On the Uniformity of Composition in Obsidians and Evidence for Magmatic Mixing

Abstract

Neutron activation measurements on several Northern California obsidian sources show chemical compositions ranging from extremely homogeneous to extremely variable. The uniformity of a source in Napa County, California was as good as the precision of the measurements even when measuring errors were less than 1%. In contrast, the flow at Borax Lake (Lake County, California) revealed variable composition of the obsidian. However, the elements showed a dramatic linear coherence which also extended through the compositions of an underlying dacite layer. The faithful relationship of the many elements suggests that mag-matic mixing occurred prior to erruption.