9811143 Cathles Cornell University Title: 3D modeling of hydrothermal venting at fast-spreading mid ocean ridges The remarkable uniformity in the spacing of mid-ocean ridge vents and volcanogenic massive sulfide deposits (thought to have formed at ancient ridges) has long been recognized but remains unexplained. Also not understood is the reason for the size distribution of deposits formed at the same time in ~40 km diameter massive sulfide "camps". Furthermore, in two well-defined cases we have studied, the spacing of both vents and deposits is about 1/3 that expected from the depth of burial of the sill-shaped heat source. However, numerical models we have constructed show that this discharge spacing can be explained if it is controlled by the thickness at which the thermal boundary above the sill- shaped intrusion first becomes convectively unstable. In this case, the spacing of hydrothermal vents (and massive sulfide deposits) provides a direct measure of crustal permeability, an important geological parameter. Our numerical modeling also shows that forced convection at sill edges (corresponding perhaps to deval offsets of mid-ocean ridges) can be an order of magnitude stronger than the free convection which occurs over the sill interior. This may be the fundamental explanation for the size distribution so commonly observed in massive sulfide deposits. These phenomena and others will be investigated by 2D and 3D numerical computer modeling under funding from the NSF. It is expected that the work will advance understanding of both convection at mid-ocean ridges and the spacing and size distribution of massive sulfide deposits, and simultaneously benefit both science and the exploration industry.
