Mesoscopic phase separation in La2CuO4.02 - a 139La NQR study

In crystals of La2CuO4.02 oxygen diffusion can be limited to such small length scales, that the resulting phase separation is invisible for neutrons. Decomposition of the 139La NQR spectra shows the existence of three different regions, of which one orders antiferromagnetically below 17K concomitantly with the onset of a weak superconductivity in the crystal. These regions are compared to the macroscopic phases seen previously in the title compound and the cluster-glass and striped phases reported for the underdoped Sr-doped cuprates.Comment: 4 pages, RevTeX, 5 figures, to be published in PR

Discs and discord: The paleontological record of Ediacaran discoidal structures in the south American continent

Discoidal sedimentary structures are commonly described in Proterozoic strata, and even more common in Ediacaran to lower Cambrian sedimentary successions. Many abiotic processes are able to produce such circular or discoidal structures in bedding planes, however, their abundance in Ediacaran strata suggests a possible correlation with the evolution and preservation of epibenthic metazoans that emerged at the end of this period. In the South American paleontological record, studies regarding the Ediacaran soft-bodied organisms are meager and restricted to few reports in Brazil, Paraguay, and Argentina. In many cases, such “fossils” were only tentatively characterized in terms of their general morphology and putative taxonomic affinity. Thus, considering the almost absence of work on these enigmatic structures in South America, this paper aims to make a critical analysis on the main occurrences of Ediacaran-Cambrian discoidal structures described in this continent. Based on a detailed review and unpublished data, it was possible to provide a general picture concerning the main paleoenvironmental and sedimentary significance of this structures, as well as on the most promising prospects in terms of the paleontological record of Ediacaran soft-bodied metazoans in South American. In this sense, it was settled that occurrences such as those in the Jaibaras and Itajaí basins should be reassessed in order to establish reliable criteria of biogenicity. In the case of the material from the Sete Lagoas and Tagatiya Guazu formations, it is considered more parsimonious to interpret the discoidal features as resulting from microbial processes. Similarly, the discoidal structures of the Cerro Negro Formation presents a series of internal laminations and textures that resembles those developed by processes of microbial grain binding and trapping suggesting that, at least part of this material, can be related to microbially induced sedimentary structures. Finally, for the ichnologically diversified Puncoviscana and Camaquã basins, two different scenarios were identified. The first presents an ichnological assemblage strongly indicative of lower Paleozoic, and possibly Cambrian affinity. Thus, the discs in association with these traces, should be viewed with caution and interpretations made in light of a Paleozoic context. The second possesses an ichnological association typical of that expected for the Ediacaran-Cambrian transition, and the diversity of discoidal forms can potentially represent imprints of macroorganisms on a microbially bounded substrate, thus deserving a more detailed approach.Fil: Inglez, L.. Universidade Estadual Paulista Julio de Mesquita Filho; BrasilFil: Warren, L. V.. Universidade Estadual Paulista Julio de Mesquita Filho; BrasilFil: Okubo, J.. Universidade Estadual Paulista Julio de Mesquita Filho; BrasilFil: Simões, M. G.. Universidade Estadual Paulista Julio de Mesquita Filho; BrasilFil: Quaglio, F.. Universidade de Sao Paulo; BrasilFil: Arrouy, Maria Julia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Centro de Investigaciones Geológicas. Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. Centro de Investigaciones Geológicas; ArgentinaFil: Netto, R. G.. Universidade do Vale do Rio dos Sinos; Brasi

Paleomagnetism of the Ediacaran Avellaneda Formation (Argentina), part I: Paleogeography of the Río de la Plata craton at the dawn of Gondwana

A paleomagnetic and rock magnetic study was carried out in the Ediacaran Avellaneda (∼570 Ma) and Cerro Negro (∼555 Ma) Formations belonging to the La Providencia Group, in the upper part of the Neoproterozoic sedimentary cover of the Tandilia region, in the Río de la Plata craton. The Avellaneda Formation was studied at outcrop level and in three drill cores, yielding a mean characteristic remanence direction of Dec: 21.4°, Inc: 67.1°, α95: 4.2°, k: 23.9, N: 51 and a paleomagnetic pole at 1.0° S, 313.4° E, A95: 5.9°. The Cerro Negro Formation yielded a mean characteristic direction of Dec: 22.0°, Inc: 68.5°, α95: 10.3°, k: 20.8, N: 11 obtained from a single drill core, from which a paleomagnetic pole at 3.6° S, 307.8°E, A95: 16.6° was computed. Rock magnetic data indicates that magnetic remanence is mainly associated with magnetite and hematite. The paleomagnetic information presented here results in a change in the previously accepted Late Ediacaran apparent polar wander path of the Río de la Plata Craton. The newly obtained poles indicate that Río de la Plata Craton experienced a rapid drift from a low latitudes location (ca. 19° S) at ca. 600 Ma to moderately higher latitudes (between 50° and 42°S) from around 580 to 550 Ma.Fil: Franceschinis, Pablo Reinaldo. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Geociencias Básicas, Aplicadas y Ambientales de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Geociencias Básicas, Aplicadas y Ambientales de Buenos Aires; ArgentinaFil: Afonso, J. W.. Universidade de Säo Paulo, Säo Paulo; BrasilFil: Arrouy, M. J.. Universidade de Sao Paulo; BrasilFil: Gómez Peral, Lucia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Centro de Investigaciones Geológicas. Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. Centro de Investigaciones Geológicas; ArgentinaFil: Poire, Daniel Gustavo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Centro de Investigaciones Geológicas. Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. Centro de Investigaciones Geológicas; ArgentinaFil: Trindade, R. I. F.. Universidade de Sao Paulo; BrasilFil: Rapalini, Augusto Ernesto. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Geociencias Básicas, Aplicadas y Ambientales de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Geociencias Básicas, Aplicadas y Ambientales de Buenos Aires; Argentin