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

A clinopyroxene record of primitive melt diversity and mantle heterogeneity beneath Italy

The young potassium-rich volcanic rocks of peninsular Italy are the products of a complex post-collisional geodynamic setting. These volcanic rocks exhibit extreme compositional variability in space and time, resulting from large variations in the subducted material in their mantle sources. The genetic relationships between distinct Italian magmatic series—shoshonitic, potassic, ultrapotassic and lamproitic, among others—that are closely related in space and time, as well as the exact nature and provenance of the metasomatic agents, are subject to active debate. The earliest crystallised silicate phases from mafic lavas—olivine and clinopyroxene—carry valuable information on the nature of mantle sources and melt extraction processes. Because Mg-rich clinopyroxene incorporates significant amounts of incompatible elements and is a ubiquitous phase in mafic Italian lavas, it potentially represents a versatile instrument for delineating the compositional complexity and regional variability of subduction-modified mantle sources in this region. Here we present the results of an extensive study of Mg-rich clinopyroxene (Mg# = 88–93 mol%) from potassium-rich mafic rocks from a chain of volcanic centres in central-southern Italy, from Tuscany down to Campania. We compare major- and trace-element data from clinopyroxenes with those from bulk rocks and olivine-hosted melt inclusions, using new estimates of trace-element partitioning between clinopyroxene and potassium-rich magmas based on cogenetic clinopyroxene-olivine crystallisation. The Mg-rich clinopyroxenes show a marked compositional diversity that reflects the nature of the (near-)primary mantle-derived melts from which they crystallised, and allow us to characterise the metasomatic agents responsible for the formation of different compositional end-members. We demonstrate that clinopyroxenes provide a detailed archive of mantle heterogeneity beneath Italy, highlighting systematic variations both regionally and beneath individual volcanic complexes

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

The South Armenian Block: Gondwanan origin and Tethyan evolution in space and time

The geodynamic evolution of the South Armenian Block (SAB) within the Tethyan realm during the Palaeozoic to present-day is poorly constrained. Much of the SAB is covered by Cenozoic sediments so that the relationships between the SAB and the neighbouring terranes of Central Iran, the Pontides and Taurides are unclear. Here we present new geochronological, palaeomagnetic, and geochemical constraints to shed light on the Gondwanan and Cimmerian provenance of the SAB, timing of its rifting, and geodynamic evolution since the Permian. We report new 40Ar/39Ar and zircon U-Pb ages and compositional data on magmatic sills and dykes in the Late Devonian sedimentary cover, as well as metamorphic rocks that constitute part of the SAB basement. Zircon age distributions, ranging from ∼3.6 Ga to 100 Ma, firmly establish a Gondwanan origin for the SAB. Trondhjemite intrusions into the basement at ∼263 Ma are consistent with a SW-dipping active continental margin. Mafic intraplate intrusions at ∼246 Ma (OIB) and ∼234 Ma (P-MORB) in the sedimentary cover likely represent the incipient stages of breakup of the NE Gondwanan margin and opening of the Neotethys. Andesitic dykes at ∼117 Ma testify to the melting of subduction-modified lithosphere. In contrast to current interpretations, we show that the SAB should be considered separate from the Taurides, and that the Armenian ophiolite complexes formed chiefly in the Eurasian forearc. Based on the new constraints, we provide a geodynamic reconstruction of the SAB since the Permian, in which it started rifting from Gondwana alongside the Pontides, likely reached the Iranian margin in Early Jurassic times, and was subject to episodes of intraplate (∼189 Ma) and NE-dipping subduction-related (∼117 Ma) magmatism

Estimates of the Temperature and Melting Conditions of the Carpathian‐Pannonian Upper Mantle From Volcanism and Seismology

What drives the formation of basaltic melts beneath intraplate volcanoes not associated with extensive thermal anomalies or lithospheric extension? Detailed constraints on the melting conditions and source region are imperative to resolve this question. Here we model the geochemistry of alkali basalts and mantle nodules brought up by young (12–0.1 Ma) intraplate volcanoes distributed across the Carpathian-Pannonian region and combine the results with geophysical observations. Rare earth element inversion and forward calculation of elemental concentrations show that the basalts require the mantle to have undergone less than 1% melting in the garnet-spinel transition zone, at depths of about 63–72 km. The calculated melt distributions correspond to a mantle potential temperature of ∼1257°C, equivalent to a real temperature of 1290°C at 65 km beneath the Pannonian Basin. The composition, modal mineralogy, and clinopyroxene geochemistry of some of the entrained mantle nodules closely resemble the basalt source, though the latter equilibrated at greater depths. The gravity anomalies and topography of the Basin reveal no large-scale features that can account for the post-extensional volcanism. Instead, the lithospheric thickness and geotherm show that melting occurs because the base of the lithosphere, at ∼50-km depth, is close to or at the solidus temperature over a large part of the Basin. Hence, only a small amount of upwelling is required to produce minor volumes (up to a few cubic kilometers) of melt. We conclude that the Pannonian volcanism originates from upwelling in the asthenosphere just below thinned lithosphere, which is likely to be driven by thermal buoyancy

The South Armenian Block: Gondwanan origin and Tethyan evolution in space and time

The geodynamic evolution of the South Armenian Block (SAB) within the Tethyan realm during the Palaeozoic to present-day is poorly constrained. Much of the SAB is covered by Cenozoic sediments so that the relationships between the SAB and the neighbouring terranes of Central Iran, the Pontides and Taurides are unclear. Here we present new geochronological, palaeomagnetic, and geochemical constraints to shed light on the Gondwanan and Cimmerian provenance of the SAB, timing of its rifting, and geodynamic evolution since the Permian. We report new 40Ar/39Ar and zircon U-Pb ages and compositional data on magmatic sills and dykes in the Late Devonian sedimentary cover, as well as metamorphic rocks that constitute part of the SAB basement. Zircon age distributions, ranging from ∼3.6 Ga to 100 Ma, firmly establish a Gondwanan origin for the SAB. Trondhjemite intrusions into the basement at ∼263 Ma are consistent with a SW-dipping active continental margin. Mafic intraplate intrusions at ∼246 Ma (OIB) and ∼234 Ma (P-MORB) in the sedimentary cover likely represent the incipient stages of breakup of the NE Gondwanan margin and opening of the Neotethys. Andesitic dykes at ∼117 Ma testify to the melting of subduction-modified lithosphere. In contrast to current interpretations, we show that the SAB should be considered separate from the Taurides, and that the Armenian ophiolite complexes formed chiefly in the Eurasian forearc. Based on the new constraints, we provide a geodynamic reconstruction of the SAB since the Permian, in which it started rifting from Gondwana alongside the Pontides, likely reached the Iranian margin in Early Jurassic times, and was subject to episodes of intraplate (∼189 Ma) and NE-dipping subduction-related (∼117 Ma) magmatism