Natsue Abe | JAMSTEC - Academia.edu (original) (raw)

Papers by Natsue Abe

Scientific Reports

The Chile Triple Junction, where the hot active spreading centre of the Chile Rise system subduct... more The Chile Triple Junction, where the hot active spreading centre of the Chile Rise system subducts beneath the South American plate, offers a unique opportunity to understand the influence of the anomalous thermal regime on an otherwise cold continental margin. Integrated analysis of various geophysical and geological datasets, such as bathymetry, heat flow measured directly by thermal probes and calculated from gas hydrate distribution limits, thermal conductivities, and piston cores, have improved the knowledge about the hydrogeological system. In addition, rock dredging has evidenced the volcanism associated with ridge subduction. Here, we argue that the localized high heat flow over the toe of the accretionary prism results from fluid advection promoted by pressure-driven discharge (i.e., dewatering/discharge caused by horizontal compression of accreted sediments) as reported previously. However, by computing the new heat flow values with legacy data in the study area, we raise ...

Japan Geoscience Union, 2019

The Chile triple junction (CTJ) is a unique place where a spreading center of mid-ocean ridge is ... more The Chile triple junction (CTJ) is a unique place where a spreading center of mid-ocean ridge is subducting near the Taitao peninsula. Around CTJ, presence of high heat flow on the continental slope (Cande et al., 1987, J. Geophys. Res.) and near-trench young granitic rocks on the Taitao peninsula (Anma and Orihashi, 2013, Geochem. J.) suggests the thermal and petrological impact of subducting ridge on the continental side. The tectonic history of the southeast Pacific since early Cenozoic to the present suggests that ridge subduction continuously occurred along the Chile trench, which migrated northward (Cande and Leslie, 1986, J. Geophys. Res.), which can contribute to growth of the continental crust (Iwamori, 2000, EPSL). In January 2019, the MR18-06 cruise Leg 2 was conducted at the Chile Triple Junction, as a part of 'EPIC' expedition by using R.V Mirai of JAMSTEC. During the leg, we completed 4 SCS lines, 6 piston coring, 6 heat flow measurements, 2 dredges, and underw...

Earth, Planets and Space, 2012

Logging data are measurements of physical properties of the formation surrounding a borehole, acq... more Logging data are measurements of physical properties of the formation surrounding a borehole, acquired in situ after completion of coring (wireline logging) or during drilling (Logging-While-Drilling, LWD). The range of data (resistivity, gamma radiation, velocity, density, borehole images,…) in any hole depends on the scientific objectives and operational constraints.

Logging data are measurements of physical properties of the formation surrounding a borehole, acq... more Logging data are measurements of physical properties of the formation surrounding a borehole, acquired in situ after completion of coring (wireline logging) or during drilling (Logging-While-Drilling, LWD). The range of data (resistivity, gamma radiation, velocity, density, borehole images,…) in any hole depends on the scientific objectives and operational constraints.

Japan Geoscience Union, 2016

IODP Expedition 360, Phase I of the Nature of the Lower Crust and Moho at Slower Spreading Ridges... more IODP Expedition 360, Phase I of the Nature of the Lower Crust and Moho at Slower Spreading Ridges (SloMo) project of a Multi-Leg Drilling Project, was carried out from 1 Dec., 2015 to 31 Jan, 2016, at Atlantis Bank that is a 5 km local high along the eastern wall of the Atlantis II Transform of the Southwest Indian Ridge. Atlantis Bank has been interpreted as an oceanic core complex, where intact lower crust and the uppermost mantle are tectonically exposed by a long-lived detachment fault. We conducted all drilling operations at a single site in a single Hole U1473A and drilled 789.7 m though gabbros. This is the deepest single-leg hard-rock drilling hole in ocean crust. Expedition 360 Hole U1473 is located at 2.2 km Northeast of 1.5 km deep Hole 735B and at 1.4 km north of 158 m deep Hole 1105A. This provides us, for the first time, a unique opportunity to explore three dimensional lower crustal characteristics beneath the slow-spreading ridges. Phase II of the SloMo has proposed ...


Scientific Drilling, 2021

For more than half a century, exploring a complete sequence of the oceanic crust from the seafloo... more For more than half a century, exploring a complete sequence of the oceanic crust from the seafloor through the Mohorovičić discontinuity (Moho) and into the uppermost mantle has been one of the most challenging missions of scientific ocean drilling. Such a scientific and technological achievement would provide humankind with profound insights into the largest realm of our planet and expand our fundamental understanding of Earth's deep interior and its geodynamic behavior. The formation of new oceanic crust at mid-ocean ridges and its subsequent aging over millions of years, leading to subduction, arc volcanism, and recycling of some Published by Copernicus Publications on behalf of the IODP and the ICDP. 70 S. Umino et al.: Workshop report: Exploring deep oceanic crust off Hawai'i components into the mantle, comprise the dominant geological cycle of matter and energy on Earth. Although previous scientific ocean drilling has cored some drill holes into old (> 110 Ma) and young (< 20 Ma) ocean crust, our sampling remains relatively shallow (< 2 km into intact crust) and unrepresentative of average oceanic crust. To date, no hole penetrates more than 100 m into intact average-aged oceanic crust that records the longterm history of seawater-basalt exchange (60 to 90 Myr). In addition, the nature, extent, and evolution of the deep subseafloor biosphere within oceanic crust remains poorly unknown. To address these fundamentally significant scientific issues, an international workshop "Exploring Deep Oceanic Crust off Hawai'i" brought together 106 scientists and engineers from 16 countries that represented the entire spectrum of disciplines, including petrologists, geophysicists, geochemists, microbiologists, geodynamic modelers, and drilling/logging engineers. The aim of the workshop was to develop a full International Ocean Discovery Program (IODP) proposal to drill a 2.5 km deep hole into oceanic crust on the North Arch off Hawai'i with the drilling research vessel Chikyu. This drill hole would provide samples down to cumulate gabbros of mature (∼ 80 Ma) oceanic crust formed at a half spreading rate of ∼ 3.5 cm a −1. A Moho reflection has been observed at ∼ 5.5 km below the seafloor at this site, and the workshop concluded that the proposed 2.5 km deep scientific drilling on the North Arch off Hawai'i would provide an essential "pilot hole" to inform the design of future mantle drilling.

Marine and Petroleum Geology, 2018

Gas hydrate-bearing layers are normally identified by a seismic imaged bottom simulating reflecto... more Gas hydrate-bearing layers are normally identified by a seismic imaged bottom simulating reflectors (BSR) or by downhole log responses because of their high acoustic velocity and electric resistivity compared to surrounding formations. These gas hydrate characteristics can also result in contrasting in-situ formation compressive strengths. Here, we describe gas hydrate-bearing layers based on equivalent strength (EST), which relates to insitu compressive strength, in five exploration boreholes drilled during the Indian National Gas Hydrate Program Expedition 02 (NGHP-02). For Site NGHP-02-23, a representative site for those that were established during NGHP-02, the EST evaluated from drilling parameters shows a constant trend of ∼2 MPa, with some strong peak values in the 0-271.4 m-below-seafloor (mbsf) interval, and a sudden increase up to 4 MPa above the BSR depth (271.4-290.0 mbsf). Below the BSR, the EST stays at ∼2 MPa to the bottom of the hole (378 mbsf). Comparing the EST with logging data and a core sample description suggests that the EST depth profiles reflect the formation lithology and gas hydrate content. The EST increases in sand-rich and gas hydrate-bearing zone. In the lower gas hydrate layers in particular, the EST curve shows the same approximate trend with that of P-wave velocity and resistivity measured during downhole logging. Similar relationships between EST, hydrate layer, and log responses are confirmed in other four sites drilled nearby in NGHP-02 Area B. These results suggest that the EST, as a proxy for in-situ formation strength, can indicate the location and extent of the gas hydrate as well as borehole logging. Although the EST was calculated after drilling, utilizing the recorded surface drilling parameters (weight on bit, top drive torque, RPM and rate of penetration) in this study, the EST can be acquired during drilling using real-time drilling parameters. In addition, the EST only requires drilling performance data without any additional tools or measurements, making it a simple and economical tool for the exploration of gas hydrates.

Geochemistry, Geophysics, Geosystems, 2016

North Pond is an isolated sedimentary pond on the western flank of the Kane area along the Mid-At... more North Pond is an isolated sedimentary pond on the western flank of the Kane area along the Mid-Atlantic Ridge. Drill-hole U1382A of IODP Expedition 336 recovered peridotite and gabbro samples from a sedimentary breccia layer in the pond, from which we collected six fresh peridotite samples. The peridotite samples came from the southern slope of the North Pond where an oceanic core complex is currently exposed. The samples were classified as spinel harzburgite, plagioclase-bearing harzburgite, and a vein-bearing peridotite that contains tiny gabbroic veins. No obvious macroscopic shear deformation related to the formation of a detachment fault was observed. The spinel harzburgite with a protogranular texture was classified as refractory peridotite. The degree of partial melting of the spinel harzburgite is estimated to be 17%, and melt depletion would have occurred at high temperatures in the uppermost mantle beneath the spreading axis. The progressive melt-rock interactions between the depleted spinel harzburgite and the percolating melts of Normal-Mid Ocean Ridge Basalt (N-MORB) produced the plagioclasebearing harzburgite and the vein-bearing peridotite at relatively low temperatures. This implies that the subsequent refertilization occurred in an extinct spreading segment of the North Pond after spreading at the axis. Olivine fabrics in the spinel and plagioclase-bearing harzburgites are of types AG, A, and D, suggesting the remnants of a mantle flow regime beneath the spreading axis. The initial olivine fabrics appear to have been preserved despite the later melt-rock interactions. The peridotite samples noted above preserve evidence of mantle flow and melt-rock interactions beneath a spreading ridge that formed at 8 Ma.

is structural geologist with interests in the formation and deformation of ocean lithosphere at m... more is structural geologist with interests in the formation and deformation of ocean lithosphere at mid-ocean ridges, mechanisms of accretion of lower ocean crust at fast-spreading ridges; lithospheric deformation at ridge axial discontinuities; rifting/detachment fault processes at slow-spreading ridges; development and application of core reorientation techniques in structural studies of borehole core; and application of robotic seabed rock drilling technologies to mid-ocean ridge studies.

Japan Geoscience Union, 2015

Metag-gabbro and some serpentinites are not only extremely altered, but also metamorphosed at hig... more Metag-gabbro and some serpentinites are not only extremely altered, but also metamorphosed at high temperature up to midamphibolite facies. Chromian spinel included in those samples, which is normally well preserved under the condition of ocean floor metamorphism, also metamorphosed into chrome-bearing magnetite and/or magnetite. The crystal orientation varies inside a grain with chemical modification of the metamorphism. We discuss the possibility of such high temperature metamorphism in the deep oceanic plate.

Annual Meeting of the Geological Society of Japan, 2016

Dunite and gabbroic materials recovered from Hole 1271B, Ocean Drilling Program (ODP) Leg 209, we... more Dunite and gabbroic materials recovered from Hole 1271B, Ocean Drilling Program (ODP) Leg 209, were examined for mineral chemistry to understand melt flow and melt-mantle reactions in the shallowest upper mantle of the Mid-Atlantic Ridge near the 15°20' Fracture Zone. Hole 1271B was drilled to 103.8 meters below seafloor on the inner corner high along the south wall of the 15°20' Fracture Zone. The total length of core collected was 15.9 m (recovery = ~15%). The dominant rock type in Hole 1271B is dunite, followed by brown amphibole gabbro, olivine gabbro, and troctolite, along with minor amounts of harzburgite and olivine gabbronorite. A large proportion of the dunite is associated with gabbroic rocks in Hole 1271B, similar to those observed in the Mohorovicic (Moho) transition zone of the Oman ophiolite, indicating significant magmatic activity in this region near the 15°20' Fracture Zone. Olivine Fo content varies from 89.2 to 91.2 in impregnated dunite and from 85.6 to 88.6 in troctolite, olivine gabbro, and olivine gabbronorite. Spinel Cr# (= 100 x Cr/[Cr + Al] molar ratio) ranges from 38.9 to 62.7 in dunite and from 46.3 to 57.6 in troctolites, olivine gabbro, and olivine gabbronorite. Compositional trends for spinel from dunite through troctolite toward olivine gabbro/gabbronorite are characterized by increases in TiO2, Cr#, and Fe3+#, very similar to those reported from Hess Deep Site 895. Olivine gabbro, olivine gabbronorite, and troctolite in Hole 1271B are considered to have formed as hybrid rocks between dunite and an evolved melt in the walls of a melt channel in the shallowest upper mantle that is tens of meters wide. The melt trapped in the wall rock crystallized plagioclase and clinopyroxene. On the other hand, dunite in the center of the melt channel became more refractory by melt-mantle reactions, increasing spinel Cr# to 62.5.

Marine and Petroleum Geology, 2018

Physical properties like porosity and density are important parameters for size estimations of fo... more Physical properties like porosity and density are important parameters for size estimations of fossil fuel reservoir in host sediments and rocks. X-ray CT based imaging and subsequent analysis of the images by using various techniques for estimations of these properties of host rocks and sediments has been attempted. However, most of those techniques tend to be biased and subjective due to complications in image processing. Here we describe a new procedure for estimating porosity and density without processing the XCT images, but using the raw data that are used for producing the images. In this study, large-size cylindrical core samples (up to 150 cm long and 6.6 cm diameter) were imaged in a matter of minutes by using a medical XCT scanner soon after their recovery from seafloor onboard drillship in order to record as realistic as possible condition of hydrate-bearing sediments. Despite some uncertainty in relative volumes of solid, liquid and gas phases at voxel level, the results obtained by this rapid and non-destructive procedure are in good agreement with those obtained by time-and sampleconsuming conventional methods of physical property measurements.

Marine and Petroleum Geology, 2018

The existence of overpressure in shallow sediments, which is often constrained by hydraulic prope... more The existence of overpressure in shallow sediments, which is often constrained by hydraulic properties, influences the gas-hydrate formation process and gas production. Porosity, permeability, and grain size measurements in laboratory experiments were conducted on core samples from gas-hydrate-bearing regions offshore from the Krishna-Godavari Basin, eastern India. Porosity was found to decrease with increasing effective stress; this is explained by the exponential decay curve along which porosity decreases from 65 % at 0 MPa to 40 % at 10 MPa. Permeability and the corresponding hydraulic diffusivity decrease from 10 −17 to less than 10 −18 m 2 and from 10 −7 to 10 −8 m 2 /s at 0.5 and 5 MPa, respectively. Grain sizes were larger and the sand fraction more scattered in channel-filled sediment sites compared to slope sediment sites. The preconsolidation stresses evaluated from consolidation curves indicate the absence of overpressure at shallow depths. In contrast, a comparison of ship-board measurements and standard compaction curves suggested that measured porosity was higher than the predicted porosity at greater depths. These porosity anomalies are interpreted as a sign of overpressure that approaches near lithostatic values at greater depths in slope sediment sites. A one-dimensional sedimentation model recreated overpressure profiles similar to those predicted by porosity gaps, under the assumption of large basial fluid influx or lower permeability than that derived from laboratory data. The modeling results suggest that near-hydrostatic pressure at shallow depths and significant overpressure at greater depths proposed by the porosity gap method is explained by the non-linearity of transport properties. The relatively small overpressure generation in channel-filled sites compared with slope sites can be explained by the higher permeability due to coarser grain size and larger sand fraction and by the smaller basal influx. On the contrary, considerably large basal influx associated with clay mineral dehydration and methane gas supply from deep sediments was expected to promote overpressure at slope sites, which is confirmed by Cl − concentrationdepth profiles.

Scientific Reports

The Chile Triple Junction, where the hot active spreading centre of the Chile Rise system subduct... more The Chile Triple Junction, where the hot active spreading centre of the Chile Rise system subducts beneath the South American plate, offers a unique opportunity to understand the influence of the anomalous thermal regime on an otherwise cold continental margin. Integrated analysis of various geophysical and geological datasets, such as bathymetry, heat flow measured directly by thermal probes and calculated from gas hydrate distribution limits, thermal conductivities, and piston cores, have improved the knowledge about the hydrogeological system. In addition, rock dredging has evidenced the volcanism associated with ridge subduction. Here, we argue that the localized high heat flow over the toe of the accretionary prism results from fluid advection promoted by pressure-driven discharge (i.e., dewatering/discharge caused by horizontal compression of accreted sediments) as reported previously. However, by computing the new heat flow values with legacy data in the study area, we raise ...

Japan Geoscience Union, 2019

The Chile triple junction (CTJ) is a unique place where a spreading center of mid-ocean ridge is ... more The Chile triple junction (CTJ) is a unique place where a spreading center of mid-ocean ridge is subducting near the Taitao peninsula. Around CTJ, presence of high heat flow on the continental slope (Cande et al., 1987, J. Geophys. Res.) and near-trench young granitic rocks on the Taitao peninsula (Anma and Orihashi, 2013, Geochem. J.) suggests the thermal and petrological impact of subducting ridge on the continental side. The tectonic history of the southeast Pacific since early Cenozoic to the present suggests that ridge subduction continuously occurred along the Chile trench, which migrated northward (Cande and Leslie, 1986, J. Geophys. Res.), which can contribute to growth of the continental crust (Iwamori, 2000, EPSL). In January 2019, the MR18-06 cruise Leg 2 was conducted at the Chile Triple Junction, as a part of 'EPIC' expedition by using R.V Mirai of JAMSTEC. During the leg, we completed 4 SCS lines, 6 piston coring, 6 heat flow measurements, 2 dredges, and underw...

Earth, Planets and Space, 2012

Logging data are measurements of physical properties of the formation surrounding a borehole, acq... more Logging data are measurements of physical properties of the formation surrounding a borehole, acquired in situ after completion of coring (wireline logging) or during drilling (Logging-While-Drilling, LWD). The range of data (resistivity, gamma radiation, velocity, density, borehole images,…) in any hole depends on the scientific objectives and operational constraints.

Logging data are measurements of physical properties of the formation surrounding a borehole, acq... more Logging data are measurements of physical properties of the formation surrounding a borehole, acquired in situ after completion of coring (wireline logging) or during drilling (Logging-While-Drilling, LWD). The range of data (resistivity, gamma radiation, velocity, density, borehole images,…) in any hole depends on the scientific objectives and operational constraints.

Japan Geoscience Union, 2016

IODP Expedition 360, Phase I of the Nature of the Lower Crust and Moho at Slower Spreading Ridges... more IODP Expedition 360, Phase I of the Nature of the Lower Crust and Moho at Slower Spreading Ridges (SloMo) project of a Multi-Leg Drilling Project, was carried out from 1 Dec., 2015 to 31 Jan, 2016, at Atlantis Bank that is a 5 km local high along the eastern wall of the Atlantis II Transform of the Southwest Indian Ridge. Atlantis Bank has been interpreted as an oceanic core complex, where intact lower crust and the uppermost mantle are tectonically exposed by a long-lived detachment fault. We conducted all drilling operations at a single site in a single Hole U1473A and drilled 789.7 m though gabbros. This is the deepest single-leg hard-rock drilling hole in ocean crust. Expedition 360 Hole U1473 is located at 2.2 km Northeast of 1.5 km deep Hole 735B and at 1.4 km north of 158 m deep Hole 1105A. This provides us, for the first time, a unique opportunity to explore three dimensional lower crustal characteristics beneath the slow-spreading ridges. Phase II of the SloMo has proposed ...


Scientific Drilling, 2021

For more than half a century, exploring a complete sequence of the oceanic crust from the seafloo... more For more than half a century, exploring a complete sequence of the oceanic crust from the seafloor through the Mohorovičić discontinuity (Moho) and into the uppermost mantle has been one of the most challenging missions of scientific ocean drilling. Such a scientific and technological achievement would provide humankind with profound insights into the largest realm of our planet and expand our fundamental understanding of Earth's deep interior and its geodynamic behavior. The formation of new oceanic crust at mid-ocean ridges and its subsequent aging over millions of years, leading to subduction, arc volcanism, and recycling of some Published by Copernicus Publications on behalf of the IODP and the ICDP. 70 S. Umino et al.: Workshop report: Exploring deep oceanic crust off Hawai'i components into the mantle, comprise the dominant geological cycle of matter and energy on Earth. Although previous scientific ocean drilling has cored some drill holes into old (> 110 Ma) and young (< 20 Ma) ocean crust, our sampling remains relatively shallow (< 2 km into intact crust) and unrepresentative of average oceanic crust. To date, no hole penetrates more than 100 m into intact average-aged oceanic crust that records the longterm history of seawater-basalt exchange (60 to 90 Myr). In addition, the nature, extent, and evolution of the deep subseafloor biosphere within oceanic crust remains poorly unknown. To address these fundamentally significant scientific issues, an international workshop "Exploring Deep Oceanic Crust off Hawai'i" brought together 106 scientists and engineers from 16 countries that represented the entire spectrum of disciplines, including petrologists, geophysicists, geochemists, microbiologists, geodynamic modelers, and drilling/logging engineers. The aim of the workshop was to develop a full International Ocean Discovery Program (IODP) proposal to drill a 2.5 km deep hole into oceanic crust on the North Arch off Hawai'i with the drilling research vessel Chikyu. This drill hole would provide samples down to cumulate gabbros of mature (∼ 80 Ma) oceanic crust formed at a half spreading rate of ∼ 3.5 cm a −1. A Moho reflection has been observed at ∼ 5.5 km below the seafloor at this site, and the workshop concluded that the proposed 2.5 km deep scientific drilling on the North Arch off Hawai'i would provide an essential "pilot hole" to inform the design of future mantle drilling.

Marine and Petroleum Geology, 2018

Gas hydrate-bearing layers are normally identified by a seismic imaged bottom simulating reflecto... more Gas hydrate-bearing layers are normally identified by a seismic imaged bottom simulating reflectors (BSR) or by downhole log responses because of their high acoustic velocity and electric resistivity compared to surrounding formations. These gas hydrate characteristics can also result in contrasting in-situ formation compressive strengths. Here, we describe gas hydrate-bearing layers based on equivalent strength (EST), which relates to insitu compressive strength, in five exploration boreholes drilled during the Indian National Gas Hydrate Program Expedition 02 (NGHP-02). For Site NGHP-02-23, a representative site for those that were established during NGHP-02, the EST evaluated from drilling parameters shows a constant trend of ∼2 MPa, with some strong peak values in the 0-271.4 m-below-seafloor (mbsf) interval, and a sudden increase up to 4 MPa above the BSR depth (271.4-290.0 mbsf). Below the BSR, the EST stays at ∼2 MPa to the bottom of the hole (378 mbsf). Comparing the EST with logging data and a core sample description suggests that the EST depth profiles reflect the formation lithology and gas hydrate content. The EST increases in sand-rich and gas hydrate-bearing zone. In the lower gas hydrate layers in particular, the EST curve shows the same approximate trend with that of P-wave velocity and resistivity measured during downhole logging. Similar relationships between EST, hydrate layer, and log responses are confirmed in other four sites drilled nearby in NGHP-02 Area B. These results suggest that the EST, as a proxy for in-situ formation strength, can indicate the location and extent of the gas hydrate as well as borehole logging. Although the EST was calculated after drilling, utilizing the recorded surface drilling parameters (weight on bit, top drive torque, RPM and rate of penetration) in this study, the EST can be acquired during drilling using real-time drilling parameters. In addition, the EST only requires drilling performance data without any additional tools or measurements, making it a simple and economical tool for the exploration of gas hydrates.

Geochemistry, Geophysics, Geosystems, 2016

North Pond is an isolated sedimentary pond on the western flank of the Kane area along the Mid-At... more North Pond is an isolated sedimentary pond on the western flank of the Kane area along the Mid-Atlantic Ridge. Drill-hole U1382A of IODP Expedition 336 recovered peridotite and gabbro samples from a sedimentary breccia layer in the pond, from which we collected six fresh peridotite samples. The peridotite samples came from the southern slope of the North Pond where an oceanic core complex is currently exposed. The samples were classified as spinel harzburgite, plagioclase-bearing harzburgite, and a vein-bearing peridotite that contains tiny gabbroic veins. No obvious macroscopic shear deformation related to the formation of a detachment fault was observed. The spinel harzburgite with a protogranular texture was classified as refractory peridotite. The degree of partial melting of the spinel harzburgite is estimated to be 17%, and melt depletion would have occurred at high temperatures in the uppermost mantle beneath the spreading axis. The progressive melt-rock interactions between the depleted spinel harzburgite and the percolating melts of Normal-Mid Ocean Ridge Basalt (N-MORB) produced the plagioclasebearing harzburgite and the vein-bearing peridotite at relatively low temperatures. This implies that the subsequent refertilization occurred in an extinct spreading segment of the North Pond after spreading at the axis. Olivine fabrics in the spinel and plagioclase-bearing harzburgites are of types AG, A, and D, suggesting the remnants of a mantle flow regime beneath the spreading axis. The initial olivine fabrics appear to have been preserved despite the later melt-rock interactions. The peridotite samples noted above preserve evidence of mantle flow and melt-rock interactions beneath a spreading ridge that formed at 8 Ma.

is structural geologist with interests in the formation and deformation of ocean lithosphere at m... more is structural geologist with interests in the formation and deformation of ocean lithosphere at mid-ocean ridges, mechanisms of accretion of lower ocean crust at fast-spreading ridges; lithospheric deformation at ridge axial discontinuities; rifting/detachment fault processes at slow-spreading ridges; development and application of core reorientation techniques in structural studies of borehole core; and application of robotic seabed rock drilling technologies to mid-ocean ridge studies.

Japan Geoscience Union, 2015

Metag-gabbro and some serpentinites are not only extremely altered, but also metamorphosed at hig... more Metag-gabbro and some serpentinites are not only extremely altered, but also metamorphosed at high temperature up to midamphibolite facies. Chromian spinel included in those samples, which is normally well preserved under the condition of ocean floor metamorphism, also metamorphosed into chrome-bearing magnetite and/or magnetite. The crystal orientation varies inside a grain with chemical modification of the metamorphism. We discuss the possibility of such high temperature metamorphism in the deep oceanic plate.

Annual Meeting of the Geological Society of Japan, 2016

Dunite and gabbroic materials recovered from Hole 1271B, Ocean Drilling Program (ODP) Leg 209, we... more Dunite and gabbroic materials recovered from Hole 1271B, Ocean Drilling Program (ODP) Leg 209, were examined for mineral chemistry to understand melt flow and melt-mantle reactions in the shallowest upper mantle of the Mid-Atlantic Ridge near the 15°20' Fracture Zone. Hole 1271B was drilled to 103.8 meters below seafloor on the inner corner high along the south wall of the 15°20' Fracture Zone. The total length of core collected was 15.9 m (recovery = ~15%). The dominant rock type in Hole 1271B is dunite, followed by brown amphibole gabbro, olivine gabbro, and troctolite, along with minor amounts of harzburgite and olivine gabbronorite. A large proportion of the dunite is associated with gabbroic rocks in Hole 1271B, similar to those observed in the Mohorovicic (Moho) transition zone of the Oman ophiolite, indicating significant magmatic activity in this region near the 15°20' Fracture Zone. Olivine Fo content varies from 89.2 to 91.2 in impregnated dunite and from 85.6 to 88.6 in troctolite, olivine gabbro, and olivine gabbronorite. Spinel Cr# (= 100 x Cr/[Cr + Al] molar ratio) ranges from 38.9 to 62.7 in dunite and from 46.3 to 57.6 in troctolites, olivine gabbro, and olivine gabbronorite. Compositional trends for spinel from dunite through troctolite toward olivine gabbro/gabbronorite are characterized by increases in TiO2, Cr#, and Fe3+#, very similar to those reported from Hess Deep Site 895. Olivine gabbro, olivine gabbronorite, and troctolite in Hole 1271B are considered to have formed as hybrid rocks between dunite and an evolved melt in the walls of a melt channel in the shallowest upper mantle that is tens of meters wide. The melt trapped in the wall rock crystallized plagioclase and clinopyroxene. On the other hand, dunite in the center of the melt channel became more refractory by melt-mantle reactions, increasing spinel Cr# to 62.5.

Marine and Petroleum Geology, 2018

Physical properties like porosity and density are important parameters for size estimations of fo... more Physical properties like porosity and density are important parameters for size estimations of fossil fuel reservoir in host sediments and rocks. X-ray CT based imaging and subsequent analysis of the images by using various techniques for estimations of these properties of host rocks and sediments has been attempted. However, most of those techniques tend to be biased and subjective due to complications in image processing. Here we describe a new procedure for estimating porosity and density without processing the XCT images, but using the raw data that are used for producing the images. In this study, large-size cylindrical core samples (up to 150 cm long and 6.6 cm diameter) were imaged in a matter of minutes by using a medical XCT scanner soon after their recovery from seafloor onboard drillship in order to record as realistic as possible condition of hydrate-bearing sediments. Despite some uncertainty in relative volumes of solid, liquid and gas phases at voxel level, the results obtained by this rapid and non-destructive procedure are in good agreement with those obtained by time-and sampleconsuming conventional methods of physical property measurements.

Marine and Petroleum Geology, 2018

The existence of overpressure in shallow sediments, which is often constrained by hydraulic prope... more The existence of overpressure in shallow sediments, which is often constrained by hydraulic properties, influences the gas-hydrate formation process and gas production. Porosity, permeability, and grain size measurements in laboratory experiments were conducted on core samples from gas-hydrate-bearing regions offshore from the Krishna-Godavari Basin, eastern India. Porosity was found to decrease with increasing effective stress; this is explained by the exponential decay curve along which porosity decreases from 65 % at 0 MPa to 40 % at 10 MPa. Permeability and the corresponding hydraulic diffusivity decrease from 10 −17 to less than 10 −18 m 2 and from 10 −7 to 10 −8 m 2 /s at 0.5 and 5 MPa, respectively. Grain sizes were larger and the sand fraction more scattered in channel-filled sediment sites compared to slope sediment sites. The preconsolidation stresses evaluated from consolidation curves indicate the absence of overpressure at shallow depths. In contrast, a comparison of ship-board measurements and standard compaction curves suggested that measured porosity was higher than the predicted porosity at greater depths. These porosity anomalies are interpreted as a sign of overpressure that approaches near lithostatic values at greater depths in slope sediment sites. A one-dimensional sedimentation model recreated overpressure profiles similar to those predicted by porosity gaps, under the assumption of large basial fluid influx or lower permeability than that derived from laboratory data. The modeling results suggest that near-hydrostatic pressure at shallow depths and significant overpressure at greater depths proposed by the porosity gap method is explained by the non-linearity of transport properties. The relatively small overpressure generation in channel-filled sites compared with slope sites can be explained by the higher permeability due to coarser grain size and larger sand fraction and by the smaller basal influx. On the contrary, considerably large basal influx associated with clay mineral dehydration and methane gas supply from deep sediments was expected to promote overpressure at slope sites, which is confirmed by Cl − concentrationdepth profiles.