Eruption age of a Pleistocene basalt from 40Ar−39Ar analysis of partially degassed xenoliths

The appendixes are available with the entire article on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W.,Washington, D.C. 20009. Document B83‐003; $2.50. Payment must accompany order. We have applied 40Ar−39Ar dating to potassium‐rich granitic xenoliths and host basalt from the Pleistocene Big Pine volcanic field, California. These xenoliths had been partially degassed upon their inclusion in the basaltic lava. Argon released from the xenoliths at extraction temperatures below ∼900°C yielded plateau ages indistinguishable from the total K‐Ar age of the basalt. The best estimate of the age of eruption was 1.18±0.05 (2σ) m.y. 40Ar extracted at higher temperatures included radiogenic argon not degassed from the late Cretaceous xenoliths 1.18 m.y. ago, causing an increase in the apparent age for the high‐temperature fractions. The agreement of the low‐temperature xenolith plateau ages and the basalt K‐Ar ages demonstrates that 40Ar−39Ar analysis of xenoliths may be used to measure the age of eruption of very young lavas. This is significant because in many instances ages cannot be reliably determined by analysis of the lavas themselves.