S Systematics of Wadalite from the Allende Meteorite

Introduction: The origin of short-lived radionuclides in the early Solar System is currently the subject of considerable debate. Their ultimate nucleosynthetic sources [c.f. 1-4] have important implications regarding the usability of these nuclides as high-resolution chronometers for dating early Solar System events. If these radionuclides originated from a nearby exploding star(s) and were well mixed into the molecular cloud from which our Solar System was born [e.g. 1,5], variations in abundances may reasonably be ascribed to the passage of time from an initial uniform inventory. However, models producing short-lived radionuclides via energetic particle bombardment [e.g., 3] accommodate widely variable abundances of radionuclide with no chronological significance. The issue is far from resolved and considerable efforts are required. Chlorine-36 (Cl) decays to Ar (98.1%, β) and S (1.9%, electron capture and β) with a half-life of 0.3 Ma. Along with Be [6], Cl is one of two shortlived radionuclides most likely produced by energetic particle irradiation within the Solar System [7]. Calculations [8] suggest that the collective fluence of protons, He and He needed to produce Be is comparable to that needed for production of Cl at the inferred initial Cl/Cl ratio reported by [7,9]. This fluence does not produce significant Al, consistent with the near absence of Al in sodalite (Na8Al6Si6O24Cl2). It is not clear, however, when, where and how volatile Cl was incorporated into the refractory mineral assemblages of Ca-Al-rich refractory inclusions (CAIs) in carbonaceous chondrites. The Pink Angel CAI studied by Hsu et al. [7] contains pure radiogenic Xe from the decay of I (t1⁄2 = 15.7 Ma) but does not contain radiogenic Ar from the decay of Cl or radiogenic Mg from decay of Al. Nor is it clear if the variable Cl/Cl initial ratios, ranging between (<1.6-4)×10 [e.g. 7,9,11], are due to temporal variations, disturbance to the Cl-S system in secondary, halogen-rich minerals, or a heterogeneous distribution of Cl in the early Solar System.