K–Ar analyses of the post-caldera lavas of Bratan volcano in Bali Island, Indonesia — Ar isotope mass fractionation to light isotope enrichment

The post-caldera lavas of Bratan volcano in Bali Island, Indonesia were collected for whole rock chemical analyses and K–Ar analyses. Major and trace element chemistry shows that the lavas are basalts to andesites and typical of subduction-related tectonic setting. The 38Ar/36Ar ratios are 0.1851 ± 3–0.1875 ± 2 and the 40Ar/36Ar, 294.3 ± 0.3–301.6 ± 0.1, which strongly suggest that the mass fractionation to light isotope enrichment took place. The effect of the groundwater on magma is common on the basis of systematic mass fractionation of the atmospheric Ar enriched in lighter isotopes. This case was under the mass fractionation law analyzed numerically, giving the mass fractionation correction ages (14 ± 15, 31 ± 6, 55 ± 22, 66 ± 23, 94 ± 32 and 125 ± 51 ka) consistent with the volcano stratigraphy though the magma composition that changed frequently in time.