Presentation Information
[PPS12-13]The origin of shock metamorphic dichotomy between ordinary chondrites and carbonaceous chondrites★Invited Papers
*Kosuke Kurosawa1,2, Gareth S. Collins3, Thomas M. Davison3, Takaya Okamoto2, Ko Ishibashi2, Takafumi Matsui2,4 (1.Graduate school of Human Development and Environment, Kobe University, 2.Planetary Exploration Research Center, Chiba Institute of Technology, 3.Impact and Astromaterials Research Centre, Department of Earth Science and Engineering, Imperial College London, 4.Institute for Geo-Cosmology, Chiba Institute of Technology)
Keywords:
Hypervelocity impacts,Chondrites,Shock metamorphism,Impact experiments,Mass spectrometry,Shock physics modeling
Most meteorites record shock metamorphism, produced by hypervelocity impacts when they were rocks on their parent bodies. Shock features have been empirically linked to peak shock pressure, and meteorites are commonly assigned to seven shock stages (S1-S7) (e.g., Stoffler+18, MaPS 53, 5-49]). The frequency distribution of shock stage differs among chondrite groups. Whereas most ordinary chondrites are classified as S3 or higher (e.g., Bischoff+19, MaPS 54, 2189-2202), nearly all carbonaceous chondrites are classified as S1 (e.g., Scott+92, GCA 56, 4281-4293). Here we investigate the origin of this shock metamorphic dichotomy.
One hypothesis is that impact-driven dehydration of phyllosilicates disperses highly shocked material from hydrated parent bodies (Tomeoka+03, Nature 423, 60-62). This mechanism cannot be applied to anhydrous carbonaceous chondrites, such as CO and CV chondrites, which are also less shocked than ordinary chondrites. We therefore extend the idea of Tomeoka et al. (2003) to reactions involving organic matter in carbonaceous meteorites.
We prepared four types of porous targets: magnetite only (Fe-O), magnetite + graphite (Fe-O-C), quartz only (Si-O), and quartz + graphite (Si-O-C). Magnetite and quartz were used as matrix analogs for hydrated and anhydrous carbonaceous chondrites, respectively. Graphite served as a proxy for insoluble organic matter (IOM), which hosts most of the organic carbon in carbonaceous chondrites (e.g., e.g., Yabuta+23, Science 379, eabn9057). Although quartz and graphite are not abundant in carbonaceous chondrites, they were selected because their hydrodynamic and thermodynamic responses to impact shocks are comparable to those of silicates and IOM, and because homogeneous powders are readily available. Powders were mixed and pressed into pellets with porosities of 50-80%.
Hypervelocity impact experiments into the target pellets were conducted with a two-stage light gas gun at the Planetary Exploration Research Center, Chiba Institute of Technology (Kurosawa+15. JGR-Planets 120, 1237-1251); the apparatus has since been relocated to Kobe University. A 2-mm-diameter Al2O3 sphere was used as the projectile, with impact velocities of 3-7 km s-1. Impact-generated gases were measured in a fully open system using the two-valve method (Kurosawa+19, GRL 46, 7258-7267) coupled to a quadrupole mass spectrometer. Calibration experiments enabled quantification of absolute gas yields.
We find that (1) adding carbon to the target increases total gas production by up to two orders of magnitude; (2) gas yield scales with impact velocity as a power law; and (3) the gas temperature reaches ~2000 K. Results (1) and (3) imply that, when shock waves propagate through carbonaceous-chondrite-like materials, organic matter reacts with minerals to generate an energetic gas-release event (an explosion) in the highest-pressure/temperature region near the top surface of a growing crater. This process can preferentially eject highly shocked material into space. With a simple consideration of energetics, on a ~100-km-diameter parent body, a mass comparable to the projectile mass that would otherwise reach strong shock metamorphism (S3 or higher) is expelled, whereas such material is retained on Ceres-sized bodies due to stronger gravity. This study has been already published [Kurosawa+25, Nature Communications, 16, 3608, https://doi.org/10.1038/s41467-025-58474-2].
Acknowledgments: This work was supported by ISAS/JAXA as a collaborative program with the Hypervelocity Impact Facility. We thank the developers of iSALE, including K. Wunnemann, B. Ivanov, J. Melosh, and D. Elbeshausen. Numerical computations and analyses were in part carried out on the general-purpose PC cluster and the analysis servers at Center for Computational Astrophysics, National Astronomical Observatory of Japan.
One hypothesis is that impact-driven dehydration of phyllosilicates disperses highly shocked material from hydrated parent bodies (Tomeoka+03, Nature 423, 60-62). This mechanism cannot be applied to anhydrous carbonaceous chondrites, such as CO and CV chondrites, which are also less shocked than ordinary chondrites. We therefore extend the idea of Tomeoka et al. (2003) to reactions involving organic matter in carbonaceous meteorites.
We prepared four types of porous targets: magnetite only (Fe-O), magnetite + graphite (Fe-O-C), quartz only (Si-O), and quartz + graphite (Si-O-C). Magnetite and quartz were used as matrix analogs for hydrated and anhydrous carbonaceous chondrites, respectively. Graphite served as a proxy for insoluble organic matter (IOM), which hosts most of the organic carbon in carbonaceous chondrites (e.g., e.g., Yabuta+23, Science 379, eabn9057). Although quartz and graphite are not abundant in carbonaceous chondrites, they were selected because their hydrodynamic and thermodynamic responses to impact shocks are comparable to those of silicates and IOM, and because homogeneous powders are readily available. Powders were mixed and pressed into pellets with porosities of 50-80%.
Hypervelocity impact experiments into the target pellets were conducted with a two-stage light gas gun at the Planetary Exploration Research Center, Chiba Institute of Technology (Kurosawa+15. JGR-Planets 120, 1237-1251); the apparatus has since been relocated to Kobe University. A 2-mm-diameter Al2O3 sphere was used as the projectile, with impact velocities of 3-7 km s-1. Impact-generated gases were measured in a fully open system using the two-valve method (Kurosawa+19, GRL 46, 7258-7267) coupled to a quadrupole mass spectrometer. Calibration experiments enabled quantification of absolute gas yields.
We find that (1) adding carbon to the target increases total gas production by up to two orders of magnitude; (2) gas yield scales with impact velocity as a power law; and (3) the gas temperature reaches ~2000 K. Results (1) and (3) imply that, when shock waves propagate through carbonaceous-chondrite-like materials, organic matter reacts with minerals to generate an energetic gas-release event (an explosion) in the highest-pressure/temperature region near the top surface of a growing crater. This process can preferentially eject highly shocked material into space. With a simple consideration of energetics, on a ~100-km-diameter parent body, a mass comparable to the projectile mass that would otherwise reach strong shock metamorphism (S3 or higher) is expelled, whereas such material is retained on Ceres-sized bodies due to stronger gravity. This study has been already published [Kurosawa+25, Nature Communications, 16, 3608, https://doi.org/10.1038/s41467-025-58474-2].
Acknowledgments: This work was supported by ISAS/JAXA as a collaborative program with the Hypervelocity Impact Facility. We thank the developers of iSALE, including K. Wunnemann, B. Ivanov, J. Melosh, and D. Elbeshausen. Numerical computations and analyses were in part carried out on the general-purpose PC cluster and the analysis servers at Center for Computational Astrophysics, National Astronomical Observatory of Japan.