¹William Herbst,²James P. Greenwood
Meteoritics & Planetary Science (in Press) Open Access Link to Article [https://doi.org/10.1111/maps.14153]
¹Department of Astronomy, Wesleyan University, Middletown, Connecticut, USA
²Department of Earth and Environmental Sciences, Wesleyan University, Middletown, Connecticut, USA
Published by arrangement with John Wiley & Sons

Chondrules probably formed during a small window of time ~1–4 Ma after CAIs, when most solid matter in the asteroid belt was already in the form of km-sized planetesimals. They are unlikely, therefore, to be “building blocks” of planets or abundant on asteroids, but more likely to be a product of energetic events common in the asteroid belt at that epoch. Laboratory experiments indicate that they could have formed when solids of primitive composition were heated to temperatures of ~1600 K and then cooled for minutes to hours. A plausible heat source for this is magma, which is likely to have been abundant in the asteroid belt at that time, and only that time, due to the trapping of ²⁶Al decay energy in planetesimal interiors. Here, we propose that chondrules formed during low-speed (≲1kms−1) collisions between large planetesimals when heat from their interiors was released into a stream of primitive debris from their surfaces. Heating would have been essentially instantaneous and cooling would have been on the dynamical time scale, 1/(Gρ) ~30 min, where � is the mean density of a planetesimal. Many of the heated fragments would have remained gravitationally bound to the merged object and could have suffered additional heating events as they orbited and ultimately accreted to its surface. This is a hybrid of the splash and flyby models: We propose that it was the energy released from a body’s molten interior, not its mass, that was responsible for chondrule formation by heating primitive debris that emerged from the collision.

¹Aaron J. Cavosie,¹Phil A. Bland,²Noreen J. Evans,²Kai Rankenburg,³Malcolm P. Roberts,^4,5Luigi Folco
Geochimica et Cosmochimica Acta (in Press) Open Access Link to Article [https://doi.org/10.1016/j.gca.2024.02.016]
¹Space Science and Technology Centre and The Institute for Geoscience Research, School of Earth and Planetary Science,Curtin University, Perth, WA 6102, Australia
²John de Laeter Centre, Curtin University, Perth, WA 6102, Australia
³Centre for Microscopy, Characterisation, and Analysis, University of Western Australia, Perth, WA 6009, Australia
⁴Dipartimento di Scienze della Terra, Università di Pisa, Pisa, 56126, Italy
⁵Centro per l’Integrazione della Strumentazione dell’Università di Pisa, Pisa 56126, Italy
Copyright Elsevier

Detection of extra-terrestrial geochemical components in melt generated during meteorite impact provides diagnostic evidence that can be used to confirm a hypervelocity impact event, and in some cases, classify the projectile. However, projectile contamination is often present at sub-percent levels, and can be difficult to detect. In contrast, meteoritic abundances in glass from small impact craters (<1 km diameter) formed by iron meteorites can be anomalously high, which has been attributed to glass originating from the projectile-target interface. Emulsion textures, immiscible liquids, metal spherules, and non-meteoritic siderophile element ratios have been cited as evidence that the projectile component is typically fractionated in impact glass. Here we present compositional data for impact glass from the Henbury crater field in Australia, where the largest crater is 145 m in diameter and the subgreywacke target rock and IIIAB iron projectile are geochemically distinct. Mixing models (Fe-Si, Ni-Co, Cr-Ir) and high platinum group element abundances indicate average projectile contributions ranging from 3 to 13 % in Henbury glass, comparable to ranges reported in glass from the Kamil (Egypt) and Wabar (Saudi Arabia) impact craters. However meteoritic siderophile element ratios (Fe:Ni, Fe:Co, Ni:Co) in Henbury glass appear nearly unfractionated, whereas Wabar and Kamil glasses have more fractionated ratios. Observed variations are attributed to fractionation of meteoritic Ni by formation of immiscible Ni-rich spherules during oxidation of meteoritic iron, and subsequent separation of Ni-rich spherules from glass during ejection. The Henbury glass sample analyzed is interpreted as an example of an interface melt that quenched prior to extensive oxidation and phase separation, and thus may represent one of the least fractionated samples of melt from the projectile-target interface described thus far, whereas Wabar and Kamil glasses record more evidence of fractionation processes. These results further highlight the influence of metal spherule formation on the composition of ejected glass from small impact structures formed by iron meteorites and provide new insights that explain textural features observed in natural impact glasses.