Evidence for extinct 36Cl from excess 36Ar in Allende sodalite

5103EVIDENCE FOR EXTINCT 36Cl FROM EXCESS 36Ar IN ALLENDESODALITEG. Turner1, R. Burgess1, S. P. Kelley2 and G. J. Wasserburg3. 1SEAES,University of Manchester, UK, E-mail: [email protected]. 2Dept. of Earth and Environmental Science, Open University, UK.3Division of Geological and Planetary Science, California Institute ofTechnology, USA.Introduction: Excesses of 36S have previously been reported insodalite (Na8Al6Si6O24Cl2) and wadalite (Ca6Al5Si2O16Cl3) in CAI andchondrules from the Allende and Ningqiang meteorites and used to inferthe presence of 36Cl (half-life 3 · 105 a) in the early solar system [1–3]. Inspite of the fact that 36Cl decays predominantly (98%) to 36Ar, allattempts to locate the very large amounts of 36Ar implied by these 36Sexcesses have been negative. Here we present results of a new method toidentify 36Ar from 36Cl decay, based on low fluence fast neutronactivation.Method: We have irradiated two batches of Allende sodalite (PinkAngel) with different fluencies of fast neutrons in the Cd-shielded CLICITfacility of the Oregon State TRIGA reactor. In the absence of trappedargon, the upper intercept of a plot of 36Ar/38Ar against 37Ar/38Ar definesa relationship between 36Ar and 38Ar from the action of cosmic rayinducedsecondary neutrons and mono-isotopic 36Ar from 36Cl decay. Bycombining data from two irradiations, the two contributions can beunambiguously distinguished. Figure 1 shows allowed combinations of36Cl/35Cl and the product of cosmogenic secondary neutron fluence and37Cl cross section for each series. The intersection corresponds to36Cl/35Cl = (1.9 ± 0.5) · 10)8 and = (1.1 ± 0.8) · 10)10.The latter is in accord with the neutron fluence observed in CAIs inAllende [4]. The 36Cl/35Cl ratio is 3 orders of magnitude lower than thehighest value inferred from 36S excesses. If interpreted chronologically,i.e., as a closure age, it would correspond to a period of 3Ma or so ofactivity involving the precipitation of sodalite, which is consistent withI-Xe data [5]. Alternatively, the low ratio may represent 36Ar loss in linewith the low activation energy for Ar diffusion in sodalite. The presenceof excess 36Ar together with 36S supports the idea that live 36Cl waspresent in the sodalite, as opposed to the 36S being inherited. Themechanism for producing 36Cl several million years after the formation ofthe solar system is still unknown.References: [1] Lin Y. et al. 2005. Proceedings of the NationalAcademy of Sciences 102:1306–1311. [2] Hsu W. et al. 2006. TheAstrophysical Journal 640:525–529. [3] Jacobsen B. et al. 2009. Abstract#2553. 40th Lunar and Planetary Science Conference. [4] WasserburgG. J. et al. 2011. Geochimica et Cosmochimica Acta, in press. [5] TurnerG. et al. Meteoritics & Planetary Science 44:A5255.