Jourdan,F., Renne,P.R. and Reimold,W.U.(2009): An appraisal of the ages of terrestrial impact structures. Earth and Planetary Science Letters, 286, 1-13.


 There are 174 confirmed impact structures known on Earth (e.g.,; late 2008) but a far smaller number of impact structures has yielded a well-constrained age. Precise and accurate age constraints are crucial for (1) correlating causes and effects on the bio- and geosphere of catastrophic processes, (2) better constraining the impactor flux through geological time and evaluation of potential inpact periodicity, (3) calibrating the absolute chronostratigraphic time scale, (4) calibrating the age of within-crater continental sedimentary deposits (e.g., for regional paleo-climatic analysis), and (5) correlating impact events and distal impact ejecta occurrences.
 Of these 174 listed impact structures only a few have precisely constrained ages (mostly obtained using radio-isotopic technique, e.g. U/Pb and 40Ar/39Ar), with only 25 ages having a stated precision better than ±2%, and a mere 16 ages with a precision better than ±1%. Yet, even the accuracy of some of these ages can be challenged and probably improved based on more detailed interpretations and statistically more rigorous data analysis. Although geochronologists are often circumspect and advise caution in accepting calculated ages, these ages tend to propagate into the literature without further critical evaluation, are considered “robust”, and become widely accepted ages. A review of the age data for the 25 short-listed structures suggests that 11 ages are accurate, 12 are at best ambiguous and should not be reported with any uncertainty, and 2 are not well characterized at all. We report detailed examples of misleading ages and/or age uncertainties (e.g., poor stratigraphic constraints, data over-interpretations, ambiguity due to inconsistent results), and highlight the robustness of the 11 well-defined ages. Based on observations and modeling, suggestions are made on how to obtain better ages by carrying out adequate sample preparation. We also indicate how to interpret ages for non-geochronologists. This brief review should be interpreted as a call for immediate, drastic qualitative and quantitative improvements of the ages of terrestrial impact structures.

Keywords: impact; crater; absolute chronology; isotopic dating; 40Ar/39Ar; U/Pb; age database』

1. Introduction
2. How many craters have been sated so far?
3. Dating tools
4. Geological, geochronological and statistical constraints on age data
 4.1. 40Ar/39Ar dating
 4.2. U/Pb dating
 4.3. Statistical constraints
5. Investigation of selected cases
 5.1. Gardnos - poor stratigraphic constraints
 5.2. Gosses Bluff - age spectrum over-interpretation
 5.3. Shoemaker - Rb/Sr chronometer and alteration
 5.4. Manson - ambiguous dates due to multi-phase age discrepancies
 5.5. Araguainha - ambiguous 40Ar/39Ar age spectra
 5.6. Popigai - the complex case
 5.7. Bedout - the hoax
 5.8. Vredefort, Chixculub, Janisjarvi(両方のaの頭に¨) and 8 others - the robust ages updated
6. Appraisal of the available impact crater ages
7. Conclusions
Appendix A. Supplementary data