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doi:10.2204/iodp.proc.324.102.2010

Igneous petrology

Physical volcanology

Previous information regarding the nature of volcanism on Shatsky Rise is scant. The best evidence comes from core recovered during Leg 198 (Hole 1213B), which reveals a series of fine-grained, massive volcanic units of several meters thickness. These units were tentatively interpreted as "sills" as a result of their intercalation with sediment and the presence of a few associated chill zones. Also present were rare, smaller volcanic features of <1 m thickness that resemble small (0.1–0.2 m diameter) glass-rimmed pillow lava units. However, there remained considerable speculation regarding the nature of volcanism on Shatsky Rise and, at that stage, it was unclear whether the volcanism conforms more readily to eruptive styles typical of either subaerial or submarine volcanism, or possibly a combination of the two. The combined occurrence of both subaerial and submarine volcanism in the development of submarine large igneous provinces (LIPs) has been previously documented in, for example, the Kerguelen Plateau (Southern Indian Ocean; Frey et al., 2000; Keszthelyi, 2002) and the accreted Wrangellia Terrane (western North America; Greene et al., 2010). Accordingly, Expedition 324 to Shatsky Rise aimed to capture key volcanological elements that would aid in determining the nature and evolution of subaerial and submarine volcanic components in this largely unstudied LIP.

The procedures for core description outlined here are similar to those developed by the ODP Leg 197 Shipboard Scientific Party (2002b), who drilled and cored the Emperor Seamounts. In our initial description of each core we used the nongenetic scheme outlined in Figure F5, but as our knowledge and understanding of Shatsky Rise basement lithostratigraphy improved through description of the core, we evolved a more relevant genetic volcanic classification based on the scheme outlined in Figure F6. The emphasis of our shipboard studies was to produce an integrated picture of the style of volcanism and environmental setting of each site. This was achieved by systematic rock descriptions and identification of key effusive, volcaniclastic, and igneous textures that are known to be diagnostic of specific modern physical volcanic processes. Accordingly, the physical description of volcanic rocks and deposits recovered during Expedition 324 required a multistage process. The first step involved defining boundaries of lithologic units by either visual identification of actual lithologic contacts, or by inference of the position of such contacts using observed changes in volcanic characteristics and/or volcaniclastic features (e.g., chill zones, flow tops, etc.). This was followed by general description of the lithology, lithologic variation, and characteristic igneous textures and vesicle distributions. All of these macroscopic observations were then combined with those from detailed petrographic microscopy of the key igneous units.

Background

Effusive basaltic volcanism in both the subaerial and submarine environment produces a range of common features associated with the development and inflation of flow lobes of varying sizes. Depending on the environment, these include pillow lavas, pahoehoe or rubbly flow surfaces and, where lava interacts directly with seawater, hyaloclastites and breccias. Unsurprisingly, the vast majority of research and literature regarding the evolution of basaltic lava morphologies relates to those observed on land. Continental flood basalt provinces (CFBPs) and other terrestrial eruptions often comprise individual eruptive units consisting of large, internally massive inflation sheets of typically 5–30 m thick, or else an accumulation of numerous stacked flow lobes that build up into single eruptive units of 5–30 m thick (commonly termed simple and compound lavas, respectively) (Walker, 1971). Hawaiian and Icelandic eruptions are also described in terms of surface and cross-sectional morphologies as either pahoehoe or a'a; the development of these morphologies is dependent upon endogenic factors (e.g., eruption rates, viscosity, strain rate, and degree of crystallinity) and exogenic factors (e.g., degree of inclination and topography of the surface upon which the lava propagates). By contrast, submarine eruptions are dominated by stacks of budding and anastomosing pahoehoe-like extrusions resulting in piles of rounded or tubular masses (Batiza and White, 2000), the most readily recognized component of these is the "pillow basalt" often referred to as "pillow lava." However, large inflation units of several meters thickness, as well as more laterally extensive extrusions, have also been observed; both these types have much more "sheetlike" morphologies (e.g., Lonsdale and Spiess, 1980; Mitchell et al., 2008).

Subaerial lava eruptions

Subaerial mafic lavas are commonly divided broadly into a'a- or pahoehoe-type flows (Macdonald, 1953, 1967), each displaying fundamentally different internal and external appearances. The morphological division is important because the mode of lava emplacement for a'a and pahoehoe flows is fundamentally different, this often being a response to differing eruptive environments. A'a flows advance like the treads on a bulldozer, contain a molten core, and are typified by thermally inefficient emplacement in open channels in which solidified lava crusts tend to become disrupted and mixed with the internal molten core. By contrast, pahoehoe inflation units (and their marine equivalent, "pillow lavas") develop an outer viscoelastic crust, and their cores are thus insulated during transport so that, prior to stagnation and cooling, they remain largely molten. The lava is internally transported by pathways within the propagating flows (or within lava tubes) to the active flow fronts, where it advances by inflating lobes with a continuous crust, "buckling," and then by sequential lobe-by-lobe emplacement (Walker, 1991; Hon et al., 1994). A wide range of intermediate flow types exists between the a'a and pahoehoe end-members. However, in drill core, these different types of lava can only be identified if centimeter-scale morphologic features such as glassy rinds, chill zones, and vesiculation patterns at the flow tops and bottoms are well preserved, recovered, and can be placed in context. In both cases, the flow cores are virtually indistinguishable from one another. Importantly, a'a-type flows are uncommon in ocean floor drill core, and the largest submarine flow types appear to be sheet flows or laterally extensive pahoehoe-like inflation units analogous to those found in terrestrial LIPs.

One feature unique to some subaerial lava stacks, as opposed to submarine examples, is the development of flow-top weathering profiles. These are usually apparent through development of oxidative alteration (reddening) that produces iron and aluminium oxyhydroxides. Weathering profiles typically develop during prolonged volcanic hiatuses or on abandoned posteruptive surfaces under suitable climatic and geomorphic conditions.

A'a flows are characterized by angular, spinose clinker at both the flow tops and bottoms (by contrast, pahoehoe flows are often characterized by smooth surfaces and sharp contacts between successive and/or adjacent units). Internal disruption during emplacement permits efficient degassing, and the crust, core, and base of a'a flows are therefore typically sparsely vesicular or nonvesicular. Transitional flow types (e.g., rubbly, slab, or toothpaste pahoehoe; Guillbaud et al., 2005) show some of the characteristics of both a'a- and pahoehoe-type flows.

Pahoehoe lavas are usually vesicular, often highly so, and frequently exhibit bulk porosities in excess of 20 vol% (typical range is from 20 to 60 vol%). Thin pahoehoe lobes (e.g., < 0.5–1 m) are often vesicular throughout and exhibit a gradual coarsening in vesicle size from the lobe margins toward the interior (Wilmoth and Walker, 1993). During development of pahoehoe lava fields, gas bubbles can frequently become trapped and entrained in the magma body as it cools from both above and below. Thus, thicker, inflated pahoehoe lobes of 5–30 m thickness, such as those documented in CFBPs, are often characterized by the threefold structure of a vesicular upper crust, a dense core, and a thinner vesicular lower crust (Aubele et al., 1988; Thordarson and Self, 1998; Jerram and Widdowson, 2005).

Subaerial eruptions entering water bodies and shallow subaqueous eruptions

Subaqueous lava flows and hyaloclastite associations have been documented where flows from land-based eruptions entered water (e.g., into the sea or freshwater lakes) or else are erupted directly into shallow submarine settings (e.g., ongoing eruption flowing from Pu'u `O'o, Hawaii; units of the Miocene Columbia River Basalt CFBP, which entered the sea; submarine magmatism associated with the emergence of the Surtsey eruptions, 1963–1967; and around the coasts of the Azores). On entering water or soft unconsolidated (e.g., wet) sediments, the eruptive style changes fundamentally and, depending on local conditions, advancement of the lava front typically generates either pillow basalt, hyaloclastites, peperites, or a combination of all three. Hyaloclastites and peperites are volcaniclastic sediments that form by quench fragmentation (Carlisle, 1963). Pillow basalts are a common product of these environments but can usually be distinguished from terrestrial pahoehoe inflation units by their much thicker glassy rinds (a consequence of a more efficient aqueous quenching process), and by the presence of lacustrine or marine sediments preserved in and between the pillows. Although less frequently documented, larger subaqueous flows may also be massive and sheetlike. Overall, the progression from pillow basalt to massive flow seems largely controlled by local flow rate, which is itself a function of effusion rate and topographic slope (Walker, 1992; Gregg and Fink, 1995).

Submarine lava eruptions

Submarine lavas are important because they represent the most abundant igneous rock on the surface of Earth, forming most of the ocean floor. As indicated, a close morphological similarity exists between pillow lava formed under water and pahoehoe lava formed on land, though the latter typically lacks, or has only a relatively thin glassy rind (i.e., glassy surface selvage). Submarine pillows occur at a variety of depths, are the product of a variety of volcanic settings (e.g., ocean spreading centers, off-ridge and isolated seamount edifices, and submarine portions of ocean islands and oceanic plateaus), and accordingly display a range of extrusive combinations and forms (Batiza and White, 2000). They are, for the most part, inaccessible to conventional observational methods (Lonsdale and Spiess, 1980; Mitchell et al., 2008) and so, by contrast with their subaerial counterparts, are relatively poorly documented.

As with subaerial examples, the eruptive styles of submarine lava flows are largely controlled by effusion rate and the local topography and slope onto which lava eruption takes place (e.g., Greg and Fink, 1995; Mitchell et al., 2008, and references therein). Basaltic magma typically erupts at relatively high temperatures and characteristically is fluid (low viscosity), allowing its relatively high gas content to be released effectively during lava emplacement. These gases may be preserved as vesicle layers and pipes that are often characteristic of submarine basaltic materials. However, because of increasing hydrostatic pressure with increasing depth of the seafloor, the degree of vesiculation decreases in deeper water eruptions (Dixon, 1995; Wallace, 1998). High hydrostatic pressure may also limit the possibility of explosivity from either gas release or steam generation caused by contact flash-heating of water upon the nascent lava. At depths deeper than 3 km, the hydrostatic pressure exceeds the critical pressure of water, effectively preventing steam explosions. Accordingly, pillow basalt formed in the upper 1000 m of the water column more commonly consists of moderately to highly vesicular lava and may show evidence of gas fracturing or fragmentation, whereas pillow basalts formed in much deeper water can usually be distinguished by a significantly lower vesicularity (<5 vol%), less fracturing and, occasionally, by the presence of pelagic rather than neritic marine sediment preserved in between the pillows.

Pillow lavas and massive inflation units

Pillow lavas are subaqueous extrusions resulting from individual budding, inflation, and separation of lava "pods" from point sources along an advancing lava front. They can accumulate in a variety of styles, depending upon effusion rate, number of point sources, and internal plumbing architectures within the growing volcanic edifice. However, pillows typically consist of discrete subrounded units of relatively small size (from ~0.2 to <1.0 m diameter), and, characteristically, their exteriors are entirely bounded by glassy rinds as a result of rapid cooling; their interiors often display internally radiating vesicle and joint patterns generated during solidification by cooling and contraction.

Larger single inflation units (~1–2 m diameter) can also develop (sometimes termed "lobate flows") by the same endogenous inflation process. Although these extrusions broadly resemble pillow lavas, they differ in that they have massive, coarser grained, and sparsely vesicular cores, often with pipe vesicle domains. The larger size permits a slower rate of cooling, more effective degassing, and maturation of the internal vesicle patterns. Accordingly, vesicle zoning is more concentrated in the upper regions of these types of inflation units and often occurs as a series of vesicle bands that develop through the inward migration of the cooling front; by contrast, the lower part of the inflation unit typically contains either sparse, poorly defined vesicle banding and/or "tear-drop" shaped vesicles at or just above the basal chill zone. Rather than having spherical or subspherical shapes, these "massive inflation units" tend to have flattened, oblate, or tubular shapes and are most probably generated as a series of interconnected semi-adjacent inflation "pods" fed simultaneously by internal plumbing. Their size and morphology are considered to be indicative of a higher effusion rate than that which generates pillow lavas. Individual units can cover an area of several square meters or significantly more where adjacent units merge either through internal plumbing conduits or as a result of rupture and coalescence. In those instances where they merge into a single entity, submarine images reveal they can develop a continuous lava flow surface displaying a dominantly botryoidal morphology.

"Massive flows" are thick (several meters or more) and are often laterally extensive, forming "sheetlike" and internally massive basalt units (van Andel and Ballard, 1979). They have a variety of surface features generated by deformation and disturbance of the solidifying lava crust while in a plastic/semiplastic state. Dimensions and internal features of massive flows can resemble those of larger subaerially erupted flows (e.g., those found in continental flood basalt provinces). These types of flows may be fed internally and achieve their size and extent by internal inflation and/or marginal budding. They often have texturally uniform massive cores, as thick as several meters, and are characterized by sparse vesicle layering and/or vertical vesicle pipes containing late-stage melt segregation material. Massive or sheetlike flows are most probably a response to particularly high effusion rates and/or local slope and other controlling topographic conditions. In the literature, these unit types are often termed "sheet flows," though this term implies a significant degree of lateral extension that cannot be determined readily from core alone. Nevertheless, in cases where recovered units are significantly thicker than that expected for the normal dimensions of pillow lava or lobate flows (~0.2–2 m) we prefer the nongenetic term "massive lava unit," an appellation that also avoids any confusion with the term "sheet flow" as employed in the CFBP literature.

Volcaniclastic deposits

Volcaniclastic deposits include a range of materials from rubbly, in situ volcanic debris to resedimented materials such as volcanic sands or tuffs (Figs. F5, F6). Volcanic materials of all sizes may be the direct products of eruptive processes (pyroclastic) or accumulations through processes involving transport, sorting, and (re)deposition (epiclastic). Pyroclastic material includes hydrovolcanic deposits formed by explosive interaction between magma and water as well as nonexplosive quench fragmentation processes (e.g., hyaloclastite and peperite). Epiclastic volcanic sediment forms by redeposition of volcanic detritus typically produced by erosion and/or transport of volcanic rocks.

Hyaloclastite literally means "glass-fragment rock" and includes all glassy fragmental debris formed by eruptions in which water has been involved. Hyaloclastite-producing environments of relevance to Shatsky Rise may include shallow-water phreatomagmatic eruptions that are generated at newly emergent volcanic islands and, most probably, volcaniclastic materials associated with the development and advancement of submarine lavas at greater water depths (Clague et al., 2009). Phreatomagmatic conditions can result in fragmentation of gas bubbles inflated by steam, producing on deposition thick layers of glass shards (i.e., syneruptive sedimentation). These shards may be sent into subaerial trajectories, or else form suspended "plumes" within the water column. In either case, the shards later settle on the surrounding land or seafloor, or are transported and deposited elsewhere (i.e., epiclastic sedimentation).

Depending on the nature of the eruption, welded and unwelded volcanic rubble and breccia also occur in these environments (Fisher and Schmincke, 1984). Submarine lavas are almost invariably associated with hyaloclastites and a variety of other fragmental volcanic debris, including breccias (Staudigel and Schmincke, 1984; Batiza and White, 2000). Hyaloclastite is rare at mid-ocean ridges but more common at seamounts where it may be the product of granulation by cooling contraction and surficial spalling of lava pillows and pods. Those environments characterized by volcanic breccias are also commonly associated with mass debris flows at various scales, and especially where the eruption occurs on significant seafloor topography.

Peperite is a distinct volcanic sediment facies occurring where submarine basaltic lava flows interact with unconsolidated sediment as they erupt into water or, more commonly, onto the seafloor (Skilling et al., 2002). The mingling of basaltic lavas and wet sediments produces distinct volcanic textures resulting from the physical interaction of lava and sediment (entrainment, baking, folding, chilling, etc.), as well as physical and chemical fragmentation and alteration through steam-rock interaction during flash heating of the water. Common features include quenched margins of basaltic lava clasts surrounded by sediment, as well as basaltic clasts with a variety of fragment morphologies, including "fluidal" (i.e., where fragments remained plastic enough to deform after deposition) or "jigsaw-fit" (i.e., where cooling and contracting fragments crack apart in situ). Lava-sediment interactions include features involving the mixing or mingling of molten or plastic lava with soft sediment, creating deformation features where lava injects or intrudes the seafloor substrate. In some instances, a lack of bedding or lamination is observed in sediments adjacent to pillow lavas or in the materials surrounding lava injections. A likely explanation is that fluidization of the fine-grained sediment, attributable to flash heating and vaporization of sediment pore water, took place during emplacement of the lava (Kokelaar, 1982). Such momentary fluidization causes sediment reconstitution, localized transport, redistribution, and, hence, the destruction of any original bedding structures.

Palagonite is a typical alteration product resulting from the interaction between water and basalt melt and is commonly associated with peperite volcanic facies where the small fragments of lava have reacted with the steam. Accordingly, the presence of peperites and their textures can provide important information regarding the nature of the eruptive environment because their occurrence is taken as evidence that the volcanism and the affected sedimentary environment existed contemporaneously.

Definition of lithologic units and volcanic successions

Our primary aim was to design an observational framework that permitted logging and recording of key data while avoiding nomenclature or terminologies that might incorrectly imply particular styles of effusive basaltic volcanism. Most lava type classifications are derived from land-based observations that consider both the lateral and vertical variations of eruptive units as well as their stacking relationships. Such refined classifications are neither realistic nor justifiable from core observation alone and may only become achievable through integration of multiple data sources alongside the core log description (e.g., petrography, downhole logging data, geochemical and paleomagnetic results, etc.). However, lithofacies associations provide one such avenue of nongenetic description that have been successfully applied to the description of a range of LIP types (e.g., Jerram and Widdowson, 2005; White et al., 2009) and therefore can assist in the interpretation of cored volcanics; the Wrangellia LIP is a particularly useful analog for Shatsky Rise (Fig. F7) because this example contains products of both submarine and subaerial eruptions, as well as instances where one passes into the other (Greene et al., 2010). Accordingly, we employ the term "stratigraphic unit" to describe successions of similar volcanic facies types such as pillow-lava stacks, packages of sheet flows, or hyaloclastic successions. Such lithologic units may often be petrogenetically or volcanically related, having developed as part of a single eruptive event. For instance, during advancement of a submarine flow, a hyaloclastite succession might evolve into a pillow basalt stack and then sheet flow successions; also, such facies associations can arise through natural lateral variation of eruption style across a single lava field. However, in the case of cored basaltic successions, these types of volcanic and petrogenetic relationships often become apparent only after consideration of additional petrographic, geochemical, or downhole logging data. In most instances, core material only permits identification of the smaller volcanic "lithologic" unit divisions (e.g., pillow unit, massive flow unit, etc.), so these are necessarily the basic unit of our descriptions.

Descriptive nomenclature

To aid efficient core description and enable later collation of the observed volcanic units into volcanologically meaningful successions (stratigraphic units), we adopted a simple two-fold numerical hierarchy. The term "lithologic unit" is adopted to define smaller features on the basis of criteria including the presence of lava contacts, chilled margins or identifiable flow tops, vesicle distributions, and intercalated volcaniclastic or sedimentary horizons. When considered together, these features typically define the key volcanic cooling or inflation units of various sizes and scales within the larger eruptive stratigraphy. Lithologic units identified during core logging are given consecutive downhole Arabic numbers (i.e., lithologic Units 1, 2, 3, etc.), irrespective of whether they are pillow, sheet, or massive flow types (i.e., a nongenetic cataloging system).

The term "stratigraphic unit" is applied where successions of consecutive cooling or inflation units of similar or shared evolutionary characteristics can be identified. In effect, these stratigraphic units combine similar types of eruptive products and provide a first step toward considering the volcanic stratigraphy in terms of packages of genetically related lithologic units and, ultimately, understanding the evolution of the volcanism. These packages are given consecutive downhole Roman numbers (i.e., stratigraphic Units I, II, III, etc.). Although we made every effort to ensure that the identified stratigraphic unit boundaries reflect individual lava packages or eruptive units, the term should not be considered as synonymous with a single eruptive event, but rather as a method of dividing the retrieved core into elements or sections displaying broadly similar volcanic characteristics (i.e., lithofacies type).

The most unequivocal evidence for dividing core into lithologic units is the presence of flow-to-flow contacts. Unfortunately, such contacts are not always common in low recovery sections. Alternatively, the presence of glassy material or fine-grained chill zones can provide a useful proxy for determining the presence or estimating the position of unit boundaries. Of particular use in sections of poor core recovery, or where the interior-most massive sections of units are preferentially recovered, are the changes in vesiculation that occur through inflation or cooling units of all sizes. To identify or estimate the position of lithologic unit boundaries using this method, we paid special attention to vesiculation characteristics by measuring or estimating (1) percentage volume of vesicles, (2) average vesicle size, (3) vesicle shape and sphericity, and (4) variations in vesicle sizes (i.e., fining or coarsening upward), these being examined at intervals appropriate for the variability shown by the core (typically across 10–100 cm intervals). The volume fraction of vesicles within the unit or section under consideration, together with modal shape and sphericity of vesicle populations, were estimated using appropriate comparison charts similar to those used by sedimentologists for determining grain size populations (i.e., Wentworth, 1922, classification scheme). Maximum and modal vesicle sizes were determined using a hand lens with a submillimeter graticule; in fact, it proved impractical to measure the minimum size since it always proved to be below the realistic limit of visual resolution.

In order to develop and refine this approach, we initially employed it for the description of archive core recovered from the previous Shatsky Rise expedition (Leg 198, Cores 198-1213B-28R through 33R). These cores were examined by the Expedition 324 shipboard party immediately prior to commencement of drilling, and the volcanologic and petrographic observations were recorded in templates that we designed and collated within the new descriptive data capture application (see "DESClogik: the new core description software for descriptive data capture"). From this application, data were uploaded into the LIMS database and used to generate hard rock VCD graphic reports (Figs. F8, F9). As far as possible, we sought to design the DESClogik templates (Fig. F10) in a manner that would ensure that the type of data recorded would be consistent with the volcanologic logging procedures described for ODP Legs 183 and 197 (Shipboard Scientific Party, 2000a, 2002b).

Lithology

Porphyritic basaltic rocks were named according to major phenocryst type, but only in those instances where the total abundance of phenocrysts was >1% (e.g., plagioclase basalt or olivine-pyroxene basalt). Aphyric rocks were not assigned any mineralogical modifier and retained instead a textural description (e.g., aphyric basalt). The term "phenocryst" was used for any crystal that was >1 mm in width and significantly (typically five times) larger than the average size of the groundmass crystals and that was euhedral or subhedral in shape. The term "microcryst" was used for crystals that were larger than the modal groundmass grain size but <1 mm. Where macroscopic observation was possible, microcrysts were also described in the DESClogik database under a suitable heading in the phenocryst table (Fig. F10A). The phenocryst abundance descriptors were further modified by including the names of phenocryst phases in order of increasing abundance. If the abundance of a particular type of phenocryst was >5% the mineral name was included without a modifier, whereas all other phenocryst names included a modifier; "olivine-bearing," for example. Thus an olivine-bearing plagioclase basalt (ol-bearing plag basalt) may contain 10% plagioclase and 1% olivine phenocrysts.

Volcanic units were identified using the criteria outlined above, and appropriate data were entered into the DESClogik database under the Volcanic Attributes tab (Fig. F10B).

Volcaniclastic deposits

We used "volcaniclastic" as a nongenetic term for any fragmental aggregate of volcanic parentage containing >60% volcaniclastic grains and <40% other types of clastic and/or biogenic material. This definition is necessarily broader than that typically used for pyroclastic deposits because the term "pyroclastic" normally applies only to products of explosive volcanic activity, including hydroclastic deposits formed by explosive interaction between magma and water/sediment and nonexplosive quench fragmentation (i.e., hyaloclastite and peperite). Our adopted definition also includes the volcanic detritus produced by erosion and/or transport of volcanic materials (i.e., epiclastic sediment). Accordingly, the term "volcaniclastic" does not necessarily imply active volcanism at the time of deposition (Fig. F6).

In fragmental lava facies we documented volcaniclastic textures using standard sedimentologic techniques (such as clast versus matrix modal proportions, clast size, shape, sorting, and lithology) (Fig. F5). Of particular interest was the presence (or absence) of features diagnostic of viscous or quench fragmentation while the lava was hot. These include clasts engulfing fragments of other (earlier) clasts, welding, glassy margins surrounding clasts, extensions of the interior of the flow into breccia, entrained clasts within the interior of the flow, and the presence of a basal breccia. We used changes in clast morphology and crystallinity as well as changes in vesicle abundance and shape to identify clast types within breccia.

The subclassification of volcaniclastic sediment followed during Expedition 324 differs from the standard ODP classification (Mazzullo et al., 1988) in that we adopt a descriptive (nongenetic) terminology similar to that employed during Leg 197 (Shipboard Scientific Party, 2002b). Unless an unequivocally pyroclastic origin for volcanogenic materials could be determined, we simply described deposits composed of these materials as being of volcanic provenance (i.e., volcaniclastic) according to the classification scheme for clastic sediment, noting the dominance of volcanic grains. We followed the clastic textural classification of Wentworth (1922) to separate the various volcanic sediment types and sedimentary rocks (according to grain size) into volcanic gravel (>2 mm), volcanic sand (2–0.0625 mm), volcanic silt (0.0625–0.0039 mm), and volcanic clay (<0.0039 mm).

For coarse-grained and poorly sorted volcaniclastic sediments, including those produced by gravity currents, we applied the terms "volcanic breccia" (angular clasts) or "volcanic conglomerate" (rounded clasts) and used lithologic or structural modifiers for further description.

Where appropriate and possible, we further classified volcaniclastic sediment by adding major and minor modifiers to the designated principal name. The principal name defines the grain size class (e.g., gravel, sand, silt, or clay). Relative proportions of vitric (glass), crystal (mineral), and lithic (rock fragment) components of the sediment are used to determine additional modifiers in the name and are placed before the principal name. For example, volcanic sand composed of 75% glass, 5% feldspar crystals, and 20% lithic fragments is named vitric-lithic volcanic sand. Where the evidence for a pyroclastic origin was compelling, we adopted the classification scheme of Fisher and Schmincke (1984). In these instances we used the grain size terms volcanic breccia (>64 mm), lapilli/lapillistone (2–64 mm), and ash/tuff (<2 mm). Sedimentary structures recorded in Shatsky Rise volcanic sediments included graded bedding, cross-bedding, planar laminations, foreset bedding, dune forms, and ripples (and bioturbation). However, although these descriptions were entered into the igneous database, the VCD for sedimentary rocks was typically employed to record the detail for core sections consisting predominantly of volcaniclastic material. This approach ensured efficiency during the logging and description of newly retrieved core material.

Core and thin section descriptions

The first step in describing the recovered core was the identification of unit boundaries on the basis of changes in lithology, including color, grain size, the presence of volcaniclastic or sedimentary intercalations, volcanological features (e.g., presence of contacts or chilled margins), vesicle distribution patterns, changes in primary mineralogy (occurrence and abundance of plagioclase, pyroxene, olivine, and oxide minerals), structure, and alteration. Unit boundaries of volcanic rocks were chosen to reflect different volcanic cooling or volcaniclastic units (see "Definition of lithologic units and volcanic successions"). In some cases, limited recovery required interpolation for the placement of unit boundaries. Of fundamental importance to the core interpretation was the additional use of digital image printouts of the archive core halves. During core processing, these printouts are generated as a matter of course on the split core using the SHIL. Annotation of the printouts provides a working record onto which observation notes, sketches of key features, or an indication of shipboard sampling or analytical requirements may easily be made. In effect, this approach mimics the type of observation and note-taking that is familiar to field-based geologists. This visual record also proved invaluable during the preparation of site reports because it provided the type of sequential context and visual cues that help in assembling and describing broader stratigrahic relationships. Scanned files of these handwritten records are available from the Expedition Project Manager/Staff Scientist.

For thin section descriptions, all minor variations in texture, grain size, vesicularity, color, and/or mineralogy, as well as important volcanic features (e.g., chilled margins, concentration of glomeroporphyritic clots, or crystal aggregates) were recorded in the LIMS database using the DESClogik software interface (see Fig. F11 for an example). The VCDs thus provide summary information regarding the position of igneous unit boundaries and also include a brief description of each section and associated lithologic boundary, if captured.

Macroscopic visual core description

We used the DESClogik data entry software to document each section of the igneous rock cores and to store the descriptions in the LIMS database. To provide consistency in approach, the procedure and database templates we designed closely follow the methods for core descriptions from previous IODP volcanic basement expeditions, including the Kerguelen Plateau, Hawaii-Emperor Seamounts, and Superfast Spreading Crust legs (Legs 183 and 197 and Expedition 309/312; Shipboard Scientific Party, 2000a, 2002b; Expedition 309/312 Scientists, 2006).

VCD graphic reports were automatically generated to summarize each section of the igneous rock cores (Fig. F8). The VCD report displays, from left to right,

1. The core depth in meters;

2. A scale for core section length of 0–150 cm, or as appropriate;

3. The sample piece number;

4. A scanned digital image of the core archive half;

5. An arrow indicating the upward direction for oriented pieces;

6. The sample type and position of intervals selected for different types of shipboard analytical study, including locations of thin sections made;

7. The number of the identified lithologic units;

8. Symbols summarizing structural information;

9. Measurements of strike and dip;

10. A line chart displaying measured and recorded percent vesicularity;

11. A stacked line chart displaying phenocryst percentage for plagioclase (pl, red line), olivine (ol, green line), and pyroxene (px, blue line);

12. A line chart displaying variation in the modal crystal size of groundmass (in millimeters), with presence of fresh glass (f) or altered glass (ag) indicated;

13. A pattern depicting alteration intensity;

14. A column-measured intensity for point-source (MS point) and whole-round (MS) magnetic susceptibility (raw and filtered); and

15. Line chart displaying recorded point-source color reflectance with the red line (L*) and blue line (b*) stacked one on top of the other.

Additionally, the VCD contains written descriptions to accompany the schematic representation of the core sections. These include the following information:

1. The expedition, site, hole, core, type, and section number (e.g., 324-U1346A-15R-3) and the top of the core section measured in meters.

2. The lithologic unit number (numbered consecutively downhole), the piece numbers, and on which piece or pieces the description was based.

3. The lithology (rock description and name). Rock description nomenclature was achieved from hand specimen observation using the eye, hand lens, and binocular microscope.

4. Volcanic description based upon the type of unit or part of unit (e.g., pillow lava, massive flow, etc.). Units were identified using the presence of glassy margins, groundmass grain size variations, and vesicle-rich bands. An interval was described as massive if there was no evidence of variation in internal structure.

5. Rock texture (whether the rock was phyric or aphyric and, if present, phenocryst types).

6. Modal color determined on wetted rock surfaces.

7. Phenocryst content and type based upon minerals identifiable by naked eye, hand lens, or binocular microscope.

8. Groundmass grain size and texture. Categories are as follows: glassy/cryptocrystalline (<0.1 mm), microcrystalline (0.1–0.2 mm), very fine grained (0.2–0.5 mm), fine grained (0.5–1 mm), medium grained (1–2 mm), coarse grained (2–5 mm), and very coarse grained (>5 mm) (Fig. F8).

9. Presence of vesicles. Data entered are (a) the percentage volume of vesicles, (b) their shape and sphericity, and (c) changes in vesicle sizes (i.e., fining or coarsening upward) or vesicle patterns within the unit. Volume fraction, modal shape, and sphericity of vesicle populations were estimated using amended comparison charts (based upon Wentworth, 1922).

10. Contact relations and boundaries in the upper and lower parts of units, as based on the physical changes observed in the core material (i.e., presence of chilled margins, changes in vesicularity, alteration, etc.) and information regarding their position within the section. "Not recovered" was entered where no direct contact was recovered (e.g., if igneous contact was otherwise inferred using proxy observations such as a chilled margin or changes in vesicularity).

11. Presence of alteration. Alteration was based on estimated percent of alteration products by volume, and categorized as follows: unaltered (<2%), slight (2%–10%), moderate (10%–40%), high (40%–80%), very high (80%–95%), and complete (95%–100%). This section includes comments regarding degree and type of vesicle infilling. See "Alteration and metamorphic petrology" for details.

12. Occurrence and type of mineral filling of veins. Additional detailed information of vein materials and orientation descriptions are reported in "Alteration and metamorphic petrology" and "Structural geology."

13. A general structural summary statement regarding the volcanic structure and/or comments regarding the nature of the igneous contact or other internal structures not described under previous headings.

Microscopic visual core description

Thin section analyses of sampled intervals were used to complement and refine macroscopic core observations. Typically, at least one thin section was examined and logged per defined lithologic unit. To capture the descriptive information, a specialized DESClogik template was created. For an example of a thin section description sheet, see Figure F11. As far as was practically possible, the same terminology and nomenclature hierarchy employed for the visual core compositional descriptions was also used for the thin section descriptions. The textural terms used were those defined by MacKenzie et al. (1982). Phenocryst assemblages and their modal percentages and sizes, groundmass, and alteration phases were determined and textural descriptions constructed.

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