Constraints on crustal recycling from boron isotopes in Italian melt inclusions

Boron represents an important tracer of crustal recycling processes in subduction zones, because it is readily mobilised from the subducted lithosphere and different components in the slab are isotopically distinct. Profiles of boron content and isotope ratio across magmatic arcs generally show that B concentrations decrease with increasing slab depth, which implies decreasing amount of slab-derived fluids. To date, however, data on continental-collision zones and post-collisional subduction settings are scarce. This study examines Plio-Quaternary Italian magmatism to quantify crustal recycling in a complex subduction setting. Magmatic products vary from (ultra)potassic along the Tyrrhenian side in the north, to calcalkaline and Na-alkaline in the south. Combined major and trace element and [B] content and δ11B values are reported in 99 Melt Inclusions (MIs), analyses from a wide range of Italian lavas. [B] vary from 4 to 298 µg/g and δ11B from -29.2 to -3.9‰. The B isotopic values are considerably lower than previously reported in arcs and other post-collisional setting magmatism. We infer a role for phengite in the source of all studied Italian magmas (with the exception of Mt. Etna lavas). This white mica is stable to high pressures in subducted sediments of altered oceanic crust and records dehydration and 11B depletion due to dehydration processes. MIs hosted in highly fosteritic olivines (Fo &gt;74; median of 89) from across Italy reveal that primary melts tap heterogeneous mantle including subducted oceanic and continental components that were introduced during the Alpine, and Adriatic and Ionian subduction phases. The combined geochemical data record the involvement of sediments that variably metasomatized the mantle wedge. We propose that slab detachment and consequent heat input from the inflow of hot asthenosphere was responsible for phengite breakdown in subducted sediments and locally produced metasomatism of the mantle wedge, imposing a characteristic B isotope signature to the overlying mantle. Continued heating due to asthenosphere inflow led to melting of the metasomatized mantle wedge and generation of the Italian magmatism. Mt. Etna represents an exception being dominated by asthenosphere upwelling through a slab window with minimal influence from active subduction.</p

Mt. Etna primary melts from 600 ka to the present day characterized by geochemistry of melt inclusions

The geochemical and isotopic variability of tholeiitic/calcalkaline volcanic products in the southern region of Italy suggest the involvement of an HFSE-enriched, OIB-type mantle component. The Sicily province includes recent to active volcanoes in eastern Sicily (Etna, Iblei), Sicily Channel, Ustica and Prometeo, which are host from tholeiitic to Na-alkaline lavas. The origin of Sicily magma's diversity is debated, but the prevailing hypothesis is that it results from melting a heterogeneous mantle influenced by subducting Ionian lithosphere and interaction with an ascending plume. To address the genesis of the Sicilian magmatism as a function of time, we study olivine-hosted melt inclusions (MIs) from Etna. Etna's magmatic evolution consists of six volcanic stages, started 600 ka ago with submarine tholeiitic lavas and continued until present days eruptions of Na-alkaline products. Here we present the geochemistry of MIs from Tholeiitic (542 & 332 ka), Timpe (154 – 126 ka), AAV (102 ka) and Mongibello (1669 AC) stages. Homogenized MIs are hosted by high-Fo olivine for Tholeiitic stage (Fo 90.5-87) and Timpe stage (Fo 90.5–74), and moderate Fo for AAV and Mongibello stages (Fo 81-72). Spinel from the Tholeiitic and Timpe stages show lower Cr# (~0.5) compare to the alkaline ones (~0.8). Studied MIs demonstrate a wide compositional diversity reflecting the variation of parental melt groups for the separate Etna magmatic stages. Tholeiitic melts differ from all other stages (alkaline melts) by low K2O, P2O5, depleted trace elements and high SiO2, with more refractory spinel suggesting a primitive mantle source for this first Etna magmatic stage. Alkaline MIs from 102 ka – 1669 have similar major and trace element compositions to recent alkaline lavas and published MIs. In contrast, the alkaline MIs from the Timpe stage (K2O 1-3 wt.%) differ from alkaline lavas and MIs from all other stages by higher TiO2, Al2O3, CaO, P2O5, SO3 and low SiO2. Our results indicate that the mantle under Etna is very heterogeneous and requires the involvement of at least two different lherzolite mantle sources for magmas of Tholeiitic and Timpe stages, and a contribution of subduction-derived components for magmas for the more recent stages

The mantle source of lamproites from Torre Alfina, Italy: Evidence from melt inclusions in olivine

The complex post-collisional subduction setting of peninsular Italy, in the central-western Mediterranean region, has given rise to an extremely diverse spectrum of potassium-rich volcanic rocks. The most primitive of these products show trace-element and radiogenic isotope signatures that point to melt derivation from upper mantle domains affected by metasomatism associated with sediment recycling. The style and extent of this metasomatism, and the metasomatic agents responsible for this modification, seem to differ significantly throughout the Italian peninsula. The lamproites of the Tuscan magmatic province, central Italy, are a peculiar and rare example of rocks that require extensive source modification that is not yet well-understood. These rocks are ultrapotassic and mafic in composition and have high compatible trace-element contents. Although bulk-rock compositions have been used to interrogate their petrogenesis, bulk lavas do not reflect the full heterogeneity of their mantle source. Here, we study the geochemistry of melt inclusions in forsterite-rich olivine, which in contrast to their host lavas are snapshots of near-primary melts that have bypassed modification on their way to the surface. The olivines (Fo88-93) from the studied lamproites of Torre Alfina host melt inclusions with major- and trace-element compositions that define two distinct groups. The first is marked by lower SiO2 (47–51 vs. 50–60 wt%) and higher K2O (11–17 vs. 8–14 wt%), CaO (3.5–6 vs. 1.5–5 wt%), TiO2 (1.8–2.4 vs. 0.3–1.8 wt%), P2O5 (1.0–1.7 vs. 0.1–0.9 wt%) and different trace-element contents. Group-1 melts are generally similar to other Tuscan lamproites, whereas group-2 melts are, in terms of trace elements, more akin to the Tuscan high-K calc-alkaline mafic rocks. We interpret these two melt types to originate from a sediment-metasomatised mantle source, which is characterised by distinct (vein) lithologies arising from superimposed metasomatic events. The Sr-Nd-Pb isotope compositions of a subset of the studied inclusions, analysed by wet chemistry and TIMS techniques, will be presented to further constrain the mantle source of these unusual and hitherto unreported primitive melt compositions, and ultimately better understand lamproite petrogenesis

Investigating combined arc and OIB signatures at a post-collisional subduction setting by geochemical and boron isotope analyses of melt inclusions from Vulture, Italy

Recent post-collisional magmatism in central-southern Italy is unique as it is strongly influenced by sediment subduction but also has an intra-plate signature. The composition of the potassium-rich magmatic products covers a wide range of compositions, from subalkaline to strongly alkaline, and from mafic to felsic. The Vulture volcanic centre, located east of the main volcanic front, is considered “anomalous” compared to the other major Quaternary volcanoes, as it shows the eruption of silica-rich and carbonatite lavas, and a magma source with both arc- and OIB-type signatures. To investigate the unique nature of this anomalous magmatism, we analysed 107 Vulture melt inclusions (MIs) trapped in high-forsterite olivine (~87-90 mol% Fo) for major and trace element composition. A subset of 27 MIs was selected for boron isotope and concentration analysis. Based on relative major and trace element enrichment we distinguish two groups of inclusions: Group 1: High CaO (10-16wt.%), TiO2 (1-3 wt.%), Na2O (~ 3wt.%), MgO (4-9 wt.%; n = 80), lower HFSE/HREE and lower LILE/LREE (n = 44); Group 2: Low CaO (6-7 wt.%), TiO2 (0.8-1.5 wt.%), high SiO2 (45-48 wt.%), Al2O3 (18-20 wt.%), K2O (5-7 wt.%; n = 27) and higher LILE/HREE and HFSE/LREE (n = 24). Group 1 MIs have more negative δ11B values (δ11Bav = -20 ‰; n = 23) and lower B concentration ([B]av = 20 ppm; n = 23) compared to Group 2 (δ11Bav = -17 ‰; [B]av = 36 ppm; n = 4). The geochemical distinction between the two groups indicates the involvement of two melt sources with diverse mineralogies. Combining major and trace elements with a more negative δ11B signature of Group 1, suggests a possible additional input of marly sediments to this group. Geophysical data confirm the presence of a slab detachment and mantle inflow under the Vulture volcanic centre, likely responsible for the OIB signature. The geochemistry of the MIs indicates that the OIB signature for this volcano is possibly derived from melts formed due to slab detachment that mix with melts from a sediment metasomatised source

Can we quantify sediment recycling in Italy's post-collisional subduction system?

Recycling of Earth's crustal components through subduction contributes to the observed geochemical heterogeneity in worldwide lavas, yet quantifying the in- and output fluxes is difficult because of the unknown compositions of subducted components and sediment transfer processes in subduction zones. Italian post-collisional magmatism is often mafic but potassiumrich, suggesting a significant contribution of subducted sediments in this complex geodynamic setting. Isotopic and elemental variability in the volcanic products across Italy likely reflects sediment recycling with variable composition and quantity from north to south. Here we report the geochemical compositions of sediments that accreted to the Apennine accretionary prism whose lateral counterparts have potentially subducted and contributed to the Italian melt source. The aim is to use the major-, trace- and Sr- Nd-Pb isotope compositions of the sediments and Italy's volcanic products to quantify subduction recycling through melt modelling. Sediments were collected from the northern-, central- and southern Apennines (Liguria, Emilia-Romagna, Umbria and Calabria) with a focus on exhumed units from below the various decollement levels. These included Triassic to Jurassic deep sea sediments in ophiolitic sequences deposited in the Ligurian- Piemonte Oceanic Basin, and Triassic to Neogene distal units of the Adria continental margin. End-member compositions are defined by deep sea clays and metapelites rich in K2O, SiO2, LILE, HFSE, REE with high 87Sr/86Sr (0.7458) and 206Pb/204Pb (19.4), and marls poor in K2O, SiO2, LILE, HFSE, REE, but rich in CaO and Sr, with low 87Sr/86Sr (0.7083) and 206Pb/204Pb (18.7). The geochemical compositions of the most primitive volcanics and olivine-hosted melt inclusions will be used to reconstruct subduction recycling processes by melt modelling of a sediment metasomatized mantle wedge. Sediment transport mechanisms, sediment/vein mineralogy, melting behavior, and melt extraction processes will be evaluated

Fuzzy petrology in the origin of carbonatitic/pseudocarbonatitic Ca-rich ultrabasic magma at Polino (central Italy)

The small upper Pleistocene diatreme of Polino (central Italy) is known in literature as one of the few monticellite alvikites (volcanic Ca-carbonatite) worldwide. This outcrop belongs to the Umbria-Latium Ultra-alkaline District (ULUD), an area characterized by scattered and small-volume strongly SiO2-undersaturated ultrabasic igneous rocks located in the axial sector of the Apennine Mts. in central Italy. Petrographic and mineralogical evidences indicate that Polino olivine and phlogopite are liquidus phases rather than mantle xenocrysts as instead reported in literature. The presence of monticellite as rim of olivine phenocrysts and as groundmass phase indicates its late appearance in magma chambers at shallow depths, as demonstrated by experimental studies too. The absence of plagioclase and clinopyroxene along with the extremely MgO-rich composition of olivine (Fo92–94) and phlogopite (average Mg# ~93) suggest for Polino magmas an origin from a carbonated H2O-bearing mantle source at depths at least of 90–100 km, in the magnesite stability field. In contrast with what reported in literature, the ultimate strongly ultrabasic Ca-rich whole-rock composition (~15–25 wt% SiO2, ~31–40 wt% CaO) and the abundant modal groundmass calcite are not pristine features of Polino magma. We propose that the observed mineral assemblage and whole-rock compositions result mostly from the assimilation of limestones by an ultrabasic melt at a depth of ~5 km. A reaction involving liquidus olivine + limestone producing monticellite + CO2 vapour + calcite is at the base of the origin of the Polino pseudocarbonatitic igneous rocks

Obsolescenza dell’umano. Günther Anders e il contemporaneo

Il pensiero e la produzione teoretica di uno dei più grandi filosofi del Novecento, Günther Anders, la cui riflessione si rivela sempre più decisiva per la comprensione della complessa fenomenologia del contemporaneo, sono il cuore dei saggi contenuti in questo libro. Essi indagano le originali idee di Anders spaziando dalle questioni politiche agli interrogativi etici che animarono il suo attivismo critico, attraversando il suo originale approccio estetico e il suo apporto nell’ambito della critica letteraria. Un pensiero originale che viene così fruttuosamente messo a confronto con quello di molti tra i più importanti intellettuali coevi, come Arendt, Adorno, Benjamin, Heidegger, Freud, Lacan, Levi, Montale, Morselli, Pasolini, Eco e altri, con l’auspicio di segnare un rilevante progresso conoscitivo e critico nel contesto della letteratura e degli studi andersiani in Italia. Il volume raccoglie contributi di Micaela LATINI, Natascia MATTUCCI, Maria Pia PATERNÓ, Francesca R. RECCHIA LUCIANI, Andrea RONDINI, Antonio TRICOMI

Experimental evidence on the origin of Ca-rich carbonated melts formed by interaction between sedimentary limestones and mantle-derived ultrabasic magmas

In this experimental study, we documented the formation of strongly ultrabasic and ultracalcic melts through the interaction of melilititic and basanitic melts with calcite. Three strongly to moderately SiO2-undersaturated volcanic rocks from the Bohemian Massif (central Europe) were mixed with 10, 30, and 50 wt% CaCO3 and melted at 1100, 1200, and 1300 °C at 2 kbar to evaluate the maximum amount of carbonate that can be assimilated by natural ultrabasic melts at shallow depths. Experiments revealed a surprisingly complete dissolution of the CaCO3, only rarely reaching carbonate saturation, with typical liquidus phases represented by olivine, spinel, melilite, and clinopyroxene. Only in the runs with the most SiO2-undersaturated compositions did abundant monticellite form instead of clinopyroxene. For all starting mixtures, strongly ultrabasic (SiO2 down to 15.6 wt%), lime-rich (CaO up to 43.6 wt%), ultracalcic (CaO/Al2O3 up to ~27) melt compositions were produced at 1200 and 1300 °C, with up to ~25 wt% dissolved CO2. When present, quenched olivine showed much higher forsterite content (Fo95–97) than olivine in the natural samples (Fo79–85). The two major results of this study are (1) silicate-carbonatite melt compositions do not necessarily imply the existence of carbonatitic components in the mantle, because they are also produced during limestone assimilation, and (2) Fo-rich olivines cannot be used to infer any primitive character of the melt nor high potential temperature (Tp