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doi:10.2204/iodp.proc.336.102.2012

Petrology, alteration, structural geology, and hard rock geochemistry

Igneous petrology

Rock description procedures during Expedition 336 generally followed those used during previous IODP expeditions (e.g., Expeditions 335, 309/312, and 304/305). The flow of core description during Expedition 336 was done as follows. First, lithologic units and subunits were defined by either visual identification of actual lithologic contacts (e.g., chilled margins) or by inference of the position of such contacts using observed changes in composition (e.g., phenocryst assemblages or volcanic and/or volcaniclastic features). Unit definition was followed by an initial description of the lithologic characteristics, igneous textures, minerals, and vesicle distributions on a piece-by-piece basis. Macroscopic observations were ultimately combined with those from detailed thin section petrographic studies of the key igneous units. Data from X-ray diffraction (XRD) analysis (see “X-ray diffraction” in “Alteration and metamorphism”) and bulk-rock chemical analyses (see “Hard rock geochemistry”) were also used to confirm constituent minerals and to interpret igneous units. Data for the macroscopic and microscopic descriptions of recovered cores were entered into the LIMS database using the DESClogik program (see DESClogik user guide in Technical Documentation [iodp.tamu.edu/​tasapps/]).

Igneous units and contact logs

Unit boundaries were identified on the basis of the presence of contacts, chilled margins, changes in primary mineralogy, color, grain size, and structural or textural variations. Lithologically and texturally similar pieces from consecutive core sections were logged as belonging to the same unit. In order to preserve important information about igneous stratigraphy without defining an unreasonable number of units in a single core, subunits were designated when there were noticeable changes in texture, color, and grain size without accompanying changes in mineralogy. Where contacts deviated from horizontal within the core reference frame, their depths were logged at their midpoints. Igneous unit and contact logs provide information about unit boundaries and a brief description of each unit. For each unit, the table lists unit number, core number, depth interval (in meters below seafloor), a description of the upper and lower boundaries, and a unit description. In addition, VCD reports (including an alteration summary) were produced for each core section. Symbols used in hard rock VCDs are shown in Figure F3.

Macroscopic core description

Lithology

Volcanic rocks were classified according to the following two definitions:

• Basalt: all igneous rocks of basaltic composition having a groundmass with a modal grain size from glassy to fine grained.
• Dolerite: holocrystalline fine- to medium-grained rocks of basaltic composition with well-developed subophitic or ophitic textures.

Basalts were further divided according phenocryst content, using the following conventions:

• Aphyric = <1% phenocrysts.
• Sparsely phyric = 1%–5% phenocrysts.
• Moderately phyric = >5%–10% phenocrysts.
• Highly phyric = >10% phenocrysts.

For phyric basalts, phenocryst phases were included as hyphenated modifiers preceding the rock name. If phenocryst abundances are <1%, however, the modifier “aphyric” precedes the rock name.

Mafic and ultramafic plutonic rocks were classified on the basis of abundance, grain size, and texture of their primary minerals (as inferred prior to alteration) using the International Union of Geological Sciences (IUGS) system (Fig. F4) (Streckeisen, 1974; Le Maitre, 1989; Le Maitre et al., 2002). This classification defines the following mafic and ultramafic rock lithologies:

• Troctolite: olivine + plagioclase > 95%, olivine > 10%, and plagioclase > 10%.
• Olivine gabbro: olivine + plagioclase + clinopyroxene, none of which are <5%.
• Gabbro or diorite: plagioclase + clinopyroxene > 95%, plagioclase > 10%, clinopyroxene > 10%, and quartz < 5%.
• Gabbronorite: plagioclase + clinopyroxene + orthopyroxene, none of which are <5%.
• Quartz diorite: quartz + alkali feldspar + plagioclase (QAP), where quartz is 5%–20% and alkali feldspar is <10%.
• Tonalite: QAP, where quartz is 20%–60% and alkali feldspar is <10%.
• Trondhjemite: tonalite with total mafic mineral content < 10%.
• Dunite: olivine > 90%.
• Harzburgite: olivine > 40% and clinopyroxene < 10%.
• Lherzolite: olivine > 40%, orthopyroxene > 10%, and clinopyroxene > 10%.
• Wehrlite: olivine > 40% and orthopyroxene < 10%.
• Olivine orthopyroxenite: olivine = 10%–40% and clinopyroxene < 10%.
• Olivine websterite: olivine = 10%–40%, orthopyroxene > 10%, and clinopyroxene > 10%.
• Olivine clinopyroxenite: olivine = 10%–40% and orthopyroxene < 10%.
• Orthopyroxenite: orthopyroxene > 90%.
• Websterite: olivine < 10%, orthopyroxene > 10%, and clinopyroxene > 10%.
• Clinopyroxenite: clinopyroxene > 90%.

In the IUGS classification, diorite is distinguished from gabbro by the anorthite content of plagioclase, with diorites having <50 mol% An. Because anorthite content cannot be observed during macroscopic description, we used the convention followed during Expedition 335: if a gabbroic rock contained quartz (<5%) or primary amphibole, indicating a high degree of fractionation, the rock was classified as diorite. If no quartz or primary amphibole was observed, the rock was classified as gabbro.

Minor modifications to the IUGS system were made as follows so that rock types could be more accurately subdivided on the basis of significant differences rather than arbitrary cutoffs based on the abundance of a single mineral.

For gabbroic rocks, the following modifiers based on modal mineralogy were used:

• Disseminated oxide: Fe-Ti oxide = 1%–2%.
• Oxide: Fe-Ti oxide > 2%.
• Olivine-bearing: olivine = 1%–5%.
• Orthopyroxene-bearing: orthopyroxene = 1%–5%.
• Troctolitic: clinopyroxene = 5%–15% and olivine > 20%.
• Olivine-rich: olivine > 70%.
• Anorthositic: plagioclase > 80%.

Additional descriptive modifiers are defined as follows:

• Leucocratic: light colored with high proportions of plagioclase.
• Micro: dominant grain size < 1 mm.
• Doleritic: fine- or medium-grained gabbroic rocks with dominant ophitic or subophitic textures.

Where alteration in ultramafic rocks is so extensive that estimation of the primary mineral assemblages is impossible, the rock is called “serpentinite.” If primary assemblages can be inferred from pseudomorphs and textures in ultramafic rocks, even though they are partially or completely replaced, the rock name is based on the reconstructed primary assemblage, preceded by the modifier “serpentinized” (e.g., serpentinized dunite).

Mineralogy

Volcanic rocks were described according to groundmass, phenocrysts (if any), and vesicles. Phenocryst abundance (in percent), shape, and maximum, minimum, and median grain size (in millimeters) were recorded for each phase. Vesicle abundance (in percent); vesicularity; size distribution; minimum, maximum, and modal size (in millimeters); roundness (rounded, subrounded, or well rounded); and sphericity (highly spherical, moderately spherical, slightly spherical, or elongate) were also recorded. Fill (in percent) and fill composition were documented in the alteration log, together with another estimation of vesicle abundance, either on a piece-by-piece basis or for a set of pieces having similar alteration characteristics. The following data for each primary silicate were recorded in the LIMS database using the DESClogik program:

• Visually estimated modal percent. Note that this estimation represents the modal mineralogy as observed in fresh rocks, whereas in altered rocks it represents the estimated igneous modes prior to alteration. Where a mineral occurs in trace quantities (i.e., it is too scarce to assign a meaningful percentage), 0.1% was recorded. Accessory phases were also noted where observed.
• Minimum, average, and maximum grain size for each mineral phase.
• Mineral shape (euhedral, subhedral, or anhedral).
• Mineral habit, defined as
• Equant = aspect ratio < 1:2,
• Subequant = aspect ratio 1:2 to 1:3,
• Tabular = aspect ratio 1:3 to 1:5,
• Elongate = aspect ratio > 1:5, or
• Interstitial, poikilitic, vermicular, skeletal, acicular, aggregates, columnar, fibrous aggregates, granular, isometric, lath-shaped, lath, oikocryst, prismatic, radial aggregates, subparallel aggregates, or vericular.

Contacts

Contacts between units were described according to contact type, definition, morphology, geometry, and interpretation. Where the contact was not recovered, this was noted as “contact not recovered.”

Contact types are defined as follows:

• Grain-size contact: units on either side have markedly different grain sizes.
• Modal contact: units on either side have markedly different mineral proportions.
• Color contact: units on either side have markedly different primary (i.e., not alteration related) color.
• Magmatic vein: thin, discontinuous intrusions of igneous material are present.
• Chilled margins: sharp gradients in grain size occur near igneous contacts.
• Dike: greater than centimeter-scale tabular intrusions of igneous material occur.

Where contacts are obscured by deformation and metamorphism, the following classification is applied:

• Sheared contact: interval with deformation fabric is in contact with an undeformed interval.
• Foliated contact: both intervals have deformation fabrics.
• Tectonic contact: contact appears to be the result of faulting.

Contacts having magmatic, crystal-plastic, or brittle deformation structures are further reported in the structure log.

Texture

The textures of volcanic rocks are defined on the basis of groundmass modal grain size, grain-size distribution, and the relationships between different grains.

Grain size was defined as follows:

• Glassy.
• Cryptocrystalline = <0.1 mm.
• Microcrystalline = 0.1–0.2 mm.
• Fine grained = 0.2–1 mm.
• Medium grained = 1–5 mm.
• Coarse grained = 5–30 mm.
• Pegmatitic = >30 mm.

Volcanic rock grain-size distribution (applied to phenocrysts only) was described using the following terms:

• Unimodal: all phenocrysts are of similar size.
• Bimodal: phenocrysts define two size populations.
• Seriate: phenocrysts form a continuous range of sizes.

To describe the textural relationships between different silicate grains, the following terms were used:

• Ophitic: total inclusion of plagioclase in clinopyroxene.
• Subophitic: partial inclusion of plagioclase in clinopyroxene.
• Intergranular: coarser touching grains form a framework of the rock with interstices filled by crystalline material.
• Intersertal: coarser touching grains form a framework of the rock with interstices filled by glass.
• Hyalophitic: predominantly glassy groundmass with little interstitial texture.

Textures of plutonic rocks were also defined according to grain size, grain-size distribution, and the relationships between different grains. Grain sizes were defined using the same terms as those for volcanic rocks. Grain-size distributions for plutonic rocks are classified as follows:

• Equigranular: all minerals are of similar size.
• Inequigranular: grain size varies significantly. Inequigranular textures are further divided using following terms:
• Seriate: a continuous range of crystal sizes is present.
• Varitextured: domains with contrasting grain size occur.
• Poikilitic: relatively large oikocrysts enclose smaller crystals, termed “chadacrysts,” of one or more other minerals.

The following terms were used for plutonic rocks to describe the textural relationships between different silicate grains: intergranular, intersertal, subophitic, ophitic, granular (i.e., aggregation of grains of approximately equal size), dendritic (branching arrangement of elongate crystal), and comb structure (a comblike arrangement of crystals growing inward from a contact).

The textures of oxide and sulfide minerals were also described in terms of grain size and their relationship to adjacent minerals. In plutonic rocks, oxides commonly occur as aggregates, and for grain size determination an aggregate is counted as a single grain.

Thin section description

Thin section observations were performed to confirm macroscopic observations and to add microscopic characteristics of core samples. Thin section description closely followed the procedure for macroscopic core description. The following data were recorded and entered into the LIMS database using separate tabs in the thin section workbook in DESClogik.

Lithology and texture

• Rock name (based on thin section observation), using the same definitions as those used for macroscopic descriptions.
• Grain size modal name (glassy, cryptocrystalline, microcrystalline, fine grained, medium grained, coarse grained, or pegmatitic).
• Grain-size distribution (equigranular, seriate, varitextured, or poikilitic).
• Texture (granular, subophitic, ophitic, granophyric, porphyritic, intergranular, intersertal, variolitic, or granoblastic).

Mineralogy

• Present abundance (in percent) of primary minerals preserved.
• Estimated abundance (in percent) of primary minerals prior to alteration.
• Estimated value of mineral replacement by alteration.
• Individual comments for primary minerals.

For primary mineral grains, the following data were recorded using the same conventions as those used during macroscopic description:

• Minimum, maximum, and median size.
• Mineral shape and habit.
• Special features of primary minerals.

Alteration and metamorphism

The characteristics of alteration and metamorphism of volcanic and plutonic rocks recovered during Expedition 336 were determined using visual inspection of the core, microscopic thin section descriptions, and XRD analyses.

Visual observations of the core were recorded in the (1) alteration log (plutonic-mantle rocks and volcanic rocks), (2) vein/halo log, and (3) breccia log. Shipboard observations of alteration and metamorphism were recorded using the DESClogik worksheet interface and uploaded to the LIMS database. Where basalt and gabbro and/or serpentinite were found in the same section, the appropriate alteration log sheet was used for intervals of each rock type. Alteration and metamorphism of rocks were described in terms of general intensity of background (groundmass and phenocrysts) alteration. The presence of localized alteration patches (zones of more intense alteration), also referred to as alteration halos when lining open cracks or veins, was also recorded. Vein and vesicle filling and breccia clasts and matrix filling were also reported together with the abundance of secondary minerals. Description of alteration of the archive half of the core also provides information on secondary mineral replacement of primary igneous features including phenocrysts and groundmass. Information on the extent of replacement of igneous minerals and groundmass by secondary minerals and, where possible, the nature and approximate modes of secondary mineral assemblages was recorded using DESClogik. The presence, description, and mineralogy of veins or vesicle infill were also recorded. Each logged interval may represent multiple pieces, core sections, or local alteration features within a piece.

The vein/halo log contains all data (e.g., shape, texture, orientation, etc.) related to vein features. The breccia log also contains data on breccia type (e.g., magmatic, hydrothermal, tectonic, and sedimentary), clast features (e.g., abundance, shape, lithology, and alteration), matrix, and cement properties. If the breccia clasts have any magmatic, crystal-plastic, or brittle structures, these features are described on the structure log (see below). Magmatic, crystal-plastic, and brittle deformation structures were also logged in the structure log.

Macroscopic core description

All volcanic, mafic, and ultramafic rocks recovered during Expedition 336 are described as follows.

Unit and subunit alteration summary

Alteration intensity plotted on the VCD reports corresponds to the background/​groundmass alteration intensity. On the lithologic unit logs (volcanic and plutonic/​mantle rocks), rocks were graded according to alteration intensity as follows based on volume of alteration products:

• Fresh = <1%.
• Rare = 1%–3%.
• Slight = >3%–10%.
• Moderate = >10%–30%.
• Abundant = >30%–50%
• High = >50%–90%.
• Complete = >90%–100%.

Alteration textures were described using the following terms in order to document variations and heterogeneities in alteration style and intensity:

• Pervasive: uniform alteration style and intensity throughout the rock.
• Patchy: pervasive background alteration with local alteration patches.
• Pseudomorph: complete replacement of primary mineral with characteristic shape.
• Corona: primary mineral crystal surrounded by an alteration rim.
• Recrystallized: pervasive background alteration, where the rocks are recrystallized to granoblastic contact metamorphic assemblages.
• Clasts: breccia clasts enclosed in a matrix of secondary minerals.
• Clasts with halos: breccia clasts that contain alteration halos and are enclosed in a matrix of secondary minerals.

The following information is displayed on the VCD forms:

• Vein summary: approximate frequency (rare = 0–5 veins/m, several = >5–15 veins/m, common = >15–30 veins/m, and very common = >30 veins/m) and average vein thickness, along with minerals in order of abundance.
• Vesicle summary: abundance and major minerals filling vesicles, together with general comments.
• Alteration summary: groundmass and phenocrysts (also recorded in lithology logs).

Volcanic rock alteration

The volcanic rock alteration log was used to record bulk volcanic rock alteration, either piece by piece or for a given group of pieces. Each entry records identifiers for core, section, piece(s), and interval (in centimeters); length of each piece or group of pieces; depth below seafloor (in meters) at the top of the piece or group of pieces; and igneous unit. Information on alteration type (as represented by rock color and secondary mineral abundances) for groundmass, alteration patches, halos, and phenocrysts is provided. A column for comments is also included.

The following features were observed and recorded for groundmass alteration, as well as for any patchy textural intervals, large alteration halos, and phenocrysts:

• Color.
• Percentage of rock altered to secondary phases.
• Percentage of each primary mineral replaced.
• Secondary minerals replacing primary minerals.

Plutonic and mantle rock alteration

The plutonic and mantle rock alteration log was used to record bulk rock alteration of gabbro and other coarse-grained holocrystalline rocks and ultramafic rocks. Total modal percentage of secondary minerals was estimated in hand specimen, as were proportions of major primary igneous minerals (olivine, clinopyroxene, plagioclase, orthopyroxene, and oxides) and the secondary minerals by which they were replaced. A column for comments is included. The following data were recorded for patchy textural intervals: pervasive background alteration information for the rock hosting the patches (as above), size (<1 cm, 1–3 cm, or >3 cm), shape (round, irregular, elongate, or network), area percentage of patch in the rock or interval, total percentage of alteration (secondary phases) in the patches, and primary mineral alteration and secondary phases present (as in background alteration).

Veins and halos

Description of veins and alteration halos is recorded in the vein/halo log using the DESClogik worksheet interface. Vein and halo descriptions were reported on a piece-by-piece and vein-by-vein scale. All tabulated information was recorded from the archive halves. Alteration halos, either associated to observed veins or along-piece edges, were tabulated to provide a consistent characterization of the extent of alteration halos of the rocks (with respect to fresher basalt) and to quantify the different alteration types. Approximate abundance of secondary minerals in veins was reported, mostly on the basis of color, hardness, HCl reactivity, and crystal habit/​morphology. Where additional mineralogical evidence is available from either thin section descriptions or X-ray diffractograms, these identifications are integrated into the summary alteration and the VCDs. Alteration halos representing zones of increased alteration adjacent to veins were described by width, color, and secondary mineral percentages in the halo comments column of the vein/halo log. Vein nets grading into breccia were reported in the breccia log.

For each vein and halo, we recorded the following information in the vein/halo log (e.g., Fig. F5):

• Top and bottom (in centimeters) of the feature and vein width (in millimeters) to estimate the apparent vein area on the cut surface.
• Piece or subpiece number and length (in centimeters) to estimate piece surface area.
• Boundary (diffuse or sharp).
• Vein boundary definition (sharp, diffuse, gradational, sutured, or sharp to gradational).
• Shape (straight, sigmoidal, irregular, pull-apart, or fault vein).
• Attitude (horizontal, subhorizontal, vertical, subvertical, inclined, steeply dipping, nonoriented, planar, planar-linear, linear, or curved).
• Connectivity (anastomosing, isolated, nonbranched or single, branched, crosscut, T-shaped, Y-shaped, splayed, wispy, or network).
• Texture (massive, cross-fiber, slip-fiber, vuggy, or polycrystalline).
• Structure (simple, composite, banded, haloed, or intravenous).
• Geometry (en echelon, ribbon, or cross fractures).
• Vein color and filling (smectite, carbonate, Fe oxyhydroxide, zeolite, etc.).
• Presence and color of a related alteration halo and width (in millimeters) of the halo on one side of the vein.
• Alteration halo properties (diffuse or multilayered) and percent of alteration of the background/​groundmass and texture.
• Orientation (dip angle and dip direction) of fracture and trend and plunge of associated lineation and slickenlines (with respect to measurement of orientation; see “Structural measurements”).
• Comments about veins and halos.

Note that if a vein had any remarkable magmatic, crystal-plastic, or brittle structure (e.g., shear vein, fault, or microfault), we also described these structural features (e.g., sense of shear, etc.) on the structure log and added a comment to the vein/halo log.

Breccia

The descriptions of brecciated units were recorded in the breccia log using the DESClogik worksheet interface. Breccia descriptions were reported on a piece-by-piece scale noting the following:

• Depth interval and piece number.
• Type:
• Hydrothermal: containing secondary matrix or vein minerals.
• Magmatic: containing glass or quench textures such as hyaloclastite and pillow breccia and primary matrix minerals.
• Tectonic: cataclasite and fault-gouge in which the matrix consists of the same material as the host rock.
• Sedimentary: matrix filling composed entirely of sedimentary materials (clays and carbonate).
• Clasts: abundance (in percent), size (minimum, maximum, and average), shape, composition (monomictic or polymictic), color, internal structures (veining, shearing, crushing, or fracturing), lithology, and alteration (fresh, altered, or devitrification).
• Matrix and cement: abundance, color, grain size, percentage of secondary minerals, and composition.

When the breccia had any orientation, dip angle, or dip direction that could be described with respect to measurement of orientation, we also recorded the structural features on the structure log.

Thin section description

Thin sections of volcanic, mafic, and ultramafic plutonic rocks recovered during Expedition 336 were examined in order to

• Confirm macroscopic identifications of secondary minerals;
• Determine their mode of occurrence in terms of vesicle and void fillings, vein composition, and primary mineral replacement;
• Identify chronological relationships between different secondary minerals;
• Establish distribution, occurrence, and abundance of secondary minerals downhole; and
• Quantify the amount of alteration, especially cryptocrystalline rocks.

Modal estimates of the secondary minerals allowed characterization of alteration intensity. A total alteration percentage was calculated using modal composition of phenocrysts and groundmass minerals and their respective percentage of alteration.

Thin section descriptions were recorded using DESClogik and subsequently uploaded to the LIMS database. A summary description of secondary mineral assemblages and replacement relations to primary phases, as well as mineralogy of veins and vesicles, was entered in DESClogik so it could be added to the thin section report. Digital photomicrographs were taken during the expedition to document features described in thin sections.

X-ray diffraction

Phase identification of whole rocks, patches, or vein material was aided by XRD analyses using a Brucker D-4 Endeavor diffractometer with a Vantec-1 detector using nickel-filtered CuKα radiation. XRD was performed on small amounts of powder (usually ~20 mg) as smear slides or pressed onto sample holders. Instrument conditions were as follows:

• Voltage = 40 kV.
• Current = 40 mA.
• Goniometer scan (bulk samples) = 4°–70°2θ.
• Step size = 0.0087°.
• Scan speed = 0.2 s/step.
• Divergence slit = 0.3°, 0.6 mm.

Structural geology

The conventions for the techniques used for macroscopic and microscopic description of structural features observed in igneous rocks used during Expedition 336 generally followed those used during Expeditions 304/305, 309/312, and 335. Definitions of structural measurements and descriptive parameters, as well as their corresponding description dictionaries, were further refined as part of the process of configuring the DESClogik core description software for hard rock descriptions.

Structural measurements

Depth intervals of structures were recorded as the distance from the top of the section to the top and bottom of the structural feature. Depth to the midpoint of structures was recorded for structures with measurable width, such as veins or intervals with magmatic foliation (Fig. F6). Where they occur, crosscutting relationships were described with core section depth. Apparent fault displacements of planar markers were recorded as they appeared on the cut face of the archive half of the core. Displacements observed on the vertical core cut face were treated as dip-slip components of movement and labeled in spreadsheets as either normal or reverse for faults inclined <090°; their displacement in millimeters was also recorded. Shear sense indicators were also marked on the spreadsheets. Slickenside or slickenfiber orientation trends and plunge measurements or the trend and plunge direction of the slip line between offset linear markers were incorporated wherever possible to determine dip-slip, oblique-slip, or strike-slip components.

We measured structures on the archive half relative to the standard IODP core reference frame (Fig. F7). The plane normal to the axis of the borehole is referred to as the apparent horizontal plane. On this plane, a 360° net is used, with pseudo-south (180°) at the bottom line of the working half and pseudo-north (000°) pointing out of the archive half (Fig. F7B). The cut surface of the split core, therefore, is a vertical plane striking 090°–270° and dips vertically. Apparent dip angles of planar features were measured on the cut face of the archive half of the core using a protractor, and its sense was indicated, whether toward 090° or 270° (Fig. F7C). A second apparent dip reading was obtained where possible in the 000°–180° plane section perpendicular to the core face (second apparent orientation) in order to find a true dip value. The two apparent dips and dip directions (or one apparent direction combined with the strike) measured for each planar feature were used to calculate the dip angle and dip direction. Mineral foliations and planar igneous contacts were measured in exactly the same way.

Macroscopic core description and terminology

The structural geologists oriented the whole cores and marked lines along which the core pieces were cut in half. Cores were marked to maximize dip on planar structures so that the dominant structure dips toward 270° in the core reference frame (i.e., toward the right when looking down at the cut surface of the archive half of the core). Where no obvious structures were present, cores were marked to maximize contiguity with adjacent core pieces.

All material from both the working and archive halves was examined. Sketches of structures and orientation measurements were made from the archive half, but observations on working-half pieces were also made for certain features that were better exposed there than in the archive half. For each section, detailed structural information was entered into the petrology worksheets in DESClogik, as described above. The worksheets contain data on location, types of structures, structural intensity, and orientation. The structural data were separated in two categories:

1. Brittle structures: breccia, faults, joints, drilling-induced fractures, serpentine foliation, cataclastic fabric intensity, and veins.
2. Magmatic and crystal-plastic structures: magmatic foliation, magmatic layering (including compositional and grain-size layering), igneous contacts, crystal-plastic foliation, and sense of shear.

Veins, defined here as fractures filled with secondary minerals, were described in the vein/halo log in the petrology worksheets, along with vein orientations. Breccia was described in the breccia log in the petrology worksheets, along with orientation and type of breccia (magmatic, hydrothermal, tectonic, or sedimentary) (see “Alteration and metamorphism”).

The most representative or prominent structural features in the cores recovered during Expedition 336 are plotted on the VCD reports. These features include intensity of magmatic and crystal-plastic fabric alignment, density of brittle fractures, and precise locations of observed prominent structures such as igneous contacts, magmatic layering and magmatic veins, vein net and breccia, cataclastic zones, shear veins, faults, joints and fractures (except for horizontal irregular fractures; see “Brittle deformation”), and folds, where recognizable.

Short explanations of the terms and abbreviations used in the above categories are given below. We followed the terminology of Ramsay and Huber (1987), Twiss and Moores (1992), Davis (1984), and Passchier and Trouw (1996).

Brittle deformation

Brittle deformation features included faults, defined as fractures with shear displacement, and joints, defined as fractures with no shear displacement. We used “fracture” as a general term indicating brittle failure with or without displacement. The term “microfault” was also used to describe faults with <1 mm width of related deformation or faults with displacement measurable at the core scale. “Shear veins,” defined here as fractures with secondary mineralization and shear-sense indicators (such as slickenlines and fibrose mineral growth), were logged independently in the vein/halo log, along with orientation of slickenlines and sense of shear, where measurable (see “Alteration and metamorphism”). The orientations of open joints/​fractures that occur along the broken surface of veins were recorded in the structure log in the petrology worksheets rather than in the vein/halo log. We also described the intensity of serpentine foliation and commented on the serpentine structural features in the structure log. Descriptions of brittle deformation include the following:

• Semibrittle vs. brittle: any feature involving both brittle and crystal-plastic deformation (i.e., melt-filled fractures).
• Type of fault rock: fault gauge, cataclasite, or pseudotachylite. Microfaults were also identified here. The type of fault rock may be accompanied by identifiers describing fabric alignment such as foliation and lineation.
• Clast/matrix ratio (in percent).
• Sense of shear: normal (n), reverse (r), dextral (d), sinistral (s), or a combination of nd, ns, rd, and rs.
• Fault offset (in millimeters), where measurable.
• Trend and plunge of slickensides, slickenlines, and slickenfibers.
• Intensity of cataclastic deformation and intensity rank (Fig. F6):
• 0 = undeformed.
• 1 = minor fracturing with no significant grain-size reduction (GSR).
• 2 = moderate fracturing without GSR.
• 3 = dense aastomosing fracturing with incipient GSR.
• 4 = well-developed GSR with evidence for clast rotation (independent particulate flow).
• 5 = cataclastic.
• Intensity of serpentine foliation and intensity rank (Fig. F6):
• 0 = massive.
• 1 = weakly foliated.
• 2 = moderately foliated.
• 3 = strongly foliated.
• Density of subhorizontal irregular fractures, identified as irregular or concave, horizontal to subhorizontal (<20°) fractures without secondary mineralization, occurring at piece ends or confined in a piece and perhaps related to drilling-induced fracturing or preexisting anisotropy of the rocks:
• 0 = no fractures.
• 1 = <1 fracture/10 cm.
• 2 = 15 fractures/10 cm.
• 3 = >5 fractures/10 cm, or rubble.
• Density of joints/​fractures other than subhorizontal irregular fractures (Fig. F6):
• 0 = no open fractures.
• 1 = <1 fracture/10 cm.
• 2 = 1–5 fractures/10 cm.
• 3 = >5 fractures/10 cm.
• Fracture morphology: planar, curved, concave, irregular, stepped, splayed, anastomosing, or Riedel array (Fig. F5).
• Fracture thickness (in millimeters). By definition, a closed fracture/joint has 0 mm thickness.
• Comments: serpentine structural features, crosscutting relationships, and morphology of fracture termination.
• Orientation (dip angle and dip direction) of fracture and trend and plunge of associated lineation.

Magmatic and crystal-plastic structures

Magmatic fabrics were defined macroscopically by magmatic layering, including compositional and grain-size layering, and shape-preferred orientation (SPO) of primary minerals where there is no evidence of crystal-plastic deformation. Descriptions of magmatic fabric include the following:

• Symmetry of magmatic fabric: linear (L), planar (S), planar-linear (L-S), or not detectable.
• Layering: grain size or compositional.
• Names of consistent layering minerals.
• Magmatic foliation, accompanied by intensity rank (Fig. F6).
• Sense of shear: normal (n), reverse (r), dextral (d), sinistral (s), or a combination of nd, ns, rd, and rs.
• Comments: name and general shape of fabric elements, together with crosscutting relationship, shape, boundary features (i.e., planar, curved, or irregular), and anything else of interest.
• SPO of mineral phases or groups of mineral phases, along with modal percent of measured mineral phases, shape ratio, and orientation of SPO long axis. Specimens were ranked according to SPO ratio:
• 0 = macroscopically isotropic (<1.05 SPO ratio).
• 1 = weak (1.05 to <1.2 SPO ratio).
• 2 = moderate (1.2 to <2.0 SPO ratio).
• 3 = strong (≥2.0 SPO ratio).
• Orientation of magmatic fabrics: dip angle and dip direction of planar fabric and trend and plunge of lineation, where measurable.

Crystal-plastic fabrics are lineations or foliations defined by grains exhibiting plastic strain. The textural criterion used for gabbroic rocks, on which this was based, was modified slightly for peridotites. Descriptions for crystal-plastic fabric include the following:

• Symmetry: linear (L), planar (S), planar-linear (L-S), or not detectable.
• Intensity, accompanied by intensity rank (Fig. F6):
• 0 = undeformed.
• 1 = weakly foliated/​lineated.
• 2 = strongly foliated/​lineated.
• 3 = porphyroclastic/​protomylonitic.
• 4 = mylonitic.
• 5 = ultramylonitic.
• Boundary geometry (i.e., shear zone boundary): planar, irregular, curved, concordant, or discordant.
• Boundary sharpness: sharp or diffused.
• Sense of shear: normal (n), reverse (r), dextral (d), sinistral (s), or a combination of nd, ns, rd, and rs.
• Comments: name and general shape of minerals that define crystal-plastic fabrics, together with crosscutting relationships, spacing, shape features, and anything else of interest.
• SPO of mineral phases or groups of mineral phases, with modal percent of measured mineral phases, shape ratio, and orientation of SPO long axis. SPO was ranked as follows:
• 0 = macroscopically isotropic (<1.05 SPO ratio).
• 1 = weak (1.05 to <1.2 SPO ratio).
• 2 = moderate (1.2 to <2.0 SPO ratio).
• 3 = strong (≥2.0 SPO ratio).
• Orientation: dip angle and dip direction of planar fabric and trend and plunge of lineation, where measurable.

Microstructures

To better characterize different types of deformation, we studied the microstructural features of interesting or prominent mesoscopic structures. Thin sections of recovered material were examined in order to

• Confirm macroscopic descriptions of structures,
• Characterize the microstructure of the rocks,
• Document crystal-plastic overprints of magmatic fabrics and brittle overprints of crystal-plastic fabrics,
• Provide information on the kinematics of brittle and brittle-ductile deformation,
• Identify temporal relationships between magmatic deformation and alteration processes, and
• Document major structural zones and downhole variations.

Microstructure descriptions followed the terminology of Passchier and Trouw (1996). Where possible, shipboard thin sections were oriented with respect to the core reference frame, and samples were cut perpendicular to the foliation and parallel to any extensional lineation, because this is the plane that best displays both shear-sense indicators and the preferred dimensional orientation of minerals. The orientation of structures measured during macroscopic core description was confirmed, and macroscopic observations were refined by microscopic description. Digital photomicrographs (available in the LIMS database) were taken and annotated to document features described in thin sections.

We generally followed the terminology used during Expedition 335. Structural domains were recorded in the structure log in the thin section description worksheet. Additional classifications and terminology were incorporated from Expeditions 304 and 305 (Expedition 304/305 Scientists, 2006). The following microscopic features were recorded for each structural domain:

• Type of microstructure: magmatic, submagmatic, crystal-plastic, cataclastic, or metamorphic.
• Morphology of grain boundary: straight, curved, serrate, polygonal, complex, or varied.
• Intensity of magmatic fabric: isotropic, weak, moderate, or strong.
• Intensity of static recrystallization: absent, weak, strong, partial, or complete.
• Presence of submagmatic fractures: absent, rare, or common.
• Intensity of crystal-plastic undulose extinction: absent, weak, moderate, strong, complete, patchy, or subgrains.
• Morphology of crystal-plastic subgrain boundaries: straight, curved, serrate, or polygonal.
• Presence of crystal-plastic deformation twinning: absent, rare, or common.
• Intensity of crystal-plastic dynamic recrystallization: absent, weak, strong, or complete.
• Intensity of overall crystal-plastic fabric:
• 0 = absent.
• 1 = weakly foliated/​lineated.
• 2 = strongly foliated/​lineated.
• 3 = porphyroclastic/​protomylonitic.
• 4 = mylonitic.
• 5 = ultramylonitic.
• Sense of shear estimated from crystal-plastic fabric, described as normal (n), reverse (r), dextral (d), or sinistral (s).
• Clast/matrix ratio (in percent) of cataclasite/brittle fracture.
• Size (in millimeters) of clasts in cataclasite/brittle fracture.
• Intensity of cataclastic fabric and intensity rank:
• 0 = undeformed.
• 1 = minor fracturing with no significant GSR.
• 2 = moderate fracturing without GSR.
• 3 = dense anastomosing fracturing with incipient GSR.
• 4 = well-developed GSR with evidence for clast rotation (independent particulate flow).
• 5 = cataclastic.

Hard rock geochemistry

Sampling and analysis of igneous rocks

Sample preparation

Representative samples of igneous rocks were analyzed for major and trace element concentrations during Expedition 336 using inductively coupled plasma–atomic emission spectroscopy (ICP-AES). Approximately 10 cm³ samples were cut from the core with a diamond saw blade. During this expedition, both the geochemistry sample and the thin section billet were taken from a quarter-cut core sample in the same interval. All outer surfaces were polished with a diamond-impregnated disk to remove surface contamination by saw marks and altered rinds resulting from drilling. Each sample was then placed in a beaker containing trace-metal-grade methanol and ultrasonicated for 15 min. After the methanol was decanted, the samples were washed in deionized water for 10 min in an ultrasonic bath and then were further ultrasonicated for 10 min in Barnstead deionized water (~18 MΩ·cm). The cleaned pieces were dried for 10–12 h at 110°C.

The cleaned, dried samples were crushed to <1 cm chips between two disks of Delrin plastic in a hydraulic press. The chips were ground to a fine powder in a tungsten carbide SPEX 8000M mixer/mill or, for larger samples, a SPEX 8515 shatterbox. According to Expedition 304/305 Scientists (2006), contamination from the tungsten carbide mills is negligible for the elements analyzed during this cruise. After the chips were ground, a ~1 g aliquot of the sample powder was weighed on a Mettler Toledo dual balance system and ignited at 1025°C for 4 h to determine weight loss on ignition (LOI). The estimated uncertainty of LOI values is ~0.2 mg (0.02 wt%).

The following protocol essentially follows the shipboard procedure described in ODP Technical Note 29 (Murray et al., 2000). After determination of LOI, 100.0 ± 0.2 mg aliquots of the ignited whole-rock powders were weighed and mixed with 400.0 ± 0.5 mg of LiBO₂ flux that had been preweighed on shore. Standard rock powders and full procedural blanks were included with unknowns in each ICP-AES run. All samples and standards were weighed on the Cahn C-31 microbalance (designed to measure on board), with weighing errors estimated to be ±0.05 mg under relatively smooth sea-surface conditions. After that, 10 mL of 0.172 mM aqueous LiBr solution was added to the mixture of flux and rock powder as a nonwetting agent to prevent the cooled bead from sticking to the crucible. Samples were then fused individually in Pt-Au (95:5) crucibles for ~12 min at a maximum temperature of 1050°C in an internal-rotating induction furnace (Bead Sampler NT-2100). After cooling, beads were transferred to 125 mL high-density polypropylene (HDPE) bottles and dissolved in 50 mL 10% HNO₃, aided by shaking with a Burrell wrist-action bottle shaker for 1 h. Following digestion of the bead, the solution was passed through a 0.45 µm filter into a clean 60 mL wide-mouth HDPE bottle. Next, 2.5 mL of this solution was transferred to a plastic vial and diluted with 17.5 mL of 10% HNO₃ to bring the total volume to 20 mL. The final solution-to-sample dilution factor was ~4000. For standards, the stock solutions were placed in an ultrasonic bath for 1 h prior to final dilution to ensure a homogeneous solution.

Analyses

Major (Si, Ti, Al, Fe, Mn, Mg, Ca, Na, K, and P) and trace (Ba, Sr, Zr, Y, V, Sc, Cu, Zn, Co, Cr, and Ni) element concentrations of standard and unknown samples were determined with a Teledyne Leeman Labs Prodigy ICP-AES instrument. Wavelengths used for sample analysis during Expedition 336 are provided in Table T2. These wavelengths were selected on the basis of the quality of calibration lines, including signal (sample)-to-noise (blank) ratios. The plasma was ignited at least 30 min before each run of samples to allow the instrument to warm up and stabilize. A zero-order search was then performed to check the mechanical zero of the diffraction grating. After the zero-order search, the mechanical step positions of emission lines were tuned by automatically searching with a 0.002 nm window across each emission peak using basalt laboratory standards BAS-140 (Sparks and Zuleger, 1995) or BAS-206 (Shipboard Scientific Party, 2003) in 10% HNO₃. During the initial setup, BAS-140 was used to select an emission profile for each peak to determine peak-to-background intensities and set the locations of background levels for each element. The Prodigy software uses these background locations to calculate the net intensity for each emission line. Photomultiplier (PMT) voltage was optimized by automatically adjusting the gain for each element using BAS-140.

The ICP-AES data presented in the site chapters were acquired using the Gaussian mode of the Prodigy software. This mode fits a curve to points across a peak and integrates the area under the curve for each element measured. Each sample was analyzed four times from the same dilute solution (i.e., in quadruplicate) within a given sample run. For elements measured at more than two wavelengths, we either used the wavelength giving the better calibration line in a given run or, if the calibration lines for both wavelengths were of similar quality, used the data from both and reported the average concentration.

Data reduction

Following each run of the instrument, the measured raw-intensity values were transferred to a data file and corrected for instrument drift and procedural blank. Drift correction was applied to each element by linear interpolation between the drift-monitoring solutions run every fourth analysis. After drift correction and blank subtraction, a calibration line for each element was calculated using the results for the certified rock standards. Element concentrations in the samples were then calculated from the relevant calibration lines.

Estimates of the accuracy and precision of major and trace element analyses during Expedition 336 are based on replicate analyses of check standards (BAS-140 and MRG-1), the results of which are presented and compared with published data in Table T3. Run-to-run relative standard deviation by ICP-AES was generally ±3% for major elements and ±10% for trace elements. Exceptions typically occurred when the element in question was near background levels.

Total carbon and nitrogen analysis

Total carbon and nitrogen contents were also obtained for the rock samples. Contents of these elements were determined using a Thermo Finnigan Flash EA 1112 carbon-hydrogen-nitrogen-sulfur (CHNS) analyzer. Aliquots of 10 mg of rock powder samples were weighed and placed in a tin container and then combusted at 900°C in a stream of oxygen. Nitrogen oxides were reduced to N₂, and the mixture of CO₂ and N₂ was separated by gas chromatography (GC3) and detected by a thermal conductivity detector. The gas chromatograph (GC) oven temperature was set at 65°C. Calibration was based on the synthetic standard sulfanilamide, which contains 41.81 wt% C, 16.27 wt% N, and 18.62 wt% S. The standard deviation of carbon, nitrogen, and sulfur concentrations was less than ±0.1%.

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