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 experimentalstudy of a high-Ca boninite suite from the northern terminationof the Tonga trench. Most samples are strongly olivine porphyriticand show a significant range of phenocryst compositions includinga very refractory olivine-spinel assemblage Fo₉₄–Cr_N =87. They are also characterized by a wide range of incompatible-elementcontents, e.g., (La/Yb)_N varies from 0.5 to 16, whereas compatiblemajor-element concentrations (Al₂O₃, FeO, CaO, SiO₂, and MgO)remain essentially the same. Primary melt compositions for thesuite were established on the basis of an experimental studyof melt inclusions in phenocrysts and numerical modelling ofthe reverse of fractional crystallization. Tongan primary meltsare characterized by high MgO contents (22–24 wt.%) andoriginated in the mantle wedge at pressures of 20–25 kbarand temperatures of 1450–1550 C. H₂O contents in primarymelts were estimated from direct measurements of melt inclusionsby ion probe, and range from 2.0 to 1.0 wt.%, and a strong correlationexists between H₂O and other incompatible element contents.The primary melts crystallized in the presence of an H₂O-richfluid in the temperature range 1390–1150 C and pressuresof 1.7–0.15 kbar. Continuous degassing of melts took placeduring crystallization. Trace-element concentrations in primarymelts were estimated using proton- and ion-probe analyses ofmelt inclusions in olivine, and whole-rock analyses. Our datasuggest that three independent components (D, E1, and E2) wereinvolved. Component D was a refractory mantle depleted in incompatibleelements, likely to be hot ‘dry’ Iherzolite producedby previous melting within a mantle plume. Component E1 wasan H₂O-rich fluid containing LILE and Th, and had an H₂O/K₂Ovalue of 20; it was probably produced by dehydration of thesubducted slab. Component E2 is thought to have been an incompatible-element-enrichedsilicate melt of plume origin. Formation of high-Ca boninitesrequires interaction of hot ‘dry’ residual mantle,associated with plumes, with a subduction-related H₂O-bearingcomponent.