Earth and Planetary Science Letters 299 (2010) 290–298

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Earth and Planetary Science Letters

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In-situ dating of the Earth's oldest trace fossil at 3.34 Ga

D. Fliegel a,⁎, J. Kosler a, N. McLoughlin a, A. Simonetti b, M.J. de Wit c, R. Wirth d, H. Furnes a a Department of Earth Science & Center for Geobiology, University of Bergen, N-5007 Bergen, Norway b Department of Civil Engineering & Geological Sciences, University of Notre Dame, Notre Dame, IN 46556, USA c AEON and Department of Geological Sciences, University of Cape Town, Rondebosch 7701, South Africa d Helmholtz Centre Potsdam, GFZ German Research Centre for Geosciences, Potsdam, D 14473 Germany article info abstract

Article history: Microbial activity in volcanic glass within the can produce micron sized pits and tunnels. Such Received 11 May 2010 biogenic textures have been described from the recent oceanic crust and mineralized equivalents in pillow Received in revised form 3 September 2010 as old as 3.47–3.45 Ga from the Barberton (BGB) of South Africa. In meta-volcanic Accepted 7 September 2010 glasses these microbial traces are preserved by titanite mineralization (CaTiSiO ) and on the basis of Available online 6 October 2010 5 morphological, textural and geochemical evidence have been argued to represent Earth's oldest trace fossils. – fi Editor: M.L. Delaney Here we report the results of in-situ U Pb dating of titanite that in lls trace fossils from the Hooggenoeg Complex of the BGB using laser ablation MC-ICP-MS (multi-collector inductively coupled, plasma mass Keywords: spectrometry). This yields a titanite age of 3.342±0.068 Ga demonstrating the antiquity of the BGB trace trace fossils fossils. This radiometric age confirms that a sub-seafloor biosphere was already established in the PaleoArchean bioalteration PaleoArchean and that it likely represented an important habitat for the emergence and evolution of early Barberton Greenstone Belt microbial life on the Earth. U–Pb titanite dating © 2010 Elsevier B.V. All rights reserved. early life sub-oceanic biosphere

1. Introduction striking morphological variations including annulated, twisted, spiral shape and branched forms (McLoughlin et al., 2009). A conceptual Sea floor pillow contain tubular and granular alteration model of how such microbial communities are thought to etch the textures in their glassy chilled margins (e.g. Fisk et al., 1998; Furnes volcanic glass has been developed in a series of papers (e.g. Furnes et al., 2008; McLoughlin et al., 2008; Staudigel et al., 2008; Thorseth et al., 2008; Staudigel et al., 2008; Thorseth et al., 1992; Thorseth et al., et al., 1995). The biogenicity of such alteration textures has been 1995). These bioalteration textures can also be regarded as trace argued for on the basis of several lines of evidence including: the fossils because they record the activities and behaviours of micro- morphology of the alteration textures (e.g. McLoughlin et al., 2009; organisms and recently an ichnotaxonomy has been proposed to Staudigel et al., 2008 and refs therein); the δ13C values of disseminate elucidate the morphological diversity of these traces and enable global carbonates found in pillow rims and cores (e.g. Furnes et al., 2001a); and stratigraphic comparisons (McLoughlin et al., 2009). microbiological staining and sequencing studies (e.g. Santelli et al., The fossil record of life in volcanic glass extends far beyond the 2008 and refs therein); and chemical imaging of trace amounts of oldest in-situ oceanic crust. The preservation of bioalteration organic material lining the textures (e.g. Furnes and Muehlenbachs, textures in glass now transformed to metamorphic mineral 2003). The body of evidence used to advance a biogenic origin for assemblages is made possible by mineralization of the micro- these glass alteration textures is extensively reviewed elsewhere (e.g. textures on the sub-seafloor prior to deformation and metamor- Furnes et al., 2001b; Staudigel et al., 2008). It is hypothesized that phism. Titanite is a common mineralizing phase and micro-textures microorganisms living in the sub-seafloor colonize fractures and with comparable morphologies to those found in the in-situ oceanic vesicles in the glass in order to obtain electron donors, nutrients, crust have now been reported from Phanerozoic and shelter and perhaps to escape predation (Cockell and Herrera, 2008; greenstone belts (reviewed by Furnes et al., 2007). The Furnes et al., 2008). In Phanerozoic volcanic glass the granular sequence of events is: microbial alteration, authigenic mineraliza- bioalteration textures are by far the most abundant and the tubular tion and subsequent ; this is summarized in Fig. 7 of textures are less common (Fig. 34 in Furnes et al., 2008) but exhibit McLoughlin et al. (2010b). The mineralization process appears to obscure some of the fine morphological detail seen in recent bioalteration textures, such as spirals and annulations, and on fi ⁎ Corresponding author. average results in laments with larger diameters than that of the E-mail address: d.j.fl[email protected] (D. Fliegel). original hollow tubes (Fig. 2 and Furnes et al., 2007). But in all other

0012-821X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2010.09.008 D. Fliegel et al. / Earth and Planetary Science Letters 299 (2010) 290–298 291 respects the mineralized tubes are comparable in morphology and sheet flows with several interlayered chert horizons that are between distribution to those found in fresh volcanic glass. In contrast, ca. 0.5 to 20 m thick and together this sequence is overlain by the granular bioalteration textures, the most abundant bioalteration sedimentary rocks of the Fig Tree and Moodies Groups (Fig. 1). Pillow texture in glassy pillow rims of the in-situ oceanic crust, that are lavas from the Hooggenoeg Complex show no or little deformation typically b0.5 μm in diameter are lacking from meta-volcanic glass, (Fig. 2). The Onverwacht Suite has been interpreted to represent probably because these are obliterated by recrystallisation of the fragments of Archean oceanic crust, termed the Jamestown host glass during metamorphism. Complex (de Wit et al., 1987), that developed in association with Trace fossils found in meta-volcanic glass of the Barberton subduction and island arc activity approximately 3.55 to 3.22 Ga (de Greenstone Belt (BGB) in South Africa are the focus of this study Ronde and de Wit, 1994). The magmatic sequence of the Onverwacht (Fig. 1). Their geological setting and key textural characteristics are Suite is exceptionally well-preserved, relatively undeformed away summarized below. Briefly, they are filamentous micro-textures that from its margins with the surrounding granitoids and has experienced are b15 μm in diameter and extend up to 200 μm from a “root zone” greenschist facies or lower degrees of metamorphism (e.g., Grosch that represents the initial crack from which the microbial alteration et al., 2009a; Tice et al., 2004). This relatively well preserved window commenced (Banerjee et al., 2006; Furnes et al., 2004). Based on onto the early Archean Earth has thus provided the focus of many textural and geochemical evidence, these tubular titanite structures effort to seek traces of life in both the sediments (e.g. Javaux et al., (Fig. 2) have been interpreted to represent the oldest physical traces 2010; Tice and Lowe, 2004) and volcanics (e.g. Furnes et al., 2004; of life on Earth (Furnes et al., 2004). The goal of this study was to Grosch et al., 2009b). obtain an in-situ U–Pb date for the titanite that infills the BGB trace The ~2700 m thick lava pile of the Hooggenoeg Complex fossils in order to constrain the timing of their mineralization and to comprises several eruptive pulses described in Furnes et al (in obtain a minimum age for the sub-seafloor microbial activity that press). The age of the Hooggenoeg Complex is bracketed by two U–Pb produced these trace fossils. zircon ages of ~3.47 and ~3.45 Ga the locations of which are shown on Fig. 1 (modified from Furnes et al., in press). A maximum age for 2. Geological setting and sample locality eruption of the Hooggenoeg Complex is provided by detrital zircons from the basal Middle Marker chert that is exposed along the The PaleoArchean BGB of South Africa contains some of the world's southeastern limb of the Onverwacht Fold where it has been dated at oldest and best-preserved pillow lavas (de Wit et al., 1987). The 3.472±0.005 Ga (Armstrong et al., 1990). Above the Hooggenoeg tectonostratigraphy of the Onverwacht Suite comprises the Thee- Complex and within the Noisy Complex appears a large dacitic spruit, Komati, Hooggenoeg, Noisy, Kromberg and Mendon Com- intrusion in the eastern part of the northern limb of the Onverwacht plexes that total 15 km in thickness (Fig. 1 and de Wit et al. in press). Fold which has yielded a concordant U/Pb SHRIMP date of 3.445± These complexes are separated by major shear zones or unconfor- 0.004 Ga (de Wit et al., 1987). The overlying Noisy Complex as mities described in detail in de Wit et al (in press). The extrusive recently defined by Furnes et al (in press) has also yielded several components include pillow lavas, minor hyaloclastite breccias and minimum age constraints upon eruption of the Hooggenoeg lavas

Fig. 1. (A) Map of South-Africa showing the location of the Barberton Greenstone Belt (BGB). (B) Sketch map of the BGB. (C) Geological map of BGB indicating the sample location on the Komati river in upper Hooggenoeg Complex (star). The eruptive age of the sample locality is bracketed between two U–Pb zircon ages (locations indicated by solid dots) of 3.472 and 3.455 Ga (see text for explanation). 292 D. Fliegel et al. / Earth and Planetary Science Letters 299 (2010) 290–298

Fig. 2. (A) Komati river with the sampling location of the pillow lavas. Pillow lavas in the photograph are well preserved and little deformed. Petrographic thin-sections of titanite mineralized trace fossils, in a greenschist facies meta-volcanic glass from Barberton Greenstone Belt South Africa. (B) Clusters of titanite mineralized filaments radiating from central “root zones” in a inter pillow hyaloclastite (IPH) sample now composed of chlorite (green) and quartz (white). (C) Combined transmitted-reflected light image showing a laser ablation track in the “root zone” of a titanite mineralized trace fossil (dotted line). (D) and (E) Examples of trace fossils showing a central titanite mineralized “root zone” (dashed line) with a medial fracture along which fluids once circulated, and from which titanite mineralized filaments radiate and are segmented (arrows) by the growth of cross-cutting chlorite. Sample number 29-BG-03. (F) Size distribution of tubular textures in the Barberton pillow lavas, redrawn from Furnes et al. (2007).

including, igneous zircons from a flow banded porphyritic lava near 3. Description of the BGB trace fossils the top of the volcaniclastic sequence that gave a concordant SHRIMP date of 3.451±0.005 Ga (de Vries et al., 2006). In addition, along the The BGB trace fossils occur in pillow rims and interpillow Komati River downstream from our sample site, zircons from a dacitic hyaloclastites. They are mineralized by micro-crystalline titanite and crystal-tuff near the top of the Noisy Complex (location shown on appear dark brown in thin-section. They occur as dense clusters and Fig. 1) yielded a magmatic U–Pb zircon age of 3455.2±7.5 Ma (Biggin bands containing a central “root zone” from which they radiate et al., submitted). (Fig. 2). Sometimes a paler-coloured band, a few microns wide can be The Hooggenoeg samples investigated here were collected from a seen at the center of the “root zone”, recording the former location of a non-vesicular and variolitic pillow lava outcrop on the banks of the fracture in the volcanic glass where the bioalteration initiated. The Komati River in the south eastern limb of the Onverwacht anticline mineralized tubular structures are curvi-linear and may taper slightly (Fig. 1). This trace fossil bearing horizon was also the target of a towards their ends, occurring in clusters of up to tens of filaments. scientific drill hole KD2b described in Grosch et al., (2009b). The They have an average width of 4 μm and are up to 200 μm in length pillow lavas are very little deformed and display well-developed with an average length of 50 μm (Fig. 7 in Furnes et al., 2007). They are chilled margins ~10 mm thick of formerly glassy material that grade commonly segmented by cross-cutting chlorite grains (Fig. 2 D and E). inwards into a variolitic zone, consisting of a mixture of altered glass and microcrystalline material (Fig. 2a and d of Banerjee et al., 2006). 4. Archean trace fossil — a summary of the evidence for biogenicity The pillow rims now comprise a greenschist facies assemblage of and antiquity chlorite, quartz, epidote, titanite and calcite. Pockets of interpillow hyaloclastites are well developed at this locality and this outcrop also There are three reported occurrences of mineralized microbial provided the material illustrated in Figs. 1 and 3 of Furnes et al. (2004) traces from Archean pillow lavas. The first and also the oldest and is referred to as locality 6 in Banerjee et al. (2006). examples yet described come from the BGB of South Africa (Furnes D. Fliegel et al. / Earth and Planetary Science Letters 299 (2010) 290–298 293 et al., 2004; Banerjee et al., 2006; Furnes et al., 2007). Morphologically McLoughlin et al. (2010a) and cannot explain the morphology, comparable textures, have also been described from the 3.35–3.31 Ga distribution and geochemical characteristics of the BGB trace fossils. pillow lavas of the Euro in Western Australia (Banerjee et al., The antiquity of the Barberton trace fossils has been argued for 2007) and 2.52 Ga pillow lavas of the Wutai Group of North West from textural observations. The micro-tunnels are segmented by China (McLoughlin et al., 2010b). The evidence for the biogenicity of metamorphic chlorite (e.g. Fig. 2), implying that the microbial traces these trace fossils has been extensively explained elsewhere (see were mineralized by titanite before growth of the cross-cutting Banerjee et al., 2006; Furnes et al., 2004; Furnes et al., 2008) and metamorphic chlorite. A low-grade, sea-floor metamorphic event is rather here, we summarise the major features that support the argued to have occurred shortly after extrusion of the Hooggenoeg biogenicity and especially antiquity of the Barberton trace fossils. pillow lava succession (Brandl and de Wit, 1997), and was followed by A biogenic origin for the titanite mineralized textures from the very low grade dynamothermal metamorphism between 3.23 and Barberton has been advanced on the basis of 3 lines of evidence 3.29 Ga (Schoene et al., 2008). The last regional deformation in the summarized below: vicinity of the BGB occurred between 3.1 and 3.2 Ga, and the rocks have not been thermally disturbed since. Maximum temperature for a. The BGB titanite mineralized tubular structures have comparable this regional metamorphism have been estimated to be between 200– morphologies, size and distribution to trace fossils found in recent 400 °C on the basis of a geothermometer involving graphite volcanic glass crystallinity measured by laser-Raman spectroscopy on carbonaceous cherts (Tice et al., 2004). The micro-textures are restricted to pillow rim and interpillow Direct dating of Archean trace fossils in volcanic glass was first breccia samples and extend away from healed fractures along which undertaken on samples from the Pilbara Craton of Western Australia seawater once flowed (Fig. 2). These tubular structures are similar in (Banerjee et al., 2007). This study reported a U–Pb age of 2.9±0.1 Ga size, shape, and distribution to features documented in glassy pillow for titanite mineralized trace fossils that is ~400 Ma younger than the rims from ophiolites such as the Troodos ophiolite and in-situ oceanic accepted eruptive age for the host pillow lavas of 3.35 Ga given by an crust (Furnes et al., 2008). Thus the BGB structures are interpreted as interbedded tuff (Nelson, 2005). The titanite date corresponds to the being the mineralized remains of microbial trace fossils. age of a regional metamorphic event related to the last phase of deformation and widespread granite intrusion in the Northern Pilbara b. The δ13C values of carbonate in the BGB pillows rims records microbial at 2.93 Ga (van Kranendonk et al., 2002). This U–Pb titanite age fractionation therefore represents a minimum estimate for the timing of titanite formation, especially given that metamorphic chlorite cross-cuts the Disseminated carbonate from bulk rock samples of pillow rims titanite tubules and so must post-date the titanite mineralization. This from the Onverwacht Suite have δ13C values of +3.9 to −16.4 per mil implies ab400 Ma post eruptive period during which the bioaltera- (‰), which are different from those of the crystalline interiors of +0.7 tion textures from the Pilbara were mineralized; but does not exclude to −6.9‰. Amygdules containing later, secondary carbonate have the possibility that the textures formed soon after eruption, remained δ13C values that cluster around zero. The δ13C values from crystalline hollow and were mineralized somewhat later. Our goal with the interior samples are bracketed between those of primary mantle CO2 Barberton samples is thus to investigate the duration of this Archean (−5to−7‰) and of Archean marine carbonate (0‰), whereas the bioalteration window and to see if we could more tightly constrain the glassy samples extend to lower δ13C values. Such isotopic contrasts antiquity of another example of Archean trace fossils. are also seen in pillow lava rims from ophiolites and oceanic crust, where the generally low δ13C values of disseminated carbonate are 5. Analytical techniques attributed to metabolic byproducts formed during microbial oxidation of dissolved organic matter in the pore waters (Fig. 2 in Furnes et al., A multi-collector ICP-MS (Thermo Finnigan Neptune) coupled to 2008). The isotopically low δ13C values of carbonate in the formerly a 213 nm Nd:YAG laser (New Wave Research UP-213) was used to glassy rims of the BGB pillows are thus interpreted to have also measure Pb/U and Pb isotopic ratios in titanite at Bergen University. formed by microbial fractionation (Banerjee et al., 2006; Furnes et al., The sample introduction system was modified to enable simulta- 2004). neous nebulisation of a tracer solution (detector cross calibration and mass bias correction) and laser ablation of the solid sample. c. X-ray element mapping of carbon on the walls of the BGB tubular NaturalTl(205Tl/203Tl=2.3871; Dunstan et al., 1980), 209Bi and structures enriched 237 Np (N99%) were used in the tracer solution which was aspirated to the plasma using a 50 μl/min PFA nebuliser, dual Furnes et al. (2004) and Banerjee et al. (2006) report X-ray cyclonic–double pass quartz glass spray chamber and a T-piece tube elemental maps of thin, b1 μm thick carbon linings on the walls of attached to the back end of the plasma torch. Helium was used as some of the BGB micro-textures. This carbon does not correlate with ablation/carrier gas. The aerosol was mixed prior to the torch via a the elemental maps of calcium, iron, and magnesium indicating that T-piece. the carbon is not bound in carbonate (e.g. Fig 7 in Banerjee et al., 2006, The laser was set up to produce energy density of 3 J/cm2 at a analytical methods explained in Banerjee and Muehlenbachs 2003). In repetition rate of 10 Hz. The laser spot size varied between 8 to 60 μmin recent volcanic glass, biofilms and organic remains containing nucleic diameter according to the sample volume available for laser ablation. acids are commonly observed along the interior surface of microbially The laser beam was focused on the sample that was mounted in ca. generated pits and tunnels (e.g. Furnes et al., 1999). Thus the BGB 3cm3 ablation cell. The “root zones” from 11 different trace fossil found carbon has been interpreted to represent decayed organic material in a polished thin section, sample numbers 101-BGB-08 were analyzed. left on the interior surfaces of microbially generated trace fossils that During ablation the stage was moved beneath the stationary laser beam was subsequently preserved during later titanite mineralization at a speed of 10 μm/s. Typical signal intensities measured for the 204Pb, (Furnes et al. 2004). 206Pb–207Pb and 238U were ca. 2000, 30000 and 20000 cps, respectively. No plausible abiotic mechanism has yet been advanced to explain The acquisition consisted of a 25 s measurement of all analytes in the gas the BGB micro-textures. Abiotic processes that have been tested and blank and aspirated tracer solution, followed by measurement of U and rejected include: purely chemical dissolution, and the migration of Pb isotope signals from titanite, along with the continuous signal from ambient inclusions. These abiotic mechanisms are discussed in detail the tracer solution for another 25 seconds. The data were acquired in in Banerjee et al. (2006); Staudigel et al. (2008); and most recently in static mode in a mixed channeltron (IC)–Faraday (FAR) detector array 294 D. Fliegel et al. / Earth and Planetary Science Letters 299 (2010) 290–298 D. Fliegel et al. / Earth and Planetary Science Letters 299 (2010) 290–298 295 for isotopes 202Hg (IC1), 203Tl (IC2), 204Hg+Pb (IC3), 205Tl (IC4), 206Pb Titanite U–Pb isotopic data obtained by LA MC ICP-MS for trace (IC5), 207Pb (IC6), 208Pb (IC7), 209Bi (FAR L4), 237 Np (FAR H4) and 238U fossils are given in Table 1 and plotted on a concordia diagram in (IC8). Solution signal of 203Tl was used to cross-calibrate the Fig. 4. The pooled concordia age for 11 trace fossil “root zones” is channeltrons before each analysis. 3.342±0.068 Ga. The mean isotopic composition of the studied The data were corrected for channeltron dead time and processed titanites plots slightly above the concordia curve. We interpret this off line in a spreadsheet-based program (LamTool; Kosler et al., 2008) as resulting from a small inaccuracy in the common lead correction and plotted on concordia diagrams using IsoplotEx (Ludwig, 2003). that was applied to the raw isotopic data. Accurate common Pb Data reduction included correction for channeltron yield, gas blank correction is crucial for obtaining correct U–Pb titanite ages but and mass discrimination using the solution signals of Tl, Bi and Np and problems in determining the initial lead isotopic composition for repeat analyses of the Khan titanite (Heaman, 2009). Correction for lavas in general, and for rocks from the BGB in particular, common Pb was based on measurement of the non-radiogenic 204Pb are well known and have been discussed in length by Brevart et al. and utilized the Pb isotope composition of Archean galena from the (1986). Briefly, the Pb isotopic composition of sulfides (mostly French Bob's mine in the Barberton Greenstone Belt; it is one of the galena) from the BGB shows a considerable spread in the 206/204Pb, galena samples used to define the two-stage model of lead isotopic 207/208Pb and 208/204Pb isotopic ratios (Brevart et al., 1986; Saager and evolution (Stacey and Kramers, 1975). Titanite sample from the Koppel, 1976). In addition, none of the published galena Pb isotopic Lillebukt Alkaline Complex in northern Norway (0.520±0.005 Ga; compositions can produce entirely concordant titanite ages and the Pedersen et al., 1989) was periodically measured for data quality galena samples come from a different part of the BGB stratigraphy control and it gave a pooled concordia age of 0.524±0.024 Ga (6 than the material studied herein. Therefore we propose that for measurements, 2 sigma). common Pb correction of the studied titanite samples, it is more Energy dispersive x-ray (SEM-EDX) images were acquired with a appropriate to use a general model for common Pb evolution, for Zeiss Supra 55VP scanning electron microscope equipped with a example, Stacey and Kramers (1975) or part of it. We therefore chose Thermo Noran EDS detector at the University of Bergen. The EDX to use the common lead composition of a galena from the French Bob's spectra were collected at 15 mm working distance using 15 keV mine in the Barberton Onverwacht Suite that defines the Archean part acceleration voltage and 60 μm aperture size. The most abundant of the 2-stage lead evolution model of Stacey and Kramers (1975). fluorescence for each element line was used for imaging. This is considered by us to be the most appropriate for the common Focused ion beam milling (FIB) and transmission electron lead correction of the titanite infilling the studied trace fossils. It microscopy were done at the GeoForschungsZentrum in Potsdam should also be noted that choice of a different initial Pb composition (GFZ) using a FEI FIB2000 TEM and a FEI TecnaiG2 F20 X-TWIN for correction of titanite data presented in this study affects the instruments. The field emission gun of the TEM was operated at concordance of the pooled concordia ages but has no significant effect 200 keV accelerating voltage. Total of four different FIB foils were on the reported age of the trace fossils. obtained from different parts of sample number 29-BGB-03 and analyzed by TEM. Datasets from all FIB foils were found to be similar. 7. Discussion Further details of the FIB sample preparation and analysis are described in Wirth (2004). The 3.342±0.068 Ga titanite age measured here records the age of metamorphic titanite growth and equilibration within these trace 6. Results fossils. This demonstrates that the trace fossils are Paleoarchean in age and provides a minimum age estimate for their formation. In other Scanning and transmission electron microscope investigations of words, this titanite age underestimates the timing of microbial the BGB sample, together with previous textural and mineralogical alteration of the BGB volcanic glasses but nonetheless, confirms observations confirm the similarities between the titanite that infill their antiquity. The Hooggenoeg lavas have never experienced the tubular structures and the titanite in the “root zones” (Figs. 2 and metamorphic temperature that exceed the closure temperature of

3). Elemental distribution maps obtained by SEM-based EDX analysis the U–Pb isotopic system in titanite (Tc =550–650 °C), thus this age indicate that there is no appreciable difference in composition represents the main phase of titanite growth within the textures (c.f. between the tubular structures and their respective “root zones” Schoene and Bowring, 2007). Furthermore, petrographic, SEM and (Fig. 3). In addition, the TEM data shows no variation at the micron or FIB-TEM observations confirm that mineralization was synchronous sub-micron scale in porosity, size and shape of the crystals between in all parts of the trace fossils i.e. in both the “root zones” and tubular the polycrystalline titanite in the micro-textures and that in the “root cavities (Figs. 2 and 3). zones” (Fig. 3G and H). Moreover, the TEM images reveal the The difference in age between the infilling titanite (3.342 ± homogeneity of the phase composition of the trace fossils, suggesting 0.068 Ga) and the eruptive age of the Hooggenoeg Complex (3.47– that mineralization of the tubular structures and the “root zones” 3.45 Ga) is attributed to either: a long-lived bioalteration window occurred simultaneously or within a very short time span. This means during which microbial alteration persisted; or a time gap after which that a U–Pb age obtained from the central “root zones” will be microbial alteration ceased and before the titanite mineralization representative of the whole trace fossil. Unfortunately, our LA MC ICP- commenced. Both of these possibilities are supported by analogy to MS requires a 40 μm spot size which is too large to date an individual the modern oceanic crust and ophiolites. An early onset to bioalteration tube. But as explained above, this is not a concern because on the Archean seafloor is supported by comparison to modern oceanic petrographic arguments suggest that mineralization of the “root spreading centres, where microbes are observed to rapidly colonize zone” and tubes was synchronous. juvenile glassy surfaces (Furnes et al., 2007). These bioalteration traces

Fig. 3. Scanning electron microscope images of the trace fossils taken in backscatter mode (A) and elemental distribution maps obtained using energy dispersive X-rays (B–F). Also transmission electron microscope, high-angle annular dark field images (TEM HAADF) of focused ion beam (FIB) foils cut from the trace fossils (G)–(H). Together these show no differences in the elemental composition or mineralogy of the “root zones” and individual tubes. (A) Segmented, filamentous trace fossils that intersect the surface of the sample and radiate from a central “root zone”. (B) Titanium elemental map. (C) Silica elemental map. (D) Magnesium elemental map. (E) Calcium elemental map. (F) Iron elemental map. (G) FIB foil from a single, tubular trace fossil that intersects the surface of the platinum coated sample. The tubes are filled with polycrystalline, porous titanite and the variation in grey contrast is due to differences in orientation of the individual titanite grains. The semi-circular cross-section of the tube is clearly seen and the interface with the chlorite matrix. (H) FIB-foil from a titanite mineralized “root zone”. No difference in the size, shape or porosity of the titanite crystals can be seen by comparison to the polycrystalline titanite in the individual trace fossil tubes (G) or “root zones” (H). Variation in grey level of (G) and (H) is attributed to the different thickness of both foils, which is also reflected in the HAADF image (note the faint dark circle visible in H is due to the a hole in the amorphous carbon foil used on the TEM grid). Images A–F are from sample 29-BGB-03 and G–H from sample 101-BGB-08. 296 D. Fliegel et al. / Earth and Planetary Science Letters 299 (2010) 290–298 σ Pb ±1 206 Pb/ 207 σ U±1 238 Pb/ 206 σ

Fig. 4. U–Pb concordia diagram showing the LA-MC-ICP-MS titanite data measured on “root zones” of microbial trace fossils from the Hooggenoeg Complex of the Barberton Greenstone Belt, South Africa. The data-point ellipses represent one sigma uncertainty; U±1 the pooled concordia age (grey ellipse) is 3.342±0.068 Ga and is shown at the 2 sigma 235 level. ). Pb/ Calculated ages Ma 207 ) 2 can remain hollow or become partially infilled with authigenic minerals

Rho principally clays and iron-oxyhydroxides over time and these may be enriched in titanium, a possible precursor to titanite (e.g. Fig. 12b in Staudigel et al., 2008). Theoretically, as long as there is still fresh glass Stacey and Kramers, 1975

σ present and seawater circulation continues, then the microbial bioalteration may go on for millions of years. The major phase of titanite mineralization is argued to occur much later during greenschist facies metamorphism, the timing of which is determined by the

Pb ±1 regionally geological history. Bioalteration textures can remain hollow 206 for long periods of time, as for example, micro-textures from the little Pb/

207 deformed 0.092 Ga Troodos ophiolite (Furnes et al., 2001c) and the 0.122 Ga Ontong Java Plateau (Banerjee and Muehlenbachs, 2003)are largely unmineralized. A summary of this sequence of events is shown in Fig. 7 of McLoughlin et al. (2010b). σ The nature and timing of metamorphism of the Hooggenoeg volcanic glasses is only sparsely characterized. The 3.342±0.068 Ga metamorphic titanite age reported here is comparable to an age

U±1 obtained by step-heating 40Ar/39Ar analysis of 3.326±0.005 Ga from 238 komatitic basalts of the underlying Komati Complex (Fig. 5 in López- Pb/

206 Martínez et al., 1992). Alternatively, the titanite age obtained here of trace fossils from sample 101-BGB-08 of the Barberton Greenstone Belt. ” may correspond to heating associated with the intrusion of gabbroic into the Onverwacht Suite that is recorded by a U/Pb baddelleyite age of 3.351±0.006 Ga obtained from a meta-gabbro σ root zones “ in the underlying Komati Complex (Kamo and Davis, 1994). Subsequent to the metamorphic event discussed here the Hooggen- oeg Complex experienced low grade dynamothermal metamorphism

U±1 during a major deformation event between 3.23 and 3.29 (de Ronde 235 and de Wit, 1994; Schoene et al., 2008). Localised deformation along Pb/ the margins of the BGB occurred between 3.1 and 3.2 Ga and the rocks 207 have not been thermally disturbed since. Whether the new U–Pb titanite age reported here represents a sub-seafloor burial metamor- ) Pb data for titanite from 11 1

– phism event, a regional low-grade metamorphic overprint, or a Pb contact metamorphic effect arising from gabbroic intrusives, it does 204 not change the fact that the trace fossils are Paleoarchean in age. Pb/

206 Moreover, this corroborates and provides an absolute age constraint on previous textural observations of metamorphic chlorite cross- cutting the titanite infilled trace fossils (Fig. 4). This is only the second study to use direct in-situ dating techniques to obtain an absolute age for a candidate biosignature from the Analysis Isotopic ratios # 1# 2# 3# 4# 5 19.342# 6 20.921# 7 19.157# 8 20.000# 9 18.248# 10 30.5638 20.040# 11 23.1769 21.097 23.8293 20.040 24.2193 18.519 4.3706 19.268 29.0922 3.1621 18.692 24.3100 3.1544 0.7161 21.2445 4.9102 0.6601 30.2134 4.4687 0.6703 22.5431 3.2647 23.2231 0.7202 2.7328 24.1641 0.0386 0.7497 5.8447 0.0335 0.7330 3.4179 0.0336 0.6750 2.9519 0.3096 0.0446 0.7422 3.2221 0.2546 0.0420 0.7086 0.2578 0.6819 0.0348 0.2439 0.6996 0.0319 0.2815 0.0145 0.0489 0.2405 0.0101 0.0352 0.2283 0.0333 0.0105 0.38 0.2952 0.0350 0.0135 0.37 0.2307 0.0152 0.2470 0.38 0.0096 0.2505 0.31 3505 0.0087 0.36 3234 0.0219 0.35 3261 0.0100 0.37 0.0092 3277 0.34 0.0102 3456 501 0.33 3281 441 0.38 3150 432 0.38 3494 3481 664 3207 3268 531 3236 3307 441 3275 3497 405 3606 676 188 3544 486 166 411 3325 166 437 3579 3518 216 3453 3214 202 3351 3233 168 3419 3145 157 3371 236 3123 172 72 164 3040 63 171 3445 64 3057 88 3165 84 3188 64 61 115 69 59 65 Table 1 Laser ablation MC-ICP-MS U Isotopic ratios corrected for initial common Pb using composition of galena from the French Bob's Mine, Onwerwacht Group ( Paleoarchean. Other traces of Archean life such as organic D. Fliegel et al. / Earth and Planetary Science Letters 299 (2010) 290–298 297

Fig. 5. Schematic overview of processes leading to mineralization of tubular alteration textures by titanite. After eruption of the lava pile at 3.47–3.45 Ga, the tubular textures were etched into the host glass. The alteration progressed until mineralization by titanite ceased at 3.34±0.07 Ga. The titanite mineralization in the non chloritized cavities was most likely related to exclusion of titanium from the chlorite during metamorphism in the presence of calcium and silicon. The two photographs show well preserved pillows from Hooggenoeg Complex (left) and tubular textures mineralized by titanite. microfossils, stromatolites and geochemical signatures are not and H. Staudigel for constructive discussions. The authors wish to possible to date directly. The age obtained here is approximately 0.4 thank J. Ellingsen for help with the artwork in Fig. 1 and L. M. Heaman billion years older than the previously published age of 2.9±0.1 Ga for providing the Khan titanite standard. This is AEON contribution from comparable trace fossils in the 3.35 Ga Euro Basalt of the Pilbara number 85. Craton in Western Australia (Banerjee et al., 2007). Furthermore, the time gap between eruption of the host lavas and the mineralization of the textures, or the co called “window for bioalteration”, is also References significantly smaller, being b0.13 Ga in the case of the Barberton trace Armstrong, R.A., Compston, W., de Wit, M.J., Williams, I.S., 1990. The stratigraphy of the fossils in contrast to ~0.44 Ga for the Pilbara trace fossils (Banerjee 3.5–3.2 Ga Barberton Greenstone Belt revisited: a single zircon ion microprobe et al., 2007) and ~0.71 Ga for the Wutai bioalteration textures study. Earth Planet. Sci. Lett. 101, 90–106. (McLoughlin et al., 2010b). This time window is controlled by the Banerjee, N.R., Muehlenbachs, K., 2003. Tuff life: bioalteration in volcaniclastic rocks from the Ontong Java Plateau. Gechem. Geophys. Geosyst. 4 (4). doi:10.1029/ regional geological history and is much shorter in the case of the 2002GC000470. Barberton trace fossils, because the greenschist facies metamorphic Banerjee, N.R., Furnes, H., Muehlenbachs, K., Staudigel, H., de Wit, M., 2006. event responsible for their mineralization and preservation occurred Preservation of similar to 3.4–3.5 Ga microbial biomarkers in pillow lavas and sooner after eruption. A schematic overview of the process is shown hyaloclastites from the Barberton Greenstone Belt, South Africa. Earth Planet. Sci. Lett. 241, 707–722. on Fig. 5. It is in these types of settings where the titanite growth Banerjee, N.R., Simonetti, A., Furnes, H., Muehlenbachs, K., Staudigel, H., Heaman, L., Van within the trace fossils was relatively early that this U–Pb dating Kranendonk, M.J., 2007. Direct dating of Archean microbial ichnofossils. Geology – technique is most powerful in constraining the antiquity of the trace 35, 487 490. 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