Radiogenic helium and argon in ultramafic inclusions from Hawaii

Isotopic studies have been made of the inert gases present in ultramafic xenoliths from two sites in Hawaii, the 1800–1801 Kaupulehu flow (Hualalai Volcano, Hawaii) and Salt Lake Crater (Oahu). Apparent ages calculated from the measurement of radiogenic argon and helium have very high values. The ratio of radiogenic helium to argon relative to natural gases and to the value expected from generation in situ is low. From gas extraction by crushing and decrepitation, it is shown that the inert gases occur in fluid and gaseous inclusions in the minerals of the nodules. The gases are believed to be representative of the magmatic environment, with the low helium-to-argon ratio resulting either from preferential loss of helium or from generation of the gas from a source similar to a chrondritic composition. The inert gas ‘ages’ then, are of uncertain significance for these samples.