The origin and fate of organic carbon in graphite-manganese bearing rocks and implications for the Lomagundi-Jatuli Event

Abstract

Our study helps to unravel the complexity of the Lomagundi-Jatuli event, the largest and longest positive carbon isotope excursion ever recorded on the Earth's surface, by providing a unique view of Paleoproterozoic graphitic rocks from the Borborema province of Northeastern Brazil. Through detailed mineralogical, textural, chemical and isotopic analyses, we bring a new perspective that provide support to elevated primary productivity and large-scale organic carbon burial during the Lomagundi-Jatuli event. Graphite crystals with distinctive textural features occur in association with silicate and oxidised manganese ores, manganese quartzites, garnetites, and gneisses. The graphites were crystallised at temperatures up to 634 °C, consistent with amphibolite facies metamorphism, according to Raman thermometry. An average total carbon content of 2.1 wt%, with δ¹³C values ranging from - 15.0 to - 21.5‰, is indicated by whole-rock geochemistry and carbon isotopic composition, respectively. Based on these results, our study proposes that these graphitic rocks may represent remnants of organic matter, possibly derived from bacterial biomass associated with manganese-rich sediments, preserved under reducing environmental conditions in a redox-stratified marine setting. Biological mediation on the origin of silicates is suggested by the close relationship between reduced manganese silicates and graphite. These constraints indicate that Paleoproterozoic graphite-rich rocks represent an important but overlooked reservoir of organic carbon that was partially degassed during the metamorphism of organic-rich sequences. Overall, this research provides new insights for the enigmatic emergence of the Lomagundi-Jatuli event, highlighting the intricate interplay among organic carbon, manganese-rich rocks and Earth's evolutionary processes during this period.

Keywords: Borborema Province; Carbon isotope; Organic matter; Paleoproterozoic era; Raman thermometry.

Fig. 1

General overview of the study area. (A) Distribution of some Paleoproterozoic organic-matter rich-successions in South America and Africa. Excepting South Africa, all deposits were included in a hypothetical Paleoproterozoic (Atlantica?) paleocontinental mass (top left corner). References for the organic-matter rich successions are as follows: (A) Santos et al., Fragomeni et al.; (B) Silva and Xavier; (C) Miranda et al.; (D) Mesquita et al.; (E) Klein et al.^,; (F) Kríbek et al.; (G) Canfield et al.. (B) Geological framework of the Northern Borborema Province highlighting study area in white star. MCD Médio Coreaú Domain, CCD Ceará Central Domain, TBL Transbrasiliano Lineament, OASZ Orós-Aiuaba Shear Zone, PSZ Patos Shear Zone. (C) Local map of the Lagoa do Riacho manganese deposit (left) and schematic cross-section constructed from the core Ocr-1 (right) showing the distribution of the manganese-graphite rich lithological groups investigated in the present study. The maps were created using QGIS v3.28 software (https://qgis.org/en/site/). Figure (A) was designed using the ETOPO Global Relief data, available at https://www.ncei.noaa.gov/products/etopo-global-relief-model.

Fig. 2

Transmitted (A,B,G,H,I); A is under cross polarized and all the others are plane polarized), reflected (E) light photomicrographs, and BSE images (C,D,F) showing petrographic aspects of the graphite and manganese-bearing rocks. (A) Manganese-pyroxene (Pxmn) porphyroblast in contact with minor graphite crystals (Gph). (B) Spessartine (Spss) porphyroblast in a matrix of manganese-oxide (Mn–O) and graphite. (C) Pentlandite (Pt) in contact with graphite flakes. (D) Euhedral cobaltite (Cb) in contact with spessartine and graphite in a silicate matrix. (E) Pyrolusite (Pyro) and todorokite (Td) crystals crosscutting a matrix of manganese-pyroxene. (F) Detailed view of the manganese-oxide-hydroxide minerals (Pyrolusite, todorokite and cryptomelane-Cml) replacing manganese-pyroxene. (G) Graphite along foliation planes in contact with spessartine and quartz (Qtz) agglomerates. At the top of the image is a manganese-rich domain. (H) Graphite (Gph) flakes aligned with a well-developed foliation in contact with spessartine. Two textural graphite types are shown, a “pocket” graphite with low crystallinity and a well-ordered graphite (more common). (I) Graphite flakes in an assemblage composed of biotite (Bt), garnet (Gt), feldspar (Fsp) and quartz along the foliation plane.

Fig. 3

Textural features and Raman spectra of the analyzed graphite crystals.

Fig. 4

General geochemistry dataset. (A) δ¹³C values of whole-rock samples from different graphite-bearing units at Lagoa do Riacho Deposit. For the box and whisker plots, the boxes extend to the interquartile range and the whiskers extend to furthest data point up to 1.5 times the interquartile range (IQR). (B) Variability of δ¹³C ratios with amount of graphitic carbon. (C) Percentage of whole-rock manganese content (wt%) in the samples with the δ¹³C ratios. (D) Comparison between Raman temperature from some selected graphite-rich lithologies and the δ¹³C ratios.

Fig. 5

δ¹³C values of whole-rock samples from different graphite-bearing units at Lagoa do Riacho Deposit compared with potential carbon sources as well as values from other flake graphite deposits. Organic, marine and mantle carbon isotope values are from Schidlowski. Carbon isotopes from fluid-deposited graphite deposits are from Luque et al.. Borborema Province (This study and Fragomeni et al.. Rio Itapicuru. Quadrilátero Ferrífero. Northeast Tocantis Province. Gurupi belt^,. West Africa Craton. Franceville basin, Gabon. The compositional range for other regions is as compiled by Zhu et al..

Fig. 6

Results of a multicomponent Rayleigh fractionation model for a starting source composition of δ¹³C_source =  − 29‰, a temperature of 654 °C, and r_CH4-CO2 = 0.99. The graphite-fluid and CH₄-CO₂ fractionation factors at this temperature are from Bottinga.