A large shock vein in L chondrite Roosevelt County 106: Evidence for a long-duration shock pulse on the L chondrite parent body

Abstract

A large shock-induced melt vein in L6 ordinary chondrite Roosevelt County 106 contains abundant high-pressure minerals, including olivine, enstatite, and plagioclase fragments that have been transformed to polycrystalline ringwoodite, majorite, lingunite, and jadeite. The host chondrite at the melt-vein margins contains olivines that are partially transformed to ringwoodite. The quenched silicate melt in the shock veins consists of majoritic garnets, up to 25 μm in size, magnetite, maghemite, and phyllosilicates. The magnetite, maghemite, and phyllosilicates are the terrestrial alteration products of magnesiowüstite and quenched glass. This assemblage indicates crystallization of the silicate melt at approximately 20-25 GPa and 2000 °C. Coarse majorite garnets in the centers of shock veins grade into increasingly finer grained dendritic garnets toward the vein margins, indicating increasing quench rates toward the margins as a result of thermal conduction to the surrounding chondrite host. Nanocrystalline boundary zones, that contain wadsleyite, ringwoodite, majorite, and magnesiowüstite, occur along shock-vein margins. These zones represent rapid quench of a boundary melt that contains less metal-sulfide than the bulk shock vein. One-dimensional finite element heat-flow calculations were performed to estimate a quench time of 750-1900 ms for a 1.6-mm thick shock vein. Because the vein crystallized as a single high-pressure assemblage, the shock pulse duration was at least as long as the quench time and therefore the sample remained at 20-25 GPa for at least 750 ms. This relatively long shock pulse, combined with a modest shock pressure, implies that this sample came from deep in the L chondrite parent body during a collision with a large impacting body, such as the impact event that disrupted the L chondrite parent body 470 Myr ago.