Noble gases in enstatite chondrites 1: Exposure ages, pairing, and weathering effects

Cosmic-ray exposure ages calculated from cosmogenic noble gas nuclides are reported for 57 enstatite (E) chondrites, 43 of them were measured for the first time. With a total of 62 individual E chondrites (literature and this data, corrected for pairing) the observed spectrum of ages ranges between 0.07 and 66 Ma. Three clusters seem to develop at about 3.5, 8, and 25 Ma, respectively. Since the uncertainty of ages is estimated to be similar to 20% (in contrast to 10 to 15% for ordinary chondrites) and the number of examined samples is still comparatively small, these peaks have to be confirmed by more measurements. Regarding the two subgroups, EH and EL chondrites, no systematic trend is apparent in the distribution of cosmic-ray exposure ages. Several E chondrites yield significantly lower Ar-38 ages compared to those calculated from cosmogenic He-3 and Ne-21. For these E chondrites, we suggest a reduction of cosmogenic Ar-38 as a result of weathering. In order to prove the possible influence of terrestrial alteration on the cosmogenic noble gas record of E-chondritic material, we simulated terrestrial weathering in an experiment of 12 weeks duration. The treatment showed that a significant amount of cosmogenic Ar-38 is lost on Earth by the influence of water.