PETROLOGY AND GEOCHEMISTRY OF BONINITES FROM THE NORTH TERMINATION OF THE TONGA TRENCH - CONSTRAINTS ON THE GENERATION CONDITIONS OF PRIMARY HIGH-CA BONINITE MAGMAS

We report here a detailed mineralogical, geochemical, and experimental study of a high-Ca boninite suite from the northern termination of the Tonga trench. Most samples are strongly olivine porphyritic and show a significant range of phenocryst compositions including a very refractory olivine-spinel assemblage Fo(94)-Cr-N=87. They are also characterized by a wide range of incompatible-element contents, e.g., (La/Yb)(N) varies from 0.5 to 16, whereas compatible major-element concentrations (Al2O3, FeO, CaO, SiO2, and MgO) remain essentially the same. Primary melt compositions for the suite were established on the basis of an experimental study of melt inclusions in phenocrysts and numerical modelling of the reverse of fractional crystallization. Tongan primary melts are characterized by high MgO contents (22-24 wt.%) and originated in the mantle wedge at pressures of 20-25 kbar and temperatures of 1450-1550 degrees C. H2O contents in primary melts were estimated from direct measurements of melt inclusions by ion probe, and range from 2.0 to 1.0 wt.%, and a strong correlation exists between H2O and other incompatible element contents. The primary melts crystallized in the presence of an H2O-rich fluid in the temperature range 1390-1150 degrees C and pressures of 1.7-0.15 kbar. Continuous degassing of melts took place during crystallization. Trace-element concentrations in primary melts were estimated using proton- and ion-probe analyses of melt inclusions in olivine, and whole-rock analyses. Our data suggest that three independent components (D, E1, and E2) were involved. Component D was a refractory mantle depleted in incompatible elements, likely to be hot 'dry' lherzolite produced by previous melting within a mantle plume. Component E1 was an H2O-rich fluid containing LILE and Th, and had an H2O/K2O value of 20; it was probably produced by dehydration of the subducted slab. Component E2 is thought to have been an incompatible-element-enriched silicate melt of plume origin. Formation of high-Ca boninites requires interaction of hot 'dry' residual mantle, associated with plumes, with a subduction-related H2O-bearing component.