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doi:10.2204/iodp.proc.309312.102.2006

Igneous petrology

Core curation and shipboard sampling

To preserve important features and structures, all core was examined before being split. Each piece was numbered sequentially from the top of each core section and labeled on the outside surface. Broken core pieces that could be fitted together were assigned the same number and lettered consecutively from the top down (e.g., 1A, 1B, and 1C). Rarely, composite pieces may have occupied more than one section. Plastic spacers were placed between pieces with different numbers. The presence of a spacer may represent a substantial interval without recovery. If it was evident that an individual piece could not have rotated about a horizontal axis during drilling, an arrow was added to the label, pointing to the top of the section. Nondestructive physical property measurements, such as magnetic susceptibility, natural gamma ray (NGR) emission, and digital imaging of the exterior of whole-core pieces were made before the core was split (see “Physical properties”). Pieces were split with a diamond-impregnated saw in such a way that important compositional and structural features were preserved in both the archive and working halves. After splitting, digital images of the core were taken using the Geotek DIS before being described on VCD forms. To minimize contamination of the core with platinum group elements and gold, describers removed jewelry from their hands and wrists before handling. After the core was split and described, the working half was sampled for shipboard physical properties, paleomagnetic studies, thin sections, X-ray diffraction (XRD), inductively coupled plasma–atomic emission spectroscopy (ICP-AES), and shore-based studies. Each section of core was examined consecutively by three teams of describers, focusing first on igneous characteristics, then on alteration, and finally on structure. Each team described all sections of hard rock cores. The igneous team recorded the depth interval and length of each piece in a piece log (Table T1).

Igneous units and contact logs

The first step in describing cores was the selection 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 (see Tables T13 and T35 in the “Site 1256” chapter). Unit boundaries of volcanic rocks were generally chosen to reflect different volcanic cooling units, although we were forced by limited recovery in some cases to arbitrarily decide the exact location of a unit boundary within an interval where the lithology above and below the interval was different. In order to preserve important information about volcanology without defining an unreasonable number of units within a single core, subunits were designated in cases where there were common changes in texture without accompanying changes in mineralogy. Where narrow dikes occur, each dike is labeled as a subunit. Within a core, if there is no significant change in dike mineralogy, all dike pieces are labeled as a single subunit. For Expedition 309 only, lithologically similar pieces from consecutive cores are labeled as different subunits. Subunits are designated in the VCDs, and their descriptions are included within the overall written description of the unit. Igneous units and contact logs (see Tables T13 and T35 in the “Site 1256” chapter) provide information about unit boundaries and a brief description of each unit. For each unit, the table lists unit number, core number, section number, interval location in the core, piece number(s), depth (in meters below seafloor) of the upper contact (calculated from the curated depth of the top of the core), physical type of upper contact, minimum thickness of the unit (calculated from piece lengths), and rock type of each unit. An additional table is used to record fresh or altered glass occurrence (see Table T14 in the “Site 1256” chapter). Units were numbered continuously from the end of Leg 206, starting with Unit 1256D-27.

Visual core description

General description

VCD forms (Fig. F3) were used to describe each section of igneous rock cores. The procedure closely follows Leg 206 core descriptions to provide consistency. A key to symbols used on the VCDs is given in Figure F4. On the VCDs, the following are displayed from left to right:

1. A scale from 0 to 150 cm,
2. Piece number,
3. Photograph of the archive half of the core,
4. Piece orientation,
5. Locations of samples selected for shipboard studies,
6. Boundaries and number of lithologic units,
7. Structures,
8. Structural measurements,
9. Presence of glass or altered glass,
10. Phenocrysts (for volcanic rocks),
11. Constituent minerals (for plutonic rocks) and constituent minerals of phenocrysts (for volcanic rocks),
12. Grain size (of groundmass for volcanic rocks and of constituent minerals for plutonic rocks), and
13. Alteration intensity.

Vertically oriented pieces are indicated on the form by an upward-pointing arrow to the right of the appropriate piece (column 4). Locations of samples selected for shipboard studies are indicated in the Shipboard Studies column (5), using the following notation:

• XRD = X-ray diffraction analysis.
• ICP = ICP-AES analysis.
• TSB = petrographic thin section.
• PP = physical property analysis.
• PMAG = paleomagnetic analysis.

The Lithologic Unit column (6) displays the locations of boundaries between units and subunits and the unit designator (consecutive downhole subunits are designated by letters after the unit number; e.g., 1, 2a, 2b, etc.). The Structure column (7) displays graphical representations of structural types from the key in Figure F4. The Structural Measurement column (8) highlights structures that have been measured. Boundaries of lithologic units and subunits were drawn on the VCDs across columns 4–13 with solid lines denoting unit boundaries.

Grain sizes are defined as follows:

• p = pegmatitic (>30 mm).
• cg = coarse grained (5–30 mm).
• mg = medium grained (1–5 mm).
• fg = fined grained (0.2–1 mm).
• µx = microcrystalline (0.1–0.2 mm).
• cx = cryptocrystalline (<0.1 mm).
• g = glassy.

Volcanic rocks

During Expedition 309, we defined igneous rocks with glassy to fine-grained (average groundmass grain size = <1 mm) characteristics as volcanic rocks and the root term “basalt” was used. During Expedition 309, for holocrystalline, relatively fine to medium-grained rocks (1–5 mm) with doleritic texture (textures transitional between intergranular and ophitic), the root word “dolerite” was used (Fig. F5).

During Expedition 312, we refined the lithologic nomenclature slightly. For the majority of Expedition 312 samples, this nomenclature is consistent with that used during Expedition 309. The only exceptions may be in the naming of basaltic rocks that are present within the gabbro section below 1406.6 mbsf. We have also refined the definition of the rarely used (during Expedition 309/312) term “dolerite.” We used the following terms:

• Basalt: All igneous rocks of basaltic composition in the size range glassy to fine grained.
• Dolerite: Holocrystalline, medium-grained rocks of basaltic composition with well-developed intersertal, subophitic, or ophitic textures. In English language usage, this term is European in origin and functionally equivalent to the North American usage of “diabase,” which is the IODP standard term. In Japanese, however, 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 and recommend its adoption by IODP as a uniquely defined term that can be readily understood worldwide.

VCDs of volcanic rocks contain a text description of each unit in each section of core that includes

• Expedition, site, hole, core number, core type, and section number;
• Depth of the top of the section in meters below seafloor;
• Unit number;
• Rock name;
• Summary description of unit as it appears in the section, including rock name and rock type (e.g., pillow basalt or sheet flow);
• Piece numbers included in the unit;
• Type of contacts when recovered;
• Munsell color;
• Phenocryst mineral abundance and size;
• Groundmass grain size and texture;
• Vesicle abundance;
• Nature of alteration;
• Information about abundance and filling of veins;
• Description of structures in the rock; and
• Any additional comments.

Units and subunits were named on the basis of groundmass texture and abundance of primary minerals. In cryptocrystalline to microcrystalline rocks, there is a clear distinction between phenocrysts and groundmass crystals. These were described based on the identification of phenocrysts in hand sample following the criteria listed below:

• Aphyric (<1% phenocrysts)
• Sparsely phyric (1%–5% phenocrysts)
• Moderately phyric (5%–10% phenocrysts)
• Highly phyric (>10% phenocrysts)

Rock names were further classified by types of phenocrysts present (e.g., sparsely plagioclase-olivine phyric, in which the amount of olivine exceeds the amount of plagioclase). In coarser grained rocks, those with seriate to equigranular textures, phenocrysts are difficult to distinguish. Therefore, in cases where grain size was fine grained or larger, we did not use these modifiers unless there was a clear distinction between phenocrysts and groundmass crystals. Rock color was determined on a wet, cut surface of the rock using Munsell color charts.

An estimate of the percentage of vesicles and their average size was included in the VCDs. Vesicularity is described according to the abundance, size, and shape (sphericity and angularity) of the vesicles. The subdivision was made according to the following:

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

Pillow lavas were identified by curved chilled margins oblique to the vertical axis of the core or, when these margins were absent, by variolitic textures, curved fractures, and microcrystalline or cryptocrystalline grain size. Sheet flows were defined as sections of core <3 m thick with the same rock type and grain size that increased toward the center of the flows. Massive units were defined as continuous intervals >3 m thick of similar lithology. Minor dikes were identified when intrusive contacts with the host rock were present. Below the last identifiable flow units (>1060.9 mbsf), all basaltic units were identified as sheeted dikes (see discussion in “Igneous petrology” in “Expedition 312” in the “Site 1256” chapter). For Expedition 309, sheeted dike units were identified as massive basalts. The term “massive” was not used for Expedition 312. Other rock types distinguished were volcanic breccias and hyaloclastites.

Igneous fabrics include layering, lamination, and lineation for rocks exhibiting a preferred orientation of mineral grains. The term “glomerocrysts” was used for mineral aggregates.

Plutonic rocks

Plutonic rock descriptions during Expedition 312 closely followed those used during IODP Expedition 304/305. These, in turn, were based on procedures from ODP Leg 209 and earlier “gabbro” legs, so there could be a relatively high degree of uniformity. Plutonic rock units were defined on the basis of primary igneous rock types and textures. Mineral modes were visually estimated, using a binocular microscope when necessary. In many cases, several subintervals with sharp to gradational changes in grain size and/or mode over a few centimeters were grouped into a single lithologic unit.

Rock classification

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) (Fig. F6). For pervasively altered rocks, the “primary assemblage” was estimated based on textural evidence, mostly in thin section.

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) and Expedition 304/305 (Blackman, Ildefonse, John, Ohara, Miller, MacLeod, et al., 2006) to facilitate intersite comparison.

For gabbros, the following modifiers based on modal mineralogy are commonly used (not all were used during Expedition 312):

• Disseminated oxide gabbro (Fe-Ti oxide 1%–2%)
• Oxide gabbro (Fe-Ti oxide > 2%)
• Olivine-bearing gabbro (olivine 1%–5%)
• Olivine gabbro (olivine > 5%)
• Orthopyroxene-bearing gabbro (orthopyroxene 1%–5%)
• Gabbronorite (orthopyroxene > 5%)
• Troctolitic gabbro (olivine 5%–15%)
• Troctolite (clinopyroxene < 5%)
• Olivine-rich troctolite (>70% olivine)

Additional descriptive modifiers include

• Leucocratic (light colored, high proportions of plagioclase)
• Anorthositic (>80% plagioclase)
• Micro (dominant grain size < 1 mm)
• Doleritic (fine- or medium-grained gabbroic rocks with dominant ophitic or subophitic textures)

Several additional rock names are also used in this classification:

• Trondhjemite: Leucocratic rocks with >20% quartz and <1% K-feldspar (a restricted part of the IUGS “tonalite” field of the system). This usage is in keeping with previous usage in the ocean crust literature.
• Tonalite: IUGS tonalitic rocks (as defined by Streckeisen, 1974) that contain >10% mafic and related minerals; in practice, tonalites commonly contained significantly >10% mafic minerals.
• Quartz diorite: Some thin dikes in which plagioclase is the dominant (or only) feldspar and quartz forms 5%–20% of (quartz + feldspar).
• Quartz-rich oxide diorite: Nonstandard term that we applied to an unusual lithology that occurs at the top of the gabbro section and in some small dikes in the sheeted dike section. Initially described as tonalites or oxide-tonalites, these rocks have ~49 wt% SiO₂, 18 wt% FeO (total), and 4 wt% TiO₂. They are compositionally identical to the most evolved FeTi basalt compositions.

On 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 F4. If the assemblage consists of secondary minerals that completely obliterate the primary 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, Fe-Ti oxide, plagioclase, quartz, 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; and
• Mineral habit: euhedral, subhedral, anhedral, and interstitial.

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

Igneous textures

Textures are defined on the basis of grain size, grain shape and habit, preferred mineral orientation, and mineral proportions. 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 (oikocrysts are relatively large crystals of one mineral, and chadacrysts enclose smaller crystals of one or more other minerals).

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 1:2–1:3)
• Tabular (aspect ratio 1:3–1:5)
• Elongate (aspect ratio > 1:5)

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

Oxides and sulfides

The abundance of primary Fe-Ti oxide and sulfide in the core is visually estimated but usually confirmed in thin section.

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

• Disseminated (scattered grains or grain clusters)
• Interstitial (interstitial to silicates)

Oxide shapes are divided into euhedral, anhedral, angular aggregates, amoeboid aggregates, and interstitial lenses. Euhedral and anhedral are used when it appears that isolated individual grains are present.

Igneous structures

Igneous structures include layering lineation, gradational grain size variations, gradational modal variations, gradational textural variations, and breccias. Layering describes planar changes in grain size, mode, or texture within a unit.

Dikes/Veins

The term dike refers to any crosscutting feature formed by injection of magma. Vein describes epigenetic mineralized fractures. Veins are described in “Alteration.” Thin dikes are often designated as subunits, as described above in “Igneous units and contact logs.”

Contacts between lithologic intervals

The most common types of contacts are those without chilled margins. These include planar, curved, irregular, interpenetrative, sutured, or gradational. Sutured refers to contacts where individual mineral grains interlock across the contact. Contacts obscured by subsolidus or subrigidus deformation and metamorphism 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 sections

Thin sections of igneous rocks were studied to complete and refine the hand specimen observations. An example of the thin section description form is given in Figure F7. Thin section descriptions are included in “Core descriptions” and are available from the IODP Janus database. Digital photomicrographs were taken to document features described in thin sections. A list of available images, any of which can be obtained from the IODP Data Librarian, is given in the photomicrograph log (see PHOTOLOG.XLS in “Supplementary material”).

Thin section descriptions include textural features; grain size of phenocrysts and groundmass (volcanic rocks) or constituent minerals (plutonic rocks); mineralogy, abundance, and type of glomerocrysts; and descriptions of inclusions. For glassy to aphanitic lavas with cryptocrystalline to microcrystalline grain sizes, modal proportions are based on visual estimates. During Expedition 309, mineral modes were determined by point counting (1000 points per thin section). During Expedition 312, modal estimation was conducted for phenocrysts only in volcanic rocks and for some plutonic rocks using whole thin section scanned images and Adobe Photoshop software. Details of this method are described below in “Estimating modal proportions in basaltic thin sections.”

Volcanic rocks

Volcanic rocks are described as holohyaline (100% glass) to holocrystalline (100% crystals). The terms phyric and glomeroporphyritic indicate the presence of phenocrysts and clusters of phenocrysts, respectively. For a continuous range in grain size, the texture is seriate. In cases where there is no significant grain size difference between groundmass crystals and somewhat larger and more euhedral crystals, which do not adhere to the definition of phenocrysts, the term microphenocryst is used.

In holohyaline to hypohyaline rocks, glasses were divided into four distinct types:

• Fresh glass (amber in transmitted polarized light and isotropic in transmitted cross-polarized light, commonly found in the outermost parts of preserved chilled margins);
• Dark (due to abundant crystallites) interstitial volcanic glass of basaltic composition termed tachylytic;
• Glass that contains abundant fibrous spherulites; and
• Glass that has been altered to clay minerals.

Spherulites form a meshwork of spheroidally arranged aggregates of acicular microcrystals emanating from a nucleus. Spherulites may crystallize directly from the melt in response to a large amount of undercooling or can be formed through devitrification of the glass in wet conditions (Fowler et al., 2002). When microlites are present, the following terms were used to describe textures:

• Variolitic (fanlike arrangement of divergent microlites)
• Intergranular (olivine and pyroxene grains between plagioclase laths)
• Intersertal (glass between plagioclase laths)
• Subophitic (partial inclusion of plagioclase in clinopyroxene)
• Ophitic (total inclusion of plagioclase in clinopyroxene)

Flow textures were described as follows:

• Trachytic (subparallel arrangement of plagioclase laths in the groundmass)
• Pilotaxitic (aligned plagioclase microlites embedded in a matrix of granular and usually smaller clinopyroxene grains)
• Hyalopilitic (aligned plagioclase microlites with glassy matrix)

Description of habits for plagioclase and clinopyroxene groundmass crystals was adapted from the practice during Leg 206 (Wilson, Teagle, Acton, et al., 2003) and in ODP Hole 896A, Leg 148 (see fig. 11 in Shipboard Scientific Party, 1993c). Four types of habits were identified:

1. Cryptocrystalline aggregates of fibrous crystals,
2. Comb-shaped or sheaflike plumose crystals,
3. Granular-acicular subhedral to anhedral crystals, and
4. Prismatic to stubby euhedral to subhedral crystals.

During Expedition 309/312, we referred to Type 1 and Type 2 textures as fibrous.

Plutonic rocks

Thin section descriptions of plutonic rocks complement and refine hand specimen observations. Data are recorded in IODP-format thin section descriptions and summarized in the thin section spreadsheet (see “Thin sections” in “Core descriptions”). Crystal size measurements are calibrated against a micrometer scale. Inclusions, overgrowths, and mineral zonation are noted, and an apparent order of crystallization may be suggested in the comments section. Relative abundances of accessory minerals such as oxides, sulfides, apatite, and zircon are noted. Modal orthopyroxene not readily identified in hand samples was observed in the thin sections of many gabbroic rocks, causing their reclassification as orthopyroxene-bearing gabbro or gabbronorite.

The same general types of data are collected from thin sections as from hand specimens and the terminology used is mostly the same.

Additional textural terms include

• Equigranular (crystals in contact with each other and similar in size)
• Intergranular (coarser touching grains form a framework with interstices filled by crystalline material [or glass for volcanic rocks])
• Ophitic (characterized by systematic enclosure of plagioclase by optically continuous clinopyroxene)
• Doleritic (used during Expedition 309 for textures transitional between intergranular and ophitic) (Fig. F5)
• Seriate (having a continuously variable range of grain size)

Crystal shape terms include

• Equant (aspect ratio < 1:2)
• Subequant (aspect ratio 1:2–1:3)
• Tabular (aspect ratio 1:3–1:5)
• Elongate (aspect ratio > 1:5)

Estimating modal phenocryst proportions in basaltic thin sections

Rationale

More than 80% of phenocryst-bearing basaltic thin sections from Hole 1256D have <1% phenocrysts. At this level, visual estimates and even formal point counts of phenocryst abundances have high absolute errors. For example, for a mineral abundance of 1%, a 1000 point count has an absolute error close to ±0.5% (50% relative) (Howarth, 1998). During Expedition 312, we developed a simple, rapid (5–10 min per thin section) method of phenocryst modal estimation using scanned thin section images. This method is both less labor intensive and significantly more accurate and precise than point counting for phenocryst contents up to ~5%. As with any method based on a single thin section, the applicability of the method to larger volumes depends on an assumption of homogeneity that may or may not be valid.

Equipment

The following equipment was routinely used during Expedition 312:

• Thin section scanner (Epson Perfection 4870 photo);
• Networked computer with Adobe Photoshop CS and Microsoft Excel; and
• Overhead projector transparency film, which takes ink evenly and crisply.

Method

Preparing and scanning acetates

Preparation of full-page, whole thin section scanned images from standard thin sections by placing them on (or between, for crossed polars) polaroid sheets in a standard scanner is a standard procedure during IODP “hard rock” expeditions. For phenocryst abundances less than ~5%, phenocryst outlines can be rapidly traced onto standard acetate film using an appropriate high-density marker pen. Not all films or pens work well for this procedure. We used Canon transparency type E film and black marker pens from the Mitsubishi Pencil Corporation, product code PA-121TEW.

Prepared acetates were then scanned and saved as native Adobe Photoshop files.

Separating phases from background

With the image open in Adobe Photoshop, initiate the threshold function: go to Image→Adjustments→Threshold. (These instructions are for Macintosh computers; the procedure for PC computers may differ slightly.) In the resulting threshold dialog box, adjust the threshold level until phenocrysts show up as solid black surrounded by a white background. For the scan in Figure F8A, in which polarizing film was seated between the scanner and thin section, the best threshold is usually just to the dark side (lower value) of the peak associated with the gray film. Click OK to return to a black-and-white image, and crop the image using the crop tool. This requires some care for irregular thin sections, as the standard tool will only crop to rectangular areas. Use the eraser tool to remove all unwanted black from the cropped area, including the line that marks the edge of the sample (Fig. F8A, F8C, F8D). Make sure that the only black that is left corresponds to the phenocrysts.

In some cases, the shape of the thin section is too irregular to crop simply without losing important phenocrysts. In such cases, draw the rectangular line around the irregular thin section boundary and fill the margin between the rectangle and thin section boundary with the black color (Fig. F8B). After that, follow the same procedure used with the normal thin section case.

Using Photoshop histogram window to perform point count

Go to Window→Histogram. Click the options button in the resulting window; this is a right-pointing squat arrow set in a small circle in the upper-right corner. Select the “Expanded View” and “Show Statistics” options. Select “Uncached Refresh.” It is important to do this before every count is taken from the image.

This procedure will create a histogram with large peaks at the extreme left and extreme right, representing black and white, respectively, and almost imperceptible against the edges of the window (Fig. F8D). If there are visible peaks in the center of the histogram, it is necessary to reselect "uncached refresh" or repeat the threshold operation from the original cropped image.

Once the histogram is ready, carefully move the crosshair across the histogram window. Note that the values of “level” and “count” should change, but “pixels” (the total number of pixels in the image) should not change. Move the crosshair to the extreme left of the histogram, where the level reads zero. Note the value of “counts” at level 0, which is the number of black pixels in the image, and calculate the total percentage of phenocrysts as the number of black pixels divided by total pixels.

In case of an irregular thin section, after erasing whole phenocrysts in the photoshop image get the number of black pixels of the margin with the procedure described above. Then, to calculate modal proportion of phenocrysts, subtract black pixels of the gap from total black pixels, which include pixels from both phenocrysts and the black margin.

Dealing with multiple phases

There are several ways to deal with the presence of more than one phenocryst phase. We experimented with color shading but were not able to find pens with sufficient color density. Separate acetate-filled sheets could be prepared for each phase of interest, but this approach is time consuming. For small numbers of phenocrysts, we found it simple to initially measure total phenocrysts for the original scan and then to remove phases one by one from the image using the Photoshop eraser tool.

If an oxide is present in a thin section, it is possible to evaluate the modal proportion of the oxide by directly applying the method described above to whole thin section scan images without additional drawing of phenocrysts on overhead transparency film and scanning (Fig. F9). This is possible because only oxides have the black color among minerals in a thin section.

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