Heat-producing radionuclides in metamorphic rocks of the Brianconnais-Piedmont Zone (Maritime Alps)

A wide set of radioactive heat-production data of gneisses, migmatites, amp hibolites, and micaschists is presented for the eastern part of the Brianco nnais-Piedmont Zone (NW Italy). They were obtained from determination of co ncentrations of uranium, thorium and potassium by means of ground gamma -ra y spectrometry. We selected as a reference site for the local background ev aluation an area with serpentinitic outcrops showing negligible radioactivi ty, and verified the possible variation of the background radiation due to changes in elevation. This allowed an assessment of more realistic radioele ment concentrations. Migmatites show the largest concentrations of U (6.1 p pm), whereas Th is more abundant in orthogneisses (17.8 ppm) as well as K ( 4.62%). Although rocks were affected by metamorphic and alteration processe s, the Th/U and K/U ratios showed differences in radioelement concentration s that mostly reflect the chemical composition of protolith. In particular, in gneisess the Th/U ratio decreases exponentially with the uranium conten t. Due to the low content in heat-producing radioelements, amphibolites sho w an average heat-production rate of 0.30 muW m(-3). The other lithotypes h ave comparable U, Th and K contents, and therefore the range of their heat production is rather narrow, varying from 2.29 (paragneiss) to 2.99 muW m(- 3) (migmatite).