Patrizia Macera

Upwelling of deep mantle material through a plate window: Evidence from the geochemistry of Italian basaltic volcanics

87 Sr/ 86 Sr– 206 Pb/ 204 Pb–eNd–eHf space. The isotopic compositions of the two end-members of these mixing arrays are assessed by least-squares regression. The mantle-derived component ( 206 Pb/ 204 Pb = 19.8, 87 Sr/ 86 Sr = 0.7025, eNd = +8, eHf = +9) is a rather homogeneous mixture of the standard high-m (HIMU) and depleted mantle (DM) components. The crust-derived component ( 206 Pb/ 204 Pb = 18.5, 87 Sr/ 86 Sr > 0.715, eNd = � 12, eHf = � 11) accounts for the enrichment of K and other large-ion-lithophile elements in the Italian volcanics. As shown by the relationship in eHf–eNd space and the lower-thanchondritic Hf/Sm ratio, this crustal component is dominated by pelagic sediments rather than terrigenous material. The overall scarcity of calc-alkaline compositions in the Italian volcanics and the presence of a HIMU component, which is the hallmark of hot spot basalts, raise the question of how plume mantle source contributes to volcanism in a subduction environment. At about 13 Ma, the Apennine collision terminated the westward subduction of the Adria plate under the European margin and rotated the direction of convergence to the northwest. The cumulative differential of subduction between the fossil plate under Tuscany and the active plate under Sicily since the opening of the Tyrrhenian Sea amounts to at least 300 km and is large enough to rift the dipping plate and open a plate window beneath the southern part of the peninsula. This model is consistent with recent high-resolution seismic tomography. We propose that the counterflow of mixed upper and lower mantle passing the trailing edge of the rifted plate is the source of Italian mafic volcanism. Alternatively, material from a so-far unidentified plume may be channeled through the plate window. The crustal signature is probably acquired by interaction of the mantle advected through the window with the upper part of the subducted plate. INDEX TERMS: 1749 History of Geophysics: Volcanology, geochemistry, and petrology; 1025 Geochemistry: Composition of the mantle; 1040 Geochemistry: Isotopic composition/chemistry; KEYWORDS: Italian volcanism, HIMU, subduction, pelagic sediments, mixing, slab window

Evidence from Sardinian basalt geochemistry for recycling of plume heads into the Earth's mantle.

Up to 10 per cent of the ocean floor consists of plateaux—regions of unusually thick oceanic crust thought to be formed by the heads of mantle plumes. Given the ubiquitous presence of recycled oceanic crust in the mantle source of hotspot basalts, it follows that plateau material should also be an important mantle constituent. Here we show that the geochemistry of the Pleistocene basalts from Logudoro, Sardinia, is compatible with the remelting of ancient ocean plateau material that has been recycled into the mantle. The Sr, Nd and Hf isotope compositions of these basalts do not show the signature of pelagic sediments. The basalts’ low CaO/Al2O3 and Ce/Pb ratios, their unradiogenic 206Pb and 208Pb, and their Sr, Ba, Eu and Pb excesses indicate that their mantle source contains ancient gabbros formed initially by plagioclase accumulation, typical of plateau material. Also, the high Th/U ratios of the mantle source resemble those of plume magmas. Geochemically, the Logudoro basalts resemble those from Pitcairn Island, which contain the controversial EM-1 component that has been interpreted as arising from a mantle source sprinkled with remains of pelagic sediments. We argue, instead, that the EM-1 source from these two localities is essentially free of sedimentary material, the geochemical characteristics of these lavas being better explained by the presence of recycled oceanic plateaux. The storage of plume heads in the deep mantle through time offers a convenient explanation for the persistence of chemical and mineralogical layering in the mantle.

Petrogenesis of contrasting hercynian granitoids from the Calabrian Arc, southern Italy

Abstract The granitoids of the southern Calabrian Arc have been investigated for whole-rock and phase chemistry, zircon typology, REE, Sr and Nd isotopes. Two distinct granitoid associations, which are related in time and partly in space, are present: a calc-alkaline one and a peraluminous one. The prevailing calc-alkaline association is compositionally expanded (SiO2=48–70%) and biotite dominated, with tonalites and granodiorites as predominant rock types. The peraluminous association is compositionally restricted (SiO2=67–76%) and contains two-mica ± Al-silicates. Distinct peraluminous typologies occur also as core facies within the calc-alkaline types. All granitoids are ilmenite-bearing. The Cittanova (CN), Villa S. Giovanni (VSG) and Capo Rasocolmo (CR) peraluminous granites display zircon typology, REE patterns, ϵSrt- (+51 to +113) and ϵNdt-values (−8.5 to −4.6) at 290 Ma, suggesting a dominantly quartzofeldspathic metasedimentary source. In more detail, the CN types and the CR-VSG types require heterogeneous and different sources. The calc-alkaline granitoids display very variable REE patterns (CeN=25–227 and YbN=3.5–18.5 in the tonalites-granodiorites) and variable age-corrected ϵNdt-values (−8.5 to −0.25), whereas Sr values vary little (+82 to +93). Thus, in terms of ϵNdtϵSrt covariation, the data points define a vertical array, which is inconsistent with a model involving crustal contamination by mantle derivatives. A more viable mechanism seems to be the melting of hydrous and heterogeneous mafic lower crust (and/or basic underplate), producing distinct magma batches evolving independently. Crustal contamination, mingling and fractionation processes may all have contributed to the observed geochemical variations within the granitoids. The peraluminous granodiorites occurring within the Serre and Capo Vaticano multipulse calc-alkaline plutons exhibit isotopic ratios (ϵNdt = −6.11 to +0.33 and ϵSrt = +93 to +97) which are similar to those of the calc-alkaline host rocks, suggesting a possible genetic link. Geologic and geochemical data indicate a continental collision setting for this plutonic activity. The magmas were produced during late-thickening to exhumation phases following collision and moderate crustal overthickening, and were emplaced subsequently during a short time span. A mantle contribution should have favoured melting, which affected various sectors of the continental crust.

A quantitative approach to trace element and Sr isotope evolution in the Adamello batholith (northern Italy)

A development of De Paolos mathematical procedure (1981) for magmatic AFC (Assimilation-Fractional Crystallization) processes is discussed with respect to both trace element and Sr isotopic ratio behaviours during the genesis and evolution of Adamello batholith (northern Italy). Resolution of a two equation-system (one relative to 87Sr/86Sr ratio variation in a magma generated by an AFC process, the other to its trace element content variations) gives the F (mass of magma at time t/mass of initial magma) and D (bulk partition coefficient) values, by which one can deduce the r (rate of assimilation/rate of crystallization) value during each step of magmatic evolution. This quantitative approach suggests that: 1) there was a common precursor magma for all the Adamello granitoids, with a Mg-rich tholeiitic composition; 2) each intrusive unit appears to have been generated by different extents of AFC; 3) the trace element distribution in the magma seems essentially influenced by mineral fractionation, rather than by the composition of the assimilated crustal material.

Geochemical and Sr–Nd isotope disequilibria during multi-stage anatexis in a metasedimentary Hercynian crust

The processes responsible for the compositional evolution of metamorphic terranes that underwent disequilibrium melting reactions control the petrology, the geochemistry and the isotopic composition of the resulting anatectic magmas. The high-grade metamorphic Tarra Padedda Complex (northern Sardinia, Italy) provides a suitable opportunity to study the isotopic (Sr–Nd) and geochemical evolution of both the anatectic magmatic products and their source. The metamorphic complex consists of migmatites (diatexites, metatexites and hornblende-bearing migmatites) and sillimanite-bearing metasediments, which were intruded by syntectonic peraluminous granitoids, aplites and pegmatites. The diatexites consist of stromatic migmatites and nebulites. Neosomes in stromatic migmatites show either granitic or trondhjemitic leucosomes, and hornblende-bearing or hornblende-free melanosomes. Petrographic characteristics of the melanosomes and hornblende-bearing migmatites suggest two distinct biotite-dehydration melting reactions: (a) biotite + sillimanite + plagioclase + quartz → garnet ±K-feldspar + melt; and (b) biotite + plagioclase (An~40) + quartz → hornblende + plagioclase (An > 60) + titanite + melt. In contrast, textural features in metatexites favour dehydration melting of muscovite: muscovite + quartz + plagioclase → sillimanite + K-feldspar + melt. A progressive variation of chemical composition coupled with a decrease of Sr and Nd crustal isotopic signatures is observed both within the metamorphic rocks and the granitoids. Field relationships, petrological, geochemical and isotope evidence of the Tarra Padedda Complex are consistent with a multi-stage anatexis of a heterogeneous metasedimentary source within the same Hercynian orogenic cycle. The variation of Sr and Nd isotope compositions within both the granitoids and the migmatites is interpreted in terms of anatexis of the various metasedimentary components, via different melting reactions involving micas plus variable amounts of accessory phases (apatite, monazite, zircon and/or xenotime).

Coexistence of calc-alkaline and ultrapotassic alkaline magmas at Mounts Cimini: evidence for transition from the Tuscan to the Roman Magmatic Provinces (central Italy)

The volcanic complex of Mts. Cimini (~0.90-1.30Ma) represents the geographical and chronological transition between the Tuscan Magmatic Province (TMP) and the Roman Magmatic Province (RMP), in central Italy. Major and trace elements, and Sr, Nd and Pb isotopes of whole-rock, as well as mineral chemistry analyses, were carried out on samples representative of the different petrographic and chronological units of Mts. Cimini. In particular, we focused on the olivine-bearing latites of Mts. Cimini that are the most mafic magmas, belong to the last phase of this volcanic activity, and are heterogeneous in highly incompatible element ratios and Sr-isotope compositions. We suggest that such heterogeneity reflects the occurrence of a heterogeneous upper mantle beneath central Italy, in which different portions, e.g., the sources of both the TMP and RMP, are characterized by distinct geochemical and petrographic features. In this scenario, about 900ka ago, the olivine-bearing latites mark the progressive decline of the TMP magma production in favour of partial melting of the RMP mantle region, thus recording the coexistence of both ultrapotassic alkaline and calc-alkaline magmas in the same volcanic region. 1

Tertiary volcanism in the Italian Alps (Giudicarie fault zone, NE Italy): insight for double alpine magmatic arc

In the Southern Alps (NE Italy), within the Giudicarie fault zone, Middle to Upper Eocene deep-water foredeep succession includes volcanic layers, pebbles from porphyric dykes and volcaniclastic sandstones, testifying the occurrence of Eocene volcanic activity from unknown volcanic centres. In this paper, we report petrographic, geochemical and chronostratigraphic characteristics.Both the volcanic layers and the pebbles share their petrographic and geochemical characteristics with calc-alkaline igneous activity of the nearby Eocene Adamello batholith, as well as with dykes from the Eastern Lombardian Alps and the Southern Alps basement, in the proximity of the Giudicarie fault zone. All of these rocks have incompatible element ratios typical of orogenic magmas (e.g., high Th/Yb and La/Nb, and low Ta/Yb, Nb/U) in contrast to that observed for the coeval anorogenic Veneto volcanic rocks. In addition, the studied Eocene calc-alkaline rocks also differ in part from the Oligocene calc-alkaline ones distributed along the E-W oriented Periadriatic fault system in terms of trace element distribution and incompatible element ratios. The Oligocene rocks are generally more enriched in incompatible elements than the Eocene ones and show higher Zr/Y but lower Nb/Y ratios. The set of data presented in this paper allows us to advance new insights on the evolution of Tertiary magmatism in the Alps, which was probably developed through a NE-SW-oriented magmatic arc during Eocene and an E-W-oriented magmatic arc during Oligocene. The two events are marked by different geochemical features of the magmatic products.

Assessment of the risk of pollution from Arsenic on human health as a function of its speciation in intestinal fluids

Investigating the risk on human health arising from exposure to As-polluted drinking water is a very important task since many area of the world has to face with this problem. Actually, chronic poisoning by arsenic carried by water has been well documented in many European and extra-European countries. Pollution of water with arsenic develops generalized symptoms of poisoning and carcino genesis. It is well known that inorganic As is rapidly absorbed after oral exposure, but little information is still available about the fate of arsenic after oral ingestion. In particular, the mechanism which affects the speciation of this element in the gastrointestinal tract is not clear, especially when other trace elements are present. In this paper we present an estimate of the health risk due to the ingestion of water containing micro-pollutants, such as As and B, using two computational codes: PHREEQC, which allowed us to determine the effect of boron on arsenic speciation in equilibrium with the fluids of the gastrointestinal tract; SADA software for assessing the risk of exposure to contaminated matrices. The use of software in the pre sent case study is mandatory since it is quite impossible to obtain experimental data. We focused on a sample area, the geothermal field of Larderello (Italy), where anomalous concentrations of arsenic and boron were found in the drinking water. The results obtained in this study prove that the risk to human health depends on the speciation of As, which is strictly related to the pH values reached at equilibrium with intestinal fluids. The pH values change as a function of the concentration of boric acid in drinking water. In waters characterized by high contents of B, the risk factor is amplified by the increase of the species As 3+ in intestinal fluids. Specifically, this study suggests that interactions between two elements in solution, such as boric acid and arsenic, both in solution in drinking water, can have a negative impact on human health when balanced with intestinal fluids, since the effects of one may magnify the toxicity of the other.

Ultra-depleted peridotites of New Caledonia: a reappraisal

Lucchi, Renata G. ... et. al.-- 87° Congresso della Societa Geologica Italiana e 90° Congresso della Societa Italiana di Mineralogia e Petrologia, The Future of the Italian Geosciences - The Italian Geosciences of the Future, 10-12 September 2014, Milan, Italy.-- 1 pageThe Montellina Spring (370 m a.s.l.) represents an example of groundwater resource in mountain region. It is a significant source of drinking water located in the right side of the Dora Baltea Valley (Northwestern Italy), SW of Quincinetto town. This spring shows a morphological location along a ridge, 400 m from the Renanchio Torrent in the lower sector of the slope. The spring was investigated using various methodologies as geological survey, supported by photo interpretation, structural reconstruction, NaCl and fluorescent tracer tests, discharge measurements. This multidisciplinary approach, necessary due to the complex geological setting, is required for the importance of the Montellina Spring. It is interesting in the hydrogeological context of Western Alps for its high discharge, relatively constant over time (average 150 l/s), and for its location outside a fluvial incision and suspended about 40 m above the Dora Baltea valley floor (Lasagna et al. 2013). According to the geological setting, the hydrogeological reconstruction of the area suggests that the large amount of groundwater in the basin is essentially favoured by a highly fractured bedrock, covered by wide and thick bodies of glacial and gravitational sediments. The emergence of the water along the slope, in the Montellina Spring, is essentially due to a change of permeability between the deep bedrock and the shallow bedrock and/or surficial sediments. The deep bedrock, showing closed fractures and/or fractures filled by glacial deposits, is slightly permeable. The shallow bedrock, strongly loosened as result of gravitational phenomena, and the local gravitational sediments are, on the contrary, highly permeable. The concentration of water at the spring is due to several reasons. a) The spring is immediately downward a detachment niche, dipping towards the spring, that essentially drains the water connected to the change of permeability in the bedrock. b) It is along an important fracture, that carries a part of the losses of the Renanchio Torrent. c) Finally, it is favored by the visible and buried morphology. Although it is located along a ridge, the spring occurs in a small depression between a moraine and a landslide body. It also can be favored by the likely concave trend of buried base of the landslide. At last, tracer tests of the Renanchio Torrent water with fluorescent tracer are performed, with a continuous monitoring in the Montellina Spring. The surveys permit to verify and quantify the spring and torrent hydrogeological relationship, suggesting that only a small fraction of stream losses feeds the spring.

Evidence for transition from the Roman to the Tuscan Magmatic Provinces in Mts. Cimini olivinlatites

Mo isotope proxy for ancient anoxia: Insights from a modern anoxic basin COREY ARCHER*, DEREK VANCE, TANYA GOLDBERG, BO THAMDRUP, GENARO ACUNA AND SIMON POULTON Dept. of Earth Sciences, University of Bristol, Bristol, BS8 1RJ, UK (c.archer@bristol.ac.uk) School of Civil Engineering and Geosciences, Newcastle University, UK Institute of Biology, University of Southern Denmark CIMAR, Universidad de Costa Rica