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doi:10.2204/iodp.proc.304305.102.2006

Igneous petrology

As during ODP Legs 176 and 209, measurements were performed by the entire igneous petrology team working in tandem. For consistency, qualitative measurements (especially the selection of igneous contacts) were made by the entire team.

Igneous units were defined on the basis of primary igneous rock types and textures. Mineral modes were visually estimated. In many cases, several subintervals with subtle centimeter to few centimeter gradational to sharp changes in grain size and/or mode were grouped into a single lithologic unit. The subtle variations within a unit are described in the text of the hard rock VCDs. Mineral habits, igneous structures, and igneous fabrics were also recorded, as well as the nature of igneous contacts. Observations were recorded in spreadsheets for each lithologic unit in the core. Details of the individual measurements are given below.

Rock classification

Igneous rocks were classified on the basis of abundance, grain size, and texture of their primary minerals based on the International Union of Geological Sciences (IUGS) system (Streckeisen, 1974; Le Maitre, 1989; Le Bas and Streckeisen, 1991) (Fig. F4). Plutonic rock names were assigned on the basis of primary phases present prior to alteration. For pervasively altered rocks, the term “primary assemblage” is used to refer to the estimated prealteration mineral assemblage. Minor modifications to the IUGS system were made to subdivide 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) to allow comparison of these records.

Basalts were subdivided according to the presence or absence of glass, grain size, and phenocryst content. If present, phenocrysts were put as modifiers in front of the rock name with a hyphen in between, according to their abundance. In the IUGS classification, “basalt” is defined on the basis of mineral mode or composition; grain sizes are variable. Because the basalts are commonly aphyric and very fine grained to aphanitic, mode determination was not feasible, so composition has been used to identify basalts. Following IUGS classification, basalts plot in a field on a SiO₂-total alkali (Na₂O + K₂O) diagram (TAS) defined by SiO₂ and total alkali coordinates 45, 0; 45, 5; 52, 0; and 52, 5 (Le Bas and Streckeisen, 1991). “Diabase” is an intrusive basaltic rock emplaced as dikes or sills and characterized by subophitic to ophitic texture of plagioclase laths and augite. Diabase occasionally displays an aphanitic to very fine grained chilled margin (grain size <<1 mm) and a fine-grained (<1 mm) interior. Even in intervals of glassy basaltic composition rocks, if chilled margins or intrusive contacts were recognized the rocks were classified as diabase.

Description of the gabbros follows that of Leg 176 (Dick, Natland, Miller, et al., 1999), where the modifier “disseminated oxide” is used when the abundance of Fe-Ti oxide is 1%–2% and the modifier “oxide” is used when the abundance is >2%. If the olivine content ranges between 1% and 5%, then “olivine-bearing” is the modifier, and if it exceeds 5%, the rock is called olivine gabbro. Gabbros with orthopyroxene content between 1% and 5% are called orthopyroxene-bearing gabbro, and samples with >5% orthopyroxene are called gabbronorite. The term “troctolitic gabbro” is used to describe olivine gabbros with 5%–15% clinopyroxene, and the rock name troctolite is used for rocks with <5% clinopyroxene. “Olivine-rich troctolite” is a rock that contains >70% olivine and relatively low modal plagioclase and clinopyroxene. Olivine-rich troctolite commonly contains subhedral to subrounded olivine and interstitial to poikilitic plagioclase and clinopyroxene in variable proportions. The term “leucocratic” is used to indicate high proportions of plagioclase. “Anorthositic” is used for gabbros with >80% plagioclase. The modifier “micro-” is used to distinguish gabbroic rocks with a dominant grain size of <1 mm (e.g., microgabbro, microtroctolite, and microgabbronorite).

Leucocratic rocks with >20% quartz and <1% K-feldspar (a restricted part of the tonalite field of the IUGS system) are called trondhjemites in keeping with previous usage in the ocean crust literature.

Where alteration in ultramafic rocks is so extensive that estimation of the primary phase assemblages is not possible, the rock is called serpentinite. If, in primary assemblages, pseudomorphs and textures can be recognized in ultramafic samples, even though they are partially or completely replaced, the rock name used is based on the reconstructed primary assemblage and is termed serpentinized (i.e., serpentinized dunite).

On the hard rock VCDs, the rock names as described above are given at the top of each interval description; the IUGS names calculated from the mode are given in the text. Symbol swatches used in the VCDs are shown in Figure F5. If a mafic rock exhibits the effects of dynamic metamorphism such that the assemblage consists of secondary hydrous minerals that completely obliterate the protolith mineralogy and texture or if the rock is made up of recrystallized primary minerals such that the original igneous protolith cannot be recognized, the appropriate metamorphic rock names are used. The methods for describing the metamorphic and structural petrology of the core are outlined in subsequent sections of this chapter.

Primary minerals

The primary rock-forming minerals recovered are olivine, orthopyroxene, clinopyroxene, spinel, Fe-Ti oxide, plagioclase, and amphibole. The following data are recorded on the VCDs (see “Core descriptions”):

• Visually estimated modal percent of the primary minerals
• Grain size
• Average crystal size for each mineral phase
• Mineral shape: equant, subequant, tabular, and elongated
• Mineral habit: euhedral, subhedral, anhedral, and interstitial

Accessory phases are also noted, and the above five classes of observations are collected. The modal percentage of each mineral includes both the fresh and altered parts of the rocks interpreted to represent that mineral.

Five major classes of rock (peridotite, pyroxenite, gabbro, diabase, and basalt) are delineated on the basis of their igneous texture. Textures are defined on the basis of grain size, grain shape and habit, preferred mineral orientation, and mineral proportions. The dominant grain size for all plutonic rocks is recorded as fine grained (<1 mm), medium grained (1–5 mm), coarse grained (5–30 mm), or pegmatitic (>30 mm).

Igneous textures

We use the following textural terms: “equigranular,” “inequigranular,” and “intergranular” (only visible in thin sections). Inequigranular textures may be further described as seriate (continuous range of crystal sizes) or poikilitic (relatively large crystals of one mineral, oikocryst, enclosing smaller crystals of one or more other minerals, chadacrysts).

The terms “euhedral,” “subhedral,” “anhedral,” and “interstitial” are used to describe the shapes of crystals interpreted to preserve their igneous morphology. Crystal shapes are divided into four classes:

• 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)

Spinel occurs in various shapes that are divided into three categories:

• Equant: the shape is equidimensional with flat and/or curved surfaces.
• Interstitial: a transitional category between vermicular and equant. The outer surfaces of these spinel grains are often concave outward and have thin tips departing from the corner of the grain.
• Vermicular: has intricate shape forming symplectic intergrowths with pyroxenes and/or olivine.

Igneous fabrics that are distinguished include “lamination” and “lineation” for rocks exhibiting a preferred orientation of mineral grains, “clusters” for mineral aggregates, and “schlieren” for lenses of igneous minerals.

The textural distinction between diabase and microgabbro is based on the presence or absence of subophitic or ophitic textures.

For basaltic rocks, the proportions and characters of phenocrysts, grain sizes, and vesicles define the following textures.

Phenocrysts are described for each mineral according to their abundance. The classifications for basalts are as follows:

• Aphyric (<1% phenocrysts)
• Sparsely phyric (1%–2% phenocrysts)
• Moderately phyric (2%–10% phenocrysts)
• Highly phyric (>10% phenocrysts)

Vesicularity is described according to the abundance, size, and shape (sphericity and angularity) of the vesicles. The subdivision was made according to the following scale:

• Nonvesicular (<1% vesicles)
• Sparsely vesicular (1%–5% vesicles)
• Moderately vesicular (5%–20% vesicles)
• Highly vesicular (>20% vesicles)

Groundmass crystallinity was classified according to the following scale:

• Aphanitic (individual grains cannot be distinguished)
• Microcrystalline (groundmass can be seen but not identified)
• Fine grained (grains <1 mm but can be identified)
• Medium grained (>1 mm grain sizes)

Oxide and sulfide minerals

The abundance of primary Fe-Ti oxide and sulfide in the core is visually estimated.

Textures of oxide and sulfide minerals are described in terms of the habit of the mineral and its relationship with adjacent minerals. Oxide habits in hand sample are divided into the following categories:

• Disseminated (scattered grain to grain clusters with no pronounced fabric)
• Interstitial network (oxides that occur interstitial to the silicates)

Oxide shapes in hand sample are divided into the following categories: euhedral, anhedral, angular aggregates, amoeboidal aggregates, and interstitial lenses. “Euhedral” and “anhedral” are used when it appears that isolated individual grains are present.

Relative sulfide abundance in hand specimen is visually estimated using a binocular microscope.

Igneous structures

Igneous structures noted in the core description include layering, gradational grain size variations, gradational modal variations, gradational textural variations, and breccias. “Layering” is used to describe planar changes in grain size, mode, or texture within a unit. Grain size variations are described as normal if the coarser part was at the bottom and as reversed if the coarser part was at the top. Modal variations are described as normal if mafic minerals are more abundant at the bottom and as reversed if mafic minerals are more abundant at the top.

Dikes/Veins

The term “dike” refers to any crosscutting feature that formed by injection of magma and/or juvenile fluids, and the word “vein” describes epigenetic mineralized fractures. Veins are described in both “Igneous petrology” and “Metamorphic petrology.”

Contacts between lithologic intervals

The most common types of contacts are those without chilled margins. These are planar, curved, irregular, interpenetrative, sutured, or gradational. “Sutured” refers to contacts where individual mineral grains are interlocking across the contact. In many cases, contacts are obscured by subsolidus or subrigidus deformation and metamorphism; they are called “sheared” if an interval with deformation fabric is in contact with an undeformed interval, “foliated” if both intervals have deformation fabrics, or “tectonic” if the contact appears to be the result of faulting. Indistinct contacts are also described as “diffuse” in some cases.

Thin section description

Thin sections of igneous rocks were examined to complement and refine the hand-specimen observations. In general, the same types of data are collected from thin sections as from hand specimens, and a similar terminology is used. All data are recorded in the thin section spreadsheet (see “Core descriptions”) and summarized in IODP-format thin-section descriptions. Crystal sizes are measured using a micrometer scale. The presence of inclusions, overgrowths, and zonation is noted, and the apparent order of crystallization is suggested in the Comments section for samples with appropriate textural relationships. The presence and relative abundance of accessory minerals such as oxides, sulfides, apatite, and zircon are noted. Abundant modal orthopyroxene was also observed in thin section in many gabbroic rocks but was not readily identified in hand sample. Thus, many rocks identified as gabbro in hand sample were subsequently classified as gabbronorite after thin section examination.

Electron beam analysis

A binocular microscope equipped with an electron beam energy-dispersive X-ray spectrometer (miniprobe) was brought on board for Expeditions 304 and 305 and used for mineral identification and characterization. The same system was used at sea during a survey cruise and proved to be a valuable aid in sample characterization (E. Hellebrand, pers. comm., 2005).

The miniprobe system consists of a binocular microscope equipped with a standard cold cathode electron source manufactured by Cambridge Image Technology Ltd., a vacuum chamber, and a silicon drift detector (Fig. F6). The silicon drift detector is a state-of-the-art high-resolution X-ray detector and is cooled by an internal Peltier element (no liquid nitrogen required). It can therefore operate at room temperature and gives excellent performance even at very high count rates. The particular detector technology was derived from the development of the Mars APXS spectrometer for the Sojourner and Pathfinder missions. In connection with a thin beryllium entrance window (8 µm), it guarantees a wide energy response to incoming radiation. The silicon drift detector in the packaged configuration is suitable for X-rays in the energy range between 1 and 30 keV. Hardware control and data analysis are performed by an energy dispersive system (EDS) computer with EMRA (version 10.4). The spectral resolution of the EDS corresponds to that of an scanning electron microscope and lies, in average, at 180 eV.

The sample chamber can hold thin sections as well as grain mounts. Samples do not require polishing or coating, as is as needed for standard electron microprobe analysis, before analysis.

An internal Ge standard is used for calibration. Measurements were performed using the following parameters:

• Current = 100 µA
• Voltage = 20 kV
• Beam diameter = 100 µm
• Measurement time = 100 s

A minimum of ~10,000 gross counts is required to maintain a good analysis.

The main use of the miniprobe onboard was to identify minerals in ship-made thin sections. In addition, some spinels from peridotites from Hole 1309B were measured to obtain their Cr# (molar Cr/[Cr + Al]), an indicator of the degree of melting (Hellebrand et al., 2001). Spinel compositions obtained from the miniprobe do not yield true values but need an additional external correction. A well-known set of homogeneous spinels from abyssal peridotites, measured on the Jeol 8900 microprobe of the University of Mainz (Germany), was used for reference (Fig. F7). These spinels cover the variational range of spinel Cr# in abyssal peridotites. There is a good linear correlation (R² = 0.90) between the analyzed values and reference values. Based on this linear correlation, the obtained Cr numbers were corrected as

Cr# = 0.9594 × Cr# (miniprobe) – 012.833.

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