Evolución química, textural y microestructural de carbonatos y sulfatos cálcicos durante la diagénesis : un estudio experimental

Tesis inédita de la Universidad Complutense de Madrid, Facultad de Ciencias Geológicas, leída el 20-01-2023Minerals have a chemical composition and crystal structure that reflect the pressure and temperature conditions of the environment in which they formed. When the conditions change, minerals usually undergo transformations to adapt themselves to these new conditions. When these transformations take place in subsurface conditions, where pressure and temperature are usually low, they are commonly driven by the action of a fluid and involve the replacement of the primary minerals by secondary phases. The presence of a fluid triggers the replacement reaction to take place through an interface coupled dissolution-precipitation (ICDP) mechanism. This mechanism makes the kinetics of the transformation to be much faster than if the transformation were developed by diffusion in solid state. Due to the ubiquity and relevance of mineral replacement reactions in subsurface geological environments, numerous experimental studies on the development of these processes in different systems have been carried out over the last two decades. However, many of the parameters controlling these fluid-driven mineral replacement processes are still poorly understood. This PhD thesis seeks to advance in this understanding. With this aim, experiments have been designed to experimentally study the replacement of calcium carbonate and sulphate phases through dissolution-crystallisation reactions that take place in the temperature range between 25°C and 200°C. The thesis is divided into 5 result chapters, which are presented as scientific article and address two major problems...Los minerales tienen una composición química y una estructura cristalina que reflejan las condiciones de presión y temperatura del entorno en el que se formaron. Cuando las condiciones cambian, los minerales sufren transformaciones para adaptarse a las nuevas condiciones del entorno. Cuando estas transformaciones tienen lugar en condiciones subsuperficiales, donde la presión y la temperatura son bajas, es común que estén catalizadas por la acción de un fluido que conduzca al reemplazamiento de los minerales primarios por fases secundarias. La presencia de un fluido hace que la reacción de reemplazamiento tenga lugar a través un mecanismo acoplado de disolución-cristalización. Este mecanismo facilita que la cinética de la transformación sea mucho más rápida que si se desarrollara por difusión en estado sólido. Debido a la ubicuidad y relevancia de las reacciones de reemplazamiento mineral en los entornos geológicos subsuperficiales, a lo largo de las últimas dos décadas se han realizado numerosos estudios experimentales acerca del desarrollo de estos procesos en distintos sistemas. Sin embargo, todavía no se comprenden bien muchos de los parámetros que controlan los procesos de reemplazamiento mineral mediados por fluidos. Esta tesis doctoral busca avanzar en esta comprensión. Para ello, se han diseñado experimentos orientados a estudiar experimentalmente el reemplazamiento de carbonatos y sulfatos cálcicos a través de reacciones de disolución-cristalización que se desarrollan en el rango de temperaturas entre 25°C y 200°C. La tesis está dividida en 5 capítulos de resultados, que se presentan en forma de artículo científico y que abordan dos grandes problemas...Fac. de Ciencias GeológicasTRUEunpu

The formation of barite and celestite through the replacement of gypsum

Barite (BaSO4) and celestite (SrSO4) are the end-members of a nearly ideal solid solution. Most of the exploitable deposits of celestite occur associated with evaporitic sediments which consist of gypsum (CaSO4·2H2O) or anhydrite (CaSO4). Barite, despite having a broader geological distribution is rarely present in these deposits. In this work, we present an experimental study of the interaction between gypsum crystals and aqueous solutions that bear Sr or Ba. This interaction leads to the development of dissolution-crystallization reactions that result in the pseudomorphic replacement of the gypsum crystals by aggregates of celestite or barite, espectively. The monitoring of both replacement reactions shows that they take place at very different rates. Millimeter-sized gypsum crystals in contact with a 0.5 M SrCl2 solution are completely replaced by celestite aggregates in less than 1 day. In contrast, only a thin barite rim replaces gypsum after seven days of interaction of the latter with a 0.5 M BaCl2 solution. We interpret that this marked difference in the kinetics of the two replacement reactions relates the different orientational relationship that exists between the crystals of the two replacing phases and the gypsum substrate. This influence is further modulated by the specific crystal habit of each secondary phase. Thus, the formation of a thin oriented layer of platy barite crystals effectively armors the gypsum surface and prevents its interaction with the Ba-bearing solution, thereby strongly hindering the progress of the replacement reaction. In contrast, the random orientation of celestite crystals with respect to gypsum guarantees that a significant volume of porosity contained in the celestite layer is interconnected, facilitating the continuous communication between the gypsum surface and the fluid phase and guaranteeing the progress of the gypsum-by-celestite replacement

Crecimiento Epitaxial de Celestina (SrSO4) sobre Anhidrita (CaSO4): El Efecto de las Impurezas

En este trabajo se analizan y discuten las relaciones epitaxiales existentes entre anhidrita (CaSO4) y celestina (SrSO4). La interacción de anhidrita y yeso con soluciones acuosas ricas en Sr es el proceso genético que se acepta para la formación de los grandes yacimientos de celestina asociados a rocas sedimentarias (Hanor, 2004). En este trabajo se busca conocer el funcionamiento de la reacción de reemplazamiento de anhidrita por celestina y el rol que juegan las posibles relaciones epitaxiales entre ambas fases en el mismo

Aproximación experimental a la formación de yacimientos de celestina: El efecto del bario

Depto. de Mineralogía y PetrologíaFac. de Ciencias GeológicasTRUEMinisterio de EconomíaMinisterio de Universidadespu

Nuevos datos sobre el origen de las mineralizaciones de celestina (SrSO4) de la cuenca de Granada

Depto. de Mineralogía y PetrologíaFac. de Ciencias GeológicasTRUEpu

Formación de Estroncianita y Witherita a través del Reemplazamiento de Cristales de Calcita

Depto. de Mineralogía y PetrologíaFac. de Ciencias GeológicasTRUEMinisterio de EconomíaMinisterio de Universidadespu

xperimental burial diagenesis of aragonitic biocarbonates: from organic matter loss to abiogenic calcite formation

Carbonate biological hard tissues are valuable archives of environmental information. However, this information can be blurred or even completely lost as hard tissues undergo diagenetic alteration. This is more likely to occur in aragonitic skeletons because bioaragonite often transforms into calcite during diagenesis. For reliably using aragonitic skeletons as geochemical proxies, it is necessary to understand in depth the diagenetic alteration processes that they undergo. Several works have recently investigated the hydrothermal alteration of aragonitic hard tissues during short-term experiments at high temperatures (T > 160 ∘C). In this study, we conduct long-term (4 and 6 months) hydrothermal alteration experiments at 80 ∘C using burial-like fluids. We document and evaluate the changes undergone by the outer and inner layers of the shell of the bivalve Arctica islandica, the prismatic and nacreous layers of the hard tissue of the gastropod Haliotis ovina, and the skeleton of the coral Porites sp. combining a variety of analytical tools (X-ray diffraction, thermogravimetry analysis, laser confocal microscopy, scanning electron microscopy, electron backscatter diffraction and atomic force microscopy). We demonstrate that this approach is the most adequate to trace subtle, diagenetic-alteration-related changes in aragonitic biocarbonate structural hard materials. Furthermore, we unveil that the diagenetic alteration of aragonitic biological hard tissues is a complex multi-step process where major changes occur even at the low temperature used in this study, well before any aragonite into calcite transformation takes place. Alteration starts with biopolymer decomposition and concomitant generation of secondary porosity. These processes are followed by abiogenic aragonite precipitation that partially or totally obliterates the secondary porosity. Only subsequently does the transformation of the aragonite into calcite occur. The kinetics of the alteration process is highly dependent on primary microstructural features of the aragonitic biomineral. While the skeleton of Porites sp. remains virtually unaltered for the entire duration of the conducted experiments, Haliotis ovina nacre undergoes extensive abiogenic aragonite precipitation. The outer and inner shell layers of Arctica islandica are significantly affected by aragonite transformation into calcite. This transformation is extensive for the prismatic shell layer of Haliotis ovina. Our results suggest that the majority of aragonitic fossil archives are overprinted, even those free of clear diagenetic alteration signs. This finding may have major implications for the use of these archives as geochemical proxies

The role of sulfate in the hydrothermal replacement of aragonite single crystals by calcite

Aragonite (CaCO3) is a stable calcium carbonate phase under high pressure conditions. However, its formation in (sub)surface environments, where calcite is the stable polymorph, is widespread. Regardless of its origin, aragonite is expected to undergo transformation into calcite under moderate pressures and temperatures. However, this transformation does not always take place, which results in the presence of abundant aragonitic relics in the geological record. Traditionally, this preservation has been explained by the presence of chemical inhibitors that prevent the conversion of aragonite to calcite. While it is widely accepted that magnesium (Mg) plays a key role in the polymorphic selection of CaCO3, the influence of other ions has also been suggested. This work evaluates the effect that different concentrations of sulfate (SO42−) in the fluid has on the progress of the aragonite-to-calcite transformation at 220 °C. Our results show that, upon reaction with deionized water or sulfate-poor solutions ([SO42−]aq 15 µm) through an interface coupled dissolution-precipitation reaction. The replacement starts at the aragonite crystal surfaces and advances inwards thanks to the development of an extensive network of fractures. Contrarily, when the solution bears higher concentrations of sulfate ([SO42−]aq > 0.1 mM), only a thin layer of smaller crystals of calcite (< 10 µm) form on the aragonite substrates, without any further transformation taking place. We interpret that these smaller crystals exert too little crystallization pressure and fail to promote the development of a network of fractures. In the absence of this network, the aragonite-calcite transformation cannot take place. The transformation does not occur neither when the experiments are conducted with deionized water and fragments of gypsum or anhydrite together with the aragonite grains. The results of this study shed light on the influence of dissolved sulfate in the kinetics of the fluid-driven transformation of aragonite into calcite. These results are useful to understand the preservation of aragonite in a variety of current geological settings and provide valuable insights for better understanding the diagenesis of sedimentary carbonates.Ministerio de Ciencia, Innovación y UniversidadesHelmholtz Recruitment InitiativeMinisterio de UniversidadesDepto. de Mineralogía y PetrologíaFac. de Ciencias GeológicasTRUEpu