The accretion and impact history of the ordinary chondrite parent bodies

Abstract

A working timeline for the history of ordinary chondrites includes chondrule formation as early as 0-2 Ma after our Solar System's earliest forming solids (CAIs), followed by rapid accretion into undifferentiated planetesimals that were heated internally by 26 Al decay and cooled over a period of tens of millions of years. There remains conflict, however, between metallographic cooling rate (Ni-metal) and radioisotopic thermochronometric data over the sizes and lifetimes of the chondrite parent bodies, as well as the timing of impact related disruptions. The importance of establishing the timing of parent body disruption is heightened by the use of meteorites as recorders of asteroid belt wide disruption events and their use to interpret Solar System dynamical models. Here we attempt to resolve these records by contributing new 207 Pb-206 Pb data obtained on phosphates isolated from nine previously unstudied ordinary chondrites. These new results, along with previously published Pb-phosphate, Ni-metal and thermometry data, are interpreted with a series of numerical models designed to simulate the thermal evolution for a chondrite parent body that either remains intact or is disrupted by impact prior to forming smaller unsorted "rubble piles". Our thermal model and previously published thermometry data limit accretion time to 2.05-2.25 Ma after CAIs. Measured Pb-phosphate data place minimum estimates on parent body diameters of ~260-280 km for both the L and H chondrite parent bodies. They also consistently show that petrologic Type 6 (highest thermal metamorphism) chondrites from both the H and L bodies have younger ages and, therefore, cooled more slowly than Type 5 (lesser metamorphism) chondrites. This is interpreted as evidence for Type 5 chondrite origination from shallower depths than Type 6 chondrites within initially concentrically zoned bodies. This contrasts metallographic cooling rate data that are inconsistent with such a simple onion shell scenario. One model that can reconcile these two data sets takes into account subtle differences in temperature to which each system responds. This working model requires that disruption occur early enough such that the Ni-metal system can record the cooling rate associated with a rubble pile (<70 Ma), yet late enough that the Pb-phosphate system can record an onion shell structure (>30 Ma). For this 30-70 Ma timeline, reaccretion into smaller rubble piles will ensure that the originally deeply buried and hot Type 6 samples will always cool faster as a result of disruption, yielding nearly uniform ages that record the time of parent body disruption. This is consistent with the available Pb-phosphate data, where all but one Type 6 chondrite (H, n=3; L, n=4) yields a cooling age within a narrow 4505 ± 5 Ma timeframe. These data collectively imply that both the H and L chondrite parent bodies were catastrophically disrupted at ~60 Ma. In addition, combined Ni-metal and Pb-phosphate models confirm that a subset of Type 4 chondrites record early rapid cooling likely ass...

Multiple impact events on the L‐chondritic parent body: Insights from SIMS U‐Pb dating of Ca‐phosphates in the NWA 7251 L‐melt breccia

“…Blackburn et al. () further suggested that the ages of Ca‐phosphates show a younging trend from ~4560 Ma (type 4) to 4500 Ma (type 6) with increasing petrologic type, which supports the onion‐skin model (e.g., Trieloff et al. ).…”

“…Several Ca-phosphates from unshocked ordinary chondrites have yielded Pb-Pb ages clustering between 4.50 and 4.56 Ga, recording the timing of cooling below the Ca-phosphate closure temperature (i.e.,~500°C) (G€ opel et al 1994;Amelin 2000;Bouvier et al 2007). Blackburn et al (2017) further suggested that the ages of Ca-phosphates show a younging trend from 4560 Ma (type 4) to 4500 Ma (type 6) with increasing petrologic type, which supports the onion-skin model (e.g., Trieloff et al 2003). Thus, ordinary chondrite parent bodies experienced a short period of heating exceeding Ca-phosphate closure temperature for <60 Myr after their formation (G€ opel et al 1994;Bennett and McSween 1996;Trieloff et al 2003).…”

Testing accretion mechanisms of the H chondrite parent body utilizing nucleosynthetic anomalies

“…; Sugiura and Fujiya ; Blackburn et al. ). As such, our upper accretion age of 1.8 ± 0.2 Myr derived from the 54 Cr signature using the same initial 26 Al/ 27 Al value is in excellent agreement with the accretion age based on thermal modeling.…”

“…; Blackburn et al. ). As such, μ 54 Cr signatures of H chondrites are consistent with rapid accretion via turbulent accretion.…”

“…Some of the data may also require a late disruption of the H chondrite parent body (Blackburn et al. ) but this does not alter the apparent link between metamorphic grade of individual H chondrite meteorites and cooling histories. If metamorphic grade does correlate with depth and, therefore, relative time of accretion, the full suite of H chondrites provides a time sequenced record of material being accreted to a planetesimal in the inner solar system during the accretion history of the H chondrite parent body.…”