Eruption Age of a ~100,000-Year-Old Basalt From ^(40)Ar-^(39)Ar Analysis of Partially Degassed Xenoliths

Abstract

We have applied ^(40)Ar-^(39)Ar dating, using stepwise thermal extraction of Ar, to five potassium-rich granitic xenoliths and their host basalt from Sawmill Canyon in the Sierra Nevada, California. Previous K/Ar analyses showed the age of the basalt to be roughly 100,000 years or less. The xenoliths, which had accumulated large amounts of radiogenic ^(40)Ar since their crystallization ∼100 m.y. ago, were partially degassed upon their inclusion in the basaltic magma. Ar released from the xenoliths in the laboratory at temperatures substantially below the melting temperature of the basalt, was created since the host magma cooled. Isotopic compositions of Ar released from the xenoliths in several extraction steps at temperatures below ∼900°C were colinear in ^(36)Ar/^(40)Ar versus ^(39)Ar/^(40)Ar diagrams and defined isochrons giving a mean age of degassing of 119,000±7000 (2σ) years. ^(40)Ar extracted at higher temperatures included ancient radiogenic ^(40)Ar that was never diffused from the xenoliths during immersion in the magma. This ^(40)Ar caused an increase in the apparent age for the high-temperature extractions. The high precision of the eruption age determined by this method is comparable to that obtained elsewhere by conventional K/Ar dating of sanidine. ^(40)Ar-^(39)Ar analysis of granitic xenoliths to date young basaltic lava flows may prove to yield results superior to those found from analysis of the lava itself. Establishing the age of eruption of the basalt flow in Sawmill Canyon establishes age limits for two Sierran glaciations which left moraines stratigraphically above and below the lava. Thus the younger glaciation must be Wisconsin; the older must be pre-Wisconsin in age.