Pumice from the ∼3460Ma Apex Basalt, Western Australia: a Natural

Precambrian Research 224 (2013) 1–10

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Precambrian Research

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∼

Pumice from the 3460 Ma Apex Basalt, Western Australia: A natural laboratory

for the early biosphere

a a b c d,e,∗

Martin D. Brasier , Richard Matthewman , Sean McMahon , Matt R. Kilburn , David Wacey

Department of Earth Sciences, University of Oxford, South Parks Road, Oxford OX1 3AN, UK

School of Geosciences, University of Aberdeen, Meston Building, Kings College, Aberdeen AB24 3UE, UK

Centre for Microscopy Characterisation and Analysis, The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia

Department of Earth Sciences & Centre for Geobiology, University of Bergen, Allegaten 41, Bergen N-5007, Norway

Centre for Core to Crust Fluid Systems, Centre for Microscopy Characterisation and Analysis & School of Earth and Environment, The University of Western Australia, 35 Stirling

Highway, Crawley, WA 6009, Australia

a r t i c l e i n f o a b s t r a c t

Article history: It has recently been hypothesised that pumice, a low-density vesicular volcanic rock, could have acted as

Received 10 April 2012

a natural ﬂoating laboratory for the accumulation and concentration of chemical reactants needed for the

Received in revised form 18 August 2012

origin of life. To test the plausibility of his hypothesis, we here turn to the earliest rock record for evidence

Accepted 4 September 2012

of pumice deposits and their associated mineralogy and biogeochemistry. We report abundant clasts of

Available online 19 September 2012

pumice from within a volcaniclastic breccia bed immediately above the ∼3460 Ma ‘Apex chert’ unit of

the Apex Basalt, Pilbara region, Western Australia. Textural and geochemical analyses reveal that the

Keywords:

body of these pumice clasts was deeply permeated by intimate associations of C, O, N, P and S. Pumice

Pumice

and scoria vesicles were also lined with carbon or with catalysts such as titanium oxide or potential

Origin of life

biominerals such as iron sulﬁde, while many were inﬁlled with aluminosilicate minerals. The latter may

Apex Basalt

Pilbara be the metamorphosed remains of potentially catalytic clay and zeolite minerals. It is not yet possible

to distinguish between chemical signals left by prokaryote biology from those left by prebiology. That

being so, then early prokaryotes may well have colonised and modiﬁed these Apex pumice clasts prior

to burial. Nevertheless, our data provide the ﬁrst geological evidence that the catalysts and molecules

needed for the earliest stages of life may be found within pumice rafts from the earliest oceans on Earth.

1. Introduction Pumice has the highest surface area to volume ratio of any rock

type. Today this allows for high levels of biological colonisation

Was there an unusual kind of geological substrate, still pre- (van Houten et al., 1995; Di Lorenzo et al., 2005) while, in a pre-

served in the early Archaean geological record, that could have biotic world, pumice would have a great capacity for adsorbing

concentrated, selected and catalysed the diversity of chemical reac- biologically relevant elements and compounds, and maximising the

tants needed for life? Was this environment not only widespread surface area available for chemical reactions. Pumice is the only

and abundant but also capable of providing reaction chambers rock type known to ﬂoat on the surface of the ocean for sustained

with maximum surface area over sufﬁciently long periods of time? periods, owing to its high pore-volume and gas-ﬁlled vesicles. It

In a recent article, Brasier et al. (2011b) have suggested that the can then come to sit within the intertidal zone of beaches for many

answer to these questions could be ‘yes’, within the vastly abun- thousands of years. This unique position at the air water interface

dant mineral vesicles of pumice and related rocks (Fig. 1). The is potentially ideal for the mixing of atmospheric and trapped vol-

special properties of pumice have been discussed in detail else- canic gases with water. Pumice would also be able to adsorb organic

where (Brasier et al., 2011b) but a summary is given here to enable complexes of the kind thought to have existed locally as oily slicks

the reader to understand the context for the ‘pumice hypothesis’ at the surface of the Archaean ocean (Nilson, 2002; Deamer et al.,

with regard to the origin of life. 2002; Bada, 2004), creating lipid lined vesicles, precursors to the

synthesis of peptides, proteins, ATP, and nucleic acids. Thermal

conditions 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.

Corresponding author at: Department of Earth Sciences & Centre for Geobiology,

Pumice also experiences an unusually wide variety of conditions

University of Bergen, Allegaten 41, Bergen N-5007, Norway. Tel.: +47 55 58 35 27;

from eruption to burial via ﬂoating (rafting) and benthic stages

fax: +47 55 58 36 60.

E-mail address: [email protected] (D. Wacey). (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 with

many small, interconnected compartments having a massive and reactive surface

area. 2. During and after eruption, the pumice exterior was exposed to UV light,

electricity and dehydration. 3. The interior compartments of pumice provided a

homeostatic 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 and

metallic compounds; it played host to catalysts such as titanium oxide, iron sulﬁde,

zeolites, and clays including halloysite nanotubes. 6. Primordial organic compounds

from extraterrestrial bombardment and from Fischer–Tropsch-type synthesis, accu-

mulated at the air–water interfaces of oceans and shorelines, alongside ﬂoating

pumice. Lipids and other polymers began to coat pumice vesicles, to form hydropho-

bic selectively permeable membranes. 7. Rhythmic oscillations in UV light (from

diurnal cycles), as well as in pH and salinity (from tidal cycles, wetting and dry-

ing), created gradients between adjacent compartments. Osmotic gradients across

membranes encouraged various kinds of pump to develop, including movements of

protons (H ions), phosphate and sodium ions.

Fig. 2. Summary diagram showing the ﬁve main stages through which pumice can

pass (modiﬁed from Brasier et al., 2011b). These stages are as follows: 1. Eruptive;

triboelectric charges, Leidenfrost boiling (see James et al., 2008) and

2. Early ﬂoating; 3. Late ﬂoating; 4. Sinking and beaching; 5. Burial. Note how the

UV light, raising the possibility of Miller–Urey syntheses (e.g. Miller, pumice (white circles) may come to ﬂoat alongside oily lipids (black ellipses). Below

1992); hydrocarbon production via electric discharges through are shown cartoons of a single pumice vesicle with suggested patterns of alteration

through these ﬁve stages. Of these, organic (carbonaceous) compounds, phosphates,

mixtures of ‘primitive’ gases (Navarro-González and Basiuk, 1996;

metals, zeolites, clays and evaporites all have some signiﬁcance for the origins of life.

Segura and Navarro-Gonzalez, 2001); and production of bio-

available phosphorus and ﬁxed nitrogen by lightning (Yamagata

et al., 1991; Schwartz, 2006; Mather and Harrison, 2006). During

its life cycle pumice would also likely experience energy-releasing

esters (Wiliams et al., 2010), while montmorillonite clays can catal-

cycles of heat, light and tides at diurnal to seasonal scales, alka-

yse the condensation of activated mononucleotides towards much

line waters, freeze–thaw conditions of the kind favoured by some

longer RNA oligomers (Ferris, 2002). Of considerable interest, too,

for nucleic acid syntheses (cf. Miller, 1992; Menor-Salvan et al.,

is the potential of pumice to convert into halloysite clay nanotubes.

2007), episodic changes in salinity (hence osmotic gradients), plus

These ultra-tiny hollow tubes occur naturally and abundantly dur-

hydration and dehydration of organic compounds held within its

ing the weathering of pumice towards kaolinite clays (Aomine and

vesicles. These types of cycle would be particularly prevalent in

Wada, 1962), producing nanotubes typically 1–30 nm in diameter,

pumice beached along marine or lake shorelines.

with lengths from c. 500 nm to over 1.2 ␮m. Such nanotubes have

Not least among the virtues of pumice and scoria for early life

noted capacities to act as molecular sieves for the uptake of aqueous

research is their capacity to host secondary minerals with known

‘pollutants’ (Du et al., 2010).

catalytic potential. Interaction with hydrothermal ﬂuids readily

The pumice hypothesis is now available for testing against the

produces microporous aluminosilicates called zeolites. These can

geological record, and that is the aim of this contribution. Volcani-

grow alongside smectite clays when siliceous pumice reacts with

clastic rocks are abundant in Archaean cratons, and some of these

alkaline waters (e.g. Tomita et al., 1993), forming amygdale inﬁll-

are now being investigated and evaluated as possible sites for early

ings. Such zeolite minerals can greatly boost the catalytic activity of

life (Walsh, 2004; Brasier et al., 2010; Westall et al., 2011). Indeed,

pumice during industrial processes today (Brito et al., 2004). Hence

pumice has been reported from a putative tidal beach setting in

zeolites (or ‘silicalite zeolites’ with periodic Ti atoms in place of Si;

rocks as old as 3490 Ma (Buick and Dunlop, 1990). Even so, no bio-

e.g. Yamashita et al., 2007) are used for the synthesis of organic

geochemical studies of Archaean pumice have yet been performed.

polymers (e.g. de Vos and Jacobs, 2001) as well as for the crack-

Below, we focus upon pumice from the 3460 Ma Apex Basalt to test

ing of hydrocarbons and the commercial liberation of hydrogen

the following postulates of the pumice hypothesis: 1. That highly

gas (van Bekkam et al., 2001). Of equal interest is the potential of

porous pumice was common on the early Earth; 2. That Archaean

pumice to generate clay minerals that could have acted as potential

pumice attracted concentrations of vital elements essential for life,

templates for replication within early protocells (e.g. Cairns-Smith,

such as C, H, O, N, P and S; 3. That Archaean pumice played host to

2005). Of these, smectite clays formed by hydrothermal alteration

secondary minerals with the potential to catalyse the formation of

of volcanic glass, provide important catalysts, bringing about abi-

organic polymers.

otic synthesis of polycyclic aromatic hydrocarbons and long-chain

M.D. Brasier et al. / Precambrian Research 224 (2013) 1–10 3

2. Materials and methods et al., 2011), it seems reasonable to suspect that the Apex cherts

were laid down in a world provided with anaerobic prokaryotes

Our pumice samples come from the ∼3460 Ma Apex Basalt of (Tice and Lowe, 2004; Wacey et al., 2011).

the Warrawoona Group in Western Australia (Fig. 3a). This unit is of The Warrawoona Group contains volcanics arranged within

great value for studies of the origins of life because it combines evi- maﬁc-to-felsic volcanic cycles. Komatiites and basalts erupted at

dence not only for komatiitic lavas, tuffs and porous volcanic rocks the start of each cycle, gradually evolving towards felsic volcanic

but it also contains some of the best preserved silica-rich hydro- eruptions towards the end of each cycle (e.g. Van Kranendonk

thermal and ﬁssure eruption systems known from the Archaean et al., 2002). Although komatiitic and basaltic volcanoes can pro-

(Brasier et al., 2002, 2005, 2011a; Van Kranendonk et al., 2002; Van duce highly porous lavas and debris (scoria), their vesicles tend to

Kranendonk, 2006; Pinti et al., 2009; Glikson et al., 2010). These be relatively large and widely separated, and the residence time of

siliceous rocks are also the source of the famous Apex chert ‘micro- these clasts at the water surface was likely to have been limited

fossils’ (Schopf, 1993). Although the presence of cyanobacteria, and owing to the higher density of the maﬁc rock. Hence we focus our

even of microfossils in these rocks has been questioned (Brasier attention upon more silicic volcanic products with notably high

et al., 2002, 2005; Pinti et al., 2009; Glikson et al., 2010; Marshall porosity and surface areas. These ancient examples were examined

Fig. 3. (a) Geological map of the Apex Basalt in the Chinaman Creek area, showing the stratiform chert with its pumice-rich pyroclastic breccia bed at the top. Also shown

are the hydrothermal dyke systems (S1 to S4 and N1 to N5) and a ﬁeld image (inset) showing the collection locality for pumice-rich sample CC139 (adapted from Brasier

et al., 2011a). (b) A suggested reconstruction of the setting for the pumice-bearing pyroclastic breccia bed.

4 M.D. Brasier et al. / Precambrian Research 224 (2013) 1–10

using a variety of analytical techniques described below, and com- 3. Results and discussion

pared with recently erupted pumice from Kone (Ethiopia) and

Tristan da Cunha (South Atlantic). Warrawoona Group volcaniclas- 3.1. Context and petrography

tic sediments studied by us have mainly come from the boundary

between two minor cycles within the Apex Basalt close to Marble Felsic rock fragments, tuff, pumice and other vesicular pyroclas-

Bar in the Pilbara of Western Australia (Brasier et al., 2005, 2011a). tic rock debris have been widely reported from the Archaean of the

Speciﬁcally, the studied samples come from a thin tuffaceous brec- eastern Pilbara (e.g. DiMarco and Lowe, 1989a,b,c). The presence of

cia bed (unit 5 of Brasier et al., 2005) within the lower portion of the features consistent with stranded pumice rafts in the Warrawoona

Apex Basalt, close to Chinaman Creek (Fig. 3a). The unit is best seen Group at North Pole, Western Australia has even been reported

in outcrop approximately 1.5 km south–southeast of the famous by Buick and Dunlop (1990), based on coarse, pure, matrix-

‘microfossil’ locality (Schopf, 1993). free pumice sand without grading of the kind associated with

Optical microscopy of polished thin sections of all samples air-fallout. Lithologies of the Apex Basalt studied here comprise

was performed using a Nikon Optiphot-pol petrological micro- pumice and other lithic clasts within a pyroclastic breccia that was

scope coupled with a QImaging QICAM CCD camera. Bubble seemingly erupted from a nearby shallow marine ﬁssure (Brasier

size-distribution analyses conﬁrmed that pumice from the Apex et al., 2005; unit 5 of Brasier et al., 2011a; ‘pyroclastic breccia’ in

Basalt is indistinguishable from modern examples arising from both Fig. 3a and b).

aerial and submarine eruptions around Kone and Tristan da Cunha In terms of geological context, this pyroclastic breccia lies imme-

∼

respectively (Matthewman, 2008). Scanning electron microscope diately above the stratiform Apex chert, a 10 m thick unit of

(SEM) imaging and preliminary geochemical analysis were carried iron-rich and iron poor chert (banded iron formation) plus carbon-

out on carbon-coated polished sections of Apex Basalt using a Jeol- rich chert layers that accumulated on the seaﬂoor above a thick

JSM840A SEM with energy dispersive X-ray (EDX) and backscatter pile of maﬁc lavas (Brasier et al., 2011a). The stratiform cherts are

electron (BSE) detectors. High-resolution elemental maps of an dilated by wedges of chert breccia with distinctly hydrothermal

identical gold-coated Apex Basalt sample were obtained using a textures and mineralogies, especially where this unit crosses syn-

NanoSIMS 50 at the University of Western Australia. For NanoSIMS depositional faults (Brasier et al., 2002, 2011a). The pyroclastic

analysis, the pumice clasts were ﬁrst identiﬁed under the optical breccia bed is thickest (>3 m near locality CC135; see Brasier et al.,

microscope in polished 30 ␮m thin sections and the fabrics were 2011a) in the South Block, having clasts up to 1 m. This unit is miss-

mapped using bright-field and reflected light. The reflected light ing from the Central Block, presumably owing to contemporaneous

images were subsequently used to locate the surface expressions of fault movements and localized uplift. The pyroclastic breccia reap-

the pumice vesicles within the NanoSIMS. Discs of 10 mm diameter pears in the North Block, where it is thins (to <1 m) over a distance

were then extracted from the thin sections, mounted on NanoSIMS of several kilometres and becomes ﬁner grained.

stubs, and coated with a thin (c. 5 nm) layer of gold to provide This unit contains sparse clasts derived from underlying litholo-

conductivity at high voltage. Details of qualitative elemental map- gies, including stratiform chert and hydrothermal vein chert. The

ping using NanoSIMS in multi-collector mode are given in Wacey bulk of the clasts (>60%), however, comprise pumice-like vesicu-

et al. (2008) and Kilburn and Wacey (2011). Brieﬂy, a focussed pri- lar volcanics that range in shape from rounded to angular (Fig. 4a).

mary Cs ion beam, with a beam current of 2–4 pA, was rastered Petrography shows that this breccia was, for the most part a coarse

over the sample surface, and the sputtered ions were extracted to sand to gravel deposit, being matrix-free and clast supported, and

a double focusing mass spectrometer. Images were acquired over having the intervening void spaces ﬁlled with early diagenetic chert

ﬁelds of view ranging from 20 ␮m × 20 ␮m to 80 ␮m × 80 ␮m in (Fig. 4a). In some samples, void spaces between the clasts are almost

size, with sub-100 nm spatial resolution obtained for most areas. lacking, seemingly owing to early compaction on the seaﬂoor. Clasts

Prior to each analysis, the sample area was pre-sputtered to remove also show little evidence for marked size sorting, nor is grading of

surface contamination, implant Cs ions into the sample matrix the kind associated with air-fallout seen.

and attain an approximate steady state of secondary ion emission As is typical for greenstone belt rocks of such great age, the orig-

(cf. Gnaser, 2003). Ion maps of carbon, nitrogen, silicon, sulfur and inal mineralogy of these volcanic clasts has largely been replaced

31 16 −

phosphate ( P O2 ) were then produced simultaneously from by cryptocrystalline silica and sericite. It must be stated at the

16 − 56 32 −

the same sputtered volumes of sample. O and Fe S were outset that a lack of pseudomorphs after phenocrysts in vesicu-

also measured in some cases to help distinguish between organic lar clasts hinders firm chemical identification (i.e. felsic or mafic)

and mineral phases. in such heavily replaced rocks (DiMarco and Lowe, 1989c). Rela-

␮

It is important to state that only relative concentrations of ele- tively small (<20 m) vesicles like those typical of modern pumice

ments can be obtained using this NanoSIMS methodology. Without may also be difﬁcult to recognise owing to later stages of inﬁll-

multiple standards, no inferences can be made from NanoSIMS ing by silicate minerals. Thus, identiﬁcation of the lithic clasts has

concerning either the absolute concentration of elements or the been undertaken on the basis of preserved fabrics, and this analy-

percentage concentration of one element compared to another. sis indicates that a wide variety of igneous rock types are present.

This is because elements have variable ion yields depending on First, accretionary lapilli – layered grains that form by accumulation

the other atoms to which they are bonded. For example, carbon of ash particles in the eruption cloud – can be readily identiﬁed in

will produce a high ion yield in NanoSIMS analysis when bonded thin section from their concentric laminations. Komatiite-type rock

to other carbon atoms within organic material. In contrast, it will fragments and xenoliths of typical Archaean type can be recognised

produce a very low ion yield when it occurs bonded to oxygen by their distinctive micro-spinifex texture. Clasts with well-formed

in carbonates. Similarly, phosphorus has a notoriously poor ion spherical vesicles (Fig. 4b, e and f), and those with pseudomorphs

yield in the NanoSIMS, in contrast to sulfur, which has a very high after plagioclase laths, most likely originated from maﬁc magmas

ion yield. NanoSIMS cannot detect the nitrogen ion in isolation, of low viscosity (cf. Klug and Cashman, 1996). Such ‘maﬁc’ vesicular

instead the afﬁnity of nitrogen ions to combine with carbon ions clasts are unlikely to have stayed aﬂoat for long, if at all, because

in the mass spectrometer is used, so that nitrogen is detected as of the relatively large and interconnected vesicles that facilitate

−

the CN ion. This is beneﬁcial for detecting nitrogen in organic water-ﬂow through the clast. In contrast, clasts with ovate to elon-

material but means that nitrogen not associated with carbon gated vesicles (Fig. 5a–c), and without plagioclase or maﬁc mineral

(e.g. in naturally occurring ammonium-bearing clays) may not be phenocrysts comprise the majority of vesicular fabrics in this unit,

detected. 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 and

geopetal fabrics. (a) Petrographic thin section showing numerous subangular to rounded pumice clasts set within a matrix of clear microcrystalline quartz, from sample

CC192. Note the dark zones of alteration and impregnation. (b) The glassy scoria matrix of this clast has been converted to cryptocrystalline quartz and aluminosilicate (grey

brown) and contains numerous spherical vesicles. Only the larger vesicles have geopetal inﬁlls of aluminosilicate (black arrow), here followed by successive generations of

quartz cement, from cryptocrystalline rims to clearly crystalline void ﬁlls (white arrow), from sample CC169. (c) Aligned geopetal inﬁlls of aluminosilicate (black arrows) and

quartz (white arrows) within numerous adjacent elliptical vesicles, from sample CC166. (d) Vesicles within scoria (of inferred maﬁc origin on the basis of the high vesicle

sphericity), 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), from

sample CC169. (e) Back scattered electron (BSE) image of a titanium-oxide rimmed vesicle from sample CC173. White titanium oxide grains (left arrow), are followed by rims

of clear silica (right arrow) and then by void-ﬁlls of silica (top arrow). Light grey areas are clay minerals of the matrix. Scale bars for (a) = 10 mm; for (b) and (c) = 1 mm; for

(d) and (e) = 100 ␮m.

theory, clasts like these could have stayed aﬂoat at the air–water obscured by factors of preferential preservation. This is not sur-

interface for periods of weeks to years (Fig. 3b). prising given the evidence obtained from modern pumice rafts. For

The potential for measurement of permeability (and hence example, glass foams found in modern pumice can be extremely

potential ﬂoatability) within these Archaean vesicular clasts is fragile, 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 (centre

of image) showing irregularly shaped vesicles, under plane polarized light, sample CC167. (b) Same view in cross polarized light, revealing vesicles completely inﬁlled

with 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 of

aluminosilicate (dark grey to black), sample CC173. (d) SEM image of modern pumice for comparison, showing the glassy matrix (black) with numerous irregularly shaped

vesicles (grey), from Tristan da Cunha. Note greyscale colouring is inverted for (c) and (d) for greater emphasis. (e) Photomicrograph in plane polarized light showing the

edge 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 of

the 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 this

␮

vein is not lined with organic material so is unlikely to have introduced post-depositional organic contamination. Scale bar for (a), (b) and (d) = 100 m; (c) = 200 ␮m, (e) and

␮

(f) = 250 m. (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of this article.)

their vesicular fabrics are usually altered or completely destroyed 1983; Van Kranendonk et al., 2002, 2007), the Apex Basalt in the

by burial compaction (Fiske, 1969; Branney and Sparks, 1990). In Marble Bar greenstone belt has only experienced very low strain

these Archaean examples, however, it seems that very early diage- conditions and low grade prehnite-pumpellyite to lower green-

netic silicification by means of hydrothermal fluids on the seafloor schist facies metamorphism during the early and mid-Archaean

was able to prevent this form of destruction. Perhaps because of (Van Kranendonk et al., 2007). This means that ancient fabrics, syn-

this, the vesicular fabrics seem remarkably well preserved. The depositional minerals and chemical signals can be decoded with

moderate degree of post-depositional burial and metamorphism more conﬁdence than in many rocks of this age.

has also been permissive for good preservation here. Although Vesicular clasts within the Apex Basalt are characterised

contact metamorphism close to granitic complexes in the East Pil- by voids that have become inﬁlled with secondary minerals

bara can be as high as amphibolite to granulite facies (Hickman, (Figs. 4b–e and 5a–c, e and f), now preserved as clay minerals

M.D. Brasier et al. / Precambrian Research 224 (2013) 1–10 7

and silica following prehnite-pumpellyite facies metamorphism. In 3.2. Biogeochemistry

places, these void-inﬁlls were eroded and exposed along the sur-

face of the pumice clast before burial took place, conﬁrming an Pumice-like clasts within the pyroclastic breccia of the Apex

early depositional context. The majority of vesicles within these Basalt can appear darkened, especially around the edges of the

grains host thin concentric linings of mineral couplets, including clast, or around vesicle margins (Fig. 5e) and also along wispy

chert (chalcedonic quartz)-aluminosilicate, pyrite-chert, titanium planes (Fig. 5f). NanoSIMS elemental mapping indicates that both

oxide-chert or combinations of these. Individual vesicles within carbon and nitrogen are widespread here. These elements show a

some clasts are commonly lined with micron-sized grains of tita- close correlation with one another both around, and more rarely

nium oxide (Fig. 4d and e). Similar coatings around volcaniclasts of within, the pumice vesicles (Figs. 6 and 7). The highest ion yields

the slightly younger Panorama Formation have been interpreted as of carbon and nitrogen tend to be found in those areas depleted

anatase crystals (Westall et al., 2006, 2011). The ﬁnal stage of void in silicon (Fig. 6), indicating that enrichments are mostly related

ﬁlling typically comprises aluminosilicate or chert ‘amygdales’, to the presence of organic material rather than to any mineralis-

some of which could have been alteration products of smectite, hal- ing silica/silicate phases. Similarly, there is no correlation between

loysite or montmorillonite clay or of zeolite minerals, all of which carbon and oxygen (the latter not shown here as it shows the same

are noted organic catalysts. distribution as silicon) which rules out a contribution to the car-

In our examples, ‘fossil spirit-level’ (geopetal) features – those bon signal from carbonate minerals. We would add that neither

that indicate the ancient way-up direction of the vesicles – can be carbon nor nitrogen layers show preferential orientation along one

clearly recognised (Fig. 4b and c), providing evidence for two or side of the pumice here, of the kind that might be expected with

more stages of inﬁlling by different mineral suites, although at this phototropism.

stage it cannot be conﬁrmed whether these stages of inﬁlling took Even without absolute concentrations, the data show that

∼

place during the pelagic, benthic or burial stages (in the sense of 3460 Ma pumice could be densely permeated by C- and N-rich

Brasier et al., 2011b; see Fig. 2, stages 2–5.). The ﬁnal inﬁlling of organic material. That the organic material detected here is of

vesicles is here attributed to the burial stage, during further hydro- early Archaean age, and not younger age, can be argued as fol-

thermal alteration of the deposit. Unfortunately, only the larger lows: 1. Organic material tends to be restricted to a limited number

vesicles can be studied in this way because vesicles smaller than of pumice and scoria clasts (Fig. 5e), which is not the pattern of

␮

c. 20 m have largely been modiﬁed by later alteration, making distribution expected from more recent organic contamination; 2.

overall porosity estimates of the original pumice difﬁcult. The carbon appears dark brown to black, unlike modern microbial

31 16 −

Fig. 6. A thin section photomicrograph (top left) together with NanoSIMS ion maps of carbon, nitrogen, silicon, sulfur, and phosphate ( P O2 ), of pumice within the

∼3460 Ma Apex Basalt (sample CC139). Note strong correlation of carbon and nitrogen within the matrix of the pumice around (and more rarely within) the vesicles,

and anti-correlation of these elements with the main mineralising silica phase. Low levels of sulfur and phosphate are found in association with the carbon and nitrogen.

␮

Distinct m-sized phosphate grains and sub-␮m-sized sulﬁde grains are indicated by the very bright areas in the phosphate and sulfur images respectively. Variations in ion

intensity are shown by the calibration bars, where brighter colours indicate higher intensity. Scale bar is 10 ␮m for the ion images and 50 ␮m for the photomicrograph. (For

interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of this article.)

8 M.D. Brasier et al. / Precambrian Research 224 (2013) 1–10

material which tends to be light brown or paler in colour; 3. Later associated with life, was being preferentially concentrated within

veins and fractures in these same rock slices are not found lined the matrix of pumice on the early Earth.

␮

with organic material (Fig. 5f), thereby discounting introduction of Sub- m-sized bright spots within the sulfur images

carbon into the pumice clasts by later fluids; 4. This carbon cannot (Figs. 6 and 7) represent iron-sulfide minerals (confirmed by

56 32 −

have come from the thin section adhesive at the base of the slide measurement of Fe S ), most likely pyrite. These tiny sulﬁde

because NanoSIMS is a surface analysis technique. Pre-sputtering crystals are commonly associated with carbon- and nitrogen-rich

with the NanoSIMS ion beam also ensures that the data obtained are organic material (Fig. 7). This is a typical association for sulﬁde

not merely surface contamination. The latter is further conﬁrmed minerals forming via microbial sulfur processing (either sulfate

by the very close correlation seen between the geochemical maps reduction or sulfur disproportionation) on the early Earth (Wacey

and the morphology of the pumice clasts as observed under the et al., 2010b, 2011; Kilburn and Wacey, 2011), and may indicate

light microscope. that sulfur-based metabolic pathways (perhaps including anoxy-

NanoSIMS reveals that low levels of sulfur are also found associ- genic photosynthesis) and heterotrophic cycles were present

ated with the carbon and nitrogen in the pumice. The co-occurrence at this time. However, in the absence of isotopic evidence (the

of these three elements on the nano-scale potentially provides a sulﬁdes are too small to analyse isotopically), a non-biological

biosignal, such as that in both modern and ancient stromatolites origin for these sulﬁdes cannot yet be ruled out. It could certainly

(Wacey, 2010; Wacey et al., 2010a), as well as in some microfossils be argued that pumice would present an ideal habitat for the

(Oehler et al., 2006; Wacey et al., 2011). In the absence of further biological formation of sulﬁde. For example, pumice could trap

morphological or geochemical evidence for biology, this cannot and concentrate photochemically produced sulfate and sulfur (cf.

yet be stated for certain here. These data do, however, lead to the Farquhar et al., 2000) that rained down from the atmosphere.

conclusion that organic material, containing elements commonly It could also be argued that the large surface area of pumice

Fig. 7. NanoSIMS ion maps of carbon, nitrogen, silicon and sulfur from a single pumice vesicle (sample CC139). Note the association of sub-␮m-sized sulﬁdes (bright areas

in the sulfur image) with organic material (bright areas in the carbon and nitrogen images) within part of the vesicle as a geopetal inﬁll (highlighted by dashed oval). The

vesicle appears to be connected to a second vesicle in the top left of the ﬁeld of view (arrowed in the silicon image). Variations in ion intensity are shown by the calibration

bars, where brighter colours indicate higher intensity. Scale bar is 10 ␮m. (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web

version of this article.)

M.D. Brasier et al. / Precambrian Research 224 (2013) 1–10 9

brought about by its numerous vesicles would also provide an photocatalyst, able to oxidise organic matter, to synthesize

ideal substrate for the attachment of elemental sulfur particles. ammonia from nitrogen in the presence of UV light, and to break

The latter could then be attractive for microbial disproportionation water into its hydrogen and oxygen components in the presence

reactions. of UV light (see above and Brasier et al., 2011b). Vesicles in the

Phosphorus is another key biological element of great interest Apex pumice and scoria clasts are often inﬁlled by silica or alumi-

for the early Archaean biosphere (e.g. Blake et al., 2010). Unfor- nosilicate, presumably during exposure to hydrothermal ﬂuids

tunately, this element is rather difﬁcult to detect by NanoSIMS after burial. Some of these inﬁllings may be the altered products

because of its relatively weak secondary ion emission. It was here of zeolites, halloysites, smectites and montmorillonite clays. Var-

−

detected instead in combination with oxygen as the PO2 ion. ious zeolites can greatly boost the synthesis of organic polymers,

This permits the inference of likely phosphate minerals but it then the cracking of hydrocarbons, or the liberation of hydrogen gas

becomes more difﬁcult to assign any phosphorus to organic mate- (see above and Brasier et al., 2011b).

rial, and to be certain of the timing of its introduction to the pumice.

Nevertheless, several phosphate hotspots can be seen within the

Before the evolution of the biosphere, the extremely high

31 16 −

pumice clasts (Fig. 6, lower right box showing P O ), indicating

2 porosity, interconnected vesicles and abundance of exposed min-

that a further essential biological element was likely freely available

erals including microporous volcanic glass, clays and zeolites, will

within pumice at this early date.

arguably have given to pumice an unusually high capacity for

adsorbing biologically important elements and compounds, and for

maximising the effective surface area needed for catalytic reactions.

4. Conclusion

In this way, pumice could have behaved as a natural laboratory that

assisted with the origins of life itself.

∼

This study of 3460 Ma pumice clasts allows us now to address

the three geologically testable aspects of the pumice hypothesis

outlined at the start of this paper. Acknowledgements

We thank Dave Waters, Graham Cairns-Smith, Harold

1. We conﬁrm that clasts with the texture and mineralogy of

Morowitz, Norman Sleep, Ian Parsons and Lynn Margulis for

pumice can be widespread in Archaean rock successions.

much appreciated encouragement and advice; Nicola McLoughlin,

Although volumetrically small when compared with the vast

Martin van Kranendonk, Cris Stoakes, Arthur Hickman, Leila Bat-

thicknesses of pillow lava and komatiite (see Fig. 3a and b),

tison, Kate Hendry, Owen Green and John Lindsay for invaluable

pumice-dominated pyroclastic deposits formed beds several

assistance with ﬁeld work in Australia and Canada; David Pyle and

metres thick and many kilometres wide. These deposits lack

Tamsin Mather for essential guidance on pumice and volcaniclastic

graded bedding of the kind associated with air-fall breccias (cf.

rocks; Scott Bryan for providing examples of modern pumice;

Buick and Dunlop, 1990). Instead, they were often matrix free,

Owen Green, John W. Still and Norman Charnley for help with

moderately size sorted, with angular to sub-rounded clasts of

the SEM facilities. We acknowledge the facilities, scientiﬁc and

pumice plus other locally derived rocks. These fabrics are consis-

technical assistance of the Australian Microscopy & Microanalysis

tent with their formation from ejecta and rafts that accumulated

Research Facility at the Centre for Microscopy Characterisation

near to felsic volcanic vents along an ancient wave-washed fore-

and Analysis, The University of Western Australia, a facility funded

shore (Fig. 3b).

by the University, State and Commonwealth Governments.

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vital elements essential for life, such as C, O, N, P and S. In support

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