Ocean Drilling Program, United States

Leg 209 of the Ocean Drilling Program will be devoted to coring mantle peridotite along the Mid-Atlantic Ridge (MAR) from 14 degrees to 16 degrees N. This area was identified at the 1996 Workshop on Oceanic Lithosphere and Scientific Drilling into the 21st Century as the ideal region for drilling of a strike line of short holes to sample the upper mantle in a magma-starved portion of a slow-spreading ridge. In this area, igneous crust is locally absent and the structure and composition of the mantle can be determined at sites over approximately 100 km along strike. A central paradigm of Ridge Interdisciplinary Global Experiments (RIDGE) program studies is the hypothesis that mantle flow, or melt extraction, or both, are focused in three dimensions toward the centers of magmatic ridge segments, at least at slow-spreading ridges such as the MAR. This hypothesis has essentially reached the status of accepted theory, but it has never been subject to a direct test. A strike line of oriented mantle peridotite samples extending for a significant distance within such magmatic segments offers the possibility of directly testing this hypothesis. Continued dredging and submersible studies cannot provide the spatial information required to make such a test. The primary aim of drilling is to characterize the spatial variation of mantle deformation patterns, residual peridotite composition, melt migration features, and hydrothermal alteration along axis. Hypotheses for focused solid or liquid upwelling beneath ridge segments make specific predictions regarding the spatial variation of mantle lineation or the distribution of melt migration features. These predictions may be directly tested by drilling.