Al 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 Al (half-life of 0.73 Myr). The decay of Al is postulated to have been an important heat source promoting asteroidal melting and differentiation. Thus, understanding the Al inventory in the accretion regions of differentiated asteroids is critical to constrain the accretion timescales of protoplanets. The current paradigm asserts that the canonical Al/Al 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 Al-Mg 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 ± 0.56 to 4565.84 ± 0.72 Ma, define statistically-significant internal isochron relationships corresponding to initial (Al/Al) ([Al/Al]0) ratios in their precursors at the time of CAI formation at 4567.3 ± 0.16 Ma ranging from (3:92 2:95 þ4:53) 10 6 to (2:74 1:09 þ1:30) 10 . These initial ratios are much lower than those predicted by the Pb-Pb ages, corresponding to age mismatches between the Pb-Pb and Al-Mg systems ranging from 0:69 0:44 þ0:54 to 2:71 0:59 þ0:66 Myr. All chondrules record Cr/Cr 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 Al in the chondrule precursors, similar to that proposed for the angrite parent body. In particular, the range of [Al/Al]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 [Al/Al]0 value of ð4:75 1:21 þ1:99) 10 , whereas a second group, with absolute ages 1 Myr younger than CAIs, record a mean mean [Al/Al]0 of ð1:82 0:40 þ0:57) 10 . We interpret this systematic variability in [Al/Al]0 values as reflecting progressive inward transport and admixing of dust of solar composition and Al content from the outer disk during chondrule recycling and remelting. Finally, a reduced [Al/Al]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 [Al/Al]0 https://doi.org/10.1016/j.gca. 2019.06. 025 0016-7037/ 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). ⇑ Corresponding author. E-mail address: bizzarro@snm.ku. dk (M. Bizzarro). J. Bollard et al. /Geochimica et Cosmochimica Acta 260 (2019) 62–83 63 ratio of (1.36 ± 0.72) 10 5 defined by the eight chondrules, thermal modelling constrains the accretion of differentiated planetesimals formed with this Al inventory from 0.1 to 0.9 Myr after Solar System formation to ensure melting by Al decay. 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/).