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- Title
REASSESSMENT OF THE EVIDENCE FOR <sup>36</sup>Ar FROM <sup>36</sup>Cl DECAY IN ALLENDE SODALITE.
- Authors
Turner, G.
- Abstract
Introduction: A reassessment of the published evidence1 for excess 36Ar from 36Cl decay in Allende sodalite, based on isotope mixing calculations, supports the former presence of 36Cl with a 36Cl/35Cl ratio of 1.9x10-8 to 2.2x10-8. While most data points, figures below, lie on correlation lines corresponding to excess 36Ar from 36Cl decay, some define a lower line with an intercept expected for irradiation by cosmogenic neutrons over the last 5.2 Myrs, i.e. from sodalite which lost its excess 36Ar or never had any. The implication that the 36Cl was introduced live into the sodalite ~7Ma after CAI formation places serious constraints on the timing of possible early solar system models combined with neutron fluence requirements. A lower limit on the neutron flux requirement, 23 times the current cosmogenic flux, is possible but only if the sodalite formation coincided closely with the end of neutron irradiation. Isotope mixing calculations: For a mixture (C) of two isotope ratios (A) and (B) the fraction f[B]of element B in the mixture is given by f[B] = [(A) - (C)]/[(A) - (B)]. Applying this expression to (36Ar/38Ar), the intercept in figs. 1a and 1b, we obtain: (36Ar/38Ar)A = (36Ar/38Ar)/(1 - [38Ar]B/[[38Ar]B + [38Ar]A]). Subscript A refers to Ar produced by cosmogenic neutron irradiation plus any additional 36Ar component. Subscript B refers to Ar isotopes produced by the reactor neutron irradiation. Note that (36Ar/38Ar)B = 0, reflecting the long half-life of 36Cl produced. The two unknowns are (36Ar/38Ar)A and [38Ar]A. [38Ar]B is the amount of 38Ar produced from 37Cl in each of the reactor irradiations, (5.43 ± 0.54)x10-10 and (2.00 ± 0.05)x10-9 respectively. (36Ar/38Ar) is the value of the intercept for each of figs 1a and 1b, 139 ± 2 and 42.5 ± 1.0. Substituting the known values and solving the two simultaneous equations we obtain, [38Ar]A/[37Cl] = (1.06 ± 0.11) x10-10 and (36Ar/38Ar)A = 855 ± 78. The proportion of (36Ar/38Ar)A which can be assigned to cosmogenic neutron production is s35/s37. 35Cl/37Cl. (1 - t36/Texp). B36, where s35 and s37 are the neutron capture cross sections, t36 the mean life of 36Cl, Texp the cosmic ray exposure duration of Allende, 5.2 Ma, and B36 the fraction of 36Cl which decays to 36Ar, 98%. Based on estimates of s35/s37 from 68 to 100, (36Ar/38Ar) from cosmogenic neutron irradiation is in the range 200 to 290, and [36Ar]/[35Cl] = (0.69 to 1.00)x10-8. The remaining excess 36Ar corresponds to (36Ar/38Ar) from 565 to 655. Relative to 35Cl this indicates [36Ar]/[35Cl] = 1.9x10-8 to 2.2x10-8, which is consistent with the value published previously for (36Cl/35Cl)1. Lower intercept: A question, not considered by Turner et al1, concerns the significance of data points below the main correlation lines in figures 1a and 1b. In particular the data from sodalite PA4 defines a separate correlation and intercept, indicated by the dashed line in fig.1b. Especially noteworthy is the intercept, (36Ar/38Ar) ~13, which is as expected from recent cosmic ray irradiation alone. This indicates that some areas of sodalite have lost or never contained excess 36Ar from 36Cl and may be a feature of previous unsuccessful attempts to locate 36Ar from 36Cl decay. Modelling, timing and neutron fluxes: The absence of neutron generated 128I and 38Ar, above the cosmogenic level, was the sine qua non used to identify 36Cl as the source of the excess 36Ar. A direct implication is that live 36Cl, with (36Cl/35Cl) ~ 2x10-8 was incorporated into the sodalite sometime after the end of the neutron irradiation. I-Xe dating indicates that the sodalite formed 7 million years after the host CAI1. The level of (36Cl/35Cl) produced by a uniform neutron irradiation of a Cl bearing fluid is: N = P/l.(1 - exp(-lT1).exp(-lT2), T1 the duration of the irradiation, T2 the time after. P = (s35.F). F is the neutron flux and s35 the 35Cl capture cross section. Solutions with P > 23 times the cosmogenic flux are required but neither the actual value nor the irradiation environment are currently known.
- Subjects
SODALITE; COSMIC rays; NEUTRON irradiation; NEUTRON flux; NEUTRON capture; FLUID-film bearings; SIMULTANEOUS equations
- Publication
Meteoritics & Planetary Science, 2022, Vol 57, p6008
- ISSN
1086-9379
- Publication type
Article