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SITE SUMMARIES (continued)

Site 1272

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 extensively dredged and surveyed during 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 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 preceded west-southwest and upslope along the dive track toward the site of 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 of these, 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). In support of the hypothesis 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, appears to 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 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) 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. F38). 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 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. F39), 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, since 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). 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 F40 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° to 16°N area in particular. Like glasses from the 14° to 16°N region, diabase and miarolitic gabbro 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 Site 1272A is a single interval of 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. F41). 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.

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 F41 and show a positive correlation between CaO and CO3. 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. 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. 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) (Collins and 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 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, 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 exception, 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.

At least four fault gouge zones were sampled between 75 and 131 mbsf in Hole 1272A (Fig. F42). Predictably, recovery was poor in these zones and other fault gouge zones may have been present but 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 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 syn-kinematic, 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.

In the case of Site 1272, half-core measurements and discrete samples all have positive inclinations, indicating that the rocks probably were normally polarized. As noted above, 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 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. 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.

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