Noble gas retention chronologies for the St Séverin meteorite

⁴⁰Ar-³⁹Ar and ¹²⁹Xe-¹²⁸Xe analyses were performed on two lithologies (light and dark) of the St Séverin (LL6) chondrite. For the light and dark fractions, respectively, we obtained ⁴⁰Ar retention ages of 4.38 and 4.42 AE and ¹²⁹Xe retention ages of 8.4 and 15.2 myr after Bjurböle. The two methods give age differences of opposite sense, and by both methods the differences are significant. Both the ⁴⁰Ar and the ¹²⁹Xe ages are interpreted as dating relaxation of metamorphic conditions. These two chronometers are decoupled, however, and do not date the same events. ⁴⁰Ar-³⁹Ar reflect chondritic metamorphisrn on a 10⁸ yr time scale. The ¹²⁹Xe-¹²⁸Xe ages reflect isotopic closure at higher temperatures and earlier times.