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doi:10.2204/iodp.proc.336.104.2012

Petrology, hard rock and sediment geochemistry, and structural geology

Basement was cored from 110 to 210 mbsf with an overall recovery of 32%. The recovered core material was divided into 8 major lithologic units comprising 17 subunits on the basis of changes in rock lithology, lava morphology, rock texture, and phenocryst occurrence (Fig. F3; Table T5). Major unit boundaries were defined by contacts between massive and pillowed flow and interlayered sedimentary units. The basalts comprise seven distinct chemical types—one highly plagioclase-olivine-phyric basalt and the rest aphyric (Fig. F4). Each basalt type consists of basaltic flows, pillow sequences, and perhaps intrusive doleritic massive flow units, ranging in thickness from 1.7 m (Unit 3) to 23.9 m (Unit 2). The sequence cored includes three massive aphyric units (Units 1, 3, and 8) and one porphyritic unit (Unit 6) lacking most contacts and glassy zones. The porphyritic basalts below the brecciated unit (Unit 5) correspond stratigraphically to basalts cored in Hole 395A. The aphyric pillow basalts in Units 2, 4, and 7 are petrologically similar, but the bulk rock chemical data indicate that Unit 7 may be derived from a parental magma with distinct Zr/Y and Ti/Zr ratios. Each major lava flow unit consists of several cooling units that are recognizable by glassy or variolitic margins or marked changes in groundmass grain size. Initial results from thin section studies reveal a wide range of grain sizes (glassy to medium grained) and textures (aphanitic to subophitic or intersertal). Basalts are either aphyric or plagioclase-olivine-phyric and have <3% vesicles. Phenocryst contents are as high as 25%, with plagioclase being more abundant than olivine. The extent of alteration ranges up to 20%, with clay (smectite, nontronite, and celadonite) being the most abundant secondary phase, followed by Fe oxyhydroxide and to a lesser extent zeolite and carbonate. Trace amounts of secondary pyrite were observed in thin sections.

The sedimentary breccia unit (Unit 5) in Cores 336-U1382A-8R and 9R (161.3–173.2 mbsf) features a variety of clasts, including plutonic and mantle rocks as well as basalts. The peridotites are weakly serpentinized harzburgites and lherzolites with a protogranular texture (Fig. F4C). The intensity of deformation of the gabbroic lithologies ranges from undeformed to mylonitic. Minor cataclastic deformation of the peridotites has led to the development of carbonate-filled vein networks, along which the rocks have been subjected to oxidative alteration, resulting in the breakdown of olivine to clay, oxide, and carbonate. The sedimentary breccia of Unit 5 consists of different lithologies, but the lack of contacts and likely disturbance during drilling preclude a determination of depth relationships between the numerous basaltic and gabbroic cobbles associated with sediment layers.

Lithologic units

Unit 1

• Depth: 110.0–119.61 mbsf
• Lithology: nonvesicular aphyric fine- to medium-grained basalt

This aphyric basalt unit is composed of four subunits that are distinguished mostly on the basis of groundmass grain size (contacts were not recovered). The basalts are nonvesicular (<1% vesicles) and are probably from the same cooling unit part of a massive flow. Alteration is generally slight to moderate at 5%–20%. The upper part of the unit (Subunits 1-1 and 1-2) is pervasively altered, whereas the lower part displays well-developed alteration halos and patches along veins and broken surfaces of the pieces (Fig. F4A). Within the grayish-brown halos, olivine microphenocrysts are extensively altered to an assemblage of secondary brown clay (smectite) and Fe oxyhydroxides (± iddingsite), whereas plagioclase and clinopyroxene remain unaltered. Minor carbonate, locally enriched in pieces (e.g., Section 336-U1382A-3R-2 [Piece 12]), and zeolite also fill voids in vugs and veins. In most recovered pieces, darker gray (freshest) basalt is surrounded by grayish-brown alteration halos, which develop due to the extensive oxidation of olivine microphenocrysts and interstitial glass (palagonitization) between fresh plagioclase laths as long as 4 mm (Fig. F5). Gray basalt displays occasional alteration of fresh olivine (<5%).

Unit 2

• Depth: 119.61–143.45 mbsf
• Lithology: sparsely vesicular aphyric cryptocrystalline pillow basalt

This aphyric basalt unit comprises 18 chilled margins (all defined as subunits) that were identified on the basis of glassy or variolitic margins (Fig. F6). The basalts are non to sparsely vesicular (0%–3% vesicles) with crypto- to microcrystalline groundmass. Recovered basalts are fresh (<1%) to slightly altered (up to 7%), with often spectacular blotchy alteration developing within the devitrified variolitic glass of pillow margins. Within alteration patches, euhedral olivine microcrysts are partly replaced by Fe-rich smectite (± iddingsite) in a slightly altered micro- to cryptocrystalline groundmass. Vesicles are filled with abundant Fe oxyhydroxides (± iddingsite), smectite (± celadonite), and minor zeolite (phillipsite) and carbonate. Some vesicles remain unfilled, with only a thin dark green (nontronite) coating. The grayish-brown alteration halos often have more unfilled or zeolite/carbonate-rich vesicles than the freshest dark gray groundmass, which contains mostly smectite-filled vesicles. In many cases, patchy alteration cannot be attributed to haloed veins on the basis of recovered specimens. Pervasive alteration is not common but rather develops as dark gray patches/​blotches in grayish-brown alteration patches. The freshest dark gray microcrystalline groundmass may have faint and irregular (1 mm wide) dark brown alteration halos along veinlets as a result of Fe oxide staining.

Volcanic breccia (hyaloclastite) was recovered in two pieces (Sections 336-U1382A-4R-2 [Piece 4], Subunit 2-5; and 5R-1 [Piece 21], Subunit 2-13) (Fig. F7). Hyaloclastite in Subunit 2-5 is poorly sorted (clast size ranges from 1 to 4 mm) and is composed of 60% angular clasts of altered micro- to cryptocrystalline basalt (aphyric and nonvesicular) with moderate alteration (20%). The matrix (40%) is composed of subequal amounts of fine-grained to cryptocrystalline Fe oxyhydroxides (± iddingsite), smectite, and light brown clay of potential sedimentary origin. Zeolite and carbonate may also be associated with matrix filling and clast alteration (no X-ray diffraction [XRD] or thin section samples were taken of hyaloclastites). The hyaloclastite in Subunit 2-13 is also poorly sorted (clast size ranges from 1 to 10 mm) but has substantially higher clast abundance (85%). It is composed of angular clasts of aphyric nonvesicular basalt displaying extensive concentric alteration halos (average alteration = 60%). Fresh glass is partially devitrified. The matrix (15%) is composed of reddish-brown clay.

Unit 3

• Depth: 143.45–145.17 mbsf
• Lithology: nonvesicular aphyric fine- to medium-grained basalt

This unit of nonvesicular aphyric fine- to medium-grained massive basalt lacks any contacts and was defined as massive/​basaltic flow. Alteration is patchy and restricted to grayish-brown alteration halos in which the groundmass is moderately altered (up to 15%) as a result of olivine and interstitial glass replacement by Fe-rich clay (iddingsite and smectite). The largest piece (Section 336-U1382A-6R-3 [Piece 1]) is 65 cm long and has centimeter-wide grayish-brown alteration halos along both sides.

Unit 4

• Depth: 145.17–154.41 mbsf
• Lithology: sparsely vesicular aphyric cryptocrystalline basalt

This unit of aphyric basalt is composed of eight subunits that were identified on the basis of glassy or variolitic chilled pillow margins. The basalts are generally sparsely vesicular (up to 5% vesicle abundance) with crypto- to microcrystalline groundmass. Recovered basalts are fresh (<1%) to slightly altered (up to 8%) with common blotchy alteration textures of devitrified variolitic glass in pillow margins. Alteration patches have mixed Fe oxyhydroxides and brown clays (smectite and iddingsite) replacing olivine and interstitial glass in the groundmass and filling vesicles. Subunit 4-1 is nonvesicular and pervasively altered (10% alteration). Aphyric fine-grained basalts (e.g., Subunit 4-3) generally develop patchy alteration patterns, whereas cryptocrystalline variolitic devitrified chilled margins develop dark brown blotches. Alteration intensity increases along radial veinlets near pillow margins.

Unit 5

• Depth: 161.30–173.24 mbsf
• Lithology: sedimentary breccia

This unit is composed of mixed lithologies of basalt (porphyritic and aphyric), sediment, breccia, plutonic rock (gabbro), and serpentinized lherzolite and harzburgite (Figs. F8, F9). Plutonic rocks and serpentinites are intercalated with pelagic nannofossil ooze having different degrees of induration and porphyritic volcanic rocks that were encountered above. Aphyric basalt at the top of Core 336-U1382A-8R is possibly from debris that fell into the hole during repeated wiper trips conducted after Core 7R was cut. The association of sediment with basalt, peridotite, and gabbroic rock is interpreted to result from mass wasting. As detailed below, eight main lithologies were identified in this unit:

1. Two short sections of soft sediment (Sections 336-U1382A-8R-2 and 8R-3) were recovered in Core 8R. These sections are composed of brownish-yellow calcareous nannofossil ooze that was disturbed by the drilling process. This unit contains volcanic clasts of grain size ranging from very fine to fine sand that could have been introduced during drilling.
2. Two intervals of more indurated calcareous nannofossil ooze, defined as chalk, were recovered in two pieces from Cores 8R and 9R (Sections 336-U1382A-8R-4 [Piece 2] and 9R-1 [Piece 18]). The piece from Core 9R was taken as a MBIO sample, and only a photograph is available for description. The chalk is light brown and contains volcanic clasts.
3. Two pieces of poorly sorted sedimentary breccia were recovered in Core 8R (Sections 336-U1382A-8R-1 [Piece 8] and 8R-4 [Piece 4]; Fig. F9). These pieces contain 30%–40% angular clasts ranging in size from 0.2 to 10 mm (fine sand to pebble). The clasts are composed of altered serpentinites (with foliated structures), possibly altered gabbroic rocks, and aphyric basalt. The matrix is composed of sedimentary light brown nannofossil clayey chalk.
4. Several pieces of coarse-grained porphyroclastic and mylonitic gabbros were recovered in Cores 8R and 9R. Sections 336-U1382A-8R-1 (Piece 4) and 8R-4 (Piece 3) are rounded pebbles of variably altered coarse- to medium-grained olivine gabbro. Altered olivine shows a foliated texture, whereas altered euhedral pyroxene is partly replaced by serpentine and chlorite. Section 336-U1382A-8R-1 (Piece 11) is characterized by completely altered olivine gabbro with pseudomorphic replacement to primary mineral assemblages by serpentine and chlorite. Section 336-U1382A-9R-1 (Piece 5) is the only piece of olivine gabbro ultramylonite recovered.
5. Several pieces of variably serpentinized harzburgite were recovered in Cores 8R and 9R (Sections 336-U1382A-8R-4 [Pieces 5 and 6], 8R-4 [Piece 9], 9R-1 [Piece 3], and 9R-1 [Pieces 12–17], Figs. F4C, F8). Contiguous pieces recovered in Section 9R-1 (Pieces 12–17) show common mesh texture of partly altered serpentine developed between/​within coarse- and medium-grained olivine grains. Coarse- and medium-grained pyroxene shows moderate alteration. Olivine (50%–70% abundance) is partially to totally (30%–100% alteration) replaced by serpentine, which is locally oxidized to Fe-rich clay assemblages in light green-brown alteration halos developed along thick (1–3 mm) carbonate (aragonite) veins (e.g., at 165.44 and 171.8 mbsf). Section 336-U1382A-9R-1 (Piece 12) is intruded by gabbroic plagioclase-rich veins, and another piece (Section 8R-4 [Piece 8]) is a fragment of dark green lherzolite having the least overall extent of serpentinization.
6. A single piece of serpentinized lherzolite (Section 336-U1382A-8R-4 [Piece 8]) recovered in Core 8R is composed of coarse- and medium-grained granular to porphyroclastic olivine (mode = 65%) and granular pyroxene (mode = 34%).
7. Several pieces of sparsely vesicular, aphyric cryptocrystalline to fine-grained basalt were recovered throughout Cores 8R and 9R (top: Section 336-U1382A-8R-1 [Piece 6]); bottom: Section 9R-1 [Piece 11]). Pieces have slightly to moderately altered cryptocrystalline groundmass with disseminated iron oxide–rich clay. As for other lithologies in Unit 5, no contact was recovered and the pieces are possibly derived from borehole wall breakouts from Unit 4 above.
8. Several pieces (some of them oriented) of highly porphyritic, sparsely to moderately vesicular (0%–10%) basalt were recovered throughout Cores 8R and 9R (Sections 336-U1382A-8R-1 [Piece 2] and 9R-2 [Piece 14]). These basalt pieces are generally similar to the plagioclase-olivine-phyric basalts encountered in Unit 6 below. The alteration is patchy, with euhedral olivine phenocrysts partly altered to iddingsite or whitish clay. Plagioclase phenocrysts are fresh. Vesicles are filled with abundant smectite (± celadonite), Fe oxyhydroxide (± iddingsite), and possibly minor zeolite and carbonate. The occurrence of porphyritic basalts in this unit may represent a basaltic flow—similar to or part of Unit 6 within Cores 8R and 9R.

Unit 6

• Depth: 173.24–191.24 mbsf
• Lithology: highly plagioclase-olivine-phyric fine-grained basalt

This unit of plagioclase-olivine-phyric basalt is composed of three subunits that are mostly distinguished from each other by groundmass grain size because no contacts were recovered (Fig. F10). The basalts are sparsely vesicular (<3% vesicles) and probably represent the same cooling unit of a massive flow. This unit is dominantly glomerophyric in texture and has microcrystalline to fine-grained groundmass. The groundmass is fine grained in Subunit 6-1 and microcrystalline with numerous microphenocrysts in Subunit 6-2. Interstitial glass is mostly devitrified and partly altered to palagonite. Euhedral plagioclase phenocryst abundance is ~20%, with crystal sizes ranging from <1 to 12 mm. Olivine phenocrysts are less abundant (mode = 3%) and smaller (0.5–4 mm). Fresh glomerocrysts of plagioclase show clinopyroxene and olivine inclusions. 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.

Unit 7

• Depth: 191.24–201.93 mbsf
• Lithology: sparsely vesicular aphyric cryptocrystalline basalt

This unit of aphyric basalt is similar to Unit 4 and comprises seven subunits identified on basis of glassy or variolitic chilled pillow margins (Fig. F11). The basalts are generally sparsely vesicular (up to 5% vesicle abundance) with crypto- to microcrystalline groundmass. Recovered basalts are fresh (<1%) to moderately altered (up to 20%; Subunit 7-5), with a common blotchy alteration texture of devitrified variolitic glass in pillow margins. Alteration patches have mixed Fe oxyhydroxide and brown clay (smectite and iddingsite) and minor carbonate replacing olivine and interstitial glass in the groundmass and filling vesicles. The cryptocrystalline groundmass has numerous olivine microphenocrysts that are variably altered (average = ~60%).

Unit 8

• Depth: 201.93–206.76 mbsf
• Lithology: nonvesicular aphyric fine- to medium-grained basalt

This last unit is composed of aphyric fine- to medium-grained basalt, which suggests a massive flow unit at the bottom of Hole U1382A (Fig. F12). The basalts are slightly to moderately altered (6%–10% alteration) with common alteration halos and patches. Because of the lack of recovered contacts and the unknown thickness of this unit, it was not possible to assign a specific flow type. Therefore, this unit has been described as basaltic flow.

Igneous petrology

Aphyric fine- to medium-grained massive basalt (Units 1, 3, and 8)

The massive basalts in Units 1, 3, and 8 are aphyric, and the groundmass texture of all of the fine- to medium-grained basalts is intersertal to subophitic. The groundmass consists of 40%–47% tabular plagioclase (1–3 mm), 36%–43% anhedral clinopyroxene (0.3–1 mm), and ~2% small (<0.1 mm) equant to elongate to skeletal Fe-Ti oxide minerals. The morphology of the olivine crystals is equant, and some of them form in inclusions in plagioclase (Fig. F13A). Anhedral clinopyroxene crystals are intergrown with plagioclase. Primary sulfide minerals are observed in some samples, but they are very small (<0.1 mm) and rare. The microcrystalline groundmass is quench-textured mesostasis, composed mainly of sheaves of intergrown plagioclase and clinopyroxene that make up to 6%–10% of the rock.

Aphyric cryptocrystalline pillowed basalt (Units 2, 4, and 7)

The cryptocrystalline pillow basalt in Units 2, 4, and 7 is generally aphyric, and significant parts (~40%) of the samples contain olivine microphenocrysts (0.1–0.3 mm) in a glassy to fine-grained groundmass. Most of the olivine microphenocrysts are euhedral equant to elongate crystals. Although it has been almost completely replaced by alteration minerals in some samples (Sections 336-U1382A-3R-2 [Piece 11], 4R-3 [Piece 8], and 5R-1 [Piece 7]), olivine can be recognized from the pseudomorph outline (Fig. F13B).

The groundmass grain size ranges from glassy/​cryptocrystalline to microcrystalline/fine grained, which corresponds to chilled margins and the interior of pillow lobes, respectively. The groundmass texture of glassy/​cryptocrystalline basalt is aphanitic to spherulitic to variolitic and that of microcrystalline/​fine-grained basalt is intersertal. Except for glassy pillow margins (described below), the groundmass generally consists of as much as 50% acicular and tabular plagioclase and up to 40% anhedral clinopyroxene intergrown with plagioclase. In all samples, plagioclase seems to be slightly more abundant than clinopyroxene, although the small grain size makes it very difficult to determine the exact proportions of plagioclase and clinopyroxene with an optical microscope. In addition to the plagioclase and clinopyroxene crystals, opaque Fe-Ti oxides commonly occur as minute (<10 µm) equant or elongate crystals making up 1%–2% of the groundmass. Rare primary sulfide grains are also very small (<10 µm).

The aphyric basalts from Units 2, 4, and 7 include ~17% pillow margins showing variolitic texture with or without fresh volcanic glass. In thin section observation, the groundmass of the pillow rims typically consists of three different parts from the outer to inner portions of the pillow (Fig. F6): (1) glassy to cryptocrystalline groundmass with skeletal olivine crystals and rare varioles, (2) a variolitic center with numerous varioles consisting of swirls of plagioclase needles and linked-chain olivine in a buff matrix lacking varioles, and (3) a microcrystalline part with faint variolitic texture and abundant sheaves of plagioclase needles. In these chilled pillow margins, the proportions of the groundmass phases cannot be accurately quantified due to the predominance of cryptocrystalline mesostasis.

Highly plagioclase-olivine(clinopyroxene)-phyric massive basalt (Unit 6)

The basalts from Unit 6 are characterized by the presence of ~20% euhedral to subhedral tabular plagioclase phenocrysts (up to 4 mm). Between 30% and 50% of the phenocrysts are glomerocrysts in which plagioclase crystals form single-phase glomerocrysts (Fig. F13C) or mixed-phase glomerocrysts with olivine or clinopyroxene (Fig. F13D). Olivine phenocrysts are also present throughout as euhedral to subhedral crystals. Most are smaller than 0.5 mm, although crystals as large as 2.5 mm were observed. On the other hand, clinopyroxene phenocrysts are relatively rare (generally <1%) in the basalt samples, although groundmass clinopyroxene crystals in coarser grained samples show more or less seriate grain-size variation and a range of shapes from granular to short prismatic to more equant, making it difficult to distinguish phenocrysts from the groundmass.

The fine-grained groundmass texture is intersertal, except for two glassy to cryptocrystalline chilled margins (Sections 336-U1382A-9R-3 [Piece 3] and 10R-3 [Piece 11]) that show hyalophitic texture in the glassy margin. In the fine-grained basalts, groundmass consists of ~35% lath-shaped plagioclase (as large as 4 mm), ~30% anhedral to subhedral clinopyroxene (as large as 0.8 mm), ~3% euhedral olivine, and 2% equant or elongate opaque Fe-Ti oxide (<0.1 mm). The chilled margin sample (Section 336-U1382A-10R-2 [Piece 10]) exhibits a variety of quench crystallization textures, ranging from spherulitic to plagioclase sheaves with or without plumose clinopyroxene. Figure F13E shows typical plagioclase spherulites surrounded by clear glass, partially replaced by clay, where acicular plagioclase forms the cores of the spherulites. As is the case in pillow margin samples, the proportions of the groundmass phases in the chilled margin sample of massive flow cannot be accurately quantified because of the predominance of glassy to cryptocrystalline mesostasis.

Biogenic/pelagic sediments and sedimentary breccia

Sediments were recovered between lava flows in three sections of Core 336-U1382A-8R, representing a total thickness of 63 cm. Smear slide identification of calcareous nannofossil assemblages is presented in a separate section (see “Micropaleontology”).

Interval 336-U1382A-8R-2, 0–15.5 cm, is composed of brownish-yellow nannofossil ooze featuring highly disturbed internal structures induced by the drilling process. It contains clasts of volcanic rocks with grain sizes ranging from very fine to fine sand. The clasts are angular and poorly sorted and could have been introduced in the course of the drilling process.

Interval 336-U1382A-8R-3, 0–14.5 cm, comprises two parts. The upper part is similar to Section 8R-2, has a color of 7.5Y 5/6, and is highly disturbed. It seems to be a bit less consolidated than the lower part, which consists of yellowish (7.5Y 4/6) nannofossil ooze. The lower part contains volcanic clasts of very fine sand (black in color). This interval does not appear to be disturbed by drilling; however, no primary sedimentary structures were observed.

Interval 336-U1382A-8R-4, 6–27 cm, is composed of light brown (10Y 8/3) nannofossil ooze. It appears to be disturbed by drilling and contains volcanic clasts that are very fine to fine in grain size.

Interval 336-U1382A-8R-4, 32–42 cm (Piece 4) (Fig. F9), is a breccia composed of several types of clasts (volcanic and sedimentary in origin). The clasts are very poorly sorted and range in size from fine sand to pebble. The coarse clasts are very angular. The matrix seems to be composed of nannofossils and may contain very fine to fine sandy clasts. This breccia does not show any sedimentary structures such as grading or cross-bedding.

Plutonic and ultramafic rocks

Overview of gabbros, serpentinized harzburgites, and lherzolites in the sedimentary breccia unit

Clasts of gabbros and serpentinized harzburgites and lherzolites were recovered from the sedimentary breccia unit (Unit 5; 161.30–173.24 mbsf) in Hole U1382A. These rocks in the sedimentary breccia are variable in size and randomly distributed. The sedimentary breccia (Section 336-U1382A-8R-4 [Piece 4]) also includes small fragments of serpentinized peridotites and plagioclase grains from gabbroic rocks. Several serpentinized harzburgites and lherzolites feature carbonate (aragonite) veins and are intruded by gabbroic veins (Fig. F14A, F14B).

Gabbros

We obtained four gabbroic rocks from the sedimentary breccia unit. The lithology of the gabbroic rocks is mainly olivine gabbro of cobble size. The olivine gabbro cobbles display subangular, subrounded, and rounded shapes. On the basis of macroscopic observation, the olivine gabbros are generally altered (background alteration varies from 20% to 70%; see “Core descriptions”). Two olivine gabbros exhibit foliation or lineation, defined by plagioclase and olivine layers or elongation of mineral grains (Fig. F15A), indicating that the rock underwent crystal-plastic deformation. Other olivine gabbros reveal a granular texture of coarse-grained plagioclase and subhedral olivine and clinopyroxene.

In meso- and microscopic observation, three olivine gabbros (Sections 336-U1382A-8R-1 [Piece 4], 8R-4 [Piece 3], and 9R-1 [Piece 5]) (Fig. F15) consist of plagioclase (50%–80%), olivine (10%–20%), and clinopyroxene (5%–20%) with minor FeTi oxide. The gabbroic rocks have either porphyroclastic or mylonitic textures depending on the percentage of matrix versus porphyroclasts (e.g., Passchier and Trouw, 2005).

Two porphyroclastic olivine gabbros (Sections 336-U1382A-8R-1 [Piece 4] and 8R-4 [Piece 3]) (Fig. F14C, F14D) consist of plagioclase, olivine, and clinopyroxene with secondary albite, serpentine, and chlorite. Plagioclase occurs as coarse-grained porphyroclasts and in the fine-grained matrix. Olivine gabbros display foliation defined by the tails of plagioclase porphyroclasts and clinopyroxene and elongated olivine. Although no foliation in Section 336-U1382A-8R-4 (Piece 3) was seen during macroscopic observation, subordinate crystal-plastic deformation was observed in thin section (Sample 336-U1382A-8R-4 [Piece 3], Thin Section 19; Fig. F15B). Grain boundaries of plagioclase are often seriated from bulging (migration) of the grain boundaries. Fine neoblastic plagioclase grains in the matrix are polygonal and occur dominantly around the plagioclase porphyroclasts. They also display intracrystalline deformation such as undulose extinction and deformation twinning (Fig. F15B). The grain size of plagioclase in the matrix varies between 30 and 200 µm. The granular shape is typical of clinopyroxene grains (5–15 mm). Plagioclase is partly replaced by chlorite, prehnite, clinozoisite, and laumontite (Fig. F15C). Olivine is partly replaced by serpentine and tremolite. The boundary between olivine and plagioclase exhibits an unidentified clay mineral and a mixture of tremolite and chlorite.

The mylonitic texture of a single olivine gabbro (Section 336-U1382A-9R-1 [Piece 5]) (Fig. F15A) consists of plagioclase, clinopyroxene, and olivine with minor ilmenite. This sample shows well-developed foliation defined by both plagioclase-dominant and clinopyroxene-olivine layers. Plagioclase porphyroclasts (1–3 mm) show features of intracrystalline deformation such as undulose extinction, subgrain boundaries, subgrains, and deformation twinning (Fig. F15A). Plagioclase grains in the matrix range in size from 50 to 150 µm and are polygonal in shape. Matrix grains also have features of intracrystalline deformation such as undulose extinction and deformation twinning. Clinopyroxenes also have features of intracrystalline deformation including undulose extinction. The grain sizes of clinopyroxene and olivine vary between 200 and 500 µm. Olivine grains are equigranular in shape and do not reveal intracrystalline deformation.

The intensity of crystal-plastic deformation in each olivine gabbro was identified on the basis of macroscopic and microscopic observations: undeformed (0), weakly foliated (0.5–1), and mylonitic (4.5), respectively (see “Magmatic and crystal-plastic structures” in the “Methods” chapter [Expedition 336 Scientists, 2012]).

Serpentinized harzburgites and lherzolites

Nine partly serpentinized peridotites were retrieved from the sedimentary breccia unit, including seven harzburgites (Sections 336-U1382A-8R-4 [Piece 6], 8R-4 [Piece 9], 9R-1 [Piece 3], 9R-1 [Piece 13], 9R-1 [Piece 14A], 9R-1 [Piece 14B], 9R-1 [Piece 15], and 9R-1 [Piece 17]) and two lherzolites (Sections 8R-4 [Piece 8] and 9R-1 [Piece 12]). Most serpentinized peridotites are green, whereas others are dominated by tangerine color, largely the product of various degrees of serpentinization and weathering (background alteration varies from 30% to 90% based on macroscopic estimation). The serpentinized peridotites do not exhibit macroscopic evidence for deformation, although some display weak foliation defined by the slightly elongated pyroxene and spinel grains. Carbonate (mostly aragonite) veins of variable thickness cut the serpentinized peridotites (Fig. F14A). These carbonate veins show strings of rounded or elongate manganese oxide/ oxyhydroxide grains in the center. One serpentinized peridotite was intruded by a gabbroic vein (Fig. F14B).

Thin section observations indicate that the protolith harzburgites consist of olivine (55%–70%), orthopyroxene (30%–44%), clinopyroxene (1%–5%), and spinel (1%) with minor plagioclase. Microstructures are characterized by coarse granular texture constituted by coarse olivine (1 to >5 mm) and medium-grained orthopyroxene (2–15 mm). Olivine grains show intracrystalline deformation, including subgrain boundaries and wavy extinction (Fig. F16A). Orthopyroxene grains are slightly bent and show wavy extinction. They also show exsolution lamellae of clinopyroxene. Serpentine replaced olivine and developed as networks of pale brown, dark brown, or black spinel grains with irregular shapes (Fig. F16B). Two serpentinized harzburgites are cut by narrow (<0.5 mm) and thick (1–4 mm) carbonate veins (Figs. F16A, F16B, F16C, F14A; Sections 336-U1382A-8R-4 [Piece 6] and 9R-1 [Piece 14B]). One serpentinized harzburgite shows two stages of carbonate veining: (1) cross-fibers sharply cutting through olivine and orthopyroxene grains, with some olivine and orthopyroxene shards preserved in the cross-fiber vein, and (2) reopening of the vein and precipitation of cracks and polygonal carbonate with manganese oxide. The polygonal carbonate grains include olivine and orthopyroxene shards originated from olivine and orthopyroxene in the adjacent harzburgite. Plagioclase grains are also present in the carbonate vein at the boundary between the cross-fiber and polygonal part of the vein (Fig. F16D).

The serpentinized lherzolite is characterized by coarse granular texture that consists dominantly of olivine (60%–68%), orthopyroxene (9%–20%), clinopyroxene (10%), spinel (1%), and plagioclase (1%) with serpentine, magnetite, and plagioclase as secondary minerals (Fig. F14B). A pervasive serpentine mesh network occupies 20%–30% of the rock. The microstructure of the serpentinized lherzolite is characterized by coarse granular olivine (0.4–10 mm) and orthopyroxene (2–8 mm). Olivine grains show features of intracrystalline deformation including subgrain boundaries. Some orthopyroxene grains are slightly bent and show wavy extinction with banding (Fig. F16E). The orthopyroxene also has exsolution lamellae of clinopyroxene (Fig. F16E). Pale brown and dark brown spinel grains have irregular shapes. Plagioclase not only occurs at the boundary between olivine grains (Fig. F16F) but also surrounding spinel grains. One serpentinized lherzolite is intruded by a gabbroic vein (~5 mm) that consists mainly of granular plagioclase (Fig. F14).

The intensity of crystal-plastic deformation in each serpentinized peridotite is mainly undeformed protogranular (0–0.5) based on macroscopic and microscopic observations (see “Magmatic and crystal-plastic structures” in the “Methods” chapter [Expedition 336 Scientists, 2012]).

Volcanic rock alteration

The basement volcanic rocks recovered from Hole U1382A are affected only by low-temperature alteration by seawater. Dark gray rocks are the most abundant and occur 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), mixed clay and Fe oxyhydroxides (iddingsite), and minor zeolite (phillipsite) and carbonate (calcite) replacing olivine and filling pore space. Alteration of the basalts is variable and ranges from fresh to moderate, manifesting as replacement of groundmass and phenocrysts, vesicle filling, glassy margin replacement, and vein formation with adjacent alteration halos. 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. We assume 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, similar to modal analyses of a thin section. Veins, halos, and breccias observed in the archive half of the core were recorded in the respective vein/halo and breccia logs (see “Alteration and metamorphism” in the “Methods” chapter [Expedition 336 Scientists, 2012]). Figures F17 and F18 provide a summary of the calculated average (per core) of the abundance, volume, width, and mineralogy of vesicles, veins, and halos.

Secondary minerals

In the basaltic rocks from Hole U1382A, secondary minerals have developed as replacement of primary phenocrysts, disseminated in the groundmass, and as vesicle and vein filling. 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 U1382A 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 the logging of alteration and veins, but these minerals were generally referred to as “smectite” or “dark green clay.” A distinction was made between minerals (carbonate and clay) associated with interflow sediments and minerals (carbonate, smectite, and iddingsite) that formed during basalt weathering. The identification of clay minerals remains tentative, however, pending further shore-based study.

Saponite is the dominant clay mineral and is present in all cores. In hand specimen, saponite occurs in black, dark green, or greenish-brown colors. When a reddish-brown to green color was observed, the mineral composition was recorded as mixed smectite–Fe oxyhydroxide. In thin section, saponite is characterized by pale brown to pale green color and mottled or fibrous form with variable pleochroism. Saponite generally evenly replaces groundmass and olivine (micro)phenocrysts, preserving the primary igneous textures. In the case of highly to extensively altered samples of pillow margin, saponite replacement is pervasive across mesostasis and groundmass microliths to form continuous mottled replacement or as blotches, revealing the original variolitic textures (Fig. F6). Commonly, saponite lines or fills vesicles (in association with other secondary minerals such as 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 observations and occurs as bright green/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 bright green (see “Vesicle filling”). Celadonite was identified by XRD in a small fragment from pillow margin (Sample 336-U1382A-5R-1, 123–129 cm [Piece 7]) and in other slightly to moderately altered basalts (Table T6).

Fe oxyhydroxide is the next most abundant secondary phase, and it occurs either as a discrete phase or mixed with saponite and other smectitic clay phases. Fe oxyhydroxides are identified by their bright orange to red color and often stain other phases (e.g., plagioclase laths in groundmass). In many cases, the mixed assemblage of Fe oxyhydroxide and clays was described as “iddingsite,” which has been previously reported in Hole 395A (Juteau et al., 1979). When present as replacement of microphenocrysts (mainly olivine) and groundmass, Fe oxyhydroxides (± iddingsite) are mixed with saponite and form intersertal hyalophitic texture. In veins and vesicles, Fe oxyhydroxides are bright orange to reddish brown and occur with or without intergrown clays. Staining of plagioclase phenocrysts and replacement of olivine with Fe oxyhydroxides are common features in the grayish-brown halos throughout all units recovered (Fig. F19).

Zeolite was found as a relatively common accessory mineral in vesicles, vugs, and vein filling, often associated with other secondary minerals (Fig. F19). Phillipsite and possibly chabazite were identified by XRD analysis in bulk sample powder (Table T6), although only phillipsite and minor stilbite were identified in altered basalts from Hole 395A (Juteau et al., 1979). In general, zeolite-filled vesicles and veins are more common in brown alteration halos of aphyric fine-grained and cryptocrystalline basalt encountered deeper in the section in Cores 336-U1382A-10R and 12R.

Carbonate is present as vug, vesicle, and vein filling and sometimes within olivine pseudomorphs (Fig. F20). Pure carbonate veins are rare in all basaltic units (e.g., Sample 336-U1382A-3R-4 [Piece 12]), but thin haloed veins often have a carbonate coating on the broken rock surfaces. As for zeolite-filled vesicles, carbonate (mostly calcite) was found essentially in grayish-brown alteration halos or pervasively altered sparsely vesicular basalts. XRD analysis of bulk rock powder did not unambiguously identify carbonate minerals. In brecciated Unit 5, large (up to 3 mm) carbonate (aragonite) veins were observed in altered serpentinized harzburgites (Fig. F8). Carbonate veins may form networks with clear crosscutting relationships, indicating different vein generations (see “Plutonic and ultramafic rocks”). In some cases, veins show spectacular aragonite druses in open voids. On the basis of thin section observation, the carbonate vein in Section 336-U1382A-9R-1 (Piece 14B), Thin Section 17, is associated with large opaque aggregates of Mn oxides (Fig. F8).

Ultratraces of secondary sulfide (pyrite) were identified only 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 1. They also occur in vesicles and voids associated with other secondary minerals such as Fe oxyhydroxides (e.g., Section 336-U1382A-7R-1 [Piece 10], Thin Section 10) or zeolite (e.g., Section 12R-1 [Piece 21], Thin Section 26). Spherules of partially oxidized pyrite were also found in altered mesostasis (Section 336-U1382A-10R-2 [Piece 10], Thin Section 23) probably as replacement of primary magmatic sulfides. In Core 336-U1382A-12R, spherical secondary pyrite (<10 µm grain size) was identified along a thin saponite vein (Fig. F21).

Phenocryst alteration

Plagioclase

Plagioclase phenocrysts (Unit 6) are fresh. 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 (Fig. F21).

Pyroxene

The augitic clinopyroxene remains essentially unaltered. When pyroxene occurs as microliths in microcrystalline groundmass, the extensive alteration (palagonitization) suggests pyroxene alteration (although the primary texture has been destroyed). Only small quantities of tiny smectite/celadonite lamellae have formed along grain boundaries around the pyroxenes of the porphyritic basalts.

Olivine

Olivine phenocrysts and microcrysts are the minerals most sensitive to alteration. In porphyritic and aphyric basalts, olivine is partially or completely replaced by reddish-brown iddingsite (a mixture of smectite and Fe oxyhydroxide) in the alteration halos (Fig. F13). In pervasively altered fine-grained basalts, olivine pseudomorphs are revealed as a result of their replacement by iddingsite (Fig. F19). In some cases, the initial shape of the mineral (skeletal or euhedral) is preserved as voids that can be partially filled by late-stage carbonate. In fresh dark gray basalts, olivine is partly replaced by saponite and possibly by celadonite.

Groundmass alteration

In the groundmass, plagioclase and clinopyroxene are quite fresh. Slight (5%) alteration of plagioclase and clinopyroxene was observed in the pervasively altered fine-grained basalt of Unit 1. In this alteration type, olivine is totally pseudomorphosed by iddingsite. Microscopic observation of fine-grained aphyric avesicular basalt of Unit 2 (e.g., Sample 336-U1382A-4R-3W, 39–42 cm, Thin Section 6) shows an overall alteration of 6%–8%, which is due to alteration of interstitial glass and olivine. Microscopic and XRD observations of microcrystalline sparsely plagioclase-olivine-phyric basalt (Unit 2) were undertaken to examine mineralogical changes with varying extents of alteration, from a fresh end-member (Sample 336-U1382A-5R-3W, 29–32 cm, Thin Section 8 [1.5% alteration]) to the most altered end-member (Sample 5R-1W, 32–36 cm, Thin Section 7 [10% alteration]). Results suggest that olivine is extensively altered and replaced by iddingsite (i.e., olivine was barely identified by XRD), whereas zeolite fills vesicles (also tentatively identified as chabazite by XRD). Plagioclase remains unaltered in both end-members.

Glass and chilled margins

Volcanic glass is generally devitrified in the groundmass of basalt. Chilled margins often show advanced palagonitization, which develops as a blotchy alteration texture following the primary variolitic texture of the mesostasis. The more altered end-members are brownish and contain vugs and veins filled with clay minerals and carbonate. Microscopic investigation of blotchy alteration texture in Unit 2 (Fig. F6) allows identification of several domains (see “Aphyric cryptocrystalline pillowed basalt (Units 2, 4, and 7)”) that are generally similar to the alteration of variolitic texture reported in Hole 395A (Natland, 1979).

In several glassy pillow margins, olivine is remarkably fresh. In Sample 336-U1382A-7R-2W, 70–73 cm, Thin Section 11, olivine occurs as euhedral to elongated skeletal microcrysts (referred to as “linked-chain morphology”; Donaldson, 1976) in cryptocrystalline groundmass. Glassy margins developing into spherulitic, mottled mesostasis also remain fresh. Note that the single thin section available for fresh glass alteration texture observation lacks microtubule textures that are considered to have formed through biological process (Fisk et al., 1998). XRD measurement of one altered pillow margin (Section 336-U1382A-5R-1 [Piece 22]) reveals significant replacement of the groundmass by zeolite (phillipsite) and celadonitic clay (Table T6).

Vesicle filling

The following filling associations were found in the vesicles: (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; and (4) carbonate, in association with other major minerals. No 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 materials. Zeolite and carbonate may also fill vesicles, in particular within dark brown halos. Many pervasively altered basalts have (remarkably) unfilled vesicles or lack saponite enrichment, which suggests different stages of vesicle filling during rock alteration. In rare cases, pyrite was found coating the interior of vesicles or as euhedral crystals with other secondary minerals. An example of composite mineral filling of vesicles is illustrated in Figure F20, which shows (1) a mixed carbonate-iddingsite vesicle (Section 336-U1382A-7R-1 [Piece 14], Thin Section 10), (2) a pure carbonate vesicle (Section 9R-3 [Piece 3], Thin Section 22), (3) a mixed iddingsite-smectite vesicle (Section 4R-3 [Piece 9], Thin Section 6), and (4) mixed Fe oxyhydroxide and smectite vesicles (Section 4R-3 [Piece 9], Thin Section 6).

Veins and halos

The following filling associations were found in the 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, in association with other major minerals; and (5) clays, possibly from interpillow sediment filling. Approximately 250 veins, including 90 haloed veins, were logged during core description of Hole U1382A, and veins make up 0.3% by volume of core (Fig. F18). The volume percent of veins for each core was estimated by calculating the volume of veins relative to the volume of the core recovered. We also logged ~170 halos in both pillow lava and massive flows that have highly variable thicknesses 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 U1382A has between 13 and 20 veins/m. The upper estimate for this hole is lower than for sections of upper volcanic basement from other holes (e.g., 27 veins/m in Ocean Drilling Program [ODP] Hole 896A, 24 veins/m in ODP Hole 801C, and 31 veins/m in Hole DSDP 504B; Alt et al., 1996; Plank, Ludden, Escutia et al., 2000). Veins range in thickness from ~0.1 to ~1 mm, but vein thickness <0.2 mm is by far the most common. The orientations of the veins are often subhorizontal to oblique in the cut face of the core, but vertical veins are not uncommon. Mixed smectite and iddingsite (Fe oxyhydroxide) veins are the most abundant, with carbonate and zeolite being less abundant. A thin (0.1 mm) iddingsite vein in an aphyric avesicular fresh basalt from Unit 2 (Sample 336-U1382A-5R-3W, 29–32 cm, Thin Section 8) and a medium-grained aphyric basalt from Unit 8 (Sample 12R-3W, 35–38 cm, Thin Section 2) are illustrated in Figure F21. In most cases, only iddingsite veins are associated with dark brown alteration halos, although most thin veins lack alteration halos.

Grayish-brown alteration halos were identified in all units in Hole U1382A (Fig. F4). Brown halos are characterized by abundant Fe oxyhydroxides disseminated in the groundmass, staining smectite-filled pores, and replacing olivine. Celadonite-nontronite may also be present. A representative piece of microcrystalline aphyric avesicular basalt with alteration halos was investigated by thin section (Sample 336-U1382A-4R-3W, 34–37 cm, Thin Section 5). The margins are more altered (~20%) with abundant clay (smectite) replacement of olivine (60% alteration) and more rarely after clinopyroxene and plagioclase (<10%). Fe oxyhydroxides are also enriched throughout the groundmass (interstitial filling) and olivine pseudomorphs. Voids and vesicles are also extensively filled with iddingsite. The less altered gray core is devoid of iddingsite, and only olivine (10% alteration) is replaced by dark green smectite. These grayish-brown alteration halos are developed throughout the entire section recovered in Hole U1382A and reflect significant oxidative weathering conditions due to alteration by oxygenated seawater. Many aphyric cryptocrystalline basalts to glassy pillow margins have very thin (<1 mm) and irregular alteration halos (Fig. F21), reflecting the formation of an oxidizing alteration front extending from the open cracks into the rock.

Breccia/hyaloclastite

The glassy fragments in hyaloclastic breccias have concentric alteration rims surrounding fresh glass preserved in the central part of the fragments (Fig. F7). The breccia cement is mainly clay (± zeolite) and carbonate. Mixed sedimentary clay and Fe iddingsite/smectite assemblages as matrix filling are apparent in Section 336-U1382A-5R-1 (Piece 21). In all hyaloclastite recovered, glass from glassy pillow rims and glass shards are variably altered to smectite, forming concentric alteration fronts with external rims of Fe oxyhydroxide. The cement is commonly smectite, carbonate, and zeolite. XRD analysis of a glassy margin in Section 336-U1382A-5R-1 (Piece 22) confirms that zeolite is common in altered glass of hyaloclastite, which is typically a result of “palagonitization” (i.e., low-temperature alteration of basaltic glass by seawater; Honnorez, 1972).

Hard rock geochemistry

Concentrations of major element oxides and several trace elements were obtained by inductively coupled plasma–atomic emission spectroscopy (ICP-AES) for a total of 19 whole-rock samples from Hole U1382A, together with total carbon and nitrogen contents by carbon-hydrogen-nitrogen-sulfur (CHNS) analyzer and LOI (see “Hard rock geochemistry” in the “Methods” chapter [Expedition 336 Scientists, 2012] for analytical procedures and precisions). Analytical values of ICP-AES were normalized to 100%, with total Fe recalculated as Fe₂O₃ (Fe₂O₃^T). Results of the ICP-AES analysis, as well as LOI and total carbon and nitrogen content data, are presented in Table T7.

Effect of plagioclase accumulation

Downhole variations of selected major element concentrations and LOI are shown in Figure F22. Although many elements show no systematic downhole trends, significant variations in concentration were observed for some elements. The downhole changes in composition include an increase in Al₂O₃, CaO, and Sr and a decrease in Fe₂O₃^T, TiO₂, Cr, Sc, V, and Y, which are associated with the presence or absence of plagioclase phenocrysts (Table T7). The significant increase of CaO and Al₂O₃ in the highly plagioclase-phyric basalts from Units 5 and 6 can be explained by the accumulation of plagioclase (Fig. F23). Likewise, the enrichment of Sr in the porphyritic basalts can very likely be attributed to accumulation of calcic plagioclase (Fig. F23). In addition, the depletions of MgO, Fe₂O₃^T and TiO₂ can reasonably be assumed to reflect dilution by plagioclase accumulation in the porphyritic basalts (Fig. F24). Because depletions of Cr, Sc, and V concentrations in the porphyritic basalts generally correlate with the depletion in Fe₂O₃^T (Fig. F25), we suggest that the decrease in concentration of these elements is also likely related to the accumulation of plagioclase.

Differences in magmatic geochemical signatures

Zr/Y and Zr/Ti ratios are not affected by crystal fractionation and accumulation of minerals in magmatic rocks. Instead, these ratios are sensitive to variations in mantle composition or melting processes during the formation of primitive magmas. The concentrations of Zr and Y are plotted in Figure F26. In this figure, two separate trends, indicating different Zr/Y ratios, are recognizable and clearly correspond to Units 1–4 (units shallower than the sedimentary breccia unit) and 6–8 (units deeper than the sedimentary breccia unit). A similar separation into two trends can also be picked in a Zr versus TiO₂ plot (Fig. F26). These results imply that the shallower and deeper units were derived from different parental magmas. On the other hand, these elements correlate well within both the shallow and deep units, suggesting that each of the two units originated from the same parental magmas. The sedimentary breccia unit (Unit 5) separates the shallow from the deep units indicating a hiatus in the volcanic activity.

The geochemical features of Hole U1382A basalts described above are similar to those reported for Hole 395A basalts at corresponding depths. For example, the difference in chemistry between aphyric and porphyritic basalts, as well as shallower and deeper units, is also shown in the Hole 395A basalts (e.g., Bougault et al., 1979). The compositional ranges of each lithology and unit also generally overlap (Figs. F23, F24, F25). These results clearly show that, although the proportion of massive and pillowed basalt flows is slightly different between the two holes, basaltic lavas recovered from Holes U1382A and 395A are essentially the same. This finding is not unexpected because Hole U1382A is only 50 m from Hole 395A.

Geochemical changes during alteration

Downhole variations of LOI and total carbon content are shown for the different lithologic units in Figure F22. Interestingly, altered samples having relatively high LOI contents (up to 2.69 wt%) occur only in the shallower units, and LOI contents of the samples from the deeper units are <1.03 wt%. Almost all basalt samples have low total carbon contents of <0.5%, except the sample from the shallowest depth, which has a total carbon content of 0.62 wt%.

In order to evaluate the geochemical exchange during alteration, the average compositions of altered samples were compared with those of fresh (i.e., least altered) samples (Fig. F27). In this calculation, basalt samples having LOI contents of <1% were defined as the least altered samples and those having LOI contents of >1% were regarded as more extensively altered ones. Compared to the fresh samples, the altered basalts are slightly enriched in K₂O, Ba, and total carbon contents (Fig. F27). The enrichment in total carbon clearly reflects the presence of secondary carbonate minerals in the altered samples. The K₂O concentrations slightly correlate with LOI values (Fig. F28), suggesting that the presence of K-rich secondary minerals (probably celadonite and phillipsite) in the altered basalt samples is responsible for the K₂O enrichment. In addition to K₂O, Ba is correlated slightly with LOI, and there is also a weak correlation between Ba and K₂O (Fig. F28). It is thus likely that Ba is also contained in K-rich minerals as a trace component.

Slight depletion of Mg was also observed in the altered basalts (Fig. F27), but the MgO depletion is very small and thus less certain. It is, however, noteworthy that olivine microphenocrysts were selectively altered to Fe oxyhydroxide in several altered basalt samples. This finding may suggest selective dissolution of Mg from altered basalts. Thus, it is likely that selective alteration of olivine causes the depletion of Mg in bulk rock chemistry of altered basalt samples.

Sediment geochemistry

Sediment was recovered in Sections 336-U1382A-8R-2, 8R-3, and 8R-4. Carbonate concentration measurements by coulometry were conducted for a total of eight samples from these sections (Table T8). Samples were taken from the split working half of the core and from two interstitial water squeeze cakes.

Calcium carbonate contents range from 20.8 to 43.9 wt%. Inorganic carbon contents range from 2.49 to 5.27 wt%.

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