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The main objective of Integrated Ocean Drilling Program (IODP) Expedition 324 was to test competing mantle plume and plate tectonic models for ocean plateau formation at Shatsky Rise (Fig. F1). In these tests, determining the timing, duration, and source of volcanism at Shatsky Rise is of pivotal importance to understand the origin of this oceanic plateau. A short time span of volcanism less than a few millions of years would imply a plume head–like eruption (e.g. Campbell, 2007; Courtillot et al., 2003), and a southwest to northeast age progression along Shatsky Rise would suggest a transition from plume head to plume tail. On the contrary, a longer duration of volcanism forming the Tamu, Ori, and Shirshov massifs at Shatsky Rise (Fig. F1) would indicate much lower rates of lava effusion that are more consistent with alternative ridge-controlled mechanisms and the effects of rapid upwelling at a ridge-ridge-ridge triple junction. In the latter cases, estimation of source temperatures and degree of partial melting will prove important in showing the possibility of shallower mantle sources and testing associated extreme mantle fertility melting models (e.g., Anderson, 1995; Foulger, 2007).

As a large igneous province (LIP), Shatsky Rise currently is an enigma in the Earth sciences because it has characteristics consistent with both a deep mantle plume and that of a shallow mantle source controlled by plate tectonic processes. Its large volume and apparently rapid eruption of the entire volcanic pile are a possible consequence of a massive plume head impinging at the base of the Pacific lithosphere at ~146 Ma (Sager and Han, 1993; Mahoney et al., 2005). Yet, lava compositions with mid-ocean-ridge basalt type signatures found in previously drilled Shatsky Rise basalt units (Mahoney et al., 2005) may provide evidence pointing to a shallow mantle source. Importantly, Shatsky Rise represents only one of the very few places in the world oceans where we can distinguish between these two end member models. Because M-series magnetic anomalies are preserved in the Jurassic to Late Cretaceous ocean crust surrounding Shatsky Rise, the buildup of this large ocean plateau can be directly compared in space and time with the formation of ancient segments of oceanic plate. Drilling of volcanic basement at five sites, from the oldest Tamu Massif (~146 Ma; Mahoney et al. 2005) to the younger Ori Massif and Shirshov Massif (~140 Ma), provides us with critical samples that will help us distinguish between the mantle plume and plate tectonic models.

Ocean Drilling Program (ODP) Leg 198, which sailed in 2001, was the first expedition to penetrate into igneous basement of Shatsky Rise at Site 1213 on the southwest flank of Tamu Massif (Fig. F1). Drilling extracted 46.6 m of basaltic rock at a high recovery of 72.3% (Shipboard Scientific Party, 2002). This site is important to the testing of the two models of formation as it may contain older eruptive sequences than lavas drilled during Expedition 324. Drilling at Site 1213 recovered three massive basalt units (8–15 m thick) originally interpreted to represent three diabase sills (Shipboard Scientific Party, 2002). During Expedition 324 and the first postcruise scientific meeting in College Station, Texas (USA), the Expedition Scientists, including igneous petrologists, volcanologists, alteration specialists, and structural geologists, redescribed all Site 1213 igneous basement cores and processed all this information using the new Laboratory Information Management System (LIMS) database. The principal outcome of this reappraisal was the reinterpretation of these diabase units as three submarine massive basalt flows, similar to those found in other LIPs and flood basalts. In this study we provide macroscopic and petrographic descriptions, some limited shipboard measurements, and a reappraisal of the basement units cored at Site 1213.