Two Colombian deposits belonging to the western emerald belt of the Eastern Cordillera, namely Coscuez and Quipama-Muzo, hosted in Lower Cretaceous black shales, have been dated for the first time by 4oAr/39Ar induction and laser microprobe methods on contemporaneous greenish Cr-V-rich K mica aggregates consisting of muscovite as a dominant phase 5 kaolinite, 5 paragonite, quartz, ? albite, and ? chlorite, pyrite, and calcite. Contamination of the K mica aggregates by wall-rock impurities is eliminated by in situ 40Ar/39Ar laser spot analysis. Two distinct plateau and spot fusion ages of 35 to 38 Ma and 31.5 to 32.6 Ma were obtained for the Coscuez and Quipama samples, respectively, i.e., a late Eocene to early Oligocene age. Concordant conventional K-Ar ages show that in spite of the small size of these micas, they did not suffer significant 39Ar loss d u e to recoil during irradiation of the samples. Internal 39Ar recoil may explain the slight disturbances observed on the age spectra. Microthermometry, Raman spectroscopy, and SEM experiments performed on fluid inclusions trapped in emerald crystals from the Coscuez deposit show that the hydrothermal fluids are complex HzO-NaC1-CaClz-KCl-CO,-N, brines (38 wt % NaCl equiv). Constrained by the 40Ar/39Ar age determinations, the Eastern Cordillera subsiding model, and the assumption of lithostatic confining pressures, isochoric extrapolations lead to a pressure-temperature estimate of 1.06 to 1.12 kbars and 290" to 360°C for the emerald deposition. The P-T evaluation is in agreement with the paragenesis accompanying the emerald deposition. A moderate-temperature epigenetic hydrothermal-sedimentary model is proposed for Colombian emerald genesis. This model makes use of the following points: (1) the depth of hydrothermal circulation within the Lower Cretaceous series, (2) a basinal origin of mineralizing fluids as inferred from oxygen and carbon isotope data, (3) an evaporitic source for the NaCl sulfate-rich brines trapped within emerald crystals, as deduced from sulfur isotope data, (4) a likely source of the metallic components, and particularly Be, Cr, and V, being the black shale reservoir, and (5) the timing of hydrothermal circulation and emerald vein formation in relation to Eastern Cordillera tectonic evolution-in particular, the strong shortening episode beginning during the Eocene. F 364 CHEILLET2 ET AL.

The isotopic analysis of micrometer sized uranium bearing particles, released from nuclear facilities, has been proven to be an efficient tool for safeguarding purposes. However these analyses are not always sufficient for identifying specifically some crucial nuclear operations, like uranium conversion, which are most of the time carried out with natural uranium. A secondary ion mass spectrometry (SIMS) methodology to detect and analyze micrometer sized particles that contain both uranium and fluorine is described. Following the particle detection, which is performed automatically, individual particles are analyzed under microbeam conditions to measure both a precise uranium isotopic composition and the relative amount of fluorine. We first confirm that the sample thermal treatment is prohibited to preserve the fluorine signature of the particles. The methodology was applied to uranium particles coming from the fuel cycle upstream from the enrichment step. This study demonstrates that, contrary to uranium isotopic measurements, the measurement of the relative amount of fluorine allows discrimination between uranium ore concentrate particles and particles coming from a conversion plant. Moreover, the results, obtained for particles which were collected 4 years ago in the surroundings of a conversion plant, show also that the sample storage in a plastic bag at ambient temperature and in the dark is enough to prevent a significant loss of fluorine and the disappearance of the conversion signature. Finally the methodology was applied successfully to an unknown real-life environmental sample.

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