Unspiked K–Ar dating of Koolau lavas, Hawaii: Evaluation of the influence of weathering/alteration on age determinations

Abstract

In order to evaluate the influence of weathering/alteration on K–Ar dating for Hawaiian tholeiites, unspiked K–Ar ages were determined for 21 samples from four lava flows with varying degrees of weathering or alteration collected from the Makapuu Head section of Koolau volcano, Hawaii. The samples were classified based on freshness of olivine phenocrysts and the groundmass olivine, and the presence of secondary minerals in vesicles. The results indicate that the ages for samples with fresh groundmass olivine are reliable, even though olivine phenocrysts may be slightly altered (thin reaction rims) or secondary minerals may have crystallized in the vesicles. The ages for the lowermost lava flow in the Makapuu Head section and the lava flow approximately 120 m above it are 2.58 ± 0.13 and 2.36 ± 0.09 Ma, respectively. The accumulation rate of this section is calculated to be 0.04–0.11 cm/year. We also report K–Ar ages for lava samples collected from the submarine flank of the Koolau volcano and the Nuuanu landslide blocks. The age for an early Makapuu-stage lava collected from the submarine flank of Koolau volcano is 2.5 Ma, similar to the age from the lower part of the subaerial Makapuu Head section. Another lava sample collected from the submarine flank of the Koolau volcano has an age of 3.3 Ma, older than any subaerial part of Koolau volcano. These results suggest that the onset of Koolau's shield-stage volcanism was no later than ~ 3.3 Ma, and the duration of the shield stage was at least 1.2 m.y.