The recovered core from Site 1200 consists of clasts of variably metamorphosed ultramafic rocks included in unconsolidated fine-grained serpentine muds. The muds that comprise the matrix of this cored material were, however, not formed by sedimentary processes. They consist of finely comminuted rock and mineral fragments and secondary products derived from the serpentinization of ultramafic rocks and metamorphism of mafic rocks. Although such fine-grained materials were described as sediments during Leg 125 (Shipboard Scientific Party, 1990), they were, in fact, igneous and metamorphic rocks. For consistency with ODP format, the descriptions of the serpentine mud were noted on barrel sheets for sedimentary material and the petrologists and sedimentologists worked together to finalize the description of the material. The clasts of serpentinized ultramafic rocks recovered from Site 1200 and the basement material recovered from Site 1201 were described on the hard rock visual core description sheets using standard ODP igneous petrology procedures.
The general procedures used during Leg 195 for describing igneous rocks from Sites 1200 and 1201 follow the outline presented in the "Explanatory Notes" chapters of Legs 125 and 176 (Shipboard Scientific Party, 1990, 1999, respectively). All qualitative and quantitative measurements (e.g., grain size, percent alteration, etc.) were made by consensus. Visual igneous, metamorphic, and structural characteristics were included in the documentation of each core to better describe important characteristics, such as alteration style, mineralogy, and the occurrence of veins. Identification of mineral phases was checked by XRD analyses according to ODP standard procedures outlined in previous Initial Reports volumes (e.g., Leg 118; Shipboard Scientific Party, 1989). At least one minicore was taken per lithologic unit for chemical and physical properties analyses, generally from the freshest part of the core. Additional thin sections, XRD, and ICP-AES analyses were performed on selected samples of specific interest (vein material, highly or less-altered intervals, and coarse-grained intervals).
Igneous VCD forms were used when describing the basement cores (Fig. F5). Where possible, the primary and secondary phases in hand specimen were noted. The volume content of the phases present was estimated and verified by examination of representative thin sections. Colors of the recovered material were determined using Munsell soil color charts on dry, cut faces of cores.
Thin section billets of igneous and metamorphic rocks were examined (1) to confirm the identity of petrographic groups in the cores, (2) to better understand the textures and interrelationships of the mineral phases, (3) to help define unit boundaries indicated by hand specimen core descriptions, and (4) to define the secondary alteration mineralogy. Percentages of individual mineral phases were visually estimated and reported on thin section description sheets. Wherever possible, approximate composition of preserved primary minerals has been determined by means of their optical properties.
Primary (olivine, orhopyroxene, clinopyroxene, spinels, feldspars, and amphibole) and secondary (serpentine, chlorite, brucite, iron oxide and hydroxide, carbonate, clay minerals, zeolites, apatite, talc, and mica) phases were identified in thin sections. Each of the minerals were recorded on thin section description forms (Fig F6). The information includes the following major observations:
Basalts are termed aphyric (<1%), sparsely phyric (1%-2%), moderately phyric (2%-10%), or highly phyric (>10%), depending upon the proportion of phenocrysts visible with a hand lens or binocular microscope (~10x magnification). Basalts were further classified by phenocryst type (e.g., a moderately plagioclase-olivine phyric basalt contains 2%-10% phenocrysts, mostly plagioclase, with subordinate olivine).
Serpentinized rocks were classified on the basis of the original phase proportions present in the protolith, using the International Union of Geological Societies classification system for ultramafic rocks. For example, "serpentinized dunite" is a rock with at least 90% modal olivine originally present.
Alteration was addressed through XRD analyses and studies of the occurrence of secondary minerals in thin sections. The XRD analyses were able to distinguish mineral groups and varieties, such as serpentines, clays, chlorite group minerals, and secondary iron oxide minerals. From thin section analyses, it was possible to identify vein-filling materials (serpentine varieties, carbonates, chlorite, and talc) and pseudomorphic textures (serpentine and brucite after olivine; serpentine and talc after orthopyroxene in serpentinized ultramafic rocks from Site 1200; clay minerals, chlorite, iron oxides and hydroxides, and carbonate after olivine; and zeolites, chlorite, and alkali feldspar after plagioclase in basement rocks from Site 1201).
Igneous rocks were classified based on their principal mineralogy. Ultramafic and mafic rock varieties were named according to the Streckeisen (1974) classification diagram based on olivine, clinopyroxene, orthopyroxene, and plagioclase abundances (Fig. F7). The volcanic rock samples were classified according to standard petrographic techniques, using a combination of modal mineralogy determinations and composition data from onboard ICP-AES analyses. The classical double triangle Streckeisen (1974) classification diagram was also used (Fig. F8).
We selected representative samples of major lithologic units for shipboard ICP-AES analysis. Large whole-rock pieces were first cut with a diamond-impregnated saw and ground on a diamond wheel to remove surface contamination. Samples were washed in an ultrasonic bath containing methanol for ~10 min. This was followed by three consecutive ~10-min washes in an ultrasonic bath containing nanopure deionized water, then the samples were dried for ~12 hr in an oven at 110°C. The cleaned whole-rock samples (~15 cm3) were reduced to fragments <1 cm in diameter by crushing between two disks of Delrin plastic in a hydraulic press and ground for ~5 min in a Spex 8510 shatterbox with a tungsten carbide barrel. The sample powders were weighed on a Scientech balance and ignited to determine the weight after loss on ignition.
We weighed 0.100 ± 0.002-g aliquots of the ignited whole-rock powders and mixed them with 0.4000 ± 0.0004 g of lithium metaborate (LiBO2) flux that had been preweighed on shore. Standard rock powders and full procedural blanks were included with the unknowns for each sample run. All samples and standards were weighed on the Scientech balance. Weighing errors were conservatively estimated to be ±0.0005 g.
Mixtures of flux and rock powders were fused in Pt-Au crucibles at 1050°C for 10-12 min in a bead sampler NT-2100. Ten microliters of 0.172-mM aqueous lithium bromide (LiBr) solution was added to the mixture before fusion as an antiwetting agent to prevent the cooled bead from sticking to the crucible. Cooled beads were transferred to 125-mL polypropylene bottles and dissolved in 50 mL of 2.3-M HNO3 by shaking with a Burrell wrist-action bottle shaker for 1 hr. After digestion of the glass bead, all of the solution was passed through a 0.45-µm filter into a clean 60-mL wide-mouth polypropylene bottle. Next, 2.5 mL of this solution was transferred to a plastic vial and diluted with 17.5 mL of 2.3-M HNO3 to bring the total volume to 20 mL. The solution-to-sample dilution factor for this procedure is 4000.
Major (Si, Ti, Al, Fe, Mn, Mg, Ca, Na, K, and P) and trace (Zr, Y, Sr, Ba, Zn, Cu, V, Cr, and Ni) element concentrations of powder samples were determined with the JY2000 Ultrace ICP-AES. The JY2000 sequentially measures characteristic emission intensities (with wavelengths between ~100 and 800 nm). ICP-AES protocols for dissolution and analysis of rock powders were developed by Murray et al. (2000). The elements analyzed, their units, and the ICP-AES detection limits for Leg 195 samples are provided in Table T1.
A typical ICP-AES run includes
A 2.3-M HNO3 wash solution was run for a minimum of 90 s between each of the samples and standards.
Following each sample run, the raw intensities were transferred to a data file and data reduction was completed using a spreadsheet to ensure proper control over standardization and drift correction. Once transferred, intensities for all samples were corrected for the full procedural blank. Drift correction was then accomplished by linear interpolation between each consecutively run drift-correcting solution and correcting the intensities of the samples run between those drift-correcting solutions. The interpolation was calculated using the lever rule. Following blank subtraction and drift correction, concentrations for each sample were calculated from the average intensity per unit concentration for the rerun certified reference material.
Estimates of accuracy and precision for major and trace element analyses were based on replicate analyses. In general, run-to-run relative precision by ICP-AES was ~2% for the major elements. Run-to-run relative precision for trace elements was generally ~5%-10%. Exceptions typically occurred when the element in question was near the detection limit of the instrument (Table T1).