Impact cratering on the H chondrite parent asteroid

This paper reports petrological data for LaPaz Icefield 02240, 03922, 031125, 031173, 031308, 04462, and 04751, which are meteoritic samples of clast-rich impact melt rocks from the H chondrite parent asteroid. The size distribution and metallographic characteristics of Fe-Ni metal in the melts indicate very rapid 1 to 40C/s cooling in the temperature range between >1500 and ∼950C when the clast-melt mixtures were thermally equilibrating. Cooling slowed to values between 10^-3 and 10^-2C/s in the temperature range between 700 and 400C when the melt rocks were cooling to their surroundings. These data suggest that the rocks cooled near the surface of the H chondrite asteroid within suevitic impact deposits. Integrating these data with the petrologic characteristics of other H chondrite melt rocks and their radioisotopic ages indicates that the H chondrite asteroid suffered at least one large impact event while still cooling from endogenous metamorphism at ∼4500 Ma; this impact must have degraded the asteroid's integrity but did not cause shattering. Impact events in the era between ∼4100 and ∼3600 Ma produced melt volumes large enough to allow segregation of metal and troilite from silicate melts, possibly within continuous impact melt sheets contained in craters. The impact record after 3600 Ma does not display such assemblages, which suggests a decrease in the rate of large impact events or a catastrophic size reduction of the H chondrite parent asteroid at around this time.