Diffusion of lead in plagioclase and K-feldspar: an investigation using Rutherford Backscattering and Resonant Nuclear Reaction Analysis (original) (raw)
Abstract
Chemical diffusion of Pb has been measured in K-feldspar (Or93) and plagioclase of 4 compositions ranging from An23 to An93 under anhydrous, 0.101 MPa conditions. The source of diffusant for the experiments was a mixture of PbS powder and ground feldspar of the same composition as the sample. Rutherford Backscattering (RBS) was used to measure Pb diffusion profiles. Over the temperature range 700–1050°C, the following Arrhenius relations were obtained (diffusivities in m2s-1):
Oligoclase (An23): Diffusion normal to (001): log D = ( − 6.84 ± 0.59) − [(261 ± 13 kJ mol −1)/2.303_RT_]Diffusion normal to (010): log D = ( − 3.40 ± 0.50) − [(335 ± 11 kJ mol −1)/2.303_RT_]
Andesine (An43): Diffusion normal to (001): log D = ( − 6.73 ± 0.54) − [(266 ± 12 kJ mol −1)/2.303_RT_] Diffusion normal to (010) appears to be only slightly slower than diffusion normal to (001) in andesine.
Labradorite (An67): Diffusion normal to (001): log D = ( − 7.16 ± 0.61) − [(267 ± 13 kJ mol −1)/2.303_RT_] Diffusion normal to (010) is slower by 0.7 log units on average.
Anorthite Diffusion normal to (010): log D = ( − 5.43 ± 0.48) − [(327 ± 11 kJ mol −1)/2.303_RT_]
K-feldspar (Or93): Diffusion normal to (001): log D = ( − 4.74 ± 0.52) − [(309 ± 16 kJ mol −1)/2.303_RT_] Diffusion normal to (010): log D = ( − 5.99 ± 0.51) − [(302 ± 11 kJ mol −1)/2.303_RT_]
In calcic plagioclase, Pb uptake is correlated with a reduction of Ca, indicating the involvement of Pb→Ca exchange in chemical diffusion. Decreases of Na and K concentrations in sodic plagioclase and K-feldspar, respectively, are correlated with Pb uptake and increase in Al concentration (measured by resonant nuclear reaction analysis), suggesting a coupled process for Pb exchange in these feldspars. These results have important implications for common Pb corrections and Pb isotope systematics. Pb diffusion in apatite is faster than in the investigated feldspar compositions, and Pb diffusion rates in titanite are comparable to both K-feldspar and labradorite. Given these diffusion data and typical effective diffusion radii for feldspars and accessory minerals, we may conclude that feldspars used in common Pb corrections are in general less inclined to experience diffusion-controlled Pb isotope exchange than minerals used in U-Pb dating that require a common Pb correction.
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- Department of Earth and Environmental Sciences, Rensselaer Polytechnic Institute, 12180-3590, Troy, NY, USA
D. J. Cherniak
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Cherniak, D.J. Diffusion of lead in plagioclase and K-feldspar: an investigation using Rutherford Backscattering and Resonant Nuclear Reaction Analysis.Contr. Mineral. and Petrol. 120, 358–371 (1995). https://doi.org/10.1007/BF00306513
- Received: 22 July 1994
- Accepted: 05 December 1994
- Issue Date: July 1995
- DOI: https://doi.org/10.1007/BF00306513