Original Isotopic Composition Research Articles

Diffusion of Oxygen Isotopes in Thermally Evolving Planetesimals and Size Ranges of Presolar Silicate Grains

Abstract Presolar grains are small particles found in meteorites through their isotopic compositions, which are considerably different from those of materials in the solar system. If some isotopes in presolar grains diffused out beyond their grain sizes when they were embedded in parent bodies of meteorites, their isotopic compositions could be washed out, and hence the grains could no longer be identified as presolar grains. We explore this possibility for the first time by self-consistently simulating the thermal evolution of planetesimals and the diffusion length of 18O in presolar silicate grains. Our results show that presolar silicate grains smaller than ∼0.03 μm cannot keep their original isotopic compositions even if the host planetesimals experienced a maximum temperature as low as 600 °C. Since this temperature corresponds to that experienced by petrologic type 3 chondrites, isotopic diffusion can constrain the size of presolar silicate grains discovered in such chondrites to be larger than ∼0.03 μm. We also find that the diffusion length of 18O reaches ∼0.3–2 μm in planetesimals that were heated up to 700–800°C. This indicates that, if the original size of presolar grains spans a range from ∼0.001 μm to ∼0.3 μm like that in the interstellar medium, then the isotopic records of the presolar grains may be almost completely lost in such highly thermalized parent bodies. We propose that isotopic diffusion could be a key process to control the size distribution and abundance of presolar grains in some types of chondrites.

Diagenetic alteration affecting δ18O, δ13C and 87Sr/86Sr signatures of carbonates: A case study on Cretaceous seep deposits from Yarlung-Zangbo Suture Zone, Tibet, China

Seep carbonates provide excellent records of past seepage activities, and have been commonly considered to preserve primary, unaltered stable carbon isotope signatures. However, late diagenetic reactions may overprint original isotopic compositions, but the mode and effect of such alterations are poorly understood. In particular, there are significant uncertainties regarding how carbon and strontium isotopic compositions of seep carbonates respond to diagenesis. This study reports recently discovered Cretaceous hydrocarbon-seep deposits from the Yarlung-Zangbo Suture Zone, Tibet, China that have experienced substantial diagenetic alteration that is shown by recrystallization and secondary veins. Unitary linear recursive analysis was applied to δ13C values and 87Sr/86Sr ratios of the seep carbonates to evaluate the degree of secondary modification and to quantitatively constrain the compositions of primary carbonates and late diagenetic fluids. The δ18O values range from −11.8‰ to −2.2‰, δ13C values from −34.1‰ to −12.9‰ and 87Sr/86Sr ratios from 0.706221 to 0.706808. The heterogeneity in isotopic compositions and the observation that the most negative δ18O values occur in samples with the most extensive recrystallization indicate significant and spatially heterogeneous modification of isotope compositions during late diagenesis. The linear correlations between δ13C values and δ18O values for matrix micrites (R2=0.54), and between bulk carbonate 87Sr/86Sr ratios and δ18O values (R2=0.85) are best explained by burial diagenetic overprinting of oxygen, strontium, and even carbon isotopic compositions rather than by meteoric water hypergenesis. Extrapolated values of δ13C and ratios of 87Sr/86Sr against a δ18O value of −2‰ (average value of calcite precipitated in isotopic equilibrium with coeval Cretaceous seawater) that would characterize the primary carbonate, give an end member δ13C value of −34‰ and an end member 87Sr/86Sr ratio of 0.7072. The end member isotopic values obtained by this extrapolation suggest that the primary seep carbonates with low δ13C values and high 87Sr/86Sr ratios were formed by anaerobic oxidation of methane near the seafloor. In contrast, the measured δ18O values and 87Sr/86Sr ratios reflect late diagenetic fluids represented by burial pore water characterized by a low 87Sr/86Sr ratio and high temperature. Our findings reveal that δ13C values can only be moderately and 87Sr/86Sr ratios can be significantly altered during late diagenesis, and show that it is possible to quantitatively assess the primary composition of diagenetically altered seep carbonates.

Isotope and hydrochemistry reveal evolutionary processes of lake water in Qinghai Lake

Abstract Qinghai Lake is the largest lake in China and is one of more than 500 brackish lakes on the Tibet Plateau. This study investigated stable isotopes (δ 18 O and δ 2 H) and hydrochemistry of lake water in Qinghai Lake and surrounding lakes to understand the lakes' evolutionary processes. These processes help explain the formation and evolution of the brackish and salt lakes on the Tibetan Plateau. The original isotopic and chemical compositions of lake water were controlled by a mixture of water from river water, groundwater, and precipitation. The values of δ 18 O and δ 2 H of most of the lake water samples were greater than zero due to continuous strong evaporation. The cations were in the order Na + > Mg 2 + > K + > Ca 2 + , and anions were in the order Cl − > SO 4 2 − > HCO 3 − . The hydrochemical types of Qinghai Lake and surrounding lakes were Na + -Cl − . The lake water, excluding Erhai Lake, was saturated with CaCO 3 . There was significant aragonite precipitation, and the Ca 2 + concentration was relatively low. Haiyanwan Lake and Gahai Lake, which have no surface water inputs, had experienced stronger evaporation than Qinghai Lake since the lakes were isolated from Qinghai Lake due to the dropping lake level and the arid climate. Erhai Lake water had relatively low salinity due to the fresh water coming in from the Daotang River. With the continued strong evaporation of lake water, Qinghai Lake and surrounding lakes should become increasingly saline. However, the salinity of lakes on the Tibetan Plateau is likely to slowly decrease in future decades due to the increasing precipitation and runoff, which will dilute lake water. In the longer term, however, Qinghai Lake and the brackish lakes will likely still become more saline, given the continued contribution of chemical ions from river water and groundwater and strong evaporation from the lake surface.

Constraints on the formation and diagenesis of phosphorites using carbonate clumped isotopes

The isotopic composition of apatites from sedimentary phosphorite deposits has been used previously to reconstruct ancient conditions on the surface of the Earth. However, questions remain as to whether these minerals retain their original isotopic composition or are modified during burial and lithification. To better understand how apatites in phosphorites form and are diagenetically modified, we present new isotopic measurements of δ18O values and clumped-isotope-based (Δ47) temperatures of carbonate groups in apatites from phosphorites from the past 265million years. We compare these measurements to previously measured δ18O values of phosphate groups from the same apatites. These results indicate that the isotopic composition of many of the apatites do not record environmental conditions during formation but instead diagenetic conditions. To understand these results, we construct a model that describes the consequences of diagenetic modification of phosphorites as functions of the environmental conditions (i.e., temperature and δ18O values of the fluids) during initial precipitation and subsequent diagenesis. This model captures the basic features of the dataset and indicates that clumped-isotope-based temperatures provide additional quantitative constraints on both the formational environment of the apatites and subsequent diagenetic modification. Importantly, the combination of the model with the data indicates that the δ18O values and clumped-isotope temperatures recorded by phosphorites do not record either formation or diagenetic temperatures, but rather represent an integrated history that includes both the formation and diagenetic modification of the apatites.

Erratum to: Variations of sulphur isotope signatures in sulphides from the metamorphosed Ming Cu(−Au) volcanogenic massive sulphide deposit, Newfoundland Appalachians, Canada

The Ming deposit is an early Ordovician, bimodal-mafic Cu–Au volcanogenic massive sulphide (VMS) deposit in the Newfoundland Appalachians that was metamorphosed to upper greenschist/lower amphibolite facies conditions and deformed in the Silurian and Devonian. The Ming deposit consists of several spatially proximal ore bodies of which the 1806 Zone, 1807 Zone, Ming South Up Plunge and Down Plunge and the Lower Footwall Zone are the focus of this paper. The ore bodies have similar stratigraphic sequences. The ore bodies can be divided into (1) a silicified horizon that caps the massive sulphides, (2) semi-massive to massive sulphides and (3) sulphide mineralization in a rhyodacitic footwall. Sulphide mineralization in a rhyodacitic footwall includes (a) sulphide stringers immediately below the semi-massive to massive sulphides and (b) chalcopyrite–pyrrhotite–pyrite stringers distally from semi-massive to massive sulphides in the Lower Footwall Zone. Pyrite, chalcopyrite, pyrrhotite, arsenopyrite and galena were analysed by in situ secondary ion mass spectrometry (SIMS) for sulphur isotope compositions. The isotopic signatures of pyrite, chalcopyrite, pyrrhotite and arsenopyrite fall within a limited range of 2.8 to 12.0 ‰ for semi-massive to massive sulphides and sulphide mineralization in the footwall. The silicified horizon capping the semi-massive to massive sulphides has higher δ 34S (5.8–19.6 ‰), especially for pyrrhotite (mean, 17.2 ± 2.2 ‰, n = 8). The sulphur isotope composition of galena is more heterogeneous, especially within semi-massive to massive sulphides and sulphide stringers, ranging from 0.8 to 17.3 ‰ (mean, 6.1 ± 4.3 ‰, n = 35) and 7.6 to 17.1 ‰ (mean, 13.7 ± 5.3 ‰, n = 3), respectively. Geothermometric calculations give insufficient formation and metamorphism temperatures for neighbouring mineral pairs, because sulphides were not in isotopic equilibrium while deposited in early Ordovician or re-equilibrated during Silurian–Devonian metamorphism, respectively. Therefore, original isotopic compositions of sulphides at the Ming deposit have been preserved. Modelling of the source of sulphur shows that: (1) reduced seawater sulphate and (2) sulphur leached from igneous wall rock and/or derived from magmatic fluids are the main sources of sulphur in the Ming deposit. The influence of igneous sulphur (igneous wall rock/magmatic fluids) increases with temperature and is an important sulphur source for the semi-massive to massive sulphides and footwall mineralization, in addition to a contribution from thermochemical sulphate reduction (TSR) of seawater. It is difficult to distinguish between sulphur leached from igneous rocks and magmatic fluid-related sulphur, and it is possible that both sources contributed to the ores at the Ming deposit. In addition to igneous sulphur, the heavy isotopes of the silicified horizon are consistent with the sulphur in this horizon being derived only from thermochemical sulphate reduction of early Ordovician seawater sulphate.

Isotopic Composition of Water in Precipitation Falling at Different Altitudes

Isotopic Composition of Water in Precipitation Falling at Different Altitudes An attempt has been made to understand the original isotopic composition of water in precipitation at different altitudes i.e., is it the same �original isotopic composition� contributing to different altitudes and modified in the transit by local climate and topology? This is popularly known as �altitude effect� of isotopic composition of water in precipitation. This has been done using available data in the studies in Guadiana Menor Basin, Southeast Spain to get plot δ18O versus δ2H and to get LMWL at different altitudes. Further new method is evolved by the plot of slope versus intercept in LMWL at different altitudes to get the variation of isotope of δ18O and δ2H with altitude and suggested that it is better approach than by in individual observations of δ18O and δ2H with altitude.

New Approach for the Analysis of Isotopic Composition in Precipitation Globally

Isotopic composition of hydrogen and oxygen of water in precipitation is important tool to analyze the water in hydrological cycle. A new method has been suggested by Singh [7] plotting slope versus intercept of Local Meteoric Water Line (LMWL) to find the original isotopic composition in precipitation as injected tracer globally. Singh [8-10] has applied these concepts to analyze the isotopic composition in precipitation in different catchment areas of river and different seasons, precipitation falling at different altitudes in a region or a place and across the continent. These studies are extended for the (a) study of changing intercept but keeping the same slope as attributed to changed conditions as the source of atmospheric moisture. The experimental data as available of two transect from Amazon to Altiplano in South America are analyzed. The results are presented assigning two components, one due to Rayleigh adiabatic condensation process rainfall and second recycled water vapour by evotranspiration. The details of the analysis are given and the results clearly indicate these two components, are discussed in detail. (b) Study of higher intercept due to Mediterranean Meteoric Water Line (MMWL), Israel samples from various inputs on mountains, coastal area, caves and valley to ascertain the precipitation is from the same source. (c) Study of LMWL across the country (India) for four different regions to obtain the LMWL of the regions by the method of Singh [7] obtained the original isotopic composition of water in all the four regions thereby get the Global Meteoric Water Line (GMWL).

Structural control and geological significance of the Zn–Pb ores formed in the Benasque Pass area (Central Pyrenees) during the post-late Ordovician extensional event of the Gondwana margin

The Benasque Pass area (Central Pyrenees) is located in the eastern part of the Variscan Lys-Caillaouas Massif, where Cambro-Ordovician rocks of the Hesperian massif crop out. In the area several Zn–Pb deposits are located in Variscan structures. Lead isotopes indicate a late Ordovician–early Devonian model age for the deposits. In the region, an unconformity between pre-late Ordovician and late Ordovician rocks has been defined by several authors and has been identified in the study area. The first structures observed in the Benasque Pass area consist of a pre-Variscan cleavage, which only affects the pre-late Ordovician rocks. In the study area, the main structures correspond to two Variscan generations: (i) E–W trending, north-verging folds, which are associated with the main foliation S1 (D1); (ii) E–W trending upright folds and associated small south-directed thrusts (D2). D2 structures are the ones hosting the zinc–lead ore. In the Benasque Pass area, D1 and D2 Variscan structures have developed under low grade metamorphic conditions. Some faults observed in the north and the south of the study area may have Alpine movement. In the Benasque Pass area, the strongly silicified and brecciated zinc–lead ore is mainly developed in the D1 short limb of the Mina Peak Syncline, within which S1 foliation is intensely folded and cross-thrust by D2 structures. Taking into account their model age, all these zinc–lead deposits seem to be stratiform, stratabound or filling open spaces in origin and related to the pre-Variscan extension described for the Central Pyrenees. However, the zinc–lead ore is hosted in strongly silicified and brecciated Variscan structures.The δ34S isotopic composition of sulfides indicates that the metamorphism and deformation did not completely obliterate the original isotopic composition, suggesting that sulfur in the Benasque Pass has mainly come from the reduction of marine sulfate extracted from the rocks of the metasedimentary sequence. In the Benasque Pass area, the lead isotope ratios are homogeneous and have a pre-Variscan model age, which is also the age of the host rocks. In the case of the Benasque Pass, the deposits occurred in Variscan structures, making it impossible to recognize their original pre-Variscan characteristics indicated by lead isotopes model age.The correlation between the Pb isotope data for deposits from the Benasque Pass and Aran Valley, also located in the Central Pyrenees, and those for other deposits from the Southern European Gondwana is evident and the isotopic data set closely fits the two-stage Pb evolution model for Sardinia terrane. The coherence of the Pb isotope trends in all the deposits suggests that Pb was extracted at various times from sources with similar time-averaged U/Pb, Th/Pb and Th/U values, through a process of leaching of the basement rocks by large volumes of hydrothermal fluids. In the case of the Pyrenees (Central and Eastern), the lead and other metals present in the deposit were extracted by hydrothermal fluids from the upper crust rocks during the post-late Ordovician extensional event of the Gondwana margin.

Composição isotópica de Sr, C e O e geoquímica de ETR das rochas carbonáticas do Bloco São Gabriel, Rio Grande do Sul

O Bloco São Gabriel, localizado na zona central do embasamento cristalino do Rio Grande Sul, apresenta extensos registros do Neoproterozóico do Ciclo Brasiliano, contidos em complexos ígneos e metamórficos. Esses complexos também guardam registros sedimentares, com sequências carbonáticas metamorfisadas, cuja evolução tectônica é pouco conhecida. Por essa razão, buscou-se nesse trabalho investigar a origem e a evolução das rochas carbonáticas ocorrentes no Bloco São Gabriel por meio da caracterização geoquímica de elementos maiores, traço, ETR e isótopos de Sr, C e O. Para tanto, foram escolhidas como áreas de pesquisa no Bloco São Gabriel a Formação Passo Feio, o Complexo Cambaí e o Complexo Coxilha do Batovi. As rochas carbonáticas da Formação Passo Feio compreendem dolomita mármores impuros, as amostras do Complexo Cambaí, calcita mármores impuros e os litotipos do Complexo Coxilha do Batovi, mármores calcíticos e calcissilicáticos, alguns ricos em grafita e com percentuais de até 10% de quartzo demonstrando contribuição siliciclástica no protólito. Idades de zircões detríticos de clorita xistos associados com os mármores, obtidos em trabalhos anteriores, indicam um intervalo entre 770 – 700 Ma para a deposição dos carbonatos do Bloco São Gabriel. Os carbonatos da Formação Passo Feio sofreram dolomitização durante ou logo após a deposição, enquanto os carbonatos do Complexo Cambaí não foram afetados pela dolomitização. A geoquímica dos elementos traço e terras raras indica que os mármores analisados preservaram as composições isotópicas originais do protólito nos diferentes graus metamórficos: Complexo Coxilha do Batovi (grau baixo), Formação Passo Feio e Complexo Cambaí (grau médio). A análise integrada dos isótopos de Sr, C e O da Formação Passo Feio e do Complexo Cambaí revelou a preservação de valores isotópicos primários ou muito próximos da composição isotópica original. Na Formação Passo Feio os valores obtidos são: 0,7074 (87Sr/86Sr), -0,26‰ e 2,44‰ (δ13CPDB) e -5,68‰ (δ18OPDB). No Complexo Cambaí, estes valores são: 0,7069 (87Sr/86Sr), 5,75‰ (δ13CPDB) e -11,64‰ (δ18OPDB). Quando comparadas com a curva de variação da razão 87Sr/86Sr e dos valores de δ13C e δ18O da água do mar ao longo do Neoproterozóico, verifica-se que os mármores da Formação Passo Feio e do Complexo Cambaí estão situados no intervalo entre 770 e 730 Ma. A contextualização dos dados isotópicos na cronologia dos eventos tectônicos reconhecidos no Bloco São Gabriel permitiu a estimativa de períodos de deposição das rochas carbonáticas da Formação Passo Feio entre 770 e 730 Ma, e entre 740 e 730 Ma para o Complexo Cambaí.

Determining the Diagenetic Conditions of Concretion Formation: Assessing Temperatures and Pore Waters Using Clumped Isotopes

Carbonate-δ^(18)O paleothermometry is used in many diagenetic studies to unravel the thermal history of basins. However, this approach generally requires an assumed pore-water δ^(18)O (δ^(18)O_(pw)) value, a parameter that is difficult to quantify in past regimes. In addition, many processes can change the original isotopic composition of pore water, which further complicates the assignment of an initial δ^(18)O_(pw) and can lead to erroneous temperature estimates. Here, we use clumped-isotope thermometry, a proxy based on the ^(13)C–^(18)O bond abundance in carbonate minerals, to evaluate the temperatures of concretion formation in the Miocene Monterey Formation and the Cretaceous Holz Shale, California. These temperatures are combined with established carbonate–water fractionation factors to calculate the associated δ^(18)O_(pw). Results demonstrate that diagenetic processes can modify the δ^(18)O of ancient pore water, confounding attempts to estimate diagenetic temperatures using standard approaches. Clumped-isotope-based temperature estimates for Monterey Formation concretions range from ∼ 17 to 35°C, up to ∼ 12°C higher than traditional δ^(18)O carbonate–water paleothermometry when δ^(18)O_(pw) values are assumed to equal Miocene seawater values. Calculated δ^(18)O_(pw) values range from +0.3 to +2.5‰ (VSMOW)—higher than coeval Miocene seawater, likely due to δ^(18)O_(pw) modification accompanying diagenesis of sedimentary siliceous phases. Clumped-isotope temperatures for the Holz Shale concretions range from ∼ 33 to 44°C, about 15 to 30°C lower than temperatures derived using the traditional method. Calculated δ^(18)O_(pw) values range from −5.0 to −2.9‰ and likely reflect the influx of meteoric fluids. We conclude that the use of clumped isotopes both improves the accuracy of temperature reconstructions and provides insight into the evolution of δ^(18)O_(pw) during diagenesis, addressing a longstanding conundrum in basin-evolution research.

Multi-stage origin of Roberts Victor eclogites: Progressive metasomatism and its isotopic effects

Xenolithic eclogites are high-pressure, high-temperature garnet–clinopyroxene rocks brought from the mantle to the surface by kimberlites and other primitive magmas. Their origins have been controversial for decades: do they represent metamorphosed subducted oceanic crust or magmatic rocks originally crystallized in the deep earth? The answer has important implications for the definition of major Earth processes.The extensive eclogite suite from the Roberts Victor kimberlite (South Africa) has previously been divided into two types (Type I and II); this study proposes five subgroups (IA, IB, IK; IIA, IIB) based on mineral assemblages and compositions. All of these eclogites were derived from depths of 170–200km, near the base of the contemporary subcontinental lithospheric mantle.The fresh, microstructurally equilibrated Type IIA eclogites are the protoliths of Type I, which were heavily metasomatized by carbonatitic–kimberlitic melts/fluids to form Types IAIBIK. All the Type I rocks show extensive textural disequilibrium, and have abundant fluid inclusions, secondary phases, and higher δ18O than Type II (IA: 5.0–8.0‰; IB: 4.1–6.8‰; IK:2.2–6.8‰; IIA=3.5‰; IIB:2.2–3.9‰). Type I are richer in LREE and LILE than Type IIA; they also are isotopically more homogeneous, with higher 87Sr/86Sr and less radiogenic Nd and Hf isotopes. Type I eclogites give gnt–cpx isochron ages (SmNd (103±10Ma); LuHf (132±16Ma)) similar to the kimberlite eruption age (128±15Ma). Type IIA and IIB eclogites have unradiogenic Sr and radiogenic Nd and Hf isotopic compositions, and give a range of Proterozoic two-mineral “ages” (SmNd: 738–1143Ma; LuHf: 1148–1544Ma), reflecting some preservation of their original isotopic compositions. Type IIB are broadly similar to Type IIA, but have lower MgO; their mutual relationships are not clear.Neither Type I nor Type II eclogites are similar to modern or Archean oceanic crust when all the chemical evidence is compared. In any case, the data from Type I eclogites cannot be used to support a “subduction model” of origin because their major- and trace-element chemistry, and radiogenic- and stable-isotope compositions, reflect the extensive metasomatism, rather than any primary characteristics.Type II eclogites are the key samples for the study of eclogite origins. Their bulk compositions and microstructures (exsolution of gnt and rutile) suggest that most or all the garnet in Type II samples might have formed by exsolution from high-Al clinopyroxenes, crystallized from mafic–ultramafic magmas in the deep mantle. Therefore, the xenolithic eclogites from Roberts Victor Mine probably were originally magmatic rocks crystallized at depth, rather than subducted oceanic crust; as a suite, they record a major metasomatic event in the deep lithospheric mantle, close to the time of kimberlite eruption.

Diagenetic alteration of the structure and δ18O signature of Palaeozoic fish and conodont apatite: Potential use for corrected isotope signatures in palaeoenvironmental interpretation

The oxygen isotopic compositions of Carboniferous conodonts and fish microfossils (ichthyoliths), from identical samples, were analysed in tandem in order to test whether these phosphatic media can be reliably used for palaeoclimatic reconstruction. The structure of conodonts and of most ichthyoliths analysed are somewhat analogous to the enamel and dentine/bone tissue, respectively, of modern mammals. Therefore, the diagenetic susceptibilities of the taxa analysed may provide important clues to other palaeoclimatic studies utilising a variety of biogenic apatite.Thermal maturation indices and scanning electron microscopy were used to characterise the preservation of biogenic apatite. Despite a high conodont colour alteration index (CAI) of ~5.5, conodont elements appear to have been only mildly affected by diagenetic alteration. In contrast, ichthyoliths were commonly recrystallised and exhibited diagenetic apatitic overgrowths containing amorphous pyrolytic carbon, interpreted as indicating that at least some overgrowth material derived from the original biogenic apatite.Diagenetic alteration has resulted in ichthyolith δ18O values being systematically lower by, on average, 2.9‰ (1σ=±0.3) relative to conodont apatite. Conodont samples yielded regionally correlatable isotope values, which can be interpreted in terms of more palaeoenvironmentally sensible palaeotemperatures relative to ichthyolith values. Densely crystalline, enamel-like conodont elements are interpreted as the more robust phosphatic medium to preserve original isotopic compositions. The strong correlation (r=0.8) of the δ18O values of the more structurally pristine conodont and commonly recrystallised ichthyolith apatite, indicates that (i) despite diagenetic lowering of absolute isotope values, original palaeoenvironmental records may be preserved and (ii) diagenetic overprinting may result in a stable offset, and therefore be correctable, locally.

Depositional and post-depositional setting of Maastrichtian limestone, Ariyalur Group, Cauvery Basin, South India: a geochemical appraisal

Major, trace, REE, and stable isotopic composition of Maastrichtian limestones of Cauvery Basin were studied to decipher their depositional and post-depositional environment. The major oxides and trace elements concentration of these limestones were normalized using Post-Archean Australian Shale (PAAS) values. The concentration of CaO is having a significant enrichment, and MgO, K2O, Co, Ni, Cu, Zn, Pb, Ba, U, and Th are moderately depleted, whereas Sr and Na2O contents are relatively similar to the PAAS values. The PAAS-normalized REE + Y pattern of Maastrichtian limestones display relatively uniform pattern among the compositionally different limestones: (1) enriched LREE (average NdSN/YbSN = 5.03 ± 0.92); (2) positive Ce anomaly (average 1.79 ± 0.16); (3) negative GdSN/Gd* anomaly (average 0.52 ± 0.02); (4) low Y/Ho ratio (average Y/Ho = 0.14 ± 0.01). The average ΣREE contents in ferruginous, arenaceous, and biostromal limestones are 65, 64, and 52 ppm, respectively, which indicate that the mineralogical variation plays a major role in incorporation of ΣREE. The Kallankurichchi limestones have δ13C values range from −3.2 to −1.1‰PDB, and the δ18O values range between −7.8 and −5.5‰PDB. The negative δ13C and δ18O values and a moderate co-variation between δ13C and δ18O (r2 = 0.35) indicate that the alteration of original isotopic composition is due to diagenesis. The positive Ce/Ce* confirms the clastic input into the system, and negative Mn* values and Th/U ratios (~0.29–1.4) in these limestones indicate their association with precipitation of carbonates in reducing (anoxic–dysoxic: Th/U ≤ 2) conditions.

Tracing sources and cycling of phosphorus in Peru Margin sediments using oxygen isotopes in authigenic and detrital phosphates

Many (bio)geochemical processes that bring about changes in sediment chemistry normally begin at the sediment-water interface, continue at depth within the sediment column and may persist throughout the lifetime of sediments. Because of the differential reactivity of sedimentary phosphate phases in response to diagenesis, dissolution/precipitation and biological cycling, the oxygen isotope ratios of phosphate (δ 18O P) can carry a distinct signature of these processes, as well as inform on the origin of specific P phases. Here, we present results of sequential sediment extraction (SEDEX) analyses combined with δ 18O P measurements, aimed at characterizing authigenic and detrital phosphate phases in continental margin sediments from three sites (Sites 1227, 1228 and 1229) along the Peru Margin collected during ODP Leg 201. Our results show that the amount of P in different reservoirs varies significantly in the upper 50 m of the sediment column, but with a consistent pattern, for example, detrital P is highest in siliciclastic-rich layers. The δ 18O P values of authigenic phosphate vary between 20.2‰ and 24.8‰ and can be classified into at least two major groups: authigenic phosphate precipitated at/near the sediment-water interface in equilibrium with paleo-water oxygen isotope ratios (δ 18O w) and temperature, and phosphate derived from hydrolysis of organic matter (P org) with subsequent incomplete to complete re-equlibration and precipitated deeper in the sediments column. The δ 18O P values of detrital phosphate, which vary from 7.7–15.4‰, suggest two possible terrigenous sources and their mixtures in different proportions: phosphate from igneous/metamorphic rocks and phosphate precipitated in source regions in equilibrium with δ 18O w of meteoric water. More importantly, original isotopic compositions of at least one phase of authigenic phosphates and all detrital phosphates are not altered by diagenesis and other biogeochemical changes within the sediment column. These findings help to understand the origin and provenance of P phases and paleoenvironmental conditions at/near the sediment-water interface, and to infer post-depositional activities within the sediment column.

Hydrogen and oxygen isotopes of water from inclusions in minerals: design of a new crushing system and on‐line continuous‐flow isotope ratio mass spectrometric analysis

An analytical line for stable isotope analyses of water recovered from fluid inclusions in minerals was built and successfully tested. The line is based on the principle of continuous-flow analysis of water via high-temperature reduction on glassy carbon. It includes a custom-designed set of high-efficiency crushers and a cryo-focusing cell. This paper provides details of the line design and discusses strategies for line conditioning and mitigation of memory effects. The line allows measurements of hydrogen and oxygen isotopes during a single acquisition. The precision of the analyses depends on the amount of water released from the inclusions. The best results are obtained for samples containing at least 0.1-0.2 microL (0.06-0.11 micromol) H(2)O. For such samples precision is better than 1.5 per thousand for deltaD and 0.5 per thousand for delta(18)O (1sigma). Smaller amounts of water can be measured but at lower precision. Analyses of modern calcite formed under stable conditions in a deep cave allowed assessment of the accuracy of the analyses. The deltaD values measured in fluid inclusions of this working standard match the deltaD value of the parent water, and the oxygen isotope values agree within ca. 0.5 per thousand. This indicates that fluid inclusions trapped in calcite at near-ambient temperatures (e.g. speleothems and low-temperatures phreatic calcite) faithfully preserve the original isotopic composition of the parent waters.

Elemental and isotope behavior of macromolecular organic matter from CM chondrites during hydrous pyrolysis

Abstract— A new insight into carbon and hydrogen isotope variations of insoluble organic matter (IOM) is provided from seven CM chondrites, including Murchison and six Antarctic meteorites (Y‐791198, Y‐793321, A‐881280, A‐881334, A‐881458 and B‐7904) as well as Murchison IOM residues after hydrous pyrolysis at 270–330 °C for 72 h. Isotopic compositions of bulk carbon (δ13Cbulk) and hydrogen (δD) of the seven IOMs vary widely, ranging from −15.1 to −7.6%0 and +133 to +986%0, respectively. Intramolecular carboxyl carbon (δ13CCOOH) is more enriched in 13C by 7.5. 11%0 than bulk carbon. After hydrous pyrolysis of Murchison IOM at 330 °C, H/C ratio, δ13Cbulk, δ13CCOOH, and δD values decrease by up to 0.31, 3.5%0, 5.5%0, and 961%0, respectively. The O/C ratio increases from 0.22 to 0.46 at 270 °C and to 0.25 at 300 °C, and decreases to 0.10 at 330 °C. δ13Cbulk‐δD cross plot of Murchison IOM and its pyrolysis residues shows an isotopic sequence. Of the six Antarctic IOMs, A‐881280, A‐881458, Y‐791198 and B‐7904 lie on or near the isotopic sequence depending on the degree of hydrous and/or thermal alteration, while A‐881334 and Y‐793321 consist of another distinct isotope group. A δ13Cbulk‐δ13CCOOH cross‐plot of IOMs, including Murchison pyrolysis residues, has a positive correlation between them, implying that the oxidation process to produce carboxyls is similar among all IOMs. These isotope distributions reflect various degree of alteration on the meteorite parent bodies and/or difference in original isotopic compositions before the parent body processes.

Stable Iron Isotopes and Microbial Mediation in Red Pigmentation of the Rosso Ammonitico (Mid-Late Jurassic, Verona Area, Italy)

The iron (Fe) isotopic composition of 17 Jurassic limestones from the Rosso Ammonitico of Verona (Italy) have been analyzed by Multiple-Collector Inductively Coupled Plasma Mass Spectrometry (MC-ICP-MS). Such analysis allowed for the recognition of a clear iron isotopic fractionation (mean -0.8 per thousand, ranging between -1.52 to -0.06 per thousand) on a millimeter-centimeter scale between the red and grey facies of the studied formation. After gentle acid leaching, measurements of the Fe isotopic compositions gave delta(56)Fe values that were systematically lower in the red facies residues (median: -0.84 per thousand, range: -1.46 to +0.26 per thousand) compared to the grey facies residues (median: -0.08 per thousand, range: -0.34 to +0.23 per thousand). In addition, the red facies residues were characterized by a lighter delta(56)Fe signal relative to their corresponding leachates. These Fe isotopic fractionations could be a sensitive fingerprint of a biotic process; systematic isotopic differences between the red and grey facies residues, which consist of hematite and X-ray amorphous iron hydroxides, respectively, are hypothesized to have resulted from the oxidizing activity of iron bacteria and fungi in the red facies. The grey Fe isotopic data match the Fe isotopic signature of the terrestrial baseline established for igneous rocks and low-C(org) clastic sedimentary rocks. The Fe isotopic compositions of the grey laminations are consistent with the influx of detrital iron minerals and lack of microbial redox processes at the water-interface during deposition. Total Fe concentration measurements were performed by Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) (confirmed by concentration estimations obtained by MC-ICP-MS analyses of microdrilled samples) on five samples, and resultant values range between 0.30% (mean) in the grey facies and 1.31% (mean) in the red facies. No correlation was observed between bulk Fe content and pigmentation or between bulk Fe content and Fe isotopic compositions. The rapid transformation of the original iron oxyhydroxides to hematite could have preserved the original isotopic composition if it had occurred at about the same temperature. This paper supports the use of Fe isotopes as sensitive tracers of biological activities recorded in old sedimentary sequences that contain microfossils of iron bacteria and fungi. However, a careful interpretation of the iron isotopic fractionation in terms of biotic versus abiotic processes requires supporting data or direct observations to characterize the biological, (geo)chemical, or physical context in relation to the geologic setting. This will become even more pertinent when Fe isotopic studies are expanded to the interplanetary realm.

Pyrite metamorphism in the devonian hunsruck slate of Germany: Insights from laser microprobe sulfur isotope analysis and thermodynamic modeling

The Devonian Hunsruck Slate of Altlay, Rhenish Massif, Germany, contains an exceptional spectrum of pyrite textures, recording progressive recrystallization and remobilization of sedimentary/diagenetic pyrite during very low grade metamorphism. Sedimentary/diagenetic pyrite is very fine-grained and forms distinct enrichments, including thin layers, elongate concretions, and pyritized fish coprolites. These pyrites show abundant idioblastic overgrowths, which are very coarse-grained and are always located at the interface between the sedimentary/diagenetic pyrite and the slate matrix. Progressive recrystallization resulted in the formation of porphyroblasts and an almost complete obliteration of the original internal structures. A detailed, texturally-resolved sulfur isotope study of the different pyrite types has been carried out, using an in situ laser microprobe technique. The δ34S values of sedimentary/diagenetic pyrite, idioblastic overgrowths, and porphyroblasts are very homogeneous and lie in the range of +7.9 to +12.8 permil (mean: +10.6 ‰), +8.6 to +12.4 permil (mean: +10.8 ‰), and +8.3 to +12.1 permil (mean: +10.3 ‰), respectively. The sulfur isotope data for each population and the isotopic relationships on the sample scale indicate that metamorphic recrystallization has not resulted in a significant modification of the original isotopic composition. Calculation of phase relations and modeling of fluid-rock equilibria place important constraints on the conditions and mechanisms of pyrite remobilization during metamorphism. The calculations show that pressure variations have no detectable influence on pyrite solubility, indicating that pressure solution was not important in the metamorphic mass transfer of pyrite. The effects of temperature and oxidation state are more pronounced, and infiltration of fluids originating from deeper crustal levels could have caused the remobilization of pyrite. Modeling of the isotopic fractionation shows that remobilization driven by interaction with such fluids having a higher temperature and oxidation state would have resulted in a significant negative shift of the δ34S values, which is not consistent with the observed isotopic pattern. We envisage an alternative model that involves fluid-assisted reduction of the grain surface area (and surface energy) per unit volume of pyrite as the dominant mechanism driving recrystallization and porphyroblast growth. The difference in surface energy between the fine-grained sedimentary/diagenetic and the idioblastic pyrite resulted in establishment of a small-scale chemical potential gradient, which evolved along a self-enhancing path during progressive idioblastic growth. The results of our modeling have implications for the preservation of primary sulfur isotopic signatures and emphasize the importance of constraining the history of fluid-rock interaction for a meaningful interpretation of sedimentary/diagenetic processes in pyrite-rich metasediments.

Implications for the Late Pleistocene climate in Finland and adjacent areas from the isotopic composition of mammoth skeletal remains

Nine samples of subfossil bone and teeth remains from woolly mammoth ( Mammuthus primigenius) discovered in Finland, Russian Karelia and Western Russia were analyzed for the oxygen isotope composition of the phosphate fraction and the carbon and oxygen isotope composition of the carbonate fraction in skeletal apatite. The samples have been radiocarbon dated in previous studies and are of late Middle Weichselian to Late Weichselian age. The preservation of the samples was tested analyzing the chemical composition and checking the isotopic equilibrium between the phosphate and carbonate components in skeletal apatite. According to these tests, five out of the nine samples were determined to have retained their original isotopic composition. These samples were used to estimate Late Pleistocene climatic conditions in Finland and neighboring areas. Based on the best-preserved enamel samples, the isotopic composition of oxygen in Late Pleistocene precipitation was 1–3‰ lower than that in the mean annual precipitation in southern and central Finland today. Using the relationship between the isotopic composition of precipitation and the ambient temperature, it can be estimated that the mean temperatures during the Middle Weichselian ice-free period were 2–6 °C lower compared to the present-day values.

Global correlation for strontium isotope curve in the Late Cretaceous of Tibet and dating marine sediments

87Sr/86Sr ratios of marine carbonate samples collected from a sedimentary section of the Late Cretaceous in the south of Tibet were measured. Based on the absence of cathodo-luminescence and a very low Mn/Sr ratio (average 0.06) of the samples, it is thought that they contain information on the original seawater strontium isotope composition. The strontium isotope evolution curve of the Late Cretaceous in Tibet we established here, is consistent with other coeval curves from Europe, North America and Antarctica, supports the notion that the strontium isotope composition of seawater is governed by global events, which provides a new approach for the inter-continental and inter-basinal correlations of Late Cretaceous in the area and is a complementarity for biostratigraphy. In addition, we attempt to determine the age of the boundaries for Campanian/Santonian and Maastrichtian/Campanian by87Sr/86Sr ratios for Gamba section in southern Tibet. The two boundaries are located in the thickness of 217 m (83.5 Ma) and 291 m (71.3 Ma), respectively.