TEXTURAL ANALYSIS AND CRYSTAL SIZE DISTRIBUTIONS OF PLAGIOCLASE AND CLINOPYROXENE FROM ATLANTIS MASSIF, MID-ATLANTIC RIDGE

Textural analysis and crystal size distributions (CSD’s) were conducted on samples from Atlantis Massif, Mid-Atlantic Ridge. The analyses of plagioclase and clinopyroxene crystals were conducted on several gabbro, microgabbro, and diabase samples. Sample depths range from about 43 to 278 mbsf. Samples contain pervasive hydrothermal alteration, however, original crystal textures are well preserved. Gabbro samples range in texture from cumulate to subophitic and exhibit weak magmatic layering. Individual plagioclase and pyroxene grains are generally euhedral and equant in shape. Crystal sizes range from 0.5 mm in length to 8 mm in length, in microgabbro and gabbro samples, respectively. The diabase sample has an intergranular texture. Plagioclase laths, up to 2 mm in length, surround smaller, equant pyroxenes. Plagioclase laths have a 1:4 aspect ration, sub-anhedral faces, and embayments, indicating rapid crystallization. CSD’s of plagioclase of diabase are steep and linear; the convex-upwards shape of the diabase sample at the smallest crystal sizes is related to the difficulty in tracing and measuring these tiny crystals. We interpret this to mean that diabase textures are controlled by rapid nucleation and growth (10’s of years). Preliminary CSD’s of plagioclase and pyroxene of gabbroic samples are linear with a gentle slope and larger maximum grain size compared to the diabase. They completely lack small crystal sizes. We interpret a multi-stage crystallization history. Plagioclase and clinopyroxene crystals grew in a sill. Compaction forced liquid out of the crystal column. The crystals experienced textural coarsening, or Ostwald ripening, which caused the small crystals to dissolve, while providing continued growth of the large crystals.