New K-Ar ages from La Montagne massif, Réunion Island (Indian Ocean), supporting two geomagnetic events in the time period 2.2-2.0 Ma

Abstract

We present new radiometric ages obtained at the type locality in La Reunion Island where palaeomagnetic records of the Reunion events have first been identified. Seven dated lava flows from the Riviere Saint Denis section, which recorded a reverse-to-normal transition, display ages from 2.12 ± 0.03 to 2.17 ± 0.03 Ma, with a mean age of 2.15 ± 0.02 Ma. Two significantly younger flows from this section, interpreted here as valley fill flows from trace elements compositions and Pb isotopic data, have been dated at 2.06 ± 0.03 and 2.08 ± 0.03 Ma. Within the Grande Chaloupe section, where a normal-to-reverse transition is recorded, two coherent ages of 2.05 ± 0.03 and 2.03 ± 0.03 Ma have been obtained. When a direct comparison was possible, our new K-Ar ages performed on separated groundmass show a rather good coherence with previous ages from La Reunion Island. When considered with continuous palaeomagnetic sedimentary records in the 2.2-2.0 Ma interval, these new results suggest that two distinct events are recorded in La Montagne lava flows at La Reunion Island, with ages of 2.15 ± 0.02 and 2.04 ± 0.02 Ma. Following recent nomenclature, the former is the Reunion event s.s., while the latter can be related to the Huckleberry Ridge event. Globally distributed volcanic and sedimentary records show that the first (Reunion s.s.; RU-1) is associated with a large dipole intensity decrease at 2.15 ± 0.02 Ma, and hence is recorded in many sequences. On the other hand, the dipole intensity decrease was not as pronounced at 2.04 ± 0.02 Ma, when the Huckleberry Ridge (RU-2) palaeomagnetic event occurred. Consequently, it is not present as a full directional change in many sections worldwide, but rather appears as a geomagnetic excursion during an episode of increased secular variation. Finally, the use of the Reunion event for magnetostratigraphic studies is recommended, while the clear identification of the Huckleberry Ridge excursion might often be difficult, preventing its use as a reliable time marker.