Saskia Erdmann
email: serdmann@dal.ca

Bruno Scaillet

Caroline Martel

Anita Cadoux

Characteristic textures of recrystallized, peritectic, and primary magmatic olivine in experimental samples and natural volcanic rocks

Keywords: olivine, crystal cluster, dendritic, peritectic, primary magmatic, secondary magmatic, recrystallization, texture

Olivine textures are potentially important recorders of olivine origin and crystallization conditions. Primary magmatic and xenocrystic origins are commonly considered for olivine from ultramafic to intermediate magmas, while secondary olivine origins (i.e. crystals formed by recrystallization or peritectic reaction) are rarely considered in the interpretation of magmatic phenocrysts. The main aim of our study was to determine textures that are characteristic for secondary magmatic olivine and non-characteristic, or at least rare, for primary magmatic olivine. To characterize the textures of the different olivine types, we review previous experimental work and present new textural data for olivine from four melanorite melting and eight basalt crystallization experiments. We (i) qualitatively characterize olivine textures using transmitted light and back-scattered electron microscopy; and (iii) semi-quantitatively characterize the 2D surface area of olivine branches and individual crystals, their 2D and calculated 3D shapes, and 2D grain boundary segment lengths. Olivine recrystallization yields crystals dendritic branches, while peritectic reaction produces olivine cluster with randomly oriented crystals. In agreement with previous studies, we find that olivine crystal and branch size, grain boundary segmentation, and inclusion relations cannot unequivocally distinguish between olivine of secondary and primary origins.

However, recrystallized olivine typically has short prismatic branches, while primary magmatic olivine dendrites commonly have elongated branches. The peritectic crystals closely compare to glomerocrysts and individual polyhedral primary magmatic crystals, but they commonly form small groups of touching crystals and cluster with crystal-poor cores.

Spatial association and a comparison with xenocrysts textures may further guide the interpretation of their origin, but detailed analyses of core compositions and zoning patterns appear necessary to firmly distinguish peritectic from primary magmatic crystals. Our comparison of experimental and natural samples suggests that dendritic olivine crystals and cluster of polyhedral olivine that commonly occur in mafic and ultramafic igneous rocks should be evaluated for possible primary as well as secondary magmatic origins.

INTRODUCTION

Olivine is a characteristic primary phase in ultramafic to intermediate magmas recrystallization (liquid1 → olivine1 + liquid2), but also commonly forms xenocrysts, thus yielding information on the open-and closed-system evolution of magma systems (e.g., [START_REF] Donaldson | Olivine crystal types in harrisitic rocks of the Rhum pluton and Archean spinifex rocks[END_REF][START_REF] Helz | Diverse olivine types in lava of the 1959 eruption of Kilauea volcano and their bearing on eruption dynamics[END_REF][START_REF] Costa | Short time scales of magmatic assimilation from diffusion modeling of multiple elements in olivine[END_REF][START_REF] Arndt | Olivine, and the origin of kimberlite[END_REF][START_REF] Qian | Formation of high-Mg diorites through assimilation of peridotite by monzodiorite magma at crustal depths[END_REF][START_REF] Erdmann | Textures of peritectic crystals as guides to reactive minerals in magmatic systems: new insights from melting experiments[END_REF][START_REF] Famin | Three differentiation stages of a single magma at Piton de la Fournaise (Reunion hotspot)[END_REF]Welsch et al., 2013). Secondary olivine formed by recrystallization (olivine1 + liquid1 → olivine2 + liquid2) or by peritectic replacement of mafic precursor minerals (e.g., orthopyroxene + liquid 1 → olivine2 + liquid 2) has been produced in experiments (e.g., [START_REF] Brearley | Dissolution rates of upper mantle minerals in an alkali basalt melt at high pressure: An experimental study and implications for ultramafic xenolith survival[END_REF][START_REF] Tsuchiyama | Experimental study of olivine-melt reaction and its petrological implications[END_REF][START_REF] Boudier | Olivine xenocrysts in picritic magmas[END_REF][START_REF] Erdmann | Textures of peritectic crystals as guides to reactive minerals in magmatic systems: new insights from melting experiments[END_REF][START_REF] Jacobs | Orthopyroxene stability within kimberlite magma: an experimental investigation[END_REF]. They form dendritic crystals and cluster of subhedral-euhedral, randomly oriented crystals, respectively (Fig. 1). Few examples of possible olivine recrystallization [START_REF] Boudier | Olivine xenocrysts in picritic magmas[END_REF][START_REF] Dunworth | Olivine melilitites of the SW German Tertiary Volcanic Province: Mineralogy and petrogenesis[END_REF][START_REF] Arndt | Olivine, and the origin of kimberlite[END_REF] and peritectic crystallization [START_REF] Erdmann | Xenocryst assimilation and formation of peritectic crystal clusters during magma contamination: an experimental study[END_REF][START_REF] Erdmann | Textures of peritectic crystals as guides to reactive minerals in magmatic systems: new insights from melting experiments[END_REF] have also been reported from natural samples, but these origins are rarely considered in interpreting the textures and compositions of olivine phenocryst.

Differences in crystal size may discriminate between olivine of various origins in mixed crystal populations (e.g., [START_REF] Dunworth | Olivine melilitites of the SW German Tertiary Volcanic Province: Mineralogy and petrogenesis[END_REF][START_REF] Amma-Miyasaka | Origin of anorthite and olivine megacrysts in island-arc tholeiites: Petrological study of 1940 and 1962 ejecta from Miyake-Jima volcano, Izu-Mariana arc[END_REF][START_REF] Costa | Short time scales of magmatic assimilation from diffusion modeling of multiple elements in olivine[END_REF][START_REF] Arndt | Olivine, and the origin of kimberlite[END_REF], but size is not an independent criterion for interpreting olivine origin. Rounded and embayed olivine grain boundary segments may be evidence for partial dissolution (Donaldson,1985;[START_REF] Tsuchiyama | Experimental study of olivine-melt reaction and its petrological implications[END_REF][START_REF] Helz | Diverse olivine types in lava of the 1959 eruption of Kilauea volcano and their bearing on eruption dynamics[END_REF][START_REF] Boudier | Olivine xenocrysts in picritic magmas[END_REF], but they may also be primary growth features [START_REF] Donaldson | An experimental investigation of olivine morphology[END_REF][START_REF] Moore | Unusual olivine zoning: evidence for complex physicochemical changes during the evolution of olivine melilitite and kimberlite magmas[END_REF]Welsch et al., 2013;[START_REF] Faure | Experimental investigation of equilibration conditions during forsterite growth and melt inclusion formation[END_REF]. Euhedral grain boundary segments may be most characteristic for primary magmatic growth, but they also form by recrystallization; peritectic growth, or late-stage epitactic overgrowth [START_REF] Boudier | Olivine xenocrysts in picritic magmas[END_REF][START_REF] Arndt | Olivine, and the origin of kimberlite[END_REF][START_REF] Erdmann | Xenocryst assimilation and formation of peritectic crystal clusters during magma contamination: an experimental study[END_REF]. Deformation features, such as microfractures, subgrains, and deformation twins have been taken to infer a xenocrystic origin of olivine crystals in many examples (e.g., [START_REF] Helz | Diverse olivine types in lava of the 1959 eruption of Kilauea volcano and their bearing on eruption dynamics[END_REF][START_REF] Boudier | Olivine xenocrysts in picritic magmas[END_REF][START_REF] Dunworth | Olivine melilitites of the SW German Tertiary Volcanic Province: Mineralogy and petrogenesis[END_REF]Vinet & Higgins, 2010), and they may identify restitic cores of recrystallized olivine. However, olivine deformation has also been inferred to take place during compaction of crystal mush or magmatic flow (e.g., Clague & Denglinger, 1994;[START_REF] Natland | Capture of helium and other volatiles during the growth of olivine phenocrysts in picritic basalts from the Juan Fernandez Islands[END_REF], and lattice mismatch as a result of dendritic growth has further been inferred to resemble deformation textures [START_REF] Sunagawa | Crystals: growth, morphology and perfection[END_REF]Welsch et al., 2013). Exotic inclusions (i.e. inclusions that differ from the cognate melt composition and/or mineral assemblage) may reflect primary crystallization resulting from local supersaturation [START_REF] Maaløe | Olivine phenocrysts of Hawaiian olivine tholeiite and oceanite[END_REF][START_REF] Faure | Experimental investigation of equilibration conditions during forsterite growth and melt inclusion formation[END_REF][START_REF] Roeder | Cotectic proportions of olivine and spinel in olivine-tholeiitic basalt and evaluation of pre-eruptive processes[END_REF], but they may also be relics of xenocrystic assemblages or dissolution and reprecipitation [START_REF] Thornber | Dissolution of olivine in basaltic liquids: experimental observations and applications[END_REF][START_REF] Helz | Diverse olivine types in lava of the 1959 eruption of Kilauea volcano and their bearing on eruption dynamics[END_REF][START_REF] Dungan | Partial assimilative recycling of the mafic plutonic roots of arc volcanoes: An example from the Chilean Andes[END_REF][START_REF] Erdmann | Textures of peritectic crystals as guides to reactive minerals in magmatic systems: new insights from melting experiments[END_REF].

It thus appears that few, if any, of the commonly considered olivine textures can unequivocally discriminate crystals of primary magmatic from those of secondary magmatic origin. The main aim of our study was to identify textures that are characteristic for secondary (recrystallized or peritectic olivine) andat leastrare for primary magmatic olivine crystallized directly from the melt. We hypothesized that such textures exist, as the components (olivine, reactive mafic phase, and/or melt) and processes (one-or multi-phase dissolution-reprecipitation versus primary crystallization) of crystallization strongly differ.

For example, "dusty-", "sieve-", or "cellular-textured" plagioclase, clinopyroxene, orthopyroxene, or spinel crystals are commonly inferred to form by dissolutionreprecipitation (e.g., [START_REF] Tsuchiyama | Dissolution kinetics of plagioclase in the melt of the system diopside-albite-anorthite, and origin of dusty plagioclase in andesites[END_REF][START_REF] Johannes | Partial melting reactions of plagioclase and plagioclase-bearing systems[END_REF][START_REF] Streck | Mineral textures and zoning as evidence for open system processes[END_REF], and they are texturally distinct from primary magmatic crystals of the same mineral types.

To examine potentially characteristic textures of secondary and primary magmatic olivine, we (i) review previous experimental studies on secondary, recrystallized and peritectic olivine; (ii) present new textural data on olivine crystal size, shape, and grain boundary segmentation for recrystallized and peritectic olivine produced in melanorite melting experiments (in which rock powders were partially melted); and (iii) compare this data to primary magmatic olivine crystallized in new and published crystallization experiments (in which glasses were partially crystallized). We propose that crystal and branch shape may be the best criteria to distinguish between secondary, recrystallized olivine and primary magmatic olivine dendrites. Cluster shape and crystal orientation within clusters appear the best textural criteria to distinguish secondary, peritectic and primary magmatic, glomerocrystic olivine. We conclude with an evaluation of possible natural examples from the Tatara-San Pedro volcanic complex in Chile, and literature examples in which olivine forms dendritic crystals and cluster of subhedral-euhedral, randomly oriented crystals.

FORMATION OF SECONDARY OLIVINE IN PREVIOUS EXPERIMENTS

Textures of experimentally recrystallized and peritectic olivine have been described by a limited number of studies. [START_REF] Tsuchiyama | Experimental study of olivine-melt reaction and its petrological implications[END_REF] has performed reaction experiments between olivine and SiO 2 -rich host melts that yielded recrystallized, Fo-rich vermicular crystals (Fig. 1a). Natural olivine crystals were used as starting material, cut into cubes with variable crystallographic orientation and then pressed into pellets with andesite and boninite rock powders. These pellets were then heated to temperatures between 1100 and 1321°C in an atmospheric gas-mixing furnace. Dissolution and recrystallization of ≤Fo 39 olivine yielded vermicular crystals resembling dusty-, sieve-, and fingerprint-textured plagioclase (Fig. 1a) [START_REF] Boudier | Olivine xenocrysts in picritic magmas[END_REF] has characterized textures of partially recrystallized olivine (Fig. 1b,c) from a plagioclase lherzolite that was partially melted and deformed using a Griggs press.

The melting experiment was performed under 5% constant strain and a confining pressure of 0.7 MPa. Partial melting and olivine recrystallization took place at ~1500 to ~1650 °C.

Inclusion-rich olivine recrystallized to undeformed, euhedral branches in optical continuity with their restitic cores. The breakdown of the cores commenced along (100) and ( 001) planes. Recrystallization proceeded by growth of (010) and {110} blades that in cases showed minor orientation mismatch between neighbouring units. Euhedral branches reached lengths of ~50-200 m and axial a:b:c ratios of ~1.2:1.0:1.5 (Fig. 2 and Table 2 of [START_REF] Boudier | Olivine xenocrysts in picritic magmas[END_REF]. Boudier highlighted the low shape anisotropy of the recrystallized olivine branches compared to primary magmatic dendrites crystallized at high growth rates, but she considered substructures (e.g, inclusions) as the best criteria for distinguishing primary dendritic olivine from dendritic crystals with a secondary origin.

We have previously presented textural data for peritectic olivine formed by partial melting of melanorite and leuconorite rock powders (Fig. 1 d;[START_REF] Erdmann | Textures of peritectic crystals as guides to reactive minerals in magmatic systems: new insights from melting experiments[END_REF]. The experiments were performed in internally heated pressure vessels at 200 MPa and temperatures between 1000 and 1150 °C. The peritectic olivine crystals replaced amphibole, orthopyroxene, and phlogopite of the starting materials. They formed clusters with euhedral to subhedral, randomly oriented crystals. Olivine replacing orthopyroxene yielded clusters with high olivine proportions and relatively large crystals (top Fig. 1d). Olivine replacing amphibole and phlogopite yielded clusters with relatively small olivine crystals and high or low olivine proportions (bottom Fig. 1d).

NEW DATA AND METHODS

As detailed in the following sections, we (i) have used transmitted light and BSE images for the qualitative comparison of olivine textures, and (ii) present measurements of the 2D surface area, 2D and 3D shape analysis, and 2D grain boundary segmentation of olivine branches and peritectic crystals. Our melting experiments used rock powders that were partially melted to produce secondary olivine. As detailed below, we report results for one new melanorite melting experiment (MN2-12) and new data for three additional melanorite melting experiments (MN-2, MN-9, and MN-12) of [START_REF] Erdmann | Textures of peritectic crystals as guides to reactive minerals in magmatic systems: new insights from melting experiments[END_REF] (Table 1). Our crystallization experiments employed a glass starting material that was then partially crystallized. For primary magmatic olivine formed in the crystallization experiments, we report phase assemblage, 2D surface area, and 2D and 3D shape analysis for eight basalt crystallization experiments (B-1 to B-12; Table 1). Experiments with variable temperature (1000-1150 °C) and H 2 O content (0.5-4.2 wt% added) were examined to compare textures of the different olivine types over a range of crystallization conditions. The new MN2-12 melting experiment was performed with a more mafic and unaltered starting material than that of the MN melting experiments to test for effects of compositional variation of the original assemblages on the resulting olivine textures.

Starting materials

Raw starting materials for our experiments were a basalt (B) from the Upper Placeta San Pedro sequence of the Tatara-San Pedro volcanic complex of the Southern Volcanic Zone in Chile, and two melanorite xenoliths (MN and MN2) recovered from dacitic lavas of the complex (whole-rock compositions are given in Table 2). Geochemical and stratigraphic details of the Tatara-San Pedro volcanic complex and important contaminants are reported by [START_REF] Dungan | Eruptive stratigraphy of the Tatara-San Pedro complex, 36°S, Southern volcanic zone, Chilean Andes: Reconstruction method and implications for magma evolution at longlived arc volcanic centers[END_REF] and [START_REF] Costa | Hornblende-and phlogopite-bearing gabbroic xenoliths from Volcan San Pedro (36°S), Chilean Andes: Evidence for melt and fluid migration and reactions in subduction-related plutons[END_REF] and references therein. The major constituents of the MN xenolith are plagioclase (~60 vol%; predominantly ~An 80 ), amphibole (~13 vol%; X Mg ~77), orthopyroxene (~7 vol%; ~En 78 , Fs 20 , Ws 2 ; X Mg ~80), phlogopite (~3 vol%; X Mg ~80), olivine (~15 vol%; ~Fo 80 ), and oxides (~2 vol%). Most olivine has iddingsite rims, microfractures, and oxide inclusions. The major constituents of the MN2 xenolith are plagioclase (~40 vol%; ~An 82 core composition), olivine (~20 vol%; ~Fo 77 ), orthopyroxene (~15 vol%; ~En 79 , Fs 19 , Ws 2 ; X Mg ~80), amphibole (~15 vol%; X Mg ~78), phlogopite (~6 vol%), and Cr-spinel (~4 vol%). The MN2 olivine has a slightly lower Fo content than in the MN starting material (~Fo 77 versus ~Fo 80 ), but is unaffected by alteration. The basalt starting rock contains phenocrysts and xenocrysts of plagioclase (~10 vol%), olivine (~5 vol%), and clinopyroxene (~3 vol%) set in a matrix composed of plagioclase, olivine, clinopyroxene, orthopyroxene, and oxides.

The coarse-grained MN and MN2 starting rocks were separately crushed to experimental starting material powders of ≤170 m that consisted of crystal and few rock fragments. The starting material of the basalt crystallization experiments was a glass fused twice for ~4 h at 1400 °C and atmospheric conditions. Between and after fusing, the glass was ground to <20 m in an agate mortar.

Experimental methods

All experiments were performed with an internally-heated pressure vessel at the Institut des Sciences de la Terre d'Orléans, France. Pressure was obtained by sequential loading of a mixture of H 2 and Ar gas at room temperature [START_REF] Scaillet | Experimental crystallization of leucogranite magmas[END_REF]. Charges were then heated at a rate of 10-15 °C/min from room temperature to ~40 °C below the experimental temperature. Final heating was performed at 2-4 °C/min to minimize overstepping of set point temperatures to ≤10 °C for ≤10 min. The experiments were ended by drop quench (e.g., Di [START_REF] Di Carlo | Experimental crystallization of high-K arc basalts: the golden pumice, Stromboli volcano (Italy)[END_REF]. Experimental fO 2 was calculated from Ni-Pd-NiO solid sensors using the calibration of [START_REF] Pownceby | Thermodynamic data from redox reactions at high temperatures. III. Activity-composition relations in Ni-Pd alloys from EMF measurements at 850-1250 K, and calibration of the NiO + Ni-Pd assemblage as a redox sensor[END_REF]. Several fragments from different parts of the experimental charges were first mounted in epoxy resin and polished for BSE imaging and microprobe analysis. Following electron beam analysis, thin sections were then prepared from the sample mounts.

Experimental conditions

All experiments were performed at ~200 MPa at temperatures of 1000 and 1150 °C with ~0.5 and 4.2 wt% H 2 O added to the charges as summarized in Figure 2 and Table 1. The analyzed MN melting experiments were selected from those presented by [START_REF] Erdmann | Textures of peritectic crystals as guides to reactive minerals in magmatic systems: new insights from melting experiments[END_REF]. The run products of three experiments were chosen for further analyses according to (i) the presence of recrystallized olivine (MN-2 and MN-12), and (ii) crystallization at a range of crystallization temperatures and intermediate to high experimental H 2 O contents (Fig. 2a,b). One run product, in which olivine showed pervasive recrystallization, was selected from a series of experiments with the MN2 starting material to test the effect of variable rock and olivine composition on olivine textures. The analyzed B crystallization experiments were chosen, because they cover a range of olivine textures.

The average experimental fO 2 was NNO+0.7. Gold capsules were used as sample containers for experiments at 1000 and 1043 °C; Au 80 Pd 20 capsules were used for experiments at 1085 and 1150 °C. Calculated experimental Fe loss was -39 to -23 % relative for the 1150 and 1085 °C experiments, but less than -15 % relative for the ≤1043 °C experiments. This limits the application of our compositional data (summarized in the Electronic Appendix) and affected the location and size of mineral stability fields (e.g., expanding the stability field of olivine), but the main insights gained from our textural analysis remain applicable, as both melting and crystallization experiments experienced Fe loss. If olivine textures were affected, it was the case for crystallization of secondary and primary magmatic olivine.

All melting experiments and the examined crystallization experiments were performed at sub-liquidus conditions (Fig. 2), which allowed us to limit experimental Fe loss.

However, the crystallization experiments therefore contained abundant nuclei when they reached the target P-T values, particularly in low-temperature, low-H 2 O experiments (in which dissolution of nuclei was inefficient). The experimental conditions that yielded specific olivine growth morphologies thus cannot be used to infer the intensive crystallization parameters (magmatic temperature or H 2 O content) of natural magmas containing olivine with equivalent morphologies. Nevertheless, in combination with textural observations from published experiments with supra-liquidus heating, we can use our results to infer characteristic features of primary versus secondary olivine.

The experimental run times were 21, 23, to 24 hours for the melting experiments and 19, 26, and 41 h for the crystallization experiments. These variations in run times preclude a direct comparison of the textures produced at specific run durations. However, the run durations of the melting and crystallization experiments overlap, which allows us to qualitatively assess the effects of time, temperature, and experimental H 2 O on olivine textures and the importance of these parameters compared to the processes of secondary versus primary crystallization on the considered textures.

Olivine classification

To quantitatively analyze olivine textures we have first classified recrystallized and peritectic olivine in MN and MN2 melting experiments as summarized in Figures 3 and4a. Olivine in MN and MN2 melting experiments with core compositions equivalent to crystals of the starting material is classified as restitic (Ol-R). Subhedral to euhedral olivine in optical continuity with restitic crystal cores is classified as recrystallized (Ol-RC). The main evidence that these olivine zones are recrystallized portions of restitic crystals is the partial preservation of original olivine compositions in the cores of some branches, though epitactic crystallization may have contributed to their crystallization (i.e. by Ol1+L1→Ol2+L2).

Inclusion-poor and randomly oriented olivine in crystal clusters or small groups are classified as peritectic (Ol-P). To further distinguish different types of peritectic olivine we have followed the classification of [START_REF] Erdmann | Textures of peritectic crystals as guides to reactive minerals in magmatic systems: new insights from melting experiments[END_REF] summarized in Figure 3, but we have renamed the olivine types. We distinguish peritectic olivine formed after amphibole (Ol-P-Amp), orthopyroxene (Ol-P-Opx), and phlogopite (Ol-P-Phl) according to olivine proportions in clusters, crystal sizes, and grain boundary segment lengths. Olivine produced in the basalt crystallization experiments is referred to as primary magmatic (Ol-PM).

Textural analysis

In addition to the qualitative descriptions, we have characterized the 2D branch and/or crystal surface area, the 3D shape, the 2D maximum major and minor axial lengths, and grain boundary segment (GBS) lengths (Fig. 4). We consider the textural data to be semiquantitative, because small crystals (commonly ≤15 m in length) were measured, the size of our datasets is limited (Table 3; n=43-305 for 2D surface area and 3D shape; n=244-468 for GBS lengths), and sectioning effects may have played a role. We nevertheless consider that the data illustrate first order similarities and differences in textures between secondary and primary experimental olivine crystals.

The textures and phase distributions of the run products of the crystallization experiments are relatively homogeneous and crystal orientations are random. Textures and phase distribution of the run products of the melting experiments are more heterogeneous, reflecting the use of rock powders as starting materials, but crystal orientations also appear random (Fig. 5). Crystal accumulation in parts of the experimental charges was not detected.

For recrystallized olivine we have measured branches of 5 to 10 crystals per charge (e.g., Fig. 4 b,c). For primary magmatic olivine we have analyzed between ~90 and 300 polyhedral crystals or 2D intersections of olivine branches per charge. For peritectic olivine we have measured crystals from 3 to 6 crystal clusters (Table 3). As peritectic crystals commonly form sub-clusters of touching grains that lack visible grain boundaries in BSE images (e.g., C2 as compared to C1 in Fig. 4b,d), we have drawn their outlines using BSE images and then used transmitted light microscopy to estimate their actual size and shape (Fig. 4d).

Crystals of each olivine type were analyzed from different parts (i.e. fragments) of the experimental charges. Drawings of 2D branches and crystals (Fig. 4c,d) were subsequently converted into binary images and analyzed using the SCION image software to quantify their 2D surface area and the 2D major and minor axial lengths. The 2D major and minor axes of olivine branches and crystals were further used to characterize their 3D shape employing the CSDslice spreadsheet of [START_REF] Morgan | On estimating crystal shape for crystal size distribution analysis[END_REF] (Fig. 4e). In addition to the calculated 3D values, we present "maximum" 2D major/minor axial ratios determined using the ten largest values derived from our image analysis. The correspondence between major, intermediate, and short axes of olivine and the crystallographic a,b,c directions are unconstrained. The 2D grain boundary segment (GBS) length, taken as the length of euhedral GBSs between inflection points, was measured in CorelDRAW (Fig. 4f).

The estimated standard errors for the 2D surface area and the 2D GBS length are <6 and <2 % of the population range. The reported reliability of the shape estimate (R 2 ) is the fractional measure of the variation in the sample explained by the best-fit shape of the database. We acknowledge that additional, unconstrained errors relate to the small crystal size and the choice of sections analyzed (e.g., we may have inadvertently analyzed more core zones of peritectic crystal clusters for one charge than for another), and that the textural data should therefore be considered as semi-quantitative.

Bulk, mineral, and glass compositions

Whole-rock starting materials reported in Table 2 were analyzed using a Phillip's PW2400 Xray spectrometer at the Regional Geochemical Centre of Saint Mary's University, Halifax.

Olivine and glass compositions reported in the Electronic Appendix were determined using a JEOL 8200 electron microprobe at Dalhousie University, Canada. Core-rim zoning with increasing and decreasing in X Mg [Mg/(Mg+Fe)x100] is referred to as reverse and normal, respectively. The mineral abbreviations used follow those of Whitney & Evans (2010).

EXPERIMENTAL RESULTS

In the following sections, we first describe the textures of restitic, partially recrystallized and peritectic olivine formed in our melting experiments and then present a textural overview of primary magmatic olivine formed in previous and in our crystallization experiments. We then summarize the effects of variable experimental temperature, H 2 O content, and olivine composition on the textures of the different olivine types.

Melanorite melting experiments

Restitic and recrystallized olivine

Partial melting of the two melanorite starting materials produced between ~12 and 3 wt % restitic olivine (Ol-R) and ~2 to 8 wt% recrystallized olivine, decreasing in abundance with increasing experimental temperature and H 2 O content and from MN2 to MN experiments (Fig. 2 a; Table 1). The restitic crystals have irregular shapes, reminiscent of the crystal fragments that were added to the charges (Figs 5,6). Their cores commonly contain some nanopores or healed microfractures (e.g., Fig. 6a,e,f) and they have on average Fo 80 and Fo 77 composition in MN and MN2 experiments, respectively, equivalent to olivine of the starting materials (the compositions are summarized in the Electronic Appendix). Zoning from restitic cores to recrystallized rims is normal and relatively sharp in 1000 °C experiments (Fig. 6d,e) to reverse and gradational in ≥1085 °C experiments (Fig. 6f-i).

The restitic crystals in the 1000 °C and 1085 °C experiments typically show minor resorption in the form of regularly embayed grain boundaries as well as subhedral grain boundary segments (e.g., Ol-R in Fig. 6d). However, some restitic crystals are rimmed by zones of optically continuous, recrystallized branches (Fig. 6a) that partially preserve the composition of the original restitic crystal and nanoinclusions in their cores. Restitic crystals in the 1150 °C experiments have sub-rounded shapes and they are partially, but pervasively resorbed (Fig. 6g-i). They show abundant glass-filled channels and pores that deeply intersect the restitic cores (e.g., Fig. 6g,h). Their grain boundary segments are curved to euhedral. As in the low-temperature experiments, some of the olivine crystals show branches that are in crystallographic continuity with the restitic cores (Fig. 6b,c). Nanopores are rare in their cores, while compositions of the original restitic olivine are partially preserved in the cores of some branches.

The axes of elongation of the recrystallized olivine branches is typically subparallel (Fig. 6g,h), although marginal branches often show a preferred shape orientation subparallel to the margin of the crystals (e.g., Fig. 6i). We also note that the major axes of elongation of the restitic fragments and their branches commonly appear to have oblique orientations (e.g., Fig. 6h versus 6g). The 2D surface area of the recrystallized branches commonly reaches up to ~200 m 2 and ~1000 m 2 in low-and high-temperature experiments, while the average 2D surface areas range between ~110 m 2 and ~940 m 2 (Fig. 7a-c; Table 3). The 2D cross-sections of the olivine branches have short prismatic shapes with calculated axial ratios of ~1.0 to 1.0-1.1 to 1.3-1.4 (Fig. 8a-c). The average 2D GBS lengths of the recrystallized branches range between ~3 and 8 m (Fig. 9a-c; Table 3).

Peritectic olivine

Partial melting of the MN and MN2 starting materials yielded between ~25 and 4 wt% peritectic olivine, decreasing in abundance with increasing experimental temperature and H 2 O content and from MN2 to MN experiments (Fig. 2b; Table 1). Crystals are largely unzoned with ~Fo 91 to ~Fo 76 composition in 1150 and 1000 °C experiments, respectively (Electronic Appendix). Crystal cluster consist of <10 to >100, sub-radially to randomly oriented crystals (Fig. 10). Crystals commonly seem to be organised around a large central glass pool (e.g., Fig. 5b,10b). Sub-groups of touching crystals that form cluster with compact to elongated shapes are common (Fig. 10a-e). The crystals have variable optical orientation, but their grain boundaries are often invisible in BSE images (e.g. Figs 4b,d and10b). The shape and size of the peritectic crystal clusters are variable, resembling the variable size and shape of crystal fragments added to the charges.

Peritectic olivine formed after orthopyroxene (Fig. 10a,b) typically forms clusters that are composed of single crystals enveloped by glass or by subgroups of two to a few touching individuals. The crystals have average 2D surface areas between ~150 and 240 m 2 and commonly reach maximum 2D crystal sizes of up to ~500 m 2 (Fig. 7d-g; Table 3). The Ol-P-Opx crystals are short prismatic with axial ratios of ~1.0 to 1.2-1.4 to 1.3-1.6 (Fig. 8d-g).

Their average 2D GBS lengths range between ~3 m and ~7 m (Fig. 9d-g). The Ol-P-Opx crystals are thus smaller than the branches of recrystallized olivine formed at 1150 and 1085 °C and larger than recrystallized olivine branches formed at 1000 °C, but their morphologies and 2D GBS lengths closely compare to those of recrystallized olivine. In contrast, olivine formed after amphibole (Fig. 10c,d) typically forms clusters of typically >50 variably oriented, touching crystals. Individual crystals have mean and maximum 2D crystal sizes of ~45 m 2 and <150 m 2 (Fig. 7h; Table 3). The crystals are short prismatic with axial ratios of ~1.0 to 1.2 to 1.3 (Fig. 8h). The mean GBS length of Ol-P-Amp branches is ~4 m and few GBSs are >12 m long (Fig. 9h). The Ol-P-Amp crystals are thus smaller and have shorter 2D GBSs than the Ol-P-Opx crystals. They also form larger subgroups of touching crystals.

Lastly, olivine formed after phlogopite (Fig. 10e,f) has 2D surface areas of <200 m 2 and typically <100 m 2 (Fig. 7i; Table 3). The crystals are acicular with axial ratios of ~1.0 to 1.1 to 3.0 (Fig. 8i). The mode of 2D GBS lengths is ~4 m, while most 2D GBSs are <12 m long (Fig. 9i). The acicular shape clearly distinguishes Ol-P-Phl from Ol-P-Opx and Ol-P-Amp crystals (Fig. 8i versus 8d-h).

Primary magmatic olivine

Published crystallization experiments with intermediate to ultramafic starting materials and sub-liquidus as well as supra-liquidus heating histories have produced olivine with polyhedral to variably skeletal and hopper-shaped to dendritic morphologies (e.g., [START_REF] Donaldson | An experimental investigation of olivine morphology[END_REF][START_REF] Lofgren | Dynamic crystallization of chondrule melts of porphyritic olivine composition: Textures experimental and natural[END_REF][START_REF] Faure | Textural evolution of polyhedral olivine experiencing rapid cooling rates[END_REF][START_REF] Shea | Kinetics of cooling-and decompression-induced crystallization in hydrous mafic-intermediate magmas[END_REF]. Typical examples reported by [START_REF] Donaldson | An experimental investigation of olivine morphology[END_REF] are summarized in Figure 11, which show that the polyhedral, skeletal and hopper-shaped crystals have short prismatic morphologies (Fig. 11a-d), while the dendritic crystals have moderately to strongly elongated branches and crystal shapes (Fig. 11e-g). The first-order branches of dendritic crystals are oriented parallel to the major elongation of the crystals, while lower-order branches are curved and may have oblique orientations (Fig. 11g).

The crystallization of basaltic glass in our 1000 to 1085 °C experiments yielded ~4 to 18 wt% olivine, decreasing in abundance with increasing experimental temperature and H 2 O content (Fig. 2c; Table 1). Olivine forms the largest crystals of the assemblage, ranging in composition from ~Fo 72 to Fo 85 . 12). The crystals have smaller sizes and apparently short prismatic and polyhedral morphologies in experiments with 0.5 wt% H 2 O added and larger sizes and elongated to dendritic morphologies in experiments with ≥2.1 wt% H 2 O added (Figs 5 and 12; Table 3). The calculated 3D shapes, however, suggest short prismatic crystal morphologies of ~1.0:1.11.2:1.5-1.9 for olivine from all experiments (Fig. 13), while the reliability of the shape estimate (R 2 ) decreases from apparently polyhedral olivine formed in low-H 2 O experiments (R 2 ≥0.82; Fig. 13a,b) to those of apparently dendritic crystals in high-H 2 O experiments (R 2 ≤0.73; Fig. 13e,h). In contrast, the calculated maximum 2D major/minor axial ratios increase from ~2.5 in low-H 2 O experiments (Fig. 10 a,b) up to ~4.0 in high-H 2 O, low-temperature experiments (Fig. 10e), consistent with the macroscopically observed textures (Figs 12 and13). We therefore conclude that the simple 2D analyses are more useful for our characterization of morphologically complex crystals than the 3D estimates.

Effects of experimental conditions and system components on olivine textures

Crystallization temperature, H 2 O content, and melanorite starting material compositions were varied to test for possible effects of these parameters on olivine textures. However, the limited number of experiments and their different run times permit only a qualitative evaluation. The relatively small variation in experimental run durations (~19-42 hours) had no first-order effect on the analyzed textures (Fig. 14a-c).

Compositional variation exerts important controls on the olivine textures. For branches of recrystallized olivine and peritectic olivine formed after orthopyroxene, the 2D surface area increases from experiment MN-12 with a partially altered assemblage to experiment MN2-12 with an unaltered assemblage and more mafic composition (e.g., Figs 7b,c,e,f and14). The textures of peritectic olivine are also strongly controlled by their precursor minerals (Figs 7g-i, 8g-i,9g-i and 10), and likely by the composition of their host melts. However, the compositions of all three olivine types overlap. Recrystallized and peritectic olivine have ~Fo 91-76 and primary magmatic olivine has ~Fo 85-72 composition. The crystallization processesprimary crystallization, dissolution-recrystallization, and peritectic crystallizationand not composition, are thus the primary controls of olivine textures.

For primary magmatic olivine, the 2D surface area tends to increase with decreasing temperature and increasing H 2 O content of the experiments (Fig. 14d,e). For peritectic olivine formed after orthopyroxene, the 2D surface tends to increase with decreasing temperature and increasing H 2 O content (Fig. 14d,e). For branches of recrystallized olivine, the 2D surface increases from olivine branches formed in low-temperature, high-H 2 O experiments to those formed at high-temperature and intermediate H 2 O contents (Fig. 14d,g). The 2D GBS lengths of all olivine types appear to increase with experimental temperature and decrease with experimental H 2 O content, although the scatter in the datasets is considerable (Fig. 14f,i). The 2D maximum/minimum axial ratios also show minor variation as a function of temperature or H 2 O content, but their values are consistently higher for the primary magmatic than for the recrystallized and peritectic olivine crystals.

DISCUSSION OF THE EXPERIMENTAL RESULTS

Experimental approach and conditions

Before proceeding with a discussion of the characteristic textures of secondary and primary olivine, we first examine possible effects of our experimental approach and conditions on olivine textures. As stated above, the variable experimental conditions (Table 1) preclude a detailed comparison of textures for primary and secondary olivine crystallized at equivalent experimental temperature, H 2 O, and run duration. However, variations in run duration (~19-41 hours) had no first-order effect on the examined olivine textures (Fig. 14a-c), and the crystallization temperatures and H 2 O contents of our melting and crystallization experiments overlap (Figs 2, 14; Table 1). The data thus allow us to qualitatively compare potentially characteristic textures of the different olivine populations.

The short run durations and sub-liquidus heating of the experiments resulted in the formation of a relatively large number of relatively small crystals (i.e. commonly ≤15 m long). As the run durations are shorter than crystallization times in typical natural systems, we anticipate that primary olivine crystals of most natural systems will have significantly larger crystal sizes than those formed in our experiments. Moreover, textures of partial recrystallization as those observed for restitic olivine in our melting experiments may not be preserved for significantly longer reaction times than those of our experiments. Possible effects of continued crystallization and ripening on olivine textures in natural systems need to be furthermore considered.

The constant experimental heat supply may have also affected the formation of specific olivine textures by controlling the rate limiting process in olivine crystallization (e.g., diffusive transfer of components versus growth of dendritic branches through melt boundary layers). These limitations imply that the textures of experimental and natural olivine crystallized at the same temperature, pressure, and H 2 O conditions will likely differ, but we maintain that the textures of experimentally produced olivine can guide the interpretation of natural examples if they are sensibly interpreted.

In the following, we first discuss characteristic and potentially distinguishing textures of secondary versus primary magmatic olivine as summarized in Table 4, and then conclude with the discussion of potential natural examples.

Characteristic textures of recrystallized olivine

Characteristic textures of the partially recrystallized olivine crystals in our melting experiments are: (i) the crystallographically continuous, dendritic rims on restitic cores that locally contain healed microfractures and inclusions; (ii) branches with small to large 2D surface areas compared to peritectic olivine; (iii) branches with short prismatic habits; (iv) outer branches with preferred shape orientation subparallel to the margin of the crystals; and

(v) commonly oblique orientations between the major axes of elongation of the restitic fragments and their branches (Table 4; Fig. 6). The textures of the partially recrystallized olivine in our experiments closely compare to those reported for recrystallized olivine from previous experimental studies of [START_REF] Tsuchiyama | Experimental study of olivine-melt reaction and its petrological implications[END_REF] and [START_REF] Boudier | Olivine xenocrysts in picritic magmas[END_REF] as summarized in Figure 1. The crystallographically continuous character of the recrystallized olivine branches unequivocally distinguishes them from clusters of peritectic olivine (e.g., Fig. 6a versus Fig. 10a). However, the continuous optical character and other features of the recrystallized, dendritic olivine closely compare those of primary magmatic dendrites. In the following, we therefore evaluate the potential of various textures to distinguish dendritic olivine of secondary, recrystallized and primary magmatic origins.

Inclusions and deformation features

Inclusions and deformation features in restitic cores of partly to largely recrystallized olivine may be a useful criterion for identifying xenocrysts as previously suggested (e.g., [START_REF] Moore | Unusual olivine zoning: evidence for complex physicochemical changes during the evolution of olivine melilitite and kimberlite magmas[END_REF][START_REF] Boudier | Olivine xenocrysts in picritic magmas[END_REF][START_REF] Dunworth | Olivine melilitites of the SW German Tertiary Volcanic Province: Mineralogy and petrogenesis[END_REF][START_REF] Dungan | Partial assimilative recycling of the mafic plutonic roots of arc volcanoes: An example from the Chilean Andes[END_REF]).

However, it has also been argued that microfractures may form by magmatic deformation or around inclusions [START_REF] Natland | Capture of helium and other volatiles during the growth of olivine phenocrysts in picritic basalts from the Juan Fernandez Islands[END_REF]Welsch et al., 2013). Infilling of skeletal cavities or space between dendritic, primary magmatic crystals during late-stage crystallization may further mimic healed microfractures [START_REF] Maaløe | Olivine phenocrysts of Hawaiian olivine tholeiite and oceanite[END_REF][START_REF] Roeder | Cotectic proportions of olivine and spinel in olivine-tholeiitic basalt and evaluation of pre-eruptive processes[END_REF], while primary magmatic dendritic growth has been invoked to explain sub-grain textures and orientation mismatch between dendritic branches [START_REF] Natland | Capture of helium and other volatiles during the growth of olivine phenocrysts in picritic basalts from the Juan Fernandez Islands[END_REF][START_REF] Sunagawa | Crystals: growth, morphology and perfection[END_REF]Welsch et al., 2013).

Abundant fluid-filled nanopores present in some restitic cores of the recrystallized olivine in our experiments (e.g., Fig. 6 e,f) are interpreted as relics of decomposed iddingsite.

Inclusions of this type are obvious and are important evidence for the replacement of an altered or hydrous precursor mineral. However, recrystallized antecrysts and most xenocrysts will be devoid of such inclusions or healed microfractures. Euhedral spinel inclusions within some recrystallized olivine branches of our charges (e.g., Fig. 6i) could be derived from the decomposition of original spinel inclusions in contact with melt. It is more likely though that they have formed by dissolution of Cr-rich olivine, orthopyroxene, amphibole, and/or phlogopite and local supersaturation as the spinel crystals show a relatively homogeneous distribution within secondary olivine and its interstitial glass (Fig. 5b). However, supersaturation and crystallization with entrapment of inclusions may equally relate to primary magmatic olivine growth [START_REF] Maaløe | Olivine phenocrysts of Hawaiian olivine tholeiite and oceanite[END_REF][START_REF] Roeder | Cotectic proportions of olivine and spinel in olivine-tholeiitic basalt and evaluation of pre-eruptive processes[END_REF]Welsch et al., 2013). In agreement with previous studies we therefore conclude that inclusions in olivine may record important information on crystallization processes, but that they rarely provide unequivocal evidence for a specific origin of their host crystal.

The size and GBS length of dendritic branches

The range in the 2D surface area of the recrystallized olivine branches produced in experiments compares to the size of branches produced in previous experiments (Figs 1 and6), but also overlaps with sizes of experimentally crystallized primary magmatic olivine (Figs 13 and 14; Table 3). Moreover, dendritic olivine of unequivocally primary origin (e.g., in crusts of pillow basalts or spinifex layers of komatiites) forms micrometre to decimetre-size crystals (e.g., [START_REF] Donaldson | An experimental investigation of olivine morphology[END_REF][START_REF] Natland | Capture of helium and other volatiles during the growth of olivine phenocrysts in picritic basalts from the Juan Fernandez Islands[END_REF][START_REF] Faure | Formation of spinifex texture in komatiites: An experimental study[END_REF][START_REF] Mattioli | Rapid ascent and contamination of magmas through the thick crust of the CVZ (Andes, Ollagüe region): Evidence from a nearly aphyric high-K andesite with skeletal olivines[END_REF]. The size of dendritic olivine is therefore not a useful criterion to per se discriminate olivine of various origins. However, as for recrystallized, cellular-textured plagioclase (cf. [START_REF] Tsuchiyama | Dissolution kinetics of plagioclase in the melt of the system diopside-albite-anorthite, and origin of dusty plagioclase in andesites[END_REF][START_REF] Johannes | Partial melting reactions of plagioclase and plagioclase-bearing systems[END_REF], we anticipate (i) that the size of recrystallized olivine branches will show a limited range, likely on the scale of micrometres to millimetres; and (ii) that the largest branches will form at high temperature, volatile-rich crystallization conditions, and for originally Fo-rich restitic olivine. For example, we interpret the small recrystallized branches formed in our low-temperature MN melting experiments to reflect the recrystallization of partially altered olivine (iddingsite rims) and the larger subunits to reflect slower dissolutionrecrystallization of unaltered olivine in the high-temperature experiments. We therefore suggest that the branch size of dendritic olivine is not a discriminating feature to determine olivine origin, but that branch sizes of secondary olivine may yield important information on recrystallization conditions and the composition of precursor crystals. The length of GBSs of the different olivine types also closely agree, and therefore cannot be used to infer their origins. However, the GBS lengths vary with experimental temperature and H 2 O conditions (Fig. 14f,i), and they therefore also record important information on crystallization conditions.

The shape of dendritic crystals

The range in morphologies of the partially recrystallized restitic olivine crystals in our experiments reflects the use of rock-and crystal-fragments as starting materials.

Recrystallized xenocrysts and antecrysts derived from fracturing of wall rocks, cumulates, and crystal mush in natural systems may likewise show a large range in shape and sizes. However, irregularly-shaped primary magmatic olivine dendrites may also form, reflecting growth in variable sub-environments or on an irregular substrate (e.g., spinifex crystals in komatiite flows; e.g., [START_REF] Faure | Formation of spinifex texture in komatiites: An experimental study[END_REF]. It is also possible that fragmentation during magmatic flow produces irregularly-shaped primary magmatic olivine dendrites (e.g., Clague & Denglinger, 1994;[START_REF] Natland | Capture of helium and other volatiles during the growth of olivine phenocrysts in picritic basalts from the Juan Fernandez Islands[END_REF]. However, it should be feasible to determine whether dendritic crystals fractured, or whether fractured crystals recrystallized into dendritic crystals (e.g., by characterizing zoning patterns). While we conclude that irregular shapes of dendritic olivine provide no unequivocal evidence for secondary versus primary magmatic origins, we consider a detailed analysis of crystal shapes as an important guide for interpreting their crystallization history and origin.

The shape of dendritic branches

The short prismatic shape of recrystallized olivine branches that is characteristic for crystals in our experiments has been previously described by [START_REF] Boudier | Olivine xenocrysts in picritic magmas[END_REF] (Fig. 1b,c), who suggested that this morphology is typical for recrystallized olivine, but not necessarily evidence for a secondary origin. We concur with this conclusion, but stress that short prismatic branches are a common feature of secondary olivine dendrites, while they appear to be a rare feature of primary magmatic dendrites (e.g., morphologies shown in Fig. 1 versus Fig. 11 or examples shown by [START_REF] Henderson | An investigation of olivine crystal growth in a picrite dyke, using the fission track method[END_REF][START_REF] Faure | Formation of spinifex texture in komatiites: An experimental study[END_REF][START_REF] O'driscoll | An origin for harrisitic and granular olivine in the Rum Layered Suite, NW Scotland: a crystal size distribution study[END_REF]. We therefore suggest that short prismatic branches of dendritic olivine should be considered as possible evidence for a secondary magmatic origin and that particular attention should be paid in the interpretation of olivine phenocrysts with such morphologies.

Optical orientation of dendritic branches

The parallel optical orientation of the main dendritic branches is a common feature of primary and secondary magmatic olivine (e.g., Figs 1a-c and11). However, dendritic olivine of both origins may also show some optical orientation mismatch between branches. Minor orientation mismatch between neighbouring branches of recrystallized olivine has been described by [START_REF] Boudier | Olivine xenocrysts in picritic magmas[END_REF]her Fig. 2b). Orientation mismatch and curvature of branches in primary magmatic olivine has also been documented, but curved branches commonly characterize dendrites crystallized at large degrees of supercooling (e.g., the feather dendrite shown in Fig. 11 g). They are characterized by highly elongated branches and should thus be distinguishable from the short prismatic branches of recrystallized olivine.

Shape anisotropy of dendritic branches

In our experiments, we further observe that the outer branches of recrystallized olivine commonly show a preferred shape orientation subparallel to the margin of the crystal (e.g., Fig. 6i). For plagioclase, similar textures have been observed, reflecting recrystallization subparallel to the dissolution surface of the crystals (e.g., [START_REF] Johannes | Partial melting reactions of plagioclase and plagioclase-bearing systems[END_REF][START_REF] Tröger | Optische Bestimmung der gesteinsbildenden Minerale[END_REF]g., their Fig. 17b). The major axes of elongation of the restitic fragments and their dendritic branches also commonly appear to have oblique orientations. While the recrystallization of irregular olivine fragments should lead to such shape anisotropy between crystals and their branches, it is nontrivial to distinguish true anisotropy from simple sectioning effects (e.g., in our experimental charges an insufficient number of crystals is exposed for statistical analysis). Moreover, primary magmatic olivine dendrites may also show such anisotropies, if they have grown on an irregular substrate, or if they have fractured following initial growth. A detailed analysis of zoning patterns should help to distinguish such primary from secondary growth, but any anisotropy of olivine dendrites needs to be cautiously interpreted.

Characteristic textures of peritectic olivine

Characteristic textures of the peritectic olivine crystals in our melting experiments are: (i) the occurrence in clusters of randomly to sub-radially oriented crystals that commonly show olivine-rich rims and olivine-poor interiors; (ii) the presence of sub-groups of touching crystals that form compact to branch-like groups, where grain boundaries are often invisible in BSE images; and (iii) distinct crystal sizes and habits for peritectic crystals formed after various precursor minerals (Figs 3 and 10). Their textures compare to those of peritectic crystals formed in other dissolution and melting experiments performed at different pressure and temperature conditions and with variable starting materials [START_REF] Brearley | Dissolution rates of upper mantle minerals in an alkali basalt melt at high pressure: An experimental study and implications for ultramafic xenolith survival[END_REF][START_REF] Erdmann | Textures of peritectic crystals as guides to reactive minerals in magmatic systems: new insights from melting experiments[END_REF][START_REF] Jacobs | Orthopyroxene stability within kimberlite magma: an experimental investigation[END_REF]. The peritectic crystal clusters are easily distinguished from primary and secondary magmatic dendritic crystals by their random orientation, but they may be mistaken for primary magmatic glomerocrysts. Moreover, peritectic crystals dispersed by magmatic flow may texturally resemble polyhedral primary magmatic crystals. et al. (2012) have suggested that peritectic olivine formed after orthopyroxene, amphibole, and phlogopite can be distinguished from each other on the basis of olivine/glass proportion in clusters, the size of the peritectic crystals, and their GBS lengths. Here, we note that their shape may be a further distinguishing criterion, ranging from acicular for olivine that replaced phlogopite (Figs 8i, 10e,f) to short prismatic forms for olivine that replaced orthopyroxene and amphibole (Figs 8d-h, 10a-d). That the boundaries of touching crystals are commonly invisible in BSE imaging mode (e.g., Figs 4b,d,10d) may indicate that these clusters commonly contain twins, but it could also reflect polishing effects.

Peritectic crystal cluster

Erdmann

The presence of olivine clusters with variable textures in natural samples may point towards the presence of peritectic crystals replacing more than one primary phase, but texturally variable glomerocrysts or peritectic and glomerocrystic crystal cluster may also be juxtaposed. It is notable that several of the experimental peritectic crystal clusters are zoned from relatively olivine-rich rims to glass-rich interiors with few touching crystals (e.g., Fig. 10a,b), reflecting nucleation of the peritectic crystals on the exterior of the decomposing phases with inwardly directed dissolution and crystallization. Glomerocrysts, in contrast, typically show outwardly directed growth or random clustering, where crystals commonly touch in the central zones of clusters (e.g., [START_REF] Jerram | Quantifying the building blocks of igneous rocks: are clustered crystal frameworks the foundation[END_REF].

Dispersed peritectic crystals

Dispersed subgroups or individual peritectic crystals in natural samples will texturally resemble primary magmatic olivine. The size, shape, or length of GBSs cannot per se be used to distinguish between peritectic and primary magmatic crystals (e.g., Fig. 14). However, the occurrence of xenocrysts or antecrysts may point towards their presence. We moreover consider a textural comparison of potential peritectic crystals and xenocrysts as a useful tool to infer olivine origin. For example, the size of recrystallized olivine branches is larger, while their shape and GBS lengths closely compare to those of experimentally produced peritectic olivine formed after orthopyroxene. If recrystallized olivine can be identified, its textures may guide the interpretation of other olivine populations in the same samples, while a combination with detailed compositional and zoning pattern analysis seems to have the best potential to distinguish peritectic from primary magmatic olivine.

In the following, we discuss possible natural occurrences of secondary magmatic olivine from natural rocks. We first report examples from the Tatara-San Pedro volcanic complex in Chile using our own data. We then review possible literature examples from picrites.

POSSIBLE NATURAL EXAMPLES

Olivine phenocrysts in lavas of the Tatara-San Pedro volcanic complex

Texturally variable olivine phenocrysts are common in many mafic to ultramafic igneous rocks (e.g., [START_REF] Helz | Diverse olivine types in lava of the 1959 eruption of Kilauea volcano and their bearing on eruption dynamics[END_REF][START_REF] Boudier | Olivine xenocrysts in picritic magmas[END_REF][START_REF] Costa | Short time scales of magmatic assimilation from diffusion modeling of multiple elements in olivine[END_REF][START_REF] Natland | Capture of helium and other volatiles during the growth of olivine phenocrysts in picritic basalts from the Juan Fernandez Islands[END_REF]Welsch et al., 2013). For instance, basaltic-andesitic and andesitic lavas of the Tatara-San Pedro (TSP) volcanic complex contain texturally and compositionally complex populations of olivine xenocrysts (Fig. 15; [START_REF] Dungan | Partial assimilative recycling of the mafic plutonic roots of arc volcanoes: An example from the Chilean Andes[END_REF][START_REF] Costa | Short time scales of magmatic assimilation from diffusion modeling of multiple elements in olivine[END_REF]. In addition, the presence of abundant peritectic olivine has been inferred [START_REF] Erdmann | Xenocryst assimilation and formation of peritectic crystal clusters during magma contamination: an experimental study[END_REF][START_REF] Erdmann | Textures of peritectic crystals as guides to reactive minerals in magmatic systems: new insights from melting experiments[END_REF].

The inferred peritectic olivine forms small clusters or individual crystals (Fig. 15a).

The crystals are characterized by a lack of deformation features that are common in olivine xenocrysts, but they commonly show crystallized, exotic melt film pseudomorphs that are interpreted to have formed by partial melting of contaminants (cf. [START_REF] Dungan | Partial assimilative recycling of the mafic plutonic roots of arc volcanoes: An example from the Chilean Andes[END_REF]. The euhedral to subhedral shape of the olivine crystals mimics the shape of experimentally produced peritectic olivine, but they are significantly larger than the experimentally produced crystals. Moreover, reactive minerals including amphibole, phlogopite, and orthopyroxene are common in xenoliths enclosed in dacitic TSP lavas [START_REF] Costa | Hornblende-and phlogopite-bearing gabbroic xenoliths from Volcan San Pedro (36°S), Chilean Andes: Evidence for melt and fluid migration and reactions in subduction-related plutons[END_REF], but largely absent from xenocrysts assemblages. The main population of the peritectic olivine in our sample set ("Type-2 Ol" of [START_REF] Erdmann | Textures of peritectic crystals as guides to reactive minerals in magmatic systems: new insights from melting experiments[END_REF] is inferred to have formed after orthopyroxene. New shape data determined using the CSDslice spreadsheet indicate that the crystals approach short prismatic shapes with axial ratios of ~1.0:1.3:1.6 (R 2 ~0.89), which is consistent with olivine formation after orthopyroxene. In our sample set, the peritectic crystals also have lower Fo core compositions than the xenocrysts (Fig. 15). Apparent olivine xenocrysts in our TSP sample set range from macro-to megacrysts (Fig. 15a-c). Most of the olivine crystals show rounded to euhedral grain boundary segments and euhedral-subhedral protrusions (Fig. 15b), which we interpret to reflect marginal dissolution and recrystallization. Few olivine megacrysts show irregular, skeletal to dendritic textures (e.g., Fig. 15c) with short prismatic subunits or branches, which we interpret as partially recrystallized xenocrysts. Rare skeletal-dendritic olivine also occurs in xenoliths, and the megacrysts could thus have recrystallized during xenolith entrainment or subsequent dispersal in the magmas. They have textures and core compositions that compare to those of olivine from cumulate xenoliths, and they locally show healed microfractures that are also typical for the xenolithic olivine that commonly do not extend into rim zones of the crystals.

Olivine dendrites and cluster in mafic-ultramafic igneous rocks: literature examples

Studies that have previously suggested the presence of secondary, recrystallized olivine in ophiolites, melilites, kimberlites, and basantite are those of [START_REF] Boudier | Olivine xenocrysts in picritic magmas[END_REF], [START_REF] Dunworth | Olivine melilitites of the SW German Tertiary Volcanic Province: Mineralogy and petrogenesis[END_REF], [START_REF] Ninomiya | Polygenetic olivine phenocrysts in Okete basanite, New Zealand[END_REF][START_REF] Ninomiya | Polygenetic olivine phenocrysts in Okete basanite, New Zealand[END_REF][START_REF] Arndt | Olivine, and the origin of kimberlite[END_REF]. [START_REF] Dunworth | Olivine melilitites of the SW German Tertiary Volcanic Province: Mineralogy and petrogenesis[END_REF] have interpreted olivine crystals with mottled textures to record possible replacement of initial xenocrystic cores. [START_REF] Ninomiya | Polygenetic olivine phenocrysts in Okete basanite, New Zealand[END_REF] have presented a detailed investigation of olivine types produced by reaction between peridotite xenoliths and basanite magma. They show that relatively small, subhedral to euhedral, and strain-free olivine crystals with minute spinel inclusions (their olivine phenocrysts I) formed in a reaction between orthopyroxene and melt (Fig. 16a), while olivine with bladed to dendritic rims (their olivine phenocrysts II) has been interpreted to have formed in a reaction between olivine xenocrysts and host melt.

The textures they describe strikingly resemble those of secondary olivine formed in our and previous experiments, and they also closely compare to textures of olivine phenocrysts that have been interpreted as primary magmatic (Fig. 16). [START_REF] Boudier | Olivine xenocrysts in picritic magmas[END_REF] and [START_REF] Arndt | Olivine, and the origin of kimberlite[END_REF] have interpreted subhedral to euhedral, commonly strain-free olivine "tablets" to have formed by recrystallization of deformed dunitic xenoliths and/or deformed olivine xenocrysts. An important point to note is that the olivine "tablets" they describe form single crystals with random orientations, while experimentally recrystallized olivine forms single crystals with dendritic branches. We consider it possible that the recrystallization of large, strongly deformed olivine xenocrysts as those described in the two studies may have resulted in the formation of randomly oriented, single olivine grains. However, it also seems possible that the natural olivine crystals formed by peritectic reaction instead of recrystallization (i.e. that they largely replaced orthopyroxene or clinopyroxene that were originally intergrown with the deformed olivine crystals).

Natural olivine crystals with textures that, in our opinion, also closely compare to those of secondary olivine formed in melting experiments and to those of secondary olivine described by [START_REF] Ninomiya | Polygenetic olivine phenocrysts in Okete basanite, New Zealand[END_REF] (Fig. 16a), have been described as primary magmatic from Kilauea volcano on Hawaii (e.g., [START_REF] Helz | Diverse olivine types in lava of the 1959 eruption of Kilauea volcano and their bearing on eruption dynamics[END_REF][START_REF] Schwindinger | Synneusis of Kilauea Iki olivines[END_REF][START_REF] Garcia | Petrography and olivine and glass chemistry of lavas from the Hawaii Scientific Drilling Project[END_REF]Vinet & Higgins, 2010), Piton de la Fournaise volcano on La Réunion [START_REF] Albarède | Modelling the recent geochemical evolution of the Piton de la Fournaise volcano, Réunion Island, 1931-1986[END_REF]Welsch et al., 2013), and several other locations worldwide (e.g., [START_REF] Larsen | Processes in high-Mg, high-T magmas: Evidence from olivine, chromite and glass in Palaeogene picrites from West Greenland[END_REF][START_REF] Natland | Capture of helium and other volatiles during the growth of olivine phenocrysts in picritic basalts from the Juan Fernandez Islands[END_REF]Roedder et al., 2006) (Fig. 16b-g). We do not attempt to evaluate the origin of olivine phenocrysts from the mentioned examples, but we would like to highlight their striking textural similarity with secondary olivine formed in melting experiments (Fig. 1 and Fig. 16) and urge that future studies consider primary as well as secondary origins for their formation. Except for their significantly larger size and tendency to show euhedral rather than subhedral-euhedral branches, olivine described as skeletal to dendritic compares to recrystallized olivine (Fig. 16b,d,f), while clusters of randomly oriented, euhedral to subhedral crystals compare to experimentally produced peritectic crystal cluster (Fig. 16c,e,g). The size difference may reflect kinetic controls and/or differences in crystallization time, while the euhedral character may reflect continued crystallization and/or ripening during prolonged magma residence. The close spatial association of the two olivine types is easily reconciled with a secondary origin, while mafic cumulates that may have been recycled to form the secondary olivine types are also known from most locations (e.g., [START_REF] Rançon | The Grand Brûlé exploration drilling: new data on the deep framework of the Piton de la Fournaise volcano. Part 1: Lithostratigraphic units and volcanostructural interpretation[END_REF][START_REF] Garcia | Petrography and olivine and glass chemistry of lavas from the Hawaii Scientific Drilling Project[END_REF][START_REF] Upton | Cumulate clasts in the Bellecombe Ash Member, Piton de la Fournaise, Réunion Island, and their bearing on cumulative[END_REF][START_REF] Helz | Processes active in mafic magma chambers: The example of Kilauea Iki Lava Lake, Hawaii[END_REF].

CONCLUSIONS

Primary as well as secondary magmatic growth produces dendritic to polyhedral olivine in experiments with textures that closely compare to each other. We suggest that (i) the shape and orientation of dendritic olivine crystals and their branches and (ii) the cluster shape and crystal distribution in clusters may be the best guides to distinguish recrystallized and peritectic olivine from primary magmatic crystals. Comparing textures of the experimental crystals with those of olivine from natural samples, we propose that dendritic phenocrysts and clusters of polyhedral crystals that are common in mafic igneous rocks worldwide should be evaluated for primary and potential secondary origins. Typical zoning patterns of recrystallized and peritectic olivine crystals or sub-microscopic textures remain unconstrained, but we suspect that they may reveal additional criteria to distinguish olivine of primary versus secondary origins. The effects of parameters such as temperature, magmatic H 2 O contents, and crystallization times on olivine textures need further detailed investigation in experiments and through numerical modelling.

FUNDING

The experimental work carried out by S.E. was supported by a postdoctoral fellowship of the "Deutscher Akademische Austauschdienst" (DAAD, German Academic Exchange Service).

We would also like to thank the Dalhousie University Electron Microprobe facility for generous access. and peritectic crystals formed after orthopyroxene and amphibole (d-g) are short prismatic, while olivine replacing phlogopite is acicular (i). n= number of analyses; R 2 = calculated reliability of the shape estimate. For comparison, typical axial ratios of forsterite-rich, polyhedral olivine are ~1.5:1:2 for a-, b-, and c-axes [START_REF] Tröger | Optische Bestimmung der gesteinsbildenden Minerale[END_REF][START_REF] Deer | An introduction to the rock forming minerals[END_REF], while elongated and dendritic crystals tend to have tabular shapes (a>c>>b) [START_REF] Donaldson | An experimental investigation of olivine morphology[END_REF]. Ol-RC = recrystallized Ol; Ol-P = peritectic Ol formed after amphibole (Amp), orthopyroxene (Opx), and phlogopite (Phl); Ol-PM = primary magmatic Ol; P = polyhedral; D = dendritic; E = elongated. n = number of analyses;  standard deviation; d = standard error (2*/n 1/2 ). min = minor, int = intermediate, and maj = major axes. * Max 2D elongation, i.e. "maximum" major/minor axial ratios were determined using the ten largest values derived from our image analysis

Figure 1 :

 1 FIGURE CAPTIONS:

Figure 2 :

 2 Figure 2: Summary of olivine occurrence and abundance (a,b) in melting experiments shown

Figure 3 :

 3 Figure 3: Summary of olivine subtypes present in our melting experiments. a, b, c = olivine

Figure 4 :

 4 Figure 4: Example of (a) olivine classification (b-e) and textural analysis. (b-d) Olivine

Figure 5 :

 5 Figure 5: Microphotographs and BSE images showing typical phase distributions and the

Figure 6 :

 6 Figure 6: Transmitted light microphotographs and BSE images of recrystallized olivine in

Figure 7 :

 7 Figure 7: Summary of 2D surface area data for (a-c) branches of recrystallized olivine, and

Figure 8 :

 8 Figure 8: Summary of 3D shape analysis for (a-c) branches of recrystallized olivine, and (d-

Figure 9 :

 9 Figure 9: Summary of 2D GBS length patterns for (a-c) branches of recrystallized olivine,

Figure 10 :

 10 Figure 10: Transmitted light microphotographs (a,c,e) and BSE images (b,e,f) of peritectic

Figure 11 :

 11 Figure 11: Line drawings of primary magmatic olivine formed in crystallization experiments

Figure 12 :

 12 Figure 12: Transmitted light (a,b) and BSE images (c) of primary magmatic olivine dendrites

Figure 13 :

 13 Figure 13: Representative line drawings and shape analysis of primary magmatic olivine

Figure 14 :

 14 Figure 14: Variation of olivine textures with experimental (a-c) run time, (d-f) temperature,

Figure 16 :

 16 Figure 16: Line drawings of dendritic olivine and clusters of polyhedral olivine from

  3D and 2D shape estimates for Ol branches (Ol-RC and Ol-PM) and crystals (Ol-

Table 4 : Summary of characteristic textures and their proposed potential to distinguish olivine of recrystallized, peritectic, and primary magmatic origins

 4

	Olivine types	Secondary magmatic olivine	Primary magmatic olivine	Textural
				potential
		Dendritic, recrystallized olivine	Dendritic, primary olivine (Ol-PM)
	Crystal size	Small to large	Small to large	*
	Inclusions	Fluid, glass, minerals possible	Fluid, glass, minerals possible	*
	Crystal shape	Euhedral to highly irregular	Elongated, possibly irregular	**
	Branch shape	Short prismatic	Elongated prismatic to tabular	***
	Elongation of	Parallel to oblique, commonly oblique	Commonly parallel, oblique for highly	**
	crystals and main	for recrystallized crystal fragments	elongated branches
	dendritic branches
	Olivine types	Peritectic, polyhedral olivine	Primary magmatic, polyhedral olivine	Textural
				potential
	Crystal size	Small to large	Small to large	*
	Inclusions	Fluid, glass, minerals possible	Fluid, glass, minerals possible	**
	Crystal shape	Euhedral-subhedral, short prismatic to	Euhedral-subhedral, short prismatic	*
		elongated	to elongated
	Cluster shape	Highly irregular, commonly crystal-poor	Highly irregular, commonly crystal-	**
		cores	rich cores
	Crystal orientation	Subradial to random, crystals rarely	Subradial to random, crystals
	in clusters	touch in center of clusters	commonly touch in center of clusters

**

Textures with low (*) to high (***) estimated potential to distinguish olivine of different origins

ACKNOWLEDGEMENTS

We greatly appreciate the detailed and constructive reviews by Benoit Welsch, Thomas Müller, and Nick Arndt which have helped us significantly to improve the presentation of our findings. We would like to thank Rémi Champallier, Philippe Teulat, and Didier Bellenoue for support with the experiments and Mike Dungan for sharing samples from the Tatara-San Pedro complex.


Salima Mosbah
email: salimacne@yahoo.fr

Ammar Khelifa Beghdouche

Lotfi Benmekhbi

Joëlle Rault Berthelot

Leila Bencharif

Comparative Electronic Properties of Two Arylene-Cyanovinylene Isomers and of Their Respective Electrogenerated Polymers

Keywords: Electropolymerization, Electroactive polymer, p-doping process, n-doping process, arylene-dicyanovinylene

Two arylene-cyanovinylene isomers formed by a phenyl unit substituted in para or meta positions by two cyanovinylene-methylthiophene were synthesized. Their main physicochemical properties were studied in order to point out the influence of the nature of the substitution (para vs meta) of the central phenyl unit on the electronic properties of the two isomers. The anodic electropolymerization of the two compounds was also studied with a main emphasis on the electrochemical behavior of the derived polymers showing that the specific behavior of the para-substituted isomer is maintained in its polymer leading to a polymer with a lower bandgap (1.95 eV) than the polymer obtained from the meta-substituted isomer (2.16 eV).

INTRODUCTION

-conjugated oligomers and polymers constitute an important class of functional materials in organic electronics (OE). They are used as active layers in organic light-emitting diodes (OLEDs), organic field-effect transistors (OFETs) or in organic solar-cells (OSC). In this latter application, efficient materials usable as p-conducting layer in bulk-heterojunction (BHJ) solar cells in association with fullerene derivatives as n-conducting materials are needed. [1,2] In fact, a material with a bandgap of 1.1 eV is able to absorb 77 % of the solar irradiation, however, semiconducting polymers or oligomers have bandgaps higher thant 2 eV and can then harvest only 30 % of the solar photons. In this field, a lot of work has been done during the last forty years to synthesize conducting polymers with very low bandgap and especially those with thiophene units. [3,4] In this area, Roncali determined five contributions that may modify -conjugated systems: [4](i) the energy related to the bond length alternation (E ), the deviation form planarity (E ), the aromatic resonance energy of the cycle (E Res ), the inductive or mesomeric electronic effects of eventual substituent (E Sub ) and the intermolecular or interchain coupling in the solid state (E int ). For example, it has been shown that polythienylenevinylene (PTV) has a lower bandgap (around 1.8 eV) [5] than polythiophene (PT) (2.2 eV). [6] The introduction in the polymer chain of the vinylic -conjugated backbone and limits the rotational disorder which plays a major role in the magnitude of the bandgap in PT. Later on, it was shown that the introduction of electron withdrawing cyano groups at the ethylene linkage leads a considerable decrease of the bandgap which reaches very low values. [7] In the present work, we studied the electrochemical behavior and the anodic electropolymerization of two arylene-cyanovinylene isomers I and II (see chart 1) consisting of a central phenyl unit substituted in para or meta positions by two cyanovinylene-methylthiophene. A specific outlook is done on the influence of the phenyl substitution (para vs meta) on the electronic properties of the polymers and their precursors.

Chart 1. Structure of the two isomers studied in this work

EXPERIMENTAL SECTION

Reagents and solvents

3-Methyl-2-thiophenecarbaldehyde, 1,3-and 1,4-phenyldiacetonitrile and potassium tertbutoxyde (t-BuOK), tetrabutylammonium hexafluorophosphate (Bu 4 NPF 6 ) and dichloromethane (CH 2 Cl 2 ), all commercially available, were used without further purification. Electrolytic solution (Bu 4 NPF 6 0.2 M in CH 2 Cl 2 ) was dried in presence of Aluminum oxide (Al 2 O 3 ) previously heated at 450°C under vacuum during 24h and keep under argon atmosphere.

Monomers synthesis

The Knoevenagel condensation [8] of 3-Methyl-2-thiophenecarbaldehyde with 1,3-or 1,4phenyldiacetonitrile in the presence of t-BuOK in C 2 H 5 OH gave the two isomers I or II respectively (see scheme 1).

Scheme 1.

In a typical experiment, 3-methylthiophenecarbaldehyde (2 eq) was added to phenyldiacetonitrile (1.15 eq) in ethanol followed by the addition of potassium tert-butoxide (0.07 eq). A yellow (isomer I) and brown (isomer II) precipitate separates immediately, but the solution was stirred for an other hour. Finally, the precipitate was filtered, washed with ethanol and dried giving a yellow or brown solid. Literature reports that the base-catalyzed reaction of aldehydes with acetonitriles forms cyano compounds only of Z-configuration. [8] (2Z,2'Z)-2,2'-benzene-

2.3.Electrochemical studies

Electrochemical experiments were performed under argon atmosphere using a Pt disk electrode (diameter 1 mm), the counter electrode was a vitreous carbon rod and the reference electrode was a silver wire in a 0.1M AgNO 3 solution in CH 3 CN. Ferrocene was added to the electrolyte solution at the end of a series of experiments. The ferrocene/ferrocenium (Fc/Fc + ) couple served as internal standard. The three electrodes cell was connected to a PAR Model 273 potentiostat/galvanostat (PAR, EG&G, USA) monitored with the ECHEM Software (in Rennes) or to a Voltalab PGZ 301 (in Constantine). Activated Al 2 O 3 was added in the electrolytic solution to remove excess moisture. For a further comparison of the electrochemical and optical properties, all potentials are referred to the SCE electrode that was calibrated at -0.405 V vs. Fc/Fc + system. Following the work of Jenekhe, [9] we estimated the electron affinity (EA) or lowest unoccupied molecular orbital (LUMO) and the ionisation potential (IP) or highest occupied molecular orbital (HOMO) from the redox data. The LUMO level was calculated from: LUMO (eV)= -[E onset red (vs SCE) +4.4] and the HOMO level from: HOMO (eV)

= -[E onset ox (vs SCE) + 4.4], based on an SCE energy level of 4.4 eV relative to the vacuum. The electrochemical gap was calculated from: ΔE el =|HOMO-LUMO| (in eV).

For large scale polymer film production for the spectroscopic characterization, platinum or ITO glass electrodes with a larger surface area were employed as working electrodes and potensiostatic method at properly chosen potential was used.

Characterization

IR spectra were recorded using FTIR-8201PC SHIMADZU spectrophotometer; the monomers and polymers were mixed with KBr powder. The absorption frequencies are in cm -1 .

UV-vis spectra were recorded using a Helios a-Unicam Spectronic spectrophotometer. Optical opt opt 1 H NMR spectra of monomers were recorded on a BRUKER ADVANCE DPX 250 (250Hz).

Chemical shifts are expressed in ppm compared to TMS. The following abbreviations have been used for the NMR assignment: s for singlet, d for doublet, t for triplet and m for multiplet.

Theoretical calculations

Full geometry optimization with Density Functional Theory (DFT) [10,11] and Time-Dependent Density Functional Theory (TD-DFT) calculations were performed with the hybrid Becke-3 parameter exchange [12][13][14] functional and the Lee-Yang-Parr non-local correlation functional [15] (B3LYP) implemented in the Gaussian 09 (Revision B.01) program suite [START_REF] Frisch | Gaussian 09, Revision B.01[END_REF] using the 6-311G+(d,p) basis set and the default convergence criterion implemented in the program. The figures were generated with GaussView 5.0.

RESULTS AND DISCUSSION

Electrochemical behavior of the two isomers I and II

Figure 1 reports the cyclic voltammetry of the two isomers recorded in oxidation and in reduction. Both compounds present an irreversible oxidation wave which maximum is recorded at 1.36 V for I and at 1.45 V for II showing that oxidation of II appears less easy than that of I. Similar shift of the oxidation of arylene-cyanovinylene isomers induced by a central para-or meta-phenyl unit was already reported in literature. [START_REF] Rault-Berthelot | [END_REF] This shift may be explained by a shorter conjugation length in the isomer with meta-substituted phenyl central core as in II. In fact, the character of the HOMO, determined on optimized geometries shows a conjugation extension involving eight double bonds in I and only seven double bond in II (see HOMO character in figure 2). It must be noted that the optimized geometry of the molecules is not totally planar. Due to the presence of the methyl group on the thienyl unit (in position C3), the thienyl is oriented with its sulphur atom pointing in the direction of the cyano group. Moreover, the two thienyl-cyanovinylene planes formed a dihedral angle of ±24° in I and of ±32° in II with the central phenyl plane as depicted in chart 1. In the cathodic range, the two compounds present irreversible reduction waves with maxima at -1.53 V (and -2.29 V not shown) for I and -1.71 V and -2.0 V for II. The 0.18 V shift of the first reduction peak (-1.53 V for I and -1.71 V for II) is twice that recorded for the shift between the first oxidation peaks (1.36 V for I and 1.45 V for II). Here also, the different reductive behavior of the two From the onset of the first oxidation and reduction waves, HOMO and LUMO levels were calculated using the Jenekhe formalism. [9] Compound I with HOMO and LUMO levels calculated at -5.67 eV and -2.95 eV presents a lowest bandgap (ΔE el : 2.72 eV) than compound II (HOMO: -5.7 eV, LUMO: -2.79 V and ΔE el : 2.91 eV). These experimental results follows the same tendency as the one observed for the theoretical HOMO, LUMO and bandgap values calculated after geometry optimization in the singlet state, using Density Functional Theory (DFT) at the Gaussian09 B3LYP/6-311+G(d,p) level of theory presented in figure 2. Isomer I is more easily oxidized and reduced than isomer II, leading to a 0.43 eV contraction of the bandgap from II to I. The two structural variables and both differences due to the different central phenyl unit are two variables that increase the bandgap from I to II.

Electropolymerization processes of the two isomers I and II

Figure 3 shows the cyclic voltammograms recorded during the oxidation of I and II along successive sweeps between 0.36 and 1.67 V for I (left) and between 0.36 and 1.57 V for II (right). Along the recurrent sweeps, one observes the appearance and the regular growth of a new reversible Ian/Icat system at potential less positive than the onset oxidation potential of the respective monomers. At the end of the ten cycles, the electrode surfaces are covered by an insoluble material showing the formation of a polymer. For I, the intensity of the new redox systems Ian (33 µA at cycle 10) is more than six times higher than the intensity recorded at E 1 ox at the first cycle (5.45 µA). For II, Ian at cycle 10 (46 µA) is in the same range as the intensity at E 1 ox at the first cycle (40 µA). This shows that the polymerization process is strongly more efficient for compound I than for compound II. This may be due to the different stability of the cation radical of the two isomers and to the different character of the SOMO of the respective cation radicals. Theoretical calculations were performed on the two radical cations and show that the Single Occupied Molecular Orbital (SOMO) is centered on the whole molecule for I .+ whereas it presents a clear conjugation breaking for II .+ (figure 4). For both compounds, the carbon-carbon coupling may occur on the carbon 5 of the thienyl units. A proposition of mechanism is presented in the following scheme 2.

Scheme 2. Electropolymerization process

In order to obtain larger amount of polymer, for physicochemical analysis (IR, UV…), electropolymerization of I and II was also performed with success along potentiostatic oxidation at a potential, slightly more positive than E 1 ox on working electrode of larger area (platinum, ITO glass electrodes). As the polymers covering the electrode are insoluble in classical solvent, the study of their elctrochemical behaviors were performed in a second electrochemical cell, in absence of monomer, using the previously modified platinum electrodes.

I .+

II .+

Electrochemical behaviour of the derived polymers

As presented figure 5, the modified electrodes were cycled repeatedly between their oxidized and reduced states without significant decomposition of the material as shown by the stable recurrent cycles. As for their respective precursors I and II, the cyclic voltammetries of the polymer allow calculating the HOMO, LUMO and electrochemical bandgap of the derived polymers. Poly(I) with onset oxidation and reduction potentials at 0.68 V and -1.27 V, respectively has its HOMO and LUMO levels at -5.08 and -3.13 eV and therefore an electrochemical bandgap ΔE el of 1.95 eV. Poly(II) is oxidized at higher potential values with onset oxidation at 0.99 V and is reduced at lower potential value with onset reduction at -1.17 V, leading to HOMO and LUMO levels at -5.39 eV and -3.23 eV respectively and a bandgap ΔE el of 2.16 eV higher than the poly(I) bandgap. The polymer bandgap contraction (0.21 eV) is in the same range than the precursors I and II bandgap contraction (0.19 eV). The anodic and cathodic responses of the polymers are associated to the well known p-and ndoping processes of electroactive polymers as presented in scheme 3. [18][19][20][21] Scheme 3. Description of the p-and n-doping processes of poly(I) and poly(II).

In oxidation, the p-doping process corresponds to an electron abstraction from the polymer backbone leading to the formation of holes, the electroneutrality of the p-doped polymer is obtained by insertion of the anion hexafluorophosphate (PF 6 -) in the polymer matrix. Under its p-doped state, mobility of holes is at the origin of the conductivity of the semi-conducting polymer. m, defined as the p-doping level, depends on the potential at which the polymer is oxidized and the p-doping process is reversible if the polymer is not over-oxidized at too positive potential value. In reduction, the n-doping process corresponds to the reduction of the polymer matrix, leading to an excess of electrons in the polymer and accompanied by the insertion of the cation tetrabutylammonium (Bu 4 N + ) for the electroneutrality. Under its n-doped state, the mobility of electrons is at the origin of the conductivity of the semi-conducting polymer. Here, m' is defined as the n-doping level and depends on the potential at which the polymer is reduced. The n-doping process is reversible since the potential do not reach the over-reduction process of the polymer. In the present study, both processes appear reversible at the scale of the cyclic voltammetry between -1.7 V and 1.1 V for poly(I) and between -1.6 V and 1.31 V for poly(II) (see figure 5). FT-IR spectra of the polymers were compared with the one of the monomers. The table 2 reports the respective vibrations of monomer I and poly(I) and monomer II and poly(II). IR spectra of the polymers present the main vibration bands observed in their precursor IR spectra. Particularly, in polymer spectra, the existence of a band at 2130-2200 cm -1 characteristic of the nitrile group is a sign of the conservation of the monomer structure after the polymerization process. As the polymers were obtained as solid film after oxidation at fixed potential, and as the reduction of thick film is not an easy task, the polymer were studied under their p-doped states. Therefore, PF 6 -anion vibration band is visible at about 840 cm -1 and the second band observed at 1470 cm -1 corresponds to the double liaison C-C between two monomeric units. [22] 3.4. Figure 6 presents the UV-visible spectra recorded in dichloromethane of I and II. Both monomers spectra present absorption bands centered at 241 and 391 nm for I and at 277 and 358 nm for II. From the edge of the absorption band measured at 445 and 403 nm, optical bandgaps of both opt is equal to 2.78 nm for I and 3.07 nm for II. The difference of 0.29 eV between the two optical bandgaps is between the one calculated from electrochemical studies (0.19 eV) and of the one obtained from theoretical calculations (0.43 eV). After polymerization on a transparent glass electrode coated by a thin indium-tin oxide (ITO) layer, poly(I) UV spectrum was recorded in the solid state under its p-doped state and after reduction under its neutral state. The neutral polymer spectrum shows a main absorption band centred at 395 nm but with a long tail up to 1000 nm, showing an extension of the conjugation from the monomer to the polymer. The optical bandgap calculated from the onset absorption wavelength (around 825 nm) is around 1.5 eV, more contracted of 1.28 eV than the bandgap of the precursor I showing an important extension of conjugation from I to poly(I). Interestingly, under its p-doped state, this absorption band decreases at the expense of a new absorption band centred at 1050 nm and assigned to bipolarons. In the wavelength range used for this study, one do not observed the edge of the absorption band of the pdoped polymer which therefore presents a large charge transfer character.

Physical characterization of monomers I and II and of their derived polymers

IR spectroscopy

For poly(II), after its synthesis as thick film on a platinum electrode, we attempt to solubilised the polymer in DMSO. Figure 6 presents the UV-visible spectrum of the solubilised fraction of polymer obtained from II. From this spectrum we note a main absorption band centred at 400 nm and a the one recorded for poly(I) in accordance with the results calculated from electrochemical studies. The 0.96 eV bandgap contraction measured between II and poly(II), is less important than the one recorded between I and poly(II) (1.5 eV) showing, as observed in the electrochemical studies, that poly(II) possesses a shorter conjugation length due to nature of the central phenyl ring which induces a conjugation breaking in monomer II as in its derived poly(II).

CONCLUSIONS

In conclusion, the main electrochemical feature of the monomers, lower bandgap for I compared to II, is also observed in their respective polymers, lower bandgap for poly(I) compared to poly(II) showing that the substitution of the central phenyl ring as also an effect on the polymer electrochemical properties. The most important result obtained in this work is the very low bandgap (1.95 eV from electrochemical studies and 1.5 eV from optical studies) recorded for poly(I). Poly(I) possesses the lowest bandgap obtained by our group by anodic oxidation of arylene-cyanovinylene derivatives. [START_REF] Rault-Berthelot | [END_REF]22] This very low bandgap confers to this new polymer a potential interest as psemiconducting layer in BHJ solar-cells. ACKNOWLEGEMENTS Salima Moshba acknowledges the University of Constantine for a grant which allows her to spend three weeks in Rennes to do most of the experiments (synthesis, electrochemical and optical studies) presented in the present publication. J. Rault-Berthelot thanks CINES (Centre d'Informatique National de l'Enseignement Supérieur, Montpellier, France) for computing time.

Figure 1 .

 1 Figure 1. Cyclic voltammetry recorded in CH 2 Cl 2 + 0.2 M Bu 4 NPF 6 of I (left) and II (right) [5 10 -3 M],Sweep-rate: 100 mV/s, working electrode: platinum disk, diameter 1 mm.

Figure 2 .

 2 Figure 2. Plot of the frontier molecular orbitals HOMO and LUMO of I and II (charge zero, singlet with a cut-of of 0.04 [e Bohr -3 ] 1/2 ) and energy levels calculated by DFT after geometry optimization at the B3LYP/6-311G+(d,p) level, (isovalue: 0.04).

  E theo :3.07 eV E theo :3.5 eV isomers comes from their different LUMO characters as presented in figure 2. For isomer I; the LUMO character shows a conjugation all along the aromatic core whereas for isomer II, a conjugation breaking, cleaving the molecule in two parts, is clearly shown as indicated by the dotted line in the LUMO of II. This conjugation breaking leads to a shorter conjugation and a more difficult reduction of II compare to I.

Figure 3 .

 3 Figure 3. Cyclic voltammograms in CH 2 Cl 2 /Bu 4 NPF 6 0.2 M of I [5.10 -3 M], 10 sweeps between 0.36 and 1.67 V (left) and of II [5.10 -3 M], 10 sweeps between 0.36 and 1.57 V . Scan rate: 100 mV.s - 1 .

Figure 4 .

 4 Figure 4. Plot of the frontier molecular orbitals SOMO of I .+ (left) and II .+ (right) (charge +1, doublet with a cut-of of 0.04 [e Bohr -3 ] 1/2 ).

Figure 5 .

 5 Figure 5. Voltammetric responses recorded in Bu 4 NPF 6 0.2 M in CH 2 Cl 2 between -1.73 and 1.08 V for poly(I) (left) and between -1.6 and 1.31 V for poly(II) (right). Scan rate: 100 mV. s -1 .

Figure 6 .

 6 Figure 6. UV-visible spectra in CH 2 Cl 2 of monomers I and II (left) and polymers (I) and (II) (right)

Table 2

 2 FTIR spectroscopy data of Monomers I and II and polymers (I) and (II)

	Assignments for IR absorption bands cm -1		Vibrations
	Monomer I Monomer II	Poly (I)	Poly (II)
	600-820	600-825	600-740	500-736	C-H out -of-plane deformation
			840	840.9	PF 6	-doping species
			1470	1334-1470	C-C between two monomer
						units
	1300-1500	1334-1570	1630	1580	C=C stretching
	2200	2202	2133	2206	CN nitrile group
	2800-2900	2900-3000	2800-2970 2800-2966	Aromatic C-H stretching