The hard rocks recovered at Site 1200 are heavily serpentinized and tectonized harzburgite and dunite clasts. Of the ultramafic rocks, 98% are harzburgites and 2% are dunites. The maximum recovered thickness of a single clast is ~1 m. Overall, the degree of serpentinization varies positively with LOI. The petrography of the serpentinized ultramafic rocks has been determined through observation and description of 50 thin sections covering all representative rock samples collected at different depths in the cored holes. The samples have been selected on the basis of visual core description to characterize different textures, primary and secondary mineral phases, crystal content and size, and vein size. All observations are reported in the thin section description forms following the outlines illustrated in "Igneous Petrology" in the "Explanatory Notes" chapter. Estimates of the primary mineralogy of the rocks are based on CIPW normative calculations (Table T3), as well as on visual and microscope observations and mineral-specific secondary textures (i.e., olivines commonly alter to mesh-textured serpentine, bastite is more common after orthopyroxene, etc.).
Clasts of similar serpentinized peridotites were recovered from the serpentine mud in Holes 1200D, 1200E, and 1200F (Fig. F16). The grit-sized fraction (0.1-1.5 cm) of the serpentine mud was inspected under a binocular microscope and handpicked for discrete lithologies. These lithologies are indicative of a high-pressure, low-temperature origin. Further microprobe and XRD analyses of the samples on shore will provide constraints on the pressure and temperature conditions.
The colors of the harzburgites range from bluish gray (5B 5/1) to dark bluish gray (5B 4/1), and the grain size is highly variable, from fine to coarse grained (0.01-5 mm). The primary mineralogy includes olivine, orthopyroxene, clinopyroxene, and chromium spinel (Fig. F17). The relict amount of olivine = 0%-35%, whereas orthopyroxene = 0%-35%, clinopyroxene = 0%-5%, and Cr spinel = 1%-3%. Estimates of the original mineral phase contents range as follows: olivine = 35%-85%, orthopyroxene = 10%-50%, clinopyroxene = 0%-5%, and Cr spinel = 1%-3%. The serpentinized harzburgites generally display typical mesh and hourglass textures (when the serpentinization involves olivine), as well as bastitic texture (when orthopyroxene is the serpentinized mafic phase) (Figs. F18, F19). The mesh texture resembles a fisherman's net, where the rim of the net is serpentine and the empty space in the mesh center is occupied by fresh olivine (Fig. F20). In the hourglass texture, olivine is completely altered, leaving no distinction between the mesh rim and the mesh center, and a typical sweeping extinction appears when the stage is rotated. Bastitic texture consists of orthopyroxene pseudomorphs, partially or completely transformed to serpentine (Fig. F21).
Evidence of penetrative deformation is extensive in the harzburgite clasts. Olivine microgranulation is abundant in the majority of the samples, leaving residual anhedral and fine-grained (average diameter = 0.05 mm) relict olivine. Kink banding is common and clearly visible in the larger grains (Fig. F22). Orthopyroxene is mainly found as equant, subhedral grains (maximum length = 4 mm and average length = 1 mm). As a result of extensive shearing, the orthopyroxene cleavage planes are often bent and exhibit undulatory extinction (Fig. F23). Clinopyroxene is present in association with or in proximity to orthopyroxene grains (Fig. F24). It also appears as small (0.2-mm-long axis), very thin, elongate exsolution lamellae parallel to (100) within orthopyroxene. Chromium spinel to magnesiochromite was identified by its translucent dark red color. Generally, it occurs as euhedral to anhedral crystals (Fig. F25).
The degree of harzburgite serpentinization is highly variable and ranges from 40% to 100% (average = ~75%). In some cases, the composition of the rock is impossible to infer from petrographic observation, due to pervasive serpentinization that obscures the primary mineralogy. In these cases, we used the chemical composition to classify the sample. Limited onboard XRD analyses indicate that lizardite is the most common serpentine group mineral, usually accompanied by brucite (Fig. F26). Where the original shape of the orthopyroxene grains is preserved, we identified bastitic serpentine pseudomorphs after orthopyroxene (Figs. F18, F19). Clinopyroxene and Cr spinel are the minerals least affected by serpentinization, but sometimes the latter is partially or even completely altered to magnetite. Frequently, magnetite is also present in veins as dustlike aggregates or rarely as small euhedral crystals, sometimes included in clinopyroxene. Amphiboles are found as orthopyroxene alteration products. They appear to have tremolite and/or anthophyllite composition and are usually <1% of the rock by volume.
Several generations of veins with variable size, mineralogy, and morphology are common in the harzburgites recovered at this site (Figs. F27, F28). Usually, a central vein with lizardite (antigorite?) mineralogy is crosscut by subsidiary fine veins of chrysotile (Fig. F29). Other veins contain secondary chlorite, magnetite, and associated scattered amphiboles. Magnetite is concentrated in the central part of the veins, in close association with chlorite and amphibole. Zones with a brownish tint in the thin sections may represent alteration of serpentine to clay minerals or brucite to hydromagnesite. Sample 195-1200A-11R-1 (Piece 5) contains not only the veining patterns and mineralogy described above, but also an additional vein with a quartz(?) ± brucite(?) ± serpentine(?) ± carbonate(?) and mica assemblage. This same vein material shows sutured grain contacts and local undulatory extinction, making the petrographic identification of this mineral assemblage difficult. In a few harzburgite samples (e.g., Sample 195-1200A-16R-1 [Piece 11B]), we observe almost completely serpentinized, rounded olivine grains included as chadacrysts within relatively fresh orthopyroxene grains, suggesting a cumulate origin (Fig. F30).
The colors of the dunites range from very dark gray (N3) to dark bluish gray (5B 4/1), and the grain size is usually fine (0.01-1 mm). The primary mineralogy consists of olivine, orthopyroxene, and Cr spinel. The preserved amount of olivine = 3%-40%, orthopyroxene = 0%-1%, and Cr spinel = 1%-3%, whereas clinopyroxene is present only in trace amounts (<1%). The estimated primary mineral contents are olivine = 90%-98%, orthopyroxene = 0%-8%, Cr spinel = 1%-4%, and trace amounts of clinopyroxene (<1%). Since extensive deformation is present in all dunite samples, they may also be classified as serpentinized ultramafic mylonites. Microgranulation and kink banding in olivine, bent cleavage planes and undulatory extinction in orthopyroxene, and granoblastic textures are widespread in all samples.
The recovered dunite clasts are all extensively serpentinized (60%-100%) and show mesh textures with hourglass extinction where olivine has been replaced by serpentine and brucite. The dunites are crosscut by several vein generations composed mainly of serpentine group minerals, chlorite, and magnetite (Fig. F28). Scattered amphibole grains (tremolite?) are present in small amounts. High concentrations of magnetite are seen in some veins. A brownish tint in the thin sections may be caused by alteration of serpentine to clay minerals.
Only one lherzolite was recovered at this site. It is visually indistinguishable from harzburgite, having the same color, texture, and mineralogy. The only difference from the harzburgites is in the clinopyroxene content, as determined from thin section estimates and CIPW normative calculations. The relict mineralogy of the lherzolite sample (195-1200A-10R-1 [Piece 6A]) consists of olivine, orthopyroxene, and clinopyroxene in a 2:3:1 ratio. Based on the petrography, we estimate the original mineralogy of the sample as 67% olivine, 25% orthopyroxene, and 7% clinopyroxene. The opaque mineral, ~1% of the rock by volume, is Cr spinel. The estimated clinopyroxene content is also confirmed by the elevated Ca and Al content (see "Geochemistry of Serpentinites") relative to the harzburgites (in the absence of carbonate minerals). The CIPW calculation gave 82% olivine, 11% orthopyroxene, and 7% clinopyroxene. The sample shows mesh and hourglass serpentine textures and contains a few thin veins of chlorite and serpentine. The degree of serpentinization is ~70%.
A quantity of wash-core material contaminated by drilling muds was recovered from Hole 1200B. We wet sieved a portion of this material using 60-µm mesh sieves and dried the sample in an oven for several hours at 100°C. Then the material was further sieved to separate the >0.1-cm fraction. The largest fraction included grains of rock material, mineral fragments, and crystals that ranged from 0.1 to ~1.5 cm in size (grit). The grit was inspected under a binocular microscope and handpicked for discrete lithologies.
Whereas most of the sieved material consists of serpentine, it also contains several distinct lithologies of metabasites. These include crossite/white mica/chlorite schist (Fig. F31), chlorite schist (Figs. F32, F33), white mica schist, amphibole schist, glaucophane schist (Fig. F34), and, possibly, jadeite schist. Glaucophane schist (Fig. F35) is ~1% by volume, and the proportion of metabasic grit (including glaucophane schist) in relation to the remaining material is ~12% by volume. These lithologies are indicative of a high-pressure, low-temperature origin (Fryer et al., 1999). We interpret these small rock fragments as derivatives of metamorphosed basic rocks from the descending slab.
There is also a trace amount (<1%) of aragonite crystals in the grit-sized fraction (Fig. F11). These are commonly found at the seafloor, where rising pore fluids from the slab interact with seawater to precipitate carbonates. In gravity cores collected in 1997 (Fryer et al., 1999), there is a higher proportion of aragonite proximal to the mudline. The percentage of aragonite in the cores drops off with penetration to a level similar to that observed in the wash cores at the bottom of the gravity cores (0.10-0.20 mbsf in most cases) (see "Lithostratigraphy").
The >0.1-mm-fraction of the sediment contains ~1% disaggregated grains of blue (sodic) amphibole (Fig. F36). Analyses of similar grains recovered in gravity cores from the same locality (Fryer et al., 1999) showed a crossitic composition, indicative of high-pressure, low-temperature metamorphism. The mineral grains show progressive zoning with blue rims and lighter blue-green cores. This suggests relatively rapid ascent within the rising serpentine muds at the site, as suggested by Fryer et al. (1999). 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 "Geochemistry") for geologically significant periods, they would probably show retrograde metamorphic effects. None of the materials studied by Fryer et al. (1999) and Todd and Fryer (1999) have ever shown any mineral zoning indicative of retrograde effects. The mineral grains that we observe in the materials from Site 1200 also show no retrograde effects.
Although the wash core cannot be assigned a specific depth, we are confident that the lithologies present in the wash core are representative of the interval drilled in Hole 1200B. We also recovered serpentine muds in Section 195-1200A-15R-CC, and a portion was sieved and inspected for comparison with the wash core and the gravity cores described by Fryer et al. (1999). The materials in Section 195-1200A-15R-CC are similar in lithology and in proportions to those of the wash core sample and to the gravity cores investigated by Fryer et al. (1999) and Todd and Fryer (2000). We thus have data indicating that the high-pressure, low-temperature lithologies discussed above are present at Site 1200 to a depth of at least 147 mbsf.
We also have data suggesting that there is not much variation in the lithology of the serpentine matrix material. P. Fryer (pers. comm., 2001) has recovered pebble- to cobble-sized rock fragments of schists from dredges on South Chamorro Seamount, but the percentage of these compared to the other dredge samples recovered is <<1%. The lack of larger-sized rock fragments in the material drilled in Hole 1200A suggests that the metabasites are preferentially smaller pieces. The abundance of phyllosilicate minerals in the schists may contribute to the comminution of the protoliths as they rise from the source region. Pressure release as the rock fragments rise may cause the protolith to become more friable as the phyllosilicates expand. The continuous mechanical interactions within the rising muds may cause grinding and breakup of these more friable rocks into small fragments. We also note that the angularity of the serpentinized peridotites varies in a general way among the rocks that were recovered from cores in Hole 1200A. The smaller fragments are generally more angular than the larger ones, suggesting active breakup as the material rose in the conduit of the mud volcano.
The discovery of glaucophane schist in the grit-sized fraction of the serpentine mud from this site (Fig. F35) provides the strongest evidence yet obtained for the deep (slab) origin of the muds that are currently actively protruding at the summit knoll of South Chamorro Seamount. Unlike Conical Seamount, where only two metabasite fragments were found in six holes drilled, the site at South Chamorro Seamount provides a significant proportion of metabasites that allow the investigation of the paragenesis of the slab-derived lithologic fraction. It will be possible to determine the pressure and temperature ranges of the metabasites by shore-based investigations using microprobe techniques and micro-X-ray diffraction.
Preliminary examination of grit from the core catcher samples as well as leftover muds from physical properties (10-cm-long whole-round sections of the cores from Holes 1200D, 1200E, and 1200F) show a diverse assemblage of lithologies similar to those recovered from the wash material. However, the total volume of metabasite grit is less in these cores (5%-8%) than in the wash material (12%). The coarse mica schists are less abundant in Holes 1200D, 1200E, and 1200F (Fig. F37). There is a predominance of crossite/chlorite/white mica schist in all of the holes. In Section 195-1200D-8H-CC, an Orbulina universa test was found that has been completely replaced by and filled in with a magnetic mineral (iron sulfide). Fossils of various types and ages occur scattered throughout the cores (see "Biostratigraphy"). The cores in the upper portion of Hole 1200D and most of the cores from Holes 1200E and 1200F were black, apparently from a reducing environment (they change color rapidly upon exposure to air) (Fig. F38). The replacement of the test by iron sulfide is, thus, not surprising. The surfaces of the grit-sized pieces of serpentinized ultramafic rocks were also black upon initial inspection.
Major, minor, and trace element contents of 27 representative samples of serpentinites from Site 1200 have been determined via ICP-AES analysis. The results are listed in Table T3, along with LOI and Mg number (= 100 x Mg2+/[Mg2+ + Fe2+]) values. Based on the olivine-orthopyroxene-clinopyroxene CIPW normative classification (Fig. F39) reported in Table T3, the serpentinite protoliths are dominantly harzburgites with minor dunites and one lherzolite, which is consistent with classification based on thin section observations. LOI values range from 11.3 to 18.2 wt%, with 14.2 wt% as the average for all serpentinites. The average Si/Mg ratio is 0.92, with both elements negatively correlated with the LOI values. The Mg-number values are high in all the rocks, ranging from 91.4 to 92.9. Our data reveal that the serpentinites are Mg rich, with Ni and Cr contents as high as 3700 and 3800 ppm, respectively. On average, the MgO, Ni, and Cr contents of the dunites are higher than in the harzburgites. Low CaO, Al2O3, and TiO2 and high Ni contents (average Ni = 2645 ppm) confirm that most of the analyzed samples are highly residual. As in other suprasubduction zone mantle rocks (Pearce et al., 2000) the Mariana forearc serpentinites from South Chamorro Seamount preserve a record of extensive partial melting. To estimate the refractory nature of these mantle peridotites, we focused on the contents of CaO, Al2O3, and the Mg-number values. In both the CaO vs. Al2O3 and CaO vs. Mg-number diagrams (Fig. F40), the analyzed samples fall between the 15% and 30% melting lines, largely overlapping the fields of similar serpentinized ultramafic rocks from Conical and Torishima Seamounts drilled during Leg 125 (Ishii et al., 1992; Parkinson and Pearce, 1998; Savov and Ryan, unpubl. data). Thus, it appears that most ultramafic samples from Site 1200 have suffered 20%-25% melt extraction. The lherzolite sample shows a significantly lower degree of depletion, slightly higher than 15%. This is in agreement with its high clinopyroxene modal abundance (>5%) and 1 wt% higher CaO content relative to the average harzburgite.
Overall, the trace element abundances of the South Chamorro serpentinites are similar to the values displayed by average serpentinites from Conical and Torishima Seamounts sampled during ODP Leg 125 at Site 780 (Fryer, Pearce, Stokking, et al., 1990), except for their higher Sr and Na and lower Ca and Al contents (Table T3).