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doi:10.2204/iodp.proc.301.106.2005

Igneous and metamorphic petrology

Basement was cored from 351.2 to 582.8 mbsf (86.0 to 317.6 m into basement in Hole U1301B). The 69.1 m of recovered core consisted of (1) basalt-hyaloclastite breccia, (2) aphyric to highly phyric pillow basalts, and (3) massive basalts. Eight units were defined on the basis of changes in lava morphology, rock texture, and phenocryst grain size, as summarized in Figure F20 and Table T3 in the "Methods" chapter. Pillow lava units (Units 1, 3, 5, 7, and 8) were subdivided based on changes in phenocryst mineralogy and abundances. Massive lava units (Units 2, 4, and 6) were subdivided into individual cooling units based on the presence of chilled margins.

Lithologic units

Pillow basalts and basalt-hyaloclastite breccias (Units 1, 3, 5, 7, and 8)

Pillow basalt was the most abundant rock type recovered from Hole U1301B. Pillow lavas were identified by the presence of curved glassy chilled margins, oblique to the vertical axis of the core, with perpendicular radial cooling cracks (Fig. F21). An almost complete section through a single pillow was recovered in interval 301-U1301B-4R-3 (Piece 1, 0–45 cm) (Fig. F22), indicating that the pillows are at least 0.5 m in diameter. The pillow fragments have dominantly hypocrystalline textures with a cryptocrystalline to microcrystalline groundmass. They are aphyric or sparsely to highly plagioclase ± clinopyroxene ± olivine phyric. Observed basalt textures vary from hyalophitic (typically with sheaf-spherultic or plumose textures) to glomeroporphyritic, seriate, and intersertal. A small number of samples contain groundmass plagioclase crystals that display a weak pilotaxitic texture. The pillows are sparsely vesicular, containing 1%–5% round gas vesicles, and are slightly to moderately altered. Alteration includes interstitial groundmass replacement, vesicle fill, vein formation (with associated alteration halos), and complete replacement of olivine phenocrysts.

The subdivision of pillow Unit 7 (Fig. F20) was based on changes in grain size and phenocryst abundance. For example, Subunit 7A is sparsely to moderately plagioclase and olivine phyric, Subunit 7B is moderately to highly phyric and also contains clinopyroxene, and Subunit 7C is aphyric to moderately plagioclase, clinopyroxene, and olivine phyric.

Intervals 301-U1301B-1R-1 (Pieces 1–7, 0–57 cm) and 35R-1 (Piece 10, 81 cm) to 35R-2 (Piece 7, 52 cm) consist of basalt-hyaloclastite breccia and are defined as Subunits 1A and 8A, respectively (Fig. F23). These thin breccias (<1 m of recovered core) are composed of clasts that are similar to the underlying basalt, some with glassy margins. Given the low recovery, it is not possible to determine the relationship between the hyaloclastite portions and underlying lavas or, specifically, whether they are part of the same cooling unit. Subunits 1B and 8B are also thin, each representing <1 m of recovered core, and lack the definitive curved glassy margins characteristic of pillow lavas. These subunits could be interpreted as either pillow flows or sheet flows (possibly with brecciated tops; Subunits 1A and 8A, respectively), and are therefore simply described as basalt lavas. Because the relationships between the basalt-hyaloclastite breccia and the underlying basalt lavas are not established, they have not been divided into separate units. Given the possibility that Subunits 1B and 8B are pillow lavas, the underlying pillow basalts are also included in Units 1 and 8 as Subunits 1C and 8C, respectively.

Massive basalts (Units 2, 4, and 6)

Massive basalt Units 2, 4, and 6 were recovered from Cores 301-U1301B-11R through 14R, 15R, and 18R, respectively (Fig. F20). They were classified as massive basalts because they consist of continuous sections of ~0.5–4.4 m of similar lithology, which increases in grain size toward the center of the flow (Fig. F24). Some massive flows have upper and/or lower planar glassy chilled margins. High recovery, up to 100% in Core 301-U1301B-12R, allows individual lava flows or cooling units to be distinguished (Subunits 2A, 2B, 4A, 4B, and 4C).

Mineralogically, the massive lavas are very similar to the sparsely to moderately phyric pillow basalts containing plagioclase, olivine, and clinopyroxene as phenocryst as well as groundmass phases. However, they are predominantly fine grained and hypocrystalline to holocrystalline, with seriate intersertal to intergranular subophitic textures. The massive basalts are sparsely to highly vesicular, with an average of 1%–5% round gas vesicles, up to 3 mm in diameter. The vesicles are generally concentrated in the upper portions of the flows, but Unit 6 has a distinct 20 cm wide vesicular band in its center in interval 301-U1301B-18R-2 (Piece 2, 75–95 cm), which is ~15% vesicles (Fig. F24). The massive flows are slightly to moderately altered and exhibit similar alteration styles to the pillow basalts: vesicle fill, vein formation (and the development of associated alteration halos), and the complete replacement of olivine phenocrysts. However, the massive basalts contain fewer fractures and veins than the pillow basalts, allowing better core recovery and the retrieval of individual pieces up to 94 cm long (e.g., interval 301-U1301B-18R-3 [Piece 2, 31–125 cm]).

Igneous petrology

The basaltic rocks recovered from Hole U1301B are divided into three types as described above (basalt-hyaloclastite breccia, pillow lavas, and massive basalts), and 55 samples were selected for petrographic analysis. Table T5 summarizes the abundances and morphologies of the phenocryst and groundmass minerals present within representative samples.

Aphyric and sparsely to highly phyric pillow basalts

The pillow lavas from Hole U1301B vary from aphyric to highly phyric basalts. Subunits 1C, 7A, and 7C consist of sparsely to moderately phyric glassy to microcrystalline basalts, with between ~1% and 12% phenocrysts, whereas Units 3 and 5 and Subunits 7B and 8C are moderately to highly phyric glassy to microcrystalline basalts with ~5%–20% phenocrysts. Plagioclase is the most abundant phenocryst phase (2%–15%), with clinopyroxene (1%–5%) and olivine pseudomorphs (<5%) also present in most of the phyric samples.

These lavas typically have hyalophitic to intersertal and intergranular textures (Fig. F25). Hyalophitic zones display sheaf-spherulitic textures, with occasional occurrences of plumose and honeycomb groundmass. Additionally, some thin sections also contain regions with glassy to variolitic and glomeroporphyritic textures, with several different textures often observed across a single thin section. A weak to strong plagioclase pilotaxitic texture was identified within the groundmass of some hypocrystalline-intersertal samples (e.g., Sample 301-U1301B-16R-1 [Piece 2, 8–10 cm]) (Fig. F25).

Massive basalt

The massive basalts of Units 2, 4, and 6 are sparsely to moderately plagioclase ± clinopyroxene ± olivine phyric (<15% phenocrysts). The groundmass grain size varies significantly from the glassy to cryptocrystalline flow margins to the fine-grained flow interiors. Texturally, the central portions of the massive flows are predominantly intersertal to intergranular or seriate, whereas the margins are dominantly hyalophitic (Fig. F25). Some samples display glomeroporphyritic to subophitic textures.

Basalt-hyaloclastite

Basalt-hyaloclastite breccia was recovered within intervals 301-U1301B-1R-1 (Pieces 1–7, 0–57 cm) and 35R-1 (Piece 10, 81 cm), which comprise Subunits 1A and 8A, respectively, to 35R-2 (Piece 7, 52 cm) (Fig. F23). These breccias consist of glassy shards and angular to subangular clasts of aphyric and plagioclase phyric basalt set within a clay matrix. The breccia is variably clast and matrix supported. The matrix is predominantly dark gray to black clay (saponite) and phillipsite, with altered glass shards and calcium carbonate. Basalt clasts are typically 0.5–6 cm, and glassy shards are generally <1 cm in length. The basaltic clasts appear similar in hand specimen to other sparsely phyric basalts from Hole U1301B and are bordered by 2–6 mm black alteration halos. Several of the basalt clasts in the Subunit 1A breccia have curved chilled margins that are glassy to cryptocrystalline and aphyric with trace plagioclase microlites, similar to those of pillow margins. The basalt fragments in the Subunit 8A breccia are more angular, sparsely plagioclase phyric, and lack glassy margins. Vesicles within the clasts are filled by pale brown and granular saponite ± iron oxyhydroxides, similar to those observed in other basalt samples from Hole U1301B.

Phenocryst phases

Plagioclase

Plagioclase phenocrysts are <4.5 mm (average = 1–2 mm) and typically comprise <15% of pillow basalts and <10% of massive lavas. They are euhedral to subhedral elongate and stubby laths, with rare skeletal or quench plagioclase crystals. The plagioclase phenocrysts occur singly and in mono- and polymineralic glomeroporphyritic clots (plagioclase ± pyroxene ± olivine) (Fig. F26). Simple to oscillatory zoning is common, and there is a sparse to dense abundance of glass inclusions in the cores of some plagioclase phenocrysts. Plagioclase is generally fresh and occasionally stained by secondary clays or replaced by saponite along cracks, cleavage planes, or crystal edges.

Olivine

Olivine is a common phenocryst phase observed in most thin sections as a pseudomorph, with an average abundance of 2%–3%. Olivine phenocrysts (<5.5 mm; commonly 0.7–1 mm) have been completely replaced by a variety of secondary hydrothermal alteration phases and are identified by their euhedral to subhedral crystal morphology and their textural relationships to surrounding minerals. The olivine is pseudomorphed by granular to fibrous saponite ± celadonite ± iddingsite ± calcium carbonate ± opaque minerals.

Pyroxene

Pyroxene (<5.5 mm; commonly 0.5–1.0 mm) is present in almost all thin sections, with an average abundance of ~3%. It is typically subhedral to euhedral forming stubby to short prismatic or round crystals and often displays simple basal twinning. The majority of crystals have a subhedral morphology and occur as solitary phenocrysts, intergrown with plagioclase, or within polymineralic glomeroporphyritic clots with plagioclase and olivine (Fig. F26). In some of the holocrystalline fine-grained basalts, the pyroxene partially encloses euhedral feldspar laths in a subophitic manner.

Groundmass

The groundmass of basalts from Hole U1301B is hypocrystalline to holocrystalline. The groundmass minerals are the same as those present as phenocrysts, but modal abundances vary between samples. Plagioclase occurs as microlaths, microlites, and quench crystals and is the most abundant groundmass crystalline phase, comprising 2%–35% of pillow lavas and 40%–60% of the holocrystalline central portions of the massive lava flows. Euhedral to skeletal microcrysts of olivine (average original abundance = 2%), subhedral to anhedral clinopyroxene (less than ~30% in pillow lavas and 7%–35% in massive flows), and trace amounts of opaque minerals are also present. Plagioclase microcrysts in some thin sections exhibit a subparallel to pilotaxitic texture. Fresh olivine was only identified as microlites within the fresh glass of some chilled margins (Fig. F27).

The remainder of the groundmass (up to 95% in pillow lavas and <55% in central portions of massive lavas) consists of a cryptocrystalline mesostasis that displays hyalophitic (sheaf-spherulitic to honeycomb) to intersertal textures (Fig. F25). Mesostasis textures are variable within a single sample, but basalts from different units of the same lithology exhibit the same range of textures. Primary magmatic opaque minerals are disseminated throughout the mesostasis, forming small (<0.02 mm), granular, euhedral to subhedral solitary grains. The mesostasis exhibits patchy alteration and is variably replaced by secondary hydrothermal clays (saponite and celadonite), disseminated pyrite, and iron oxyhydroxide.

Almost all basalt samples are sparsely vesicular, with 1%–3% vesicles in most samples. The majority of vesicles are round and are 0.05–3 mm in diameter. The upper portions of massive flows are typically particularly vesicular, with up to 7% vesicles that are <5 mm in diameter. However, the central portion of the Unit 6 massive flow contains a 20 cm wide highly (~15%) vesicular band in the interval 301-U1301B-18R-2 (Piece 2, 75–95 cm) (Fig. F24). The majority of vesicles have been partially to completely filled by mono- to polymineralic secondary assemblages, which include saponite, celadonite, iron oxyhydroxides, hematite, pyrite, and carbonate. Many of the vesicles display sequential concentric fills that are described in more detail in "Basement alteration." In some samples, vesicles are partially to completely filled with mesostasis, with some vesicles having a geopetal fill (for example, Sample 301-U1301B-15R-1 [Piece 14, 118–129 cm]).

Hard rock geochemistry

Forty-four representative basalt samples from Hole U1301B were analyzed for major and trace elements on the JY2000 inductively coupled plasma–atomic emission spectrometer (ICP-AES). Effort was taken to collect the freshest material from the cores of rock pieces in order to obtain a downhole record of primary magmatic compositions. Rocks with thin fresh glassy margins and altered samples will be analyzed postcruise. Loss on ignition (LOI) values are variable, with some samples losing <1 wt% (with the exception of a hyaloclastite sample [Unit 8B] that loses 2.6 wt%) but the majority gaining 0–1.7 wt%. We report the data without an LOI correction factor (Table T6).

During the seven shipboard ICP-AES runs, within-run reproducibility was assessed based on multiple analyses of BAS-148 and BAS-206, basaltic standards from ODP Legs 148 and 206, respectively (Table T12 in the "Methods" chapter). Precision on the JY2000 was typically <4% for major elements and <10% for trace elements, although for elements with near-background concentrations, the analytical error is artificially high and the actual analytical error is much better than suggested by the numbers. However, Ni and Ba were reproducible only to <30%–40% and are reported in the data table but should be used only with caution.

Results

Hole U1301B penetrated 230 m of basement, which primarily consisted of slightly to highly olivine-clinopyroxene-plagioclase phyric pillow basalts, with three massive flow units. The chemical compositions of the basalts are shown in Table T6. All of them are normal depleted MORB, with MgO = 6.48–8.10 wt%, Fe₂O₃ = 8.73–12.89 wt%, Mg# = 52.4%–65.8%, Cr = 47–272 ppm, and Zr = 91–178 ppm. Figure F28 is a plot of TiO₂ (wt%) versus Zr (ppm) for all the analyzed samples; the good correlation indicates that all the Hole U1301B basalts have the same source. Figure F29 shows all the measured major and trace elements plotted against the Mg# (Mg/Mg + Fe). CaO and Al₂O₃ show a weak correlation with Mg#; however, when viewed as a total batch, the majority of elements show no significant variation with Mg#.

Figure F30 shows the variation in Mg# and major and trace elements with depth. The most significant vertical variation is in Mg#, which varies between units (Fig. F30). Massive flows have the lowest Mg#, 54.8% and 52.4% in Units 2 and 4, respectively. The Mg# is highest (65.8%) toward the base of Unit 7 and decreases to 59.9% upward through this ~80 m thick pillow lava unit. Values within the upper thick pillow lava unit (Unit 1) are much more uniform (average = 62.5%), with no obvious decrease up through the unit. The incompatible trace elements (Sr, Y, Zr, and Nb) all show similar variations with depth, increasing slightly down through Unit 1, and are most abundant in massive lava flows. Subunit 7B has lower concentrations of incompatible trace elements than Subunits 7A and 7C; this may reflect the high phenocryst content of these pillow lavas. Other elements do not show systematic behavior downhole.

Basement alteration

All of the basement rocks recovered from Hole U1301B have undergone alteration. Most pieces are slightly to moderately altered, with secondary minerals (1) lining or filling vesicles and cavities, (2) filling fractures and veins and present in adjacent alteration halos, (3) replacing phenocrysts, and (4) replacing interstitial mesostasis and glass (Fig. F31). The alteration and vein logs (see "Site U1301 alteration log" and "Site U1301 vein log" in "Core Descriptions") quantify these different alteration types in hand specimen, on a piece-by-piece scale. Thin section observations indicate that the degree of alteration varies from ~5% to 25%, excluding the hyaloclastite breccia, which is ~60% altered (Table T5). The freshest rocks are the interior cores of most pieces, which have a dark gray, saponitic background alteration. Fresh olivine occurs only as microphenocrysts in some glass margins, and elsewhere is completely replaced by clay minerals.

Secondary minerals

Secondary minerals were identified primarily on the basis of hand specimen descriptions, and specific secondary minerals are not generally distinguished in the vein and alteration logs. Observations were calibrated by thin section observations by X-ray diffraction (XRD) for six samples (five veins and a matrix of the hyaloclastite breccia) (Table T7).

Clay minerals are the most abundant secondary minerals and are the principal constituent of all four styles of alteration (vesicle fill, vein fill, phenocryst replacement, and background mesostasis alteration). The clay minerals were distinguished primarily on the basis of color in hand specimen and by their optical properties in thin section. Saponite is the most abundant of the clay minerals, identified in every thin section. It occurs as cryptocrystalline granular or fibrous aggregates and varies in color from black to dark greenish brown to pale blue in hand specimen and tan-brown to olive-green in thin section. Saponite lines or fills vesicles, is the most common olivine phenocryst replacement, occurs in mono- and polymineralic veins, replaces mesostasis and glassy margins, and forms the matrix of the hyaloclastite breccia. Celadonite, bright blue-green in hand specimen and bright green in thin section, also fills vesicles and veins and replaces olivine phenocrysts and mesostasis. However, celadonite is typically restricted to the alteration halos, frequently occurring as intergrowths with saponite and/or iron oxyhydroxide.

Iddingsite, a mixture of clay minerals and iron oxyhydroxide, is the second most abundant alteration product identified in Hole U1301B cores, producing a characteristic red-orange or reddish brown color in both hand specimen and thin section. It fills veins and vesicles, stains primary minerals, and is intergrown with the clays that replace olivine.

Calcium carbonate was observed in only six cores, filling vesicles and veins, and as a minor component of the basalt-hyaloclastite breccia matrix. Secondary pyrite was observed lining vesicles, as fine grains within saponite vesicle linings, with saponite ± calcium carbonate in veins, and as disseminated fronts bounding some alteration halos. Zeolites (analcime and phillipsite) were tentatively identified by XRD in Samples 301-U1301B-1R-1 (Piece 15, 121–124 cm) and 18R-4 (Piece 7, 132–136 cm) in veins as well as the matrix of the hyaloclastite breccia.

Basalt-hyaloclastite breccia

The basalt-hyaloclastite breccias recovered in Cores 301-U1301B-1R and 35R are moderately to highly altered. The glass shards and basalt clasts are partially to completely replaced by green saponite and minor calcium carbonate and zeolite. Basalt clasts generally have altered glass margins or alteration halo rims and are freshest in the center (Fig. F23). XRD of the breccia matrix from Sample 301-U1301B-35R-1 (Piece 15, 112–118 cm) indicates the zeolite identified in the saponitic matrix is phillipsite.

Vesicle filling

Basalts from Hole U1301B are sparsely to moderately vesicular, containing 0.05–3.0 mm (commonly 0.1–0.5 mm) round vesicles. Almost all vesicles are partially or completely filled by one or more secondary minerals (saponite, celadonite, iron oxyhydroxide, hematite, pyrite, carbonate, and, possibly, zeolites). The type and sequence of filling is interpreted as a sensitive record of successive changes in the chemical microenvironments affecting the host rock. Vesicle fillings vary systematically, depending on whether the vesicles are in the alteration halos, transitional zones, or the less altered, gray interiors. Microscopic examples are shown in Figure F32. In the slightly altered gray rock interiors vesicles are either empty or lined/filled with fibrous to granular saponite. The vesicles within alteration halos (black, green, and brown) are filled with mono- or polymineralic assemblages that include saponite, celadonite, iron oxyhydroxide, hematite, pyrite, and carbonate. The particular vesicle filling assemblage reflects the secondary mineralogy of the halo, discussed below.

In many cases, these minerals form concentric fills, with the mineral-sequence varying in and between samples. In some cases, early fibrous saponite is followed by iron oxyhydroxide, hematite, or celadonite, although other samples contain saponite-filled vesicles that are lined with celadonite, indicating variable alteration sequences. The vesicles within the massive basalts are typically lined with fibrous saponite or fibrous saponite with microgranular pyrite. Rare, small vesicles (<0.5 mm diameter) adjacent to pyrite-bearing veins are filled with pyrite. Some of the vesicles within massive flows contain calcium carbonate; these vesicles are lined with carbonate ± pyrite, with an inner fibrous carbonate layer or radial/coarse crystalline carbonate fill (e.g., Sample 301-U1301B-18R-2 [Piece 2, 75–95 cm]). The carbonate is always the innermost vesicle-filling mineral, indicating that it is a later stage alteration product.

Veins

A total of 2301 veins were identified in the core recovered from Hole U1301B, with an average frequency of 31 veins/m of recovered core (Table T8). Saponite is the most abundant vein-filling mineral, present in 98% of the veins. Iron oxyhydroxide was documented in 1010 of the veins, equivalent to 44% of the total in the core, typically occurring with saponite. Celadonite was identified in only 93 of the veins, equivalent to 4% of the total in the core, typically occurring with iron oxyhydroxide ± saponite. Pyrite was observed in 59 veins (2.6% of the total in the core) and is typically associated with saponite. Calcium carbonate was observed in only 38 veins (1.6% of the total in the core), with saponite ± pyrite. Two zeolite-bearing veins were identified by XRD (Table T7). Examples of the different vein types, in thin section, are given in Figure F33. The majority of veins described in samples from Hole U1301B are flanked by alteration halos.

Clay-bearing veins are ubiquitous in the rocks recovered from Hole U1301B and vary in width from 10 µm to 6 mm (average = 0.2 mm). The maximum width of the simple dark green saponite veins is 2 mm. These predominantly narrow veins are prolific in pillow fragments, with saponite filling many of the radial cooling cracks along pillow margins. Iron oxyhydroxide- and celadonite-bearing clay veins vary in width from 10 µm to 6 mm, and average 0.2 mm. They are most common in the pillow lavas (Units 1, 3, 5, 7, and 8). However, the most spectacular iron oxyhydroxide-bearing vein occurs in a massive lava flow, spanning the interval 301-U1301B-15R-1 (Pieces 10 and 11, 70–97 cm). This vein is 6 mm wide with a 10–25 mm wide alteration halo (Fig. F34). Goethite and minor celadonite were identified within this vein by XRD (Samples 301-U1301B-15R-1 [Piece 11, 121–124 cm] and 15R-1 [Piece 11, 87–89 cm]).

Calcium carbonate-bearing veins were documented only in Cores 301-U1301B-1R, 4R, 5R, 11R, 12R, 18R, and 35R, and they are restricted to the massive flows (Table T8). Carbonate veins vary in width from 0.1 to 2 mm and average 0.3 mm. The veins are typically lined with a thin layer of saponite ± pyrite, with blocky to fibrous carbonate spanning the gap (Fig. F33). The carbonate-bearing veins are generally not flanked by alteration halos.

Alteration halos

The rocks throughout Hole U1301B exhibit a pervasive dark gray background alteration with saponite replacing mesostasis and olivine phenocrysts and lining vesicles, but many pieces also contain differently colored alteration halos. The halos border rock fragments and flank 56% of the veins. The relative proportions of background alteration and halos are described in Table T6 in the "Methods" chapter. In hand specimen, alteration halos are generally associated with saponite, iron oxyhydroxide, and celadonite-bearing veins.

The alteration halos termed "black" range in color from very dark gray to dark green to black and flank 54% of the veins in Hole U1301B (e.g., Fig. F31). The black halos range in width from 1 to 30 mm, but the majority are 3–12 mm wide. The percentage of secondary minerals is similar or slightly greater than in the adjacent dark gray host rock (5%–15% secondary minerals), but the mineralogy is distinct, with celadonite typically present in addition to saponite. These minerals have a similar distribution to the saponite in the background alteration, replacing mesostasis and olivine phenocrysts and lining vesicles (Fig. F35). Approximately half of the halos described as black in the alteration log contain vesicles filled with a mixture of iron oxyhydroxide and saponite or celadonite (e.g., Fig. F36) that give the halos an orange or greenish tinge, depending on the vesicularity of the rock. Because the iron oxyhydroxide is primarily restricted to the vesicles, these halos are still termed black, but a note of which halos contain iron oxyhydroxide-filled vesicles is made in the vein log where they are described as "mixed." A narrow band (~0.1 mm) of disseminated secondary pyrite separates the black halo of saponite + iron oxyhydroxide veins from the adjacent dark gray host rock in three cases: intervals 301-U1301B-5R-2 (Piece 11, 80–102 cm), 18R-2 (Piece 2, 80–102 cm), and 18R-3 (Piece 2, 79–80 cm) (e.g., Fig. F37).

Rarely, brown and green halos were described. The green halos, 2–35 mm wide, are mineralogically similar to the black alteration halos but have a greater percentage of secondary minerals (up to 25% saponite + celadonite), which impart a stronger green color to the rock. Brown alteration halos, ranging from 2 to 12 mm in width, are associated with only 0.2% of the veins described from Hole U1301B. The orange-red-brown coloration of these halos results from the filling of vesicles and fractures and the staining of primary minerals with iron oxyhydroxides (Fig. F35). Six "multi" halos with impressive color zonation were also observed. These halos consist of two or three different-colored halos, most commonly an inner brown halo and outer black halo, or with a transitional green zone separating the two (Fig. F38). In such cases the outer black and inner brown halos are mineralogically similar to those described above, and the transitional zone contains minor iron oxyhydroxides with celadonite and saponite, although there is a variation in the modal compositions of the different zones between samples.

Spatial variations

Although all samples recovered from Hole U1301B have undergone some alteration, there is a significant variation in alteration style and intensity between the identified lithologic units. The pillow lavas (Units 1, 3, 5, 7, and 8) contain between 25 and 36 veins/m recovered core (Table T8); because the most fractured material is likely to be lost during coring, this is considered a minimum estimate of the actual vein density. The principal vein-filling minerals in these units are saponite, iron oxyhydroxides, and celadonite. In contrast, the massive lavas (Units 2, 4, and 6) contain between 14 and 18 veins/m in recovered core, with up to 95% and 100% recovery in Cores 301-U1301B-18R and 12R.

Low-temperature seafloor basalt alteration is often described in terms of two contrasting styles: oxidative and nonoxidative (Laverne et al., 1996; Teagle et al., 1996). Iron oxyhydroxides and celadonite are characteristic of oxidative alteration, and saponite and sulfides are indicative of nonoxidative alteration. This may account for the differences in the alteration styles observed at Site U1301 because the pillow basalts provide a highly fractured permeable aquifer for circulation of oxidative evolved seawater and consequently have an oxidative alteration assemblage, whereas the massive flows are less permeable, allowing limited fluid circulation that results in less oxidative alteration.

Basement structures

Basalts from Hole U1301B were cut during different episodes by veins and fractures of various origins. They include fractures formed during eruption (such as the radial cooling cracks of pillow lavas), hydrothermal alteration (such as veins), later tectonic fractures (such as shear veins), and those that have been induced during coring.

The dips of 647 veins and fractures were measured in the recovered cores from Hole U1301B. Four types of fractures were distinguished in the cores: (1) veins flanked by alteration halos (termed haloed veins), (2) veins not flanked by alteration halos but filled with secondary minerals (termed nonhaloed veins), (3) shear veins with slickenfibers (microfaults with contemporaneous displacement and secondary mineral growth), and (4) microveins (<0.05 mm wide) that are only identifiable in thin sections.

Haloed veins were the most frequently observed structures in rocks from Hole U1301B. The radial cracks observed perpendicular to some chilled pillow margins also have associated alteration halos that are often truncated by perpendicular (pillow margin-parallel) haloed veins. Nonhaloed veins were identified in the massive lavas and some pillow lava pieces and represent ~30% of the vein structures documented in Table T8.

Shear veins or faults were identified in three of the recovered pieces (e.g., Sample 301-U1301B-23R-2 [Piece 18, 139–149 cm]). These are always steeply dipping structures and have calcite slickenfibers or overlapping fibers. It is not possible to determine the offset across the shear veins, as they occur on the sides of pieces for which the adjoining pieces were not recovered. However, the fibers define a steeply plunging lineation with asymmetrical calcite crystals, indicating contemporaneous dip-slip motion and calcite precipitation. This extensional style of deformation may relate to regional normal faulting.

Microveins (<0.05 mm wide) were identified in several thin sections. They are filled with the same minerals as the macroscopic veins observed in hand specimen: saponite, iron oxyhydroxide, and celadonite. They have pinch and swell, curved, and anastomosing structures.

Figure F39 shows the distribution of measured fracture dips and indicates a general progressive increase in occurrence with increasing dip angle. Slight bimodal peaks are identified at 55°–65° and 80°–90°, which include >70% of the total of counted veins. Fractures and veins are not uniformly distributed through Hole U1301B and occur more frequently (per meter of recovered core) in the pillow lava units relative to the massive lava units (Fig. F40). However, shallow and deep pillow lava units (e.g., Units 1 and 7) show similar distributions of vein and fracture dips. Histograms of vein/fracture dip for haloed structures are compared to those of nonhaloed veins in the massive and pillow lavas (Fig. F41) and indicate that massive and pillow lavas have similarly dipping fractures. Haloed fractures in massive flows and pillow lavas are predominantly steeply dipping, with frequency increasing with dip angle. The nonhaloed fractures have a more uneven distribution of orientations, with peaks at 15°–30°, 50°–65°, and 80°–90° within massive flows and at 20°–35° and 80°–85° in pillow lavas. The expected sampling bias during coring is toward horizontal structures that are more likely to be intersected by a vertical hole. The observed predominance of steeply dipping haloed veins is therefore interpreted to reflect the actual distribution of fracture orientations in the basement at Site U1301.

Hand specimen observations indicate that the sequence of structure formation in rocks from Hole U1301B is (1) formation of radial cooling cracks perpendicular to pillow margins, (2) formation of vertical cracks with associated hydrothermal alteration halos during seafloor spreading/normal faulting, and (3) development of younger fractures without halos. The observed dominance of vertical, extensional related structures is consistent with the location of Site U1301 on an abyssal, normal-faulted basement high.

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