[1] The issue of the duration of Martian magmatic activity is one of the controversial debates recently: whether the young radiometric ages of $180 Ma for shergottites, a class of Martian meteorites, reflect the timing of crystallization from a magma or rather record later events related to shock metamorphism. In order to address the timescale over which Mars was geologically active, we have undertaken U-Pb isotopic studies on baddeleyite in the Roberts Massif (RBT) 04261 shergottite. Baddeleyite in RBT 04261 is usually associated with ilmenite and shows monoclinic structure, suggesting that the baddeleyite could have formed by crystallization from a residual liquid and not by shock metamorphism. In situ U-Th-Pb isotopic analyses of baddeleyite yield a young 238 U-206 Pb age of $200 Ma. Since the U-Pb system of baddeleyite is considered to be more resistant to resetting during reheating events, and olivine in RBT 04261 preserves an igneous calcium zoning, the young age of baddeleyite could be interpreted as a crystallization age of RBT 04261. The present results imply that Martian magma was still forming only 200 Ma ago, and that Mars had been geologically active until the recent past.

Shock metamorphism of minerals in meteorites provides insights into the ancient Solar System.Calcite is an abundant aqueous alteration mineral in carbonaceous chondrites. Return samples from the asteroids Ryugu and Bennu are expected to contain calcite-group minerals. Although shock metamorphism in silicates has been well studied, such data for aqueous alteration minerals are limited.Here, we investigated the shock effects in calcite with marble using impact experiments at the Planetary Exploration Research Center of Chiba Institute of Technology. We produced decaying compressive pulses with a smaller projectile than the target. A metal container facilitates recovery of a sample that retains its pre-impact stratigraphy. We estimated the peak pressure distributions in the samples with the iSALE shock physics code. The capability of this method to produce shocked grains that have experienced different degrees of metamorphism from a single experiment is an advantage over conventional uniaxial shock recovery experiments. The shocked samples were investigated by polarizing microscopy and X-ray diffraction analysis. We found that more than half of calcite grains exhibit undulatory extinction when peak pressure exceeds 3 GPa. This shock pressure is one order of magnitude higher than the Hugoniot elastic limit (HEL) of marble, but it is close to the HEL of a calcite crystal, suggesting that the undulatory extinction records dislocation-induced plastic deformation in the crystal. Finally, we propose a strategy to re-construct the maximum depth of calcite grains in a meteorite parent body, if shocked calcite grains are identified in chondrites and/or return samples from Ryugu and Bennu.