Diffusive retention of carbon and nitrogen in a microcrystalline quartzdominated chert: Implications for reconstructing Earth's ancient atmosphere

摘要

We investigate the retention of volatile species and we analyze the volatile species released on low temperature crushing of a microcrystalline quartz (< 5 μm)-dominated chert from the Petrified Forest National Park, known to be ca. 215 million years old, in order to learn more about the atmospheric composition during the Late Triassic. Diffusion laws for nitrogen and carbon in the Petrified Forest chert were determined using both indiffusion of C (believed to diffuse as CO_2) and N (believed to diffuse as N_2) from a C-O-H-N vapor and ion implantation followed by heating. Arrhenius relationships in the form of D = D_0exp(?Ea/RT) were established: the log (D_0, m~2/s) values are ?14.1 and ?17.2 and the Ea (kJ/mol) values are 68.9 and 37.9 for nitrogen and carbon, respectively. Although this chert sample has not experienced temperatures above 100 °C, the diffusion results indicate that it is ~70% retentive to carbon and ~92% retentive to nitrogen at 100 °C for over 1 billion years, given a diffusion domain size of a sphere with a 1 cm radius. This suggests that at ambient conditions this material is a very good container for volatiles over geologically long timescales. In addition, the major atmospheric gases (N_2, O_2, Ar, CO_2) are released when this sample is crushed incrementally into a quadrupole mass spectrometer, indicating that microcrystalline quartz dominated phases can provide a record of past atmospheric compositions. The combination of the retentiveness of atmospheric gases as well as the release of inclusion gas upon crushing makes this a reliable proxy for Earth's volatile history extending at least to the Late Triassic and potentially even earlier Earth history.