Site 1272 is located along the track of Faranaut 15°N Dive 2 on the western flank of the Mid-Atlantic rift valley, near the summit of the inside corner high just south of the 15°20´ Fracture Zone. The Faranaut dive recovered 10 peridotite samples from 3399 to 2484 meters below sea level (mbsl), with samples of basaltic lava and diabase at 3143 and 2490 mbsl. The inside corner high, with two summits known informally as Mt. Bougault (15°4.63´N, 45°59.10´W) and Mt. Dmitriev (15°5.58´N, 45°58.49´W) has been surveyed via dredging and submersible dives (Bougault et al., 1988, 1993; Cannat, 1993; Cannat et al., 1992, 1997b; Cannat and Casey, 1995; Dosso et al., 1993; Fujiwara et al., 2003), which recovered numerous samples of dunite, orthopyroxene-poor harzburgite, and basalt.

The camera survey for Site 1272 began at 2677 mbsl near the site of Faranaut 15°N peridotite sample FR-02-11 (2650 mbsl), where cliffs interspersed with sedimented slopes were observed during the dive. Cliffs, a few meters high, were also observed near the beginning of the camera survey, but the intervening slopes were steep and talus covered, so the camera survey proceeded west-southwest and upslope along the dive track toward the site of peridotite sample FR-02-12 (2484 mbsl), also close to a cliff outcrop. A large cliff (~5 m high) along the camera survey track at ~2600 mbsl is topped by a relatively flat area covered by mixed talus and sediment. We proceeded to the west-southwest, searching for a talus-free area. However, at 15°5.6645´N, 44°58.3060´W (2567 mbsl) there was an abrupt change from talus-covered sediment to completely smooth sediment along a sharp line striking approximately northwest. We feared that this feature is the surface expression of a major fault, so we moved back downslope 30 m to a relatively flat area, which—despite the presence of talus on the surface—we chose as the site of Hole 1272A, at 2571 mbsl. A push-in test with the drill string indicated that the sediment cover was only ~1 m thick.

During drilling, numerous faults, zones of poor recovery, and diverse lithologies were encountered in the top 55 m of the hole. Two lithologies, a fine-grained diabase or basalt flow and a medium-grained hypabyssal gabbro-diorite-diabase, were recovered over depth intervals of several meters. At the time, our favored interpretation was that we were drilling along the margin of a large dike or sill emplaced into weathered peridotite. However, in retrospect, it may be that the top portion of Hole 1272A was in a tectonic breccia (fault or landslide) with individual blocks exceeding 5 m in maximum dimension (based on the size of the cliff observed during the camera survey at ~2600 mbsf and ~50 m downslope from Hole 1272A and on the lengths of recovered intervals composed of homogeneous lithologies). This hypothesis is supported by the observation/inference that the top 55 m of the hole sampled a tectonic breccia, a carbonate-cemented breccia recovered at ~23 mbsf, with clasts of serpentinized peridotite, may have formed on or near the seafloor.

Paleomagnetic data on samples from the upper 55 m of the hole yield variable remanent inclinations, which could also indicate that this section is a tectonic breccia. However, measurements of inclination in the medium-grained intrusive rock between ~15 and 25 mbsf (termed "gabbro" in this report and in the "Igneous and Mantle Petrology" and "Paleomagnetism" sections in each site chapter) are internally consistent and parallel to consistent measurements of inclination in homogeneous peridotites at depths of 55–131 mbsf. Highly variable inclinations in other lithologies could have arisen as a result of drilling through rubble that had fallen from higher in the hole. Thus, the magnetic data do not clearly indicate whether the section is a tectonic breccia or an intact block.

In any case, below a fault gouge zone at 55 mbsf, we drilled ~75 m of homogeneous, green, serpentinized mantle peridotite with consistent magnetic inclinations. From a tectonic perspective, we are confident that this lower part of the hole is intact and in place.

Proportions of Igneous Rocks

The igneous and residual mantle protoliths of core recovered from above 55 mbsf were dunite, harzburgite, aphanitic diabase or basalt, and medium-grained gabbro-diorite-diabase (Fig. F37). The uncertain tectonic situation in the upper part of Hole 1272A, discussed in the previous section, makes it difficult to be certain if the fine-grained basaltic rocks are lavas or dikes. Because the gabbro-diorite lithology may be part of a thick dike, and because it includes miarolitic cavities indicative of shallow crystallization, some workers might call it a diabase. However, based on grain size and texture, we refer to this lithology as gabbro in this report. In addition to the igneous and mantle rocks, 0.3 m of carbonate-cemented tectonic breccia was recovered in this part of the hole. There were also intervals with low recovery and abundant fine-grained fault gouge.

Core recovered from below 55 mbsf consists of 93% harzburgite, 3.4% dunite, and 3.5% fine-grained mafic rocks. The fine-grained mafic rocks are generally within a few centimeters of the top of each cored interval and are interpreted as fragments of blocks from above 55 mbsf that fell to the bottom of the hole during core recovery. In addition, at ~90 mbsf there is a short interval of coarse-grained oxide gabbronorite. The downhole Formation MicroScanner (FMS) log (see "Downhole Measurements" in the "Explanatory Notes" chapter) suggests that this gabbronorite is far more resistive than the surrounding serpentinized peridotites and that it is ~1 m thick. The gabbronorite is very much unlike the quickly cooled diabase and miarolitic gabbro in the upper 55 m, and—given the presence of exsolution lamellae in pyroxene and evidence for high-temperature ductile deformation in this lithology—it must have cooled slowly.

Because the bottom 75 m of Hole 1272A is almost completely composed of harzburgite and lesser amounts of dunite (Fig. F38), it is very different from Sites 1268, 1270, and 1271 and from overall dredging statistics for the Mid-Atlantic Ridge from the 15°20´ Fracture Zone to 14°S. In each of these other data sets, the peridotite to gabbro proportions are ~75/25. The upper 55 m of Hole 1272A is so different from the lower 75 m that it may be potentially misleading to group them. However, if we combine the 2 m of medium-grained gabbro and the 3 m of peridotite recovered from the upper 55 m of Hole 1272A with the 28 m of peridotite from the lower 75 m, we obtain "average" proportions of 94% peridotite to 6% gabbroic rocks, still lower than at any previous site on this leg. Although our sample set is small, the overall lithologic proportions recovered in Hole 1272A are consistent with previous dive and dredging results, which yielded mainly peridotite and basalt with very little gabbro on Mts. Bougault and Dmitriev.

The proportion of dunite to harzburgite in Hole 1272A is ~4/96, lower than at any previous site on this leg. In contrast, the proportion of dunite to harzburgite at Sites 1268, 1270, and 1271 are 15/85, 10/90, and 90/10, respectively. The low proportion of dunite in Hole 1272A is somewhat surprising because dunites compose ~50% of all previous dive and dredging samples from Mt. Dmitriev and Mt. Bougault (Bougault et al., 1988, 1993; Cannat, 1993; Cannat et al., 1992, 1997b; Cannat and Casey, 1995; Dosso et al., 1993; Fujiwara et al., 2003). We infer that the low proportion of dunite/harzburgite in Hole 1272A and the high proportion of dunite at Site 1271 are not representative of the inside corner in this region. Instead, we believe that the distribution of dunites is not uniform on the scale of our drilling.

Highly Depleted Mantle Peridotites

Geochemical analyses of all Site 1272 peridotites show low concentrations of nominally immobile incompatible elements such as Al, Sc, and V. For example, the Al₂O₃ concentrations in Site 1272 peridotites range from 0.1 to 0.8 wt% (average = 0.6 wt%), whereas the Al₂O₃ concentrations in peridotites from Site 920 along the Mid-Atlantic Ridge at 23°20´N range from 1 to 2 wt% (Casey, 1997) and the median Al₂O₃ in abyssal peridotites worldwide is 1.4 wt% (Bodinier and Godard, in press). Thus, the average Al₂O₃ concentration in 12 peridotites from Site 1272 is lower than at that Site 1271 (average = 0.9 wt%) and 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, 1271, and 1272 are among the most depleted residual mantle peridotites yet obtained from the mid-ocean ridges.

Petrogenesis of Plutonic Rocks

There are two types of gabbroic rocks in core from Hole 1272A. The more abundant type, found in the upper 55 m of the hole, are the miarolitic gabbros or diorites. These have textures and mineral assemblages indicative of rapid cooling. For example, they include both olivine and quartz. The miarolitic cavities indicate that they rapidly became saturated in fluid without efficient degassing on the scale of the core samples, which is more likely during rapid cooling at low pressure. In fact, they may have crystallized in the central part of a large dike rather than in a "magma chamber." As such, they could retain liquid compositions on the scale of the core samples rather than the compositions of cumulate minerals in equilibrium with a melt from which the remaining liquid was later extracted. Figure F39 shows that the compositions of two miarolitic gabbros and three diabase samples lie within the range of SiO₂ content, incompatible element concentrations (illustrated using Zr) and Mg# observed for Mid-Atlantic Ridge basalt glasses in general, and for glasses from the 14°–16°N area in particular. Like glasses from the 14°–16°N region, diabase and miarolitic gabbro from Hole 1272A have high SiO₂ and Zr at a given Mg#, compared to average glasses from the Mid-Atlantic Ridge. The medium-grained miarolitic gabbros have higher SiO₂ contents than the diabases, but (1) their SiO₂ contents are not outside the range observed in glasses and (2) high SiO₂ is not expected for cumulate plutonic rocks. Thus, all appear to be close to liquid compositions.

The other type of plutonic rock from Hole 1272A is a single interval of oxide gabbronorite at ~90 mbsf. This gabbronorite is texturally and mineralogically similar to gabbronorites sampled elsewhere during Leg 209, particularly in Hole 1270B. Oxide gabbronorites will be useful in postcruise research, as oxide-oxide relationships record temperature and oxygen fugacity during magmatic and/or metamorphic processes and oxide-silicate relationships can be used to constrain equilibration pressures.

Hydrothermal Alteration, Metamorphism, and Metasomatism

Many peridotites from Site 1272 are completely altered to serpentine plus brucite and/or talc. Unlike the SiO₂-rich metaperidotites from Sites 1268 and 1270, the compositions of most metaperidotites from Site 1272 are similar to the compositions of fresh dunites and harzburgites with 0%–25% orthopyroxene and thus do not require large metasomatic increases in Si/(Mg + Fe) during alteration (Fig. F40). Probably as a result of the low Si/(Mg + Fe), brucite is an important accessory phase in the background alteration of many peridotites from Sites 1271 and 1272. The implications of the presence of brucite are discussed in more detail in "Metamorphic Petrology" in the "Site 1271" chapter.

Two samples of metaperidotite from the upper 55 m of Hole 1272A contain carbonate-bearing alteration veins. The presence of these veins is reflected in the bulk rock compositions, which lie well off the SiO₂ to MgO + FeO mixing line in Figure F40 and show a positive correlation between CaO and CO₃ (see "Geochemistry" in the "Site 1272" chapter). Addition of carbonate to these metaperidotites also led to high Sr concentrations. This type of metasomatism—which may be common in serpentinized peridotites worldwide—is discussed more extensively in "Site 1274" below.

Fault gouge and some serpentinized peridotites from Site 1272 include substantial proportions of iowaite, a magnesium hydroxide–ferric oxychloride, whose very fine grain size may contribute to the clayey appearance of the rocks. Iowaite has been previously reported in metaperidotites from the Iberian margin and the Izu-Bonin forearc, as discussed in "Metamorphic Petrology" in the "Site 1272" chapter. It may form in oxidizing conditions, where Fe³⁺ in the brucite structure is charge balanced by Cl^– in interlayers (Heling and Schwarz, 1992) or during alteration associated with transport of high-Cl brines (Gibson et al., 1996).

Alteration in harzburgite from Hole 1272A becomes less intense with increasing depth. Decreasing intensity in alteration is mirrored by a downhole increase in seismic velocity, density, and thermal conductivity and a concomitant decrease in porosity and magnetic susceptibility measured on core samples (see "Physical Properties" in the "Site 1272" chapter). For example, P-wave velocity increases from ~3 km/s between 60 and 80 mbsf to ~3.5 km/s between 110 and 130 mbsf. This gradient, ~0.01 km/s/m, is much steeper than gradients observed in seismic studies of the Mid-Atlantic Ridge between 15° and 16°N, which show a relatively gradual increase from 3 km/s near the seafloor to 4 km/s at ~1 km depth at 16°N and from 3.5 km/s near the seafloor to 5.5 km/s at 1 km depth at 15°37´N (0.001–0.002 km/s/m) (J.A. Collins and R.S. Detrick, pers. comm., 1998). Thus, the relatively steep gradient in serpentinization in samples from Hole 1272A, with related gradients in physical properties, must be a local feature, not representative of the regional alteration gradient.

Structural Geology

Orthopyroxene and clinopyroxene in Site 1272 harzburgites show mainly protogranular textures. As in Site 1268 and 1271 harzburgites and orthopyroxene-bearing dunites, orthopyroxene crystals are intergrown with vermicular spinel. This intergrowth suggests that high-temperature exsolution of spinel or formation of spinel during reaction of migrating melt with orthopyroxene postdated deformation in these rocks. Leg 209 peridotites from Sites 1268, 1270, 1271, and 1272 are surprisingly undeformed, unlike typical porphyroclastic harzburgites from ophiolites. Mylonitic shear zones within and near gabbroic veins in peridotite and gabbroic rocks are common at Sites 1268, 1270, and 1271. In Hole 1272A, the relative scarcity of gabbroic rocks intruding the harzburgite section is accompanied by a lack of mylonitic shear zones. The sole gabbroic intrusion into harzburgite, a small oxide gabbronorite, does show signs of crystal-plastic deformation at moderate temperature (between ~600° and 1000°C), but no mylonites were recovered. We infer that this block of peridotite may have been largely insulated from far-field stresses by shear zones with a spacing larger than the scale of the hole, or that continued brittle deformation along long-lasting shear zones converted high-temperature mylonites into fault gouge.

As discussed in "Structural Geology" in the "Site 1272" chapter, at least four fault gouge zones were sampled between 75 and 131 mbsf in Hole 1272A (Fig. F41). Predictably, recovery was poor in these zones and other fault gouge zones may have been present but was not sampled. We tentatively infer the presence of 10 or more gouge zones from the FMS images generated during downhole logging. These gouge zones must be parts of major faults with substantial brittle offsets. They generally strike parallel to the rift axis or have northwest strikes intermediate between the rift axis and the 15°20´ Fracture Zone, in keeping with the position of Site 1272 atop the inside corner high just south of the fracture zone. Dips inferred from the FMS image are both toward and away from the rift and transform valleys.

The fault gouge recovered from Hole 1272A was strikingly plastic when water saturated, as it was when it first came on board. This plasticity may be due to the presence of abundant clay, particularly iowaite, in the gouge. Laboratory experiments show that serpentine muds, like those in the Hole 1272A fault gouge, have low plastic yield strengths (Phipps and Billotti, 1992). Thus, fault zones containing serpentine mud could yield at low stresses and potentially creep aseismically. Additionally, since serpentine muds are much less dense than serpentinites and have yield strengths lower than salt, the muds could rise diapirically along faults, lubricating previously stronger structures. With this said, the fault gouge zones sampled in Hole 1272A clearly had a cataclastic origin because they contain angular serpentinite and gabbroic fragments. Alteration of gouge or serpentinite protoliths to clay may have been pre-, post-, or synkinematic, but there is no doubt that the faults record brittle failure and cataclasis as well as plastic deformation.

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 1272, half-core measurements and discrete samples all have positive inclinations, indicating that the rocks were probably normally polarized. As noted above and in "Structural Geology" in the "Site 1272" chapter, the upper 55 m of Hole 1272A may have sampled a tectonic breccia, though this is uncertain. The average remanent inclination for 14 discrete samples of harzburgite from depths below 55 mbsf is 45.2° (95% CI = +5.4°/–6.7°), and the mean inclination for archive half cores is 42.7° (95% CI = +1.4°/–3.5°). (Two miarolitic gabbro samples from the upper 55 m of the core also have magnetic inclinations in this range.) These values are significantly higher than the expected inclination of 28°. These data suggest that block rotation has affected the section sampled at Site 1272, as also found for Sites 1268 and 1270, where remanent magnetic inclinations require substantial rotation of the section. For Sites 1268 and 1270, we assumed that rotation axes were probably near horizontal and parallel to the normal faults that form steep slopes along the rift valley. At Site 1272, near the eastern inside corner formed by the Mid-Atlantic Ridge and the 15°20´ Fracture Zone, the choice of a tectonically reasonable rotation axis is less clear. Increasing the inclination from 28° to ~44° requires a minimum of 16° of counterclockwise rotation around a horizontal rotation axis with an azimuth of 270° or larger rotations (clockwise or counterclockwise) around axes parallel to the trend of the fracture zone (~290°), the rift valley (~020°), or the northwest-striking foliation, veins, cracks, and faults observed in FMS images from Hole 1272A.

Site 1273 is the southernmost drilling target on our transect of sites north of the 15°20´N Fracture Zone. Video tapes from a precruise submersible survey during Faranaut 15°N Dive 16 revealed extensive steep east-facing scarps on the western wall of the Mid-Atlantic Ridge axial valley with virtually continuous exposure, and serpentinized peridotite was sampled from a water depth just below our intended drilling location. Based on our assessment of precruise survey information, we considered the outcrop at Site 1273 to be the most favorable for drilling of all our primary targets.

Unfortunately, as at Site 1269, unforeseen complications resulted in abandonment of drilling at this site. Low recovery, predominantly of basalt (albeit with three small fragments of serpentinized peridotite), and unstable hole conditions led us to the conclusion that locations where drilling was possible (on flat, sedimented terraces above steep outcrops) were covered with talus shed from farther upslope and were unsuitable for deeper penetration.

Only a few pieces of the basalt recovered from Site 1273 exhibit even incipient seafloor weathering. For the most part the rocks are fresh. Several of the samples appear to be fragments of pillows, based on subtriangular morphology and the occurrence of glassy rims and hyaloclastite on piece edges. These basalts are nearly aphyric, with rare small (length < 5 mm) plagioclase phenocrysts. Unlike the basalts recovered from Site 1269, fragments of basalt recovered from Site 1273 are only slightly to moderately vesicular (maximum < 8 vol%) with a maximum vesicle size of 1.5 mm. In thin section, we can see that the groundmass of the Site 1273 basalts contains acicular plagioclase laths and quench-textured clinopyroxene, with minor amounts of fresh brown glass and skeletal opaque minerals.

A thin section from one of the small fragments of serpentinized peridotite recovered with basalt pieces from Hole 1273C is a completely altered protogranular harzurgite. Olivine in the harzburgite has been completely replaced by serpentine and brown clay, and orthopyroxene is altered to talc, serpentine, and minor chlorite and tremolite.

Site 1274 is located along the track of Shinkai 6500 Dive 416 on the western flank of the Mid-Atlantic rift valley at ~15°39´N. Shinkai 6500 Dive 416 recovered eight peridotite samples from 4434 to 3915 mbsl from weathered outcrops along a gentle slope. Concerned about the possibility that some of these outcrops are large landslide blocks, we chose a drill site near the uppermost peridotite outcrop observed during the dive (sample 416R008) on a relatively small flat spot in the midst of a relatively steep slope. Site 1274 is the northernmost of our transect of drill sites along the rift valley and is ~31 km north of the northwestern intersection of the Mid-Atlantic Ridge and the 15°20´ Fracture Zone. Drilling of Hole 1274A penetrated to a depth of 156 mbsf with a total recovery of 35 m of core. At this point, to continue drilling we would have had to place the entire bottom-hole assembly below seafloor. Because the core had been relatively uniform and drilling conditions were not optimal, we elected to move on to Site 1275. At this point, the pipe became stuck and could only be recovered after dropping the bit.

Proportions of Igneous Rocks

The igneous and residual mantle protoliths of recovered core at Site 1274 were 77% harzburgite, 20% dunite, and 3% gabbro [N7] (Fig. F42). In this way, Site 1274 is similar to the lower portion of Site 1272 and different from Sites 1268, 1270, and 1271, where we recovered ~75% peridotite and ~25% gabbro. The relative lack of gabbroic rocks at Site 1272 could have been viewed as indicating that plutons emplaced in mantle peridotite become rare as the 15°20´ Fracture Zone is approached [N8], in keeping with the idea that magma supply from the mantle is focused to the centers of slow-spreading ridge segments, with lateral transport of melt to segment ends in shallow dikes and lava flows (e.g., Dick, 1989). However, in view of the fact that Site 1274 is not located near a fracture zone or other obvious discontinuity in the Mid-Atlantic Ridge and sits on the flank of a Mantle Bouguer Anomaly low centered at ~16°N (Escartin and Cannat, 1999; Fujiwara et al., 2003), the relative lack of gabbroic rocks in Hole 1274A suggests that gabbroic rocks are not homogeneously distributed in exposed mantle peridotites along the ridge at the scale of our drill, dredge, and submersible sampling.

The proportion of dunite to harzburgite in Hole 1274A is 21/79, much higher than at Site 1272 (4/96). The proportion of dunite to harzburgite at Sites 1268, 1270, and 1271 are 15/85, 10/90, and 90/10, respectively. We infer that dunite proportions are also not uniform at the scale of our drilling, dredging, and submersible sampling.

Highly Depleted Mantle Peridotites

Geochemical analyses of all Site 1274 peridotites show low concentrations of nominally immobile incompatible elements such as Al, Sc, and V. For example, the Al₂O₃ concentrations in Site 1274 peridotites range from 0.2 to 0.9 wt% (average = 0.6 wt%), whereas Al₂O₃ concentrations in peridotites from Site 920 along the Mid-Atlantic Ridge at 23°20´N range from 1 to 2 wt% (Casey, 1997) and the median Al₂O₃ concentration in abyssal peridotites worldwide is 1.4 wt% (Bodinier and Godard, in press). Thus, the average Al₂O₃ concentration in 9 peridotites from Site 1274 is lower than at Site 1271 (average = 0.9 wt%) and as low as the average of 0.6 wt% Al₂O₃ in 12 peridotites from Site 1272, 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, 1271, 1272, and 1274 are among the most depleted residual mantle peridotites yet obtained from the mid-ocean ridges. This result is not consistent with previous inferences, based on the composition of dredged peridotite samples, that the degree of depletion of mantle peridotites decreases from south to north across the 15°20´ Fracture Zone from 14°40´ to 15°40´N along the Mid-Atlantic Ridge (e.g., Fig. F32, accompanying text, and references therein). Instead, our data taken alone suggest that the mantle in this region is uniformly depleted, perhaps as the result of a prior melt depletion event. Resolving the discrepancy between prior results from dredged peridotite samples and our results on peridotite drill core will be an important topic of postcruise research.

Hole 1274A harzburgites include appreciable amounts of clinopyroxene and CaO, generally more than is observed at Sites 1268, 1270, 1271, and 1272. In "Geochemistry" in the "Site 1274" chapter, we show that CaO contents of Hole 1274A peridotites are weakly correlated with Al₂O₃ contents, although these elements do not show correlated variations in peridotites from previous sites. The low CaO in peridotites from previous sites may be due, in part, to hydrothermal metasomatism with removal of Ca during serpentinization. Thus, it could be that the relatively high CaO contents of Hole 1274A harzburgites are representative of the original CaO contents of all harzburgites sampled during Leg 209, preserved at Site 1274 because of the lower extent of hydrothermal alteration there.

It is unclear how much of the clinopyroxene observed in Hole 1274A harzburgites is residual or igneous. In most samples, clinopyroxene is spatially associated with orthopyroxene and, in particular, clinopyroxene is interstitial to broken and partly recrystallized orthopyroxene (Fig. F43). In this association, we believe that some of the clinopyroxene could have been exsolved from orthopyroxene during cooling and recrystallization. However, other clinopyroxene is interstitial to olivine, far from any orthopyroxene (e.g., Fig. F22, in the "Site 1274" chapter), and is probably a residual or igneous phase. We infer that the clinopyroxene is not residual. In view of the highly depleted Al, Sc, and V contents of the peridotites, Site 1274 harzburgites probably represent residuum of partial melting that extended beyond the exhaustion of clinopyroxene as a residual phase (>20%–25% melting and melt extraction). If this inference is correct, the observed interstitial clinopyroxene (and, perhaps, some of the clinopyroxene that is spatially associated with orthopyroxene) is igneous. It probably crystallized from cooling melt migrating along grain boundaries after the peridotites had been incorporated into the thermal boundary layer beneath the Mid-Atlantic Ridge. Trace element analyses of whole rocks and clinopyroxene grains during postcruise research will help to evaluate this hypothesis.

Finally, we caution readers that the relatively high CaO in Site 1274 harzburgites may not be entirely due to the presence of clinopyroxene in the rocks. Site 1274 harzburgites also contain more carbonate alteration than most peridotites from previous sites (see "Hydrothermal Alteration, Metamorphism, and Metasomatism" below).

Petrogenesis of Plutonic Rocks

Deformed oxide gabbronorites intrude the peridotite in Hole 1274A. These gabbronorites, while altered and recrystallized, are texturally and mineralogically similar to gabbronorites sampled elsewhere during Leg 209, particularly in Holes 1270B, 1275B, and 1275D.

Hydrothermal Alteration, Metamorphism, and Metasomatism

Metasomatic Changes in Peridotite

Peridotites from Site 1274 include the freshest mantle samples recovered during Leg 209, with up to 35% of the original mantle minerals preserved. However, the peridotites may have been affected by appreciable major element metasomatism during hydrothermal alteration. Figure F44 shows that Hole 1274A peridotites may have been affected by SiO₂ gain or MgO + FeO loss. Dunites have ~5–13 wt% normative orthopyroxene, and five of the six harzburgites have ~26–29 wt% normative orthopyroxene. These proportions of orthopyroxene could be primary [N9]. However, these orthopyroxene proportions seem high given the very low Al, Sc, and V contents of the rocks, which reflect high degrees of partial melting.

In addition to possible metasomatic shifts in SiO₂/(MgO + FeO) and despite their apparently high degrees of melt depletion, Site 1274 peridotites generally contain higher proportions of components other than MgO, FeO, and SiO₂, compared to peridotites from previous sites sampled during Leg 209. This may be due to several factors. First, as noted in "Geochemistry" in the "Site 1274" chapter, Site 1274 harzburgites contain appreciable clinopyroxene and thus have more CaO than harzburgites from other sites. However, it is not clear that this fully explains the relatively high CaO in Site 1274 peridotites. Figure F51, p. 86, in the "Site 1274" chapter, shows that Site 1274 dunites have less CaO than harzburgites, as expected from their primary mineralogy. However, within the group of harzburgites, CaO is poorly correlated with Al₂O₃ and other indices of pyroxene content and degree of melt depletion.

In addition to CaO in clinopyroxene, peridotites from Site 1274 may have undergone metasomatic enrichment in CaO as a result of carbonate metasomatism. They contain an average of 0.35 wt% CO₂ (1 = ±0.03), significantly higher than in peridotites from Site 1268 (average CO₂ = 0.07 wt%), Site 1270 (average CO₂ = 0.13 wt%, excepting an outlier with 2.2 wt%), Site 1271 (average CO₂ = 0.23 wt%, excepting an outlier with 1.5 wt%), and Site 1272 (average CO₂ = 0.24 wt%, excepting outliers with 7.3 and 9.3 wt%). Figure F45 summarizes our data on CaO and CO₂ contents of peridotites from Leg 209. In the data set as a whole, carbonate addition clearly accounts for most of the variation in CaO contents of these rocks. As discussed in this and previous site summaries, the poor correlation of CaO with CO₂ in samples with low concentrations may reflect analytical uncertainty, CaO in primary mantle pyroxenes, and CaO in impregnations of igneous pyroxene and plagioclase. However, we emphasize that almost all peridotite samples from Leg 209 contain measurable quantities of CO₂, mostly in aragonite and calcite. This also appears to be true of peridotite samples from Site 920, drilled along the Mid-Atlantic Ridge at 23°20´N during Leg 153 (Cannat, Karson, Miller, et al., 1995; Casey, 1997), though some may contain magnesite instead of CaCO₃. Thus, some of the CaO in most of the ODP samples of metaperidotite from the Mid-Atlantic Ridge is probably in carbonate minerals.

It is well established that many metaperidotites lose CaO during serpentinization, apparently because serpentine (as well as talc and brucite) do not readily accept Ca in their crystal structures and other minerals are not stable in the hydrothermal alteration environment. However, although this is not as commonly considered, it is equally clear from our data that metaperidotites sometimes gain CaO during carbonate metasomatism. This may be true of Site 1274 peridotites, despite their relatively low degree of serpentinization.

Aragonite, calcite, and magnesite may contain substantial quantities of Sr and perhaps other trace elements that can form carbonate minerals (e.g., Pb and Ba). Figure F46 shows that Leg 209 metaperidotites with the highest CO₂ also have the highest Sr contents and among the highest Ba contents [N10]. During postcruise research, it will be interesting to determine the trace element contents of metasomatic carbonate minerals in peridotites from Leg 209 and the extent to which these minerals influence the whole-rock trace element budget. Meanwhile, interested readers should note that elements that are highly incompatible during partial melting of mantle peridotite may be added to metaperidotites during carbonate metasomatism. As a consequence, whole-rock trace element analyses of hydrothermally altered peridotites, even where they still contain tens of percent of fresh, primary minerals, should be interpreted with caution.

On a different but related topic, Site 1274 samples include ultramafic mud, forming the matrix of fault gouge in several horizons. These muds have high TiO₂ and Al₂O₃ as well as CaO and CO₂ compared to Site 1274 peridotites. Since the fault gouge includes clasts of gabbroic rock as well as peridotite, we infer that the elevated TiO₂ and Al₂O₃ in the muds is due to mechanical mixing of gabbroic material with peridotite, whereas the high CaO may be a consequence of both this mixing process and addition of metasomatic carbonate.

Downward Increase in Serpentinization

There was a clear increase in the extent of alteration of peridotites with increasing depth in Hole 1274A. In general, olivine shows >80% serpentinization, whereas pyroxenes are only 50%–60% altered in the uppermost 60 m, with a gradual increase to nearly 100% alteration of both olivine and pyroxene near the bottom of the hole at 156 mbsf. This gradient of increasing serpentinization away from the seafloor is probably due to alteration associated with large faults, represented in the core by extensive fault gouge zones sampled between ~95 and ~145 mbsf. In fact, the degree of alteration may decrease again beneath 145 mbsf, though it is difficult to be sure given our limited sampling below this depth.

The downhole increase in the extent of serpentinization is accompanied by a sharp increase in the proportion of secondary magnetite, reflected in a correlation between magnetic susceptibility and the intensity of alteration as estimated during visual core description (Fig. F47). Density and seismic P-wave velocity decrease downhole (see "Physical Properties" in the "Site 1274" chapter) and are significantly correlated with the intensity of alteration (Fig. F47). These relationships could provide a useful tool in geophysical studies of near-seafloor, partially serpentinized peridotites.

Metamorphic Parageneses

As at Sites 1271 and 1272, brucite is abundant in core from Hole 1274A. This is in keeping with the relatively low SiO₂/(MgO + FeO) for peridotites from all these sites, which are substantially different from the relatively talc-rich, high-SiO₂/(MgO + FeO) peridotites exemplified by samples from Site 1268, but also observed at Site 1270.

Clinopyroxene is relatively abundant in peridotite from Hole 1274A. Where it has been incipiently altered, it is associated with talc and tremolite. This association may be explained by hydrothermal metasomatism, involving substantial input of Mg (±Fe) and extraction of Ca, as explained in "Metamorphic Petrology" in the "Site 1274" chapter. However, we have no independent evidence for addition of MgO to Site 1274 peridotites and Ca extraction—if any—was limited. As an alternative, formation of tremolite can be explained as a result of reactions involving H₂O and primary phases present in the peridotites, such as

forming tremolite + olivine. Similarly, talc formation in metaperidotites is generally ascribed to reactions such as

All of these reactions require only H₂O addition from hydrothermal fluid, and thus may occur even in cases where independent evidence for metasomatism is lacking.

Brucite is observed in Site 1274 peridotites that also contain pyroxene and/or talc. This assemblage is almost certainly metastable because under most metamorphic conditions brucite and talc break down to form serpentine [N11] via

Thus, the presence of brucite and talc together in serpentinized peridotites from Site 1274 attests to the presence of local disequilibrium, with fluid composition and/or reaction kinetics varying on the scale of a few millimeters. In turn, this suggests that fluid fluxes and fluid/rock ratios may have been low during alteration. As discussed in "Metamorphic Petrology" in the "Site 1274" chapter, formation of brucite, serpentine, and magnetite is interpreted to reflect local hydration and oxidation of olivine. If so, brucite may be more abundant in the more altered, deeper parts of Hole 1274A that show high magnetic susceptibility associated with relatively large proportions of magnetite.

Ultramafic fault gouge recovered from Site 1274 includes abundant clay, mainly nontronite (Na_0.3Fe³⁺₂[Si, Al]₄O₁₀[OH₂] · nH₂O). As discussed in "Metamorphic Petrology" in the "Site 1274" chapter, the presence of ferric iron–rich nontronite, as well as the development of aragonite veins with oxidation halos present to depth of 90 mbsf, suggests that water-rock reactions continued at low temperatures under oxidizing conditions. This is striking because the main fault gouge zones are well below the seafloor at depths of ~95 to ~145 mbsf; indications of oxidizing alteration and weathering have been restricted to shallower depths at all previous sites on Leg 209. In addition, this result is somewhat unexpected because the dark green to black color of the fault gouge muds initially suggested to us that they lack hematite and Fe oxyhydroxides and, thus, that they formed under reducing conditions. It will be interesting to further investigate the redox conditions during fault gouge formation during postcruise research.

Structural Geology

Two main structural features were observed in Hole 1274A. First, as in the peridotites at Sites 1272, 1271, 1270, and 1268, protogranular textures are abundant and porphyroclastic textures are nearly absent. Second, the hole intersected large fault zones, as represented by fault gouge at depths of ~95, 100, 110, 125–135, 140, and 145 mbsf. We discuss each of these in turn.

In Hole 1274A peridotites, it was difficult to even detect and measure a high-temperature foliation. During Leg 209, we increasingly focused on this apparent lack of deformation. In this, as in previous holes, orthopyroxene in mantle peridotites shows little sign of crystal-plastic deformation. Orthopyroxene is commonly interstitial to olivine, with long, narrow projections along olivine grain boundaries. Orthopyroxene crystals are intimately intergrown with complex skeletal spinels that have no discernible shape fabric in three dimensions. Large skeletal spinels, several millimeters long and just a few hundred micrometers wide, with branches at high angles to each other, are also interstitial to olivine where no orthopyroxene is present. Although these observations are qualitative, it is difficult to believe that these textures can have survived significant shear strains.

In the Oman ophiolite, where residual mantle peridotites have been studied extensively on scales of meters to tens of kilometers, high-temperature protogranular textures are restricted to small zones—often interpreted as "diapirs"—with near-vertical spinel lineation. Outside these zones of vertical spinel lineation, residual mantle peridotites have lower-temperature porphyroclastic textures that record pervasive ductile deformation of the upper mantle at 1000°–1200°C, with foliation and spinel lineation that are subhorizontal, roughly parallel to the plane of the crust/mantle boundary (e.g., Boudier and Coleman, 1981; Ceuleneer, 1991; Ceuleneer and Rabinowicz, 1992; Jousselin et al., 1998; Nicolas and Rabinowicz, 1984; Nicolas and Violette, 1982).

In the region investigated during Leg 209, protogranular textures appear to be pervasive and both porphyroclastic textures and well-developed spinel lineations are rare. This observation needs to be checked and quantified by extensive shore-based research. However, it appears that pervasive Moho-parallel deformation in the upper mantle was rare or absent beneath the Mid-Atlantic Ridge from 14°43´ (Site 1270) to 15°39´N (Site 1274). We believe that this cannot be due to an absence of deformation at the plate scale. Substantial shear stresses must have developed in the uppermost mantle in this region as a result of (1) corner flow associated with plate spreading, (2) possible diapiric upwelling of buoyant mantle, and (3) uplift of mantle peridotite from the base of the thermal boundary layer to the seafloor. However, these stresses must have been accommodated by strain localization along the mylonitic shear zones, many of which developed within and near gabbroic veins in peridotite and within gabbroic rocks, as observed at Sites 1268, 1270, and 1271. If this hypothesis is correct, strain localization must have been initiated at temperatures >1200°C.

Tellingly, a single piece of mylonitic peridotite was recovered from Hole 1274. Although this fragment was in Core 209-1274A-2R, we believe that it represents debris that fell from higher in the hole during core recovery. As at Site 1270, where we spudded into a mylonitic fault surface that had been sampled during the dive survey in 1998, we suspect that there is a mylonitic surface at the top of the seafloor outcrop at Site 1274 that was not sampled in the first core, which recovered only 1.1 m of core in 11.9 m of penetration. In general, we have the impression that several Leg 209 holes penetrated little deformed blocks overlain by poorly sampled, narrow, high-temperature peridotite and gabbro mylonites that form the outcrop surface.

As at Sites 1268, 1270, 1271, and 1272, Hole 1274A encountered numerous zones of poorly consolidated serpentinite + clay fault gouge (Fig. F48). Recovery of gouge was poor at several sites; therefore, the widths of the associated fault zones are uncertain. However, we recovered several meters of continuous gouge at various depths in Holes 1272A and 1274A. These thick gouge zones must represent major faults. The gouge had an unusual rheology, unfamiliar to us, with a pliable claylike texture when it first came on board. We suspect that this type of material has an extremely low plastic yield strength and—once formed as a result of cataclastic deformation during brittle failure—it could undergo continuous aseismic creep. Faults lined with serpentinite + clay fault gouge probably have very low strength and could remain active even at low dip angles.

Peridotite outcrops along the flanks of the Mid-Atlantic rift valley are commonly interpreted to be footwall exposures that were originally overlain by normal fault surfaces (e.g., Cannat and Casey, 1995). The ubiquity of subsurface faults lined with gouge, found in all holes in peridotite that penetrated deeper than 35 mbsf, suggests that peridotite outcrops along the flanks of the Mid-Atlantic rift valley are also generally underlain by weak faults. Thus, we have formed a tentative picture in which the rift valley walls are underlain by numerous anastomosing fault zones parallel to the regional slope on the order of 100 m apart, perpendicular to the fault plane. Some of these fault zones, particularly in Hole 1268A, clearly overprint earlier high-temperature mylonitic shear zones. Poor recovery associated with most of the fault zones makes it impossible to determine whether this relationship is common. In any case, each of the fault gouges was formed during brittle failure and each represents a weak plane of potential future deformation, whether by normal sense displacement during tectonic extension or as the sole of a landslide.

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").

Half-core measurements and discrete peridotite samples all have positive inclinations that are presumably of normal polarity, although the mean inclination (13°) is significantly shallower than the expected dipole inclination at the site (28°). It was difficult to discern high-temperature foliations in peridotite from Hole 1274A, as discussed above. Those that could be identified could only be measured in the cut face of the core, yielding apparent dips in this plane. Since the cut face was generally oriented perpendicular to the serpentine foliation, rotation of the apparent dips of high-temperature foliation into a common remanent magnetization azimuth must yield a cluster of poles with east-northeast strikes. However, in addition to the predictable clustering, the apparent dip data seem to form a girdle on a stereographic projection (see "Structural Geology" and Fig. F41, in the "Site 1274" chapter"), consistent with folding of the high-temperature foliations striking parallel to the serpentine foliation.

This result is intriguing but remains uncertain. In addition to the difficulties outlined in the previous paragraph, microscopic examination of core yielded foliation directions, based on the presence of elongate orthopyroxene grains and faint spinel trains, that were nearly perpendicular to the serpentine foliation and to the macroscopic fabric marked by alternating light and dark patches visible in core from afar. Our preferred interpretation is that the macroscopic foliation simply represents less serpentinized lenses within the serpentine foliation and is not really a high-temperature feature, in keeping with similar observations and interpretations at ODP Site 920 (Ceuleneer and Cannat, 1997), but uncertainty about this persists. It will be very helpful to measure olivine lattice fabrics in these rocks during postcruise research.

The average remanent inclination for 17 discrete samples of harzburgite is 13.4° (95% CI = +7.4°/–8.1°), and the mean inclination for archive half cores is 18° (95% CI = +2°/–3°). These values are significantly lower than the expected inclination of 28°. These data suggest that block rotation has affected the section sampled at Site 1274, as was also found for Sites 1268, 1270, and 1272, where remanent magnetic inclinations require substantial rotation of the section. For Sites 1268 and 1270, we inferred that rotation axes were probably near horizontal and parallel to the normal faults that form steep slopes along the rift valley. At Site 1272, near the eastern inside corner formed by the Mid-Atlantic Ridge and the 15°20´ Fracture Zone, the choice of a tectonically reasonable rotation axis is less clear. Near Site 1274, there are many south-facing slopes, quite distinct from the east-facing slopes that make up most of the western flank of the rift valley in this region. Thus, the choice of an axis for the rotation of the Site 1274 section is not well constrained. Decreasing the inclination from 28° to ~15° requires a minimum of 13° of clockwise rotation around a horizontal rotation axis with an azimuth of 270° or larger rotations around axes parallel to the trend of the Fracture Zone (~290°) or the rift valley (~020°).

Site 1275 is located along the track of Shinkai 6500 Dive 422 on the summit of a topographic dome, known informally as Mt. Mike, ~10 mi west of the Mid-Atlantic rift valley at ~15°44´N. This dome has been interpreted on the basis of bathymetric data as an oceanic core complex, or "megamullion" (Escartin and Cannat, 1999; Escartin et al., 2003; Fujiwara et al., 2003; MacLeod et al., 2002). Dive 422 recovered seven samples of gabbro from ~2290 to ~1750 mbsl, mostly from steep blocky outcrops along the southern flank of the dome, and then two samples of peridotite and one sample of gabbro from nearly flat, striated outcrops and overlying talus across the summit of the dome at ~1650–1540 mbsl. In 2001, the James Clark Ross made a survey of this dome using the BRIDGE portable wireline rock drill and dredging to sample (Escartin et al., 2003; MacLeod et al., 2002). In addition to gabbro and peridotite, short (~1 m) drill cores from the summit of the dome recovered abundant diabase.

Holes 1275A and 1275B were located in an area with horizontal, striated outcrop surfaces extending for ~100 m², ~300 m north of the top of the dome and 90 m north of the northernmost Shinkai 6500 sample, 422-R010 (described as highly altered plagioclase-bearing dunite and peridotite, recovered from talus resting on a flat outcrop surface). In an attempt to recover as much material as possible from the upper 5 m of the outcrop, we retrieved the core barrel after 5 m of drilling. Core 209-1275A-1R was composed of a small amount of diabase and carbonate-cemented diabase breccia. Recovering this core involved lifting the drill bit above seafloor, so we immediately spudded Hole 1275B a few meters away. Diabase composed 100% of the rocks from Cores 209-1275B-1R and 2R and >50% of Core 3R, extending to 18 mbsf. Below this depth, core was dominated by gabbroic rocks intruded by a few diabase sills or dikes, except within an interval of "troctolite" or impregnated peridotite from ~30 to ~35 mbsf. Given the ultramafic lithology in Shinkai 6500 sample 422-R010, we were surprised that the first ultramafic rocks in Hole 1275B were not recovered until the hole reached a depth of 30 mbsf.

After 109 m of drilling in Hole 1275B, it was necessary to change the bit. We marked Hole 1275B for possible reentry but also used the opportunity to move to a position directly above sample 423-R010 to test the resistivity-at-the-bit with coring (RAB-C) coring equipment. The RAB-C test in Hole 1275C recovered very little core, but this included two small pieces of troctolite. Thus, we elected to spud Hole 1275D just a few meters away from Hole 1275C to determine the extent of lateral variability of the lithologies at this site and to provide a complementary rotary core barrel (RCB) hole to fully evaluate the coring potential of the RAB-C equipment. The first three cores in Hole 1275D recovered almost 8 m of core over ~17.6 m of penetration, the best recovery for the first three cores in any hole drilled during Leg 209. By comparison, Hole 1270C, drilled just a few meters away with the RAB-C bit, recovered just a few tens of centimeters of core over ~21 m of penetration, which was the worst recovery for the first three cores in any hole during Leg 209 (Fig. F49).

In striking contrast to Hole 1275B, "troctolite," or impregnated peridotite, was recovered in the first 11 cores of Hole 1275D, extending to ~56 mbsf, though diabase was also present in the first core. Below 56 mbsf, the hole passed into gabbroic rocks intruded by diabase sills or dikes, with a second interval of troctolite between ~80 and 95 mbsf. Thus, although Hole 1275D was only ~90 m south of Hole 1275B, the lithostratigraphy is quite different in the two holes. Correlation of rock units from the two holes is still in an initial stage. However, the simplest interpretation seems to be that the lower troctolite in Hole 1275D, flanked by gabbroic rocks, is correlative to the first (and only) troctolite in Hole 1275B, which is also surrounded by gabbroic rocks. If so and assuming a roughly planar upper contact between troctolite and overlying gabbro, the apparent dip of this contact is ~30° to the south in the plane formed by the two drill holes (Fig. F50). Intriguingly, this dip is parallel to the south slope of the dome along the track of the Shinkai 6500 Dive 422 from sample 422-R003 to 422-R008 (see Fig. F2B, in the "Site 1275" chapter).

Proportions of Igneous Rocks

The igneous and (possibly) residual mantle protoliths of recovered core at Site 1275, weighting the proportions of recovered lithologies by the depths of each hole, were 14% "troctolite" or impregnated peridotite, 74% gabbroic rocks (gabbro, gabbronorite, oxide gabbro, oxide gabbronorite, and olivine gabbro), 10% diabase, and 2% granophyre (Fig. F50). The large proportion of gabbroic rocks and the presence of diabase were no surprise, given past submersible sampling and more recent dredge and BRIDGE drilling results from Mt. Mike (e.g., Fig. F2B, in the "Site 1275" chapter). Obviously, Site 1275 is very different from all previous sites drilled during this leg. Drilling at Sites 1274 and the lower portion of Site 1272 recovered just a few percent gabbroic rocks, and at Sites 1268, 1270, and 1271 we recovered ~75% peridotite and ~25% gabbro. Considering all these sites together, without weighting for the different depths of drilling, we can tentatively infer that the crust from 14°39´ to 15°44´ is composed of 20%–40% variably altered gabbroic rocks and 80%–60% partially serpentinized peridotite. As discussed in "Site 1268," it is possible that these proportions of gabbro to peridotite extend to depths below the Moho.

Petrogenesis of Troctolites or Impregnated Dunites

As mentioned above, a lithology termed "troctolite" in lithologic logs and site chapters was recovered in Holes 1275B, 1275C, and 1275D. Several meters of troctolite were recovered from between ~30 and 35 mbsf in Hole 1275B, and tens of meters of "troctolite" were recovered from the upper 95 m of Hole 127D (Fig. F51). The rocks are highly altered, but relict primary phases remain and the alteration was static so that pseudomorphic assemblages allow unambiguous determination of the primary mineralogy. The protolith of this rock type was composed of 50% to almost 100% olivine with 1%–3% chromian spinel surrounded by poikilitic orthopyroxene and clinopyroxene, and by interstitial plagioclase. Minor brown hornblende in these rocks could be igneous but is probably of metamorphic origin. Olivine crystals, ranging from 1–8 mm (average = 4 mm), are generally rounded and locally embayed, but no reaction rims are present between any of the primary phases.

Because the rock is coarse grained and the proportions of the minerals vary on a centimeter scale, different parts of the "troctolite" could be classified as dunite, harzburgite, wehrlite, troctolite, or lherzolite. Nonetheless, the different rock types grade into one another and are spatially continuous. Also, chemical analyses reveal that they are relatively homogeneous from one part of the core to another when sample sizes are a few cubic centimeters. For simplicity, throughout this volume we refer to all these rocks as troctolites.

The troctolite is cut by numerous centimeter-scale dikes and veins of gabbroic to felsic material. These are almost all 100% altered to talc + amphibole assemblages. However, the presence of pyroxene oikocrysts that cross contacts between troctolite and intruding gabbroic rock suggests that the two rock types are genetically related. Although the gabbroic dikes and veins that cut the troctolite may be coarser than most gabbroic rocks elsewhere in Holes 1275B and 1275D, the two are probably related. Abundant magnetite is observed in most gabbroic rocks from Site 1275, including the dikes and veins cutting troctolite (see description of thin section 209-1275D-8R-1 [Piece 18, 128–133 cm]), and zircon is also present in some dikes, indicating an evolved parental melt composition (thin section 209-1275B-6R-2 [Piece 5, 57–61 cm]) (see "Site 1275 Thin Sections"). We have observed qualitatively in ophiolites that grain size in gabbro increases near intrusive contacts with peridotite (P.B. Kelemen, H.J.B. Dick, and J.F. Casey, pers. comm., 2003). Thus, we interpret the troctolite lenses at 30–35 mbsf in Hole 1275B and 80–95 mbsf in Hole 1275D as rafts of older rock included within a gabbroic pluton and the thick troctolite unit that forms the upper 56 m of Hole 1275D as another raft, cut by the fault that forms the top of the dome, or as a part of the roof of the pluton.

The troctolite is of great interest for two main reasons. First, the apparent equilibrium between igneous olivine, pyroxenes, plagioclase, and spinel places important constraints on the pressure at which this assemblage crystallized. Second, some of the olivine in the troctolite may be relict, residual mantle olivine and thus the presence of the troctolite may indicate that the gabbroic rocks from Site 1275 were intruded into mantle peridotite. We discuss each of these topics in turn in the following paragraphs.

First, we discuss constraints on equilibrium between basaltic liquid, olivine, orthopyroxene, clinopyroxene, spinel, and plagioclase. As is well known to igneous petrologists, the five solid phases in this assemblage are related by reactions such as

which consumes olivine plus the anorthite component in plagioclase, stable at relatively low pressure, to produce orthopyroxene, clinopyroxene, and spinel at higher pressures. Because of constrained Fe/Mg exchange between all these phases, this assemblage is nearly univariant in Ca-Mg-Al-Si-Fe. However, because albite (NaAlSi₃O₈) is an important component in plagioclase and Cr spinel (e.g., MgCr₂O₄) is an important part of spinel solid solutions in these rocks, there is actually a broad range of pressure and temperature conditions over which this assemblage can be stable. Because this assemblage also contains several geothermometers (e.g., pyroxene solvus and Cr/Al exchange between orthopyroxene and spinel) and because of recent improvements in the understanding of spinel solid solutions, during postcruise research it will be possible to constrain the pressure and temperature at which all these solid phases last equilibrated.

Meanwhile, we can infer likely pressures at which basaltic liquid could have been in equilibrium with olivine, orthopyroxene, clinopyroxene, spinel, and plagioclase [N12]. The troctolites have whole-rock Mg#s of 86%–89%, similar to and slightly lower than typical values for oceanic residual mantle peridotites. This is fortunate, because an extensive body of experimental work has been devoted to determining the relationships between liquid compositions, pressures, and temperatures for equilibrium between melts and the mineral assemblages in mantle peridotites. The presence of plagioclase in this assemblage indicates that the pressure was less than ~1.2 GPa (e.g., Green and Hibberson, 1970). Conversely, it is well known that the minimum pressure for orthopyroxene saturation in primitive melts is a function of silica content in the melt (e.g., Elthon and Scarfe, 1984; O'Hara, 1965; Stolper, 1980; recent summary figure in Braun and Kelemen, 2002). Thus, primitive tholeiitic basalts with ~49 wt% SiO₂ are last saturated in orthopyroxene at a pressure of ~1 GPa (e.g., Baker and Stolper, 1994), whereas primitive andesites with ~54 wt% SiO₂ can be saturated in orthopyroxene at ~ 0.5 GPa (e.g., Jaques and Green, 1980) and boninitic melts with ~58 wt% SiO₂ can crystallize orthopyroxene at 0.1 MPa (e.g., Crawford, 1989). In this context, the relatively SiO₂-rich nature of MORB from the 14°–16°N region along the Mid-Atlantic Ridge, first noted in "Site 1268" is significant.

Kinzler and Grove (1992, 1993) developed a relatively simple algorithm that relates the composition of melts saturated in olivine, orthopyroxene, clinopyroxene, and plagioclase (± spinel) to pressure and temperature. Although their main goal was to calculate melt compositions given a source peridotite composition, pressure, and temperature, their method can be inverted to estimate pressure and temperature from a melt composition. Figure F52 shows predicted pressures vs. experimental pressures for the suite of olivine + orthopyroxene + clinopyroxene + plagioclase + melt experiments that was used to calibrate the Kinzler and Grove expressions. The agreement between predicted and experimental values is good, with an uncertainty of approximately ±0.2 GPa, similar to the uncertainty in pressure calibrations for the piston cylinder apparatus used in the experiments.

Accordingly, we have taken all published compositions of basaltic glasses from 14° to 16°N along the Mid-Atlantic Ridge and calculated the pressures and temperatures at which each could be saturated in olivine, orthopyroxene, clinopyroxene, and plagioclase. The results are illustrated in Figure F53. Estimates for primitive glass compositions (molar Mg# = 60%–73%) form a tight cluster, averaging 0.54 GPa (2 = ±0.14 GPa) and 1220°C (2 = ±16°C), though a few glasses yield higher temperature and pressure values. Because the data are tightly clustered, the standard errors of the mean for the average pressure and temperature are very small (~15 MPa and 2°C). It should be noted that these calculations are based on the assumption that the MORB glasses could be saturated in olivine, orthopyroxene, clinopyroxene, and plagioclase at some pressure and temperature and does not prove that they were in equilibrium with all these phases at any time. Nevertheless, the more primitive basalt glass compositions are close to Fe/Mg exchange equilibrium with the minerals in the troctolite, whose composition can be inferred from the whole-rock Mg#. Because some of the glasses are very primitive, we conclude that they probably were close to saturation in a mantle phase assemblage including olivine, two pyroxenes, and plagioclase.

Thus, we believe that the data in Figure F53 indicate that the Site 1275 troctolites probably formed in equilibrium with a melt at 0.40–0.68 GPa. Because the orthopyroxene, clinopyroxene, and plagioclase in these rocks clearly crystallized from a melt, it follows that conductive cooling and crystallization of rising magma began at a depth of 12–20 km beneath the Mid-Atlantic Ridge, at least in this region. This result is consistent with thermal models for slow-spreading ridges, which predict that the thermal boundary layer extends to depths of ~20 km beneath the Mid-Atlantic Ridge (Braun et al., 2000; Reid and Jackson, 1981; Sleep, 1975). This result is also consistent with fractional crystallization models, which require pressures of 0.4–0.8 GPa to account for observed variation in the composition of genetically related basalts from the Mid-Atlantic Ridge in general (Grove et al., 1992; Meurer et al., 2001; Michael and Chase, 1987) and from the 14°–16°N region in particular (C. Xia et al., unpubl. data). Finally, in keeping with the observation that residual mantle peridotites are extensively exposed in outcrop in the 14°–16°N region, this result reminds us that rocks that were incorporated into the plate at great depth have been tectonically uplifted and unroofed on the seafloor. And finally, since the troctolites lack evidence for viscous deformation, we can infer that at depths above 12–20 km the tectonic exhumation process involves localized deformation at a temperature less than ~1220°C.

More evolved glass compositions (molar Mg# = 50%–60%) from basalts sampled between 14° and 16°N yield estimated pressures ranging from ~0.6 to ~0.3 GPa and temperatures ranging from 1220° to 1175°C. As can be seen in Figure F53, the pressure-temperature estimates for the evolved glasses form a cluster at significantly lower pressures and temperatures compared to the cluster for more primitive glasses. Although further analysis is necessary, this fact suggests that crystallization of melt in the thermal boundary layer beneath the Mid-Atlantic Ridge is a polybaric process in which increasingly evolved melts crystallize at increasingly shallow depths.

We turn now to the question of whether the olivine in the troctolites is igneous or is relict, residual olivine from mantle peridotite. If the olivine is igneous, the troctolites would be the most primitive igneous cumulates yet recovered from a mid-ocean ridge. If the olivine is derived from mantle peridotite, then the troctolites should be viewed as impregnated peridotites, similar to those sampled at Site 1271 and to impregnated peridotites dredged from the mid-ocean ridges and common in some ophiolites. In thin section, some spatially isolated olivine crystals appear to be optically continuous with other nearby olivine grains, though they are now separated by intervening pyroxene and plagioclase. Thus, some rocks may preserve an olivine lattice-preferred orientation, despite the undeformed nature of the pyroxene, plagioclase, and alteration assemblages. If so, this would be consistent with a polygenetic origin for the rocks, composed of older, previously deformed olivine grains that have been partially disaggregated and incorporated in a matrix of younger igneous minerals.

In "Site 1271," we argued that covariation of olivine Mg# and Ni contents can be used to distinguish some impregnated peridotites from cumulate igneous peridotites. Where olivine Mg#s of impregnated or cumulate peridotites are similar to typical values for mantle peridotites, olivine cumulates cannot be distinguished from mantle dunites. However, where the Mg#s of impregnated or cumulate peridotites are lower, in the range of 88%–85%, then high Ni contents in olivine at a given Mg# are indicative of an origin via melt/rock reaction and impregnation (e.g., Godard et al., 2000; Kelemen, 1986, 1990), whereas lower Ni contents at the same Mg# are consistent with an origin via fractional crystallization (e.g., Hart and Davis, 1978). Because the Site 1275 troctolites are highly altered (some contain up to 9 wt% CO₂), it is difficult to be sure about the composition of the remaining olivine. However, we have inferred the olivine compositions, based on the bulk compositions of samples with 0.2–3 wt% CO₂. To do this, we calculated CIPW norms for the anhydrous bulk composition. These yielded normative proportions ranging from 57 to 78 wt% olivine (with four of six compositions between 70 and 72 wt%), 11 to 30 wt% pyroxene (four of six between 11 and 15 wt%), and 4 to 12 wt% plagioclase (four of six from 9 to 12 wt%). We then estimated the original Ni concentration in olivine, assuming Ni partitioning between olivine and pyroxenes was governed by partitioning at 1250°C, using relationships derived by Kelemen et al. (1998a), and that there is no Ni in plagioclase (or any other phase, including sulfide). The calculated Ni concentrations in olivine are plotted in Figure F54, together with similar estimates for Ni in olivine from mantle harzburgites and dunites recovered at other sites during Leg 209 and with published olivine compositions from dunites, impregnated dunites, and wehrlites in the mantle transition zone of the Oman ophiolite.

It is apparent that the estimated Ni contents of olivine in the Site 1275 troctolites are higher than those predicted for crystal fractionation of olivine. This is consistent with the hypothesis that the troctolites are the products of melt-rock reaction, in which Fe/Mg exchange with migrating melt lowered olivine Mg#s but the original, high Ni concentrations were retained in the olivine. Therefore, we tentatively infer that the Site 1275 troctolites are impregnated peridotites, containing relict, residual olivine derived from mantle peridotites plus pyroxenes and plagioclase that crystallized from a cooling melt migrating along olivine grain boundaries.

Petrogenesis of Gabbroic Rocks and Diabase

Gabbroic rocks, including gabbro, gabbronorite, oxide gabbro, oxide gabbronorite, and olivine gabbro, compose ~74% of the core from Site 1275. Diabase composes an additional 10% of the core, particularly in a thick interval at the top of Hole 1275B. The gabbroic rocks are characterized by large variations in mineral proportions and textures on short length scales (Fig. F55), such that the longest interval with homogeneous grain size and mineral proportions is only a few meters. Contacts between contrasting lithologies are generally sharp and planar, though "crenelated" contacts are also observed in dozens of intervals (Fig. F56). Diabase contacts with gabbroic rocks are very sharp. However, there is a gradation in grain size between coarse-grained diabase and fine-grained gabbroic rocks. Thus, the range of grain sizes appears to represent a continuum and we tentatively infer that the gabbroic sections were constructed by multiple intrusions that continued over a range of temperatures and cooling rates. Although steep contacts are locally observed, the great majority of contacts between contrasting gabbro and diabase units have dips <30°, so that—unless the section has undergone large tectonic rotations—most of the intrusions may have been subhorizontal sills.

Diabase and fine-grained gabbros have compositions that resemble basaltic liquid compositions. If they are liquid compositions, the melt was relatively primitive, with Mg# = ~60%. This is striking, because the coarser gabbroic rocks from Site 1275 are probably cumulates formed by crystallization of substantially more evolved melts. We interpret the coarser gabbroic rocks as cumulates because they have low incompatible element abundances (e.g., low Zr) compared to evolved MORB at the same, or even higher, Mg#. However, there is little sign of the complementary evolved melts that formed these cumulates, either in core from Site 1275 or among erupted lavas from the 14°–16°N region.

Granophyric dikes and veins are a minor but ubiquitous part of the gabbroic core. These conceivably represent melts in equilibrium with the evolved cumulate gabbros, but if so there must be a much larger volume of lava or intrusive rocks with the same composition somewhere else. The granophyres have sharp to completely gradational contacts with host gabbros and probably represent small amounts of highly evolved melt that were extracted from the gabbros in their last stages of crystallization and then ascended to intrude other gabbros higher in the section.

Granophyres locally show complex mingling relationships with host gabbros, and even host diabase. Similarly, some contacts between contrasting gabbroic units also show complex mingling and some fine-grained gabbroic units contain coarse pyroxene crystals that are tentatively interpreted as xenocrysts derived locally from coarser gabbroic host rocks. Thus, the multiple intrusions that constructed the Site 1275 gabbro section occurred while earlier crystallized gabbroic rocks were still partially molten. This process of repeated intrusion led to mingling and, probably, to magma mixing.

Olivine gabbros were only observed in lower portions of Hole 1275D. Initial postcruise analyses show that two olivine gabbros have Mg#s of 79% and 85% and ~690–1050 ppm Ni. These values are higher than those observed in primitive MORB glasses and higher than those for melts in equilibrium with residual mantle olivine in abyssal peridotites. Thus, we infer that the olivine gabbros are primitive cumulates. As such, along with the primitive(?) gabbronorites from Hole 1268A, these are among the most primitive cumulates yet sampled from the mid-ocean ridges.

Aside from the olivine gabbros, the diabases, and one or two fine-grained gabbros with high Mg#s, the gabbroic rocks from Site 1275 that we analyzed have evolved compositions with low Mg# and high incompatible element concentrations. Such evolved compositions appear to be typical of gabbroic intrusions into peridotite that have previously been sampled by dredging and submersibles along the mid-ocean ridges (e.g., Cannat, 1993; Cannat et al., 1992). The common observation of evolved gabbroic intrusions into mantle peridotite poses an interesting conundrum. Evolved lavas with compositions that could crystallize such iron- and titanium-rich gabbroic cumulates have not been sampled in the 14°–16°N area. By the same token, basalt compositions from the Mid-Atlantic Ridge in this area record fractional crystallization of a relatively primitive cumulate assemblage.

In fact, this problem is more general. As shown in Figures F57 and F58, truly primitive cumulates are rare among oceanic samples, even in the thick gabbroic sequences at ODP Sites 735, 921, and 923 that have been interpreted as parts of a lower crustal gabbro section beneath sheeted dikes in "typical" layered oceanic crust (e.g., Cannat et al., 1995; Dick et al., 2002; Natland and Dick, 2002). Because average MORB has about twice as much zirconium and other incompatible elements as primitive MORB, the average lavas record fractional crystallization of ~50% mafic cumulates, of which at least one-half must have Mg#s > 70%. In contrast, the average Mg# of rocks recovered from Sites 735, 921, and 923 is 67%–71%. It is clear that a substantial proportion of more primitive cumulates has formed beneath the mid-ocean ridges. However, these primitive cumulates have rarely been sampled.

During Leg 209, only the gabbronorites from Site 1268 and olivine gabbros from Hole 1275D have the high Mg# and low incompatible element concentrations required for the primitive cumulates complementary to fractionation of typical MORB. This is intriguing because, as for the troctolites at Site 1275, the Site 1268 gabbronorites include igneous orthopyroxene. MORB is saturated in orthopyroxene only at subcrustal pressures (see discussion of troctolite genesis, above). As we outlined in "Site 1268," if the Site 1268 gabbronorites formed from primitive basalts, they must have crystallized at relatively high pressure (perhaps ~0.3–0.4 GPa) within an upper mantle thermal boundary layer beneath the Moho. However, we caution readers that the Site 1268 gabbronorites are highly altered, and alteration may have modified the bulk rock Mg#, so that full evaluation of this hypothesis awaits shore-based analysis of relict clinopyroxene crystals.

The depth of crystallization of the relatively evolved gabbroic rocks at Site 1275 is not well constrained. If the earliest formed gabbroic rocks are temporally related to the melts that crystallized impregnations of plagioclase and pyroxene in the troctolites, then they must have crystallized at depths of 12–20 km, driven by slow conductive cooling near the base of the thermal boundary layer, with initial magmatic temperatures of ~1200°C. The aphanitic texture of the diabase intrusions, on the other hand, suggests that they cooled relatively quickly at shallow depths. Since some diabases show mingling with hornblende-bearing granophyre, these diabases must have intruded while the host rocks were near the fluid-saturated granite/trondjhemite solidus at ~700°C. Other massive diabases, particularly those at the top of Hole 1275B, may have intruded even colder wallrocks, perhaps at shallow depths. Again, we infer that the igneous construction of the gabbroic section took place via multiple intrusions, with wallrock temperatures varying over time from ~1200°C to <700°C. Probably, this process also took place over a range of depths, beginning at 12–20 km and ending near the seafloor. Alternatively, if the crystallization of the gabbroic rocks at Site 1275 was not temporally related to the impregnation of the troctolites, then crystallization of the gabbroic section could have taken place over a more restricted range of temperature and pressure. We hope that oxide-silicate thermobarometry on oxide gabbro and gabbronorite samples from Site 1275 will help to distinguish between these different possibilities.

Hydrothermal Alteration, Metamorphism, and Metasomatism

Metasomatic Changes in Troctolite

As for carbonate-altered peridotites from previous sites on this leg, some troctolites from Site 1275 include abundant carbonate veins. CO₂ contents in these rocks range up to 9 wt% and are correlated with CaO contents, as demonstrated in "Geochemistry," in the "Site 1275" chapter. As was proposed in "Hydrothermal Alteration, Metamorphism, and Metasomatism," in "Site 1274" in the discussion of metasomatism of altered peridotites, it could be that the troctolites have been modified by metasomatic addition of Ca as well as CO₂. However, this is unlikely. The troctolites contain substantial Al₂O₃, abundant plagioclase and plagioclase pseudomorphs, and only minor amounts of spinel. Thus, most of the Al₂O₃ in these rocks must have originally been in plagioclase. If so, CaO is required to form the anorthite component in the plagioclase. CIPW norms for the anhydrous, CO₂-free compositions of the troctolites include very little clinopyroxene, and some have trace amounts of normative corundum. Thus, most or all of the calcium currently present in the troctolites is required to balance the Al₂O₃ in plagioclase. As a result, we tentatively conclude that CO₂ was added to the troctolites during hydrothermal metasomatism but that calcium was relatively immobile during this process.

Mainly Static Alteration

Alteration intensity in Holes 1275B and 1275D decreases with increasing depth. Cataclastically deformed gabbroic rocks and diabase and a talc amphibole schist with an ultramafic protolith are abundant in the first five cores of Hole 1275D, and these show 50%–100% amphibolite to greenschist facies alteration. The cataclastic deformation is probably related to the fault(s) that formed the top of the bathymetric dome at Site 1275. However, below this fault-related cataclastic deformation, hydrothermal alteration of the troctolites and gabbroic rocks at Site 1275 took place almost entirely under static conditions along microcracks that lack a systematic preferred orientation and may have formed during cooling. This static alteration is very different from what has been observed in the thick gabbroic sections at ODP Sites 735, 921, and 923, where alteration intensity is highest in zones of penetrative crystal-plastic deformation that occur throughout the section.

Structural Geology

Crystal-plastic deformation is rare in samples from Site 1275. Only a few gabbros show evidence for dynamic recrystallization of plagioclase. Evidence of minor strain, in the form of bent plagioclase with undulatory extinction and deformation twinning, is present throughout Holes 1275B and 1275D, but this reflects minor high-temperature deformation of the type that may be common during igneous compaction of gabbroic intrusions.

Brittle deformation, in the form of cataclastic breccias and semibrittle schists, is common in the upper 15 m of Hole 1275B and the upper 50 m of Hole 1275D. The difference in depth of cataclastic deformation in the two holes may be related to the greater thickness of relatively weak, altered troctolite in Hole 1275D compared to Hole 1275B. Cataclastic breccias are commonly (always?) developed from gabbroic protoliths, whereas semibrittle schists have mainly ultramafic protoliths. The intensity of brittle deformation decreases markedly downhole in Holes 1275B and 1275D. This observation is consistent with the hypothesis that the brittle deformation is mainly related to the fault zone that likely forms the top of the topographic dome at Mt. Mike.

Two fascinating observations are that there are fine-grained diabase clasts—and perhaps even chilled diabase material within the matrix—in some cataclastic breccias and that diabase intrusions form deformed lenses, possibly preserving chilled margins, within semibrittle schists. These observations—whose interpretations are still uncertain—may be consistent with the presence of a thick and largely undeformed diabase intrusion in the top 18 m of Hole 1275B. We suggest that diabase sills intruded along the fault zone at the top of Mt. Mike, penecontemporaneously with continued slip on the fault, as has also been proposed by Escartin et al. (2003).

Metamorphic veins in core from Site 1275 do not show any preferred orientation and thus may have followed cracks that formed as a consequence of nearly hydrostatic stresses during cooling of the igneous rocks.

Magmatic banding or layering in gabbroic rocks is defined by sharp contacts between lithologies with different proportions of minerals and/or different grain sizes and by plagioclase shape fabrics. This magmatic banding is subhorizontal. Contacts between granophyres and gabbroic host rocks are irregular, but a significant subset (>49% of measurements in Hole 1275B) have steep dips (>60°). Since the statistical probability of sampling steeply dipping features in a vertical drill hole is very small compared with the probability of intersecting equally abundant horizontal features (i.e., for features with the same width and density the probability of intersections in a vertical borehole of fixed depth is equal to the cosine of the dip), we can infer that near-vertical granophyre veins are much more abundant within the Site 1275 gabbroic rocks than subhorizontal granophyre veins (<88%, normalized for probability, in Hole 1275B). Because the evolved melt that formed the granophyre was less dense than the host gabbros, one possible interpretation is that the steeply dipping granophyres crosscut the gabbros as near-vertical dikes and veins (as, for example, in the "transgressive granophyres" of the Skaergaard intrusion [Wager and Brown, 1968, figs. 101 and 105 and accompanying text; Hirschmann, 1992]) and thus that the section has not undergone large rotations around near-horizontal axes since the formation of the granophyres. If so, this places an important constraint on the direction and magnitude of possible tectonic rotations of the gabbroic section at Site 1275. Alternatively, the majority of the granophyres with steep dips could have formed as subhorizontal "segregation sheets" (e.g., review in Boudreau and Philpotts, 2002). In this case, one would infer that the section could have undergone almost 90° of rotation around a nearly horizontal axis.

Five of six diabase/gabbro contacts are subhorizontal. If the gabbroic rocks have not undergone substantial tectonic rotations around a horizontal axis, this indicates that the diabases were intruded as sills emplaced in gabbro. In this case, many of the other subhorizontal magmatic layers could also be sills, intruded into preexisting gabbroic rocks while the growing plutonic complex was at relatively high temperatures. These observations and inferences are consistent with the idea that gabbroic sections beneath some mid-ocean ridges, and perhaps some layered intrusions, form via multiple injection of "sheeted sills" (Bédard et al., 1988; Bernstein et al., 1992, 1996; Browning, 1984; Browning et al., 1989; Kelemen and Aharonov, 1998; Kelemen et al., 1997b; Korenaga and Kelemen, 1997, 1998). Alternatively, if the steeply dipping granophyres at Site 1275 were emplaced as subhorizontal segregation sheets, then most of the contacts between diabase and gabbro and between different gabbro units probably formed with a near-vertical orientation. Whereas dominantly vertical contacts between different gabbroic units are rare in large intrusions, such structures are observed in specific plutons such as the large, shallow-level Lilloise intrusion, intruding Tertiary flood basalts in East Greenland (Chambers and Brown, 1995). And, finally, we emphasize to readers that in presenting two alternative interpretations of these structures, we do not mean to imply that this is an exhaustive list of possibilities. Instead, we simply present two ideas which seem plausible based on our preliminary data.

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 Hole 1275B, half-core measurements and discrete samples all have negative stable inclinations, suggesting that the rocks are reversely polarized. Rotation of structural features around a vertical axis into a common remanent magnetization direction yields two important results. First, magmatic layering in the gabbroic rocks forms a relatively tight cluster on a lower hemisphere stereo plot, with an average dip of ~20°. Second, although many granophyre/gabbro contacts also have shallow dips, there is a distinct population with near-vertical dips. These are roughly perpendicular to the average attitude of layering in the gabbros. As explained in the previous section of this summary, these observations could be consistent with two different end-member scenarios. In the first scenario, we interpret the steeply dipping granophyre/gabbro contacts as defining the paleovertical direction during granophyre intrusion. In this view, the nearly horizontal present-day attitude of the gabbro layering probably indicates that these formed as subhorizontal layers. Alternatively, the steeply dipping granophyres could have formed as subhorizontal sheets, in which case the section has undergone large tectonic rotations.

The average remanent inclination for 26 discrete samples of gabbro and diabase from Hole 1275B is –9.5° (95% CI = +3.9°/–3.7°), which agrees well with data on inclination for archive half cores. The average remanent inclination for 34 discrete samples from Hole 1275D is –3.5° (95% CI = +7.0°/–6.7°), also in reasonable agreement with data on inclination for archive half cores. The values for gabbroic rocks are not statistically different.

Discrete sample data for Hole 1275B were all quite similar, but in Hole 1275D there were several samples (particularly troctolites and diabases in the upper 50 m of the hole) that have scattered but positive inclinations. One sample of troctolite contains two nearly antipodal magnetization components. These data suggest that both normal and reversed magnetizations are recorded in Hole 1275D samples. Acquisition of an early reversed magnetization probably occurred via cooling of igneous magnetite through 500°–600°C. Acquisition of a later normal magnetization could have occurred during crystallization of secondary finer-grained metamorphic magnetite associated with serpentinization of olivine in the troctolites and/or during local reheating of fine-grained magnetite in the rocks associated with late intrusion of some of the diabase sills and dikes. Removal of troctolite and diabase samples with positive inclinations from the data set for Hole 1275D yields an average inclination of approximately –12° for the remaining samples, which is even more consistent with the data from Hole 1275B than the average for the entire Hole 1275D data set.

The average inclination of approximately –10° to 12° in gabbroic rocks from Site 1275 is significantly lower than the expected inclination of –28° for rocks with a reversed polarity magnetization. These data suggest that block rotation has affected the section sampled at Site 1275, as found for Sites 1268, 1270, 1272, 1274, and 1275, where remanent magnetic inclinations also require substantial rotation of the section. For Sites 1268 and 1270, we inferred that rotation axes were probably near horizontal and parallel to the normal faults that form steep slopes along the rift valley. At Sites 1272, 1274, and 1275, the choice of a tectonically reasonable rotation axis is less clear.

Decreasing the inclination from –28° to –11° in the gabbroic rocks from Site 1275 requires a minimum of 17° of counterclockwise rotation around a horizontal rotation axis with an azimuth of 270° or larger rotations around axes parallel to the trend of the rift valley (~020°). One option is that the section has indeed been rotated ~17° around a near-horizontal east-west axis. This is consistent with the first structural interpretation discussed above, in which the steeply dipping granophyres formed as near-vertical intrusions into horizontally layered gabbros. However, we cannot rule out the possibility that the steeply dipping granophyres were originally subhorizontal and the section has undergone large rotations, for example, around a nearly horizontal axis parallel to the rift valley walls, as inferred for Sites 1268 and 1270. And again, we emphasize that in listing two alternatives we do not mean to imply that this list is exhaustive. There may be many other interpretations that are consistent with our data.

We are confident that continuing analysis of the structural and paleomagnetic data will place additional constraints on the tectonic history of Mt. Mike. For example, the Hole 1275 data may offer the opportunity for another refinement. There seems to be a significant difference between the magnetic inclinations in gabbroic rocks from ~50 to 140 mbsf, which averages approximately –6°, and magnetic inclinations of approximately –15° in gabbroic rocks from 150 to 209 mbsf (Fig. F59). This difference may reflect smaller amounts of tectonic rotation in the bottom of Hole 1275D compared to the middle of Hole 1275D and the entire section sampled in Hole 1275B.