Ocean Drilling Program, United States

Ocean Drilling Program Leg 192 will attempt to penetrate a minimum of 100-150 m of igneous basement in each of four sites on the Ontong Java Plateau (OJP), the largest of the world's large igneous provinces. Primary objectives of the leg are to determine the age and duration of emplacement of the plateau, the range and diversity of magmatism, and the environment and style of eruption. Objectives at all sites are aimed at investigating the causes of plateau formation and fundamental processes governing the crustal development of oceanic plateaus. Additional insight will be accrued on the relation of plateau emplacement to Cretaceous oceanographic, climatic, and biological events. Drilling will provide samples for geochronological, petrological, chemical, and isotopic studies of widely distributed, previously unsampled areas across this 1.6X10 (super 6) km (super 2) plateau, as well as information on paleodepths. Data from these studies will be combined with results from the three previous drill sites that reached basement, and from obducted slivers of OJP crust exposed in the eastern Solomon Islands, to determine the chronology of the plateau's formation, the nature of its mantle sources, the characteristics of magmatic evolution, and the water depth of basement at the time of volcanism. This information will be used to test models for plateau formation, particularly the currently preeminent plume-head model. Contrary to the predictions of this model, basement lavas from existing sites document the presence of two, rather than one, major eruptive episodes (at 122+ or -3 Ma and 90+ or -4 Ma), have an unexpectedly limited range of chemical and isotopic compositions, and appear to have been emplaced under considerable depths of water. Data from key regions of the OJP to be sampled during Leg 192, including the large eastern lobe and the central dome of the main plateau, will provide a much more complete picture of the range of ages, compositions, and eruptive environments. Together with paleodepth data, information on the content of volatiles and the extent of post-magmatic hydrothermal modification of the upper levels of basement crust will be used to assess the environmental consequences of plateau magmatism.