Optical characterization of the growth and spatial structure of a substorm onset arc

We present a detailed case study of the characteristics of auroral forms that constitute the first ionospheric signatures of substorm expansion phase onset. Analysis of the optical frequency and along-arc (azimuthal) wave number spectra provides the strongest constraint to date on the potential mechanisms and instabilities in the near-Earth magnetosphere that accompany auroral onset and which precede poleward arc expansion and auroral breakup. We evaluate the frequency and growth rates of the auroral forms as a function of azimuthal wave number to determine whether these wave characteristics are consistent with current models of the substorm onset mechanism. We find that the frequency, spatial scales, and growth rates of the auroral forms are most consistent with the cross-field current instability or a ballooning instability, most likely triggered close to the inner edge of the ion plasma sheet. This result is supportive of a near-Earth plasma sheet initiation of the substorm expansion phase. We also present evidence that the frequency and phase characteristics of the auroral undulations may be generated via resonant processes operating along the geomagnetic field. Our observations provide the most powerful constraint to date on the ionospheric manifestation of the physical processes operating during the first few minutes around auroral substorm onset.

Novel bilayer graphene structures produced by arc-discharge

A new form of carbon is described, which consists of hollow, three-dimensional shells bounded by bilayer graphene. The new carbon is produced very simply, by passing a current through graphite rods in a commercial arc-evaporation unit. Characterisation of the carbon using high resolution transmission electron microscopy is described, and the possible formation mechanism discussed.

Statistical characterisation of the growth and spatial scales of the substorm onset arc

We present the first multi-event study of the spatial and temporal structuring of the aurora to provide statistical evidence of the near-Earth plasma instability which causes the substorm onset arc. Using data from ground-based auroral imagers, we study repeatable signatures of along-arc auroral beads, which are thought to represent the ionospheric projection of magnetospheric instability in the near-Earth plasma sheet. We show that the growth and spatial scales of these wave-like fluctuations are similar across multiple events, indicating that each sudden auroral brightening has a common explanation. We find statistically that growth rates for auroral beads peak at low wavenumber with the most unstable spatial scales mapping to an azimuthal wavelength λ≈1700 − 2500 km in the equatorial magnetosphere at around 9-12 RE. We compare growth rates and spatial scales with a range of theoretical predictions of magnetotail instabilities, including the cross-field current instability and the shear-flow ballooning instability. We conclude that, although the cross-field current instability can generate similar magnitude of growth rates, the range of unstable wavenumbers indicates that the shear-flow ballooning instability is the most likely explanation for our observations.

Genesis of quantum nonlocality

We revisit the problem of an otherwise classical particle immersed in the zero-point radiation field, with the purpose of tracing the origin of the nonlocality characteristic of Schrodinger`s equation. The Fokker-Planck-type equation in the particles phase-space leads to an infinite hierarchy of equations in configuration space. In the radiationless limit the first two equations decouple from the rest. The first is the continuity equation: the second one, for the particle flux, contains a nonlocal term due to the momentum fluctuations impressed by the field. These equations are shown to lead to Schrodinger`s equation. Nonlocality (obtained here for the one-particle system) appears thus as a property of the description, not of Nature. (C) 2011 Elsevier B.V. All rights reserved.

The Sphene-Centered Ocellar Texture: An Effect of Grain-Supported Flow and Melt Migration in a Hyperdense Magma Mush

The sphene-centered ocellar texture consists of leucocratic ocelli with sphene (titanite) crystals at the center, enclosed in a biotite-rich matrix. This texture has been recognized worldwide in hybrid intermediate rocks. On the basis of structural, petrological, and geochronological data from selected outcrops of the Variscan Ribadelago pluton (NW Iberian Massif), we propose that the ocelli were formed by migration and accumulation of a residual melt through a plagioclase- and biotite-dominated crystalline framework. At the late stage of crystallization, the magma acted as a hyperdense suspension and reacted to the pressure gradient caused by the regional stress field, entering the domain of grain-supported flow. Microstructures reveal that aligned crystal domains arose in the crystal framework from the shearing and compaction of the crystal mush and behaved as magmatic microshears. Relative displacement of adjacent crystal clusters along these microshears corresponded to the onset of Reynolds dilatancy that generated an expansion of the crystal mush, involving melt migration and pore aperture. The mineralogy of the ocelli, dominated by andesine and sphene, represents the composition of the migrating melt. The chemistry of this late, Ti-rich melt stems from the incongruent melting of biotite. Magmatic sphene from the ocelli yields a U-Pb age of 317 +/- 1 Ma, which represents the final crystallization of the hybridized magmatic system. Moreover, this texture offers an opportunity to better understand the rheological behavior of highly crystallized magmas.

Evaluation of petrogenetic models for intermediate and silicic plutonic rocks from the Sierra de Valle Fertil-La Huerta, Argentina: Petrologic constraints on the origin of igneous rocks in the Ordovician Famatinian-Puna paleoarc

The whole Valle Fertil-La Huerta section appears as a calc-alkaline plutonic suite typical of a destructive plate margin. New Sr and Nd isotopic whole-rock data and published whole-rock geochemistry suggest that the less-evolved intermediate (dioritic) rocks can be derived by magmatic differentiation, mainly by hornblende + plagioclase +/- Fe-Ti oxide fractional crystallization, from mafic (gabbroic) igneous precursors. Closed-system differentiation, however, cannot produce the typical intermediate (tonalitic) and silicic (granodioritic) plutonic rocks, which requires a preponderant contribution of crustal components. Intermediate and silicic plutonic rocks from Valle Fertil-La Huerta section have formed in a plate subduction setting where the thermal and material input of mantle-derived magmas promoted fusion of fertile metasedimentary rocks and favored mixing of gabbroic or dioritic magmas with crustal granitic melts. Magma mixing is observable in the field and evident in variations of chemical elemental parameters and isotopic ratios, revealing that hybridization coupled with fractionation of magmas took place in the crust. Consideration of the whole-rock geochemical and isotopic data in the context of the Famatinian-Puna magmatic belt as a whole demonstrates that the petrologic model postulated for the Sierra Valle Fertil-La Huerta section has the potential to explain the generation of plutonic and volcanic rocks across the Early Ordovician paleoarc from central and northwestern Argentina. As the petrologic model does not require the intervention of old Precambrian continental crust, the nature of the basement on which thick accretionary turbiditic sequences were deposited remains a puzzling aspect. Discussion in this paper provides insights into the nature of magmatic source rocks and mechanisms of magma generation in Cordilleran-type volcano-plutonic arcs of destructive plate margins. (C) 2010 Elsevier Ltd. All rights reserved.

Petrogenesis of Upper Paleozoic post-collisional peraluminous leucogranites, Sierra de Ancasti, northwest Argentina

The Santa Rosa and Sauce Guacho plutons are two post-collisional peraluminous Late Devonian to Early Carboniferous leucogranites that intruded the banded schists of the Ancasti Formation. The leucogranites are composed of microcline phenocrysts along with quartz, plagioclase, muscovite, biotite, ilmenite, tourmaline, apatite, monazite and zircon. Their geochemical composition is consistent with S-type granites and mineralogically they belong to MPG granites (muscovite-peraluminous granites). It is proposed that granite magma generation was related to shear zones that concentrated fluids in the metasedimentary crust during a collision or transcurrent tectonics. U-Pb analyses on monazite gave an age of 369.8 +/- 5.3 Ma, while Sm/Nd isotopic data yield epsilon(Nd(t)) values of -5.3 for Sauce Guacho and -5.7 for Santa Rosa indicating crustal provenance. Nd model ages between 1,544 and 1,571 Ma are within the range of magmatic rocks from the Lower Ordovician Famatinian Arc in the Central Sierras Pampeanas.

Petrogenesis of Early Cretaceous silicic volcanism in SE Uruguay: The role of mantle and crustal sources

Early Cretaceous (similar to 129 Ma) silicic rocks crop out in SE Uruguay between the Laguna Merin and Santa Lucia basins in the Lascano, Sierra Sao Miguel. Salamanca and Minas areas They are mostly rhyolites with minor quartz-trachytes and are nearly contemporaneous with the Parana-Etendeka igneous province and with the first stages of South Atlantic Ocean opening A strong geochemical variability (particularly evident from Rb/Nb, Nb/Y trace element ratios) and a wide range of Sr-Nd isotopic ratios ((143)Nd/(144)Nd((129)) = 0.51178-0.51209, (87)Sr/(86)Sr((129)) = 0.70840-0.72417) characterize these rocks Geochemistry allows to distiniguish two compositional groups, corresponding to the north-eastern (Lascano and Sierra Sao Miguel, emplaced on the Neo-Proterozoic southern sector of the Dom Feliciano mobile belt) and south-eastern localities (Salamanca, Minas, emplace on the much older (Archean) Nico Perez teriane or on the boundary between the Dom Feliciano and Nico Perez termites) These compositional differences between the two groups are explained by variable mantle source and crust contributions. The origin of the silicic magmas is best explained by complex processes involving assimilation and fractional crystallization and mixing of a basaltic magma with upper crustal lithologies, for Lascano and Sierra Sao Miguel rhyolites. In the Salamanea and Minas rocks genesis, a stronger contribution from lower crust is indicated.

Sr and Nd isotope composition of the metamorphic, sedimentary and ultramafic xenoliths of Lanzarote (Canary Islands): Implications for magma sources

The lavas produced by the Timanfaya eruption of 1730-1736 (Lanzarote, Canary Islands) contain a great many sedimentary and metamorphic (metasedimentary), and mafic and ultramafic plutonic xenoliths. Among the metamorphosed carbonate rocks (calc-silicate rocks [CSRs]) are monomineral rocks with forsterite or wollastonite, as well as rocks containing olivine +/- orthopyroxene +/- clinopyroxene +/- plagioclase: their mineralogical compositions are identical to those of the mafic (gabbros) and ultramafic (dunite, wherlite and lherzolite) xenoliths. The (87)Sr/(16)Sr (around 0.703) and (143)Nd/(144)Nd (around 0.512) isotope ratios of the ultramafic and metasedimentary xenoliths are similar, while the (147)Sm/(144)Nd ratios show crustal values (0.13-0.16) in the ultramafic xenoliths and mantle values (0.18-0.25) in some CSRs. The apparent isotopic anomaly of the metamorphic xenoliths can be explained in terms of the heat source (basaltic intrusion) inducing strong isotopic exchange ((87)Sr/(86)Sr and (143)Nd/(144)Nd) between metasedimentary and basaltic rocks. Petrofabric analysis also showed a possible relationship between the ultramafic and metamorphic xenoliths. (C) 2009 Elsevier B.V. All rights reserved.

Trace element geochemistry and Sr-Nd characteristics of Mesoproterozoic mafic intrusive rocks from Rondonia, Brazil, SW Amazonian Craton: Petrogenetic and tectonic inferences

The Amazonian Craton comprises an Archean domain surrounded by four successively younger Proterozoic tectonic provinces. Within the Rio-Negro-Juruena province the Serra da Providencia Intrusive Suite (1.60 and 1.53 Ga) consists of A-type rapakivi granites, charnockites and mangerites genetically associated with diabase dikes, gabbros and amphibolites lites. The original mafic melts were derived from a depleted mantle source (epsilon(Nd(T)) + 2.5 to +2.8; epsilon(Sr(T)) - 12.1). Underplated mafic magma induced melting of a short-lived fielsic crust, thus originating coeval felsic-inafic magmatism in a continental intraplate setting. The Colorado Complex, assigned to the Rondonian-San Ignacio province, comprises 1.35-1.36 Ga intrusive bimodal magmatism represented by monzonite gneisses associated with amphibolite, gabbro and metadiabase dikes intercalated with metasediments with detrital zircon that yield U-Pb ages of 1.35 to 1.42 Ga. Mafic samples display juvenile signatures (epsilon(Nd(T)) 0.0 to +5.2; epsilon(Sr(T)) -5.0 to -30.7) and are less contaminated than the Serra da Previdencia and Nova Brasiladndia ones. The generation of the basaltic magma is related to the subduction of an oceanic slab below the peridotite wedge (intraoceanic arc setting). Fluids and/or small melts from the slab impregnated the mantle. The Nova Brasilandia Sequence (Sunsas-Aguapei province) comprises a metasedimentary sequence intruded by 1.10-1.02 Ga metadiabases, gabbros, meta-gabbros, and amphibolites associated with granitic plutons (bimodal magmatism). The original tholeiitic magmas, derived from a depleted source (epsilon(Nd(T)) = +3.1 to +5.0), in a proto-oceanic setting, underwent subsequent contamination by the host rocks, as indicated by the isotopic and trace element data.