Four holes were drilled at Site 1270, reaching between 17.5 and 57.3 mbsf. The recovery ranged from 10.6% (Hole 1270C) to 37.4% (Hole 1270B). Holes 1270A and 1270B are located at a water depth of 1951 and 1910 m, respectively. Drill core from Hole 1270A is mainly composed of completely serpentinized harzburgite and dunite, whereas slightly to moderately altered gabbro was recovered from Hole 1270B. Holes 1270C (1822 meters below sea level [mbsl]) and 1270D (1817 mbsl) are <20 m apart and were drilled at a location ~250 m east of Holes 1270A and 1270B. Drill core from Hole 1270C is dominated by serpentinized harzburgite and dunite. Serpentinized harzburgite with rodingitized gabbro veins or dikes was recovered from Hole 1270D.
Hole 1270A reached a maximum depth of 26.9 mbsf (average recovery = 17.6%). The predominant rock type is serpentinized harzburgite with minor serpentinized dunites, completely altered gabbros, and rare breccia. The entire succession is completely altered.
Harzburgites and dunites in Hole 1270A are dark green and are completely altered to serpentine and magnetite (Fig. F27A). This macroscopic observation was confirmed by X-ray diffraction (XRD) analyses (Fig. F27B; Table T2). In some intervals, the core is reddish black, suggesting the presence of relict primary minerals (<2%). Serpentinite microtextures in two thin sections (Samples 209-1270A-1R-1, 35–38 cm, and 1R-1, 96–99 cm) are transitional with serrate chrysotile veins. Orthopyroxene is altered to bastite with minor talc and tremolite rims. Sample 209-1270A-1R-1, 35–38 cm, contains a serpentine mylonite with interpenetrating textures of presumed antigorite (Fig. F28A, F28B). Late chrysotile veins are deformed into the mylonite, indicating that deformation took place during or after late-stage serpentinization (Fig. F28C). The abundance of pyrite in the serpentine mylonite is unusually high. Pyrite and magnetite appear to replace spinel (Fig. F28D). Furthermore, there is minor weathering and formation of trace amounts of clay, Fe oxyhydroxide, and carbonate after olivine in Hole 1270A.
Two ultramafic dikes or veins are present in Section 209-1270A-4R-1 (intervals 4R-1, 43–46 cm, and 4R-1, 48–56 cm). Interval 209-1270A-4R-1 (Piece 12, 43–36 cm) is an amphibolized clinopyroxenite, whereas interval 4R-1, 48–56 cm, contains two pieces of harzburgite with a pyroxenite dike or vein. A thin section of Sample 209-1270A-4R-1 (Piece 16, 54–56 cm) reveals that these pyroxenite veins are ~50% altered to serpentine, chlorite, talc, amphibole, and oxides. Olivine is partly replaced by serpentine and oxides, whereas the pyroxenes are partially altered to serpentine, talc, chlorite, and amphibole.
Small pieces of gabbro were recovered in intervals 209-1270A-2R-1, 81–85 cm; 2R-1, 113–117 cm; and 4R-1, 0–5 cm. The pyroxenes in these gabbros are altered to talc and serpentine, whereas the plagioclase is largely fresh.
A fault gouge in Section 209-1270A-3R-1 appears to consist entirely of serpentine mud. Another small piece of a fault breccia (interval 209-1270A-2R-1, 101–104 cm) is composed of angular clasts of serpentinized harzburgite embedded in a cement of green clay and abundant carbonate.
Metamorphic veins in Hole 1270A account for 1.2 vol% of the recovered core and consist predominantly of serpentine (Fig. F29; Table T3). Two generations of chrysotile veins can be distinguished. The first generation is paragranular and has coarse magnetite grains; the younger generation is white, magnetite-free, and transgranular. Transgranular chrysotile veins are bent into the foliation planes of mylonite zones in Section 209-1270A-1R-1. Talc-serpentine veins are crosscut by the younger generation of chrysotile veins; a crosscutting relationship to the earlier chrysotile-magnetite veins could not be established. Talc veins in Section 209-1270A-4R-1 are present in proximity to ultramafic dikes and are oriented subparallel to these dikes.
Hole 1270B had a core recovery of 37.4% and reached a maximum depth of 45.9 mbsf. The drill core consists of slightly to moderately altered oxide-rich gabbro and microgabbro. There is no systematic variation in alteration intensity with depth (Fig. F30). Three pieces of completely talc-altered harzburgite were recovered in Cores 209-1270B-6R and 7R. These show the highest intensity of alteration among rocks from Hole 1270B.
Alteration is dominated by chlorite–green amphibole assemblages at the contacts of pyroxene and plagioclase crystals (Fig. F31). Brown amphibole of hydrothermal or magmatic origin is commonly associated with Fe-Ti oxides and replaces pyroxene at the margins and along internal cracks (Fig. F32). Brown amphibole is also present in veins/dikelets of chlorite and amphibole that locally crosscut the gabbro (Fig. F33). Two types of chlorite may be distinguished in some thin sections from Hole 1270B. Chlorite with gray to brown interference color is commonly formed at the expense of pyroxene crystals, whereas chlorite with anomalous blue interference color replaces plagioclase. Some of the plagioclase was altered to chlorite and fine-grained quartz aggregates, similar to the alteration of gabbro in Hole 1268A (cf. Fig. F34 in the "Site 1268" chapter). Alteration of igneous calcic plagioclase to secondary sodic plagioclase took place along grain boundaries and internal cracks. Further, plagioclase experienced grain size reduction due to deformation. A relationship between deformation and intensity of hydrothermal alteration could not be established.
Sections of moderate alteration intensity (Fig. F30) show partial replacement of pyroxene by talc-chlorite-amphibole assemblages in hand specimen. Talc replacement is particularly well developed in a deformed zone of exceptional olivine-bearing gabbro in interval 209-1270B-8R-1, 23–31 cm (Fig. F34A, F34B). However, talc replacement was not complete, and thin bands (<1 mm in width) of fresh olivine were observed in a thin section of this interval (Fig. F34C). Furthermore, three pieces of completely talc-altered harzburgite were recovered (intervals 209-1270B-6R-1, 51–57 cm; 7R-1, 4–11 cm; and 7R-1, 19–24 cm) that are similar in appearance to specimens recovered from Hole 1268A (see Fig. F21 in the "Site 1268" chapter). A microgabbro in interval 209-1270B-7R-1, 114–118 cm, contains a nodular, talc-rich patch surrounded by a 1-cm-wide light green to brown halo, which may represent a halo to an exceptionally thick talc vein (Fig. F35).
In addition to the oxide minerals, which are locally abundant in the gabbro (cf. "Igneous and Mantle Petrology"), there are noticeable amounts (<1%) of disseminated sulfides, especially in intervals 209-1270B-7R-2, 36–146 cm; 7R-3, 0–46 cm; 8R-1, 83–136 cm; and 10R-1, 80–106 cm. These sulfides are mainly pyrite with minor chalcopyrite and may represent recrystallized igneous phases. In some instances, primary Fe-Ti oxides underwent partial subsolidus exsolution/oxidation and subsequent hydrothermal alteration to aggregates of trellislike ilmenite lamellae with secondary oxides and rare titanite.
Veining is weakly developed in Hole 1270B, which is reflected in the comparatively low calculated value for the volume percent of veins in the core (0.5%) (Table T3). The mineralogical composition of the veins in Hole 1270B differs significantly from that in Hole 1270A, which is dominated by serpentine (Fig. F29). In Hole 1270B, talc and chlorite are the principal phases followed by serpentine and amphibole (Fig. F36). However, sulfide-bearing veins are locally prominent.
In general, thin (<1 mm in width), irregular chlorite-amphibole veins can be distinguished from talc-chlorite veins. However, a relative age relationship could not be established. Both types of veins crosscut the foliation of the gabbro if present (e.g., interval 209-1270B-2R-1, 24–27 cm). Exceptional talc-chalcopyrite veins (~5 mm wide) were observed in interval 209-1270B-7R-1, 49–66 cm (Fig. F37). In sections of the core below this interval, noticeable amounts of disseminated sulfides were observed in the gabbro (see "Hydrothermal Alteration" above).
Hole 1270C was drilled to a final depth of 18.6 mbsf (recovery = 10.6%). The predominant rock types are serpentinized harzburgite and dunite. Intermittently, there are intervals containing irregular, completely altered intrusions of gabbro.
The harzburgites and dunites have mostly been completely serpentinized and display ribbon texture background alteration. XRD (Table T2) and magnetic susceptibility measurement (see "Paleomagnetism") show that completely serpentinized ultramafic rocks consist of lizardite ± chrysotile and magnetite. Orthopyroxene is commonly pseudomorphed by bastite. However, minor green amphibole rims are locally present (e.g., intervals 209-1270C-1R-1, 36–39 cm, and 3M-1, 44–48 cm).
Relict fresh olivine and orthopyroxene are preserved in a few places (Fig. F38). Here, orthopyroxene is partially replaced by talc along the margins and along internal fractures. Subsequent serpentinization replaced most of the olivine. However, some fresh olivine is preserved adjacent to the orthopyroxene. During low-temperature alteration (seafloor weathering), clay and Fe oxyhydroxides formed at the expense of relict olivine and orthopyroxene (Fig. F38). These assemblages are easily recognized at the hand specimen scale because of their rusty orange-brown color (Fig. F38A). Spotty to pervasive weathering is present in all three cores from Hole 1270C.
Gabbroic intrusions were observed intermittently throughout Hole 1270C (Fig. F39A). They are typically deformed and metamorphosed to talc-serpentine ± chlorite ± brown amphibole and host porphyroclasts of serpentinized harzburgite (Fig. F39B, F39C). Brown amphibole partially replaces and rims orthopyroxene within the deformed zones (Fig. F39E). However, amphibole is absent in the "boudins" of fresher peridotites located above and below the deformed bands, suggesting that brown amphibole pseudomorphs were formed from high-temperature fluids infiltrating along the deformed band.
Metamorphic veins are more abundant in Hole 1270C than in Holes 1270A and 1270B and account for 2 vol% of the core. As in Hole 1270A, serpentine is the most common mineral in veins (86%) (Fig. F40; Table T3). However, the vein mineralogy in Hole 1270C is distinctive because talc is only a minor component and iron oxides are common (12.5%). Furthermore, there are rare carbonate veins.
The earliest generation of veins are sigmoidal, paragranular, cross-fiber chrysotile veins, common in Core 209-1270C-2R. These are crosscut by transgranular serpentine veins that show variable orientations with respect to the foliation, varying from perpendicular to subparallel. These serpentine veins show complex interrelationships and represent several generations. Networks of iron oxide veinlets postdate the serpentine and are particularly common in Core 209-1270C-1R. A single carbonate vein in Sample 209-1270C-1R-1, 83–85 cm, cuts the massive serpentine and cross-fiber chrysotile veins, representing a late stage of vein alteration, possibly associated with seafloor weathering (Fig. F41).
Hole 1270D reached a depth of 57.3 mbsf (average recovery = 13.4%). The core consists mainly of a complex association of deformed serpentinized harzburgite that hosts irregular, schlieren- to network-like, hydrothermally altered and partly rodingitized gabbro. Furthermore, there are minor serpentinized dunites throughout the drill core. The alteration is mainly complete; however, there are minor occurrences of less strongly altered harzburgite (Fig. F42).
Harzburgite and dunite in Hole 1270D were affected by three different styles of alteration: serpentinization, alteration to talc (in proximity to gabbro veins), and local weathering to orange clay and Fe oxyhydroxide. The morphology of the serpentine is dominated by ribbon texture with poorly developed core and rim structures remaining (e.g., Sample 209-1270D-3R-1, 63–65 cm). Occasional pockets of mesh texture alteration are preserved (e.g., Sample 209-1270D-5R-1, 23–28 cm) where serpentinization of olivine has not been complete. Bastite pseudomorphs after orthopyroxene are locally present but less common than at Site 1268 (e.g., Sample 209-1270D-10R-1, 4–6 cm). Fine-grained magnetite forms networks within the serpentinized matrix and was also detected by XRD analyses (Table T3). The highest visual estimates for magnetite are ~3 vol% in interval 209-1270D-4R-1, 19–22 cm. Oxide veining, which is common in Cores 209-1270D-1R, 2R, 3R, and 4R, locally grades into oxide-rich background alteration (e.g., interval 4R-1, 38–51 cm). Weathering of harzburgite to clay and Fe oxyhydroxides occurs throughout the length of the core and is particularly strongly developed in Core 209-1270D-1R (Fig. F43). Talc ± tremolite alteration locally overprints serpentinization but is largely restricted to gabbro zones and deformed zones.
The harzburgites and dunites are intruded by numerous gabbroic veins or dikes that were mylonitized, imparting a gray, foliated appearance to a large proportion of the core recovered from Hole 1270D. Locally, completely serpentinized harzburgite is present as boudins within the deformed gabbroic intrusions (Fig. F44).
The gabbro intrusions are altered to albite, tremolite, talc, chlorite ± smectite, and, locally, to calcium silicates (e.g., hydrogrossular and prehnite in thin section of Sample 209-1270D-10R-1, 11–17 cm). Formation of calcium silicate mineral assemblages in gabbro is evidence of calcium metasomatism. Fluids enriched in calcium are generated during serpentinization of ultramafic rocks, and calcium metasomatism of adjacent lithologies is a common phenomenon referred to as rodingitization. This indicates synchronous alteration of the ultramafic rocks and the enclosed gabbro intrusions.
In addition to these mineral assemblages, some of the gabbros show replacement of pyroxene by brown amphibole similar to Sample 209-1270C-1R-1, 84–87 cm, illustrated in Figure F39. Furthermore, the gabbro of Sample 209-1270D-10M-1, 142–144 cm, contains minor biotite and an amphibole replacing orthopyroxene in Sample 4R-1, 33–37 cm, contains minor biotite and rare epidote inclusions.
The abundance of metamorphic veins in Hole 1270D is similar to that in Hole 1270C. Veins in Hole 1270D account for 1.9 vol% of the core, and serpentine is the most common mineral in veins (83.5%) (Fig. F45; Table T3). Hematite and other iron oxides (including Fe oxyhydroxides, which are common in the weathered zones) account for 9.7 vol% of the vein minerals, followed by talc (5.4 vol%). Minor minerals include carbonate, amphibole, magnetite, and sulfides.
In general, vein alteration in Hole 1270D is dominated by serpentine and Fe oxyhydroxide. Fe oxyhydroxides can account for as much as 50% of some serpentine veins, but the modal percentage is highly variable. These variations account for the variable colors of serpentine veins in Core 209-1270D-1R.
Serpentine veins often show sigmoidal shapes forming semicontinuous, en echelon arrays. Locally, they cut across the deformed gabbroic intrusions (e.g., Section 209-1270D-3R-1) (Fig. F46). In some of the cores, sigmoidal chrysotile veins are oriented subparallel to the foliation and may represent the earliest stage of serpentine veining (e.g., Section 209-1270D-5R-1).
Overall, the serpentine veins in Hole 1270D show complex interrelationships, and three different generations can be distinguished locally based on crosscutting relationships (Fig. F47). However, it is often difficult to separate the different generations of serpentine veins in hand specimen because of variations in their textural relationships. For example, veins that appear as discrete generations in the upper part of Core 209-1270D-4R become indistinguishable at ~130 cm. Furthermore, there are textural variations within single veins. In Section 209-1270D-3R-2, occasional pockets of chrysotile fibers are present in the tips of otherwise massive serpentine veins. In Section 209-1270D-4R-1, these veins are accompanied by as much as 10% magnetite. Elsewhere in Section 4R-1, fine networks of chrysotile veins extend from the tips of sigmoidal serpentine veins. This differs from higher in the core, where these vein types are two separate generations.
Networks of submillimeter hematite veins commonly crosscut serpentine veins throughout Hole 1270D and are particularly abundant in the upper cores. Iron oxide veins may also follow the margins of earlier chrysotile veins or propagate along their interior. This feature is particularly prominent in interval 209-1270D-3R-2, 33–114 cm. Hematite veins crosscut all other features and are therefore one of the latest veining events. Locally, the fine hematite veining is intense. In its extreme form, the individual veins are no longer discernable and the veining grades into the background alteration (e.g., interval 209-1270D-4R-1, 40–50 cm).
Other features of vein alteration in Hole 1270D are minor replacement of serpentine veins by talc, rare carbonate veining, and rare sulfide veining. An aragonite vein, as wide as 3 mm, is present in interval 209-1270D-6R-1, 41–50 cm (Fig. F48). Carbonate veins crosscut the surrounding serpentine veins and may be related to the late-stage weathering of the serpentinized harzburgite. A massive sulfide vein in interval 209-1270D-5R-1, 5–9 cm, consists of pyrite, chalcopyrite, and hematite (Fig. F49). It has a banded structure with a hematite core and sulfide along the margins.
Veining within the gabbro is infrequent. Minor chrysotile veins with virtually no Fe oxyhydroxides are present. A few serpentine veins occur within the gabbroic shear zones.
Drill core recovered from Hole 1270A represents a typical example of completely serpentinized harzburgite altered under static conditions. Within the deformed zones, the mesh-textured serpentine is recrystallized to serpentine with an interpenetrating texture (antigorite?) and late chrysotile veins have been transposed into the foliation. These observations indicate that deformation occurred during late-stage serpentinization or after serpentinization was largely complete.
The alteration of the gabbro in Hole 1270B is fairly monotonous over the entire 45.9 m drilled. The chlorite-amphibole alteration on grain boundaries of pyroxene and plagioclase crystals is characteristic of greenschist facies conditions. There are fairly minor variations in the intensity of alteration and no apparent correlation with the degree of rock deformation. This is consistent with textural observations that suggest alteration was mainly static. However, in rare highly deformed shear zones there are bands with substantial talc replacement of pyroxene.
Hydrothermal veining is comparatively weak in Hole 1270B and is dominated by chlorite-amphibole and talc-chlorite veins. This mineralogy differs substantially from the serpentine veins observed in Hole 1270A. This difference is clearly related to the host rock composition, indicating that the fluids forming the veins in Hole 1270B have not interacted with ultramafic material or were saturated with serpentine and quickly evolved to chlorite-amphibole saturation by interaction with the gabbro.
Holes 1270C and 1270D were drilled within 30 m of each other and show many similarities, suggesting that they underwent the same deformation-alteration paths (see "Structural Geology"). There is an intimate association of serpentinized harzburgite and dunite with deformed, schlieren-shaped intrusions of completely altered gabbro. Within these deformed zones there is evidence for highly localized, synkinematic high-temperature alteration. The vein assemblages in Holes 1270C and 1270D show complex relationships, indicating that multiple episodes of veining took place after the deformation.
In the following sections, we will discuss the possible origin of key alteration features and their implications in terms of the relationship between alteration and deformation. A model for the relative timing of alteration-deformation events is proposed based on overprinting relationships of serpentinization, serpentine veins, and spatial correlation of alteration types with shear zones.
In Hole 1270B static alteration of gabbro under greenschist facies postdates the deformation. In contrast, gabbroic intrusions in Holes 1270A, 1270C, and 1270D are replaced by amphibole-chlorite-talc assemblages and are generally deformed. Locally, these assemblages are dominated by talc, especially in boudins located in shear bands. This style of alteration may be related to deformation under greenschist facies conditions. In addition, static alteration of gabbro within and outside of the shear zones occurs in Holes 1270C and 1270D, indicating that gabbro alteration was not restricted to zones of localized strain.
In the shallow part of Holes 1270C and 1270D (<20 mbsf), some of the gabbros are altered to tremolite-talc schists. These schists display penetrative schistosity defined by oriented amphibole, talc, and chlorite crystals (e.g., intervals 209-1270C-1R-1, 83–85 cm, and 1R-1, 16–18 cm). Similar talc-amphibole-chlorite schists were described in fault rocks from the Mid-Atlantic Ridge at 15°45´N and were interpreted as alteration of a peridotite protolith (Escartin et al., 2003). However, it is apparent that the schists in Holes 1270C and 1270D developed as a result of alteration of gabbro boudins in shear zones hosted by serpentinized ultramafic rocks in Holes 1270C and 1270D. This relationship indicates that deformation continued under greenschist facies conditions.
It is uncertain why the gabbroic rocks in Holes 1270B, 1270A, 1270C, and 1270D have been affected by different styles of alteration. Potential factors are the timing of gabbro emplacement (pre-, syn-, or postkinematic), the size of the gabbro intrusions, and the location of gabbro emplacement with regard to the tectonically active zones.
Overprinting serpentinization textures are locally observed in completely serpentinized peridotites from Holes 1270A, 1270C, and 1270D. Replacement of pseudomorphic texture is locally present in Holes 1270C and 1270D where a relict mesh texture is crosscut by trains of magnetite grains to form transitional ribbon textures. Mesh textures are mainly preserved in pressure shadows of serpentine pseudomorphs after orthopyroxene. The transitional serpentinite textures may be interpreted as evidence for syntectonic serpentinization associated with the deformation of mesh textures (Wicks, 1984).
Sample 209-1270A-1R-1, 35–37 cm, contains firm evidence for synkinematic to postkinematic serpentine recrystallization, where the serpentinite mesh texture is overprinted by shear zones that recrystallized to interpenetrating serpentine (antigorite[?]) (Fig. F28). In this sample, chrysotile veins are transposed into the foliation, indicating that deformation occurred during late-stage serpentinization (see "Structural Geology").
Pre- to synkinematic serpentine alteration in Holes 1270C and 1270D is reflected by the prominent development of paragranular serpentine veins relative to transgranular serpentine veins (Fig. F50). Furthermore, some en echelon cross-fiber serpentine veins are transposed and rotated in talc shear zones. However, late generations of serpentine veins crosscut talc-rich shear zones and earlier generations of serpentine veins (Fig. F47). This observation indicates that serpentinization took place in several stages and that deformation continued after the initial high-temperature ductile deformation. Hence, in different sections of cores from Site 1270, there is evidence for serpentinization prior to, during, and after the deformation recorded by amphibole-chlorite-talc schists.
One of the prominent characteristics of shear zones in Holes 1270C and 1270D is the common presence of brown amphibole in composite microshear zones containing boudins of harzburgite and gabbro (Fig. F39). Minerals in harzburgitic and gabbroic boudins display evidence of high-temperature ductile deformation (see "Structural Geology"). Talc is common in these shear zones and postdates the development of brown amphibole. Although Holes 1270C and 1270D also contain brown amphibole as millimeter-wide layers of pyroxenites (cf. "Igneous and Mantle Petrology"), brown amphibole occurrences in shear zones are clearly distinct in terms of mineral assemblages and textures. Brown amphiboles in shear zones form discrete amphibole-rich bands oriented subparallel to the high-temperature fabric defined by porphyroclasts of orthopyroxene and plagioclase (Fig. F39).
Several textural observations suggest that the formation of brown amphibole was related to synkinematic infiltration of high-temperature fluids/melts into the shear zones. Fluid inclusions in amphibole range from three-phase inclusions to vapor-dominated inclusions, suggesting that the fluids underwent supercritical phase separation. Amphibole crystals in the shear bands are euhedral with a well-developed cleavage and lack any indication of stress-induced syntectonic deformation or recrystallization, in contrast to the minerals in harzburgitic and gabbroic boudins. In addition, amphibole replaces and overgrows plastically deformed minerals of the harzburgitic and gabbroic boudins. The style of amphibole formation appears to be controlled by the nature of the enclosing host rock. Where peridotite wallrock is present, the brown amphibole replaces orthopyroxene porphyroclasts along the margins of peridotitic boudins (Fig. F39E). Where the amphibole-rich zones are in contact with gabbroic wallrock, the brown amphibole preferentially replaces clinopyroxene while plagioclase is replaced by dark green amphibole. This observation suggests that amphibole was not in equilibrium with all the minerals in the shear zone. Furthermore, brown amphibole is exclusive to the margins and internal fractures of peridotitic or gabbroic boudins but absent in their interiors.
Synkinematic brown amphibole veins with textural characteristics similar to those of Holes 1270C and 1270D have been reported from lower crustal rocks formed at slow-spreading mid-ocean ridges (e.g., the Kane Fracture Zone [MARK] area [Leg 153; Dilek et al., 1997b] and Atlantis Bank, Southwest Indian Ridge [Hole 735B; Stakes et al., 1991]). These brown amphiboles were interpreted to be the product of crystallization from high-temperature fluids/melts circulating along shear zones. In Holes 1270C and 1270D the brown amphibole veins occur in shear bands composed of deformed gabbros and harzburgites, but there is no direct evidence to invoke a spatial association with a crystallizing pluton. However, the local presence of zircon and apatite crystals (cf. "Igneous and Mantle Petrology") in the shear bands suggests that the brown amphibole may have been precipitated from high-temperature (800°–1200°C) fluids/melts (e.g., Ayers and Watson, 1991). The observations discussed above may be explained by a process in which fluids or melts penetrated shear zones rooted in a differentiating magmatic plumbing system.
One peculiarity of Holes 1270C and 1270D is the restriction of talc alteration to microshear zones and talc veinlets along shear zones. In these shear zones, talc clearly overprints minerals formed during higher-temperature fluid circulation, such as brown amphibole. Despite being restricted to shear zones, talc-rich zones display no compelling textural evidence of syntectonic talc crystallization. The fine grain size of talc shear bands makes it difficult to prove that talc crystallization was syntectonic. However, the fact that talc alteration is mainly restricted to associations with shear zones and the apparently low crystallinity of talc veins associated with the shear zones may indicate that the formation of talc was synkinematic.
In addition to the main types of alteration described above, Holes 1270C and 1270D contain minor carbonate and oxide veins. Networks of Fe oxyhydroxide veins are generally restricted to the shallower cores of these holes and probably formed during seafloor weathering. The predominance of hematite and general lack of sulfides indicates oxidizing conditions and low aqueous H2S activities of the fluids. Carbonate veins with red clays also formed during shallow seawater infiltration. The carbonate commonly displays a prismatic habit and is probably aragonite that is formed by interaction with cold seawater (e.g., Bonatti et al., 1980)
However, carbonate is locally present in the deeper part of Hole 1270D, where carbonate veins are spatially related to talc shear zones and cut serpentine textures. These veins do not cut across the talc-rich shear zones but appear to grade into them. Moreover, carbonate replaces orthopyroxene in Sample 209-1270D-6R-1, 29–32 cm. Potentially, this relationship may indicate that the formation of carbonate is related to the circulation of carbonate-rich fluids along the shear zones.
Figure F51 illustrates the inferred temporal relationships between background alteration, alteration veins, and deformation based on our textural observations at Site 1270. Peridotite and gabbro intrusions record ductile deformation in the granulite facies prior to deformation. Shear zones display evidence of synkinematic formation of brown amphibole by interaction with supercritical fluids in the amphibolite facies. Finally, talc and tremolite replace the previous mineral assemblages along shear zones during a greenschist facies overprint. This sequence of textural and mineralogical features suggests that shear zones were active over a large temperature range from granulite to greenschist facies conditions. It is likely that anastomosing talc-bearing shear zones are the result of increasing shear localization in previously wider shear zones. Away from these shear zones, serpentinization of peridotites and alteration of gabbro took place statically under greenschist facies conditions.