Jinichiro Maeda

A long in situ section of the lower ocean crust: results of ODP Leg 176 drilling at the Southwest Indian Ridge

Ocean Drilling Program Leg 176 deepened Hole 735B in gabbroic lower ocean crust by 1 km to 1.5 km. The section has the physical properties of seismic layer 3, and a total magnetization sufficient by itself to account for the overlying lineated sea-surface magnetic anomaly. The rocks from Hole 735B are principally olivine gabbro, with evidence for two principal and many secondary intrusive events. There are innumerable late small ferrogabbro intrusions, often associated with shear zones that cross-cut the olivine gabbros. The ferrogabbros dramatically increase upward in the section. Whereas there are many small patches of ferrogabbro representing late iron- and titanium-rich melt trapped intragranularly in olivine gabbro, most late melt was redistributed prior to complete solidification by compaction and deformation. This, rather than in situ upward differentiation of a large magma body, produced the principal igneous stratigraphy. The computed bulk composition of the hole is too evolved to mass balance mid-ocean ridge basalt back to a primary magma, and there must be a significant mass of missing primitive cumulates. These could lie either below the hole or out of the section. Possibly the gabbros were emplaced by along-axis intrusion of moderately differentiated melts into the near-transform environment. Alteration occurred in three stages. High-temperature granulite- to amphibolite-facies alteration is most important, coinciding with brittle^ductile deformation beneath the ridge. Minor greenschist-facies alteration occurred under largely static conditions, likely during block uplift at the ridge transform intersection. Late post-uplift low-temperature alteration produced locally abundant smectite, often in previously unaltered areas. The most important features of the high- and low-temperature alteration are their respective

Drilling constraints on lithospheric accretion and evolution at Atlantis Massif, Mid‐Atlantic Ridge 30°N

Expeditions 304 and 305 of the Integrated Ocean Drilling Program cored and logged a 1.4 km section of the domal core of Atlantis Massif. Postdrilling research results summarized here constrain the structure and lithology of the Central Dome of this oceanic core complex. The dominantly gabbroic sequence recovered contrasts with predrilling predictions; application of the ground truth in subsequent geophysical processing has produced self-consistent models for the Central Dome. The presence of many thin interfingered petrologic units indicates that the intrusions forming the domal core were emplaced over a minimum of 100-220 kyr, and not as a single magma pulse. Isotopic and mineralogical alteration is intense in the upper 100 m but decreases in intensity with depth. Below 800 m, alteration is restricted to narrow zones surrounding faults, veins, igneous contacts, and to an interval of locally intense serpentinization in olivine-rich troctolite. Hydration of the lithosphere occurred over the complete range of temperature conditions from granulite to zeolite facies, but was predominantly in the amphibolite and greenschist range. Deformation of the sequence was remarkably localized, despite paleomagnetic indications that the dome has undergone at least 45 degrees rotation, presumably during unroofing via detachment faulting. Both the deformation pattern and the lithology contrast with what is known from seafloor studies on the adjacent Southern Ridge of the massif. There, the detachment capping the domal core deformed a 100 m thick zone and serpentinized peridotite comprises similar to 70% of recovered samples. We develop a working model of the evolution of Atlantis Massif over the past 2 Myr, outlining several stages that could explain the observed similarities and differences between the Central Dome and the Southern Ridge.

Primitive layered gabbros from fast-spreading lower oceanic crust

Three-quarters of the oceanic 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 sea floor. Despite the importance of these rocks, sampling them in situ is extremely challenging owing to the overlying dykes and lavas. This means that models for understanding the formation of the lower crust are based largely on geophysical studies and ancient analogues (ophiolites) that did not form at typical mid-ocean ridges. Here we describe cored intervals of primitive, modally layered gabbroic rocks from the lower plutonic crust formed at a fast-spreading ridge, sampled by the Integrated Ocean Drilling Program at the Hess Deep rift. Centimetre-scale, modally layered rocks, some of which have a strong layering-parallel foliation, confirm a long-held belief that such rocks are a key constituent of the lower oceanic crust formed at fast-spreading ridges. Geochemical analysis of these primitive lower plutonic rocks—in combination with previous geochemical data for shallow-level plutonic rocks, sheeted dykes and lavas—provides the most completely constrained estimate of the bulk composition of fast-spreading oceanic crust so far. Simple crystallization models using this bulk crustal composition as the parental melt accurately predict the bulk composition of both the lavas and the plutonic rocks. However, the recovered plutonic rocks show early crystallization of orthopyroxene, which is not predicted by current models of melt extraction from the mantle and mid-ocean-ridge basalt differentiation. The simplest explanation of this observation is that compositionally diverse melts are extracted from the mantle and partly crystallize before mixing to produce the more homogeneous magmas that erupt.

Petrology of local concentration of chromian spinel in dunite from the slow-spreading Southwest Indian Ridge

This is the first detailed report on local concentration of chromian spinel in a dunite from an ultraslow spreading ridge, the Southwest Indian Ridge (SWIR). The sample was collected from an outcrop with detailed observations using submersible SHINKAI 6500 of the Japanese Marine Science Technology Center. The dunite occurs as a tabular-shaped layer in a lherzolite host outcrop. Spinel is found as a string of small micropods 2–3 centimeters in size. These spinel micropods make a layer in the middle part of a spinel-poor dunite ( 0.4) compared with other peridotite samples in the studied area (lherzolites to harzburgite with low-Cr# spinel, typically Cr# ≤ 0.3). The occurrence and chemical compositions of clinopyroxene in the enstatite-poor harzburgite suggest that some clinopyroxenes crystallized from infiltrated interstitial melts. The host peridotites are interpreted as a residue of relatively low degrees of partial melting consistent with a location along the SWIR far from a mantle hot spot. This was then followed by crystallization of clinopyroxene from interstitial melt in the dunite. Irrespective of their small size, the lithological relationships between the spinel micropods and the host peridotites are the same as those for podiform chromitite in ophiolites and orogenic peridotites. The spinel Cr# in the micropods (0.3) is compatible with the lower range of those in basalts from SWIR far from hot spot. The spinel micropods were mainly formed by interaction between relatively depleted peridotite and a locally significant volume of basaltic melt traversing the upper mantle. This study coupled with the previous works on chromitites suggest that podiform chromites occur in every geodynamic setting, though economic concentrations of chromite (Cr-rich spinel) are unlikely to occur in the mantle at ultraslow spreading ridges.

Magmatic srilankite (Ti2ZrO6) in gabbroic vein cutting oceanic peridotites: An unusual product of peridotite-melt interactions beneath slow-spreading ridges

Abstract We report srilankite in a gabbroic vein cutting a serpentinized peridotite collected from the Atlantis II Fracture Zone, the slow-spreading Southwest Indian Ridge, using submersible SHINKAI 6500 of the Japanese Marine Science Technology Center. Srilankite occurs in small patches, <30 μm across, always coexisting with ilmenite and rutile. Zircon, apatite, and phlogopite also occur as accessory minerals in the vein. The Zr/Ti ratio of the srilankite is close to the stoichiometric value of one-half (Ti2.00Zr0.98Hf0.01Fe0.01O6). Based on petrography, the srilankite appears to have co-crystallized with ilmenite and rutile from melts rather than through metamorphic recrystallization. Mineral assemblages and mineral compositions in the vein indicate that melts that produced the vein have high concentrations of compatible elements (MgO and Cr2O3) as well as incompatible elements (high-field strength elements, K2O, and H2O). On the other hand, TiO2-enrichment of minerals in the peridotite host on the periphery of the gabbroic vein may have resulted from interaction with the melts. Geochemical interactions between peridotite and melt in the upper mantle may effectively concentrate incompatible elements in a modified melt, which may precipitate srilankite directly. Physical conditions under slow-spreading ridges, characterized by a highly attenuated magma supply and high rock/melt ratio, favor peridotite-melt interactions

Lower oceanic crust formed at an ultra-slow-spreading ridge: Ocean Drilling Program Hole 735B, Southwest Indian Ridge

Ocean Drilling Program ODP Hole 735B, drilled on Legs 118 and 176, 1508 m of oceanic layer 3 on a transverse ridge adjacent to the Atlantis II Fracture Zone, Southwest Indian Ridge. The cored sequence consists predominantly or olivine gabbro and troctolite and lesser amounts of gabbro, and gabbronorite rich in oxides. The section contains live major blocks of relatively primitive olivine gabbro and troctolite, composed of many smaller igneous bodies. Each Of these composite blocks shows a small upward decrease in Mg# [defined as 100 x Mg/(Mg + Fe 2+)] and contains more fractionated Fe- and Ti-rich gabbros near the top.Small, crosscutting bodies of olivine gabbro and troctolite with diffuse boundaries may represent conduits through crystal mushes for melts migrating upward and feeding individual intrusions. Oxide gabbros and gabbronorites are commonly associated with shear zones of intense deformation, which crosscut the section at all levels, However, oxide-rich rocks decrease in abundance downward and are nearly absent in the lower 500 m of the section. The gabbros and gabbronorites appear to have formed from late-stage, Fe- and Ti-rich, intercumulus melts that were expelled out of fractionating olivine gabbros into the shear zones. The fabrics of the recovered gabbros are consistent with synkinematic cooling and extension of the crustal section in a mid-ocean ridge environment. However, thick intervals of the core have only a weak magmatic foliation. The magmatic foliation is commonly overprinted by a weak, parallel, deformational fabric probably reflecting the transition from a largely magmatic to a largely crystalline state. Deformation in this crustal section decreases markedly downward. Metamorphism and alteration also decrease downward, and much of the core has less than 5% background alteration. Major zones of crystal-plastic (ductile by dislocated creep) deformation in the upper part of the core probably formed under conditions equivalent to granulite-facies conditions when there was little or no melt present. Late-magmatic and hydrothermal fluids produced a variety of plagioclase, amphibole, and diopside veins. Late-stage, low-temperature veins of calcite, smectite, zeolite, prehnite are present in a few intervals. The fact that the cored is unlike ophiolite as defined by the Penrose Conference Participants suggests that no ophiolite representing an ultra-slow-spreading-ridge environment like the Southwest Indian Ridge may be preserved.

Exploring the plutonic crust at a fast-spreading ridge:new drilling at Hess Deep

Integrated Ocean Drilling Program (IODP) Hess Deep Expedition 345 was designed to sample lower crustal primitive gabbroic rocks that formed at the fast-spreading East Pacific Rise (EPR) in order to test 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 ODP Leg 147 as well as more recent submersible, remotely operated vehicle, and near-bottom 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 gabbros and troctolites 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 along the southern slope of an intrarift ridge within the Hess Deep Rift between 4675 and 4850 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 subsurface seismic 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 two failed attempts to establish reentry capability (Holes U1415K and U1415M) 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 troctolite, clinopyroxene oikocryst-bearing gabbro, and gabbronorite. These rocks exhibit cumulate textures similar to those found in layered basic intrusions and some ophiolite complexes. All lithologies are primitive, with Mg# between 0.76 and 0.89, falling within the global range of primitive oceanic gabbros. Spectacular modal and/or grain size layering was prevalent in >50% of the recovered core, displaying either simple or diffuse boundaries. Magmatic foliation largely defined by the shape-preferred orientation of plagioclase and olivine is moderate to strong in intervals with simple modal layering but weak to absent in the troctolite series and largely absent in the multitextured lay-ered series. The abundance of orthopyroxene in these primitive rocks was unexpected and deviates from the standard model for mid-ocean-ridge basalt crystallization. Pres-ervation of delicate skeletal olivine grains suggests that at least part of the recovered section of the lower crust was not subjected to significant hypersolidus or subsolidus strain. The metamorphic mineral assemblages record the cooling of primitive gabbroic lithologies from EPR magmatic conditions (>1000°C) to zeolite facies conditions ( 2 km beneath the sheeted dike-plutonic transition and thus represents the lower half to a third of the EPR plutonic crust. The orientation of the magmatic fabrics and magnetic inclinations of the core suggest that Site U1415 is composed of a series of 30-65 m thick blocks that likely formed by mass wasting. Sampling three or four blocks of relatively fresh rocks proved advantageous, as it facilitated observations of two distinct types of layering and troctolite units with varying grain size, lithologic associations, and textures. The mineralogical and textural relationships show that in several respects the Oman ophiolite is not the ideal model for fast-spreading ocean crust and call into question some aspects of both of the end-member accretion models that were to be tested. The results of the IODP Hess Deep Expedition 345 provide a reference section for primitive fast-spreading lower crust that did not exist before. This highlights the necessity of ocean drilling to address questions related to the origin and evolution of the lower ocean crust.