HR: 14:25h
AN: V22E-04 INVITED     [PDF]
TI: The Geochronology of Terrestrial Meteorite and Cometary Impacts
AU: * Kelley, S P
EM: s.p.kelley@open.ac.uk
AF: Open University, Walton Hall, Milton Keynes, MK7 6AA United Kingdom
AB: Geochronology has become a crucial part of the debate over the influx of extraterrestrial material and its long term importance to terrestrial life. Many of the known terrestrial craters have ages attached to them, but all too often the ages are imprecise and unfortunately some are inaccurate. Despite these problems the database of measured ages has been used to support hypotheses of clustering and periodicity in the impact record, and compare ages with those for mass extinctions in the fossil record. Over 170 craters have been identified on the Earth's surface, but the ages of less than half are known to better than 10 million years.\\ The crucial question of peak eruption ages for large igneous provinces (LIPs) formed during the Palaeozoic, such as Deccan and the Siberian Traps, has been resolved using radiometric dating techniques such as Ar-Ar and U-Pb dating. The precision of measured ages for LIPs is better than 1% in most cases, but the precision and accuracy of ages determined for impact events is very variable. The ages of the largest 5 known terrestrial impact craters ($>$100 km diameter) have been established using radiometric dating techniques such as Ar-Ar and U-Pb and are known to precisions of better than 1%. However, the ages of many smaller craters, even some over 50 km in diameter, are less well constrained. It is the record of these smaller impacts which is littered with low precision ages, inaccurate ages and impacts whose age is constrained only by the age of the target rock and the youngest overlying sediments.\\ Why is the record of smaller impacts so poorly constrained? The main reason is the scarcity of samples and post impact alteration. The largest impacts form significant quantities of melt and which remain liquid for sufficiently long to coalesce and form conventional igneous bodies. It is these bodies which have been dated using radiometric dating techniques, often U-Pb dating of zircon crystallized from the melt. Smaller impacts form little or no melt and where present, they are heterogeneous mixtures of melt and host rock clasts of all sizes. The task of dating such samples is often made more difficult by alteration as a result of post impact hydrothermal activity. In addition, unlike LIPs, samples of impact melt are often rare even within the crater. Ar-Ar dating has become the technique of choice for these samples since both furnace heating and laser heating extraction techniques lend themselves to analysis of small heterogeneous samples. Ar-Ar analysis has been used to separate and identify components such as host rock clasts and the effects of later alteration, to reveal precise ages for the impacts. The Ar-Ar technique can also be used to determine argon loss in the host rock as a result of heating during the impact, and measure the remnants of host rock Ar dissolved in melt which can also indicate the rapidity of melt formation and freezing.\\ In fact the geochronology of impacts is fast improving area and although more work is required before we can determine whether impacts fall randomly, with a periodicity, or in clusters, there are some strong indications of future directions. Some apparent clusters of impacts appear to warrant further study, whereas others may disappear as new geochronological data is acquired. In particular the long known 450-500 Ma asteriod break-up event seems to have a terrestrial corollary in a mid-Ordovician cluster of impacts.
DE: 1035 Geochronology
DE: 5420 Impact phenomena (includes cratering)
DE: 6210 Comets
DE: 6240 Meteorites and tektites
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting