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doi:10.2204/iodp.proc.335.102.2012

Igneous petrology

Rock description procedures during Expedition 335 closely followed those used during IODP Expeditions 304/305 (Blackman, Ildefonse, John, Ohara, Miller, MacLeod, and the Expedition 304/305 Scientists, 2006) and 309/312 (Teagle, Alt, Umino, Miyashita, Banerjee, Wilson, and the Expedition 309/312 Scientists, 2006). These, in turn, were based on ODP Leg 209 and earlier “gabbro” legs (ODP Legs 118, 147, 153, and 176) to maintain a relatively high degree of uniformity. As during Legs 176, 206, and 209 and Expeditions 304/305 and 309/312, core descriptions were performed by the entire igneous petrology team working together. Each member was responsible for one or more aspects of the description (igneous contacts, textures, mineral modes, and habits) to ensure consistency of recorded observations throughout the core, but commonly the entire team would work together, particularly when defining units and contacts.

Recovered core was described macroscopically and microscopically, and its characteristics were entered into the LIMS database through the DESClogik portal. Key information was entered into the Section-Unit summary tab in DESClogik. This information was then used for the production of VCDs, which summarize the description of each section of core (see “Core descriptions”). A key to symbols used on the VCDs is given in Figure F6.

Igneous units and contact logs

The first step in describing core was the identification of unit boundaries on the basis of the presence of contacts, chilled margins, changes in primary mineralogy, color, grain size, and structural or textural variations. Igneous units in Hole 1256D were numbered continuously from the end of Expedition 312, starting with Unit 1256D-96. Lithologically and texturally similar pieces from consecutive core sections were curated as belonging to the same unit. In order to preserve important information about igneous stratigraphy without defining an unreasonable number of units within a single core, subunits were designated in cases where there were marked changes in texture without accompanying changes in mineralogy, or vice versa. In addition, crosscutting veins and thin dikes were generally designated as subunits. Where contacts deviated from horizontal within the core reference frame, their depth was logged at their midpoint. The igneous unit and contact log (see Table T4 in the “Site 1256” chapter) provides information about unit boundaries and a brief description of each unit. For each unit, the table lists unit number, depth (in meters below seafloor) of its top, core-section-interval and piece number of the top of the unit, unit thickness, lithology, a description of the upper and lower boundaries, and a unit description.

Macroscopic core description

Macroscopic descriptions were divided into the following categories.

Lithology

Plutonic rocks

Plutonic rocks were classified on the basis of abundance, grain size, and texture of their primary minerals (as inferred prior to alteration), based on the International Union of Geological Sciences (IUGS) system (Streckeisen, 1974; Le Maitre, 1989; Le Maitre et al., 2002). This classification defines the following rocks (Figs. F7, F8):

• Troctolite: olivine + plagioclase >95%, olivine >10%, and plagioclase >10%.

• Olivine gabbro: olivine + plagioclase + clinopyroxene, none of which is <5%.

• Gabbro or diorite: plagioclase + clinopyroxene >95%, plagioclase >10%, clinopyroxene >10%, and quartz <5%.

• Gabbronorite: plagioclase + clinopyroxene + orthopyroxene, none of which is <5%.

• Quartz diorite: quartz 5%–20% of quartz + alkali feldspar + plagioclase (QAP), with alkali feldspar <10% of QAP.

• Tonalite: quartz 20%–60% of QAP, with alkali feldspar <10% of QAP.

• Trondhjemite: tonalite with total mafic mineral content <10%.

In the IUGS classification, diorite is distinguished from gabbro by the anorthite content of plagioclase, with diorites having plagioclase containing <50 mol% An and gabbros having plagioclase containing >50 mol% An. Because this cannot be characterized during macroscopic description, we used the following convention: if a gabbroic rock contained quartz (<5%) or primary amphibole, indicating a relatively 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 to divide the rock types more accurately on the basis of significant differences rather than arbitrary cutoffs based on the abundance of a single mineral. We have attempted to follow as closely as possible the descriptions from Leg 209 (Kelemen, Kikawa, Miller, et al., 2004) and Expeditions 304/305 (Blackman, Ildefonse, John, Ohara, Miller, MacLeod, and the Expedition 304/305 Scientists, 2006) and 309/312 (Teagle, Alt, Umino, Miyashita, Banerjee, Wilson, and the Expedition 309/312 Scientists, 2006) to facilitate intersite comparison.

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

• Disseminated oxide = 1%–2% Fe-Ti oxide.

• Oxide = >2% Fe-Ti oxide.

• Olivine-bearing = 1%–5% olivine.

• Orthopyroxene-bearing = 1%–5% orthopyroxene.

• Troctolitic = 5%–15% clinopyroxene; >20% olivine.

• Olivine-rich = >70% olivine.

• Anorthositic = >80% plagioclase.

Additional descriptive modifiers are defined as follows:

• Leucocratic = light colored, high proportions of plagioclase.

• Micro = dominant grain size < 1 mm.

• Doleritic = fine- or medium-grained gabbroic rocks with dominant ophitic or subophitic textures.

One rock type recovered during Expedition 335 contains high proportions (85%–95%) of apparently albitic plagioclase associated with minor amounts of Fe-Ti oxides, titanite, amphibole, and epidote; quartz appears to be absent. These rocks have been classified as albitite.

Volcanic rocks

For volcanic and hypabyssal rocks, we used the following definitions:

• Basalt: all igneous rocks of basaltic composition in the grain size range glassy to fine grained.

• Dolerite: holocrystalline, fine- to medium-grained rocks of basaltic composition with well-developed subophitic or ophitic textures.

In English language usage, the term “dolerite” is European in origin and functionally equivalent to the North American usage of diabase, which is the IODP standard term. However, in Japanese, the term diabase has a distinctly different meaning, referring to strongly altered (green) basaltic rocks, and is expressed differently in Kanji script. This usage of diabase is also prevalent in Europe. We therefore agreed to use dolerite.

Basalt was divided according phenocryst content, using the following convention:

• Aphyric = <1% phenocrysts.

• Sparsely phyric = 1%–5% phenocrysts.

• Moderately phyric = >5%–10% phenocrysts.

• Highly phyric = >10% phenocrysts.

If present, phenocryst phases were placed as modifiers in front of the rock name with a hyphen in between. If <1% phenocrysts is present the rock is given the modifier “aphyric.”

Mineralogy

Plutonic rocks

In oceanic plutonic rocks, the primary rock-forming minerals are olivine, plagioclase, clinopyroxene, orthopyroxene, amphibole, Fe-Ti oxide, sulfide, and quartz. The following data are recorded in the LIMS database for each primary silicate:

• Visually estimated modal percent: in fresh rocks this represents the modal mineralogy as observed; in (partially) altered rocks this represents the estimated igneous modes prior to alteration. Where a mineral occurs in trace quantities (i.e., too low to assign a meaningful percentage), 0.1% is recorded. Accessory phases are also noted where observed. Modal estimates were made independently for each phase by a different team member and summed. If the total deviated significantly from 100%, the unit was reexamined by the team and estimates were adjusted. Where totals were close to 100%, the mode of the most abundant mineral (generally plagioclase) was adjusted, retaining the original estimates of phases that occur in minor abundance (generally oxides, olivine, and/or orthopyroxene). The rationale behind this procedure was that the absolute uncertainty in estimating modal proportions is largest for the most abundant minerals.

• Grain size: minimum, median, and maximum for each mineral phase.

• Mineral shape: euhedral, subhedral, and anhedral. Where oxides and sulfides form aggregates, they are divided into angular aggregates, amoeboid aggregates, and interstitial aggregates.

• Mineral habit:

• 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.

• Interstitial.

• Poikilitic.

The first four terms apply predominantly to subhedral or euhedral grains, the latter two generally to anhedral grains.

Volcanic rocks

In volcanic and hypabyssal rocks, the groundmass, phenocrysts (if any), and vesicles were described. For the groundmass, grain size was recorded using the following definitions:

• 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.

For phenocrysts, the abundance (in percent); maximum, minimum, and median grain size (in millimeters); and shape were recorded for each phase. For vesicles, abundance (in percent); vesicularity; size distribution; minimum, maximum, and modal size (in millimeters); roundness (rounded, subrounded, or well rounded); sphericity (highly spherical, moderately spherical, or slightly spherical or elongate); filling (in percent); and fill composition were documented. If vesicles are elongate, the direction is noted.

Contacts

For contacts between units, the type, definition, geometry, and interpretation were described. Where the contact was not recovered this was noted.

Contact types are

• Grain size: units on either side have markedly different grain sizes,

• Modal: units on either side have markedly different mineral proportions, or

• Color: units on either side have markedly different primary (i.e., not alteration-related) color.

If contacts were characterized by combinations of the above parameters, the terms were combined (e.g., “grain size and modal contact”).

Where contacts are obscured by deformation and metamorphism, they are called

• Sheared: an interval with deformation fabric is in contact with an undeformed interval,

• Foliated: both intervals have deformation fabrics, or

• Tectonic: the contact appears to be the result of faulting.

Contact definitions describe how well defined a contact is, using the terms sharp, gradational, and sutured (contacts where individual mineral grains interlock across the contact). Contact geometry can be planar, curved, or irregular.

Following description, contacts are interpreted as being extrusive, intrusive, or igneous. The latter term is used for contacts in plutonic rocks where the units on either side of the contact were interpreted to form part of the same igneous package (e.g., a modal contact between cumulate layers or a grain size contact in a graded sequence).

The term dike refers to any sharp, well-defined, and relatively thick (>1 cm) crosscutting feature formed by injection of magma. This contrasts with igneous vein, which describes a thin (<1 cm) crosscutting feature formed by injection of magma with generally less well defined contacts. Dikes and veins are generally designated as subunits, as described above in “Igneous units and contact logs.”

Texture

Textures are defined on the basis of three categories: grain size, grain size distribution, and the relationships between different grains.

Grain sizes were 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.

For plutonic rocks, grain size distributions are classed as equigranular where all minerals are of similar size and inequigranular where grain size varies significantly. Inequigranular textures are further divided into seriate (continuous range of crystal sizes), varitextured (domains with contrasting grain size), or poikilitic (relatively large oikocrysts enclosing smaller crystals, termed chadacrysts, of one or more other minerals). For volcanic rocks, grain size distribution applies to phenocrysts only, using the terms unimodal where all phenocrysts are of similar size, bimodal where they define two size populations, or seriate where they form a continuous range of sizes.

The following terms were used to describe the textural relationships between different silicate grains (Fig. F9):

• Granular: aggregation of grains of approximately equal size.

• Intergranular: coarser grains (typically plagioclase) form a touching 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.

• Subophitic: partial inclusion of plagioclase in clinopyroxene.

• Ophitic: total inclusion of plagioclase in clinopyroxene.

• Comb structure: comblike arrangement of crystals growing inward from a contact.

• Dendritic: branching arrangement of elongate crystals.

Similar to silicate minerals, the textures of oxide and sulfide minerals are 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.

Layering, where present, is divided into modal layering and grain size layering; when neither term describes the observations well, the term “layering (other)” is used, and the nature of layering is described in the comments. In all cases, the geometry of layering is described as being sharp, gradational, or irregular.

Thin section descriptions

Each thin section was photographed in both plane-polarized light (PPL) and cross-polarized light (XPL) (see TS_ORIGINAL_IMAGES_335_335(312) in IMAGES in “Supplementary material”). Thin section descriptions closely follow the procedure for macroscopic core descriptions. Where a thin section contained areas with different primary (i.e., not alteration-related) lithology, mineralogy, and/or texture, these were defined as domains (e.g., Domain 1, Domain 2, and so on). For thin sections with multiple igneous domains, a map of the domains is shown in the full thin section photomicrograph (see Fig. F10 for an example). Domains were described separately and their relative abundance was noted.

The following data were recorded and entered into the LIMS database through separate tabs within the Thin Section workbook in DESClogik.

Lithology and texture

• Rock name (based on thin section observations), using the same definitions as those for macroscopic descriptions.

• Number of igneous domains within the thin section.

• Nature of igneous domains, if any (e.g., contact between two units, mix of two lithologies in one section, or presence of texturally different regions within one thin section).

• Igneous domain relative abundance (in percent).

• Igneous domain number within the thin section (igneous Domain 1, igneous Domain 2, igneous Domain 3, and so on); this parameter identifies each domain described in the mineralogy tab.

• Igneous domain lithology name. If only one domain is present, this is identical to the rock name.

• Igneous domain grain size modal name (glassy, cryptocrystalline, microcrystalline, fine grained, medium grained, coarse grained, or pegmatitic).

• Igneous domain grain size distribution (equigranular, seriate, varitextured, or poikilitic).

• Igneous domain texture (granular, subophitic, ophitic, granophyric, porphyritic, intergranular, intersertal, variolitic, or granoblastic. The latter refers to a fine-grained granular metamorphic texture describing a high-grade metamorphic overprinted, as defined during Expedition 312 [Teagle, Alt, Umino, Miyashita, Banerjee, Wilson, and the Expedition 309/312 Scientists, 2006]).

Mineralogy

• Igneous domain number: the igneous domains defined in the lithological-textural description.

• Igneous domain mineralogy: abundance (in percent) of primary minerals preserved; estimated abundance (in percent) of primary minerals prior to alteration; computed value of mineral replacement by alteration; minimum size, maximum size, median size, shape, habit, and special features of primary minerals, using the same conventions as during macroscopic description; individual comments for primary minerals; and absorption colors/pleochroism for clinopyroxene, orthopyroxene, and amphibole.

For plagioclase, a qualifier for zoning was recorded, using the following convention:

• 0 = none.

• 1 = zoning is rare and weakly developed.

• 2 = abundant zoning that can range from weak to strong.

• 3 = nearly ubiquitous, generally strong zoning.

The type of zoning in plagioclase was also documented:

• Continuous: zoning is optically continuous from core to rim.

• Discontinuous: zoning occurs from core to rim, but with distinct break(s).

• Patchy: zoning occurs in patches randomly throughout the grain.

• Oscillatory.

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