A New Single Vacuum Furnace Design for Cosmogenic ³He Dating

Abstract

Helium‐3 is a stable cosmogenic isotope that can be used to determine the time interval during which a rock sample has been at or close to the Earth’s surface. As a result of the high production rate of ‘cosmogenic’ 3 He (≈ 130 at g −1 year −1 ) and the low detection limit of modern mass spectrometers, it is possible to date exceptionally young surfaces (≈ 1000 years). The precision and accuracy of cosmogenic 3 He measurements depend critically on the passive helium blank (produced by the metalwork of the extraction furnace) which can be significant relative to the sample signals. We have developed and constructed, at the CRPG (Nancy, France), a new high temperature furnace (< 1500 °C) to extract helium in minerals such as apatite, pyroxene and olivine at 1050, 1350 and 1450 °C, respectively. The furnace demonstrated an excellent helium extraction yield (> 99% for olivine and pyroxene for heating times of 20–30 min and temperatures in the range 1050–1450 °C) and low residual helium contributions (the blank, obtained under the same analytical conditions as for sample extraction: 1 × 10 −15 mole 4 He and < 4 × 10 −21 mole 3 He). This is approximately an order of magnitude lower than those reported by other laboratories using conventional furnaces.