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doi:10.2204/iodp.proc.330.107.2012

Alteration petrology

Four stratigraphic units were identified at Site U1376. The sequence includes two sedimentary units and two units of volcanic breccia and lava flows (see “Igneous petrology and volcanology”). The entire section of basaltic rock has undergone some degree of alteration by low-temperature water-rock interactions or weathering, but large intervals are only slightly altered. Lithologies from Hole U1376A are relatively well preserved compared to those at all other Expedition 330 sites.

Overall alteration of the volcanic rocks from Hole U1376A ranges from fresh to complete (2%–95%; Fig. F20), as estimated from core descriptions and thin section observations. Alteration at Site U1376 has resulted in replacement of olivine and volcanic glass in some of the rocks. In contrast, augite is generally well preserved as phenocrysts and in the groundmass throughout the igneous portion of the core. Olivine is altered to iddingsite and hematite only in an interval from 55 to 70 mbsf (Unit III) and even here the transformation is sporadic. Throughout the hole, olivine is either unaltered (<2% alteration; 72–106 mbsf) or moderately to completely altered to green clay and calcite (Fig. F21). Fresh olivine is especially abundant in the massive moderately olivine-augite-phyric basalt of Unit III (lithologic Unit 15; Fig. F10; Table T6).

Alteration phases

We distinguished three main groups of alteration minerals in Hole U1376A:

1. Clay minerals (e.g., smectite, saponite, nontronite, and illite) and celadonite are abundant and were identified using optical microscopy and X-ray diffraction spectroscopy (XRD).

2. Carbonates are also abundant secondary minerals as infillings in vesicles and veins. XRD analyses of whole rocks, veins, and vesicles suggest a predominance of Mg calcite, with evidence also of aragonite, siderite, and ankerite (Figs. F22, F23). Aragonite was identified in one interval of highly to completely altered basalt (Sample 330-U1376A-6R-1, 96–98 cm).

3. Other secondary phases are minor zeolites, probably phillipsite, various Fe oxyhydroxides, probably hematite, and goethite. Pyrite is rare but was observed in some fractures (Section 330-U1376A-19R-2).

Overall alteration characteristics

Overall alteration of groundmass ranges from 5% to 95%, as estimated from visual inspection (Fig. F20). However, alteration in most of the core is slight to moderate. Relatively fresh basalt (<20% alteration) is present throughout the core but is particularly abundant in Unit III. Units below the massive, moderately olivine-augite-phyric basalt (lithologic Unit 15 of stratigraphic Unit III) are mainly composed of volcanic breccia and hyaloclastites, and even those that are moderately altered tend to contain significant amounts of unaltered glass fragments.

On the basis of core descriptions and thin section observations we defined two general alteration groups of basalt: slightly altered gray basalt and more highly altered greenish basalt. Only minor amounts of reddish-brown alteration are present in restricted locations of the core. Representative logs displaying the distribution of alteration colors with depth are given in Figures F20 and F24.

Gray basalt

Slightly altered gray basalt occurs throughout the core. The groundmass of the gray basalt is slightly altered to fine-grained brown and green minerals difficult to identify in thin section, referred to simply as clay minerals (Fig. F25E). Pyroxene is relatively fresh in this basalt. Olivine is often unaltered, but some grains show partial replacement by green clay and carbonates (Figs. F21, F25A–F25D; Table T6). Fresh glass is present in several sections (discussed below).

Grayish-brown/reddish-brown alteration

Reddish-brown alteration is limited to very localized portions of the core (Fig. F20) and usually forms zones <20 cm wide between clasts. This alteration is characterized by hematite, carbonates, saponite, and perhaps bannisterite (Fig. F22). Large (>1 cm) prismatic carbonate crystals (Fig. F22) were determined to be aragonite.

Basalt with greenish alteration

Basalt with greenish alteration is present throughout the core and is the characteristic type of alteration at Site U1376. Clay minerals (saponite, nontronite, and montmorillonite) are the dominant alteration minerals of the groundmass and appear to be most abundant in the upper part of the basement (42–70 mbsf) and in an interval from 144 to 150 mbsf (Fig. F20). The degree of alteration varies in rocks lower in the basement, but fragments of fresh glass are generally still present.

Vesicle infillings

Most of the basaltic flows and clasts from Hole U1376A have low vesicularity. Vesicle abundance varies from 0.5% to 25%, with the high percentages being very rare and found mainly in clasts in volcanic breccia (see “Igneous petrology and volcanology”). The massive flow (Section 330-U1376A-8R-4 through Core 13R; lithologic Unit 15; stratigraphic Unit III) shows only 0%–0.5% vesicle abundance, and most of these vesicles are partly filled with secondary minerals (Fig. F26), predominantly carbonate and clay minerals, throughout the core (Figs. F27, F28). Zeolite infillings are rare. Clay minerals (e.g., saponite) often line vesicle walls, together with carbonate and/or other clay minerals that further fill vesicle interiors (Fig. F27). More oxidized zones within the core also contain Fe oxyhydroxides (goethite/hematite; Fig. F28D–F28F). Botryoidal infillings on vesicle walls in interval 330-U1376A-18R-1, 95–108 cm, were identified as siderite from XRD analysis (Fig. F23).

Vein infillings

Volcanic rocks from Site U1376 are penetrated by numerous veins. A total of 1190 veins and 280 vein networks were counted, yielding an average vein density of 8.1 veins per meter, which is at least twice as high as that observed at previous sites (see “Structural geology”). Throughout the core these veins are mostly filled with carbonate (Figs. F29, F30), especially the wider veins (as wide as 20 mm). Smaller veins are often filled with a mixture of carbonate and dark green and black clay. The highest abundance of carbonate veins occurs in Cores 330-U1376A-5R (38.6–47.12 mbsf), 7R (57.40–66.97 mbsf; within stratigraphic Unit III), and 22R (163.60–171.80 mbsf). The massive 33 m thick, moderately olivine-augite-phyric basaltic flow of Unit III (Section 330-U1376A-8R-5 through Core 13R) shows fewer veins. The secondary mineralogy changes in the lowermost two cores of the hole (Cores 22R and 23R), interpreted as an aphyric basaltic sheet intrusion, where the common occurrence of zeolite fillings in small veins was first noted (Fig. F29). Additionally, tiny pyrite grains occur along fractures in Section 330-U1376A-19R-2 (olivine-phyric basalt) in the middle of Unit IV. A blue coating on the vein rims was occasionally found in Sections 330-U1376A-7R-1, 7R-2, 7R-3, 15R-3, 15R-4, 23R-5, and 23R-6.

Olivine alteration

A summary of olivine (and glass) preservation observed in thin section is given in Table T6. Most of the olivine observed in the archive-half sections or thin sections shows varying degrees of alteration, as discussed above. Nevertheless, several intervals contain olivine with <50% alteration. In the uppermost 75 m of core, iddingsite replaces olivine, but it is minor compared to the green clay alteration. Deeper than 75 mbsf, no iddingsite was observed. From 75 mbsf to the bottom of the hole, the only olivine alteration noted was replacement by green clay and carbonates, which coincides with the overall greenish alteration observed in the rock, indicating persistent reducing conditions throughout the hole.

Glass alteration

Large amounts of fresh glass are present in the rocks from Site U1376, especially in the hyaloclastites from Unit IV. Significant amounts of this unaltered glass were observed in Cores 330-U1376A-15R through 19R and in Thin Sections 260 (Sample 15R-3, 41–44 cm) and 262 (Sample 17R-1, 140–143 cm). However, more highly altered basalt has groundmass glass completely altered to clay minerals or palagonite (Fig. F25E, F25F).

Interpretation of alteration

Igneous units throughout Hole U1376A are affected by multistage alteration that is mainly produced by low-temperature fluid-rock interactions. The abundance of smectite (saponite and nontronite), celadonite, and carbonates throughout Hole U1376A indicates low temperatures (30°–150°C) typical of the lowest stages of ocean crust alteration (Alt, 1995). The overall paucity of zeolites and the lack of evidence of alteration under oxidizing conditions suggest fluids of different compositions from those that generated the abundant zeolites at Site U1374. Moreover, in contrast to all other sites drilled during Expedition 330, we did not observe any reddish-brownish alteration in the upper part of the basement, with the exception of a few intervals with hematite. This observation complements those of the igneous petrology group (see “Igneous petrology and volcanology”) and supports the suggestion that the volcanic units were mostly formed submarine. We therefore conclude that cores from Hole U1376A did not experience weathering and alteration under oxidizing conditions. Nevertheless, the sedimentology group observed some evidence of sedimentary deposition in shallow-water environments and discussed the presence of an erosional surface between the sedimentary cover and underlying volcanic basement (see “Sedimentology”). We assume that the upper part of the volcanic basement, which probably experienced weathering and alteration under oxidizing conditions, is therefore missing because of erosion.

Additionally, Site U1376 is characterized by a large number of (relatively thick) carbonate veins, reflecting the strong circulation of Ca-rich fluids within the volcanic basement. This feature was not observed in such abundance at the other Expedition 330 sites. It is possible that the relatively smooth basement topography combined with the algal reef limestone cap (see “Sedimentology”) could have restricted the access of oxygenated seawater and influenced the composition of downward-percolating fluids. Additionally, early low-temperature hydrothermal alteration and mineral precipitation in fractures could have reduced permeability and restricted the flow of oxygenated seawater (Alt and Teagle, 2003). The massive basalt lava flow observed in Unit III (~72–106 mbsf) may also have acted as an impermeable layer, aiding in preserving the lower part of Hole U1376A from pervasive alteration. The precipitation of carbonate is one of the later-stage processes and usually precipitates directly from seawater at temperatures below 50°C. Carbonate precipitation could be the result of the percolation of Ca-rich fluids that acquired their Ca-rich compositions from the overlying limestone unit (Subunit IIA).

In addition to Mg calcite, XRD analysis identified aragonite, especially in Sample 330-U1376A-6R-1, 96–98 cm (Fig. F22). The abundance of aragonite in marine sediments is higher in deposits that are younger than the early Cenozoic (Stanley and Hardie, 1998). This could be a result of aragonite being metastable and inverting to calcite in older sediments, or it could reflect an increase in the Mg/Ca ratio of seawater since the early Cenozoic because high Mg/Ca ratios favor precipitation of aragonite (Stanley and Hardie, 1998). Sr isotopic age determinations for carbonates at ODP Site 707 in the northwest Indian Ocean indicate a temporal change in fluid compositions that led to a progression of aragonite to siderite and to calcite precipitation, although there is some overlap in the precipitation intervals (Burns et al., 1992). This progression in carbonate precipitation might represent an evolution of fluid composition over several million years. A similar age progression in fluid compositions and sequence of carbonate mineral precipitation might also have occurred at Site U1376, but confirmation of this hypothesis will require more extensive shore-based studies.

We also observed that many veins have fibrous carbonate crystals that grew perpendicular to the vein walls (Fig. F30). Similar features observed during ODP Leg 148 in 6 m.y. old oceanic crust in the equatorial East Pacific were interpreted as a result of repeated opening of filled cracks and sealing by precipitation of additional material from Ca-rich solutions (see “Structural geology”).

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