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doi:10.2204/iodp.proc.331.103.2011 LithostratigraphyFour lithostratigraphic units were identified at Site C0013, Iheya North hydrothermal field (Fig. F3; Table T2).
Hydrothermal processes have significantly altered and/or replaced detrital siliciclastic and volcanic components in all units. We use the phrase “hydrothermally altered mud” to emphasize the importance of hydrothermal processes in controlling mineralogy, either by providing detrital components or by altering the primary mineralogy. Description of unitsUnit I
Unit I comprises three main sediment types: hydrothermally altered mud, sand, and grit with unconsolidated medium- to coarse-grained hydrothermal sulfide lithoclasts. Within Holes C0013B and C0013E, hydrothermal clay is volumetrically subordinate to sand and grit. Grit predominates in Holes C0013C and C0013D, but top intervals were not cored in these holes, and thus the true thickness of mud is not represented. Coarser units occur irregularly (Fig. F4), and no correlations could be made between holes, indicating an irregular sedimentary architecture and interbedding of sediment types. The rapid lateral variation between holes at distances of <10 m, particularly in coarse sulfide content, suggests a proximal source for this material. This is difficult to reconcile with a source at the South Big Chimney Mound, some 100 m distant. Sediment typesHydrothermally altered mud and clayThe detrital clay size fraction has hydrothermally altered and detrital hydrothermal minerals as major sediment components. Sediment is unconsolidated. Major mineral phases determined by X-ray diffraction (XRD) are kaolinite, illite, and barite. Accessory components include illite; detrital muscovite, some as small mica books 2–5 mm thick; angular quartz silt; detrital sphalerite and pyrite grains as large as 1 mm; disseminated wurtzite, sphalerite, pyrite, and galena; anhydrite, including euhedral anhydrite crystals; and acicular and partially devitrified volcanic spicules. Hydrothermal grit/gravelGrit or gravel layers contain discrete mineral grains and hydrothermal vein fragments or hydrothermal lithoclasts as major components. Detrital hydrothermal grains tend to be poorly sorted. Accessory components include quartz silt; discrete grains of wurtzite, sphalerite, pyrite, galena, and covellite; clasts of clear anhydrite veins with inclusions of galena and sphalerite; and opaque anhydrite, in some cases intergrown with talc and calcite. Clasts of metalliferous detrital sulfideClasts of metalliferous detrital sulfide consist of sphalerite crystals intergrown with anhydrite and quartz, as well as discrete pyrite grains. Clasts are typically subrounded and irregularly spherical to oblate (Fig. F5A). With increasing textural maturity, the size of the sulfide grains decreases and their sphericity increases because of abrasion. Abrasion preferentially erodes sulfide in fragments with siliceous gangue, increasing their quartz volume fraction, as shown in Figure F5C. However, secondary euhedral anhydrite overgrowths are present on many sulfide-rich gravel and grit clasts (Fig. F5B). Unit II
Unit II consists of hydrothermally altered mud with some heavily veined intervals, as well as coarser lithologies containing anhydrite and detrital metalliferous massive sulfide lithoclasts. Sediment types and lithotypesHydrothermally altered mudThe mud is pale gray with intense mottling and is often indurated. The detrital clay size fraction has hydrothermally altered or hydrothermal minerals as major components. Major minerals determined by XRD are kaolinite, illite, and anhydrite. Accessory minerals and components include discrete sulfide veins as wide as 1 mm (pyrite, sphalerite, and infrequent galena), vein anhydrite with sulfide mineralization, euhedral anhydrite crystals up to 2 mm in size, barite (detected by XRD only), fine acicular partially devitrified volcanic glass spicules, and lithoclasts of hydrothermal sulfide. Detrital metalliferous sulfide depositsThe detrital metalliferous sulfide deposits consist of poorly sorted detrital sulfide grains (see below). Layers are thin and generally in the grit- or sand-size fractions in Unit II (in Unit I they are coarser). Thin black bands containing sulfide mineral sand were also noted but were not logged as individual units. Components include angular quartz silt; aggregate detrital sulfide grains and discrete grains of wurtzite, sphalerite, pyrite, galena, and covellite; and clasts of clear vein anhydrite with inclusions of galena and sphalerite. Clast- or matrix-supported anhydrite brecciaThis lithotype comprises matrix- to clast-supported breccias with clasts of clear or white anhydrite (Fig. F6) in soft pale gray hydrothermal clay. Many are euhedral crystal fragments, up to 2 cm in size, and clearly formed in veins; others are more rounded but still an approximate euhedral form. Intervals with abundant euhedral anhydrite crystals exhibiting jigsaw fit, observed particularly well in X-ray computed tomography (CT) images, are interpreted as coarse veins fractured by drilling. A progression in clast shape can be seen from clear veins with inclusions of sulfide minerals (pyrite and sphalerite) to rounded anhydrite clasts that are opaque (Fig. F7). Unit III
Unit III comprises hydrothermally altered mud with layers rich in nodular anhydrite. It is distinguished from Unit II by the presence of large polycrystalline anhydrite nodules and reduced abundances of clear vein anhydrite. Nodular anhydrite horizons were encountered in Holes C0013C and C0013D at similar depths, suggesting formation at a common depth or via a common sedimentary mechanism. Sediment types and lithotypesHydrothermally altered mudThe mud is pale gray with intense mottling and is generally indurated. The detrital clay size fraction has alteration minerals as major components. Major minerals as determined by XRD are anhydrite, illite, and Mg chlorite. Minor talc is also present. Accessory minerals include sulfide minerals as discrete veins up to 1 mm in width (pyrite, sphalerite, and infrequent galena), euhedral anhydrite crystals up to 2 mm in length, and coarse euhedral vein anhydrite, partially replaced by sulfide minerals. Nodular anhydrite horizonsThe nodular anhydrite horizons are similar to hydrothermal mud layers but contain white, opaque anyhydrite nodules up to 5 cm in diameter (Fig. F8). The dominant component is always anhydrite, but minor dolomite, calcite, quartz, and talc, as well as 1%–2% fine-grained disseminated sphalerite and pyrite, are also typically present. Nodules commonly exhibit complex internal zonation, including coarsely crystalline veining, and many show pitted surfaces and truncation of internal zonation—clear evidence of erosion following precipitation. Unit IV
In Hole C0013E, Unit IV comprises silicified volcanic breccia from 23 mbsf (Section 331-C0013E-6X-CC, 8 cm) to 45 mbsf (interval 9X-1, 0–49 cm); it was also encountered at the base of Hole C0013D at 26 mbsf (Sections 331-C0013D-4X-1, 0 cm, to 4X-CC, 42.5 cm). Although described here as single unit, additional geochemical analysis of volcanic clast types and thin section description may further help subdivide the unit. A greater variety of clast types was encountered in Hole C0013E than in Hole C0013D, in which we only encountered a single silicified volcanic breccia horizon. Resedimented clasts of volcanic breccia predominate as the main clast type, and both monomictic and polymictic types are present (Fig. F9). All lithoclasts are now matrix supported and cemented by quartz (in interval 331-C0013D-4H-1, 0–2 cm) and quartz ± anhydrite (from Core 6H through Section 9H-CC). Other volcanic components in the breccia include clasts of pumiceous to vesicular flow-banded lavas. The major mineral in all cases is quartz, which may be coarsely crystalline or fine grained, accompanied by Mg chlorite, muscovite, and illite. Domains of volcanic glass are commonly well preserved (Fig. F10). Clay minerals replace other primary volcanic phases such as muscovite. Correlation of individual volcaniclastic lithologies from hole to hole was not practical because Unit IV was only encountered in the deepest parts of Holes C0013E and C0013D. However, it is notable that at Site C0014, substantial thicknesses of hydrothermally altered mud stratigraphically separate intervals dominated by volcaniclastic lithologies at 26–30 mbsf (Holes C0014B and C0014G), 54 to ~60 mbsf, and 126 mbsf (Hole C0014G). Hence, it is possible that distinct stratigraphic horizons are present and could be correlated on the basis of elemental composition between the two sites. Grain size distributionSeven sediment samples, ranging in depth from 0.54 to 4.92 mbsf, were selected from Hole C0013F for grain size measurement; data are summarized in Table T3 and Figure F11. It should be noted that this sample set is sourced entirely from Unit I and is not representative of the drilled interval at Site C0013. The mean grain size of the measured sediments shows large variation downcore, ranging from 7.13Φ to 2.89Φ (7 to 135 µm). Clay fractions in the sediment samples are mostly <10%, whereas sand fractions are mostly >40%. According to the grain size classification of Folk and Ward (1957), five of the measured sediment samples are silty sand and only two samples are sandy silt (Fig. F12). Grain size data indicate that most of the sediments are poorly sorted to very poorly sorted, with the standard deviation ranging from 1.33Φ to 2.68Φ. Skewness data suggest that only one sample (331-C0013F-1H-5, 15–16 cm) is coarse skewed and the others are all very fine skewed. Measured samples show platykurtic and leptokurtic to mesokurtic kurtosis according to classification by Blott and Pye (2001). Interestingly, two fine-grained samples (331-C0013F-1H-2, 45–46 cm, and 1H-5, 15–16 cm) show distinctly different distribution patterns when compared with the five coarser samples (Fig. F13), probably indicating different sedimentary provenance. InterpretationAt Site C0013, sediment lithologies are dominated by hydrothermal activity, either by in situ formation of hydrothermal minerals or through resedimentation of local hydrothermal deposits. Notable are the anhydrite-rich units, as anhydrite is undersaturated in cold seawater and thus dissolves. The presence of substantial anhydrite- and magnesium-rich clay horizons is evidence for subsurface temperatures greater than ~150°C. Were temperatures cooler than this for prolonged periods of time, anhydrite would not persist. Clay and sulfate minerals predominate in the lithologies found at Site C0013. These minerals display replacement textures in hand specimen and under the binocular microscope, and/or they have hydrothermal associations in terms of their genesis. Key minerals include Mg chlorite and talc. The hydrothermally altered sediments at Site C0013 likely sequester large quantities of sulfate and magnesium from interstitial water. The source of dissolved Mg cannot be hydrothermal fluids, as acidic rocks such as rhyolite are magnesium poor, especially in pumice (Kato, 1987), and high-temperature hydrothermal fluids are depleted in Mg in any case. Both sulfate and Mg doubtless derive from seawater, which is a major component of the fluids that have altered the sediment at Site C0013. Pumice is a common seafloor sediment in the vicinity of knolls and seamounts in the middle Okinawa Trough area and is abundant within sedimentary sequences at other sites drilled during IODP Expedition 331. These pumices often contain parallel tubelike vesicles (Kato et al., 1987). Recent studies infer that tube pumice is formed in subaqueous eruptions at 500–1000 m water depth when seawater that is rapidly imbibed into the elongated vesicles reduces clast density and inhibits floating (Allen et al., 2010). According to this model, the pumiceous clasts accumulated by volcanic debris flows must originate from nearby seamounts, as they are buoyant for only a short time. It is reasonable to expect abundant pumice sedimentation at Site C0013 because it is located within the volcanic complex. Little if any pumice persists in the hydrothermally altered sediment at Site C0013. The relative rarity of pumice at Site C0013 is interpreted to be due to its destruction during intense hydrothermal alteration. |
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