Pietro Armienti

The Phlegraean Fields: magma evolution within a shallow chamber

Abstract A systematic petrological and chemical study of the volcanic products of the Phlegraean Fields has been accomplished based on the new stratigraphy described by Rosi et al. (this volume). The majority of Phlegraean rocks belong to the “potassic” series of the Roman province. The compositional spectrum ranges from trachybasalts to latites, trachytes, alkali trachytes, and peralkaline phonolitic trachytes. Trachybasalts are extremely rare and there is a sharp compositional gap between them and the latites. The series between latites and the trachytic varieties is complete. Trachytic rocks are much more voluminous than latites. The order of appearance of the main solid phases is: olivine, clinopyroxene, plagioclase, alkali feldspar, biotite, and oxides. Mineral compositions obtained by microprobe analyases are compatible with the evolution of the rock chemistry. However, primitive compositions of plagioclase (An 80 ) and clinopyroxene (diopside) persist in the cores of phenocrysts, even at the trachytic and alkali trachytic stage. Fractional crystallization within a shallow magma chamber has been the dominant process for the generation of Phlegraean rock series. The volume of the magma chamber is estimated to have been at least 240 km 3 at the moment of the eruption of the Campanian Ignimbrite, nearly 35,000 yr. ago. This event was followed by a large caldera collapse. The depth of the chamber cannot be precisely evaluated. However, its top must have been very shallow, probably at 4–5 km, as suggested by contact metamorphic rocks obtained from deep geothermal wells within the caldera. Volcanological and petrological data favor a model of upward migration of lighter liquids produced mostly by fractionation along the cool walls of the chamber, the deeper part of which is occupied by a convecting trachybasaltic magma. Progressive migration of eruptive vents toward the caldera center and the contemporaneous strong reduction in the volume of the erupted products, suggest that the chamber behaved as a closed system. The volume of magma was progressively reduced by both cooling and extraction to the surface.

Effects of magma storage and ascent on the kinetics of crystal growth

The size distributions of crystals of olivine, plagioclase and oxides of the 1991/93 eruption at Mt. Etna (Italy) are analyzed. The simultaneous collection of this information for different minerals gives precious insight into the cooling history of lavas. Three distinct episodes are detectable: a storage of the magma in a deep reservoir, characterized by nearly constant and low nucleation and growth rates (near to equilibrium); an ascent phase, with an ever increasing nucleation rate related to volatile exsolution; and finally a quenching phase. In addition to geochemical and geophysical evidence, the similarity of the crystal size distributions of the present eruption with those of previous ones of this century makes it possible to exclude that crystal size distributions of Etnean lavas are due to mixing of different populations. This strongly suggests that the main features of the volcano feeding system have not changed despite observed variations in the magma output rates.

Subduction-like fluids in the genesis of Mt. Etna magmas: evidence from boron isotopes and fluid mobile elements

Abstract New whole-rock B, Sr, Nd isotope ratios and 87Sr/86Sr on clinopyroxenes have been collected to study the enrichment of fluid mobile elements (FMEs) observed in Mt. Etna volcanics. Etna volcano, one of the most active in the world, is located in an extremely complex tectonic context at the boundary between colliding African and European plates. The analytical work focuses on current (1974–1998) and historic (1851–1971) eruptive activity, including some key prehistoric lavas, in order to interpret the secular shift of its geochemical signature to more alkaline compositions. Boron is used as a tool to unravel the role of fluids in the genesis of magmas, revealing far-reaching consequences, beyond the case study of Mt. Etna. Small variations are observed in δ11B (−3.5 to −8.0‰), 87Sr/86Sr (0.70323–0.70370), and 143Nd/144Nd (0.51293–0.51287). Moreover, temporal evolution to higher δ11B and 87Sr/86Sr, and to lower 143Nd/144Nd, is observed in the current activity, defining a regular trend. Sr isotopic equilibrium between whole-rock and clinopyroxene pairs indicates the successive introduction of three distinct magma types into the Etna plumbing system over time; these are characterized by differing degrees of FME enrichment. In addition, certain lavas exhibit evidence for country rock assimilation, magma–fluid interaction, or magma mixing in the shallow feeding system; at times these processes apparently lowered magmatic δ11B and/or induced Sr isotopic disequilibrium between whole rock and clinopyroxene. The regular increase of δ11B values is correlated with Nb/FME and 87Sr/86Sr ratios; these correlations are consistent with simple mixing between the mantle source and aqueous fluids derived from nearby Ionian slab. The best fit of Mt. Etna data is obtained using an enriched-MORB mantle source and a fluid phase with δ11B of about −2‰ and 87Sr/86Sr of 0.708. We argue that the slab window generated by differential roll-back of subducting Ionian lithosphere (with respect to Sicily) allows the upwelling of asthenosphere from below the subduction system and provides a suitable path of rise for subduction-related fluids. The increasing geochemical signature of fluid mobile elements enrichment to Mt. Etna lavas is considered a consequence of the progressive opening of slab window through time.

Petrology and Sr-Nd isotope geochemistry of recent lavas from Mt. Etna: bearing on the volcano feeding system

Abstract The volcanic activity of Mt. Etna during the period 1971–1987 has been investigated, based on new volcanological, petrographic, geochemical and isotopic data. The magma output rate since 1971 is much higher than that of the last 200 years, marking a significant change in the feeding regime of Mt. Etna. The highest output rate has been observed for the 1981 eruption. Sub-aphyric lavas, emitted during the excentric activity of 1974, are the most primitive of those studied ( Mg # ∼ 57−55 ). The major- and trace-element chemistry of the lavas indicates limited differentiation by crystal fractionation, mainly involving mafic phases. Nevertheless, the observed variability of some ratios between highly incompatible elements suggests possible source heterogeneities. The Sr-Nd isotopic compositions range between 0.70362–0.70377 and 0.512829–0.512892 respectively, with the lowest and highest 87Sr/86Sr ratios measured in the 1981 and 1974 lavas. The observed isotopic variability is not due to late contamination processes, but probably to the mantle source. Variations in Sr and Nd isotopic composition could indicate the lack of large magma chambers at any level in Mt. Etna feeding system, suggesting a model in which magma reaches the surface through a network of dikes, where complete homogenization of different batches cannot be attained.

GEOCHEMICAL AND ISOTOPIC STRUCTURE OF THE EARLY PALAEOZOIC ACTIVE MARGIN OF GONDWANA IN NORTHERN VICTORIA LAND, ANTARCTICA

Abstract The regional distribution of geochemical and isotopic compositions of granitoid rocks from a Gondwana continental margin is studied to highlight its structure and geodynamic evolution. The intrusive rocks emplaced during the early Palaeozoic Ross Orogeny in northern Victoria Land (Antarctica) constitute a high-K calc-alkaline association. The geographic patterns of isotope and geochemical data on granitoid rocks allow the distinction of two portions of the continental margin, separated by a sharp discontinuity. The portion towards the palaeo-Pacific Ocean (Oceanward Side) displays strongly regular inland increase of Sr- and decrease of Nd-isotope ratios, coupled with analogous variations in major and trace elements; on this basis we infer a NW-SE-trending margin affected by SW-directed subduction. The portion towards the East Antarctic Craton (Continentward Side) shows a similar regular variation only for Nd isotope compositions, consistent with a hypothesis of a N-S-striking margin with west-ward subduction. In the Oceanward Side, isotope and trace-element characteristics suggest that the granites were generated by extensive interaction of mantle-derived magmas with high-level crustal melts. The origin of Continentward Side intrusives is compatible with a process of interaction between mantle-derived melts and a mafic granulite lower crust. The granitoids of the two crustal sectors share the same Proterozoic Sm-Nd model ages, suggesting that they both belong to the same crustal province. We interpret this arrangement of crustal segments as due to the shift and rotation of a forearc sliver of the Gondwana margin. This movement was likely enhanced by oblique subduction under an irregular margin weakened by the presence of a magmatic arc.

Cenozoic climatic change in Antarctica recorded by volcanic activity and landscape evolution

A long-lasting Cenozoic record (∼50 m.y.) of alkaline igneous rocks characterizes northern Victoria Land, Antarctica. Landscape analysis allows distinction between older volcanic and intrusive rocks with well-developed alpine topography sculptured by wet-based glaciers, and younger volcanic cones lacking these features. Many K-Ar and Rb-Sr dates testify that the erosion that formed the alpine landscape ceased between ca. 8.2 and 7.5 Ma. Since ca. 8 Ma, morphological evolution has been driven by cold-based glaciers; warm-based glaciers were no longer active. That this change affected a 300-km-long coastal area suggests a persistent cause of global significance. Glacier dynamics control landscape shaping as a function of ice thickness and temperature, which are driven by climatic conditions. In this view, a significant climatic change occurred in northern Victoria Land between 8.2 and 7.5 Ma. The perfectly preserved serrated alpine ridges, with their delicate spires, testify that no warm-based ice sheets overrode the region after 7.5 Ma and that polar conditions held sway in the Pliocene and Pleistocene Epochs.

Phenocryst/matrix trace-element partition coefficients for hawaiite-trachyte lavas from the Ellittico volcanic sequence (Mt. Etna, Sicily, Italy)

SummaryA set of phenocryst/matrix partition coefficients was obtained for up to 29 trace elements (ICP-MS analyses) in hawaiite to trachyte lavas from the Ellittico volcanic sequence (Mt. Etna system, Sicily). Partition coefficients were determined for plagioclase, clinopyroxene, olivine, kaersutite and Ti-magnetite. These phases, along with apatite (not analysed in this work), constitute the common fractionating solid assemblage of alkaline magmas feeding Mt. Etna volcanic system. The obtained data set forms the first attempt to characterise the solid/melt trace-element partitioning for Etnean magmas, and can be usefully applied in other sites of alkaline volcanism. The partition coefficients are here used to define the scale of incompatibility of 29 trace elements and to asses the extent of differentiation processes and the prevailing oxygen fugacity of Ellittico magmas.ZusammenfassungEin Satz von Phänokristall/Matrix Verteilungs-Koeffizienten für bis zu 29 Spurenelemente wurde mittels ICP-MS Analytik in hawaiitischen bis trachytischen Laven der Ellittico Abfolge (Ätna-System, Sizilien) erhalten. Die Verteilungs-Koeffizienten wurden für Plagioklas, Klinopyroxen, Olivin, Kaersutit und Ti-Magnetit bestimmt. Diese Phasen, zusammen mit Apatit (in dieser Arbeit nicht analysiert), stellen die fraktionierende Assoziation von festen Bestandteilen in alkalischen Magmen, die das vulkanische System des Ätna versorgen, dar. Der Datensatz ist ein erster Versuch die Verteilung von Spurenelementen zwischen Festphasen und Schmelze für Magmen des Ätna zu charakterisieren und kann ebenso nutzbringend auf andere Gebiete mit alkalischem Vulkanismus angewandt werden. Die Verteilungs-Koeffizienten werden hier benützt, um das Ausmaß der Inkompatibilität von 29 Spurenelementen zu erfassen, und das Ausmaß der Differentiationsprozesse und der vorherrschenden SauerstoffFugazität der Ellittico-Magmen zu bestimmen.

A model of the Phlegraean Fields magma chamber in the last 10,500 years

Volcanological and petrological data suggest that the Phlegraean Fields volcanic activity has been fed, at least in the last 10,500 years, by a not-refilled magma chamber where trachytic residual liquids were produced by fractionation of a trachybasaltic magma. Using estimated volumes of the erupted products andP–T data obtained through petrological studies, a conductive thermal model of the chamber was built up in order to estimate its past and present size. Results suggest a volume decrease from approximately 14 to 1.4 km3 of the trachybasaltic magma in 10,500 years. Trachytic liquid would also be present in the chamber in a minimum amount of 0.4 km3. The model allowed some insights on the petrogenesis of the Phlegraean trachytes, suggesting that they were erupted as liquids because thermally buffered within the magma chamber.

Power law olivine crystal size distributions in lithospheric mantle xenoliths

Olivine crystal size distributions (CSDs) have been measured in three suites of spinel- and garnet-bearing harzburgites and lherzolites found as xenoliths in alkaline basalts from Canary Islands, Africa; Victoria Land, Antarctica; and Pali Aike, South America. The xenoliths derive from lithospheric mantle, from depths ranging from 80 to 20 km. Their textures vary from coarse to porphyroclastic and mosaic–porphyroclastic up to cataclastic. Data have been collected by processing digital images acquired optically from standard petrographic thin sections. The acquisition method is based on a high-resolution colour scanner that allows image capturing of a whole thin section. Image processing was performed using the VISILOG 5.2 package, resolving crystals larger than about 150 μm and applying stereological corrections based on the Schwartz–Saltykov algorithm. Taking account of truncation effects due to resolution limits and thin section size, all samples show scale invariance of crystal size distributions over almost three orders of magnitude (0.2–25 mm). Power law relations show fractal dimensions varying between 2.4 and 3.8, a range of values observed for distributions of fragment sizes in a variety of other geological contexts. A fragmentation model can reproduce the fractal dimensions around 2.6, which correspond to well-equilibrated granoblastic textures. Fractal dimensions >3 are typical of porphyroclastic and cataclastic samples. Slight bends in some linear arrays suggest selective tectonic crushing of crystals with size larger than 1 mm. The scale invariance shown by lithospheric mantle xenoliths in a variety of tectonic settings forms distant geographic regions, which indicate that this is a common characteristic of the upper mantle and should be taken into account in rheological models and evaluation of metasomatic models.

Experimental determination of plagioclase and clinopyroxene crystal growth rates in an anhydrous trachybasalt from Mt Etna (Italy)

Plagioclase and clinopyroxene crystal growth rates were experimentally determined in an anhydrous potassic trachy-basalt lava from the 1991–93 Mt Etna eruption. Experiments were performed at atmospheric pressure, in air, at temperatures variable from 1160 to 1240 °C. Growth rates, in potassic trachybasalt to shoshonitic melts, were calculated measuring both the width of overgrowth rims on crystal seeds and the sizes of microlite crystals: they are in the order of 10 −8 cm s −1 at low under-cooling ( −9 cm s −1 at higher undercoolings. The growth rates, determined for plagioclase are three-four orders of magnitude lower than those determined for this mineral in the same lava sample by other authors under water-saturated condition. Combining experimental data on plagioclase growth rates, water solubility in trachybasalt magmas, and crystal size distributions of Etnean lavas, we modelled the final ascent of magmas from shallow reservoirs located within the basement of the volcano in correspondence with known geological discontinuities. Calculations suggest that the final ascent of magmas occurs in a time span of 1–2 h with final velocities of 0.2–0.8 m s −1 , values compatible with actual output rates observed during the quiet effusion of degassed lavas at Mt Etna.