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doi:10.2204/iodp.proc.344.104.2013 Lithostratigraphy and petrologySite U1414 was drilled to investigate the lithostratigraphy and pore water of the sedimentary sequence on top of the oceanic basement and in the uppermost portions of the underlying igneous basement. In Hole U1414A, 375.25 m of sediment and 96.35 m of oceanic basement were recovered. The cored interval was divided into three major sedimentary units with four subunits and eight igneous basement units (Fig. F3; Table T2). Overall core recovery was 86%: 96% for APC coring, 86% for XPC coring, and 34% for RCB sediment coring. The top of Hole U1414A is characterized by a predominantly monotonous sequence of soft, light greenish gray hemipelagic silty clay to clay. The uppermost part of Unit I (Subunit IA) contains several thin sand layers, whereas calcareous nannofossils gradually increase in the lower part (Subunit IB). Unit I contains terrigenous material (lithic fragments, glass shards, and minerals) that decreases with depth. Biogenic material such as foraminifers and radiolarians is abundant. Tephra layers make up ~1% of Unit I and are distributed into 16 well-sorted, discrete tephra horizons. The Unit I/II boundary at 145.34 mbsf (Section 344-U1414A-17H-1, 94 cm) is marked by a change from greenish gray nannofossil-rich clay sediment to brownish to whitish nannofossil-rich calcareous ooze. Unit II (145.34 mbsf; Sections 344-U1414A-17H-1, 94 cm, to 35X-CC, 29 cm) is a 164.03 m thick, moderately consolidated, white to dark grayish to yellowish brown clayey to silty interval that is divided into two subunits. Unit II is generally composed of nannofossil-rich calcareous ooze and variable amounts of sponge spicules, foraminifers, and diatoms. Subunit IIA (145.34–200.01 mbsf; Sections 344-U1414A-17H-1, 94 cm, to 22H-CC, 30 cm) is dominated by calcareous nannofossil ooze, whereas Subunit IIB is characterized by meter-scale alternating calcareous nannofossil ooze and biosilica-rich calcareous ooze. Tephra layers comprise ~1% of the unit and are found throughout Unit II. Ten dark brown to gray and sometimes light gray tephra horizons are preserved as discrete layers or individual and/or layered ash pods. Unit III is a 65.88 m thick interval between 309.37 and 375.25 mbsf (Sections 344-U1414A-35X-CC, 29 cm, to 45R-1, 65 cm). It is a sequence of lithified, calcareous, and siliceous cemented silt- and sandstone with well-preserved original sedimentary structures such as bedding and bioturbation. The biogenic components in this unit have been lost, most likely because of diagenetic remobilization. Tephra layers and pods comprise <1% of this interval, and nine dark brown layers are recorded here. Based on preliminary descriptions of igneous basement at Site U1414, one thin flow unit (4) and six massive flow units (1–3, 5, 6, and 8) separated by intercalated sediment (Unit 7) were identified. Subunits were defined based on changes in grain size, vesicle abundance, textural relationships, and phenocryst variation. Unit distribution is summarized in Figures F3 and F4, with greater detail on definitions in “Lithostratigraphy and petrology” in the “Methods” chapter (Harris et al., 2013b). Detailed textural description, mineralogy, and their relationships throughout basement were not possible on the ship because of time constraints that prevented analyses such as thin sections, inductively coupled plasma spectroscopy analyses, and X-ray diffraction (XRD). However, thin sections were made and analyzed following the expedition, and the results are incorporated into this report. Description of unitsCores recovered from Hole U1414A are divided into three sedimentary and eight basement lithostratigraphic units (Fig. F3; Table T2). These units comprise the 375.25 m cover sequence above igneous basement and 96.35 m of basalt. Unit I
Unit I is massive dark greenish gray soft (silty) clay sediment with variable proportions of clay, silt, and sand and variable concentrations of calcareous nannofossils. Unit I is divided into two subunits based on compositional changes. Subunit IA contains centimeter-sized, normally graded sand layers and a relatively high component of terrigenous material, mainly lithic fragments, quartz, and feldspar (Fig. F5). Subunit IB contains abundant calcareous nannofossils within the silty clay, whereas terrigenous components such as quartz and feldspar are much diminished relative to Subunit IA (Fig. F6). The 16 disseminated tephra layers in Unit I range between 1 and 71 cm in thickness (Fig. F7). Subunit IA (0.00–78.3 mbsf)In addition to greenish gray silty clay, Subunit IA contains centimeter-sized, normally graded sand layers (Fig. F5). The sandstone layers contain abundant foraminifers that are very poorly consolidated. Smear slides, together with XRD analyses, indicate a large quantity of terrigenous material dominated by lithic fragments, quartz, and feldspar but also common to abundant amphibole, pyroxene, chlorite, glauconite, and chert. Biogenic components such as radiolarians, sponge spicules, foraminifers, and diatoms are common. Four mostly gray to dark gray tephra layers, ranging in thickness from 1 to 68 cm, were found in Subunit IA, and they appear to become more common in the sediment just above the boundary with Subunit IB. Subunit IB (78.3–145.34 mbsf)Subunit IB consists of light greenish gray calcareous nannofossil-rich silty clay (Fig. F6). Smear slides and XRD analyses show that abundances of lithic fragments, quartz, and feldspar decrease with depth. Components such as chert, chlorite, pyroxene, amphibole, opaque minerals, calcite, glauconite, and glass are also much reduced compared to Subunit IA. In contrast, biogenic material and fragments such as radiolarians, foraminifers, and sponge spicules become more dominant in the matrix. Twelve well-sorted, light gray to gray tephra layers ranging in thickness from 1 to 10 cm are present in normally graded layers. Unit II
Unit II is distinguished from Unit I by its abundant biogenic content, also reflected in the change from dark greenish gray to light greenish gray or white and yellowish brown sediment color. The sediment contains >70% biogenic components, such as spicules, diatoms, radiolarians, and nannofossils, but calcareous components decrease with depth. The abundance of clay, siliceous biogenic material, and silica cement (opal) partly increases with depth, but from Core 344-U1414A-31R downward, only diagenetic silica is observed in the smear slides. Mineral components that occur in trace amounts include feldspar, pyroxene, calcite, and opaque minerals. Ten tephra horizons, ranging from 1 to 49 cm in thickness, are soft to moderately lithified, show normal gradation from medium sand to silt, and occur as discrete layers, individual ash pods, or ash pod layers. The glass shards are mainly transparent, and preliminary observations suggest they are felsic in origin and contain heavy mineral assemblages dominated by amphibole and biotite. Subunit IIA (145.34–200.01 mbsf)Subunit IIA consists of strongly bioturbated, massive, light green calcareous ooze (Fig. F8). Abundant pyrite is disseminated throughout the sediment, and the most common detrital grains are quartz and feldspar. Common discoloration indicative of glauconite and sometimes glauconite agglomerated grains can also be found throughout Subunit IIA. XRD analyses of Subunit IIA indicate this subunit contains the last significant occurrences of smectite and zeolite, specifically heulandite and clinoptilolite, before they become absent in the underlying sedimentary sequences. Five tephra layers are distributed throughout the sedimentary interval. Subunit IIB (200.01–309.37 mbsf)Subunit IIB consists mainly of dark grayish greenish to yellowish brown, soft to hardened silty clayey calcareous ooze alternating with calcareous ooze containing abundant sponge spicules (Fig. F9). The amount of foraminifers is variable in both types of ooze. Detrital grains (quartz and plagioclase), sponge spicules, radiolarians, diatoms, and zeolite and smectite gradually fade out. Although the biogenic material (radiolarians and nannofossils) shows evidence of recrystallization during the uppermost tens of meters of Subunit IIB, diagenetic opal increases remarkably. Five mostly dark gray to reddish gray tephra layers, ranging in thickness from 1 to 49 cm, were found in Subunit IIB. Unit III
The boundary between Units II and III is marked by a noticeable increase in the lithification state of the sediment, a change to a dark reddish brown color, and the occurrence of preserved sedimentary structures, such as bedding and bioturbation, and deformational features like foliation and faults (Figs. F10, F11). The matrix of the strongly lithified calcareous and siliceous (chert?) silt- to sandstone is replaced by cement, and the sediment is often crosscut by calcite veins. Bedding and foliation planes are also filled by calcite, although foliation is evident from the flow of matrix around (siliceous) clasts. Biogenic material is absent. The main mineralogic components appear to be the recrystallized sedimentary components of former silt and sandstones. Nine tephra horizons, ranging from 2 to 11 cm in thickness, are mostly lithified, show normal gradation from medium sand to silt, and occur as discrete layers, individual ash pods, or ash pod layers. Next to some tephras dominated by brownish glass shards, the glass shards in the majority of tephras are mainly transparent with a high amount of pyrite between the shards and growing on the glass shard surfaces, making the entire tephra layer dark gray to black. In Section 344-U1414A-42R-CC, a sharp inclined contact to completely lithified limestone breccia made out of centimeter-sized clasts is nicely preserved (Fig. F12). Apart from dolomite grains, rare pyrite, and other chert-like components, smear slide observations and XRD analyses do not show evidence of other detrital or biogenic components. Tephra layersEvaluating whether volcanism shut down because of the arrival of the Cocos Ridge and how volcanism evolved in Central America was a major goal at Site U1414; thus, recovering the tephra records between the mid- to late Pleistocene and mid-Miocene that were missing at Site U1381 (see “Paleontology and biostratigraphy” in the “Input Site U1381” chapter [Harris et al., 2013a]) was important. Onboard biostratigraphic results (see “Paleontology and biostratigraphy”) indicate that sediment cored within this time interval was recovered, along with ~20 felsic tephra layers. These tephra layers will give new insights into Central American volcanism during this time period. A total of 35 intercalated tephra layers are recognized throughout Hole U1414A. Unit I contains 16 well-defined tephras, Unit II has 10 tephra layers, and Unit III contains 9 tephras. Individual tephra layers range in thickness from 1 to 71 cm. Unconformable and/or inclined bedding is rare. Most tephra layers are well sorted and have a sharp basal contact to underlying sediment but a gradual transition with overlying ash-bearing sediment. Below Subunit IIA, tephra layer horizons become more disseminated and smeared because of drilling disturbance. Normal grading is commonly observed in both the tephra horizons and the larger ash pods. Some tephra layers are well lithified (Figs. F13, F14). In some cores, localized bioturbation is observed at tephra layer boundaries. Compositions of the 36 identified tephra layers are variable and show mainly gray to pinkish and some gray to brown-black colors. Nearly all tephra layers are dominated by transparent glass shards that are fresh with few signs of alteration and by pumiceous clasts. Devitrification structures within glass shards increase with depth, reflecting differing levels of alteration. In Unit III, pyrite can be identified as an alteration product growing on and between the transparent glass shards. Grain size ranges from very fine to coarse ash (up to several millimeters in size). Mineral assemblages consist of plagioclase, pyroxene, hornblende, and biotite. Plagioclase is the dominant phenocryst phase, but some tephras are dominated by amphibole and biotite. X-ray diffraction analysisPreliminary XRD analysis of sediment samples from Hole U1414A suggests that there are gradational variations from one lithostratigraphic unit to another (Figs. F14, F15). X-ray diffractograms of Unit I indicate that the major mineral components are phyllosilicates, including chlorite and smectite, plagioclase, and quartz. Calcite, zeolite (heulandite and laumontite), amphibole (hornblende), and pyrite peaks are also present. Relative peak intensities of silicates decrease throughout Subunit IB, whereas those of calcite increase. Glauconite is present in some samples in Subunit IB, mainly in the lower part of the unit. Samples from Unit II produce spectra dominated by calcite. Chlorite and amphibole are not detected in the unit. Relative peak intensities of quartz, plagioclase, zeolite, and smectite decrease throughout Subunit IIA, but these minerals are absent in Subunit IIB. In the lower part of Subunit IIB, calcite is the only significant mineral and is associated with silica (most likely opal) and minor pyrite. Zeolite (clinoptilolite type) has been identified, sometimes in large amounts, in few samples. X-ray diffractograms of Unit III indicate the major mineral component is dolomite. It is associated with calcite, silica (most likely opal), and zeolite (clinoptilolite type). Igneous basementIn Hole U1414A, igneous basement was recovered from 375.25 to 471.6 mbsf (0 to 96.4 meters subbasement [msb]), of which 61.6 m was recovered (66%). Recovered basement broadly comprises aphyric to highly phyric massive basaltic flows and thin flows that are divided into seven units, with one additional unit of intercalated calcareous sandstone. Igneous lithologic units were defined based on changes in lava morphology, flow boundaries, texture, and phenocryst occurrence. The distribution of lithologic units is summarized in Figure F4. Greater detail regarding how units are defined may be found in “Lithostratigraphy and petrology” in the “Methods” chapter (Harris et al., 2013b). LithostratigraphyMassive and thin sheet flow units (Units 1–6 and 8) were divided based on changes in phenocryst abundance, the presence of olivine, grain size, vesicle abundance, and the presence of chilled margins. Vesicle infill is described in “Basement alteration.” Unfortunately, actual contacts between flows were not recovered. Within a given unit, the lack of chilled margins or structures associated with rapid cooling or separate flows (sharp textural changes), together with textural similarities above and below unrecovered intervals, was interpreted as indicating a continuous individual flow. Unit 1, from Section 344-U1414A-45R-1, 65 cm (375.25 mbsf), to 46R-2, 124 cm (378.74 mbsf), is a 3.5 m thick massive basaltic lava flow that ranges from sparsely phyric to moderately phyric with a groundmass that consists of plagioclase, clinopyroxene, Fe-Ti oxides, and ~3% olivine. Textures observed include intersertal and intergranular. Grain size ranges from cryptocrystalline to microcrystalline, and cryptocrystalline groundmass is restricted to mesostasis between plagioclase and clinopyroxene grains (intersertal). Although intersertal texture occurs throughout Unit 1, it is most prevalent near the top and bottom. Vesicles occur discretely throughout Unit 1 but are least abundant in the central portion (Section 344-U1414A-45R-2). Phenocrysts (in increasing order of abundance) include rare olivine, clinopyroxene, and plagioclase and range in size from 0.1 to 0.7 mm. Phenocrysts are most abundant in the uppermost 40 cm (10% by volume) and lowermost 30 cm (1.5% by volume) of Unit 1. Subunits 1a and 1b were defined by the relative paucity of phenocrysts (~0.1%–2% abundance) below 375.5 mbsf. Chilled margins within a rubble interval at 378.74 mbsf define the bottom of Unit 1. Unit 2, from Section 344-U1414A-46R-2, 125 cm (379.04 mbsf), to 49R-2, 54 cm (395.94 mbsf), is a 16.9 m thick massive basaltic lava flow. Groundmass composition is similar to Unit 1, with phenocryst abundances that range from aphyric to porphyritic. Texture throughout Unit 2 is typically massive and intersertal but also includes vesicular, subophitic, and intergranular. Variolitic and glassy textures were observed in the uppermost 5 cm of Unit 2. Vesicle abundance varies from absent to 30% vesicles within interval 344-U1414A-47R-1, 13–97 cm, and is highest in the upper 8 m of Unit 2. Phenocrysts include olivine, clinopyroxene, and plagioclase and range in size from 0.1 to 3 mm. Phenocryst abundance broadly increases from 1% at the top of Unit 2 to 55% within Section 344-U1414A-48R-4 but then sharply decreases to ~2% overall abundance at the bottom of Unit 2. Subunits 2a–2c were broadly defined based on the change from sparsely phyric (Subunit 2a), aphyric to sparsely phyric and vesicular (Subunit 2b), to fine-grained highly phyric to porphyritic and sparsely vesicular (Subunit 2c). Upper and lower contacts were not recovered; however, a chilled margin observed at the bottom of Subunit 2c is inferred to represent the bottom of Unit 2. Unit 3, from Section 344-U1414A-49R-2, 54 cm (395.94 mbsf), to 51R-2, 114 cm (405.9 mbsf), is a 9.69 m thick massive flow that exhibits intersertal, hypocrystalline, and seriate textures. In addition, intergranular texture was observed in thin section. Phenocryst abundance ranges from sparsely phyric to aphyric and consists of clinopyroxene and plagioclase. Overall phenocryst abundance decreases toward the middle of Unit 3 (Section 344-U1414A-50R-2; 400.9 mbsf). A slight increase in plagioclase phenocryst size and abundance is observed toward the bottom of Unit 3. Olivine is absent from Unit 3. Vesicles are rare (0.2% by volume of core) throughout and absent in the central portion of Unit 3 (396.71–401.91 mbsf). The upper and lower contacts of Unit 3 were not recovered. Unit 4 (Sections 344-U1414A-51R-3, 0 cm, to 51R-1, 91 cm; 405.9–409.41 mbsf) is a 3.51 m thick “thin” flow that exhibits porphyritc texture throughout. Unit 4 is microcrystalline to fine grained with a groundmass that comprises plagioclase, clinopyroxene, and Fe-Ti oxides. Intersertal, seriate, and hypocrystalline textures are observed in thin section. Phenocrysts make up ~30% of the groundmass and consist of plagioclase (2.5 mm in size; 23% of groundmass) and clinopyroxene (0.8 mm in size; 7% of groundmass). Olivine is absent from both the phenocrysts and groundmass. Vesicle abundance ranges from 1% in the top of Unit 4 (Section 344-U1414A-51R-3) to 0.1% in the lowermost 1 m of Unit 4. No contacts were recovered. Unit 5 (Sections 344-U1414A-52R-1, 91 cm, to 54R-5, 83 cm; 409.41–423.87 mbsf) is defined as a 14.46 m thick massive basaltic lava flow. Groundmass grain size is microcrystalline and comprises plagioclase, clinopyroxene, Fe-Ti oxides and ~3% olivine. Groundmass textures observed in thin section include intersertal, seriate, subophitic, and intergranular. Phenocryst abundance ranges from 0.6% to 1.3% (sparsely phyric) and phenocrysts comprise olivine (in Core 344-U1414A-54R), clinopyroxene, and plagioclase. Phenocryst size and abundance gradually increase throughout Unit 3. An isolated piece in interval 344-U1414A-54R-1, 0–11 cm, comprises 30% phenocrysts and exhibits textures similar to Unit 4; thus, it has been interpreted as a dropstone from the above unit. Vesicles are rare in Unit 5, and overall vesicle abundance decreases from 0.1% to 0.05% from top to bottom, respectively. The upper and lower unit contacts were not recovered. Unit 6 (Sections 344-U1414A-55R-1, 0 cm, to 58R-1, 10 cm; 423.87–437.7 mbsf) is a 13.83 m thick massive basaltic lava flow composed of plagioclase, clinopyroxene, and Fe-Ti oxides arranged in a groundmass of variable textures. Textures observed in hand specimen and thin section include intersertal, intergranular, and seriate in Section 344-U1414A-55R-1 to porphyritic intergranular in the center of Unit 6 (424.42–429.91 mbsf) and seriate holocrystalline massive at the bottom of Unit 6 (429.91–437.69 mbsf). Phenocrysts include clinopyroxene and plagioclase (with plagioclase most abundant), and their abundance varies from 1.6% at the top of Unit 6 (423.87 mbsf) to 25% between 427.9 and 429.91 mbsf. Between 429 and 437.7 mbsf, phenocryst abundance decreases to 0.5%. Olivine is absent in both phenocrysts and groundmass. Vesicles at the top of Unit 6 (Section 344-U1414A-55R-1) make up 40% by volume of recovered material; however, their abundance rapidly decreases below 424.29 mbsf to 2% and then gradually falls to 0.5% at the bottom of Unit 6. Unit contacts were not recovered; however, a chilled margin observed at 437.6 mbsf (interval 344-U1414A-58R-1, 0–2 cm) is inferred to represent the base of the lava flow. Unit 7 (Sections 344-U1414A-58R-1, 10 cm, to 58R-2, 33 cm; 437.7–439.37 mbsf) is a partially recrystallized intercalated calcareous silty sandstone with 5%–10% baked benthic foraminifers. Layered schlieren of greenish sandstone and reddish calcareous siltstone and rare clay horizons are observed. Unit 7 is partially to strongly foliated with a preferred orientation observed within the groundmass. Normal faults, present at the bottom of Section 344-U1414A-58R-1, are filled with carbonate veins. The lowermost 10 cm of Unit 7 is silicic in composition and exhibits relatively strong recrystallization compared to the upper and middle portion of Unit 7. No unit contacts were recovered. Unit 8 (Sections 344-U1414A-58R-2, 32 cm, to 63R-4, 29 cm; 439.37–466.24 mbsf) is at least 26.87 m thick and is defined as a massive basaltic lava flow. Texturally, Unit 8 ranges from massive, spherulitic in the uppermost 8 cm to intersertal, seriate, subophitic, and intergranular in the remainder of Unit 8. Grain size in the uppermost 17 cm is cryptocrystalline but then grades to microcrystalline (439.54 mbsf) and fine grained by 461.9 mbsf. Clinopyroxene and plagioclase phenocryst abundance is variable (0%–8%). Two intervals (439.45–451.42 and 457.06–465.95 mbsf) within Unit 8 exhibit increased phenocryst abundances of 1%–3% and 3%–8%, respectively. Olivine is observed as a phenocryst phase in Section 344-U1414A-62R-1, 6 cm (457.03 mbsf). Large “megacrysts” of plagioclase as large as 20 mm are observed between 457 and 464.75 mbsf. Within Unit 8, vesicle abundance varies from 0% to 15%. Vesicle abundance tends to be higher in intervals that exhibit relatively low phenocryst abundance. A chilled margin with devitrified glass observed at the top of Unit 8 is inferred to represent the flow top, and immediately below the glassy margin is 47 cm of in situ basalt-clastic breccia. No other contact was observed below the breccia. Unit 8 is most likely thicker because drilling stopped at 466.24 mbsf because of time constraints. PetrologyAll basement units at Site U1414 were divided into massive lava flows, one thin flow, and one intercalated sediment. A total of 19 thin sections were selected for petrographic analysis. The thin section images are available under Images in the Laboratory Information Management System (LIMS) database (web.iodp.tamu.edu/UWQ/). The mineralogy of the thin flow and massive flow units at Site U1414 (Units 1–6 and 8) is typical for seafloor basalt. Units 1, 2, 4, 5, and 8 contain olivine within their groundmass and phenocrysts. However, because the majority of olivine is completely replaced by secondary minerals (saponite/smectite), identification often relied on the identification of six-sided crystal outlines. For each igneous unit, the variation in texture, principal igneous textures, and distribution of mineralogy, where recovered, are interpreted to relate to the variation in texture associated with cooling rates and the relative position within a cross section of an individual lava flow. An example could be the central portion of a lava flow, where relatively slow cooling rates permit the formation of larger crystals (coarser groundmass) than crystals formed near the flow boundary that cool rapidly. Figure F16 demonstrates the relationship between the abundance of phenocrysts and vesicles with units in Hole U1414A. Basalt within each flow unit may be broadly termed flow top, flow center, and flow base, an example of which is shown in Figure F17. Recovered basalt inferred to be near or at the flow top of each unit in Hole U1414A typically exhibits the following features:
The flow centers for each unit in Hole U1414A are characterized by the following features:
Basalt recovered from intervals inferred as being near or in the lowermost portion of the flow typically exhibit the following features:
Groundmass summaryBasaltic groundmass at Site U1414 varies from hypocrystalline to holocrystalline and is composed primarily of plagioclase and clinopyroxene with minor accessory Fe-Ti oxides and olivine. Plagioclase typically occurs as microlaths, microlites, and acicular crystals. The range of groundmass composition for each igneous unit is given in Table T3. However, because composition ranges are based on a limited number of thin sections, further detailed study is required to resolve the compositional variation of Site U1414A basement. Plagioclase is the most common crystalline phase, comprising between 44% and 63% of the groundmass. Clinopyroxene comprises 14% and 31% of the groundmass and occurs as interstitial growths between plagioclase. Microlaths, microlites, and aggragates of clinopyroxene are common. Anhedral to subhedral microcrysts of olivine that range from partially altered to completely replaced are occasionally present. We estimate olivine to range from 0.1% to 8.6% in abundance. However, these abundance estimates should be treated with caution because of the uncertainty introduced by the extensive replacement of olivine. Mesostasis textures include hyalophitic and intersertal and are present throughout the basement. Spherulitic, variolitic, and glassy textures are rare but present. Almost all mesostasis is typically subject to alteration, resulting in a patchy texture when observed in thin section and hand specimen. Secondary minerals within the groundmass include saponite, smectite, pyrite, rare carbonate, and, in Sections 344-U1414A-48R-4 and 49R-1, rare chlorite. Primary magmatic opaque abundance ranges from 4.2% to 36.2%. Vesicles are typically filled with secondary assemblages (see “Basement alteration”). Phenocryst summaryPhenocrysts at Site U1414 consist of, in order of increasing abundance, olivine, clinopyroxene, and plagioclase (Fig. F18). The majority of phenocrysts range in size from 0.1 to 3 mm; however, very large megacrysts of plagioclase as long as 20 mm are present in Unit 8. Plagioclase typically exhibits complex zoning and twinning patterns and may include multiple small inclusions (tecoblasts) within the cleavage planes. Plagioclase can be skeletal within cryptocrystalline and glassy intervals. Minor alteration within cracks and along phenocryst edges is common and ranges from 0% to 15%. Replacement minerals include saponite and smectite. Clinopyroxene, present throughout Hole U1414A, is typically subhedral and smaller than plagioclase, and it typically ranges from 0.1 to 2.5 mm in size. However, between Sections 344-U1414A-48R-4 and 49R-1, larger 5–6 mm clinopyroxene phenocrysts are observed. Crystals are typically anhedral to subhedral, with simple basal twining present throughout. Clinopyroxene is typically intergrown with plagioclase and often occurs as subophitic crystals around plagioclase. Alteration of clinopyroxene phenocrysts varies from 0% to 40%. Replacement minerals include saponite and smectite, and rare chlorite is observed partially replacing clinopyroxene in Sections 344-U1414A-48R-4 and 49R-1. Olivine is present in Units 1, 2, 4, 5, and 8, and its abundance (as a phenocryst phase) varies from 0% to 6.1%. Olivine is almost always replaced by secondary minerals, including saponite and smecite. Rare chlorite is observed partially replacing olivine in Sections 344-U1414A-48R-4 and 49R-1. As indicated earlier, their identification relied on their crystal morphology and textural relationships with surrounding minerals. Basement alterationAll basement rocks at Site U1414 have been subjected to alteration by interaction with either seawater or modified “hydrothermal” fluids. Alteration varies from slight to high. Basement alteration at Site U1414 is characterized by
An example of each alteration type observed at Site U1414 is shown in Figure F19. Macroscopic and microscopic observation of alteration distribution implies an overall slight to moderate alteration intensity in the groundmass, with more intense alteration concentrated around veins and vesicles. Alteration assemblages include saponite, smectite, carbonate, silicates, and sulfides. Zeolite and chlorite are rare. Only one XRD analysis of secondary clay minerals was made during Expedition 344; thus, most mineralogical descriptions rely on macroscopic and microscopic visual observation only. As a result, the term “smectite” has been used in place of a more definitive mineralogical definition. Future detailed clay mineral identification will be needed to resolve the clay mineral phases at Site U1414, and it is likely that a continuum of expanding clay minerals are present. We therefore describe saponite and smectite together. Saponite is present in Cores 344-U1414A-45R through 53R (374.6–418.2 mbsf). Saponite coloration ranges from black to greenish brown to greenish gray. In microscopic observation, it is typically pale brown. Smectite is observed starting with Core 344-U1414A-46R (376.4 mbsf); however, it is interpreted as ubiquitous from Core 344-U1414A-53R (413.3 mbsf) to the bottom of the hole. Smectite color ranges from black to olive-green to dark green-gray in macroscopic observation and green-brown to green-gray in microscopic observation. Slight to moderate replacement is pervasive throughout basement, with alteration concentrated around mesostasis forming a patchy texture observed in thin section. In intervals where high alteration is observed, the majority of groundmass is partially to completely replaced, partially obscuring the original igneous textures and mineralogy. Saponite and smectite are the most common phases replacing olivine, and they are the most common secondary minerals filling vesicles and forming veins. Smectite is the most common secondary mineral within breccia. Pyrite is present throughout Hole U1414A as an accessory phase, although it is most abundant in Sections 344-U1414A-45R-1 through 53R-1 (374.4–413.3 mbsf). It is identified by gold coloration in macroscopic observation and bright yellow in thin sections observed with reflected light. Pyrite occupies veins and alteration halos, but it is also present as a minor phase within some vesicles and background alteration. Crystal structure ranges from blocky to amorphous, and weak cleavage is occasionally observed. Other mineral phases present throughout Hole U1414A include calcite, zeolite, quartz, and rare chlorite (observed in Sections 344-U1414A-48R-4 and 49R-1). Carbonate, quartz, and zeolite are very rare within the groundmass. Rare chlorite was observed partially replacing mesostasis, olivine, and clinopyroxene. Alteration features are described in order of alteration intensity. Breccia represents the most intense alteration feature. Veins, followed by vesicles, represent the most ubiquitous alteration features. Halos are the least pervasive form of alteration, flanking relatively few veins and vesicles. Basalt clastic brecciaIn situ basalt-clastic breccia, recovered in interval 344-U1414A-58R-2, 51–97 cm (439.54–440.0 mbsf) (Fig. F20), represents 0.7% of the recovered core. Our estimate of the percentage of breccia at Site U1414 likely represents an underestimate of the true breccia proportion because core recovery is likely to favor rheologically stronger lithologies. Breccia consists of highly altered subangular basaltic clasts within a matrix of smectite. Clasts range in size from 1 to 20 mm; on average they are 10 mm. Clast groundmass is microcrystalline to cryptocrystalline and sparsely clinopyroxene plagioclase phyric. Primary textures observed include intersertal, seriate, and spherulitic. Primary igneous mineral phases (in order of decreasing abundance) include plagioclase, clinopyroxene, and Fe-Ti oxides. Phenocrysts make up ~0.5% of the clasts. Alteration with the clasts is high (~55%) and consists of saponite and accessory pyrite throughout the groundmass. Narrow silicate and saponite veins protrude into the clasts and in turn are flanked by a green-brown smectite halo. The intensity of brecciation decreases rapidly in the bottom 10 cm of the brecciated interval to a vein net. This, combined with the compositionally identical subangular clasts, suggests the breccia formed in situ. In addition, the composition and primary igneous texture is similar to the remainder of Unit 4. VeinsA total of 1159 veins were recorded in the Site U1414 basement. Average vein density is 19.99 veins/m of recovered core, and vein fill makes up 3.5% by volume of the recovered core (Table T4). Figure F21 shows the downhole distribution and abundance of veins. Vein thickness ranges from <0.1 mm to spectacular 70 mm thick veins (Fig. F19). Only veins thicker than 0.05 mm that were visible by macroscopic observation on a wet cut surface were recorded. Vein morphology is highly variable with morphologies that range from planar, curved, branching, anastomosing, irregular, en eschelon, and crosscutting. Secondary minerals filling veins include saponite, smectite, carbonate, pyrite, quartz, and zeolite. Additional postcruise study will be required to resolve the mineralogy of the expanding clay minerals (smectite) and zeolite. Veins may be monominerallic or polyminerallic and may contain a combination of any of the above mentioned filling phases. A few veins (5.8%) are flanked by halos. Saponite and smectite are the most common vein-filling phases, making up 1.7% and 0.9% of the core, respectively; together they make up 73.6% of all veins. Saponite- and smectite-bearing veins range from 0.05 to 70 mm thick and on average are 0.2 mm thick. Saponite and smectite are commonly associated with pyrite, carbonate, and quartz but are observed with all other secondary mineral phases. Saponite veins were identified between 374.6 and 413.3 mbsf. Smectite is observed from 376.4 mbsf downhole but occurs in greater abundance from 403.6 mbsf downhole. Both saponite and smectite are crosscut by all other mineral phases, and they are typically located on the inner flanks of multiminerallic veins. In larger veins, multiple phases of saponite and smectite precipitation occur and occasionally crosscut each other. Slickenlines within saponite and smectite in some of the larger veins (Fig. F17) suggest precipitation during vein opening together with displacement. Quartz is the next most abundant vein mineral, making up 0.5% of the recovered core and 13.2% of all veins. Veins containing quartz range from 0.5 to 70 mm thick, but veins containing solely quartz are restricted to a narrow ≤0.1 mm thickness. Quartz typically occurs with and crosscuts saponite, smectite, carbonate, and pyrite. Quartz typically occurs in the central portion of a given multiminerallic vein. Following quartz, carbonate makes up 0.4% of the total recovered core volume and 11.2% of all veins. Carbonate occurs in veins of all thicknesses, and it is typically located within the central portion and/or within one of the inner flanks of a multiminerallic vein (Fig. F18). In addition, carbonate may occur as monominerallic veins, crosscutting saponite and smectite veins. Crystals are typically elongate and are either angled or perpendicular to the vein wall, indicating crystal growth during fracture expansion. Pyrite in veins occurs throughout the recovered basement, and it makes up 0.1% of the core (3.6% of all veins). Pyrite is typically observed within multiminerallic veins in conjunction with saponite and smectite but may also occur with carbonate and quartz. Few veins (<0.1%) are 100% pyrite; however, pyrite typically overprints and/or crosscuts saponite. Crystals within veins occur either as discrete irregular to blocky growths or as larger agglomerations filling voids. Pyrite-bearing veins between 375.2 and 376.4 mbsf are often flanked by pale yellow–bearing to dark yellow–bearing halos of varying thickness (see “Halos”). Zeolite occurs in Sections 344-U1414A-51R-1 and 51R-2 only as a minor phase within narrow (0.1–0.2 mm thick) smectite ± quartz veins. Zeolite was not identified in thin section, and there are no XRD analyses available to confirm its presence; thus, its identification relies on macroscopic observation of texture only. Large veins (Fig. F19) often exhibit pressure-induced brecciated margins, where basement is completely replaced by saponite/smectite and “clasts” are surrounded by numerous carbonate and silica microveins. VesiclesAll units at Site U1414 contain vesicles, the abundance of which varies from <0.1% to 40%. Typical abundances range from 0.1% to 10%. The abundance of vesicles varies considerably on a unit scale; however, increased vesicle abundances tend to coincide with the tops of unit boundaries (Fig. F16). Vesicle fill ranges from 0% to 100%, with most vesicles ≥90% filled with secondary minerals (saponite, smectite, sulfides, quartz, carbonate, and zeolite) (Table T4). The vast majority of vesicles are 100% filled with saponite/smectite, with either sulfides (pyrite), quartz, or carbonate filling the central portion of the vesicle as a late-stage infill. Common vesicle-filling assemblages, outlined by order of filling, are listed below:
Variation in vesicle fill, however, is typically restricted to discrete intervals, reflecting perhaps a restrictive fluid regime within Site U1414 basement. HalosHalos are present in varying abundance from 0% to 20% (by volume of core) throughout Hole U1414A basement and make up 1.38% (by volume). The abundance of each type of halo versus background is shown in Figure F22. Dark yellow halos are the most abundant, making up 0.6% of Hole U1414A basement. Dark yellow halos are typically associated with pale yellow halos (0.2% of the core by volume) forming a complex halo flanking pyrite-bearing veins. Both dark yellow and pale yellow halos contain pyrite and saponite, but pale yellow halos exhibit higher abundances of pyrite that are typically concentrated as a pyrite-rich halo front (Fig. F19). Pale yellow and dark yellow halos are absent below 374.6 mbsf. Dark gray halos make up 0.5% of the core and occur in small proportions in most Hole U1414A cores, with the exception of Cores 344-U1414A-46R, 52R, 57R, 59R, 61R, and 63R. Dark gray halos flank saponite- and smectite-bearing veins and comprise saponite and smectite alteration of groundmass. Halo widths range from 0.1 to 25 mm. Boundaries are diffuse and irregular. Pale gray halos are present in Core 344-U1414A-54R only, flanking smectite veins as part of a complex 10 mm thick dark gray–light gray halo. No thin section is currently available, so mineralogy is difficult to determine. Alteration summaryLow-temperature hydrothermal alteration at Site U1414 is typical of a regime in which access to seawater is restricted, giving rise to oxygen-starved alteration (Laverne et al., 1996; Teagle et al., 1996, 2006). Figure F23 provides a tentative summary for the alteration history, outlining the ingress of modified, restricted fluids into Site U1414 basement. No oxidative alteration features or mineralogy were observed either in macroscopic observation or in thin section (Table T4). The extent of alteration is low, and the vast majority of basaltic groundmass is only slightly altered. Secondary minerals in veins, breccia, and vesicles make up 3.8% by volume of the recovered core. Alteration intensity tends to be focused on or near igneous unit boundaries, implying that ingress of seawater and/or modified fluid, secondary mineral precipitation, and wall-rock interaction is restricted to interflow regions. Localized complex alteration and veins that exhibit multiple episodes of reopening and infill suggest that fluid flow occurred over several episodes, possibly throughout the ~15 m.y. history of Site U1414 basement. The abundance of pyrite and carbonate within the uppermost portion of basement and the presence of carbonate-bearing veins and silicates in the sediment immediately overlying the basement imply lateral fluid flow at this boundary. The presence of small amounts of chlorite in Sections 344-U1414A-48R-4 and 49R-1 suggests possible localized upflow or lateral flow of warm modified hydrothermal fluids at subgreenshist facies temperatures. No other hydrothermal mineral phase was observed (e.g., anhydrite, actinolite, prehnite, or epidote). In addition, the chlorite observed at Site U1414 may actually represent a chlorite/smectite mix, which can form at temperatures below 100°C. Chlorite or chlorite/smectite observed in the basement may have formed either early during axial hydrothermal circulation or recently. The relative timing of these fluids is hard to discern because no overprinting or crosscutting relationship with chlorite was observed. Correlation to Site U1381The lithology at Site U1414 generally shows a very nice continuous transition from a mid-Miocene mid-ocean ridge crest origin—and subsequent pelagic sedimentation in an abyssal environment—to a step-by-step increasing terrigenous component as the oceanic plate approaches the Central American subduction zone in the Pleistocene. Subunit IA in the sediment cover recovered from Hole U1414A consists of a sequence of hemipelagic silty clay and clay that contains some tephra horizons. This sequence correlates well with the lithology of Unit I from Holes U1381A (Expedition 334 Scientists, 2012) and U1381C (Harris et al., 2013a). The subsequent sedimentary succession (Subunit IB) consists of calcareous silty clay with more felsic ash layers. Together with the major parts of Unit II (nannofossil and sponge spicule–rich ooze), those recovered lithologies represent the missing sediment portion at Site U1381. Both the lithology and biostratigraphy support this hypothesis, which will be investigated in more detail with shore-based studies (see “Paleontology and biostratigraphy”). At Site U1414, recovered basement consists of a series of sheet and massive flows. This finding is in contrast to basement at Site U1381, which consists of 69.6 m of pillow lavas. A lack of features typically associated with pillow lavas (e.g., curved chilled margins, cryptocrystalline to glassy textures, and cooling fractures) and the recovery of massive uninterrupted basaltic sections attests to the dominance of lava flows at Site U1414. Intercalated sediment observed in basement at Site U1414 and Site U1381 may indicate that the timing of lava flow/pillow lava emplacement may be related; however, further investigation is required to define the nature of lava emplacement and its relationship between lavas erupted on the Cocos Ridge (Site U1381) and the ridge–seamount transition (Site U1414). Alteration at Sites U1381 and U1414 is broadly similar, with saponite, sulfides, and quartz forming the major secondary mineral assemblages. In addition, both sites exhibit a lack of oxidative secondary mineral phases (e.g., iron-oxyhydroxide and celadonite), which suggests that basement at Sites U1414 and U1381 shared a similar, restricted fluid regime. |
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