What we have learned about Shatsky Rise Oceanic Plateau from IODP Expedition 324

In fall 2009, Integrated Ocean Drilling Program (IODP) Expedition 324 drilled igneous cores at five sites (U1346 to U1350) to study the origin of Shatsky Rise oceanic plateau, one of the large igneous provinces of the Pacific Ocean. Since we are now three years post-cruise and seeing results of studies come to light, we present here what we have learned about Shatsky Rise. Located ~1500 km east of Japan, Shatsky Rise consists of three massifs, Tamu, Ori, and Shirshov, which were generated along the trace of a mid-ocean ridge triple junction during the latest Jurassic to Early Cretaceous times. Radiometric dating of cored basalts using the 40Ar/39Ar technique is turning out to be challenging due to low-K content of the basalts, but preliminary ages show that eruptions are age progressive across Shatsky Rise. Eruptions apparently began at the southwest, with Tamu Massif, then moved to the northeast with time. Geochemistry data indicate that four magma types exist on the plateau; namely normal, low-Ti, high-Nb, and U1349 types. The normal type constitute ~94% lava units of the oldest volcano (Tamu Massif) and the other three types comprise ~57% of samples from the younger Ori Massif, implying that geochemical compositions may have become heterogeneous with time. Phase equilibria modeling and our new experimental data both indicate the low pressure magma fractionation (<200-300MPa). Model calculations of immobile trace elements estimate that melting fraction of Shatsky Rise is similar to normal mid-ocean ridge basalt (N-MORB), but there is greater effect of residual garnet inferring the depth of melting is probably higher. Sr-Nd-Pb-Hf isotopic results for the normal, low-Ti, and U1349 type basalts are almost identical and fall within the N-MORB range, but the high-Nb type basalts have distinctly enriched compositions. The normal type basalts show uniform and lower 3He/4He than N-MORB, suggesting the presence of recycled material in the magma source of Shatsky Rise. Detailed examination of secondary mineral assemblages at the four basaltic sites drilled at Shatsky reveals variations in fluid-rock interaction conditions within the plateau. These variations, mainly related to composition of fluids, temperature and redox conditions, occur spatially: laterally from one site to another, sites U1346 and U1349 being strongly more altered than sites U1347 and U1350, and also vertically within a single site (e.g., Site U1349).