SITE SUMMARIES

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.

Proportions of Igneous Rocks

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 Fig. F1, 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. F5E). 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 in altered gabbroic rocks 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°–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.

Highly Depleted Mantle Peridotites

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) and reported Al₂O₃ in abyssal peridotites worldwide has a median value of 1.4 wt% (Bodinier and Godard, in press). 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 + FeO_total), 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.

Petrogenesis of Intrusive Gabbronorites

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 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°–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). Nevertheless, these primitive glasses are still too poor in SiO₂ to form orthopyroxene at pressures <0.4 GPa (see the extensive discussion in "Petrogenesis of Gabbroic Rocks and Diabase" in "Site 1275").

An alternative hypothesis is that the primitive gabbronorites from Hole 1268A crystallized at depths corresponding to pressures of 0.4–1 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). Specifically, 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 cool conductively 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 saturated in orthopyroxene at low to moderate pressure. In "Petrogenesis of Gabbroic Rocks and Diabase" 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 = 5.4 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.

Hydrothermal Alteration, Metamorphism, and Metasomatism

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 postkinematic 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. Note that in order to retain a high Mg#, close to the original Mg# in residual peridotite, requires removal of Mg and Fe in identical proportions. Because this is unlikely, we prefer the hypothesis that the rocks have undergone SiO₂ addition rather than MgO + FeO removal. The addition of SiO₂ or loss of MgO + FeO 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.

Structural Geology

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 intrusion 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 synkinematic 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:

1. Partial melting of mantle peridotite together with formation of dunite bands and protogranular textures in residual harzburgite;
2. Cooling of peridotite at the base of the thermal boundary layer;
3. Intrusion of gabbroic veins and gabbronorite bodies while peridotites were still at high temperature;
4. Formation of high-temperature mylonite zones with associated ductile deformation in surrounding gabbroic and ultramafic rocks while temperatures were >600°C and probably >900°C [N1];
5. Cooling of peridotites and gabbroic rocks to <400°C;
6. Static, pervasive, nearly complete serpentinization of peridotites, perhaps with continued deformation in highly localized shear zones;
7. Static replacement of serpentine in peridotites and pyroxenes in gabbronorites by talc, with associated alteration of plagioclase to chlorite + quartz; and
8. Continued brittle deformation.

Paleomagnetic Data and Tectonics

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) (see "Structures in Peridotite and Gabbroic Intrusions" in "Mantle Upwelling, Melt Transport, and Igneous Crustal Accretion"). 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; see "Paleomagnetism" in the "Site 1268" chapter) 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 inclinations that do 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 and in "Metamorphic Petrology" and "Structural Geology" in the "Site 1268" chapter, 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°–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, 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, predicated on the assumptions outlined in "Structures in Peridotite and Gabbroic Intrusions" in "Mantle Upwelling, Melt Transport, and Igneous Crustal Accretion" for rotation into a geographical reference frame.

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 could 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°–120°) 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 account for the observed paleomagnetic data, and that the model results are subject to the numerous uncertainties outlined in the previous paragraphs.

Microbiology

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.

Although serpentinized peridotite and gabbro were sampled from nearby outcrops during precruise submersible dives, the only rocks recovered from three different hole locations at Site 1269 are basalts. Since the outcrops were too steep to be viable drilling targets, we chose to attempt to drill through the flat, lightly sedimented terraces above the outcrops and into the oceanic basement below. We interpret the low recovery of only basaltic rock and poor hole conditions at this site to indicate that the terrain between outcrops exposed by faulting is covered by basalt flows or talus.

The basalts recovered from Site 1269 are fresh and range from aphyric to slightly plagioclase-olivine phyric. Glomerocrysts of plagioclase and olivine are present but rare, and acicular plagioclase laths and quench clinopyroxene with minor amounts of fresh brown glass and skeletal opaque minerals make up the groundmass. The most striking characteristic of these basalts is their high vesicularity, commonly >15 vol% (Fig. F19; also see Fig. F5, in the "Site 1269" chapter).

MORB erupted deeper than 2500 m water depth typically has <2 vol% vesicles because of the high hydrostatic pressure (Moore et al., 1977). One exception is the 14°–15°N section of the Mid-Atlantic Ridge where MORB with high volatile abundances and uncommon noble gas compositions has been recovered in dredge hauls (Staudacher et al., 1989; Sarda and Graham, 1990; Javoy and Pineau, 1991; Burnard et al., 1997; Moreira et al., 1998; Pineau et al., 1976).

Based on rare gas abundance, high CO₂, and vesicle size distribution analysis, high-vesicularity basalts previously recovered from 14° to 15°N have been interpreted to represent undegassed MORB magma (Sarda and Graham 1990). The basalts recovered from Site 1269 have the same high vesicularity, and analysis of images of the Site 1269 basalts yields similar trends in vesicle size distribution (see Fig. F6, in the "Site 1269" chapter). However, a higher abundance of larger vesicles and a lack of intermediate-sized vesicles (possibly due to coalescence) in the Site 1269 basalts compared to previous studies may represent a more mature stage in the vesicle forming process in MORB.

Site 1270 is located along the track of Shinkai 6500 submersible Dive 425 from 4 July 1998, which recovered samples of peridotite and gabbro from nearly planar, striated outcrops along the eastern flank of the Mid-Atlantic Ridge. The slope of the outcrop surfaces was nearly constant throughout most of the dive, leading to the inference that the slope of the rift valley along the dive track represents a single, kilometer-scale fault surface. All holes at Site 1270 were begun in exposed outcrop surfaces. Recovery averaged ~20%, with 37% recovery over 46 m in Hole 1270B.

Hole 1270A was begun near the site of Shinkai 6500 sample 425-R007 (Fig. F20), a mylonitic peridotite taken from a planar fault surface. Poor drilling conditions, especially the intersection of a zone of unconsolidated fault gouge at 17 mbsf, caused us to abandon this hole. Hole 1270B was situated ~100 m upslope to the east, near another striated, planar outcrop observed during the Shinkai 6500 dive. This hole had better recovery but became unstable after 46 m of penetration and had to be abandoned. Hole 1270C was sited ~200 m upslope from Hole 1270B, in an outcrop ~20 m upslope from the top of a steep, 10- to 20-m-tall cliff, near the site of Shinkai 6500 sample 425-R008, a weathered harzburgite. The outcrop on and above the cliff is more weathered than those at Holes 1270A and 1270B. Poor drilling conditions caused us to abandon this hole after ~19 m. We infer that a slope-parallel, low-angle fracture along the base of the nearby cliff might have interfered with circulation in the hole. Thus, we sited Hole 1270D in outcrop ~30 m farther upslope. Again, however, poor drilling conditions forced us to abandon this Hole after 57 m of penetration. Overall, fault gouge, altered shear zones, poor recovery, and/or anomalously rapid progress in drilling were encountered at 15–20 mbsf in each hole at Site 1270. Our impression is that the planar fault surfaces exposed on the seafloor are underlain by parallel brittle fault zones over the entire slope sampled by the Shinkai 6500 and our drilling program at this site.

Proportions of Igneous Rocks

The igneous and residual mantle protoliths of recovered core are harzburgite, dunite, gabbro, gabbronorite, and minor pyroxenite (Fig. F21). Gabbro, gabbronorite, and pyroxenite are intrusive into the peridotite in Holes 1270A, 1270C, and 1270D. Hole 1270B includes only a few small fragments of completely altered peridotite and no intrusive contacts, so it is not certain that the gabbroic rocks intrude peridotite. However, by analogy to relationships in Holes 1270A, 1270C, and 1270D and at Sites 1268, 1271, 1272, and 1274, we infer that the gabbros and gabbronorites in Hole 1270B were also intrusive into peridotite.

About 89% of the core from Hole 1270A (total depth = 27 mbsf) is composed of harzburgite, 5% is dunite, and gabbroic rocks compose ~4%, with the remainder being serpentinite and fault breccias with unknown protoliths. Core from Hole 1270B (total depth = 46 mbsf) is 98% gabbro and gabbronorite, with only 2% completely altered peridotite. Core from Hole 1270C (total depth = 19 mbsf) is 81% harzburgite, 17% dunite, 1% gabbroic dikes or veins, and 1% pyroxenite dikes or veins. Core from Hole 1270D (total depth = 57 mbsf) is composed of 91% harzburgite, 7% dunite, and 2% gabbroic dikes or veins.

The overall proportion of rock types in all the holes at Site 1270, weighted by length of recovered core, is ~41% harzburgite, 4% dunite, and 55% gabbro. Weighted by depth of penetration in each hole and using the proportions of rock types recovered in core in each hole, the proportion of lithologies is ~62% harzburgite, 6% dunite, and 32% gabbro and gabbronorite, similar to the proportion of gabbro to peridotite in Hole 1268A (63% harzburgite, 11% dunite, and 26% gabbronorite and gabbro), the proportion of lithologies (2 gabbros/6 peridotites) collected during Shinkai Dive 427, the proportion (2 gabbros/6 peridotites) collected during Shinkai Dive 425, 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 Fig. F1, Fujiwara et al., 2003, and references cited therein).

As discussed in "Proportions of Igneous Rocks" in "Site 1268" above, the data on proportions of different lithologies at Site 1270 suggest that much of the seismic crust and even parts of the uppermost mantle may be composed of ~75% peridotite and 25% gabbroic rocks, with crack density and alteration in both lithologies decreasing with depth. However, shipboard measurements of seismic P-wave velocities for the fresh gabbroic rocks from Hole 1270B range from 4.6 to 5.8 km/s. These velocities are too high for these rocks to compose a substantial proportion of the seismic crust in this region at depths from <2 to 3 km. Instead, if gabbroic rocks compose a significant proportion of the shallow crust, they must be more altered or there must be a relatively large proportion of cracks compared to the samples from Hole 1270B whose P-wave velocities were measured on board.

Highly Depleted Mantle Peridotites

With some notable exceptions, peridotites from Site 1270 are 100% altered, mainly to serpentine. However, geochemical analyses of these peridotites show exceptionally low concentrations of nominally immobile incompatible elements such as Al, Sc, and V. For example, with the exception of a peridotite sample including a gabbroic dike or vein, Al₂O₃ concentrations in Site 1270 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) and the median concentration of Al₂O₃ in abyssal peridotites worldwide is 1.4 wt% (Bodinier and Godard, in press). The average Al₂O₃ concentration in 7 peridotites from Site 1270 is the same as that determined for 20 peridotites from Site 1268. If these values have not been modified by hydrothermal metasomatism, then the peridotites from Sites 1268 and 1270 are among the most depleted residual mantle peridotites yet obtained from the mid-ocean ridges.

Molar Mg# of the peridotites from Site 1270 varies from 90.0% to 91.5%. This is a narrower range of Mg# than in Site 1268 peridotites (88%–94%) and lies within the normal range of whole-rock Mg# in residual mantle peridotites from mid-ocean ridges (~89%–92%; average = ~90%–91%) (e.g., Dick, 1989). By comparison, we infer that Mg#s <89% and >92% in Site 1268 peridotites are due to minor metasomatic changes in Fe/Mg during the extensive hydrothermal metasomatism at that site.

Petrogenesis of Intrusive Gabbronorites

Most of the gabbroic rocks in Hole 1270B are Fe-Ti oxide–bearing gabbronorites (Fig. F22) with substantial proportions of igneous plagioclase, clinopyroxene, orthopyroxene, and Fe-Ti oxide minerals preserved. The question arises, how much of the oxide is igneous? It has been proposed that some gabbronorites in the Haylayn massif of the Oman ophiolite were originally oxide-free olivine gabbros that were oxidized at near-solidus conditions via the reaction

consuming olivine and forming orthopyroxene + magnetite (Boudier et al., 2000). The compositions of Hole 1270B gabbronorites lie within the volume olivine + plagioclase + pyroxene on most major element projections (e.g., Fig. F23). However, the gabbronorites are rich in TiO₂. Even if they are assumed to contain no ferric iron, and thus no magnetite, CIPW norms for these compositions yield 2–12 wt% Fe-Ti oxide in the form of normative ilmenite (FeTiO₃). In addition, the freshest, least deformed samples of oxide gabbronorite contain coarse discrete oxide and pyroxene crystals separated from each other by plagioclase, suggesting that both oxide and pyroxene crystallized directly from a melt. Thus, we infer that the magma(s) that crystallized the Hole 1270B gabbronorites were saturated in Fe-Ti oxide minerals.

Fe-Ti oxides also form interstitial crystals, even within deformed and recrystallized plagioclase. This fact indicates that the gabbronorites may have been partially molten and that igneous Fe-Ti oxides may have continued to crystallize during high-temperature crystal-plastic deformation. However, interstitial textures can form during metamorphism as well as igneous crystallization. Thus, we view the hypothesis that melt was present during deformation of the gabbronorites as intriguing but uncertain. For a more extensive discussion, please see "Structural Geology" (below) and Figure F24.

Analyzed samples have Mg#s from 39% to 68%, similar to the range observed in gabbronorites from ODP Sites 735, 894, and 923 (e.g., Cannat et al., 1997a; Casey, 1997; Dick et al., 1991, 2002; Natland and Dick, 1996). Evolved crustal gabbronorites formed at pressures <0.3 GPa 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 0.3 GPa or less. However, the coarse igneous textures and scarcity of optical zoning in plagioclase and pyroxene, together with their generally low incompatible element contents, suggest that the oxide gabbros and gabbronorites are "cumulates" formed by partial crystallization of a melt followed by extraction of the remaining liquid (also see Figure F58 and associated text).

The inference that evolved melts were extracted after crystallization of Hole 1270B gabbronorites presents an interesting puzzle because no lavas with appropriate compositions have been found along the Mid-Atlantic Ridge from 14° to 16°N. We do not know the igneous Mg# of the pyroxenes in Hole 1270B gabbronorites because the samples contain substantial amounts of magnetite. Thus, the whole-rock Mg# must be substantially lower than the pyroxene Mg#. Also, postcrystallization Fe/Mg exchange between pyroxenes and oxides—plus possible oxidation reactions—may have modified the original igneous Mg#s. However, we can estimate that the clinopyroxenes have Mg#s of 70% or 75% in the Hole 1270B gabbronorites with the highest Mg# and lowest proportion of oxide minerals. Using an Fe/Mg pyroxene/liquid K_d of 0.23 (Sisson and Grove, 1993a, 1993b) and FeO/(FeO + Fe₂O₃) of 100%–70%, we infer that liquids coexisting with these clinopyroxenes must have had an Mg# less than ~41%. In contrast, the lowest reported Mg# in glass from lavas dredged along the Mid-Atlantic Ridge from 14° to 16°N is 50%. Indeed, Mg#s <50% are rare along the entire Mid-Atlantic Ridge (Fig. F25). Thus, the final destination of evolved liquids beneath mid-ocean ridges, once they are extracted after crystallizing evolved gabbronorites, is not yet known.

Gabbroic veins intruding peridotite in Holes 1270A, 1270C, and 1270D are heavily altered and have been recrystallized during extensive ductile deformation. They include plagioclase and clinopyroxene, interpreted as igneous phases. Clinopyroxene is replaced by high-temperature pleochroic amphibole, which could be igneous or could be a high-temperature alteration product. Locally, the veins contain oxide minerals, zircon, and apatite (Fig. F26). Natural gamma emissions recorded on the multisensor track show high radioactivity in the intervals containing these veins. As might be expected from the mineral assemblages in the veins and their gamma ray emissions, the whole-rock composition of a peridotite containing one of these veins (interval 209-1270D-3R-1, 63–66 cm) is enriched in incompatible elements; for example, this sample contains 96 ppm zirconium. Thus, we infer that the melt parental to the veins was enriched in highly incompatible trace elements. Perhaps these veins crystallized from liquids extracted from evolved gabbronorites similar to those recovered from Hole 1270B.

Hydrothermal Alteration, Metamorphism, and Metasomatism

Most peridotites from Site 1270 are completely altered to serpentine with minor talc. As is typical for hydrated peridotites worldwide, most of the talc alteration was early replacement of pyroxene, followed by serpentinization of olivine and remaining pyroxene. Serpentinization occurred at temperatures <400°C under static conditions in most of the core.

Although talc alteration is subdued in Site 1270 peridotites compared to Site 1268 peridotites, samples from both sites show a similar kind of metasomatism with enrichment in SiO₂ and/or loss of MgO + FeO. The normative orthopyroxene contents of all but two of the altered peridotites from Sites 1268 and 1270 are >30 wt% (Fig. F27), in contrast to the 20–25 wt% generally observed in fresher residual mantle harzburgites from mid-ocean ridges (e.g., Cannat et al., 1995; Dick, 1989; Dick et al., 1984). It is unlikely that MgO and FeO are geochemically similar in the hydrothermal alteration environment; variable proportions of FeO and MgO loss during hydrothermal alteration would probably modify the Mg#. In view of the fact that most peridotites from both Sites 1268 and 1270 have Mg#s within the normal range for abyssal peridotites (see above and "Site 1268"), we infer that MgO and FeO were comparatively immobile during metasomatism and the high SiO₂ contents of peridotite samples from both sites are due mainly to SiO₂ enrichment.

An unexplained but consistent observation is that peridotites within a few centimeters of gabbroic veins in Holes 1270C and 1270D and peridotite inclusions within the veins are much less altered than peridotites elsewhere in this hole.

Gabbronorites and gabbros in Hole 1270B were extensively recrystallized during deformation but are relatively free of hydrous alteration phases. Given the extensive recrystallization of pyroxene and oxide phases during subsolidus or near-solidus high-temperature deformation, it is likely that these minerals approached chemical equilibrium. Both quartz and olivine are absent from these rocks, whereas ilmenite and Ti-bearing magnetite are common. Thus, metamorphic conditions during deformation must be bracketed by the reactions

Exsolution of ilmenite from titanomagnetite also constrains the temperature and oxygen fugacity. And in addition, two pyroxene thermometry will provide independent constraints on temperature. Thus, it may be possible to constrain the pressure and temperature conditions of high-temperature recrystallization and reequilibration.

Hydrothermal alteration of gabbros and gabbronorites in Hole 1270B is limited mainly to formation of chlorite-amphibole assemblages along pyroxene/plagioclase contacts. Brown hornblende of high-temperature metamorphic or low-temperature magmatic origin is associated with Fe-Ti oxides and locally replaces pyroxene along grain boundaries.

Gabbroic veins in Holes 1270C and 1270D are extensively altered, though they include fresh recrystallized plagioclase, pyroxene, and even olivine in mylonitic zones. Altered portions of veins include talc + serpentine + albite + tremolite ± chlorite ± brown amphibole. Brown amphibole replaces pyroxene within veins and along contacts between peridotite boudins and veins, but not within the boudins, suggesting that circulation of high-temperature fluids and/or hydrous melts was restricted almost entirely to the veins. Locally, a rodingite assemblage (Ca silicates including prehnite and hydrogrossular) replaces the veins.

Structural Geology

Intrusion of gabbroic rocks into peridotite occurred while the peridotite was at high temperature, >600°C and probably close to 900°C. This inference is based on the presence of coarse-grained relict igneous pyroxene in the veins and in tiny anastomosing branches from igneous veins, the association of veins with high-temperature hornblende replacing pyroxene, and high-temperature deformation structures in mylonite zones within gabbroic veins.

Ductile deformation of the peridotites away from gabbroic veins was relatively subdued, with most samples preserving protogranular textures. Hole 1270A peridotites are an end-member in this regard, with protogranular to weakly porphyroclastic textures, very few magmatic veins, and little evidence anywhere in the core for ductile deformation under subsolidus conditions. This is particularly striking given the peridotite mylonite (Shinkai 6500 sample 425-R007) (Fig. F20) recovered from a planar outcrop surface just tens of meters away from the site of Hole 1270A.

In contrast, all of the gabbroic veins in Holes 1270C and 1270D were the locus of high-temperature mylonitic deformation. Additional deformation of peridotites surrounding these mylonitized veins is evident in the form of olivines with well-developed subgrain boundaries and bent pyroxene crystals, but the entire gradient from protogranular to mylonitic textures is present over a few millimeters and true mylonites were not developed in peridotites free of gabbroic veins. This indicates that the gabbroic veins were much weaker than surrounding peridotites during deformation because they were partially molten during the onset of deformation, or because they were hydrous while surrounding peridotites were dry, or simply because gabbroic mineral assemblages had a lower viscosity than peridotites at the conditions of deformation.

High-temperature ductile deformation of the gabbroic rocks in Hole 1270B is extensive and irregularly distributed. The top of the hole had relatively poor recovery, perhaps due in part to ductile and brittle deformation features associated with the fault that formed the planar outcrop surface on which we started the hole. In any case, crystal-plastic deformation intensity in the top 10–15 m of the hole is relatively high. Below this depth, crystal-plastic deformation is concentrated in a second zone at 40–45 mbsf, underlain by less deformed gabbronorites. The intensity of ductile deformation of the gabbroic rocks appears to be correlated with the abundance of oxide minerals (Fig. F24). This correlation is consistent with the tentative hypothesis, based on preliminary data, that there is a correlation between deformation intensity and the proportion of oxide minerals in oceanic crustal gabbros from Holes 735B and 1105A from Atlantis Bank on the Southwest Indian Ridge (e.g., Dick et al., 1991, 2000, 2002; Natland and Dick, 2002; Natland et al., 1991; Niu et al., 2002; Ozawa et al., 1991; Pettigrew, Casey, Miller, et al., 1999).

The correlation between deformation intensity and magnetite proportion in Hole 1270B gabbronorites is poorly quantified and uncertain because of incomplete recovery, a lack of continuous measurements of oxide proportion (magnetic susceptibility is heavily weighted to ferromagnetic phases) in the recovered core [N4], a "nugget effect" resulting from coarse-grained, irregularly distributed oxide crystals, and the presence of anastomosing ductile shear zones, including less deformed lenses, leading to irregular variations in deformation intensity on the centimeter to decimeter scale. The apparent relationships in Figure F24 should provide an excellent starting point for postcruise research to test and fully quantify the hypothesis that there is a correlation between oxide proportion (measured in detail by image analysis of thin sections) and deformation intensity (quantified, for example, by grain size analysis of recrystallized plagioclase).

Brittle deformation features are relatively uncommon in the core, compared to Site 1268. However, there is a fault gouge zone—probably several meters thick—below ~17 mbsf in Hole 1270A, and we encountered poor recovery and difficult drilling at this depth in the other holes at Site 1270 as well. We believe that this indicates the presence of large faults parallel to planar outcrop surfaces and to the overall slope that forms the eastern wall of the rift valley in this region.

Crosscutting relationships define the following sequence of events for the core from Site 1270:

1. Partial melting of mantle peridotite, together with formation of dunite bands and protogranular textures in residual harzburgite;
2. Cooling of peridotite at the base of the thermal boundary layer;
3. Intrusion of gabbroic veins and gabbronorite bodies while peridotites were still at high temperature;
4. Formation of high-temperature mylonite zones in gabbroic veins within peridotite in Holes 1270C and 1270D, perhaps with synchronous ductile deformation in gabbroic rocks in Hole 1270B, while temperatures were >600°C and probably >900°C;
5. Cooling of peridotites and gabbroic rocks to <400°C;
6. Static, pervasive, nearly complete serpentinization of peridotites; and
7. Continued brittle deformation in large fault zones, probably parallel to exposed fault surfaces in outcrops observed during Shinkai 6500 Dive 425 and to the entire slope of the eastern flank of the Mid-Atlantic rift valley in this region.

This history is similar to the sequence of events inferred for Site 1268, except that mylonitic deformation at Site 1270 took place entirely within gabbroic rocks (within massive gabbros in Hole 1270B and within gabbroic veins in peridotite in Holes 1270C and 1270D), whereas at Site 1268 mylonitic deformation was concentrated in zones of mixed peridotite and gabbroic veins and affected both lithologies. Also, replacement of serpentine by talc is common at Site 1268 but rare or absent at Site 1270 and small brittle deformation features are relatively uncommon at Site 1270 compared to Site 1268.

Dips of crystal-plastic foliation planes in gabbros from Hole 1270B and in mylonitized gabbroic veins in Holes 1270C and 1270D dip at 30°–60° (average = ~45°). Thus, foliations in the core are significantly steeper than the striated planar outcrop surfaces with slopes of ~20° observed on the seafloor during Shinkai 6500 Dive 425 and during the camera surveys on this cruise. Also, we did not recover peridotite mylonites (matrix grain size < 10 µm) in core from Site 1270, despite the fact that Shinkai 6500 sample 425-R007 is a peridotite mylonite recovered from a striated outcrop surface just a few tens of meters from the site of Hole 1270A (Fig. F20). Thus, it is apparent that the faults represented by the striated outcrop surfaces are not related in a simple way to the coarser-grained, higher-temperature mylonitic shear zones observed in core from Site 1270. Instead, the fault surface that forms seafloor outcrops dipping ~20° to the west must cut the higher-temperature fabrics at an average angle >20°. In the following section, we argue that the high-temperature foliation in the core probably dips ~45° to the east and thus intersects the west-dipping fault(s) represented by outcrop surfaces at an angle of ~65°.

Paleomagnetic Data and Tectonics

Paleomagnetic data were collected on half cores and individual discrete samples. Some samples retain 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 an orientation with a common azimuth for the remanent magnetization vector (see "Structures in Peridotite and Gabbroic Intrusions" in "Mantle Upwelling, Melt Transport, and Igneous Crustal Accretion").

In the case of Site 1270, this procedure was complicated because of the low inclinations of the remanent magnetization. Mainly negative stable inclinations in gabbroic rocks from Hole 1270B (average = –14°; see below for details) suggest that these rocks acquired their magnetization during a period of reversed polarity. Hole 1270B is 13–18 km from the rift axis, indicative of 1.0–1.4 m.y. of spreading at a half-rate of 12.8 km/m.y. (Fujiwara et al., 2003), so it could have formed and been magnetized during the Matuyama Reversed Chron. This hypothesis is consistent with—but not required by—the sea-surface magnetic data presented by Fujiwara et al. (2003). Thus, the negative inclinations in Hole 1270B may indicate that the rocks were reversely magnetized. Based on this interpretation, rotation of structural features around a vertical axis into a common magnetic orientation—together with the assumption that tectonic rotations have not substantially modified the azimuth of the remanent magnetization vector—yields the result that high-temperature foliations in the gabbroic rocks from Hole 1270B dipped ~45° to the east when the remanent magnetization vector pointed south.

Some peridotites in Holes 1270C and 1270D retain a stable remanent magnetization. Inclinations of this magnetization are nearly horizontal (average = –3°; see below). Thus, the polarity of the Earth's magnetic field when these rocks acquired their magnetization is highly uncertain. One possibility as that both peridotites and gabbroic rocks acquired a remanent magnetization with the same polarity of the Matuyama Reversed Chron. Assuming that the peridotites were reversely magnetized like the gabbroic rocks in Hole 1270B yields the result that mylonite zones in gabbroic veins dipped ~45° to the west when the remanent magnetization vector pointed south, assuming that tectonic rotations have not substantially modified the azimuth of the remanent magnetization vector (Fig. F28A). Alternatively, assuming that both peridotites and gabbroic rocks were normally magnetized implies that the crystal-plastic foliation in the gabbroic rocks dipped to the west and the mylonite zones in the peridotites dipped to the east (Fig. F28B) when the remanent magnetization vector pointed north, again with the assumption that the azimuth of the remanent magnetization vector has not been modified by tectonic rotations.

Holes 1270C and 1270D are only a few hundred meters from Hole 1270B, and the deformation features in all three holes record similar conditions (localized deformation at ~900°C, possibly in partially molten rocks). Thus, the inference that the fabrics have dips that differ by 90° seems problematic. Another possibility is that the polarity of remanent magnetization in the peridotites is different from the polarity in the gabbroic rocks. For example, perhaps the peridotites were magnetized during the Brunhes Normal Chron (or the Jaramillo, or even some older normal polarity chron), while the gabbroic rocks in Hole 1275B were reversely magnetized. In this case, rotation of the high-temperature foliations in Holes 1270C and 1270D yields ~45° dips to the east, parallel to the high-temperature foliation in Hole 1270B (Fig. F28C). This scenario is similar to our interpretation of remanent magnetization in Hole 1268A, in which we inferred that gabbroic rocks acquired their magnetization during cooling of igneous magnetite through 500°–600°C, whereas peridotites acquired their magnetization significantly later as a result of metamorphic growth of magnetite during serpentinization at ~300°C.

The average remanent inclination for 10 discrete samples from the gabbroic rocks in Hole 1270B is –14° (95% CI = +10°/–10°), much shallower than the expected inclination of ±28°. The average inclination of 10 discrete samples of peridotite from Holes 1270C and 1270D is –3° (95% CI = +13°/–13°), which is also significantly shallower than the expected value of 28° but not significantly different from the inclinations in the gabbroic rocks of Hole 1270B.

One possible explanation for the statistically significant difference between the expected and observed inclinations in both gabbroic rocks and peridotites is that tilting occurred after cooling and blocking of the remanence. As discussed in "Paleomagnetic Data and Tectonics" in "Site 1268," 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. The stepwise alternation of steep rift-facing slopes and nearly horizontal benches along the flanks of the Mid-Atlantic Ridge is generally interpreted to result from the presence of tilted normal fault blocks. Back-tilting of fault blocks on the east side of the rift valley in the region around Site 1270 is probably top-to-the-east (clockwise around a northeast-striking axis). In this region, the west-facing slopes strike ~010°–020°, parallel to the strike of the rift valley. We infer that tilting may have been produced by clockwise rotation of normal fault blocks along an approximately rift-parallel axis. For a horizontal rotation axis striking 020° and assuming that the gabbroic rocks are reversely magnetized, ~35° of clockwise rotation is required to change an original inclination of –28° to the observed average inclination of –14° in reversely magnetized gabbros from Hole 1270B (Fig. F29). Similarly, if we adopt the interpretation that the peridotites are normally magnetized (Fig. F28C), the peridotites must have undergone ~60° of clockwise rotation to change an original inclination of +28° to the observed average inclination of –3°. Given the uncertainties in the data, both gabbro and peridotite inclinations can be explained, within error, using a model in which there was ~45°–50° of clockwise rotation around a horizontal axis striking 020° after both lithologies acquired their magnetization.

Alternatively, the differences between magnetic inclination in the gabbros (approximately –14°) and in the peridotites (approximately –3°) suggest that the two lithologies might record different amounts of rotation. If we adopt the interpretation in Figure F28B, in which both peridotites and gabbroic rocks are normally magnetized, we can explain this difference in the same manner as for Site 1268. For a horizontal rotation axis striking 020°, ~85° of clockwise rotation is required to change an original inclination of +28° to the observed average inclination of –14°. And, as already stated above, the peridotites must have undergone ~60° of clockwise rotation. In this scenario, clockwise rotation of the Site 1270 section around a horizontal axis striking 020° began before magnetization of the peridotites and continued after magnetization of the peridotites [N5].

An additional uncertainty in inferring the amount of tilting is introduced by the unknown plunge of the rotation axis. Figure F30 illustrates the effect of gently plunging rotation axes. For a rotation axis with a fixed azimuth of 020°, if the plunge were 10° along 020° and gabbroic rocks are normally magnetized, ~115° of clockwise rotation would be required to change an original inclination of 28° to the observed average inclination of the gabbroic rocks in Site 1270B of –14°. Conversely, if the plunge were 10° along 200°, ~75° of counterclockwise rotation would be required for the same change in inclination [N6].

In summary, if the hypothesis outlined here is correct, the gabbroic rocks in Hole 1270B underwent a large amount of clockwise rotation (~45°–100°) around a nearly horizontal, rift-parallel axis after acquisition of the magnetic remanence. The peridotites record ~50°–65° of this rotation.

These results and hypotheses are similar to our interpretation of paleomagnetic and structural data from Site 1268. At Sites 1270 and 1268, the magnetic inclination data, together with the assumption that tectonic rotations occurred around rift-parallel, nearly horizontal rotation axes, require tectonic rotations of 45°–120°. Also, at both sites the peridotites may record smaller tectonic rotations than the gabbroic rocks, perhaps due to early magnetization of the gabbroic rocks compared to the peridotites.

It must be emphasized again that our tectonic interpretations of structural and magnetic data are nonunique, forward models, chosen from among many other models that could potentially account for the data. However, in any case, large tectonic rotations are required to explain the low paleomagnetic inclinations at Sites 1268 and 1270.

Site 1271 is located along the track of Faranaut 15°N Dive 7 on the western flank of the Mid-Atlantic rift valley. The dive, using the Nautile submersible, recovered basaltic samples at water depths >3700 m and three samples of peridotite and two samples of gabbro higher on the rift valley walls. Hole 1271A was initiated on a smooth, sedimented slope uphill from gabbro sample FR07-10. After a relatively promising beginning, relatively high penetration rates and poor recovery from 13 to 28 mbsf suggested that we had encountered a near-surface fault zone, similar to those encountered at Site 1270. We continued to drill to 44.8 mbsf, but the hole was unstable and ultimately had to be abandoned. Recovery averaged ~13%. We then moved 74 m southwest and spudded Hole 1271B into a similar sedimented slope. Although drilling conditions were far from ideal, we were able to drill to a depth of 104 mbsf with 15% recovery. At this point, because of poor recovery combined with great lithologic diversity, we decided to log the hole, anticipating that density and resistivity contrasts in downhole data would help us to map the distribution of gabbroic intrusions and peridotite host rocks. However, even after the bit was released, it was very difficult to raise the drill string. Apparently, the hole collapsed as the pipe was withdrawn. The logging string encountered obstructions just a few meters below the base of the open pipe, ending operations at Site 1271.

Proportions of Igneous Rocks

The igneous and residual mantle protoliths recovered from Hole 1271B are dunite, harzburgite, troctolite, gabbro, minor gabbronorite, and a small but significant amount of chromitite in three 1- to 3-cm-thick lenses (Fig. F31). Gabbroic rocks are intrusive into the peridotite in Holes 1271A and 1271B. The process of igneous intrusion was probably variable; brown amphibole gabbros (BAGs) apparently were injected into sharp-sided dikes, whereas olivine gabbros, troctolites, and impregnated peridotites apparently crystallized from melt migrating along peridotite grain boundaries.

Among igneous and mantle rocks, dunite is abundant in Holes 1271A (98%) and 1271B (56%), whereas harzburgite composes <1% of recovered samples from Hole 1271A and only 9% of the rocks from Hole 1271B. BAG composes <2% of the igneous and mantle material in Hole 1271A and 21% in Hole 1271B, whereas olivine gabbro and troctolite compose 14% of igneous and mantle rocks recovered from Hole 1271B. Weighting the proportions of rocks in Holes 1271A and 1271B by the length of each hole yields overall proportions of 68% dunite, 6.5% harzburgite, 15% BAG, and 10% olivine gabbro and troctolite. Although the ratio of dunite to harzburgite at Site 1271 is much higher than that observed at Sites 1268 and 1270, the ratio of peridotite (75%) to gabbroic rocks (25%) at Site 1271 is remarkably similar to the proportions in Hole 1268 (74% peridotite/26% gabbroic rocks), in Hole 1270 (68% peridotite/32% gabbroic rocks), and recovered from dives and dredging along the Mid-Atlantic Ridge from 14° to 16°N (75.5% peridotite/24.5% gabbroic rocks).

Highly Depleted Mantle Peridotites

With some notable exceptions, peridotites from Site 1271 are 100% altered, mainly to serpentine and brucite. Many peridotites—both harzburgites and dunites—contain 1%–15% interstitial gabbroic material interpreted as the crystallization products of melt migrating through the peridotite along grain boundaries. However, despite this "impregnation," geochemical analyses of all but one of the Site 1271 peridotites show low concentrations of nominally immobile incompatible elements such as Al, Sc, and V that are thought to be immobile during hydrothermal alteration. For example, the Al₂O₃ concentrations in all but one of the Site 1271 peridotites are <1 wt% (average = 0.9 wt%), whereas Al₂O₃ concentrations in peridotites from Site 920 on the Mid-Atlantic Ridge at 23°20´N range from 1 to 2 wt% (Casey, 1997) and the median concentration of Al₂O₃ in abyssal peridotites worldwide is 1.4 wt% (Bodinier and Godard, in press). Thus, despite the presence of gabbroic impregnations, the average Al₂O₃ concentration in 8 peridotites from Site 1271 is low, though not as low as the average of 0.6 wt% Al₂O₃ in 7 peridotites from Site 1270 and 20 peridotites from Site 1268. If Al₂O₃ concentrations have not been modified by hydrothermal metasomatism, then the peridotites from Sites 1268, 1270, and 1271 are among the most depleted residual mantle peridotites yet obtained from the mid-ocean ridges.

Origin of Site 1271 Dunites

Molar Mg# of the peridotites from Site 1271 ranges from 85% to 89% in five dunites and from 90% to 92% in three harzburgites. Most of the Site 1271 dunite Mg#s are lower than Mg#s in most harzburgites from Sites 1268, 1270, and 1271 and below the normal range of whole-rock Mg# in residual mantle peridotites from mid-ocean ridges (~89%–92%; average = ~90%–91%) (e.g., Dick, 1989). The low Mg# in dunite could be due to metasomatic changes in Fe/Mg during hydrothermal metasomatism, olivine crystal fractionation, or reaction of migrating melt with residual mantle peridotites.

Figure F32 shows that two dunites have bulk compositions very close to the composition of pure olivine. Other dunites lie along mixing lines between olivine and pyroxene, as do two of the harzburgites. Still other dunites, including a sample of impregnated dunite with 15% interstitial gabbroic material (probably composed of plagioclase and pyroxene ± hornblende, prior to alteration), show significant compositional shifts away from olivine-pyroxene mixing lines toward mixtures including plagioclase ± Cr-Al spinel ± igneous amphibole. Because of the compositional shifts resulting from the presence of interstitial gabbroic material in most peridotite samples from Site 1271, it is difficult to discern if the peridotites were also modified by subsolidus metasomatism. Nonetheless, there is no compelling evidence for SiO₂ gain or MgO loss from these samples, so we tentatively rule out metasomatic changes as the cause of the low Mg#s in Site 1271 dunites.

We now wish to distinguish between olivine crystal fractionation and melt-rock reaction as explanations for the composition of Site 1271 dunites. Figure F33 shows that Site 1271 dunites have Ni contents in olivine that are comparable to those in harzburgites from Sites 1268, 1270, and 1271, despite the fact that Site 1271 dunites have lower Mg#s than the harzburgites. Similar relationships are seen in dunites from the crust–mantle transition zone in the Oman ophiolite (Godard et al., 2000; Koga et al., 2001; Korenaga and Kelemen, 1997). The low Mg# of the migrating melts probably indicates that they were undergoing gradual conductive cooling, with decreasing magma mass due to reaction and crystallization near the base of the thermal boundary layer beneath the Mid-Atlantic Ridge. Nearly constant concentrations of compatible elements accompanied by decreasing Mg# are the hallmark of dunites formed by combined crystal fractionation and reaction between relatively low Mg#, migrating melt, and residual mantle olivine (e.g., DePaolo, 1981; Kelemen, 1986; Kelemen et al., 1998a; Navon and Stolper, 1987). In contrast, olivine crystal fractionation alone leads to a steep decline in olivine Ni contents with decreasing Mg#, as seen in Figure F33. Therefore, we conclude that the Site 1271 dunites are the product of combined olivine crystallization and reaction between residual peridotite and migrating melts with relatively low Mg#.

Chromitites from Mid-Ocean Ridges

To our knowledge, the three chromitites recovered at Site 1271 are the first to be sampled from the Mid-Atlantic Ridge. In fact, Site 1271 is only the second chromitite locality that has been found along any mid-ocean ridge, the first being near the East Pacific Rise at Hess Deep at ODP Site 895 (Arai and Matsukage, 1996). The chromitite recovered from Hess Deep is a single, elongate, discontinuous train of chromite grains <1 cm wide. It resembles chromite "trails" observed in dunites worldwide almost as much as it resembles massive chromitites in ophiolites. In contrast, the Site 1271 chromitites are rounded rather than elongate, have sharp contacts with surrounding peridotite, and are clearly massive (Fig. F34).

It has been proposed that the formation of chromitite occurs only during subduction-related arc magmatism because the formation process requires hydrous magmas (Matveev and Ballhaus, 2002). Based on our results, confirming the Hess Deep observation, it is clear that chromitites are not restricted to arc magmatism. However, there is abundant high-temperature amphibole in core from Site 1271, some of which could be igneous. If the presence of amphibole is genetically related to the formation of chromitite at Site 1271, then Matveev and Ballhause (2002) may be correct in inferring that hydrous magmas are important in chromitite genesis.

Based on the observation of unusually high molar Cr/(Cr + Al), or Cr#, in spinels from harzburgite and dunite dredged from the Mid-Atlantic Ridge in the 14°–16°N region (Bonatti et al., 1992; Dick and Kelemen, 1992; Sobolev et al., 1992b), together with the general observation that spinels in chromitites have higher Cr# than spinels in residual mantle peridotites (e.g., Dick and Bullen, 1984), we anticipate that the Cr#s in the Site 1271 chromitites will be the highest yet observed in spinel from mid-ocean ridges. High Cr#s would provide insight into the processes that form chromitite. In addition, such a result would change the interpretation of ophiolite provenance. It has been proposed that spinel Cr#s >60 are only found in mantle peridotites from subduction-related settings (e.g., Dick and Bullen, 1984), and high Cr#s have been cited as evidence that most ophiolites with high Cr# in mantle spinels do not form at normal mid-ocean ridges.

Petrogenesis of Intrusive Rocks

There appear to be two types of gabbroic rocks in core from Site 1271. Brown amphibole–bearing metagabbros with relics of igneous plagioclase (and locally some relict clinopyroxene) always contain ~50% plagioclase and 50% mafic minerals in their igneous protolith (Fig. F35). Contacts of BAG with peridotite were not recovered, but its consistent phase proportions suggest that the BAG forms dikes with sharp contacts in peridotite. In contrast, olivine gabbros and troctolites (>15% plagioclase, pyroxene, amphibole, and alteration products derived from these phases) are gradational into host peridotites with 1%–15% interstitial gabbroic material. As a result, we interpret much of the olivine in olivine gabbros and troctolites as xenocrysts, derived from a partially disaggregated residual mantle protolith.

The origin of amphibole in the BAG is uncertain. Some of the amphibole is tremolite-actinolite, clearly metamorphic in origin, but other crystals appear to be idiomorphic brown hornblende and could be igneous (Figs. F35, F36). We analyzed a fragment of this amphibole obtained from a coarse-grained sample of the BAG. Its composition indicates that the amphibole is a hornblende solid solution composed of ~71 mol% tremolite-actinolite and ~29 mol% pargasite. In detail, the chemical analysis combined with stoichiometric constraints suggests that there is also ferric iron in a hastingsite component. If this amphibole formed by subsolidus reaction of igneous minerals with H₂O, it could have formed from olivine, pyroxene, and plagioclase via reactions such as

+ 4 CaAl2Si2O8 + 4 NaAlSi3O8 = 4 NaCa2(Mg, Fe)4AlSi6Al2O22(OH)2
(anorthite)	(albite)		(pargasite)

 

+ 10 Ca2(Mg, Fe)5Si8O22(OH)2.
	(tremolite)

Alternatively, this type of amphibole could be igneous. However, the large proportion of tremolite-actinolite component in the solid solution suggests a relatively low, near-solidus temperature of crystallization.

A fragment of altered white material believed to be relict plagioclase and alteration products replacing plagioclase were analyzed separately, yielding a composition close to that of plagioclase with 81 mol% anorthite. However, the composition includes appreciable Mg, Fe, and Ti and 0.5 wt% K₂O, so the sample probably incorporates minerals other than plagioclase and plagioclase alteration products.

One BAG was analyzed for major and trace element contents. It has Mg# = 84, higher than that in any of the gabbroic rocks from Sites 1268 and 1270 (maximum = 83% at Site 1268). This Mg# is indicative of crystallization from a relatively primitive melt, close to Fe/Mg equilibrium with residual mantle peridotite. The BAG also has higher incompatible element concentrations than any gabbroic rocks from the previous sites. For example, the BAG has Y = 48 ppm and Zr = 123 ppm, compared with a maximum of Y = 17 ppm and Zr = 48 ppm in gabbroic rocks from Sites 1268 and 1270. Although the high Y and Zr contents may be due, in part, to the (possible) presence of igneous hornblende in the BAG, it is likely that the parental melt also had high Y and Zr contents. The combination of high Mg# together with high incompatible element concentrations in magmas is the signature of decreasing melt mass resulting from partial crystallization combined with buffering of the Mg# at high values from reaction of melt with residual mantle peridotite (e.g., DePaolo, 1981; Kelemen, 1986). Thus, we infer that the melt parental to the BAG was derived from a primary basalt in melt but was modified by conductive cooling, partial crystallization, and reaction with peridotite wallrocks.

We also analyzed an olivine gabbro from Hole 1271B. It has Mg# = 67, rather low for a rock that is thought to include xenocrysts of mantle olivine. If our interpretation of these rocks as hybrids is correct, then this sample must have formed with a relatively high melt/rock ratio.

Hydrothermal Alteration, Metamorphism, and Metasomatism

Many peridotites from Site 1271 are completely altered to serpentine. Unlike the SiO₂-rich metaperidotites from Sites 1268 and 1270, the compositions of metaperidotites from Site 1271 are similar to the compositions of fresh dunites and harzburgites and thus do not require large metasomatic increases in Si/(Mg + Fe) during alteration (Fig. F32). Brucite is an important accessory phase in the background alteration of many peridotites from Site 1271. The presence of brucite probably reflects the olivine-rich, low Si/(Mg + Fe) dunite protoliths for most Site 1271 peridotites (see Fig. F32). Brucite is not stable in bulk compositions with molar Si/(Mg + Fe) greater than that in serpentine (0.67). Most peridotites from Sites 1268 and 1270 have Si/(Mg + Fe) greater than 0.67, consistent with observed low-temperature alteration to serpentine + talc rather than serpentine + brucite. Lack of pyroxene in dunite protoliths may have allowed fluid compositions to reach low aqueous Si activities and relatively high pH compared to fluid compositions during hydrothermal alteration of harzburgites at Sites 1268 and 1270. These fluid compositions favor formation of brucite + serpentine (see "Metamorphic Petrology" and Fig. F34, both in the "Site 1271" chapter).

Spinel in many metaperidotites from Site 1271 is surrounded by chlorite. The chlorite could be a hydrothermal alteration product of plagioclase, with the implication that crystallization of melt, migrating along peridotite grain boundaries, formed plagioclase halos around spinel crystals. However, chlorite around spinel may also have formed via subsolidus reactions involving olivine, pyroxene, spinel, and fluid, such as

H2O + (Mg, Fe)2SiO4 + (Mg, Fe)2Si2O6 + (Mg, Fe)(Al, Cr)2O4
(olivine)	(orthopyroxene)		(spinel)

= (Mg, Fe)5(Al, Cr)2Si3O10(OH)4.
	(chlorite)

Similar chlorite rims around spinel are common in amphibolite-grade metaperidotites worldwide, even in the absence of plagioclase.

A strikingly large proportion of impregnated peridotites from Site 1271 with 1%–15% interstitial gabbroic material contains fresh olivine and pyroxene. In this way, they resemble the relatively fresh peridotite along contacts with gabbroic veins in cores from Holes 1270C and 1270D. The presence of relict clinopyroxene and plagioclase may have buffered fluid compositions to low pH, inhibiting the formation of serpentine from adjacent olivine and orthopyroxene (see "Metamorphic Petrology" in the "Site 1272" chapter).

Interstitial gabbroic material in impregnated peridotites, olivine gabbros, and troctolites from Site 1271 includes a large proportion of high-temperature hornblendic amphibole. As for the amphibole in the BAGs, it is not clear whether any of the amphibole in these other lithologies is igneous. Sharp contacts between olivine and high-temperature amphibole in some samples suggest that at some point the two phases were in equilibrium. Sharp contacts between olivine and plagioclase in peridotites, olivine gabbros, and troctolites likewise suggest that olivine and plagioclase were in equilibrium, probably at igneous temperatures. However, spinel + chlorite(?) symplectites separate olivine from amphibole and olivine from plagioclase in other parts of the core.

In addition to the presence of high-temperature metamorphic or igneous amphibole, gabbroic rocks at Site 1271 underwent secondary alteration to tremolite-actinolite, talc, chlorite, sericite, metamorphic plagioclase, and, locally, quartz. In the BAGs, rutile intergrown with quartz appears to replace Fe-Ti oxides. The presence of rutile rather than titanite suggests that this replacement occurred under amphibolite rather than greenschist facies conditions.

Structural Geology

Orthopyroxene in orthopyroxene-poor harzburgites and orthopyroxene-bearing dunites from Site 1271 has transitional protogranular to porphyroclastic textures. In some particularly intriguing examples, elongate porphyroclastic orthopyroxene crystals showing incipient recrystallization are rimmed by undeformed vermicular spinels. This texture suggests that high-temperature exsolution of spinel or formation of spinel during reaction of migrating melt with orthopyroxene postdated deformation in these rocks.

Lower-temperature, more localized deformation at Site 1271 occurred preferentially in gabbroic veins and surrounding peridotite wallrocks, as observed at Sites 1268 and 1270. Shear zones in Hole 1271B show a well-developed progression from granulite facies recrystallization of dunite and gabbroic rocks into porphyroclastic mylonites, through amphibolite facies replacement of deformed minerals with amphibole that was itself deformed, to formation of greenschist facies, chlorite-amphibole, and serpentine schists. High-temperature shear zones, although locally crosscut by later features, apparently continued to be the primary locus of deformation throughout most of the cooling history of the Site 1271 core to temperatures <300°C.

Fault gouge and fault breccia zones sampled at several depths appear to be parts of major faults with substantial brittle offsets.

Paleomagnetic Data and Tectonics

Paleomagnetic data were collected on half cores and individual discrete samples. 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 an orientation with a common azimuth for the remanent magnetization vector (see "Structures in Peridotite and Gabbroic Intrusions" in "Mantle Upwelling, Melt Transport, and Igneous Crustal Accretion").

In the case of Site 1271, split cores and discrete samples all have positive inclinations, indicating that the rocks are normally polarized. The average remanent inclination for 15 discrete samples is 25° (95% CI = +11°/–13°), and the mean inclination for archive-half cores is 29° (95% CI = +3°/–4°). These inclinations are statistically indistinguishable from the expected inclination of 28°, which is consistent with the hypothesis that no significant block rotation has affected the section sampled at Site 1271. This result is quite different from the paleomagnetic data from Sites 1268 and 1270, where remanent magnetic inclinations also require substantial rotation of the section. Alternatively, large rotations may have coincidentally restored the rocks to an apparently normal magnetic inclination. For example, a 70° counterclockwise rotation about a horizontal axis along 020° would restore the inclination to 28°, and so would be consistent with the data.

("Site Summaries" continued next file)