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doi:10.2204/iodp.proc.336.105.2012

Petrology and hard rock geochemistry

Basement in Hole U1383C was cored from 69.5 to 331.5 mbsf with an overall recovery of 19.2%. The recovered core material was divided into three major lithologic units on the basis of changes in rock lithologies and contacts between pillow lavas and sheets or more massive flows (Fig. F4; Table T5). Each unit was further divided into subunits on the basis of sedimentary and brecciated contacts, including interflow sediments, tectonic or sedimentary breccias, and hyaloclastites. In contrast to Hole U1382A, subunits were not defined on the basis of chilled margins because of the large number of pillow margins recovered.

The basalts comprise three distinct petrologic types: (1) sparsely plagioclase-olivine-phyric basalt (Unit 1), (2) highly plagioclase-olivine-phyric basalt (Unit 2), and (3) aphyric cryptocrystalline to fine-grained basalt (Unit 3). The units consist of pillow sequences (Units 1 and 3) and perhaps a sheet or massive flow (Unit 2). They range in thickness from 37 m (Unit 2) to 57 m (Unit 1) to 161 m (Unit 3). The core sequence comprises 247 chilled margins and several brecciated glassy zones, featuring spectacular hyaloclastites with variably palagonitized glass clasts. Interflow sediments and sedimentary breccias were recovered in 12 different intervals representing a total of 1% of recovered basement rocks. Interflow materials are present in Units 1 and 2 (to 165 mbsf) and are classified as limestone on the basis of micritic calcite filling. They lack sedimentary structure and well-preserved micro- or nannofossils. Each major lava flow unit consists of several cooling units, which are recognized by glassy or variolitic margins or marked changes in groundmass grain size. Initial results from thin section studies reveal a range of groundmass grain size, from glassy to fine grained with aphanitic, hyalophytic to intersertal textures. Trace amounts of primary sulfides were observed in thin sections of fine-grained basalts.

Basalts of Unit 1 are either aphyric or sparsely plagioclase-olivine-phyric and have <3% vesicles. Basalts of Unit 2 are highly plagioclase-olivine-phyric, with phenocryst contents ranging up to 13% and plagioclase being more abundant than olivine. The phyric basalts in Unit 2 are petrographically similar to the plagioclase-olivine-phyric basalts cored in Hole U1382A (Unit 6), although geochemical analysis suggests that these basalts formed from different parental magma sources. The aphyric pillow basalts in Unit 3 are petrologically and geochemically similar to each other and include several discrete intervals of hyaloclastites. These intervals are most abundant between Cores 336-U1383C-20R and 24R and probably represent different successive cooling units. Unit 3, although much thicker, is also petrographically similar to the pillow lava units in Hole U1382A, but the bulk rock chemical data indicate different magma sources.

In all units, the extent of alteration ranges from fresh (<1%) to 60%, with brown to dark green clay followed by iddingsite being the most abundant secondary phases replacing primary minerals and mesostasis. Vesicles and veins, representing <2% of the recovered core material, are variably filled by the following assemblages: (1) smectite (e.g., nontronite and saponite) and celadonite; (2) reddish-brown iddingsite, generally logged as mixed Fe oxyhydroxides and smectite assemblages; (3) zeolite, identified as mainly phillipsite; (4) carbonate, identified as mainly calcite; and (5) micritic limestone similar to the calcareous interflow sediment. The volcanic basement in Hole U1383C has an average vein density of 33 veins/m (Fig. F5), which is significantly higher than in Hole U1382A (average of 20 veins/m) but is similar to other sections from the upper volcanic basement (Alt et al., 1996).

Lithologic units

Unit 1

• Depth: 69.5–126.7 mbsf
• Lithology: nonvesicular to sparsely vesicular aphyric to sparsely phyric pillow lava

Unit 1 is composed of 11 subunits that are distinguished mostly on the basis of brecciated contacts such as interflow sediments and sedimentary breccia. The basalts are nonvesicular (<1% vesicles) to sparsely vesicular (maximum of 4% vesicles) and sparsely plagioclase-olivine-phyric, with plagioclase phenocrysts (1%–3%) being more abundant than olivine phenocrysts (<0.1%–1%). Plagioclase phenocrysts range from <0.1 to 2 mm in size and are generally fresh. Only pervasively altered pieces show slight plagioclase alteration (<10%). Olivine phenocrysts are <1 mm in size and are moderately to completely altered to an assemblage of secondary brown clay (smectite) and Fe oxyhydroxides (± iddingsite) in grayish-brown alteration patches.

Groundmass grain size ranges from cryptocrystalline to microcrystalline and is rarely fine grained; glass is encountered only within chilled margins. Groundmass alteration is generally slight to moderate (5%–20%) but reaches 40% in pervasively altered basalts (Fig. F6). The average extent of alteration in Unit 1 is ~8%. Altered basalts generally display well-developed alteration patches and halos along veins and broken surfaces of pieces (Fig. F7). Vesicles remain mostly unfilled, with only 25% vesicle filling, on average, composed of mixed smectite (possibly celadonite) and Fe oxyhydroxide (possibly iddingsite) mineral assemblages (Fig. F6).

Unit 1 features five subunits of interflow sediments composed of micritic limestone (Fig. F8). In some cases, interflow sediments are defined as sedimentary breccias because of the occurrence of angular basaltic clasts (mainly altered glass) in the matrix. Glassy margins are variably palagonitized (up to 80%) with a reddish-brown alteration color and are occasionally in direct contact with interflow sediments (e.g., Section 336-U1383C-3R-2 [Piece 2]). The sharp and irregular contact between the limestone and altered glassy margin is indicative of direct contact with soft sediment, presumably nannofossil ooze, during lava emplacement.

The average vein density in Unit 1 is ~20 veins/m, and vein width ranges from 0.1 to 5 mm. The larger veins, typically >1 mm, are composed of carbonate (micrite) of sedimentary origin (i.e., from interflow limestone). Thinner veins are filled with a mixed mineral assemblage of smectite, iddingsite (± Fe oxyhydroxides), and zeolite. Microcrystalline carbonate occurrence in this assemblage is considered to form from micritic carbonate dissolution and recrystallization. Grayish-brown halos, and more rarely dark green halos, line veins, with larger halos lining veins exposed at the outer surface or side of a piece.

Unit 2

• Depth: 126.7–163.7 mbsf
• Lithology: sparsely vesicular phyric basalt sheet to massive flow

This unit of plagioclase-olivine-phyric basalt is composed of nine subunits distinguished from each other on the basis of sedimentary breccias and interflow sediments. About 25 chilled glassy margins with planar glassy contacts were recovered, together with limestone intervals, suggesting that this unit represents several sheet flow cooling units (Fig. F9).

The basalts are sparsely vesicular (<5% vesicles), show dominantly glomerophyritic texture, and have a cryptocrystalline to fine-grained groundmass. Tabular to elongated euhedral plagioclase phenocrysts have an overall abundance between 4% and 7% on average, with crystal sizes ranging from <1 to 2.5 mm. Olivine phenocrysts are less abundant (~1% modal abundance) and smaller in size (<1.4 mm). Euhedral olivine phenocrysts are partly altered (~40% alteration) to brown clay (smectite ± iddingsite) in the alteration patches, whereas plagioclase remains fresh or is merely tainted with Fe oxides. Glass from chilled margins is extensively devitrified and altered to palagonite (up to 95%), whereas groundmass alteration averages 7%–15% and is always <25%. Note that the highly altered aphyric chilled margin (up to 70% alteration) in Section 336-U1383C-9R-1 (Piece 1), together with basalt in Section 9R-1 (Piece 2), is possibly derived from borehole wall breakouts from Unit 1 above.

Vesicles are filled with abundant Fe oxyhydroxides (± iddingsite), smectite (± celadonite), and minor zeolite (phillipsite) and calcite. Some vesicles remain unfilled (50% on average), with only thin coatings of dark green clay (e.g., nontronite). The grayish-brown alteration halos have more unfilled or iddingsite-rich vesicles than the freshest dark gray groundmass. In many cases, patchy alteration cannot be attributed to haloed veins on the basis of recovered specimens (Fig. F10). Pervasive grayish-brown alteration was also encountered in several pieces and is the result of the complete alteration of olivine phenocrysts and slight plagioclase alteration. Sedimentary breccias were recovered in four intervals (Sections 336-U1383C-10R-1 [Piece 8], 10R-2 [Pieces 9 and 19], and 12R-1 [Piece 11]) and are composed of >80%–95% micritic carbonate matrix with minor angular and highly altered glass clasts ranging from <0.1 to 8 mm (Fig. F8).

Unit 3

• Depth: 163.7–324.8 mbsf
• Lithology: nonvesicular aphyric pillow lava

This unit of aphyric basalt is composed of 30 subunits that were identified on the basis of hyaloclastites (13 subunits) and tectonic breccias (2 subunits). A total of 173 glassy or variolitic chilled pillow margins were recovered (Fig. F4), suggesting that Unit 3 is composed of multiple pillow lava sequences. The basalts are generally nonvesicular (<0.5% vesicle abundance), with the exception of irregular vugs as wide as 5 mm encountered in Sections 336-U1383C-20R-1 (Pieces 25 and 26) (Subunit 3-14; see “Core descriptions”). In Core 336-U1383C-28R, vesicle abundance increases substantially, up to 3%, also because of the occurrence of irregular vugs. Basalt groundmass generally ranges from glassy to cryptocrystalline, although microcrystalline to fine-grained groundmass with intersertal texture is not uncommon. Fine-grained basalts feature plagioclase laths up to 1 mm, with olivine <0.3 mm.

Recovered basalts are mostly fresh (<1%) to moderately altered (up to 15% alteration), with a common blotchy alteration texture of devitrified variolitic glass in pillow margins (Fig. F11). Alteration halos are also common and feature mixed Fe oxyhydroxides, brown to green clay (smectite), and minor carbonate replacing olivine and interstitial glass in the groundmass and filling vesicles. The cryptocrystalline groundmass has numerous microphenocrysts of olivine that are variably altered (~60% alteration on average).

Volcanic breccias (hyaloclastite) were recovered throughout Unit 3 but are most abundant in Cores 336-U1383C-20R and 22R (Fig. F4). The hyaloclastite subunits are rather similar to each other, and all display poorly sorted angular clasts of glass ranging in size from 0.1 to 15 mm (Fig. F12). Clasts are essentially composed of glassy basalt fragments covering the whole spectrum of alteration intensity from fresh obsidian-like glass to completely palagonitized and devitrified glass. In some cases, the contact of hyaloclastite with a cryptocrystalline basalt margin was recovered. In other cases, highly fractured chilled margins grade into hyaloclastite in the outer rim (Fig. F12). The matrix of hyaloclastites is often lacking, leading to numerous open vugs partly filled by reddish Fe-rich clays and zeolite. Zeolite generally constitutes the cement between basalt clasts.

The top of Unit 3 features a remarkable tectonic breccia (Section 336-U1383C-11R-1 [Piece 9]) showing a sharp contact between two different basalt lithologies from Unit 3 (microcrystalline and fine-grained aphyric basalt) separated by micritic sedimentary filling (Fig. F13). This interval is considered to represent a fault or large fissure that was filled by debris from overlying sediment and basalts. Several contacts with tectonic breccias were also recovered on the outside of basalt pieces (e.g., Cores 336-U1383C-26R and 32R; Fig. F13). These breccias are composed of poorly sorted angular clasts of altered basalts and light to dark brown clay filling.

Igneous petrology

Sparsely plagioclase-olivine-phyric basalt (Unit 1)

The uppermost part of Hole U1383C (Unit 1) is composed of sparsely (~3%) plagioclase-olivine-phyric basalt. Plagioclase phenocrysts (up to ~1 mm) in Unit 1 basalts are euhedral to subhedral and typically well elongated. Some of the phenocrysts have hollow cores parallel to an elongated direction filled by glass or cryptocrystalline groundmass material (Fig. F14). About 30% of the phenocrysts occur as glomerocrysts in which tabular plagioclase crystals form an assemblage with olivine. Olivine phenocrysts are present throughout as euhedral to subhedral crystals and are generally polyhedral to granular in shape. Most of them are <0.2 mm in size, although crystals as large as 1 mm were observed.

The groundmass texture of microcrystalline to fine-grained basalts in Unit 1 is intersertal, although glassy to cryptocrystalline chilled margin samples (e.g., Sections 336-U1383C-2R-2 [Piece 3] and 6R-1 [Pieces 7 and 17]) have hyalophitic to aphanitic textures. In the microcrystalline to fine-grained basalts, groundmass mainly consists of ~40% lath-shaped plagioclase (as large as 2 mm), ~30% anhedral to subhedral pyroxene, and 2% equant or elongate opaque Fe-Ti oxide minerals (as large as 0.04 mm). Plagioclase crystals are skeletal, showing “swallow-tail” and rectangular “belt-buckle” forms. In the microcrystalline mesostasis that typically makes up 20%–40% of the rock, plumoselike intergrowth between acicular plagioclase needles and interstitial anhedral clinopyroxene were also commonly observed with Fe-Ti oxide mineral grains.

In the chilled margin samples, a variety of quench crystallization textures were observed, including spherulitic textures with glassy groundmass and plagioclase sheaves with or without plumose clinopyroxene. From the glassy margin to the more crystallized interior, three zones showing different quench crystallization textures were typically recognized in the samples: (1) a glassy spherulite zone consisting of a glass matrix in which tiny and skeletal acicular plagioclase, tabular plagioclase phenocrysts, and granular olivine microphenocrysts form the interior of dark spherules (Fig. F14B); (2) a dense spherulite zone with only a few acicular plagioclase in dark groundmass; and (3) a crystallized zone composed of well-developed plagioclase fan-spherulites with interstitial cryptocrystalline mesostasis (Fig. F14C). Very fine equant to elongate titanomagnetite crystals in mesostasis and in interstices between plagioclase were also observed in the crystalline zone (Fig. F14C).

Highly plagioclase-olivine-phyric basalt (Unit 2)

The basalts from Unit 2 are characterized by abundant (up to 10%), large (up to 6 mm) euhedral plagioclase phenocrysts (some of which can be regarded as “megacrysts”), with ~3% of olivine up to 0.5 mm. The plagioclase phenocrysts often contain glass inclusions as blebs in their cores (Fig. F14D), and some parts of the phenocrysts also show internal skeletal structures with irregularly shaped inclusions of groundmass or glass (Fig. F14E). The edges of phenocrysts often exhibit dendritic overgrowth (Fig. F14F), reflecting undercooling upon extrusion. Euhedral to subhedral olivine phenocrysts were also observed throughout Unit 2 basalts as polyhedral to granular crystals. Most are <0.3 mm in size, although crystals as large as 1 mm also occur. Between 30% and 50% of the plagioclase phenocrysts are glomerocrysts, in which tabular plagioclase crystals form single-phase glomerocrysts or mixed-phase glomerocrysts with olivine.

The groundmass grain size of Unit 2 basalts is microcrystalline to fine grained. In the microcrystalline basalts, plagioclase laths are skeletal and elongated, and swallow-tail and belt-buckle forms were commonly observed. The microcrystalline groundmass also contains quench-textured mesostasis composed mainly of sheaves of acicular plagioclase needles and anhedral clinopyroxene forming plumose intergrowth. Equant or elongate tiny opaque Fe-Ti oxides (as large as 0.02 mm) occur in interstices between plagioclase laths and needles with clinopyroxene. Plagioclase laths in the fine-grained basalts exhibit a tabular shape and less skeletal intergrowth with anhedral prismatic clinopyroxene crystals. Fe-Ti oxide mineral grains are coarser (>0.06 mm) than those in the microcrystalline basalts and occur in interstices between plagioclase lath as equant to elongate grains.

Aphyric cryptocrystalline basalt (Unit 3)

The pillow basalts in Unit 3 are aphyric with a glassy to fine-grained groundmass. In altered samples, small olivine crystals (equant to elongate, skeletal, and up to 1 mm) are pseudomorphed by brownish clay minerals. The groundmass is composed of olivine, plagioclase, clinopyroxene, Fe-Ti oxide, and crypto- to microcrystalline mesostasis. The groundmass grain size of the basalts generally ranges from glassy/​cryptocrystalline to microcrystalline/​fine grained, which corresponds to chilled margins and the interior of pillow lobes, respectively.

In microcrystalline basalts, plagioclase is present as skeletal laths or acicular sheaves (<1 mm in size). The skeletal plagioclase laths often exhibit rectangular belt-buckle and swallow-tailed forms (Fig. F15A), depending on the cut directions of the crystals. Several samples contain radiating acicular plagioclase bundles enclosing tiny clinopyroxene and titanomagnetite (Fig. F15B). On the other hand, plagioclase crystals in fine-grained samples exhibit tabular shapes and are as large as 2 mm. Olivine crystals in Unit 3 basalts show a variety of quench crystal morphologies. In finer grained samples, characteristic olivine morphologies of the skeletal and elongated olivine crystals were commonly observed, including hopperlike (Fig. F15C), lanternlike (Fig. F15D), and swallow-tailed (Fig. F15E) forms. Clinopyroxene crystals are generally intergrown with plagioclase. In microcrystalline samples, plumose intergrowth of acicular plagioclase needles and anhedral clinopyroxene was commonly observed in the groundmass. In fine-grained samples, on the other hand, coarser grained prismatic clinopyroxene crystals are intergrown with tabular plagioclase laths. Fe-Ti oxide mineral grains in the samples are very tiny (generally <0.01 mm) and equant or elongated in shape. These minerals occur in the mesostasis and interstices between acicular plagioclase needles.

Unit 3 is characterized by well-recognizable variolitic-textured chilled margins with or without fresh volcanic glass. Thin sections made from variolitic-textured chilled margin samples (e.g., Sections 336-U1383C-16R-1 [Piece 14], 16R-2 [Piece 6], and 19R-1 [Piece 6]) are mainly composed of a glassy outer zone and variolitic inner zone. The glassy zone is composed of “linked chain” and “lantern and chain” olivine crystals (Fig. F15F) and devitrified glassy to cryptocrystalline matrix. Acicular plagioclase sheaves or swirls occur in the center of each variole, together with intergrown clinopyroxene and Fe-Ti oxide. The frequency of varioles increases from the glassy part to the variolitic zone, which suggests that the variolitic texture (or the variole itself) occurs at the transition from glass solidification to mineral crystallization.

Interflow sediments and sedimentary breccias

The lithology of interflow sediments recovered in Hole U1383C is essentially micritic limestone (i.e., very fine calcite crystals) with light brown-beige color. The limestones also contain poorly sorted volcanic clasts of different sizes, from silt to sand size and gravel. Thin section observations of Section 336-U1383C-7R-2 (Piece 13), Thin Section 37, show that the limestones are made of very fine calcite crystals with two domains separated by a layer of palagonitized glass clasts. These domains are differentiated on the basis of clay concentration, which resulted in slightly different colors. No sedimentary structure or micro- to nannofossils were identified in thin section. According to the classification of Dunham (1962) for limestone, interflow sediments represent mudstone with very rare nannofossils and clasts. Smear slides allowed identification of some very rare nannofossils such as Discoaster spp.

Some contacts between chilled margins and limestone show that lava emplacement occurred after deposition of the limestones or its ooze precursor. The limestone described above may represent a highly lithified version of the nannofossil ooze and chalk described in the sedimentary section of Hole U1383E. The unconsolidated ooze could have undergone high-temperature dissolution and recrystallization during the emplacement of overlying lava flows. This possibility would explain the highly lithified state of the limestone, which is unusual for young (<10 Ma) sediments overlying basaltic oceanic crust.

Volcanic rock alteration

All basement volcanic rocks recovered from Hole U1383C are affected only by low-temperature alteration by seawater. Dark gray rocks with grayish-brown alteration patches are the most abundant type and are present throughout the basaltic section. These rocks are the least altered, generally containing <3% by volume of secondary minerals such as dark green clays (saponite and celadonite) and Fe oxyhydroxides (iddingsite) replacing olivine phenocrysts, groundmass, and filling vesicles. In addition, glass from pillow margins and hyaloclastite is variably replaced by palagonite. Zeolite, identified as phillipsite in thin section and by X-ray diffraction (XRD), is a common secondary mineral filling vesicles and veins and forming hyaloclastite cement. Carbonate (calcite) was also encountered throughout all units, although Units 1 and 2 contain mostly micritic carbonate of sedimentary origin as vein fill and breccia cement. Carbonate precipitate was mostly encountered in the lower part of Unit 3 as vein and vug filling. Alteration intensity of the basalts is variable and ranges from fresh to moderate (up to 20%), manifested as replacement of groundmass and phenocrysts, vesicle filling, glassy margin replacement, and vein formation with adjacent alteration halos. Highly altered rocks with up to 50% alteration were also recovered, but such high extents of alteration were found only in pervasively altered basalt and resulted in extensive replacement of interstitial glass and cryptocrystalline groundmass by smectitic clay minerals. Thin section estimation of groundmass alteration is generally consistent with results from visual core description, except in chilled pillow margins where microscopic observations suggest a lower extent of alteration, with both cryptocrystalline and olivine microphenocrysts often remaining unaltered (Table T6). Throughout the following sections we refer to volume percentages of alteration types, breccias, vesicles, and veins by assuming that the surface areas of these features on the cut faces of the core, when converted to area percent, are equivalent to volume percent of the core. In Figures F6 and F5 we report a summary of calculated averages per core of abundance, volume, width, and mineralogy of vesicles, veins, and halos.

Secondary minerals

In basaltic rocks from Hole U1383C, secondary minerals have developed as replacement of primary phenocrysts, disseminated in the groundmass as replacement of mesostasis, and as vesicle and vein filling. The identification of secondary minerals was primarily made in hand specimen on the basis of color, habit, and texture, with subsequent verification by thin section observations and XRD (Table T6). The most abundant secondary minerals in Hole U1383C are clay minerals, which are present in all types of alteration. Specific secondary clay minerals (e.g., saponite, nontronite, and celadonite) were characterized for a few examples during logging of alteration and veins, but they were generally referred to as “smectite” or “dark green” or “brown” clays. A distinction was made between micritic carbonate derived from interflow sediments (limestone) and veins of sparry carbonate away from palagonitized glass clasts.

Saponite is the dominant clay mineral and is present throughout the cores. In hand specimen, saponite occurs in black, dark green, and greenish-brown colors. When brown color was observed, the mineral composition was recorded as mixed smectite–Fe oxyhydroxide assemblages. In thin section, saponite is characterized by pale to dark brown and pale green colors and a mottled or fibrous form with variable pleochroism. Saponite generally replaces groundmass and olivine (micro)phenocrysts uniformly, preserving the primary igneous textures. In the case of variolitic pillow margins, saponite replacement is pervasive across the mesostasis and groundmass, leading to the formation of mottled replacement or blotches and revealing the original variolitic texture (Fig. F15). Commonly, saponite lines or fills vesicles (in association with other secondary formations such as iddingsite, Fe oxyhydroxide, carbonate, and zeolite) and forms a lining along thin (0.1 mm) veins.

Celadonite is the only other clay mineral identified in hand specimen and thin section, and it appears bright green to blue in hand specimen. Celadonite is also present in all four types of alteration but is less abundant than saponite. In hand specimen, celadonite (mainly in vesicles) is green-blue, whereas in thin section it is pale green (see “Vesicle filling”).

Zeolite is the next most abundant secondary phase and is found as a major mineral filling vesicles, vugs, and veins and as hyaloclastite cement (Fig. F16). It is often associated with other secondary minerals such as calcite and smectite. Phillipsite was identified by XRD analysis in bulk rock powder (Table T6). In general, zeolite-filled vesicles and veins are more common in Unit 3 in aphyric fine-grained and cryptocrystalline basalts. Zeolite occurs as either fibrous aggregates in druses (sometimes overgrown by carbonate), as radiating or colloform aggregates showing fan-shaped extinction (under cross-polarized light), or as microcrystalline euhedral granular filling.

Fe oxyhydroxide mainly occurs as a discrete phase or mixed with saponite and other smectitic clay phases. In many cases, the mixed assemblage of Fe oxyhydroxide and clays was described as “iddingsite,” which was previously reported for Hole 395A (Juteau et al., 1979). Fe oxyhydroxides were identified in thin section by their deep red color to nearly opaque appearance, whereas iddingsite (showing weak pleochroism) was identified by its bright orange to red color. When present as replacement of microphenocrysts (mainly olivine) and groundmass, Fe oxyhydroxides (± iddingsite) are mixed with saponite and preserve the intersertal to hyalophitic textures. In veins and vesicles, iddingsite is bright orange to reddish brown and occurs with or without intergrown clays and may contain aggregates of Fe oxyhydroxides. Staining of plagioclase phenocrysts and replacement of olivine with Fe oxyhydroxides is a common feature in the grayish-brown halos in all recovered units.

Carbonate is present principally as vug, vesicle, and vein filling (Fig. F17). Within the upper part of Unit 1, carbonate (calcite) occurs mainly as crypto- to microcrystalline aggregates hosting clay minerals and fine palagonitized glass shards. This type of carbonate filling was interpreted to result from interflow sediment (limestone) infill. In the lower part of Unit 1 and in Unit 2, calcite is generally recrystallized to coarser aggregates, usually anhedral to acicular, with much less clay material. In the lower part of Unit 3, calcite (with possibly minor aragonite) veins are also common, either as pure carbonate veins or associated with zeolite, forming acicular to granular crystal aggregates. As for zeolite-filled vesicles, calcite was found essentially within grayish-brown alteration halos or pervasively altered sparsely vesicular variolitic basalts. XRD analysis of bulk rock powder did not allow unambiguous identification of carbonate minerals.

Ultratraces of secondary sulfide (pyrite) were identified in thin section or under the binocular microscope. They occur as <10 µm crystals along cracks in plagioclase or disseminated in the groundmass of the fine-grained aphyric basalt of Unit 3. They also occur in vesicles and voids associated with other secondary minerals such as pale green saponite or celadonite (e.g., Section 336-U1383C-13R-1 [Piece 5], Thin Section 44). In Section 336-U1383C-28R-1 (Piece 7), Thin Section 54, spherical secondary pyrite (<10 µm grain size) extensively oxidized to Fe oxyhydroxide was identified along a thin saponite vein.

Phenocryst alteration

Plagioclase

Plagioclase phenocrysts and microphenocrysts are fresh, except in pervasively altered grayish-brown basalt (alteration <10%). In the only transformations observed, plagioclases of aphyric basalt samples in the immediate vicinity of veinlets filled with smectite have been partially replaced by clay or tainted by Fe oxyhydroxides.

Pyroxene

The augitic clinopyroxene remains essentially unaltered. When pyroxene occurs as microliths or plumose in microcrystalline groundmass, extensive brown staining suggests incipient pyroxene alteration. However, the exact extent of clinopyroxene alteration is difficult to estimate on the basis of thin section observation (e.g., Section 336-U1383C-10R-1 [Piece 1], Thin Section 41).

Olivine

Olivine phenocrysts and microcrysts are the minerals most sensitive to alteration. Olivine is partially or completely replaced by reddish-brown iddingsite (a mixture of smectite and Fe oxyhydroxide) in the alteration halos (Section 336-U1383C-31R-2 [Piece 6], Thin Section 57). In grayish-brown alteration halos, olivine pseudomorphs are revealed as a result of replacement by iddingsite. In fresh dark gray basalts and devitrified chilled margins, olivine is remarkably fresh.

Groundmass alteration

In crypto- to microcrystalline groundmass, plagioclase microliths (e.g., sheaves) are quite fresh, although the extent of alteration of clinopyroxene plumose is difficult to estimate. In thin section, the highest alteration intensity of the groundmass is observed in pervasively altered basalts. For example, Section 336-U1383C-3R-1 (Piece 2), Thin Section 30, in Unit 1 displays up to 40% groundmass alteration from extensive replacement of microcrystalline mesostasis by brown clay. The pervasively altered phyric basalt of Unit 2 generally shows more modest alteration (15%; Section 336-U1383C-10R-1 [Piece 7], Thin Section 41) because of less abundant mesostasis. In Unit 3, several pieces of pervasively altered crypto- to microcrystalline basalt display groundmass alteration as high as 25% (e.g., Sections 336-U1383C-16R-2 [Piece 7], Thin Section 48, 20R-1 [Piece 9], Thin Section 51, and 31R-2 [Piece 6], Thin Section 57). Microscopic observation of fine-grained aphyric avesicular basalt at the bottom of Unit 3 (e.g., Sections 336-U1383C-23R-1 [Piece 10], Thin Section 52, and 32R-2 [Piece 9], Thin Section 59) shows an overall alteration of 6%–8% that is attributed to alteration of interstitial mesostasis and olivine. Microscopic and XRD observations (as well as geochemical studies; see “Hard rock geochemistry”) of several fresh/​altered pairs were undertaken to examine mineralogical changes with varying extents of alteration. Although not systematic, results show that olivine is readily altered (i.e., olivine was barely identified by XRD) in most altered samples.

Glass and chilled margins

Chilled margins often show advanced palagonitization, which develops as an irregular alteration front. Glass devitrification developing as a spherulitic texture is generally more abundant in palagonite alteration rinds but is also not uncommon in fresh glass around plagioclase and olivine microliths (Fig. F18; Section 336-U1383C-2R-2 [Piece 3], Thin Section 28). The cryptocrystalline mesostasis of chilled margins commonly consists of coalesced spherules. In fresh glass, the spherules show weak birefringence. Altered glass is revealed by a zone of typically yellowish-brown material commonly referred to as palagonite (Honnorez, 1972). The boundary between spherules or palagonite and fresh glass is generally characterized by a granular texture and also develops channel-like or branchlike structures. Such microtubules have been suggested to form by microbial process (Fisk et al., 1998) and appear to be widespread in glassy margins.

Microscopic investigation of the blotchy alteration texture in pillow margins allowed identification of several domains that are generally similar to the alteration of variolitic texture reported for Hole U1382A (see the “Site U1382” chapter [Expedition 336 Scientists, 2012b]) and Hole 395A (Natland, 1979).

Vesicle filling

All basalts recovered are sparsely vesicular to nonvesicular, and vesicle abundances per core are <1% (Fig. F6). The following filling associations were found in the vesicles: (1) dark brown to dark green smectite (nontronite and smectite); (2) reddish-brown iddingsite, generally logged as mixed Fe oxyhydroxides and smectite assemblages; (3) zeolite, mainly phillipsite; and (4) carbonate (calcite). No clear trend with depth was found, but dark green clay (saponite and celadonite) mixed with variable proportions of Fe oxyhydroxides forms the vast majority of vesicle filling material in the upper section of Hole U1383C (Cores 336-U1383C-2R through 20R), whereas zeolite- and carbonate-filled vesicles are more abundant in the lower section.

Interestingly, vesicle filling in all units is very limited (25% on average), and larger vesicles are mainly void except in the phyric basalt of Unit 2. Vesicle filling colors range from blue-green in the freshest grayish diabase to dark brown and white in the brown alteration halos, which suggests different stages of vesicle filling during rock alteration. An example of a composite mineral filling of vesicles is illustrated in Figure F19, showing (1) mixed saponite and possible celadonite vesicle filling in gray groundmass, (2) smectite stained with Fe oxyhydroxide in dark brown alteration, and (3) mixed iddingsite smectite zeolite-filled vesicles in grayish-brown halos. An example of thin section observation of vesicle filling is illustrated in Figures F15 and F16. In variolitic, sparsely vesicular pillow basalt from Unit 3 (e.g., Section 336-U1383C-16R-2 [Piece 6], Thin Section 47), vesicles often display bimodal filling (i.e., carbonate or zeolite filling), although most vesicles remain unfilled.

Cores 336-U1383C-23R and 24R feature several pieces with 1–3 mm vugs filled by carbonate. In rare cases, pyrite is found as euhedral crystals associated with saponite or other secondary minerals in vugs of microcrystalline to fine-grained basalt.

Veins and halos

The following filling associations are found in veins: (1) Fe-rich dark green smectite (nontronite and smectite); (2) reddish-brown iddingsite, generally logged as mixed Fe oxyhydroxides and smectite assemblages; (3) zeolite, mainly phillipsite; (4) carbonate; and (5) micritic sediment, possibly from interflow limestone filling (Fig. F5). As with Hole U1382A, the volume percent of veins for each core was estimated by calculating the volume of veins relative to the volume of core recovered. Approximately 780 veins, including 500 haloed veins, were logged during core description of Hole U1383C (Fig. F17). The average vein abundance is ~1%, which is three times higher than the vein abundance in Hole U1382A, reflecting in most part the occurrence of carbonate(micrite)-filled veins wider than 1 mm. We also logged 203 halos in both pillow lava and massive flows that have highly variable widths and are not always clearly associated with a vein. For this reason, we logged nonveined halos as having 0 mm vein thickness. Both veins (with or without halos) and nonveined halos were logged for the total vein count, which gives an estimation of the minimum and maximum vein abundance per core. Using this approach, we estimated that the recovered section in Hole U1383C has between 22 and 50 veins/m, averaging 33 veins/m for the entire drilled section. This estimate is higher than that for Hole U1382A (average of 20 veins/m) and similar to that for sections of other upper volcanic basement (e.g., 27 veins/m in Ocean Drilling Program [ODP] Hole 896A, 24 veins/m in ODP Hole 801C, and 31 veins/m in DSDP Hole 504B; Alt et al., 1996; Plank, Ludden, Escutia, et al., 2000). Veins range in thickness from ~0.1 to ~6 mm, but vein thickness of <0.2 mm is by far the most common. Mixed smectite and iddingsite (Fe oxyhydroxide) veins are the most abundant. Carbonate and zeolite veins also represent a significant fraction of veins greater than that in Hole U1382A.

In Unit 1, zeolite veins are often associated with micritic and recrystallized calcite, as confirmed by thin section observations and XRD analysis of a vein filling Section 336-U1383C-2R-1 (Piece 7) (Table T6). An example of a composite zeolite-carbonate vein is illustrated in Figure F17A, which shows three domains: (1) micritic carbonate filling mixed with altered glass shards, (2) later stage recrystallized calcite lacking glass shards, and (3) cryptocrystalline zeolite mixed with clay with low birefringence. Pure zeolite veins were also observed in thin (<0.2 mm) veins, especially in association with glassy margins, as illustrated in Figure F16.

In Unit 3, mixed carbonate and zeolite veins were also commonly identified, but they lack micritic fillings. As shown in Figure F17A, mineral habit and paragenesis suggest the following sequence: (1) zeolite first crystallized in open vein fractures, forming radiated acicular to fibrous clusters; (2) calcite partly replaced zeolite or form pseudomorphic overgrowth; and (3) granular calcite, showing radial extinction, finally filled the remaining vein space. Pure zeolite veins were commonly observed throughout Unit 3, as shown in Sections 336-U1383C-16R-1 (Piece 14), Thin Section 47, 29R-1 (Piece 14), Thin Section 55, and 31R-2 (Piece 6), Thin Section 57, and phillipsite was identified in bulk rock powders by XRD analysis (Table T6).

Grayish-brown alteration halos, and to a lesser extent dark (green) halos, were identified in all volcanic units in Hole U1383C (Fig. F5). Brown halos are characterized by the presence of abundant iddingsite disseminated in the groundmass, staining smectite-filled pores, and replacing olivine and mesostasis. Dark green halos are characterized by higher green clay (e.g., celadonite) abundance in the groundmass relative to iddingsite. In most cases, grayish to dark brown alteration halos develop after, or are superimposed onto, dark green halos (Fig. F8B). In Unit 1, brown halos represent ~20% of the recovered material, whereas dark halos represent >8%. In the highly phyric basalt of Unit 2, alteration halos are even more abundant (27% of brown halos and 14% of dark halos). Aphyric pillow basalts from Unit 3 also display mixed dark and brown halos (15% and 7%, respectively) but differ from other units in by the abundant occurrence of blotchy alteration textures making up as much as 33% of the recovered material.

A representative piece of microcrystalline aphyric avesicular basalt with alteration halos was investigated by thin section (Section 336-U1383C-28R-1 [Piece 7], Thin Section 54). The margins are more altered (~20% alteration) with abundant Fe-rich clay (smectite) replacement of skeletal olivine sheaves and mesostasis. Iddingsite is also common throughout the groundmass and as vesicle filling. The less altered gray core (<1% alteration) is devoid of iddingsite, and only olivine (10% alteration) is replaced by dark green smectite.

Hyaloclastite

Hyaloclastites were recovered in all units in Hole U1383C and represent ~2.5% of the total basement recovered (Fig. F4). Cores 336-U1383C-20R and 22R feature the highest amount of hyaloclastite intervals, representing ~17% and 10%, respectively, of these cores. The glassy fragments in hyaloclastic breccias have typical concentric alteration rims surrounding fresh glass preserved in the central part of the fragments (Fig. F12). In all of the hyaloclastite recovered, glass clasts are variably altered to palagonite, forming layered, bright reddish-brown to yellowish-brown alteration fronts (Fig. F20). In the single thin section of hyaloclastite investigated (Section 336-U1383C-20R-1 [Piece 28], Thin Section 50), the transition between palagonite and fresh glass lacks granular or fibrous textures, contrasting remarkably with the pillow margin textures (Fig. F18) and showing ubiquitous microtubules extending into the fresh glass. The fresh glass also displays more limited spherulites associated with glass devitrification. The hyaloclastite cement is mainly zeolite (phillipsite) forming an external rind around glass clasts. Numerous open vugs between glass clasts feature drusy euhedral to fibrous phillipsite druses. The matrix of the hyaloclastites is often missing and is mostly composed of mixed zeolite and palagonite to smectitic clay without carbonate.

Hard rock geochemistry

Concentrations of major element oxides and several trace elements, together with weight loss on ignition (LOI), were determined for 25 whole-rock samples from Hole U1383C. The results are presented in Table T7, where major element oxide concentrations were normalized to 100% and total iron was recalculated as Fe₂O₃ (Fe₂O₃^T).

Basalts from Hole U1383C are typical depleted mid-ocean-ridge tholeiites with low K/Ti ratios and Mg numbers between 35 and 63. Whole-rock compositions do not show any systematic downhole trends (Fig. F21). The relationship between Al₂O₃ and CaO concentrations (Fig. F22) suggests that the accumulation of plagioclase could affect the composition of plagioclase-phyric basalts in Units 1 and 2. CaO concentrations also correlate with Sr and Ba contents, which is consistent with plagioclase accumulation. Compositional variations in MgO, Al₂O₃, Fe₂O₃^T, and TiO₂ suggest, however, that the effect of plagioclase accumulation on bulk-rock chemistry of the plagioclase-phyric basalts from Units 1 and 2 is quite limited (Fig. F23), which contrasts significantly with the same type of basalt from Holes U1382A (see the “Site U1382” chapter [Expedition 336 Scientists, 2012b]) and 395A (Bougault et al., 1979; Rhodes et al., 1979). Negative correlations in the MgO vs. Al₂O₃ and MgO vs. CaO plots suggest a lack of clinopyroxene and plagioclase fractionation (i.e., only olivine fractionation was involved) on the magma evolution of Hole U1383C basalts (Fig. F23).

Differences in magmatic geochemical signatures

In order to determine if there are differences in the composition of primitive magmas unaffected by crystal fractionation and mineral accumulation, the correlation between Zr, Y, and TiO₂ was examined. Figure F24 shows two separate trends with different Zr/Y and Zr/TiO₂ ratios. Rocks from Units 1 and 2 show generally higher Zr/Y and Zr/TiO₂ ratios than those from Unit 3 (except one sample in the Y/Zr panel). These results imply that, despite the petrographic difference, both the sparsely plagioclase-olivine-phyric basalts in Unit 1 and the highly plagioclase-olivine-phyric basalts in Unit 2 are derived from the same parental magma. In contrast, the aphyric basalts from Unit 3 may be derived from a different magma source.

Given the Zr-Y-TiO₂ systematics, it is interesting to note that the highly plagioclase-phyric basalt unit in Hole U1383C (Unit 2) belongs to the shallower unit, whereas the same type of basalt in Unit 6 in Hole U1382A belongs to the deeper unit. Moreover, in Hole U1382A the values of Zr/Y and Zr/TiO₂ in shallower units are lower than those in deeper units (including the porphyritic basalt unit) (Fig. F25). In contrast, shallower units in Hole U1383C (including the porphyritic basalt unit) have higher Zr/Y and Zr/TiO₂ ratios than the deeper unit (Fig. F25). These facts lead us to consider that the two sequences of basalt units in Holes U1382A and U1383C are chemically different, despite the striking similarities in petrographic features.

Alteration

LOI values of Hole U1383C basalts are as high as 3.66 wt%, with an average of 1.63 wt%. The values are generally higher than those observed in Hole U1382A (2.69 wt%; 0.90 wt% on average). High extents of groundmass alteration, observed by macroscopic and microscopic observations in Hole U1383C basalts, are consistent with the higher LOI values, although generally low LOI values indicate that the degree of bulk-rock alteration is still relatively small. Downhole variation of LOI values suggests that the degree of alteration is significantly higher in the shallower (<200 mbsf) part than in the deeper part (Fig. F21), which is similar to the pattern observed in Hole U1382A. However, unlike Hole U1382A, there is no correlation between LOI and K₂O contents (Fig. F26), suggesting a lack of K enrichment during alteration of Hole U1383C basalts. On the other hand, CaO and Ba concentrations are weakly correlated with LOI values. It is likely that the presence of secondary minerals (possibly carbonate minerals or zeolite) also affects the bulk-rock compositions of CaO and Ba in some degree, although a significant amount of variation in these elements is attributed to igneous processes, as shown above. In addition, there is also a weak negative correlation between LOI and MgO (Fig. F26), suggesting that depletion of MgO is associated with alteration. Macroscopic and microscopic observations indicate selective replacement of olivine by Fe oxyhydroxide in the Hole U1383C basalts. It is thus possible that alteration of olivine causes the depletion of MgO in bulk-rock chemistry of altered basalt samples.

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