87 research outputs found
Geologia e geomofologia dellâisola di Pantelleria: il settore di Cala Tramontana-Cala Levante
Evidence of basaltic magma intrusions in a trachytic magma chamber at Pantelleria (Italy)
In the last 50 ka basalts have erupted outside the margin of the young caldera on the island
of Pantelleria. The inner portion of the caldera has instead been filled by trachyte lavas,
pantellerite lavas and pumice fall deposits. This paper focuses on a low-volume benmoreite
lava topping the trachyte lava pile in the middle of the young caldera.
The mineral chemistry, including trace elements in clinopyroxene (LA-ICP-MS), suggests
that benmoreite is a hybrid product resulting from mixing between a trachytic magma and a
basaltic end member even more primitive than those erupted during the past 50 ka. The
principal inference is that basaltic magmas intruded the trachytic magma chamber below the
caldera and were erupted in recent times within the caldera and not only beyond, as the
distribution of basaltic centers would suggest. Data are used to discuss the relationship
between felsic and mafic magmas at Pantelleria
Petrology of some amphibole-bearing volcanics of the pre-ellittico period (102-80 ka) Mt. Etna
We present here petrological and
geochemical data on volcanics cropping out in
southern and northeastern walls of the Valle del
Bove (Mt. Etna), belonging to the Rocche, Serra
Giannicola Grande and Canalone della Montagnola
Units. These units constitute the remnants of several
volcanoes that were active in the time span 102-80
ka, i.e. before the growth of the Ellittico-Mongibello
strato-volcano. Their products, range in composition
from hawaiites to benmoreites. Amphibole
(kaersutite) is present as phenocryst in all the studied
rocks, and commonly shows breakdown coronas
of rhönite ± clinopyroxene and plagioclase formed
during magma ascent. Nevertheless, in mafic rocks,
amphibole occurs as an early liquidus phase enclosed
in a Ca-rich plagioclase (up to An87). We propose that
early cotectic crystallization of amphibole and Caplagioclase
may reflect H2O-rich melts. Variations in
major and trace elements among lavas erupted from
coeval centres, suggest that fractional crystallization
was the principal evolutionary process but at the same
time magmas feeding the various volcanoes belonging
to the Rocche Unit were more heterogeneous with
respect to the younger Units studied here
Cognate xenoliths in Mt. Etna lavas: witnesses of the high-velocity body beneath the volcano
Various xenoliths have been found in lavas of the
1763 (âLa Montagnolaâ), 2001, and 2002â03 eruptions at Mt.
Etna whose petrographic evidence and mineral chemistry
exclude a mantle origin and clearly point to a cognate nature.
Consequently, cognate xenoliths might represent a proxy to
infer the nature of the high-velocity body (HVB) imaged
beneath the volcano by seismic tomography. Petrography
allows us to group the cognate xenoliths as follows: i) gabbros
with amphibole and amphibole-bearing mela-gabbros, ii)
olivine-bearing leuco-gabbros, iii) leuco-gabbros with amphibole,
and iv) Plg-rich leuco gabbros. Geobarometry estimates
the crystallization pressure of the cognate xenoliths between
1.9 and 4.1 kbar. The bulk density of the cognate xenoliths
varies from2.6 to 3.0 g/cm3. P wave velocities (VP), calculated
in relation to xenolith density, range from 4.9 to 6.1 km/s. The
integration of mineralogical, compositional, geobarometric data,
and density-dependent VP with recent literature data on 3D
VP seismic tomography enabled us to formulate the first
hypothesis about the nature of the HVB which, in the depth
range of 3â13 km b.s.l., is likely made of intrusive gabbroic
rocks. These are believed to have formed at the âsolidification
frontâ, a marginal zone that encompasses a deep region (>5 km
b.s.l.) of Mt. Etnaâs plumbing system, within which magma
crystallization takes place. The intrusive rocks were afterwards
fragmented and transported as cognate xenoliths by the
volatile-rich and fast-ascending magmas of the 1763 âLa
Montagnolaâ, 2001 and 2002â03 eruptions
Experimental Crystallization of a High-K Arc Basalt: the Golden Pumice, Stromboli Volcano (Italy)
International audienceThe near-liquidus crystallization of a high-K basalt (PST-9 golden pumice, 49·4 wt % SiO2, 1·85 wt % K2O, 7·96 wt % MgO) from the present-day activity of Stromboli (Aeolian Islands, Italy) has been experimentally investigated between 1050 and 1175°C, at pressures from 50 to 400 MPa, for melt H2O concentrations between 1·2 and 5·5 wt % and {Delta}NNO ranging from â0·07 to +2·32. A drop-quench device was systematically used. AuPd alloys were used as containers in most cases, resulting in an average Fe loss of 13% for the 34 charges studied. Major crystallizing phases include clinopyroxene, olivine and plagioclase. FeâTi oxide was encountered in a few charges. Clinopyroxene is the liquidus phase at 400 MPa down to at least 200 MPa, followed by olivine and plagioclase. The compositions of all major phases and glass vary systematically with the proportion of crystals. Ca in clinopyroxene sensitively depends on the H2O concentration of the coexisting melt, and clinopyroxene Mg-number shows a weak negative correlation with {Delta}NNO. The experimental data allow the liquidus surface of PST-9 to be defined. When used in combination with melt inclusion data, a consistent set of pre-eruptive pressures (100â270 MPa), temperatures (1140â1160°C) and melt H2O concentrations is obtained. Near-liquidus phase equilibria and clinopyroxene Ca contents require melt H2O concentrations <2·7â3·6 and 3 ± 1 wt %, respectively, overlapping with the maximum frequency of glass inclusion data (2·5â2·7 wt % H2O). For olivine to crystallize close to the liquidus, pressures close to 200 MPa are needed. Redox conditions around {Delta}NNO = +0·5 are inferred from clinopyroxene compositions. The determined pre-eruptive parameters refer to the storage region of golden pumice melts, which is located at a depth of around 7·5 km, within the metamorphic arc crust. Golden pumice melts ascending from their storage zone along an adiabat will not experience crystallization on their way to the surface
High-resolution 40Ar/39Ar chronostratigraphy of the post-caldera (<20 ka) volcanic activity at Pantelleria, Sicily Strait
The island of Pantelleria (Sicily Strait), the type locality for pantellerite, has been the locus of major calderaforming
eruptions that culminated, ca. 50 ka ago, in the formation of the Cinque Denti caldera produced by the
Green Tuff eruption. The post-caldera silicic activity since that time has been mostly confined inside the caldera
and consists of smaller-energy eruptions represented by more than twenty coalescing pantelleritic centers
structurally controlled by resurgence and trapdoor faulting of the caldera floor. A high-resolution 40Ar/39Ar
study was conducted on key units spanning the recent (post-20 ka) intracaldera activity to better characterize
the present-day status (and forecast the short-term behavior of) the system based on the temporal evolution of
the latest eruptions. The new 40Ar/39Ar data capture a long-term (N15 ka) decline in eruption frequency with a
shift in eruptive pace from 3.5 kaâ1 to 0.8 kaâ1 associated with a prominent paleosol horizon marking the only
recognizable volcanic stasis around 12â14 ka. This shift in extraction frequency occurswithoutmajor changes in
eruptive style, and is paralleled by a subtle trend of decreasingmelt differentiation index. We speculate that this
decline probably occurred (i) without short-term variations in melt production/differentiation rate in a steadystate
compositionally-zoned silicic reservoir progressively tapped deeper through the sequence, and (ii) that it
was possibly modulated by outboard eustatic forcing due to the 140 m sea level rise over the past 21 ka. The
intracaldera system is experiencing a protracted stasis since 7 ka. Coupled with recent geodetic evidence of
deflation and subsidence of the caldera floor, the system appears today to be on a wane with no temporal
evidence for a short-term silicic eruption
The December 2018 eruption at Etna volcano: a geochemical study on melt and fluid inclusions
This study focus on the Mt Etna December 2018 eruption with the aim of investigating the geochemical characteristics of the feeding magma. New data on major and trace element geochemistry of olivine-hosted melt inclusions (MI) in volcanic products are presented together with the noble gas geochemistry of fluid inclusions (FI) in olivines. The noble gas geochemistry of fluid inclusions (FIs) in olivines was also investigated. The major element composition of MIs is variable from tephrite/trachybasalt to phonotephrite/basaltic trachyandesite, with SiO2 = 45.51â52.72Â wt%, MgO = 4.01â6.02Â wt%, and CaO/Al2O3 = 0.34â0.72. Trace element patterns of MIs present a typical enrichment in LILE and LREE, depletion in HFSE, and relatively fractionated REE patterns: (La/Lu) N= 18.8â41.08, with Eu/Eu* = (0.5â1.8). Positive anomalies in Sr (Sr/Sr* = 0.8â2.3) and Ba can be ascribed to the assimilation of plagioclase-rich cumulates in the magmatic reservoir. The variable Ba/La (9.8â15.8), K/Nb (260â1037), Ce/Nb (1.9â3.4), Rb/La (0.4â1.6), and Ba/Nb (10.8â25.8) ratios reveal mixing between two types of end-member magmas comparable to those emitted from 1) the 2001 Upper Vents and 2002â03 Northern Fissures (Type-1) and 2) the 2001 Lower Vents and 2002â03 Southern Fissures (Type-2), respectively. Type-2 represents a magma that was under the influence of a crustal component, whereas Type-1 is compatible with a HIMUâMORB-type heterogeneous mantle source. It appears that the 2018 MIs have captured the two different types of magmas, and the lack of homogenization may imply a very fast ascent (a few months). Compatible with the contemporary presence of primordial HIMUâMORB and crust-contaminated end-members are the data on noble gases from FI that highlighted an 3He/4He value of 6.5â6.6Ra. The hypothesis of two different types of magmas, identified by the trace element geochemistry in MIs, is, thus, reinforced by helium isotopic data on FI of the 2018 eruption together with data from other Etnean eruptions and allows the inference of a bicomponent magma mixing
Phase equilibrium constraints on the production and storage of peralkaline silicic magmas: insights from Kenya and Pantelleria.
The origin of peralkaline silicic rocks is still obscure and stands perhaps as one of the last major unsettled issues in classic igneous petrology. The debate goes back to the end of the 18th century and despite intensive petrological, geochemical and laboratory efforts the consensus has yet to emerge as to which mechanisms produce peralkaline derivatives. Bowen (1937) first proposed that the shift from metaluminous to peralkaline field was due to extensive fractionation of calcic plagioclase. Perhaps the best illustration of such an hypothesis is provided by the Boina rock series in the Ethiopian rift studied by Barberi et al. (1975). However, such an hypothesis still awaits experimental confirmation. A different view has been expressed for the origin of peralkaline rhyolites erupted in the central part of the Kenya Rift Valley. There, a partial melting of crustal protoliths has been advocated on the basis of geochemical arguments (Macdonald et al., 1987). The origin of peralkaline rocks at Pantelleria, the type locality of peralkaline rhyolites, is also a matter of debate. Mahood et al (1990) have proposed an origin via partial melting of Fe-rich differentiates of transitional basalts, whilst Civetta et al. (1998) have argued that pantellerites could be produced via extensive fractionation of their putative parent basalts. The diversity of opinions reflects in part that, presumably, there is not only one mechanism at work. But it is also due to the fact that most experimental studies devoted to the clarification of this problem have failed in producing decisive arguments during more than one century of intense debate
Evolution of the magma system of Pantelleria (Italy) from 190 ka to present
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, silicicmagmatismwas 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 (20â7 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
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