Reassessing the biogenicity of Earth's oldest trace fossil with implications for biosignatures in the search for early life

Abstract

Microtextures in metavolcanic pillow lavas from the Barberton greenstone belt of South Africa have been argued to represent Earth's oldest trace fossil, preserving evidence for microbial life in the Paleoarchean subseafloor. In this study we present new in situ U-Pb age, metamorphic, and morphological data on these titanite microtextures from fresh drill cores intercepting the type locality. A filamentous microtexture representing a candidate biosignature yields a U-Pb titanite age of 2.819 ± 0.2 Ga. In the same drill core hornfelsic-textured titanite discovered adjacent to a local mafic sill records an indistinguishable U-Pb age of 2.913 ± 0.31 Ga, overlapping with the estimated age of intrusion. Quantitative microscale compositional mapping, combined with chlorite thermodynamic modeling, reveals that the titanite filaments are best developed in relatively low-temperature microdomains of the chlorite matrix. We find that the microtextures exhibit a morphological continuum that bears no similarity to candidate biotextures found in the modern oceanic crust. These new findings indicate that the titanite formed during late Archean ca. 2.9 Ga thermal contact metamorphism and not in an early ca. 3.45 Ga subseafloor environment. We therefore question the syngenicity and biogenicity of these purported trace fossils. It is argued herein that the titanite microtextures are more likely abiotic porphyroblasts of thermal contact metamorphic origin that record late-stage retrograde cooling in the pillow lava country rock. A full characterization of low-temperature metamorphic events and alternative biosignatures in greenstone belt pillow lavas is thus required before candidate traces of life can be confirmed in Archean subseafloor environments.

Keywords: Archean Earth; Archean habitats; astrobiology; bioalteration; ichnofossil.

Fig. 1.

Titanite microtextures in Archean metavolcanic pillow lavas now comprising a chlorite dominated matrix, compared with partially mineralized microtunnels from younger oceanic crust. (A–F) A continuum of titanite microtextures from spheres with or without filamentous projections, to well-developed clusters with radiating filaments, LA tracks for U–Pb titanite dating are shown (white bands in C). (G) Partially mineralized microtubules radiating at high angles from a fracture in volcanic glass from Ocean Drilling Program hole 418A in the West Atlantic. (H) Curvilinear and spiral-shaped (Inset) microtunnels in volcanic glass of the Troodos ophiolite Cyprus (drill core CY-1A). (I) Histogram of measured Archean titanite filament widths (n = 303) in 12 samples from the Barberton drill core showing that they are much larger in diameter (dark green line = mean of 12 µm and light green band = SD) and span a wider range compared with microtubules in younger volcanic glass (dark purple line = mean of 1.3 µm and light purple band = SD, replotted from ref. 2). (Scale bars: 50 µm A–H except G and H, Insets, which are 10 µm.)

Fig. 2.

U–Pb isotopic data for hornfelsic titanite in pillow lava metabasalt sample B77 of the Hooggenoeg Formation, South Africa, determined by single-collector LA-ICP-MS. (A) Measured ²⁰⁷Pb:²⁰⁶Pb and ²³⁸U:²⁰⁶Pb isotopic ratios plotted on a Tera–Wasserburg Concordia diagram yielding a lower intercept age of ca. 2.913 ± 0.31 Ga. (B and C) Monte Carlo statistical calculation plotted as a histogram gives an average lower intercept age of 2.871 ± 0.31 Ga. These titanite ages overlap with late Archean mafic dyke intrusion ages and not with early seafloor hydrothermal alteration.

Fig. 3.

U–Pb isotopic data for Archean titanite microtextures in sample B74, previously interpreted as trace fossils, determined by LA-ICP-MS. (A) Measured ²⁰⁷Pb:²⁰⁶Pb and ²³⁸U:²⁰⁶Pb isotopic ratios determined by single-collector LA-ICP-MS and plotted on a Tera–Wasserburg Concordia diagram, yielding a lower intercept age of 2.828 ± 0.440 Ga. (B) A more precise age of 2.819 ± 0.200 Ga determined using a multicollector LA-ICP-MS, and the Wetherill Concordia method.

Fig. 4.

Quantitative microscale maps of candidate titanite biosignatures and surrounding metamorphic conditions in Archean metabasaltic pillow lavas from the BGB. (A) Map of TiO₂ (wt%) showing the image of titanite filaments of interest (arrows) highlighted in the dashed-line box. (B) Calculated metamorphic conditions in the chlorite matrix using a chlorite thermodynamic modeling approach (SI Materials and Methods). The dashed-line box highlights a typical area with well-developed titanite microfilaments surrounded by chlorite pixels recording relatively low temperature conditions of less than T = 350 °C (pixel group 2). (C and D) Image pixel grouping above and below the mean value of T = 350 °C, indicating a higher proportion of pixels recording conditions below T = 350 °C surrounding the titanite microfilaments (dashed-line box in C). In D the y axis variable Al(iv) = atoms per formula unit of Al⁴⁺ in tetrahedral coordination modeled in the chlorite crystal structure.