Island-arc ankaramites: Primitive melts from fluxed refractory lherzolitic mantle

The distinctive island-arc ankaramites exemplified by the active Vanuatu arc may be produced by melting of refractory lherzolite under conditions in which melting is fluxed by H₂O + CO₂. Parental picritic ankaramite magmas with maximum CaO/Al₂O₃ to ≥1.5 are produced by melt segregation from residual chromite-bearing harzburgite at 1.5 GPa, ∼ 1320-1350°C. A pre-condition for derivation of such high CaO/Al₂O₃ melts from orthopyroxene-bearing sources/residues is that pyroxenes have low Al₂O₃ (< 3 wt %), high Cr₂O₃ (≥1 wt %), and spinel, if it occurs, has Cr number > 70. Bulk compositions have CaO/Al₂O₃ ≥1.3, i.e. much higher than chondritic values. The effects of both (CO₃)^2- and (OH)^- dissolved in the silicate melt combine with the refractory wedge composition to produce ankaramitic picrite magmas that segregate from residual harzburgite at pressures of spinel stability. Other primitive arc and back-arc magmas such as boninites (low Ca and high Ca) share the primitive signatures of island-arc ankaramites (liquidus olivine Mg number ≥90, spinels with Cr number > 70). Consideration of the relative proportions of Na₂O, CaO and Al₂O₃ in these primitive arc magmas leads to the inference of a common factor of refractory mantle fluxed by differing agents. H₂O-rich fluid alone carries these refractory major element characteristics into the primitive melts (high-CaO boninites, tholeiitic picrites). Fluxing with dolomitic carbonatite melt, which may develop from C-O-H-fluids within the mantle wedge, generates high CaO/Al₂O₃ sources and thus facilitates the formation of picritic ankaramites. Alternatively, melting may be fluxed by hydrous dacitic to rhyodacitic melt derived from the subducted slab (garnet amphibolite or eclogite melting). In this case, higher Na₂O/CaO, lower CaO/Al₂O₃ and higher SiO₂ contents characterize the low-CaO boninites.