Island-arc Ankaramites: Primitive Melts from Fluxed Refractory Lherzolitic Mantle


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Abstract

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.

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