GROUND RADIOMETRIC SURVEY OF U, TH AND K ON THE LIPARI ISLAND, ITALY

The island of Lipari consists of four distinct volcanic episodes from the earliest two are calc,alkalic basaltic to andesitic flows and the latter two are high-K alkalic rhyolitic flows and related pyroclastic rocks. The distribution of radioactive elements which are important to heat-flux studies, i.e. concerning uranium (eU), thorium (eTh) and po tassium (K), was determined using in situ gamma-ray spectrometric meas urements from key locations throughout the island. Uranium is spatiall y related to both thorium and potassium. The K (eU/eTh) ratio equals 0 .6-1.7, while the Th/U ratio is on average 3.2. The magmatic evolution is reflected clearly by the concentrations of the three radioelements , as they are more abundant within the more felsic units of the volcan ic series. Both in the northeastern and in the southernmost part of Li pari, the higher values of eU (on average 16-19 ppm) coincide with hig her eTh and K concentrations associated with rhyolitic volcanic rocks. Earlier andesitic to basaltic volcanic rocks comprise much of the rem ainder of the island and have a lower concentration of these elements (3-5 ppm eU and 10-15 ppm eTh). Potassium is more abundant in felsic r ocks, but generally has low concentrations, ranging on average from 1. 9 to 5.3%. The pattern of some petrophysical properties resembles the variation in spatial distribution of the radioelements observed on the island. (C) 1998 Elsevier Science B.V.