Ages of the second caldera of Piton de la Fournaise volcano (Réunion) determined by cosmic ray produced 3He and 21Ne

Abstract

Cosmic ray produced 3He and 21Ne in olivine separates from the Plaine des Rempart, close to the second caldera of Piton de la Fournaise, Réunion, indicate identical exposure ages of ∼ 62,000 yrs. This age is only slightly less than the formation age of the basalt, which is 65,000 ± 2000 yrs, and represents the final stage of eruption phase 2. The lower part of the wall of the second caldera, just above the Plaine des Sables, shows a cosmic ray exposure age of 23,800 ± 2000 yrs. This is probably the age for the collapse of the second caldera and represents the first direct dating of a fault. In addition, this age agrees with the range of 41,000–19,000 yrs given by K Ar ages of different lava flows. Further, six rocks that represent the final filling of the second caldera show low exposure ages of between 3300 and 5100 yrs. These ages date the final stage of eruption phase 3 at 3400 ± 1000 yrs and set an upper limit on the formation of the third caldera, the Enclos Fouqué. This work shows that 21Ne and 3He exposure age dating are powerful techniques for determining young ages in mafic samples and for obtaining ages for geological phenomena such as faulting. Because neon has three isotopes, cosmogenic, initial and MORB-type neon can be effectively resolved, and combined use of 21Ne c and 3He c represents a significant improvement in exposure age dating.