Figure 2. Scatterplot of the number of ages involved in the calculation of the concordance score for all 4,875 records, including the linear regression and R2 value. Note that each datapoint may represent any number of records with the same concordance score and number of age determinations. est concordance with 238U-206Pb (concordance score of 0.79). Like 238U-206Pb, its concordance scores with K-Ar (0.27) and FT (0.14) are extremely low, indicating very little agreement between these methods. 235U-207Pb yields consistently greater ages than K-Ar (77.5% of the time), 238U-206Pb (85.0%), and FT (85.7%), but consistently lower ages than Pb-Pb (84.7% of the time). Perhaps unsurprisingly it appears to be very similar to 238U-206Pb with respect to concordance scores. 4. Uranium-lead (238U-206Pb) For all 238U-206Pb age determinations for the same record, 238U-206Pb has a 0.76 average concordance score with itself (that is, compared with other 238U-206Pb estimations for the same records). For “Three Methods Comparisons,” the average score is similar (0.73). Out of every record with a 238U-206Pb concordance score, 66.3% (330 out of 498) are concordant (have a score of 1.00) and 13.7% (68) are discordant (have a score of 0.00). When compared with other methods, 238U-206Pb shows the highest degree of concordance with 235U-207Pb (concordance score of 0.79), Its concordance scores with K-Ar and FT are extremely low (0.24 and 0.06 respectively), indicating very little agreement between these methods. 238U-206Pb yields consistently greater ages than K-Ar (greater 71.9% of the time) and FT (87.1%), but consistently lower ages than Pb-Pb (lower 85.0% of the time) and 235U-207Pb (83.2%). 5. Thorium-lead (232Th-208Pb) For all 232Th-208Pb age determinations for the same record, 232Th-208Pb has a 0.62 average concordance score with itself (that is, compared with other 232Th-208Pb determinations for the same records). For “Three Methods Comparisons,” the average score is similar (0.60). BEACHY, KINARD AND GARNER How often do radioisotope ages agree? 2023 ICC 391
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