-
H2O + olivine
serpentine + brucite.
Two holes were cored at Site 1271, which is located at the inside corner high of the Mid-Atlantic Ridge segment south of the 15°20´N Fracture Zone.
The drill core from Hole 1271A consists predominantly of completely serpentinized dunite. Locally, the serpentinites include brucite. Some intervals also experienced low-temperature seafloor weathering. Gabbroic intrusions are a minor component in Hole 1271A.
The drill core from Hole 1271B consists of a complex association of completely altered mafic (gabbros and amphibolites) and ultramafic rocks, dominated by dunite and harzburgite. These are intruded and infiltrated by amphibole-bearing gabbroic lithologies and have experienced variable degrees of syndeformational hydrothermal alteration. Relatively fresh dunite is also present, particularly in proximity to amphibolites.
The recovered core consists of highly to completely serpentinized dunite and minor intercalated, highly to completely altered gabbro and microgabbro. Veining is generally well developed throughout the core, and several generations of veins can be discriminated. Locally, the dunite shows signs of low-temperature oxidative alteration, whereas serpentine-brucite alteration prevails near the bottom of the hole. The degree of downhole alteration is illustrated in Figure F21.
Hole 1271A is dominated by light gray to black completely serpentinized dunites. Microtextures in the serpentinized dunite range from pseudomorphic mesh texture through transitional texture to nonpseudomorphic ribbon texture. Unaltered orthopyroxene and traces of fresh olivine are restricted to interval 209-1271A-1R-1, 13–19 cm. In Sections 209-1271A-1R-1 and 1R-2, the dunites have a yellowish green to gray groundmass hosting abundant transgranular, black serpentine-magnetite veins, mainly oriented parallel to the long axis of the core (Fig. F22; Table T2).
Within the gray-green to black dunites of Sections 209-1271A-4R-1 and 4R-2, there are intermittently distributed centimeter-scale, occasionally branched veinlike domains and patches that consist mainly of chlorite-talc-amphibole. These are interpreted as the alteration products of intrusions of gabbroic material along olivine grain boundaries (see "Igneous and Mantle Petrology").
Chromite is present in Section 209-1271A-4R-1 as a nodular, centimeter-sized chromitite pod (interval 4R-1, 80–83 cm). Within this pod, chlorite, talc, and smectite form the interstitial material between chromite grains and probably represent the alteration products of pyroxene, olivine, and/or plagioclase.
At least some parts of the dark gray, completely altered dunite in Section 209-1271A-4R-2 include a substantial proportion of brucite. This style of alteration is macroscopically indistinguishable from serpentine without brucite and has been recognized during the course of X-ray diffraction analyses and thin section observations (Fig. F23; Table T2). Brucite forms aggregates of fine elongate crystals with anomalous brown interference colors that are surrounded by and intergrown with serpentine, indicating that the brucite formed at the same time as serpentine. This represents the first major occurrence of brucite in altered ultramafic rocks identified during the course of Leg 209.
Some of the dunite experienced variable degrees of low-temperature seafloor weathering with orange to brownish spots containing clays, iron oxyhydroxides, and minor carbonate. This style of alteration is particularly common in Section 209-1271A-3R-1. A fault breccia in interval 209-1271A-1R-1, 7–13 cm, is composed of weathered serpentinized dunite clasts in a carbonate-rich matrix with minor green clay.
Intrusions of variably altered gabbroic material in the serpentinized dunite are present in intervals 209-1271A-1R-1, 37–51 cm; 1R-1, 23–32 cm; 2R-1, 4–14 cm; and 6R-1, 6–25 cm. Plagioclase in these gabbros and microgabbros was commonly replaced by sericite, quartz, and secondary sodic plagioclase, and pyroxene experienced slight to moderate amphibole-chlorite alteration.
Interval 209-1271A-1R-1, 37–51 cm, is exceptional and represents an olivine-bearing, pegmatitic gabbro dike. Pyroxene and plagioclase were completely altered to an assemblage of amphibole, chlorite, and secondary plagioclase. Olivine is largely unaffected by alteration except for minor talc replacement along its margins.
Metamorphic veins in Hole 1271A account for 6.8 vol% of the recovered core (Table T3) and consist predominantly of serpentine (79 vol%) (Fig. F24). Other vein minerals are talc (8.6 vol%), amphibole (5.3 vol%), magnetite (2.5 vol%), carbonates (2.1 vol%), and minor zeolites, clays, and iron oxides (other than magnetite).
Several different transgranular vein types were recognized in the serpentinized dunite of Hole 1271A: serpentine-magnetite veins, green picrolite veins, white chrysotile veins, talc-amphibole veins, and carbonate (mainly aragonite)-clay veins. Serpentine-magnetite veins are crosscut by all other vein generations and, therefore, represent the earliest veining event. Other crosscutting relationships indicate that the white chrysotile veins postdate green picrolite and talc-amphibole veins. The carbonate veins represent the last generation of metamorphic veins.
In Hole 1271A, dunite was mainly replaced by serpentine; however, noticeable amounts of brucite are present in Section 209-1271A-4R-2. The textural relationships indicate that brucite formed from the olivine breakdown reaction:
serpentine + brucite.
Chemical analyses of two brucite-bearing dunites reveal (Fe + Mg)/Si molar ratios of 1.98 and 2.01, respectively, basically identical to that of olivine (cf. "Geochemistry"). This result suggests that olivine reacted with water to form serpentine and brucite without significant Si, Mg, and Fe mass transfer.
Chlorite-amphibole assemblages of the subordinate gabbroic intrusions in Hole 1271A represent static greenschist facies alteration, similar to the style of alteration observed in gabbros from Sites 1268 (lower part of Hole 1268A) and 1270 (Hole 1270B). An exceptional pegmatitic olivine gabbro dike contains abundant fresh olivine that has been only marginally replaced by talc, whereas pyroxene and plagioclase were completely replaced by amphibole, chlorite, and secondary plagioclase. Apparently, the olivine stability and reaction kinetics during alteration were strongly dependent on the primary lithology of the rocks.
Veining in Hole 1271A is intense (6.8 vol% of the core), suggesting that fracture-controlled hydrothermal fluid flow was relatively abundant in this area. All veins show transgranular geometries, which may indicate that even the earliest vein generation (serpentine-magnetite) formed at a late stage of serpentinization. The late-stage formation of carbonate veins probably took place during low-temperature seafloor weathering of the succession. This is consistent with the observation of carbonate cement in a fault zone (interval 209-1271A-1R-1, 7–13 cm) that contains weathered serpentinized dunite clasts.
Three distinct lithologies, harzburgites/dunites, BAGs, and amphibole-bearing dunites, are present in Hole 1271B. Each lithology is variably altered and shows characteristic features that are described separately. However, the different features within each lithology may in fact be linked by common processes (see "Discussion" below).
Serpentinized dunite composes a greater proportion of the recovered core than serpentinized harzburgite in Hole 1271B. This differs from the relative proportions recovered at Sites 1268 and 1270, where serpentinized harzburgites were dominant. Serpentinized dunite and harzburgite range from light or dark green (Sections 209-1271B-1R-1 and 2R-1) to gray (Section 19R-1) to black (Sections 7R-1 and 19R-1) and are composed mainly of serpentine and magnetite (Table T2). Brucite forms ~5% of a thin section from Sample 209-1271B-6R-1, 9–12 cm, as brown aggregates in serpentine mesh texture. Noticeable amounts of brucite are also present near the bottom of the hole in Sections 209-1271B-17R-1 to 19R-1. The amount of brucite in these intervals is lower than that in Section 209-1271A-4R-2. Serpentinization textures vary from relict mesh texture to ribbon textures to shear zones in which a distinct foliation is present. Locally, bastite pseudomorphs after orthopyroxene are present (e.g., Section 209-1271B-6R-1). Talc alteration is minor in Hole 1271B and is present only in three short intervals related to deformation. For example, in talc-tremolite schists found in intervals 209-1271B-12R-1, 0–12 cm, 24–33 cm, and 42–55 cm, and 3R-1, 17–22 cm, talc accounts for as much as 35% of the schist. Additional talc alteration is present in Section 209-1271B-10R-1 (intervals 7–10 cm, 21–24 cm, 26–30 cm, 36–39 cm, and 106–118 cm), where as much as 15% of the rock is altered to talc. In relatively fresh dunites (e.g., Sections 209-1271B-12R-1 and 13R-1), rare orthopyroxene is replaced by talc, which has potential implications for reconstructing the conditions of serpentinization (see "Discussion" below).
Orange clay and iron oxyhydroxide weathering is minor; however, it is locally present throughout the core (e.g., Sections 209-1271B-1R-1, 37–41 cm, and 58–64 cm, and all of Section 4R-1, in particular, intervals 1–3 cm, 10–19 cm, and 50–63 cm). This low-temperature alteration affects relict primary phases much more than it affects hydrous phases produced by serpentinization. Therefore, where clay and iron oxyhydroxide are developed, they have probably replaced relict olivine (and to lesser extent orthopyroxene).
Brown Amphibole Gabbro. Highly to completely altered BAG is present throughout the core, with the exception of the upper 18 and the lower 15 m of Hole 1271B (Fig. F25). The secondary mineral assemblages in the BAG do not show significant variation with depth. BAG is altered to secondary amphibole, talc, chlorite, and sericite with a trace of zeolite. Brown amphibole and minor talc replace former clinopyroxene, and plagioclase is altered to quartz, sericite, secondary plagioclase, and minor amphibole. A remarkable feature of the BAG is the presence of rutile (up to 2%). Rutile intergrown with quartz appears to replace primary Fe-Ti oxides in a few instances, suggesting that rutile may form hydrothermally. The presence of rutile, rather than titanite, may indicate alteration under amphibolite facies conditions.
Tremolite-talc schists probably represent highly sheared and retrogressed BAG. A tremolite-talc schist from interval 209-1271B-3R-1, 18–22 cm, is composed of acicular to prismatic tremolite (50%–60%) with fibrous talc (30%–40%) and minor chlorite. Anastomosing gabbroic veins or dikes are present in amphibole-bearing dunite (intervals 209-1271B-11R-1, 44–49 cm, 72–76 cm, and 82–108 cm) and exhibit similar talc-amphibole–dominated secondary mineral assemblages while showing complex relationships between hydrothermal alteration and deformation (see "Discussion").
Gabbros and Microgabbros. In addition to the BAG, a second type of variably altered gabbro and microgabbro is present in various sections of Hole 1271B (e.g., interval 209-1271B-10R-1, 11–20 cm) (Fig. F25). These rocks are volumetrically less significant than the BAG. They are moderately to completely altered to secondary plagioclase, green amphibole, chlorite, and minor talc and quartz.
Alteration of Hybrid Rocks. Two rock types have tentatively been described as affected by crystallization of interstitial gabbroic material from intergranular melt (see "Igneous and Mantle Petrology"): (1) the development of diffuse vein networks and patches of chlorite, talc, and minor green amphibole within ultramafic rocks and (2) amphibolites that appear to penetrate into dunite and locally engulf dunite fragments.
The chlorite + talc–rich veinlets were described above. Their origin is unknown. They are present in fresh dunite as well as in completely serpentinized dunite, and they contrast markedly in style and geometry with other veins that are clearly metamorphic (see below). A preliminary interpretation of these features is that plagioclase and pyroxene crystallized from melts migrating along grain boundaries in dunite and harzburgite and subsequently were hydrothermally altered to talc, chlorite, and minor amphibole.
Amphibole-bearing dunites in which the dunite is commonly relatively fresh first appear in Section 209-1271B-10R-1. Green amphibole with minor chlorite and traces of quartz is interstitial to olivine. A few samples include relict coarse-grained plagioclase, suggesting that the protolith of the amphibole + chlorite + quartz assemblage was gabbro. The contacts between dunite and amphibole-bearing assemblages are variable and complex. The general impression is that of intrusions of mafic material into dunite, locally disaggregating it and engulfing centimeter-sized dunite fragments, while forming apophyses and veins in other sections of the core. Alteration of the dunite to serpentine begins along irregular cracks that form a mesh network, usually with fresh olivine in the mesh centers (Fig. F26). Olivine is also replaced by amphibole, particularly in proximity to the contacts with mafic material. Amphibolitization of olivine is pseudomorphic and preserves the magnetite rims of mesh texture in olivine (Fig. F27). Orthopyroxenes with vermicular spinel are altered to talc, even in relatively fresh dunite (e.g., Sample 209-1271B-10R-1, 40–43 cm) (Fig. F28). In Section 209-1271B-13R-1 the relationship between the presence of amphibolite and the extent of olivine preservation is best developed. It appears that dunite in contact with mafic material is much less serpentinized than dunite in sections of the core that do not contain mafic material. A sharp reaction front between completely serpentinized dunite and relatively fresh dunite with amphibole veins is present in Section 209-1271B-10R-1 (Fig. F29).
Alteration in Fault Zones. Fault breccia is present in interval 209-1271B-3R-1, 48–54 cm, which is composed of completely serpentinized clasts of dunite with a small amount of talc in a carbonate/green clay matrix. However, similar to intervals 209-1270A-2R-1, 101–104 cm, and 1R-1, 7–13 cm, fault breccia is volumetrically insignificant. Green serpentine mud, presumably representing a fault zone, is present in intervals 209-1271B-19R-1, 4–10 cm, 50–57 cm, 73–83 cm, and 96–103 cm.
Serpentine and magnetite dominate the vein mineralogy of Hole 1271B, jointly accounting for 77% of the volume of the veins and 3.1% of the volume of the core (Fig. F30; Table T3). Other vein minerals are talc, carbonate, amphibole, quartz, and minor chlorite, epidote, iron oxide, and clay.
Black serpentine + magnetite veins are the dominant vein type in Hole 1271B. Locally, they form anastomosing networks (e.g., Section 209-1271B-6R-1) and persist through areas of intense orange clay and iron oxyhydroxide weathering (see "Alteration of Harzburgite and Dunite" in "Hydrothermal Alteration" above). They are crosscut by a second generation of serpentine + magnetite veins and are subsequently crosscut by white sigmoidal chrysotile veins (e.g., Section 209-1271B-3R-1). All of these generations are crosscut by late serpentine + magnetite veins in interval 209-1271B-6R-1, 9–12 cm, and green and white picrolite veins in Sections 209-1271B-1R-1 and 3R-1. Given the complex crosscutting relations between the various generations of serpentine veins, it is difficult to interpret the exact timing of each of these generations, and some may be emplaced synchronous to others. All generations of serpentine veins are developed throughout the core.
Late, vuggy aragonite veins crosscut all other vein generations (as in Hole 1271A). They are composed of coarse-grained, acicular to prismatic aragonite aggregates with minor green clay (Sections 209-1271B-2R-1, 4R-1, 7R-1, and 15R-1). Aragonite veins are particularly abundant in interval 209-1271B-15R-1, 37–68 cm, where the serpentinized harzburgite is strongly weathered.
Quartz, quartz-amphibole-rutile, quartz-epidote-zeolite, quartz-zeolite, and epidote veins are present in the BAG (e.g., Sections 209-1271B-3R-1 and 5R-1). Whereas these veins are volumetrically insignificant (Table T3), their variable mineral compositions may indicate multiple episodes of veining and fluid flow under a range of conditions. Talc veins are especially well developed in the uppermost 17 m of Hole 1271B.
Three major distinct lithologies are present in Hole 1271B: (1) serpentinized harzburgite and dunite, (2) BAG, and (3) amphibole-bearing dunite. In addition, there are subsidiary gabbros distributed throughout the core that are volumetrically insignificant. The distribution of these lithologies is outlined above in the summaries of vein alteration and background alteration where, for clarity, they were discussed separately. Here, we combine their metamorphic features into a coherent history of alteration and deformation for Hole 1271B.
The earlier parageneses that can be recognized in the BAG are brown amphibole, plagioclase, and Fe-Ti oxides. On textural evidence, it is not possible to discern if the amphibole is igneous or is an alteration product replacing primary pyroxene. The absence of pyroxene, even in BAG that has abundant apparently fresh plagioclase, could indicate that (1) brown amphibole is a primary phase or (2) the BAG underwent pervasive amphibolite-facies alteration during which plagioclase was stable. The partial breakdown of primary Fe-Ti oxides to rutile and quartz in interval 209-1271B-5R-1, 12–14 cm, is consistent with relatively high temperature alteration but does not rule out a magmatic origin for the brown amphibole. Late, secondary green amphibole along with chlorite and minor talc is present in varying proportions in BAG.
In mylonitic BAG the brown amphibole has been deformed in a ductile manner. In these mylonites, brown amphibole was deformed as large porphyroclasts and recrystallized to neoblasts (Fig. F31A, F31B) or as neoblast bands with intense grain size reduction (in the margins of Fig. F31A and F31B). Although plagioclase in these samples is completely altered, textural observations indicate that plagioclase underwent a similar type of deformation before static breakdown to sericite, quartz, and secondary plagioclase (the secondary plagioclase must have relatively high CaO to account for the whole-rock CaO of 11.7 wt% compared to the amphibole CaO content of 12.5 wt%). This is suggested by large altered plagioclase grains that retain the polysynthetic twinning of plagioclase porphyroclasts and are embedded in a matrix of smaller neoblasts of (altered) plagioclase (interval 209-1271B-14R-1, 25–29 cm) (Fig. F31). These observations indicate that the BAG underwent ductile deformation localized in shear bands affecting brown amphibole and plagioclase. Following ductile deformation, plagioclase in these shear bands was statically altered to sericite, quartz, and secondary plagioclase.
Most BAG samples record later pervasive, brittle, retrograde alteration. This event led to the breakdown of brown amphibole along schistose bands or veins in BAG. In places this breakdown is static, but more commonly it is pervasive, in particular in schistose amphibole-chlorite bands. Retrogression of brown amphibole formed fine-grained, fibrous aggregates of green amphibole and chlorite that were synkinematically deformed, likely under greenschist facies conditions (Fig. F31D, F31E). The amphibole-chlorite schist locally shows a well-developed foliation and folding of the schistose fabric.
Rare troctolites show textural evidence for static plagioclase breakdown to chlorite and amphibole in the proximity of olivine. We tentatively relate plagioclase breakdown to retrograde greenschist facies assemblages.
Amphibole-bearing dunites are the most common olivine-bearing rock type other than peridotite. They are found in close proximity to BAG. The earliest secondary phase that can be observed in these rocks is coarse amphibole crystals in textural equilibrium with olivine (Fig. F32A). These large amphibole crystals fill the intergranular space between round olivine grains and display continuous extinction (Fig. F32A). There is no evidence that the amphibole is replacive after a specific primary igneous phase.
Amphibole-bearing dunites underwent high-temperature ductile deformation, manifest in elongated porphyroclasts of deformed amphibole surrounded by a mantle of fine-grained amphibole neoblasts (Fig. F32B). The ductile overprint in olivine is weak (wavy extinction and kink bands), indicating deformation temperatures <900°C. Sharp boundaries between olivine and amphibole suggest that both phases were in equilibrium during ductile deformation. We tentatively relate ductile deformation in amphibole-bearing dunites to the same deformation event recorded in BAG because amphibole did not break down during deformation in either lithology. These observations indicate that BAG and amphibole-dunite underwent ductile deformation in the high-temperature amphibolite facies, with variable strain localization into mylonitic bands, in both lithologies.
Most amphibole-bearing dunites record greenschist facies retrogression. This resulted in the alteration of coarse, early amphibole to a retrograde assemblage of fibrous green amphibole and chlorite. As this retrograde assemblage formed, it propagated along grain boundaries and along fractures in the surrounding olivine (Fig. F32C, F32D). These textural relations indicate brittle conditions during greenschist overprint. Greenschist facies metamorphism appears to be contemporaneous with serpentinization because the partial breakdown of early amphibole to secondary amphibole + chlorite and serpentinization are both developed near mylonitic bands composed of the retrograde assemblage (Fig. F32C, F32D). During retrogression, serpentinized olivine porphyroclasts were partially replaced by coronas of secondary amphibole and chlorite (Fig. F32D). In addition, olivine porphyroclasts are locally transformed to amphibole needles growing topotactically on serpentine (Fig. F32E). Early amphibole porphyroclasts were transformed to fibrous amphibole and chlorite along retrograde, sheared amphibole-chlorite bands (Fig. F32F). However, elsewhere in the amphibole-bearing dunite (e.g., interval 209-1271B-13R-1, 20–24 cm) there is evidence for static replacement of olivine by the retrograde mineral assemblage. Here, former grain boundaries of olivine, outlined by rims of magnetite, are still visible in the retrograde mineral assemblage of secondary amphibole, chlorite, and sericite (Fig. F27). This suggests that in zones of limited resistance to deformation dynamic alteration processes actively altered early amphibole and olivine. Where shear strength was limited, for example in the strong, semirigid matrix of fresh olivine, alteration and overprinting was static.
In terms of texture and mineralogy, this retrograde event in amphibole-bearing dunites is consistent with the alteration and deformation history preserved in amphibole-chlorite bands replacing BAG. The strain displayed in both units is consistent with high-temperature shearing followed by, or continuing as, synkinematic to static greenschist facies alteration, resulting in the breakdown of early amphibole into discrete zones of retrograde secondary amphibole that locally form amphibole-chlorite schists.
The relationships between serpentinization in harzburgite and dunite and the metamorphic events preserved in the other lithologies are not clear. Some serpentinization clearly predates the alteration of former gabbroic dikelets in the amphibole-bearing dunite, as veins composed of secondary amphibole + chlorite + sericite are observed crosscutting serpentine veins (Fig. F33). However, these amphibole veins are subsequently cut by serpentine veins that resemble the earlier serpentine vein generation. The relative timing of serpentinization and synkinematic greenschist facies alteration is difficult to distinguish, and these events may be synchronous.
Evidence for syndeformational serpentinization is limited to discrete areas that show deformation fabrics sharply juxtaposed with essentially strain-free areas of serpentinite. Early serpentinization followed by greenschist facies alteration, which was contemporaneous with a second serpentinization event, could account for the textural relationships observed in the serpentine and the complex crosscutting relationships observed between serpentine and the greenschist facies assemblages. Multiphase serpentinization is also consistent with the complex crosscutting relationships between the various generations of serpentine veins in Hole 1271B (cf. "Metamorphic Veins" above).
The presence of incompletely serpentinized dunites and minor harzburgites at Site 1271 provides the opportunity to speculate about potential serpentinization reaction paths. Field and experimental studies suggest that serpentinization at low to moderate temperature (<250°C) is a nonequilibrium process during which dissolution of olivine proceeds at rates faster than precipitation of talc and serpentine (e.g., Nesbitt and Bricker, 1978; Martin and Fyfe, 1970).
Figure F34A displays a Mg-Ca-Si-O-H mineral-fluid phase diagram for 200°C and 500 bar. The fluid composition will be driven toward the stability fields of serpentine and brucite, which represent the lowest-energy mineral assemblage (e.g., Hemley et al., 1977). The proposed reaction path (black arrow in Fig. F34A) is strongly curved, owing to rapid dissolution of olivine and consumption of acidity, followed by a shoaling of the trend due to dissociation of orthosilicate at high pH. The last part of the proposed reaction path is controlled by the forsterite dissolution boundary. At pH = 10 and silica concentrations similar to that of seawater, the Lost City vent fluids (Kelley et al., 2001), when respeciated at 200°C and 500 bar, have aqueous silica activities that put them right on the serpentine/brucite boundary, suggesting that the Lost City fluid is saturated in both phases. Also shown in Figure F34A is a schematic reaction path for fluids from a harzburgite-seawater interaction experiment (Seyfried and Dibble, 1980) that trends toward the chrysotile/brucite boundary. In contrast, fluids from a lherzolite-seawater experiment at 200°C and 500 bar (Janecky and Seyfried, 1986) do not evolve toward brucite saturation because the high abundance of pyroxenes keeps the fluid pH low and the silica activity of the fluids high (Fig. F34A).
The effect of pyroxene on solution chemistry is even stronger at higher temperatures (>250°–300°C), where pyroxenes react faster than olivine (Martin and Fyfe, 1970; Allen and Seyfried, 2003). Figure F34B demonstrates that high-temperature (365°C) black smoker fluids from the Logatchev and Rainbow peridotite-hosted hydrothermal systems (e.g., Charlou et al., 2002), when respeciated at 400°C and 500 bar, are pinned by fluid saturation in metastable pyroxene and that talc and possibly tremolite should form at the expense of pyroxene. As long as pyroxenes are present (and dissolve fast) the pH of the interacting fluids is low and the silica activity high (cf. Allen and Seyfried, 2003). Only when the fluid is no longer pyroxene saturated can it evolve to higher pH and lower silica activity.
These mineral-fluid phase relationships are relevant for the interpretation of the serpentine-brucite and olivine-amphibole-talc assemblages observed in cores from Site 1271. A possible interpretation of the relatively fresh dunites with rare talc-altered orthopyroxene pseudomorphs is that these rocks reacted with hydrothermal fluids at high temperatures (>300°C) where orthopyroxene reacts faster than olivine. The proximity of mafic material (amphibolites) to intervals of unaltered dunite may suggest that the presence of pyroxene and amphibole kept the silica activity of the interacting fluids high so that serpentinization of olivine was inhibited. Formation of talc after olivine is thermodynamically favored (Fig. F34B) but kinetically sluggish (e.g., Nesbitt and Bricker, 1978). Replacement of orthopyroxene by talc, on the other hand, is commonly observed in alpine and abyssal serpentinites (e.g., Aumento and Loubet, 1971; Hostetler et al., 1966). As the system moves toward lower silica activity after pyroxenes are exhausted, talc that had formed during initial alteration will react to serpentine. This explains the scarcity—or complete lack—of talc as part of the background alteration in completely serpentinized rocks from Site 1271.
The brucite-bearing dunites (and minor harzburgites) from the bottom of Holes 1271A and 1271B require a different genetic model (see Fig. F34A). The phase relations indicate that these lithologies reacted with fluids of high pH and low silica activity. Such fluids are generated when alteration is controlled by rapid olivine dissolution and relatively sluggish hydrous mineral precipitation. Thermodynamically and kinetically, conditions for brucite-serpentine formation are favorable at low temperatures and in the absence of pyroxenes. The abundance of brucite at Site 1271 may suggest that the circulating fluids are dominantly high pH and low silica activity, which could be explained if the basement at Site 1271 consists dominantly of dunite.
