ENI Norge AS, Norway

IODP drilling of conjugate volcanic margins in the North Atlantic is proposed by the IODP Norge team. The NE Atlantic conjugate passive margins are characterized by extensive rifting and continental break-up related magmatic activity recorded by basalt flows, magmatic underplates, and intrusive complexes. To a large extent the amount of magmatism cannot be explained by passive decompression melting of sub-lithospheric mantle with a normal potential temperature. Three competing end-member hypotheses are proposed for the formation of this excess magmatic activity: 1) excess magmatism results from elevated mantle potential temperatures resulting from mantle plume processes, 2) rifting induced small scale convection at the base of the lithosphere enhances the flux of material through the melt window during rifting and mid-oceanic ridge spreading, and 3) mantle heterogeneities with lower melt temperatures of the melt source may produce larger than expected magmatic productivity. The mantle plume mechanism is essentially external to the rifting process and coincidence with the rifting process appears fortuitous. Small scale convection on the other hand is inherently connected to and produced by the rifting process. Although in the NE Atlantic we have unsurpassed constraints on conjugate crustal structure between the Norwegian-Jan Mayen-Greenland rifted margins, the relation between rifting and the anomalous excess magmatic productivity is still unresolved. New constraints on 1) melting conditions, 2) timing of magmatism, 3) spatial and temporal variations, and, 4) eruption environment, are required to resolve the controversy. Systematic IODP conjugate drilling is the only way to provide these constraints and will allow the development of a quantitatively testable framework for conjugate volcanic rifted margin formation. The proposed drilling will be closely coordinated with industry, in particular for geophysical surveys and drilling of a 4-km deep sub-basalt hole. Transects along the conjugate Jan Mayen-Norwegian rifted margins provide an outstanding possibility to test the causes of excess magmatism during breakup across the NE Atlantic. The first order questions addressed by the proposed drilling initiative will allow unraveling the distinct end-member models for the formation of excess volcanism during continental breakup (source composition, temperature anomaly related to a mantle plume, small scale convection).