Florent Monie

Bruno Grignard

Jean-Michel Thomassin

Raphael Mereau

Thierry Tassaing

Christine Jerome

Christophe Detrembleur
email: christophe.detrembleur@uliege.be[

to cyclic carbonates for the preparation of self-blowing non-isocyanate polyurethane foams

Keywords: non-isocyanate polyurethane, foams, CO2, selfblown reactive formulation. Regioselective ring-opening

Chemo-and regio-selective additions of nucleophiles

Introduction

Polyurethanes (PUs) foams, with a world-wide production expected to reach 12.7 million tons by 2024, 1 are key materials of our modern life with a broad range of applications for comfort and wellness (furniture, mattresses, seats…), shock absorption, thermal/acoustic insulation or as sealants. Industrially, PUs are made by polyaddition of di-or polyisocyanates and di-or polyols. 2 Their foaming is generally induced by hydrolysis of the isocyanates with concomitant formation of CO2. The in-situ release of this gas expands the polymer during its formation, leading to foams with properties that depend on the composition of the reactive formulation. 2 Due to the large diversity of polyols that can be used, rigid to soft foams can be easily produced by this simple process. However, the health concerns and regulation changes associated to the use of toxic isocyanates are motivating the industries and the scientists to explore novel routes to design isocyanate-free PU foams. The recent advances in the chemical transformations of CO2 opened new opportunities to solve this challenge and multiple pathways are now accessible to fabricate PUs by directly using CO2 as a comonomer or by copolymerizing CO2-based building blocks. 3 Amongst them, the step-growth copolymerization of di-or polyamines with 5-membered polycyclic carbonates, easily synthesized by the [3+2] coupling of CO2 with epoxides, 4 is growing rapidly and gives access to poly(hydroxyurethane)s (PHUs). 5 Unlike isocyanates, cyclic carbonates however do not decompose spontaneously by simple addition of water to release CO2 as the blowing agent, rendering impossible the formation of self-blowing PHU foams. To date, foaming procedures for PHUs are rare and very limited in scope. The first strategy was reported by Caillol by thermal decomposition of Momentive MH15, i.e. poly(methylhydrogenosiloxane), into dihydrogen (a highly flammable gas) that expanded the PHU matrix during its formation. 6 Another approach reported by Mulhaupt consisted in adding Solkane, i.e. a fluorocarbon physical blowing agent, that foamed the PHU matrix by evaporation during the polymerization. 7 Self-blown rigid PHU foams with open cells porosity were designed by using sodium bicarbonate as chemical blowing agent at high temperature (>200°C) 8 , or by the addition of maleic acid and glutaraldehyde to a PHU formulation (the foaming mechanism is however not clear in this case). 9 Only two relevant works have described the use of CO2 as blowing agent for the production of PHU foams without requiring the addition of external reagents. The first one reported the fabrication of microcellular PHU foams with promising heat insulating properties by the supercritical CO2assisted batch foaming technology. 10 In that case, presynthesized PHUs thermoplastics were impregnated by CO2 at the supercritical state and, upon release of the pressure, CO2 was expanding the polymer matrix. The foam morphology, the density and the pore size were adjusted by a fine control of the CO2 pressure, the impregnation and foaming conditions. In 2018, the first CO2 self-blowing PHU foam was reported by North by exploiting a specific bis(cyclic carbonate) monomer, i.e. sorbitan bis-carbonate, that underwent an intramolecular side-reaction provoking the partial decarboxylation of the monomer during thermally driven aminolysis. 11 The CO2 release was however observed at a low level and this strategy was limited to sorbitan biscarbonate, therefore strongly limiting the scope of the process. The quest for a universal strategy to producing self-blowing PHUs that is versatile, simple, compatible to industrial processes (reactive extrusion foaming, reactive molding, foaming in place, etc.) and that mimics the conventional PUs foaming methodology (by the in-situ generation of CO2) is highly desirable for the next generation of greener PU foams.

To tackle this challenge, we took inspiration from the dialkylcarbonates chemistry responding to the Pearson's Hard-Soft Acid-Base (HSBA) theory. Dialkylcarbonates display two electrophilic sites, the carbonyl and methylene carbons, with a reactivity that can be discriminated by the Hard-Soft character of the nucleophile. 12 Hard nucleophiles (e.g. amines) are undergoing carbonyl attack whereas methylene attack occurs with soft nucleophiles (e.g. thiols) (Scheme 1). The same situation is also noted for 5membered cyclic carbonates (5CCs) but under harsher reaction conditions. We therefore postulated that combining two types of nucleophiles with distinct hard and soft characters within a single PHU formulation should lead to two competitive and selective 5CC ring-opening pathways, i.e. the acylation of the amine to form β-hydroxyurethane linkages (via carbonyl attack, Scheme 1) and the alkylation of the soft nucleophile with concomitant insitu release of CO2 (via methylene attack, Scheme 1). Although the S-alkylation of thiols was sparsely reported with dialkylcarbonates, 12c we identified thiols as ideal soft nucleophiles to combine with amines (hard nucleophiles) for fabricating PHU foams from reactive formulations. In this work, we explore the formation of self-blown PHU foams by directing the concomitant selective aminolysis of the cyclic carbonate and the decarboxylative S-alkylation of a thiol from reactive formulations containing amines, thiols and cyclic carbonates. First, we investigate the organocatalyzed regio-selective addition of model thiols to five-membered cyclic carbonates that leads to the decarboxylative formation of hydroxythioethers or poly(hydroxythioether)s, and we approach the reaction mechanism by combining kinetics studies and DFT modeling. We then report how to exploit this reaction for the preparation of microcellular PHU foams and we illustrate the scope of this process. We also show that, beside the generation of the blowing agent during the PHU formation, the thiol/cyclic carbonate reaction provides hydroxythioether linkages that contribute to the polymer matrix construction and improve the thermal properties of the final material.

Results and Discussion

Model reactions

The self-blowing PHU foaming method lies on two competitive reactions, i.e. the aminolysis of 5CC to form the urethane linkages and to construct PHU, and the S-alkylation of thiols to generate the blowing agent (CO2) while creating thioether linkages. The rates and selectivity of the reactions have to be adjusted for controlling both the PHUs synthesis and foaming. Ideally, both reactions have to take place in a range of temperature between 80 °C and 120 °C, which are common operative conditions to synthesize PHUs. 5 The main challenge in self-blowing processes is to form polymer chains with a sufficient viscosity and a crosslinking degree prior to the release of the blowing agent (CO2) in order to avoid the collapse of the foam. In our case, this means that the 5CC ring-opening by the amine should be faster that the decarboxylation promoted by the thiol. The S-alkylation of thiols with dialkylcarbonates has been reported but was slow and generally realized under demanding conditions (T = 100-180 °C, K2CO3 as catalyst). 12c These operative conditions are not compatible with the PHU chemistry as they will favor the occurrence of side-reactions such as the urea formation and provoke a deviation of the stoichiometry which is detrimental for the step-growth polymerization. 5i, 13 Identifying a single catalyst that is able to catalyze both the S-alkylation and the aminolysis is then a prerequisite for the formation of self-blowing PHU. Organobases such as DBU, DBN, TBD, DMAP, DABCO are potential candidates and some of them are known to catalyze the aminolysis of 5CC. 14 We therefore investigated and compared the organobase catalyzed additions of a diamine (2,2'-(Ethylenedioxy)bis-ethylamine) and a dithiol (2,2'(ethylenedioxy)diethanethiol) of similar structure on propylene carbonate (PrC) as a model 5CC (Figure 1). These reactions were carried out under equimolar conditions between the reactive groups without any solvent and under ambient atmosphere at 80 °C in order to approach reactions conditions that will be implemented for the foaming. The reactions were monitored by 1 H-NMR spectroscopy as described in Supporting Information (Figure S1 andS2). Under catalyst-free conditions, the thiol did not react with PrC, which is in sharp contrast to the amine that provided the expected two hydroxy-urethane regioisomers (80 % PrC conversion in 1h). 14 By adding weak bases such as DMAP or DABCO (5 mol% compared to PrC), the S-alkylation of the thiol by decarboxylative addition onto PrC was observed, however was slow with respectively 10 or 30 % conversion in 2 h. Importantly, this reaction was accelerated by using superbases with conversions as high as 90 % in 2 h with DBN

O O O R' Nu-R carbonyl attack methylene attack Nu = NH 2 Nu = SH R H N O OH O R' O H N R O R' OH + hydroxyurethane regioisomers R S OH R' + CO 2 hydroxythioether O O O Nu-R carbonyl attack methylene attack Nu = NH 2 Nu = SH O H N R O R' + R S R' + CO 2 R' R' R' OH R' OH + urethane alcohol thioether alcohol

Additions to dialkylcarbonates

Additions to cyclic carbonates or 96 % with DBU in 1 h, with the selective formation of the Salkylated regioisomer that originates from the thiol attack at the electrophilic methylene site of PrC. Although DMAP did not affect the PrC aminolysis, the addition of DABCO, DBU and DBN significantly accelerated the reaction with the production in all cases of the two hydroxyurethane regioisomers. The comparison of the kinetics profiles for the two considered reactions, i.e. the S-alkylation of thiol and the aminolysis of PrC, gave clear insights into the feasibility of the PHU foaming methodology. First, whatever the catalyst, the aminolysis of PrC was always faster than the S-alkylation of the thiol which is a prerequisite to tailor PHU chains before the in-situ formation of the blowing agent (CO2). Second, by the choice of the catalyst, the rates of these two competitive reactions can be adjusted, which constitutes an additional lever for tuning the viscosity of the reaction medium prior PHU expansion. From the comparative kinetic studies, DBU was selected in the next studies as it catalyzed both reactions under reasonable period of time (Figure S3). It is also operative over a broad range of temperature (25-100 °C) for the S-alkylation of the thiol (Figure S4).

DFT calculations

Model reactions carried out in the presence of DBU were confronted to mechanistic DFT calculations at the M062X/6-311G(d,p) level of theory (see ESI5 for details). In the case of the aminolysis, the overall reaction proceeds in 2 steps. From the van der Walls complex (-2.3 kcal.mol -1 ), the initial step corresponds to the nucleophilic attack of the amine onto the carbonate group of 5CC accompanied by a proton transfer from the amine towards the sp 2 N atom of DBU. This step is the rate determining step with a barrier height of 22.3 kcal.mol -1 . The resulting cyclic amino alkoxide anion (interacting with DBUH + by H-bond) is then encountering a ring opening by C-O bond cleavage and a simultaneous proton transfer between DBUH + and the adjacent O atom of the carbonate to product the target hydroxyurethane and the regeneration of DBU with a barrier height of 6.9 kcal.mol -1 . The S-alkylation reaction is also proceeding in two elementary steps. The first one consists in the initial van der Walls complex (-1.6 kcal.mol -1 ) that reacts by a methylene attack of the thiol onto the non-substituted carbon atom of the 5CC while the thiol proton is transferred to the sp 2 N atom of DBU. This step exhibits a barrier height of 30.1 kcal.mol -1 , making it the rate determining step of the S-alkylation reaction. In a second step, a decarboxylation of the resulting linear carbonate anion occurs together with the formation of the hydroxythioether by proton transfer from DBUH + and then the regeneration of DBU with a barrier height of 5.5 kcal.mol -1 .

Therefore, the barrier heights of the rate determining step of the aminolysis (22.3 kcal.mol -1 ) and the S-alkylation (30.1 kcal.mol -1 ) reactions obtained from DFT calculations are in good agreement with the kinetic trends observed, i.e the 5CC ring-opening by the amine is easier and faster than the decarboxylation promoted by the thiol.

Copolymerization of bis(cyclic carbonate) with dithiols and/or diamines

We then investigated the influence of adding a dithiol to a formulation producing PHU, thus containing a mixture of a bis(cyclic carbonate) and a diamine. Indeed, although the step-growth copolymerization of a diamine with a bis(cyclic carbonate) is largely described in the literature for the synthesis of PHU, the terpolymerization with dithiols or even the direct copolymerization of dithiols with bis(cyclic carbonate)s was never reported. Series of DBU-catalyzed polymerizations were performed under solvent-free conditions at 80 °C under ambient atmosphere for 24 h (5 mol% DBU vs the bis(cyclic carbonate)). Note that ambient atmosphere was used here to fit the conditions that are conventionaly used for foaming PUs.

Scheme 2 illustrates the structure of the monomers used for this study, i.e. the bis(cyclic carbonate) C1 (4,4'-[1,4-Butanediylbis(oxymethylene)]bis(1,3-dioxolan-2-one)), the diamine A1 (1,2-bis(2-aminoethoxy)ethane) and the dithiol T1 (2,2'-(ethylenedioxy)diethanethiol). Various comonomer molar ratios [C1]/[A1]/[T1] were tested from 1/1/0 (to produce PHU) to 1/0/1 (to yield poly(hydroxythioether)), with intermediate compositions in order to tentatively synthesize unprecedented poly(hydroxyurethane-co-hydroxythioether)s. The results are collected in Table 1.

Scheme 2. Synthesis of poly(hydroxyurethane), poly(hydroxythioether) and poly(hydroxyurethane-co-hydroxythioether) by DBU-catalyzed step-growth copolymerization.

The macromolecular characteristics of the polymers were determined by size exclusion chromatography (SEC) on the crude product in order to avoid any polymer fractionation during purification (Figure S5). As expected the poly(hydroxyurethane) PC1A1 was formed by copolymerizing C1 with A1, however the relative number average molar mass was rather low (Mn = 4,200 g/mol) as the result of the uncomplete C1 conversion (Table 1, entry 1). By adding dithiol T1 to the C1/A1 formulation, almost full C1 conversion was noted under identical conditions, with the formation of polymers of higher molar masses for all compositions (Mn = 8,400-8,800 g/mol; Table 1, entries 2-4). Importantly, the copolymerization of the dithiol T1 with the bis(cyclic carbonate) C1 was also efficient with the formation of a polymer characterized by a Mn of 9,200 g/mol (Table 1, entry 5). No C1/T1 copolymerization was observed in the absence of DBU, highlighting the importance of the organobase for the thiol/cyclic carbonate reaction in agreement with the previous model reactions. The microstructure of the poly(hydroxyurethane) PC1A1, poly(hydroxythioether) PC1T1 and a representative poly(hydroxyurethane-co-hydroxythioether) PC1A1T1 were elucidated by 1 H-(Figure 3) and 13 C-NMR (Figure S6) spectroscopy. PC1A1 displayed the typical resonances of the two methylene groups -CH2-N(C(O)O-and -CH2-O(C(O)Nadjacent to the urethane moiety at δ = 4.10-4.17 ppm and 3.37 ppm, as well as the methylene and the methyne groups close to the alcohol function at 3.65 and 4.88 ppm, respectively. The microstructure of PC1A1 was further confirmed by the presence of the carbonyl signal of the urethane at 157 ppm in the 13 C-NMR spectrum (Figure S6). The 1 H-NMR spectrum of PC1T1 highlighted the formation of thioether moieties with the two signals at 2.67 and 2.78 ppm corresponding to the methylenes linked to the sulfur atom as well a methyne resonance (-CH-OH) at 3.9 ppm. The Salkylation of T1 via decarboxylative addition onto C1 was further confirmed by the absence in the 13 C-NMR spectrum of a resonance typical of a carbonyl group that could have been produced via the formation of a thiocarbonate linkage. This characterization confirmed the selective addition of the thiol onto the methylene site of the cyclic carbonate ring. The organocatalyzed S-alkylation of dithiols with bis(cyclic carbonate) is therefore highly attractive to construct regioregular poly(hydroxythioether)s that are currently only accessible by the exothermic LiOH driven thiol-epoxy click reaction. 15 PC1A1T1 displayed the typical signals of the methylene or methyne groups of both the hydroxyurethane and hydroxythioethers units at similar chemical shifts as the one observed in PC1A1 and PC1T1. This observation confirms that both A1 and T1 were inserted in the growing chains when the terpolymerization of C1, A1 and T1 was considered.

H 2 N R 2 NH 2 HS R 2 SH R 1 = R 2 = O O OH R 1 OH O H N O R 2 H N n S R 1 S R 2 OH OH S m O O OH R 1 OH O H N O R 2 H N R 2 S R 1 OH O O OH n

Thermal analysis of the copolymers

The thermal stability and glass transition temperature of the polymers were investigated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC), respectively (Figure 4). Interestingly, the introduction of thioether units within the chains strongly improved the thermal stability of the polymers. The thermal degradation temperature (Td5%) increased progressively with the thioether linkages content and evolves from 222 °C for the pure PHU PC1A1 to 289 °C for the pure poly(hydroxythioether) PC1T1 (Table 1, Figure 4A). This observation is consistent with a recent study reporting the higher thermal stability of sulfurcontaining polyurethanes prepared by the isocyanate chemistry. 16 DSC curves (Figure 4B) showed clear endothermic transitions corresponding to the Tg of the different polymers. The Tg values were dependent on the A1/T1 molar content of the reactive formulation and evolved from -15 °C for the pure PHU (PC1A1) to -46 °C for the poly(hydroxythioether) (PC1T1) (Table 1). This observation is linked to the interchain hydrogen-bonding interactions favored at high content of urethane linkage which reduced the chain mobility. The presence of only one Tg for each polymer, whatever its A1/T1 composition, suggests that the urethane and thioether linkages were relatively randomly distributed in the polymer chains.

Preparation and characterizations of PHU foams

In order to prepare CO2 self-blowing PHU foams with dimensional stability, crosslinking of the polymer chains has to occur during the expansion of the foam. We therefore added a tris(cyclic carbonate) C2 (trimethylolpropane triscarbonate) to the PHU formulation containing the diamine A1 and the dithiol T1. We worked under stoichiometric conditions in the complementary reactive groups

([C2]/[A1]/[T1] = 1/1.125/0.375; thus [5CC]/[NH2]/[SH] = 1).
After homogenization of the mixture under solvent-free conditions with DBU (5 mol% compared to C2) at room temperature for 2 min, the formulation was poured in a silicone mold. By heating this formulation at 100 °C, a tackyfree foam was collected after 6 h. The foam was however heterogeneous with a large distribution of pores (from 800 µm to 2.7 mm) (Figure 5), which was assigned to a too low initial viscosity of the reactive formulation. Analysis of the PHU foam by Raman spectroscopy evidenced the disappearance of the S-H stretching band around 2550 cm -1 of the thiol T1 and carbonate band at 1800 cm -1 of the cyclic carbonate C2 present in the initial formulation (Figure S7). This analysis also demonstrated the presence of two specific linkages in the foam, thioether and urethane, with the new bands at 500 cm -1 for the C-S-C thioether bending and at 1715 cm -1 for the C=O urethane one.

To improve the foaming, the formulation was prereacted at 25 °C for a longer time in order to pre-polymerize PHU and increase the viscosity prior to the decarboxylation promoted by the thiol. Figure S8 shows that the viscosity indeed increased progressively up to 715 Pa.s in 16 h due to the slow aminolysis of C2 with the formation of oligohydroxyurethanes. The absence of a gel point (corresponding to the crossover point in the time evolution of G' and G" values) after 16 h suggested that the polymer was not crosslinked under these conditions. The foaming was then thermally induced by a curing for 2 h at 100 °C followed by a temperature increase to 120 °C in 30 min and an additional 1 h post-curing treatment at 120 °C (this optimal procedure has been identified and selected after screening different conditions). This foaming methodology enabled us to design self-blown PHU foams in a reproducible manner (Figure 6). Although some shrinkage was observed at the end of the foaming process as soon as the foam was removed from the oven, the material recovered its expended shape either spontaneously after 1 to 2 h at 25 °C or by heating the sample at 80 °C for 1 min. At this stage, we do not understand this phenomena but the process is perfectly reproducible and systematically furnished homogeneous PHU foams. To demonstrate the robustness of the process but also to tune the characteristics and properties of the foams, some additives were added to the formulation, such as a synthetic clay (Laponite S482) or an epoxy-functionalized PDMS oligomer (TEGOMER E-Si 2330). Laponite is a hydrophilic nanosheet clay that was chosen to reinforce the polymer matrix and to act as a nucleating agent that is expected to favor the formation of homogeneous foams with small cells size. The PDMS oligomers was selected as a surface acting agent for the stabilization of the cells during their growth. All formulations added by clay (1.5 wt%), PDMS (0.01 eq compared to C2) or by the two additives (clay + PDMS) provided foams after curing b Ncell = (nM/A)3/2ρs/ρp with n = the number of cells in the SEM image (average value on 6 SEM images), M the magnification, A the surface area of the image (mm²) and ρs and ρp the solid and the foamed sample densities. The solid density was estimated to 0.98 +/-0.02 g/cm³ on non-foamed samples c at 50 % of strain d for a 99 % shape recovery following the procedure described above. The foams morphology, the cells size and distribution, as well as the open-to-close cell ratio were determined by scanning electron microscopy (SEM) characterizations. Figure 6 illustrates the morphologies of the self-blown PHU foams and their characteristics are summarized in Table 2. In all cases, homogeneous microcellular foams with an open cell morphology were formed with a density ranging from 0.166 to 0.207 g/cm 3 . In the absence of additives, foams with a homogeneous cells size distribution and a large average cell diameter of 0.98  0.35 mm were obtained. The addition of PDMS reduced the pore size to 0.87  0.23 mm and their distribution. As expected, loading the formulation with clay further decreased the cell size to 0.62 +/-0.23. PHU foams loaded with both clay and PDMS were also highly homogeneous with a pore size of 0.60  0.21 mm (Figure S9). The PHU foam with the lowest density of 0.166 g/cm³ was obtained with the additive-free formulation. The other foams were characterized with slightly higher densities ranging from 0.207 g/cm 3 (with clay) to 0.180 g/cm 3 (with clay + PDMS).

The open-to-close cells ratio was then evaluated by SEM images treatment following the methodology reported by Fitzgeral. 17 Cell-faces area Ac and cell-face holes area Ah were measured, and the Ah/Ac ratio was calculated [31] and is reported in Table 2 The thermal degradation of the foams was evaluated by TGA. All samples depicted a similar degradation profile (Figure S10) with a degradation temperature (Td5%) at around 245 °C (Table 2). There was therefore no significant influence of the additive on the degradation temperature of the foam, at least within the additive loading range that was considered in this work. DSC analysis showed an endothermic transition for all samples, with Tg values between 2.4 and 7.9 °C. The rather similar values are not surprising as the Tg is mainly guided by the choice and content of the comonomers that are almost identical in all formulations. Preliminary mechanical properties were realized by dynamic mechanical analysis (DMA). Figure S11 illustrates the compression stress-strain Creep experiments were then realized to evaluate the ability of the foams to recover their initial shape after compression. For each sample, three compression-release cycles at a fixed strain of 50 % were applied and the time for the foams to recover their initial shape was evaluated (Figure 7). After the third compression, all samples recovered over 99 % of their initial shape in less than 30 s, qualifying these materials as flexible (Table 2). The PHU foams containing the PDMS additive displayed the fastest shape recovery rate in only 8.4 to 10 seconds, in line with the high flexibility of silicones, even Finally, other commercially available diamines, i.e. 1,6hexamethylene diamine (HMDA, A2) and m-xylylenediamine (MXDA, A3) were tested in combination with C2 and T1 in order to illustrate the robustness of the new methodology and the possible variations of the PHU structures. As the intrinsic reactivity of the amine with the cyclic carbonate is dependent on their respective structural features, the foaming protocols (curing steps) were adjusted to identify the most appropriate foaming window of the reactive PHU formulations (these procedures are detailed in Supporting information, ESI 13). Under optimized foaming conditions, the reactive mixture [C2]/[A2]/[T1] = 1/1.125/0.375 added by 5 mol% of DBU (compared to the carbonate moieties) provided a PHU foam that did not collapse after preparation. The PHU material displayed an open cell microcellular morphology with a cell size average of 0.26  0.085 mm (Figure 8A , Table S1, entry 1) and a foam density of 0.320  0.012 g/cm 3 . For a similar formulation with A3 in replacement of A2 ([C2]/[A3]/[T1] = 1/1.125/0.375), a foam with an open cells size gradient from 0.3 mm to 3.5 mm was collected with a density of 0.210 +/-0.041 g/cm³ (Figure 8B, Table S1, entry 2). Blending A3 with A2 (with a [A3]/[A2] molar ratio of 80/20) enabled to preparing homogeneous foams that displayed open cells of 0.62 +/-0.11 mm and a density of 0.250 +/-0.02 g/cm³ (Figure 8C, Table S1, entry 3). All PHU foams made from A2, A3 or a blend of A2 and A3 showed similar thermal behavior with Tg and Td5% values of 20.5-24.6 °C and 232-238 °C, respectively (Table S1). Note that the Tg values close to the room temperature prevented us to determine the mechanical properties of the foams with sufficient accuracy.

Conclusion

This work describes a general strategy to prepare microcellular self-blown isocyanate-free polyurethane foams by exploiting chemo-and regio-selective additions of amines and thiols to 5-membered cyclic carbonates (5CCs), resulting in an attractive alternative to conventional PU foams. The main challenge of this work was to demonstrate that poly(cyclic carbonate)s used in the preparation of poly(hydroxyurethane)s (PHU) by copolymerization with diamines could also be involved in the matrix foaming by generation of the blowing agent (CO2) by decarboxylation. We first investigated on model compounds the reaction conditions that promoted the formation of urethane linkages by aminolysis of 5CC and the release of CO2 by the attack of the thiol to the methylene site of 5CC with the formation of thioether linkages. While the aminolysis was possible at 80°C without any catalyst, the decarboxylation promoted by the thiol required the use of an organobase (e.g. DBU) to occur. When this organocatalyst was added at 5 mol%, the two chemoselective additions were possible, with a faster aminolysis. DFT calculations of the mechanism of the two competitive reactions enabled to rationalize the difference of reactivity between the amine and the thiol for the addition to 5CC. Applied to di(cyclic carbonate)s, these reactions give access to new polymers containing both urethane and thioether linkages by DBU-catalyzed copolymerization with diamines and dithiols.

The PHU foaming was then considered from reactive formulations containing a tri(cyclic carbonate), a diamine and a dithiol for the proof of concept. In the presence of DBU and an appropriate mild thermal curing, the aminolysis of 5CC provided the PHU matrix that expanded by the decarboxylation of 5CC promoted by the thiol. The formation of thioether bonds during the formation of the blowing agent participated in the polymer network construction and fixed the thiol to the polymer matrix. Homogeneous microcellular PHU foams with open cell morphology were easily prepared under ambient atmosphere. Highly flexible foams were prepared as a proof of concept, and we demonstrate that the process is compatible to the addition of additives such as clay and/or PDMS. The main components of the formulation (the amines and the thiols) are accessible at large scale and low cost, and the cyclic carbonates are easily prepared by coupling CO2 to the corresponding epoxides in a cost effective fashion. Due to the impressive diversity and accessibility of these components and all possible additives that can be employed in the reactive formulation, this new solvent-free process is offering new avenues for constructing more sustainable PU foams with various properties that could fit a broad range of applications.

Scheme 1 .

 1 Scheme 1. (Regio)selective addition of amines or thiols to dialkylcarbonates or 5-membered cyclic carbonates

Figure 1 .

 1 Figure 1. Conversion of propylene carbonate (PrC) with the reaction time for (a) the S-alkylation of thiol and (b) the aminolysis catalyzed by organobases (DMAP, DABCO, TBD, DBN or DBU). Conditions: [SH] or [NH2]/[PrC] = 1, organobase = 5 mol% compared to PrC, 80 °C, bulk.

  Figure 2 depicts the two competitive pathways (i.e. aminolysis and S-alkylation) catalyzed by DBU using simplified model amine and thiol of similar structure (CH3-O-CH2-NH2 and CH3-O-CH2-SH).

Figure 2 .

 2 Figure 2. Mechanistic insight of the DBU-catalyzed aminolysis (black) and S-alkylation (red) of propylene carbonate at the M062X/6-311G(d,p) level. The Gibbs free energies are expressed in kcal.mol -1

Figure 3 .

 3 Figure 3. 1 H-NMR spectrum of poly(hydroxyurethane) PC1A1, poly(hydroxythioether) PC1T1 and a representative poly(hydroxyurethaneco-hydroxythioether) PC1A1T1 with [A1]/[T1] = 1.

Figure

  Figure 4. (A) Thermogravimetric (TGA) and (B) dynamic scanning calorimetry (DSC) analyses of poly(hydroxyurethane) PC1A1, poly(hydroxythioether) PC1T1 and poly(hydroxyurethane-cohydroxythioether) PC1A1T1 with various [A1]/[T1] compositions.

Figure 5 .

 5 Figure 5. PHU formulation and SEM characterizations of self-blown foams made from C2, A1 and T1. Conditions: [C2]/[A1]/[T1] = 1/1.125/0.375, DBU(5 mol% compared to 5CCs), mixing for 2 min at rt, curing at 100 °C for 4 h.

Figure 6 .

 6 Figure 6. SEM characterization of self-blown PHU foams a) additives-free PHU foam, b) PHU foam loaded with 1.5 wt% clay, c) PHU foam loaded with 0.1 eq compared to C2 of Tegomer Si-C 2230 and d) PHU foam loaded with both PDMS and clay.

  . The Ah/Ac ratio values show that PDMS reduced the open porosity of the foams from 17.2 for an additive-free PHU foam to 9.7 in the presence of PDMS. At the opposite, clay increased the content of open pores to 23.7. For PHU foams containing both additives, the Ah/Ac ratio was estimated to 11.2 showing that the increase of open porosity induced by clay was counter-balanced by PDMS.

Figure 7 .

 7 Figure 7. Shape recovery rate of the PHU foams after a 50% deformation as determined by dynamic mechanical analysis (DMA).

Figure 8 .

 8 Figure 8. SEM characterizations of self-blown PHU foams. A) PHU foam made from [C2]/[A2]/[T1] = 1/1.125/0.375, B) PHU foam made from [C2]/[A3]/[T1] = 1/1.125/0.375, C) PHU foam made from [C2]/[Ax]/[T1] = 1/1.125/0.375 with [Ax] being a blend of A3 and A2 with a molar ratio [A3]/[A2] = 80/20.

Table 1 .

 1 Poly(hydroxyurethane) PC1A1, poly(hydroxyurethane-co-hydroxythioether) PC1A1T1 and poly(hydroxythioether) PC1T1 prepared at 80 °C in the presence of DBU: reaction conditions, macromolecular characteristics and thermal properties. Determined by SEC using a polystyrene standard calibration and DMF with LiBr as eluant.

	Entry	[C1]/[A1]/[T1]	Conv. a	Mn b	Mw	b	Đ	Td5%	Tg
			(%)	(g/mol)	(g/mol)		(°C)	(°C)
	1	1/1/0	95	4,200	6,000	1.44	222	-15
	2	1/0.75/0.25	99	8,400	14,000	1.69	220	-18
	3	1/0.5/0.5	99	8,600	14,700	1.70	230	-28
	4	1/0.25/0.75	99	8,800	15,400	1.74	242	-38
	5	1/0/1	99	9,200	17,400	1.89	289	-46
	Conditions: 80 °C, 24 h, catalyst loading = 5 mol% compared to the carbonate moiety, bulk.
	a Cyclic carbonate conversion determined by	1 H-NMR spectroscopy on the crude samples

b

Acknowledgements

The authors of Liège thank the Fonds National de la Recherche Scientifique (FNRS) and the Fonds Wetenschappelijk Onderzoek -Vlaanderen (FWO) for funding the EOS project n°O019618F (ID EOS: 30902231). C. Detrembleur is FNRS Research Director. The authors thank Cedric Malherbe for providing the Raman spectra.


Méderic Amann

Marc Ulrich

Gianreto Manatschal

Eric Pelt

Marie-Eva Epin

Julia Autin

Daniel Sauter

Geochemical characteristics of basalts related to incipient oceanization: The example from the Alpine-Tethys OCTs

Keywords: Ocean-Continent Transition, Alpine-Tethys, basalt, inheritance, subcontinental mantle, OCTbasalt

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

Introduction

The geochemical composition of mid-ocean ridge basalts (MORB) is due to the partial melting of a depleted mantle source (DMM). One remaining key question is whether basalts from magma-poor oceancontinent transitions (OCTs) are formed by similar melting processes as MORB or if they reflect more complex mantle processes associated with lithospheric thinning of a sub-continental lithospheric mantle (SCLM).

Basaltic samples derived from present-day magma-poor OCTs are rare and limited to few samples drilled at the Iberia-Newfoundland [START_REF] Chazot | Lithospheric Mantle Evolution during Continental Break-Up: The West Iberia Non-Volcanic Passive Margin[END_REF][START_REF] Cornen | Basalts, underplated gabbros and pyroxenites record the rifting process of the West Iberian margin[END_REF][START_REF] Robertson | Evidence of continental breakup from the Newfoundland rifted margin (Ocean Drilling Program Leg 210): Lower Cretaceous seafloor formed by exhumation of subcontinental mantle lithosphere, and the transition to seafloor spreading[END_REF], the South China Sea [START_REF] Sun | Expedition 367/368 scientific prospectus: South China Sea rifted margin[END_REF] or the Diamantine OCT [START_REF] Beslier | Une large transition continent-océan en pied de marge sud-ouest australienne: premiers résultats de la campagne MARGAU/MD110[END_REF][START_REF] Chatin | La zone Diamantine (océan Indien oriental), témoin de la séparation entre l'Australie et l'Antarctique: arguments pétrologiques et géochimiques[END_REF]. In contrast, Alpine-Apennine ophiolites expose remnants of the OCTs of the ancient magma-poor Jurassic Alpine Tethys, which provide information not only on the basalts but also on the underlying mantle [START_REF] Picazo | Mapping the nature of mantle domains in Western and Central Europe based on clinopyroxene and spinel chemistry: Evidence for mantle modification during an extensional cycle[END_REF]. Previous studies on basalts derived from the fossil Alpine-Tethys OCT show that they are characterized by a wide range of trace element compositions, from depleted basalts [START_REF] Bill | Major, minor, trace element, Sm-Nd and Sr isotope compositions of mafic rocks from the earliest oceanic crust of the Alpine Tethys[END_REF][START_REF] Desmurs | Onset of magmatic accretion within a magma-poor rifted margin: a case study from the Platta ocean-continent transition, eastern Switzerland[END_REF][START_REF] Durand-Delga | Arguments en faveur de la position, au Jurassique, des ophiolites de Balagne (Haute-Corse, France) au voisinage de la marge continentale europeenne[END_REF][START_REF] Kramer | Geochemistry of metabasalts from ophiolitic and adjacent distal continental margin units: Evidence from the Monte Rosa region (Swiss and Italian Alps)[END_REF][START_REF] Montanini | Petrogenesis of basalts and gabbros from an ancient continent?ocean transition (External Liguride ophiolites, Northern Italy)[END_REF][START_REF] Piccardo | The Jurassic Ligurian Tethys, a fossil ultraslow spreading ocean: the mantle perspective[END_REF][START_REF] Saccani | Corsica ophiolites: geochemistry and petrogenesis of basaltic and metabasaltic rocks[END_REF] to enriched basalts [START_REF] Frisch | The Arosa Zone and Platta Nappe ophiolites (Eastern Swiss Alps): geochemical characteristics and their meaning for the evolution of the Penninic Ocean[END_REF][START_REF] Renna | Role of melting process and melt-rock reaction in the formation of Jurassic MORB-type basalts (Alpine ophiolites)[END_REF][START_REF] Steinmann | Geochemical evidence for the nature of the crust beneath the eastern North Penninic basin of the Mesozoic Tethys ocean[END_REF][START_REF] Vannucci | Ophiolitic magmatism in the Ligurian Tethys: an ion microprobe study of basaltic clinopyroxenes[END_REF] leading to various interpretations concerning the parental sources of these basalts. Over the last decade, a considerable data set has been acquired on the mantle rocks from Alpine-Tethys OCTs [START_REF] Müntener | Plagioclase Peridotites in Ocean-Continent Transitions: Refertilized Mantle Domains Generated by Melt Stagnation in the Shallow Mantle Lithosphere[END_REF][START_REF] Picazo | Mapping the nature of mantle domains in Western and Central Europe based on clinopyroxene and spinel chemistry: Evidence for mantle modification during an extensional cycle[END_REF][START_REF] Piccardo | Evolution of the lithospheric mantle during passive rifting: Inferences from the Alpine-Apennine orogenic peridotites[END_REF][START_REF] Rampone | Multi-stage melt-rock interaction in the Mt. Maggiore (Corsica, France) ophiolitic peridotites: microstructural and geochemical evidence[END_REF]. This new knowledge, combined with the geological observations, enable to better discriminate between the various interpretations and to understand the processes involved in magma-production at deeper parts of the lithosphere during mantle exhumation and lithospheric breakup.

In this paper, we present a compilation of Alpine-Tethys basalt chemistry including new data from the Platta and Tasna nappes (SE Switzerland, Fig. 1a). The lack of a major Alpine metamorphic overprint, the excellent exposure and the understanding of the mantle evolution prior and during breakup of these two nappes allowed to propose a tectono-magmatic model that may explain the peculiar geochemistry of the basalts of the Alpine-Tethys OCT.

Geological setting

Remnants of OCTs in the Alps

The Tasna and Platta nappes represent well preserved remnants of a fossil OCT of the ancient European and Adriatic conjugate rifted margins [START_REF] Manatschal | What is the tectono-metamorphic evolution of continental break-up: The example of the Tasna Ocean-Continent Transition[END_REF][START_REF] Manatschal | A continent-ocean transition recorded in the Err and Platta nappes (Eastern Switzerland)[END_REF] that were related to the opening of the Jurassic Liguro-Piemonte/Alpine Tethys ocean (Fig. 1b). This ocean probably was a narrow basin (<600km-wide) that developed as a result of an ultra-slow spreading activity [START_REF] Marroni | The architecture of an incipient oceanic basin: a tentative reconstruction of the Jurassic Liguria-Piemonte basin along the Northern Apennines-Alpine Corsica transect[END_REF][START_REF] Li | New isotopic constraints on age and magma genesis of an embryonic oceanic crust: The Chenaillet Ophiolite in the Western Alps[END_REF]. The Tasna nappe is part of the northeastern European OCT preserving a wedge of continental crust that is laterally replaced by exhumed subcontinental mantle and grades into proto-oceanic crust with first magmatic additions [START_REF] Manatschal | What is the tectono-metamorphic evolution of continental break-up: The example of the Tasna Ocean-Continent Transition[END_REF]. The Penninic Platta nappe is subdivided into two Alpine tectonic units, the Upper and the Lower Platta units (Supplementary material S1). The Upper Platta nappe is bounded at its top by the Lower Austroalpine Err nappe (part of the Adria distal continental margin) and overlies Middle and Lower Penninic nappes (remnant of the European distal margin; [START_REF] Manatschal | A continent-ocean transition recorded in the Err and Platta nappes (Eastern Switzerland)[END_REF]. Both the Platta and Tasna nappes, as well as the majority of Alpine-Apennine ophiolites, consist of highly serpentinized peridotites intruded by large gabbroic bodies and overlain locally by massive basaltic lava flows, continent-derived extensional allochthons and post-rift sediments (supplementary material S2, [START_REF] Desmurs | Mantle evolution and magmatism in an evolving ocean-continent transition: The Platta nappe, eastern Switzerland[END_REF][START_REF] Desmurs | The Steinmann trinity revisited: mantle exhumation and magmatism along an ocean-continent transition: the Platta nappe, eastern Switzerland[END_REF][START_REF] Peters | Geologischer Atlas der Schweiz Blätt 1256[END_REF].

Nature of the exhumed mantle

Two main types of mantle have been distinguished all around the Alpine-Tethys and more particularly in the Tasna and Platta nappes (Fig. 2a, [START_REF] Picazo | Mapping the nature of mantle domains in Western and Central Europe based on clinopyroxene and spinel chemistry: Evidence for mantle modification during an extensional cycle[END_REF]. The first type identified in the Tasna and Upper Platta units consists of spinel lherzolites containing abundant garnet-bearing pyroxenites [START_REF] Desmurs | Mantle evolution and magmatism in an evolving ocean-continent transition: The Platta nappe, eastern Switzerland[END_REF][START_REF] Müntener | Plagioclase Peridotites in Ocean-Continent Transitions: Refertilized Mantle Domains Generated by Melt Stagnation in the Shallow Mantle Lithosphere[END_REF][START_REF] Peters | Geologischer Atlas der Schweiz Blätt 1256[END_REF]. Major and trace element compositions in clinopyroxene demonstrate the fertile character of these mantle rocks [START_REF] Picazo | Mapping the nature of mantle domains in Western and Central Europe based on clinopyroxene and spinel chemistry: Evidence for mantle modification during an extensional cycle[END_REF]. This mantle, referred to as the inherited subcontinental mantle, is similar to the mantle exhumed in the proximal parts of the present-day Iberia-Newfoundland OCTs [START_REF] Müntener | High degrees of melt extraction recorded by spinel harzburgite of the Newfoundland margin: The role of inheritance and consequences for the evolution of the southern North Atlantic[END_REF].

The second type of mantle composing the lower Platta unit consists of spinel lherzolites with minor harzburgites and dunites and shows only few occurrences of pyroxenites [START_REF] Desmurs | Mantle evolution and magmatism in an evolving ocean-continent transition: The Platta nappe, eastern Switzerland[END_REF][START_REF] Müntener | Plagioclase Peridotites in Ocean-Continent Transitions: Refertilized Mantle Domains Generated by Melt Stagnation in the Shallow Mantle Lithosphere[END_REF][START_REF] Müntener | Refertilization of mantle peridotite in embryonic ocean basins: trace element and Nd isotopic evidence and implications for crust-mantle relationships[END_REF][START_REF] Peters | Geologischer Atlas der Schweiz Blätt 1256[END_REF]. This peridotite is pervasively impregnated by ascending tholeiitic melts [START_REF] Müntener | Plagioclase Peridotites in Ocean-Continent Transitions: Refertilized Mantle Domains Generated by Melt Stagnation in the Shallow Mantle Lithosphere[END_REF][START_REF] Müntener | Refertilization of mantle peridotite in embryonic ocean basins: trace element and Nd isotopic evidence and implications for crust-mantle relationships[END_REF]) that pre-date the Middle Jurassic mantle exhumation [START_REF] Piccardo | Evolution of the lithospheric mantle during passive rifting: Inferences from the Alpine-Apennine orogenic peridotites[END_REF][START_REF] Rampone | Petrology, mineral and isotope geochemistry of the External Liguride peridotites (Northern Apennines, Italy)[END_REF]. This mantle type has also been described from the distal part of the Iberia margin [START_REF] Müntener | High degrees of melt extraction recorded by spinel harzburgite of the Newfoundland margin: The role of inheritance and consequences for the evolution of the southern North Atlantic[END_REF] and is considered to be the refertilized part of exhumed subcontinental mantle (see type 2 mantle in [START_REF] Picazo | Mapping the nature of mantle domains in Western and Central Europe based on clinopyroxene and spinel chemistry: Evidence for mantle modification during an extensional cycle[END_REF].

Basalts from the Platta and Tasna OCTs

Petrology of extrusive magmatic rocks and basaltic intrusions

Extrusive basaltic rocks exposed in the Platta and Tasna nappes (supplementary material S3) occur as hyaloclastites, pillow breccias, pillow lavas and massive lava flows that can reach up to 300 m thickness (Fig. 2a-b). Basalts lie on exhumed mantle and are covered by Upper Jurassic to Lower Cretaceous marine sediments (Fig. 2c). Basalts exhibit intersertal to porphyric textures and contain plagioclase phenocrysts embedded in a fine-grained matrix of plagioclase, chlorite, epidote, pumpellyite and calcite (Fig. 2d). Hyaloclastite breccias consist of fragments of lava flows, which probably derive from the dislocation of former volcanic deposits. A Jurassic age (167-161 Ma) is based on the dating of radiolarian cherts that are either interlayered or overlie the basalts [START_REF] Bill | Radiolarite ages in Alpine-Mediterranean ophiolites: Constraints on the oceanic spreading and the Tethys-Atlantic connection[END_REF] and U-Pb on gabbros that are genetically linked to the basalts [START_REF] Schaltegger | The transition from rifting to sea-floor spreading within a magma-poor rifted margin: field and isotopic constraints[END_REF].

A younger magmatic event consisting of sills of 10 to 20 m thickness (Fig. 2e) has been found in the Platta nappe. These sills intrude limestones and shales of Lower Cretaceous age [START_REF] Peters | Geologischer Atlas der Schweiz Blätt 1256[END_REF], representing the youngest pre-Alpine magmatic event identified so far in the Alpine OCT. The sills are characterized by finegrained textures at the rims and coarse-grained at the center. They are composed of plagioclase, chlorite, pumpellyite and epidote (Fig. 2f).

Major and trace element composition of basalts

A comprehensive description of analytical methods for chemical analysis are given in supplementary material S4. Major element content of our samples is similar to those of other basalts from the Platta nappe [START_REF] Desmurs | Onset of magmatic accretion within a magma-poor rifted margin: a case study from the Platta ocean-continent transition, eastern Switzerland[END_REF][START_REF] Frisch | The Arosa Zone and Platta Nappe ophiolites (Eastern Swiss Alps): geochemical characteristics and their meaning for the evolution of the Penninic Ocean[END_REF]. Considering both their mineralogical and geochemical compositions, they range between tholeiite and alkali basalt in nature (SiO2: 45-50 wt.%; Al2O3: 15-19 wt.%; Na2O+K2O < 5 wt.%) and display relatively low Mg# (< 0.65) showing that they represent differentiated melts rather than primitive liquids (Table 1).

On the basis of trace element compositions, two different types of Jurassic basalts can be distinguished (Fig. 3). Type-1 is depleted in incompatible elements, shows a positive Zr anomaly, a low (La/Sm)N and is comparable to "normal" MORB. Type-1 shows a large fractionation of HREE over MREE, (Sm/Yb)N=0.82-1.76, and a widespread Nb/La ratio (0.55 to 3.25). Samples characterized by medium to high HREE over MREE and a low Nb/La ratio are volumetrically predominant in the Platta-Tasna nappes as well as in the entire Alpine Tethys margins (Fig. 5). The type-2 is enriched in incompatible elements (>10 times PUM concentrations) and high field strength elements (HFSE, Nb, Ta, Zr, Hf; Fig. 3). Type-2 basalts have high (La/Sm)N > 1.3 and high (Sm/Yb)N>1.5 ratios, and are similar to present-day enriched MORB (E-MORB; [START_REF] Gale | The mean composition of ocean ridge basalts: MEAN MORB[END_REF]. Although of a different apparent age, trace element patterns of Cretaceous sills (type-3) are similar to those of Jurassic E-MORB. This geochemical diversity is not restricted to Platta and Tasna basalts but have been described in several ophiolites of the Alpine-Tethys domain (e.g. [START_REF] Bill | Major, minor, trace element, Sm-Nd and Sr isotope compositions of mafic rocks from the earliest oceanic crust of the Alpine Tethys[END_REF] and Supplementary material S6).

Origin of OCT-basalts

Basalts from this study are representative of basalts from the Alpine-Tethys (Fig. 4). The Cr vs. Y diagram shows that all these basalts result from 5% to 15% partial melting followed by various degrees of fractional crystallization (Fig. 4). However, their variable trace element patterns (Fig. 3), illustrate that the Platta-Tasna basalts (types-1, 2 and 3) as well as other basalts from the Alpine Tethys may not derived from the melting of a depleted mantle source but rather may reflect the melting of another mantle source.

The depleted "N-MORB"-like basalts (type-1)

Type-1 basalts depletion in LREE makes them to a first order comparable to N-MORB (Erreur ! Source du renvoi introuvable.a). However, most of these basalts as well as most of the Alpine-Tethys basalts substantially differ from present-day MORB by lower Nb/La ratios and higher (Sm/Yb)N ratios (Erreur ! Source du renvoi introuvable. and supplementary material S5). High (Sm/Yb)N ratios are classically used as a proxy to identify the presence of a garnet-bearing component in the MORB source [START_REF] Hellebrand | Garnet-field melting and late-stage refertilization in 'residual' abyssal peridotites from the Central Indian Ridge[END_REF][START_REF] Montanini | Petrogenesis of basalts and gabbros from an ancient continent?ocean transition (External Liguride ophiolites, Northern Italy)[END_REF][START_REF] Saccani | Continental Margin Ophiolites of Neotethys: Remnants of Ancient Ocean-Continent Transition Zone (OCTZ) Lithosphere and Their Geochemistry, Mantle Sources and Melt Evolution Patterns[END_REF][START_REF] Sobolev | The amount of recycled crust in sources of mantle-derived melts[END_REF][START_REF] Stracke | Assessing the presence of garnet-pyroxenite in the mantle sources of basalts through combined hafnium-neodymium-thorium isotope systematics[END_REF]. The melting of a DMM-like source in the spinel stability field hardly produces basalts with (Sm/Yb)N>1.2, even at low degrees of melting, while most of the Alpine basalts display (Sm/Yb)N>1.2 (see supplementary material S5). Such high (Sm/Yb)N ratios are easily simulated by melting a depleted mantle source in the garnet stability. However, both modeling approaches fail to reproduce the low Nb/La ratio that characterize most of the Alpine basalts (Erreur ! Source du renvoi introuvable.b). Since Nb is more incompatible than La (DNb<DLa), Nb/La ratios tend to decrease in both the residual mantle and the extracted melts with increasing melt degree. Thus, basalts with Nb/La<0.7 must be derived from the melting of a mantle source more refractory than the asthenospheric DMM.

The inherited SCLM can hardly be considered as a potential source for type-1 basalts since it is too fertile (mantle type 1a from [START_REF] Picazo | Mapping the nature of mantle domains in Western and Central Europe based on clinopyroxene and spinel chemistry: Evidence for mantle modification during an extensional cycle[END_REF] or too depleted (mantle type 1b from [START_REF] Picazo | Mapping the nature of mantle domains in Western and Central Europe based on clinopyroxene and spinel chemistry: Evidence for mantle modification during an extensional cycle[END_REF]. Similarly, the oceanic mantle (type 3 from [START_REF] Picazo | Mapping the nature of mantle domains in Western and Central Europe based on clinopyroxene and spinel chemistry: Evidence for mantle modification during an extensional cycle[END_REF] is too depleted and occurs too oceanward from the location where basalts erupted. However, the refertilized mantle section (mantle type 2; [START_REF] Picazo | Mapping the nature of mantle domains in Western and Central Europe based on clinopyroxene and spinel chemistry: Evidence for mantle modification during an extensional cycle[END_REF]) may be a plausible source. The geochemical composition of this refertilized mantle is calculated following the modelling approach of [START_REF] Müntener | Plagioclase Peridotites in Ocean-Continent Transitions: Refertilized Mantle Domains Generated by Melt Stagnation in the Shallow Mantle Lithosphere[END_REF]. The starting mantle composition is the PUM, which has undergone 4% partial melting in the garnet (4%) stability field, followed by 4% in the spinel stability field. This residual mantle is then refertilized by ascending melts (up to 12% in volume) deriving from the melting of an asthenospheric depleted source (DMM) in the spinel stability field. Fig. 5a,b shows that the melting of such a refertilized source is able to produce melts similar to type-1 basalts with low Nb/La and high (Sm/Yb)N. The calculated melting degree of the refertilized mantle is rather low (<15%), which is consistent with previous estimates made on Alpine basalts [START_REF] Desmurs | Onset of magmatic accretion within a magma-poor rifted margin: a case study from the Platta ocean-continent transition, eastern Switzerland[END_REF]. This model is very similar to the one proposed previously by [START_REF] Meyzen | A discontinuity in mantle composition beneath the southwest Indian ridge[END_REF] to explain the peculiar REE signature of some MORB from the northern magma-poor section of the ultraslow-spreading Southwest Indian Ridge. More importantly, this model is consistent with geological observations made by previous studies [START_REF] Müntener | Plagioclase Peridotites in Ocean-Continent Transitions: Refertilized Mantle Domains Generated by Melt Stagnation in the Shallow Mantle Lithosphere[END_REF][START_REF] Picazo | Mapping the nature of mantle domains in Western and Central Europe based on clinopyroxene and spinel chemistry: Evidence for mantle modification during an extensional cycle[END_REF].

The formation of "E-MORB"-like basalts (types-2 and 3)

Aside from type-1 basalts, a few percent of extrusive basalts can be ascribed as enriched-basalts (E-MORB, types-2 and 3). Additional occurrences of E-MORBs have been reported previously in other fossil OCT in the Alps (External Ligurides and Corsica; [START_REF] Montanini | Petrogenesis of basalts and gabbros from an ancient continent?ocean transition (External Liguride ophiolites, Northern Italy)[END_REF][START_REF] Renna | Role of melting process and melt-rock reaction in the formation of Jurassic MORB-type basalts (Alpine ophiolites)[END_REF][START_REF] Saccani | Corsica ophiolites: geochemistry and petrogenesis of basaltic and metabasaltic rocks[END_REF]. The enriched nature of these basalts was attributed to the simultaneous melting of the Jurassic ascending asthenosphere (i.e. the DMM) and garnet-pyroxenites. This hypothesis was tested by applying a similar modeling approach to that of [START_REF] Montanini | Petrogenesis of basalts and gabbros from an ancient continent?ocean transition (External Liguride ophiolites, Northern Italy)[END_REF]: variable amounts of garnet-pyroxenite derived melts were mixed with (i) aggregated melts of a depleted spinel peridotite source (red mixing lines in Erreur ! Source du renvoi introuvable.c, d) and (ii) type-1 basalts extracted from the refertilized SCLM (blue mixing lines in Erreur ! Source du renvoi introuvable.c, d). Three different partial melting degrees are considered for the formation of depleted melts (F=0.05, 0.1 and 0.15), while a high partial melting degree of 0.25 is considered for the pyroxenite melt [START_REF] Montanini | Petrogenesis of basalts and gabbros from an ancient continent?ocean transition (External Liguride ophiolites, Northern Italy)[END_REF]. Other modeling features (partition coefficients, pyroxenite source composition, etc.) are given in Erreur ! Source du renvoi introuvable. caption. Results highlight that E-MORB melts may derive from the mixing between 80-90% of depleted MORB and 10-20% partial melts of garnet-pyroxenite, these results being in good agreement with previous calculations of [START_REF] Sobolev | The amount of recycled crust in sources of mantle-derived melts[END_REF] and [START_REF] Montanini | Petrogenesis of basalts and gabbros from an ancient continent?ocean transition (External Liguride ophiolites, Northern Italy)[END_REF].

Geodynamic constraints and conclusions

Ophiolites from the Alpine-Tethys domain have provided strong evidence for processes of interaction between inherited SCLM and the ascending asthenosphere [START_REF] Bodinier | Continental to oceanic mantle transition-REE and Sr-Nd isotopic geochemistry of the Lanzo lherzolite massif[END_REF][START_REF] Montanini | Petrogenesis of basalts and gabbros from an ancient continent?ocean transition (External Liguride ophiolites, Northern Italy)[END_REF][START_REF] Müntener | Refertilization of mantle peridotite in embryonic ocean basins: trace element and Nd isotopic evidence and implications for crust-mantle relationships[END_REF][START_REF] Müntener | Melt migration in ophiolitic peridotites: the message from Alpine-Apennine peridotites and implications for embryonic ocean basins[END_REF]Piccardo et al., 2004b[START_REF] Piccardo | Melt/peridotite interaction in the Southern Lanzo peridotite: Field, textural and geochemical evidence[END_REF][START_REF] Tribuzio | Origin of the Gabbro-Peridotite Association from the Northern Apennine Ophiolites (Italy)[END_REF], which is different from basalts formed at steady state, mature MORs. Almost all extrusive basalts from the Alpine-Tethys OCT overlie an exhumed, refertilized SCLM [START_REF] Picazo | Mapping the nature of mantle domains in Western and Central Europe based on clinopyroxene and spinel chemistry: Evidence for mantle modification during an extensional cycle[END_REF]. Only a few overlie the inherited SCLM in Central Alps (Upper Platta, Malenco, Totalp, Picazo et al., 2016). We therefore suggest here that the specific geochemistry of basalts from the Alpine-Tethys OCT is related to their emplacement over the exhumed, refertilized SCLM. The upward melt infiltration coming from the partial melting of the DMM in the garnet stability field creates the widespread refertilized SCLM (type 2 mantle of [START_REF] Picazo | Mapping the nature of mantle domains in Western and Central Europe based on clinopyroxene and spinel chemistry: Evidence for mantle modification during an extensional cycle[END_REF]. This melting episode is enhanced by a near-adiabatic decompression regime [START_REF] Mckenzie | The volume and composition of melt generated by extension of the lithosphere[END_REF] during the very first stages of lithospheric thinning and asthenospheric uplift (Piccardo et al., 2004a, Fig. 5a). Melt percolation occurred during lithospheric/crustal thinning and was controlled by tectonic processes and the thermal structure of the extending lithosphere, both of which are not well constrained. Kaczmarek and Müntener (2010) proposed, using the example of the Lanzo shear zone, that percolation occurred in the footwall of a major extensional shear zone that acted as a permeability barrier. The thermal advection related to both asthenospheric upwelling and melt percolation is likely able to increase the thermal gradient in order to reach the solidus of the refertilized SCLM at shallower depths in the spinel stability field (Piccardo et al., 2004a(Piccardo et al., , 2004b)). Integrating this inherited history of the SCLM may explain the peculiar geochemical signature characterizing most of basalts from the Alpine-Tethys OCTs that were erupted on exhumed SCLM in the distal part of the hyperextended margin (i.e. type-1 basalts, Fig. 5b). In contrast, the enriched basalts may correspond to low degree melts of the DMM with small amounts of garnet-pyroxenite, both before and after the onset of seafloor spreading (Fig. 5b-c). This study supports the idea that basalts erupting in the Alpine-Tethys OCT have long-lived, poly-phase and variable mantle sources. The peculiar trace element geochemistry of basalts and the well-established geodynamic setting of OCT in the Alps with complex mantle inheritance enable to explain the differences between OCT and MOR basalts. Therefore, basalts formed along OCTs should be referred to as OCT-basalts rather than MOR-basalts. the Late Jurassic (redrawn after [START_REF] Picazo | Mapping the nature of mantle domains in Western and Central Europe based on clinopyroxene and spinel chemistry: Evidence for mantle modification during an extensional cycle[END_REF]. The reconstruction is based on structural [START_REF] Manatschal | A type sequence across an ancient magma-poor ocean-continent transition: the example of the western Alpine Tethys ophiolites[END_REF] as well as mantle geochemical [START_REF] Picazo | Mapping the nature of mantle domains in Western and Central Europe based on clinopyroxene and spinel chemistry: Evidence for mantle modification during an extensional cycle[END_REF][START_REF] Rampone | A global overview of isotopic heterogeneities in the oceanic mantle[END_REF] considerations. Ophiolite with (*) highlight ambiguous paleo-location. CE: Cecina Valley, CH: Chenaillet, EL: External Ligurides, ET: Erro-Tobbio, GE: Gets Nappe, IL: Internal Ligurides, LA: Lanzo, LP: Lower Platta, MA: Malenco, MM: Monte Maggiore, MV: Mont-Viso, UP: Upper Platta, TA: Tasna, TO: Totalp, ZS: Zermatt-Saas.

List of figures and tables

Br: Briançonnais, Sa: Sardinia. [START_REF] Gale | The mean composition of ocean ridge basalts: MEAN MORB[END_REF] and other basalts from Platta are from [START_REF] Desmurs | Onset of magmatic accretion within a magma-poor rifted margin: a case study from the Platta ocean-continent transition, eastern Switzerland[END_REF]. [START_REF] Workman | Major and trace element composition of the depleted MORB mantle (DMM)[END_REF]. The arrows indicate the calculated trends of the liquids for fractional crystallization of clinopyroxene + plagioclase (cpx + plg) assemblages starting from the relatively primitive basaltic sample 13-04 (residual liquid fraction F up to 0.5). Mineral mass fractions are 0.2cpx+0.8plg. Partition coefficients sources for Y and Cr are from [START_REF] Ionov | Mechanisms and sources of mantle metasomatism: major and trace element compositions of peridotite xenoliths from Spitsbergen in the context of numerical modelling[END_REF] and from [START_REF] Hart | Experimental cpx/melt partitioning of 24 trace elements[END_REF] respectively.

Note that the starting mantle composition does not dramatically change the partial melting models nor the crystal fractionation trends. 1 : Gale et al., 2013, 2 : Other Platta basalts are from [START_REF] Desmurs | Onset of magmatic accretion within a magma-poor rifted margin: a case study from the Platta ocean-continent transition, eastern Switzerland[END_REF] and [START_REF] Frisch | The Arosa Zone and Platta Nappe ophiolites (Eastern Swiss Alps): geochemical characteristics and their meaning for the evolution of the Penninic Ocean[END_REF]. Alpine-Apennine basalts (little blue circles) are the overall Alpine-Tethys domain. Internal Ligurides [START_REF] Ferrara | Ocean-floor affinity of basalts from north Apennine ophiolites: geochemical evidence[END_REF][START_REF] Ottonello | Rare earth and 3d transition element geochemistry of peridotitic rocks: I. Peridotites from the Western Alps[END_REF][START_REF] Rampone | Isotopic contrasts within the Internal Liguride ophiolite (N. Italy): the lack of a genetic mantle-crust link[END_REF][START_REF] Venturelli | Rare earth and trace element characteristics of ophiolitic metabasalts from the Alpine-Apennine belt[END_REF], External Ligurides [START_REF] Marroni | The association of continental crust rocks with ophiolites in the Northern Apennines (Italy): implications for the continent-ocean transition in the Western Tethys[END_REF][START_REF] Montanini | Petrogenesis of basalts and gabbros from an ancient continent?ocean transition (External Liguride ophiolites, Northern Italy)[END_REF][START_REF] Vannucci | Ophiolitic magmatism in the Ligurian Tethys: an ion microprobe study of basaltic clinopyroxenes[END_REF][START_REF] Venturelli | Rare earth and trace element characteristics of ophiolitic metabasalts from the Alpine-Apennine belt[END_REF], Corsica [START_REF] Beccaluva | The trace element geochemistry of Corsican ophiolites[END_REF][START_REF] Durand-Delga | Arguments en faveur de la position, au Jurassique, des ophiolites de Balagne (Haute-Corse, France) au voisinage de la marge continentale europeenne[END_REF][START_REF] Padoa | Structural and geochemical data on the Rio Magno Unit: Evidence for a new 'Apenninic' ophiolitic unit in Alpine Corsica and its geodynamic implications[END_REF][START_REF] Renna | Role of melting process and melt-rock reaction in the formation of Jurassic MORB-type basalts (Alpine ophiolites)[END_REF][START_REF] Saccani | Corsica ophiolites: geochemistry and petrogenesis of basaltic and metabasaltic rocks[END_REF][START_REF] Venturelli | Rare earth and trace element characteristics of ophiolitic metabasalts from the Alpine-Apennine belt[END_REF][START_REF] Venturelli | Rare-earth and other element distribution in some ophiolitic metabasalts of Corsica, Western Mediterranean[END_REF], Western Alps [START_REF] Bodinier | Geochemistry of basic dikes in the Lanzo massif (Western Alps): petrogenetic and geodynamic implications[END_REF][START_REF] Chalot-Prat | No significant element transfer from the oceanic plate to the mantle wedge during subduction and exhumation of the Tethys lithosphere (Western Alps)[END_REF][START_REF] Lewis | The Montgenevre ophiolite (Hautes Alpes, France): Meta-morphism and trace-element geochemistry of the volcanic sequence[END_REF][START_REF] Pognante | Petrology of ultramafic and mafic rocks from the Lanzo peridotite body (Western Alps)[END_REF], Central Alps [START_REF] Desmurs | Onset of magmatic accretion within a magma-poor rifted margin: a case study from the Platta ocean-continent transition, eastern Switzerland[END_REF][START_REF] Frisch | The Arosa Zone and Platta Nappe ophiolites (Eastern Swiss Alps): geochemical characteristics and their meaning for the evolution of the Penninic Ocean[END_REF], Tuscany [START_REF] Ferrara | Ocean-floor affinity of basalts from north Apennine ophiolites: geochemical evidence[END_REF][START_REF] Mazzeo | New constraints on the origin of the ophiolitic rocks within sin-orogenic turbiditic sequences at Cilento region (southern Italy)[END_REF], Calabre [START_REF] Liberi | Geodynamic significance of ophiolites within the Calabrian Arc[END_REF] and inferred Valais domain [START_REF] Bill | Major, minor, trace element, Sm-Nd and Sr isotope compositions of mafic rocks from the earliest oceanic crust of the Alpine Tethys[END_REF][START_REF] Dürr | Geochemistry and geodynamic significance of North Penninic ophiolites from the Central Alps[END_REF][START_REF] Kramer | Geochemistry of metabasalts from ophiolitic and adjacent distal continental margin units: Evidence from the Monte Rosa region (Swiss and Italian Alps)[END_REF][START_REF] Mugnier | The tholeiites of the Valaisan domain (Versoyen, western Alps): a Carboniferous magma emplaced in a small oceanic basin[END_REF][START_REF] Steinmann | Geochemical evidence for the nature of the crust beneath the eastern North Penninic basin of the Mesozoic Tethys ocean[END_REF]. [START_REF] Mcdonough | The composition of the Earth[END_REF] experienced 4% partial melting in the garnet stability field (A) followed by 4% of additional melting in the spinel stability field (B). The residue is refertilized by adding a 4% fractional melt derived from a spinel DMM source (1% increment, C). A volume of 1%, 3%, 6%, 9% and a maximum of 12% of melts stored in the peridotite is considered [START_REF] Müntener | Plagioclase Peridotites in Ocean-Continent Transitions: Refertilized Mantle Domains Generated by Melt Stagnation in the Shallow Mantle Lithosphere[END_REF]. The red curves correspond to melts extracted from the calculated refertilized mantle in the spinel stability field at F up to 20%. Partition coefficients are from [START_REF] Johnson | Experimental determination of partition coefficients for rare earth and high-fieldstrength elements between clinopyroxene, garnet, and basaltic melt at high pressures[END_REF], starting mineralogy and modes are from [START_REF] Johnson | Melting in the oceanic upper mantle: an ion microprobe study of diopsides in abyssal peridotites[END_REF]. (b) (Sm/Yb)N vs. YbN of alpine basalts. Red curves are calculated in the same way as in Fig. 5a. (c) Nb/La vs. La of E-MORB alpine basalts, assuming that they derive from the mixing of aggregated non-modal fractional melts (F=5, 10, 15%) from a spinel DMM source (in red) or a refertilized SCLM (8% refertilization, in blue) with garnet pyroxenite derived melts (F=25%; [START_REF] Montanini | Petrogenesis of basalts and gabbros from an ancient continent?ocean transition (External Liguride ophiolites, Northern Italy)[END_REF]. The garnet pyroxenite source derives from high degrees of batch melting (F=40%; [START_REF] Montanini | Garnet clinopyroxenite layers from the mantle sequences of the Northern Apennine ophiolites (Italy): Evidence for recycling of crustal material[END_REF] of a recycled oceanic crust (i.e. an eclogite; [START_REF] Sobolev | The amount of recycled crust in sources of mantle-derived melts[END_REF]. The eclogite composition (Ceclogite) is calculated assuming that eclogite corresponds to basaltic, N-MORB type crust (CN-MORB) that experienced element mobility during subduction prior being recycled into the mantle: Ceclogite= CN-MORB -retention factors where retention factors = 1 -mobility coefficients [START_REF] Kogiso | Trace element transport during dehydration processes in the subducted oceanic crust: 1. Experiments and implications for the origin of ocean island basalts[END_REF]. The calculated eclogite has a high Nb/La ratio (~2, not shown here), consistent with values calculated by [START_REF] Rudnick | Rutile-Bearing Refractory Eclogites: Missing Link Between Continents and Depleted Mantle[END_REF]. Gt-pyroxenite mineralogy: 60% cpx, 30% opx, 10% gt [START_REF] Hirschmann | A possible role for garnet pyroxenite in the origin of the "garnet signature[END_REF]; melting modes: cpx=gt=50%. Eclogite mineralogy: 50% gt, 50% cpx.

Fig. 1 :

 1 Fig. 1: (a) Location of major ophiolite massifs in the Alps-Apennine-Corsica (redrawn after Rampone and

Fig. 2 :

 2 Fig. 2: Macroscopic field and microscopic observations of basalts. (a) Panoramic view of the Err-Platta Ocean-

Fig. 3 :

 3 Fig. 3: Whole-rock incompatible trace element (normalized to the primitive mantle, PM, McDonough and

Fig. 4 :

 4 Fig. 4: Cr (ppm) versus Y (ppm) of whole-rock Platta-Tasna basalts. The partial melting trend starts from the

Fig. 5 :

 5 Fig. 5: (a) Nb/La vs. La showing the compositions of Alpine basalts. The modelling consists of the calculation of

  Numbers in italic represent the proportion of mantle derived melts in the mixing. (d) (Sm/Yb)N vs. YbN of alpine basalts. Red and blue curves are calculated in the same way as in Fig.5c. N-star: N-MORB average ± 2σ (grey area); Present-day MORBs and N-MORB average are from[START_REF] Gale | The mean composition of ocean ridge basalts: MEAN MORB[END_REF].

Fig. 5 :

 5 Fig. 5 : Schematic representation of the geodynamic model leading to the OCT-basalts. (a) Melt percolation.

Acknowledgements

We greatly thank Thibault Wiedemann, Nicolas Mattioni, Jean Zielinski and René Boutin for their help during laboratory works. We also thank John Moine for the realization of thin sections. This work is a part of M. Amann Ph.D thesis and has benefited of the financial support provided by the French Ministry of Research and INSU-CNRS through the SYSTER program. None of the authors of the above manuscript has declared any conflict of interest.

partial melting in the garnet stability field (pink circles), and initial percolation of melts towards the subcontinental mantle (light brown area) during the stretching of the lithosphere. Porous-flow melt percolation induces (i) the refertilization of Permian (?) depleted SCLM, (ii) the formation of plagioclasebearing peridotite at shallower levels, and (iii) the formation of a weakened mantle wedge for the lithospheric breakup [START_REF] Piccardo | Evolution of the lithospheric mantle during passive rifting: Inferences from the Alpine-Apennine orogenic peridotites[END_REF]. The distal subcontinental mantle is melt-impregnated (up to 12%) and refertilized.

Major crustal/lithospheric thinning is followed by mantle exhumation and onset of sea-floor exposure of the melt-modified peridotites. (b) Creation of OCT-basalts. Thermal advection [START_REF] Piccardo | The Ligurian Tethys: Mantle processes and geodynamics[END_REF] enhanced by porous-flow melt percolation moves the solidus upwards allowing the melting of the refertilized subcontinental mantle in the spinel stability field (light blue circles) forming the type-1 basalts (light blue pipes and volcanoes). Mixing of melts from the partial melting of the DMM (dark blue circles) and garnet-pyroxenite (black dashes, orange circles) creates type-2 basalts (red pipes and volcanoes). (c) Formation of type-3 off-axis basalts (yellow volcanoes) by the same process as type-2. The mixing between the two sources is located at the two edges of the upwelling asthenosphere and is probably related to thermal readjustments. Active spreading ridge followed by MOR-basalt formation caused by the partial melting of the DMM under spinel (shallow) stability field. Supplementary material S2: Schematic section representing the lithologies and their stratigraphic position in the Platta nappe. Note that the shear-zone in mantle and the sulfide deposit are truncated by the detachment fault. Redrawn after [START_REF] Peters | Geologischer Atlas der Schweiz Blätt 1256[END_REF] and own fieldwork observations. Supplementary material S3: Locations and mineralogical assemblage of studied magmatic samples.

Supplementary material

Coordinates grid is WGS84 UTM 32N, decimal degrees. Fds (Ab): feldspar (albite), Cal: calcite, Chl: chlorite, Cpx: clinopyroxene, Ep: epidote. Crosses indicate relative mineral proportions. Note that samples 13-03 and 13-04 are from the Tasna nappe. Supplementary material S4: Analytical method Major and trace elements were measured using the analytical method of [START_REF] Chauvel | Comprehensive Chemical and Isotopic Analyses of Basalt and Sediment Reference Materials[END_REF] at Laboratoire d'Hydrologie et de Géochimie de Strasbourg (LHyGeS, University of Strasbourg, France). For major elements, ~50 mg of sample powder was precisely weighted and dissolved in closed Savilex® beakers using a mixture of 0.3 mL HF 24N and 0.85 mL HNO3 14N in a hot plate at 90°C for 5 days. 20 ml of boric acid (H3BO3 at 20 g.l -1 ) was added to the solutions after cooling to neutralize excess HF and were further diluted with 250 ml of Milli-Q® water. The solutions were then stored in a refrigerator for at least 2 days to ensure complete neutralization of HF by H3BO3 and analyzed within a week using an ICP-AES (Thermo Scientific ICAP 6500). For trace elements, 100 mg of sample powder were dissolved in a HF-HClO4 mixture (5:1) in closed teflon beaker on a hot plate at 140°C for 7 days. The mixture was then completely evaporated at 150°C, and the residue was taken up in concentrated HNO3. After another complete evaporation, the residue is finally diluted in 40 mL of HNO3 7N. The international standard PM-S (gabbro) was analysed repeatedly as unknown to ensure the validity of our measurements. Results show that PM-S concentrations are consistent with the preferred reference values from GeoREM [START_REF] Jochum | Geostandards and Geoanalytical Research bibliographic review 2007[END_REF]. Differences are less than 0.6% for SiO2 and less than 2% for all other major oxides except for Na2O, K2O and P2O5 which are present in very low concentrations. In addition, sample 24-04 has been analyzed twice as a duplicate and results show no relevant differences in trace elements between the two aliquots. It is noticed that the anomalous composition of one sample (20-03), which is characterized by very low SiO2 (36.92 wt.%), high CaO (15.33 wt.%) and high LOI (9.33 wt.%), is explained by the amount of calcite veins and is not used in the discussion. Supplementary material S5: (a) (La/Sm)N vs. (Sm/Yb)N of Alpine basalts compared to present-day MORBs [START_REF] Gale | The mean composition of ocean ridge basalts: MEAN MORB[END_REF]. Sample position reflects their shape of REE patterns, which depends on melting processes and/or the source composition. Most of present-day MORBs (N-MORBs) are explained by moderate to high degrees of partial melting [START_REF] Gale | The mean composition of ocean ridge basalts: MEAN MORB[END_REF] of a depleted mantle source in the spinel stability field [START_REF] Workman | Major and trace element composition of the depleted MORB mantle (DMM)[END_REF] as represented by the small graph in the lower left corner. In contrast, most Alpine basalts (including basalts from the Platta and Tasna nappes) are characterized by high (Sm/Yb)N, suggesting the involvement of garnet in the mantle source. Samples that plot in the upper right part of the graph (i.e. E-MORB) can be related to low degrees of melting of a garnet depleted mantle source although they more likely derive from the melting of an heterogenous, enriched source. N-star: N-MORB average ± 2σ (grey area); E-star: E-MORB average [START_REF] Gale | The mean composition of ocean ridge basalts: MEAN MORB[END_REF] [START_REF] Johnson | Experimental determination of partition coefficients for rare earth and high-fieldstrength elements between clinopyroxene, garnet, and basaltic melt at high pressures[END_REF], starting mineralogy and modes are from [START_REF] Johnson | Melting in the oceanic upper mantle: an ion microprobe study of diopsides in abyssal peridotites[END_REF].