Kinetic Zr isotope fractionation on Mars recorded in ancient Zr-rich minerals

A model for kinetic isotope fractionation during redox reactions

Copper shows limited isotopic variation in equilibrated mantle-derived silicate rocks, but large isotopic fractionation during kinetic processes. For example, lunar and terrestrial samples that have experienced evaporation were found to have an isotopic fractionation of up to 12.5‰ in their 65Cu/63Cu ratios, while komatiites, lherzolites, mid-ocean ridge and ocean island basalts show negligible Cu isotope fractionation as a result of equilibrium partial melting and crystal fractionation. The contrast between the observed magnitudes of equilibrium and kinetic isotope fractionation for Cu calls for a better understanding of kinetic Cu isotope fractionation. One of the mechanisms for creating large kinetic isotopic fractionation even at magmatic temperatures is diffusion. In this study, we performed Cu isotopic measurements on Cu diffusion couple experiments to constrain the beta factor for Cu isotopic fractionation by diffusion. We demonstrate a Monte Carlo approach for the regression and error estimation of the measured isotope profiles, which yielded beta values of 0.16 ± 0.03 and 0.18 ± 0.03 for the two experimental charges measured. Our results are subsequently applied to a quantitative model for the evaporation of a molten sphere to discuss the role of diffusion in affecting the bulk Cu isotopic fractionation between liquid and vapor during evaporation. We apply the model to Cu evaporation experiments and tektite data to show that convection primarily governs mass transport for evaporation during tektite formation. In addition, we show that Cu isotopes can be used as a tool to test the role of kinetics during various magmatic processes such as magmatic sulfide ore deposit formation, porphyry-type ore deposit formation, and fluid-rock interactions.

Si stable isotope fractionation during adsorption and the competition between kinetic and equilibrium isotope fractionation: Implications for weathering systems

Kinetic isotopic fractionation during carbonate dissolution in laboratory experiments: Implications for detection of microbial CO 2 signatures using δ 13C-DIC

In recent years, stalagmites have become an important archive for paleoclimate. Several studies about stable isotope records in stalagmites show a simultaneous enrichment of d18O and d13C along individual growth layers, which is associated with kinetic isotope fractionation. However, to deduce paleoclimatic information from calcite which is precipitated under these non-equilibrium-conditions,it is important to improve the understanding of kinetic isotope fractionation in dependence of local conditions like temperature and drip rate. Within this research work, laboratory experiments with synthetic carbonates were carried out under controlled conditions. The d18O and d13C evolution of the precipitated calcite were studied for different experiment parameters such as the initial composition of the solution, temperature and drip rate. In addition, in situ experiments were carried out in two cave systems in Sauerland (Bunkerhohle and B7-Hohle). The modern calcite collected at three drip sites was compared with the calcite obtained from the laboratory experiments. All experiments show a distinct isotopic enrichment along the precipitated calcite. Lower drip rates, higher temperatures and higher initial supersaturation with respect to calcite result in a greater total isotopic enrichment and in a lower slope of the linear correlation d18O(d13C). The latter indicates a larger oxygen isotope buffering from the water reservoir. From a comparison with theoretical models it can be concluded that the conversion reactions between the bicarbonate and the carbon dioxid and the exchange reactions between the oxygen isotopes in the bicarbonate and the water reservoir occur faster than predicted from present publications, particularly in case of higher temperatures (23°C). Thus, for higher temperatures other effects might play a role not yet considered in theoretical models.

Experimental investigation of nitrogen isotopic effects associated with ammonia degassing at 0–70 °C

The analytical method to determine the stable oxygen isotope (18 O/16 O) composition of carbonates via phosphoric acid digestion leads to temperature- and solid-dependent kinetic isotope fractionation. Values for the double carbonate norsethite (BaMg(CO3 )2 ) have been unknown so far. The temperature dependence of kinetic oxygen isotope fractionation during the reaction of synthetic and natural BaMg(CO3 )2 with orthophosphoric acid (H3 PO4 ) according to the overall reaction BaMg(CO3 )2 + 2H3 PO4 = Ba2+ + Mg2+ + 2HPO4 2- + 2CO2 + 2H2 O has been examined for the first time using separate carbonate decomposition via fluorination or phosphoric acid digestion, with the resulting gases analyzed by isotope ratio monitoring mass spectrometry. In the temperature range between 25 and 70°C the kinetic fractionation factor between acid-generated CO2 and artificial and natural norsethite is described by (T in K): [Formula: see text] with A= 4.15 and B= 6.47 for natural norsethite, and A= 4.77 and B= 5.94 for synthetic norsethite. The fractionation factor measured for a poorly crystallized synthetic carbonate agrees with those for the other samples at 25°C, but is slightly lower at 50 and 70°C. No carbon isotope fractionation was found during the unidirectional acid dissolution. The kinetic oxygen isotope fractionation during phosphoric acid liberation of CO2 from BaMg(CO3 )2 is quantified. Based on published results for endmember carbonates, the results at 25°C for other double carbonates are estimated.

High-temperature kinetic isotope fractionation of calcium in epidosites from modern and ancient seafloor hydrothermal systems

Kinetic carbon isotope fractionation links graphite and diamond precipitation to reduced fluid sources

Silicon isotope variations in hydrothermal systems at Yellowstone National Park, Wyoming, U.S.A.

'Clumped isotope' thermometry of carbonates in the carbonaceous chondrites (CM, CI, CR and Tagish Lake) demonstrates that aqueous alteration of their parent bodies occurred from -31 to 71°C and involved reaction with fluids having δ18OVSMOW values of -29.7‰ to 11.8‰ and δ17OVSMOW of -14.9‰ to 7.6‰. Estimated carbonate formation temperatures decrease in the order: calcite > dolomite > breunnerite. Based on independent constraints on the ages of these carbonates and models of the evolution of the oxygen isotope compositions of parent body waters, I estimate that carbonate precipitation during aqueous alteration of the carbonaceous chondrite parent bodies started within 1-2 million years after the accretion of those parent bodies, and that the alteration temperatures decreased from 34°C to 18°C in the first ~4 million years and further to -20°C after a total of ~6.5 million years. Our results provide the first direct measurements of the low-temperature cooling histories of C1 and C2 carbonaceous chondrite parent bodies. Within the CM chondrite group itself, I observe a negative correlation between the δ13C values of CM carbonates and the δ18O of their formation waters, suggesting formation and escape of 13C-depleted CH4 during aqueous alteration on the CM chondrite parent bodies. I apply ab initio transition state and statistical thermodynamics theory to study the kinetic isotope fractionations of clumped isotopologues (i.e., multiply-substituted isotopologues; I also consider singly-substituted isotopologues) associated with phosphoric acid digestion of carbonate minerals and with the degassing of CO2 from aqueous solutions. Assuming that H2CO3 is the reaction intermediate during phosphoric acid digestion of carbonate minerals, I predict at 25°C that CO2 derived from acid digestion of carbonate minerals will be 10.72‰ and 0.220‰ higher, respectively, in 18O/16O ratios and 13C-18O clumped isotope anomaly than the reactant carbonate. These predicted kinetic isotope fractionations associated with phosphoric acid digestion and their temperature dependences (for both oxygen isotope and clumped isotopologues) compare favorably with independent experimental constraints for phosphoric acid digestion of calcite. I evaluate the effect of carbonate cation compositions on phosphoric acid digestion fractionations using cluster models in which disproportionating H2CO3 interacts with adjacent cations. These cluster models underestimate the magnitude of isotope fractionations, but do successfully reproduce the general trend of variations and temperature dependences of oxygen isotope acid digestion fractionations among different carbonate minerals (suggesting I have correctly identified the basic mechanism responsible for a dependence on cation chemistry, but not the exact structural model for cation—H2CO3 clusters). I further integrate our acid digestion fractionation model with previous theoretical evaluations of abundances of 13C-18O bonds in carbonate minerals, and predict the relationship between Δ47 values for CO2 extracted from carbonate minerals and the growth temperatures of those carbonates, including witherite, calcite, aragonite, dolomite and magnesite. I observe reasonable agreement between these predictions and available experimental determinations (e.g., difference of less than 0.05‰ over 0-50°C, for calcite). Kinetic isotope fractionation associated with HCO3- dehydration and HCO3- dehydroxylation reactions (the two pathways of CO2 degassing from aqueous solutions) are estimated with ab initio transition state theory calculations. Coupled with models of isotopic fractionations accompanying carbonate precipitation, I predict that kinetic isotope fractionation associated with CO2 degassing reactions will increase the δ13C and δ18O but decrease the relative proportion of 13C-18O bonds in carbonate minerals that precipitate from degassing solutions. Furthermore, these kinetic isotope effects are correlated with each other. For example, I predict the 13C/12C ratio of carbonate increases by 1.1-3.2‰ and its Δ47 value decreases by 0.017-0.026‰ for every 1‰ kinetic enrichment in its 18O/16O at 25°C, with the exact values depending on the pathway for CO2 degassing (i.e., HCO3- dehydration vs. HCO3- dehydroxylation) and on the amount of carbonate formation that accompanies CO2 degassing. These predictions compare favorably with the experimental constraints from laboratory synthesized cryogenic carbonates and speleothem-like carbonates and with the isotopic compositions of natural modern speleothems.

Stalagmites (carbonate deposits in caves) are excellent archives of palaeoenvironmental changes, as they are not only amenable to high-precision radiometric dating but host numerous proxy variables that reflect past changes in climate conditions related to hydrology, temperature, and atmospheric dynamics. In most stalagmites, the convex shape of the apex leads to rapid runoff of dripwater, which results in a thin film of water on top of the stalagmite, fast CO2 degassing, and depletion of the dissolved inorganic carbon (DIC) pool. These kinetic dynamics lead to carbon and oxygen isotope fractionation, hampering quantitative estimation of the carbonate formation temperature and reconstruction of original dripwater composition using, e.g., clumped isotope systematics (Affek et al. 2008; Affek & Zaazur 2014). Equilibrium conditions can be achieved however in subaqueously-formed speleothems (Daëron et al. 2019).Whether stalagmites that possess a ‘drip cup’, i.e., a concave apex, show near-equilibrium conditions and constitute archives of past temperature should be testable using isochronous clumped isotope analyses. We hypothesized that the deeper water body inside the ‘drip cup’ limits CO2 degassing and DIC depletion, enabling near-equilibrium conditions compared to the outer slope of the rim wall surrounding the ‘drip cup’ that is likely affected by kinetic fractionation. We tested this hypothesis using multiple isochronous samples from stalagmite MAYA-22-7, taken at Cenote Ch’en Mul, Mayapán, Yucatán (México). The actively growing stalagmite was collected in 2022. Air temperature in the cave chamber is very stable, at 25.7 ± 0.6°C year-round. The stalagmite shows a prominent and several-mm-deep ‘drip cup’ in its lower growth section. U-series dating showed that this interval is ca. 500 years old. Very thin growth layers inside the ‘drip cup’ and thick growth layers at the rim wall indicate different growth conditions across the stalagmite at the time of formation, likely with suppressed DIC depletion and reduced kinetic isotope fractionation in the ‘drip cup.’ Δ47 values were predicted to be highest inside the drip cup (if kinetic isotope fractionation increases with distance from the apex) or to remain constant (if kinetic effects are negligible). We milled seven subsamples from growth layers of the same age for Δ47 analysis. Each sample integrates several years (< 10) because of the required sample mass (~ 4.5 mg). Clumped isotopes were measured at Northumbria University on a NU Instruments Perspective IRMS. We discuss the clumped isotope dynamics along a single growth interval and implications for palaeotemperature reconstructions. Finding near-equilibrium conditions inside the ‘drip cup’ would offer the opportunity to reconstruct past temperatures, if not continuously, then at least for some time intervals.ReferencesAffek, H.P., Bar-Matthews, M., Ayalon, A., Matthews, A., Eiler, J.M., 2008. Glacial/interglacial temperature variations in Soreq cave speleothems as recorded by ‘clumped isotope’ thermometry. Geochim. Cosmochim. Acta 72, 5351–5360Affek, H.P., Zaarur, S., 2014. Kinetic isotope effect in CO2 degassing: insight from clumped and oxygen isotopes in laboratory precipitation experiments. Geochim. Cosmochim. Acta 143, 319–330Daëron, M., Drysdale, R.N., Peral, M., Huyghe, D., Blamart, D., Coplen, T.B., Lartaud, F., Zanchetta, G., 2019. Most Earth-surface calcites precipitate out of isotopic equilibrium. Nature Comms. 10:429

Quantifying kinetic fractionation in Bunker Cave speleothems using Δ47

Isotope fractionation of alkaline and alkaline-earth elements (Li, K, Rb, Mg, Ca, Sr, Ba) during diffusion in aqueous solutions

Determination of the mass-dependence of cadmium isotope fractionation during evaporation