Petrology and geochemistry of a CESAR bedrock sample: implications for the origin of the Alpine Ridge.

The sample is a highly altered fragmental volcanic rock, which was rimmed with a crust of manganese oxide, dredged from the scarp of the major graben of the Ridge. Clasts form approx 90% of the rock, are up to 1 cm, and are subrounded to angular. The rock is heterolithic, comprising 92% aphyric, 5% clinopyroxene-phyric, and 3% plagioclase-microphyric clasts. Plagioclase microlites display skeletal form. Clasts commonly contain 50-60% vesicles up to 4 mm in size and are spherical to irregular in shape. Some vesicles may be relict spherulites. The skeletal form of plagioclase microlites, lack of abundant relict crystals, and possible relict spherulites suggest volcanic fragments were glassy to very fine grained. The combined textural evidence (quench textures, high vesicularity, fragmental nature and small clast size) suggest that the volcanic fragments were erupted in shallow water during a phreatomagmatic or Plinian-type eruption. Rare clinopyroxene phenocrysts comprise the only unaltered portion of the rock. They have the compositional range Wo_51-53En_32-37Fs_12-16, with significant Ca, Al and Ti. Clinopyroxene chemistry suggests a within-plate environment.-T.F.