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Alteration and metamorphism

Summary of Expedition 312 results

Alteration of the plutonic section cored during Expedition 312 was reexamined during Expedition 335 by visual observation of the archive section halves, with estimations of secondary mineral abundances and primary mineral alteration in hand specimen calibrated by examination of thin sections and aided by previously published works (Teagle, Alt, Umino, Miyashita, Banerjee, Wilson, and the Expedition 309/312 Scientists, 2006; Koepke et al., 2008; France et al., 2009; Alt et al., 2010). These observations are recorded in the Expedition 335 alteration log and vein and halo log (see DESCLOGIK_WORKBOOKS _335(312) in DESCRIPTIONS in “Supplementary material”). The rocks display pervasive alteration as well as localized alteration effects. These are divided into

  • Pervasive background alteration,

  • Localized alteration patches,

  • Recrystallized granoblastic basalt and recrystallized domains (xenoliths),

  • Secondary mineral veins, and

  • Alteration halos along veins.

Figure F36 summarizes the distribution of alteration textures and recrystallized domains (xenoliths) versus depth in the plutonic section, and Figure F37 summarizes the overall distribution of secondary minerals versus depth.

The alteration history of the plutonic section penetrated during Expedition 312 involves hydrothermal alteration of sheeted dikes in a mid-ocean-ridge axial hydrothermal system, intrusion of gabbro bodies into hydrothermally altered sheeted dikes, and contact metamorphism of the intruded dikes, followed by crystallization of the gabbros, cooling, injection of late-stage dikes, and hydrothermal alteration of gabbro and early and late-stage dikes, with superimposition of greenschist-grade alteration on previously metamorphosed dikes (Teagle, Alt, Umino, Miyashita, Banerjee, Wilson, and the Expedition 309/312 Scientists, 2006; Koepke et al., 2008; France et al., 2009; Alt et al., 2010).

Background alteration

Coarser grained (gabbroic) and finer grained (dikes) rocks exhibit pervasive background alteration (Fig. F38). Gabbroic rocks are typically dark gray and moderately (10%–50%) altered but range from highly to completely altered locally. More intense alteration is commonly associated with coarser grained areas, igneous contacts, and leucocratic rocks.

Figure F39 shows the major mineralogy of primary phase replacement in the background alteration. Olivine is partially to totally altered to smectitic phyllosilicates ± red Fe oxyhydroxide, talc, and magnetite, with local outer rims of pale blue-green and colorless amphibole and minor chlorite. Clinopyroxene is typically moderately altered to green amphibole with common small (~1–10 µm) inclusions of magnetite. Slightly altered clinopyroxenes have clear cores that give way outward to dusty corroded rims and then amphibole replacement and overgrowth. Brown amphibole occurs locally in interstitial areas and as patches within clinopyroxene and may be of late igneous origin. Plagioclase is generally less altered than clinopyroxene and is partially altered to secondary plagioclase (albite), amphibole, chlorite, prehnite, laumontite, and epidote. Orthopyroxene, where present, is partially altered to smectitic phyllosilicates and green and colorless amphibole. Titanomagnetite exhibits ilmenite exsolution lamellae and is typically partially to highly replaced by titanite. Recrystallized igneous sulfide globules are common in interstitial areas and as inclusions in plagioclase and rarely in amphibole replacing clinopyroxene.

The last pieces of core recovered during Expedition 312 (Section 312-1256D-234R-1 [Pieces 4–8]) are dark greenish gray, with clinopyroxene altered to dusty clinopyroxene, amphibole, chlorite, and secondary plagioclase. The presence of primary intergranular igneous texture and the absence of granoblastic recrystallization indicate that the core represents a late dike.

Alteration patches

Alteration patches are zones that are more intensely altered than the background alteration and are typically 50%–90% altered (average = 60%) (Fig. F40). The patches range from <1 to >3 cm across and range from round to irregular, elongate, and network shaped. Most of the gabbros exhibit patchy alteration, averaging ~8% of the recovered gabbro cores, but patches are more abundant in Gabbro 1, where they commonly form interconnected networks in the uppermost 5 m. Patches range from dark to light gray or green to white for more felsic patches (Fig. F41). Secondary mineralogy of the patches is generally similar to background alteration (Fig. F42), but patches can locally contain abundant epidote.

Recrystallized basalts and recrystallized domains

The dike screens consist of fine-grained basalts that are strongly to completely recrystallized to granoblastic assemblages of clinopyroxene, plagioclase, orthopyroxene, ilmenite, and magnetite ± olivine. Fragments of these recrystallized rocks are incorporated into the gabbro units (Fig. F36). These fragments are xenoliths or stoped blocks of recrystallized dike rock, may be variably resorbed by the magma, and are referred to here as recrystallized domains. These domains are strongly to completely recrystallized to granoblastic assemblages as in the recrystallized dikes. They range from centimeter-scale streaks and areas to several centimeter–sized angular blocks and are most common near the margins of Gabbro 2. Recrystallized domains exhibit slight to locally strong postrecrystallization hydrous alteration to amphibole, albite, and smectite.

Veins and vein-related alteration halos

The gabbros average 10 veins/m recovered core, compared to ~35 veins/m in the sheeted dikes (Figs. F43, F44, F45). Among the earliest generation of veins are green amphibole veins with diffuse boundaries. These veins are narrow (<0.1 mm) and are defined by amphibole replacement of clinopyroxene in a narrow (1–2 mm) amphibole-rich halo in the wall rock. These veins are cut by discrete 0.5–1 mm green amphibole veins that also have amphibole alteration halos. Also early are narrow (typically ~1 mm but as wide as 5 mm) quartz ± amphibole ± plagioclase veins. In some cases these veins may be hydrothermal in origin, but in others they are related to late magmatic fluids (Alt et al., 2010). Veins of epidote, quartz, and prehnite, with intensely altered chloritic margins, are present locally. Relationships are not always clear in hand specimen, but these veins cut or fill centers of earlier amphibole veins (Teagle, Alt, Umino, Miyashita, Banerjee, Wilson, and the Expedition 309/312 Scientists, 2006; Alt et al., 2010).

The dike screens contain ~15–30 veins/m, most with halos. The first generation of post-contact metamorphic veins is wispy <0.1 mm amphibole veins, with diffuse 1–2 mm amphibole-rich halos. These veins are cut by later amphibole veins bounded by actinolite-rich alteration halos. Later chlorite-actinolite, quartz-chlorite, and quartz veins with 1–2 mm chloritic margins cut the earlier veins.

Expedition 335 Sections 335-1256D-235R-1 through 239R-1

Sections 335-1256D-235R-1 through 239R-1 comprise dark gray, fine-grained, recrystallized basalt and are essentially identical to the granoblastic basalts from Dike Screen 2 in the Expedition 312 plutonic section. Also present are local pieces of coarser grained plutonic material. Similar recrystallized basalt in the junk baskets (see “Expedition 335 junk baskets,” below) retains chilled contacts and textures of brecciated dike margins and most likely came from near the bottom of the hole, indicating that Expedition 335 cores and junk basket samples are part of a dike screen within the plutonic section, most likely Dike Screen 2.

Granoblastic basalts

The dike screens are recrystallized material consisting of fine-grained to very fine grained basalt recrystallized to granoblastic assemblages of clinopyroxene, orthopyroxene, plagioclase, magnetite, ilmenite, and rare brown hornblende and quartz. These rocks are essentially identical to similar rocks described from the Expedition 312 plutonic section (Teagle, Alt, Umino, Miyashita, Banerjee, Wilson, and the Expedition 309/312 Scientists, 2006; Koepke et al., 2008; Alt et al., 2010) and exhibit a similar degree of recrystallization (Figs. F46, F47). The rocks are completely recrystallized mineralogically but exhibit variable degrees of development of granoblastic textures and concomitant loss of original igneous textures (Fig. F48). The degree of development of granular texture ranges from partial to complete and is highly variable from one sample to another (Fig. F47). Several factors influenced recrystallization, including initial grain size, the type and extent of hydrous alteration of the protolith, and the temperature and duration of reheating.

Magnetite and ilmenite occur as equant, discrete grains, and ilmenite is also present as segregations or lamellae within magnetite in some samples. Local 100–200 µm wide veins of orthopyroxene are present and probably represent preexisting hydrothermal veins in the protolith that were subsequently recrystallized during contact metamorphism (Fig. F48C).

Sulfide minerals, including pyrite, chalcopyrite, and pyrrhotite, are present in trace amounts. Sulfides, which may be recrystallized igneous grains or preexisiting secondary sulfides that were recrystallized during contact metamorphism, occur in interstitial areas and as tiny inclusions in plagioclase and rarely in clinopyroxene and appear adjacent to orthopyroxene veins. Sulfide minerals that are likely related to post-contact metamorphism hydrothermal alteration occur along grain boundaries, in coronae around and in amphibole replacing clinopyroxene and orthopyroxene, and disseminated outside alteration halos along amphibole veins.

The recrystallized basalts are generally only slightly altered by subsequent hydrous alteration but are locally moderately to highly altered (e.g., Samples 335-1256D-236R-1, 0–4 cm, and 238R-1, 13–15 cm). Clinopyroxene is variably altered to amphibole plus variable but minor amounts of magnetite and to local chlorite (Fig. F49). Orthopyroxene is partly altered to amphibole and local talc and smectite. Plagioclase is less altered than pyroxenes but is slightly altered to actinolite, secondary plagioclase, clays, and local chlorite veinlets and to rare epidote, prehnite, zoisite, and quartz. Ilmenite is variably replaced by titanite.

Sample 335-1256D-236R-1, 0–4 cm, contains a contact between fine-grained felsic material and typical granoblastic basalt. The felsic material is highly altered, with pyroxene replaced by amphibole, magnetite, and chlorite. Plagioclase is altered to secondary plagioclase and minor chlorite and clays. Magnetite is altered to titanite. Also present are poikiloblasts of epidote. Alteration of granoblastic material is generally more intense around intrusions (Fig. F49B).

Veins and halos

The most common veins contain amphibole (actinolite and/or hornblende) and range from ~10 µm to 1.5 mm wide (Figs. F50, F51, F52). Small (~10 µm) veins of amphibole (predominantly actinolite) are common throughout most thin sections. A characteristic of these tiny amphibole veins is that clinopyroxene and in some cases plagioclase are replaced by amphibole where cut by the veins. Larger amphibole veins (as wide as 0.5 mm) display ~1–5 mm wide amphibole-rich alteration halos in the wall rock, where clinopyroxene and orthopyroxene are replaced by amphibole (Fig. F51). In some cases, the larger amphibole veins also contain chlorite and titanite. In some samples, the ~10 µm actinolite veins cut across the larger amphibole veins. Sample 335-1256D-236R-1, 0–4 cm, contains a 300–400 µm wide vein of actinolite + epidote + chlorite that grades into a 500 µm wide vein lined with chlorite and filled with prehnite enclosing needles of actinolite (Fig. F52B). This vein cuts across and offsets a contact between felsic and granoblastic materials in this sample. Quartz + amphibole is present in a 1–2 mm wide vein in Section 335-1256D-235R-1 (Piece 6). Carbonate is present with quartz in a 1 mm wide vein along the edge of Sample 335-1256D-236R-1, 31–32 cm.

Coarse-grained material

Only limited coarse-grained material was recovered in the Expedition 335 cores. This material (e.g., Sample 335-1256D-235R-1, 23–35 cm [Piece 5]) consists of foliated diorite hosting a leucocratic tonalite vein that is in turn cut by an amphibole vein (Fig. F53). The background consists of moderately altered foliated diorite, where amphibole replaces clinopyroxene and igneous amphibole. Plagioclase appears cloudy and is altered to secondary plagioclase, chlorite, and rare epidote and zeolite. Quartz and apatite are also present. Titanomagnetite is highly altered to titanite. The main part of the section consists of a leucocratic magmatic vein that is similarly moderately altered to amphibole and plagioclase, but plagioclase is more cloudy and altered than in the host rock. A ~1 mm vein is present in the central part of the leucocratic vein and is composed of amphibole + quartz + epidote + plagioclase + magnetite. A 3 mm clot of epidote is present in one area of the vein.

Expedition 335 junk baskets

Rock samples were recovered in junk basket Runs 11–22. The rocks are mainly pebbles to cobbles of fine-grained basalt recrystallized to granoblastic assemblages. Also present are fine-grained platy fragments exhibiting granoblastic recrystallization; coarser grained fragments of olivine gabbronorite, oxide gabbro, and quartz diorite; and dark gray to black fine-grained basalt fragments.

The dark gray to black basalt fragments have igneous textures of basalt lava, and smectite coating fracture surfaces on one piece was identified by X-ray diffraction. These observations are consistent with these samples falling to the bottom of the hole from the volcanic section.

Volcanic basalt aside, the rocks in the junk baskets are similar to those recovered from the lower granoblastic dikes and dike screens during Expedition 312. The rocks are mainly granoblastic basalts, including dike margins (see below), with local small (0.1–3 cm) intrusions of coarse-grained gabbro and more differentiated material. The lithology, angularity, and large size (>10 cm) of many blocks in the junk baskets suggest that the majority of this material fell into the hole from the lowermost ~10 m of the hole. These rocks thus provide a greater sampling and further view of the rocks from these depths than a single one-dimensional drill core.

Recrystallization of granoblastic basalts

The main lithology from the junk baskets consists of basalt recrystallized to granoblastic assemblages of clinopyroxene, orthopyroxene, plagioclase, magnetite, ilmenite, and rare brown hornblende and quartz. These are essentially identical to the granoblastic basalts cored in Dike Screens 1 and 2 during Expeditions 312 and 335. Clinopyroxene and orthopyroxene are totally recrystallized and contain common round magnetite inclusions. Magnetite and ilmenite are completely recrystallized and are present as equant, discrete mineral grains or as domains of one mineral within the other. Sulfide minerals (pyrite, chalcopyrite, and pyrrhotite) are present in interstitial areas, in coronas around pyroxenes, as inclusions in plagioclase, and, rarely, in clinopyroxene and orthopyroxene. Linear trails or bands (tens to ~100 µm wide) of orthopyroxene and plagioclase or orthopyroxene and clinopyroxene are present locally (e.g., in Sample 335-1256D-Run11-EXJB-J1) and are probably recrystallized hydrothermal veins.

Orthopyroxene veins (100–200 µm wide), similar to those in the Expedition 335 cores, are present in several samples from the junk baskets (e.g., Figs. F48C, F49C, F50A). These orthopyroxene veins are commonly interconnected, similar to the hydrothermal vein networks present in some upper dike samples from Expedition 312 (Teagle, Alt, Umino, Miyashita, Banerjee, Wilson, and the Expedition 309/312 Scientists, 2006; Alt et al., 2010).

Sample 335-1256D-Run12-RCJB-Rock S consists of a fine-grained dike chilled against a relatively coarser grained dike (Fig. F54). The entire sample is recrystallized to a granoblastic assemblage, and later fine amphibole veins cut across the contact and both dikes. Local 0.5 mm thick plagioclase-rich banding in the chilled margin may represent previous hydrothermal veins or possibly relate to flow structures and alteration of the chilled margin protolith. Aggregates of sulfide minerals (as large as 250 µm of pyrite, chalcopyrite, pyrrhotite, ± magnetite) are common in the chilled margin and in the plagioclase-rich bands and may be recrystallized hydrothermal grains.

Two samples (335-1256D-Run12-RCJB-Rock Q and 335-1256D-Run14-EXJB-Foliated) exhibit granoblastic recrystallization but also display a 1–10 mm banded texture. The bands comprise plagioclase-rich, orthopyroxene-rich, and oxide-rich zones. Sample 335-1256D-Run14-EXJB-Foliated comprises recrystallized breccia (Fig. F55). Angular 5–10 mm clasts consist of granoblastic plagioclase with minor clinopyroxene, orthopyroxene, magnetite, and ilmenite. The protolith of this material was probably altered fine-grained chilled margin material. The matrix is orthopyroxene rich, with minor clinopyroxene, plagioclase, and oxides, and forms ~5 mm wide bands separating the plagioclase-rich clasts. The orthopyroxene-rich material likely recrystallized from hydrothermal minerals (e.g., amphibole and chlorite) veining and cementing the breccia protolith. More typical granoblastic material forms one end of the thin section, and the other consists of coarser grained orthopyroxene- and plagioclase-rich granoblastic material with typical magnetite and ilmenite and with abundant intergrown pyrite, chalcopyrite, pyrrhotite, and magnetite. The texture and abundance of sulfide minerals are similar to those in mineralized dike margins from shallower in the Expedition 309/312 dike sections (e.g., figs. F61 and F62 in Expedition 309/312 Scientists, 2006; Alt et al., 2010). Other samples (335-1256D-Run12-RCJB-Rock C and 335-1256D-Run12-RCJB-Rock T) exhibit less distinct compositional banding, which may be related to intense prerecrystallization hydrothermal veining and alteration of the protoliths.

Rocks from Sample 335-1256D-Run11-EXJB comprise equant to platy basalt fragments, 1–3 cm thick by as much as 7 cm × 10 cm across. Several samples have stepped edges defined by veins and fractures. Aside from a few stray basalt fragments from the volcanic section, these rocks are recrystallized to granoblastic assemblages with slight later hydrous alteration and amphibole veins, as described above. Some samples consist of several centimeter–sized platy fragments of extremely fine grained, hard, microcrystalline material (possibly basaltic). The abundance of veins and fractures and the platy morphology of most of these rocks suggest two possibilities: (1) there is a highly fractured and veined interval in the wall rock at approximately this depth or (2) this material may be typical granoblastic dike material at this site, which has low recovery because the abundant fractures cause the rock to break up and be ground up during drilling.

Postrecrystallization alteration of granoblastic basalt

The granoblastic rocks typically exhibit at most only slight alteration (generally <15%) to hydrous minerals, mainly amphibole (Figs. F45, F46). Clinopyroxene is locally partly altered to amphibole, orthopyroxene is variably altered to amphibole and local talc and smectite, and plagioclase is locally slightly altered to trace chlorite and actinolite and to secondary plagioclase and clays (likely smectite). Fe-Ti oxides are partly altered to titanite. The granoblastic rocks are more highly altered in 1 mm wide alteration halos along amphibole veins. Clinopyroxene and orthopyroxene in these alteration halos are highly to completely replaced by amphibole, whereas plagioclase is generally unaltered or may be slightly altered to actinolite and trace chlorite. Sulfide minerals (pyrite, pyrrhotite, and minor chalcopyrite) are locally common in minor amounts disseminated in the wall rock along amphibole veins. In some cases, the granoblastic rocks are more altered where associated with intrusions of coarser grained rock (Fig. F49B). For example, Sample 335-1256D-Run11-EXJB-J6 contains diorite intruding granoblastic rock, and the host rock is highly altered, with clinopyroxene highly altered to amphibole.

The granoblastic material sampled by the junk baskets contains common hydrothermal veins, the most common being filled with amphibole (actinolite and/or hornblende). Amphibole veins range from ~10 µm to 1 mm in width. The most abundant veins occur as tiny cracks filled with amphibole, but the distinguishing characteristic is the presence of amphibole replacing clinopyroxene and orthopyroxene that are cut by the tiny cracks (Fig. F50). These micrometer-sized actinolite veins cut recrystallized (granoblastic) veins and mixed amphibole-chlorite veins. The larger amphibole veins have 1 mm wide (rarely as wide as 4 mm) amphibole-rich alteration halos. Chlorite and rarely later prehnite are also present in veins. The density of veins is variable, but vein nets occur in several samples (e.g., in Sample 335-1256D-Run12-RCJB-Rock F). Epidote and quartz are present in three veins in Sample 335-1256D-Run11-EXJB, as 1.5–2 mm wide veins of quartz + epidote ± chlorite and in a 0.4 mm vein of quartz + amphibole + epidote.

Alteration of coarser grained lithologies

Gabbro is present as coarser grained fragments of amphibole-rich oxide-gabbronorite in Samples 335-1256D-Run11-EXJB-J5 and 335-1256D-Run11-EXJB-TS#29 as a centimeter-scale intrusion into a granoblastic host (Sample 335-1256D-Run12-RCJB-Rock B) and as equant to elongate and irregular intrusions linked by amphibole veins in large cobbles in Sample 335-1256D-Run14-FTJB. Similar coarse-grained intrusions are present in some small fragments (e.g., Sample 335-1256D-Run13-RCJB). Clinopyroxene is highly altered to amphibole plus minor magnetite and chlorite (Fig. F56). Plagioclase in the gabbroic rocks is partly altered to secondary plagioclase and minor amphibole, chlorite, clay, prehnite, and epidote. Sulfide minerals (chalcopyrite and pyrrhotite) are locally common in interstitial areas as inclusions in ilmenite and plagioclase, intergrown with amphibole, and in tiny veinlets. Ilmenite-magnetite is altered to titanite. Olivine is abundant in Sample 335-1256D-Run11-EXJB and is altered to magnetite, talc, amphibole, chlorite, and smectite in coronitic textures. Rare smectite associated with iron oxide in hand specimens (Sample 335-1256D-Run14-EXJB) indicates the presence of altered olivine or orthopyroxene.

Oxide-rich quartz-diorite is present locally as 0.5–2 cm wide intrusions (patches and veins; Samples 335-1256D-Run11-EXJB-J6 and 335-1256D-Run13-RCJB-Rock A). A few coarse-grained leucocratic veins and pebbles, cuttings and pebbles of tonalite, and a coarse-grained quartz diorite pebble are present in Samples 335-1256D-Run13-RCJB, 335-1256D-Run14-EXJB, and 335-1256D-Run17-BSJB. These felsic rocks are highly altered, with plagioclase highly altered to secondary plagioclase, minor chlorite, epidote, and local prehnite and quartz. Clinopyroxene and primary amphibole are highly altered to amphibole + magnetite, and minor orthopyroxene is replaced by talc and smectite. Fe-Ti oxides are slightly altered to titanite. Interstitial quartz contains needles of actinolite. Zircon, apatite, and rare biotite are also present.

Thin section observations reveal that some pebbles of leucocratic material are plagioclase rich (>90%), consisting of granular plagioclase with minor Fe-Ti oxide, apatite, and clay and local larger crystals of phenocryst-like plagioclase. The unusual textures and mineralogy suggest that these rocks may be metasomatized dikes.

Veins within the coarser grained lithologies are rare. In thin section these are only observed where coarse material intrudes granoblastic material, with amphibole veins that extend through the granoblastic host rock and cut across the coarser grained intrusive material.

Expedition 335 rocks

The cores and rocks recovered during Expeditions 312 and 335 sample the transition from sheeted dikes to the gabbroic section of oceanic crust. The dikes underwent hydrothermal alteration in a mid-ocean-ridge hydrothermal system at the spreading axis, at temperatures reaching >400°C, and with fluid compositions similar to those of black smokers venting at the seafloor (Alt et al., 2010). The hydrothermally altered dikes were then intruded by the two gabbro bodies cored during Expedition 312 and underwent contact metamorphism at temperatures of ~900°–1000°C, with the degree of recrystallization influenced by the effects of prior hydrothermal alteration (Teagle, Alt, Umino, Miyashita, Banerjee, Wilson, and the Expedition 309/312 Scientists, 2006; Koepke et al., 2008; France et al., 2009; Alt et al., 2010). Crystallization of the gabbro bodies and cooling of the gabbros and recrystallized dikes allowed penetration of fluids and hydrothermal alteration of the rocks, with formation of amphibole veins and later retrograde minerals (actinolite, quartz, epidote, chlorite, prehnite, and late smectite and iron oxyhydroxides).

The granoblastic dikes represent the conductive boundary layer between mafic magma and the overlying hydrothermal system, and the rocks from Hole 1256D are similar to those observed in ophiolites and elsewhere in oceanic crust (e.g., Gillis, 2008; France et al., 2009). The granoblastic basalts sampled beneath Gabbro 2 during Expedition 312 are part of a dike screen within the transition from sheeted dikes to gabbros. The rocks are more strongly recrystallized than Dike Screen 1 and similar to the granoblastic dikes above the gabbros.