Search for Extinct Chlorine-36 in an Allende Cai

Introduction: The search for the former existence in the Early Solar System of the now extinct radionuclide Cl (T1⁄2 = 3x10 a) has proven to be difficult [e.g., 1]. This is at least partly due to the volatility of chlorine and the fact that in primitive meteorites Cl occurs in high abundance only in alteration phases such as sodalite (Na8Al6Si6O24Cl2). 36-Cl decays to both Ar by β decay (98.1 %) and to S by β decay and electron capture (1.9 %). Early attempts to establish its presence focused on Ar in fine-grained CaAl-rich inclusions of the Allende CV3 meteorite that contain several thousand ppm of Cl in bulk [2]. Excesses of Ar observed at low to intermediate gas extraction temperatures, with Ar/Ar ranging as high as ~90 [2,3], if interpreted as due to extinct Cl, correspond to Cl*/Cl ratios on the order of ~ 10, but could be explained as due to neutron irradiation during the ~5 Ma recent cosmic ray exposure of the meteorite. A positive result was reported by [4] who observed apparent Ar overabundances in bulk measurements of the CV3 meteorite Efremovka. Their inferred Cl*/Cl ratio of ~1.4x10 has been widely cited in the subsequent literature and used in models that try to provide a coherent model for the origin of extinct radionulcides (e.g., [5, 6]), but has been shown to have arisen from an analytical artifact [7]. Only recently there appears to be unambiguous evidence for the presence of Cl in the ESS. This is based on excesses of S observed by SIMS that correlate with Cl/S ratio in a CAI from the Ningqiang anomalous carbonaceous chondrite [8] and, more recently in the “Pink Angel” inclusion as well as a porphyritic olivine chondrule in Allende [9]. From a Cl*/Cl ratio of ~4x10 at the time of sodalite formation the authors infer a ratio at the time of CAI formation of >10. Using the NanoSIMS with its higher sensitivity and multi-collection capability we have begun a study of the Cl-S system in CAIs hoping to confirm the results of [8, 9] with higher accuracy. Sample: A thick section was prepared from the Allende meteorite containing a “typical” fine-grained inclusion (CAI MP-1) similar to those studied by [2] for Cl abundance and noble gases. Seven areas of some 200 to 700 μm size within the CAI were mapped for Cl abundances leading to the identification of numerous grains of sodalite, with a typical size ~20 μm (Figs. 1, 2). Notably, we also found sodalite grains in Allende matrix, contrary to the case of [9] who did not observe sodalite in Allende matrix nor in a second chondrule close to the Pink Angel.