Evaluación de la precisión del examen petrográfico de arenas en la detección de agregados reactivos

Se realizó el estudio petrográfico de una arena natural silícea proveniente del río Paraná, con el propósito de evaluar la incidencia de la experiencia del petrógrafo en la detección de materiales potencialmente reactivos trabajando con grano suelto. Para ello se tamizó la muestra según lo establecido en la norma IRAM 1649. El retenido en cada fracción se dividió en cuatro partes. Cada cuarto fue analizado por un petrógrafo diferente con distinta experiencia en el análisis litológico sobre grano suelto. Se calculó la composición del material retenido en cada tamiz y la composición en la muestra total ponderada según su abundancia (0,4 % - #10; 1,2 % - #18; 12,4 % - #30; 79,4 % - #60; 6,0 % - #100 y 0,6 % - < #100). Considerando que casi el 80 % en peso de la muestra se encuentra en el retenido #60, y por lo tanto este análisis será el que tenga mayor influencia en el cálculo de la composición total, se impregnó la arena en resina y se determinó el porcentaje de cada litología mediante conteo sobre un corte delgado para comparar los resultados con los obtenidos con grano suelto. Se puso especial énfasis en los materiales potencialmente reactivos como es el caso de la calcedonia, que aún en muy baja proporción y un error de estimación < 1 % puede modificar la calificación entre inocuo o potencialmente reactivo. Con los resultados obtenidos, se evaluó la incidencia de la experiencia de los petrógrafos en la precisión de las determinaciones

Sr, C and O isotope composition of marbles from the Sierra de de Ancasti, Eastern Sierras Pampeanas, Argentina: age and constraints for the Neoproterozoic–Lower Paleozoic evolution of the proto-Gondwana margin

The Sierra Brava Complex on the eastern flank of the Sierra de Ancasti consists of marbles, metabasites, calc-silicate rocks, psammo-pelitic schists and gneisses. In the central part of this sierra a thick succession of banded schists (Ancasti Formation) crops out. Regional metamorphism of these rocks is attributed to the Famatinian orogeny (Ordovician), metamorphic grade increasing westwards and southwards and culminating in a migmatite complex on the western side of the Sierra. The meta-carbonate rocks are subdivided into a northeastern group (low-grade calcite marbles), and a southeastern group (high-grade calcite and calcite-dolomite marbles). Twenty-three marble samples were analysed for Sr isotope composition and Rb, Mn, Mg and Ca contents, and six for C and O isotope composition. An Ediacaran depositional age of 570 –590Ma is inferred by reference to the trend of 87Sr/86Sr in Neoproterozoic seawater. Thus the metacarbonates are probably slightly older than the Ancasti Formation (equivalent to the Puncoviscana Formation of northern Argentina), which has a maximum sedimentation age of ca. 570Ma. Ediacaran depositional ages have also been reported for metacarbonates elsewhere in Argentina, Uruguay and Brazil. We propose that the Sierra de Ancasti carbonates on one hand, and those in the Western Sierras Pampeanas (Difunta Correa Sequence) and –tentativelythe Corumbá Group of Brazil on the other, represent platforms on opposite margins of the extinct Clymene Ocean, whereas Neoproterozoic carbonate successions such as the Loma Negra Formation (Tandilia, southern Argentina) and the Arroyo del Soldado Group (Uruguay) were deposited on the eastern side (present coordinates) of the Rio de la Plata craton, which at the time occupied a position farther to the north.Peer reviewe

Cryogenic bath-type heat exchangers for ultra-pure noble gas applications

Two cryogenic bath-type heat exchangers for ultra-pure noble gas applications were developed with particular emphasis on noble gas liquefaction in cryogenic distillation systems. The main objective was to construct heat exchangers for xenon from materials that do not emanate radon and that fulfill ultra-high vacuum standards. Therefore, only high-quality copper and stainless steel materials were used. Especially, large-area oxygen-free copper fins with high conductivity in a new design ensure efficient heat transfer. One bath-type Xe-Xe heat exchanger was designed with a diameter of 50 cm to achieve a xenon condensing capacity of at least 100 kg/h. In order to guarantee the necessary heat transfer between the two xenon reservoirs, this heat exchanger features a specially manufactured stainless steel flange with a copper plate welded inside. We first tested our concept on a dedicated bath-type heat exchanger with a reduced diameter of 30 cm using liquid nitrogen to liquefy the xenon. A model based on conservative assumptions such as film boiling on the nitrogen side and film condensation on the xenon side was developed and applied to caluclate the expected heat transfer for our design. We were able to demonstrate an adjustable xenon liquefaction rate of up to 113 kg/h limited only by our measurement procedure at a cooling efficiency of (0.98 ± 0.03) for the LN$_{2}$-Xe heat exchanger

Delivering on a hog futures contract

1 online resource (PDF, 4 pages)This archival publication may not reflect current scientific knowledge or recommendations. Current information available from the University of Minnesota Extension: https://www.extension.umn.edu

Evaluación de la precisión del examen petrográfico de arenas en la detección de agregados reactivos

Se realizó el estudio petrográfico de una arena natural silícea proveniente del río Paraná, con el propósito de evaluar la incidencia de la experiencia del petrógrafo en la detección de materiales potencialmente reactivos trabajando con grano suelto. Para ello se tamizó la muestra según lo establecido en la norma IRAM 1649. El retenido en cada fracción se dividió en cuatro partes. Cada cuarto fue analizado por un petrógrafo diferente con distinta experiencia en el análisis litológico sobre grano suelto. Se calculó la composición del material retenido en cada tamiz y la composición en la muestra total ponderada según su abundancia (0,4 % - #10; 1,2 % - #18; 12,4 % - #30; 79,4 % - #60; 6,0 % - #100 y 0,6 % - < #100). Considerando que casi el 80 % en peso de la muestra se encuentra en el retenido #60, y por lo tanto este análisis será el que tenga mayor influencia en el cálculo de la composición total, se impregnó la arena en resina y se determinó el porcentaje de cada litología mediante conteo sobre un corte delgado para comparar los resultados con los obtenidos con grano suelto. Se puso especial énfasis en los materiales potencialmente reactivos como es el caso de la calcedonia, que aún en muy baja proporción y un error de estimación < 1 % puede modificar la calificación entre inocuo o potencialmente reactivo. Con los resultados obtenidos, se evaluó la incidencia de la experiencia de los petrógrafos en la precisión de las determinaciones.Fil: Locati, F. Universidad Nacional de Córdoba. Consejo Nacional de Investigaciones Científicas y Técnicas. CICTERRA; Argentina.Fil: Maiza, P. Universidad Nacional del Sur. Departamento de Geología; Argentina.Fil: Marfil, S. Universidad Nacional del Sur. Departamento de Geología; Argentina.Fil: Murra, J. Universidad Nacional de Córdoba. Consejo Nacional de Investigaciones Científicas y Técnicas. CICTERRA; Argentina.Geociencias multidisciplinari

The continental assembly of SW Gondwana (Ediacaran to Cambrian): a synthesis

SW Gondwana resulted from complex interplay between continental amalgamation and dispersal between ~ 650 and 490 Ma. The main cratons involved were Laurentia, Amazonia– MARA (Proterozoic Maz–Arequipa–Rio Apa, Casquet et al., 2012), Kalahari, Rio de la Plata (RPC), Congo and East Antarctica (Mawson block). Several collisional orogenic belts resulted, notably the East Africa–Antarctica, Brasiliano–Panafrican, Pampean–Saldania, and Ross– Delamerian orogens. East-Antarctica broke away from the western margin of Laurentia in Rodinia. After a long drift and counter-clockwise rotation (Dalziel, 2013) it collided with Congo and Kalahari to produce the southern part of the left-lateral transpressional East Africa–Antarctica orogen between 580 and 550 Ma, completing the amalgamation of East Gondwana. The Trans-Antarctic margin became an active one in the Ediacaran and subduction of the Pacific Ocean lithosphere occurred throughout the Paleozoic, forming a tract of the Terra Australis orogen. NW–SE directed compression in Late Cryogenian and Early Ediacaran times promoted closure of the Adamastor Ocean, resulting in the left-lateral transpressional Brasiliano–Pan African orogeny between 650 and 570 Ma. The Pampean orogenic belt to the west of the RPC resulted from right-lateral collision between Laurentia and its eastern extension MARA on the one hand and Kalahari–RPC on the other. Ocean opening started at ~ 630 Ma and subduction and further collision took place between 540 and 520 Ma, coeval with the northward drift of Laurentia (~ 540 Ma) away from MARA and the consequent formation of the proto-Andean margin of Gondwana. The margins of the intervening Puncoviscana ocean were covered by Laurentia-derived siliciclastic sediments and carbonates on the MARA side between 630 and ~ 540 Ma (Rapela et al, 2014; this symposium), and by the marine siliciclastic Puncoviscana Formation on the other. The latter formation, deposited between a 570 and ~530 Ma, received input from large alluvial fans descending from juvenile Mesoproterozoic and Neproterozoic sources (new Hf isotope evidence) largely located in the southern East Africa–Antarctica orogen. The Pampean orogen extended into the Saldania–Gariep orogen of southern South Africa (545–520 Ma) and was apparently discordant to the earlier Brasiliano–Pan African orogen. In late-Early to late Cambrian times the Pampean–Saldania realm evolved into a passive margin with siliciclastic platform sedimentation. The Pampean-Saldania realm was separated from the active Trans-Antarctic margin of East Antarctica by an inferred transform fault in Ediacaran to Cambrian times. Regional NW–SW shortening in the Ediacaran became N–S directed in the Cambrian, suggesting a major plate reorganization at this time.Peer reviewe

Review of the Cambrian Pampean orogeny of Argentina; a displaced orogen formerly attached to the Saldania Belt of South Africa?

The Pampean orogeny of northern Argentina resulted from Early Cambrian oblique collision of the Paleoproterozoic-Mesoproterozoic MARA block, formerly attached to Laurentia, with the Gondwanan Kalahari and Rio de la Plata cratons. The orogen is partially preserved because it is bounded by the younger Córdoba Fault on the east and by the Los Túneles-Guacha Corral Ordovician shear zone on the west. In this review we correlate the Pampean Belt with the Saldania orogenic belt of South Africa and argue that both formed at an active continental margin fed with sediments coming mainly from the erosion of the Brasiliano–Pan-African and East African–Antarctica orogens between ca. 570 and 537 Ma (Puncoviscana Formation) and between 557 and 552 Ma (Malmesbury Group) respectively. Magmatic arcs (I-type and S-type granitoids) formed at the margin between ca. 552 and 530 Ma. Further right-lateral oblique collision of MARA between ca. 530 and 520 Ma produced a westward verging thickened belt. This involved an upper plate with high P/T metamorphism and a lower plate with high-grade intermediate to high P/T metamorphism probably resulting from crustal delamination or root foundering. The Neoproterozoic to Early Cambrian sedimentary cover of MARA that was part of the lower plate is only recognized in the high-grade domain along with a dismembered mafic–ultramafic ophiolite probably obducted in the early stages of collision. Uplift was fast in the upper plate and slower in the lower plate. Eventually the Saldania and Pampean belts detached from each other along the right-lateral Córdoba Fault, juxtaposing the Rio de la Plata craton against the internal high-grade zone of the Pampean belt.Centro de Investigaciones Geológica

Necessary fictions: indigenous claims and the humanity of rights

Indigenous right insistently challenges the surpassing arrogations of sovereign right. In so doing, it affirms dimensions of being-together denied or stunted in sovereign modes of political formation. This force of Indigenous right is amplified here through legal and literary instantiations. These, in turn, uncover the continuously created and fictional quality of rights, revealing them to be necessary fictions

Isotope (Sr, C) and U–Pb SHRIMP zircon geochronology of marble-bearing sedimentary series in the Eastern Sierras Pampeanas, Argentina : Constraining the SW Gondwana margin in Ediacaran to early Cambrian times

The Sierras de Cordoba Metasedimentary Series consists of marbles and metasiliciclastic rocks of Ediacaran to early Cambrian age (ca. 630 and 540 Ma) that underwent high-grade metamorphism during the collisional Pampean orogeny in the early Cambrian. The ages of the marbles were determined from the Sr-isotope composition (blind dating) of screened samples of almost pure calcite marble and were further constrained with C- and O-isotope data and U–Pb SHRIMP detrital zircon ages of an interbedded paragneiss. Two groups of samples are recognised with Sr-isotope composition ca. 0.7075 and 0.7085 that are considered stratigraphically significant. The first is inferred early Ediacaran, the second late Ediacaran to early Cambrian. The Sierras de Cordoba Metasedimentary Series is correlated for the first time with marble-bearing metasedimentary series in several sierras to the west and north of Sierras de Cordoba (e.g., the Difunta Correa Sedimentary Sequence and the Ancajan Series), implying that all were probably parts of an originally extensive sedimentary cover. These series bear evidence of sedimentary sources in the Mesoproterozoic (and Paleoproterozoic) basement of the Western Sierras Pampeanas (part of the large MARA continental block) and farther west (Laurentia?). In terms of the age of limestones/marbles and detrital zircon patterns, the Sierras de Cordoba Metasedimentary Series differs strongly from the older section of late Ediacaran to early Cambrian Puncoviscana Formation of northwestern Argentina, which mostly outcrops in northern Sierra Chica and Sierra Norte, with sedimentary input from western Gondwana sources. The Sierras de Cordoba Metasedimentary Series and the Puncoviscana Formation were probably juxtaposed during the Pampean orogeny along a complex suture zone that was further folded and/or imbricated at mid-crustal depths. The peak of metamorphism was attained at 527 ± 2 Ma. According to the evidence found here most of the Sierras Pampeanas to the west of the Sierras de Cordoba were part of the lower colliding plate during the final amalgamation of SW Gondwana.Facultad de Ciencias Naturales y MuseoCentro de Investigaciones Geológica