SCIENTIFIC OBJECTIVES

Leg 183 was the first Ocean Drilling Program leg dedicated to sampling the igneous basement of an oceanic plateau with the goals of understanding its origin and evolution. An overall objective of Leg 183 was to evaluate the hypothesis that the Kerguelen Plateau and Broken Ridge LIP formed as a result of volcanism associated with ascent of a plume head with subsequent volcanism resulting from the plume stem. To accomplish this objective, Leg 183 focused on three major problems related to the formation and evolution of the LIP:

1. Magmatic and tectonic development of the Kerguelen Plateau and Broken Ridge: we investigated how magma flux varied as a function of time to test the plume model for magma production rates during the formation of LIPs. We also examined relationships between magmatism and tectonism to understand how LIP development was related to the breakup of Gondwana and seafloor spreading.
2. Petrogenesis of basement igneous rocks: we sought to constrain the mineralogy and composition of the mantle sources that contributed to the magmatism, the melting processes that created the magmas, and the postmelting magmatic evolution. A specific objective was to evaluate the relative roles of a plume, asthenosphere, and continental lithosphere in the magmatism that formed the different domains of this LIP.
3. Environmental impact: what were the environmental effects of LIP magmatism? In particular, we sought to answer the following questions. During periods of high magma flux was it likely that (a) released volatiles and particulates affected the climate and (b) hydrothermal alteration affected the geochemical characteristics of seawater?

Important advances in solving these problems arose from shipboard observations that are summarized in Shipboard Scientific Party (2000) and Frey et al. (2000a). In this synthesis of postcruise results we summarize the progress made in addressing these three problems. Furthermore, we summarize advances in our understanding of Cretaceous and Cenozoic paleoenvironments and paleoceanography yielded by the sedimentary sections overlying igneous basement. The overall objectives and results of Leg 183 are complemented by ongoing studies of the Cenozoic subaerial lavas that form the Kerguelen archipelago (Damasceno et al., 2002; Doucet et al., 2002; Frey et al., 2002a; Mattielli et al., 2002).