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191.
Textural and chemical changes occurring in illite and chlorite concomitant with pressure solution of limestone were studied in samples from the Kalkberg Formation of Catskill, New York, using XRD and TEM/AEM. Samples on one limb of an anticline are massive shaly limestones, but those on the other have undergone extensive pressure solution and well-developed cleavage is present. Illite and chlorite from the uncleaved shaly limestone are found in small individual packets (100–800 Å thick) dispersed throughout the carbonate matrix with crystal morphologies characteristic of burial diagenesis. Phyllosilicates from the limb more affected by pressure solution occur in larger units (>1 μm thick) as stacks of subparallel packets (150–500 Å thick). Such stacks are inferred to represent coalescence of smaller packets. These data imply that the phyllosilicates are largely passive during pressure solution of limestone; however, localized solution-recrystallization is required by the coherent to semi-coherent packet boundaries and the crystal morphologies present in the pressure solution sample. The largely passive role is in contrast with the more active role of phyllosilicates in many shales and slates.XRD data for illite show an increase in crystallinity in the pressure solution sample under isothermal conditions. Differences in illite crystallinity are adequately explained in large part by differences in crystal size with some contribution due to strain. The data demonstrate that illite crystallinity cannot be unambiguously used in determining absolute or even relative temperatures. 相似文献
192.
Jacob Mulder Graham Hagen-Peter Teresa Ubide Rasmus Andreasen Ellen Kooijman Melanie Kielman-Schmitt Yue-Xing Feng Bence Paul Andreas Karlsson Christian Tegner Charles Lesher Fidel Costa 《Geostandards and Geoanalytical Research》2023,47(2):311-336
Laser ablation multi-collector mass spectrometry (LA-MC-ICP-MS) has emerged as the technique of choice for in situ measurements of Sr isotopes in geological minerals. However, the method poses analytical challenges and there is no widely adopted standardised approach to collecting these data or correcting the numerous potential isobaric inferences. Here, we outline practical analytical procedures and data reduction strategies to help establish a consistent framework for collecting and correcting Sr isotope measurements in geological materials by LA-MC-ICP-MS. We characterise a new set of plagioclase reference materials, which are available for distribution to the community, and present a new data reduction scheme for the Iolite software package to correct isobaric interferences for different materials and analytical conditions. Our tests show that a combination of Kr-baseline subtraction, Rb-peak-stripping using βRb derived from a bracketing glass reference material, and a CaCa or CaAr correction for plagioclase and CaCa or CaAr + REE2+ correction for rock glasses, yields the most accurate and precise 87Sr/86Sr measurements for these materials. Using the analytical and correction procedures outlined herein, spot analyses using a beam diameter of 100 μm or rastering with a 50–65 μm diameter beam can readily achieve < 100 ppm 2SE repeatability ("internal") precision for 87Sr/86Sr measurements for materials with < 1000 μg g-1 Sr. 相似文献
193.
Lina Seybold Claudia A. Trepmann Stefan Hölzl Kilian Pollok Falko Langenhorst Fabian Dellefant Melanie Kaliwoda 《Meteoritics & planetary science》2023,58(9):1287-1305
Shock-related calcite twins are characterized in calcite-bearing metagranite cataclasites within crystalline megablocks of the Ries impact structure, Germany, as well as in cores from the FBN1973 research drilling. The calcite likely originates from pre-impact veins within the Variscan metagranites and gneisses, while the cataclasis is due to the Miocene impact. Quartz in the metagranite components does not contain planar deformation features, indicating low shock pressures (<7 GPa). Calcite, however, shows a high density (>1/μm) of twins with widths <100 nm. Different types of twins (e-, f-, and r-twins) crosscutting each other can occur in one grain. Interaction of r- and f-twins results in a-type domains characterized by a misorientation relative to the host with a misorientation angle of 35°–40° and a misorientation axis parallel to an a-axis. Such a-type domains have not been recorded from deformed rocks in nature before. The high twin density and activation of different twin systems in one grain require high differential stresses (on the order of 1 GPa). Twinning of calcite at high differential stresses is consistent with deformation during impact cratering at relatively low shock pressure conditions. The twinned calcite microstructure can serve as a valuable low shock barometer. 相似文献