『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.
Keywords: K-Ar dating;
Weathering; Alteration; Basalt; Koolau volcano; Hawaii』
1. Introduction
2. Geological setting and analytical samples
2.1. Koolau volcano
2.2. Makapuu head section
2.3. Submarine samples
3. Analytical procedures
3.1. Classification for weathering/alteration of the lava
samples
3.2. Whole rock major-element analysis
3.3. K-Ar dating
4. Results and discussion
4.1. Evaluation of the influence of weathering/alteration
on K-Ar dating
4.2. Accumulation rate of lava in the Makapuu head section
4.3. Unspiked K-Ar ages for submarine Koolau lavas
5. Summary
Acknowledgments
Appendix I. Photographs of the thin sections
Appendix II. Unspiked K-Ar dating
References