Radiometric dating of young MORB using the 40Ar‐39Ar incremental heating method

Abstract

Determination of reliable crystallization ages by K‐Ar methods for young (<1 Ma), fresh basalts from the seafloor has been frustrated by several effects. The small amounts of radiogenic 40Ar developed over these timescales in such low‐K rocks are difficult to resolve from predominantly atmospheric 40Ar. An additional concern is that mantle‐derived 40Ar may not be totally outgassed when magmas quench at seafloor hydrostatic pressures. We have developed a successful strategy for partially separating atmospheric from radiogenic 40Ar in 40Ar‐39Ar incremental heating experiments on mid‐ocean ridge basalts, from the East Pacific Rise near the Clipperton Fracture Zone. Pre‐heating samples to 400°C removes surficial atmospheric 40Ar, while subsequent heating at 600°–1000°C produces favorable proportions of radiogenic 40Ar. Experiments using “zero‐age” glassy and holocrystalline samples show that mantle‐derived 40Ar is retained only in the outermost few cm of rapidly cooled pillow basalts.