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Igneous petrology

During Expedition 333, basement rocks were recovered in Holes C0012E, C0012F, and C0012G.

Hole C0012E

The sediment/basement boundary was recovered at 525.815 mbsf (Section 333-C0012E-3X-7, 114 cm). In total, 1.53 m pillow lavas were cored in this hole with a recovery rate of 100%.

Hole C0012F

The sediment/basement boundary was recovered at 520.46 mbsf (Section 333-C0012F-1R-1, 46 cm). In total, 3.45 m igneous rocks were cored with a recovery rate of ~68%. All the igneous rocks recovered in this hole are classified as pillow lavas.

Hole C0012G

The sediment/basement boundary was recovered at 525.69 mbsf (Section 333-C0012G-2R-2, 80 cm). In total, 100.75 m basement rocks were cored in this hole with a recovery rate of ~20%. The 100.75 m igneous section was divided into two igneous units: pillow lavas (Unit I) and sheet flows with pillow lava interlayers (Unit II) (Fig. F52).

Pillow lavas of Unit I (Sections 333-C0012G-2R-2, 80 cm, to 6R-1, 22 cm) are composed of pillow basalts. The pillows are predominantly phyric, and grain size ranges from glassy at the chilled margins to cryptocrystalline or microcrystalline. The groundmass of pillow basalts generally consists of plagioclase and clinopyroxene microlites, with interstitial titanomagnetite and altered glass. Phenocrysts in these rocks are mainly plagioclase, clinopyroxene, and olivine, in order of decreasing abundance, commonly clustered in a glomeroporphyritic texture. Pillow basalts were moderately to heavily altered. The dominant alteration product is saponite. Celadonite and analcime occur as void fillings. Other zeolites and pyrite occur as minor components. This assemblage indicates a reducing low temperature alteration environment.

Unit II sheet flows with pillow lava interlayers (Sections 333-C0012G-6R-1, 22 cm, to 15R-CC, bottom) are dominantly composed of sheet flows. Pillow lavas occurred as interlayers (Fig. F52), indicating a greater prevalence of massive flows deeper in the section. The individual cooling units of sheet flows range from tens of centimeters to several meters thick. Individual flows are commonly separated by chilled margins. Where contacts were not recovered, individual flows were distinguished by systematic changes in grain size. The sheet flows have more or less the same petrology as pillow basalts but show interstitial texture and are generally less vesicular. Thin section observations show that the most finely grained rocks have intergranular to interstitial groundmass textures. In this unit, Fe oxyhydroxide is found as alteration products in veins and alteration halos. Celadonite and saponite and, locally, pyrite are present in the rock mass. This indicates that alteration in Unit II occurred both under iron oxidizing conditions and iron reducing conditions. These most probably occurred at different times but the sequence of events is not yet known.

Visual core descriptions and thin sections

Digital images and visual core descriptions as well as thin section descriptions are included in “Core descriptions.”