Aspects of meteorite impact: Petrogenesis of breccias, stratigraphic record of impact, and impact craters as probes of planetary crusts.

摘要

Roter Kamm: The rocks exposed in the rim of the 2.5 km wide and 3.7 Ma old Roter Kamm crater in southwest Namibia are cut by breccia veins that macroscopically resemble pseudotachylytes. The veins were shown to be cataclasites with no evidence for melting [1]. 40 Ar/39Ar data for vein and host rock samples indicate a low-grade metamorphic event at around 300 Ma but provide no evidence for an impact age. The samples have suffered 5--7% Ar loss, which we associate with the impact event. Modeling the 40Ar/39Ar data, assuming instantaneous impact heating followed by extended cooling, and coupling these results to published data on fluid inclusions, quartz precipitation, shock effects and crater degradation, suggest that the veins reached maximum temperatures of 230--290°C during impact. The rocks from Roter Kamm carry a complex paleomagnetic signal, which cannot be completely interpreted at this time, primarily due to sampling limitations. The formation of the vein breccias led to decreased magnetic susceptibility, increased AMS and disruption of the regional AMS pattern.; Lunar maria: Maria Tranquillitatis and Fecunditatis have been mapped in detail based on Clementine image mosaics and derived iron and titanium maps and using impact craters to serve as stratigraphic probes. The new data indicate that volcanism in both these maria started with low-Ti basalts and evolved toward medium- and high-Ti basalts. However, some of the high-Ti basalts in Mare Traquillitatis, erupted early, contemporaneous with the low- and medium-Ti basalts, and form the oldest units exposed on the mare surface. Mare Tranquillitatis is mostly covered with high-Ti basalts. In Mare Fecunditatis the volume of erupting basalts clearly decreased as the titanium content increased and the high-Ti basalts form only a few patches on the mare surface.