Five holes were cored at Site 1200. Unconsolidated serpentine successions were recovered from Holes 1200D, 1200E, and 1200F. All holes are characterized by a succession of silty clay-sized serpentine with dispersed clasts to clast-poor serpentine diamicton with clast-rich intervals that all belong to the same lithostratigraphic unit (Fig. F4).
Unit I extends through all recovered serpentine sections at Site 1200. It is characterized by poorly sorted material with dispersed clasts. Two distinct sedimentary facies are identified (Fig. F4). Facies I is composed of silty clay-sized serpentine with dispersed clasts and clast-poor serpentine diamicton (Figs. F5, F6). The observation of silty clay-sized serpentine with dispersed clasts (<5% clasts >2 mm in size) (Fig. F3 in the "Explanatory Notes" chapter) or clast-poor serpentine diamicton (5%-10% clasts >2 mm in size) seems to depend on the core-splitting technique rather than on changes in lithology. Thus, clasts are visibly less abundant in holes and intervals split using the soft-sediment wireline method (Hole 1200D [0-21.9 mbsf] and interval 195-1200E-1H-1, 0 cm, to 6H-CC, 40 cm [0-25.93 mbsf]). Small pebbles and granules are pushed into the sediment and not split in half by the wireline. The rock saw was used to split the remaining intervals of Hole 1200E (interval 195-1200E-7H-1, 0 cm, to 10H-CC, 33 cm [25.93-56.45 mbsf]) and Hole 1200F (0-16.3 mbsf). The rock saw enables small pebbles and granules to be split in half, resulting in an apparently higher concentration of clasts. Facies II is composed of clast-rich serpentine diamicton (10%-30% clasts >2 mm) (Fig. F7). It should be noted that this facies was only recognized and described in cores cut by the rock saw.
Facies I is the predominant facies and is composed of silty clay-sized serpentine with dispersed clasts and clast-poor serpentine diamicton (Figs. F5, F6). White chrysotile fibers and fragments are visible throughout all described sections.
Downcore variability in sediment composition is evident in color reflectance data (Fig. F4). All sediment cores were dark blue gray to black when first split but lightened in color once exposed to the aerobic environment of the core lab. Calcareous intervals (Fig. F4) oxidized extremely rapidly after the cores were split, revealing hues of light yellowish brown, light bluish gray, and light pink (Fig. F8). Recorded calcareous intervals (195-1200D-1H-1, 10-120 cm [0.10-1.20 mbsf], 195-1200E-1H-1, 10-133 cm [0.10-1.33 mbsf], 195-1200E-3H-1, 0 cm, to 4H-CC, 36 cm [11.00-17.55 mbsf], and 195-1200F-1H-1, 10-142 cm [0.10-1.42 mbsf]) and the location of clasts coated with calcareous precipitates (intervals 195-1200D-1H-4, 98.5-101 cm [5.48-5.51 mbsf], 1H-5, 33 cm [6.33 mbsf], 2H-2, 21-23.5 cm [8.61-8.64 mbsf], and 195-1200E-1H-2, 44 cm [1.94 mbsf]) and calcareous nodules (Sample 195-1200D-7H-1, 88 cm [24.38 mbsf]) correlate well with microfossil content and abundance (see "Biostratigraphy").
Whole-core photography and color spectral analysis were completed as quickly as possible after splitting. A downcore trend from dark bluish gray to lighter greenish gray is an artifact of oxidation. Scraping core surfaces revealed the underlying dark blue-gray to black color of the material characteristic of its natural anoxic environment. Thin (<2 mm) bands of faint and diffuse greenish gray were observed in intervals 195-1200E-10H-1, 60-85 cm (53-53.25 mbsf), 195-1200F-1H-5, 16-22.5 cm, and 60-93 cm (6.16-6.22 and 6.60-6.93 mbsf), 2H-4, 35-49 cm (12.05-12.19 mbsf), and 3H-CC, 40-60 cm (16.01-16.21 mbsf). These color bands are not associated with a change in lithology, clast content, or texture. It is therefore surmised that they are not indicative of depositional processes but rather may represent intervals of "staining" from nearby altered clasts or differential oxidation rates.
Clasts identified in Facies I sediments are predominantly altered serpentinized ultramafic varieties (see "Smear Slide Analysis," "X-Ray Diffraction," and "Igneous and Metamorphic Petrology of Ultramafics"). The degree of alteration varies. Highly altered clasts range from green to greenish brown to red in color, are soft, and are easily fragmented. They are often present as a concentration of small (<2 mm) angular fragments in the footprint of a former large clast. Clast shape varies from rounded to angular (see "Structural Geology").
Facies II is composed of clast-rich serpentine diamicton and extends through intervals 195-1200E-7H-3, 68-97 cm, and 10H-2, 61-114 cm (29.58-29.87 and 54.51-55.04 mbsf) (Figs. F4, F7). With the exception of increased clast content, these sediments are identical to Facies I; however, no calcareous intervals are present within recovered intervals of clast-rich diamicton.
Microscopic inspection of smear slide samples and two thin sections was conducted on the matrix of the serpentine deposits, including the clay-, silt-, and fine to medium sand-sized fractions. Some samples were wet sieved to obtain the fine sand fraction (grain diameter = 63-344 µm), which provided clean grains for optical mineral identification and photographic documentation. All smear slide samples and thin sections are dominated by serpentine minerals associated with small amounts of heavy minerals, whereas some samples also include authigenic phases (see "Site 1200 Smear Slides").
Most serpentine species appear as silt- to sand-sized flaky particles, consisting of single crystals or microcrystalline aggregates (Figs. F9, F10A, F10B, F10E). Occasionally, they show hexagonal outlines or internal lamellae. They exhibit colors in various shades of dull green to greenish brown. No pleochroism is evident. They have low refractive indices (n = ±1.56) and low birefrigence with first-order interference colors (gray to yellow). Their optical character is biaxial negative with mostly low angles between the optic axes. From the optical features, they represent members of the lizardite and/or antigorite groups. Because a reliable distinction between the two serpentine groups cannot be made, serpentine species were defined by their degree of alteration. Thus, fresh and translucent serpentines can be distinguished from dusky serpentines that include dispersed "dustlike" opaque particles that give the serpentines a dirty appearance. A third species consists of strongly altered serpentines that were replaced to a great extent by opaque materials. The latter species makes up the major constituent of the black serpentine muds frequently encountered in Holes 1200E and 1200F.
Chrysotile fibers were recognized as another distinct serpentine species, which is present in minor amounts throughout the investigated sections (Fig. F10C, F10D). Apart from their typical fibrous habit, they show pleochroism from greenish to brownish green and have slightly higher birefringence (multicolored first-order interference colors) than the other serpentine species.
Larger sand-sized clasts embedded in the silty matrix are frequently composed of rock fragments that were replaced pseudomorphously by serpentine minerals and brucite (Fig. F9). Distinction of serpentine and brucite arises from the brownish interference colors of the latter (Fig. F9). In addition to altered rock pieces, fresh rock fragments of greenschists and blueschists represent ubiquitous components of the serpentine muds (see detailed description in "Igneous and Metamorphic Petrology of Ultramafics").
In most samples, the serpentines are associated with low abundances of pale bluish amphiboles, particularly in the fine sand fraction (Fig. F11A). The elongated prismatic crystals show weak pleochroism between shades of blue and pinkish violet. Under crossed polarizers, internal zonation becomes visible (Fig. F11B). Moreover, they have low extinction angles and negative biaxial optical character, similar to common hornblendes. However, the distinct color and pleochroism features show affinities to amphiboles of the crossite to glaucophane group. Possibly, the blue amphiboles originate in deep blueschists that formed in the high-pressure, low-temperature environment of the Mariana subduction zone (Fryer et al., 1999).
Other identified accessory minerals are spinels, which appear as small isotropic grains of high relief and whiskeylike color, as well as garnet and greenish flakes of chlorite with anomalous interference colors.
The uppermost sections (0-1 mbsf) of Holes 1200D, 1200E, and 1200F include abundant authigenic aragonites. Exceptionally, they are also present farther downcore in Cores 195-1200E-1H through 3H, where they are associated with an interval of contorted and oxidized greenish silty clays and with parts of the underlying black silty clays. Aragonites appear as up to 1-mm-long lath-shaped crystals or form "mikado-like" bunches of small needles (Fig. F11C, F11D, F11E, F11F). Common features of both varieties are high refraction indices and high birefringence with high-order interference colors. Broken and rounded pieces of aragonite are present in trace amounts (one per smear slide) in other silty clay samples, where they likely represent reworked particles from the aragonite-bearing strata.
In Section 195-1200E-1H-2, black serpentine silty clays contain white millimeter-sized nodules exhibiting cauliflower structures. In smear slides, these concretions appear as radiating fibers attached to opaque nuclei (Fig. F10F). From its habit and according to X-ray diffraction (XRD) findings, this mineral likely represents a species of the zeolite group.
To infer the source materials of framework clasts and fine-grained matrix in the serpentine mud deposits, XRD analyses were carried out on both rock pieces and mud samples on bulk powder mounts (Table T2). Similar to smear slides, the identification of distinct serpentine minerals by XRD is ambiguous and should be reexamined on shore by sophisticated X-ray microbeam or electron optical techniques.
Rock samples were taken from Hole 1200A, which was drilled with the RCB. Two major rock types can be distinguished from XRD results: altered harzburgites and strongly altered fine-grained ultramafic rocks (Fig. F12). Though dominated by serpentine, the harzburgites still include portions of the original rock-forming minerals olivine (forsterite), orthopyroxene (enstatite), and traces of clinopyroxene. Whereas brucite represents a subordinate mineral in the harzburgites, it is abundant in the fine-grained ultramafic rocks, which are otherwise dominated by serpentine minerals and contain few primary minerals. Some ultramafic rocks show signs of silica gel precipitation, as indicated by the presence of opal-CT (Fig. F12). Opal-CT is made up of recrystallized opal, including cristobalite and tridymite (Jones and Segnit, 1971). It thus represents a transitional phase between amorphous opal and microquartz and is usually formed under high-temperature conditions, but it may form discrete early diagenetic chert layers in shallowly buried deep-sea sediments that have not been affected by elevated temperatures (e.g., Bohrmann et al., 1994). One exotic greenschist fragment was analyzed, which shows no signs of serpentinization and consists of clinopyroxene and magnesium-rich chlorite (Fig. F12).
XRD measurements of silty clay samples support results from smear slide analyses and point to serpentine minerals and brucite as dominant constituents (Fig. F12). In contrast to the analyzed rock pieces, accessory amounts of amphibole, together with talc and chlorite, are evident in many samples. This assemblage indicates the presence of exotic greenschist and blueschist detritus within the serpentine muds.
The downcore semiquantitative distribution of minerals, as identified by XRD measurements, illustrates the distribution of aragonite in Holes 1200D, 1200E, and 1200F, as also deduced from smear slide analyses (Figs. F13, F14, F15). Aragonite is often associated with calcite. The downcore patterns suggest that aragonite/calcite and brucite tend to mutually exclude each other. Pore water conditions in the upper sediment layers may favor aragonite precipitation and brucite dissolution (cf. "Geochemistry"). From the top of Hole 1200D, fragments of a carbonate chimney consisted predominantly of calcite with minor aragonite.
Another mineral with a major XRD peak at 5.44 Å possibly represents the zeolite-like mineral found in one of the smear slides (Sample 195-1200E-1H-2, 116 cm). The XRD results, however, show that this authigenic mineral is actually encountered in most intervals composed of black serpentine silty clays. The only zeolite that shows a strong XRD peak near 5.44 Å is analcime (5.61 Å). This interpretation has to be regarded with caution because other diagnostic peaks of analcime at 3.43 and 2.93 Å seem to be suppressed in the Site 1200 XRD records. Halite was recognized as a minor component in all XRD samples as an artifact from sea salt remains in the sediment pore spaces.
The serpentine muds at Site 1200 show little variability in lithology, providing little insight into the depositional processes taking place on the knoll of South Chamorro Seamount. However, a few observations can be made. The overall poorly sorted nature of the muds is suggestive of a tectonically active area with a high rate of mud supply in a small area, consistent with the environmental setting of a mud volcano. The diamictic texture is consistent with the protrusion of unsorted serpentine mud onto the surface of the seamount and its transportation and deposition by debris flows on the flanks of the mud volcano. Clast-rich intervals are rare but may suggest crude bedding.
High abundances of aragonite and calcite and the absence of brucite in the near-surface muds point to geochemical pore water gradients that control the dissolution and precipitation of solid phases. These gradients are caused by rising pore fluids that originated at depth and mix with seawater near the surface. Where these pore fluids are released as springs, they give rise to the formation of carbonate chimneys at the seafloor, as observed at the top of Hole 1200A (see "Geochemistry"). Moreover, the top layers of carbonate-bearing muds are mostly oxidized.
These observations lead us to conclude that the carbonate-rich and partly oxidized interval between 11.00 and 17.55 mbsf encountered in Hole 1200E may represent preserved paleosurface material, which was not affected by changing geochemical gradients while buried under younger material. This interval is characterized by deformed and folded color bands but is not associated with a change in lithology, texture, clast content, or clast concentration, which may suggest disaggregation and redeposition via debris flows or turbidity currents. Instead, sediment structures suggest downslope gravitational motion via slow creep or sliding. This process likely thickened the paleosurface deposits and protected them from chemical alteration during later burial. This might help to explain the presence of carbonate-bearing material, which was also found at depth on the flanks of Conical Seamount during Leg 125 (Shipboard Scientific Party, 1990b).
The interpretation of reworked "paleosurface" deposits suggests that these holes have been drilled into the flanks of the mud volcano, immediately adjacent to the present-day active-flow conduit. It is noted that camera surveys prior to drilling provided visual confirmation of conduit activity at the site of Hole 1200E; however, the conduit "pipe" may be oriented oblique to the drilled hole, enabling us to penetrate flank material downhole. It is also possible that calcareous material from the surface has been incorporated into material at depth by falling into fractures or crevasses observed at the surface by camera surveys during this leg (see "Operations") and earlier ROV and submersible surveys (Fryer et al., 1990). Serpentine mud volcano environments are very active and dynamic systems, and it is likely that the location of the active conduit changes over time. The data presented in this section confirm this complexity and extend our limited knowledge of the processes involved in fluid and sediment transport.