Distribution of thorium and uranium in three early Paleozoic plutonic series of New Hampshire

Abstract

Chemical uranium and thorium data were determined for the calc-alkaline Highlandcroft (Late Ordovician), Oliverian (Middle or Late Devonian(?)) and New Hampshire (Late Devonian(?)) Plutonic Series. Ten rock specimens were chemically and modally analyzed, and their constituent minerals analyzed chemically for uranium and thorium and spectrochemically for minor elements. All series show normal radioactivities, progressive increases in uranium and thorium with increased content of felsic minerals, and no marked trends in thorium to uranium ratios. Highlandcroft rocks show a range of 2.6-5.0 ppm (parts per million) uranium, and 10.6-14.8 ppm thorium; Oliverian rocks 0.6-15.9 ppm uranium and 2.9-57 ppm thorium; New Hampshire rocks 1.5-39 ppm uranium and 2.8-28.5 ppm thorium. Pegmatites have abnormally low thorium to uranium ratios (but abnormally high uranium content), and aplites have high but variable uranium and thorium contents. A linear relationship exists between K20 and thorium, but not between K 20 and uranium, supporting the inference that uranium may be relatively volatile. An inverse nonlinear (but irregular) relation also exists between CaO and uranium (or thorium). High uranium and thorium accompany high Si02 and K20 and low CaO. In rocks of the Highlandcroft and Oliverian Series, both of which are regionally metamorphosed, as much as 67 percent of the uranium and 79 percent of the thorium may be held by sphene and epidote. Both minerals are absent or rare in the higher temperature unmetamorphosed New Hampshire Series, in which monazite and xenotime are characteristic accessories, and in which most of the uranium and thorium are dispersed among the major silicates. Scavenging of dispersed actinides by sphene and epidote is evidently one of the major side effects of regional metamorphism. Despite these differences in the distribution patterns of uranium and thorium, statistical analysis of the chemical and spectrochemical data demonstrates close associations, within. the minerals of all the series, between uranium and thorium on the one hand, and rare earth or transition group metals on the otheK. These results, in turn, imply that there was no extensive addition or subtraction of uranium and thorium subsequent to the formation of the rocks. Enrichment of the plutonic residuum in elements rejected by early crystallized silicates appears to be the simplest and best explanation of the uranium and thorium distribution pattern. Fl F2 CONTRIBUTIONS TO GEOCHEMISTRY