Accretion of a large LL parent planetesimal from a recently formed chondrule population

Abstract

Chondritic meteorites, derived from asteroidal parent bodies and composed of millimeter-sized chondrules, record the early stages of planetary assembly. Yet, the initial planetesimal size distribution and the duration of delay, if any, between chondrule formation and chondrite parent body accretion remain disputed. We use Pb-phosphate thermochronology with planetesimal-scale thermal models to constrain the minimum size of the LL ordinary chondrite parent body and its initial allotment of heat-producing 26Al. Bulk phosphate 207Pb/206Pb dates of LL chondrites record a total duration of cooling ≥75 Ma, with an isothermal interior that cools over ≥30 Ma. Since the duration of conductive cooling scales with parent body size, these data require a ≥150-km radius parent body and a range of bulk initial 26Al/27Al consistent with the initial 26Al/27Al ratios of constituent LL chondrules. The concordance suggests that rapid accretion of a large LL parent asteroid occurred shortly after a major chondrule-forming episode.

A Non‐Magnetized Chondrite Parent Body Revealed by Paleomagnetic Investigation of LL6 Chondrite NWA 14180

“…The difference between the two mechanisms lies in whether chondrules first form or are planetesimal. Our findings are also consistent with the mechanism of LL chondrule formation from nebular shocks rather than planetesimal collisions (Edwards & Blackburn, 2020; Fu et al., 2014). Based on the Pb‐Pb age of LL6 chondrites, the radius of the LL chondrite planetesimal is estimated at 150 km (Edwards & Blackburn, 2020), which is still below the threshold value (>250 km) of dynamo generation for later accretion scenarios (Elkins‐Tanton et al., 2011).…”

“…Our findings are also consistent with the mechanism of LL chondrule formation from nebular shocks rather than planetesimal collisions (Edwards & Blackburn, 2020; Fu et al., 2014). Based on the Pb‐Pb age of LL6 chondrites, the radius of the LL chondrite planetesimal is estimated at 150 km (Edwards & Blackburn, 2020), which is still below the threshold value (>250 km) of dynamo generation for later accretion scenarios (Elkins‐Tanton et al., 2011). Compared to the dynamo record in other meteorites (Figure 12), this suggests that the parent bodies of different classes of meteorites may undergo diverse accretion processes.…”

“…Thus, chondrule formation by transient heating events in the protoplanetary disk (Desch et al., 2012; Fu et al., 2014; Weiss et al., 2021) could not have taken place after that time. This timeline also suggests that planetesimal accretion was active during this period, with chondrules serving as planetary building blocks (Edwards & Blackburn, 2020). Subsequently, these planetesimals could have experienced heating by radioactive decay of short‐lived nuclides like 26 Al, leading to metamorphism or differentiation processes (Elkins‐Tanton et al., 2011).…”

Upper limits of water contents in olivine and orthopyroxene of equilibrated chondrites and several achondrites

“…Recent size estimates by Pb-phosphate thermochronology suggest a minimum diameter of 300 km (Edwards & Blackburn, 2020). The latter corresponds to a pressure of ~322 bar in the body's center when assuming a bulk Final H 2 O is the calculated H 2 O content screened for the detection limits at 99.7% and 95.5% confidence, either stating the value (if above the 99.7% limit), the 99.7% upper limit (if above the 95.5% limit) or the 95.5% upper limit (if below).…”

“…The LL parent body has been linked to the Flora asteroid family, for which a pre‐disruptive diameter of its parent planetesimal of 150–220 km has been estimated (Brož et al., 2013; Vokrouhlický et al., 2017). Recent size estimates by Pb‐phosphate thermochronology suggest a minimum diameter of 300 km (Edwards & Blackburn, 2020). The latter corresponds to a pressure of ~322 bar in the body's center when assuming a bulk density of 3200 kg m −3 .…”