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\DOI{10.5802/crgeos.264}
\datereceived{2024-04-06}
\daterevised{2024-05-27}
\dateaccepted{2024-05-28}
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\begin{document}

\begin{noXML}

\CDRsetmeta{articletype}{research-article}

\title{The first application of Re--Os dating on Paleoproterozoic
Francevillian sediments (Gabon)}

\alttitle{Premi\`{e}re application de la datation Re--Os sur des
s\'{e}diments pal\'{e}oprot\'{e}rozo\"{i}ques du Francevillien (Gabon)}

\author{\firstname{Julie Ngwal'ghoubou} \lastname{Ikouanga}}
\address{University of Poitiers, UMR 7285 CNRS, IC2MP, F-86073,
Poitiers Cedex 9, France}

\author{\firstname{Laurie} \lastname{Reisberg}\CDRorcid{0000-0002-9551-7752 }}
\address{Centre de Recherches P\'{e}trographiques et G\'{e}ochimiques,
UMR 7358 CNRS - Universit\'{e} de Lorraine, F-54500
Vandoeuvre-l\`{e}s-Nancy, France}

\author{\firstname{Anne-Catherine}\nobreak \lastname{ Pierson-Wickmann}\CDRorcid{0000-0002-7715-3772}}
\address{University of Rennes, CNRS, G\'{e}osciences Rennes - UMR 6118,
F-35000 Rennes, France}

\author{\firstname{Anna} \lastname{El Khoury}\CDRorcid{0009-0003-7323-6049}}
\addressSameAs{1}{University of Poitiers, UMR 7285 CNRS, IC2MP,
F-86073, Poitiers Cedex 9, France}
\address{Nanoscopium Beamline, Synchrotron Soleil, 91192
Gif-sur-Yvette, France}

\author{\firstname{Claude} \lastname{Fontaine}\CDRorcid{0000-0001-5686-6725}}
\addressSameAs{1}{University of Poitiers, UMR 7285 CNRS, IC2MP,
F-86073, Poitiers Cedex 9, France}

\author{\firstname{Abderrazak} \lastname{El Albani}\CDRorcid{0000-0001-9598-955X}\IsCorresp}
\addressSameAs{1}{University of Poitiers, UMR 7285 CNRS, IC2MP,
F-86073, Poitiers Cedex 9, France}
\email[A. El Albani]{abder.albani@univ-poitiers.fr}

\thanks{La R\'egion Nouvelle Aquitaine, French government program
(Investissements d'Avenir, EUR INTREE, ANR-18-EURE-0010)}

\shortrunauthors

\begin{abstract}
Understanding the age of geological formations is essential to
reconstruct Earth's history. Nevertheless, dating Proterozoic
formations is a real challenge because they are often impacted by
tectonic, magmatic or metamorphic phenomena. The sedimentary sequences
of the Francevillian Basin are well preserved and have been dated
previously using many methods (U--Pb, Ar--Ar, Rb--Sr,\,{\ldots}). Here, we
applied the Re--Os dating method for the first time, specifically on the
``FB'' and ``FD'' formations containing a high organic matter (OM)
content (up to10\%). The age obtained, 2103 ${\pm}$ 11~Ma, is coherent
with the previous studies. This data confirms the unusual quality of OM
preservation and the chronology of the emergence of multicellular life
occurring during the Lomagundi event.\looseness=-1
\end{abstract}

\begin{altabstract}
Il est essentiel de comprendre l'\^{a}ge des formations g\'{e}ologiques
pour reconstituer l'histoire de la Terre. N\'{e}anmoins, la datation
des formations prot\'{e}rozo\"{i}ques est un v\'{e}ritable d\'{e}fi car
elles sont souvent affect\'{e}es par des ph\'{e}nom\`{e}nes
tectoniques, magmatiques ou m\'{e}tamorphiques. Les s\'{e}quences
s\'{e}dimentaires du bassin de Franceville sont bien pr\'{e}serv\'{e}es
et ont \'{e}t\'{e} dat\'{e}es pr\'{e}c\'{e}demment \`{a} l'aide de
nombreuses m\'{e}thodes (U--Pb, Ar--Ar, Rb--Sr,\,{\ldots}). Ici, nous avons
appliqu\'{e} la m\'{e}thode de datation Re--Os pour la premi\`{e}re
fois, sp\'{e}cifiquement sur les formations
\guillemotleft{}~FB~{}\guillemotright{} et
\guillemotleft{}~FD~{}\guillemotright{} contenant une teneur
\'{e}lev\'{e}e en mati\`{e}re organique (MO) (jusqu'\`{a} 10\%).
L'\^{a}ge obtenu, 2103 ${\pm}$ 11 Ma, est coh\'{e}rent avec les
\'{e}tudes pr\'{e}c\'{e}dentes. Ces donn\'{e}es confirment la
qualit\'{e} inhabituelle de la pr\'{e}servation de la MO et la
chronologie de l'\'{e}mergence de la vie multicellulaire survenue lors
de l'\'{e}v\'{e}nement Lomagundi. 
\end{altabstract}

\keywords{\kwd{Dating}
\kwd{Re--Os}
\kwd{Francevillian}
\kwd{Paleoproterozoic}
\kwd{Gabon}}

\altkeywords{\kwd{Datation}
\kwd{Re--Os}
\kwd{Francevillien} 
\kwd{Pal\'{e}oprot\'{e}rozo\"{i}que}
\kwd{Gabon}}

\maketitle

{\vspace*{12pt}}

\twocolumngrid

\end{noXML}

\section{Introduction}

Dating geological formations is a major challenge in the study of
ancient basins, in particular those of Precambrian age. In fact, these
basins are generally impacted by tectonic
{\citep{Caswelletal2021,DAgrella-Filhoetal2016},} magmatic
{\citep{Kabengeleetal1991}} metamorphic
{\citep{Chatiretal2022,Prochaskaetal1992},} or diagenetic phenomena,
that may severely alter isotopic signatures. Nevertheless, in some
cases, these effects are minor 
{\citep{Gabbottetal2004,OssaOssaetal2013}}, allowing the history of the
basin since its formation to be traced. 

In Gabon, the Francevillian Basin is well known for the quality of
preservation of its sedimentary formations. These host both uranium
deposits {\citep{Bankoleetal2016,Gauthier-Lafaye1986},} which include
the only known case of a natural nuclear reactor ever found in the
world  {\citep{Naudet1991,Neuillyetal1972},} and the huge manganese
deposits of Moanda  {\citep{Pambo2004,Weber1997}.} These sediments are
also renowned as the rocks hosting the oldest multicellular life forms
ever observed, which are present in an exceptional state of
conservation
{\citep{ElAlbanietal2010,ElAlbanietal2014,ElAlbanietal2023,Ikouangaetal2023}.}  


A variety of approaches have been used to date the various formations
of the Francevillian Basin\break (Figure~\ref{fig1}). The FA Formation was dated using
U--Pb analyses of uraninite and Ar--Ar analyses of illite, giving ages of
2050 ${\pm}$ 30 Ma  {\citep{Gancarz1978},} and 2040 Ma--2010 Ma 
\citep{OssaOssaetal2014}, respectively. As the FB Formation includes
the well-known manganese and uranium deposits, several dating attempts
have been attempted. The Sm--Nd method was applied on clay fractions of
the black shales, giving an age between 2099 ${\pm}$ 115 Ma and 2036
${\pm}$ 79~Ma \citep{Brosetal1992}. The age of the FB Formation was
also constrained indirectly through Rb--Sr dating of intrusive syenites,
yielding 2143  ${\pm}$ 143 Ma {\citep{Bonhommeetal1982}.} In addition,
U--Pb dating of zircons from these syenites provided ages varying
between 2027 ${\pm}$ 55 Ma  {\citep{MoussavouEdou-Minko2006}} and 2191
${\pm}$ 13 Ma {\citep{Sawakietal2017}.}   {\citet{Horieetal2005}} dated
the FD Formation using the U--Pb method of zircons from cinerites,
giving an age of 2083 ${\pm}$ 6 Ma. Similarly, U--Pb laser-MC-ICP-MS
dating of zircons from this formation yielded an age of 2072 ${\pm}$ 29
Ma  {\citep{Thieblemontetal2009}.} All of these dates show that the
Francevillian Basin was formed around 2.1 Ma ago, a period that
witnessed the maturation of organic matter associated with argilite
sedimentation {\citep{Mossmanetal2005}.}

\begin{figure}
\includegraphics{fig01}
\caption{\label{fig1}Comparison of different ages obtained on
Francevillian formations, using 1/U--Pb (a), Sm--Nd (b) on clay fractions
of black shales, Rb--Sr (c) on syenites 2/U--Pb (d) on uraninite, 
3/Ar--Ar (e) on apatites and 4/ Re--Os (f) on organic matter.}
\end{figure}

In this new study, we present, for the first time, results from Re--Os
dating of the Francevillian Formations, where the percentage of organic
matter may exceed 10\% (FB and FD Formations). Though the technique was
applied to whole rock powders, the extracted Re and Os are hosted
almost exclusively by organic material, especially when the digestion
is done using a
CrO$_{3}$--H$_{2}$SO$_{4}$ solution, as
described below. The date obtained, 2103 ${\pm}$ 11 Ma, corresponds to
the closing of the Re--Os system in organic matter at the time of the
deposition of the Francevillian sediments. Thus, the age of these
formations constitutes strong evidence of the emergence of
multicellular life forms starting at 2103 ${\pm}$ 11 Ma.

\section{Study area}

The study area is located in southwestern Gabon, in the Francevillian
Basin (Figure~\ref{fig2}). This area is\break situated more than 80 km from
the site of the Oklo natural nuclear reactor, so this unusual feature
should have no effect on the studied radiometric systems. Four
sedimentary formations of Paleoproterozoic age are described 
{\citep{AzzileyAzzibrouck1986,Weber1968}.} The FA Formation is formed
of detrital and conglomeratic sediments. The uranium mineralization is
found at the summit of this\break formation, at the boundary with the FB
Formation {\citep{Bankoleetal2016,Gauthier-Lafaye1986,Gauthier-LafayeWeber1989}.} 
The FB Formation is subdivided into two sub-groups:
FB1 and FB2 {\citep{Reynaudetal2018,Weber2016}} with the manganese
deposit found in between  {\citep{Pambo2004,Weber1997}.}  The FB1
formation is composed of green marly sediments and turbiditic pelites,
siltstones (FB1a), dolomitic and silty (FB1b) black shales, dolomites,
and black shales very rich in organic matter. The FB2 formation is
constituted of alternating pelites and sandstones (FB2a) and silty
black shales (FB2b). It is the FB2b unit that hosts the oldest
multicellular organisms 
{\citep{ElAlbanietal2010,ElAlbanietal2014,ElAlbanietal2019,Ikouangaetal2023}.} 
The FC Formation is mainly formed of cherts and massive dolomites 
{\citep{Preatetal2011,Weberetal2016}.}  The FD Formation englobes
mainly black shales of high organic matter content 
{\citep{Ngombi-Pembaetal2014,Thieblemontetal2009}} and constitutes the
summit of the sedimentary pile of the Francevillian Basin.

\begin{figure}
\vspace*{4pt}
\includegraphics{fig02}
\vspace*{4pt}
\caption{\label{fig2}Lithostratigraphic log and geological map of the
Francevillian Basin established from the sub-basin of Franceville,
showing sampling zones \citep[modified from][]{ElAlbanietal2010}.}
\vspace*{2pt}
\end{figure}

\section{Method}

Re--Os dating was performed to obtain a depositional age of the black
shales that host the Francevillian biota. A first series of six
samples, taken from a vertical section from FB1b to FD, was analysed.
About 0.2 g of powdered bulk sample was spiked with an appropriate
quantity of a mixed $^{185}$Re--$^{190}$Os
isotopic tracer and digested in a solution of inverse aqua regia (2 ml
HCl: 5 ml HNO$_{3}$) at 300~\textdegree C in a high-pressure
Asher (Anton Paar HPA-S). Osmium was extracted from the resulting
solution into Br$_2$ and purified by microdistillation, using techniques
adapted from {\citep{Bircketal1997}.} Rhenium was extracted from the
remaining solution, after drying and redissolution in 0.8 N
HNO$_{3}$, using chromatographic columns filled with anion
exchange resin (AG1 X8). In order to reduce the scatter of the results,
a series of four samples was replicated using a modified digestion
technique recommended by  {\citet{SelbyCreaser2003}.} After spiking
with the mixed isotopic tracer, samples were digested at
300~\textdegree C in 7 ml of a CrO$_{3}$ solution in 4 M
H$_{2}$SO$_{4}$. The benefit of this latter
method, relative to digestion in aqua regia, is that it is less likely
to dissolve detrital phases, thus ensuring that the measured Re and Os
are derived almost exclusively from organic matter. After sample
digestion, Os was extracted into Br$_{2}$ and purified by
microdistillation. Because of the presence of high contents of
Cr$^{6+}$, separation of Re from the residual solution
using anion exchange columns was not possible. Instead, Re was
extracted by liquid--liquid exchange into iso-amylol, as recommended by
{\citet{Bircketal1997}.}\looseness=1

The purified Os fractions were analysed as OsO$^{3-}$ ions
by negative thermal ionization mass \mbox{spectrometry} (NTIMS) 
{\citep{Creaseretal1991,Volkeningetal1991}} using a Finnigan MAT262
instrument, at the CRPG laboratory (CNRS-University of Lorraine).
Measurements were made by peak jumping in ion counting mode using an
ETP electron multiplier. Data corrections for heavy oxides, mass
fractionation and spike contribution were performed off-line, assuming
the oxygen isotope composition of Nier ($^{18}$O/$^{16}$O $=$ 0.002045
and $^{17}$O/$^{16}$O $=$ 0.0003708) and a $^{192}$Os/$^{188}$Os
normalizing ratio of 3.08271. Over the analytical period, the value of
the in-house Os standard was 0.17400 ${\pm}$ 0.00065 (82 analyses),
consistent with the long-term value of this standard from the CRPG
laboratory. Rhenium concentrations were determined by isotope dilution
calculations after measurement of the isotope compositions by MC-ICP/MS
(ThermoScientific Neptune). Mass fractionation was corrected by
standard bracketing. Total Os blanks were 0.51 ${\pm}$ 0.24 (1s) pg
during the analytical period. Total Re blanks were about 20 ${\pm}$ 16
(1s) pg for the samples digested in inverse aqua regia and 140~pg for
the blank associated with the CrVI digestions, reflecting the commonly
reported contamination of this reagent in Re. Nevertheless, as the same
amount of Cr$^{\mathrm{VI}}$ solution was added to each sample, it
was possible to correct for this\break contamination. 

\section{Results}

The Re--Os data are presented in Table~\ref{tab1}. For the three samples for
which both techniques (aqua regia and CrO$_{3}$--H$_{2}$SO$_{4}$) were
applied, the results are quite similar. Nevertheless, while the
isotopic ratios of all of the samples are roughly aligned in a Re--Os
isochron diagram (Figure~\ref{fig3}a) the samples digested in the
CrO$_{3}$--H$_{2}$SO$_{4}$ solution define a tighter correlation, in
agreement with the findings of  {\citet{SelbyCreaser2003}. As noted by
these authors, digestion in CrO}$_{3}$--{H}$_{2}${SO}$_{4}$ {solution
accesses only hydrogeneous Os hosted by organic matter, while digestion
in aqua regia may in addition liberate small amounts of Os from
detrital phases, thus adding scatter to the isochron
(}Figure~\ref{fig3}b). The well-defined correlation line obtained from
the samples digested in CrO$_{3}$--H$_2$SO$_{4}$ solution, which includes
four samples from the FB and FD Formations, indicates a Re--Os age of
2103 ${\pm}$ 11 Ma (MSWD $=$ 2.6) with an initial $^{187}$Os/$^{188}$Os
ratio close to 0.108 ${\pm}$ 0.007 (all uncertainties 2s). We note that
this age was obtained from only four samples and that the MSWD value is
higher than the value of 1 expected for true isochrons. This means that
the apparently tight limits on the age should be viewed with some
caution. Nevertheless, the value obtained places a strong constrain on
the age of the organic matter that hosts the Re and Os in these rocks.
The isotope ratios $^{187}$Os/$^{188}$Os and $^{187}$Re/$^{188}$Os are
higher in the black shales of the FB1b and FD Formations than in the
FB1c Formation.

\begin{sidewaystable*}[p!]
\vspc{}
\caption{\label{tab1}Re--Os isotopic data obtained by digestion with
Cr$^{\mathrm{VI}}$--H$_2$SO$_{4}$  and aqua regia solutions}
\tabcolsep=4pt
\fontsize{8.5}{10}\selectfont
\begin{tabular}{cccccccccccc}  
\thead
\xmorerows{1}{Sample name} 
& \multicolumn{4}{c}{{CrVI: Samples digested in CrO$_3$ dissolved in 4N H$_2$SO$_4$}} & & \multicolumn{6}{c}{{AR: Samples digested in inverse aqua regia}} \\ 
\cline{2-5}\cline{7-12}
&   BA 37-64 & BACOM-85 & BMB27 & B15 &    & BA 37-105 & BA 37-64 & BACOM-85 & BA 74-36 & BB26 & B15 \\ 
\endthead
Sample wreight (g) &  0.23570 & 0.23391 & 0.16365 & 0.14748 & & 0.20093 & 0.19992 & 0.19763 & 0.19883 & 0.20050 & 0.20242 \\ 
$^{187}$Os/$^{188}$Os &  11.471 & 0.3064 & 11.225 & 5.974 & & 7.022 & 11.481 & 0.3191 & 0.343 & 5.274 & 5.9987 \\ 
${\pm}$2s   & 0.009 & 0.0003 & 0.012 & 0.006 &  & 0.013 & 0.013 & 0.0004 & 0.0005 & 0.003 & 0.0004 \\ 
{[Os] ppb}   & 0.728 & 6.222 & 2.467 & 2.919 &  & 0.093 & 0.727 & 5.529 & 1.332 & 3.155 & 2.920 \\ 
${\pm}$2s (ppb) &   0.002 & 0.165 & 0.008 & 0.007 &  & 0.002 & 0.003 & 0.094 & 0.004 & 0.007 & 0.009 \\ 
{[$^{188}$Os]} (mole/g)  & 2.05 $\times$ 10$^{-13}$ & 4.24 $\times$ 10$^{-12}$ & 7.03 $\times$ 10$^{-13}$ & 1.16 $\times$ 10$^{-12}$ &  & 3.426 $\times$ 10$^{-14}$ & 2.05 $\times$ 10$^{-13}$ & 3.76 $\times$ 10$^{-12}$ & 9.04 $\times$ 10$^{-13}$ & 1.32 $\times$ 10$^{-12}$ & 1.15 $\times$ 10$^{-12}$ \\ 
${\pm}$2s (mole/g)  & 6.7 $\times$ 10$^{-16}$ & 1.1 $\times$ 10$^{-13}$ & 2.3 $\times$ 10$^{-15}$ & 2.6 $\times$ 10$^{-15}$ &  & 7.2 $\times$ 10$^{-16}$ & 9.2 $\times$ 10$^{-16}$ & 6.4 $\times$ 10$^{-14}$ & 2.1 $\times$ 10$^{-15}$ & 3.0 $\times$ 10$^{-15}$ & 3.6 $\times$ 10$^{-15}$ \\ 
{[Re]} (ppb)  & 19.51 & 7.04 & 65.76 & 56.25 &  & 1.93 & 19.02 & 6.30 & 1.77 & 56.53 & 57.99 \\ 
${\pm}$2s (ppb) &  0.25 & 0.14 & 0.64 & 0.37 &  & 0.16 & 0.16 & 0.16 & 0.16 & 0.16 & 0.16 \\ 
$^{187}$Re (moles/g)  & 6.56 $\times$ 10$^{-11}$ & 2.37 $\times$ 10$^{-11}$ & 2.21 $\times$ 10$^{-10}$ & 1.89 $\times$ 10$^{-10}$ &  & 6.49 $\times$ 10$^{-12}$ & 6.39 $\times$ 10$^{-11}$ & 2.12 $\times$ 10$^{-11}$ & 5.94 $\times$ 10$^{-12}$ & 1.90 $\times$ 10$^{-10}$ & 1.95 $\times$ 10$^{-10}$ \\ 
${\pm}$ 2s (mole/g)  & 8.4 $\times$ 10$^{-13}$ & 4.7 $\times$ 10$^{-13}$ & 2.2 $\times$ 10$^{-12}$ & 1.2 $\times$ 10$^{-12}$ &  & 5.38 $\times$ 10$^{-13}$ & 5.41 $\times$ 10$^{-13}$ & 5.47 $\times$ 10$^{-13}$ & 5.44 $\times$ 10$^{-13}$ & 5.41 $\times$ 10$^{-13}$ & 5.35 $\times$ 10$^{-13}$ \\ 
$^{187}$Re/$^{188}$Os  & 320.2 & 5.58 & 314.3 & 163.7 &  & 189.3 & 312.7 & 5.6 & 6.6 & 144.3 & 169.0 \\ 
${\pm}$2s   & 4.2 & 0.18 & 3.2 & 1.1 &  & 16.2 & 3.0 & 0.17 & 0.60 & 0.52 & 0.70 \\ 
TOC (\%) &  7.6 & 2.8 & N.D & N.D & & 3.3 & 7.6 & 2.8 & 6.7 & 8.6 & N.D 
\botline
\end{tabular}
\tabnote{Listed uncertainties include blank variability as well as
analytical uncertainties.}
\end{sidewaystable*}



\begin{figure}[t!]
\includegraphics{fig03}
\vspace*{-2pt}
\caption{\label{fig3} (a) Re--Os isochron obtained from organic matter
from the FB-FD sequence of the Francevillian (Gabon) (b)
Deviation in \% of the measured $^{187}$Os/$^{188}$Os ratio of each
sample from the correlation line regressed through the samples
dissolved in Cr$^{\mathrm{VI}}$ solution. It is clear that the
samples dissolved in aqua regia display considerably more scatter,
possibly indicating minor release of Os from detrital material in
samples digested by this method.}
\vspace*{-2pt}
\end{figure}


\section{Discussion}

Very ancient sedimentary systems, such as the one in this study, are
often well-adapted to dating by the Re--Os geochronometer
{\citep{Hannahetal2008}.} Transition metals, including Re and Os, are
stabilized when associated with organic matter by forming
organometallic complexes  {\citep{ShockKoretsky1995}.}  Rhenium and
osmium are therefore concentrated in sedimentary organic matter,
explaining their high concentrations in black shales 
{\citep{RavizzaTurekian1989, SelbyCreaser2005}.}  It~has also been
shown that the Re--Os system is usually not perturbed by organic matter
maturation {\citep{Creaseretal2002}.} Nevertheless, to guarantee a
reliable result, the studied system must not have been disturbed by
external influences such as hydrothermalism, metamorphism and many
others. This is the case of the studied formations 
{\citep{ElAlbanietal2010,ElAlbanietal2014,ElAlbanietal2019,Ikouangaetal2023,Neuillyetal1972,Weberetal2016}.} 
Finally, the use of a CrO$^{3}$--H$_{2}$SO$_{4}$ digestion medium
greatly limits contamination with Re and Os derived from detrital
material  {\citep{SelbyCreaser2003},} as shown by the better alignment
of samples dissolved using this medium compared to those digested in
aqua regia ({Figure~\ref{fig3}}).


The FB Formation is composed primarily (around 80\%) of marine shales,
with organic matter contents varying between 0.5 and 10 wt\%
(total organic carbon or TOC) {\citep{Mossmanetal2005}.} The organic
matter of the Francevillian sediments originated mainly from
cyanobacteria remains {\citep{Mossmanetal2005}.} This material played
an important role in the early stages of diagenesis
{\citep{Mossmanetal2005}.} During burial, the organic matter is usually
transformed into kerogen by polymerization
{\citep{Stankiewiczetal2000}.} During this process, the preservation of
organic matter may be granted by clay-polymer interactions 
{\citep{Christidis2014}.} The TOC contents cited above show that
organic matter is not negligible in the Francevillian Formations and
could provide material for\break dating. 

The different dating techniques that have been used in the
Francevillian Basin address the specific events to which they are
sensitive: uranium mineralization  {\citep{Gancarz1978},} manganese
mineralization {\citep{Brosetal1992},} the setting up of the N'goutou
volcanic complex  {\citep{MoussavouEdou-Minko2006,Sawakietal2017},} and
localized hydrothermal processes  {\citep{OssaOssaetal2014}.}
Nevertheless, some studies present ages that can be linked to
sedimentary processes, mainly illitization during early diagenesis
{\citep{Brosetal1992}.}

The various types of dating that have been done for the Francevillian
Basin yield consistent ages (between 2.191 Ma and 2.036 Ma). In the FA
Formation, dating was done by the Ar--Ar method on illites 
{\citep{OssaOssaetal2014}} and by the U--Pb method on uraninite 
{\citep{Gancarz1978}.} In the first case, the origin of the dated
illites is not clearly defined, as they are formed during diagenetic or
hydrothermal processes, and would postdate the depositional age. The
obtained age corresponds to a lower limit, because the illites were
formed during diagenesis or hydrothermalism. Concerning the second
case, the uranium in the conglomeratic rocks is mobilized by oxidizing
fluids  {\citep{Gauthier-LafayeWeber1989}.} Consequently, the uranium
is brought in by oxidizing fluids and its age (2050 ${\pm}$ 30 Ma)
defines a lower limit for the FA Formation. 

Syenite intrusion affects solely at the base of the FB Formation, which
is already in place  \citep{MoussavouEdou-Minko2006,Sawakietal2017}.
The ages of these intrusions (respectively 2027 ${\pm}$ 55 Ma and 2143 
${\pm}$ 143) thus mark a lower time limit for the deposition. 
\citet{Brosetal1992} applied the Sm--Nd technique to date the clay
fraction of black shales. To avoid possible detrital contribution, they
separated clay fractions by centrifugation, then observed the phases
using a transmission electron microscope, before proceeding to leaching
of the black shales. However, the results of this dating are associated
with the different illitization episodes during early diagenesis. The
obtained ages (2099 ${\pm}$ 115 Ma and 2036 ${\pm}$ 79 Ma) do not
correspond directly to the deposit of FB Formation sediments, but to
their early diagenesis. The Rb--Sr dating of intrusive syenites (2143 
${\pm}$ 143 Ma) proposed by  \citet{Bonhommeetal1982} represents only
the limit between the FA and FB Formations without providing a precise
time for the deposition of the FB Formation.

Zircons of tuffaceous sandstones of the FD Formation analysed by
Laser-ICP/MS mark the most recent event  {\citep{Thieblemontetal2009}}.
Therefore, the depositional age is older than 2072 ${\pm}$ 29. 
{\citet{Horieetal2005}} also dated this formation (2083 ${\pm}$ 6 Ma).
As the pyroclastic layer overlies the FD sediments, this well-defined
age strongly constrains the minimal depositional age of these
sediments. 


Re--Os dating of Francevillian formations indicated an age of 2103
${\pm}$ 11 Ma, corresponding to the age of closure of the Francevillian
sedimentary system (from FB to FD). Thus, the fossilization age of the
first multicellular life forms would have occurred during this period.
The new Re--Os age for organic matter agrees with the age inferred from
the ${\delta}^{13}$C record of marine carbonates in the FB2 member
{\citep{Canfieldetal2013a,ElAlbanietal2010}}, around 2.1 Ga ${\pm}$
0.03. The high burial rate of organic matter associated with the
deposition of the black shale was correlated with the Lomagundi Event,
which represents the longest positive isotopic excursion of carbon in
Earth history  {\citep{KarhuHolland1996,Praveetal2022}.} Some authors
link this event to the increase of oxygen in the atmosphere
{\citep{Canfieldetal2013a,KarhuHolland1996}}, whereas 
{\citet{Melezhiketal1999}} argue that it is due to \mbox{paleo-environmental}
phenomena including organic burial. This event took place between 2110
Ma and 2060 Ma {\citep{Martinetal2004,Melezhiketal2013}.}  In
consequence, the Re--Os age of organic matter of the Francevillian
formations confirms the timing of the Lomagundi event.

\section{Conclusion}

A new depositional age of the Francevillian formations was obtained
using the Re--Os dating method. The reliability of this age is supported
by the relatively low MSWD value (2.6) determined for the isochron,
which indicates that there has been only minimal perturbation of these
samples in the intervening time. These Francevillian formations were
deposited at 2103 ${\pm}$ 11 Ma, a period during which the Earth
registered a high rate of carbon burial during the Lomagundi Event. The
age obtained also confirms the date of the emergence of multicellular
life forms that were identified in the FB2b Formation.

\section*{Declaration of interests}

The authors do not work for, advise, own shares in, or receive funds
from any organization that could benefit from this article, and have
declared no affiliations other than their research organizations.

\section*{Acknowledgments}

We thank the Gabonese government, the Region Nouvelle-Aquitaine, the
Centre National pour la Recherche Scientifique and Technique du Gabon
(CENAREST), the General Direction of Mines and Geology of Gabon, the
Agence Nationale des Parcs Nationaux du Gabon, the French government
program (Investissements d'Avenir, EUR INTREE, ANR-18-EURE-0010), the
University of Masuku, the COMILOG and SOCOBA Companies, the French
Embassy at Libreville and the Institut Fran\c{c}ais du Gabon, for their
support. We thank A. Ngomanda, R. Oslisly, A. Meunier, O. Bankole, F.
Weber, F. Gauthier-Lafaye, F. Pambo and J.L. Albert, for geological
information and scientific discussion, and C. Lebailly, L. Tromas and
C. Laforest, for technical assistance. We also thank the two anonymous
reviewers and C.~Zimmermann for assistance with the Re--Os analyses.

\CDRGrant[ANR]{ANR-18-EURE-0010}

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