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

Abstract

Abstract 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\u202fMyr). 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 ∼\u202f5\u202f×\u202f10−5 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 CV3OxA (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\u202f±\u202f0.56 to 4565.84\u202f±\u202f0.72\u202fMa, 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\u202f±\u202f0.16\u202fMa ranging from ( 3.92 + 4.53 - 2.95 )\u202f×\u202f10−6 to ( 2.74 + 1.30 - 1.09 )\u202f×\u202f10−5. 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 \u202fMyr. 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 )\u202f×\u202f10−6, whereas a second group, with absolute ages ∼1\u202fMyr younger than CAIs, record a mean mean [26Al/27Al]0 of ( 1.82 + 0.57 - 0.40 )\u202f×\u202f10−5. 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\u202f±\u202f0.72)\u202f×\u202f10−5 defined by the eight chondrules, thermal modelling constrains the accretion of differentiated planetesimals formed with this 26Al inventory from ∼0.1 to ∼0.9\u202fMyr after Solar System formation to ensure melting by 26Al decay.