Three-quarters of the ocean crust formed at fast-spreading ridges is composed of plutonic rocks whose mineral assemblages, textures and compositions record the history of melt transport and crystallization between the mantle and the seafloor.Despite the significance of these rocks, sampling them in situ is extremely challenging due to the overlying dikes and lavas. This means that our models for

The Murotomisaki Gabbroic Complex is a sill like layered intrusion of up to 220 m in thickness and is located at Cape Muroto, Kochi Prefecture, Japan. There are several olivine rich zones within the intrusion, which may have been formed through accumulation of olivine crystals. However, up to now it has not been clear as to whether all of the olivine rich zones formed in this way. To clarify this, we reinvestigated the layered structure by collecting a consistent data set of modal composition, crystal size, and crystal number density of olivine from throughout the intrusion. It was found out that nearly all of the olivine crystals, in terms of crystal numbers, occur in the basal olivine rich zone (within 40 m of the base of the intrusion), and the average value of the crystal number density of olivine throughout the entire intrusion coincides with the crystal number density of olivine in the outermost parts of the lower and upper chilled margins. Hence, we conclude that most primary olivine phenocrysts within the magma settled under the influence of gravity and accumulated to form the basal olivine rich zone. The crystal number density of olivine within the mid level zones (40 100 m from the base of the intrusion) is much less than the initial values, as indicated by values recorded in the chilled margins. It is proved that the increase of the olivine mode within this zone is attributed not to the crystal accumulation of olivine but to the increase of the crystal size of olivine, i.e., the crystal growth. In this way, considering the mode, crystal size, and crystal number density of olivine throughout the intrusion, the olivine rich zones within the intrusion can be classified, in terms of their origin, as either crystal accumulation zone (AC zone) or crystal growth zone (GR zone). The growth of olivine crystals in the GR zone was apparently accompanied by an increase in MgO, FeO, and MnO concentrations to levels well above initial (i.e., the chilled marginal) values. This enrichment suggests that crystal growth occurred within a chemically open system in the sense that the increase in MgO content within the GR zone arose from material transfer between the boundary layer (the GR zone) and the overlying magma.

This study presents a forward model to quantify the P‐wave velocity (VP), S‐wave velocity (VS), and electrical conductivity (σ) of the solid‐liquid mixtures for a given set of pressure, temperature, lithology, liquid phase (aqueous fluid or melt), liquid fraction, and geometrical parameters in relation to the aspect ratio and connectivity of the liquid phase. This is based on previous experimental and theoretical studies on seismic velocity and electrical conductivity of solid rocks and liquid phases. A total of 78 lithologies, an aqueous fluid with NaCl (∼0–10 wt.%), and mafic to felsic melt appropriate for the crust and the uppermost mantle conditions were described in terms of VP, VS, and σ, as per previous experimental measurements and molecular dynamics simulation. This forward model is provided as a Windows executable program, and generates synthetic VP, VS, and σ, referring to the seismic velocities and electrical conductivity observed in the northeast Japan arc. After generation of the synthetic VP, VS, and σ, the original lithology and liquid parameters (phase, fraction, aspect ratio, and connectivity) were searched by implementing the grid search algorithm to map the misfit over the broad parameter space. The mapping shows the presence of a global misfit minimum around the optimized solution and the possibility of resolving the lithology and the liquid phase parameters based on the observed VP, VS, and σ by using the forward model presented in this study.

Integrated Ocean Drilling Program (IODP) Hess Deep Expedition 345 was designed to sample lower crustal primitive gabbroic rock that formed at the fast-spreading East Pacific Rise (EPR) in order to test competing models of magmatic accretion and the intensity of hydrothermal cooling at depth. The Hess Deep Rift was selected to exploit tectonic exposures of young EPR plutonic crust, building upon results from Ocean Drilling Program Leg 147 as well as more recent submersible, remotely operated vehicle, and nearbottom surveys. The primary goal was to acquire the observations required to test end-member crustal accretion models that were in large part based on relationships from ophiolites, in combination with mid-ocean ridge geophysical studies. This goal was achieved with the recovery of primitive layered olivine gabbro and troctolite with many unexpected mineralogical and textural relationships, such as the abundance of orthopyroxene and the preservation of delicate skeletal olivine textures. Site U1415 is located within the Hess Deep Rift along the southern slope of the intrarift ridge between 4675 and 4850 m water depths. Specific hole locations were selected in the general area of the proposed drill sites (HD-01B-HD-03B) using a combination of geomorphology, seafloor observations, and shallow acoustic subbottom profiling data. A total of 16 holes were drilled. The primary science results were obtained from coring of two ~110 m deep reentry holes (U1415J and U1415P) and five single-bit holes (U1415E and U1415G-U1415I). Despite deep water depths and challenging drilling conditions, reasonable recovery for hard rock expeditions (15%-30%) was achieved at three 35-110 m deep holes (U1415I, U1415J, and U1415P). The other holes occupied during this expedition included three failed attempts to establish reentry capability (Holes U1415K, U1415M, and U1415P) and six jet-in tests to assess sediment thickness (Holes U1415A-U1415D, U1415F, and U1415L). Olivine gabbro and troctolite are the dominant plutonic rock types recovered at Site U1415, with minor gabbro, clinopyroxene oikocryst-bearing gabbroic lithologies, and gabbronorite. These rocks exhibit cumulate textures similar to those found in layered mafic intrusions and some ophiolite complexes. All lithologies are primitive, with Mg# between 76 and 89, falling within the global range of primitive oceanic gabbros. Spectacular modal and grain size layering was prevalent in >50% of the recovered core, display

The cored interval is measured in meters below rig floor (mbrf) and reported in meters below seafloor (mbsf). Depth below seafloor is determined by subtracting the seafloor depth measured Methods 1