Ultra-calcic magmas generated from Ca-depleted mantle: An experimental study on the origin of ankaramites

Ultra-calcic ankaramitic magmas or melt inclusions are ubiquitous in arc, ocean-island and mid-ocean ridge settings. They are primitive in character (X_Mg > 0.65) and have high CaO contents (>14 wt %) and CaO/Al₂O₃ (>1·1). Experiments on an ankaramite from Epi, Vanuatu arc, demonstrate that its liquidus surface has only clinopyroxene at pressures of 15 and 20 kbar, with X_CO2 in the volatile component from 0 to 0·86. The parental Epi ankaramite is thus not an unfractionated magma. However, forcing the ankaramite experimentally into saturation with olivine, orthopyroxene and spinel results in more magnesian, ultra-calcic melts with CaO/Al₂O₃ of 1·21-1·58. The experimental melts are not extremely Ca-rich but high in CaO/Al₂O₃ and in MgO (up to 18.5 wt %), and would evolve to high-CaO melts through olivine fractionation. Fractionation models show that the Epi parent magma can be derived from such ultra-calcic experimental melts through mainly olivine fractionation. We show that the experimental ultra-calcic melts could form through low-degree melting of somewhat refractory mantle. The latter would have been depleted by previous melt extraction, which increases the CaO/Al₂O₃ in the residue as long as some clinopyroxene remains residual. This finding corrects the common assumption that ultra-calcic magmas must come from a Ca-rich pyroxenite-type source. The temperatures necessary for the generation of ultracalcic magmas are ≥1330°C, and their presence would suggest melting regimes that are at the upper temperature end of previous interpretations made on the basis of picritic magmas.