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Alteration petrology

One sedimentary unit containing different types of volcanic clasts was identified in Hole U1375A, and one igneous unit (microgabbro) was recovered in Hole U1375B (see “Igneous petrology and volcanology”). Sections from both holes have undergone alteration by low-temperature water-rock interactions or weathering.

Alteration phases

Alteration at Site U1375 in both the volcanic clasts from Hole U1375A and the microgabbro from Hole U1375B has mainly resulted in the replacement of olivine. In contrast, plagioclase and augite are generally well preserved, both as phenocrysts and in the groundmass within the clasts. Plagioclase shows minor alteration to sericite/illite in some rocks but characteristically is fresh. Olivine is typically completely altered to iddingsite (Fig. F8B), hematite, Fe oxyhydroxides, and carbonates.

Overall alteration characteristics of Hole U1375A

The overall alteration of the volcanic clasts in sedimentary units from Hole U1375A ranges from slight to high (10%–60%), as estimated from core descriptions and thin section observations. Pervasive alteration gave a yellow/orange to brownish-gray color to the clasts. Nevertheless, two clasts are dark gray (Sample 330-U1375A-2R-1, 42–46 cm) to light greenish gray (Sample 2R-1, 81–100 cm).

No vesicles were identified in these clasts. Veins are mainly filled with carbonate and brown clay. Some of the clasts have brown halos associated with veins and fractures. Such halos, commonly found in permeable basaltic formations such as breccia, are related to the alteration of basalts by bottom seawater percolating through the oceanic basement (halmyrolysis), which takes place at water temperatures of <2°C and under large water/rock ratios and oxidizing conditions (Mahoney, Fitton, Wallace, et al., 2001).

Overall alteration characteristics of Hole U1375B

The moderately olivine-augite-phyric microgabbro (dolerite) from Hole U1375B shows moderate to high (55%) alteration and has an orange to dark gray color. Olivine is altered to hematite, iddingsite, and some oxyhydroxides (Fig. F8B). The microgabbro contains strongly oxidized millimeter-thick veins (Fig. F8A). Two types of veins were identified. The thicker ones are mostly filled with goethite (Fig. F9E, F9F), whereas the thinner veins are coated by goethite on the walls and filled with carbonate (Figs. F8C, F9C, F9D). A few veins were first filled with green clay minerals and then later with goethite (Fig. F9A, F9B).