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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]

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»190 ka, silicic magmatismwas dominated by comenditic trachyte to comendite compositions, evolving along generally similar liquid lines of descent (LLOD). The final ignimbrite, the Green Tuff (»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. [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]

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]

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]

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]

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]