Research ArticleASTRONOMY

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

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Science Advances  15 Apr 2020:
Vol. 6, no. 16, eaay8641
DOI: 10.1126/sciadv.aay8641
  • Fig. 1 Model chondrite parent body thermal structure, corresponding model Pb-phosphate cooling dates, and measured LL chondrite Pb-phosphate dates.

    (A) Schematic diagram of the distribution and peak metamorphic temperatures (Tmax) of chondrite petrologic types in a concentrically zoned onion shell planetesimal. Type 3.0 samples are unmetamorphosed, type 6 samples experienced considerable metamorphism at silicate and FeS subsolidus conditions, and type 7 chondrites reflect heating to suprasolidus temperatures (21, 22, 32, 51). Depth ranges of petrologic types 4 to 6 are identified on the basis of peak metamorphic temperature ranges summarized in (44). We identify the depth range of type 7 formation following relationships between body size and maximum crater depths permitting planetesimal survival (49), assuming a 180-km radius body. (B) Pb-phosphate cooling dates simulated by coupled planetesimal thermal and Pb production-diffusion in phosphate models at depths corresponding to petrologic types 4 to 7 in (A). (C) Summary of measured LL chondrite Pb-phosphate model cooling dates from this and previous studies (19, 23, 24). Pb-phosphate dates are calculated using the revised bulk chondritic 238U/235U of (14). St. Severin and NWA 6990 reflect shallow samples affected by an early impact event and, thus, deviate from onion shell model behavior.

  • Fig. 2 Relationship between parent body radius and Pb-phosphate dates in a conductively cooling planetesimal.

    Coupled thermal and Pb production-diffusion in phosphate models predict the Pb-phosphate cooling dates at the center of the simulated planetesimal and the type 5–type 6 boundary as identified by the depth at which the average LL6 temperature (~900°C) (51) is reached. The Pb-phosphate date in the center of the body (blue) becomes increasingly younger for larger planetesimal radii, while increasing radius results in larger differences between this youngest LL6 age and the oldest LL6 chondrite at the LL5-LL6 boundary (ΔLL6-age, gray). This simulation assumes a 50-μm phosphate grain radius and instantaneous accretion 2.1 Ma after CAIs for (26Al/27Al)o = 5.23 × 10−5.

  • Fig. 3 Concordia diagrams of LL chondrite phosphate U-Pb compositions.

    Individual phosphate fractions are plotted with gray points and 2σ uncertainty ellipses traced in black. U-Pb Concordia is traced in blue with concordant ages (in Ma) identified by white circles. Fractions exhibit pervasive discordance, especially negative discordance. In all cases, U-Pb measurements plot on chords that project lower intercepts nearly within uncertainty (2σ) of a nil age. Regressions are plotted with light gray lines (uncertainty envelopes dashed). Excluded measurements (red) are rejected following criteria discussed in the text. Regressions are calculated for two groups of phosphate fractions from ALH 83070 (light blue and black). NWA 6990 is excluded because the two phosphate fractions exhibit overlapping U-Pb compositions and a chord cannot be regressed. Linear regressions are calculated (MSWD: Mean squared weighted deviation) and plotted using the algorithms of U-Pb Redux (42).

  • Fig. 4 Pb-phosphate thermochronologic and petrologic constraints on LL parent planetesimal size and 26Al/27Al at the time of accretion.

    Contours identify the date range of the modeled type 6 region (ΔLL6-age). The measured minimum ΔLL6-age of 30 Ma is traced in gray (see Supplementary Methods). The blue curve traces conditions that yield a date of 4485 Ma (ALH 83070) at the planetesimal center (labeled “Central date”), although younger dates are permissible (blue arrow, Fig. 2). The red curve traces the maximum solidus temperature (1140°C) for LL chondrite compositions (32). The teal curve traces the minimum possible metamorphic temperature (800°C) recorded by any LL6 chondrite (51). The purple kernel density estimation reflects the initial 26Al/27Al composition of Semarkona chondrules (n = 24) with individual chondrule (26Al/27Al)o means denoted by white dots and corresponding uncertainties plotted as overlying gray Gaussian bells, whereby taller bells reflect lower uncertainty measurements (10, 5254). One chondrule (26Al/27Al)o ratio (“n = 1”) exceeds the upper bound of the plotted 26Al/27Al range, and four (“n = 4”) are below the lower bound. Time line is calculated from (26Al/27Al)o = 5.23 × 10−5.

Supplementary Materials

  • Supplementary Materials

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

    Graham H. Edwards and Terrence Blackburn

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    • Supplementary Methods
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    • Figs. S1 to S7
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