2011
● IFREE MC seminar
Place: Meeting Room at 3F, YOKOSUKA HQ
Time: 16:30-18:00 on 22nd, December(Thursday)
Speaker: Alex Nichols（IFREE 4A)
Title:
Melt inclusions: little 'presents' wrapped in crystals, delivered by volcanoes…not necessarily at Christmas
Abstract:
I will talk about two ongoing studies dealing with the geochemistry of glassy silicate melt inclusions hosted by olivine crystals.
The first study, on inclusions from the Higashi-Izu Monegenetic Volcano Field, Izu Peninsula, at the northern tip of the Izu-Bonin Arc, I have talked about before. During this part of the seminar I will remind you of the data and then present a new interpretation to explain the intra- and inter-volcano variability that the inclusions reveal.
In the second part of the seminar, I will present new major and volatile element data from melt inclusions found in lavas recovered from the West Zealandia Seamount, in the southern Mariana Arc. These inclusions are enclosed within olivines that are part of dunite and wehrlite xenoliths and, considering the textures exhibited by the xenoliths, are surprisingly glassy. Crystal chemistry from this volcano was presented by Shukuno-san a few months ago when he combined it with data from neighbouring Zealandia Bank to provide evidence for cross-arc variability in the erupted magmas. I will use the inclusions to show that the two types of xenolith are from different depths in the crust beneath West Zealandia.
● IFREE MC seminar
Place: Meeting Room at 3F, YOKOSUKA HQ
Time: 16:30-18:00 on 24th, November(Thursday)
Speaker: CHANG QING（IFREE 4B)
Title:
Development of precise and high spatial resolution in-situ Pb isotope ratio analysis of geological glasses by laser ablation-multiple ion counter-ICP-MS (LA-MIC-ICP-MS)
Abstract:
The aim of this work is to establish in-situ Pb isotopic ratio analysis for geological glass samples with limited size and low Pb content. Such samples include basaltic melt inclusions in olivine ranging from a few tens to one hundred micrometers in size with Pb contents of ~10 ppm. For this purpose we investigated the capability of laser ablation sampling (LA) coupled to an ICP-MS equipped with multiple Faraday cup (MFC) and multiple ion counter (MIC) detection system. Pb isotope ratios of 207Pb/206Pb and 208Pb/206Pb can be precisely determined by applying MFC detector array for samples with high Pb (> 40ppm) at large crater size (>100 micrometer in diameter), but this approach is not applicable to small melt inclusions. In contrast to ion current measurement by MFC, direct ion counting using MIC provides~100 times better signal-to-noise ratio allowing measurement of Pb isotopes with lower intensities. However, use of MIC detection system is a big challenge due to its poor performance in both linearity and stability. We found that mass bias factor obtained by MIC is signal intensity dependent due to different responses between individual MICs. Dead time correction improves linearity, but time-dependent decay of the small MIC is rapid and enigmatic. Standard bracketing method using controlled ion intensity between standard and sample can correct for both the linearity and time dependent decay of MICs. However, control of signal intensity is not always easy for sample with unknown Pb content, especially for melt inclusion with limited sample volume. To overcome this problem, we developed a dual intensity standard bracketing method.
Different laser repetition rate was utilized for measuring bracketing standard glass in order to cover expected intensity range of Pb signal of unknown sample. Response of each MIC is calibrated by using linear regression of a set of different standard signal intensities and isotope abundance in the standard. The regression curves determined for individual MICs are then used to determine the isotope abundance of unknown samples. This method was tested by analyzing Pb isotopes of well characterized reference material BHVO-2G (1.7 ppm Pb) using the 200 nm UV-femtosecond-LA coupled to a sensitivity enhanced MIC-MC-ICP-MS installed at IFREE. The obtained analytical precision and accuracy of 207Pb/206Pb and 208Pb/206Pb are 0.3–0.4% (2sd) for a spatial resolution of 20 micrometer.
The precision and accuracy are, so far, comparable to other geochemical laboratories, but spatial resolution is well improved.
● IFREE MC seminar
Place: Meeting Room at 3F, YOKOSUKA HQ
Time: 16:30-18:00 on 10th, November(Thursday)
Speaker: Yoshiyuki Tatsumi (IFREE4)
Title:
Why is this planet to be the Earth?
Abstract:
One characteristic feature of the Earth is the bimodal height distribution at the surface. This is caused by the difference both in density and thickness of the oceanic and continental crusts. Two types of crust on the Earth are created at convergent and divergent plate boundaries, i.e., via. plate tectonics. The essential cause of operation of plate tectonics is the temperature difference within the mantle, which triggers mantle convection. The upper thermal boundary layer of this convection corresponds to the lithospheric plate, but behaves as a stagnant-lid, i.e., the plate should not move. The presence of liquid water at the surface, on the other hand, strongly reduces the yielding stress and could cause the fracture within the stagnant-lid, triggering the plate motion. Parameters controlling the presence and absence of liquid water on the terrestrial planets are: the distance from the Sun and the mass of a planet.
A conclusion of this consideration would be that the Earth is a shore planet because of the presence of the ocean, which seems to be a conclusion of a Zen dialog.
● IFREE MC seminar
Place: Meeting Room at 3F, YOKOSUKA HQ
Time: 16:30-18:00 on 27th, October(Thursday)
Speaker: Hiroshi Shukuno (IFREE4A)
Title:
Petrochemical cross-arc variations of the Pagan and Diamante cross-arc volcanoes and the Sarigan-Zealandia multi-volcano complex, the southern Mariana arc
Abstract:
ROV Hyper-Dolphin dives in the Southern Mariana region were carried out in NT09-08 and NT10-12 cruises (R/V Natsushima).
We focused on the submarine volcanoes within the Pagan-Daon cross-arc chain, the Diamante cross-arc chain and the Sarigan-Zealandia Bank multi-volcano complex. The newly obtained petrologic and geologic data, together with data from previous studies on the IBM arc, will provide insights into understanding processes occurring in the subduction factory. Here preliminary results of our recent mineral and bulk analyses will be presented. We have collected relatively undifferentiated basaltic samples in the submarine parts of the cross-arc chain volcanoes. Cross-arc chemical variations were revealed about several trace element ratios such as Zr/Y and Ba/Zr, probably indicating that effect of slab-derived component and melting proceeds could be different between volcanic front and rear-arc side.
Mineral chemistries, the Fo-An relation of plagioclase and olivine and the Al-Ti relation of spinel, also exhibit cross-arc variations. Magmatic front lavas show arc signature, whereas rear arc side lavas show MORB-like signature. However rear arc side lavas have arc signature on oxygen fugacity calculated by spinel inclusions and olivine phenocrysts, indicating that rear arc side lavas probably are reflect by MORB source affected by subduction-related magmas or fluids, back-arc basin basalts source. These cross-arc variations resemble those of the Guguan cross-arc chain volcanoes (Stern et al., 2006) and might be explained by sequential melting in wedge mantle as proposed by Stern et al. (2003).
● IFREE MC seminar
Place: Meeting Room at 3F, YOKOSUKA HQ
Time: 16:30-18:00 on 13th, October(Thursday)
Speaker: Takeshi Hanyu(IFREE4B)
Title:
Along- and across-arc geochemical variation in the Sangihe
arc: Mode of element transport from subducted slab to volcanic front and backarc
Abstract:
The Sangihe arc is a 500 km-long volcanic chain between Sulawesi in Indonesia and Mindanao in Philippines. It is an ordinary arc with the convergent rate and dip angle of the subducting slab within the ranges defined by many oceanic arcs. However, the Sangihe arc is situated in unique tectonic settings in that (i) backarc opening has not occurred and (ii) arc-arc collision is ongoing in the northern arc. Since the mantle wedge depletion associated with backarc opening would be minimal, the lavas in the Sangihe arc should provide primary geochemical record of the slab-derived components in response to compositions and P-T conditions of the subducted slab. The compositional diversity for lavas at volcanic front (VF) and backarc (BA) reflects various influx of slab-derived component to the mantle wedge.
The slab-derived component exhibit systematic along-arc geochemical variation. Although the slab-derived component beneath VF is isotopically uniform, that in the northern arc is mildly enriched in fluid-immobile elements (e.g., Th) relative to that in the southern arc. This is best explained by along-arc temperature variation in the sub-arc mantle; as fluid is hotter, apparently fluid-immobile elements become more partitioned into fluid. Moreover, we propose that isotopically uniform fluid source for VF is rehydrated peridotite layer immediately above the subducting slab. This layer would have acquired fluid and supercritical liquid liberated from AOC and sediment at any depth in the forearc region, and therefore it could buffer potential compositional heterogeneity in the slab flux. On the other hand, the across-arc geochemical variation demonstrates that the proportion of sediment in the slab-derived flux is various and tends to be lower for BA compared to VF.
Significant enrichment in fluid-immobile elements for BA suggests that not only fluid from rehydrated peridotite but also slab-derived melt or supercritical liquid play an essential role in generating BA magmas.
● IFREE MC seminar
Place: Meeting Room at 3F, YOKOSUKA HQ
Time: 16:30-18:30 on 22nd, September (Thursday)
Speaker: Hiroshi Kawabata
Title:
Geochemically distinct magmas erupted in en echelon volcanic chains in Samoa
Abstract:
The bathymetry of the eastern Samoan archipelago is characterized by two en echelon ridges. These ridges, called the VAI and MALU volcanic trends, are contemporaneously produced by magma eruption from two major eruptive centers after 1.5 Ma. Workman et al. (2004) proposed that lavas from these ridges are isotopically distinct, and thus spatially distinct geochemical distribution observed in the eastern Samoa has been considered as an equivalent of the Hawaiian "Loa" and "Kea" trends. However, this idea should be revised, because our new geochemical data obtained from the MALU volcanic trend (Tutuila Island) plot overlapped with the field defined by the VAI trend in common Pb isotope space. We propose that the variety of Pb isotope ratios primarily reflect the difference in the contribution of the three mantle components of depleted mantle (DM), less degassed mantle (FOZO), and enrich mantle (EM2). Lavas from the VAI trend mainly sampled FOZO mantle, whereas lavas from the MALU trend and older Samoan islands (>1.5 Ma) sampled
EM2 and DM mantles in addition to FOZO mantle. The observed geochemical variations and the systematics of isotopic array are explained by the progressive magma mixing during ascent of a heterogeneous mantle plume.
● IFREE MC seminar
Place: Meeting Room at 3F, YOKOSUKA HQ
Time: 16:30-18:30 on 1st, September (Thursday)
Speaker: Tatsuo Nozaki
Title:
Re-Os ages of Besshi-type massive sulfide deposits associated with in-situ basalt as a new age constraint for the ridge subduction
Abstract:
We report three Re-Os isochron ages for the Makimine, Minenosawa and Shimokawa (Kyusyu, Tokai and Hokkaido districts, Japan, respectively) Besshi-type massive sulfide deposits in the Northern Shimanto Belt. These Besshi-type deposits are characterized by close association with an in-situ basalt whose geochemical composition is similar to those of mid-ocean ridge basalts. Terrigenous clastic materials such as sandstone and mudstone directly overlie massive sulfide layers, indicating that the Makimine, Minenosawa and Shimokawa deposits were formed in the shelf sea, not so far from the continental crust.
The Re-Os ages obtained for the Makimine, Minenosawa and Shimokawa deposits are ca. 89 Ma, 68 Ma and 48 Ma, respectively. Based on the stratigraphic and geochemical features of three Besshi-type deposits, we interpret these Re-Os ages as a timing of sulfide deposition on a paleo-seafloor when the Kula-Ridge subducted to the paleo-Japanese island arc since the Late Cretaceous. The Re-Os age of the Makimine deposit (89 Ma) is generally consistent with the timing of the ridge subduction determined by microfossils in the sedimentary rocks. The plate motion model has documented the northeastward migration of ridge subduction occurred at the Late Cretaceous Japanese island arc. Considering that the distance between three Besshi-type deposits, the northward migration speed of the Kula ridge is estimated to be 3.0 cm/yr from 89 to 68 Ma and 4.8 cm/yr from 68 to 48 Ma, respectively. This temporal speed variation of northward movement of the Kula Ridge was attributable to the different direction of the plate subduction, which is very consistent with the model estimation.
● IFREE MC seminar
Place: Meeting Room at 3F, YOKOSUKA HQ
Time: 16:30-18:30 on 25th, August (Thursday)
Speaker: Shigeaki Ono
Title:
Spin transitions of iron and iron alloys
Abstract:
The properties of the major constituents of the Earth's inner core, i.e., iron and its alloys, have long been of great interest to geophysicists. The spin and magnetic state of iron has a major influence on the physical properties of iron and its alloys, including the relative stability of their polymorphs. We made use of high-pressure experiments and quantum mechanics computations using the first-principles method, to investigate the physical properties of iron and its alloys at high pressures and high temperatures. The structural variations of iron and its alloys on the spin transition were confirmed by the experiments. These structural variations were in good agreement with those predicted by the quantum mechanics computations. As the spin state of iron atom in minerals could be predicted accurately using our calculations, we also calculated elastic properties of different spin states in minerals. For example, our calculations predict that the bulk modulus of the high-spin state of iron is ~10% less than that of the low-spin state. Such change in elastic property on the spin transition should be considered to investigate the dynamic of the Earth's and planets' cores.
● IFREE MC seminar
Place: Meeting Room at 3F, YOKOSUKA HQ
Time: 16:30-18:30 on 22nd, August (Monday)
Speaker: Pete Tollan (Durham University, UK)
Title:
Cumulate and mantle xenoliths: an alternative sampling approach to island arc investigation
Abstract:
Sampling of arc magmatic systems is overwhelmingly dominated by lavas. Whilst lavas can provide essential insights into our understanding of subduction zone processes, they carry an inherent problem. Magma erupted at the surface represents the integration of a number of different melting and differentiation processes, which reduce the resolution with which any particular process or stage can be investigated.
In this seminar, I will describe my study of mantle and cumulate xenoliths recovered from an arc environment.
Studying xenoliths allows particular stages of magmatic evolution to be investigated in more detail than is possible with lavas. In the first part of the seminar, I will detail my work at JAMSTEC, measuring the water content of nominally anhydrous minerals picked from mantle xenoliths from the Bismarck microplate. In the second part, I will present my recently published work concerning the petrology and geochemistry of a diverse set of cumulate xenoliths from the Lesser Antilles island arc.
● IFREE MC seminar
Place: Meeting Room at 3F, YOKOSUKA HQ
Time: 16:30-18:30 on 4th, August (Thursday)
Speaker: Tetsuya Sakuyama
Title:
Melting of the stagnant slab in the mantle transition zone
Abstract:
Cold oceanic plates, which include igneous and sediment layers, sink along subduction zones. The layers dehydrate and melt as they undergo subduction, which feeds slab components to arc magmas. After it has been subducted, the remaining slab stagnates in the mantle transition zone (at depths of 400–660 km), where the minerals undergo pressure-induced transformations, and the resulting density turnover prevents further slab penetration and instead it stagnates. Experimental data predict that the stagnant slab melts due to conductive heating from the ambient mantle and then the residual slab becomes dense and can sink deeper.
Here we present evidence that melts from the igneous layer in the stagnant Pacific plate slab have contributed to the source composition of basalts erupted in eastern China. Fe-rich Si-poor (CO2-rich) basalts only occur above the leading edge of the stagnant slab. Their source has Nd-Hf isotope compositions akin to the igneous layer in the Pacific slab. Extremely low Rb and Pb contents suggest that this source has been modified during subduction. Together these characteristics lead us to conclude that the source of these basalts includes a contribution from melts derived from the dehydration of carbonate-bearing oceanic crust at the leading edge of the stagnant slab, and thus the basalts provide evidence of melting of the stagnant slab.
● IFREE MC seminar
Place: Meeting Room at 3F, YOKOSUKA HQ
Time: 16:30-18:30 on 21st, July (Thursday)
Speaker: Teruhiko Kashiwabara
Title:
Geochemistry of molybdenum and tungsten at solid/water interface: its implication on their concentrations and isotopic compositions in seawater
Abstract:
Geochemistry of Mo and W at solid/water interface is important because it is closely associated with evolution of atmosphere, in particular for oxygenation of the atmosphere and possibly with evolution of life in the Earth history. In this study, distribution and isotopic fractionation of Mo and W between seawater and ferromanganese oxides are focused. These processes may have great impacts on concentrations and isotopic compositions of Mo and W in modern oxic seawater. However, basic chemistry of Mo and W at the surface of ferromanganese oxides is still not clear. The purpose of this study is to clarify molecular mechanisms of distribution and isotopic fractionation of Mo and W. In this seminar, I will introduce a part of my study about Mo.
Molybdenum shows large mass-dependent isotopic fractionation during adsorption on ferromanganese oxides, which is responsible for isotopic composition of Mo in modern oxic seawater. Due to this fractionation process, isotopic ratio of Mo in marine environment is expected to be used as a paleocean redox proxy. To reveal the molecular-scale mechanism of this isotopic fractionation, we investigated surface complex structures of Mo on various synthetic Fe/Mn (oxyhydr)oxides and natural ferromanganese oxides, and compared them with previously-reported isotopic fractionation.
Adsorption experiments of Mo were conducted under various conditions using synthesized ferrihydrite and d-MnO2 as adsorbents. Local structures of Mo adsorbed on a series of synthetic Fe/Mn (oxyhydr)oxides and hydrogenetic ferromanganese nodules were investigated by XAFS analyses. These measurements were performed at BL01B1 in SPring-8 and at BL-9A in Photon Factory, Japan.
Molybdenum L3-edge XANES and K-edge EXAFS revealed that Mo species adsorbed on the surface of ferrihydrite was a tetrahedrally coordinated outer-sphere complex, while that on δ-MnO2 was an octahedrally coordinated inner-sphere complex. Additionally, it was also revealed that δ-MnO2 was the dominant host phase of Mo in the hydrogenetic ferromanganese nodules from the comparison of their XAFS spectra. Previous studies reported that lighter isotopes of Mo were preferentially incorporated into ferromanganese oxides from seawater. This fractionation can be explained based on the structural difference between tetrahedral MoO42– (=a major species in seawater) and the octahedral species adsorbed on the Mn oxide phase in ferromanganese nodules. In contrast, little change in Mo local structures during its adsorption onto ferrihydrite also suggested the smaller fractionation of Mo isotopes in the presence of Fe hydroxides without Mn oxides. These facts imply that the Mo isotopic composition in ancient marine sediments can distinguish redox boundaries of Fe2+/Fe(OH)3 and of the more oxic Mn2+/MnO2.
● IFREE MC seminar
Place: Meeting Room at 3F, YOKOSUKA HQ
Time: 16:30-18:30 on 7th, July (Thursday)
Speaker: Yoshihiko Tamura
Title:
Primitive basalts from Pagan volcano, Mariana arc
Abstract:
I'd like to present preliminary results of XRF studies of Pagan and Daon
volcanoes, which are situated in latitude ~18 degree north in the Mariana
arc. We have recovered several basalts (48-50 wt % SiO2) with 7-11 wt % MgO
and Mg-number of 60-70 from Pagan volcano and basalts (50-52 wt % SiO2) with
8-10 wt % MgO and Mg-number of 60-70, from Daon volcano. Basalts from both
volcanoes are primitive, indicating little fractionation. These rocks are
compared with the COB and POB of the NW Rota-1 volcanoes (Tamura et al.,
2011) and with the magnesian Mariana trough basalts from 17 degrees north
section recovered by Fujiwara et al (2008).
---------------------------------------------
Pagan Island is one of the active volcanoes in the Central Island Province
of the Mariana magmatic arc. Pagan is elongate roughly NE-SW and the
southern end of the island is inactive, steep, and eroded. The northern end
is active and was producing small clouds of steam and ash during operations
for Hyper-Dolphin dives of NT10-12 in 2010. The northeastern slopes of Pagan
show a NNE trending rift zone with several small parasitic cones. This rift
and the associated cones were the targets for HPD#1147, in water depths of
2000-1500 m, which were found to mainly consist of basaltic pillow lavas.
The south end of the Pagan Island is inactive, with steep eroded slopes. As
with dive HPD#1147,
HPD#1148 was planned to investigate small parasitic cones and a ridge to the
southwest of Pagan, east of the cross-chain Daon Seamount.
Daon seamount is a "behind-the-magmatic-front" (cross-chain or rear-arc)
volcano associated with Pagan. No volcanic or hydrothermal activity is
known. Daon's summit rises from a base ~3000-3200 m to a summit that lies
less than 900 m b.s.l. Bloomer et al. (1989) calculate a volume of 150 km3,
~10% of the size of the largest Mariana volcanoes such as Pagan and Agrigan.
Daon merges across a ~2500 m deep saddle with the SW extension of S. Pagan.
Daon has an unusual morphology, with many ridges radiating from it, which
may reflect the presence of radiating dikes. No studies are reported for
Daon, although Bloomer et al. (1989) dredged the SW part of the edifice,
recovering dacites with phenocrysts of plagioclase, hornblende,
clinopyroxene, and orthopyroxene.
● IFREE MC seminar
Place: Meeting Room at 3F, YOKOSUKA HQ
Time: 16:30-18:30 on 30th, June (Thursday)
Speaker: Wei Tian (Associate Prof., Peking Univ.)
Title:
Spatio-temporal Distribution and Chemical Structure of the Tarim Flood Basalt, an Early Permian Flood Basalt Province from Northwest China
Abstract:
The Tarim Flood Basalt is a newly identified continental flood basalt
province (CFBP) that occurs across the Tarim plate, northwest China,
including a suite of voluminous basalt subcrops mostly under a thick
succession of sedimentary layers. Based on petroleum industrial drilling,
down-hole physical logging data and high quality seismological data, the
minimum areal extent of the extrusive part of the Tarim Flood Basalt has
been estimated to be 265,600 km2. If taking the intrusive dolerite dykes and
layered intrusions into account, the province extends across an area of at
least 0.34 Mkm2. The basaltic lava flow is thickest in northwestern Tarim
(>3000 m). The original pre-erosion eruptive volume of the province is
difficult to estimate. However it is very likely to have exceeded 0.4 Mkm3
(based on an areal extent of >0.34 Mkm2 and an average thickness of 1.2 km
calculated from 163 drilling wells) and certainly in excess of the minimum
of 0.1 Mkm3 suggested by Bryan and Ernst (2008) for a LIP.
Basaltic magma activities from northern, central and western Tarim have been
dated by zircon U-Pb or baddeleyite Pb-Pb method. The dating results are in
a time span of 291±1 Ma to 283±2 Ma, certainly older than the suggested
LIP age (~275 Ma) based mainly on felsic intrusion dating (eg. Zhang et al,
2010). It is suggested in this study that the main pulse of the basaltic
activity occurs at ~290Ma.
Nearly all the eruptive products of the Tarim CFB are olivine tholeiitic
(ol- and hy- norm) to quartz tholeiitic (qtz- norm), which are typical for
all the Phanerozoic CFBs on the Earth. High Nb/La ratios (0.91-1.34) and low
initial 187Os/188Os ratios (low to 0.1308) of basalts and picrites from
northern Tarim attest that they are mantle-derived magma with negligible
crustal contamination and strongly suggest that they were very likely
derived from the plume center. Relative low Nb/La ratios and high initial
187Os/188Os ratios of basalts from central and southwestern Tarim suggest
that they may have experienced more crustal contamination or in the distal
margin of the mantle plume.
● IFREE MC seminar
Place: Meeting Room at 3F, YOKOSUKA HQ
Time: 16:30-18:00 on 9th, June (Thursday)
Speaker: Takashi Sano(National Museum of Nature and Science,Tokyo)
Title:
Magma variety and stratigraphy of Ori Massif, Shatsky Rise
Abstract:
In order to examine magma genesis and evolution of large
igneous provinces (LIPs), Integrated Ocean Drilling Program
Expedition 324 cored ~470 m of igneous basement at four
holes on Shatsky Rise, located ~1500 km east of Japan. The
four basement holes are distributed on three volcanic
massifs of Shatsky Rise (they are called Tamu, Ori, and
Shirshov massifs from southwest to northeast); two holes on
Ori Massif (Hole U1349A and U1350A) and one each hole on
Shirshov (Hole U1346A) and Tamu (Hole U1347A) massifs.
Geochemistry of ~130 fresh glass samples from the basement
holes has indicated that Shatsky magmas are divided into
three groups (Shimizu et al., AGU 2010 Fall Meeting Abstract
U51A-0019); normal, low-Ti, and high-Nb (K) groups. Chemical
compositions of the normal group are similar to those of
normal mid-ocean ridge basalt (N-MORB), but slightly
enriched (e.g., higher La/Yb than N-MORB). The fresh glasses
and fresh whole rocks (LOI <1 wt %) of the normal group show
an obvious chemical trend, which can be explained by a
fractional crystallization of three phenocryst phases
(olivine, plagioclase and augite) under low pressure (<<200
MPa) and nearly dry (water, 0.2-0.6 wt %) conditions. The
crystallization temperatures are estimated to be 1130-1210
degree C. The low-Ti group has slightly lower Ti, Fe, Mn, V,
Sr and Zr concentrations at the same MgO. The high-Nb
basalts are characterized by their distinctly higher
concentrations of incompatible trace elements (e.g., K, Nb,
REEs) than the normal basalts, indicating that they were
likely affected by enriched components. Examinations of the
rock chemistry show that the three groups are simply
discriminated by Nb/Ti ratio; normal group has low Nb/Ti
(less than 0.00058), low-Ti group has middle Nb/Ti
(0.00058-0.00071), high-Nb has high Nb/Ti (more than
0.00071). The discrimination method has strong merit that
can be applied to altered rocks because both Ti and Nb are
highly resistant to alteration processes.
The longest basement section (~173 m) among the Expedition
324 holes was recovered from Hole U1350A on flank site of
Ori Massif. All three geochemical groups are present in the
hole. On the basis of the basement morphology, the thick
section is divided into three units; pillow and massive
flows (unit II), hyaloclastite and breccias (unit III), and
pillow lavas set in a matrix intercalated micritic limestone
(unit IV). Unit II is subdivided into three subunits; upper
massive-flow dominant unit IIa, transitional unit IIb, and
lower pillow dominant unit IIc. The Nb/Ti of ~100 basement
basalts (fresh glasses, lava flows and breccias) indicates
that the normal group is the most abundant and occupies 64%
of Hole 1350A. The high-Nb group is the second abundant
(28%) and constitutes middle portion of unit IIa and most
part of unit IIb. The low-Ti group (8% of the basement
section) often appears adjacent to the high-Nb, implying
that the origins of the two groups are closely related. One
important note is that unit IIb consists only high-Nb and
low-Ti groups (normal group basalt is absent). The
geochemical examination shows that about 1/3 of the basement
section is composed of non-normal basalts (high-Nb and
low-Ti groups), which may indicate that the involvement of
enriched components are important factors to discuss magma
genesis of Shatsky Rise.
● IFREE MC seminar
Place: Meeting Room at 3F, YOKOSUKA HQ
Time: 16:30-18:00 on 2th, June (Thursday)
Speaker: Toshihiro Suzuki
Title:Pressure effect on element partitioning between minerals and
silicate melt
Abstract:
It is known that partitioning behaviors of elements between
minerals and silicate melt are generally controlled by ionic
radius, cation charge and crystal structure of minerals.
When partition coefficients are plotted against ionic radius
(PC-IR diagram), partition coefficients with the same charge
vary smoothly with ionic radius. In recent years, these
parabolic curves appear in the PC-IR diagram are explained
by Lattice Strain Model (Blundy and Wood, 1994). Scince the
peak positions of these curves represent the "optimum size"
of the cation sites in the crystal structure, the peak
positions corresponding to the same cation site must be
identical. Indeed, the curves described in PC-IR diagrams
show the similar peak positions in many cases. However,
PC-IR diagrams obtained from high pressure minerals, such as
Majorite – melt system at 25 GPa (Corgne and Wood, 2004),
show different aspect. The peak positions of the curves
change with cation charge. Such observation can be caused by
the difference in compressibility of cations. Thus we
performed high pressure melting experiments on alkali basalt
and "Ca-rich material" up to 20GPa, and observed systematic
change in the partition curves of Garnet-melt system and
Merwinite (MgCa3Si2O8) -melt system. As a result, we found
that an obvious change in the peak position with pressure is
found in monovalent cation curve on PC-IR diagram, while the
peak position of trivalent cation curve is almost the same
in all pressure conditions. Since it is believed that the
compressibility of monovalent cations is larger than other
cations, the observed peak shift in monovalent cation curve
may be explained by the difference of compressibility among
cations.
● IFREE MC seminar
Place: Meeting Room at 3F, YOKOSUKA HQ
Time: 16:30-18:00 on 12th, May (Thursday)
Speaker: Junichi Kimura
Title:
What stays in the slab and what returns to the surface? A geochemical mass balance model perspective
Abstract:
We have developed the Arc Basalt Simulator (ABS), a quantitative forward model to calculate the mass balance of slab dehydration and melting, and slab fluid/melt-fluxed mantle melting, in order to quantitatively evaluate magma genesis beneath arcs. ABS models can reproduce magma compositions in many arcs.
The model suggests that the slab-derived component at volcanic fronts (VF) is mostly generated by dehydration, but successful models for most VF and all rear arc (RA) magmas also require the slab to melt. The compositions of slab fluids and melts are controlled primarily by the breakdown of amphibole and lawsonite beneath the VF and by the breakdown of phengite beneath the RA in addition to residual eclogite mineral phases including garnet, clinopyroxene, and quartz.
In the model, about 78-98% of relatively fluid-immobile elements including Nd and Hf in the arc lavas come from mantle peridotite. However, most liquid-mobile elements come from the slab. Modeled residual peridotite compositions are similar to those in some supra-subduction zone ophiolites and mantle xenoliths, providing constraints on reactions in the mantle wedge.
Altered oceanic crust (AOC) and sediment in the residual slab are modified by the subtraction of melt- and fluid-mobile elements. Unmodified AOC potentially becomes the EM I mantle component after 1 Ga, whereas melted AOC can have extremely fractionated U-Pb and become the HIMU source after 1-2 Ga. Element re-distribution beneath arcs can form the recycled materials that have been detected in ocean island basalts. However, there is a question on the role of the deep mantle processes that further modify the subducted slab. The question still remains open.
● IFREE MC seminar
Place: Seminar Room, the Annex 1F, Yokosuka HQ
Time: 16:00-17:30 on 27th, April (Wednesday)
Speaker: Yusuke Sawaki
Title:
Surface environmental change in the latest Proterozoic;
evidences from isotope and elemental analyses of sedimenta ry rocks
Abstract:
Ediacaran (latest Proterozoic) to Cambrian period was one of the most important intervals through the Earth history,
because life on the Earth drastically grew in size and diversified.
However, the scarcity of well-preserved outcrops of Ediac aran and Cambrian rocks still leaves ambiguity in decoding ambient surface environmental changes and biological evolution.
The Ediacaran to Cambrian strata in South China are almost continuously exposed,
comprise mainly carbonate rocks with subo rdinate black shales and sandstones,
and they contain many fossils,
suitable for study of environmental and biological chang es in the Ediacaran and Cambrian.
Our group (Maruyama laboratory at Tokyo Institute of Technology) conducted drilling at three sites in the Three Gorges area to obtain continuous,
fresh samples without surface alteration and oxidation.
We focus on unraveling causes of evolution of life in the latest Proterozoic.
In order to reveal links between surface environmental changes and life evolution comprehensively,
we are systematically analyzing several isotopes ratios (δ13Ccarb, δ13Corg, δ18O, 87Sr/86Sr, δ15Norg and Mo) and elemental concentrations (Mn, Fe, P and Ce) in carbonate more detail than previous works.
In this talk, I will present a brief summary of works in Maruyama laboratory and one of my doctoral studies.
The weathering influx from continents is thought to be a major influence on the seawater compositional change and on biolo gical evolution.
Its flux can be estimated from the 87Sr/86Sr ratio of carbonate rocks.
I analyzed 87Sr/86Sr ratios of the fresh carbonate rocks,
selected on the basis of microscopic observations and the geochemical signatures of Sr contents,
Mn/Sr and Rb/Sr ratios, and δ18O values,
with a multiple collector-inductively coupled plasma-mass spectrometer (MC-ICP-MS@ Kyoto university).
Chemostratigraphy of the 87Sr/86Sr ratios of the drilled core samples displays two step increases and two sho rt-term positive anomalies.
Although it is difficult to compare accurately, it seems that these changes correlate with some of the stage of life evolution.
This indicates that nutrients supplied from this enhanced continental erosion have influenced surface environment.
● IFREE MC seminar
Place: Meeting Room at 3F, YOKOSUKA HQ
Time: 16:30-18:00 on 14th, April (Thursday)
Speaker: Kenichiro Tani
Title:Unraveling the continental basement beneath the Izu-Bonin
arc: Preliminary results of the YK10-04 cruise and aim of the YK11-08 cruise
Abstract:
The Izu-Bonin arc has been regarded as a typical intra-oceanic arc,
where the oceanic Pacific plate is subducting beneath the Philippine Sea plate.
The current Philippine Sea plate is a complex of active and inactive arcs and
back-arc basins.
It is dominated by oceanic crust forming three large back-arc
basins; Shikoku, Parece Vela,
and West Philippine Basins, making the present
Philippine Sea plate look like an “oceanic” plate.
However, all of these back-arc basins were formed after the inception of subduction
at Izu-Bonin arc,
which began at ~52 Ma. Little is known about the proto-Philippine Sea
plate,
which existed as a counterpart to the Pacific plate during subduction initiation and
before the formation of back-arc basins.
To understand the crustal structures of the proto-Philippine Sea plate,
we have conducted SHINKAI6500 and Deep-Tow surveys during the YK10-04 cruise
at the Amami Plateau, Daito Ridge, and Okidaito Ridge (ADO) region.
The ADO region comprises the current northwestern Philippine Sea plate and
considered to represent the remnants of the proto-Philippine Sea plate.
The submersible observations and rock sampling conducted during the YK10-04 cruise and
the preliminary petrographycal and geochronological studies revealed that
ADO region dominantly expose deep crustal section of Jurassic to Cretaceous metamorphic
and plutonic rocks, indicating that a part of the proto-Philippine Sea plate is composed of older,
non-oceanic, possibly continental, crust.
This suggests that subduction of the Izu-Bonin arc initiated at the continental margin,
and later acquired "intra-oceanic"-like setting through formation of the backarc basins.
The detrital zircon studies conducted at the northern Izu-Bonin forearc,
counterpart of the ADO region, indicate that such continental basement
may even exist beneath the present Izu-Bonin arc.
In this talk, I will present a summary of YK10-04 cruise, introducing the general geology of the ADO
region, along with the preliminary results of the zircon U/Pb geochronology of the recovered samples.
I will also introduce the scope of the YK11-08 cruise, scheduled in September 2011,
aiming to find the pre-existing continental basement underneath the present northern Izu-Bonin arc.
● IFREE MC seminar
Place: Meeting Room at 3F, YOKOSUKA HQ
Time: 16:30-18:00 on 10th, March (Thursday)
Speaker: Alex Nichols (IFREE 3A) and the Expedition 330Scientific Party
Title: IODP Expedition 330 to the Louisville Seamount Trail a tale of mantle plumes, volcanic breccia, free-fall funnels and (controlled) explosions - an igneous petrologist's perspective
Abstract:
The Louisville Seamount Trail is a 4,300 km chain of
seamounts located north-east and east of New Zealand in the
south-western Pacific Ocean. It is one of the longest
seamount trails in the Pacific Ocean and is generally
inferred to have been built as the Pacific Plate moved over
a mantle plume located at the south-eastern end of the
trail, 3,700 km from New Zealand. The seamount trail extends
up to the Kermadec Arc where the oldest seamount, Osbourn
Seamount, 77-79 Ma, is currently being subducted into the
trench. It is the southern hemisphere counterpart of the
much better studied Hawaiian-Emperor Seamount Chain. To date
very few geophysical and sampling surveys have been
conducted along the Louisville Seamount Trail. The recent
IODP Expedition 330, which ran from December 2010 to
February 2011 using the D/V JOIDES Resolution, sought to
investigate the Louisville Seamount Trail further, by
drilling several hundred metres into seamounts of varying
age, from 80 to 50 Ma.
The primary objective of IODP Expedition 330 is to
investigate whether the Louisville mantle plume has moved
during its history. Drilling, during ODP 197, of some of the
Emperor Seamounts has provided evidence to suggest that
rather than being fixed, the Hawaiian mantle plume has moved
~15° south during the last 80 Ma. The geodynamic models of
mantle flow patterns that reproduce this shift for the
Hawaiian mantle plume predict that the Louisville mantle
plume has essentially remained in the same place, and is
thus independent of the Hawaiian mantle plume. The second
major objective is to examine the geochemical and
volcanological evolution of the seamount trail. The rocks
recovered in the only previous three dredging campaigns have
revealed the erupted magmas to be relatively isotopically
homogeneous, alkali basalts. Notably there is no evidence of
the tholeiitic shield-building stage that dominates Hawaiian
volcanoes. IODP Expedition 330 will provide more material to
constrain further the magmatic evolution and melting
processes in the older Louisville volcanoes.
As one of the igneous petrologists on IODP Expedition 330,
my job was to describe the core as it was recovered. In
total we drilled 1,113 metres into the seafloor, recovering
806 metres of mostly volcanic rocks, at six sites on five
different seamounts. The proportion of the drilled rock that
we recovered was a record for igneous rock drilling over the
history of scientific ocean drilling. In this talk I will
present a summary of the rocks we recovered, focusing on the
igneous rocks, their geochemistry (based on the preliminary
shipboard measurements), and the initial findings arising
from the expedition. I will also share with you some of the
adventures, surprises and difficulties we had over the 2 months.
● IFREE MC seminar
Place: Meeting Room at 3F, YOKOSUKA HQ
Time: 16:30-18:00 on 24th, February (Thursday)
Speaker: Q. Chang (IFREE 3B)
Title: Standalone analysis of major and trace elements in geological samples by an ultraviolet femtosecond laser ablation coupled to sector field ICP-MS (UV-fsLA-SF-ICP-MS)
Abstract:
We present a new analytical technique on in situ
multi-element analysis by a state-of-the-art ultraviolet
femtosecond laser ablation (UV-fsLA) coupled to a sector
field (SF)-ICP-MS. Using this technique, a full set of
major, minor, and trace elements can be determined
simultaneously. Unlike any other conventional techniques,
which need to determine at least one major element (e.g. Ca
or Si) externally by other techniques for correction of
ablation efficiency, this technique can determine all the
elements at once from one single ablation spot. The ablation
system consists of a combination of a Ti-sapphire 200/266 nm
UV femto-second laser ablation (UV-fsLA) system, coupled to
a high precision sample stage and to a sector field
(SF)-ICP-MS. This UV-fsLA-SF-ICP-MS system has high
sensitivity (> 1.2 Gcps/ppm at 115In by solution analysis)
and wide dynamic range (greater than 109 cps) of ion
detection, allowing simultaneous determination of the
elements from tens of % to sub-ng/g levels. A stable
ablation signal profile over several minutes can be obtained
by optimizing the UV-fsLA system using a rotation raster
technique by the high precision stage controller.
Matrix-matched calibration strategy is still required here
using sample standard bracketing. Although UV-fsLA provides
greater sampling efficiency than any other LA systems, the
level differs between different samples. Thus, instead of
internal standardization, ablation efficiency of each
ablated spot is corrected by 100% normalization of the sum
of the major oxides (in term of wt.%, SiO2, TiO2, Al2O3,
FeO, MnO, MgO, CaO, Na2O, K2O, and P2O5) obtained directly
from calculations against the bracketing standard. Relative
ablation efficiency against the standard was then applied to
the rest of the elements. This approach favors precise in
situ analysis because all the major elements were measured
from the same sampling spot where trace element
concentrations were determined. The method was applied to
analyze synthetic and natural glass standards, NIST SRM 612,
610, USGS basalt BHVO-2G, BCR-2G, BIR-1G and MPI-DING
glasses. Igneous silicate minerals plagioclase, garnet,
clinopyroxene, and zircon were also analyzed by the same
method, but with particular care for isobaric interferences
on some elements. The typical crater size used was 20-50 μm
in diameter with depths ranging from 20 to 30 μm. The best
spatial resolution we could obtain, so far, was 10 μm
dependent on which element was analyzed. With the normal
analytical conditions, the analytical precision is better
than 3% for most elements. Accuracy is usually within 5%
when matrix matched standard was used (e.g., basalts
analysis using BHVO-2G). Detection limits are down to <0.01%
for major elements and sub-ppm to ppb levels for minor and
trace elements except Ni and Cr which are at 1-10 ppm level.
Obtained major and trace element concentrations for minerals
were comparable to those measured by EPMA and by SIMS. These
results indicate that this new UV-fsLA-SF-ICPMS technique is
versatile for most geochemical applications.
● IFREE MC seminar
Place: Meeting Room at 3F, YOKOSUKA HQ
Time: 16:30-18:00 on 10th, February (Thursday)
Speaker: H. Ozawa (IFREE 3C)
Title: Oxygen as a light element in the Earth’s outer core and implications for stratified layers near the CMB and the ICB
Abstract:
Is the Earth's outer core a chemically homogeneous and
adiabatic fluid? A number of seismological observations
reveal complex structures involving low-velocity layers near
the top (e.g., Tanaka, 2007; Helffrich & Kaneshima, 2010)
and bottom of the outer core (e.g., Souriau & Poupinet,
1991). Such stratified layers may be caused by chemical
reaction or phase transition of core material near the
boundaries.
Chemical reaction at the core mantle boundary (CMB) should
be a possible process to incorporate light elements such as
oxygen and silicon in the outer core. We investigated
element partitioning between molten iron and mantle minerals
to 146 GPa by a combination of laser-heated diamond-anvil
cell and analytical transmission electron microscope. Our
result demonstrated that molten iron should include oxygen
and silicon more than required to account for the core
density deficit of below 10% when co-existing with both
perovskite and ferropericlase at the CMB. This suggests that
the bulk outer core liquid with <10% density deficit is not
in direct contact with the mantle. Dissolutions of light
elements from the mantle can produce a gravitationally
stratified liquid layer at the topmost outer core, which can
be responsible for the low-P wave velocity layer observed
there (Ozawa et al., 2009).
Very recently, we have examined the stable phases in FeO up
to 330 GPa and 4180 K based on in situ X-ray diffraction
measurements. Our experiments showed for the first time that
phase transition from NiAs-type (B8) to CsCl-type (B2)
structure in FeO above 290 GPa at 3100 K. FeO transforms
directly from NaCl-type (B1) to B2 structure above 4000 K
with negative Clapeyron slope. When the similar transition
occurs within the oxygen-rich liquid outer core, the dense
liquid may form and resist convection at the lowermost outer
core. It results in the formation of thermal boundary layer
with hotter liquid that is seismologically observed as
low-velocity layer above the inner-core boundary (ICB).
● IFREE MC seminar
Place: International Exchanging Room at 5F, YOKOSUKA HQ
Time: 16:30-18:00 on 27th, January (Thursday)
Speaker: Y. Hirahara (IFREE 3B)
Title: Determination of Ce isotope ratio (138Ce/142Ce) using the three-step chemical separation procedure and thermal ionization mass spectrometry (TIMS)
Abstract:
The Rare earth elements (REE) have similar chemical and
physical properties. Although REE exhibit a gradual change
in the ionic radius, they are the most useful tools for the
studies of geological process. The REE have two long-lived
decay systems, the 147Sm-143Nd and 138La-138Ce decay
systems. A combination of those isotope ratios has provide
us information on ages, initial ratios, and enable
estimation of the time-integrated REE evolution of complex
geological reservoirs in the mantle or crust.
However, the variation of Ce isotopic ratio due to secular
radiogenic decay to 138Ce is small because of the low
abundance of 138La and its long half-life. For example, the
total variation in reported global oceanic samples show less
variation than 3 units of epsilon Ce. Therefore, highly
precise determination of Ce isotopic ratios is necessary to
obtain valuable data for application of geochemical studies.
We report a price determination method of Ce isotope using
the three-step chemical separation procedure and TIMS. The
three-step chemical separation procedure excluded the
interfering elements from Ce to undetectable levels with the
lower procedural total blank. The Ce isotopic ratios were
measured using TIMS (TRITON TIR, Thermo Fisher Scientific
Co., Germany) equipped with nine Faraday cups at IFREE,
JAMSTEC. The analytical reproducibility and precision in JMC
304, Ce isotope reference material, were obtained ± 0.004 %
(2SD) and ± 0.001-0.003 % (2σm). The Ce isotope measurements
with normalization to JMC 304 for Ames Laboratory Ce metal
reference material and BCR-1 were carried out with an
analytical reproducibility of ± 0.004 % (2SD). We also
performed the measurement of Ce isotopic compositions in
seven geological reference materials (JB-1a, JB-3, JA-1,
JA-2, JR-1, JG-1a, and JMn-1) with normalization to JMC 304
in order to eliminate any inter-laboratory biases. The Ce
isotope ratios (epsilon Ce) were -1.06 ± 0.22 (2σm) , -1.65
± 0.29, -1.12 ± 0.20, 0.05 ± 0.30, -0.95 ± 0.19, -0.45 ±
0.22, and -0.65 ± 0.20, respectively. The results agree well
with reported values and improved the analytical precision
of Ce isotope ratios in those samples.
● IFREE MC seminar
Place: Meeting Room at 3F, YOKOSUKA HQ
Time: 16:30-18:00 on 13th, January (Thursday)
Speaker: K. Ueki (Tokyo Institute of Technology)
Title: Mantle wedge structure and origin of arc magmas in the Sengan region, Northeastern Japan
Abstract:
Subduction zone is considered to be a major site of material
flux into the mantle and also a major site of material
fractionation of the Earth. Understanding chemical and
physical processes in subduction zone is a key to
investigate material/thermal evolution of the Earth.
Recently, existence of a three-dimensional melting structure
in a mantle wedge beneath the northeastern Japan arc has
been suggested based on geological studies (e.g.; Tamura et
al., 2002) and seismic studies (e.g.; Nakajima et al.,
2001). The melting and non-melting zone in a mantle wedge
beneath the northeastern Japan exhibits an along-arc
periodical distribution with wavelength of 50 km, in
addition to the across arc structure including volcanic
front. The melting region represents an above solidus area.
Then, it suggests that there exists a three dimensional
distribution of temperature and/or H2O content in the mantle
wedge.
To investigate a thermal structure and distribution of H2O
in the three dimensional melting zone, detailed analyses of
volcanic rocks have been carried out in a volcanic cluster
called Sengan region, the northeastern Japan. The Sengan
region is 30 km-30 km in area and consists of 45 Quaternary
volcanoes, including those aligned on the volcanic front.
Development of thick seismic low velocity zone surrounded by
relatively high velocity and non-volcanic zone is observed
beneath the Sengan region. We first investigate a spatial
distribution of chemical composition of volcanic rocks in
the Sengan region. Major element compositions of volcanic
rock in the Sengan region show systematics between central
part and outer rim. To investigate melting condition in the
mantle wedge, compositions of primary melts are estimated by
correcting an effect of fractionation or magma mixing. Then,
melting condition of each volcano is investigated by
optimizing temperature, pressure, H2O-content to minimize a
difference between composition of the primary melt estimated
from the observed volcanic rocks and partial melts
calculated by thermodynamic model pMELTS (Ghiorso et al.,
2002). Estimated H2O content are higher beneath the central
part of the volcanic area and lower beneath the outer rim.
H2O content beneath the central part is estimated to be 0.7
wt. % whereas H2O content beneath the outer rim is estimated
to be 0.3 to 0.1 wt. %. Variation of trace element
compositions also suggests a fluid enrichment in a mantle
beneath the central part compared to the outer rim. Melting
temperatures are estimated to be 1268 to 1388 degree.
Estimated Melting depth ranges 1 to 2 GPa.
Driving mechanism of melting in a mantle wedge is discussed
based on the estimated melting conditions and previously
reported phase diagram of mantle peridotite. Across arc
variation of H2O concentration is necessary to produce
observed along-arc three-dimensional melting structure
beneath the northeastern Japan. The H2O content is about 0.7
wt. % beneath the volcanic zone and lowers toward the
surrounding non-volcanic zone. H2O content of sub solidus
mantle beneath the non-volcanic zone is estimated to be
lower than 0.1 wt. %.
Temperature and H2O distributions in a mantle wedge are in
general associated with dynamic aspects such as relative
motion and consequent chemical reaction between melt and
solid. Internally consistent model to discuss mantle
dynamics and melting is necessary to discuss an origin of
the inferred three-dimensional H2O distribution. However,
there exist a number of problems with the previous
thermodynamic models for melting of mantle rocks such as
pMELTS and Perplex (Connolly, 2005). We have constructed a
thermodynamic model for partial melting of peridotite by
system energy minimization. Minimization algorithm,
thermodynamic equations, parameters and calculated results
of the thermodynamic model will be briefly presented.
