Massimo D'Orazio

Slab window-related magmatism from southernmost South America: the Late Miocene mafic volcanics from the Estancia Glencross Area (∼52°S, Argentina–Chile)

Abstract The Estancia Glencross Area (EGA) volcanic rocks form a series of five isolated buttes located at the southern end (∼52°S) of the discontinuous belt of Cenozoic basaltic lava formations occurring in the extra-Andean Patagonia. EGA volcanics are subalkaline basalts and basaltic andesites erupted at 8.0–8.5 Ma in a region closely behind the Andean Cordillera. EGA volcanism predated by about 4–5 my the onset of the volcanism in the nearby Pali Aike Volcanic Field, which produced highly primitive, alkaline lavas. Incompatible trace-element distributions and Sr–Nd isotope compositions of EGA rocks are those typical of within-plate OIB-type basalts and are indicative of minimal interaction of sub-lithospheric magmas with enriched reservoirs. The geochemical characteristics of EGA volcanics, as well as their age and location are consistent with a model of slab window opening beneath this region. The high silica content and the garnet signature of the estimated EGA primary magma are explained by a two-stage process involving the initial production of melts from a garnet lherzolite source followed by the reaction of these melts with harzburgite country rocks during their ascent through the mantle lithosphere. The melt/harzburgite reaction, favoured by a slow melt ascent rate, as well as the low magma production at EGA, are likely related to the dominantly compressive stress regime operating in this area during Late Miocene.

Spatial distribution of cones and satellite-detected lineaments in the Pali Aike Volcanic Field (southernmost Patagonia): insights into the tectonic setting of a Neogene rift system

Abstract The relationships between the distribution and morphometric features of eruptive structures (scoria and spatter cones, maar, tuff rings) and the fracture network were investigated in the Pliocene–Quaternary Pali Aike Volcanic Field (southernmost Patagonia, Argentina–Chile). The alkali basaltic/basanitic magmas which erupted in this area have nearly primary magma compositions and often bear mantle xenoliths; hence magma ascent from deep-seated reservoirs was probably very fast, with no significant stagnation at crustal levels. Field surveys and satellite image analysis led to the identification of up to 467 eruptive structures and four main NW–SE, NE–SW, E–W and N–S fracture systems. The spatial distribution of eruptive cones and fractures was investigated through the computation of power-law exponents ( Df ) for self-similar clustering. The self-similarity of cones and fractures was defined between lower and upper cut-offs which were in turn related to the thickness of the fractured mechanical layer. The fractal character of cones and fracture distribution (clustering) in the Pali Aike Volcanic Field area was thus correlated with crustal thickness. The self-similarity of fractures was used to establish the relative chronology of the detected fracture systems. The self-similar clustering exponent is highest in the E–W and NW–SE fracture systems ( Df =1.78 and 1.77, respectively), and lowest in the N–S system ( Df =1.65). The self-similar clustering of eruptive structures is well defined ( Df =1.45). The intense volcano-tectonic activity in the Pali Aike area marks a major Pliocene–Quaternary phase in the development of the Magellan Neogene Rift System.

The Quaternary calc-alkaline volcanism of the Patagonian Andes close to the Chile Triple Junction: geochemistry and petrogenesis of volcanic rocks from the Cay and Maca volcanoes (~ 45°S, Chile)

Abstract Major- and trace-element, Sr–Nd isotopes, and mineral chemistry data were obtained for a collection of volcanic rock samples erupted by the Cay and Maca Quaternary volcanoes, Patagonian Andes (∼45°S, Chile). Cay and Maca are two large, adjacent stratovolcanoes that rise from the Chiloe block at the southern end of the southern volcanic zone (SVZ) of the Andes. Samples from the two volcanoes are typical medium-K, calc-alkaline rocks that form two roughly continuous, largely overlapping series from subalkaline basalt to dacite. The overall geochemistry of the samples studied is very similar to that observed for most volcanoes from the southern SVZ. The narrow range of Sr–Nd isotope compositions (87Sr/86Sr=0.70389–0.70431 and 143Nd/144Nd=0.51277–0.51284) and the major- and trace-element distributions indicate that the Cay and Maca magmas differentiated by crystal fractionation without significant contribution by crustal contamination. This is in accordance with the thin ( Notwithstanding the proximity of the Cay and Maca magma sources to the northern edge of the slab window generated by the subduction of the Chile ridge under the South American plate, we did not find any geochemical evidence for a contribution of a subslab asthenospheric mantle. However, this mantle has been used to explain the peculiar geochemical features (e.g. the mild alkalinity and relatively low ratios between large ion lithophile and high field strength elements) of the Hudson volcano, which is located even closer to the slab window than the Cay and Maca volcanoes are.

The Kamil Crater in Egypt

An unusually well-preserved 45-meter-diameter crater provides ground truth for small-scale meteorite impacts on Earth. We report on the detection in southern Egypt of an impact crater 45 meters in diameter with a pristine rayed structure. Such pristine structures are typically observed on atmosphereless rocky or icy planetary bodies in the solar system. This feature and the association with an iron meteorite impactor and shock metamorphism provides a unique picture of small-scale hypervelocity impacts on Earth’s crust. Contrary to current geophysical models, ground data indicate that iron meteorites with masses of the order of tens of tons can penetrate the atmosphere without substantial fragmentation.

Microtektites from Victoria Land Transantarctic Mountains

We report on the discovery of a microtektite (microscopic impact glass particles) strewn field from the Victoria Land Transantarctic Mountains, Antarctica. Microtektites were found trapped in the local detritus accumulated in weathering pits and in joints of several glacially eroded summits (~2600 m above sea level [asl]) distributed latitudinally for 520 km. Their physical and chemical properties define a coherent population with a geochemical affinity to Australasian microtektites and compatible Quaternary 40Ar-39Ar formation age. We therefore suggest that Transantarctic Mountain microtektites (TAMM) define the southern extension of the Australasian strewn field. The margin of the Australasian strewn field is thus shifted southward by ~3000 km and the maximum distance from the putative parent impact site in Indochina by ~2000 km. This emphasizes the paradox of the missing parent crater of the largest (>10% of the Earths surface) and youngest tektite strewn field discovered on Earth. Furthermore, TAMM are depleted in volatile elements (i.e., Pb, Na, K, Rb, Sr, Rb, and Cs) relative to Australasian ones, suggesting a possible relationship between high-temperature–time regimes in the microtektite-forming process and high-angle trajectories in the ejecta plume.

Kamil Crater (Egypt): Ground truth for small-scale meteorite impacts on Earth

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Phenocryst/matrix trace-element partition coefficients for hawaiite-trachyte lavas from the Ellittico volcanic sequence (Mt. Etna, Sicily, Italy)

SummaryA set of phenocryst/matrix partition coefficients was obtained for up to 29 trace elements (ICP-MS analyses) in hawaiite to trachyte lavas from the Ellittico volcanic sequence (Mt. Etna system, Sicily). Partition coefficients were determined for plagioclase, clinopyroxene, olivine, kaersutite and Ti-magnetite. These phases, along with apatite (not analysed in this work), constitute the common fractionating solid assemblage of alkaline magmas feeding Mt. Etna volcanic system. The obtained data set forms the first attempt to characterise the solid/melt trace-element partitioning for Etnean magmas, and can be usefully applied in other sites of alkaline volcanism. The partition coefficients are here used to define the scale of incompatibility of 29 trace elements and to asses the extent of differentiation processes and the prevailing oxygen fugacity of Ellittico magmas.ZusammenfassungEin Satz von Phänokristall/Matrix Verteilungs-Koeffizienten für bis zu 29 Spurenelemente wurde mittels ICP-MS Analytik in hawaiitischen bis trachytischen Laven der Ellittico Abfolge (Ätna-System, Sizilien) erhalten. Die Verteilungs-Koeffizienten wurden für Plagioklas, Klinopyroxen, Olivin, Kaersutit und Ti-Magnetit bestimmt. Diese Phasen, zusammen mit Apatit (in dieser Arbeit nicht analysiert), stellen die fraktionierende Assoziation von festen Bestandteilen in alkalischen Magmen, die das vulkanische System des Ätna versorgen, dar. Der Datensatz ist ein erster Versuch die Verteilung von Spurenelementen zwischen Festphasen und Schmelze für Magmen des Ätna zu charakterisieren und kann ebenso nutzbringend auf andere Gebiete mit alkalischem Vulkanismus angewandt werden. Die Verteilungs-Koeffizienten werden hier benützt, um das Ausmaß der Inkompatibilität von 29 Spurenelementen zu erfassen, und das Ausmaß der Differentiationsprozesse und der vorherrschenden SauerstoffFugazität der Ellittico-Magmen zu bestimmen.

Time-evolution of magma sources in a continental back-arc setting: The Cenozoic basalts from Sierra de San Bernardo (Patagonia, Chubut, Argentina)

East of the Patagonian Andes, mafic volcanic rocks (mainly lava flows and scoriae) are exposed in the Sierra de San Bernardo fold belt and neighbouring areas (central Patagonia; 44.5–46° S, 69–71° W). They were erupted over a wide interval of time (late Eocene–Pleistocene; 14 new K–Ar ages), and show systematic chemical and Sr–Nd–Pb isotopic variations in time. The alkaline lavas (Mg number 57–66) erupted during the late Eocene and early Miocene, have an intraplate geochemical affinity, and have the highest 143 Nd/ 144 Nd and 206 Pb/ 204 Pb and the lowest 87 Sr/ 86 Sr ratios of the dataset. Their compositions indicate that their depth of equilibration in the mantle was greater than that of subsequent lavas. In contrast, the Plio-Pleistocene alkaline lavas (Mg number 58–71) are the most enriched in incompatible elements, still showing an intra-plate signature, and have the lowest 143 Nd/ 144 Nd and 206 Pb/ 204 Pb and the highest 87 Sr/ 86 Sr ratios. A distinctive group of early Miocene subalkaline lavas is characterized by slightly more evolved compositions (Mg number 56–59), coupled with very low incompatible element contents, flat LREE and fractionated HREE patterns (‘kinked’ pattern), and intermediate Sr–Nd–Pb isotope compositions. The Pleistocene basanites (Mg number 71–72) from the Cerro Ante monogenetic cone, on the easternmost slopes of the Patagonian Andes, have a marked orogenic geochemical signature and Sr–Nd–Pb isotope ratios that overlap with those of volcanic rocks from the adjacent active Andean arc. They originated in a mantle source extensively modified by the addition of materials from the subducting Pacific oceanic plates. We suggest that the wide chemical and isotopic variability of the Sierra de San Bernardo lavas reflects the upwelling of asthenospheric mantle beneath the study area, which induced lithospheric erosion and progressive involvement of enriched mantle domains in the genesis of magmas. In this context, late Eocene and early Miocene alkaline magmatism was dominantly sourced from the asthenospheric mantle, whereas Plio-Pleistocene alkaline magmas contain the largest proportion of an enriched lithospheric component. The peculiar compositional features of the early Miocene subalkaline lavas are interpreted in terms of high-degree mantle melting followed by melt–lithospheric mantle reaction processes. Based on current knowledge about the relative movement and decoupling between lithosphere and asthenosphere, we propose that the asthenosphere below the study area rose up to compensate for the westward drift of the mantle wedge coupled with the South American lithosphere.

Experimental determination of plagioclase and clinopyroxene crystal growth rates in an anhydrous trachybasalt from Mt Etna (Italy)

Plagioclase and clinopyroxene crystal growth rates were experimentally determined in an anhydrous potassic trachy-basalt lava from the 1991–93 Mt Etna eruption. Experiments were performed at atmospheric pressure, in air, at temperatures variable from 1160 to 1240 °C. Growth rates, in potassic trachybasalt to shoshonitic melts, were calculated measuring both the width of overgrowth rims on crystal seeds and the sizes of microlite crystals: they are in the order of 10 −8 cm s −1 at low under-cooling ( −9 cm s −1 at higher undercoolings. The growth rates, determined for plagioclase are three-four orders of magnitude lower than those determined for this mineral in the same lava sample by other authors under water-saturated condition. Combining experimental data on plagioclase growth rates, water solubility in trachybasalt magmas, and crystal size distributions of Etnean lavas, we modelled the final ascent of magmas from shallow reservoirs located within the basement of the volcano in correspondence with known geological discontinuities. Calculations suggest that the final ascent of magmas occurs in a time span of 1–2 h with final velocities of 0.2–0.8 m s −1 , values compatible with actual output rates observed during the quiet effusion of degassed lavas at Mt Etna.

Human exposure to thallium through tap water: A study from Valdicastello Carducci and Pietrasanta (northern Tuscany, Italy).

A geological study evidenced the presence of thallium (Tl) at concentrations of concern in groundwaters near Valdicastello Carducci (Tuscany, Italy). The source of contamination has been identified in the Tl-bearing pyrite ores occurring in the abandoned mining sites of the area. The strongly acidic internal waters flowing in the mining tunnels can reach exceptional Tl concentrations, up to 9000μg/L. In September 2014 Tl contamination was also found in the tap water distributed in the same area (from 2 to 10μg/L). On October 3, 2014 the local authorities imposed a Do Not Drink order to the population. Here we report the results of the exposure study carried out from October 2014 to October 2015, and aimed at quantifying Tl levels in 150 urine and 318 hair samples from the population of Valdicastello Carducci and Pietrasanta. Thallium was quantified by inductively coupled plasma - mass spectrometry (ICP-MS). Urine and hair were chosen as model matrices indicative of different time periods of exposure (short-term and long-term, respectively). Thallium values found in biological samples were correlated with Tl concentrations found in tap water in the living area of each citizen, and with his/her habits. Thallium concentration range found in hair and urine was 1-498ng/g (values in unexposed subjects 0.1-6ng/g) and 0.046-5.44μg/L (reference value for the European population 0.006μg/L), respectively. Results show that Tl levels in biological samples were significantly associated with residency in zones containing elevated water Tl levels. The kinetics of decay of Tl concentration in urine samples was also investigated. At the best of our knowledge, this is the first study on human contamination by Tl through water involving such a high number of samples.