Carbon Diffusion in Zircon
Abstract
With the recent discovery of potentially biogenic carbon in 4.1 billion-year-old zircon (Bell et al., 2015), it is important to understand diffusion of C in zircon in order to assess the potential of these carbonaceous inclusions to preserve isotopic evidence of Earth's early history. In this study, we have characterized carbon diffusion in natural zircon at 1-atm and under high-pressure conditions in the presence of hydrous species. The 1-atm experiments were conducted by implanting 13C into polished, oriented slabs of zircon, with implanted samples heated in air or under buffered conditions in silica glass ampoules sealed under vacuum. High-pressure experiments (1 GPa) were conducted in a piston-cylinder apparatus, using 13C labeled glycine as the starting source (this breaks down at run conditions). In all cases, 13C distributions in zircon were measured by Nuclear Reaction Analysis (NRA) using the reaction 13C(p,γ)14N. In addition to the NRA measurements, we have also conducted analyses of carbon in zircon by ion microprobe. For diffusion parallel to c we obtain a well-constrained Arrhenius relation over the temperature range 600-1100°C. Diffusion normal to c is similar. Data from the high-pressure experiments and those run under buffered conditions also yield similar diffusivities, suggesting that carbon diffusion is not significantly affected by these parameters over the range of conditions studied. Carbon diffuses more rapidly than most cations in zircon, with the exception of lithium, but several orders of magnitude more slowly than helium. Zircon will therefore be moderately retentive of carbon. For example, diffusion distances on Gyr timescales would be on order of nm at 200°C, μm at 350°C and mm at 650°C, suggesting that carbon isotopic signatures contained within carbonaceous inclusions in zircon may be retained over long timeframes if zircons experience only moderate heating. E.A. Bell, P. Boehnke, T. M. Harrison, W.L. Mao (2015) PNAS 47, 14518-14521
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2016
- Bibcode:
- 2016AGUFMMR42A..02C
- Keywords:
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- 3904 Defects;
- MINERAL PHYSICSDE: 3630 Experimental mineralogy and petrology;
- MINERALOGY AND PETROLOGYDE: 5120 Plasticity;
- diffusion;
- and creep;
- PHYSICAL PROPERTIES OF ROCKSDE: 5139 Transport properties;
- PHYSICAL PROPERTIES OF ROCKS