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The liquid line of descent from trachyte to pantellerite is controlled primarily by fractional crystallization of alkali feldspar, with whole-rock compositions following a fractionation path along the 'thermal valley' in the peralkaline haplogranite system Qz-Ab-Or-Ac-Ns and terminating at a minimum on the feldspar-quartz cotectic. Although whole-rock compositions for different pantelleritic suites follow nearly identical paths in a Qz-Ab-Or projection that terminate near the experimental minimum (Qz40.5Or34.5Ab25 at 100 MPa, projected from Ac-Ns), matrix glass from samples with near-minimum compositions record extreme differentiation and form a 'cotectic delta' beyond the terminus of the 'thermal valley'. Although each glass trend shows a continuing increase in Zr to >3000 μg/g, the most evolved compositions in each suite differ in peralkalinity (mol [Na + K] / Al) and in the proportions of FeOT, Qz, Ab, Or, and other components, which are related to subtle variations in the mafic phases controlled mainly by differences in oxygen fugacity (f O2) and pressure (P). To determine the controls over mafic mineral crystallization in pantelleritic magmas and the various paths these suites take beyond the apparent (whole-rock) minimum, amphibole-phyric suites from the ~159 ka Cala dell'Altura and Cala Gadir volcanic centres and the ~8–10 ka Cuddia Mida volcanic centre on Pantelleria have been analyzed and compared with each other and with the well-characterized and amphibole-free, compositionally zoned Green Tuff, the ~46 ka caldera-forming ignimbrite of the Cinque Denti caldera. Differences between the extended fractionation trends may be ultimately attributed to variations in oxygen fugacity, depth of emplacement, and water saturation. Shallower (lower pressure) magma reservoirs such as the one for the Green Tuff are water-saturated and undergo degassing, which leads to an increase in relative oxygen fugacity. Deeper (higher pressure) magma reservoirs remain water-undersaturated and retain water in the melt, which both maintains lower relative oxygen fugacities and enables the crystallization of amphibole. Amphibole formation appears to require melt water contents >4 wt%, low oxygen fugacity (<ΔNNO-1.5), and low temperatures (<700°C), although fluorine may stabilize it at higher temperatures in some rocks. [ABSTRACT FROM AUTHOR]

Solubility experiments were performed on a trachyte and a pantellerite from Pantelleria. The trachyte has $\mathrm{SiO}_{2}=65.2$ wt%, $\mathrm{Al}_{2}\mathrm{O}_{3}=15.2$ wt% and a peralkaline index $(\mathrm{P.I.} = \mathrm{molar}[(\mathrm{Na}_{2}\mathrm{O}+ \mathrm{K}_{2}\mathrm{O})/\mathrm{Al}_{2}\mathrm{O}_{3}]) \sim 1$ while the pantellerite has $\mathrm{SiO}_{2}=72.2$ wt%, $\mathrm{Al}_{2}\mathrm{O}_{3}=11$ wt% and a $\mathrm{P.I.}=1.3$. Solubility experiments were performed in the pressure range of 50–300 MPa at ${T}=950~\text{°C}$ for the trachyte and 50–200 MPa at ${T}=850~\text{°C}$ for the pantellerite. The water content of experimental glasses was determined by Karl Fischer titration, elemental analyser and FT–IR spectroscopy. Water content appears similar in both compositions for analogous pressure conditions, varying from ${\sim }2.5$ wt% at 50 MPa to ${\sim }5.8$ at 200 MPa. Comparison of the experimental data with the thermodynamic models shows that the empirical model of Moore et al. [1998] better matches the experimental data for both compositions, while the thermodynamic models of Papale et al. [2006] and Ghiorso and Gualda [2015] tend to overestimate melt water content, probably due to the paucity of peralkaline melt compositions used for calibration. One inference of these new solubility data is on MI-derived depth of felsic reservoirs at Pantelleria which deepens from 2.4 to 3.5 km.

The eruptive history of Pantelleria has been marked by the eruption of nine peralkaline ignimbrites, with inter-ignimbrite episodes from small, local volcanic centres. New whole-rock geochemical data are presented for seven ignimbrites and used with published data for younger units to track compositional changes with time. From ${\sim }190$ ka, silicic magmatism was dominated by comenditic trachyte to comendite compositions, evolving along generally similar liquid lines of descent (LLOD). The final ignimbrite, the Green Tuff (${\sim }46$ ka), was tapped from a compositionally zoned pantelleritic upper reservoir to a trachytic mush zone. Younger (20–7 ka) silicic magmatism has been relatively small scale, with compositions similar to the earliest pre-Green Tuff pantelleritic ignimbrite (Zinedi). These data suggest that the comenditic reservoirs may have been emplaced at deeper levels than the pantelleritic reservoirs. While both types of series evolved along similar LLOD dominated by fractionation of alkali feldspar, it is the fractionation of iron that determines whether comendite or pantellerite is produced. The deeper reservoirs were more oxidizing and wetter, thus leading to the crystallization of magnetite and therefore the fractionation of iron.

The recent (<190 ka) volcanic history of Pantelleria is characterized by the eruption of nine peralkaline ignimbrites, ranging in composition from comenditic trachyte to comendite to pantellerite. The ~46 ka Green Tuff (GT) was the last of these ignimbrites, which was followed by many effusive and explosive low-volume eruptions of pantellerite from vents within the caldera moat and along the caldera rim. Although recent studies have shed additional light on the age, petrochemistry, and volcanology of the older ignimbrites, there is very little knowledge of magmatism that occurred between these older ignimbrites, primarily due to the very scarce exposures. In this paper, we present new field descriptions and geochemical data for three local peralkaline centers never studied before, two pre-GT and one post-GT, which share a similar setting with respect to the caldera scarps but differ in terms of their age, composition, and eruptive style. These centers include: (i) the older (~125 ka) Giache center (comenditic trachyte), (ii) the ~67 ka Attalora center (comendite, pantellerite), and (iii) the younger (~14 ka) Patite center (pantellerite). [ABSTRACT FROM AUTHOR]

Silicic volcanoes are increasingly understood to be underlain by crystal-rich and vertically extensive magma reservoirs within which disequilibrium is widespread. Observations from ignimbrite deposits demonstrate that silicic magma reservoirs are often compartmentalised and compositionally stratified. However, it is currently unclear whether the small (i.e., < 0.1 km3 dense rock equivalent) eruptions that dominate activity at many volcanoes, and peralkaline volcanoes in particular, are fed from similarly complex magma reservoirs as their larger counterparts. Here I report petrographic and geochemical observations from the products of a small peralkaline eruption on Pantelleria, Italy, with the aims of unravelling peralkaline magma assembly processes and evaluating the complexity of reservoirs feeding small silicic eruptions. Matrix glass and whole-rock compositions from the Khaggiar lava flow and Cuddia Randazzo pumice cone reveal that erupted magmas were assembled from at least three distinct magma types stored within a compartmentalised magma reservoir: trachytes, less-evolved pantellerites and evolved pantellerites. Chemical variability in the Khaggiar lava flow was created by at least three distinct processes: the accumulation of evolved macrocrysts into evolved pantellerites; the injection of trachytic magmas into less evolved pantellerites; and the accumulation of relatively primitive macrocrysts into trachytic magmas. Macrocryst textures indicate that both trachytic and pantelleritic domains of the magma reservoir experienced numerous recharge events prior to the one that ultimately triggered eruption. Overall, magmas forming the Khaggiar lava flow and Cuddia Randazzo pumice cone appear to have been assembled in analogous ways to those erupted in much larger events. My observations are in good agreement with those from other peralkaline volcanoes and confirm that magma mingling, crystal cannibalism and macrocryst entrainment are as ubiquitous in peralkaline systems as they are in their calc-alkaline counterparts. [ABSTRACT FROM AUTHOR]

The relationships between trachytes and peralkaline rhyolites (i.e. pantellerites and comendites), which occur in many continental rift systems, oceanic islands and continental intraplate settings, is unclear. To fill this gap, we have performed phase equilibrium experiments on two representative metaluminous trachytes from Pantelleria to determine both their pre-eruptive equilibration conditions (pressure, temperature, H2O content and redox state) and liquid lines of descent. Experiments were performed in the temperature range 750-950 °C, pressure 0·5-1·5 kbar and fluid saturation conditions with XH2O [=H2O/(H2O+CO2)] ranging between zero and unity. Redox conditions were fixed below the nickel-nickel oxide buffer (NNO). The results show that at 950 °C and melt water contents (H2Omelt) close to saturation, trachytes are at liquidus conditions at all pressures. Clinopyroxene is the liquidus phase, being followed by iron-rich olivine and alkali feldspar. Comparison of experimental and natural phases (abundances and compositions) yields the following pre-eruptive conditions: P=160·5 kbar, T=925±25 °C, H2Omelt = 261 wt %, and fO2 between NNO- 0·5 and NNO- 2. A decrease in temperature from 950 °C to 750 °C, as well as of H2Omelt, promotes a massive crystallization of alkali feldspar to over 80 wt %. Iron-bearing minerals show gradual iron enrichment when T and fO2 decrease, trending towards the compositions of the phenocrysts of natural pantellerites. Despite the metaluminous character of the bulk-rock compositions, residual glasses obtained after 80 wt % crystallization evolve toward comenditic compositions, owing to profuse alkali feldspar crystallization, which decreases the Al2O3 of the melt, leading to a consequent increase in the peralkalinity index [PI = molar (Na2O+K2 O)/Al2 O3 ]. This is the first experimental demonstration that peralkaline felsic derivatives can be produced by low-pressure fractional crystallization of metaluminous mafic magmas. Our results show that the pantelleritic magmas of basalt-trachyte-rhyolite igneous suites require at least 95 wt % of parental basalt crystallization, consistent with trace element evidence. Redox conditions, through their effect on Fe-Ti oxide stabilities, control the final iron content of the evolving melt. [ABSTRACT FROM AUTHOR]

The viscosity of two series of synthetic alkali silicates, corresponding to Al-bearing pantelleritic and Al-free tri-silicate compositions, has been investigated as a function of temperature, iron redox and Na/(Na + K) ratio. Low temperature (708–1000 K) viscosities were determined by the micropenetration technique in the 10 9.6 to 10 13.6 Pa s range. The effect of Fe 2 + /Fe tot , from 0.15 to 0.86, was explored for [Na/(Na + K)] ratios from 0 to 1. The results demonstrate a strong decrease of viscosity with the replacement of Na for K in both the Al-bearing and Al-free compositions. In the Al-bearing pantelleritic glasses (Ebu) the viscosity as well as the activation energy for viscous flow decrease with an increase of Fe 2 + /Fe tot from 0.15 to 0.76. In contrast, no measurable changes in viscosity occur in the Al-free tri-silicate glasses (NFS and KFS) as Fe 2 + /Fe tot ratio varies from 0.15 to 0.86. The comparison of the pantelleritic glasses with peralkaline compositions from the literature stresses the strong influence of iron redox on the overall viscosity of such melts. This suggests that the contribution of iron species should be accounted for in the calculation of agpaitic index (AI) if magma Fe 3 + contents are known. One peculiarity of peralkaline magmas is the excess of alkali to alumina, which generally results in lower viscosities of pantelleritic liquids compared with more common metaluminous rhyolites and exerts a primary control of the rheological behaviour of these melts. Our new viscosity data, combined with existing literature, allow more accurate constraints on the nature and eruption of the pantelleritic magmas. Moreover, the present experimental study can be used to improve the viscosity models that mostly do not take in consideration parameters as iron redox in the viscosity prediction. [ABSTRACT FROM AUTHOR]

To develop Raman spectroscopy as a quantitative tool in both geosciences and planetary sciences the effect of iron oxidation state (Fe3+/Fetot) on the Raman spectra of basaltic and pantelleritic glasses has been investigated. We have used remelted pantellerite from Pantelleria Island and synthetic iron-rich basaltic glasses [from Chevrel et al. (2014)]. The Raman spectra of pantelleritic glasses reveal dramatic changes in the high wavelength region of the spectrum (800-1200 cm-1) as iron oxidation state changes. In particular the 970 cm-1 band intensity increases with increasing oxidation state of the glass (Fe3+/Fetot ratio from 0.24 to 0.83). In contrast, Raman spectra of the basaltic glasses do not show the same oxidation state sensitivity (Fe3+/Fetot ratio from 0.15 to 0.79). A shift, however, of the 950 cm-1 band to high wavenumber with decreasing iron oxidation state can be observed. We present here two empirical parameterizations (for silica- and alkali-rich pantelleritic glasses and for iron-rich basaltic glasses) to enable estimation of the iron oxidation state of both anhydrous and hydrous silicate glasses (up to 2.4 wt% H2O). The validation of the models derived from these parameterizations have been obtained using the independent characterization of these melt samples plus a series of external samples via wet chemistry. The "pantelleritic" model can be applied within SiO2, FeO, and alkali content ranges of ~69-75, ~7-9, and ~8-11 wt%, respectively. The "basaltic" model is valid within SiO2, FeO, and alkali content ranges of ~42-54, ~10-22, and ~3-6 wt%, respectively. The results of this study contribute to the expansion of the compositionally dependent database previously presented by Di Genova et al. (2015) for Raman spectra of complex silicate glasses. The applications of these models range from microanalysis of silicate glasses (e.g., melt inclusions) to handheld in situ terrestrial field investigations and studies under extreme conditions such as extraterrestrial (i.e., Mars), volcanic, and submarine environments. [ABSTRACT FROM AUTHOR]

The Plinian eruption of the Wendo Koshe crater within the Corbetti Caldera occurred around 396 BC. The pumice lapilli deposit, with a thickness exceeding 10 cm, dispersed over an area of over 1000 km 2 around the towns of Hawasa and Shashemene. Most of the pumice was deposited by fall-out; however, minor local pyroclastic density currents also occurred. The calculated volume of preserved pumice fall deposit (approximately 1.2 km 3 ), combined with the estimated volume of dispersed fine ash distributed further from the volcano, corresponds to an estimated volume of 0.4 km 3 (dense rock equivalent) of erupted magma. The age of the pumice eruption (396 ± 38 BC) was determined by 14 C radiometric dating of a paleosoil that developed on previous pyroclastic deposits buried by the pumice. The majority of the post-caldera volcanic products are characterized by a relatively uniform chemical composition (TiO 2 = 0.24–0.27 wt.%, Zr = 1300–1600 ppm, ƩREE = 920–1150 ppm) without any significant development in composition. Despite the negligible variations in composition of the magmas that erupted during the last 2500 years within the Corbetti Volcanic System, a significant change in composition was documented prior to the 396 BC Wendo Koshe younger pumice eruption. The caldera stage ignimbrite of Corbetti (TiO 2 = 0.34 wt.%, Zr = 500 ppm, ƩREE = 370 ppm) and the early post-caldera obsidians are (TiO 2 = 0.34 wt.%, Zr = 800 ppm, ƩREE = 410 ppm) characterized by a commenditic composition, and the character of the rhyolitic magmas shifted towards pantellerites in the post-caldera stage. The compositional contrast is confirmed also by Sr isotope ratios. The Corbetti ignimbrite is characterized by being more radiogenic ( 87 Sr/ 86 Sr = 0.70678) than the post-caldera obsidians ( 87 Sr/ 86 Sr = 0.7046–0.7047). In contrast to the trace-element concentrations, the early Chabi obsidian does not differ from younger obsidians in isotope composition. Similarly to other silicic volcanic systems of the Main Ethiopian Rift, the rhyolitic magmas of the recently active volcanoes within the Corbetti Volcanic System are most likely produced by extreme fractional crystallization of basaltic melts. [ABSTRACT FROM AUTHOR]

This study focuses on the comenditic trachyte enclaves hosted in pantelleritic lava and pumices emplaced during one of the most recent eruptive events at Pantelleria (~ 6 ka). Enclaves range from mm-sized fragments to dm-sized blocks with spheroidal to amoeboid shapes and characteristic globular surfaces; they are crystal-rich with ~ 30 vol% large anorthoclase, less abundant Fe-rich olivine, clinopyroxene and Fe–Ti oxides. Vesicles ranging from a few mm to 1–2 cm in size are distributed throughout the enclave and are commonly filled with microlite-free vesicular glass. The groundmass presents spectacular textures, including fine to coarse spherulites and hopper and skeletal microlites (mostly anorthoclase) with 10–35 vol% residual glass and 10–15 vol% small vesicles (< 50 μm to 200 μm). Residual glass has a pantelleritic composition with Na + K/Al > 2. Accordingly, the microlite composition is close to that of the host pantellerite. These textures were acquired during a magma mixing event prior to the eruption: (i) a vapor-saturated comenditic trachyte magma intruded the shallower, cooler pantellerite magma body, triggering a first degassing event ( first boiling ) and rapid crystallization with a high degree of undercooling (ΔT = 100–150 °C); (ii) vapor exsolution induced by rapid crystallization ( second boiling ) produced the microvesicular groundmass; and (iii) vapor pressure forced the residual pantelleritic liquid to migrate into the large vesicles and/or outside the enclaves. Based on this interpretation, these enclaves can be used as a natural laboratory for identifying the chemical and physical processes driving the evolution of silicic magmas at Pantelleria. In particular, they can be used to assess whether pantellerite melts can be obtained from comenditic trachytes after extensive crystallization followed by gas-driven filter-pressing and consequent (pantellerite) melt segregation. [ABSTRACT FROM AUTHOR]

Abstract: The viscosity of pantelleritic melts from the Khaggiar lava flow (5.5ka — Pantelleria Island) was investigated as a function of temperature and water content. High (1673–1323K) and low (973–613K) temperature dry and hydrous liquid viscosities were determined by a combination of concentric cylinder (102.44 to 104.56 Pas) and micropenetration (108.67 to 1011.37) viscometric techniques. The effect of water, from 0.02 to 3.55wt.% H2O, was explored in the temperature range of 973–613K. Liquid viscosities have been parameterized by means of a modified Vogel–Fulcher–Tammann equation (VFT) which describes viscosities and derivative properties (glass transition temperature Tg, fragility m) of silicic liquids as a function of T–X (H2O). The results yield the expected strong decrease of viscosity with temperature and water content. Fragility, as a measure of the deviation from Arrhenian behavior, increases with H2O content from m =23 to m =28. The peralkalinity (molar alkali excess over alumina), which characterizes the pantelleritic magmas, exerts a primary control over the rheological behavior of these melts. The excess of alkalies over alumina content is responsible for the peculiarly low viscosities of pantelleritic liquids compared to their metaluminous counterpart in the high temperature range, and leads to an even more dramatic decrease in the viscosities of these rhyolites in the low temperature range. Comparison with available models shows substantial differences between the measured and calculated viscosities. Based on our new viscosity results combined with existing literature data, we propose a new parameterization based on a modified Vogel–Fulcher–Tammann equation, accounting for the effect of water and composition, which can be used to determine the viscosity of pantelleritic melts. The derived relationship reproduces the experimental data (58 in total) in the viscosity range from 102.44 to 1011.37 Pas and in the temperature range from 613 to 1673K within a root-mean-square-error (RMSE) of 0.12 log units. Application of this calculation model for high temperature low viscosity hydrous melt is limited by the lack of experimental data and needs verification by additional measurements. Volcanological implications for welding and rheomorphic processes as well as conduit dynamic modeling are also discussed in light of the results presented in this work. [Copyright &y& Elsevier]

We present the results of the first systematic study of melt compositions at Pantelleria, based on both melt inclusions and matrix glasses in pantellerites from 10 eruptions during the last eruptive cycle (<45 kyr). We present major and trace element compositions, as well as data on the volatiles sulphur (S), fluorine (F), chlorine (Cl), water (H2O), carbon dioxide (CO2) and lithium (Li) Rare earth element (REE) compositions were inverted using the program INVMEL to establish the melt fraction vs depth relationship in the Pantellerian mantle source region. Inversion indicates that melts are generated by ∼1·7% melting of a light rare earth element (LREE)-enriched mantle source. The source lies principally within the spinel–garnet transition zone, which, on the basis of trace element ratios, shows some affinity to the source of North African magmatism. Major and trace element data indicate a gap in melt compositions at intermediate compositions, consistent with previously published whole-rock data. This gap rules out the possibility of explaining chemical variability in the Pantelleria lavas merely by changes in the crystal content of the magmas. Principal component analysis of major element glass compositions shows that the liquid line of descent for mafic melt compositions is controlled by clinopyroxene, plagioclase, magnetite and olivine crystallization. Alkali feldspar, clinopyroxene, ilmenite and olivine or aenigmatite crystallization controls the liquid line of descent for the silicic melt compositions, with aenigmatite broadly replacing olivine in the most evolved magmas. Trace element modelling indicates that 96% fractional crystallization is required to generate pantellerites from alkali basalts at Pantelleria (through trachytes, generated after 76% fractional crystallization). We have measured pantellerite volatile concentrations in melt inclusions and in matrix glasses from a variety of eruptions. Melt inclusions, on average, contain 350 ppm S, 3500 ppm F and 9000 ppm Cl. We have measured up to 4·9 wt % H2O and 150 ppm CO2 in melt inclusions. Li–H2O systematics and Cl abundances in melt inclusions are consistent with partitioning of Li and Cl into a subcritical hydrosaline fluid at low pressures. The volatiles H2O and CO2 are used to estimate melt equilibration pressures, which reach a maximum of 1·5 kbar. Temperatures of 800°C are calculated for the most evolved pantellerites, using published feldspar–melt geothermometers, and up to 870°C for the least evolved samples. Low melt viscosities are calculated for the range of pantellerite compositions observed and may account for rapid differentiation by crystal settling. Stable density stratification of the magma chamber is reflected in the eruption of generally progressively more fractionated compositions after the Green Tuff eruption during the last eruptive cycle. Some anomalies in this trend may be explained by variation in the relative rates of eruption vs fractionation. The density stratification is expected to be enhanced and further stabilized by the efficient migration of a fluid phase to the roof of the magma chamber. The sulphur data are used in combination with published experimental partitioning data for peralkaline rhyolites to estimate the sulphur yield to the atmosphere for a large pantelleritic eruption similar to the Green Tuff. This is expected to be markedly higher than for a similar-sized metaluminous rhyolitic or dacitic eruption, mainly owing to the higher bulk sulphur content, lower fluid–melt partition coefficients, and rapid differentiation and vapour phase segregation in the magma chamber. [ABSTRACT FROM PUBLISHER]

The Marie Byrd Land province includes 18 large (up to 1,800 km) central volcanoes distributed across an active volcano-tectonic dome. The typical volcano structure consists of a basal 1,000-5,000 m of basanite surmounted by trachyte and subordinate intermediate rocks, plus phonolite, or pantellerite, or comendite. The volumes of felsic sections are large (~30-700 km), but these rocks probably make up <10% of volcanic rock in the province. This paper describes pantellerite volcanoes in the Ames and Flood Ranges, which include a large and varied suite of these iron-rich, silica-poor rhyolites. Isotopic and trace element data, maintenance of isotopic equilibrium throughout the basalt-felsic range, and the results of modeling, all exclude significant crustal contamination and point to fractional crystallization as the process that controls magmatic evolution. The most unusual feature of these volcanoes is the apparent need to derive pantellerites from basanite, the long interval of fractionation at the base of the lithosphere and crust, involving kaersutite as the key phase in developing pantellerite, and a plumbing system that permitted coeval eruption of pantellerite and phonolite from the same edifice. Peralkalinity most likely developed in upper crustal reservoirs during the final 4-5% of magmatic history, by fractionating a high proportion of plagioclase under low pHO. Mantle plume activity appears to drive doming and volcanism. This, a stationary plate, and continental lithospheric structure seem to provide an optimal environment for the evolution of a diverse, large volume suite of felsic rocks by fractional crystallization. [ABSTRACT FROM AUTHOR]

We present experimental phase equilibria carried out on a pantelleritic bulk-rock composition with a peralkalinity index [PI = molar (Na2O + K2O)/Al2O3] = 1·4, which is representative of the most energetic pumice fall eruption of the resurgent post-caldera volcanism on Pantelleria island. For the explored conditions (P = 25–150 MPa; T = 680–800°C; H2Omelt up to 6 wt %; fO2 &leq; NNO, where NNO is nickel–nickel oxide buffer) clinopyroxene is the liquidus phase followed by alkali feldspar and then quartz. The crystallization of amphibole is limited to temperatures below 700°C. Aenigmatite crystallizes near the liquidus for P &geq; 100 MPa. When clinopyroxene is the sole liquidus phase, the coexisting melt is characterized by a decrease in peralkalinity index to 1·2, if compared with the starting material, approaching comendite composition. In contrast, when alkali feldspar joins the liquidus phase assemblage, residual melts become extremely peralkaline (PI = 3·2), with FeO contents in excess of 14 wt %. Comparison of our experiments with natural phase abundances and compositions constrains the depth of the Fastuca pantellerite reservoir to be around 5 ± 1 km (or P = 120 ± 20 MPa) at T = 730 ± 10°C, with H2Omelt around 4 wt %, and fO2 1·5 log unit below NNO. Such conditions correspond to the dominant phase assemblage of feldspar, clinopyroxene and aenigmatite. Amphibole occurrence requires both higher melt water contents and lower temperatures (up to 6·5 wt % at 680°C), and may reflect provenance from a deeper and wetter portion of the tapped reservoir. In contrast, quartz is more probably related to crystallization during magma ascent, which explains the lower melt water content of its melt inclusions. Overall, the results confirm that pantellerites may be both water-rich and reduced magmas. These characteristics imply that any coexisting fluid phase in the reservoir will be dominated by water and hydrogen species, the latter amounting to a few mole per cent. Magma unrest at Pantelleria may thus be heralded by emissions of hydrous and reduced gases (i.e. CO2-poor and H2-rich), if the present-day reservoir is felsic in composition. [ABSTRACT FROM PUBLISHER]

Abstract: The position, scaled intensity and topology of the Raman stretching envelope at ~1000 cm−1 (HF) have been established in complex anhydrous glasses having variable Fe3+/ΣFe ratios and contrasting compositions of peralkaline rhyolite (pantellerite; FeOT: 7.8 wt.%), iron-poor and iron-rich basalts (FeOT: 7.6 and 10.1 wt.%). Sensitivity of glass structure to iron redox decreases with increasing glass basicity. Raman spectra of pantellerite glasses (Fe3+/ΣFe: 0.04–0.97) evidence the sharp increase of a band at ~970 cm−1 with the progressive entry of 4-fold Fe3+ in alkali-bonded Q3 units. The spectral evolution of basalts with increasing iron oxidation (Fe3+/ΣFe: 0.05–0.87) is more subtle and is likely determined by formation of less polymerized units. The moderate to low influence of redox state changes on the structure of natural basalt glass, compared with the high sensitivity of their synthetic analogues, can be attributed to strong competition between network-forming cations (Al, Fe) in the absence of an excess of alkalis. We demonstrate that Raman calibration against reference glasses allows precise determination of Fe2+/Fe3+ratio but is highly composition dependent. As a consequence of the variable link existing between Fe2+/Fe3+ and glass polymerization, specific calibration procedures based on deconvolution, shift or intensity of HF band are required for compositions with high (acid glasses), moderate or poor (basic glasses) sensitivity to iron redox changes, respectively. [Copyright &y& Elsevier]

Abstract: The Quaternary Eburru volcanic complex in the south-central Kenya Rift consists of pantelleritic trachytes and pantellerites. The phenocryst assemblage in the trachytes is sanidine+fayalite+ferrohedenbergite+aenigmatite±quartz±ilmenite±magnetite±pyrrhotite±pyrite. In the pantellerites, the assemblage is sanidine+quartz+ferrohedenbergite+fayalite+aenigmatite+ferrorichterite+pyrrhotite±apatite, although fayalite, ferrohedenbergite and ilmenite are absent from more evolved rocks (e.g. with SiO2 >71%). QUILF temperature calculations for the trachytes range from 709 to 793 °C and for the pantellerites 668–708 °C, the latter temperatures being among the lowest recorded for peralkaline silicic magmas. The QUILF thermobarometer demonstrates that the Eburru magmas crystallized at relatively low oxidation states (ΔFMQ +0.5 to −1.6) for both trachytes and pantellerites. The trachytes and pantellerites evolved along separate liquid lines of descent, the trachytes possibly deriving from a more mafic parent by fractional crystallization and the pantellerites from extreme fractionation of comenditic magmas. [Copyright &y& Elsevier]

Abstract: The phase relationships and compositions of a pantellerite from the Eburru complex in the Kenya Rift Valley have been determined at 150 MPa and under reducing conditions, 2 log units below the Ni–NiO solid buffer. The effects of temperature and melt water content on phase relationships have been explored. Alkali feldspar and quartz crystallise alone at temperatures above 700 °C, irrespective of melt water content. Below 700 °C, sodic amphibole and clinopyroxene also crystallise; the amphibole being the liquidus phase under water-rich conditions. The coexistence of amphibole phenocrysts with alkali feldspar and quartz in a crystal-poor pantellerite implies temperatures below 700 °C and melt water contents higher than 4 wt.%, possibly up to 5–6 wt.%. Pantellerites have lower liquidus temperatures than associated comendites, which supports a parent–daughter relationship between the two magma types. The melts produced in the experiments extend the compositional trend displayed by the natural rock series, and reproduce some extreme compositions occasionally observed in alkaline volcanic series, with FeO⁎ contents above 12 wt.% and Na2O contents approaching 10 wt.%. Pantellerites are therefore the true near-minimum melt compositions of alkaline oversaturated magma series. [Copyright &y& Elsevier]

Abstract: A 30-m long piston core from the floor of Pantelleria Trough penetrated an 18 m thick volcaniclastic interval under- and overlain by hemipelagic sediments. The distribution and thickness of the interval is mapped in 3.5 kHz profiles across the trough. Core sediments have been analysed for grain size, carbonate content, detrital petrology, and physical properties such as bulk density and magnetic susceptibility. Chemical analyses of tephra and included minerals have been made by electron microprobe. The volcaniclastic interval is correlated with the youngest large explosive eruption on Pantelleria, the 50 ka Green Tuff ignimbrite eruption, on the basis of the sedimentation rate of overlying sediment, calcareous nannofossil and foraminiferal biostratigraphy, a calibrated radiocarbon age of 43–47±2 ka (possibly infinite) in the underlying marls, and many similarities in tephra and mineral chemistry and abundance. The lower 8 m of the volcaniclastic interval consists of poorly sorted massive units of lapilli tuff, interpreted as resulting from the direct flow of a pyroclastic flow into the sea, with the energetic hyperconcentrated flows crossing the length of Pantelleria Trough and eventually “freezing” on the low gradients. The overlying 10 m thick turbidite probably developed from the dilute turbulent pyroclastic flow of the co-ignimbrite cloud and the resulting turbulent subaqueous flow eroded bioclastic material from the shelf and deposited a well-graded bed from sand through silt to mud. The poorer sorting of the volcaniclastic interval compared with the Green Tuff ignimbrite on land supports the concept that ignimbrite deposits are sorted by aggrading bed processes. [Copyright &y& Elsevier]

Pantelleria, the type locality of pantellerite, is a Late-Pleistocene magmatic system with a bimodal association of alkali basalts and peralkaline rocks showing a large compositional gap at SiO2=50–67 wt.%. Major and trace element data show a strong change in all element patterns with two distinct liquid lines of descent for the mafic and the felsic end-members for many elements (Al2O3, FeOtot, Ba, and Cr). The mineral chemistry data also show compositional gaps for many mineral phases, similar to those observed in the whole-rock data. Trace element models show that a trachytic melt can be generated from a basaltic parental magma either by a two step fractional crystallisation process, or by 30% partial melting of gabbroic cumulate. Pantelleritic magmas are derived by fractional crystallisation of mainly K-feldspar from a trachytic end-member. The crystal-chemical parameters of the clinopyroxene crystals show two clearly distinct patterns for the mafic and felsic rocks, which are unlikely to be generated by continuous crystal fractionation process from basalt to pantellerite. Clinopyroxene crystals in the mafic rocks show trends comparable with patterns observed for some Italian potassic suites (e.g., Sabatini volcano), with an increase of Al in the T site and Ca in M2 with differentiation. On the other hand, crystals in the peralkaline felsic rocks follow the patterns of the African Na-rich products (e.g., Nyambeni, Boseti) with Si increasing in T and Na in M2. Chemical parameters of the host rocks plotted vs. structural data of the clinopyroxene crystals divide mafic and felsic rocks into two well distinguished groups of undersaturated and oversaturated magmas, but do not explain the transition between the two groups.The data suggest that simple fractional crystallisation by itself cannot explain the generation of peralkaline magmas in Pantelleria starting from parental basalts. Other processes such as partial melting of gabbroic cumulates are more viable and have to be considered in the genesis of peralkaline magmas. [Copyright &y& Elsevier]