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Tonarini S.,CNR Institute of Geosciences and Earth Resources | Leeman W.P.,National Science Foundation | Leat P.T.,British Antarctic Survey
Earth and Planetary Science Letters | Year: 2011

The South Sandwich volcanic arc is sited on a young oceanic crust, erupts low-K tholeiitic rocks, is characterized by unexotic pelagic and volcanogenic sediments on the down-going slab, and simple tectonic setting, and is ideal for assessing element transport through subduction zones. As a means of quantifying processes attending transfer of subduction-related fluids from the slab to the mantle wedge, boron concentrations and isotopic compositions were determined for representative lavas from along the arc. The samples show variable fluid-mobile/fluid-immobile element ratios and high enrichments of B/Nb (2.7 to 55) and B/Zr (0.12 to 0.57), similar to those observed in western Pacific arcs. δ11B values are among the highest so far reported for mantle-derived lavas; these are highest in the central part of the arc (+15 to +18%) and decrease toward the southern and northern ends (+12 to +14%) δ11B is roughly positively correlated with B concentrations and with 87Sr/86Sr ratios, but poorly coupled with other fluid-mobile elements such as Rb, Ba, Sr and U. Peridotites dredged from the forearc trench also have high δ11B (ca. +10%) and elevated B contents (38-140ppm). Incoming pelagic sediments sampled at ODP Site 701 display a wide range in δ11B (+5 to -13%; average=-4.1%), with negative values most common. The unusually high δ11B values inferred for the South Sandwich mantle wedge cannot easily be attributed to direct incorporation of subducting slab materials or fluids derived directly therefrom. Rather, the heavy B isotopic signature of the magma sources is more plausibly explained by ingress of fluids derived from subduction erosion of altered frontal arc mantle wedge materials similar to those in the Marianas forearc. We propose that multi-stage recycling of high-δ11B and high-B serpentinite (possibly embellished by arc crust and volcaniclastic sediments) can produce extremely 11B-rich fluids at slab depths beneath the volcanic arc. Infiltration of such fluids into the mantle wedge likely accounts for the unusual magma sources inferred for this arc. © 2010 Elsevier B.V.

Beltrando M.,University of Turin | Compagnoni R.,University of Turin | Lombardo B.,CNR Institute of Geosciences and Earth Resources
Gondwana Research | Year: 2010

The Pressure-Temperature-time-deformation evolution of the high- to ultra-high-pressure units of the Western Alps has been progressively refined in the last 40 years, leading to several paradigm shifts in the understanding of orogenesis. This set of information, combined with Cretaceous-Tertiary plate kinematic reconstructions and Mesozoic palaeogeography, indicates that the Western Alps represent the amalgamation of a Cretaceous and an Eocene orogen, which developed at the expense of the Adriatic and European rifted margins, respectively. In the Cretaceous, NNE-directed drift of the Adriatic plate, parallel to the inherited Jurassic structural trends, led to the development of a highly oblique subduction boundary. In this setting, the Canavese Zone and parts of the Sesia Zone, derived from the hyper-extended Adriatic margin, underwent deformation and metamorphism at conditions ranging from sub-greenschist to eclogite facies. A Lower Eocene switch to NNW-directed motion, perpendicular to the rift-related Jurassic structural trends, culminated in the collision between the proto-Alps and the Briançonnais block at ca. 44 Ma. As a result, the Briançonnais Domain and parts of its hyper-extended margin, preserved in the (U)HP Piemonte Units, were accreted to the orogen. Continued convergence was subsequently accommodated by subduction of the more external Valaisan basin and of the thinned European margin. The arrival of thick European continental crust at the subduction zone at ca. 35-30 Ma marked the onset of the final continental collision. To a first order, the orogen grew through the progressive episodic accretion of units located towards north-westerly positions. Accretion of subducting units at the front of the orogen was coeval with kinematic reworking of tectonic contacts in the hangingwall units, locally resulting in renewed deformation/metamorphism. © 2010 International Association for Gondwana Research.

Scambelluri M.,University of Genoa | Tonarini S.,CNR Institute of Geosciences and Earth Resources
Geology | Year: 2012

Serpentinites formed by alteration of oceanic and forearc mantle are major volatile and fl uid-mobile element reservoirs for arc magmatism, though direct proof of their dominance in the subduction-zone volatile cycles has been elusive. Boron isotopes are established markers of fl uid-mediated mass transfer during subduction. Altered oceanic crust and sediments have been shown to release in the subarc mantle 11B-depleted fl uids, which cannot explain 11B enrichment of many arcs. In contrast to these crustal reservoirs, we document high 11B values retained in subduction-zone Alpine serpentinites. No 11B fractionation occurs in these rocks with progressive burial: the released 11B-rich fl uids uniquely explain the elevated 11B of arc magmas. B, O-H, and Sr isotope systems indicate that serpentinization was driven by slab fl uids that infi ltrated the slab-mantle interface early in the subduction history. © 2012 Geological Society of America.

Steinhardt P.J.,Princeton University | Bindi L.,University of Florence | Bindi L.,CNR Institute of Geosciences and Earth Resources
Reports on Progress in Physics | Year: 2012

The concept of quasicrystals was first introduced twenty-eight years ago and, since then, over a hundred types have been discovered in the laboratory under precisely controlled physical conditions designed to avoid crystallization. Yet the original theory suggested that quasicrystals can potentially be as robust and stable as crystals, perhaps even forming naturally. These considerations motivated a decade-long search for a natural quasicrystal culminating in the discovery of icosahedrite (Al 63Cu 24Fe 13), an icosahedral quasicrystal found in a rock sample composed mainly of khatyrkite (crystalline (Cu,Zn)Al 2) labeled as coming from the Koryak Mountains of far eastern Russia. In this paper, we review the search and discovery, the analysis showing the sample to be of extraterrestrial origin and the initial results of an extraordinary geological expedition to the Koryak Mountains to seek further evidence. © 2012 IOP Publishing Ltd.

Nimis P.,University of Padua | Nimis P.,CNR Institute of Geosciences and Earth Resources | Grutter H.,BHP Billiton
Contributions to Mineralogy and Petrology | Year: 2010

Mutual relationships among temperatures estimated with the most widely used geothermometers for garnet peridotites and pyroxenites demonstrate that the methods are not internally consistent and may diverge by over 200°C even in well-equilibrated mantle xenoliths. The Taylor (N Jb Min Abh 172:381-408, 1998) two-pyroxene (TA98) and the Nimis and Taylor (Contrib Mineral Petrol 139:541-554, 2000) single-clinopyroxene thermometers are shown to provide the most reliable estimates, as they reproduce the temperatures of experiments in a variety of simple and natural peridotitic systems. Discrepancies between these two thermometers are negligible in applications to a wide variety of natural samples (≤30°C). The Brey and Köhler (J Petrol 31:1353-1378, 1990) Ca-in-Opx thermometer shows good agreement with TA98 in the range 1,000-1,400°C and a positive bias at lower T (up to +90°C, on average, at T TA98 = 700°C). The popular Brey and Köhler (J Petrol 31:1353-1378, 1990) two-pyroxene thermometer performs well on clinopyroxene with Na contents of ~ 0.05 atoms per 6-oxygen formula, but shows a systematic positive bias with increasing Na Cpx (+150°C at Na Cpx = 0.25). Among Fe-Mg exchange thermometers, the Harley (Contrib Mineral Petrol 86:359-373, 1984) orthopyroxene-garnet and the recent Wu and Zhao (J Metamorphic Geol 25:497-505, 2007) olivine-garnet formulations show the highest precision, but systematically diverge (up to ca. 150°C, on average) from TA98 estimates at T far from 1,100°C and at T < 1,200°C, respectively; these systematic errors are also evident by comparison with experimental data for natural peridotite systems. The older O'Neill and Wood (Contrib Mineral Petrol 70:59-70, 1979) version of the olivine-garnet Fe-Mg thermometer and all popular versions of the clinopyroxene-garnet Fe-Mg thermometer show unacceptably low precision, with discrepancies exceeding 200°C when compared to TA98 results for well-equilibrated xenoliths. Empirical correction to the Brey and Köhler (J Petrol 31:1353-1378, 1990) Ca-in-Opx thermometer and recalibration of the orthopyroxene-garnet thermometer, using well-equilibrated mantle xenoliths and TA98 temperatures as calibrants, are provided in this study to ensure consistency with TA98 estimates in the range 700-1,400°C. Observed discrepancies between the new orthopyroxene-garnet thermometer and TA98 for some localities can be interpreted in the light of orthopyroxene-garnet Fe 3+ Partitioning systematics and suggest localized and lateral variations in mantle redox conditions, in broad agreement with existing oxybarometric data. Kinetic decoupling of Ca-Mg and Fe-Mg exchange equilibria caused by transient heating appears to be common, but not ubiquitous, near the base of the lithosphere. © Springer-Verlag 2009.

Bonini M.,CNR Institute of Geosciences and Earth Resources
Earth-Science Reviews | Year: 2012

This study examines the use of specific mud volcano features (i.e., elongated calderas, aligned vents and elongated volcanoes) as potential indicators of tectonic stress orientation. The stress indicator principles, widely recognised for magmatic systems, have been discussed and applied to mud volcano settings such as in the Northern Apennines and the Azerbaijan Greater Caucasus, as well as in other instances where the analysis was fully based on a remote sensing study. The results of these applications are promising, the obtained maximum horizontal stress (S H) directions generally showing a good correlation with those determined in the upper crust by classical methods (i.e., earthquake focal mechanism solutions, well bore breakouts). Therefore, stress information from mud volcanoes could be used as a proxy for stress orientation (1) where stress data is lacking, (2) where settings are inaccessible (i.e., underwater or the surface of planets), or simply (3) as supplementary stress indicators. This study also pays special attention to structural elements that may control fluid expulsion at various length scales, and pathways that should have spawned the mud volcanoes and controlled their paroxysmal events and eruptions. Different types of sub-planar brittle elements have been found to focus fluid flow rising up-through fold cores, where the vertical zonation of stresses may take part in this process by creating distinctive feeder fracture/fault sets. On a regional scale, mud volcanoes in active fold-and-thrust belts may occur over wider areas, such as the prolific mud volcanism in Azerbaijan, or may cluster along discrete structures like the steep Pede-Apennine thrust in the Northern Apennines, where the generation of overpressures is expected to establish a positive feedback loop allowing for fault movement and mud volcanism. © 2012 Elsevier B.V.

The evolution and characteristics of narrowcontinental rifting are illustrated in this paper through a review of recent lithospheric-scale analog models of continental extension compared with selected examples from the East African Rift System. Rift location is controlled by reactivation of lithospheric-scale pre-existing weaknesses; in these areas, the initial phases of rifting correspond to the activation of few, large-offset boundary faults that accommodate basin subsidence, which can be at places strongly asymmetric. The plan-view geometry of rift faults is primarily related to the relative orientation of the lithospheric weakness with respect to the extension direction: orthogonal rifting gives rise to long, extension-orthogonal boundary faults with associated pronounced subsidence, whereas oblique rifting results in a general en-echelon arrangement of faults and basins with less subsidence. Inherited fabrics having variable orientation with respect to the rift trend may control rift architecture at both regional and local scales. In these initial phases, widespread magmatism may encompass the rift, with volcanic activity localized along major boundary faults, transfer zones and limited portions of the rift shoulders (off-axis volcanism). Progressive extension leads to a change in deformation style from the few, large-offset boundary faults at the rift margins to dense fault swarms - with limited vertical motions - affecting the rift floor where the magmatic activity is concentrated. In these areas of focused tectono-magmatic activity (the so-called magmatic segments) the thinned lithosphere is strongly modified and weakened by the extensive magma intrusion, and extension is facilitated and accommodated by a combination of magmatic intrusion, dyking and faulting. The feedback between strain localization, magma injection and lithospheric weakening is selfreinforcing, facilitating the rupture of the continental lithosphere. At this stage, magmatic segments (as for instance in the Northern Main Ethiopian Rift) act as incipient slow-spreading mid-ocean ridges, developing within a lithosphere that is transitional between continental and oceanic. © 2011 Published by Elsevier B.V.

This paper discusses the palaeoenvironmental significance of the "Orbitolina Level", the microbial carbonates and the Salpingoporella dinarica-rich deposits encased in the Aptian/Albian shallow water carbonate platform strata of Monte Tobenna and Monte Faito (Southern Italy). These facies show a peculiar field appearance due to their color and/or fossil content. In the shallow water carbonate strata, the Late Aptian "Orbitolina Level" was formed during a period of decreasing accommodation space. Microbial carbonates occur in different levels in the composite section. They reach their maximum thickness around the sequence boundaries just above the "Orbitolina Level" and close to the Aptian-Albian transition, and were not deposited during maximum flooding. S. dinarica-rich deposits occur in the lower part of the Monte Tobenna-Monte Faito composite section, in both restricted and more open lagoonal sediments. S. dinarica has its maximum abundance below the "Orbitolina Level" and disappears 11m above this layer. On the basis of δ13C and δ18O values recorded at Tobenna-Faito, the succession has been correlated to global sea-level changes and to the main volcanic and climatic events during the Aptian. Deterioration of the inner lagoon environmental conditions was related to high trophic levels triggered by volcano-tectonic activity. Microbial carbonates were deposited especially in periods of third-order sea level lowering. In such a scenario, periods of increased precipitation during the Gargasian induced the mobilization of clay during flooding of the exposed platform due to high-frequency sea-level changes, with consequent terrigenous input to the lagoon. This and the high nutrient levels made the conditions unsuitable for the principle carbonate producers, and an opportunistic biota rich in orbitolinids (Mesorbitolina texana and M. parva) populated the platform. In the more open marine domain, the increased nutrient input enhanced the production of organic matter and locally led to the formation of black shales (e.g. the Niveau Fallot in the Vocontian Basin). It is argued that the concomitant low Mg/Ca molar ratio and high concentration of calcium in seawater could have favoured the development of the low-Mg calcite skeleton of the S. dinarica green algae. During third-order sea-level rise, no or minor microbial carbonates formed in the shallowlagoonal settings and S. dinarica disappeared. Carbonate neritic ecosystems were not influenced by the environmental changes inferred to have been induced by the mid-Cretaceous volcanism. The "Orbitolina Level", the microbial carbonates and the Salpingoporella dinarica-rich deposits in the studied Aptian/Albian shallow water carbonate strata are interpreted to be the response to environmental and oceanographic changes in shallow-water and deeper-marine ecosystems. © Author(s) 2012. CC Attribution 3.0 License.

The relations between fluid seeps and tectonic structures have been targeted in some key areas of the axial sector, and partly at the edge of the exposed Northern Apennines (Pede-Apennine margin). In the axial zone, fluid seepage is dominated by methane venting, which may occur in the form of dry seeps or associated with mineral springs. Surface venting is linked to the presence of foreland-dipping normal faults, or related to reservoirs localised at inactive anticlines. The Pede-Apennine margin is instead dominated by thrusting and mud volcanism. The two different categories of fluid seepage appear strongly coupled to the dissimilar stress fields (compressional or extensional) operating in these sectors. Pressure data inferred from deep wells delineate an overall fluid pressure increase from the axial zone toward the Pede-Apennine margin, possibly as a result of the growth of tectonic compaction in this direction. The increase of fluid pressure at the Pede-Apennine margin is thus interpreted as the primary control on the transition from dry vents to mud volcanism. The intimate association between seepage modes and distinct tectonic structures involves relevant consequences on fluid-fault interactions and seismotectonics, and also shows connections with processes dictating the internal deformation of evolving fold-and-thrust belts. © 2012 Elsevier B.V.

Cavazzini G.,CNR Institute of Geosciences and Earth Resources
International Journal of Mass Spectrometry | Year: 2012

The exponential law is an empirical model which is used to describe the isotopic fractionation both in instrumental (thermal ionization- and induced coupled plasma-source mass spectrometry) and in natural processes. It is shown that isotopic fractionation which occurs according to this law can be interpreted in terms of a Rayleigh-type distillation process, in which the value of the vapour/residue distribution coefficient for the various isotopes of an element is a natural logarithmic function of the isotope mass:D= D0+alnmm0. In this equation, D0 is the value of the vapour/residue distribution coefficient for a reference mass m0 anda=p(1+lnf), where p is the instantaneous value of the so called "fractionation exponent" (or "fractionation coefficient") of the exponential law, and f is the residual mass fraction of the sample at the same instant. © 2011 Elsevier B.V. All rights reserved.

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