the quartz–sulfide stage in skarn. The high-grade W–Cu mineralization in the SQM veins formed by multiple mechanisms, including fracturing, and fluid immiscibility, boiling, and mixing.

Influence of Host Marble Rocks on the Formation of Intrusive Alkaline Rocks and Carbonatites of Sangilen (E. Siberia, Russia)

The study of the O and C isotope composition of calcite from nepheline syenites, ijolites and carbonatites of the Chik intrusion and the intrusions of the Erzin–Tarbagatay group of Sangilen (Eastern Siberia, Russia) showed derivation from alkaline melts enriched with a carbonate component from the host marbleized sedimentary rocks. The calculations showed that about 40% of the initial mass of carbonates involved in the interaction with silicate melts have remained after decarbonation. During the assimilation of the carbonate, an oxygen isotope exchange took place between the residual carbonate material and the silicate phase. Crystallization products of such hybrid magmas are carbonatite veins, calcite-rich nepheline rocks and their pegmatites with a calcite core.

Geological Characteristics and Genesis of the Jiamoshan MVT Pb–Zn Deposit in the Sanjiang belt, Tibetan Plateau

AbstractThe carbonate‐hosted Pb–Zn deposits in the Sanjiang metallogenic belt on the Tibetan Plateau are typical of MVT Pb–Zn deposits that form in thrust‐fold belts. The Jiamoshan Pb–Zn deposit is located in the Changdu area in the middle part of the Sanjiang belt, and it represents a new style of MVT deposit that was controlled by karst structures in a thrust–fold system. Such a karst‐controlled MVT Pb–Zn deposit in thrust settings has not previously been described in detail, and we therefore mapped the geology of the deposit and undertook a detailed study of its genesis. The karst structures that host the Jiamoshan deposit were formed in Triassic limestones along secondary reverse faults, and the orebodies have irregular tubular shapes. The main sulfide minerals are galena, sphalerite, and pyrite that occur in massive and lamellar form. The ore‐forming fluids belonged to a Mg2+–Na+–K+–SO42––Cl−–F−–NO3−–H2O system at low temperatures (120–130°C) but with high salinities (19–22% NaCl eq.). We have recognized basinal brine as the source of the ore‐forming fluids on the basis of their H–O isotopic compositions (–145‰ to –93‰ for δDV‐SMOW and –2.22‰ to 13.00‰ for δ18Ofluid), the ratios of Cl/Br (14–1196) and Na/Br (16–586) in the hydrothermal fluids, and the C–O isotopic compositions of calcite (–5.0‰ to 3.7‰ for δ13CV‐PDB and 15.1‰ to 22.3‰ for δ18OV‐SMOW). These fluids may have been derived from evaporated seawater trapped in marine strata at depth or from Paleogene–Neogene basins on the surface. The δ34S values are low in the galena (–3.2‰ to 0.6‰) but high in the barite (27.1‰), indicating that the reduced sulfur came from gypsum in the regional Cenozoic basins and from sulfates in trapped paleo‐seawater by bacterial sulfate reduction. The Pb isotopic compositions of the galena samples (18.3270–18.3482 for 206Pb/204Pb, 15.6345–15.6390 for 207Pb/204Pb, and 38.5503–38.5582 for 208Pb/204Pb) are similar to those of the regional Triassic volcanic‐arc rocks that formed during the closure of the Paleo‐Tethys, indicating these arc rocks were the source of the metals in the deposit. Taking into account our new observations and data, as well as regional Pb–Zn metallogenic processes, we present here a new model for MVT deposits controlled by karst structures in thrust–fold systems.

Experimental constraints on magnesium isotope fractionation during abiogenic calcite precipitation at room temperature

Magnesium (Mg) isotopes in carbonate minerals are a useful proxy for paleoclimate studies, but interpretations are often limited by an inadequate understanding of the various factors controlling Mg isotopic fractionation during carbonate formation. Previous work has studied a number of parameters including aqueous chemistry, mineralogy, temperature, and precipitation rate. However, little is known about the impact of solid/solution ratio, calcite growth mechanism, and crystal morphology on isotope fractionation. In this work, two groups of seeded chemo-stat calcite precipitation experiments were conducted at 25 °C to explore the potential impact of crystal growth and morphology on the fractionation of Mg isotopes. Group-1 experiments (G1) contained nine individual runs that were performed under identical physicochemical conditions, except for solid/solution ratio and the length of an experiment. The isotope fractionation between precipitated calcite and aqueous solution is limited, with Δ26Mgcal-sol ranging from −2.58 to −2.40‰ and an average of −2.49 ± 0.12‰ (2SD, n = 9). The Group-2 experiments (G2) contained 3 paired runs with solution Mg/Ca molar ratios of 0.5, 2.0, and 5.0, and yielded Δ26Mgcal-sol values that ranged from −2.69 to −2.36‰ with an average of −2.62 ± 0.25‰ (2SD, n = 6).The average Δ26Mgcal-sol value for both sets of experiments is −2.54 ± 0.22‰ (2SD, n = 15), and it is independent of precipitation rate, solution Mg/Ca molar ratio, solid/solution ratio, amount of overgrowth, and mol% Mg content in overgrowth. The form and texture of the calcite overgrowths in our experiments range from {104} rhombohedra with smooth crystal faces containing few macrosteps to {104} rhombohedra containing extensive evidence for 2-D nucleation on crystal faces, to more steeply sided rhombohedra {0kl}. While significant changes in crystal morphology are related to solid/solution ratio and solution composition in the G1 and G2 experiments, respectively, there was no difference in Mg isotope systematics, suggesting that crystal morphology does not affect the Mg isotopic composition of calcite within the range of features investigated and 2-D nucleation may be less affected by calcite growth kinetics than a spiral growth mechanism. Integrating our results with previous published values, an equilibrium isotopic fractionation factor of −2.47 ± 0.09‰ (weighted average ± weighted 2SD, n = 70) between calcite and aqueous solutions is derived at room temperature.

Trace elements and C-O isotopes of calcite from Carlin-type gold deposits in the Youjiang Basin, SW China: Constraints on ore-forming fluid compositions and sources

The Carlin-type gold deposits in the Youjiang Basin, SW China can be further classified into three sub-types; strata-bound, fault-controlled, and diabase-associated. The compositions and sources of the ore-forming fluids for each deposit type are evaluated using the texture of calcite by CL (Cathode luminescence), its chemistry by EMPA (Electron probe microanalysis) and LA-ICP-MS (Laser ablation inductively coupled plasma mass spectrometry), and its C-O isotopic compositions by SIMS (Secondary ion mass spectroscopy). The studied deposits are the stratabound Zimudang deposit, the fault-controlled Lannigou deposit, and the diabase-associated Badu deposit. Calcite in the Zimudang and Lannigou deposits formed at three different stages: pre-ore stage pore-filling and massive calcite (Cal 1), late-ore stage vein calcite (Cal 2) associated with realgar and pyrite, and post-ore stage vein calcite (Cal 3). Calcite in the Badu deposit formed in two stages: ore stage vein and replacement calcite (Cal 2) associated with pyrite and post-ore stage calcites (Cal 3). Calcite with brighter red CL color has higher Mn and lower Fe content than calcite with darker red CL color. Furthermore, the higher the total REE content, the brighter the CL color. The host rocks are characterized by light REE (Rare earth element) enrichments. In contrast, the ore stage calcite shows middle REE enrichments. The ore stage calcite has higher concentrations of Fe, Mn, Mg, Si and Sr, and lower concentrations of Cu, Sb and Zn than both the pre- and post-ore calcite. The ranges of carbon (PDB) and oxygen (SMOW) isotope compositions of calcite are from −11.29 to 6.89‰ and from 16.49 to 22.60‰ for the Zimudang deposit, from −13.31 to 2.80‰ and from 9.12 to 29.71 for the Lanningou deposit, and from −11.94 to 0.49‰ and from 10.51 to 18.97‰ for the Badu deposit. Generally, the ore stage calcite has a wider range of δ13C and δ18O values than the other stage calcite. The isotope data indicate that a deep crustal fluid was involved in the formation of all of these three deposits. Meteoric water containing abundant organic matter was also involved in the formation of the Badu deposit as well as the Zimudang deposit, whereas a basin brine was also involved in the formation of the Lanningou deposit.

Calcite and dolomite formation in the CM parent body: Insight from in situ C and O isotope analyses

Abstract To constrain the conditions of aqueous alteration in early planetesimals, we carried out in situ C and O isotope analyses of calcite and dolomite and O isotope analyses of magnetite from the highly altered CM chondrites ALH 83100, ALH 84034, and MET 01070. Petrographic and isotopic analyses of these samples support previous findings of multiple generations of carbonate growth. We observe wide ranges in the C and O isotope compositions of carbonates of up to 80‰ and 30‰, respectively, that span the full range of previously reported bulk carbonate values for CM chondrites. Variations in the Δ17O values indicate that fluid evolution varied for each chondrite. ALH 83100 dolomite-magnetite δ18O fractionation of 23‰ ± 7‰ (2SD) corresponds to dolomite formation temperature of 125 °C ± 60 °C. δ13C vs δ18O values fall into two groups, one consisting of primary calcite and the other consisting of dolomite and secondary calcite. The positive correlation between δ13C and δ18O for primary calcite is consistent with the precipitation of calcite in equilibrium with a gas mixture of CO (or CH4) and CO2. The isotopic composition of calcite in CM1s and CM2s overlap significantly; however, many CM1 calcite grains are more depleted in δ18O compared to CM2s. Altogether, the data indicate that the fluid composition during calcite formation was initially the same for both CM1s and CM2s. CM1s experienced more episodes of carbonate dissolution and reprecipitation where some fraction of the carbonate grains survive each episode resulting in a highly disequilibrium assemblage of carbonates on the thin-section scale.

The rapid resetting of the Ca isotopic signatures of calcite at ambient temperature during its congruent dissolution, precipitation, and at equilibrium

This study provides direct experimental evidence of the resetting of the calcium (Ca) isotope signatures of calcite in the presence of an aqueous fluid during its congruent dissolution, precipitation, and at equilibrium at ambient temperatures over week-long timescales. Batch reactor experiments were performed at 25 °C in aqueous NaCl solutions; air or CO2-gas mixtures were bubbled through this fluid to fix pH. During congruent calcite dissolution, the fluid became enriched in isotopically heavy Ca, and the Ca isotope composition continued to become heavier after the fluid attained bulk chemical equilibrium with the mineral; the δ44/42Ca composition of the fluid was up to 0.8‰ higher than the dissolving calcite at the end of the dissolution experiments. Calcite precipitation was provoked by increasing the reactor fluid pH after chemical equilibrium had been attained via dissolution. Rayleigh isotope fractionation effects were observed immediately after the pH was increased and rapid calcite precipitation occurred. However, isotopic exchange continued after the system chemically equilibrated, eradicating this Rayleigh signal. Taken together, these observations 1) confirm dynamic mineral-fluid equilibrium (i.e. dissolution and precipitation occur at equal, non-zero rates at equilibrium), and 2) indicate that isotopic compositions of calcite can readily equilibrate even when this mineral is in bulk chemical equilibrium with its coexisting fluid. This latter observation suggests the preservation of paleo-environmental isotopic signatures in calcite may require a combination of the isolation of the fluid-mineral system from external chemical input and/or the existence of a yet to be defined calcite dissolution/precipitation inhibition mechanism.

Effect of organic ligands on Mg partitioning and Mg isotope fractionation during low-temperature precipitation of calcite in the absence of growth rate effects

Calcite growth rate has been previously shown to be the dominating parameter controlling both Mg partitioning and Mg isotope fractionation during calcite growth. In natural calcite precipitation environments - characterized by abundant organic material - the presence of dissolved organic molecules may affect these two parameters. In order to assess the role of organic molecules, steady state calcite growth experiments have been performed at 25°C, 1bar pCO2 and constant, within analytical uncertainty growth rate (rp=10−7.4molm−2s−1) in the presence of aqueous Mg and six organic ligands in the concentration range from 0.01 to 10mM. The organic ligands used in this study are: (i) acetic acid, (ii) citric acid, (iii) glutamic acid, (iv) salycilic acid, (v) glycine, and (vi) ethylenediaminetetraacetic acid (EDTA). These contain one or more carboxyl- and amino-groups that are commonly present in natural organic substances found in lacustrine, fluvial, soil, cave, as well as in marine and earliest diagenetic porewater environments. Results shown here indicate that the presence of these carboxyl- and amino-groups promotes an increase in the partition coefficient of Mg in calcite DMg=(Mg/Ca)calcite(Mg/Ca)fluid that can be attributed to their adsorption onto the calcite surfaces and the subsequent reduction of the active sites of growth. This increase of DMg values as a function of the supersaturation degree of calcite in the fluid phase can be described by the linear equation: LogDMg=0.3694(±0.0329)×SIcalcite-1.9066(±0.0147)R2=0.92 In contrast, data shown here suggest that the presence of organic ligands, with exception of citric acid, does not significantly affect the Mg isotope fractionation factor between calcite and reactive fluid. Citric acid likely exhibits larger fractionation between the Mg-ligand complexes and free aqueous Mg, compared to the other organic ligands studied in this work, as evidenced from the smaller Δ26Mgcalcite-fluid values. The main result shown here is that in Earth’s surface precipitation environments the presence of organic ligands is not a major factor affecting the Mg content or Mg-isotope composition of inorganic calcite. But in the presence of higher contents of organic macromolecules from byproducts of metabolic activities, humid, and fulvic acids and/or exopolymeric substances and matrices, an impact on Mg content or Mg isotope composition of calcite cannot be ruled out.

Early Cretaceous tungsten mineralization in Southeastern China: the Wuning example

ABSTRACTThe Wuning tungsten deposits in northwestern Jiangxi are located in the Middle-Lower Yangtze River metallogenic belt of SE China. This article decodes the large tungsten-mineralized ore field by analysing three typical deposits: Shimensi, Dahutang, and Shiwendong. The Shimensi deposit shows three mineralization styles, and the associated hydrothermal alteration mainly includes K-feldspathization, greisenization, chloritization, silicification, and carbonatization. The sulphur isotopic compositions of sulphides in the Dahutang deposit indicate that the sulphur source was homogeneous and closely related to magmatic sulphur. The lead isotopic compositions of sulphides suggest multiple sources that were also closely related to magmatism. The carbon isotopic compositions of calcite indicate that hydrothermal fluids were derived from the granitic magma. Hydrogen and oxygen isotopic compositions of quartz veins indicate that the ore-forming fluids had a magmatic-hydrothermal origin, but mixed with meteoric water. Fluid inclusions from the Shiweidong deposit are low density, indicating medium-high T and low-medium salinity, further suggesting that it was a shallow magmatic-hydrothermal deposit, and the metallogenic hydrotherm mingled with the surface water late in the evolution of the system. Mineralization was related to Early Cretaceous magmatism in eastern China. The subduction of the palaeo-Pacific plate and the subsequent lithospheric delamination reflected transition from a compressional to extensional tectonic regime. The upwelling of the hot asthenosphere triggered partial melting of the Neoproterozoic Shuangqiaoshan Group, with high W background concentrations. The ascent of the magma resulted in the exsolution of the ore-bearing magmatic hydrothermal fluid.

Tracing formation and durability of calcite in a Punic–Roman cistern mortar (Pantelleria Island, Italy)

Ancient hydraulic lime mortar preserves chemical and isotopic signatures that provide important information about historical processing and its durability. The distribution and isotopic composition of calcite in a mortar of a well-preserved Punic–Roman cistern at Pantelleria Island (Italy) was used to trace the formation conditions, durability, and individual processing periods of the cistern mortar. The analyses of stable carbon and oxygen isotopes of calcite revealed four individual horizons, D, E, B-1 and B-2, of mortar from the top to the bottom of the cistern floor. Volcanic and ceramic aggregates were used for the production of the mortar of horizons E/D and B-1/B-2, respectively. All horizons comprise hydraulic lime mortar characterized by a mean cementation index of 1.5 ± 1, and a constant binder to aggregate ratio of 0.31 ± 0.01. This suggests standardized and highly effective processing of the cistern.The high durability of calcite formed during carbonation of slaked lime within the matrix of the ancient mortar, and thus the excellent resistance of the hydraulic lime mortar against water, was documented by (i) a distinct positive correlation of δ18Ocalcite and δ13Ccalcite; typical for carbonation through a mortar horizon, (ii) a characteristic evolution of δ18Ocalcite and δ13Ccalcite through each of the four mortar horizons; lighter follow heavier isotopic values from upper to lower part of the cistern floor, and (iii) δ18Ocalcite varying from −10 to −5 ‰ Vienna Pee Dee belemnite (VPDB). The range of δ18Ocalcite values rule out recrystallization and/or neoformation of calcite through chemical attack of water stored in cistern.The combined studies of the chemical composition of the binder and the isotopic composition of the calcite in an ancient mortar provide powerful tools for elucidating the ancient techniques and processing periods. This approach helps to evaluate the durability of primary calcite and demonstrates the importance of calcite as a proxy for chemical attack and quality of the ancient inorganic binder.

Coherent pan‐Asian climatic and isotopic response to orbital forcing of tropical insolation

AbstractThe oxygen‐18 isotope composition of calcite in stalagmites across southern and eastern Asia are highly correlated to one another on orbital time scales: large negative excursions are coincident with maxima in summer insolation in the subtropics of the Northern Hemisphere (NH). These isotopic excursions reflect changes in the precipitation‐weighted isotopic composition of precipitation, δ18Op. We present results from two core experiments using an isotope‐enabled climate model—the“high‐insolation” and “low‐insolation” experiments—in which the model is forced by extrema in NH summer insolation. Compared to the low‐insolation experiment, the high‐insolation climate features profound, large‐scale changes in the pattern of monsoon precipitation spanning from Africa to Southeast Asia that are due to changes in the relative contributions of temperature and moisture to the near‐surface equivalent potential temperature θe. Under high insolation, a more rapid increase in land surface temperature in early summer causes the greatest θe (and hence precipitation) to shift from the oceans in low insolation (such as today) to be over land in high insolation (such as the early Holocene). The model captures the general pattern of isotopic excursions seen in caves spanning from Israel to western China, including large drops in δ18Op over eastern Tibet (−7‰), the Arabian Peninsula, and northeast Africa (−4‰). Although there are large changes in precipitation over Tibet, the change in δ18Op is due to changes in the δ18O of water vapor that is delivered and subsequently precipitated; it does not inform on local precipitation amount or intensity.

Vermiculation patterns in Coiba Mare Cave, Bihor Mountains, Romania

Materials from the vermiculations developed on passage walls of the Coiba Mare Cave, Bihor Mountains, Romania were analysed using powder X-ray diffraction (PXRD), Fourier transform infrared (FTIR), energy dispersive analyses (EDS). The isotope composition of calcite, δ18O and δ13C was determined as well. The material consists mainly of calcite with traces of quartz, muscovite, chlinochlore, kaolinite, potassium feldspar and organic material. Mineralogical evidence indicates that the source of material is related to slow dissolution of wall rock. Subsequent crystallisation and concentration of the calcite particles in vermiculations patterns are the result of evaporation and shrinking water spots.

Sulfur, Carbon, Hydrogen, and Oxygen Isotope Geochemistry of the Idaho Cobalt Belt

Cobalt-copper ± gold deposits of the Idaho cobalt belt, including the deposits of the Blackbird district, have been analyzed for their sulfur, carbon, hydrogen, and oxygen isotope compositions to improve the understanding of ore formation. Previous genetic hypotheses have ranged widely, linking the ores to the sedimentary or diagenetic history of the host Mesoproterozoic sedimentary rocks, to Mesoproterozoic or Cretaceous magmatism, or to metamorphic shearing. The δ 34 S values are nearly uniform throughout the Blackbird district, with a mean value for cobaltite (CoAsS, the main cobalt mineral) of 8.0 ± 0.4‰ ( n = 19). The data suggest that (1) sulfur was derived at least partly from sedimentary sources, (2) redox reactions involving sulfur were probably unimportant for ore deposition, and (3) the sulfur was probably transported to sites of ore formation as H 2 S. Hydrogen and oxygen isotope compositions of the ore-forming fluid, which are calculated from analyses of biotite-rich wall rocks and tourmaline, do not uniquely identify the source of the fluid; plausible sources include formation waters, metamorphic waters, and mixtures of magmatic and isotopically heavy meteoric waters. The calculated compositions are a poor match for the modified seawaters that form volcanogenic massive sulfide (VMS) deposits. Carbon and oxygen isotope compositions of siderite, a mineral that is widespread, although sparse, at Blackbird, suggest formation from mixtures of sedimentary organic carbon and magmatic-metamorphic carbon. The isotopic compositions of calcite in alkaline dike rocks of uncertain age are consistent with a magmatic origin. Several lines of evidence suggest that siderite postdated the emplacement of cobalt and copper, so its significance for the ore-forming event is uncertain. From the stable isotope perspective, the mineral deposits of the Idaho cobalt belt contrast with typical VMS and sedimentary exhalative deposits. They show characteristics of deposit types that form in deeper environments and could be related to metamorphic processes or magmatic processes, although the isotopic evidence for magmatic components is relatively weak.

Catagenesis of terrigenous rocks in the neoproterozoic intracratonic sedimentary basins and its effect on the formation of unconformity-type uranium mineralization

Mineral transformation of host rocks and localization of orebodies at the unconformity-type uranium deposits are considered for the Karku deposit in the northern Ladoga region. It is shown that the great depth of uranium mineral formation and the peculiar composition of host rocks, along with temperature and chemistry of fluids, played a critical role in variation of lithostatic and fluid pressure, porosity, and permeability. The compaction of quartz sandstone and gravelstone, which are typical host rocks at unconformity-type deposits, the development of microstylolithic sutures, conformal structures, pressure solution and deposition of quartz in free pores gave rise to the closure or constraint of pore space and to increase in pore pressure of fluids in the deep part of the Riphean troughs with approaching lithostatic loading. A transitional zone between hydrostatic and lithostatic pressure controlled localization of orebodies and was decisive for uranium mineral formation. This zone coincided with the Riphean-Paleoproterozoic unconformity and sank somewhat into the crystalline basement. Below this transitional zone, the intergranular fluid was under a pressure that was close to the pressure on solid phases, i.e., Ptot ≈ Pfl. The reliability of this phenomenon is confirmed by cessation of pressure solution-redeposition of quartz and distinct deceleration of dehydration of hydrous minerals. As is shown for the Karku deposit, the highly hydrated clay minerals of the illite-smectite series are widespread in its subore portion and lacking at the supraore levels along with termination of quartz regeneration. It is suggested that a zone of superhigh fluid pressure in deep parts of sedimentary basins constrains localization of uranium orebodies by structural and stratigraphic unconformity between Riphean and Paleoproterozoic rocks. It is stated that altered wall rocks at the unconformity-type uranium deposits cannot be identified with products of hydrothermal phyllic and argillic alteration of host rocks at the medium- and low-temperature endogenic deposits. The main distinctions consist in lack of wall-rock metasomatic reaction zoning and acid-alkaline evolution of solutions. All transformations of host rocks should be classified as diverse manifestations of deep catagenesis of sedimentary sequences and buried regolith. The carbon and oxygen isotopic compositions of calcite from host rocks at the Karku deposit are far from those of magmatic and hydrothermal carbonates. They are characterized by a high δ18O = +17 to +25‰ and a high dispersion of δ13C = −1.5 to almost −15‰. No granitoid magmatism is known in the regions, where the unconformity-type uranium deposits occur. Therefore, the rocks of the crystalline basement are the most probable source of uranium, which precipitated on the reductive barrier as a product of interaction with bitumen contained in the Riphean basal beds.

How well do sediment indicators record past climate? An evaluation using annually laminated sediments

The reliability of paleoclimatic inferences from lake-sediment records rests on the understand- ing of how various sediment indicators respond to environmental changes. Despite the recent prolifera- tion of paleoclimatic records, only a limited number of studies have rigorously evaluated potential indi- cators by comparing lake-sediment records with instrumental weather data. We analyzed annually laminated sediments of the past 100 years from a lake in north-central Minnesota for a suite of variables commonly used for climatic reconstructions. Results were compared with time series of climatic or climate-derived variables, as well as with indices of climate modes thought to influence the regional climate of the midwestern United States. The oxygen- isotopic composition of calcite (d 18 Oc) shows trends

Chronology of fracture sealing under a meteoric fluid environment: Microtectonic and isotopic evidence of major Cainozoic events in the eastern Paris Basin (France)

A detailed reappraisal of the sequence of tectonic stages that has affected the eastern part of the Paris Basin from the late Jurassic to the end of Cainozoic is presented in order to improve knowledge of the relationships between tectonics and fluid/mass palaeo-transfers in that basin. This goal was performed in two steps: i) establishing a relative tectonic chronology on the basis of numerous observations and detailed analysis of geometrical relationships between striated planes, vertical and bedding stylolites, and tension gashes, and ii) discussing the geochemical signatures ( δ 18O and δ 13C) of calcite from fracture infillings well positioned within the relative tectonic chronology. The resulting tectonic sequence agrees with the main events described by previous papers, but the present work details the Pyrenean s.l. (Eocene–Oligocene) and Alpine (Miocene–Pliocene) convergence stages and explains the mechanisms of the Oligocene–Miocene transition between the two previous events. The different tectonic stages are as follows: i) late Jurassic to early Cretaceous extension starting with a WNW–ESE direction but changing to E–W; ii) the Pyrenean convergence (Eocene), changing from NNE–SSW to NE–SW and finally ENE–WSW compression; iii) Oligocene transition from Pyrenean to Alpine convergence, marked by a drastic evolution of main stress which swaps from a horizontal position to a vertical one and radial extension marked by a strong reactivation of bedding stylolitisation; iv) the Alpine convergence (late Miocene to present-day), changing from WNW–ESE to NNW–SSE direction of shortening. δ 18O- and δ 13C-values of calcite from tension gashes associated to each tectonic stage spread from 18‰ to 27‰ (SMOW) and from 0.8‰ to 3‰ (PDB), respectively. According to the maximum thermal history of this part of the basin ( T MAX = 40 °C or 60 °C according to the available published estimates), the calculated δ 18O-values of fluids (− 7‰ to − 4‰) involved in mass transfer processes are interpreted as meteoric. The lowest δ 18O-values (18–20‰) are located around the main regional faults and associated with δ 13C-values that are rather different from those of host rocks. In contrast, calcites from non-faulted area present a higher δ 18O- (> 20‰) and δ 13C-values close to those of host rocks. These data are explainable in terms of differences in the fluid/rock ratios prevailing during carbonate redistribution, the highest ratios being associated to discontinuities (faults, sometimes with hydraulic breccias) that were active during the Tertiary tectonic events. Outside the fault zones, the isotopic compositions of calcite from tension gashes are similar, at the regional scale, to those of calcite from geodes or cements analysed in previous studies on localized areas. As a result, we emphasize the fact that a significant part of the carbonate redistribution in the Middle to Upper Jurassic limestones from the eastern Paris Basin occurred during the Tertiary s.l., enlightening the influence of distant orogenic systems (extensive or compressive) in the matter redistribution within sedimentary basins, at variance of models inferring a porosity cementation predominantly during burial.

δ13C profiles along growth layers of stalagmites: Comparing theoretical and experimental results

The isotopic carbon ratio of a calcite-precipitating solution flowing as a water film on the surface of a stalagmite is determined by Rayleigh distillation. It can be calculated, when the HCO3--concentration of the solution at each surface point of the stalagmite and the fractionation factors are known. A stalagmite growth model based entirely on the physics of laminar flow and the well-known precipitation rates of a supersaturated solution of calcite, without any further assumptions, is employed to obtain the spatial distribution of the HCO3--concentration, which contributes more than 95% to the dissolved inorganic carbon (DIC). The δ13C profiles are calculated along the growth surface of a stalagmite for three cases: (A) isotopic equilibrium of both CO2 outgassing and calcite precipitation; (B) outgassing of CO2 is irreversible but calcite precipitation is in isotopic equilibrium. (C) Both CO2 outgassing and calcite precipitation are irreversible. In all cases the isotopic shift δ13C increases from the apex along the distance on a growth surface. In cases A and B, calcite deposited at the apex is in isotopic equilibrium with the solution of the drip water. The difference between δ13C at the apex and the end of the growth layer is independent of the stalagmite’s radius, but depends on temperature. For case A, it is about half the value obtained for cases B and C. In case C, the isotopic composition of calcite at the apex equals that of the drip water, but further out it becomes practically identical with that of case B. The growth model has been applied to field data of stalagmite growth, where the thickness and the δ13C of calcite precipitated to a glass plate located on the top of a stalagmite have been measured as function of the distance from the drip point. The calculated data are in good agreement to the observed ones and indicate that deposition occurred most likely under conditions B, eventually also C. A sensitivity analysis has been performed, which shows that within the limits of observed external parameters, such as drip rates and partial pressure of carbon dioxide PCO2 in the cave, the results remain valid.