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Fluid inclusions size and distribution in Stalagmites applying NMR techniques

AuthorsLópez-Elorza, M.; Muñoz-García, M. Belén; Fernández-Valle, Encarnación; González Acebrón, Laura; Martín-Chivelet, J. CSIC ORCID
Issue Date22-May-2017
Citation8th International Conference Climate Change: The Karst Record (2017)
AbstractNuclear magnetic resonance (NMR) techniques are powerful non-destructive tools to understand the internal porosity and fluid inclusions distribution of carbonate rocks. In the case of speleothems, relaxometry experiments and the reconstruction of 2D magnetic resonance images (MRI) help to complement the results obtained by petrographic analysis in microscope, where different type of fluid inclusions have been classified. Relaxometry test of 1 H nuclei of water in a porous media allows to quantify isolated cavities as fluid inclusions and, based on so called “surface effects”, the size distribution can be achieved. It is known that T1 (longitudinal) and T2 (transversal) relaxation times, indicators of pore size when a single fluid is present, are shorter in pores with a high surface-to-volume ratio (S/V). On the other hand, high intensity pixels (bright areas) are associated to water filled fluid inclusions in 2D MRI images, while low intensity pixels (black areas) are related to calcite. The technique shows a high potential to analyze different fluid inclusions bear in mind their magnitudes, allowing to classify and estimate the amount of intracrystallite and intercrystallite inclusions. Moreover, the 2D and 3D visualization of fluid inclusions distribution inside the sample by MRI images and the pre-estimation of enclosed water amount in each sample by relaxometry curves, turn this technique into a useful tool previous to stable isotope or noble gas crushing measurements of fluid inclusions under high vacuum line. In this work, two stalagmites from Cueva del Tortero (Valencia, Spain), Tortero-2 and Tortero-4, have been analyzed by this method. T1 and T2 curves of fully dried samples reveal that fluid inclusions could be classified in three groups taking their size into consideration, where short relaxation times represent small fluid inclusions and larger T1 and T2 values characterize enlarged cavities. Furthermore, the greater sensitivity that T2 displays to inclusions morphology allows to identify more deeply different types of fluid inclusions. In both samples, intracrystallite fluid inclusions are very tiny (3-15 μm) and correspond to shorter T2 values (0,2-3 ms for Tortero-4 slabs and 1-4 ms for Tortero-2). On the other hand, intercrystallite inclusions are bigger in size (20-100 μm) and they provoke T2 relaxation times between 4 and 500 ms for Tortero-4 slabs and from 8-50 ms for Tortero-2. T2 relaxation times bigger than 1000 ms are induced by enlarged porosity in both samples. Moreover, MRI images confirm that lots of intracrystallite inclusions are located in bands and in the hiatus, where the signal given by water is brighter.
DescriptionTrabajo presentado en el 8th International Conference Climate Change: The Karst Record, celebrado en Austin, Texas (Estados Unidos), del 21 al 24 de mayo de 2017
Appears in Collections:(IGEO) Comunicaciones congresos
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