Hyun-Ok Cho

Sung Hi Cho
email: chois@cnu.ac.kr

Pierre Schiano

Moonsup Cho

Nicolas Cluzel

Jean-Luc Devidal

Kyoochul Ha

Hyun-Ok Choi

Sung Hi Choi

Jean- Luc Devidal

inclusions in the Baekdusan (Changbaishan) basalts: Implications for recycling of oceanic crustal materials into the mantle source

Keywords: Baekdusan, mantle transition zone, melt inclusions in olivine phenocrysts, recycled oceanic crustal materials, plume

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

Introduction

Silicate melt inclusions (hereafter 'melt inclusions') are small droplets entrapped in phenocrysts during their growth within magmatic systems (e.g., [START_REF] Lowenstern | Applications of silicate-melt inclusions to the study of magmatic volatiles[END_REF][START_REF] Roedder | Fluid Inclusions[END_REF]. Melt inclusions entrapped by phenocrysts that formed early in the process of magma evolution may represent snapshots of primitive magmatic conditions [START_REF] Kent | Melt inclusions in basaltic and related volcanic rocks[END_REF][START_REF] Schiano | Primitive mantle magmas recorded as silicate melt inclusions in igneous minerals[END_REF][START_REF] Shimizu | The geochemistry of olivine-hosted melt inclusions in a FAMOUS basalt ALV519-4-1[END_REF][START_REF] Sobolev | Melt inclusions in minerals as a source of principle petrological information[END_REF]. Studies of such melt inclusions can provide important insights into the nature of mantle sources and the melting and melt transport processes that occur in primitive magma and are subsequently erased from the basaltic rock by fractional crystallization, magma mixing, or crustal contamination during its evolution. Meanwhile, it is well recognized that the original composition of a melt inclusion may be altered by postentrapment processes during natural cooling, such as crystallization of host minerals on the walls of inclusions, crystallization of other daughter minerals inside the inclusions, formation of shrinkage bubbles, or diffusive re-equilibration between host minerals and the external magma (e.g., [START_REF] Chen | The rate of water loss from olivinehosted melt inclusions[END_REF][START_REF] Cottrell | Consequences of diffusive reequilibration for the interpretation of melt inclusions[END_REF][START_REF] Danyushevsky | Re-equilibration of melt inclusions trapped by magnesian olivine phenocrysts from subduction-related magmas: petrological implications[END_REF]Gaetani and Watson, 2000;[START_REF] Qin | Diffuse reequilibration of melt and fluid inclusions[END_REF][START_REF] Roedder | Fluid Inclusions[END_REF]. Such modification, however, can be reversed by rehomogenization experiments and/or numerical reconstruction (e.g., [START_REF] Danyushevsky | Experimental and petrological studies of melt inclusions in phenocrysts from mantle-derived magmas: an overview of techniques, advantages and complications[END_REF][START_REF] Kent | Melt inclusions in basaltic and related volcanic rocks[END_REF][START_REF] Schiano | Primitive mantle magmas recorded as silicate melt inclusions in igneous minerals[END_REF].

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Late Cenozoic intraplate volcanism is widespread in the eastern North China Craton (NCC; Fig. 1a,b). Numerous petrological and geochemical studies have shown that Cenozoic basaltic rocks have oceanic island basalt (OIB)-like patterns of trace element distribution, including enrichment of light rare earth elements (LREE) and a lack of depletion of high field strength elements (HFSE), and thus might be derived from a peridotitic source in the subcontinental lithospheric mantle or asthenosphere [START_REF] Basu | Major element, REE, and Pb, Nd and Sr isotopic geochemistry of Cenozoic volcanic rocks of eastern China: implications for their origin from suboceanic-type mantle reservoirs[END_REF][START_REF] Chen | Geochemistry of Cenozoic volcanic rocks and related ultramafic xenoliths from the Jilin and Heilongjiang provinces, northeast China[END_REF][START_REF] Chu | Source of highly potassic basalts in northeast China: Evidence from Re-Os, Sr-Nd-Hf isotopes and PGE geochemistry[END_REF][START_REF] Kuritani | Intraplate magmatism related to deceleration of upwelling asthenospheric mantle: Implications from the Changbaishan shield basalts, northeast China[END_REF][START_REF] Kuritani | Transition zone origin of potassic basalts from Wudalianchi volcano, northeast China[END_REF][START_REF] Li | Geochemistry of Cenozoic basalts from the Bohai Bay Basin: implications for a heterogeneous mantle source and lithospheric evolution beneath the eastern North China Craton[END_REF][START_REF] Sakuyama | Melting of dehydrated oceanic crust from the stagnant slab and of the hydrated mantle transition zone: constraints from Cenozoic alkaline basalts in eastern China[END_REF][START_REF] Xu | Role of lithosphereasthenosphere interaction in the genesis of Quaternary alkali and tholeiitic basalts from Datong, western North China Craton[END_REF]Xu et al., , 2012a; [START_REF] Yan | Cenozoic alkali basalts from Jingpohu, NE China: the role of lithosphere-asthenosphere interaction[END_REF][START_REF] Zhang | Potassic volcanic rocks in NE China: geochemical constraints on mantle source and magma genesis[END_REF][START_REF] Zhang | Late Cenozoic intraplate volcanism in Changbai volcanic field, on the border of China and North Korea: insights into deep subduction of the Pacific slab and intraplate volcanism[END_REF][START_REF] Zou | U-Th systematics of dispersed young volcanoes in NE China: asthenosphere upwelling caused by piling up and upward thickening of stagnant Pacific slab[END_REF]. However, there is a general consensus that the peridotitic source contains a significant proportion of mafic lithologies such as pyroxenite, eclogite, or hornblendite, which may play an important role in generating OIB-like intraplate basaltic magmatism (e.g., [START_REF] Eisele | The role of sediment recycling in EM-1 inferred from Os, Pb, Hf, Nd, Sr isotope and Griffin[END_REF]Gao et al., 2004Gao et al., , 2008;;[START_REF] Hauri | Major-element variability in the Hawaiian mantle plume[END_REF][START_REF] Hirschmann | Alkalic magmas generated by partial melting of garnet pyroxenite[END_REF][START_REF] Keshav | High-pressure melting experiments on garnet clinopyroxenite and the alkalic to tholeiitic transition in ocean-island basalts[END_REF][START_REF] Kogiso | High-pressure partial melting of garnet pyroxenite: possible mafic lithologies in the source of ocean island basalts[END_REF][START_REF] Kogiso | Partial melting experiments of bimineralic eclogite and the role of recycled mafic oceanic crust in the genesis of ocean island basalts[END_REF]Pertermann and Hirschmann, 2003a, b;[START_REF] Rehkämper | Recycled ocean crust and sediment in Indian Ocean MORB[END_REF][START_REF] Sobolev | An olivine-free mantle source of Hawaiian shield basalts[END_REF][START_REF] Sobolev | The amount of recycled crust in sources of mantle-derived melts[END_REF][START_REF] Yaxley | Reactions between eclogite and peridotite: mantle refertilisation by subduction of oceanic crust[END_REF]. Recycled oceanic crust or meltperidotite reaction products may provide the mafic source rocks (e.g., [START_REF] Hauri | Major-element variability in the Hawaiian mantle plume[END_REF][START_REF] Herzberg | Identification of source lithology in the Hawaiian and Canary Islands: Implications for origins[END_REF][START_REF] Sobolev | An olivine-free mantle source of Hawaiian shield basalts[END_REF][START_REF] Sobolev | The amount of recycled crust in sources of mantle-derived melts[END_REF]Straub et al., 2008), and some Cenozoic basaltic rocks from the eastern NCC (e.g., Abaga-Dalinuoer, Chifeng, Jilin, Hebei, Hannuoba, Liaoning, Bohai Bay, Shandong, Anhui, Jiangsu) may be derived from a source lithology containing pyroxenite in addition to peridotite [START_REF] Hong | Constraints from melt inclusions and their host olivines on the petrogenesis of Oligocene-Early Miocene Xindian basalts, Chifeng area, North China Craton[END_REF][START_REF] Li | Olivine and melt inclusion chemical constraints on the source of intracontinental basalts from the eastern North China Craton: discrimination of contributions from the subducted Pacific slab[END_REF][START_REF] Liu | Recycled crust controls contrasting source compositions of Mesozoic and Cenozoic basalts in the North China Craton[END_REF][START_REF] Qian | Chemical and Pb isotope composition of olivine-hosted melt inclusions from the Hannuoba basalts, North China Craton: implications for petrogenesis and mantle source[END_REF]Zhang et al., 2009;Zhang and Guo, 2016).

The Baekdusan volcanic field (also called Changbaishan in China; N41°20′-42°40′, E127°00′-129°00′), located on the border between China and North Korea, represents the largest exposure of intraplate volcanic rocks on the NCC (Fig. 1b,c). The volcanoes in this area have erupted repeatedly in historical times. The most powerful eruption, called "the Millennium Eruption", occurred with a Volcanic Explosivity Index (VEI) of 6 or 7 in ca.

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938-946 AD [START_REF] Horn | Volatile emission during the eruption of Baitoushan Volcano (China/North Korea) ca. 969 AD[END_REF][START_REF] Iacovino | Quantifying gas emissions from the "Millennium Eruption" of Paektu volcano, Democratic People's Republic of Korea/China[END_REF][START_REF] Wei | Review of eruptive activity at Tianchi volcano, Changbaishan, northeast China: implications for possible future eruptions[END_REF][START_REF] Hong | Constraints from melt inclusions and their host olivines on the petrogenesis of Oligocene-Early Miocene Xindian basalts, Chifeng area, North China Craton[END_REF]Yin et al., 2012;[START_REF] Zou | Rapid development of the great Millennium eruption of Changbaishan (Tianchi) Volcano, China/North Korea: evidence from U-Th zircon dating[END_REF]. Voluminous Plinian fallout, ignimbrite, lahar, and other erupted materials covered 33,000 km 2 of northeastern China and Korea (Stone, 2011;Sun et al., 2014), and extended as far as northern Japan, ~1200 km from the volcano [START_REF] Machida | Extensive ash falls in and around the Sea of Japan from large late quaternary eruptions[END_REF]. After the Millennium Eruption, minor volcanic activity continued, with eruptions in 1413, 1597, 1668, 1702, 1898, and 1903 AD, exhibiting a roughly 100-year periodicity [START_REF] Cui | A textual research on the historic records of the eruptions from Tianchi Volcano, Changbaishan[END_REF]Stone, 2011;Xu et al., 2012b[START_REF] Hong | Constraints from melt inclusions and their host olivines on the petrogenesis of Oligocene-Early Miocene Xindian basalts, Chifeng area, North China Craton[END_REF]. Although the volcano has not erupted in the past 100 years, all available geological data including seismicity, ground deformation, and geochemical monitoring of springs indicate that Baekdusan is an active volcano with the potential for eruption in the near future [START_REF] Ramos | U-series and 40 Ar/ 39 Ar ages of Holocene volcanic rocks at Changbaishan volcano, China[END_REF][START_REF] Wei | Three active volcanoes in China and their hazards[END_REF][START_REF] Wei | Review of eruptive activity at Tianchi volcano, Changbaishan, northeast China: implications for possible future eruptions[END_REF]Xu et al., 2012b;Yun and Lee, 2012). Thus, it is of the utmost importance to understand the nature of the magmatism to anticipate its behaviour at this site. Previous studies have primarily focused on the history of Baekdusan volcanic activity [START_REF] Wei | Three active volcanoes in China and their hazards[END_REF][START_REF] Wei | Timescale and evolution of the intracontinental Tianchi volcanic shield and ignimbrite-forming eruption, Changbaishan, Northeast China[END_REF][START_REF] Wei | Review of eruptive activity at Tianchi volcano, Changbaishan, northeast China: implications for possible future eruptions[END_REF], magma evolution and the eruptive mechanism (Liu et al., 2015a;[START_REF] Wang | Late Pliocene-recent tectonic setting for the Tianchi volcanic zone, Changbai Mountains, northeast China[END_REF][START_REF] Zhang | Late Cenozoic intraplate volcanism in Changbai volcanic field, on the border of China and North Korea: insights into deep subduction of the Pacific slab and intraplate volcanism[END_REF][START_REF] Zou | U-Th systematics of dispersed young volcanoes in NE China: asthenosphere upwelling caused by piling up and upward thickening of stagnant Pacific slab[END_REF][START_REF] Zou | Rapid development of the great Millennium eruption of Changbaishan (Tianchi) Volcano, China/North Korea: evidence from U-Th zircon dating[END_REF]Zou et al., , 2014)), and geochemical lines of evidence of enriched mantle sources [START_REF] Basu | Major element, REE, and Pb, Nd and Sr isotopic geochemistry of Cenozoic volcanic rocks of eastern China: implications for their origin from suboceanic-type mantle reservoirs[END_REF][START_REF] Chen | Geochemistry of Cenozoic basalts and mantle xenoliths in Northeast China[END_REF][START_REF] Hsu | Geochemistry of Cenozoic volcanic rocks from Kirin Province, northeast China[END_REF][START_REF] Kuritani | Intraplate magmatism related to deceleration of upwelling asthenospheric mantle: Implications from the Changbaishan shield basalts, northeast China[END_REF]Liu et al., 2015a). Here, we describe for the first time the major and trace element compositions of re-heated melt inclusions entrapped in olivine phenocrysts from the Baekdusan basaltic rocks. Combining these data with whole-rock major element, trace element, and host olivine compositions, we shed light on (1) the particular mineral phases and lithologies of the mantle source, (2) the nature of source heterogeneities, and (3) the geodynamic processes that gave rise to volcanism at Baekdusan.

Geological setting and sampling

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The NCC is one of the oldest cratons in the world (~3.8 Ga; [START_REF] Liu | Remnants of ≥3800 Ma crust in the Chinese part of the Sino-Korean craton[END_REF][START_REF] Zheng | 3.6 Ga lower crust in central China: new evidence on the assembly of the North China craton[END_REF], and the Baekdusan volcanic field is located on the northeastern margin of the NCC (Fig. 1a,b). The basement of the eastern NCC consists primarily of Archean tonalitetrondhjemite-granodiorite (TTG) gneisses, granitoids, and supracrustal rocks, which are locally overlain by Proterozoic to Paleozoic strata (e.g., [START_REF] Wang | Late Pliocene-recent tectonic setting for the Tianchi volcanic zone, Changbai Mountains, northeast China[END_REF], Zhang et al., 2015[START_REF] Zhang | Decoding Neoarchean to Palaeproterozoic tectonothermal events in the Rangnim Massif, North Korea: regional correlation and broader implications[END_REF]. The eastern NCC experienced widespread tectonothermal reactivation during the Late Mesozoic to Cretaceous, as indicated by the emplacement of voluminous silicic intrusions and volcanic rocks (e.g., [START_REF] Meng | What drove late Mesozoic extension of the northern China-Mongolia tract?[END_REF]. The Central Asian Orogenic Belt (CAOB; Fig. 1a) bounds the NCC to the north. Disappearance of the Paleo-Asian Ocean between the NCC and the southern accretionary margin of the Siberian Craton was completed in the Late Paleozoic to Early Mesozoic (Eizenhöfer et al., 2014), and the paleo-Pacific plate began to subduct under the Asian continent in the Early Jurassic (Sun et al., 2013). The Baekdusan volcanic field lies ~1,300 km away from the Japan Trench. Recent studies of seismic tomography show that the subducting Pacific slab flattens and stagnates in the mantle transition zone (~410 to 660 km depth) under northeastern Asia [START_REF] Duan | Seismic structure and origin of active intraplate volcanoes in Northeast Asia[END_REF][START_REF] Guo | Seismic evidence of on-going sublithosphere upper mantle convection for intra-plate volcanism in Northeast China[END_REF][START_REF] Huang | High-resolution mantle tomography of China and surrounding regions[END_REF][START_REF] Lei | P-wave tomography and origin of the Changbai intraplate volcano in Northeast Asia[END_REF]Liu et al., 2015b;[START_REF] Zhao | Seismic image and origin of the Changbai intraplate volcano in East Asia: role of big mantle wedge above the stagnant Pacific slab[END_REF]. The leading edge of the stagnant slab reaches a longitude of 120 o E (e.g., [START_REF] Huang | High-resolution mantle tomography of China and surrounding regions[END_REF].

The Baekdusan field has >100 volcanic centers, including three main mountains: Tianchi in the China-North Korea border region, Wangtian'e in China, and Namphothe in North Korea (Fig. 1c). Tianchi is the youngest and tallest cone with a 5-km-wide and 384-mdeep crater lake at its summit. These mountains are the source of the headwaters of the Songhua, Yalu, and Tumen Rivers (Fig. 1b). The eruptions of these three volcanoes are geochemically indiscernible (e.g., Liu et al., 2015a), and the volcanoes each consist of an early-stage basaltic plateau, a middle-stage trachytic cone, and a late-stage explosive comenditic ignimbrite (Liu et al., 2015a;[START_REF] Wei | Three active volcanoes in China and their hazards[END_REF][START_REF] Wei | Timescale and evolution of the intracontinental Tianchi volcanic shield and ignimbrite-forming eruption, Changbaishan, Northeast China[END_REF][START_REF] Wei | Review of eruptive activity at Tianchi volcano, Changbaishan, northeast China: implications for possible future eruptions[END_REF] (Fig. 1c). The main

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early shield-forming eruptions took place between 22.6 and 1.5 Ma [START_REF] Wei | Timescale and evolution of the intracontinental Tianchi volcanic shield and ignimbrite-forming eruption, Changbaishan, Northeast China[END_REF][START_REF] Wei | Review of eruptive activity at Tianchi volcano, Changbaishan, northeast China: implications for possible future eruptions[END_REF], and the cone-construction stage lasted from ~1.0 Ma to ~20 ka, followed by mostly comenditic eruptions [START_REF] Wei | Review of eruptive activity at Tianchi volcano, Changbaishan, northeast China: implications for possible future eruptions[END_REF]. Eruption of trachytic and comenditic magmas reflects a long period of differentiation from the parent basaltic magma (e.g., [START_REF] Wei | Timescale and evolution of the intracontinental Tianchi volcanic shield and ignimbrite-forming eruption, Changbaishan, Northeast China[END_REF].

Samples used in this study were collected from Wangtian'e volcano and Tianchi volcano along the Tumen River (Fig. 1b,c). The K-Ar whole-rock ages for the basalts range from ca. 5.5 to 0.2 Ma (Fan et al., 2007). Most samples exhibit a porphyritic texture with plagioclase phenocrysts (>0.1 mm in size) in a groundmass of olivine, clinopyroxene, plagioclase, ilmenite, and/or titanomagnetite. One sample (BDH-01) contains large olivine and plagioclase phenocrysts (up to 2-3 mm in size) in a groundmass of olivine, clinopyroxene, and plagioclase. Melt inclusions are randomly distributed throughout the olivine grains, and are generally rounded ellipsoidal shapes ~20 to ~200 μm in size. We carefully selected melt inclusions far from crack planes, which usually contain a shrinkage bubble and daughter minerals grown from the trapped melt during slow cooling.

Analytical procedures

All samples used in this study were freshly collected. Samples for the whole-rock analysis were crushed into small pieces (< 5 mm in diameter) in a tungsten carbide mortar, and cleaned in an ultrasonic bath containing Milli-Q water. Fresh fragments were pulverized in an agate ball mill prior to geochemical analysis. Whole-rock major element contents were analyzed by X-ray fluorescence spectrometry (XRF) at Pukyong National University in Pusan, South Korea. The data were reduced using a weighted regression line created from standards BIR-1 and MO-5. The precision of the technique for preparing and analyzing

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standards was within 5% (Table S1). Whole-rock trace element concentrations were determined using inductively coupled plasma mass spectrometry (ICP-MS) at Act Labs, Ontario, Canada. Precision was estimated to be ±10% based on replicate analyses of international rock standards (BIR-1, JR-1, and DNC-1; Table 1).

Heating experiments were conducted at the Laboratoire Magmas et Volcans (Clermont-Ferrand, France) with a Vernadsky-type heated microscope stage [START_REF] Sobolev | The formation conditions of the high-magnesium olivines from the monomineralic fraction of Luna 24 regolith[END_REF] to determine the composition of melt at the time of its entrapment. The contents of the inclusion (glass, gas, and daughter minerals) were homogenized by heating the host olivines in a 6-mm-long vertical Pt 90 Rh 10 tube in a purified He atmosphere under redox conditions that correspond to the iron-wüstite buffer. The temperature was monitored by a Pt-Pt 90 Rh 10 thermocouple welded to the sample holder. The accuracy of the temperature measurements was approximately ± 20ºC, and the system was calibrated at the melting temperature of Au (1064ºC). All experiments were performed with heating rates of 0.9ºC/s from 20ºC to 900ºC and 0.4ºC/s for T > 900ºC in order to access the effects caused by variations in the rate of the transformations in the inclusions. The homogenization temperatures of melt inclusions varied from 1216ºC to 1313ºC (Table S2). After quenching, the samples were polished to expose the inclusions for electron microprobe analyses. Textural evidence of quench crystallization was absent in back-scattered electron images of the inclusions studied. Some homogenized melt inclusions, however, contain tiny sulfide globules (Fig. S1).

Major element concentrations of homogenized melt inclusions and host olivines were determined using a CAMECA SX-100 electron microprobe at the Laboratoire Magmas et Volcans. Analyses for glass samples were performed using an accelerating voltage of 15 kV and a probe current of 8 nA for major elements (Na, Al, Si, Ca, Fe, Mg, Ti, Mn, K, and P) and 80 nA for S, with a defocused (5-10 μm) beam. For host minerals, the beam size was 1 μm and the beam current was 15 nA. Counting time was 10 s for peaks and 5 s for the

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background. Sulfur was analyzed in integrated mode, using 5 steps of 10 s each. The detection limit for S was ~50 ppm and analytical uncertainties were better than 10%. Natural and synthetic minerals and oxides were used for routine calibration, except for S, which was calibrated to VG2 basaltic glass according to the Smithsonian Microbeam Standards. For the glass analysis, an inter-laboratory check was performed using the international glass standard A99 (Kilauea basalt glass; [START_REF] Jarosewich | Electron microprobe reference samples for mineral analyses[END_REF]Thornber et al., 2002). These results are provided in Tables S2 andS3.

Trace element concentrations of reheated melt inclusions were analyzed by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) at the Laboratoire Magmas et Volcans. This instrument comprises an Excimer ArF laser (193 nm) coupled with an Agilent 7500 CS quadrupole mass spectrometer. A spot size smaller than the area of each exposed inclusion was used for analysis, which varied from 10 to 20 µm. The laser pulsed at 2 Hz, and data acquisition time was 30 s for background and 60 s for peaks. Before melt inclusion ablation and after every five batches, the reference materials NIST-610 and NIST-612 were analyzed to test for and correct any signal variation. The United States Geological Survey (USGS) Basalt of Columbia River (BCR) glass standard was analyzed as an unknown to check the accuracy of the analysis. The calibration values for NIST-610 and NIST-612 used in data reduction were from Gagnon et al. (2008). Trace element concentrations were corrected for variations in ablation efficiency between samples and standards using minor isotopes of Ca ( 43 Ca and 44 Ca) as an internal standard. CaO concentrations in the melt inclusions had been determined previously using an electron microprobe. Multiple analyses of BCR indicated that analytical reproducibility and accuracy were better than 10% for most reported elements. No corrections for interfering molecular or isobaric species were required due to the selection of appropriate analytical isotopes and minimal formation of molecular species, with + ThO/ + Th <0.5% (Table S4).

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Results and interpretation

Host basaltic rocks

Major and trace element concentrations in the host rocks are given in Table S1. S1; Fig. 3). The Ni, Co, and Cr contents of the samples were 50-110, 34-39, and 70-150 ppm, respectively (Table S1). The Mg# and the Ni and Cr contents of the host basaltic rocks were lower than those of primitive basalt (Mg# >70; Ni >250-350 ppm, Cr > 500-600 ppm; Ulmer, 1989;Wilkinson and Le Maitre, 1987) and McDonough, 1989). The samples reveal overall enrichment in highly incompatible elements compared with less incompatible ones. They exhibit Nb and Ta enrichment relative to LREE, resembling a typical OIB (Fig. 4c andd). However, some samples have small negative anomalies in Nb and Ta (Fig. 4c andd). Furthermore, they were distinguished by marked positive anomalies in Ba and K, and possibly P, compared with the OIB (Fig. 4c andd). Also note that these host rocks have higher fractionated (Zr/Hf) N ratios than OIB (Fig. 4c andd).

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Olivine phenocrysts

Olivines are present as euhedral to subhedral phenocrysts in thin section. Major element concentrations of inclusion-bearing olivines are given in Table S3. They have Fo values ranging from 71.5 to 82.7 (Fig. 5). The CaO contents range from 0.17 to 0.28 wt%, much higher than those of mantle peridotite xenoliths (CaO < 0.1 wt%; Thompson and Gibson, 2000) (Fig. 5), indicating that these olivines are phenocrysts crystallized from basaltic magmas.

Melt inclusions in olivine phenocrysts

The major element concentrations of reheated melt inclusions are presented in Table S2. Owing to their mobility, volatiles can rapidly diffuse out of inclusions when they are breached during reheating. Thus, the sulfur content was determined in order to monitor inclusion integrity during rehomogenization experiments [START_REF] Nielsen | Chemical and physical indicators of compromised melt inclusions[END_REF]. The S contents obtained varied from 240 to 1380 ppm (Table S2). Reported S contents of quenched mid-ocean ridge basalt (MORB) and submarine OIB (e.g., Mauna Kea, Pitcairn, Society, and Galapagos) glasses range from ~400 to 1800 ppm and ~700 to 2200 ppm, respectively [START_REF] Kendrick | Contrasting behaviours of CO 2 , S, H 2 O and halogens (F, Cl, Br, and I) in enrichedmantle melts from Pitcairn and Society seamounts[END_REF][START_REF] Mathez | Sulfur solubility and magmatic sulfides in submarine basalt glass[END_REF][START_REF] Nielsen | Chemical and physical indicators of compromised melt inclusions[END_REF]Stolper et al., 2004;Yi et al., 2000). We note that none of the reheated inclusions reported in this study appeared to be breached based on visual inspection. However, some homogenized melt inclusions contain tiny sulfide globules (Fig. S1), which may result in artificially low S measurements when the entire melt inclusion cannot be ablated during analysis [START_REF] Danyushevsky | Experimental and petrological studies of melt inclusions in phenocrysts from mantle-derived magmas: an overview of techniques, advantages and complications[END_REF]. To test olivine-liquid equilibria, we calculated the Mg-Fe exchange coefficient, K d = (Fe 2+ /Mg) Ol /(Fe 2+ /Mg) Liq , between inclusions (Liq) and host olivines (Ol). The calculated K d values ranged from 0.38 to 0.65, significantly higher than the nominal value of 0.30 ± 0.03 at 1 atm [START_REF] Roeder | Olivine-liquid equilibrium[END_REF], indicating that the melt inclusions are in disequilibrium

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with their host olivines. Trapped melt composition can be modified by re-equilibration with host olivines after entrapment and prior to natural quenching [START_REF] Danyushevsky | Re-equilibration of melt inclusions trapped by magnesian olivine phenocrysts from subduction-related magmas: petrological implications[END_REF][START_REF] Danyushevsky | Experimental and petrological studies of melt inclusions in phenocrysts from mantle-derived magmas: an overview of techniques, advantages and complications[END_REF], particularly where magma cools significantly before eruption. When re-equilibration occurs, elements that are incompatible with host minerals are unlikely to be significantly exchanged, but compatible elements can be modified and the change in composition of the trapped melt is unlikely to be reversed experimentally [START_REF] Cottrell | Consequences of diffusive reequilibration for the interpretation of melt inclusions[END_REF][START_REF] Danyushevsky | Re-equilibration of melt inclusions trapped by magnesian olivine phenocrysts from subduction-related magmas: petrological implications[END_REF][START_REF] Sobolev | Melt inclusions in minerals as a source of principle petrological information[END_REF]Spandler et al., 2007). For olivine-hosted melt inclusions, a reequilibration process called "Fe-loss" has been recognized in several studies (e.g., [START_REF] Danyushevsky | Re-equilibration of melt inclusions trapped by magnesian olivine phenocrysts from subduction-related magmas: petrological implications[END_REF][START_REF] Sobolev | Petrology and geochemistry of boninites from the North Termination of the Tonga Trench: constraints on the generation conditions of primary High-Ca boninite magmas[END_REF], whereby Fe diffuses from a melt inclusion into the host olivine while Mg diffuses from the host into the melt inclusion, increasing the K d value. The S solubility in a melt is positively correlated with its FeO content (e.g., [START_REF] Mathez | Sulfur solubility and magmatic sulfides in submarine basalt glass[END_REF][START_REF] Wallace | Sulfur in basaltic magmas[END_REF]. Therefore, the sulfide globules observed in some homogenized melt inclusions could be a consequence of Fe-loss [START_REF] Danyushevsky | Experimental and petrological studies of melt inclusions in phenocrysts from mantle-derived magmas: an overview of techniques, advantages and complications[END_REF]. We have corrected for Fe-loss using the computer program "Petrolog3" introduced by [START_REF] Danyushevsky | Petrolog3: Integrated software for modeling crystallization processes[END_REF]. This calculation requires an independent estimate of the initial FeO* contents of melt inclusions. The FeO* contents of Baekdusan basalts do not show a meaningful variation attributed to magmatic differentiation (Fig. S2). We thus assumed the initial FeO* contents of melt inclusions to be a mean value of the host basaltic rocks. The Fe 2 O 3 contents of trapped melts were calculated using (Fe 2+ /Fe 3+ ) melt = 9. Corrected melt compositions are reported in Table S5. The K d (Fe 2+ -Mg) values between host olivines and corrected melt inclusions are between 0.29 and 0.32 (Table S5), within the range (0.30 ± 0.03; [START_REF] Roeder | Olivine-liquid equilibrium[END_REF] determined experimentally.

The SiO 2 contents of melt inclusions range from 41.7 to 53.1 wt% (Fig. 2). Based on the SiO 2 and trace element contents (see below), we divided the melt inclusions into two groups: a low-Si group (SiO 2 <45 wt%) and a high-Si group (SiO 2 >45 wt%). The low-Si group is compositionally picrobasalt to basanite in the alkaline series (Fig. 2). The high-Si group consists of (trachy)basalt to basaltic (trachy)andesite, which can be further divided into two sub-groups: an alkaline (trachy)basalt-basaltic trachyandesite group and a sub-alkaline basalt-basaltic andesite group (Fig. 2). The MgO contents of the low-Si group are limited to ~7.0 wt%, but those of the high-Si group vary from 3.8 to 7.3 wt% (Fig. 3). The low-Si group has higher TiO 2 , CaO, and P 2 O 5 concentrations, but lower Al 2 O 3 , Na 2 O, and K 2 O contents at a given MgO than the high-Si group (Fig. 3). The sub-alkaline group has higher SiO 2 , and lower Al 2 O 3 , Na 2 O, and K 2 O contents than the alkaline group at a given MgO concentration (Fig. 3). The TiO 2 , Na 2 O, and K 2 O contents of the high-Si group increase with decreasing MgO, but the SiO 2 , Al 2 O 3 , CaO, and P 2 O 5 contents do not correlate well with MgO (Fig. 3).

The trace element concentrations of reheated melt inclusions are given in Table S4.

They exhibit LREE-enriched patterns [(La/Yb) N = 7.8-30.4] similar to a typical OIB (Table S4; Fig. 4a andb). However, most of the high-Si group are distinguished by positive Eu anomalies [(Eu/Eu * ) N =1.0-1.3) compared with the OIB. Furthermore, in terms of primitivemantle-normalized multi-element patterns (Fig. 4c andd), the high-Si group is generally similar to the host basalt, characterized by significant enrichments in Ba, Rb, K, Pb, P, and Ti, and exhibits fractionated (Zr/Hf) N ratios compared with the OIB. The low-Si group also exhibits distinctive positive anomalies in Ba and P and fractionated (Zr/Hf) N ratios, but has relatively low abundances of Ba and Rb and a significantly negative anomaly in K compared with the high-Si group or host basalt. Also note that sample BD-07-5a exhibits pronounced Nb and Ta depletion relative to LREE.

Discussion

Source mineralogy and lithology

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Melt inclusions exhibit a wider diversity of compositions than their host basalts (Fig. 3). For example, TiO 2 , K 2 O, and P 2 O 5 contents of melt inclusions varied from 2.4 to 5.3, 0.2 to 3.0, and 0.2 to 5.3 wt%, respectively, but those species in the host basalt are lower, from 2.1 to 3.0, 1.3 to 1.8, and 0.3 to 0.7 wt%, respectively. Significant compositional diversity has often been observed in melt inclusions from a single sample (e.g., [START_REF] Choi | Melt inclusions in olivine and plagioclase phenocrysts from Antarctic-Phoenix Ridge basalts: Implications for origins of N-and E-type MORB parent magmas[END_REF][START_REF] Gurenko | Enriched and depleted primitive melts included in olivine from Icelandic tholeiites: origin by continuous melting of a single mantle column[END_REF][START_REF] Kamenetsky | Calcic melt inclusions in primitive olivine at 43°N MAR: evidence for melt-rock reaction/melting involving clinopyroxene-rich lithologies during MORB generation[END_REF][START_REF] Kent | Assimilation of seawaterderived components in an oceanic volcano: evidence from matrix glasses and glass inclusions from Loihi seamount, Hawaii[END_REF][START_REF] Li | Olivine and melt inclusion chemical constraints on the source of intracontinental basalts from the eastern North China Craton: discrimination of contributions from the subducted Pacific slab[END_REF]Sobolev and Shimizu, 1993). In these cases, melt inclusions have been interpreted as sampling a range of liquids reflecting a range of source compositions, extents of melting and/or melting processes, and trapped before mixing and homogenization in magma chambers diluted or averaged them to produce the compositions of erupted lavas [START_REF] Kent | Melt inclusions in basaltic and related volcanic rocks[END_REF][START_REF] Schiano | Primitive mantle magmas recorded as silicate melt inclusions in igneous minerals[END_REF]Sobolev and Shimizu, 1993).

Low-Si group melt inclusions are typified by much higher MgO and lower SiO 2 concentrations than the high-Si group or host basalts, implying that they record an early stage in the evolution of magma, prior to mixing and blending at a shallow depth. The low-Si group has notably elevated CaO and P 2 O 5 contents (Fig. 3d,g), and these two elements are positively correlated (Fig. 6). Major element concentrations in the high-Si sub-alkaline group generally resemble those of the host basalts, but the high-Si alkaline group exhibits significantly higher compositional diversity (Fig. 3). MgO concentrations of the host basalts apparently lie in the middle of the compositional range of high-Si alkaline group (Fig. 3).

However, this group is characterized by generally lower SiO 2 and higher TiO 2 , Al 2 O 3 , K 2 O, and P 2 O 5 values at a given MgO content than host basalts (Fig. 3).

Trace element concentrations of melt inclusions may be modified by diffusive equilibration with host minerals or external melts (e.g., [START_REF] Kent | Melt inclusions in basaltic and related volcanic rocks[END_REF]. Diffusive exchange occurs more readily for elements with higher diffusivity in host minerals, higher partition coefficients between the host mineral and melt, and smaller melt inclusions and host minerals ACCEPTED MANUSCRIPT [START_REF] Qin | Diffuse reequilibration of melt and fluid inclusions[END_REF]. Olivine is a favorable host for isolation of melt inclusions from the external media, because this phase possesses minimal concentrations of most elements that characterize basaltic melt, with the exceptions of Mg, Fe, and Si (e.g., [START_REF] Cottrell | Consequences of diffusive reequilibration for the interpretation of melt inclusions[END_REF][START_REF] Qin | Diffuse reequilibration of melt and fluid inclusions[END_REF][START_REF] Sobolev | Melt inclusions in minerals as a source of principle petrological information[END_REF]. Experimental result of [START_REF] Cherniak | REE diffusion in olivine[END_REF] suggests that timescales ranging from tens of thousands to a few million years are necessary for REE in a melt inclusion with a 50-μm radius within a 1-mm radius olivine to reach 90% reequilibration with the external melt at a temperature of 1300°C. Note that our study is limited to inclusions (> 20 μm in size; Table S2) located at the center of host olivines (> 1 mm in size). Given that estimated times for the processes of basaltic melt formation, extraction, ascent, and residence in the crust range from ~10 to 10 5 years [START_REF] Reid | Timescales of magma transfer and storage in the crust[END_REF], it is thus likely that incompatible elements in olivine-hosted melt inclusions record the signatures of primary melts from the mantle. Despite this assumption, we use incompatible element abundances and ratios in order to minimize the possibility of any modification.

Compared with a typical OIB, high-Si group melt inclusions and host basalts exhibit significantly positive anomalies in Eu, Ba, Rb, K, and Pb (Fig. 4a,c). These anomalies are also present in near-primary (i.e., highly magnesian and low SiO 2 ) melts hosted in Fo-rich olivine (e.g., sample BD-07-2: Fo = 82.7; MgO = 7.3 wt%; SiO 2 = 49.5 wt%) (Fig. 4a,c), suggesting a lack of crustal contamination. K-hollandite [KAlSi 3 O 8 ], a high-pressure (above ~10 GPa) polymorph of potassium feldspar, is characterized by relatively high concentrations of K, Rb, Ba, and Pb concentrations [START_REF] Rapp | Subduction recycling of continental sediments and the origin of geochemically enriched reservoirs in the deep mantle[END_REF][START_REF] Schmidt | Experimental constraints on recycling of potassium from subducted oceanic crust[END_REF][START_REF] Urakawa | Synchrotron radiation study on the high-pressure and high-temperature phase relations of KAlSi 3 O 8[END_REF][START_REF] Wang | Subsolidus and melting experiments of a K-rich basaltic composition to 27 GPa: Implication for the behavior of potassium in the mantle[END_REF][START_REF] Yagi | High pressure transitions in the system KAlSi 3 O 8 -NaAlSi 3 O 8[END_REF]. The positive anomalies, along with the elevated Al 2 O 3 contents of the high-Si alkaline group at a given MgO (Fig. 3c), suggest a possible presence of K-hollandite in the source [START_REF] Kuritani | Intensive hydration of the mantle transition zone beneath China caused by ancient slab stagnation[END_REF]. This K-hollandite signature, however, is not present in the low-Si group (Fig. 4b,d).

Instead, the low-Si group together with some high-Si alkaline group have a remarkable spike

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in P (Fig. 4d). Also note that the P 2 O 5 contents exhibit a positive correlation with CaO contents (Fig. 6). These observations indicate the presence of a calcium phosphate in the source. Apatite is one of the major phosphates in terrestrial rocks, including the garnet lherzolite (e.g., [START_REF] Konzett | The high PT stability of apatite and Cl partitioning between apatite and hydrous potassic phases in peridotite: an experimental study to 19 GPa with implications for the transport of P, Cl and K in the upper mantle[END_REF][START_REF] Konzett | The high P-T stability of hydroxyl-apatite in natural and simplified MORB-an experimental study to 15 GPa with implications for transport and storage of phosphorus and halogens in subduction zones[END_REF][START_REF] Murayama | A dense polymorph of Ca 3 (PO 4 ) 2 : a high pressure phase of apatite decomposition and its geochemical significance[END_REF]. However, experimental data and thermodynamic calculations indicate that apatite is transformed to tuite [γ-Ca 3 (PO 4 ) 2 ] at pressures above ~8 GPa [START_REF] Konzett | The high PT stability of apatite and Cl partitioning between apatite and hydrous potassic phases in peridotite: an experimental study to 19 GPa with implications for the transport of P, Cl and K in the upper mantle[END_REF][START_REF] Konzett | The high P-T stability of hydroxyl-apatite in natural and simplified MORB-an experimental study to 15 GPa with implications for transport and storage of phosphorus and halogens in subduction zones[END_REF][START_REF] Murayama | A dense polymorph of Ca 3 (PO 4 ) 2 : a high pressure phase of apatite decomposition and its geochemical significance[END_REF]Sugiyama and Tokonami, 1987;Thompson et al., 2013). Xie et al. (2002) reported the coexistence of tuite and Na-hollandite [NaAlSi 3 O 8 ] in the shock-produced veins of the Suizhou chondrite. Natural K-hollandite may exist as a solidsolution phase containing NaAlSi 3 O 8 [START_REF] Yagi | High pressure transitions in the system KAlSi 3 O 8 -NaAlSi 3 O 8[END_REF]. Thus, we consider tuite and/or Khollandite to be essential phases in the mantle source of Baekdusan magmatism.

Low-Si melts and some high-Si melts exhibit a slight negative anomaly in Nb-Ta relative to the neighboring La (Fig. 4c andd). This apparent anomaly could be accounted for by crustal contamination during magma differentiation, because continental crust has a distinct negative Nb-Ta anomaly (e.g., [START_REF] Rudnick | Composition of the continental crust[END_REF]. We note, however, that (Nb/La) N ratios do not show a meaningful correlation with the Fo values of host olivines (Fig. 7a), implying an insignificant role for AFC (assimilation and fractional crystallization) processes. This has also been substantiated by previous studies (e.g., [START_REF] Kuritani | Intensive hydration of the mantle transition zone beneath China caused by ancient slab stagnation[END_REF]Liu et al., 2015a), which found no discernible correlations between SiO 2 and 87 Sr/ 86 Sr or Ba/Th ratios in the Baekdusan basalts. Instead, we observe a meaningful negative correlation between (Nb/La) N and (P/P*) N ratios (Fig. 7b), where (P*) N = (Nd N + Sm N )/2. The partition coefficient (D) of La between apatite and basaltic melt is significantly higher than that of Nb (D La =1.9-13.3, D Nb = 0.0011-0.0035, D Ta = 0.0010-0.0026; [START_REF] Prowatke | Trace element partitioning between apatite and silicate melts[END_REF]. Thus, inasmuch as partitioning coefficients for tuite are unknown, we suggest that the

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Nb-Ta depletion might be attributed to relative enrichment in La by presence of tuite in the source.

The magmatism at Baekdusan is characterized by positively fractionated (Zr/Hf) N ratios, in contrast to OIB (Fig. 4c andd). Recycled oceanic crust (e.g., eclogite or garnet pyroxenite) has been proposed as a reservoir with a suprachondritic Zr/Hf ratio [START_REF] Klemme | Experimental constraints on major and trace element partitioning during partial melting of eclogite[END_REF][START_REF] Pertermann | Experimental determination of trace element partitioning between garnet and silica-rich liquid during anhydrous partial melting of MORB-like eclogite[END_REF]. Experimental studies [START_REF] Hauri | Experimental and natural partitioning of Th, U, Pb and other trace elements between garnet, clinopyroxene and basaltic melts[END_REF][START_REF] Pertermann | Experimental determination of trace element partitioning between garnet and silica-rich liquid during anhydrous partial melting of MORB-like eclogite[END_REF][START_REF] Van Westrenen | Crystal-chemical controls on trace element partitioning between garnet and anhydrous silicate melt[END_REF] show complementary D Zr and D Hf values in clinopyroxene (D Zr < D Hf ) and garnet (D Zr > D Hf ). The partition coefficient for Zr and Hf in tuite might be about one order of magnitude lower than that determined for clinopyroxene and garnet [START_REF] Prowatke | Trace element partitioning between apatite and silicate melts[END_REF]. The Zr/Hf ratio in melt can thus be controlled by the modal proportions of clinopyroxene and garnet in the source. Fig. 8a shows the Zr/Hf versus La/Yb relationship in partial melts derived from eclogite and peridotite, calculated using the [START_REF] Shaw | Trace element fractionation during anatexis[END_REF] equation. The peridotite is assumed to melt non-modally, but the eclogite in modal proportions. The partition coefficients between minerals and melts, and the concentrations of select trace elements used in the calculations are listed in Table S6. Notably, partial melting of garnet peridotite alone is unlikely to produce the elevated Zr/Hf ratios observed in the Baekdusan melts. The Baekdusan data are best approximated by admixture of partial melts derived from clinopyroxene-rich eclogite and garnet peridotite (Fig. 8a).

To examine the relative contributions of garnet-or spinel-facies sources, we calculated Dy/Yb ratios and Yb concentrations for melts (Fig. 8b). Garnet preferentially incorporates heavy rare earth elements over LREE relative to spinel [START_REF] Mckenzie | Partial melt distributions from inversion of rare earth element concentrations[END_REF][START_REF] Pertermann | Experimental determination of trace element partitioning between garnet and silica-rich liquid during anhydrous partial melting of MORB-like eclogite[END_REF]. Therefore, garnet peridotite produces a melt with a higher Dy/Yb ratio and lower Yb content than a spinel peridotite source (Fig. 8b). Partial melts derived from spinel-garnet peridotite contain slightly elevated Yb and a reduced Dy/Yb compared with those of garnet peridotite (Fig. 8b). The Baekdusan melts have Dy/Yb ratios

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overlapping with those of partial melts from a garnet peridotite, but their Yb contents are too high for modeled melts of garnet peridotite (Fig. 8b). Clinopyroxene-rich eclogites can produce melts with Yb contents higher than a garnet peridotite source (Fig. 8b), which further supports a mixed garnet peridotite-eclogite source.

The sub-alkaline group exhibits lower abundances of incompatible trace elements than the high-Si alkaline group (Fig. 4a,c). Meanwhile, both groups show similar geochemical characteristics such as positive anomalies in Eu, Ba, Rb, K, Pb, and P, and positively fractionated (Zr/Hf) N ratios compared with OIB (Fig. 4a,c), indicating different degrees of partial melting from the same source. The sub-alkaline melts represent a higher degree of partial melting than the alkaline melts (Fig. 8a,b).

In summary, there are at least two distinct source materials (i.e., altered oceanic crust component (tuite-bearing eclogite) and sediment component (metasomatic agent from Khollandaite) together with a garnet peridotite component for the Baekdusan magmatism. The concentrations of incompatible elements were modeled in order to investigate the nature of the mantle source of Baekdusan magmatism (Fig. 9). The partition coefficient between minerals and melts, and concentrations of trace elements used in the calculations are provided in Table S7. We used an "enriched" depleted MORB source (E-DMM; Workman and Hart, 2005) for modeling. For high-Si melts, we first assumed a metasomatized mantle [a mixture of 0.5% K-hollandite [START_REF] Rapp | Subduction recycling of continental sediments and the origin of geochemically enriched reservoirs in the deep mantle[END_REF] and 99.5% E-DMM], and P-rich melt I [a mixture of 5% tuite melt (Zhai et al., 2014) and 95% of a 1% eclogite melt]. We used two approaches to determine high-Si melt compositions: (1) a 3% partial melt of a hybrid source [95% metasomatized mantle and 5% P-rich melt I] (Model 1), and (2) a mixture containing 90% of a 1% partial melt of metasomatized mantle and 10% P-rich melt I (Model 2). Phase proportions (by weight) in the solid and melt modes for the metasomatized mantle are Ol 55 Opx 25 Cpx 10 Grt 10 and Ol 5 Opx 5 Cpx 45 Grt 45 , respectively, whereas those for eclogite are the

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characteristics of the lithospheric mantle may record the removal of Archean lithospheric mantle during the Phanerozoic, possibly via delamination (Gao et al., 2002;[START_REF] Menzies | Palaeozoic and Cenozoic lithoprobes and the loss of >120 km of Archaean lithosphere, Sino-Korean craton, China[END_REF][START_REF] Menzies | Geodynamics of the North China Craton[END_REF]. Removal of thickened lower continental crust comprising eclogitic rocks may be accompanied by lithospheric mantle delamination (Gao et al., 2004(Gao et al., , 2008)). Lower crustal materials foundered in the convecting mantle could provide a mafic source for Baekdusan's magmatism. However, it is still unclear whether the delaminated lower crust reaches a depth near the upper boundary of the subducted Pacific plate, and its duration in the asthenospheric mantle is also unknown. It should be noted that melts derived from foundered eclogitic lower continental crust are characterized by strong depletion of HFSEs, a highly radiogenic initial 87 Sr/ 86 Sr ratio (0.7053 to 0.7101), and negative ε Nd values (-14 to +2) (e.g., Gao et al., 2004Gao et al., , 2008;;Xu et al., 2002[START_REF] Xu | Mesozoic adakitic rocks from the Xuzhou-Suzhou area, eastern China: evidence for partial melting of delaminated lower continental crust[END_REF][START_REF] Xu | Interaction of adakitic meltperidotite: implications for the high-Mg# signature of Mesozoic adakitic rocks in the eastern North China Craton[END_REF]. As discussed above, the Baekdusan melts do not show any obvious geochemical characteristics of crustal contamination. Furthermore, the Sr-Nd isotopic compositions of the Baekdusan basaltic rocks are generally close to bulk silicate Earth values ( 87 Sr/ 86 Sr = 0.7046 to 0.7062; ε Nd = -4 to +3; [START_REF] Basu | Major element, REE, and Pb, Nd and Sr isotopic geochemistry of Cenozoic volcanic rocks of eastern China: implications for their origin from suboceanic-type mantle reservoirs[END_REF][START_REF] Kuritani | Intraplate magmatism related to deceleration of upwelling asthenospheric mantle: Implications from the Changbaishan shield basalts, northeast China[END_REF]Liu et al., 2015a;[START_REF] Zhang | Late Cenozoic intraplate volcanism in Changbai volcanic field, on the border of China and North Korea: insights into deep subduction of the Pacific slab and intraplate volcanism[END_REF]. We thus rule out the possibility of foundered lower continental crustal materials in the source.

Instead, we propose recycled oceanic crust as a possible source of eclogitic materials. Oceanic crust may have been recycled into the mantle beneath northeastern China in dual subduction episodes: (1) the southward subduction of the Mongol-Okhotsk oceanic plate until the Jurassic (Tang et al., 2014;Zhang et al., 2016;[START_REF] Zhou | The onset of Pacific margin accretion in NE China: evidence from the Heilongjiang high-pressure metamorphic belt[END_REF] and (2) the northwestward subduction of the Pacific plate since the Cretaceous [START_REF] Duan | Seismic structure and origin of active intraplate volcanoes in Northeast Asia[END_REF]Guo et al., 2016;[START_REF] Huang | High-resolution mantle tomography of China and surrounding regions[END_REF][START_REF] Lei | P-wave tomography and origin of the Changbai intraplate volcano in Northeast Asia[END_REF]Liu et al., 2015b;[START_REF] Zhao | Seismic image and origin of the Changbai intraplate volcano in East Asia: role of big mantle wedge above the stagnant Pacific slab[END_REF].

Subduction of the Mongol-Okhotsk oceanic plate ceased about 150 Ma ago, and the subducted slab penetrated down to the lower mantle (from depths of ~1,500 km to at least 2,500 km) [START_REF] Van Der Voo | Mesozoic subducted slabs under Siberia[END_REF]. No high-velocity seismic anomalies, which would

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signify the presence of slab remnants from the Mongol-Okhotsk oceanic plate, have been detected in the upper mantle beneath northeastern China (e.g. [START_REF] Huang | High-resolution mantle tomography of China and surrounding regions[END_REF]Wei et al., 2012). Intraplate volcanism in northeastern Asia is thus considered to be closely associated with deep subduction of the Pacific plate (e.g., [START_REF] Lei | P-wave tomography and origin of the Changbai intraplate volcano in Northeast Asia[END_REF]Tang et al., 2014;Tian et al., 2016;Wei et al., 2012Wei et al., , 2015;;[START_REF] Zhao | Seismic image and origin of the Changbai intraplate volcano in East Asia: role of big mantle wedge above the stagnant Pacific slab[END_REF]. Seismic tomography of northeastern Asia has revealed a prominent low-velocity anomaly beneath the Baekdusan volcano, extending down to the 410-km discontinuity [START_REF] Duan | Seismic structure and origin of active intraplate volcanoes in Northeast Asia[END_REF][START_REF] Guo | Seismic evidence of on-going sublithosphere upper mantle convection for intra-plate volcanism in Northeast China[END_REF][START_REF] Huang | High-resolution mantle tomography of China and surrounding regions[END_REF][START_REF] Lei | Seismic imaging of the deep structure under the Chinese volcanoes: An overview[END_REF][START_REF] Lei | P-wave tomography and origin of the Changbai intraplate volcano in Northeast Asia[END_REF]Tian et al., 2016;[START_REF] Zhao | Seismic image and origin of the Changbai intraplate volcano in East Asia: role of big mantle wedge above the stagnant Pacific slab[END_REF], which may indicate a wet plume from the mantle transition zone (MTZ) [START_REF] Kuritani | Intensive hydration of the mantle transition zone beneath China caused by ancient slab stagnation[END_REF][START_REF] Richard | Stagnant slab, wet plumes and Cenozoic volcanism in East Asia[END_REF][START_REF] Zhao | Seismic image and origin of the Changbai intraplate volcano in East Asia: role of big mantle wedge above the stagnant Pacific slab[END_REF].

Continent-derived sediments subducted along with the oceanic lithosphere can be stored in the MTZ. K-hollandite may be the most dominant phase within stagnant subducted continent-derived sediments, and is also a major repository of LILEs (e.g., K, Ba, Sr, Pb, LREE, etc.) [START_REF] Irifune | Subduction of continental crust and terrigenous and pelagic sediments: an experimental study[END_REF][START_REF] Murphy | Lamproites from Gaussberg, Antarctica: possible transition zone melts of Archaean subducted sediments[END_REF][START_REF] Ono | Stability limits of hydrous minerals in sediment and mid-ocean ridge basalt compositions: implications for water transport in subduction zones[END_REF][START_REF] Rapp | Subduction recycling of continental sediments and the origin of geochemically enriched reservoirs in the deep mantle[END_REF]. The thermal stability of K-hollandite is dependent upon the amount of H 2 O in the system [START_REF] Rapp | Subduction recycling of continental sediments and the origin of geochemically enriched reservoirs in the deep mantle[END_REF]; it decomposes around 1600°C at 16 GPa under relatively dry conditions, but breaks down at ~1400°C under wet conditions [START_REF] Rapp | Subduction recycling of continental sediments and the origin of geochemically enriched reservoirs in the deep mantle[END_REF][START_REF] Wang | Subsolidus and melting experiments of a K-rich basaltic composition to 27 GPa: Implication for the behavior of potassium in the mantle[END_REF]. Subducting sediments can carry H 2 O to the mantle transition zone (Kerrick and Connolly, 2001a;[START_REF] Ono | Stability limits of hydrous minerals in sediment and mid-ocean ridge basalt compositions: implications for water transport in subduction zones[END_REF]. The H 2 O fluid released from the subducting slab migrates into the overlying mantle peridotite, and can be trapped to form hydrous phases in peridotites or even incorporated into nominally anhydrous minerals (e.g., wadsleyite, ringwoodite) [START_REF] Iwamori | Transportation of H 2 O beneath the Japan arcs and its implications for global water circulation[END_REF][START_REF] Kawamoto | Experimental evidence for a hydrous transition zone in the early Earth's mantle[END_REF][START_REF] Luth | Mantle volatiles-distribution and consequences[END_REF][START_REF] Ohtani | Water transport into the deep mantle and formation of a hydrous transition zone[END_REF]. Hydrous peridotite is likely to be dragged down by the descending slab, and can also transport H 2 O into the transition zone [START_REF] Ohtani | Water transport into the deep mantle and formation of a hydrous transition zone[END_REF][START_REF] Ono | Stability limits of hydrous minerals in sediment and mid-ocean ridge basalt compositions: implications for water transport in subduction zones[END_REF]. The exact amount of H 2 O transported to the transition zone depends on the thermal structure of the subducting slab (Kerrick and Connolly, ACCEPTED MANUSCRIPT 2001a, b;[START_REF] Luth | Mantle volatiles-distribution and consequences[END_REF][START_REF] Ohtani | Water transport into the deep mantle and formation of a hydrous transition zone[END_REF][START_REF] Ono | Stability limits of hydrous minerals in sediment and mid-ocean ridge basalt compositions: implications for water transport in subduction zones[END_REF]. If the subducted stagnant slab and its accompanying peridotite warm to ambient mantle temperatures, thermal decomposition of Khollandite can occur. Fluids or melts expelled from the breakdown of K-hollandite may facilitate the transfer of LILEs into the mantle above the stagnant slab, where they can metasomatize the ambient mantle peridotite [START_REF] Kuritani | Intensive hydration of the mantle transition zone beneath China caused by ancient slab stagnation[END_REF]. In addition, the presence of H 2 O in peridotite may induce Rayleigh-Taylor instabilities and thus generate wet plumes [START_REF] Richard | Water-induced convection in the Earth's mantle transition zone[END_REF][START_REF] Richard | Stagnant slab, wet plumes and Cenozoic volcanism in East Asia[END_REF], which may pick up stagnant slab materials (Fig. 10). Alternatively, seismic images and waveform modelling show that focused mantle upwelling may rise through a gap in the deep stagnant slabs beneath northeast China [START_REF] Lei | Seismic imaging of the deep structure under the Chinese volcanoes: An overview[END_REF]Tang et al., 2014). Tuite can exist within peridotitic or MORB-like bulk compositions [START_REF] Konzett | The high PT stability of apatite and Cl partitioning between apatite and hydrous potassic phases in peridotite: an experimental study to 19 GPa with implications for the transport of P, Cl and K in the upper mantle[END_REF] and is stable over a wide range of subduction zone temperature regimes, but not in convecting asthenospheric mantle [START_REF] Konzett | The high P-T stability of hydroxyl-apatite in natural and simplified MORB-an experimental study to 15 GPa with implications for transport and storage of phosphorus and halogens in subduction zones[END_REF][START_REF] Konzett | The high PT stability of apatite and Cl partitioning between apatite and hydrous potassic phases in peridotite: an experimental study to 19 GPa with implications for the transport of P, Cl and K in the upper mantle[END_REF]. When plumes ascend through the upper mantle, metasomatized mantle and recycled oceanic crustal materials (tuite-bearing eclogite or garnet pyroxenite) may undergo partial melting as soon as they surpass the solidus temperature (Fig. 10). The solidus for recycled oceanic crust falls at temperatures ~50 to 150°C lower than that of typical peridotite, and is therefore encountered first during adiabatic upwelling of mantle [START_REF] Hirschmann | A possible role for garnet pyroxenite in the origin of the "garnet signature[END_REF][START_REF] Kogiso | High-pressure partial melting of garnet pyroxenite: possible mafic lithologies in the source of ocean island basalts[END_REF]. High concentrations of alkali, especially K 2 O, in peridotites have a great influence on solidus temperatures, which decrease with increasing alkali content, possibly due to incompatible behavior during partial melting [START_REF] Herzberg | New experimental observations on the anhydrous solidus for peridotite KLB-1[END_REF][START_REF] Hirschmann | Mantle solidus: experimental constraints and the effects of peridotite composition[END_REF]. For example, the solidus temperature can decrease by ~150°C per percent alkali in the bulk composition [START_REF] Herzberg | New experimental observations on the anhydrous solidus for peridotite KLB-1[END_REF][START_REF] Hirschmann | Mantle solidus: experimental constraints and the effects of peridotite composition[END_REF]. Thus, the difference between the solidus temperatures for LILE-enriched peridotites created by K-hollandite breakdown and recycled oceanic crust may be minor. Krich melts from metasomatized mantle, P-rich melts from recycled oceanic crust, and ACCEPTED MANUSCRIPT heterogeneous mixtures of these two components may result in compositionally diverse melt inclusions in olivines from the Baekdusan basalts (Fig. 10). We note that the TiO 2 and P 2 O 5 contents of the melt inclusions are higher than those of the host basalts (Fig. 3b,g). This observation suggests that there must have been another component having low TiO 2 and P 2 O 5 in the host magma, probably produced by prevailing peridotites. The absence of melt inclusions having low TiO 2 and P 2 O 5 suggests that the olivine phenocrysts were grown in the mantle source before significant melting of peridotite component. This is consistent with the observation of relatively low Fo values (72-83; Table S3) of the inclusion-bearing olivines.

Conclusions

Olivine-hosted melt inclusions in Baekdusan basalts can be divided into two major groups in terms of their major and trace element compositions: a low-Si group and a high-Si group. The low-Si group is characterized by a distinct positive spike for P in the spidergram, as well as a positive correlation between CaO and P 2 O 5 . The high-Si group exhibits pronounced positive anomalies in Eu, Ba, Rb, K, Pb, and P. They are characterized by fractionated (Zr/Hf) N ratios higher than the OIB. Geochemical modeling indicates that the high-Si melts may be produced by mixing K-rich melt from the "enriched" depleted MORB source (E-DMM) metasomatized by K-hollandite breakdown with P-rich melt from tuitebearing eclogite. Meanwhile, the low-Si melts may have been formed by admixture of a melt from E-DMM with a P-rich melt from tuite-bearing eclogite. Partial melting of recycled oceanic crustal materials entrained within a plume ascending from the mantle transition zone or from depths near 700 km through a gap in the stagnant slab along with garnet peridotites may have played an important role in the genesis of volcanism at Baekdusan. [START_REF] Le Maitre | A classification of igneous rocks and glossary of terms[END_REF] for corrected melt inclusions in olivines and host basalts. The boundary line dividing alkaline and subalkaline series is from [START_REF] Irvine | A guide to the chemical classification of the common volcanic rocks[END_REF]. The gray shaded area represents published data for Baekdusan basaltic rocks [START_REF] Chen | Geochemistry of Cenozoic basalts and mantle xenoliths in Northeast China[END_REF]Fan et al., 2006;[START_REF] Guo | Temporal variations and petrogenetic implications in Changbai basaltic rocks since the Pliocene[END_REF][START_REF] Kuritani | Intraplate magmatism related to deceleration of upwelling asthenospheric mantle: Implications from the Changbaishan shield basalts, northeast China[END_REF]Liu et al., 2015a;Yun and Koh, 2014;[START_REF] Zhang | Late Cenozoic intraplate volcanism in Changbai volcanic field, on the border of China and North Korea: insights into deep subduction of the Pacific slab and intraplate volcanism[END_REF].

Figure Captions

  On a total alkali versus silica (TAS) diagram (Le Maitre et al., 1989) (Fig. 2), they are compositionally basalt to basaltic andesite, belonging to the sub-alkaline suite. The host rocks have a limited range of SiO 2 (51.1-52.3 wt%) with Mg#s [=100Mg/(Mg+Fe 2+ )] varying from 46.0 to 59.0 (Table

  , reflecting fractionation of ferromagnesian minerals such as olivine and clinopyroxene. They exhibit a light rare earth element (LREE)-enriched pattern [(La/Yb) N = 8.8-10.2] on chondrite-normalized REE plots (Fig. 4a and b), with a slight positive anomaly in Eu [(Eu/Eu * ) N =1.1-1.2, where (Eu*) N = (Sm N + Gd N )/2]. Extended primitive-mantle-normalized trace element patterns (also called spidergrams) are provided in Fig. 4c and d, along with the composition of typical OIBs (Sun

Fig. 1 .

 1 Fig. 1. (a) Simplified geological map showing major tectonic units of the North China Craton

Fig. 2 .

 2 Fig. 2. Total alkali versus silica plots (Le Maitre et al., 1989) for corrected melt inclusions in

Fig. 3 .

 3 Fig. 3. Major oxide patterns of corrected melt inclusions in olivines and host basalts. MI =

Fig. 4 .

 4 Fig. 4. Rare earth element concentrations normalized to chondrite values (Sun and

Acknowledgements

This work was funded by the Korea Meteorological Administration Research and Development Program under Grant KMIPA2015-7090. We would like to thank to Nadja Omara Cintron Franqui for her help during the experimental work. Insightful reviews by Takeshi Kuritani and an anonymous reviewer greatly improved the manuscript. We thank Sun-Lin Chung for the editorial handling.

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trace element systematics of the Pitcairn hotspot. Earth and Planetary Science Letters 196, 197-212. Eizenhöfer, P.R., Zhao, G., Zhang, J., Sun, M., 2014. Final closure of the Paleo-Asian Ocean along the Solonker Suture Zone: Constraints from geochronological and geochemical data of Permian volcanic and sedimentary rocks. Tectonics 33, 441-463. Fan, Q.C., Sui, J.L., Wang, T.H., Li, N., Sun, Q., 2006. Eruption history and magma evolution of the trachybasalt in the Tianchi volcano, Changbaishan. Acta Petrologica Sinica 22, 1449-1457 (in Chinese with English abstract). Fan, Q.C., Sui, J.L., Wang, T.H., Li, N., Sun, Q., 2007. History of volcanic activity, magma evolution and eruptive mechanisms of the Changbai Volcanic Province. Geological Journal of China Universities 13, 175-190 (in Chinese with English abstract). Gaetani, G.A., Watson, E.B., 2000. Open system behavior of olivine-hosted melt inclusions. Earth and Planetary Science Letters 183, 27-41. Gao, S., Rudnick, R.L., Carlson, R.W., McDonough, W.F., Liu, Y.-S., 2002. Re-Os evidence for replacement of ancient mantle lithosphere beneath the North China craton. Earth and Planetary Science Letters 198, 307-322. Gao, S., Rudnick, R.L., Xu, W.-L., Yuan, H.-L., Liu, Y.-S., Walker, R.J., Puchtel, I.S., Liu, X., Huang, H., Wang, X.-R., Yang, J., 2008. Recycling deep cratonic lithosphere and generation of intraplate magmatism in the North China Craton, Earth and Planetary Science Letters 270, 41-53. Gao, S., Rudnick, R.L., Yuan, H.-L., Liu, X.-M., Liu, Y.-S., Xu, W.-L., Ling, W.-L., Ayers, J., Wang, X.-C., Wang, Q.-H., 2004. Recycling lower continental crust in the North China craton. Nature 432, 892-897. ACCEPTED MANUSCRIPT Yi, W., Halliday, A.N., Alt, J.C., Lee, D.-C., Rehkämper, M., Garcia, M.O., Langmuir, C.H., Su, Y., 2000. Cadmium, indium, tin, tellurium, and sulfur in oceanic basalts: Implications for chalcophile element fractionation in the Earth. Journal of Geophysical Research Solid Earth 105, 18927-18948. Yin, J., Jull, A.J.T., Burr, G.S., Zheng, Y., 2012. A wiggle-match age for the Millennium eruption of Tianchi Volcano at Changbaishan, Northeastern China. Quaternary Science Reviews 47, 150-159. Yun, S.-H., Koh, J.S., 2014. Petrochemical characteristics of volcanic rocks of historic era at Mt. Baekdusan. Journal of the Geological Society of Korea 50, 753-769 (in Korean with English abstract). Yun, S.-H., Lee, J.H., 2012. Analysis of unrest signs of activity at the Baegdusan Volcano. Journal of the Petrological Society of Korea 21, 1-12 (in Korean with English abstract). Zhai, S., Xue, W., Yamazaki, D., Ma, F., 2014. Trace element composition in tuite decomposed from natural apatite in high-pressure and high-temperature experiments. Science China Earth Science 57, 2922-2927. Zhang, F.-F., Wang, Y.-H., Liu, J.-J., Wang, J.-P., Zhao, C.-B., Song, Z.-W., 2016. Origin of the Wunugetushan porphyry Cu-Mo deposit, Inner Mongolia, NE China: constraints from geology, geochronology, geochemistry, and isotopic compositions. Journal of Asian Earth Science 117, 208-224. Zhang, J.-J., Zheng, Y.-F., Zhao, Z.-F., 2009. Geochemical evidence for interaction between oceanic crust and lithospheric mantle in the origin of Cenozoic continental basalts in east-central China. Lithos 110, 305-326. Zhang, M., Guo, Z., 2016. Origin of Late Cenozoic Abaga-Dalinuoer basalts, eastern China: implications for a mixed pyroxenite-peridotite source related with deep subduction of the Pacific slab. Gondwana Research 37, 130-151.

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same as Cpx 89 Grt 11 . Eclogite compositions were determined using the average oceanic crust values from the PetDB database and the mobility of elements during slab dehydration [START_REF] Becker | Trace element fractionation during dehydration of eclogites from high-pressure terranes and the implications for element fluxes in subduction zones[END_REF][START_REF] Kogiso | Trace element transport during dehydration processes in the subducted oceanic crust: 1. Experiments and implications for the origin of ocean island basalts[END_REF]. Model 1 is strongly enriched in large-ion lithophile elements (LILE) and exhibits positive anomalies in Pb and P, but no meaningful anomaly in K (Fig. 9a). On the other hand, Model 2 has characteristic spikes in K along with Pb and P, and a positive (Zr/Hf) N ratio, as observed in high-Si melts (Fig. 9a). For the low-Si group, we assumed P-rich melt II [a mixture of 7% tuite melt and 93% of a 1% eclogite melt], and a hypothetical mantle [90% E-DMM with 10% P-rich melt II]. We modeled low-Si melt compositions in two ways: (1) a 2% partial melt of a hypothetical mantle (Model 3), and (2) 75% of a 1% melt of E-DMM with 25% P-rich melt II (Model 4). Model 3 yield melt compositions significant enriched in LILEs compared with low-Si melts (Fig. 9b). Also note that Model 3 has a positive anomaly in Pb, contrary to observations in low-Si melts (Fig. 9b).

Meanwhile, the elemental abundances in Model 4 match those of the low-Si melts, exhibiting a positive anomaly in P and negative anomalies in K and Rb (Fig. 9b). Taken together, we suggest that the high-Si melts may have originated from an enriched MORB mantle source together with subordinate amount of K-hollandite and tuite-bearing eclogite, and the low-Si melts came from the same source but with insignificant contribution from K-hollandite.

Tectonic implications

To account for the mineral phases (K-hollandite, tuite) present in the source, the Baekdusan magmatism must be derived from a mantle source deeper than ~10 GPa. Recently, replacement of thick (~150-220 km), cold, refractory Archean lithospheric mantle by thin (~60-120 km), hot, fertile Cenozoic lithospheric mantle has been reported from the eastern NCC (Gao et al., 2002;Griffin et al., 1998;[START_REF] Menzies | Palaeozoic and Cenozoic lithoprobes and the loss of >120 km of Archaean lithosphere, Sino-Korean craton, China[END_REF][START_REF] Menzies | Geodynamics of the North China Craton[END_REF][START_REF] Xu | Thermo-tectonic destruction of the Archaean lithospheric keel beneath the Sino-Korean craton in china: evidence, timing and mechanism[END_REF][START_REF] Yang | Diachronous decratonization of the Sino-Korean craton: Geochemistry of mantle xenoliths from North Korea[END_REF]. Dramatic changes in the mechanical, thermal, and chemical ACCEPTED MANUSCRIPT Sobolev, A.V., Shimizu, N., 1993. Ultra-depleted primary melt included in an olivine from the Mid-Atlantic Ridge. Nature 363, 151-154. Spandler, C., O'Neill, H.S.C., [START_REF] Sobolev | The amount of recycled crust in sources of mantle-derived melts[END_REF] Survival times of anomalous melt inclusions from element diffusion in olivine and chromite. Nature 447, 303-306. Stolper, E., Sherman, S., Garcia, M., Baker, M., Seaman, C., 2004. Glass

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Wei, W., Xu, J., Zhao, D., Shi, Y., 2012. East Asia mantle tomography: new insight into plate subduction and intraplate volcanism. Journal of Asian Earth Science 60, 88-103. Wei, W., Zhao, D., Xu, J., Wei, F., Liu, G., 2015. P and S wave Zou, H., Fan, Q., Zhang, H., Schmitt, A.K., 2014. U-series zircon age constraints on the plumbing system and magma residence times of the Changbai volcano, China/North Korea border. [169][170][171][172][173][174][175][176][177][178][179][180]. S3. The "enriched"-depleted MORB mantle (E-DMM; Workman and Hart, 2005) and natural eclogite compositions [START_REF] John | Trace element fractionation during fluid-induced eclogitization in a subducting slab: trace element and Lu-Hf-Sm-Nd isotope systematics[END_REF]

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Fig. 9. Primitive-mantle-normalized incompatible trace element concentrations of melts modeled using non-modal batch melting [START_REF] Shaw | Trace element fractionation during anatexis[END_REF] for the high-Si group (a) and the low-Si group (b). See text for details of the modeled melt compositions. Gray lines represent olivine-hosted melt inclusions from Baekdusan basaltic rocks. MI = melt inclusion. Fig. 10. A schematic illustrating the melting process in the upper mantle beneath Baekdusan.


M Chauvet

P T Hoa

And F B Assignot

Voltage-induced waveguides in lithium niobate films on silicon substrates

Electrically tunable channel waveguides have been fabri-cated by domain inversion of a LiNbO 3 thin film bonded to a silicon wafer. The electro-optic effect is used to induce the waveguides and to alter its guiding properties. A low amplitude electric signal can tune the voltage-induced structure from an efficient waveguide to an antiguide giving a waveguide transmission that varies over more than 25 dB. The frequency response of the components is given.

Waveguides whose properties can be modified with the application of an electric field are key elements to process optical signals. They are part of guided optics devices such as modulators, variable attenuators, and switches that rely on various physical phenomena. Among others, we can cite electroabsorption [1], thermal effect [2] or electro-optic effect [3,4]. The latter is, by far, the most widely used one thanks to its fast and nonresonant response. These electro-optic devices are usually made of permanent waveguides whose guiding properties are slightly altered by application of a voltage. However, the ability to make a waveguide appear or disappear at will could find applications. For instance, full control over coupled waveguide arrays would bring great flexibility to route and shape an input signal beam. Such voltage-induced optical waveguides have been investigated since electro-optic crystal with strong electro-optic coefficients became available. A first demonstration was done by Channin [5] in the early 1970s taking advantage of the electro-optic properties of LiTaO 3 , but application of a voltage of hundreds of volt was necessary to induce efficient waveguides in a bulk medium. A similar device based on electrodes deposited at the surface of a substrate was later developed by optimization of the electrode design [6] and by choosing a material with a larger electro-optic effect [7].

In the present study, a hybrid structure that takes advantage of domain inversion in LiNbO 3 thin films adhered to silicon is fabricated to form voltage-operated waveguides. The waveguides properties can be tuned from guiding to antiguiding with a low 10 V amplitude signal. Characterizations performed at telecom wavelengths reveal the good performances of these original hybrid devices that offer an alternative for silicon photonics modulators.

The first stage of the fabrication process is to locally invert ferroelectric domains in a 500 μm thick 4 in. (10.16 cm) diameter Z-cut congruent LiNbO 3 wafer, supplied by Gooch and Housego. To do so, a standard technique involving the application of an intense electric field opposite to the initial Z -axis direction [8] through a photo-resist pattern is used. The mask generating the poling pattern consists in a series of 50 mm long parallel lines 1 mm apart whose width varies from 4 to 16 μm. Observation of the poled wafer with a polarizing microscope reveals the successful poling as witnessed by the presence of 50 mm long narrow stripes oriented along the y axis of the LiNbO 3 wafer. As discussed later in this Letter, the width of the poled stripes is found to be significantly larger than the ones of the mask which is attributed to an overpoling [9]. In a second stage, a 350 nm-thick SiO 2 layer is deposited by ICPECVD onto one face of the poled wafer followed by the sputtering of a 300 nm-thick gold layer. Then, a high flatness 4 in. (10.16 cm) diameter silicon wafer is also coated onto one face with a 300 nm-thick gold layer. The metalized faces of both the PPLN and the silicon wafers are then placed into contact and pressed in an EVG wafer bonding machine. For this bonding process, a pressure of 65 N • cm -2 is applied under vacuum. This metal diffusion bonding process is realized at room temperature which prevents mechanical stress that could occur due to the dissimilar temperature coefficients of the two wafers. The bonding is finalized by applying a strong pressure to the stack which yields more than 98% of the surface bonded as observed by an ultrasound characterization technique [START_REF] Ballandras | Ferroelectrics-Applications[END_REF]. A 1 mm thick hybrid structure composed of a silicon substrate bonded to a poled LiNbO 3 wafer is thus obtained. The next step is to thin the LiNbO 3 layer down by grinding and polishing to reach a 6 μm thickness. At last, a 350 nm-thick SiO 2 layer is deposited at the surface of the LiNbO 3 film followed by the sputtering of a 300 nm-thick gold electrode. Finally, this hybrid structure is diced with a precision saw to give samples of different propagation lengths with polished input and output faces. Each sample thus forms a slab waveguide thanks to the step index between silica (n SiO 2 1.45) and the 6 μm thick LiNbO 3 (n LN ≈ 2.2) layer as illustrated in Fig. 1(a). The LiNbO 3 layer is Z oriented except along the stripes where the domains have been inverted. Characterization of the samples surface flatness with an optical profilometer reveals that the LiNbO 3 film is slightly thicker along these poled stripes. To illustrate this excess thickness, Fig. 1(b) presents the transverse profile measured across a stripe poled with a 6 μm wide line of the mask. A 120 nm high and 18 μm (FWHM) wide hump is observed. An exhaustive analysis of all poled stripes shows similar profiles but with a height that increases from 100 nm for the narrowest mask lines (4 μm) up to 180 nm for the widest lines (16 μm). In addition, we found that the hump widths (FWHM) are systematically 12 μm wider than the mask line which corroborates a fixed offset due to an overpoling. It suggests that the characteristics of the electrical pulse [9] used for the poling can be optimized. The surface corrugation observed in the -Z oriented area is attributed to the disparate mechanical properties of -Z and Z oriented LiNbO 3 surfaces [START_REF] Chauvet | [END_REF]. Indeed, -Z surfaces are harder than Z surfaces which explain that the poled area become slightly thicker than the surrounding areas during the polishing process.

The components described above are then optically characterized. Light from a laser diode emitting at a wavelength of 1550 nm is collimated and linearly polarized. This light beam is focused with a 40 mm focal length lens to be end-fired coupled in the waveguide. Beam overlap with the fundamental mode is optimized to avoid excitation of higher order modes. The image at the output face of the waveguide is formed on a Vidicon camera thanks to a microscope objective. Two electrical wires are then connected to each component. One is attached to the gold layer located at the surface, and the other one is connected to an aluminum electrode deposited on the edge of the component which is in contact with the gold bonding layer. With this configuration, the voltage is directly applied across the SiO 2 and LiNbO 3 layers giving rise to an electric field oriented along the Z axis.

To test the component response, a DC voltage supply is first used. When no driving voltage is present, the light launched in the y direction is weakly guided along the poled stripe thanks to the presence of a rib waveguide induced by the hump present at the surface. To illustrate this effect, Fig. 2(c) depicts the light distribution observed at the exit face of a 3 cm long waveguide whose measured width W is 16 μm. Note that no coupling to higher order modes is observed experimentally. When the electric potential of the surface electrode is lower than the one of the bonding electrode (negative voltage) the Z-polarized guided mode becomes narrower as observed in Figs. 2(a) and 2(b). This behavior is due to the refractive index change induced by the Z-oriented electric field E LN applied in the LiNbO 3 layer. For a negative voltage, the refractive index increases in the core of the waveguide, while it decreases on both sides. This refractive index deepness Δn along the x axis that depends on light polarization is given by

Δn E LN 2 4 n 3 o r 13 E LN 00 0 n 3 o r 13 E LN 0 00 n 3 e r 33 E LN 3 5 ; (1)
where n 0 and n e are, respectively, the ordinary and extraordinary refractive index of LiNbO 3 . r 33 and r 13 are coefficients of the LiNbO 3 electro-optic tensor. n e 2.203, n o 2.286 [12], r 13 9.6 × 10 -12 mV -1 , r 33 30.9 × 10 -12 mV -1 [13].

A step-index waveguide is thus formed for a negative applied voltage. Note that for a given applied field the refractive index change given by Eq. ( 1) is twice the one obtained with standard voltage-induced waveguides because it benefits from the two oppositely poled areas.

As expected, the waveguide becomes an antiguide when a positive voltage is applied. The light is repelled from the central part of the waveguide because the index of refraction is lower in the inverted area than on each side as seen in Figs. 2(d) and 2(e). A similar behavior is observed for an X -polarized beam, but the voltage has to be increased accordingly to the lower electro-optic coefficient r 13 .

To calculate the electric field E LN induced by the voltage V , we consider that the multilayer dielectric structure is composed of SiO 2 and high resistivity LiNbO 3 . The line integral relation between the voltage and the static electric field along with the electric displacement continuity then yield

E LN -V d LN 1 d SiO 2 ϵ LN ∕d LN ϵ SiO 2 -V d eff ; (2)
where d LN 6 μm and d SiO 2 0.7 μm are, respectively, the thickness of the LiNbO 3 and silica layers, and ϵ Sio 2 3.9 and ϵ LN 35 are their respective relative dielectric permittivity [14,15]. For the present structure, we find d eff 12 μm which is twice the thickness of the LiNbO 3 layer. This effective thickness d eff can be reduced to obtain a larger electric field E LN for a given applied voltage V . Indeed, the SiO 2 layers thickness could be minimized or, preferentially, material with larger dielectric constant than SiO 2 could to be used so that the effective thickness d eff approaches d LN . Moreover, a LiNbO 3 film of submicron thickness is also a viable option to improve the component.

The measured mode width as a function of applied voltage is presented in Fig. 3 for extraordinary light. As expected, the behavior is highly nonlinear. For large negative voltage, the guided mode width (FWHM) saturates to a minimum width of approximately 15 μm FWHM corresponding to Fig. 2(a) which is close to the measured width of the poled stripe (16 μm). It shows that the width of the poled region defines the waveguide width contrary to standard voltage-induced waveguides, where the electric field distribution dictates the waveguide index profile. For the positive applied field, we observe that the guiding effect due to the surface rib is cancelled for an applied voltage of approximately 6 V. Above this latter amplitude, light is repelled from the central region of the waveguide, and the mode width cannot be determined anymore. The tested component is thus in a partial on state in idle position (no applied voltage). Note that a device in low transmission regime (off state) in idle position could have been realized with a slight change in the fabrication process. Indeed, by simply bonding the silicon wafer to the opposite face of the LiNbO 3 wafer, the polishing process would have yielded to the presence of a dip at the surface.

From Eqs. ( 1) and ( 2), we find that an applied voltage of 50 V leads to a step index waveguide with a refractive index difference Δn 1.3 × 10 -3 between the core and the cladding for extraordinary light. In reference [START_REF] Kogelnik | Guided-Wave Optoelectronic[END_REF], it is found that a waveguide width of approximately 5 μm would be best to obtain an optimally confined mode at a wavelength of 1.55 μm. A better control of the poling process to form narrower waveguides is thus expected to improve the characteristics of these voltage-induced waveguides.

The large change in mode width depicted in Fig. 3 for an applied voltage between 20 V and -20 V allows envisioning that the tested device can be used as an efficient intensity modulator controlled with a moderate amplitude signal. As a demonstration, a single-mode fiber is butt-coupled to the output of a 13 mm long voltage-controlled waveguide whose other dimensions are similar to the previously tested waveguide. A 50 MHz bandwidth photodiode is connected to the fibered components. Light coupling in the waveguide is still realized with the previous optical spatial set-up. The component transmission response to various electrical signals is then analyzed. As an example, the response to a 10 kHz positive driving square signal is presented in Fig. 4. As expected, the transmitted optical signal is out of phase with the electrical signal. For a positive driving signal of about 10 V amplitude, the contrast between the on and off states reaches 25 dB.

At last, the electrical frequency response of the voltagecontrolled waveguide is characterized by applying an AC voltage. The generator is set to apply a weak modulation signal V ac superimposed to a positive offset voltage to work in a linear part of the component transmission response. The response is defined as the ratio of the amplitude of the modulated detected light to the amplitude of the modulation signal. Figure 5 presents the response of the previously used 13 mm long waveguide normalized to the low frequency response. A low-pass frequency response is obtained with an experimental cutoff frequency at 3 dB of 600 kHz. If we consider that LiNbO 3 and SiO 2 are perfectly dielectric (insulator), the equivalent electrical circuit is well described by a series RC model. The resistor value R is due to the generator 50 Ω internal resistance, and the capacitor is given by the LiNbO 3 and the SiO 2 layers. The total capacitance C eq can be calculated by the respective capacitance of each layer C LN and C SiO 2 :

1 C eq 1 C LN 1 C SiO 2 d LN ϵ 0 ϵ LN S d SiO 2 ϵ 0 ϵ SiO 2 S ; (3)
where S is the surface of the gold electrodes (see Fig. 1). The signal V ac supplied by the generator is applied to this RC circuit, while the transmission variation of the waveguide is proportional to the voltage V eff present across the LiNbO 3 capacitor. The transfer function, defined as T f V eff ∕V , is thus given by

T f 1 1 d SiO 2 ϵ LN ∕d LN ϵ SiO 2 1 2πRC eq f : (4)
T f corresponds to the transfer function of a first order lowpass filter with a cutoff frequency 1∕2πRC eq . For the component characterized in Fig. 5, the surface of the electrode is 1.3 cm 2 which gives a calculated cutoff frequency of about 900 kHz. It is in fair agreement with the experimental results of 600 kHz. The discrepancy may be due to the uncertainties on the characteristics of the deposited SiO 2 layers (thickness and dielectric constant). Moreover, it is important to note that the electrode surface is not ideal because it extends 5 mm on both sides of the tunable waveguide. To gain some bandwidth, the electrode should instead be slightly wider than the waveguide. According to Eq. ( 3), a 50 μm wide electrode would lead to a frequency cutoff of 200 MHz. Modulators based on voltage-induced waveguides could be achieved with even higher bandwidths by using carefully designed traveling wave electrodes as often used for Mach-Zehnder modulators [START_REF] Noguchi | [END_REF].

The presented voltage-induced waveguides could be at the heart of a variable optical attenuator with a fast response. As stated above, applications such as large bandwidth modulators could also be envisioned. A great advantage of the structure comes from its straightforwardness because it is composed of a single straight waveguide. The simple fabrication process should give high yields at low cost. Alternatives to the described fabrication can be envisioned to simplify it even more. For instance, domain reversal of the LiNbO 3 could be realized after the thinning stage so that the same electrodes could be used for the poling and to apply the electric signal for tuning the waveguides. Optimally aligned electrodes would thus be obtained in a simple fabrication stage. In addition, fabrications of thinner LiNbO 3 films are in progress to both obtain single-mode waveguide and improve the electrical response.

In conclusion, the concept of voltage-controlled waveguides based on the local poling of a LiNbO 3 film has been presented. These components are fabricated by a combination of wafer bonding and grinding/polishing techniques to form few μm thick locally repoled LiNbO 3 films. Optical characterizations show that the voltage-induced waveguides can be tuned from efficient waveguides to antiguide waveguides with a low 10 V amplitude electric signal. This electrical signal can vary the waveguides transmission over more than 25 dB. Electrical bandwidth can reach a few hundred MHz by simple optimization of the electrode dimensions, while faster response could be envisioned with traveling wave electrodes. This first demonstration of an electro-optic induced waveguide with such a manufacturing process opens the way for the fabrication of electrically variable attenuators, fast modulators or even tunable waveguide arrays.

Funding. Région Franche-Comté; Agence Nationale de la Recherche (ANR) (ANR-16-CE24-0024-01).

Fig. 1 .

 1 Fig. 1. (a) Schematic cross section of the structure of the voltageinduced waveguides along with (b) the line profile measured with an optical profilemeter transversely to a waveguide.

Fig. 2 .

 2 Fig. 2. Output light distribution at the exit face of a 3 cm long waveguide for different applied DC voltage V. Waveguide width Wi s1 6μm. Light is extraordinary polarized.

Fig. 3 .

 3 Fig. 3. Mode width versus applied voltage at the exit face of a 3 cm long waveguide versus applied DC voltage. Waveguide width W is 16 μm.

Fig. 4 .

 4 Fig. 4. Chronogram of the transmitted signal (channel 2) with a single-mode fiber butt-coupled at the exit face of a 13 mm long waveguide driven by a 10 kHz square positive voltage (channel 1). Channel origins are indicated by arrows placed on the left side.

Fig. 5 .

 5 Fig. 5. Normalized frequency response of the 13 mm long voltage-induced waveguide.

Acknowledgment. This work was also partly supported by the RENATECH network and its FEMTO-ST MIMENTO technological facility.