Chronological variations in concentrations and isotopic compositions of anthropogenic atmospheric lead in sediments of a remote subalpine pond     

H. Shirahata  R.W. Elias  C.C. Patterson  M. Koide 《Geochimica et cosmochimica acta》1980,44(2):149-162

Lead concentrations decrease 4-fold in going from the surface of sediments in a mountain pond to relatively small values in layers 130 yr old. There is a corresponding change in the $\text{Pb}{}^{206}\text{Pb}{}^{207}$ ratio in the sediments from industrial-like values of 1.18 near the surface to natural values of 1.24 at depth. Concentrations of Ca, Sr, and Ba remain relatively constant with depth. The excess Pb is shown to be of eolian anthropogenic origin, through isotopic, chemical, and mass balance relationships with metals in aerosols, dry deposition, and precipitation. This proves that inputs of contamination Pb were nearly absent centuries ago in a remote non-domesticated subalpine canyon, but are present today in that ecosystem in amounts more than 20 times the natural inputs and are irrefutably linked with industrial sources. As a consequence of these inputs, present Pb concentrations have been elevated 5-fold in plants and 50-fold in animals above natural levels. These Pb contamination effects are being caused by present-day atmospheric concentrations of ~10 ng Pb/m³. The ecosystem studied here characterizes the vast remote non-domesticated regions of North America, and these findings indicate that such regions are highly polluted by industrial Pb aerosols.  相似文献   

Manganese carbonate overgrowths on foraminifera tests     

Edward A. Boyle 《Geochimica et cosmochimica acta》1983,47(10):1815-1819

Below the zone where manganese is remobilized as Mn²⁺(aq), reductively cleaned foraminifera in deep sea sediments have much higher $\text{Mn}\text{Ca}$ than those in core tops and sediment traps. $\text{Mn}\text{Ca}$ ranges from less than 20 × 10^?6 in and above the MnO₂ maximum to as high as 700 × 10^?6 in reducing Panama Basin sediments. The most plausible explanation for this enrichment is that the tests are coated with Mn carbonate overgrowths. These coatings can account for a significant proportion of the Mn in reduced deep-sea sediments. Uptake of manganous ion by carbonate may explain the absence of Mn nodules in areas of high carbonate accumulation. Extreme degrees of overgrowth can alter foram trace element values, but this artifact can be avoided by avoiding foraminifera with high Mn/Ca.  相似文献   

Precise age determinations and petrogenetic studies using the KCa method     

B.D. Marshall  D.J. DePaolo 《Geochimica et cosmochimica acta》1982,46(12):2537-2545

High precision mass spectrometric determination of calcium isotope ratios allows the ⁴⁰K → ⁴⁰Ca radioactive decay to be used for dating a much broader range of geologic materials than is suggested by previous work. ${}^{40}\text{Ca}{}^{42}\text{Ca}$ is used to monitor enrichments in ⁴⁰Ca and can be measured to ±0.01% (2σ) using an exponential mass discrimination correction (Russell et al., 1978) and large ion currents. The earth's mantle has such a low $\text{K}\text{Ca}$ (～0.01) that it has retained “primordial” ${}^{40}\text{Ca}{}^{42}\text{Ca}\text{= 151.016}$ ± 0.011 (normalized to ${}^{42}\text{Ca}{}^{44}\text{Ca}\text{= 0.31221}$), as determined by measurements on two meteorites, pyroxene from an ultramafic nodule, metabasalt, and carbonatite. ${}^{40}\text{Ca}{}^{42}\text{Ca}$ ratios can be conveniently expressed relative to this value as ?_Ca in units of 10^?4. To test the method for age dating, a mineral isochron has been obtained on a sample of Pikes Peak granite, which has been shown to have concordant KAr, RbSr, and UPb ages. Plagioclase, K-feldspar, biotite, and whole rock yield an age of 1041 ± 32 m.y. (2σ) in agreement with previous age determinations (λ_K = 0.5543 b.y.^?1, $\text{λ}{}_{\mathrm{\beta ?}}\text{λ}{}_{\mathrm{<mtext>K</mtext>}}\text{= 0.8952}$, ⁴⁰K = 0.01167%). The initial ${}^{40}\text{Ca}{}^{42}\text{Ca}$ of 151.024 ± 0.016 (?_Ca = +0.5 ± 1.0), indicates that assimilation of high K/Ca crust was insufficient to affect calcium isotopes. Measurements on two-mica granite from eastern Nevada indicate that the magma sources had K/Ca ≈ 1, similar to intermediate-composition crustal rocks. These results show that the KCa system can be used as a precise geochromometer for common felsic igneous and metamorphic rocks, and may prove applicable to sedimentary rocks containing authigenic K minerals. The relatively short half-life of ⁴⁰K, the non-volatile daughter, and the fact that potassium and calcium are stoichiometric constituents of many minerals, make the KCa system complementary to other dating methods, and potentially applicable to a variety of geologic problems.  相似文献   

Isotopic and incompatible element constraints on the genesis of island arc volcanics from Cold Bay and Amak Island,Aleutians, and implications for mantle structure     

J.D Morris  S.R Hart 《Geochimica et cosmochimica acta》1983,47(11):2015-2030

Cold Bay and Amak Island, two Quaternary volcanic centers in the eastern Aleutians, are orthogonal relative to the trench and separated by ~50 km. Sr, Nd and Pb isotopic compositions of the calc-alkaline andesite magmas show no sign of contamination from continental crust (average ${}^{87}\text{Sr}{}^{86}\text{Sr}\text{= 0.70323}$, ${}^{143}\text{Nd}{}^{144}\text{Nd}\text{= 0.51301}$, ${}^{206}\text{Pb}{}^{204}\text{Pb}\text{= 18.82}$, ${}^{207}\text{Pb}{}^{204}\text{Pb}\text{= 15.571}$). These samples plot within the mantle arrays for Sr-Nd and for Pb and are similar to arcs such as the Marianas and New Britain (Sr-Nd) and Marianas and Tonga (Pb). Incompatible element ratios for the Aleutian andesites (K/Rb ~ 332, K/Cs ~ 10,600, K/Sr ~ 22.4, K/Ba ~ 18.3, Ba/La ~ 60) are within the range reported for arc basalts, despite the difference in degree of fractionation.Average K content, K/Rb, K/Ba and K/Sr are approximately the same for basalts from arcs and from oceanic islands (OIB); K/Cs is a factor of 4 lower and Ba/La almost 3 times higher in arcs. Abundance ratio correlations indicate that arcs are enriched in Cs and depleted in La relative to OIB, with other incompatible element abundances very similar. Histograms of Sr and Nd isotopic compositions for MORB, OIB, and intraoceanic arcs show remarkably similar peaks and distribution patterns for intraoceanic arcs and OIB.A “plum pudding” model for the upper mantle best accommodates a) geochemical coherence of OIB and IAV, b) the existence of mantle plumes at some oceanic islands, and c) the presence of a MORB-type source at back arc spreading centers. In this model, OIB plums are imbedded in a MORB matrix; small degrees of melting generate OIB-type magmas while larger degrees of melting dilute the OIB magma with MORB matrix melts.OIB plums are merely less robust lower mantle plumes (i.e., blobs) which are distributed throughout the upper mantle by convection. The existence of at least two types of OIB, as indicated by Sr, Nd, and Pb isotopes, suggests that nuggets of recycled oceanic lithosphère may coexist with lower-mantle plums and that both may be tapped in arcs and intraplate environments.  相似文献   

Determination of the ⁸⁷Rb decay constant     

D.W. Davis  J. Gray  G.l. Gumming  H. Baadsgaard 《Geochimica et cosmochimica acta》1977,41(12):1745-1749

The decay constant ⁸⁷Rb has been redetermined by measuring the amount of radiogenic ⁸⁷Sr produced over a period of 19 years, in 20 g samples of purified RbClO₄, using isotope dilution techniques. The rubidium sample was spiked with ⁸⁴Sr and the nanogram quantities of strontium separated by coprecipitation with Ba(NO₃)₂. Analyses were carried out on a 25cm, 90° sector mass spectrometer equipped with a Spiraltron electron multiplier. Measurement of three independent ratios permitted continuous monitoring of the ion beam fractionation. The average of nine determinations gives a value for the decay constant of $\text{1.419(±0.012) × 10}{}^{\mathrm{?11}}\text{yr}{}^{\mathrm{?1}}\text{(2σ)}$. [$\text{τ}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{= 4.89(±0.04) × 10}{}^{10}\text{yr}$.]  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   

Geochemical behavior of inorganic germanium in an unperturbed estuary     

Philip N Froelich  Gordon A Hambrick  Lisa W Kaul  James T Byrd  Odile Lecointe 《Geochimica et cosmochimica acta》1985,49(2):519-524

Eleven monthly estuarine profiles of dissolved inorganic germanium (Ge_i) and silica (Si) in a natural, pristine river/bay system demonstrate that Ge-removal and -input parallel the seasonal silica cycle, reflecting Ge-uptake by and -dissolution from diatoms. The Ge/Si atom ratio of the river is 0.6 ± 0.15 × 10^?6, which is near the average value for continental granites and for uncontaminated, remote, natural rivers (0.7 ± 0.3 × 10^?6). The $\text{Ge}\text{Si}$ ratio escaping this estuary to the ocean is 0.8 × 10^?6, reflecting some estuarine enhancement of the fluvial Ge-flux, probably due to release of Ge_i from fluvial particulates. Nevertheless, the post-estuarine $\text{Ge}\text{Si}$ ratio is not significantly different from the continental crustal ratio but is very different from the ratio in sea-floor hot springs and mid-ocean ridge hydrothermal plumes (4 ± 2 × 10^?6) and in oceanic basalts (2.6 × 10^?6). Thus natural estuarine processes do not obscure the contrasting $\text{Ge}\text{Si}$ signatures entering the ocean from dissolution of continental and sea-floor silicates.  相似文献   

Pb,Sr and ¹⁰Be isotopic studies of volcanic rocks from the Northern Mariana Islands. Implications for magma genesis and crustal recycling in the Western Pacific     

Jon D. Woodhead  Donald G. Fraser 《Geochimica et cosmochimica acta》1985,49(9):1925-1930

We report Sr and Pb isotope analyses for an extensive suite of volcanic rocks from the N. Mariana arc together with Sr and Pb isotope analyses of sediments from the nearby Mariana and Nauru basins. In addition ten of the most recent volcanic samples were analysed for ¹⁰Be.The Sr isotope compositions cluster tightly around ${}^{87}\text{Sr}{}^{86}\text{Sr}$ = 0.7035 being slightly but significantly higher than the Pacific ocean floor basalts on either side of the arc and agreeing well with previous data. In contrast, the large number of new Pb isotopic data presented significantly extends the observed range of Pb isotope compositions for volcanic rocks from the Mariana arc to more radiogenic compositions. The concentrations of ¹⁰Be were very low (< 0.5 × 10⁶ atom g^?1).These new data require either that the Pb and Sr isotopic compositions of the Mariana sub-arc mantle be substantially different from those of the mantle source of ocean floor basalts on either side of the arc, or that the enrichment in radiogenic Pb and Sr relative to the values observed in Pacific ocean floor basalts be related to the subduction process. We prefer the latter hypothesis in which radiogenic Sr and Pb in ocean floor sediments are added to M.O.R.B. type mantle either by direct assimilation of the sediments in partial melts or, more probably, by transfer in a fluid phase into the zone of magma production. The low ¹⁰Be concentrations observed suggest the removal of at least the top few metres of sediment during subduction.  相似文献   

UThPb systematics of the eucrite “Juvinas”: Precise age determination and evidence for exotic lead     

G Manhes  C.J Allegre  A Provost 《Geochimica et cosmochimica acta》1984,48(11):2247-2264

The U-Th-Pb isotope systematics of the eucrite “Juvinas” have been studied in whole rock fragments as well as in plagioclases and pyroxenes. The results show that this monomict breccia crystallized with a very high $\text{U}\text{Pb}$ initial ratio at T = 4.539 ± 0.004 AE ago. There is evidence for a less radiogenic Pb component (${}^{206}\text{Pb}{}^{204}\text{Pb}\text{= 13.0}$; ${}^{207}\text{Pb}{}^{204}\text{Pb}\text{= 13.5}$; ${}^{208}\text{Pb}{}^{204}\text{Pb}\text{= 32.71}$) interpreted as “exotic lead” induced by a meteoritical impact at the surface of the Juvinas parent body, 1.92 ± 0.06 AE ago.  相似文献   

Behaviour of strontium in subsurface calcium chloride brines: Southern Israel and Dead Sea rift valley     

E. Sass  A. Starinsky 《Geochimica et cosmochimica acta》1979,43(6):885-895

Calcium chloride brines are, as a rule, relatively rich in strontium, but the enrichment is usually limited and is found to be related to the concentration of calcium. The limiting mechanisms were evaluated as a model which comprises simple interactions between minerals and solutions. Based on the known ranges of strontium concentration in minerals, mineral solubilities and partition coefficients of strontium (both poorly known in certain cases), six fields of $\text{Sr}\text{Ca}$ molar ratios were defined in terms of participating minerals and processes: (a) 0.38?1.56 × 10^{? 3} by dolomitization of calcite; (b) 1.5?2.2 × 10^{? 2} due to dolomitization of aragonite; (c) 0.4?1.4 × 10^{? 2} as a result of solution-reprecipitation of calcite; (d)0.12?0.20 through transformation of aragonite to calcite; (e)0.10?0.60 through equilibrium of the pair calcite-strontianite; and (f)0.01?0.08 by equilibrium with gypsum and celestite.The model was applied to the analysis of two groups of brines from southern Israel which are originated in the coastal plain (group C) and in the rift valley (group R). The low $\text{Mg}\text{Ca}$ ratios of both water groups point to dolomitization as the main subsurface modifying process. $\text{Sr}\text{Ca}$ ratios of brines belonging to group C are consistent with dolomitization of aragonitic surface sediments at the beginning of their evolution. Brines of group R bear evidence to a similar pathway at the beginning of their evolution, but most of them were further affected by interaction with limestone.  相似文献   

Strontium and its isotopes in Canadian rivers: Fluxes and global implications     

Moire A. Wadleigh  Ján Veizer  Christopher Brooks 《Geochimica et cosmochimica acta》1985,49(8):1727-1736

Systematic sampling of the 39 largest Canadian rivers shows that the weighted average ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratio in the dissolved load is 0.7111, similar to previous measurements on such large rivers as the Amazon and Mississippi. Consequently, we believe that the above estimate is likely representative of the global average. This imposes a limit of 6.5 × 10¹¹g yr^?1 on the amount of Sr exchanged with basalts in hydrothermal cells on mid-oceanic ridges. Evaluation of geological information suggests that Sr from silicate sources is of considerable importance for all but the largest Canadian rivers. The latter have chemical and isotopic composition consistent with ~4:1 carbonate vs. silicate derivation of Sr, but such interpretation is not unique. In terms of their water discharge, the 39 Canadian rivers studied account for 4.2% of the world total and their weighted average concentrations for other dissolved solutes are: TDS 176 ppm, Ca 18 ppm, Cl 6.8 ppm and Sr 84 ppb.  相似文献   

Rb-Sr and U-Th-Pb systematics of alkaline rocks: the alkaline rocks from Italy     

R Vollmer 《Geochimica et cosmochimica acta》1976,40(3):283-295

The isotopic composition of Pb and Sr and the abundances of Rb, Sr, U, Th, and Pb were determined for whole rock samples from all major volcanic centres of the Cenozoic alkaline volcanism of Central and South Italy, together with some samples from the contemporaneous anatectic Tuscan volcanism. The Sr and Pb isotopic compositions of the alkaline rocks show a negative correlation combined with a regional trend: the ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios decrease from 0.711 in the north-west to 0.704 in the south-east, while the ${}^{206}\text{Pb}{}^{204}\text{Pb}$ ratios increase from 18.7 to 20.0. Variations in both isotopic compositions are generally small throughout erupted rock sequences for any volcanic centre.The Pb and Sr isotopic abundance variations are interpreted on the basis of two alternative models, which correspond to two groups of geological processes: variations can result (i) from a time dependent development in subsystems with different $\text{Rb}\text{Sr}$ or $\text{U(Th)}\text{Pb}$ ratios or, (ii) from mixing of Sr or Pb with different isotopic compositions. Combining both Pb and Sr isotope abundance measurements it is shown that the source of each volcanic centre is formed by various degrees of mixing between two components. One component and the most southern Tuscan anatectic rocks most likely have a common source, whereas the other component of the mixing process is suggested to be a liquid fraction derived from a small degree of partial fusion of a hydrous mantle. Thus at least a two-stage evolution of the Italian alkaline rocks is indicated: first a mixing process leading to the formation of the parental material followed by differentiation processes leading to the formation of the erupted rock sequences.The geodynamic model which explains the data best is that of a lateral inhomogeneous mantle. The lateral inhomogeneities in the mantle would be the result of mixing between originally mantle and crustal derived material. The mixing process itself would not have any primary connection with the Quarternary volcanic activity.  相似文献   

Interstellar organic matter in meteorites     

Jongmann Yang  Samuel Epstein 《Geochimica et cosmochimica acta》1983,47(12):2199-2216

Hydrogen which is highly enriched in deuterium is present in organic matter in a variety of meteorites including non-carbonaceous chondrites. The concentrations of this hydrogen are quite large. For example Renazzo contains 140 μmoles/g of the 10,000‰ δD hydrogen. The $\text{D}\text{H}$ ratios of hydrogen in the organic matter vary from 8 × 10^?5 to 170 × 10^?5 (δD ranges from ? 500‰ to 10,000‰) as compared to 16 × 10^?5 for terrestrial hydrogen and 2 × 10^?5 for cosmic hydrogen. The majority of the unequilibrated primitive meteorites contain hydrogen whose $\text{D}\text{H}$ ratios are greater than 30 × 10^?5. If the $\text{D}\text{H}$ ratios in these compounds were due to enrichment relative to cosmic hydrogen by isotope exchange reactions, it would require that these reactions take place below 150 K. In addition the organic compounds having $\text{D}\text{H}$ ratios above 50 × 10^?5 would require temperatures of formation of < 120 K. These types of deuterium enrichments must take place by ion-molecule reactions in interstellar clouds where both ionization and low temperatures exist. Astronomically observed $\text{D}\text{H}$ ratios in organic compounds in interstellar clouds are typically 180 × 10^?5 and range between about 40 × 10^?5 and 5000 × 10^?5. The $\text{D}\text{H}$ values we have determined are the lower limits for the organic compounds derived from interstellar molecules because all processes subsequent to their formation, including terrestrial contamination, decrease their $\text{D}\text{H}$ ratios.In contrast, the $\text{D}\text{H}$ ratios of hydrogen associated with hydrated silicates are relatively uniform for the meteorites we have analyzed with an average value of 14 × 10^?5; very similar to the terrestrial value. These phyllosilicates values suggest equilibration of H₂O with H₂ in the solar nebula at temperatures of about 200 K and higher.The ${}^{13}\text{C}{}^{12}\text{C}$ ratios of organic matter, irrespective its $\text{D}\text{H}$ ratio, lie well within those observed for the earth. If organic matter originated in the interstellar medium, our data would indicate that the ${}^{13}\text{C}{}^{12}\text{C}$ ratio of interstellar carbon five billion years ago was similar to the present terrestrial value.Our findings suggest that other interstellar material, representing various inputs from various stars, in addition to the organic matter is preserved and is present in the meteorites which contain the high $\text{D}\text{H}$ ratios. We feel that some elements existing in trace quantities which possess isotopic anomalies in the meteorites may very well be such materials.  相似文献   

A low-contamination method for hydrothermal decomposition of zircon and extraction of U and Pb for isotopic age determinations     

T.E Krogh 《Geochimica et cosmochimica acta》1973,37(3):485-494

A simple procedure for the decomposition of zircon and the extraction of U and Pb for isotopic age determinations has been developed and tested (Krogh, 1971a,b). Samples are decomposed at 220°C with 48 per cent HF in a Teflon® capsule confined by a self-sealing stainless steel jacket. Uranium and lead are isolated on a Teflon® anion exchange column using Dowex 1 resin. Measured lead contamination levels range from 0.5 to 5.0 ng. In contrast, lead blanks for the borax fusion technique used in all previous zircon analyses are typically 0.2 to 1.0 μg.Eight small samples weighing 0.3 to 7 mg contained 30 to 260 ng of Pb²⁰⁶. The average value for the total amount of common lead present in the lead isotopic composition analysis, contributed from both the sample and the chemical procedures, was 1.4 ng. The highest $\text{Pb}{}^{206}\text{Pb}{}^{204}$ ratio measured to date (126,000) was obtained on a 0.1-g sample that contained 50 ppm of Pb²⁰⁶. An exact determination of the amount of common lead in zircons is now possible. A maximum value of 0.3 ppm was found for fourteen nonmagnetic zircon fractions from granites and rhyolites. Higher values reported in the literature suggest that lead contamination levels are often underestimated in the analysis of zircons by the borax fusion technique.The silica-gel loading technique for lead provides stable emission for small samples as well as limited isotopic fractionation in the mass spectrometer. These features, combined with the low levels of lead contamination and the high precision of mass spectrometric analyses, make possible an average reproducibility (for duplicate decompositions of the same finely ground sample) of 0.3 m.y. for lead $\text{207}\text{206}$ ages of 2750 m.y. The new method requires fewer reagents and is much easier than the borax fusion technique.  相似文献   

The isotopic composition of silver and lead in two iron meteorites: Cape York and Grant     

J.H. Chen  G.J. Wasserburg 《Geochimica et cosmochimica acta》1983,47(10):1725-1737

Silver in the metal phases of Cape York (IIIA) and Grant (IIIB) has been determined after an extensive surface cleaning process. The ${}^{107}\text{Ag}{}^{109}\text{Ag}$ was found to be enriched over that found in terrestrial Ag by ~7%. to 19%., demonstrating the presence of excess ¹⁰⁷Ag (¹⁰⁷Ag¹) in this class of meteorites. An effort was made to find schreibersite with a distinctive ¹⁰⁸Pd/¹⁰⁹Ag ratio in order to establish a three-point isochron, but the results are not markedly different from those obtained for the bulk metal. The Ag isotopic ratio of sulfides from the same meteorites were nearly normal in composition. These results demonstrate correlations of ${}^{107}\text{Ag}{}^{109}\text{Ag}$ with ${}^{108}\text{Pd}{}^{109}\text{Ag}$ between coexisting phases of two iron meteorites that are associated with planetary differentiation processes. The ratios ${}^{107}\text{Ag}{}^1{}^{108}\text{Pd}$ were found to be 1.7 × 10^?5 and 1.2 × 10^?5 for Cape York and Grant, respectively. These observations are in support of the widespread presence of ¹⁰⁷Pd in the early solar system. The difference in isotopic composition between metal and sulfide phases demonstrates that silver diffusion was small (over 6.5 × 10⁶ y) indicating a cooling rate much greater than 150°C/my for meteorites which have been attributed to small planetary cores. Uranium determinations were carried out on the metal phases and concentrations of ~ $\text{1 × 10}{}^{12}$ g U/g and 2 × 10^?10g U/g were found for Cape York and Grant, respectively. The Pb in these meteorites was determined using the improved cleaning procedures and chemical separations with low blank levels. The results confirm the presence of variable proportions of radiogenic Pb in both the metal and sulfide phases of iron meteorites. No simple explanation for the presence of radiogenic lead is apparent; while terrestrial contamination may appear to be the obvious explanation, it is possible that this effect could result from relatively recent metamorphism in the meteorite parent body.  相似文献   

Lead orthophosphates—II. Stability of cholopyromophite at 25°c     

Jerome O. Nriagu 《Geochimica et cosmochimica acta》1973,37(3):367-377

The conversion of secondary lead orthophosphate [PbHPO₄] into chloropyromorphite [Pb₅(PO₄)₃Cl] in ca. 10^?1 M NaCl solutions has been investigated at 25°C. From the composition of the supernatant solutions, the solubility product constant for Pb₅(PO₄)₃Cl has been calculated to be 10^?84.4±0.1, corresponding to $\text{ΔG}{}_{\mathrm{?}}\text{°}$ of ?906.2 kcal mol^?1. The solution equilibria and phase relationships in the system PbCl₂-PbO-P₂O₈-H₂O are discussed along with the geological implications.  相似文献   

Pb isotope geochemistry of a massif-type anorthositic-charnockitic body: The Hidra Massif (Rogaland,S.W. Norway).     

D Weis  D Demaiffe 《Geochimica et cosmochimica acta》1983,47(8):1405-1413

The Hidra Massif (Rogaland complex, S.W. Norway) is a massif-type anorthositic-charnockitic body. It consists of undeformed anorthosites and leuconorites, grading into fine-grained jotunites at the contact with the granulite facies gneisses of the metamorphic envelope. A stockwork of charnockitic dykes cross-cuts the massif. The Pb isotopic compositions of the anorthosites and leuconorites are comparable or slightly less radiogenic than those of the jotunites (${}^{206}\text{Pb}{}^{204}\text{Pb}$ from 18.079 to 19.307,(${}^{207}\text{Pb}{}^{204}\text{Pb}$ from 15.568 to 15.657 and ${}^{208}\text{Pb}{}^{204}\text{Pb}$ from 37.617 to 38.493). These values are compatible with an upper mantle origin for the parental magma of jotunitic composition and for the plagioclasic cumulates, but show the incorporation of lower crustal material (U-depleted and thus less radiogenic). The charnockitic dykes have significantly less radiogenic Pb isotopic compositions (${}^{206}\text{Pb}{}^{204}\text{Pb}$ from 17.472 to 19.171, ${}^{207}\text{Pb}{}^{204}\text{Pb}$ from 15.489 to 15.620and ${}^{208}\text{Pb}{}^{204}\text{Pb}$ from 36.991 to 40.922) which can be explained by a larger proportion of lower crustal contamination material. The contaminant could be the granulite facies gneisses of the metamorphic envelope. This interpretation is compatible with the K-Rb relationships of these rocks and with the O and Sr isotopic geochemistry.The proportion of contaminating lead in the charnockitic dykes can be estimated at 55 ± 15% considering the border facies jotunite as the uncontaminated parental magma and the least radiogenic gneiss of the metamorphic envelope as the contaminant.  相似文献   

Single-grain 207Pb206Pb and U/Pb age determinations with a 10-μm spatial resolution using the ion microprobe mass analyzer (IMMA)     

J.R. Hinthorne  C.A. Andersen  R.L. Conrad  J.F. Lovering 《Chemical Geology》1979,25(4):271-303

Analytical techniques have been developed for using a secondary ion mass spectrometer, the ion microprobe mass analyzer (IMMA), to determine, in situ, ${}^{207}\text{Pb}{}^{206}\text{Pb}$ and U/Pb ages on approximately 10-μm areas of individual mineral phases containing relatively abundant radiogenic Pb isotopes. Standard samples of known age and U, Th and Pb contents, together with the Andersen-Hinthorne local thermal equilibrium (LTE) model for predicting ionization parameters are used to establish a semi-empirical relationship for correcting observed U/Pb intensities to atom ratios. Measurements of isotope standards show that mass fractionation corrections are not required and that the accuracy and precision of analysis are generally limited by Poisson counting statistics.Many U-rich accessory minerals yield spectra which consist only of Pb at masses 204, 206, 207 and 208; thus the measurement of ${}^{207}\text{Pb}{}^{206}\text{Pb}$ ages is accomplished by simply measuring the intensities of these peaks and the background. An excellent correspondence of the ages determined using the IMMA with those from more conventional techniques is demonstrated for terrestrial, lunar and meteoritic phases. For example, the following ${}^{207}\text{Pb}{}^{206}\text{Pb}$ ages were obtained from polished thin sections of crystalline lunar samples: 10047, 3.75 ± 0.05 (2σ) Ga; 14310, 3.96 ± 0.03 Ga; and 15555, 3.36 ± 0.06 Ga. From small U-rich phases in CaAl-rich inclusions in the Allende carbonaceous chondrite, seven ${}^{207}\text{Pb}{}^{206}\text{Pb}$ ages were obtained, averaging 4.60 ± 0.09 (2σ) Ga.A method of correcting low-resolution Pb-isotope data for molecular ion interferences in zircon and baddeleyite is presented and shown to be practical through the analysis of terrestrial and lunar samples.  相似文献   

Li,Sr, Mg,and Na in foraminiferal calcite shells from laboratory culture,sediment traps,and sediment cores     

Margaret Lois Delaney  Allan W.H.Bé  Edward A. Boyle 《Geochimica et cosmochimica acta》1985,49(6):1327-1341

Constant-temperature laboratory culture experiments of the planktonic foraminiferal species Globigerinoides sacculifer (Brady) suggest that the ratios of Li and Sr to Ca in the shells are a function of these ratios in the culture solutions. $\text{Mg}\text{Ca}$ and $\text{Na}\text{Ca}$ in the shells did not vary with changes of these ratios in the culture solution. These are the first direct determinations of the relationship between foraminiferal shell chemistry and solution composition.The possibility of temperature dependence for the minor elemental composition of foraminiferal shells was also investigated in the laboratory and by analysis of several planktonic and one benthic foraminiferal species from sediment trap and sediment core samples. The $\text{Sr}\text{Ca}$, $\text{Mg}\text{Ca}$, and $\text{Na}\text{Ca}$ ratios in the natural samples roughly correlate with calcification temperature, whereas differences in the Li/Ca ratios are small and not systematically related to temperature. However, laboratory culture experiments at 20°C and 30°C showed no variation in the $\text{Li}\text{Ca}$, $\text{Sr}\text{Ca}$, $\text{Mg}\text{Ca}$, and $\text{Na}\text{Ca}$ ratios with calcification temperature for the planktonic foraminifera G. sacculifer and Orbulina universa. Therefore, observed differences in the $\text{Sr}\text{Ca}$, $\text{Mg}\text{Ca}$, and $\text{Na}\text{Ca}$ ratios for the sediment trap and core foraminiferal samples cannot be ascribed to direct effects of calcification temperature, but may be due to some other environmental factor which is correlated with temperature.  相似文献   

Geochronology of archean gneisses in the Lake Helen area,Southwestern Big Horn Mountains,Wyoming     

J.G Arth  F Barker  T.W Stern 《Precambrian Research》1980,11(1):11-22

The RbSr and UPb methods were used to study gneisses in the $\text{7}\text{1}\text{2}\text{-}\text{minute}$ Lake Helen quadrangle of the Big Horn Mountains, Wyoming. Two episodes of magmatism, deformation and metamorphism occurred during the Archean. Trondhjemitic to tonalitic orthogneisses and amphibolite of the first episode (E-1) are cut by a trondhjemite pluton and a calc-alkaline intrusive series of the second episode (E-2). The E-2 series includes hornblende-biotite quartz diorite, biotite tonalite, biotite granodiorite and biotite granite.A RbSr whole-rock isochron for E-1 gneisses indicates an age of 3007 ± 34 Ma (1 sigma) and an initial ⁸⁷Sr/⁸⁶Sr of 0.7001 ± 0.0001. UPb determination on zircon from E-1 gneisses yield a concordia intercept age of 2947 ± 50 Ma. The low initial ratio suggests that the gneisses had no significant crustal history prior to metamorphism, and that the magmas from which they formed had originated from a mafic source.A RbSr whole-rock isochron for E-2 gneisses gives an age of 2801 ± 31 Ma. The ⁸⁷Sr/⁸⁶Sr initial ration is 0.7015 ± 0.0002 and precludes the existence of the rocks for more than 150 Ma prior to metamorphism. The E-2 magmas may have originated from melting of E-1 gneisses or from a more mafic source.  相似文献