Mineralogy, Chemistry, and Genesis of the Boninite Series Volcanics, Chichijima, Bonin Islands, Japan

The Bonin archipelago represents an uplifted fore-arc terrainwhich exposes the products of Eocene supra-subduction zone magmatism.Chichijima, at the centre of the chain, represents the typelocality for the high-Mg andesitic lava termed boninite. Therange of extrusives which constitute the boninite series volcanicsare present on Chichijima, and are disposed in the sequenceboninite-andesite-dacite with increasing height in the volcano-stratigraphy.Progression to evolved compositions within the Chichijima boniniteseries is controlled by crystal fractionation from a boniniteparental magma containing 15% MgO. Olivine and clinoenstatiteare the initial liquidus phases, but extraction of enstatiticorthopyroxene, followed by clinopyroxene and plagioclase, isresponsible for the general evolution from boninite, throughandesite, to dacite. Some andesites within the overlying MikazukiyamaFormation are petrographically distinct from the main boniniteseries in containing magnetite phenocrysts and a high proportionof plagioclase. As such, these andesites have affinities withthe calc-alkaline series. Major and trace element data for 74 boninitic series rocks fromChichijima are presented. Although major element variation isdominantly controlled by high-level crystal fractionation, thelarge variations in incompatiable trace element concentrationsat high MgO compositions cannot be explained by this mechanism.Nd, Pb, and Sr isotopic data indicate the following: (1) a strongoverprint on ⁸⁷Sr/⁸⁶Sr by seawater alteration; (2) Pb isotopeslie above the northern hemisphere reference line (NHRL) andare thus similar to the <30-Ma are and basin lavas of theIzu—Bonin system, and (3) _Nd(40 Ma) ranges between 2.8and 6.8 within the boninite series volcanics. Differences inrare-earth elements (REE), Zr, Ti, and ¹⁴³Nd/¹⁴⁴Nd at similardegrees of fractionation can be explained by the addition ofa component of fixed composition from the down-going oceaniccrustal slab to a variably depleted source region within theoverlying wedge. Data presented for Sm/Zr and Ti/Zr indicatethat boninite series volcanics are characterized by low valuesfor both of these ratios. In particular, boninites appear tohave uniquely low Sm/Zr ratios. These characteristics may bethe result of slab melting in the presence of residual amphibole;the resultant melt could combine with typical slab dehydrationfluids and infiltrate the overlying mantle wedge. Such a fluid—meltcomponent could mix either with shallow mantle or directly withprimitive melts from depleted mantle. Trace elements, REE, andisotope data thus point to a model for boninite genesis whichrequires tightly constrained pressure—temperature conditionsin the slab combined with melting of a variably depleted sourcein the overlying wedge. Such constraints are rarely met exceptduring the subduction of juvenile oceanic crust beneath a young,hot overriding plate.