Book contents
- Frontmatter
- Dedication
- Contents
- Preface
- Acknowledgments
- 1 Basics
- 2 Nuclear Transformations
- 3 Nucleosynthesis
- 4 Isotopics
- 5 Radioactivity and Radiometric Dating
- 6 Mass Spectrometry and Isotope Geochemistry
- 7 Error Analysis
- 8 Meteorites: Link between Cosmo- and Geochemistry
- 9 Chronology of Meteorite History
- 10 Chemical Evolution of the Earth
- 11 Chronology of Earth History
- References
- Index
9 - Chronology of Meteorite History
Published online by Cambridge University Press: 21 October 2017
- Frontmatter
- Dedication
- Contents
- Preface
- Acknowledgments
- 1 Basics
- 2 Nuclear Transformations
- 3 Nucleosynthesis
- 4 Isotopics
- 5 Radioactivity and Radiometric Dating
- 6 Mass Spectrometry and Isotope Geochemistry
- 7 Error Analysis
- 8 Meteorites: Link between Cosmo- and Geochemistry
- 9 Chronology of Meteorite History
- 10 Chemical Evolution of the Earth
- 11 Chronology of Earth History
- References
- Index
Summary
The problem then is the identification, within that piece of matter, those observations which can establish the nature and chronology of events whose memory has persisted. Such an approach requires the use of concepts, observations and measurement techniques from rather diverse fields and together define a new field, that of cosmochemistry or cosmophysics
G J WasserburgINTRODUCTION
The last chapter contained a brief description of the processes leading up to the formation and subsequent changes of meteorites in the early Solar System. Figure 9.1 recapitulates the same, and, in addition, includes the later stages or events in the history of meteorites terminating with their capture by the earth.
A plausible list of distinguishable stages/events is:
1. Separation of a parcel of gas and dust from the interstellar medium, and its collapse into the proto-sun and proto-planetary nebular disc.
2. Formation from the cooling solar nebular disc of the precursor materials of chondrites, like CAIs, AOAs, low temperature compounds, and finally chondrules in a cool environment.
3. Accretion of mineral grains in various proportions, and chondrules into km-sized planetesimals (parent bodies of meteorites) including presolar grains directly from the interstellar medium
4. Metamorphism, fluid alteration, recrystallization on a local scale on the surface of, and even melting inside of parent bodies that escaped further growth in size into embryos and eventually planets.
5. Impact brecciation and shock reheating, presumably a few times.
6. Collisional ejection of metre-sized fragments (meteorites) from parent bodies, and their exposure to galactic cosmic rays in space.
7. Capture of meteorites by the earth, and their recovery later by chance (Finds).
8. Capture of meteorites by the earth and their prompt recovery (Falls).
This sequence of events is, of course, very general and meant as a useful framework for discussion. For example, not all stages may be recorded in a single meteorite. Some processes, like metamorphism, may be missing in primitive carbonaceous chondrites. Some events may have overlapped in time. The relative order of at least some of the events is obvious. For example, a carbonaceous chondrite is a nonequilibrium assemblage of refractory inclusions, metal, sulfide, and matrix, each of which formed independently. So, the chondrite should postdate these constituents. A long-range goal of meteorite research is to quantitatively date every stage precisely and accurately.
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- Principles of Radiometric Dating , pp. 121 - 139Publisher: Cambridge University PressPrint publication year: 2017