40Ar/39Ar dating of olivine hosted melt inclusions

Abstract

Olivine is generally considered to incorporate extraneous argon and is therefore routinely removed from basalt prior to ⁴⁰Ar/³⁹Ar dating. However, the mechanisms by which K-free olivine hosts and retains this argon remain unexplained. In this study, individual olivine grains were analyzed by the laser total-fusion ⁴⁰Ar/³⁹Ar dating method to investigate the source of extraneous argon within olivine. The results indicate that >92% of the grains showed no detectable argon signal, demonstrating that the majority of olivine grains are free of extraneous argon. Of the 282 grains analyzed, 21 yielded detectable argon signals, and 11 of these provided viable ⁴⁰Ar/³⁹Ar ages ranging from 105.6 ± 32.7 Ma to 162.6 ± 11.5 Ma (2σ). Within this subset, 6 grains exhibited relatively high ⁴⁰Ar contents and yielded meaningful ⁴⁰Ar/³⁹Ar ages with analytical uncertainties better than 10%, which are consistent within error (160.2 ± 3.2 Ma, 2σ). Major element analysis indicates that potassium (K) in olivine is predominantly hosted within K-rich melt inclusions, and the proportion of olivine grains containing such inclusions is ∼8.6%. This percentage is comparable to the proportion of grains with detectable argon signals (∼7.5%), suggesting that K-rich melt inclusions represent the primary source of extraneous argon in the olivine grains. In addition, the consistent ⁴⁰Ar/³⁹Ar ages obtained from these K-rich melt inclusions demonstrate that some melt inclusions in olivine remain as closed systems after capture, and that the extraneous argon they contain is inherited argon with geological significance, rather than excess argon of ambiguous origin.