Call for an improved set of decay constants for geochronological use (original) (raw)
Related papers
Geochronology and radiogenic isotope research
Reviews of Geophysics, 1979
Introduction Sm-Nd Dating During the four year period 1975-1978 research Probably the most important advance in the activity in the fields of geochronology and radio-field of geochronology and related isotopic geogenic isotopes has been alive and quite healthy. chemistry in the past four years has been the de-Geochronological laboratories have added greatly velopment and application of the Sm-Nd technique. to our knowledge of time relations and patterns in Until recently the long half-life of 147Sm (•the crust and others have applied studies of radio-decay to 143Nd, 1.06 x 1011 years) and the genergenic isotopes to problems of the sources, genesis, ally low Sm/Nd ratio in nature (4 0.3) have preand history of terrestrial and extraterrestrial cluded application of this system. These problems materials. In this paper we will review progress are compounded by the similar geochemical behavior in areas that are, in our judgment, the most ex-of these rare earth elements, so that ranges in citing and that represent the greatest advances Sm/Nd ratios are also generally small. Thus, rain earth science through application of isotopic diogenic enrichments in 143Nd/l•Nd are commonly studies. Much of the paper will be devoted to the only 1-2 percent over the age of the solar system. development of the Sm-Nd system and to the dis-With the use of the high-precision mass speccovery of long-term heterogeneity in the mantle. trometry developed over the past several years, Other topics discussed include the new decay con-Lugmair (1974) reported the first successful destant conventions, the application of Sr and Pb termination of the Sm-Nd age of a rock: the baisotopes to understanding of continental and con-saltic achondrite (eucrite) Juvinas (internal tinent-marginal igneous activity, advances in tech-mineral isochron, 4.56 • 0.08 b.y.). This study nique in U-Pb geochronometry in zircons, the dis-was also important because the sample chosen recovery of rocks with ages close to 3.7 b.y. and presents a system that crystallized very soon (< isotopic constraints on the evolution of the crust, 50 m.y.) after the formation of the solar system and progress in extraterrestrial chronology. and has basically a chondritic (flat) rare earth pattern. As a result, the isotopic parameters New Decay Constant Convention defined by Juvinas for the Sm-Nd method now provide the principal reference to which other re-One major problem in geochronology over the sults may be referred (see below). past two decades has been a lack of unanimity with These techniques have been adopted by several regard to certain physical constants, particularly other laboratories, but this has also resulted in the half-life of 8?Rb. This problem became coma problem not covered by the IUGS Subcommission pounded recently by proposed revisions or new de-on Geochronology report (see above): What values terminations of half-lives and isotopic abundances of the Sm and Nd isotope ratios are "best" and for geochronologic systems. Thus, recent literature contains results based on various sets of physical constants, with attendant systematic differences in ages. A Subcommission on Geochronology of the IUGS was formed under the direction of R. H. Steiger and E. J•ger to evaluate various alternatives and to decide upon a "best" set of constants. After considerable study, the subcommission reached a consensus in 1976 at the 25th IGC in Sydney with the results published shortly thereafter (Steiger and J•ger, 1977). There is currently a strong tendency for general adoption of ti•ese recommended values. Furthermore, systematic differences between different which should be adopted as principal reference values? For example, data reported from La Jolla (Lugmair, 1974; Lugmair et al., 1975b, 1976) are normalized to the equivalent of 142Nd/l•6Nd = 1.5817; Caltech (DePaolo and Wasserburg, 1976a, b; 1977) has used the equivalent of l$0Nd/l•2Nd = 0.2096; and Lamont-Doherty (O'Nions et al., 1977; Carter et al., 1978a; Hamilton et al., 1977) has used l•6Nd/l•4Nd = 0.7219. The Caltech normalization is not equivalent to the others and results in a value of 0.50598 for the isochron intercept value of Juvinas whereas the La Jolla and Lamont-Doherty value for this intercept is 0.50677 (Lugmair, 1974; Lugmair et al., 1976). This isoparent-daughter systems will also be small (on the chron intercept represents our best estimate of order of 1 percent or less). To be sure, there may still be some refinements in the exact values of interest, but such revisions will probably be minor and entail less than a 1 percent change in absolute ages, making it not worthwhile to change from the set of adopted values. Such remaining uncertainties are more than offset by the advantage of having all data reported on a uniform basis.
Geochronology is concerned with determining the absolute age of rocks and minerals, and thereby geological events that shaped these materials. This is generally done using the natural decay of unstable (radioactive) isotopes ("parent" or "mother" nuclide) to daughter nuclides, which may either be radioactive themselves, or stable. Each radioactive isotope has a characteristic half-life (T ½ ), which is the time in which half of the parent nuclides decay to their daughter nuclide. The rate of radioactive decay can also be expressed as the decay constant l, which is related to the half-life by T ½ ¼ ln2/l.
138La β-decay constant estimated from geochronological studies
Earth and Planetary Science Letters, 1988
LaCe ages are reported for two sets of Finnish pegmatites, 1.6~b61e and Mustikkamfiki. and for an Amitsoq gneiss, Greenland. When X/~I3~La value (2.29 × 10 12 yr 1) obtained by radioactivity measurement [1] is used for the chronological calculation, the La-('e ages (2129, 2325, 3271 Myr) evaluated for these rocks are 18-35% older than the Sm-Nd ages for the same samples. To make the La-('e age fit to the Sm-Nd age for the same sample, a new value of (2.77 z 0.21)× 10 ~2 yr ~ is evaluated for ,~/~ l~xLa. In this calculation, the La-('e and Srn-Nd ages reported for a Bushveld gabbro [2] have been also taken into account together with those for the Li~vb61e pegmatite and the Mustikkamaki pegmatite, while the Am'itsoq gneiss ((KiU110999) has been omitted because of the complicated thermal history of this sample.
Radioisotopes and the Age of the Earth
Institute for Creation Research …, 2000
RATE is an acronym applied to a research project investigating radioisotope dating sponsored by the Institute for Creation Research and the Creation Research Society. It stands for Radioisotopes and the Age of The Earth. This article summarizes the purpose, history, and intermediate findings of the RATE project five years into an eight-year effort. It reports on the latest status of the research on helium diffusion through minerals in granitic rock, accelerated nuclear decay theory, radiohalos, isochron discordance studies, case studies in rock dating, and carbon-14 in deep geologic strata. Each of the RATE scientists will present separate technical papers at the Fifth International Conference on Creationism on the details of this research.
Journal of Geophysical Research, 1995
The concentration of cosmogenic isotopes produced within mineral grains varies with both the exposure age and erosion rate of the rock surface. In principle therefore, exposure age and erosion rate may be determined by analyzing two cosmogenic isotopes from the same sample, provided the erosion rate is constant. It is also possible to find either age or erosion rate from one isotope if the other parameter can be determined independently. Simple mathematical models predict the precision and accuracy of the exposure ages and erosion rates. The results provide insight into the use of cosmogenic isotopes and a framework to optimize dating experiments and predict tractable geologic questions. The precision and accuracy of the exposure ages and erosion rates depend on the precision of the measured isotope concentrations, the half-lives of the isotopes, and the age and erosion rate of the sampled rock surface. They also vary with the analytic strategy. Exposure age estimates from some isotope pairs approach measurement precision for late-Pleistocene surfaces eroding at <1 cm kyr-•. Uncertainties in erosion rate for the same surfaces may be better than _+0.2 cm kyr-•. For older surfaces the upper limits for both age and erosion rate estimates become infinite. Pairing of isotopes with different half-lives may give misleading results unless the erosion rate is constant because they record different erosion histories. Analyzing a single isotope removes this source of error but is appropriate only if either the age or erosion rate can be well determined independently. If the erosion rate is poorly constrained, ages for some realistic situations may be inaccurate by 50% or more even though their precision may approach the measurement precision. Although it is important to understand the limitations of cosmogenic dating, it is equally important not to lose sight of the potential of this powerful tool for quantitative geomorphologic studies. Introduction Determining the ages and erosion rates of landforms is important to studies of long-term geomorphic processes. These parameters of landscape evolution can be estimated from the abundance of cosmogenic isotopes produced by cosmic rays in samples collected from Earth's surface. Cosmogenic analysis has stimulated widespread interest among Quaternary geologists because landscape stability and age are typically difficult to determine by other means. In this paper, simple mathematical models are used to calculate theoretical precisions for different strategies designed to estimate exposure ages and erosion rates from measurements of cosmogenic isotope abundance. While straightforward, the calculations for the general case are more complicated than simple error propagation, mainly because two nonlinear equations must be solved simultaneously. The results are important both for optimizing experimental design (picking which isotopes to use, for example) and for predicting or evaluating the significance of geologic conclusions drawn from cosmogenic isotope analysis. Cosmogenic isotopes are produced in terrestrial rocks by a variety of nuclear interactions occurring primarily within 1Now at Department of Geology, University of Vermont, Burlington.
Report On the Workshop On the Calibration of the Radiocarbon Dating Time Scale
Radiocarbon, 1980
and other Participants of the Workshop Last winter, Tucson was warmed by some lively and very useful group discussions concerning calibration of the radiocarbon time scale. Thanks to support from the USA National Science Foundation, it was possible to bring together representatives of the major USA laboratories working on the calibration problem as well as statisticians and some observers representing users and the f under. The workshop was convened by P E
Radiocarbon dating and its applications in Quaternary studies
E&G Quaternary Science Journal, 2008
This paper gives an overview of the origin of 14 C, the global carbon cycle, anthropogenic impacts on the atmospheric 14 C content and the background of the radiocarbon dating method. For radiocarbon dating, important aspects are sample preparation and measurement of the 14 C content. Recent advances in sample preparation allow better understanding of long-standing problems (e.g., contamination of bones), which helps to improve chronologies. In this review, various preparation techniques applied to typical sample types are described. Calibration of radiocarbon ages is the fi nal step in establishing chronologies. The present tree ring chronology-based calibration curve is being constantly pushed back in time beyond the Holocene and the Late Glacial. A reliable calibration curve covering the last 50,000-55,000 yr is of great importance for both archaeology as well as geosciences. In recent years, numerous studies have focused on the extension of the radiocarbon calibration curve (INTCAL working group) and on the reconstruction of palaeo-reservoir ages for marine records. [Die Radiokohlenstoffmethode und ihre Anwendung in der Quartärforschung] Kurzfassung: Dieser Beitrag gibt einen Überblick über die Herkunft von Radiokohlenstoff, den globalen Kohlenstoffkreislauf, anthropogene Einfl üsse auf das atmosphärische 14 C und die Grundlagen der Radiokohlenstoffmethode. Probenaufbereitung und das Messen der 14 C Konzentration sind wichtige Aspekte im Zusammenhang mit der Radiokohlenstoffdatierung. Gegenwärtige Fortschritte in der Probenaufbereitung erlauben ein besseres Verstehen lang bekannter Probleme (z.B. die Kontamination von Knochen) und haben zu verbesserten Chronologien geführt. In diesem Überblick werden verschiedene Aufbereitungstechniken für typische Probengattungen beschrieben. Der letzte Schritt beim Erstellen einer Chronologie ist die Kalibration der Radiokohlenstoffalter. Die gegenwärtige auf Baumringzeitreihen basierende Kalibrationskurve wird stetig über das Holozän und Spätglazial hinaus erweitert. Eine zuverlässige Kalibrationkurve für die letzten 50.000-55.000 Jahre ist von herausragender Bedeutung sowohl für die Archäologie als auch die Geowissenschaften. In den letzten Jahren haben zahlreiche Studien an der Erweiterung der Radiokohlenstoff-Kalibrationskurve (INTCAL working group) und an der Rekonstruktion des Paläo-Reservoireffekts in marinen Archiven gearbeitet.