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doi:10.2204/iodp.proc.331.107.2011

Petrology

Site C0017, which was one of the contingency sites for Expedition 331, targeted a potential recharge zone ~1.5 km east of the principal hydrothermal field at Iheya North Knoll. Four holes were drilled within a 10 m radius (Fig. F1) at Site C0017, where the intervals from 0.0 to 37.3 mbsf (Holes C0017A–C0017C) and 60 to 150.7 mbsf (Hole C0017D) were cored. The four holes are considered to represent a single drilled section for purposes of petrological interpretation.

Hydrothermal alteration

The sequence drilled at Site C0017 comprises calcareous clay, volcaniclastic and foraminiferal silt and sand, and pumiceous grit and gravel (see “Lithostratigraphy”). With the exception of two intervals discussed below and the weak scattered development of green staining of pumice fragments in the upper parts of the sequence (interpreted as glauconitic “greensand,” although glauconite was not identified by XRD), the extensive hydrothermal alteration seen at Sites C0013 and C0014 is almost absent in the drilled interval at Site C0017.

In Hole C0017C, the interval from ~26 to 35 mbsf (Sections 331-C0017-1H-6 through 2H-6) shows weak to moderate oxidation, similar in nature but more intense than that seen at Site C0015. Oxidation is expressed as yellow to brown coloration of clay, silt, and sand, the development of orange to brown iron oxide staining on pumice fragments (Fig. F6), and the occurrence of 1–2 mm orange-brown botryoidal aggregates of Fe-Si oxyhydroxides. XRD analyses of samples from this interval show similar mineral assemblages to those that were identified for unaltered material from the site (see below), with the exception that primary igneous feldspar is absent in some samples. It is likely that some degree of feldspar destruction accompanies the oxidation of the pumice. Geochemical results (see “Geochemistry”) indicate that this zone of oxidation within the otherwise reduced sedimentary sequence is the result of an influx of seawater at this depth, potentially representing recharge flow into the Iheya North hydrothermal system.

The only other visual evidence of hydrothermal alteration at Site C0017 was seen in Core 331-C0017D-11X (140.2–149.7 mbsf), the deepest core from the site, which shows development of pale gray clay alteration of more permeable pumiceous grit horizons, intercalated with apparently unaltered indurated dark gray calcareous clay (Fig. F7). XRD analyses of this altered material (Table T3) identify the same quartz-glass ± anorthite assemblage as for unaltered pumiceous horizons (consistent with the visual observation that much of the pumice retains its original texture and mineralogy), with the addition of sylvite and halite in one sample (see “Mineralogy and pore fluid chemistry,” below).

The remaining sequence drilled at Site C0017 shows no visual evidence of alteration, with routine XRD identifying those phases that would be expected for a sequence of calcareous and volcaniclastic marine sediments, namely calcite, dolomite (discussed in “Mineralogy and pore fluid chemistry,” below), quartz, feldspar, muscovite, chlorite, and volcanic glass (Table T3). Halite is also commonly detected using XRD in samples from below ~25 mbsf, but it is possible that this may be due to seawater contamination from drilling.

Sulfide mineralization

When compared with Sites C0013 and C0014, the drilled interval at Site C0017 is notable for the absence of sulfide mineralization at shallow depth. Pyrite was only observed consistently at depths below 94 mbsf in Hole C0017D. In these deeper portions of the sequence, pyrite occurs in trace quantities as very fine grained disseminations and as rare scattered 1–3 mm aggregates with quartz and occasional discontinuous millimeter-scale bands or veins within indurated clay sediment.

Mineralogy and pore fluid chemistry

Site C0017 is the only location cored during Expedition 331 where dolomite is present in sediments (it also occurs in anhydrite nodules at Site C0013). At Site C0017, the occurrence of dolomite is limited to two discrete depth intervals (Table T3): ~10–20 mbsf in Holes C0017B and C0017C and ~110–140 mbsf in Hole C0017D. These intervals correlate well with intervals of low-Mg, high-alkalinity pore fluids (Figs. F8, F9; see “Geochemistry”).

The detection of the evaporitic minerals sylvite (KCl) and halite at ~143 mbsf in Sample 331-C0017D-11X-3, 27–29 cm, also correlates well with the pore water chemistry, corresponding to a sharp increase in dissolved K at that depth (Fig. F8; see “Geochemistry”).

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