Dating of submarine volcanism based on combined K-Ar and 40Ar/39Ar methods: example from the Eastern Mayotte Volcanic Chain (Comoros Archipelago).

Dating submarine volcanic rocks is essential for understanding the relationship between tectonics and volcanism, as well as for characterizing the evolution of volcanic systems, for instance. The K-Ar chronometer is generally used but dating young rocks (<50 ka) remains a significant challenge, particularly because their amount of radiogenic ⁴⁰Ar is low. Here, we present a comparison of K-Ar and the ⁴⁰Ar/³⁹Ar ages obtained from submarine samples from the Eastern Mayotte Volcanic Chain (EMVC; Comoros Archipelago). This volcanic chain consists of monogenetic pyroclastic cones, lava domes and lava flow fields with basanitic to phonolitic compositions. It includes the Fani Maoré, a new submarine volcano that formed a few years ago, and the Horseshoe, a potentially active U-shaped morphological structure, located only 10 km offshore Mayotte. A total of 19 samples were collected in the Horseshoe and Fani Maoré regions, by dredging and/or using the Victor6000 remotely operated vehicle (ROV), during 6 oceanographic campaigns. These samples were dated using both the K-Ar and the ⁴⁰Ar/³⁹Ar methods, on a carefully selected groundmass fraction obtained within a narrow density range with heavy liquids. Both methods used here allow dating of very young samples with ages as young as 3 ± 1 ka for phonolitic samples, with radiogenic content as low as 0.1%. The ⁴⁰Ar /³⁹Ar inverse isochrons confirm the atmospheric initial trapped ⁴⁰Ar/³⁶Ar component, suggesting that no argon fractionation affected the ages from either method, and flat ⁴⁰Ar/³⁹Ar age spectra indicate that the K-Ar system remained closed. In order to further check both methods with zero-age basanitic lavas, we have analysed samples from the 2018 and 2020 eruptions of the Fani Maoré volcano.  The relatively precise ages obtained by both methods confirmed that, following a carefully sample selection and preparation, both K-Ar and ⁴⁰Ar /³⁹Ar methods are well suited for dating submarine Holocene volcanics such as the phonolitic lavas and pyroclasts. Finally, the comparison between K-Ar and ⁴⁰Ar/³⁹Ar shows coherent results in most cases, enabling a precise temporal framework to be established for the Horseshoe region.