Out of every record with a 232Th-208Pb concordance score, 53.3% (80 out of 150) are concordant (have a score of 1.00) and 28.7% (43) are discordant (have a score of 0.00). When compared with other methods, 232Th-208Pb shows the highest degree of concordance with 238U-206Pb (concordance score 0.55). Its scores with Pb-Pb (0.18) and FT (0.12) are extremely low, indicating very little agreement between these methods. 232Th-208Pb typically yields the lowest ages of all the U-Th-Pb methods, and significantly lower ages than Pb-Pb (which gives lower ages only 15.4% of the time). However, it yields greater ages than FT 82.4% of the time. Thus, 232Th-208Pb tends to give lower ages than other methods, with the exception of FT. 6. Potassium-argon (K-Ar) For all K-Ar age determinations for the same record, K-Ar has a 0.62 average concordance score with itself (that is, compared with other K-Ar determinations for the same records). For “Three Methods Comparisons,” the average score is similar (0.63). Out of every record with a K-Ar concordance score, 57.6% (1,228 out of 2,131) are concordant (have a score of 1.00) and 33.2% (708) are discordant (have a score of 0.00). When compared with other methods, K-Ar shows the highest degree of concordance with Rb-Sr (concordance score of 0.51). This is slightly higher than with FT (0.49 concordance). K-Ar has significantly less concordance with each U-Pb method, the highest being 0.34 (232Th-208Pb). K-Ar yields consistently lower age estimations when compared with most other methods for the same records. K-Ar ages are lower than those given by every U-Th-Pb method at least 67.1% of the time. The only method that yields consistently lower ages than K-Ar is FT (72.1% of the time). This suggests that K-Ar typically underestimates ages in relation to other methods, with the exception of FT. 7. Fission track (FT) For all FT age estimations for the same record, FT has a 0.56 average concordance score with itself (that is, compared with other FT estimations for the same records), which is the lowest of all the methods. For “Three Methods Comparisons,” the average score is similar (0.33). Out of every record with an FT concordance score, 49.3% (150 out of 304) are concordant (have a score of 1.00) and 35.5% (108) are discordant (have a score of 0.00). When compared with other methods, FT shows the highest degree of concordance with K-Ar (concordance score 0.49). The next highest score is with Rb-Sr (0.21), and the lowest is with Pb-Pb (0.00). The lack of concordance between FT and Pb-Pb ages is very striking, especially given that the FT ages are greater 45.1% of the time, which is far more often than in any of the other pairwise comparisons with FT. FT typically yields the lowest ages of all the methods, giving significantly lower ages at least 54.9% of the time. This result is perhaps unsurprising given that fission tracks can be thermally reset and are therefore typically regarded as minimum ages. 8. Samarium-neodymium (Sm-Nd) As noted previously, only 32 records included Sm-Nd ages. Interestingly, all came from five adjacent counties in north-central California and together yielded only three numerically unique ages (either 178.00, 314.00, or 575.00 million years). Moreover, only seven of the 32 records included age calculations for other methods. We omitted these records from our analysis since any conclusions drawn would be statistically insignificant. B. “Three Methods Comparison” Only 34 records in the database (0.18%) included ages calculated using three or more different methods (when all the U-Th-Pb methods are counted as one method). The distribution of concordance scores for this subset of records shows fewer extreme values but is concentrated toward lower values (Fig. 3). Appendix C provides the full data for all of our “Three Methods Comparisons.” The concordance scores for this dataset are shown in Table 4. The average concordance score for our “Three Methods Comparisons” is 0.39, with only one of the 34 records (2.9%) having a score of 1.00. This suggests that records with ages calculated by multiple methods tend to have lower concordance scores. Figure 3. The distribution of concordance scores for all 34 records in the National Geochronological Database with ages determined using three or more radioisotope methods, at least two of which were not in the U-Th-Pb decay chain. This is the “Three Methods Comparison” dataset. BEACHY, KINARD AND GARNER How often do radioisotope ages agree? 2023 ICC 392
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