LITHOLOGY

Site 1224 lies in the pelagic clay province of the North Pacific Ocean (Leinen, 1989). The sediments in this part of the Pacific Ocean are eolian in origin, consisting primarily of dust blown eastward from the arid regions of central Asia. This region of the Pacific Ocean is below the calcite compensation depth (~3500 m), and little or no biogenic calcite is thought to reach the seafloor (Leinen, 1989). Nevertheless, calcareous nannofossils are present in Core 200-1224E-2R below 17.5 mbsf. Calcium carbonate was found during DSDP Leg 5 in many of the sites drilled, although in the red clays, carbonate content was low (Vallier, 1970) (Fig. F8). Coccoliths and discoasters were recorded in samples from some of the cores. They were present in all but 34 of the 168 samples collected specifically for this purpose (Bukry and Bramlette, 1970). At Site 172, a calcareous ooze was detected 48 cm below the top of Core 18-172-4. Preservation of the nannofossils was generally poor (Wise, 1973). The overlying 22 m of brown clay was unfossiliferous.

Siliceous biogenic material is rapidly dissolved by the silica-poor bottom waters; nevertheless, some radiolarians are recorded in the sediment column at Site 1224 (see "Site 1224 Smear Slides").

Sediments were obtained from four holes drilled at Site 1224. The sediments consist mostly of abyssal clays of varying color. Occasionally coarser horizons are present as are horizons with varying densities of microfossils, both siliceous (radiolarians and sponge spicules) and calcareous (coccoliths and discoasters) (Figs. F26, F27).

Core 200-1224A-1X had zero recovery and Core 2X recovered <0.1 m of pebbles and granules in the core catcher. These do not effervesce in acid and are thought to be infilled burrows. They are <1 cm in diameter, roughly circular in cross section, and some are nearly cylindrical in form. They are indurated but not hard, and XRD analysis indicates the presence of phillipsite (a member of the zeolite group) (see "Geochemistry"). Core 200-1224A-3X contains only a few grams of soupy, very dark brown clay. Section 200-1224A-4X-1 contains ~117 cm of highly disturbed, soupy red-brown clay along with a 6-cm-long piece of basalt in the base of the core catcher and other related pebbles and granules. Sections 200-1224A-5X-1 and 6N-1 were cored into basaltic basement, and they contain only small amounts of basaltic granules and pebbles mixed in with clay that is likely to have been washed downhole. Radiolarians are found in varying abundance throughout the sediments. Sponge spicules are much less common.

Core 200-1224B-1H contains only ~20 cm of homogenous brown clay. Radiolarians are rare.

A single piston core was collected in Hole 1224C. It contained massive clay throughout the entire 6.53 m recovered. Sections 200-1224C-1H-1 and 1H-2 are massive homogenous brown pelagic clay. Section 200-1224C-1H-3 is also massive pelagic clay, but it gradually darkens in color from brown at the top to dark brown at 70 cm to very dark brown at 147 cm. Sections 200-1224C-1H-4, 1H-5, and 1H-CC are also very dark brown massive clay. Radiolarians and sponge spicules are common in Sections 200-1224C-1H-4, 1H-5, and 1H-CC.

In Hole 1224D, only basalt was cored.

The cored sediment interval in Hole 1224E was 27.1 m thick, and we recovered 10.52 m of highly disturbed massive clay. The sediments were acquired by punch cores using the RCB bit and coring system but without rotating. The clay varies in color between dark brown, very dark brown, black, and dark yellowish brown. The high disturbance due to the punch coring process causes the colors to be streaked and mottled throughout the hole. Most color changes are gradual. Some are sharp, however, especially in the lower part of the core. Light-colored granules and pebbles are found in the top few centimeters of Core 200-1224E-1R (~8 mbsf). Like the burrows in Hole 1224A, they do not effervesce and are thought to be infilled burrows. These sediments also contain small manganese nodules. They are as wide as 2 mm and are irregular or elongated in shape. The surface is either relatively smooth or is mammillated on a very small scale. Radiolarians are abundant at the top of Core 200-1224E-1R (~8 mbsf) and occur less frequently throughout the rest of the hole. Calcium carbonate fossils such as coccoliths and discoasters are present below 17.5 mbsf.

Samples taken at regularly spaced intervals (except the interval from 8.0 to 8.8 mbsf) from Hole 1224E effervesce with acid. Smear slides from samples taken along the core did not indicate a uniform presence of coccoliths and discoasters.

In Hole 1224F, no sediments were cored.

Visual Description

Basalts were recovered in Holes 1224A, 1224D, 1224E, and 1224F. Basaltic basement was tagged with the XCB in Hole 1224A (Core 200-1224A-4X), and two additional short cores were taken with the MDCB (Core 200-1224A-6N). Basalt was rotary cored for 31 m in reentry Hole 1224D. Hole 1224E was intended as a single-bit follow-up ~20 m southwest of Hole 1224D (Fig. F3). It was also rotary cored, but, because of equipment malfunction, it was cored without the advantage of AHC. After one core's length was drilled into basalts, the pipe was pulled out of basement but not above the mudline in order to change out some pipe at the top of the string. Upon lowering the drill bit back into basaltic basement, however, the driller was unable to reenter the hole into basalt because (1) the short section of soft sediments was insufficient to support the end of the drill string laterally and (2) the diameter of the hole through the sediment had widened.

The top of the basalts was cored again in Hole 1224F, probably <1 m away from Hole 1224E. The new hole was cored for 146.4 m into basalts. Site 1224 thus accomplished a novel, if inadvertent, experiment—sampling the top of the same basaltic section three times within ~20 m. However, unlike triple piston coring in sediments, which is deliberately done to recover duplicate sections, the result was to discover substantial lateral variability in the basalts over a very short distance.

The general attributes of the cores with basalts are summarized in Table T5 and Figure F28. In Figure F28, except for the uppermost cores into basement, all depths of recovered rock are based on the assumption, by ODP convention, that the top of a core is assigned to the top of the cored interval, as measured from drill string length and reported in the Janus database.

We assume that the top of basaltic basement lies at a constant depth of 28 mbsf for all four holes. The basement depth of 28 mbsf is probably uncertain by about a meter, and there may be some slight relief to the top of basalts as well. But this method avoids assigning basalt recovery to depths that actually are above the point where basement was touched by the drill string. Drilling in all four holes cored into the same fairly massive basalt flow. The recovered basalt is placed at this fixed depth of 28 mbsf for Holes 1224A and 1224E from which a glassy flow top was recovered. The basalt contact was not cored in Holes 1224D and 1224F, and an uncertain but arbitrarily small gap of ~30 cm is shown between sediments and the top of basalts for these two holes.

In Hole 1224D, a second substantial flow was encountered at 50.7 mbsf, near the base of the hole (Unit 2, lower flow). This lower flow was not reached in the short 8.7 m of coring into basalt below 28 mbsf in Hole 1224E. Based on chemical analyses (see "Geochemistry"), it almost certainly was reached and drilled completely through in Hole 1224F. Hole 1224F was cored for 146.5 m below the presumed top of basement at 28 mbsf. The massive Unit 1 basalts extend for 34.7 m (62.7 mbsf) in Hole 1224D. Below 62.7 mbsf, the percentage of recovery dropped off sharply. This may be related to the change in morphology to thinner flows and pillows that are more intensely fractured and altered from the massive rocks sampled above. Somewhat thicker flows were encountered again at 133.5 mbsf (Core 200-1224F-13R). Core 200-1224F-15R reached a depth of 161.8 mbsf and was the last core in which basalt was recovered. The following two cores were empty, probably because of a jammed bit. We cannot say this for sure, because the bit was released in the hole in preparation for logging after Core 200-1224F-17R. The total penetration in Hole 1224F was 174.5 m; we cored 146.5 m into basement, making it the deepest hole cored into basaltic basement of the Pacific plate younger than 100 Ma since DSDP Leg 65. For all holes, we recovered a total of ~58.14 m of core from ~191 m of penetration.

Based on morphology, physical properties, and downhole log correlations, the basalts recovered from Site 1224 are divided into three lithologic units:

Unit 1 (28-62.7 mbsf): two massive basalt flows. This unit includes all the basalt cores from Holes 1224A, 1224D, and 1224E and cores down to 62.7 mbsf in Hole 1224F. The thickness of Unit 1 in Hole 1224F is curated as 34.7 m. Recovery in Unit 1 was 52.6% for all four holes. The two flows have different compositions (see "Geochemistry").

Unit 2 (62.7-133.5 mbsf): thin flows and pillows. This unit extends from interval 200-1224F-4R-6 (top of Piece 2, 10 cm) to 200-1224F-12R-1 (Piece 15, 129 cm). The base of the unit is curated at 133.5 mbsf, and its thickness as curated is 70.7 m. Recovery in Unit 2 was 14.6%. These basalts are chemically similar to the lower of the two massive flows in Unit 1.

Unit 3 (133.5-161.7 mbsf): basalt flows of intermediate thickness alternating with thin flows and pillows. This unit includes Core 200-1224F-13R through the end of Core 15R. The base of the unit is taken to be the bottom of Core 200-1224F-15R at 161.7 mbsf, and its curated thickness is 28.1 m. Recovery in Unit 3 was 21.4%. The rocks are ferrobasalts, the most strongly differentiated of any rocks cored at the site.

Based on downhole logs, the base of Unit 1 corresponds to a change in porosity and density at 63 mbsf. This is almost exactly the curated depth (62.7 mbsf). The base of Unit 2 in the logs is less precise, but probably deeper than the curated depth of 133.5 mbsf. A zone of very high porosity (high ) is present between 135 and 140 mbsf (see "Downhole Measurements") (shown as large, red arrows on Fig. F28). The temperature in the hole also increased at 136 mbsf (see "Downhole Measurements") (Fig. F80), suggesting that seawater flowed down the hole or was introduced by drilling operations. A permeability barrier was probably encountered at this depth. The relatively massive rock in Core 200-1224F-13R is unlikely to have been cored above this depth. In the formation, then, the base of Unit 2 appears to be ~2-4 m deeper than the curated depth.

Detailed descriptions of these units are given below. Locations of cooling unit boundaries are inferred from the presence of glass, changes in grain size, fractures, vein presence, and alteration.

Unit 1

For Unit 1, the descriptions are based mainly upon observations on cores from Holes 1224D and 1224F, but they apply to all four holes. The Unit 1 basalt is fresh, massive, and fine grained, but grain size coarsens slightly in the middle of flows. The Unit 1 basalt of Hole 1224D is unusually fresh, with only incipient local alteration. Based on variations in grain size, Unit 1 consists of two flows. The upper flow of Unit 1 is 22.7 m thick in Hole 1224D and 35.2 m thick in Hole 1224F. The bottom of the second flow was not reached in ~8 m of coring in Hole 1224D, but it was reached after 13.6 m of coring in Hole 1224F. These thicknesses are based on cores with incomplete recovery. The actual thicknesses of the flows are somewhat greater.

In Hole 1224A, the glassy top of the upper flow is a rim on a small piece of basalt. In Hole 1224E, it consists of a thin flow-top palagonitized hyaloclastite breccia. The rocks are slightly vesicular, and the vesicles are small, irregularly distributed, usually round, and widely spaced. Some entire 1.5-m sections are nearly without vesicles. Many vesicles are segregation vesicles, with macroscopically visible linings of darker basaltic material. The vesicles are usually lined or filled with bluish gray clays, and some are filled with calcite (Fig. F29). There are several gas pipes contained in coarser-grained basalt that have segregation vesicles filled with calcite (Fig. F30) near the top of the deeper flow in Unit 1.

Pieces of continuously cored basalt approximately several tens of centimeters long with few fractures were recovered from Unit 1. The few fractures are generally straight and at high angles to the core. Some are vertical and may be columnar-type contraction fractures. A few are irregular vein networks. Almost all fractures in Unit 1 of Hole 1224D and many in Hole 1224F are lined with dark-green clays, calcite, and pyrite. These have only narrow zones of ~1 cm width of darker, slightly altered basalt adjacent to them.

Whereas basalts of Unit 1 from Hole 1224D are quite fresh, a substantial proportion of Unit 1 basalts from Hole 1224F are moderately to strongly altered (Fig. F28). The alteration is present in two forms. The first is prominent dark-gray to light reddish brown or orange-brown alteration zones. The second alteration manifestations are halos that are either adjacent to iron oxyhydroxides and calcite-lined fractures or are located on the edges of pieces that once had fractures but were lost during coring (Fig. F31). Where the fracture fillings are intact, broken bits of iron oxyhydroxides can be seen embedded in calcite lining the same veins (Fig. F32). The iron oxyhydroxides thus lined the fractures first, and the calcite came later. Cores, tens of centimeters to meters in length, are veined and altered in this way (Fig. F28). Even interiors of some pieces of rock with prominent alteration halos are affected, being light tan rather than dark gray in color. Plagioclase crystals, especially, are bleached in contrast to their appearance in equally coarse-grained but darker fresh basalt at the immediate edge of the altered zone. In such bleached rock the halos branch in intricate swirls into the centers of the pieces (Fig. F33), evidently following microfractures. In the cores from Hole 1224D, however, the only basalts altered to this extent are fine grained at the base of the uppermost thick flow (Fig. F28).

Unit 2

Ascertaining the character of the basalts in Unit 2 was hindered by the poor recovery. Few of the rocks were obtained in pieces in which the original orientation could be determined. Many of the smallpieces retain almost circumferential alteration halos (Fig. F34) and were thus not significantly rounded by churning in rubble broken from the hole by the bit. They entered the core barrel at about their original size and shape, which was often smaller than the diameter of the core. As in Unit 1, the halos are presumed to have formed next to fractures lined with iron oxyhydroxides, but few pieces were recovered with fractures. We hypothesize that the host formation was highly jointed or fractured and laced with veins.

Several pieces still retain fresh or altered glass. Ten cooling-unit boundaries were defined on the basis of glass presence in Unit 2 (Hole 1224F) (Table T5). Unit 2 probably contains dozens of small pillows and flows. Two hyaloclastites were recovered. One is cemented by clays and is entirely palagonitized. Interestingly, the other is cemented by calcite (Fig. F35). In the latter, angular glass chips are partially altered to orange palagonite.

The basalts are either nonvesicular or they have only tiny pinhole-sized vesicles, partially filled with clays (dark specks in Fig. F34) or iron oxyhydroxides. The usually dark-gray color of the basalts has been dulled by pervasive minor oxidative alteration.

Unit 3

Unit 3 basalts combine attributes from Units 1 and 2 in that they are fairly coarse grained and have alteration halos adjacent to inclined fracture networks that alternate with small pieces of fine-grained basalt with circumferential alteration halos. Portions of at least two cooling units were recovered through Unit 3 (Table T5). There is a small percentage of vesicles that are filled with secondary minerals. Fracturing and alteration are similar to those of Unit 1. Reddish brown stains are present as either halos on small pieces, or, in larger pieces, the stains are parallel to fractures lined with iron oxyhydroxides and calcite. Vein calcite intergrown with zeolite is present in thin section. One rock has a spectacular closely spaced parallel set of fractures (Fig. F36) lined with iron oxyhydroxides and subsequent calcite. The same sequence is present in crossing fractures in some of the finer-grained basalts of Unit 3 (Fig. F37).

Basalts from Holes 1224A, 1224D, 1224E, and 1224F have a relatively homogeneous mineral paragenesis, but with subtle distinctions in crystal morphologies between basalts of different composition, especially in the quickly quenched margins of pillows and flows. The main phases are plagioclase, clinopyroxene, two oxide minerals, ilmenite and magnetite, globular sulfides, and rare pigeonite. Based on the presence of the pigeonite in the coarsest-grained basalts, the rocks can be classified as tholeiitic basalts. Olivine is rare, and only a few small iddingsitized euhedral to anhedral groundmass crystals in fine-grained rocks have been found. Iddingsite is a typical alteration product of olivine and is made up of a mixture of goethite and layer-lattice silicates (e.g., smectite). The massive basalts are holocrystalline (almost 100% crystals) to hypocrystalline (crystals >50%) and can be described as Unit 1 lava flows. Below the two upper massive flows, Unit 2 basalts have hypohyaline (crystals <50% and glass >50%) texture and volcanic glass contents >90% are common. The increase in glass content indicates the presence of chilled pillow margins. The Unit 3 basalts (~153 mbsf) are holocrystalline massive lava flows. The basalts range from aphanitic (difficult to distinguish the crystals in the groundmass with the naked eye) to aphyric (absence of phenocrysts). Rare plagioclase or plagioclase-clinopyroxene sparsely phyric (<2% phenocrysts) basalts are also present. The crystals in the groundmass are equigranular and are euhedral to anhedral. The texture of the massive basalts of Unit 1 is intergranular (with clinopyroxene in interstitial relationships with plagioclase) to subophitic (with plagioclase laths partially enclosed in clinopyroxene) and, more rarely, intersertal (with microcrystalline to glassy material between plagioclase). Rare vermicular structures in the groundmass between plagioclase laths can be considered as myrmekitic intergrowths of sodic plagioclase and quartz. The presence of segregation vesicles has been commonly reported. Hyalopilitic (with plagioclase laths and clinopyroxene crystals in a glassy matrix) to, more rarely, intersertal textures are found in the pillow lavas of Unit 2. The grain size of the groundmass ranges from very fine grained (0.001-0.5 mm) to fine grained (0.5-1 mm).

Hole 1224A

In Hole 1224A, basement was encountered at 28 mbsf, and a total length of <1 m was recovered (see "Site 1224 Visual Core Descriptions"). This represents the quenched and fine-grained top of the upper massive flow of Unit 1. Six thin sections (15, 24, 25, 26, 52, and 59) have been examined for this hole. The topmost thin section (Sample 200-1224A-4X-CC, 16-21 cm) is made up of plagioclase-clinopyroxene sparsely phyric basalt with <3% phenocrysts (Fig. F38A). Typical phenocrysts are (1) fresh euhedral bytownite (percent An = ~80) with columnar habit and a maximum length of 1.2 mm and (2) partially fractured pale-yellow euhedral Ca- and Mg-rich clinopyroxene (Augite; c^ = ~46°), with prismatic and pseudo-octagonal habit and a maximum size of 3 mm. The clinopyroxene is usually intergrown with smaller, tabular plagioclase crystals. These rare phenocrysts are embedded in an intersertal, intergranular, and rarely subophitic groundmass of plagioclase, clinopyroxene, and opaque minerals (Fig. F38B). The groundmass plagioclase is often skeletal, whereas the clinopyroxene is anhedral and equant. Opaque minerals (Fig. F38C) include acicular, skeletal or equant, and globular sulfide (pyrrhotite). Noteworthy is the variable amount of partially devitrified brownish volcanic glass in the groundmass (generally >50%). Abundant tiny, acicular plagioclase spherulites are present in the glassy matrix. Alteration is generally <6%. The secondary mineralogy is composed of clay minerals and iron oxyhydroxides, the latter replacing olivine. Vesicles are ~2% and in some cases are partially filled with an unidentified yellowish globular, highly birefringent clay.

Brownish to yellowish palagonitized volcanic glass from chilled pillow margins is present in >90% of the two thin sections (15 and 24) from Core 200-1224A-4X. There are a few (~1%) fresh, euhedral, and tabular plagioclase microphenocrysts (maximum size = ~0.5 mm) present in thin section 59 (taken 2 cm below thin section 24). Vesicles filled by brownish to yellowish clay minerals are present in minor amounts (~1%). Reddish granules (~4%), some with rhombic shape, may be olivines replaced by iddingsite.

We recovered aphyric basalts with <1% plagioclase microphenocrysts (maximum size = ~1.1 mm) from Hole 1224A (Cores 200-1224A-5X and 6N). The groundmass is composed of plagioclase, clinopyroxene, and opaques but is only 60% in volume. The remainder is mainly made up of segregation vesicles (Fig. F38D, F38E). These are brown to black and show acicular to skeletal plagioclase, anhedral equant clinopyroxene, and tiny elongate opaque minerals (ilmenite?) in a devitrified glassy matrix. These vesicles represent late-stage interstitial melt. The skeletal and acicular shape of plagioclase and opaques is a result of rapid cooling. Approximately 5% of glass and clinopyroxene is replaced by brownish clay minerals that are often associated with segregation vesicles. A green nontronite (clay mineral) is present (~1%) as cavity and vesicle filling in the bottom of Section 200-1224A-5X-1 (thin section 25) (Fig. F38D). Nontronite is distinguished from chlorite by its color and higher birefringence and from glauconite by weaker pleochroism (green to pale yellow). Cavities are always <3% of the total volume. They are either empty (~60%), partially filled (~15%), or totally filled (~25%). The filling material is either (1) nontronite; (2) a microlitic intergrowth of plagioclase, clinopyroxene, and opaques, all <0.1 mm (segregation cavity); (3) a sulfide mineral (pyrite?); or (4) combinations of all three. Microlitic intergrowths that partially fill some of the cavities are the same material as the segregation vesicles and may represent late-stage magmatic melts. In one case, a large pyrite crystal (~1 mm) almost entirely fills one such cavity.

Hole 1224D

In Hole 1224D, a total of 15.65 m of basalt was recovered in 31 m of coring (see "Site 1224 Visual Core Descriptions"). Eighteen thin sections (34-51) were examined for this hole. Only the massive basalts of Unit 1 were recovered, reaching into the top few meters of the lower flow. Approximately 55% of the recovered rocks are aphanitic, aphyric basalts. The average dimension of the groundmass crystals is <0.5 mm; ~15% are clinopyroxene-plagioclase sparsely phyric (<2% phenocrysts) basalts (average crystal dimension = <0.5 mm), and the remaining 30% are fine-grained (average crystal dimension = ~0.5 mm) basalts. All but one of the thin sections are holocrystalline with hypidiomorphic, equigranular, and isotropic textures (Fig. F39A, F39B, F39C). Petrographic textures are typical for relatively thick lava flows. An alteration zone at 47-48 mbsf (in Section 200-1224D-3R-3) (Fig. F28) is the transition zone between the two cooling units. Unfortunately, the contact between these two flows was not recovered.

The rare phenocrysts are pale-yellow subhedral, equant, partially fractured clinopyroxenes (maximum size = ~1.5 mm), and/or euhedral tabular to anhedral bytownitic plagioclase (maximum size = ~3 mm). Glomerocrysts of plagioclase and clinopyroxene with two mineral intergrowths are present in thin sections 36, 48, and 50. The groundmass is composed of plagioclase, clinopyroxene, and opaque oxide minerals for all Hole 1224D basalts. Pigeonite, Ca-poor clinopyroxene, is characteristically found in subalkaline volcanic rocks (Fig. F39D). It is present in the groundmass in only two thin sections (44, 45). These thin sections were taken from the upper flow in lithologic Unit 1 (Samples 200-1224-D-3R-1, 50-53 cm, and 3R-1, 54-57 cm). Plagioclase with tabular to acicular habit has intergranular to subophitic textures with anhedral pale-yellow clinopyroxene (augite; 2V = ~60°) and euhedral to anhedral equant or skeletal opaque minerals (Fig. F39E, F39F). In some cases, plagioclase is skeletal and the voids are filled by microlites of clinopyroxene and skeletal opaque minerals. Pigeonite is a clinopyroxene characteristic of subalkaline volcanic rocks, and its presence confirms the petrographic classification of tholeiitic basalts. It is distinguished from Ca-rich clinopyroxene mainly on the basis of its pseudomonoassic character (2V = ~0°) and its euhedral shape. Plagioclase and pyroxenes are usually fresh with only minor signs of alteration. The plagioclase, clinopyroxene, opaques, and pigeonite groundmass ranges from 40% to 90% in volume and is generally >70%. The presence of "open" plagioclase spherulites in fine-grained interstitial patches in the groundmass is relatively common (Fig. F39E). These are composed of columnar to acicular plagioclase radiating from a center. The space between each plagioclase lath is occupied by smaller clinopyroxene and plagioclase spherulites. Many interstitial patches contain one or more large euhedral or skeletal titanomagnetite crystals (Fig. F39E, F39F) suggesting crystallization from very iron-rich late-stage in situ liquids. Some also contain round sulfide globules that segregated immiscibly from such liquids (Fig. F39E).

The second most abundant feature is irregular segregation vesicles similar to those already described in basalts from Hole 1224A (~5% to 40% but generally >25%). These are crystallization products of late-stage melts that leaked into ruptured vesicles. The segregation vesicles have acicular to skeletal plagioclase, subhedral to anhedral, equant to prismatic clinopyroxene, and needlelike or trellised iron titanium oxides in a devitrified glassy matrix. Plagioclase and clinopyroxene are smaller in the segregation vesicles than in the groundmass, but the oxide minerals can be much coarser. The following features are present in only some thin sections or are present in association with only one segregation vesicle: (1) a tiny elongated, often acicular (i.e., needlelike) opaque mineral (ilmenite?); (2) a large (up to 0.6 mm) equant, skeletal, opaque mineral (Ti magnetite?); (3) small (~0.1 mm) spherical drops of a sulfide mineral (pyrrhotite?); (4) an acicular low-birefringent mineral with roughly parallel extinction, 2V = ~90°, and rosette texture (albite-oligoclase?); and (5) a high-birefringent mineral with a columnar habit (epidote?). Segregation vesicles (variable from ~5% to 50% but generally <30%) are altered to brownish to yellowish clay minerals and iron oxyhydroxides. Interstitial volcanic glass is easily altered to clays. Alteration only affects the glass in the segregation vesicles, not the plagioclase and clinopyroxene. There are also patches of a brownish clay mineral that is not related to the segregation vesicles. Here, the clay minerals have an interstitial relationship with the groundmass plagioclase and are possibly a result of clinopyroxene or interstitial glass alteration.

Vesicularity is variable in the basalts. Both virtually vesicle-free zones (e.g., Core 200-1224D-2R; ~5 m thick with much less than 1% cavities) and relatively vesicle-free zones (e.g., Core 200-1224D-1R; ~4.4 m thick, with cavities ranging from 3% to 6%) are present. Generally, the percentage of vesicles decreases with depth above ~60 mbsf, and vesicles are concentrated mainly in the first 5 m of the cored basement, although the number of cavities increases in the deeper part of Hole 1224F. The cavities are either empty (5%-50%), partially filled (35%-60%), or completely filled (20%-100%). All three types of vesicles with subspherical shape often coexist in the same thin section. The filling material may be (1) brownish clay minerals; (2) microlitic intergrowth of plagioclase, clinopyroxene, acicular opaques, plus possibly glass and clay minerals; (3) chalcedony?; (4) aragonite; (5) calcite; (6) brownish to pale greenish clay minerals; (7) sulfide minerals (pyrite?); or (8) a combination. The border of the cavities is often made of plagioclase or elongated opaque minerals tangent to the outer surface. In a few cases, vermicular structures are associated with carbonate-filled cavities. These have been interpreted as possible evidence of biological processes (see "Microbiology").

There are only a few thin sections of veined basalts (36, 39, 41, 48, 50). The filling material ranges from Ca carbonate (either calcite or aragonite) to anhedral to cubic euhedral sulfide minerals (pyrite?), lowbirefringent minerals (zeolites?), iron oxyhydroxides, acicular colorless minerals with high birefringence and parallel extinction (lawsonite?), chlorite, and clay minerals. In hand specimen, the iron oxyhydroxide-filled cavities have a pale brownish halo ~1.5 cm wide on both sides of the vein. This halo is due to the presence of a brownish film (iron oxyhydroxide?) coating the groundmass minerals.

The top of the lower lava flow in Unit 1 (Section 200-1224D-4R-1) has textures typical of gas pipes (Fig. F30); vertical bands (Fig. F30A) have the same minerals as the groundmass but with coarser size (maximum plagioclase size = ~0.6 mm). Some of them contain their own internal segregation vesicles (Fig. F30B). The gas pipes are preferential escape routes for volcanic gases (Fig. F30C). Their segregation vesicles contain plagioclase, brown clinopyroxene, and trellised iron titanium oxides.

Hole 1224E

In Hole 1224E, ~4.4 m of basalt was recovered over a total cored section of ~9.6 m (~46% recovery) (see "Site 1224 Visual Core Descriptions"). Three thin sections (53-55) of volcanic rocks were examined from Hole 1224E. In two of the thin sections (53 and 54) we observed hyaloclastitic breccias composed of variably sized (0.5 to 3 cm) subangular to subrounded glass clasts set in carbonate cement. The glass clasts are almost always altered and have an orange palagonite rim of variable thickness (Fig. F40A, F40B). The palagonite is embayed into the glass along fractures (Fig. F40A) and has alternating dark- and light-orange bands concentric to the surfaces of the glass, with which it forms a sharp contact (Fig. F40B). It penetrates into the glass along small microfractures (Fig. F40C).

The carbonate cementing the glass shards is fine to very fine grained, but larger crystals (~0.3 mm) are present at the contact with the glass clasts. A colorless biaxial mineral with low relief, extremely low birefringence, a rhombic to prismatic habit, and multiple twins is only present in the carbonate veins and where it is in contact with the glass clasts. This mineral may be either tridymite or one of the zeolite-group minerals. Alternatively, the mineral may be a zeolite.

The hyaloclastite flow top is the only portion of the upper flow in Unit 1 to reveal the quench mineralogy of the flow. Quench crystals include rare prismatic, tabular, and equant plagioclase microphenocrysts ~0.1 to 0.3 mm in length (Fig. F40C). The sections also contain numerous smaller acicular plagioclase crystals <0.1 mm in width, similarly tiny euhedra of clinopyroxene, and these two minerals intergrown (Fig. F40C, F40D, F40E). In some larger shards, these minerals are coated with dark-brown plagioclase spherulites set in pale-brown glass (Fig. F40D). In some palagonitized glass, there are rare skeletal olivine crystals ~0.1 mm long (Fig. F40F). These are altered to iddingsite (clays plus iron oxyhydroxides).

The third thin section (55) from Hole 1224E is from an aphyric holocrystalline basalt with hypidiomorphic texture and <1% plagioclase phenocrysts (maximum size = ~2.2 mm). The groundmass and texture are similar to the basalts from Holes 1224A and 1224D. Cavities compose ~5% of the rock, and they are almost totally filled. The material infilling the cavities is mainly calcite. Microlitic intergrowth of plagioclase, opaque minerals, and clinopyroxene (segregation material) fills ~30% of the cavities; rarely (~2%-3%) the cavities are filled by a brownish clay mineral.

Hole 1224F

In Hole 1224F, a total of 37.7 m of basalt was recovered from a cored section of 147 m (recovery ~25%) (see "Site 1224 Visual Core Descriptions"). The low recovery of this hole is related to the morphology of the volcanic rocks that, below Core 200-1224F-4R (63 mbsf), are mainly pebble-sized pillow lava fragments with a few thin massive lava flows near the bottom of the hole. Twenty-seven thin sections (56-60, 62, 64-69, and 71-85) were examined from Hole 1224F. There are pillow basalts (74, 76, and 78) (10% have glassy chilled margins), volcanic breccias (65 and 66), and hyaloclastites (79 and 80). Seventy percent of the rock recovered is aphanitic, aphyric massive lava flows. Sixty-five percent of the massive lavas are designated as very fine grained (<0.5 mm), and 35% are defined as fine grained (~0.5 mm). All of the massive basalts are holocrystalline with hypidiomorphic, equigranular, and isotropic textures. Pillow lava fragments were cored from ~62 to ~130 mbsf. They have holocrystalline (with intergranular to subophitic textures) to hypohyaline (with intersertal to hyalopilitic textures and >90% glass) textures.

Massive flows and pillow lavas have subhedral to euhedral skeletal to tabular plagioclase in an intergranular to subophitic relationship with anhedral to subhedral clinopyroxene (possibly augite; 2V = ~60°) and subhedral to euhedral equant opaque minerals in the goundmass. A few (<1%) euhedral tabular plagioclase microphenocrysts (maximum size = <1.4 mm) are present. Groundmass minerals compose 55% to 80% of the rock, and the abundance of segregation vesicles is 10% to 30%. These have a paragenesis similar to the segregation vesicles of Holes 1224A and 1224D, with acicular to skeletal plagioclase, anhedral to subhedral equant clinopyroxene, tiny acicular opaques (ilmenite?), and skeletal equant Ti magnetite set in a partially devitrified glassy matrix. Cavities and vesicles are generally rare (all but one sample [81] have <10%) and empty (80%-100%). In a few cases, the cavities are partially (~10%) to totally (100%) filled. The filling materials are brownish clay minerals and possibly iron oxyhydroxides.

Pillow lava fragments cored from ~62 to ~130 mbsf are holocrystalline (with intergranular to subophitic textures) to hypohyaline (with intersertal to hyalopilitic textures and >90% of glass). From Cores 200-1224F-5R to 200-1224F-11R, the rocks have compositions similar to that of the lower flow in Unit 1. Therefore, these rocks reveal the quench mineralogy of the lower massive flow. The quench mineralogy is seen in glassy pillow rims extending into zones of coalesced and sheaf spherulites within ~2 cm of pillow rims (Fig. F41A). The spherulites at the pillow rims are dark-brown globules that coalesce within ~1 cm of wholly glassy rims. The glass itself is light grayish brown except where it has been altered to yellowish palagonite. The most abundant quench mineral is acicular plagioclase, usually present as isolated crystals ~0.5 to 1.0 mm long or as intergrowths with smaller anhedral clinopyroxene (Fig. F41A, F41B). More rarely, small euhedral olivine (Fig. F41C) and zoned euhedral clinopyroxene (Fig. F41D) also are present. Even in palagonite, the plagioclase and clinopyroxene are fresh. Olivine, however, is usually altered to orange-red iron oxyhydroxides and clay minerals (iddingsite).

Pillow fragments show a large textural variation, from virtually holohyaline (glass = >90%) through hypohyaline (crystals = <50%) and hypocrystalline (glass = <50%) to virtually holocrystalline (glass = <10%). The coarser-grained fragments are from the inner parts of the pillows. These experienced a lower undercooling than quenched pillow rims; therefore, they had the possibility to develop larger crystal sizes. Groundmass phases of the pillows are the same as the massive flows, except that large equant opaque minerals, such as those found in segregation vesicles of the massive lavas, are not found here. Plagioclase is often arranged in isolated round or coalesced sheaf and fan spherulites, with fibers of plagioclase becoming coarser in this progression toward pillow interiors. In the more glassy samples, small glomerules of plagioclase and clinopyroxene (average size = <0.5 mm) give the pillow rims a spotted appearance.

In a few cases, gabbroic clots or xenoliths ~1 cm in diameter (Sample 200-1224F-3R-1, 90-92 cm) also are present (Fig. F41E, F41F). These indicate scavenging of phases that crystallized in the lower gabbroic portion of the ocean crust. The mineralogy of the xenoliths is similar to the rare phenocrysts—subhedral to euhedral tabular plagioclase and anhedral to euhedral equant prismatic clinopyroxene. Absence of the magmatic oxides, ilmenite and magnetite, is a characteristic of gabbroic cumulates from the ocean crust (e.g., Natland and Dick, 1996).

The increase in size and morphology of plagioclase spherulites from the pillow rims to their interiors is associated with the disappearance of fresh olivine in the pillow interiors. The mineral evidently was better preserved in nearly impermeable glass. The chilled margins vary from vesicle-free to slightly vesicular (~3%); vesicles and cavities are generally partially filled with calcium carbonates.

Two hyaloclastite breccias were recovered at 102 and 129 mbsf, both lying within the interval of ferrobasaltic pillows and flows comprising Unit 3. These consist of subangular glassy shards of variable size from 0.5 to ~3 cm set in a calcitic cement (Fig. F42A, F42B, F42D). In some places, the cement can also be reddish to dark-brown iron oxyhydroxides. The glassy fragments are almost always altered with a thin to thick orange palagonite rim. The glassy shards of the sample at 102 mbsf (Sample 200-1224F-12R-1, 61-64 cm) are almost totally glassy, with the percentage of plagioclase, clinopyroxene, and olivine crystallites even in the most crystalline shards <2%. This aspect is quite different from the chilled margins of the pillow fragments of Unit 2 and the glass flow top of the upper flow of Unit 2, which always have several percent of plagioclase crystallites, dark spherulitic rims to those crystals, and a few microphenocrysts of plagioclase, clinopyroxene, and olivine (Figs. F40C, F41A, F41B). The more numerous small crystallites in the pillow rims thus nucleated and grew after eruption at a smaller undercooling than was experienced by the purely glassy hyaloclastites.

The calcitic cement is fine to very fine grained. Larger calcite crystals (average size = ~0.3 mm) formed only at the contact with the glassy shards. A colorless mineral with low relief and extremely low birefringence, rhombic to prismatic habit, and multiple twinning (possibly tridymite or zeolite) is intergrown with the calcite in the veins at the contact with the glassy shards.

The partly crystalline glass shards in the Unit 3 hyaloclastite have crystals of olivine, plagioclase, and clinopyroxene that are at once smaller and more numerous than the most comparable glassy portions of pillow rims of Units 1 and 2 (Fig. F42D, F42E). Near the pale-yellow palagonitized glassy rims of these shards, short prismatic to acicular and partially skeletal plagioclase crystallites and stellate intergrowths of plagioclase and clinopyroxene are set in the altered glass (Fig. F42D). This photomicrograph also shows two small, skeletal olivine crystals altered to orange secondary minerals. Farther away from the glassiest rims, many of the crystallites and stellate intergrowths are coated with brown fibrous spherulitic material (Fig. F42E). All of these crystallites and crystal fibers formed after eruption at high undercooling during quenching of the glass.

There are no larger tabular plagioclases or well-formed clinopyroxene microphenocrysts. These basalts therefore arrived at the seafloor as almost completely aphyric magma, in contrast to the basalts of Units 1 and 2. However, the nucleation density of plagioclase nearest the quenched edges of the shards was greater (compare Fig. F42D, F42E to Figs. F40, F41A, F41B, F41C, F41D). This probably reflects the more differentiated composition and accordingly higher viscosity of the ferrobasaltic liquids when they were so suddenly and strongly cooled.

The interiors of pillows and flows of this basalt show a mineral paragenesis similar to that of the two upper flows of Unit 1. The differences are (1) these portions of the rocks are coarser grained (average dimension of the crystals = >0.5 mm), (2) these have a higher proportion of segregation vesicles (~50%), (3) the acicular opaque minerals in the segregation vesicles are much longer (up to 0.6 mm), and (4) they are more altered to brownish clay minerals (~7%-10%).

All the petrographic features of volcanic basement at Site 1224 can be found in eight Microsoft PowerPoint files (see the "Supplementary Materials" contents list). The first four files (IA to ID) deal mainly with petrographic definitions and the classification of the massive lava flows (phenocrysts and groundmass); the fifth and sixth files (IIA and IIB) deal with pillow-related features; and the seventh and eighth files (IIIA and IIIB) deal mainly with alteration. Tables with detailed petrographic descriptions (of which there are 52 samples) are available in the "Site 1224 Thin Sections".