APPLICATION OF ELEMENT CONCENTRATION VARIABILITY TO THE STUDY OF BASALT ALTERATION (FANGATAUFA ATOLL, FRENCH-POLYNESIA)

Lavas sampled in a drill hole at Fangataufa atoll, French Polynesia, have been studied to investigate trace-element behaviour during alteration processes. While most of these elements are not affected by alteration, U, Cs, Rb and Ba show various degrees of mobility due to alteration phenomena. Mobility is different for the different lithological units: hyalotuffs, hyaloclastites, submarine lava flows, and basaltic dykes and sills. Gain or loss of trace elements are studied using the properties of both variability, V (V = sigma/m, where m is the mean concentration of an element and sigma its standard deviation) and correlation coefficient between elements. The results allow a distinction to be made between open-system alteration, with chemical pins or losses, and closed-system alteration, in which dissolution-precipitation mechanisms keep concentrations unchanged: Cs is removed from subaerial basaltic intrusions. In submarine lava flows, alteration leads to U and Ba losses. In hyaloclastites, we notice Cs gain, U and Ba losses, and Rb redistribution. In hyalotuffs, alteration leads to U and Cs gains, and Ba loss.