Site 1268 was located along the track of the Shinkai 6500 submersible Dive 427 from 1998, which recovered samples of peridotite, pyroxenite, and gabbro from a series of steep outcrops separated by flat, sedimented seafloor along the western flank of the rift valley. The specific drill site was in flat terrain upslope and west of a steep scarp from which samples of dunite and peridotite with a gabbroic dike or vein were recovered during the dive survey. Recovery averaged ~53%, with 79% recovery from Cores 209-1268A-12R through 20R.
The igneous and residual mantle protoliths of recovered core were harzburgite, dunite, gabbro, gabbronorite, and minor pyroxenite (Fig. F7). Gabbro, gabbronorite, and pyroxenite are intrusive into the peridotite and become increasingly common toward the bottom of the hole. About 63% of the Hole 1268A core is composed of harzburgite, 11% is dunite, and gabbroic rocks compose ~26%. Strikingly, the proportion of peridotite (harzburgite + dunite) to gabbroic rocks is the same as the proportion of gabbro to peridotite (6 peridotites/2 gabbros) collected during Shinkai Dive 427, and the proportion of peridotite to gabbro in all dredge and dive samples along the Mid-Atlantic Ridge south of the 15°20' Fracture Zone to 14°40'N (~77 peridotites/~25 gabbros, based on the compilation presented in the Leg 209 Scientific Prospectus, data in Fujiwara et al., 2003, and references cited therein).
In some regions, the seismic structure of "crust" beneath slow-spreading ridges exhibits a P-wave velocity gradient from values <4 km/s in the upper kilometer to values near 8 km/s at 7 km depth (e.g., Canales et al., 2000; Detrick and Purdy, 1980; Detrick et al., 1993). This is in contrast to nearly constant lower crustal P-wave velocities of ~7 km/s with a sharp increase to ~8 km/s at 7 km depth in seismic profiles from the Pacific (e.g., Vera et al., 1990; compilation by White et al., 1992; see also Fig. F4E). The velocity gradient beneath some slow-spreading ridges is often interpreted to indicate that the "crust" is composed mainly of serpentinized mantle peridotite with proportions of serpentine and/or open cracks decreasing from the seafloor to ~7 km depth. However, the P-wave velocities we measured on altered gabbros from Hole 1268A during Leg 209 range from 3.7 to 5 km/s, very similar to those for serpentinite. Thus, gabbroic intrusions could compose 25% or more of the seismic crust in this region.
It is interesting to speculate on the depth of emplacement of the gabbroic rocks into peridotite at Site 1268. Abundant gabbroic intrusions into peridotite have been sampled by dredging along the Mid-Atlantic Ridge, in particular in the 14° to 16°N area (e.g., Cannat, 1996; Cannat et al., 1992, 1997a, 1997b; Cannat and Casey, 1995). Some of these record high-temperature plastic deformation (~600°C or more), leading Cannat and coworkers to propose that some intrusions are emplaced, solidified, and cooled at depths of 15 km or more. Our onboard compilation of P-wave velocities for the "upper mantle" in oceanic plates at depths of 7 km or more shows a large range (from 7.6 to >8.4 km/s) (Fig. F8), even for near-axis regions.
The observed range of P-wave velocities in oceanic plates at depths of 7 km or more may arise, in part, from differences in temperature and degree of alteration as well as from variable experimental methods and uncertainties in the data. However, the range of velocities could also be due in part to the presence of variable proportions of gabbroic intrusions within mantle peridotite. If unaltered, crack-free, cold, olivine-rich peridotites in the shallow mantle beneath slow-spreading ridges have P-wave velocities of 8.1 km/s and gabbroic rocks under the same conditions have P-wave velocities of ~7.2–7.3 km/s (e.g., Behn and Kelemen, in press; Hacker et al., 2003; Korenaga et al., 2000, 2002), then the presence of 25% gabbro within 75% residual mantle harzburgite at depths of 7–20 km would yield a P-wave velocity of 7.9 km/s, well within the observational range of "mantle" velocities beneath oceanic crust in general and slow-spreading mid-ocean ridges in particular.
With very few exceptions, peridotites from Site 1268 are 100% altered to a mixture of serpentine and talc (Figs. F9, F10). However, geochemical analyses of these peridotites show exceptionally low concentrations of nominally "immobile" incompatible elements such as Al, Sc, and V. For example, Al2O3 concentrations in Site 1268 peridotites are <1 wt% (average = 0.6 wt%), whereas Al2O3 in peridotites from Site 920 along the Mid-Atlantic Ridge at 23°20'N ranges from 1 to 2 wt% (Casey, 1997). If these values have not been modified by hydrothermal metasomatism, then the peridotites from Site 1268 are among the most depleted residual mantle peridotites yet obtained from the mid-ocean ridges.
Molar MgO/(MgO + FeOtotal), or Mg#, of the peridotites varies from ~88% to 94%. This includes the normal range of whole-rock Mg# in residual mantle peridotites from mid-ocean ridges (~89%–92%; average = ~90%–91%) (e.g., Dick, 1989), but values <89% and >92% may be indicative of metasomatic changes in Fe/Mg during hydrothermal alteration.
Most of the gabbroic rocks, in two thick sequences toward the bottom of the hole, were probably gabbronorites, based on the presence of relict clinopyroxene together with texturally distinct, altered orthopyroxene. (Orthopyroxene is completely replaced by talc in all samples). Because the pyroxenes have undergone substantial alteration and metasomatism, interpretation of their whole-rock compositions in terms of igneous petrogenesis is highly uncertain. Nevertheless, some aspects of their bulk composition are intriguing from an igneous perspective.
The gabbronorites from Hole 1268A are unusually primitive. Molar Mg# is 75%–83% in the large gabbbronorite bodies near the base of the hole. In contrast, gabbronorites from East Pacific Rise, Mid-Atlantic Ridge, and Southwest Indian Ridge crust at ODP Sites 735, 894, and 923,respectively, are comparatively evolved, with whole-rock and orthopyroxene Mg# < 72% (e.g., Cannat et al., 1997a; Casey, 1997; Dick et al., 1991, 2002; Natland and Dick, 1996). Such evolved crustal gabbronorites must form by nearly complete crystallization of orthopyroxene-normative basalts because MORB lavas very rarely contain orthopyroxene crystals and phase equilibria confirm that primitive and normal MORB are not saturated in orthopyroxene at crustal pressures.
Primitive gabbbronorites are found in the crustal section of the northern massifs in the Oman ophiolite. For example, the lower crustal section of the Fizh massif has abundant orthopyroxene-rich, primitive cumulates, including pyroxenites (websterites) as well as gabbronorites (orthopyroxene Mg# = 80%–90%) (e.g., Smewing, 1981). We interpret the presence of high-Mg# orthopyroxene in the plutonic section of the northern Oman massifs to indicate an important role for crystallization of primitive andesite, with a higher SiO2 content than MORB, at crustal depths in these massifs.
In the southern Oman ophiolite massifs (Samail and Wadi Tayin), orthopyroxene is essentially absent from the lower crustal section (e.g., Pallister and Hopson, 1981), but undeformed, primitive orthopyroxene-bearing cumulates (Mg# = 80%–93%) form small, isolated intrusions and dikes in the mantle section (e.g., Amri et al., 1996; Benoit et al., 1996; Kelemen et al., 1997b), cutting ductile fabrics in surrounding peridotite. These gabbronorite and websterite intrusions into the mantle section have been attributed to crystallization of late, hydrous, incompatible element–depleted, primitive andesite magmas in the uppermost mantle. Such magmas could have formed by partial melting of depleted, serpentinized, shallow mantle peridotite at a mid-ocean ridge (Benoit et al., 1996) or by fluid-fluxed melting above a subduction zone during the initial stages of ophiolite obduction (Kelemen et al., 1997b). These melts must have been rare compared to the MORB-like basalts that formed crustal gabbros, dikes, and lavas in the southern Oman ophiolite massifs.
If the relatively high Mg#s of the Hole 1268A gabbronorites are indeed representative of their primary igneous composition, they would be distinct from evolved gabbronorites previously recovered from mid-ocean ridges and relatively similar to primitive gabbronorites that intrude the mantle section of the Oman ophiolite. Thus, it is possible that primitive gabbronorite cumulates crystallized from depleted hydrous magmas formed by hydrous partial melting of shallow, already depleted, hydrothermally altered peridotite, as has been proposed for the Oman gabbronorites (Benoit et al., 1996). If this occurred, it would give rise to relatively SiO2-rich melts. Indeed, primitive glasses (Mg# > 60%) from the 14° to 16°N region of the Mid-Atlantic Ridge (Melson et al., 1977; C. Xia et al., unpubl. data) extend to >52 wt% SiO2. These are among the most SiO2-rich primitive glasses that have been recovered from the mid-ocean ridges (Fig. F11).
An alternative hypothesis is that the relatively primitive gabbronorites from Hole 1268A crystallized at depths corresponding to pressures of 0.4 to 1 GPa, where orthopyroxene forms relatively early during crystallization of primitive MORB. This idea is consistent with the hypothesis that some Mid-Atlantic Ridge lava suites lie along a liquid line of descent, indicative of crystal fractionation at 0.4–0.8 GPa (Grove et al., 1992; Michael and Chase, 1987; Meurer et al., 2001). C. Xia et al. (unpubl. data) showed that lavas from the Mid-Atlantic Ridge from 14°N to the 15°20' Fracture Zone fall along a liquid line of descent for crystallization of MORB at ~0.6 GPa. Thus, magmas beneath the Mid-Atlantic Ridge in this region begin to conductively cool and crystallize at a depth of ~20 km. This inference is consistent with slow magma transport into the base of a conductive boundary layer with a thickness of 20 km, as predicted by thermal models of slow-spreading ridges (e.g., Braun et al., 2000; Reid and Jackson, 1981; Shen and Forsyth, 1995; Sleep, 1975). However, by itself, crystal fractionation at 0.6 GPa would not explain the relatively SiO2-rich nature of some lavas from this region.
An intermediate hypothesis is that magmas entering a thick thermal boundary layer begin to cool, crystallize, and interact with surrounding mantle peridotite at 0.4–1 GPa in a process of "combined assimilation and fractional crystallization" (DePaolo, 1981; Gaetani et al., 1995; Kelemen, 1986, 1990; Kelemen et al., 1997a, 1997b). This process would give rise to relatively incompatible element–enriched, primitive basaltic andesites or andesites that were saturated in orthopyroxene at low to moderate pressure. In "Site 1275," we calculate olivine–orthopyroxene–clinopyroxene–plagioclase saturation pressures for lavas from 14°14' to 16°N in order to constrain the likely pressures of gabbronorite crystallization in this region. For primitive lavas (Mg# > 60%, in equilibrium with pyroxene having an Mg# > 75%), saturation pressures range from ~0.4 to ~0.9 GPa (average = 0.54 GPa).
To summarize, if they record their igneous Mg#s, the Hole 1268A gabbronorites are the most primitive gabbronorites yet recovered from a mid-ocean ridge. As such, they probably record cooling and partial crystallization of ascending MORB near the base of the thermal boundary layer beneath the Mid-Atlantic Ridge (perhaps at 15–25 km), followed by tectonic uplift and unroofing at the seafloor together with host peridotites. This is an exciting hypothesis. However, since these rocks have undergone compositional changes during hydrothermal alteration, it may be that their Mg#s increased during metasomatism and are not representative of the original igneous values. Postcruise, electron probe analyses of Mg# in unaltered clinopyroxene will provide additional constraints on the igneous composition of these rocks.
Hydrous alteration of peridotites to serpentine and talc is substantial in most peridotites from Hole 1268A. In a very unusual paragenesis, talc replaces serpentine in a distinct metamorphic event that occurred after complete serpentinization (Fig. F12). Serpentinization and later replacement of serpentine by talc occurred at temperatures <400°C under static conditions in most of the core. However, there are a few cataclastic shear zones with syn- and post-kinematic talc and serpentine. Talc alteration is most pronounced near contacts with gabbroic intrusions. Gabbros were also altered under static conditions, with pyroxenes pseudomorphed mainly by talc and chlorite and plagioclase pseudomorphed mainly by chlorite and quartz. Some gabbros show 100% replacement of pyroxene by talc pseudomorphs. Fresh plagioclase and clinopyroxene are increasingly well preserved toward the bottom of the hole.
In addition to the unusual replacement of serpentine by talc, the alteration of Hole 1268A peridotites and gabbronorites was accompanied by remarkably extensive metasomatism. Talc-altered peridotites with pseudomorphic textures that unambiguously indicate a harzburgite protolith have whole-rock compositions corresponding to the stoichiometric composition of talc itself (Fig. F13), indicating large amounts of SiO2 gain and/or MgO + FeO loss. This compositional change is similar to but more extensive than metasomatism of peridotites during seafloor weathering documented by Snow and Dick (1995). In fact, the normative orthopyroxene contents of all the altered peridotites in Hole 1268A, including talc-free serpentinites, are generally >30 wt%, in contrast to the 20–25 wt% generally observed in fresher residual mantle harzburgites (e.g., Cannat et al., 1995; Dick, 1989; Dick et al., 1984). Therefore, we infer that even serpentinites without talc alteration have undergone SiO2 addition or MgO + FeO loss. Transforming a harzburgite with an Mg# of 90% and 20 wt% orthopyroxene (SiO2 = ~44 wt%) to a rock with the composition of talc (SiO2 = ~66 wt%) by SiO2 addition requires adding ~40 wt% SiO2 to the original rock mass. Performing the same transformation by removing MgO and FeO from the harzburgite (MgO = ~46 wt% and FeO = ~9 wt%) to form talc with the same Mg# requires removal of 30 wt% of the MgO and FeO from the original rock, corresponding to removal of 16% of the original rock mass. This is in addition to possible removal of Al2O3, Cr2O3, CaO, Na2O, and other oxides, which together total 0.4–3 wt% of the altered peridotites from Hole 1268A (average = 1.2 wt%). Conversion of a multicomponent polymineralic rock to a monomineralic rock during metasomatism is typical of "blackwall" reaction zones composed of pure talc and pure chlorite that form between peridotite and relatively silicic wallrocks in metamorphic belts (e.g., Chidester, 1962; Hanford, 1982; Thompson, 1959).
Whole-rock compositions of altered gabbronorites also show dramatic metasomatic effects, with low CaO and high MgO + FeO in the most altered rocks (Fig. F14). The compositions of the most altered rocks approach those of talc + chorite + quartz mixtures. These minerals are abundant in the alteration assemblage. Again, the style of metasomatism and the mineral assemblages involved recall the formation of metasomatic "blackwall" between peridotite and felsic wallrock. In settings other than blackwall reaction zones, relative CaO loss from gabbroic rocks in contact with serpentinized peridotites is unusual, as serpentinization of host peridotite produces Ca-rich fluids that commonly react with gabbroic rocks to form metasomatic calc-silicate assemblages known as rodingites. However, in Hole 1268A, both peridotites and gabbronorites apparently lost CaO. Although we hypothesize that hydrothermal alteration of the gabbros produced the fluids that added SiO2 to the talc-altered peridotites, there is no obvious sign of SiO2 loss from the altered gabbronorites.
As discussed in more detail in "Structural Geology" below, it is important to add that hydrothermal alteration and metasomatism of both peridotite and gabbro occurred at low temperature (<400°C) after intrusion of gabbro into relatively hot peridotite (>600°C) and after ductile deformation of both gabbro and peridotite. Thus, although the gabbroic rocks may have acted as the source of SiO2 during silica enrichment of peridotites, the intrusion of the gabbro did not cause the hydrothermal alteration and metasomatism of Site 1268 peridotites.
Intrusion of gabbroic rocks into peridotite probably occurred while the peridotite was at high temperature (>600°C). This inference is based on two observations, neither of which is definitive. First, the presence of coarse-grained gabbronorite crosscutting peridotite in a contact preserved in Section 209-1268A-21R-1 (Fig. F15) suggests but does not prove that the contact was at high temperature during intrusion. Second, mylonite zones are present at depths of 15, ~48–53, 65, 76–79, and 88–89 meters below seafloor (mbsf). The largest two are within zones of abundant millimeter- to centimeter-scale gabbroic veins or dikes in peridotite, termed magmatic breccias. The mylonites record crystal-plastic deformation under granulite facies conditions at temperatures >600°C and probably >900°C.
Deformation intensity in both gabbro and peridotite varied substantially over length scales of <1 m throughout the hole, with protogranular, porphyroclastic, and mylonitic textures in peridotites and undeformed to mylonitic textures in gabbroic rocks. Strain was concentrated in and around the mylonite zones discussed in the previous paragraph and thus occurred mainly in zones of mixed gabbroic intrusions and peridotite.
Brittle deformation features are also common throughout the core. Cataclastic shear zones cutting peridotite and intrusive rocks are present at several depths. Fault gouge zones were recovered in core at depths at ~60, 65, and 80 mbsf. That two of the fault gouge zones are immediately beneath the mylonitic zones discussed in the previous two paragraphs is probably indicative of continued localization of deformation in these zones over a wide range of temperatures. Some brittle faults and shear zones show syn-kinematic serpentine and talc mineralization, demonstrating that deformation continued to temperatures less than ~300°C.
We used crosscutting relationships to reconstruct the following sequence of events for the core from Hole 1268A:
Paleomagnetic data were collected on half cores and individual discrete samples. Some samples retained a stable remanent magnetization. Using these data, we rotated the measured orientations of foliations, faults, veins, and dikes in individual core pieces around a vertical axis, thereby restoring core pieces to a common orientation (assuming the remanent magnetization vector in all core pieces had a common azimuth prior to drilling). Rotation of brittle shear zones, faults, and cracks into a common magnetization orientation produces a systematic girdle of poles. Furthermore, rotation of crystal-plastic foliations into the same common magnetization orientation produces a fairly tight cluster of poles.
The average remanent inclination for 11 discrete samples from the gabbros is 15° (95% confidence interval [CI] = +7°/–8°), significantly shallower than the expected inclination of 28°. The average inclination of the peridotites calculated from 38 discrete samples is 40° (95% CI = +5°/–9°), significantly steeper than the expected value of 28°. The most reliable peridotite magnetizations (9 high-coercivity talc-altered samples) have an average inclination of 36° (95% CI = +12°/–14°).
One possible explanation for the statistically significant difference between the expected and observed inclinations in the gabbroic rocks is that they cooled quickly through their dominant unblocking temperature range (~500°–570°C) and therefore do not average secular variation. If so, they may record an inclination that does not correspond to that expected from a geocentric axial dipole. However, the observed range of inclinations in discrete samples (6°–36°) is larger than would be expected solely from orientation errors. Also, as explained above, geologic evidence (high-temperature contacts between gabbro and peridotite and high-temperature ductile fabrics cutting some gabbros) suggests that wallrocks were at temperatures greater than ~600°C during gabbroic intrusion, higher than the upper thermal stability limits for serpentine and talc at lithospheric pressures. Thermal modeling and inferences from igneous petrology both suggest that the conductive boundary layer beneath slow-spreading ridges in general, and the 14° to 16°N region in particular, is ~20 km thick. Therefore, cooling of the gabbroic rocks through the Curie temperature by conduction coupled with tectonic uplift to the seafloor was probably slow. For these reasons, we infer that the magnetization directions from the gabbros likely represent some degree of time averaging.
A second possible explanation for the statistically significant difference between the expected and observed inclinations in the gabbroic rocks is that tilting of the gabbroic rocks occurred after cooling and blocking of the remanence. If so, the magnetization of the peridotites must have occurred later, during alteration that postdated some of this tilting. In the following paragraphs we outline the quantitative implications of this hypothesis. It should be emphasized, however, that this is simply a forward model.
With additional assumptions, paleomagnetic inclination data may be used to infer the direction and magnitude of tectonic rotations that may have occurred between the time the sample recorded remanent magnetization and the present. Restoration of the core to its pretilt attitude has no unique solution. Unless other independent information on core orientation is available, the uncertain azimuth of cores precludes an estimation of the strike and plunge of the rotation axis along which tilting was produced. If the rotation axis is assumed to be horizontal, the azimuth of this axis dictates the amount of rotation required to rotate an original inclination of 28° to produce an observed inclination of 15°. Figure F16 illustrates this point. If a horizontal rotation axis strikes 000°, the required rotation is ~60°. Alternatively, if a horizontal rotation axis strikes 270°, the required rotation is only 13°.
Some constraints can be derived from seafloor topography and inferences about the processes that produce it. The stepwise alternation of steep, east-facing slopes and nearly horizontal benches along the western flank of the Mid-Atlantic Ridge is generally interpreted to result from the presence of numerous tilted normal fault blocks. Back-tilting of fault blocks on the west side of the rift valley in the region around Site 1268 is probably top-to-the-west (counterclockwise around a northeast-striking axis). In this region, the east-facing slopes strike ~020°, parallel to the strike of the rift valley. We infer that tilting was produced by counterclockwise rotation of normal fault blocks along an approximately rift-parallel axis. For a horizontal rotation axis striking 020°, ~90° of counterclockwise rotation is required to change an original inclination of 28° to the observed average inclination of 15° in gabbros from Hole 1268A.
The differences between magnetic inclination in the gabbros (~15°) and the peridotites (~36°) require that the two lithologies record different amounts of rotation. The inclinations in the peridotite are not statistically different from the expected inclination of 28° at 95% CI. Thus, one option is that gabbroic rocks were rotated prior to magnetization of the peridotites.
However, the average inclinations are 40° for all the peridotites and 36° for the more reliable data from talc-altered peridotites. The relatively steep average inclinations in the peridotites can be explained as the result of smaller counterclockwise rotations around a horizontal axis striking 020° because small counterclockwise rotations around this axis produce a steepening of the magnetic inclination (Fig. F16). This hypothesis is consistent with relatively early magnetization of the gabbroic rocks during cooling through their discrete blocking temperature range (~500°–570°C) and later magnetization of the peridotites during magnetite crystallization accompanying serpentinization at ~300°C. In this scenario, counterclockwise rotation of the Site 1268 section around a horizontal axis striking 020° began before magnetization of the gabbroic rocks and continued after magnetization of the peridotites [N2].
An additional uncertainty in inferring the amount of tilting is introduced by the unknown plunge of the rotation axis. Figure F17 illustrates this problem. For a rotation axis with a fixed azimuth of 020°, if the plunge were 10° along 020°, ~120° of counterclockwise rotation would be required to change an original inclination of 28° to the observed average inclination of the gabbroic rocks of 15°. Conversely, if the plunge were 10° along 200°, ~80° of counterclockwise rotation would be required for the same change in inclination [N3].
In summary, if the hypothesis outlined here is correct, the gabbroic rocks in Hole 1268A underwent a large amount of counterclockwise rotation (~60°–90°) around a nearly horizontal, rift-parallel axis after acquisition of the magnetic remanence. The magnetic remanence in the gabbroic rocks was probably acquired when they passed through their dominant blocking temperature at >500°C. The peridotites record only ~30° of this rotation.
This is a potentially important result because it is consistent with the independent hypothesis that the large fault planes forming the top surfaces of oceanic core complexes rotate from steep normal faults at depth to a subhorizontal attitude as they reach the seafloor. However, the rotations we infer from the paleomagnetic data are larger than the rotations predicted in most models of oceanic core complex formation. Finally, it must be emphasized again that this is a nonunique forward model, one of many which could potentially account for the observed paleomagnetic data, and that the model results are subject to the numerous uncertainties outlined in the previous paragraphs.
One sample of metaperidotite was taken from interval 209-1268A-2R-1, 38–47 cm, to characterize the microbial community inhabiting this environment. Aliquots of surface and bottom water were also sampled and prepared for deoxyribonucleic acid (DNA) analysis. Results of cultures and DNA analysis of rock and water samples will not be available for this report. Direct counts of microorganisms were performed on the seawater samples. Atmospheric dust and air samples were obtained, and both bacterial and viral growth were measured in these samples.