Experimental constraints on amphibole stability in primitive alkaline and calc-alkaline magmas

Equilibrium crystallization experiments were carried out on two primitive basaltic rocks (APR16: Na2O+K2O=4.40 wt%; CM42: Na2O+K2O=2.59 wt%) with the aim to investigate the amphibole stability in the differentiation processes at deep crustal level, of primitive alkaline (APR16) and calc-alkaline (CM42) magmas. The experiments were performed with different initial H2O contents (0-5 wt%), at pressure of 800 MPa, in the temperature range of 975-1225 °C. For the explored conditions, amphibole crystallization occurs in both compositions at H2O in the melt >7wt% while the temperature of their occurrence is lower in the alkaline composition (<1050 °C in APR16 and ≥1050 °C in CM42). Moreover, amphibole crystallization seems to be influenced by the Na2O/K2O ratio rather than the absolute Na2O content in the melt. This is evident when experimental results on the APR16 and CM42 are compared with experimental data obtained from a primitive ultrapotassic composition (leucite-basanite: Na2O+K2O=4.58 wt%) and with thermodynamic modelling by the Rhyolite-MELTS algorithm. The comparison shows that amphibole never saturates the leucite-basanite at any of the investigated/modelled conditions, even when an extended crystallization increases the Na2O of melts up to contents like those of calc-alkaline experimental glasses. We conclude that, at pressure of 800 MPa and hydrous conditions, only primitive liquids with Na2O/K2O ratio ≥0.9 are more prone to crystallize amphibole

Impulsive supply of volatile-rich magmas in the shallow plumbing system of Mt. Etna volcano

Magma dynamics at Mt. Etna volcano are frequently recognized as the result of complex crystallization regimes that, at shallow crustal levels, unexpectedly change from H2O -undersaturated to H2O-saturated conditions, due to the impulsive and irregular arrival of volatile-rich magmas from mantle depths. On this basis, we have performed hydrous crystallization experiments for a quantitative understanding of the role of H2O in the differentiation of deep-seated trachybasaltic magmas at the key pressure of the Moho transition zone. For H2O = 2.1–3.2 wt %, the original trachybasaltic composition shifts towards phonotephritic magmas never erupted during the entire volcanic activity of Mt. Etna. Conversely, for H2O = 3.8–8.2 wt %, the obtained trachybasalts and basaltic trachyandesites reproduce most of the pre-historic and historic eruptions. The comparison with previous low pressure experimental data and natural compositions from Mt. Etna provides explanation for (1) the abundant release of H2O throughout the plumbing system of the volcano during impulsive ascent of deep-seated magmas; (2) the upward acceleration of magmas feeding gas-dominated, sustained explosive eruptions; (3) the physicochemical changes of gas-fluxed magmas ponding at shallow crustal levels; and (4) the huge gas emissions measured at the summit craters and flank vents which result in a persistent volcanic gas plume

Geological constraints for a conceptual evolutionary model of the slope deformations affecting Mt. Nuovo at Ischia (Italy)

ischia island was the scenario of several Holocene slope in- stability events occurred at different scales, from shallow mass movements, triggered by meteo-climatic forcing, up to massive rock slope failures such as large debris avalanches these last ones related to the volcano-tectonic dynamics of a resurgent caldera. the present study focuses on the gravitational deformation that in- volves Mt. nuovo, located in the western portion of Mt. epomeo resurgent block. a high-resolution engineering-geological model was reconstructed according to a multi-modelling approach sup- ported by field geo-structural evidences and constrained by pas- sive seismic investigations. it revealed a complex morpho-struc- tural setting and led to the identification of a multiple compound mechanism, involving a rock mass volume of about 190 million of cubic meters. the obtained geological model shows a partial structural control of the pre-existing tectonic pattern on slope deformation mechanisms, highlighting geometric and volumetric similarities between the Mt. nuovo ongoing deformation and an already oc- curred rock avalanche. the defined conceptual evolutionary mod- el allows to hypothesize the role of inner pressures constraining the shear zone initiation and propagation and making reliable a future scenario of generalized collapse. Starting from these new field and laboratory data, numerical models will be reconstructed in order to depict the evolution of the gravitational slope deformation, evaluate its sensitivity and constrain future evolutionary instability scenarios

Cenozoic magmatism of north Victoria Land, Antarctica: an experimental study on the mantle source of a primary basanite from the McMurdo Volcanic Group

Volcanoes of the McMurdo Vocanic Group (MMVG) (Antarctica) dot the eastern shoulder of Ross Sea Rift System giving rise to alkaline transitional volcanic suites which in north Victoria Land are emplaced since Early Cenozoic. Geochemical geological, geophysical and geochronological data on Cenozoic volcanic activity in NVL suggest that the region is a site of passive astenospheric rise, rather than affected by a thermally active mantle plume. Furthermore the comparison of geochemical and isotopical data of basic lavas with those provided by mantle xenoliths they carry to the surface, document the compositional heterogeneity of sublithospheric mantle caused by the coupled action of partial melting and metasomatism. In particular the metasomatic episode is probably linked to the amagmatic extensional event that affected the West Antarctic Rift System in the Late Cretaceous. The astenospheric melts generated during this event, moving through the upper mantle, can have crystallized as veins or may have led to the formation of metasomatic minerals such as amphibole or phlogopite. In this scenario the mineralogical and chemical composition of sources responsible for Cenozoic magmatism, amphibole-bearing spinel-peridotite versus pyroxenite in the garnet stability field, it is still a matter of debate. To shed light on this argument a previous experimental study on a basanite of MMVG, representative of primary magma (Orlando et al., 2000) has been integrated with new experimental investigation on the same basanitic composition. The preliminary experiments were conducted to pressures of 1.0 - 2.0GPa in the presence of 0-1% of added water and indicate olivine on the liquidus at 1.0 GPa that is substitute by clinopyroxene at 2.0GPa. The addition of 1% of water induces a decrease of liquidus temperature of about 40°C shifting its value in the T range (1280-1310°C) the same that was inferred by melt inclusions hosted in the olivine phenocrysts of the studied basanit

The italian quaternary volcanism

The peninsular and insular Italy are punctuated by Quaternary volcanoes and their rocks constitute an important aliquot of the Italian Quaternary sedimentary successions. Also away from volcanoes themselves, volcanic ash layers are a common and frequent feature of the Quaternary records, which provide us with potential relevant stratigraphic and chronological markers at service of a wide array of the Quaternary science issues. In this paper, a broad representation of the Italian volcano logical community has joined to provide an updated comprehensive state of art of the Italian Quaternary volcanism. The eruptive history, style and dynamics and, in some cases, the hazard assessment of about thirty Quaternary volcanoes, from the north ernmost Mt. Amiata, in Tuscany, to the southernmost Pantelleria and Linosa, in Sicily Channel, are here reviewed in the light of the substantial improving of the methodological approaches and the overall knowledge achieved in the last decades in the vol canological field study. We hope that the present review can represent a useful and agile document summarising the knowledege on the Italian volcanism at the service of the Quaternary community operating in central Mediterranean area

Cenozoic thermal evolution of lithospheric mantle in northern Victoria Land (Antarctica): Evidences from mantle xenoliths

Thermobarometry of spinel peridotites collected in northern Victoria Land in Cenozoic basalts of the Mt Melbourne Volcanic Province, reveals warming of local lithospheric mantle, marked by a shift of geothermal gradient from 0.5. °C/km to ̃3. °C/km during the development of Cenozoic magmatism related to the opening of the Ross Sea rift system. We suggest that a significant uplift fraction of the rift shoulder on the West Antarctic margin is due to the change in olivine molar volume induced by mantle heating. The heat source is provided by enhancement of asthenospheric convection induced by an "edge effect" in the mantle circulation, following the opening phase of the Ross Sea. Besides explaining the asymmetric uplift of the rift shoulder corresponding to the Transantarctic Mountains, this mechanism satisfactorily accounts for the time scale (̃10. Ma) and most of total uplift (̃3000m) of the western border of the Ross Sea in northern Victoria Land. © 2010 Elsevier B.V

Clinopyroxene growth and dissolution rate: constraints on the deep level ascent rate of a K- basaltic magma from the Campi Flegrei Volcanic District

The estimation of the magma ascent rate in the Campi Flegrei Volcanic District (CFVD, south Italy) is of paramount significance in terms of volcanic hazard. Indeed, deeplevel ascent rates may be the key to understand the triggering mechanisms of volcanic eruptions and are essential for understanding the rates at which magmas are supplied to volcanic complexes. Thus, to investigate the CFVD magmas transport at Moholower crust depth and provide an estimate of magma recharge of the deep reservoirs, we assessed the cooling rate and the deeplevel ascent rates of Kbasaltic magmas by combining the clinopyroxene growth rate determined by highpressure crystallization experiments with data from crystal size distribution analyses and thermobarometry of clinopyroxenes occurring in the most primitive scoria clasts of the CFVD. In addition, since only few studies have considered the role played by crystal dissolution phenomena during the crystalmelt interaction and crystal growth, we investigated the role of crystal dissolution in the estimation of magma ascent rate by performing a series of dissolution experiments. In particular, clinopyroxene growth and dissolution rates were experimentally determined in a Kbasaltic rock from Procida island (CFVD) through a series of experiments performed at 0.8 GPa by using the piston cylinder apparatus available at the HPHT Laboratory of the Department of Earth Sciences of Sapienza University of Rome (Italy). Crystallization experiments were carried out at 10301250 °C, 1 ≤ H2O ≤ 4 wt.% and dwell times of 0.25, 3, 6 and 9 hours. Overall, growth rate reaches a maximum value in the shortest experiments (~ 3·107 cm s1) decreasing to ~ 1x108 cm s1 in the longest duration runs. Partition coefficients based on the crystalliquid exchange show that mineral chemistry progressively approaches equilibrium with increasing run duration. Furthermore, the combination of the determined growth rates with data from thermobarometry and from crystal size distribution analyses of clinopyroxenes in the most primitive scoria clasts of the CFVD, suggests that recharge by primitive magma in the deep reservoirs occurs with a relatively high ascent rate of ~ 0.5x104 m s1. Dissolution experiments, instead, were performed at superliquidus temperatures of 1300 and 1350 °C and dwell times between 0.5 and 2 hours. The calculated dissolution rates are in the order of ~105106 cm s1 and results significantly controlled by temperature, while they are not pressure and time dependent. The role of crystal dissolution in the estimation of magma ascent rate has been tested for a natural magmatic system, by interpolating the obtained dissolution rates with the textural data of clinopyroxene crystals from the AgnanoMonte Spina pyroclastic deposits at Campi Flegrei (Italy). Calculations indicate that the time required for partial or complete resorption of these clinopyroxene crystals varies from ~0.5 to ~40 hours, and that the effect of crystal dissolution may be relevant on the estimates of magma residence times if significant dissolution occurs during magma mixing processes

Cumulate xenoliths from Mt. Overlord, northern Victoria Land, Antarctica: A window into high pressure storage and differentiation of mantle-derived basalts

The alkaline basaltic magmas at Mt. Overlord (northern Victoria Land, Antarctica) entrained abundant ultramafic xenoliths (wehrlites, clinopyroxenites and hornblendites). Textures, bulk rock compositions, mineral chemistry and thermobarometric calculations indicate that the xenoliths represent cumulates that crystallised at the mantle–crust boundary. In particular, the major and trace element compositions of the bulk rocks and minerals indicate that the Mt. Overlord cumulates were formed through processes of crystal fractionation that affected hydrous basanitic magmas. Some of the xenoliths have textural features that suggest a lengthy (> 13 Myr) post-emplacement history at relatively low temperatures (1050–1100 °C) and high pressures (0.8 to 1.4 GPa) and that their primary parental melts were therefore related to the earliest phases of Cenozoic magmatism. These processes produced a “wet and hot deep zone” that had a strong influence on the thermochemical evolution of the lower crust beneath Mt. Overlord