Nanoscale petrographic and geochemical insights on the origin of the Palaeoproterozoic stromatolitic phosphorites from Aravalli Supergroup, India

Abstract

Stromatolites composed of apatite occur in post-Lomagundi-Jatuli successions (late Palaeoproterozoic) and suggest the emergence of novel types of biomineralization at that time. The microscopic and nanoscopic petrology of organic matter in stromatolitic phosphorites might provide insights into the suite of diagenetic processes that formed these types of stromatolites. Correlated geochemical micro-analyses of the organic matter could also yield molecular, elemental and isotopic compositions and thus insights into the role of specific micro-organisms among these communities. Here, we report on the occurrence of nanoscopic disseminated organic matter in the Palaeoproterozoic stromatolitic phosphorite from the Aravalli Supergroup of north-west India. Organic petrography by micro-Raman and Transmission Electron Microscopy demonstrates syngeneity of the organic matter. Total organic carbon contents of these stromatolitic phosphorite columns are between 0.05 and 3.0 wt% and have a large range of δ(13) Corg values with an average of -18.5‰ (1σ = 4.5‰). δ(15) N values of decarbonated rock powders are between -1.2 and +2.7‰. These isotopic compositions point to the important role of biological N2 -fixation and CO2 -fixation by the pentose phosphate pathway consistent with a population of cyanobacteria. Microscopic spheroidal grains of apatite (MSGA) occur in association with calcite microspar in microbial mats from stromatolite columns and with chert in the core of diagenetic apatite rosettes. Organic matter extracted from the stromatolitic phosphorites contains a range of molecular functional group (e.g. carboxylic acid, alcohol, and aliphatic hydrocarbons) as well as nitrile and nitro groups as determined from C- and N-XANES spectra. The presence of organic nitrogen was independently confirmed by a CN(-) peak detected by ToF-SIMS. Nanoscale petrography and geochemistry allow for a refinement of the formation model for the accretion and phototrophic growth of stromatolites. The original microbial biomass is inferred to have been dominated by cyanobacteria, which might be an important contributor of organic matter in shallow-marine phosphorites.

Evidence of oxygenic phototrophy in ancient phosphatic stromatolites from the Paleoproterozoic Vindhyan and Aravalli Supergroups, India

“…Previous studies of the mineralogy of the Jhamarkotra stromatolites by Papineau et al. () show the primary apatite phase to be carbonate fluorapatite (francolite), consistent with the EDS data.…”

“…Many of these morphologies have been previously described by Banerjee (1971) and Chauhan (1979) Figure S1). Previous studies of the mineralogy of the Jhamarkotra stromatolites by Papineau et al (2016) show the primary apatite phase to be carbonate fluorapatite (francolite), consistent with the EDS data.…”

“…Recent dating of the Lower Aravalli Jhamarkotra Formation suggests that the Aravalli phosphorites are likely closer in age to the 1.6 Ga phosphatic Vindhyan stromatolites than previously recognized (see discussions in McKenzie et al., , ; Melezhik et al., ; Papineau et al., ). This new age was based on a large population of detrital zircon grains giving U–Pb ages of 1.7–1.9 Ga.…”

“…in a way resembling the Aravalli globules (e.g., Figures 4a-c and 5). Tufted filamentous fabrics in the Vindhyan mats were sometimes found in association with larger irregular fenestrae infilled with diagenetic dolomite (Figure 11a-c).Recent dating of the Lower Aravalli Jhamarkotra Formation suggests that the Aravalli phosphorites are likely closer in age to the 1.6 Ga phosphatic Vindhyan stromatolites than previously recognized (see discussions inMcKenzie et al, 2013McKenzie et al, , 2014Melezhik et al, 2014;Papineau et al, 2016). This new age was based on a large populationF I G U R E 5 Globular objects in Jhamarkotra phosphatic stromatolites.…”

Widespread occurrences of variably crystalline 13C-depleted graphitic carbon in banded iron formations

“…The association of isotopically-light organic carbon and apatite is a common feature of sediments incorporating biomass (Papineau et al, 2016;She et al, 2014). Apatite [Ca5(PO4)3(F, Cl, OH)] requires phosphorus, which can be derived from the decomposition of phosphorus-bearing biological organic matter (biomass) in sediments.…”