METAMORPHIC PETROLOGY

Metamorphic characteristics of the drill core were determined using visual core descriptions (macroscopic and mesoscopic), microscopic thin section descriptions, and X-ray diffraction (XRD) analyses. Metamorphism, metasomatism, and hydrothermal alteration are used loosely and synonymously without making implications about open- vs. closed-system behavior. For each piece of core the following information was recorded: leg, site, hole, core number, core type, section number, piece number (consecutively downhole), and position in the section. All metamorphic descriptions and measurements were made on the archive halves of the cores, except where otherwise noted. The depth of a given feature was defined as the point where the center of a given structure intersects the center of the cut face of the archive half of the core or, if the feature does not appear in the center of the core, the depth of the centroid of the feature in the archive half of the core. The metamorphic petrology group worked together during the same shift to minimize measurement inconsistencies. Each member of the team was responsible for making a specific set of observations throughout the entire core.

The visual core descriptions provide information (on a piece-by-piece basis) on the extent of replacement of igneous minerals by secondary minerals, as well as the nature and approximate modes of secondary mineral assemblages. These data are used to estimate the total extent of rock alteration that is reported for each site in the alteration log (see the "Supplementary Material" contents list). Further, the extent to which metamorphic minerals contribute to any subsolidus fabric is recorded in the comments. Alteration styles are background, patchy, and vein halos. The presence of veins is noted in the alteration log, and detailed vein descriptions and vein abundance estimates are provided in the vein log, which records style, mineralogy, shape, and connectivity of veins (see the "Supplementary Material" contents list). Thin section descriptions and XRD results were integrated with visual core descriptions to ensure internal consistency and maximize accuracy of the core descriptions.

Visual Core Descriptions

The metamorphic mineral assemblages as well as style and intensity of alteration were recorded in the alteration log. Primary phases (olivine, clinopyroxene, orthopyroxene, spinel, and plagioclase) and the secondary minerals that replace them were noted and, where possible, the volume percent of each phase was estimated in hand specimen and confirmed by observation of representative thin sections. In the alteration log, there is one entry for each piece, unless pyroxenite or gabbro dikes/segregates are developed, in which case their metamorphic characteristics were logged separately. Pervasively altered, calcium-metasomatized rocks are called rodingites if the alteration has destroyed the primary igneous composition and textural relationships. Similarly, pervasively serpentinized rocks with unknown protolith are named serpentinite. If the nature of the protolith can be established, the adjective serpentinized or rodingitized is added to the rock name. Alteration intensity is classified as follows:

  1. Fresh = <2%.
  2. Slight = 2%–10%.
  3. Moderate = 10%–40%.
  4. High = 40%–80%.
  5. Very high = 80%–95%.
  6. Complete = >95%.

This estimated alteration intensity is added to the rock name (e.g., completely serpentinized harzburgite, highly altered metagabbro, etc.). In addition, the estimated alteration intensity was recorded in the alteration log as percentage values. The alteration mineralogy was tentatively described (and confirmed by thin section and XRD) and the degree of alteration of each primary phase was recorded in percent. The proportions of phases in the alteration assemblage replacing olivine, orthopyroxene, clinopyroxene, and plagioclase were estimated and reported normalized to 100%. In some instances, it is appropriate to distinguish between pseudomorphic and nonpseudomorphic alteration styles, in which case this was noted in the comments (e.g., bastite after orthopyroxene preserved kink banding). Both alteration styles are pervasive but pseudomorphic indicates that primary features of the protolith are preserved, whereas nonpseudomorphic indicates these were destroyed. Replacement textures (e.g., coronas, overgrowths, and pseudomorphs) were also recorded in the comments.

Vein mineralogy and mineral abundance, the top and the bottom location of the vein in the piece(s), vein type, shape, connectivity, structure, texture, color, and associated vein halos were recorded in the vein log. The criteria for distinguishing different vein types (paragranular and transgranular), vein shapes (straight, sigmoidal, kinked, and irregular), connectivities (single, branched, and network), textures (cross-fiber, slip-fiber, massive, and vuggy), and structures (simple, composite, and banded) are outlined in Table T1 and illustrated in Figure F4. For an individual piece, multiple entries were made in the vein log if several vein types are developed. The length and width of each vein as well as its orientation were measured by the structural geology group. The percent area of the veins within a piece or interval was estimated visually. These data were used to calculate the volume percentage of each vein type within a given interval and within the entire core.

Acronyms used in the alteration and vein logs and in the thin section description forms are listed in Table T2. We used number schemes (see Fig. F4) to enter vein descriptions in the vein logs. If a vein could not be appropriately characterized using this classification, we marked it "mixed" (number 9) and entered a description in the comments.

Thin Section Descriptions

Detailed petrographic thin section descriptions were made to aid in identification and characterization of metamorphic and vein mineral assemblages. Stable mineral parageneses were noted, as were textural features of minerals indicating overprinting events (e.g., coronas, overgrowths, and pseudomorphs). Secondary mineral assemblages and replacement relations to primary phases were described, and secondary modes were estimated visually. The nature of minor and trace phases (carbonates, sulfides, and oxides) as well as the extent of Cr spinel alteration could usually not be established by visual core description. Particular emphasis was therefore placed on the identification and description of minor and trace phases. The area of a thin section occupied by different vein types and relationships between veins and rock alteration were also recorded. These data are summarized, together with igneous and structural descriptions, in the thin section description sheets (see the "Core Descriptions" contents list). The modal estimates allowed characterization of the intensity of metamorphism and aided in establishing the accuracy of the macroscopic and microscopic visual estimates of the extent of alteration. Thin section descriptions also include microtextural information on serpentinization (Fig. F5). The distinction between pseudomorphic, nonpseudomorphic, and transitional serpentinization (see Table T1) provides useful information, as only pseudomorphic textures are primary alteration features, whereas the latter two represent recrystallization phenomena. Microtextural criteria were combined with optical orientation of serpentine fibers (or apparent fibers) to accomplish a tentative identification of chrysotile and lizardite (Table T3).

X-Ray Diffraction

Phase identification in selected samples of whole-rock shipboard powders and metamorphic vein material was aided by XRD analyses using a Philips model PW1729 X-ray diffractometer with CuK radiation (Ni filter). Each sample was freeze-dried, crushed, and mounted with random orientation in an aluminum sample holder. Instrument conditions were as follows:

Voltage = 40 kV,
Current = 35 mA,
Goniometer scan (bulk samples) 2–702,
Step size = 0.022,
Scan speed = 1.22/min, and
Count time = 1 s.

Peak intensities were converted to values appropriate for a fixed slit width. An interactive software package (MacDiff version 4.1.1. PPC) was used on a Macintosh computer to identify the primary minerals (public domain software is available at www.pangaea.de/Software). Identifications were based on multiple peak matches, using the mineral database provided with MacDiff. Relative proportions of serpentine (a distinction between the different polytypes was usually not possible), brucite, zeolites, carbonates, magnetite, calcium silicates, pyroxene, amphibole, chlorite, and clay minerals were estimated using peak intensity ratios. Relative abundances reported in this volume (trace, minor, and major components) are useful for general characterization but are not precise.

NEXT