< 80°, indicating Ca-rich composition, close to bytownite.In Hole 1223A, we cored a 41.0-m interval and recovered 23.5 m of core that ranges in age from Pliocene to recent and consists primarily of unconsolidated sediments, weakly consolidated claystones and siltstones, and crystal vitric tuffs (Fig. F2; Table T3). The recovered sequence is divided into 14 units on the basis of lithology, induration, and alteration. Several units (1, 2, 4, and 11) are divided into subunits (A, B, C, etc.) (Fig. F2). Sediments and units that are labeled "volcaniclastic" contain >30% volcanic material and do not refer to the environment or method of deposition.
In the following, ages are inferred from magnetic stratigraphy (see "Paleomagnetism"). The uppermost part of the lithologic sequence consists of alternating layers of clay and normally graded volcaniclastic fine sands and muds. At least eight separate turbidites were identified as part of the top unconsolidated units. A massive dark-brown 0.58-m-thick layer of clay (Unit 3) underlies the turbidite layers. Under this clay is unconsolidated black sand (Unit 4) (another possible turbidite). Although a significant amount of the sand was lost during the coring process as a result of its coarse and unconsolidated nature, 2.88 m was recovered. We estimate that ~2 m of material was not recovered from this interval, and it is likely that most of the coring gap is part of this unconsolidated black sand.
An indurated crystal vitric tuff (Unit 5) lies beneath the unconsolidated black sand. This unit marks the top of a series of volcanic and volcaniclastic units that continue to at least 38.7 of the 41 mbsf and is one of two vitric tuff layers recovered. The two crystal vitric tuffs (Unit 5 and Subunit 11B) are separated by almost 17 m. Both are very fine to medium grained and have olivine, glassy shards, vitric fragments, and lithic fragments. Vesicles are common in the glass shards of both tuffs, but the lower tuff is significantly more altered. Below both tuffs are a series of very fine grained, weakly indurated volcaniclastic claystone and siltstone units. A few units are bioturbated, one heavily (~80%), and the rare laminae are commonly indistinct. White effervescing granules are present in several of these units. Fine-grained crystals, altered coarse grains, and glass fragments are found throughout the units.
Interval: 200-1223A-1H-1, 0 cm, to 1H-4, 67 cm
Unit 1 contains alternating layers of clay of pelagic origin (Subunits 1A, 1C, 1E, and 1G) and normally graded volcaniclastic sand, silt, and clay turbidites (Subunits 1B, 1D, 1F, and 1H). At least four turbidite sequences are present with a minimum thickness of 10 cm (Fig. F3) (Subunits 1B, 1D, 1F, and 1H). Numerous small turbidite deposits may be present as laminae within the clay and larger turbidite sequences (Fig. F4). Alternatively, some of these may represent the parallel laminae portion of a Bouma sequence, division D (Bouma, 1962), and may be part of the next turbidite below. Bioturbation and burrows are present below three of the four turbidites. Sponge spicules are common in both the clay and turbidite layers. Subunit 1B includes ~30 cm of alternating beds and laminae of clay and volcaniclastic grains that may represent additional smaller turbidites or division D in the Bouma sequence.
Interval: 200-1223A-1H-4, 67 cm, to 1H-CC, 9 cm
Unit 2 includes several turbidites directly overlying one another. The sharp basal contacts indicate that erosion of the turbidite below has occurred (Fig. F5). At least four turbidites are represented in this unit, and each is labeled as individual subunits in Figure F2. Additional smaller beds may also be turbidites.
Although each of the turbidite sequences includes layers of fine black sand grading upward to silt and clay, all four large turbidites are composed primarily of very fine sand. Beds of granules made of calcite (identified by XRD) are located at the bottom of Subunit 2C (Fig. F5). Rare radiolarians are present in these layers.
Interval: 200-1223A-1H-CC, 9 cm, to 2H-1, 20 cm
Unit 3 is a 0.58-m-thick massive dark-brown clay layer (Fig. F6).
Interval: 200-1223A-2H-1, 20 cm, to 2H-4, 90 cm
Unit 4 is divided into two subunits, 4A and 4B. Subunit 4A is the upper 0.75 m of Unit 4 and consists of fining-upward black sand (Fig. F7). Subunit 4B (2.13 m) is unconsolidated fine black sand, made of the same material, but highly disturbed during drilling. Coarse fresh glass is abundant throughout this unit. Approximately 40% of the volume of this unit may have been lost during recovery of Core 200-1223A-2H.
Interval: 200-1223A-3X-1, 0 cm, to 3X-2, 86 cm
Unit 5 is an angular to subrounded clast-supported lithified very fine to medium-grained crystal vitric tuff in a brown clayey matrix (Fig. F8). The grain size increases with depth from medium to coarse. Section 200-1223A-3X-1 is predominantly medium-grained sand. This normally graded tuff also contains radiolarians in the clay matrix and structures currently interpreted to be gas pipes (Fig. F9). The length and width of the pipes increase toward the middle of Unit 5 and then decrease above and below.
Petrographic observations indicate an equigranular texture. Abundances of olivine, glassy shards, and vitric fragments decrease from the top toward the bottom of the unit (13%-6%, 20%-10%, and 16%-10%, respectively). The proportion of lithic fragments, however, increases downward from 8% to 20%; grain size also increases downward.
Glassy shards have a thin rim of clay in the top of the unit, but the glass in the lower half of the unit is missing this rim. Lithic fragments have subophitic to intergranular textures with plagioclase, clinopyroxene, olivine, and Fe-Ti oxides. There are two types of vitric fragments, one that has black to brown glass and the other with pale-brown glass. Both have spherulitic plagioclase and rare euhedral phenocrysts of olivine and plagioclase. They are cemented in a brown clayey matrix that ranges from 20% to 40% in volume. Many vesicles and cavities (5%-7%) are filled with zeolites. Chlorite grains are rare (<1%).
Interval: 200-1223A-3X-2, 86-109 cm
An erosional surface separates this unit into two parts. The upper part is a 4-cm-thick massive and heavily bioturbated (~80%) interval, and the lower part is a 19-cm-thick intermittently laminated interval with minor bioturbation (~5%) (Fig. F10). Sharp contacts are present at the top and bottom. The fine-grained, dark-gray, <0.5-cm-thick top and bottom contacts are horizontal and parallel with their adjacent units. The thin laminae below the inclined erosional surface are <0.5 cm thick with cross-bedding (101 cm) (Fig. F10). Both the clay matrix and the burrow fillings contain glass, olivine, and plagioclase. The burrows range from 0.1 to 1.0 cm in diameter and from 0.5 to 2 cm in length; they are not oriented; and ~10% are curved.
Interval: 200-1223A-3X-2, 109 cm, to 3X-CC, 40 cm
In Unit 7, layers of sand-sized grains consisting of clay are interlayered with volcaniclastic silt (Fig. F11). The sediments are weakly indurated clast-supported sandy siltstone with poorly oriented silt particles. Coherent clumps of clay-sized particles are well rounded and between ~0.06 and 2 mm in size. The silt is angular to subrounded. Cross-bedding and planar laminations are present in the larger pieces that have not been significantly disturbed by drilling. The laminations may be due to concentrations of clay clumps in parallel, linear orientation. This unit was highly disturbed by drilling and broken into biscuits surrounded by slurry. Components of the 0.2-mm (fine sand) fraction are ~5% olivine, ~2% plagioclase, >10%-20% glass shards, ~10% vitric fragments, and >10% lithic fragments.
Interval: 200-1223A-4X-1, 0 cm, to 4X-4, 50 cm
Overall, the volcaniclastic silty claystone fines upward with an intermediate layer containing <0.5-cm subangular to subrounded cavities filled with white material, some of which effervesce (Fig. F12). Cavities decrease in size and abundance with depth. The shape of the cavities changes from round to elongated in the upper part of the unit to almost entirely elongated in the lower portion. Based on macroscopic observations, crystals increase in size and abundance with depth (~30%-40% crystals in the lower section and ~15%-30% at the top). The unit is disturbed by drilling and is "biscuited." A sharp erosional contact with a rip-up clast >1 cm in size marks the lower boundary of this unit (Fig. F13).
Interval: 200-1223A-4X-1, 50-61 cm
Unit 9 is a massive weakly indurated, well-sorted, matrix-supported claystone with ~1% fine sand sized crystals. A few burrows are present. Their diameters are all <0.5 cm, with most <0.2 cm. Observations of thin sections show that some cavities are partially filled with zeolites, possibly phillipsite. A sharp erosional contact in a dark layer marks the top of this unit. Another dark 0.5-cm-thick layer marks the gradational base of the unit (Fig. F14).
Interval: 200-1223A-4X-1, 61 cm, to 4X-CC, 37 cm
Unit 10 is a moderately well sorted silty claystone with small amounts of fine-grained crystals (7%-10%) and glass (~2%). Although most of the unit is massive, a few minor laminations are present (Fig. F15). Coarse lithic clasts are scattered throughout the unit. An area (10 cm thick) of granule and pebble breccia is present in the middle of the unit (Fig. F16). One large (>5 cm) clast is located near the top of the unit. The percentage of crystals and the size of crystal grains increase significantly with depth. Glass shards are fresh, angular, and range in size from fine sand to silt. Crystals are rounded to subrounded.
Interval: 200-1223A-5X-1, 0 cm, to 5X-CC, 39 cm
Unit 11 is divided into two subunits, 11A and 11B. The components are similar for the two subunits, but Subunit 11A is more altered and was highly disturbed by drilling. Subunit 11A is a matrix-supported medium- to very coarse grained palagonitized crystal vitric tuff (Fig. F17). The main constituents are glassy shards altered to palagonite (26%), vitric (8%) and lithic (4%) fragments, and olivine (~7%), with minor amounts of plagioclase (1%). Rounded cavities that resemble vesicles increase in size and abundance upward. The glassy shards appear to have experienced two types of alteration. The glass first altered to palagonite, and then the palagonite altered to chlorite and clay. Alteration of the glass decreases with depth.
Interval: 200-1223A-6X-1, 0 cm, to 6X-3, 105 cm
Subunit 11B is a lithified, altered, very fine to fine-grained palagonitized vitric tuff with subangular to subrounded volcanic fragments in a brown clayey matrix (Fig. F18). The main constituents are glassy shards, lithic fragments, vitric shards, olivine, and plagioclase. Grain size decreases with depth from fine to very fine. For the entire subunit, glassy shard content increases (30%-80%), whereas spherulitic glassy abundance (10.5%-6.5%) decreases with depth. Palagonitization of the glassy shards is more complete with depth. The lithic fragment content is low (~5%) and constant. Some vesicles and cavities (~5%-7%) in glass shards are filled with material identified as zeolites. The main difference between Unit 11 and Unit 5 is based on the alteration extent of glass: the glass is highly palagonitized in Unit 11, whereas it is less altered in Unit 5. Almost all of the glass in Subunit 11B is either replaced by or has rims of palagonite (see "Alteration").
Interval: 200-1223A-6X-3, 105 cm, to 6X-4, 8 cm
A sharp contact marks the top of Unit 12. It is a weakly indurated, matrix-supported massive silty claystone with irregular to spherical cavities (<0.1-0.4 cm) filled with white material that effervesces vigorously and leaves a white platy residue (Fig. F19). Preliminary electron microprobe analyses indicate that this material is anhydrite. XRD analysis shows that the surrounding claystone contains paragonite, wairakite, kaolinite, and illite (see "X-Ray Diffraction Analyses"). Large angular clasts (~1 cm) at the base of the unit are very soft and can be easily scratched with a fingernail. The clasts increase in abundance with depth.
Interval: 200-1223A-6X-4, 8-89 cm
Unit 13 is a weakly indurated, matrix-supported clayey siltstone. It contains <7% angular to subrounded lithic clasts (average size = 0.3 mm) in the lower half of the unit (Fig. F20). It has a higher percentage of silt than the bottom of Unit 12. There are two distinct intervals with anhydrite and/or calcite fillings; some are in cavities, and some are more veinlike. Two zones (~5-6 cm) of highly disturbed material (drill slurry) between drilling biscuits contain broken pieces of similar white material that probably originally was veins (Fig. F21). Faint planar laminations are present in the lower part of the unit. Small claystone clasts, <0.1 to 0.5 cm in size, with darker angular to subrounded edges (sometimes oval shaped), form ~5% of the unit. The unit is more massive toward the bottom. The regions with the laminations and lithic clasts are less disturbed than other parts of the unit (Fig. F22).
Interval: 200-1223A-6X-CC, 0-39 cm
Unit 14 is a weakly indurated, matrix-supported clayey siltstone (Fig. F23). A few coarse clasts (<1%; 2 mm) in the siltstone are very soft and easily scratched with a fingernail. Angular to subrounded clasts measuring ~1 cm x 0.5 cm are present at the base (Fig. F23).
The rocks recovered in Hole 1223A have three main lithologies, all of which have volcanic components varying from ~1% to >99% in volume. The three unit types are (1) barely consolidated to unconsolidated sediments, (2) weakly consolidated claystones and siltstones, and (3) crystal vitric tuffs.
The weakly consolidated to unconsolidated sands have a volcanic fraction close to 99% in volume and, therefore, can be called volcaniclastic sand or volcaniclastic ash. The main constituents are (in order of abundance) fresh glassy shards, olivine, vitric fragments, plagioclase, lithic fragments, palagonitized glass, and clinopyroxene. In a few cases, radiolarians (<1% in volume) are found.
The claystones and siltstones have a "nonvolcanic" fraction (60%-99%) composed mainly of clay minerals and, more subordinately, of carbonate/anhydrite granules. The main difference between these two groups of rocks is the silty fraction, which is represented by volcanic material—less than ~20% in volume in claystone and generally between 20% and 30% in volume in siltstones. Alteration products such as zeolites, anhydrite, and chlorite are relatively common and found in vesicles; they are concentrated in vugs or associated with glass shards. The volcanic fraction is represented by small olivine and plagioclase clasts together with glassy shards, which are altered to palagonite in varying amounts.
The indurated crystal vitric tuffs are composed of >50% (in volume) volcanic material, mostly as clasts of minerals and lithic/vitric fragments. The relative abundance of the volcanic material is variable, but the overall distribution is (in order of abundance) glass shards (fresh to altered), lithic and vitric fragments, olivine, plagioclase, and clinopyroxene clasts. The clayey matrix ranges from 15% to 50% in volume (in all but one case it is <40%). Relatively common authigenic minerals are zeolite (from 1% to 15% in volume; generally <10%), chlorite (from 1% to 10%; generally <5%), Fe oxyhydroxides (5%-20%), and sulfide minerals (<1%).
A more detailed petrographic description for each lithology is presented below, with a special emphasis on the crystal vitric tuffaceous layers. More information on the alteration products can be found in "Alteration". Table T4 represents a general review of the most important petrographic features of the different units of Site 1223. A detailed table for each individual thin section is available in "Site 1223 Thin Sections".
The unconsolidated sediments in the nine thin sections examined (18, 19, 27, 28, 29, 30, 31, 32, and 33) are either volcaniclastic sand layers or the coarse fraction of a turbidite sequence. Volcaniclastic sand (>60% volcanic material) is designated volcanic ash when it consists of particles <2 mm in diameter (Mazzullo et al., 1988). The volcanic ash layers of Unit 1 and the black volcanic ash of Unit 4 are normally graded (from 0.1 to 0.3 mm), with an average size of ~0.2 mm (Fig. F24). The most common components in these ash layers (Fig. F25) are
Associated with this paragenesis, claystone and carbonate clasts are found in the volcaniclastic sand from the turbidite sequence (Unit 2) (Fig. F26). The claystone clasts (~50% in volume) range from 0.1 to 4 mm, with an average size of 1.8 mm, and are subangular to spherical in shape. In some cases these clasts have a relatively high percentage of carbonate inside. The carbonate clasts (~5% in volume) are made up of calcite (micrite) and range in size from 0.3 to 3.5 mm, with an average size of 0.6 mm (Fig. F26).
Nine thin sections from Site 1223 were examined (6, 8, 10, 11, 13, 16, 17, 21, and 23). The silty to sandy fraction of these sediments is represented by volcanic material; other components are claystone and carbonate (micritic) granules. The most common volcanic material in silty claystones is represented by anhedral to subhedral relatively fresh olivine clasts with a size of <0.15 mm and euhedral to anhedral plagioclase grains generally <0.1 mm in size. The maximum extinction angle measured in plagioclase is 35°, which implies labradoritic or more Ca-rich compositions. Volcanic material is more abundant in coarser lithologies, such as sandy siltstones. In this case the maximum size of the volcanic material ranges from ~0.7 (olivine clasts) to ~0.3 mm (glassy shards). The most abundant are
< 80°, indicating Ca-rich composition, close to bytownite.The silty claystones (Units 12 and 13), below the lower tuffaceous layer, have 2% to 20% vugs (Fig. F27). The size of the vugs across the sections range from 0.4 to 1.7 mm, but with a maximum diameter of 3.5 mm. They are spherical to subrounded and are filled by a mineral that is white in hand specimen and colorless in thin section. This mineral has a columnar to hexagonal shape (Fig. F28), parallel extinction, high birefringence, three pronounced cleavage directions, and uniaxial negative character. Preliminary electron microprobe analysis on shore shows that Ca and S are present, indicating that the mineral is a calcium sulfate, either gypsum or anhydrite. Shipboard XRD patterns display peaks of anhydrite (see "X-Ray Diffraction Analyses").
Two tuffaceous layers were examined in the nine thin sections examined from Cores 200-1223A-3X (Unit 5) and 5X-6X (Subunits 11A and 11B). A brief petrographic description of these layers is presented below.
This tuff is medium to very coarse grained, with a maximum grain size of <2.2 mm and an average size of <1.1 mm (Fig. F29). The texture is equigranular. The main constituents are
> 80°, which indicates bytownitic to labradoritic composition. The crystals are anhedral to subhedral and have a columnar to equant habit. They are between 0.1 and 0.3 mm in size.
(~60°) and the color and birefringence. It is found as fresh euhedral to anhedral twinned crystals in glassy shards as well as in the clayey matrix (Fig. F32).All of the above components are cemented in a brown matrix made up of clay minerals and Fe oxyhydroxides that range from 20% to 40% in volume. The olivine, glassy shard, and vitric fragment content decrease downward in the unit (~13%-6%, ~20%-10%, and ~16%-10% in volume, respectively). The proportion of lithic fragments, however, increases downward (8%-20% in volume); grain size also increases downward. Lithic and vitric fragments are interpreted to be different portions of pillows and/or lava flows. Vitric fragments represent the outer glassy quenched rim of pillows, whereas lithic fragments are pillows or flow interiors that cooled more slowly. Many vesicles and cavities (~5%-7% in volume) are filled with zeolites. Rounded grains of chlorite are rare (<1%); in some cases, chlorite replaces all or part of the glassy shards.
This tuff is very fine to medium grained, with a maximum grain size of <1.6 mm and an average size <1.0 mm. The texture is equigranular. The main constituents are
= 90°). (~90°) and the maximum extinction angle measured, ~35°, suggest bytownitic to labradoritic composition (An65-85).All of the components are cemented in a brown clayey matrix (~45% in volume). Vesicles and cavities (~20%-25% in volume) are filled by zeolites and chlorite. As in Core 200-1223A-3X, chlorite is present both as alteration of glass and as discrete granules.
This tuff is very fine to fine grained, with a maximum grain size <1.5 mm, and the average grain size is <0.6 mm (Fig. F39). The texture is equigranular. The main constituents are
(= 90°) suggests a chrysolitic composition. The average size is ~0.4 mm. (>80°) and maximum extinction angle measured, ~40°, suggest bytownitic to labradoritic composition. The average size is <0.3 mm.All of these components are cemented in a brown clayey matrix whose abundance ranges from 30% to 40% in volume. The grain size decreases toward the bottom of the unit. Glassy shard content increases, whereas vitric fragment content decreases toward the bottom of the unit (~30%-80% and ~10.5%-6.5% in volume, respectively); the lithic fragment percentage is always low (<5% in volume). As in the Unit 5 tuffs, the lithic and vitric fragments can be considered to have formed from different portions of pillows and/or lava flows. Vesicles and cavities (~5%-7% in volume) are sometimes filled with zeolites. Rare rounded grains of chlorite (<1% in volume) also are present.
The essential features of the tuffaceous layers can be summarized as follows:
XRD analyses of granules, vein fillings, and veinlets were performed to determine the presence and composition of major and minor phases in order to gain a more thorough understanding of possible secondary mineralization. Material from the following were analyzed by X-ray diffraction (Table T5): (1) yellow and brown granules recovered from Unit 2 in the coarse-grained layer of the turbidite (Fig. F42), (2) an inclusion in Subunit 11B, the palagonitized crystal vitric tuff (Fig. F43), and (3) both vein and vug filling material from Unit 12 (Fig. F44).
Two types of granules in the Unit 2 turbidite were analyzed by XRD. The brown granules (Sample 200-1223A-1H-5, 94-95 cm) have a complex XRD pattern. The main peaks correspond to phillipsite (zeolite) with minor peaks that match the clay minerals (montmorillonite). Peaks for plagioclase are present at 3.20, 2.51, and 2.46 Å, but are not labeled on Figure F42A. Based on the XRD analyses, the yellowish granules (Sample 200-1223A-1H-5, 114-115 cm) have a simple spectrum that corresponds to calcite plus some low-intensity peaks that match plagioclase (Fig. F42B).
The palagonitized crystal vitric tuff (Subunit 11B) has a fine-grained inclusion (interval 200-1223A-6H-3, 69-75 cm) that has a greenish rim and a white core. The white and green materials were separated and sampled for XRD analyses (Fig. F43). Both XRD analyses have similar patterns. Major peaks, at 13.6, 4.46, 2.56, and 1.69 Å, are characteristic of montmorillonite, whereas peaks at 15.4, 4.6, 3.13, 2.64, 1.74, and 1.54 Å indicate the presence of saponite. The identified peaks are listed in Table T6.
XRD analyses were conducted on material sampled from thin veins (Samples 200-1223A-6X-3, 117-118 cm) and white filled vugs (Samples 200-1223A-6X-3, 129-130 cm, and 6X-4, 0-20 and 34-35 cm) in the volcaniclastic silty claystone of Unit 12 located directly below the palagonitized crystal vitric tuff. Apparently, both vug fillings and a portion of their surroundings were incorporated in the XRD samples. Thus both of the samples have complex but similar d-spacing patterns indicating the presence of several minerals (Fig. F44).
The upper part of Unit 12 (Sample 200-1223A-6X-3, 117-118 cm) has many thin white veins. XRD patterns of this sample have some small peaks that are distinctive from the white vug filling material. These peaks are identified as zeolite-group minerals such as phillipsite, thomsonite, and natrolite (natrolite not shown) (Fig. F44B) (Miyashiro, 1973).
The characteristics and interpretation of these patterns are as follows:
Postdepositional mineralogical transformation of sedimentary material is usually described either as diagenesis or as metamorphism, depending on the conditions and the degree of the transformation with respect to pressure and temperature. Because the two terms encompass all combinations of temperature and pressure affecting crustal rocks, there should be no need for others. However, the word "alteration" has come to be applied to chemical and mineralogical transformations of igneous rock of the ocean crust that take place in response to interactions with seawater at low pressure, where there is still sufficient porosity in the rocks to allow ready flow of hydrous fluids through them. The engine of fluid flux is temperature, whether it is provided by proximity to igneous intrusions at, for example, a ridge axis or seamount or it is derived from the subsequent cooling of any part of the oceanic lithosphere. Alteration at high temperature is commonly described as hydrothermal alteration; alteration that occurs near the ambient temperature of bottom water is low-temperature alteration. High- and low-temperature alteration are often linked through a continuum of processes that occur during particular regimes of hydrologic flow. The agent of mineralogical transformation in either case is the driven fluid, originally seawater, which both dissolves soluble constituents of the rocks and deposits those constituents with which it is saturated. Dissolution and deposition depend on the conditions of the fluid, especially temperature, oxidation state, fugacities of H2O and CO2, alkalinity, and the degree of acidity. It is also usual to include among alteration mineral assemblages those materials that now line or fill original void spaces such as fractures or vesicles in addition to the replacement products of primary phases. Alteration in the marine realm is not the same as weathering, which takes place on land mainly in response to the flow of groundwater.
The volcaniclastic sediments and rocks recovered in Hole 1223A provide a special case of alteration. Although the agent of transformation appears to have been heated seawater and seawater likely flowed through the rock as well, chemical and mineralogical changes that led to induration were accomplished far from the usual sources of heat in the ocean crust and virtually at the seafloor. The effects of alteration were also extremely restricted, occurring in the two lithified crystal vitric tuffs, each only a few meters thick. Indeed, the induration is one of the most important consequences of the alteration. However, there were additional contact metamorphic effects in subjacent sediment, although the effects did not extend very far. Most of the rest of the sedimentary section has experienced an incipient diagenesis that produced partial lithification. Even this is surprising in view of the insignificant thickness of sediment above them. The terms alteration, contact metamorphism, and diagenesis thus have specific meanings in the context of the short sedimentary section cored in Hole 1223A, and it is important to bear in mind their distinctions in considering the descriptions that now follow.
There are four main protoliths to the partially and largely transformed sediments and rocks of Hole 1223A. These are
Hole 1223A is below the calcium carbonate compensation depth; thus, the seafloor received no pelagic calcareous material in the history represented by the cores. However, there are minor amounts of other biogenic material such as sponge spicules and traces of authigenic material and metamorphosed basalt in some of the sediments.
These protoliths are combined in different proportions in the lithologies recovered in Hole 1223A. No sediment or rock appears to be without at least some fraction of all four components, including radiolarians. This is because all of the volcaniclastic material traveled across the seafloor in some kind of density flow and entrained pelagic material as it flowed. Some mixing also resulted locally from bioturbation. The pelagic protolith is most important in the upper 7 m of the section, lending the soft sediment a distinctive brownish hue. Several meters of unconsolidated lithic-vitric sand underlies this and overlies Unit 5.
Unit 5 is an indurated crystal vitric tuff composed mainly of sand-sized submarine volcanogenic glass shards, associated igneous minerals (olivine, plagioclase, and clinopyroxene in decreasing order of abundance), and angular lithic grains set in a finer-grained matrix. The mystery of this rock is that it is well indurated, or cemented, even though it lies beneath a minimal thickness of surficial sediment. The usual pattern of lithification during burial diagenesis has not occurred here. To accomplish the same induration in similar materials simply by burial normally requires hundreds of meters of overburden, as attested by many holes drilled in the western Pacific Ocean and elsewhere (e.g., Hussong, Uyeda, et al., 1982; Taylor, Fujioka, et al., 1990).
Along the basal contact with the crystal vitric tuff of Unit 11, the underlying silty claystone (Unit 12) is partially recrystallized for a thickness of some 40 cm. It contains narrow veins lined with a mineral that is either wairakite or analcime, and vugs or cavities that are completely filled with white secondary minerals, including anhydrite. The cause of the induration of the deeper vitric tuff is no less mysterious than that of the upper one because it is covered with only 32 m of sediment and rock.
Alteration is easier to identify in thin sections than it is to define macroscopically. Chiefly it occurs when some primary material such as volcanic glass becomes something else, such as clays and iron oxyhydroxides. It is essential to recognize what the material was originally, by its shape, textural relationship with other substances, or the presence of relict phases, before inferring that it has become something else.
The two indurated vitric tuffs qualify as altered because in both tuffs much of the fine-grained matrix that originally contained sand-sized vitric, lithic, and mineral fragments has been replaced by clay minerals and zeolites. The clays and zeolites are the principal agents in the cementation of the rocks. In the lower tuff, nearly 90% of the original sand-sized glass has been replaced by palagonite. Palagonite is an unusual secondary substance to encounter in shallowly buried volcanogenic sediments. It usually replaces basaltic volcanic glass that is found in hyaloclastites and aquagene tuffs, which combine lobes of pillow lava with hyaloclastitic breccia carapaces (e.g., Peacock, 1926; Carlisle, 1963; Bonatti, 1965). It can also form during the flow of low-temperature groundwater through vitric tuff (Hay and Iijima, 1968) and more rapidly in tuffaceous materials under hydrothermal conditions, between 50° and 200°C (Hoppe, 1940; Surdam, 1973; Furnes, 1975). It involves a fairly extensive change in the composition of the original glass, a complementary formation of zeolites, and a reduction in density. Peacock (1926) described two types of palagonite. Both are orange or reddish brown. Gel palagonite is isotropic in cross-polarized light. Fibro-palagonite contains smectites with distinctive birefringence. Peacock considered the gel palagonite to be a mineraloid; it has a distinctive appearance but lacks obvious crystallographic attributes. In the Icelandic tuffs he studied, gel palagonite formed first and, so Peacock believed, at a higher temperature than fibro-palagonite. Icelandic tuffs that were transformed mainly to gel palagonite tend to be well indurated and break with a conchoidal fracture. Peacock (1926) also noted the common presence of spherical structures in palagonite consisting in many cases of bundles of needles radiating from a common center. These he termed spherulites. This term was later used to describe radiating needles of plagioclase formed from the melt at very high cooling rates in experiments on basalt that are observed near the rims of rapidly quenched submarine pillow basalt (e.g., Kirkpatrick, 1979).
Most of these attributes of palagonite can be seen in the thin sections of Subunit 11B. Figure F45 depicts palagonite in Sample 200-1223A-6X-3 (Piece 1C, 86-89 cm) taken 3.64 m from the top of the tuff and 1.35 m above its base. In this sample, matrix material accounts for 40% of the mode. It is largely inchoate, but most of it is colored the same amber red as coherent palagonitized glass shards, and it contains small broken bits of unaltered igneous silicate minerals, chiefly olivine and lesser plagioclase. A good deal of it is also clay, and radiolarians can be seen here and there; thus, it must include some fraction of pelagic material even if that material is now transformed to other substances.
The proportion of definable glass and palagonitized glass shards averages 38.1% as determined by 1000-point counts for each of the three thin sections. The proportion of glass exclusive of the matrix is 64.6%, and of this 91% is palagonitized (Fig. F45A). The small proportion of unaltered glass in the shards is always enclosed in a rim of palagonite ~0.1 mm thick (Fig. F45B). The rims are separated from the glass by sharp boundaries. Their nearly uniform thickness shows that they formed after deposition, after all movement and fracturing of glass in the tuff had ceased. Crystals of olivine are unaltered in both palagonitized glass shards and the matrix throughout most of the lower tuff (Fig. F45A, F45B, F45C). Near the top of this tuff, however, olivine is almost entirely replaced by fairly strongly birefringent clear clays.
Many former glass shards are vesicular, with large bubbles forming as much as 50% of the fragments. Two examples shown in Figure F45C and F45D are completely palagonitized. Original palagonite rims appear to be intact and are present on all vesicle rims, whether they are open to the external matrix material or not. Darker islets of palagonite between the vesicles may be relics of original fresh glass interiors, as in Figure F45A and F45B. The palagonite shard depicted in Figure F45A has two types of spherical structure, both of which are circular in the plane of the section. These spherical structures are open vesicles that are filled with matrix material and rimmed zones of darker radiating fibers (Peacock, 1926). These probably are radiating bundles of plagioclase fibers that grew at high cooling rates as the glass was quenched and well before the palagonite formed. They now have sharply defined rims of palagonite, as if the bundles of plagioclase fibers served as impermeable barriers to the fluids that otherwise reconstituted the glass. Figure F45D also shows a lithic grain with small acicular plagioclases at the lower right. The grain is completely untouched by palagonitization, as are all of the darker lithic grains in Figure F45A and F45B. The alteration process thus affected only the glass.
Alteration is more extensive near the top of the lower tuff in Subunit 11A. Once glassy interiors surrounded by palagonite rims are now altered to green clays (Fig. F46A), and some of the palagonite rims themselves have also been replaced by green clays (Fig. F46B). Large void spaces between palagonite grains are now lined with rosettes of zeolite (Fig. F46C, F46D, F46E, F46F). There are at least three zeolites. One is fibrous, perhaps natrolite. The second has a columnar shape with flat terminations resembling thomsonite. The most abundant zeolite has pyramidal terminations on individual crystals; therefore, it is probably phillipsite. A second stage of alteration, or at least a more continued alteration, thus appears to have followed the formation of palagonite in the upper part of this tuff.
The upper indurated tuff in Unit 5 is unlike the tuff in Unit 11 in that it has almost no palagonite, although it is cemented by clay minerals and zeolites and almost all of its original glass is still fresh. Most glass shards are still quite fresh, as is olivine. The grains simply are cemented by a dull greenish brown clay mineral of globular habit (Fig. F47A, F47B, F47C). Some vesicles and voids are lined with radiating zeolite rosettes that are probably phillipsite (Fig. F47D).
An unusual aspect of some olivine grains may be related to alteration. In several thin sections of the vitric tuffs, fractured grains of the mineral are intricately decorated with exsolved reddish iron oxides (Fig. F48). In some grains, this is so extensive that transmitted light is almost entirely occluded. In other grains, the decoration is present as closely spaced swirls or fine lines (Fig. F48A, F48C, F48D, F48E, F48F). In still other grains, the cross-sectional pattern is that of cuspate stacks that alternate between being concave upward and downward (Fig. F48C, F48D, F48E, F48F). The birefringent olivine between the stacks is highlighted by different amounts of a faint oxidative stain, with boundaries present precisely where the stacks change from concave upward to concave downward. Extinction is uniform throughout, so these are not kink bands, although there are olivine grains in the thin sections with kink-banding subgrains (Fig. F48B).
The cuspate arrangement of these swirls resembles that revealed in olivine grains in deformed cumulate dunite xenoliths from Hualalai Volcano, Hawaii, using a procedure described by Kirby and Green (1980). They produced the identical pattern by heating thin sections in an oxygen-rich environment. This was done in order to reveal the intricate patterns of dislocation in the deformed olivine crystals that otherwise would not be evident during routine petrographic examination. The dislocations are manifestations of stresses acting on the dunite in the deep crust or upper mantle beneath the volcano. In the vitric tuffs of Hole 1223A, nature evidently supplied a similarly oxidizing and warm environment with the same result on deformed olivine grains. Most of the olivine in the tuffs is phenocrystic, thus it is not deformed. The scattered grains of olivine with the decorated patterns revealed by oxidative heating are scavenged bits of deformed dunite that, like the Hualalai xenoliths, came from the deep lower crust or upper mantle. The decorated patterns show up only because of the general alteration experienced by the surrounding tuffs.
The basal contact of the upper tuff (Unit 5) with the sediments is marked by a narrow lighter zone about 1 cm wide in the tuff itself. The underlying sediment is compact, bioturbated, and lighter in color for some 30 cm more than siltstones and mudstones farther away.
The contact of the lower tuff (Subunit 11B) with the underlying sediments of lithologic Unit 12 is marked by a fairly high concentration of subspherical white patches that appear to be cavity or vug fillings. Narrow irregular veinlets filled with white material link the cavities, crossing the core diagonally every few millimeters (Fig. F49A, F49B, F49C). The veins and filled cavities are set in a moderately indurated light-brown fine-grained matrix. X-ray diffractograms show that if this matrix once contained clay minerals, it has them no longer. In thin section, the veinlets are lined with two minerals, one with moderate birefringence that is present in sprays and the other with very low birefringence that is present in blocks (Fig. F50A, F50B). From X-ray diffractograms, the latter is either wairakite or analcime, both of which have almost indistinguishable powder diffraction patterns. A similar blocky mineral fills many circular relict tests of radiolarians (Figs. F49H, F50C, F50D, F50E). The white infilling in the patches (Fig. F49D, F50E) is a bladed biaxial negative mineral that has intermediate to high birefringence, parallel extinction, and three prominent orthogonal cleavage directions; these features match the orthorhombic sulfate mineral anhydrite. Some grains have pseudohexagonal cross sections (Fig. F49E; embossed image). Several anhydrite peaks are present in the diffractogram of this material shown in Figure F44A. The same diffractogram produced peaks for a multimineral assemblage including paragonite, wairakite/analcime, and possibly pumpellyite; these were evidently incorporated from the claystone directly surrounding the assemblage. The mineral with low birefringence in the vein fillings likely is wairakite or analcime, and pumpellyite may be the other. The matrix surrounding the white patches is very fine grained, with tiny relict grains of pyroxene and titanomagnetite. Much of this matrix is recrystallized to very tiny crystals having low birefringence, probably paragonite; some of them have square cross sections (Fig. F49G). Some tiny voids also contain tiny stellate intergrowths of sharply pointed crystals, perhaps cristobalite (Fig. F49I).
The material beneath the lower Subunit 11B tuff is recrystallized, and its mineralogy suggests a contact facies equivalent to the prehnite-pumpellyite facies of regional metamorphism (e.g., Miyashiro, 1973). Wairakite is present in some hydrothermal assemblages and is stable at temperatures of perhaps 150° to 250°C (Liou, 1971; Seki, 1973; Tomasson and Kristmannsdóttir, 1972; Elders et al., 1979; Kristmannsdóttir, 1976). Paragonite, the sodic equivalent of muscovite, is an unusual mineral to find at a recrystallized contact, but its identification is based on both X-ray diffractograms and optical properties. It is usually found in regionally metamorphosed metaluminous rocks of the greenschist or blueschist facies associated with andalusite or kyanite (Deer et al., 1992). It is also known to replace chlorite in hydrothermally altered basalts in the stockwork beneath high-temperature sulfide mounds on the Mid-Atlantic Ridge (Honnorez et al., 1998). These basalts lie ~100 m below the seafloor just beneath breccias veined with anhydrite, pyrite, and quartz. Temperatures determined from oxygen isotope analyses of quartz associated with the paragonite and fluid inclusion studies of the anhydrite range from 212° to 390°C (Alt and Teagle, 1998; Petersen et al., 1998; Teagle et al., 1998).
About 50 cm below the contact with the tuff, the contact zone gives way to a darker, less consolidated greenish gray silty claystone (Unit 13). This material was recovered as drilling biscuits that are zones of coherent and undisturbed core, each several centimeters thick but separated by narrower zones of highly disturbed sediment and distorted by rotation of the core barrel during the coring process. The biscuits themselves are laminated to slightly cross-bedded, with small rip-up clasts of fine-grained claystone. Two of the intervening disturbed zones contain broken bits of a white secondary mineral resembling that of the white patches near the contact with the tuff. X-ray diffractograms revealed a similar mineralogy. Whether these were originally circular patches, veins, or both is unknown. Their presence in softer claystones does establish that contact effects persisted, but diminished, with distance from the base of the tuff.
Alteration and cementation in the lower tuff (Subunit 11B) and the transformations in sediments at its lower contact probably occurred at an elevated temperature of at least 150°-250°C. This is consistent with prehnite-pumpellyite to zeolite metamorphic conditions and the presences of wairakite or analcime described above. This may have been a retrograde sequence. The formation of palagonite and its associated zeolites in the lower tuff was probably synchronous with the formation of these secondary minerals. Paragonite and anhydrite in underlying sedimentary rocks, however, may well have formed at significantly higher temperatures, based on comparison to the Trans-Atlantic Geotraverse (TAG) hydrothermal stockwork.
The formation of palagonite was not isochemical. In this case, it involved loss of CaO and Sr and addition of K2O, Na2O, and Ba to the bulk compositions of the rocks (see "Geochemistry"). These exchanges required substantial flow of fluids derived from seawater through the porous tuffs. The presence of the Na-mica, paragonite, also suggests that some of the fluids were saline brines (cf., Honnorez et al., 1998). The alteration differed from that at the TAG hydrothermal mound, however, because it was exclusively oxidative in character. There is no sulfide mineralization at Site 1223. Alteration at high temperature under oxidative conditions had surprisingly little affect on olivine, except for those grains containing deformation dislocations that become loci for exsolution possibly of iron oxides. In contrast, under nonoxidative hydrothermal conditions, olivine is usually completely transformed to clays and other secondary minerals.
The sources of heat for the alteration and contact metamorphism may have been either distant or local. If local, they were presumably produced by igneous activity, either intrusion or extrusion, that has not yet been documented for this part of the Hawaiian Arch. Neither lava fields nor fissures associated with such hypothetical volcanism are evident in the smoothly sedimented crest of the arch near Site 1223 that has been seismically profiled, although the site is downslope from a small seamount. The nearest known young volcanism on the arch occurred at the North Arch volcanic field, some 200 km to the northwest. There, lavas and tuffs are alkalic olivine basalts, basanites, and olivine nephelinites (Dixon et al., 1997), unlike the tholeiitic precursors to the indurated tuffs and other sedimentary rocks of Hole 1223A. Similar volcanism near the drill site thus would only coincidentally have driven hydrothermal fluids through the sediments that we cored.
The distant alternative is that the tuffs themselves were pyroclastic in origin and were deposited containing some heat. Nevertheless, the amount of heat in a few meters of such materials is unlikely to have driven fluid flow for very long, at least if the deposit was small. On the other hand, a widespread blanket of hot material deposited suddenly might have acted abruptly as a compressive load on uncompacted surface sediments, and, where sufficiently thick, as a permeability barrier to fluids mobilized by sudden compaction. The fluids were forced to flow laterally and may have sustained high temperatures at the base of the tuff for some time. The result is that the most concentrated effects of alteration and contact metamorphism are present at the base. A similar effect was postulated for the pattern of fluid flow at the top of the basaltic basement that lies beneath ~100 m of volcaniclastic turbidites in the eastern Mariana Trough at DSDP Site 456 (Natland and Hékinian, 1982). There, greenschist-facies hydrothermal conditions were reached in the basalts, and both wairakite and cristobalite formed in the sediments at the contact.
This hypothesis provides a mechanism for directing fluid flow and thus concentrating the most pronounced alteration effects in the tuff beds themselves, rather than in adjacent sediments. This probably would not have been the case if hydrothermal flow was directed along vertical fissures associated with local igneous action. Admittedly, the geometry of fluid flow in variably permeable sediments is difficult to extrapolate over long distances from the vantage of a single hole.
At Site 1223, the two tuffs experienced alteration that was different either in type or in degree. The upper tuff is not palagonitized, although it is cemented by clay minerals and zeolites, and almost all of its original glass is still fresh. The essential difference was probably temperature—lower for the upper tuff—although the upper tuff may have experienced less fluid flow as well. Lower temperature and reduced fluid flow may mean the same thing—less heat was available to drive fluids, whether or not it was derived locally or from a more distant source.
We consider two origins for the crystal vitric tuffs—a Hawaiian Islands source and a local source. If the source of the tuffs was local, they were presumably produced by igneous activity, either intrusion or extrusion, that has not yet been documented for this part of the Hawaiian Arch. Neither lava fields nor fissures associated with such hypothetical volcanism are evident in seismic data of the smoothly sedimented crest of the arch near Site 1223, but no high-resolution bathymetry data have been taken of the surrounding area. The nearest known young volcanism on the arch occurred at the North Arch volcanic field, some 200 km to the northwest. There, lavas and tuffs are alkalic olivine basalts, basanites, and olivine nephelinites (Dixon et al., 1997), which are very unlike the tholeiitic precursors to the indurated tuffs and other sedimentary rocks of Hole 1223A. Similar volcanism near the drill site, thus, would only have coincidentally driven hydrothermal fluids through the sediments that we cored.
A possible scenario for the Hawaiian Islands source hypothesis is that a very large eruption of primitive Hawaiian tholeiite occurred when a deep magma reservoir was breached by the catastrophic failure of the flank of a volcano, similar to the 1980 eruption of Mount Saint Helens (Fig. F51) (Moore and Albee, 1981). This may have occurred on Oahu when the northeast flank of Koolau Volcano collapsed, producing the giant Nuuanu debris avalanche. Sudden decompression caused pressure release, vesiculation, and expansion of the magma. The magma erupted as a directed blast and passed over the collapsing blocks now strewn on the seafloor as a submarine pyroclastic debris flow that reached over the Hawaiian Arch. In this scenario, if the material reached the area of Site 1223 (300 km away) quickly enough in a bottom-hugging density flow, it may have retained enough heat to cause the alteration and induration of the two crystal vitric tuffs. Water surrounding a hot pyroclastic flow may become vaporized, creating a water vapor barrier around the flow that helps to insulate the flow and prevent mixing (Kato et al. 1971; Yamazaki et al., 1973).
Subaerial pyroclastic flows have observed velocities ranging from 14 km/hr (Tsuya, 1930) to 230 km/hr (Moore and Melson, 1969) and travel great distances (>100 km) moving over and around obstacles (surmount >600 m) (Fisher and Schmincke, 1984). Their ability to move has been attributed to several factors, namely exsolution of gas from glassy particles, gas being released when particles are broken, and the heating of the medium causing thermal expansion (e.g., Sparks, 1979). The gas reduces the friction between particles allowing the flow to travel faster. In addition to moving at tremendous speeds, pyroclastic flows are very good at retaining heat. Boyd (1961) calculated that cold air has a minimal effect on a hot pyroclastic flow. Therefore, a hot pyroclastic flow may remain at almost magmatic temperatures during transport and even after deposition.
The above applies to subaerial pyroclastic eruptions; subaqueous pyroclastic flows are less well understood. A massive coarse-grained subaqueous pyroclastic flow deposit >4.5 m thick and extending as far as 250 km from its source was recovered in the Grenada Basin, Lesser Antilles. Carey and Sigurdsson (1980) interpreted the deposit to be a debris flow that originated when a hot subaerially erupted pyroclastic flow entered the ocean. The debris flow incorporated pelagic sediment and seawater, decreasing internal friction and giving the flow great mobility and the ability to suspend large fragments. Thermal remanent magnetism of a similar deposit (Pliocene-Miocene in age) was interpreted by Kato et al. (1971) to have been deposited at temperatures ~500°C.
Both the whole-rock ICP-AES analyses and the preliminary electron microprobe analyses of individual glass fragments within both crystal vitric tuffs (Unit 5 and Subunit 11B) support a Hawaiian Islands source. Unlike the whole-rock ICP-AES analyses, the effects of olivine accumulation and alteration do not complicate the glass geochemistry. Therefore, it is easier to compare the glass geochemistry from the tuffs with the geochemistry from other potential sources (i.e., Hawaiian Islands and North Arch volcanic field). The glass geochemistry suggests that the tuffs have tholeiitic basalt compositions and are similar to Hawaiian tholeiitic basalts.
Futhermore, the glasses in both tuffs have low S concentrations (majority <300 ppm) (S. Sherman, unpubl. data). Sulfur concentrations can be used to estimate depths of eruption (e.g., Moore and Fabbi, 1971; Dixon et al., 1991). Low S concentrations (<200 ppm) are indicative of significant degassing and predominantly result from subaerial eruptions. Deep submarine glasses with low S contents were recovered from Puna Ridge, which is a submarine extension of Kilauea Volcano. The low S abundances for these glasses were attributed to mixing between lavas that had degassed at or near the subaerial surface with the rift lavas that were erupted underwater (Clague et al., 1995). Therefore, the low S concentrations in the glasses in the tuffs suggest that they were erupted subaerially or at least mixed with subaerial lavas. A subaerial eruption supports a Hawaiian rather than a North Arch source for the tuffs. Not only is the North Arch volcanic field submarine, the majority of the glasses from the North Arch volcanic field have S contents >300 ppm (Dixon et al., 1997). It may be difficult to imagine the source for the tuffs from Site 1223 as one of the Hawaiian Islands, but the geochemical evidence indicates it is the most likely option.
