Sr–Nd–Pb–O isotopic evidence for decreasing crustal contamination with ongoing magma evolution at Alicudi volcano (Aeolian arc, Italy): implications for style of magma-crust interaction and for mantle source compositions

Abstract

New isotopic (Sr–Nd–Pb–O) data are reported for a suite of basalts (MgO up to 10 wt.%) to andesites and associated mineral separates from the Island of Alicudi, Aeolian arc, Southern Tyrrhenian Sea, with the aim of investigating the effects of magma-wall rock interaction in continental magmatic systems. Major and trace elements at Alicudi exhibit smooth variations, suggesting derivation of the entire suite from a single type of parental magma that underwent fractional crystallisation. However, 87Sr/ 86Sr and δ 18O values decrease, while 143Nd/ 144Nd increases with increasing SiO 2 and decreasing MgO. Moreover, phenocryst phases show isotopic disequilibrium relative to whole rocks and groundmass. Such an unusual isotope vs. major and trace element variation and isotopic disequilibria suggest interaction between magmas and crustal wall rocks, but the decrease of 87Sr/ 86Sr with increasing differentiation excludes assimilation-fractional crystallisation processes. We suggest that mafic calcalkaline magmas with primitive Sr–Nd–O isotopic signatures were intruded into a deep reservoir, where they underwent fractional crystallisation. Contamination occurred shortly before eruption, as the magmas ascended to the surface. Interactions between magmas and wall rocks were more extensive for the basalts than basaltic andesites and andesites, generating a positive correlation between MgO and 87Sr/ 86Sr ratios. The variable degrees of magma-wall rock interaction are suggested to depend on the differing capabilities of magmas to dissolve crustal rocks due to variable temperature and viscosity. The present study, therefore, provides Sr–Nd–Pb–O isotopic evidence on an uncommon modality of magma contamination by upper crust. The Alicudi andesites have the most primitive isotope signatures over the entire Aeolian arc. In plots of 206Pb/ 204Pb vs. 87Sr/ 86Sr or 143Nd/ 144Nd isotopes, the bulk of the Aeolian volcanoes fall along a curved trend that connects HIMU mantle reservoir and the upper crust. In contrast, the Alicudi andesites fall outside this trend and plot on a binary trend connecting HIMU and EM1 mantle reservoirs. It is suggested that the Alicudi source consisted of a hybrid HIMU-EM1 mantle that was modified by fluids or melts coming from an oceanic slab. These fluids did not change significantly the isotopic signatures of the wedge but generated high LILE/HFSE ratios, which were inherited by the Alicudi magma. In contrast, the compositions of other Aeolian volcanoes and of central Italian magmatism reflect addition of upper crust to a HIMU or FOZO mantle. Collectively, the data record the complex interplay between different mantle reservoirs and the addition of various amounts and types of subduction-related components in the southern Tyrrhenian Sea.