A two-component mantle source feeding Mt. Etna magmatism; insights from the geochemistry of primitive magmas.

The major elements, trace elements and Sr and Nd isotopes of selected Etnean primitive rocks (b15 ky BP) werestudied in order to characterize their mantle source. The noble-gas geochemistry of fluid inclusions in mineralsfromthe same lavaswas also investigated. Themajor element compositions ofwhole rocks and minerals showedthat these products are among the most primitive atMt. Etna, comprising 6.3–17.5 wt.% MgO. The variable LREE(Light Rare Earth Elements) enrichment relative to MORB (Mid-Ocean Ridge Basalt) (Lan/Ybn = 11–26), togetherwiththe patterns of certain trace-element ratios (i.e., Ce/Yb versus Zr/Nb and Th/Y versus La/Yb), can be attributedto varying degrees of melting of a common mantle source. Numerical simulations performed with theMELTS program allowed the melting percentages associated with each product to be estimated. This led us to recalculatethe hypothetical parental trace-element content of the Etneanmantle source, whichwas common to allof the investigated rocks. The characteristics of the Sr, Nd and He isotopes confirmed the primitive nature of therocks,with themost-depleted and primitive lava being that ofMt. Spagnolo (SPA; 143Nd/144Nd = 0.512908 87Sr/86Sr = 0.703317–0.703325 and 3He/4He = 7.6 Ra), and highlighted the similarity of the mantle sources feedingthe volcanic activity of Mt. Etna and the Hyblean Plateau (a region to the south of Mt. Etna and characterized byoldermagmatismthan Mt. Etna). The coupling of noble gases and trace elements suggests an origin for the investigatedEtnean lavas from melting of a Hyblean-like mantle, consisting of a two-component source where aperidotitic matrix is veined by 10% pyroxenite. A variable degree of mantle contamination by crustal-like fluids,probably related to subduction, is proposed to explain the higher Sr-isotope and lowerNd-isotope values in somerocks (143Nd/144Nd up to 0.512865 and 87Sr/86Sr up to 0.703707). This process probably occurred in the sourceprior tomagma generation, refertilizing some portions of themantle. Accordingly, the estimated degree of meltingresponsible for each magma appears to be related to its 87Sr/86Sr enrichment. In contrast, the decouplingbetween 3He/4He and 87Sr/86Sr ratios requires the occurrence in the crustal reservoirs of further processes capableof shifting the He isotope ratio towards slightly more radiogenic values, such as magma aging or a contributionof shallow fluid. Therefore, different residence times in the Etnean reservoir and/or various rates of magmaascent could be key parameters for preserving the original He isotope marker of the Etnean mantle source.© 2013 Published by Elsevier B