The 147.6-m-long section drilled at Site 1268 is composed of completely altered harzburgite and dunite intersected by mylonitic shear zones, late-stage magmatic dikes, and gabbronorite bodies that are moderately to completely altered. The high core recovery rate in Hole 1268A (53.3%) provides a unique opportunity to link the metamorphic evolution of the shallow mantle to tectonic and magmatic events. The most important first-order observation is that two alteration events have affected the rocks: (1) pervasive serpentinization and (2) pervasive talc alteration overprinting serpentinization in some sections. Both serpentinization and talc alteration were static and apparently unrelated to the intensity of crystal-plastic deformation. Cataclastic deformation and veining correlate with the intensity of gabbro alteration.
The basement at Site 1268 is heavily veined, with a remarkably high abundance of sulfide and sulfide oxide–bearing talc and serpentine veins. Sulfides and oxides are also abundant (up to a few percent of individual cores) in the halos along these metamorphic veins as well as along gabbroic dikes and veins. Different generations of serpentine veins are developed throughout Hole 1268A. These are early microscopic chrysotile + magnetite veins and later wispy chrysotile veins that were overprinted by talc alteration and are, hence, only preserved in the green serpentinites. The main stage of sulfide veining postdates the talc alteration. Late, usually subvertical serpentine veins are developed in all lithologies; although they may be transformed into talc veins, they intersect talc-altered rocks. The relative timing of crosscutting talc and serpentine veins is uncertain, but they both appear late in the sequence of veining.
During serpentinization, harzburgite and dunite are completely altered (secondary minerals = >95%) to dark green to green serpentinite consisting of serpentine ± magnetite ± pyrite (completely replacing both olivine and orthopyroxene) (Fig. F18; Table T2). Cr spinel is fresh in most instances, although minor alteration to magnetite and rare ferritchromite is present locally. The microtextures of serpentinized harzburgite and dunite range from pseudomorphic mesh and hourglass textures (Fig. F19) to transitional ribbon textures to nonpseudomorphic interlocking textures with serrate chrysotile veins (Fig. F20).
Talc alteration is common in the recovered drill core. Talc-altered dunite is massive and gray; macroscopic primary textures and serpentinite textures are variably preserved in pervasively talc-altered rock. Dissolution of orthopyroxene in talc-altered harzburgite imposed a vuggy appearance to the rock, with vuggy orthopyroxene pseudomorphs lined with talc (Fig. F21). Talc-altered harzburgites reveal variable replacement of serpentine and bastite by talc (Fig. F22). In talc-altered rock with complete bastite replacement, dark green millimeter-wide rims of relict serpentine of unusually intense green color and high birefringence outline the original shape of the orthopyroxene (Fig. F23). Macroscopic and thin section observations show that serpentine is replaced by talc in the talc-altered rocks. Incipient replacement of serpentine by talc is initiated along serpentine bands defining ribbon textures and mesh rims (Fig. F24). Even in highly talc-altered rock the original serpentinite microtexture is commonly preserved (Fig. F25). In some completely talc-altered rocks recrystallization resulted in coarse-grained nonpseudomorphic talc patches. Textural evidence shows that talc alteration postdates serpentinization, early chrysotile + magnetite veining, pyrite + serpentine veining, and later generations of chrysotile veins (Figs. F20, F23, F26). Hence, talc alteration is overprinting advanced serpentinization. Talc-altered rocks generally have lower oxide contents than the serpentinite precursors. Sulfides are also generally less abundant in talc-altered rock. However, talc-altered rock is locally enriched in sulfide, particularly along talc-oxide-sulfide veins.
In rare incompletely altered harzburgites and orthopyroxenites, olivine is replaced by serpentine and magnetite along mesh rims and hourglass cells and orthopyroxene is partly altered to serpentine and minor amounts of talc and tremolite. A sample of a highly altered orthopyroxenite (Sample 209-1268A-18R-4, 88–91 cm) has relict primary sulfide (pyrrhotite/pentlandite) and incipient breakdown products (heazlewoodite[?] and millerite[?]), indicating reducing conditions during the early stages of serpentinization.
Mylonitic shear zones formed under granulite facies conditions (see "Structural Geology") and are statically altered under greenschist facies conditions. The mylonitic shear zone with highest intensities of crystal-plastic deformation is between 75 and 80 mbsf. At 78.6 mbsf this rock is pervasively altered, with amphibole, quartz, chlorite, oxides, and sulfides making up a significant fraction of the rock along with serpentine and varying proportions of talc (cf. thin section description of Sample 209-1268A-15R-1, 75–77 cm; see "Site 1268 Thin Sections"). The abundance of amphibole in the mylonite, as well as significant proportions of Fe-Ti oxides and apatite, suggests the presence of evolved magmatic material in this shear zone. The shear zone could represent a mylonitized gabbro-veined harzburgite or high-temperature metamorphic mineralization (e.g., brown amphibole) in harzburgite.
A mylonitized harzburgite at 75.6 mbsf lacks mafic and oxide-rich material and shows static alteration to almost pure serpentinite with minor amounts of magnetite (cf. thin section description of Sample 209-1268A-14R-2, 130–133 cm; see "Site 1268 Thin Sections"). Protomylonites and rare mylonites in other sections of Hole 1268A do not differ in alteration style or intensity from their undeformed analogs.
Crosscutting features interpreted as completely altered gabbroic dikes or veins are common, in particular between 60 and 100 mbsf. They are composed of talc, chlorite, amphibole, biotite, and white mica (Fig. F27). Gabbroic material also seems to be represented by heavily talc-altered rocks with a brecciated appearance that are distinguished from serpentinized harzburgites by the presence of pseudomorphs after large pyroxene and plagioclase crystals and/or a relict fine-grained texture. There is some suggestion, in particular for the section below 50 mbsf, that talc alteration is spatially associated with the presence of gabbroic veins (Fig. F28), small (meter scale) gabbro intrusions (Fig. F29), and contacts of large (tens of meters scale) gabbro intrusions. One possible explanation for this relationship is that silica-rich fluids, generated by gabbro-seawater interaction, metasomatized the surrounding serpentinites, causing the formation of talc at the expense of serpentine (see "Silica Metasomatism of Serpentinites" in "Discussion").
The large gabbroic intrusions appear to be gabbronorite in which orthopyroxene is completely altered to talc ± chlorite, while clinopyroxene is partly to completely altered to green amphibole and chlorite (Figs. F30, F31). Replacement of plagioclase and pyroxene by chlorite and amphibole along grain boundaries has created pronounced dark coronas (Figs. F32, F33). Plagioclase is variably altered to albite, quartz, and chlorite (Fig. F34; Table T2). The intensity of hydrothermal alteration of gabbronorite decreases downsection (Fig. F35). With decreasing alteration intensity, the relative proportion of chlorite in the secondary mineral assemblages decreases whereas that of amphibole increases. The lowermost gabbronorite unit has abundant fresh clinopyroxene and plagioclase, whereas orthopyroxene is completely altered in gabbroic rock throughout the entire hole. There appears to be a positive correlation between alteration intensity and intensity of cataclastic deformation as well as vein abundance (Fig. F36; see "Structural Geology"). Although alteration of the gabbros is static, this relationship may indicate that large fractures had some control on alteration intensity by providing pathways for fluids that then penetrated the rock along microcracks and caused the moderate to complete alteration. A remarkable difference between gabbros from Hole 1268A and the gabbros and gabbroic veins drilled or dredged at other sites of mantle exposure on the seafloor (e.g., Bideau et al., 1991; Früh-Green et al., 1996; Dilek et al., 1997b) is the complete lack of any Ca metasomatic and zeolite facies overprints in the gabbronorites and gabbroic veins in Hole 1268A.
A <1-cm-thick, highly altered amphibolite band is developed at 23.2 mbsf (cf. thin section description of Sample 209-1268A-3R-3, 6–9 cm; see "Site 1268 Thin Sections"). Apart from a few percent of relict brown amphibole, the amphibolite is completely altered to green amphibole, chlorite, serpentine, and white mica. This amphibolite is sandwiched in a centimeter-wide zone of highly altered orthopyroxene-rich harzburgite that shows significant amounts of weakly deformed relict olivine and orthopyroxene.
In the following sections we discuss the mineralogy of the veins as well as their characteristic macroscopic features and crosscutting relationships in the harzburgites, dunites, and gabbroic lithologies in Hole 1268A.
As part of the visual core description, we estimated the volumetric fraction of the core that is composed of different vein minerals (talc, serpentine, oxides, sulfides, and chlorite) (see the "Supplementary Material" contents list). Figure F36 illustrates the relationships between vein volume, composition, and lithology. Vein volume is low in zones of relatively high crystal-plastic deformation, as is the vein density (cf. "Crystal-Plastic Deformation" in "Structural Geology"). Sulfide veins are volumetrically important in Unit I between 20 and 30 mbsf, in the intrusion breccia zone (Unit II), and locally in Unit III. Oxide abundance in veins shows a similar distribution, but there is an overall trend of decreasing vein oxide downsection. Talc veins and serpentine veins exhibit spiky downhole distribution patterns that correlate with deformation intensity (lows in both serpentine and talc) and intensity of background talc alteration (highs in talc vein volume).
The distribution of the constituent minerals in different vein populations identified visually and verified by shipboard X-ray diffraction (XRD) analyses and thin section observations is shown in Figure F37. Figure F38 shows the different (a) shapes, (b) textures, (c) structures, and (d) connectivities of the identified vein populations. The percentage of the recovered core is shown in Table T3 along with breakdowns of the relative contribution of each vein population to this total. The distribution of various vein minerals among the vein populations and their relative contributions to the volume of core recovered is also calculated in Table T3.
The rocks of Hole 1268A have experienced a complex history of subseafloor metamorphism and associated vein alteration. Four distinct populations of veins are present within the harzburgite and dunite lithologies. These are described in more detail in the following sections. Chlorite and other populations of veins are subsidiary to these four.
Veins composed entirely of sulfides with varying proportions (as much as 50%) of iron oxides (dominantly magnetite and hematite) are described here as massive, referring to their dominant texture (Fig. F38A). This vein type commonly has irregular or sigmoidal shapes (Fig. F38B) and composite or uniform structures (Fig. F38C). Relative to other vein populations, the massive sulfide veins show the highest connectivity (Fig. F38D). Mineralogically, they are dominated by pyrite, magnetite, and hematite, although traces of chalcopyrite and millerite are also present. Hematite is easily identified because of its reddish contrast against the dark background alteration. In thin section it appears that this red oxide is intergrown hematite and magnetite. Finely anastomosing sulfide veinlets are so abundant in places (e.g., Sections 209-1268A-2R-2 [Pieces 4A, 4B] and 19R-1 [Pieces 2A, 2B, 2C]) that they grade into the pervasive background alteration. A similar network of sulfide veinlets is present in the halos around pyroxenite of Section 209-1268A-13R-1 (Pieces 6A, 9). These sulfide veins crosscut chrysotile veins that were subsequently altered to talc (Fig. F39A). The sum of sulfide and iron oxide contents of the massive sulfide veins and the talc pyrite veins accounts for 10.6% of vein mineralization and ~0.9% of the entire core (for summaries of the relative proportions of veins and their mineralogy see Table T3; Figs. F38, F40).
Talc veins with as much as 40% disseminated pyrite and iron oxides (mostly hematite) dominate the vein alteration at this site. The shape of the talc veins is irregular and occasionally sigmoidal, whereas the texture is massive and the internal structure is uniform. Half of this vein generation is nonbranched and the other half is composed of locally branched veins. Volumetrically, talc veins compose 4.2% of the core and talc itself accounts for 48% of all vein mineralogy (see Table T3). The talc is usually white or light green and contains irregular clusters of sulfide minerals (pyrite, rare marcasite, and millerite) and minor oxide (hematite and rare magnetite). Talc-pyrite veins cut both the serpentinized harzburgite/dunite and talc-altered lithologies. However, in proximity to areas of heavy talc alteration the connectivity of the veins drops (see Fig. F40C, F40D) and the pyrite and hematite content diminish over a short distance (several centimeters) to the point where the veins are composed solely of talc. This phenomenon is particularly well developed in intervals 209-1268A-4R-1, 80–116 cm, and 4R-2, 74–81 cm. Conversely, in serpentinized harzburgite/dunite lithologies the modal proportion of pyrite ± hematite increases when talc-pyrite veins crosscut relict orthopyroxene grains, commonly to the point where talc is absent. The relationships observed in Section 209-1268A-3R-2 suggest that two separate generations of this type of vein may be present because crosscutting relationships between talc veins with differing modal percentages of sulfide ± hematite are present here. Talc-pyrite veins crosscut massive sulfide veins (see Fig. F39), and in Sections 209-1268A-12R-3 and 22R-1 they crosscut chrysotile veins. Talc-pyrite veins also crosscut the gabbroic veins or dikes of Section 209-1268A-24R-2.
A generation of massive talc veins with only traces of sulfide cuts the earlier generations of talc-pyrite and massive sulfide veins. Massive talc veins are dominantly of irregular shape (Fig. F38B). Commonly, they have a uniform or composite structure (Fig. F38C) and a dominantly massive texture (Fig. F38B). In contrast to the massive sulfide and talc-pyrite veins, they exhibit cross and slip fibers. The connectivity ranges from single to branched (Fig. F38D). Crosscutting is particularly well developed in Sample 209-1268A-11R-1 (Piece 24, 126–137 cm) and in Section 16R-2, where the massive talc veins cut earlier vein generations. Notably, talc overprints chrysotile veins (Fig. F39B) and talc veins generally cut chrysotile veins. In Section 209-1268A-24R-2 massive talc veins contain small amounts of pyrite, hematite, and trace proportions of needlelike millerite. This is the only section in Hole 1268A where significant amounts of sulfides and iron oxides could be observed within the massive talc veins. Massive talc veins cross lithologic boundaries between serpentinized harzburgite/dunite and talc-altered ultramafics. They also crosscut talc-pyrite veins and, in some examples, may be associated with normal faulting (Fig. F40D). Therefore, this generation of veining was probably late in the sequence of alteration events that occurred at Site 1268.
After talc, serpentine is the most abundant vein mineral in Hole 1268A. It constitutes 39.1% of the vein mineralization, which in turn represents 3.4% of the core volume (see Table T3; Figs. F38, F40). Monomineralic serpentine veins that range from white through light to dark green are seen throughout Hole 1268A. This vein population occurs with irregular, sigmoidal, and straight shapes (Fig. F38B), a dominantly uniform structure (Fig. F38C), and mostly massive textures (Fig. F38A), as well as cross- and slip-fiber types. Serpentine veins are either single or show connectivities ranging from branched to nonbranched (Fig. F38C). They are generally sigmoidal and run subparallel to lithologic boundaries in poorly connected or unconnected semicontinuous en echelon arrays. For example, they are subparallel to the lithologic boundary between harzburgite and dunite in Sections 209-1268A-13R-2 (Pieces 4, 12, 19, 20) and are often present adjacent to the talc-altered transition zones between harzburgite/dunite and various igneous intrusions (e.g., Section 209-1268A-15R-2 [Piece 2B, 34 cm]). In many cases these veins have been replaced by talc (e.g., Section 209-1268A-3R-1 [Piece 21]) and are crosscut by talc-pyrite veins (e.g., Section 22R-1). Talc overprinting of chrysotile veins is identified by the powdery, weathered look of some chrysotile veins that only locally retain patches of their former fibrous nature. Elsewhere, however (e.g., in Section 209-1268A-18R-2), chrysotile veins of this type crosscut talc-pyrite veins, supporting the hypothesis that at least one of these vein types has multiple generations.
Serpentine and magnetite veins occur throughout the dunite-rich section between 90 and 105 mbsf and constitute a significant fraction of the magnetite present in this lithologic unit (0.2 vol% of veins and 0.02 vol% of the recovered core, not including magnetite present in background serpentinite alteration).
A final generation of serpentine veins crosscuts both talc-pyrite veins and talc veins, suggesting that the serpentine veins probably represent the last significant alteration and veining event at Site 1268. Sulfide is not usually present in these veins; however, chalcopyrite is present in Section 209-1268A-18R-3. This serpentine vein system is usually branching with individual veins >1 cm wide. It is present in substantial volumes of core within areas of serpentinized dunite (e.g., Sections 209-1268A-24R-1 and 24R-2). Rotated clasts of former chrysotile veins and angular rock fragments are commonly enclosed within these veins, so some of these vein systems are associated with minor cataclastic deformation. The serpentine veins in Section 209-1268A-24R-2 were identified by XRD analysis as lizardite.
Chlorite veins, or veins containing a significant proportion of chlorite, compose only 2% of the total volume of the veins logged and only 0.14% of the volume of core recovered (Table T3). Rare, small, wispy veins thought to be composed of chlorite are present in areas of dunite (particularly in Section 209-1268A-18R-4). Chlorite is also present in small mixed talc-chrysotile veins in Section 209-1268A-8R-1 (Piece 30).
Talc veins dominate areas of serpentinized harzburgite/dunite that were subsequently overprinted by pervasive talc alteration. These veins can be traced into areas of serpentinized peridotites and dunites where the talc overprint is absent. They exhibit increasing modal proportions of sulfide and hematite with distance away from the zones of talc alteration. We interpret this to indicate that sulfide was preferentially precipitated in areas of serpentinized harzburgite. Alternatively, talc alteration was superimposed upon a preexisting generation of sulfide-rich talc veins, mobilizing sulfide and reprecipitating it in areas away from the talc alteration front.
Small, wispy, sigmoidal chrysotile veins are present in some areas of heavy talc alteration. These are usually subparallel single or semicontinuous en echelon fibrous chrysotile veins. Their orientation is commonly parallel to the leading edge of the talc alteration front.
The occurrence and frequency of veins in the gabbroic units is low, and crosscutting relationships are rare, making the construction of an alteration history for this lithology difficult.
Rare small talc veins are present throughout the massive gabbros and are volumetrically insignificant. Small talc veins crosscut clasts of gabbro within the gabbroic intrusion breccias and are present within the gabbro from Section 209-1268A-22R-2 downhole. Further, a small chlorite-amphibole-serpentine vein network is developed in Section 209-1268A-20R-3.
Serpentine veins run subparallel to, or split, gabbro veins or dikes and gabbroic intrusion breccias, contributing to their extensive and multistage hydrothermal alteration. In Sections 209-1268A-16R-1 and 18R-4, serpentine veins crosscut gabbroic veins or dikes at a high angle. Cross fractures typically associated with gabbro veinlets are filled with serpentine. Serpentine veins in intrusion breccias do not cut peridotite clasts. This type of relationship is also seen in the gabbroic veins of Section 209-1268A-16R-2, where serpentine veins within the gabbro terminate at the gabbro/harzburgite contact.
A single pyrite-chalcopyrite-hematite vein is present in Section 209-1268A-18R-3, which hosts a large aggregate of chalcopyrite.
Pyroxenite is present only in very small volumes in cores from Hole 1268A, and accordingly, the veins within this lithology account for an insignificant volume of the whole core. A halo surrounding the pyroxenite in Section 209-1268A-13R-1 contains a small network of pyrite-hematite veins similar to those that are well developed in the serpentinized harzburgite/dunite. Chrysotile veins crosscut the pyroxenite in Section 209-1268A-18R-1, whereas in Section 18R-4 small serpentine-chlorite-amphibole veins are present.
The rocks recovered from Site 1268 have experienced a complex history of veining and hydrothermal alteration, as is demonstrated by the variety of veins present and their complex crosscutting relationships. Recognizing that the succession of alteration and veining events follows a systematic trend toward more oxidizing conditions, we have grouped the veins simply as early, main, and late stages of vein formation. A simplified overview of the relative timing of alteration and veining is provided in Figure F41.
Serpentine veins, in their various incarnations (see "Serpentine Veins" in "Veins in Harzburgite and Dunite," above; Fig. F38), are common throughout the alteration history of Hole 1268A. The crosscutting relationships of various populations of veins outlined above yield the following relative sequence of veining. A first generation of serpentine veins was synchronous with the serpentinization of the harzburgites and dunites. These serpentine-magnetite veins are present chiefly in serpentinized dunite (90–105 mbsf). Later generations of pyrite ± serpentine veins followed by fibrous chrysotile veinlets within the serpentinites are part of the early background serpentine alteration. The presence of magnetite in veins as well as the occurrence of reduced iron and nickel sulfides (relict pyrrhotite/pentlandite and possible partial replacement by heazlewoodite) in a moderately altered orthopyroxenite (Sample 209-1268A-18R-4, 88–91 cm) suggest fairly reducing conditions during early serpentinization and veining (Fig. F42A). Phases indicating even lower oxygen fugacities, found in peridotites from Hess Deep and the Kane Fracture Zone (MARK) area (Alt and Shanks, 1998, 2003), were not identified in the course of our shipboard studies.
The occurrence of massive pyrite veins with variable hematite modal percentages represents the most easily recognizable aspect of the main stage of alteration and veining. The coexistence of pyrite, magnetite, hematite, and trace millerite requires very specific oxygen and sulfur fugacities, provided that temperature and pressure can be estimated (Fig. F42A). This effectively pinpoints the emplacement of the massive sulfide veins at the position indicated on Figure F42A. When compared to other mid-ocean-ridge serpentinites, these conditions are unusual in that hematite is not developed in MARK and Hess Deep serpentinites (Alt and Shanks, 1998, 2003) (Fig. F42B).
The conditions recorded in veins that postdate emplacement of the massive sulfide veins (see above) also differ from the late-stage alteration conditions observed in other abyssal peridotites. In previous studies (Alt and Shanks, 1998, 2003), pyrite veins characterize late-stage alteration and hematite is absent (Fig. F42B). In the late-stage oxide sulfide–bearing veins in Hole 1268A, hematite is a major vein constituent, indicating more oxidizing conditions (and/or lower H2S activities) during late-stage circulation than at Hess Deep and MARK.
Talc-pyrite veins crosscut the fine anastomosing sulfide-hematite veins in several locations throughout Hole 1268A (see above), and where this occurs sulfide is mobilized and appears more concentrated in the talc-dominated veins. This suggests that hematite formation may have occurred during the talc alteration stage. It is conceivable that increasing modal percentages of hematite in the late-stage talc-pyrite-hematite veins are associated with decreasing H2S(aq) and H2(aq) activities of the interacting hydrothermal solution. Such a trend can be expected in long-lived hydrothermal systems where the reducing capacity of the rock is exhausted by reduction of seawater sulfate (e.g., Seyfried and Ding, 1995). This scenario is consistent with the observation that the peridotites in Hole 1268A underwent multiple episodes of intense and pervasive alteration.
The last stage of veining in the Hess Deep and the MARK areas is the formation of aragonite veins under ambient conditions at or near the seafloor (e.g., Bluzstajn and Hart, 1996). Aragonite veins were not identified in core from Hole 1268A, suggesting that present-day veining and seawater ingress may be minimal—or that exposure at the seafloor was recent.
In contrast to serpentinized ultramafic rocks from the Hess Deep and MARK areas (Früh-Green et al., 1996; Dilek et al., 1997b), which contain noticeable amounts of early talc and tremolite (after orthopyroxene) and brucite (after olivine), serpentinized peridotites in Hole 1268A do not contain these assemblages, except in one highly serpentinized orthopyroxenite. In other, more common localities, it is thought that early talc forms after orthopyroxene and is replaced by lizardite either during retrograde serpentinization (Wicks, 1984) or due to a combination of increased H2O activity and decreased silica activity of the fluid (Frost, 1985). Hydrothermal experiments (at 400°C and 500 bar) confirm that fluids are talc-saturated as long as fresh pyroxene is left in the serpentinizing peridotite (Allen and Seyfried, 2003). Similarly, brucite is believed to be a product of incipient serpentinization which, due to reaction with aqueous SiO2 released by the breakdown of orthopyroxene to lizardite, forms lizardite and magnetite (e.g., Toft et al., 1990). There is no relict talc, tremolite, or brucite in the rocks from Hole 1268A, suggesting that serpentinization reached an advanced stage at which only serpentine and magnetite were present. The high proportion of magnetite in the dunite-rich unit between 90 and 105 mbsf (cf. Fig. F92) is consistent with this scenario. The molar (Mg + Fe)/Si ratios of completely serpentinized rocks from Hole 1268A are close to 1.5 (cf. "Geochemistry"; Table T5), which is higher than the likely (Mg + Fe)/Si ratio of the protolith (i.e., [Mg + Fe]/Si = 1.8 for a harzburgite with 20% orthopyroxene). The comparatively low magnetic susceptibilities in the harzburgites and dunites from the upper 90 m in Hole 1268A may indicate destruction of magnetite during subsequent alteration that did not affect the dunite-rich section between 90 and 105 mbsf of Unit III. The (Mg + Fe)/Si ratios decrease from 1.62 ± 0.05 (1
; N = 4) in the strongly magnetized harzburgites/dunites of Unit III to 1.41 ± 0.06 (1; N = 11) in the serpentinites in the upper 90 mbsf to 0.79 ± 0.06 (1; N = 5) in the talc-altered serpentinites, suggesting increasing extent of relative Si gains or Mg + Fe losses.
Another indication of the advanced degree of serpentinization in Hole 1268A is the general scarcity of pseudomorphic mesh textures and the common development of transitional hourglass and ribbon textures (O'Hanley, 1996). Furthermore, interlocking textures, indicating recrystallization of mesh-textured serpentine, are common and serrate chrysotile veins formed at the expense of early lizardite.
The most unusual aspect of the hydrothermal alteration at Site 1268 is the widespread replacement of serpentine by talc. In previously drilled and dredged sections of upper mantle (Dilek et al., 1997b; Früh-Green et al., 1996; Bideau et al., 1991), gabbros are Ca metasomatized, whereas at Site 1268, serpentinites are Si metasomatized. Furthermore, zeolite facies alteration and veining, common in metagabbros from Hess Deep and MARK areas, is absent in core from Hole 1268A. Hence, we infer that the relative timing of deformation, magmatism, and hydrothermal alteration at Site 1268 is different from otherwise similar, previously investigated settings.
Instead of Ca metasomatism of gabbro, the serpentinites at Site 1268 reveal replacement of serpentine by talc. Serpentine has a molar (Mg + Fe)/Si = 1.5, whereas that of talc is 0.75. Talc alteration may hence be a result of Si metasomatism. One possible reaction between serpentine and fluid to form talc is
Mg3Si4O10(OH)2 + H2O,
which proceeds to the right if the silica activity of the fluid is increased and/or if the water activity is lowered. In some cases, the formation of talc-magnesite rocks after serpentinite have been a consequence of lowering the water activity of the fluid by adding CO2 (e.g., Peabody and Einaudi, 1992). However, we did not observe magnesite (or other carbonates) in detectable amounts and conclude that carbonatization cannot account for the abundant development of talc after serpentine. It is hence more likely that an increase in silica activity is responsible for the conversion of serpentine to talc.
The close spatial association of talc alteration and gabbro intrusions suggests that gabbro emplacement and talc alteration are intimately linked. Gabbro intrusion and dike formation in serpentinite could revive hydrothermal circulation by providing a heat source and creating permeability. However, hydrothermal circulation long after gabbro emplacement could also transport SiO2 from gabbro into host peridotite. Theoretical geochemical models suggest that at 350°C and 500 bar mafic rock–seawater reactions form hydrothermal fluids much higher in silica and lower in pH than hydrothermal fluids produced by reaction of seawater with ultramafic rocks (Wetzel and Schock, 2000).
Hydrothermal reaction between gabbro and seawater under greenschist facies conditions transformed pyroxene and plagioclase in the gabbros to chlorite, amphibole, and talc, releasing silica and acidity and causing Si metasomatism and talc formation in the serpentinites. The activity coefficient for aqueous SiO2 is strongly pH dependent and becomes larger as the pH decreases. Silica activity and pH are hence strongly coupled, in particular at near-neutral conditions (Johnson et al., 1992).
Ca metasomatism takes place when gabbros and ultramafic rocks undergo the same metamorphic history (e.g., Schandl et al., 1989). Ca-rich fluids generated during serpentinization metasomatize the gabbros replacing pyroxene and plagioclase by clinopyroxene, tremolite, clinozoisite, prehnite, and hydrogrossular (e.g., O'Hanley et al., 1992). The lack of rodingitization—even in small gabbroic veins—may suggest that the gabbros intruded after serpentinization was largely completed and the Ca of the peridotites had already been leached out. Experimental studies show that Ca is lost most rapidly in the early stages of high-temperature serpentinization when the reactions rates of pyroxenes are fast (Allen and Seyfried, 2003). However, other considerations (see "Crosscutting Relationships and Deformation Paths" in "Structural Geology") suggest that many or most of the gabbroic intrusions formed while the peridotite wallrocks were still quite hot (>500°C).
