Filippo Ridolfi

Iron-bearing chlor-fluorapatites in crustal xenoliths from the Stromboli volcano (Aeolian Islands, Southern Italy): an indicator of fluid processes during contact metamorphism

Chlor-fluorapatite crystals, with an unusually high Fe content (up to 0.715 apfu), have been found in cordierite- and feldspar-bearing hornfels metapelitic xenoliths included in calcalkaline basaltic andesite lavas from the Stromboli volcano. These xenoliths originated in a mid-crustal (200–350 MPa) contact aureole (800°C) surrounding the magmatic plumbing system of the volcano. The high and somewhat variable Cl, Fe, Na, and Mn content in the chlor-fluorapatite can be interpreted as CaCl2, FeCl2, FeCl3, NaCl, and MnCl2 hydrous chloride melts present in the calcalkaline basaltic andesite magma responsible for the metamorphism of the metapelites to hornfels facies in the contact aureole. During the partial melting and dehydration of the metapelites, H2O was strongly partitioned into the partial melt and subsequently into the calcalkaline basaltic andesite magma. This H2O-rich partial melt was partly replaced by hydrous chloride melts, which metasomatised the original fluorapatite to chlor-fluorapatite with variable amounts of Cl, Fe, Na, and Mn. This scenario is supported by the patchy enrichment of Cl, Fe, Na, and Mn in the chlor-fluorapatite grains and the correspondingly low water activities present during the formation of the hornfels as confirmed by the absence of hydrous phases in the modal mineralogy of the xenoliths.

Evolutionary stages of crystallization of weakly peralkaline syenites: evidence from ejecta in the plinian deposits of Agua de Pau volcano (São Miguel, Azores Islands)

Abstract Weakly peralkaline syenite ejecta within the plinian fall deposits of Agua de Pau volcano (São Miguel, Azores Islands) are divisible into (1) silica-undersaturated and (2) silica-saturated/oversaturated groups. The first group occurs in the pre-Fogo-A pyroclastic falls (15.2 ka) whereas the second occurs in the Fogo-A pumices (5 ka). The petrography indicates that the syenites of both groups crystallized at shallow depths and whole-rock geochemistry shows that they have compositions reflecting those of the Agua de Pau trachyte magmas. The syenites are inferred to be derived from solidification zones around the margins of the parental magma chamber. Mineral parageneses in both syenite groups permit distinction of three magmatic stages of crystallization: (1) an early crystallization stage; (2) a main crystallization stage during which most of the feldspar growth occurred; and (3) a late interstitial stage. Crystallization of the trachytic magmas yielded peralkaline residuals, highly enriched in volatiles, alkalis, HFSE and REE. In the silica-undersaturated syenites, the complex Zr-Ti silicate minerals, eudialyte and a låvenite-like mineral, crystallized interstitially from the residual magmas. In contrast, the zircono-silicate dalyite, and REE-rich minerals (chevkinite-(Ce), britholite-(Ce) and pyrochlore), together with thorite, were late-stage interstitial products of the silica-saturated/oversaturated syenites.

Xenopumice erupted on 15 October 2011 offshore of El Hierro (Canary Islands): a subvolcanic snapshot of magmatic, hydrothermal and pyrometamorphic processes

On 15 October 2011, a submarine eruption offshore of El Hierro Island gave rise to floating volcanic products, known as xenopumices, i.e., pumiceous xenoliths partly mingled and coated with the juvenile basanitic magma. Over the last few years, no consensus in the scientific community in explaining the origin of these products has been reached. In order to better understand the formation of xenopumice, we present a textural, mineralogical, and geochemical study of the possible magmatic, hydrothermal, and pyrometamorphic processes, which usually operate in the plumbing systems of active volcanoes. We carried out a comprehensive SEM investigation and Sr-Nd-Pb isotope analyses on some samples representative of three different xenopumice facies. All the data were compared with previous studies, new data for El Hierro extrusives and a literature dataset of Canary Islands igneous and sedimentary rocks. In the investigated xenopumices, we emphasize the presence of restitic magmatic phases as well as crystallization of minerals (mainly olivine + pyroxene + magnetite aggregates) as pseudomorphs after pre-existing mafic phenocrysts, providing evidence of pyrometamorphism induced by the high-T juvenile basanitic magma. In addition, we identify veins consisting of zircon + REE-oxides + mullite associated with Si-rich glass and hydrothermal quartz, which indicate the fundamental role played by hydrothermal fluid circulation in the xenopumice protolith. The petrological data agree with a pre-syneruptive formation of the xenopumice, when El Hierro basanite magma intruded hydrothermally altered trachyandesite to trachyte rocks and triggered local partial melting. Therefore, the El Hierro xenopumice represents a snapshot of the transient processes at the magma-wall rock interface, which normally occurs in the feeding system of active volcanoes.