Meteorite evidence for partial differentiation and protracted accretion of planetesimals

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Science Advances  24 Jul 2020:
Vol. 6, no. 30, eaba1303
DOI: 10.1126/sciadv.aba1303
  • Fig. 1 IIE meteorites contain evidence for all the expected layers of a partially differentiated body.

    Diagrams illustrate how IIE meteorites contain both chondritic and achondritic silicate inclusions exhibiting different degrees of metamorphism/differentiation depending on the meteorite. Mont Dieu is one of the least heated and contains chondrules. Techado and Watson show more evidence of thermal alteration. In addition, some IIE irons contain achondritic silicates, exhibiting different degrees of differentiation ranging from Miles with pyroxene-enriched basaltic silicates to Colomera with silicates of andesitic composition (56, 57). This study provides the missing evidence that the IIE parent body had a metallic core. Direct petrologic evidence for achondritic materials on the CV and H chondrite parent bodies remains to be confirmed.

  • Fig. 2 Reflected light images of the four K-T interfaces analyzed in Colomera and Techado.

    (A) K-T interface D in Colomera. Interface D is located on the other side of the sample with respect to A, B, and C ~5 mm away from K-T interface C; the reference frame indicates how the sample is rotated with respect to (B). The subscript “C” of the reference frame refers to “Colomera.” (B) K-T interfaces A, B, and C in Colomera. The CZs are clearly recognizable by their dark gray color, with the surrounding light gray tetrataenite rim and embedded in the kamacite medium gray matrix. Neumann bands (shock-induced twinning of the kamacite crystal) are shown. The subscript C of the reference frame refers to Colomera. (C) K-T interfaces 1 and 2 in Techado. Plessite is visible. The subscript “T” of the reference frame refers to “Techado.” The two meteorite samples (Colomera and Techado) are not mutually oriented. For each image, the sample had been polished and etched between 15 and 25 s with nital (98% ethanol, 2% nitric acid).

  • Fig. 3 Measuring the three components of NRM using XPEEM.

    (A to C) Single-polarization XPEEM images of one region of K-T interface 2 in Techado taken at three different orientations of the sample with respect to the x-ray beam. The gray scale quantifies the flux of electron captured in the optics. The beam hits the sample at 30° with respect to the plane of the image (its in-plane direction is shown with an arrow). The reference frame refers to Fig. 2. (D to F) Corresponding XMCD images obtained when combining the corrected XPEEM images obtained with left- and right-circular polarizations.

  • Fig. 4 Ancient field directions recovered from each K-T interface in Techado and Colomera.

    (A) Average paleofield directions estimated from the two K-T interfaces in Techado (1 and 2). (B) Average paleofield directions estimated from the four K-T interfaces in Colomera (A to D). Shown in (A) and (B) are equal area projection in the reference frames of Fig. 2. Open (closed) symbols and dashed (solid) lines denote upper (lower) hemisphere. Ellipses represent the 95% confidence interval accounting for the measurement uncertainty, the small-number statistical uncertainty, and the uncertainty associated with the mutual orientations of the CZs.

  • Fig. 5 Cooling profiles of a partially differentiated IIE parent body.

    (A) Time-temperature constraints. Circle and square denote 40Ar/39Ar ages for 0.1- to 1-mm feldspar grains for Techado and Colomera, respectively (30). Curves represent the temperature of material on a 170-km radius, partially differentiated planetesimal at depths between 33 and 66 km below the surface. (B) Time-cooling rate constraints on same body shown in (A). Circle and square denote cooling rates at ~350°C of Techado and Colomera, respectively. Curves show the evolution of the cooling rate at same depths as in (A). In (A) and (B), the dotted line shows when the core of the planetesimal is completely crystallized and error bars indicate 2 SD.

  • Fig. 6 Initial and final states of representative IIE-forming, 2D impact simulations.

    (A) Initial state of a 170-km radius target with a 60-km radius iron core and a 110-km-thick dunite layer before impact. Impactor is 30 km in radius, is solid, and is made of either iron or dunite. The left half of the figure shows the constituent materials, and the right half shows the temperature obtained from the thermal evolution model at 30 Ma after CAI formation. (B) End of the impact simulation after 24,600 s for a 40-km radius, dunite impactor impacting vertically at 5 km s−1. Small amounts of core material are placed in the upper half of the target’s silicate layer. A finite number of tracers were used to identify the different materials on the left half of the figure; this can result in regions with no tracers (white) but does not mean that no material is present at these locations. (C) End of the impact simulation (after 24,600 s) for a 30-km-radius iron impactor impacting vertically at 1 km s−1.

Supplementary Materials

  • Supplementary Materials

    Meteorite evidence for partial differentiation and protracted accretion of planetesimals

    Clara Maurel, James F. J. Bryson, Richard J. Lyons, Matthew R. Ball, Rajesh V. Chopdekar, Andreas Scholl, Fred J. Ciesla, William F. Bottke, Benjamin P. Weiss

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