Clay Minerals in an Active Hydrothermal Field at Iheya‐North‐Knoll, Okinawa Trough

Abstract

Iheya-North-Knoll is one of the small knolls covered with thick sediments in the Okinawa Trough back-arc basin. At the east slope of Iheya-North-Knoll, nine hydrothermal vents with sulfide mounds are present. The Integrated Ocean Drilling Program (IODP) Expedition 331 studied Iheya-North-Knoll in September 2010. The expedition provided us with the opportunity to study clay minerals in deep sediments in Iheya-North-Knoll. To reveal characteristics of clay minerals in the deep sediments, samples from the drilling cores at three sites close to the most active hydrothermal vent were analyzed by X-ray diffraction, scanning electron microscope and transmission electron microscope. The sediments are classified into Layer 0 (shallow), Layer 1 (deep), Layer 2 (deeper) and Layer 3 (deepest) on the basis of the assemblage of clay minerals. Layer 0 contains no clay minerals. Layer 1 contains smectite, kaolinite and illite/smectite mixed-layer mineral. Layer 2 contains chlorite, corrensite and chlorite/smectite mixed-layer mineral. Layer 3 is grouped into three sub-layers, 3A, 3B and 3C; Sub-layer 3A contains chlorite and illite/smectite mixed-layer mineral, sub-layer 3B contains chlorite/smectite and illite/smectite mixed-layer minerals, and sub-layer 3C contains chlorite and illite. Large amounts of dioctahedral clay minerals such as smectite, kaolinite, illite and illite/smectite mixed-layer mineral are found in Iheya-North-Knoll, which is rarely observed in hydrothermal fields in mid-ocean ridges. Tri-octahedral clay minerals such as chlorite, corrensite and chlorite/smectite mixed-layer mineral in Iheya-North-Knoll have low Fe/(Fe + Mg) ratios compared with those in mid-ocean ridges. In conclusion, the characteristics of clay minerals in Iheya-North-Knoll differ from those in mid-ocean ridges; di-octahedral clay minerals and Fe-poor tri-octahedral clay minerals occur in Iheya-North-Knoll but not in mid-ocean ridges.

Defining boundaries for the distribution of microbial communities beneath the sediment-buried, hydrothermally active seafloor

“…These discontinuities in K concentrations could be explained by the possible occurrence of impermeable layers in the subseafloor environment ( Figure 1) and hydrothermal fluid flows just below the layers. The distribution pattern of the hydrothermally altered mud supports this interpretation (Supplementary Table S1; Miyoshi et al, 2015). In addition, because potassium was enriched in the endmember 310°C hydrothermal fluids (80 mM) but low in the ambient 4°C seawater (10 mM; Kawagucci et al, 2011), the K concentration is an indicator of the degree of mixing between hydrothermal fluids and infiltrated seawater and/or ambient pore-water.…”

“…In addition, because potassium was enriched in the endmember 310°C hydrothermal fluids (80 mM) but low in the ambient 4°C seawater (10 mM; Kawagucci et al, 2011), the K concentration is an indicator of the degree of mixing between hydrothermal fluids and infiltrated seawater and/or ambient pore-water. Although the magnesium concentration is also an excellent indicator of mixing between ambient seawater and hydrothermal fluids, the pore-water magnesium concentration could be affected by not only mixing but also association and dissociation processes related to hydrothermally altered minerals Miyoshi et al, 2015). Thus, in this study, the pore-water K concentration is likely more reliable than the magnesium concentration.…”

“…These values indicated that the clay minerals formed at 4138°C (Supplementary Table S1). At Site C0014, the shallow sediments were not affected by hydrothermal alteration at depths above~12 mbsf, whereas the deeper 412 mbsf sediments were hydrothermally altered (Miyoshi et al, 2015). The oxygen isotopic values of the hydrothermally altered clays at 12.8 and 15.1 mbsf in Hole C0014B were +8.7 and +12.5‰, respectively (Miyoshi, 2013).…”

“…Hydrothermally altered mud with elemental sulfur and sulfide grit was found in shallow sediments at 0.8 and 4.5 mbsf in Holes C0013B and C0013D, respectively (Miyoshi et al, 2015). The clay minerals in the hydrothermal mud yielded oxygen isotopic values of +8.0 and +9.6‰ (Miyoshi, 2013).…”

The origin of hydrothermal chlorite- and anhydrite-rich sediments in the middle Okinawa Trough, East China Sea

“…More detailed information about the cores is included in the IODP 331 report (Expedition 331 Scientists, 2011). A substantial amount of hydrothermally-formed secondary clay minerals, including chlorite, kaolinite-muscovite (intergrowth) and illite/smectite were recovered during IODP 331 Expedition (Expedition 331 Scientists, 2011;Miyoshi et al, 2015;Shao et al, 2015).…”