The Marquesas archipelago is a short. NW-SE trending cluster of islands and seamounts that formed as a result of volcanic activity over a weak hotspot. This volcanic chain lies at the northern margin of a broad region of warm and compositionally diverse mantle that melts to build several other subparallel...
We have measured ³He/⁴He ratios and He and Ne concentrations on a suite of 24 basalt glasses from the neovolcanic zone of the Juan de Fuca Ridge (JdFR) from 44.6°N near the Blanco Transform up to 48.0°N on the Endeavour Segment. The helium isotope ratios exhibit a clear geographic variation,...
Within the altered parts of the glass rim of pillow lavas of Hole 896A, at depths at least 432 m below seafloor (237 m below the top of volcanic basement), microbes have been identified. This is indicated by the size and shapes of alteration textures and verified by the presence...
Determination of reliable crystallization ages by
K-Ar methods for young (<1 Ma), fresh basalts from the
seafloor has been frustrated by several effects. The small
amounts of radiogenic ⁴⁰Ar developed over these timescales in
such low-K rocks are difficult to resolve from predominantly
atmospheric ⁴⁰Ar. An additional concern is that...
The majority of the Earth's shallow crust is composed of basalt that erupted on the seafloor and was subsequently altered by chemical exchange with seawater. One aspect of this alteration is the replacement of glass by secondary minerals, including clays. Petrographic thin sections from ODP Holes 504B and 896A revealed...
Within the Australian-Antarctic Discordance (AAD), a boundary exists between isotopically defined “Pacific-type” and “Indian-type” mid-ocean ridge basalt (MORB) erupted along the Southeast Indian Ridge (SEIR). This boundary has migrated westward beneath the easternmost AAD spreading segment at a minimum rate of 25 mm/yr since 4 Ma; however, its long-term history...
The basement of the Tonga intraoceanic forearc comprises Eocene arc volcanic crust formed during the earliest phases of subduction. Volcanic rocks recovered from the forearc include boninites and arc tholeiites, apparently erupted into and upon older mid-oceanic ridge tholeiites. Rock assemblages suggest that the forearc basement is a likely analog...