Radioactivity of the rocks of the batholith of southern California

Determination of radioactivity has been made on 43 rocks, carefully selected from the different mapped units of the complex Cretaceous batholith of Southern California; they range from gabbro to granite. The activity of the gabbro averages about 0.3 alphas/mg./hr., that of the tonalites, 0.8, the granodiorite 1.3, and the granites over 2.0. The activity of the average rock of the batholith is about 0.9 alphas/mg./hr. Most of the rocks have been analyzed, and modal compositions are given. The radioactivity and the percentages of K (sub 2) O, SiO (sub 2) and PbO are plotted on a variation diagram. The variation curve for activity nearly parallels that for K (sub 2) O, and the ratio of activity to K (sub 2) O is about 0.5. The radioactivities of some rocks fall far from the variation curve, and degree of activity correlates with K (sub 2) O content. A less-marked correlation between activity SiO (sub 2) and PbO was also noted. The K (sub 2) O, SiO (sub 2) , PbO, and radioactivity concentrate in the residual liquid during crystallization and differentiation. PbO follows K (sub 2) O closely, as it is concentrated in the K (sub 2) O minerals, but radioactivity is very low in the K (sub 2) O minerals and is greatly concentrated in the accessory minerals.