The U-series ages of carnotites and implications regarding their formation

Abstract

The U-series nuclides, 230Th, 234U, 238U, 227Ac, 231Pa and 210Pb, were analyzed in up to 75 carnotites from a group of trenches in southwestern Israel. Though 230Th234U had the large range of 0.14 to 0.99, its spatial distribution was not random and the ratio never exceeded unity. 231Pa230Th was at least 91% concordant with 230Th234U in four of five cases. These facts, together with the details of the 230Th234U distribution, especially among ten samples from one small block of rock, indicate that 230Th234U ages of carnotites are at least a reasonable approximation of true ages. The unexpectedly widespread deficiencies in both 227Ac and 210Pb indicate that although carnotites are nearly closed to the migration of uranium, they are open to those of actinium and to lead, radon or radium. A model of carnotite precipitation is presented which attributes the large range in 230Th234U to a continual shift in the site of carnotite precipitation, in keeping with a continual change in the geometry of joints. This model may apply to other minerals as well.