Noble gases in mafic phenocrysts from extrusives and ultramafic nodules of Stromboli: A Comparison to presently-released fluids

Noble gas compositions (He, Ne, Ar) and isotopic ratios (He, Ar) in olivine- and clinopyroxene-hosted fluid inclusions were measured for the (i) present-activity basalts, i.e. Low Porphyritic (hereafter LP) pumices and High Porphyritic (hereafter HP) scorias, and (ii) ultramafic nodules within the ~2 ka old San Bartolo basaltic andesite lavas. The ultramafic nodules (mostly wehrlites) are igneous cumulates consisting of clinopyroxene and olivine crystals with abundant fluid and melt inclusions. Textures and mineral chemistry of the wehrlites suggest that they are the products of early crystallization of primary basaltic magmas at mantle conditions. In addition, ultramafic cumulate and LP pumice crystals show similar major oxide composition (olivine and clinopyroxene) and trace element patterns (clinopyroxene). Strontium and neodymium isotope ratios of wehrlite clinopyroxenes indicate a crystallization from some of the most primitive batches of basaltic magmas feeding the Stromboli volcanism throughout its history. The gas content in mafic crystals decreases from ultramafic nodules to LP pumice and finally to HP scoria. This is probably due to the decreasing pressure (and depth) of fluid entrapment. In agreement with previous knowledge, the HP crystals are severely degassed and, as a result, the measurement of their 3He/4He ratio was unsuccessful. The 3He/4He of LP pumices and San Bartolo ultramafic nodules converge to a range of 4.2-4.7 Ra that we interpret as the magmatic value of the least degassed actual feeding system of Stromboli. This also allows the helium isotopes of the thermal waters of the Stromboli basal aquifer to be considered a mix between mantle-derived and atmospheric fluids. Therefore, the 3He/4He values measured in the Stromboli rocks represent the upper limit that should be expected in thermal fluids during long-term monitoring in case of events such as lava effusions and/or paroxysms