Earth and Planetary Science Letters 184 (2001) 523^528
www.elsevier.com/locate/epsl
Direct evidence of late Archean to early Proterozoic anoxic
atmosphere from a product of 2.5 Ga old weathering
Takashi Murakami a; *, Satoshi Utsunomiya a , Yoji Imazu a , Nirankar Prasad b
a
Department of Earth and Planetary Science, Graduate School of Science, The University of Tokyo, Hongo (Science Bldg. 5),
Bunkyo-ku, Tokyo 113-0033, Japan
b
Geological Survey of Canada, 601 Booth St., Ottawa, Ont., Canada K1A 0E8
Received 22 December 1999; accepted 8 November 2000
Abstract
Because Precambrian paleosols (ancient soils formed by weathering) are usually subjected to later alteration, the
evidence gleaned from chemical studies has provided inconclusive evidence on the atmospheric O2 evolution. In a 2.6^
2.45 Ga paleosol developed on Archean granite near Pronto mine, Canada, we found that Ce-rich rhabdophane formed
directly during weathering, replaced primary apatite, and has survived for about 2.5 Ga because of its low solubility and
high resistance to heat. Our data show that La, Ce, and Nd behaved similarly in both rocks and rhabdophane, i.e., most
Ce existed as Ce3‡ in the weathering solution, unlike the younger weathering profiles where Ce3‡ oxidizes and forms
cerianite, CeO2 . The presence of rhabdophane with Ce3‡ throughout the Pronto paleosol provides compelling evidence
of an anoxic atmosphere 2.6^2.45 Ga ago. Because apatite is a common accessory mineral in granitic rocks, Ce content
of the replaced rhabdophane can be a useful indicator for tracing O2 evolution in the Precambrian. ß 2001 Elsevier
Science B.V. All rights reserved.
Keywords: paleoatmosphere; weathering; rhabdophane; Paleosols; Precambrian
1. Introduction
There still remain many uncertainties about
how Earth's atmosphere evolved [1^7]. The evolution of CO2 and O2 levels in the Precambrian
has been of major concern for the past 30 years
* Corresponding author. Fax: +81-3-3816 5714;
E-mail: [email protected]
because of the close relationship of this evolution
to chemical reactions at the Earth's surface, formation of ore deposits, climate, biological activity, human activity and future, and even life on
other planets [6]. In the past, the evidence of O2
evolution in the atmosphere has been derived primarily from rock chemistry and ¢eld observations, e.g., the presence or absence of detrital uraninite, pyrite, redbeds, banded iron formations,
type of biological activity [1,8]. Weathering is a
near-surface process and occurs in contact with
the atmosphere in the past as well as in recent
times. Consequently the mineralogy and chemistry of weathering products re£ect the nature of
0012-821X / 01 / $ ^ see front matter ß 2001 Elsevier Science B.V. All rights reserved.
PII: S 0 0 1 2 - 8 2 1 X ( 0 0 ) 0 0 3 4 4 - 7
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T. Murakami et al. / Earth and Planetary Science Letters 184 (2001) 523^528
the atmosphere at the time of weathering. Paleosols (ancient soils formed by weathering) are
therefore potential indicators of early atmospheric
O2 evolution. However, the original weathering
information of paleosols can be lost due to postweathering episodes of diagenesis and metamorphism. The loss or modi¢cation of the original
information usually leads to inconsistent conclusions regarding O2 evolution (e.g., [2^4,6,7,9,10]).
Such inconsistent conclusions have resulted
mainly from the interpretation of whole-rock
analyses of paleosols which have been altered
since their formation. Secondary weathering minerals in paleosols, if not altered by later changes,
can provide de¢nitive criteria of the atmospheric
conditions, because such secondary minerals re£ect the weathering conditions directly. We here
report the ¢rst ¢nding of a direct product of late
Archean to early Proterozoic weathering, and discuss its implication for the evolution of atmospheric oxygen.
2. Geology and mineralogy
The paleosol at Pronto mine, Ontario, Canada
(46³12.5P N, 82³42.5P W) was formed between 2.6
and 2.45 Ga, occurring between the underlying
Archean granite and the overlying Paleoproterozoic Matinenda Formation. This age constraint is
based on the ages, determined elsewhere in the
Huronian basin, of the Archean granite and the
overlying volcanic rocks, which are considered to
be coeval with the Matinenda Formation [7,10^
14]. The paleosol was later subjected to very lowgrade metamorphism [11,15]. Previous studies of
the Pronto paleosols, exposed in the old Pronto
mine and in nearby areas, were mainly based on
their bulk chemistry [1,11,15^18]. The color and
texture of the rock and the absence of sedimentary structures indicate that the pro¢le is a residual
deposit formed by weathering of the parent granite [11]. The constituent minerals in the parent
granite are quartz, microcline, plagioclase (largely
altered to sericite), and biotite (largely altered to
chlorite) with accessory apatite and zircon. The
paleosol consists mainly of quartz, sericite and
minor microcline and chlorite.
3. Samples and experimental methods
We collected intensely weathered, moderately
weathered, and relatively fresh unaltered samples
from the Pronto paleosol zone, which were located 6 1, 10, and 12 m below the unconformity
(samples 206, PR10 and 200A respectively). The
intensity of weathering of the samples was qualitatively determined based on their mineral constituents and on the chemical composition of the
samples.
Backscattered electron imaging as well as energy dispersive X-ray spectrometry (EDX) were
applied to thin sections of the samples to obtain
mineralogical and chemical data of the samples
using a ¢eld-emission scanning electron microscope (FESEM) equipped with EDX. Transmission electron microscopy (TEM) and analytical
electron microscopy were employed for further
mineralogical investigations. TEM specimens
were made by Ar ion milling.
The La, Ce, and Nd concentrations of the rock
samples were determined by inductively coupled
plasma mass spectrometry, and those of mineral
grains by FESEM^EDX. Semi-quantitative analysis of mineral grains was used to analyze La, Ce,
Nd, and P to estimate the relative abundances of
La, Ce, and Nd. Praseodymium was ignored because of its low concentration.
4. Results and discussion
In the intensely weathered sample we found at
the rim of apatite (Ca-hydroxy, £uor phosphate),
a mineral species with light rare earths (La, Ce,
and Nd), P, and O (Fig. 1A,B) crystallographically identical to rhabdophane, (La, Ce,
Nd)PO4 WnH2 O (Fig. 1C). The mineral was identi¢ed as rhabdophane based on its chemistry and
structure. Twelve apatite grains out of 14 in a thin
section of the intensely weathered sample were
rimmed by rhabdophane; only two apatite grains
remained unaltered. There were ¢ve examples
where rhabdophane also occurred as a ring surrounding sericite (Fig. 1D); the formation of this
texture is explained below. In a thin section of the
moderately weathered sample, rhabdophane was
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525
Fig. 1. Texture and chemistry of rhabdophane. A: Backscattered electron image of rhabdophane (R) replacing apatite (Ap) in an
intensely weathered sample (sample 206). The brightest contrast corresponds to rhabdophane formed at the rim of an apatite
grain. B: EDX spectrum of the rhabdophane of A. La, Ce, and Nd are obviously present, and possibly a small amount of Pr.
C: Selected area electron di¡raction pattern of rhabdophane at the rim of apatite by TEM. The crystallographic axes are given
in the ¢gure. Rhabdophane was identi¢ed by its chemistry and di¡raction pattern. The crystallographic data of rhabdophane are
based on Mooney [31]. D: Backscattered electron image of rhabdophane (R) in a sericite (S) matrix in an intensely weathered
sample (sample 206). E: Backscattered electron image showing rhabdophane (R) formed at only part of the rim of an apatite
grain (Ap) in a moderately weathered sample (sample PR10). F: Backscattered electron image showing that rhabdophane was
not formed at the rim of an apatite grain (Ap) in a relatively fresh unaltered sample (sample 200A). The other abbreviations are:
Ab, albite; Kf, K-feldspar; and Q, quartz. The scale bars denote 10 Wm.
found only as a part of the rim of apatite (Fig.
1E); we found three examples with this texture,
but 12 apatite grains remained unaltered. In contrast, all of 10 apatite grains in the fresh sample
lacked rhabdophane alteration (Fig. 1F). Thus,
the replacement of apatite by rhabdophane did
not take place in fresh granite, but increased in
amount with the intensity of weathering.
The solubility product of rhabdophane is quite
low (e.g., log K0 = 324.5 for LaPO4 WnH2 O at
25³C) [19], and apatite can be a substrate for reaction with dissolved rare earth elements (REE)
to form rhabdophane [19]. The modern weathering of granite, syenite and gneiss also produces
rhabdophane in both tropical and temperate
zones [20^24]. In agreement with our observations, Ban¢eld and Eggleton [24] showed the formation of rhabdophane as a result of granite
weathering by the replacement of apatite rims.
Their rhabdophane, however, did not contain
Ce. They concluded that the formation of rhabdophane by replacement of apatite results from
weathering and not hydrothermal alteration.
These studies show that rhabdophane can easily
be formed during weathering, if a phosphate substrate such as apatite and a light REE source are
present. Rhabdophane-type minerals lose water at
190^250³C, are rehydrated in a day, and persist
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T. Murakami et al. / Earth and Planetary Science Letters 184 (2001) 523^528
up to 500³C [25]. Very low-grade metamorphism
with maximum temperatures far below 500³C
such as at Pronto [11,15] cannot change the rhabdophane structure. Our studies, therefore,
strongly suggest that rhabdophane in the Pronto
paleosol is a product of 2.6^2.45 Ga old weathering. It survived post-weathering diagenesis and
metamorphism, and has retained its original form
and composition. This is signi¢cant, because secondary weathering products provide much more
accurate information on the nature of weathering
and of the atmosphere rather than the bulk rock
chemistry, which can be altered much more readily during later diagenetic and metamorphic processes [15], and thus lead to inconsistent conclusions regarding atmospheric evolution (e.g.,
[9,26,27]). The fact that the rhabdophane was a
product of 2.6^2.45 Ga old weathering is shown
by the texture in Fig. 1D. While the rhabdophane
at the rim of an apatite grain was formed during
weathering, the unweathered apatite core was
subsequently replaced by sericite during postweathering alteration. Rhabdophane, however, remained unaltered because of its lower solubility
[19] and high resistance to heat [25].
Rhabdophane is strongly or in some case moderately depleted in Ce compared with that in
whole rocks in modern, oxidizing weathering pro¢les [20^24]. Ban¢eld and Eggleton [24] suggested
that the strong fractionation of Ce during weathering of the Bemboka granodiorite is due to the
oxidation of Ce3‡ to Ce4‡ , and the subsequent
precipitation of ¢nely crystalline material under
oxidizing conditions, rather than adsorption
onto clays as previously reported (e.g., [28]).
This has been con¢rmed by Braun et al. [20,21],
who found both Ce-poor rhabdophane and cerianite (Ce4‡ O2 ) in laterite. Thus, modern, oxidizing weathering results in the precipitation of Cefree or Ce-poor rhabdophane and cerianite, with
a di¡erence in Ce fractionation between rhabdophane and weathered rocks. In contrast, the relative abundance of La, Ce, and Nd in rhabdophane grains in the intensely weathered Pronto
sample reveals that the rhabdophane was rich in
Ce (Fig. 2A). The concentrations of Ce and the
other light REEs were similar in rhabdophane
and in the whole rock, and these elements were
rather constant throughout the weathering pro¢le
(Fig. 2B). We did not ¢nd any Ce-bearing minerals without La and Nd such as cerianite in the
weathered samples. In addition to the Ce abundance in rhabdophane, no anomalously high or
low Ce concentrations were observed in the
present study (Fig. 2C). The comparison of rhabdophane composition and light REE abundances
in an ancient weathering pro¢le with those in
modern, oxidizing weathering pro¢les indicates
that the light REE in the Pronto pro¢le originated
from the parent Archean Pronto granite. The fundamental di¡erence in the Pronto case is that the
Ce3‡ remained unoxidized and hence was incorporated in rhabdophane grains. This was possible
only if the weathering solution was anoxic. Modern, oxidizing pro¢les, however, contain ample O2
to oxidize Ce3‡ to Ce4‡ and to form cerianite.
Taking into account the thermodynamic equilibrium between Ce3‡ and cerianite [29], we provide
direct evidence for the presence of O2 -de¢cient
solutions even in the uppermost Pronto weather-
Fig. 2. Chemical characteristics of rock samples and rhabdophane. A: Relative abundances of La, Ce, and Nd in three rhabdophane grains of sample 206 based on a La concentration in each grain. B: Relative abundances of La, Ce, and Nd in the three
rock samples based on a La concentration in each sample. C: C1-chondrite-normalized [32] La, Ce, and Nd pattern of the rock
samples. Circle, sample 200A; square, PR10; diamond, 206.
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T. Murakami et al. / Earth and Planetary Science Letters 184 (2001) 523^528
ing pro¢le, indicating an anoxic atmosphere during 2.6^2.45 Ga old weathering.
Finding a product of late Archean to early Proterozoic weathering is signi¢cant because such
weathering products provide direct evidence for
the evolution of O2 in the atmosphere. Although
the presence of Ce3‡ in the groundwater does not
unequivocally determine the concentration of atmospheric O2 , the combination with other weathering products such as greenalite [30], and/or with
data for the bulk chemistry of constituents such
as Fe2‡ at Pronto [11] enable us to determine
more accurately the O2 concentrations in the Precambrian atmosphere [7,8]. Apatite is a common
accessory mineral in granites. Therefore, it is quite
easy to examine paleosols formed by late Archean
to early Proterozoic weathering and to check the
presence of rhabdophane and its Ce content in the
rim of apatite grains. This approach could be useful for determining the change in concentration of
atmospheric O2 in during the Proterozoic era, and
might provide a more precise estimate of the time
of inversion from a reducing to an oxidizing atmosphere.
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
Acknowledgements
We thank H. Ohmoto and M. Nedachi for useful suggestions, J.F. Ban¢eld, T. Kogure, and T.
Ohnuki for discussions, M. Hailstone of Ontario
Geological Survey for help in sampling, and N.
Yanase for the use of ICP-MS. This manuscript
was improved greatly by the comments of H.D.
Holland, J.A. Donaldson, and an anonymous reviewer. The electron microprobe analysis and
electron microscopy were performed in the Electron Microbeam Analysis Facility of the Mineralogical Institute, the University of Tokyo. This
work was partly supported by a Scienti¢c Grant
of the Ministry of Education, Science and Culture
of Japan.[EB]
[11]
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[13]
[14]
[15]
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