Compositional variation and 226Ra-230Th model ages of axial lavas from the southern Mid-Atlantic Ridge, 8°48S

We present geological observations and geochemical data for the youngest volcanic features on the slow spreading Mid-Atlantic Ridge at 8 degrees 48S that shows seismic evidence for a thickened crust and excess magma formation. Young lava flows with high sonar reflectivity cover about 14 km(2) in the axial rift and were probably erupted from two axial volcanic ridges each of about 3 km in length. Three different lava units occur along an about 11 km long portion of the ridge, and lavas from the northern axial volcanic ridge differ from those of the southern axial volcanic ridge and surrounding lava flows. Basalts from the axial rift flanks and from a pillow mound within the young flows are more incompatible element depleted than those from the young volcanic field. Lavas from this volcanic area have Ra-226-Th-230 disequilibria model ages of 1000 and 4000 years whereas the older lavas from the rift flank and the pillow mound, but also some of the lava field, are older than 8000 years. Glasses from the northern and southern ends of the southern lava unit indicate up to 100 degrees C cooler magma temperatures than in the center and increased assimilation of hydrothermally altered material. The compositional heterogeneity on a scale of 3 km suggests small magma batches rising vertically from the mantle to the surface without significant lateral flow and mixing. The observations on the 8 degrees 48S lava field support the model of low-frequency eruptions from single ascending magma batches that has been developed for slow spreading ridges.