Ocean Drilling Program, United States

Ocean Drilling Program Hole 899B on the Iberia Abyssal Plain penetrated a thick serpentinite breccia flow underlain by smaller breccia flows, all separated by sediments containing weathered basalt and diabase (microgabbro) clasts. The basalt and diabase clasts, some with discolored weathering rinds, recovered with Early Cretaceous sediments below the thick upper serpentinite breccia, have the compositional characteristics of mid-ocean ridge basalt (MORB) modified by weathering on the seafloor. Seafloor weathering has added MgO and K (sub 2) O and removed CaO from the clasts, resulting in strongly modified major element compositions. However, the more immobile trace elements remain largely unaltered, and all basalt and diabase clasts have rare-earth element and spidergram patterns similar to transitional to enriched MORB (E-MORB). The E-MORB clasts are similar to the type of mid-ocean ridge volcanism that later produced the Atlantic Ocean floor in the region of the present Azores Plateau. The incompatible element enrichment necessary to produce E-MORB indicates a plume contribution and suggests that Early Cretaceous rifting on the Iberia Abyssal Plain was plume initiated despite the lack of extensive volcanism.