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doi:10.2204/iodp.proc.345.107.2014

Metamorphic petrology

Background alteration

The lithologies in Unit I consist mainly of slightly altered gabbro and olivine gabbro. Samples exhibit metamorphism over greenschist to subgreenschist facies and show a broad range of alteration intensities, but because they are not in situ, patterns of alteration cannot be established. Primary mineral replacement ranges from 10% to 60%. Most of the secondary minerals are visible to the naked eye; some particularly fine grained minerals were identified only in thin section.

In the recovered gabbroic rock, pyroxene is slightly to moderately altered to pale green amphibole along rims and cleavage planes. Olivine is replaced by serpentine in seams parallel to the magmatic foliation but with little development of corona textures. Serpentine was subsequently altered to clay minerals in some grains. Orthopyroxene is altered to bright green chlorite-smectite aggregates. In most of the rocks, plagioclase is slightly altered to prehnite and secondary plagioclase. Plagioclase is also replaced by chlorite in rims surrounding relict olivine. Alteration within vein halos is more intense but with the same mineralogy as the dominant alteration.

Veins

Three of the five recovered pieces contain veins. Veins are mostly thin (<1–2 mm wide) and isolated and are dominated by subgreenschist facies minerals including clay minerals, quartz, actinolite, and minor prehnite. Vein shapes are regular and characterized by sharp contacts with the host rock (Fig. F5). Alteration of mafic minerals is more complete in vein halos (2–5 mm wide); within vein halos olivine is completely altered to serpentine and clay. Plagioclase is very fresh and does not appear more altered within vein halos. See also “Alteration veins.”

Metamorphic conditions

The alteration observed in Hole U1415G is variable, as would be expected in rocks from discontinuous intervals. The dominant alteration was at subgreenschist to greenschist facies conditions. The alteration predominantly affects pyroxene and olivine; primary plagioclase is largely unaltered except where in contact with relict olivine or in vein halos.

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