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Japan Geoscience Union Meeting 2016

Advent of Continent: Evidence from the Nishinoshima Volcano in the Ogasawara Arc*Yoshihiko Tamura1, Osamu Ishizuka2,1, Tomoki Sato1, Alexander Nichols1
1.R & D Center for Ocean Drilling Science, Japan Agency for Maine-Earth Science and Technology,
2.Geological Survey of Japan/AIST
Nishinoshima, one of the submarine volcanoes in the Ogasawara Arc, ~1,000 km south of Tokyo, Japan,
suddenly erupted in November 2013, after 40 years of dormancy. The Nishinoshima volcano might
represent the missing link between the mantle and the continental crust because (1) Nishinoshima,
whose underlying crust is only 21 km thick, is one of the world’s closest volcanoes to the mantle,
and (2) the lavas have been andesites and were similar in composition to the continental crust.
Here we report the scientific results of our endeavours to collect lavas from the currently
erupting lava flows on the surface to the submarine lavas of the Nishinoshima volcano. Using
olivine-bearing phenocryst-poor andesite samples, we developed a mantle-derived andesite model forthe genesis of the Nishinoshima volcano. Shallow and hydrous mantle melting is necessary to produce
primary andesite magmas, and thus it is only achieved beneath Nishinoshima and submarine volcanoes
in the Ogasawara arc, where the crust is thin. We also show that the primary magma compositionchange from basalt produced at a considerable depth beneath the old thick lithosphere to andesite
produced beneath the present curst corresponds to the thermal evolution of the mantle wedge in the
Ogasawara arc.
Keywords: andesite, continental crust, oceanic arc©2016. Japan Geoscience Union. All Right Reserved.

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Japan Geoscience Union Meeting 2016

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Japan Geoscience Union Meeting 2016

Izu-Bonin-Mariana Arc basement from IODP Exp. 351(Amami Sankaku Basin)*Osamu Ishizuka1, Rosemary Hickey-Vargas2, Gene Yogodzinki3, Masafumi Sudo4, Anders McCarthy5, Ivan
Savov6, Yuki Kusano1, Richard Arculus7, Kara Bogus8
1.Geological Survey of Japan, AIST, 2.Florida International University, 3.University of South
Carolina, 4.Universitaet Potsdam, 5.University of Lausanne, 6.University of Leeds, 7.Australian
National University, 8.Texas A&M University
IODP (International Ocean Discovery Program) Exp.351 targeted, in particular, evidence for theearliest evolution of the Izu-Bonin-Mariana (IBM) arc system following inception. This can beobtained by identification and exploration of regions adjacent to an arc, where unequivocally
pre-arc crust (basement) overlain by undisturbed arc-derived materials exists. The drill site
(U1438) is located in Amami Sankaku Basin (ASB), west of the Kyushu-Palau Ridge (KPR), i.e.,
paleo-IBM arc. Exp.351 successfully recovered pre-Izu-Bonin-Mariana arc basement, a volcanic andgeologic record spanning pre-Arc, arc initiation to remnant arc stages. In this contribution, age
and geochemical characteristics of arc basement and its tectonic significance will be discussed.
At U1438, 1611m thick section in total was cored, comprising 1461m of sediment section and 150m of
igneous basement. Numerous flow contacts were identified in the basement, several chilled margins
could be identified but low recovery in basement rock makes their interpretation in terms of
volcanic stratigraphy difficult. Paleomagnetic measurements show consistent shallow plunges ofmaximum anisotropy axes, and that implies these basalts were emplaced as sheet flows rather thanpillow lavas.
Basalts have diverse textures that include aphyric to subophitic/ophitic. Phenocrysts are present
in approximately half of the basalts and consist of plagioclase, clinopyroxene, titanomagnetite,spinel and minor olivine.
Age interpretations based on biostratigraphy (Arculus et al., Nat Geosci, 2015) determined that thebasement section is between 64 and 51 Ma, and direct age determination of basement basalt by Ar/Ar
dating is under way.
Whole rock geochemical analysis of the basement basalt shows that these basalts are low K
tholeiitic basalts. They mostly have high-MgO (generally >8 wt%), low-TiO2 (0.6–1.1 wt%), Ti/V,
low-Zr (mostly <50 ppm). On Ti/V plot, the U1438 basalts and forearc basalt (FAB) which is supposed
to be associated with subduction initiation from IBM arc are similar to each other and clearly
distinct from Philippine Sea backarc basin basalts and normal MORB by having low Ti/V.
One prominent characteristic of the basalts is their depletion of immobile highly incompatible
elements compared with MORB, e.g., they are strikingly light REE (LREE) depleted. But La/Nd ratio
and Th/LREE increases upcore in the uppermost part of the basement, and these trace element
variations with depth is accompanied by variation of major element composition such as Si and Ti.
Hf-Nd isotopes for the basement basalts show a significant range of compositions, and relatively
radiogenic Hf compared to Nd indicates an Indian Ocean-type MORB source. However, the dominantsignature, with eHf >16.5, is more radiogenic than most Indian MORB.
Preliminary data suggests that the basement basalts are relatively primitive melts and likely
derived from Indian Ocean-type MORB sources that are more strongly depleted in terms ofincompatible trace elements than typical MORB or backarc basin basalt in the Philippine Sea.
Geochemical variation in the uppermost part of the core might imply variability of slab-derived
enrichment and/or fertility of mantle at the onset of subduction.
Geochemical characteristics of arc basement from Exp.351 imply that depleted MORB-like basalt(“FAB”) appears to have formed in wider area than previously thought, i.e., including “reararc”side©2016. Japan Geoscience Union. All Right Reserved.

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Japan Geoscience Union Meeting 2016of ancient IBM arc. However, critical assessment of genetic relation between FAB and basement
basalt of ASB requires precise age determination to constrain tectonic setting where the ASB
basalts formed.
Keywords: arc basement, Izu-Bonin-Mariana arc, geochemical characteristics, Ar/Ar age©2016. Japan Geoscience Union. All Right Reserved.

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Japan Geoscience Union Meeting 2016

Physical properties of fore-arc basalt and boninite recovered by IODP EXP352 and its
significance for the seismic velocity structure in the oceanic crust*Katsuyoshi Michibayashi1, Mutsumi Honda2, Yuzuru Yamamoto3, Nana Kamiya4, Tetsuya Sakuyama5, Tohru
Watanabe6
1.Institute of Geosciences, Shizuoka University, 2.Graduate School of Integrated Science andTechnology, Shizuoka University, 3.Department of Mathematical Science and Advanced Technology,
JAMSTEC, 4.Graduate School of Integrated Sciences, Nihon University, 5.Faculty of Science, Osaka
City University, 6.Gradudate School of Science and Engineering, University of Toyama
Physical properties of the cores recovered by IODP EXP352 were characterized through a set ofmeasurements on whole core sections and discrete samples. Gamma ray attenuation density (GRA),
magnetic susceptibility (MS) and P-wave velocity (PWL) of the cores were obtained using theWhole-Round Multisensor Logger (WRMSL). Natural gamma radiation (NGR) was measured by the Natural
Gamma Radiation Logger (NGRL). Point magnetic susceptibility (PMS), reflectance spectroscopy andcolorimetry (RCS) data were acquired using the SHMSL. Thermal conductivity (TCON) and moisture anddensity (MAD) were obtained on sections and discrete samples, respectively. All raw data were
subsequently “filtered” to remove spurious points that correspond to empty intervals in the liner
or broken pieces. We have mainly studied the relationships among P-wave velocity, density, prosityand magnetic suscepetibility. We show that the physical properties are useful dataset for theinterpretation of the layer 2 (i.e. volcanic rocks) in the seismic velocity structure of theoceanic crust.
Keywords: IODP EXP352, Physical property©2016. Japan Geoscience Union. All Right Reserved.

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Japan Geoscience Union Meeting 2016

Evolution process of volcano-bounded basin revealed by mapping of seismic reflectors
associated with geological boundary from drilling results in Izu rear arc*Mikiya Yamashita1, Yoshihiko Tamura2, Narumi Takahashi1, Seiichi Miura1, Shuichi Kodaira1
1.Japan Agency for Marine-Earth Science and Technology, 2.ODS, Japan Agency for Marine-Earth
Science and Technology
International Ocean Discovery Program (IODP) has successfully conducted the first rear-arc drillingin the Izu-Ogasawara (Bonin) intra-oceanic arc at Site U1437 in 2014. The drilling purpose of Site
U1437 is to reveal the formation of oceanic arc crust and its evolution into continental crust
about the history of “the missing half” of the subduction factory. Site U1437 is located at the
volcano-bounded basin between the Manji and Enpo backarc seamount chains in the Izu rear arc and~90 km west of the arc-front volcanoes Myojinsho and Myojin Knoll, at 2117 m below sea level. Site
U1437 had excellent core recovery in Holes U1437B and U1437D, and we succeeded in hanging thelongest casing ever in the history of R/V JOIDES Resolution scientific drilling (1085.6 m) in Hole
U1437E and cored to 1806.5 mbsf.
In order to evaluate the crustal structure of this proposed site before the IODP drilling, Japan
Agency for Marine-Earth Science and Technology carried out many seismic reflection and refraction
surveys using R/V Kaiyo and Kairei in the Izu rear arc during 2006 to 2008. Five clear seismicreflection profiles consisting of three kinds of survey environments and one seismic velocity image
by seismic refraction survey are obtained across the drilling site U1437. Five sedimentary units
consisting of volcaniclastics are identified from our interpretations around the drilling site in
the seismic reflection profiles over the 5 km/s and 6 km/s iso-contours of P-wave velocity obtainedby the velocity image of seismic refraction survey in order to evaluate structure of the drilling
location before drilling. However, some unit boundary is not recognized from the drilling core. Itmeans the difficulties for identification the geological target from only seismic images involcanic regions. According to the drilling results, the acquired geological core is consisted ofseven lithological units (I, II, III, IV, V, VI and VII). Units I to V was produced at age of 0-9
Ma. The Unit VI and VII, 1320-1806.5 mbsl, have the ages ranging from 9 to ~14 Ma. P-wave velocity
calculated from obtained core samples increases downhole from ~1500 to ~4500 m/s which agree withthe range of our velocity analysis. Tops of unit II, V and VII correspond to the strong reflectorof seismic profiles. We interpreted and mapped using grid survey data around the drilling site. The
top of unit II which corresponds to the volcaniclastics has south dipping trend. The top of unit V
which corresponds to the mudstone with volcaniclastics layer has south dipping trend. The top ofunit VII which corresponds to volcaniclastics estimated by near-vent deposit has southeast dipping
trend. Each reflector is interrupted by igneous basement near seamount chains. We also calculate
the thickness from unit VII to V and unit II to seafloor from mapped reflectors. These features
suggest the different activity of various directions in the Izu rear arc. We will discuss about theevolution process in Izu rear arc deduced from our mapping results.
Keywords: site survey, volcano-bounded basin, rear arc©2016. Japan Geoscience Union. All Right Reserved.

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Japan Geoscience Union Meeting 2016

Preliminary reports on the Nature of the Lower Crust and Moho at Slower Spreading Ridges(SloMo)Henry J.B. Dick2, Christopher J. MacLeod3, Peter Blum4, *Tomoaki Morishita1, Natsue Abe5, Toshio
Nozaka8, Alessio Sanfilppo1,6, Scientific Party Expedition 3607
1.Kanazawa University, 2.Woods Hole Oceanographic Institute, 3.Cardiff University, 4.Texas A & M
University, 5.JAMSTEC, 6.University of Pavia, 7.Expedition 360 Scientific Party, 8.Okayama
University
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 IIof the SloMo has proposed to drill 6 km through MOHO by the CHIKYU. In the meeting, we will
introduce preliminary results of IODP Expedition 360 and the future perspective leading to Phase IIof the SloMo, a mantle drilling into ultraslow-spreading ridges.
Expedition 360 Scientific Party: Donna K. Blackman, Julie A. Bowles, Michael J. Cheadle, Kyungo
Cho, Jakub Ciazela, Jeremy R. Deans, Virginia P. Edgcomb, Carlotta Ferrando, Lyderic France,
Biswajit Ghosh, Benoit M. Ildefonse, Mark, A. Kendrick, Juergen H. Koepke, James A.M. Leong,
Chuanzhou Liu, Qiang Ma, Antony Morris, James H. Natland, Oliver Pluemper, Jason B. Sylvan, Maurice
A. Tivey, Riccardo Tribuzio, Gustavo Viegas
Keywords: Ultraslow-spreading ridge, Oceanic Core complex, Oceanic Lower Crust©2016. Japan Geoscience Union. All Right Reserved.

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Japan Geoscience Union Meeting 2016

Lord Howe Rise Drilling: Deep stratigraphic record for the Cretaceous eastern Gondwana
margin*Saneatsu Saito1, Ron Hackney2, Yasuhiro Yamada1, Kliti Grice3, Junichiro Kuroda1, Jessica Whiteside4, Marco Coolen3, Fumio Inagaki1, Richard Arculus5, Dietmar Müller6
1.Japan Agency for Marine-Earth Science and Technology, 2.Geoscience Australia, 3.Curtin
University, 4.University of Southampton, 5.Australian National University, 6.University of Sydney
The Lord Howe Rise (LHR) is an elongate ribbon of submerged and extended continental crust that
separated from Australia during the Late Cretaceous. Current knowledge of the LHR is based only on
widely-distributed marine and satellite geophysical data, limited dredge samples, and sparseshallow drilling into Cenozoic pelagic sediments. Building more detailed knowledge of LHR geology
and the evolution of the eastern Gondwana margin requires drilling into rocks that record thetectonic and climatic history of the region.
Geoscience Australia and JAMSTEC are leading an international effort to promote an IODP project(871-CPP) to drill a deep stratigraphic hole through a LHR rift basin using D/V CHIKYU that will
recover Cretaceous and older sediments and basement rocks. The objectives of this proposal are to:1) define the role and importance of continental crustal ribbons, like the LHR, in plate tectonic
cycles and continental evolution; 2) recover new high-latitude data in the southwest Pacific tobetter constrain Cretaceous paleoclimate and linked changes in ocean biogeochemistry; and 3) test
fundamental evolutionary concepts for sub-seafloor microbial life over a 100-million-year
timeframe.
Keywords: IODP, Lord Howe Rise, Chikyu, Cretaceous, Gondwana, Zealandia©2016. Japan Geoscience Union. All Right Reserved.

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Japan Geoscience Union Meeting 2016

Seismological structure of oceanic lithosphere inferred from Po/So waves*Azusa Shito1, Daisuke Suetsugu2, Takashi Furumura3
1.Institute of Seismology and Volcanology, Kyushu University, 2.Department of Deep Earth Structure
and Dynamics Research, Japan Agency for Marine-Earth Science and Technology, 3.Earthquake ResearchInstitute, University of Tokyo
Oceanic lithosphere, comprising oceanic crust and upper mantle, plays an important role in plate
tectonics. Knowledge of the detailed structure of oceanic lithosphere is key to understanding its
origin.
The propagation of Po/So waves over large distances across oceanic lithosphere provides information
about lithospheric structure. Po/So waves, characterized by a high frequency, large amplitude, andlong duration, were identified as early as 1935 [Linehan, 1940]. Many previous studies haveattempted to quantitatively explain the generation and propagation mode of Po/So phases. Although
it is generally accepted that Po/So phases are guided waves traveling very efficiently throughout
the oceanic lithosphere, the generation and propagation processes of Po/So phases remain unclear.
In the past two decades, there have been great technological advances in computer simulations ofhigh-frequency seismic waves in heterogeneous structures and in broadband seismic observations on
the seafloor. These gains motivated us to further investigate the processes relating to guided
waves in oceanic lithosphere and the generation of Po/So waves, using broadband seismic data andthe numerical Finite Difference Method (FDM) to simulate high-frequency seismic waves. In thispresentation, we outline recent progress in the study of Po/So waves.
Shito et al. [2013] reported that Po/So waves are generated by multiple forward scattering of Pand S-waves due to small-scale heterogeneities in oceanic lithosphere. The laterally elongatedheterogeneities are described by a von Karman distribution function with a correlation length of 10
km in the horizontal and 0.5 km in thickness, with a velocity perturbation of 2%.
Kennett and Furumura [2013; 2014] and Shito et al. [2015] found that the propagation efficiency of
Po/So waves depends on the age of oceanic lithosphere, and that this relationship can bequalitatively explained by thickening of oceanic lithosphere that contains small-scale
heterogeneities and a reduction in intrinsic attenuation. Recently, Kennett and Furumura [2015]
proposed a new model that the amplitude of such heterogeneities increases with depth to the bottomof the lithosphere. These results suggest that small-scale heterogeneities may form continuously inoceanic lithosphere, from the time of its formation at a spreading ridge, via the solidification ofmelts in the asthenosphere.
The petrological and mineralogical processes that cause the small-scale heterogeneities remain
poorly known. Future studies that combine seismological observations with petrological analyses
will yield a greater understanding of the origin of oceanic lithosphere.
Keywords: Po/So waves, oceanic lithosphere, small-scale heterogeneities©2016. Japan Geoscience Union. All Right Reserved.

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Japan Geoscience Union Meeting 2016

The effect of melt density, assimilation, and eruption location on volatile and halogen
contents in MORB glasses: an example from IODP Site 1256
*Takashi Sano1, Shigeru Yamashita2
1.Department of Geology and Paleontology, National Museum of Nature and Science, 2.Institute forStudy of the Earth's Interior, Okayama University
A continuous section of ~700 m-thick lava pile at Integrated Ocean Drilling Program Site 1256
provides important depth variation of volatile (H2O, CO2, S) and halogen (Cl) contents in freshglasses to understand style of eruption, condition of magma storage, and degree of hydrothermal
assimilation. The lava pile is divided into two groups based on eruption location: off-axis basalts
of upper portion (250–534 m beneath seafloor, mbsf) and on-axis basalts of lower portion (534–941
mbsf). Majority of the lava pile is composed of pillow, sheet, and massive lavas, but both groups
have short (1–2 m) intervals of hyaloclastite layers (e.g., Wilson et al., 2006, Science).
H2O contents of hyaloclastite samples are distinctly higher than those of lavas, suggesting thatthe high H2O would induce explosive eruption to produce the hyaloclastite materials. CO2/Nb andS/Dy in most fresh glasses are smaller than degassed ratios (e.g., Saal et al., 2002, Nature), andthese facts indicate shallow degassing and CO2 and S losses during transport to the seafloor.
Saturation pressures calculated by dissolved H2O and CO2 contents are wide range from pressure ofseafloor (~25 MPa) to pressure of magma chamber (~60 MPa). The saturation pressure positively
correlates with melt density, but obvious differences between off-axis and on-axis samples are not
identified. These observations may imply that the melt density is important factor to estimate
degassed pressure. When the melt density is low, the melt can ascent from melt lens (top of magma
chamber) to shallow place within oceanic crust and highly degassed before reach to seafloor.
Conversely, dense melt would not ascent to the shallow place and less degassed at deep level near
the melt lens.
The most distinctive character for the Site 1256 glasses is higher Cl/Nb and Cl/K than any other
MORB glasses. The strong Cl enrichment is explained by assimilation of highly hydrothermally
influenced crust (e.g., Sano et al., 2008, 2011, G-cubed). Beneath the Site 1256, melt lens wasvery shallow (<1.5 km) and hydrothermal circulation of high-salinity brines would easily reach toroof crust of the melt lens.
Keywords: IODP, volatile, melt density©2016. Japan Geoscience Union. All Right Reserved.

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Japan Geoscience Union Meeting 2016

A seamount on top of Ontong Java Plateau was created by remelting of plateau lithosphereby plate flexure*Takeshi Hanyu1, Maria Luisa Tejada1, Kenji Shimizu1, Osamu Ishizuka2, Jun-Ichi Kimura1, Qing CHANG1, Ryoko Senda1, Takashi Miyazaki1, Kosuke T. Goto2, Akira Ishikawa3
1.Japan Agency for Marine-Earth Science and Technology, 2.Geological Survey of Japan, AIST, 3.The
University of Tokyo
The Ontong Java Plateau (OJP) was formed by a main volcanism occurred at ca. 120 Ma, followed byseveral pulses of the late-stage volcanism on and along the margins of the plateau. The origin of
the late-stage volcanism is poorly understood because of limited rock sampling. We present the age
and geochemical composition of basalts dredged from Nuugurigia Atoll that stands on the plateau
basement, possibly erupted in the late-stage volcanism. The 40Ar/39Ar age of dredged basalts is ca.
20 Ma, younger than any other known late-stage volcanism on OJP (down to 34 Ma), and is coincidedwith the collision of the plateau with the proto-Solomon arc. These basalts have enriched isotopic
signatures pointing towards EM1 distinct from any other rocks thus far collected from OJP.
Moreover, they show unusual trace element composition with Sr enrichment and Zr-Hf depletion
relative to the elements with similar incompatibility. Such isotopic and trace elemental featureare shared with quartz-bearing garnet pyroxenite xenoliths rarely found from Solomon Islands(Ishikawa et al., 2007). Ishikawa et al. inferred that such pyroxenite was derived from delaminatedgranulitic lower crust and was part of the OJP lithosphere underplated beneath the plateau
lithosphere via the mantle upwelling responsible for the main plateau-forming volcanism. We suggestthat the basalts in the late-stage volcanism at Nuugurigia were products of rejuvenated melting ofsuch pyroxenite. Melt transport through the lithosphere may have been facilitated by plate flexure
occurred just south of the atoll when the plateau collided with the proto-Solomon arc around 20 Ma.
Keywords: oceanic plateau, lithosphere, recylced material©2016. Japan Geoscience Union. All Right Reserved.

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Japan Geoscience Union Meeting 2016

Understanding continental crust emplacement: A continental scientific drilling perspective*Thomas Wiersberg1, Ulrich Harms1
1.Helmholtz Centre Potsdam GFZ German Research Centre for Geosciences
Scientific drilling is a critical tool for Earth science to understand crustal evolution andprocesses. In the recent past, the International Continental Scientific Drilling Program, ICDP,
supported several drilling operations in hard rock addressing the formation and emplacement ofcontinental crust from Archean volcanism (Peering into the Cradle of Life, Barberton, South
Africa), Paleoproterozoic volcanism (FAR-DEEP, Baltic Shield, Russia), rift volcanism (Krafla,
Iceland), hot spot volcanism (Hawaii, USA, Snake River Plain, Idaho, USA) and Himalyan-style thrust
sheet emplacement (COSC, Sweden). The results of these studies have significantly improved ourunderstanding in past and present formation of continental crust.
ICDP supports scientific drilling operations that facilitate outstanding science at globally
important sites. ICDP brings together scientists and stakeholders from 24 nations to work together
at the highest scientific and technical level. Since its founding in 1996, more than 40 drilling
projects and 75 planning workshops have been supported by ICDP worldwide. The outcomes ofscientific drilling operations supported by ICDP cover the full range of the Earth sciences fromclimate change, natural hazards and earth resources to the origins and evolution of life on Earth,effectively addressing the needs of our growing population for energy, sustenance, and quality oflife.
Forthcoming ICDP drilling into the Samail ophiolite complex in Oman will provide key data on melt
extraction processes from the mantle, igneous accretion of oceanic crust, and hydrothermal
modification of that crust. Drilling will also investigate present day alteration processes ofmantle peridotites and their relationship to the deep biosphere.
The Surtsey volcano drilling in fall 2016 aims to investigating processes of rift zone volcanism,hydrothermal alteration and biological colonization of basaltic tephra. It will refine ourunderstanding of seawater- interactions with magma and rock that influence diagenetic and microbial
alteration of tephra to produce lithified tuff and shed new light on how rift zone volcanic islands
form, lithify, and are ultimately destroyed.
ICDP is also funding drilling for Reservoir Triggered Seismicity near the Koyna dam located close
to the west coast of India. The basement rocks of the Koyna-Warna region consists of Precambrianbasement overlain by a more than 1 km thick Deccan Trap cover that erupted about 65 Ma ago. Twopilot boreholes will provide critical information on the in-situ stress regime, pore fluidpressure, fluid/gas properties and hydrological parameters of basement rocks, and the geothermal
regime. These 3 km deep wells will lay the ground for two deep main boreholes for penetrating into
the source of induced seismicity at more than 5 km depth.
Keywords: ICDP, Scientific Drilling, Continental Crust emplacement©2016. Japan Geoscience Union. All Right Reserved.

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Japan Geoscience Union Meeting 2016

From Birth to Death: Fate of The Extremely High-T Subduction Zone of The Oman Ophiolite*Susumu Umino1, Yuki Kusano2, Keitaro Kitamura3, Kyoko Kanayama4, Kazuyha Nagaishi5, Tsuyoshi
Ishikawa6, Ryo Otsuka1
1.Department of Earth Sciences, Kanazawa University, 2.Geological Survey of Japan, AIST, 3.Asia Air
Survey Co., Ltd., 4.Tottori Prefectural Museum, 5.Marine Works Japan Ltd., 6.Kochi Institute forCore Sample Research, JAMSTECThe Oman Ophiolite preserves the entire geological records of intra-oceanic subduction zone
formation and arc evolution, where 98-96 Ma MORB-like basalt magmatism (V1) was followed by 96-94
Ma arc tholeiitic and low-silica boninitic volcanism (V2) [1,2,3,4]. The remanents of the subductedslab are now preserved as the high-grade metamorphic soles beneath the ophiolite sheets [5]. Aftera quiescence period, alkali basalt flows (V3) were extruded at about 90 Ma, which has anintermediate geochemical characteristics between OIB and EMORB [6]. The V3 trace element
compositions can be reproduced by pooled partial melts in the stability field of garnet and spinellherzolite.Through the V2 magmatism, the source mantle shows progressive depletion by stepwise melt
extraction, as shown by the lower Nb/Ta ratios for the younger volcanic rocks (V2 boninite < V2
tholeiite < V1) [2] with identical εHf(t) values. V2 glasses have higher B, Pb, and LILEs with age,
indicating an increasing contribution of slab fluids from earlier arc tholeiite to later boninite.
Boninite magma was generated with the supply of high-T hydrous fluid and sedimentary melt liberated
from the metamorphic sole as demonstrated by the Sr-Nd isotopic compositions of the amphibolite andmetachert in the sole and clinopyroxene separates from boninites [1,2,7]. Therefore, themetamorphic sole beneath the ophiolite sheets are responsible for the generation of V2 arc magmas.
Melt inclusions in Cr spinel derived from boninite comprise homogeneous glass of mostly low-Si
boninitic [3,4] and slightly differentiated composition, with SiO2 ranging in 52-62 wt% and MgO up
to 16 wt%. The primary boninite magma assumed as the most magnesian melt inclusion can coexist withmantle olivine and orthopyroxene [9] at 0.4-0.6 GPa and 1350°C. This T-P conditions indicate a
segregation depth of ~17 km from the mantle with a potential T of 1400°C. Meanwhile, the peakmetamorphic conditions for the subducted slab that liberated high-T fluids to form boninite and arc
tholeiite magmas are 770–900°C and 1.1-1.3 GPa [1,6,7].
The keys of the Oman subduction zone are 1) the preservation of diapiric structures in the mantle[10], 2) short time interval of the V1 and V2 magmatism <2 m.y., 3) T-P conditions for a primary
low-Si boninite (Umino et al., 2014); and 4) extremely high T & low-P metamorphic conditions forthe sole that liberated the fluids generating the V2 magmas [1,5]. These lines of evidence are most
readily explained by intraoceanic thrusting initiated near the ridge axis that developed into ashallow and hot subduction zone [1,2,8,10].
Forced subduction of such an extremely high-T, buoyant slab suppressed convection in the mantle
wedge, resulted in the progressive depletion of the source mantle through the V2 arc magmatism.
Numerical modeling suggests that melting of the slab and mantle wedge occurrs only in the earlystage and ceases as the mantle wedge cools because of the absence of convection [10]. Consequently,
the Oman arc volcanism terminated in only a few million years. After several million years, partsof the subducted slab delaminated and induced upwelling and adiabatic melting of DMM asthenosphere,
resulted in the generation of alkalic magmas of V3 extruded onto thick pelagic sediments before
colliding onto the Arabian Peninsula.
[1] Ishikawa et al., doi:10.1016/j.epsl.2005.09.049 [2] Kusano, Y. et al.
http://dx.doi.org/10.1144/SP392.9 [3] Kanayama et al., doi: 10.1111/iar.12000 [4] Umino et al.,doi: 10.1130/G36191.1 [5] Cowan et al., http://dx.doi.org/10.1144/SP392.8 [6] Umino,©2016. Japan Geoscience Union. All Right Reserved.

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Japan Geoscience Union Meeting 2016

doi:10.1029/2012JB009198 [7] Kusano et al., 2015. Goldschmidt, Prague, p. 1725 [8] Putirka, doi:
10.2138/rmg.2008.69.3. 2008 [9] Nicolas, 1989. Kluwer Academic Publication [10] Gutcher et al.,doi:10.1130/0091-7613(2000)28<535:CSMBCB>2.0.CO;2
Keywords: Oman Ophiolite, boninite, high-T subduction zone, metamorphic sole, subduction
initiation, obduction©2016. Japan Geoscience Union. All Right Reserved.

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Japan Geoscience Union Meeting 2016

The redox state in the Fizh mantle section, the northern Oman ophiolite as an analog ofmantle wedge in subduction zone
Masafumi Tani1, *Eiichi TAKAZAWA1,2, Madhusoodhan Satish-Kumar1, Masahide Akasaka3, Yoshihiko Tamura2
1.Department of Geology, Faculty of Science, Niigata University, 2.R & D Center for Ocean Drilling
Science, Japan Agency for Maine-Earth Science and Technology, 3.Department of Geoscience, Shimane
University
The redox state of the upper mantle is important because it controls phase equilibrium andpartitioning of some trace elements. Previous studies of mantle peridotites show mantle xenoliths
in arc lavas are more oxidized than abyssal peridotites on the ocean floor because subducting slab
is dehydrated to supply ‘oxidized fluid’ such as water and carbon dioxide to the mantle wedge inhanging wall (e.g., Parkinson and Pearce, 1998).
The mantle section of the Oman ophiolite is a good analog for a mantle wedge because it
experienced supra-subduction zone setting during oceanic thrusting prior to the obduction. In thisstudy, we focus on the Fizh mantle section in the north Oman ophiolite as a proxy of mantle wedge
in a subduction zone and evaluate the redox state recoded in the peridotites. We calculated delta
logfO2 (FMQ) using equation of Ballhaus et al. 1991, chemical composition of olivine and spinelwith ferric and ferrous iron ratio assuming stoichiometory. The spatial distribution of delta log
fO2 (FMQ) in the Fizh mantle section indicates that the basal part of the mantle section is more
reduced whereas the upper part of the mantle section beneath the Moho is more oxidized. This may
indicate that the basal part of mantle wedge above the contact to subducting slab is most reduced.
We compared ferric and ferrous iron ratio calculated from spinel composition assuming perfectstoichiometory to those directly obtained by Mössbauer spectrometry. The reslts show that thedifference is minor and does not significantly affect the tendency mentioned above. As independent
approach, we used abundance ratio of vanadium against scandium in whole rock peridotites. Vanadium
partitioning between peridotite and melt is sensitive to the change in oxygen fugacity whereas
scandium doesn’t. Thus the ratio of vanadium against scandium is a good proxy for estimating oxygen
fugacity. As a result, delta log fO2 (FMQ) shows negative correlation with the ratios of vanadium
against scandium for whole rock peridotites. This result confirms that the basal part of the Fizh
mantle section is more reduced than the upper part of the mantle section.
One possible factor for the reduced signature in the basal part of Fizh mantle section is areaction of peridotite with a reduced melt produced by melting of sediment with reduced carbon
subducted beneath the Fizh mantle section. This hypothesis is supported by high Th/Ce ratio in
whole rock peridotites and clinopyroxenes with low delta log fO2 (FMQ). The high Th/Ce ratio is oneof the features which indicate contribution of oceanic sediments and suggests those influence. The
Raman peak of methane has also been detected in the fluid inclusions in the peridotite from thebasal part. On the other hand, a peak of methane has not been detected from the fluid inclusion in
the peridotites beneath the Moho Transition Zone. The presence of methane in the fluid inclusions
from the basal part of mantle section also supports reduced condition at the base. We consider thatthe methane was derived from reduced carbon in the subducted oceanic sediment during oceanic
thrusting of the ophiolite.
Keywords: Oman ophiolite, Redox state, mantle peridotite, mantle wedge, oxygen fugacity, subduction
zone©2016. Japan Geoscience Union. All Right Reserved.

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