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Metamorphic petrology

Background alteration

The lithologies recovered in Hole U1415H consist mainly of relatively unaltered olivine gabbro and olivine gabbronorite. Samples exhibit metamorphism over a wide range of grades and intensities, but because they are from a rubble unit, no patterns of alteration can be established. Primary mineral replacement ranges from 10% to >90%; the typical range is 10%–30%. Pyroxene is slightly to moderately altered to pale green amphibole, and olivine is strongly replaced by serpentine with mesh textures. Corona textures are developed in some of the pieces. Clinopyroxene and orthopyroxene are altered to poorly crystalline green amphibole. In most of the lithologies, plagioclase is slightly altered (typically <10%) to prehnite and secondary plagioclase. Plagioclase is replaced by chlorite where it contacts relict olivine along microfractures. A number of the pieces from this core appear to be from cataclastic zones (see “Structural geology”), with intense fracturing of minerals and enhanced alteration of plagioclase to prehnite and zeolites. Cataclastic cements include zeolites, prehnite, and clay minerals; this assemblage appears to replace fine-grained cataclastic material in some cases (Fig. F6). Zeolites identified by X-ray diffraction include thomsonite and chabazite (Table T2).


Veins are composed of several mineralogies, including monomineralic pale green amphibole, prehnite, and clay minerals, as well as amphibole-prehnite, amphibole-clay minerals, and amphibole-secondary plagioclase. Pieces in this core typically have one or two thin (<1–2 mm wide), isolated veins. Vein shapes range from regular to anastomosing single veins to relatively dense networks of thin veins (e.g., Sample 345-U1415H-1R-1 [Piece 7]). Halos are 1–3 mm thick and are characterized by more complete alteration of mafic minerals, particularly olivine to serpentine, and enhanced alteration of plagioclase to chlorite.

Metamorphic conditions

The alteration observed in these samples is variable, as would be expected in rocks from discontinuous intervals. The dominant alteration of all recovered rocks is greenschist to zeolite facies. The alteration predominantly affects pyroxene and olivine, except in cataclastic zones where prehnite, secondary plagioclase, and zeolite pervasively replace plagioclase.