Suchergebnisse

Methods based on X-ray diffraction commonly used to decipher diagenetic/low- grade metamorphic conditions are revised and evaluated according to our current knowledge of related geological processes obtained via transmission electron microscopy. A lack of evident qualitative changes in phyllosilicates, including their mineral chemistry, from deep diagenetic to low-epizone conditions explains the scarcity of valid grade criteria. Throughout this range of diagenetic-metamorphic conditions a progressive increase in the crystalline domain size together with a decrease in the number of defects are the only significant changes to be observed. Therefore indices related to crystallinity still remain the most successful methods based on the crystal- chemical characteristics of phyllosilicates. ; Se revisan y evalúan los métodos basados en difracción de rayos X comúnmente usados para conocer las condiciones diagenéticas/metamórficas de bajo grado, de acuerdo con el conocimiento actual de los procesos geológicos relacionados obtenido por microscopía electrónica de transmisión. La falta de cambios cualitativos evidentes en los filosilicatos, incluida su química mineral, desde condiciones diagenéticas profundas hasta condiciones de epizona débil, explica la escasez de criterios de grado válidos. En toda esta gama de condiciones diagenético-metamórficas, un aumento progresivo en el tamaño del dominio cristalino junto con una disminución en el número de defectos son los únicos cambios significativos que se pueden observar. Por lo tanto, los índices relacionados con la cristalinidad siguen siendo los métodos basados en características cristaloquímicas de los filosilicatos más adecuados .

Samples were obtained in the framework of TASYO (MAR-98-0209), MOUNDFORCE—EUROCORE-EUROMARGINS 01-LEC-EMA06F and INDEMARES-LIFE projects, on board of RVs Hespérides and Cornide de Saavedra. The authors thank M. Mar Abad of the Centro de Instrumentación Científica (CIC) of Granada, Andalusian project RNM-1897 and PAIDI-groups RNM-328 and RNM-363 of the Andalusia Government for the financial support, as well as the Supercomputational Center of the Granada University (UGRGRID), the Centro Técnico de Informática of CSIC and the Spanish institute of Oceanography for providing samples of mud volcanoes. We acknowledge the Arizona State University National SIMS Facility supported by the US National Science Foundation grant (EAR-0948878). ; Clay dehydration at great depth generates fluids and overpressures in organic-rich sediments that can release isotopically light boron from mature organic matter, producing 10B-rich fluids. The B can be incorporated into the tetrahedral sites of authigenic illite during the illitization of smectite. Therefore, the crystal-chemical and geochemical characterization of illite, smectite or interlayered illite–smectite clay minerals can be an indicator of depth (temperature) and reactions with the basin fluids. The aim of this study was to determine the detailed clay mineralogy, B-content and isotopic composition in illite–smectite rich samples of mud volcanoes from the Gulf of Cádiz, in order to evaluate interactions of hydrocarbon-rich fluids with clays. Molecular modeling of the illite structure was performed, using electron density functional theory (DFT) methods to examine the phenomenon of B incorporation into illite at the atomic level. We found that it is energetically preferable for B to reside in the tetrahedral sites replacing Si atoms than in the interlayer of expandable clays. The B abundances in this study are high and consistent with previous results of B data on interstitial fluids, suggesting that hydrocarbon-related fluids approaching temperatures of methane generation (150 °C) are the likely source of B-rich illite in the studied samples. ; CADHYS project RNM-3581 ; Doctoral scholarship of the Junta de Andalucia (Spain)

This research used samples from the ODP. We thank the staff at the Gulf Coast core repository for assistance in ODP Leg 113 core handling and shipping. ; We acknowledge the help of Dr. María del Mar Abad, Dr. Isabel Sánchez Almazo, Dr. Miguel Angel Hidalgo, Dr. Miguel Angel Salas, and Isabel Nieto (Scientific Instrumentation Center of the University of Granada, CIC) for their help along different phases of the laboratory work. We also acknowledge the constructive comments of two anonymous reviewers that have helped to improve this paper. We wish to thank Prof. C. Hans Nelson for their constructive comments and improvement of our English, which contributed greatly toward clarification of the text. Thanks are also given to Dr. Francisco J. Lobo and Dr. Fernando Bohoyo for their helpful comments related to the study area and regional tectonics. ; Three types of glaucony grains were identified in the late Eocene (~35.5–34.1 Ma) sediments from Ocean Drilling Program (ODP) Hole 696B in the northwestern Weddell Sea (Antarctica). The grains are K2O-rich (~7 wt%) and formed by smectite-poor interstratified ~10 Å glauconite-smectite with flaky/ rosette-shaped surface nanostructures. Two glaucony types reflect an evolved (types 1 and 2 glaucony; less mature to mature) stage and long term glauconitization, attesting to the glaucony grains being formed in situ, whereas the third type (type 3 glaucony) shows evidences of alteration and reworking from nearby areas. Conditions for the glaucony authigenesis occurred in an open-shelf environment deeper than 50 m, under sub-oxic conditions near the sediment-water interface. These environmental conditions were triggered by low sedimentation rates and recurrent winnowing action by bottomcurrents, leading to stratigraphic condensation. The condensed glaucony-bearing section provides an overview of continuous sea-level rise conditions pre-dating the onset of Antarctic glaciation during the Eocene-Oligocene transition. Sediment burial, drop of O2 levels, and ongoing reducing (postoxic to sulphidic) conditions at Hole 696B, resulting in iron-sulphide precipitation, were a key limiting factor for the glauconitization by sequestration of Fe2+. ; Funding for this research is provided by the Spanish Ministry of Science and Innovation (grants CTM2014-60451-C2-1-P and CTM2017-89711-C2-1-P, CGL2016-75679P) cofunded by the European Union through FEDER funds and RNM-208 group (Discontinuidades estratigráficas, Junta de Andalucía, Spain).