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doi:10.2204/iodp.proc.345.102.2014 Igneous petrologyRock description procedures during Expedition 345 closely followed those used during IODP Expeditions 335, 304/305, and 309/312 (Expedition 335 Scientists, 2012; Expedition 304/305 Scientists, 2006; Expedition 309/312 Scientists, 2006). These, in turn, were based on Ocean Drilling Program (ODP) Leg 209 and earlier “gabbro” legs (ODP Legs 118, 147, 153, and 176) to maintain a relatively high degree of consistency of description between legs (Shipboard Scientific Party, 1989, 1993a, 1995, 1999a). 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 (Shipboard Scientific Party, 2003, 2004). Each member was responsible for one or more aspects of the description (igneous contacts, textures, mineral modes, and habits) and data entry on the entire core to ensure consistency of recorded observations throughout. Commonly, the entire team worked together, particularly when defining units and contacts. Recovered core was described both 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 the DESClogik Macroscopic template. This information was then used for the production of VCDs, which summarize the description of each section of core (see “Core descriptions”). The key to symbols used on the VCDs is given in Figure F7. Piece descriptions and igneous unitsCore characterization was based on the description of individual pieces from a recovered section of a core. If neighboring pieces were homogeneous with respect to magmatic features, they were combined to one lithologic interval and accordingly described macroscopically in the corresponding DESClogik tab. Lithologically and texturally similar pieces from consecutive core sections were curated as belonging to the same lithologic interval. The upper and lower ends of each piece were directly taken from the curated piece log available within DESClogik. If within a given piece a contact (e.g., chilled margin, change in primary mineralogy, color, grain size, and structural or textural variation) was recognized, a new lithologic interval was defined and described. In this way, all of the important information about the igneous stratigraphy was preserved on a truly descriptive basis. Where contacts and/or boundaries deviated from horizontal within the core reference frame, their depth was logged at the midpoint on the cut face. A summary of the lithologic intervals, including the description of the igneous contacts, was logged into the corresponding DESClogik tab. The numbering of lithologic intervals started with “1” for each hole and consecutively continued throughout the whole core. Within a given interval, no subdivisions were made. Based on the description of the different lithologic intervals and their contacts, igneous units of coherent rock type were defined on a broader, more interpretative scale after discussions within the whole scientific party. Description, characterization and explanations, and reasons why lithologic intervals were combined into units for each hole separately at the beginning of each igneous hole report. For those cores for which units were defined, tables are presented at beginning of the descriptions of the hole, listing the lithologic intervals in each hole and their classification into units. For each lithologic interval, core-section-interval, interval depth (in mbsf) of top and bottom, piece numbers, a complete lithologic name (name of the principal lithology and modifiers, if used), and a description of the contacts is given. Average grain sizes and modal contents of the principal minerals for individual intervals can be obtained from the VCDs (see “Core descriptions”). Macroscopic core descriptionLithologyPlutonic rockPlutonic rock was classified on the basis of mineral abundance, grain size, and texture (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 rock types (Figs. F8, F9):
In the IUGS classification, diorite is distinguished from gabbro by the anorthite content of plagioclase, with diorite having plagioclase containing <50 mol% An and gabbro having plagioclase containing >50 mol% An. Because the anorthite content of was not determined from 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. For plutonic rock rich in chromian spinel, we followed the classification of Leg 209 (Shipboard Scientific Party, 2004): a sample that contains >10% chromian spinel was called chromitite. We use the rock name “oikocryst gabbro” for troctolite or, less commonly, gabbro containing >40% clinopyroxene oikocrysts. This term describes gabbroic rock characterized by a high concentration of centimeter-scale clinopyroxene oikocrysts, which makes this rock unique and easily recognizable macroscopically. Because of the uniqueness of this rock, we introduced this name to distinguish from normal gabbro, based wholly on the mineral mode, in order to identify oikocryst gabbro in the VCDs and lithology distribution diagrams. For the definition of oikocryst, see below. 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 attempted to follow as closely as possible the descriptions from Leg 209 and Expeditions 304/305, 309/312, and 335 to facilitate intersite comparison. For gabbroic rock, the following modifiers based on modal mineralogy were used:
Additional descriptive modifiers were defined as
Volcanic and hypabyssal rockFor volcanic and hypabyssal rock, we used the following definitions:
In English language usage, the term dolerite is European in origin and functionally equivalent to the North American usage of the term 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 to phenocryst content, using the following convention:
If present, phenocryst phase names were used as modifiers in front of the rock name with a hyphen. If <1% phenocrysts, the rock was given the modifier “aphyric.” MineralogyPlutonic rockIn oceanic plutonic rock, the primary rock-forming minerals are olivine, plagioclase, clinopyroxene, orthopyroxene, amphibole, Fe-Ti oxide, sulfides, and rarely quartz. The following data were recorded in the LIMS database for each primary silicate:
Some troctolite and lesser gabbro contains spectacular clinopyroxene oikocrysts. These oikocrysts are a macroscopic feature with a distinctive texture clearly observable in the core. The oikocrysts have a well-defined spherical or augen-like habit that shows a clear boundary to the troctolitic or, less commonly, gabbroic matrix. Chadacrysts (enclosed grains) within the oikocrysts are clearly visible and typically are plagioclase. Olivine only occurs at the outermost rim. Because of their well-defined habit and clear relationships with other minerals, the oikocrysts are easily identified with the naked eye. Clinopyroxene is defined as poikilitic if characterized by interstitial with the matrix. Poikilitic clinopyroxene grains are often very irregular in shape, forming large irregular clusters, with much less defined contact relations relative to the matrix. In more olivine rich gabbros, these clinopyroxenes also include olivine as chadacryst, irrespective of whether they are located in the central part or in the rim of the clinopyroxene. The macroscopic observations concerning the oikocrysts are consistent with microscopic observations. Volcanic rockIn volcanic and hypabyssal rock, the groundmass, phenocrysts (if any), and vesicles were described. For the groundmass, grain size was recorded using the following definitions:
For phenocrysts, the abundance (in percent); maximum, minimum, and median grain size (in millimeters); and shape were recorded for each phase. Phenocrysts and groundmass crystals were described based on the identification of phenocrysts in hand sample following the criteria listed below:
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). 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 was noted. ContactsFor igneous contacts between units, the type, definition, geometry, and interpretation were described. We noted when the contact was not recovered. For more details on contacts see “Structural geology.” Contact types include
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/or metamorphism, they were called
Contact definitions describe how well defined a contact is, using the terms sharp, gradational, and sutured. “Sutured” refers to contacts in which individual mineral grains interlock across the contact. Contact geometry can be planar, curved, or irregular. Following description, contacts were interpreted as being extrusive, intrusive, or igneous. The latter term is used for contacts in plutonic rocks where the lithologies on either side of the contact were interpreted to form part of the same igneous sequence (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 individual intervals. TexturePlutonic rockTextures were defined on the basis of three categories: grain size, grain size distribution, and the relationships between different grains. Grain sizes were defined as
For plutonic rock, grain size distributions (Fig. F10) were classed as
For defining textures, we used the terms
Similar to the silicate minerals, the textures of oxide and sulfide minerals are described in terms of grain size and their relationship to adjacent minerals. In plutonic rock, 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. If layering is present, the geometry of layering is described in the comments (e.g., sharp, gradational, or irregular). For more details see also “Structural geology.” Volcanic rockTextures were defined on the basis of three categories: grain size, grain size distribution, and the relationships between different grains. Grain sizes were defined as follows:
For volcanic rock, grain size distribution applies to phenocrysts only, using the terms “unimodal” when all phenocrysts are of similar size, “bimodal” when two size populations are defined, or “seriate” when they form a continuous range of sizes. The following terms were used to describe the textural relationships between different silicate grains (Fig. F10):
Similar to the silicate minerals, the textures of oxide and sulfide minerals were described in terms of grain size and their relationship to adjacent minerals. Thin section descriptionsEach thin section was photographed in both plane light and under crossed polars (see thin section images in WEBIMAGE in “Supplementary material”). Thin section descriptions closely follow the procedure for macroscopic core description. 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, etc.). For thin sections with multiple igneous domains, a map of the domains is shown in the full thin section photomicrograph (Fig. F11), with different domains described separately and their relative abundance noted. For each hole, a table is provided listing the corresponding thin sections, the number and nature of the individual domains, the characteristics of the contact between the domains, and a link to the corresponding image of the thin section with the domain boundaries marked. The following data were recorded and entered into the LIMS database through separate tabs within the thin section workbook in DESClogik. Lithology and textureThe following definitions were used for plutonic, volcanic, and ultramafic rock:
Textural definitions were used for the three different rock types (plutonic, volcanic, and ultramafic) individually, as outlined in the following sections. Plutonic rockFor the domain grain size distribution in plutonic rock, we used the terms equigranular and seriate. For the description of the texture of each domain, the terms granular, subophitic, ophitic, granophyric, and poikilitic were used. Volcanic rockFor the textures in volcanic rock, we followed the definitions from Expedition 309/312 (Expedition 309/312 Scientists, 2006). Volcanic rock is 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 rock, glass was divided into four distinct types:
As grain size distribution, the terms equigranular or seriate were used. For groundmass, the following terms were used to describe textures:
Flow textures were described as
MineralogyIgneous domain mineralogy was described using 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, maximum, and 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:
The type of zoning in plagioclase was also documented as
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