The geological implications of the basalts and sediments of the Lucky Strike segment

The first chapter focuses on the basalts of the Lucky Strike segment on the Mid-Atlantic Ridge, near the Azores. This is an area of known hydrothermal activity with water column studies, sampling of hydrothermal sulfides and biota by dredging, ROV studies, and Alvin dives.; The variations in composition of the basaltic rocks within the segment are described in the first chapter and a striking cluster of relatively enriched samples is found at the bathymetric high at the segment center. The preferred model described involves mixing melts of two different materials: a regional mantle melt and a low degree melt of a more enriched mantle material. Enriched samples at bathymetric highs are found within other segments near the Azores and have also been found at segments away from the influence of hotspots, indicating that the processes leading to these central enrichments are pervasive under the world's ocean ridges.; The second chapter concentrates on the sediment from the Lucky Strike segment and other segments from 33-41°N Mid-Atlantic Ridge. The main focus of this chapter is the description of the sediments' composition compared to the frequency the hydrothermal vents are found along the MAR.; The third chapter examines ridge crest sediments from around the world. The input rate of hydrothermal material into the sediment is determined and compared to regional spreading rate. The area with the fastest accumulation rate of hydrothermal material is from 13-20°S East Pacific Rise (EPR). This area coincides with large helium and thermal anomalies in the water column that extend westward from the ridge. Also in this region, high concentrations of dissolved Mn are present in the water column above the ridge crest, and a blanket of metal-rich sediments covers the region. All of these are expressions of the high temperature hydrothermal activity in this region.; In previous studies hydrothermal output has been related to spreading rate. With the regions of fastest spreading would then have the most intense evidence of hydrothermal output, but the most intense expressions of hydrothermal activity are found at 13-20°S exclusively which is not the world's fastest spreading ridge. There are many complicating issues concerning the hydrothermal mass accumulation rate observations including the possible redistribution of sediment by bottom currents and the difficulty in sampling the fastest spreading ridge from 20-30°S EPR. Despite the complications there is a confluence of evidence including the hydrothermal mass accumulation rates, thermal and He anomalies, Mn in water column, inflated ridge crest indicating that the greatest output of hydrothermal material is taking place at 13-20°, and a simple relationship between spreading rate and hydrothermal output does not exist. (Abstract shortened by UMI.)...