Simone Tommasini

INCOMPATIBLE TRACE-ELEMENTS IN OIB AND MORB AND SOURCE ENRICHMENT IN THE SUB-OCEANIC MANTLE

The concentrations of incompatible trace elements in ocean island basalts (OIB) from the central Atlantic extend to relatively enriched and fractionated compositions in regions of older oceanic lithosphere. Certain trace element ratios normally considered to be uniform in the mantle, such as CePb, are particularly variable. However, other trace element ratios that are expected to be variable because of differences in bulk distribution coefficient, such as CeU, are relatively uniform. The CePb ratios in enriched OIB are correlated with unusually high UPb and low KU. These UPb ratios would have generated excessively radiogenic Pb if they were long-term (109 yr) features of the source such as might result from core formation or recycling of hydrothermally altered ocean floor basalts. However, volcanic centers with high UPb do have high 206Pb204Pb for their 207Pb204Pb, a feature that is most easily modelled by enrichment in U relative to Pb about 108 yr prior to melting, a time similar to the age of the lithosphere. We propose that the source regions of these magmas are enriched by the introduction of small degree partial melts soon after the formation of the oceanic lithosphere. Metasomatism of the uppermost mantle by small degree partial melts produced in equilibrium with a combination of residual upper mantle major silicate phases, together with minor amphibole (≤ 2%), sulfide (≤ 0.2%) and phlogopite (≤ 0.2%) at about the time of formation of the lithosphere, would generate a ‘near-surface fractionated’ (NSF) source with low KU and high UPb, Δ206Pb204Pb and CePb, while maintaining CeU, NbU, BaCe and BaNb that are only slightly fractionated relative to other OIB. An important feature of the modelling of NSF mantle is that U is more incompatible than Ba or Rb. This is confirmed by the variability in incompatible trace element ratios with U concentration for enriched OIB. However, this contrasts with the relative incompatibility deduced from Uue5f8Thue5f8Ra disequilibrium data for MORB and OIB, endorsing the view that the variability in highly incompatible trace element ratios in enriched OIB is dominated by source enrichment effects that are distinct from the fractionation that takes place during the production of the erupted magmas. n nThe CeU, BaCe and UPb ratios of all OIB, including enriched OIB from regions of old lithosphere, are uniform relative to data for MORB. This appears inconsistent with the degree of isotopic variability in OIB relative to MORB and is difficult to explain unless the variations in incompatible trace element ratios in MORB are dominated by effects other than melting. Ratio-element plots provide evidence that the incompatible element ratios of MORB are affected by OIB-component contamination in the source or in transit to the surface and this is consistent with covariation between trace element ratios and some isotopic compositions in MORB. The ratios and concentrations of highly incompatible trace elements in MORB vary as a consequence of this contamination, as well as degree of partial melting. The relative uniformity and near chondritic proportions of CeU and, to a lesser extent, BaCe in OIB compared with MORB are difficult to reconcile with recycling models that advocate material resembling present-day MORB or hydrothermally altered MORB as the dominant component of OIB sources but are consistent with NSF mantle recycling. Similarly, the Ba/U/Ce ratios are inconsistent with models in which the OIB source was affected by Ca perovskite fractionation in a magma ocean on the early Earth.

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.

Volcanology and Magma Geochemistry of the Present‐Day Activity: Constraints on the Feeding System

Stromboli volcano is famous in the scientific literature for its persistent state of activity, which began about 1500 years ago and consists of continuous degassing and mild intermittent explosions (normal Strombolian activity). Rare lava emissions and sporadic more violent explosive episodes (paroxysms) also occur. Since its formation, the present-day activity has been dominated by the emission of two basaltic magmas, differing chiefly in their crystal and volatile contents, whose characteristics have remained constant until now. The normal Strombolian activity and lava effusions are fed by a crystal-rich, degassed magma, stored within the uppermost part of the plumbing system, whereas highly vesicular, crystal-poor light-colored pumices are produced during paroxysms testifying to the ascent of volatile-rich magma batches from deeper portions of the magmatic system. Mineralogical, geochemical, and isotopic data, together with data on the volatile contents of magmas, are presented here with the aim of discussing (1) the relationships between the different magma batches erupted at Stromboli, (2) the mechanisms of their crystallization and transfer, (3) the plumbing system and triggering mechanisms of Strombolian eruptions.

High-precision 87Sr/86Sr analyses in wines and their use as a geological fingerprint for tracing geographic provenance.

The radiogenic isotopic compositions of inorganic heavy elements such as Sr, Nd, and Pb of the food chain may constitute a reliable geographic fingerprint, their isotopic ratios being inherited by the geological substratum of the territory of production. The Sr isotope composition of geomaterials (i.e., rocks and soils) is largely variable, and it depends upon the age of the rocks and their nature (e.g., genesis, composition). In this study we developed a high-precision analytical procedure for determining Sr isotopes in wines at comparable uncertainty levels of geological data. With the aim of verifying the possibility of using Sr isotope in wine as a reliable tracer for geographic provenance, we performed Sr isotope analyses of 45 bottled wines from four different geographical localities of the Italian peninsula. Their Sr isotope composition has been compared with that of rocks from the substrata (i.e., rocks) of their vineyards. In addition wines from the same winemaker but different vintage years have been analyzed to verify the constancy with time of the (87)Sr/(86)Sr. Sr isotope compositions have been determined by solid source thermal ionization mass spectrometry following purification of Sr in a clean laboratory. (87)Sr/(86)Sr of the analyzed wines is correlated with the isotopic values of the geological substratum of the vineyards, showing little or no variation within the same vineyard and among different vintages. Large (87)Sr/(86)Sr variation is observed among wines from the different geographical areas, reinforcing the link with the geological substratum of the production territory. This makes Sr isotopes a robust geochemical tool for tracing the geographic authenticity and provenance of wine.

Geochemical modeling of acid–basic magma interaction in the Sardinia–Corsica Batholith: the case study of Sarrabus, southeastern Sardinia, Italy

Abstract The Hercynian intrusive rocks outcropping in the Sarrabus area, southeastern Sardinia, Italy, consist of microgranular mafic enclave (MME)-bearing granites (>95%) associated with coeval stratified gabbroic complexes and basic septa (BS) (

Conservation of 87Sr/86Sr isotopic ratios during the winemaking processes of ‘Red’ wines to validate their use as geographic tracer

(87)Sr/(86)Sr has been determined in wines, musts grape juices, soils and rocks from six selected vineyards of Cesanese wine area. Cesanese is a monocultivar wine from a small region characterised by different geologic substrata, a key locality to test the influence of both substratum and winemaking procedure on the (87)Sr/(86)Sr of wines. Experimental work has been performed on wines from different vintage years to check possible seasonal variations. The data reveal that (87)Sr/(86)Sr does not change through time, to validate the selection of wineries performed, and in addition no isotopic variations are observed during winemaking. Indeed, no significant isotopic variations have been observed in musts and wines. These findings reinforce the hypothesis that the isotopic signature of wines is strongly related to the bioavailable fraction of the soil rather than to its bulk. The data corroborate the possibility that Sr-isotopes of high-quality wines can be used as a reliable tool for fingerprinting wine geographic provenance.

Armouring effect on Sr-Nd isotopes during disequilibrium crustal melting: the case study of frozen migmatites from El Hoyazo and Mazarrón, SE Spain

Crustal melting is responsible for the production of large volumes of rhyolitic melt and therefore is central to understand the rheology of the crust and the mechanisms of crustal differentiation. The attainment of isotopic equilibrium during melting of crustal rocks is implicitly assumed in most isotopic dating and tracing studies. This assumption considers the melting event as an instantaneous process and does not take into account the duration of anatexis. To assess the critical role of the timescale of crustal melting, we have studied the unique occurrence of erupted migmatites enclosed as xenoliths in the El Hoyazo and Mazarron dacites of the Neogene Volcanic Province of SE Spain. These xenoliths represent the residue after some 30-60 % rhyolitic melt extraction at P-T conditions of 5-7 kbar and ∼850 °C, and consist of biotite, plagioclase, sillimanite, garnet, cordierite, graphite and abundant glass inclusions (i.e., not extracted rhyolitic melt) within each mineral phase. The timescale of melt extraction was ∼3 Myr and <0.8 Myr at El Hoyazo and Mazarron, respectively, resembling the duration of melting events during rapid anatexis caused by basalt underplating and crustal assimilation processes. In both localities, the minerals and glass inclusions of erupted migmatites preserve a significant Sr and minor Nd isotope disequilibrium. At Mazarron the isotopic disequilibrium is most marked owing to the shorter residence time of the melt within the source. The isotopic disequilibrium is not caused by the major xenolith-forming minerals but rather by the accessory phosphate inclusions (apatite ± monazite ± xenotime) hosted in garnet and biotite. The preservation of isotopic disequilibrium in these accessory phases has been facilitated by both their intrinsically low Sr and Nd diffusion coefficients and the armouring effect caused by their occurrence within biotite and garnet crystals, which acted as chemical barriers to Sr and Nd diffusion. This result implies that modelling of radiogenic isotope equilibration in natural systems should consider elemental diffusion in a composite medium with a resistance at the interface, i.e. different partition coefficients between adjacent mineral phases.

Petrology of the late-Carboniferous Punta Falcone gabbroic complex, nothern Sardinia, Italy

The Punta Falcone gabbroic complex represents an evolved high-alumina basalt which rose from the mantle through the lower crust, and subsequently intruded a granite magma in middle crustal levels, during the calc-alkaline magmatic activity which took place in the Sardinian and Corsican islands in the Carboniferous. The gabbroic complex has a stratified sub-vertical structure, and consists of three zones developing from the bottom to the top of the magma chamber. An interaction zone can be recognized along contacts with the surrounding granite stock, and it is characterized by finer-grained and more evolved rocks than the interior of the gabbroic complex. Processes occurring in its interior zone have been substantially different from those occurring in its marginal interaction zone. Petrographical and geochemical features indicate that the differentiation of the interior of the gabbroic complex can be accounted for by low pressure, closed-system in-situ crystallization. The different gabbroic units represent mixtures between cumulus phases and trapped liquid. Plagioclase + pyroxenes, and successively plagioclase + calcic amphibole + oxides nucleated and grew in-situ on the floor and walls of the chamber. Floating of plagioclase towards the top of the magma chamber resulted in the accumulation of the denser liquid at the bottom. Compaction phenomena and convective fractionation processes permitted the development of the pile of cumulus crystals with their trapped liquid, and the migration of part of this evolved liquid towards the top of the magma chamber. On the basis of major and trace element modelling a mathematical artifice has been developed to evaluate cumulus-intercumulus processes that occurred in the interior of the gabbroic complex. Accordingly, the formation of the different units can be modelled by mixtures between the parental magma and different percentages of minerals formed during the first stages of crystallization. Contemporaneously with the differentiation of the interior zone, the envelope of fine-grained rocks enclosing and grading into the coarser inner part of the gabbroic complex experienced both chemical and physical processes. Chemical processes resulted in the evolution of the marginal interaction zone by crystal fractionation plus contamination by the acid magma. Physical processes were closely related to the thermodynamic instability of this marginal zone, and consisted of mingling and back veining phenomena which developed interdigitations of granite veins along contacts. In addition, an increase of the melt fraction of the granite magma, superheated by the latent heat of crystallization of the mafic magma, caused the occurrence of tilting of the mafic magma chamber, and resulted in the development of the sub-vertical structure of the gabbroic complex.

Unraveling the hidden origin and migration of plagioclase phenocrysts by in situ Sr isotopes: the case of final dome activity at Nisyros volcano, Greece

This contribution reports a detailed study on in situ Sr isotope analyses, along with textural and compositional characteristics, of plagioclase phenocrysts occurring in the rhyodacitic dome-lavas and associated mafic enclaves, erupted during the last magmatic activity at Nisyros volcano (Greece). Dome-lavas and enclaves have a paragenesis dominated by plagioclase. We recognize five different types of plagioclase based on their specific textures and composition. Dome-lava plagioclases (Type-1) are mainly large (1–5xa0mm), subhedral, clear, and poorly zoned crystals with low An content (An25–35). The plagioclase phenocrysts (Type-4 and Type-5) and groundmass microlites crystallizing in the enclaves, and found in dome-lavas as xenocrysts, have high An content (An75–95). In both dome-lavas and enclaves, two other types of plagioclase do also occur: (1) plagioclase phenocrysts with size and core composition similar to those of Type-1 having a dusty sieve zone (DSZ) at the rims (Type-2); (2) plagioclases with a DSZ affecting the entire crystal but a thin rim (Type-3). The drilled plagioclases have 87Sr/86Sr negatively correlated with their An content. Low An cores of Type-1 and Type-2 have quite homogeneous 87Sr/86Sr (0.7044–0.7046), whose values are more radiogenic than their host magmas (0.70403–0.70408) and similar to those of the previous Upper Pumice (UP) rhyolite magma (0.70438–0.70456). The DSZs of Type-2 and Type-3 show lower and scattered 87Sr/86Sr (0.70397–0.70426) with intermediate and variable An content. High An cores of Type-4 and Type-5 have the least radiogenic Sr isotope composition (0.70379) in equilibrium with that measured in the enclaves (0.70384–0.70389). We demonstrate that Type-1 plagioclase crystallizes in the previous UP rhyolitic magmasxa0representing the silica-rich magma from which the dome-lava melts derived by open system evolutionary processes (e.g., mixing, mingling, and crystal migration), caused by successive refilling of mafic enclave-forming magma. The Type-2 plagioclase derives from entrainment of Type-1 into the still molten enclave magma. The DSZs originated in response to the interaction between the low An plagioclase and the enclave mafic melt in which dissolution and re-crystallization acted together as function of the interaction time. Type-1 and Type-2 plagioclases record, therefore, a long-lived timescale of events starting from their crystallization in the UP rhyolite. Instead, the different width of DSZsxa0(Type-2 and Type-3) seems to indicate shortxa0different interaction timescales between the single crystals and the enclave melt (from few hours to some 40xa0days). These microanalytical data contribute to the understanding of the origin of the rhyodacitic dome-lavas at Nisyros volcano and to set robust constraints on the dynamics of mingling/mixing processes in terms of crystal exchange pathways and enclave disaggregation.

Tracing 87Sr/86Sr from rocks and soils to vine and wine: an experimental study on geologic and pedologic characterisation of vineyards using radiogenic isotope of heavy elements. Science of the Total Environment

In this paper we report an experimental study to assess the process of Sr-isotope uptake from the soil and its transfer to the grapevine and then to the wine made through micro-vinification. The experimental work has been carried out with a deep control of the boundary conditions (i.e., type of soil, geologic substratum, ground water supply, etc.) on 11 selected vine-plant sites over a period of four harvest years. Sr-isotopes have been determined on grape-bunches, grapevine sap, on the bioavailable fraction of the soil, on bulk soil, and on the rocks of the substratum. No significant Sr-isotope variability has been observed among micro-vinifications from different harvest years. A slight but significant Sr-isotope variability occurred among wines from rows embedded on different soil type. The Sr-isotope data on micro-vinifications well match those of grapevine sap and bioavailable fraction of soils, all of them falling well within the whole geological range of the bedrock, despite an evident decoupling between bioavailable fraction, whole soils and bedrocks does exist. This decoupling has been ascribed to differential geochemical behaviour of minerals in response to pedogenetic processes. The findings of our experiments indicate that the biological activity of the vine is not able to change the original 87Sr/86Sr composition up-taken from the bio-available fraction of the soil. Thus, the 87Sr/86Sr of the wine is an unadulterated feature of the terroir.