Inversion of cosmogenic-nuclide data from iron meteorites

The analysis also yields new estimates of the exposure ages that are consistent with the time-varying flux. That is, one does not need to presuppose any particular functional form for the flux. The application of inverse theory to such problems also makes available additional benefits that have been widely recognized in geophysical analysis. For example, it forces one to explicitly acknowledge the inherent assumptions, it provides quantitative estimates of the uncertainty of the results, and it provides a nieans of anticipating the value of new experimental data. We have based our demonstration on published data from four individual iron meteorites, since these data apparently provide a self-consistent set. Smoothed estimates are derived of the long-term prehistory of the particle flux at similar depths within these bodies (1000 kg m-' as inferred from the cosniogenic nuclides "Ne and 'OK). With our preferred choice for the present value of the particle flux, the results independently support a model in which the magnitude of the particle flux 10' years ago was about 40% less than the present value; the individual estiniatcs for the four meteorites of the ratio of the present flux to the flux 10' years ago are 1.75 (Mount Ayliff), 1.66 (Yardymly), 1.64 (Williamstown), and 1.72 (Carbo). The exposure ages consistent with this history arc . smaller, by some 10-20% , than previous estimates. One can choose a smaller value for the present particle flux to bring the ages into agrcement with previously estimates values, but then a substantially larger change in flux with time is required. Thus our analysis provides a sophisticated test of the possibility of a constant-flux model.