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Precambrian Research
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umice from the ∼3460 Ma Apex Basalt, Western Australia: A natural laboratoryor the early biosphere
artin D. Brasiera, Richard Matthewmana, Sean McMahonb, Matt R. Kilburnc, David Waceyd,e,∗
Department of Earth Sciences, University of Oxford, South Parks Road, Oxford OX1 3AN, UKSchool of Geosciences, University of Aberdeen, Meston Building, Kings College, Aberdeen AB24 3UE, UKCentre for Microscopy Characterisation and Analysis, The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, AustraliaDepartment of Earth Sciences & Centre for Geobiology, University of Bergen, Allegaten 41, Bergen N-5007, NorwayCentre for Core to Crust Fluid Systems, Centre for Microscopy Characterisation and Analysis & School of Earth and Environment, The University of Western Australia, 35 Stirlingighway, Crawley, WA 6009, Australia
r t i c l e i n f o
rticle history:eceived 10 April 2012eceived in revised form 18 August 2012ccepted 4 September 2012vailable online 19 September 2012
eywords:umicerigin of life
a b s t r a c t
It has recently been hypothesised that pumice, a low-density vesicular volcanic rock, could have acted asa natural floating laboratory for the accumulation and concentration of chemical reactants needed for theorigin of life. To test the plausibility of his hypothesis, we here turn to the earliest rock record for evidenceof pumice deposits and their associated mineralogy and biogeochemistry. We report abundant clasts ofpumice from within a volcaniclastic breccia bed immediately above the ∼3460 Ma ‘Apex chert’ unit ofthe Apex Basalt, Pilbara region, Western Australia. Textural and geochemical analyses reveal that thebody of these pumice clasts was deeply permeated by intimate associations of C, O, N, P and S. Pumiceand scoria vesicles were also lined with carbon or with catalysts such as titanium oxide or potential
pex Basaltilbara
biominerals such as iron sulfide, while many were infilled with aluminosilicate minerals. The latter maybe the metamorphosed remains of potentially catalytic clay and zeolite minerals. It is not yet possibleto distinguish between chemical signals left by prokaryote biology from those left by prebiology. Thatbeing so, then early prokaryotes may well have colonised and modified these Apex pumice clasts priorto burial. Nevertheless, our data provide the first geological evidence that the catalysts and molecules
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needed for the earliest sta
. Introduction
Was there an unusual kind of geological substrate, still pre-erved in the early Archaean geological record, that could haveoncentrated, selected and catalysed the diversity of chemical reac-ants needed for life? Was this environment not only widespreadnd abundant but also capable of providing reaction chambersith maximum surface area over sufficiently long periods of time?
n a recent article, Brasier et al. (2011b) have suggested that thenswer to these questions could be ‘yes’, within the vastly abun-ant mineral vesicles of pumice and related rocks (Fig. 1). Thepecial properties of pumice have been discussed in detail else-here (Brasier et al., 2011b) but a summary is given here to enable
he reader to understand the context for the ‘pumice hypothesis’ith regard to the origin of life.
∗ Corresponding author at: Department of Earth Sciences & Centre for Geobiology,niversity of Bergen, Allegaten 41, Bergen N-5007, Norway. Tel.: +47 55 58 35 27;
Pumice has the highest surface area to volume ratio of any rocktype. Today this allows for high levels of biological colonisation(van Houten et al., 1995; Di Lorenzo et al., 2005) while, in a pre-biotic world, pumice would have a great capacity for adsorbingbiologically relevant elements and compounds, and maximising thesurface area available for chemical reactions. Pumice is the onlyrock type known to float on the surface of the ocean for sustainedperiods, owing to its high pore-volume and gas-filled vesicles. Itcan then come to sit within the intertidal zone of beaches for manythousands of years. This unique position at the air water interfaceis potentially ideal for the mixing of atmospheric and trapped vol-canic gases with water. Pumice would also be able to adsorb organiccomplexes of the kind thought to have existed locally as oily slicksat the surface of the Archaean ocean (Nilson, 2002; Deamer et al.,2002; Bada, 2004), creating lipid lined vesicles, precursors to thesynthesis of peptides, proteins, ATP, and nucleic acids. Thermalconditions would not have been severe on the surface of the ocean,and the vesicular nature of pumice would have provided protection
from harsh UV radiation.
Pumice also experiences an unusually wide variety of conditionsfrom eruption to burial via floating (rafting) and benthic stages(Fig. 2; see also Brasier et al., 2011b). These include: exposure to
2 M.D. Brasier et al. / Precambrian Research 224 (2013) 1– 10
Fig. 1. Pumice as a substrate for the emergence of life, showing some of the attrac-tions of the hypothesis. 1. Pumice provided a cellular, open system provided withmany small, interconnected compartments having a massive and reactive surfacearea. 2. During and after eruption, the pumice exterior was exposed to UV light,electricity and dehydration. 3. The interior compartments of pumice provided ahomeostatic environment, comparatively insulated from UV light and major tem-perature variations. 4. Pumice floated on the surface of the sea in huge rafts,eventually accumulating on beaches where it sat for many thousands of years. 5.During this phase, pumice rapidly adsorbed organic molecules, phosphate ions andmetallic compounds; it played host to catalysts such as titanium oxide, iron sulfide,zeolites, and clays including halloysite nanotubes. 6. Primordial organic compoundsfrom extraterrestrial bombardment and from Fischer–Tropsch-type synthesis, accu-mulated at the air–water interfaces of oceans and shorelines, alongside floatingpumice. Lipids and other polymers began to coat pumice vesicles, to form hydropho-bic selectively permeable membranes. 7. Rhythmic oscillations in UV light (fromdiurnal cycles), as well as in pH and salinity (from tidal cycles, wetting and dry-imp
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Fig. 2. Summary diagram showing the five main stages through which pumice canpass (modified from Brasier et al., 2011b). These stages are as follows: 1. Eruptive;2. Early floating; 3. Late floating; 4. Sinking and beaching; 5. Burial. Note how thepumice (white circles) may come to float alongside oily lipids (black ellipses). Beloware shown cartoons of a single pumice vesicle with suggested patterns of alterationthrough these five stages. Of these, organic (carbonaceous) compounds, phosphates,metals, zeolites, clays and evaporites all have some significance for the origins of life.
ng), created gradients between adjacent compartments. Osmotic gradients acrossembranes encouraged various kinds of pump to develop, including movements of
rotons (H+ ions), phosphate and sodium ions.
riboelectric charges, Leidenfrost boiling (see James et al., 2008) andV light, raising the possibility of Miller–Urey syntheses (e.g. Miller,992); hydrocarbon production via electric discharges throughixtures of ‘primitive’ gases (Navarro-González and Basiuk, 1996;
egura and Navarro-Gonzalez, 2001); and production of bio-vailable phosphorus and fixed nitrogen by lightning (Yamagatat al., 1991; Schwartz, 2006; Mather and Harrison, 2006). Duringts life cycle pumice would also likely experience energy-releasingycles of heat, light and tides at diurnal to seasonal scales, alka-ine waters, freeze–thaw conditions of the kind favoured by someor nucleic acid syntheses (cf. Miller, 1992; Menor-Salvan et al.,007), episodic changes in salinity (hence osmotic gradients), plusydration and dehydration of organic compounds held within itsesicles. These types of cycle would be particularly prevalent inumice beached along marine or lake shorelines.
Not least among the virtues of pumice and scoria for early lifeesearch is their capacity to host secondary minerals with knownatalytic potential. Interaction with hydrothermal fluids readilyroduces microporous aluminosilicates called zeolites. These canrow alongside smectite clays when siliceous pumice reacts withlkaline waters (e.g. Tomita et al., 1993), forming amygdale infill-ngs. Such zeolite minerals can greatly boost the catalytic activity ofumice during industrial processes today (Brito et al., 2004). Henceeolites (or ‘silicalite zeolites’ with periodic Ti atoms in place of Si;.g. Yamashita et al., 2007) are used for the synthesis of organicolymers (e.g. de Vos and Jacobs, 2001) as well as for the crack-
ng of hydrocarbons and the commercial liberation of hydrogenas (van Bekkam et al., 2001). Of equal interest is the potential ofumice to generate clay minerals that could have acted as potential
emplates for replication within early protocells (e.g. Cairns-Smith,005). Of these, smectite clays formed by hydrothermal alterationf volcanic glass, provide important catalysts, bringing about abi-tic synthesis of polycyclic aromatic hydrocarbons and long-chain
esters (Wiliams et al., 2010), while montmorillonite clays can catal-yse the condensation of activated mononucleotides towards muchlonger RNA oligomers (Ferris, 2002). Of considerable interest, too,is the potential of pumice to convert into halloysite clay nanotubes.These ultra-tiny hollow tubes occur naturally and abundantly dur-ing the weathering of pumice towards kaolinite clays (Aomine andWada, 1962), producing nanotubes typically 1–30 nm in diameter,with lengths from c. 500 nm to over 1.2 �m. Such nanotubes havenoted capacities to act as molecular sieves for the uptake of aqueous‘pollutants’ (Du et al., 2010).
The pumice hypothesis is now available for testing against thegeological record, and that is the aim of this contribution. Volcani-clastic rocks are abundant in Archaean cratons, and some of theseare now being investigated and evaluated as possible sites for earlylife (Walsh, 2004; Brasier et al., 2010; Westall et al., 2011). Indeed,pumice has been reported from a putative tidal beach setting inrocks as old as 3490 Ma (Buick and Dunlop, 1990). Even so, no bio-geochemical studies of Archaean pumice have yet been performed.Below, we focus upon pumice from the 3460 Ma Apex Basalt to testthe following postulates of the pumice hypothesis: 1. That highlyporous pumice was common on the early Earth; 2. That Archaeanpumice attracted concentrations of vital elements essential for life,such as C, H, O, N, P and S; 3. That Archaean pumice played host to
secondary minerals with the potential to catalyse the formation oforganic polymers.
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. Materials and methods
Our pumice samples come from the ∼3460 Ma Apex Basalt ofhe Warrawoona Group in Western Australia (Fig. 3a). This unit is ofreat value for studies of the origins of life because it combines evi-ence not only for komatiitic lavas, tuffs and porous volcanic rocksut it also contains some of the best preserved silica-rich hydro-hermal and fissure eruption systems known from the ArchaeanBrasier et al., 2002, 2005, 2011a; Van Kranendonk et al., 2002; Vanranendonk, 2006; Pinti et al., 2009; Glikson et al., 2010). These
iliceous rocks are also the source of the famous Apex chert ‘micro-ossils’ (Schopf, 1993). Although the presence of cyanobacteria, andven of microfossils in these rocks has been questioned (Brasiert al., 2002, 2005; Pinti et al., 2009; Glikson et al., 2010; Marshall
ig. 3. (a) Geological map of the Apex Basalt in the Chinaman Creek area, showing the stre the hydrothermal dyke systems (S1 to S4 and N1 to N5) and a field image (inset) shot al., 2011a). (b) A suggested reconstruction of the setting for the pumice-bearing pyrocl
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et al., 2011), it seems reasonable to suspect that the Apex chertswere laid down in a world provided with anaerobic prokaryotes(Tice and Lowe, 2004; Wacey et al., 2011).
The Warrawoona Group contains volcanics arranged withinmafic-to-felsic volcanic cycles. Komatiites and basalts erupted atthe start of each cycle, gradually evolving towards felsic volcaniceruptions towards the end of each cycle (e.g. Van Kranendonket al., 2002). Although komatiitic and basaltic volcanoes can pro-duce highly porous lavas and debris (scoria), their vesicles tend tobe relatively large and widely separated, and the residence time of
these clasts at the water surface was likely to have been limitedowing to the higher density of the mafic rock. Hence we focus ourattention upon more silicic volcanic products with notably highporosity and surface areas. These ancient examples were examined
ratiform chert with its pumice-rich pyroclastic breccia bed at the top. Also shownwing the collection locality for pumice-rich sample CC139 (adapted from Brasier
astic breccia bed.
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sing a variety of analytical techniques described below, and com-ared with recently erupted pumice from Kone (Ethiopia) andristan da Cunha (South Atlantic). Warrawoona Group volcaniclas-ic sediments studied by us have mainly come from the boundaryetween two minor cycles within the Apex Basalt close to Marblear in the Pilbara of Western Australia (Brasier et al., 2005, 2011a).pecifically, the studied samples come from a thin tuffaceous brec-ia bed (unit 5 of Brasier et al., 2005) within the lower portion of thepex Basalt, close to Chinaman Creek (Fig. 3a). The unit is best seen
n outcrop approximately 1.5 km south–southeast of the famousmicrofossil’ locality (Schopf, 1993).
Optical microscopy of polished thin sections of all samplesas performed using a Nikon Optiphot-pol petrological micro-
cope coupled with a QImaging QICAM CCD camera. Bubbleize-distribution analyses confirmed that pumice from the Apexasalt is indistinguishable from modern examples arising from botherial and submarine eruptions around Kone and Tristan da Cunhaespectively (Matthewman, 2008). Scanning electron microscopeSEM) imaging and preliminary geochemical analysis were carriedut on carbon-coated polished sections of Apex Basalt using a Jeol-SM840A SEM with energy dispersive X-ray (EDX) and backscatterlectron (BSE) detectors. High-resolution elemental maps of andentical gold-coated Apex Basalt sample were obtained using aanoSIMS 50 at the University of Western Australia. For NanoSIMSnalysis, the pumice clasts were first identified under the opticalicroscope in polished 30 �m thin sections and the fabrics wereapped using bright-field and reflected light. The reflected light
mages were subsequently used to locate the surface expressions ofhe pumice vesicles within the NanoSIMS. Discs of 10 mm diameterere then extracted from the thin sections, mounted on NanoSIMS
tubs, and coated with a thin (c. 5 nm) layer of gold to provideonductivity at high voltage. Details of qualitative elemental map-ing using NanoSIMS in multi-collector mode are given in Waceyt al. (2008) and Kilburn and Wacey (2011). Briefly, a focussed pri-ary Cs+ ion beam, with a beam current of 2–4 pA, was rastered
ver the sample surface, and the sputtered ions were extracted to double focusing mass spectrometer. Images were acquired overelds of view ranging from 20 �m × 20 �m to 80 �m × 80 �m inize, with sub-100 nm spatial resolution obtained for most areas.rior to each analysis, the sample area was pre-sputtered to removeurface contamination, implant Cs+ ions into the sample matrixnd attain an approximate steady state of secondary ion emissioncf. Gnaser, 2003). Ion maps of carbon, nitrogen, silicon, sulfur andhosphate (31P16O2
−) were then produced simultaneously fromhe same sputtered volumes of sample. 16O− and 56Fe32S− werelso measured in some cases to help distinguish between organicnd mineral phases.
It is important to state that only relative concentrations of ele-ents can be obtained using this NanoSIMS methodology. Withoutultiple standards, no inferences can be made from NanoSIMS
oncerning either the absolute concentration of elements or theercentage concentration of one element compared to another.his is because elements have variable ion yields depending onhe other atoms to which they are bonded. For example, carbonill produce a high ion yield in NanoSIMS analysis when bonded
o other carbon atoms within organic material. In contrast, it willroduce a very low ion yield when it occurs bonded to oxygen
n carbonates. Similarly, phosphorus has a notoriously poor ionield in the NanoSIMS, in contrast to sulfur, which has a very highon yield. NanoSIMS cannot detect the nitrogen ion in isolation,nstead the affinity of nitrogen ions to combine with carbon ionsn the mass spectrometer is used, so that nitrogen is detected as
he CN− ion. This is beneficial for detecting nitrogen in organic
aterial but means that nitrogen not associated with carbone.g. in naturally occurring ammonium-bearing clays) may not beetected.
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3. Results and discussion
3.1. Context and petrography
Felsic rock fragments, tuff, pumice and other vesicular pyroclas-tic rock debris have been widely reported from the Archaean of theeastern Pilbara (e.g. DiMarco and Lowe, 1989a,b,c). The presence offeatures consistent with stranded pumice rafts in the WarrawoonaGroup at North Pole, Western Australia has even been reportedby Buick and Dunlop (1990), based on coarse, pure, matrix-free pumice sand without grading of the kind associated withair-fallout. Lithologies of the Apex Basalt studied here comprisepumice and other lithic clasts within a pyroclastic breccia that wasseemingly erupted from a nearby shallow marine fissure (Brasieret al., 2005; unit 5 of Brasier et al., 2011a; ‘pyroclastic breccia’ inFig. 3a and b).
In terms of geological context, this pyroclastic breccia lies imme-diately above the stratiform Apex chert, a ∼10 m thick unit ofiron-rich and iron poor chert (banded iron formation) plus carbon-rich chert layers that accumulated on the seafloor above a thickpile of mafic lavas (Brasier et al., 2011a). The stratiform cherts aredilated by wedges of chert breccia with distinctly hydrothermaltextures and mineralogies, especially where this unit crosses syn-depositional faults (Brasier et al., 2002, 2011a). The pyroclasticbreccia bed is thickest (>3 m near locality CC135; see Brasier et al.,2011a) in the South Block, having clasts up to 1 m. This unit is miss-ing from the Central Block, presumably owing to contemporaneousfault movements and localized uplift. The pyroclastic breccia reap-pears in the North Block, where it is thins (to <1 m) over a distanceof several kilometres and becomes finer grained.
This unit contains sparse clasts derived from underlying litholo-gies, including stratiform chert and hydrothermal vein chert. Thebulk of the clasts (>60%), however, comprise pumice-like vesicu-lar volcanics that range in shape from rounded to angular (Fig. 4a).Petrography shows that this breccia was, for the most part a coarsesand to gravel deposit, being matrix-free and clast supported, andhaving the intervening void spaces filled with early diagenetic chert(Fig. 4a). In some samples, void spaces between the clasts are almostlacking, seemingly owing to early compaction on the seafloor. Clastsalso show little evidence for marked size sorting, nor is grading ofthe kind associated with air-fallout seen.
As is typical for greenstone belt rocks of such great age, the orig-inal mineralogy of these volcanic clasts has largely been replacedby cryptocrystalline silica and sericite. It must be stated at theoutset that a lack of pseudomorphs after phenocrysts in vesicu-lar clasts hinders firm chemical identification (i.e. felsic or mafic)in such heavily replaced rocks (DiMarco and Lowe, 1989c). Rela-tively small (<20 �m) vesicles like those typical of modern pumicemay also be difficult to recognise owing to later stages of infill-ing by silicate minerals. Thus, identification of the lithic clasts hasbeen undertaken on the basis of preserved fabrics, and this analy-sis indicates that a wide variety of igneous rock types are present.First, accretionary lapilli – layered grains that form by accumulationof ash particles in the eruption cloud – can be readily identified inthin section from their concentric laminations. Komatiite-type rockfragments and xenoliths of typical Archaean type can be recognisedby their distinctive micro-spinifex texture. Clasts with well-formedspherical vesicles (Fig. 4b, e and f), and those with pseudomorphsafter plagioclase laths, most likely originated from mafic magmasof low viscosity (cf. Klug and Cashman, 1996). Such ‘mafic’ vesicularclasts are unlikely to have stayed afloat for long, if at all, becauseof the relatively large and interconnected vesicles that facilitatewater-flow through the clast. In contrast, clasts with ovate to elon-
gated vesicles (Fig. 5a–c), and without plagioclase or mafic mineralphenocrysts comprise the majority of vesicular fabrics in this unit,and are here inferred to have originated from felsic magmas. In
M.D. Brasier et al. / Precambrian Research 224 (2013) 1– 10 5
Fig. 4. Pumice and scoria clasts from the 3460 Ma pyroclastic breccia unit within the Apex Basalt near Chinaman Creek, Western Australia, showing vesicle veneers andgeopetal fabrics. (a) Petrographic thin section showing numerous subangular to rounded pumice clasts set within a matrix of clear microcrystalline quartz, from sampleCC192. Note the dark zones of alteration and impregnation. (b) The glassy scoria matrix of this clast has been converted to cryptocrystalline quartz and aluminosilicate (greybrown) and contains numerous spherical vesicles. Only the larger vesicles have geopetal infills of aluminosilicate (black arrow), here followed by successive generations ofquartz cement, from cryptocrystalline rims to clearly crystalline void fills (white arrow), from sample CC169. (c) Aligned geopetal infills of aluminosilicate (black arrows) andquartz (white arrows) within numerous adjacent elliptical vesicles, from sample CC166. (d) Vesicles within scoria (of inferred mafic origin on the basis of the high vesiclesphericity), here lined with dark titanium oxide grains (left arrow), and later infilled with rims of clear silica (right arrow) followed by void-fills of silica (top arrow), froms esicleo eas ar(
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ample CC169. (e) Back scattered electron (BSE) image of a titanium-oxide rimmed vf clear silica (right arrow) and then by void-fills of silica (top arrow). Light grey ard) and (e) = 100 �m.
heory, clasts like these could have stayed afloat at the air–waternterface for periods of weeks to years (Fig. 3b).
The potential for measurement of permeability (and henceotential floatability) within these Archaean vesicular clasts is
from sample CC173. White titanium oxide grains (left arrow), are followed by rimse clay minerals of the matrix. Scale bars for (a) = 10 mm; for (b) and (c) = 1 mm; for
obscured by factors of preferential preservation. This is not sur-prising given the evidence obtained from modern pumice rafts. Forexample, glass foams found in modern pumice can be extremelyfragile, and these foams are provided with very thin walls. Hence
6 M.D. Brasier et al. / Precambrian Research 224 (2013) 1– 10
Fig. 5. Pumice clasts from the 3460 Ma Apex Basalt near Chinaman Creek, Western Australia, showing vesicle veneers and secondary infillings. (a) Pumice clast (centreof image) showing irregularly shaped vesicles, under plane polarized light, sample CC167. (b) Same view in cross polarized light, revealing vesicles completely infilledwith aluminosilicate (paler colours) or infilled with quartz (dark). (c) Scanning electron microscope (SEM) image from a polished surface, showing the once glassy matrix(black, now altered to aluminosilicate) separating irregularly shaped vesicles infilled with an outer layer of microcrystalline quartz (light grey) followed by an infilling ofaluminosilicate (dark grey to black), sample CC173. (d) SEM image of modern pumice for comparison, showing the glassy matrix (black) with numerous irregularly shapedvesicles (grey), from Tristan da Cunha. Note greyscale colouring is inverted for (c) and (d) for greater emphasis. (e) Photomicrograph in plane polarized light showing theedge of a pumice clast from sample CC183. Note that the organic material is confined to the pumice clast (lower part of image) and is not found within the remainder ofthe rock matrix (upper part of image). (f). Photomicrograph in plane polarized light of a cross-cutting silica vein (arrow) within an organic-rich pumice clast. Note that thisv tional( read
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ein is not lined with organic material so is unlikely to have introduced post-deposif) = 250 �m. (For interpretation of the references to colour in this figure legend, the
heir vesicular fabrics are usually altered or completely destroyedy burial compaction (Fiske, 1969; Branney and Sparks, 1990). Inhese Archaean examples, however, it seems that very early diage-etic silicification by means of hydrothermal fluids on the seaflooras able to prevent this form of destruction. Perhaps because of
his, the vesicular fabrics seem remarkably well preserved. The
oderate degree of post-depositional burial and metamorphism
as also been permissive for good preservation here. Althoughontact metamorphism close to granitic complexes in the East Pil-ara can be as high as amphibolite to granulite facies (Hickman,
organic contamination. Scale bar for (a), (b) and (d) = 100 �m; (c) = 200 �m, (e) ander is referred to the web version of this article.)
1983; Van Kranendonk et al., 2002, 2007), the Apex Basalt in theMarble Bar greenstone belt has only experienced very low strainconditions and low grade prehnite-pumpellyite to lower green-schist facies metamorphism during the early and mid-Archaean(Van Kranendonk et al., 2007). This means that ancient fabrics, syn-depositional minerals and chemical signals can be decoded with
more confidence than in many rocks of this age.
Vesicular clasts within the Apex Basalt are characterisedby voids that have become infilled with secondary minerals(Figs. 4b–e and 5a–c, e and f), now preserved as clay minerals
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nd silica following prehnite-pumpellyite facies metamorphism. Inlaces, these void-infills were eroded and exposed along the sur-ace of the pumice clast before burial took place, confirming anarly depositional context. The majority of vesicles within theserains host thin concentric linings of mineral couplets, includinghert (chalcedonic quartz)-aluminosilicate, pyrite-chert, titaniumxide-chert or combinations of these. Individual vesicles withinome clasts are commonly lined with micron-sized grains of tita-ium oxide (Fig. 4d and e). Similar coatings around volcaniclasts ofhe slightly younger Panorama Formation have been interpreted asnatase crystals (Westall et al., 2006, 2011). The final stage of voidlling typically comprises aluminosilicate or chert ‘amygdales’,ome of which could have been alteration products of smectite, hal-oysite or montmorillonite clay or of zeolite minerals, all of whichre noted organic catalysts.
In our examples, ‘fossil spirit-level’ (geopetal) features – thosehat indicate the ancient way-up direction of the vesicles – can belearly recognised (Fig. 4b and c), providing evidence for two orore stages of infilling by different mineral suites, although at this
tage it cannot be confirmed whether these stages of infilling tooklace during the pelagic, benthic or burial stages (in the sense ofrasier et al., 2011b; see Fig. 2, stages 2–5.). The final infilling ofesicles is here attributed to the burial stage, during further hydro-
hermal alteration of the deposit. Unfortunately, only the largeresicles can be studied in this way because vesicles smaller than. 20 �m have largely been modified by later alteration, makingverall porosity estimates of the original pumice difficult.
ig. 6. A thin section photomicrograph (top left) together with NanoSIMS ion maps of
3460 Ma Apex Basalt (sample CC139). Note strong correlation of carbon and nitrogennd anti-correlation of these elements with the main mineralising silica phase. Low leveistinct �m-sized phosphate grains and sub-�m-sized sulfide grains are indicated by the
ntensity are shown by the calibration bars, where brighter colours indicate higher intensnterpretation of the references to colour in this figure legend, the reader is referred to th
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3.2. Biogeochemistry
Pumice-like clasts within the pyroclastic breccia of the ApexBasalt can appear darkened, especially around the edges of theclast, or around vesicle margins (Fig. 5e) and also along wispyplanes (Fig. 5f). NanoSIMS elemental mapping indicates that bothcarbon and nitrogen are widespread here. These elements show aclose correlation with one another both around, and more rarelywithin, the pumice vesicles (Figs. 6 and 7). The highest ion yieldsof carbon and nitrogen tend to be found in those areas depletedin silicon (Fig. 6), indicating that enrichments are mostly relatedto the presence of organic material rather than to any mineralis-ing silica/silicate phases. Similarly, there is no correlation betweencarbon and oxygen (the latter not shown here as it shows the samedistribution as silicon) which rules out a contribution to the car-bon signal from carbonate minerals. We would add that neithercarbon nor nitrogen layers show preferential orientation along oneside of the pumice here, of the kind that might be expected withphototropism.
Even without absolute concentrations, the data show that∼3460 Ma pumice could be densely permeated by C- and N-richorganic material. That the organic material detected here is ofearly Archaean age, and not younger age, can be argued as fol-
lows: 1. Organic material tends to be restricted to a limited numberof pumice and scoria clasts (Fig. 5e), which is not the pattern ofdistribution expected from more recent organic contamination; 2.The carbon appears dark brown to black, unlike modern microbial
carbon, nitrogen, silicon, sulfur, and phosphate (31P16O2−), of pumice within the
within the matrix of the pumice around (and more rarely within) the vesicles,ls of sulfur and phosphate are found in association with the carbon and nitrogen.
very bright areas in the phosphate and sulfur images respectively. Variations in ionity. Scale bar is 10 �m for the ion images and 50 �m for the photomicrograph. (Fore web version of this article.)
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aterial which tends to be light brown or paler in colour; 3. Latereins and fractures in these same rock slices are not found linedith organic material (Fig. 5f), thereby discounting introduction of
arbon into the pumice clasts by later fluids; 4. This carbon cannotave come from the thin section adhesive at the base of the slideecause NanoSIMS is a surface analysis technique. Pre-sputteringith the NanoSIMS ion beam also ensures that the data obtained areot merely surface contamination. The latter is further confirmedy the very close correlation seen between the geochemical mapsnd the morphology of the pumice clasts as observed under theight microscope.
NanoSIMS reveals that low levels of sulfur are also found associ-ted with the carbon and nitrogen in the pumice. The co-occurrencef these three elements on the nano-scale potentially provides aiosignal, such as that in both modern and ancient stromatolitesWacey, 2010; Wacey et al., 2010a), as well as in some microfossils
Oehler et al., 2006; Wacey et al., 2011). In the absence of further
orphological or geochemical evidence for biology, this cannotet be stated for certain here. These data do, however, lead to theonclusion that organic material, containing elements commonly
ig. 7. NanoSIMS ion maps of carbon, nitrogen, silicon and sulfur from a single pumice vn the sulfur image) with organic material (bright areas in the carbon and nitrogen imagesicle appears to be connected to a second vesicle in the top left of the field of view (arrars, where brighter colours indicate higher intensity. Scale bar is 10 �m. (For interpretatiersion of this article.)
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associated with life, was being preferentially concentrated withinthe matrix of pumice on the early Earth.
Sub-�m-sized bright spots within the sulfur images(Figs. 6 and 7) represent iron-sulfide minerals (confirmed bymeasurement of 56Fe32S−), most likely pyrite. These tiny sulfidecrystals are commonly associated with carbon- and nitrogen-richorganic material (Fig. 7). This is a typical association for sulfideminerals forming via microbial sulfur processing (either sulfatereduction or sulfur disproportionation) on the early Earth (Waceyet al., 2010b, 2011; Kilburn and Wacey, 2011), and may indicatethat sulfur-based metabolic pathways (perhaps including anoxy-genic photosynthesis) and heterotrophic cycles were presentat this time. However, in the absence of isotopic evidence (thesulfides are too small to analyse isotopically), a non-biologicalorigin for these sulfides cannot yet be ruled out. It could certainlybe argued that pumice would present an ideal habitat for the
biological formation of sulfide. For example, pumice could trapand concentrate photochemically produced sulfate and sulfur (cf.Farquhar et al., 2000) that rained down from the atmosphere.It could also be argued that the large surface area of pumice
esicle (sample CC139). Note the association of sub-�m-sized sulfides (bright areases) within part of the vesicle as a geopetal infill (highlighted by dashed oval). Theowed in the silicon image). Variations in ion intensity are shown by the calibrationon of the references to colour in this figure legend, the reader is referred to the web
brian
biTr
ftbdTbrNptw
4
to
1
2
3
M.D. Brasier et al. / Precam
rought about by its numerous vesicles would also provide andeal substrate for the attachment of elemental sulfur particles.he latter could then be attractive for microbial disproportionationeactions.
Phosphorus is another key biological element of great interestor the early Archaean biosphere (e.g. Blake et al., 2010). Unfor-unately, this element is rather difficult to detect by NanoSIMSecause of its relatively weak secondary ion emission. It was hereetected instead in combination with oxygen as the PO2
− ion.his permits the inference of likely phosphate minerals but it thenecomes more difficult to assign any phosphorus to organic mate-ial, and to be certain of the timing of its introduction to the pumice.evertheless, several phosphate hotspots can be seen within theumice clasts (Fig. 6, lower right box showing 31P16O2
−), indicatinghat a further essential biological element was likely freely availableithin pumice at this early date.
. Conclusion
This study of ∼3460 Ma pumice clasts allows us now to addresshe three geologically testable aspects of the pumice hypothesisutlined at the start of this paper.
. We confirm that clasts with the texture and mineralogy ofpumice can be widespread in Archaean rock successions.Although volumetrically small when compared with the vastthicknesses of pillow lava and komatiite (see Fig. 3a and b),pumice-dominated pyroclastic deposits formed beds severalmetres thick and many kilometres wide. These deposits lackgraded bedding of the kind associated with air-fall breccias (cf.Buick and Dunlop, 1990). Instead, they were often matrix free,moderately size sorted, with angular to sub-rounded clasts ofpumice plus other locally derived rocks. These fabrics are consis-tent with their formation from ejecta and rafts that accumulatednear to felsic volcanic vents along an ancient wave-washed fore-shore (Fig. 3b).
. We find that Archaean pumice played host to concentrations ofvital elements essential for life, such as C, O, N, P and S. In supportof this, we have found an association between autochthonouscarbon and nitrogen throughout the body of some Apex pumiceclasts, concentrated in irregularly shaped patches and wisps.These patches likely relate to penetration of organic matter alongmicropores. Because they lie beneath early mineral coatingsand void infillings, they are likely to have formed early, prob-ably during stages 2–3 (Fig. 2). This leads us to the conclusion,important for the origins of life, that the highly absorbent prop-erties of pumice and scoria will have allowed them to hostcarbon–nitrogen polymers not only in the biotic world but alsoin the prebiotic world. Tiny spots of pyrite and phosphate arealso scattered throughout much of the matrix of Apex pumiceand scoria clasts. We suspect, but cannot prove, that these crys-tals originated from early microbial processes. If so, this has theclear implication that the Apex pumice was colonised by earlyprokaryotic microbes, most likely during stages 2–4, meaningthat biological signals will have overprinted and clouded anypotentially prebiotic chemical signals. It is not possible, there-fore, to be sure that the Apex pumice can provide us with a ‘clean’prebiotic analogue.
. We conclude that Archaean pumice also played host to sec-ondary minerals with the potential to catalyse the formationof organic polymers. In support of this view, we report thin
coatings of titanium oxide around the vesicles and marginsof Apex pumice and scoria. Such titanium oxide coatings arelikely to have formed during the later stages of rafting to earlyburial. Intriguingly, titanium oxide is known to be a versatile
Research 224 (2013) 1– 10 9
photocatalyst, able to oxidise organic matter, to synthesizeammonia from nitrogen in the presence of UV light, and to breakwater into its hydrogen and oxygen components in the presenceof UV light (see above and Brasier et al., 2011b). Vesicles in theApex pumice and scoria clasts are often infilled by silica or alumi-nosilicate, presumably during exposure to hydrothermal fluidsafter burial. Some of these infillings may be the altered productsof zeolites, halloysites, smectites and montmorillonite clays. Var-ious zeolites can greatly boost the synthesis of organic polymers,the cracking of hydrocarbons, or the liberation of hydrogen gas(see above and Brasier et al., 2011b).
Before the evolution of the biosphere, the extremely highporosity, interconnected vesicles and abundance of exposed min-erals including microporous volcanic glass, clays and zeolites, willarguably have given to pumice an unusually high capacity foradsorbing biologically important elements and compounds, and formaximising the effective surface area needed for catalytic reactions.In this way, pumice could have behaved as a natural laboratory thatassisted with the origins of life itself.
Acknowledgements
We thank Dave Waters, Graham Cairns-Smith, HaroldMorowitz, Norman Sleep, Ian Parsons and Lynn Margulis formuch appreciated encouragement and advice; Nicola McLoughlin,Martin van Kranendonk, Cris Stoakes, Arthur Hickman, Leila Bat-tison, Kate Hendry, Owen Green and John Lindsay for invaluableassistance with field work in Australia and Canada; David Pyle andTamsin Mather for essential guidance on pumice and volcaniclasticrocks; Scott Bryan for providing examples of modern pumice;Owen Green, John W. Still and Norman Charnley for help withthe SEM facilities. We acknowledge the facilities, scientific andtechnical assistance of the Australian Microscopy & MicroanalysisResearch Facility at the Centre for Microscopy Characterisationand Analysis, The University of Western Australia, a facility fundedby the University, State and Commonwealth Governments.
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