Geochemistry and oxidation state of Lau Basin basalts

The backarc glasses recovered during Ocean Drilling Program Leg 135 are unique among submarine tholeiitic glasses with respect to their oxygen fugacity and sulfur concentrations. Unlike mid-ocean-ridge basalt glasses, fO2 in these samples (inferred from ratios Fe3+/Fe2+) is high and variable, and S variations (90-1140 ppm) are not coupled with FeO concentration. Strong correlations occur between the alkali and alkaline-earth elements and both fO2 (positive correlations) and S concentrations (negative correlations). Correlations between fO2 and various trace elements are strongest for those elements with a known affinity for hydrous fluids (perhaps produced during slab dehydration), suggesting the presence of a hydrous fluid with high fO2 and high alkali and alkaline earth element concentrations in the Lau Basin mantle. Concentrations of S and fO2 are strongly correlated; high fO2 samples are characterized by low S in addition to high alkali and alkaline earth element concentrations. The negative correlations between S and these trace elements are not consistent with incompatible behavior of S during crystallization. Mass balance considerations indicate that the S concentrations cannot result simply from mixing between low-S and high-S sources. Furthermore, there is no relationship between S and other trace elements or isotope ratios that might indicate that the S variations reflect mixing processes. The S variations more likely reflect the fact that when silicate coexists with an S-rich vapor phase the solubility of S in the silicate melt is a function of fO2 and is at a minimum at the fO2 conditions recorded by these glasses. The absence of Fe-sulfides and the high and variable vesicle contents are consistent with the idea that S concentrations reflect silicate-vapor equilibria rather than silicate-sulfide equilibria (as in MORB). The low S contents of some samples, therefore, reflect the high fO2 of the supra-subduction zone environment rather than a low-S source component.

Mineralogy and geochemistry of a basalt from ODP Hole 119-738C

The Lower Cretaceous tholeiitic basalt cored at Site 738, on the southernmost part of the Kerguelen Plateau, shows anomalous Sr, Nd, and Pb isotopic compositions compared to other lavas from Kerguelen Island and the Kerguelen Plateau. The strongly negative value of eNd (- 8.5) and high 207Pb/204Pb ratio (15.71) reflect a long-term evolution in a source high in Nd/Sm and µ. These geochemical properties, not observed in the Indian Ocean mantle plumes (St. Paul, Kerguelen Islands), have been reported for alkali lavas erupted in East Antarctica, thus suggesting that they originate from the Gondwana subcontinental lithosphere.

Geochemistry, mineralogy, and 40K-40Ar radiometric dating of DSDP Leg 84 basalts - Guatemala Trench

Geochemical (atomic absorption, neutron activation analyses), mineralogical (microprobe), and radiometric (40K - 40Ar) data are presented for five basalts from the Guatemala Trench area (Deep Sea Drilling Project, Leg 84). Strong geochemical and mineralogical differences distinguish two types among these basalts: (1) One basalt (Sample 567A-19,CC), recovered below Upper Cretaceous limestone has the following characteristics: it is quartz normative and has low TiO2, content, as well as low amounts of Cr, Ni and other transition metals, an LREE depleted pattern, and affinities of clinopyroxene phenocryst plotted into the field of tholeiitic and calc-alkalic pyroxenes. (2) Four alkaline basalts, recovered from the mafic and ultramafic acoustic basement, are nepheline normative and show high TiO2 content, high amounts of Cr, Ni and so on, an LREE enriched pattern and compositions of clinopyroxene phenocryst plotted close to or within the field of alkali basalt pyroxenes. These basalts are comparable to those recognized in the lower part of the Santa Elena complex and are clearly different from the oceanic basalts of the Cocos Plate. The radiometric age of the orogenic basalt seems to be close to 80 Ma. The alkaline basalts are clearly older, even if a discrepancy appears between the results of different analyses because of the secondary effects of alteration.

Geochemistry and petrogenesis of basalts from DSDP Leg 92 holes

Basalts recovered on DSDP Leg 92 include all the major basalt types so far recovered from the ocean crust of the eastern Pacific. Basalts from Holes 597, 597A, 597B, 597C, and 599B are tholeiites exhibiting all the mineralogical and geochemical characteristics of N-type mid-ocean ridge basalts (MORB). Fragments of ferrobasalts and alkali basalts were also obtained, however, from Holes 60IB and 602B, respectively. Hole 597C, which penetrated 91 m into basement and is the deepest hole so far drilled in fast-spreading crust, yielded basalts that can be divided into three major lithologic units. The lowest unit, Unit III, contains modal olivine and comprises basalts which, at about 8 to 10% MgO, are as basic as any sampled from fast-spreading crust. The middle unit, Unit II, is the most evolved; its basalts are olivine free and contain between 6 and 7.5% MgO. The upper unit, Unit I, is intermediate in composition between Units II and III; it is characterized by both modal olivine and glomerocrysts made up of plagioclase and rare olivine. Unit I is probably a massive flow, whereas Units II and III may be massive flows or sills. The basalts appear to have undergone three stages of alteration ("deuteric," "relatively reducing," and "oxidizing"), the intensity of alteration decreasing markedly downcore. Hole 597B, at 26.4 m of basement penetration the only other "deep" hole, contains just one lithologic unit, which closely resembles Unit I of Hole 597C. Petrogenetic modeling reveals that the three lithologic units in Hole 597C are cogenetic and that they were derived from a depleted mantle source similar to the source of the tholeiites and ferrobasalts sampled in other holes; the alkali basalts are the only rocks derived from enriched mantle. Lavas of Unit III probably lay on the olivine-plagioclase cotectic, whereas the other lavas lay on an olivine-plagioclase-clinopyroxene peritectic. Some 60% of closed-system crystallization is needed to generate the most-evolved from the last-fractionated tholeiite, and a further 50% crystallization (80% overall) is needed to generate the ferrobasalts. Xenocrysts of calcic plagioclase and pseudomorphosed olivine in tholeiites from Hole 597B and Unit I of Hole 597C, and in the ferrobasalts from Hole 601B, provide evidence, however, that some magma mixing may have taken place.

Contribution to Theme A of the Benchmark Workshop: Effect of Concrete Swelling on the Equilibrium and Displacements of an Arch Dam

The Kariba dam is undergoing concrete expansion as a result of an alkali-aggregate reaction. The model adopted to simulate the process is explained in the paper; it is based on the model first proposed by Ulm et al, as later modified by Saouma and Perotti. It has been implemented in the commercial finite element code Abaqus and applied to solve the benchmark problem. The parameters of the model were calibrated using the data recorded up to 1995. The calibrated model was then used for predicting the evolution of the dam up to the present date. Apart from this prediction the paper offers a number of conclusions, such as the fact that the stress level appears to have a major influence on the expansion process; and it presents some suggestions to improve the formulation of the benchmark, such as providing temperature data and widening the locations and conditions of the data employed in the calibration

Effect of Concrete Swelling on the Equilibrium and Displacements of an Arch Dam

The Kariba dam is undergoing concrete expansion as a result of an alkali-aggregate reaction. The model adopted to simulate the process is explained in the paper; it is based on the model first proposed by Ulm et al, as later modified by Saouma and Perotti. It has been implemented in the commercial finite element code Abaqus and applied to solve the benchmark problem. The parameters of the model were calibrated using the data recorded up to 1995. The calibrated model was then used for predicting the evolution of the dam up to the present date. Apart from this prediction the paper offers a number of conclusions, such as the fact that the stress level appears to have a major influence on the expansion process; and it presents some suggestions to improve the formulation of the benchmark, such as providing temperature data and widening the locations and conditions of the data employed in the calibration

Estudio de la reactividad álcali-sílice originada por componentes reactivos minoritarios

El objetivo de la presente tesis se enmarca dentro del estudio del estado de hormigones de presas, desarrollado en los últimos años en el Laboratorio Central del CEDEX, en el que se ratifica que una de las causas más importantes del deterioro de obras hidráulicas en España es la reacción álcali-sílice. La tesis que se presenta pretende contribuir al mejor conocimiento de la reacción álcali sílice con fines normativos preventivos, abordando los aspectos relativos a la identificación de áridos reactivos en el hormigón. El conocimiento de los áridos reactivos en España (origen de la reactividad, tipos de reacción y su comportamiento, así como las herramientas disponibles para su detección) es imprescindible para evitar la futura aparición de esta patología en nuevas estructuras, ya sea evitando el uso de áridos reactivos o tomando las medidas preventivas necesarias si su utilización es inevitable. A partir del Estudio Bibliográfico realizado se han detectado diversas lagunas en la identificación y caracterización de áridos de reacción rápida, cuya característica principal es que son reactivos con concentraciones muy bajas de diferentes componentes reactivos. Para resolver las lagunas identificadas se ha planeado un estudio experimental, consistente en el análisis de áridos cuya reactividad es conocida porque han sido empleados en obras afectadas por la reacción álcali sílice. Sobre el árido grueso extraído de estas estructuras se han realizado una serie de ensayos normalizados (estudio petrográfico, ensayo acelerado de probetas de mortero, ensayo Gel Pat y ensayos químicos). El análisis de los resultados experimentales ha permitido conocer las limitaciones reales en áridos reactivos españoles de las diferentes técnicas existentes, tratando de minimizarlas para áridos cuya reactividad es debida a componentes minoritarios (áridos de reacción rápida). Además, se ha evaluado la utilización de la difracción de rayos X (no normalizada) y la creación de un nuevo ensayo (Gel Pat Modificado). Finalmente, el estudio experimental ha permitido fijar una metodología de ensayo para el estudio de áridos reactivos por su contenido en componentes minoritarios (áridos de reacción rápida). The objective of this Thesis fits into the research program developed in CEDEX the last years and focused on the durability of concrete in Dams. This research work confirms that one of the main problems related to the deterioration of hydraulic structures is the alkali silica reaction. This Thesis aims to contribute to a better understanding of alkali-silica reaction, for preventive regulation purposes, considering the aspects related to the identification of reactive aggregates. The knowledge of Spanish reactive aggregates (origin of the reactivity, types of reaction and their behavior, and the tools available to detect and describe them) is essential to avoid the appearance of this pathology in new structures, either not using the reactive aggregate or taking the necessary preventive measures available in bibliography if the use of the reactive aggregate is inevitable. From the State-of –the-Art developed, several gaps have been detected in the detection and description of rapid reactive aggregates, which main characteristic if that they are reactive with low content of some reactive components. An experimental programme has been designed to solve these gaps, consisting on studying the reactivity of aggregates used in Spanish structures affected by the alkali silica reaction. Several Standard Tests have been carried out on coarse aggregates removed from the affected structures (Petrographic description, Accelerated Mortar Bar Test, Gel Pat Test and Chemical Tests). The analysis of the results obtained in Spanish reactive aggregates allows to know the advantages and limitations of each test, trying to minimize the disadvantages to detect Spanish reactive aggregates because of the minority content of rapid reactive components (rapid reactive aggregates). Moreover, X ray diffraction (not standardized) has been tested to detect rapid reactive aggregates and also a new test has been developed for the same purpose (Optimized Gel Pat Test). Finally, the experimental programme has made possible to define a methodology for detection of Spanish rapid reactive aggregates.

Selección de métodos estadísticos para la interpretación de intercomparaciones en ensayos de áridos finos

El objeto de este proyecto es seleccionar los métodos estadísticos adecuados para el tratamiento de los resultados obtenidos en un ejercicio de intercomparación para ensayos de áridos para construcción con el fin de cumplir la normativa para dicho ejercicio. Los ensayos estudiados son: - Ensayo Azul de Metileno - Equivalente de arena - Contenido total en azufre - Contenido en humus - Reactividad álcali-carbonato Los criterios aplicables para el desarrollo de los trabajos son los exigidos por la Entidad Nacional de Acreditación (ENAC) para laboratorios acreditados conforme a la Norma UNE-EN ISO/IEC 17025 y para proveedores acreditados conforme a la Norma UNE-EN ISO/IEC 17043. ABSTRACT: The purpose of this Project is to select appropriate statistical methods for the treatment of the results obtained from an intercomparison exercise for testing of aggregates for construction with the target of comply the rules for this exercise. The studied assays are: - Methylene Blue Assay - Sand equivalent - Total Sulfur Content - Humus content - Alkali-carbonate reactivity The criteria for the development of the work are those required by the National Accreditation Body (ENAC) for laboratories accredited in accordance with the UNE-EN ISO / IEC 17025 and for suppliers in accordance with the UNE-EN ISO / IEC 17043.

Concrete Swelling in a Double Curvature Dam

Se describe el problema del hinchamiento del hormigón en las presas de doble curvatura. Several chemical reactions are able to produce swelling of concrete for decades after its initial curing, a problem that affects a considerable number of concrete dams around the world. The object of the work reported is to simulate the underlying mechanisms with sufficient accuracy to reproduce the past history and to predict the future evolution reliably. Having studied the available formulations, that considered to be more promising was adopted and introduced via user routines in a commercial finite element code. It is a non isotropic swelling model,compatible with the cracking and other non-linearities displayed by the concrete. The paper concentrates on the work conducted for a double-curvature arch dam. The model parameters were determined on the basis of some parts of the dam’s monitored histories, reliability was then verified using other parts and, finally, predictions were made about the future evolution of the dam and its safety margin.