@Article{Tang:2017, author = {H. Tang, M.-C. Liu, K.D. McKeegan, F.L.H. Tissot and N. Dauphas}, title = {In situ isotopic studies of the U-depleted Allende CAI Curious Marie: Pre-accretionary alteration and the co-existence of $^{26}$Al and $^{36}$Cl in the early solar nebula}, journal = {Geochim. Cosmochim. Acta.}, booktitle = {}, editor = {}, publisher = {}, month = {}, year = {2017}, volume = {207}, number = {}, pages = {1--18}, note = {}, annote = {}, keywords = {Curious Marie CAI; 26Al - 26Mg systematics; 36Cl - 36S systematics; Aqueous alteration; Origin}, url = {http://sims.ess.ucla.edu/PDF/Tang_et_al_2017.pdf}, doi = {10.1016/j.gca.2017.03.001}, isbn = {}, abstract = {The isotopic composition of oxygen as well as $^26}$Al-$^{26}$Mg and $^{36}$Cl-$^{36}$S systematics were studied in Curious Marie, an aqueously altered Allende CAI characterized by a Group II REE pattern and a large $^{235}$U excess produced by the decay of short-lived $^{247}$Cm. Oxygen isotopic compositions in the secondary minerals of Curious Marie follow a mass-dependent fractionation line with a relatively homogenous depletion in $^{16}$O ($\Delta$$^{17}$O of -8$\permil$) compared to unaltered minerals of CAI components. Both Mg and S show large excesses of radiogenic isotopes ($^{26}$Mg* and $^{36}$S*) that are uniformly distributed within the CAI, independent of parent/daughter ratio. A model initial $^{26}$Al/$^{27}$Al ratio [(6.2 $\pm$ 0.9) \times $10^{-5}$], calculated using the bulk Al/Mg ratio and the uniform $\delta$$^{26}$Mg* ~ +43$\permil$, is similar to the canonical initial solar system value within error. The exceptionally high bulk Al/Mg ratio of this CAI (~95) compared to other inclusions is presumably due to Mg mobilization by fluids. Therefore, the model initial $^{26}$Al/$^{27}$Al ratio of this CAI implies not only the early condensation of the CAI precursor but also that aqueous alteration occurred early, when $^{26}$Al was still at or near the canonical value. This alteration event is most likely responsible for the U depletion in Curious Marie and occurred at most 50 kyr after CAI formation, leading to a revised estimate of the early solar system $^{247}$Cm/$^{235}$U ratio of (5.6 $\pm$ 0.3) \times $10^{-5}$. The Mg isotopic composition in Curious Marie was subsequently homogenized by closed-system thermal processing without contamination by chondritic Mg. The large, homogeneous $^{36}$S excesses ($\Delta$$^{36}$S* ~ +97$\permil$) detected in the secondary phases of Curious Marie are attributed to $^{36}$Cl decay ($t_{1/2}$ = 0.3 Myr) that was introduced by Cl-rich fluids during the aqueous alteration event that led to sodalite formation. A model $^{36}$Cl/$^{35}$Cl ratio of (2.3 $\pm$ 0.6) \times $10^{-5}$ is calculated at the time of aqueous alteration, translating into an initial $^{36}$Cl/$^{35}$Cl ratio of ~1.73 \times $10^{-5}$ at solar system birth. The Mg and S radiogenic excesses suggest that $^{26}$Al and $^{36}$Cl co-existed in the early solar nebula, raising the possibility that, in addition to an irradiation origin, $^{36}$Cl could have also been derived from a stellar source.}, }