In Hole U1367F, basement was cored from 20 to 50.5 mbsf (0 to 35.5 meters subbasement [msb]), of which 3.39 m was recovered (11.2% recovery).
The recovered basement consists of aphyric to sparsely phyric, cryptocrystalline to microcrystalline fragments with large quench structures, a few of which contain glassy margins and a limited number of in situ pieces of more massive units with some chill margins. These lithologies were divided into two basement units based on changes in lava morphology and texture (Fig. F12). Further detail regarding the definition of igneous units may be found in “Lithostratigraphy, igneous petrology, alteration, and structural geology” in the “Methods” chapter (Expedition 329 Scientists, 2011a).
Several small fragments of basalt were recovered in the core catcher of Hole U1367B. These were not assigned an igneous unit because they are altered to the point of groundmass replacement and they were recovered from a different hole with no continuity to basement material recovered from Hole U1367F.
Fractured pillow fragments occupy 3.4 m of the recovered core (86.4% by volume) and are the most abundant lithology at Site U1367. Classification was based on the presence of curved chilled margins, glassy rinds, and quenching structures that flank cooling fractures (Fig. F13). Vesicles are rare and entirely filled with secondary minerals (see “Igneous petrology” and “Basement alteration”). Although the features used to distinguish pillow lava (glassy rinds, quench structures, and curved chill margins) are not observed in every fragment, they are present throughout Unit 1. The groundmass is composed of plagioclase, clinopyroxene ± olivine, and Fe-Ti oxide. Textures observed include subophitic, spinifex, and spherulitic. Plagioclase phenocrysts that are 0.1–0.4 mm wide are present within the groundmass; however, their rarity (<0.2% by volume) classifies the rock as aphyric. Most glass observed in Unit 1 is only very slightly altered, with some minor celadonite and iron oxyhydroxide filling vesicles. Glassy margins range from fresh to slightly altered, but overall alteration in 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 comparatively large chilled margins, which are dark and have a sharp front when viewed in hand specimen.
The thin flow occupies the lowermost 54 cm of Hole U1367F (13.6% of the core). Classification was based on the reduction of fractures and joints (leading to the only significant section of recovered in situ core), a lack of curved glassy margins and fractures, and an overall decrease in alteration within the lowermost 54 cm of Hole U1357F compared to the rest of the recovered material. Unit 2 basalt comprises plagioclase, clinopyroxene ± olivine, and Fe-Ti oxide with grain sizes that range from cryptocrystalline to microcrystalline. No phenocrysts are present (aphyric), and texturally Unit 2 is predominantly porphyritic to intergranular. However, intersertal, glomeroporphyritic, variolitic, and subophitic textures are also observed. Subvertical planar fractures occur within Unit 2 and they are filled with iron oxyhydroxides and saponite. Alteration in Unit 2 ranges from slight to moderate, and it primarily consists of brown to dark gray alteration halos around the chilled margins and veins.
As described above, the basaltic rocks recovered from Hole U1367F are divided into fractured pillow basalt and thin flows. Four samples that best represent the primary igneous textures and alteration effects were selected for petrographic analyses by thin section (see Site U1367 thin sections in “Core descriptions”).
The mineralogy of Unit 1 is typical of seafloor basalt. 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. The fragments are cryptocrystalline to glassy. Phenocryst abundance ranges from 0% to 0.2% (aphyric). Phenocrysts are composed of blocky to prismatic plagioclase and clinopyroxene crystals that typically range from 0.1 to 0.2 mm in size. The dominant textural pattern within the quench structures is spinifex to intersertal, which is defined as fine radiating laths of plagioclase. Between the plagioclase laths, the texture ranges from glassy to partially formed microcrysts of pyroxene and opaque minerals. Vesicle content is relatively low, but vesicles occur near the glassy margins and within the chilled, partially crystallized zone.
Unit 2 is a thin flow and contains plagioclase (60%–67%), clinopyroxene (26%–35%), Fe-Ti oxides (2%–5%), and rare olivine (approximately <1%). Plagioclase forms small prismatic crystals that are intergrown with anhedral to subhedral clinopyroxene and Fe-Ti oxide (titanomagnetite). The groundmass ranges from cryptocrystalline to microcrystalline, and the most common textures are intersertal to intergranular. Grain size within the flow is homogeneous throughout 90% of the recovered material, except near chilled margins, where grain size reduces to cryptocrystalline. In Unit 2, a number of curved and irregular quenching structures flank cooling fractures. Within the quench structure, textures range from spinifex to intersertal. Phenocrysts are rare (<0.1%, aphyric) and composed of lathlike 0.3 mm long plagioclase. Vesicles were not observed in thin section or hand specimen. This was the only unit in which several pieces were recovered in situ at the base of the hole.
Plagioclase
Plagioclase phenocrysts are rare throughout the basement at Site U1367. Although they comprise <0.1% of the fractured pillow lavas and the more massive basaltic unit below, plagioclase is the most abundant phenocryst phase. Plagioclase phenocrysts are euhedral to subhedral with a bladed to prismatic crystal structure. Plagioclase phenocrysts range from 0.1 to 0.4 mm in length and are typically fresh, with only minor replacement by secondary minerals located within alteration halos. Replacement minerals include clays, saponite, and iron oxyhydroxides that occur along cracks, cleavage planes, and edges of plagioclase crystals (see “Basement alteration”).
Pyroxene
Clinopyroxene phenocrysts are very rare (<<0.1% of the recovered core), range from 0.1 to 0.2 mm in length, and are commonly associated with plagioclase phenocrysts. The clinopyroxene phenocrysts are subhedral with simple basal twinning present throughout. Alteration of clinopyroxene phenocrysts ranges from 0% to 80% and documents replacement by secondary clays, saponite, iron oxyhydroxides, and oxides along cracks, cleavage planes, or crystal edges.
Groundmass
The basaltic groundmass at Site U1367 is composed of plagioclase and clinopyroxene, with minor accessory Fe-Ti oxides, whereas grain size varies from glassy to cryptocrystalline. Plagioclase is the most abundant groundmass crystalline phase, comprising between 58% and 70% of the groundmass, and it occurs as microlaths, microlites, or spinifex texture within quench structures. Clinopyroxene comprises ~35% of the fractured pillow lavas (Unit 1) and ~30% of the more massive basaltic Unit 2. It occurs as interstitial growths between plagioclase, microlaths, microlites, and aggregates of fibrous or plumose crystals. Within the quench zones, clinopyroxene crystals are either not present or partially grown between spinifex plagioclase. Olivine was not detected within the groundmass; however, the lack of fresh olivine and the difficulty in identifying olivine pseudomorphs (based on relict crystal structure) may have adversely affected our efforts to determine its presence and estimate its abundance. Mesostasis textures include hyalophitic and intersertal; they are present throughout the recovered basement.
In the thin flow unit at Site U1367 the most common texture is intergranular. Mesostasis texture is typically subject to patchy alteration and is preferentially altered relative to the plagioclase and clinopyroxene groundmass. Almost all patchy alteration observed at Site U1367 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% by volume of Unit 1) are present in all units. These form small (<0.2 mm), granular, subhedral crystals of titanomagnetite, which are partially replaced.
Hard rock geochemistry
Under the same analytical techniques and conditions outlined in “Biogeochemistry” in the “Site U1365” chapter (Expedition 329 Scientists, 2011b), five representative samples of basement were analyzed for major and trace elements by inductively coupled plasma–atomic emission spectroscopy (ICP-AES). These samples included relatively fresh basalt groundmass (gray to green) and basalt within chilled margins. Samples were selected to examine the chemistry of primary igneous features and alteration. Although rocks recovered at Site U1367 are relatively fresh, alteration is visible within the quench structures. Two samples were selected from the pale gray interiors to represent the least altered basalt. To avoid unrepresentative analyses of whole rock, all samples were selected, where possible, to avoid veins and fractures. Details of the methods for preparation and analyses are in “Lithostratigraphy, igneous petrology, alteration, and structural geology” in the “Methods” chapter (Expedition 329 Scientists, 2011a). The international standard BCR-2 was analyzed 24 times over 3 runs. The 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
Trace element ranges and averages include
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Sr = 115–167 ppm (average = 125 ppm),
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V = 427–473 ppm (average = 461 ppm), and
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Zr = 161–181 ppm (average = 166 ppm).
For the least altered basalts (three samples), total alkaline (K2O + Na2O) content ranges from 3.02 to 3.16 wt% and SiO2 content ranges from 49 to 50 wt%. Al2O3 ranges from 13.2 to 14.6 wt%, and CaO ranges from 10.1 to 11.3 wt%. Because of the profound uncertainty in the location of these samples within Hole U1367F, no downhole geochemical trends can be inferred. K2O/TiO2 at Site U1365 ranges from 0.05 to 0.16, indicating depleted basaltic compositions. The potential for emplacement of potassium-rich secondary minerals (celadonite and saponite) within portions of the least altered samples imply that true primary K2O is even lower.
Basement alteration
All basement rocks at Site U1367 have been subject to alteration by interaction with seawater. Alteration varies from slight to moderate. However, the majority of recovered basement material at Site U1367 is only very slightly altered. Basement alteration at Site U1367 consists of
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Replacement of mesostasis in the groundmass by secondary minerals,
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Filling of veins and formation of halos by emplacement of secondary minerals, and
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Lining and filling of vesicles.
Rock visibly altered in macroscopic view or thin section constitutes between 2% and ~25% of individual samples, with most alteration concentrated around veins and chilled margin contacts. Alteration products include saponite, celadonite, iron oxyhydroxides, quartz, and rare carbonate. No breccia was recovered from Site U1367.
Saponite is present throughout the core. Like the other secondary minerals reported at Site U1367, it contains similar properties to those observed in Site U1365 basement samples (see “Igneous lithostratigraphy, petrology, alteration, and structural geology” in the “Site U1365” chapter [Expedition 329 Scientists, 2011b]). Replacement of the groundmass is usually slight and discontinuous, only replacing mesostasis and a minor portion of the groundmass. In areas of moderate alteration, saponite (green/brown in hand specimen and thin section) replaces mesostasis and a varying proportion of groundmass crystals. Saponite also frequently fills vesicles and forms monomineralic or polymineralic veins. Celadonite (bright green in thin section) is present as discrete infills of vesicles and replacement of mesostasis throughout the recovered basement and it is often found in conjunction with (or overprinted by) iron oxyhydroxides. Iron oxyhydroxides (red to red-brown) in hand specimen and thin section) are present throughout the core and may typically fill or partially fill veins and commonly form iron oxyhydroxide–dominated halos. Other minor secondary minerals identified in hand specimen and thin section include quartz and rare vein calcite. A detailed analysis of secondary mineral phases at Site U1367 will require postexpedition research.
Vein- and halo-related alteration
The most visible manifestation of alteration at Site U1367 is vein- and halo-related alteration. Dark gray/brown saponitic and celadonitic background alteration occurs throughout the recovered basement at Site U1367. However, vein-related alteration is also present throughout the site as localized, narrow (~1 mm) to wide (~20 mm), brown to dark gray or mixed alteration halos that flank the veins. Alteration halos are often preserved where the vein was not recovered. Veins are typically polymineralic with the principal constituents iron oxyhydroxides, celadonite, and saponite. Quartz is also sometimes present as a minor phase in some veins. Two rare 0.1 mm thick carbonate veins are present at intervals 329-U1367F-2R-3, 24–28 cm, and 6R-1, 129–134 cm.
Halos
Dark green/black halos are present throughout Site U1367; however, they are most concentrated in Unit 2, flanking veins within the massive basaltic pieces. Dark green/gray halos occur in Unit 1 as 1–5 mm wide halos around cooling fractures and as a component in mixed halos (Fig. F13). Dark green halos are characterized by celadonite replacing mesostasis material and filling vesicles; however, secondary mineral abundance within these dark halos is low (~1%–5%). Within the dark green/black halos, iron oxyhydroxide may be present in the form of discontinuous bands or discrete alteration patches. Vein- and vesicle-filling sequences (discussed below) indicate that the saponite phase arrived after celadonite. Within the lowermost section of Unit 2 (interval 329-U1367F-6R-1, 114–140 cm), dark gray halos produce unusual mottled fronts that propagate within the rock.
Brown halos occur throughout the recovered core at Site U1367. Unit 1 contains thin (1–2 mm) brown halos and occasional more pervasive alteration halos that extend beyond the quenching structures. Brown halos also form a component of the mixed halos, frequently overprinting the dark green/black alteration. Brown halos flank iron oxyhydroxide, saponite, celadonite, and polymineralic veins (e.g., Sample 329-U1367F-4R-1, 51–53 cm; Fig. F14). Thin section observations of the brown halos indicate that the dominant secondary mineral filling vesicles and replacing interstitial and glassy material is saponite. Replacement of the groundmass varies from <0.5% to 10%. Frequently present within these halos are iron oxyhydroxides, which stain the saponite orange-brown. Iron oxyhydroxide covers between 0.5% and 2% of the rock within the brown halos.
Red halos are present in discrete areas near chilled margins and within mixed halos. These halos are dominated by iron oxyhydroxides that replace interstitial zones, fill vesicles, and partially alter the groundmass. Typically, red halos are discontinuous and patchy with diffuse halo fronts. Red halos are 1 to 3 mm thick and occur flanking celadonite, saponite, and iron oxyhydroxide veins. Iron oxyhydroxide tends to overprint celadonitic (dark green/black halos).
Mixed halos occur near cooling fractures and veins and are the most abundant halo type at Site U1367. Mixed halos are the result of multiple overprinting stages from dark green/black halos, red halos, and green-brown halos. Typically, two halo zones co-occur. However, several samples exhibit the complete sequence of halos. Although halos range in thickness from 2 to 20 mm, most are 2 to 5 mm thick. 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. In a number of mixed halos, earlier alteration halos are partially overprinted by later alteration halos that extend well beyond the boundary of the previous halo.
Veins
A total of 66 veins were identified in the basement cores from Hole U1367F, with an average density of 20 veins/m of recovered core (Table T3). Vein thickness varies from <0.1 to 1 mm, although most veins are in the 0.1–0.2 mm range. Veins observed in basement at Site U1367 exhibit curved, planar, and irregular morphologies. Secondary minerals that fill veins include saponite, celadonite, iron oxyhydroxides, sulfides, and rare carbonate. Silicates and zeolite are also present in discrete quantities. A lack of distinct crosscutting veins and indistinct vein relationships in basement recovered at Site U1367 means that order of emplacement remains speculative. Veins may be monomineralic or polymineralic, with any combination of the major secondary minerals. Veins at Site U1367 are usually flanked by mixed alteration halos that propagate outward from the vein. A summary table of all veins is provided in (Table T3).
Saponite is present in nearly all of the veins (31% of all vein material) and makes up 0.06% by volume of the recovered basement. Although saponite is most commonly associated with iron oxyhydroxides, its relationship to this mineral phase remains indistinct. Iron oxyhydroxide makes up 27.3% of the veins and 0.06% by volume of the recovered rock. Although iron oxyhydroxide commonly occurs with celadonite and saponite, many veins are exclusively composed of iron oxyhydroxide. Celadonite makes up 9% by volume of all vein material (0.03% of the recovered rock). Celadonite occurs intergrown with iron oxyhydroxide (e.g., Sample 329-U1367F-4R-1, 51–53 cm; Fig. F14) and saponite or on its own in narrow (0.1 mm) veins. In a similar fashion to the overprinting of celadonite in halos discussed earlier celadonite is overprinted by iron oxyhydroxides and saponite. Secondary silicates occur in 14% of all veins and 0.03% of the total recovered basement rock. Although silicates occur with all other secondary phases at Site U1367, the lack of clear evidence for overprinting and crosscutting features implies that the timing of silicate emplacement remains unknown. Secondary sulfide veins are typically monomineralic, planar, and narrow (0.1 mm wide). However, one example also contains celadonite (Sample 329-U1367F-6R-1, 50–51 cm). Sulfides make up 0.01% of the core and 3% of recovered vein material. Carbonate at Site U1367 is rare, since it is observed in only two veins (intervals 329-U1367F-2R-3, 19 cm, and 6R-1, 52 cm).
Only one vein in Unit 2 contained enough material for XRD analysis (see Sample 329-U1367F-6R-1, 88–102 cm, diffractogram in Fig. F15). The major peaks are indicative of an iron-rich mineral, possibly glauconite, celadonite, or nontronite. Slight contamination of plagioclase also appears to give rise to a peak indicative of albite or anorthite. The counts per acquisition on the XRD instrument for this sample are very low, with a maximum peak intensity of 280. Therefore, any conclusions drawn from this analysis must be treated with caution.
Vesicles
Vesicles are rare in basement rocks recovered at Site U1367. Saponite and iron oxyhydroxide vesicles in halos and near chilled margins make up <0.01% of the recovered rock. Because vesicles were not observed in thin section, their detailed alteration history remains open to further study.
Alteration geochemistry
Of the five samples selected for basement geochemical analyses, one is from relatively altered basalt (Sample 329-U1367F-4R-1, 21–31 cm; altered basaltic chips from the sediment/basement interface) and one is sourced from a chill margin (Sample 4R-1, 51–53 cm). A simple comparison between the average of the least altered background samples and the slightly more altered samples (Fig. F16) indicates increases in Fe2O3(T), MnO, K2O, Ba, TiO2, and LOI and decreases in Al2O3, Na2O, P2O5, Cu, and Sr. Despite the large apparent changes in iron content and LOI, overall change is relatively small. The increases in Fe2O3(T), MnO, K2O, and LOI may reflect the incorporation of secondary minerals (saponite, celadonite, and iron oxyhydroxides) that contain Fe, K, and LOI into the groundmass. Increased TiO2 may be the result of incorporation of secondary Fe-Ti oxides, such as titanomagnetite. Although the chilled-margin sample (Sample 329-U1367F-4R-1, 51–53 cm) is generally similar to the other least altered samples (Fig. F16), it exhibits slightly elevated Fe2O3(T), MnO, and K2O. These changes indicate slight alteration along the margins, which is supported by the presence of brown alteration halos. Despite the location of Sample 329-U1367F-4R-1, 21–31 cm, at the sediment/basement interface, Ca and Mg show very little variation, implying only minimal Ca/Mg exchange with seawater. The lack of a relatively complete section, however, means that true alteration extent and Ca/Mg exchange at Site U1367 may be much greater. Detailed postexpedition work, including future drilling, would be required to fully compositionally characterize rock alteration at Site U1367.
Alteration summary
The style of low-temperature hydrothermal alteration at Site U1367 is similar to the slight alteration observed in the uppermost portion of the oceanic basement at other areas where in situ ocean crust is recovered (e.g., Ocean Drilling Program (ODP) Holes 504B and 1256D and the nearest sites to Site U1365 [DSDP Sites 595/596]) (Shipboard Scientific Party, 1987; Laverne et al., 1996; Teagle et al., 1996; Teagle, Alt, Umino, Miyashita, Banerjee, Wilson, and the Expedition 309/312 Scientists, 2006).
Alteration extent was recorded by visual observation from core descriptions and by natural gamma radiation (NGR) core logging (using NGR-based potassium concentration as a proxy for alteration extent). The visual record and the NGR-based potassium show strong correspondence. NGR measurements indicate greater alteration at the top and bottom portions of the hole, with limited alteration in the rest of the recovered material (see “Physical properties” for a detailed description of NGR).
Secondary minerals celadonite, saponite, iron oxyhydroxide, and pyrite suggest evolution from open circulation (celadonite and iron oxyhydroxides) to a more restricted environment (saponite and pyrite). Because few vein mineral, vesicle, and halo relationships are present in the recovered rock, the timing of secondary mineral emplacement and alteration regimes is not fully known. Celadonite is replaced by iron oxyhydroxides, and saponite halos may overprint iron oxyhydroxide and celadonite. More detailed analyses of the alteration affects at Site U1367 will be required to determine alteration history.
Low recovery of only small basaltic fragments means that the distribution of alteration cannot be placed precisely. Only Cores 329-U1367F-6R and 2R can be placed in situ because of their large oriented core samples (Core 6R) and position at the top of the hole (2R). Basaltic fragments recovered from Cores 2R and 6R are the most altered, as determined by NGR measurement.
The lack of dissolved Mg in the lowermost sediment and a dearth of carbonate veins may indicate that Ca/Mg exchange is limited (see “Biogeochemistry” in the “Site U1365” chapter [Expedition 329 Scientists, 2011b]). However the poor recovery of basement Site U1367 suggests that the recovered section may not be representative of the true extent of alteration. As such, interpretation of the alteration regime at Site U1367 remains speculative.
Structural geology
Structural features at Site U1367 are mainly composed of synmagmatic to postmagmatic structural features that include curved or irregular fractures, chilled margins, and pillow structure textures (radial fracturing, curved glassy margins, and vesicles). Cooling fracture fills include saponite, celadonite, and iron oxyhydroxides. The majority of these structures are preserved in basaltic fragments that fell from the sides of the hole during drilling; as such, they are not oriented, not in situ, and therefore unsuitable for measuring. Only three intervals of the entire recovered material at Site U1367 were in situ, two of which contained steeply dipping planar joint fractures filled with vein material (saponite, celadonite, and iron oxyhydroxide). One of these intervals exhibited a conjugate vein pair. These measured structural features were described and entered into the IODP Laboratory Information Management System (LIMS) database with DESClogik software application (see “Lithostratigraphy, igneous petrology, alteration, and structural geology” in the “Methods” chapter [Expedition 329 Scientists, 2011a]). Sample 329-U1367F-6R-1, 60–71 cm, contains a conjugate saponite and iron oxyhydroxide vein with a measured dip and dip direction of 40°/004° and 35°/180° for each vein in the conjugate pair (measured as 60°/184° and 65°/000° when reoriented to true vertical position).
Most of the observed structure at Site U1367 appears to result from cooling and subsequent fracturing by ingress of seawater and precipitation of secondary minerals. The curved and/or irregular fractures are indicative of fractured or brecciated pillow lava. As such, they represent a marked difference from the horizontal flow fractures and veins observed at Site U1365. The vertical fractures within Unit 2 suggest that later tectonic fracturing took place.
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