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
∗
Corresponding author. Tel.: +47 5558 3530; fax: +47 5558 3660.
E-mail address: [email protected] (H. Furnes).
0301-9268/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.precamres.2007.04.012
Life on Earth may have evolved well before the oldest preserved rocks, prior to 3.8 Ga, and most likely in
the vicinity of hydrothermal vents in the oceanic crust
(Nisbet and Sleep, 2001; Canfield et al., 2006). Evidence
H. Furnes et al. / Precambrian Research 158 (2007) 156–176
for the earliest (∼3.8 Ga) purported life on Earth is solely
geochemical and consists of isotopically light carbon
in graphite contained within amphibolite- to granulitegrade supracrustal rocks in the Itsaq Gneiss Complex
of the North Atlantic Craton in southwest Greenland
(Schidlowski, 1988, 2001; Mojzsis et al., 1996; Rosing,
1999), but this evidence has been controversial (van
Zuilen et al., 2002; Lepland et al., 2005). The earliest
candidate fossilized microorganisms, on the other hand,
are found in rocks ∼3.5 Ga from the Pilbara Craton in
Australia (Walter et al., 1980; Schopf, 1993; Hoffman et
al., 1999; Ueno et al., 2001; Van Kranendonk et al., 2003;
Allwood et al., 2006) and the Barberton Greenstone
Belt in South Africa (Muir and Grant, 1976; Knoll and
Barghoorn, 1977; Walsh and Lowe, 1985; Walsh, 1992;
Westall et al., 2001, 2006), but many of these claims have
likewise proved controversial (e.g. Lowe, 1994; Brasier
et al., 2002, 2005; Garcia-Ruiz et al., 2003).
Until recently, only sediments were considered to
provide habitats for microbial activity, leaving volcanic
rocks largely unexplored by biogeoscience research.
Recent studies have shown that submarine glassy basaltic
rocks also provide habitats for microbial life, first convincingly shown by Thorseth et al. (1992). Moreover,
it has been suggested that soon after eruption, when
the ambient temperature (<113 ◦ C) is tolerable for life
to exist (Stetter et al., 1990; Stetter, 2006), colonization of the glassy rim of pillow lavas by microorganisms
occurs contemporaneously wherever seawater has access
(Thorseth et al., 2001).
Microbial colonization of the glassy selvages of pillow lavas is most commonly observed along fractures,
leaving behind several traces of their former presence.
The most ubiquitous are microscopic alteration textures found within the fresh glass at the interface with
altered glass. These are empty or mineral-filled pits and
channels with sizes and shapes that are comparable to
modern microbes. Furthermore, samples with these petrographic alteration textures commonly show very low
δ13 C values (e.g. Furnes et al., 1999, 2001a; Banerjee and
Muehlenbachs, 2003), elevated concentrations of elements such as C, N, P, K and S (e.g. Furnes et al., 2001b;
Banerjee and Muehlenbachs, 2003), and in younger samples the presence of DNA (e.g. Torsvik et al., 1998;
Furnes et al., 2001a; Banerjee and Muehlenbachs, 2003),
all of which are strongly suggestive of a biogenic origin.
Several studies have documented alteration textures,
element distributions and carbon isotope compositions
of pillow lavas from in situ oceanic crust world-wide
that are indicative of microbial alteration processes
(Thorseth et al., 1995a, 2001, 2003; Furnes et al.,
1996, 1999, 2001a,b; Fisk et al., 1998, 2003; Torsvik
157
et al., 1998; Furnes and Staudigel, 1999; Banerjee
and Muehlenbachs, 2003). Furthermore, Furnes and
Staudigel (1999) have demonstrated that the bioalteration process can be traced as deep as ∼550 m into the
oceanic crust and that these biotic alteration processes
dominate in the upper ∼350 m of the volcanic crust.
Thus, a large body of evidence collected over the last
decade has convincingly demonstrated that the upper
volcanic part of the in situ oceanic crust is a habitat for
microbial life. Moreover, the bioalteration of pillow lava
glass is a widespread and common process that may have
a profound effect on the chemical reactions, fluxes and
products of seawater–rock interactions (e.g. Staudigel
and Furnes, 2004; Staudigel et al., 2004).
Textural studies of pillow lavas from in situ oceanic
crust can only record bio-interaction with pillow lavas
dating back to the oldest intact example of ∼170 Ma
(Fisk et al., 1999). This represents only a small fraction of
Earth’s history and to extend the record of bioalteration
further back in Earth’s history, it is therefore necessary
to investigate pillow lavas of former oceanic crust represented by ophiolites and greenstone belts. These studies
have so far confirmed that similar microbe-rock interactions have taken place within formerly glassy volcanic
rocks since the Mesoarchean (Furnes and Muehlenbachs,
2003; Furnes et al., 2004, 2005; Banerjee et al., 2006a).
Thus bioalteration textures provide a new search tool for
the earliest signs of life on Earth and other planetary surfaces (e.g. Banerjee et al., 2004a,b, 2006b; McLoughlin
et al., 2007).
In this paper, we first describe the range of alteration
textures that are found within the glassy rims of pillow
lavas from in situ oceanic crust. We then present a model
for the biotic alteration of oceanic pillow lavas and proposed criteria for testing the antiquity and biogenicity of
these bioalteration textures. We then proceed to demonstrate that similar bioalteration textures are preserved in
ancient pillow lavas from ophiolites and greenstone belts
back to 3.5 billion years ago. We outline the petrographic
characteristics of mineralised bioalteration structures in
ancient pillow lava rims and hyaloclastites and review
what is currently known about how these biostructures
are preserved. Lastly, we explore how bioalteration textures found in terrestrial pillow lavas may be sought in
extraterrestrial rocks.
2. Alteration textures in pillow lava of the
modern oceanic crust
There are two fundamentally different modes of alteration of basaltic glass in modern seafloor setting, i.e.
abiotic and biotic alteration. The abiotic alteration results
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H. Furnes et al. / Precambrian Research 158 (2007) 156–176
in the formation of the long-recognized, but enigmatic
material termed palagonite. The other alteration mode
is the more recently-recognized biotic etching generated by the microbial colonization of rock surfaces.
The two alteration processes may be contemporaneously
active within the temperature limits of life. Below we
briefly comment on the products of abiotic alteration
and provide a comprehensive description of biogenic
alteration.
2.1. Abiotic alteration
The alteration of basaltic glass is traditionally viewed
as a purely physio-chemical process and commonly
yields a pale yellow to dark brown material referred to
as palagonite. It appears as banded material of approximately equal thickness on both sides of fractures, with
smooth alteration fronts between the fresh and altered
glass that are symmetric with respect to the fracture.
Peacock (1926) divided palagonite into two types, gelpalagonite (amorphous) and fibro-palagonite (consisting
of clays, zeolites and ferrohydroxides). Palagonitization,
however, involves a continuous aging process from the
amorphous to crystalline stage involving complicated
processes of incongruent and congruent dissolution
and contemporaneous precipitation, hydration, and pronounced chemical exchange, that takes place at low to
high-temperature (e.g. Thorseth et al., 1991; Stroncik
and Schmincke, 2001; Walton and Schiffman, 2003;
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
2.2. Biotic alteration
Two types of textural development have been
observed and related to microbial alteration (subsequently referred to as bioalteration). These are granular
and tubular textures which are texturally and geochemically distinguishable from the smooth and/or banded
palagonite alteration fronts which result from solely abiotic alteration.
2.2.1. Granular alteration textures
The granular alteration type consists of individual,
spherical bodies filled with cryptocrystalline to very
fine-grained phyllosilicate phases. In the initial stages
of bioalteration the granular type appears as isolated
spherical bodies along fractures in the glass (Fig. 1A
and B). At more advanced stages of bioalteration these
become more numerous and coalesce to define granular
aggregates along fractures (Fig. 1C), or at the intersection between fractures (Fig. 1D). These granular
aggregates can develop into irregular bands that protrude into the fresh glass from one or several fractures
(Fig. 1E). In this way the development of granular alteration generally shows no symmetry on opposite sides
of fractures (Fig. 1C–F), and the thickness and distri-
159
bution of granular alteration fronts around fractures is
commonly variable (Fig. 1F). In some cases it is evident
that abiotic palagonitization started before the onset of
granular bioalteration (Fig. 1E).
2.2.2. Tubular alteration textures
The tubular alteration type is defined by tubes
which are invariably rooted on surfaces where water
has permeated. The tubes may be empty but are most
commonly filled with similar material to the granular
type. They may occur as straight and/or curved tubes
and develop from tiny individuals into dense bundles of
long, numerous tubes, attached to fractures in the glass
(Fig. 2). Individual tubes may show segmentation structures and swelling to bud-like structures in various stages
of growth. These eventually develop into new tubes and
bifurcating branches (Fig. 3). In most cases the tubules
propagated perpendicular to the alteration front (Fig. 2),
although random orientations are also seen (Fig. 4A).
Tube alignment independent from the orientation of
fractures has also been observed (Fig. 4B). In rare cases
parallel tubes may abruptly change growth direction by
180◦ when they meet another tube or fracture (Fig. 4C).
In cases where vesicles are present it is common to see
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.
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H. Furnes et al. / Precambrian Research 158 (2007) 156–176
Fig. 3. Tubules showing budding and branching (b) (A), and segmentation (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 concentric 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,
from crust varying in age from ∼6 Ma to 110 Ma, have
been measured. The diameter of the granules, regardless
of age, location, as well as depth into the crust, varies
from 0.1 ␮m up to rare examples of 1.5 ␮m. The most
common size is around 0.4 ␮m (Fig. 6). In comparison,
the diameters of the tubular structures are substantially
larger and there is only a minor overlap in their size distributions (Fig. 7A). The smallest and largest diameters
measured are ∼0.4 ␮m and 6 ␮m, respectively, and the
most common size range is ∼1–2 ␮m (Fig. 7A), the average diameter being 1.4 ␮m. The lengths of the tubes are
highly variable, from a few micrometers up to several
hundred micrometers (Fig. 2).
Fig. 4. Tubules showing random growth orientation (A), preferred
alignment independent from the orientation of the fractures (B), and
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:
from DSDP sample 70–504B, 47-2, piece 889, 123–124 cm. Picture
C: from sample 417D, 53-2, 61–65 cm.
2.2.4. Textural type versus depth
The variation in measured bioalteration as a percentage of total alteration (i.e. abiotic plus biotic) and how
this changes with depth and temperature into the volcanic
crust is shown in Fig. 8. The data are predominantly from
the oceanic crustal sections collected at Sites 417 and
418 of the 110 Ma Western Atlantic and at Sites 504B
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,
1999; Furnes et al., 2001b. The temperature data was
collected at Site 504B, where the volcanic basement is
overlain by 275 m of sediments). Fig. 8 shows that the
total amount of alteration within the uppermost part of
the volcanic sequence is generally low and in these locations the granular type of bioalteration is completely
dominant relative to abiotic alteration. With increasing depth and temperature however, abiotic alteration
take over as the dominant alteration process. Of the two
bioalteration morphologies the granular type is by far
the most common and can be found at all depths into
the volcanic basement where the presence of fresh glass
allows bioalteration to be traced (down to ∼550 m). In
the upper ∼350 m of the crust the granular alteration
type is dominant, though most pronounced in the upper
200 m at temperatures less than ∼80 ◦ C, and decreases
steadily to become subordinate at temperatures of about
115 ◦ C. The tubular alteration textures meanwhile, constitute only a small fraction of the total alteration and
show a clear maximum at ∼120–130 m depth at temperatures of around 70 ◦ C. At the surface, as well as below
∼350 m the tubular textures are generally absent or rare.
This dataset documenting the depth distribution of
bioalteration textures is the time-integrated product of
microbial bioalteration in the oceanic crust as it cools,
subsides and is buried (e.g. Crosby et al., 2006). The
build up of oceanic crust may take 10–100 s of 1000
years depending on the spreading rate. The time required
to build the ∼650 m thick volcanic sequence found at
the intermediate-spreading rate Costa Rica Rift is estimated to be between 15,000 and 20,000 years (Pezard
et al., 1992). The stratigraphy of volcanic successions
described from in situ oceanic crust and ophiolites indicates construction during two to seven main volcanic
cycles each with several sub-cycles (see summary in
Furnes et al., 2001c, 2003). This implies that during
the build-up of the volcanic pile microbial colonization
and bio-corrosion may have commenced immediately
after a new eruption and subsequently waned several
times.
Reports of bioalteration textures in samples dredged
from the ocean floor suggest that bioalteration starts very
early (Thorseth et al., 2001), but it is as yet unknown as
to when and where in the volcanic pile bioalteration is
most vigorous. Changes in fluid flux, nutrient supply and
temperature are likely to be important controls on the
location of bioalteration in the oceanic crust that could
be investigated by detailed downhole studies. It should
be remembered, however, that most drill holes in the volcanic rocks of relatively young oceanic crust have rather
low (∼20%) recoveries. In addition, this net distribution pattern includes preservational variables such as the
differential precipitation of minerals within the bioalteration textures that may act to enhance their chance of
surviving in the rock record.
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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
bioalteration (Fig. 8) has been constructed are mainly
derived from the 5.9 Ma Costa Rica Rift and the 110 Ma
Western Atlantic oceanic crustal segments. The original
data sets from these two oceanic crustal segments show
exactly the same maximum bioalteration as a percentage
of total alteration despite their very different ages (see
Fig. 11 of Furnes et al., 2001b). This would indicate
that a substantial part of the bioalteration happens very
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.
In broad terms this is consistent with the estimates of
microbial biomass production from oxidative alteration
and hydrolysis within the upper oceanic crust (e.g. Bach
and Edwards, 2003). However, as long as fresh glass
is present and seawater circulation occurs, the bioalteration process may continue. Hydrothermal convection
as a result of convective heat loss in the oceanic crust
may occur for time periods up to ∼65 Ma (e.g. Stein
et al., 1995). This age may put some constrains on the
upper time limit of bioalteration in the oceanic crust.
2.2.5. Microbiological constraints on the
bioalteration of volcanic glass
Microbiological investigations have shown that
microorganisms are associated with the alteration of
volcanic glass and the aim of this section is to give a
brief overview of current knowledge of these organisms
and the metabolisms that may be involved. During
the alteration of basaltic glass it is envisaged that
oxidized compounds (e.g. SO2 4− , CO2 , Fe3+ and Mn4+ )
supplied by circulating seawater may be used as electron
acceptors and carbon sources, and that reduced Fe and
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H. Furnes et al. / Precambrian Research 158 (2007) 156–176
Fig. 8. Relationship between bioalteration (granular (blue field) and
tubular (red field)) of total alteration, depth and temperature into the
volcanic crust. For the construction of this diagram the modes of biotic
(granular and tubular) and abiotic alteration were point counted on
72 thin sections. Original data published in Furnes et al. (2001b), and
diagram modified from Staudigel et al. (2006).
Mn in the volcanic glass may provide electron donors.
Staining for nucleic acids, bacterial and archeal RNA
has revealed that biological material is concentrated
at the interface between fresh and altered glass, and is
localized within granular and tubular alteration textures
(e.g. Giovannoni et al., 1996; Torsvik et al., 1998,
Fig. 2; Banerjee and Muehlenbachs, 2003, Fig. 14).
In addition, cells have been observed by SEM on the
surface of altered glass with morphologies that included
filamentous, coccoid, oval, rod and stalked forms (e.g.
Thorseth et al., 2001). Furthermore, these often occur
in or near etch marks in the glass that exhibit forms and
sizes that resemble the attached microbes (e.g. Thorseth
et al., 2003). Along fractures in basaltic glass that are
now altered to palagonite bacterial moulds encrusted in
iron and manganese rich oxides are found with coccoid
forms, also branched and twisted filaments that resemble
the Fe-oxidizing bacteria Gallionella; e.g. Thorseth
et al. (2001, 2003). This is not surprising, given that
Gallionella ferruginea and Leptothrix discophora are
considered to be classic examples of organisms capable
of lithotrophic Fe-oxidation at circum neutral pH. In
addition, diverse manganese oxidizing bacteria have
been isolated from basaltic seamounts and are argued to
enhance the rate of Mn oxidation during the bioalteration
of basaltic glass (e.g. Templeton et al., 2005).
Culture independent molecular profiling studies
meanwhile, have found that basaltic glass is colonized
by microorganism that are distinct from those found in
both deep seawater and seafloor sediments. For example, microbial sequences obtained from samples dredged
from the Arctic seafloor are affiliated to eight main
phylogenetic groups of bacteria and a single marine Cre-
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 communities 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 Marinobacter 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 laboratory culture experiments with volcanic glass substrates
have had mixed success. Early studies conducted
at room temperature, with high nutrient levels and
relatively short incubation times of ca. 1 year produced
etch pits and altered surfaces (Thorseth et al., 1995b).
Some of these etch pits exhibited features interpreted as
“growth rings” which were taken to suggest that these
pits may develop into tubular structures and so a model
for the bioalteration of volcanic glass was advanced
(Thorseth et al., 1992; see also Section 5). Such extended
tubular morphologies however, have yet to be produced
in the laboratory. More recent microcosm experiments
designed to mimic natural, oligotrophic seafloor environments with temperatures of 10 ◦ C, low concentrations of
N, P and Fe and only 6–1206 ppm total organic carbon
content failed to produce however, enhanced bioalteration rates relative to sterile controls (Einen et al., 2006).
Thus in summary it appears, that Fe and Mn oxidation are
important microbial metabolisms that likely contribute to
the bioalteration of volcanic glass. However, the optimal
conditions under which this occurs and the diversity of
microorganisms that may be involved are yet to be fully
documented.
3. Alteration textures in pillow lava of ophiolites
and greenstone belts
Ophiolites and greenstone belts represent fragments
of ancient ocean crust that enable studies of alteration
processes in pillow lavas and hyaloclastites which predate in situ oceanic crust. Below we will demonstrate that
the methods developed for studying textural biomarkers
from in situ oceanic crust have also been successfully
applied to ophiolites and greenstone belts. These allow
us to search for petrographic traces of life during periods of the Earth’s history in which relatively undeformed
or little-deformed submarine, formerly glassy volcanic
rocks can be found. Suitable pillow lava sequences have
been found in ∼3500 million-years-old rocks South
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 SolundStavfjord ophiolite complex (SSOC) in western Norway
(e.g. Furnes et al., 2003); and the 1953 Ma Jormua ophiolite 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, however, from which we present bioaltered material, lack a
prominent sheeted dike complex (though discrete dike
swarms occur in places), and pillow lavas rest on gabbroic 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 experienced 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
tubular texture is much thicker (up to 20 ␮m thick) and
shows well-developed segmentation structures (Fig. 9C).
Both granular and tubular bioalteration textures have
also been found in rare patches of fresh glass in pillow
lava rims of the MOC (Fig. 10). The tubular biotextures are enclosed in zeolites (Fig. 10B). It is uncertain
however, whether the zeolites represent replacement of
basaltic glass or merely fill spaces between originally
glass fragments. Chemical and isotopic characteristics
associated with the tubular textures in the glassy rim
of pillows also strongly support their biological origin
(see Furnes et al., 2001d; Furnes and Muehlenbachs,
2003).
Fig. 9. Granular (A) and tubular (B and C) textures in glassy pillow lava
rims from the Troodos Ophiolite Complex, Cyprus. Note the segmented
nature of the tubular structure shown in image (C). Abbreviations—FG:
fresh glass; GT: granular texture; ST: segmented tube.
In the older pillow lavas from the least deformed
part of the SSOC which is of lower greenschist facies,
possible traces of biogenerated structures were recorded
(Furnes et al., 2002a). Whilst even in the strongly recrystallized glassy rims of the lower amphibolite facies
JOC, within areas of high carbon content, we have
also found mineralized features that strongly resemble
organic remains (Furnes et al., 2005).
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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 predominantly basaltic and komatiitic extrusive (pillow lavas,
sheet flows, and minor hyaloclastite breccias) and intrusive rocks. These rocks are generally well preserved,
locally little-deformed, and have undergone prehnitepumpellyite 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).
Fig. 10. Granular (A) and tubular (B) textures in glassy pillow lava rims
from the Mirdita Ophiolite Complex, Albania. Abbreviations—FG:
fresh glass; GT: granular texture; T: tubular textures in zeolite; Ze:
zeolite.
3.2. Greenstone belts
Pillow lavas from two of the oldest and best preserved greenstone belts in the world, the mesoarchean
Barberton Greenstone Belt (BGB) of South Africa, and
the Pilbara Craton (PC) of Western Australia, have been
investigated for biosignals. In both the BGB and PC, the
outermost 10–20 mm of most pillows is defined by a dark
zone that represents the chilled, originally glassy rim. In
many cases part of the glassy margin spalled off during pillow growth and formed interpillow hyaloclastite.
Due to the pervasive prehnite-pumpellyite to greenschist
facies metamorphic overprint in both of these localities,
these rims now consist of extremely fine-grained chlorite
with scattered grains of quartz, epidote, and amphibole.
3.2.1. Barberton Greenstone Belt
The 3480–3220 Ma old magmatic sequences (de
Ronde and de Wit, 1994) of the BGB comprises 5–6 km
of submarine komatiitic and basaltic pillow lavas, with
interbedded sheet flows and related intrusions that are
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 succession of low-grade metamorphic, dominantly volcanic
supracrustal rocks (Pilbara Supergroup) that were
deposited in four autochthonous groups from 3.52–
3.165 Ga (Van Kranendonk, 2006; Van Kranendonk et
al., 2007) These include, from base to top, the 3.52–
3.43 Ga Warrawoona Group, the 3.42–3.31 Ga Kelly
Group, the 3.27–3.23 Ga Sulphur Springs Group, and
the 3.23–3.165 Ga Soanesville Group. Pillows and interpillow hyaloclastites were collected from the Dresser
Formation and Apex Basalt of the Warrawoona Group,
and the Euro Basalt of the Kelly Group. The pillow samples that have so far been found to display bioalteration
textures come from the lower part of the 5–8 km thick
Euro Basalt (Staudigel et al., 2006; Banerjee et al., 2007).
3.2.3. Mineralized bioalteration textures
Mineralized tubular structures from the BGB and
Euro Basalt (Pilbara Craton) pillow lavas and hyaloclastites are now 1–9 ␮m in width (averages of 4 ␮m
for the BGB samples and 2.4 ␮m for the Euro Basalt)
(Fig. 7), up to 200 ␮m in length (average ∼50 ␮m),
are infilled by extremely fine-grained titanite with some
also containing minor amounts of chlorite and quartz
(Figs. 11 and 12). These structures are observed to
extend away from healed fractures and/or grain boundaries along which seawater may once have flowed. Some
of these tubular structures exhibit segmentation into subspherical bodies caused by chlorite overgrowths formed
during metamorphism (Figs. 11 and 12). In addition to
H. Furnes et al. / Precambrian Research 158 (2007) 156–176
Fig. 11. (A) Tubular textures in interpillow hyaloclastites from the
Hooggenoeg Formation of the Barberton Greenstone Belt, South
Africa. The black zone (consisting of titanite) across the middle part
of the picture, and in which tubular structures are rooted, marks the
healed boundary between originally glassy fragments. The fine grained
green mineral is chlorite, and the white, stubby mineral grains below
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.
167
Fig. 12. (A) Tubular textures in interpillow hyaloclastites from the
Euro Basalt of the Pilbara Supergroup, Western Australia. Black patchy
zones of titanite mark the healed boundary between originally glassy
fragments, in which tubular structures are rooted. The fine grained
green mineral is chlorite, and the white to light brownish mineral is
calcite. The boxed areas (B and C) show the positions of the enlarged
pictures. Note the segmented structure of all the tubules.
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H. Furnes et al. / Precambrian Research 158 (2007) 156–176
the morphological similarities between the ancient structures and modern tubular bioalteration textures, C, N
and P enrichments are localized in the linings of the
Archean tubular structures. X-ray element maps of these
linings are given in Fig. 3 of Furnes et al. (2004) for
the Hooggenoeg examples, and Fig. 2 of Banerjee et
al. (2007) from the Euro Basalt examples. By analogy
to modern examples, these enrichments in biologically
significant elements, which coincide with the microtube
margins have been taken to represent decayed biological
remains.
The tubular structures occur within apparently narrow windows of these thick Archean submarine lava
sequences. However, when they do occur they are generally present in dense populations. Based on their
similarity to textures observed in recent glassy pillow
basalts we interpret these structures to represent ancient
mineralized traces of microbial activity formed during
biogenic etching of the originally glassy pillow rims and
hyaloclastites as microbes colonized the glass surface,
i.e. that they were originally hollow tubular structures.
In contrast to bioalteration textures from in situ
oceanic crust where granular textures dominate (Fig. 8),
those from investigated Archean greenstone belts are
largely titanite-filled tubular textures (Figs. 11 and 12).
(Although, one possible occurrence of putative granular
textures has been identified along fractures in originally
glassy fragments of hyaloclastites from the Hoeggenoeg Formation, see Fig. 6 of Banerjee et al., 2006a).
This predominance of the tubular alteration textures in
Archean lavas may be attributed to the masking of the
typically smaller granular textures by titanite crystallization. Whereas conversely, the early precipitation of
titanite to infill many of the larger tubular texture may
have enhanced their preservation by limiting morphological changes caused by recrystallization of the host
rock (Figs. 11 and 12).
4. Establishing antiquity and biogenicity
Several lists of criteria have been proposed for
establishing the biogenicity of ancient microfossil and
stromatolite remains (see for example Buick et al.,
1981; Schopf and Walter, 1983; Brasier et al., 2005).
These have provided a useful framework for discussions
surrounding the early fossil record and have also generated much controversy (see Rose et al., 2006). To date
there have been only preliminary attempts to outline a
comparable list of biogenicity criteria for bioalteration
textures in volcanic rocks; see for example McLoughlin
et al. (2007). Here, we draw upon studies of modern
and ancient endolithic organisms in volcanic glasses,
carbonates and sandstones to propose the following criteria for investigating volcanic bioalteration textures:
(1) an outcrop to thin-section scale geological context
that demonstrates the syngenicity and antiquity of the
bioalteration textures; (2) morphologies and distribution of the bioalteration textures that are consistent with
biogenic behaviour; (3) geochemical evidence that is
suggestive of biological processing.
To demonstrate that candidate bioalteration textures
are syngenetic with the volcanic substrate and are not
later contaminants relies upon fabric relationships. At
the outcrop scale this involves mapping to show that
the phases containing the bioalteration textures are syneruptive and not younger veins or dyke filling phases.
At the thin-section scale the bioalteration textures themselves should also be seen to predate cross-cutting
fractures, veins and cements. They should be concentrated along paths of early fluid migration and/or
weaknesses in the glass and occur as asymmetric masses
across fractures that are distinct from symmetric, abiotic
palagonite alteration fronts (see Fig. 13A). Further support for their antiquity can be gained from the infilling
mineral and/or organic phases that should have experienced degrees of metamorphism comparable to the host
rock. For example, in Archean greenschist facies terranes
one might hope to find bioalteration textures infilled with
chlorite and graphite bearing phases. A new approach
that may enable confirmation of such relative age estimates is the direct dating of titanite phases which infill
the bioalteration textures, see Section 6.2 and Banerjee
et al. (2007).
The range of morphologies displayed by candidate
bioalteration textures in ophiolites and greenstones is
more restricted than that seen in recent volcanic glasses,
none-the-less there are number of useful morphological
indicators and distribution patterns that can be sought
to test their biogenicity. For instance, many examples
of ancient tubular bioalteration show branching patterns
and sharp changes in direction when they encounter
another tube or fracture and both of these features are difficult to explain by purely abiotic alteration (cf. Fig. 4).
The size range of these ancient bioalteration textures
(see Fig. 6) is consistent with microbial involvement,
but is not a strong criterion for inferring their biogenicity
and probably reflects significant taphonomic modification. The distribution and abundance of bioalteration
textures, especially their concentration around vesicles
and varioles (see Fig. 5), is also suggestive of biological behaviour and suggests the selection of sites with
localised chemical gradients or concentrated strain that
provides weakness in the glass. The key controls on their
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 microcavities 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
primary circulating fluids may exert a strong control on
their distribution. For comparison, it has been observed
that morphologically similar microtubular structures in
Archean sandstones preferentially occur in clasts rich in
metal inclusions (Brasier et al., 2006; Wacey et al., 2006)
and this type of apparent substrate selection may yet be
found between volcanic clasts of varying composition.
Fine-scale geochemical analyses of the phases that
infill candidate bioalteration textures provide our third
approach for testing their biogenicity. Thin (less than
1 ␮m wide) linings of C, N, and P have been detected
within modern and ancient bioalteration textures and are
interpreted to represent preserved organic matter (e.g.
Giovannoni et al., 1996; Furnes and Muehlenbachs,
2003). Ideally, accompanying depletions in metabolically significant elements in the surrounding rock matrix
would provide further support for their biogenicity.
For example, depletions in Mg, Fe, Ca, and Na have
been described around bioalteration textures from in
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H. Furnes et al. / Precambrian Research 158 (2007) 156–176
situ oceanic crust (Alt and Mata, 2000). This type of
elemental data may be further strengthened by carbon
isotopic measurements on phases associated with the
bioalteration textures. For instance, it is reported that
disseminated carbonate preserved in altered pillow basalt
rims is 13 C-poor, typically between +3.9‰ and −16.4‰
compared to carbonate within the unaltered pillow interiors, which has δ13 C values of +0.7‰ to −6.9‰ that
are similar to mantle values. This much greater range
exhibited in the pillow rims is interpreted to reflect the
microbial oxidation of organic matter that gives the more
negative values and perhaps the loss of 12 C-enriched
methane from Archea to give the more positive values
(e.g. Fig. 9 in Furnes et al., 2002b; Banerjee et al., 2006a,
and references therein). Corroboration of these findings
may be possible by direct in situ analysis of carbon
within the bioalteration textures themselves using nanoscale secondary ion mass spectrometry (NanoSIMS;
cf. Kilburn et al., 2005). We remind the reader that all
such analyses need to be conducted in conjunction with
petrographic studies to confirm that the phases analysed
are syngenetic and not the result of later water–rock
interaction.
5. Model of bioalteration
Microbial colonization is known to produce pits and
channels during etching of basaltic glass. This process
has been known since Mellor (1922) described church
glasses with surface pitting at locations where lichens
grew (see Krumbein et al., 1991 for review). The first
description of etching of natural glasses by microorganisms was by Ross and Fisher (1986). However, no
convincing mechanism for how microbes may actually
facilitate glass dissolution was provided. Later, in a
study of subglacial hyaloclastites in Iceland the presence of bacteria hosted within basaltic glass aleration
textures was reported (Thorseth et al., 1992). On this
basis Thorseth et al. (1992) suggested that microbes
may cause local variations in pH and/or secrete ligands that allow them to chemically “drill” into a silicate
substrate. This process was subsequently verified experimentally by Thorseth et al. (1995b) and Staudigel et al.
(1995, 1998) who demonstrated that glass alteration was
enhanced in the presence of microbes.
During microbially driven glass dissolution the total
surface area of fresh glass available will progressively
increase as the process proceeds. Staudigel et al. (2004)
calculated that the fresh surface area would increase by
factors of 2.4 and 200 during the formation of tubular
and granular textures, respectively. In contrast, abiotic
alteration causes the surface area of fresh glass to pro-
gressively decrease (Fig. 13A). In some samples we see
that these processes are not mutually exclusive with the
typical yellow to brown, smooth palagonite zones adjacent to fractures in the core of the alteration zone forming
the first alteration product, which is then overtaken by
granular and tubular alteration textures that occur on
the outer edges adjacent to the fresh glass. Hence, from
the numerous samples we have investigated, it is clear
that the conditions under which bioalteration takes place,
allows for a faster dissolution of the glass than the formation of abiotically-formed palagonite.
Based on the textural characteristics shown in
Figs. 1–12 we present a biotic alteration model for
basaltic glass as shown in Fig. 13B and C. In this model
the glass is congruently dissolved by chemical etching caused by microorganisms. The important roles of
microorganisms and biofilms in the breakdown and dissolution of minerals and glass is well-established (e.g.
Welch et al., 1999; Brehm et al., 2005). The sizes of the
alteration textures are of the same order as the size range
of candidate microbes. The size variations (Figs. 6 and 7)
show log-normal distributions, a common phenomenon
observed in biological systems (van Dover et al., 2003).
A similar model is here advanced to that proposed by
Thorseth et al. (1992) for alteration structures found in
the 6–7 mm thick, light-exposed part of subglacial Icelandic hyaloclastites, a phenomenon that was attributed
to light-dependent cryptroendolithic cyanobacteria creating a highly alkaline micro-environment which caused
the etching. Here, we extend the model of Thorseth et al.
(1992) to the deep biosphere.
The granular and tubular structures that formed during bioalteration of the fresh glass of pillow rims are
commonly filled with authigenic minerals, even in young
pillow lavas. Subsequent to the congruent dissolution of
the glass adjacent to the microorganisms, some of the
chemical components precipitate on the cavity walls.
Detailed studies of the authigenic minerals indicate that
they are primarily clay minerals, Fe-hydroxides, zeolites
and titanite (Storrie-Lombardi and Fisk, 2004; Furnes
and Muehlenbachs, 2003; Banerjee et al., 2006a). As
long as water continues to flow through the cavities
removing waste products and perhaps also supplying
nutrients then the bioalteration will proceed, providing
also that the ambient temperature is sufficiently low for
life to exist. When the voids are completely sealed the
bioalteration process will cease (Fig. 13B and C at t4 ).
The biogenicity of structures claimed to represent fossilized microorganisms, of which filamentous features
in the 3460 Ma Apex chert of the Pilbara Craton are a
prime example (Schopf, 1993; Schopf et al., 2002), have
been questioned (Brasier et al., 2002, 2005) and heav-
H. Furnes et al. / Precambrian Research 158 (2007) 156–176
ily debated (see Dalton, 2002). Recent experiments have
also generated filamentous microfossil-like structures,
strikingly similar to those of the Apex chert, by abiotic
mechanisms (Garcia-Ruiz et al., 2003). Hence, morphology alone may not be sufficient to argue for a biogenic
origin of microbe-looking structures. In light of this
discussion there is, however, a fundamental difference
between the structures claimed to be fossilized microorganisms in sedimentary rocks and the granular and
tubular bioalteration textures presented here. These alteration textures are micro-tracefossils in volcanic rocks;
i.e. originally hollow traces that were produced as a
result of microbial etching of the original glass and that
have a higher preservation potential than the constructing organisms. Once a hollow tube has been produced in
the glass and subsequently filled with secondary minerals, it may, in the absence of penetrative deformation and
high-grade metamorphism, be preserved for billions of
years. The organisms that produced the tube, on the other
hand, will easily decay, and only traces of their chemical
components may remain associated with the structures.
In this context, it should be mentioned that somewhat similar looking structures to those presented above
have been observed in Precambrian organic-rich cherts
(e.g. Gruner, 1923). The formation of these structures
was attributed to a process in which gas produced by
the metamorphic heating and/or decay of organic matter
drives tiny mineral grains through the rock matrix that
act as ‘millstones’ producing the hollow tubular structures (Tyler and Barghoorn, 1963; Knoll and Barghoorn,
1974). These so-called ambient inclusion trails (AITs)
are most commonly found in microcrystalline cherts and
phosphorites and may still contain the terminal crystal
and sometimes, longitudinal striae along the tube margins. The tubular textures presented here are all hosted
in formerly glassy rocks, but do not contain traces of
mineral grains at their tips that could have acted as
a millstone. Also, the commonly developed budding
observed along the stem of the tubes would require several millstones within the same tube. In no way can we
see that the textures we report from volcanic glasses,
especially the granular textures, are compatible with a
pressure solution mechanism and hence we refute this
mechanism to account for the generation of the tubular structures presented here (see also Banerjee et al.,
2006b; Brasier et al., 2006). Further, the enrichment of
typical bio-elements, such as carbon, nitrogen, phosphorous, sulphur and potassium associated with the granular
and tubular structures, as well as low (13 C values, are all
signatures that strongly indicate a microbiological origin of these textures, and would be extremely difficult
to account for by an abiotic AIT mechanism. Lastly, it
171
is difficult to explain how the elevated pore fluid pressure necessary to generate AITs concentrated at volcanic
clast margins, could be maintained within the complex
and partially open network of fractures found in modern
oceanic crust.
6. Significance of textural evidence for microbial
alteration as a biomarker
Presently, we are not aware of any feasible abiotic
mechanism that can explain the origin of the granular and tubular textures described herein. Rather, their
size distribution, morphological features and inferred
growth patterns, as well as the concentration of biologically significant elements (e.g. Furnes et al., 1999,
2001a; Banerjee and Muehlenbachs, 2003; Banerjee et
al., 2006a) and associated C-isotope characteristics (e.g.
Furnes et al., 2001b, 2002a, 2004, 2005; Banerjee and
Muehlenbachs, 2003; Banerjee et al., 2006a) are all suggestive of a microbiological origin. This has a number of
important implications for the mapping of bioalteration
in submarine, formerly glassy volcanic rocks throughout
the terrestrial rock record and provides a new signature
for the search for life beyond earth.
6.1. Mapping the deep biosphere in oceanic crust
In the in situ oceanic crust where fresh glass is still
commonly present, the granular and tubular textures can
be very easily observed by ordinary light microscopy.
The dark, commonly mineral-filled structures (Figs. 1–5)
appear in strong contrast to the light yellowish-brown,
isotropic glass. This makes it possible to identify and
quantitatively estimate the extent of bioalteration as a
function of depth by reinvestigating DSDP/ODP cores.
The only study of this kind to date is that of Furnes
and Staudigel (1999), which documented the relative
proportions of alteration types in basaltic glass as a function of depth and temperature (Fig. 8) to ∼500 m into
the volcanic basement of the oceanic crust. This study
(op.cit.) showed that the tubular and granular bioalteration is dominant (up to 80%) of the total alteration
in the upper 300 m of the volcanic pile. Whereas the
granular alteration type occurs throughout the upper
∼500 m, the tubular type has only been found in the
upper ∼300 m, has its maximal occurrence at a depth
range of ∼100–200 m at a present temperature range
of ∼65–75 ◦ C (Fig. 8). However, there are still many
unanswered questions related to the controls upon the
distribution of bioalteration with depth. Important geological factors will include the permeability at a give
place and a given time and hence the flux of nutrients, and
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H. Furnes et al. / Precambrian Research 158 (2007) 156–176
further, the nature of the environment (reducing or oxidizing). There is a wealth of drill core samples of pillow
lavas from in situ oceanic crust yet to be investigated for
bioalteration.
6.2. Tracing back the record of microbial
colonization of volcanic rocks
Building on the work reviewed herein we can contribute to intriguing questions about the early signs of
life on Earth. When did microbial colonization of volcanic rocks begin, and do we see the same bioalteration
patterns in pillow lavas of the ancient oceanic crust as in
the young, in situ oceanic crust?
As to the first question it has been convincingly
demonstrated that 3.3–3.5 Ga bioalteration textures do
occur in the submarine, originally glassy rocks (pillow
lavas and hyaloclastites) of the Barberton Greenstone
Belt (Furnes et al., 2004; Banerjee et al., 2006a) and the
Euro Basalt of the Pilbara Craton (Banerjee et al., 2007).
However, these submarine volcanic rocks are not the
world’s oldest, and the petrographic search for bioalteration textures or other biotraces in even older pillow lavas
(e.g. the >3.8 Ga pillow lavas of the Isua supracrustal
belt, Greenland) may contribute to the ongoing debate
about the earliest signs of life on Earth.
Concerning the age constraints upon the formation of
biotextures, a crucial question is: when did they form?
In some cases, as for example the above-mentioned pillow lavas from the Barberton Greenstone Belt, it can be
demonstrated that metamorphic mineral assemblages of
know age (3486 Ma by 40 Ar/39 Ar dating of amphibole;
Lopez-Martinez et al., 1992) have overgrown the tubular biotextures, thus providing a minimum age for their
formation. This age overlaps with the igneous U/Pb ages
(3482 Ma) of the complex (de Ronde and de Wit, 1994;
Dann, 2000) and suggests that bioalteration occurred
very soon after eruption. The petrographic relationships
between the tubular textures and the metamorphic minerals are not however, always obvious and ubiquitous, thus
leaving an uncertainty as to the age of their formation.
Another avenue is provided by titanite which partially
or completely infills the granular and tubular textures
found in ancient pillow lavas (and to some extent in the
young examples). Direct dating of the titanite tubules
in an interpillow hyaloclastite sample from the 3350 Ma
Euro Basalt of the Pilbara Craton by in situ laser ablation
multi-collector-ICP-MS in thin sections yielded a minimum age of 2921 ± 110 Ma (Banerjee et al., 2007). This
age corresponds with regional metamorphism and intrusive activity in the region (Van Kranendonk et al., 2002,
2004). The dating of the titanite thus yields a metamor-
phic age, giving a minimum age of the formation of the
tubular structures. Even with the high age uncertainty
this date clearly shows that the tubules are of Archean
age. Our present knowledge on colonization of microorganisms on glass surfaces (e.g. Thorseth et al., 1995b,
2001) would indicate that the bio-corrosion that resulted
in the cavities (see Fig. 13C) formed shortly after eruption, during cooling, and early burial diagenesis of the
volcanic pile. Since the granular and tubular textures of
the ancient pillow lavas commonly are filled with titanite,
this dating technique holds great potential for providing
minimum age estimates of bioalteration.
6.3. Mapping the pattern of early bioalteration
Textural biosignals in the formerly glassy rims of pillow basalts have so far been found in selected units in the
thick volcanic successions of the Barberton Greenstone
Belt and East Pilbara Terrane. Since the bio-signals that
we look for have been created by viable microorganisms,
they can only develop when the ambient temperature
that allows life is below ∼113 ◦ C (Stetter et al., 1990).
It is therefore unlikely that bioalteration occurs continuously throughout such thick volcanic sequences, some
of which were probably deposited relatively quickly and
buried at temperatures that were too high for life to
exist. For example, the biotextures found in the interpillow hyaloclastite from the Euro Basalt (Fig. 12) are
from the lower part of the 5–8 km thick basalt succession. The basal flows of the Euro Basalt are dated at
3350 ± 2 Ma, whereas a thin felsic volcaniclastic unit
in the upper part of the formation has yielded an age
of 3346 ± 6 Ma (see Fig. 2 in Van Kranendonk, 2006),
suggesting that most of this very thick volcanic succession was erupted within a few million years, probably
exceeding temperature viable for life.
6.4. Bioalteration on Mars?
Several hypotheses for life on other planetary surfaces involve endolithic organisms that may produce
bioalteration textures like those described herein (see
for example Friedmann and Koriem, 1989; McKay et al.,
1992). It has been proposed that an endolithic mode of
life may be best adapted to the intense UV flux, absence
of liquid water and freezing temperatures that exist today
on Mars and have for much of Martian history. The observation of palagonite-like material on Mars (see Bishop
et al., 2002) is significant because it suggests extended
exposure of basalts to water on the Martian surface. We
know that palagonitization of basaltic glass on Earth proceeds by both biotic and abiotic mechanisms, and that
H. Furnes et al. / Precambrian Research 158 (2007) 156–176
glass altering microbes may have existed on Earth since
at least Mesoarchean (Furnes et al., 2004; Banerjee et
al., 2006b; Staudigel et al., 2006). The possibility of
subaqueous basaltic glass alteration on early Mars both
by biotic and abiotic processes has been suggested by
Banerjee et al. (2004a,b, 2006b) amongst others. The
recent discovery in the Nakhla meteorite of carbonaceous vein filling material with tubular and bleb shaped
microstructures that are similar to terrestrial bioalteration textures have renewed interest in these hypotheses
(McKay et al., 2006; Gibson et al., 2006). In addition,
microtubular weathering channels in olivines and pyroxenes from this same class of meteorite have also recently
been described (Fisk et al., 2006). To establish a Martian
age for these microstructures these authors are seeking
the same types of fabric relationships as those described
herein, particularly with reference to the fusion crusts
formed during transport of the meteorite. Furthermore,
to test their biogenicity, the morphologies, chemical
and isotopic composition of these micro-textures are
being documented in attempts to eliminate an origin
from impact related processes that caused fracturing and
alteration of the Nakhla meteorite. Much remains to be
learned before it will be possible to unambiguously identify bioalteration textures in meteoritic samples.
173
number of reasons, including: (1) mapping of the deep
oceanic biosphere; (2) tracing the earliest microbial
colonization of volcanic rocks and thus adding to our
understanding of the origin of early life on Earth; (3)
mapping the pattern of volcanic bioalteration which we
may one day be able to use as a proxy for oceanic crustal
conditions; and (4) providing a new search image for
life in extraterrestrial rocks.
Acknowledgements
Financial support to carry out this study was provided
by the Norwegian Research Council, the National Sciences and Engineering Research Council of Canada, the
US National Science Foundation, the Agouron Institute,
the National Research Foundation of South Africa, and
the Geological Survey of Western Australia. We thank
Fred Daniel of Nkomazi Wilderness for hospitality and
the Mpumalanga Parks Board for access during field
work in the Barberton Mountain Land of South Africa.
This work has greatly benefited from the constructive
comments of two anonymous reviewers. Jane Ellingsen
kindly helped with the illustrations. This paper is published with the permission of the Executive Director of
the Geological Survey of Western Australia.
7. Conclusions
Bioalteration textures in the glassy margin of pillow
lavas and hyaloclastites are produced by biologically
driven dissolution of volcanic glass. This process is
widespread within in situ oceanic crust of any age
and produces granular and tubular textures that are
invariably rooted on surfaces where water was available
(in fractures, vesicles and on the edges of fragments),
and appear as individuals, or, more commonly, as a
myriad of coalesced bodies. Their size, form, and
irregular growth patterns along fractures are all features
compatible with microbially mediated dissolution. This
supposition is supported by their common association
with enrichments in biologically significant elements
such as carbon, nitrogen, phosphorous and sulfur, as
well as the carbon-isotope signatures of the altered
material in which they occur.
Comparable mineralized alteration textures comprising titanite-filled tubes have been identified in the
original glassy part (now greenschist to amphibolite
mineral assemblages) of ancient, submarine pillow lavas
and hyaloclastites from ophiolites and greenstone belts,
for which we attribute a similar biogenic origin. These
bio-generated textures have so far been detected as far
back as 3.5 billion years, and are most important for a
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