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doi:10.2204/iodp.proc.330.105.2012 Alteration petrologyNineteen stratigraphic units were identified at Site U1374 on Rigil Guyot. The sequence includes 3 sedimentary units and 16 units of volcanic breccia, lava flows, and dikes (see “Igneous petrology and volcanology”). The entire section of basaltic rock has undergone alteration by low-temperature water-rock interactions or weathering. The overall alteration of the volcanic rocks ranges from slight to complete (5%–95%; Fig. F31), as estimated from core descriptions and thin section observations. Alteration at Site U1374 resulted primarily in replacement of olivine and volcanic glass. In contrast, plagioclase and augite are generally well preserved, both as phenocrysts and groundmass, throughout the entire igneous portion of the core. Plagioclase shows minor alteration to sericite/illite in some rocks but characteristically is fresh. Olivine is typically completely altered to iddingsite, hematite, Fe oxyhydroxides, and carbonates in the uppermost 350 m of core, and at greater depths it is primarily replaced by green clay (Fig. F32). Nevertheless, unaltered olivine was found in Unit XIII, and moderately fresh olivine occurs in Units XII–XIII (Fig. F32). In some cases fresh olivine and glass were also observed in thin sections (Table T8). Alteration phasesWe distinguished four main groups of alteration minerals in Hole U1374A:
Overall alteration characteristicsThe overall alteration of groundmass ranges from 5% to 95%, as estimated by core description (Fig. F31). The entire hole is very heterogeneous, with slightly to highly altered intervals occurring intercalated from top to bottom. A gradual increase in the degree of groundmass alteration was observed down to 60 mbsf (Units III–VIII), followed by a sequence of highly variable alteration from 60 to ~240 mbsf, below which an overall poorly defined downhole trend to slightly less alteration was observed. Throughout this lower interval, the majority of alteration is slight to moderate, but several intervals of high alteration are present even at these depths (Fig. F31). Relatively fresh basalt (<20% alteration) is present throughout the core but becomes more abundant below 400 mbsf in the intrusive sheets of Units XVIII and XIX. On the basis of core descriptions and thin section observations, we defined three general alteration groups: slightly altered gray basalts and two types of pervasive alteration. The main characteristic used to define these pervasive alteration groups is either grayish-brown/reddish-brown or greenish alteration colors (discussed below). Representative logs displaying the distribution of alteration colors with depth are given in Figures F38 and F39. Gray basaltSlightly altered gray basalt occurs throughout Hole U1374A. This basalt tends to be in thin units in the uppermost 300 m and becomes relatively abundant only lower in the hole in Units XVIII and XIX. The groundmass of this gray basalt is slightly altered to fine-grained brown and green minerals that are difficult to identify in thin section and will be referred to simply as clay minerals. Pyroxene and plagioclase are relatively fresh. Olivine is often partially replaced by green clay, iddingsite, and Fe oxides (Figs. F32, F40, F41), but some has moderate to high degrees of preservation (Table T8). Some fresh glass is present in several sections (discussed below). In particular, the intrusive sheets in Units XVII and XIX are only slightly altered, yet they exhibit tiny grains of pyrite in the groundmass, veins, and sometimes in vesicles. Grayish-brown to reddish-brown alterationGrayish-brown to reddish-brown alteration was observed mainly in the uppermost 350 mbsf of Hole U1374A in a series of basalt breccias extending downhole to Unit XVI. This alteration is not persistent throughout the entire portion of the core and is interrupted in places by greenish alteration that occurs sporadically throughout this interval. Slightly altered basalt occurs throughout this interval. This alteration group is characterized by carbonates, zeolites, and minor brown clay minerals. Olivine is mostly altered to iddingsite, hematite, Fe oxyhydroxides, and brown clay (Fig. F32). Basalts with greenish alterationGreenish alteration is present sporadically in the uppermost 340 m of Hole U1374A, with occurrences in only a few thin intervals. However, it becomes dominant below 360 mbsf, where the clay minerals (saponite, nontronite, and montmorillonite) are the dominant alteration minerals of the groundmass. Degree of alteration varies in these rocks, and zeolites are abundant in the more highly altered basalts. Pyrite is relatively common, especially along vein walls in association with clay minerals. Brown halosThroughout Hole U1376A we observed the presence of brown halos, typically 1–5 mm thick, sometimes associated with veins. The halos also appear on chilled margin contacts or surrounding the least altered basalt clasts. Their color ranges from light yellowish brown to dark reddish brown. The brown halos formed by the alteration of basalts by bottom seawater percolating through the oceanic basement (halmyrolysis; Mahoney, Fitton, Wallace, et al., 2001), which takes place at water temperatures of <2°C and under large water/rock ratios and oxidizing conditions. Such alteration is commonly found in permeable basaltic formations such as hyaloclastite and breccia. Vesicle infillingsMost of the basaltic flows and clasts from Site U1374 contain up to 10% vesicles, with abundance decreasing with depth. The highest percentage of vesicles, reaching 50%, was found in clasts in Cores 330-U1374A-55R and 66R (Units XVI and XVII). Below 384 mbsf, vesicles are rarer. Throughout the entire core most vesicles are partly filled with secondary minerals (Fig. F42), predominantly carbonates and zeolites (Fig. F34). Although carbonates are more common in the uppermost 370 mbsf, zeolites are the most abundant infilling mineral at greater depths (Figs. F34D, F42). Clay minerals (e.g., saponite) often line the vesicle walls, with carbonate and zeolite filling the interiors of the vesicles (Figs. F43, F44). Some intervals have clay formation followed by the crystallization of well-formed acicular or botryoidal crystals of zeolite (Fig. F44B–F44F). The clay becomes progressively darker with depth, with green (smectite) and white clay to ~300 mbsf, followed by black clay to the bottom of the hole. We note an important interval of green to blue-green clay and phyllosilicate minerals (nontronite and celadonite) filling vesicles and voids in Cores 30R through 56R (Units XIII–XVII; Fig. F45). On the basis of microscopic observations, it appears that the green clay formed later than carbonates, zeolites, and brown clay. Iron oxyhydroxides are sporadically present in vesicles in the uppermost 120 mbsf (down to sedimentary conglomerate Unit XI; Fig. F33F). They are most abundant in Unit VIII (olivine-phyric basalt). After a gap without abundant Fe oxyhydroxides, they appear again in vesicles in Units XVIII and XIX. Another phase present in some vesicles is tiny grains of native copper (interval 330-U1374A-30R-2, 53–54 cm). Native copper is thought to form either from precipitation from vapors in the initial volcanic rock (Hunter, 2007; Baxter, 2008) or from the release of Cu from Fe-Ti oxides during subaerial weathering, followed by remobilization and precipitation from seawater (LeHuray, 1989). Vein infillingsA total of 1229 filled or partly filled veins (and 3225 veinlets in vein networks; for details see “Structural geology”) were counted, yielding an average of 2.4 (8.5 when including veinlets) veins per meter. These veins are mostly small fractures (~1 mm) filled with clay minerals and carbonates in the uppermost 300 m of core (Figs. F46, F47A–F47B). Below 370 mbsf zeolite is the more common vein-filling material (Fig. F47C), but carbonates persist throughout the core and we occasionally observed coexistence of both species in the same vein (Fig. F47E, F47F). Clay is present in veins, with brown clay (Fig. F47D) being limited to the uppermost 360 m of the hole. The presence of brown clay in veins coincides with the reddish and brown alteration colors of the rock (Fig. F46). Both green and black clay minerals are present throughout the hole, but they become the only clay minerals present below ~360 mbsf. In the intrusive sheets of Units XVIII and XIX we observed the common occurrence of pyrite associated with zeolites in some veins (Fig. F48A, F48B). We also noted the occasional presence of colored halos (brown-red) associated with some veins throughout the hole (distinguishable in the wet, cut surfaces of the core; Fig. F37D). VoidsHole U1374A consists mainly of numerous units of volcanic breccia, portions of which contain a large number of voids, now (partially) filled in with minerals. For example, void space in the sedimentary conglomerate cover (Unit II) is predominantly strongly cemented by carbonates (see “Sedimentology”). The same cement is present in voids down to the basaltic breccia of Unit XIII, below which the voids become less abundant and smaller in size. Above 160 mbsf and between 280 and 360 mbsf, carbonates and zeolites are void-filling minerals (Figs. F49, F50). Between 150 and 280 mbsf, carbonate dominates, whereas below Core 58R (Unit XVIII), the voids are almost exclusively filled with zeolites (Figs. F49, F50). On the basis of a limited number of XRD analyses (Figs. F35, F36), it appears that there may be a progressive change from phillipsite to analcite in the voids with depth. At shallower depths, phillipsite is the main zeolite mineral; at deeper intervals analcite, natrolite, gmelinite, and possibly stilbite and thaumasite are also present. We identified phillipsite in Samples 330-U1374A-10R-4, 80–82 cm (52.5 mbsf); 22R-2, 140–143 cm (113.9 mbsf); 25R-2, 8–10 cm (126.8 mbsf); 28R-2, 0–4 cm (141.4 mbsf); and 30R-3, 56–58 cm (157 mbsf), whereas analcite was found in Sample 67R-4, 58–60 cm (483.6 mbsf), and natrolite and gmelinite were found in Sample 68R-5, 73–74 cm (483.6 mbsf). A zeolite tentatively identified as stilbite was observed optically in Cores 43R and 44R. Moreover, we identified thaumasite by XRD analysis (Samples 59R-6, 74–76 cm; 67R-4, 58–60 cm; and 68R-5, 90–91 cm). Thaumasite is a hydrothermal alteration mineral occurring after geothermal alteration of basalt and is commonly associated with zeolites, carbonates, and pyrite. Giret et al. (2003) indicated that in the volcanic rocks of Kerguelen archipelago, phillipsite formed at a temperature range of 40°–80°C, whereas analcite crystallized between 70° and 110°C. Stilbite requires temperatures between 100° and 140°C. A similar alteration thermal gradient therefore might also apply to the rocks at Site U1374. Goethite was found in association with zeolites in some sections (Sample 330-U1374A-9R-2, 109–110 cm; Fig. F33F) and was also observed in Samples: 47R-1, 133–135 cm (Thin Section 200); 48R-8, 20–23 cm (Thin Section 205); 49R-1, 92–94 cm (Thin Section 206); and 54R-4, 38–40 cm (Thin Section 210) (Fig. F41E, F41F). Olivine alterationMost of the olivine observed in split-core sections or thin sections is altered to varying degrees, as discussed above. Several intervals have olivine with <50% alteration. A summary of olivine (and glass) preservation as observed in thin section is given in Table T8. In the uppermost 80 m of the hole, iddingsite is the dominant alteration mineral after olivine. From 110 to 350 mbsf, there is no systematic pattern of olivine alteration. Throughout this interval, olivine is typically partly to completely altered to iddingsite, Fe oxides and oxyhydroxides, green and brown clay minerals, and carbonates. The rims are often altered to iddingsite, and the interiors subsequently altered to clay and iron oxides. The variation from iddingsite to green clay and Fe oxide alteration suggests some degree of variable oxidation conditions throughout this interval (Fig. F32). Below 350 mbsf, green clay minerals are the dominant alteration minerals after olivine, which coincides with the overall greenish alteration observed in the rock, indicating more persistent reducing conditions in the deepest portion of the hole. Glass alterationGroundmass 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-U1374A-3R-1, 7R-4, 21R-2, 29R-4, 34R-2, and 39R-2 (Table T8). Fresh to moderately altered glass is most abundant in Units XII and XIII. Fresh glass occurs only sporadically in these units because the rocks are highly variable in freshness themselves. Glass was visually observed in Sections 45R-2, 47R-1, 47R-2, 50R-1, 50R-2, 50R-3, 50R-4, 50R-6, and 66R-1. Interpretation of alterationIgneous units throughout Hole U1374A at Rigil Guyot are characterized by multistage alteration, mainly dominated by low-temperature fluid-rock interactions. One of the first indicators of alteration conditions is the distinctive colors observed in the core (Figs. F38, F39). Three main alteration colors characterize Hole U1374A, similar to Hole U1372A at Canopus Guyot. Gray reflects less altered basaltic units, whereas reddish brown and green could be directly related to the oxidation state of alteration processes, with the upper reddish-brown units indicating oxidizing conditions and the lower greenish units indicating reducing conditions. Oxidizing zones are characterized mostly by iddingsite, Fe oxyhydroxides, and carbonates replacing groundmass olivine and olivine phenocrysts, whereas reducing zones are characterized by green clay (nontronite and saponite) replacing olivine. Moreover, the presence of sulfides (pyrite) in the groundmass of intrusive sheets is also a good indicator of reducing conditions. Although the upper part of the hole seems to be dominated by oxidizing conditions, the oxidation state fluctuates in the uppermost 300 mbsf, reflecting environmental changes from submarine to subaerial. We observed that both environments were sometimes coeval in the uppermost 300 mbsf, which could reflect episodic changes in the eustatic variations of sea level or different phases of construction of the seamount in different environments. The contact between the oxidizing and reducing zones is gradational between 290 and 360 mbsf (see “Color reflectance spectrometry” in “Physical properties”; Fig. F28). Deeper than 360 mbsf, reducing conditions prevailed, yet even within this interval a portion of Core 330-U1374A-64R shows minor brownish alteration. Our observations are consistent with those of shipboard igneous petrologists (see “Igneous petrology and volcanology”), who propose that magmatism started in a submarine environment (Units XIX–XVII), which corresponds to the greenish reducing zone. Later, the magmatic activity progressed in a shallow-submarine environment (Units XVI–XII), which can explain the coeval existence of green and reddish-brown alteration. From Units XII to III, volcanic activity took place mainly in subaerial or wet, and therefore oxidizing, environments. The abundance of smectite (saponite and nontronite), illite, Fe oxyhydroxides, celadonite, and zeolites throughout Hole U1374A indicates low temperatures (30°–150°C) typical of the lowest stages of ocean crust alteration (Alt, 1995), as also observed in Holes U1372A and U1373A. Moreover, the presence of significant numbers of veins, vesicles, and voids filled with carbonates in the uppermost 300 mbsf indicates interaction with CO2-rich seawater-derived hydrothermal fluids at a relatively low temperature (<100°C; Honnorez, 2003). Nevertheless, with depth we also observed increased occurrences of zeolites and especially the transition between zeolite species from phillipsite to analcite and possibly to stilbite. This transition suggests an alteration temperature gradient, perhaps ranging from 40°–80°C to as high as 140°C (Walker, 1951; Neuhoff et al., 1997). |
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