Four holes were drilled at Site 1275 (15°44.47´N, 46°54.21´W; mean water depth = 1567.4 mbsl). Hole 1275A (15°44.489´N, 46°54.211´W; 1562.3 mbsl) is 5 m deep and recovered 10 cm of slightly altered diabase with minor chlorite and amphibole replacing mesostasis and a piece of matrix-supported, carbonate-cemented breccia with slightly altered angular diabase clasts. Hole 1275B (15°44.486´N, 46°54.208´W; 1561.7 mbsl) is 108.7 m deep and recovered 46.9 m of core (recovery = 43.1%). Lithologies from Hole 1275B include slightly to highly altered diabase, gabbro, microgabbro, and troctolite cut by variably altered gabbroic and felsic dikes and veins. Metamorphic veining is dominated by amphibole and chlorite-amphibole veinlets. Alteration is static, and macroscopic amphibole and amphibole-chlorite veins, as well as abundant amphibole-filled microcracks, acted as conduits for seawater-derived hydrothermal fluids. Replacive formation of chlorite after clinopyroxene and plagioclase is limited mostly to the uppermost 35 m of Hole 1275B. Coronitic replacement of olivine and plagioclase by talc, chlorite, and tremolite characterizes the alteration of troctolitic rocks in the interval from 28.2 to 34.1 mbsf. Below 34 mbsf, alteration is uniformly dominated by partial replacement of clinopyroxene by green and minor brown amphibole. Plagioclase is <5% altered to secondary plagioclase and green amphibole along cracks. Oxides are partly replaced by titanite and hematite. Felsic dikes and veins in Hole 1275B are variably altered to secondary plagioclase, amphibole, and, locally, carbonate. Talc replacing pyroxene is present in the host gabbro adjacent to some of these veins. Felsic veins have noticeable amounts of quartz, titanite, apatite, and zircon, and patches of titanite, quartz, and apatite are present in some gabbro in proximity to these felsic veins. Metamorphic veins are dominated by amphibole, amphibole-chlorite, and clay-oxide veins and make up 0.43% of the core volume.
Hole 1275C (15°44.440´N, 46°54.218´W; 1552.8 mbsl) recovered <1% of the 20.8 m of basement drilled. Rocks from Hole 1275C comprise highly to completely altered gabbro, microgabbro, and troctolite. Alteration assemblages in gabbroic rocks are dominated by amphibole with minor chlorite and secondary plagioclase. Troctolites are variably serpentinized and talc-altered. Given the low core recovery, alteration of rocks from Holes 1275A and 1275C will not be further discussed in this chapter.
Hole 1275D is 209 m deep, and 104.6 m of core was recovered (recovery = 50.1%). Similar to Hole 1275B, the lithologies encountered in Hole 1275D comprise gabbro, microgabbro, diabase, mafic and felsic magmatic veins, and troctolitic rocks with variable amounts of orthopyroxene and clinopyroxene. The alteration style in Hole 1275D is also similar to that in Hole 1275B. Green amphibole is the most abundant secondary mineral, replacing clinopyroxene (20%–100% alteration) and plagioclase (<1%–15% alteration) in the gabbroic rocks. Alteration of the troctolitic rocks is characterized by serpentine and magnetite replacing olivine (50%–100% alteration) and chlorite, clay, and carbonate replacing plagioclase (50%–90% alteration). Pyroxenes in troctolitic rocks are altered to talc, serpentine, and amphibole. The major difference in alteration between Holes 1275B and 1275D is the higher abundance of lower greenschist facies minerals (chlorite/smectite after plagioclase and clinopyroxene) and low-temperature oxidative alteration (red clay/Fe oxyhydroxide and carbonate after olivine; clay and carbonate after plagioclase) in Hole 1275D. In comparison with Hole 1275B, amphibole, picrolite, and carbonate are more abundant and clay-oxide veins are less abundant in Hole 1275D. In both holes, late-stage metamorphic veins commonly follow preexisting magmatic veins and amphibole veins.
Hydrothermal alteration at Site 1275 started under amphibolite facies conditions (500°–700°C) with the formation of brown amphibole and continued under greenschist, zeolite, and brownstone facies conditions, as indicated by the development of green amphibole, chlorite, chlorite/smectite, clay, Fe oxyhydroxide, and carbonate. Hydrothermal alteration is almost exclusively static, and greenschist facies assemblages with green amphibole and chlorite or chlorite/smectite (in Hole 1275D) are the most abundant secondary minerals. Small veins and microcracks provided the main fluid pathways. These fractures and cracks are either related to cooling or to unroofing of the footwall of a normal fault.
Alteration intensities in Hole 1275B range from 10% to 100% (Fig. F34). We distinguish between an upper sequence of gabbro and diabase dikes (0–28 mbsf; Sections 209-1275B-1R-1 to 6R-1) and a lower sequence (34–105 mbsf; Sections 7R-1 to 22R-2) of gabbro and felsic magmatic dikes. In the upper sequence, diabase is 10%–40% altered, manifest in the replacement of mesostasis and clinopyroxene microphenocrysts by fibrous amphibole and chlorite. Gabbro and microgabbro (Fig. F35A) are 20%–50% altered. Clinopyroxene is highly to completely altered, mostly to green amphibole, although noticeable amounts of chlorite are generally present. Plagioclase is slightly altered (5%–10%) to secondary plagioclase and minor chlorite. Alteration of plagioclase is generally limited to narrow zones along grain boundaries and internal cracks. Pre- to synmetamorphically deformed contacts between dikes and gabbro consist of very fine grained felty chlorite and amphibole with minor quartz (e.g., Sample 209-1275B-3R-1 [Piece 9, 57–59 cm]). Fe-Ti oxides show breakdown to titanite and hematite (e.g., intervals 209-1275B-3R-1, 57–59 cm, 84–87 cm, and 105–107 cm). Titanite accounts for as much as 2%–3% by volume in these rocks and is detectable by X-ray diffraction (XRD) (Fig. F35C; Table T2). Titanite usually surrounds hematite grains and is intergrown with amphibole needles (Fig. F35B). In some instances, patches of quartz ± titanite intergrown with acicular amphibole are developed in pockets. These patches are particularly abundant in proximity to, and within, felsic veins.
The lower gabbroic unit (34–108.7 mbsf) is uniform in terms of alteration style and intensity. The extent of alteration varies between 10% and 40% and is dominated by replacement of clinopyroxene by amphibole. Coarse, prismatic brown amphibole can be as abundant as green amphibole in individual thin sections. In most samples, however, green amphibole is more abundant than brown amphibole. Commonly, felty green amphibole is developed along the margins of clinopyroxene veins (Fig. F36). Green amphibole also replaces plagioclase, although this replacement is usually limited to narrow, feathery areas that follow internal cracks in plagioclase (Fig. F37). Also along cracks and grain margins, plagioclase is altered to secondary plagioclase. Overall, plagioclase alteration is <4% throughout the lower gabbro unit. Fe-Ti oxides are mostly fresh, except for subsolidus exsolution/oxidation to trellislike ilmenite lamellae in magnetite (Fig. F38). In contrast to the upper gabbro/diabase sequence, titanite is rare in the lower gabbro unit and is present only in felsic veins and in the host gabbro directly adjacent to felsic veins. Felsic dikes and veins (granophyres) make up a significant proportion of the core (0.8 vol%), in particular in Cores 209-1275B-18R and 19R (Fig. F39A). They are variably altered to amphibole and secondary plagioclase. Locally, the felsic veins are rich in carbonate that appears to replace plagioclase and has needlelike inclusions of green amphibole or zeolite (Fig. F39B, F39C).
Troctolitic rocks with variable amounts of orthopyroxene and clinopyroxene are present in the interval from 209-1275B-6R-1, 56 cm, to 7R-1, 81 cm (28.2–33.1 mbsf). Olivine in the troctolite is moderately to completely altered (20%–98%) to talc, serpentine, and magnetite. Alteration of olivine starts along irregular internal cracks with the formation of magnetite and serpentine networks. In an advanced stage of alteration, these networks form a characteristic mesh texture. Near magmatic veins, which are particularly abundant in Section 209-1275B-7R-1, olivine is dominantly replaced by talc and magnetite. The textural development of this talc alteration is similar to that of serpentinization. Commonly, both serpentinization and talc alteration are developed in a single sample. In these cases, talc surrounds partially serpentinized olivine grains. Along former olivine/plagioclase grain boundaries, coronitic replacement of olivine and plagioclase imposes a characteristic texture to the core (Fig. F40A). These reaction coronas are banded and consist of talc and/or tremolite (after olivine) and chlorite (after plagioclase) (Fig. F40B, F40C). Plagioclase in troctolitic rocks is usually more altered than olivine. Replacement of plagioclase is primarily by fibrous chlorite, although minor amounts of secondary plagioclase, talc, and green amphibole are also present. Orthopyroxene oikocrysts in Section 209-1275B-6R-2 are variably altered (10%–70%) to talc and chlorite. Clinopyroxene, where present, is moderately to highly altered (30%–70%) to chlorite and minor amphibole. Low-temperature oxidative alteration of olivine to Fe oxyhydroxide, red clay, and carbonate (iddingsite) is developed locally, in particular along clay-oxide veinlets. Mafic and felsic magmatic veins in troctolite are abundant in Sections 209-1275B-6R-2 and 7R-1. They are completely altered to amphibole, chlorite, talc, and titanite. The presence of zircon suggests that these veins represent late-stage magmatic products.
Metamorphic veins in Hole 1275B make up 0.43% of the core volume (Table T3). Amphibole, amphibole-chlorite, and clay-oxide veins are the most common vein types (Fig. F41), along with rare quartz, zeolite, and carbonate veins. The abundance of veins and the vein mineralogy show no systematic correlation with depth (Fig. F42). However, picrolite and minor chrysotile veins are restricted to the troctolitic unit between 28.2 and 33.1 mbsf. These veins account for ~6% of the total vein volume (Figs. F41, F42). Amphibole and amphibole-chlorite veins are the most common vein type in Hole 1275B (52.7%). They are usually <2 mm wide and appear to be predominantly dipping at moderate to steep angles (Fig. F43). In thin section, sets of subparallel, en echelon microscopic amphibole veinlets are apparent (Fig. F44). Veins that consist exclusively of chlorite are rare but tend to be thicker than amphibole and mixed amphibole-chlorite veins. These chlorite veins are particularly prominent between 89 and 97 mbsf. Apart from this interval and rare chlorite veins in the diabase of the uppermost 30 m, chlorite in Hole 1275B is associated with green amphibole. Clay-oxide veins make up ~25% of the vein volume. They are usually <1 mm wide and form irregular networks with pronounced orange-brown alteration halos. In thin section they appear dark brown to opaque. Associated alteration halos have minor staining of plagioclase by oxides and replacement of clinopyroxene and plagioclase by brown clay.
The style of alteration in Hole 1275D is broadly similar to that in Hole 1275B. However, some particular lithologies and alteration styles are exclusive to Hole 1275D (Fig. F45). The following sections provide a brief overview of alteration features in Hole 1275D with particular emphasis on the differences in alteration style and intensity between Holes 1275B and 1275D.
Intervals with predominantly gabbroic rocks in Hole 1275D comprise oxide gabbro (53–81 mbsf; Unit II), oxide gabbronorite (91–151 mbsf; Unit IV), and a succession of olivine gabbro and oxide gabbronorite (151–209 mbsf; Unit V). The oxide gabbro is very similar in alteration style and intensity to the oxide gabbro in Hole 1275B (see above). Except for rare intervals of high alteration intensities, alteration of the oxide gabbros and gabbronorites is usually moderate. The most common alteration mineral is green amphibole replacing clinopyroxene (20%–100%) and, to a lesser extent, plagioclase (<10%). Brown amphibole is common as rims around clinopyroxene that is partly to completely replaced by green amphibole in the center. Locally, brown amphibole completely replaces clinopyroxene and is variably overprinted by fibrous green amphibole. Secondary plagioclase appears to be a minor component and generally composes <5% of the rock. Unlike clinopyroxene, alteration of plagioclase is limited to areas along cracks and grain boundaries. Titanite is a breakdown product of Fe-Ti oxides, but the abundance of secondary titanite is somewhat lower than that in Hole 1275B. However, there is abundant titanite in vein networks in the brecciated diabase in Section 209-1275D-1R-1.
A marked difference between Holes 1275B and 1275D is the common development of chlorite and chlorite/smectite after plagioclase and clinopyroxene in Hole 1275D. Locally, these phases can make up 5%–10% of the rock volume. Their presence imposes a greenish to brownish color to the rock in hand specimen (Fig. F45C). In thin section, replacement of primary minerals by chlorite/smectite is patchy and affects clinopyroxene more intensely than plagioclase (Fig. F46).
Olivine-bearing gabbros and gabbronorites of Unit V are moderately to highly altered. Alteration of olivine is characterized by the development of fibrous talc, acicular green amphibole, and subhedral magnetite along the grain margins (Fig. F47). Alteration of olivine starts along cracks with the formation of talc and magnetite. Plagioclase adjacent to olivine is partly replaced by chlorite and talc (Fig. F47). Pyroxene in olivine-bearing gabbros is variably altered to felty green amphibole and minor talc. Locally, oxidative alteration leads to partial alteration of olivine relics to clay, Fe oxyhydroxide, and carbonate (Fig. F45D).
Troctolitic rocks are highly to completely altered to serpentine, magnetite, clay, talc, carbonate, and Fe oxyhydroxide (after olivine) and chlorite, clay, and carbonate (after plagioclase). Troctolitic units in Hole 1275D are cut by a large number of magmatic veins (both felsic and mafic) that are variably altered to amphibole, chlorite, and talc (mafic veins) or secondary plagioclase, amphibole, and carbonate (felsic veins) (Fig. F48). Away from these magmatic veins and dikes, olivine is completely altered to serpentine and trails of magnetite forming mesh textures. Pyroxenes are variably altered to serpentine (bastite after orthopyroxene), talc, and green amphibole. Locally, troctolitic rocks are heavily talc-altered (Fig. F45A, F45B). Talc alteration is limited to millimeter-thick halos near magmatic veins and larger-scale contacts with gabbroic rocks (Fig. F48), where talc appears to postdate serpentinization. However, talc alteration at boundaries between troctolite and gabbro is by no means universal. Where talc alteration is weak, noticeable amounts of fresh plagioclase and olivine are preserved in the former grain centers. Plagioclase relics are variably affected by a late-stage low-temperature overprint, resulting in the formation of clay and carbonate (Fig. F49). Olivine, in particular, is commonly well preserved in proximity to magmatic veins, although it is variably affected by low-temperature alteration to carbonate, clay, and Fe oxyhydroxide (Fig. F50). This type of oxidative alteration is most pronounced along irregular networks of carbonate and carbonate-clay veinlets that cut serpentinized olivine grains and follow former plagioclase-olivine grain boundaries (Figs. F49, F50). XRD analyses of these domains indicate that the carbonate phase is calcite (Fig. F50; Table T2). Calcite can make up 3–10 vol% of individual pieces of core in the troctolites of Unit I (0–53 mbsf).
Alteration intensity in Hole 1275D varies considerably within given intervals (Fig. F51). This is in part due to differences in the primary lithology. For instance, diabase is usually less altered than gabbros, fine-grained gabbros and micrograbbros are less altered than coarse-grained gabbros, and troctolitic rocks are the most altered. In general, the intensity of troctolite and gabbro alteration decreases with depth (Fig. F51).
Metamorphic veins account for 0.65 vol% of the core in Hole 1275D. The most abundant vein types are amphibole, amphibole-chlorite, carbonate, and picrolite (Table T3; Fig. F52). Minor minerals in metamorphic veins are talc, iron oxides, quartz, and clay. The distribution of vein types is partially controlled by the host lithology. Picrolite and talc veins are restricted to troctolite in Units I and III, whereas amphibole ± chlorite veins are usually present in gabbro. The downhole distribution of metamorphic vein minerals (Fig. F53) indicates that carbonate, chlorite, and serpentine veins are particularly abundant in the upper 50 m of the hole. Below 50 mbsf carbonate veins are only locally developed. Amphibole ± chlorite veins are common in gabbro below 50 mbsf but relatively scarce above this level and absent in the troctolite of Units I and III. Compared to Hole 1275B, veining is more intense in Hole 1275D. In particular, carbonate veins are substantially more common in Hole 1275D. Overall, metamorphic veining at Site 1275 is weak. Amphibole ± chlorite veins are 0.2–1 mm thick and make up 0.1–0.2 vol% of the core. The average amphibole vein density at Site 1275 is 1–10 veins/m of core, similar to the amphibole vein densities at Sites 921 and 923 in the Kane Fracture Zone (MARK) area (Dilek et al., 1997).
Gabbros from Site 1275 show little evidence for crystal-plastic deformation and granulite facies metamorphism. Plagioclase-amphibole veins that splay off felsic dikes are probably transitional magmatic–hydrothermal veins that initially form during late-stage magmatic activity. However, they include hydrothermal amphibole, secondary plagioclase, chlorite, and carbonate, indicating that hydrothermal reactions continued over a range of temperature conditions from amphibolite to zeolite facies.
High-temperature, static background alteration by hydrothermal fluids is pervasive throughout the core. This alteration is generally manifest in coronitic replacement of clinopyroxene by brown amphibole. In gabbronorites, magnesium-amphibole ± talc typically replaces orthopyroxene. The intensity of this amphibolite facies (500°–700°C) alteration is usually low (<5%–10%) and does not appear to correlate with the grain size of the rock. Pervasive background alteration continued at lower temperatures facilitated by the ingress of seawater-derived hydrothermal fluids along fractures (now green pleochroic amphibole veins) that are probably related to cooling and cracking of the rocks in the axial environment at temperatures of ~400°–500°C (e.g., Kelley, 1997). At a smaller scale, microcracks (<100 µm), representing smaller-scale fracturing and fluid penetration, are abundant throughout the core in Holes 1275B and 1275D. These microcracks are filled with serpentine, talc, and magnetite (in troctolites) and secondary plagioclase and amphibole ± chlorite in gabbroic rocks. Troctolitic rocks reacted more rapidly with the hydrothermal fluids than gabbros, as suggested by the intense coronitic replacement of olivine and plagioclase along former grain boundaries. Fracturing and fluid percolation continued under lower greenschist facies and zeolite facies conditions as indicated by the development of chlorite/smectite ± fibrous actinolite, clay-oxide, and carbonate veins and by replacement of olivine and plagioclase by clay and carbonate, particularly in Hole 1275D.
The following section provides a brief comparison of the alteration of plutonic rocks from Site 1275 with those from other locations, namely Hess Deep, MARK, and Atlantis Bank. It is commonly suggested that at slow-spreading ridges alteration of lower ocean crust is facilitated by fluid penetrating the lower crust along shear zones (Ito and Clayton, 1983; Mével and Cannat, 1991; Stakes et al., 1991), whereas static cracking provides pathways for fluids at fast-spreading ridges (Manning et al., 1996; Nehlig, 1994). In Hole 735B (Atlantis Bank, Southwest Indian Ridge) deformation intensity is strongly correlated with the extent of high-temperature alteration and amphibole veining (Dick et al., 2000; Cannat et al., 1991). In the uppermost 200 m of Hole 735B, alteration intensities are generally high to complete and amphibole veins make up ~2% of the core volume (Dick et al., 2000). Gabbros recovered from Sites 921–924 (MARK area, Mid-Atlantic Ridge 23°N) also show a systematic relationship between the intensity of crystal-plastic deformation and alteration intensity (Dilek et al., 1997; Kelley, 1997). At Sites 921–924, static alteration is also abundant (average = ~50%) (Fletcher et al., 1997) away from zones of crystal-plastic deformation, unlike in Hole 735B, where alteration intensity is usually very low in undeformed zones (Dick et al., 2000). Average amphibole vein density at Sites 921 and 923 is 1–10 veins/m of core (Dilek et al., 1997), significantly less than that in the highly deformed uppermost 200 m of Hole 735B and similar to that in the deeper, less deformed section of that hole (Dick et al., 2000). In Hess Deep gabbros, alteration is exclusively static and controlled by a downward-propagating cracking front that developed as the lower crust cooled to 700°–800°C (Gillis, 1995; Manning et al., 1996).
These examples clearly demonstrate that deformation and hydrothermal alteration are strongly related. In areas of pronounced crystal-plastic deformation, alteration intensities and amphibole vein densities tend to be higher than in undeformed zones. Brittle and semibrittle deformation also results in increased alteration intensities, although alteration is controlled by brittle veins rather than by cooling fractures that tend to form at higher temperatures (Magde et al., 1995). The relationship between crystal-plastic deformation and alteration is best illustrated in rocks from Hole 735B that are primarily altered under granulite to amphibolite facies conditions (550°–800°C) with alteration intensities correlated with the extent of crystal-plastic deformation. The amount of amphibolite and greenschist facies alteration in undeformed rocks is <5%–10%, except for more intense alteration related to a few brittle faults (Dick et al., 2000). In the gabbros from the MARK area, a much larger proportion of the total alteration can be related to brittle features, although alteration intensities and amphibole vein densities increase markedly in rare crystal-plastic shear zones (Dilek et al., 1997; Kelley, 1997). Consistent with the notion that alteration started in the ductile region in Hole 735B and mainly in the semibrittle region at MARK, amphibole-plagioclase geothermometry indicates that temperatures of the earliest metamorphic assemblages at MARK were 560°–680°C, whereas amphibolitization in Hole 735B took place at 770°–820°C (Gillis and Meyer, 2001). Crystal-plastic deformation at Site 1275 is very rare, even compared to the MARK area, and hence fluid ingress was largely facilitated by brittle deformation and cracking. The presence of secondary brown amphibole indicates that fluid ingress occurred at temperatures >500°C. Brittle fractures in the Hess Deep gabbros form at temperatures of ~770°C (Manning et al., 1996), which may also be the upper temperature limit (at pressures <4 kbar) for the amphibole veins at Site 1275. The brittle fracturing and high-temperature alteration at Site 1275 might be a result of cooling in an axial or near-axial environment or it could be related to unroofing in the footwall of a normal fault. The variability in metamorphic mineral assemblages, ranging from brown amphibole to green, fibrous amphibole and chlorite/smectite to carbonate and clay, suggests that fluid circulation and water-rock interactions continued throughout a period of progressive cooling and uplift. The abundance of low-temperature alteration is similar to that in a zone between 500 and 600 mbsf in Hole 735B (Bach et al., 2001) and provides further evidence for significant low-temperature exchange between seawater and tectonically exposed gabbroic crust.