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doi:10.2204/iodp.proc.331.104.2011

Petrology

Seven holes were drilled at Site C0014, which was targeted at a low heat flow site ~0.5 km east of the main high-temperature chimneys of the Iheya North hydrothermal field (Takai et al., 2010). Of these seven holes, three (Holes C0014A, C0014C, and C0014F) were drilled no deeper than 6.5 mbsf and did not encounter significant hydrothermal alteration. The four deeper holes—Holes C0014B (cored interval 0.0–44.5 mbsf), C0014D (cored interval 0.0–16.0 mbsf), C0014E (cored interval 16.0–35.0 mbsf), and C0014G (cored interval 0.0–136.7 mbsf)—penetrated a sequence of hydrothermal alteration that is broadly consistent with the alteration assemblages seen at Site C0013. Although trace concentrations of sulfide minerals, most commonly pyrite, were present in almost all cores recovered from Site C0014, no significant mineralization was encountered.

The following description of hydrothermal alteration and mineralization from Site C0014 is based primarily on visual descriptions of cores from Holes C0014A–C0014G, supplemented by XRD analyses, scanning electron microscope (SEM) imaging, and energy dispersive spectrometry (EDS) characterization of representative samples. Most of the rocks cored at the site are composed of very fine grained material. With the exception of pumiceous volcanic glass, anhydrite, quartz, and sulfide minerals, most mineralogical determinations were made or confirmed using XRD analysis. This is particularly true of Mg chlorite, talc, kaolinite, and muscovite within the altered sequence.

The seven holes at Site C0014 were drilled within a 15 m × 15 m area and over a change in seafloor elevation of 2 m (Fig. F1), meaning that they can be considered to represent a single stratigraphic hole for petrographic interpretation.

Overview of hydrothermal alteration and mineralization at Site C0014

Hydrothermal alteration and mineralization at Site C0014 can be spatially divided into three assemblages that exhibit broadly consistent vertical distribution beneath the seafloor (Fig. F15):

1. From 0 to 8–10 mbsf, hemipelagic ooze and pumiceous volcaniclastic sediments exhibit little evidence of hydrothermal alteration.
2. From ~8–10 to ~25–30 mbsf, the sedimentary sequence is hydrothermally altered to pale gray mottled clay, with detrital quartz and muscovite the dominant phases detected by XRD, along with alteration products illite/montmorillonite and, less commonly, kaolinite. The uppermost 1–2 m of this zone shows a gradational increase in alteration intensity, with the replacement of the fine-grained matrix occurring at shallower depths than that of coarser pumiceous clasts. Unlike Site C0013, no significant anhydrite is associated with illite/montmorillonite alteration at Site C0014, implying less direct seawater input into the system at this site. Pyrite occurs in trace amounts as fine-grained disseminations and as very rare coarser veins throughout the sequence.
3. At depths below ~25–30 mbsf to the maximum drilling depth of 136.7 mbsf in Hole C0014G, XRD indicates that Mg chlorite becomes an important alteration phase, irrespective of lithology. Unlike Site C0013, detrital muscovite persists within this alteration zone, as does quartz. Anhydrite is present, mostly in millimeter-scale irregular veinlets with halite, below 57 mbsf in Hole C0014G but in much lower abundance than was seen at Site C0013. Pyrite occurs in trace amounts as fine-grained disseminations and very rare coarser veins throughout the sequence.

Near-surface unaltered sediment

As noted above, the sequence of mud, sand, and pumiceous gravel cored in the three shallow holes—Holes C0014A (0–6.5 mbsf), C0014C (0–6.5 mbsf), and C0014F (0–4.2 mbsf)—shows no evidence of hydrothermal alteration. Additionally, the uppermost portions of Holes C0014B (0–10.4 mbsf), C0014D (0–8.4 mbsf), and C0014G (0–7.9 mbsf) comprise similar unaltered material.

Sediment from just below the seafloor is a combination of pumiceous felsic volcaniclastic material, bioclastic fragments, and hemipelagic sediment. Based on XRD analyses (Table T7), quartz, muscovite, and calcite are the most abundant phases, with opaline silica, crystobalite, halite, feldspars, and chlorite also commonly detected. Fine-grained disseminated and framboidal pyrite (interpreted to be formed primarily by microbial processes) was also seen in most samples, both visually and by XRD.

Although there is no evidence of hydrothermal alteration in the near-surface sediments at Site C0014, the top few centimeters of Holes C0014F and C0014G show weak oxidation, expressed as brown coloration. In addition, the lower part of Hole C0014C, below ~1.5 mbsf, shows slight green staining of pumiceous gravel, interpreted to be glauconitic “greensand,” which implies an anoxic marine environment that is rich in organic detritus and low in sedimentary input (Odin, 1988). However, this interpretation is tentative, and no evidence of glauconite was found in XRD samples from the interval.

Illite/montmorillonite alteration

At the base of the unaltered sedimentary sequence in Holes C0014B, C0014D, and C0014G, a gradation toward increasing hydrothermal alteration is observed over a distance of 1–5 m in coarse pumiceous sediments. At the top of this zone, pumice clasts show incipient devitrification, and the fine-grained matrix is partially altered to pale clay (Fig. F16A). With increasing depth, pumice clasts increasingly lose their original texture and clay alteration becomes more intense (Fig. F16B, F16C). When fully altered, there is no textural difference between the remnant pale pseudoclasts after pumice and the darker clay that surrounds them (Fig. F16D). Hole C0014E, which began coring at 16.0 mbsf, commences in strongly altered material.

XRD analyses reveal an assemblage of quartz and muscovite, with variable occurrence of illite/montmorillonite and much less widespread kaolinite (Table T7). Illite/montmorillonite is detected in Holes C0014B and C0014D–C0014F, whereas kaolinite is only detected in Holes C0014B and C0014D and is significantly less abundant than at Site C0013.

It is likely that much of the quartz and muscovite within the kaolinite-illite/montmorillonite alteration zone at Site C0014 is detrital in origin, as both phases are abundant in fresh sediments at the site. The intensity of alteration, although estimated visually as “very high” (defined as >80% alteration products) in the lower parts of the interval, based primarily on the destruction of pumice clasts and the pale color of the sediment, is clearly lower than that observed at Site C0013. Additionally, the absence of anhydrite in this alteration zone at Site C0014 is suggestive of a much lower degree of fluid-rock interaction than at Site C0013.

The base of the illite/montmorillonite alteration zone at Site C0014 is defined via XRD by the appearance of Mg chlorite within the alteration assemblage. It occurs at 29.9 mbsf in Hole C0014B, 25.5 mbsf in Hole C0014E, and 28.2 mbsf in Hole C0014G. Hole C0014D, which was cored to a total depth of 16 mbsf, ends in illite/montmorillonite-altered material.

There is little mineralization associated with illite/montmorillonite alteration at Site C0014. Sulfide is mostly limited to fine-grained disseminated pyrite that occurs at low abundances (typically visually estimated at <<1%) throughout the sequence. A handful of slightly coarser pyrite-sphalerite veins, up to 2 mm in width, were also observed. A handful of XRD traces were also interpreted to indicate covellite and “copper sulfide” (Table T7), although none was identified visually in core.

Mg chlorite alteration

As was noted above, Mg chlorite–bearing alteration persists from ~25–30 mbsf to the maximum depth of drilling in Holes C0014B (44.5 mbsf), C0014E (35.0 mbsf), and C0014G (136.7 mbsf, with very poor recovery below ~115 mbsf).

A broad diversity of lithologies is present within the Mg chlorite–altered interval (see Site C0014 visual core descriptions in “Core descriptions”), including unconsolidated soft pale gray clay; heavily indurated dark gray clay that is verging on mudstone; hard fragmentally textured volcanic clasts featuring pale clay–altered fragments in a dark quartz-rich matrix; and sugary pale gray silicified fragmentally textured, flow-banded, and pumiceous volcanic clasts in a soft greenish-gray clay matrix. Despite the wide lithological diversity, the mineralogical assemblage shows remarkable consistency throughout the drilled sequence. As in the overlying kaolinite-illite/montmorillonite–altered interval, quartz and muscovite are present throughout the Mg chlorite–altered interval at Site C0014 and likely reflect the primary mineralogy of the volcaniclastic and pelagic sediments that comprised the sequence prior to alteration. Mg chlorite is present in almost all samples analyzed (Table T7). Anhydrite is found only below depths of ~57 mbsf in Hole C0014G and in low abundances. Although a small number of coarse 1 cm crystals were noted, most anhydrite occurs in irregular submillimeter-scale veinlets with halite. SEM imaging of a coarse anhydrite crystal (Fig. F17A) shows corrosion of the crystal surface, indicating partial dissolution of the mineral following precipitation, presumably by fluids at temperatures <150°C (Bischoff and Seyfried, 1978).

As was noted for the overlying kaolinite-illite/montmorillonite alteration, the lack of significant concentrations of anhydrite at Site C0014 is in stark contrast to Site C0013, where anhydrite is a major component in the Mg chlorite alteration zone. This difference is interpreted to reflect a relative lack of direct seawater influence on hydrothermal alteration at Site C0014.

Little mineralization is associated with Mg chlorite alteration at Site C0014. Sulfide is mostly limited to fine-grained disseminated pyrite that occurs at low abundances (typically visually estimated at <<1%) throughout the sequence. A handful of slightly coarser pyrite-sphalerite veins, up to 2 mm in width, were also observed, and 1% euhedral pyrite is also associated with the rare coarse anhydrite veining (Fig. F17B). In the lower part of the sequence, chalcopyrite was also identified visually in Cores 331-C0014G-24T (hosted in a vein with sphalerite and galena) (Fig. F18) and 331-C0014G-30X (disseminated in siliceous altered volcanic fragments, with pyrite). A handful of XRD traces were also interpreted to indicate covellite and “copper sulfide” (Table T7), although none was identified visually in core.

Interpretation of alteration at Site C0014

Hydrothermal alteration at Site C0014 shows a change with increasing depth from illite/montmorillonite to Mg chlorite–bearing rocks. This is similar to the change from kaolinite-muscovite to Mg chlorite alteration seen at Site C0013. As discussed in “Petrology” in Expedition 331 Scientists (2011c), this transition is consistent with the generally accepted compositional evolution of upwelling hydrothermal fluids in seawater-dominated subseafloor systems (Seyfried et al., 1999), so it is likely that the alteration observed at Site C0014 was produced by a single hydrothermal event.

Compared with Site C0013, the predominance of illite/montmorillonite over kaolinite at Site C0014 and the absence of native sulfur at shallow depths imply higher fluid pH for the altered sequence at Site C0014, which is consistent with an absence at Site C0014 of high-temperature H₂S-rich fluids that form sulfide chimneys in the Okinawa Trough (cf. Marumo and Hattori, 1999).

Significantly, transitions in alteration mineralogy occur at much greater depth at Site C0014 than at Site C0013, which is closer to current hydrothermal venting, consistent with the lower temperature gradient observed at the more distant site. As was discussed for Site C0013, temperature estimation is difficult, based on phyllosilicate and clay assemblages. Illite in particular may be stable over a very broad temperature range from <100°C to ~300°C (Srodon and Eberl, 1984), whereas, as previously noted, Mg chlorite implies alteration temperatures of 220°–300°C (Browne, 1978; Árkai, 2002) below ~25 mbsf.

Anhydrite is much less abundant at Site C0014 than Site C0013, and primary volcanosedimentary mineralogy is partially preserved throughout the altered sequence, implying a significantly lower fluid flux than at Site C0013. This observation is consistent with the interpretation that Site C0013 represents a site of recent high-temperature discharge, whereas Site C0014 is a “background” location within the hydrothermal system at Iheya North Knoll.

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