Late formation of a comet Wild 2 crystalline silicate particle, Pyxie, inferred from Al-Mg chronology of plagioclase

We examined the Al-Mg isotope systematics of plagioclase in a FeO-poor ferromagnesian Wild 2 particle (C2092,7,81,1,0; named Pyxie) using a ~2 μm spot. Three analyses show average ²⁷Al/²⁴Mg ratio of ~65 and excess δ²⁶Mg^* value of +0.1±4.5‰ (2σ), indicating no resolvable ²⁶Mg excess in the particle. The inferred initial (²⁶Al/²⁷Al)₀ ratio of plagioclase in Pyxie is estimated as (-0.6±4.5)×10^-6 with an upper limit of 4×10^-6. The result is very similar to that of the FeO-rich ferromagnesian particle "Iris" (Ogliore et al., 2012). Assuming homogeneous distribution of ²⁶Al in the early solar system, Pyxie formed at least 2.6 Ma after the oldest Ca-Al-rich inclusions. This minimum formation age is marginally younger than formation ages of most chondrules in type ~3.0 chondrites but comparable with those of Mg#<98 chondrules in CR3 chondrites. Considered in conjunction with similar oxygen isotope ratios between Pyxie (and Iris) and Mg#<98 chondrules in CR3 chondrites, it is inferred that the ferromagnesian Wild 2 particles and Mg#<98 chondrules in CR3 chondrites formed late in local disk environments that had similar oxygen isotope ratios and redox states.