par Merlin, Oplinia Hieke;Picciotto, Edgardo ;Wilgain, S.
Référence Geochimica et cosmochimica acta, 11, 3, page (171-188)
Publication Publié, 1957-03
Article révisé par les pairs
Résumé : The granodiorite from Adamello (Italy) shows a distribution of the α-radioactivity similar to that found in granitic rocks. This distribution is characterized by its heterogeneity. More than 95% of the activity is concentrated in accessories which amount to 0.1% of the volume of the rock. The most active accessories are 100,000 times more active than the essential minerals. In some cases, more than 90% of the α-radioactivity, apparently connected to the essential minerals is in fact due to microscopic inclusions of accessories. The apparent enrichment in uranium and thorium of the ferro-magnesian minerals, pointed out by several authors, is due to the abundance of radioactive inclusions in those minerals and not to the substitution by uranium and thorium of certain elements of their crystalline lattice. Very radioactive inclusions have been found. Their uranium and thorium content exceeds 10%. Their exact identification is not possible, but there are good reasons to believe that the most active are uraninite and torbernite. Allanite, the most active among the usual accessories, emits 0·47Tα/cm2/sec. corresponding to an uranium content of 0·16%. Following order of decreasing activity, then come: titanite, zircon, epidote, apatite. The "real activity" of the essential minerals, quartz, feldspar, biotite, is of the order of 3. 10-3 α/cm2/sec which would correspond to an uranium concentration of the order of 10-7 gr/gr. In contradiction to what has been observed in other granitic rocks, Adamello's granodiorite does not show any concentration of radioactivity into boundaries between crystals. Relationships between the intensity of the pleoehroic haloes and the activity of the inclusions-which produce them are discussed. © 1957.