IMMISCIBILITY IN NEOVOLCANIC FERROBASALTS IN THE AXIAL VALLEY OF THE SOUTHERN SEGMENT OF THE JUAN-DE-FUCA RIDGE

Neovolcanic ferrobasalts in the axial rift valley in the southern sect or of the Cleft segment of the Juan de Fuca Ridge are studied. It has been found that, besides drop-like sulfide impregnations in the quench ed crusts of vitreous and variolitic ferrobasalts, the inner parts of cover flows have structurally different magmatic-ore domains consistin g of aluminosilicate and sulfide-oxide fractions as well as drop-like impregnations, rims on large gas pores, taxite domains in ferrobasalts , microdikes, and cement in magmatic breccias. Such ore formations com prise 1-5 vol.% of all dragged ferrobasalts. By composition, the sulfi de-oxide fraction in vitreous aluminosilicate liquates differs from th e sulfide impregnations in the quenched flow domains mainly by a large amount of sphalerite and the presence of high-Zn magnetite and titano magnetite. The vitreous aluminosilicate fractions in the ore domains a re compositionally different: They are characterized by extremely high contents of alkalies. The development of the above-mentioned immiscib le structural forms in ferrobasalts and the revealed compositions of t he ''ore melt'' fractions are explained by a combination of two proces ses - subliquidus liquation of the melt in an intermediate chamber (su lfide globules in quenched glasses and variolitic barren basalts) and ''regeneration'' of ferrobasite liquid in a shallow magmatic chamber u nder the axial rift valley during its bubbling by a gas flow which sup plied chlorine, alkalies, silica, and zinc (high-Zn liquates enriched in Cl and alkalies).