Hydrogen,oxygen and carbon isotopic evidence for the origin of rodingites in serpentinized ultramafic rocks     

David B. Wenner 《Geochimica et cosmochimica acta》1979,43(4):603-614

$\text{D}\text{H}$, ${}^{18}\text{O}{}^{16}\text{O}$ and ${}^{13}\text{C}{}^{12}\text{C}$ analyses were made of 14 whole rock and 28 mineral samples of rodingites associated dominantly with lizardite-chrysotile serpentinites from the West Coast of the U.S.A., New Zealand, and the Northern Appalachian Mtns. The δD values of the rodingite minerals are in three groupings: 5 monomineralic veins of pectolite, ?281 to ?429; 8 monomineralic veins of xonotlite, ?112 to ?135; all other minerals, including hydrogarnet, idocrase, prehnite, actinolite, nephrite, and chlorite, ?34 to ?80. Most calcites in rodingites have δ¹⁸O (+9.3 to +14.4) and (δ¹³C (?6.7 to +0.9) values similar to calcites in other Franciscan rocks, but distinct from the very low temperature calcite veins in serpentinites. The $\text{D}\text{H}$ data, combined with δ¹⁸O values of xonotlite (+5.7 to +10.9) and pectolite (+8.9 to +12.4) suggest formation from meteoric-type waters at low temperatures; the $\text{D}\text{H}$ depletion of pectolite, however, is anomalous. Rodingite whole rock values range from δ¹⁸O = +4.1 to +11.5 and δD = ?50 to ?86; one sample containing minor amounts of lizardite-chrysotile serpentinite has δD = ?92, outside this range. However, most rodingites of basaltic or gabbroic parentage are more restricted in δ¹⁸O (+4.1 to +8.6). Such a wide range in δ¹⁸O is consistent with the idea that most rodingites form over a relatively broad range of hydrothermal temperatures. Hydrogen isotopic data for most rodingite minerals (except xonotlite and pectolite) and for whole rocks are suggestive of non-meteoric waters. These $\text{D}\text{H}$ data overlap those observed for veins of hydrous minerals found in Franciscan igneous rocks studied by Margaritz and Taylor (1976, Geochim. Cosmochim. Acta40, 215–234), possibly suggesting evolved D-enriched, connate type metamorphic waters generated during high P, low T Franciscan-type metamorphism at temperatures (250–500°C) comparable to estimates based on mineral stabilities. Such an interpretation is supported by the ${}^{18}\text{O}{}^{16}\text{O}$ and ${}^{13}\text{C}{}^{12}\text{C}$ data for calcite in rodingites.The isotope data appear to contradict some of the conclusions derived from geologic and petrologic studies that indicate concomitant metasomatism and serpentinization of their presently observed host rock. These data appear most consistent with the interpretation that most rodingite minerals, with the exception of late-stage veins of xonotlite and possibly pectolite, may involve metasomatism in association with antigorite serpentinization of ultramafic rock. Subsequent upward tectonic transport in many instances may result in incorporation of the rodingites into their presently observed lizarditechrysotile host rock during or subsequent to pervasive shallow level serpentinization by meteoric waters.  相似文献   

Isotopic abundances of water of crystallization of gypsum from the Miocene evaporite formation,Carpathian Foredeep,Poland     

S. Hałas  H.R. Krouse 《Geochimica et cosmochimica acta》1982,46(2):293-296

For sulfates of Miocene evaporites in the Carpathian Foredeep, the waters of crystallization of gypsum (w.c.g.) have $\text{δD = ?38 to ?113\%.}$ and $\text{δ}{}^{18}\text{O = 0 to ?11\%.}$ (SMOW). The $\text{δ}{}^{34}\text{S}$ and $\text{δ}{}^{18}\text{O}$ values of the sulfates are uniform and consistent with a marine origin. It is proposed that the original w.c.g. was equilibrated with marine water. Subsequently, it re-equilibrated towards very isotopically light water (δD ～ ?100%., δ¹⁸O ～ ?14%) during a glacial or postglacial period and is now trending towards current waters circulating through the deposits (δD ～ ?50%., δ¹⁸ ～ ?7%). The extent of reequilibration increased with decreasing crystal size.  相似文献   

Hydrogen and carbon isotopes of petroleum and related organic matter     

Hsueh-Wen Yeh  Samuel Epstein 《Geochimica et cosmochimica acta》1981,45(5):753-762

$\text{D}\text{H}$ and ${}^{13}\text{C}{}^{12}\text{C}$ ratios were measured for 114 petroleum samples and for several samples of related organic matter. δD of crude oil ranges from ?85 to ?181‰, except for one distillate (?250‰) from the Kenai gas field; δ¹³C of crude oil ranges from ?23.3 to ?32.5‰, Variation in δD and δ¹³C values of compound-grouped fractions of a crude oil is small, 3 and 1.1%., respectively, and the difference in δD and δ¹³C between oil and coeval wax is slight. Gas fractions are 53–70 and 22.6–23.2‰ depleted in D and ¹³C, respectively, relative to the coexisting oil fractions.The δD and δ¹³C values of the crude oils appear to be largely determined by the isotopic compositions of their organic precursors. The contribution of terrestrial organic debris to the organic precursors of most marine crude oils may be significant.  相似文献   

A stable isotope study of serpentinization and metamorphism in the Highland Border Suite,Scotland, U.K.     

Nicholas P Ikin  Russell S Harmon 《Geochimica et cosmochimica acta》1983,47(2):153-167

$\text{D}\text{H}$ and ${}^{18}\text{O}{}^{16}\text{O}$ ratios have been measured for whole-rock samples and mineral separates from the mafic and ultramatic rocks of the Cambro-Ordovician Highland Border Suite. The H- and O- isotopic compositions of these rocks record individual stages in a relatively complex 500 Myr old hydrothermal/metamorphic history. Lizardite serpentinites (δD ～ ? 105‰; δ¹⁸O ～ + 6.2‰) record a premetamorphic history and indicate that parent harzburgites, dunites, and pyroxenites were serpentinized through low-temperature interaction with meteoric waters during cooling. The other rocks of the Highland Border Suite record subsequent interaction with metamorphic fluids. Amphibolite facies hornblende schists were produced through thrust-related (dynamothermal) metamorphism of spilitic pillow lavas. During dehydration, D-enriched fluids were driven off from the spilites thus leaving the hornblende schists to equilibrate with a relatively D-depleted internal fluid reservoir (δD ～ ? 45‰). The expelled D-enriched fluids may have mixed with more typical Dalradian metamorphic waters which then exchanged with the remaining mafic rocks and lizardite serpentinites during greenschist facies regional metamorphism to produce antigorite serpentinites (δD ～ ? 62‰; δ¹⁸O ～ + 8‰) and greenschist metaspilites (δD ～ ? 57‰; δ¹⁸O ～ + 7.3‰) with similar H- and O-isotopic compositions. Serpentinites which have been only partially metamorphosed show intermediate H-isotopic compositions between that of metamorphic antigorite (δD ～ ? 62‰) and non-metamorphic lizardite δD ～ ? 105‰) end members.  相似文献   

The geochemistry of the stable isotopes of silicon   总被引：1，自引：0，他引：1  

C.B Douthitt 《Geochimica et cosmochimica acta》1982,46(8):1449-1458

One hundred thirty two new measurements of the relative abundances of the stable isotopes of silicon in terrestrial materials are presented. The total variation of ^δ30Si found is 6.2%., centered on the mean of terrestrial mafic and ultramafic igneous rocks, δ³⁰Si = ?0.4%.. Igneous rocks show limited (1.1%.) variation; coexisting minerals exhibit small, systematic silicon isotopic fractionations that are roughly $\text{1}\text{3}$ the magnitude of concomitant oxygen isotopic fractionations at 1150°C. In both igneous minerals and rocks, δ³⁰Si shows a positive correlation with silicon content, as does δ¹⁸O. Opal from both sponge spicules and sinters is light, with $\text{}\text{?}\text{gd}{}^{30}\text{Si}\text{= ?2.3}$ and ?1.4%., respectively. Large ^δ30Si values of both positive and negative sign are reported for the first time from clay minerals (?2.3 to +1.8%.), opaline phytoliths (?1.4 to +2.8%.), and authigenic quartz (+ 1.4%.). All highly fractionated samples were precipitated from solution at low temperatures; however, aqueous silicon is not measurably fractionated relative to quartz at equilibrium. A kinetic isotope fractionation of ≈3.5%. is postulated to occur during the low temperature precipitation of opal and, possibly, poorly ordered phyllosilicates, with the silicate phase being enriched in ²⁸Si. This fractionation, coupled with a Rayleigh precipitation model, is capable of explaining most non-magmatic δ³⁰Si variations. Chert ^δ30Si values are largely inherited, but the primary opal ^δ30Si values can be modified by isotopic equilibration of silicate silicon and dissolved silicon during the transformation of opal into quartz.  相似文献   

The geochemical evolution of the Pleistocene Lake Lisan-Dead Sea system     

Amitai Katz  Yehoshua Kolodny  Arie Nissenbaum 《Geochimica et cosmochimica acta》1977,41(11):1609-1626

The geochemical history of Lake Lisan, the Pleistocene precursor of the Dead Sea, has been studied by geological, chemical and isotopic methods.Aragonite laminae from the Lisan Formation yielded (equivalent) Sr/Ca ratios in the range 0.5 × 10^?2?1 × 10^?2, Na/Ca ratios from 3.6 × 10^?3 to 9.2 × 10^?3, $\text{δ}{}^{18}\text{O}{}_{\mathrm{PDB}}$ values between 1.5 and 7%. and $\text{δ}{}^{13}\text{C}{}_{\mathrm{PDB}}$ from ?7.7 to 3.4%..The distribution coefficient of Na⁺ between aragonite and aqueous solutions, $\text{λ}{}^{\mathrm{A}}{}_{\mathrm{Na}}$, is experimentally shown to be very sensitive to salinity and nearly temperature independent. Thus, Na/Ca in aragonite serves as a paleosalinity indicator.Sr/Ca ratios and $\text{δ}{}^{18}\text{O}$ values in aragonite provide good long-term monitors of a lake's evolution. They show Lake Lisan to be well mixed, highly evaporated and saline. Except for a diluted surface layer, the salinity of the lake was half that of the present Dead Sea (15 vs 31%).Lake Lisan evolved from a small, yet deep, hypersaline Dead Sea-like, water body. This initial lake was rapidly filled-up to its highest stand by fresh waters and existed for about 40,000 yr before shrinking back to the present Dead Sea. The chemistry of Lake Lisan at its stable stand represented a material balance between a Jordan-like input, an original large mass of salts and a chemical removal of aragonite. The weighted average depth of Lake Lisan is calculated, on a geochemical basis, to have been at least 400, preferably 600 m.The oxygen isotopic composition of Lake Lisan water, which was higher by at least 3%. than that of the Dead Sea, was probably dictated by a higher rate of evaporation.Na/Ca ratios in aragonite, which correlate well with $\text{δ}{}^{13}\text{C}$ values, but change frequently in time, reflect the existence of a short lived upper water layer of varying salinity in Lake Lisan.  相似文献   

O18O16 of feldspars in carbonate rocks     

J.R Lawrence  M Kastner 《Geochimica et cosmochimica acta》1975,39(2):97-102

Six authigenic feldspars and three detrital feldspars in limestones and dolostones of Eocene to Preeambrian ages were analyzed for their $\text{O}{}^{18}\text{O}{}^{16}$ content. The difference in δO¹⁸ between the authigenic feldspars (δO¹⁸range = + 18.2 to + 24.7%.) and carbonate host rocks, both limestones and dolostones, was found to be ?0.5 to ?1.4%. Detrital feldspars (δO¹⁸ = + 11.2, + 22.5 and + 17.0%.) exhibit Δ^feldspar_carbonate values of ?12.0, ?2.4 and ?1.6‰, respectively, and appear to have undergone increased isotopic exchange as a function of decreased grain size under solid-state conditions.  相似文献   

Geochimie et teneurs isotopiques des systèmes saisonniers de dissolution de la calcite dans un sol sur craie     

L. Dever  R. Durand  J.Ch. Fontes  P. Vachier 《Geochimica et cosmochimica acta》1982,46(10):1947-1956

In a soil developed on the Cretaceous chalk of the Eastern Paris basin, calcite dissolution begins at the surface. The soil water is rapidly saturated in calcite. Calcite dissolution follows two different pathways according to seasonal pedoclimatic conditions.During winter: the soil is only partly saturated in water and the CO₂ partial pressure is low (Ca 10^?3 atm.). As a consequence total inorganic dissolved carbon (TIDC) is a hundred times the carbon content of the gaseous phase. Equilibrium is usually observed between the two phases. It is a closed system. The measured carbon 14 activity (87,5%) and ¹³C content ($\text{δ}{}_{\mathrm{tidc}}{}^{13}\text{C = ?12,2\%0}$) of the drainage water are very close to theoretical values calculated for an ideal mixing system between gaseous and mineral phases (respectively characterized by the following isotopic values: $\text{δ}{}_{\mathrm{G}}{}^{13}\text{C = ?21,5\%0}$; $\text{A}{}_{\mathrm{G}}{}^{14}\text{C = 118\%}$; $\text{δ}{}_{\mathrm{M}}{}^{13}\text{C = +2,9\%0}$; $\text{A}{}_{\mathrm{M}}{}^{14}\text{C = 28\%}$).During spring and summer: the soil moisture decreases, the input of biogenic CO₂ induces an increase of the soil CO₂ partial pressure (Ca from 3.10^?3 atm to 7.10^?3 atm). The carbon content of the gaseous phase is higher by an order of magnitude compared to winter conditions. Therefore the aqueous phase is undersaturated in CO₂ with respect to the latter. This disequilibrium occurs as a result of unbalanced rates of CO₂ dissolution and CO₂ effusion toward atmosphère. It is an open system. The carbon isotopic ratio of the aqueous phase is regulated by that of the gaseous phase, as demonstrated by the agreement between measured and calculated isotopic compositions (respectively δ_{L mes} = from ?9,4%0 to ?11,5%0, δ_{l calc} = from ?9,8%0 to ?13,9%0 A_{L mes} = 119%, A_{L calc} = from 119% to 125%).The solutions originating from both systems (open and closed) move downwards without significant mixing together. It has also been observed that no significant variation of the TIDC isotopic composition occurs during precipitation of secondary calcite.  相似文献   

Fractionation of stable carbon isotopes by marine phytoplankton     

William W. Wong  William M. Sackett 《Geochimica et cosmochimica acta》1978,42(12):1809-1815

Stable carbon isotope fractionation by seventeen species of marine phytoplankton, representing the classes of Bacillariophyceae, Chlorophyceae, Prasinophyceae, Chrysophyceae, Haptophyceae and Dinophyceae have been determined in laboratory culture experiments using bicarbonate enriched artificial sea water. The values (Δ_HCO3^? = δ¹³C of algae vs HCO₃^?) range from ?22.1 to ?35.5%. Nitzschia closterium shows the smallest fractionation of ? 22.1% and Isochrysis galbana, the greatest of ?35.5%,. Since these algae were cultured under identical laboratory conditions, the wide range of Δ_HCO3^? values is seemingly due to the presence of different metabolic pathways within these organisms.A temperature dependent fractionation of 0.36% per °C with decreasing temperatures was measured for Skeletonema costatum whereas, smaller temperature dependencies of ?0.13, +0.15 and ?0.07%. per °C were observed for Dunaliella sp., Monochrysis lutheri and Glenodinium foliaceum, respectively.The consistency of values of Skeletonema costatum, Dunaliella sp. and Monochrysis lutheri grown at salinities of 22, 26, 32 and 36% indicates that natural salinity variations have negligible effects on the isotopic composition of marine phytoplankton.  相似文献   

Annual periodicity of the 18O16O and 13C12C ratios in the coral Montastrea annularis     

Richard G. Fairbanks  Richard E. Dodge 《Geochimica et cosmochimica acta》1979,43(7):1009-1020

The isotopic ratios ${}^{18}\text{O}{}^{16}\text{O}$ and ${}^{13}\text{C}{}^{12}\text{C}$ show an annual periodicity in the coral Montastrea annularis from Bermuda, Jamaica and Barbados. The abundances of ¹⁸O and ¹³C are positively correlated in the Jamaica and Barbados samples, but inversely related in the Bermuda sample. Annual high density growth bands are formed during the season of warmest water temperature at all 3 sites and are enriched in ¹⁶O. M. Annularis has a constant displacement from oxygen isotopic equilibrium and accurately records seasonal temperature variations via the temperature-dependent aragonite-water fractionation factor. Light intensity, through the activity of the coral's endosymbiotic algae, regulates the depth-dependent and seasonal variations in the skeletal carbon isotopic composition.  相似文献   

High-¹⁸O granitic plutons from the Frontenac Axis,Grenville province of ontario,Canada     

Yuch-Ning Shieh 《Geochimica et cosmochimica acta》1985,49(1):117-123

  相似文献   

Experimental observations on carbon isotope exchange in carbonate-water systems     

A.A. Mozeto  P. Fritz  E.J. Reardon 《Geochimica et cosmochimica acta》1984,48(3):495-504

This study presents data from experiments investigating carbon isotope exchange between carbonate solution and solid calcite using carbon-13 as a tracer. All experiments were done with calcite saturated solutions and results show that a two-step adsorption-recrystallization reaction takes place. Isotope effects are caused by exchange by carbonate on the solid surface with carbon in the aqueous phase. Adsorption reactions are characterized by a maximum isotopic exchange capacity (IEC) on crystal surfaces of about 10¹¹ reaction sites per cm², following a second order rate law with respect to ¹³C concentration in solution ($\text{constant}\text{kex ? 10}{}^6\text{cm}{}^5\text{mole}{}^{\mathrm{?1}}\text{s}{}^{\mathrm{?1}}$ and $\text{half-life}\text{t}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{= 700}\text{s}$). The adsorption reaction was followed by a first order recrystallization which is characterized by a rate constant of the order of 10^?8 s^?1 and a $\text{t}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ of 10⁷ s. Negative isotopic gradient experiments and runs with calcite crystals in Mg²⁺ spiked solutions provided the preliminary basis for the characterization of the mechanisms of both proposed reactions.  相似文献   

Sedimentary iron monosulfides: Kinetics and mechanism of formation     

Albert J Pyzik  Sheldon E Sommer 《Geochimica et cosmochimica acta》1981,45(5):687-698

The reaction between hydrous iron oxides and aqueous sulfide species was studied at estuarine conditions of pH, total sulfide, and ionic strength to determine the kinetics and formation mechanism of the initial iron sulfide. Total, dissolved and acid extractable sulfide, thiosulfate, sulfate, and elemental sulfur were determined by spectrophotometric methods. Polysulfides, S₄^2? and S₅^2?, were determined from ultraviolet absorbance measurements and equilibrium calculations, while product hydroxyl ion was determined from pH measurements and solution buffer capacity.Elemental sulfur, as free and polysulfide sulfur, was 86% of the sulfide oxidation products; the remainder was thiosulfate. Rate expressions for the reduction and precipitation reactions were determined from analysis of electron balance and acid extractable iron monosulfide vs time, respectively, by the initial rate method. The rate of iron reduction in moles/liter/minute was given by $\text{d(}\text{reduction}\text{Fe)}\text{dt}\text{= kS}{}_{\mathrm{t}}{}^{0.5}\text{(J}{}^{\mathrm{+}}\text{)}{}^{0.5}\text{A}{}_{\mathrm{FeOOH}}{}^1$ where S_t was the total dissolved sulfide concentration, (H⁺) the hydrogen ion activity, both in moles/ liter; and A_FeOOH the goethite specific surface area in square meters/liter. The rate constant, k, was 0.017 ± 0.002m^?2 min^?1. The rate of reduction was apparently determined by the rate of dissolution of the surface layer of ferrous hydroxide. The rate expression for the precipitation reaction was $\text{d(FeS)}\text{dt}\text{= kS}{}_{\mathrm{t}}{}^1\text{(H}{}^{\mathrm{+}}\text{)}{}^1\text{A}{}_{\mathrm{FeOOH}}{}^1$ where $\text{d(FeS)}\text{dt}$ was the rate of precipitation of acid extractable iron monosulfide in moles/liter/minute, and k = 82 ± 18 mol^?1l²m^?2 min^?1.A model is proposed with the following steps: protonation of goethite surface layer; exchange of bisulfide for hydroxide in the mobile layer; reduction of surface ferric ions of goethite by dissolved bisulfide species which produces ferrous hydroxide surface layer elemental sulfur and thiosulfate; dissolution of surface layer of ferrous hydroxide; and precipitation of dissolved ferrous specie and aqueous bisulfide ion.  相似文献   

The marine oxygen isotope record in Pleistocene coral,Barbados, West Indies     

Richard G. Fairbanks  R.K. Matthews 《Quaternary Research》1978,10(2):181-196

The reef-crest coral Acropora palmata from late Pleistocene reefs on Barbados has recorded the same global variations in oxygen isotopes as planktonic and benthonic foraminifera. Although the record of oxygen isotopes in Acropora palmata is discontinuous, it offers several advantages over the isotope records from deep-sea sediments: (1) the coral grows at water depths of less than 5 m; (2) the samples are unmixed; (3) specimens may be sampled from various elevations of paleo-sea level; and (4) aragonitic corals are suitable for ${}^{230}\text{Th}{}^{234}\text{U}$ and $\text{He}\text{U}$ dating techniques. The latter advantage means that direct dating of the marine oxygen isotope record is possible. Oxygen isotope stage 5e corresponds to Barbados III, dated at 125,000 ± 6000 yr BP. Petrographic and geochemical evidence from five boreholes drilled into the south coast of Barbados indicates a major eustatic lowering (greater than 100 m below present sea level) occurred between 180,000 and 125,000 yr BP. The age and isotopic data suggest correlation of this change in sea level to Emiliani's oxygen isotope stage 6. Acropora palmata deposited at various elevations of sea level during oxygen isotope stage 6 vary by 0.11 ‰ δ¹⁸O for each 10 m of change in sea level. We further hypothesize a minimum drop of 2°C in the average temperature occurred during the regressive phase of oxygen isotope stage 6. These data indicate that temperature lowering of surface water near Barbados lagged behind a major glacial buildup during this time period. Using the δ¹⁸O vs sea level calibration herein derived, we estimate the relative height of sea stands responsible for Barbados coral reef terraces in the time range 80,000 to 220,000 yr BP.  相似文献   

Etude systématique des isotopes de l'oxygène,du carbone et du strontium dans les carbonatites     

Françoise Pineau  Marc Javoy  Claude J Allegre 《Geochimica et cosmochimica acta》1973,37(11):2363-2377

${}^{18}\text{O}{}^{16}\text{O}$, ${}^{13}\text{C}{}^{12}\text{C}$ and ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios have been measured on the same samples for carbonatite complexes. The results show that besides the ‘carbonatite box’ of Tayloret al. (1967) there exist higher δ¹⁸O and δ¹³C values than can be explained by late magmatic or deuteric processes. These processes correspond to high concentrations of CO₂ and lead to big enrichments in ¹⁸O and ¹³C as well as in some ‘volatile’ elements. Strontium results are consistent with a model of selective contamination of deep-seated material by highly radiogenic strontium. The whole study leads to the opinion that parent magmas of carbonatites differentiated in a crustal environment with or without significant contamination.  相似文献   

Source rock-crude oil correlation by isotopic type-curves     

W.J. Stahl 《Geochimica et cosmochimica acta》1978,42(10):1573-1577

An isotopic type-curve has been defined based on the ${}^{13}\text{C}{}^{12}\text{C}$ ratios of the saturated, aromatic, heterocomponent (NOSs), and asphaltene fractions of crude oils. These fractions show ¹³C enrichments with increasing polarity or polarizability. This systematic pattern can be used to estimate the ${}^{13}\text{C}{}^{12}\text{C}$ ratio of the kerogen from which the oil had been generated. Genetically associated source rock oil pairs have been used to show that the difference between the measured and the estimated δ-values of kerogen is about ?0.5%., and between the δ-values of the kerogen and the asphaltene fraction is approximately +0.6%.  相似文献   

Stable isotopic investigations of early development in extant and fossil chambered cephalopods I. Oxygen isotopic composition of eggwater and carbon isotopic composition of siphuncle organic matter in Nautilus     

Kimberley C. Crocker  Michael J. DeNiro  Peter D. Ward 《Geochimica et cosmochimica acta》1985,49(12):2527-2532

Eggwaters from the chambered cephalopod Nautilus are depleted in both ¹⁸O and deuterium relative to ambient seawater. Eggwaters from six other species, including the related chambered cephalopod Sepia, do not show such depletion. These observations indicate that the previously observed step towards more positive $\text{δ}{}^{18}\text{O}$ values in calcium carbonate laid down after Nautilus hatches, relative to carbonate precipitated prior to hatching, can be explained by equilibration of the carbonate with water in the egg before hatching and with seawater after hatching. The presence of an oxygen isotope difference between eggwater and seawater for Nautilus and its absence for Sepia suggest that hatching will be recorded in the $\text{δ}{}^{18}\text{O}$ values of shell carbonates for some but not all extinct and extant chambered cephalopods.The $\text{δ}{}^{13}\text{C}$ values of the organic fraction of the siphuncle in Nautilus do not show any consistent pattern with regard to the time of formation before or after hatching. This observation suggests that the minimum in $\text{δ}{}^{13}\text{C}$ values previously observed for calcium carbonate precipitated after Nautilus hatches is not caused by a change in food sources once the animal becomes free-swimming, as has been suggested.  相似文献   

On the observation of 92Nb and 94Nb in nature     

K.E Apt  J.D Knight  D.C Camp  E.W Perkins 《Geochimica et cosmochimica acta》1974,38(9):1485-1488

Radioactivity measurements have shown evidence for long-lived ⁹²Nb and $\text{2.03 × 10}{}^4\text{yr}{}^{94}\text{Nb}$ in natural niobium. The specific activity of ⁹⁴Nb was observed to be 0.32 ± 0.03 dis/min. kg Nb and that of ⁹²Nb to be 0.058 ± 0.035 dis/min. kg Nb. With $\text{t}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ taken as ≈ 1.7 × 10⁸yr, the isotopic abundance of ⁹⁸Nb is 1.2 × 10^?10 per cent.  相似文献   

A corundum-rich inclusion in the Murchison carbonaceous chondrite     

Miryam Bar-Matthews  Ian D Hutcheon  Glenn J MacPherson  Lawrence Grossman 《Geochimica et cosmochimica acta》1982,46(1):31-41

A corundum-hibonite inclusion, BB-5, has been found in the Murchison carbonaceous chondrite. This is the first reported occurrence of corundum as a major phase in any refractory inclusion, even though this mineral is predicted by thermodynamic calculations to be the first condensate from a cooling gas of solar composition. Ion microprobe measurements of Mg isotopic compositions yield the unexpected result for such an early condensate that ²⁶Mg excesses are small: δ_N²⁶Mg = 7.0 ± 1.6%. for hibonite and 5.0 ± 4.8%. for corundum, despite very large ${}^{27}\text{Al}{}^{24}\text{Mg}$ ratios, 130 and 2.74 × 10⁴, respectively. Within the errors, δ_N²⁶Mg does not vary over this exceedingly large range of ${}^{27}\text{Al}{}^{24}\text{Mg}$ ratios. The extreme temperature required to melt this inclusion makes a liquid origin unlikely, except possibly by hypervelocity impact involving refractory bodies. If, instead, BB-5 is a direct gas-solid condensate, textural evidence implies that corundum formed first and later reacted to produce hibonite. In this model, BB-5's uniform enrichment in ²⁶Mg must be a characteristic of the reservoir from which it condensed. Because severe difficulties are encountered in making such a reservoir by prior decay of ²⁶Al, nebular heterogeneity in magnesium isotopic composition is a preferred explanation.  相似文献   

Stable sulfur and nitrogen isotopic compositions of crude oil fractions from Southern Germany     

A.V Hirner  W Graf  R Treibs  A.N Melzer  P Hahn-Weinheimer 《Geochimica et cosmochimica acta》1984,48(11):2179-2186

Eleven samples of crude oil from the Molasse Basin of Southern Germany were fractionated and their contents of sulfur and nitrogen as well as the stable isotope compositions of these elements (${}^{34}\text{S}{}^{32}\text{S}$ and ${}^{15}\text{N}{}^{14}\text{N}$, resp.) investigated.According to the δ³⁴S determinations, all crude oils from the Tertiary base of the Western and Eastern Molasse belong to one oil family and differ significantly from the Triassic and Liassic oils in the Western Molasse.An enrichment of ³⁴S was observed with increasing polarity of crude oil fractions. The isotope distributions of sulfur in the polar constituents of the biodegraded oils from the sandstones of Ampfing, however, approach a homogeneous distribution.The nitrogen isotope distribution is rather uniform in Southern German oils. A regional differentiation can be recognized, although the overall isotopic variation is small. The δ¹⁵N values of the crudes and asphaltenes do not correlate.  相似文献