This project had two major goals.  The first was to do a synthesis of survey data and samples collected from a non-volcanic sunken ocean island in the Indian Ocean that exposes the lower ocean crust directly to the seafloor.  The crest of this seamount is a flat-topped wave cut platform that exposes the lower ocean crust where it can be drilled and studied directly as tectonic faulting at the ocean ridge where it originally formed removed the upper ocean crust made of brittle hard lavas that are dificult to drill through, and uplifted it above seasurface to form an ancient ocean island.  This island subsequently sunk beneath the waves to its current depth of less than half a mile - very shallow for a piece of seafloor formed at an ocean ridge.  Thus, the sunken island provides the best opportunity to explore the lower ocean crust, and to drill through it through the Moho - the first major seismic boundary in the earth.  This has been a long-held dream of the Earth science community as the nature of this boundary up till now can only be inferred from seismic studies, and might be the boundary between the Earth's crust and the mantle, or it could be an alteration front in the mantle.  Before drilling, however, it was necessary to make a geologic map of the region to be drilled to evaluate it's suitability for this endeavor, and to identify the best location for drilling.  This project succesfully brought together the bathymetric and geophysical data, submarine and remotely operated vehicle observations, and the composition and physial condition of the rock samples collected to accomplish these goals.  As a consequence the International Ocean Discovery Program drillship JOIDES Resolutiion occupied a site on the bank and started drilling the hole down to Moho, reaching a depth of a half mile.  The project will continue with future drilling legs until it reaches some 500-m below Moho.

Several new results came out of this project, one of which was the first careful study of the dike-gabbro transition.  Much of the ocean crust is layered.  The deepest layer consisting of the lower crust made of a rock know as gabbro - the fossil remains of magma chambers.  The gabbros crystallized directly from magmas rising out of the mantle, which the lower crust overlies.  Above the gabbro layer are sheeted dikes - a continuous layer that forms by solidification of magma left in fissures as the earth as it periodically cracks above the magma chamber in the lower ocean crust at an ocean ridge.  The uppermost layer is made of lavas that erupted out of the fissures onto the seafllor.  The dike-gabbro transition represents the contact or transition from the fossil magma chamber to the layer of dikes.  We found that unlike fast spreading ridges, where the magma chamber continuously erodes the dikes, here at crust formed at a slow spreading ridge, the dikes cut through the gabbros.  What this means is that at fast spreading ridges, like the East Pacific Rise, the lower crust  consists of a layer beneath the ridge axis that is nearly always partially molten, whereas at a slower spreading ridge the gabbros crystallize deeper than the dike-gabbro transition, and are then pulled up as the tectonic plates spread apart until they reach the shallow zone of dike formation.  This, in part, explains why slow spreading ocean ridges have deep rift valleys, typically a mile deep, while fast spreading ridges have axial ridges with only a very small narrow rift valley generally around 50-100 feet deep. 

The project has an important societal component in that the lower ocean crust and mantle are potentially a major resevoir for carbon sequestration from the oceans as sea water percolates down into it and crystallizes carbonate minerals (e.g.: calcite) by reaction of CO2 in seawater with calcium released from the rocks.  How much and how important this process might be, and hence its role in cotrolling ocean and atmospheric carbon depends on the nature of the rocks with which the oceans interact - and that is what this project seeks to understand.  Another component of potential societal impact is the search for economic mineral deposits on the seafloor.  Regions like Atlantis Bank, are know to have often quite large deposits of massive sulfide cntaining significant amounts of zinc, copper, gold and silver.  Where the best candidates may be found depends directly on understanding the composition of the rocks in the lower ocean crust and how it varies.  Our work is demonstrating that the composition of the ocean crust is highly varied, and hence the economic potential, and is essential to understanding where to look for the best candidates for eventual seafloor mining as the appropriate technology is developed.

Last Modified: 01/14/2017
Modified by: Henry J Dick