A chemical study of vitrophyric rocks from Boavista Island (Cape Verde) using synchrotron radiation X-ray fluorescence

Volcanic glasses with phonolitic composition from Boavista Island (Cape Verde Archipelago) display lower silica and higher water contents, plus a calculated composition indicating an excess of normative albite, when compared to vitreous rocks with similar composition reported for other Atlantic islands. A complete chemical study of vitrophyric rocks from lava flows and dikes focused on the distribution of trace heavy metals and on the sulphur content was undertaken using synchrotron-induced X-ray fluorescence (SRXRF). Comparatively high contents in Rb, Sr, Y, Zr, Nb were measured, along with a normal and quite homogeneous distribution pattern for the rare earths. Simultaneously, glass forming metals (particularly Zn) and heavy metals (like Mo, Pb, Th, U) are uniformly present. SRXRF results are discussed taking into account geological and mineralogical data and brief prospective comments on sulphur speciation are presented.