RADIOMETRIC DATING OF YOUNG MORB USING THE 4039ARAR 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 Ar-40 developed over these timescales in such low-K rocks are difficult to resolve from predominantly atmospheric Ar-40. An additional concern is that mantle-derived Ar-40 may not be totally outgassed when magmas quench at seafloor hydrostatic pressures. We have developed a successful strategy for partially separating atmospheric from radiogenic Ar-40 in Ar-40-Ar-39 incremental heating experiments on mid-ocean ridge basalts, from the East Pacific Rise near the Clipperton Fracture Zone. Pre-heating samples to 400-degrees-C removes surficial atmospheric Ar-40, while subsequent heating at 600-degrees-1000-degrees-C produces favorable proportions of radiogenic Ar-40. Experiments using ''zero-age'' glassy and holocrystalline samples show that mantle-derived Ar-40 is retained only in the outermost few cm of rapidly cooled pillow basalts.