Abstract

Spectrochemical determinations of rubidium and potassium in common igneous rock types (granite, diabase, basalt, gabbro and ultramafics) and in chondrites are given. These determinations show that the ratio K/Rb remains virtually unchanged when preceding from rock type to rock type, and in crustal igneous rocks %K/%Rb is 90 on the average. In chondrites this ratio remains remarkably uniform (av. 100) and to within an assumed tolerance of ±20% is equal to that in terrestrial igneous rocks. As neither K nor Rb enters metal and sulphide phases to a significant extent, it is concluded that to within ±20% the ratio K/Rb in the earth as a whole is equal to that in meteoritic matter as a whole: it is believed that this is the first recorded example of such a relationship. The abundance of potassium in chondrites is relatively uniform and appears to be considerably less than that usually given in the literature. The present determination shows that for the ultramafic rocks (mainly dunite and serpentine) the estimated abundance of potassium has to be drastically reduced. As a result, heat production by K ⁴⁰ in a dunite mantle may be a small fraction, perhaps only 1/30, of that contributed by the U + Th series. The abundance of Rb is 0-035% in crustal igneous rocks and 0-0009% in chondrites.