Uranium content and fission track ages of some basalts from the FAMOUS area

The uranium contents of basalts from the rift valley in the Atlantic ocean near 37 deg N range between 75 ppb and 450 ppb. The fission track ages of these basalts range between 3x103 years and 6x105 years. They increase with distance from the axis of the median valley. Therefore, the locus of accretion of new crust seems to be restricted to a relatively narrow zone along the valley floor. In addition, the ages indicate that during the last 105 years the rate of accretion has been slower to the west than to the east. This indicates a migration of the active spreading center to the west by at least 1 km. the sea-floor spreading rates are high, up to 9cm/year, near the center of actual magmatic activity. They decrease with distance from the valley axis to 0.7 cm/year at about 2 km in the west respectively 1.5 cm/year at about 4 km in the east