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doi:10.2204/iodp.proc.327.103.2011

Petrology, hard rock geochemistry, and structural geology

Basement was cored from 346.0 to 496.0 mbsf (110–260 msb) in Hole U1362A with 45.27 m of core recovered (30%). The recovered core consisted of (1) aphyric to moderately phyric pillow basalt, (2) aphyric to sparsely phyric sheet flows, and (3) sparsely to highly phyric basalt flows. These lithologies were divided into eight units on the basis of changes in lava morphology, rock texture, and phenocryst occurrence (Fig. F5; Table T4; see Site U1362 visual core descriptions in “Core descriptions”). Pillow lava units (Units 1, 3, and 5) were divided according to changes in phenocryst abundance and mineralogy. Sheet flow units (Units 4, 6, and 8) were divided on the basis of the presence of chilled margins and variations in phenocryst mineralogy. In this hole breccia recovery is limited to one 7 cm piece of a magmatic breccia and a single centimeter-sized cataclastic zone, neither of which is defined as an individual unit.

In addition to cores recovered from Hole U1362A, small millimeter- to centimeter-sized chips were recovered from the drill bit in Holes U1362A and U1362B. In both cases the drill bit only penetrated a few meters below the sediment/basalt interface before being pulled to the rig floor, where the drill cuttings were removed. The source of the drill cuttings can be constrained to a short interval at this interface and represents the only recovery of basement material at the sediment/basalt interface at Site U1362. The cuttings were rinsed and separated on the basis of color or mineralogy, where possible. A range of variably altered clinopyroxene plagioclase phyric microcrystalline basalt was recovered (see “Basement alteration”). A chilled margin 3 cm long was also recovered from the bit in Hole U1362B. No basement units were assigned because of the limited recovery of chips from the drill bits.

Lithologic units

Pillow basalt (Units 1, 3, and 5)

Pillow basalt forms the most abundant flow morphology of Hole U1362A, with Units 1, 3, and 5 accounting for 78.85 m of the stratigraphy. Pillow basalt was primarily identified by the occurrence of curved glassy chilled margins. In larger pieces perpendicular radial cooling cracks were present (Fig. F6), but often the pieces recovered with glassy chilled margins were too small to determine if the cracks were radial. The largest piece of pillow basalt recovered was 45 cm in length. The pillow basalt core with the highest recovery had 35% recovery for a 5.5 m advance and contained nine chilled margins (Core 327-U1362A-5R). The sizes of individual pillow lava pieces can be inferred to be between 45 and 61 cm in diameter. The pillow basalt is dominantly hyalophitic and glomeroporphyritic in texture and varies between cryptocrystalline and microcrystalline in groundmass grain size. It is sparsely to highly phyric, with olivine, clinopyroxene, and plagioclase phenocrysts present. Thin section observations identified spherulitic, hyalophitic, intersertal, and glomeroporphyritic igneous textures. Pillow basalt ranges from sparsely to moderately vesicular, with a range of secondary minerals filling the vesicles. Alteration in the pillows is variable and ranges from slight to high, manifesting as replacement of mesostasis and phenocrysts, vesicle filling, glassy margin replacement, and vein formation with adjacent alteration halos.

Sheet flows (Units 4, 6, and 8)

Sheet flows are the second most common lava morphology in Hole U1362A and were classified on the basis of continuous sections of the same lithology with increasing grain size downhole from the top of the flow and fewer chilled margins. Core recovery in these units averaged 43% and was as high as 112%. Recovery of >100% in Core 327-U1362A-17R is an artifact of tidal fluctuations during coring, which resulted in the depth penetrated appearing to be smaller than the recovered core length. Two nearly continuous sheet flows recovered in Cores 17R and 18R were divided into two subunits on the basis of a change in phenocryst mineralogy.

Primary mineralogy of the sheet flows is very similar to that of the pillow basalt, with a range from aphyric to moderately phyric basalt with olivine, clinopyroxene, and plagioclase as phenocryst phases and forming the groundmass. Grain size within the sheet flows is variable, ranging from cryptocrystalline to fine grained in Subunit 4C, and textures vary from intersertal to intergranular. The sheet flows range from nonvesicular to highly vesicular, with some flows exhibiting similar abundance throughout (e.g., Subunits 6A and 6B); others are highly variable within the flow (e.g., Subunit 4C). Alteration within the sheet flows varies from slight to high and is associated with groundmass replacement, vesicle fill, vein formation, and alteration halos. Alteration halos within the sheet flows are highly variable and not always clearly associated with a recovered vein (see “Basement alteration”). The lower fracture and vein intensity in the sheet flows compared to in the pillow basalt resulted in improved core recovery and larger individual pieces (maximum piece length of 140 cm in Section 327-U1362A-17R-1).

Basalt flows (Units 2 and 7)

Classification of basalt flow units is based on the absence of definitive morphological features associated with either pillow lava or sheet flows, allowing only a general “basalt flow” interpretation to be made. When present, glassy chilled margins lack definitive pillow structure and could represent either a pillow margin or the margins of a thin sheet flow. These basalt units are aphyric to moderately phyric crypto- to microcrystalline with the same primary mineralogy as basalt from Hole U1362A. The basalt flows are generally sparsely vesicular, with secondary minerals filling vesicles and textures varying from hyalophitic to variolitic. Alteration is moderate to high and is present as groundmass replacement (mesostasis and phenocrysts), vesicle fill, vein formation, and halos.

Igneous petrology

The basaltic rocks recovered from Hole U1362A are divided into three types, as described above (pillow lava, sheet flows, and thin basalt flows), and 27 samples were selected for petrographic analysis (see Site U1362 thin sections in “Core descriptions”).

Pillow lava (sparsely to highly phyric basalt)

The pillow lava from Hole U1362A varies from sparsely to highly phyric basalt. Subunit 5B is sparsely phyric basalt with glassy to fine-grained groundmass and <4% phenocrysts. Unit 1 and Subunit 5A are moderately to highly phyric basalt with glassy to microcrystalline grain size and ~6%–15% phenocrysts. Plagioclase is the most abundant phenocryst phase (~3%–11%), with clinopyroxene (~1%–10%) and olivine pseudomorphs (<5%) also present in most of the phyric samples.

This lava is predominantly hyalophitic and glomeroporphyritic to subophitic (partial inclusion of plagioclase in clinopyroxene) with frequent examples of intersertal and spherulitic texture. A glassy chilled margin sample (327-U1362A-2R-1, 121–126 cm; Fig. F7) displays a transition from holohyaline to spherulitic texture in which spheroidally arranged aggregates of acicular microcrystals become increasingly abundant away from fresh amber glass. Several textures were frequently observed across a single thin section (Fig. F8C–F8E) and also within a single (sub)unit.

Sheet flows

The sheet flows of Units 4, 6, and 8 vary from aphyric to moderately phyric basalt. Subunit 4C is aphyric fine-grained basalt with <2% phenocrysts, and Units 6 and 8 and Subunits 4A, 4B, and 4D are sparsely to moderately phyric microcrystalline basalt with 1%–7% phenocrysts. Overall, plagioclase is the most abundant phenocryst phase (<5%), with clinopyroxene (<3%) and olivine pseudomorphs (<2%) also present. Almost all sheet flow interiors display intersertal texture with frequent glomeroporphyritic clots and infrequent sections of high vesicularity.

Basalt flows

The thin basalt flows of Units 2 and 7 are moderately olivine clinopyroxene phyric (~8% phenocrysts). Samples close to flow margins exhibit cryptocrystalline grain size and variolitic to hyalophitic textures, and samples from flow interior locations are microcrystalline to fine grained and have intersertal to intergranular and seriate textures.

Phenocryst phases

Plagioclase

Plagioclase is the most abundant phenocryst phase and typically makes up <10% of pillow lava and <5% of sheet and basalt flows. Phenocrysts range from 0.2 to 4.1 mm in length and are mostly euhedral in shape. More than 60% of crystals are found in clots with mainly clinopyroxene and in rare cases olivine. Euhedral platy to stubby discrete crystals are present throughout, and rare skeletal or quench plagioclase crystals are found in lower pillow lava units. Simple to oscillatory zoning is uncommon but is present in some crystals (e.g., Sample 327-U1362A-9R-2, 74–75 cm). Glass and clinopyroxene inclusions are occasionally present as blebs and microlites parallel with plagioclase twin planes. Alteration of crystals varies from 0% to 20% and generally manifests as replacement by secondary clays, saponite, iron oxyhydroxides, and oxides along cracks, cleavage planes, or crystal edges (Fig. F9).

Clinopyroxene

Clinopyroxene is present in almost all thin sections and typically makes up <1.5% of pillow lava, <1.0% of sheet flows, and <8.0% of thin basalt flows. Phenocrysts are generally <1.2 mm long (rarely up to 5.5 mm) and are typically anhedral to euhedral, stubby to short prismatic or round crystals and are predominantly intergrown with plagioclase in glomeroporphyritic clots. Solitary subhedral to euhedral crystals are uncommon and tend toward seriate texture. Simple basal twinning is common throughout. Alteration of clinopyroxene varies from 0% to 80% and manifests as replacement by secondary clays, saponite, iron oxyhydroxides, and oxides along cracks, cleavage planes, or crystal edges (Fig. F9).

Olivine

Olivine is only observed in thin sections as a pseudomorph, with an average abundance of 1%–2%. Olivine phenocrysts (0.1–1.7 mm; average width = 0.6 mm) are completely replaced by a variety of secondary phases and are identified by their subhedral crystal morphology and textural relationships with surrounding minerals. Larger olivine crystals (as large as 1.7 mm in diameter) from sheet flow interiors tend to be skeletal and coexist with more equant, smaller crystals. Identified pseudomorphs of olivine are granular saponite, celadonite, iddingsite, and opaque minerals (Fig. F9).

Groundmass

The groundmass of rocks recovered from Hole U1362A varies from hypocrystalline to holocrystalline. Constituent minerals are the same as those present as phenocrysts, but modal abundances vary between samples. The groundmass is composed primarily of plagioclase and clinopyroxene. Plagioclase occurs as microlaths, microlites, and quench crystals and is marginally the more abundant groundmass crystalline phase, composing 2%–43% of pillow lava, 27% of thin basalt flows, and 34%–41% of sheet flow interiors. Clinopyroxene occurs as microlaths, microlites, and aggregates of fibrous or plumose crystals and composes 1%–36% of pillow lava, 31% of thin basalt flows, and 24%–38% of sheet flows. Anhedral to subhedral microcrysts of pseudomorphed olivine are present in low abundances in pillow lava and thin basalt flows (original abundance of 0.5%–8%) and are slightly more abundant in thick sheet flows (8%–12%). Trace amounts of opaque minerals are also present throughout all units.

Cryptocrystalline mesostasis makes up the remainder of the groundmass in the basalt. Abundances are highly variable in pillow basalt, ranging from 14% to 80%. Sheet flows and thin basalt flows are less variable, with mesostasis abundances of 10%–22% and 28%, respectively. Mesostasis textures are variable within a single sample, but basalt from different units of the same lithology exhibits the same range of textures. Nearly all lithologies display hyalophitic and intersertal textures. Spherulitic and variolitic textures are common in pillow lava and thin basalt flows (Figs. F8, F10). Primary magmatic opaque minerals are disseminated throughout the mesostasis, forming small (<0.1 mm) granular euhedral to subhedral solitary grains. The mesostasis exhibits patchy alteration in which discrete areas of groundmass are more intensely altered than the background replacement of original host rock to secondary hydrothermal clays (saponite and celadonite) and iron oxyhydroxides (Fig. F11).

Almost all units are sparsely to moderately vesicular (3%–9% vesicles in most samples). The majority of these are slightly to moderately spherical and 0.1–9.0 mm in diameter. There is little to distinguish between units except for a large contrast in vesicle abundances between Subunits 4C and 4D, which range between nonvesicular (<0.1%) and highly vesicular (20%). The majority of vesicles are >50% filled by mono- to polymineralic secondary assemblages that include saponite, celadonite, iron oxyhydroxides, pyrite, and mixed clays. The presence of concentrically filled vesicles is related to the overall alteration of the lithology and is described in more detail in “Basement alteration.”

Basement alteration

All of the basement rocks recovered from Hole U1362A have undergone alteration by interaction with seawater. The extent of alteration varies from slight to completely altered, and most pieces are moderately altered. The rocks manifest four types of alteration: (1) replacement of groundmass, (2) replacement of phenocrysts, (3) hydrothermal veins and alteration halos, and (4) lining and filling vesicles. In thin section, alteration is observed to range from 8% to 91%. Away from vesicles and veins, background alteration is generally moderate to high in pillow lava and predominantly moderate in sheet and basalt flows and is dominated by saponitic background alteration. Olivine is present only as completely replaced pseudomorphs.

Secondary minerals

The identification of secondary minerals was primarily made in hand specimen, with subsequent partial verification by thin section observations and XRD. The distinction of specific secondary clay minerals was made for only a few examples during logging of alteration and veins.

The most abundant secondary minerals in Hole U1362A are clay minerals, which are present in all four types of alteration. Clay minerals were identified primarily by color and were verified where possible by thin section observations. Saponite is the dominant clay mineral and is present throughout all cores. In hand specimen saponite occurs in black, dark green, greenish-brown, and pale blue colors. In thin section it is characterized by pale brown color and mottled or fibrous form. Saponite generally evenly replaces groundmass and phenocrysts, preserving the primary igneous textures. In the case of highly to completely altered samples, saponite replacement is pervasive across mesostasis and groundmass crystals to form continuous mottled replacement, destroying the original textures (Fig. F12). Commonly, saponite lines or fills vesicles, forms a lining along vein edges, and replaces both groundmass and phenocrysts (Fig. F12). Saponite occurs occasionally as pale blue, either as alteration of glassy chilled margins or lining vesicles. In Unit 6 saponite occurs as a dark green waxy coating on most fractures and veins. Celadonite is the only other clay mineral confidently identified in hand specimen and thin section observations and occurs as bright green/blue in hand specimen. Celadonite is also present in all four types of alteration but is less abundant than saponite. In thin section celadonite is bright green, and within some vesicles the intensity of the color varies (see “Vesicle filling”).

Iron oxyhydroxide is the next most abundant secondary phase, and it occurs either alone as iron oxyhydroxides or mixed with saponite and other clay phases. Iron oxyhydroxides are easily identified by their bright orange to red color and often stain other phases present. When present as replacement of phenocrysts and groundmass, iron oxyhydroxides are mixed with saponite and clays (iddingsite) and form hyalophitic texture. In veins and lining or filling vesicles, iron oxyhydroxides are bright orange to red-brown and occur with or without intergrown clays. Staining and replacement of phenocrysts with iron oxyhydroxides is a common feature in the dark gray/black halos present throughout the hole.

The zeolite phillipsite was identified by XRD analysis of altered chilled margins (Sample 327-U1362A-3R-1, 22–24 cm) (Table T5). Other veins analyzed by XRD contained montmorillonite (smectite group) and sepiolite clay phases. The sepiolite is interpreted to be contamination from drilling mud. Carbonate is present as vesicle fill in veins and within chilled margins. XRD analysis of carbonate-bearing samples identified the carbonate as calcite (Samples 327-U1362A-2R-1, 43–45 cm; 14R-2, 53–54 cm; and 16R-2, 56–119 cm). Anhydrite is present in veins from Core 18R, where it occurs as pure white crystals and was identified by bright third-order interference colors in thin section.

In addition to cores recovered from Hole U1362A, small millimeter- to centimeter-sized chips were recovered from the drill bit in Holes U1362A and U1362B. Basalt exhibiting a wide variety of hydrothermal alteration was recovered in these chips, and similar compositions were recovered in both Holes U1362A and U1362B. Alteration types included pervasive green and red alteration, iron oxyhydroxides, pale gray sulfide-bearing mud, and basalt chips with tentatively identified epidote crystals. Spot analysis of the epidote crystals undertaken postcruise at the National Oceanography Centre, Southampton, confirmed epidote to be present within the basalt chips recovered from the sediment/basalt interface in Hole U1362A (Table T6). XRD analysis identified saponite, sepiolite, and phillipsite in the green alteration and pyrite with gismondine in the sulfide mud. The sepiolite is interpreted to be a contaminant from the drilling mud. A 3 cm long chilled margin was also recovered from Hole U1362B.

Breccia

Breccia is not common in this hole and is only present as small localized features. No unit-size hyaloclastite breccia was recovered. A single occurrence of magmatic breccia (hyaloclastite) present in Section 327-U1362A-13R-1 (Piece 8) consisted of two large basaltic clasts and multiple small <1–5 mm clasts in a saponite and altered-glass matrix (Fig. F13). The large clasts are subangular and as large as 42 mm and form 75% of the piece. Alteration within these clasts is high: black halos dominate the clasts with small 10 mm slightly altered cores. Multiple vein compositions in the clasts formed both before and after brecciation, the latter observed as veins continuing from the matrix into the clast itself. The saponitic matrix (16 mm wide at the broadest point) contains a complete range of smaller clasts that vary from moderately altered to completely replaced and pseudomorphed by secondary phases. These clasts are predominantly replaced by saponite, with clast rims highlighted by pale green clays. Internal structure in these clasts is often preserved; one example shows a filled vesicle still present. The wide range of alteration of the smaller clasts of the same size requires multiple sources of material to form the hyaloclastite. A piece of hyaloclastite from Section 327-U1362A-13R-1 (Subunit 5A, pillow basalt) contains a wide variety of alteration features, including altered chilled margins, black alteration halos, and large iron oxyhydroxide veins with multilayered halos.

A second example of breccia in Hole U1362A is a tectonic breccia vein in Subunit 4A (Section 327-U1362A-9R-2 [Piece 10, 74–87 cm]), interpreted to represent a cataclastic zone. This cataclastic zone forms approximately half of Piece 10, oriented diagonally through the piece (Fig. F14). A 2–3 mm dark green vein of saponite and pulverized basalt marks the edge of the cataclastic zone, with subparallel saponite veins and adjacent green-brown alteration halos. The alteration of the host basalt (background and halos) is consistent with the alteration both within Section 327-U1362A-9R-2 and the remainder of the subunit. All of the clasts show <0.1 mm light to dark brown alteration along their edges. The matrix in this breccia is dark green and in thin section appears to be predominantly pulverized hydrothermally altered host rock with no evidence of later cementation by secondary phases.

Glass

Volcanic glass is a common feature throughout the hole and is predominantly found within the pillow lava units on chilled margins. Glassy chilled margins vary from <1 mm to several centimeters and are altered to varying degrees. Of the 61 chilled margins recovered, some have retained fresh glass in addition to replacement by secondary phases (palagonite and saponite), whereas others are completely replaced by saponite. In examples where the alteration of glassy chilled margins is not complete, hydrothermal veins are commonly observed crossing through and along the margin. Compositionally, these veins include both green and pale blue saponite and carbonate. Continuation of these veins into the pillow core perpendicular to the chilled margin was also observed, indicating that the same hydrothermal fluids moved through the pillow core and margin. In thin section, volcanic glass is amber in color and contains microlites of plagioclase and clinopyroxene, multiple subparallel and crosscutting microveins, and aggregates of opaque microcrystals emanating from nuclei. Volcanic glass recovered within the sheet flow and basalt flow units has a similar range of features and differs only in abundance and absence of the characteristic radial cooling cracks observed in the pillow lava examples.

Vesicle filling

Most units from Hole U1362A are sparsely to moderately vesicular, containing 0.1–9.0 mm diameter vesicles. The majority of vesicles are partially to completely filled with one or more secondary minerals, and those observed in thin section are generally filled with >50% secondary phases. Secondary minerals present within vesicles include saponite (pale green, pale brown, or pale blue), celadonite, iron oxyhydroxides, carbonate, unspecified clay, and pyrite. Saponite and iron oxyhydroxides are the most common secondary minerals, and either or both occur in every unit. On both unit and piece scales the variability of vesicle-filling minerals is high, with a typical assemblage for a unit containing seven or more different secondary minerals (Fig. F15). The mineralogy and order of filling within the vesicles can be used as a record of changes in fluid chemistry and can be combined with crosscutting vein relationships to document the order of secondary mineral formation. Hand specimen observations of sequential relationships from Hole U1362A vesicles include the following:

• Saponite lining with cores of

• Iron oxyhydroxides

• Pyrite

• Pale green clay

• Carbonate

• Celadonite

• Mixed brown and white (clay + carbonate?)

• Iron oxyhydroxide lining with cores of

• Saponite

• Celadonite

• Pale green clay

• Celadonite lining with cores of

• Saponite

• Iron oxyhydroxides

The overlap between saponite, iron oxyhydroxides, and celadonite as lining and filling vesicles (e.g., saponite with celadonite core and celadonite with saponite core) indicates continual evolution of secondary mineral formation. Conclusive relationships can only be made for carbonate and pyrite, which must form later because they are always present within the cores of previously lined vesicles or as monomineralic vesicles.

The continual evolution of the early secondary minerals is highlighted in thin section, where concentrically multilayered vesicles are evident. Figure F16 shows a multilayered vesicle with bright green celadonite lining and subsequent filling of mixed celadonite with saponite, resulting in varied shades of green.

When vesicles are present within alteration halos the composition of the halo strongly influences the secondary minerals that fill the vesicles. For example, vesicle fill is dominated by iron oxyhydroxides in orange-brown halos.

Veins

A total of 1230 hydrothermal veins were identified and logged in Hole U1362A, with an average frequency of 27 veins per meter of recovered core (see VEINLOG in “Supplementary material”). Vein thickness varies from <0.1 to 4.0 mm, and veins recovered include planar, straight, curved, branched, crosscutting, stepped, kinked, and irregular shapes. More than 75% of the veins are <0.1 mm in width. The most prevalent vein mineralogy is saponite, which is present in at least 76% of veins. Clay minerals too fine grained to identify occur in 50% of the total veins (Fig. F17). The next most abundant vein mineral is iron oxyhydroxides, with 392 occurrences (32%). Carbonate was identified in 119 veins (~10%) but is present as the main component (>50% abundance) in only 34 of these veins. Pyrite is present in 107 veins and only rarely by itself (7 monomineralic pyrite veins). Celadonite is present in 21 veins (~2%) and is, overall, much more prevalent in the background alteration. Anhydrite is present in at least 2 veins. Of the recovered veins, 40% occur as monomineralic veins, with the remaining 60% of veins filled with two or more secondary minerals. Alteration halos directly associated with an adjacent vein are present in only ~15% of the observed veins (see “Alteration halos”).

Multilayered veins can be used to determine the sequence in which secondary minerals formed. In Hole U1362A layered veins are a common feature, and saponite generally forms the outermost lining. Vein core compositions include iron oxyhydroxides, carbonate, and celadonite, although iron oxyhydroxides are also present in the outermost layers in some examples (Fig. F18). Crosscutting relationships between different generations of veins were identified in 11 intervals.

Carbonate veins are the fourth most abundant in Hole U1362A, although they predominantly occur with carbonate as a minor phase. Carbonate is commonly found within the altered chilled margins and is mixed with pale green clays. When carbonate is the dominant mineral, it occurs with saponite and pyrite as accessory minerals. An anhydrite vein was identified in one thin section from Sample 327-U1362A-18R-2, 100–102 cm (Fig. F19), and on the basis of this calibration at least one other vein was also identified, also in Core 18R. Anhydrite is present as a white vein with good crystal structure and is associated with saponite as a lining. In addition to pyrite within veins, chalcopyrite was tentatively identified on two exposed vein surfaces.

Alteration halos

In addition to the generally pale to dark gray background alteration exhibited in the basalt of Hole U1362A, alteration halos are also a prevalent feature throughout the hole. The halos are either adjacent to hydrothermal veins, bordering individual rock pieces, or are not visibly associated with any defining hydrothermal or structural feature. Alteration halos flanking veins (15% of the total veins) vary from 0.2 to >35 mm wide and are associated with both monomineralic veins (saponite, clay, and iron oxyhydroxides) and polymineralic veins (celadonite, carbonate, sulfides, and talc). In two places alteration halos are present around predominantly carbonate veins (>50% carbonate).

Classification of halos was based on the color of the halo. When more than one color is present the halo was described as mixed. The dominant single-color halo is dark gray and ranges from 1 to 20 mm in width (Fig. F20). Compositionally, these halos represent the alteration of mesostasis and groundmass with comparatively more intense replacement and overprinting by saponite than pervasive background alteration. Additional secondary minerals that may or may not also be present within halo alteration zones include celadonite and unidentified oxides. The distribution of secondary minerals within the halos is similar to the background alteration and replaces mesostasis and phenocrysts. Orange halos are also reasonably abundant and mineralogically represent host rock extensively replaced by iron oxyhydroxides. The dark green halos are the rarest of the single-color halos and are a mix of saponite and celadonite (Fig. F21).

Mixed halos are the most abundant halo type and form a wide range of multilayered and patchy halos. We frequently observed orange alteration spots within the dark gray halos where iron oxyhydroxides replaced vesicles and phenocrysts (Fig. F22). Orange alteration spots also occur in the dark green and pale gray halos. Distinct bands of colors can be clearly seen in hand specimen in multilayered halos. Color combinations in this type of halo include dark gray and pale gray; pale gray, orange, and dark gray; orange and dark gray; dark gray and pale brown; dark green and light green; dark gray, dark green, and light green; and dark green, orange, pale brown, and dark gray (Fig. F23). Mixed halos occur flanking veins and occasionally as piece borders and are the only type where no visible association with veins/structures is present (Fig. F24). The nonassociated halos are several centimeters wide, are generally larger than vein-flanking halos, and are abundant within Sections 327-U1362A-9R-1 and 11R-2. The range of colors in the layered halos represents changes in the dominant secondary minerals present. Those with green color contain celadonite, whereas orange-brown halos contain iron oxyhydroxides.

Alteration summary

Low-temperature (<100°C) hydrothermal alteration of basalt from the ocean crust has been documented in numerous boreholes (e.g., ODP Holes 504B and 1256D) and is generally described in terms of two components: open circulation of seawater causing oxidative alteration and restricted circulation causing nonoxidative alteration (Laverne et al., 1996; Teagle et al., 1996; Teagle, Alt, Umino, Miyashita, Banerjee, Wilson, and the Expedition 309/312 Scientists, 2006). Each of these alteration styles can be associated with the formation of different secondary minerals; iron oxyhydroxides and celadonite are typical of oxidative alteration, whereas saponite and sulfides are indicative of nonoxidative alteration.

The range of secondary minerals present in Hole U1362A is consistent with hydrothermal alteration in both oxidative and nonoxidative environments. The extent of hydrothermal alteration is variable on a unit-by-unit scale, but overall comparison of the three types of units identifies a few differences. For example, pillow lava has the same range in total alteration as the sheet flows. However, detailed observations of thin sections demonstrate that the pillow lava is predominantly highly altered, whereas the sheet and basalt flows are moderately altered. Vein density clearly differs by unit type, with ~32–37 veins per meter of recovered core in the basalt flow and pillow lava units and ~17 veins per meter of recovered core in the sheet flows. There are no clear correlations between secondary mineral occurrence in veins and the different unit lithologies, although variations exist. For example, sulfide veins are more prevalent in the lower units (Subunit 5B to Unit 8), and celadonite does not appear deeper than Subunit 6B. It is difficult to reconcile the vein mineral assemblages in each unit to define dominance of oxidative or nonoxidative alteration.

In addition to secondary minerals characteristic of typical hydrothermal alteration of upper oceanic basement, Hole U1362A also contains some secondary minerals that indicate more complex hydrothermal processes. The recovery of epidote at the sediment/basalt interface is unusual. Epidote is a greenschist facies mineral generally associated with high-temperature hydrothermal alteration and has been recovered from in situ ocean crust in only a few locations (e.g., Holes 1256D [Teagle, Alt, Umino, Miyashita, Banerjee, Wilson, and the Expedition 309/312 Scientists, 2006] and 894G [Shipboard Scientific Party, 1993] and the Tonga forearc [Banerjee et al., 2000]). Epidote has been linked to upflowing hydrothermal fluids in ophiolite studies. The presence of anhydrite veins in Hole U1362A is a new discovery from Juan de Fuca Ridge-flank drilling. Anhydrite has been found in other upper crustal sites and can be related to mixing of hydrothermal fluids and seawater. The occurrence of epidote with pyrite at the seafloor, combined with anhydrite at depth, may indicate that Hole U1362A was formally a location of hydrothermal upflow.

Hard rock geochemistry

Thirty-one representative samples of igneous rocks were analyzed for major and trace elements using a Teledyne-Leeman (model Prodigy) Inductively Coupled Plasma–Atomic Emission Spectrometer (ICP-AES). Sample selection for ICP-AES analysis was targeted toward the freshest material from Hole U1362A in order to obtain a downhole record of primary magmatic compositions. The methods of preparing and analyzing the samples are described in detail in “Petrology, hard rock geochemistry, and structural geology” in the “Methods” chapter. The international standard BCR-2 was analyzed nine times over three runs and indicates that the analytical precision was <3% for major elements and <10% for trace elements.

The major and trace element data for the 31 selected samples is shown in Table T7. Loss on ignition (LOI) values and calculated Mg number (Mg#) of the samples are also included in the table. All of the samples are characterized as N-MORB. The good linear correlation between TiO₂ (weight percent) and Zr (parts per million) for all 31 samples indicates that the different basalt units of Hole U1362A originated from a similar, if not the same, source (Fig. F25).

The samples are grouped into three unit types: basalt flow, pillow, and sheet flow. Plots of major and trace element abundance versus Mg# of the three unit types are shown in Figures F26 and F27. Generally, the samples do not show significant correlations between major or trace elements and Mg#; however, some trends can be distinguished for the different unit types. For example, for all unit types MnO decreases with increasing Mg#. Similarly, for basalt flow and sheet flow units, Na₂O, TiO₂, P₂O₅, Sc, V, and Zr are positively correlated with Mg#.

The variations of major and trace elements and Mg# with depth are shown in Figure F28. Although there are some fluctuations, Fe₂O₃ shows a weak increasing trend with depth, MgO slightly decreases, and Mg# decreases. Cu decreases with depth, and Cr decreases with excursions at Unit 5 (pillow lava) and Unit 7 (basalt flow). The vertical variations of TiO₂, V, and Zr are consistent with the rock type; usually the contents from pillow lava samples (Units 1 and 5) are lower than those from basalt flows (Units 2 and 7) and sheet flows (Units 4, 6, and 8).

Structural geology

Basalt from Hole U1362A documents a range of structural features related to different processes. These include fractures formed during eruption, such as the radial cooling cracks of pillow lava; hydrothermal alteration preserved as veins; later tectonic fractures such as any planar features and cataclastic zones; and fractures induced by coring.

The dips of 519 veins and fractures were measured in the recovered cores from Hole U1362A. Three types of veins and 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”), and (3) joints sometimes flanked by alteration halos but not filled with minerals. Nonhaloed veins were the most frequently observed structures in rocks from Hole U1362A. Nonhaloed veins were identified mainly in the massive lava and some pillow lava pieces. No clear faults or shear veins showing any evidence of displacement were found.

Figure F29 shows the distribution of measured fracture and vein dips and indicates an overall progressive increase in frequency with increasing dip angle. More than 50% of the measured veins form slight trimodal peaks identified at 40°–50°, 65°–75°, and 80°–90°. Fractures and veins are not uniformly distributed but occur more frequently in the pillow lava units relative to the massive lava units (Fig. F30). The deeper pillow lava and massive units (e.g., Units 5 and 6) have similar distributions of vein and fracture dips. However, these similarities may be artifacts of the uneven number of samples in each unit. Histograms of vein/fracture dip for haloed structures compared to those of nonhaloed veins (Fig. F31) show that haloed and nonhaloed features have different structural characteristics. Haloed fractures are predominantly steeply dipping, with frequency increasing with dip angle. Nonhaloed veins have peaks at 20°–30°, 40°–45°, 50°–55°, 60°–70°, and 80°–90°. For massive lava, 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 dips in the basement at Site U1362. Recovery was very high in Core 327-U1362A-18R, where a near-continuous sheet flow was recovered. This whole-round core was imaged multiple times, and the images were stitched together to produce a continuous image of the core exterior (see CORE18R in “Supplementary material”; Fig. F32).

The cataclastic zone recovered in Subunit 4A (Section 327-U1362A-9R-2 [Piece 10, 74–87 cm]), was classified as tectonic breccia because the matrix is composed of ground up basalt. This cataclastic zone forms approximately half of Piece 10, oriented diagonally through the piece (Fig. F14). The cataclastic zone is up to 28 mm long and is composed of subangular pieces of fine-grained host rock as large as 18 mm that were fractured in situ perpendicular to the vein orientation. Open spaces of 1–2 mm are present between the larger clasts, but generally the cataclastic zone is fully consolidated with matrix and clasts. Smaller clasts are also present and exhibit varying degrees of clast rotation and separation. The good fit between adjacent clasts, the examples of clast rotation of the smaller clasts, and the composition of the matrix are evidence for classifying this as a tectonic breccia. The steep angle and presence of alteration halos suggest this feature formed at a similar time as the vertical cracks with alteration halos.

Hand specimen observations support the following sequence of structure formation in rocks from Hole U1362A:

1. Formation of radial cooling cracks perpendicular to pillow margins;

2. Formation of vertical cracks and cataclastic zones with associated hydrothermal alteration halos during normal faulting near the ridge axis;

3. Development of younger fractures without halos; and

4. Formation of planar fractures in massive lava by tectonic stress.

The observed dominance of vertical extensional-related structures is consistent with the location of Site U1362 on an abyssal, normal-faulted basement high.

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