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\DOI{10.5802/crgeos.292}
\datereceived{2024-12-19}
\daterevised{2025-04-08}
\dateaccepted{2025-05-05}
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{Sample} &amp; {22CM01} &amp; {22CM02} &amp; {22CM03} &amp; {22CM04} &amp; {22CM05} &amp; {22CM06} &amp; {22CM07} &amp; {22CM08} &amp; {22CM09A} &amp; {22CM11} \\
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\dateposted{2025-05-26}
\begin{document}

\begin{noXML}

\CDRsetmeta{articletype}{research-article}
\TopicFR{Volcanologie}
\TopicEN{Volcanology}

\shortrunauthors

\title{Reconstruction of the volcanic history of Grande Comore and
volcano-tectonic implications for the Comoros Archipelago}

\alttitle{Reconstruction de l'histoire volcanique de la Grande Comore
et implications volcano-tectoniques pour l'archipel des Comores}

\author{\firstname{Sandy} \lastname{Rougeau}}
\address{Universit\'{e} Paris-Saclay, CNRS, GEOPS, Orsay, 91405, France}

\author{\firstname{Xavier} \lastname{Quidelleur}\CDRorcid{0000-0002-9964-5161}\IsCorresp}
\addressSameAs{1}{Universit\'{e} Paris-Saclay, CNRS, GEOPS, Orsay, 91405, France}
\email[Q. Xavier]{xavier.quidelleur@universite-paris-saclay.fr}

\author{\firstname{Vincent} \lastname{Famin}\CDRorcid{0000-0001-7451-5363}}
\address{Universit\'{e} de Paris, Institut de physique du globe de
Paris, CNRS, UMR 7154, Paris, 75005, France}
\address{Universit\'{e} de La R\'{e}union, Laboratoire G\'{e}oSciences R\'{e}union, Saint Denis, 
97744, France}

\author{\firstname{Laurent} \lastname{Michon}\CDRorcid{0000-0002-4486-5378}}
\addressSameAs{2}{Universit\'{e} de Paris, Institut de physique du globe
de Paris, CNRS, UMR 7154, Paris, 75005, France}
\addressSameAs{3}{Universit\'{e} de La R\'{e}union, Laboratoire
G\'{e}oSciences R\'{e}union, Saint Denis,  97744, France}

\author{\firstname{Fran\c{c}ois} \lastname{Nauret}\CDRorcid{0000-0002-1475-0377}}
\address{Universit\'{e} Clermont Auvergne, CNRS, IRD, OPGC, Laboratoire
Magmas et Volcans, F-63000 Clermont-Ferrand, France}

\author{\firstname{Ana\"{i}s} \lastname{Rusquet}\CDRorcid{0000-0002-3352-6776}}
\addressSameAs{2}{Universit\'{e} de Paris, Institut de physique du globe
de Paris, CNRS, UMR 7154, Paris, 75005, France}
\addressSameAs{3}{Universit\'{e} de La R\'{e}union, Laboratoire
G\'{e}oSciences R\'{e}union, Saint Denis,  97744, France}

\author{\firstname{Andrea} \lastname{Di Muro}\CDRorcid{0000-0002-7635-1283}}
\address{Laboratoire de G\'{e}ologie de Lyon : Terre, Plan\`{e}tes et
Environnement, Universit\'{e} Claude Bernard Lyon 1, 69622
Villeurbanne, France}

\thanks{French ANR (Agence Nationale de la Recherche) project COYOTES
(ANR-19-CE31-0018, \url{https://anr.fr/Projet-ANR-19-CE31-0018}), BRGM,
European Union project INTERREG HATARI,  French oceanographic campaigns
SISMAORE and SCRATCH}

\begin{abstract} 
Since the onset of Fani Maore submarine eruption, the volcanism of the
Comoros Archipelago has gained renewed interest. Geochronological
constraints have been recently obtained for Moh\'{e}li, Anjouan and
Mayotte islands and nearby seamounts. Here we provide new groundmass
K--Ar ages for Grande Comore. Along with major and trace element
geochemistry and geomorphological analyses, these ages enable us to
propose a three stages reconstruction of the island. The first, or
M'Badjini stage, is much older than previously thought, with ages as
old as 2.2 Ma. Following a period of quiescence and dismantling, the
second phase occurred between 280 and 230 ka, probably on both La
Grille and Karthala volcanoes. Then, volcanic activity resumed for a
third phase from ca. 130 ka until present time. The Comoros
geochronological database, updated with our new results, confirms that
the construction history of the archipelago occurred as pulses and
follows a northward propagation of volcanism (i.e. a widening of the
volcanically active region).
\end{abstract}

\begin{altabstract} 
Depuis le d\'{e}but de l'\'{e}ruption sous-marine d\textbf{u} Fani
Maor\'{e}, le volcanisme de l'archipel des Comores suscite un regain
d'int\'{e}r\^{e}t. Des contraintes g\'{e}ochronologiques ont
r\'{e}cemment \'{e}t\'{e} obtenues pour les \^{i}les de Moh\'{e}li,
Anjouan et Mayotte, ainsi que pour les monts sous-marins voisins. Nous
pr\'{e}sentons ici de nouveaux \^{a}ges K--Ar obtenus sur matrice pour
la Grande Comore. Associ\'{e}s \`{a} des analyses g\'{e}ochimiques
(\'{e}l\'{e}ments majeurs et en traces) et \`{a} une \'{e}tude
g\'{e}omorphologique, ces \^{a}ges nous permettent de proposer une
reconstruction en trois phases de l'\^{i}le. La premi\`{e}re phase,
dite de M'Badjini, est bien plus ancienne qu'estim\'{e}e auparavant,
avec des \^{a}ges pouvant atteindre 2,2 millions d'ann\'{e}es.
Apr\`{e}s une p\'{e}riode de quiescence et de d\'{e}mant\`{e}lement,
une seconde phase s'est produite entre 280 et 230 mille ans,
probablement \`{a} la fois sur les volcans La Grille et Karthala.
L'activit\'{e} volcanique a ensuite repris pour une troisi\`{e}me
phase, depuis environ 130 mille ans jusqu'\`{a} aujourd'hui. La base de
donn\'{e}es g\'{e}ochronologique des Comores, enrichie de nos nouveaux
r\'{e}sultats, confirme que l'histoire de la construction de l'archipel
s'est faite par impulsions successives, suivant une propagation du
volcanisme vers le nord (autrement dit, un \'{e}largissement de la zone
d'activit\'{e} volcanique).
\end{altabstract} 

\keywords{\kwd{Comoros Archipelago}
\kwd{Grande Comore}
\kwd{K--Ar dating}
\kwd{Geochemistry}}

\altkeywords{\kwd{Archipel des Comores}
\kwd{Grande Comore}
\kwd{Datation K--Ar}
\kwd{G\'{e}ochimie}}

\maketitle

\vspace*{.5pc}

\twocolumngrid

\end{noXML}

\let\rmmu\upmu

\defcitealias{EmerickDuncan1982}{ibid.}

\section{Introduction}\label{sec1}

\begin{figure*}
\includegraphics{fig01}
\caption{\label{fig1}Map of the Comoros Archipelago, with an insert
showing its location relative to Africa and Madagascar. Image was made
using the QGIS software, compilating a 30-m gridded shaded digital
elevation model, drawn with topographic data from the Japan Aerospace
Exploration Agency \citep{Takakuetal2020}, and bathymetry 
\citep[from][]{Tzevahirtzianetal2021,
Thinonetal2022,BerthodZaragosiFamin2021}. The Fani Maore active
submarine volcano is represented by a red star.}
\end{figure*}


The submarine eruption of the Fani Maore volcano on the submarine
slopes of Mayotte in 2018
\citep{Cescaetal2020,Lemoineetal2020,Feuilletetal2021,Berthodetal2021a}
has revived scientific interest for the volcanism of the Comoros
Archipelago (Mozambique Channel; Figure~\ref{fig1}). Before recent
studies, the Comoros Archipelago was \mbox{interpreted} as due to reactivation
of deep \mbox{lithospheric} fractures \citep{Nougieretal1986} or as a hotspot
trail of volcanoes progressing westward 
\citep[e.g.,][]{HajashArmstrong1972,EmerickDuncan1982}, 
which would have
progressively formed Mayotte (to the east), Anjouan, Moh\'{e}li and up
to the volcanically active island of Grande Comore (to the west). The
submarine eruption of Fani Maore volcano located east of Mayotte,
together with the new Holocene ages of volcanic eruptions in Anjouan
and Moh\'{e}li \citep{Quidelleuretal2022,Rusquetetal2023}, challenges
this latter scenario and questions the origin of Comorian volcanism.
More recently, Comorian volcanism has been related to the East African
Rift System \citep{Michonetal2022} and to the boundary between the
Somali and Lwandle plates \citep{Stampsetal2018,Faminetal2020}. In an
effort of better understanding the volcanic history of the Comoros,
numerous radiometric dating have been recently carried out on Mayotte,
Anjouan, and Moh\'{e}li
\citep{Pelleteretal2014,Quidelleuretal2022,Rusquetetal2023,Rusquetetal2025}. 
Available ages for Grande Comore Island in the literature are limited
to two ages with the K--Ar method, and ten radiocarbon ages restricted
to the second half of the Holocene
\citep{HajashArmstrong1972,EmerickDuncan1982,BacheleryCoudray1993}.
Therefore, the main objective of this study is to develop an absolute
geochronological framework by new K--Ar dating of Grande Comore
volcanoes, allowing us to complete the database of the Comorian
radiometric ages and, if necessary, to check the validity of previous
ages. By combining geochronological, geochemical and geomorphological
data from both subaerial and submarine terrains, we revise the history
of Grande Comore's edification and the scenario of volcanic progression
in the Comoros, offering insights into the geodynamics, and into the
volcanic hazards of the northern Mozambique\break Channel.


\section{Geological setting}\label{sec2}

\subsection{Comoros archipelago}\label{sec21}

The Comoros Archipelago is located in the Mozambique Channel, between
the northern tip of Madagascar and the Mozambican coast. This area
results from the break-up of the Gondwana supercontinent ${\sim}$160 Ma
ago and the southward drift of Madagascar relative to Africa 
\citep[e.g.,][]{Chatterjeeetal2013,Pique1999}. The archipelago
(\mbox{10--13\textdegree S}; \mbox{43--46\textdegree~E})~is composed of four
islands (Figure~\ref{fig1}), from east to west: Mayotte (or Maore),
Anjouan (Nzwani), Moh\'{e}li (Mwani) and Grande Comore (Ngazidja).
Below sea level, the Comoros is a volcanic chain that also comprises
seamounts, banks and atolls extending toward the east and up to the
Cenozoic volcanoes of northern Madagascar \citep{Rusquetetal2025}. This
area has a seismic and volcanic activity among the most intense of the
western Indian Ocean \citep{Bacheleryetal2016}, the Karthala being the
second most active volcano after Piton de la Fournaise (La R\'{e}union
Island).

Consensus is emerging that the Comoros Volcanic Chain is built over an
oceanic basement
\citep{Phetheanetal2016,Rolandoneetal2022,Masqueletetal2022,Masqueletetal2024a},
approximately 150 Ma old, along a nascent border between the Lwandle
and Somalia plates \citep{Faminetal2020} also interpreted as a transfer
area between the East African Rift System and Madagascar's grabens
\citep{Feuilletetal2021}. The volcanic ridges connecting the Comoros
Islands are considered to be lithospheric fractures at the dextral
transform frontier between the Lwandle and Somalia plates, controlling
the volcanism \citep{Faminetal2020}. 

The magmas of the Comoros Volcanic Chain share strong similarities in
major and trace element compositions and isotopic signatures with those
of the Cenozoic volcanism of northern Madagascar, suggesting similar
melting sources and/or ascent processes 
\citep{Cuccinielloetal2011,Cuccinielloetal2016,Cuccinielloetal2022,Rusquetetal2025}.
Comorian magmas have been produced by partial melting at different
degrees of a mantle source composed of a mix between a HIMU (high
${\upmu} ={}^{238}{\mathrm{U}}/^{204}\mathrm{Pb}$) and a DMM (Depleted
MORB Mantle) part, as inferred from their Sr--Nd--Pb isotopic ratios
and trace elements compositions 
\citep{Spathetal1996,Pelleteretal2014,BacheleryHemond2016}. A possible
explanation to the HIMU contribution in Comorian magmas is delamination
of a continental lithosphere in the mantle induced by Gondwana breakup
\citep{Spathetal1996,Pelleteretal2014,BacheleryHemond2016}.

Mayotte is considered to be the oldest island of the Comoros, its
construction on the seafloor would have started 27~Ma ago 
\citep{Masqueletetal2022}. Episodic subaerial volcanism in this island
is documented over the past 6~Ma, with 3 different phases: 6--5~Ma and
4--3~Ma, occurred in the southern and northwestern parts, and the last
one since about 2.4~Ma in the northeastern part. The subaerial
volcanism of Anjouan has been recently dated between 900 ka to 11 ka
\citep{Quidelleuretal2022} and the ages of Moh\'{e}li range from 3.25
Ma to 8 ka \citep{Rusquetetal2023}. 


\subsection{Grande comore}\label{sec22}

Grande Comore is an elongated island along an overall N--S axis (NW--SE
in the south), with a length of 65 km and a width of around 20 km. It
is composed of two main volcanic complexes: Karthala, a shield basaltic
volcano forming the southern two thirds of the island, and La Grille, a
shield volcano forming the northern part of the island.
\citet{BacheleryCoudray1993} have also identified the relict reliefs of
a third eroded edifice beneath the Karthala volcano, south of Grande
Comore, which they called ``M'Badjini''. Whether this eroded M'Badjini
relief is a third independent edifice truly older than Karthala and
whether it has a distinct geochemical signature is unknown. Grande
Comore is prolongated to the north by a series of submarine volcanic
ridges called N'Dround\'{e}, to the west by the Vailheu Bank, an
elongated reef outcropping occasionally at low tide, and finally to the
southeast by the submarine volcanic ridge of Domoni, prolonging the
southwest rift zone of Karthala and/or M'Badjini at sea and connecting
Grande Comore and Moh\'{e}li
\citep{Tzevahirtzianetal2021,Thinonetal2022}.



The Karthala volcano, rising at 2361 m, is the highest summit of the
archipelago. The Karthala volcano displays two rift zones extending
from the summit caldera, one toward the north, the other toward the
southeast. The steep flanks of Karthala and the presence of hummocky
surfaces on the seafloor \unskip\break \citep{Audruetal2006,Tzevahirtzianetal2021},
notably in the east and to a lesser extent in the south of the island,
are evidence of ancient landslides 
\citep[e.g.][]{BacheleryCoudray1993,Tzevahirtzianetal2021}, as commonly
observed for basaltic shield volcanoes \citep[e.g.,][]{Mooreetal1994}.
The volcanism of Karthala is only temporally constrained by a series of
$^{14}$C ages near the summit between 625 and 5045 years BP
\citep{BacheleryCoudray1993}, and by two K--Ar age on whole rock at 
$10 \pm 10$~ka \citep{HajashArmstrong1972} and $130 \pm 20$~ka
\citep{EmerickDuncan1982}. The Karthala volcano is the conundrum of
the controversy about the geodynamic origin of the Comorian volcanism,
because it is the only edifice to show a mantle plume signature
according to Sr--Nd--Pb--Os isotopes, distinct from the other magmatic
products of the Comoros Volcanic Chain 
\citep{Classetal2005,Classetal2009,BacheleryHemond2016}.

North of Karthala, the La Grille edifice forms an elongated ridge
toward the north. La Grille is covered by many large scoria cones, with
alignments linked to its rift zone, and tuff-rings, remnants of violent
hydromagmatic eruptions. Recent eruptions on La Grille are documented
by two $^{14}$C ages of $1300 \pm 65$ years BP and $740 \pm 130$ years
BP \citep{BacheleryCoudray1993}, but there is no geochronological
constrain on the construction history of La Grille. The La Grille
volcano has a HIMU-DMM Sr--Nd--Pb--Os isotopic signature also observed on
the other islands, but at variance with the Karthala signature 
\citep{Classetal2005,Classetal2009,BacheleryHemond2016,Bordencaetal2023}.

\section{Methods}\label{sec3}

\subsection{Sampling}\label{sec31}

Ten volcanic rock samples were collected in June 2022
(Figure~\ref{fig2}; Table~\ref{tab1}). Samples 22CM01, 22CM02 and
22CM03 are from lava flows, southeast of the island, in an area mapped
as belonging to the M'Badjini massif \citep{BacheleryCoudray1993}.
Samples 22CM07 and 22CM08 are from dykes, also from the M'Badjini
massif, located to the west of the above samples. Sample 22CM04
(Figure~\ref{fig2}) is from an ankaramite lava flow (a melanocrate and
porphyric clinopyroxene-dominated basalt) located in the same area.
Samples 22CM05 and 22CM06, which belong to the massif of Karthala, are
ankaramite lava flows located on the eroded flanks in the eastern part
of Grande Comore. In the northern part of the island, 22CM09A and
22CM11 were sampled from lava flows belonging to La Grille volcano. As
K--Ar dating requires a relatively large amount of fresh groundmass with
low vesicularity, some of the above samples were not considered
suitable for geochronological analysis. It is the case of samples
22CM04, which was too porphyric and vesiculated, and 22CM11, which
showed evidence of secondary mineralization in vesicles (see
Supplementary Material). The eight remaining samples underwent K--Ar
dating at the GEOPS laboratory (Orsay, France).

\begin{figure*}
\includegraphics{fig02}
\caption{\label{fig2}Location map showing the 3 volcanic massifs of
Grande Comore, their summit and altitude, with colors derived from the
geological map of \citet{BacheleryCoudray1993}: La Grille (in green),
Karthala (in beige) and M'Badjini (in purple). Contour lines every 
100~m. All ages now available for Grande Comore are shown. Eight K--Ar ages
(in Ma) are from this study (red plain circles) and four ages are from
literature, with two K--Ar ages (in Ma) (orange squares) from
\citet{HajashArmstrong1972} and \citet{EmerickDuncan1982} and two
$^{14}$C ages (orange triangles) from \citet{BacheleryCoudray1993}. Our
two undated but geochemically analyzed samples are also shown (red and
white circles). The main cities (in italic) are also located on the
map, as well as the main roads (light brown). Image was made using the
QGIS software, compilating a 30-m gridded shaded digital elevation
model, drawn with topographic data from the Japan Aerospace Exploration
Agency \citep{Takakuetal2020}.}
\end{figure*}

\begin{table*}[t!]
\caption{\label{tab1}New K--Ar ages performed on groundmass separates}
\begin{tabular}{cccccccccc}
\thead
{Sample} &amp; {Lat. (\textdegree S)} &amp; {Long. (\textdegree E)} &amp; {K (\%)}
&amp; \parbox[t]{.9cm}{\centering $^{40}$Ar* (\%)} &amp; 
\parbox[t]{1.6cm}{\centering $^{40}$Ar* ($10^{11}$~at/g)} 
&amp; {Age (Ma)} &amp; {Un. (Ma)} &amp; 
\parbox[t]{1cm}{\centering  Mean Age~(Ma)} &amp; 
\parbox[t]{.9cm}{\centering ${\pm}$1 ${\upsigma}$ (Ma)}
\vspace*{2pt}\\
\endthead
\textbf{22CM01}  &amp; 11.886964 &amp; 43.510652 &amp; 1.306 &amp;       \08.5 &amp; 3.161\0 &amp; 0.232 &amp; 0.004 &amp; \textbf{0.230} &amp; \textbf{0.004} \\
{Duplicate}  &amp;  &amp;  &amp;  &amp;       \09.7 &amp; 3.117\0 &amp; 0.228 &amp; 0.004 &amp;  &amp;  \\
\textbf{22CM02} &amp; 11.897837 &amp; 43.496377 &amp; 1.048 &amp;       \07.2 &amp; 2.897\0 &amp; 0.265 &amp; 0.005 &amp; \textbf{0.268} &amp; \textbf{0.005} \\
{Duplicate} &amp;  &amp;  &amp;  &amp;       \07.8 &amp; 2.957\0 &amp; 0.270 &amp; 0.005 &amp;  &amp;  \\
\textbf{22CM03}    &amp; 11.907782 &amp; 43.500077 &amp; 1.144 &amp;       \06.9 &amp; 3.380\0 &amp; 0.283 &amp; 0.006 &amp; \textbf{0.283} &amp; \textbf{0.006} \\
{Duplicate} &amp;  &amp;  &amp;  &amp;       \07.7 &amp; 3.390\0 &amp; 0.284 &amp; 0.005 &amp;  &amp;  \\
\textbf{22CM05}    &amp; 11.684658 &amp; 43.421778 &amp; 1.371 &amp;       \0\00.37 &amp; 0.1285 &amp; 0.009 &amp; 0.002 &amp; \textbf{0.007} &amp; \textbf{0.002} \\
{Duplicate} &amp;  &amp;  &amp;  &amp;       \0\00.27 &amp; 0.0831 &amp; 0.006 &amp; 0.002 &amp;  &amp;  \\
\textbf{22CM06}    &amp; 11.782472 &amp; 43.452428 &amp; 1.088 &amp;       \0\00.46 &amp; 0.2056 &amp; 0.018 &amp; 0.004 &amp; \textbf{0.014} &amp; \textbf{0.004} \\
{Duplicate} &amp;  &amp;  &amp;  &amp;       \0\00.26 &amp; 0.1107 &amp; 0.010 &amp; 0.004 &amp;  &amp;  \\
\textbf{22CM07}    &amp; 11.913931 &amp; 43.469972 &amp; 0.900 &amp;     35.3 &amp; 20.521\0\0 &amp; 2.183 &amp; 0.031 &amp; \textbf{2.167} &amp; \textbf{0.031} \\
{Duplicate} &amp;  &amp;  &amp;  &amp;     38.1 &amp; 20.241\0\0 &amp; 2.153 &amp; 0.031 &amp;  &amp;  \\
\textbf{22CM08}    &amp; 11.9146\0 &amp; 43.470439 &amp; 0.925 &amp;     32.6 &amp; 21.234\0\0 &amp; 2.197 &amp; 0.032 &amp; \textbf{2.209} &amp; \textbf{0.032} \\
{Duplicate} &amp;  &amp;  &amp; 1.166 &amp;     35.7 &amp; 27.057\0\0 &amp; 2.220 &amp; 0.032 &amp;  &amp;  \\
\textbf{22CM09A}   &amp; 11.411742 &amp; 43.408786 &amp; 0.859 &amp;       \04.6 &amp; 2.402\0 &amp; 0.268 &amp; 0.007 &amp; \textbf{0.280} &amp; \textbf{0.008} \\
{Triplicate} &amp;  &amp;  &amp;  &amp;       \04.7 &amp; 2.604\0 &amp; 0.290 &amp; 0.007 &amp;  &amp;  \\
{Triplicate} &amp;  &amp;  &amp;  &amp;       \02.2 &amp; 2.554\0 &amp; 0.285 &amp; 0.013 &amp;  &amp; 
\botline
\end{tabular}
\tabnote{Column headings indicate sample names; latitude and longitude
(in degrees); mean potassium (K) concentration in percent;
concentration of radiogenic $^{40}$Ar ($^{40}$Ar$^*$) in percent and in
number of atoms per gram (${\times}10^{11}$); age (in Ma);
1${\upsigma}$ uncertainty (Un., in Ma); weighted mean age (in Ma)
calculated from triplicate (22CM09A) or duplicate (other samples)
analyses shown as different lines; 1${\upsigma}$ weighted, by
$^{40}$Ar$^*$ (\%), uncertainty (in Ma).}
\vspace*{-3pt}
\end{table*}

\subsection{K--Ar dating}\label{sec32}

The Cassignol--Gillot technique \citep{Gillotetal2006}, described in
detail along with uncertainties calculation in \citet{Germaetal2011},
was applied to perform K and Ar measurements. Thin sections of each
sample selected were first examined to confirm their freshness (i.e.,
absence of secondary phases) and to determine the groundmass fraction
size (typically 125--250~${\upmu}$m) that should be analyzed. Then,
samples were crushed and sieved in order to isolate the groundmass for
dating. Samples were then washed with diluted nitric acid (10\%
concentrate) in an ultrasonic bath to eliminate any possible
alteration. A heavy liquid separation using diiodomethane was performed
to isolate the groundmass from the remaining dense phenocrysts and the
light phases. When needed magnetic sorting was realized to further
separate crystals from the groundmass.

The K content was measured by Atomic Absorption Spectroscopy, using an
Agilent AA240 spectrometer. Solutions with K concentrations from 1 to 
2~mg/l were used to calibrate measurements, then compared to references
standards BCR-2 \citep[$K = 1.481\%$;][]{Raczeketal2001} and MDO-G 
\citep[$K = 3.510\%$;][]{Gillotetal1992}. $^{40}$K content was
calculated using decay constants and K isotopic ratio of
\citet{SteigerJager1977}.

The sample gases were released from another aliquot of each sample
using a high frequency induction furnace above 1400\textdegree C. They
were purified on a titanium foam heated at 800\textdegree C, which
\mbox{eliminates} all the non-noble gases, leaving argon nearly as the only
remaining gas. The sample underwent further purification stages using
Al--Zr getters. \mbox{Argon} measurement was performed using a
180\textdegree~sector  multi-collector mass spectrometer 
\citep[e.g.,][]{Gillotetal2006}. Age determination by the Cassignol--Gillot
technique relies on the comparison of argon 36 and 40 from the samples
and atmospheric aliquots, with a detection limit of 0.1\% for
radiogenic argon \citep[$^{40}$Ar*;][]{Quidelleuretal2001}. A
calibration of the $^{40}$Ar signal was made with an air pipette, based
on repeated measurements of HD-B1 standard \citep{Fuhrmannetal1987}
using the age of 24.18~Ma \citep{SchwarzTrieloff2007}.

Each K and Ar measurement was at least duplicated. Finally, the age was
obtained by calculating a weighted mean of replicated measurements of K
and Ar. Age uncertainties presented in this study are given at the
1${\upsigma}$ level.

\subsection{Major and trace element geochemistry}\label{sec33}

The ten samples underwent geochemical analyses, including whole-rock
major and trace elements and REE measurements. The analyses were
performed at the University of Bretagne Occidentale, in the
\mbox{G\'{e}osciences} Oc\'{e}an laboratory (Brest, France), by ICP-AES
(Inductively Coupled Plasma-Atomic Emission Spectrometry) for major
elements and by ICP-MS (Inductively Coupled Plasma--Mass Spectrometry)
for trace elements. Relative uncertainties are ${&lt;}$2\% for major
elements and ${&lt;}$5\% for trace elements at 2${\upsigma}$ levels. To
compare our samples with the existing literature, geochemical
compositions of rocks from all the archipelago were extracted from the
GEOROC database: for Grande Comore 
\citep{Reisbergetal1993,Spathetal1996,Classetal1998,Classetal2005,
Classetal2009,Deniel1998,Rusquetetal2025,ClassGoldstein1997},
Mayotte \citep{Debeuf2004,Pelleteretal2014,Rusquetetal2025}, Moh\'{e}li
\citep{Rusquetetal2023,Rusquetetal2025}, and Anjouan
\citep{Quidelleuretal2022,Rusquetetal2025}.\looseness=-1

\begin{figure*}
\vspace*{-4pt}
\includegraphics{fig03}
\vspace*{-4pt}
\caption{\label{fig3}Total alkali versus silica (TAS) diagram 
\citep{LeBasetal1986} for Grande Comore rocks, from this study (in color, with
the sample names) and from the literature (in grey shades), and for the
Comoros Archipelago (small triangles), with data from the GEOROC
database (see text for references). Samples from La Grille are shown in
green, those from the Karthala are in orange and M'Badjini samples are
in purple. TB: Trachy-basalt.}
\vspace*{-6pt}
\end{figure*}

\subsection{Geomorphology}\label{sec34}

A geomorphological investigation was conducted on the edifices of La
Grille, Karthala and M'Badjini where the eight dated rocks were sampled
(22CM01, 22CM02, 22CM03, 22CM05, 22CM06, 22CM07, 22CM08, 22CM09A). The
analysis is based on the bathymetry data from
\citet{Tzevahirtzianetal2021} and a digital elevation model derived
from the ALOS World 3D--30~m program of the Japan Aerospace Exploration
Agency \citep[JAXA;][]{Takakuetal2020}.

\section{Results}\label{sec4}

\subsection{K--Ar ages}\label{sec41}

The eight new K--Ar ages obtained in this study range from  $0.007 \pm
0.002$ to $2.209 \pm 0.032$~Ma (Table~\ref{tab1} and
Figure~\ref{fig2}). The groundmass contains between 0.859\% and 1.371\%
of potassium and between 0.3\% and 38.1\% of $^{40}$Ar$^*$. The ages can be
divided into three stages. (1) Two ages are close to 10 ka (22CM05 and
22CM06). They are both ankaramites, from the base of the steep east
flank of the Karthala massif. (2)~Four ages range between 230 and 283
ka (22CM01, 22CM02, 22CM03, and 22CM09A). Three of these samples
(22CM01, 22CM02, and 22CM03) were collected on the M'Badjini massif
according to the map of \citet{BacheleryCoudray1993}, while the fourth
sample (22CM09A) was taken on the lowermost, presumably oldest
outcropping units of La Grille volcano. (3) Two samples (22CM07 and
22CM08) display much older ages ($2.167 \pm 0.031$ and $2.209 \pm
0.032$~Ma),  undistinguishable from each other at the 1${\upsigma}$
level. Note that the mean age of sample 22CM08 (Table~\ref{tab1}) was
obtained from dating two different density fractions (3.16--3.08 and
2.93--3.08), with different K content (0.925 and 1.166\%,
respectively), which further reinforces the confidence in our K--Ar
groundmass ages.\looseness=-1

\begin{figure*}
\includegraphics{fig04}
\vspace*{-3pt}
\caption{\label{fig4}(a) Spider diagram of trace elements normalized to
primitive mantle \citep[after][]{LyubetskayaKorenaga2007} for the 10 samples
analyzed in this study. Patterns from previous studies from the GEOROC
database (see text for references) are also shown (in beige for Grande
Comore, and in grey for Moh\'{e}li). (b) Spider diagram of the Rare
Earth Elements (REE) normalized to chondrite values  
\citep[after][]{SunMcdonough1989} for the same samples (same colors)
and with the data from literature (same as above).}
\vspace*{-5pt}
\end{figure*}

\subsection{Major and trace elements}\label{sec42}

Samples from this study are basalts, basanites and trachybasalts
(Figure~\ref{fig3}), characterized by {phenocrysts} (olivine,
clinopyroxene, and plagioclase) of various sizes and proportions, in
crystalline to micro-crystalline groundmass (see Supplementary
Material).~Major and trace elements contents have been measured for the
ten samples of this study and are given in Table~\ref{tab2} and shown
in Figures~\ref{fig3} and~\ref{fig4} along with previous data. Note
that all samples have losses on ignition (LOI) lower than 0.5\%.
Measurements are displayed in a total alkali versus silica (TAS)
diagram (Figure~\ref{fig3}), highlighting their low SiO$_{2}$ content
(${&lt;}$48\%). All the samples are relatively close to each other in this
diagram. Rocks from Karthala and M'Badjini belong to the alkaline
series, while samples from La Grille belong to the hyperalkaline
series, with primitive magmas  \citep{BacheleryHemond2016}. Samples
from La Grille fall in the field of basanites (22CM09A and 22CM11),
those from M'Badjini fall in the basalt field (22CM07, 22CM08, and
22CM04), and samples from Karthala are also basalts (22CM06, 22CM01,
22CM02, and 22CM03), except for 22CM05, which falls in the
trachy-basalt field. These results are consistent with previous data
for Grande Comore
\citep{HajashArmstrong1972,EmerickDuncan1982,BacheleryCoudray1993}.
There is a clear separation between the hyperalkaline tephrites of La
Grille and the (trachy-) basalts from Karthala and M'Badjini, with
Karthala samples slightly above M'Badjini's in the TAS diagram,
although sample 22CM06 is very close to the M'Badjini group.

\begin{table*}[p!]
\caption{\label{tab2}Major (in wt\%) and trace elements (in ppm)
composition of whole-rock samples from this study}
\tabcolsep=3.2pt
\begin{tabular}{ccccccccccc}
\thead
{Sample} &amp; {22CM01} &amp; {22CM02} &amp; {22CM03} &amp; {22CM04} &amp; {22CM05} &amp; {22CM06} &amp; {22CM07} &amp; {22CM08} &amp; {22CM09A} &amp; {22CM11} \\
\endthead
\multicolumn{11}{c}{(wt\%)}  \\
SiO$_{2}$ &amp; 47.4 &amp; 47.5 &amp; 47.0 &amp; 46.1 &amp; 47.1 &amp; 46.1 &amp; 45.2 &amp; 46.1 &amp; 42.5 &amp; 42.6 \\
TiO$_{2}$ &amp; 2.38 &amp; 2.24 &amp; 2.25 &amp; 1.08 &amp; 2.74 &amp; 1.79 &amp; 1.45 &amp; 1.72 &amp; 1.55 &amp; 1.92 \\
Al$_{2}$O$_{3}$ &amp; 14.4 &amp; 13.5 &amp; 13.8 &amp; 4.96 &amp; 15.2 &amp; 9.61 &amp; 7.40 &amp; 8.95 &amp; 10.5 &amp; 10.8 \\
Fe$_{2}$O$_{3}$ &amp; 12.5 &amp; 12.8 &amp; 12.2 &amp; 10.8 &amp; 13.0 &amp; 12.5 &amp; 13.0 &amp; 12.1 &amp; 12.7 &amp; 13.4 \\
MnO &amp; 0.20 &amp; 0.19 &amp; 0.18 &amp; 0.16 &amp; 0.19 &amp; 0.18 &amp; 0.18 &amp; 0.18 &amp; 0.20 &amp; 0.18 \\
MgO &amp; 8.61 &amp; 9.24 &amp; 9.04 &amp; 21.9 &amp; 6.46 &amp; 16.6 &amp; 20.8 &amp; 15.4 &amp; 18.0 &amp; 15.8 \\
CaO &amp; 10.8 &amp; 10.6 &amp; 10.7 &amp; 12.6 &amp; 10.2 &amp; 12.4 &amp; 10.7 &amp; 11.7 &amp; 10.7 &amp; 10.6 \\
Na$_{2}$O &amp; 3.20 &amp; 3.15 &amp; 3.32 &amp; 1.06 &amp; 3.80 &amp; 2.03 &amp; 1.33 &amp; 1.66 &amp; 2.80 &amp; 2.69 \\
K$_{2}$O &amp; 1.33 &amp; 1.24 &amp; 1.29 &amp; 0.33 &amp; 1.57 &amp; 0.75 &amp; 0.42 &amp; 0.63 &amp; 0.74 &amp; 0.95 \\
P$_{2}$O$_{5}$ &amp; 0.48 &amp; 0.42 &amp; 0.39 &amp; 0.15 &amp; 0.52 &amp; 0.25 &amp; 0.23 &amp; 0.26 &amp; 0.57 &amp; 0.42 \\
LOI &amp; 0.48 &amp; ${-}$0.29 &amp; ${-}$0.32 &amp; ${-}$0.02 &amp; ${-}$0.46 &amp; ${-}$0.39 &amp; 0.41 &amp; 0.21 &amp; ${-}$0.38 &amp; ${-}$0.07 \\
Total &amp; 102 &amp; 101 &amp; 99.9 &amp; 99.1 &amp; 100 &amp; 102 &amp; 101 &amp; 99.0 &amp; 99.8 &amp; 99.3\vspace*{6pt} \\
\multicolumn{11}{c}{(ppm)} \\
K &amp; 5500 &amp; 5134 &amp; 5347 &amp; 1364 &amp; 6521 &amp; 3114 &amp; 1758 &amp; 2612 &amp; 3071 &amp; 3949 \\
Li &amp; 9.80 &amp; 6.73 &amp; 7.06 &amp; 3.33 &amp; 8.42 &amp; 4.60 &amp; 4.47 &amp; 4.64 &amp; 6.71 &amp; 5.70 \\
Be &amp; 2.13 &amp; 1.93 &amp; 2.00 &amp; 0.68 &amp; 2.28 &amp; 1.06 &amp; 1.05 &amp; 1.14 &amp; 1.53 &amp; 1.37 \\
Sc &amp; 47.9 &amp; 33.7 &amp; 32.4 &amp; 52.1 &amp; 26.6 &amp; 51.6 &amp; 84.1 &amp; 30.3 &amp; 31.5 &amp; 29.6 \\
Ti &amp; 14255 &amp; 13398 &amp; 13504 &amp; 6448 &amp; 16394 &amp; 10728 &amp; 8718 &amp; 10306 &amp; 9296 &amp; 11512 \\
V &amp; 272 &amp; 286 &amp; 285 &amp; 206 &amp; 299 &amp; 269 &amp; 221 &amp; 291 &amp; 252 &amp; 282 \\
Cr &amp; 440 &amp; 652 &amp; 616 &amp; 2577 &amp; 227 &amp; 1403 &amp; 2186 &amp; 1028 &amp; 849 &amp; 685 \\
Mn &amp; 1702 &amp; 1658 &amp; 1683 &amp; 1424 &amp; 1651 &amp; 1529 &amp; 1563 &amp; 1552 &amp; 1727 &amp; 1659 \\
Co &amp; 54.7 &amp; 60.4 &amp; 61.8 &amp; 94.0 &amp; 50.3 &amp; 80.9 &amp; 97.2 &amp; 77.8 &amp; 85.0 &amp; 86.8 \\
Ni &amp; 189 &amp; 225 &amp; 215 &amp; 750 &amp; 108 &amp; 525 &amp; 775 &amp; 476 &amp; 719 &amp; 625 \\
Cu &amp; 80.7 &amp; 99.2 &amp; 94.5 &amp; 62.7 &amp; 52.6 &amp; 125 &amp; 79.3 &amp; 111 &amp; 100 &amp; 108 \\
Zn &amp; 140 &amp; 123 &amp; 163 &amp; 89.8 &amp; 133 &amp; 95.8 &amp; 99.5 &amp; 97.0 &amp; 108 &amp; 122 \\
Ga &amp; 21.6 &amp; 20.3 &amp; 20.1 &amp; 8.69 &amp; 23.8 &amp; 14.5 &amp; 12.4 &amp; 14.7 &amp; 15.8 &amp; 17.9 \\
Rb &amp; 57.2 &amp; 31.8 &amp; 29.6 &amp; 7.86 &amp; 40.9 &amp; 18.7 &amp; 9.73 &amp; 15.8 &amp; 20.3 &amp; 22.4 \\
Sr &amp; 637 &amp; 515 &amp; 523 &amp; 181 &amp; 565 &amp; 332 &amp; 266 &amp; 329 &amp; 659 &amp; 517 \\
Y &amp; 28.8 &amp; 24.3 &amp; 24.7 &amp; 10.6 &amp; 29.0 &amp; 17.6 &amp; 14.4 &amp; 18.3 &amp; 20.8 &amp; 18.1 \\
Zr &amp; 259 &amp; 238 &amp; 227 &amp; 77.2 &amp; 274 &amp; 140 &amp; 117 &amp; 131 &amp; 156 &amp; 155 \\
Mo &amp; 2.88 &amp; 2.62 &amp; 2.09 &amp; 1.10 &amp; 2.72 &amp; 1.64 &amp; 1.35 &amp; 1.55 &amp; 2.96 &amp; 2.87 \\
Nb &amp; 62.3 &amp; 54.7 &amp; 52.7 &amp; 13.8 &amp; 69.8 &amp; 35.3 &amp; 21.7 &amp; 27.3 &amp; 70.2 &amp; 53.6 \\
Cs &amp; 0.67 &amp; 0.26 &amp; 0.20 &amp; 0.09 &amp; 0.30 &amp; 0.14 &amp; 0.50 &amp; 0.15 &amp; 0.22 &amp; 0.23 \\
Ba &amp; 435 &amp; 353 &amp; 363 &amp; 109 &amp; 440 &amp; 227 &amp; 156 &amp; 216 &amp; 487 &amp; 358 \\
La &amp; 50.4 &amp; 39.1 &amp; 37.8 &amp; 9.94 &amp; 46.9 &amp; 24.3 &amp; 15.1 &amp; 19.7 &amp; 55.9 &amp; 32.0 \\
Ce &amp; 77.2 &amp; 63.7 &amp; 61.7 &amp; 17.8 &amp; 76.1 &amp; 40.1 &amp; 26.0 &amp; 33.2 &amp; 84.3 &amp; 52.1 \\
Pr &amp; 10.8 &amp; 8.81 &amp; 8.62 &amp; 2.75 &amp; 10.5 &amp; 5.71 &amp; 3.95 &amp; 4.93 &amp; 10.9 &amp; 7.37 \\
Nd &amp; 41.0 &amp; 34.6 &amp; 33.9 &amp; 12.0 &amp; 41.2 &amp; 23.0 &amp; 16.9 &amp; 21.0 &amp; 40.6 &amp; 29.6 \\\tabtwosplittabular
Sm &amp; 7.67 &amp; 6.71 &amp; 6.59 &amp; 2.82 &amp; 8.01 &amp; 4.72 &amp; 3.82 &amp; 4.75 &amp; 7.05 &amp; 5.86 \\
Eu &amp; 2.43 &amp; 2.13 &amp; 2.11 &amp; 0.93 &amp; 2.54 &amp; 1.52 &amp; 1.26 &amp; 1.57 &amp; 2.21 &amp; 1.89 \\
Gd &amp; 7.36 &amp; 6.33 &amp; 6.30 &amp; 2.69 &amp; 7.62 &amp; 4.51 &amp; 3.67 &amp; 4.55 &amp; 6.66 &amp; 5.46 \\
Tb &amp; 1.01 &amp; 0.89 &amp; 0.88 &amp; 0.40 &amp; 1.07 &amp; 0.65 &amp; 0.54 &amp; 0.68 &amp; 0.84 &amp; 0.75 \\
Dy &amp; 5.43 &amp; 4.85 &amp; 4.77 &amp; 2.28 &amp; 5.83 &amp; 3.57 &amp; 3.03 &amp; 3.79 &amp; 4.30 &amp; 3.93 \\
Ho &amp; 1.03 &amp; 0.91 &amp; 0.90 &amp; 0.42 &amp; 1.08 &amp; 0.66 &amp; 0.56 &amp; 0.70 &amp; 0.78 &amp; 0.70 \\
Er &amp; 2.63 &amp; 2.31 &amp; 2.28 &amp; 1.03 &amp; 2.75 &amp; 1.66 &amp; 1.41 &amp; 1.75 &amp; 1.91 &amp; 1.69 \\
Tm &amp; 0.38 &amp; 0.33 &amp; 0.33 &amp; 0.14 &amp; 0.39 &amp; 0.23 &amp; 0.20 &amp; 0.24 &amp; 0.27 &amp; 0.23 \\
Yb &amp; 2.35 &amp; 2.05 &amp; 2.02 &amp; 0.87 &amp; 2.42 &amp; 1.41 &amp; 1.20 &amp; 1.49 &amp; 1.62 &amp; 1.40 \\
Lu &amp; 0.33 &amp; 0.28 &amp; 0.28 &amp; 0.12 &amp; 0.33 &amp; 0.20 &amp; 0.17 &amp; 0.20 &amp; 0.23 &amp; 0.19 \\
Hf &amp; 5.38 &amp; 5.00 &amp; 4.81 &amp; 1.87 &amp; 5.68 &amp; 3.17 &amp; 2.73 &amp; 2.98 &amp; 3.07 &amp; 3.38 \\
W &amp; 0.86 &amp; 0.78 &amp; ${&lt;}$LD &amp; ${&lt;}$LD &amp; ${&lt;}$LD &amp; ${&lt;}$LD &amp; ${&lt;}$LD &amp; ${&lt;}$LD &amp; 0.9 &amp; ${&lt;}$LD \\
Ta &amp; 3.40 &amp; 2.86 &amp; 2.75 &amp; 0.75 &amp; 3.74 &amp; 1.83 &amp; 1.20 &amp; 1.49 &amp; 3.04 &amp; 2.66 \\
Pb &amp; 3.42 &amp; 3.23 &amp; 3.46 &amp; 0.98 &amp; 3.83 &amp; 1.74 &amp; 1.38 &amp; 2.02 &amp; 3.19 &amp; 1.97 \\
Th &amp; 5.42 &amp; 3.92 &amp; 3.70 &amp; 0.96 &amp; 4.89 &amp; 2.53 &amp; 1.86 &amp; 1.71 &amp; 6.14 &amp; 3.08 \\
U &amp; 1.09 &amp; 0.95 &amp; 0.93 &amp; 0.24 &amp; 1.16 &amp; 0.60 &amp; 0.38 &amp; 0.39 &amp; 1.42 &amp; 0.76 \\
P &amp; 1050 &amp; 916 &amp; 847 &amp; 319 &amp; 1141 &amp; 544 &amp; 494 &amp; 567 &amp; 1236 &amp; 908
\botline
\end{tabular}
\tabnote{L.O.I: loss on ignition.\vspace*{-5pt}}
\end{table*}

Figure~\ref{fig4}a shows spider diagrams of trace elements normalized
to primitive mantle \citep{LyubetskayaKorenaga2007}. Patterns of the
new data in the spider diagrams are similar to those obtained
previously for Grande Comore 
\citep{Reisbergetal1993,Spathetal1996,Classetal1998,Classetal2005,Classetal2009,Deniel1998,
Rusquetetal2025,ClassGoldstein1997}, with lower values for U, K, Pb,
and P. In detail, however, samples from La Grille (22CM09A and 22CM11)
are more enriched than those of Karthala (22CM01, 22CM02, 22CM03,
22CM05, and 22CM06), consistent with previous data, presenting a small
enrichment in most trace elements. On the contrary, samples 22CM04,
22CM07, and 22CM08 from M'Badjini massif show values slightly depleted
in most trace elements, even more than previously published data from
Grande Comore. In a Rare Earth Elements (REE) diagram normalized to
chondrite \citep{SunMcdonough1989}, all samples show a much higher
light-REE contents than heavy-REE contents (Figure~\ref{fig4}b), which
may indicate a low degree of partial melting \citep{Rollinson1993}.
The distribution of samples is the same as above, with similar behavior
for Karthala and La Grille and lower values for M'Badjini. Note that in
major and trace elements diagrams, sample 22CM06 presents
concentrations in between those of the M'Badjini and Karthala 
samples.\looseness=-1 

\subsection{Geomorphology}\label{sec43}

Grande Comore is known to present contrasted morphologies between La
Grille and Karthala volcanoes  \citep{Bacheleryetal2016}. La Grille is
capped by numerous, large scoriascoria cones scattered on its flat
summit and most of its subaerial and submarine flanks
(Figure~\ref{fig5}a). The maximum concentration of cones along a
NNW--SSE axis delineates the summit rift zone of the volcano extending
from the saddle between Karthala and La grille to the northern flank.
The rift zone western limit corresponds to a NNE--SSW trending break in
slope that separates the summit and the western flank, which is covered
by few scoria cones (Figure~\ref{fig5}a). In submarine domain, the
flanks of La Grille present regular slopes on the edifice northwestern,
northern and northeastern parts. Conversely, they show a concave
geometry in the west (Figure~\ref{fig5}c).

\begin{figure*}
{\vspace*{-3pt}}
\includegraphics{fig05}
{\vspace*{-4pt}}
\caption{\label{fig5}Morphology of La Grille (a) and Karthala (b)
edifices marked by cone concentrations and/or edifice elongation due to
the volcanic activity rift zones, and morphological discontinuities
related to dismantling processes. Topographic profiles characterizing
the main morphological features of La Grille~(c), Karthala (d), and
M'Badjini (e). Contour lines every 10 m and 200 m in subaerial and
submarine domains, respectively.}
\vspace*{-6pt}
\end{figure*}


Compared to La Grille, the morphology of Karthala is marked by
well-developed coalescent collapse calderas dissecting the flat summit,
steep flanks with average slope of 15\textdegree~(average of
8\textdegree~for La Grille), and small scoria cones mostly
concentrated along two rift zones north and south of the summit
(Figure~\ref{fig5}b). Although most of Karthala edifice is composed of
regular homogeneous slopes, the subaerial east and southwest flanks of
Karthala present obvious morphological discontinuities \mbox{characterized} by
steep slopes of 30--40\textdegree. In the southwest, these steep
slopes form two clear U-shaped scars intersecting each other, and a
third arcuate structure parallel to the SE rift zone
(Figure~\ref{fig5}b). East of the summit, the flank shows two main
slope breaks at high (1500--1800~m) and low (100--300~m) elevations
(Figure~\ref{fig5}d). The lower slope break is composed of a curved
scarp opened toward the submarine east flank between our samples 22CM05
and 22CM06, and, south of 22CM06, a second scarp that progressively
merges with the east slope of the SE rift zone (Figure~\ref{fig5}b).
Noteworthily, these scarps and the flank between the upper and lower
slope breaks are incised by a network of narrow valleys
(Figure~\ref{fig5}a). East of the lower slope breaks, the edifice slope
is formed by coalescent lava deltas.

The southern part of the SE rift zone, mapped as belonging to the
M'Badjini massif \citep{BacheleryCoudray1993}, is characterized by
contrasting flank geometries (Figure~\ref{fig5}e). The apex and the
eastern flank of the SE rift zone are covered by large scoria cones
scattered on a regular slope. Conversely, the western flank presents a
partially drowned dissected relief limited by steep slopes along which
samples 22CM04, 22CM07, and 22CM08 were collected (Figure~\ref{fig5}b
and e).

\section{Discussion}\label{sec5}

\subsection{Construction of the radiometric ages database}\label{sec51}

Besides K--Ar, other dating methods can be applied for volcanic rocks
but they need specific mineral phases, such as zircon or apatite for
(U--Th)/He, or charcoal for $^{14}$C, that are seldom found in basalts.
The $^{40}$Ar/$^{39}$Ar technique provides high-precision ages when
applied to K-rich minerals, but it is challenging when used for low-K
high-Ca samples. This is due to the requirement of sample irradiation
prior to analyses in order to transform $^{39}$K into $^{39}$Ar.
Unfortunately, it also produces $^{36}$Ar from Ca, making it less
precise for basic rocks, especially when they are young and have a high
natural $^{36}$Ar content. Therefore, we prefer to use the K--Ar
technique applied to the groundmass for young Quaternary basaltic rocks
\citep{QuidelleurFamin2024}. The groundmass is preferred because, as
this phase is the last to crystallize (i.e., during the eruption), it
contains the largest amount of potassium, which is an incompatible
element. In addition, the removal of phenocrysts prevents possible age
bias due to excess argon that would be introduced by minerals
crystallized prior to the eruption \citep{Germaetal2023}. This is the
reason why K--Ar dating applied to whole rock must be considered with
caution in crystal-bearing rocks, because this technique often yields
``too old'' ages compared to other techniques 
\citep[e.g.,][]{Samperetal2007,Quidelleuretal2022}.

\subsubsection{Grande Comore}\label{sec511}

Two whole-rock K--Ar ages (Karthala) were previously published for
Grande Comore. The K--Ar ages suggested a volcanic activity restricted
to the Holocene \citep[$10 \pm 10$~ka;][]{HajashArmstrong1972} and Late
Pleistocene \citep[$130 \pm 20$~ka;][]{EmerickDuncan1983}. The former age
from the west of Karthala (Figure~\ref{fig2}) is in agreement with the
historic activity of Grande Comore and with our new groundmass K--Ar
ages of $14 \pm 4$ and $7 \pm 2$~ka obtained for sample 22CM06 and 22CM05,
respectively, both located on the eastern part of the Karthala.
However, the late Pleistocene whole-rock age of $130 \pm 20$~ka
\citep{EmerickDuncan1982} is strikingly different from the age of 
$283 \pm 6$ obtained for our nearby sample 22CM02 (Table~\ref{tab1}).
Unfortunately, \citetalias{EmerickDuncan1982} neither provide the precise
location of their sample nor its petrography and major element
chemistry. It is thus not possible to assess if the date discrepancy
reflects a true temporal gap or an analytical bias of the K--Ar
technique on whole-rock. This ambiguity illustrates the difficulty of
interpreting K--Ar ages on whole rock in absence of other information,
such as the xenocryst content and the freshness of the dated samples.
We are thus forced to keep this age of $130 \pm 20$~ka in the
geochronological database, even though it might be prone to inaccuracy
due to the incorporation of excess $^{40}$Ar from phenocrysts.

The two $^{14}$C ages previously available ($1300 \pm 65$ and $740 \pm
130$ years BP, \xcitealp{BacheleryCoudray1993}{1993}) confirm that La Grille
(Figure~\ref{fig2}) has been very active in the recent past as
evidenced by its morphology and historical eruptions. These ages belong
to the subset of recent Holocene ages in Grande Comore, including the
age of $10 \pm 10$~ka of \citet{HajashArmstrong1972} and our new
groundmass, more accurate, K--Ar ages of $14 \pm 4$ and $7 \pm 2$~ka
(Table~\ref{tab1}).

\begin{figure*}
\includegraphics{fig06}
\caption{\label{fig6}Location map showing all the K--Ar and
$^{40}$Ar/$^{39}$Ar ages considered reliable (see Section~\ref{sec512}) for
the Comoros Archipelago, including this study. The symbols geometry and
colors refer to the authors and ages of each sample, respectively. The
Fani Maore active submarine volcano is represented by a red star.
Dashed lines represent the location of volcanic structures with similar
age and the arrow highlights the progressive northward propagation of
the volcanism. The map background is the same as in Figure~\ref{fig1}.}
\end{figure*}

\subsubsection{Whole comoros archipelago}\label{sec512}

\citet{Rusquetetal2025} have updated the age database for the whole
archipelago by evaluating the quality of each radiometric data,
including whole-rock K--Ar ages. Thus, the following part is a summary
of the selection carried out by these authors, but with the more severe
criterion of rejecting most whole-rock ages for reasons explained
above. We realize that we might reject valid ages among erroneous data,
but we prefer here to build a very robust ages dataset, albeit limited,
for the whole Comoros Archipelago. Mayotte Island has been subjected
to the greatest number of dating, with 25 groundmass
$^{40}$Ar/$^{39}$Ar ages \citep{Debeuf2004,Pelleteretal2014}, 33~K--Ar
whole-rock ages \citep{HajashArmstrong1972,EmerickDuncan1982,EmerickDuncan1983,Nougieretal1986}
and 4~K--Ar groundmass ages
\citep{Rusquetetal2025}. From these, the 4~K--Ar groundmass ages can
be considered as reliable, as they were obtained after a severe
selection in the field and thorough mineral preparation in the
laboratory. Among the 25 groundmass $^{40}$Ar/$^{39}$Ar ages, only 3
satisfy the quality criterions of this technique, namely presenting a
plateau age or a linear isochron with three continuous steps. Thus,
only 7 ages remain for Mayotte (Figure~\ref{fig6}). 8~K--Ar whole-rock
ages are available for Anjouan
\citep{HajashArmstrong1972,EmerickDuncan1982,EmerickDuncan1983,Nougieretal1986}, 
but are not further considered here for the
above reasons. Recently, 13~K--Ar groundmass ages and one $^{14}$C age
\citep{Quidelleuretal2022} completed the database. These 14 remaining
ages make Anjouan the island with the highest number of reliable ages
for the Comoros Archipelago. The island of Moh\'{e}li has 10~K--Ar
groundmass ages \citep{Rusquetetal2023}, all considered reliable. Only
2~K--Ar whole-rock ages and 2 very young $^{14}$C ages were available
for Grande Comore before this study
\citep{HajashArmstrong1972,EmerickDuncan1982,EmerickDuncan1983,Nougieretal1986},
making it the least geochronologically
constrained island of the Comoros. The island has now 8 reliable K--Ar
groundmass ages available (Figure~\ref{fig6}). Recently, submarine
volcanic edifices have been identified during oceanic campaigns 
\citep{Thinonetal2021,Thinonetal2022,Berthodetal2021a}, some of which have been
dated, making them of great importance to understand the whole Comorian
volcanism. It is the case for the Domoni Ridge, between Grande Comore
and Moh\'{e}li, dated by\unskip\break \citet{Rusquetetal2025}, with two
$^{40}$Ar/$^{39}$Ar groundmass plateau ages around 1.23 and 1.43~Ma. One
K--Ar groundmass age has been obtained for the Chistwani Ridge
\citep{Rusquetetal2023}. Submarine samples have also been dated on
Mwezi (two K--Ar groundmass ages), Jumelles (two K--Ar groundmass ages
and one $^{40}$Ar/$^{39}$Ar groundmass age) and Anjouan (one
$^{40}$Ar/$^{39}$Ar groundmass age) \citep{Rusquetetal2025}. All ages from
our selection are shown in Figure~\ref{fig6}.

\subsection{Volcanic history of Grande Comore}\label{sec52}

As presented above, the volcanism of Grande Comore presents three main
distinct stages, each evidenced by the age relative probability plot of
\citet[Figure~\ref{fig7}]{DeinoPotts1992}. 
The most recent phase is of Holocene age, with 3
ages around 10 ka for the Karthala massif
(\xcitealp{HajashArmstrong1972}{1972}; samples 22CM05 and 22CM06 of this
study) and the 12 very recent ages around ${&lt;}$5 ka for La Grille and
Karthala volcanoes ($^{14}$C ages). These data shows that both
volcanoes are still active, with the last eruption of Karthala in 2007
AD. Our results show that a widespread volcanic activity was present on
the island between around 130 ka and 280 ka. Note that ages from our
study show a coeval period of activity for Karthala (samples 22CM01,
22CM02, and 22CM03) and La Grille (sample 22CM09A) during this
interval. No subaerial activity has been recorded in our study and
earlier works between 280 ka and 2.2~Ma, which could represent a period
of volcanic quiescence of nearly 2~Ma for Grande Comore. However, the
ages between 1.2 and 1.4~Ma obtained for the N155\textdegree~E Domoni
Ridge \citep{Rusquetetal2025} could be related to the volcanism of
Grande Comore, as suggested by the trace element diagrams
(Figure~\ref{fig4}a and b), and therefore would question the hypothesis
of volcanic quiescence during this interval. Our study is the first to
report a phase of volcanism around 2.2~Ma on Grande Comore
(Table~\ref{tab1}). The break in slope in the geomorphology of the
southern area (Figure~\ref{fig5}b and e) suggests that our two samples
22CM07 and 22CM08 were collected on the edges of an ancient structure
now eroded and partly covered by younger volcanic units. These two
samples are dykes belonging to a series of intrusions oriented
N120\textdegree~E and following the break in slope. These pieces of
evidence further corroborate the existence of a M'Badjini edifice as
identified on the geologic map \citep{BacheleryCoudray1993},
preexisting to the Karthala and covered by it.

\begin{figure}
\includegraphics{fig07}
\caption{\label{fig7}Volcanic activity phases in Grande Comore showed
on an age-probability distribution spectrum \citep{DeinoPotts1992},
based on 8~K--Ar ages from this study and the whole-rock K--Ar ages of
$130 \pm 20$~ka \citep{EmerickDuncan1982}. Color of the samples is
attributed according to the new edifice classification described below
(Karthala in yellow, M'Badjini in purple, and La Grille in green). The
map background is the same as in Figure~\ref{fig1}.}
\end{figure}

Major and trace elements geochemistry also show a clear difference
between samples from La Grille and Karthala and samples from M'Badjini,
the latter being less enriched in incompatible elements than the others
(Figure~\ref{fig4}). Olivine and clinopyroxene enrichment may partly
explain this relative depletion in two samples (22CM04 and 22CM07) but
not in the third one (22CM08). These results strengthen the idea that
M'Badjini and Karthala are two distinct massifs, erupting at a
different time with a different chemistry. Finally, the submarine
continental shelf prolongating the south of the M'Badjini is the most
developed one of the whole island
\citep[Figure~\ref{fig1};][]{Tzevahirtzianetal2021},  which may
represent the continuation of
this old massif completely eroded today. These arguments support the
existence of an old, eroded massif in the southern Grande Comore, which
is clearly distinct from the Karthala massif.

In contrast, our three samples (22CM01, 22CM02, and 22CM03) were taken
from volcanic units mapped as part of the M'Badjini massif
\citep{BacheleryCoudray1993}. The ages and chemistry of these samples,
and the absence of any topographic difference with the modern Karthala
(Figure~\ref{fig5}e), show that they in fact belong to the Karthala
massif. This result implies that the history of the modern Karthala
volcano may date back to at least 280 ka, the modern subaerial edifice
being built on the remnants of a much older eroded volcano, emerged
${&gt;}$2.2~Ma ago. A consequence of our geochronological results is that
some samples collected by previous workers on the southeastern slope of
Grande Comore and purported representative of the M'Badjini massif
\citep{Classetal1998,Classetal2005}, in fact belong to the 130 ka and
230--280~ka  units of Karthala. It is thus very likely that the true
M'Badjini edifice has not yet been subjected to any detailed
geochemical investigation, as inferred from the range of values shown
in Figure~\ref{fig4}.

The evolution of the La/Sm ratio as a function of the age is given in
Figure~\ref{fig8}a for Grande Comore. It presents a slight increase of
the incompatible \mbox{element} concentration ratios with time, which might be
interpreted as a decrease in the degree of partial melting of the
lithosphere with time. This situation is reminiscent of Moh\'{e}li's
geochemical evolution, also characterized by gradual increases in
incompatible element ratios such as Ba/Ti and La/Yb within the last
${\sim}$3.8~Ma, and interpreted as decreasing partial melting
\citep{Rusquetetal2023}. Figure~\ref{fig8}b shows that such overall
increase is also observed for Mayotte and Anjouan islands. Whatever the
cause of this temporal geochemical evolution, it is thus a common
pattern of Grande Comore and other islands from the Comoros
archipelago.\looseness=-1

\begin{figure}
\vspace*{3pt}
\includegraphics{fig08}
\vspace*{2pt}
\caption{\label{fig8}(a) La/Yb ratio of dated samples as a function of
time (in Ma), calculated after normalization to chondrite values 
\citep[after][]{SunMcdonough1989}. 
The color chart of the symbols is associated
with the K--Ar age of the sample, from purple (oldest) to red (most
recent). (b) Same as (a) including published data from other islands
and submarine rocks within the archipelago 
\citep{Pelleteretal2014,Quidelleuretal2022,Rusquetetal2023,Rusquetetal2025}.}
\end{figure}

Finally, Grande Comore seems to have had a discontinuous volcanic
activity. First an old volcanic phase at more than 2~Ma built the
M'Badjini edifice, in agreement with the onset of Grande Comore
volcanism inferred from seismic data at $2 \pm 1$ Ma 
\citep[][accepted]{Masqueletetal2024b}. This volcano likely
experienced a series of dismantling processes to make it a remnant
subsequently drowned by lava flows of the 200--300 ka volcanic phase.
Karthala and La Grille volcanoes likely built during this latter period
as suggested by widespread ages. Whether La Grille experienced a
westward flank destabilization after this period of activity to explain
the morphological discontinuity and the concave geometry of the
submarine flank is still unclear (Figure~\ref{fig5}c). In contrast, the
obvious scars on the Karthala's east and southwest flanks indicate that
several successive flank failures occurred after this building period.
The presence of a network of narrow valleys in the east flank points to
a lull of activity after the flank failure long enough to enable
fluvial incision. Sporadic eruptions may have occurred during this
period as suggested by the $130 \pm 20$ ka age along the SE rift zone.
The activity definitely resumed in the last 15 ka in the two massifs of
Karthala and La Grille. Radiometric ages reveal that eruptions fed lava
flows channelized down to the seashore on the flanks of Karthala and
built scoria cones of the summit of La Grille.\looseness=-1


\begin{figure*}
\vspace*{-3pt}
\includegraphics{fig09}
\vspace*{-3pt}
\caption{\label{fig9}(Top) Volcanic activity phases in the Comoros
Archipelago showed on an age-probability distribution spectrum
\citep{DeinoPotts1992}, based on reliable ages, onshore and offshore,
from the literature (see text for references) and the 8~K--Ar ages from
this study. Grande Comore probability peaks are shown in brown, Mayotte
in beige, Moh\'{e}li in blue, Anjouan in green and the submarines in
grey. (Bottom)~Closed-up for the last 1 Myr.}
\vspace*{-3pt}
\end{figure*}

\subsection{\mbox{Volcanic evolution of the Comoros} Archipelago}\label{sec53}

Relative ages probability plots \citep{DeinoPotts1992} obtained for
Grande Comore are shown in Figure~\ref{fig9} along with the literature
data of the Comoros Archipelago selected with the criteria discussed
above. The close-up for the last 1 Myr allows us to clearly distinguish
several phases of Comorian volcanism. The recent phase starting at
${\sim}$10~ka described for Grande Comore matches the recent volcanism of
the Comoros Archipelago, with volcanism up to ${\sim}$30 ka on Moh\'{e}li
and Anjouan, and the rather recent activity suggested for Petite Terre
at Mayotte \citep[e.g.,][]{Lacombeetal2024}. The period without volcanism
evidenced on Grande Comore between ${\sim}$50 and ${\sim}$130 ka is also
observed for all the other Comoros islands. Then, between ${\sim}$200~ka
and ${\sim}$300~ka, the volcanic activity on Grande 
\mbox{Comore} \mbox{occurs}
concomitantly with volcanic activity on Moh\'{e}li and Anjouan, as well
as submarine volcanism. The archipelago undergoes several other
distinct periods of volcanism: a submarine episode at ${\sim}$400 ka,
activity on Moh\'{e}li and Anjouan around ${\sim}$530~ka, and then a more
diffuse stage nearly uninterrupted of volcanism from ${\sim}$700~ka to
${\sim}$1.5~Ma on Mayotte, Anjouan, Moh\'{e}li and submarine volcanism.
The M'Badjini stage identified in Grande Comore at ${\sim}$2.2~Ma
slightly precedes the ${\sim}$1.8~Ma volcanism on Moh\'{e}li. Finally, a
volcanic episode occurs between ${\sim}$3.2--3.5~Ma on Moh\'{e}li and
Mayotte, and another one is recorded in Mayotte at ${\sim}$5.9~Ma.
Altogether, these ages show that the emersion of Grande Comore is older
than that of Anjouan, and thus that Grande Comore is not the youngest
island of the Comoros Archipelago as previously thought.

We note that Mayotte and Moh\'{e}li islands have carbonate platforms
developed on edifice slopes aged of less than 2~Ma
\citep{Rusquetetal2023,Rusquetetal2025}, whereas Anjouan and Grande
Comore are nearly devoid of shelves despite being of similar age.
Mayotte and Moh\'{e}li also differ from Anjouan and Grande Comore by
showing evidence of subsidence whereas the two later islands do not
\citep{Debeuf2004,Tzevahirtzianetal2021,Rusquetetal2023,Rusquetetal2025}. This
comparison suggests that subsidence is the primary factor of coral
growth and shelf development around volcanic islands, whereas activity
rate and age of the volcano is only a secondary factor. Thus, we think
that Anjouan and Grande Comore did not develop a large carbonate
platform because they never subsided, or if they did, the eroded
reliefs were too small to allow a carbonate platform to develop.


\subsection{\mbox{Tectonic evolution of the Comoros} Archipelago}\label{sec54}

An element of answer for the origin of the Comorian volcanism could be
found in the repartition of the volcanism in the archipelago over time.
In Figure~\ref{fig6}, samples with similar age ranges are represented
together on a line. This enables us to distinguish several alignments
of volcanic periods. Volcanism older than 3~Ma is present on Moh\'{e}li
and Mayotte. The south of Grande Comore, Anjouan, and the Jumelles
Ridges have their oldest volcanism ranging from 2 to 1~Ma. The northern
part of Grande Comore and the Mwezi Volcanic Province have a volcanism
younger than 200--300 ka. These observations could imply that the
volcanism is propagating northward in the Comoros Archipelago,
resulting in a presently wider active volcanic area than it was before
3~Ma.

Recently, new hypotheses have been raised to link volcanism with a
transtensive \citep{Feuilletetal2021} or strike-slip
\citep{Faminetal2020} tectonic context, resulting in fractures and
faults between Somali and Lwandle plates. In both cases, there could be
a reactivation of deep lithospheric fractures
\citep{Nougieretal1986,Feuilletetal2021,Boymondetal2025},  facilitating
the magma ascent from the base of the lithosphere to the surface,
possibly in link with changes in motion of the Somalia/Lwandle/Rovuma
plate system \citep{Rusquetetal2025}. Our results suggest that the
Comorian volcanism is widening with time as in volcanic zones caused by
lithospheric thinning (Figure~\ref{fig6}), which is an additional
argument supporting these hypotheses. 

\section{Conclusions}\label{sec6}

Our new groundmass K--Ar ages suggest that the volcanism of Grande
Comore is much older than 0.13~Ma as previously proposed
\citep{EmerickDuncan1982}. Indeed, we report here an earlier subaerial
volcanic phase on the island around 2.2~Ma implying that the volcanic
history of Grande Comore now includes at least three stages: stage 1
around 2.2~Ma corresponds to the construction of a M'Badjini volcano.
Stage 2 between 230--280~ka or 130--280~ka is related to the main
construction of La Grille and Karthala volcanoes, and stage 3 since 
15~ka is the latest period of activity in La Grille and Karthala.
During a protracted period of quiescence between 2.2~Ma and 280~ka, the
island has undergone strong weathering and probably flank collapses, as
inferred by the numerous debris avalanche deposits found around Grande
Comore \citep{Tzevahirtzianetal2021}. These processes have affected the
strongly eroded M'Badjini since 2~Ma and together with its new covering
by Karthala products, this prevents us from reconstructing its original
shape.~Major and trace elements geochemistry, combined with
geomorphological analyses, show a clear separation between the Karthala
and the M'Badjini. The emersion of Grande Comore is thus probably older
than that of Anjouan. Grande Comore, Anjouan, Moh\'{e}li and Mayotte
have thus all experienced volcanic activity in the interval 2 to 0~Ma.
However, Moh\'{e}li and~Mayotte have experienced subsidence whereas
Grande Comore and Anjouan have not. The old ages obtained in this
study, along with the recent active submarine volcanism off the coast
of~Mayotte and the subaerial ages recently obtained 
\citep{Quidelleuretal2022,Rusquetetal2023,Rusquetetal2025},  reveals
that the Comoros Archipelago does not follow a simple scenario of
westward propagating volcanism as in the hotspot hypothesis. Rather,
volcanism appears to widen from south to north, as in magmatic zones
induced by lithospheric deformation. The fact that the volcanic phases
identified here on Grande Comore match those of the entire Comoros
Archipelago, and that changes in the geodynamics of the area observed
between 3 and 2~Ma from volcanic alignments \citep{Rusquetetal2025} and
further supported by our new ages, suggest a causal relationship
between volcanism and tectonics within the whole volcanic chain from
the Comoros to northern Madagascar \citep{Rusquetetal2023}.

\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*{Acknowledgements}

This work was supported by several national projects focused on the
Mozambique Channel, including the project COYOTES (ANR-19-CE31-0018,
\url{https://anr.fr/Projet-ANR-19-CE31-0018}) funded by the French ANR
(Agence Nationale de la Recherche) and the BRGM, the project INTERREG
HATARI funded by the European Union, and the French oceanographic
campaigns SISMAORE and SCRATCH.

\CDRGrant[ANR]{ANR-19-CE31-0018}

\back{}

\section*{Supplementary data}
Supporting information for this article is available on the journals
website under \printDOI\ or from the author.

\CDRsupplementaryTwotypes{supplementary-material}{\cdrattach{crgeos-292-suppl.pdf}}

\printbibliography
\refinput{crgeos20241039-reference.tex}

\end{document}
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