Original Isotopic Composition Research Articles

Stable oxygen isotopes of crocodilian tooth enamel allow tracking Plio-Pleistocene evolution of freshwater environments and climate in the Shungura Formation (Turkana Depression, Ethiopia)

Abstract. This study adopts a new approach describing palaeohydrology and palaeoclimates based on the interpretation of stable oxygen isotopes (δ18Op) recorded in fossil crocodilian teeth. They represent an archive of prime interest for tracking freshwater palaeoenvironmental change, applicable to many palaeontological localities in the world: crocodilian teeth are abundant in continental basins and have been widely distributed since their diversification during the Mesozoic; the enamel phosphate is resistant to diagenesis and retains its original isotopic composition over geological timescales; and their δ18Op mainly relies on that of the crocodilian's home waterbody (δ18Ow), which in turn reflects waterbody types, regional climate, and evaporation conditions. This study presents the first application of this theoretical interpretative model to the Shungura Formation (Lower Omo Valley, Ethiopia), a key witness of the important environmental change in eastern Africa during the Plio-Pleistocene that impacted the evolution of regional faunas, including humans. In this complex and variable environmental context, the δ18Op of coexisting crocodilians allows for the fingerprinting of the diversity of aquatic environments they had access to at a local scale. This study sheds light on two important results: the δ18Op of crocodilian teeth (1) indicates stable aquatic environments in the northern Turkana Depression from 2.97 to ca. 2.57 Ma but a decline in local waterbodies diversity after 2.32 Ma, suggesting increasing aridity, and (2) shows, like previous geochemical studies on palaeosols and bivalves in the area, a significant increase in δ18Ow from 2.97 to ca. 1.14 Ma, likely due to the shifting air stream convergence zones between the West African and Indian Summer Monsoons and/or reduced rainfall over the Ethiopian Highlands.

Oxygen Isotope Alterations during the Reduction of U3O8 to UO2 for Nuclear Forensics Applications.

The fabrication of UO2 from U3O8 is an essential reaction in the nuclear fuel cycle. The oxygen isotope fractionation associated with this reaction has significant implications in the general field of nuclear forensics. Hence, the oxygen isotope fractionation during the reduction of U3O8 to UO2 was determined in the temperature range of 500-700 °C and for a duration of 2 to 6 h under a high-purity H2 atmosphere. Three U3O8 samples, possessing a different oxygen isotopic composition, were used to investigate key parameters involved with the fractionation during the reduction process. All UO2 products did not maintain the original isotope composition of the starting U3O8 under all conditions. The results show that the system UO2-H2O attains isotope equilibrium at 600 °C, provided the reduction process lasts at least 4 h or more. At 600 °C, UO2 was isotopically depleted by 2.89 ± 0.82‰ compared to the U3O8 from which it was produced. We find that the H2O formed during the reduction plays a major role in determining the final δ18O of UO2 prepared from U3O8. The isotope equilibrium of the system UO2-H2O at 600 °C was calculated, indicating that δ18O of the H2O was enriched by about 11‰ relative to the UO2 due to the uranium mass effect. These findings could potentially have important implications for nuclear forensics, as they provide a new method for determining the history of UO2 samples and tracing back their production process.

Utilizing Geochemical Analysis for Provenance Determination in Sedimentary Systems

Geochemical and isotopic analysis represents a potent approach for investigating sediment provenance, a crucial aspect for paleogeographic reconstructions, understanding the evolution of the Earth's crust and characterizing geology in areas that are no longer exposed at the surface. The sediment composition can be influenced by multiple processes including the weathering and erosion of the source rock, mixing with other sediment sources, grain size sorting and hydrodynamic processes such as water or wind transport. Additionally, climate and topography can impact the sediment composition by affecting the rate and nature of weathering and erosion as well as the transport and deposition of sediments. Moreover, post-depositional processes such as diagenesis and metamorphism have the potential to modify the original isotopic and geochemical composition of sediments. The objective of the present investigation is to conduct an overview of diverse geochemical proxies frequently employed in the evaluation of provenance as well as to assess their capabilities and limitations in estimating the origin of sediment. Additionally, statistical methodologies might be advantageous for sediment source determination, although it is crucial to acknowledge their restrictions and to merge them with traditional geochemical and isotopic techniques. A multidisciplinary strategy that encompasses several techniques and methodologies is probable to yield the most sturdy and dependable outcomes for identifying sediment provenance.

Preservation and modification of the isotopic composition (18O/16O and 2H/1H) of structurally-bound hydration water of gypsum (CaSO4·2H2O) in aqueous solution

Early investigations on stable isotopes (δ18O and δD) of structurally-bound gypsum (CaSO4·2H2O) hydration water (GHW) suggested that soon after gypsum precipitation, its primary isotopic composition could be altered via gypsum re-crystallization or by isotope exchange by diffusion of water into the intact crystals. If this occurs, the use of stable isotopes of GHW as a paleoclimate proxy is compromised. Here we investigated the long-term (up to 6 years) stability of GHW in contact with aqueous solution by conducting isotopic exchange experiments at different temperatures and using varying gypsum grain size. We placed gypsum with known hydration water isotopic composition in 18O/D-enriched and 18O/D-depleted aqueous solutions and monitored any change in the stable isotopes of GHW with time. At low temperature (25 °C) and when the solution is in chemical equilibrium with the mineral, GHW preserves its primary isotopic composition after 2 years. In contrast, when the gypsum-solution system is out of chemical equilibrium, the δ18OGHW and δDGHW values are altered, first via gypsum re-crystallization from the solution and later by isotope exchange via diffusion. Importantly, δ18OGHW and δDGHW stabilized after 2 years to values that represent only an 8 % change relative to the original GHW and remain constant during the subsequent 4 years. Our results suggest that at low temperature the effect of isotope exchange on GHW is limited. At higher temperature (65 °C), chemical equilibrium is not attained after 2 years for fine-grained synthetic gypsum (<250 μm) and its isotopic values continues to change. In contrast, the δ18OGHW and δDGHW in experiments using fine-grained natural gypsum at 45 °C stabilized after 1 year and values changed less than 3 % with respect to initial conditions. Experiments with coarser-grained natural gypsum (1–2 mm) at 45 °C did not show measurable isotopic changes of the GHW, indicating the lack of gypsum dissolution/re-crystallization or diffusion isotopic exchange. We conclude that microcrystalline gypsum crystals (<250 μm) are more readily affected by diagenesis resulting from changes in the gypsum saturation state of the solution, whereas larger gypsum crystals are more likely to preserve their original isotopic compositions. Our results indicate that under certain conditions, GHW can preserve the isotopic composition of its parent fluid and provide valuable information about paleoclimate and paleo-hydrologic conditions.

Carbon and Oxygen Isotopic Characteristics of Longmendong Formation in Muxu River Section of Qishan, Shanxi Province and Its Significance

The lower and upper Longmen cave formation of Late Ordovician in the Muxu river section of Qishan in the southwest margin of Ordos are Sandaogou formation of Middle Ordovician and beiguoshan formation of Upper Ordovician respectively, with continuous and complete strata. The average values of carbon and oxygen isotopes of carbonate rocks are 0.57‰ and -10.73‰ respectively, with poor correlation, which basically preserves the characteristics of original isotopic composition. The paleosalinity Z is between 120 and 130, which reflects that the late Ordovician was a marine sedimentary environment, which is consistent with the geological research results. According to the base value characteristics and drift changes of carbon isotope curve of Longmenshan formation, it can be divided into XH1-XH4 cycle segments. Generally speaking, the early carbon isotope values are mainly positive drift, with large scale and long time; The late stage is dominated by negative drift, with small scale and short time. This reflects that the sea level fluctuated greatly in the early Late Ordovician, with strong activity, and the sea level fluctuated slightly in the late Ordovician, showing a downward trend as a whole. Carbon and oxygen isotope variation curve of longmendong formation and global Ordovician δ13C variation curves shows that longmendong formation is deposited in the late Ordovician, which is equivalent to Sandbian and Katian periods. There is a positive drift event of GICE carbon isotope, which is regionally comparable.

Calibrating the triple oxygen isotope composition of evaporite minerals as a proxy for marine sulfate

The triple oxygen isotope composition of seawater sulfate, as recorded in marine sulfate evaporites and barites, is commonly used to interpret past changes in atmospheric pO2/pCO2 and gross primary production (GPP). In practice, the most-negative measured triple oxygen isotope value (Δ'17O) of sulfate from a marine evaporite deposit is thought to most closely represent contemporaneous seawater sulfate and is used to calculate atmospheric composition. However, a range of triple oxygen isotope compositions are typically measured within a single marine evaporite basin. Here, we characterize in detail the variability in the triple oxygen isotope composition of the sulfate in gypsum sampled from three Messinian (5-6 Ma) marine evaporite sub-basins from the Western Mediterranean Basin. Evaporite sulfate is offset from contemporaneous seawater sulfate and reflects mixing between two end-member sulfate populations: the original seawater sulfate and sulfate that has been isotopically reset after basin restriction. The combined Δ'17O and δ18O compositions of sulfate within a stratigraphic context offer the opportunity to better constrain the degree to which marine sulfate evaporites preserve the original isotopic composition of open ocean seawater sulfate. This study encompasses an exploration of mass-dependent fractionation, isotope equilibrium with water, and various scenarios of mixing. Our results calibrate the utility of marine sulfate evaporites in constraining the contemporaneous, open ocean triple oxygen isotope composition of seawater sulfate.

Determining the stable isotope composition of porewater using low temperature multi-step extraction for low water content soils

The δ2H and δ18O values (isotope values) of porewater in soils have been used as natural tracers in many research disciplines. Conventional methods to determine the isotope values of porewater require extraction of water from the soil. To extract soil porewater cryogenic vacuum distillation is a commonly used technique. Based on extraction techniques porewater can be considered to be composed of ‘extractable’ and ‘non-extractable’ two components. Based on the goal of most studies, porewater consists of ‘active’ and ‘inactive’ two components, of which only the active part is of interest. Because different porewater components are interrelated through isotope exchange, in principle the conventional distillation technique cannot extract the active porewater without changing its original isotopic composition, regardless of whether recovery of the active porewater is complete or not. Thus, using the isotope results determined on the extracted water to represent the active porewater in soil may lead to variable results. Here, we describe a new distillation strategy for extraction of porewater for determining active porewater isotope values. The extraction uses a lower extraction temperature that is comparable to natural conditions. The isotope values are determined on the extracted water corresponding to different recoveries resulted from multi-step extraction. The final results of the active porewater can be derived indirectly from the multi-step extraction. This study demonstrates that in some cases the conventional one-step extraction technique may not be capable for accurate determination of isotope values of the active porewater. The multi-step extraction technique described in this paper can be used to give more reliable results. Multi-step extraction is similar to direct equilibration methods in that it determines the porewater component which plays an active part in the water cycle, regardless of whether extraction of the porewater is complete or not. Multi-step extraction method could provide an alternative approach to direct equilibration methods in some special cases, or provide a way to test the reliability of other methods for the determination of δ2H and δ18O values of soil porewater.

A triple oxygen isotope perspective on the origin, evolution, and diagenetic alteration of carbonatites

Carbonatites are unique magmatic rocks that are essentially composed of carbonates, and they usually host a diverse suite of minor and accessory minerals. To provide additional insights on their petrogenesis, triple oxygen isotope analyses were carried out on carbonatites from sixteen localities worldwide in order to assess the behaviour of oxygen isotopes (mass-dependent fractionation) during their formation. The study evaluates the mineralogical differences, i.e., calcite, dolomite, ankerite, and Na-carbonates, and the mode of emplacement (intrusive or extrusive) in the mantle-derived carbonatites to further constrain the triple oxygen isotopic composition (Δ′17O) of the upper mantle. Δ′17O values in the intrusive calcite carbonatites vary between −0.003 to −0.088‰ (n = 20) and −0.024 to −0.085‰ (n = 5) in the dolomite varieties. We surmise that the magnitude of isotopic fractionation in the different carbonate phases during their formation is similar and thus, the observed variations are independent of mineralogy and may be related to alteration in the rocks. Taking the samples that classify as primary igneous carbonatites altogether, the average Δ′17O value of the mantle is estimated as −0.047 ± 0.027‰ (1SD, n = 18) which overlaps those of other mantle rocks, minerals and xenoliths, indicating that the mantle has a relatively homogenous oxygen isotope composition. Two ankerite carbonatites have identical Δ′17O values as calcite but a few samples, together with pyroclastic tuffs have significantly lower Δ′17O values (−0.108 to −0.161‰). This deviation from mantle Δ′17O signature suggests diagenetic alteration (dissolution and recrystallisation) and mixing of carbonate sources (juvenile and secondary carbonates) which is consistent with the high δ18O and clumped isotope (Δ47) values recorded in the pyroclastic and ankeritic rocks. In summary, diagenetic alteration driven by fluid-rock interaction at low temperatures, sub-solidus re-equilibration with magmatic waters, and the incorporation of secondary carbonates altogether facilitate the alteration of original isotopic compositions of carbonatites, obliterating their primary mantle signatures.

Soil CO2 flux baseline in Planchón – Peteroa Volcanic Complex, Southern Andes, Argentina - Chile

The Planchón – Peteroa Volcanic Complex exhibits two active degassing areas: Peteroa crater, PC, and peripheral hydrothermal areas, analyzed in three sampling sites, namely Valle Baños del Azufre, VBA, Valle del Peñón, VP and Valle Valenzuela, VZ. This prospective study was designed to investigate the soil diffuse emissions of CO2 at this volcanic complex. The survey consisted of 560 measurements of CO2 soil diffuse flux and temperature carried out at PC, VBA, VP. The results of this work show that diffuse CO2 is emitted at PC through two well-defined diffuse degassing structures and flux distribution that can be modeled by combining two log-normal populations. Here, the main source of soil CO2 is a shallow aquifer recharged by steam condensation and meteoric fluids that interact with rising magmatic gases. The carbon isotopic signature reveals that CO2 diffuse degassing is affected by secondary soil processes during its ascent. The total diffuse CO2 output at the surveyed area of PC, which covers ̴77,000 m2 around the crater fumarolic field, is 6.49 ± 0.32 t d−1. Soil diffuse emissions at the hydrothermal peripheral sites, VBA and VP, were studied covering a total area of ̴590,000 m2. These emissions are characterized by poorly defined diffuse degassing structures and flux distributions that can be modeled by combining three log-normal populations, which suggest that multiple sources feed CO2 diffuse emissions. In concordance, the isotopic analysis also suggests a mixed source of CO2 and degassing affected by secondary processes that modify the original isotope composition. CO2 soil degassing at VZ shows a clear relation with the vegetated areas of the soil. The minimum endogenous CO2 output estimation here proposed for the Planchón – Peteroa Volcanic Complex is that of PC, ̴6.5 t d−1. The thermal energy release was computed for PC, and the estimation is 2.57 ± 0.53 MW.

Elemental and isotopic geochemistry of carbonate rocks from the Pila Spi Formation (Middle–Late Eocene), Kurdistan Region, Northern Iraq: implication for depositional environment

Major, trace, rare earth elements (REE), and stable isotopic compositions in carbonate rocks of Pila Spi Formation were measured to constrain the source of rare earth elements and depositional environment. The low ΣREE content (average = 6.53 ppm, n = 14) indicates that the carbonates seawater-like REE patterns with (1) LREE depletion (average (Nd/Yb)n = 0.81), (2) HREE enrichment, (3) negative Ce anomaly (average = 0.91), and (4) superchondritic Y/Ho ratio (average = 30.07); slightly lower average value of Y/Ho ratio than the typical seawater value (~ 44–74) suggests modification of the seawater by input of fresh water. The (Nd/Yb)n and (Dy/Yb)n ratios of the Pila Spi carbonates have similarity with that of the Arabian sea carbonate. The low concentrations of U (0.35–1.0 ppm) suggest the deposition under oxygen-rich environment. This study indicates that the carbonates still preserve their original seawater-like REE pattern, provided that the contamination with terrigenous materials was small, and they serve as a seawater proxy. The Pila Spi carbonates have δ13C values ranged from − 6.17 to 0.76‰ PDB, and the δ18O values range between − 3.85 and 0.46‰ PDB. The positive correlation between δ13C and δ18O (r = 0.949, n = 14) values indicates the alteration of original isotopic compositions due to diagenesis. Finally, it is conceived that the Pila Spi carbonate was deposited in the lagoon environment with seawater invading and mixing of the continental (terrigenous) materials to the basin is viable.

Original composition and formation process of slab-derived deep brine from Kashio mineral spring in central Japan

Brine samples from the wells in the Kashio mineral spring (an “Arima-type” hot spring at Ooshika-Mura, central Japan) were analyzed to determine the original chemical and isotopic compositions of the deep brine end-member before its dilution by meteoric water and to elucidate the origin of the end-member. The trends of variation between Cl, δD, and δ18O indicated the existence of a two-component mixing system and a systematic variation in the mixing ratio, which were mentioned in previous studies. By carefully tracking the variation in tritium (3H) and atmospheric noble gas in the brine, the Cl concentration in the end-member was determined to be 24,000 mg/L. This value is consistent with the result of previous studies. Based on the estimated composition and other related data, we inferred that the end-member originated from slab-derived fluid, which in turn may have undergone oxygen isotope exchange reactions with minerals. Although both the Arima and Kashio brines are considered to be derived from fluid dehydrated from the Philippine Sea slab, the chemical and isotopic compositions of the Kashio end-member are different from those of the Arima end-member. In particular, the Kashio end-member is characterized by low Cl concentration (~ 40% lower than that in the Arima end-member), low hydrogen isotope ratio, and low 3He/4He ratio (1.4 Ra). These results indicate that the chemical and isotopic compositions of the slab-derived fluid are different for each location. The significant difference in δD could reflect the difference in the dehydration depth. Finally, the low temperature and relatively low 3He/4He ratio of the brine end-member could be explained by its long residence time within the crust.

Preservation of primordial signatures of water in highly-shocked ancient lunar rocks

Spurred by the discovery of water in lunar volcanic glasses about a decade ago, the accessory mineral apatite became the primary target to investigate the abundance and source of lunar water. This is due to its ability to contain significant amounts of OH in its structure, along with the widespread presence of apatite in lunar rocks. There is a general understanding that crustal cumulate rocks of the lunar magnesian (Mg) suite are better candidates for recording the original isotopic compositions of volatile elements in their parental melts compared to eruptive rocks, such as mare basalts. Consequently, water-bearing minerals in Mg-suite rocks are thought to be ideal candidates for discerning the primary hydrogen isotopic composition of water in the lunar interior. Mg-suite rocks and most other Apollo samples that were collected at the lunar surface display variable degrees of shock-deformation. In this study, we have investigated seven Apollo 17 Mg-suite samples that include troctolite, gabbro and norite lithologies, in order to understand if shock processes affected the water abundances and/or H isotopic composition of apatite. The measured water contents in apatite grains range from 31 to 964 ppm, with associated δD values varying between −535 ± 134‰ and +147 ± 194‰ (2σ). Considering the full dataset, there appears to be no correlation between H2O and δD of apatite and the level of shock each apatite grain has experienced. However, the lowest δD was recorded by individual, water-poor (<∼100 ppm H2O) apatite grains that are either directly in contact with an impact melt or in its proximity. Therefore, the low-δD signature of apatite could be a result of interactions with D-poor regolith (solar wind derived H), facilitated by shock-induced nanostructures that could have provided pathways for migration of volatiles. In contrast, in relatively water-rich apatites (>∼100 ppm H2O), regardless of the complexity of the shock-induced nanostructures, there appears to be no evidence of water-loss or alteration in their δD. The weighted average δD value of 24 such water-rich apatites is −192 ± 71‰, and, of all 36 analyzed spots is −209 ± 47‰, indistinguishable from that of other KREEPy lunar lithologies or the Earth's deep mantle. Despite experiencing variable degrees of shock-deformation at a later stage in lunar history, water-rich apatite in some of the earliest-formed lunar crustal material appears to retain the original isotopic signature of H in the Moon.

Lithium isotope compositions of U.S. coals and source rocks: Potential tracer of hydrocarbons

Kerogen in organic-rich rocks contains trace amounts of lithium (Li) that has been overlooked as a contributor to the global Li geochemical cycle. This study examined a variety of coals where kerogen is concentrated (>50% organic carbon) and hydrocarbon source rocks of different ages, depositional environments and thermal maturity to determine their range of Li isotopic compositions (δ7Li‰) and factors that influence their compositions.Using Secondary Ion Mass Spectrometry (SIMS), we analyzed 22 coals and 4 hydrocarbon source rocks (Types I, II, III), to determine the δ7Li of kerogen in situ, without chemical isolation of phases that can alter their original isotopic compositions. The δ7Li values of the coals surveyed are distinctly isotopically light (<0‰) compared to most natural minerals and fluids. In immature coals, with a vitrinite reflectance in oil (VRo) of ≤0.5%, kerogen δ7Li values average –23.4 ± 1.1‰ and become heavier with increasing thermal grade to temperatures of gas generation (VRo ~1.3%). The linear correlation between δ7Li and VRo suggests that 6Li may be preferentially released to pore fluid from kerogen during thermal maturation. Notably, authigenic clays forming at diagenetic temperatures substitute Li from pore fluids into silicate layers, therefore, the Li isotopic composition of the clays may record fluid isotopic compositions influenced by organic-Li sources.NanoSIMS isotopic maps of the Lower Bakken Shale, and SIMS measurements of the Green River Shale show isotopically light Li associated with C-dominated areas, and heavier δ7Li with Si-dominated areas of the hydrocarbon source rocks. We conclude that kerogen is a source of isotopically light Li that contributes to fluids during thermal maturation and hydrocarbon generation. Kerogen may be a significant contributor of Li to pore fluids and its distinctly light Li isotopic composition relative to other terrestrial waters and minerals could provide a tracer of organic inputs to the global geochemical cycle.

Impact of early hydrocarbon charge on the diagenetic history and reservoir quality of the Central Canyon sandstones in the Qiongdongnan Basin, South China Sea

The Central Canyon sandstones, located in the Ledong–Lingshui Depression, Qiongdongnan Basin, South China Sea, are the main reservoirs of the deep-water area. The present study aimed to examine the diagenetic processes and reservoir quality of these sandstones. Using an integrated approach of examination of casting thin sections, X-ray diffraction, porosity and permeability measurements, scanning electron microscopy, carbon and oxygen stable isotope analyses, fluid inclusion analyses and electron microprobe analysis, we analysed the diagenetic features of the sandstones and the reservoir quality of the LD30-1 and LS17-2 sandstones, respectively. The results showed that the sandstones consist of lithic arkoses and sublitharenites, the main pore systems consist of primary intergranular pores and secondary pores, and the main cements consist of authigenic quartz, clay minerals, and carbonates. By combining the analyses of burial history and hydrocarbon charging history, we discovered that the LD30-1 sandstone reservoirs and the LS17-2 sandstone reservoirs are quite different. A significant negative shift in δ13C and δ18O (ranging from −4.88‰ to −2.27‰ and from −11.83‰ to −5.96‰, respectively) is evident in the carbonate cements of the LD30-1 sandstones, and oxidation of organic matter had an important role in the occurrence of the carbonate cements. However, relatively high and concentrated δ13C and δ18O values in the LS17-2 sandstones, ranging from −1.76‰ to −0.48‰ and −5.04‰ to −2.95‰, respectively, are closer to the original isotopic compositions. Based on casting thin sections, the abundance of quartz overgrowths in the LD30-1 sandstones is higher than that in the LS17-2 sandstones. The development of each clay mineral in the LD30-1 and LS17-2 sandstones is quite different. The LS17-2 sandstones have higher reservoir quality than the LD30-1 sandstones. Early hydrocarbon charge is an important factor causing the differences of diagenetic minerals and reservoir quality between the LS17-2 sandstones and the LD30-1 sandstones.

In situ LA-ICP-MS apatite and zircon U–Pb geochronology of the Nicholson Lake impact structure, Canada: Shock and related thermal effects

In situ laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) has been deployed to determine the U–Pb ages of impact metamorphosed apatite and zircon associated with the ∼12.5 km diameter Nicholson Lake impact structure, Northwest Territories, Canada. The dated phases occur within both impact melt-bearing breccias and clast-laden impact melts. A total of 84 laser ablation analyses from 57 apatite grains within seven rock samples yield a minimum refined lower intercept at 387 ± 5 Ma (MSWD = 0.87, 2σ, n=26), and maximum age with lower intercept of ∼1740 Ma. A total of 90 laser ablation analyses on 52 zircon grains from two rock samples yield a minimum intercept age of 384 ± 8 Ma (MSWD = 1.3, 2σ, n=22), with a maximum upper intercept age of 2679 ± 14 Ma, and a second discordia with a ∼1740 Ma upper intercept age. The results are consistent with the target rocks comprising Archean Snow Island Suite (∼2.7 Ga) and Paleoproterozoic Nueltin plutonic suite (∼1.74 Ga), with the impact event occurring at approximately 385 Ma. The degree of resetting of inherited apatite can be related to its proximity to impact melt (now partly devitrified glass) within the host impact melt-bearing breccias and clast-laden impact melt rocks. Those grains in direct contact with melt bodies are reset, while those occurring as inclusions in lithic or mineral clasts partly or wholly retain their original isotopic compositions. The youngest (impact-reset) apatite ages are most closely associated with the highest shock levels (i.e., granular textured and thermally dissociated zircons), which we relate to juxtaposition of apatite with superheated melt. Due to accumulated radiation damage prior to impact, many of the relic zircons are metamict, which facilitated enhanced Pb diffusion and their resetting to the impact age. Granular zircons record impact ages while those exhibiting planar fractures or experiencing negligible shock record basement ages. We link apatite and zircon geochronology to the textural and structural states of ZrSiO4 and its associated shock levels via field emission scanning electron microscopy and micro-Raman spectrometry.

Oxygen Isotopic Exchange between Amorphous Silicate and Water Vapor and Its Implications for Oxygen Isotopic Evolution in the Early Solar System

Abstract Meteoritic evidence suggests that oxygen isotopic exchange between 16O-rich amorphous silicate dust and 16O-poor water vapor occurred in the early solar system. In this study, we experimentally investigated the kinetics of oxygen isotopic exchange between submicron-sized amorphous forsterite grains and water vapor at protoplanetary disk-like low pressures of water vapor. The isotopic exchange reaction rate is controlled either by diffusive isotopic exchange in the amorphous structure or by the supply of water molecules from the vapor phase. The diffusive oxygen isotopic exchange occurred with a rate constant D (m2 s−1) = (1.5 ± 1.0) × 10−19 exp[−(161.5 ± 14.1 (kJ mol−1))R −1(1/T−1/1200)] at temperatures below ∼800–900 K, and the supply of water molecules from the vapor phase could determine the rate of oxygen isotopic exchange at higher temperatures in the protosolar disk. On the other hand, the oxygen isotopic exchange rate dramatically decreases if the crystallization of amorphous forsterite precedes the oxygen isotopic exchange reaction with amorphous forsterite. According to the kinetics for oxygen isotopic exchange in protoplanetary disks, original isotopic compositions of amorphous forsterite dust could be preserved only if the dust was kept at temperatures below 500–600 K in the early solar system. The 16O-poor signatures for the most pristine silicate dust observed in cometary materials implies that the cometary silicate dust experienced oxygen isotopic exchange with 16O-poor water vapor through thermal annealing at temperatures higher than 500–600 K prior to their accretion into comets in the solar system.

Revised interpretations of stable C and O patterns in carbonate rocks resulting from meteoric diagenesis

A positive correlation between the δ13C and δ18O values of carbonate rocks is a screening tool widely used to identify the overprint of meteoric diagenesis on the original isotopic composition of a sample. In particular, it has been suggested that systematic change from negative to positive δ13C and δ18O values with increasing depth in the core is an indicator of alteration within the zone of mixing between meteoric and marine waters. In this paper, we propose that such covariance is not generated within the traditionally defined mixing zone, and that positive correlations between δ13C and δ18O values in marine carbonates are not necessarily indicators of meteoric alteration. This new interpretation is based on data collected from the shallow sub-surface of the Bahamas, a region unequivocally influenced by meteoric waters to depths of at least 200m below the current sediment-water interface. The classic interpretation of the diagenetic environments, based on changes in the δ13C and δ18O values, would suggest the maximum penetration of freshwater occurs between 65 and 100m below seafloor. Below these depths, a strong positive covariation between the δ13C and δ18O values exists, and would traditionally be defined as the mixing zone. However, based upon known changes in sea level, the penetration of the freshwater lens extends significantly below this limit. We contend that the zone showing covariance of δ13C and δ18O values is actually altered within the freshwater lens, and not the mixing zone as previously proposed. The co-varying trend in δ13C and δ18O values is the result of diagenetic processes occurring at the interface between vadose and phreatic zones. Significantly greater rates of recrystallization and neomorphism are driven by the increased rates of oxidation of organic matter at this transition with progressively less alteration occurring with increasing depth. As sea level oscillates, the position of this interface moves through the deposit, causing cumulative alteration throughout the section. Hence, we propose that the covariation between δ13C and δ18O values is a consequence of varying degrees of alteration, rather than the result of diagenesis occurring within the zone where marine and freshwater fluids mix. Furthermore, within the pervasively altered vadose zone, there is little correlation between δ13C and δ18O values, and therefore covariation between δ13C and δ18O values is not an unequivocal indicator of meteoric diagenesis.

Nonlinear relationship between preservation of microstructure and geochemical composition of Pleistocene brachiopod shells from the Ryukyu Islands, southwestern Japan

AbstractStable carbon and oxygen isotope composition of fossilized brachiopod shells serves as an important source to delineate Earth's paleoenvironmental evolution in the Phanerozoic. However, the original isotopic composition is potentially modified by various kinds of diagenesis. To evaluate the extent to which the original isotopic composition of fossilized brachiopod shells is modified by meteoric diagenesis, microstructure, cathodoluminescence (CL) images, and carbon and oxygen isotope composition of fossilized Kikaithyris hanzawai (rhynchonellate brachiopod) shells were examined. The shells were collected from Pleistocene shallow marine carbonates exposed on the Ryukyu Islands, southwestern Japan. The extent of diagenetic alteration is quantitatively evaluated here as both the preservation state of the original shell microstructure and the luminescence/non‐luminescence of shells. Although altered fibers were commonly observed in the brachiopod shells, the original isotopic composition was almost retained. There are no significant differences in the isotopic composition between the luminescent and non‐luminescent shells. There is no direct relationship between the preservation state of the original shell microstructure and the luminescence/non‐luminescence of shells at three of four horizons, indicating that CL images are not necessarily useful for the detection of diagenetic alteration of shells or shell portions. Applying multiple criteria to assessing diagenetic alteration and cross‐checking them are required to distinguish between diagenetically altered and unaltered brachiopod shells.

Chromatographic formation of a triadic band of lithium in hydrated LTA zeolite: An investigation on lithium isotope separation effects by ion exchange

Lithium concentrations [Li] and isotopic ratios [7Li]/[6Li] were measured for effluent fractions from a biphasic zeolite column. The biphasic state was ascribed to a mixture of hydrated Linde Type A (LTA) zeolites, [Li0.08(NH4)0.92]A and [Li0.33(NH4)0.67]A, which were formed by Li ion exchange from hydrated ammonium in the form (NH4)12[Al12Si12O48]·nH2O (NH4A). The biphasic Li band of the column was displaced by ion exchange with a solution of NH4NO3. A constant [Li] with a much lower level than the concentration of NH4+ in the displacer (NH4NO3) was observed for the effluent from a short column. This constant lower level of [Li] was attributable to the biphasic state. On this [Li] plateau of the effluent, the level of [7Li] shifted higher than the original isotopic composition of the Li feed, whereas 6Li was concentrated on the biphasic zeolite solid. The accumulation of 6Li in the zeolite proceeded by a mechanism of differential elution of 7Li from the biphasic zeolite. For the long column experiment, a significant enrichment of 6Li in the zeolite was observed, whereby a triadic band of Li was probably formed in the column. Two monophasic and a biphasic state were assigned. The biphasic band was deemed to push the monophasic bands forward, thereby enriching the monophasic bands with 7Li, while 6Li accumulated at the end of the biphasic band. The trio structure of the Li band and isotopic discrimination in the band were analyzed.

Magmatic recharge buffers the isotopic compositions against crustal contamination in formation of continental flood basalts

Isotopic compositions of continental flood basalts are essential to understand their genesis and to constrain the character of their mantle sources. Because of potential crustal contamination, it needs to be evaluated if and to which degree these basalts record original isotopic signals of their mantle sources and/or crustal signatures. This study examines the Sr, Nd, Hf, and Pb isotopic compositions of the late Cenozoic Xinchang–Shengzhou (XS) flood basalts, a small-scale continental flood basalt field in eastern China. The basalts show positive correlations between 87Sr/86Sr and 143Nd/144Nd, and negative correlations between 143Nd/144Nd and 176Hf/177Hf, which deviate from compositional arrays of crustal contamination and instead highlight variations in magmatic recharge intensity and mantle source compositions. The lava samples formed by high-volume magmatic recharge recorded signals of recycled sediments in the mantle source, which are characterized by moderate Ba/Th (91.9–106.5), excess 208Pb/204Pb relative to 206Pb/204Pb, and excess 176Hf/177Hf relative to 143Nd/144Nd. Thus, we propose that magmatic recharge buffers the original isotopic compositions of magmas against crustal contamination. Identifying and utilizing the isotope systematics of continental flood basalts generated by high volumes of magmatic recharge are thus crucial to trace their mantle sources.