Most boninite
magma is formed by second stage melting in
forearcs via
hydration of previously depleted
mantle within the mantle wedge above a
subducted slab, causing further melting of the already depleted
peridotite. A forearc environment is ideal for boninite genesis, but other tectonic environments, such as
backarcs, might be able to form boninite. The content of titanium (an incompatible element within melting of peridotite) is extremely low because previous melting events had removed most of the incompatible elements from the residual mantle source. The first stage melting typically forms island arc
basalt. The second melting event is partly made possible by hydrous fluids being added to the shallow hot depleted mantle, leading the enrichment in large ion lithophile elements in the boninite. Boninite attains its high magnesium and very low titanium content via high degrees of partial melting within the
convecting mantle wedge. The high degrees of partial melting are caused by the high water content of the mantle. With the addition of slab-derived volatiles, and incompatible elements derived from the release of low-volume partial melts of the subducted slab, the depleted mantle in the mantle wedge undergoes melting. Evidence for variable enrichment or depletion of incompatible elements suggests that boninites are derived from refractory peridotite which has been metasomatically enriched in
LREE, strontium, barium, and
alkalis. Enrichment in Ba, Sr and alkalis may result from a component derived from subducted oceanic crust. This is envisaged as contamination from the underlying subducted slab, either as a sedimentary source or as melts derived from the dehydrating slab. Boninites can be derived from the peridotite residue of earlier arc tholeiite generation which is metasomatically enriched in LREE before boninite
volcanism, or arc tholeiites and boninites can be derived from a variably depleted peridotite source which has been variably metasomatised in LREE. Areas of fertile peridotite would yield tholeiites, and refractory areas would yield boninites. == Examples ==