Geocheminar spring-2016

36Cl-36S and 26Al-26Mg systematics in the U-depleted Curious Marie CAI: New insights into early aqueous alteration and the co-existence of 26Al and 36Cl in the early solar nebula

Curious Marie is an intensely altered Allende CAI with Group II REE pattern. It was characterized as U-depleted with high d235U produced from 247Cm decay (t1/2=15.6 Myr). Here we analyzed oxygen isotopes, 26Al-26Mg and 36Cl-36S systematics to understand the origin and evolution of this CAI. Oxygen isotopic compositions in the secondary minerals of Curious Marie are consistent with the common CAIs. However, unlike the terrestrial 26Mg isotopic composition in the secondary sodalite observed in previous studies, an elevated, yet uniform 26Mg excess was observed. The model 26Al/27Al ratio calculated using bulk Al/Mg ratio and the uniform 26Mg excess is identical to the canonical value. Since the aqueous alteration involving an external oxidizing fluid can result in chemical exchange and the redistribution of Mg isotopes, the canonical model 26Al/27Al ratio implies not only the early condensation of its precursor but the extremely early aqueous alteration when 26Al was still close to the canonical value. Subsequently, a closed system thermal process is required to homogenize Mg isotopic composition without the contamination from chondritic Mg. In addition, homogeneous 36S excesses attributed to 36Cl decay (t1/2=0.3 Myr) is also detected in the secondary phases of Curious Marie, although the bulk Cl/S ratio is unknown, hindering a reliable estimate on the model ratio of 36Cl/35Cl. However, given that 36Cl can be introduced through Cl-rich fluid in the aqueous alteration related to sodalite formation, we propose the possibility that 26Al and 36Cl co-existed in the early solar nebula, and therefore, 36Cl in the extraterrestrial objects can not only be produced from late local irradiation but also inherited from the stellar source.