42 research outputs found
Geologia e geomofologia dell’isola 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 (“La Montagnola”), 2001, and 2002–03 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 3–13 km b.s.l., is likely made of intrusive gabbroic
rocks. These are believed to have formed at the “solidification
front”, a marginal zone that encompasses a deep region (>5 km
b.s.l.) of Mt. Etna’s 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 “La
Montagnola”, 2001 and 2002–03 eruptions
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 ka−1 to 0.8 ka−1 associated with a prominent paleosol horizon marking the only
recognizable volcanic stasis around 12–14 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
Hydrothermal processes with REE enrichment in Fuerteventura carbonatites: evidences in accessory minerals
Los diques carbonatíticos que afloran en la parte centro-occidental del Complejo Basal de Fuerteventura (Ajuy, Punta de la Nao),
fueron afectados por el evento intrusivo mioceno responsable del
metamorfismo de contacto y fusión parcial de parte del complejo
alcalino-carbonatítico. Las características texturales, los procesos
de formación de monacita enriquecida en LREE (La, Ce, Nd) en microfracturas que afectan a los cristales de zircón, evidencian interacciones de las carbonatitas con fluidos hidrotermales y removilización parcial de REE. El pirocloro es el principal mineral reservorio
de tierras raras (suma REE=216694 ppm), con valores elevados de
LREE (LaN=185816 y CeN=165209) respecto a las HREE (YbN=2589
y LuN=1814) con un espectro bastante inclinado (La/Yb)N=72 en el
diagrama de abundancia de REE. La paragénesis mineral representada por los interestratificados illita/esmectita, clorita y vermiculita
permite encuadrar el evento hidrotermal en facies de las ceolitas (Ta
≈ 100-200 °C)Carbonatitic dykes that crop out in the central western part of
Fuerteventura Basal Complex (Ajuy, Punta de la Nao), were affected by the Miocene intrusive event responsible for contact metamorphism and partial melting of part of the alkaline-carbonatitic
complex. Textural characteristics, monazite enriched in LREE (La, Ce,
Nd) formation processes in microfractures that affect zircon crystals,
show carbonatite interactions with hydrothermal fluids and partial
REE remobilization. Pyrochlore is the main mineral carrier of rare
earth elements (sum REE=216694 ppm), with high LREE (LaN=185816
y CeN=165209) with respect to HREE (YbN=2589 y LuN=1814) with a
rather steep pattern (La/Yb)N=72 in REE diagram. The mineral paragenesis represented by the interstratified illite/smectite, chlorite and
vermiculite allows to frame the hydrothermal event in zeolite facies
(T° ≈ 100-200 °C
A two-component mantle source feeding Mt. Etna magmatism; insights from the geochemistry of primitive magmas.
The major elements, trace elements and Sr and Nd isotopes of selected Etnean primitive rocks (b15 ky BP) were
studied in order to characterize their mantle source. The noble-gas geochemistry of fluid inclusions in minerals
fromthe same lavaswas also investigated. Themajor element compositions ofwhole rocks and minerals showed
that these products are among the most primitive atMt. Etna, comprising 6.3–17.5 wt.% MgO. The variable LREE
(Light Rare Earth Elements) enrichment relative to MORB (Mid-Ocean Ridge Basalt) (Lan/Ybn = 11–26), togetherwith
the patterns of certain trace-element ratios (i.e., Ce/Yb versus Zr/Nb and Th/Y versus La/Yb), can be attributed
to varying degrees of melting of a common mantle source. Numerical simulations performed with the
MELTS program allowed the melting percentages associated with each product to be estimated. This led us to recalculate
the hypothetical parental trace-element content of the Etneanmantle source, whichwas common to all
of the investigated rocks. The characteristics of the Sr, Nd and He isotopes confirmed the primitive nature of the
rocks,with themost-depleted and primitive lava being that ofMt. Spagnolo (SPA; 143Nd/144Nd = 0.512908 87Sr/
86Sr = 0.703317–0.703325 and 3He/4He = 7.6 Ra), and highlighted the similarity of the mantle sources feeding
the volcanic activity of Mt. Etna and the Hyblean Plateau (a region to the south of Mt. Etna and characterized by
oldermagmatismthan Mt. Etna). The coupling of noble gases and trace elements suggests an origin for the investigated
Etnean lavas from melting of a Hyblean-like mantle, consisting of a two-component source where a
peridotitic matrix is veined by 10% pyroxenite. A variable degree of mantle contamination by crustal-like fluids,
probably related to subduction, is proposed to explain the higher Sr-isotope and lowerNd-isotope values in some
rocks (143Nd/144Nd up to 0.512865 and 87Sr/86Sr up to 0.703707). This process probably occurred in the source
prior tomagma generation, refertilizing some portions of themantle. Accordingly, the estimated degree of melting
responsible for each magma appears to be related to its 87Sr/86Sr enrichment. In contrast, the decoupling
between 3He/4He and 87Sr/86Sr ratios requires the occurrence in the crustal reservoirs of further processes capable
of shifting the He isotope ratio towards slightly more radiogenic values, such as magma aging or a contribution
of shallow fluid. Therefore, different residence times in the Etnean reservoir and/or various rates of magma
ascent could be key parameters for preserving the original He isotope marker of the Etnean mantle source.
© 2013 Published by Elsevier
A two-component mantle extending from Hyblean Plateau to Mt Etna (Eastern Sicily) as inferred by an integrated approach with noble gases, trace elements and isotope geochemistry.
We carried out a geochemical investigation of the mantle beneath Hyblean and Etnean area through ultramafic xenoliths (peridotites and pyroxenites) retained in Miocenic age Hyblean volcanics and primitive Etnean lavas and tephra, respectively. Major and trace elements and Sr-Nd isotopes (whole rock and /or minerals) were analysed together with noble gases entrapped in fluid inclusions hosted in olivines and pyroxenes phenocrysts.
The geochemical results from Hyblean xenoliths study highlighted the presence of two distinct compositional groups: the peridotites, featured by a more enriched geochemical fingerprint (3He/4He ∼7 Ra, 143Nd/144Nd ∼0.5129 and Zr/Nb ∼ 4) whereas the pyroxenites, characterized by a more primitive character (3He/4He up to 7.6 Ra, 143Nd/144Nd ∼0.5130 and Zr/Nb ∼30). Our interpretation is that metasomatic processes interested the Hyblean lithosphere and the pyroxenites (former primitive mantle melts) represent the metasomatizing agent. During their ascent these primitive melts permeated the peridotitic mantle at different levels, producing a variable degree of refertilization. The metasomatic processes affected distributions of both trace elements and noble gases, even though these geochemical tracers displayed very different sensitivity to the effects of metasomatic mixing between two end-members.
The investigated primitive Etnean magmas showed a variable REE enrichment respect to MORB (Lan/Ybn =11-26) and isotopic values of Sr, Nd and He in the following ranges: 143Nd/144Nd= 0.512869 -0.512896; 86Sr/87Sr=0.70330 - 0.70370; 3He/4He = 7-7.6 Ra. A variable melting degree of a common mantle source together with a variable level of crystallization and crustal contamination is hypothesised to explain the variations exhibited by the above Etnean dataset. Numerical simulation performed on MELT code allowed to estimate the trace elements content of the Etnean mantle source. These results, joined to the most primitive isotopic values of He- Sr- Nd among the investigated products helped to geochemically characterize the mantle beneath the Etnean area suggesting a strict relation with that Hyblean. Indeed, the modeled Etnean source locates on the mixing zone between the Hyblean peridotite and pyroxenite, so testifying a simultaneous contribution of two components in the genesis of the investigated lavas and supporting the hypothesis of an heterogeneous and metasomatized lithosphere common to both areas