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doi:10.2204/iodp.proc.327.106.2011

Petrology

Basalt recovery near Grizzly Bare outcrop

Recovery of basalt from the upper ocean crust near Grizzly Bare outcrop occurred during XCB coring in Holes U1363B and U1363D but also occurred in Hole U1363F during APC coring. Basalt at Site U1363 was designated as lithologic Unit 4 (see “Lithostratigraphy”). Although only a few basalt samples were recovered, this material provides an opportunity to assess the extent of crustal alteration and microbiological activity associated with the upper crust near an area of hypothesized hydrothermal recharge.

Hole U1363B

A single 9 cm long piece of basement was recovered in Core 327-U1363B-10X. This piece is a highly clinopyroxene plagioclase phyric cryptocrystalline basalt with glomeroporphyritic texture visible in hand specimen. Phenocrysts are large (up to 8 mm) and are anhedral to euhedral in shape. The basalt is sparsely vesicular, with highly variable vesicle size and shape. Secondary minerals replace background groundmass, form alteration halos, fill vesicles, and line hydrothermal veins. Background alteration is characterized by a pale gray color, and alteration halos are dark gray and flank the hydrothermal vein and occur along the edge of the piece. Vesicle filling is variable and includes saponite or celadonite lining vesicles, completely carbonate filled to iron oxyhydroxide filled within the alteration halos and euhedral open-space, filling crystals within the larger irregular vesicles. Within this piece a 0.2 mm saponite vein network forms a mosaic-like texture in the background alteration. Down the center of the piece is a single 1 mm mixed-saponite vein lined with clay with an iron oxyhydroxide core and a 2 mm dark gray halo. This piece is interpreted to be part of a basalt pillow.

Hole U1363D

One large 6.9 cm piece taken for microbiology and three rubble chips (3.0–3.5 cm in size) were recovered from Core 327-U1363D-6X. Rubble chips are sparsely plagioclase phyric cryptocrystalline basalt. Small millimeter-sized chilled margins are present on two of the chips. Iron oxyhydroxide and green clay occur as a patchy coating on several exterior surfaces on all three pieces.

Hole U1363F

One 1 cm piece taken for microbiology and numerous rubble chips (1–20 mm in size) were recovered from Core 327-U1363F-4H. These rubble chips are a mixture of highly plagioclase phyric glassy basalt with a manganese crust, fragments of manganese crust, and sedimentary material. Plagioclase phenocrysts are up to 3 mm in size. In some pieces an iron oxyhydroxide vein is present at the interface between the basalt and manganese crust.