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Principal Results

The principal objective at Site 1200 was to install a borehole geochemical observatory at the summit of South Chamorro Seamount to sample fluids from the dècollement below the Mariana forearc. Although this objective was achieved, no data will be recovered from the observatory until it is revisited by an ROV in 2002. As was to be expected in such an exotic environment, however, the drilling and coring undertaken to install the observatory and document its setting produced unexpected and often spectacular results.

Perhaps the most fundamental achievement at Site 1200 was the documentation of the muds, xenoliths, and fluids rising to the surface from the mantle and the dècollement zone through the central conduit of the mud volcano. The recovery in all cored holes consisted of poorly sorted, dark blue-gray to black serpentine mud breccia (Fig. F9) in which the muds are composed predominantly of silty clay-sized serpentine, and the clasts, which range up to a meter across, consist largely of serpentinized ultramafics.

Well-preserved and diversified subtropical assemblages of planktonic foraminifers and calcareous nannofossils were found in the top 0.1-0.3 m of the holes that were APC cored (1200D, 1200E, and 1200F), indicating that the surface of the summit is blanketed with a veneer of calcareous microfossil-bearing deposits. A few species of benthic foraminifers are also present in small quantities in all holes. Samples farthest away from the vent communities contain more abundant, diversified, and better-preserved microfossil fauna. The downcore sections in these three holes are virtually barren of microfossils, except for a peculiar interval with folded color bands between 11 and 13 mbsf in Hole 1200D, which is interpreted as a paleosurface that has been covered or folded into the mud by the flow of serpentine. This interval contains abundant and diversified calcareous microfossils comparable to the core tops. The major difference is that these fossils tend to be robust species overgrown by calcite on the original structures, whereas the more delicate species have been dissolved. All the fossils are late Quaternary in age.

With the exception of the two calcareous intervals noted above, which are light yellow-brown to pink due to their microfossil content, and a 10- to 20-m-thick zone of strongly reduced, black serpentine muds starting about a meter below the seafloor, the mud breccias in the conduit display no stratigraphy, which is consistent with their diapiric origin. They have been divided into two facies, however, on the basis of the abundance of clasts (<10% and 10%-30%); based on recovery, the average is about 7%. The smaller ultramafic clasts tend to be angular with planar external surfaces along early serpentine veins, whereas the larger clasts are subrounded to rounded, suggesting comminution by collisions during ascent.

The mineralogy and composition of the muds and clasts almost all attest to a deep origin along the dècollement zone or the overlying mantle, regardless of the particle or clast size, from silty clay to boulder. With the exception of aragonite produced at the mudline, where rising pore fluids interact with seawater to produce carbonates and rare zeolites (analcime) found further down in the section, X-ray diffraction (XRD) analysis shows that the muds are dominated by serpentine minerals (lizardite/antigorite > chrysotile > brucite) caused by the serpentinization of ultramafics + accessory glaucophane, spinel, garnet, chlorite, and talc derived from the metamorphism of mafic rocks along the dècollement.

The dual origin of the materials making up the mud breccia is even more clearly revealed in the grit fraction (0.1-1.5 cm). About 90% of the grit fraction by volume consists of partially to completely serpentinized ultramafic rocks, but 10% consists of metabasites, including glaucophane schist (Fig. F10), crossite/white-mica/chlorite schist, chlorite schist (Fig. F11), white-mica schist, amphibolite schist containing blue-green to black amphibole and interstitial mica, and possibly jadeite and eclogite. These lithologies, especially the blueschists, are indicative of a high-pressure, low-temperature origin (Fryer et al., 1999; Fryer and Todd, 1999; Todd and Fryer, 1999), and we interpret them as metamorphosed basic rocks from the descending slab. About 1% of the grit fraction consists of blue sodic amphibole. Analysis of similar grains in gravity cores from South Chamorro Seamount showed a crossitic composition (Fryer et al., 1999). The mineral grains are zoned with blue rims and lighter blue-green cores, implying relatively rapid ascent with the rising serpentine muds. The rationale for this interpretation is that if the grains had been in contact with rising fluids having the extreme compositions observed in the pore water analyses (see below) for geologically significant periods of time, they would have likely back-reacted and would show retrograde metamorphic effects.

Although retrograde reactions are generally sluggish, the primary reason for this is the lack of reactive fluids in a system that has previously experienced prograde regional metamorphism. Such metamorphism drives volatiles out of the rocks, resulting in a dry system, which is far less likely to undergo retrograde metamorphism despite changes in pressure and temperature. The presence of highly reactive fluids in intimate contact with the serpentine muds at Site 1200, however, would make the possibility of retrograde reactions far more likely. None of the materials studied previously by Fryer and colleagues (Fryer et al., 1999; Todd and Fryer, 1999) have ever shown any indication of retrograde effects. The mineral grains from Site 1200 also lack retrograde effects.

Although the grit fraction contains a rich and varied population of high P-T samples from the dècollement, such samples appear to be absent from the large clasts, suggesting that the metabasites reaching the surface are preferentially smaller pieces. The abundance of phyllosilicate minerals in the schists may contribute to the comminution of these samples as they rise from the source region. Pressure release as the rock fragments' rise may cause the phyllosilicates to expand and disintegrate, and the continual collisions and mechanical grinding within the rising muds may cause the more friable rocks to break up into small fragments.

As noted earlier, about 7% of the material recovered at Site 1200 consists of large clasts of partially to completely serpentinized ultramafic rocks from the mantle wedge under the Mariana forearc. Whereas serpentinization has been extensive in all samples (40%-100%; average = ~75%), there are sufficient relict minerals present in many of the samples to assess the original grain size (0.01-5.0 mm) and to determine that harzburgite is the dominant protolith, dunite is much less common, and lherzolite is rare. This is consistent with the whole-rock chemistry of the samples, which suggests that the ultramafics underwent 20%-25% melt extraction at some point during the formation of the arc (Fig. F12). The actual percentage of relict minerals is extremely variable, with olivine ranging from 0% to 40%, orthopyroxene (enstatite) ranging from 0% to 35%, clinopyroxene from 0% to 5%, and chrome spinel from 0% to 3%, depending on the original mineralogy and the degree of serpentinization. In general, olivine and enstatite were the least stable minerals, with olivine altering readily to serpentine (lizardite) or brucite + magnetite and enstatite altering to serpentine tremolite/actinolite, whereas clinopyroxene and spinel were usually the most resistant to alteration. Olivine often developed mesh and hourglass textures during serpentinization, whereas enstatite is commonly replaced by bastitic textures. The serpentinization appears to have occurred in stages, because lizardite veins are often cut orthogonally by chrysotile veins, producing spectacular "Frankenstein veins" consistent with uniform dilation during late-stage serpentinization along grain boundaries. Interestingly, most of the ultramafics and all of the dunites show evidence of deformation prior to serpentinization: the olivines commonly show kink banding and granulation and the enstatites (or their bastite replacements) often display undulatory extinction.

The pore waters from Site 1200 revealed two distinct phenomena, a deep-sourced fluid that is believed to be upwelling from the top of the subducting slab 25-30 km below the seafloor and a new and exotic extremophile microbial community at 0-20 mbsf that is chemically manipulating its environment to suit its own needs. As can be seen in Figure F13, most pore water vs. composition profiles for the site represent nearly ideal advection-diffusion curves and the gradients in the top few meters are so steep that they can only be maintained by upwelling from below. The deep fluid is similar in many ways to that sampled at Conical Seamount to the north during Leg 125 (Fryer, Pearce, Stokking, et al.,1990). It has a pH of 12.5 because it is in equilibrium with brucite, making it, along with the Conical Seamount fluids, the most alkaline pore water ever sampled in the deep sea. The pore water is also enriched in (mainly carbonate) alkalinity (60 mmol/kg), Na (610 mmol/kg), Na/Cl (1.2), K (19 mmol/kg), B (3.2 mmol/kg), ammonia (0.22 mmol/kg), methane (2 mmol/kg), and C2 through C6 hydrocarbons, all components that are virtually absent in depleted harzburgites and therefore require a different source. The pore water is highly depleted in Mg, Ca, Sr, and Li and has low concentrations of Si, Mn, Fe, Ba, and phosphate. It is slightly depleted in chloride (510 mmol/kg in seawater) and enriched in sulfate (by 7% relative to chloride). This chloride depletion is much smaller than in the deep fluid from Conical Seamount, suggesting that the Conical conduit is more heavily serpentinized and less reactive, allowing more of the H2O from the deep source to arrive at the seafloor without being lost to serpentinization along the way, or that the fluids are rising more rapidly at Conical Seamount and have had less time to react.

Pore water composition vs. depth profiles also reveal that these deep fluids feed an active microbial community that is oxidizing light hydrocarbons from the fluid while reducing sulfate within the black serpentine mud in the upper 20 mbsf. This is a true extremophile community, operating at and probably driving the pH to 12.5 to perpetuate its own ecosystem. Sulfate reduction is most active at two levels. Microbes within the upper level at 3 mbsf reduce seawater sulfate that diffuses downward against the ascending flow. Those within the lower level at 13 mbsf reduce sulfate that is supplied from the subducting slab by the upwelling fluid. As organic carbon is virtually absent within the depleted, serpentinized harzburgite, the microbes rely on methane and the C2 through C6 thermogenic hydrocarbons for their source of organic carbon and ammonia for their source of nitrogen. Both are supplied by the upwelling fluid. The microbial community intercepts these nutrients and effectively traps them within the ecosystem, where they can be recycled and continually enriched. This process explains the enrichment in organic carbon in the uppermost sediment. Iron sulfides and CaCO3 in the form of aragonite needles and chimneys are also enriched there by reaction between the ascending fluid, the microbial community, and the overlying seawater.

As would be expected, the physical properties of the serpentine muds and clasts at Site 1200 are quite different and both are strongly influenced by the properties of serpentine. The velocities of the clasts, for example, range from 3.8 to 5.5 km/s and average 4.9 km/s, consistent with extensive serpentinization. Whereas the mud and the clasts have the same average grain density (2.64 g/cm3), the average bulk densities of the clasts and the muds are lower and quite different, 2.49 and 1.87 g/cm3, respectively. This is due primarily to differences in porosity between the clasts, which have low porosities, and the muds, which range from 40% to 60% porosity. Assuming the mud constitutes 93% of the breccia, the material in the conduit has an average density of 1.91 g/cm3. If the average density of the crust is 2.75 g/cm3, then the buoyancy of the serpentine mud breccia in the upper crust would be ~0.8 g/cm3 before consolidation, or four times the density contrast between the salt in diapirs and most sedimentary rocks. Similarly, the buoyancy of completely consolidated serpentine mud breccia with a bulk density of 2.64 g/cm3 (the grain density) in fresh ultramafics (~3.2 g/cm3) would be ~0.5 g/cm3, or more than twice the buoyancy of salt in sedimentary rocks. Whereas the average shear strength of the serpentine mud (52.5 kPa) is high for sediments, it is orders of magnitude lower than that for rocks, which is consistent with their extrusion on the seafloor as mud volcanoes.

Interestingly, although vent communities are observed on the summit, the borehole temperatures are very low in the upper 50 m in all of the holes measured at Site 1200, ranging from 2° to 3°C, or ~1.3°C above seafloor values. The heat flow values are quite variable, however, with those measured in Holes 1200A and 1200E near the springs averaging 15 mW/m2, considerably below the global average of 50 mW/m2, whereas the value measured in Hole 1200F, which was further away, was ~100 mW/m2. The thermal conductivities of the muds are not unusual, ranging from 1.04 to 1.54 W/(m•K) with an average of ~1.32 W/(m•K), but the hydraulic conductivities are extremely low, ~0.6 cm/yr.

Unlike the physical properties discussed above, the magnetic properties of the clasts and muds are similar: the average natural remanent magnetization (NRM) intensities of both are high (0.49 and 0.44 A/m for the clasts and muds, respectively), as are their average susceptibilities (5.58 x 10-3 and 6.81 x 10-3, respectively). In both cases, the magnetization disappears at a Curie temperature of 585°C, indicating that the dominant magnetic mineral is magnetite produced by serpentinization. The only significant difference is that the NRM in the serpentine muds is unstable, whereas that in the clasts tends to be stable, with a high Koenigsberger ratio (average = 2.4). The NRM inclination and declination vary randomly with depth in single long pieces, however, indicating that the magnetization was acquired (i.e., serpentinization occurred) over a relatively long period of time or when the rock was being deformed or tumbled in the dècollement or the conduit of the seamount.

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