Early planetary metamorphism in chondritic meteorites

Abstract

Lead isotope relations were studied in whole rock and separated phases of Mezö-Madaras (L3) and Sharps (H3) chondrites in order to study the record of early events in the solar system and to seek further information on the isotopic composition of primordial lead. The internal 207Pb/ 206Pb ages are4.480 ± 0.011AE (1AE= 10 9 years) for Mezö-Madaras and4.472 ± 0.005AE for Sharps. The ages are not significantly changed when Canyon Diablo troilite lead is included in the data sets, suggesting that the initial Pb isotopic composition in both meteorites was the same as that in the troilite. U Pb data from both meteorites plot along chords in concordia diagrams that indicate recent disturbances inU/Pb ratios. The chords are poorly defined owing to the relatively non-radiogenic character of the lead isotopes. Rb Sr measurements on Sharps likewise fail to yield an isochron, in agreement with the U Pb data. Data from the literature indicate a similar disturbance in the Rb Sr system for Mezö-Madaras. The 4.48 AE ages could be caused by pre-analysis contamination with terrestrial lead, however statistical comparison of isotope correlations between the acid-washes of analyzed samples and the residual washed samples suggests that the ages are real and not due to terrestrial contamination. The 4.48 AE age, which is distinctly younger than the well-established ages of 4.54–4.56 AE for the Allende chondrite and Angra dos Reis achondrite, appears to date an early metamorphic event rather than the formation of the chondrites. Rb Sr, Sm Nd and K Ar ages in support of the 4.48 AE metamorphic event are reviewed. Such a metamorphic age is not necessarily in conflict with 129I/ 129Xe data which indicate that the parent material of most chondrites, including those of type 3, cooled through temperatures sufficient to retain radiogenic Xe within a time interval of ca. 0.02 AE.