Ages of Hawaiian Volcanoes — Funkhouser et al. 
371 
The basaltic dike (HK-119) cutting through 
the rhyodacite (HK-121) yields a high age 
relative to its host formation. The low air 
argon contamination in this specimen precludes 
any convenient overlap of ages within experi- 
mental error. The single determination on the 
feldspar (HK-143) from the flow overlying 
the rhyodacite should also be less than 2.3 ± 
0.4 my. The feldspar phenocrysts were hand- 
picked from a weathered specimen and treated 
with hydrofluoric acid to remove the more 
easily soluble altered layers. Microscopic exam- 
ination of these indicated pure plagioclase 
crystals with no alteration or contamination 
evident. The high atmospheric argon correction 
creates a large potential uncertainty in the 4.3 
my date, and it is possible that an even greater 
error than acknowledged exists due to altera- 
tion not revealed by examination. It is also 
possible that these samples (HK-119 and HK- 
143) contain an "excess” of radiogenic argon. 
Damon, Laughlin, and Percious (1967) noted 
0.56 X 10- 7 std. cc/g excess argon in large 
plagioclase phenocrysts from a very young 
basalt, and 2.3 X 10“ 7 std. cc/g excess argon 
from the chilled border of a young basaltic dike. 
It is believed that the discordance in age shown 
between whole-rock sample B-7 and its constitu- 
ent mica, B-8, is an example of mica giving an 
anomalously older age due to excess radiogenic 
argon present in inclusions, as in the samples 
from the Mauna Kuwale rhyodacite noted 
above. 
HK-144, a sample from a core taken from 
the 1081 -foot level of the Ewa I drill hole at 
Barber’s Point, Oahu, is listed as a Lower 
Waianae sample, although H. T. Stearns has 
stated, on topographic grounds, that it might 
come from a flow from Koolau Volcano, the 
other major volcano of the island of Oahu. 
The variability in the degree of air argon 
contamination among the different samples is 
worth noting. Bake-out temperatures and time 
cannot be correlated with the quantity of air 
argon in each sample. A relatively large amount 
of air argon was present in the apparatus during 
the initial gas extractions but any corrections 
for this would lower the atmospheric correction 
of HK-121 (whole rock) to a minimum of 
approximately 80%. In general, certain miner- 
als have been found to hold inherently less air 
than others. This phenomenon apparently holds 
true for different whole-rock samples as well, 
and is probably related to the degree of micro- 
vesiculation of the basalts. The differences be- 
tween feldspar separates and whole-rock 
samples is mostly attributable to the residual 
air argon in the extraction system which would 
predominate when smaller total quantities of 
radiogenic argon are released. 
The two specimens in which feldspar and 
whole-rock ages can be compared indicate a 
possible loss of radiogenic argon from the 
plagioclase, yet Evernden and James (1964), 
and Livingston, Damon, Manger, Bennett, and 
Laughlin (1965) have reported that volcanic 
feldspars are quite retentive, especially for 
young rocks. It is most probable that during 
the 23-28 hours of bake-out at 210°-240°C 
some of the radiogenic argon was lost by dif- 
fusion, or that the lower ages are coincidental 
and result from experimental error. 
Nihoa (Bird) Island is approximately 150 
miles west-northwest of Kauai while Necker 
Island is about 180 miles beyond Nihoa. The 
ages of 7.5 my and 11.3 my, respectively, for 
these island remnants are not surprising con- 
sidering the progression of age of the islands 
(McDougall, 1964). 
The paucity of data and the uncertainty in 
the age determinations preclude any generaliza- 
tions regarding the duration of island growth or 
volcanism except to note that volcanic activity 
of the Waianae Volcano appears to have 
spanned well over two million years. 
REFERENCES 
Damon, P. E., A. W. Laughlin, and J. K. 
Percious. 1967. The problem of excess 
argon-40 in volcanic rocks. Symposium on 
Radioactive Dating and Methods of Low- 
Level Counting, IAEA, Monaco. Paper SM- 
87/45. 
Evernden, J. F., and G. H. Curtis. 1965. 
The potassium-argon dating of late Cenozoic 
rocks in East Africa and Italy. Current 
Anthropology 6:343-385. 
Evernden, J. F., and G. T. James. 1964. 
Potassium-argon dates and the Tertiary floras 
of North America. Am. J. Sci. 262:945-974. 
