Mount Etna lies near the boundary between two regions that exhibit significantly different types of volcanism. To the north, volcanism in the Aeolian island arc is thought to be related to subduction of the Ionian lithosphere. On Sicily itself, however, no chemical or seismological evidence of subduction-related volcanism exists, and so it is thought that the volcanism-including that on Mount Etna itself-stems from the upwelling of mantle material, associated with various surface tectonic processes. But the paucity of geological evidence regarding the primary composition of magma from Mount Etna means that its source characteristics remain controversial. Here we characterize the trace-element composition of a series of lavas emitted by Mount Etna over the past 500 kyr and preserved as melt inclusions inside olivine phenocrysts. We show that the compositional change in primary magmas from Mount Etna reflects a progressive transition from a predominantly mantle-plume source to one with a greater contribution from island-arc (subduction-related) basalts. We suggest that this is associated with southward migration of the Ionian slab, which is becoming juxtaposed with a mantle plume beneath Sicily. This implies that the volcanism of Mount Etna has become more calc-alkaline, and hence more explosive, during its evolution.

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Melt inclusions in olivine and apatite, and REE distribution of apatite were studied in one of the least differentiated members of the oldest alkaline succession of Mt. Etna. Apatite occurs both as microphenocrysts and as inclusions in olivine crystals, even in the most Mg-rich ones (Fo 82 ). In addition phenocrysts and groundmass are composed of plagioclase, clinopyroxene, olivine and magnetite.Apatite is¯uor-apatite, with rather homogeneous major element (measured by electron microprobe, EMP) and REE (measured by laser-ablation microprobe, LAM, and by secondary ion mass spectrometer, SIMS) contents. REE are enriched when compared to the whole rock, with contents in olivine-hosted apatite lower than or the same as those of the microphenocryst cores; these in turn show lower REE values than their edges. Distribution coef®cients, calculated from LAM data of microphenocryst edges and whole rock analyses, are higher for LREE (8±12) than for HREE (5±4).In the SiO 2 vs. P 2 O 5 diagram melt inclusions and whole rock samples de®ne a trend that is consistent with continuous apatite extraction from a``high P'' basalt magma. Finally, whole rock data show LREE/HREE (La/Lu) n enrichment ratios from hawaiites to mugearites ( 1.14), consistent with apatite fractionation, lower than those documented for lavas of the``low P'' type (enrichment ratio 1.34±1.37), where conditions for apatite saturation were not established.

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