Kyushu University Academic Staff Educational and Research Activities Database
List of Presentations
Junichiro Ishibashi Last modified date:2019.06.14

Associate Professor / Material Science of Solar Planets / Department of Earth and Planetary Sciences / Faculty of Sciences

1. Jun-ichiro Ishibashi, Saki Tsutsumi, Youko Miyoshi, Hydrothermal clay minerals recognized beneath and around seafloor sulfide mounds in the Okinawa Trough, 16th International Clay Conference, 2017.07, Seafloor massive sulfides recognized in hydrothermal fields in arc-backarc settings in the western Pacific have been considered a modern analogue for ancient volcanogenic massive sulfide (VMS) deposits. A series of scientific drilling campaigns were conducted at Iheya North Knoll in mid-Okinawa Trough, where active venting of high temperature fluid (T>300ºC) from sulphide mounds had been located at some sites. In September 2010, five sites were drilled at Original Site (27°47.5' N, 126°53.8' E; water depth = 1000 m) during IODP (Integrated Ocean Drilling Program) Expedition 331 [1]. Two drilling campaigns CK14-04 in July 2014 and CK16-01 in March 2016 followed, which targeted at another active field named Aki Site (27°46.1' N, 126°54.1' E; water depth = 1070 m). Although massive body of sulphide deposit was not recognized, most of sediment cores obtained from beneath and around actively forming hydrothermal mounds were characterized by intense and diverse hydrothermal alteration [1-4]. We conducted mineralogical and geochemical studies of the hydrothermal clay minerals to clarify their occurrence and characteristics.

Occurrence of hydrothermal clay minerals at the Original Site was characterized by zonal distribution of distinctive clay mineral assemblage; Zone 1 consists mainly of montmorillonite and sometimes of kaolinite, Zone 2 of chlorite and chlorite-smectite mixed-layer mineral, and Zone 3 represented by co-occurrence of illite (sericite) and chlorite, from shallow to deep [2]. Similar sequence of clay mineral assemblages was recognized also at the Aki Site. Occurrence of kaolin minerals was minor but notable in both sites, as previously reported for other active hydrothermal fields in the Okinawa Trough [5]. Especially, at the peripheral locality of the Aki Site, focused formation of kaolin minerals was recognized in rather shallow depth (from 8.5 to 11.0 meters below the seafloor), where dominant kaolin minerals changed along the depth from halloysite, kaolinite to dickite. The zonal distribution of clay minerals may reflect hydrothermal structure developed in piles of volcanoclastic sediment, and the drastic change of clay mineral assemblage along the depth could be attributed to distinctive fluid-sediment interactions.

We recognize occurrence and characteristics of hydrothermal clay minerals in Iheya North Knoll show strong similarity to those had been reported for Kuroko-type VMS deposits in the northeast Japan. For example, significantly Mg-rich composition of the chlorite is commonly notable in spite of Mg-poor composition of the surrounding felsic volcanic materials. Comparative studies of hydrothermal clay minerals would provide keys to understand common hydrothermal interactions among modern and ancient systems which could play an important role in formation of VMS deposits..
2. Jun-ichiro Ishibashi, Hydrothermal clay minerals recognized in subseafloor of active hydrothermal fields in the Okinawa Trough, 45th Underwater Mining Conference, 2016.10, Diverse occurrence of hydrothermal clay minerals has been documented for seafloor hydrothermal fields in the Okinawa Trough, where the fluid discharge zone develops within thick sediment layer (Marumo and Hattori, 1999; Miyoshi et al., 2015a). Occurrence of clay minerals would provide important clues to understanding the physicochemical condition above and/or below an active seafloor hydrothermal field, because the formation of clay minerals is controlled by specific temperature and chemical conditions. Furthermore, in the case that occurrence of sulfide minerals can be related to occurrence of specific clay mineral, we may discuss physicochemical condition for hydrothermal mineralization process.
Recently, some scientific drilling campaigns were conducted in the Okinawa Trough (e.g., Takai et al., 2011; Ishibashi et al., 2015). They provide us a unique opportunity to directly access the subseafloor of active hydrothermal fields. Most of the obtained sediment cores are characterized by intense hydrothermal alteration, suggesting hydrothermal interactions are ongoing within the sediment layer (Takai et al., 2012). In this report, results of mineralogical and geochemical studies of the hydrothermal clay mineral in the Okinawa Trough are reviewed.
3. Jun-ichiro Ishibashi, Tomoharu Miyamoto, Practical class "Water geochemistry" composed of fieldwork, analysis and lecture, Goldschmidt Conference 2016, 2016.06, A practical class "Water geochemistry" is composed of pre-training (6 hours), fieldwork to collect water samples and observe rocks (one day), analysis of the collected water samples (6 hours), classroom lecture on chemical equilibrium of water-rock interactions (12 hours), and report writing (students' own effort). Sampling stations are located along streams around a karst tableland called Hirao-dai (Fig.1). The streams are selected from different geologic bodies (limestone region, granite body, metamorphic rocks), so that students easily recognize diversity of chemical compositions among the collected waters and ruminate factors controlling water chemistry (Fig. 2). Experiences through the class should be valuable for the third grade undergraduates as good training to switch their mind from passive lecture-based learning to active thesis studies..
4. Jun-ichiro Ishibashi, Are Seafloor Massive Sulfides a modern analogue for VMS deposit ?, Goldschmidt Conference 2016, 2016.06, Seafloor massive sulfides recognized in hydrothermal fields in arc-backarc settings in the western Pacific are commonly represented by coexisting occurrence of zinc- and lead-enriched polymetallic sulfides and abundant sulfate minerals. The mineralogy and geochemical signatures present has led researchers to suggest these areas may be a modern analogue for the formation of ancient Kuroko-type volcanogenic massive sulfide (VMS) deposits. Seafloor drilling during IODP (Integrated Ocean Drilling Program) Expedition 331 documented the subseafloor hydrothermal system at the Iheya North Knoll in the Okinawa Trough. Mineral textures and assemblages present in the drilled cores obtained from a hydrothermal mound in the proximal area were consistent with those recognized in ancient Kuroko-type mineralization. Moreover, stratabound occurrences of base-metal mineralization and widespread hydrothermal alteration were recognized across an area of over 500 m extent, which are comparable to ancient Kuroko-type deposits. On the other hand, geochemical studies of hydrothermal fluid venting from the seafloor and pore fluid within the sediment demonstrated diverse range of sulfide and sulfate mineralization could be related to subseafloor geochemical processes and fluid flows. Geochemical studies on present seafloor hydrothermal activities would provide a rare window into the dynamic processes for formation of VMS deposits..
5. Jun-ichiro Ishibashi, Carbon flux related to submarine volcanic and hydrothermal activities, Deep Carbon Observatory Symposium, 2016.06, CO2 flux transported by hydrothermal fluid circulation systems is important for discussion on global carbon flux to the oceans. Global CO2 flux can be estimated considering analytical results of CO2 and 3He concentrations in vent fluids. CO2 flux from an individual volcano can be estimated by monitoring pH (and CO2) anomalies in the water column above the volcano.
6. Jun-ichiro Ishibashi, Hydrogeological structure of a seafloor hydrothermal system related to backarc rifting in a continental margin setting, EGU General Assembly 2016, 2016.04, Seafloor hydrothermal systems in the Okinawa Trough backarc basin are considered as related to backarc rifting in a continental margin setting. Since the seafloor is dominantly covered with felsic volcaniclastic material and/or terrigenous sediment, hydrothermal circulation is expected to be distributed within sediment layers of significantly high porosity. Deep drilling through an active hydrothermal field at the Iheya North Knoll in the middle Okinawa Trough during IODP Expedition 331 provided a unique opportunity to directly access the subseafloor. While sedimentation along the slopes of the knoll was dominated by volcanic clasts of tubular pumice, intense hydrothermal alteration was recognized in the vicinity of the hydrothermal center even at very shallow depths. Detailed mineralogical and geochemical studies of clay minerals in the altered sediment suggest that the prevalent alteration is attributed to laterally extensive fluid intrusion and occupation within the sediment layer. Onboard measurements of physical properties of the obtained sediment revealed drastic changes of the porosity caused by hydrothermal interactions. While unaltered sediment showed porosity higher than 70%, the porosity drastically decreased in the layer of anhydrite formation. However, the porosity remained high (~50%) in the layer of only chlorite alteration. Cap rock formation caused by anhydrite precipitation inhibit the ascent of high temperature fluids to the seafloor. Moreover, an interbedded nature of pelagic mud units and matrix-free pumice deposits may prompt formation of a tightly layered architecture of aquifers and aquicludes. This sediment architecture should be highly conducive to lateral flow pseudo-parallel to the surface topography. Occurrence of sphalerite-rich sulfides was usually recognized as associated with detrital and altered sediment, suggesting mineralization related to subsurface chemical processes. Moreover, the vertical profiles of mineral occurrence documented for the obtained cores could be directly compared to those found within economically important Kuroko-type volcanogenic massive sulfide deposits (VMSD). High porosity within volcaniclastic sediment led to the development of a laterally extensive hydrothermal reservoir, favorable for formation of these large polymetallic ore deposits..
7. Jun-ichiro Ishibashi, Review of Japanese Taiga project part 2: Okinawa Trough, Third InterRidge Theoretical Institute, 2015.09.
8. Ishibashi, J., N. Ebina, S. Tsutsumi, S. Kawagucc, Fluid geochemistry of vent fields at submarine arc volcanoes in the Izu-Bonin arc, Goldschmidt Conference 2015, 2015.08, We conducted geochemical studies on vent fluid samples which were collected from three submarine arc volcanoes in the Izu-Bonin arc; clear fluid up to 280ºC venting from chimney structure along the caldera slope of Myojin Knoll, clear fluid up to 200ºC emanating from mound structure along the caldera slople of Bayonnaise Knoll, and clear fluid up to 300ºC emanating from flat hydrothermal precipitates or directly from sandy seafloor within the summit crater depression of Suiyo Seamount. The samples were collected by ROV (remotely operated vehicle) Hyper Dolphin (JAMSTEC) during NT14-06, NT12-10 and NT07-08 expeditions.
From analytical results of the collected samples, chemical composition of the hydrothermal fluid enedmember was estimated based on so called magnesium diagrams. For each vent field, single hydrothermal end member represents all the collected samples. For the three vent fields, major elements composition of the hydrothermal fluid is likely to be controlled by fluid-mineral equilibria in the reservoir.
Enrichment in Ca is notable for the three vent fields in the Izu-Bonin arc compared with a trend among mid-ocean ridge hydrothermal fluids. This signature is contrary to that the hydrothermal fluids from the Brothers Seamount in the Kermadec arc showed K-rich signature. Since both Izu-Bonin arc and Kermadec arc are known as an intraoceanic arc, the different trend in fluid chemistry is considred reflecting different chemical composition of host rocks. Actually, volcanic rocks from submarine volcanoes in the Izu-Bonin arc are classified in low-K series, while those from Brothers Seamount showed mid-K signature..
9. Junichiro Ishibashi, Youko Miyoshi, Hydrothermal alteration process in active seafloor hydrothermal systems in the Okinawa Trough, from a viewpoint of a modern analog for the Kuroko-type VMS deposits, PACRIM2015, 2015.03.
10. Junichiro Ishibashi, Dating hydrothermal minerals in active hydrothermal fields in the southern Mariana Trough, Goldshcmidt Conference, 2014.06.
11. Junichiro Ishibashi, Youko Miyoshi, Hiroyasu Inoue, C. Yeats, S.P. Hollis, J.C. Corona, S. Bowden, S.Y. Yang, G. Southam, Y. Masaki, H. Hartnett, Subseafloor structure of a submarine hydrothermal system within volcaniclastic sediments: a modern analogue for ‘Kuroko-type’ VMS deposits, 12th Biennial SGA Meeting, 2013.08.
12. Junichiro Ishibashi, Shingo Hirao, Tatsuo Ono, Toshiro Yamanaka, Chemical evolution within a hydrothermal fluid circulation system at the Aira caldera, Kyushu, Japan, IAVCEI 2013 Scientific Assembly, 2013.07.
13. Geochemistry of hydrothermal fluids from south Mariana backarc spreading center.
14. Geochemical studies of hydrothermal systems in western Pacific.
15. Fluid geochemistry of South Mariana Spreading Center.
16. Hydrothermal plumes along South Mariana Spreading Center.
17. Fluid geochemistry of Suiyo SMt. hydrothermal system.
18. Hydrothermal interaction with volcaniclastic sediment beneath the Suiyo Seamount submarine caldera, Izu-Bonin Arc.
19. Concentration of Biologically Important Chemical Species in Hydrothermal Fluids from Submarine Arc Volcano Suiyo Seamount.
20. Metallogenic and geobiologic outputs thorough seafloor hydrothermal systems related with Izu-Bonin Mariana arc-backarc magmatism.
21. Periodic Sampling of Diffuse Flow to Monitor Chemical Fluctuation.