Abstract
Organic-rich rocks of the 2.0 Ga Zaonega Formation, Karelia, Russia, have been studied extensively to gain understanding of the global carbon cycle and reconstruction of paleo-environments, directly after the Great Oxidation Event (GOE). This formation has a complex history of alteration, involving pervasive hydrothermal circulation, hydrocarbon generation/migration, and mineral authigenesis. Several previous studies have focused on the description of these secondary effects, and the identification of primary geochemical signals in the carbonaceous phases. Migration and infiltration of organic-rich fluids appear to have had only limited effect on the primary carbon isotope record (δ13Corg). However, the structural variability of carbonaceous material (CM) appears to have been strongly affected, with a range of reported structures including carbon onion-shaped nanostructures and mineral-templated graphite films. Here we present a systematic Raman spectroscopy-based study of the structural variability of CM in a drill core representing the middle and upper strata of the Zaonega Formation. The Raman spectra of CM show a systematic difference in structural order between the bulk carbonaceous matrix (Matrix-CM) and the CM occurring near mineral contacts (Contact-CM), indicating that mineral templating was an important process affecting structural order in the formation. The templating effect was observed on the surface of a wide range of minerals. The difference in structural order between Matrix-CM and Contact-CM can be traced throughout the ca. 400 m stratigraphy. The structural order varied with the degree of alteration and hydrothermal circulation, from highly ordered structures directly above a large gabbro intrusion at the bottom of the stratigraphy to less ordered structures higher up in the sequence. This trend directly correlates with the δ18O trend of secondary calcite, and can be attributed to the decreasing influence and temperature regime of hydrothermal circulation upward in the stratigraphy. The results presented here suggest that organic-rich hydrothermal fluids can locally strongly enhance graphitization of carbonaceous materials, and cause sample-scale heterogeneities in the structural order of organic materials. This has implications for the interpretation of carbonaceous materials in other ancient rocks experiencing circulation of organic-rich hydrothermal fluids.
Highlights
Organic-rich rocks of the 2.0 Ga Zaonega Formation, Karelia, Russia (Figs. 1 and 2), have been studied extensively to gain understanding of the global carbon cycle and reconstruction of paleo-environments, directly after the Great Oxidation Event (GOE)
The three Raman spectral indicators used here to quantify the structural order of carbonaceous material (CM): I-1350/1600, R2 and full width at half maximum (FWHM)-G, all show this difference between Contact-CM and Matrix-CM, and reveal small differences in Contact-CM at the different types of mineral surfaces (Fig. 6G–I and Table S2)
Minor mineral specific variability of Contact-CM structural order may indicate slightly different mineral templating effects depending on the mineral surfaces
Summary
Organic-rich rocks of the 2.0 Ga Zaonega Formation, Karelia, Russia (Figs. 1 and 2), have been studied extensively to gain understanding of the global carbon cycle and reconstruction of paleo-environments, directly after the Great Oxidation Event (GOE). 1 and 2), have been studied extensively to gain understanding of the global carbon cycle and reconstruction of paleo-environments, directly after the Great Oxidation Event (GOE). The Zaonega Formation has a complex history of alteration, involving syndepositional gabbro intrusions that caused pervasive hydrothermal circulation, hydrocarbon generation/migration and extensive mineral recrystallization and authigenesis in the adjacent organic-rich sediments. Studying micro-drilled samples throughout the Zaonega Formation, Qu et al (2012) showed that the isotopic difference between the organicrich veins and their host rocks appeared to be rather small (up to 1.1‰, typically < 0.5‰), indicating the remobilized CM in the veins largely originated from the directly surrounding sediments. Migration and infiltration of organic-rich fluids appear to have had only limited effect on the primary stratigraphic δ13Corg trend
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