Jean-François Pastre
email: jean-francois.pastre@lgp.cnrs.fr

Évelyne Debard
email: evelyne.debard@free.fr

Catherine Ferrier
email: catherine.ferrier@u-bordeaux.fr

Michel Fialin
email: michel.fialin@upmc.fr

Bernard Gély
email: bernard.gely@culture.gouv.fr

Bertrand Kervazo
email: bertrand.kervazo@orange.fr

Frédéric Maksud
email: frederic.maksud@culture.gouv.fr

Fatima Mokadem
email: fatima.mokadem@lgp.cnrs.fr

Sébastien Nomade
email: sebastien.nomade@lsce.ipsl.fr

Nicolas Rividi
email: nicolas.rividi@upmc.fr

Ségolène Saulnier-Copard
email: segolene.saulniercopard@lgp.cnrs.fr

A cryptotephra from the Upper Pleistocene volcanism of the Bas-Vivarais in the sedimentary infilling of the Chauvet-Pont d'Arc

Keywords: Chauvet-Pont d'Arc cave, Cryptotephra, Phreatomagmatism, Bas-Vivarais volcanism, Upper Pleistocene

Introduction

The Chauvet-Pont d'Arc cave is situated in France in the Ardèche valley near 4' 25' 07" E, Figure 1A,B,D). Discovered in 1994, it is universally known for the richness and the age of its Paleolithic paintings [START_REF] Clottes | La Grotte Chauvet[END_REF][START_REF] Clottes | La Grotte Chauvet[END_REF]Geneste, 2007, Delannoy andGeneste, 2020). The 14 C datings of charcoal and paintings indicate two periods of occupation attributed to the Aurignacian and to the Gravettian (from 37.5 to 33.5 ka cal BP and from 31 to 28 ka cal BP; [START_REF] Clottes | Les peintures préhistoriques de la Grotte Chauvet-Pont d'Arc (Ardèche, France) : datations directes et indirectes par la méthode du radiocarbone[END_REF][START_REF] Cuzange | Radiocarbon intercomparaison program for Chauvet Cave[END_REF][START_REF] Quiles | A high-precision chronological model for the decorated Upper Paleolithic cave of Chauvet-Pont d'Arc, Ardèche, France[END_REF][START_REF] Valladas | Bilan des datations carbone 14 effectuées sur des charbons de bois de la grotte Chauvet[END_REF]. These dates coincide with one of the activity phases of the Bas-Vivarais volcanism. Several recent 40 Ar/ 39 Ar datings have actually dated these eruptions between 29 ± 10 ka and 35 ± 8 ka (95% probability uncertainty [START_REF] Nomade | A 36,000-Year-Old Volcanic Eruption Depicted in the Chauvet-Pont d'Arc Cave (Ardèche, France) ?[END_REF][START_REF] Sasco | Développement d'un outil chronostratigraphique pour les archives climatiques : datations absolues (K/Ar, 40 Ar/ 39 Ar) et paléomagnétisme appliqués aux laves[END_REF][START_REF] Sasco | 40 Ar / 39 Ar and unspiked 40 K-40 Ar dating of upper Pleistocene volcanic activity in the Bas-Vivarais (Ardèche, France)[END_REF]. These dates fully cover the 14 C AMS datings [START_REF] Quiles | A high-precision chronological model for the decorated Upper Paleolithic cave of Chauvet-Pont d'Arc, Ardèche, France[END_REF] and the thermoluminescence (TL) datings [START_REF] Guibert | When were the walls of the Chauvet Cave heated? A chronological approach by thermoluminescence[END_REF] which correspond to the human settlements responsible for numerous parietal representations in the cave. Among them, spray-shape signs (Figure 1C) were hypothetically ascribed to depiction of strombolian volcanic eruptions [START_REF] Nomade | A 36,000-Year-Old Volcanic Eruption Depicted in the Chauvet-Pont d'Arc Cave (Ardèche, France) ?[END_REF].

In the context of the pluridisciplinary study of the cave financed by the French Culture Ministry the analysis of the heavy minerals of the sediments was undertaken. This study led to the discovery of volcanic minerals attributed to a cryptotephra subcontemporaneous with the sedimentary infilling of the entrance area of the cave. These results, presented here, corroborate the contemporaneity of the recent volcanism of the Bas-Vivarais with the history of the cave during a period close to its occupation by Aurignacians.

Stratigraphical and sedimentological context

The volcanic minerals were identified in the sediments of a limited archeological pit located at about twenty meters from the old entrances which are now filled by deposits related to several processes (Figure 2). Four phases were distinguished in the clogging of the entrance [START_REF] Debard | Grotte Chauvet-Pont d'Arc (Ardèche) : évolution morphosédimentaire de l'entrée. Implication sur les occupations et sur la conservation des vestiges[END_REF]:

-The first one constitutes the origin of a gravity scree (= lower scree) mainly due to the gelifraction of the porch and entry area, to the mechanical expansion of the vaults as well as to the reworking of old superficial deposits. The spreading of the deposits on about forty meters up to the entrance of the Chamber of the Bear Hollows ("Salle des Bauges") is attributed to mobilization by creep and solifluction.

-The second one also corresponds to a gravity scree (intermediate scree) less expanded than the lower one. Their very similar components suggest a same origin.

-The third phase corresponds to blocks generated by the collapse of the cliff. This collapse mainly occurred in two main phases determined by 36 Cl cosmogenic dating (23 500 ± 1 200 and 21 500 ± 1 000 years - [START_REF] Sadier | Further constraints on the Chauvet cave artwork elaboration[END_REF] and gave rise to the definitive cave closure. This set was later covered by fine brown sediments originating from reworking of superficial deposits.

-After the closing of the porch, the ultimate registered events correspond to the desquamation of the equilibrium vaults resulting in small sedimentary clasts, to water runoff, to recent concretions growth, to the settling down of the scree.

The archeological pit (named GE1) carried out between 2006 and 2010 [START_REF] Gély | Sondage GE-1 dans la galerie d'entrée -Travaux 2009 et synthèse des résultats 2006-2009[END_REF][START_REF] Gély | Rapport triennal 2010-2012[END_REF] is located in a small gallery connecting the old porch to the Chamber of the Bear Hollows ("Salle des Bauges") and corresponding to one of the natural paths used by large vertebrates and prehistoric humans (Figure 2). Of small dimensions (50 x 50 cm), 80 cm deep, the pit intersected the lower scree and, near the surface, a gravel of clasts issued from the desquamation of the vault.

Six beds have been defined (Figure 3a, b):

-Bed 1: near the surface, this layer is mainly composed of small plates of desquamation (c.1a). Underneath is found dry gravel of elements righted in counterslope (c.1b).

Among the clasts of the first level was a fistulous concretion which was dated by U/Th to 13,315 ± 79 years [START_REF] Genty | Datations et analyses isotopiques des stalagmites de la grotte Chauvet du Secteur Rouge[END_REF]. This dating as well as the one obtained on the stalagmite Chau-stm 8 which seals the scree near the archaeological pit and whose basis is dated at 11,500 ± 170 years [START_REF] Genty | Apport des stalagmites pour l'étude de la grotte Chauvet: datations absolues U/Th (TIMS) et reconstitution paléoclimatique par les isotopes stables de la calcite[END_REF] allow to date this bed between the Bølling-Allerød warming and the beginning of the Holocene (Lateglacial). Level c.1b supplied a flint attributed to the Upper Palaeolithic, a trace of the last human incursion before the porch was finally clogged up and attributed to the Gravettian (31,000-28,000 cal BP; [START_REF] Quiles | A high-precision chronological model for the decorated Upper Paleolithic cave of Chauvet-Pont d'Arc, Ardèche, France[END_REF].

-Bed 2: this bed is a small gravel secondarily packed in a sandy-silty reddish matrix. Some clasts show a soft surface identified as bears' polish. Bed 2b corresponds to the reworking of the uppermost part of bed 3.

-Bed 3: this bed is about 10 cm thick, it is less gravely, with a sandy and micaceous matrix very rich in charcoals responsible of its dark color. This layer contrasts with the reddish color of the remaining part of the sequence. It corresponds to the reworking of hearths probably at the origin of the significant thermal impacts observed close to the archeological pit on the ceiling and the walls [START_REF] Ferrier | Les parois chauffées de la grotte Chauvet-Pont d'Arc (Ardèche, France) : caractérisation et chronologie[END_REF]. Rubefied clasts or with blackish marks have been found in this bed. Dating of several charcoals (i.e. ranging from 36,000-37,000 cal BP; [START_REF] Quiles | A high-precision chronological model for the decorated Upper Paleolithic cave of Chauvet-Pont d'Arc, Ardèche, France[END_REF] attributes these hearths to the Aurignacian occupancy.

-Bed 4: this bed is divided in two, the upper part corresponding to a small dense gravel with angular granules due to run-off, while its lower part is coarser. The matrix is sandysilty reddish brown becoming more yellow towards the bottom.

-Bed 5: this bed is made of an angular gravel packed in a red sandy sediment, more silty towards the base. At its base, a charcoal dated from between 36,361 and 34,797 cal. BP [START_REF] Quiles | A high-precision chronological model for the decorated Upper Paleolithic cave of Chauvet-Pont d'Arc, Ardèche, France[END_REF] is contemporaneous to the first Aurignacian occupation of the cave.

-Bed 6: also, coarse, contains wall elements (one block and clasts) polished by bears.

The matrix is more sandy, brownish.

-In beds 2 to 5, the presence of sorted lamellar microstructures and occasionally layered caps at the surface of the clasts shows the role of cold weather (mainly frost periods) in this part of the scree and its spreading by solifluction towards the Chamber of the Bear Hollows.

The volcanic minerals characterizing the Chauvet cryptotephra were identified in the beds 6 to 1b, indicating a reworking of the initial tephra fall from the outside.

Material and methods

The mineralogical study was carried out on the 63µm-2mm sand fraction separated by wet sieving in an aqueous solution of sodium hexametaphosphate (dispersant) and removing of carbonates (post-deposition calcite coatings) by HCl 1N/2 hours treatment. Results are based on the study of ten samples distributed in the various beds of the archeological pit. The percentages between the silty-clayey fraction and the clayey fraction characterize clayey-silty sands and clayey-sandy silts. The examination of the light sandy fraction reveals quartz-rich sands (angular or slightly blunt quartz) poor in feldspars, issued from the regional granitic and metamorphic basement. Heavy minerals were separated with bromoform (samples CHAU 1-4 = beds 1b, 2, 3, 4) or with sodium polytungstate (samples CHAU 8-9 = beds 5a and 6, samples ARD 25-28 = beds 3, 4, 5, 6). Thus, they represent the heavy fraction (d > 2.8), including some minerals of the basement and all the volcanic minerals potentially present, except possible plagioclases of basaltic origin, not analyzed. These volcanic minerals either correspond to minerals typical of basaltic magma or to mantellic minerals issued from ultramafic xenoliths (peridotites/lherzolites). The percentages of heavy minerals vary between 0.85 and 3% of the sand fraction. The minerals have been identified with a polarization microscope and a binocular lens. In order to establish their frequency, which is slightly variable due to the grain-size of the samples, several samples were counted on a squared grid. To specify their respective characterizations and origins, the various minerals identified by optical study were then analyzed with an electron microprobe. The handpicked minerals were included in epoxy resin and polished with abrasive papers and diamond powders. They were then analyzed with a WDS Cameca SX 100 microprobe at the Camparis center (Sorbonne University, Paris VI/CNRS) with column conditions 15 keV, 10 nA. These results were then compared to those of the analysis performed on minerals issued from tephras or peridotitic nodules from the Upper Pleistocene volcanism of Bas-Vivarais (Ardèche).

Mineralogy and characterization of the cryptotephra

The results of the heavy minerals counting of six samples are reported in Table 1 andFigure 4. They make it possible to appraise the frequencies of the various species and possibly to link them to the modal composition of the original host-rocks. All the samples are largely dominated by olivine (73.8 to 89.7%). This one is mainly composed of xenomorph grains which can result from the mechanical fragmentation of peridotites nodules or from basaltic magma. Some automorphic specimens clearly originate from the last. The orthopyroxenes (xenomorph enstatite 3.9-7.6%) and the green clinopyroxenes (xenomorph chromiferous diopside, 2.5-6.6%) then represent minerals characteristic of peridotites xenoliths (e.g. [START_REF] Berger | Enclaves ultramafiques, mégacristaux et leurs basaltes-hôtes en contexte océanique (Pacifique sud) et continental (Massif central français)[END_REF]. The brown clinopyroxene (diopside) only plays a minor role (0.7-2%). It probably originates from a basaltic magma together with other pyroxenes such as those occurring in the alkali basalts from the Massif Central. The brown amphiboles, also little abundant (0.2-2%) are probably peridotites amphiboles as those described by [START_REF] Berger | Enclaves ultramafiques, mégacristaux et leurs basaltes-hôtes en contexte océanique (Pacifique sud) et continental (Massif central français)[END_REF].

These minerals are attributable to a basaltic volcanism rich in peridotites nodules (lherzolites) as well as lower crustal material which can only correspond to the Upper Pleistocene volcanism of the Bas-Vivarais located about 35 km away (e.g. [START_REF] Berger | Le volcanisme récent de l'Ardèche[END_REF][START_REF] Berger | Enclaves ultramafiques, mégacristaux et leurs basaltes-hôtes en contexte océanique (Pacifique sud) et continental (Massif central français)[END_REF][START_REF] Sasco | 40 Ar / 39 Ar and unspiked 40 K-40 Ar dating of upper Pleistocene volcanic activity in the Bas-Vivarais (Ardèche, France)[END_REF].

They come from a tephra brought by the wind at the probable origin of most of the other drifts (basement silts and sands) and scattered in the cave environment. The homogeneity of the minerals assemblage and the freshness of the minerals (e.g. automorphic olivine) lead one to think that this slightly reworked tephra is subcontemporaneous with the sediments including the latter. Thus, it can be defined as a cryptotephra. This fall probably results from a phreatomagmatic eruption associated with formation of a maar. The explosive products (basesurges and falls) are actually usually rich in peridotites minerals (see discussion). The origin of the fall from a strombolian eruption is much less probable. In fact, this type of eruption effectively produces basaltic ashes and scoriae which are not present in the Chauvet samples and are also generally poor in minerals.

The other heavy silicates are basement minerals which play only a minor role (Table 1). These include garnets (0.7 to 5.9%), andalusite, sillimanite-fibrolite, tourmaline and rutile. They could be alluvial minerals from the Ardèche river, coming from the Massif Central and reworked from old fluvial terraces by run-off or gravity process. But they were more probably brought by the wind. Some could also come from the cryptotephra, the explosive products of the maars being mainly rich in basement minerals. The opaque minerals, little represented (2.5 to 3.6%), correspond either to Fe-Ti oxides issued from the regional basaltic volcanism (magnetite, titanomagnetite), or to silicates and oxides of unknown origin (work in progress), or to rare chromium spinels (about 23% of Cr2O3) probably issued from peridotites.

Microprobe geochemical study of minerals

In order to precisely characterize the mineralogy and chemical composition of the minerals, a mineralogical study using an electron microprobe was undertaken and compared with the available analyses [START_REF] Berger | Enclaves ultramafiques, mégacristaux et leurs basaltes-hôtes en contexte océanique (Pacifique sud) et continental (Massif central français)[END_REF] and new analyses of proximal volcanic falls (base-surges) as well as peridotites from the Ardèche volcanism. Various characteristic analyses are reproduced on Table 2.

Olivines are mainly characterized by the variation of their MgO contents. The analysis of automorphic grains reveals compositions comprised between 43.67 and 46.30% of MgO (Fo = 83.1-86.6%). Such compositions, poor in Mg, are typical of basaltic phenocrystals as confirmed by their automorphic habitus. After all the cross-checking we have done, it seems that the limit of composition between basaltic and peridotite olivines is at about 47% MgO. The lowering of magnesium in these basaltic olivines (e.g. Table 2, 2, OL3-4, Figure 3). However, these olivines are slightly less magnesian than the olivine from peridotites of the Vestide du Pal and Chambon maars analysed by [START_REF] Berger | Enclaves ultramafiques, mégacristaux et leurs basaltes-hôtes en contexte océanique (Pacifique sud) et continental (Massif central français)[END_REF] which mostly exceed 49% of MgO (48.71-49.82%); but based on six analyses performed on the phreatomagmatic tuff of Vestide. We have noticed that they were less magnesian and compatible with the compositions of the Chauvet cryptotephra . For comparison, the olivines from the Ray-Pic maar peridotites have compositions varying between 47.89 and 49.50% MgO. The olivines of micro-peridotite enclaves found within the strombolian tuff of Gravenne de Montpezat,were also analyzed. They are slightly more magnesian and mainly exceed 49% MgO. Although small, these variations seem to indicate significant slight differences of composition for the olivines of peridotites xenoliths from the Ardèche volcanism.

The orthopyroxenes necessarily come from peridotites of which they are one of the characteristic minerals with MgO contents near 32-33%. These are enstatites En87.8-89.8, Wo0.93-1.27 (Figure 6). Their analytical results (e.g. Table 2, EN1-2) are close to those of [START_REF] Berger | Enclaves ultramafiques, mégacristaux et leurs basaltes-hôtes en contexte océanique (Pacifique sud) et continental (Massif central français)[END_REF] concerning the orthopyroxenes of peridotites from the Vestide du Pal and Chambon maars. Like them, they are slightly chromiferous (Cr2O3 = 0.22-0.34%). The other orthopyroxenes we analyzed (Ray-Pic maar, Borée maar, Gravenne de Montpezat scoria cone, Moula rockshelter tephra) showed relatively similar compositions.

The green clinopyroxene corresponds to the common clinopyroxene of peridotites [START_REF] Berger | Enclaves ultramafiques, mégacristaux et leurs basaltes-hôtes en contexte océanique (Pacifique sud) et continental (Massif central français)[END_REF]. This is chromiferous diopside (calcic clinopyroxene Wo45-48, Figure 5). The results of the ten analyses performed (e.g. Table 2, CD1-2) show slightly less chromium-rich pyroxenes than those analysed by [START_REF] Berger | Enclaves ultramafiques, mégacristaux et leurs basaltes-hôtes en contexte océanique (Pacifique sud) et continental (Massif central français)[END_REF] for peridotites of the Vestide du Pal and Chambon maars: 0.54-0.81% compared to 0.75-1.36%. This difference is not explained. Only one sample differs with 1.34/1.48% Cr2O3.

The brown clinopyroxenes correspond to diopsides Wo44.97-48.30 -En40.01-42.34 -Fs10.68-13.87 (e.g. 6). These are basaltic pyroxenes common in the alkali basalts from the Massif Central and attributed to the basaltic magma pulverized by the phreatomagmatic eruption. They show typical amounts of Al2O3 (8.51-9.46%), TiO2 (2.15-2.53%) and FeOt (6.14-7.86%).

Analysis of some opaque minerals revealed the existence of several minerals of unspecified origin. The first one presents a chemical composition with 27-34.5% SiO2, 5-7% K2O, 15.4-20.3% FeOt, 13.9-17.7% Al2O3, 5.8-8.8% MgO, 3.8-4.5% TiO2. The second one is mainly composed of FeO (67-75% FeOt), but contains low percentages of SiO2, Al2O3, CaO, MnO and P2O5 and cannot be ascribed to a true oxide.

Discussion, origin and possible age of the cryptotephra

As previously seen the mantellic minerals (peridotites minerals = magnesian olivine + orthopyroxene + chromiferous diopside) and the basaltic minerals (olivine + brown diopside) characterizing the cryptotephra of the Chauvet-Pont-d'Arc cave can be ascribed without any doubt to a phreatomagmatic eruption related to a maar of the Upper Pleistocene volcanism of the Bas-Vivarais. They were pulverized and mixed by the phreatomagmatic explosions. The absence of basaltic ash (microscoriae) attributable to a strombolian eruption, the mixing of mantellic and basaltic minerals and the lower explosivity of this type of eruption led us to exclude a strombolian eruption at the origin of the cryptotephra.

The volcanism of the Bas-Vivarais is known since the work of Faujas de Saint-Fond (1778). Its volcanic products are mainly alkaline basalts derived from a unique enriched mantle source [START_REF] Chauvel | Nd-St isotope and REE geochemistry of alkali basalts from the Massif Central, France. Geochim[END_REF][START_REF] Downes | Relationship between geochemistry and textural type in spinel peridotite xenoliths, Massif Central, France[END_REF]. It benefited in particular from the works of Berger (e.g. [START_REF] Berger | Le volcanisme récent de l'Ardèche[END_REF][START_REF] Berger | Enclaves ultramafiques, mégacristaux et leurs basaltes-hôtes en contexte océanique (Pacifique sud) et continental (Massif central français)[END_REF][START_REF] Berger | Les jeunes volcans d'Ardèche[END_REF] and is distinguished by the frequency of its ultramafic xenoliths [START_REF] Berger | Enclaves ultramafiques, mégacristaux et leurs basaltes-hôtes en contexte océanique (Pacifique sud) et continental (Massif central français)[END_REF][START_REF] Berger | Réactions des basaltes alcalins des Causses, du Vivarais et du Velay (France) avec les xénocristaux et les minéraux des enclaves homéogènes[END_REF]. It comprises 17 eruptive centers. Its activity is characterized by phreatomagmatic eruptions (maars) preceded or generally followed by a strombolian activity associated with the emission of basaltic flows filling pre-existing valleys.

The karst of the Bas-Vivarais, situated to the southeast, is characterized by the importance of its Upper Pleistocene infillings [START_REF] Debard | Le Quaternaire du Bas-Vivarais d'après l'étude des remplissages d'avens, de porches de grottes et d'abris sous-roche. Dynamique sédimentaire, paléoclimatologie et chronologie[END_REF][START_REF] Debard | Les remplissages karstiques du Bas-Vivarais : karstogénèse, sédimentogénèse et archéologie[END_REF]. The signature of volcanic eruptions from the Bas-Vivarais was evidenced in three of these karstic infillings: the Moula rockshelter in Soyons [START_REF] Debard | Nouvelles données sur les téphras pléistocènes piégés dans les remplissages karstiques ardèchois (S-E France)[END_REF][START_REF] Pastre | Un téphra-repère du volcanisme phréatomagmatique du Vivarais dans la séquence Pléistocène supérieur de l'abri Moula (Soyons, Ardèche, France)[END_REF], the Marzal II aven (or Flahault aven) and the Devès du Reynaud aven in Saint-Remèze [START_REF] Debard | Nouvelles données sur les téphras pléistocènes piégés dans les remplissages karstiques ardèchois (S-E France)[END_REF]. Among them, the insitu tephra found in Soyons is the more representative example of a phreatomagmatic fall comparable to that detected in Chauvet. Among the recent maars of the Bas-Vivarais four present an explosive activity and can be at the origin of the cryptotephra we described above: the Ray-Pic, the Vestide du Pal, the Chambon and the Sapède [START_REF] Berger | Enclaves ultramafiques, mégacristaux et leurs basaltes-hôtes en contexte océanique (Pacifique sud) et continental (Massif central français)[END_REF]. This last explosion crater can a priori be excluded, due to its essentially basaltic type [START_REF] Berger | Enclaves ultramafiques, mégacristaux et leurs basaltes-hôtes en contexte océanique (Pacifique sud) et continental (Massif central français)[END_REF] as confirmed by our own observations and analyses. Derived tephras are therefore mainly composed of basalt clasts and their olivines are mainly basaltic in composition with 44.01-46.55% MgO (Fo = 86.3-87%). The Chambon maar emitted a base-surge and lahars visible in the Fau valley above the hamlet. The mineralogical and microchemical study shows that these products both contain peridotite and basaltic minerals similar to Chauvet' ones. The peridotites olivines (48.43-48.97% MgO) are however more abundant than the basaltic ones in the sample we analyzed. This maar cut in the basement could have been large enough to disperse widespreadly its products, but, due to little extended dispersal in the proximal area, it was probably less extended than those of the following maars. Among the following eruptions, the Vestide du Pal maar constitutes one of the largest explosive craters in the Massif Central (diameter = 1700 m, [START_REF] Berger | Le volcanisme récent de l'Ardèche[END_REF][START_REF] Berger | Enclaves ultramafiques, mégacristaux et leurs basaltes-hôtes en contexte océanique (Pacifique sud) et continental (Massif central français)[END_REF]. It emitted important surges visible in its eastern and southern parts (Suc du Pal). The mineralogical study of these products rich in basement minerals pulverized by explosions, show that they content both peridotite and basaltic minerals.

These ones however comprise peridotite olivines (47.05-50% MgO) and peridotite pyroxenes.

The basaltic olivines (44.57-45.37% MgO) are rare. However, this composition makes them compatible with the composition of Chauvet cryptotephra ones. The Ray-Pic maar also emitted large surges visible along the road D 215 to the east of the Ray-Pic cascade. The mineralogical study of these tephras also rich in basement minerals shows that they contain basaltic and peridotite minerals. The olivine shows a composition varying between 44.63 and 49.78% MgO (Fo = 85.2-90.9%) which clearly argues for these two origins and compositions close to those of Chauvet.

The cryptotephra-bearing beds are well dated due to the presence of micro-charcoals ascribed to frequenting of the site by Aurignacians. 14 C AMS datings gave ages comprised between 36, 361-34,797 (bed c5) and 36,100-34,880 (bed c2a) years cal. BP [START_REF] Quiles | A high-precision chronological model for the decorated Upper Paleolithic cave of Chauvet-Pont d'Arc, Ardèche, France[END_REF]. Thus, the cryptotephra must have an age greater than or close to 36 ka. The Bas-Vivarais volcanism benefited from 14 C datings [START_REF] Berger | Le volcanisme récent de l'Ardèche[END_REF], Berger et al., 1975[START_REF] Berger | Les jeunes volcans d'Ardèche[END_REF], from thermoluminescence datings [START_REF] Guérin | La thermoluminescence des plagioclases. Méthode de datation du volcanisme, applications au domaine volcanique français : Chaîne des Puys[END_REF][START_REF] Guérin | Nouveaux éléments de chronologie du volcanisme pléistocène du Bas-Vivarais, (Ardèche, France) par thermoluminescence[END_REF][START_REF] Sanzelle | Datation par thermoluminescence : étude d'une corrélation possible entre le maar de la Vestide-du-Pal et un niveau de téphra de la Baume-Moula-Guercy (Ardèche, France)[END_REF], from paleomagnetism measurements [START_REF] Rochette | La province volcanique Pléistocène Supérieur du Bas-Vivarais (Ardèche, France) : propagation de fentes crustales en échelons ?[END_REF][START_REF] Sasco | Développement d'un outil chronostratigraphique pour les archives climatiques : datations absolues (K/Ar, 40 Ar/ 39 Ar) et paléomagnétisme appliqués aux laves[END_REF][START_REF] Sasco | 40 Ar / 39 Ar and unspiked 40 K-40 Ar dating of upper Pleistocene volcanic activity in the Bas-Vivarais (Ardèche, France)[END_REF] and from K-Ar and 40 Ar/ 39 Ar datings [START_REF] Sasco | Développement d'un outil chronostratigraphique pour les archives climatiques : datations absolues (K/Ar, 40 Ar/ 39 Ar) et paléomagnétisme appliqués aux laves[END_REF][START_REF] Sasco | 40 Ar / 39 Ar and unspiked 40 K-40 Ar dating of upper Pleistocene volcanic activity in the Bas-Vivarais (Ardèche, France)[END_REF]. Among them, the 40 Ar/ 39 Ar datings (ibid.) seem to be the most accurate. The 40 Ar/ 39 Ar age of the Vestide du Pal maar would thus be comprised between 25.8 ± 7.7 and 27.2 ± 8.2 ka (ibid.). It would thus belong to the recent phase of the Ardèche volcanism. This age seems too young to fit with the potential age of the Chauvet cryptotephra. Nevertheless, it was performed on a basalt following the phreatomagmatic eruption and not on the tephras coming from the maar explosions. It is also the same for the age of the Chambon maar dated at 18.8 ± 9.6 ka (ibid.). On the other hand the age of the Ray-Pic maar comprised between 36.2 ± 11.3 and 32.2 ± 11.1 ka (ibid) is well compatible with the 14 C datings of the beds containing the minerals of the Chauvet cave. This maar belonging to the middle phase of the Bas-Vivarais volcanism could be at the origin of the cryptotephra we discovered. Nevertheless, its products do not contain the peculiar minerals which are currently studied to define their origin (terrestrial or extraterrestrial).

The correlation with the other tephras identified in the karstic infillings of the region can be examined. The tephra of the Devès du Reynaud aven [START_REF] Debard | Nouvelles données sur les téphras pléistocènes piégés dans les remplissages karstiques ardèchois (S-E France)[END_REF] is characterized by the presence of basaltic glass and its richness in biotite to which a peculiar provenance can be attributed. This composition is clearly different from the Chauvet cryptotephra. In the Marzal II (or Flahault) the basaltic and mantellic minerals reworked in set 5 [START_REF] Debard | Nouvelles données sur les téphras pléistocènes piégés dans les remplissages karstiques ardèchois (S-E France)[END_REF] can also be attributable to a phreatomagmatic emission of the Upper Pleistocene volcanism of Ardèche. The data obtained by biochronology suggest that this fall could belong to the recent or middle phase of Ardèche volcanism. The minerals of this infilling could originate from the same fall as the minerals of the Chauvet cave's cryptotephra or originate from another eruption like those of Vestide du Pal or Chambon. The tephra identified in the Moula rockshelter corresponds to a beautiful in situ phreatomagmatic fall (Debard andPastre, 2008, Pastre et al., 1994). Rich in basement minerals, it contains both peridotite and basaltic minerals as illustrated by the composition of its olivines varying between 43.97 and 48.87% MgO. Its initial dating around 40 ka [START_REF] Pastre | Un téphra-repère du volcanisme phréatomagmatique du Vivarais dans la séquence Pléistocène supérieur de l'abri Moula (Soyons, Ardèche, France)[END_REF] was contested according to a TL dating of 72 ± 12 ka [START_REF] Sanzelle | Datation par thermoluminescence : étude d'une corrélation possible entre le maar de la Vestide-du-Pal et un niveau de téphra de la Baume-Moula-Guercy (Ardèche, France)[END_REF]. It is indeed worth noticing that this older age is hardly reconcilable with the sedimentological and paleoclimatic data suggesting a periglacial context rather than a temperate one such as the Saint-Germain 2 episode [START_REF] Debard | Nouvelles données sur les téphras pléistocènes piégés dans les remplissages karstiques ardèchois (S-E France)[END_REF]. In the hypothesis of an age close to 40 ka, its correlation with the phreatomagmatic eruption of the Ray-Pic maar is possible. This hypothesis is in any case compatible with the composition of the products identified in these two entities.

Conclusion

The cryptotephra from the Chauvet-Pont d'Arc cave provides a new example of tephra-fall from the Upper Pleistocene volcanism of the Bas-Vivarais, trapped in the karst of this area. Its possible attribution to the phreatomagmatic eruption of the Ray-Pic maar could be supported by the mineralogical and geochemical compositions of its products containing mantellic and basaltic minerals. It is also in agreement with the recent 40 Ar/ 39 Ar datings of the Ardèche volcanism [START_REF] Sasco | Développement d'un outil chronostratigraphique pour les archives climatiques : datations absolues (K/Ar, 40 Ar/ 39 Ar) et paléomagnétisme appliqués aux laves[END_REF][START_REF] Sasco | 40 Ar / 39 Ar and unspiked 40 K-40 Ar dating of upper Pleistocene volcanic activity in the Bas-Vivarais (Ardèche, France)[END_REF], giving an age subcontemporaneous with the beds containing the charcoals well dated by 14 C [START_REF] Quiles | A high-precision chronological model for the decorated Upper Paleolithic cave of Chauvet-Pont d'Arc, Ardèche, France[END_REF] and contemporaneous with the Aurignacian occupation and art, that is slightly older than or close to 36 ka. In the other hand, the cryptotephra could also originate from the Vestide du Pal maar, as it also contains peculiar minerals whose origin is still unknown and which were also found in sedimentary infilling of the Chauvet cave. The discovery of this slightly reworked tephra in this remarkable Paleolithic cave provides an additional evidence of the contemporaneity of the Ardèche volcanism with the Aurignacian occupation of this region. Our findings are however not the bulletproof that will legitimate the hypothesis according which the spray-shape signs of the cave can be ascribed to the first depiction of a volcanic eruption [START_REF] Nomade | A 36,000-Year-Old Volcanic Eruption Depicted in the Chauvet-Pont d'Arc Cave (Ardèche, France) ?[END_REF], but they surely reinforced the hypothesis that Aurignacians have been the witnesses of volcanic activity in the Ardèche region and that volcanic derived material did fall in the vicinity of Chauvet-Pont d'Arc cave about 36 ka ago. team. Warm thanks are addressed to Jane Reilly for her help in the first English translation of this paper. We are grateful to Hélène Paquet for her review of this paper.

Figures and tables captions

  OL1-2) results in their iron enrichment (FeOt = 12.65-15.39% for the most ones) as shown by the diagram of the figure 5. Except for visibly broken specimens from basaltic origin with MgO contents < 47%, the other xenomorphic olivines are more magnesian (MgO = 47.40-48.96%) and characterize olivines from peridotites (e.g. Table

Fig. 1 .

 1 Fig. 1. A) Location of the Chauvet cave in France, B) Location of the Chauvet cave in Ardèche area, C) Example of spray-shape sign, D) Perspective view of the Bas-Vivarais volcanism seen from Chauvet cave.

Fig. 2 .

 2 Fig. 2. Location of the archeological pit GE1 located about twenty meters from the Palaeolithic entrances. Équipe Grotte Chauvet (Ministère de la Culture, CNRS, Universités).

Fig. 3a .

 3a Fig. 3a. Archaeological pit GE1: view of the southern and western sections and dating elements. Picture by C. Ferrier (Equipe Grotte Chauvet -Ministère de la Culture, CNRS, Universités).

Fig. 3b .

 3b Fig. 3b. Log of the archeological pit GE1.

Fig. 4 .

 4 Fig. 4. Heavy minerals composition of the beds containing the cryptotephra.

Fig. 5 .

 5 Fig. 5. MgO/total FeO diagram of the olivines of Chauvet cryptotephra.

Fig. 6 .

 6 Fig. 6. Wo-En-Fs diagram of the pyroxenes of Chauvet cryptotephra.

  Fig. 3b

Fig. 4

 4 Fig.4

Fig. 5

 5 Fig. 5

  Fig. 6

Table 2

 2  Figure

Acknowledgments

We thank the French Culture Ministry and the ARPA (Association de Recherche Paléoécologique en Archéologie) for their financial support. We are especially grateful to Jean-Michel Geneste, former director of the scientific team of the Chauvet cave who as agreed to fund some of the analyzes and to Carole Fritz (CNRS), actual research manager of the Chauvet


Yousf Islem Bourezg

Hiba Azzeddine

Thierry Baudin

Anne-Laure Helbert

Yi Huang

Djamel Bradai

Terence G Langdon

Texture and microhardness of Mg-Rare Earth (Nd and Ce) alloys processed by high-pressure torsion

Keywords: Mg-Rare Earth alloy, High-Pressure Torsion, Microhardness, Twinning, Second phase, Texture

The influence of high-pressure torsion (HPT) processing on the texture and microhardness of two binary Mg-RE (RE=Nd and Ce) alloys was investigated using X-ray diffraction and Vickers microhardness measurements. Disks cut from the alloys were processed by HPT at room temperature for up to 10 turns. The precipitation products of both alloys were identified using synchrotron radiation. The results show that both alloys exhibit a weak basal texture where the c-axis of most grains is shifted 15° from the shear direction. An Mg-1.44Ce (wt. %) alloy showed a continuous decrease in the texture strength which may be due to the effect of second precipitation phases (Mg 17 Ce 2 and MgCe 2 ). The microhardness of both alloys increased significantly with increasing HPT turns but levelled-off beyond about one HPT turn. Maximum values of ~65 and ~96 Hv were achieved which are significantly higher than the hardness of the undeformed Mg-Ce and Mg-Nd alloys.

Introduction

Despite their favorable thermal stability and strength, magnesium alloys have a disadvantage because of their poor room temperature formability and ductility which is caused by the basal texture in the microstructure and basal slip during deformation [START_REF] Sandlobes | On the role of non-basal deformation mechanisms for the ductility of Mg and Mg-Y alloys[END_REF]. This disadvantage may be limited or even avoided through microstructural changes by the addition of alloying elements, through appropriate mechanical processing or combinations of both. A review of the literature indicates that additions of small amounts of rare earth (RE) elements, such as Nd, Ce, Y, Gd and La, provide the potential for improving the room temperature formability, modifying the recrystallization kinetics and the plastic deformation behavior as well as influencing the texture evolution of Mg-based alloys [START_REF] Yi | Improvement of magnesium sheet formability by alloying addition of rare earth elements[END_REF][START_REF] Hantzsche | Effect of rare earth additions on microstructure and texture development of magnesium alloy sheets[END_REF][START_REF] Mishra | Influence of cerium on the texture and ductility of magnesium extrusions[END_REF][START_REF] Kainer | Development of the Microstructure and Texture of RE Containing Magnesium Alloys during Hot Rolling[END_REF][START_REF] Cottam | Materials Processing and Texture[END_REF][START_REF] Stanford | The effect of Gd on the recrystallisation, texture and deformation behaviour of magnesium-based alloys[END_REF][START_REF] Liu | The Theory Foundation of Magnesium-Based Lightweight Alloys and their Applications[END_REF][START_REF] Stanford | The Origin of "Rare Earth" Texture Development in Extruded Mg-Based Alloy and Its Effect on Tensile Ductility[END_REF][START_REF] Stanford | Effect of composition on the texture and deformation behaviour of wrought Mg alloys[END_REF].

It has been reported that, contrary to conventional Mg-based alloys with non-RE elements, either small (Mg-X (X=Nd, Ce, Y, La)) or heavy additions of RE (WE alloys) can develop more random-type texture during deformation processing [START_REF] Stanford | Effect of composition on the texture and deformation behaviour of wrought Mg alloys[END_REF][START_REF] Ball | Tensile-compressive yield asymmetries in high strength wrought magnesium alloys[END_REF][START_REF] Bohlen | The texture and anisotropy of magnesium-zinc-rare earth alloy sheets[END_REF][START_REF] Mackenzie | The deformation, recrystallisation and texture of three magnesium alloy extrusions[END_REF]. It was suggested that the modified texture in Mg-RE alloys was due to particle-stimulated nucleation of recrystallization (PSN) following deformation [START_REF] Ball | Tensile-compressive yield asymmetries in high strength wrought magnesium alloys[END_REF]. Nerveless, PSN is not the single mechanism responsible for RE-texture modification, especially in the case of Mg-based alloys containing RE micro-alloying with a very small amount of second phase. The promotion of non-basal slip such as <a> prismatic and/or <c + a> pyramidal slip via a solute strengthening mechanism and the suppression of grain boundary mobility through solute drag were proposed as solid solution-based mechanisms responsible for the texture weakening in Mg-RE alloys [START_REF] Mackenzie | The deformation, recrystallisation and texture of three magnesium alloy extrusions[END_REF][START_REF] Cottam | Dynamic recrystallization of Mg and Mg-Y alloys. Crystallographic texture development[END_REF][START_REF] Hadorn | Role of solute in the texture modification during hot deformation of Mg-rare earth alloys[END_REF].

The mechanical processing of metals through the application of intense deformation provides the potential for achieving significant grain refinement [START_REF] Valiev | Bulk nanostructured materials from severe plastic deformation[END_REF]. Specifically, severe plastic deformation (SPD) techniques, such as equal-channel angular pressing (ECAP) [START_REF] Valiev | Principles of equal-channel angular pressing as a processing tool for grain refinement[END_REF], accumulative roll bonding (ARB) [START_REF] Saito | Ultra-fine grained bulk aluminum produced by accumulative roll-bonding (ARB) process[END_REF] and high-pressure torsion (HPT) [START_REF] Smirnova | Evolution of structure of fcc single crystals subjected to large plastic deformation[END_REF][START_REF] Zhilyaev | Using high-pressure torsion for metal processing: Fundamentals and applications[END_REF], have the capacity to produce ultrafine-grained (UFG) microstructures [START_REF] Khereddine | Microstructures and Textures of a Cu-Ni-Si Alloy Processed by High-Pressure Torsion[END_REF][START_REF] Tirsatine | Texture and microstructure evolution of Fe-Ni alloy after Accumulative Roll Bonding[END_REF][START_REF] Hadj Larbi | Microstructure and texture evolution in a Cu-Ni-Si alloy processed by equal-channel angular pressing[END_REF] and also significantly affect the size/distribution of precipitates within the crystalline matrix [START_REF] Gubicza | The effect of severe plastic deformation on precipitation in supersaturated Al-Zn-Mg alloys[END_REF].

It is well known that processing of metals through the application of intense deformation provides a potential for achieving important additional microstructural modifications such as very high dislocation densities and high densities of point defects [START_REF] Valiev | Bulk nanostructured materials from severe plastic deformation[END_REF].

To date, many available scientific reports demonstrate the successful achievement of grain refinement down to the sub-micrometer level and significant improvements of the mechanical properties in various Mg-based alloys processed by SPD techniques [START_REF] Bai | Microstructural Evolution of Mg-4Nd Alloy Processed by High-Pressure Torsion[END_REF][START_REF] Estrin | Microstructure, texture and mechanical properties of the magnesium alloy AZ31 processed by ECAP[END_REF][START_REF] Horita | A two-step processing route for achieving a superplastic forming capability in dilute magnesium alloys[END_REF][START_REF] Matsubara | Developing superplasticity in a magnesium alloy through a combination of extrusion and ECAP[END_REF].

Frequently, SPD processing (such as by ECAP) of Mg-based alloys were performed at a relatively high temperature (200-300 °C) [START_REF] Mukai | Ductility enhancement in AZ31 magnesium alloy by controlling its grain structure[END_REF]. Consequently, the grain refinement was rather limited due to the occurrence of recovery and even dynamic recrystallization during processing. Surprisingly, the hydrostatic stresses developed during HPT processing prevented cracking and allowed the processing of magnesium alloys even at room temperature [START_REF] Figueiredo | Processing magnesium alloys by severe plastic deformation[END_REF].

Moreover, it has been confirmed that HPT processing gave more significant grain refinement compared to ECAP processing of the same AZ31 alloy and at a similar temperature (200 °C) [START_REF] Serre | Influence of strain rate on the characteristics of a magnesium alloy processed by high-pressure torsion[END_REF][START_REF] Figueiredo | Grain refinement and mechanical behavior of a magnesium alloy processed by ECAP[END_REF].

The crystallographic texture of Mg-based alloys such as pure Mg, AZ31and ZK60A alloys after HPT processing was characterized by a basal texture where the c-axis rotates from the disc plane towards the torsion axis [START_REF] Kocich | Texture, deformation twinning and hardening in a newly developed Mg-Dy-Al-Zn-Zr alloy processed with high pressure torsion[END_REF][START_REF] Qiao | Hardening mechanism of commercially pure Mg processed by high pressure torsion at room temperature[END_REF][START_REF] Lee | Evolution in hardness and texture of a ZK60A magnesium alloy processed by high-pressure torsion[END_REF][START_REF] Huang | Microstructure and Texture Evolution in a Magnesium Alloy During Processing by High-Pressure Torsion[END_REF]. The deviation of the c-axis from the torsion axis depends strongly on the HPT processing temperature [START_REF] Huang | Microstructure and Texture Evolution in a Magnesium Alloy During Processing by High-Pressure Torsion[END_REF]. Detailed texture analysis revealed a significant difference in the evolution of texture between the overall texture taken throughout the disk and the local texture at the disk edges up to 5 HPT turns [START_REF] Qiao | Hardening mechanism of commercially pure Mg processed by high pressure torsion at room temperature[END_REF][START_REF] Lee | Evolution in hardness and texture of a ZK60A magnesium alloy processed by high-pressure torsion[END_REF].

The microstructures and thermal stability of an Mg-Gd alloy processed by HPT were investigated using positron annihilation spectroscopy (PAS) and transmission electron microscopy (TEM) [START_REF] Čížek | Microstructure and thermal stability of ultra-fine grained Mg and Mg-Gd alloys prepared by high-pressure torsion[END_REF][START_REF] Čížek | Microstructure development and ductility of ultra-fine grained Mg-Gd alloy prepared by high pressure torsion[END_REF]. The results showed that the Mg-Gd alloy exhibited a homogeneous UFG microstructure with a high density of uniformly distributed dislocations and a grain size of about 100 nm with a concomitant significant rise of hardness [START_REF] Čížek | Microstructure and thermal stability of ultra-fine grained Mg and Mg-Gd alloys prepared by high-pressure torsion[END_REF]. An appreciable thermal stability up to 300°C was also reported [START_REF] Čížek | Microstructure development and ductility of ultra-fine grained Mg-Gd alloy prepared by high pressure torsion[END_REF].

It was also demonstrated that HPT processing strongly affects the precipitation kinetics in Mg-RE alloys [START_REF] Čížek | Precipitation effects in ultra-fine-grained Mg-RE alloys[END_REF]. The occurrence of precipitation was shifted to a lower temperature in the HPT-deformed sample and this shift was associated with a high fraction of grain boundaries that act as potential nucleation site of a second phase and the enhanced difficulty of the solute elements to diffuse along grain boundaries [START_REF] Čížek | Precipitation effects in ultra-fine-grained Mg-RE alloys[END_REF].

To the present time, little information is available on the crystallographic texture and mechanical properties of Mg-RE alloys subjected to HPT processing [START_REF] Kocich | Texture, deformation twinning and hardening in a newly developed Mg-Dy-Al-Zn-Zr alloy processed with high pressure torsion[END_REF] and this contrasts with the extensive published data about Mg-based alloys subjected to conventional deformation [START_REF] Basu | Shear band-related recrystallization and grain growth in two rolled magnesium-rare earth alloys[END_REF][START_REF] Wu | Grain growth and texture evolution during annealing in an indirect-extruded Mg-1Gd alloy[END_REF][START_REF] Xia | Microstructure and texture of coarse-grained Mg-Gd-Y-Nd-Zr alloy after hot compression[END_REF][START_REF] Elfiad | Investigation of texture, microstructure, and mechanical properties of a magnesium-lanthanum alloy after thermomechanical processing[END_REF][START_REF] Azzeddine | On the texture and grain growth in hot deformed and annealed WE54 alloy[END_REF][START_REF] Azzeddine | Texture and microstructure of WE54 alloy after hot rolling and annealing[END_REF]. Accordingly, the objective of the present research was to investigate the influence of HPT processing at room temperature on the texture and the microhardness of two Mg-RE alloys (Mg-1.43Nd and Mg-1.44Ce (wt. %)) in the presence of precipitates and residual solute elements.

Experimental materials and procedure

The materials used in this investigation were alloys of Mg-1.43Nd and Mg-1.44Ce (wt.%) supplied in an as-cast state by colleagues from the Institutfür Metallkunde und Metallphysik (IMM), Aachen, Germany. Discs with diameter of 10 mm were machined from the as-cast materials and then homogenized in sealed glass tubes at 535 °C for 5 h.

After homogenization, disks with thicknesses of 1.5 mm were sliced from the cylindrical samples and then carefully polished with abrasive papers to final thicknesses of ~0.85 mm. The HPT processing was conducted at room temperature with a rotational speed of 1 rpm up to 1/2, 1, 5 and 10 turns using an imposed pressure of 6.0 GPa. All disks were processed by HPT under quasi-constrained conditions [START_REF] Figueiredo | Using finite element modeling to examine the temperature distribution in quasi-constrained high-pressure torsion[END_REF] where there is a limited out-flow of material around the periphery of the disk during processing.

The textures were determined at the centers and edges of the disks processed by HPT by measuring the incomplete pole figures using a Phillips X-ray texture goniometer. A set of six measured pole figures {10 ̅ 0}, {0002}, {10 ̅ 1}, {10 ̅ 2}, {11 ̅ 0} and {10 ̅ 3} was used to calculate the orientation distribution function using the MTEX toolbox [START_REF] Hielscher | A novel pole figure inversion method: specification of the MTEX algorithm[END_REF]. Figure 1 shows a representation of the axis system to assist the texture representation, where SD, RD and CD refer to the shear, rotation and compression directions, respectively. X-rays diffraction patterns were obtained using the high photon flux synchrotron Xray beam at MaxLab, Lund, Sweden (beamline I711). A wavelength of 0.9938 Å was calibrated using a Si (111) single-crystal monochromator. A large area Titan CCD detector from Agilent with a radius of 165 mm was used in the range 12-60° in 2θ. The volume fraction of precipitates was determined from a quantitative analysis using MAUD (for Material Analysis Using Diffraction, http://www.ing.untin.it/ ~luttero/) software.

The Vickers microhardness was measured on the cross-section in the center and edge of the deformed disk. At least five indentations were recorded to obtain the average hardness values with a load of 100 g (Hv 0.1 ) and dwell time of 10 s.

Experimental results

The microstructures of the solution-annealed Mg-1.43Nd and Mg-1.44Ce alloy are given in Fig. 2(a) and (b), respectively. Inspection shows that the microstructures of both alloys are characterized by elongated large columnar grains with average transverse grain sizes of ~ 400 µm. It was not possible to effectively characterize the initial texture due to the very poor statistics with records of only 22 grain orientations for Mg-1.43Nd and 13 grain orientations for Mg-1.44Ce. Nevertheless, it is reasonable to assume that the texture is random.

The evolution of the texture measured at the center and edge of the HPT processed disks of both alloys are presented in Figs 3 and4 in terms of the basal (0002) recalculated pole figures, respectively. As can be seen, the developed texture at the center and edge of the HPT processed disks of both alloys was globally similar and shows an asymmetric split of the basal planes shifted towards SD.

The intensity distributions of the basal (0002) poles of the processed Mg-1.43Nd and Mg-1.44Ce alloys are shown in Fig. 5 as a function of the polar angular tilt towards SD. It is readily apparent in Figs. 5(a) and 5(b) for Mg-1.43Nd that in the center and edge of the ½ turn HPT processed disks there are two intensity maxima around 15° and 40° shifts towards SD.

However, the Mg.144Ce alloy after ½ HPT turn shows in both the center and edge of the HPT-processed disks a strong basal texture (~8.8 mrd) where the basal (0002) planes are parallel to the shear (SD, RD) plane or the c-axis is parallel to CD. From Figs. 5 (c) and (d), the basal maximum intensity is located at 15° towards SD. With increasing strain, the splitting shown in Mg-1.43Nd disappears and a basal texture appears. This basal texture is visible in the form of a crescent unilaterally shifted towards SD (15°) in the basal (0002) images of Figs 3 and4. Figure 5 demonstrates clearly that all samples exhibit an asymmetric basal texture, with the distribution of the basal poles lying towards CD.

It is interesting to note that a similar texture evolution was reported in the WE54 alloy after hot uniaxial compression [START_REF] Azzeddine | On the texture and grain growth in hot deformed and annealed WE54 alloy[END_REF]. The present results show that the intensity of the basal texture of the Mg-1.43Nd alloy is essentially stable during HPT processing except after 5 HPT turn, as demonstrated in Fig. 3(c and Mg-1.44Ce HPT-processed disks with increasing numbers of HPT turns. Thus, the microhardness increases significantly in the initial stages with increasing HPT turns but after about 1 turn it essentially saturates for both alloys. The maxima values are ~65 and ~96 Hv for the Mg-Ce and Mg-Nd alloys and these values are ~57 and ~100 % more than the hardness values of the undeformed Mg-Ce and Mg-Nd alloys, respectively. From Fig. 7, it can be seen also that the hardness as well as the hardening rate at the beginning of HPT for Mg-Nd are obviously higher than those for Mg-Ce.

Discussion

Texture evolution

The texture results demonstrate that the Mg-1.44Ce alloy develops a weaker texture than the Mg-1.43Nd alloy during HPT processing. It is known that the Ce element exerts a stronger effect on texture and grain size compared to other RE elements such as Y and Gd and this is due to the strong Ce interaction with dislocations and grain boundaries [START_REF] Stanford | Micro-alloying Mg with Y, Ce, Gd and La for texture modification-A comparative study[END_REF].

Nevertheless, another investigation on the effect of RE elements (Ce, La, Nd, Gd) on the sheet texture modification during warm rolling of a ZEK 100 alloy showed that the Ce and Nd elements led to similar texture evolutions [START_REF] Al-Samman | Sheet texture modification in magnesium-based alloys by selective rare earth alloying[END_REF]. The texture weakening after addition of these RE elements is associated with the grain boundary pinning effects caused by solute segregation and precipitate particles and the suppression of twinning [START_REF] Bohlen | New perspectives for wrought magnesium alloys[END_REF][START_REF] Laser | The influence of calcium and cerium mischmetal on the microstructural evolution of Mg-3Al-1Zn during extrusion and resulting mechanical properties[END_REF][START_REF] Stanford | KU editor Magnesium: Proceedings of the 8th International Conference on Magnesium Alloys and Their Applications[END_REF]. In this context, it is important to consider the effect of residual Nd and Ce solutes and possible precipitated Mg x RE y particles.

As is apparent from inspection of Fig. 6, the XRD patterns of the HPT-processed alloys show that the Mg-1.44Ce alloy contains large volume fractions, proportional to their intensities, of two second phases identified as MgCe 2 (space group I4/mmm, a = b= 1.036 nm and c = 0.596 nm) and Mg 17 Ce 2 (space group P63/mmc, a = b= 1.035 nm and c = 1.026 nm).

By contrast, smaller amounts of the Mg 12 Nd phase (space group I4/mmm, a = b = 1.031 nm and c = 0.593 nm) exist in the Mg-1.43Nd alloy as shown in Fig. 6(a) and (b). The volume fraction of precipitates determined from a quantitative analysis using MAUD software were as follows: Fv (Mg 17 Ce 2 ) = 4-4.5 % and Fv (Mg 12 Nd) = 1-1.5 % and obviously there was no variation upon straining. Additionally, a close inspection of the matrix (Mg) diffraction peaks revealed no shift of their 2θ angular positions.

Consequently, the heat treatment at 535 °C for 5 h failed to ensure a fullyhomogenized solid solution in both alloys. From a close inspection of the precipitate peaks, it is concluded that more residual Nd atoms remain in the Mg-Nd alloy than Ce atoms in the Mg-Ce alloy.

It was reported that the presence of a large volume fraction of second-phase particles in the AZ91 alloy leads to weak deformation textures at high strains where this is attributed to the reduction of the twinning activity and the promotion of dynamic recrystallization [START_REF] Li | Influence of second-phase precipitates on the texture evolution of Mg-Al-Zn alloys during hot deformation[END_REF].

However, the suppression or the activation of twining by interaction with particles depends strongly on their size and their distribution within the microstructure [START_REF] Stanford | Effect of composition on the texture and deformation behaviour of wrought Mg alloys[END_REF][START_REF] Clarke | Transmission electron microscopy study of age hardening in a Mg-5 wt.% Zn alloy[END_REF][START_REF] Mchargue | Effect of interstitial elements on twinning in columbium[END_REF]. For example, a particle spacing of ~100 nm was found to lead to the suppression of twins in Mg-1.62 Mn and Mg-5% Zn alloys [START_REF] Stanford | Effect of composition on the texture and deformation behaviour of wrought Mg alloys[END_REF][START_REF] Clarke | Transmission electron microscopy study of age hardening in a Mg-5 wt.% Zn alloy[END_REF]. Unfortunately, in the present research the size and distribution of the Mg 17 Ce 2 , MgCe 2 and Mg 12 Nd second phases, as well as the Nd and Ce residual supersaturations in both alloys, are not known precisely. It is also worth noting that these characteristics may be strongly affected by the HPT processing. Moreover, in addition to the lack of direct evidences for the size and distribution of precipitates, it is not clear whether or not there is a suppression of twinning in the two alloys and whether the precipitates influence the recovery and recrystallization. Consequently, it is believed that the texture difference between the two alloys may be partly due to the effect of second phase particles and residual solutes.

The texture evolutions of both alloys are different from the typical torsion texture where most of the (0002) planes of grains are oriented parallel to the shear (SD-RD) plane [START_REF] Guo | On the Swift effect and twinning in a rolled magnesium alloy under free-end torsion[END_REF]. Furthermore, the texture in this investigation is different from that in other Mg-based alloys processed by HPT [START_REF] Qiao | Hardening mechanism of commercially pure Mg processed by high pressure torsion at room temperature[END_REF][START_REF] Lee | Evolution in hardness and texture of a ZK60A magnesium alloy processed by high-pressure torsion[END_REF][START_REF] Huang | Microstructure and Texture Evolution in a Magnesium Alloy During Processing by High-Pressure Torsion[END_REF]. Indeed, the textures obtained are not only weaker in the Mg-RE alloy but also the qualitative characters of the textures were altered as well. For example, the texture of an AZ31 alloy after HPT processing at room temperature exhibited two fibres, a rotated basal texture (10° towards SD) and a second fibre ( 1 = 87°,  = 68° and  2 = 0 to 60°) [START_REF] Huang | Microstructure and Texture Evolution in a Magnesium Alloy During Processing by High-Pressure Torsion[END_REF]. It was also found that the HPT processing temperature affects the angle between the c-axis and the torsion axis such that the c-axis is 10° from the torsion axis at 25 and 100 °C but 5° from the torsion axis at 200 °C [START_REF] Huang | Microstructure and Texture Evolution in a Magnesium Alloy During Processing by High-Pressure Torsion[END_REF]. For commercially pure Mg processed by HPT at room temperature up to 8 turns there was a typical torsion texture and it was stable after one HPT turn [START_REF] Qiao | Hardening mechanism of commercially pure Mg processed by high pressure torsion at room temperature[END_REF]. A weak basal fibre texture that saturated with increasing numbers of HPT turns (up to 5 turns) was reported in a ZK60A (Mg-6Zn-0.5Zr wt.% alloy) after HPT processing at room temperature [START_REF] Lee | Evolution in hardness and texture of a ZK60A magnesium alloy processed by high-pressure torsion[END_REF].The formation of a typical torsion texture was attributed to the activation of {10 ̅ 2}<10 ̅ 1>tension twinning that caused a rotation of the basal planes about 86.3° to SD [START_REF] Qiao | Hardening mechanism of commercially pure Mg processed by high pressure torsion at room temperature[END_REF]. During further HPT processing, and since the grains are now oriented in positions where dislocation slip on basal <a> or prismatic <a> planes may dominate over the twinning, the c-axis became perpendicular to SD and the Schmid factor of <a> dislocation slip was not zero.

It is well known that the deformation texture depends strongly on the initial texture of the alloy with respect to the loading and torsional plane [START_REF] Lee | Evolution in hardness and texture of a ZK60A magnesium alloy processed by high-pressure torsion[END_REF]. The initial texture was assumed random in the present investigation. For this situation, the c-axis of the grains may be parallel, perpendicular or may have any orientation relatively to SD, so that the basal <a>, prismatic <a> and/or pyramidal <c + a> slip can be activated and probably contribute to the torsional deformation during HPT processing [START_REF] Huang | Microstructure and Texture Evolution in a Magnesium Alloy During Processing by High-Pressure Torsion[END_REF]. The rare earth content in the Mg may also contribute to the activation of non-basal slip by modifying the stacking-fault energy (SFE) of the Mg matrix [START_REF] Jung | Role of RE in the deformation and recrystallization of Mg alloy and a new alloy design concept for Mg-RE alloys[END_REF].

The Nd and Ce elements may play a crucial role in the resulted texture after HPT processing and the precise role of rare earth elements was investigated for the texture modification of Mg alloys [START_REF] Jung | Role of RE in the deformation and recrystallization of Mg alloy and a new alloy design concept for Mg-RE alloys[END_REF]. It was assumed that the slow diffusivity of the rare earth elements in magnesium, together with the segregation of RE element on the grain boundaries and dislocations, will slow down the movement of grain boundaries and dislocations (solute drag) and hence this will influence the texture development during deformation [START_REF] Jung | Role of RE in the deformation and recrystallization of Mg alloy and a new alloy design concept for Mg-RE alloys[END_REF]. The dominant part to texture weakening is, however, quiet difficult to determine. Moreover, it was reported that RE inter-metallic precipitates appear to be less effective than RE segregation to GBs or dislocations for producing texture modification.

There is also another factor which may explain the texture evolution during HPT at both the disk centers and edges but very little published information exists [START_REF] Huang | Microstructure and Texture Evolution in a Magnesium Alloy During Processing by High-Pressure Torsion[END_REF][START_REF] Figueiredo | Development of structural heterogeneities in a magnesium alloy processed by high-pressure torsion[END_REF]. This is associated with the deformation strain at the central region of the HPT disk that is very As shown above, HPT processing neither induced a shift to lower angles (i.e. larger lattice parameter) for the α-Mg diffraction peaks nor decreased the intensities of the intermetallic diffraction peaks. Therefore, any dissolution of the Mg 12 Nd and Mg 17 Ce 2 particles occurred upon increasing strain. Thus, the solute content of the α-Mg remains reasonably constant and causes no supplementary hardening. There is evidence for such solution hardening in an Mg-Gd-Y-Zn-Zr alloy after HPT processing [START_REF] Sun | Evolution of microstructure and mechanical properties of an as-cast Mg-8.2Gd-3.8Y-1.0Zn-0.4Zr alloy processed by high pressure torsion[END_REF]. Furthermore, this research also revealed a mechanism of dispersion hardening by which the Mg-RE intermetallics in the Mg-Gd-Y-Zn-Zr alloy were broken into small particles with sizes of ~1-10 μm and were then homogeneously dispersed in the matrix after HPT for 16 turns [START_REF] Sun | Evolution of microstructure and mechanical properties of an as-cast Mg-8.2Gd-3.8Y-1.0Zn-0.4Zr alloy processed by high pressure torsion[END_REF].

It is well known that metals and alloys strengthen through the four basic mechanisms of solid solution, precipitation, dislocation and grain boundary strengthening. Conventionally age-hardened Mg-RE alloys often exhibit a mixture of precipitate phases that have different morphologies, sizes and distributions. Then the overall strengthening of these alloys is totally determined by multi-form interactions between precipitates with dislocations and twins during the deformation process [START_REF] Nie | Precipitation and hardening in magnesium alloys[END_REF]. Basically, the strengthening is progressive up to the peak value and it is proportional to the volume fractions of the precipitates.

It is known that there are two main causes of hardening upon processing by SPD.

These are the effects of a strong grain refinement which is accompanied by a steep evolution of the dislocation density [START_REF] Qiao | Hardening mechanism of commercially pure Mg processed by high pressure torsion at room temperature[END_REF]. However, these two effects may not apply in a simple manner in f.c.c metals owing to the simultaneous occurrence of recovery or dynamic recrystallization.

In practice, it was shown that dynamic recrystallization leads to a bimodal distribution of small recrystallized grains embedded within the original large grains [START_REF] Figueiredo | Processing magnesium alloys by severe plastic deformation[END_REF] and this development of a bimodal microstructure is fully consistent with the principles of grain refinement developed for Mg alloys processed by other SPD techniques [START_REF] Figueiredo | Principles of grain refinement in magnesium alloys processed by equal-channel angular pressing[END_REF][START_REF] Figueiredo | The nature of grain refinement in equal-channel angular pressing: a comparison of representative fcc and hcp metals[END_REF].

Earlier results showed that dynamic recrystallization takes place in the temperature range of approximately 147 -327 °C for magnesium and its alloys that are usually processed by ECAP [START_REF] Ion | Dynamic recrystallization and the development of microstructure during high temperature deformation of magnesium[END_REF][START_REF] Galiyev | Correlation of plastic deformation and dynamic recrystallization in magnesium alloy ZK60[END_REF][START_REF] Myshlyaev | Twinning, dynamic recovery and recrystallization in hot worked Mg-Al-Zn alloy[END_REF][START_REF] Beer | Influence of initial microstructure on the hot working flow stress of Mg-3Al-1Zn[END_REF]. By contrast, in the present study the Mg-RE alloys were processed by HPT at room temperature and these conditions will probably prevent, or at least hinder, any dynamic recrystallization. A DSC analysis of the Mg-1.43Nd and Mg-1.44Ce alloys suggested that recrystallization may occur in the temperature range of ~180-220 °C.

Moreover, it has been reported that defect recovery is stronger in Mg by comparison with Al, Cu, Ag and a range of metals with higher melting temperatures [START_REF] Qiao | Hardening mechanism of commercially pure Mg processed by high pressure torsion at room temperature[END_REF].

Finally, it is necessary to note that the presence of precipitate particles may also influence the defect recovery process and hence the overall hardening of the Mg-RE alloys.

The processes of dislocation generation, recovery and the interaction of dislocations with precipitation products and the possibility of particle breakage are areas requiring further exploration. Nevertheless, the very weak evolution of hardness upon HPT processing between ½ and 10 turns appears to be due to a steady-state dislocation generation in work hardening and the annihilation or recovery of dislocations as proposed also in another report [START_REF] Straumal | Phase transitions induced by severe plastic deformation: steady-state and equifinality[END_REF].

Summary and conclusions

• Mg-1.43Nd and Mg-1.44Ce alloys were processed by HPT at room temperature for up to 10 turns.

• The experimental textures at the centers and edges of the disks of both alloys were characterized by an asymmetric basal texture shifted 15° towards the shear direction with the appearance and disappearance of some minor components. The Mg-1.44Ce alloy developed a weaker texture than the Mg-1.43Nd alloy during HPT processing.

• Whereas for the Mg-1.43Nd alloy the texture was reasonably stable, there was a continuous decrease in the texture intensity in Mg-1.44Ce with increasing number of HPT turns and this was associated with the effect of Mg 17 Ce 2 and MgCe 2 phases.

• The microhardness increased with increasing HPT turns but ultimately leveled off for both alloys.

  ) and Figs. 5(a), 5(b) where the intensity increases to reach a maximum of 8.8 mrd in the edge of the 10 HPT sample. The Mg-1.44Ce alloy developed a similar type of basal texture in the center and edge of the sample, but the intensity decreased continuously with increasing numbers of HPT turns from ~8.8 mrd after ½ HPT turn to ~ 4.2 mrd after 10 HPT turns in the center and from ~8.7 mrd after ½ HPT turn to ~5.6 mrd after 10 HPT turns in the edge. It is interesting to note that the texture in the center of the Mg-1.44Ce alloy is different after 10 HPT turns because symmetrical splitting of the weak basal poles is formed from CD towards SD and a new weak component also appears that is a basal (0002) pole 45° tilted from SD. The 1D-XRD patterns from the high photon flux synchrotron for the HPT-processed Mg-1.43Nd and Mg-1.44 Ce alloys are shown in Fig. 6 after 1 and 10 turns. Inspection of these patterns shows that the use of a high beam easily reveals the different precipitate peaks that are often obscured in the background when using more conventional XRD techniques.

Figure 7

 7 Figure 7 illustrates the microhardness evolution in the center and edge of Mg1.43Nd

Figure captions Figure 1 :

 captions1 Figure captions

Figure 2 :

 2 Figure 2: IPF maps showing the microstructure of annealed: a) Mg-1.43Nd and Mg-1.44Ce.

Figure 3 :

 3 Figure 3: Basal (0002) recalculated pole figures near the centre and the edge of the disks of

Figure 4 :

 4 Figure 4: Basal (0002) recalculated pole figures near the centre and the edge of the disks of

Figure 5 :

 5 Figure 5: Basal (0002) pole intensity distribution in shear direction (SD) of HPT deformed: a)

Figure 6 :

 6 Figure 6: XRD patterns of HPT deformed: Mg-1.43Nd alloy to a) 1 turn, b) 10 turns and Mg-

Figure 7 :

 7 Figure 7: Evolution of Vickers microhardness with number of HPT turn of Mg-1.43Nd and

Acknowledgements

DB gratefully acknowledges Dr. Olivier Balmes and Dr. Matej Zdeneck from Max IV synchrotron facility, Lund Sweden, for their kind invitation and assistance during his short stay. The work of two of us was supported by the European Research Council under Grant Agreement No. 267464-SPDMETALS (YH and TGL).

complex, and varies with the position relative to the disk center. In the present study, the XRD macrotexture measurements were performed at the disk center and, unlike the disk edge, the sample texture was then complicated by the axial symmetric strain around the central axis.

Hardness evolution

The considerable increase in the hardness shown in Fig. 7 within 1/2 turn of HPT processing may be due not only to conventional dislocation and grain size hardening but also to texture strengthening. It is plausible that texture strengthening may play an important role in this early hardening during HPT where the strengthening is a direct consequence of the basal plane reorientations from a stochastic arrangement in the as-cast condition to an arrangement parallel to the shear direction (and thus vertical to the loading direction of the microhardness tests) [START_REF] Qiao | Hardening mechanism of commercially pure Mg processed by high pressure torsion at room temperature[END_REF].

Many authors have reported similar significant grain refinement within one HPT turn [START_REF] Qiao | Hardening mechanism of commercially pure Mg processed by high pressure torsion at room temperature[END_REF][START_REF] Lee | Evolution in hardness and texture of a ZK60A magnesium alloy processed by high-pressure torsion[END_REF][START_REF] Khereddine | An examination of microstructural evolution in a Cu-Ni-Si alloy processed by HPT and ECAP[END_REF]. In addition, the very slight change between 1 and 10 HPT turns is due in part to a concomitant saturation of grain subdivision and the dislocation density and in part to a recovery of dislocations which begins even at room temperature in HPT-deformed Mg [START_REF] Cizek | Defects in ultra-fine grained Mg and Mg-based alloys prepared by high pressure torsion studied by positron annihilation[END_REF].

Values of the hardness achieved in this investigation are no more than 20 % higher than the peak age-hardening values reported for conventionally aged Mg-Ce and Mg-Nd alloys [START_REF] Rokhlin | Magnesium alloys containing rare earth metals[END_REF][START_REF] Saito | TEM study of real precipitation behavior of an Mg-0.5at%Ce agehardened alloy[END_REF][START_REF] Su | Effect of Nd content and heat treatment on the thermal conductivity of Mg-Nd alloys[END_REF][START_REF] Nie | Precipitation and hardening in magnesium alloys[END_REF][START_REF] Kawasaki | Evolution of hardness in ultrafine-grained metals processed by high-pressure torsion[END_REF][START_REF] Edalati | High-pressure torsion of pure magnesium: Evolution of mechanical properties, microstructures and hydrogen storage capacity with equivalent strain[END_REF]. Furthermore, the microhardness evolution follows a trend that is similar to many metals processed by HPT or other SPD techniques [START_REF] Qiao | Hardening mechanism of commercially pure Mg processed by high pressure torsion at room temperature[END_REF][START_REF] Kawasaki | Evolution of hardness in ultrafine-grained metals processed by high-pressure torsion[END_REF][START_REF] Edalati | High-pressure torsion of pure magnesium: Evolution of mechanical properties, microstructures and hydrogen storage capacity with equivalent strain[END_REF]. The higher hardness and hardening rate at the beginning of the HPT processing observed for Mg-Nd by comparison with Mg-Ce may be associated with residual solute/dislocation and precipitate/dislocation effects as well as the concomitant influence of the complex Mg x RE y phase size, distribution and morphology.

It is worth noting that there is very little if any data about such complex inter-correlations available in the literature and especially after severe plastic deformation.