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Department of Solid Earth Geochemistry

Seminar Schedule

[D-SEG Seminar]

Date:
2018/12/ 20(Thursday)16:00-17:00
Place:
JAMSTEC HQ/ Adm. Building, 3F Meeting Room
Speaker:
Tatsu Kuwatani (D-SEG)
Title:
Geochemical Data-processing for Extracting Geological Processes
Abstract:
Multivariate analysis, such as principal component analysis, regression analysis and discriminant analysis, has been performed in order to extract useful information about geological processes from multi-element data sets. They are the basis of data-driven analysis, which is a general term for powerful mathematical methods for extracting useful information at the maximum extent from an available dataset and is recently applied in various natural sciences, engineering fields and industry. Collaboration between information scientists and natural scientists leads to the development of new powerful data-driven analyses for specific and general problems in each field of sciences. In this presentation, we will introduce the concept of data-driven analysis and some application examples in geochemistry. Recently, we have developed a new method which quantitatively estimates material transfer during metasomatic alteration process by sparse modeling. We will share the mathematical formulation and preliminary results of the new method.

[D-SEG Seminar]

Date:
2018/11/29 (Thursday) 16:00-17:00
Place:
JAMSTEC HQ/ Marine Ecosystem Research Building, 1F Seminar Room
Speaker:
Satoru Haraguchi (D-SEG)
Title:
Inconsistency of the format of archived geochemical data and re-analysis of the archive sample as the method of improvement of data
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:
2018/10/25 (Thursday) 16:00-17:00
Place:
JAMSTEC HQ/ Marine Ecosystem Research Building, 1F Seminar Room
Speaker:
Natsue Abe (ODS)
Title:
Initial results of the physical property measurements on Chikyu for the 3200m cores from upper crust to the mantle section of the Oman ophiolite, ICDP Oman Drilling Project
Authors:
Natsue Abe (ODS, JAMSTEC), Keishi Okazaki (KCC, JAMSTEC), Ikuo Katayama, Kohei Hatakeyama, Yuya Akamatsu (Hiroshima Univ.), James Andrew Leong (Arizona State Univ.), Ole Ivar Ulven (Univ. Oslo), Gilbert Hong (Saul Univ.), Annika Greve (ODS, JAMSTEC), Wenlu Zhu (Univ. Melyland), Benoit Cordonnier (Univ. Oslo), Benoit Ildefonse (Montpellier Univ.), Katsuyoshi Michibayashi (Nagoya Univ.), Yamato Tateishi (Okayama Univ.) and The Oman Drilling Project Science Party
Abstract:
More than 3200m length of hard rock cores drilled from Samail Ophiolite, Sultanate of Oman by ICDP Oman Drilling Project were analyzed using on-board facilities on Chikyu. Totally 4 legs (leg 1 – 4) for 4 months in total from July 15 to September 15, 2017 and from July 5 to September 3, 2018, and about 150 scientists from 21 countries were working separately in 6 teams (Igneous and Metamorphic petrologists, Structural geologist, Geochemist, Paleomagnetist and Physical properties). I will report the initial results on the physical property measurements of core samples from the Holes GT1A, GT2A, GT3A and BT1B for Phase I (ChikyuOman2017) and Holes CM1A, CM2B, BA1B, BA3A and BA4A for Phase II (Chikyuoman2018). Nearly 100% core recovery allowed us to take a large data set of petrophysical data on the cores from Oman ophiolite including fault zones and highly altered intervals.

All core sections were scanned and imaged the XCT, followed by the MSCL-W as same as ChikyuOman Phase 1 that measures natural gamma radiation (NGR), magnetic susceptibility (MS), gamma ray attenuation (GRA) density, Noncontact resistivity (NCR). P-wave velocity (Vp) were measured by using MSCL-W only for three holes (Hole BT1B, GT3A and CM1A). After those whole round measurements, all sections were split into halves and the color reflectance on the split surface of half-round cores are measured by MSCL-C with point magnetic susceptibility measurement. The discrete sample measurements were undertaken on 20 x 20 x 20 mm sample cube cut from the working half of the cores. In addition to compressional wave (P-wave) velocity and moisture and density (MAD) measurements (that are used to determine bulk density, grain density, and porosity), electrical resistivity (complex impedance: only during ChikyuOman 2018) measurements were also undertaken. Some highlights of the physical properties during ChikyuOman on-board measurements are below;

[Phase I] Cores are mainly composed of basalt and diabase, followed by gabbros (gabbro, olivine gabbro and oxide gabbro), with minor felsic, trondhjemite and tonalite, dikes intruded into the mafic rocks. Shipboard measurements of physical properties were undertaken to characterize recovered core material.

Mafic rocks from the Hole GT3A are hydrothermal altered to a range of different mineral assemblages. Compared to the gabbros from the Hole GT1A and GT2A of the Oman Drilling Project, which were described and measured during Leg 1 of ChikyuOman2017 expedition, GT3A cores show much wider range of physical properties. For example, P-wave velocity of the samples from GT3A show a range from 2.2 to 7.1 km/s. There is a clear negative correlation between P-wave velocity and porosity. It is noteworthy that there is a positive correlation between the densities and porosity in more than 1/4 of the samples. Highly altered samples from the GT3A cores show high porosity (up to 12%) and high both bulk and grain densities (up to 3.2 and 3.4 g/cm^3, respectively). It is probably because that the larger volume of epidote with high density (3.39 – 3.48 g/cm^3) in the more altered samples. The color spectrums, especially yellowness of the core samples, clearly have correlations with the rock physics of the GT3A samples. The yellowness of the mafic rocks in the GT3A imply their degree of alteration reflected the mode of epidote. In other words, the epidote precipitation during the hydrothermal alteration reduces the volume of a part of oceanic crust and probably causes the higher porosity as previously proposed on a qualitative basis.

[Phase II] Two CM holes were drilled through the crust-mantle transition zone, from gabbroic lower crust, through mixed gabbro and dunite, to residual mantle harzburgite. BA holes were taken from the mantle section and are composed of dunite and harzburgite intruded by minor gabbroic and pyroxenite dikes. Ultramafic rocks at both sites were extensively serpentinized, but the names of igneous protoliths are used here. Generally, gabbroic lithologies show higher density, P-wave velocity, and electrical resistivity, and lower porosity than ultramafic lithologies. Serpentinized dunite shows lower density, P-wave velocity and electrical resistivity, and higher porosity than serpentinized harzburgite, and these physical properties are correlated with magnetic susceptibility, probably due to crystallization of magnetite during alteration. Average XCT values are also correlated with many other physical properties in most lithologies. This is the opposite feature to that expected “normal” Moho in the oceanic floor. It is probably due to the sever serpentinization related to the emplacement of the ophiolite.

[D-SEG Seminar]

Date:
2018/10 /11(Thursday)16:00-17:00
Place:
JAMSTEC HQ/ Adm. Building, 3F Meeting Room #305
Speaker:
Morihisa Hamada (D-SEG)
Title:
Toward understanding behavior of volatiles in back-arc magmatism: A case study of Myoko volcano
Abstract:
Studying volcanic rocks from back-arc volcanoes provides fundamental information necessary to map the distribution of volatiles across the arc. However, data of volatiles in melt inclusions from the back-arc volcanoes are extremely limited due to the lower number of back-arc volcanoes compared to frontal-arc volcanoes and the relative scarcity of melt inclusions as quenched glasses from the back-arc volcanoes. In order to improve these situation, we will analyze quenched melt inclusions in scoriae collected from the Myoko Volcano, a back-arc volcano in the central Japan. This is extended study performed by Professor Yoshiaki Yamaguchi (now retired from Shinshu University), who has been studying melt inclusions from Myoko Volcano with his students for almost 10 years. He found heterogeneities (high-K2O and high-Cl domains) in both the groundmass of volcanic rocks and melt inclusions. He interpreted the origin of such domains to be small magma batches enriched in K2O and Cl injected into the main magma reservoir beneath the Myoko Volcano. We will analyze volatiles in the melt inclusions from Myoko Volcano by using SIMS and trace elements by using LA-ICP-MS to constrain their origin and source. In this seminar, I will talk the outline (not the analytical result) of this study. This study is a collaborative study among DSEG, KCC and French scientists.

[D-SEG Seminar]

Date:
2018/08/02 (Thursday) 16:00-17:00
Place:
JAMSTEC HQ/ Adm. Building, 3F Meeting Room #305
Speaker:
Dr. Eun Young Lee (Chonnam National University, Republic of Korea)
Title:
Basin analysis and applications to sedimentary basins of offshore Western Australia
Abstract:
Since the breakup between the Australian and Greater Indian plates, an extensional rift system formed sedimentary basins along the offshore Western Australia. Evolution of the sedimentary basins resulted from interplay of tectonics, sedimentation and paleoenvironmental changes in Australia and Indian Ocean, therefore understanding the basin evolution is crucial to investigate the regional furthermore global geologic history. IODP Expedition 356 Indonesian Throughflow drilled six Sites U1459-U1464 which are located in the northern Perth Basin, Northern Carnarvon Basin and Roebuck Basin along the northwest shelf, Australia. IODP Expedition 369 Australian Cretaceous Climate and Tectonics drilled four Sites U1513-U1516 in the Mentelle Basin on the southwest shelf of Australia. Using the data from Expedition 356 and pre-existing industry wells, the subsidence analysis techniques have been examined and improved, which are applied to a MATLAB-based software BasinVis 2.0 for basin analysis and visualization. The techniques are being applied to study the evolution of the Mentelle Basin. This presentation introduces the basin analysis techniques and preliminary basin studies conducted in the Mentelle Basin.

[D-SEG Seminar]

Date:
2018/07/26 (Thursday) 16:00-17:00
Place:
JAMSTEC HQ/ Adm. Building, 3F Meeting Room #305
Speaker:
Takeshi Hanyu (D-SEG)
Title:
Deep mantle volatile cycle; a case study for chlorine
Abstract:
Volatile cycle between the mantle and surface layers (hydrosphere and crust) is poorly constrained. Oceanic crust and sediment scavenge volatiles from seawater and potentially transport them to the mantle via subduction. Although the greater part of volatiles in subducting oceanic crust and sediment are liberated during subduction and returned to the surface through arc magmatism, recent studies suggest that some volatiles could remain and be carried to the mantle after subduction dehydration/melting. If it is the case, some mantle domain should be enriched in volatiles derived from the surface layers.
In this talk, we focus on chlorine as a tracer of seawater-derived volatiles and present Cl composition in olivine-hosted melt inclusions in oceanic island basalts derived from the deep-rooted mantle plume, the source of which is recognized to involve ancient subducted oceanic crust in the previous studies. Cl enrichment in combination with lithophile elements and isotopes in the melt inclusions suggest the transport of seawater-derived Cl to the deep mantle via subduction of altered oceanic crust. The implication from this study is that mantle domain involving subducted oceanic crust would be a significant reservoir for Cl and that subduction process may have buffered the salinity condition in the Earth’s surface environment.

[D-SEG Seminar]

Date:
2018/07/12 (Thursday) 16:00-17:00
Place:
JAMSTEC HQ/ Adm. Building, 3F Meeting Room #305
Speaker:
Qing Chang (D-SEG)
Title:
A rapid bulk-rock element analysis using XRF glass beads and fsLA-ICPMS
Abstract:
Bulk-rock analysis of naturally heterogeneous rock samples by ICP-MS requires laborious sample preparation including acid/alkali digestion and solution nebulization. Analysis of crystalline plutonic rocks or sediments remains challenging because of the difficulty in complete digestion of acid resistant minerals which contain significant fraction of high field strength elements (HFSE) or rare earth elements (REE). In this study we tested bulk-rock analysis using laser ablation (LA) ICP-MS with glass beads prepared for X-ray fluorescence spectrometry (XRF). This method maintains the benefits of no sample digestion by wet chemistry, high-sensitivity, high-sample throughput, and low costs. A comprehensive examination of measurement parameters was made on wave length and repetition rate of the laser beam. Choice of internal standard and external calibration method were also examined. Using high-dilution XRF glass beads (1:10 sample to lithium tetraborate ratio) and optimized in-house developed femtosecond (fs) LA system coupled to a quadruple-type ICP-MS (iCAP Qs), the parameters and methods were validated by determining 48 elements including ten major (Si, Ti, Al, Fe, Mn, Mg, Ca, Na, K, P), refractory lithophile (Sr, Ba, REE and HFSE), and volatile (Ga, Rb, Cs, Tl, Pb) elements in well-established geological reference materials (GSJ JB-1, JG-1a, JG-2 and JR-1) and synthetic reference glasses (NIST SRM 612, USGS GSD-1G). Laboratory bias (or trueness by ISO 5725-1) for most elements was better than 10% when compared to the reference or consensus values. Poorly determined or failed elements were P, Sc, V, Ni, Sn, Sb, and W, whose data deviated from the reference values >20%. Volatile elements such as Rb, Cs and Ga showed no substantial loss; however, Tl and Pb exhibited always lower values suggesting their loss during fusion in the glass bead preparation. Repeatability and intermediate precision (ISO 5725-1) of all measured elements were better than 5–7% RSD except Cu, Sn, Sb, Cs and Tl which showed 10–40% RSD due to extremely low abundances or strong spectral interferences. The low limits of detection by this method were comparable to those obtained by solution ICP-MS for most elements of geochemical interests. Our results demonstrate that the fsLA-ICPMS using XRF glass bead can be used for rapid bulk-rock analyses of trace, minor and major elements.

[D-SEG Seminar]

Date:
2018/07/05 (Thursday) 16:00-17:00
Place:
JAMSTEC HQ/ Marine Ecosystem Research Building, 1F Seminar Room No.3 #105
Speaker:
Danielle McLean (JSPS summer program fellow, Oxford University)
Title:
Resolving the tempo of volcanism in East Asia using the Lake Suigetsu record (central Honshu)
Abstract:
The Lake Suigetsu core, obtained from a small tectonic lake in Fukui Prefecture, is the most precisely dated sediment record for Japan. Due to its high sedimentation rates, small catchment and incredibly low energy environment, the core resolves a very comprehensive record of ash (tephra) fall events over the last 150 ka. By using density-extraction techniques, we have identified that four times more tephra layers are recorded as non-visible (cryptotephra) markers in the sediments. Glass compositions of these tephra layers reveal that they are sourced from volcanoes across Japan, Ulleungdo (South Korea) and Changbaishan (North Korea and China border). These detailed investigations are able to significantly extended the known ash dispersal of many key markers and provide an improved chronology of the tempo and frequency of eruption events.

[DSEG-ODS seminar]

Date:
2018/06/28 (Thursday) 16:00-17:00
Place:
JAMSTEC HQ/ Adm. Building, 3F Meeting Room #305
Speaker:
Shigeaki Ono (ODS)
Title:
Density and seismic velocities of Ca-perovskite from first-principles simulation
Abstract:
Physical properties of Ca-perovskite are of great importance to interpret the structure and chemical composition in the mantle. First-principles molecular dynamics simulations were performed to investigate the equation of state and the elasticity of Ca-perovskite under high pressure and high temperature corresponding to the mantle. The estimated density and shear wave velocity were higher than those of the lower mantle. In contrast, the longitudinal wave velocity was lower than the mantle. This indicated that at least three minerals, Ca-perovskite, Mg-perovskite and ferropericlase, are necessary to explain elastic properties of the lower mantle.

[D-SEG Seminar]

Date:
2018/06/21 (Thursday) 16:00-17:00
Place:
JAMSTEC HQ/ Adm. Building, 3F Meeting Room #305
Speaker:
Kenta Ueki (D-SEG)
Title:
Machine learning-based geochemical discrimination and feature selection of magmatic tectonic settings
Abstract:
Geochemically discriminating between magmatism in different tectonic settings is a fundamental problem for the geology and geochemistry. It enables us to understand the processes of magma generation within the Earth’s mantle, based on the identification of similarities and differences between the rock-forming conditions and geological processes that are prevalent in different tectonic settings.
Here, we present an approach where machine-learning (ML) methods are used for quantitative tectonic discrimination and feature selection using global geochemical datasets containing data for volcanic rocks generated in eight different tectonic settings (Back−arc basin, Continental arc, Continental flood, Island arc, Intra−oceanic arc, Mid−ocean ridge, Oceanic island and Oceanic plateau)with data for 20 elements and 5 isotopic ratios.
This study uses three ML methods; support vector machine, random forest, and sparse multinomial regression (SMR).
All Three ML methods allowed the successful geochemical discrimination between magmas formed in eight different tectonic settings. The results show that magmas formed in different tectonic settings have unique geochemical signatures, indicating that magma generation processes are closely connected to the tectonic setting.
We found that SMR is a particularly powerful and interpretable method because it quantitatively identifies geochemical signatures that characterize the tectonic settings of interest and the characteristics of each sample. We also present the most representative basalt composition for each tectonic setting based on SMR.

[DSEG-ODS seminar]

Date:
2018/06/14 (Thursday) 16:00-17:00
Place:
JAMSTEC HQ/ Adm. Building, 3F Meeting Room #305
Speaker:
Yoshihiko Tamura (ODS)
Title:
Origin of Moho in the Oceans: a new hypothesis
Abstract:
Oceanic Moho is defined by multi-channel seismic reflection image because there is a strong difference of acoustic impedance between the crust and mantle, which results in strong reflection at the boundary. A contradiction between Moho and the crust-mantle boundary appears here, that is, Moho reflections are not always observed. If Moho reflection represents the crust-mantle boundary, it should be universal. Moreover, we found an interesting relationship between the existence of Moho (reflection) and crustal thickness. We present here a new hypothesis about the origin of Moho based on seismological and petrological observations.

[D-SEG seminar]

Date:
2018/05/31 (Thursday) 16:00-17:00
Place:
Meeting Room at 3F, YOKOSUKA HQ
Speaker:
Jun-Ichi Kimura (D-SEG)
Authors:
Jun-Ichi Kimura, James Gill, Peter van Keken, Hiroshi Kawabata, and Susanne Skora
Title:
Origin of geochemical mantle components: Role of spreading ridge, subduction zone, and thermal evolution of mantle
Abstract:
We explore the element redistribution at mid-ocean ridges (MOR) and subduction zones (SZ) using a numerical model to evaluate the roles of decompression melting at MOR and fluxed melting at SZ of the mantle in Earth’s geochemical cycle, with focus on the formation of the depleted and enriched mantle components. Our model uses a trace element mass balance based on an internally consistent thermodynamic-petrologic computation to explain the composition of MOR basalt (MORB) and SZ magmas and their residual mantles and slab components. Model results for MORB-like basalts from 3.5 to 0 Ga indicate a high mantle potential temperature (Tp) of 1650–1500°C during 3.5–1.5 Ga before decreasing gradually to ~1300°C today. The source mantle composition changed from primitive (PM) to depleted as Tp decreased, but this source mantle is variable with an early depleted reservoir (EDR) mantle periodically present. We examine a two-stage Sr-Nd-Hf-Pb isotopic evolution of mantle residues from melting of PM or EDR at MORs. At high-Tp (3.5–1.5 Ga), the MOR process formed extremely depleted DMM. This event coincided with formation of the majority of the continental crust (CC), the sub-continental lithospheric mantle (SCLM), and the enriched mantle components EM1, EM2, and HIMU formed from the slab components at the Archean to early Proterozoic high-Tp SZs and stored in the lowermost mantle now found in ocean island basalts (OIBs) through plume upwelling. During cooler mantle conditions (1.5–0 Ga), the MOR process formed most of the modern ocean basin DMM. Changes in the mode of mantle convection from vigorous deep mantle recharge before ~1.5 Ga to less vigorous afterwards is suggested to explain the thermochemical mantle evolution. These EM and DM components appear to form large domains forming N-S hemispheric lower mantle EM and E-W hemispheric upper mantle DM structures, respectively. Mantle convection appears to be sluggish for these mantle components perhaps due to density stratification of the source materials.

[D-SEG seminar]

Date:
2018/05/10 (Thursday) 16:00-17:00
Place:
Meeting Room at 3F, YOKOSUKA HQ
Speaker:
Dr. Hetu Sheth (Department of Earth Sciences, Indian Institute of Technology
Title:
Deccan flood basalt volcanism: An overview
Abstract:
The 65 Ma Deccan Traps of India constitute one of the largest and best-studied flood basalt provinces of the world, implicated in the K/Pg Boundary mass extinctions. This talk will present the physical geology of the Deccan Traps as a virtual field tour. I will discuss the volcanological features of its “compound” and “simple” lava flows (dominantly pāhoehoe and rubbly pāhoehoe and rarely ‘a’ā) and discuss their comparisons to Hawaiian and Icelandic lava flows. I will also discuss the main geological features of the tholeiitic dyke swarms of the Deccan, and the search for feeder dykes of the Deccan lavas based on geochemical and Sr-Nd-Pb isotopic criteria. I will also talk briefly about larger intrusive complexes and non-tholeiitic (alkalic and silicic) rock types, tectonic deformation, secondary minerals (mainly zeolites), ferricrete duricrusts forming from basalt weathering, and past studies of the Deccan by international (especially Japanese) researchers.

[D-SEG seminar]

Date:
2018/04/19 (Thursday) 16:00-17:00
Place:
Third Seminar Room on 1F, Marine Ecosystem Research Lab.
Speaker:
Ryosuke Oyanagi (JSPS PD, D-SEG)
Title:
Progress of Si-metasomatism during serpentinization in the oceanic lithosphere
Abstract:
Serpentinization is a hydration process that causes significant changes in physical and chemical properties of the oceanic lithosphere. Serpentinization occurs via interactions between mantle peridotite and water that commonly passes through the crust. Given that such a fluid has a high silica activity compared with mantle peridotite, it is thought that serpentinization and silica metasomatism occur simultaneously at the crust-mantle boundary. In this study, we conducted hydrothermal experiments in the olivine (Ol)-quartz (Qtz)-H2O system at 250°C and vapor-saturated pressure under highly alkaline conditions (NaOHaq, pH = 13.8 at 25°C) to clarify the mechanism of silica metasomatism at the crust-mantle boundary. Based on the hydrothermal experiments, we will discuss the progress of Si-metasomatism during serpentinization of oceanic lithosphere.

[D-SEG seminar]

Date:
2018/03/22 (Thursday) 16:00-17:00
Place:
Meeting Room at 3F, YOKOSUKA HQ
Speaker:
Bogdan Vaglarov (D-SEG)
Authors:
B. Vaglarov, T. Miyazaki, J-I. Kimura, Q. Chang, S. Haraguchi
Title:
Deployment of highly effective Hf-isotope auto-separation and details on improved method procedures
Abstract:
Hf-isotope ratios have proved to be indispensable geochemical tool.
In the past 20 years intensive research on the separation techniques as well as advent of multiple collector inductively coupled plasma mass spectrometry (MC-ICPMS) has led to high precision isotope ratio measurement of the high field strength elements (HFSE: Zr-Hf-Nb-Ta).
I will revise the achievements in this area and show our lab’s detailed and improved purification methods based on Ln-Spec selective resin. We deployed second fully automatic open-column chemical separation system (CPS100) particularly aimed at Hf-isotope separation.
Our results for several reference rock standards agree well with other published data.
I will also compare the different procedures used after acid digestion (AC) and Alkali flux fusion (AF) and will reveal more details from our laboratory and measurement procedures.

[D-SEG seminar]

Date:
2018/03/15 (Thursday) 16:00-17:00
Place:
Third Seminar Room on 1F, Marine Ecosystem Research Lab, Yokosuka HQ.
Speaker:
Takashi Miyazaki (D-SEG)
Title:
Investigation of shallow magma chamber processes using clinopyroxene and bulk rock Sr-Nd-Hf-Pb isotope compositions: A case study on the Raivavae ocean island basalts
Abstract:
Basalt lavas vary in composition because of differences in their formation processes and differences in the source materials. The formation processes include mantle melting, fractionation, mixing, and assimilation. Except mantle melting, all these processes take place in magma chambers. The importance of shallow magma chamber processes has been highlighted in the study of ocean island basalts (OIBs), including such fields as mineralogy and petrology, isotope geochemistry, and volatile geochemistry. Decoding the shallow magma chamber processes is important for any further discussion on source composition and mantle processes.
In many cases, isotopic and elemental variations (i.e., trends) of OIBs have been attributed to the variations in the source compositions and mantle processes. However, here we focus on the shallow magma chamber processes because of the clues provided by isotopic variations between the strongly leached bulk rock powders and the clinopyroxene separates. Clinopyroxene is a suitable OIB constituent for our study, because it is relatively resistant to alteration and was crystallized early in the magma differentiation process, and thus should have recorded the early history of a less fractionated magma.
The Raivavae ocean island, one of the Austral Islands in the South Pacific, consists of two volcanic units: the Rairua (7.4-10.6 Ma) and the Anatonu (5. 4-6.4 Ma) basalts (Maury et al., 2013). The Rairua basalts show typical HIMU isotopic signatures, while the Anatonu basalts have far more unradiogenic Pb than the typical HIMU basalts (Lassiter et al., 2003; Maury et al., 2013). In this study, the Sr-Nd-Hf-Pb isotopic compositions of the strongly leached bulk rock powders and the clinopyroxene separates are carefully examined to identify the shallow magma chamber processes that control the geochemical variations observed in the Raivavae basalts.
In this seminar, I will first talk about the analytical protocols of Sr-Nd-Hf-Pb isotope analysis optimized for this study, and then I will discuss the origin of the isotopic variations found in these basalts and clinopyroxenes. These variations are likely caused by the interactions between different basalt melts generated from different mantle sources in the shallow magma chambers.

[D-SEG seminar]

Date:
2018/02/22 (Thursday) 16:00-17:00
Place:
Meeting Room at 3F, YOKOSUKA HQ
Speaker:
Tatsu Kuwatani (D-SEG)
Title:
Probabilistic reconstruction of metamorphic P-T paths using zoned minerals
Abstract:
Understanding the pressure-temperature (P-T) evolution of metamorphic rocks is one of the fundamental goals of metamorphic petrology. Knowledge of continuous P-T paths for individual samples provides significant insight into the tectonic processes responsible for orogenesis and material recycling. Minerals often preserve information about P-T history within the chemically zoned structure. By the thermodynamic calculation, the changes of mineral composition from core to rim are translated into the evolution of P-T paths. However, the precise estimation has been difficult due to large uncertainty and noise from several unclear sources. In this study, we construct the Bayesian inversion methods, which can probabilistically reconstruct the P-T path from a zoned mineral. Here, we will introduce the basic concept of the Bayesian estimation and the data-driven approach with several applications to P-T inversion problems using differential thermodynamic method (Spear 1993) and inclusion geothermobarometry (St-Onge 1987).

[D-SEG seminar]

Date:
2018/02/08 (Thursday) 16:00-17:00
Place:
Meeting Room at 3F, YOKOSUKA HQ
Speaker:
Iona McIntosh (D-SEG)
Title:
Investigating silicic submarine eruption styles using volatile contents of volcanic glasses
Abstract:
Although silicic submarine volcanoes are common in island arcs like the Izu-Bonin, there have been few direct observations of their eruptions, and these observations have been limited to where a submarine eruption plume creates surface impacts, such as subaerial ash jets or floating pumice rafts. Submarine pyroclasts obtained by ROV and dredge surveys are therefore crucial records of eruption dynamics that occur unobserved beneath the waves.

The solubility of magmatic volatiles such as H2O and CO2 is pressure dependent, so measuring the volatile contents of pyroclast glasses can reveal the pressure, hence water depth, at which the magma was quenched to glass. Silicic glasses are very vulnerable to secondary hydration, where water from the surrounding environment diffuses into the glass in the time following deposition and alters its original water content. However a new method using FTIR spectroscopy enables us to identify such ‘hydrated’ glasses and to estimate their original water contents (McIntosh et al 2017 American Mineralogist). Using FTIR it is therefore now possible to find the quench pressures of silicic submarine pyroclasts glasses.

I will present volatile data from two silicic submarine volcanoes, which are shedding new light on our understanding of silicic submarine eruption styles. The first example, Oomurodashi, is a shallow active volcano in the northern Izu-Bonin arc that lies ~20 km off Izu-Oshima and is now recognised to have caused subaerial tephra fall on nearby islands. Volatile data from the subaerial tephra fall imply that its eruption is linked to disequilibrium degassing processes, perhaps due to rapid magma ascent. Other pyroclasts from the submarine crater wall appear to be effusive in origin and offer the tantalising prospect of identifying their approximate eruption ages by linking their volatile contents to sea level rise during the Holocene. The second example, Havre, is a volcano in the Kermadec Arc that erupted in 2012, creating a subaerial plume and large floating pumice raft from a deep vent 900 metres below sea level. Despite this deep eruption source many of these pyroclasts have volatile contents implying degassing to near-surface pressures, including giant pumice clasts measuring several metres across. In addition, almost all clasts show evidence of hydration, despite their young age. I will discuss the implications of these novel data for our current understanding of submarine eruption plumes.

[ODS&D-SEG seminar]

Date:
2018/02/01 (Thursday) 16:00-17:00
Place:
Meeting Room at 3F, YOKOSUKA HQ
Speaker:
Yasuhiro Hirai (ODS)
Title:
Geochemical variations in primitive lavas from the Kibblewhite volcano, Kermadec arc
Abstract:
Kibblewhite is one of the submarine volcanoes in the volcanic front of the southern segment of the Kermadec arc. Samples were collected from this volcano by dredging during the RV SONNE Vitiaz (SO-255) cruise in March-April 2017. The recovered lavas range from basalt to rhyolite (SiO2 = 49.9-70.9 wt.%; Mg# = 78.2-28.8) and belong to the Low-K to Medium-K series of Gill (1981), consistent with previous observations (Wright et al., 2006). Here, we focus on mafic lavas with Mg# >55, and those geochemical compositions are consistent with the presence of at least three types of primary magmas beneath the Kibblewhite volcano.

The primitive lavas (Mg# >55) can be divided into ankaramites, rift-type basalts, and magnesian andesites. Ankaramites contain abundant (~40 vol.%), centimeter-sized clinopyroxene (cpx) and olivine phenocrysts and show high CaO/Al2O3 ratio (0.9 to 1.4), which is similar to "island arc ankaramite" of Vanuatu Arc (Barsdell and Berry, 1990). Rift-type basalts (containing ~3 vol.% olivines and ~1 vol.% plagioclase and cpx) have higher wt.% TiO2, Al2O3, and lower FeO* and Ba/Nb values (~20.8) compared with the other types (Ba/Nb = 155-570) and are similar to basalts from Havre Trough (Todd et al., 2011), suggesting smaller contributions from subduction components. Magnesian andesites (SiO2 = 57.4-58.8 wt.%; MgO = 5.3-5.7 wt.%; Mg# = 56-58) are aphyric with skeletal olivine and cpx microphenocrysts and sometimes contain olivine xenocrysts.

Olivines in the ankaramites are forsteritic (Fo = ~92), but have unusually low NiO contents (~0.15 wt.%), and are not in equilibrium with mantle peridotite. On the other hand, olivines in the rift-type basalts show positive correlation between NiO (0.11-0.28 wt.%) and Fo contents (85-89). This trend is consistent with olivine maximum fractionation model, which suggests that ~12% olivine should have been fractionated from the primary melts in equilibrium with MORB residual mantle (Fo = ~91; Warren et al., 2016). Olivine microphenocrysts in the magnesian andesites have restricted compositions (Fo = 84-85; NiO = 0.13-0.17), which are in equilibrium with host rocks. The olivine xenocrysts extend to more forsteritic compositions (Fo = 86-93) and have higher NiO contents (0.19-0.35 wt.%) that can be in equilibrium with residual arc mantle (Ishii et al., 1992). Fractionation of 15% olivine with the same amount of cpx from the primary melts explains the continuous trends in major and compatible trace element composition.

The high CaO/Al2O3 of the ankaramites and the unusually low NiO contents in their olivines cannot be explained by partial melting of a peridotite source, however, experimental studies suggest that olivine-clinopyroxene cumulate could have been the source. Estimated primary melt of the magnesian andesite is also andesitic (SiO2 = 54.8 wt.%) with 12.7 wt.% MgO, and its normative composition is consistent with melting experiment of hydrous lherzolite at 1 GPa (Hirose and Kawamoto, 1995). All three types of lavas have a very restricted range of Nd and Pb isotopic compositions (e.g. 143Nd/144Nd = 0.51295-0.51300; 206Pb/204Pb = 18.84-18.89), suggesting that they have been derived from a similar source.

[D-SEG seminar]

Date:
2018/01/18 (Thursday) 16:00-17:00
Place:
Third Seminar Room on 1F, Marine Ecosystem Research Lab, Yokosuka HQ.
Speaker:
Maria Luisa G. Tejada (D-SEG)
Title:
A Journey Back in Time to Gondwanaland: IODP Expedition 369
Abstract:
I recently joined IODP Expedition 369 to the Great Australian Bight, Naturaliste Plateau and Mentelle Basin, the three basins that developed during the breakup of Gondwanaland. Australia, Antarctica and India were once continuous and formed the eastern part of Gondwanaland 140 million years ago and the break up led to the extension of the Indian Ocean to the southeast, forming the SEIR and the Southern Oceans. It is a major tectonic event that affected the oceans and opened marine gateways between the Pacific and the Indian Ocean. The goal of the expedition is to find out how oceanographic conditions changed as a result of this tectonic event and how it was triggered. The expedition successfully recovered recent to pre-Jurassic sedimentary sequences from the combined cores recovered at five sites, U1512, U1513, U1514, U1515, and U1516. These cores contain important records of paleoclimatic perturbations during the Late Cretaceous and into the Paleogene-Neogene transitions. The other highlight of the expedition is the recovery of pre-breakup sequences at Site U1515 and volcaniclastic sediments and basalt sequences at the deepest site, U1513. These materials will address questions about the pre-breakup conditions and the possible triggering mechanisms for the breakup. I will present some of the highlights of the expedition, with emphasis on the volcanic sequence and its possible association with the initiation of the Kerguelen Large Igneous Province.

[D-SEG seminar]

Date:
2018/01/11 (Thursday) 16:00-17:00
Place:
Third Seminar Room on 1F, Marine Ecosystem Research Lab, Yokosuka HQ.
Speaker:
Satoru Haraguchi (D-SEG)
Title:
Geochemical database of Japanese islands for basement rocks: compilation of domestic articles
Abstract:
A geochemical database covering the basement rocks of Japanese islands is constructed, the data of which are published in Japanese domestic journals and bulletins. Although there are some international geochemical databases available on the internet, most of the data published in Japanese have not been included. Increasing momentum towards the data-driven science shed light on the availability of dense dataset of the geochemical database with detailed location information. The new database project based on the domestic publications performed geocoding of the published data and introduced a unified format that is useful for the data-driven geochemistry. I introduce a technique of the database compilation mainly on the digitizing of positional information and collecting data, petrological and geochemical characteristics.