-
6 CaMgSi2O6 + 6 MgSiO3 + 5 Mg2+ + 9 H2O
3 Mg3SiO5(OH)4
A total of 34.65 m of highly to completely serpentinized harzburgite and minor very highly to completely serpentinized dunite was recovered from Hole 1274A. Altered gabbroic intrusive rocks are present only in some short intervals.
The ultramafic rocks show a systematic downhole increase in the total extent of serpentinization. In the upper 60 mbsf, up to 35% of the primary igneous phases (olivine, orthopyroxene, and clinopyroxene) are preserved, representing the least altered peridotite unit drilled at Sites 1268–1274. In some intervals, the serpentinized ultramafic rocks have been affected by cataclastic deformation, resulting in the formation of fault gouges. Oxidative carbonate–iron oxyhydroxide–clay alteration is prominently developed along aragonite veins and extends to ~90 mbsf. Brucite alteration is particularly well developed in dunite. Thin section observations indicate that olivine is replaced by brucite-serpentine-magnetite assemblages.
Vein alteration is dominated by black serpentine-magnetite and white chrysotile veins. The chrysotile veins are transgranular as well as paragranular. The paragranular vein type is particularly common in proximity to shear zones. Carbonate and iron oxide veins are limited to the upper part of Hole 1274A and are related to late-stage circulation of cold, oxygenated seawater.
The grayish black to black harzburgites and dunites of Hole 1274A are highly to completely serpentinized. Alteration products include serpentine after olivine and orthopyroxene, brucite, serpentine, and magnetite after olivine and minor talc and tremolite after clinopyroxene and orthopyroxene. Spinel is commonly fresh or slightly altered to magnetite. In partially serpentinized rocks, olivine is generally more altered than orthopyroxene. Clinopyroxene is only moderately altered to talc and tremolite. In the uppermost 60 m of Hole 1274A, the degree of orthopyroxene alteration averages 50%–60%, while that of olivine is 80%. Olivine alteration in this interval produced only trace amounts of magnetite. Very highly to completely altered harzburgite and dunite from deeper in Hole 1274A show noticeable amounts of magnetite, commonly in diffuse magnetite-serpentine veinlike networks that appear to trace former olivine grain boundaries and cracks (mesh-rim texture). Qualitatively, the downhole increase in the proportion of magnetite observed in thin sections appears to correlate with an increase in magnetic susceptibility downhole (see "Paleomagnetism").
X-ray diffraction (XRD) results indicate that the secondary mineral assemblage is dominated by serpentine, minor brucite, and magnetite (Table T2). Brucite appears more abundant in dunites (e.g., Sample 209-1274A-20R-1, 141–144 cm) (Fig. F27), whereas in harzburgites, brucite may be absent or represent only a trace component (e.g., Sample 17R-2, 16–19 cm) (Fig. F28).
The ultramafic rocks from Hole 1274A show an interesting variety of microtextures that involve relict orthopyroxene and olivine as well as the alteration products serpentine, magnetite, and brucite. Specifically, three different types of mesh textures are developed: (1) mesh rims of serpentine and magnetite, (2) relict kernels of olivine surrounded by serpentine and magnetite and/or brucite (Fig. F29A, F29B), and (3) brucite kernels surrounded by serpentine and a trace of magnetite (Fig. F29C). Individual thin sections commonly show various combinations of these types of mesh texture. Unlike the fibrous brucite in Holes 1271A and 1271B, which could be clearly distinguished from fibrous serpentine based on its optical orientation (length-fast), apparent fibers of brucite in thin sections of serpentinites from Hole 1274A are length-slow. Similar brown kernels were observed in mesh-textured serpentinites from Site 920 and were identified by electron microprobe analyses as finely intergrown brucite and serpentine (Dilek et al., 1997). The optical orientation of these aggregates suggests that the serpentine is lizardite. Direct replacement of olivine by lizardite and brucite (magnetite) is common during serpentinization at relatively low temperatures of ~200°C (e.g., Sanford, 1981; O'Hanley, 1996). Nonpseudomorphic ribbon textures are also very common in Hole 1274A, and both mesh and ribbon textures are commonly developed in single thin sections. Ribbon textures consisting dominantly of chrysotile (apparent fibers are length-slow) replace mesh textures, in particular in sections of the hole where paragranular chrysotile veins are abundant. Ribbon textures and paragranular chrysotile veins locally impose a foliated fabric on the rock.
Systematic downhole variations in the degree of serpentinization of ultramafic rocks were observed in Hole 1274A (Fig. F30A, F30B). In the uppermost 100 m of the hole, the degree of alteration increases from 60%–70% to >95%. Between 60 and 95 mbsf, alteration intensities vary between 70% and 100%. Below a prominent fault gouge zone in Section 209-1274A-18R-2 at ~95 mbsf, alteration intensity is generally very high to complete (>95%). In the lowermost 2 m recovered from Hole 1274A, alteration intensities drop again slightly to values between 85% and 95%. These variations in the estimated degrees of alteration are mirrored by changes in rock density, thermal conductivity, compressional velocity, and porosity (see Figs. F55, F30A, F30B).
The uppermost 90 m of Hole 1274A is affected by oxidative seawater alteration forming prominent centimeter-wide reddish to brownish halos along carbonate veins (Fig. F31), which account in some intervals for as much as 15 vol% of the hole (Fig. F30C). This type of alteration is commonly restricted to pieces with white carbonate veins, although these are not always preserved and may have been broken off the margins of individual pieces during drilling.
The fault gouges of Sections 209-1274A-22R-1 to 26R-1 are composed mainly of serpentine and minor magnetite. Two different varieties, one greenish and the other grayish black, can be distinguished. XRD analyses of two fault gouge samples from Section 209-1274A-23R-2 (Table T2) identified serpentine and magnetite in the greenish variety of interval 23R-2, 0–3 cm (Fig. F32A, F32B) and serpentine, magnetite, and nontronite in the grayish black variety of interval 23R-2, 67–70 cm (Fig. F32C, F32D). Intervals of gravel within these fault gouges include centimeter-sized fragments of white asbestiform chrysotile (e.g., Sample 209-1274A-18R-2, 6–13 cm) (Table T2).
An oxide gabbro in Section 209-1274A-11R-1 is moderately altered to green amphibole, chlorite, prehnite, talc, and serpentine. Fibrous green amphibole is the most abundant secondary mineral, replacing plagioclase, clinopyroxene, and orthopyroxene. Minor replacement of plagioclase by blocky, bowtie-structured prehnite is patchy (Fig. F33A). Olivine is 50% altered to serpentine.
Gabbros in the lowermost 70 m of Hole 1274A are completely altered and show variable degrees of rodingitization (e.g., see hand specimen shown in Fig. F33B), which is most pronounced at the contacts between gabbro and harzburgite/dunite. Chlorite and amphibole are the most abundant secondary phases and replace plagioclase and pyroxenes. Fe-Ti oxides are replaced by magnetite and titanite, although exsolution/oxidation lamellae of ilmenite are usually preserved. Plagioclase is variably altered to calcium silicates, including prehnite, zoisite (Fig. F33C), hydrogrossular andradite (Fig. F33D), and vesuvianite.
In Hole 1274A metamorphic veins account for 1.8 vol% of the recovered drill core (Table T3). Figure F34 shows the distribution of mineral phases identified in the veins by macroscopic observation. As at Site 1272, serpentine minerals (87.2%) dominate the mineralogy of veins in Hole 1274A. The relative proportions of chrysotile and picrolite vary downhole (Fig. F35A). Picrolite veins are rare in the uppermost 40 m of Hole 1274A. Black picrolite-magnetite veins are more abundant than chrysotile veins between 40 and 55 mbsf and in the lowermost 70 m of the hole (Fig. F35A, F35C). Paragranular chrysotile veins are most abundant between 25 and 35 mbsf and from 57 to 70 mbsf (Fig. F35B). Carbonates, mainly aragonite, account for 11.3% of the veins. These veins are present in variable abundance in the uppermost 90 m of Hole 1274A, with a maximum at ~60 mbsf (Fig. F35C). Minor phases in metamorphic veins are magnetite (1.0%) as well as iron oxyhydroxide, clay, and sulfide (combined = 0.4%) (Fig. F34).
Two major serpentine vein generations, (1) black serpentine-magnetite veins and (2) white cross-fiber chrysotile veins, are present in varying proportions throughout Hole 1274A. Black serpentine-magnetite veins are clearly the earliest generation and form either diffuse vein networks or sets of subparallel, transgranular veins. White, thin chrysotile veins crosscut the black serpentine-magnetite veins. In some intervals (e.g., interval 209-1274A-15R-1, 8–17 cm), two different chrysotile vein types are present: (1) paragranular, nonbranched, sigmoidal veins and (2) transgranular, irregular, thin, occasionally branched veins. Both transgranular and paragranular serpentine veins occur throughout Hole 1274A. Locally present, green, massive picrolite and white and green composite picrolite-chrysotile veins cut black serpentine-magnetite and are cut by chrysotile veins.
Gray to white carbonate veins are present in the upper 90 m of Hole 1274A (Figs. F31, F35C). They are mainly composed of aragonite and contain minor clays and iron oxyhydroxides. These veins have centimeter-wide orange to brownish halos in the serpentinized harzburgites and dunites, which are, in the uppermost 40 m, strictly associated with aragonite veins. The aragonite veins and associated oxidized halos represent a late stage in the metamorphic evolution of the crust at Site 1274 and can be related to circulation of oxygenated seawater at low temperatures.
The short intervals of gabbro in Hole 1274 host only minor green picrolite veins.
The variations in the degree of serpentinization of ultramafic rocks provide the opportunity to examine possible reaction paths of serpentinization at Site 1274. Where serpentinization is incomplete (as low as 60%), magnetite is rare, suggesting that significant magnetite formation does not begin until >60% of the primary rock is serpentinized. This observation is consistent with the sequence of serpentinization reactions proposed by Toft et al. (1990) to explain density–magnetic susceptibility relationships in variably serpentinized harzburgites from ophiolites. These authors suggested that initial serpentinization produces iron-rich serpentine and iron-rich brucite and that magnetite forms during recrystallization of these phases as serpentinization proceeds.
Temperature-pressure stabilities of serpentine phases are not well constrained. Thermodynamic calculations predict that antigorite is the stable serpentine phase >200°–300°C and pressures <2 kbar and that chrysotile and lizardite form from antigorite during low-temperature recrystallization (e.g., Evans, 1977; Dilek et al., 1997). Consistent with this view, an early oxygen isotope study of serpentinites suggests formation of lizardite and chrysotile at temperatures <235°C (Wenner and Taylor, 1971). However, more recent stable isotope investigations (Agrinier et al., 1995; Früh-Green et al., 1996; Agrinier and Cannat, 1997), field studies (O'Hanley and Wicks, 1995; Wicks and Whittacker, 1977), and hydrothermal experiments (Normand et al., 2002; Janecky and Seyfried, 1986; Allen and Seyfried, 2003) indicate that lizardite and chrysotile form directly from olivine at temperatures well above 300°C. The phase relations proposed by Evans (1977) assume that P(H2O) = Ptotal. However, Sanford (1981) and O'Hanley (1996) demonstrated that the lizardite + brucite stability field extends toward higher temperatures if P(H2O) < Ptotal. Whereas P(H2O) = Ptotal is expected in prograde metamorphism, during retrograde serpentinization of oceanic peridotites P(H2O) is likely to be less than Ptotal because H2O is consumed, H2 and CH4 are produced, and the fluid may be under hydrostatic rather than lithostatic pressure. Moreover, the nucleation energy of (sheetlike) lizardite after olivine is lower than that of (needlelike) antigorite because of a larger interfacial (olivine-lizardite) surface area (Normand et al., 2002). Lizardite may hence form at lower levels of supersaturation than antigorite (and chrysotile), and metastable replacement of olivine by lizardite may be kinetically more favorable than the formation of antigorite, even if formation of the latter is predicted thermodynamically.
In Hole 1274A alteration of clinopyroxene to talc and tremolite is consistent with results of hydrothermal reaction experiments at 400°C and 500 bar (Allen and Seyfried, 2003):
3 Mg3SiO5(OH)4
These authors also suggest that pyroxenes react with aqueous fluids faster than olivine does at temperatures >300°C. Breakdown of pyroxene and the associated release of Ca2+ (see equation above) to the interacting hydrothermal fluids may be related to the partial rodingitization of the gabbroic units in Hole 1274A. Rodingitization probably takes place at temperatures between 300° and 400°C (e.g., Bideau et al., 1991; Dilek et al., 1997), which coincides with the temperature interval at which pyroxene breakdown is expected to proceed rapidly.
As discussed in "Metamorphic Petrology" in the "Site 1271" chapter, the formation of brucite is not expected to be associated with the breakdown of pyroxene because the high silica activity of fluids reacting with pyroxene would prevent brucite formation and would cause previously formed brucite to react to serpentine according to the reaction:
The presence of both brucite and partially serpentinized pyroxene may indicate local disequilibria during the main stage of serpentinization. However, it is more likely that the formation of serpentine + brucite + magnetite after olivine took place at a later stage, at temperatures <250°C. In contrast to the main stage of serpentinization, magnetite that accompanies serpentine and brucite appears to form directly from olivine breakdown. The following reaction can possibly account for this observation:
The presence of nontronite in serpentine muds indicates that water-rock reactions continued at low temperatures and under oxidizing conditions. This is also suggested by the development of aragonite veins with oxidation halos in the uppermost 90 m of Hole 1274A. The aragonite veins disappear abruptly below the first fault gouge at 95 mbsf. Either the fault gouge represents a hydrogeological barrier that prevents cold seawater from penetrating deeper into the basement or the fault zone accommodates the strain so that fracturing and circulation of cold seawater is limited to the hanging wall of the gouge.
