Combined U-corrected Pb-Pb dating and 26Al-26Mg systematics of individual chondrules–Evidence for a reduced initial abundance of 26Al amongst inner Solar System chondrules

Chondrites are fragments of asteroids that avoided melting and, thus, provide a record of the material that accreted to form protoplanets. The dominant constituent of chondrites are millimeter-sized chondrules formed by transient heating events in the protoplanetary disk. Some chondritic components, including chondrules, contain evidence of the extinct short-lived radionuclide ²⁶Al (half-life of 0.73 Myr). The decay of ²⁶Al is postulated to have been an important heat source promoting asteroidal melting and differentiation. Thus, understanding the ²⁶Al inventory in the accretion regions of differentiated asteroids is critical to constrain the accretion timescales of protoplanets. The current paradigm asserts that the canonical ²⁶Al/ ²⁷Al ratio of ∼ 5 × 10 ⁻⁵ recorded by the oldest dated solids, calcium-aluminium refractory inclusions, represents that of the bulk Solar System. We report, for the first time, the ²⁶Al- ²⁶Mg systematics of chondrules from the North West Africa (NWA) 5697 L 3.10 ordinary chondrite and Allende CV3 _OxA (Vigarano type) carbonaceous chondrite that have been previously dated by U-corrected Pb-Pb dating. Eight chondrules, which record absolute ages ranging from 4567.57 ± 0.56 to 4565.84 ± 0.72 Ma, define statistically-significant internal isochron relationships corresponding to initial ( ²⁶Al/ ²⁷Al) ([ ²⁶Al/ ²⁷Al] ₀) ratios in their precursors at the time of CAI formation at 4567.3 ± 0.16 Ma ranging from (3.92 _+4.53 ^-2.95) × 10 ⁻⁶ to (2.74 _+1.30 ^-1.09) × 10 ⁻⁵. These initial ratios are much lower than those predicted by the Pb-Pb ages, corresponding to age mismatches between the Pb-Pb and ²⁶Al- ²⁶Mg systems ranging from 0.69 _+0.54 ^-0.44 to 2.71 _+0.66 ^-0.59 Myr. All chondrules record ⁵⁴Cr/ ⁵²Cr compositions indicating an origin from inner Solar System precursor material and, as such, we interpret the age mismatch to reflect a reduced initial abundance of ²⁶Al in the chondrule precursors, similar to that proposed for the angrite parent body. In particular, the range of [ ²⁶Al/ ²⁷Al] ₀ ratios essentially defines two groups, which are apparently correlated with the absolute ages of the chondrules. A first group, charactertized by chondrules with absolute Pb-Pb ages identical to CAIs, defines a mean [ ²⁶Al/ ²⁷Al] ₀ value of (4.75 _+1.99 ^-1.21) × 10 ⁻⁶, whereas a second group, with absolute ages ∼1 Myr younger than CAIs, record a mean mean [ ²⁶Al/ ²⁷Al] ₀ of (1.82 _+0.57 ^-0.40) × 10 ⁻⁵. We interpret this systematic variability in [ ²⁶Al/ ²⁷Al] ₀ values as reflecting progressive inward transport and admixing of dust of solar composition and ²⁶Al content from the outer disk during chondrule recycling and remelting. Finally, a reduced [ ²⁶Al/ ²⁷Al] ₀ ratio in chondrule precursors impacts our understanding of the accretion timescales of differentiated planetesimals if chondrules are indeed representative of inner disk material. Using the average [ ²⁶Al/ ²⁷Al] ₀ ratio of (1.36 ± 0.72) × 10 ⁻⁵ defined by the eight chondrules, thermal modelling constrains the accretion of differentiated planetesimals formed with this ²⁶Al inventory from ∼0.1 to ∼0.9 Myr after Solar System formation to ensure melting by ²⁶Al decay.