Organo-mineral associations in chert of the 3.5 Ga Mount Ada Basalt raise questions about the origin of organic matter in Paleoarchean hydrothermally influenced sediments

Abstract

Hydrothermal and metamorphic processes could have abiotically produced organo-mineral associations displaying morphological and isotopic characteristics similar to those of fossilized microorganisms in ancient rocks, thereby leaving false-positive evidence for early life in the geological record. Recent studies revealed that geologically-induced alteration processes do not always completely obliterate all molecular information about the original organic precursors of ancient microfossils. Here, we report the molecular, geochemical, and mineralogical composition of organo-mineral associations in a chert sample from the ca. 3.47 billion-year-old (Ga) Mount Ada Basalt, in the Pilbara Craton, Western Australia. Our observations indicate that the molecular characteristics of carbonaceous matter are consistent with hydrothermally altered biological organics, although significantly distinct from that of organic microfossils discovered in a chert sample from the ca. 3.43 Ga Strelley Pool Formation in the same area. Alternatively, the presence of native metal alloys in the chert, previously believed to be unstable in such hydrothermally influenced environments, indicates strongly reducing conditions that were favorable for the abiotic formation of organic matter. Drawing definitive conclusions about the origin of most Paleoarchean organo-mineral associations therefore requires further characterization of a range of natural samples together with experimental simulations to constrain the molecular composition and geological fate of hydrothermally-generated condensed organics.

Pushing Raman spectroscopy over the edge: purported signatures of organic molecules in fossil animals are instrumental artefacts

“…As a result, Raman spectra of organic materials preserved in (meta)sedimentary rocks are dominated by the socalled graphite (G) and defect (D1-D4) bands, which provide information about the structural organization of the aromatic skeleton [23] . Consistently, Raman spectra of geologically altered organic materials can be similar even when they have significantly different elemental and molecular compositions [13,14,[24][25][26] . Second, under continuous illumination, luminescence occurs concurrently with Stokes Raman scattering and generates a signal that overlaps with the Raman spectral window [21,27,28] .…”

“…[ 23 ] Consistently, Raman spectra of geologically altered organic materials can be similar even when they have significantly different elemental and molecular compositions. [ 13,14,24–26 ] Second, under continuous illumination, luminescence occurs concurrently with Stokes Raman scattering and generates a signal that overlaps with the Raman spectral window. [ 21,27,28 ] Cross sections of Raman (the probability that Raman scattering takes place) are typically eight to 10 orders of magnitude smaller than that of luminescence.…”

Triple Oxygen Isotope Trend Recorded by Precambrian Cherts: A Perspective from Combined Bulk and in situ Secondary Ion Probe Measurements

“…As Precambrian cherts are almost exclusively made of quartz, they generally lack mineral assemblages that can be used for mineral thermometry. Yet, these rocks usually contain carbonaceous matter that has been often characterized using Raman spectroscopy in order to inform on its thermal history (Tice et al 2004;Schopf and Kudryavtsev 2005;Wacey et al 2012;Sugitani et al 2013;Alleon et al 2016Alleon et al , 2019.…”

Inherited geochemical diversity of 3.4 Ga organic films from the Buck Reef Chert, South Africa

“…In fact, because liquid water enhances their reactivity [47][48][49][50][51][52] , the circulation of fluids can only be detrimental to the molecular preservation of organic compounds. Yet, there is no organic carbon at microquartz crystal boundaries in the chert matrix which would testify to the infiltration of oil or to the fluid-deposition of organic carbon, in contrast to what was described in the 340 Ma Red Dog Zn-Pb deposit (Alaska) 53 or in the 3.5 Ga chert of the Mount Ada Basalt (Pilbara, Australia) 54 .…”

“…A black chert sample was selected for this study, because Paleoarchean organic structures interpreted as fossilized biofilms were previously reported in similar rocks of the same Kromberg Formation 29,31,34,35,37 . The chert sample investigated here was only mechanically fractured to expose fresh surfaces for further Raman, SEM, TEM, and STXM experiments, as previously done for similar Paleoarchean cherts 12,13,54 , thereby avoiding organic contamination issues related to resin embedding and polishing.…”