Hypervelocity collisions as drivers of planetary differentiation: Evidence from the Boutel Fil (b) H chondrite impact melt breccia

IntroductionMeteorites are commonly divided into two principal groups: chondrites, which represent unmelted accretionary aggregates, and achondrites, which are products of melting and planetary differentiation [1]. The coexistence of chondrites and achondrites is traditionally interpreted as the result of a spatial and/or temporal dichotomy in the population of planetesimals in the early solar system [2–4], and it is consequently envisaged by meteorite classification schemes that no single planetary body could be the parent body of both chondritic (i.e., unmelted) and achondritic (i.e., melted) meteorites. Yet, evidence of protracted accretion and partial differentiation of planetary bodies exists in the meteorite inventory, as recently illustrated for the parent body of the IIE iron meteorites [5]. While initially refuted based on the view that hypervelocity impacts did not facilitate widespread melting of cold planetesimals [6], hypervelocity impacts onto chondritic parent bodies during early time steps of solar system history, when heat-producing decay of short-lived radionuclides was ongoing, might have governed partial differentiation of chondritic planetary bodies [7–9]. A few individual H chondrites such as Pultusk [10] and Gao-Guenie [11], several meteorites recovered from LaPaz Icefield [12], and anomalous achondrites such as Dar al Gani 896 [13] might indeed show direct evidence of this process. However, it has remained unclear if hypervelocity impacts could have facilitated partial planetary differentiation as envisaged, for example, for the asteroid 6 Hebe [7]. Here we report on the H5 melt breccia Boutel Fil (b), which next to unmelted domains of H5 chondrite composition contains abundant achondritic domains of impact-melt origin in which metals and silicates separated in the liquid state.ResultsBoutel Fil (b) was found while searching for meteorites in the Boutel Fil strewn field, Chari-Baguirmi, Chad. It was recognized because it looks strikingly different from the fresh LL6 Boutel Fil chondrite individuals that form the strewn field. It is also distinct from the nearby Djermaia H3–6 chondrite. Petrographic analysis revealed that Boutel Fil (b) is a partly fusion-crusted, chondritic melt breccia composed of H5 regions in abundant melt rock matrix (Fig. 1A). The H5 regions are crosscut by shock melt veins and contain partly flattened chondrules with a mean diameter of about 0.3 mm (Fig. 2A). Olivine in the chondrules is Fa18.1.Fig. 1 Boutel Fil (b). A Overview images of two different slices of the meteorite. B µXRF element distribution map of one of the meteorite sections, showing the distribution of Mg, Ca, Ni, and S.Fig. 2 Backscattered electron images of the different domains in Boutel Fil (b). A Unmelted H5 domain with chondrules and shock veins. B Fine-grained, metal-rich impact melt rock with zoned olivine and low-Ca pyroxene, interstitial mesostasis, and metal blebs and veins. C Transition between the fine-grained, metal-rich impact melt rock (left) and the coarse-grained, metal-poor impact melt rock (right). D Coarse-grained, metal-poor impact melt rock with zoned olivine quench crystals and interstitial mesostasis. Scale bars are 100 µm.