K-Ar geochronology of Marion Island, Southern Ocean

Marion Island is a youthful intraplate shield volcano in the Southern Ocean , generally considered to represent the location of a mantle plume. New K-A r age determinations on 37 whole rock samples of so-called older grey basal t from the island range from 450 +/- 10 to similar to 30 ka. The lavas are remarkably fresh and holocrystalline, appearing to be ideal for dating. The age results from two almost vertical sections, where relative age relation ships are not in doubt, are in accord with the stratigraphy. A third sectio n gave aberrant ages that indicate the presence of inherited or excess radi ogenic argon. This is highly unusual for subaerial flows on an oceanic isla nd. We therefore urge caution in accepting K-Ar ages on samples without str atigraphic control. Volcanic activity on Marion could have been essentially continuous during the last half million years but on the basis of local un conformities and gaps between the measured ages it is considered more likel y to have been episodic. Eight periods of effusive activity at approximatel y 450, 350, 240, 170, 110, 85, 50 and < 10 ka ago can be distinguished with varying degrees of confidence. Glacigene deposits that are intercalated wi th the lavas can be correlated with glacial stages 2, 4, 6, 8 and 12 of the Pleistocene time scale based on the marine oxygen isotopic record. Indicat ions are that at least some of the earlier volcanic activity coincided with glacial stages but the two most recent episodes, including the Holocene on e extending up to the present, are clearly interglacial.