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Images: Melt Extraction from the Mantle Beneath Mid-Ocean Ridges

Photographs for studies of a Washington Cascades ophiolite were taken from BOLO (Blimp for On Land Oceanography) tethered at a height of about seven meters

Diagram of rising mantle, convection, and melt region.

Red lines show known ophiolites, slices of oceanic crust that have been exposed on land by tectonic forces. Ophiolites allow scientists to study the three-dimensional structure of the oceanic crust and shallow mantle directly rather than by remote means beneath the seafloor. (After W. P. Irwin and R. G. Coleman, U.S. Geological Survey, 1974.)

Figure 1. Schematic illustration of a porous dissolution channel network coalescing downstream, based on theoretical work of MIT/WHOI graduate student Einat Aharonov and others.

Figure 2. Results of numerical modeling of the formation of porous dissolution channels in the mantle by Lamont-Doherty Earth Observatory scientist Marc Spiegelman and others.

Figure 3. Plots illustrating the relationship between the number of channels and flux in coalescing flow networks. Plot A is based on the schematic network in Figure 1. Plot B illustrates results of numerical modeling with points used in fitting the curve marked in red. Plot C is based on the work of Theodore Wilson (University of Minnesota), who showed that coalescing human bronchial tubes conserve cross-sectional area as well as flux. This is the optimally efficient geometry for respiration.

Figure 4. Maps made using BOLO, the Blimp for On Land Oceanography. The color illustration is an example of a single photograph of an area about 3 meters across, and the grayscale image is a map of an outcrop about 30 meters across, created from a mosaic of about 50 photographs. Note that, though the dunites channels appear small here, they are actually several meters wide.

Figure 5. Plot showing results for all the dunites mapped in the Washington Cascades ophiolite in terms of the number of dunites with a particular width. As for coalescing channel networks, the dunites show a power law relationship with a negative slope between their width and their number. Points used in fitting the curve are shown in red. The blue lines illustrate that the observed slope can be explained as the result of a network of interconnected, porous dunite channels, with porosity varying from a maximum of 5 percent in the largest channels to a minimum of about 0.2 percent in the smallest ones.

Figure 6. At left, a schematic cross section of the mantle section of the Oman ophiolite illustrates the relationships between residual mantle peridotites (gray), dunites (red), and dikes (black). Light lines show the trend of deformation features ("foliation") in peridotite, which are sub-parallel to the paleo-seafloor. At right, constraints from the mantle sections of ophiolites, and theoretical studies of reactive porous flow of melt through partially soluble rocks, can be used to infer that melt extraction beneath mid-ocean ridges occurs in a coalescing network of porous dunite channels.