Book contents
- Frontmatter
- Contents
- List of contributors
- Preface
- Acknowledgments
- List of abbreviations and acronyms
- Part I INTRODUCTION
- Part II CONCEPTS AND METHODS
- 3 Biostratigraphy: time scales from graphic and quantitative methods
- 4 Earth's orbital parameters and cycle stratigraphy
- 5 The geomagnetic polarity time scale
- 6 Radiogenic isotope geochronology
- 7 Strontium isotope stratigraphy
- 8 Geomathematics
- PART III GEOLOGIC PERIODS
- Part IV SUMMARY
- Appendix 1 Recommended color coding of stages
- Appendix 2 Orbital tuning calibrations and conversions for the Neogene Period
- Appendix 3 Geomathematics
- Bibliography
- Stratigraphic Index
- General Index
6 - Radiogenic isotope geochronology
Published online by Cambridge University Press: 05 June 2012
- Frontmatter
- Contents
- List of contributors
- Preface
- Acknowledgments
- List of abbreviations and acronyms
- Part I INTRODUCTION
- Part II CONCEPTS AND METHODS
- 3 Biostratigraphy: time scales from graphic and quantitative methods
- 4 Earth's orbital parameters and cycle stratigraphy
- 5 The geomagnetic polarity time scale
- 6 Radiogenic isotope geochronology
- 7 Strontium isotope stratigraphy
- 8 Geomathematics
- PART III GEOLOGIC PERIODS
- Part IV SUMMARY
- Appendix 1 Recommended color coding of stages
- Appendix 2 Orbital tuning calibrations and conversions for the Neogene Period
- Appendix 3 Geomathematics
- Bibliography
- Stratigraphic Index
- General Index
Summary
The isotopic systems used for the chronometric calibration of the time scale are reviewed. Multiply concordant analyses based on U—Pb dates from the Thermal Ionization Mass Spectrometric (TIMS) method are regarded as the most robust and accurate measure of age for the Mesozoic and Paleozoic, followed by dates that imply only minor episodic Pb loss or inheritance of pre-existing material. For the Mesozoic and Cenozoic, 40Ar/39Ar mineral analyses of biotite, sanidine, and hornblende from volcanic rocks that show no evidence for thermal alteration or of excess 40 Ar also provide excellent material for time scale calibration.
INTRODUCTION
The chronometric calibration of stratigraphic boundaries underpins the geologic time scale. It was the discovery of radioactive decay and the recognition that its measurement in minerals gave temporal information (Holmes, 1937, 1947) that provided the impetus for the development of time scales calibrated in millions of years. Updated time scales were published as new calibration points slowly, but regularly, appeared (Holmes, 1947, 1960; Kulp, 1961; Armstrong, 1978; Harland et al., 1982; Palmer, 1983).
The last quarter of a century has witnessed an explosive growth in data, an increased precision of measurement, the inter-laboratory standardization of decay constants (Steiger and Jäger, 1978) and a better understanding of the isotope systematics of analyzed materials.
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- A Geologic Time Scale 2004 , pp. 87 - 95Publisher: Cambridge University PressPrint publication year: 2005
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