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SITE SUMMARIES (continued)

Site 1274

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 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. F43). 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 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). 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. Instead, our data suggest that the mantle in this region is uniformly depleted, perhaps as the result of a prior melt depletion event.

Hole 1274A harzburgites include appreciable amounts of clinopyroxene and CaO, generally more than is observed at Sites 1268, 1270, 1271, and 1272. CaO contents of Hole 1274A peridotites are weakly correlated with Al2O3 contents, although these elements do not show correlated variation 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. F44). 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, 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 F45 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, 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. 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 (CO2 = 0.07 wt%), Site 1270 (CO2 = 0.13 wt%, excepting an outlier with 2.2 wt%), Site 1271 (CO2 = 0.23 wt%, excepting an outlier with 1.5 wt%), and Site 1272 (CO2 = 0.24 wt%, excepting outliers with 7.3 and 9.3 wt%). Figure F46 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. Thus, some of the CaO in all these metaperidotites is probably in carbonate minerals.

It is well established that metaperidotites sometimes 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 F47 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 fault zones, 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. F48). Density and seismic P-wave velocity decrease downhole and are significantly correlated with the intensity of alteration (Fig. F48). 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 may be explained by hydrothermal metasomatism, involving substantial input of Mg (±Fe) and extraction of Ca. 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 primary phases present in the peridotites, such as

2 (Mg, Fe)CaSi2O6 + 3 (Mg, Fe)2Si2O6 + 3 H2O = Ca2(Mg, Fe)5Si8O22(OH)2 + (Mg, Fe)3Si2O5(OH)4,

forming tremolite + serpentine from pyroxene or, at higher temperature,

4 (Mg, Fe)CaSi2O6 + 5 (Mg, Fe)2Si2O6 + 3H2O = 2 Ca2(Mg, Fe)5Si8O22(OH)2 + 2 (Mg, Fe)2SiO4,

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

3 (Mg, Fe)2Si2O6 + 3 H2O = (Mg, Fe)3Si4O10(OH)2 + (Mg, Fe)3Si2O5(OH)4,

or, at higher temperature,

5 (Mg, Fe)2Si2O6 + 2 H2O = 2 (Mg, Fe)3Si4O10(OH)2 + 2 (Mg, Fe)2SiO4.

All of these reactions require only H2O addition from hydrothermal fluid, and thus may be preferable 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

3 (Mg, Fe)(OH)2 + (Mg, Fe)3Si4O10(OH)2 = 2 (Mg, Fe)3Si2O5(OH)4.

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. 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 (Na0.3Fe3+2[Si, Al]4O10[OH2] · nH2O). 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 since 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, may 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 into 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. F49). Recovery of gouge was poor at several sites; therefore, the width of the associated fault zones is 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. 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.

Half-core measurements and discrete peridotite samples all have positive inclinations that are presumable of normal polarity, although the mean inclination (13°) is significantly shallower than the expected dipole inclination at the site (28°), indicating that the rocks are normally polarized. 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 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°).

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