Argon-argon age measurements and calculations of temperatures resulting from asteroidal break-up

Abstract

In addition to providing chronological information, 40 Ar— 39 Ar measurements on meteorites can be used as a geothermometer providing a record of the thermal history of the fragmentation events that have led from asteroidal parent body to museum specimen. A simple method of treating the experimental data is in terms of effective outgassing temperature. This is the temperature required to produce, in the laboratory, a fractional release of neutron induced 39 Ar equal to the loss of radiogenic 40 Ar determined from the 40 Ar- 39 Ar age spectrum. The effective temperature, T e , of a meteorite heated to a temperature T for a time t during a single parent body fragmentation or cratering event is given by 1/ T e =1/ T - R/E In( t/t 0 ), where t 0 is the duration of the laboratory heating, E the activation energy for argon diffusion and R the gas constant. An attempt has been made to relate the experimentally observed distribution of T e for meteorites to the unknown distribution of fragmentation temperatures, T , by using a Monte Carlo model to predict the distribution of fragment cooling times, t . It is concluded that for ordinary chondrites the mean temperature rise during at least one such event is of the order of 200 to 400 K, corresponding to a mean energy dissipation of more than 2 x 10 5 Jkg- -1 . This conclusion is relatively insensitive to details of the model because of the logarithmic dependence of T e on t . Two mechanisms are suggested to account for these high values, which are two orders of magnitude larger than the minimum required to produce fragmentation in laboratory experiments. One possibility is that we are seeing the effects of the break-up of a large asteroid, with a mass of the order of 10 19 kg, where such a large energy input would be required to overcome the gravitational potential energy. A second explanation is that the mechanism for transferring material from the asteroid belt to Earth-crossing orbit selects only the high-velocity impact ejecta. This material is expected also to have the highest internal energy following impact. The two mechanisms lead to quite distinct predictions concerning meteorite age histograms that are susceptible to future tests.