Evaluation of the 129I Half-Life Value Through Analyses of Primitive Meteorites.

The preserved record of decay of now-extinct 129I into 129Xe forms the basis of the I-Xe chronometer. Comparison of the high precision I-Xe and Pb-Pb ages of chondrules and pure mineral phases separated from eight meteorites suggests the 17.5 ÷ 14.6 Ma range for the 129I half-life, assuming that the 235U and 238U half-lives are correct. The mean value of 16 Ma indicates that the 15.7 Ma half-life of 129I used here for the I-Xe age calculations is most probably correct. Since the 129I half-life value only affects the relative I-Xe ages, the few Ma relative to the Shallowater standard, the absolute I-Xe ages are almost immune to this uncertainty in the 129I half-life.

1. Introduction

There are four experimental values available for the 129I half-life [Table I]. The recommended 129I half-life of 17 ± 1 Ma was derived from them as an unweighted average [5] and later as a weighted average, 16.1 ± 0.7 Ma [6], when experimental details for the more precise measurements [2, 3] became available. The half-life value of 15.7 ± 0.6 Ma [3] is also routinely used in the literature.

Table I

The experimental half-life values of 129I.

Author (year of publication)	T1/2 Ma
[1] Katcoff (1951)	17.2 ± 0.9
[2] Russel (1957)	15.6 ± 0.6
[3] Emery (1972)	15.7 ± 0.6
[4] Kuhry (1973)	19.7 ± 1.4

Here we independently evaluate the 129I half-life using the precise I-Xe ages of chondrules and different mineral phases separated from the primitive meteorites.

2. Experimental

2.1 I-Xe chronometry

Discovery of nucleosynthetic input into the Solar System within 107–108 years of its formation was made based on observations of an excess of stable 129Xe in a meteorite due to β decay of now-extinct 129I [7]. The preserved record of the 129I decay forms the basis of I-Xe radiometric dating, capable of deciphering the early Solar System processes with a high degree of precision [8].

In I-Xe dating the 129I/127I ratio at closure of the iodine host mineral is the value of interest. A tracer for 127I is iodine-derived *128Xe, produced by neutron capture on stable 127I in a reactor

where Φ is neutron fluence, σ - effective cross section.

It is measured along with radiogenic *129Xe produced by 129I decay, so the *129Xe/*128Xe ratio provides the chronometry, avoiding the problems associated with measurements of absolute quantities of either parent, typically at 10–100 ppb level, or daughter. The direct monitoring of the neutron capture probability in 127I is avoided by use of a meteoritic standard, which is irradiated along with the sample. The I-Xe age Δt of the sample is then determined relative to the standard [8]: where τ is the mean lifetime of 129I.

2.2 Comparison of the I-Xe and Pb-Pb ages

When two chronometers experience concordant evolution and all the decay constants are correct, the corresponding ages, measured in the same samples, fall on a correlation line with the slope of 1 (Fig. 1). For the evaluation of the 129I half-life value, I-Xe ages should be compared to a high precision chronometer, Pb-Pb being a logical choice, with the implicit assumption that both chronometers closed at the same time and date the same event. Unfortunately, minerals rich in iodine (hence radiogenic *129Xe) usually have concentrations of uranium that are too low for the high precision Pb-Pb dating, and vice versa. Very few meteoritic materials fit the requirements for both good Pb-Pb and good I-Xe ages.

Fig. 1

Comparison of I-Xe and U-corrected P-Pb ages measured in a range of materials from the early Solar System [Table II]. The dotted free-fit correlation line is based on all available data. The solid free fit correlation line is based on chondrules data only (solid symbols). Both are plotted using a weighted total least-squares algorithm. All I-Xe ages are calculated assuming the 129I half-life of 15.7 Ma.

Samples used for the comparison of I-Xe and Pb-Pb ages are combined in Table II. I-Xe ages are calculated using the 129I half-life of 15.7 Ma. The Pb-Pb ages are corrected using the latest 238U/235U ratio for the Earth and the Solar System of 137.794 ± 0.027 and meteorite specific 238U/235U ratios for Richardton (137.711 ± 0.008) and Acapulco (137.796 ± 0.013) [10]. An average of two Allende bulk 238U/235U ratios [10] was applied for the correction of the Pb-Pb ages of the earliest chondrules. The 238U/235U ratio of 137.794 ± 0.014 for Gujba [9] was used here as a proxy for the HH 237 238U/235U ratio value.

Table II

Samples used for the I-Xe Pb-Pb ages correlation plots and their respective ages. I-Xe ages are relative to the meteoritic standard Shallowater.

Meteorite (Type)	I-Xe age, 106 yearsShallowater ≡ 0		Pb-Pb age, 106 years
Chainpur, LL3.4Semarkona, LL 3.0Allende, CV3	chondrules [11, 12]	4.3 ± 0.6	chondrules [13]	4565.6 ± 1.0
HH 237 (CB)	chondrule [14]	−0.29 ± 0.16	silicates [15]	4561.9 ± 0.9
			Gujba chondrules[16]	4562.5 ± 0.2
Richardton (H4)	chondrule 2 [17]	−0.1 ± 0.1	chondrule 2 [17]	4561.5 ± 0.8
	chondrule 6 [17]	−4.1 ± 0.6	chondrule 6 [17]	4557.3 ± 1.4
	pyroxene [18]	1.1 ± 2.0	pyroxene [19]	4560.3 ± 0.6
Acapulco	feldspar [20]	−3.8 ± 1.5	phosphate [21]	4556.2 ± 1.0
Ste Marguerite(H4)	feldspar [20]	0.7 ± 0.4	phosphate [21]	4561.8 ± 0.3

The slopes of the I-Xe – Pb-Pb correlation lines range from 1.12 ± 0.19 for the whole set of data points from Table II to 0.91 ± 0.21, when only chondrule data are considered [solid symbols, Fig. 1]. In order to bring the slope values to 1, the half-life of 129I should be ~17.4 Ma when all data are considered, or ~ 14.6 Ma for the chondrules- only correlation.

Additional data points (not listed in Table II) consistently bring the slope of the regression bellow 1.12. However, the underlying problem here is the divergence of minerals suitable for dating by both chronometers. In fact, HH237 chondrule is the only sample in this compilation with strong isotopic and mineralogical evidence for the simultaneous closure of the I-Xe and Pb-Pb systems [14], in line with the proposed formation of CB chondrules from a melt fraction of the impact-generated plume [15].

3. Conclusion

Comparison of precise I-Xe and Pb-Pb ages measured in chondrules and mineral phases separated from eight meteorites suggests the 17.5 ÷ 14.6 Ma range for the 129I half-life value, assuming that the 235U and 238U half-lives are correct. The mean value of 16 Ma indicates that the 15.7 Ma half-life of 129I used here for the I-Xe age calculations is most probably correct. Since the 129I half-life only affects the relative I-Xe ages, the absolute I-Xe ages, calculated relative to the Shallowater standard age of 4562.4 ± 0.2 Ma, are almost immune to this uncertainty in the 129I half-life.