¹C. Fry, ¹C. Samson, ^2,3P. J. A. McCausland,¹M. Ralchenko,¹T. K. McLeod
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13067]
¹Department of Earth Sciences, Carleton University, Ottawa, Ontario, Canada
²Department of Earth Sciences, Western University, London, Ontario, Canada
³Centre for Planetary Science and Exploration, Western University, London, Ontario, Canada
Published by arrangement with John Wiley & Sons

This study tested the feasibility of using 3‐D laser imaging to measure the bulk density of iron meteorites. 3‐D laser imaging is a technique in which a 3‐D model of an object is built after aligning and merging individual detailed images of its surface. Assuming that the mass of the object is known, the volume of the model is calculated by software and an estimate of bulk density can be obtained by dividing mass by volume. The 3‐D laser imaging technique was used to determine the density of 46 fragments from 11 different iron meteorites. The technique proved to be robust and was applied successfully to study samples ranging close to four orders of magnitude in mass (8 g to 156 kg) and exhibiting a variety of surface textures (e.g., cracks, regmaglypts), reflectivities (e.g., polished surfaces, fusion crust, rust), and morphologies (e.g., sharp angular edges, shrapnel tendrils). Three metrics were considered to estimate the error associated with density measurements: the range accuracy of the laser camera, image alignment error, and inter‐operator variability during model building. Inter‐operator variability was the largest source of error and was highest when assembling models of samples which either lacked distinctive features or were very complex in shape. Excluding two anomalous Zagora samples where silicate inclusions might have lowered density, the densities measured using 3‐D laser imaging ranged from 6.98 to 7.93 g cm−3, consistent with previous studies. There is overlap between bulk density and iron meteorite class, and therefore bulk density cannot be used in isolation as a classification criterion. It is a good indicator, however, of weathering effects and of the potential presence of low‐density inclusions.

¹Gregory A.Brennecka, ²Yuri Amelin, ¹Thorsten Kleine
Earth and Planetary Science Letters 490, 1-10 Link to Article [https://doi.org/10.1016/j.epsl.2018.03.010]
¹Institut für Planetologie, University of Münster, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany
²Research School of Earth Sciences, The Australian National University, Canberra ACT 0200, Australia
Copyright Elsevier

The crystallization ages of planetary crustal material (given by basaltic meteorites) and planetary cores (given by iron meteorites) provide fiducial marks for the progress of planetary formation, and thus, the absolute ages of these objects fundamentally direct our knowledge and understanding of planet formation and evolution. The lone precise absolute age of planetary core material was previously obtained on troilite inclusions from the IVA iron meteorite Muonionalusta. This previously reported Pb–Pb age of 4565.3 ± 0.1 Ma—assuming a 238U/235U =137.88—only post-dated the start of the Solar System by approximately 2–3 million years, and mandated fast cooling of planetary core material. Since an accurate Pb–Pb age requires a known 238U/235U of the sample, we have measured both 238U/235U and Pb isotopic compositions of troilite inclusions from Muonionalusta. The measured 238U/235U of the samples range from ∼137.84 to as low as ∼137.22, however based on Pb and U systematics, terrestrial contamination appears pervasive and has affected samples to various extents for Pb and U. The cause of the relative 235U excess in one sample does not appear to be from terrestrial contamination or the decay of short-lived 247Cm, but is more likely from fractionation of U isotopes during metal–silicate separation during core formation, exacerbated by the extreme U depletion in the planetary core. Due to limited Pb isotopic variation and terrestrial disturbance, no samples of this study produced useful age information; however the clear divergence from the previously assumed 238U/235U of any troilite in Muonionalusta introduces substantial uncertainty to the previously reported absolute age of the sample without knowledge of the 238U/235U of the sample.

Uncertainties associated with U isotope heterogeneity do not allow for definition of a robust age of solidification and cooling for the IVA core. However, one sample of this work—paired with previous work using short-lived radionuclides—suggests that the cooling age of the IVA core may be significantly younger than previously thought. This work indicates the metallic cores of protoplanetary bodies solidified no earlier than the first ∼5–10 million years of the Solar System.