B Zhao

J Gong

T Tong

Y Xu

E Semail

N. -K Nguyen

F Gillon

N.-K Nguyen

A Novel Five-Phase Fractional Slot Concentrated Winding with Low Space Harmonic Contents

Keywords: phase permanent magnet (PM) machine, finite element analysis, low-harmonic contents

In this article, a novel five-phase fractional-slot concentrated winding (FSCW) with 20-slot/22-pole is presented. It benefits not only the advantages of conventional FSCW but also weak space harmonics of magnetomotive force (MMF). The winding allows eliminating the first sub-order harmonic. The new layout of the winding topology is obtained by a combination of stator shift technique of the winding in the slots with a special coupling of the windings (star-pentagon), using winding function theory. The high performances of the new winding layout are validated using the finite element method (FEM). Compared to the conventional winding, the winding factor and the total harmonic distortion (THD) of MMF are improved by 1.3% and 2.2%, respectively. With the same injection of current density, the average output torque is increased by 1% and the torque ripple is decreased by 60%. The eddy current losses in the permanent magnets (PMs) at rated speed (600 r/min) and 2100 r/min speed are improved by 67% and 56%, respectively.

F RACTIONAL-SLOT concentrated winding (FSCW) are widely applied in electrical machines, due to the advantages of short end windings, high power density, high efficiency, high slot-filling factor, small cogging torque, etc., [START_REF] El-Refaie | Fractional-slot concentrated-windings synchronous permanent magnet machines: Opportunities and challenges[END_REF], [START_REF] Gong | Design considerations of five-phase machine with double p/3p polarity[END_REF]. Numerous commercial three-phase traction motors with FSCW can be found, such as Honda (24 slots/16 poles), Bosch (36 slots/ 24 poles), Hyundai (24 slots/16 poles), and Toyota (12 slots/8 poles). In the case of multiphase machines, more choices of slot/pole combinations of FSCW can be made, which provides more possibilities for the requirement of the machine performance [START_REF] El-Refaie | Fractional-slot concentrated-windings synchronous permanent magnet machines: Opportunities and challenges[END_REF].

One of the main challenges of FSCW is the rich content of space harmonics of magnetomotive force (MMF), including both sub-and high-orders. These parasite harmonics can lead to local core saturation [START_REF] Bianchi | Design considerations for fractional-slot winding configurations of synchronous machines[END_REF], Eddy current loss, vibration and noise, etc., which deteriorate the performance of the machine, especially for high-speed applications. Therefore, many research studies have been made to avoid the bad selection of slot/pole combinations [START_REF] Aslan | Slot/pole combinations choice for concentrated multiphase machines dedicated to mildhybrid applications[END_REF], [START_REF] Aslan | General analytical model of magnet average eddy-current volume losses for comparison of multiphase PM machines with concentrated winding[END_REF]. In addition, to furtherly reduce or eliminate parasitic harmonics of classical slot/pole combinations of FSCW, various techniques have been proposed [START_REF] Dajaku | Advanced optimization methods for fractional slot concentrated windings[END_REF]. Stator shifting method is the most common one, which can significantly reduce the loss-producing harmonics and improve the flux weakening performance by extending the one slot coil pitch to two slots [START_REF] Reddy | Generalized approach of stator shifting in interior permanent-magnet machines equipped with fractional-slot concentrated windings[END_REF]- [START_REF] Abdel-Khalik | A six-phase 24-slot/10-pole permanent-magnet machine with low space harmonics for electric vehicle applications[END_REF]. The effectiveness of this method was also validated with multiphase machines in [START_REF] Patel | Six-phase fractionalslot-per-pole-per-phase permanent-magnet machines with low space harmonics for electric vehicle application[END_REF] and [START_REF] Wang | Synthesis of high performance fractional-slot permanent-magnet machines with coil-pitch of two slotpitches[END_REF]. Unfortunately, the end-winding is overlapped, which makes the winding production more complex. Unequal tooth widths technique is proposed in [START_REF] Zheng | Investigation of a novel 24-slot/14-pole six-phase fault-tolerant modular permanentmagnet in-wheel motor for electric vehicles[END_REF] and [START_REF] Dajaku | A novel 24-slots/10-poles winding topology for electric machines[END_REF] and integrated with an optimization algorithm for the winding design in [START_REF] Bekka | A novel methodology for optimal design of fractional slot with concentrated windings[END_REF] and [START_REF] Tang | A systematic approach for developing electric machine windings with suppressed MMF space harmonics[END_REF]. A higher number of layers (≥3) can also be used for the reduction of dominant loss-producing harmonics [START_REF] Tang | Framework and solution techniques for suppressing electric machine winding MMF space harmonics by varying slot distribution and coil turns[END_REF], [START_REF] Cistelecan | Three phase tooth-concentrated multiple-layer fractional windings with low space harmonic content[END_REF]. However, with the increase of the layer number, the winding becomes more complex to realize. In this article, based on the stator shift technique but with the endwinding non-overlapping, a new topology of five-phase FSCW with selective harmonic elimination is proposed by combining the star and pentagon coils connection [START_REF] Zhao | Star and delta hybrid connection of a FSCW PM machine for low space harmonics[END_REF]. This method can also be extended to any number of phase winding design. This article will be organized as follows. In Section II, the basic structure of the traditional five-phase 20-slot/ 22-pole winding with MMF harmonics presentation is introduced, and the derivation process of the proposed new winding topology layout using winding shift technology is introduced. In Section III, the finite element method (FEM) is used to verify the high performance of machines with the proposed winding topology.

II. DERIVATION PROCESS OF NEW WINDING WITH LOW MMF HARMONIC CONTENTS

A. Conventional 20-Slot/22-Pole Winding

For electrical machines with FSCW, the relationship between the slot number (Q) and the pole number (2 p), i.e., 2 p = Q ± 2, is often satisfied to obtain high flux linkage and torque density. Therefore, a five-phase winding topology in [START_REF] Sui | Research on a 20-slot/22-pole five-phase fault-tolerant PMSM used for fourwheel-drive electric vehicles[END_REF] is chosen here as a reference to present the new method, which can be used to eliminate the parasite harmonics of MMF. The method can be easily extended to other similar slot/pole combinations of different phases. Fig. 1(a) shows the structures of the outer-rotor 5-phase/20-slot/22-pole permanent magnet synchronous machines (PMSMs) with conventional winding configuration, which can be used for in-wheel traction application. The windings of different phases are distinguished by color and the A-phase winding is marked as an example. It can be noted that the winding of each phase consists of four coils which can be connected in series or parallel. The MMF of this winding topology is analyzed using winding function theory and shown in Fig. 1(b). It can be seen that the amplitude of the sub-and sup-harmonics are important compared to the working harmonic (11th). These harmonics of MMF rotate in the air gap with different speeds inducing eddy-currents and causing losses not only on the stator but also on the permanent magnets (PMs) on the rotor, local saturation and torque ripple, etc., which are expected to be as small as possible.

B. Proposal of New Layout of 20-Slot/ 22-Pole Winding

To eliminate or reduce the parasitic harmonics, the conventional winding configuration of each phase can be split into two sub-windings A1 and A2 as shown in Fig. 1(a), and each sub-winding consists of two coils. Considering the injection of fundamental current, the vth MMF harmonic of sub-winding A1 is expressed as

f 1v = 5k w N I πv cos(ωt ∓ vθ) (1)
where k w is the winding factor, N is the effective number of turns of each coil around tooth, I is the rms value of the current, ω is the angular velocity, θ is the initial angle, the harmonics rotating in the same direction as the working harmonic as are with "-," and the reverses are with "+." Now the new stator shift technique is applied to the conventional configuration. Supposing that the sub-winding A2 is shifted by a mechanical angle δ, and the injected current is shifted by an electrical angle φ, the vth MMF harmonic of the sub-winding 2 can be expressed as

f 2v = 5k N I πv cos[(ωt -φ) ∓ v(θ -δ)]. ( 2
)
Combining ( 1) and ( 2), the vth harmonic of the total MMF can be then expressed as

f v = 5k N I πv •2•cos φ ∓ vδ 2 cos ωt - φ 2 -v θ - δ 2 .
(3)

According to (3), a distribution factor of the vth harmonic of MMF is defined in the following equation:

k v = cos φ ∓ vδ 2 . ( 4
)
The distribution factor of working harmonic can be designed to be 1, i.e., k 11 = 1, and the ones of the other parasite harmonics can be selectively eliminated by designing k v . In this article, the first harmonic is first chosen to eliminate, i.e., k 1 = 0, and the following relationship can be obtained:

φ -11δ = 2kπ, with k 11 = 1 ( 5 ) φ -δ = (2k + 1)π, with k 1 = 0. ( 6
)
Since the shifted mechanical angle δ should be integral multiple of the slot angle (360 • /20 = 18 • ), the optimal angle δ optim is equal to 126 • , i.e., the sub-winding 2 is shifted seven slots compared to the sub-winding 1. In this case, the phase difference of the currents between these two sub-windings is φ optim = -54 • , i.e., the injected current in sub-winding A2 is lagged 54 • to the sub-winding A1.

In the case of the elimination of the ninth harmonic, the relationship in ( 7) should be respected

φ -δ = (2k + 1)π, with k 9 = 0. (7)
However, the obtained angle δ from ( 5) and ( 7) cannot be achieved practically due to the condition of the integral multiple of the slot angle.

The proposed new winding configuration is shown in Fig. 2(a), which consists of two sub-winding groups: A1 and A2 and shifted by seven slots in the anti-clockwise direction.

C. Supply Strategy of the New Winding

The phase difference of the injected currents between the two sub-windings can be obtained by using two five-leg voltage source inverter (VSI) inverter but also in the precise case by achieving a star and pentagon connection between the windings as shown in Fig. 2(b). In a five-phase machine supplied in steady-state operation, the shift angle between two successive voltages is effectively (180 • -360/5)/2 = 54 • . To keep the balance MMF distribution between these two subwindings, the number of turns of A1 is equal to ( √ 5/2) times of the one of A2. Meanwhile, the section of the conductor should be also adjusted in the same ratio to have the same current density.

D. Winding Factors and MMF Analysis

For the new winding layout, the winding distribution coefficient k d = 1 and a short-pitch coefficient k p = 0.9877, and the winding factor is 0.988 instead of 0.9755 for the initial layout. Hence, the higher flux linkage and the higher torque density can be obtained with the new layout.

The main advantage of the new winding configuration is the elimination of selected space harmonics of MMF, while the short-end characteristic of FSCW is kept. Using the theory of winding functions, Fig. 3 shows the comparison of the MMF

TABLE I MAIN SPECIFICATIONS OF THE TWO MACHINES

and the harmonic spectrum between the two winding configurations. It can be seen that the first sub-space harmonic of the new winding is eliminated, the 11th one is slightly increased, and the total harmonic distortion (THD) is reduced by 2.2%. The method has been presented for a particular five-phase machine but it can be used for other multiphase machines.

III. PERFORMANCE EVALUATION USING FEM

A. Machine Design

To verify the previous analysis, the 5-phase/20-slot/22-pole machines with both winding layouts are designed and analyzed using FEM. Table I presents the main specifications of the machine.

B. Comparison of Winding-Produced Air-Gap Flux Density and Spectrum

Fig. 4 shows the winding flux line distribution of the proposed machine. To remove the interference of the rotor magnetic field on the MMF of the windings, the PMs on the rotor are considered as laminated iron.

The flux density of the two machines under no-load is shown in Fig. 5. Only one flux density is shown due to the two are the same because the two machines have a similar rotor except for coil turns.

The MMF harmonic spectrum can be verified by analyzing the winding produced air-gap flux density [START_REF] Sui | Research on a 20-slot/22-pole five-phase fault-tolerant PMSM used for fourwheel-drive electric vehicles[END_REF]. The waveforms and the harmonic spectrum of the two configurations are compared in Fig. 6. It can be seen that the first sub-order space harmonic is eliminated and the THD is improved by 2.1%. Hence, similar results are obtained compared to the ones obtained by winding function theory in Section II-C.

C. Back Electromagnetic Force

The FEM predicted back electromagnetic forces (EMFs) of the conventional and the proposed new windings are shown in Fig. 7

D. Comparison of PM Loss and Efficiency

The PM losses of the two machines at rated speed (600 r/min) and 2100 r/min speed are calculated, respectively, and shown in Fig. 8. It can be seen that there are less PM losses for the proposed winding due to the less parasite MMF harmonics, which shows the effectiveness of the new method for winding design and also the advantages of the new structure of the winding. Fig. 9 shows the efficiency of the two machines at rated speed (600 r/min) and 2100 r/min speed, under the same injected current, respectively. It can be seen that the efficiency both speeds has been improved. At the 2100 r/min speed, the efficiency has increased from 85.3% to 86.5%.

E. Electromagnetic Torque Characteristic

Fig. 10 shows the electromagnetic torque characteristics with the same current density injection J = 5 A/mm 2 . It can be noted that the average torque is increased by 1% and the torque ripple is reduced by 60% with the new winding layout. Fig. 11 shows the torque-speed curves under the same condition. It can be noted that with the new winding layout, the average torque is slightly increased during the entire operating range.

IV. CONCLUSION

In this article, a novel 5-phase/20-slot/22-pole winding topology is proposed by a stator shift technique combined with a star/pentagon winding connection. Compared to the conventional configuration, the first harmonic of MMF in the new winding is eliminated. Moreover, the non-overlapping characteristics of FSCW are kept. The winding factor of new winding is improved, and the THD of MMF is reduced by 2.1%. The output torque is increased by 1%, and the torque ripple is reduced by 60%. The FEM results proved the effectiveness of the proposal.

Fig. 1 .

 1 Fig. 1. 5-phase/20-slots/22-poles conventional winding. (a) Conventional layout. (b) MMF harmonics [20].

Fig. 2 .

 2 Fig. 2. 5-phase/20-slots/22-poles new winding. (a) New winding layout. (b) Supply strategy.

Fig. 3 .

 3 Fig. 3. Comparison of MMF waves and harmonics. (a) MMF wave. (b) MMF harmonics.

Fig. 4 .

 4 Fig. 4. Flux line distribution with the new winding layout.

Fig. 5 .

 5 Fig. 5. Flux density of two machines under un-load.

Fig. 6 .

 6 Fig. 6. Air gap flux density waves and harmonics without PMs. (a) MMF wave. (b) MMF harmonics.

  (a). It can be seen that the same value of back EMF is obtained for both configurations (53.14 and 53.08 V for the conventional and the proposed new windings, respectively), which can avoid the disadvantage of the conventional stator shift technique. The back-EMFs of the two sub-windings of the new winding are shown in Fig. 7(b). The different values are obtained due to the different number of turns, which can keep the balance MMF distribution between the two sub-windings with the current supply strategy shown in Fig. 2(b).

Fig. 7 .Fig. 8 .Fig. 9 .

 789 Fig. 7. Back-EMFs at 600 r/min. (a) Comparison of the back EMF of two machines under no-load. (b) Comparison of the back EMF of two subwindings under no-load with √ 5/2 ratio.

Fig. 10 .

 10 Fig. 10. Comparison of electromagnetic torque.

Fig. 11 .

 11 Fig. 11. Comparison of torque-speed

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ACKNOWLEDGMENT

This work was supported by the Natural Science Foundation of Shandong Province of China through the Project under Grant ZR2020ME207.


Maksym Dekhtiarenko

Simon Pascal

Mourad Elhabiri

Valérie Mazan

David Canevet

Magali Allain

Vincent Carré

Frédéric Aubriet

Zoia Voitenko

Marc Sallé

Valerie Mazan

Olivier Siri

Sébastien Goeb

Reversible pH-Controlled Catenation of a Benzobisimidazole-based Tetranuclear Rectangle

Keywords: Benzobis(imidazole), Host-guest systems, Metalla-cycles, Self-assembly, pH, Catenation

niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Herein, we report the synthesis of two self-assembled metalla-rectangles Oxa2BBI2 and Naph2BBI2 featuring the pH sensitive benzobisimidazole (BBI) unit which also incorporate two pyridine fragments able to coordinate the metal centers. These structures differ essentially by the distances separating both BBI ligands, a parameter which depends on the size of the bisruthenium complex spacer (RuOxa or RuNaphto) (Erreur ! Source du renvoi introuvable.). On this basis, it is worth mentioning that whilst the smaller rectangle Oxa2BBI2 exists only as a discrete monomer species, the larger Naph2BBI2 can dimerize to form the catenated (Naph2BBI2)2 structure. Remarkably, this responsive interlocked species is able to dissociate in response to three different types of chemical or physical stimuli: i) dilution, ii) addition of pyrene and iii) protonation of the benzobisimidazole sidewalls (Erreur ! Source du renvoi introuvable.).

The synthesis of ligand BBI was achieved by the condensation of two equivalents of 4-pyridinecarboxaldehyde with tetraaminobenzene tetrahydrochloride in presence of piperidine and under aerobic conditions. After 16 h in refluxing acetonitrile, the ligand was isolated as a yellow-orange powder in a 80% yield. The reaction between the RuOxa [20] complex and the BBI ligand was carried out in MeOD at C = 10 -3 M (T = 50 °C) and followed by 1 H NMR spectroscopy. After 2 h, the reaction converged to a single species that could be isolated upon precipitation with tert-butyl methyl ether (MeOtBu). The resulting set of 1 H NMR signals illustrates the symmetry of the resulting self-assembled structure (Erreur ! Source du renvoi introuvable.b). A 1 H DOSY NMR experiment (Figure S3) revealed the presence of a single species in solution with a diffusion coefficient of D = 3.9 × 10 -10 m 2 s -1 in MeOD-d4 at C = 10 -3 M. A hydrodynamic radius of ca. 10 Å was estimated from the Stokes-Einstein equation, [21] a value which is compatible with the formation of the expected Ru4L2 complex Oxa2BBI2. Similarly, the reaction carried out between the RuNaphto [22] complex and ligand BBI in MeOD afforded the metalla-rectangle Naph2BBI2 characterized by a symmetric 1 H NMR spectrum (Figure 2c). As expected, a slightly larger Stokes radius of ca. 11 Å was extracted from the 1 H DOSY NMR experiment carried out in MeOD at C = 10 -3 M (Figure S6). andS32). While both Ru atoms from a RuOxa complex are separated by 5.5 Å, π-π interactions occurring between face-to-face benzobisimidazole motifs lead to an interplanar distance of only 3.4 Å between the two central BBI units. Consequently, a significant bending occurs around the peripheral pyridine units, with a torsion of ca. 12° of the benzobisimidazole-pyridine junction. In contrast, Naph2BBI2 shows a quasi-rectangular solid structure. Here, the larger distance which separates both BBI ligands (7.3 Å) does not allow π-π interactions between facing π-systems and is compensated by the accommodation of two Et2O molecules in the central pocket.

High-resolution ESI-FTICR-HRMS spectrometry experiments were also carried out on Oxa2BBI2 and Naph2BBI2 in MeOH at C = 2 × 10 -3 M (Figures S10-S12). While the reaction with RuOxa shows only signals corresponding to the monomer system Oxa2BBI2 at m/z = 630.7315 (3+) and 1020.5756 (2+) (main contributions, Figure S10), the species isolated from the reaction with the larger RuNaphto acceptor exhibits, under the same conditions, two sets of signals. They correspond to the monomeric metalla-rectangle Naph2BBI2 with characteristic isotopic patterns at m/z = 697.4194 (3+) and 1120.6059 (2+) (main contributions), and to a dimeric (Naph2BBI2)2 structure with signals recorded at m/z = 1543.4649 (3+) and 1120.6059 (4+) (main contribution, Figures S11 andS12), respectively. In all cases, experimental isotopic distributions are in good agreement with theoretical ones. On this basis, the ability of metalla-rectangle Naph2BBI2 to dimerize was further investigated by concentration dependent 1 H NMR spectroscopy. Following Le Chatelier's principle, changing the concentration promotes the reversible conversion between the interlocked structure and the monomeric constitutive unit, thanks to the labile nature of the metal-ligand bonds and throughspace interactions between the subunits. [11b] While only one set of 1 H NMR signals is observed at C = 10 -3 M (Erreur ! Source du renvoi introuvable.c), the spectrum recorded at C = 4 × 10 -2 M (Erreur ! Source du renvoi introuvable.d and S7) shows the splitting of the pyridine units (a and b protons), the para-cymene moieties (d and e protons) and the naphtoquinonato groups (i protons) signals, in accordance with the quantitative formation of the dimeric interlocked species (Naph2BBI2)2. [11a] The corresponding 1 H DOSY NMR measurement confirmed the larger size of the resulting structure with a calculated hydrodynamic radius of ca. 15 Å (T = 298 K) (Figure S9). Formation of Borromean rings can be ruled out because of the non-symmetrical character of the 1 H NMR spectra, [11a] and the distances ratio between the long and the short sides of the metalla-rectangle, which do not match with the rules proposed by G.-X. Jin. [11b] The slow equilibrium between the monomer and the dimer species at the NMR timescale, allowed for a straightforward quantification of the dimerization constant Kdim (MeOD) = 4.4(± 0.2) × 10 2 (Figure S13 andS14). [16a, 23] Despite numerous attempts, no crystal of (Naph2BBI2)2 of sufficient quality to be analyzed by X-ray diffraction has been obtained. Molecular force field (MM+) studies were therefore undertaken to determinate the structure of this interlocked species (Figure 3c). This modeling shows that the two metallacyles interlock without distortion with inter-ligand distances of about 3.4 Å as expected for such π-π stacking interactions. [11a, 16a] The stability of the dimeric (Naph2BBI2)2 assembly was further investigated upon pyrene complexation. Such an electron rich planar species is expected to fill the cavity of the metallarectangle while interacting with both facing electron-deficient benzobisimidazole units, to afford a more thermodynamically stable host-guest complex. The progressive addition of pyrene to a solution of (Naph2BBI2)2 at 10 -2 M in MeOD was followed by 1 H NMR (Figure S15). Remarkably, addition of one equivalent of pyrene per metalla-rectangle unit was sufficient to drastically simplify the NMR spectrum, as a consequence of the dissociation of the dimeric structure attributed to the intercalation of pyrene between both BBI panels of Naph2BBI2 (Figure 4a). This hypothesis was confirmed by 1 H DOSY NMR measurement (Figure S16) which clearly demonstrates a higher diffusion value of the resulting species (3.55 × 10 -10 m² s -1 ) when compared with (Naph2BBI2)2, a value which now corresponds to the one reported for isolated Naph2BBI2 ring (Figures 2c andS6). The binding capability of Naph2BBI2 for pyrene was evaluated by a 1 H NMR Job plot analysis (Figures S20 andS22a) and a titration experiment at C = 10 -3 M in MeOD-d4 (Figures S21 andS22b). [24] These measurements revealed the complexation of one pyrene unit in the cavity of Naph2BBI2 with an association constant Ka = 3.0(±0.2) × 10 3 . The optical properties of the ligand and both metallamacrocycles were then investigated in methanol solutions. While BBI shows an absorption maximum at 358 nm (Figure 5a), the corresponding ligand-centered transitions are found at 393 and 386 nm for Oxa2BBI2 and Naph2BBI2, respectively. These bands show more than doubled molar extinction coefficients (89 000 < ε < 101 000 M -1 cm -1 ) compared to the free ligand (ε = 38 000 M -1 cm -1 ). The red shift triggered by the coordination of the terminal pyridine units to ruthenium atoms is consistent with the one measured upon protonation of the BBI ligand in acidic medium (λmax = 386 nm, Figure S33). To gain information into the stability of the complexes, their electronic absorption spectra were monitored over time at different concentrations (Figure S34). At C = 2 × 10 -4 M, both Oxa2BBI2 and Naph2BBI2 complexes show unaltered absorptions after several days in solution. In contrast, for C  2 × 10 -5 M, an hypsochromic shift is observed after 24 h, corresponding to the release of uncomplexed ligand in solution. A strong emission of the BBI ligand was measured in the UV-blue region (Figure 5a), with a maximum centered at 430 nm and a fluorescence quantum yield of 40%. For both metallamacrocycles recorded in fresh solutions at C = 2 × 10 -5 M, weaker emissions are observed at around 460 nm, which is consistent with the fluorescence of recently reported tetranuclear Znbenzobisimidazoles grid complexes. [25] However, when decreasing the concentration to C = 10 -6 M, the emission is blueshifted to 430 nm and most likely corresponds to the free ligand in solution (Figure S35) as already observed by absorption spectrophotometry. [26] Complexation properties of Naph2BBI2 were further investigated by UV-vis and fluorescence spectroscopy. The absorption and emission spectra of 1:1 mixture solutions of BBI, Oxa2BBI2 or Naph2BBI2 with pyrene were compared at C = 2 × 10 -5 M (Figures 5b, S36 and S37). As expected, all absorption spectra show an additive effect between the two constituents. On the other hand, while the emission spectra corresponding to the mixture of Naph2BBI2 and pyrene reveal a quasi-complete quenching of the pyrene emission (85 % decrease in the total fluorescence), a much weaker decrease of 21% and 33% is observed in the presence of Oxa2BBI2 or BBI ligand respectively (Figures 5b andS37) illustrating the interaction of pyrene with the cavity of Naph2BBI2. [START_REF]Decrease in emission was evaluated by comparing the emission aera of a 1:1 mixture of pyrene and BBI, Oxa2BBI or Naph2BBI, with the additiion of emission aeras of pyrene and BBI, oxa2BBI and Naph2BBI respectively[END_REF] The acid-base properties of the BBI ligand and the Naph2BBI2 metalla-rectangle were then investigated using UVvisible absorption titrations as a function of pH (Figures S38-S44). The BBI ligand is characterized by 6 pH sensitive reactive sites, i.e. two on each imidazole units and one per pyridyl fragment (Figure S38). Figure S39 illustrates the spectral variations recorded during absorption spectrophotometric titration of BBI as a function of pH, which are in agreement with reported results on 4-pyridylbenzimidazole (4PBI). [28] Under neutral pH conditions (MeOH/H2O 80/20 w/w, Figure S39), BBI is characterized by an intense absorption band in the UV region centered at 357 nm ( 357 = 3.50 × 10 4 M -1 cm -1 ) in agreement with the data recorded in methanol. Increasing the pH from 6.5 to 11.9 induces a bathochromic shift of 23 nm of the π-π* transitions. The statistical processing of these data led to the determination of a pKa value of 11.2 ± 0.1 that is ascribed to the deprotonation of one of the two N1-imidazole sites (i.e., the pKa value for the deprotonation of the second N1-H function was estimated to be > 12 and could not be determined under our experimental conditions). On the other hand, the decrease in pH from 6 to  2.3 also leads to a bathochromic shift. Two pKa values of 3.7 ± 0.1 and 2.7 ± 0.1 were calculated (Figure S38). The two other pKa values were estimated to be < 2 in agreement with repulsive interactions between the various positively charged species. The neutral BBI ligand thus predominates over a broad range of pH (from 4.5 to 10.2) and can lead to BBI 4+ and BBI 2-under strongly acidic and basic conditions, respectively. Since protons can compete for Lewis base pyridine sites in coordination self-assembled structures, acidic conditions often lead to a disassembly process. [17] We therefore investigated the pH-responsive behavior of the Naph2BBI2 metalla-rectangle with the hypothesis that the presence of the imidazole moieties would allow for a selective protonation of the ligands without disassembly. Under neutral pH conditions (MeOH/H2O 80/20 w/w), Naph2BBI2 is characterized by an intense absorption in the UV region centered at 386 nm ( 386 = 8.88 x 10 4 M -1 cm -1 ) assigned to -* ligand-localized transitions as well as a set of much weaker absorptions at 643 and 693 nm that correspond to LMCT transitions (i.e., signature of the metalla-rectangle). [22] Upon complexation to RuNaphto, the π-π* transitions of the two BBI ligands undergo significant bathochromic and hyperchromic shits ( 29 nm; BBI:  357 = 3.50 × 10 4 M -1 cm -1 ; Naph2BBI2:  386 = 8.88 × 10 4 M -1 cm -1 ). Similarly to the observations in MeOH, the effect of complexation to the Ru centers by the terminal pyridines is comparable to that of their protonation (Figure S41,  = 389 nm and  389 = 2.68 × 10 4 M -1 cm -1 ). The absorption versus pH titrations demonstrate that no decomposition of Naph2BBI2 occurs over a large range of pH (2. 20 -12.05) as assessed by the ongoing presence of the LMCT signature. The statistical analysis of the data allowed evidencing two pH regions of interest (Figures S42-S44). Under strongly acidic conditions, the Naph2BBI2 metallarectangle can undergo four protonation equilibria at its four equivalent N3-imidazole sites and three protonation constants could be accurately determined (pKa4 = 2.7 ± 0.2 and pKa3 + pKa2 = 4.5 ± 0.2). The protonation of these units results in only small spectral variations both of the π-π* and LMCT transitions. In addition, the calculated pKa values are lowered compared to those of the free BBI ligand (pKa3 = 2.7 ± 0.1 and pKa4 = 3.7 ± 0.1) due to the coordination of the pyridine units to the RuNaphto metal centers. These data therefore demonstrate that the Naph2BBI2 meta-rectangle can undergo stepwise protonation on each of its N3-imidazole units without any apparent signs of structural disassembly. Likewise, each of the N1-imidazole units can undergo a deprotonation reaction without also affecting the architecture of the metallo-rectangle itself. Four deprotonation constants were determined under basic conditions (Figure S44). In contrast to acidic conditions, deprotonation of N1-imidazole units is accompanied by strong spectral variations in both intraligand π-π* transitions ( = 426 nm and  426 = 7.22 × 10 4 M -1 cm - 1 for Naph2BBI2-4H) and LMCT transitions (hypsochromic shifts of 60 nm; Figure S43).

On the basis of this in-depth investigation and considering the stability of cage Naph2BBI2 in acidic media, we then studied by 1 H NMR and DOSY NMR the behavior of interlocked (Naph2BBI2)2 in presence of trifluoromethanesulfonic acid (HOTf). We hypothesized that protonation of the BBI moieties may lead to intermolecular electrostatic repulsions that could strongly affect the stability of the interlocked structure. Remarkably, addition of HOTf (6 equivalents per metallacycle) into a solution of (Naph2BBI2)2 in MeOD-d4 at C = 2 × 10 -2 M produces quantitatively a new species (Figures 4c andS23). It shows a higher D value and non-splitted NMR signals that are downfield shifted in comparison to those of the dimeric structure (Figure 4b). These NMR characteristics support the dissociation of the protonated (Naph2BBI2)2 interlocked dimer and formation of the Naph2BBI2H4 monomer. This was unambiguously demonstrated through a comparative NMR study with Naph2BBI2 recorded in a 0.12 M solution of HOTf in MeOD at C = 10 -3 M (Figures S24 andS25) and ESI-FTICR HRMS measurements (Figure S26). Finally, it is worth noting that this original dissociation phenomenon is reversible, since addition of 6 equivalents of N,N-diisopropylethylamine (DIPEA) reforms quantitatively the starting interlocked species (Figure S23). Finally, we investigated the complexation ability of Naph2BBI2H4 toward the electron rich pyrene unit. Since protonation of the BBI units increases their electron deficient character, we expected Naph2BBI2H4 to be a better host for pyrene than Naph2BBI2. This was confirmed by a 1 H NMR titration recorded at C = 10 -3 M in a solution of HOTf (C = 6 × 10 -3 M) in MeOD-d4 (Figures S22 andS27). An association constant Ka = 1.32 (± 0.09) × 10 4 was measured, a value four times higher than the one calculated with the non-protonated Naph2BBI2 host analog. Interestingly, this tunable binding affinity can be followed by 1 H NMR and DOSY NMR upon successive addition of HOTf and DIPEA (Figures S28-S31).

In summary, two novel self-assembled metalla-rectangles incorporating a pH sensitive benzobis(imidazole)-based ligand were designed. Thanks to the reversible nature of the coordination bonds and favorable electronic interactions, the larger Naph2BBI2 which shows an interplanar distance of 7.3 Å between both ligand units can be quantitatively transformed to the interlocked molecular structure (Naph2BBI2)2 in MeOH at C = 2 × 10 -2 M. Remarkably, this stimuli responsive interpenetrated catenane can be dissociated in three different ways: i) upon dilution to a concentration of C = 1 mM, ii) in the presence of pyrene, leading the 1:1 host-guest complex pyrene⊂Naph2BBI2 and iii) by selectively protonating the N3-imidazole basic sites with HOTf to form the charged Naph2BBI2H4 metalla-rectangle. Importantly this can be accomplished without any trace of disassembled species as usually observed under acidic conditions and reversibly through addition of DIPEA.

Deposition numbers 2096156 (Oxa2BBI2), and 2096158 (for Naph2BBI2) contain the supplementary crystallographic data for this paper. These data are provided free of charge by the joint Cambridge Crystallographic

Figure 2 .

 2 Figure 2. 1 H NMR (298 K, downfield region) of a) ligand BBI in DMSO-d6, b) Oxa2BBI2 in MeOD-d4 (C = 10 -3 M), c) Naph2BBI2 in MeOD-d4 (C = 10 -3 M)

Figure 3 .

 3 Figure 3. Single crystal X-ray diffraction structures of a) Oxa2BBI2, b) Naph2BBI2 (including two solvent molecules, Et2O) and c) MM+ simulation of (Naph2BBI2)2. For a) and b) anions are omitted for clarity.

Figure 4 .

 4 Figure 4. 1 H NMR spectra recorded in MeOD at C = 2 × 10 -2 M of a) (Naph2BBI2)2 in presence of 1 equiv. of pyrene, b) (Naph2BBI2)2 and c) (Naph2BBI2)2 in presence of 6 equiv. of HOTf.

Figure 5 .

 5 Figure 5. a) Electronic absorption (solid line) and emission (dashed line) spectra of BBI (C = 2 x 10 -5 M), Oxa2BBI2 and Naph2BBI2 (C = 2 x 10 -4 M for absorption and C = 2 x 10 -5 M for emission) in methanol and b) fluorescence spectra recorded under the same experimental conditions of pyrene, Naph2BBI2 and 1:1 mixture of both compounds in methanol (C = 2 x 10 -5 M and λex = 330 nm).

Acknowledgements

The authors gratefully acknowledge the French Embassy in Kiev (Ukr) for PhD grant (MD). They also acknowledge the ASTRAL platform (SFR MATRIX, Univ. Angers) for their assistance in spectroscopic analyses and the French national FT-ICR network for its financial support (FR 3624 CNRS).

Data Centre and Fachinformationszentrum Karlsruhe Access Structures service www.ccdc.cam.ac.uk/structures.


Mélodie-Neige Cornu

Raphaël Paris
email: raphael.paris@uca.fr

Régis Doucelance

Patrick Bachélery

Chantal Bosq

Delphine Auclair

Mhammed Benbakkar

Abdel-Mouhcine Gannoun

Hervé Guillou

Exploring the links between volcano flank collapse and the magmatic evolution of an ocean island volcano: Fogo, Cape Verde

des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

New insights on the volcanic history of Fogo Island

The Cape Verde Islands are considered as the surface expression of a mantle plume at 500-800 km west of the African continental margin [START_REF] Gerlach | Magma sources of the Cape Verdes archipelago: Isotopic and trace element constraints[END_REF][START_REF] Doucelance | Pb-Sr-He isotope and trace element geochemistry of the Cape Verde Archipelago[END_REF][START_REF] Pim | Crustal structure and origin of the Cape Verde rise[END_REF] . The spatial and chronological evolution of volcanic activity, from East to West, is consistent with the slow progression of the African plate over the hotspot since at least the Oligocene, with a decoupling between two distinct branches at ca. 5 Ma [START_REF] Holm | An 40Ar-39Ar study of the Cape Verde hot spot: Temporal evolution in a semistationary plate environment[END_REF] . All islands exhibit Quaternary volcanism except Boa Vista and Maio, but Fogo was the only active volcano of the archipelago for the last five centuries [START_REF] Ribeiro | A Ilha do Fogo e as suas erupcoes[END_REF][START_REF] Machado | Vulcanismo das islas de Cabo Verde e das outras ilhas atlantidas[END_REF][START_REF] Torres | Carta geologica das erupcoes historicas da Ilha do Fogo : Revisao e actualizacao In A erupcao vulcanica de 1995 na Ilha do Fogo, Cabo Verde[END_REF] . Fogo Island has the morphology of a shield volcano overtopped by a steeper stratovolcano that is truncated by an 8 km-wide horseshoe-shaped caldera opened to the east (Fig. 1: Bordeira). The caldera is bounded on its northern, western and southern sides by steep walls up to 1000 m high offering an open book on the eruptive and intrusive evolution of the volcanic edifice. The origin of the caldera is still debated: a pure gravitational model implying at least one massive flank collapse of the eastern flank of the volcano [START_REF] Foeken | Cosmogenic 3He exposure dating of the Quaternary basalts from Fogo, Cape Verdes: Implications for rift zone and magmatic reorganisation[END_REF][START_REF] Marques | Caldera or flank collapse in the Fogo volcano? What age? Consequences for risk assessment in volcanic islands[END_REF][START_REF] Day | A past giant lateral collapse and present-day flank instability of Fogo, Cape Verde Islands[END_REF] , and a hybrid model combining central subsidence (caldera collapse s.s.) and flank collapse [START_REF] Martínez-Moreno | Investigating collapse structures in oceanic islands using magnetotelluric surveys: The case of Fogo Island in Cape Verde[END_REF][START_REF] Brum Da Silveira | A estrutura da Ilha do Fogo, Cabo Verde. In A Erupção Vulcânica de 1995 na Ilha do Fogo[END_REF][START_REF] Madeira | The role of mass movements on the geomorphologic evolution of island volcanoes: Examples from Fogo and Brava in the Cape Verde archipelago[END_REF] . The caldera floor is filled by post-caldera volcanism, including the growth of the Pico do Fogo central cone and historical lavas. The volcanic history of Fogo Island is poorly documented, although different stages of growth separated by major unconformities were distinguished [START_REF] Foeken | Cosmogenic 3He exposure dating of the Quaternary basalts from Fogo, Cape Verdes: Implications for rift zone and magmatic reorganisation[END_REF][START_REF] Day | A past giant lateral collapse and present-day flank instability of Fogo, Cape Verde Islands[END_REF][START_REF] Machado | Vulcanismo das islas de Cabo Verde e das outras ilhas atlantidas[END_REF] . Uplifted lavas of the seamount stage are exposed in valleys of the western part of the island together with carbonatite dated 5.1 to 3.2 Ma [START_REF] Foeken | Carbonatite Seamount Formation and First Subaerial Exposure of Fogo[END_REF][START_REF] Madeira | K/Ar ages of carbonatites from the Island of Fogo (Cape Verde)[END_REF] . The early subaerial stage (lower shield), exposed in the northern valleys and in the lower part of the caldera walls, is not chronologically well constrained, with only one lava flow dated 212 ± 20 ka (K-Ar) at the base of the caldera wall [START_REF] Marques | Caldera or flank collapse in the Fogo volcano? What age? Consequences for risk assessment in volcanic islands[END_REF] . The lavas produced during this stage range from nephelinite and melilitite, to phonolite. Eroded remnants of the lower shield are overlapped by an upper shield, the Monte Amarelo edifice [START_REF] Day | A past giant lateral collapse and present-day flank instability of Fogo, Cape Verde Islands[END_REF] , thus forming a major erosional unconformity that is clearly visible on the walls of the caldera (Fig. S1). The Monte Amarelo stage is characterised by the growth of a ~ 3000 m high central edifice with poorly-defined peripheral rift-zones [START_REF] Day | A past giant lateral collapse and present-day flank instability of Fogo, Cape Verde Islands[END_REF] . The central part of this edifice is densely intruded by dykes and sills, as observed all along the western caldera wall.

Our new K-Ar and [START_REF] Brum Da Silveira | A estrutura da Ilha do Fogo, Cabo Verde. In A Erupção Vulcânica de 1995 na Ilha do Fogo[END_REF] Ar/ [START_REF] Torres | Carta geologica das erupcoes historicas da Ilha do Fogo : Revisao e actualizacao In A erupcao vulcanica de 1995 na Ilha do Fogo, Cabo Verde[END_REF] Ar ages of lava flows (Tables S2 &S3) allow the dating of the early activity of the Amarelo shield at 160 ± 4 ka on the northern caldera wall (Fig. S1), and 135 ± 4 ka on the western wall (Fig. S2). The northern rift-zone was particularly active between 160 and 122 ka. Waning of activity to the north at ~ 120 ka coincides with enhanced activity of the central edifice, including explosive activity. Indeed, thick units of ignimbrites represent half of the material accumulated along the western wall, which contrasts with the absence of explosive products on the northern wall (Fig. S2). Interestingly, it is possible to correlate the ignimbrite units with tephra layers observed in deep-sea cores [START_REF] Eisele | Pleistocene to Holocene offshore tephrostratigraphy of highly explosive eruptions from the southwestern Cape Verde Archipelago[END_REF] . A major tephra layer dated 145 ka [START_REF] Eisele | Pleistocene to Holocene offshore tephrostratigraphy of highly explosive eruptions from the southwestern Cape Verde Archipelago[END_REF] might in fact correspond to the ignimbrite observed at the base of the western caldera wall (sample Fo-22 > 135 ka on Fig. S2). Other marine tephra layers at 117 and 88 ka are also concordant with the stratigraphic position of the main ignimbrites found on the western caldera wall. The latest ignimbrite is found on the caldera rim or intercalated with the uppermost pre-collapse lava flows (Figs. S2 &S3). The recurrence of eruptions producing ignimbrites at Fogo, as reported here for the first time, corroborates the existence of caldera collapse structures in the central part of the volcanic edifice [START_REF] Martínez-Moreno | Investigating collapse structures in oceanic islands using magnetotelluric surveys: The case of Fogo Island in Cape Verde[END_REF][START_REF] Brum Da Silveira | A estrutura da Ilha do Fogo, Cabo Verde. In A Erupção Vulcânica de 1995 na Ilha do Fogo[END_REF][START_REF] Madeira | The role of mass movements on the geomorphologic evolution of island volcanoes: Examples from Fogo and Brava in the Cape Verde archipelago[END_REF] .

Cape Verde

New constraints on the age of the Monte Amarelo collapse

The eastern flank of the Monte Amarelo edifice was destroyed by a massive flank failure leading to the formation of the horseshoe-shaped caldera [START_REF] Masson | Flank collapse and large-scale landsliding in the Cape Verde Islands, off West Africa: Large-scale landsliding, Cape Verde Islands[END_REF][START_REF] Paris | Tsunami deposits in Santiago Island (Cape Verde archipelago) as possible evidence of a massive flank failure of Fogos volcano[END_REF][START_REF] Martínez-Moreno | Investigating collapse structures in oceanic islands using magnetotelluric surveys: The case of Fogo Island in Cape Verde[END_REF][START_REF] Day | A past giant lateral collapse and present-day flank instability of Fogo, Cape Verde Islands[END_REF] . Multibeam bathymetry of the eastern flanks of Fogo Island [START_REF] Masson | Flank collapse and large-scale landsliding in the Cape Verde Islands, off West Africa: Large-scale landsliding, Cape Verde Islands[END_REF][START_REF] Barrett | Revisiting the tsunamigenic volcanic flank collapse of Fogo Island in the Cape Verdes, offshore West Africa[END_REF] confirmed the existence of a large debris avalanche deposit covering 6820 km 2 with a volume > 160 km [START_REF] Carracedo | Giant Quaternary landslides in the evolution of La Palma and El Hierro, Canary Islands[END_REF] and a thickness of 100-400 m. The age of the Monte Amarelo collapse was previously inferred from (a) [START_REF] Carracedo | Giant Quaternary landslides in the evolution of La Palma and El Hierro, Canary Islands[END_REF] He exposure ages of precollapse (~ 123 ka) and early post-collapse (~ 62 ka) lava flows on Fogo 27 ; (b) [START_REF] Carracedo | Giant Quaternary landslides in the evolution of La Palma and El Hierro, Canary Islands[END_REF] He exposure ages of megaclasts transported by the collapse-generated tsunami on Santiago Island (65-84 ka [START_REF] Ramalho | Hazard potential of volcanic flank collapses raised by new megatsunami evidence[END_REF] ), (c) and K-Ar ages of pre-collapse (~ 60 ka) and post-collapse (43 ka) lava flows of the caldera walls [START_REF] Marques | Caldera or flank collapse in the Fogo volcano? What age? Consequences for risk assessment in volcanic islands[END_REF] .

Our K-Ar and [START_REF] Brum Da Silveira | A estrutura da Ilha do Fogo, Cabo Verde. In A Erupção Vulcânica de 1995 na Ilha do Fogo[END_REF] Ar/ [START_REF] Torres | Carta geologica das erupcoes historicas da Ilha do Fogo : Revisao e actualizacao In A erupcao vulcanica de 1995 na Ilha do Fogo, Cabo Verde[END_REF] Ar ages give a more precise age of ~ 68 ka for the Monte Amarelo collapse (Fig. 1). Indeed, the uppermost pre-collapse lava flow of the southern caldera wall, which is K-Ar dated 69 ± 2 ka (Fig. S3 & Table S2), is overlain by a post-collapse lava flow which is [START_REF] Brum Da Silveira | A estrutura da Ilha do Fogo, Cabo Verde. In A Erupção Vulcânica de 1995 na Ilha do Fogo[END_REF] Ar/ 39 Ar dated 66.9 ± 8.3 ka (Table S3). A postcollapse lava flow overlapping the northern caldera rim has an age of 59 ± 10 ka (Fig. S1), that is concordant with the ages previously obtained on similar wall top lavas [START_REF] Marques | Caldera or flank collapse in the Fogo volcano? What age? Consequences for risk assessment in volcanic islands[END_REF] . Two post-collapse lava flows cascading from the steep slopes of the caldera wall are dated at 21 ± 2 ka (southern wall), and 20 ± 2 ka (western wall, above a lava previously dated 43 ka [START_REF] Marques | Caldera or flank collapse in the Fogo volcano? What age? Consequences for risk assessment in volcanic islands[END_REF] ). The oldest post-collapse lava flow exposed along the eastern coast is 35 ± 4 ka old (Table S2). Using a first-order reconstruction of the Monte Amarelo edifice before and after its collapse, we estimate a precollapse growth rate of 2.2 km 3 /ka (~ 200 km 3 of lavas produced between 160 and 68 ka), which is relatively high compared to growth rates calculated for the Canary Islands [START_REF] Paris | Volcanic and morphological evolution of La Gomera, based on new K/Ar ages and magnetic stratigraphy: implications for oceanic islands evolution[END_REF][START_REF] Carracedo | The later stages of the volcanic and structural evolution of La Palma, Canary Islands: The Cumbre Nueva giant collapse and the Cumbre Vieja volcano[END_REF][START_REF] Carracedo | Evolution of ocean island rifts: The northeast rift-zone of Tenerife, Canary Islands[END_REF] . Post-collapse volcanism represents a volume of 74 km 3 filling the collapse scar at a lower rate of 1.1 km 3 /ka (< 68 ka).

Sources and processes controlling the composition of the magmas

Silicate lavas of the Cape Verde Islands are ranging from foidites and basanites, to phonolites and trachytes [START_REF] Gerlach | Magma sources of the Cape Verdes archipelago: Isotopic and trace element constraints[END_REF][START_REF] Doucelance | Pb-Sr-He isotope and trace element geochemistry of the Cape Verde Archipelago[END_REF][START_REF] Kogarko | Characteristics of alkali magma differentiation at the Cape Verde Islands[END_REF] . Lavas of the southern islands are slightly more alkaline than those of the northern islands [START_REF] Doucelance | Pb-Sr-He isotope and trace element geochemistry of the Cape Verde Archipelago[END_REF] . Intrusive and extrusive carbonatites found on São Vicente, Maio, Santiago, Fogo and Brava are interpreted as resulting or deriving from the partial melting of a carbonated oceanic crust [START_REF] Hoernle | Geochemistry of oceanic carbonatites: mantle recycling of crustal carbonate[END_REF][START_REF] Doucelance | Geochemical constraints on depth of origin of oceanic carbonatites: The Cape Verde case[END_REF][START_REF] Doucelance | What coupled cerium and neodymium isotopes tell us about the deep source of oceanic carbonatites[END_REF][START_REF] Amsellem | Calcium isotopic evidence for the mantle sources of carbonatites[END_REF] . Mixing models accounting for the spatial variability of the isotopic signature (Sr-Nd-Pb-He-Os) of the Cape Verde Islands have been proposed [START_REF] Gerlach | Magma sources of the Cape Verdes archipelago: Isotopic and trace element constraints[END_REF][START_REF] Doucelance | Pb-Sr-He isotope and trace element geochemistry of the Cape Verde Archipelago[END_REF][START_REF] Davies | A combined chemical and Pb-Sr-Nd isotope study of the Azores and Cape Verde hot-spots: the geodynamic implications[END_REF][START_REF] Christensen | argon and lead isotopic composition of volcanics from Santo Antao and Fogo, Cape Verde Islands[END_REF][START_REF] Escrig | Os isotope systematics in Fogo Island: Evidence for lower continental crust fragments under the Cape Verde Southern Islands[END_REF][START_REF] Millet | Mantle plume heterogeneity versus shallow-level interactions: A case study, the São Nicolau Island, Cape Verde archipelago[END_REF][START_REF] Barker | A 5 million year record of compositional variations in mantle sources to magmatism on Santiago, southern Cape Verde archipelago[END_REF][START_REF] Barker | Magmatic evolution of the Cadamosto Seamount, Cape Verde: Beyond the spatial extent of EM1[END_REF] . Isotopic and trace element compositions of the lavas of the southern islands suggest a mixture between a moderate HIMU-like (High µ = high 238 U/ 204 Pb) end-member and an EM1-like (Enriched Mantle 1) end-member during magma differentiation at shallow depth [START_REF] Doucelance | Pb-Sr-He isotope and trace element geochemistry of the Cape Verde Archipelago[END_REF][START_REF] Escrig | Os isotope systematics in Fogo Island: Evidence for lower continental crust fragments under the Cape Verde Southern Islands[END_REF] . The HIMU component is commonly explained by the recycling of a Proterozoic oceanic crust by the mantle plume, while the origin of the enriched component has been debated (lithospheric [START_REF] Escrig | Os isotope systematics in Fogo Island: Evidence for lower continental crust fragments under the Cape Verde Southern Islands[END_REF][START_REF] Millet | Mantle plume heterogeneity versus shallow-level interactions: A case study, the São Nicolau Island, Cape Verde archipelago[END_REF] or asthenospheric [START_REF] Barker | A 5 million year record of compositional variations in mantle sources to magmatism on Santiago, southern Cape Verde archipelago[END_REF] ). The existence of a third component similar in composition to the Depleted MORB Mantle (DMM) was proposed [START_REF] Millet | Mantle plume heterogeneity versus shallow-level interactions: A case study, the São Nicolau Island, Cape Verde archipelago[END_REF] , but in a very small proportion in the southern islands. Some Cape Verde basalts have also experienced shallow-level contamination by oceanic crustal material (i.e., Jurassic MORB basement) [START_REF] Doucelance | Pb-Sr-He isotope and trace element geochemistry of the Cape Verde Archipelago[END_REF] , especially in São Nicolau Island, but this interaction is negligible (possibly non-existent) for Fogo lavas [START_REF] Millet | Mantle plume heterogeneity versus shallow-level interactions: A case study, the São Nicolau Island, Cape Verde archipelago[END_REF] . The overall isotopic (Sr-Nd-Pb) composition of the 48 lava samples collected in Fogo (Fig. S4) is concordant with the model of two main components, with an EM1 component reflecting the assimilation of fragments of continental lithosphere incorporated in the oceanic lithosphere during the opening of the Atlantic Ocean [START_REF] Millet | Mantle plume heterogeneity versus shallow-level interactions: A case study, the São Nicolau Island, Cape Verde archipelago[END_REF] . Interestingly, 206 Pb/ 204 Pb vs. 208 Pb/ 204 Pb and 87 Sr/ 86 Sr vs. 143 Nd/ 144 Nd plots of lavas preceding the Monte Amarelo collapse (160-68 ka) do not fully overlap with post-collapse lavas (Fig. S4).

Fogo lavas range from foidites and tephriphonolites (Fig. S5). The ignimbrites observed along the caldera walls have a phonotephritic-to-tephriphonolitic composition, which is concordant with the composition of offshore tephra units [START_REF] Eisele | Pleistocene to Holocene offshore tephrostratigraphy of highly explosive eruptions from the southwestern Cape Verde Archipelago[END_REF] . Major-element compositions are controlled by (a) minor variations in the primitive melt composition, and (b) fractionation/accumulation of crystals, mainly pyroxene and olivine. This was previously demonstrated [START_REF] Hildner | Barometry of lavas from the 1951 eruption of Fogo, Cape Verde Islands: Implications for historic and prehistoric magma plumbing systems[END_REF][START_REF] Hildner | Magma storage and ascent during the 1995 eruption of Fogo, Cape Verde Archipelago[END_REF][START_REF] Mata | The 2014-15 eruption and the short-term geochemical evolution of the Fogo volcano (Cape Verde): Evidence for small-scale mantle heterogeneity[END_REF] , and it is also confirmed by the negative correlation between incompatible elements and MgO (Fig. S6), in agreement with highly variable phenocryst content inferred from petrographic observations. Megacrystals are most often not in equilibrium with the melt, and they can be classified as antecrysts from a former magmatic stage. Mass balance calculations (Fig. S7) indicate that the whole rock composition of the lavas implies a crystal fractionation of up to 50% (mostly involving clinopyroxene and amphibole, with lesser proportions of Fe-Ti oxides, apatite, and olivine), and an accumulation of up to 40% (mostly clinopyroxene, olivine ± Fe-Ti oxides). This is in good agreement with the modal compositions of the most commonly observed lavas, where clinopyroxene is often the most abundant megacrysts phase, and with values reported for fractional crystallization of lavas produced by the 1995 eruption [START_REF] Hildner | Magma storage and ascent during the 1995 eruption of Fogo, Cape Verde Archipelago[END_REF] . Magma storage and differentiation occurs in the uppermost mantle at depths between 18 and 28 km [START_REF] Hildner | Magma storage and ascent during the 1995 eruption of Fogo, Cape Verde Archipelago[END_REF][START_REF] Mata | The 2014-15 eruption and the short-term geochemical evolution of the Fogo volcano (Cape Verde): Evidence for small-scale mantle heterogeneity[END_REF] , with short-term secondary storages in the crust [START_REF] Hildner | Barometry of lavas from the 1951 eruption of Fogo, Cape Verde Islands: Implications for historic and prehistoric magma plumbing systems[END_REF][START_REF] Hildner | Magma storage and ascent during the 1995 eruption of Fogo, Cape Verde Archipelago[END_REF][START_REF] Mata | The 2014-15 eruption and the short-term geochemical evolution of the Fogo volcano (Cape Verde): Evidence for small-scale mantle heterogeneity[END_REF] .

Pre-collapse lavas (160-68 ka)

Detailed logging, sampling and dating along the Bordeira caldera walls provide a unique opportunity to address the evolution of magma composition through time, and its possible links with the Monte Amarelo flank collapse (68 ka). Because the major element composition of the primary magmas is almost independent of the source mantle components [START_REF] Hildner | Barometry of lavas from the 1951 eruption of Fogo, Cape Verde Islands: Implications for historic and prehistoric magma plumbing systems[END_REF][START_REF] Hildner | Magma storage and ascent during the 1995 eruption of Fogo, Cape Verde Archipelago[END_REF][START_REF] Mata | The 2014-15 eruption and the short-term geochemical evolution of the Fogo volcano (Cape Verde): Evidence for small-scale mantle heterogeneity[END_REF] , trends of major element composition of the lavas through time indicate a progressive increase of magma differentiation during the 60 kyrs (~ 130-68 ka) preceding the Monte Amarelo collapse (Fig. 2). Lavas become slightly more alkaline with time, as observed on the SiO 2 /(Na 2 O + K 2 O) trend (Fig. 2A). This temporal evolution of magma composition is likely due to increasing pyroxene (± amphibole) fractionation rather than crystal accumulation, as suggested by the Na 2 O/MgO and CaO/Al 2 O 3 trends (Fig. 2B,C). Variation of K 2 O/TiO 2 with time displays distinct patterns (Fig. 2D) with high (> 2%) K 2 O lavas on western caldera wall, and low (< 1.5%) K 2 O lavas on the southern flanks. Major element composition somehow varies geographically, with lavas being more differentiated near the paleo-centre of the Monte Amarelo edifice (Fig. S9).

The distinct components in the source of the Fogo magmas can be distinguished based on their Pb-Sr isotope compositions [START_REF] Doucelance | Pb-Sr-He isotope and trace element geochemistry of the Cape Verde Archipelago[END_REF][START_REF] Escrig | Os isotope systematics in Fogo Island: Evidence for lower continental crust fragments under the Cape Verde Southern Islands[END_REF] . The HIMU component is characterised by a relatively high 206 Pb/ 204 Pb but low 87 Sr/ 86 Sr, and the EM1 component has low 206 Pb/ 204 Pb and high 87 Sr/ 86 Sr. The Pb-Sr-Nd isotopic composition of the lavas displays a significant temporal evolution during the last 160 kyrs (Fig. 3), but unlike the major and trace element trends, these variations are not clearly linked to the Monte Amarelo collapse (Fig. 3). The 87 Sr/ 86 Sr ratio decreases from 160 to 35 ka, whereas the 143 Nd/ 144 Nd ratio shows a slight increase during the same period. The 206 Pb/ 204 Pb is characterised by a progressive increase from 160 to 20 ka, with possibly a more pronounced one after the Monte Amarelo flank collapse (68 ka). Near-collapse lavas are characterised by higher 143 Nd/ 144 Nd and low 87 Sr/ 86 Sr ratios compared to other pre-collapse lavas (Fig. 3). There is thus a long-term increase of the proportion of HIMU component or decline of the EM1 component in the magma source during the pre-collapse period, and this trend persists during 40-50 kyrs after the collapse.

Post-collapse lavas (< 68 ka)

Lavas produced soon after the collapse (68-59 ka) share some similarities with the latest pre-collapse lavas in terms of differentiation (Fig. 2A) and isotopic composition (Fig. 3). However, most of these early post-collapse lavas are considerably enriched in incompatible elements (Fig. 4). All these observations converge to a short-term (~ 10 kyrs after the collapse) evacuation of residual magmas characterized by a low degree of partial melting, as observed on a La/Sm vs La plot (Fig. S8). The eruptive vents of these lavas are all located on the rim of the caldera and their products (lava flows) are found in discontinuity on the pre-collapse lavas, although both preand post-collapse lavas are cut by retrogressive erosion of the caldera walls.

Late post-collapse lavas (< 59 ka) are characterized by a degree of differentiation that is similar to early (> 100 ka) pre-collapse lavas, i.e. significantly less differentiated than near-collapse lavas (Fig. 2A). This is not directly linked to crystal fractionation, as indicated by similar Na 2 O/MgO for late pre-collapse (< 100 ka) and late post-collapse lavas (Fig. 2B). Peripheral post-collapse lavas filling the collapse embayment on the eastern coast are characterized by olivine rather than pyroxene fractionation (e.g. high MgO and CaO/Al 2 O 3 , but low Na 2 O lavas at 30-40 ka on Fig. 2B,C, see also Fig. S8). Since 20-30 ka, the 87 Sr/ 86 Sr ratio increased slightly faster than it was decreasing before the collapse (Fig. 3). The 206 Pb/ 204 Pb ratio shows a very fast decline from 19.4 at 20 ka (Table S2: sample Fo-60 dated 20 ka) to < 19.0 for historical lavas, although Mata et al. (2017) reported smaller and shorter-term variations between historical lavas, interpreted as small-scale mantle heterogeneities. After at least 140 kyrs of constant augmentation, the proportion of HIMU component compared to EM1 thus abruptly decreases since 20-30 ka. Although it cannot be definitively proved that the Sr-Nd-Pb isotopic variations illustrated here are connected to the collapse, they most likely correspond to the response to pressure changes at lithospheric depth 20 favouring EM1-like materiel assimilation that becomes then a dominant process at the magma source.

Discussion and conclusions

Timescales of astenospheric, lithospheric, and volcanic processes are different, but their respective histories are not disconnected, and the Fogo example provides an original example of interactions between the source and storage of the magmas, and a major volcano flank collapse. In the case of Fogo volcano, major element composition of lavas evidence a progressive magma differentiation due to increasing crystal fractionation during the 60 kyrs preceding the Monte Amarelo flank collapse. This temporal evolution of magma storage in the lithospheric mantle is coeval with the development of shallower reservoirs, including a plutonic complex inside the volcanic edifice itself, and recurrent explosive activity forming ignimbrites. The increasing load of the growing volcano may have promoted magma stagnation, as previously proposed for Teno volcano, Canary Islands [START_REF] Longpré | Volcanic and geochemical evolution of the Teno massif, Tenerife, Canary Islands: Some repercussions of giant landslides on ocean island magmatism[END_REF] . A similar pre-collapse evolution towards more differentiated products was observed on Taburiente volcano, La Palma, Canary Islands 62 , but this trend persisted during the early post-collapse stage (Bejenado lavas) until volcanic activity moved to the South, thus building a new shield (Cumbre Vieja). The central edifice of Tenerife provides another example of such correlation between volcano growth, storage conditions, eruptive processes shifting from effusive to explosive, and flank instability of an ocean island [START_REF] Carracedo | Evolution of ocean island rifts: The northeast rift-zone of Tenerife, Canary Islands[END_REF][START_REF] Bryan | Stratigraphy of the Bandas del Sur Formation: An extracaldera record of Quaternary phonolitic explosive eruptions from the Las Cañadas edifice, Tenerife (Canary Islands[END_REF][START_REF] Deegan | Crustal versus source processes recorded in dykes from the Northeast volcanic rift zone of Tenerife, Canary Islands[END_REF][START_REF] Paris | Explosive eruption, flank collapse and megatsunami at Tenerife ca. 170 ka[END_REF] .

However, the unique dataset presented here for Fogo volcano highlights two different scales of consequences of the flank collapse on lava composition. The delayed response to the unloading effect of the collapse takes ~ 50 kyrs at the lithospheric scale (i.e., the depth at which the EM1-like component is incorporated into the moderate-HIMU-like plume magmas), compared to ~ 10 kyrs at the depth of the main magma storage and crystal fractionation. The shallow and short-term impact (~ 10 kyrs) is evidenced by the composition of early post-collapse lavas (68-59 ka) that are incompatible element-rich. The collapse-induced unloading upset magma storage conditions in the lithospheric mantle (~ 20-30 km) and favoured the release of highly fractionated magma. Major and trace element composition of lavas generated during the last 60 kyrs indicates a degree of differentiation that is similar to that of early (> 100 ka) pre-collapse lavas. The temporal and spatial distribution of post-collapse volcanism supports and refines the model of eastward vent shifting [START_REF] Maccaferri | The effect of giant lateral collapses on magma pathways and the location of volcanism[END_REF] , with early post-collapse lavas (68-59 ka) being located along the head of a large failure plane corresponding to the present-day caldera rim (Fig. 5), lavas cascading on the caldera walls between 43 and 20 ka, and later lavas (< 20 ka) on the caldera floor (Fig. 1). This eastward migration may also reflect the retrogressive dynamics of the eastward collapse [START_REF] Paris | Tsunami deposits in Santiago Island (Cape Verde archipelago) as possible evidence of a massive flank failure of Fogos volcano[END_REF][START_REF] Marques | Caldera or flank collapse in the Fogo volcano? What age? Consequences for risk assessment in volcanic islands[END_REF][START_REF] Barrett | Revisiting the tsunamigenic volcanic flank collapse of Fogo Island in the Cape Verdes, offshore West Africa[END_REF] . The main collapse is now accurately dated at 68 ka, but secondary collapses may have occurred between 59 and 43 ka. The multistage nature of Fogo flank instability is confirmed by offshore data showing two successive debris avalanche deposits [START_REF] Barrett | Revisiting the tsunamigenic volcanic flank collapse of Fogo Island in the Cape Verdes, offshore West Africa[END_REF] and it is concordant with what is observed on many other ocean islands [START_REF] Hunt | Turbidite record of frequency and source of large volume (>100 km 3) Canary Island landslides in the last 1.5 Ma: Implications for landslide triggers and geohazards: Record of Canary Island Landslides[END_REF][START_REF] Paris | Explosive eruption, flank collapse and megatsunami at Tenerife ca. 170 ka[END_REF] .

The model proposed here for Fogo should now be compared with other studies using a similar approach, i.e. combining structural, chronological and compositional data [START_REF] Watt | From catastrophic collapse to multi-phase deposition: Flow transformation, seafloor interaction and triggered eruption following a volcanic-island landslide[END_REF] . The analytical cost of such studies is high but it is the only way to understand the causal links between volcano growth, instability, and magma storage down to the lithospheric mantle.

Methods

Sampling strategy and description of the lavas. A total of 48 lava flows, 11 intrusions (dykes, sills, and intrusive complex of Monte Amarelo), and 2 ignimbrite samples were collected along the caldera walls and coastal cliffs (Table S1, Figs. 1, S1, S2 and S3). The majority of the samples (75%) predate the Monte Amarelo collapse, but almost all the accessible post-collapse lavas were sampled. The geographic distribution of the sampling allows the volcano-stratigraphic units to be dated directly (K-Ar and Ar-Ar ages from this work, and published K-Ar ages [START_REF] Marques | Caldera or flank collapse in the Fogo volcano? What age? Consequences for risk assessment in volcanic islands[END_REF] ) or indirectly (considering their stratigraphic position compared to the dated samples). Six stratigraphic sections were extensively sampled: (1) the northern caldera wall and the Montinho post-collapse lava flow on the caldera rim (Fig. S1); (2) the intrusive complex (dykes and plutonic bodies) of the Monte Amarelo (Fig. S1); (3) the western caldera wall along a 500 m-high via ferrata (Fig. S2); (4) the southeastern caldera wall (Fig. S3); (5) the paleocliff forming the southeastern tip of the collapse scar (Fig. 1); (6) and the cliff of Corvo on the eastern coast (Fig. 1). Four types of lavas were distinguished: aphyric lavas (< 5% of phenocrysts), porphyric lavas (5-40% of phenocrysts, clinopyroxene ± olivine ± amphibole), ankaramites (> 40% of pyroxene and olivine phenocrysts), and intrusive lavas (basanite dykes, nepheline syenites). The mesostatis is micro-crystallised in oxides, pyroxene, olivine, plagioclase and rare felspathoïdes (nepheline, leucite, haüyne). The syenites of the Monte Amarelo intrusive complex are composed of clinopyroxenes, alkali feldspars, nepheline, oxides, and rare biotite. The pyroxene phenocrysts are typically euhedral but often fragmented, and their rims are not in equilibrium with the melt. They display a weak normal zonation (although inverse zonation is also present), with textures of dissolution and recrystallization. Thus, these pyroxenes were most probably captured from a crystal mush during magma storage and ascent.

Geochronology: unspiked K-Ar technique. Splits of purified groundmass separates from 4 pre-collapse (Fo-36, Fo-43, Fo-31, and Fo-48) and 6 post-collapse samples (Fo-28, Fo-73, Fo-44, Fo-60, Fo-54, and Fo-70) were prepared following the procedure described by [START_REF] Guillou | Effectiveness of combined unspiked K-Ar and 40Ar/39Ar dating methods in the 14C age range[END_REF] . Alteration may cause gain and/or loss of K and Ar and as a consequence may lead to under or overestimated K-Ar ages [START_REF] Guillou | Dating young MORB of the Central Indian Ridge (19°S): Unspiked K-Ar technique limitations versus 40Ar/39Ar incremental heating method[END_REF] . To decide if a sample should be used for dating, it is crucial to estimate its degree of alteration. Indeed, after macroscopic and microscopic observations many pre-collapse samples collected along the walls of the caldera were discarded due to their high degree of alteration. The loss-on-ignition values (L.O.I.) range from 0 to 0.4% (Table S1). They are indicative of unaltered samples. The absence of alteration phases in the samples was also verified by observations with a binocular microscope. Isotopic compositions of Ar were determined via the unspiked method [START_REF] Charbit | Cross calibration of K-Ar standard minerals using an unspiked Ar measurement technique[END_REF] and achieved using the new argon extraction, purification and transfer line recently developed at the LSCE (Laboratoire des Sciences du Climat et de l'Environnement 69 ). Groundmass splits (1.0-2.5 g) of samples were wrapped into 99.5% copper foil packets, loaded in the sample holder, which had been turbo-molecular pumped for about 20 h. During the last two hours of that stage, the molybdemium (Mo) crucible was degassed at about 1500 °C until the pressure decreased to 10 -9 Torr. The sample was then dropped into the Mo crucible and molten at full power of the induction furnace. During the melting stage (i.e. 20 min), the extracted gas was adsorbed by an active charcoal finger at liquid nitrogen temperature. Next to the melting, the gas was released by heating the charcoal at 110 °C and purified via the mutual action of a titanium sublimation pump and a SAES 10 GP-MK3 Zr-Al getter operated at 400 °C. This first step of gas clean-up generally lasted 40 min and was followed by three consecutive exposures of five minutes each of the gas to SAES 10 GP-MK3 Zr-Al getters also operated at 400 °C. The gas was then adsorbed 5 min by a second active charcoal maintained at liquid nitrogen temperature. After adsorption, this sector is isolated. The gas, released at room temperature, was finally cleaned up by a SAES APGP-10 Zr-Al getter operated at 400 °C during another 5 min and introduced into a 180°, 6 cm radius mass spectrometer, equipped with a double Faraday collector. Isotopic analysis was performed on total [START_REF] Brum Da Silveira | A estrutura da Ilha do Fogo, Cabo Verde. In A Erupção Vulcânica de 1995 na Ilha do Fogo[END_REF] Ar contents ranging between 0.5 and 2.5 × 10 -11 mol. One or multiple manometrically-calibrated doses of atmospheric argon were used to convert beam intensities into atomic abundances and to monitor the atmospheric correction. The manometric calibration is based on periodic, replicate determinations of the international dating standard HD-B1 (24.21 ± 0.32 Ma [START_REF] Hess | Compilation of K-Ar measurements on HD-B1 standard biotite[END_REF][START_REF] Schwarz | Intercalibration of 40Ar-39Ar age standards NL25, HB3gr hornblendes, GA-1550, SB-3, HD-B1 biotite and BMus/2 muscovite[END_REF] ). Uncertainties for the K and Ar data reported in Table S2 are 1σ (analytical only), and consist of propagated and quadratically averaged experimental uncertainties arising from the K, [START_REF] Brum Da Silveira | A estrutura da Ilha do Fogo, Cabo Verde. In A Erupção Vulcânica de 1995 na Ilha do Fogo[END_REF] Ar (total), and [START_REF] Brum Da Silveira | A estrutura da Ilha do Fogo, Cabo Verde. In A Erupção Vulcânica de 1995 na Ilha do Fogo[END_REF] Ar* determinations. All uncertainties on the ages are given at 2σ. www.nature.com/scientificreports/ Geochronology: [START_REF] Brum Da Silveira | A estrutura da Ilha do Fogo, Cabo Verde. In A Erupção Vulcânica de 1995 na Ilha do Fogo[END_REF] Ar/ [START_REF] Torres | Carta geologica das erupcoes historicas da Ilha do Fogo : Revisao e actualizacao In A erupcao vulcanica de 1995 na Ilha do Fogo, Cabo Verde[END_REF] Ar method. In addition to K-Ar dating and to better constrain the age of the collapse, we have achieved a [START_REF] Brum Da Silveira | A estrutura da Ilha do Fogo, Cabo Verde. In A Erupção Vulcânica de 1995 na Ilha do Fogo[END_REF] Ar/ [START_REF] Torres | Carta geologica das erupcoes historicas da Ilha do Fogo : Revisao e actualizacao In A erupcao vulcanica de 1995 na Ilha do Fogo, Cabo Verde[END_REF] Ar age determination on sample Fo-01 (Table S3), previously K-Ar dated at 86 ± 3 ka [START_REF] Paris | Tsunami deposits in Santiago Island (Cape Verde archipelago) as possible evidence of a massive flank failure of Fogos volcano[END_REF] . Indeed, the K-Ar age of this post-collapse sample was questioned [START_REF] Ramalho | Hazard potential of volcanic flank collapses raised by new megatsunami evidence[END_REF] . [START_REF] Brum Da Silveira | A estrutura da Ilha do Fogo, Cabo Verde. In A Erupção Vulcânica de 1995 na Ilha do Fogo[END_REF] Ar/ [START_REF] Torres | Carta geologica das erupcoes historicas da Ilha do Fogo : Revisao e actualizacao In A erupcao vulcanica de 1995 na Ilha do Fogo, Cabo Verde[END_REF] Ar measurements on similar groundmass samples of FO-01 may allow verifying if the K-Ar age was in error by excess. Irradiation, extraction, gas clean-up procedures, mass spectrometric measurements and blank corrections, are detailed in [START_REF] Guillou | Effectiveness of combined unspiked K-Ar and 40Ar/39Ar dating methods in the 14C age range[END_REF] .

The total decay constants given by [START_REF] Min | A test for systematic errors in 40Ar/39Ar geochronology through comparison with U-Pb analysis of a 1.1 Ga rhyolite[END_REF] and the [START_REF] Brum Da Silveira | A estrutura da Ilha do Fogo, Cabo Verde. In A Erupção Vulcânica de 1995 na Ilha do Fogo[END_REF] Ar/ [START_REF] Holm | An 40Ar-39Ar study of the Cape Verde hot spot: Temporal evolution in a semistationary plate environment[END_REF] Ar atmospheric ratio at 298.56 [START_REF] Lee | A redetermination of the isotopic abundances of atmospheric Ar[END_REF] were used for age calcula-

  Exploring the links between volcano flank collapse and the magmatic evolution of an ocean island volcano: Fogo, Cape Verde Mélodie-Neige Cornu, Raphael Paris, Régis Doucelance, Patrick Bachèlery, Chantal Bosq, Delphine Auclair, Mhammed Benbakkar, Abdelmouhcine Gannoun, Hervé Guillou To cite this version: Mélodie-Neige Cornu, Raphael Paris, Régis Doucelance, Patrick Bachèlery, Chantal Bosq, et al.. Exploring the links between volcano flank collapse and the magmatic evolution of an ocean island volcano: Fogo, Cape Verde. Scientific Reports, 2021, 11 (1), 10.1038/s41598-021-96897-1. hal-03335357

Figure 1 .

 1 Figure 1. Location map of sampling sites on Fogo Island (Cape Verde archipelago). (1) Western caldera wall; (2) northern caldera wall; (3) southern caldera wall; (4) south-eastern coast; (5) north-eastern coast. Two types of samples are distinguished: samples pre-dating the Monte Amarelo flank collapse (68 ka), and post-collapse samples found on (1) top of the caldera rim, (2) cascading on the wall, or (3) on the caldera floor. Blue and red squares correspond respectively to pre-collapse and post-collapse samples. Shaded relief view derived from the DEMFI (2010) 5 m digital elevation model. Maps realised using QGIS 3.14 (https:// www. qgis. org).

Figure 2 .

 2 Figure 2. Trends of major element composition of lavas through time, with pre-collapse lavas in blue, and postcollapse lavas in red. Bold dashed line indicates the Monte Amarelo collapse at 68 ka and its inferred short-term impact (thin dashed line at 10 ka after the collapse).

Figure 3 .

 3 Figure 3. Temporal variations of the 87 Sr/ 86 Sr, 143 Nd/ 144 Nd, and 206 Pb/ 204 Pb isotopic ratios. Bold dashed line indicates the Monte Amarelo collapse at 68 ka and its inferred long-term impact at the lithospheric level (thin dashed line at 40-50 ka after the collapse).

Figure 4 .

 4 Figure 4. Trends of trace element composition of lavas through time, with pre-collapse lavas in blue, and postcollapse lavas in red. Note the peak of trace elements in early post-collapse lavas (68-59 ka).

  https://doi.org/10.1038/s41598-021-96897-1

Figure 5 .

 5 Figure 5. Summary sketch of the magmatic evolution of Fogo island since 160 ka, and possible interactions with Monte Amarelo collapses (68 ka and 59-43 ka).

Scientific Reports | (2021) 11:17478 | https://doi.org/10.1038/s41598-021-96897-1

Scientific Reports | (2021) 11:17478 | https://doi.org/10.1038/s41598-021-96897-1

Acknowledgements

This research was financed by the French Government Laboratory of Excellence initiative No. ANR-10-LABX-0006, the Region Auvergne and the European Regional Development Fund. We thank Claire Fonquernie, Jean-Luc Devidal, and Christophe Constantin (LMV Clermont-Ferrand), and the members of the PhD jury of M.N. Cornu for helpful discussion (Meritxell Aulinas Junca, Maud Boyet, José Madeira and Georges Boudon). This work also benefited from a wise review of our regretted colleague Hervé Martin, to whom we wish to dedicate this contribution. We are also grateful to Takeshi Kuritani, Ricardo Ramalho and an anonymous reviewer for their relevant comments. This is Laboratory of Excellence ClerVolc contribution No. 500.

Data availability

All data generated or analyzed during this study are included in this published article (and its Supplementary Information files).

www.nature.com/scientificreports/ tions. The precision of the mass discrimination correction was monitored by daily measurements of air argon and is about 0.15% (2σ standard deviation for multiples of experiments). Neutron fluence (J) was monitored by co-irradiation of Acs crystals [START_REF] Nomade | Alder creek sanidine (ACs-2): A quaternary 40Ar/39Ar dating standard tied to the Cobb mountain geomagnetic event[END_REF] placed in three pits encircling each sample. The J value for each sample was determined from 6 single crystal laser fusion analyses of Acs. Corresponding J values were calculated using an age of 1.1840 ± 0.0007 Ma [START_REF] Phillips | Astronomical calibration of 40Ar/39Ar reference minerals using highprecision, multi-collector (ARGUS VI) mass spectrometry[END_REF] .

Major and trace elements chemistry. Major element compositions (Table S1) were analysed by ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry) at LMV (Clermont-Ferrand, France) after alkaline melting with lithium borate and nitric acid dissolution (100 mg of sample) [START_REF] Elahpour | The petrogenesis and geochemistry of Tabas Black Land volcanic field: Implications for volcanic activity along the Nayband fault, East Iran[END_REF] . BHVO-1 standard was also analysed during the analytical session. For trace elements analysis (Table S1), samples were dissolved in a HNO 3 -HF mixture, heated for 24 h and then evaporated. After dissolution, fluoride precipitates were dissolved with several cycles of additions of 7 N HNO 3 and 6 N HCl and evaporations. Additionally, major element composition of ignimbrite and pumice samples (Table S1) was determined using an electronic microprobe (SX100 CAMECA) at LMV. An accelerating voltage of 15 kV was combined with a beam current of 10 nA for a beam diameter of 1 μm. Whole-rock trace elements were obtained by solution Inductively-Coupled Plasma Mass-Spectrometry (ICP-MS, Agilent 7500) at LMV. The BHVO-2 standard was measured every five samples, and the reproducibility was determined with BCR-2 and CMS standards [START_REF] Vlastélic | Magma degassing during the April 2007 collapse of Piton de la Fournaise: The record of semi-volatile trace elements (Li, B, Cu[END_REF] .

Sr-Nd-Pb isotopic composition. One hundred milligrams of bulk-rock powder were leached with hot 6 M-HCl (75 °C) during 3 h following the protocol described in [START_REF] Millet | Mantle plume heterogeneity versus shallow-level interactions: A case study, the São Nicolau Island, Cape Verde archipelago[END_REF] , acid-digested with HF-HNO 3 , and passed through the chromatography procedure of [START_REF] Pin | Rapid, simultaneous separation of Sr, Pb, and Nd by extraction chromatography prior to isotope ratios determination by TIMS and MC-ICP-MS[END_REF] . Strontium, Nd and Pb analytical blanks were all negligible with respect to sample contents. Strontium isotopic measurements were performed on a Finnigan Triton thermoionization mass spectrometer (TIMS), Pb measurements on a Thermo Fisher Neptune Plus Multiple Collector-Inductively Coupled Plasma-Mass Spectrometer (MC-ICP-MS), and Nd measurements used both instrumentations (Table S1). Strontium isotope ratios were mass-fractionation-corrected to 86 Sr/ 88 Sr = 0.1194 and normalized to 87 Sr/ 86 Sr = 0.71025 for the NIST SRM987 standard. Neodymium isotope ratios were massfractionation-corrected to 146 Nd/ 144 Nd = 0.7219 and normalized to 143 Nd/ 144 Nd = 0.512100 for the JnDi-1 standard. Repeated TIMS analyses of the two standards during the course of the study gave 87 Sr/ 86 Sr = 0.710242 ± 18 (2sd, n = 33) and 143 Nd/ 144 Nd = 0.512104 ± 6 (2sd, n = 17). Lead mass discrimination was corrected by standard bracketing of samples with NBS-981 standard using values of [START_REF] Doucelance | Reevaluation of precise lead isotope measurements by thermal ionization mass spectrometry: Comparison with determinations by plasma source mass spectrometry[END_REF]

Competing interests

The authors declare no competing interests.

Additional information

Supplementary Information

The online version contains supplementary material available at https:// doi. org/ 10. 1038/ s41598-021-96897-1.

Correspondence and requests for materials should be addressed to R.P.

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