Deciphering Degassing and Source Effects in Cl Isotopes in Melt Inclusions: The Possible Role of Amphibole in the Magma Source of Stromboli (Aeolian Island Arc)

co-auteur étrangerInternational audienceChlorine isotopes have emerged as a new geochemical tool over the past 15 years. Most of the data consist of bulk rock data, with a minority carried out in situ on melt inclusions using secondary ion mass spectrometry. More data are necessary to understand the relationship between δ 37 Cl measured in melt inclusions and that in bulk rocks from the same volcanic center. Here we have analyzed a suite of melt inclusions entrapped in olivine Fo 63-85 , as well as some from clinopyroxene crystals, from a single hand-sample from the Vancori unit of Stromboli, Aeolian Islands. The 27 selected melt inclusions have major element compositions ranging from high potassium alkali basalt to evolved shoshonite. Their δ 37 Cl vary from −2.6 ± 0.1‰ to +1.2 ± 0.2‰, a far larger range than for Stromboli bulk rocks. In this dataset, the δ 37 Cl variation in melt inclusions is not related to Cl degassing, or to fractional crystallization. Instead, correlations between δ 37 Cl and S/Cl, K 2 O and trace element ratios suggest mixing of two Cl endmembers with distinct δ 37 Cl signatures. A first endmember is characterized by high potassium alkali basalt compositions, high Ba/La (∼28), high S/Cl, and high δ 37 Cl (>1‰), confirming the influence in the mantle source of an aqueous fluid and providing a new constraint on its composition: that it derives from the breakdown of amphibole. The second endmember has a more evolved composition, high La/Yb, low S/Cl, and low δ 37 Cl (<−2‰), identifying the influence of a solute-rich component derived from subducted sediments. The δ 37 Cl data thus help refine the two sources initially identified from bulk rock studies and δ 37 Cl proves to be a potential tracer for amphibole