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Metamorphic petrology

Metamorphic characteristics of the drill core were determined using VCDs (macroscopic and mesoscopic), microscopic thin section descriptions, and X-ray diffraction (XRD) analyses. In this report, the terms “metamorphism,” “alteration,” and “hydration” are used loosely and interchangeably without making implications about open- versus closed-system behavior. The term “metasomatism” will apply to rocks, such as the tremolite-chlorite schists, the composition of which have clearly changed during metamorphism. Hydration reactions rarely involve the simple addition of H2O. At the level of this study, however, we are incapable of detecting minor changes in bulk composition, and we will therefore consider hydration reactions to be mostly related to the addition of H2O.

For each entry into any table, the following information was recorded: leg, site, hole, core number, core type, section number, piece number(s) (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 metamorphic petrology team 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, but in the initial phase of each cruise, members worked together to make maximum use of the varied experiences of the team. For definitions of terms used in the VCDs and logs, see Tables T1 and T2.

Visual core descriptions

The VCDs (Fig. F2) provide information (on intervals) on the extent of replacement of igneous minerals by secondary minerals, as well as the nature and approximate modes of secondary mineral assemblages. Except for simple cases, quantification of individual mineral modes was only possible where thin section or XRD data were available. Where several pieces showed similar characteristics, secondary mineral modes were assigned to the whole group on the basis of thin section or XRD analysis. More detailed information on an interval basis is presented in the alteration logs (see “Supplementary material”), which were used to generate the summary information in the VCDs. Total alteration intensity was classified as shown in Figure F8 and entered graphically into the VCD (Fig. F2).

The VCDs record overprinting alteration relationships and any association between alteration and deformation fabrics. The presence of characteristic textures (vein halos, pseudomorphs, alteration zones, corona textures, and porphyroblasts) and the lithological controls on alteration are noted. The presence, approximate abundance, and mineralogy of veins were recorded on the VCDs, with more detailed information on an interval basis given in the vein log (see “Supplementary material”).

Alteration log

The alteration log was used to describe alteration assemblages and textures on an interval basis. The intervals on this log do not necessarily correspond to the lithologic intervals defined by the igneous team. Alteration assemblages were described by color and estimated mineralogy and given a numerical code (refined by thin section and XRD study; see Table T3). A distinction was made between overprinting alteration assemblages and assemblages localized by preexisting lithology changes. For example, a core of serpentinized dunite containing a gabbro dike altered to a chlorite-tremolite assemblage would be described as three discrete intervals on three rows in the log. On the other hand, a core of serpentinized dunite containing a subinterval with vein halos would be described using two rows: one long interval of serpentinized dunite and a shorter subinterval containing the overprinting vein halo assemblage. For Expedition 305, this type of observation was given as a percent and vein data would generally be logged separately unless the vein had an extensive halo. In the same way, overprinting zones of breccia, cataclasite, or ductile shear containing new alteration assemblages or textures were recorded as subintervals. The total alteration percentage recorded is an estimate of the percentage of that interval affected by the assemblage at the time of formation (i.e., a long interval might be recorded as 100% serpentine, whereas a subinterval might contain 20% vein halos of overprinting talc).

The alteration log was used to record the presence or absence of textures (pervasive alteration, pseudomorphs, coronas, foliation, clasts, halos around clasts or veins, and the approximate percentage of vein material in the interval). During Expedition 305, the approximate percent of vein material was recorded in the vein log. Approximate mineral proportions and the possible identity of unidentified mineral phases were recorded and refined following thin section observation and/or XRD analysis. Where pseudomorphic textures are present, the suggested primary and secondary phases were noted as comments. Pervasively serpentinized rocks with unknown protolith are named “serpentinite.” If the nature of the protolith can be established, the adjective “serpentinized” is added to the rock name. This estimated alteration intensity is added to the rock name (e.g., completely serpentinized harzburgite, highly altered metagabbro, etc.).

Vein log

During Expedition 304, a combined vein log was recorded in conjunction with the structural geology team (see “Supplementary material”). Veins and vein sets were recorded on an interval basis, and vein-free intervals were also noted. Where several vein types were present in an interval, these were numbered and data were recorded on separate rows. In the metamorphic/​alteration part of the vein log, we recorded vein color and vein textures using a series of codes (Fig. F9). Textures were recorded in terms of vein shapes (straight, sigmoidal, irregular, pull-apart, and fault vein), connectivities (isolated, single, branched, and network), textures (massive, cross-fiber, slip-fiber, vuggy, and polycrystalline), and structures (simple, composite, banded, haloed, and intravenous). Any vein that did not fit into the classification scheme was entered as a separate code (9) and described in a comment. The length and width of each vein, as well as its orientation, were measured by the structural geology team. The percent volume of each vein type within a piece or interval was estimated visually. Acronyms used in the alteration and vein logs and in the thin section description forms are listed in Table T1. Table T4 lists a classification of vein types that was developed in light of experience in Hole U1309B. This was used in the vein log for Hole U1309D in a “vein type” column but not in the Hole U1309B vein log. Minor changes were also made in the vein mineralogy columns between Cores 304-U1309D-1R to 22R and Cores 304-U1309D-23R to 78R. Hence, two separate vein log spreadsheets for the uppermost 400 m of Hole U1309D are archived (see “Supplementary material”).

During Expedition 304, a separate log was also made of the intensity of vein-related alteration, together with the relationship between this and breccia zones and late magmatic leucocratic dikelets (see vein alteration logs in “Supplementary material”). In this log, the lithology, interval, and approximate percent of the interval consisting of veins and altered vein halos was estimated, concentrating on greenschist-facies veins (including talc-tremolite veins in ultramafic rocks but not hornblende veins or fracture-controlled serpentine/prehnite alteration). This log, as well as the alteration log (see “Supplementary material”) was used to create alteration-depth diagrams.

During Expedition 305, the vein log (see “Supplementary material”) was recorded independently of the structural geology team and no vein alteration log was recorded. Classifications used in the vein log were the same as for Expedition 304 with the exception of the vein types, where a simpler scheme was adopted (Table T4). The percent volume of each vein type within a piece or interval was calculated from width and length data on Expedition 305 only.

Thin section descriptions

Thin section descriptions were made using the standard IODP template (see “Thin Sections” for all sites). 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 and, where possible, grain sizes were estimated visually. The nature of minor and trace phases (carbonates, sulfides, and oxides) and the extent of Cr spinel alteration could rarely be established by visual core description. In some cases, phases were identified using a microscope-mounted electron beam energy-dispersive X-ray spectrometer (miniprobe) supplied by E. Hellebrand. See “Electron beam analysis” in “Igneous petrology” for a description. The modal estimates allowed characterization of the intensity of alteration and aided in establishing the accuracy of the macroscopic and microscopic visual estimates of the extent of alteration. Whole thin section images were used to guide microtextural studies, and for Hole U1309B they were then compiled with photomicrographs into annotated photo sheets (see “Supplementary material”).

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 CuKα 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°–70°2θ
  • Step size = 0.02°2θ
  • Scan speed = 1.2°2θ/min
  • Count time = 1 s

Peak intensities were converted to values appropriate for a fixed slit width. An interactive software package (MacDiff version 4.2.5 PPC) was used to identify the primary minerals (public domain software is available from​Software). Identifications were based on multiple peak matches, using the mineral database provided with MacDiff. Relative abundances reported in this volume (trace, minor, and major components) are useful for general characterization but are not precise.