Precambrian Research 158 (2007) 156–176

Comparing petrographic signatures of bioalteration in recent to Mesoarchean pillow lavas: Tracing subsurface life in oceanic igneous rocks Harald Furnes a,∗, Neil R. Banerjee b, Hubert Staudigel c, Karlis Muehlenbachs d, Nicola McLoughlin a, Maarten de Wit e, Martin Van Kranendonk f a Centre for Geobiology and Department of Earth Science, University of Bergen, Allegt. 41, 5007 Bergen, Norway b Department of Earth Sciences, University of Western Ontario, London, Ontario, Canada N6A 5B7 c Scripps Institution of Oceanography, University of California, La Jolla, CA 92093-0225, USA d Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, Canada T6G 2E3 e AEON and Department of Geological Sciences, University of Cape Town, Rondebosch 7701, South Africa f Geological Survey of Western Australia, 100 Plain St. East Perth, Western Australia 6004, Australia Received 31 October 2006; received in revised form 12 March 2007; accepted 28 April 2007

Abstract Bioalteration of basaltic glass in pillow lava rims and glassy volcanic breccias (hyaloclastites) produces several distinctive traces including conspicuous petrographic textures. These biologically generated textures include granular and tubular morphologies that form during glass dissolution by microbes and subsequent precipitation of amorphous material. Such bioalteration textures have been described from upper, in situ oceanic crust spanning the youngest to the oldest oceanic basins (0–170 Ma). The granular type consists of individual and/or coalescing spherical bodies with diameters typically around 0.4 ␮m. These are by far the most abundant, having been traced up to ∼550 m depths in the oceanic crust. The tubular type is defined by distinct, straight to irregular tubes with diameters most commonly around 1–2 ␮m and lengths exceeding 100 ␮m. The tubes are most abundant between ∼50 m and 250 m into the volcanic basement. We advance a model for the production of these bioalteration textures and propose criteria for testing the biogenicity and antiquity of ancient examples. Similar bioalteration textures have also been found in hyaloclastites and well-preserved pillow lava margins of Phanerozoic to Proterozoic ophiolites and Archean greenstone belts. The latter include pillow lavas and hyaloclastites from the Mesoarchean Barberton Greenstone Belt of South Africa and the East Pilbara Terrane of the Pilbara Craton, Western Australia, where conspicuous titanite-mineralized tubes, have been found. Petrographic relationships and age data confirm that these structures developed in the Archean. Thus, these biologically generated textures may provide an important tool for mapping the deep oceanic biosphere and for tracing some of the earliest biological processes on Earth and perhaps other planetary surfaces. © 2007 Elsevier B.V. All rights reserved.

Keywords: Bioalteration textures; Volcanic glass; Oceanic crust; Ophiolites; Greenstone belts; Evidence for early life

1. Introduction

Life on Earth may have evolved well before the old- est preserved rocks, prior to 3.8 Ga, and most likely in ∗ Corresponding author. Tel.: +47 5558 3530; fax: +47 5558 3660. the vicinity of hydrothermal vents in the oceanic crust E-mail address: [email protected] (H. Furnes). (Nisbet and Sleep, 2001; Canfield et al., 2006). Evidence

0301-9268/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.precamres.2007.04.012 H. Furnes et al. / Precambrian Research 158 (2007) 156–176 157 for the earliest (∼3.8 Ga) purported life on Earth is solely et al., 1998; Furnes and Staudigel, 1999; Banerjee geochemical and consists of isotopically light carbon and Muehlenbachs, 2003). Furthermore, Furnes and in graphite contained within amphibolite- to granulite- Staudigel (1999) have demonstrated that the bioalter- grade supracrustal rocks in the Itsaq Gneiss Complex ation process can be traced as deep as ∼550 m into the of the North Atlantic Craton in southwest Greenland oceanic crust and that these biotic alteration processes (Schidlowski, 1988, 2001; Mojzsis et al., 1996; Rosing, dominate in the upper ∼350 m of the volcanic crust. 1999), but this evidence has been controversial (van Thus, a large body of evidence collected over the last Zuilen et al., 2002; Lepland et al., 2005). The earliest decade has convincingly demonstrated that the upper candidate fossilized microorganisms, on the other hand, volcanic part of the in situ oceanic crust is a habitat for are found in rocks ∼3.5 Ga from the Pilbara Craton in microbial life. Moreover, the bioalteration of pillow lava Australia (Walter et al., 1980; Schopf, 1993; Hoffman et glass is a widespread and common process that may have al., 1999; Ueno et al., 2001; Van Kranendonk et al., 2003; a profound effect on the chemical reactions, fluxes and Allwood et al., 2006) and the Barberton Greenstone products of seawater–rock interactions (e.g. Staudigel Belt in South Africa (Muir and Grant, 1976; Knoll and and Furnes, 2004; Staudigel et al., 2004). Barghoorn, 1977; Walsh and Lowe, 1985; Walsh, 1992; Textural studies of pillow lavas from in situ oceanic Westall et al., 2001, 2006), but many of these claims have crust can only record bio-interaction with pillow lavas likewise proved controversial (e.g. Lowe, 1994; Brasier dating back to the oldest intact example of ∼170 Ma et al., 2002, 2005; Garcia-Ruiz et al., 2003). (Fisk et al., 1999). This represents only a small fraction of Until recently, only sediments were considered to Earth’s history and to extend the record of bioalteration provide habitats for microbial activity, leaving volcanic further back in Earth’s history, it is therefore necessary rocks largely unexplored by biogeoscience research. to investigate pillow lavas of former oceanic crust repre- Recent studies have shown that submarine glassy basaltic sented by ophiolites and greenstone belts. These studies rocks also provide habitats for microbial life, first con- have so far confirmed that similar microbe-rock interac- vincingly shown by Thorseth et al. (1992). Moreover, tions have taken place within formerly glassy volcanic it has been suggested that soon after eruption, when rocks since the Mesoarchean (Furnes and Muehlenbachs, the ambient temperature (<113 ◦C) is tolerable for life 2003; Furnes et al., 2004, 2005; Banerjee et al., 2006a). to exist (Stetter et al., 1990; Stetter, 2006), coloniza- Thus bioalteration textures provide a new search tool for tion of the glassy rim of pillow lavas by microorganisms the earliest signs of life on Earth and other planetary sur- occurs contemporaneously wherever seawater has access faces (e.g. Banerjee et al., 2004a,b, 2006b; McLoughlin (Thorseth et al., 2001). et al., 2007). Microbial colonization of the glassy selvages of pil- In this paper, we first describe the range of alteration low lavas is most commonly observed along fractures, textures that are found within the glassy rims of pillow leaving behind several traces of their former presence. lavas from in situ oceanic crust. We then present a model The most ubiquitous are microscopic alteration tex- for the biotic alteration of oceanic pillow lavas and pro- tures found within the fresh glass at the interface with posed criteria for testing the antiquity and biogenicity of altered glass. These are empty or mineral-filled pits and these bioalteration textures. We then proceed to demon- channels with sizes and shapes that are comparable to strate that similar bioalteration textures are preserved in modern microbes. Furthermore, samples with these pet- ancient pillow lavas from ophiolites and greenstone belts rographic alteration textures commonly show very low back to 3.5 billion years ago. We outline the petrographic δ13C values (e.g. Furnes et al., 1999, 2001a; Banerjee and characteristics of mineralised bioalteration structures in Muehlenbachs, 2003), elevated concentrations of ele- ancient pillow lava rims and hyaloclastites and review ments such as C, N, P, K and S (e.g. Furnes et al., 2001b; what is currently known about how these biostructures Banerjee and Muehlenbachs, 2003), and in younger sam- are preserved. Lastly, we explore how bioalteration tex- ples the presence of DNA (e.g. Torsvik et al., 1998; tures found in terrestrial pillow lavas may be sought in Furnes et al., 2001a; Banerjee and Muehlenbachs, 2003), extraterrestrial rocks. all of which are strongly suggestive of a biogenic origin. Several studies have documented alteration textures, 2. Alteration textures in pillow lava of the element distributions and carbon isotope compositions modern oceanic crust of pillow lavas from in situ oceanic crust world-wide that are indicative of microbial alteration processes There are two fundamentally different modes of alter- (Thorseth et al., 1995a, 2001, 2003; Furnes et al., ation of basaltic glass in modern seafloor setting, i.e. 1996, 1999, 2001a,b; Fisk et al., 1998, 2003; Torsvik abiotic and biotic alteration. The abiotic alteration results 158 H. Furnes et al. / Precambrian Research 158 (2007) 156–176 in the formation of the long-recognized, but enigmatic as palagonite. It appears as banded material of approx- material termed palagonite. The other alteration mode imately equal thickness on both sides of fractures, with is the more recently-recognized biotic etching gener- smooth alteration fronts between the fresh and altered ated by the microbial colonization of rock surfaces. glass that are symmetric with respect to the fracture. The two alteration processes may be contemporaneously Peacock (1926) divided palagonite into two types, gel- active within the temperature limits of life. Below we palagonite (amorphous) and fibro-palagonite (consisting briefly comment on the products of abiotic alteration of clays, zeolites and ferrohydroxides). Palagonitization, and provide a comprehensive description of biogenic however, involves a continuous aging process from the alteration. amorphous to crystalline stage involving complicated processes of incongruent and congruent dissolution 2.1. Abiotic alteration and contemporaneous precipitation, hydration, and pro- nounced chemical exchange, that takes place at low to The alteration of basaltic glass is traditionally viewed high-temperature (e.g. Thorseth et al., 1991; Stroncik as a purely physio-chemical process and commonly and Schmincke, 2001; Walton and Schiffman, 2003; yields a pale yellow to dark brown material referred to Walton et al., 2005).

Fig. 1. Progressive development of granular texture from the incipient stage along fractures (A and B), to a more advanced stage along fracture (C), at intersection between fractures (D), to advanced stages (E and D). Note the regular palagonite rim adjacent to the fracture in (E), and different stages of alteration along the fractures in (F). The images are from the following samples: (A) ODP Site 648B-1R-1, unit 3, piece 7, 37–40 cm; (B) detail from A (central fracture); (C) DSDP Site 418A-52-5, 75–80 cm; (D) DSDP/OPD (Leg 70) Site 504B-46-3, unit 30A, piece 803, 105–106 cm; (E) DSDP Site 417D, 30-6, 20–24 cm; (F) DSDP Site 418A-55-4, 112–114 cm. H. Furnes et al. / Precambrian Research 158 (2007) 156–176 159

2.2. Biotic alteration bution of granular alteration fronts around fractures is commonly variable (Fig. 1F). In some cases it is evident Two types of textural development have been that abiotic palagonitization started before the onset of observed and related to microbial alteration (subse- granular bioalteration (Fig. 1E). quently referred to as bioalteration). These are granular and tubular textures which are texturally and geochem- 2.2.2. Tubular alteration textures ically distinguishable from the smooth and/or banded The tubular alteration type is defined by tubes palagonite alteration fronts which result from solely abi- which are invariably rooted on surfaces where water otic alteration. has permeated. The tubes may be empty but are most commonly filled with similar material to the granular 2.2.1. Granular alteration textures type. They may occur as straight and/or curved tubes The granular alteration type consists of individual, and develop from tiny individuals into dense bundles of spherical bodies filled with cryptocrystalline to very long, numerous tubes, attached to fractures in the glass fine-grained phyllosilicate phases. In the initial stages (Fig. 2). Individual tubes may show segmentation struc- of bioalteration the granular type appears as isolated tures and swelling to bud-like structures in various stages spherical bodies along fractures in the glass (Fig. 1A of growth. These eventually develop into new tubes and and B). At more advanced stages of bioalteration these bifurcating branches (Fig. 3). In most cases the tubules become more numerous and coalesce to define granular propagated perpendicular to the alteration front (Fig. 2), aggregates along fractures (Fig. 1C), or at the inter- although random orientations are also seen (Fig. 4A). section between fractures (Fig. 1D). These granular Tube alignment independent from the orientation of aggregates can develop into irregular bands that pro- fractures has also been observed (Fig. 4B). In rare cases trude into the fresh glass from one or several fractures parallel tubes may abruptly change growth direction by (Fig. 1E). In this way the development of granular alter- 180◦ when they meet another tube or fracture (Fig. 4C). ation generally shows no symmetry on opposite sides In cases where vesicles are present it is common to see of fractures (Fig. 1C–F), and the thickness and distri- tubular growth from the vesicle walls radially outwards

Fig. 2. Various stages in the development of tubular texture protruding into fresh glass (yellow in A, C, D, and greenish in B), showing beginning growth stage with only few tubules (A), relatively dense population of tubules (B) to extremely dense concentration of tubules (C and D). Picture A: from DSDP sample 70-504B, 35-1, 24, piece 274, 106–113 cm. Note the buds on some of the tubules. Picture B: from DSDP sample 46-396B, 20R-4, piece 5, 32–40 cm. Note that the tubules have grown predominantly on only one side of the fractures. Picture C: from DSDP sample 418A, 62-4, 64–70 cm. Picture D: detail from (C) showing the long and thin tubules. 160 H. Furnes et al. / Precambrian Research 158 (2007) 156–176

Fig. 3. Tubules showing budding and branching (b) (A), and segmen- tation (B). Picture A: from DSDP sample 46-396B, 20R-4, piece12F, 112–122 cm. Picture B: sample from Site 1184A. into the fresh glass (Fig. 5A). Tubular textures are also observed around varioles and phenocrysts, where con- centric cooling/stress fractures have developed (Fig. 5B).

2.2.3. Size distribution of granular and tubular textures The size distribution of bioalteration textures from six different DSDP/ODP Sites in the Atlantic Ocean, Pacific Ocean (at the Costa Rica Rift), and the Lau Basin, Fig. 4. Tubules showing random growth orientation (A), preferred ∼ alignment independent from the orientation of the fractures (B), and from crust varying in age from 6 Ma to 110 Ma, have ◦ been measured. The diameter of the granules, regardless a change of 180 in the growth orientation of tubules (C). Picture A: from ODP sample 48-896A, 11R-1, piece10, 111–113 cm. Picture B: of age, location, as well as depth into the crust, varies from DSDP sample 70–504B, 47-2, piece 889, 123–124 cm. Picture from 0.1 ␮m up to rare examples of 1.5 ␮m. The most C: from sample 417D, 53-2, 61–65 cm. common size is around 0.4 ␮m(Fig. 6). In comparison, the diameters of the tubular structures are substantially 2.2.4. Textural type versus depth larger and there is only a minor overlap in their size dis- The variation in measured bioalteration as a percent- tributions (Fig. 7A). The smallest and largest diameters age of total alteration (i.e. abiotic plus biotic) and how measured are ∼0.4 ␮m and 6 ␮m, respectively, and the this changes with depth and temperature into the volcanic most common size range is ∼1–2 ␮m(Fig. 7A), the aver- crust is shown in Fig. 8. The data are predominantly from age diameter being 1.4 ␮m. The lengths of the tubes are the oceanic crustal sections collected at Sites 417 and highly variable, from a few micrometers up to several 418 of the 110 Ma Western Atlantic and at Sites 504B hundred micrometers (Fig. 2). and 896A of the 5.9 Ma Costa Rica Rift. (The percentage H. Furnes et al. / Precambrian Research 158 (2007) 156–176 161

Fig. 5. Tubular texture developed and rooted around a vesicle (A and B), and varioles (C and D). Picture A: from DSDP sample 418A, 62-4, 64–70 cm. Note the radial arrangement of the tubular texture. Picture B: detail from A showing straight to highly irregular tubules. Picture C: from DSDP sample 46-396B, 20R-4, 112–122 cm. Picture D: detail from C. Note the segmented nature of the tubules. bioalteration is recalculated from Furnes and Staudigel, years depending on the spreading rate. The time required 1999; Furnes et al., 2001b. The temperature data was to build the ∼650 m thick volcanic sequence found at collected at Site 504B, where the volcanic basement is the intermediate-spreading rate Costa Rica Rift is esti- overlain by 275 m of sediments). Fig. 8 shows that the mated to be between 15,000 and 20,000 years (Pezard total amount of alteration within the uppermost part of et al., 1992). The stratigraphy of volcanic successions the volcanic sequence is generally low and in these loca- described from in situ oceanic crust and ophiolites indi- tions the granular type of bioalteration is completely cates construction during two to seven main volcanic dominant relative to abiotic alteration. With increas- cycles each with several sub-cycles (see summary in ing depth and temperature however, abiotic alteration Furnes et al., 2001c, 2003). This implies that during take over as the dominant alteration process. Of the two the build-up of the volcanic pile microbial colonization bioalteration morphologies the granular type is by far and bio-corrosion may have commenced immediately the most common and can be found at all depths into after a new eruption and subsequently waned several the volcanic basement where the presence of fresh glass times. allows bioalteration to be traced (down to ∼550 m). In Reports of bioalteration textures in samples dredged the upper ∼350 m of the crust the granular alteration from the ocean floor suggest that bioalteration starts very type is dominant, though most pronounced in the upper early (Thorseth et al., 2001), but it is as yet unknown as 200 m at temperatures less than ∼80 ◦C, and decreases to when and where in the volcanic pile bioalteration is steadily to become subordinate at temperatures of about most vigorous. Changes in fluid flux, nutrient supply and 115 ◦C. The tubular alteration textures meanwhile, con- temperature are likely to be important controls on the stitute only a small fraction of the total alteration and location of bioalteration in the oceanic crust that could show a clear maximum at ∼120–130 m depth at temper- be investigated by detailed downhole studies. It should atures of around 70 ◦C. At the surface, as well as below be remembered, however, that most drill holes in the vol- ∼350 m the tubular textures are generally absent or rare. canic rocks of relatively young oceanic crust have rather This dataset documenting the depth distribution of low (∼20%) recoveries. In addition, this net distribu- bioalteration textures is the time-integrated product of tion pattern includes preservational variables such as the microbial bioalteration in the oceanic crust as it cools, differential precipitation of minerals within the bioalter- subsides and is buried (e.g. Crosby et al., 2006). The ation textures that may act to enhance their chance of build up of oceanic crust may take 10–100 s of 1000 surviving in the rock record. 162 H. Furnes et al. / Precambrian Research 158 (2007) 156–176

Fig. 6. Relationship between diameter of granular structures and percentage in size classes from different DSDP/ODP Sites.

The data sets on which the depth distribution of exactly the same maximum bioalteration as a percentage bioalteration (Fig. 8) has been constructed are mainly of total alteration despite their very different ages (see derived from the 5.9 Ma Costa Rica Rift and the 110 Ma Fig. 11 of Furnes et al., 2001b). This would indicate Western Atlantic oceanic crustal segments. The original that a substantial part of the bioalteration happens very data sets from these two oceanic crustal segments show early and that the alteration pattern is established at H. Furnes et al. / Precambrian Research 158 (2007) 156–176 163

Fig. 7. Comparison between diameters of granular and tubular structures in the glassy margin of pillow lavas from in situ oceanic crust (A), and tubular structures from former glassy hyaloclastites of the Euro Basalt (B), Kelly Group, Pilbara Craton (Western Australia), and the Hoeggenoeg Formation (C) of the Barberton Greenstone Belt (South Africa). an early stage in the crustal history, i.e. within ∼6 Ma. 2.2.5. Microbiological constraints on the In broad terms this is consistent with the estimates of bioalteration of volcanic glass microbial biomass production from oxidative alteration Microbiological investigations have shown that and hydrolysis within the upper oceanic crust (e.g. Bach microorganisms are associated with the alteration of and Edwards, 2003). However, as long as fresh glass volcanic glass and the aim of this section is to give a is present and seawater circulation occurs, the bioalter- brief overview of current knowledge of these organisms ation process may continue. Hydrothermal convection and the metabolisms that may be involved. During as a result of convective heat loss in the oceanic crust the alteration of basaltic glass it is envisaged that 4− 3+ 4+ may occur for time periods up to ∼65 Ma (e.g. Stein oxidized compounds (e.g. SO2 ,CO2,Fe and Mn ) et al., 1995). This age may put some constrains on the supplied by circulating seawater may be used as electron upper time limit of bioalteration in the oceanic crust. acceptors and carbon sources, and that reduced Fe and 164 H. Furnes et al. / Precambrian Research 158 (2007) 156–176

narchaeota group (Lysnes et al., 2004). With ageing of the basaltic glass it is reported that autotrophic microbes which tend to dominate the early colonizing commu- nities are replaced by heterotrophic microbes in older, more altered samples (Thorseth et al., 2001). Moreover, it has recently been discovered that a group of bacteria distantly related to the heterotrophic organisms Mari- nobacter sp. and Hyphomonas sp. are also capable of chemolithoautrophic growth and employ Fe-oxidation at circum neutral pH on a range of reduced substrates that include basaltic glass (Edwards et al., 2003). Attempt to generate bioalteration textures in labora- tory culture experiments with volcanic glass substrates Fig. 8. Relationship between bioalteration (granular (blue field) and have had mixed success. Early studies conducted tubular (red field)) of total alteration, depth and temperature into the at room temperature, with high nutrient levels and volcanic crust. For the construction of this diagram the modes of biotic (granular and tubular) and abiotic alteration were point counted on relatively short incubation times of ca. 1 year produced 72 thin sections. Original data published in Furnes et al. (2001b), and etch pits and altered surfaces (Thorseth et al., 1995b). diagram modified from Staudigel et al. (2006). Some of these etch pits exhibited features interpreted as “growth rings” which were taken to suggest that these Mn in the volcanic glass may provide electron donors. pits may develop into tubular structures and so a model Staining for nucleic acids, bacterial and archeal RNA for the bioalteration of volcanic glass was advanced has revealed that biological material is concentrated (Thorseth et al., 1992; see also Section 5). Such extended at the interface between fresh and altered glass, and is tubular morphologies however, have yet to be produced localized within granular and tubular alteration textures in the laboratory. More recent microcosm experiments (e.g. Giovannoni et al., 1996; Torsvik et al., 1998, designed to mimic natural, oligotrophic seafloor environ- Fig. 2; Banerjee and Muehlenbachs, 2003, Fig. 14). ments with temperatures of 10 ◦C, low concentrations of In addition, cells have been observed by SEM on the N, P and Fe and only 6–1206 ppm total organic carbon surface of altered glass with morphologies that included content failed to produce however, enhanced bioalter- filamentous, coccoid, oval, rod and stalked forms (e.g. ation rates relative to sterile controls (Einen et al., 2006). Thorseth et al., 2001). Furthermore, these often occur Thus in summary it appears, that Fe and Mn oxidation are in or near etch marks in the glass that exhibit forms and important microbial metabolisms that likely contribute to sizes that resemble the attached microbes (e.g. Thorseth the bioalteration of volcanic glass. However, the optimal et al., 2003). Along fractures in basaltic glass that are conditions under which this occurs and the diversity of now altered to palagonite bacterial moulds encrusted in microorganisms that may be involved are yet to be fully iron and manganese rich oxides are found with coccoid documented. forms, also branched and twisted filaments that resemble the Fe-oxidizing bacteria Gallionella; e.g. Thorseth 3. Alteration textures in pillow lava of ophiolites et al. (2001, 2003). This is not surprising, given that and greenstone belts Gallionella ferruginea and Leptothrix discophora are considered to be classic examples of organisms capable Ophiolites and greenstone belts represent fragments of lithotrophic Fe-oxidation at circum neutral pH. In of ancient ocean crust that enable studies of alteration addition, diverse manganese oxidizing bacteria have processes in pillow lavas and hyaloclastites which pre- been isolated from basaltic seamounts and are argued to date in situ oceanic crust. Below we will demonstrate that enhance the rate of Mn oxidation during the bioalteration the methods developed for studying textural biomarkers of basaltic glass (e.g. Templeton et al., 2005). from in situ oceanic crust have also been successfully Culture independent molecular profiling studies applied to ophiolites and greenstone belts. These allow meanwhile, have found that basaltic glass is colonized us to search for petrographic traces of life during peri- by microorganism that are distinct from those found in ods of the Earth’s history in which relatively undeformed both deep seawater and seafloor sediments. For exam- or little-deformed submarine, formerly glassy volcanic ple, microbial sequences obtained from samples dredged rocks can be found. Suitable pillow lava sequences have from the Arctic seafloor are affiliated to eight main been found in ∼3500 million-years-old rocks South phylogenetic groups of bacteria and a single marine Cre- Africa and Western Australia, and perhaps even in con- H. Furnes et al. / Precambrian Research 158 (2007) 156–176 165 siderably deformed sequences as far back as ∼3800 million-years-ago in southwest Greenland.

3.1. Ophiolites

Pillow lavas from four ophiolite sequences ranging in age from Cretaceous to Early Proterozoic, have been investigated for biosignals: the 92 Ma Troodos ophiolite complex (TOC) in Cyprus (e.g. Schmincke and Bednarz, 1990); the 160 Ma Mirdita ophiolite complex (MOC) in Albania (e.g. Dilek et al., 2005); the ∼443 Ma Solund- Stavfjord ophiolite complex (SSOC) in western Norway (e.g. Furnes et al., 2003); and the 1953 Ma Jormua ophio- lite complex (JOC) in Finland (e.g. Kontinen, 1987). The entire sequences of the TOC, SSOC and JOC display a Penrose-type pseudostratigraphy, i.e. a layered-cake structural organization of oceanic crust components (e.g. Dilek et al., 1998). The western parts of the MOC, how- ever, from which we present bioaltered material, lack a prominent sheeted dike complex (though discrete dike swarms occur in places), and pillow lavas rest on gab- broic and serpentinized ultramafic mantle rocks. This type of development is comparable to the slow-spread Hess type oceanic crust (Dilek, 2003). In the youngest two investigated ophiolites, i.e. the TOC and MOC, which are both at zeolite to lowest greenschist facies metamorphic grade and have expe- rienced minor to no deformation, there are spectacular textural features of purported microbiological origin. Figs. 9 and 10 show a collage of textures for which we attribute biologic origin. Fig. 9 shows granular and tubular biotextures within patches of still preserved fresh glass from pillow rims of the TOC. The most common biotexture is the granular type which may appear as symmetrical or asymmetrical patches rooted in original, now clay-filled fractures (Fig. 9A). Associated with the granular alteration type are also straight to curved, thin (1–2 ␮m thick), empty to mineral-filled tubes that may attain length up to 100 ␮m(Fig. 9B). Another type of Fig. 9. Granular (A) and tubular (B and C) textures in glassy pillow lava tubular texture is much thicker (up to 20 ␮m thick) and rims from the Troodos Ophiolite Complex, Cyprus. Note the segmented shows well-developed segmentation structures (Fig. 9C). nature of the tubular structure shown in image (C). Abbreviations—FG: Both granular and tubular bioalteration textures have fresh glass; GT: granular texture; ST: segmented tube. also been found in rare patches of fresh glass in pillow lava rims of the MOC (Fig. 10). The tubular biotex- tures are enclosed in zeolites (Fig. 10B). It is uncertain In the older pillow lavas from the least deformed however, whether the zeolites represent replacement of part of the SSOC which is of lower greenschist facies, basaltic glass or merely fill spaces between originally possible traces of biogenerated structures were recorded glass fragments. Chemical and isotopic characteristics (Furnes et al., 2002a). Whilst even in the strongly recrys- associated with the tubular textures in the glassy rim tallized glassy rims of the lower amphibolite facies of pillows also strongly support their biological origin JOC, within areas of high carbon content, we have (see Furnes et al., 2001d; Furnes and Muehlenbachs, also found mineralized features that strongly resemble 2003). organic remains (Furnes et al., 2005). 166 H. Furnes et al. / Precambrian Research 158 (2007) 156–176

interlayered with cherts and overlain by cherts, banded iron formations (BIF) and shales (de Wit et al., 1987). The magmatic sequence consists of the Theespruit, Komati, Hooggenoeg and Kromberg Formations (the Onverwacht Group) and comprises 5–6 km of predom- inantly basaltic and komatiitic extrusive (pillow lavas, sheet flows, and minor hyaloclastite breccias) and intru- sive rocks. These rocks are generally well preserved, locally little-deformed, and have undergone prehnite- pumpellyite to greenschist facies metamorphism (de Wit et al., 1987). Samples were collected from the Komati, Hooggenoeg and Kromberg Formations. Bioalteration textures in pillow rims have so far been found in the upper part of the Hooggenoeg Formation and the lower part of the Kromberg Formation (Furnes et al., 2004; Banerjee et al., 2006a).

3.2.2. Pilbara Craton The Pilbara Craton contains some of the best preserved geological record of the early Earth (Van Kranendonk and Pirajno, 2004; Van Kranendonk et al., 2002, 2004, 2007). The East Pilbara Granite-Greenstone Terrane of the craton contains a 20 km thick succes- sion of low-grade metamorphic, dominantly volcanic supracrustal rocks (Pilbara Supergroup) that were deposited in four autochthonous groups from 3.52– Fig. 10. Granular (A) and tubular (B) textures in glassy pillow lava rims 3.165 Ga (Van Kranendonk, 2006; Van Kranendonk et from the Mirdita Ophiolite Complex, Albania. Abbreviations—FG: al., 2007) These include, from base to top, the 3.52– fresh glass; GT: granular texture; T: tubular textures in zeolite; Ze: 3.43 Ga Warrawoona Group, the 3.42–3.31 Ga Kelly zeolite. Group, the 3.27–3.23 Ga Sulphur Springs Group, and the 3.23–3.165 Ga Soanesville Group. Pillows and inter- 3.2. Greenstone belts pillow hyaloclastites were collected from the Dresser Formation and Apex Basalt of the Warrawoona Group, Pillow lavas from two of the oldest and best pre- and the Euro Basalt of the Kelly Group. The pillow sam- served greenstone belts in the world, the mesoarchean ples that have so far been found to display bioalteration Barberton Greenstone Belt (BGB) of South Africa, and textures come from the lower part of the 5–8 km thick the Pilbara Craton (PC) of Western Australia, have been Euro Basalt (Staudigel et al., 2006; Banerjee et al., 2007). investigated for biosignals. In both the BGB and PC, the outermost 10–20 mm of most pillows is defined by a dark 3.2.3. Mineralized bioalteration textures zone that represents the chilled, originally glassy rim. In Mineralized tubular structures from the BGB and many cases part of the glassy margin spalled off dur- Euro Basalt (Pilbara Craton) pillow lavas and hyalo- ing pillow growth and formed interpillow hyaloclastite. clastites are now 1–9 ␮m in width (averages of 4 ␮m Due to the pervasive prehnite-pumpellyite to greenschist for the BGB samples and 2.4 ␮m for the Euro Basalt) facies metamorphic overprint in both of these localities, (Fig. 7), up to 200 ␮m in length (average ∼50 ␮m), these rims now consist of extremely fine-grained chlorite are infilled by extremely fine-grained titanite with some with scattered grains of quartz, epidote, and amphibole. also containing minor amounts of chlorite and quartz (Figs. 11 and 12). These structures are observed to 3.2.1. Barberton Greenstone Belt extend away from healed fractures and/or grain bound- The 3480–3220 Ma old magmatic sequences (de aries along which seawater may once have flowed. Some Ronde and de Wit, 1994) of the BGB comprises 5–6 km of these tubular structures exhibit segmentation into sub- of submarine komatiitic and basaltic pillow lavas, with spherical bodies caused by chlorite overgrowths formed interbedded sheet flows and related intrusions that are during metamorphism (Figs. 11 and 12). In addition to H. Furnes et al. / Precambrian Research 158 (2007) 156–176 167

Fig. 12. (A) Tubular textures in interpillow hyaloclastites from the Fig. 11. (A) Tubular textures in interpillow hyaloclastites from the Euro Basalt of the Pilbara Supergroup, Western Australia. Black patchy Hooggenoeg Formation of the Barberton Greenstone Belt, South zones of titanite mark the healed boundary between originally glassy Africa. The black zone (consisting of titanite) across the middle part fragments, in which tubular structures are rooted. The fine grained of the picture, and in which tubular structures are rooted, marks the green mineral is chlorite, and the white to light brownish mineral is healed boundary between originally glassy fragments. The fine grained calcite. The boxed areas (B and C) show the positions of the enlarged green mineral is chlorite, and the white, stubby mineral grains below pictures. Note the segmented structure of all the tubules. the titanite zone are epidote crystals. The boxed areas (B and C) show the positions of the enlarged pictures. Note the segmented nature of the tubules, and the overgrowth of chlorite. 168 H. Furnes et al. / Precambrian Research 158 (2007) 156–176 the morphological similarities between the ancient struc- carbonates and sandstones to propose the following cri- tures and modern tubular bioalteration textures, C, N teria for investigating volcanic bioalteration textures: and P enrichments are localized in the linings of the (1) an outcrop to thin-section scale geological context Archean tubular structures. X-ray element maps of these that demonstrates the syngenicity and antiquity of the linings are given in Fig. 3 of Furnes et al. (2004) for bioalteration textures; (2) morphologies and distribu- the Hooggenoeg examples, and Fig. 2 of Banerjee et tion of the bioalteration textures that are consistent with al. (2007) from the Euro Basalt examples. By analogy biogenic behaviour; (3) geochemical evidence that is to modern examples, these enrichments in biologically suggestive of biological processing. significant elements, which coincide with the microtube To demonstrate that candidate bioalteration textures margins have been taken to represent decayed biological are syngenetic with the volcanic substrate and are not remains. later contaminants relies upon fabric relationships. At The tubular structures occur within apparently nar- the outcrop scale this involves mapping to show that row windows of these thick Archean submarine lava the phases containing the bioalteration textures are syn- sequences. However, when they do occur they are gen- eruptive and not younger veins or dyke filling phases. erally present in dense populations. Based on their At the thin-section scale the bioalteration textures them- similarity to textures observed in recent glassy pillow selves should also be seen to predate cross-cutting basalts we interpret these structures to represent ancient fractures, veins and cements. They should be con- mineralized traces of microbial activity formed during centrated along paths of early fluid migration and/or biogenic etching of the originally glassy pillow rims and weaknesses in the glass and occur as asymmetric masses hyaloclastites as microbes colonized the glass surface, across fractures that are distinct from symmetric, abiotic i.e. that they were originally hollow tubular structures. palagonite alteration fronts (see Fig. 13A). Further sup- In contrast to bioalteration textures from in situ port for their antiquity can be gained from the infilling oceanic crust where granular textures dominate (Fig. 8), mineral and/or organic phases that should have experi- those from investigated Archean greenstone belts are enced degrees of metamorphism comparable to the host largely titanite-filled tubular textures (Figs. 11 and 12). rock. For example, in Archean greenschist facies terranes (Although, one possible occurrence of putative granular one might hope to find bioalteration textures infilled with textures has been identified along fractures in originally chlorite and graphite bearing phases. A new approach glassy fragments of hyaloclastites from the Hoegge- that may enable confirmation of such relative age esti- noeg Formation, see Fig. 6 of Banerjee et al., 2006a). mates is the direct dating of titanite phases which infill This predominance of the tubular alteration textures in the bioalteration textures, see Section 6.2 and Banerjee Archean lavas may be attributed to the masking of the et al. (2007). typically smaller granular textures by titanite crystal- The range of morphologies displayed by candidate lization. Whereas conversely, the early precipitation of bioalteration textures in ophiolites and greenstones is titanite to infill many of the larger tubular texture may more restricted than that seen in recent volcanic glasses, have enhanced their preservation by limiting morpho- none-the-less there are number of useful morphological logical changes caused by recrystallization of the host indicators and distribution patterns that can be sought rock (Figs. 11 and 12). to test their biogenicity. For instance, many examples of ancient tubular bioalteration show branching patterns 4. Establishing antiquity and biogenicity and sharp changes in direction when they encounter another tube or fracture and both of these features are dif- Several lists of criteria have been proposed for ficult to explain by purely abiotic alteration (cf. Fig. 4). establishing the biogenicity of ancient microfossil and The size range of these ancient bioalteration textures stromatolite remains (see for example Buick et al., (see Fig. 6) is consistent with microbial involvement, 1981; Schopf and Walter, 1983; Brasier et al., 2005). but is not a strong criterion for inferring their biogenicity These have provided a useful framework for discussions and probably reflects significant taphonomic modifica- surrounding the early fossil record and have also gener- tion. The distribution and abundance of bioalteration ated much controversy (see Rose et al., 2006). To date textures, especially their concentration around vesicles there have been only preliminary attempts to outline a and varioles (see Fig. 5), is also suggestive of biolog- comparable list of biogenicity criteria for bioalteration ical behaviour and suggests the selection of sites with textures in volcanic rocks; see for example McLoughlin localised chemical gradients or concentrated strain that et al. (2007). Here, we draw upon studies of modern provides weakness in the glass. The key controls on their and ancient endolithic organisms in volcanic glasses, distribution are yet to be fully documented, but it is envis- H. Furnes et al. / Precambrian Research 158 (2007) 156–176 169

Fig. 13. Model of alteration modes (abiotic and bioalteration) of basalt glass. (A) Abiotic alteration in which the typically yellowish brown palagonite develops around the glass fragments with approximately equal thickness. With progressive alteration the empty spaces between the grains become filled with authigenic minerals and finally sealed, thus preventing water circulation and thus slowing down the alteration process. (B and C) Biotic alteration of granular (B) and tubular (C) types. In our model microbes attach to the glass surface where water can get access (along fractures and on the outer surface of grains) and start etching. With progressive alteration cell division occurs and the granular and/or segmented tubular structures develop as long as water is accessible. With progressive alteration there is also continuous growth of authingenic minerals in the empty micro- cavities and micro-tubules. When the authigenic minerals have sealed the structures preventing water access, the bioalteration growth eventually stops. Modified from Staudigel et al. (2006). aged that the temperature, redox state and composition of approach for testing their biogenicity. Thin (less than primary circulating fluids may exert a strong control on 1 ␮m wide) linings of C, N, and P have been detected their distribution. For comparison, it has been observed within modern and ancient bioalteration textures and are that morphologically similar microtubular structures in interpreted to represent preserved organic matter (e.g. Archean sandstones preferentially occur in clasts rich in Giovannoni et al., 1996; Furnes and Muehlenbachs, metal inclusions (Brasier et al., 2006; Wacey et al., 2006) 2003). Ideally, accompanying depletions in metaboli- and this type of apparent substrate selection may yet be cally significant elements in the surrounding rock matrix found between volcanic clasts of varying composition. would provide further support for their biogenicity. Fine-scale geochemical analyses of the phases that For example, depletions in Mg, Fe, Ca, and Na have infill candidate bioalteration textures provide our third been described around bioalteration textures from in 170 H. Furnes et al. / Precambrian Research 158 (2007) 156–176 situ oceanic crust (Alt and Mata, 2000). This type of gressively decrease (Fig. 13A). In some samples we see elemental data may be further strengthened by carbon that these processes are not mutually exclusive with the isotopic measurements on phases associated with the typical yellow to brown, smooth palagonite zones adja- bioalteration textures. For instance, it is reported that cent to fractures in the core of the alteration zone forming disseminated carbonate preserved in altered pillow basalt the first alteration product, which is then overtaken by rims is 13C-poor, typically between +3.9‰ and −16.4‰ granular and tubular alteration textures that occur on compared to carbonate within the unaltered pillow inte- the outer edges adjacent to the fresh glass. Hence, from riors, which has δ13C values of +0.7‰ to −6.9‰ that the numerous samples we have investigated, it is clear are similar to mantle values. This much greater range that the conditions under which bioalteration takes place, exhibited in the pillow rims is interpreted to reflect the allows for a faster dissolution of the glass than the for- microbial oxidation of organic matter that gives the more mation of abiotically-formed palagonite. negative values and perhaps the loss of 12C-enriched Based on the textural characteristics shown in methane from Archea to give the more positive values Figs. 1–12 we present a biotic alteration model for (e.g. Fig. 9 in Furnes et al., 2002b; Banerjee et al., 2006a, basaltic glass as shown in Fig. 13B and C. In this model and references therein). Corroboration of these findings the glass is congruently dissolved by chemical etch- may be possible by direct in situ analysis of carbon ing caused by microorganisms. The important roles of within the bioalteration textures themselves using nano- microorganisms and biofilms in the breakdown and dis- scale secondary ion mass spectrometry (NanoSIMS; solution of minerals and glass is well-established (e.g. cf. Kilburn et al., 2005). We remind the reader that all Welch et al., 1999; Brehm et al., 2005). The sizes of the such analyses need to be conducted in conjunction with alteration textures are of the same order as the size range petrographic studies to confirm that the phases analysed of candidate microbes. The size variations (Figs. 6 and 7) are syngenetic and not the result of later water–rock show log-normal distributions, a common phenomenon interaction. observed in biological systems (van Dover et al., 2003). A similar model is here advanced to that proposed by 5. Model of bioalteration Thorseth et al. (1992) for alteration structures found in the 6–7 mm thick, light-exposed part of subglacial Ice- Microbial colonization is known to produce pits and landic hyaloclastites, a phenomenon that was attributed channels during etching of basaltic glass. This process to light-dependent cryptroendolithic cyanobacteria cre- has been known since Mellor (1922) described church ating a highly alkaline micro-environment which caused glasses with surface pitting at locations where lichens the etching. Here, we extend the model of Thorseth et al. grew (see Krumbein et al., 1991 for review). The first (1992) to the deep biosphere. description of etching of natural glasses by microor- The granular and tubular structures that formed dur- ganisms was by Ross and Fisher (1986). However, no ing bioalteration of the fresh glass of pillow rims are convincing mechanism for how microbes may actually commonly filled with authigenic minerals, even in young facilitate glass dissolution was provided. Later, in a pillow lavas. Subsequent to the congruent dissolution of study of subglacial hyaloclastites in Iceland the pres- the glass adjacent to the microorganisms, some of the ence of bacteria hosted within basaltic glass aleration chemical components precipitate on the cavity walls. textures was reported (Thorseth et al., 1992). On this Detailed studies of the authigenic minerals indicate that basis Thorseth et al. (1992) suggested that microbes they are primarily clay minerals, Fe-hydroxides, zeolites may cause local variations in pH and/or secrete lig- and titanite (Storrie-Lombardi and Fisk, 2004; Furnes ands that allow them to chemically “drill” into a silicate and Muehlenbachs, 2003; Banerjee et al., 2006a). As substrate. This process was subsequently verified exper- long as water continues to flow through the cavities imentally by Thorseth et al. (1995b) and Staudigel et al. removing waste products and perhaps also supplying (1995, 1998) who demonstrated that glass alteration was nutrients then the bioalteration will proceed, providing enhanced in the presence of microbes. also that the ambient temperature is sufficiently low for During microbially driven glass dissolution the total life to exist. When the voids are completely sealed the surface area of fresh glass available will progressively bioalteration process will cease (Fig. 13BandCatt4). increase as the process proceeds. Staudigel et al. (2004) The biogenicity of structures claimed to represent fos- calculated that the fresh surface area would increase by silized microorganisms, of which filamentous features factors of 2.4 and 200 during the formation of tubular in the 3460 Ma Apex chert of the Pilbara Craton are a and granular textures, respectively. In contrast, abiotic prime example (Schopf, 1993; Schopf et al., 2002), have alteration causes the surface area of fresh glass to pro- been questioned (Brasier et al., 2002, 2005) and heav- H. Furnes et al. / Precambrian Research 158 (2007) 156–176 171 ily debated (see Dalton, 2002). Recent experiments have is difficult to explain how the elevated pore fluid pres- also generated filamentous microfossil-like structures, sure necessary to generate AITs concentrated at volcanic strikingly similar to those of the Apex chert, by abiotic clast margins, could be maintained within the complex mechanisms (Garcia-Ruiz et al., 2003). Hence, morphol- and partially open network of fractures found in modern ogy alone may not be sufficient to argue for a biogenic oceanic crust. origin of microbe-looking structures. In light of this discussion there is, however, a fundamental difference 6. Significance of textural evidence for microbial between the structures claimed to be fossilized micro- alteration as a biomarker organisms in sedimentary rocks and the granular and tubular bioalteration textures presented here. These alter- Presently, we are not aware of any feasible abiotic ation textures are micro-tracefossils in volcanic rocks; mechanism that can explain the origin of the granu- i.e. originally hollow traces that were produced as a lar and tubular textures described herein. Rather, their result of microbial etching of the original glass and that size distribution, morphological features and inferred have a higher preservation potential than the construct- growth patterns, as well as the concentration of bio- ing organisms. Once a hollow tube has been produced in logically significant elements (e.g. Furnes et al., 1999, the glass and subsequently filled with secondary miner- 2001a; Banerjee and Muehlenbachs, 2003; Banerjee et als, it may, in the absence of penetrative deformation and al., 2006a) and associated C-isotope characteristics (e.g. high-grade metamorphism, be preserved for billions of Furnes et al., 2001b, 2002a, 2004, 2005; Banerjee and years. The organisms that produced the tube, on the other Muehlenbachs, 2003; Banerjee et al., 2006a) are all sug- hand, will easily decay, and only traces of their chemical gestive of a microbiological origin. This has a number of components may remain associated with the structures. important implications for the mapping of bioalteration In this context, it should be mentioned that some- in submarine, formerly glassy volcanic rocks throughout what similar looking structures to those presented above the terrestrial rock record and provides a new signature have been observed in Precambrian organic-rich cherts for the search for life beyond earth. (e.g. Gruner, 1923). The formation of these structures was attributed to a process in which gas produced by 6.1. Mapping the deep biosphere in oceanic crust the metamorphic heating and/or decay of organic matter drives tiny mineral grains through the rock matrix that In the in situ oceanic crust where fresh glass is still act as ‘millstones’ producing the hollow tubular struc- commonly present, the granular and tubular textures can tures (Tyler and Barghoorn, 1963; Knoll and Barghoorn, be very easily observed by ordinary light microscopy. 1974). These so-called ambient inclusion trails (AITs) The dark, commonly mineral-filled structures (Figs. 1–5) are most commonly found in microcrystalline cherts and appear in strong contrast to the light yellowish-brown, phosphorites and may still contain the terminal crystal isotropic glass. This makes it possible to identify and and sometimes, longitudinal striae along the tube mar- quantitatively estimate the extent of bioalteration as a gins. The tubular textures presented here are all hosted function of depth by reinvestigating DSDP/ODP cores. in formerly glassy rocks, but do not contain traces of The only study of this kind to date is that of Furnes mineral grains at their tips that could have acted as and Staudigel (1999), which documented the relative a millstone. Also, the commonly developed budding proportions of alteration types in basaltic glass as a func- observed along the stem of the tubes would require sev- tion of depth and temperature (Fig. 8)to∼500 m into eral millstones within the same tube. In no way can we the volcanic basement of the oceanic crust. This study see that the textures we report from volcanic glasses, (op.cit.) showed that the tubular and granular bioalter- especially the granular textures, are compatible with a ation is dominant (up to 80%) of the total alteration pressure solution mechanism and hence we refute this in the upper 300 m of the volcanic pile. Whereas the mechanism to account for the generation of the tubu- granular alteration type occurs throughout the upper lar structures presented here (see also Banerjee et al., ∼500 m, the tubular type has only been found in the 2006b; Brasier et al., 2006). Further, the enrichment of upper ∼300 m, has its maximal occurrence at a depth typical bio-elements, such as carbon, nitrogen, phospho- range of ∼100–200 m at a present temperature range rous, sulphur and potassium associated with the granular of ∼65–75 ◦C(Fig. 8). However, there are still many and tubular structures, as well as low (13C values, are all unanswered questions related to the controls upon the signatures that strongly indicate a microbiological ori- distribution of bioalteration with depth. Important geo- gin of these textures, and would be extremely difficult logical factors will include the permeability at a give to account for by an abiotic AIT mechanism. Lastly, it place and a given time and hence the flux of nutrients, and 172 H. Furnes et al. / Precambrian Research 158 (2007) 156–176 further, the nature of the environment (reducing or oxi- phic age, giving a minimum age of the formation of the dizing). There is a wealth of drill core samples of pillow tubular structures. Even with the high age uncertainty lavas from in situ oceanic crust yet to be investigated for this date clearly shows that the tubules are of Archean bioalteration. age. Our present knowledge on colonization of microor- ganisms on glass surfaces (e.g. Thorseth et al., 1995b, 6.2. Tracing back the record of microbial 2001) would indicate that the bio-corrosion that resulted colonization of volcanic rocks in the cavities (see Fig. 13C) formed shortly after erup- tion, during cooling, and early burial diagenesis of the Building on the work reviewed herein we can con- volcanic pile. Since the granular and tubular textures of tribute to intriguing questions about the early signs of the ancient pillow lavas commonly are filled with titanite, life on Earth. When did microbial colonization of vol- this dating technique holds great potential for providing canic rocks begin, and do we see the same bioalteration minimum age estimates of bioalteration. patterns in pillow lavas of the ancient oceanic crust as in the young, in situ oceanic crust? 6.3. Mapping the pattern of early bioalteration As to the first question it has been convincingly demonstrated that 3.3–3.5 Ga bioalteration textures do Textural biosignals in the formerly glassy rims of pil- occur in the submarine, originally glassy rocks (pillow low basalts have so far been found in selected units in the lavas and hyaloclastites) of the Barberton Greenstone thick volcanic successions of the Barberton Greenstone Belt (Furnes et al., 2004; Banerjee et al., 2006a) and the Belt and East Pilbara Terrane. Since the bio-signals that Euro Basalt of the Pilbara Craton (Banerjee et al., 2007). we look for have been created by viable microorganisms, However, these submarine volcanic rocks are not the they can only develop when the ambient temperature world’s oldest, and the petrographic search for bioalter- that allows life is below ∼113 ◦C(Stetter et al., 1990). ation textures or other biotraces in even older pillow lavas It is therefore unlikely that bioalteration occurs contin- (e.g. the >3.8 Ga pillow lavas of the Isua supracrustal uously throughout such thick volcanic sequences, some belt, Greenland) may contribute to the ongoing debate of which were probably deposited relatively quickly and about the earliest signs of life on Earth. buried at temperatures that were too high for life to Concerning the age constraints upon the formation of exist. For example, the biotextures found in the inter- biotextures, a crucial question is: when did they form? pillow hyaloclastite from the Euro Basalt (Fig. 12) are In some cases, as for example the above-mentioned pil- from the lower part of the 5–8 km thick basalt succes- low lavas from the Barberton Greenstone Belt, it can be sion. The basal flows of the Euro Basalt are dated at demonstrated that metamorphic mineral assemblages of 3350 ± 2 Ma, whereas a thin felsic volcaniclastic unit know age (3486 Ma by 40Ar/39Ar dating of amphibole; in the upper part of the formation has yielded an age Lopez-Martinez et al., 1992) have overgrown the tubu- of 3346 ± 6 Ma (see Fig. 2 in Van Kranendonk, 2006), lar biotextures, thus providing a minimum age for their suggesting that most of this very thick volcanic succes- formation. This age overlaps with the igneous U/Pb ages sion was erupted within a few million years, probably (3482 Ma) of the complex (de Ronde and de Wit, 1994; exceeding temperature viable for life. Dann, 2000) and suggests that bioalteration occurred very soon after eruption. The petrographic relationships 6.4. Bioalteration on Mars? between the tubular textures and the metamorphic miner- als are not however, always obvious and ubiquitous, thus Several hypotheses for life on other planetary sur- leaving an uncertainty as to the age of their formation. faces involve endolithic organisms that may produce Another avenue is provided by titanite which partially bioalteration textures like those described herein (see or completely infills the granular and tubular textures for example Friedmann and Koriem, 1989; McKay et al., found in ancient pillow lavas (and to some extent in the 1992). It has been proposed that an endolithic mode of young examples). Direct dating of the titanite tubules life may be best adapted to the intense UV flux, absence in an interpillow hyaloclastite sample from the 3350 Ma of liquid water and freezing temperatures that exist today Euro Basalt of the Pilbara Craton by in situ laser ablation on Mars and have for much of Martian history. The obser- multi-collector-ICP-MS in thin sections yielded a mini- vation of palagonite-like material on Mars (see Bishop mum age of 2921 ± 110 Ma (Banerjee et al., 2007). This et al., 2002) is significant because it suggests extended age corresponds with regional metamorphism and intru- exposure of basalts to water on the Martian surface. We sive activity in the region (Van Kranendonk et al., 2002, know that palagonitization of basaltic glass on Earth pro- 2004). The dating of the titanite thus yields a metamor- ceeds by both biotic and abiotic mechanisms, and that H. Furnes et al. / Precambrian Research 158 (2007) 156–176 173 glass altering microbes may have existed on Earth since number of reasons, including: (1) mapping of the deep at least Mesoarchean (Furnes et al., 2004; Banerjee et oceanic biosphere; (2) tracing the earliest microbial al., 2006b; Staudigel et al., 2006). The possibility of colonization of volcanic rocks and thus adding to our subaqueous basaltic glass alteration on early Mars both understanding of the origin of early life on Earth; (3) by biotic and abiotic processes has been suggested by mapping the pattern of volcanic bioalteration which we Banerjee et al. (2004a,b, 2006b) amongst others. The may one day be able to use as a proxy for oceanic crustal recent discovery in the Nakhla meteorite of carbona- conditions; and (4) providing a new search image for ceous vein filling material with tubular and bleb shaped life in extraterrestrial rocks. microstructures that are similar to terrestrial bioalter- ation textures have renewed interest in these hypotheses (McKay et al., 2006; Gibson et al., 2006). In addition, Acknowledgements microtubular weathering channels in olivines and pyrox- enes from this same class of meteorite have also recently Financial support to carry out this study was provided been described (Fisk et al., 2006). To establish a Martian by the Norwegian Research Council, the National Sci- age for these microstructures these authors are seeking ences and Engineering Research Council of Canada, the the same types of fabric relationships as those described US National Science Foundation, the Agouron Institute, herein, particularly with reference to the fusion crusts the National Research Foundation of South Africa, and formed during transport of the meteorite. Furthermore, the Geological Survey of Western Australia. We thank to test their biogenicity, the morphologies, chemical Fred Daniel of Nkomazi Wilderness for hospitality and and isotopic composition of these micro-textures are the Mpumalanga Parks Board for access during field being documented in attempts to eliminate an origin work in the Barberton Mountain Land of South Africa. from impact related processes that caused fracturing and This work has greatly benefited from the constructive alteration of the Nakhla meteorite. 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