Approximation of terrestrial lead isotope evolution by a two-stage model
TL;DR: In this paper, a two-stage model for terrestrial lead isotope evolution is proposed, which permits the age of the earth to be that of the meteorite system and also yields good model ages for samples of all ages.
About: This article is published in Earth and Planetary Science Letters.The article was published on 1975-06-01. It has received 8218 citations till now. The article focuses on the topics: Age of the Earth & Meteorite.
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TL;DR: In this paper, the authors compared the relative abundances of the refractory elements in carbonaceous, ordinary, and enstatite chondritic meteorites and found that the most consistent composition of the Earth's core is derived from the seismic profile and its interpretation, compared with primitive meteorites, and chemical and petrological models of peridotite-basalt melting relationships.
10,830 citations
TL;DR: In this paper, a new calculation of the crustal composition is based on the proportions of upper crust (UC) to felsic lower crust (FLC) to mafic lower-crust (MLC) of about 1.6:0.4.
5,317 citations
TL;DR: In this paper, the results of a study to develop natural zircon geochemical standards for calibrating the U-(Th)-Pb geochronometer and Hf isotopic analyses are reported.
Abstract: We report here the results of a study to develop natural zircon geochemical standards for calibrating the U-(Th)-Pb geochronometer and Hf isotopic analyses. Additional data were also collected for the major, minor and trace element contents of the three selected sample sets. A total of five large zircon grains (masses between 0.5 and 238 g) were selected for this study, representing three different suites of zircons with ages of 1065 Ma, 2.5 Ma and 0.9 Ma. Geochemical laboratories can obtain these materials by contacting Geostandards Newsletter.
4,845 citations
TL;DR: In this paper, a laser ablation-inductively coupled plasma-mass spectrometer (LA-ICP-MS) was used for in situ U-Pb zircon geochronology.
4,092 citations
Additional excerpts
...(7) Corrected for fractionation, spike, blank and initial common Pb (estimated from Stacey and Kramers (1975) model)....
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University of Bergen1, Charles University in Prague2, Academy of Sciences of the Czech Republic3, University of Nottingham4, Boise State University5, Goethe University Frankfurt6, Memorial University of Newfoundland7, Stockholm University8, University of Vienna9, University of Geneva10, Swedish Museum of Natural History11
TL;DR: The Plesovice zircon as discussed by the authors has a concordant U-Pb age with a weighted mean Pb-206/U-238 date of 337.13 +/- 0.37 Ma (ID-TIMS, 95% confidence limits, including tracer calibration uncertainty).
3,694 citations
Cites background or methods from "Approximation of terrestrial lead i..."
...A common-Pb correction based on the interference- and background-corrected 204Pb signal and a model Pb composition (Stacey and Kramers, 1975) was carried out, where necessary....
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...Common Pb was monitored using the 204Pb signal and corrections were made using the modern terrestrial Pb composition from the model of Stacey and Kramers (1975), assuming that the common Pb is largely surface contamination introduced during sample preparation....
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...A common-Pb correction based on the interference- and background-corrected (204)Pb signal and a model Pb composition (Stacey and Kramers, 1975) was carried out, where necessary....
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...For this, time-integrated alpha fluences Dα (i.e., the number of α-decay events per gram) were calculated from Da ¼ 8 cU NA 0:9928 M238 106 ek238t 1 þ 7 cU NA 0:0072 M235 106 ek235t 1 þ 6 cTh NA M232 106 ek232t 1 (Murakami et al., 1991; Nasdala et al., 2001), where cU and cTh are the present actinide concentrations (in ppm), NA is Avogadro's number, M238, M235, and M232 are the molecular weights of the parent isotopes, λ238, λ235, and λ232 are the respective decay constants (Firestone and Shirley, 1996), and t is the integration time (assumed to be ca. 337 Ma, i.e. the isotopic age of the zircon)....
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...(b) Percentage of common Pb detected, calculated from measured 204Pb and assuming 0 Ma, Stacey and Kramers (1975) average terrestrial Pb. Figures in parentheses indicate, where no common Pb corrections have been applied....
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References
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TL;DR: New determinations of the half-lives of half-life values of $ −235 and −238 have been made improved techniques have allowed the halflife values to be measured with greater accuracy than has been heretofore achieved as discussed by the authors.
Abstract: New determinations of the half-lives of $^{235}\mathrm{U}$ and $^{238}\mathrm{U}$ have been made Improved techniques have allowed the half-life values to be measured with greater accuracy than has been heretofore achieved Samples were prepared by molecular plating and counted in a intermediate-geometry $\ensuremath{\alpha}$-proportional counter with an extremely flat pulse-height plateau The small amount of residual nonplated uranium was counted in a $2\ensuremath{\pi}$ counter Energy analysis with a silicon-junction detector was used to measure the presence of "foreign" activities For $^{235}\mathrm{U}$, the measured specific activity was (47981\ifmmode\pm\else\textpm\fi{}33) (dis/min)/(mg $^{235}\mathrm{U}$), corresponding to a half-life of (70381\ifmmode\pm\else\textpm\fi{}00048) \ifmmode\times\else\texttimes\fi{} ${10}^{8}$ yr For $^{238}\mathrm{U}$, the specific activity was measured as (74619\ifmmode\pm\else\textpm\fi{}041) (dis/min)/(mg $^{238}\mathrm{U}$), corresponding to a half-life of (44683\ifmmode\pm\else\textpm\fi{}00024) \ifmmode\times\else\texttimes\fi{} ${10}^{9}$ yr Errors quoted are statistical (standard error of the mean), based upon the observed scatter of the data This scatter exceeds that expected from counting statistics alone We believe that systematic errors, if present, will no more than double the quoted errors
2,543 citations
TL;DR: Measurements of the lead isotopic composition and the uranium, thorium, and lead concentrations in meteorites were made in order to obtain more precise radiometric ages of these members of the solar system.
Abstract: Measurements of the lead isotopic composition and the uranium, thorium, and lead concentrations in meteorites were made in order to obtain more precise radiometric ages of these members of the solar system. The newly determined value of the lead isotopic composition of Canyon Diablo troilite is as follows: (206)Pb/(204)Pb = 9.307, (207)Pb/(2O4)Pb = 10.294, and (208)Pb/(204)Pb = 29.476. The leads of Angra dos Reis, Sioux County, and Nuevo Laredo achondrites are very radiogenic, the (206)Pb/(204)Pb values are about 200, and the uranium-thorium-lead systems are nearly concordant. The ages of the meteorites as calculated from a single-stage (207)Pb/(206)Pb isochron based on the newly determined primordial lead value and the newly reported (235)U and (838)U decay constants, are 4.528 x 10(9) years for Sioux County and Nuevo Laredo and 4.555 x 10(9) years for Angra dos Reis. When calculated with the uranium decay constants used by Patterson, these ages are 4.593 x 10(9) years and 4.620 x 10(9) years, respectively, and are therefore 40 to 70 x 10(6) years older than the 4.55 x 10(9) years age Patterson reported. The age difference of 27 x 10(6) years between Angra dos Reis and the other two meteorites is compatible with the difference between the initial (87)Sr/(86)Sr ratio of Angra dos Reis and that of seven basaltic achondrites observed by Papanastassiou and Wasserburg. The time difference is also comparable to that determined by (129)1-(129)Xe chronology. The ages of ordinary chondrites (H5 and L6) range from 4.52 to 4.57 x 10(9) years, and, here too, time differences in the formation of the parent bodies or later metamorphic events are indicated. Carbonaceous chondrites(C2 and C3) appear to contain younger lead components.
502 citations
TL;DR: In this article, a steady-state model for the evolution of the earth's crust and upper mantle has been proposed, where the oceans, continents, and mantle have been nearly constant for at least the last 2.5 b.y.
Abstract: Contrasting interpretations of existing models of Sr and Pb isotope evolution can be eliminated with a model in which crustal material is recycled through the mantle. In this model the earth's crust and upper mantle (above approximately 500 km depth) are in a steady-state system, and the volumes and bulk compositions of ocean, continent, and mantle have been nearly constant for at least the last 2.5 b.y. and probably for most of the earth's history.
Sialic material is continuously eroded from continents into ocean basins and, as a consequence of this process, is isotopically homogenized. In continental-margin orogenic belts and island arcs, the ocean basin, rise, and trench sediments are dragged into the mantle. Isotopic equilibration between sialic and simatic material takes place within the mantle, and the sialic material is returned to the continents or island arcs as juvenile-appearing volcanics, thus completing the geochemical cycle. Most of the radioactive parent isotopes reside within the continental sial, whereas the mantle remains depleted and unable to sustain its observed isotope evolution.
With this model it is possible to explain Pb isotope evidence of widespread ancient continents and common Pb evolution in a system which appears to have a very uniform U/Pb and Th/U ratio, even though most of the U and Th are highly enriched in the heterogeneous sialic crust. At the same time the model provides an explanation for Sr isotope evidence of continual addition of material to continents. Sr isotope evolution is dominated by the reservoir of Sr in the mantle; in contrast, Pb isotope evolution is dominated by isotopic mixing during erosion and sedimentation.
The apparent differences in the evolutions of Sr and Pb isotopes are due to differing responses to various parts of the steady-state cycle as a consequence of the differences in parent to daughter ratios in the sialic crust as compared with the upper mantle and in the degree of enrichment of parent and daughter products in the crust. Identical mathematical models may be used to describe the evolution of both isotope systems.
412 citations
TL;DR: In this article, a refolded syncline of mainly basic rocks containing banded ironstones and other rocks of supracrustal origin is enclosed by granitic gneisses.
Abstract: SOME 150 km north-east of Godthaab, on the edge of the inland ice cap, a refolded syncline of mainly basic rocks containing banded ironstones and other rocks of supracrustal origin is enclosed by granitic gneisses (Fig. 1). These gneisses have yielded an Rb-Sr whole-rock isochron age of 3,700 ± 140 m.y., indistinguishable from that of the Amitsoq gneisses of the Godthaab area1. The present metamorphic grade of both the gneisses and the iron formation is upper greenschist to amphibolite facies. There is no published map of the area, but the following geological details are taken from reports by Keto2. The total succession of supracrustal rocks is 2 to 3 km thick. The lower part of the succession consists of quartzites and meta-greywackes. These are overlain by garnet–chlorite schists and banded iron formation, including both Fe-oxides and carbonates. The top of the succession consists of greenschist facies metabasites, Bridgwater et al.3 suggest that the Isua supracrustals may represent a shallow-water shelf facies.
286 citations
TL;DR: Amphibolite-facies feldspathic gneisses from the Godthaab area of West Greenland give hitherto unique age and isotope data, including a Rb-Sr whole-rock isochron age of 3980 ± 170 my as discussed by the authors.
209 citations