Age and nature of deposits on the submarine flanks of Piton de la Fournaise (Reunion Island)

Debris-avalanche deposits are by far the most extensive and voluminous formations in the submarine domain around Reunion Island. 21 sites sampled offshore of Piton de la Fournaise were studied. All the dredged rocks have a volcanic origin and often consist of blocks buried in a finer-grained matrix. Both massive and vesiculated samples were found, but only rare fragments of pillow lavas were encountered.All the lavas are basalts with variable amounts of olivine. A few plagioclase-bearing basalts were also found. These lithologies are known among Piton de la Fournaise subaerial lavas. A set of fresh samples representing the various rock types was further studied. The microscopic characteristics of these lavas confirm their great similarity with lavas from the subaerial volcano. Major elements show that all the submarine lavas are transitional basalts, with compositions very similar to those of Piton de la Fournaise subaerial lavas. The petrological and geochemical similarity of the dredged samples with Piton de la Fournaise subaerial lavas indicates that they belong to this volcano.The sulfur contents of the dredged samples and of various subaerial lavas from Piton de la Fournaise were determined on whole rocks. The results suggest that massive or slightly vesiculated dredged lavas (0.005–0.010 wt.% S) probably correspond to the internal parts of slowly cooled and subaerially degassed lava flows. The dredged pillow lavas display sulfur contents (0.013–0.020 wt.% S) similar to those of the external parts of recent subaerial lavas. This indicates that they were primarily degassed in the atmosphere, as also suggested by noble gases data, or in shallow water. Only one dredged sample has a relatively high S content (0.034 wt.%) suggesting that it may have been an early lava of a subaerial eruption or a lava directly emplaced under water, though at shallow depth. Thus, both the nature of the dredged samples and their sulfur contents indicate that these lavas were subaerially emitted. These results thus give strong evidence for landslide deposits having their source areas on land.K–Ar and 230Th–238U dating were also performed on some samples. The dredged lavas involved in the landslides of the volcanic edifice are mostly in the range of 110 ka to < 10 ka. These ages agree well with the relative stratigraphy available for these debris-avalanche deposits. A sample dated at 1.8 Ma would belong to Les Alizés palaeovolcano.