Tyrrhenian magmatism and mantle exhumation: first results from IODP Exp. 402

We report the first results of the International Ocean Discovery Program Expedition 402 in the Tyrrhenian Sea (February 9 to April 8, 2024).  Almost 40 years have passed since the discovery of exposed mantle west of Iberia by ODP but key questions remain unanswered, such as the nature of the mantle, whether it is subcontinental or formed by ultraslow seafloor spreading, or how models can explain the apparent lack of melting. Since then, the mantle has only been probed at mid-ocean ridges, because obtaining samples and data from drilling in magma-poor COTs is a major challenge, as the exposed mantle is typically buried under kilometers of sediment.

The Tyrrhenian Basin is the youngest of the western Mediterranean, formed from the Middle Miocene to recent times by continental extension associated with the ESE-SE rollback of the subducting slab and with the migrating Apennine subduction system. The Tyrrhenian is an outstanding location to test COT formation models by drilling because of its thin sedimentary cover, the presence within a few tens of kilometers of the conjugate pair of COT margins, and the availability of high-quality geophysical data suggesting the presence of serpentinized mantle rocks in its center. A surprising key feature of the basin is the lack of seafloor spreading after mantle exhumation, which allows for the first time a close look into the early stages of the exhumation process.

The main objective of the IODP Exp. 402 was to determine the nature of the geological basement in the central part of the Tyrrhenian Basin and in the conjugate margins to the west and east. The objectives included the kinematics of the opening, the deformation mechanisms of the crust and mantle, and the relationship of the melted products to the exhumed mantle.

The samples and data collected during Exp.402 provide an extensive new data set to determine mantle heterogeneity, the nature and history of melt production and impregnation, and the extent and evolution of serpentinization and carbonate formation; to constrain the geometry and timing of deformation that led to mantle exhumation; to study fluid-rock interactions between seawater, sediment, and the serpentinizing mantle; and to constrain geodynamic models of rifting and COT formation.