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doi:10.2204/iodp.proc.324.103.2010

Alteration and metamorphic petrology

The entire section of basaltic basement recovered from Hole U1346A has undergone low-temperature water-rock interactions resulting in near-complete replacement of pyroxene, olivine, and glassy pillow rinds and complete replacement of glassy mesostasis. In contrast, plagioclase is generally well preserved. The overall alteration of the basalt pieces ranges from 25% to 90%, estimated visually using the binocular microscope on the archive half and the optical microscope on discrete thin section samples, without taking into account veins and vein halos.

Clay minerals are the most abundant secondary minerals and were principally identified by optical microscopy and X-ray diffraction (XRD) patterns on whole-rock, vesicle, and vein samples. Identification of clay species on XRD patterns is difficult because of the high proportion of calcite in the samples. Nontronite is the most readily identifiable clay mineral in the sequence with a green color and pleochroism in transmitted light. Saponite is identifiable by its translucent brown color in transmitted light and its third-order birefringence in cross-polarized light. Calcite is the second most abundant secondary mineral in these basaltic rocks, both pervasively altering the rocks and filling vesicles, voids, and veins. XRD spectra of whole rock, vesicles, and veins suggest that calcite is dominantly magnesium calcite. Other alteration minerals identified in the cores include pyrite, Fe oxyhydroxides, very rare marcasite and chalcopyrite, and zeolite only in one sample.

We compared the alteration degree and mineralogy of basaltic rocks recovered from Hole U1346A with previously well studied portions of ocean crust (Alt, 1995, 2004) and with basalts recovered from the Ontong Java Plateau (ODP Leg 192) (Mahoney, Fitton, Wallace, et al., 2001; Banerjee et al., 2004), Deep Sea Drilling Project (DSDP) Site 454 in the Mariana Trough (Natland and Hekinian, 1982), and DSDP Site 543 in Cretaceous crust of the Mid-Atlantic Ridge, near the Barbados accretionary prism in the western North Atlantic (Natland et al., 1984).

Low-temperature pervasive alteration processes

Based on core descriptions and thin section observations, we identified three types of pervasive alteration and two principal types of veining. Alteration colors range from dark gray to brown, with green alteration recovered at the top of the hole. The distribution of alteration colors is given in Figure F26 and in 324ALT.XLS in LOGS in "Supplementary material." Color classification was made on wet cut surfaces of the archive section half.

Green alteration

Extensive green alteration was only encountered in the first two igneous sections recovered 18 m above the onset of continuous igneous lithology interpreted as basement contact in Hole U1346A (from Section 324-U1346A-4R-1, 0 cm, through 4R-2, 39 cm; 119.5–121 mbsf). This sequence, defined as volcaniclastic debris by the igneous petrologists (stratigraphic Unit II), contains vesicular basaltic clasts interspersed with carbonate sediments. The basaltic clasts show a high degree of alteration ranging from 80% to 95%. Plagioclase is the only primary igneous phase remaining, although it has been moderately altered to brown clay. By contrast, pyroxene and glassy mesostasis are completely altered to green and brown clays, including nontronite and pyrite. Vesicles are rimmed by dark clays overlain by nontronite and are infilled with calcite, pyrite, and very rare marcasite and chalcopyrite (Fig. F27).

Dark gray alteration

Dark gray alteration is the most common alteration type encountered in the basement basalts in Hole U1346A (Fig. F26). Overall alteration of these variably vesicular basalts ranges from 30% to 70%, based on petrographic observations of thin sections. Secondary minerals in gray to dark gray basalts are brown clay minerals, which are typically very fine grained and difficult to identify either by optical microscopy or in XRD patterns. During this expedition, groundmass clays rarely have been identified as a fibrous brown saponite. Brown clays completely replace the glassy mesostasis, and pyroxenes are nearly completely replaced by calcite. Relics of primary pyroxenes are observed in some samples (e.g., Sample 324-U1346A-7R-1, 52–54 cm) (Fig. F28). The presence of olivine is inferred from the observation of calcite pseudomorphs that commonly contain fresh spinel (Sample 324-U1346A-8R-1, 56–60 cm). Calcite is also present as a minor phase throughout the clay-dominated altered groundmass.

Brown alteration

Brown alteration is found in short intervals alternating with the dark gray basalts (Fig. F26). The brown alteration overprints the gray alteration and many of the veins (Fig. F29). Overall alteration of these brown, variably vesicular basalts ranges from 30% to 60%, based on petrographic observations of thin sections. Secondary minerals in these brown basalts are brown clay minerals completely replacing the glassy mesostasis and almost completely replacing the pyroxenes, whereas plagioclase is moderately affected by alteration. Calcite pseudomorphs after olivine are also present in these basalts. Fine-grained Fe oxyhydroxides are disseminated in the groundmass and together with the brown clays likely give the rocks their brown color.

Glass alteration

Alteration of glass to phyllosilicates is virtually complete throughout the entire hole. No glassy mesostasis in the interiors of pillows and no pillow rim glass on the external rind of the pillows were preserved. Glass shards in hyaloclastites were nearly completely altered to phyllosilicates. Small segments of basaltic glass were only observed in two places in the core: in brown altered basalt in Sample 324-U1346A-8R-1, 56–60 cm, and in gray altered basalt in Sample 14R-1, 122–124 cm. In both cases glass is present as shards in hyaloclastites, which were preserved in a calcite cement (Fig. F17).

Vesicles

Basaltic rocks recovered from Hole U1346A are virtually all highly vesicular. The vesicles are filled with calcite and clays, irrespective of the alteration type, and commonly show a rim of dark brown clay minerals and fine-grained oxides, interpreted to be altered segregation melt within the vesicles (Fig. F30). Open vesicles or vesicles partially filled with clays are rare and only observed in fine-grained pillow rims. Pyrite is a major component, associated with little or no Fe oxyhydroxides, of the vesicle fillings in Unit II (Fig. F27) and in the upper part of the basement succession (above 143 mbsf in Cores 324-U1346A-6R and 7R). Below this depth, Fe oxyhydroxides are occasionally found in the vesicles in the gray alteration zones. Rarely, vesicles contain fibrous clays including nontronite and saponite (e.g., Sample 324-U1346A-13R-1, 102–105 cm) (Fig. F31).

Vein fillings

Eleven veins were counted in the green altered basaltic volcaniclastic debris (Unit II) and 159 veins in the ~53 m of basement rocks recovered from Hole U1346A (see Fig. F32 and 324VEIN.XLS in LOGS in "Supplementary material"), making an average of 3 veins/m. Veins in Unit II are isolated within the vesicular basalt and do not appear to extend into the enclosing sediment. Nevertheless, the mineralogy of the Unit II veins is nearly identical to that in the basement pillow lavas. All veins will therefore be considered together in the following description.

Most of the veins result from symmetrical infilling of open cracks with minor or no replacement of the wall rock. Two main vein types occur: (1) massive calcite veins and (2) nontronite-bearing veins associated with calcite, with or without either Fe oxyhydroxides or pyrite. Cross-cutting relationships, shown in Thin Section 51 (Sample 324-U1346A-14R-2, 66–70 cm) (Fig. F29), indicate that calcite veins are later than nontronite-bearing veins. In the nontronite-bearing veins, nontronite typically occurs along the rims and the interior of the veins is cemented with calcite, indicating that nontronite precipitated before calcite.

Pyrite occurs only as a major phase in the veins in the upper portion of the succession (Figs. F32, F33) with little or no Fe oxyhydroxides, whereas below 142 mbsf veins contain significant Fe oxyhydroxides (Fig. F34) in both brown and gray alteration styles. The veins range from fine fractures <0.5 mm to zoned veins of up to 3 cm in width, and vein width shows no systematic variation with depth (Fig. F32).

Sulfides

The volcaniclastic debris (Unit II; Core 324-U1346A-4R), recovered on top of ~60 m of sediment and extensively altered to green clays, are the only rocks in Hole U1346A to carry abundant pyrite and other sulfide minerals. Several percent of sulfides are present in the green basalt clasts in Core 324-U1346A-4R, which are set in indurated limestone that still shows sedimentary deformation around the clasts. Sulfides diminish in abundance in Core 324-U1346A-6R, are present but sparse in rocks at the top of Core 7R, and do not occur between Thin Section 22 (Sample 7R-2, 91–102 cm) and Core 11R. They are then minor through Core 14R and absent thereafter to the base of the hole.

Figure F27 shows features of the sulfides as seen in reflected light. In Thin Section 6 (Sample 324-U1346A-4R-1, 48–51 cm), sulfides are concentrated mainly in calcite veins and calcite-filled vesicles. The principal sulfide is pyrite, which occurs both in blocky and porous forms (Fig. F27A). Two small crystals of chalcopyrite (darker bronze) are also present. In Figure F27B, the blocky sulfide is marcasite (different colors in partially cross-polarized light and also anisotropic when rotating the stage) and the porous sulfide again is pyrite. Figure F27C shows a small vesicle in combined transmitted and reflected light. Green nontronite lines the vesicle and occurs as stringers within it. Both calcite and pyrite fill the remainder of the vesicle and are distributed across it. Other vesicles contain swarms of small pyrite crystals concentrated in calcite but to one side of the amygdule (Fig. F27D). An interpretation is that green clays and three different sulfides formed first and that all were cemented together by calcite in the veins and vesicles.

In some places, where calcite veins directly abut the basalt, the two forms of sulfide penetrate and replace materials between acicular plagioclase (Fig. F27E). Penetration of sulfide into basalt in this manner also occurs at the top of the pillow sequence in Core 324-U1346A-6R (Fig. F27F). Only small amounts of pyrite are disseminated in the groundmass of rocks at the top of Core 324-U1346A-7R, and none exists below section 324-U1346A-7R-2.

Interpretations of alteration

The basaltic rocks recovered in Hole U1346A are all moderately to highly altered with different types of alteration, green, dark gray, and brown, reflecting chemically variable fluid flow through the different stratigraphic units. The predominance of clay minerals (nontronite and saponite), with no high-temperature alteration minerals (e.g., chlorite), suggests relatively low temperature alteration (<100°C) (Alt, 1995).

Green alteration: interaction with S-rich low-temperature hydrothermal fluids at the basalt/sediment interface

Green alteration of the basaltic rocks recovered from the top of Hole U1346A relates to extensive replacement of primary minerals to green clays (i.e., nontronite) and precipitation of calcite and sulfide minerals. Altering fluids likely entered the rock along microfractures and other microscopic porosity structures. These fluids caused the replacement of existing glass and primary minerals in the groundmass to green clays, as well as the precipitation of calcite and sulfides in vesicles. Fluid circulation in larger fractures formed veins of green clays associated with abundant reddish Fe oxyhydroxides and calcite.

Evidence for in situ alteration of volcanic rock in sediment is provided by a strikingly zoned altered clast in Thin Section 7 (Sample 324-U1346A-4R-2, 33–39 cm). A greenish alteration rind surrounds a darker gray interior with two abrupt alteration fronts, the outer one ~1 cm into the rock and the inner one ~4 cm into the rock. The rock was cold when it combined with the enclosed indurated limestone, likely as either talus or as part of a debris flow, and it was subsequently heated and altered by circulating sulfide-precipitating hydrothermal fluids.

It is possible to correlate the green alteration of Core 324-U1346A-4R (Unit II) with the gray alteration in the basement pillow lavas of Core 6R (Unit V), sulfide mineralization being characteristic of the most pervasively altered green rocks at the basalt/sediment interface. Occurrence of three sulfides (pyrite, chalcopyrite, and marcasite) and green clays in the most pervasively altered rocks, with these changing downward to a combination of Fe oxyhydroxides and green clay, is reminiscent of mineral assemblages observed at margins of hydrothermal mounds at the Galapagos Spreading Center and elsewhere (e.g., Borella et al., 1983). The zonation suggests that nonoxidative hydrothermal fluid flow was concentrated at the basalt/sediment interface, probably related to impermeability of the sediments, and allowed precipitation of sulfides and green clays. In the porosity structure of the rocks, the fluids mixed with ambient seawater, which diluted and oxygenated them, favoring precipitation of a more oxidative secondary mineral assemblage (e.g., Fe oxyhydroxides).

Similar downward zonations in authigenic mineral paragenesis, with sulfide mineralization concentrated at the basalt/sediment interface and transitions from basalt colored green to basalt colored brown or reddish brown occur in pillow lavas drilled beneath a thin volcaniclastic blanket at DSDP Site 456 in the Mariana Trough (Natland and Hekinian, 1982) and at Site 543 in Cretaceous crust of the Mid-Atlantic Ridge near the Barbados accretionary prism in the western North Atlantic (Natland et al., 1984). At both places, sediment clearly provided permeability barriers through which circulating fluids in the more porous basalts could not penetrate. The sediments thus directed fluid flow horizontally, at the very top of basement.

At this juncture, temperatures of the mineralizing fluids are difficult to estimate. Both the sulfide mineral assemblage and the pervasive green alteration of the basalts of Core 324-U1346A-4R suggest that fluids were warmer, perhaps by tens of degrees in the volcanic rocks that were enclosed into sediment, than they were only 60 m deeper in the basement pillow lavas (Unit V) from Core 6R downward. By comparison, uppermost basalt at Site 456 is chloritized and secondary mineral assemblages in recrystallized sediment suggest a temperature >200°C. The sequence at Site 543, on the other hand, produced the greenish magnesian clay mineral saponite together with pyrite in the most altered basalts and occurred at much lower temperature. Further study of authigenic minerals in both the sedimentary rock and basalt of Core 324-U1346A-4R is warranted. Indications of low-temperature alteration in the bulk of the pillow sequence of Unit V do not indicate whether a very sharp hydrothermal temperature gradient once existed over only a few meters of core at Site U1346.

Gray and brown alteration: interaction with CO2-rich low-temperature fluids

Gray alteration is the most abundant alteration type in the basement pillow lavas (Unit V) in Hole U1346A and is interspersed with brown alteration throughout the hole. Secondary mineralogy for both alteration types is dominantly clay minerals (i.e., nontronite and saponite, an Fe-Al-rich and Mg-rich clay, respectively) and calcite. Clay minerals to various extents replace glassy mesostasis, plagioclase, and pyroxene, whereas calcite only replaces pyroxene and olivine, when present. Saponite and calcite associated with pyrite are commonly observed in the ocean crust, as filling veins and cracks or replacing the groundmass and olivine, and reflect low-temperature anoxic conditions at relatively low water-rock ratios (see Honnorez, 2003, for a review). Dark gray alteration with similar secondary mineralogy has also been observed in basalts from the OJP (Banerjee et al., 2004). The particularity of alteration in Hole U1346A, compared to other portions of ocean crust and to basalts at OJP, is the extensive replacement of pyroxene and glassy mesostasis by calcite, suggesting interaction of the basalts with CO2-rich seawater-derived fluids at relatively low temperature (<70°C) (Honnorez, 2003).

Brown alteration occurs in short intervals throughout Hole U1346A; petrographic observations indicate that this alteration overprinted the dark gray alteration (Fig. F29), with the secondary mineralogy being very similar to that of the dark gray alteration. Occurrences of disseminated Fe oxyhydroxides in the groundmass suggest circulation and interaction of the basalts with oxidative seawater-derived fluid at low temperature (<70°C). The low magnetic susceptibility in the basalts affected by brown alteration relative to the gray alteration (see "Physical properties") likely reflects the oxidation of magnetite in the gray alteration to Fe oxyhydroxides in the brown alteration. Chemical analyses on these two types of alteration indicate that there is no clear variation in chemistry related to the alteration type (see "Geochemistry").