Geochemistry of Tungsten in Basalts and Andesites

Abstract

<p>Few methods exist to analyse tungsten at the ppm level in rocks. Consequently, the amount of data is very scarce and often conclusions are drawn from a very small number of samples. In this study, a NAA radiochemical separation method was used to obtain information on tungsten contents in some 125 volcanic rocks. Besides the major oxide data, which were in most cases obtained from the donors of the samples, additional information was gathered with XRF methods on the following trace elements: Rb, Sr, Ni, Y, Zr, Nb, Zn, Pb, U and Th. Factor analysis was applied in order to process this vast array of newly obtained data. Knowledge of the local geology is necessary if not essential for the interpretation of these data.</p> <p>The wide variety of rocks involved in this study comprises predominantly basalts (alkaline and subalkaline) and andesites, but also includes some felsic differentiates as well as some ultramafic rocks. An attempt, although not always successful, was made to obtain as fresh samples as possible. These rocks are derived from differing tectonic areas - (1) ocean floors and islands, (2) island arcs and continental margins and (3) continental areas.</p> <p>Tungsten is an incompatible element accumulating in residual liquids together with elements such as K, Rb, Zr, etc. Existing tungsten estimates for both basalts and andesites are too high. Tungsten contents for basalts and andesites vary according to their petrotectonic environment. Among these rocks, ocean floor basalts have the lowest values followed by island arc basalts and andesites and ocean island basalts. Tungsten contents generally are higher in alkaline basalts and their range is wider than for subalkaline basalts. No differences seem to exist between subalkaline ocean island and continental basalts.</p>