Initial Isotopic Compositions Research Articles

K-Ar Ages and Geochemical + Sr-Nd Isotopic Compositions of Adakitic Volcanic Rocks, Western Shandong Province, Eastern China: Foundering of the Lower Continental Crust

K-Ar age, geochemical, and Sr-Nd isotopic data are reported for western Shandong volcanic rocks with the objectives of deciphering the origin and dynamic significance of Mesozoic magmatic events within the North China craton. K-Ar ages indicate eruption of the volcanic rocks between 116.2 ± 1.1 and 120.3 ± 1.1 Ma. The studied rocks include andesite and dacite that have SiO2 contents ranging from 59.3 to 65.9 wt% and Al2O3 contents from 15.3 to 16.8 wt%; K2O and Na2O contents range from 3.4 to 5.1 wt% and 3.1 to 3.8 wt%, respectively, indicating that they belong to the K-rich series. These rocks are depleted in heavy rare-earth elements (HREE) and variably enriched in light rare-earth elements (LREE). They have positive Ba, Sr, and K and negative Nb, Ta, and Ti anomalies. The rocks have high Sr ranging from 492 to 1665 ppm and relatively low Yb (0.77-1.75 ppm) and Y contents (9.5-17.1 ppm), resulting in high Sr/Y (52-102) and La/Yb ratios (28-45), characteristic of adakites. εNd(t) and initial Sr isotopic compositions range from -13.2 to -10.3 and from 0.7089 to 0.7098, respectively. These geochemical features are consistent with an origin from adakitic magmas that were likely derived from dehydration melting of a delaminated lower crust. The relatively high MgO, Cr, and Ni contents in the analyzed rocks suggest that interaction between melts and peridotite may have occurred before eruption. This study provides further evidence for foundering of lower crust accompanied by lithosphere thinning beneath the North China craton (NCC).

Nd, Pb, and Sr isotope composition of Late Mesozoic to Quaternary intra-plate magmatism in NE-Africa (Sudan, Egypt): high-μ signatures from the mantle lithosphere

The isotopic composition of mafic small-volume intra-plate magmatism constrains the compositions of the sub-continental mantle sources. The Nd, Pb, and Sr isotope signatures of widespread late Mesozoic to Quaternary intra-plate magmatism in NE Africa (Sudan, South Egypt) are surprisingly uniform and indicate the presence of a high-μ (μ = 238U/204Pb) source in the mantle. The rocks are characterized by small ranges in the initial isotopic composition of Nd, Pb, and Sr and most samples fall within ε Nd ca. 3–6, 206Pb/204Pb ca. 19.5–20.5, 207Pb/204Pb ca. 15.63–15.73, 208Pb/204Pb ca. 39–40 and 87Sr/86Sr ca. 0.7028–0.7034. We interpret this reservoir as lithospheric mantle that formed beneath the Pan-African orogens and magmatic arcs from asthenospheric mantle, which was enriched in trace elements (U, Th, and light REE). Combining our new data set with published data of intra-plate magmatic rocks from the Arabian plate indicates two compositionally different domains of lithospheric mantle in NE-Africa–Arabia. The two domains are spatially related to the subdivision of the Pan-African orogen into a western section dominated by reworked cratonic basement (NE-Africa; high-μ lithospheric mantle) and an eastern section dominated by juvenile Pan-African basement (easternmost NE-Africa and Arabia; moderate μ lithospheric mantle). The compositions of the Pan-African lithospheric mantle and the MORB-type mantle of the Red Sea and Gulf of Aden spreading centers could explain the Nd–Pb-Sr isotopic compositions of the most pristine Afar flood basalts in Yemen and Ethiopia by mixtures of the isotopic composition of regional lithospheric and asthenospheric sources.

Rhenium-Osmium Isotope and Platinum-Group Element Constraints on the Origin and Evolution of the 1{middle dot}27 Ga Muskox Layered Intrusion

Platinum-group element (PGE: Os, Ir, Ru, Pt, Pd) and Re^Os isotope systematics determined for the entire preserved stratigraphy of the 1 27 Ga Muskox intrusion provide an exceptional view of magma chamber processes and mineralization in the main plutonic system of the Mackenzie large igneous province (LIP).We present new Re^Os isotope data for the intrusion, together with PGE and trace element abundances, and oxygen and Sm^Nd isotope data on samples that include local crustal materials, layered series peridotites, stratiform chromitites, marginal and roof zone rocks, and the Muskox Keel feeder dyke. Intrusive rocks span wide ranges in initial isotopic compositions (gOsi1⁄4þ1 0 to þ 87 6; eNdi1⁄4 0 4 to 6 6; dOOl1⁄4þ 5 5 to þ 6 9o) and highly siderophile element abundances (HSE: PGE and Re; Re1⁄4 0 02^105 ppb; Pt1⁄4 0 23^ 115 ppb; Os1⁄4 0 02 to4 200 ppb). HSE and fluid-immobile trace element abundance variations are consistent with relative compatibilities expected for cumulate rocks. The most radiogenic Os and unradiogenic Nd isotope compositions occur in the Muskox marginal and roof zones. Negative gOsi values in these rocks and their nonisochronous relations result from mobilization of Re in the intrusion through post-magmatic hydrothermal processes.The most significant process causing Os and Nd isotope variations in the layered series of the intrusion is crustal contamination of mantle-derived magma batches feeding individual cyclic units.This process may be directly responsible for formation of chromitite horizons within the intrusion. Accounting for crustal assimilation, the Muskox intrusion parental magma has gOsi1⁄4þ1 2 0 3, eNdi 4 ^1 0 0 4, dO þ5 5o and HSE abundances similar to those expected from 15% partial melting of the Mackenzie LIP mantle source. This composition is similar to that calculated for 1 27 Ga primitive upper mantle. Parental magmas were probably derived from a mantle source unaffected by long-term, large-scale melt depletion, with no appreciable input from recycled crust and lithosphere, or putative core contributions.

Stable carbon (δ13C) and nitrogen (δ15N) isotopes as natural indicators of live and dry food in Piaractus mesopotamicus (Holmberg, 1887) larval tissue

This study proposed the use of the stable isotope technique to track the type of food utilized by pacu Piaractus mesopotamicus larvae during their development, and to identify the moment when the larvae start using nutrients from the dry diet by retaining its carbon and nitrogen atoms in their body tissues. Five-day-old pacu larvae at the onset of exogenous feeding were fed Artemia nauplii or formulated diet exclusively; nauplii+formulated diet during the entire period; or were weaned from nauplii to a dry diet after 3, 6 or 12 days after the first feeding. δ13C and δ15N values for Artemia nauplii were −15.1‰ and 4.7‰, respectively, and −25.0‰ and 7.4‰ for the dry diet. The initial isotopic composition of the larval tissue was −20.2‰ and 9.5‰ for δ13C and δ15N respectively. Later, at the end of a 42-day feeding period, larvae fed Artemia nauplii alone reached values of −12.7‰ and 7.0‰ for δ13C and δ15N respectively. Larvae that received the formulated diet alone showed values of −22.7‰ for δ13C and 9.6‰ for δ15N. The stable isotope technique was precise, and the time at which the larvae utilized Artemia nauplii, and later dry diet as a food source could be clearly defined.

Extreme deuterium enrichment of organic radicals in the Orgueil meteorite: Revisiting the interstellar interpretation?

The insoluble organic matter of the carbonaceous meteorites contains radicals having a polyaromatic structure and a heterogeneous distribution. By using Hyperfine Sublevel Correlation spectroscopy (HYSCORE) in pulsed Electron Paramagnetic Resonance (pulsed-EPR), whereby nuclear frequencies of magnetic nuclei and their hyperfine interaction with electron spin of radicals are detected with high resolution, the radicals are shown to be considerably enriched in deuterium in the Orgueil meteorite, with a D/H ratio of 1.5 ± 0.5 × 10 −2. These radicals hold 3.6 ± 1.2 × 10 −3 H relative to total organic H. Analysis of hydrogen and deuterium hyperfine interactions indicates that the deuterium atoms are localized in the benzylic position, on aliphatic carbons bonded to aromatic radical moieties. This type of C–H bond exhibits one of the smallest bond energy, reinforcing the recent finding that the lower the C–H bond energy the higher the deuterium-enrichment (Remusat L., Palhol F., Robert F., Derenne S. and France-Lanord C. (2006) Enrichment of deuterium in insoluble organic matter from primitive meteorites: a solar system origin? Earth Planet. Sci. Lett. 243, 15–25). Such a behavior is difficult to reconcile with the usual interpretation according to which high D/H ratios represent survivals of interstellar grains. More likely, the deuterium-enrichment process took place after the formation of organic grains whose initial isotopic compositions was close to the protosolar D/H ratio. These grains were possibly loaded at the surface of the protosolar disk where they exposed to the intense solar UV irradiation, triggering an isotopic exchange with deuterium-rich highly reactive ions.

Osmium, tungsten, and chromium isotopes in sediments and in Ni‐rich spinel at the K‐T boundary: Signature of a chondritic impactor

Abstract— It is now established that a large extraterrestrial object hit the Earth at the end of the Cretaceous period, about 65 Ma ago. We have investigated Re‐Os, Hf‐W, and Mn‐Cr isotope systems in sediments from the Cretaceous and the Paleogene in order to characterize the type of impactor. Within the Cretaceous‐Tertiary (K‐T) boundary layer, extraterrestrial material is mixed with terrestrial material, causing a dilution of the extraterrestrial isotope signature that is difficult to quantify. A phase essentially composed of Ni‐rich spinel, formed in the atmosphere mainly from melted projectile material, is likely to contain the extraterrestrial isotopic signature of the impactor. We show that the analysis of spinel is indeed the best approach to determine the initial isotope composition of the impactor, and that W and Cr isotopes confirm that the projectile was a carbonaceous chondrite.

First compound-specific chlorine-isotope analysis of environmentally-bioaccumulated organochlorines indicates a degradation-relatable kinetic isotope effect for DDT

Compound-specific chlorine-isotope analysis (CSIA-Cl) of 1,1,1-trichloro-2,2-bis( p-chlorophenyl)ethane ( p, p′-DDT) and 1,1-dichloro-2,2-bis( p-chlorophenyl)ethene ( p, p′-DDE) in blubber from Baltic Grey seal ( Halichoerus grypus) was performed in order to investigate if a kinetic isotope effect (KIE) could be observed concomitant to environmental degradation of DDT. The δ 37Cl of p, p′-DDT and p, p′-DDE were −0.69 ± 0.21‰ and −2.98 ± 0.57‰ (1σ, n = 3), respectively. Both samples were enriched relative to the hypothesized initial isotope composition (−4.34‰), thus indicating a composite KIE associated with the degradation mechanisms pertaining to DDT. An isotope fractionation factor for degradation of dichloromethane, from the literature, was adapted and modified for use in the calculation of DDT degradation. A subsequent simplified Rayleigh distillation model of the DDT chlorine-isotope composition yielded an estimated fraction ( f) of 7 ± 2% of released DDT presently remaining as undegraded compound in the environment. The consistency between the result of the Rayleigh model ( f ∼ 7%) and the use of the DDT/(DDT + DDE) ratio as a measure of DDT degradation (∼10% undegraded DDT) suggests that the KIE of DDT degradation may be significant, and that the novel approach of CSIA-Cl may be a valuable tool for degradation/persistence studies of lipophilic organochlorines in the environment.

Zircon Hf isotope composition of metamorphic eclogite from Xindian, Dabie Terrain

Hf isotope measurement has been carried out for UHP metamorphic eclogite from Xindian by using LA-MC-ICP-MS technique. The result indicates that metamorphic growth zircon has high 176Hf/177Hf (0.282544–0.282612) and low 176Lu/177Hf (0.000004–0.000211) ratio, inherited and recrystallized protolith zircon has low 176Hf/177Hf (0.282266–0.282466) and high 176Lu/177Hf (0.000090–0.002144) compositions. The low 176Lu/177Hf of growth zircon comes from its decreasing of Lu and increasing of Hf during UHP process. The high 176Hf/177Hf deduced from high radiogenic 176Hf, which was produced from long-term evolution of high Lu/Hf ratio minerals. Partial recrystallization of protolith zircon would not cause reworking of Lu/Hf isotope in zircon. Compared to U-Pb, zircon Lu-Hf system has better stability. The initial Hf isotope composition of metamorphic growth zircon may represent the Hf isotope composition of whole rock system at the same time. The initial e Hf of +3.0 for metamorphic precursor formation of Xindian eclogite indicates that the source material mainly derived from weak depleted mantle or mixing of depleted mantle with old crust.

Isotopically selective collisional vibrational energy transfer in CF3H

The authors investigate here the mechanism of collisionally enhanced isotopic selectivity observed in infrared multiple photon dissociation (IRMPD) of vibrationally preexcited CF3H by Boyarkin et al. [J. Chem. Phys. 118, 93 (2003)]. For both the carbon-12 and carbon-13 isotopic species they measure the dependence of the IRMPD yield on the time delay between the preexcitation and the dissociation pulses at different dissociation frequencies as well as its dependence on the initial isotopic composition of the sample. The results reveal that the collisional increase in isotopic selectivity originates not only from that of IRMPD itself but also from the isotopic selectivity of vibrational energy transfer, with the latter making the major contribution under their experimental conditions. They suggest that the observed isotopic selectivity in collisional energy transfer arises from the difference in overlap between the absorption spectra of the nu5 mode in the 12CF3H acceptor molecule with emission spectra of the same mode in the two isotopically different donors. Understanding the origin of this collisional effect has important implications for optimization of laser isotope separation processes.

Investigations of the photochemical isotope equilibrium between O<sub>2</sub>, CO<sub>2</sub> and O<sub>3</sub>

Abstract. Contrary to tropospheric CO2 whose oxygen isotopic composition follows a standard mass dependent relationship, i.e. δ17O~0.5δ18O, stratospheric CO2 is preferentially enriched in 17O, leading to a strikingly different relation: δ17O~1.7δ18O. It has been shown repeatedly that the isotope anomaly is inherited from O3 via photolytically produced O(1D) that undergoes isotope exchange with CO2 and the anomaly may well serve as a tracer of stratospheric chemistry if details of the exchange mechanism are understood. We have studied the photochemical isotope equilibrium in UV-irradiated O2-CO2 and O3-CO2 mixtures to quantify the transfer of the anomaly from O3 to CO2 at room temperature. By following the time evolution of the oxygen isotopic compositions of CO2 and O2 under varying initial isotopic compositions of both, O2/O3 and CO2, the isotope equilibria between the two reservoirs were determined. A very strong dependence of the isotope equilibrium on the O2/CO2-ratio was established. Equilibrium enrichments of 17O and 18O in CO2 relative to O2 diminish with increasing CO2 content, and this reduction in the equilibrium enrichments does not follow a standard mass dependent relation. When molecular oxygen exceeds the amount of CO2 by a factor of about 20, 17O and 18O in equilibrated CO2 are enriched by (142±4)‰ and (146±4)‰, respectively, at room temperature and at a pressure of 225 hPa, independent of the initial isotopic compositions of CO2 and O2 or O3. From these findings we derive a simple and general relation between the starting isotopic compositions and amounts of O2 and CO2 and the observed slope in a three oxygen isotope diagram. Predictions from this relation are compared with published laboratory and atmospheric data.

Effects of Mother Lode-Type Gold Mineralization on 187Os/188Os and Platinum Group Element Concentrations in Peridotite: Alleghany District, California

Osmium isotope compositions and concentrations of Re, platinum group elements (PGE), and Au were determined for host peridotites (serpentinites and harzburgites) and hydrothermally altered ultramafic wall rocks associated with Mother Lode-type hydrothermal gold-quartz vein mineralization in the Alleghany district, California. The host peridotites have Os isotope compositions and Re, PGE, and Au abundances typical of the upper mantle at their presumed formation age during the late Proterozoic or early Paleozoic. The hydrothermally altered rocks have highly variable initial Os isotope compositions with γOs values (% deviation of 187Os/188Os from the chondritic average calculated for the approx. 120 Ma time of mineralization) ranging from ‐1.4 to ‐8.3. The lowest Os isotope compositions are consistent with Re depletion of a chondritic source (e.g., the upper mantle) at ca. 1.6 Ga. Most of the altered samples are enriched in Au and have depleted and fractionated abundances of Re and PGE relative to their precursor peridotites. Geochemical characteristics of the altered samples suggest that Re and some PGE were variably removed from the ultramafic rocks during the mineralization event. In addition to Re, the Pt and Pd abundances of the most intensely altered rocks appear to have been most affected by mineralization. The 187Os-depleted isotopic compositions of some altered rocks are interpreted to be a result of preferential 187Os loss via destruction of Re-rich phases during the event. For these rocks, Os evidently is not a useful tracer of the mineralizing fluids. The results do, however, provide evidence for differential mobility of these elements, and mobility of 187Os relative to the initial bulk Os isotope composition during hydrothermal metasomatic alteration of ultramafic rocks.

Heterogeneity of Ar and Sr initial isotopic composition in the coexisting minerals from Miocene hypabyssal granitoids in the Caucasian Mineral Waters region

Heterogeneity of Ar and Sr initial isotopic composition in the coexisting minerals from Miocene hypabyssal granitoids in the Caucasian Mineral Waters region

Fast chemical and isotopic exchange of nitrogen during reaction with hot molybdenum

Molybdenum crucibles are commonly used to extract nitrogen from geological samples by induction heating. Because nitrogen is known to be reactive with certain metals (e.g., Ti and Fe), we have tested the reactivity of gaseous nitrogen with a Mo crucible held at 1800°C. The consumption of nitrogen, determined by monitoring the N2/40Ar ratio of the gas phase, varied between 25 and 100%, depending on the reaction duration. Nitrogen of the reacted gas was found to be systematically enriched in 15N relative to 14N by 10‰ compared to the initial isotopic composition, without any correlation with nitrogen consumption. We propose that a rapid isotopic exchange occurs between nitrogen originally trapped in the crucible and nitrogen from the gas phase, which modifies the isotopic composition of the reacted gas. This process can significantly bias the isotopic determination of nitrogen in rocks and minerals when a Mo furnace is used for gas extraction. Meanwhile, the rate of N‐Mo chemical bonding may be controlled by the formation of nitride (rather than solid solution), a process slower than the isotopic exchange. The use of a Mo furnace for the extraction of trace nitrogen from rocks and minerals should therefore be avoided.

Isotope geochemistry and FOZO mantle component of the alkaline–carbonatitic association of Fuerteventura, Canary Islands, Spain

Carbonatites are very rare in oceanic environments, where they have been reported only at the Canary and Cape Verde islands in the Atlantic Ocean. In the Canary archipelago, calciocarbonatites occur only on the island of Fuerteventura, in clear spatial and temporal association to alkaline silicate rocks, ranging from ultramafic to salic, and comprising alkaline clinopyroxenites, melteigites–ijolites–urtites, nepheline-bearing gabbros and nepheline–syenites, together with their volcanic equivalents. Selected isotopic (Sr, Nd and Pb) data from representative samples of this alkaline–carbonatitic association show a very restricted range of isotopic compositions, with those of the calciocarbonatites overlapping those of the alkaline silicate rocks. Despite the fact that all these rocks display cumulate textures and cover a wide range of SiO 2 and Sr, Nd, Pb contents, initial isotopic compositions are approximately constant and mantle-like, indicating that evolution of the alkaline–carbonatitic association of Fuerteventura must have taken place in the absence of crustal contamination processes. Regarding the mantle source for the alkaline–carbonatitic association, its depleted initial Sr ( 87Sr/ 86Sr = 0.7032–0.7036) and Nd ( 143Nd/ 144Nd = 0.5128–0.5129) compositions, fall on the same trend as the East African young (< 40 Ma) carbonatites. In 143Nd/ 144Nd and 87Sr/ 86Sr vs. 206Pb/ 204Pb diagrams, the alkaline–carbonatitic association of Fuerteventura shows FOZO compositions for Sr and Nd, and slightly higher (towards HIMU) Pb compositions. Furthermore, these data fit the projection of the mantle plane defined by Sr, Nd and Pb isotopic compositions of the DMM, HIMU and EMI components, this fit not being found in sources containing crustal components. In view of this, we propose that the alkaline–carbonatitic association of Fuerteventura was derived mainly from a heterogeneous mantle plume with a deep-seated (FOZO) signature, mixed with HIMU component.

Nd isotope variation across the Archaean-Proterozoic boundary in the North Ladoga Area, Russian Karelia

In order to investigate the boundary between Archaean crust of the Karelian Craton and Paleoproterozoic crust of the Svecofennian Orogen in the area north and west of the lake Ladoga (the North Ladoga Area) 24 samples of mostly granitoid rocks, collected along the 100 km long profile across the inferred suture, were analysed for their Nd isotopic composition. Ten previously published Nd data were incorporated in the dataset. It was established that gneisses from so-called “mantled domes” north of the suture have very low εNd values and represent reworked Archaean basement. North of the Kirjavalahti Dome, the presence of Archaean basement under Kalevian sedimentary cover is registered by abundant Archaean cores in zircons, revealed by a SHRIMP study, and by low εNd values in the rocks of the 1874±13 Ma-old Alattu dyke complex. The transition from Archaean to Proterozoic crust is registered by a shift from very low to Bulk-Earth-type εNd values and occurs within a 10-20 km-wide zone north of the inferred suture. The rocks of the Svecofennian Orogen south of the suture are characterized by relatively uniform initial Nd isotopic composition (average εNd value at 1880-1860 Ma +0.7, n = 17). In terms of Nd isotopic composition, this domain, the Lahdenpohja Domain, is comparable to the Svecofennian terranes to the west: the Central Finland Granitoid Complex and the Accretionary Arc Complex of Southern Finland. The Primitive Arc Complex of Central Finland has, in contrast, significantly more juvenile Nd isotopic composition.

Chondritic osmium isotopic composition of late Archean convecting upper mantle: Evidence from Zunhua podiform chromitites, Hebei, North China

Podiform chromite deposits are a characteristic feature of the mantle sequences of harzburgitic ophiolites. The chromites usually have very low Re and high Os contents, which makes it the most resistant phase remaining from the primary magmatic history of the ultramafic sections of ophiolites. The podiform chromite is one of the robust indicators of initial Os isotopic compositions of the ophiolites where podiform chromites were derived from, which provides strong evidence for the origin and evolution of oceanic lithosphere. The Re and Os contents and the Os isotopic compositions of seven podiform chromitites from Zunhua ophiolitic melange belt, North China are reported in this study. The Re contents range from 0.019 to 0.128 ng/g, Os from 8.828 to 354.0 ng/g, and the187Os/l88Os ratio from 0.11003 to 0.11145. Three massive chromitites among the sample set have very high Os contents (> 300 ng/g), and their187Os/188Os ratios range from 0.11021 to 0.11030, averaging 0.11026±0.00005 (σ), equivalent to a γOs=∼-0.12 ±0.06 at 2.6 Ga, which means that the Os isotopic composition of convecting upper mantle is chondritic in late Archean. It is the Os isotopic composition of podiform chromitites that are derived from the oldest ophiolite in the world till now.

Kinetic-simulating experiment combined with GC-IRMS analysis: application to identification and assessment of coal-derived methane from Zhongba Gas Field (Sichuan Basin, China)

The main purpose of this study is to model the δ 13C values of methane derived from coal by combining kinetic-simulating experiment with the gas chromatography–isotope ratio mass spectrum (GC-IRMS) analysis. The stable carbon isotopic variation of methane in pyrolysates with heating temperature indicates that the assumptions for both a constant kinetic isotope effect ( α) and a uniform initial isotopic composition ( δ 13C o) are impractical for explaining the carbon isotopic fractionation during coalification. For purposes of simplification, two approaches are used in this paper to deal with the heterogeneity of terrestrial organic matter. One is that, assuming a uniform initial isotopic composition (i.e., δ 13C i, o= δ 13C o) for all methane-generating precursors in coal, the isotopic variation of methane is fitted by adjusting Δ E a, i ( E a 13C, i − E a 12C, i ) for each hypothetical reaction. The other is that, assuming a constant kinetic isotope effect during the whole gas formation, that is all Δ E a, i values are identical, the modeling of methane isotopic composition is achieved by changing the 13CH 4 generation potential of each reaction ( f i, 13C ), namely, by adjusting the initial δ 13C value ( δ 13C i, o) for each methane-generating precursor. Results of the kinetic calculation shows that the two simulating methods can yield a similar result at a geological heating rate of 2 °C/My, which further demonstrates that those natural gases with methane δ 13C value being approximately −36‰ are possibly sourced from the upper Triassic coal measure strata in the Northwestern Sichuan Basin.

Geochemistry of late Archaean stromatolites from Zimbabwe: evidence for microbial life in restricted epicontinental seas

We report comprehensive trace element and Sr-isotope data for microbial carbonates from the Archaean Mushandike limestone, Masvingo Greenstone Belt, Zimbabwe. The stromatolites have very coherent REE+Y patterns and share the essential shale-normalised characteristics of other Archaean marine precipitates (positive La and Gd anomalies, absence of a negative Ce anomaly and a strongly superchondritic Y/Ho ratio). Mixing models constrain the maximum amount of shale contamination to 0.25–1% and calculated detritus-free carbonate REE+Y systematics require precipitation from seawater. In terms of light-REE over heavy-REE depletion, however, the studied samples are very different from all other known Archaean marine precipitates. In shale-normalised plots, the Mushandike samples yield a negative slope. A very restricted, regional input source of the dissolved load is indicated because normalisation with locally occurring tonalite gneiss REE+Y data yields a pattern closely resembling typical shale-normalised Archaean marine chemical sediments. The disappearance of a negative Eu anomaly when patterns are normalised with local tonalite gneiss strengthens this interpretation. Sr-isotope ratios are strongly correlated with trace element contents and ratios, which explains the modest scatter in Sr-isotope ratios as representing (minor) clastic contamination. Importantly, even the least contaminated samples have very radiogenic initial 87 Sr/ 86 Sr ratios (0.7184) implying Sr input from an ancient high Rb/Sr source, such as the early Archaean gneisses of south-central Zimbabwe. A local ancient (3.5–3.8 Ga) source is also indicated by previously published Pb-isotope datasets for the Mushandike stromatolites. This is entirely compatible with the occurrence of 3.7–3.8 Ga zircons in quartzites and metapelites from comparably old greenstone belts within less than 150 km of the studied locality. Comparison of the Pb-isotope ratios of the Mushandike stromatolites with 2.7 and 2.6 Ga old stromatolites from the neighbouring Belingwe Greenstone Belt demonstrates differences in initial isotope composition that relate to the extent of exchange with the open ocean. The development of numerous basins on old continental crust, with water masses variably restricted from the open ocean, suggests a lack of strong vertical topography on this late Archaean craton.

Modelling the isotopic composition of snow using backward trajectories: a particular precipitation event in Dronning Maud Land, Antarctica

AbstractWe consider a specific accumulation event that occurred in January 2002 in western Dronning Maud Land, Antarctica. Snow samples were obtained a few days after accumulation. We combine meteorological analyses and isotopic modelling to describe the isotopic composition of moisture during transport. Backward trajectories were calculated, based on European Centre for Medium-Range Weather Forecasts operational archive data so that the history of the air parcels transporting water vapour to the accumulation site could be reconstructed. This trajectory study showed that the air masses were not (super)saturated along most of the transport path, which is in contrast with assumptions in Lagrangian fractionation models and probably true for most precipitation events in Antarctica. The modelled fractionation along the trajectories was too limited to explain the measured isotopic content of the snow. It is shown that the observed isotopic composition of precipitation resulted from fractionation of initially more depleted water. This lower initial isotopic composition of water vapour might result from atmospheric mixing with more depleted air along the trajectory or from earlier condensation cycles, not captured by the trajectories. This is in accordance with isotope fields resulting from general circulation models, indicating a gradient in isotopic composition from the Equator to Antarctica.

Experimental investigation of oxygen isotope exchange between CO2 and O(1D) and its relevance to the stratosphere

Stratospheric CO2 is found to be enriched in heavy oxygen isotopes relative to tropospheric CO2 values caused by photo‐induced isotopic exchange with stratospheric ozone, a gas highly enriched in heavy isotopes relative to the ambient oxygen. The transfer of isotopic enrichment from the ozone pool to the CO2 pool takes place through CO*3 formed by reaction of CO2 with O(1D) derived from O3 photolysis. Recent stratospheric measurements indicate that the transfer process results in CO2 getting more enriched in 17O compared to 18O. Experiments, reported here, investigate this exchange and determine the effects of the initial isotopic composition of CO2 and O3 on the final isotopic composition of the exchanged CO2. It is seen that the slope relating 17O and 18O enrichments in CO2 depends on the initial isotopic composition of the two gases. Interestingly, with CO2 of tropospheric composition and O3 of stratospheric composition the observed slope (1.82 ± 0.04) is close to the value (1.71) found for stratospheric CO2. The apparent extra 17O enrichment in CO2 resulting in slope higher than that of O3 can be explained by simple mixing if one assumes that O(1D) originates mainly from the asymmetric ozone species and if additional fractionation occurs because of differences in collision frequencies of isotopic species and possibly also during dissociation of CO*3 to CO2 and O atom.