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Alteration petrology

Seventeen stratigraphic units were identified at Site U1372, comprising eight units of lava flows and seven units of volcaniclastics in the volcanic basement (see “Igneous petrology and volcanology”), overlain by two units of sediment (see “Sedimentology”). The entire section of mafic lavas and volcaniclastic units has undergone alteration by low-temperature water-rock interaction or weathering. Overall alteration of the volcanic rocks from Hole U1372A ranges from slight to complete (5%–100%), as estimated from core description and thin section observation. Alteration at Site U1372 resulted in replacement of olivine and volcanic glass. Olivine is typically completely altered to iddingsite, and in some portions of the cores the original phenocrysts have been replaced by Fe oxyhydroxide, green clay, serpentine, or carbonate minerals (calcite, Mg calcite) (Figs. F31, F32). In some intervals, however, olivine and glass remain unaltered. The least altered intervals, with abundant unaltered olivine, occur from Sections 330-U1372A-9R-5 through 9R-6 at ~60 mbsf (Unit IV) and in the moderately olivine-augite-plagioclase-phyric basalt from 225 mbsf to the base of the hole (Unit XVII). In contrast to olivine and glass, primary plagioclase and augite are generally well preserved throughout the entire igneous portion of Hole U1372A. Augite is almost always unaltered, whereas plagioclase is occasionally partially altered to sericite/illite.

Alteration phases

We distinguished three main groups of alteration phases in Hole U1372A:

  1. Clay minerals (saponite, nontronite, glauconite, and montmorillonite) and celadonite are abundant secondary phases and were principally identified using optical microscopy and X-ray diffraction. Clay minerals are present mainly in the groundmass as a result of alteration of interstitial glass, in the coating of vesicle walls, and in veins as intergrowths with other secondary minerals such as carbonates or zeolites.

  2. Carbonates are abundant secondary minerals and occur as infillings in vesicles, vugs, and veins. X-ray diffraction analyses of whole rocks, veins, and vesicles suggest a predominance of Mg calcite (Fig. F33) and minor siderite (interval 330-U1372A-26R-1, 123–124 cm).

  3. Other secondary phases are mostly zeolites (especially in vesicles and vugs), iddingsite, glauconite, Fe oxyhydroxides, and some pyrite. In a few sections at the base of the hole, low-temperature serpentine phases (chrysotile and lizardite; Section 330-U1372A-31R-1) and talc are present as a matrix surrounding clasts in volcanic sands (Unit XV).

Overall alteration characteristics

On the basis of core and thin section descriptions, we identified four broad alteration types, with the main distinguishing feature being color (which is highly dependent on alteration mineralogy). The four types recognized are light to dark gray (generally corresponding to fresh basalt), brown (generally clays), red (correlating to minerals such as iddingsite or iron oxides), and green (e.g., green clays and serpentine). Representative logs displaying the distribution of alteration colors with depth are given in Figures F34 and F35. These differences in color, particularly the distribution of red and green alteration types, were also quantitatively identified by color reflectance data collected by the Section Half Multisensor Logger on the archive halves (see “Physical properties”).

Gray basalt

Aphyric gray basalt, as well as well-crystallized groundmass within phyric basalt flows, was encountered throughout Hole U1372A in the inner, more massive parts of thick lithologic units. Within this gray basalt and groundmass, pyroxene and plagioclase are relatively fresh and olivine is often partially replaced by iddingsite, commonly along rims and fractures (Fig. F32A, F32B). However, fresh olivine was observed (Fig. F31; Table T7; see “Igneous petrology and volcanology”). Groundmass glass is generally altered to fine-grained brown minerals difficult to identify in thin section and therefore is simply referred to as palagonite.

Brown alteration

Brown alteration was observed mainly in the uppermost 50 m of Hole U1372A in the basalt breccia (Subunits IIA and IIC) and conglomerate (Subunits IID and IIE). Brown alteration is also present down to a maximum of 130 m in lithologic Units 4–5, 7, 15–19, 26, and 40–44. This alteration type is characterized by brown and white clay minerals and carbonates. Olivine is mostly destabilized to iddingsite, Fe oxyhydroxides (Fig. F32C), and brown clay (Fig. F32D).

Red alteration

The red alteration type was observed mainly from 45 to ~90 mbsf in Hole U1372A (Units III–VI) but is also occasionally present at shallower depths of <35 mbsf (Units I and II) and at depths of ~100 mbsf (Units VII and VIII) and ~140 mbsf (Unit XIII). The main color is given by the strong alteration of olivine phenocrysts and microcrystals in the groundmass to iddingsite (Fig. F32C) and Fe oxyhydroxides. Altered olivine crystals occasionally have brown oxidation halos, especially in areas associated with more intense veining. Regions with red oxidization also have abundant secondary clay minerals and carbonates that replace minerals in the groundmass, as well as infilling veins and vesicles (Figs. F33, F36). Volcanic glass is mainly altered to a mixture of brown clay and Fe oxyhydroxides. The red alteration occurs almost exclusively in the subaerially erupted units (Units IV–VI; see “Igneous petrology and volcanology”), indicating alteration by fluid-rock interaction in an oxidizing environment (Fig. F34).

Green alteration

The green alteration type was observed in the lower portions of Hole U1372A (90–232 mbsf; Units VI–XVII). These units were interpreted as resulting from submarine volcanism (see “Igneous petrology and volcanology”). This interval is characterized by a dominance of green clay minerals and a decrease in carbonate filling vesicles and veins (Figs. F36, F37); however, at least one vein (interval 330-U1372A-26R-1, 123–124 cm) contains siderite and clay minerals. Blue phyllosilicates (smectite and celadonite) are present as coatings in many vesicles and voids (Fig. F38D). Between 180 and 190 mbsf, hyaloclastite breccia and intercalated lava flows have matrix glass that is often (but not always) highly altered to palagonite. Olivine is moderately to completely altered to brown and green clay (e.g., smectite; Fig. F32) and perhaps serpentine group minerals (chrysotile and lizardite) and chlorite. We note the abundance of green alteration minerals such as nontronite (and perhaps serpentine) down to 200 mbsf (Core 31R). We also note the presence of fibrous minerals in the volcanic sand (interval 31R-2, 0–29 cm), which probably are a mixture of fibrous nontronite, talc, chrysotile, or needlelike zeolites, although this is difficult to determine. Such a paragenesis would suggest the circulation of moderately hot fluids (100°–200°C; Alt, 1995, 2004).

Vesicle infillings

The basaltic rocks from Site U1372 vary in vesicularity. Cores recovered between 50 and 70 mbsf (Units III–V) contain bands with high abundances of vesicles (up to 50%). At greater depths, the abundance of vesicles decreases to the point where nearly no vesicles are present at the bottom of the hole in lithologic Units 70–81.

The vesicles are mainly filled with carbonates, clay minerals, and zeolites (Figs. F33, F36, F38, F39). Clay minerals were found throughout the core (Figs. F38, F40). They appear alone or more often as coatings of vesicle walls that were subsequently filled with carbonates or zeolites (Fig. F40A, F40B, F40E, F40F). The carbonates show blocky to fibrous crystal habits, as well as botryoidal crystals (Fig. F40E, F40F). X-ray diffraction data indicate that calcite dominates, with minor siderite also present in interval 330-U1372A-26R-1, 123–124 cm. In larger vesicles and vugs, different generations of carbonates, sometimes associated with clay minerals, are present (Figs. F38, F40). Such associations and the botryoidal textures can be relics of multistage fluid flow through the rocks. In lava flows of Units III–VI, which formed subaerially, we occasionally found patches and vesicles filled first with sediment and then with red micritic limestone. We observed a slight decrease in the abundance of carbonates with depth (most of the occurrences in veins and vesicles are from 10 to ~90 mbsf).

Zeolite infillings are scarce to absent in the aphyric lava flows of stratigraphic Unit VI downhole to the upper hyaloclastites (Unit XII). Where present, they commonly form small, well-shaped crystals (Fig. F38E, F38F). Bluish minerals (smectite or celadonite; Fig. F38D, F38E) line voids and vesicles in several hyaloclastite sections. Fe oxyhydroxide infillings appear randomly downhole to Unit XI (Fig. F38C).

Vein infillings

Within the basement units (Units III–XVII) of Hole U1372A, 422 filled or partly filled veins were counted, yielding an average of 2 veins per meter (see “Structural geology”). These veins are mostly narrow (~1 mm) and are filled with clay minerals and carbonates, as well as minor zeolites (Fig. F37). Massive carbonate veins are the major vein type for Units III–VI. Carbonate-filled veins continue until Unit XIV. In the lower aphyric basalt, veins also contain Fe oxyhydroxides. Veins in the hyaloclastites (beginning with Unit XI) contain green clay minerals or zeolites, minor carbonates, and occasionally palagonite.

Many veins contain clay minerals at their margins and later stage infillings of carbonate or zeolite. Another vein type is filled with botryoidal crystals of carbonate or zeolite; the remaining space is filled with clay minerals (Fig. F41; interval 330-U1372A-26R-1, 117–128 cm).

High vein abundances at flow boundaries were sometimes observed, especially in the hyaloclastites of Unit XIV (see “Structural geology”). Here green clay infillings associated with zeolites or carbonates are common, but recovery was not very high in this unit.

Carbonates are abundant in the basalt breccia of Units I and II. The breccia is partly cemented with well-crystallized carbonates that show different generations of carbonate precipitation (see “Sedimentology”).

Olivine alteration

Most of the olivine observed in core samples or thin sections experienced varying degrees of alteration, as discussed above. Nevertheless, some occurrences of fresh to moderately altered olivine can be distinguished in hand specimen, especially in Sections 330-U1372A-9R-5 and 9R-6 at ~60 mbsf. A summary of olivine (and glass) preservation based on thin section observations is given in Table T7.

Glass and groundmass alteration

Most of the cored basalt has groundmass of varying degrees of alteration. Groundmass plagioclase and augite are fresh, whereas olivine and glass in the groundmass are extensively altered. A few sections of basalt above the hyaloclastite described below have small portions of preserved fresh glass (Table T7). Some of these more massive, fine-grained aphyric basalt sections contains <10%–20% unaltered glass in the groundmass, with the remainder altered to palagonite. Examples of massive basalt with minor fresh glass can be found in Sections 330-U1372A-13R-1 and 13R-4. All other basalt sections above the hyaloclastite sequences have glass completely altered to palagonite.

The hyaloclastites (Units XI–XVI) contain as much as 30%–50% unaltered glass (Fig. F42). The vitric clasts in these volcaniclastics show palagonization at their margins, with larger clasts (>1 cm) being mostly preserved. The matrix material between the glass shards is completely palagonitized. In this matrix zeolites and minor calcite are also present. Petrographic examination revealed excellent examples of fresh glass in the hyaloclastites in Sections 330-U1372A-18R-1, 19R-3, 27R-3, and 29R-3 (Table T7).

Interpretation of alteration

Igneous units throughout Hole U1372A are characterized by multistage alteration, mainly dominated by low-temperature fluid-rock interactions. One of the main indicators of alteration is the distinctive color observed throughout the core (Figs. F34, F35). Four alteration colors characterize Hole U1372A. Gray colors reflect less altered lithologic units. Red, brown, and green colors could be directly related to the oxidation state of alteration processes, with oxidizing conditions for the reddish and brownish upper units and reducing conditions for the greenish lower units. Oxidizing zones are characterized by iddingsite (replacing groundmass olivine and olivine phenocrysts) and Fe oxyhydroxides (in groundmass and vesicles) in the massive lava units. In contrast, reducing zones are characterized by green clay (nontronite and saponite) and a few occurrences of serpentine (lizardite and chrysotile), chlorite, and talc occurring principally in volcaniclastic breccia. Vesicles and veins present more or less the same alteration characteristics. The contact between the subaerial and submarine alteration zones is gradational, with the boundary at ~95 mbsf (see “Color reflectance spectrometry” in “Physical properties”; Fig. F43) and indicates a change in the oxidation state of circulating fluids, mainly related to the environment (submarine versus subaerial). These oxidation changes are consistent with typical alteration in the oceanic crust (Alt, 1995, 2004), where low-temperature moderately oxidative alteration occurs in the upper oceanic crust and is associated with the formation of clay minerals (e.g., nontronite) or celadonite, and more reducing conditions occur deeper in the oceanic crust where seawater-derived fluids are hotter (>150°C).

The overall alteration sequence in Hole U1372A is mostly dominated by carbonates, clay minerals (brown and green), and zeolites. The presence of these minerals, especially clay and zeolite, throughout the entire core indicates that the temperature was relatively low and constant (Honnorez, 2003). For example, Alt (1995) determined the formation temperature to be 30°–60°C for celadonite and nontronite and 15°–170°C for saponite. Moreover, the presence of an important amount of veins filled with carbonate minerals indicates interaction with CO2-rich, seawater-derived hydrothermal fluids at a relatively low temperature (<100°C; Honnorez, 2003). The slight change in the alteration sequence with a decrease in carbonate occurrences below 90 mbsf could also reflect more reducing conditions in the deeper part of Hole U1372A. Despite the dominance of low-temperature fluid circulation, we note the presence of minerals such as talc, serpentine, and maybe chlorite at ~200 mbsf and deeper. Such paragenesis may indicate that the deeper part of Hole U1372A reacted with higher temperature fluids (150°–300°C).