Volcanism and metallogenesis of axial and off-axial structures on the East Pacific Rise near 13 degrees N

Abstract

A fast-spreading segment (12 cm/yr (super -1) ) of the East Pacific Rise near 12 degrees 50' N, explored in detail by surface ship and manned submersible, displays intense hydrothermal activity. The rise crest (about 1,500 m in width) with a regional depth of 2,600 m is occupied at its center by an axial graben ( 1.3%); titania-alkali-enriched olivine (Ti-K) basalts with high TiO 2 (>1.4%) and high alkali content (Na 2 O > 3.0%; K 2 O > 0.3%) occur on the southeastern seamount. The fractionated tholeiites are believed to have been derived from primitive melts during differentiation which took place within a magma reservoir--asymmetrical with respect to the axis of the graben--which supplied both axial and off-axial lava flows. The Ti-K-enriched basalts are believed to have been produced from an alkali-enriched magmatic source.