Geochemistry and oxygen and iron isotope ratios of mafic rocks

Recycling of oceanic crust into the deep mantle via subduction is a widely accepted mechanism for creating compositional heterogeneity in the upper mantle and for explaining the distinct geochemistry of mantle plumes. The oxygen isotope ratios (d18O) of some ocean island basalts (OIB) span values both above and below that of unmetasomatised upper mantle (5.5 ± 0.4 per mil) and provide support for this hypothesis, as it is widely assumed that most variations in d18O are produced by near-surface low-temperature processes. Here we show a significant linear relationship between d18O and stable iron isotope ratios (d57Fe) in a suite of pristine eclogite xenoliths. The d18O values of both bulk samples and garnets range from values within error of normal mantle to significantly lighter values. The observed range and correlation between d18O and d57Fe is unlikely to be inherited from oceanic crust, as d57Fe values determined for samples of hydrothermally altered oceanic crust do not differ significantly from the mantle value and show no correlation with d18O. It is proposed that the correlated d57Fe and d18O variations in this particular eclogite suite are predominantly related to isotopic fractionation by disequilibrium partial melting although modification by melt percolation processes cannot be ruled out. Fractionation of Fe and O isotopes by removal of partial melt enriched in isotopically heavy Fe and O is supported by negative correlations between bulk sample d57Fe and Cr content and bulk sample and garnet d18O and Sc contents, as Cr and Sc are elements that become enriched in garnet- and pyroxene-bearing melt residues. Melt extraction could take place either during subduction, where the eclogites represent the residues of melted oceanic lithosphere, or could take place during long-term residence within the lithospheric mantle, in which case the protoliths of the eclogites could be of either crustal or mantle origin. This modification of both d57Fe and d18O by melting processes and specifically the production of low-d18O signatures in mafic rocks implies that some of the isotopically light d18O values observed in OIB and eclogite xenoliths may not necessarily reflect near-surface processes or components.

Nd, Pb, and Be isotopes of ferromanaganese crusts of the South Pacific

The application of radiogenic isotopes to the study of Cenozoic circulation patterns in the South Pacific Ocean has been hampered by the fact that records from only equatorial Pacific deep water have been available. We present new Pb and Nd isotope time series for two ferromanganese crusts that grew from equatorial Pacific bottom water (D137-01, 'Nova', 7219 m water depth) and southwest Pacific deep water (63KD, 'Tasman', 1700 m water depth). The crusts were dated using 10Be/9Be ratios combined with constant Co-flux dating and yield time series for the past 38 and 23 Myr, respectively. The surface Nd and Pb isotope distributions are consistent with the present-day circulation pattern, and therefore the new records are considered suitable to reconstruct Eocene through Miocene paleoceanography for the South Pacific. The isotope time series of crusts Nova and Tasman suggest that equatorial Pacific deep water and waters from the Southern Ocean supplied the dissolved trace metals to both sites over the past 38 Myr. Changes in the isotopic composition of crust Nova are interpreted to reflect development of the Antarctic Circumpolar Current and changes in Pacific deep water circulation caused by the build up of the East Antarctic Ice Sheet. The Nd isotopic composition of the shallower water site in the southwest Pacific appears to have been more sensitive to circulation changes resulting from closure of the Indonesian seaway.

Nd, Sr and Pb isotopic composition of eolian dust deposited in the central North Pacific

The silicate fractions of recent pelagic sediments in the central north Pacific Ocean are dominated by eolian dust derived from central Asia. An 11 Myr sedimentary record at ODP Sites 885/886 at 44.7°N, 168.3°W allows the evaluation of how such dust and its sources have changed in response to late Cenozoic climate and tectonics. The extracted eolian fraction contains variable amounts (>70%) of clay minerals with subordinate quartz and plagioclase. Uniform Nd isotopic compositions (epsilon-Nd =38.6 to 310.5) and Sm/Nd ratios (0.170-0.192) for most of the 11 Myr record demonstrate a well-mixed provenance in the basins north of the Tibetan Plateau and the Gobi Desert that was a source of dust long before the oldest preserved Asian loess formed. epsilon-Nd values of up to 36.5 for samples 62.9 Ma indicate <=35 wt% admixture of a young, Kamchatka-like volcanic arc component. The coherence of Pb and Nd in the erosional cycle allows us to constrain the Pb isotopic composition of Asian loess devoid of anthropogenic contamination to 206Pb/204Pb =18.97 +/- 0.06, 207Pb/204Pb =15.67 +/- 0.02, 208Pb/204Pb =39.19 +/- 0.11. 87Sr/86Sr (0.711-0.721) and Rb/Sr ratios (0.39-1.1) vary with dust mineralogy and provide an age indication of ~250 Ma. 40Ar/39Ar ages of six dust samples are uniform around 200 Ma and match the K-Ar ages of modern dust deposited on Hawaii. These data reflect the weighted age average of illite formation. Changes from illite- smectite with significant kaolinite to illite- and chlorite-rich, kaolinite-free assemblages since the late Pliocene document changes in the intensity of chemical weathering in the source region. Such weathering evidently did not disturb the K-Ar systematics, and only induced scatter in the Rb-Sr data. We propose that when smectite forms at the expense of illite, K and Ar are quantitatively lost from what becomes smectite, but are quantitatively retained in adjacent illite layers. 40Ar/39Ar age data, therefore, are insensitive to smectite formation during chemical weathering but date the diagenetic growth of illite, the major K-bearing phase in the dust. Over the past 12 Myr, the dust flux to the north Pacific increased by more than an order of magnitude, documenting a substantial drying of central Asia. This climatic change, however, did not alter the ultimate source of the dust, and neoformational products of chemical weathering always remained subordinate to assemblages reworked by mechanical erosion in dust deposited in eastern Asia and the Pacific Ocean.

La gramática en la enseñanza de la lengua materna

Desde hace varias décadas se viene cuestionando la utilidad de la gramática en la enseñanza. La mayoría de los argumentos pueden sintetizarse en que no enseña a leer más comprensivamente ni a producir textos más apropiados. A esto se suma que, a diferencia del estructuralismo, que tuvo una adecuada transposición didáctica, las teorías actuales proponen desarrollos sofisticados que las alejan de las aulas. Como reacción, suele pensarse que solo marcos teóricos centrados en el uso y desentendidos del tratamiento formal de los fenómenos pueden ser útiles. Se asume así que, dado que todos nos servimos de la lengua, es posible dar cuenta de su funcionamiento sin los conceptos y herramientas apropiados, es decir, se pasa por alto la especificidad del dominio. Sin embargo, "sin una teoría de las palabras - es decir, una gramática - no hay manera de hacer explícita la propia interpretación del significado de un texto" (Halliday: 1985). Por tanto, buscando un equilibrio entre el "saber instrumental" del hablante nativo y el altamente especializado, en nuestros proyectos de investigación venimos impulsando un enfoque centrado en la palabra, como unidad esencial de la lengua. En este planteo, el léxico constituye la interfaz que, por un lado, vincula los distintos componentes de la estructura lingüística (Jackendoff 1990) y, por el otro, pone en relación el sistema lingüístico con el conocimiento de mundo. De este modo, es el hilo conductor que facilita el aprendizaje y habilita la "reintroducción del significado", como elemento fundamental en el estudio de la gramática (Camps y Zayas: 2006). Según nos proponemos mostrar en esta ponencia, un enfoque léxico-gramatical hace posible avanzar desde el reconocimiento de las estructuras morfosintácticas, como conjunto de opciones que ofrece la lengua, hasta la identificación de los valores comunicativos que los hablantes les otorgan en diferentes contextos. El estudio de la gramática queda así englobado en una perspectiva integradora que, junto con la reflexión sobre el sistema, permite considerar su uso

La contribución de algunos rasgos prosódicos a la coherencia de fragmentos narrativos en conversaciones informales

En este trabajo abordamos el análisis de algunas estrategias utilizadas por los hablantes en conversaciones coloquiales para citar discursos propios y de terceros. Desde una perspectiva sistémicofuncional, centramos nuestra atención en las cláusulas paratácticas de proyección (Halliday 1994, Thompson 2004) utilizadas por los participantes en fragmentos narrativos (relatos, anécdotas, chismes). Los datos utilizados en esta investigación surgen de un corpus de 60 conversaciones informales grabadas en audio y/o video entre estudiantes universitarios argentinos de entre 18 y 28 años de edad, recolectado en el marco del proyecto Cohesión y Coherencia en la Conversación Coloquial de la UNLP. En este estudio analizamos las estrategias léxicogramaticales y prosódicas frecuentemente imitaciones y parodias de los hablantes a quienes se citaempleadas en las cláusulas proyectadas para lograr diferentes efectos pragmáticos. Los resultados preliminares obtenidos parecen indicar que los recursos en estudio, sumados al conocimiento del mundo y del contexto de situación que aportan los participantes, contribuyen a la coconstrucción de la coherencia en el tipo de interacción verbal que estudiamos. Entendemos que es importante que los alumnos de español como lengua segunda y extranjera tomen conciencia de la importancia que los recursos analizados en este trabajo tienen para lograr una interacción eficaz

Funciones interpersonales de las metáforas gramaticales textuales en conversaciones coloquiales entre estudiantes universitarios

Un tema central en la Lingüística Sistémico funcional -perspectiva teórica desde la que abordaremos este estudio- es el de la metáfora gramatical que fuera introducido por M.K.A. Halliday quien la define como un tipo de metáfora que complementa a la metáfora léxica, más trabajada hasta ese momento. En trabajos anteriores (Ferrari, 2011; Ferrari y Granato, 2011) hemos determinado que ciertas estructuras del español, las oraciones hendidas y pseudo-hendidas, pueden ser consideradas metáforas gramaticales textuales en español y que tienen funciones específicas en la conversación coloquial como por ejemplo enfatizar la idea de repetición, o de obligatoriedad y crear en el discurso el efecto de exclusividad (Ferrari, 2011). También servirían como un elemento anticipatorio en posición temática que indicaría al interlocutor que se va a introducir información nueva (Piatti y Granato, 2010). Estas funciones de las metáforas textuales se relacionan con la metafunción textual, que otorga a la cláusula su carácter como mensaje y una organización específica por medio de la cual encaja y contribuye con el flujo del discurso (Halliday y Matthiessen (2004), y la metafunción ideacional, que permite representar el mundo en el discurso. En el presente trabajo nos ocuparemos de la incidencia de las metáforas textuales en el plano interpersonal del lenguaje. Los significados interpersonales tienen dos aspectos principales. El primero se refiere al tipo de interacción que tiene lugar: cuando decimos algo lo hacemos con un propósito, los propósitos más fundamentales son dar o pedir información o realizar o pedir que se lleve a cabo una acción. El segundo se refiere a la posición que el hablante toma en relación con lo que está diciendo. Es en este segundo aspecto en el que creemos que influyen estos tipos de metáforas textuales

Isotopic and chemical compositions of rocks and minerals from the TAG hydrothermal mound, ODP Site 957

Strontium- and oxygen-isotopic measurements of samples recovered from the Trans-Atlantic Geotraverse (TAG) hydrothermal mound during Leg 158 of the Ocean Drilling Program provide important constraints on the nature of fluid-rock interactions during basalt alteration and mineralization within an active hydrothermal deposit. Fresh Mid-Ocean Ridge Basalt (MORB), with a 87Sr/86Sr of 0.7026, from the basement beneath the TAG mound was altered at both low and high temperatures by seawater and altered at high temperature by near end-member black smoker fluids. Pillow breccias occurring beneath the margins of the mound are locally recrystallized to chlorite by interaction with large volumes of conductively heated seawater (>200°C). The development of a silicified, sulfide-mineralized stockwork within the basaltic basement follows a simple paragenetic sequence of chloritization followed by mineralization and the development of a quartz+pyrite+paragonite stockwork cut by quartz-pyrite veins. Initial alteration involved the development of chloritic alteration halos around basalt clasts by reaction with a Mg-bearing mixture of upwelling, high-temperature (>300°C), black smoker-type fluid with a minor (<10%) proportion of seawater. Continued high-temperature (>300°C) interaction between the wallrock and these Mg-bearing fluids results in the complete recrystallization of the wallrock to chlorite+quartz+pyrite. The quartz+pyrite+paragonite assemblage replaces the chloritized basalts, and developed by reaction at 250-360°C with end-member hydrothermal fluids having 87Sr/86Sr ~0.7038, similar to present-day vent fluids. The uniformity of the 87Sr/86Sr ratios of hydrothermal assemblages throughout the mound and stockwork requires that the 87Sr/86Sr ratio of end-member hydrothermal fluids has remained relatively constant for a time period longer than that required to change the interior thermal structure and plumbing network of the mound and underlying stockwork. Precipitation of anhydrite in breccias and as late-stage veins throughout most of the mound and stockwork, down to at least 125 mbsf, records extensive entrainment of seawater into the hydrothermal deposit. 87Sr/86Sr ratios indicate that most of the anhydrite formed from ~2:1 mixture of seawater and black smoker fluids (65%±15% seawater). Oxygen-isotopic compositions imply that anhydrite precipitated at temperatures between 147°C and 270°C and require that seawater was conductively heated to between 100°C and 180°C before mixing and precipitation occurred. Anhydrite from the TAG mound has a Sr-Ca partition coefficient Kd ~0.60±0.28 (2 sigma). This value is in agreement with the range of experimentally determined partition coefficients (Kd ~0.27-0.73) and is similar to those calculated for anhydrite from active black smoker chimneys from 21°N on the East Pacific Rise. The d18O (for SO4) of TAG anhydrite brackets the value of seawater sulfate oxygen (~9.5?). Dissolution of anhydrite back into the oceans during episodes of hydrothermal quiescence provides a mechanism of buffering seawater sulfate oxygen to an isotopically light composition, in addition to the precipitation and dissolution of anhydrite within the oceanic basement during hydrothermal recharge at the mid-ocean ridges.