Dating Carbonaceous Matter in Archean Cherts by Electron Paramagnetic Resonance

Abstract

Ancient geological materials are likely to be contaminated through geological times. Thus, establishing the syngeneity of the organic matter embedded in a mineral matrix is a crucial step in the study of very ancient rocks. This is particularly the case for Archean siliceous sedimentary rocks (cherts), which record the earliest traces of life. We used electron paramagnetic resonance (EPR) for assessing the syngeneity of organic matter in cherts that have a metamorphic grade no higher than greenschist. A correlation between the age of Precambrian samples and the shape of their EPR signal was established and statistically tested. As thermal treatments impact organic matter maturity, the effect of temperature on this syngeneity proxy was studied; cyanobacteria were submitted to cumulative short thermal treatment at high temperatures followed by an analysis of their EPR parameters. The resulting carbonaceous matter showed an evolution similar to that of a thermally treated young chert. Furthermore, the possible effect of metamorphism, which is a longer thermal event at lower temperatures, was ruled out for cherts older than 2 Gyr, based on the study of Silurian cherts of the same age and same precursors but various metamorphic grades. We determined that even the most metamorphosed sample did not exhibit the lineshape of an Archean sample. In the hope of detecting organic contamination in Archean cherts, a "contamination-like" mixture was prepared and studied by EPR. It resulted that the lineshape analysis alone does not allow contamination detection and that it must be performed along with cumulative thermal treatments. Such treatments were applied to three Archean chert samples, making dating of their carbonaceous matter possible. We concluded that EPR is a powerful tool to study primitive organic matter and could be used in further exobiology studies on low-metamorphic grade samples (from Mars for example).

Nuclear Magnetic Biosignatures in the Carbonaceous Matter of Ancient Cherts: Comparison with Carbonaceous Meteorites

“…This analysis of the evolution of resonance lines by cw-EPR can be also extended to the lineshape, which evolves from a Lorentzian line toward a stretched Lorentzian line upon natural and artificial aging Bourbin et al, 2013). This joined evolution of the g-factor, DB pp , and the lineshape upon natural and accelerated aging clearly indicates that the EPR of radicals in fossil CM would be strongly influenced by any thermal event that occurs during metamorphic episodes subsequent to the formation of the sediment.…”

“…The parameters for the molecules encapsulated in silica are given in Table 1 for the thermal step 550°C, a temperature for which the EPR lines are very similar to that of Gunflint chert. In addition to DB pp and g, the EPR lineshape of radicals is an interesting proxy that depends on the age of the CM and thus provides a test for syngenicity of the organic matter in Archean cherts (Bourbin et al, 2013). It may be argued that the slight distortion of some EPR lines in Fig.…”

EPR of Primitive Organic Matter: A Tool for Astrobiology

“…Data below ~500 °C are not reported because the poor signal-to-noise ratio at lower temperature introduced too much error in R 10 data processing. It appears that upon increasing step annealing temperature, R 10 decreases linearly from R 10 ~3 (Lorentzian with Gaussian character) to 0 (Lorentzian, D = 3) and next to negative values corresponding to D = 2 and D = 1 for stretched Lorentzian [41]. The variation of R 10 with age of Fig.…”

“…For the limit case of diluted electron spin system without hyperfine broadening by 1 H nuclei, the limit values of R 10 are R 10 = 0, − 1.78 and − 2.95 for dimensionality of the spin distribution D = 3, 2 and 1, respectively. Details of data processing are given in [41]. dimensional distribution (2 ≥ D ≥ 1) for Archean IOM (2500-3500 Myr).…”

“…15.5. With the help of the age-temperature correlation, we obtain the following expression for the variation of R 10 factor with the age of the IOM [41]:…”