Correlated cosmogenic W and Os isotopic variations in Carbo and implications for Hf–W chronology. (15th March 2015) - Qin, Liping
Abstract: An obstacle for establishing the chronology of iron meteorite formation using 182 Hf– 182 W systematics ( t 1/2 = 8.9 Myr) is to find proper neutron fluence monitors to correct for cosmic ray modification of W isotopic composition. Recent studies showed that siderophile elements such as Pt and Os could serve such a purpose. To test and calibrate these neutron dosimeters, the isotopic compositions of W and Os were measured in a slab of the IID iron meteorite Carbo. This slab has a well-characterized noble gas depth profile reflecting different degrees of shielding to cosmic rays. The results show that W and Os isotopic ratios correlate with distance from the pre-atmospheric center. Negative correlations, barely resolved within error, were found between ε 190 Os–ε 189 Os and ε 186 Os–ε 189 Os with slopes of −0.64 ± 0.45 and −1.8(+1.9/−2.1), respectively. These Os isotope correlations broadly agree with model predictions for capture of secondary neutrons produced by cosmic ray irradiation and results reported previously for other groups of iron meteorites. Correlations were also found between ε 182 W–ε 189 Os (slope = 1.02 ± 0.37) and ε 182 W–ε 190 Os (slope = −1.38 ± 0.58). Intercepts of these two correlations yield pre-exposure ε 182 W values of −3.32 ± 0.51 and −3.62 ± 0.23, respectively (weighted average ε 182 W = −3.57 ± 0.21). This value relies on a large extrapolation leading to a large uncertainty but gives a metal–silicate segregation age of −0.5 ± 2.4 Myr after formation of the solar system. Combining the iron meteorite measurements with simulations of cosmogenic effects in iron meteorites, equations are presented to calculate and correct for cosmogenic effects on 182 W using Os isotopes.