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Alteration petrology

Alteration characteristics were described from visual observation of the archive halves, and the general degree of alteration is presented on the igneous VCDs (see “Igneous petrology and volcanology”). Petrographic description of thin sections and X-ray diffraction (XRD) analyses (bulk rock, veins, vesicles, and void infillings) were carried out on samples from the working halves.

All igneous rocks recovered during Expedition 330 underwent some degree of alteration or weathering in subaerial, shallow-subaqueous, hydrothermal, or deep-marine environments. Primary igneous phases such as glass and olivine have therefore often been altered to a variety of secondary phases, including clays, zeolites, carbonates, and iron oxides that were identified and described by the alteration petrology group. Methods used include hand sample descriptions, inspection of polished thin sections, and XRD spectroscopy. Alteration mineralogy was defined by color, habit and shape, association with primary minerals (if distinguishable), and hardness. Complications arise in the identification and description of the secondary phases because many minerals produced in the different alteration environments (e.g., low-temperature submarine alteration and subaerial weathering) are visually similar and indistinguishable in the cores. Also, some cores may have experienced multiple alteration events (e.g., subaerial weathering overprinted by seafloor alteration), producing a more complex mineral assemblage. Hence the identification of some alteration phases remains tentative, pending more detailed shore-based XRD studies and electron microprobe analyses.

Core logging

The types, forms, and distribution of secondary minerals were determined during core description, as was the abundance of veins and vesicles and their related infilling material. Descriptions were based mostly on hand specimen observations of cut, dry, and wet surfaces. These observations provided information on the alteration of primary igneous features, including the alteration of phenocrysts and groundmass minerals and volcanic glass. Information was recorded on the extent of replacement of igneous minerals and groundmass by secondary minerals and, where possible, the nature and approximate modes of secondary mineral assemblages.

The secondary mineral assemblages were largely composed of clay minerals that were difficult to identify macroscopically; therefore, a general descriptive term such as “brown clay” was used for these cores. Similarly, the specific zeolite and carbonate minerals are not generally distinguished, except when their crystal morphology allowed unequivocal identification. Visual estimates of alteration degree, type, color, and textures (e.g., halos and patches); abundance (%) of minerals filling veins and vesicles; and the proportion of altered groundmass, volcanic glass, and phenocrysts were recorded.

The degree of alteration for groundmass and glass is defined and reported graphically on the VCDs according to various ranges of intensity (Fig. F7):

  • Fresh = <2 vol%.

  • Slight = 2–10 vol%.

  • Moderate = >10–50 vol%.

  • High = >50–95 vol%.

  • Complete = >95–100 vol%.

Note that for some intervals, especially those containing clasts with different degrees of alteration, mixed alterations are defined.

Alteration color was defined using Munsell Soil Color Charts (Munsell Color Company, Inc., 1994) and converted to a more intuitive color name (e.g., very dark gray, greenish gray, etc.). Quantification of individual mineral modes was estimated by investigating the archive halves under a binocular microscope or using hand lenses with graticules of 0.1 mm. A distinction was made between overprinting alteration assemblages and assemblages localized by preexisting lithology changes.

During Expedition 330, veins were first recorded by the shipboard structural geologists to identify the location, orientation, and width of the veins; alteration petrologists then recorded the mineralogy of veins and vein halos. All features were recorded in DESClogik (see below) using a series of codes (Fig. F8) for vein shape (straight, sigmoidal, irregular, pull-apart, and fault vein), connectivity (isolated, single, branched, and network), texture (massive, cross fiber, slip fiber, vuggy, and polycrystalline), structure (simple, composite, banded, haloed, and intravenous), and geometry (en echelon, ribbon, and cross fractures). Likewise, vesicles were first recorded by the igneous petrology group for shape, abundance, size, and density, after which the infilling minerals were identified by the alteration petrologists. Voids were described by the alteration petrologists in terms of size, abundance, and infilling minerals.

DESClogik: descriptive data capture

During Expedition 330 the DESClogik application (see DESClogik user guide) was used to enter data from visual core description, thin section description, and compilation into the LIMS database. Before drilling operations began, spreadsheet templates were constructed and customized in DESClogik to record alteration characteristics of Expedition 330. These templates were based on the methods and observations of ODP Leg 192 and IODP Expeditions 304/305 and 324 (Mahoney, Fitton, Wallace, et al., 2001; Blackman, Ildefonse, John, Ohara, Miller, MacLeod, and the Expedition 304/305 Scientists, 2006; Sager, Sano, Geldmacher, and the Expedition 324 Scientists, 2010). A first template was used to record general alteration characteristics observed in the igneous rocks (first column of Table T5), and a second template, shared with the igneous petrologists and structural geologists, was used to record alteration characteristics of veins and vesicles (second and third columns of Table T5).

Thin section description

Thin section descriptions were also recorded using DESClogik and subsequently uploaded to the LIMS database. Secondary mineral assemblages and replacement relations to primary phases were described, as were secondary modes. Mineralogy of veins and vesicles, as well as cement and voids present in basaltic breccia, was also reported. Modal estimates of secondary minerals allowed characterization of alteration intensity. Total alteration (%) was calculated using the modal proportions of phenocrysts and groundmass minerals and their respective percentages of alteration.

X-ray diffraction

Mineral identification was aided by XRD analyses using a Bruker D-4 Endeavor diffractometer with a Vantec-1 detector using nickel-filtered CuKα radiation. Instrument conditions were as follows:

  • Voltage = 40 kV.

  • Current = 40 mA.

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

  • Step size = 0.0087°.

  • Scan speed = 0.2 s/step.

  • Divergence slit = 0.3°, 0.6 mm.

When available, additional mineralogical evidence from thin section descriptions or X-ray diffractograms was integrated into the alteration, vein, and vesicle DESClogik template and into the summary text field of the VCDs.

Clay mineral identification using XRD remains tentative, notably because of the absence of well-marked peaks in XRD patterns resulting from the low crystallinity of the clay minerals and the high proportion of associated calcite, which obscures the clay signal. For some samples with high proportions of clay, two other analyses of immediately adjacent material were performed in order to obtain a better resolution for clay mineral peaks. One of the two analyses was conducted after carbonates were dissolved from the sample with diluted hydrochloric acid. The second analysis was performed after ethylene glycolation of the powder (60°C; 6–12 h). This method expands the crystal network of smectite and results in a d-spacing shift toward lower 2θ (Moore and Reynolds, 1997), which allowed better identification of clay minerals that are present mainly in groundmass resulting from the alteration of interstitial glass, in coatings of vesicle walls, and in veins as intergrowths with other secondary minerals such as carbonate or zeolite.