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doi:10.2204/iodp.proc.345.106.2014

Metamorphic petrology

All of the recovered lithologies show some degree of hydration and exhibit metamorphism over greenschist to subgreenschist facies conditions; because the core is from a rubble unit, no patterns of alteration can be established.

Background alteration

Primary mineral replacement ranges from 10% to 60%. Most of the secondary minerals are visible to the naked eye. In the recovered gabbroic rock, pyroxene was moderately to strongly altered to pale green amphibole, and olivine in these samples was strongly (60% to >90%) replaced by serpentine with mesh textures. Troctolite (Sample 345-U1415E-1R-1, 29.5–39.5 cm [Piece 4]) recovered from this hole was strongly (60% to >90%) altered, showing tremolite-chlorite corona textures between olivine and plagioclase, later serpentinization and replacement of plagioclase by prehnite and hydrogarnet, and finally late replacement of olivine by talc. Incipient corona textures (concentric zones of chlorite replacing plagioclase and tremolite ± talc ± chlorite replacing olivine) are present. The olivine corona textures appear to result in a volume increase during the serpentinization of olivine. Anorthositic gabbro (Sample 345-U1415E-2R-1, 0–6 cm [Piece 1]) was extensively replaced by prehnite, clinozoisite, and chlorite associated with a vein of similar mineralogy. In most of the lithologies recovered in Hole U1415E, plagioclase is slightly altered to secondary plagioclase, prehnite, and chlorite. Plagioclase is commonly altered to chlorite along grain boundaries with relict olivine. Some of the rock shows evidence for cataclasis, and adjacent to these zones plagioclase is highly fractured with minor replacement by secondary plagioclase and prehnite along the fracture surfaces.

In one orthopyroxene-bearing gabbro sample (345-U1415E-1R-1, 21–23 cm [Piece 3]; Table T2), alteration was minimal, although a fine-grained patch appeared to be developing grain boundary migration within the ophitic texture. In this case, the ophitic texture is still visible but the grain boundaries are strongly sutured (Fig. F6). This texture may represent incipient granoblastic recrystallization.

Veins

Veins are mostly thin (1–2 mm wide) and isolated and are dominated by subgreenschist facies minerals including clay minerals, actinolite, and prehnite. In olivine-rich lithologies, serpentine veins are also present. Microscopic zeolite veins often crosscut plagioclase crystals. Vein shapes are irregular and are characterized by sharp contacts with the host rock, but several parallel veins of identical nature may be present in one single piece. Most of the pieces recovered in Hole U1415E contain one or two veins (see “Structural geology”). A small amount of material collected in Sample 345-U1415E-2R-1, 6–9 cm (Piece 2) is identified as gabbro but may in fact be fragments of material sampling a larger vein. The composition of this material includes abundant prehnite, chlorite, and secondary plagioclase.

Rock coatings

The uppermost two pieces of olivine-bearing gabbro (0–0.2 mbsf) have a thin manganese-bearing phyllosilicate (ganophyllite) surface coating confirmed by X-ray diffraction, suggesting possible seafloor weathering.

Metamorphic conditions and degrees of alteration

The alteration observed in these samples is variable, as would be expected in rocks from a rubble unit. The dominant alteration of all of the rock recovered is subgreenschist to possibly lower amphibolite facies. The alteration predominantly affects pyroxene and olivine; primary plagioclase is largely unaltered, except where it is in contact with relict olivine or where fractured in cataclastic zones. The presence of zeolite veins indicates that some of the hydration occurred at or below zeolite facies conditions.

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