Petrological and geochemical results from Integrated Ocean Drilling Program (IODP, 2003-2013) analyses of deep-seated mid-ocean ridge rocks (gabbros and peridotites) are summarized. Three important contributions to our understanding of the nature and origin of oceanic core complexes were obtained from: (1) a >1400 m gabbroic section from the Atlantis Massif of the Mid-Atlantic Ridge (site U1309, drilled by Expeditions 304/305), (2) sampling of intact oceanic gabbro after drilling through basaltic and sheeted dike complex layers from the fast-spreading Cocos Plate (leg 206 of site 1256D, drilled by Expeditions 309, 312 and, 335), and (3) sampling of layered gabbro from the Hess Deep of the East Pacific Rise (site U1415, drilled by Expedition 345). Future deep-seated hard-rock drilling projects by the research drilling vessel CHIKYU, including mantle drilling projects, are introduced.

In the first decade of the 21st century, two oceanic large igneous provinces (LIPs), the Ontong Java Plateau (OJP) and the Shatsky Rise, were drilled to recover basement lavas. Coring data show that basement rocks comprise massive sheet flows, similar to continental flood basalts. The high effusion rate to generate the massive sheet flows cannot be explained by normal plate tectonic processes. Because the two LIPs are considered to have formed at plate boundaries, previous workers have suggested that thermal plume heads underlie plate boundaries, generating voluminous magmas. ⁴⁰Ar-³⁹Ar data show that the duration of LIP magmatism exceeds that of the predictive model for plumes. Minor volcanic episodes between 90 and 44 Ma occurred in the OJP, with the major event occurring at ~122 Ma. For the Shatsky Rise, a ~10 Myr hiatus separates the final massive flows from main volcanism. Petrological data suggest the temperature of the magma source (potential temperature) was ~300°C, higher than the ambient mantle of the OJP, ~150-200°C higher than the Shatsky Rise, and lower than that of the plume model (>400°C). Geochemical data show that lavas from the two LIPs are variably enriched relative to normal upper mantle, suggesting they were derived from a plume source. However, unequivocal plume signatures (e.g., lower-mantle geochemical signatures with high ³He/⁴He) have not been identified in the volcanic products of the LIPs. Similarly, some lava compositions suggest the contribution of subducted slab materials to their magmatic source. Furthermore, the amount of subsidence following emplacement of the LIPs is less than that predicted by the thermal plume model. Thus, our study shows that a simple thermal plume model does not account for the genesis of the LIPs, indicating that more complex (e.g., thermochemical) models should be developed. Recent oceanic crust drilling programs have improved our understanding of LIP magmatism, but further work is required to constrain its characteristics and source.

In September 2010, Expedition 331 of the Integrated Ocean Drilling Program (IODP) was conducted to investigate active subseafloor microbial ecosystems associated with hydrothermal activity in the Iheya North field of the Okinawa Trough. We drilled and cored five sites in total, and created four artificial hydrothermal vents for post-cruise research. The artificial vents were drilled using specialized guide bases and lined with casing pipes and steel mesh at specific depths. The geological, geochemical, and microbiological characteristics of the core samples are interpreted and summarized elsewhere in this issue. Here we present the results of the post-drilling investigations. After the expedition, the seafloor hydrothermal fluid discharging pattern and chemistry, the hydrothermal mineral deposition pattern and chemistry and the seafloor chemosynthetic community pattern and composition were investigated for more than 3 years using a remotely operative vehicle (ROV). The post-drilling investigations yielded the following findings: (1) hydrothermal minerals that formed at the artificial hydrothermal vents are physically and chemically different from those at natural vents, and have great potential as seafloor metal resources; (2) hydrothermal fluids from the artificial hydrothermal vents are derived from a deep fluid reservoir underlying impermeable rock layers below the seafloor; and (3) anthropogenic disturbance affects both the seafloor landscape and the habitation behavior of vent-endemic chemosynthetic megafauna. The scientific drilling operation and construction of new hydrothermal vents during IODP Expedition 331 provide a test bed for investigating the ecological responses of deep-sea biological communities to anthropogenic disturbance, and for advances in deep-sea engineering.

Integrated Ocean Drilling Program (IODP) Expedition 331 was conducted in September 2010 and provided an unique opportunity to directly access the subseafloor environment beneath an active hydrothermal field in the Iheya North Knoll in the mid-Okinawa Trough. Localities of five drilled sites (C0013-C0017) along the slope of a knoll cover the hydrothermal field, represented by vigorous high temperature fluid venting from the active hydrothermal mound named NBC. Drilling at Site C0016 at the flank part of the mound revealed hydrothermal sulfide/sulfate mineralization comparable to that in Kuroko-type volcanic massive sulfide deposit. From Sites C0013 and C0014 located 100 and 450 m east from the mound, evidence for lateral intrusion of the hydrothermal component was observed by intense and widespread hydrothermal alteration, stepwise change in profiles of pore fluid chemistry, and drastic change in physical properties of the sediment. On the other hand, downward infiltration of seawater was recognized even at 50 m below the seafloor at Site C0017 located 1550 m apart from the mound. These results suggest the mixing region between the high-temperature and reductive hydrothermal environment and the low-temperature and oxidative seawater-dominant environment outspreads over a km scale beneath the active hydrothermal field. Furthermore, microbiological studies revealed that habitat of subvent microorganism significantly overlaps this mixing region. Expedition 331 was successful in providing a new model for a hydrothermal fluid circulation system that develops in a geologic setting dominated by volcanic sediment.

This review considers the progress in rock and paleomagnetism since the start of the Integrated Ocean Drilling Program (IODP) in 2003. Here, we focus on geomagnetic paleointensity and related topics, although many other studies have applied rock and paleomagnetic techniques to achieve various aims on IODP expeditions. High-quality, relative paleointensity records derived from North Atlantic cores have led to the establishment of a high-resolution paleointensity stack for the last 1.5 m.y. Several relative paleointensity records of the Eocene to Miocene have been obtained from equatorial Pacific cores; previously, few records were available before 3 Ma. These records show large-amplitude paleointensity variations on timescales of 10⁴ year during a stable polarity, including paleointensity minima similar to those at polarity transitions. These are similar to the variations known for the last 800 kyr, and are considered to be characteristic of geomagnetic field variations. Inter-core correlation and age estimation using relative paleointensity variations can now be applied back to the Eocene. It has been proved that variations in the magnetic properties of sediments, which are induced by paleoenvironmental changes, influence estimates of relative paleointensity. In particular, fluctuations in the relative abundances of biogenic and terrigenous magnetic minerals and sedimentation rates contaminate relative paleointensity records. To overcome this problem, we propose two possible strategies: (1) extraction of a real geomagnetic component using principal component analysis from a global set of relative paleointensity records, obtained from various oceanographic settings, and (2) independent estimation of paleointensity using beryllium isotope ratios. It is also important to gain a better understanding of the biogeochemical remanent magnetization carried by biogenic magnetite. The successful recovery of uncontaminated paleointensity records will enable us to explore controversial issues in paleomagnetism, such as correlations between geomagnetic field intensity and polarity length, and possible linkages among geomagnetic field variations, the Earth's orbit, and paleoclimatic change.