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Potassium argon dating principles techniques and applications to geochronology

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The 1.1-billion-year Rb-Sr isochron date for the Cardenas Basalt is widely regarded by geologists as the best “age” yet obtained for Grand Canyon strata.12 That “age” agrees with what most geologists have come to believe about other Precambrian strata and the pervasive mafic intrusive and extrusive magmatism of the Middle Proterozoic of the southwestern United States.35 Although we might suppose that radioisotopes could definitely date Cardenas Basalt, a significant problem has arisen.Published potassium-argon (K-Ar) “ages” for Cardenas Basalt and Precambrian diabase are significantly younger than their associated rubidium-strontium (Rb-Sr) “ages.”6 Why are K-Ar dates significantly younger than the “accepted” Rb-Sr age?Before alteration, this mediumgrained basalt was similar in texture and mineralogy to the sills and dikes, while petrologic and chemical data suggest to some that this lower member is a spilitic hyaloclastite which is the altered effusive equivalent of those mafic intrusives.10 However, the spheroidal masses may simply have resulted from onion-skin weathering rather than being suggestive of pillow structures, and therefore, the field evidence could indicate subaerial extrusion of these lower member basalts in a series of low-viscosity pahoehoe flows.

Beds of siltstone and sandstone 1.5 to 3 m thick occur between some of the flows.

The dramatic change to the resistant, finer-grained flows of the upper member coincides with the abrupt change in magma chemistry.

Individual flows vary in thickness from about 20 to 50 m, each flow being separated from adjacent flows by laterally persistent siltstone and sandstone beds which generally range in thickness from 0.3 to 3 m.

The sills are not found in direct association with the Cardenas Basalt, but are confined to the lower part of the Unkar Group, particularly near the boundary between the Bass Formation and the Hakatai Shale, while the related dikes are intruded above the sills along faults that predate or are contemporaneous with the sills.

These mafic sills crop out in seven locations along a 70–80 km length of the Grand Canyon (fig. They are composed chiefly of medium-grained ophitic diabase.

This paper was originally published in the Proceedings of the Fourth International Conference on Creationism, pp.

35-51 (1998) and is reproduced here with the permission of the Creation Science Fellowship of Pittsburgh. 1) is a succession of lava flows over 300-m-thick occurring deep within a thick succession of strata in the eastern Grand Canyon, Arizona.

For more than twenty years it has been known that the Rb-Sr and K-Ar systems give discordant “ages” for Cardenas Basalt and associated Proterozoic diabase sills and dikes of Grand Canyon.

Thirteen new K-Ar analyses of Proterozoic mafic rocks of Grand Canyon are added to nine published K-Ar analyses.

The dikes have a similar composition but are finer grained, as are the chilled margins of the sills.

Early differentiation and crystal settling in the sills is evidenced by granophyre layers up to 10-m thick and felsite dikes, and by olivinerich layers.