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Time:May 26 16:15-16:30

Applications of state of the art downhole logging for hard-rock
SANADA, Yoshinori1∗ ; SAITO, Saneatsu1 ; MOE, Kyaw1
1

JAMSTEC

Downhole logging has been one of standard measurements in scientific drilling. Its basic data are gamma-ray, spectral gammaray, electrical resistivity, density, porosity, sonic velocity, seismic velocity, and borehole image. These data has being applied tophysical properties, lithology, sedimentology, structural geology, rock mechanics, hydrogeology, mineralogy, geochemistry etc.
State of the art downhole logging leads us higher resolution, more data variation and volume, and then to new scientific results.
We here discuss basics of wireline logging and LWD (Logging Whole Drilling), their standard/extensive applications, drilling
strategy, potential applications for hard rocks with new technology with some case studies.
One of the advantages of downhole logging over coring is continuous data. Poor core recovery is one of the major issues in hardrock drilling. Continuous logging data covers the missing core intervals. Another advantage is in-situ measurement in downhole.
It is important to measurements physical properties like electrical resistivity, sonic velocity, density, porosity, before physicalchanging and developing contamination of cores with drilling fluid. New geochemical fluid analyzer allows us to measure fluid
compositions, for example optical spectrometer (20ch), C1, C2-C5, C6+, CO2, pH, fluorescence, density, viscosity, flowline
pressure, temperature, electrical resistivity, gas-oil ratio. Downhole fluid sampler allows us to sample formation fluid with lesscontamination. In addition, large diameter side-wall coring tool allows us to take 3.8 cm diameter and 6.3 cm length cores from
borehole wall.
New sensor technology brings us higher accuracy/resolution and more data volume. Latest borehole image tools give us muchhigher resolution than previous generations. It helps to descript small structure and fractures. New gamma-ray spectral tools
allows us to identify minerals: Al, Ba, C, Ca, Cl, Fe, Gd, K, Mg, Mn, Na, S, Si, Ti, Cu, Ni. Some of these new tools requirelarger guide pipes to low the tools into borehole. Chikyu’s riser drilling gives more chances to use new tools with large diameter
riser pipes.
Chikyu’s riser drilling allows us to approach deeper in safer approach. While it expands our leading edge of science, it requires
longer days and huge cost. It drives us to consider spot coring (at selected intervals) from coring of the whole interval. Under
this situation, it is more important for downhole logging to compensates the missing coring intervals. While wireline logging iscarried out after coring, LWD before coring suggests us best spot coring intervals or best position for observatory installation.
New LWD technology provides various high resolution data same as those of wireline logging. These above change paradigm ofdrilling strategies.
Riser drilling circulates cuttings (small pieces of formation rocks during drilling) and formation gas to surface with drilling
fluid. They are valuable samples and information from subsurface, too, and utilized oil and gas industries long time, but we justhave started our studies for science researches. Integration analysis with core, logging, seismic, and cuttings must maximize ourscience products.

Keywords: logging, hard rock, Chikyu, riser drilling, IODP

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SIT05-02

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Time:May 26 16:30-16:45

Young basaltic volcanism as a key for understanding serpentinite-hosted vents in the Challenger Deep Forearc
OHARA, Yasuhiko1∗ ; OHARA, Yasuhiko2 ; STERN, Robert3 ; MARTINEZ, Fernando4 ; ISHII, Teruaki5
1

Hdyrographic and Oceanographic Department of Japan, 2 Japan Agency for Marine-Earth Science and Technology, 3 Universityof Texas at Dallas, 4 University of Hawaii, 5 Fukada Geological Institute
The Izu-Bonin-Mariana forearc is a typical nonaccretionary convergent plate margin; the inner trench slope exposes lithologies found in many ophiolites. In particular, serpentinized peridotite crops out and has been sampled from the inner trench wallalong the southernmost Mariana forearc facing the Challenger Deep. Our studies there indicate that this is a region of forearc
rifting unusually close to the trench axis, as manifested by the Southeast Mariana Forearc Rift [SEMFR; Ribeiro et al., 2013,
G3]. Convergent margin igneous activity is generally limited to beyond 100-200 km from the trench, so the presence of SEMFR
is an unusual characteristic of the southernmost Mariana forearc. We have also discovered more evidence of young basaltic
volcanism from ˜100 km west of SEMFR. DSV Shinkai 6500 dives during YK13-08 cruise recovered volcaniclastics from 5.5
to 6 km deep in the inner wall of the Mariana Trench, ˜50 km northeast of the Challenger Deep [Stern et al., 2014, Island Arc].
The volcaniclastics include fresh basaltic glasses that are similar to basalts from SEMFR as well as to Mariana Trough backarc
basin basalts and we conclude that they formed by recent eruptions on the inner trench wall. Earthquake foci also indicate that
the Challenger Deep forearc is a region of strong extension, and bathymetric data indicate that multiple tectonic rifts dissect it,
indicating that diffuse extension occurs in the forearc.
We have discovered and have been studying a serpentinite-hosted ecosystem, the Shinkai Seep Field [SSF; Ohara et al., 2012,
PNAS] in the inner wall of the Mariana Trench, ˜80 km northeast of the Challenger Deep. SSF is a diffuse cold seep, serpentinitehosted system that hosts ecosystem mainly consisted of vesicomyid clams. We have tried to find more SSF-type seeps along thesouthernmost Mariana forearc during YK13-08 and YK14-13 cruises, but no such seeps were found so far. The origin of thefluid of SSF may originate in the shallow subducting slab, unrelated to igneous activity. Another possibility, based on the factthat YK13-08 volcaniclastics are found ˜5 km west of SSF, is that SSF vent fluid originated from seawater circulated within
the shallow crust driven by the heat of young magmatic intrusion, as is proposed for the Lost City hydrothermal field in the
Mid-Atlantic Ridge. Our results suggest that identifying sites of recent forearc igneous activity may help locate other sites ofseafloor venting on the inner trench wall of the Challenger Deep Forearc.

Keywords: Challenger Deep, forearc, serpentinite, young basalt

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SIT05-03

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Time:May 26 16:45-17:00

Volatile studies through drilling of oceanic plateaus and hotspot seamounts
HANYU, Takeshi1∗
1

Japan Agency for Marine-Earth Science and Technology

Oceanic plateaus and hotspot seamounts are the manifestation of magmatism unrelated to plate boundary processes. Suchoceanic volcanism is a key issue in mantle geochemistry as it is a probe of the source mantle in depth. Collecting submarine
volcanic rocks is essential to geochemical and petrological studies because most part of volcanic bodies exist under water.
Moreover, mantle volatiles can be only studied with submarine glasses quenched under high water pressure. A problem forstudying submarine rocks would be that they are easily altered by hydrothermal and low-temperature fluids, but here I would
show some examples in which noble gas compositions were successfully determined using fresh core samples from aged oceaic
plateau and hotspot seamounts. This is probably the first study to determine noble gas compositions of oceanic plateau basalts.
Louisville seamount chain was formed by a long-lived hotspot in the southern Pacific. Moderately to highly altered rocks havebeen collected by previous dredge hauls. The IODP Expedition 330 drilled and cored seamounts at the age between 50 and 74
Ma. Although the cored rocks were variously altered, we occasionally found fresh basalts in which olivine phenocrysts were
well preserved. Such olivines are good container of mantle volatiles including noble gases. The 3 He/4 He ratios of the studied
olivines range from a value similar to those of MORB (˜8 Ra) to slightly elevated ratios up to 10.6 Ra (Hanyu, 2014). Moreover,some olivines exhibit a primordial Ne isotopic signature that can be discriminated from MORB Ne ratios. These noble gas
compositions document a deep origin of the Louisville mantle plume from less-depleted mantle.
Shatsky Rise in the northern Pacific is an oceanic plateau constructed by intense volcanism around 140 Ma. This plateau wasrecently revisited by IODP Expedition 324, in which amazingly fresh glasses were cored in two of the drill holes at such aged
oceanic plateau. Well-preserved quenched glasses on pillow basalts and massive flows allowed us to determine reliable major
and trace elements (Sano et al., 2012), volatile compositions (Shimizu et al., 2013), and noble gases (Hanyu et al., in press).
Fortunately, the effect of radiogenic ingrowth was minimal for He isotopes because of high He abundance and low U and Th
concentrations in tholeiitic basalts. Glasses from a drill core at Ori Massif show a narrow range in 3 He/4 He between 5.5 and
5.9 Ra, which is lower than the MORB value. Such low and uniform 3 He/4 He is assigned as a feature for their mantle source,suggesting the involvement of recycled slab material in the source of Shatsky Rise.
Our understanding of the time of volcanism, crustal structure, magma sources, and melting processes could be deepened
through drilling of oceanic plateaus and hotspot seamounts. Emission of volatiles from oceanic plateaus might have caused
drastic change of Earth’s surface environment, such as mass extinction, a hypothesis of which must be proved by ocean drilling.
Ontong Java Plateau could be one of potential targets of future drilling, as its crustal structure is partly getting uncovered byon-going geophysical surveys.
Keywords: oceanic plateaus, hotspots, ocean drilling, volatiles, mantle recycling

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SIT05-04

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Time:May 26 17:00-17:15

Evolutionary processes of initial arc magma yield from hot subduction zone reference
from the Oman Ophiolite
KUSANO, Yuki1∗ ; UMINO, Susumu1 ; KITAMURA, Keitaro1 ; ADACHI, Yoshiko2 ; SHINJO, Ryuichi3 ;MIYASHITA, Sumio2
1

Kanazawa University, 2 Niigata University, 3 University of Ryukyus

Tethys ophiolite zone consisted of fragments of Jurassic to Cretaceous oceanic crusts is considered to be produced on forearc
setting (e.g. Dilek and Furnes, 2009). However, it is questioned that do initial arc always develop a long-survived subduction
zone as explained by evolution processes of the Izu-Ogasawara-Mariana arc (e.g. Stern, 2004). We present volcanic, magmaticand tectonic evolutionary process of short-lived juvenile arc from the northern Oman ophiolite.
The Oman ophiolite belonging to the Tethys ophiolite zone is one of the best places to investigate magmatic and volcanic
developing processes of an infant arc. The Ophiolite had formed on a spreading axis and followed by subduction stage magmatismat approximately 100 Ma. Latest U-Pb age of zircon in plutonic bodies shows that there is only 0.5 m.y. time gap between the
spreading and subduction stages (Riuox et al., 2014). Based on the radiolarian fossil age, the initial subduction volcanism ceased<2 m.y. after the ridge stage (Agui et al., 2014), therefore, it seems to record short-spanned island arc magmatism.
The subduction stage volcanic rocks extending 1100 m thick consist of the lower arc tholeiite (LV2) and upper boninite (UV2).
Pahoehoe and sheet flows are dominate in the LV2, while 50 m thick pyroclastic rocks are partly distributed upward. Since theupper part consists mainly of sheet flows and pyroclastic rocks with intervening some pelagic sedimentary layers, the LV2 volcano
was developed quickly at the beginning and the volcanism became explosive and intermittently later. The UV2 magma intrudes
into lower plutonic and extrusive sequences and erupted as pyroclastic fall and lava flows through fissure vents. The UV2 isoverlying the LV2 with interbedded sedimentary layer and distributed >350 km along the Oman Ophiolite. Geochemically it issuggested that the LV2 magma is generated by wet partial melting containing hydrous fluids while the UV2 magma is generated
by accretion of sedimentary melt. Estimated degree of melting in the LV2 and UV2 indicates that both are explained by remeltingof the residual mantle after the spreading magmatism and the difference of magma was controlled by involved fluid compositions
rather than progressive source depletion. The boninite magma genesis is well supported by ˜1400 ℃ mantle potential temperature
calculated from primary magma composition of glass inclusion in boninite Cr-spinel (Kitamura et al., 2014).
Keywords: Initial arc, Hot subduction zone, Boninite, Oman Ophiolite

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Time:May 27 09:00-09:15

IODP Exp. 351 Izu-Bonin-Mariana Arc Origins
ISHIZUKA, Osamu1∗ ; ARCULUS, Richard2 ; BOGUS, Kara3 ; EXP. 351, Scientific party3
1

GSJ/AIST, 2 ANU, 3 IODP, TAMU

Understanding how subduction zones are initiated and continental crust forms in intraoceanic arcs requires knowledge of the
inception and evolution of a representative intraoceanic arc, such as the Izu-Bonin-Mariana (IBM) arc system. 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. IODP Exp.351 targeted, in particular, evidence for the earliest evolution of the IBM
system following inception.
The Exp. 351 drill site (U1438) is located in Amami Sankaku Basin (ASB), west of the Kyushu-Palau Ridge (KPR), i.e.,
paleo-IBM arc. Seismic reflection profiles suggest that sediment thickness of the Basin is about 1.3 km thick, and igneous crust,
presumed to be oceanic, is about 5.5 km thick. This igneous crust seemed likely to be the basement of the IBM arc.
Primary objectives of this Expedition were: 1) determine the nature of the crust and mantle preexisting the IBM arc; 2) identify
and model the process of subduction initiation and initial arc crust formation; 3) determine the compositional evolution during
the Paleogene of the IBM arc; 4) establish geophysical properties of the ASB.
Exp. 351 lasted 2 months from May 30, 2014 aboard the JOIDES Resolution. Site U1438 (in 4700m water depth) consistedof 4 cored holes with overlapping recoveries; igneous basement was reached after coring the entire sediment section. The coredinterval comprises 5 units: uppermost Unit I is hemipelagic sediment with intercalated ash layers, presumably recording explosive
volcanism mainly from the Ryukyu and Kyushu arcs; Units II and III host a series of volcaniclastic gravity-flow deposits, likely
recording the magmatic history of the IBM Arc from arc initiation until 25 Ma; Siliceous pelagic sediment (Unit IV) underlies
these deposits with minimal coarse-grained sediment input, and could pre-date arc initiation. Sediment-basement contact occurs
at 1461 mbsf. A basalt to dolerite section dominantly composed of plagioclase and clinopyroxene with rare chilled margins
continues to the bottom of the Hole at 1611 mbsf. Preliminary assessment of the results suggests that basaltic basement is earlymiddle Eocene (or older) and geochemically similar to forearc basalts from IBM forearc.
Exp.351 is regarded as successful because :1)Sedimentary record preserving subduction initiation, arc maturation to shutdown
was recovered: 2)Igneous basement of Amami Sankaku Basin, i.e., basement of IBM arc was recovered. The outcome of thisexpedition permits hypothesis testing for subduction initiation and subsequent Arc evolution.

Keywords: Izu-Bonin-Mariana arc, IODP, Exp.351, subduction initiation, oceanic island arc

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SIT05-06

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Overview of IODP Expedition 352 - Testing subduction initiation and ophiolite models
by drilling the outer IBM fore-arcs
SHIMIZU, Kenji1∗ ; MICHIBAYASHI, Katsuyoshi2 ; SAKUYAMA, Tetsuya3 ; PYTHON, Marie4 ;IODP, Expedition 352 scientists5
1

JAMSTEC, 2 Institute of Geosciences, Shizuoka University, 3 Dept. Geosciences, Osaka City Univ., 4 Natural History Science,
Hokkaido Univ., 5 IODPThe Izu-Bonin-Mariana (IBM) arc is the ideal locality for studying subduction initiation, arc magmatism and the earliest stagesof continental crust formation. To gain a better understanding of the evolution of subduction zones, three IODP expeditions(Expedition 350, 351 and 352) were conducted at the IBM arc system (rare-arc, proto-arc and fore-arc) between March and
September 2014 by the JOIDES Resolution drilling vessel. Expedition 352 was targeted to drill the entire magmatic sequencecomprising the outer Bonin fore-arc to elucidate early subduction dynamics and test ophiolite formation models posit formation
upon subduction initiation. During the expedition, a total of 1.22 km of igneous basement related to subduction initiation and0.46 km of overlying sedimentary rocks were cored from four sites (U1439, U1440, U1441, U1442).
Two sites (U1440 and U1441) located nearer to the trench, recovered igneous rocks at the basement that are mostly fore-arc
basalts (FABs) manifest as pillow lavas, sheet flows and hyaloclastites. At the lowermost part of Hole U1440B, FABs are overlain
by dolerites, which are interpreted as feeder dikes for the upper FAB lava units. Compositions of FABs are similar to those of
mid oceanic ridge basalts, and exhibit little evidence of subduction influence.
From the two sites (U1439 and U1442) located ˜15 km west from U1440 and U1441, pillow lavas, massive lavas, hyaloclastites
and pyroclastic flow deposits of boninite were recovered. Boninites with doleritic texture were also recovered from the lowermost
part of Hole U1439C, which may represent a dike complex. No FAB was found beneath boninite in these sites. Boninites are
chemically distinct from FAB by virtue of higher SiO2, MgO and K2O and lower TiO2. These chemical criteria dictate thatboninites formed by partial melting of a more depleted mantle source enriched in slab-derived subduction components.
The presence of feeder dikes at the bases of FAB and boninite holes (U1440B and U1439C, respectively) indicates that theoccurrence of boninitic and FAB lavas was offset horizontally as opposed to vertically. At a relatively early stage of subduction
formation, conditions of magma genesis changed drastically from predominantly decompression melting (formation of FAB) to
flux melting (formation of boninite).

Keywords: IODP, Fore-arc basalt, boninite, subduction zone, ophiolite

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Water in glassy volcaniclastics recovered during IBM IODP Exp. 350: can it help fingerprint eruption sites?
NICHOLS, Alexander1∗ ; TAMURA, Yoshihiko1 ; BUSBY, Cathy2 ; BLUM, Peter3 ; JUTZELER, Martin4 ;SCHINDLBECK, Julie5 ; MCINTOSH, Iona1 ; IODP, Expedition 350 scientists6
1

JAMSTEC, Yokosuka, Japan, 2 Department of Earth and Planetary Sciences, UC Davis, USA, 3 IODP, Texas A&M University,
USA, 4 National Oceanography Centre, Southampton, UK, 5 GEOMAR, Helmholtz Centre for Ocean Research, Kiel, Germany,
6n/a
The Izu-Bonin-Mariana arc system (IBM) extends over 2800 km south from the Izu Peninsula (Japan) to beyond Guam (USA).
During 2014 the Izu segment was the focus of three IODP drilling expeditions (350, 351 and 352) that investigated different aspects of this intra-oceanic arc system. This work focuses on material recovered in the first of these expeditions that took placefrom March 30 to May 30 2014.
IODP Expedition 350 drilled at two sites (Tamura et al., In Press). Site U1436 in the fore-arc at 1776 meters below sea level(mbsl), ∼60 km east of the arc-front volcano Aogashima, is a geotechnical hole drilled in preparation for proposed deep drillingat site IBM-4. This was followed by Site U1437 at 2117 mbsl in the rear-arc, the main focus of the expedition, located in a
volcano-bounded basin between the Manji and Enpo rear-arc seamount chains, ∼90 km west of the arc-front volcanoes Myojinsho and Myojin Knoll. Hole U1436A reached 150 meters below seafloor (mbsf), recovering 71.6 m of Pleistocene to Pliocene
sediments consisting of a single lithostratigraphic unit of tuffaceous mud (∼60%) intercalated with ∼150 volcaniclastic layers.
Volcaniclastics range from ash to lapilli-ash size, and record mafic (∼80 layers, ∼60% of the recovered volcaniclastics) to more
evolved volcanism. A distinctive glassy mafic ash layer that may record a large-volume eruption was recovered at ∼50 mbsf; inorder to recover the layer less disturbed by coring, three additional holes (U1436B, C, and D) were drilled at the site to better
constrain its thickness and thus its origin. Drilling at Site U1437 across Holes U1437B, D and E, reached 1806.5 mbsf andrecovered 1120.76 m of Pleistocene to Miocene tuffaceous mud and mudstone (∼60%) intercalated with volcaniclastic layers(∼2500 layers) that were divided into seven lithostratigraphic units. The proportion of the volcaniclastics and their dominantly
fine grain size (ash/tuff) is surprisingly low for an intra-volcano basin and suggest distal sources. In Units VI and VII, below
1320 mbsf, the volcaniclastics included a greater proportion of coarser material (lapilli-tuff to tuff-breccia) that may originate
more proximally. Within Unit I a record of mafic and more evolved volcanism could be identified, but this became more difficult
with depth.
Water is being measured in glassy material from the volcaniclastic layers. As water solubility increases with increasing pressure, if water is saturated in the melt on eruption, the water left in the glass provides a means to estimate the pressure at the timeof quenching. This can then be used to infer whether or not the eruption that generated the volcaniclastics was submarine, and if
so at what water depth it occurred. We will examine whether water can be used to constrain eruption depths and help to locate
possible sources of the volcaniclastics recovered in IODP Expedition 350. One possible issue that may overprint the water signal
even in fresh-looking glass is post-eruption hydration of the glass by seawater at ambient temperature at their site of deposition.
Water added in this way enters the glass as molecular water because at sub-magmatic temperatures the species interconversion
reaction between molecular water and hydroxyl species is negligible. This results in anomalously high concentrations of molecular water compared to the speciation expected at eruption temperature. If this is recognized in the glassy volcaniclastics fromIODP Expedition 350, we will examine whether it is possible to restore eruption water contents using their measured hydroxyl
content and water speciation models. If successful, we will then infer from the restored water contents whether the eruptions
that generated the glasses were submarine, and if so the depth of eruption required for the melts to be saturated at the time ofquenching.
Tamura, Y. et al., In Press. Proceedings of the International Ocean Discovery Program, Expedition 350. doi:10.14379/iodp.proc.350.2015

Keywords: Izu-Bonin-Mariana drilling, volcaniclastics, glass, water content, water speciation, vent conditions

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SIT05-08

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Time:May 27 09:45-10:00

Andesite Magmas are Produced Along Oceanic Arcs Where the Crust is Thin: A New
Hypothesis
TAMURA, Yoshihiko1∗
1

JAMSTEC

The straightforward but unexpected relationship presented here relates crustal thickness and magma type in the Izu-Ogasawara(Bonin) Oceanic arc. Volcanoes along the Ogasawara segment of the arc are underlain by thin crust (16-21 km)?in contrast to
those along the Izu segment, where the crust is ˜35 km thick. Interestingly, andesite magmas are dominant products from theformer volcanoes and mostly basaltic lavas erupt from the latter. Moreover, andesite magmas have been similarly dominant in
the Oligocene Izu-Ogasawara-Mariana arc, when the arc was immature and their crust must have been thin. Why and how dovolcanoes on the thin crust erupt andesite magmas? An introductory petrology textbook might answer this question by suggesting
that, under decreasing pressure and hydrous conditions, the liquidus field of forsterite expands relative to that of enstatite, with theresult that, at some point, enstatite melts incongruently to produce primary andesite melt. According to the hypothesis presented
here, however, rising mantle diapirs stall near the base of the oceanic crust at depths controlled by the thickness of the overlying
crust. Where the crust is thin, as along the Ogasawara segment of the arc, pressures are relatively low, and magmas producedin the mantle wedge tend to be andesitic. Where the crust is thick, as along the Izu segment, pressures are greater, and only
basaltic magmas tend to be produced. Implications of this hypothesis include the following: (1) A ’stockpile’ of continental
crust (andesitic magma) was produced during the Archean and Proterozoic, when most crust was thin. (2) Most andesite magmas
erupted on continental crust could be recycled from ’primary’ andesite originally produced in oceanic arcs. The rate of continental
crust accumulation would therefore have been greatest early in Earth’s history, soon after subduction was initiated.
Keywords: continental crust, andesite, oceanic arc, crust, mantle, primary magma

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SIT05-09

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Is high-silica boninite of recycled slab origin?
UMINO, Susumu1∗ ; KITAMURA, Keitaro1 ; KANAYAMA, Kyoko1 ; KUSANO, Yuki1 ; SENDA, Ryoko2 ;ISHIZUKA, Osamu3
1

Division of Natural System, Kanazawa University, 2 Department of Solid Earth Geochemistry, JAMSTEC, 3 Geological Surveyof Japan, AIST
Primitive melt inclusions in chrome spinel from the Ogasawara Archipelago comprise two discrete groups of high-SiO2 , MgO(high-Si) and low-SiO2 , MgO (low-Si) boninitic suites with ultra-depleted dish- and V-shaped, and less depleted flat rare earth
element (REE) patterns. The most magnesian melt inclusions of each geochemical type were used to estimate the genetic T-P
conditions for primary boninites by using [1], which range from 1345 degC-0.56 GPa to 1421 degC-0.85 GPa for the 48-46
Ma high-Si and low-Si boninites, and 1381 degC-0.85 GPa for the 45 Ma low-Si boninite. These T-P conditions for the low-Si
boninites lie on an adiabatic melting path of depleted mid-ocean ridge basalt mantle (DMM) with a mantle potential T (MPT)of 1420 degC, which is in agreement with that of the primary proto-arc basalt (PAB) magma preceding boninites estimated by
PRIMELT2 [2]. This is consistent with the previous model of the subduction initiation in which the onset of the Pacific Slabsubduction at 52 Ma forced upwelling of DMM from the depth of ca. 100 km to yield PAB. The residue of PAB was subsequently
fluxed by slab fluids to yield the low-Si boninite at 48-46 Ma [3]. On the contrary, the higher temperatures for the high-Si boninite
magma generation cannot be explained by this scheme, but has been ascribed to the involvement of a mantle plume with a MPT>1500 degC [4]. However, the ascent of such high-T peridotite to <1 GPa should cause extensive decompression melting to
produce picritic magmas, which have never been found among the pre-boninite PAB. This discrepancy can be reconciled if thedepleted proto-boninite source already existed below the DMM-like PAB source before the subduction began. With the rise of
DMM, refractory harzburgite ascended without melting, and hence retained its high temperature. At 48-46 Ma, introduction ofslab fluids caused remelting of the PAB residue and high-T harzburgite, resulted in the low-Si and high-Si boninites, respectively.
Meanwhile, convection within the mantle wedge brought the less depleted residue of PAB and DMM into the region fluxed byslab fluids, which melted to yield the less depleted low-Si boninite at 45 Ma, and fertile arc basalts, respectively.
The presence of refractory high-Si boninite source is supported by the unradiogenic Os isotopic compositions of chrome spinel
derived from high-Si boninite in Ogasawara [5] and harzburgite drilled in the Izu Forearc [6], which experienced melt extractionin Proterozoic age and became the source for the boninite magmas. Such Proterozoic depleted harzburgites are also known toexist below the lithosphere of the Ontong Java [7] and Kerguelen Plateau [8], and are considered to be remnants of recycled slab
subducted below the Rodinia supercontinent [7]. The residual mantle experienced up to 25% melting below the Proterozoic midocean ridges descended and stagnated in the transition zone below Rodinia. The refractory harzburgite slab was then brought up
to the base of the continental lithosphere at a depth of around 100 km with the ascent of the super plume either during the riftingof Rodinia, or later Gondwana, and was drifted away with the continental fragments and is now spread sporadically below the
Pacific and Indian plates. The globally limited occurrence of high-Si boninite is only possible when the remnants of harzburgitic
slabs are tapped by a descending slab after subduction initiation and brought upward to the region of flux melting.
[1] Putirka, 2008. Reviews in Mineralogy and Geochemistry, 69, 61-120.
[2] Herzberg and Asimow, 2008. Geochem. Geophys. Geosys., 9, Q09001, doi:10.1029/2008GC002057.
[3] Kanayama et al., 2012. Island Arc, 21, 288-316.
[4] Macpherson and Hall, 2001. Earth Planet. Sci. Lett., 186, 215-230.
[5] Suzuki et al., 2011. Geology, 39, 999-1002.
[6] Parkinson et al., 1998. Science, 281, 2011-2013.
[7] Ishikawa et al., 2011. Earth Planet. Sci. Lett., 301, 159-170.
[8] Hassler and Shimizu, 1998. Science, 280, 418-421.

Keywords: Izu-Ogasawara-Mariana Arc, boninite, subduction initiation, melt inclusions, Cr spinel, recycled slab

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Reexamination of genetic relationship between mantle peridotite and volcanic rocks in
the Oman ophiolite
TAKAZAWA, Eiichi1∗ ; KANKE, Nami2 ; FUJII, Satoru2 ; NOMOTO, Yuki2 ; KUSANO, Yuki3 ; UMINO, Susumu3 ;TAMURA, Yoshihiko1
1

R & D Center for Ocean Drilling Science, Japan Agency for Maine-Earth Science and Technology, 2 Graduate School of Scienceand Technology, Niigata University, 3 Division of Natural systems, College of Science and Engineering, Kanazawa University
The volcanic sequence in the Oman ophiolite consists of MORB, island arc tholeiite and boninite from the bottom to the top(Ishikawa et al., 2002; Yamazaki, 2012, Kusano et al., 2012, 2014). These evidences support the change in tectonic setting for
the Oman ophiolite from spreading ridge to incipient subduction zone (Umino et al., 1990; Arai et al., 2006). The boninitic dikes
and lavas in the Oman ophiolite are high-Ca boninite and require melting of cpx-bearing peridotite such as lherzolite (Kusano etal, 2014). However, boninite occupies only about 13 % of the lavas in the V2 unit (Kusano et al, 2014). It’s left as an unsolved
problem what and where the residue after extraction of island arc tholeiite is. Harzburgite with spinel Cr# (=Cr/(Cr+Al) atomic
ratio) lower than 0.65 is widely distributed inside of the mantle section. We speculate that these harzburgites may be the residues
after extraction of arc tholeiitic melt produced by flux melting of lherzolite during intra-oceanic thrusting. Clinopyroxenes in such
harzburgite are highly depleted in LREE relative to HREE in the C1 chondrite-normalized REE patterns. Before flux melting
the mantle section may have consisted of moderately depleted lherzolite which composition is similar to the basal lherzolites
in the northern Oman ophiolite (Takazawa et al., 2003; Khedr et al., 2013, 2014). Highly refractory harzburgites and dunites
locally occur in the lower part of the mantle section above basal thrust and have spinel Cr# greater than 0.7. Emplacement of
hot young oceanic lithospheric mantle caused thermal metamorphism and dehydration of subducting oceanic crust (Hacker andMosenfelder, 1996; Ishikawa et al., 2005). Both fluid and sediment-derived melt may have infiltrated into the mantle section
caused flux melting of harzburgite and produced boninitic melt together with highly refractory dunite (Arai et al., 2006; Nomoto
and Takazawa, 2013; Kanke and Takazawa, 2014).
Keywords: Oman ophiolite, mantle section, peridotite, island arc tholeiite, boninite, flux melting

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SIT05-11

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Time:May 27 10:30-10:45

Evolutional process of fast-spreading lower oceanic crust: an example of troctolites at the
Hess Deep Rift
AKIZAWA, Norikatsu1∗ ; GODARD, Marguerite2 ; ILDEFONSE, Benoit2 ; ARAI, Shoji1
1

Department of Earth Sciences, Kanazawa University, 2 Geosciences Montpellier, Universite Montpellier & CNRS

Troctolites were recovered during IODP Expedition 345 at Hess Deep Rift (Dec 2012 - Feb 2013), which targeted plutonic
rocks from fast-spread lower ocean crust. The troctolites are divided into two groups based on textural differences; fine-grained
troctolite (including a skeletal olivine-bearing troctolite sample),and coarse-grained troctolite.
The major-element compositions of olivine, plagioclase and clinopyroxene in coarse-grained troctolites are intermediate between those in olivine gabbros/olivine-bearing gabbros and peridotites recovered from the Hess Deep Rift. Fo content and NiOof olivine range from 87 to 89, and 0.2 to 0.3 wt.%, respectively. An content of plagioclase ranges from 85 to 90. Mg# and Cr2 O3
of clinopyroxene range from 0.88 to 0.91, and 0.5 to 1.2 wt.%, respectively. In contrast, fine-grained troctolites partly overlap
with olivine gabbros/olivine-bearing gabbros in mineral chemistry. Fo content and NiO of olivine range from 83 to 86, and 0.08
to 0.2 wt.%, respectively. An content of plagioclase ranges from 77 to 84. Mg# and Cr2 O3 of clinopyroxene range from 0.82 to
0.89, and nearly nil to 1.0 wt.%, respectively. Trace-element analyses of olivine and plagioclase show progressive enrichment in
REE from coarse-grained to fine-grained troctolites. In contrast, clinopyroxenes show scattered trace-element compositions in
the fine-grained troctolites, even in a single thin section.
The changes in chemical composition of olivine and plagioclase from coarse-grained to fine-grained (and skeletal olivinebearing) troctolites may be ascribed to variable degrees of reequilibration with crystallizing melts during cooling. Fine-grained
troctolites possibly record melt/rock interactions that would be responsible for the variable chemical compositions of clinopyroxenes. At Hess Deep, lower crustal troctolites possibly underwent several stages of evolution, combining fractional crystallizationof MORB (mid-ocean ridge basalt) melts, combined with melt/troctolite interaction during migration. Melt migration processes
in the lower oceanic crust would result in enhanced regional diversity of MORB chemistry.
Keywords: Troctolite, Fast-spreading ridge, Melt/troctolite interaction, Hess Deep, Trace-element composition

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SIT05-12

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Time:May 27 11:00-11:15

Earthquake activity in the Pacific plate near the Japan Trench axis after the 2011 TohokuOki Earthquake
OBANA, Koichiro1∗ ; FUJIE, Gou1 ; NAKAMURA, Yasuyuki1 ; KODAIRA, Shuichi1
1

JAMSTEC

Intra-plate normal-faulting earthquakes near oceanic trenches likely associate with bending of the incoming/subducting plates.
Focal mechanisms of the intra-plate normal-faulting earthquakes in trench-outer slope area suggest tensional stress at shallow
depths, which may promote infiltration of seawater several tens of kilometers into the oceanic lithosphere. Recent seismic
structural studies in the trench-outer slope area present seismic velocity changes in the oceanic plate approaching the trench
accompanied by the development of bending-related faults cutting the oceanic crust (e.g., Fujie et al., 2013, Grevemeyer et al.,2007). However, details on hypocenter locations, especially in depths, of outer-trench normal-faulting earthquakes and relation
to the crustal structures have not been well understood due to less frequent activity than inter-plate earthquakes and lack ofnear-field observations. After the 2011 Tohoku-Oki Earthquake (Mw 9.0), shallow normal-faulting seismicity has been activein the incoming/subducting Pacific plate near the Japan Trench (e.g., Asano et al., 2011). To investigate the stress state in
the incoming/subducting Pacific plate near the trench axis and relations between earthquakes and crustal structures, we haveconducted a series of ocean bottom seismograph (OBS) observations near the trench axis since the occurrence of the 2011
Tohoku-Oki earthquake. These OBS observations provide accurate hypocenter locations and focal mechanisms of earthquakes
occurred in the Pacific plate. Earthquakes with a normal-faulting focal mechanism occurred at depths of shallower than 40 km
beneath the outer slope of the Japan Trench. The normal-faulting earthquakes in the oceanic crust coincide with normal-faults
cutting the oceanic crust and forming horst and graben structures. The hypocenter distributions and T-axes directions suggest
earthquakes activity along pre-existing structures in the oceanic crust in addition to the trench-parallel normal faults. Both thepre-existing structures, such as fracture zones, and trench-parallel normal faults formed in the trench outer slope area could act
as faults of the shallow normal-faulting earthquakes. Furthermore, the normal-faulting earthquakes occurred at deeper depths
compared with the OBS observations before the 2011 earthquake by Hino et al. (2009). The 2011 Tohoku-Oki Earthquake likely
changed the stress state in the Pacific plate. These observations suggest that stress regime in the oceanic lithosphere, which
could change in temporal and spatial, and both pre-existing and newly created faults in the oceanic crust are important factor tounderstand the hydration of the oceanic plate prior to the subduction.
Keywords: Intra-plate earthquake, horst and graben, normal faulting, OBS

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SIT05-13

Room:303

Time:May 27 11:15-11:30

Ultradepleted olivine and spinel sands in Challenger Deep, Mariana Trench
SHIMADA, Yuka1 ; MICHIBAYASHI, Katsuyoshi1∗ ; TERUMINE, Naonobu1 ; UEHARA, Shigeki1 ; OYA, Shoma1 ;OHARA, Yasuhiko3 ; ISHII, Teruaki4 ; HARIGANE, Yumiko5 ; NUNOURA, Takuro2 ; MIYAZAKI, Junichi2 ;TAKAI, Ken2 ; MANNEN, Kazutaka6
1

Institute of Geosciences, Shizuoka University, 2 JAMSTEC, 3 Hydrographic and Oceanographic Department of Japan, 4 Fukada
Geological Insitute, 5 AIST/GSJ, 6 Hot Springs Institute of Kanagawa Prefecture

Peridotite has been studied extensively as a clue to understand the uppermost mantle structure. Abysal peridotite is known to
be exposed to the plate spreading axes such as mid-ocean ridges and the plate convergence margin such as trenches. Many studies
have had interested in peridotites outcropped at the landside slope in the southern Mariana Trench. Challenger Deep (10,911 m
depth) of Mariana Trench is the deepest in the Earth. However, it is difficult even today to sample rocks exposed at deeper slopes
than 7,000 m depth due to technical problem. In 2008, JAMSTEC (Japan Agency for Marine-Earth Science and Technology)
sampled a sediment core at 10,350 m in Challenger Deep by ABISMO (Automatic Bottom Inspection and Sediment Mobile).
Mafic minerals such as olivine and spinel have been identified in this core. They may be derived from peridotites that could
be exposed at deeper slopes than 7,000 m below the sea surface. Therefore, these mafic minerals may give us an opportunity
to explore mantle peridotite at the bottom of Challenger Deep. We have chosen relatively coarse mineral grains from this core.
These grains were analysed their chemical compositions by EPMA (Electron Probe Micro Analyzer). As a result, olivine, spinel,
pyroxene, plagioclase, quartz and magnetite were identified. The olivine CaO are less than 0.07 wt%. Moreover, assuming thatboth spinel and olivine grains were derived from the same peridotites, spinel Cr# and olivine Mg# indicated that the peridotite
could be in the mantle origin. The spinel Cr# are highly depleted up to 0.8, suggesting their origin from the forearc mantle.
Olivine Mg# in the sediment core have been compared with those in peridotites occurred at the landside slope. It shows that
olivine Mg# increase toward deeper slopes from 3,500 m depth. As a result, it suggests that these olivine and spinel grains could
be derived from peridotites exposed at the deeper slopes than 7,000 m depth, possibly at very bottom of Challenger Deep, where
unknown peridotites could have been highly depleted.
Keywords: Challenger Deep, olivine sand, spinel sand, forearc, boninite

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SIT05-14

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Time:May 27 11:30-11:45

Metasomatism during subduction initiation recorded in basal peridotites of the northern
Fizh massif, Oman ophiolite
YOSHIKAWA, Masako1∗ ; PYTHON, Marie2 ; ARAI, Shoji3 ; TAMURA, Akihiro3 ; SHIBATA, Tomoyuki1 ;TAKAZAWA, Eiichi4 ; UEDA, Akira5 ; SATO, Tsutomu6
1Inst. Geotherm. Sci., Kyoto University, 2 Dep. Natural History Sci., Hokkaido University, 3 Dep. Earth Sci., Kanazawa University, 4 Dep. Geol., Niigata University, 5 Faculty of Science, University of Toyama, 6 Faculty of Engineering, Hokkaido University
The Oman ophiolite is one of the best preserved sections of oceanic lithosphere worldwide. It lies on more than 400 km along
the north eastern coast of the Arabian Peninsula. The Oman ophiolite and its underling metamorphic sole are regarded as being
direct analogues of obducted oceanic lithosphere and subducted oceanic crust that formed by overthrusting (e.g. Boudier et al.,
1989, Ishikawa et al., 2002). This idea is supported by the presence of granitic rocks and boninite dykes in the northern partof the ophiolite, which were generated by the partial melting of the subducting plate sediment cover during metamorphic soleformation (Cox et al., 1999) or by mantle metasomatism by fluid dehydrated from metamorphic sole during subduction initiation(Ishikawa et al., 2002).
We focus on the basal clinopyroxene (Cpx)-rich peridotites in the northernmost Fizh massif in order to discuss the origin of
the metasomatic agent and the degree of metasomatism, and to estimate the Cpx trace element and Nd-Sr isotopic compsositions.
The chondrite-normalised multi-element patterns for Cpxs in these rocks are significantly depleted in incompatible elements.
The multi-element Cpx patterns were basically reproduced by 4-12% of melt extraction from a spinel peridotite source. However, the highly incompatible element (e.g., Ba, Nb, La, Ce, and Pb) characteristics of the basal Cpx-rich peridotites could not be
reproduced by simple melting modelling. Ishikawa et al. (2005) proposed a trace element compositions for fluids released fromthe metamorphic sole beneath the Oman ophiolite. The enrichment of highly incompatible elements is generally reproduced by
the addition of very small amounts of these fluids (≤ 0.3 %) to the residual peridotites.
The Sm-Nd isotopic data plotted on the gabbro isochron (100 Ma) of the Fizh block (as given by McCulloch et al., 1980,1981) suggests that the basal Cpx-rich peridotites were formed by partial melting contemporaneously with the generation ofoceanic crust. Initial Sr isotopic compositions of the Cpxs within the basal Cpx-rich peridotites cover a wide range (87 Sr/86 Sr= 0.7030-0.7074), in contrast to the rather constant initial Nd isotopic compositions. The initial Sr-Nd isotopic compositions
consistently plot on the mixing line between Cretaceous seawater and MORB-type oceanic crust (presented by McCulloch et al.,1980), suggesting a contribution of seawater from the metamorphic sole.
Based on these observations, we propose that small amounts of fluids derived from the metamorphic sole (amphibolites andquartzose rocks) were added to the overlying residual peridotites during the initial stages of subduction.
Keywords: mantle metasomatism, Oman ophiolite, basal peridotite, slab-derived fluid, trace element compositions, Nd-Sr isotopic compositions

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SIT05-15

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Time:May 27 11:45-12:00

Investigation into hydrology along Bending-induced faults by off-Tohoku Incoming Plate
Sampling
MORISHITA, Tomoaki1∗ ; MORGAN, Jason2 ; FUJIE, Gou3 ; ABE, Natsue3 ; ONO, Shigeaki3 ;SAITO, Saneatsu3
1

Kanazawa University, 2 University of London, 3 JAMSTEC

The magnitude and spatial distribution of hydration of the oceanic plate is a key to understand water-carbon circulation at
subduction zone, and also trigger earthquakes and arc magmatism. Hydration of oceanic plate at the outer rise region due tobending and faulting before subduction has been conceptually proposed (e.g., Peacock, 2001 Geolgy) and has been recently
supported by seismic observations (e.g., Ranero et al., 2003 Nature; Worzewski et al., 2010 Nature Geosci.). It is, however, notclear yet, how, how deep and how much water can infiltrate into the bending oceanic plate. Dredging and submersible studies
cannot provide the deep and spatial information of oceanic plate. We are going to propose a drilling project to obtain the spatial
information of altered oceanic plate along bending-induced faults in off-Tohoku incoming plate, which is an old and cold end
member of the oceanic plate. Although trenchward-dipping seismic reflections have not been observed in the studied region,
recent multbeam bathymetric analyses show horst and graben structures parallel to subparallel to the Japan trench axis, which
are newly formed from plate bending induced faults (Nakanishi, 2011 in Accretionary Prisms and Convergent Margin Tectonics;
Nakamura et al., 2013 Geophys. Res. Lett.). Epicenters of the earthquakes in this region show lineations parallel to trend of
topographic lineations of these horst and graben structures (Obana et al., 2012, Geophys. Res. Lett.) Ocean bottom seismographobservations suggest that intraplate earthquakes after the 2011 Tohoku earthquake occurred in the oceanic crust and uppermost
mantle at depths <40 km, and have normal-faulting focal mechanisms (Obana et al., 2012). Hydration along these bendinginduced faults should cause serpetinization in the incoming plate prior to the subduction that has been linked to the existence ofthe lower part of the double Wadati-Benioff seismic zone in this region (e.g., Peacock, 2001; Reynard et al., 2010 Geophys. Res.
Lett.; Nakajima et al., 2011 Geophys. Res. Lett.; Garth and Rietbrock, 2014 Geology). In the meeting, we will present drilling
strategies to achieve the scientific objectives.
Keywords: Subducting Plate, Outerrise, Hydrology, Earthquake, Oceanic Drilling, Serpentinization

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SIT05-16

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Time:May 27 12:00-12:15

Structural variation of oceanic Moho at southeast of the Shatsky Rise in the Northwestern
Pacific
OHIRA, Akane1∗ ; KODAIRA, Shuichi2 ; NAKAMURA, Yasuyuki2 ; FUJIE, Gou2 ; ARAI, Ryuta2 ;MIURA, Seiichi2
1

Yokohama National University, 2 Japan Agency for Marine-Earth Science and Technology

In the northwestern Pacific magnetic anomaly lineations are identified by many studies. It is revealed that the oceanic crustin southeast of Shatsky Rise was formed on a paleo-ridge between the Pacific Plate and Farallon Plate from Late Jurassic to
the Early Cretaceous (e.g. Nakanishi et al., 1989), and considered to consist of the typical oceanic crust and mantle. In sucharea, understanding of crustal and mantle seismic structure and nature of the Moho is an important clue to reveal structures andformation/alternation process of the typical oceanic lithosphere. However, there are few studies which covered wide range ofages of the oceanic plate continuously by using latest seismic techniques.
In 2014 we conducted an active-source refraction/reflection survey along 1130km-long line in southeast of Shatsky Rise. Fiveocean bottom seismometers (OBSs) were deployed and recovered by R/V Kairei of Japan Agency for Marine-Earth Science andTechnology (JAMSTEC). We used an airgun array with a total volume of 7,800 cubic inches with firing at intervals of 200m ascontrolled sources. Multi-channel seismic reflection (MCS) data were also collected with a 444-channel, 6,000-m-long streamer
cable.
On MCS sections strong variation of the Moho were imaged. The clear and sharp Moho was imaged up to about 50km fromsouthwest end, then the Moho was changed to be ambiguous from this point. In some areas, the Moho was not identified. The
thickness of the sedimentary layer was about ≤0.3km except area around northeast end of survey line in which sediments fromthe Emperor seamounts may be supplied. The apparent velocity of uppermost mantle refractions (Pn) observed on the OBS
record was about ≤8.6km/sec. We also identified reflected waves from the upper mantle at large offsets in records (170-440km
offsets), which are similar to mantle reflection phases observed in northwest Pacific Basin (Kaneda et al., 2010). As a result of
forward modeling (Fujie et al., 2008) of the mantle reflection phases, depths of these reflectors were about 40km-65km, some ofwhich may correspond to the lithosphere-asthenosphere boundary.

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SIT05-17

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Time:May 27 12:15-12:30

Petrological characteristics of Opx-bearing primitive gabbros from the East Pacific Riseand the Oman ophiolite
CHATTERJEE, Syantani1∗ ; PYTHON, Marie1 ; AKIZAWA, Norikatsu2
1

Hokkaido University, 2 Kanazawa University, 3 Toulouse University

The Mid Oceanic Ridge Basalt is the tectonic window, which provides the geochemical and petrological character of the lower
oceanic crust and the process associated with the melt-rock interaction and crystallization. The Hess Deep rift is located in
the vicinity of the Galapagos triple junction between the fast spreading East Pacific Rise and the Cocos-Nazca Ridge. Lower
crust of Hess Deep is exposed along the southern slope of the intrarift ridge between 4675 and 4800 m depth and was sampledduring IODP Expedition 345. Primitive troctolites and olivine-rich gabbros are the dominant recovered lithologies and shipboard
data showed a high Mg# whole rock chemistry in concordance with their primitive nature. In a MOR system, olivine is atypical primitive mineral and orthoyroxene (Opx) usually appear late in the crystallisation sequence, when the magma alreadyreached a significant degree of differentiation. In spite Opx is not expected in any primitive lithology, this mineral is commonly
present in Hess Deep gabbros and may be associated with olivine. This curious association of cumulate Opx with olivine andother primitive minerals was also observed at a lower extent in some gabbros from ODP/IODP Hole 1256D, in the upper Hess
Deep crustal section (ODP Hole 894G), and in the crustal section of the Oman ophiolite (Kahwad and Maqsad massifs) where,in particular, Opx-bearing troctolites coexist with clinopyroxene oikocrysts and Opx-bearing troctolites and amphibole-bearing
primitive olivine gabbros.
Three types of Opx textures may be distinguished in Opx-bearing olivine gabbros and troctolites: (1) recrystallised coronaaround olivine, (2) exsolution within clinopyroxene and (3) large prismatic or poikilitic grains. Prismatic or poikilitic Opx arepresent at all level of the gabbroic crust, while exsolutions and corona were observed only in the lower crust. The mineralchemical compositions vary more with the structural level than with the lithological type and (Opx-bearing) olivine gabbros
from Holes 894G, 1256D and from the upper crust of the Oman ophiolite show more differentiated characteristics than the samelithology in the Site 1415 and in the Oman lower crust. Pyroxenes in all samples from the lower crust show a relatively narrow
range of Mg# (from 84 to 86% for Opx and 86 to 89% for Cpx) with large variation of minor elements (Ti, Al, Cr) suggesting astrong influence of melt-rock reaction during their formation. On the other hand, the upper crust samples show a large variationin their ferro-magnesian Mg# (72-87% for Cpx and 70-85% for Opx) together with a relatively weak scatter in minor elements.
Poikilitic Opx are more differentiated and associated with lower Fo-olivine. Magmatic crystallisation were then the dominant
event in the upper crust, so that Opx is likely to be directly crystallised from magma. In contrast, in the lower crust, magmatic
processes were dominated by melt-rock reaction, and the chemical composition and habitus of Opx show that they have been
probably formed by reaction between previously abundant olivine and melt.
Keywords: Hess Deep, IODP exp. 345, Primitive gabbro, Orthopyroxene, Ocean lower crust, Oman ophiolite

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Time:May 27 12:30-12:45

Petrological and chemical evolution of oceanic crust above a spreading axis: an example
from Wadi Mahram, Oman Ophiolite
ITO, Natsumi1 ; PYTHON, Marie1∗ ; ARAI, Shoji2
1

Hokkaido University, 2 Kanazawa University

The Maqsad (or Sumail) massif composes the largest block of the Oman ophiolite. It is characterised by a structural mantle
diapir elongating along an axis orientated 120o to the North. This mantle diapir represents the axis of the former Oman spreading
centre. Previous geological mapping revealed right above the diapir and from the mantle up to the sheeted dyke, a gabbroic
crustal section mainly composed of olivine-rich, probably primitive, lithologies. Troctolites are particularly abundant in thissection, and crop out at any level of the lower and the upper crustal section. We sampled in Wadi Mahram a transect of thistroctolite-rich oceanic crust right above the diapir, thus of the last crust that was formed precisely above the spreading axis.
The lower most section, below 800 m above the mantle-crust transition zone (MCTZ), is mainly composed of a layered dunitetroctolites series characterized by strong variation in their modal composition. In spite of the local presence of olivine-poorgabbroic to anorthositic layers, the section remains dominated rich in olivine-rich facies. Locally, the layered section is cross-cut
by discordant olivine gabbro dykes and veins.
From 800 to 2000 m above MCTZ, a stronger diversity is observed in the lithological facies, which varies in the range ofplagioclase-free dunite-wehrlite to troctolite and olivine-free isotropic gabbro. Dolerite dykes are observed in the uppermost
lithologies whatever their nature (troctolitic, wehrlitic or gabbroic) showing that the sheeted dyke complex roots equally inprimitive and differentiated formations. Orthopyroxene is almost absent from the lower most section but appears and becomesabundant above 800 m above MCTZ, its coronitic to poikilitic texture suggest that it comes partly as the reaction product between
melt and olivine and as the crystallisation product of late stagnant melts.
The mineral chemistry show that the lower section exhibit on average more primitive characteristics than the upper section.
However, the most differentiated lithologies are found below 500 m above the mantle-crust transition zone where they crop out
as dykes cross cutting troctolite layered blocks, showing that differentiated melt may be injected directly from the mantle in
the lower crust. The upper half section is characterized by a great chemical scatter also reflected by the lithological variability
that may be compatible with magma mixing and differentiation. Olivine gabbros are sometimes more primitive than troctolites,
which may overlie them, showing that these two lithologies are not strictly linked by a same parental magma. The variation of
the mineral chemical composition with depth shows that injection of primitive or differentiated magmas occurred at various levelin the crust. The genesis of the crust directly above the spreading axis was driven by complex processes involving successive
injections of primitive and differentiated magmas within a crystallising mush, magma mixing and more or less strong melt-rockreaction.
Keywords: Oman Ophiolite, Gabbro, Oceanic crust genesis, Spreading axis, Ocean ridge, Troctolite

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