Metal veins in the Kernouvé (H6 S1) chondrite: Evidence for pre- or syn-metamorphic shear deformation

Abstract Kernouve is an H6 chondrite that experienced a very small degree of late stage shock loading (S1). However, Kernouve contains Fe–Ni metal vein-like structures, whose formation have been attributed to an early impact event. To establish the formation conditions of metal veins in Kernouve, we examined the three dimensional (3D) arrangement of metal vein-like structures and typical metal grains in two samples of Kernouve with X-ray microtomography (μCT) at resolutions of ∼11 μm/voxel. We additionally investigated the 3D structure of the porosity present in Kernouve using μCT at two different resolutions (∼3 and ∼11 μm/voxel). These data and optical microscopy support the hypothesis that Kernouve has experienced little post-metamorphic shock. However, the moderate 5.8 vol.% porosity of Kernouve is in the form of intergranular voids rather than cracks, which indicates any cracking that may have existed in the relatively brittle silicate grains was annealed. We estimate that 70–80% of the primordial porosity in Kernouve was removed by impact-related compaction. Moreover, we found no collective orientation of metal grains, so high metamorphic temperatures following compaction erased any common orientation of metal grains due to compaction. We propose that the metal vein structures can be explained as a pre- or syn-metamorphic shock-induced process, which we infer was primarily shear deformation, with some uniaxial compaction also occurring. The coarse metal veins probably formed by accumulation of ductile metal grains along shear zones, a process that would have been facilitated by having an already warm H chondrite parent body when shock occurred (i.e., syn-metamorphic shock). The complexity of shape, including numerous tendrils expanding from the primary structure, of the veins in Kernouve is likely due to the coalescence of metal by metamorphic growth after the shear event. The metal veins in Kernouve thus appear to record evidence for early, shock-induced metal mobilization and the maximum shock pressure experienced by Kernouve may have been ⩽21 GPa. Our study suggests that collision-induced segregation of metal occurred at an early stage in low-gravity planetesimals, consistent with the idea that this process could have been important for the differentiation of such objects.