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doi:10.2204/iodp.proc.335.102.2012

Alteration and metamorphism

The characteristics of hydrothermal alteration and contact metamorphism of rocks recovered during Expedition 335 were determined using visual inspection of the core, microscopic thin section descriptions, and X-ray diffraction (XRD) analyses. Visual observations on the core were recorded in the alteration log and the vein and halo log (Tables T6, T7) and are provided as Excel files in DESCLOGIK_WORKBOOKS_335 in DESCRIPTIONS in “Supplementary material.” Shipboard observations of alteration and metamorphism were recorded using the DESClogik worksheet interface and uploaded to the LIMS database. Alteration and metamorphism of igneous rocks were described in terms of pervasive background alteration, localized alteration patches (zones of more intense alteration), hydrothermal veins and alteration halos, presence of recrystallized domains (xenoliths of contact metamorphosed rock fragments in gabbro), breccia clasts, and breccia clasts with alteration halos. Each logged interval may represent multiple pieces or core sections.

Macroscopic core description

Background alteration

Alteration textures were recorded in the alteration log, and features were described using the following terms in order to document variations and heterogeneities in alteration style and intensity:

• Pervasive: uniform alteration style and intensity throughout the rock.

• Patchy: pervasive background alteration with local alteration patches.

• Recrystallized: pervasive background alteration where the rocks are recrystallized to granoblastic contact metamorphic assemblages.

• Clasts: breccia clasts enclosed in a matrix of secondary minerals.

• Clasts with halos: breccia clasts that contain alteration halos and are enclosed in a matrix of secondary minerals.

For each alteration textural type, the following features were observed and recorded for background alteration (the entire interval or the host rock for patches and recrystallized domains):

• Color,

• Percentage of rock altered to secondary phases,

• Percentage of each primary mineral replaced, and

• Secondary minerals replacing primary minerals.

For each textural category, the volume percentage of alteration was estimated and classified using the following scale:

• Fresh = <2% by volume alteration products.

• Slight = 2%–9%.

• Moderate = 10%–49%.

• High = 50%–95%.

• Complete = >95%.

The following data were recorded for patchy textural intervals:

• The pervasive background alteration information for the rock hosting the patches (as above);

• The size (<1, 1–3, or >3 cm), shape (round, irregular, elongate, or network), and area percentage of patch in the rock or interval;

• The total percentage of alteration (secondary phases) in the patches; and

• The primary mineral alteration and secondary phases present (as in the background alteration).

Recrystallized alteration texture comprises pervasive background alteration where the rock is recrystallized to granoblastic contact metamorphic assemblages (based on examination of thin sections and previous work) (Teagle, Alt, Umino, Miyashita, Banerjee, Wilson, and the Expedition 309/312 Scientists, 2006; Koepke et al., 2008; France et al., 2009; Alt et al., 2010). For recrystallized intervals, background alteration was recorded as described above for pervasive background alteration. Recrystallization degree was classified on a scale from 0 to 6, based on the classification from Teagle, Alt, Umino, Miyashita, Banerjee, Wilson, and the Expedition 309/312 Scientists (2006) (Fig. F11). Recrystallization begins with the growth of isolated micrometer-sized pyroxene granules replacing clinopyroxene and developing as small inclusions in clinopyroxene and reaches “partial” replacement where the igneous texture begins to disappear because of recrystallization to granular pyroxene. Samples were classified as strongly recrystallized if all minerals are replaced but the igneous texture is not completely erased. In completely recrystallized samples, the igneous texture is obliterated and overprinted by an equigranular granoblastic assemblage of secondary pyroxene, plagioclase, ilmenite, and magnetite.

Xenoliths of dike rock are variably recrystallized to granoblastic contact metamorphic assemblages and were described as recrystallized domains. The abundance of these was estimated, and the pervasive background alteration of the host rock was recorded as described above.

An alteration and metamorphism summary description was entered in DESClogik so it could be added to the VCDs. The alteration intensity plotted on the VCDs corresponds to the background alteration intensity. The VCDs also contain a summary statement of the alteration characteristics for each unit in a section.

Veins and alteration halos

The frequency of veins per 10 cm interval of core was recorded in the alteration log as vein density on a scale of 0 to 5:

• 0 = no veins,

• 1 = <1 vein per 10 cm,

• 2 = 1–5 veins per 10 cm,

• 3 = 6–10 veins per 10 cm,

• 4 = 11–20 veins per 10 cm, and

• 5 = >20 veins per 10 cm.

For each vein, the depth interval, width, attitude (vertical or inclined), connectivity, boundary (diffuse or sharp), and color and mineralogy (with each mineral estimated as a percentage of the vein) were recorded in the vein and halo log. The classification used for vein connectivity is shown in Figure F6. The orientations of veins in oriented core pieces were systematically measured (see “Structural geology”). Alteration halos representing zones of increased alteration adjacent to veins were described by width, color, and secondary mineral percentages in the Halo comments column of the vein and halo log. Vein nets and breccia were recorded on the vein and halo log in the Alteration feature column, noting the depth interval, total volume percent secondary minerals, and percentages of individual secondary phases. Breccia clasts were to be described in the alteration log, but such features were not encountered in the Expedition 335 cores.

Thin section description

Thin sections of basement rocks recovered during Expedition 335 were examined to confirm macroscopic identifications of secondary minerals and establish the distribution, occurrence, and abundance of secondary minerals with depth in the core. The amount of alteration/recrystallization was determined, along with the mode of occurrence of vesicle and void fillings, vein composition, and primary mineral replacement. Chronological relationships between different secondary minerals or parageneses were recorded.

Data were recorded in the thin section log in DESClogik for each alteration domain present within a thin section, including background, patch, recrystallized domain, and vein. Data recorded for each secondary mineral present include

• Estimates of secondary mineral percentage,

• The primary mineral(s) replaced, and/or

• Texture as a comment.

Digital photomicrographs were taken to document features described in thin sections. A summary description was entered in DESClogik so it could be added to the thin section report. The recrystallization degree (from 0 for not recrystallized to 6 for complete recrystallization; details are given in Fig. F11) was also added in the thin section log.

X-ray diffraction

Phase identification of whole rocks, patches, or vein material was aided by XRD analyses using a Brucker D-4 Endeavor diffractometer with a Vantec-1 detector using nickel-filtered CuKα radiation. XRD was performed on small amounts of powder (usually ~20 mg) as smear slides or pressed onto sample holders. Instrument conditions were

• Voltage = 40 kV.

• Current = 40 mA.

• Goniometer scan (bulk samples) 2θ = 4°–70°.

• Step size = 0.0087°.

• Scan speed = 0.2 s/step.

• Divergence slit = 0.3°, 0.6 mm.

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