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Department of Solid Earth Geochemistry

Seminar Schedule

[DSEG-ODS seminar]

Date:
2019/2/14(Thursday)16:00-17:00
Place:
JAMSTEC HQ/ Adm. Building, 3F Meeting Room #305
Speaker:
Yasuhiro Hirai (ODS)
Title:
Generation of primary andesitic magma in the oceanic arcs: evidence from Kibblewhite Volcano, Kermadec arc
Abstract:
Melting of hydrous mantle at low pressure (<1 GPa) produces primary andesitic melts, suggesting that primary andesitic melts will be produced in oceanic arcs where the crust is thin. Recently, Tamura et al. (2018) discovered olivine-bearing andesites derived from primary andesitic melts at the Nishinoshima volcano in the Ogasawara arc, Japan, where the crustal thickness is only 21 km. Here, we will present another example for generation of primary andesitic melts in the southern Kermadec arc.

During R/V SONNE expedition in 2017 (SO-255) phenocryst-poor andesites had been recovered from Kibblewhite volcano, one of the volcanic complexes in the southern Kermadec arc. These andesites contain skeletal olivine and clinopyroxene microphenocrysts showing similar and restricted Mg-numbers and a few euhedral olivine xenocrysts having very magnesian core. No plagioclase is found even in groundmasses. The andesites are not primitive but magnesian and are relatively high in Ni and Cr. Trace elemental characteristics in these andesites, which are highlighted by depleted high-field strength element and enriched large-ion lithophile element compositions, are similar to the coeval basalts from the same volcano and typical for the southern Kermadec arc lavas. Sr-Nd-Hf-Pb isotope compositions for the andesites and basalts of the Kibblewhite volcano also cannot be distinguished from each other and are consistent with the range of published data.

Calculation of equilibrium olivine compositions and olivine fractionation trends indicate that olivine microphenocrysts in the andesites are in equilibrium with host rocks and that the andesites have experienced 10% olivine fractionation from mantle-derived primary melts. Primary melt compositions for the andesites (estimated by addition of 10 wt.% equilibrium olivines) are still andesitic in composition and similar to the estimated primary andesite melt of the Nishinoshima volcano. Thermo-barometer of Lee et al. (2009) estimates that the primary melts for the andesites are segregated from mantle peridotite at T-Ps of 1190–1220℃ and 0.45–0.55 GPa where primary H2O contents are assumed as 1.0–3.0 wt.%. The low primary magma segregation pressures, corresponding to 15–18 km depths at sub-arc mantle, may be consistent with that the southern Kermadec arc has thin crustal thickness (9–12 km).

[D-SEG Seminar]

Date:
2019/2/7(Thursday)16:00-17:00
Place:
Meeting Room at 3F, YOKOSUKA HQ
Speaker:
Iona M. McIntosh (D-SEG)
Title:
Past eruption styles of Oomurodashi, an active shallow silicic submarine volcano near Tokyo Bay, Japan
Abstract:
Oomurodashi is a recently discovered active, shallow (~140 mbsl), silicic submarine volcano  in the northern Izu-Bonin arc, only 60 km from the entrance to Tokyo Bay. Past activity at Oomurodashi spans a wide range of eruption styles and includes small lava effusions, submarine pumice, and subaerial tephra deposits on nearby Izu-Oshima and Toshima islands. Here I present volatile and textural data that reveal contrasting degassing, quench and fragmentation processes occurring in the shallow conduit and overlying water column.

H2O data for pumiceous pyroclasts and lavas were obtained using new Fourier Transform Infrared spectroscopy (FTIR) analytical techniques for vesicular and hydrated glasses. Although the Oomurodashi glasses are extensively hydrated, the interconversion of H2O species (i.e. molecular water, H2Om, and hydroxyl groups, OH) stops at the glass transition temperature, and low temperature hydration occurs via diffusive addition of H2Om only, leaving OH contents unchanged. OH data can therefore be interpreted according to the known pressure-dependence of H2O solubility and temperature dependence of H2O speciation.

Pumice clasts within the subaerial tephra layer have OH contents that are consistent with quench fragmentation at depths of ~60 – 150 m depth within the shallow edifice. Moreover, the majority of this tephra deposit consists of ‘mosslike’ particles formed by annealing of extremely fine-grained material, typically formed by explosive phreatomagmatic activity. We therefore propose that this tephra represents the distal deposits from the explosive formation of the shallow (~100 m depth) summit crater by a shallow submarine, phreatomagmatic maar-type eruption.

By contrast, two in situ submarine lavas have lower OH contents than expected for their current water depths. Comparison with past sea level variation suggests that these effusive eruptions occurred at ~8-9 and ~12 ka during times of lower sea level. Loose submarine pumice also sampled from the edifice has a range of OH contents and vesicularities, reflecting different quench depths and dispersal histories within the water column.

[D-SEG Seminar]

Date:
2019/1/24(Thursday)16:00-17:00
Place:
Meeting Room at 3F, YOKOSUKA HQ
Speaker:
Suguru Yabe (DSEG)
Title:
Core-Log integration through XCT data analysis
Abstract:
X-ray Computed Tomography (XCT) scanner is equipped on D/V Chikyu, and XCT images have been acquired for all cores obtained by D/V Chikyu during IODP expeditions. As XCT scanning can image interior of drilled cores without any destruction, onboard scientists can visually know the internal structure (faults, fractures, veins, etc.) before splitting the core. Although such a qualitative use of XCT image is very useful, XCT data (or CT number) itself must be useful in quantitative sense as well. CT number represents the X-ray absorption of the scanned material, which depends on the composition and density of the material. This study compiled XCT data and onboard physical-properties measurements during IODP expeditions of Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE). A good linear relation was observed between CT number and bulk density of the sedimentary material, which enables us to calculate bulk density and porosity in any portions of cores, where onboard moisture and density measurements were not conducted. With this technique, empirical relations between porosity and other physical properties were constructed. Those empirical relations were used to calculate thermal structure and in-situ porosity from Logging-While-Drilling (LWD) resistivity data.

[D-SEG Seminar]

Date:
2019/1/17(Thursday)16:00-17:00
Place:
Meeting Room at 3F, YOKOSUKA HQ
Speaker:
Maria Luisa Tejada (D-SEG)
Title:
Emplacement history of an Early Cretaceous volcanic sequence at IODP Site U1513, Naturaliste Plateau
Abstract:
Many data of the former geochemical studies are compiled in geochemical databases such as GEOROC and PetDB. It become able to handle data of enormous quantity recorded in a database by improvement of the data-processing capacity of the computer. However, analysis data have much "differences of format" from the analyzed time, limitation of the technique and social situation. Therefore, it may not compare even the same parameter simply because standards are different. New geochemical database "DODAI" compiling chemical data published by domestic journals and bulletins with highly precise positional data is going to produce by the Journal of the Geological Society of Japan. I introduce these "inconsistency of the format" that became clear in a process of the database production and introduce "the re-analysis of the archive sample" as the means to solve the problem of the format.

[DSEG-ODS seminar]

Date:
2019/1/10(Thursday)16:00-17:00
Place:
JAMSTEC HQ/ Adm. Building, 3F Meeting Room
Speaker:
Kenta Yoshida (ODS/D-SEG)
Title:
Chemical analysis of a tiny fluid inclusion by FIB-microsampling and cryo-FIB observation
Abstract:
Fluid inclusion, a fluid-filled cavity in a solid phase, is a remnant of paleo fluids. Chemical compositions of fluids are determined using conventional methods such as microthermometry and Raman microscopy. While most gas species can be determined by Raman analysis, solute species in aqueous fluids can be roughly determined by eutectic melting point of ice. Although these analytical approaches are powerful, some species in multicomponent aqueous solution can be hardly detected by conventional methods as shown by Yoshida et al. (2018). Main cause of the difficulty in the chemical analysis of fluid inclusions is always its small size. For large fluid inclusions, many previous researchers succeeded in determining chemical composition precisely using either LA-ICP-MS analysis or adventurous cryo-cleaving SEM analysis. However, fluid inclusions in metamorphic rocks (especially high-pressure type) are generally small up to 10 microns.For the chemical analysis of tiny fluid inclusions, precise machining technique is required, such as focused ion beam (FIB). In the seminar, an outline of a new analytical scheme combining FIB-microsampling and cryo-FIB-SEM chemical analysis is shown. The new analytical scheme consists of two steps: 1) a fluid inclusion is cut out with is host phase by using FIB micro-excavation and put on a pre-tilt stage; 2) fluid-containing sample is cooled under cryogenic temperature (ca. -130 degC) and cut in the SEM chamber, then chemical composition of ice (or amorphous phase) is analyzed by EDX.Although micro-sampling approach turned out to clear many problems pointed out in previous studies, small size of fluid inclusion still prevented the single-phase analysis of fluid, i.e. we shall obtain a mixed EDX spectra of fluid and host phase. However, the new approach will be a powerful tool to detect minor solute component in aqueous fluid in tiny fluid inclusions.