The petrogenesis of ocean-floor basalts : an experimental and geochemical study

Parts I and II are an experimental study concerned with the current controversy regarding the evolved versus primary nature of primitive MORB glasses. In part I equilibrium partial melt compositions under anhydrous conditions are presented on four peridotite compositions at 10kbar. The four peridotite compositions are: 'MORB pyrolite', 'Hawaiian pyrolite', Tinaquillo lherzolite and the spinel lherzolite KLB-1. In part II equilibrium partial melt compositions are presented for MORB pyrolite from 8 to 35kbars and for Tinaquillo lherzolite at 15 to 20kbars. The equilibrium liquids were determined by 'sandwich' experiments. The results of the experiments are used to test a 10kbar melt model for the generation of primitive MORB glasses. The melt compositions from the four peridotites at 10kbar are significantly different from primitive MORB glasses in major element chemistry and plot away from the field of primitive MORB glasses in the CIPW molecular normative 'Basalt tetrahedron'. The results suggest primary MORB melts segregate from source diapirs at pressures of 8 to 25kbars in equilibrium with either lherzolite or harzburgite residues. MgO contents of primary MORB melts range from 10-17 wt% while primary melts >17 wt% MgO are of minor importance. Parts III, IV and V consist of a petrographic, mineral chemical, geochemical and petrological study of dredged lavas from the North Tongan forearc recovered during the 1984 cruise of the research vessel 'Natsushima'. In part III petrographic, wholerock major and trace element data is presented and the geochemical affinities of all the dredge suites discussed. Part IV looks in detail at the chemistry of large (up to 0.2mm) glass inclusions contained in unusually magnesian olivine phenocrysts (Fo94 ) within the high-mg lavas. Part V is concerned with the petrogenesis of the high-mg lavas and associated island arc tholeiites from North Tonga. The mineral chemistry of phenocryst and groundmass phases combined with wholerock, glass and glass inclusion major element chemistry give unequivocal evidence of mixing of two or more distinct parental magmas. The major, trace and isotopic (Sr, Nd) geochemistry of the lavas can be explained by partial melting of refractory mantle peridotite (depleted after extraction of primary MORB picrite) at shallow depths (