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Alteration petrology

Three stratigraphic units (including six lithologic units) were identified in cores from Hole U1377A, and another three stratigraphic units (comprising 18 lithologic units) were recognized in cores from Hole U1377B (see “Igneous petrology and volcanology”). The entire section of trachybasaltic rocks at Site U1377 has undergone some degree of alteration by low-temperature water-rock interaction.

The overall alteration of volcanic rocks from Hole U1377A ranges from nearly fresh to highly altered (10%–95%) and from slightly to completely altered for rocks from Hole U1377B (Fig. F13). No evidence of greenish alteration indicative of reducing conditions was observed in either Holes U1377A or U1377B. Rather, it appears that oxidizing conditions were prevalent. Alteration at Site U1377 resulted in replacement of olivine and volcanic glass in some rocks. Olivine is altered to green and white clay in Hole U1377A but to iddingsite and Fe oxyhydroxides in Hole U1377B (Fig. F14).

Alteration phases

We distinguished three main groups of alteration minerals in Holes U1377A and U1377B:

  1. Carbonates are abundant secondary minerals as infillings in vesicles and veins. X-ray diffraction analyses on veins and vesicles suggest Mg calcite is predominant, with siderite and ankerite also being present (Fig. F15).

  2. Clay minerals (mostly white clay) are also abundant secondary phases and were principally identified using optical microscopy.

  3. Other secondary phases are pyrite (especially in lithologic Units 4–6 in Hole U1377A; Fig. F16), Mn oxides, and various Fe oxyhydroxides, likely hematite and goethite.

Overall alteration characteristics of Hole U1377A

The overall alteration of volcanic rocks in Hole U1377A is dominantly slight to moderate but has an overall range from fresh to high (10%–75%), as estimated from core descriptions and thin section observations (Fig. F13). Pervasive alteration imparted a brownish to brownish-gray color to the rocks (Figs. F13, F17). Moderately fresh rock with <20% altered groundmass is present in Sections 330-U1377A-3R-2, 6R-2, and 6R-3.

Olivine is typically completely altered to white clay in the trachybasalt from Hole U1377A (Fig. F14). Augite is not abundant and may have minor amounts of clay along fractures (Sample 330-U1377A-3R-2, 2–4 cm [Thin Section 270]). Vesicles are mostly filled with carbonate and white clay (Fig. F18). Abundant veins are present in these rocks. These veins are mostly filled with Fe oxyhydroxides in the uppermost 16 mbsf and with carbonates, white clay, and minor amounts of Fe oxyhydroxides in the remainder of the core (Fig. F19).

Overall alteration characteristics of Hole U1377B

The overall degree of alteration of the 18 lithologic units in Hole U1377B is high (>50%). Only a few intervals are slightly to moderately altered (e.g., Section 330-U1377B-4R-2 and interval 4R-3, 21–141 cm), and a few are completely altered (e.g., interval 4R-1, 0–12 cm). Alteration colors are brown to reddish brown, with strong evidence of oxidation. Olivine is altered to iddingsite and Fe oxyhydroxides. Plagioclase shows minor alteration to sericite/illite in some rocks but characteristically is fresh.

The overall high amount of alteration generally has completely transformed glass, but moderately fresh glass is present along what appear to be pillow rinds in intervals 330-U1377B-4R-3, 120–135 cm; 4R-4, 1–10 cm; 4R-5, 6–15 cm; and 5R-2, 96–110 cm. These are elongated glassy bands up to 15 cm long and ~1–2 cm wide.

Two dominant types of vein-fill minerals were identified. Carbonates and Fe oxyhydroxides form their own distinct veins, but many are composite, having both minerals present. Composite veins are typically >1 mm thick.

Vesicle infillings

Vesicles in rocks from Hole U1377A contain mainly carbonates and green and white clay minerals (Fig. F18). Thin section observations indicate a mixture of both in many vesicles, with smectite at the rim and carbonate in the interiors (Fig. F16A, F16B). The carbonate infilling may be zoned (Fig. F20), which may indicate precipitation of carbonate from different fluid compositions or temperatures. In this interval, we also observed pyrite in the groundmass and as coatings in vesicles that were later filled with carbonates (Fig. F20). X-ray diffraction patterns of the infilling of these vesicles identified the carbonates as Mg calcite, ankerite, and siderite (Fig. F15).

Vesicles in the rocks from Hole U1377B are quite similar, with carbonates and clay minerals as the dominant infilling minerals. Fe oxyhydroxide minerals are also present.

Vein infillings

In the cores from Hole U1377A, a total of 178 veins and vein networks were measured by the structural geologists, yielding an average of 4 veins per meter (see “Structural geology”). Most veins are quite thin and consist of carbonate and clay (Fig. F19). For Hole U1377B, the structural geologists measured 283 veins and vein networks, with an average of 9 veins per meter. The veins contain mainly carbonate interiors and reddish Fe oxyhydroxides (goethite) margins (Figs. F16C, F16D, F19). Many of the veins and fractures have reddish alteration halos. Other veins contain small dark minerals and dendrites, suggesting the presence of Mn oxides.

Interpretation of alteration

Igneous units throughout Holes U1377A and U1377B are affected by multistage alteration, mainly dominated by low-temperature fluid-rock interactions. The abundance of smectite and carbonates throughout Site U1377 indicates low temperatures (30°–150°C) typical of the lowest stages of ocean crust alteration (Alt, 1995). Ankerite and siderite were observed in Hole U1377A. These minerals are stable over a wide range of temperatures but can form at relatively low temperatures (<85°C). Nevertheless, they are not very common in the oceanic crust because they are metastable and can be easily dissolved (Laverne, 1993). Additionally, the number of veins filled with Fe oxyhydroxides (e.g., goethite) in Hole U1377B suggests alteration under oxidizing conditions (and probably weathering) or at least alteration in a shallow-water environment.

Combining observations from the two holes at Site U1377, we propose a two-stage alteration process similar to that discussed by Laverne (1993). The first stage would have taken place under reducing conditions, where the hydrothermal fluids penetrated the basalt and precipitated pyrite and siderite and probably Mn oxides. In a second stage under more oxidizing conditions, Fe oxyhydroxides precipitated and red halos formed.