Radiometric Dating of Young MORB Using the ⁴⁰Ar³⁹Ar Incremental Heating Method

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 4øAr developed over these timescales in such 10w-K rocks are difficult to resolve from predominantly atmospheric 4øAr. An additional concern is that mantle-derived 4øAr may not be totally outgassed when magmas quench at seafloor hydrostatic pressures. We have developed a successful strategy for partially separating atmospheric from radiogenic 4øAr in 4øAr-39Ar incremental heating experiments on midocean ridge basalts, from the East Pacific Rise near the Clipperton Fracture Zone. Pre-heating samples to 400øC removes urficial atmospheric 4øAr, while subsequent heating at 600ø-1000øC produces favorable proportions of radiogenic 4øAr. Experiments u ing "zero-age" glassy and holocrystalline samples how that mantle-derived 4øAr is retained only in the outermost few cm of rapidly cooled pillow basalts.