Lorella Francalanci

Geochemical and isotopic variations in the calc-alkaline rocks of Aeolian arc, southern Tyrrhenian Sea, Italy: constraints on magma genesis

New geochemical and isotopic data are reported for calc-alkaline (CA) volcanics of the Aeolian arc. Three main groups are recognized: the Alicudi and Filicudi volcanics in the western part of the arc; the Panarea, Salina and Lipari (henceforth termed PSL) volcanics in the central part of the arc and the Stromboli suite which makes up the eastern part of the arc. Each group is characterized by distinctive isotopic ratios and incompatible element contents and ratios. 87Sr/86Sr values (0.70352–0.70538) increase from west to northeast, and are well correlated with 143Nd/144Nd (ɛNd from +4.8 to -1.5). Pb isotope ratios are fairly high (6/4=19.15–19.54; 7/4=15.61–15.71; 8/4=38.97–39.36), with a general increase of 7/4 and 8/4 values from Alicudi to PSL islands and Stromboli. LILE contents and some incompatible element ratios (e.g. Ba/La, La/Nb, Zr/Nb, Rb/Sr) increase from the western to the central part of the arc, whereas HFSE and REE abundances decrease. Opposite variations are often observed in the volcanics toward the north-east from PSL islands. To account for these features and the decoupling observed between isotopic compositions and incompatible element abundances and ratios, it is suggested that a mantle source with affinities to the MORB source is “metasomatized” by slab-derived, crustal components. The proportion of crustal material entrained in the mantle source increases from Alicudi to Stromboli, according to the Sr and Nd isotope variations. It is also proposed that slab derived hydrous fluids play an important role, but which is variable in different sectors of the arc. This is attributed to the metasomatizing agent having variable fluid/melt ratios, reflecting different types of mass transfer from the subducted contaminant (probably pelagic sediments) to the mantle wedge. Thus, it is suggested that the slab derived end-member has a high hydrous fluid/melt ratio in the PSL mantle source and a correspondingly lower ratio in the Alicudi and Stromboli sources.

Volcanological and magmatological evolution of Stromboli volcano (Aeolian Islands): The roles of fractional crystallization, magma mixing, crustal contamination and source heterogeneity

The present paper reports the results of a detailed stratigraphical, petrological and geochemical investigation on the island of Stromboli, Aeolian arc, Southern Tyrrhenian sea. Major and trace element data determined on a large quantity of samples from well-established stratigraphic positions indicate that the magmatological evolution of the island through time was more complex than previously known. The activity of the exposed part of Stromboli, which occurred over a time span of about 100 000 years, started with the emission of high-K calc-alkaline (HKCA) volcanics, which were covered by calc-alkaline (CA), shoshonitic (SHO), high-K calc-alkaline (HKCA) and potassic (KS) products. The most recent activity consists of HKCA lavas and the present-day SHO-basaltic volcanics emitted by mildly explosive “strombolian” activity. Most of the products are lavas, with minor amounts of pyroclastic rocks emplaced mainly during the early stages of activity. The transition from the SHO to the KS cycle was associated with the collapse of the upper part of the volcanic apparatus; the transition from KS to the present-day SHO activity has been found to have occurred at the time of the sliding of the western portion of the volcano that generated the “Sciara del Fuoco” depression. The rock series cropping out at Stromboli show variable enrichment in potassium, incompatible trace elements and radiogenic Sr which increase from CA through HKCA, and SHO up to KS rocks. Major, trace element and Sr-isotopic data agree in indicating that the HKCA and SHO series evolved by crystal/liquid fractionation starting from different parental liquids, whereas crustal assimilation appears to have been the leading process during the evolution of KS volcanics. Mixing processes also played a role although they can be well documented only when they occurred between magmas with different isotopic and geochemical characteristics. Geochemical modelling based on trace element and isotopic data indicates that the mafic magmas of the different volcanic series may be generated by melting of an upper mantle heterogeneously enriched in incompatible elements and radiogenic Sr by addition, via subduction, of different amounts of crustal material. Geochemical data, however, are also in agreement with the alternative hypothesis that the most mafic magmas of the different series have been generated by combined processes of fractional crystallization, assimilation and mixing of a CA magma in a deep-sited magma chamber; the mafic magmas formed by these complex processes were successively emplaced in a shallow reservoir where they evolved by simple fractional crystallization (HKCA and SHO series) and by assimilation of crustal material (KS). The occurrence of changes in the geochemical signatures of the magmas at the time of the structural modification of the volcano is believed to favour the hypothesis that the variable composition observed in the volcanic rocks of Stromboli is the result of processes occurring within the volcanic system.

The volcanic activity of Stromboli in the 1906–1998 AD period: mineralogical, geochemical and isotope data relevant to the understanding of the plumbing system

Volcanic activity of Stromboli in the last 100 years was characterised by regular Strombolian eruptions with some occurrences of major explosions and paroxysms at the summit crater and lava flows down into the Sciara del Fuoco. Two types of juvenile fragments, shoshonitic to high-K basalts in composition, are peculiarly outpoured during major explosions: black scoriae, similar to those erupted by the normal Strombolian activity, representing a highly porphyritic (45–60 vol%), volatile-poor magma (HP magma) and a small volume of light pumice, representing a low-porphyritic (<10 vol%), volatile-rich magma (LP magma). Lava flows are constituted by the HP magma. The LP magma has a less evolved composition, lower incompatible trace element contents and Sr isotope ratios (0.70610) than the HP magma. Mineral phases in equilibrium with the LP and HP magmas have distinct compositions (Fo% in olivine: 80–85 and around 70, Mg# in clinopyroxene: 0.83–0.91 and around 0.75, An% in plagioclase: 80–90 and around 65, respectively), in spite of syn-eruptive mingling, and disequilibrium processes lead to large compositional ranges in minerals of both LP and HP magmas. Olivine and clinopyroxene of pumice were equilibrated at higher temperatures (and probably higher pressures) than the same crystals of scoriae. All these data provide evidence that the two magmas are characterised by sharply distinct physico-chemical conditions. It has also been pointed out that the plumbing system is under overall steady-state conditions. The HP magma resides at shallow level and evolves in a continuously erupting, crystallising (olivine+clinopyroxene+plagioclase) and replenished reservoir, which is fed by the LP magma. The LP magma derives from a deeper reservoir in which it undergoes limited crystallisation of femic phases. Plagioclase begins to crystallise only at lower depth. Significant geochemical, mineralogical and isotope variations with time in scoriae and lavas of the 20th century are observed. The MgO, V, Ni and Cr contents of magmas slightly increase from 1906 to 1930, then decrease from 1965 towards the present. Mineral chemistry data also vary accordingly. An increase of incompatible trace element contents after 1930 and a decrease of Sr isotope ratios after 1980–85 (from 0.70626 to 0.70616) have also been pointed out. These variations are thought to be due to changes in the crystallisation/replenishment equilibrium of the magma reservoir or to compositional modifications of the feeding magma. Some correlations between the main compositional variations and the type of eruptive activity also seem to be present. The occurrence of relatively frequent paroxysms during the first part of the 20th century, associated with the observed decrease of magma evolution, seems to indicate that more energetic explosions are associated with the rise of a higher volume of refreshing magma.

Sr isotope evidence for new magma inputs and short residence times in the XX century activity of Stomboli Volcano.

Abstract Stromboli is the type locality of continuous and moderately explosive volcanic activity. Monitoring temporal variations in the composition of the material erupted allows constraints to be made on the magma chamber dynamics and volcanic hazard. Here we present an Sr isotope survey of scoriae and lavas erupted from Stromboli volcano during this century. The material erupted is transitional between shoshonite and High-K basalt, with relatively constant major and trace element composition. This implies no substantial physical separation of minerals during crystallization (ca. 50 vol% minerals in both lavas and scoriae) and hence a relatively homogeneous reservoir. 87Sr/86Sr values are constant from 1900 to ca. 1980, then, beginning prior to the major lava flow eruption of December 1985, there is a smooth decrease. The Sr isotope decrease records the arrival of a new feeding magma, and allows estimation of the magma residence time and the volume of the reservoir beneath Stromboli volcano. Our results, along with a critical assessment of magma flux estimates, are best reconciled with steady state conditions, and establish the existence of a relatively small, ∼0.3 to 0.04 km3, reservoir with a magma residence time (τ) of ca. 19 years. Monitoring the temporal variation of isotopic ratios in active volcanoes appears a successful tool in forecasting some major volcanic eruptions.

Petrology and geochemistry of the ultrapotassic rocks from the Sabatini Volcanic District, central Italy: the role of evolutionary processes in the genesis of variably enriched alkaline magmas

Abstract The Sabatini Volcanic District (SVD) is a large volcanic field characterised by the lack of any major volcanic center. Its activity, spread over a wide area, started at 0.6 Ma and developed through five main phases, during which several calderas and the Bracciano lake volcano-tectonic depression were formed. All the volcanic rocks belong to the Roman-type ultrapotassic series (HKS), ranging from leucite tephrites to leucite and hauyne phonolites. Although the major- and compatible-element contents indicate a single series of evolution, there are differences in the incompatible trace-element abundances. A high-Ba series (HBaS) has been distinguished from a low-Ba series (LBaS), with the former also enriched in all other incompatible elements (e.g., REE, Nb, Zr, Th) except Rb. The HBaS rocks are plagioclase-free, leucite-bearing lavas and were abundantly outpoured from the Bracciano volcanoes during the third and fifth phase of activity. Plagioclase- and phlogopite-bearing rocks constitute the LBaS and were erupted during the other phases generally from smaller and eccentric volcanic centers. The initial 87 Sr/ 86 Sr values are higher in the HBaS rocks and do not vary significantly with magma evolution (0.71047–0.71080), but cover a wider range in the LBaS rocks (0.70944–0.71038), with the lowest Sr isotope ratios occurring in the least evolved lavas. The higher Ca content in the olivine and Ti and Al IV in the clinopyroxene, and the lower ulvospinel content of the Ti-magnetites of the HBaS rocks suggest an evolution at lower pressure and higher temperature for this magma. The observed petrologic characteristics suggest that the HBaS magma evolved at lower depths by processes of refilling, tapping, fractionation and probably assimilation (RTFA), where the crystallisation rate of clinopyroxene+leucite±olivine dominates over the input rate of the fresh magma. The LBaS magma evolved at slightly higher pressure, in separate and small magma bodies, by fractional crystallisation of clinopyroxene+plagioclase±phlogopite±olivine that was often associated with crustal assimilation (AFC). It has been suggested that RTFA processes with high input rate/crystallisation rate ratios could also be responsible for the differentiation between the HBaS and LBaS. The different processes of evolution undergone by the HBaS and LBaS could have been related to the different volumes of magma rising from the source.

Generation of oceanic-island basalt–type volcanism in the western Trans-Mexican volcanic belt by slab rollback, asthenosphere infiltration, and variable flux melting

Mantle plumes or decompression melting of a heterogeneous mantle have been proposed to explain oceanic-island basalt (OIB) type lavas found in the western Trans-Mexican volcanic belt. We show that mantle plumes cannot account for several geologic observations, whereas decompression melting is unrealistic given the low extension rate in the volcanic arc. On the basis of new geologic and geochemical studies, we propose an alternative model that reconciles geologic data, plate tectonic history, and petrology. Since 8.5 Ma, the volcanic front has migrated trenchward ∼80 km, and OIB began to be emplaced in the rear half of the arc after a period of stalled subduction of the Rivera plate between 7.2 and 4.8 Ma. As a whole, OIB accounts for ∼5% of the total volume of volcanism. Their trace element signatures require an enriched-mantle source, akin to the deep asthenosphere, but also indicate a minor involvement of a subduction component. We propose that during the period of very low convergence at the end of the Miocene, the leading edge of the Rivera slab started to sink into the mantle and continued rolling back until it reached the present dip of 45°. The sinking of the slab may have induced small amounts of asthenosphere to flow laterally into the opening mantle wedge, from the Gulf of California rift on the west and through the subducted Rivera-Cocos plate boundary on the east. This mechanism would produce veins of relatively enriched mantle, which would be able to produce melts with an OIB composition once the flux of fluids released from the subducted slab resumed.

Origin of magmas feeding the Plinian phase of the Campanian Ignimbrite eruption, Phlegrean Fields (Italy) : constraints based on matrix-glass and glass-inclusion compositions

Abstract Glass inclusions hosted by clinopyroxene and matrix glasses of pumices from the Plinian fall deposit associated with the Campanian Ignimbrite eruption were analysed in order to investigate processes that occurred at various stages of the magmatic evolution and to characterise the magma-withdrawal dynamics during the eruption. Petrographic, SEM–EDS, and electron microprobe analyses were performed on pumice samples, glass inclusions, and host minerals. Primary non-evolved glass inclusions were found both in homogeneous salite and in zoned diopside-salite clinopyroxenes. All the studied glasses are peralkaline alkali-trachytic in composition. The variation trends of glass inclusions and matrix glasses overlap for most elements except for alkalies and Cl. The least evolved compositions, however, are shown by glass inclusions found in salitic clinopyroxene at interfaces with diopside from the early erupted Plinian pumices (Lower Fall), whereas the most evolved compositions were found in the matrix glasses of the late-erupted Plinian pumices (Upper Fall). Excluding the most mafic glass inclusions, the observed compositional range is closely comparable to the range for ignimbrite juvenile clast compositions, indicating that the two eruptive phases drained the same magmatic reservoir. The compositional evolution of the Campanian Ignimbrite magmatic system seems to have resulted from different processes occurring at different stages: pre-eruptive magma mixing, due to an input of mafic magma into the reservoir, and crystal fractionation starting from different hybrid parental magmas to produce the most evolved compositions. These processes could have generated a vertically and laterally zoned magma chamber with multiple layers of variably hybrid magma extending from the input points of mafic magma. This type of zoning seems to better account for all the complex chemical and mineralogical characteristics of the Campanian Ignimbrite magmatic system, instead of a simple vertically zoned magmatic reservoir. A syn-eruptive mingling between the least evolved hybrid magmas and the most evolved melts characterised the first phase of Plinian eruption. This uncommon emptying of the magma chamber could be explained by the particular geometry of the reservoir.

Unusual coexistence of subduction-related and intraplate-type magmatism: Sr, Nd and Pb isotope and trace element data from the magmatism of the San Pedro–Ceboruco graben (Nayarit, Mexico)

Subduction-related and continental intraplate type magmatism coexisted in the San Pedro–Ceboruco (SPC) graben in western Mexico in a rather unusual close association. The magmatic systems belong to the Trans-Mexican Volcanic Belt (TMVB), the active volcanic arc linked to subduction of the Rivera and Cocos plates beneath the North American plate. Three different magmatic series are recognized in the San Pedro–Ceboruco graben: calc-alkaline, Na-alkaline, and transitional. The transitional series can be further subdivided into: Low-Ti; High-Ti and Amado Nervo groups. The different magmatic series are characterized by variable LILE/HFSE and LREE/HFSE ratios. The Na-alkaline series has the most radiogenic Nd compositions and the lowest 87 Sr/ 86 Sr (0.70320–0.70344). The calc-alkaline series shows the lowest eNd and the highest 87 Sr/ 86 Sr (0.70395– 0.70402) overlapping the values for the transitional High-Ti group (0.70390–0.70404). Sr isotopic compositions for transitional Low-Ti (0.70367–0.70394) and transitional Amado Nervo (0.70351–0.70389) groups are intermediate between those of the Na-alkaline and calc-alkaline rocks. All the studied rocks show similar 207 Pb/ 204 Pb (15.58–15.61) and 208 Pb/ 204 Pb (38.41– 38.65), but 206 Pb/ 204 Pb discriminates well the Na-alkaline series (18.90–19.03) from all the others (18.68–18.75). Compositional and isotopic data suggest that the different series derive from distinct parental magmas, which were generated by partial melting of a heterogeneous mantle source characterized by two different components. A depleted mantle (DM) component with low 206 Pb/ 204 Pb and an enriched (EM) component characterized by high 206 Pb/ 204 Pb. These two components

Crystal retention, fractionation and crustal assimilation in a convecting magma chamber, Nisyros Volcano, Greece

Nisyros island is a calc-alkaline volcano, built up during the last 100 ka. The first cycle of its subaerial history includes the cone-building activity with three phases, each characterized by a similar sequence: (1) effusive and explosive activity fed by basaltic andesitic and andesitic magmas; and (2) effusive andextrusive activity fed by dacitic and rhyolitic magmas. The second eruptive cycle includes the caldera-forming explosive activity with two phases, each consisting of the sequence: (1) rhyolitic phreatomagmatic eruptions triggering a central caldera collapse; and (2) extrusion of dacitic-rhyolitic domes and lava flows. The rocks of this cycle are characteized by the presence of mafic enclaves with different petrographic and chemical features which testify to mixing-mingling processes between variously evolved magmas. Jumps in the degree of evolution are present in the stratigraphic series, accompanied by changes in the porphyritic index. This index ranges from 60% to about 5% and correlates with several teochemical parameters, including a negative correlation with Sr isotope ratios (0.703384–0.705120). The latter increase from basaltic andesites to intermediate rocks, but then slightly decrease in the most evolved volcanic rocks. The petrographic, geochemical and isotopic characteristics can be largely explained by processes occurring in a convecting, crystallizing and assimilating magma chamber, where crystal sorting, retention, resorption and accumulation take place. A group of crystal-rich basaltic andesites with high Sr and compatible element contents and low incompatible elements and Sr isotope ratios probably resulted from the accumulation of plagioclase and pyroxene in an andesitic liquid. Re-entrainment of plagioclase crystals in the crystallizing magma may have been responsible for the lower 87Sr/86Sr in the most evolved rocks. The gaps in the degree of evolution with time are interpreted as due to liquid segregation from a crystal mush once critical crystallinity was reached. At that stage convection halted, and a less dense, less porphyritic, more evolved magma separated from a denser crystal-rich magma portion. The differences in incompatible element enrichment of pre-and post-caldera dacites and the chemical variation in the post-caldera dome sequence are the result of hybridization of post-caldera dome magmas with more mafic magmas, as represented by the enclave compositions. The occurrence of the quenched, more mafic magmas in the two post-caldera units suggests that renewed intrusion of mafic magma took place after each collapse event.

A West-East Traverse along the magmatism of the south Aegean volcanic arc in the light of volcanological, chemical and isotope data

Abstract The volcanic rocks of the South Aegean arc (SAAVA) form a chain from the Gulf of Saronikos (Susaki, Egina, Poros, Methana) at West, to an area close to the Anatolian coast at East (Kos, Nisyros and minor islands), through the central part (Milos and Santorini island groups). The volcanic activity began in the Lower Pliocene at Egina (4.7 Ma) and lasted until present days, with the still active Methana, Milos, Santorini and Nisyros volcanoes. The beginning of volcanism is younger in the central sector of the arc. Volcanic center location was controlled by large tectonic lineaments, most of them still active, trending E-W to NW-SE for the western part and mainly NE-SW for the central and eastern parts of the arc. Volcanic fields developed along ellipse shaped areas with the longest axis oriented perpendicular to the subduction front. In the western volcanic fields (Susaki, Egina-Poros-Methana and Milos), volcanic centers are mostly monogenetic and no composite volcanic structures are present. In the eastern sector of the arc, Santorini and Nisyros are important composite volcanoes with caldera structures. Volcanic rocks belong to the calc-alkaline and high-K calc-alkaline (mainly Pliocenic in age) series. Basalts are mainly present in Santorini island group. Magmas underwent to complex differentiation processes, dominated by crystal fractionation, often associated to crustal contamination and mixing-mingling. Large compositionally zoned magma chambers often fed highly explosive eruptions, especially in the central and eastern sectors of the arc. In the western and eastern parts, potassium content of erupted magmas decreased with time, probably due to the increase of partial melting degree of the mantle source. Trends of evolution tend to pass from calc-alkaline to tholeiitic from West to the Santorini volcanic field, back again to calc-alkaline toward the Nisyros volcanic field. Incompatible trace element contents are lower in Santorini mafic magmas. From West to East, Sr and Pb isotope ratios decrease, whereas Nd isotope ratios increase. Partial melting of a MORB-like astenospheric mantle, metasomatised by prevailing subducted sediments, is thought to produce the entire spectrum of parental magmas of SAAVA. Slab-derived fluids are generally reduced. Low Ba/La values suggest the occurrence of even lower fluid contents during magma genesis at Santorini. Total amount of subducted sediments involved in the magma genesis decreases from West to East. Alternatively, the West-East 87 Sr/ 86 Sr and 144 Nd/ 143 Nd variation of the Mediterranean sediments leads to suggest a similar variation in the subducted sediment composition. Most of the geochemical characteristics of the Santorini magmas can be explained by higher partial melting degrees of the mantle source, probably triggered by the greater lithosphere extension, inducing adiabatic upwelling of the mantle. The higher lithosphere extension also caused Santorini magmas to stop at shallower levels, thus preventing amphibole crystallisation and allowing a higher amount of mafic magmas to reach the surface. From the beginning of SAAVA magmatism, occurred in the external parts of the arc, the partial melting degrees of the magma mantle source seem to have increased with decreasing time and going towards the central sector of the arc.