Dipartimento di Scienze della Terra e Geoambientali, Università degli Studi di Bari, via Orabona 4, I-70125 Bari, Italy

Fluorine-, boron- and magnesium-rich metamorphosed xenoliths occur in the Campanian Ignimbrite deposits at Fiano (southern Italy), at ∼50 km northeast of the sourced volcanic area. These rocks originated from Mesozoic limestones of the Campanian Apennines, embedded in a fluid flow. The Fiano xenoliths studied consist of ten fluorophlogopite-bearing calc-silicate rocks and five carbonate xenoliths, characterized by combining mineralogical analyses with whole-rock and stable isotope data. The micaceous xenoliths are composed of abundant idiomorphic fluorophlogopite, widespread fluorite, F-rich chondrodite, fluoborite, diopside, Fe(Mg)-oxides, calcite, humite, K-bearing fluoro-richterite and grossular. Of the five mica-free xenoliths, two are calcite marbles, containing subordinate fluorite and hematite, and three are weakly metamorphosed carbonates, composed only of calcite. The crystal structure and composition of fluorophlogopite approach that of the end-member. The Fiano xenoliths are enriched in trace elements with respect to the primary limestones. Comparisons between the rare-earth element (REE) patterns of the Fiano xenoliths and those of both Campanian Ignimbrite and Somma-Vesuvius marble and carbonate xenoliths showthat the Fiano pattern overlaps that of Somma-Vesuvius marble and carbonate xenoliths, and reproduces the trend of Campanian Ignimbrite rocks. Values of δ13C and δ18O depict the same trend of depletion in the heavy isotopes observed in the Somma-Vesuvius nodules, and is related to thermometamorphism. Trace-element distribution, paragenesis, stable isotope geochemistry and data modelling point to infiltration of steam enriched in F, B,Mg and As into carbonate rocks at a temperature of ∼300–450°C during the emplacement of the Campanian Ignimbrite.

This study reports the first crystal chemical database resulting from a detailed structural investigation of trioctahedral micas found in xenolithic ejecta produced during the AD 1631, 1872 and 1944 eruptions, three explosive episodes of recent volcanic period of Vesuvius volcano (Southern Italy). Three xenolith types were selected: metamorphic/metasomatic skarns, pyrometamorphic/hydrothermally altered nodules and mafic cumulates. They are related to different magma chemistry and effusive styles: from sub-plinian and most evolved (AD 1631 eruption) to violent strombolian with medium evolution degree (AD 1872 eruption) to vulcanian-effusive, least evolved (AD 1944 eruption) event, respectively. Both xenoliths and micas were investigated employing multiple techniques: the xenoliths were characterized by X-ray fluorescence, inductively-coupled plasma-mass spectrometry, optical microscopy, X-ray powder diffraction, and quantitative energy-dispersive microanalysis; the micas were studied by electron probe microanalysis and single crystal X-ray diffraction. The mica-bearing xenoliths showvariable texture and mineralogical assemblage, clearly related to their different origin. Based on the major oxide chemistry, only one xenolithic sample falls in the skarn compositional field fromthe Somma-Vesuvius literature, some fall close to the skarns and cumulate fields, others plot close to the syenite/foidolite/essexite field. A subgroup of the selected ejecta does not fall or approach any of the compositional fields. Trace and rare earth element patterns show some petrological affinity between studied xenoliths and erupted magmas with typical Eu, Ta and Nb negative anomalies. Strongly depleted patterns were detected for the 1631 metamorphic/metasomatic skarns xenoliths. Three distinct mica groups were distinguished: 1) Mg-, Al-rich, low Ti-bearing, low to moderate F-bearing varieties (1631 xenolith), 2) Al-moderate, F- and Mg-rich, Ti-, Fe-poor varieties (1872 xenolith), and 3) Al-, Ti- and Fe-rich, F-poor phases (1944 xenolith). All the analyzed mica crystals are 1Mpolytypes with the expected space group C2/m. Micas from xenoliths of the 1631 Vesuvius eruption are phlogopites characterized by a combination of low extent of oxy-type and variable extent OH−→F− substitutions, as testified by the range of F concentration (from ~0.20 to 0.80 apfu). Micas from xenoliths of the 1872 Vesuvius eruption exhibit structural peculiarities typical of fluorophlogopites, i.e. OH−→F− substitution is predominant. Micas from the xenolith of the 1944 Vesuvius eruption display features typical of oxy-substituted micas. The variability of the crystal chemical features of the studied micas is consistentwith the remarkable variation of their host rocks. Micas from1631 nodules are related to metasomatic, skarn-type environment, deriving from the metamorphosed wall-rocks hosting the magma reservoir. The fluorophlogopites from the 1872 xenoliths testify for strongly dehydrated environmental conditions compared to those of the 1631 and 1944 hosts. Finally, magma storage condition at depth, associated to a decreasing aH2O may have promoted major oxy-type substitutions in 1944 biotites.

The thermal behavior of melanterite from the Fornovolasco mine (Tuscany, Italy) has been investigated via differential thermal analysis (DTA), thermogravimetry (TG), in situ high-temperature X-ray powder diffraction (XRPD) and Fourier-transform infrared spectroscopy (FTIR). The DTA curve showed endothermic peaks at 70, 100, 260, 500–560 and 660 °C whereas the TG curve evidenced a total mass decrease of ~68%, in keeping with the loss of all H2O and SO4 groups. Rietveld refinements were performed for all the collected patterns in the 25–775 °C range and converged at 1.57 ≤ R (%) ≤ 2.75 and 1.98 ≤ Rwp (%) ≤ 3.74. The decomposition steps FeSO4·7H2O → FeSO4·4H2O (25 ≤ T ≤ 50 °C) → FeSO4·H2O (50 < T ≤ 100 °C) → FeOHSO4 (75 < T ≤ 200 °C) → Fe2(SO4)3 (400 < T ≤ 500 °C) → Fe2O3 (500 < T ≤ 775 °C) were obtained. The high-temperature infrared analysis confirmed that melanterite undergoes a three-step dehydration in the 25–300 °C temperature range. The FeOHSO4 phase is stable over a wide range of temperature and transforms partially to Fe2(SO4)3 without the formation of Fe2O(SO4)2. The findings highlight a different behavior of the studied sample with respect to the synthetic salt.