Petrofabrics in the CM chondrite Kolang: Evidence for non-spherical chondrules in the protoplanetary disk

Abstract

The alignment of non-spherical “flattened” chondrules into a petrofabric is a common feature of hydrated carbonaceous chondrite meteorites. This texture can form as a result of impacts at peak shock pressures exceeding 10 GPa. However, many carbonaceous chondrites with petrofabrics are unshocked. While several processes have been proposed to explain this incongruency, including erasure of shock effects by alteration (both aqueous and thermal), none have yet been confirmed. Kolang is a brecciated Mighei-like carbonaceous chondrite wherein analysis of chondrule shape and orientation shows that it has a pronounced petrofabric defined by elongate chondrules that is shared between clasts with differing aqueous and thermal alteration histories. Its petrofabric, therefore, must have developed after the altered clasts had been juxtaposed; any sign of shock associated with impact-driven deformation cannot have been erased. We have investigated the shock experienced by Kolang with a combination of traditional optical methods and electron backscatter diffraction. We find that the peak shock pressure experienced by Kolang was likely ~4–5 GPa, too low to generate an impact-induced petrofabric. Kolang has not experienced sufficient shock, whether by a single or multiple impacts, to deform its chondrules from spheres into elongate chondrules. The most likely explanation, therefore, is that Kolang accreted elongate chondrules that were aligned under relatively low pressure.