Concentrazioni: in 238U, in 232Th, in 226Ra libero nel magma vesuviano

Imbò, Giuseppe

Atti della Accademia nazionale dei Lincei. Rendiconti della Classe di scienze fisiche, matematiche e naturali, Série 8, Tome 57 (1974) no. 5, pp. 404-417

Résumé

The escape at the moment of the eruption in Vesuvian lava flow of a Thorium volatile component has been deduced on the basis that the presence of Mesothorium I(228Ra) in lava flows is limited to only few years after the eruption. In the same way, the excess of 226Ra ($m_{Ra}$) in the specific mass relative to the 238U content, is a proof of the escape also of a volatile component of the Uranium. The presence of volatile components of the 238U and 232Th series in magmas was already called for in order to justify some peculiar behaviour in the magmatic evolution and in the eruptive dynamics. The experimental data consist of measurements of total gamma ray radioactivity and recently of gamma ray spectrometry of lava samples erupted from 1631 to 1944. The time-variation of a fraction of the 226Ra excess was considered as indicating a continuous decreasing of the 238U magmatic content. The above results suggest, on the basis of justified assumptions, the magmatic contents of 238U, 232Th, free 236Ra, in equilibrium with the other components of their families. The obtained values, relative to 1944, are: \begin{equation*}m_{U} = 3.28 \times 10^{-5} gr/gr \quad,\quad m_{Th} = 4.90 \times 10^{-5} gr/gr,\end{equation*}\begin{equation*}\frac{m_{Th}}{m_{U}} = 1.50 \quad,\quad m_{Ra} = 1.47 \times 10^{-12} gr/gr.\end{equation*}