Clumped Isotope Thermometry Research Articles

Comparative clumped isotope temperature relationships in freshwater carbonates

AbstractLacustrine, riverine and spring carbonates represent archives of terrestrial climates and their geochemistry has been used to study palaeoenvironments. Clumped isotope thermometry is an emerging tool that has been applied to freshwater carbonates. Limited work has been done to evaluate comparative relationships between clumped isotopes and temperature in different types of modern freshwater carbonates. This study assembles an extensive calibration data set with 135 samples of modern freshwater carbonates from 96 sites and constrains the relationship between independent observations of water temperature and the clumped isotopic composition of carbonates (denoted by Δ47), including new measurements, and recalculates published data in accordance with current community‐defined standard values. For temperature reconstruction, the study reports a composite freshwater calibration and material‐specific calibrations for biogenic carbonates (freshwater gastropods and bivalves), fine‐grained carbonate (e.g. micrites), biologically mediated carbonates (microbialites and tufas) and travertines. Material‐specific calibration trends show a convergence of slopes that are in agreement with recently published syntheses, but statistically significant differences in intercepts occur between some materials (e.g. some biogenics, fine‐grained carbonates). These differences may arise due to unresolved seasonal biases, kinetic isotope effects and/or varying degrees of biological influence. The impact of different calibrations is shown through application to new data for glacial and deglacial age travertines from Austria and published data sets. While material‐specific calibrations may yield more accurate results for biogenic and fine‐grained carbonate samples, the use of material‐specific and the composite freshwater calibrations generally produces values within 1.0–1.5°C of each other, and typically fall within calibration uncertainty given limitations of precision.

Temperatures of Vein Formation Associated With Plate Interface Deformation Constrained by Oxygen and Clumped Isotope Thermometry

AbstractTectonic mélanges, characterized by conditions reflective of modern subduction fault zones, preserve mineral veins formed through mass transfer, a mechanism influencing the slip behavior of subduction megathrusts. In this study, we apply secondary ion mass spectrometry quartz‐calcite oxygen isotope thermometry and clumped isotope thermometry to examine the temperatures of vein formations in six mélange units in the Cretaceous Shimanto belt and one mélange in the Kodiak accretionary prism. Calcite in the veins exhibits δ13CPDB values ranging from −17.2‰ to −6.8‰, indicative of a carbon source mixing with sedimentary carbonate and organic matter. δ18OSMOW values of calcite range from +11.1‰ to +17.2‰; quartz yields δ18OSMOW values of +14.9‰ to +21.7‰. Oxygen isotopic signatures in minerals reveal that most vein‐forming fluids are significantly affected by rock buffering, while some retain isotopic compositions of seawater and meteoric water. Temperature estimates, derived from both thermometers, fall within the range of 100–250°C. Notably, vein temperatures remain constant across diverse vein types and mélange units with distinct maximum temperatures. The combined temperature records and fluid isotopic compositions imply vein formations at shallower depths linked to the incorporation of seawater, meteoric water, and fluid released from early dehydration reactions. At greater depths, vein formations are associated with fluid released from clay dehydration and long‐distance fluid flow. Reduced vein formations between 250 and 350°C may correlate with a shift to fluid‐unsaturated conditions resulting from clay hydration reactions. Our study highlights potential mechanical and hydraulic variations within the thermal conditions of 100–350°C along the plate boundary driven by fluid‐mineral interactions.

Mineralogical and environmental effects on the δ13C, δ18O, and clumped isotope composition of modern bryozoans

Extensive bryozoan fossil records date back to the early Ordovician, forming an important part of sedimentary archives, yet the applicability of classical 18‑oxygen thermometry to bryozoan carbonate is still a matter of debate, with mineralogical and biological issues still hindering paleoclimate reconstructions. Complementing other methods (i.e., δ18O, Mg/Ca), clumped-isotope thermometry (Δ47) could provide more accurate paleotemperature estimates and help identify potential biotic factors influencing the isotopic composition of bryozoans. Here we report on the first investigation of clumped-isotope thermometry applied to bryozoan carbonate, spanning a broad range of modern species living in different environments from two localities (Atlantic Ocean and Mediterranean Sea). We confirm that bryozoan δ13C and δ18O records are affected by biotic and abiotic factors susceptible to bias in growth condition estimates. Our Atlantic bryozoans yield Δ47 derived temperatures (T47) consistent with spring/fall seawater temperature (but only after correcting for minor mineralogical effects), reflecting seasonal growth bias or, more likely, moderate isotopic disequilibrium. By contrast, Mediterranean samples display large positive offsets from Δ47 equilibrium values. We propose that this stronger disequilibrium is related to the higher salinity at this site, decreasing Carbonic Anhydrase activity, favoring CO2 hydroxylation over hydration, and slowing down DIC (dissolved inorganic carbon) equilibration reactions at the precipitation site. Our findings highlight how “vital effects” in bryozoans is not only species-specific as often assumed for other biocalcifiers, but could also depend on mineralogy and environmental factors.

Clumped isotopes reveal relationship between mussel growth and river discharge

Freshwater mussels preserve valuable information about hydrology, climate, and population dynamics, but developing seasonal chronologies can be problematic. Using clumped isotope thermometry, we produced high-resolution reconstructions of modern and historic (~ 1900) temperatures and δ18Owater from mussel shells collected from an impounded river, the Brazos in Texas, before and after damming. We also performed high-resolution growth band analyses to investigate relationships between mussel growth rate, rainfall, and seasonal temperature. Reconstructed δ18Owater and temperature vary little between the modern (3R5) and historic shell (H3R). However, a positive relationship between reconstructed δ18Owater and growth rate in H3R indicates that aside from diminished growth in winter, precipitation and flow rate are the strongest controls on mussel growth in both modern and pre-dam times. Overall, our results demonstrate (1) the impact, both positive and negative, of environmental factors such as flow alteration and temperature on mussel growth and (2) the potential for clumped isotopes in freshwater mussels as a paleohydrology and paleoclimate proxies in terrestrial environments.

The thermal history of Permian carbonate strata reconstructed with clumped isotopes and U–Pb dating: Eastern Sichuan Basin, SW China

The lack of effective paleogeothermometers restricts the study of the thermal history of carbonate strata. Carbonate clumped isotope thermometry and in-situ U–Pb dating are appropriate for reconstructing the thermal history of carbonate strata. Herein, this study analyzed clumped isotope compositions and U–Pb ages of calcite fabrics, including micrite matrix and veins, from Permian carbonate strata in the eastern Sichuan Basin. U–Pb dating of the micrite matrix was performed to rule out the influence of recrystallization and determine the diagenetic age of the samples. Clumped isotope temperatures of the four micrite matrix samples ranged from 102 °C to 137 °C. The maximum paleotemperature derived from a solid-state reordering model was between 210 °C and 220 °C. From all four samples, three calcite veins yielded crystallization temperatures corresponding to 160 ± 10 °C, 60 ± 10 °C, and 29 ± 4 °C, as determined by clumped isotope and solid-state reordering model analyses. In-situ U–Pb dating provided corresponding crystallization ages of 159 ± 17 Ma, 256.6 ± 5.5 Ma, and 17.86 ± 0.8 Ma. The crystallization ages and temperatures of the three calcite veins align well with the thermal history of the Permian strata, indicating a high degree of accuracy. This study introduces a new strategy for reconstructing the thermal history of carbonate strata. The thermal history can be quantified using the constraints of the crystallization ages and temperatures from different stages of calcite veins or cements. This approach is suitable for determining the maturation progress of the source rock and conducting oil and gas exploration in petroliferous basins.

Subaqueous carbonate speleothems as paleotemperature archives – clumped isotope thermometry and stable isotope compositions of inclusion-hosted water

Clumped isotope measurements of carbonates and stable isotope analyses of water trapped in fluid inclusions are both promising techniques to determine carbonate formation temperatures. Cave-hosted carbonate deposits (speleothems) are excellent targets for such studies, but kinetic fractionations and diagenetic influences frequently deteriorate the temperature data obtained from these methods. However, subaqueous carbonate deposits may provide reliable data, as kinetic fractionations are less significant in underwater environments. In this study, subaqueous speleothems, whose formation temperatures were directly measured in the water, were investigated. Additionally, temperatures calculated from the oxygen isotope fractionations between calcite and fluid inclusion-hosted water were compared with clumped isotope temperatures obtained for subaqueous carbonate formations in cave-hosted lakes. The clumped isotope temperatures fit the measured and calculated fluid inclusion temperatures within the analytical precisions. Carbonate deposits formed at elevated temperatures (~50°C or above) may undergo post-formational calcite-water oxygen isotope exchange, altering the composition of the inclusion-hosted water. In contrast, subaqueous speleothems formed at about 20-25°C appear to preserve the primary isotopic compositions. Our study shows that subaqueous carbonate speleothems are useful targets for clumped isotope and inclusion water analyses, making them valuable paleotemperature archives.

Burial History and Hydrocarbon Potential of the Harudi and Fulra Limestone of Kachchh, Western India Constrained using Carbonate Clumped Isotope Thermometry

Abstract The clumped isotope paleothermometry is used to estimate minimum burial depth of the mid Eocene Harudi and Fulra Limestone Formations of Kachchh, western India. The Nummulites and other fossil foraminiferal genera were separated for conventional (δ18O, δ13C) and clumped isotopic (Δ47) measurements. The measurements were also made on whole rock (WR) to visualize the different phases of diagenetic alteration, expected depth of burial and capacity to generate hydrocarbon. The measured δ18O and δ13C values of WR samples vary between -3.09 and -2.21‰ and -0.78 and 0.96‰ respectively, with average values of -2.88‰ and 0.16‰. The δ18O and δ13C values of benthic foraminifera vary between -3.52 and -3.20‰ (Average: -3.35±0.013) and -1.04 and 0.81‰ (Average: 0.26±0.76) respectively. The measured Δ47 for the foraminifera translates to temperature of 35 and 49°C and between 26 and 48°C for WR indicating minor diagenetic alteration. The minimum estimates for the burial depth of the studied formations are up to 2 km which is based on the temperature difference between the ecological preference temperature of benthic foraminifera and the diagenetically altered foraminifera and δ13C of WR and foraminifera, indicating that rocks have potential to generate methane.

A record of overpressure and Sevier tectonics within beef calcite of the Heath Formation, Central Montana Trough

The Central Montana Trough is an important sedimentary basin for petroleum resources and as a record of North American tectonism. The thermal, overpressure, and tectonic histories of the trough are investigated through a study of beef calcite veins taken from the Heath Formation, the primary hydrocarbon source rock of the basin. Results from beef calcite analysis include petrographic characterization, U-Pb geochronology via LA-ICP-MS, clumped isotope thermometry, and conventional carbon and oxygen stable isotopes. U-Pb ages and clumped isotope temperatures are 121.8 ± 19.2 Ma at 43.1 ± 2.3 °C; 96.2 ± 25.9 Ma at 52.1 ± 3.7 °C; and 101.6 ± 24.2 Ma at 50.2 ± 2.4 °C. Large errors on U-Pb ages result from very low U contents (<0.03 ppm). Ages and temperatures indicate staged vein precipitation at burial depths between 600 m to 1.3 km during the major burial event of the basin and a normal geothermal gradient of 25 °C/km. Beef calcite emplacement and basin burial are the direct result of regional tectonism of the Sevier Orogeny. Calculated fluid δ18O using clumped isotope temperatures revealed δ18O values of 3–7 ‰ (SMOW), indicating beef calcite precipitation from connate formation brine. Analysis of median-symmetrical beef carbonate δ18O and δ13C values indicates precipitating fluids were diagenetically modified by bacterial sulfate reduction and methanogenesis of organic matter. A thin (0.1 mm) second generation of growth is found on the outermost vein margin by a sharp chemical boundary and a trail of inclusions. This second growth stage could not be dated separately, but significantly lower δ18O and δ13C values of −2 ‰ and −5.5 ‰, respectively, indicate precipitating fluids were hotter and further diagenetically altered by thermal organic matter breakdown.Supplemental data: https://osf.io/39znd/.

Hot carbonates deep within the Chicxulub impact structure.

Constraining the thermodynamic conditions within an impact structure during and after hypervelocity impacts is extremely challenging due to the transient thermal regimes. This work uses carbonate clumped-isotope thermometry to reconstruct absolute temperatures of impact lithologies within and close to the ∼66 Myr old Chicxulub crater (Yucatán, México). We present stable oxygen (δ18O), carbon (δ13C), and clumped-isotope (Δ47) data for carbonate-bearing impact breccias, impact melt rock, and target lithologies from four drill cores on a transect through the Chicxulub structure from the northern peak ring to the southern proximal ejecta blanket. Clumped isotope-derived temperatures (T(Δ47)) are consistently higher than maximum Late Cretaceous sea surface temperatures (35.5°C), except in the case of Paleogene limestones and melt-poor impact breccias outside of the crater, confirming the influence of burial diagenesis and a widespread and long-lived hydrothermal system. The melt-poor breccia unit outside the crater is overlain by melt-rich impact breccia yielding a much higher T(Δ47) of 111 ± 10°C (1 standard error [SE]), which likely traces the thermal processing of carbonate material during ejection. Finally, T(Δ47) up to 327 ± 33°C (1 SE) is determined for the lower suevite and impact melt rock intervals within the crater. The highest temperatures are related to distinct petrological features associated with decarbonation and rapid back-reaction, in which highly reactive CaO recombines with impact-released CO2 to form secondary CaCO3 phases. These observations have important climatic implications for the Cretaceous-Paleogene mass extinction event, as current numerical models likely overestimate the release of CO2 from the Chicxulub impact event.

Dual carbonate clumped isotope (Δ47-Δ48) measurements constrain different sources of kinetic isotope effects and quasi-equilibrium signatures in cave carbonates

Cave carbonate minerals are an important terrestrial paleoclimate archive. A few studies have explored the potential for applying carbonate clumped isotope thermometry to speleothems as a tool for constraining past temperatures. To date, most papers utilizing this method have focused on mass-47 clumped isotope values (Δ47) at a single location and reported that cave carbonate minerals rarely achieve isotopic equilibrium, with kinetic isotope effects (KIEs) attributed to CO2 degassing. More recently, studies have shown that mass-47 and mass-48 CO2 from acid digested carbonate minerals (Δ47 and Δ48) can be used together to assess equilibrium and probe KIEs. Here, we examined 44 natural and synthetic modern cave carbonate mineral samples from 13 localities with varying environmental conditions (ventilation, water level, pCO2, temperature) for (dis)equilibrium using Δ47-Δ48 values, in concert with traditional stable carbon (δ13C) and oxygen (δ18O) isotope ratios. Data showed that 19 of 44 samples exhibited Δ47-Δ48 values indistinguishable from isotopic equilibrium, and 18 (95 %) of these samples yield Δ47-predicted temperatures within error of measured modern temperatures. Conversely, 25 samples exhibited isotopic disequilibria, 13 of which yield erroneous temperature estimates. Within some speleothem samples, we find Δ47-Δ48 values consistent with CO2 degassing effects, however, the majority of samples with KIEs are consistent with other processes being dominant. We hypothesize that these values reflect isotopic buffering effects on clumped isotopes that can be considerable and cannot be overlooked. Using a Raleigh Distillation Model, we examined carbon and oxygen isotope exchange trajectories and their relationships with dual clumped isotope disequilibria. Carbon isotope exchange is associated with depletion of both Δ47 and Δ48 relative to equilibrium, while oxygen isotope exchange is associated with enrichment of both Δ47 and Δ48 relative to equilibrium. Cave rafts collected from proximate locations in Mexico exhibit the largest average departures from equilibrium (ΔΔ47¯ = −0.032 ± 0.007, ΔΔ48¯ = −0.104 ± 0.035, where ΔΔi is the measured value – the equilibrium value). This study shows how the Δ47-Δ48 dual carbonate clumped isotope framework can be applied to a variety of tcave carbonate mineral samples, enabling identification of isotopic equilibria and therefore quantitative application of clumped isotope thermometry for paleoclimate reconstruction, or alternatively, constraining the mechanisms of kinetic effects.

A correction scheme for calcium carbonate clumped isotope (Δ47) thermometric equation depending on sample preparation technique

The CO2 preparatory methods implemented during carbonate-clumped isotope analysis for the acid digestion of carbonate with the goal of high sample throughput yielded multiple empirical relationships for the thermometry. These methods varied significantly from its original practice of carbonate reaction at 25 °C using sealed vessel method to automated quick performance acid drip or common acid bath method at 70 °C or 90 °C temperatures, respectively; these approaches differed noticeably. Technical development replacing primitive reaction protocols, introduced different reaction conditions, causing significant differences in the chemical reaction procedure and CO2 trapping, which caused variation in the values of slope and intercept for the linear regression equations governing carbonate-clumped isotope(Δ47) distribution with carbonate growth temperatures. These studies include the Δ47 measurement and analysis of either laboratory-grown or natural carbonates with precise knowledge of their precipitation/depositional temperatures. However, the discrepancies in the existing universal calibration schemes remained poorly understood despite adopting an identical data correction protocol. This is explained here by the reaction kinetics and CO2 collection methodologies adopted during experimentation. The present study investigated the slope and intercept values of the published carbonate clumped isotope thermometry equations expressed in the accepted Absolute Reference Frame (ARF in CDES) at 25 °C after accounting for the acid correction factor. We observe a systematic shift in the mean slope and intercept values of 0.0154(±0.007) and 0.153(±0.0686) ‰ for 70 °C reaction experiments and offset of 0.0181(±0.008) and 0.197(±0.079) ‰ for the experiments conducted at 90 °C by using the acid drip and/or Common Acid bath method, respectively from the slope and intercept values of the calibration equations proposed using sealed vessel method at 25 °C reaction temperature. The mean values for slope and intercept are compared using ANOVA and paired f-test. These correction factors for slopes and intercepts will allow the transformation of clumped isotope values at different temperatures into ARF scale at 25 °C and enable accurate deduction of temperature for carbonate samples. Correction factors proposed here account for variations in the sample preparation techniques arising due to different reaction temperatures, mechanisms and vapor pressure in the chamber for isotopic exchange reaction to happen for smaller or prolonged time intervals.

Isotopic disequilibrium in brachiopods disentangled with dual clumped isotope thermometry

The stable oxygen and clumped isotope composition of brachiopod calcite are important proxies for the reconstruction of Phanerozoic seawater temperatures and δ18O values. The utility of brachiopods as a temperature archive is nonetheless challenged by indications that their shells precipitate out of isotopic equilibrium with ambient seawater, though the origin of disequilibrium effects has remained elusive. Here, we apply the recently developed dual clumped isotope thermometer (i.e., the simultaneous measurement of Δ47 and Δ48 values in CO2 derived from the acid digestion of carbonates) to modern and fossil brachiopods to investigate disequilibrium signatures recorded in brachiopod calcite. We present evidence that disequilibrium signatures are derived from the combination of two rate-limiting processes: the hydration/hydroxylation of CO2, and, potentially, the diffusion of HCO3−/CO32− in water. An empirical correction for clumped isotope disequilibrium is presented, based on correlation between measured disequilibrium offsets in the Δ47 and Δ48 values of modern brachiopods (Δ47disequilibrium = –0.88 × Δ48disequilibrium + 0.02). Dual clumped isotope thermometry of Eocene age brachiopods from Seymour Island (∼65 °S) yields temperatures in agreement with previously reported Δ47 derived temperatures from coeval bivalves. In contrast, uncorrected Δ47 derived temperatures from the same brachiopods are 6–11 °C colder. Measurement of dual clumped isotope composition alongside δ18O values of brachiopod carbonate should also enable more accurate reconstruction of seawater-δ18O. In comparison with group-specific Δ47-temperature calibrations, which correct for an average kinetic bias in Δ47, dual clumped isotope thermometry accounts for the magnitude of disequilibrium inherent to an individual sample, facilitating more reliable paleotemperature and seawater-δ18O reconstructions.

Transient Deep Ocean Cooling in the Eastern Equatorial Pacific Ocean at the Eocene‐Oligocene Transition

AbstractAt the Eocene‐Oligocene Transition (EOT), approximately 34 million years ago, Earth abruptly transitioned to a climate state sufficiently cool for Antarctica to sustain large ice sheets for the first time in tens to hundreds of millions of years. Oxygen isotope records from deep‐sea benthic foraminifera (δ18Ob) provide the foundation of our understanding of this pivot point in Cenozoic climate history. A deeper insight, however, is hindered by the paucity of independent deep‐sea temperature reconstructions and the ongoing challenge of deconvolving the temperature and continental ice volume signals embedded in δ18Ob records. Here we present records of deep‐sea temperature change from the eastern equatorial Pacific for the EOT using clumped isotope thermometry, which permits explicit temperature reconstructions independent of seawater chemistry and continental ice volume. Our records suggest that the deep Pacific Ocean cooled markedly at the EOT by 4.7 ± 0.9°C. This decrease in temperature represents the first direct and robust evidence of deep‐sea cooling associated with the inception of major Cenozoic glaciation. However, our data also indicate that this major cooling of the deep Pacific Ocean at the EOT was short‐lived (∼200 kyrs), with temperatures rebounding to values close to pre‐EOT levels by 33.6 Ma. Our calculated record of seawater δ18O suggests that this rebound in ocean temperature occurred despite the continued presence of a large‐scale Antarctic ice sheet. This finding suggests a degree of decoupling between deep ocean temperatures in the eastern equatorial Pacific Ocean and the behavior of the newly established Antarctic ice sheet.

Chemostratigraphic contributions to paleoenvironmental assessment of microbialites from neoproterozoic Capiru Formation, Southern Ribeira Belt – Brazil

The Capiru Formation records supracrustal metassedimentary rocks within the Southern Ribeira Belt. These rocks underwent heterogeneous deformation and metamorphism, resulting in an incomplete greenschist facies paragenesis. They are tectonically interbedded with strata that exhibit well-preserved primary structures. In the Morro Grande synform region, there is a continuous 150 m outcrop of metadolomites with preserved sedimentary records and was described in a stratigraphic profile (1:500) subsequently detailed (1:50). Dolomite, minor amounts of quartz, traces of ilite, graphite and zircon constitute the mineral assembly, defined by XRD, petrography and SEM-EDS. Distinct preserved facies are recognized by morphostructures: (1) lamina (parallel, discontinuous, crenulated); (2) stromatolites (homogeneous columnar, club-shaped, conical conophyton-like, pseudocolumnar, parallel-branching, divergent-branching, delicate-branching); (3) thrombolite (rugous) and (4) flat pebble conglomerate. Single-isotope data revials different microbial patterns, particularly in stromatolites (−1.57 to −0.40‰ δ13C; −8.21 to - 3.94‰ δ18O) and lamina (−1.89 to 1.29‰ δ13C; −7.32 to −3.55‰ δ18O). Facies associations suggest a shallow sea with a regressive trend. Tectonofacies are characterized by massive, venulated or brecciated carbonates. Chemostratigraphic profile is divided into two major intervals. At the bottom, the first does not exhibit a specific isotopic trend and can be subdivided in heterogeneous post-deformation venulas (Unit I, −1.75 to 0.40 δ13C; −2.34 to −8.16 δ18O), thrombolythic signals (Unit II, −1.43 to 0.40‰ δ13C; −6.35 to −4.11‰ δ18O), and facies variation, mainly supratidal (Unit III, 1.89 to 1.29‰ δ13C, −7.32 to −3.31‰ δ18O). Upwads in the profile, the second interval consists of microbial facies, displaying a more uniform signature with depleted values of δ13C and δ18O along with a slight enrichment marking the transition from intratidal to supratidal environments (Unit IV; −1.32 to −0.81‰ δ13C; - 8.17 to −6.25‰ δ18O) and supratidal lagoons (Unit V; −1.35 to −0.41‰ δ13C; −8.20 to −5.49‰ δ18O). Preserved EPS, graphite, and ilite confirm low-grade metamorphism, with clumped isotope thermometry temperatures ranging from 206.07 to 307.58 °C. Calcite is secondary, filling porosity and present in recrystallized facies. Isotopic signature of recrystallized facies differs from microbial ones, with the most depleted values (−2.16‰ δ13C; −14.02‰ δ18O) and lower formation temperature (122.29 ± 7.07 °C), indicating a late restricted event. This dolomitic succestion within Capiru Formation provides evidence of a shallow marine paleoenvironment with diverse microbial activity during its deposition.

Applicability of carbonate clumped isotope thermometry in the Tuchang-Jentse geothermal field

Temperature and oxygen isotope composition of fluids, which are critical for reconstructing thermal structures and characterizing hydrothermal fluids in a geothermal system, can be obtained using carbonate clumped isotope analysis. Ideally, carbonate clumped isotope compositions (Δ47) recording the thermal fluid temperatures and compositions for fault-associated veins and rhombic calcite crystals inside open cracks demonstrate previous tectonic activities and present reservoir conditions, respectively. We studied the clumped isotope geochemistry of veins and travertines in outcrops and cuttings from exploration wells in the Tuchang–Jentse geothermal field in Taiwan. The Δ47-based temperatures (Δ47-T; 161−8+8 to 238−14+16 °C) and calculated δ18Ofluid values (+2.0 to +5.0‰) of vein calcites were the highest, indicating mixing of the magmatic/metamorphic fluids with the meteoric water after water–rock interaction in the deep thermal fluid cycle and migration to shallow areas by fault activity. Rhombic calcites with high Δ47-T (142−6+6 to 185−4+5 °C) and low calculated δ18Ofluid (−9.1 to −5.3‰) correspond to downhole temperatures and measured δ18Ofluid of thermal fluids in the exploration wells, respectively, suggesting that the latest thermal fluid was of meteoric water origin. Travertines near faults have low calculated δ18Ofluid (−8.9 to −1.0‰) and Δ47-T that are above air temperatures (32−9+9 to 95−9+10 °C), implying that the local groundwater was subsequently heated by or mixed with deep fluid. Carbonate clumped isotope thermometry application demonstrates that the carbonate clumped isotope thermometer not only precisely estimates temperature in the carbonate-dominated system using drill cutting samples during the exploration phase, but also reveals the spatial and temporary evolution of geothermal fluids used as a reference for future site selection planning.

Fluid flow evolution revealed by carbonate clumped isotope thermometry along the fractures in a complex salt dome setting: Study case (Jebel Madar, Oman)

A better understanding of constraining paleo fluid circulation along fractures in tectonically complex regions is essential from the scientific perspective and in exploiting sustainable geo-energies such as geothermal resources, but it remains a challenge. Jebel Madar in Oman as a salt dome is an ideal case study because of the complexity of its tectonic evolution and diagenetic process history. Here, we investigated the Cretaceous carbonate-encasing rock of salt domes and calcite-infilling fractures, making them an ideal case study for paleo fluid flow. The objectives were to test two different hypotheses: (1) to verify the previous study based on seismic data that the Hawasina Nappe had been priorly emplaced at the study location and was subsequently completely eroded, and (2) that the complex history of paleo-fluid flow existed in the fractures of the salt dome, and that this system was not yet fully constrained by previous studies, and also including accounting for the presence of any kinetic fractionation. To achieve our objectives, we applied carbonate clumped isotopes on different generations of veins as well as on the carbonate cement of the host rock itself to reconstruct paleo temperatures and the isotopic composition of the paleo-fluids. The carbonate clumped isotope temperatures indicate that calcite vein precipitation occurred between 48 and 171 °C. Assuming that our carbonate clumped isotopes data faithfully capture the recrystallization temperature of the limestone, the geothermal gradient during the Campanian can be estimated at 33.5 °C km−1, implying that the Hawasina Nappe must indeed have been present at the location. We also argue that the evolution of fluid flow in the study area comprises two distinct episodes of fluid circulation. The first episode led to the precipitation of high-temperature calcites followed by closed-system recrystallization during the local emplacement of the Hawasina Nappe. The second episode of fluid circulation occurred at lower temperatures with calcite members representing meteoric diagenetic fluids with evidence of the kinetic fractionation of the clumped isotopes signature in our samples. Our result demonstrates that the new paleotemperature data present new constraints of paleo fluid circulation along the fractures in a salt-domed setting and, importantly, confirms for the first time that the Hawasina Nappe was present at the site of Jebel Madar in the Late Cretaceous.

Physicochemical Conditions of the Devonian-Jurassic Continental Deep Biosphere Tracked by Carbonate Clumped Isotope Temperatures of Granite-Hosted Carbonate Veins

Previous studies have shown that microorganisms thrive in oligotrophic fracture systems, and metabolisms include consumption and production of methane. In the Laxemar, Götemar, and Forsmark areas of Sweden, ancient microbial activity has previously been demonstrated by large δ13CVPDB variability of carbonate vein infillings within granitic host rocks. Here, we apply carbonate clumped isotope thermometry to reconstruct the temperature of precipitation of the carbonate within these veins. The carbonate clumped isotope temperatures indicate that mineralization took place between 46°C and 108°C, in line with previously published fluid inclusion data (&lt;50°C to 113°C). The new carbonate clumped isotope data more accurately narrows temperatures at the lower end of this range as fluid inclusions are not easily applicable below 80°C, a temperature regime of high importance for paleobiological reconstructions. Our results demonstrate that large volumes of biogenic carbonate cement were formed from low-temperature microbial processes in low-salinity water, succeeding calcite of deep brine origin. The known burial and thermal history of the region combined with our carbonate clumped isotope data place the microbial activity at the end of the Variscan orogeny and later events (e.g., Jurassic). Thus, carbonate clumped isotope thermometry reduces uncertainties in deep biosphere studies by providing more accurate temperature constraints in the low-temperature regime, the biogenic processes, and the origin of the diagenetic fluids.

Clumped Isotope Thermometry in Plant‐Derived Carbonates

AbstractThe genus Celtis includes widespread trees that produce drupes with aragonite endocarps, or “hackberries.” These carbonate endocarps are preserved in the fossil record, often in cave deposits or packrat middens, and thus are targets for paleoclimate reconstructions. Stable oxygen isotopes in Celtis endocarps have been used as proxies for oxygen isotopic composition of past stream water and for paleothermometry. Here, we explore the suitability of hackberry carbonates for paleoclimate reconstructions based on carbonate clumped‐isotope thermometry. We sampled modern hackberries grown at sites across North America (n = 37) for stable and clumped isotope analyses. Measured clumped‐isotope temperatures are found to be within the range of measured local modern growing season surface temperatures and typically in dual clumped‐isotope equilibrium. As such, we propose that hackberry clumped‐isotope measurements can be used to reconstruct past Earth‐surface air temperatures.

Frontiers of Carbonate Clumped Isotope Thermometry

Carbonate minerals contain stable isotopes of carbon and oxygen with different masses whose abundances and bond arrangement are governed by thermodynamics. The clumped isotopic value Δi is a measure of the temperature-dependent preference of heavy C and O isotopes to clump, or bond with or near each other, rather than with light isotopes in the carbonate phase. Carbonate clumped isotope thermometry uses Δi values measured by mass spectrometry (Δ47, Δ48) or laser spectroscopy (Δ638) to reconstruct mineral growth temperature in surface and subsurface environments independent of parent water isotopic composition. Two decades of analytical and theoretical development have produced a mature temperature proxy that can estimate carbonate formation temperatures from 0.5 to 1,100°C, with up to 1–2°C external precision (2 standard error of the mean). Alteration of primary environmental temperatures by fluid-mediated and solid-state reactions and/or Δi values that reflect nonequilibrium isotopic fractionations reveal diagenetic history and/or mineralization processes. Carbonate clumped isotope thermometry has contributed significantly to geological and biological sciences, and it is poised to advance understanding of Earth's climate system, crustal processes, and growth environments of carbonate minerals. ▪ Clumped heavy isotopes in carbonate minerals record robust temperatures and fluid compositions of ancient Earth surface and subsurface environments. ▪ Mature analytical methods enable carbonate clumped Δ47, Δ48, and Δ638 measurements to address diverse questions in geological and biological sciences. ▪ These methods are poised to advance marine and terrestrial paleoenvironment and paleoclimate, tectonics, deformation, hydrothermal, and mineralization studies.

An empirical calibration of the serpentine-water oxygen isotope fractionation at T = 25–100 °C

The ability to precisely constrain the physical–chemical conditions of serpentinization, such as temperature and fluid source, is limited by the accuracy of calibrations for oxygen isotope fractionation between serpentine and water – i.e., 1000 lnα(serpentine-water) – which disagree by up to 20‰ when extrapolated to T < 200 °C. In this study, we present a new empirical calibration of 1000 lnα(serpentine-water) aiming to improve applications of oxygen isotope geochemistry to very low-T serpentinization (T < 100 °C). We used the high-spatial resolution capabilities of Secondary Ion Mass Spectrometry (SIMS) to analyze oxygen isotope ratios in mineral pairs of calcite + serpentine, quartz + serpentine and talc + serpentine co-crystallized at scales ≤ 50 μm in six serpentinite samples from the Samail ophiolite (Oman). SIMS analysis shows that the mineral pairs are relatively homogenous in oxygen isotope ratios with variability in δ18O values ≤2‰ (2 s). Clumped isotope thermometry and petrological constraints indicate crystallization temperatures from 25 to 100 °C for the investigated samples. These independent constraints on temperature allowed us to derive 1000 lnα(serpentine-water) by combining Δ18O(quartz-serpentine), Δ18O(talc-serpentine) and Δ18O(calcite-serpentine) measured by SIMS in the investigated samples with Δ18O(quartz-water), Δ18O(talc-water) and Δ18O(calcite-water) calibrations available in the literature. Our empirical calibration of 1000 lnα(serpentine-water) = 1.04 ± 0.20 (2SE) × 106/T2 (T in K), from T = 25 to 100 °C, is within uncertainty of former high-temperature empirical calibrations extrapolated to T < 100 °C and matches experimental calibrations when extrapolated to T = 250 °C. The new serpentine-water calibration enables more accurate reconstructions of fluid-rock interactions occurring during low-temperature serpentinization processes in various tectonic settings.