The history of the Ontong Java Plateau (OJP) in the western Pacific, which is the largest oceanic large igneous province (LIP) in the world (Fig. F1), was investigated during Ocean Drilling Program (ODP) Leg 192. Understanding the origin and evolutionary history of the OJP is of particular importance because it is the best manifestation of mid-Cretaceous volcanism (~124–90 Ma) in the Pacific (Larson, 1991). Results from previous Deep Sea Drilling Project (DSDP) and ODP drilling expeditions to the OJP have shown that the OJP exhibits characteristics that are inconsistent with formation by mantle plume heads (e.g., Tarduno et al., 1991; Bercovici and Mahoney, 1994), a bolide impact (e.g., Rogers, 1982), or mantle avalanche (e.g., Muller, 2002). Other scientists (e.g., Ingle and Coffin, 2004) have recently argued that an extraterrestrial impact model is more consistent with existing data and suggested a large object (>10 km in diameter) may have struck the deep ocean basins and resulted in emplacement of the OJP. Despite the huge size of the OJP (larger than Alaska) and its potential role in contributing to our understanding of mantle circulation and environmental change in the past, the origin and evolution of the OJP is still poorly understood (Mahoney and Coffin, 1997).
Basement and sediment cores were recovered during Leg 192 at five widely spaced sites in previously unsampled areas of the OJP (Fig. F1). Four drill sites (1183, 1185, 1186, and 1187) successfully reached basaltic basement, which comprises a thick sequence of submarine pillow and massive lavas. The lava flows of the OJP have undergone low-temperature seawater-mediated alteration (Banerjee et al., 2004). Because responses to varying oceanographic conditions can result in changes in alteration regimes and in physical properties of the basalt, physical property parameters of the basaltic flows are useful for studying petrogenesis and environment of the OJP formation. Standard physical properties, such as P-wave velocity, density, porosity, magnetic susceptibility, and thermal conductivity, are useful indicators of alteration and can therefore be used to understand the relationship of physical properties to the alteration regime at the Leg 192 sites.
In this paper, we investigate the relationship between physical properties and alteration and vein formation in basaltic cores recovered from the four Leg 192 basement sites. We will first briefly mention the newly available age information about the basement sites and then outline the physical property data and explore their implications for understanding the interplay of primary volcanic emplacement and subsequent alteration.