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doi:10.2204/iodp.proc.329.106.2011

Igneous lithostratigraphy, petrology, alteration,
and structural geology

In Hole U1368F, basement was cored from 11.8 to 115.1 mbsf, of which 31.74 m was recovered (27.6% recovery).

The recovered basement at Site U1368 is predominantly composed of very fine grained (cryptocrystalline to microcrystalline) aphyric to phyric basaltic fragments that contain large quench structures and glassy margins. Several intervals contain larger in situ fragments that texturally exhibit similar features to the basaltic fragments. In addition, more massive microcrystalline aphyric to sparsely phyric intervals that resemble sheet flows were also recovered. One interval includes a hyaloclastite breccia composed of variably altered glass shards and basaltic fragments cemented in quartz. These lithologies were divided into 13 basement units based on changes in lava morphology, flow boundaries, the presence of pillow basalt textures, and phenocryst occurrence. The distribution of lithologic units is summarized in Figure F15. Further detail regarding the definition of igneous units is reported in “Lithostratigraphy, igneous petrology, alteration, and structural geology” in the “Methods” chapter (Expedition 329 Scientists, 2011a).

Basement Units 1, 3, 5, 7, 9, and 11 were interpreted to represent flowlike units. Units 2, 4, 6, 8, 10, and 12 are interpreted to represent variably fractured pillow lavas. No subunits were defined at Site U1368. The presence of pillow lavas at Site U1368 is discussed in “Lithologic units.” One volcaniclastic hyaloclastic breccia (Unit 13) was recovered at the base of the hole in interval 329-U1368F-14R-1, 43–150 cm, and is interpreted to represent a collapse event, in which partially altered glassy rinds and pillow basalt fragments were deposited into a topographic low. Samples of basement were also recovered in Holes U1368B and U1368D; however, their relationship with the units defined from the basement recovered in Hole U1368F is not known, and therefore we did not assign units to these samples. These samples include a glassy margin recovered in interval 329-U1368D-2H-CC, 0–8 cm, in which the glass is moderately altered with a few fresh glassy interiors exposed. In addition, basaltic samples within the core catcher are cryptocrystalline and aphyric, with pervasive alteration and a ropy pahoehoe texture, suggesting surface contact with cold seawater. In Holes U1368B–U1368D, basalt at the sediment/basement interface has been 100% altered to form dark green sand (see “Lithostratigraphy” for more details).

Lithologic units

Units 2, 4, 6, 8, 10, and 12 (fractured pillow lavas)

Fractured pillow lavas occupy 20.2 m (58.8% by volume) of the recovered core, making fractured pillow lavas the most abundant lava morphology at Site U1368. Classification of fractured pillow lavas was based on the presence of curved chilled margins and glassy rinds, conchoidal fracturing, abundant vesicles near the chilled margins, quenching structures that flank cooling fractures, and concentric cooling zones within some fragments (Figs. F16, F17). The basaltic groundmass is composed of plagioclase, clinopyroxene ± olivine, and Fe-Ti oxides. We observed subophitic, spinifex, and spherulitic textures. Phenocrysts of plagioclase and rare clinopyroxene are present in Units 2 and 4, with abundances from 0.1% to 3% by volume of the recovered core. Glassy margins are present throughout the fractured pillow lava units, most of which are either devitrified or slightly to moderately altered with saponite and iron oxyhydroxides. The distribution of fresh and altered glassy margins is shown in Figure F15. Alteration within Units 2, 4, 6, 8, 10, and 12 varies from very slight to slight. Many vesicles within the central portions of the fragments are only partially filled by secondary minerals. Alteration is most pervasive within the spinifex and interstitial textures within the quench structures and glassy margins.

Units 1, 3, 5, 7, 9, and 11 (sheet and massive flows)

Sheet flows occupy 38 m (~38% by volume) of the recovered core and are the second most abundant lava morphology at Site U1368. Recovery of complete or near-complete flows occurred in Cores 329-U1365E-5R, 6R, 8R, and 12R. Classification was based on the presence of continuous sections of the same lithology, very slight coarsening of grain size away from the top of the flow, and vesicle variation within the lava flow. In terms of flow thickness, Units 1 and 3 are larger than Units 5, 7, 9, and 11, and they are tentatively classified as massive sheet flows. Units 1 and 3 share the same characteristics as Units 5, 7, 9, and 11; therefore, we group Units 1, 3, 5, 7, 9, and 11 as sheet flows. The sheet and massive flows are also noted for their low abundance of chilled margins.

The sheet and massive flows are aphyric, with groundmass composed of plagioclase, clinopyroxene ± olivine, and Fe-Ti oxides. Unit 3 contains rare (<0.2%) plagioclase phenocrysts that range from 0.1 to 0.4 mm in size. Major textural features within the lava flows vary from intergranular, subophitic, hyalophitic, and glomeroporphyritic. However, textural observations of thin sections include spinifex, spherulitic, ophitic, subophitic, hyalophitic, intersertal, and glomeroporphyritic textures. Grain sizes range from cryptocrystalline toward the flow margins to fine grained in the central portions of some flows. Vesicle abundance in the flow basalt units varies from none (Unit 3) to 10%; however, modal abundance is ~1%. Vesicle abundance within an individual flow unit varies from 0% to 10%, with most vesicles typically near the unit boundaries. Almost all vesicles are partially to completely filled with secondary minerals (see “Basement alteration”). Rare flow margins are usually altered and exhibit cryptocrystalline and sometimes glassy textures.

Alteration within the sheet and massive flows is generally slight but ranges from very slight to moderate. Alteration may include groundmass replacement, vesicle fill, veins, halos, and alteration patches (see “Basement alteration”). Unit 1 is the most pervasively altered, with a gray-brown background that shares some similarity to the obliterated basaltic sediment in Core 329-U1368B-2R (Fig. F18). Although no real changes in alteration were observed near the top or base of the flow units, the actual contacts were not recovered, implying that alteration may be under-represented. The flows within Units 7, 9, and 11 contain large brown alteration halos that relate to steeply dipping curved fractures. The more massive flow within Units 1, 3, 5, 7, 9, and 11 are better recovered than the fractured pillow lava units, with many more oriented individual pieces than the fractured pillow lavas (Units 2, 4, 6, 8, 10, and 12).

Unit 13 (volcaniclastic breccia)

The lowermost 1.07 m (3.1%) of Hole U1368F is volcaniclastic breccia that consists of partially to completely altered glassy clasts and altered basaltic clasts with quartz cement. Alteration of the glassy clasts gives this breccia a characteristic pale orange-brown color. The breccia is described in greater detail in “Basement alteration.”

Igneous petrology

As described above, the basaltic rocks recovered from Hole U1368F are divided into massive sheet flows and thin basalt flows. A total of 18 samples were selected for petrographic analyses by thin section (see Site U1368 thin sections in “Core descriptions”).

Sheet and massive flow basalt

The mineralogy of the sheet and massive flow units at Site U1368 (Units 1, 3, 5, 7, 9, and 11) is typical of seafloor basalt. The groundmass is composed of plagioclase (58%–75%), clinopyroxene (26%–35%), Fe-Ti oxides (2%–5%), and rare olivine (approximately <1%). The plagioclase typically forms bladed crystals that are intergrown with anhedral to subhedral clinopyroxene and Fe-Ti oxides (titanomagnetite). Overall grain size of the groundmass ranges from cryptocrystalline to microcrystalline, and the most common textures are intergranular and subophitic to hyalophitic. The sheet flows identified at Site U1368 show no variation in grain size from the top to the center to the bottom of the units. Only Unit 3 contains some minor plagioclase phenocrysts (0%–0.2% modal abundance) only plagioclase and rare clinopyroxene phenocrysts are present. Plagioclase is blocky to prismatic and ranges in size from 0.2 to 0.8 mm. Clinopyroxene crystals are lathlike to prismatic and range in size from 0.2 to 0.6 mm. Olivine is rare and only observable as saponite/iddingsite pseudomorphs recognized by a rough, six-sided outline. Some pseudomorphs of olivine are identified by the presence of sphene intergrown contemporaneously with olivine prior to replacement of the olivine by saponite/iddingsite.

Vesicle abundance within the flow units varies from 0% to ~11%. Vesicle morphology is rounded, subrounded, angular, and interconnected. In Units 1, 5, and 11, vesicles are concentrated near the top of the units, whereas vesicles in Unit 7 are concentrated in the middle portion. Unit 9 contains no vesicles. Vesicles vary from 0.1 to 3 mm in diameter; most are ~0.4 mm wide. Vesicles are predominantly filled with silicates, zeolite, celadonite, and iron oxyhydroxides. Less common vesicle fillings include saponite, calcite, and pyrite. The secondary mineral fills of vesicles are discussed in “Basement alteration.”

Fractured pillow basalt

The fractured pillow basalt of Units 2, 4, 6, 8, 10, and 12 has groundmass compositions that are typical of seafloor basalts. Plagioclase and clinopyroxene are the most abundant primary mineral phases. Minor phases include Fe-Ti oxides (titanomagnetite), rare olivine, and sphene. Primary igneous textures and mineralogical differences within the lava are defined by the rapid nature of cooling, including glassy margins, quench structures, and changes in crystal size. Grain size ranges from microcrystalline to glassy; most of the fractured pillow basalt, especially within the chill margins, is cryptocrystalline to glassy. The most common textures observed include intergranular, spinifex, and porphyritic. Other textures observed include glomeroporphyritic and spherulitic. Within chilled margins, grain size is irregular, with patches of glassy and spinifex textures surrounded by cryptocrystalline to microcrystalline intergranular and glomeroporphyritic groundmass.

Units 6, 8, 10, and 12 are aphyric, whereas phenocryst abundance in Units 2 and 4 ranges from 0.1% to 3%. Phenocrysts in Unit 2 are composed of plagioclase (75% of phenocrysts) and clinopyroxene (25% of phenocrysts), with crystal sizes ranging from 0.2 to 2 mm (mode = 0.8 mm) and 0.2 to 0.8 mm (mode = 0.4 mm), respectively. Plagioclase is relatively fresh with crystal shapes that range from prismatic to blocky and having endured only minor corrosion around the edges and partial replacement by saponite. Clinopyroxene is subhedral to blocky and has endured slightly greater alteration to saponite than plagioclase. Phenocrysts in Unit 4 are very similar to those in Unit 2 but range in crystal size from 0.2 to 1 mm (mode = 0.4 mm) for plagioclase and 0.3 to 0.5 mm (mode = 0.4 mm) for clinopyroxene. Rare altered olivine phenocrysts are also present. Glassy margins range from fresh to slightly altered, whereas overall alteration within Unit 1 is slight. Small portions of each fragment contain brown and dark gray complex halos that propagate inward from the fracture and chill margins. These alteration halos are not to be confused with the large chilling margins, which are dark and have a sharp front when viewed in hand specimen. Alteration is discussed in more detail in “Basement alteration.”

Phenocryst phases

Plagioclase

Although plagioclase phenocrysts at Site U1368 are present only in Units 2, 3, and 4, they remain the most abundant phenocryst phase. Plagioclase phenocrysts make up <0.1% of the sheet and massive flows and 0.2% of the fractured pillow lavas. Their shape ranges from subhedral to euhedral and is commonly blocky to prismatic. Bladed and acicular plagioclase phenocrysts are also observed. Plagioclase phenocrysts range from 0.1 to 2 mm in length, with most between 0.1 and 0.8 mm long. Rarely, plagioclase forms skeletal or quench plagioclase crystals within the pillow lava units (Figs. F19A, F19B) and zoning of plagioclase occasionally occurs in the larger phenocrysts (Fig. F19B). Although plagioclase phenocrysts are typically fresh or very slightly altered, replacement by secondary minerals can vary from 0% to 50%. Replacement minerals that include clays, saponite, and iron oxyhydroxides occur along cracks, cleavage planes, or crystal edges. In addition, secondary Fe-Ti oxides occasionally form around the edges of some plagioclase phenocrysts.

Pyroxene

Clinopyroxene phenocrysts are present throughout the Site U1368 basalt but make up <0.1% of the recovered core. These phenocrysts range from 0.2 to 1 mm in length, although most are 0.2 to 0.4 mm long. They are typically anhedral to subhedral and either irregular, prismatic, or lathlike, with simple basal twinning present throughout. Clinopyroxene is typically intergrown with plagioclase in glomeroporphyritic clots or as subophitic to ophitic crystals around plagioclase (Figs. F19C, F19D). 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.

Olivine

Fresh olivine is extremely rare, and olivine pseudomorphs make up <0.01% of the recovered material. Olivine phenocrysts are, on average, 0.4 mm wide and blocky to prismatic in shape. They are almost always completely replaced by secondary minerals (Fig. F20), including saponite, celadonite, iddingsite, and opaques (sphene and Fe-Ti oxides). Olivine’s identification, therefore, relied on crystal morphology (subhedral to euhedral) and textural relationships with surrounding minerals. Additionally, the presence of sphene next to phenocryst pseudomorph is used as an indicator of olivine.

Groundmass

The basaltic groundmass at Site U1368 varies from hypocrystalline to holocrystalline and is composed primarily of plagioclase and clinopyroxene with minor accessory Fe-Ti oxides. Olivine in the groundmass is extremely rare. Plagioclase is the most abundant groundmass crystalline phase, comprising between 58% and 70% of the groundmass; it occurs as microlaths, microlites, acicular crystals in a spinifex texture or microcrysts in chill margins. Within chilled margins and alteration halos, plagioclase may be partially corroded by secondary mineral phases.

Clinopyroxene comprises between 25% and 30% of the pillow lavas and 30% and 35% of the flow units. It occurs as interstitial growths between plagioclase, microlaths, microlites, and aggregates of fibrous or plumose crystals. Anhedral to subhedral microcrysts of pseudomorphed olivine are very rare within the massive lavas and thin basalt flows. The lack of fresh olivine and the difficulty in identifying olivine pseudomorphs (based on relict crystal structure) hampers efforts to estimate its abundance; however, our observations imply that it occupies <0.1% of the recovered core.

Mesostasis in Hole U1368F ranges from 0.5% to 20% by volume. Mesostasis textures include hyalophitic and intersertal. These textures are present throughout the recovered basement but are most common within quenching structures and chilled margins. Spherulitic and variolitic textures are most common in the thin basalt flows and close to chill margins. Mesostasis is typically subject to patchy alteration, in which it is preferentially altered relative to the plagioclase and clinopyroxene groundmass. Almost all patchy alteration observed at Site U1368 is the result of altered mesostasis. Replacement minerals in the groundmass include clay (saponite and celadonite), iron oxyhydroxides, and, rarely, carbonate. Primary magmatic opaques (<1%–4%) are present in all units. These form small (<0.2 mm), granular, subhedral crystals of sphene and secondary hematite and titanomagnetite.

Hard rock geochemistry

Twenty-three representative samples of the basaltic basement and one whole breccia sample were analyzed for major and trace elements using a Teledyne-Leeman (Prodigy) inductively coupled plasma–atomic emission spectrometer (ICP-AES). Analyses were carried out in the same was as for samples from Sites U1365 and U1367. Samples analyzed from Site U1368 include relatively unaltered basalt groundmass (gray to green) and variably altered halos (red to brown). The least altered samples were chosen based on (1) the lowest abundance of secondary mineral phases present in thin section and (2) the least number of veins, halos, and filled vesicles. Altered samples were chosen so that each alteration phase is represented. Details of the methods for preparation and analyses are given in “Lithostratigraphy, igneous petrology, alteration, and structural geology” in the “Methods” chapter (Expedition 329 Scientists, 2011a). International Standard BCR-2 was analyzed 24 times over 3 runs. Analytical precision and accuracy is reported in Table T2 in the “Methods” chapter (Expedition 329 Scientists, 2011a).

Results

Major and trace element data and loss on ignition (LOI) for the selected samples are shown in Table T2. For all basaltic samples, the ranges of major element oxides include

• SiO₂ = 45.6–52.6 wt%,
• Al₂O₃ = 13.1–17.2 wt%,
• Fe₂O₃^(T) = 8.9–15.4 wt%,
• MgO = 4.9–14.8 wt%,
• Na₂O = 2.4–3.2 wt%,
• TiO₂ = 1.47–2.26 wt%, and
• K₂O = 0.14–0.76 wt%.

Trace element ranges and averages include

• Sr = 117–376 ppm (average = 172 ppm),
• V = 208–479 ppm (average = 379 ppm), and
• Zr = 113–172 ppm (average = 149).

Trends in least altered basalt

Twelve least altered samples that ranged in color from gray to gray/green were selected, based on their low abundance of secondary minerals, for primary whole-rock chemical analyses. Total alkaline (K₂O + Na₂O) concentration ranges from 2.94 to 3.56 wt%, and SiO₂ concentration ranges from 49.6 to 52.6 wt%. Al₂O₃ ranges from 13.1 to 14.9 wt%, and CaO ranges from 9.9 to 11.9 wt%.

Vertical distribution of major and trace elements for all samples, including alteration halos and breccia, is shown in Figure F21. Tie lines are included to indicate halo/background pairs. The altered rocks on this plot are discussed in “Lithostratigraphy, igneous petrology, alteration, and structural geology” in the “Methods” chapter (Expedition 329 Scientists, 2011a). Only the background and least altered rocks are described below. Overall geochemical trends for Site U1368 basement indicate increasing MgO and possibly increased Fe₂O₃^(T) with depth and decreased K₂O with depth. The basalt samples from the uppermost 10 m of basement appear to show markedly different chemistry relative to the rest of the basalt, with notably lower Fe₂O₃^(T) and TiO₂ and higher Sr and LOI than the rest of the basalt. In addition, in these samples from the uppermost 10 m both one of the least altered samples and two altered samples are strongly enriched in MgO compared to the rest of the basement. Although the basaltic clast sample (329-U1368F-14R-1, 141–142 cm) compositionally behaves similarly to the other whole-rock samples, the presence of halos indicates that this sample was subject to alteration.

On plots of MgO versus Na₂O, Fe₂O₃^(T), TiO₂, and K₂O for the least altered samples (Fig. F22), increasing Na₂O, TiO₂, and K₂O may indicate primary magmatic evolution. However, Fe₂O₃^(T) exhibits no distinctive trends. K₂O/TiO₂ for the least altered basalts at Site U1368 ranges from 0.063 to 0.179 and falls within the accepted range for depleted basaltic compositions. All trends observed in Figures F21 and F22 provide limited insight into the primary magmatic evolution of the basement at Site U1368. Because all rocks at Site U1368 are altered (especially the uppermost 10 m of basement), the deviation from the primary igneous chemistry remains uncertain. The extent to which downhole variation in chemical composition of these least altered samples is due to magma evolution or basalt alteration will be addressed by postexpedition research.

Basement alteration

All basement rocks at Site U1368 have been subjected to alteration by interaction with seawater. Alteration varies from slight to high. However, the majority of recovered basement material at Site U1368 is only slightly altered. Basement alteration at Site U1368 consists of

• Replacement of phenocrysts by secondary mineral assemblages,

• Replacement of mesostasis in the groundmass by secondary minerals,

• Filling of veins and the formation of halos by emplacement of secondary minerals, and

• Lining and filling of vesicles.

Visible alteration in macroscopic view or thin section constitutes between 2% and ~60% of individual samples, with most alteration concentrated around veins and vesicles. The most intense alteration is present within chilled margins and the volcaniclastic breccias, where alteration can be nearly complete. Alteration products include saponite, celadonite, iron oxyhydroxides, quartz, carbonate, and accessory zeolite (laumontite by XRD) and sulfides (chalcopyrite and pyrite).

Secondary minerals were identified by macroscopic observation and thin section observation. XRD analyses were carried out on two samples at this site, the results of which are shown in Figure F22. Clay minerals are predominantly in the saponite group or celadonite group and were distinguished by color variations.

Saponite is present throughout the recovered core. In macroscopic observation, it is black, dark green, greenish brown, or pale blue. In thin section, it is characterized by pale brown color and it may be mottled, fibrous, or botryoidal in form. Replacement of the groundmass is usually even and slight, only replacing olivine mesostasis and some of the groundmass. In areas of moderate alteration, saponite replaces mesostasis and a varying proportion of groundmass crystals. High to completely altered basalt exhibits continuous mottled replacement, destroying most or all original textures. Saponite also frequently fills vesicles, forms monomineralic or polymineralic veins, and variably alters glass within the volcaniclastic breccia.

Celadonite is present throughout the recovered basement and is the next most common secondary mineral at Site U1368 after saponite. In hand specimen and thin section, celadonite is bright green/blue. It fills veins and vesicles and replaces primary interstitial zones in basaltic groundmass to form a pervasive dark gray to dark green background.

Iron oxyhydroxides are present throughout the recovered core. However, they are less abundant than celadonite. They occur alone or, more commonly, intermixed with saponite, imparting red, brown, and orange-brown staining. They are identifiable by a bright red-orange color and they often stain other secondary mineral phases (e.g., saponite stained by iron oxyhydroxide becomes brown/red-brown). In addition, iron oxyhydroxide typically replaces phenocrysts as a mixture with saponite (iddingsite) to form a hyalophitic texture. Iron oxyhydroxides may also fill or partially fill veins and they commonly form iron oxyhydroxide–dominated halos.

Less dominant secondary minerals at Site U1368 identified by XRD (Fig. F23) include stevensite, zeolite (laumontite and phillipsite), and clay (montmorillonite and sepiolite). Carbonate at Site U1368 is relatively uncommon with only a few veins present in Cores 329-U1368F-2R, 3R, 9R, and 10R. Pyrite was observed only in one vein in interval 329-U1368F-5R-2, 26–31 cm. Pyrite was identified by macroscopic and microscopic observation (gold in hand specimen and bright yellow in reflected light) and partially fills vesicles, veins, and patches. The crystal structure of pyrite at Site U1368 ranges from blocky to irregular.

Alteration features within the basement at Site U1368 are described below in order of alteration intensity. At Site U1368, breccia exhibits the greatest degree of alteration, albeit at only one very concentrated zone at the bottom of Hole U1368F (see below). The most pervasive forms of alteration at Site U1368 are halos and veins. Like the alteration at Sites U1365 and U1367, halos and veins represent the evolution of low-temperature secondary mineral emplacement within the basalt. Although vesicle fill is the least pervasive form of alteration at Site U1368, it provides a clear indication of the relative timing of secondary mineral emplacement; as such, it is described separately.

Breccias

One breccia unit (Unit 13) was recovered at the very bottom of Hole U1368F at interval 329-U1368F-14R-1, 43–150 cm. The breccia is a volcaniclastic hyaloclastite that comprises 3.1% of the total recovered rock at Site U1368. Given that core recovery is 26.3% and rheologically stronger units (sheet flows and massive flows) are preferentially recovered, it is likely that the recovered breccia percentage underestimates the true proportion of brecciated basement at Hole U1368F.

The hyaloclastite at interval 329-U1368F-14R-1, 43–150 cm, consists of variably altered glassy and basaltic clasts encased in quartz cement. Variably altered glass makes up 90% of the clasts, which range in size from 0.1 to 8 mm with most clasts between 1 and 3 mm in size. Basaltic fragments make up 10% of all the clasts and range from 0.1 to 39 mm in size, although the typical size range is 1 to 15 mm. Clasts make up 80% of the total volume of the breccia, whereas quartz and void space between the clasts each contribute 10% to the total breccia volume. All clasts within the breccia are angular and poorly sorted. Steeply dipping grading of some clasts is present, in which the clasts vary from coarse to fine (Fig. F24).

Alteration of the breccia is dominated by variable alteration within the glassy clasts and quartz cement. Glassy clasts exhibit zoned alteration with highly altered brown to orange saponitic zones on the outside edge and a less altered zone in the center (Fig. F24). Many of the larger glass clasts contain a fresh, unaltered glass interior (fresh glass makes up ~15% of the total breccia). The basaltic clasts resemble the fractured pillow lavas, such as Units 4 and 12, in that they are cryptocrystalline and aphyric and contain chilled margins and quench structures. Alteration of the basaltic clasts ranges from slight to moderate. They are characterized by brown to dark gray alteration halos within the quench structures and flanking microveins of celadonite, saponite, and iron oxyhydroxide. Halos appear to terminate abruptly at the edge of some larger clasts. In addition, relatively fresh portions of basaltic clasts are adjacent to the cement. These observations suggest that alteration within the larger basaltic clasts took place prior to brecciation. Smaller (<10 mm) clasts are all highly altered to saponite, secondary silicates, and iron oxyhydroxides; however, the timing of alteration relative to brecciation within the smaller clasts is not known (Fig. F24).

The highly angular nature of the clasts combined with steeply dipping particle gradients and the lack of alteration within the larger basaltic and glassy clasts suggests that the breccia represents infill from collapse from higher ground. The presence of nearby seamounts and the variable depth at which basement was tagged in Holes U1368A–U1368E (see “Lithostratigraphy”) imply that the bottom surface topography is irregular, further increasing the likelihood of breccias forming by collapse of nearby pillow lavas. In addition, two thin (1 mm) lamellae of sediment are present at intervals 329-U1368F-14R-1, 125–125 cm, and 14R-1, 133–133 cm, and imply that brecciation was not a single event and that there was elapsed time between successive lava collapses.

Vein- and halo-related alteration

Vein- and halo-related alteration at Site U1368 is similar in style to that of Site U1365, albeit with notably less abundant carbonate veins. Similarities include the presence of dark gray/brown saponitic and celadonitic background alteration throughout recovered basement and the presence of monomineralic and polymineralic veins flanked by variably colored alteration halos. Veins may contain any combination of the secondary minerals saponite, celadonite, iron oxyhydroxides, and silicates (quartz and chalcedony). Less common vein-filling phases include carbonate (calcite and aragonite), zeolite (possibly laumontite or phillipsite), and secondary sulfides (pyrite). Alteration halos at Site U1368 share the same nomenclature and styles as those from Site U1365. Alteration halos along the vein margins at Site U1368 include dark green/black halos, green/brown halos, and mixed halos. The most abundant halo type is dark gray/green halos, followed by red-brown to orange-brown halos. Red halos and complex (mixed) halos are the least common and primarily occur near chilled margins within the fractured pillow lava units and occasionally near veins. Halos and veins are discussed in detail below.

Halos

Dark green/black halos are present throughout the basaltic basement of Site U1368, flanking veins and fractures as individual halos and as part of mixed halos (Fig. F25). Dark halos range in width from 1 to 20 mm but are most commonly 1 to 10 mm wide. Secondary mineral abundance in the dark green/black halos is slightly greater than in the gray background basalt, with celadonite replacing olivine and interstitial material and filling vesicles. Like at Sites U1365 and U1367, celadonite is identified by its green color in thin section or its blue-green color and brittle texture in hand specimen; it typically replaces between 2% and 5% of the rock within the halo. As with celadonite observed at Sites U1365 and U1367, this phase may either be celadonite, nontronite, or a mixed-layer celadonite-nontronite. Therefore, detailed postcruise XRD analyses will be required to further refine our mineral definitions. Within these halos, saponite is occasionally observed replacing olivine, interstitial material, and vesicles, as well as overprinting celadonite. Vein- and vesicle-filling sequences (discussed later) indicate that the saponite phase arrived after celadonite. Iron oxyhydroxides may also be present in small amounts.

Green/brown halos occur throughout the recovered basalt at Site U1368. However, the greatest intensity of these halos occurs at the top of the basaltic basement, in Cores 329-U1368F-2R and 3R. Although green/brown halos range in width from 1 to 20 mm, the majority range from 1 to 13 mm. These halos are typically associated with saponite veins, but they also flank polymineralic veins and veins of celadonite, iron oxyhydroxides, and/or carbonate (Fig. F24). Thin section observation indicates that the dominant secondary mineral is saponite, which is green to brown in plane-polarized light and dark green/brown in hand specimen. Saponite fills vesicles and replaces olivine phenocrysts and interstitial material. Overall, saponite replaces between 3% and 80% of the rock within the halo. Most halos, however, exhibit only slight to moderate replacement (3%–20% replacement by saponite). Iron oxyhydroxides that stain the saponite to an orange-brown color are frequently present within these halos. Iron oxyhydroxide covers between 0.5% and 2% of the rock within the halo. One example of a brown saponitic halo sequence exhibits multiple banding in which the iron-rich saponite has formed concentric halos within coarser grained bands (Fig. F17) of the groundmass. Red halos represent any halo in which iron oxyhydroxides are the dominant mineral.

Other mineral phases that are sometimes present include saponite and celadonite. At Site U1368, iron oxyhydroxide–dominated halos are not common and most “red” halos actually contain iron-rich saponite. Iron oxyhydroxide halos occur in greatest concentrations at the top of the basalt in Hole U1368F; in the rest of the core, only discrete iron oxyhydroxide patches and individual crystals are present. Approximately 1% of the rock within the halo is composed of iron oxyhydroxides, which fill vesicles, replace olivine, and fill interstitial areas. An example of an iron oxyhydroxide halo is shown in Figure F25. In a style similar to the oxidation halos at Site U1365, iron oxyhydroxide occurs as narrow strands that propagate between individual grain boundaries, staining the background rock to form the halo. Red halos range in thickness from 0.5 to 5 mm. Halo margins are typically irregular and diffuse, with iron oxyhydroxide forming concentrated zones within the halo. Red halos are typically associated with veins of iron oxyhydroxide or iron oxyhydroxide and celadonite, but they can also flank saponite and carbonate veins.

Mixed halos are uncommon at Site U1368, but they are evenly distributed throughout the recovered core. Mixed halos are typically found flanking multimineralic veins. However, they are also found flanking veins observed to contain only carbonate and quartz. Careful observation of core samples and thin sections reveals that mixed halos are the result of overprinting by green/brown halos over dark green/black halos (Fig. F25). Mixed halos range in width from 3 to 20 mm but are usually between 5 and 10 mm wide, and the mineralogy of each individual halo within each mixed halo essentially falls into any one of the dark green/black, red, or green/brown halo categories. However, because of overprinting, typically the innermost halo contains mineralogy that relates to two or more alteration phases; therefore, the coloration is mixed. As with the simple “single alteration phase” halos, the intensity of coloration reflects the level of alteration. In mixed halos, the dark green/black celadonitic halos are partially to completely overprinted by iron-rich saponite and often have indistinct or diffuse boundaries. In most mixed halos, only discrete patches of celadonite remain. In a number of mixed halos, earlier sequences are overprinted by later alteration halos that extend well beyond the boundary of the previous halo.

Veins

A total of 308 veins were identified in the basement core recovered from Hole U1368F with an average density of 10 veins/m of recovered core. Vein fill makes up 0.16% by volume of recovered core. Vein thickness varies from <0.1 to 4 mm, although the thickness of most veins is in the 0.1–1 mm range. Vein morphology observed in basement at Site U1368 exhibits planar, straight, curved, branching, anastomosing, kinked, sinusoidal, and irregular forms. Secondary minerals that fill veins include saponite, celadonite, iron oxyhydroxides, carbonate, and accessory phases (other unidentified clays, quartz, chalcedony, zeolite, and secondary sulfides). Veins may be monomineralic or polymineralic with any combination of the major secondary minerals. Veins may be flanked by alteration halos or may simply penetrate the groundmass with no alteration halo. Crosscutting relationships and vein-filling orders, relative to each vein mineral, are described below.

Iron oxyhydroxide is by far the most abundant secondary mineral at Site U1368. By volume, it makes up 56% of the vein fill and 0.09% of the recovered rock. Iron oxyhydroxide veins range from <0.1 to 4 mm thick (0.3 mm thick on average). Although a number of veins are exclusively iron oxyhydroxide (e.g., in Fig. F26), most iron oxyhydroxide is present with saponite and celadonite. Iron oxyhydroxide is typically overprinted or crosscut by saponite and, rarely, calcite; however, it is often overprinted or intergrown with celadonite. Iron oxyhydroxide is present throughout Hole U1368E. Although iron oxyhydroxide is most abundant in Core 329-U1368F-2R, the overall trend is toward increasing iron oxyhydroxide abundance downhole.

Celadonite is the next most abundant vein-filling mineral, filling 16% of the total veins and forming 0.03% by volume of the recovered core. Celadonite-filled vein thicknesses vary from <0.1 to 2 mm. However, most celadonite veins are between <0.1 and 0.4 mm thick. Pure (100%) celadonite veins tend to be narrow (<0.1–0.3 mm thick). Most celadonite veins are either intergrown with or overprinted by iron oxyhydroxides. In addition, celadonite is overprinted by saponite and carbonate. In many veins, only discrete patches of celadonite remain. Celadonite was identified in thin section by its green color and in hand specimen by its blue-green color and brittle texture. XRD analyses of celadonite indicate intergrowths of saponite.

Saponite is present throughout the basalt of Site U1368, albeit at much lower abundance than iron oxyhydroxide and celadonite (just 0.003% by volume of the core). Saponite-bearing veins range from <0.1 to 0.8 mm thick and their thickness averages ~0.21 mm. Although saponite is observed to occur with every other secondary mineral, it is most commonly associated with iron oxyhydroxides. Saponite typically crosscuts celadonite and iron oxyhydroxide and is itself crosscut by rare carbonate and zeolite. XRD analyses of a saponite vein indicate a trioctahedral smectite structure. The brown-red color of saponite at Site U1368 indicates that it is iron rich.

Carbonate veins make up 5.5% of all veins and 0.01% of the total volume of recovered core. Carbonate is present in its own veins; crosscutting celadonite, iron oxyhydroxide, and saponite; or, more frequently, as a late-stage infill in polymineralic veins (Fig. F26). The proportion of carbonate in a given polymineralic vein ranges from trace to almost 100%. In almost all situations where overprinting/replacement relationships can be discerned, carbonate replaces all other major secondary phases. Veins bearing carbonate can be as thin as <0.1–2 mm. Most 100% carbonate veins are not flanked by halos. A number of vertical to subvertical veins with only carbonate infilling occur with no halos. These veins usually have no halos flanking them and they appear to crosscut all other subhorizontal veins, including carbonate. The majority of the carbonate veins are present near the top of Hole U1368F (Core 329-U1368F-2R).

Quartz, zeolite, and pyrite make up the remainder of the vein-filling minerals, with each comprising 3.6%, 0.27%, and 0.1% of the total vein-filling mineral percentage, respectively. Although a number of quartz-only veins are present, these minerals typically make up discrete portions of polymineralic veins. Quartz appears to crosscut celadonite and iron oxyhydroxides, making it one of the last phases to precipitate. The timing of emplacement for zeolite and pyrite was not ascertained because of a dearth of crosscutting features. Examples of these secondary minerals are shown in Figure F26.

Vesicles

All units from Hole U1368F contain vesicles, which vary in abundance from <0.1% to 10%. Most vesicles at Site U1368 are partially filled, with secondary minerals lining the inside edge of the vesicle; these vesicles are typically found in the central portion of recovered pieces. Totally filled or near–totally filled vesicles typically occur within alteration halos and chilled margins near fractures. Secondary mineral fill consists of celadonite, iron oxyhydroxides, saponite, zeolite, silicates, and calcite, in order of occurrence. On both a unit scale and a piece scale, the variability of vesicle-filling minerals is high, with the typical assemblage of each unit containing several different secondary minerals. Zeolite-filled vesicles are exclusively present near the top of the hole. Vesicles within alteration halos are usually filled with the dominant phase of that halo (e.g., iron oxyhydroxides in a red halo). However, earlier fill (lining at the edge of a vesicle) may also be present (Fig. F27). In order of filling, common mineralogical relationships within polymineralic vesicles observed at Site U1368 are

1. Celadonite, saponite, and calcite;

2. Iron oxyhydroxide, celadonite, and calcite;

3. Saponite and iron oxyhydroxide;

4. Iron oxyhydroxide and calcite;

5. Iron oxyhydroxide and saponite; and

6. Iron oxyhydroxide, zeolite, celadonite, and calcite.

The high variability of vesicle fill history and vesicle fill distribution indicates that continuous, localized fluid evolution and secondary mineral emplacement has taken place. The lack of vesicle fill in the central portions of much of the recovered core indicates relatively limited alteration extent. This limited extent of alteration is in contrast to vesicles observed at Sites U1365 and U1367, where vesicles are almost always filled. Within a number of examples, for example in interval 329-U1368F-12R-1, 2–3 cm (Fig. F27C), saponite appears to precipitate before iron oxyhydroxides.

Compositional comparison of alteration features to least altered material

A suite of 12 samples, including one whole-rock breccia and one basaltic clast, was selected for shipboard study of compositional alteration at Site U1368. The ICP-AES results are presented in Table T2. The altered samples were selected based on visual observation of secondary minerals within the groundmass, either as alteration halos or as alteration present within the groundmass. Samples were also selected adjacent to samples of least altered background; comparisons of these “pairings” are discussed in this section.

Ranges and averages of some key elements for altered samples include

• Fe₂O₃^(T) = 10.37–14.38 wt% (average = 13.65 wt%),
• MgO = 4.94–14.83 wt% (average = 6.70 wt%),
• CaO = 8.14–11.98 wt% (average = 10.49 wt%),
• K₂O = 0.33–0.76 wt% (average = 0.55 wt%),
• TiO₂ = 1.51–2.21 wt% (average = 1.99 wt%), and
• Sr = 117–376 wt% (average = 164 wt%).

Overall differences between the ranges and averages of the altered and the least altered basalt are relatively small. On average, there is an overall increase from least altered basalt to altered basalts in Fe₂O₃^(T), MnO, K₂O, and LOI. Overall, on average, decreases occur in SiO₂, Al₂O₃, MgO, Ca, Co, and Sr. These average differences between least altered and altered basalt may reflect variable replacement of groundmass by secondary minerals and scavenging of metals (including Fe, Al, and Mg) to form secondary minerals within veins. Examples of possible mechanisms include corrosion and/or replacement of titanomagnetite to supply iron oxyhydroxides and alteration of plagioclase and clinopyroxene to release SiO₂, Al, and Mg to form saponite in veins.

The decrease in Mg in the more altered samples and lack of observed change in Ca suggest that Ca/Mg exchange between seawater and wall rock has been either modest but pervasive in the recovered basalt.

For basic assessment of elemental mobility within the whole rock, 10 of the alteration halo samples were selected to have direct contact with a measured least altered background. Changes of all the sample pairs are shown as ratios of altered versus unaltered (Fig. F28). Values that are greater or less than 1 (outside of error) indicate chemical change. All altered samples exhibit increased K₂O and Fe₂O₃^(T) and most exhibit variable increases in MnO₂ and LOI compared to the least altered portion of the whole rock. Observed decreases include variably decreased Ba (despite large error), Cu, Ni, and SiO₂. The increases in Fe₂O₃^(T), MnO₂, K₂O, and LOI may reflect the incorporation of secondary minerals (saponite, celadonite, and iron oxyhydroxides) that contain Fe, Mn, K, and LOI into the groundmass.

The reduction in MnO₂ in Samples 329-U1368F-9R-1, 54–65 cm, versus 9R-1, 42–53 cm, and 11R-1, 65–67 cm, may have resulted from either Mn scavenging from primary oxides within the groundmass followed by subsequent precipitation within veins and fractures or variation in primary composition on a scale larger than that of the sample. The changes observed in the chilled margin sample (329-U1368F-7R-1, 31–36 cm) are similar to those observed within the alteration halos, which suggests that chilled margins are more susceptible to alteration effects than the interiors. The relatively altered sample pairs are plotted as concentrations versus depth in Figure F21. Vertical variation of chemical change is variable, with overall decreasing Fe₂O₃^(T) and K₂O enrichment. MgO and TiO₂ exhibit relatively similar changes. In all plots on Figure F21, breccia exhibits the greatest alteration with very high concentrations of Fe₂O₃^(T) (17.21 wt%), K₂O (3.65 wt%), TiO₂ (2.42 wt%), and LOI (10.9 wt%). These values reflect the high to intense replacement of the glassy clasts to saponite and other clay minerals. Although these high values are in contrast to the least altered samples and the basaltic clast sampled at Site U1368, the origin of the basaltic clasts and, therefore, the chemical relationship between the breccia and the rest of the basement remains uncertain.

Overall, the relative changes observed between altered samples and least altered samples are consistent with (1) incorporation of the secondary minerals saponite, celadonite, and iron oxyhydroxides and (2) partial chemical exchange with the basement. The high variability of trends associated with alteration suggests that alteration varies on a localized scale. The level of uncertainty of LOI means that only the largest changes are detected. It is possible that all rock at Site U1368, including the least altered rock, has undergone some degree of alteration; however, detailed postexpedition work will be required to fully compositionally characterize rock alteration at Site U1368.

Alteration summary

Alteration extent was recorded by visual observation from core descriptions and by NGR logging (using NGR-based potassium concentration as a proxy for alteration extent). The style of low-temperature hydrothermal alteration at Site U1368 is similar to that of Site U1365, albeit at a lower intensity (lower volumes of halos and veins), but higher than that of Site U1367. Given that basement at Sites U1368 and U1367 is poorly recovered (26.3% and 11.2%, respectively) when compared to Site U1365 (74.6%), any comparisons of overall alteration intensity must be treated with caution. Alteration intensity at Site U1368, as measured by visual observation and NGR potassium content, is greatest in the top (Core 329-U1368F-2R) and bottom (Cores 12R and 13R) cores.

Similar to alteration observed at Sites U1365 and U1367, alteration at Site U1368 can be divided into two components: (1) open circulation of seawater causing oxidative alteration and (2) restricted fluid circulation giving rise to oxygen-starved alteration (Laverne et al., 1996; Teagle et al., 1996; Teagle, Alt, Umino, Miyashita, Banerjee, Wilson, and the Expedition 309/312 Scientists, 2006). Secondary minerals iron oxyhydroxide and celadonite are typical of oxidative alteration, whereas the presence of saponite and minor secondary sulfides suggest oxygen-poor alteration. It is not yet clear how these stages are distributed within the basement. However, observations of individual veins and halos indicate that, at least on a local level, fluid flow becomes restricted as voids are filled by secondary phases. Complex alteration at Site U1368 is relatively uncommon. Halos usually represent a single alteration phase or sometimes two phases.

Polymineralic veins indicate that some areas underwent more than one phase of secondary mineral emplacement. The extent to which renewed oxidative alteration has taken place in zones with more than one phase of secondary mineral emplacement remains unclear. The occurrence of late-stage carbonate, however, indicates that reopening must have taken place. Saponite followed by iron oxyhydroxide precipitation in vesicles in interval 329-U1368F-12R-1, 2–3 cm, suggests that at least some renewed oxidation took place. In a similar situation to Site U1365, the presence in the lowermost sediment of dissolved Mg at below-deepwater concentrations and dissolved Ca at above-deepwater concentrations indicates that basalt-water interaction in the form of Mg exchange for Ca persists today (see “Biogeochemistry” in the “Site U1365” chapter [Expedition 329 Scientists, 2011b]). This exchange may continue to drive late-stage calcite precipitation. In addition, the overall lower abundance of carbonate precipitate at the younger site (0.01% calcite at Site U1368; 13.5 Ma) than that of the older site (1.05% calcite at Site U1365; 84–120 Ma) supports the hypothesis that seafloor weathering continues (intermittently or continuously) for tens of millions of years after crustal formation. Our shipboard studies of hand specimens and thin sections provide no evidence that late-stage fills are oxidative (i.e., we observed no late-stage alteration halos, celadonite, or iron oxyhydroxide). However, in similar style to Site U1365, the presence of dissolved oxygen in the lowermost sediment at below-deepwater concentrations indicates that oxidation or loss to overlying sediment along the flow path continues to take place, albeit perhaps at a very low rate (see “Biogeochemistry” in the “Site U1365” chapter (Expedition 329 Scientists, 2011b).

Structural geology

Basalt recovered from Site U1368 has been subject to a small variety of synmagmatic and postmagmatic structural changes, with structural features that include flow laminations, planar flow margins, jointing, volcaniclastic breccia, and veins. Nongeological structural features include joints induced by the coring and core-handling process. Structural features were described and entered into the Laboratory Information Management System (LIMS) database using DESClogik software (see “Lithostratigraphy, igneous petrology, alteration, and structural geology” in the “Methods” chapter [Expedition 329 Scientists, 2011a]). In addition, breccia units were described in terms of their textural features and composition.

As with Sites U1365 and U1367, only geological features were recorded in the structural log and only planar features were entered. This practice restricted the number of records to only a few measurements.

Veins

Veins constitute the most pervasive and numerous structural features observed in basement recovered at Site U1368. Structurally, veins are extensional fractures that have been filled with secondary minerals (see “Basement alteration”). Measurements were made on planar veins that were present on oriented pieces. In addition, to minimize unreliable data, veins with geometries that appeared to represent thermal contraction were excluded from measurement; these included veins with Y-shaped intersections and sinuous, steeply dipping veins that are intersected by radiating veins. This restricted useful measurements to <1% of all veins at Site U1368.

Most veins at Site U1368 represent fracture fills within the fractured pillow lava units. As such, these veins form carves around the chilled margins and quench structures. A number of narrow irregular branching veins that postdate the cooling features are present; these veins occasionally have splayed ends or kinks where the vein has propagated along weaker portions of the rock (e.g., along quench margins, glassy zones, or changes in grain size). Planar veins are present in the massive and thin flow units and are typically inclined to vertical. Horizontal veins are not common at Site U1368; however, they are the most common within the more massive units. These veins tend to be irregular or ribbonlike and crosscut by vertical veins. Most veins do not have known orientation; therefore we cannot be certain of the overall orientation trends.

Structural orientation

A lack of orientation of the recovered core means that only dip can be determined, with dip direction being defined relative to an arbitrary north (see “Lithostratigraphy, igneous petrology, alteration, and structural geology” in the “Methods” chapter [Expedition 329 Scientists, 2011a]). Shore-based analyses utilizing paleomagnetic data and wireline logging results will need to be carried out to reorient some of the veins. A summary of the dips and apparent strikes of veins and joints for basement at Site U1368 is shown in Figure F29. Most structures are vertical features.

Structural summary

Hand-specimen observations support the following sequence of structure formation in rocks from Hole U1368F:

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

2. Formation of horizontal cracks that were possibly formed at or near lava-flow boundaries from which fluid flow was focused; and

3. Development of younger vertical fractures without halos, which was possibly related to tectonic stresses caused by rotational movement of the basement within a fault block as the crust moved away from the spreading axis.

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