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Alteration petrology

Seven stratigraphic units were identified at Site U1373, comprising two sedimentary and five igneous units (see “Sedimentology” and “Igneous petrology and volcanology”). Overall alteration of volcanic rocks from Hole U1373A ranges from slight to high (10%–95%; Fig. F18), as estimated from core descriptions and thin section observations. Alteration at Site U1373 resulted in variable amounts of replacement of olivine and volcanic glass. Plagioclase phenocrysts and groundmass crystals have minor alteration to sericite/illite in some rocks but characteristically are unaltered. Augite is almost always unaltered. Olivine is typically completely altered to iddingsite and hematite in the uppermost 30 m of core, and at greater depths is primarily replaced by Fe oxyhydroxide, green clay, and carbonate minerals (Figs. F19, F20, F21, F22). Nevertheless, fresh olivine was found in a Type 7 clast in Unit I, and moderately fresh olivine occurs in Units IV–VI. No fresh glass was encountered in Hole U1373A.

Alteration phases

We distinguished three main groups of alteration phases in Hole U1373A:

  1. Phyllosilicate minerals (saponite, nontronite, montmorillonite, celadonite, and bannisterite) are abundant secondary phases and were identified using optical microscopy and X-ray diffraction (XRD).

  2. Carbonates are also abundant secondary minerals as infillings in vesicles, vugs, and veins. XRD analyses on whole rocks, veins, and vesicles suggest a predominance of Mg calcite and minor aragonite (interval 330-U1373A-10R-2, 108–110 cm; Figs. F23, F24).

  3. Other secondary phases are mostly zeolites (chabazite, phillipsite, and natrolite, especially in vesicles and vugs; Figs. F25, F26) and iddingsite, hematite, Fe oxyhydroxides, clinochlore, some pyrite/chalcopyrite, and goethite.

Overall alteration characteristics

The overall alteration of groundmass ranges from 10% to 95%, as estimated by visual observations (Fig. F18). In the uppermost 5 m of Hole U1373A (Units I and III), the groundmass alteration of basaltic clasts is quite homogeneous, regardless of the heterogeneity of these clasts in the sedimentary units. The volcanic breccia of Unit II is highly altered (70%–95%) with a brownish color. Basalt flows from Unit IV are characterized by slightly altered groundmass (10%), whereas the deeper Units V and VI are more altered (20%–70%) and brownish. Units V and VI are separated by a thin oxidized weathering surface. The lowermost recovered Unit VII consists of two massive aphyric basalt lava flows that are nearly fresh (10% alteration) with a gray groundmass.

On the basis of core descriptions and thin section observations, two types of pervasive alteration were identified on the basis of alteration color. In addition to the relatively unaltered light to dark gray basalt cored from Hole U1373A, two general color groups were identified: one with a slight greenish alteration color and one with a slight brown/red color. Representative logs displaying the distribution of alteration colors with depth are given in Figures F27 and F28.

Grayish-brown/reddish-brown alteration

Grayish-brown to reddish-brown alteration occurs mainly in the uppermost 45 m of Hole U1373A in the basalt breccia (volcanic clasts from Units I and III) and through the succession of peperitic flows from Units V–VI. This alteration is characterized by carbonates, zeolites, and minor brown clay minerals. Olivine is mostly altered to iddingsite, hematite, Fe oxyhydroxides, and brown clay minerals (Fig. F21). However, in Units III and IV olivine is mostly altered to green clay and Fe oxides, with thin rims of iddingsite.

Gray basalt with slight greenish alteration

Gray basalt is encountered in Unit IV and below 45 mbsf in Unit VII. This basalt is only slightly altered, and because of the massive nature of the unit, alteration appears to be quite consistent. The groundmass of this gray basalt is slightly altered to fine-grained brown and green minerals that are difficult to identify in thin section are referred to simply as clay minerals or palagonite. Pyroxene and plagioclase are relatively fresh, and olivine is often partially replaced by green clay and Fe oxides (Fig. F20). Blue clay minerals (smectite) are present as coatings in many vesicles and voids throughout the lower portion of the core (Fig. F25).

Vesicle infillings

The basaltic rocks from Site U1373 vary in vesicularity (see “Structural geology” and “Igneous petrology and volcanology”). In general, vesicularity peaks at 20%–50% abundance in the lower part of Unit III through Unit VI. Deeper than ~48 mbsf (Unit VII) the vesicle abundance becomes very low (<10%).

Many vesicles are partially filled with secondary minerals (Figs. F29, F30, F31), but the vesicles in clasts from Units I and II and in the aphyric basalt from Unit VII are typically fully filled. Optical microscopic observations as well as XRD data indicate that the vesicle infilling material is often a mixture of secondary minerals (Fig. F30). Basalt conglomerates and breccia (Units I and III) generally have calcite and minor zeolite vesicle infillings. Deeper than 37 mbsf, the amount of calcite in the vesicles decreases, with zeolites and green clay being the dominant vesicle-filling minerals. In this interval, many vesicles are composite, with clay minerals coating the walls of vesicles (Fig. F30A, F30B) that were subsequently infilled by well-formed crystals of zeolite (Fig. F30C, F30D). Occasionally, Fe oxyhydroxides appear in these lower units, and we notably observed goethite in interval 330-U1373A-7R-4, 108–110 cm (Fig. F31F). Blue clay linings of vesicles are especially abundant in the peperitic parts of Units V–VII (Figs. F23, F30F). XRD analyses of vesicle infilling material of Units IV–VII showed a mixture of different minerals such as zeolite (chabazite, phillipsite, and natrolite) and Mg calcite. Other minerals in these vesicles are usually smectite (Section 330-U1373A-8R-2, 29 cm) as well as chlorite (clinochlore; Sections 330-U1373A-7R-4, 39 cm, and 9R-1, 102 cm). Selected XRD spectra are shown in Figures F23, F25, and F26.

Vein infillings

A total of 291 filled or partly filled veins were counted, yielding an average of 4.4 veins per meter (see “Structural geology”). These veins are mostly small fractures (~1 mm) filled with clay minerals and carbonates. An exception is a higher abundance of slightly thicker veins (up to 9 mm) in Unit IV. Additionally, Unit IV has a higher abundance of veins in general. Massive carbonate veins are the major vein type for Unit IV and downhole to the upper and middle parts of Unit VII (Fig. F32C). In the upper sedimentary units of Hole U1373A (Units I and III), carbonate, minor brown clay, and green clay, as well as some sediment, are the most important vein infillings. From Unit IV to the bottom of this hole the abundance of green and black clay increases, as do Fe oxyhydroxides (Fig. F33). Many veins show a thin layer of clay at their margins and a later stage filling of carbonates (Fig. F32D), shown in XRD spectra as Mg calcite, saponite, bannisterite, and chalcopyrite for Sample 330-U1373A-7R-2, 141–142 cm (Fig. F24A). Another vein shows Mg calcite and aragonite (Sample 330-U1373A-10R-2, 108–110 cm; Fig. F24B). Zeolite is more common in the lowermost two lava flow Units VI and VII. In the lower part of Unit VII (~60 mbsf and below) cross-fibrous carbonate veins are the dominant vein type.

Olivine alteration

Most of the olivine observed in core samples or thin sections presents varying degrees of alteration, as discussed above. The only interval with significant amounts of fresh to moderately altered olivine is a Type 7 clast (see “Igneous petrology and volcanology” for further definition) in Section 330-U1373A-1R-2 at 1.69–2.35 mbsf in Unit I (Fig. F20A, F20B) and a highly olivine-augite phyric massive basalt in Unit VI (Samples 330-U1373A-7R-2, 129–130 cm [Thin Section 90], and 7R-3, 109–111 cm [Thin Section 92]; Fig. F20C, F20D). A summary of olivine (and glass) preservation in thin section is given in Table T6.

Several types of olivine alteration were observed in Hole U1373A. From Units I through III, olivine is mainly altered to iddingsite and hematite (Figs. F19, F21A, F21B), with some occurrences of mixtures of Fe oxides, clay minerals (Fig. F21C), and carbonates. From Unit IV to the bottom of Hole U1373A, olivine is mainly altered into green and brown clay minerals (montmorillonite and bannisterite; Figs. F19, F22). Some olivine phenocrysts have thin rims of iddingsite, with carbonate and clay in the interior portions of the grains (Fig. F21A). Alteration was commonly initiated in cracks to form sawtooth patterns of green clay, followed by a colorless mineral in the central portion of the veins. This mineral could be a later infilling of chrysotile (Fig. F20E, F20F).

Glass and groundmass alteration

Rocks from Hole U1373A have varying degrees of groundmass alteration in the basaltic rocks. Groundmass augite is always unaltered, plagioclase in some samples is altered to sericite, and olivine and glass are extensively altered. Groundmass glass in most of the thin sections is completely altered to clay minerals or palagonite. Nevertheless, slightly altered groundmass glass is present in thin sections from Sections 330-U1373A-1R-2, 1R-3, 7R-3, 7R-4, 9-1, 9R-2, and 13R-1 (Table T6).

Interpretation of alteration

The abundant occurrence of smectite (saponite and nontronite), celadonite, Fe oxyhydroxides, and zeolites in Hole U1373A indicates low temperatures (30°–150°C) typical of the lowest stage of ocean crust alteration (Alt, 1995). The association of these secondary minerals suggests circulation of large volumes of oxidizing fluids in the lava pile. If interpretation of the XRD data is correct, chlorite appears in Units IV and VI, which would be interpreted as alteration at higher temperatures (250°–300°C). The abundance of sulfides (pyrite) in the lower part of Hole U1373A below ~45 mbsf points to a change from oxidizing to reducing conditions consistent with the change of alteration zones at the same depth.

From the bottom to the top of Hole U1373A we observed gray massive aphyric basalt (Unit VII) that experienced slight alteration. The presence of green clay after olivine indicates reducing conditions for these basalts and reflects a submarine environment. The upper portions of core with a variety of grayish-brown to reddish-brown colors are indicative of oxidizing conditions, as shown by the alteration of olivine to iddingsite. Vesicles and veins present more or less the same characteristics with depth for the upper Units I–VI (with the exception of Unit IV). We interpret these units to have been emplaced and weathered under subaerial conditions. Units V and VI are separated by a thin oxidized layer, likely representing an ancient weathering horizon.