Alireza Kavand

Christian Blanck

Frédéric Przybilla

Yves Mély

Nicolas Anton

Thierry Vandamme

Christophe A Serra

Delphine Chan-Seng

Investigating the growth of hyperbranched polymers by self-condensing vinyl RAFT copolymerization from the surface of upconversion nanoparticles

Hyperbranched poly(N-(2-hydroxypropyl) methacrylamide)s grafted on upconversion nanoparticles (UCNPs) were prepared by surface-initiated self-condensing vinyl RAFT copolymerization from the surface of UCNPs modified with a RAFT chain transfer agent. The effect of the grafting density in RAFT chain transfer agent on UCNPs and the concentration in transmer were investigated. The propagation of the polymerization was strongly affected by termination and transfer reactions associated to the high local concentration in propagating chains due to the immobilization of the polymer chains at the surface of UCNPs and increased number of active centers during the polymerization, but also to the hopping and rolling mechanisms related to the surface-initiated RAFT polymerization. The development of such nanohybrids opens the way to novel nano-objects for biomedical imaging and theranostics.

INTRODUCTION

Hybrid materials are attractive to respond to the increasing demand for novel materials with well-defined properties. The preparation of hybrid materials based on polymers and inorganic nanoparticles has been investigated notably by encapsulation of inorganic nanoparticles in polymer particles and attachment of polymers at the surface of inorganic nanoparticles. 1 In the latter case, the two main methods are the "grafting to" consisting of the reaction between orthogonal functional groups present on the polymer and at the surface of the nanoparticles and "grafting from" involving the polymerization of monomers from the surface of the nanoparticles. A wide range of inorganic nanoparticles 2 have been used to grow polymers from their surface using different polymerization techniques such as cationic, 3,4 anionic, 5,6 and radical 7 polymerizations. These hybrid materials provide access to the compatibilisation of inorganic particles in polymer matrices, 8 and enhancement of the nanoparticles stealthiness for biomedical applications. 9 Lanthanide-doped upconversion nanoparticles (UCNPs), a new family of luminescent nanoparticles, have attracted considerable interest for the development of probes for bioimaging. 10 UCNPs are able to convert near-infrared (NIR) excitation into visible and ultraviolet (UV-vis) emission upon excitation of a low intensity laser. 11 This property, specific to UCNPs, is of interest for biomedical applications as it leads to low phototoxicity, no conflict with the autofluorescence of biological systems, and high optical penetration depth of light in biological tissues. 12 Compared to organic dyes and quantum dots, UCNPs show sharp emission bandwidth, long luminescence decays, no photobleaching, and no photoblinking. Their properties, especially their dispersity in solution, can be tuned through the production of a polymer corona around UCNPs through ligand exchange, 13,14 encapsulation in a polymer shell, 15 and surface-initiated polymerization. The latter approach requires the introduction of functional groups at the surface of the nanoparticles to allow the growth from the surface. The functionalities at the surface of UCNPs have been tuned by either choosing the proper ligand during the synthesis of UCNPs to introduce the functional group promoting the polymerization (e.g. ricinoleic acid as ligand to introduce hydroxyl groups as initiating groups for the cationic ring-opening polymerization of glycidol 16,17 ) or by modifying the functionalities present at the surface to insert the suitable functional group to induce polymerization from the surface (e.g. chain transfer agents for photoenergy/electron transfer-reversible additionfragmentation chain transfer radical polymerization affording linear poly(oligo(ethylene glycol) methacrylate)s, 18 and α-bromoisobutyryl groups to initiate the atom transfer radical polymerization (ATRP) of 4,5-dimethoxy-2-nitrobenzyl methacrylate 19 ).

Self-condensing vinyl polymerization (SCVP) was first introduced by the group of Fréchet 20 consisting of using a vinyl (co)monomer possessing an initiating or chain transfer agent (inimer or transmer respectively) on its side chain to create branching points. The degree of branching (DB) can be tuned by changing the ratio between monomer and inimer/transmer. SCVP has been investigated through the three main controlled radical polymerization techniques, 21 i.e. nitroxide-mediated polymerization, ATRP, and reversible addition-fragmentation chain transfer (RAFT) polymerization. The use of SCVP to grow hyperbranched polymers (HBPs) on nanoparticles has been mostly performed using ATRP from silica, 22 and ZnO 23 nanoparticles. 24 Herein we report the growth of HBPs prepared by SCVP under RAFT conditions from the surface of UCNPs. To the best of our knowledge, SCVP under RAFT conditions has not been attempted on nanoparticles. The surface of UCNPs was modified by silanization to introduce functional groups permitting to covalently attach the anchoring groups that will mediate the polymerization. Various approaches have been reported in the literature to functionalize the surface to conduct surface-initiated RAFT polymerization including anchoring the initiator or RAFT chain transfer agent (Z-C(=S)S-R) through its R-or Z-group to the surface with a strong preference for the anchoring of the chain transfer agent through its R group 7 that was adopted here. SCVP was investigated by varying the grafting density in chain transfer agent at the surface of UCNPs and the ratio of transmer to monomer and comparing to the results to those for the preparation of linear polymers.

RESULTS AND DISCUSSION

Preparation of UCNPs with a well-defined density of RAFT chain transfer agents at their surface

Monodispersed spherical β-NaYF4:Yb 3+ , Er 3+ nanocrystals (UCNP@OA) with a diameter of 27.2 ± 0.4 nm were prepared by co-precipitation method using oleic acid as capping agent according to the protocol described by Li et al. 25 and characterized by high-resolution transmission electron microscopy (HR-TEM), dynamic light scattering (DLS), X-ray diffraction and luminescence measurements (Fig. S1). The surface of UCNP@OA was modified to create a silica shell possessing amine groups at their surface (UCNP@NH2) by reverse microemulsion technique 26 through the successive condensation of tetraethylorthosilicate and (3-aminopropyl) triethoxysilane (APTS). The formation of the silica shell was confirmed by i) HR-TEM showing a silica shell of 8.1 ± 0.5 nm around the UCNPs, ii) DLS showing monomodal peaks without aggregates with an increase in hydrodynamic diameter (Dh) from 22 to 45 nm (values in number, Fig. S2a), and iii) Fouriertransform infrared (FT-IR) spectroscopy exhibiting the characteristic stretching peak of -Si-O-Si-groups at 1062 cm -1 . The presence of amine groups on the nanoparticles was identified at 1650 cm -1 (NH in-plane stretching) and 3420 cm -1 (NH2) on the FT-IR spectrum and supported by a positive Kaiser test 27 (Fig. S3). The surface specific area of UCNP@NH2 was determined by establishing the nitrogen adsorption/desorption isotherms according to the Brunauer-Emmet-Teller (BET) theory (Fig. S4) and resulted in a value of 45.44 ± 0.15 m² g -1 (for 30 mg of UCNP@NH2, the surface area could be estimated to 1.4 m 2 ). These nanoparticles, dispersible in water, ethanol, acetone, and DMF, were prepared with different amounts of APTS (i.e. 38, 76, and 152 µmol of APTS for 80 mg of UCNP@OA) to vary the grafting density in amine groups at the surface of UCNPs. A carboxylic acidterminated RAFT chain transfer agent, suitable for the polymerization of methacrylates, 4-cyano-4-(thiobenzoylthio)pentanoic acid (CPABD), was coupled to the amine groups present at the surface of UCNPs using N,N,N′,N′-tetramethyl-O-(1H-benzotriazol-1yl)uronium hexafluorophosphate as coupling agent in the presence of 1-hydroxybenzotriazole hydrate and diisopropylethylamine in DMF (Scheme 1). The color of the UCNPs changed from white to red corresponding to the color of CPABD (Fig. 1c). UCNP@CPABD were characterized by FT-IR spectroscopy (Fig. 1a) showing the characteristic stretching absorption band of nitrile groups at 2160 cm -1 and UV-vis spectroscopy as a suspension in ethanol exhibiting the strong absorption of the thiocarbonyl group at 315 nm (Fig. 1b) confirming the presence of CPABD at the surface of UCNPs. 28 The grafting density in CPABD attached to UCNPs was determined by TGA 18,29 (Fig. S5) as 0.6, 1.4 and 3.1 CPABD per square nanometer of UCNPs for UCNP@CPABDlow, UCNP@CPABDmedium, and UCNP@CPABDhigh respectively (Table S1).

Scheme 1. Synthesis of UCNP@CPABD and its use to prepare linear (UCNP@polyHPMA) and hyperbranched (UCNP@HBP) poly(N-(2-hydroxypropyl) methacrylamide) grafted on UCNPs.

Polymerization of N-(2-hydroxypropyl) methacrylamide from the surface of UCNP@CPABD

Poly(N-(2-hydroxypropyl) methacrylamide) is commonly used to prepare drug conjugates 30,31 leading our choice to N-(2-hydroxypropyl methacrylamide) (HPMA) as monomer for this study. The nature of the solvent is crucial especially because the polymerization was performed at the surface of UCNPs. Various reports highlight the effects of surface confinement and diffusion of monomers towards the propagating chains grafted on a surface emphasizing the importance of the solvent quality to perform a surface-initiated controlled radical polymerization. 32,33 Linear poly(N-(2-hydroxypropyl) methacrylamide) (polyHPMA) were first prepared from the surface of UCNP@CPABD (Scheme 1) considering DMF, 1,4-dioxane, and MeOH as solvent for the polymerization.

The polymerization of HPMA from UCNP@CPABD was conducted in an oil bath thermoregulated at 65 °C fixing the concentration in nanoparticles to 10 mg mL -1 and the monomer concentration to 0.57 M using a molar ratio between HMPA, CPABD and 2,2'-azobis(isobutyronitrile) (AIBN) as initiator of 200:1:0.2. HPMA conversion was determined by 1 H NMR spectroscopy by comparing the integrations of the vinylic proton at 5.7 ppm to the methyl protons of the polymer backbone at 0.5-1.1 ppm. The polymerization of HPMA seemed to be more suitable when conducted in 1,4-dioxane rather than in MeOH and DMF. HPMA conversion after 21 h of polymerization from UCNP@CPABDhigh reached 18% in 1,4-dioxane compared to 7% in DMF and MeOH. TGA indicated a content in polyHPMA of 25 and 6 wt% using UCNP@CPABDhigh and UCNP@CPABDlow respectively for the polymerization in 1,4-dioxane and almost no polymer formation on UCNPs for the polymerizations in DMF and MeOH (Fig. S6). No significant visual differences were observed for the dispersions of UCNP@CPABD in 1,4-dioxane and DMF, however their dispersibility in methanol was visually observed to be poor. The formation of aggregates could lead to the low accessibility of HPMA to the chain transfer agents grafted at the surface of UCNPs. Pan et al. investigated the polymerization of HMPA in different organic solvents including DMF. 34 The kinetic plot (ln([M]0/[M] vs. time) exhibited a deviation from linearity that was attributed to a decrease in propagating chains due to termination reactions. It seems that using 1,4-dioxane led to a lower probability of termination reactions.

Fig. 1 Characterization of UCNP@CPABD by (a) FT-IR, and (b) UV-vis spectroscopies, along with the (c) observation of the color change of the UCNP powders according to the grafting density in CPABD (from left to right: UCNP@NH2, UCNP@CPABDlow, UCNP@CPABDmedium, UCNP@CPABDhigh).

Self-condensing vinyl copolymerization from the surface of UCNP@CPABD

HBPs grafted on UCNPs were prepared by self-condensing vinyl copolymerization of HPMA with a methacrylate-based transmer bearing CPABD, 2-(methacryloyloxy)ethyl-4-cyano-4-((phenylcarbonothioyl)thio)pentanoate 35 (MA-CPABD) inducing the formation of branching points (Scheme 1c). The polymerization was conducted in 1,4-dioxane in an oil bath thermoregulated at 65 °C for 24 h using UCNP@CPABDhigh (10 mg mL -1 ), HPMA (0.56 M) and a molar ratio between HMPA, MA-CPABD and AIBN of 200:1:0.2. The consumption of MA-CPABD to form the HBP was observed by the decrease in the pink color of the polymerization solution (Fig. 2, inset photo) and reached a monomer conversion of 18% determined gravimetrically (compared to 21% for the preparation of polyHPMA under the same conditions). After cleavage of the polymer from the surface of UCNPs by treatment with HF, its number-average molecular weight was determined by size-exclusion chromatography (SEC) as 220 kg mol -1 with a dispersity (Đ) of 6.75 as compared to 1794 kg mol -1 and 3.15 respectively for polyHMPA. As the same ratio of UCNP@CPABDhigh and HPMA were used in both cases, it would have been expected that similar molecular weights would have been observed. However, the HBP had a lower molecular weight and higher dispersity, which could be attributed to the mechanism of RAFT polymerization from a surface and the increased steric hindrance due to the synthesis of HBP on a surface.

If the polymerization was living, all the terminal points of the HBP should possess a CPABD group and the number of CPABD groups should be correlated to the number of branching points (i.e. for n CPABD terminal groups, the HBP would have (n-1) branching points). Due to i) the steric hindrance induced by the presence of polymer chains on UCNPs hindering the access to all CPABD, ii) the necessity of the monomer to diffuse towards the active centers present at the surface of UCNPs, and iii) the transfer of CPABD between active and dormant chains that are in close proximity, the number of branching points may be underestimated by considering the amount of CPABD. However, it is not possible to determine directly the number of branching points. The determination of the amount of CPABD incorporated in the HBP grafted on UCNPs (UCNP@HBP) was first attempted by 1 H NMR spectroscopy on the cleaved HBPs. However, the characteristic peaks of the branching points and the terminal CPABD groups were difficult to identify due to their low content on the HBP and the potential hydrolysis of the thioester groups during the cleavage of the HBPs from the surface of UCNPs with HF. Thiocarbonyl end groups show π→π* and n→π* absorption bands in the UV and visible range respectively (300 and 515 nm for CPABD). 36 As the direct quantification of CPABD groups present on UCNP@HBP by UV-vis spectroscopy could not be achieved due to the scattering interference of UCNP@polymer, the number of CPABD groups in the supernatant after 24 h of polymerization was determined using this technique and compared to a control solution consisting in the same initial concentration of CPABD in MeOH (i.e. 3 mM). The decrease of the absorption at 515 nm (Fig. 2a) was associated to the incorporation of 140 μmol of MA-CPABD on the grafted HBP per gram of UCNPs and thus the formation of branching points on the polymer. To confirm our observation, the same experiment was conducted for the synthesis of a linear polymer on UCNPs (Fig. 2b), indicating a constant concentration in CPABD and confirming the stability of the RAFT chain transfer under the polymerization conditions used. The number of micromoles of CPABD on UCNP@HBP per gram of UCNP (m), the number of branching points per polymer chain assuming a constant branching length, 37 the degree of branching (DB), and the grafting density (number of polymer chains per nm 2 of UCNP, σ) were estimated using Equations 1, 2, 3 and 4 38, 39 respectively as 140, 122, 0.17, and 0.014 chain per nm 2 using UCNP@CPABDhigh.

m = �n 0 -n s �n control × A2 A1 �� m UCNP Equation 1
where m is the molality of CPABD for UCNP@HBP (i.e. number of moles of CPABD on UCNP@HBP per gram of UCNP), n0 and ns the number of moles of MA-CPABD in solution at t = 0 and 24 h respectively, ncontrol the number of moles of CPABD in the control experiment, A1 and A2 the absorbance of MA-CPABD for the control solution and supernatant after polymerization respectively, and mUCNP the mass of UCNP used. 2where Mn is the number-average molecular weight of the polymer grafted on UCNPs determined by SEC, n1 the number of moles of HPMA per gram of UCNPs calculated from TGA, n2 the number of moles of MA-CPABD per gram of UCNPs calculated from UV-Vis spectroscopy and M the molecular weight of HPMA as repeating unit (143.1 g mol -1 ). DB = 2D 2D+L Equation 3where D is the total number of branching units derived from MA-CPABD, and L the total number of linear repeat units derived from HPMA consumption. The factor of 2 in Equation 3 is related to the fact that each branching unit induces the formation of two active centers and thus two branches. 37,40 The value for D was determined by UV-vis spectroscopy considering the absorbance at 515 nm corresponding to the thiocarbonyl group of MA-CPABD (based on its number of moles per gram of UCNPs), while L was calculated from TGA measurement (based on its number of moles of HPMA per gram of UCNPs).

branching point per chain = M n n 1 n 2 ×M Equation
σ = (w/M n,UCNP )×N a m UCNP ×S UCNP Equation 4
where w is the weight loss determined by TGA measurement, Mn,UCNP the number-average molecular weight of the polymer grafted on UCNP determined by SEC analysis, Na the Avogadro number, mUCNP the weight of UCNP and SUCNP the specific surface area of UCNP calculated by BET analysis. DLS characterization of UCNP@HBP indicated a Dh of 141 nm (PDI = 0.087) corresponding to a polymer corona of 27 nm considering the Dh of UCNP@NH2 of 87 nm. UCNP@polyHPMA exhibited a larger polymer corona of 82 nm (Dh = 251 nm, PDI = 0.258, Fig. S7) that could be associated to the higher molecular weight observed for this system as compared to the hyperbranched one. TGA revealed a lower content in polymer when preparing HBP from the surface of UCNPs as compared to the linear ones (Fig. 3) with an estimation of 1.4 and 3.4 mmol of HPMA units per gram of UCNPs for the HBP and linear polymer respectively. The dispersion of UCNP@polyHPMA and UCNP@HBPhigh at a concentration of 0.5 mg mL -1 seemed both stable without observation of sedimentation within 24 h. Fig. 3 TGA of UCNP@CPABD (red dotted line), UCNP@polyHPMA (black solid line), and UCNP@HBP (green dashed line). The polymers were prepared from UCNP@CPABDhigh using 3 mM of CPABD and MA-CPABD for the linear and hyperbranched polymers respectively.

Effect of the grafting density in CPABD on UCNPs

The results from SEC and TGA measurements both suggested that the growth of HBPs from UCNPs under these conditions were hindered as compared to linear polymers. The migration of chain transfer agents when performing surface-initiated RAFT polymerization to obtain linear polymers has been reported to lead to termination and transfer reactions. [41][42][43] Furthermore, the synthesis of hyperbranched structures induces steric hindrance that can affect the proper propagation of the polymer chains and thus enhance the propensity of termination and transfer reactions especially because the concentration of CPABD at the surface of UCNPs is locally high. The grafting density in CPABD on UCNPs was therefore investigated considering the selfcondensing vinyl copolymerization from UCNP@CPABDmedium and UCNP@CPABDlow in the presence of 3 mM of MA-CPABD. According to UV-vis spectroscopy, the number of branching points incorporated on the HBP grafted on UCNPs was 140, 90 and 34 μmol per gram of UCNP@HBP for HBP prepared from UCNP@CPABDhigh, UCNP@CPABDmedium and UCNP@CPABDlow respectively. TGA of UCNP@HBPs (Fig. S8 andS9) indicated that the weight percentage of organic content grafted on UCNP from UCNP@CPABDmedium was higher than for the nanohybrids obtained from UCNP@CPABDhigh indicating a higher consumption of HMPA and thus a lower DB (i.e. 0.14 and 0.05 for the HBP obtained from UCNP@CPABDhigh and UCNP@CPABDmedium respectively). The termination and transfer reactions seemed less pronounced with the decrease of the grafting density in CPABD affording a higher conversion of HMPA. The number of branching points per HBP chain was determined as 148 and 6 for UCNP@CPABDmedium and UCNP@CPABDlow respectively as compared to 122 for UCNP@CPABDhigh. The calculation of the grafting density ( revealed that UCNP@CPABDhigh and UCNP@CPABDmedium had very low densities estimated as 0.014 and 0.007 chain per nm 2 respectively, while the one of UCNP@CPABDlow was higher (0.06 chain per nm 2 ). These results suggested that transfer reactions for UCNP@CPABDhigh and UCNP@CPABDmedium were more pronounced at the surface of UCNPs leading to a higher proportion of dead chains. 42 As initiation takes place in solution in our system, the propagating chains can either consume HMPA and MA-CPABD in solution leading to HBP formed in solution or go through a chain transfer reaction with the CPABD present at the surface of UCNPs creating an active center to grow a HBP at the surface of UCNPs. The HBP formed in solution is expected to be less affected by termination and transfer reactions as the concentration in active centers will be lower as compared to the HBP immobilized on UCNPs. To better understand the polymerization behavior, the polymer grafted on UCNPs and formed in solution during the polymerization were both characterized by SEC. The polymer formed in solution had a lower number-average molecular weight as compared to the polymer formed at the surface of UCNPs (Fig. 4), i.e. 20 and 13 kg mol - 1 (Đ = 3.88 and 12.86) respectively for polymers obtained from UCNP@CPABDmedium and UCNP@CPABDhigh in solution compared to 839 and 220 kg mol -1 (Đ = 4.17 and 6.75) respectively for polymers grafted on UCNPs. The molecular weights of the polymers formed in solution were lower than the calculated ones with relatively low dispersity, which could be attributed to cyclization reactions as previously reported in the literature. 44,45 However, the polymers formed at the surface of UCNPs had a higher molecular weight and a broader molecular weight distribution that could be due to the high concentration in radicals per polymer chain leading to irreversible termination reactions notably between neighboring propagating chains. The higher the grafting density in CPABD on UCNPs, the more pronounced the termination reactions. Further lowering of the grafting density in CPABD, i.e. using UCNP@CPABDlow, afforded polymers grafted on UCNPs with a lower molecular weight (Mn = 19 kg mol -1 ) and narrower molecular weight distribution (Đ = 1.65), closer to the characteristics of the polymer obtained in solution. It indicated a lower contribution of the termination and transfer reactions associated to the surface-initiated self-condensing vinyl RAFT copolymerization, for which CPABD were anchored at the surface of UCNPs inducing a high localized concentration in RAFT chain transfer agents. In solution, this phenomenon is less pronounced due to the homogeneous and lower localized concentration in RAFT chain transfer agent. 46 Fig. 4 Normalized SEC traces for HBPs (a) grafted on UCNPs and (b) formed in solution prepared using 3 mM of MA-CPABD from UCNP@CPABDlow (black solid line), UCNP@CPABDmedium (red dotted line), and UCNP@CPABDhigh (blue dashed line).

Effect of the concentration in MA-CPABD transmer

Considering UCNP@CPABDlow, the concentration of MA-CPABD used during the polymerization was increased to 6 mM to determine its effect on the HBP formed at the surface of UCNPs. According to UV spectroscopy, the number of MA-CPABD incorporated as branching points increased from 34 to 67 μmol per gram of UCNP for 3 and 6 mM of MA-CPABD used for the polymerization. TGA measurement permitted to determine the content of HPMA as 0.52 mmol per gram of UCNP, which was close to the value obtained for the HBP prepared with 3 mM of MA-CPABD (i.e. 0.68 mmol). Thus, DB increased from 0.08 to 0.19 when doubling the amount of MA-CPABD used. SEC measurements (Fig. 5) indicated a decrease of the number-average molecular weight with the increase of the concentration in MA-CPABD, i.e. 19 kg mol -1 in the case of 3 mM of MA-CPABD compared to 11 kg mol -1 when using 6 mM of MA-CPABD, with a decrease of the dispersity from 1.65 to 1.55 respectively. Under the same polymerization conditions (3 mM of CPABD), polyHPMA had a lower number-average molecular weight (9 kg mol -1 ) and narrower molecular weight distribution (Đ = 1.46). The number of branches per polymer chain was calculated as 6 and 9 for 3 mM and 6 mM of MA-CPABD respectively. The grafting density of polymer was determined as 0.06 chain per nm 2 for 3 mM of MA-CPABD, which was similar to the one using 6 mM of MA-CPABD (i.e. 0.07 chain per nm 2 ). However, when preparing linear polymer from UCNPs under the same conditions, the number of polymer chains was 0.11 chain per nm 2 , which was twice as compared to the HBP. The three-dimensional structure of HBPs affected strongly the propagation of the polymer chains from the surface of UCNPs inducing steric hindrance and enhancing the probability of termination and transfer reactions. These phenomena illustrated in Fig. 6 would not only decrease the grafting density, but also increase the dispersity of the polymer formed 33,47 (Đ = 1.46 and 1.65 for polyHPMA and UCNP@CPABDhigh, and 44 and 19 wt% for UCNP@CPABDmedium respectively) that was attributed to the enhancement of termination and transfer reactions due to the increased grafting density in CPABD 46,48,49 on UCNPs and the higher amount of terminal active centers. In summary, the ratio between the molecular weight of the HBP formed on UCNPs and in solution significantly increased with the increase in grafting density of CPABD on UCNPs (Fig. 7a). The effect of the concentration of MA-CPABD (Fig. 7b) had a less pronounced influence on the difference in molecular weight between the HBP formed on UCNPs and in solution, while for polyHMPA ([MA-CPABD] = 0 mM) this difference was minimal. This phenomenon could be attributed to the higher probability of coupling reactions between propagating chains for HBPs as compared to linear polymers due to their higher number of terminal active centers and their closer spatial proximities. To further investigate these coupling reactions, the grafting density in polymer chains per surface area of UCNPs was determined after 6 and 24 h of polymerization. The grafting density of the HBP was estimated as 0.35 chain per nm 2 after 6 h (Mn = 2.6 kg mol -1 , Đ = 1.16) and 0.10 chain per nm 2 after 24 h (Mn = 13.4 kg mol -1 , Đ = 2.72). This decrease in grafting density suggested the high propensity of termination and transfer reactions. Tsujii et al. proposed for the growth of linear polymers by RAFT polymerization from a flat surface that the decrease of the grafting density could be attributed to the high rate of termination reaction at the early stage of the polymerization. 42 However, for the synthesis of HBP, this phenomenon could be worsened due to steric hindrance. Furthermore, hopping and rolling mechanisms for surface-initiated RAFT polymerization and the coupling between hyperbranched propagating chains (e.g. between HBP formed in solution and those growing at the surface of UCNPs) could also contribute to this phenomenon (Fig. 8). 41

Fig. 8 Summary highlighting all the reactions involved in self-condensing vinyl RAFT polymerization from the surface of UCNPs.

Luminescence properties of UCNP@HBP

The optical activity of UCNP@HBP dispersed at a concentration of 2 mg mL -1 in water upon a 980 nm excitation was confirmed. The expected characteristic peak maxima of UCNPs doped with Er 3+ were observed at 409 nm ( 4 H9/2→ 4 I15/2), 542 nm ( 4 H11/2→ 4 I15/2), 552 nm ( 4 S3/2→ 4 I15/2), and 654 nm ( 4 F9/2→ 4 I15/2). The comparison of the normalized luminescence intensity of UCNP@HBP with UCNP@OA dispersed in cyclohexane (Figure 9a) revealed that the red-to-green ratio of the red emission (657 nm) to the green emission (542 nm) intensity was higher for UCNP@HBP as compared to the one of UCNP@OA. This phenomenon is characteristic of the quenching of the green emissive state of the Er 3+ ions by water molecules that establish non-radiative relaxation channels. 50 Surprisingly, the dispersion of UCNP@OA in cyclohexane was globally less bright (by a factor of 3) as compared to aqueous dispersion of UCNP@NH2 and UCNP@HBP (Fig. S10). The heterogeneity of UCNP@NH2 and UCNP@HBP dispersions in water observed by DLS (Fig. S11) that could be attributed to the aging of the dispersions may contribute to this observation.

Fig. 9 Luminescence properties of UCNPs solutions at a concentration of 2 mg mL -1 in water (except UCNP@OA in cyclohexane) excited at 980 nm: (a) luminescence spectrum of dispersion of UCNP@OA (blue solid line), UCNP@NH2 (black dashed line), and UCNP@HBP (red dotted line) in solution normalized at 657 nm and (b) on the left, representative luminescence microscopy images (46x46 µm 2 ) and on the right, the corresponding intensity distribution of luminescent spots obtained from luminescence microscopy images for UCNP@OA, UCNP@NH2, and UCNP@HBP.

Single particle microcopy experiments (Figure 9b) were conducted by immobilizing the nanoparticles on microscopy cover glasses. For UCNP@OA a sample highly diluted in cyclohexane was dried on a cover glass, whereas for UCNP@NH2 and UCNP@HBP experiments were performed in aqueous conditions by electrostatically immobilizing the particles on poly(ethylene imine)-coated cover glasses using highly diluted solutions. For the dispersions in water (i.e. UCNP@NH2 and UCNP@HBP), 1 mM of NaF was added to prevent the potential dissolution of UCNPs during their imaging. 51 To decrease the number of aggregates in the field of view (which severely limit the dynamic range of the images due to saturation and bleeding effects) the supernatant of a 1000-fold dilution centrifuged at 2000 g for 5 min was used. All samples were imaged on the same wide-field upconversion microscope using the same experimental parameters. For each sample, forty-nine images were acquired and analyzed to extract the intensity of diffraction limited luminescent spots. The histogram of UCNP@OA showed a single population with a luminescence intensity of ~10 5 a.u. with few brighter outliers attributed to the drying process, and notably the coffee-ring effect, producing a non-homogeneous distribution of UCNPs over the cover surface, and formation of small bright aggregates. The histograms of UCNP@NH2 and UCNP@HBP were very similar a main population with a luminescence intensity of ~2.10 4 a.u and a sub-population with a luminescence intensity of ~4.10 4 a.u. It is hypothesized that the brighter subpopulation consisted of either UCNP dimers, or two single particles that adsorbed close to each other, whereas the brighter tail could be attributed to higher order multimers. Overall it seems that the brightness of UCNP@NH2 and UCNP@HBP were similar and about five times lower than the brightness of UCNP@OA. However, such a direct comparison may not be fully valid due to the fact that the direct environment affects the optical properties of nano-emitters. [START_REF] Novotny | Principles of Nano-Optics[END_REF] UCNP@NH2 and UCNP@HBP were at the interface water-poly(ethylene imine), while UCNP@OA were at the interface air-glass. Moreover, the centrifugation step used for the preparation of water dispersible samples, may also bias the results by favoring small and less bright UCNPs. Nevertheless, our data clearly showed that UCNP@HBP had a strong emission and single UCNPs could be easily detected by wide-field upconversion microscopy.

SUMMARY

Surface-initiated self-condensing vinyl RAFT copolymerization of HMPA from the surface of UCNP was investigated. Homopolymerization of HPMA was first conducted to identify the suitable polymerization conditions before attempting the synthesis of HBPs by copolymerization with MA-CPABD. The preparation of HBPs from UCNPs was challenging yielding to grafted HBPs of molecular weight and dispersity higher than expected. These phenomena were attributed to a) the increased concentration in propagating chains at the surface of UCNPs due to the formation of HBPs by SCVP, b) the close proximity of the propagating chains due to their immobilization on the nanoparticles, and c) the fact that RAFT polymerization from a surface was more prone to migration of the active center through hopping and rolling mechanisms. These UCNP@HBP exhibited strong emission comforting their potential use for biomedical imaging and theranostics.

EXPERIMENTAL Materials

Yttrium(III) chloride hexahydrate (99.99%), ytterbium(III) chloride hexahydrate (99.99%), erbium(III) chloride hexahydrate (99.99%), oleic acid (OA, technical grade, 90%), 1-octadecene (technical grade, 90%), sodium hydroxide (anhydrous, ACS reagent, ≥97%), ammo-nium fluoride (ACS reagent, ≥98.0%, IGEPAL® CO-520, tetraethyl-orthosilicate (TEOS, 98%),

(3-aminopropyl)triethoxysilane (APTES, 98%), 1-hydroxybenzotriazole hydrate (HOBt, ≥97%), 2,2'-azo-bis(isobutyronitrile) (AIBN, 98%), N,N'-dicyclohexylcarbodiimide (DCC, 99%), 4-(dimethylamino)pyridine (DMAP, 99%), 4-cyano-4-(thiobenzoylthio)pentanoic acid (CPABD, >98%), ammonium hydroxide (28%), silica gel (high-purity grade, pore size 60 Å, 230-400 mesh), N,N-dimethylformamide anhydrous, (DMF, 99.8%), anhydrous methanol (MeOH, 99.8 %), anhydrous dichloromethane (DCM, ≥99.8%) were purchased from Sigma-Aldrich. N,N,N′,N′-Tetramethyl-O-(1H-benzotriazol-1-yl)uronium hexafluorophosphate (HBTU, 99%) was purchased from Iris Biotech. 2-Hydroxyethyl methacrylate (HEMA, 98%) was purchased from ABCR. N-Ethyldiisopropylamine (DIPEA, 99%), 1,4-dioxane (≥99.5%) were purchased from Alfa Aesar. Hydrofluoric acid (HF, 40% in H2O) was purchased from Ridel de Haen. Ethanol (absolute, 99.99%), cyclohexane (99.8%), and acetone (99.8%) were purchased from Carlo Erba. Dialysis tubing (regenerated cellulose, MWCO of 1 kDa, SpectrumLab) was purchased from Roth. All chemicals were used as received, except if noted otherwise. AIBN was recrystallized twice from ethanol. N-(2-Hydroxypropyl) methacrylamide [START_REF] Ulbrich | [END_REF] (HPMA) and 2-(methacryloyloxy)ethyl 4-cyano-4-(phenylcarbonothioylthio) pentanoate 35 (MA-CPABD) were synthesized as reported in the literature.

Characterizations

Dynamic light scattering (DLS) experiments were performed on a Malvern Zetasizer Nano ZS at 633 nm (He-Ne laser beam) with a scattering angle fixed at 173° at 25 °C. Before measurement, the solutions were filtered through a 0.45 μm syringe filter.

Transmission electron microscopy (TEM) images were obtained using a Technai G2 microscope (FEI) at 200 kV. 5 µL of the UCNP suspension was deposited onto a freshly glow discharged carbon-covered copper grid (400 mesh). The suspension was left for 2 min and the excess removed by using a filter paper. Images were acquired with an Eagle2K ssCCD camera (FEI). The size and size distribution of UCNPs were determined based on at least 29 nanoparticles using the ImageJ software (Version 1.51n, Wayne Rasband, National Institutes of Health, USA).

Fourier transform infrared (FTIR) spectra were recorded on a Bruker Vertex 70 spectrometer using the attenuated total reflectance (ATR) technique between 500 and 4000 cm -1 .

Nitrogen adsorption/desorption isotherms according to the Brunauer-Emmett-Teller (BET) theory were recorded on a Micromeritics TriStar 3000 V6.07A analyzer. The samples were degassed at 150 °C for 3 h in the degassing port of the adsorption apparatus.

UV-vis spectroscopy was carried out using a Perkin Elmer Lambda 25 spectrophotometer.

Thermogravimetric analyses (TGA) were performed on a Mettler-Toledo TGA2 thermogravimeter at a heating scan rate of 10 °C min -1 under argon (flow rate: 100 mL min -1 ) using alumina crucibles.

1 H and 13 C NMR spectra were recorded in CDCl3, DMSO-d6, MeOD, or D2O on a 400 MHz Bruker Avance III HD spectrometers equipped with a BBO type probe at 25 °C.

Size-exclusion chromatography (SEC) was performed on a DIONEX Ultimate 3000 system equipped with a guard column and four Shodex OH-pak columns (7.5 x 300 mm, 803HQ, 804HQ, 806HQ, 807HQ), a Wyatt OPTILAB rEX differential refractometer, and a Wyatt DAWN HELEOS II light scattering detector and was operated at 30 °C using 60/40 water/acetonitrile with 0.1 M NaNO3 as eluent at a flow rate of 0.5 mL min -1 .

Luminescence spectroscopy and luminescence imaging of UCNPs were performed using homemade setups as described in details in our previous work. 54 For luminescence spectroscopy, the continuous-wave laser beam of a single mode fiber coupled laser diode (Qphotonics, QFBGLD-980-350) was focused in a quartz cuvette (Hellma QS Semi-Micro Cells) with an estimated average excitation intensity in the beam waist of 8 kW cm -2 . The emission of the sample was collected by a 10x/0.25 numerical aperture objective and detected by a fiber spectrometer (Avantes, AvaSpec-3648). For luminescence microscopy, the same laser source was coupled to an inverted microscope (Olympus IX71) equipped with a high numerical aperture objective (Olympus, UApo N 100x/1.49 Oil) and allowed to excite the UCNPs with an excitation power density of 8 kW cm -2 in epi illumination. Luminescence emission was separated from the excitation beam by using a short pass dichroic mirror (Chroma, T875spxrxt), while the residual laser light was removed by a low pass filter (Chroma, E700SP). Emission was detected by an electron multiplying CCD camera (Hamamatsu, ImagEM X2 C9100-23B). Acquisition was fully automated and controlled by scripts within the MicroManager framework. 55 All static images shown in the article were recorded as an averaged stack of 100 images, each with 30 ms exposure time. Wide-field microscopy images were treated with ImageJ 1.51h as part of the FIJI package, 56 spot fitting was performed with the Gaussian Fit module of the GDSC SMLM package. [START_REF]GDSC ImageJ plugins[END_REF] Synthesis of NaYF4; Yb 20%; Er 2% upconversion nanoparticles with amine groups at their surface (UCNP@NH2)

NaYF4:20% Yb 3+ , 2% Er 3+ nanocrystals having a diameter of 25-30 nm were prepared by the co-precipitation method according to a previously reported procedure 33 with some modifications. Briefly, 1 mmol of lanthanide chloride (Y:Yb:Er = 78:20:2) were dissolved into a solution of oleic acid (6 mL) in 1-octadecene (15 mL) under vigorous stirring and heated to 150 °C to obtain a homogeneous solution that was then cooled to 30 °C under argon. A mixture of NaOH (0.10 g, 2.5 mmol) and NH4F (0.14 g, 4.0 mmol) dissolved in 5 mL of MeOH was then added and stirred for 30 min at 30 °C. The mixture was then heated at 110 °C for 50 min to remove MeOH, then at 290 °C for 180 min under argon. The solution was cooled to room temperature and 25 mL of acetone was added. The precipitate was centrifuged, isolated and redispersed in 20 mL of cyclohexane. The concentration of the dispersion of NaYF4:20% Yb 3+ , 2% Er 3+ nanocrystals capped with oleate ligands (UCNP@OA) in cyclohexane was around 40 mg mL -1 .

UCNP@OA were modified with silanol (Si-OH) in the presence of TEOS under basic conditions by reverse microemulsion method 26,[START_REF] Gnanasammandhan | [END_REF] followed by the introduction of primary amine groups by treating with APTS. 34 Briefly, 1.5 g of IGEPAL® CO-520 (3.33 mmol) was charged in a 50 mL centrifuge tube followed by 21 mL of cyclohexane and 4 mL of UCNP@OA dispersion (20 mg mL - 1 in cyclohexane) were added into the tube. After 1 min of sonication, the mixture was poured to a 100 mL flask under vigorous stirring (750 rpm). 0.165 mL of ammonium hydroxide (28%) was added slowly and stirred for 30 min. 50 µL of TEOS (0.22 mmol) were added to the reaction mixture that was stirred for 24 h. 30 µL of APTS (152 μmol) was then added and stirred for 24 h. 50 mL of a mixture of acetone and ethanol (50:50) was added to the reaction tube. The mixture was centrifuged at 6500 g for 30 min at room temperature and the supernatant was discarded. The UCNPs were washed several times with redispersion by vortex and sonication in ethanol and finally with acetone. The nanoparticles were dried under vacuum yielding 140 mg of aminefunctionalized UCNPs. UCNP@NH2 with a lower amount of amine groups were prepared similarly by adding 7.5 or 15 µL of APTS (38 and 76 μmol respectively) yielding to 121 and 129 mg of UCNP@NH2,low and UCNP@NH2,medium respectively.

Synthesis of CPABD-functionalized UCNPs (UCNP@CPABD)

120 mg of UCNP@NH2,high were dispersed in 5 mL of DMF. A solution of HBTU (154 mg, 0.4 mmol), HOBt (55 mg, 0.4 mmol), CPABD (113 mg, 0.4 mmol) and DIPEA (142 µL, 0.8 mmol) in 2 mL of DMF was added to the dispersion of UCNP@NH2 under argon. The reaction mixture was stirred vigorously overnight at room temperature. The UCNPs were then collected by centrifugation and washed five times with DMF and three times with acetone by redispersion in the solvent followed by sonication for 10 s. UCNP@CPABD with a high CPABD grafting density (UCNP@CPABDhigh) was finally dried under vacuum at room temperature for 48 h affording 115 mg of UCNP@CPABDhigh (95% yield). The preparation of UCNP@CPABD with a medium (UCNP@CPABDmedium) and low (UCNP@CPABDlow) CPABD grafting density were carried out according to the same protocols using 77 mg of HBTU (0.2 mmol), 22.5 mg of HOBt (0.2 mmol), 57 mg of CPABD (0.2 mmol) and 71 µL of DIPEA (0.4 mmol) for UCNP@CPABDmedium and 39 mg of HBTU (0.1 mmol), 12 mg of HOBt (0.1 mmol), 29 mg of CPABD (0.1 mmol) and 40 µL of DIPEA (0.2 mmol) for UCNP@CPABDlow.

Preparation of polyHPMA from UCNP@CPABD by surface-initiated RAFT polymerization from UCNPs (UCNP@polyHPMA)

In a 5 mL round bottom flask, 30 mg of UCNP@CPABD, 250 mg of HPMA (1.7 mmol), 2.52 mg of CPABD (9 μmol, 3 mM), and 3 mL of solvent (1,4-dioxane, DMF or MeOH) were added and dispersed under ultrasonication for 6 min (three times 2 min in a cold-water bath). 0.33 mg of AIBN (2 μmol) dissolved in 50 L of the solvent used for the polymerization was added to the reaction flask. The reaction mixture was degassed by bubbling argon into the solution for 30 min. The reaction mixture was stirred in an oil bath thermostated at 65 °C. For kinetic studies, aliquots were withdrawn at predetermined intervals during the polymerization to determine the monomer conversion of the polymer formed in solution. HPMA conversion was determined by 1 H NMR spectroscopy by comparing the integration of the vinyl protons at 5.7 ppm of HPMA to the integration of the methyl protons at 0.5-1.1 ppm of the backbone of polyHPMA. After 24 h of reaction, the reaction mixture was then quenched by placing the reaction flask in an ice bath. UCNP@polyHMPA were isolated by centrifugation. The supernatant was collected and precipitated in Et2O to isolate the polymer formed in solution. UCNP@CPABD were further purified by performing cycles of centrifugation and redispersion in MeOH (five times) and acetone (twice) to remove any ungrafted polymer. The polymers formed in solution and at the surface of UCNPs (UCNP@polyHMPA) were dried under vacuum at room temperature for 48 h.

Preparation of HBP from UCNP@CPABD by surface-initiated RAFT copolymerization (UCNP@HBP) 30 mg of UCNP@CPABD, 250 mg of HPMA (1.7 mmol), 3.52 mg of MA-CPABD (90 μmol), and 3 mL of 1,4-dioxane were placed in a 5 mL bottom flask and dispersed under ultrasonication for 6 min (2 min × 3) in a water bath at 10 °C. 0.33 mg of AIBN (2 μmol) dissolved in 50 µL of 1,4-dioxane was added the reaction mixture. The reaction mixture was degassed by bubbling argon into the solution for 30 min. The reaction mixture was stirred in an oil bath thermostated at 65 °C. For kinetic studies, aliquots were withdrawn at predetermined intervals during the polymerization to determine the monomer conversion of the polymer formed in solution. After 24 h of reaction, the reaction mixture was then quenched by placing the reaction flask in an ice bath. UCNP@HBP were isolated by centrifugation, MeOH was added to redisperse the nanoparticles and centrifuged again. The supernatant was collected after each washing step. The degree of branching (DB), monomer conversion and molecular weight of the polymer formed in solution were determined by UV-vis spectroscopy, 1 H NMR spectroscopy, and SEC measurement respectively. UCNP@HBP were further purified by performing cycles of centrifugation and redispersion three times in successively 1,4-dioxane, MeOH, and acetone to remove any ungrafted polymer. The polymers formed in solution (HBPsol) and at the surface of UCNPs (UCNP@HBP) were dried under vacuum at room temperature for 48 h. The UCNPs were dried under vacuum at room temperature for 48 h.

Cleavage of the polymers from the surface of UCNPs 30 mg of UCNP@polymer dispersed in 2.5 mL of deionized water were sonicated for 2 min before adding 0.5 mL of 40 % HF (Caution: HF is highly corrosive). The solution was shaken for 4 h and then neutralized by adding very slowly a saturated solution of NaHCO3. The mixture was dialyzed against water for 2 days and then centrifuged to collect the supernatant that was lyophilized.

Monitoring of CPABD concentration by UV-vis spectroscopy

For accurate comparison, a stock solution of CPABD was prepared using one part for the polymerization and another part as control. After polymerization, the supernatant was collected from the washing steps and concentrated by rotary evaporation. The pink residue was dissolved in 5 mL of MeOH and the solutions were characterized by UV-vis spectroscopy.

CONFLICTS OF INTEREST

There are no conflicts to declare.

Fig. 2

 2 Fig. 2 UV-spectra of decanted solution after 24 h of polymerization (red dashed line) and control solution (blue solid line) using UCNP@CPABDhigh for the preparation of (a) HBP with HPMA:MA-CPABD:AIBN = 200:1:0.2 (inset photo of the control and polymerization solution after 24 h) and (b) linear polymer with HPMA:CPABD:AIBN = 200:1:0.2.

Fig. 5

 5 Fig. 5 Normalized SEC traces for polymers (a) grafted on UCNPs and (b) formed in solution prepared from UCNP@CPABDlow. HBPs were synthesized using 3 mM (red dotted line) and (b) 6 mM (blue dashed line) of MA-CPABD, while the preparation of linear polymers was conducted with 3 mM of CPABD (black solid line).

Fig. 6 Fig. 7

 67 Fig.6 Growing linear vs hyperbranched polymers from UCNP@CPABDlow and the influence of steric hindrance, and termination and transfer reactions on their chain propagation.

ACKNOWLEDGEMENTS

This work was financially supported by the CNRS and the University of Strasbourg. The doctoral position of AK is supported by the University of Strasbourg through a doctoral contract from the Physics and Chemistry-Physics doctoral school. The authors are thankful to C. Foussat, M. Legros, M. Lenertz, F. Vigneron for SEC, TGA, XRD and BET measurements respectively, and M. Schmutz for fruitful discussions. The authors thank the polymer characterization facilities at the Institut Charles Sadron, the electron microscopy facilities at the Institut Charles Sadron, and the Cronenbourg NMR core facilities. YM is grateful to the Institut Universitaire de France (IUF) for support and providing additional time to be dedicated to research.


Keywords: Oceanic core complex Seismic properties of ocean crust International Oocean Ddiscovery Pprogram JOIDES Rresolution Eexpeditions 304, 305, 340T, and

The physical properties of rock within Oceanic Core Complexes (OCC) provide information about the history of magmatism, deformation and alteration associated with detachment faulting and unroofing of their gabbroic sequences. New core and logging data from Atlantis Bank (AB) OCC are compared with prior deep sea drilling data from this site on the Southwest Indian Ridge, as well as from Atlantis Massif (MFAM) OCC on the Mid-Atlantic Ridge. The average seismic velocity at each site is typical for gabbroic rock. Downhole core and log variations are found to depend on both porosity and alteration. Porosity at millimeter to meter scale impacts P-wave velocities (6.0-6.4 km/s) in the upper several hundred meters at both OCC. Below ~ 600-800 m depth, in situ velocities are higher (6.6-6.9 km/s) for the remainder of the 1.5 km drilled to date, except in zones where alteration is significant. Variability 1 2 3 4 5 6 e.Proofing https://eproofing.springer.com/journals_v2/printpage.php?toke... 2 of 34 4/26/19, 14:19

of 0.5-1 km/s over depth intervals of a few meters is persistent where mineral alteration exceeds 10-20%, whereas variability drops to ± 0.1 km/s for intervals affected by little to no alteration. Seismic anisotropy is negligible overall, with just a few intervals showing consistent fast azimuth and magnitude of a few percent. The depth extent of alteration, probably related to the zone deformed below the detachment when active, is ~ 600-800 meters at both AB and AM.

Introduction

Deep sea drilling provides unique access to ocean crustal properties at a scale that allows geologic processes to be addressed. The intrusive portion of the oceanic crust is typically inaccessible at the seafloor so seismic imaging and velocity analysis are applied to infer geologic structure of the basement. Averaging that is inherent in seismic survey methods means that narrow intervals marking key aspects of the geologic evolution of the crust may not be recognized. Here we take advantage of drilling and borehole logging results at two oceanic core complexes (OCCs) to illustrate inherent physical properties and to assess what the downhole variability might indicate about how they formed and the extent to which seismic surveys can and cannot detect such evidence of interplay between magmatism, faulting, and fluid circulation.

OCCs expose intrusive crustal sections at/near the seafloor through sustained slip along detachment fault(s), which can be active for periods on the order of 10 year within the axial zone of slow spreading centers (e.g., [START_REF] Dick | Tectonic evolution of the Atlantis II Fracture, Zone[END_REF][START_REF] Cann | Corrugated slip surfaces formed at ridge-transform intersections on the Mid-Atlantic Ridge[END_REF][START_REF] Tucholke | Megamullions and mullion structure defining oceanic metamorphic core complexes on the Mid-Atlantic ridge[END_REF][START_REF] Blackman | Geology of the Atlantis Massif (MAR 30°N): implications for the evolution of an ultramafic oceanic core complex[END_REF][START_REF] Escartin | Central role of detachment faults in accretion of slow-spreading oceanic lithosphere[END_REF][START_REF] Escartín | Tectonic structure, evolution, and the nature of oceanic core complexes and their detachment fault zones (13°20′N and 13°30′N, Mid Atlantic Ridge)[END_REF][START_REF] Macleod | Life cycle of oceanic core complexes[END_REF][START_REF] John | Deformation and alteration associated with oceanic and continental detachment systems: Are they similar?[END_REF]. Understanding the inherent physical properties of the unroofed domal 'core' of these complexes can elucidate the footwall evolution during/following fault activity and illuminate aspects of the magmatic accretion that built the section that is penetrated by deep sea drilling.

AQ1

Two OCCs have been targeted for deep penetration by the Ocean Drilling Program (ODP,Legs 118,176,179), the Integrated Ocean Drilling Program (IODP Phase 1, Expeditions 304, 305, and 340T) and the International Ocean Discovery Program (IODP Phase 2, Expeditions 360 and 362T). Logging and selected shipboard description/measurement data for these deep penetrations are the main focus of this paper. ODP Hole 735B [START_REF] Dick | A long in situ section of the lower ocean crust: results of ODP Leg 176 drilling at the Southwest Indian Ridge[END_REF] and IODP Hole U1473A [START_REF] Macleod | Southwest Indian Ridge Lower Crust and Moho[END_REF] were drilled into the footwall exposed at Atlantis Bank (AB), on the flank of the Southwest Indian Ridge (SWIR); IODP Hole U1309D [START_REF] Blackman | Drilling constraints on lithospheric accretion and evolution at Atlantis Massif[END_REF]) was drilled into the Central Dome of Atlantis Massif (AM), on the flank of the Mid-Atlantic Ridge (MAR). AB developed as new crust accreted 11 Ma and the OCC subsequently rafted off axis to its current position ~ 80 km from the spreading center [START_REF] Dick | Tectonic evolution of the Atlantis II Fracture, Zone[END_REF]. AM developed 1-2 Ma and the east slope of its Southern Ridge currently forms the western rift valley wall near the ridge-transform intersection [START_REF] Cann | Corrugated slip surfaces formed at ridge-transform intersections on the Mid-Atlantic Ridge[END_REF][START_REF] Blackman | Geology of the Atlantis Massif (MAR 30°N): implications for the evolution of an ultramafic oceanic core complex[END_REF]. Regional seafloor morphology shows that detachment faulting and core complex formation in these areas is an episodic process (Fig. 1), with more typical faulting/magmatism developing linear abyssal hills in the intervening periods. At AB, transtension across the transform also contributed to the high-relief morphology [START_REF] Dick | Tectonic evolution of the Atlantis II Fracture, Zone[END_REF]. Half spreading rate in these sections of the SWIR and MAR average 8 km/Myr [START_REF] Dick | Tectonic evolution of the Atlantis II Fracture, Zone[END_REF][START_REF] Hosford | Crustal magnetization and accretion at the Southwest Indian Ridge near the Atlantis II fracture zone, 0-25 Ma[END_REF] and 12 km/Myr [START_REF] Zervas | Morphology and crustal structure of a small transform fault along the Mid-Atlantic Ridge: the Atlantis fracture zone[END_REF], respectively.

Fig. 1

Ridge-transform areas where the AB (left) and AM (right), oceanic core complex occur, at the same scale. Brackets on AM map mark the Southern Ridge to the south of the Central Dome Both oxide-and olivine-gabbro are also prevalent in the 157 m deep Hole 1105A drilled at AB during ODP Leg 179, about a km NE of Hole 735B (Pettigrew et al. 1999). At AM, Hole U1309D extends 1415 m into the domal core of the complex and it was logged in stages during IODP Expeditions 304, 305, and 340T [START_REF] Blackman | Expedition 304/305 Scientists[END_REF]Expedition 340T Scientists 2012). The recovered core is somewhat more variable than at AB and consists of gabbro (56%), olivine gabbro (25%) and several percent each of olivine-rich troctolites, troctolites, oxide gabbros, and diabase. Details of the physical property logging and corresponding core sample results for Holes 735B and U1309D have been published previously [START_REF] Iturrino | Velocity structure of the lower oceanic crust: results from Hole 735B, Atlantis II Fracture Zone[END_REF][START_REF] Collins | Seismic and drilling constraints on velocity structure and reflectivity near Hole U1309D on the central dome of Atlantis Massif, Mid-Atlantic Ridge 30[END_REF][START_REF] Blackman | of 34 4/26/19, 14:19 signatures of past and present hydration within a young oceanic core complex[END_REF][START_REF] Harding | Velocity structure near IODP Hole U1309D, Atlantis Massif, from waveform inversin of streamer data and borehole measurements[END_REF]) and we summarize those findings below. New AB results from Hole U1473A are reported in more detail. Also, new determinations of U1473A and U1309D (at AM) core sample velocities at pressures up to 200 MPa are reported. Following the description of the logging and relevant core measurements, we compare and contrast the results and discuss possible implications for recognizing core complex evolution at AB and AM.

Observations

Hole U1473A, AB good borehole condition away from recognized fractured zones. Velocities are generally lower, ~ 6.4 km/s, at depths less than 580 mbsf, whereas Vp averages ~ 6.9 km/s below that depth. Mean Vp/Vs ratio for the full section is 1.8 ± 0.08, with variability about the mean dropping from ± 0.1 above 580 mbsf to ± 0.03 at greater depths.

The presence of alteration veins correlates with the amplitude of density and velocity variability, as expected given the observed (MacLeod et al. 2017) clay, carbonate, or secondary amphibole fill compositions. The mean thickness of these veins averages 0.13 cm, with just two instances where a vein exceeds 2 cm in the recovered core. Veins up to 2 cm thick were not uncommon in the upper 100 m; mean thickness of 0.3-1 cm is not uncommon in the 100-300 mbsf interval. Below that, almost all veins in the recovered core were less than 0.2 cm in mean thickness. Most of the core from shallower than 580 mbsf was characterized as having an intensity of 1-5 veins per 10 cm. Below that depth, typical intensity drops to < 1 vein per 10 cm, except for an increase at 630-650 mbsf, below which the section is free of clay and carbonate veins [START_REF] Macleod | Southwest Indian Ridge Lower Crust and Moho[END_REF]. In this deepest section, logged velocities are more constant and the variation that does occur coincides with isolated vein(s) identified in the core and/or logged as spikes in gamma radiation (5-7 gAPI relative to background level of < 2 gAPI). Density in this deepest region shows some relation to core sample oxide content, although it is not simple. Oxide content was typically estimated by macroscopic core description, with occurences of locally increased concentration guided by along-core magnetic susceptibility plots. Density values somewhat greater than 2900 kg/m are typical from 580 to 685 mbsf, they drop below 2900 kg/m for the olivine gabbro interval 685-710 mbsf, and then increase to almost 3000 kg/m where oxide content is notable, to ~ 740 mbsf (Fig. 2). This downhole pattern is not mirrored in Vp, which would not be surprising if oxide dominates the density signal, since oxide gabbro velocity is lower than that of olivine gabbro [START_REF] Iturrino | Velocity structure of the lower oceanic crust: results from Hole 735B, Atlantis II Fracture Zone[END_REF].

The U1473A cross dipole sonic log data were processed to determine anisotropy using the post-processing module in Schlumberger's Techlog software. Shear waves and their slowness are identified/determined by that algorithm, and rotation to obtain the polarization direction of the fast shear wave is solved. Not surprisingly, intervals that might have been most interesting in terms of anisotropy signature were those where fracturing degraded the borehole condition, so reliable results were not obtained.

Outside of these zones, several intervals that are 10′s m in length display relatively constant azimuth of fast shear wave propagation (~ N20°W, Figure S1) with low anisotropy magnitude ~ 1%. Just a few meter-scale intervals indicate larger anisotropy, 3-4%, at subseafloor depths of 335, 410, and 545 mbsf. The two shallower intervals are associated/bounded by faults documented in the core so deformation-controlled porosity likely is responsible for the anisotropy. Between 541 and 548 mbsf, visual core description [START_REF] Macleod | Southwest Indian Ridge Lower Crust and Moho[END_REF] indicates a fault, underlain by a zone of increased veining, both within olivine gabbro, which overlies oxide gabbro for which the upper contact was not recovered. So, deformation related orientation of porosity probably is responsible for the logged seismic anisotropy.

Core sample measurements at STP [START_REF] Macleod | Southwest Indian Ridge Lower Crust and Moho[END_REF]) document an expected general decrease in Vp as porosity increases in Hole U1473A but there is considerable scatter (Fig. 3a). Similarly, primary mineralogy is not a consistent factor in the measured core sample velocity, as indicated by the lack of clean trend of Vp with change in olivine mode (Fig. 3b), although olivine content does play a role. Gabbro samples (olivine mode < 5) have average Vp of 6.59 km/s whereas the average for olivine gabbros (mode 5-17 in the U1473A sample set) is 6.74 km/s. Alteration plays a more important role in reducing velocity for these samples, mean Vp values dropping on the order of 0.18 km/s for every 10% increase in overall alteration, as visually estimated [START_REF] Macleod | Southwest Indian Ridge Lower Crust and Moho[END_REF].

Fig. 3

Hole U1473A core sample physical properties measured at STP (MacLeod et al. 2017). a Velocity shows broad decrease with increasing porosity, also ). In the upper, drilled section, an overall increase in seismic velocity with depth is punctuated by local reductions, where alteration is locally high and/or deformation structures are more intense [START_REF] Blackman | of 34 4/26/19, 14:19 signatures of past and present hydration within a young oceanic core complex[END_REF]. Vp increases from ~ 4 to 6 km/s from the seafloor to 350 mbsf, showing significant variability at meter(s) scale (Fig. 4). Average Vp is 6.0 km/s in the 350-800 mbsf interval where variation is still significant (as much as ± 1 km/s). The lower part of the section, 800-1400 mbsf, has Vp of 6.6 km/s on average, with generally smaller meter-scale variability.

The olivine-rich troctolite interval at 1080-1220 mbsf averages ~ 1 km/s slower than the surrounding section due to generally much greater alteration in that interval (Figs. 4 and5), although local variation in the extent of serpentinization is significant (Fig. 5). The boundaries between petrologic units tend to show drops in Vp and, as expected, intervals where serpentinization is nearly complete (total alteration 80-100%) are associated with local dip in Vp.

Fig. 4

Downhole Relation between primary and secondary lithology and P-wave velocity, shown for the deeper olivine-rich troctolite interval in Hole U1309D. Lithology shown at left is based on petrologic units described during Exp 304/305, as was the total alteration of core pieces (black symbols in right panel). Red curve shows the sonic log data with no downhole averaging. Correspondence of depths between core and log may be uncertain by up to 1-2 m, still a common correlation between Vp variability and alteration is evident. Figure S2 shows the same data with depth scale expanded to aid core-log comparison A fault at 746 mbsf has a damage zone extending at least 20 m below, and possibly as much as that above (depending how much borehole degradation has impacted data quality) where Vp is 1-2 km/s lower than surrounding wallrock (Fig. 4). Low return and the presence of ultracataclasite (thin black interval in lithology column of Fig. 4) in the 17% that was recovered, along with analysis of images of borehole structure document this fault [START_REF] Michibayashi | Hydration due to high-T brittle failure within in situ oceanic crust, 30°N Mid-Atlantic Ridge[END_REF]. Diabase (gray at 760 mbsf in Fig. 4) intruded the section right at the base of the reduced Vp zone associated with this fault.

Cross dipole sonic log data for Expedition 340T covers the lower ~ half of the section and these data were processed the same way as the Hole U1473A data. For most of this lower section, the polarization direction of the fast shear wave is variable, a given value consistent only along intervals < ~ 5 m in length; the magnitude of anisotropy is low overall (generally < 2%; Figure S1). Only in the 900-1080 mbsf interval, where a fairly uniform gabbroic sequence occurs, is there a constant fast azimuth (N ~ 70°W) with anisotropy magnitude ~ 1.5%. Two meter-scale intervals where anisotropy reaches 3-6% occur in the olivine-rich troctolite zone 1090-1200 mbsf.

In-situ velocity/density of OCC gabbroic rocks at lower crustal depth

Selected core samples from Hole U1309D were analyzed at pressures simulating conditions throughout the crust, to estimate in situ Vp for a multi-km thick gabbroic sequence. Results for 200 MPa (~ Moho depth for a standard ocean crustal section) were part of the seismic results reported by [START_REF] Collins | Seismic and drilling constraints on velocity structure and reflectivity near Hole U1309D on the central dome of Atlantis Massif, Mid-Atlantic Ridge 30[END_REF]. An error in the calibration used to compute those values was recognized recently, so we report corrected values here, along with showing how measured Vp on these samples varies as confining pressure is increased from 0 to 200 MPa. We analyze these results together with prior measurements made on gabbroic samples from ODP Holes 735B Hole U1309D rocks that were originally olivine-rich, either troctolite or olivine gabbro with more than 20% modal olivine, show a linear trend of increasing Vp (5.6-6.5 km/s) with increasing density (2650-2800 kg/m , Fig. 6a). Most of the samples from this and the other sites scatter at higher value, in the ranges 6.5-7.5 km/s and 2800-3040 kg/m , although the broad increase of Vp with density does continue the same basic trend as the originally olivine-rich samples. There is no clear pattern in Vp at 200 MPa with respect to gabbro sample depth. All but the U1309D highly altered olivine-rich samples, which occurred within the olivine-rich troctolite zone ~ 290-350 mbsf, vary throughout the 6.5-7.5 km/s range (Fig. 6b).

Fig. 6

Properties of core samples from gabbroic sections obtained by deep sea drilling, at room temperature. The increase in velocity with pressure was determined for each sample suite [START_REF] Carlson | Influence of pressure and mineralogy on seismic velocities in oceanic gabbros: implications for the composition and state of lower oceanic crust[END_REF][START_REF] Collins | Seismic and drilling constraints on velocity structure and reflectivity near Hole U1309D on the central dome of Atlantis Massif, Mid-Atlantic Ridge 30[END_REF]. Curves predicting the average increase in Vp with the natural log of estimated depth [START_REF] Carlson | Influence of pressure and mineralogy on seismic velocities in oceanic gabbros: implications for the composition and state of lower oceanic crust[END_REF] were fit assuming an increase of 1 km per 20 MPa pressure increase. For Hole U1309D, gabbroic samples alone have average velocity about 0.1 km/s higher than the full sample set tested (Fig. 7a).

Predicted curves fit each sample set well and display ~ 0.5 km/s higher velocity in the upper 1-2 km of the crust that characterizes shallow gabbroic sequences, relative to the more typical upper crustal diabase (e.g. Hole 504B in crust formed at the East Pacific Rise), as shown in Fig. 7b. Velocity of core samples as a function of depth, as measured in a series of lab runs at TAMU where confining pressure was increased incrementally and then values were measured again as pressure was incrementally decreased (Supplementary Material, Tables S1 andS2). Differences between the velocity-depth curves for the different gabbroic sequences that have been drilled (Fig. 7b) indicate that alteration experienced by young intrusive oceanic crust would have a long-term velocity reduction effect as great as 0.25 km/s. Both the AM (U1309D) full dataset and the Hess Deep (894) dataset include more highly altered samples than the other datasets and the curves that fit these data predict lower velocity. Results to date suggest that the thickness of the affected interval in young ocean lithosphere may be on the order of 800-1000 m, although drilling coverage is not currently sufficient to constrain the actual penetration of hydrothermal circulation and associated alteration. If there are fractures that extend to full crustal depths (several km) and there are intervals with olivine content 20% or greater, the velocity reduction due to associated seawater alteration would persist at depth as the plate ages.

Comparison of AB and AM

The core complex data from AM and AB display differences in physical properties, for which primary lithology appears to play some role but deformation and alteration history are controlling factors. The shipboard sample data (Fig. 8) indicate that seismic velocity is consistently lower at AM than at AB, despite bulk and grain density having significant overlap between the sites. Post expedition measurements of selected U1473A and U1309D samples using instrumentation aboard the Chikyu [START_REF] Abe | IODP Expedition 360 postcruise research presentation, Sicily[END_REF]) confirm that the prior shipboard results do not simply reflect an unrecognized change in lab calibration on the JOIDES Resolution.

Fig. 8

Core sample porosity and Vp. a Comparison of prior [START_REF] Iturrino | Velocity structure of the lower oceanic crust: results from Hole 735B, Atlantis II Fracture Zone[END_REF][START_REF] Blackman | Expedition 304/305 Scientists[END_REF]) and new [START_REF] Macleod | Southwest Indian Ridge Lower Crust and Moho[END_REF][START_REF] Abe | IODP Expedition 360 postcruise research presentation, Sicily[END_REF] properties at STP for AB and AM. Red curve should be red, not gray, in Porosity of the core samples is a major factor in the reduced Vp values at AM, compared to AB samples. Shipboard determinations of sample porosity are several percent higher in some of the AM section and this is one reason for the difference (Fig. 8a). Importantly, samples from deeper than ~ 800 mbsf in Hole U1309D show a steady decreasing Vp trend with depth, which is not observed in the borehole log (Fig. 8b). Hole 735B samples do not exhibit such a decrease in Vp for the lower part of the section (no log data are available for comparison at this site). We infer that Hole U1309D samples are currently under a combination of lithostatic and tectonic loads which, when released through drilling and recovery, results in microcracking that reduces Vp from the in situ value. Such effect has been noted in prior investigations of deep, hard rock core from the Pacific [START_REF] Carlson | The effects of alteration and porosity on seismic velocities in oceanic basalts and diabases[END_REF]. Current conditions at Site 735 in the 800-1500 mbsf interval must be such that sample unloading effects are minor. Larger fractures affect in situ Vp at depths to several hundred meters below seafloor, where core sample values are almost always higher than log values (Fig. 8c, U1309D; [START_REF] Macleod | Southwest Indian Ridge Lower Crust and Moho[END_REF]. These are not a factor at greater depths where lab Vp tracks logged value quite well (e.g. Fig. 8d; [START_REF] Macleod | Southwest Indian Ridge Lower Crust and Moho[END_REF]. Logged velocity is sensitive not only to effective properties that reflect distributed small cracks/porosity across the 2-m tool length, but also to features with size ranging up to many cm. Core samples, on the other hand, are chosen to avoid visible fractures and measurement of velocity on these cubic-2-cm pieces is sensitive to µm to sub-mm scale porosity.

In the upper ~ 800 m, differences in primary composition and/or the respective alteration histories probably also contribute to the reduced sample Vp at AM compared to AB. The latter is dominated by olivine gabbro (modal percent olivine > 5%) whereas the upper kilometer and a half of AM comprises dominantly gabbro. [START_REF] Carlson | The olivine enigma: or why alteration controls the seismic properties of oceanic gabbros[END_REF] showed that the influence of olivine content on seismic velocities in gabbros depends on the Mg# as well as the degree of alteration to serpentine. In light of the comparatively high Mg numbers of the OCC samples (75-85), a greater olivine content would correspond to higher Vp in unaltered samples. While amounts are modest, AM contains several troctolite intervals and significant fractions of the olivine within these units are extensively altered, thereby lowering Vp overall despite (primary) olivine content being locally high.

The upper ~ 600-800 mbsf at both the AB and AM OCCs show greater variability in Vp than the underlying sections penetrated by seafloor drilling and this pattern probably reflects detachment-related processes (Fig. 9). The thickness of the deformation zone associated with detachment faulting at OCCs has been shown to be several hundred meters [START_REF] Dick | A long in situ section of the lower ocean crust: results of ODP Leg 176 drilling at the Southwest Indian Ridge[END_REF][START_REF] John | Deformation and alteration associated with oceanic and continental detachment systems: Are they similar?[END_REF][START_REF] Blackman | of 34 4/26/19, 14:19 signatures of past and present hydration within a young oceanic core complex[END_REF][START_REF] Harding | Velocity structure near IODP Hole U1309D, Atlantis Massif, from waveform inversin of streamer data and borehole measurements[END_REF], with the intensity of impacts being greatest in the 200-400 m immediately beneath the fault [START_REF] Schroeder | Strain localization on an oceanic detachment fault system[END_REF]Karson et al. 2006;[START_REF] Escartín | Constraints on deformation conditions and the origin of oceanic detachments, the Mid-Atlantic Ridge core complex at 15°45′N[END_REF]. The association that we find between Vp variability, on the Since AB is a wave-cut platform covered by carbonate deposits (Robinson and von Herzen 1989), it is unclear how much of the original section from just below the exposed detachment fault has been eroded away.

Relationships between the core recovered from Hole U1473A and Hole 735B and 1105A suggest that faulting within the bank may vertically offset these sequences by a up to a few hundred meters (MacLeod et al. 2017) so only rough depth correlation between the AB Sites is possible. Regardless, removal of a couple hundred meters of the detachment deformation zone could explain why cataclastic structures in the core from AB drop notably at a shallower depth than at AM-somewhat < 600 mbsf versus ~ 800 mbsf, respectively. We cannot rule out the possibility that the detachment fault systems at AB and AM simply had different deformation distributions with depth. The occurrence of troctolites in the AM sequence and its proximity to a serpentinite-dominated domal core (the middle and western portion of the Southern Ridge, [START_REF] Henig | Downward continued multichannel seismic refraction analysis of Atlantis Massif oceanic core Ridge[END_REF][START_REF] Harding | Velocity structure near IODP Hole U1309D, Atlantis Massif, from waveform inversin of streamer data and borehole measurements[END_REF]Früh-Green et al. 2016), may indicate that magmatic conditions and possibly the interplay with faulting differed between the two OCCs [START_REF] Escartín | Constraints on deformation conditions and the origin of oceanic detachments, the Mid-Atlantic Ridge core complex at 15°45′N[END_REF].

Certainly, the extent of high temperature plastic deformation in the upper couple hundred meters of AB at Hole 735B is greater than that observed in the upper part of Hole U1309D (Ildefonse et al. 2007).

The similarity of depth to which Vp variability extends at AB and AM and the association of this variability with alteration veins suggests that effects of long-term aging on a ridge flank do not overwhelm signatures that develop as an OCC forms. Differences in the balance of magmatic/tectonic activity between the sites might be expected. While Despite AB having formed at an ultraslow spreading rate, [START_REF] Dick | A long in situ section of the lower ocean crust: results of ODP Leg 176 drilling at the Southwest Indian Ridge[END_REF] conclude that melt supply in this segment center was higher than typical for most the SWIR. AM formed at the end of a slow-spreading segment and magmatism is interpreted to be episodic due to reduced melt supply far from the segment center [START_REF] Blackman | Drilling constraints on lithospheric accretion and evolution at Atlantis Massif[END_REF][START_REF] Henig | Downward continued multichannel seismic refraction analysis of Atlantis Massif oceanic core Ridge[END_REF]. Recent petrologic, geochemical, and structural analysis reported by [START_REF] Dick | Dynamic accretion beneath a slow spreading ridge segment: IODP Hole U1473A and the Atlantis Bank oceanic core complex[END_REF] support a fundamental difference in melt, crystallization, and accretion dynamics between AB and AM. However, while the associated differences in igneous stratigraphies might explain a modest offset in Vp between the sites, it cannot explain a the observed consistent depth extent of variabilty in Vp.

Borehole logs indicate that Vp of 6.9 km/s would likely characterize gabbroic bodies within the upper kms of OCC if faulting and alteration have not significantly affected them. For each of Holes 735B, U1473A, and U1309D, this value dominates the log at depths where cataclastic structures and alteration are generally quite low (< 1 vein per 10 cm, minor fracturing). The uncertainty in the full waveform inversion also collapses for this part of the section at AM (Fig. 9, center panel), suggesting that advanced processing of multi-channel seismic data [START_REF] Harding | Velocity structure near IODP Hole U1309D, Atlantis Massif, from waveform inversin of streamer data and borehole measurements[END_REF][START_REF] Canales | Seismic evidence for large-scale compositional heterogeneity of oceanic core complexes[END_REF][START_REF] Canales | Small-scale structure of the Kane oceanic core complex, Mid-Atlantic Ridge 23°30′N, from waveform tomography of multichannel seismic data[END_REF][START_REF] Xu | Seismic imaging of the Kane oceanic core complex using early-arrival full waveform inversion and pre-stack depth migration of multichannel seismic streamer data[END_REF]) can provide some constraint on the extent of hydration within undrilled portions of OCC gabbro bodies.

  significant scatter. b Velocity shows very slight dependence on olivine mode in this 0-800 mbsf section Hole U1309D, AM Site U1309 in the footwall to the Central Dome detachment at AM comprises a gabbroic sequence that is probably ~ 4 km thick[START_REF] Harding | Velocity structure near IODP Hole U1309D, Atlantis Massif, from waveform inversin of streamer data and borehole measurements[END_REF][START_REF] Blackman | Lower crustal variability and the crust/mantle transition at the atlantis massif oceanic core complex[END_REF]. Results of post-drilling seismic survey data analysis, employinged downward continuation of streamer refraction data and waveform inversion, are consistent with the borehole data at the scale of imaging resolution (order 100-200 m,[START_REF] Harding | Velocity structure near IODP Hole U1309D, Atlantis Massif, from waveform inversin of streamer data and borehole measurements[END_REF]

Fig. 5

 5 Fig. 5

Fig. 7

 7 Fig. 7 legend in Figure 7b has incorrect year in Cary & Chapman reference-it should be 1988

Figure 7a curve

 7a Figure 7a curve shows Vp: Porosity prediction following formulation developed by ((Carlson 2010; Carlson 2014); see Supplementary Material). b Core sample (black dot) and logged Vp (10-m average, red) in Hole U1309D. (c-d) Raw log (red, magenta) and sample Vp for 2 intervals at lab pressure of 0 (black dot), 10 (blue +), and 100 MPa (green o)

  ). Circles show location of drill holes and Site numbers for which core/log data are discussed. Gray arrows indicate relative plate motion at spreading axes (opposing arrows) and transform faults (single side arrow). Index map also shows locations (+) of Kane core complex (Atlantic) and Hess Deep tectonic window (Pacific) where gabbroic sequences were also recovered

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  data for IODP Hole U1309D, at AM, MAR. Lithology determined by visual core description. Logging runs show borehole width/condition, and 2-m running average of wallrock density and seismic velocity. Exp 305 sonic log shown by purple curve and Exp 340T, 7 years later, is shown by red curve. Plot scales and lithology key are same as in Fig.2

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a

  Corrected calibration for U1309D gabbro sample values with standard error shown, and curve fit solution for all rock types tested from that hole. b Curve fit solutions for sample suites from several gabbroic sections drilled by (I)ODP (735B, 894G, and 923A data from[START_REF] Carlson | Influence of pressure and mineralogy on seismic velocities in oceanic gabbros: implications for the composition and state of lower oceanic crust[END_REF]. Seismic refraction data from various Atlantic studies are shown in gray curves, with crustal age and sources identified in legend. These selected expanding spread profile data have good velocity control and are located away from fracture zones

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  -scale downhole, and the intensity of alteration veins indicates that seawater circulation is enabled by the detachment-induced deformation and footwall exposure. The fluid flow extends to depths of at least ~ 800 mbsf and persists long enough for reaction with the rock to leave a physically (and chemically) detectable record.

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	Fig. 9
	Comparison of AB Holes 735B, U1473A, and AM Hole U1309D downhole
	data. Lithology shown as 20-m running average of recovered core.
	Alteration and deformation determined by visual core description on each
	expedition, with same intensity scale. All velocity depth curves are plotted
	together in center panel: dark green U1473A sonic log (10-m average); light
	green 735B sonic log (no ave, Iturrino et al. 2002); purple-U1309D sonic
	log (10-m ave.); gray-histogram of FullWave Inversion model within 1 km
	of U1309D (Harding et al. 2016); magenta-representative FWI curve for
	Southern Ridge of AM for comparison with Central Dome at Hole U1309D.
	Faulted intervals are marked by thick gray-green lines for U1473A and
	U1309D
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Acknowledgements

This research used samples and data provided by the International Ocean Drilling Program and the International Ocean Discovery Program. Funding for this work was provided by the National Science Foundation, via the US Science Support Program (DKB, AH).

Publisher's Note

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Electronic supplementary material

Below is the link to the electronic supplementary material.

Supplementary material 1 (DOCX 108 kb) Supplementary material 2 (EPS 7759 kb) Supplementary material 3 (EPS 3040 kb)


K Neep

T Nikolopoulos

R Papaevangelou

Ward

Fast Neutron Spectroscopy with a Nitrogen-Based Gaseous Detector

I. Katsioulas, I. Giomataris, P. Knights, T.

Fast Neutron Spectroscopy with a Nitrogen-Based Gaseous Detector I. Katsioulas, I. Giomataris, P. Knights, T. Neep, K. Nikolopoulos, T.Papaevangelou, R. Ward

Abstract-A simple, e cient and safe to use but also a ordable fast neutron spectroscopic system is the Holy Grail in many scientific and industrial communities. 3 He based detectors provide a solution that fulfils most of the requirements but its high demand combined with its scarcity has created a worldwide shortage and e orts are focused on finding an alternative solution. Several detectors are proposed for this task, such as BF 3 proportional counters, plastic scintillators, and recoil detectors, all with disadvantages that make their adoption far from ideal. We propose a system based on a spherical proportional counter filled with nitrogen-based gas mixtures to exploit the 14 N(n,a)B 11 and 14 N(n,p)C 14 reactions for the detection of fast neutrons. 3 He and N 2 have comparable Q-values and absorption cross-sections for fast neutron capture, o ering similar detection capabilities. In the future, such a detector could be utilised in industrial applications such as reactor monitoring, medical physics, accelerator facilities, homeland security, fuel searches and serve as a tool in scientific research.

I. I

A simple, e cient, yet a ordable fast neutron spectroscopy system is sought after by many scientific and industrial communities but with little success. Several attempts were made for an e cient spectroscopy system based on various detector technologies [START_REF] Brooks | Neutron spectrometry-historical review and present status[END_REF]. However, neutron measurements remain cumbersome, as demonstrated by the sparsity of detailed neutron spectra in many laboratories and industrial sites.

The most popular method is that of 3 He based detectors, as it has high e ciency for thermal neutrons through the 3 He(n,p)He 3 ( th =5330 b) reaction and very low e ciency in g-ray detection. 3 He is not flammable and non-toxic, while detectors using it are radiation hard and have a long lifetime. For fast neutrons detection using 3 He, usually cylindrical proportional counters are employed operated in pulsed mode for g-ray discrimination [START_REF] Knoll | Radiation detection and measurement[END_REF] in high pressure or 3 He gaseous scintillators in pressure close to 150 bar.

Because of its popularity, 3 He demand is disproportionate to production, dramatically increasing its price. Consequently, 3 He based detectors require expensive retrieval systems, leading governments and industries to search for alternatives. Existing alternatives [START_REF] Kouzes | Neutron detection alternatives to 3He for national security applications[END_REF], [START_REF] Simpson | INMM 52nd Annual Meeting[END_REF] have disadvantages prohibiting them replacing 3 He detectors: i) BF 3 proportional counters-BF 3 is a toxic and corrosive gas; ii) 10 B lined tubes-Poor e ciency, and a high cost; iii) Bulk scintillators-Complicated response functions and an insu cient /n separation along with limited radiation hardness; iv) 6 Li coated argon based detectors-6 Li(n, a)H 3 reaction has a high Q-value (2.79 MeV) a ecting resolution.

II. T

A nitrogen-based neutron detection system aspires to provide an alternative to 3 He based detectors for neutron spectroscopy. The detection relies on the 14 N(n, a) B 11 and 14 N(n, p)C 14 reactions, with the neutron energy determined through the measurement of the reaction charged products with a gaseous proportional counter. Early explorations of this idea are discussed in Ref. [START_REF] Bougamont | Neutron spectroscopy with the Spherical Proportional Counter based on nitrogen gas[END_REF]. The cross-section of the 3 He(n,p)He 3 reaction although higher is comparable to the 14 N(n, a) B 11 and 14 N(n, p)C 14 cross sections for fast neutrons as displayed in Fig. 1 and can be compensated by the higher number of atoms in N 2 and the much-reduced wall-e ect. Also, although nitrogen has a much lower thermal neutron absorption crosssection (1.83 b), due to the large number of atoms in a sensitive volume it could nonetheless prove to be e cient for thermal neutron counting.

Traditionally, the operation of nitrogen-filled gaseous detectors in atmospheric pressure or above has been challenging due to the high voltage required to reach proportional mode operation. Equally daunting has been the task of ensuring containment of the resulting MeV-range protons or alpha particles within a detector's sensitive volume. That is the reason why 3 He-filled cylindrical proportional counters are used mainly as flux monitors and rarely for spectroscopy. These challenges are addressed by the use of a Spherical Proportional Counter (SPC).

III. T S P C

The SPC, shown in Fig. 2, consists of a grounded spherical shell (cathode) and a small sphere (anode) placed at the centre of the shell, supported by a grounded metallic rod, to which high voltage is applied and from which the signal is read-out. The 1/r 2 dependence, where r is the radius from the centre, of the electric field results in large charge multiplication factors with anode diameters of O(mm), and thus is more robust and reliable detector construction compared to cylindrical counters. Details on the SPC and its advantages compared to other detector geometries can be found in [START_REF] Katsioulas | NEWS-G, Light dark matter search with a Spherical Proportional Counter, First results and Future prospects[END_REF], [START_REF] Arnaud | Spherical Proportional Counter: A review of recent developments[END_REF], [START_REF] Knights | Recent developments in the read-out electrodes for Spherical Proportional Counters[END_REF]. The pulseshape dependence on the nature of the interaction (interaction radius, track length, orientation, clustering) provides handles to discriminate events using a small number of parameters (pulse height, rise time or width) [START_REF] Savvidis | Low energy recoil detection with a spherical proportional counter[END_REF]. The background discrimination capability greatly improves the performance for neutron detection through rejection of g-induced background and partial energy depositions. These features and the inherent low g-ray e ciency of gaseous detectors make such a system ideal for mixed g/n environments.

IV. O U B

The detector used in the University of Birmingham (UoB) is a 30-cm-diameter stainless steel SPC. The detector is operated in sealed mode after being filled with pure nitrogen which has been filtered to reduce contaminants (O 2 , H 2 O) to parts per trillion level. The detector is equipped with resistive sensors developed for high-gain, high-pressure operation [START_REF] Katsioulas | A sparkless resistive glass correction electrode for the spherical proportional counter[END_REF] and can withstand high voltages applied over 6 kV. The experimental setup is displayed in Fig. 3. The detector calibration, shown in Fig. 4, is done using radiation sources such as 239 Pu, 241 Am, 244 Cm that emit alpha particles with energy of a few MeV and characterised using fast neutrons sources such as Cf 252 and 241 Am-9 Be. Furthermore, the detector capability to discriminate g-rays from neutron-induced events using g-sources such as 137 Cs is studied, before the system can be moved to mixed radiation environments.

The response of the detector is simulated using a framework developed at UoB based on GEANT4 and Garfield++ for high energy physics applications, more details can be found in [START_REF] Ward | Development of a Simulation Framework for Spherical Proportional Counters[END_REF]. As an example, shown Fig. 5 is the rise time versus amplitude distribution of simulated pulses of events induced by 4 MeV neutrons interacting with the nitrogen nuclei in a 30-cmdiameter detector. The simulated response is being compared with measurements for validation. Finally, a series of measurements are under consideration in the UoB's MC40 cyclotron and in underground laboratories, attempting to emulate hadron therapy environments and to improve the neutron background estimation of rare-event experiments respectively. Green points are events coming from the 14 N(n,a)B 11 reaction, yellow points are events from the 14 N(n,p)C 14 reaction, and red points are elastic scattering events.

V. A This neutron detection system is aimed to be applied initially to measurements of neutron backgrounds in underground laboratories (where rare event searches take place), reactor driven experiments and to measurements of fast neutrons in hadron therapy environments. Developments that could improve the sensitivity in a large number of experiments and create a safer environment for patients and medical sta .

Nevertheless, the main goal is the development of a system that could to be used in industrial applications: reactor monitoring, medical physics, accelerator facilities, homeland security, fuel searches, space science and other.

VI. C

A nitrogen-filled SPC presents a simple, robust, and reliable fast neutron spectroscopy system. The use of a large volume gaseous detector, such as the SPC, filled with a nitrogenbased mixture, provides an ideal alternative to a 3 He-based fast neutron spectrometer. A gaseous nitrogen mixture can provide good e ciency for the detection of fast neutrons, without moderation, and limited influence from g-rays. The nitrogen-based detector exhibits the desired features of 3 He counters and can pioneer a line of detectors with alternatives designs, more compact or larger to cover di erent needs. The nitrogen-based spectroscopic system is being developed and tested at the University of Birmingham. The detector response is studied using simulations based on GEANT4 and Garfield++ validated through measurements with alpha and fast neutron sources. Such a fast neutron spectroscopy system will facilitate neutron measurements in laboratories, sensitive environments and industrial sites. R

  This project has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement no 841261 (DarkSphere) I. Katsioulas, T. Neep, K. Nikolopoulos, and R.Ward are with the School of Physics and Astronomy, University of Birmingham, B15 2TT, United Kingdom I. Giomataris and T.Papaevangelou are with IRFU, CEA, Universite Paris-Saclay, Gif-sur-Yvette, F-91191, France P. Knights is with the School of Physics and Astronomy, University of Birmingham, B15 2TT, United Kingdom and with IRFU, CEA, Universite Paris-Saclay, Gif-sur-Yvette, F-91191, France

Fig. 1 .

 1 Fig. 1. Comparison of the 3 He(n,p)He 3 , 10 B(n,a)Li 7 , 14 N(n,p)C 14 and 14 N(n,a)B 11 neutron reaction cross sections for energy up to 20 MeV [9].

Fig. 2 .

 2 Fig. 2. The spherical proportional counter design and its principle of operation.

Fig. 3 .

 3 Fig. 3. The experimental setup in the University of Birmingham (Photo credit: R. Owen).

Fig. 4 .

 4 Fig. 4. Rise time versus amplitude distribution displaying the detector calibration using alphas sources. The detector was filled with 300 mbar of nitrogen.

Fig. 5 .

 5 Fig. 5. Rise time versus amplitude distribution of simulated events from 4 MeV neutrons interacting with the nitrogen nuclei in a 30-cm-diameter detector.Green points are events coming from the 14 N(n,a)B 11 reaction, yellow points are events from the 14 N(n,p)C 14 reaction, and red points are elastic scattering events.