GLASS-BEARING XENOLITHS FROM CAPE-VERDE - EVIDENCE FOR A HOT RISING MANTLE JET

Peridotitic xenoliths from melanephelinites of Sal Island, Cape Verde Archipelago, have a compositional range from moderately depleted Iherz olites to refractory harzburgites. Most xenoliths have protogranular t extures but porphyroclastic and mylonitic textures are not uncommon. S mall amounts of glass are present in the intergranular space of these rocks which possibly, at least in part, represent quenched silicate me lt which invaded these rocks just before they were excavated. These gl asses contain microphenocrysts of olivine, clinopyroxene, and spinel, as well as small grains of sulphides and metallic Fe-Ni alloys. Metall ic phases were most likely produced by the desulphurization of sulfide s, which also resulted in very low oxygen fugacities (several logarith mic units below QFM buffer) in the interstitial glasses and associated microphenocrysts. This is reflected in the chemical composition of th e newly formed spinels which are characterised by low amounts of ferri c iron. In contrast, primary spinel-bearing mineral assemblages of the peridotites were formed at much higher f(O2), which were similar to t hose estimated for the host nephelinites which have high titanomagneti te contents.