Constraints on the Origin of Nepheline-Normative Primitive Magmas in Island Arcs Inferred from Olivine-hosted Melt Inclusion Compositions


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Abstract

Here we address the question of the origin of rare nepheline-normative arc magmas through a systematic study of major, minor and trace elements in primitive olivine-hosted melt inclusions, together with literature data. The host-rocks of the inclusions are Mg-rich basalts to ankaramite lavas and scoria from several intra-oceanic volcanic arcs (Vanuatu, Lesser Antilles, Indonesian, Luzon and Aeolian arcs). The studied melt inclusions display trace element patterns typical of subduction-related calc-alkaline basalts, with variable enrichments in large ion lithophile elements and Sc (20–91 ppm), and La/Yb and Nb/Y ratios ranging from 1 to 18 and from 0·1 to 0·3, respectively. In CMAS projections, the melt inclusions delineate a trend linking two well-defined end-members, which are strongly and weakly enriched in the diopside component, respectively. The melt inclusions provide snapshots of the compositions of instantaneous melts, recording compositional diversity in the primitive magma batches, which requires mixing between melts generated by partial melting of peridotite and amphibole-bearing clinopyroxene-rich lithologies, possibly at a pressure of ∼1 GPa and between 1200 and 1300°C. This hypothesis is supported by trace element modeling, and particularly by the correlation of Sc with incompatible element ratios. We propose that amphibole-bearing clinopyroxenites, occurring as cumulates at the base of the crust and/or as metasomatic veins in the upper mantle of island arcs, represent a suitable source for the Ne-normative melt inclusions variably enriched in CaO, observed in arc environments.

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