KAr geochemistry of carnallite from salt-cycle six of the Paradox Formation in Utah, U.S.A.

Abstract

A technique was developed which yielded KAr apparent ages for whole-rock evaporite specimens and KAr apparent ages for carnallite which was too finely disseminated to be separated from the evaporite-mineral matrix. Camallite was selectively decomposed within the evaporite-mineral matrix by controlled heating. Water and radiogenic Ar released on carnallite decomposition were measured and used to calculate an apparent age. Whole-rock apparent ages, based on the total radiogenic Ar and on the K content, could be calculated following vaporization of the entire specimen. This technique was applied to selected specimens from three levels in salt-cycle six of the Pennsylvanian Paradox Formation. Apparent ages of dated evaporite specimens from salt-cycle six of the Paradox Formation in Utah range from 5 to 53 Ma. Most of the whole-rock apparent ages as well as the older half of the carnallite apparent ages range from 17 to 30 Ma. This apparent-age range overlaps with radiometric apparent ages of 17 to 31 Ma from rocks associated with igneous activity near the Colorado Plateau. This overlap suggests closure of some of the evaporites with respect to radiogenic Ar following a period of steepened geothermal gradient associated with regional igneous activity. The younger half of the carnallite apparent ages indicates that not all of the carnallite has been closed to radiogenic Ar since the thermal event. No age can be assigned to a thermal event affecting the evaporites of salt-cycle six based on data presented here. However, the data do suggest that KAr whole-rock apparent ages of carnallite-bearing evaporite specimens as well as apparent ages of carnallite can be sensitive indicators of periods of disturbance in an evaporite body.