Radiometric ages of Kodiak Seamount and Giacomini Guyot, Gulf of Alaska: Implications for circum-pacific tectonics

Kodiak Seamount and Giacomini Guyot have been dated at 22.6 ± 1.1 and 19.9 ± 1.0 [2σ (standard deviation)] x 10⁶ years, respectively. Concordant whole-rock and plagioclase potassium-argon dates and fission-track apatite ages demonstrate that significant quantities of excess radiogenic ⁴⁰Ar are not present in the dated samples. These seamounts are the northwesternmost edifices of the Pratt-Welker chain, which cuts obliquely across magnetic anomaly patterns in an older northeastern Pacific sea floor. The older of the two dated seamounts is in the Aleutian Trench, apparently about to be subducted. If one assumes that seamounts are generated by plate motion over a fixed hot spot in the mantle, a Pacific-plate motion of 6.6 centimeters per year during early Miocene time may be calculated.