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Department of Solid Earth Geochemistry

Seminar Schedule

[D-SEG seminar]

Date:
2016/12/22 (Thursday) 16:00-17:00
Place:
Meeting Room at 3F, YOKOSUKA HQ
Speaker:
QING CHANG (DSEG)
Title:
Laser ablation ICP-MS analysis of major, minor, and trace elements of an olivine reference material
Abstract:
Olivine is the earliest mineral phase crystallizing from mantle-derived melts and is the most ubiquitous mineral in ultramafic rocks. Trace element geochemistry of olivine offers important implications to the origin of these rocks including pressure, temperature and other crystallization conditions (i.e., oxygen fugacity). Due to the unique crystallographic structure, olivine usually hosts only few transition metals which have similar ionic radii and charge with the divalent magnesium and iron. With the advance of laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), simultaneous determination of the major, minor and trace element compositions of olivine become available. However, high precision analysis is limited by lack of matrix-matched reference materials even by using the state-of-the-art femtosecond laser ablation technique. Predominant Mg and Fe matrices in olivine largely differ from that of the reference standard materials commonly used in microanalyses (e.g., NIST soda glass and USGS basalt glass standards). To overcome this problem, an olivine sample (MongOl sh11-2) is being proposed for reference standard for various microanalyses, such as EPMA, LA-ICP-MS and SIMS. MonOl sh11-2 is separated from a mantle peridotite xenolith in a basaltic breccia at Shavaryn-Tsaram volcano (48.046N, 99.994E) in the north-central Mongolia. Five leading geoscience laboratories, including JAMSTEC, are now working together to determine reference values of minor to trace element compositions of this olivine using EPMA, XRF, SIMS, LA-ICP-MS and ICP-AES. In this talk I will report analytical results of our femtosecond LA-ICP-MS and inter-laboratory and inter-method comparisons.

[D-SEG seminar]

Date:
2016/12/08 (Thursday) 16:00-17:00
Place:
Meeting Room, Guest House 2F, YOKOSUKA HQ
Speaker:
Marial Luisa Tejada(DSEG)
Title:
Magmatism and basin evolution in the South China Sea
Abstract:
The South China Sea is the largest marginal basin in the world, with its tectonic history largely shaped by its unique position at the confluence of three major tectonic plates. Two of the important objectives of IODP Expedition 349 in the South China Sea (SCS) was to (1) measure the geochemical composition, magnetization, and physical properties of the oceanic crust and seamount volcanism to understand crustal accretion and deep mantle processes associated with the tectonic development of the basin and test the different proposed driving mechanisms that led to continental breakup and seafloor spreading, and (2) investigate the sedimentary sequences to understand the past environmental, climatic, and oceanographic conditions related to the tectonic evolution of the SCS. To address these objectives, my post-cruise studies on samples recovered by drilling are aimed at understanding the nature of the deep mantle processes during the late stages of spreading and investigating the sedimentary response to tectonic and magmatic events. In this seminar, I will be presenting some results of the Os isotope study of the oceanic crust and sedimentary sequences and the interpretations that can be derived about mantle sources and tectonic models for the SCS. The second part of the seminar will focus on the first ever Re-Os isotope study of the sediments and the implications on the post-spreading evolution of the basin.

[D-SEG seminar]

Date:
2016/12/01 (Thursday) 16:00-17:00
Place:
Meeting Room, Guest House 2F, YOKOSUKA HQ
Speaker:
Tatsu Kuwatani (DSEG)
Title:
Simultaneous estimation of melting degrees and a source composition of MORB using Markov chain Monte Carlo (MCMC) method
Abstract:
Bulk compositions of igneous rocks directly reflect physical and chemical processes in the earth's interior, such as melting of original rocks, magma mixing and rock-fluid interaction. However, it is difficult to extract quantitative information due to many unknown factors and insufficient quality of data sets. We develop a new methodology for inversion analyses of geochemical processes based on a Bayesian estimation framework and Markov chain Monte Carlo (MCMC) optimization. Using MCMC method, we can deal with complex processes including nonlinearity and complex compositional distributions. In this presentation, we will present the results of synthetic inversion tests with an preliminary application to global MORB data sets in order to discuss the validity, effectiveness and the applicability of the proposed method.

[D-SEG seminar]

Date:
2016/11/10 (Thursday) 16:00-17:00
Place:
Meeting Room at 2F, Marine Research Building, YOKOSUKA HQ
Speaker:
Takashi Miyazaki (DSEG)
Title:
Magma genesis conditions for the juvenile magma source of N-Izu suggested by the volcaniclastic geochemistry in the deepest part of Site U1437, IODP Expedition 350
Abstract:
Site U1437 is located in the Izu rear arc, ~330 km west of the Izu-Bonin Trench axis and ~90 km west of the arc-front volcanoes, at 2117 mbsl. Stratigraphic Units VI and VII, below 1320 mbsf, contain volcaniclastics and hyaloclastites with coarse lava clasts. One of the scientific objectives of the IODP Expedition 350 is the clarification of the geochemical characteristics of the Izu rear arc not accessed by dredging. The geochemical characteristics of the Unit VII volcaniclastics are expected to reflect the mantle source of the early N-Izu rear-arc magmatism soon after the Shikoku Basin opening, which occurred between 24-15 Ma (Tamura et al., 2015). Major and trace elements of the Unit VII lava clasts differ from those of the Neogene rear-arc seamounts or Quaternary arc-front volcanoes. Most lava clasts from Unit VII have trace element characteristics indicating weak influences from the slab (fluid or melt) (Sato et al., in prep). Sr-Nd-Pb-Hf isotope ratios of the same samples are consistent with the trace element characteristics. Conventional mixing calculations using these isotope ratios suggest the mixing ratio between the wedge mantle and fluid or melt from the sediment and altered oceanic crust. However, several hypothetical conditions, such as the conditions during slab melting or dehydration, must be determined before the calculation. Therefore, the calculated result is not unique but is set by the hypothetical conditions. In this study, ABS4 (Arc Basalt Simulator ver.4 coded by Kimura et al., 2014) and PRIMACALC2 (Primary Magma Calculator ver.2 coded by Kimura and Ariskin, 2014) are used to determine the conditions of magma genesis from the major element, trace element, and Sr-Nd-Pb-Hf isotope ratios to provide a unique representation of early N-Izu rear-arc magmatism. In this seminar, I will talk about the ongoing calculation results.

[ODS&D-SEG seminar]

Date:
2016/11/04 (Friday) 16:30-17:30
Place:
Seminar Room at 1F, Marine Ecosystem Research Building,YOKOSUKA HQ
Speaker:
Natsue Abe (ODS)
Title:
Pre-subduction metasomatic enrichment of the oceanic lithosphere induced by plate flexure: Implications from the multidisciplinary petit-spot research
Abstract:
Oceanic lithospheric mantle is generally interpreted as depleted mantle residue after mid-ocean ridge basalt extraction. Several models have suggested that metasomatic processes can refertilize portions of the lithospheric mantle before subduction. Here, we report mantle xenocrysts and xenoliths in petit-spot lavas that provide direct evidence that the lower oceanic lithosphere is affected by metasomatic processes. Chemical similarity of clinopyroxene observed in petit-spot mantle xenoliths and clinopyroxene from melt-metasomatized garnet or spinel peridotites sampled by intracontinental basalts and kimberlites indicate that the metasomatic processes affecting oceanic and continental lithospheric mantle are similar. We suggest that extensional stresses in oceanic lithosphere such as plate bending in front of subduction zones allowing low degree melts from the seismic low-velocity zone to percolate, interact, and weaken the oceanic lithospheric mantle indicating that percolation and metasomatism could be initiated by tectonic processes. Since plate flexure is a global mechanism in subduction zones, a significant portion of oceanic lithospheric mantle is likely to be metasomatized. Recycling of metasomatic domains into the convecting mantle is fundamental to understanding the generation of small-scale mantle isotopic and volatile heterogeneities sampled by oceanic island and mid-ocean ridge basalts. These results present direct evidence of complex structure of the lower oceanic lithosphere, which seems to be subject to metasomatic processes.
The outline of the multidisciplinary petit-spot research and the motivation of the oceanic plate study toward to the Mantle Drilling by D/V Chikyu will be introduced in this seminar.

[D-SEG seminar]

Date:
2016/10/27 (Thursday) 15:30-16:30
Place:
Meeting Room at 3F, YOKOSUKA HQ
Speaker:
Hans-Ulrich Schmincke and Mari Sumita (GEOMAR)
Title:
Evolutionary, compositional and geotectonic contrasts between well-studied prototype Pacific (Hawaii) and prototype Atlantic (Gran Canaria) islands and their volcanic aprons
Abstract:
Oceanic volcanic islands consist of a submarine/subaerial edifice and a submarine volcanic apron comprising a seismically chaotic proximal and a layered basin facies, some extending >>100 km from shore. Seaward transfer from fast-growing, fast sinking and compositionally relatively monotonous mafic Hawaii, grown on young elastic lithosphere covered by thin marine sediments, appears dominantly by slumps and debris flows, lack of datable tephra compromising apron drilling proposals. Gran Canaria, focus of this talk, in contrast, developed on thick stable lithosphere covered by thick hemipelagic sediments at the upwelling African margin from a ca. 15-16 Ma old mafic shields to the late Holocene. Major volcanic and compositional stages comprise a 20 km diameter caldera, source of ca. 20 rhyolitic to late stage phonolitic ignimbrites (14 Ma - ca. 8.5 Ma), totaling > ca. 1000 km3 magma (DRE). Following a 4-5 m.y. long erosional hiatus, a >500 m high stratocone made of alkalic mafic to highly undersaturated phonolite lavas and abundant lahars (ca. 4.3-3.7 Ma) was characterized by terminal radially transported huge debris avalanche deposit generated during wet Pliocene climate. Younger thick alkalic basalts-basanites and phonolitic tephra/domes mark a third stage.
ODP drilling (Leg 157) penetrated the apron as far as 75 km from shore as deep as 1200 m bsf with high recovery, recovering all major developmental island stages. Initial hyaloclastite debrites reflect the submarine island stage, followed by basaltic flank collapse breccias, subaerial-submarine basaltic land-sea delta deposits, thin basaltic scoria/sideromelane turbidites, tens of highly evolved syn-ignimbrite turbidites significantly exceeding the volume of their subaerial source ignimbrites. Several of them correlating perfectly temporally and compositionally to widespread subaerial ignimbrite sheets. Evolved magmas are interpreted as fractionated, rise of mafic parents being hindered by density contrast while erupting on the submarine flanks. Sedimentation rates in the drilled basin apron are at a minimum during the major nonvolcanic hiatus during which the island was deeply eroded by >>1000 m. The bulk sediment transfer throughout the islands history into the adjacent basins was thus principally governed by voluminous, instant volcanic events:
Submarine hyaloclastite debrites, shield stage flank collapse, land-sea lava flow delta deposits, syn-ignimbrite turbidites and very voluminous far-inland-sourced debris avalanche deposits during mature stages.

Selected literature:
Schmincke H-U, Sumita M (1998) Volcanic evolution of Gran Canaria reconstructed from apron sediments: Synthesis of VICAP project drilling (ODP Leg 157). In: Weaver PPE, Schmincke H-U, Firth JV, Duffield WA (eds) Proc ODP, Sci Results, 157: College Station, TX (Ocean Drilling Program): 443-469

Schmincke & Sumita Schmincke H-U, Sumita M (2013) Instability of Oceanic Volcanic Edifices: Examples of Sector Collapse, Debris Avalanches, and Debris Flows from Gran Canaria (Canary Islands). In: Krastel S (Ed) Submarine mass movements and their consequences: 6th International Symposium: Advances in Natural and Technological Hazards Research, 37:
Springer International, pp 603-614. DOI: 10.1007/978-3-319-00972-8_54

[D-SEG seminar]

Date:
2016/10/6 (Thursday) 16:00-17:00
Place:
Meeting Room at 3F, YOKOSUKA HQ
Speaker:
Hitomi Nakamura (DSEG)
Title:
Compositional variations of the Arima-type and associated spring waters in the Kinki district, southwest Japan
Abstract:
Rare earth elements (REEs) of the spring waters upwelling in the non-volcanic fore-arc region of the Kinki district in southwest Japan have been investigated to assess their upwelling processes and deep-seated origins [Nakamura et al., 2014; 2015]. In this study, a principal component analysis of the REE data has identified three principal components (PCs) that cover 89% of the entire sample variance: (1) PC-01, which corresponds to a dilution process by which fluids are introduced at low concentrations, previously represented by major solute binary trends, including δ18O?δD systematics; (2) PC-02, which is a precipitation process of REEs from the brine; and (3) PC-03, which is an incorporation of REEs from country rock by carbonic acidity, although the types of country rocks may also have a significant impact on the spring water compositions. Based on these three PCs, together with the major solute concentrations and hydrogen, oxygen, and helium isotopic compositions determined in previous studies, five distinct types of spring waters in the Arima and Kii areas have been identified: (i) “Tansansen”, (ii) “Kinsen”, (iii) “Ordinary Arima”, (iv) “Ginsen”, and (v) “Eastern Kii”. These five types probably represent (ii) a deep brine, (iii) an evolved deep brine that precipitated REE-bearing minerals, (iv) a mixture of (iii) and meteoric water, (v) a meteoric water carbonated by deep gas derived from (ii), and (i) a spring water similar to (v) with a more significant influence of the country rock constituting the aquifer. A comparison of the spring waters in the Arima and Kii areas revealed a systematic geographic distribution. The “Ordinary Arima”-type occurs along the Median Tectonic Line, and the “Eastern Kii”-type occurs in the eastern part of the Kii area. The latter seems to upwell in the restricted region where deep low-frequency tremors are observed. We suggest that the geographical distributions are linked to the tectonic setting and/or temporal evolution of fluid upwelling.

[D-SEG seminar]

Date:
2016/9/29 (Thursday) 16:00-17:00
Place:
Seminar Room at 1F, Marine Ecosystem Research Building,Yokosuka HQ
Speaker:
Morihisa Hamada (DSEG)
Title:
Systematics of volatiles in melt inclusions from the paleo- IBM arc (30-40 Ma)
Abstract:
IODP Expedition 351 (June-July 2014) drilled the Amami Sankaku Basin, west of the Kyushu-Palau Ridge (KPR) as a paleo-Izu-Bonin-Mariana (IBM) arc, and recovered cores which record the temporal evolution of the IBM rear-arc from 50 Ma to 25 Ma. We have been analyzing melt inclusions collected from there as a post-cruise study. In this seminar, I will report analytical results of volatiles using SIMS at the Kochi Institute for Core Sample Research. As a whole, concentration of volatiles increases from 40 Ma to 30 Ma, while concentration of incompatible elements (such as K2O) also increases as well. Therefore, for example, Cl/K2O and F/K2O are almost constant during the early history of the IBM arc over 10 Ma. These analytical results suggest that flux of volatiles had been constant but degree of partial melting had decreased through time. We confirm that Cl-rich melt inclusions (up to 1% Cl) found from two levels do not mean intermittent injection of volatile-rich magmas into the paleo-IBM at least twice arc but are assimilated by brine, based on systematics of volatiles in melt inclusions.

[D-SEG seminar]

Date:
2016/7/21 (Thursday) 16:00-17:00
Place:
Seminar Room at 1F, Marine Ecosystem Research Building,Yokosuka HQ
Speaker:
Shigeaki Ono (ODS)
Title:
Phase transition between coesite and stishovite: Implications for the seismic X-discontinuity
Abstract:
Silica has several important polymorphs that have attracted great interest in earth science. For expample, the X- discontinuity is observed at 270-330 km depths corresponding to the pressure of around 10 GPa. This discontinuity has been often explained by the phase transition of silica. In this study, the phase boundary between coesite and stishovite in silica was determined using a multi-anvil high-pressure apparatus combined with the synchrotron X-ray diffraction technique. The phase transition determined in our study occurs at around 10 GPa at the normal mantle geotherm, coinciding with the depth of the seismic X-discontinuity.

[D-SEG seminar]

Date:
2016/6/16 (Thursday) 16:00-17:00
Place:
Meeting Room at 3F, YOKOSUKA HQ
Speaker:
Jun-Ichi Kimura (DSEG)
Title:
Origin of geochemical mantle reservoirs: Role of subduction filter, mid ocean ridge, and thermal evolution of mantle
Abstract:
We quantitatively explore element redistribution at subduction zones and mid ocean ridges using numerical mass balance models to evaluate the roles of the subduction zone filter and the spreading ridge factory in the Earth’s geochemical cycle. Our models of differs from previous works by being internally consistent with geodynamic models of modern arcs and ocean ridges that successfully explains magma geneses and include element fluxes in the residual slab and mantle components. We estimated that the mantle potential temperature (Tp) was ~1650 °C at 3.5-2.0 Ga and gradually decreased to ~1300 °C today. Hot subduction zones with Tp ~1650 °C have a thermal structure like modern SW Japan where high-Mg andesite is formed that is like continental crust. After 3.0-2.0 Gyr of storage in the mantle, the residual igneous oceanic crust from hot subduction zones can evolve isotopically to the HIMU mantle reservoir, the residual base of the mantle wedge to EMI, the residual sediment becomes an essential component of EMII, and the residual top of the mantle wedge can become the subcontinental lithosphere reservoir. The Common or Focal Zone component is a stable mixture of the first three residues occasionally mixed with early depleted mantle. Slab residue that recycled earlier (~2.5 Ga) can form the DUPAL anomaly in the southern hemisphere, whereas more recent recycling (~1.7 Ga) focused in the northern hemisphere. The east-west heterogeneity of the depleted upper mantle involves sub continental mantle except in the Pacific. We also examined Sr-Nd-Hf-Pb isotopic evolution of the Earth’s depleted mantle using a melting model for mid ocean ridges coupled with subsequent isotopic evolutions of the residual mantle. The two-stage mantle evolution model, including primitive mantle formation occurred at ~4.57 Ga followed by MORB melt depletion at ~2.0 Ga (average age), accounted for generation of the depleted MORB source mantle today. The high-mantle Tp age by ~2.0 Ga coincides with formation period of the majority continental crust and enriched mantle reservoirs from the hot Archean subduction zones. All above results suggest changes in mode of mantle convection, vigorous before ~2.0 Ga and gradually sluggish after that time. This change may characterize the thermochemical evolution of the Earth’s mantle.

[D-SEG seminar]

Date:
2016/5/19 (Thursday) 16:00-17:00
Place:
Meeting Room at 3F, YOKOSUKA HQ
Speaker:
Hikaru Iwamori (DSEG)
Title:
Role of water in subduction zone dynamics
Abstract:
In this talk we will discuss how water affects subduction zone dynamics and geochemistry. Within the solid Earth system, water plays crucial roles in both physical and chemical aspects, e.g., reducing rock strength and density, decreasing melting temperature, and redistributing elements and isotopes effectively during water-rock interactions. Although each effect has been vigorously studied, less attention has been paid to their interplay. As an example in subduction zone, dehydration of a subducting slab hydrates the overlying mantle wedge, reducing the viscosity. Then, the flow-thermal field is modified to accommodate the extent and the locus of dehydration and melting, which in turn affects again the viscosity and thermal-flow field, forming a non-linear feedback system. We investigate such systems by numerical simulation of two-phase mantle convection models, involving water-element transport, hydrous phase relation, fluid generation and migration, and fluid-rock reactions [1]. The water transport and the mantle convective flow are interactive through rheology, density and phase relation for hydrated-dehydrated rocks. Based on the numerical models, we demonstrate how the influences of water on phase relation, rheology, density, and flow- thermal structure appear as observable variables and phenomena, such as the location of arc volcanic zone, the subduction velocity and angle of plate, slab morphology, stress field, trench migration and back-arc spreading. One of the key parameters is effective viscosity of hydrous minerals, which is not tightly constrained at present: e.g., development of serpentinite just above the subducting slab critically controls the mechanical coupling between the slab and the mantle, leading to various flow-thermal structures depending on the viscosity. It has been also found that rheological weakening of lithosphere by fluid upwelling, as well as effective density reduction of the hydrated slab, has significant impacts on the trench mobility, back-arc basin formation and convergence rate, which determines the subsequent evolution of slab morphology over the global scale [2]. The observed variations in subduction mode, at least a part of them, could be attributed to different degrees of water effects on subduction dynamics.

References: [1] Horiuchi&Iwamori, 2016, JGR; Iwamori et al., 2016, submitted. [2] Nakao et al., 2016 EPSL, in revision.

[D-SEG seminar]

Date:
2016/5/12 (Thursday) 16:00-17:00
Place:
Meeting Room at 2F, Guest House, YOKOSUKA HQ
Speaker:
Junichiro Ohta(DSEG)
Title:
Determination of depositional age of extremely REY-rich mud by Osmium-isotope stratigraphy
Abstract:
REY-rich mud, a new deep-sea mineral resource for REY recently reported by Kato et al. (2011), has a potential to solve problems associated with its uneven supply structure. REY- rich mud is deep-sea sediment with high REY content (total REY content = 400-2000 ppm) characterized by a huge ore reserve, low concentration of radioactive elements, and ease of extraction by dilute acids. In 2013, seven sediment cores (PC01-PC07) were collected from the seafloor in the Japanese Exclusive Economic Zone around Minamitorishima Island by the Cruise KR13-02 of R/V Kairei for the purpose of an exploration of REY-rich mud. Bulk chemical composition analyses of the sediment cores showed that one of these cores (PC05) contained extremely REY-rich mud (total REY content >5000 ppm). Detailed mineralogical observations showed that the extremely REY-rich mud contains abnormally huge amount (> 20%) of biogenic calcium phosphate (BCP) which highly concentrate REY. This study made a paleoceanographic approach for the purpose of revealing a formation mechanism of the extremely REY-rich mud. It is crucial for this purpose to understand depositional ages and components of the mud. In this study, Os isotope stratigraphy, a recently developed dating method, was applied to date the extremely REY-rich mud. Os isotope ages determined by fitting to the Cenozoic Os isotope ratio curve in seawater indicated that the depositional ages of the extremely REY-rich mud were ~34 Ma. The formation mechanisms of REY-rich mud were discussed on the basis of the Os age, calculated mass accumulation rate (MAR), paleoclimate at the depositional age, and data of previously studied sediment cores from North and South Pacific. The extremely REY-rich mud is characterized by extremely high REY content, very high P MAR, and large grain size of BCP. A climate at ~34 Ma was marked by a rapid global cooling which probably enhanced the ocean circulation. Therefore, the very high P MAR is considered to reflect a very high biological productivity caused by an enhanced upwelling over seamounts which supplied nutrients from deep to surface of the ocean. In addition, very large grain size of BCP suggests a strong sorting of sediment components by enhanced bottom current around seamounts. Fine grained components which mainly consist of low-REY-content components were removed by the sorting, resulting in higher concentration of BCP in the sediment.

[D-SEG seminar]

Date:
2016/4/28 (Thursday) 16:00-17:00
Place:
Seminar Room at 1F, Marine Ecosystem Research Building,Yokosuka HQ
Speaker:
Kenta Yoshida(DSEG)
Title:
Stretching and freezing processes of fluid inclusion density during the exhumation of metamorphic rocks: a potential geobarometer
Abstract:
Density and volume of fluid inclusions (FIs) are known to change during the retrogression of high-grade metamorphic rocks. Visible evidence of the deformation of FIs are recognized as decrepitation or implosion, which are often accompanied by satellite inclusions. However, existence of “cryptic reequilibration” of FIs are also indicated by natural samples (Küster and Stöckhert, 1997). Such cryptic reequilibration is caused by a ductile flow of the host phase driven by a pressure difference between inside and outside of FIs, and thus, occurs at lithostatic conditions. Recent study investigated the modification of FIs during ductile deformation of the host rock, revealing the compression of FIs by tectonic over-pressure taken place at approximately greenschist facies conditions (Diamond and Tarantola, 2015). These previous studies suggest the existence of “closure temperature” of the density of FIs, where the host minerals behave strong enough to freeze the density of enclosing FIs.

This study constructs the deformation model of FIs during the exhumation of high-grade metamorphic rocks on the basis of flow law and physical properties of representative rock-forming minerals. We consider the deformation process of a spherical fluid inclusion filled with pure water emplaced within a homogeneous solid phase. Only differential pressure between fluid and host minerals are considered as the driving force of the deformation. The deformation follows the simple power-flow law. Given the P-T paths passing at lower-P side of the isochores of FIs, stretching of FIs are modelled. The results clearly predict a closure temperature of the density of FIs for each mineral, which can be defined by the temperature where flow-rate of the host mineral and expanding rate of the constituent fluid become similar in log-scale. Quartz gives a closure temperature ranging 300-400 ºC which is similar to the previous study, whereas garnet and olivine show the range of 700-800 ºC and 900-1100 ºC, respectively. These results suggest a potential of FI-geobarometer deciphering the exhumation and cooling P-T trajectories of high-grade metamorphic rocks.

[D-SEG seminar]

Date:
2016/3/9(Wednesday)14:30−15:30
Place:
Meeting Room at 2F, Marine Research Building YOKOSUKA HQ
Speaker:
CHIAKI TOYAMA(DSEG)
Title:
Behavior of fluorine, chlorine, and boron in arc magmatism of Southern Volcanic Zone, Chile
Abstract:
Recently, halogens are revealed as one of the powerful tracers of water cycling in subduction zones (e.g., Sumino et al., 2010). The detail behavior of halogens during subduction processes and their fate in the earth’s mantle are known only a little. Therefore, we newly analyzed halogen contents in regional representative lavas obtained from 10 volcanoes on the Quaternary volcanic front of the Southern Volcanic Zone (SVZ) of Andean arc in Chile. In addition, we carried out estimation of the melting condition based on the major and trace element composition of samples reported by Shinjoe et al. (2013) and the fitting model reported by Nakamura and Iwamori (2013). And, we estimated the fluorine, chlorine and boron contents in magma source of samples using the rock data analyzed and the degree of partial melt calculated.
In this presentation, we focus on fluorine and chlorine and report the first finding of decoupling of along-arc variation in fluorine, chlorine and boron.

[D-SEG seminar]

Date:
2016/02/18(Thursday)16:00-17:00
Place:
Meeting Room at 3F, YOKOSUKA HQ
Speaker:
HAMADA MORIHISA (DSEG)
Title:
Heterogeneity of source materials beneath an ocean island --A preliminary case study of Rarotonga Island
Abstract:
Geochemical endmembers of Earth’s mantle, such as HIMU and enriched mantles (EM1 and EM2), have been identified based on the systematics of radiogenic isotopes such as Sr, Nd and Pb isotopes in ocean island basalts (OIBs). Instead of previous whole-rock geochemical studies of OIBs, melt inclusions in OIBs are intensively analyzed by many geochemists now to constrain the concentration and isotopic composition of volatiles in primitive melt derived from each geochemical endmember in the mantle. Recently, I joined collaborative research project in DSEG to analyze melt inlcusions in OIBs. The primary objective of this project is to link various elements in melt inclusions such as volatiles, major elements, trace elements and Pb isotopes, to further understand geochemical structure of Earth's mantle and the origins of OIBs. In this seminar, I will report the preliminary analytical results of melt inclusions collected from Rarotonga Island of the Cook-Austral archipelago in the South Pacific.

[D-SEG seminar]

Date:
2016/2/4(Thursday)16:00-17:00
Place:
Meeting Room at 3F, YOKOSUKA HQ
Speaker:
Chang Qing (DSEG)
Title:
High resolution analysis of REE and minor elements in apatite using laser-ablation (LA) ICP-MS
Abstract:
Biogenic apatite (fossil bones and teeth) is considered as the main carrier of rare earth elements and yttrium (REY) in deep-sea mud. Some of the mud contain REY as high as up to ~2% (wt), arousing wide interests of economic and high-tech industrials due to global REE crisis. However, when and how the metal ions were incorporated into these hard bio-tissues is not clearly understood. Spatial resolved elemental analysis at micron scale on surface of biogenic apatite and fossil tissues will be an approaches shedding light on these issues. Elemental distribution pattern, ratio and concentration gradient of fossil tissues are all important implications of elemental diffusion, postmortem diagenesis and provenance of the elements. We attempted to determine REY, minor and major elements composition at ~20 μ m scale on phosphate reference standard (Durango apatite) and fish teeth collected from sea-floor of Minami-Torishima by state-of-the-art femto-second LA-ICP-MS. The major issue in precise and accurate analysis of these elements in apatite is normalization of laser ablation efficiency on unknown and calibration standard. In this seminar I’ll discuss the progress on analytical method development and preliminary results of REY and minor elements of fish teeth analyzed by LA-ICP-MS.

[D-SEG seminar]

Date:
2016/01/28 (Thursday) 16:00-17:00
Place:
Meeting Room at 3F, YOKOSUKA HQ
Speaker:
M. L. G. Tejada (DSEG)
Title:
Spreading and tectonics in the South China Sea: Some geochemical and geophysical results from IODP Expedition 349
Abstract:
International Ocean Discovery Program (IODP) Expedition 349 (26 January 2014 to 30 March 2014) drilled five sites in the central basin of the South China Sea to study regional geodynamics and oceanic crustal evolution. Three of the five sites (Sites U1431, U1433, and U1434) were cored into the igneous basement near the fossil spreading center where seafloor spreading terminated, and another Site U1435 penetrated through the breakup unconformity proximal to the northern continent-ocean boundary where seafloor spreading started [Expedition 349 Scientists, 2014]. Site U1431E in the East Subbasin recovered the deepest cored basement section with 108.4 m of penetration. The upper section is ~70 m thick while the lower section is at least 30 m-thick, each of which contain ~26 m of massive flows and separated by an interflow claystone unit 3.7 to 9.5 m thick, representing hiatus in magmatism. Two sites were cored at the Southwest Subbasin. Site U1433B cored 60.8 m into igneous basement recovering 37.5 m thick succession of small, pillowed flows, with the bottom 23.3 m section consisting mostly of massive flows. Site U1434 closer to the spreading axis penetrated 30.3 m into igneous basement but with only 10% recovery. The oceanic crust at all sites is overlain by a hemipelagic claystone with a paleontological age of ~15 at Site U1431E, ~18 Ma at Site U1433B, and ~12 Ma at Site U1434.

Geochemical results yielded Re-Os isotope isochron ages of 18 1.8 Ma for Site U1433B and a less precise age of ~15-20 Ma for Site U1431E, consistent with paleontological ages. Isotope data indicate three isotopically distinct magmatic types, apparently representing at least three stages of magmatism in the South China Sea: 1) the lower section at Site U1431E; 2) the upper section at Site U1431E and the whole section at Site U1433B, and 3) the whole section at Site U1434. The most radiogenic Os isotopic compositions occur toward the end of spreading at all sites while the influence of continental crustal input is strongest in the source of Site U1434 basalts in the Southwest Subbasin. Spreading at Site U1431may have occurred in two stages, with an earlier episode dominated by mixed sources involving oceanic-ridge type magmatism, with altered oceanic crustal input, and later evolved into more enriched ocean ridge-type magmatism. This interpretation is consistent with a rifting model initiated at the East Subbasin and progressed toward the Southwest Subbasin.

Basement structures and magnetic anomaly observations support multiple periods of spreading with variable, generally decreasing rates from the East Subbasin to the Southwest Subbasin [Li et al., 2014]. Magnetic anomaly modeling suggest that the best fit to the basement ages and IODP 349 drilling results is a model wherein spreading initiated at 33 Ma along the present-day continent-ocean boundary, with a ridge jump occurring at ~23 Ma, coinciding with initiation of spreading at the Southwest Subbasin, and the same or about one m.y. earlier cessation of spreading in the Southwest than in the East Subbasin at ~15-16 Ma.

[D-SEG seminar]

Date:
2016/01/21(Thursday)16:00-17:00
Place:
Meeting Room 2F, Guest House YOKOSUKA HQ
Speaker:
Iona McIntosh (ODS)
Title:
Investigating degassing and eruption styles of silicic submarine volcanoes in the Izu-Bonin Arc
Abstract:
The Izu-Bonin arc contains several submarine volcanoes that have produced voluminous deposits of silicic pyroclasts, including highly vesicular pumice. Unfortunately these glasses are typically affected by secondary hydration after deposition, which has previously prevented volatile-based investigation of how they were formed. I will present a new method for finding accurate H2O contents of volcanic glasses using FTIR, and demonstrate how this can be used to estimate the original eruptive H2O content of hydrated glasses, which in turn can be used to find the depth at which pyroclasts quenched. Preliminary data from Kurose Nishi and Oomurodashi volcanoes show that reconstructed H2O contents, hence quench depths, vary with pyroclast textures. In my future research I will expand these datasets to include full volatile contents of melt inclusions and pyroclast glasses, in order to constrain degassing histories and pyroclast formation mechanisms of silicic submarine volcanoes.
Doing so will improve our understanding of the potential hazards of future eruptions for maritime traffic and coastal communities.