Petrology and geochemistry of boninite series volcanic rocks from the Mariana trench

Abstract

Boninite series volcanic rocks have been recovered from three dredge hauls on the inner slope of the Mariana Trench. These hauls included olivine boninites, boninites, boninitic andesites and boninitic dacites, as well as island arc tholeiitic basalts and andesites. The boninite series volcanics range from 52 to 68% SiO₂, and are characterized by very low abundances of high-field-strength cations and heavy-rare-earth elements. Boninites and olivine boninites have phenocrysts of olivine and orthopyroxene, the andesites phenocrysts of orthopyroxene and clinopyroxene, and the dacites orthopyroxene, clinopyroxene, and plagioclase. Most of the major and trace element variation in the series from boninite to boninitic dacite can be modelled by fractionation of olivine, orthopyroxene, clinopyroxene, and plagioclase in the proportions 2.5∶4∶1∶2, leaving 47% residual liquid. The fractionation must be in part open-system: reverse zoned phenocrysts, resorbed olivine and plagioclase xenocrysts, and bulk rock compositions which cannot be fit by simple closed system crystallization indicate some magma mixing and phenocryst accumulation. Two boninitic magma stems can be identified, with similar high-field-strength element abundances, but different amounts of Ca, Na, Al and light-rare-earth elements. There is also evidence for a magma stem transitional in chemistry from the boninites to arc tholeiites. The compositions of these boninites are consistent with hypotheses for boninite formation by partial melting of a depleted mantle mixed with an incompatible element enriched fluid. The Mariana forearc boninite series lacks a strong iron enrichment, but produces andesites with lower Ti, Al and Y/Zr, and higher Mg, Ni and Cr than typical calcalkaline arc andesites and dacites. Boninites in the Mariana system were erupted only in the earliest phases of subduction zone activity.