Site 1200 is located in the Mariana arc, a nonaccretionary convergent margin with a pervasively faulted forearc. The Mariana system contains numerous large mud volcanoes, which are composed principally of unconsolidated flows of serpentine muds with clasts of serpentinized mantle peridotite. Site 1200 is situated on the knoll of South Chamorro Seamount, the only known site of active blueschist mud volcanism in the world with associated megafaunal assemblages. The primary objectives at Site 1200 were to establish a long-term geochemical observatory and to obtain core samples to examine geological and biological processes associated with subduction materials from great depth.
Hole 1200A was cored with the rotary core barrel (RCB) to a depth of 147.2 meters below seafloor (mbsf) and recovered serpentinized ultramafic clasts and minor amounts of the host serpentine matrix. Hole 1200B was washed down to 98 mbsf and had to be abandoned because of hole stability problems before coring could be initiated. Hole 1200C is cased to 202.3 mbsf and instrumented with a long-term borehole observatory equipped with a reentry cone, a thermistor string with pressure cells, and two osmotic water samplers. Three holes, 1200D, 1200E, and 1200F were cored with the advanced hydraulic piston corer/extended core barrel (APC/XCB) to a maximum depth of 54.4 mbsf and recovered soft serpentine matrix with hard rock clasts.
The clasts consist of serpentinized ultramafic rocks embedded in serpentine mud. Most clasts are angular to subangular, whereas the larger clasts are subangular to subrounded. Harzburgites are the dominant original rock type, with minor dunites and a few lherzolites. The degree of serpentinization varies between 40% and 100% (average = ~70%), and loss on ignition (LOI) values vary between 11% and 18% (average = 14%). Based on preliminary geochemical analyses, it is likely that the serpentinized ultramafic rocks represent portions of the forearc mantle wedge that suffered 20%-25% melt extraction (overlapping with the ultramafic rock assemblages found on Conical Seamount during Ocean Drilling Program [ODP] Leg 125). The average Si/Mg ratio is 0.92, and the average Mg number is 92.3.
Relative to depleted mantle values, two trends are distinguished: one of Sr and Na enrichments and the other of Ca and Al depletions. In general, geochemical data from Site 1200 are well correlated with the values obtained for serpentinites from Site 780 (Conical Seamount summit) during Leg 125 (Shipboard Scientific Party, 1990c).
All holes at Site 1200 are characterized by a succession of silty clay-sized serpentine containing dispersed clasts or clast-poor serpentine diamicton with clast-rich intervals, all belonging to the same lithostratigraphic unit.
This unit is characterized by two distinct poorly sorted facies with dispersed clasts. Facies I is predominant and is composed of silty clay serpentine with dispersed clasts and clast-poor serpentine diamicton. Facies II is present in two short intervals in Hole 1200E and is characterized by clast-rich serpentine diamicton. With the exception of the increased clast content, the two facies are identical; however, no calcareous intervals occur within recovered intervals of clast-rich diamicton.
Core variability is evident in color reflectance data. All cores were dark blue gray to black when first split but became lighter in color once exposed to the aerobic laboratory environment. Calcareous intervals oxidized rapidly after cores were split, revealing hues of light yellowish brown, light bluish gray, and light pink. Recorded calcareous intervals and the location of clasts coated with a calcareous precipitate and calcareous nodules correlate well with microfossil content and abundance. Although most of the mud consisted of serpentine minerals, examination of the grit fraction revealed a rich population of high-pressure, low-temperature rocks and minerals from the décollement, including glaucophane, white mica/crossite/chlorite, talc, and chlorite schists.
Well-preserved and diversified subtropical assemblages of planktonic foraminifers and calcareous nannofossils were found in the top 0.1-0.3 m of Holes 1200D, 1200E, and 1200F, indicating that the surface of the summit is blanketed with calcareous microfossil-bearing deposits. Benthic foraminifers in small quantity and low diversity are also present in all holes. Samples farthest away from Hole 1200A contain more abundant, diversified, and better-preserved microfossil faunas. The downcore sections of these three holes are virtually barren of microfossils, except for a peculiar interval between 11 and 13 mbsf in Hole 1200D. This interval contains abundant and diversified calcareous microfossils comparable to those in the core tops. The major difference is that these fossils were overgrown by calcite on the original structures. All the fossils are late Quaternary in age.
Pore waters from Site 1200 reveal 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 has reduced sulfate in this fluid to zero and produced high alkalinity (up to 130 meq/kg) and dissolved sulfide in the process, possibly by oxidizing the C1-C6 hydrocarbons detected in headspace gas samples. The deep fluid is similar in many ways to that sampled on Conical Seamount to the north during Leg 125. It has a pH of 12.5, making it, along with the Conical Seamount fluids, the most alkaline pore water ever recovered from the deep sea. Relative to seawater, it is depleted in chloride, Mg, and Ca and greatly enriched in alkalinity, Na and Na/Cl, K, sulfate, and light hydrocarbons.
Ultramafic rocks were easily demagnetized by alternating-field techniques and yielded mostly stable magnetic directions. The stable magnetization represents a chemical remanent magnetization acquired by magnetite particles as they formed during serpentinization. As expected from their emplacement history, the serpentine muds did not display a stable magnetic component. Thermal demagnetizations reveal Curie temperatures of 585°C, which suggests that the dominant magnetic mineral is pure magnetite. Koenigsberger ratios (mean = 2.4) indicate that the in situ magnetization of the ultramafic clasts is dominated by remanent magnetization rather than an instantaneous magnetization induced by the Earth's magnetic field.
The results of the physical properties measurements show that the material recovered from South Chamorro Seamount is very similar to that recovered from Conical Seamount during Leg 125 (Fryer, Pearce, Stokking, et al., 1990). The average bulk density of the clasts is 2.50 g/cm3; the average grain density of the clasts is 2.66 g/cm3. The average bulk density of the matrix is 1.87 g/cm3; the average grain density of the matrix is 2.64 g/cm3, nearly identical to that of the clasts. The hydraulic conductivity and diffusivity of the matrix were measured during a consolidation test in a Manheim squeezer cell. The initial results of the tests showed the matrix to have a hydraulic conductivity ranging between 10-11 and 10-10 m/s. These values are comparable in magnitude and range to those of clays and silts.
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2Shipboard Scientific Party addresses can be found under "Shipboard Scientific Party" in the preliminary pages of the volume.
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