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Ne chronometry
A196
Goldschmidt Conference Abstracts 2006
(U–Th)/Ne chronometry C.E. GAUTHERON1, L. TASSAN-GOT2, K.A. FARLEY3
Alkaline earth uranyl compounds—from solution to mineral phases G. GEIPEL, G. BERNHARD
1
UMR Interactions et Dynamique des Environnements de Surface, UMR 8148, Bat 504, Universite´ Paris XI, Orsay 91405, France ([email protected]) 2 IPN Institut Physique Nucle´aire, Bat 102, Universite´ Paris XI, Orsay 91405, France ([email protected]) 3 Division of Geological and Planetary Sciences, MS100-23, California Institute of Technology, Pasadena, CA 91125, USA ([email protected]) Nuclear production of 21Ne, like 4He, in U and Th rich minerals such as apatite, zircon, monazite and titanite can potentially be used for chronometry. To test the possibility, a review of the available cross-section data was done, permitting a reevaluation of the 21Ne production from this reaction. The important factors of the simulation are the cross section and the stopping distance values for a mineral characterized by its chemical composition and density. Additionally, 21Ne has a stopping range of about 1 lm compared to about 20 lm for a particles; thus the (21Ne/4He) production ratio also depends on crystal size when the crystals are small enough that a-ejection is important. We also present measurements of the (21Ne/4He) ratio on few mg aliquots of well-dated volcanic apatites and zircons. The measured (21Ne/4He) are in agreement with the theoretical values for apatite and zircon. Based on our production rate estimates the Durango apatite and Fish Canyon Tuff zircon give Ne ages of 34.2 ± 8.6 Ma and 28.0 ± 12.2 Ma, respectively, which are in agreement with independently known ages. Additionally, the 4 He and 21Ne content of zircons from the deeply exhumed crustal section in Gold Butte, Nevada (crystallization age of 1.4 Ga) imply (U–Th)/Ne ages of 963 ± 164 and 777 ± 122 Ma, far older than their He ages of 16.7 ± 1.3 and 19.1 ± 1.5 Ma, respectively. To explain the age difference, a neon closure temperature for zircon around 400 °C is derived. This study demonstrates that (U–Th)/Ne chronometry can be a powerful new tool permitting access to a higher closure temperature than with (U–Th)/He. Additionally, the physical and mineralogical properties of the minerals of interest can be a great advantage and can help when K–Ar or Ar–Ar dating is difficult. doi:10.1016/j.gca.2006.06.395
Forschungszentrum Rossendorf, Institute of Radiochemistry, Germany ([email protected]; g.bernhard@ fz-rossendorf.de) The uranyl tricarbonato complex is one of the most important uranyl species under environmental conditions. The tendency to form stable metal-uranyl tricarbonato complexes was found particularly for the interaction with alkaline earth elements. We studied chemical behavior of these compounds in aqueous solution by time-resolved laser-induced fluorescence spectroscopy (TRLFS). However, under comparable chemical conditions the formation of these complexes is very different. While magnesium tends mainly to the formation of a MgUO2 ðCO3 Þ3 2þ —complex, in the case of calcium the Ca2UO2(CO3)3(aq.) complex is the most stable. The stability constant for the Ca2UO2(CO3)3—complex is derived to be log b 213 ¼ 30:90 0:25 (Bernhard et al., 2001). In the corresponding systems with strontium as well as for barium only the MeUO2 ðCO3 Þ3 2þ —complex is formed. The stability constants of the MeUO2 ðCO3 Þ3 2þ —complexes are determined to be log b 113 ¼ 26:13 0:27 and 26.24 ± 0.31 for the alkaline earth elements Sr and Ba, respectively. The Me2UO2(CO3)3—complexes for Mg and Ca form stable natural minerals as bayleyite and liebigite. However several other mineral modifications as zellerite, fontanite, sharpite and rabbittite underline the geochemical importance of this class of compounds. Analogous phenomena can be expected in the alkaline earth uranyl phosphate systems. Therefore we studied the interaction of alklaine earth metal ion with UO2(PO4) at pH 7.0. From the fluorescence data the formation of MeUO2(PO4)+ complexes in solution can be concluded. The stability constants are derived to be b 111 ¼ 16:85 0:16, 16.62 ± 0.15, 17.4 ± 0.4 and 16.9 ± 0.4 for Mg, Ca, Sr and Ba, respectively. The formation of complexes with the common formula Me(UO2)2(PO4)2(aq.) has not yet been observed due to the low solubility of these compounds. In the case of Mg and Ca the fluorescence data will be compared to the corresponding minerals saleiite and autunite (Geipel et al., 2000).
References Bernhard, G., Geipel, G., Reich, T., Brendler, V., Amayri, S., Nitsche, H., 2001. Radiochim. Acta 89, 511–518. Geipel, G., Bernhard, G., Rutsch, M., Brendler, V., Nitsche, H., 2000. Radiochim. Acta 88, 757–762. doi:10.1016/j.gca.2006.06.396
Goldschmidt Conference Abstracts 2006
(U–Th)/Ne chronometry C.E. GAUTHERON1, L. TASSAN-GOT2, K.A. FARLEY3
Alkaline earth uranyl compounds—from solution to mineral phases G. GEIPEL, G. BERNHARD
1
UMR Interactions et Dynamique des Environnements de Surface, UMR 8148, Bat 504, Universite´ Paris XI, Orsay 91405, France ([email protected]) 2 IPN Institut Physique Nucle´aire, Bat 102, Universite´ Paris XI, Orsay 91405, France ([email protected]) 3 Division of Geological and Planetary Sciences, MS100-23, California Institute of Technology, Pasadena, CA 91125, USA ([email protected]) Nuclear production of 21Ne, like 4He, in U and Th rich minerals such as apatite, zircon, monazite and titanite can potentially be used for chronometry. To test the possibility, a review of the available cross-section data was done, permitting a reevaluation of the 21Ne production from this reaction. The important factors of the simulation are the cross section and the stopping distance values for a mineral characterized by its chemical composition and density. Additionally, 21Ne has a stopping range of about 1 lm compared to about 20 lm for a particles; thus the (21Ne/4He) production ratio also depends on crystal size when the crystals are small enough that a-ejection is important. We also present measurements of the (21Ne/4He) ratio on few mg aliquots of well-dated volcanic apatites and zircons. The measured (21Ne/4He) are in agreement with the theoretical values for apatite and zircon. Based on our production rate estimates the Durango apatite and Fish Canyon Tuff zircon give Ne ages of 34.2 ± 8.6 Ma and 28.0 ± 12.2 Ma, respectively, which are in agreement with independently known ages. Additionally, the 4 He and 21Ne content of zircons from the deeply exhumed crustal section in Gold Butte, Nevada (crystallization age of 1.4 Ga) imply (U–Th)/Ne ages of 963 ± 164 and 777 ± 122 Ma, far older than their He ages of 16.7 ± 1.3 and 19.1 ± 1.5 Ma, respectively. To explain the age difference, a neon closure temperature for zircon around 400 °C is derived. This study demonstrates that (U–Th)/Ne chronometry can be a powerful new tool permitting access to a higher closure temperature than with (U–Th)/He. Additionally, the physical and mineralogical properties of the minerals of interest can be a great advantage and can help when K–Ar or Ar–Ar dating is difficult. doi:10.1016/j.gca.2006.06.395
Forschungszentrum Rossendorf, Institute of Radiochemistry, Germany ([email protected]; g.bernhard@ fz-rossendorf.de) The uranyl tricarbonato complex is one of the most important uranyl species under environmental conditions. The tendency to form stable metal-uranyl tricarbonato complexes was found particularly for the interaction with alkaline earth elements. We studied chemical behavior of these compounds in aqueous solution by time-resolved laser-induced fluorescence spectroscopy (TRLFS). However, under comparable chemical conditions the formation of these complexes is very different. While magnesium tends mainly to the formation of a MgUO2 ðCO3 Þ3 2þ —complex, in the case of calcium the Ca2UO2(CO3)3(aq.) complex is the most stable. The stability constant for the Ca2UO2(CO3)3—complex is derived to be log b 213 ¼ 30:90 0:25 (Bernhard et al., 2001). In the corresponding systems with strontium as well as for barium only the MeUO2 ðCO3 Þ3 2þ —complex is formed. The stability constants of the MeUO2 ðCO3 Þ3 2þ —complexes are determined to be log b 113 ¼ 26:13 0:27 and 26.24 ± 0.31 for the alkaline earth elements Sr and Ba, respectively. The Me2UO2(CO3)3—complexes for Mg and Ca form stable natural minerals as bayleyite and liebigite. However several other mineral modifications as zellerite, fontanite, sharpite and rabbittite underline the geochemical importance of this class of compounds. Analogous phenomena can be expected in the alkaline earth uranyl phosphate systems. Therefore we studied the interaction of alklaine earth metal ion with UO2(PO4) at pH 7.0. From the fluorescence data the formation of MeUO2(PO4)+ complexes in solution can be concluded. The stability constants are derived to be b 111 ¼ 16:85 0:16, 16.62 ± 0.15, 17.4 ± 0.4 and 16.9 ± 0.4 for Mg, Ca, Sr and Ba, respectively. The formation of complexes with the common formula Me(UO2)2(PO4)2(aq.) has not yet been observed due to the low solubility of these compounds. In the case of Mg and Ca the fluorescence data will be compared to the corresponding minerals saleiite and autunite (Geipel et al., 2000).
References Bernhard, G., Geipel, G., Reich, T., Brendler, V., Amayri, S., Nitsche, H., 2001. Radiochim. Acta 89, 511–518. Geipel, G., Bernhard, G., Rutsch, M., Brendler, V., Nitsche, H., 2000. Radiochim. Acta 88, 757–762. doi:10.1016/j.gca.2006.06.396