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1.
A combined in situ SIMS and LA-(MC)-ICPMS study of U-Pb ages, trace elements, O and Lu-Hf isotopes was conducted for zircon from eclogite-facies metamorphic rocks in the Sulu orogen. The two microbeam techniques sampled various depths of zircon domains, revealing different element and isotope relationships between residual magmatic cores and new metamorphic rims and thus the geochemical architecture of metamorphic zircons which otherwise cannot be recognized by the single microbeam technique. This enables discrimination of metamorphic growth from different subtypes of metamorphic recrystallization. Magmatic cores with U-Pb ages of 769 ± 9 Ma have positive δ18O values of 0.1-10.1‰, high Th/U and 176Lu/177Hf ratios, high REE contents, and steep MREE-HREE patterns with negative Eu anomalies. They are interpreted as crystallizing from positive δ18O magmas during protolith emplacement. In contrast, newly grown domains have concordant U-Pb ages of 204 ± 4 to 252 ± 7 Ma and show negative δ18O values of −10.0‰ to −2.2‰, low Th/U and 176Lu/177Hf ratios, low REE contents, and flat HREE patterns with weak to no Eu anomalies. They are interpreted as growing from negative δ18O fluids that were produced by metamorphic dehydration of high-T glacial-hydrothermally altered rocks during continental subduction-zone metamorphism. Differences in δ18O between different domains within single grains vary from 0.8‰ to 12.5‰, suggesting different degrees of O isotope exchange between the positive δ18O magmatic core and the negative δ18O metamorphic fluid during the metamorphism. The magmatic zircons underwent three subtypes of metamorphic recrystallization, depending on their accessibility to negative δ18O fluids. The zircons recrystallized in solid-state maintained positive δ18O values, and REE and Lu-Hf isotopes of protolith zircon, but their U-Pb ages are lowered. The zircons recrystallized through dissolution exhibit negative δ18O values similar to the metamorphic growths, almost completely reset U-Pb ages, and partially reset REE systems. The zircons recrystallized through replacement show variably negative δ18O values, and partially reset REE, and U-Pb and Lu-Hf isotopic systems. Therefore, this study places robust constraints on the origin of metamorphic zircons in eclogite-facies rocks and provides a methodological framework for linking the different types of metamorphic zircons to petrological processes during continental collision.  相似文献   

2.
The calcite fossils of the Derbyhaven Beds, Isle of Man, have δ13C values (+ 1·8 PDB) similar to modern, shallow-water marine skeletons, but the δ18O values (?6·1 PDB) are much lighter than modern skeletons. The light oxygen values indicate either re-equilibration with isotopically light water before cementation started, or Carboniferous sea water with δ18O of ?6‰. Aragonite dissolution was followed by precipitation of zoned calcite cement. In this cement, up to six intracrystalline zones, recognized in stained thin sections, show isotopic variation. Carbon varies from + 3-8 to + 1-2‰. and oxygen from ? 2-6 to ? 12-4‰. with decreasing age of the cement. This trend is attributed to increasing temperature and to isotopic evolution of the pore waters during burial. The zoned calcite is sequentially followed by dolomite and kaolinite cements which continue the trend towards light isotopic values. This trend is continued with younger, fault-controlled dolomite, and is terminated by vein-filling calcite and dolomite. The younger calcite, interpreted as a near-surface precipitate from meteoric waters, is unrelated to the older sequence of carbonates and has distinctly different carbon isotope ratios: δ13C ? 6-8‰.  相似文献   

3.
Unusual 18O depletion, with δ18O values as negative as −10‰ to −4‰ relative to VSMOW, was reported in zircons from ultrahigh-pressure eclogite-facies metamorphic rocks in the Dabie-Sulu orogenic belt, China. But it is critical for the negative δ18O zircons to be distinguished between magmatic and metamorphic origins, because the 18O depletion can be acquired by high-T eclogite-facies metamorphism of meteoric-hydrothermally altered low δ18O rocks. While zircon O diffusion kinetics has placed a reasonable constraint on this, zircon trace element compositions can provide a straightforward distinction between the magmatic and metamorphic origins. This paper reports our finding of unusual 18O depletion in zircon from granitic gneiss in the northeastern end of the Sulu orogen. Zircon δ18O values vary from −7.8‰ to −3.1‰ along a profile of 50 m length at Zaobuzhen. They are close to extremely low δ18O values of −9.0‰ to −5.9‰ for metagranite at Qinglongshan and adjacent areas in the southwestern end of the Sulu orogen. CL imaging suggests that the low δ18O zircons at Zaobuzhen are primarily of magmatic origin, but underwent different degrees of metamorphic modification. Zircon U-Pb dating yields middle Neoproterozoic ages of 751 ± 27 to 779 ± 25 Ma for protolith crystallization and Triassic ages of 214 ± 10 to 241 ± 33 Ma for metamorphic resetting. However, no metamorphic modification occurs in zircon REE patterns that only indicate magmatic recrystallization and hydrothermal alteration, respectively. Thus, the negative δ18O zircons are interpreted as crystallizing from negative δ18O magmas due to melting of meteoric-hydrothermally altered negative δ18O rocks in an active rift setting at about 780 Ma. The variation in zircon δ18O values indicates considerable O isotope heterogeneity in its granitic protolith. Zircon Lu-Hf isotope analyses give positive εHf(t) values of 1.6-4.1 and Hf model ages of 1.18-1.30 Ga. This suggests that the granitic protolith was derived from the mid-Neoproterozoic reworking of late Mesoproterozoic juvenile crust. The metagranites at Zaobuzhen and Qinglongshan, about 450 km apart, are two known occurrences of the unusually low δ18O zircons below −6‰ so far reported in the Sulu orogen. They are similar to each other in both protolith and metamorphic ages, so that they share the same nature of both Neoproterozoic protolith and Triassic metamorphism. Therefore, the locally negative δ18O zircons may register centers of low δ18O magmatism during the supercontinental rifting.  相似文献   

4.
This work considers petrogenesis of the largest Holocene basaltic fissure eruptions of Iceland, which are also the largest in the world: Laki (1783-84 AD, 15 km3), Eldgjá (934 AD, 18 km3), Veidivötn (900, 1480 AD, multiple eruptions, >2 km3), Núpahraun (ca. 4000 BP, >1 km3) and Thjórsárhraun (ca 8000 BP, >20 km3). We present oxygen isotope laser fluorination analyses of 55 individual and bulk olivine crystals, coexisting individual and bulk plagioclase phenocrysts, and their host basaltic glasses with average precision of better than 0.1‰ (1SD). We also report O isotope analyses of cores and rims of 61 olivine crystals by SIMS with average precision on single spots of 0.24‰ (1SD) in 13 samples coupled with electron microprobe data for major and trace elements in these olivines. Within each individual sample, we have found that basaltic glass is relatively homogeneous with respect to oxygen isotopes, plagioclase phenocrysts exhibit crystal to crystal variability, while individual olivines span from the values in equilibrium with the low-δ18O matrix glass to those being three permil higher in δ18O than the equilibrium. Olivine cores with maximum value of 5.2‰ are found in many of these basalts and suggest that the initial magma was equilibrated with normal-δ18O mantle. No olivines or their intracrystalline domains are found with bulk or spot value higher than those found in MORB olivines. The δ18O variability of 0.3-3‰ exists for olivine grains from different lavas, and variable core-to-rim oxygen isotopic zoning is present in selected olivine grains. Many olivines in the same sample are not zoned, while a few grains are zoned with respect to oxygen isotopes and exhibit small core-to-core variations in Fe-Mg, Ni, Mn, Ca. Grains that are zoned in both Mg# and δ18O exhibit positive correlation of these two parameters. Electron microprobe analysis shows that most olivines equilibrated with the transporting melt, and thin Fe-richer rim is present around many grains, regardless of the degree of olivine-melt oxygen isotope disequilibrium.The preservation of isotopic and compositional zoning in selected grains, and subtle to severe Δ18O (melt-olivine) and Δ18O (plagioclase-olivine) disequilibria suggests rather short crystal residence times of years to centuries. Synglacially-altered upper crustal, tufaceous hyaloclastites of Pleistocene age serve as a viable source for low-δ18O values in Holocene basalts through assimilation, mechanical and thermal erosion, and devolatilization of stoped blocks. Cumulates formed in response to cooling during assimilation, and xenocrysts derived from hyaloclastites, contribute to the diverse δ18O crystalline cargo. The magma plumbing systems under each fissure are likely to include a network of interconnected dikes and sills with high magma flow rates that contribute to the efficacy of magmatic erosion of large quantities (10-60% mass) of hyaloclastites required by isotopic mass balance.Olivine diversity and the pervasive lack of phenocryst-melt oxygen isotopic equilibrium suggest that a common approach of analyzing bulk olivine for oxygen isotopes, as a proxy for the basaltic melt or to infer mantle δ18O value, needs to proceed with caution. The best approach is to analyze olivine crystals individually and demonstrate their equilibrium with matrix.  相似文献   

5.
《Gondwana Research》2001,4(3):377-386
The Kerala Khondalite belt is a Proterozoic metasupracrustal granulite facies terrain in southern India comprising garnet-biotite gneiss, garnet-sillimanite gneiss and orthopyroxene granulites as major rock types. Calc-silicate rocks and marbles, occurring as minor lithologies in the Kerala Khondalite Belt, show different mineral assemblages and reaction histories of which indicate a metamorphic P-T-fluid history dominated by internal fluid buffering during the peak metamorphism, followed by external fluid influx during decompression. The carbon and oxygen isotopic compositions of calcite from three representative metacarbonate localities show contrasting evolutionary trends. The Ambasamudram marbles exhibit carbon and oxygen isotope ratios (δ13C ∼ 0‰ and δ18O ∼ 20‰) typical of middle to late Proterozoic marine carbonate sediments with minor variation ascribed to the isotopic exchange due to the devolatilization reactions. The δ13C and δ18O values of ∼ −9‰ and 11‰, respectively, for calcite from calc-silicate rocks at Nuliyam are considerably low and heterogeneous. The wollastonite formation here, possibly corresponds to an earlier event of fluid infiltration during prograde to peak metamorphism, which resulted in decarbonation and isotope resetting. Further, petrologic evidence supports a model of late carbonic fluid infiltration that has partially affected the calc-silicate rocks, with subsequent isotope resetting, more towards the contact between calc-silicate rock and charnockite. At Korani, only oxygen isotopes have been significantly lowered (δ18O ∼ 13‰) and the process involved might be a combination of metamorphic devolatilization accompanied by an aqueous fluid influx, supported by petrologic evidence. The stable isotope signatures obtained from the individual localities, thus indicate heterogeneous patterns of fluid evolution history within the same crustal segment.  相似文献   

6.
The Callovian-Oxfordian (COx) clayey unit is being studied in the Eastern part of the Paris Basin at depths between 400 and 500 m depth to assess of its suitability for nuclear waste disposal. The present study combines new mineralogical and isotopic data to describe the sedimentary history of the COx unit. Petrologic study provided evidence of the following diagenetic mineral sequence: (1) framboidal pyrite and micritic calcite, (2) iron-rich euhedral carbonates (ankerite, sideroplesite) and glauconite (3) limpid calcite and dolomite and celestite infilling residual porosity in bioclasts and cracks, (4) chalcedony, (5) quartz/calcite. Pyrite in bioturbations shows a wide range of δ34S (−38‰ to +34.5‰), providing evidence of bacterial sulphate reduction processes in changing sedimentation conditions. The most negative values (−38‰ to −22‰), measured in the lower part of the COx unit indicate precipitation of pyrite in a marine environment with a continuous sulphate supply. The most positive pyrite δ34S values (−14‰ up to +34.5‰) in the upper part of the COx unit indicate pyrite precipitation in a closed system. Celestite δ34S values reflect the last evolutionary stage of the system when bacterial activity ended; however its deposition cannot be possible without sulphate supply due to carbonate bioclast dissolution. The 87Sr/86Sr ratio of celestite (0.706872-0.707040) is consistent with deposition from Jurassic marine-derived waters. Carbon and oxygen isotopic compositions of bulk calcite and dolomite are consistent with marine carbonates. Siderite, only present in the maximum clay zone, has chemical composition and δ18O consistent with a marine environment. Its δ13C is however lower than those of marine carbonates, suggesting a contribution of 13C-depleted carbon from degradation of organic matter. δ18O values of diagenetic chalcedony range between +27‰ and +31‰, suggesting precipitation from marine-derived pore waters. Late calcite crosscutting a vein filled with chalcedony and celestite, and late euhedral quartz in a limestone from the top of the formation have lower δ18O values (∼+19‰), suggesting that they precipitated from meteoric fluids, isotopically close to present-day pore waters of the formation. Finally, the study illustrates the transition from very active, biotic diagenesis to abiotic diagenesis. This transition appears to be driven by compaction of the sediment, which inhibited movement of bacterial cells by reduction of porosity and pore sizes, rather than a lack of inorganic carbon or sulphates.  相似文献   

7.
Nine marble horizons from the granulite facies terrane of southern India were examined in detail for stable carbon and oxygen isotopes in calcite and carbon isotopes in graphite. The marbles in Trivandrum Block show coupled lowering of δ13C and δ18O values in calcite and heterogeneous single crystal δ13C values (? 1 to ? 10‰) for graphite indicating varying carbon isotope fractionation between calcite and graphite, despite the granulite facies regional metamorphic conditions. The stable isotope patterns suggest alteration of δ13C and δ18O values in marbles by infiltration of low δ13C–δ18O‐bearing fluids, the extent of alteration being a direct function of the fluid‐rock ratio. The carbon isotope zonation preserved in graphite suggests that the graphite crystals precipitated/recrystallized in the presence of an externally derived CO2‐rich fluid, and that the infiltration had occurred under high temperature and low fO2 conditions during metamorphism. The onset of graphite precipitation resulted in a depletion of the carbon isotope values of the remaining fluid+calcite carbon reservoir, following a Rayleigh‐type distillation process within fluid‐rich pockets/pathways in marbles resulting in the observed zonation. The results suggest that calcite–graphite thermometry cannot be applied in marbles that are affected by external carbonic fluid infiltration. However, marble horizons in the Madurai Block, where the effect of fluid infiltration is not detected, record clear imprints of ultrahigh temperature metamorphism (800–1000 °C), with fractionations reaching <2‰. Zonation studies on graphite show a nominal rimward lowering δ13C on the order of 1 to 2‰. The zonation carries the imprint of fluid deficient/absent UHT metamorphism. Commonly, calculated core temperatures are > 1000 °C and would be consistent with UHT metamorphism.  相似文献   

8.
Oxygen and iron isotope analyses of low-Ti and high-Ti mare basalts are presented to constrain their petrogenesis and to assess stable isotope variations within lunar mantle sources. An internally-consistent dataset of oxygen isotope compositions of mare basalts encompasses five types of low-Ti basalts from the Apollo 12 and 15 missions and eight types of high-Ti basalts from the Apollo 11 and 17 missions. High-precision whole-rock δ18O values (referenced to VSMOW) of low-Ti and high-Ti basalts correlate with major-element compositions (Mg#, TiO2, Al2O3). The observed oxygen isotope variations within low-Ti and high-Ti basalts are consistent with crystal fractionation and match the results of mass-balance models assuming equilibrium crystallization. Whole-rock δ56Fe values (referenced to IRMM-014) of high-Ti and low-Ti basalts range from 0.134‰ to 0.217‰ and 0.038‰ to 0.104‰, respectively. Iron isotope compositions of both low-Ti and high-Ti basalts do not correlate with indices of crystal fractionation, possibly owing to small mineral-melt iron fractionation factors anticipated under lunar reducing conditions.The δ18O and δ56Fe values of low-Ti and the least differentiated high-Ti mare basalts are negatively correlated, which reflects their different mantle source characteristics (e.g., the presence or absence of ilmenite). The average δ56Fe values of low-Ti basalts (0.073 ± 0.018‰, n = 8) and high-Ti basalts (0.191 ± 0.020‰, n = 7) may directly record that of their parent mantle sources. Oxygen isotope compositions of mantle sources of low-Ti and high-Ti basalts are calculated using existing models of lunar magma ocean crystallization and mixing, the estimated equilibrium mantle olivine δ18O value, and equilibrium oxygen-fractionation between olivine and other mineral phases. The differences between the calculated whole-rock δ18O values for source regions, 5.57‰ for low-Ti and 5.30‰ for high-Ti mare basalt mantle source regions, are solely a function of the assumed source mineralogy. The oxygen and iron isotope compositions of lunar upper mantle can be approximated using these mantle source values. The δ18O and δ56Fe values of the lunar upper mantle are estimated to be 5.5 ± 0.2‰ (2σ) and 0.085 ± 0.040‰ (2σ), respectively. The oxygen isotope composition of lunar upper mantle is identical to the current estimate of Earth’s upper mantle (5.5 ± 0.2‰), and the iron isotope composition of the lunar upper mantle overlaps within uncertainty of estimates for the terrestrial upper mantle (0.044 ± 0.030‰).  相似文献   

9.
High-precision measurements of the oxygen isotopic compositions of carbonates (calcite and dolomite) from five CM2 chondrites are presented and put into context of the previously determined mineralogic alteration index (MAI), which places these meteorites into an alteration sequence. The carbonate oxygen isotopic compositions range from +20.0 to +35.7‰ for δ18O, +8.0 to +17.7‰ for δ17O, and −0.7 to −2.7‰ for Δ17O. Carbonate Δ17O values are inversely correlated with MAI and track the evolution of fluid composition from higher to lower Δ17O values with increasing alteration on the CM parent body. Similar Δ17O values for calcite and dolomite fractions from the same splits of the same meteorites indicate that calcite and dolomite in each split precipitated from a single fluid reservoir. However, reversed calcite dolomite fractionations (δ18Odol − δ18Occ) indicate that the fluid was subject to processes, such as freeze-thaw or evaporation, that fractionated isotopes in a mass-dependent way. Consideration of the carbonate isotopic data in the context of previously proposed models for aqueous alteration of carbonaceous chondrites has provided important insights into both the evolving alteration conditions and the utility of the models themselves. The data as a whole indicate that the isotopic evolution of the fluid was similar to that predicted by the closed-system, two-reservoir models, but that a slightly larger matrix-water fractionation factor may apply. In the context of this model, more altered samples largely reflect greater reaction progress and thus probably indicate more extended times of fluid exposure. Petrographic observations of carbonates reveal a trend of variable carbonate morphology correlated with alteration that is also consistent with changes in the duration of fluid-rock interaction. The data can also be reconciled with fluid-flow models in a restricted region of the parent body, which is consistent with assertions that the different types of carbonaceous chondrites derive from different regions of their parent bodies. In this case, the model results for a 9-km-radius body, and our data place the location of the CM chondrite formation in a 100-m-thick zone 1 km from the surface. The size of this zone could be increased if the model parameters were adjusted.  相似文献   

10.
Here we present Sr, C, and O isotope curves for Ordovician marine calcite based on analyses of 206 calcitic brachiopods from 10 localities worldwide. These are the first Ordovician-wide isotope curves that can be placed within the newly emerging global biostratigraphic framework. A total of 182 brachiopods were selected for C and O isotope analysis, and 122 were selected for Sr isotope analysis. Seawater 87Sr/86Sr decreased from 0.7090 to 0.7078 during the Ordovician, with a major, quite rapid fall around the Middle-Late Ordovician transition, most probably caused by a combination of low continental erosion rates and increased submarine hydrothermal exchange rates. Mean δ18O values increase from −10‰ to −3‰ through the Ordovician with an additional short-lived increase of 2 to 3‰ during the latest Ordovician due to glaciation. Although diagenetic alteration may have lowered δ18O in some samples, particularly those from the Lower Ordovician, maximum δ18O values, which are less likely to be altered, increase by more than 3‰ through the Ordovician in both our data and literature data. We consider that this long-term rise in calcite δ18O records the effect of decreasing tropical seawater temperatures across the Middle-Late Ordovician transition superimposed on seawater δ18O that was steadily increasing from ≤−3‰ standard mean ocean water (SMOW). By contrast, δ13C variation seems to have been relatively modest during most of the Ordovician with the exception of the globally documented, but short-lived, latest Ordovician δ13C excursion up to +7‰. Nevertheless, an underlying trend in mean δ13C can be discerned, changing from moderately negative values in the Early Ordovician to moderately positive values by the latest Ordovician. These new isotopic data confirm a major reorganization of ocean chemistry and the surface environment around 465 to 455 Ma. The juxtaposition of the greatest recorded swings in Phanerozoic seawater 87Sr/86Sr and δ18O at the same time as one of the largest marine transgressions in Phanerozoic Earth history suggests a causal link between tectonic and climatic change, and emphasizes an endogenic control on the O isotope budget during the Early Paleozoic. Better isotopic and biostratigraphic constraints are still required if we are to understand the true significance of these changes. We recommend that future work on Ordovician isotope stratigraphy focus on this outstanding Middle-Late Ordovician event.  相似文献   

11.
The stable carbon and oxygen isotope compositions of fossil ostracods are powerful tools to estimate past environmental and climatic conditions. The basis for such interpretations is that the calcite of the valves reflects the isotopic composition of water and its temperature of formation. However, calcite of ostracods is known not to form in isotopic equilibrium with water and different species may have different offsets from inorganic precipitates of calcite formed under the same conditions. To estimate the fractionation during ostracod valve calcification, the oxygen and carbon isotope compositions of 15 species living in Lake Geneva were related to their autoecology and the environmental parameters measured during their growth. The results indicate that: (1) Oxygen isotope fractionation is similar for all species of Candoninae with an enrichment in 18O of more than 3‰ relative to equilibrium values for inorganic calcite. Oxygen isotope fractionation for Cytheroidea is less discriminative relative to the heavy oxygen, with enrichments in 18O for these species of 1.7 to 2.3‰. Oxygen isotope fractionations for Cyprididae are in-between those of Candoninae and Cytheroidea. The difference in oxygen isotope fractionation between ostracods and inorganic calcite has been interpreted as resulting from a vital effect. (2) Comparison with previous work suggests that oxygen isotope fractionation may depend on the total and relative ion content of water. (3) Carbon isotope compositions of ostracod valves are generally in equilibrium with DIC. The specimens’ δ13C values are mainly controlled by seasonal variations in δ13CDIC of bottom water or variation thereof in sediment pore water. (4) Incomplete valve calcification has an effect on carbon and oxygen isotope compositions of ostracod valves. Preferential incorporation of at the beginning of valve calcification may explain this effect. (5) Results presented here as well as results from synthetic carbonate growth indicate that different growth rates or low pH within the calcification site cannot be the cause of oxygen isotope ‘vital effects’ in ostracods. Two mechanisms that might enrich the 18O of ostracod valves are deprotonation of that may also contribute to valve calcification, and effects comparable to salt effects with high concentrations of Ca and/or Mg within the calcification site that may also cause a higher temperature dependency of oxygen isotope fractionation.  相似文献   

12.
Applications of speleothem calcite geochemistry in climate change studies require the evaluation of the accuracy and sensitivity of speleothem proxies to correctly infer paleoclimatic information. The present study of Harrison’s Cave, Barbados, uses the analysis of the modern climatology and groundwater system to evaluate controls on the C and O isotopic composition of modern speleothems. This new approach directly compares the δ18O and δ13C values of modern speleothems with the values for their corresponding drip waters in order to assess the degree to which isotopic equilibrium is achieved during calcite precipitation. If modern speleothems can be demonstrated to precipitate in isotopic equilibrium, then ancient speleothems, suitable for paleoclimatic studies, from the same cave environment may also have been precipitated in isotopic equilibrium. If modern speleothems are precipitated out of isotopic equilibrium, then the magnitude and direction of the C and O isotopic offsets may allow specific kinetic and/or equilibrium isotopic fractionation mechanisms to be identified.Carbon isotope values for the majority of modern speleothem samples from Harrison’s Cave fall within the range of equilibrium values predicted from the combined use of (1) calcite-water fractionation factors from the literature, (2) measured temperatures, and (3) measured δ13C values of the dissolved inorganic carbon of drip waters. Calcite samples range from ∼0.8‰ higher to ∼1.1‰ lower than predicted values. The 13C depletions are likely caused by kinetically driven departures in the fractionation between HCO3− (aq) and CaCO3 from equilibrium conditions, caused by rapid calcite growth. 13C enrichments can be accounted for by Rayleigh distillation of the HCO3− (aq) reservoir during degassing of 13C-depleted CO2.Modern speleothems from Harrison’s Cave are not in O isotopic equilibrium with their corresponding drip waters and are 0.2‰ to 2.3‰ enriched in 18O relative to equilibrium values. δ18O variations in modern calcite are likely controlled by kinetically driven changes in the fractionation between HCO3− (aq) and CaCO3 from equilibrium conditions to nonequilibrium conditions, consistent with rapid calcite growth. In contrast to δ13C, δ18O values of modern calcite may not be affected by Rayleigh distillation during degassing because CO2 hydration and hydroxylation reactions will buffer the O isotopic composition of the HCO3− (aq) reservoir. If the effects of Rayleigh distillation manifest themselves in the O isotopic system, they will result in 18O enrichment in the HCO3− (aq) reservoir and ultimately in the precipitated CaCO3.  相似文献   

13.
We report a study of the oxygen isotope ratios of chondrules and their constituent mineral grains from the Mokoia, oxidized CV3 chondrite. Bulk oxygen isotope ratios of 23 individual chondrules were determined by laser ablation fluorination, and oxygen isotope ratios of individual grains, mostly olivine, were obtained in situ on polished mounts using secondary ion mass spectrometry (SIMS). Our results can be compared with data obtained previously for the oxidized CV3 chondrite, Allende. Bulk oxygen isotope ratios of Mokoia chondrules form an array on an oxygen three-isotope plot that is subparallel to, and slightly displaced from, the CCAM (carbonaceous chondrite anhydrous minerals) line. The best-fit line for all CV3 chondrite chondrules has a slope of 0.99, and is displaced significantly (by δ17O ∼ −2.5‰) from the Young and Russell slope-one line for unaltered calcium-aluminum-rich inclusion (CAI) minerals. Oxygen isotope ratios of many bulk CAIs also lie on the CV-chondrule line, which is the most relevant oxygen isotope array for most CV chondrite components. Bulk oxygen isotope ratios of most chondrules in Mokoia have δ18O values around 0‰, and olivine grains in these chondrules have similar oxygen isotope ratios to their bulk values. In general, it appears that chondrule mesostases have higher δ18O values than olivines in the same chondrules. Our bulk chondrule data spread to lower δ18O values than any ferromagnesian chondrules that have been measured previously. Two chondrules with the lowest bulk δ18O values (−7.5‰ and −11.7‰) contain olivine grains that display an extremely wide range of oxygen isotope ratios, down to δ17O, δ18O around -50‰ in one chondrule. In these chondrules, there are no apparent relict grains, and essentially no relationships between olivine compositions, which are homogeneous, and oxygen isotopic compositions of individual grains. Heterogeneity of oxygen isotope ratios within these chondrules may be the result of incorporation of relict grains from objects such as amoeboid olivine aggregates, followed by solid-state chemical diffusion without concomitant oxygen equilibration. Alternatively, oxygen isotope exchange between an 16O-rich precursor and an 16O-poor gas may have taken place during chondrule formation, and these chondrules may represent partially equilibrated systems in which isotopic heterogeneities became frozen into the crystallizing olivine grains. If this is the case, we can infer that the earliest nebular solids from which chondrules formed had δ17O and δ18O values around -50‰, similar to those observed in refractory inclusions.  相似文献   

14.
The oxygen and hydrogen isotopic composition of Eocene and Miocene freshwater cherts in the western United States records regional climatic variation in the Cenozoic. Here, we present isotopic measurements of 47 freshwater cherts of Eocene and Miocene age from the Great Basin of the western United States at two different sites and interpret them in light of regional climatic and tectonic history. The large range of δ18O of terrestrial cherts measured in this study, from 11.2‰ to 31.2‰ (SMOW: Standard Mean Ocean), is shown to be primarily the result of variations in δ18O of surface water. The following trends and patterns are recognized within this range of δ18O values. First, in Cenozoic rocks of northern Nevada, chert δ18O records the same shift observed in authigenic calcite between the Eocene and Miocene that has been attributed to regional surface uplift. The consistent covariation of proxies suggests that chert reliably records and retains a signal of ancient meteoric water isotopic composition, even though our analyses show that chert formed from warmer waters (40°C) than coexisting calcite (20°C). Second, there is a strong positive correlation between δ18O and δD in Eocene age chert from Elko, Nevada and Salina, Utah that suggests large changes in lake water isotopic composition due to evaporation. Evaporative effects on lake water isotopic composition, rather than surface temperature, exert the primary control on the isotopic composition of chert, accounting for 10‰ of the 16‰ range in δ18O measured in Eocene cherts. From authigenic mineral data, we calculate a range in isotopic composition of Eocene precipitation in the north-central Great Basin of −10 to −14‰ for δ18O and −70 to −100‰ for δD, which is in agreement with previous estimates for Eocene basins of the western United States. Due to its resistance to alteration and record of variations in both δ18O and δD of water, chert has the potential to corroborate and constrain the cause of variations in isotope stratigraphies.  相似文献   

15.
We report high precision SIMS oxygen three isotope analyses of 36 chondrules from some of the least equilibrated LL3 chondrites, and find systematic variations in oxygen isotope ratios with chondrule types. FeO-poor (type I) chondrules generally plot along a mass dependent fractionation line (Δ17O ∼ 0.7‰), with δ18O values lower in olivine-rich (IA) than pyroxene-rich (IB) chondrules. Data from FeO-rich (type II) chondrules show a limited range of δ18O and δ17O values at δ18O = 4.5‰, δ17O = 2.9‰, and Δ17O = 0.5‰, which is slightly 16O-enriched relative to bulk LL chondrites (Δ17O ∼ 1.3‰). Data from four chondrules show 16O-rich oxygen isotope ratios that plot near the CCAM (Carbonaceous Chondrite Anhydrous Mineral) line. Glass analyses in selected chondrules are systematically higher than co-existing minerals in both δ18O and Δ17O values, whereas high-Ca pyroxene data in the same chondrule are similar to those in olivine and pyroxene phenocrysts.Our results suggest that the LL chondrite chondrule-forming region contained two kinds of solid precursors, (1) 16O-poor precursors with Δ17O > 1.6‰ and (2) 16O-rich solid precursors derived from the same oxygen isotope reservoir as carbonaceous chondrites. Oxygen isotopes exhibited open system behavior during chondrule formation, and the interaction between the solid and ambient gas might occur as described in the following model. Significant evaporation and recondensation of solid precursors caused a large mass-dependent fractionation due to either kinetic or equilibrium isotope exchange between gas and solid to form type IA chondrules with higher bulk Mg/Si ratios. Type II chondrules formed under elevated dust/gas ratios and with water ice in the precursors, in which the ambient H2O gas homogenized chondrule melts by isotope exchange. Low temperature oxygen isotope exchange may have occurred between chondrule glasses and aqueous fluids with high Δ17O (∼5‰) in LL the parent body. According to our model, oxygen isotope ratios of chondrules were strongly influenced by the local solid precursors in the proto-planetary disk and the ambient gas during chondrule melting events.  相似文献   

16.
《Geochimica et cosmochimica acta》1999,63(13-14):1981-1989
In order to better understand environmental factors controlling oxygen isotope shifts in autochthonous lacustrine carbonate sequences, we undertook an extensive one-year study (March, 1995 to February, 1996) of water-column chemistry and daily sediment trap material from a small lake in Central Switzerland. Comparisons between calculated equilibrium isotope values, using the fractionation equation of Friedman and O’Neil, (1977) and measured oxygen isotope ratios of calcite in the sediment-traps reveal that oxygen isotopic values of autochthonous calcite (δ18O) are in isotopic equilibrium with ambient water during most of the spring and summer, when the majority of the calcite precipitates. In contrast, small amounts of calcite precipitated in early-spring and again in late-autumn are isotopically depleted in 18O relative to the calculated equilibrium values, by as much as 0.8‰. This seasonally occurring apparent isotopic nonequilibrium is associated with times of high phosphorous concentrations, elevated pH (∼8.6) and increased [CO32−] (∼50 μmol/l) in the surface waters. The resulting weighted average δ18O value for the studied period is −9.6‰, compared with a calculated equilibrium δ18O value of −9.4‰. These data convincingly demonstrate that δ18O of calcite are, for the most part, a very reliable proxy for temperature and δ18O of the water.  相似文献   

17.
Here we calibrate the carbonate clumped isotope thermometer in modern deep-sea corals. We examined 11 specimens of three species of deep-sea corals and one species of a surface coral spanning a total range in growth temperature of 2-25 °C. External standard errors for individual measurements ranged from 0.005‰ to 0.011‰ (average: 0.0074‰) which corresponds to ∼1-2 °C. External standard errors for replicate measurements of Δ47 in corals ranged from 0.002‰ to 0.014‰ (average: 0.0072‰) which corresponds to 0.4-2.8 °C. We find that skeletal carbonate from deep-sea corals shows the same relationship of Δ47 (the measure of 13C-18O ordering) to temperature as does inorganic calcite. In contrast, the δ13 C and δ18O values of these carbonates (measured simultaneously with Δ47 for every sample) differ markedly from equilibrium with seawater; i.e., these samples exhibit pronounced ‘vital effects’ in their bulk isotopic compositions. We explore several reasons why the clumped isotope compositions of deep-sea coral skeletons exhibit no evidence of a vital effect despite having large conventional isotopic vital effects.  相似文献   

18.
In the Schwarzwald area, southwest Germany, more than 400 hydrothermal veins hosting different gangue and ore mineral assemblages cross-cut the crystalline basement rocks. Many of the post-Variscan fluorite-barite-quartz veins are considered to have precipitated through mixing of a deep saline brine with meteoric, low salinity waters. This hypothesis was tested using carbon, sulfur, and oxygen isotope data of sulfides, sulfates and calcite, coupled with fluid inclusion studies. Primary hydrothermal calcites from the deposits show a positive correlation of their δ13C (V-PDB) and δ18O (V-SMOW) values, which range from −12 to −3‰ and from 12 to 18.5‰, respectively. Carbon and oxygen isotope compositions of paragenetically young, remobilized calcite types are shifted towards higher values and range from −12 to −1‰ and from 20 to 25‰, respectively. We developed an improved calculation procedure for modeling the covariation of carbon and oxygen isotopes in calcite resulting from mixing of two fluids with different isotopic compositions and total carbon concentrations. In our model, the carbon speciation in the two model fluid end-members and the fluid mixtures are calculated using a speciation and reaction path code. The carbon and oxygen isotope covariation of primary Schwarzwald calcites can effectively be modeled by a mixing trend of a deep saline brine and a meteoric, low salinity water. Sulfur isotope data of barites from 44 hydrothermal fluorite-barite-quartz veins vary from 9 to 18‰ (CDT), sulfide ore minerals show δ34S values between −14.4 and 2.9‰. Calculated sulfide-sulfate equilibrium temperatures are in the range between 300 and 350 °C. These temperatures differ significantly from the formation temperatures of 150 to 200 °C of most of the deposits as estimated from fluid inclusions, and are interpreted as preserved paleotemperatures of the deep aquifer. This assumption has been carefully checked against possible contamination of an equilibrated sulfide-sulfate system from the deep aquifer with sulfate from surface-derived sources, considering also the kinetics of the sulfide-sulfate isotope exchange. A combination of the S isotopic results with microthermometric fluid inclusion data and constraints on the temperature of the meteoric water was used to calculate mixing ratios of the two fluid end-members. The results indicate that mass fractions of the deep saline brine in the mixed fluid were between 0.5 and 0.75. Considering all geologic, geochemical and isotopic information, we propose that the majority of the post-Variscan hydrothermal veins in the Schwarzwald area were precipitated by district-scale mixing of a homogeneous deep saline brine with meteoric waters.  相似文献   

19.
Oxygen isotopic compositions of silicates in eclogites and whiteschists from the Kokchetav massif were analyzed by whole‐grain CO2‐laser fluorination methods. Systematic analyses yield extremely low δ18O for eclogites, as low as ?3.9‰ for garnet; these values are comparable with those reported for the Dabie‐Sulu UHP eclogites. Oxygen isotopic compositions are heterogeneous in samples of eclogite, even on an outcrop scale. Schists have rather uniform oxygen isotope values compared to eclogites, and low δ18O is not observed. Isotope thermometry indicates that both eclogites and schists achieved high‐temperature isotopic equilibration at 500–800 °C. This implies that retrograde metamorphic recrystallization barely modified the peak‐metamorphic oxygen isotopic signatures. A possible geological environment to account for the low‐δ18O basaltic protolith is a continental rift, most likely subjected to the conditions of a cold climate. After the basalt interacted with low δ18O meteoric water, it was tectonically inserted into the surrounding sedimentary units prior to, or during subduction and UHP metamorphism.  相似文献   

20.
Using secondary ion mass spectrometry (SIMS) we looked at the natural variability in the oxygen isotope ratio of the shallow water, symbionts-bearing foraminiferan Amphistegina lobifera. Live foraminifera were collected in February 2005 in the Gulf of Eilat, Israel. Vertical section exposing the knob area of this species represents the growth history of this species from August 2004 to February 2005. SIMS profile at a resolution of ∼15 μm (representing about 2 weeks considering the size of the knob area and the life span of ≈6 months of this foraminifera species) yielded δ18O changes of ∼1.5‰ that are compatible with the known temperature changes for the Gulf of Eilat for this period (21-27 °C). Natural variability between primary and secondary calcite at the knob area were obtained on horizontal section of the upper knob area. This section is semi-tangential to the growth lines and exposes relatively wide belts of the primary calcite which could be analysed using the SIMS (beam size of 10 × 20 μm). The primary calcite δ18O value is on average more than 3‰ lower than the secondary calcite that represents the bulk of the skeleton (more than 95% by weight). A vertical profile at the knob was obtained by rastering an area of 50 × 50 μm at vertical steps of roughly 1 μm. The profile revealed a narrow zone of lower δ18O compared to the higher values above and below it. The difference between the lowest δ18O and the highest one was also close to 2‰. The δ18O in the margin - keel area of A. lobifera is also lower compared to the bulk secondary calcite. Specimens that were cultured in the laboratory at a constant temperature and inorganic carbon but at different pH have increased their CaCO3 weight by roughly a factor of 8. Single specimen from each pH (ranging between 7.90 and 8.45) were investigated with the SIMS at the knob area. While there is some variability within each specimen (perhaps related to the primary calcite), the general trend was a decrease in δ18O with increasing pH (or CO32− concentration), in agreement with previous studies on planktonic foraminifera. Some other specimens grown at different temperatures (between 21 and 33 °C) were also measured with the SIMS at the knob area. For each temperature, we observed also some variability, nevertheless the trend of −0.2‰/°C in δ18O is observed.  相似文献   

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