Marine Paquet
email: mpaquet@ucsd.edu

Paterno D Day

R Castillo

James M D Day

Paterno R Castillo

Osmium isotope evidence for a heterogeneous 3He/4He mantle plume beneath the Juan Fernandez Islands

Keywords: Osmium isotopes, highly siderophile elements, Juan Fernandez, mantle geochemistry, Ocean Island Basalts

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

can occur over short-timescales (<2 Ma) and can impact lithospheric compositions, leading to eruption of rejuvenated lavas with unusual isotopic characteristics.

INTRODUCTION

Ocean island basalts (OIB) with high 3 He/ 4 He ratios (e.g., Hawaii, Iceland, Galapagos, Samoa, Reunion and the Juan Fernandez Islands) are widely considered to originate from primordial lower mantle via upwelling plumes (e.g. [START_REF] Kurz | Helium isotopic systematics of oceanic islands and mantle heterogeneity[END_REF][START_REF] Hart | Mantle plumes and entrainment: isotopic evidence[END_REF][START_REF] Farley | Binary mixing of enriched and undegassed (primitive?) mantle components (He, Sr, Nd, Pb) in Samoan lavas[END_REF]. Hence, these high-3 He/ 4 He OIB are probes to better constrain the composition of the deep mantle. High-3 He/ 4 He lavas are linked to conventional radiogenic isotope systematics in that they converge to a common mantle component in Sr-Nd-Pb isotope space, variably called FOZO (Focus Zone: [START_REF] Hart | Mantle plumes and entrainment: isotopic evidence[END_REF], PHEM (Primitive Helium Mantle: [START_REF] Farley | Binary mixing of enriched and undegassed (primitive?) mantle components (He, Sr, Nd, Pb) in Samoan lavas[END_REF], or C (Common: [START_REF] Hanan | Lead and helium isotope evidence from oceanic basalts for a common deep source of mantle plumes[END_REF], which only differ in detail from their isotope compositions in Sr, Nd and Pb isotopes, if at all [START_REF] Hofmann | Sampling mantle heterogeneity through oceanic basalts: isotopes and trace elements[END_REF].

The Juan Fernandez volcanic chain in the southeastern Pacific is an important location for investigating geochemical variations in OIB in so far as its lavas show a large range of 3 He/ 4 He compositions, between 7.8 to 18.0 R A ( 3 He/ 4 He normalized to air; Farley et al., 1993), from normal MORB-like ratios (8 ± 1 R A ; [START_REF] Graham | Noble gas isotope geochemistry of mid-ocean ridge and ocean island basalts: Characterization of mantle source reservoirs[END_REF] to higher ratios, which have previously been interpreted as a deep-mantle source signature (e.g. [START_REF] Kurz | Helium isotopic systematics of oceanic islands and mantle heterogeneity[END_REF][START_REF] Hart | Mantle plumes and entrainment: isotopic evidence[END_REF]. In contrast to their variable He isotope compositions, Juan Fernandez lavas span a limited range of Sr-Nd-Pb isotope ratios [START_REF] Gerlach | Mantle heterogeneity beneath the Nazca plate: San Felix and Juan Fernandez islands[END_REF][START_REF] Baker | Geochemical evolution of the Juan Fernandez islands, SE Pacific[END_REF]Farley et al., 1993;Truong et al., 2018). The Juan Fernandez volcanic island and seamount chain is located at 34°S on the Nazca Plate, between 76° and 83°W, approximately 660 km west of Chile (Figure 1). The chain is composed of two main islands, 180 km apart from each other, Robinson Crusoe (also called Mas a Tierra, closer to the land, at approximately 78°50'W), and Alexander Selkirk (Mas Afuera, away from the land, at approximately 80°50'W). A small island, Santa Clara, to the southwest of Robinson Crusoe, and numerous seamounts rise along the aseismic ridge oriented east-west at 34°S (Baker et al., 1983;Farley et al., 1993;[START_REF] Devey | Metasomatic reactions between carbonated plume melts and mantle harzburgite: the evidence from Friday and Domingo Seamounts (Juan Fernandez chain, SE Pacific)[END_REF][START_REF] Rodrigo | Plate tectonics and the origin of the Juan Fernández Ridge: analysis of bathymetry and magnetic patterns[END_REF]Lara et al., 2018a[START_REF] Lara | 40 Ar/ 39 Ar constraints on the age progression along the Juan Fernández Ridge, SE Pacific[END_REF]. Radiometric ( 40 Ar/ 39 Ar) ages for shield lavas range between 8.41-9.26 Ma for O'Higgins Guyot (e.g. Lara et al., 2018b), 3.40-4.10 Ma for Robinson Crusoe Island [START_REF] Reyes | Contrasting P-T paths of shield and rejuvenated volcanism at Robinson Crusoe Island, Juan Fernández Ridge, SE Pacific[END_REF][START_REF] Lara | 40 Ar/ 39 Ar constraints on the age progression along the Juan Fernández Ridge, SE Pacific[END_REF], and 0.83-0.94 Ma for Alexander Selkirk Island [START_REF] Lara | 40 Ar/ 39 Ar constraints on the age progression along the Juan Fernández Ridge, SE Pacific[END_REF]. Previous studies reported K-Ar ages up to 5.8 ± 2.1 Ma for Santa Clara, from 3.1 ± 0.9 to 4.2 ± 0.2 Ma for Robinson Crusoe, and from 0.85 ± 0.3 Ma to 2.4 ± 0.1 Ma for Alexander Selkirk [START_REF] Booker | Palaeomagnetism and age of rocks from Easter Island and Juan Fernandez[END_REF][START_REF] Stuessy | Botanical and geological significance of potassium-argon dates from the Juan Fernandez Islands[END_REF][START_REF] Baker | Geochemical evolution of the Juan Fernandez islands, SE Pacific[END_REF]. All available age data suggest a difference between Robinson Crusoe and Alexander Selkirk Islands of ~2 to 3 Ma, which is in good agreement with the Nazca Plate moving eastward at a spreading rate of about 74 mm.yr -1 [START_REF] Demets | Geologically current plate motions[END_REF]. Previous studies described fresh basalts from Friday and Domingo seamounts (Farley et al., 1993;[START_REF] Devey | Metasomatic reactions between carbonated plume melts and mantle harzburgite: the evidence from Friday and Domingo Seamounts (Juan Fernandez chain, SE Pacific)[END_REF], indicating that these seamounts are younger than Alexander Selkirk. The progressively younger ages to the west support the hypothesis of an age-progressive hotspot volcanism.

Early studies argued that the eastward movement of the Nazca Plate over a stationary plume formed the Juan Fernandez volcanic chain, making new volcanoes in the west while the magma supply to the older volcanoes was decreasing [START_REF] Baker | Geochemical evolution of the Juan Fernandez islands, SE Pacific[END_REF]Farley et al., 1993). Farley et al. (1993) proposed that the observed variations in helium isotopic compositions might be attributed to the mixing of the plume component, with a heterogeneous distribution of volatiles or having suffered extraction of small degree partial melts prior to mixing, and an asthenospheric component. The Juan Fernandez plume is now thought to sample both FOZO and a relatively depleted component (with MORB-like 3 He/ 4 He ratios) during the shield building stage, whereas the rejuvenated phase of the plume sampled mostly the FOZO component (Truong et al., 2018). These two stages, shield and rejuvenated volcanism in the Juan Fernandez Islands, have contrasting patterns of magma ascent and storage [START_REF] Reyes | Contrasting P-T paths of shield and rejuvenated volcanism at Robinson Crusoe Island, Juan Fernández Ridge, SE Pacific[END_REF] and correspond to a classical example of intraplate magmatism fed by a mantle plume; mechanisms also proposed for other OIB, like Hawaii or Réunion. Conversely, [START_REF] Natland | Capture of helium and other volatiles during the growth of olivine phenocrysts in picritic basalts from the Juan Fernandez Islands[END_REF] suggested that the helium contents measured in olivine crystals in Juan Fernandez lavas were indicative of mantle-derived volatiles, not derived from a plume, that pervaded magma conduits and storage reservoirs at shallow levels to be trapped in crystallizing olivine.

Highly siderophile elements (HSE: Os, Ir, Ru, Rh, Pt, Pd, Re and Au) and Os isotopes are potential arbitrators for understanding shallow versus deep mantle processes. In basaltic magmas, the HSE show highly compatible (Os, Ir, Ru), to compatible (Pt and Pd) or moderately incompatible (Re) behavior. These elements typically have parallel liquid lines of descent during fractional crystallization, which are controlled by spinel, olivine or sulfide fractionation (e.g. [START_REF] Puchtel | Platinum group element fractionation in a komatiitic basalt lava lake[END_REF]Ireland et al., 2009;[START_REF] Day | Hotspot volcanism and highly siderophile elements[END_REF]. In particular, the partition coefficient for Ru between spinel and melt is one to two orders of magnitude higher than for the other HSE (e.g. [START_REF] Day | Hotspot volcanism and highly siderophile elements[END_REF]. Therefore, HSE signatures in OIB can be dominated by mantle components, due to the corresponding low abundances of these elements in crustal rocks, but can also record shallow level fractionation processes [START_REF] Day | Hotspot volcanism and highly siderophile elements[END_REF]. This behavior contrasts with other lithophile isotopic systems (Sr-Nd-Pb-Hf) usually documented in OIB; these elements tend to be enriched in crustal and sedimentary rocks (>>ppm level), which can be assimilated by melts and obscure the isotopic and elemental signature of the mantle source (<ppm level).

The long-lived 187 Re-187 Os decay system ( 187 Re → 187 Os + β -; λ = 1.6668 x 10 -11 a -1 ; [START_REF] Selby | Assessment of the 187 Re decay constant by cross calibration of Re-Os molybdenite and U-Pb zircon chronometers in magmatic ore systems[END_REF] is an important tool for characterizing the geochemical history of mantle reservoirs.

Rhenium and Os have contrasting incompatibilities during melting, with Os being retained within the residue and Re being incompatible, resulting in large isotopic variations generated in crustal reservoirs ( 187 Os/ 188 Os ≥ 1; e.g. Day, 2013 and references therein), and smaller, generally subchondritic 187 Os/ 188 Os variations ( 187 Os/ 188 Os = 0.13) in peridotites (e.g. Day, 2013 and references therein). Abundances of the HSE, coupled with the long-lived 187 Re-187 Os decay system, are therefore important tools for characterizing the geochemical history of mantle reservoirs and shallow petrogenetic processes affecting magmas and their volcanic products (e.g. [START_REF] Rehkämper | Platinumgroup element abundance patterns in different mantle environments[END_REF][START_REF] Brandon | The debate over core-mantle interaction[END_REF][START_REF] Day | Hotspot volcanism and highly siderophile elements[END_REF].

We present whole-rock and olivine separate Os isotope compositions and major-(for olivine separates), trace-and highly siderophile-element abundances for "shield stage" alkali basalts and "post-shield stage or rejuvenated" basanites from the two main islands of the Juan Fernandez Archipelago, Robinson Crusoe and Alexander Selkirk that were previously analyzed by Farley et al. (1993) and Truong et al. (2018) (see Table S1 for the compilation of all data available for the Juan Fernandez lavas and olivine grains), as well as in situ major and highly siderophile-element compositions of the spinel inclusions identified in the olivine grains. The objectives of this study are: (1) to complement the global OIB dataset with lavas from the Juan Fernandez Islands for HSE abundances and Os isotope compositions and compare them with other primitive plume-derived lavas from Hawaii, Réunion or Canary Islands, and (2) investigate the origin of the Juan Fernandez Islands, and in particular the differences in mantle source signature for the lava series from Robinson Crusoe and Alexander Selkirk. Combining Os and He isotopes has the potential to better explain the source contributions in Juan Fernandez lavas, and the origin of their variable 3 He/ 4 He ratios.

SAMPLES AND METHODS

The samples examined in this study were collected in 1988, during Leg I of the Scripps Institution of Oceanography Hydros expedition. We analyzed 17 basalts (11 samples from Robinson Crusoe; 6 samples from Alexander Selkirk), as well as olivine separates from three of these samples (two from Robinson Crusoe; one from Alexander Selkirk), for their Os isotope compositions, highly siderophile element (HSE: Re, Pd, Pt, Ru, Ir, Os) and trace element abundances. Samples from Robinson Crusoe include 'main shield' olivine tholeiites and alkali basalts, previously termed Group I lavas by Farley et al. (1993), and younger basanite lavas (termed Group II by Farley et al., 1993) that correspond to the rejuvenated post-shield stages of Robinson Crusoe Island [START_REF] Reyes | Contrasting P-T paths of shield and rejuvenated volcanism at Robinson Crusoe Island, Juan Fernández Ridge, SE Pacific[END_REF]Truong et al., 2018). Samples from Alexander Selkirk comprise mostly olivine tholeiites (termed Group III by Farley et al., 1993), and correspond to the main shield stage during formation of the island [START_REF] Reyes | Contrasting P-T paths of shield and rejuvenated volcanism at Robinson Crusoe Island, Juan Fernández Ridge, SE Pacific[END_REF]Truong et al., 2018).

Whole-rock powders and olivine separates were obtained from the study of Troung et al. (2018). The olivine separates were 100% pure olivine, however, dark inclusions were observed in virtually all olivine grains and could not be removed from the separate due to the requirement for sufficient analyte for Os isotope and HSE abundance analyses. Osmium isotope and HSE abundance analyses were performed at the Scripps Isotope Geochemistry Laboratory (SIGL).

One gram of finely ground and homogenized sample powder was precisely weighed and then digested in sealed 20 cm long borosilicate Carius tubes using a mixture of multiply Teflon distilled HCl (4 mL) and 'purged' HNO 3 (7 mL; expunged of Os using H 2 O 2 ), with isotopically enriched multi-element spikes ( 99 Ru, 106 Pd, 185 Re, 190 Os, 191 Ir, 194 Pt). Digestions lasted for 72 hours in an oven at a maximum temperature of 240°C.

After digestion, Os was extracted from aqua regia in three steps using CCl 4 and then backextracted using HBr (Cohen and Water, 1996), before being purified by micro-distillation [START_REF] Birck | Re-Os isotopic measurements at the femtomole level in natural samples[END_REF]. The other HSE (Re, Pd, Pt, Ru, Ir) were recovered and purified from the residual solutions using anion exchange column chemistry [START_REF] Day | Oxygen isotope systematics of South African olivine melilitites and implications for HIMU mantle reservoirs[END_REF][START_REF] Day | Use of Hydrofluoric Acid Desilicification in the Determination of Highly Siderophile Element Abundances and Re-Pt-Os Isotope Systematics in Mafic-Ultramafic Rocks[END_REF]. Osmium isotopic measurements were performed using a Thermo Scientific Triton thermal ionization mass spectrometer in negative ion mode, and Re, Pd, Pt, Ru and Ir abundances were measured using a Thermo Scientific iCaP Qc ICP-MS coupled to a Cetac Aridus II desolvating nebulizer. Osmium isotope and abundance data were appropriately oxide-, fractionation-, spike-and blank corrected [START_REF] Day | Use of Hydrofluoric Acid Desilicification in the Determination of Highly Siderophile Element Abundances and Re-Pt-Os Isotope Systematics in Mafic-Ultramafic Rocks[END_REF]. Precision for 187 Os/ 188 Os, determined by repeated measurements of the 35 pg UMCP Johnson-Matthey standard, was better than ± 0.2% (2SD; 0.11357 ± 23; n = 7). Rhenium, Pd, Pt, Ir and Ru isotopic ratios were corrected for mass fractionation using the deviation of standards measured during the run over the natural ratio of the element (Table S2). External reproducibility on HSE analyses was better than 0.9% for 0.5 ppb solutions and all reported values are blank corrected. Total procedural blanks run with samples had 187 Os/ 188 Os = 0.22 ± 0.09 (n = 4; 1 s.d.), with quantities (in pg) of 0.5 ± 0.6 [Re], 11.9 ± 10.5 [Pd], 1.3 ± 0.7 [Pt], 10.2 ± 8.5 [Ru], 1.6 ± 1.3 [Ir] and 2.9 ± 1.1 [Os] (n=3; 1 s.d.) (see Table S3 for details). Repeat measurements of BHVO-2 and OKUM-1 standards gave 187 Os/ 188 Os ratios of 0.15317 ± 0.02522 (n = 3) and 0.26471 ± 0.02373 (n = 3) respectively. HSE abundances are (in ng) 0.7 ± 0.02 [Re],

3.2 ± 0.3 [Pd], 14.2 ± 4.9 [Pt], 0.3 ± 0.02 [Ru], 0.1 ± 0.01 [Ir] and 0.3 ± 0.07 [Os] for BHVO-2 (n = 3; 1 s.d.) and 0.6 ± 0.002 [Re], 12.7 ± 0.8 [Pd], 11.1 ± 0.08 [Pt], 4.7 ± 0.02 [Ru], 1.0 ± 0.06

[Ir] and 1.2 ± 0.2 [Os] for OKUM-1 (n = 3; 1 s.d.). The HSE abundances and 187 Os/ 188 Os ratios for OKUM-1 are consistent with those reported by [START_REF] Savard | Comparison between nickel-sulfur fire assay Te co-precipitation and isotope dilution with high-pressure asher acid digestion for the determination of platinum-group elements, rhenium and gold[END_REF], [START_REF] Chen | Platinum-group element abundances and Re-Os isotopic systematics of the upper continental crust through time: Evidence from glacial diamictites[END_REF] and [START_REF] Maier | The concentration of platinum-group elements and gold in southern African and Karelian kimberlite-hosted mantle xenoliths: implications for the noble metal content of the Earth's mantle[END_REF][START_REF] Maier | Platinum-group element contents of Karelian kimberlites: Implications for the PGE budget of the sub-continental lithospheric mantle[END_REF]. Information regarding run statistics can be found in Tables S2 andS3.

Major and trace elements analyses were performed on 50 mg of homogenized whole rock powder or powdered olivine, using a Thermo Scientific iCaP Qc ICP-MS using the method outlined in [START_REF] Day | Oxygen isotope systematics of South African olivine melilitites and implications for HIMU mantle reservoirs[END_REF]. Samples were prepared and analyzed with powdered rock standards (BHVO-2, BCR-2 and BIR-1a), to confirm accuracy, with reproducibility on most of trace element abundances better than ± 5% (with the exception of Ba, Zr and Mo; see Tables S7 andS8 for more information). We find that our new trace element data are in good agreement and expand the available trace elements measured in previous studies (Baker et al., 1983;Farley et al., 1993;Truong et al., 2018) using both XRF and ICP-MS methodologies. Trace element abundances are reported in Table S6. Major element analyses were performed for the olivines to complement the whole-rock dataset from Farley et al. (1993). We note that Juan Fernandez olivine separates have high Al 2 O 3 and TiO 2 content (0.4 -1.2 wt% and 0.08 -1.9 wt% respectively) compared to other OIB olivine separates like those from Samoa (Jackson & Shirey, 2011; olivine Al 2 O 3 content between 0.04 and 0.07 wt%, TiO 2 content between 0.01 and 0.03 wt%), likely reflecting impurities in the measured separates, including spinel grains. The Robinson Crusoe rejuvenated olivine is enriched in incompatible elements compared to the main shield olivine from the same island (Figure 3). Moreover, the olivine grains in the three different samples display strong positive anomalies in titanium. This is explained by the abundance of chromian titanomagnetite inclusions in the olivine grains, as described by [START_REF] Natland | Capture of helium and other volatiles during the growth of olivine phenocrysts in picritic basalts from the Juan Fernandez Islands[END_REF].

Characterization of the spinel inclusions in the olivine grains was performed using a Field Emission Gun Scanning Electron Microscopy (FEG-SEM) coupled with an EDS detector at the University of California San Diego. Map acquisition was performed using one frame count and a pixel dwell time of 100 µs. Spectra were acquired for 20 s. The major element compositions of the spinel inclusions in the olivine grains are reported in Table S4. In-situ HSE abundance analyses of spinel inclusions and olivine grains were performed at the SIGL using a Thermo Scientific iCaP Qc ICP-MS coupled with a New Wave Research UP213 Laser Ablation System. Ablation analysis took place in a 3 cm 3 ablation cell. The cell was flushed with a He-gas flow mixed with Ar as a carrier at ~ 1 L.min -1 . Signals were acquired in Time Resolved Acquisition, devoting around 15s for the blank and 30s for measurement of the analyses. Wash time between each analysis was 120 s. The laser was fired using an energy density of ~3.4 J.cm -2 at a frequency of 5Hz. Standardization was performed using iron meteorites Hoba, Filomena and Coahuila as certified reference material, using preferred values reported by [START_REF] Day | The potential for metal contamination during Apollo lunar sample curation[END_REF] for the HSE. Because the spinel inclusions were mostly smaller than the beam size, olivine grain compositions were also measured and proportionally subtracted from the spinel measurements.

Even though HSE were detected in some of the olivine grains and their spinel inclusions, results were limited by the detection efficiency of the instrument and the sample size. The HSE abundances in the spinel inclusions and olivine grains are reported in Table S5.

RESULTS

Characterization of inclusions in Juan Fernandez olivine grains

Juan Fernandez olivine grains contain numerous inclusions that are mostly spinels (Figure 2 and Table S4), as previously described by [START_REF] Natland | Capture of helium and other volatiles during the growth of olivine phenocrysts in picritic basalts from the Juan Fernandez Islands[END_REF]. Spinel inclusions in Robinson Crusoe and Alexander Selkirk main shield olivine grains have higher Cr# (55 ± 7; Cr/[Cr+Al] AFU) and lower Mg# (43 ±4; Mg/[Mg+Fe] AFU; n = 16) than spinel grains in the rejuvenated lava (Cr# = 12 ± 6; Mg# = 66 ± 2; n = 4). Abundances of the HSE for olivine and spinel are reported in Table S5. Spinel inclusions have generally elevated abundances of Re (~0.3 × CI chondrite), Ru, Os and Pd (~0.12 × CI chondrite) and low Ir and Pt compared to their olivine host.

Trace element abundances

Trace element abundance measurements for Juan Fernandez Islands lavas and olivine separates are reported in Table S6 andFigures 3 andS1. These new data expand upon -and are in excellent agreement with -previously published Juan Fernandez datasets (Baker et al., 1983;Farley et al., 1993;Truong et al., 2018). Juan Fernandez lavas show enrichment relative to primitive mantle in high field strength elements (HFSE) Ti, Ta and Nb, as reported by Truong et al. (2018), which are common features of most OIB [START_REF] Jackson | Globally elevated titanium, tantalum, and niobium (TITAN) in ocean island basalts with high 3 He/ 4 He[END_REF][START_REF] Peters | Assessment of relative Ti, Ta, and Nb (TITAN) enrichments in ocean island basalts[END_REF].

The majority of Robinson Crusoe and Alexander Selkirk main shield lavas show positive anomalies in Nb, Ta, Zr, and Sr, and slight negative anomalies in Y. Main shield lavas also have similar rare earth element (REE) patterns with light REE (LREE) enrichments (Figure 3). In contrast, Robinson Crusoe basanite lavas have higher absolute trace element abundances than those from the main shield stage and higher Nb/Y at a given Zr/Y (Figures 3 and4). This characteristic is also observed in the olivine separates (Figure 3), which also display strong positive anomalies in titanium (see Supplementary Information). Olivine separates have low Mo and W abundances (<0.2 and <0.1 ppm, respectively) compared with main shield lavas from Robinson Crusoe (Mo = 0.72 ± 0.09 ppm; W = 0.23 ± 0.09 ppm) and Alexander Selkirk (Mo = 0.84 ± 0.15 ppm; W = 0.44 ± 0.34 ppm), which in turn, have lower abundances of Mo and W than the basanite lavas from Robinson Crusoe (Mo = 2.23 ± 0.18 ppm; W = 0.88 ± 0.13 ppm) (Figure S2).

Osmium isotope compositions and highly siderophile element abundances

Rhenium-osmium isotope and HSE (Re, Pd, Pt, Ru, Ir, Os) abundance measurements for Juan Fernandez Islands lavas and olivine separates are reported in Table 1. Ratios of 187 Os/ 188 Os were appropriately age-corrected to 4 Ma and 1 Ma, respectively, for the islands of Robinson Crusoe and Alexander Selkirk and we refer to age-corrected values even though age-corrections are negligible (Table 1). Low-Os lavas (10-50 ppt; PV-2, PF-16 Rpt, MF-16, MF-6 Rpt and MF-C4) have variable to radiogenic isotopic compositions (0.1291 to 0.1435). For samples (including olivine separates) with Os >50 ppt, 187 Os/ 188 Os ratios for Robinson Crusoe main shield lavas are 0.1313 ± 0.0022 (Os = 65-910 ppt; 1 s.d., n = 7), with slightly less radiogenic although overlapping values for Alexander Selkirk main shield lavas (0.1291 ± 0.0023; Os = 50-276 ppt; 1 s.d., n = 5) (Figure 5). The least radiogenic lavas are the basanite samples from Robinson Crusoe (0.1279 ± 0.0046; Os = 180-800 ppt; 1 s.d., n = 4). The osmium isotopic compositions of Juan Fernandez lavas are generally less radiogenic than lavas from many other OIB, and in the range of primitive mantle 187 Os/ 188 Os (0.1296 ± 0.0008; [START_REF] Meisel | Osmium isotopic compositions of mantle xenoliths: a global perspective[END_REF].

Olivine separates tend to be less radiogenic than their host lavas ([ 187 Os/ 188 Os Host -187 Os/ 188 Os Olivine] × 1000 values range between 3.7 and 6.4), with 187 Os/ 188 Os ratios between 0.12709 and 0.13032 (0.1283 ± 0.0017), with Os concentrations between 21 and 138 ppt (Figure S3). The 187 Os/ 188 Os composition of the olivine separates are akin to those from Samoa (Jackson and Shirey, 2011) and from Iceland and Jan Mayen (Debaille et al., 2009), which also display high He isotope values.

Robinson Crusoe main shield lavas have relatively flat HSE patterns, approximately ten times lower than Primitive Mantle estimates [START_REF] Day | 186 Os-187 Os and highly siderophile element abundance systematics of the mantle revealed by abyssal peridotites and Os-rich alloys[END_REF], with elevated Re contents (Figure 6a). Alexander Selkirk main shield lavas have generally lower absolute HSE abundances than Robinson Crusoe main shield lavas, with prominent Ru anomalies, and one sample also shows Pd and Ir enrichment (MF-16; Figure 6c). Sample MF-S1 also has anomalously high concentrations of Pt (>100 ppb); for clarity this datum is not shown in Figure 6. Robinson Crusoe basanites have a range of HSE abundances spanning those observed in the Robinson Crusoe and Alexander Selkirk main shield lavas (Figure 6b). Olivine separates tend to have lower HSE abundances compare to their host lava, with the exception of a strong positive Ru anomaly. Absolute HSE abundances of Juan Fernandez lavas are generally lower than for lavas from Hawaii (Ireland et al., 2009), Réunion (Peters et al., 2016) and the Canary Islands (Day et al., 2010), but show similar inter-element HSE variations (Figures 7 andS4).

DISCUSSION

Rejuvenated and main shield lavas and clarification of prior groupings

Flexural uplift of lithosphere in response to far-field stresses can lead to rejuvenated volcanism. On mature oceanic plate, such as around Hawaii, uplift of the elastic lithosphere occurs up to 200 km downstream from the active load, leading to decompression of the underlying asthenosphere (e.g., [START_REF] Bianco | Secondary Hawaiian volcanism formed by flexural arch decompression[END_REF] and generation of rejuvenated basaltic lavas characterized by depleted isotopic signatures (e.g., [START_REF] Bizimis | The composition and distribution of the rejuvenated component across the Hawaiian plume: Hf-Nd-Sr-Pb isotope systematics of Kaula lavas and pyroxenite xenoliths[END_REF][START_REF] Garcia | Petrology and geochronology of lavas from Ka 'ula Volcano: Implications for rejuvenated volcanism of the Hawaiian mantle plume[END_REF] and higher Nb/Y ratios at a given Zr/Y ratio than for main shield stage basalts (Figure 4), suggesting distinct mantle sources for the main shield stage and rejuvenated stage lavas.

Post-erosional lavas from Robinson Crusoe bear many of the hallmarks of rejuvenated lavas (see also, [START_REF] Reyes | Contrasting P-T paths of shield and rejuvenated volcanism at Robinson Crusoe Island, Juan Fernández Ridge, SE Pacific[END_REF]Truong et al., 2018). These lavas are low-degree partial melts (basanites; Figure S5) that are enriched in incompatible elements compared to the main shield lavas from Robinson Crusoe, and also have relatively depleted Sr and Nd isotope signatures [START_REF] Baker | Geochemical evolution of the Juan Fernandez islands, SE Pacific[END_REF]Farley et al., 1993;[START_REF] Reyes | Contrasting P-T paths of shield and rejuvenated volcanism at Robinson Crusoe Island, Juan Fernández Ridge, SE Pacific[END_REF]Truong et al., 2018). As with Hawaii, Robinson Crusoe rejuvenated lavas also have higher Nb/Y ratios at a given Zr/Y than those from the main shield stage. We also find that these rejuvenated lavas span a larger range of HSE abundances and have generally lower and more variable Os isotopes ratios than main shield lavas (Figure 5). Because there is no correlation between HSE abundances with fractional crystallization indicators like MgO (Figure S4), these variations likely reflect a greater degree of lithospheric interaction, where the oceanic lithosphere has relatively unradiogenic Os isotopic compositions and high HSE abundances (e.g., [START_REF] Day | 186 Os-187 Os and highly siderophile element abundance systematics of the mantle revealed by abyssal peridotites and Os-rich alloys[END_REF].

Generation of rejuvenated lavas on Robinson Crusoe at ~ 0.9 Ma ( 40 Ar/ 39 Ar age reported for one basanite from Santa Clara Island by [START_REF] Reyes | Contrasting P-T paths of shield and rejuvenated volcanism at Robinson Crusoe Island, Juan Fernández Ridge, SE Pacific[END_REF] are consistent with loading of the lithosphere by the formation of Alexander Selkirk (between 0.85 ± 0.3 Ma and 2.4 ± 0.1 Ma) some 180 km away at a plate velocity of 6 km Ma -1 . Since the Robinson Crusoe basanites can be classified as rejuvenated lavas, we clarify that the previous compositional grouping scheme of Farley et al. (1993) for the basalts (Group I = Robinson Crusoe main shield; Group II = Robinson Crusoe basanites; Group III = Alexander Selkirk main shield) is consistent with the distinct volcanism mechanisms for their formation.

Crystal-liquid fractionation defines Juan Fernandez lava HSE compositions

During ascent through the oceanic crust towards the surface, mantle-derived magmas can assimilate significant volumes of crustal material, which can alter the HSE abundances and Os isotopic composition of the melt (e.g. Roy-Barman and [START_REF] Roy-Barman | 187 Os/ 186 Os in oceanic island basalts: tracing oceanic crust recycling in the mantle[END_REF][START_REF] Lassiter | Osmium-isotope variations in Hawaiian lavas: evidence for recycled oceanic lithosphere in the Hawaiian plume[END_REF][START_REF] Widom | Os isotope systematics in the Canary Islands and Madeira: lithospheric contamination and mantle plume signatures[END_REF]Day et al., 2010). As Os is more compatible than Re during partial melting, large isotopic variations between mantle-derived materials ( 187 Os/ 188 Os < 0.18) and crustal or sedimentary materials occur with time ( 187 Os/ 188 Os ≥ 0.4; [START_REF] Day | Hotspot volcanism and highly siderophile elements[END_REF]. OIB with Os contents lower than 50 ppt are considered to be more susceptible to contamination by high 187 Os/ 188 Os crustal materials (e.g. [START_REF] Day | Hotspot volcanism and highly siderophile elements[END_REF]. A few samples among Juan Fernandez lavas have low Os concentrations (≤ 50 ppt), associated with radiogenic Os isotope compositions for two samples (MF-C4 = 0.13772 and PV-2 = 0.14349). Aside from these samples, assimilation processes play a limited role in Juan Fernandez main shield lava HSE compositions.

To quantify the role of fractional crystallization in the HSE fractionation for Juan Fernandez lavas, we used a model assuming olivine-melt and spinel-melt control using a calculated parental melt composition. The parent melt composition was obtained using a log-log variation diagram of Al 2 O 3 versus MgO which allows discrimination between samples that witnessed olivine and clinopyroxene accumulation or removal by a break in slope at ~11 to 12 wt.% Al 2 O 3 and ~14 to 16 wt.% MgO. The parent melt composition was then derived from linear regressions of HSE versus MgO trends at 15 ± 1 wt.%. These calculations indicate that the parental melts of lavas from Robinson Crusoe and Alexander Selkirk have broadly similar HSE contents. The amounts of early-formed spinel and olivine crystallization permissible for Juan Fernandez lavas were calculated with Petrolog [START_REF] Danyushevsky | Petrolog3: Integrated software for modeling crystallization processes[END_REF] for fractional crystallization of olivine and spinel at a range of pressures (1, 2, 5 and 10 kbar). Starting compositions were chosen as closest to the parent melt composition (MgO = 15 wt.%) for each of the three groups of lavas (PF-10, PF-16 and MF-C2 for Robinson Crusoe main shield lavas, Robinson Crusoe rejuvenated lavas and Alexander Selkirk main shield lavas respectively). Major element compositions for Juan Fernandez main shield lavas (e.g. Al 2 O 3 or CaO contents in function of MgO) can be explained by up to 30% of olivine fractionation (together with spinel crystallization of up to 5%) between 1 to 5 kbar. On the other hand, Robinson Crusoe rejuvenated lavas compositions can be reproduced at higher-pressure fractional crystallization (up to 10 kbar): these mantle-derived melts experienced rapid ascent to the surface with a deeper crystallization history. These results are consistent with [START_REF] Baker | Geochemical evolution of the Juan Fernandez islands, SE Pacific[END_REF] and [START_REF] Reyes | Contrasting P-T paths of shield and rejuvenated volcanism at Robinson Crusoe Island, Juan Fernández Ridge, SE Pacific[END_REF] who suggested shallow storage conditions for the shield stage lavas (~ 1-3 kbar) and rapid ascent with polybaric crystallization for the basanitic lavas (starting ~10 kbar).

Fractionation of olivine (up to 30%) alone from estimated parental melts produces residual melts with HSE abundances similar to the observed compositions for Juan Fernandez Islands but does not explain the shape of the observed HSE patterns, with a positive anomaly in Ru (ol/melt partition coefficients from [START_REF] Puchtel | Platinum group element fractionation in a komatiitic basalt lava lake[END_REF]. Ruthenium, Ir and Os are similarly compatible in olivine but Ru is highly compatible in spinel compared to the other HSE, and in particular to Ir and Os (e.g. [START_REF] Brenan | Experimental constraints on the partitioning of Ru, Rh, Ir, Pt and Pd between chromite and silicate melt: the importance of ferric iron[END_REF]. A notable feature of the Juan Fernandez olivine separates that we measured is a strong positive Ru anomaly (Figure 6). This can be explained by spinel crystallization and, then, inclusion in the olivine grains (Figure 2), further suggesting fractionation of both early-formed spinel and olivine. The method used to estimate the parent melt composition is associated with large uncertainties for HSE abundance estimates, which could explain why we are not able to reproduce the compositional range shown in Figure 8a. The incorporation of early-formed spinel (up to 5%) to the olivine grains can explain the generation of anomalously high Ru contents in olivine, due to the high compatibility of Ru in spinel (partition coefficients for sp/melt from [START_REF] Righter | Partitioning of Ru, Rh, Pd, Re, Ir, and Au between Cr-bearing spinel, olivine, pyroxene and silicate melts1[END_REF] for Re and [START_REF] Brenan | Experimental constraints on the partitioning of Ru, Rh, Ir, Pt and Pd between chromite and silicate melt: the importance of ferric iron[END_REF] for Pd, Pt, Ir, Ru (see compilation in [START_REF] Day | Hotspot volcanism and highly siderophile elements[END_REF]).

Fractionation of up to 30% of olivine (ol/melt partition coefficients for REE from [START_REF] Fujimaki | Partition coefficients of Hf, Zr, and REE between phenocrysts and groundmasses[END_REF] from the most primitive lava from each of the three groups of the Juan Fernandez lavas produces REE patterns similar to those of the Juan Fernandez olivines for the HREE but that are too depleted for the LREE. The addition of 1 to 5% spinel (sp/melt partition coefficients for REE from [START_REF] Nagasawa | Experimental mineral/liquid partition coefficients of the rare earth elements (REE), Sc and Sr for perovskite, spinel and melilite[END_REF] is also insufficient to reproduce the REE patterns we observe for Juan Fernandez olivine grains. However, the contamination by a small fraction (1 to 2% for the main shield lavas from both islands, up to 5% for the rejuvenated lavas) of the host lava (melt) is sufficient to enrich the olivine in LREE to the observed abundances, implying that silicate melt inclusions are also present in some olivine grains (Figure 9), which is consistent with petrographic observations [START_REF] Natland | Capture of helium and other volatiles during the growth of olivine phenocrysts in picritic basalts from the Juan Fernandez Islands[END_REF]. Contamination by the melt inclusions of the HSE signature of the olivine grains tends to lower the negative Pd anomaly of the calculated composition, although overall, it has a limited effect on the HSE signature of the olivine separates (Figure 8).

Comparison of OIB main shield and rejuvenated lavas

The Alexander Selkirk main shield lavas are lithologically and compositionally similar in terms of major and trace element abundances to Robinson Crusoe main shield basalts [START_REF] Baker | Geochemical evolution of the Juan Fernandez islands, SE Pacific[END_REF]Farley et al., 1993), whereas the Robinson Crusoe basanite lavas (rejuvenated lavas) have higher abundances of incompatible trace elements, interpreted as resulting from lower degrees of partial melting than for the main shield stage [START_REF] Baker | Geochemical evolution of the Juan Fernandez islands, SE Pacific[END_REF]Farley et al., 1993;[START_REF] Reyes | Contrasting P-T paths of shield and rejuvenated volcanism at Robinson Crusoe Island, Juan Fernández Ridge, SE Pacific[END_REF] and Figure S5). Samples from these three groups exhibit a large range in 3 He/ 4 He ratios (7.8 to 18.0 R A ): Robinson Crusoe shield basalts have 3 He/ 4 He ratios between 13.6 and 18.0 R A , whereas Robinson Crusoe post-shield basanites range from 11.2 to 12.9 R A (Farley et al., 1993;Truong et al., 2017). Main shield basalts from Alexander Selkirk show lower and more uniform 3 He/ 4 He ratios with an average value of 8.3 ± 0.5 R A (Farley et al., 1993).

Previous studies have shown limited variations for 87 Sr/ 86 Sr (0.70340-0.70378) and 143 Nd/ 144 Nd ratios (0.512835-0.512918) [START_REF] Gerlach | Mantle heterogeneity beneath the Nazca plate: San Felix and Juan Fernandez islands[END_REF][START_REF] Baker | Geochemical evolution of the Juan Fernandez islands, SE Pacific[END_REF]Farley et al., 1993;Truong et al., 2018). On average, Robison Crusoe main shield basalts are the most isotopically heterogeneous in terms of Sr and Nd isotopic composition, while the rejuvenated basanites lie at the most depleted end of the trend defined by Robinson Crusoe main shield basalts (lower 87 Sr/ 86 Sr and higher 143 Nd/ 144 Nd). Main shield basalts from Alexander Selkirk show homogeneous compositions, with higher 143 Nd/ 144 Nd at a given 87 Sr/ 86 Sr than those from Robinson Crusoe. Robinson Crusoe basalts and basanites have slightly more radiogenic common, albeit slightly heterogeneous mantle source [START_REF] Gerlach | Mantle heterogeneity beneath the Nazca plate: San Felix and Juan Fernandez islands[END_REF][START_REF] Baker | Geochemical evolution of the Juan Fernandez islands, SE Pacific[END_REF]Farley et al., 1993;Truong et al., 2018).

Alexander Selkirk main shield lavas have been dated between 0.83-0.94 Ma [START_REF] Lara | 40 Ar/ 39 Ar constraints on the age progression along the Juan Fernández Ridge, SE Pacific[END_REF], ca. 2 to 3 Ma younger than Robinson Crusoe main shield lavas. A 40 Ar/ 39 Ar age of 0.9 ± 0.03 Ma has also been reported by [START_REF] Reyes | Contrasting P-T paths of shield and rejuvenated volcanism at Robinson Crusoe Island, Juan Fernández Ridge, SE Pacific[END_REF] for a basanite from Santa Clara Island.

Even though no radiometric ages are available for Robinson Crusoe rejuvenated lavas, they are clearly younger as they intruded or cross-cut the main shield lavas [START_REF] Reyes | Contrasting P-T paths of shield and rejuvenated volcanism at Robinson Crusoe Island, Juan Fernández Ridge, SE Pacific[END_REF]. They are most likely to be contemporaneous with the creation of Alexander Selkirk Island. Robinson Crusoe rejuvenated basanites have slightly less radiogenic and more variable 187 Os/ 188 Os ratios than Alexander Selkirk main shield lavas, associated with higher 3 He/ 4 He values (11.2 to 12.9 R A for Robinson Crusoe rejuvenated lavas, and 7.8 to 9.5 R A for Alexander Selkirk main shield lavas).

Konter and Jackson (2012) reported 187 Os/ 188 Os ratios for Samoan rejuvenated lavas that are similar to 187 Os/ 188 Os ratios for Samoan peridotite xenoliths [START_REF] Jackson | Peridotite xenoliths from the Polynesian Austral and Samoa hotspots: Implications for the destruction of ancient 187 Os and 142 Nd isotopic domains and the preservation of Hadean 129 Xe in the modern convecting mantle[END_REF], and exhibit lower those for the Samoan main shield lavas (Figure 10). The Samoan xenoliths most likely originate from within the mantle lithosphere [START_REF] Hauri | Constraints on melt migration from mantle plumes: a trace element study of peridotite xenoliths from Savai'i, Western Samoa[END_REF], such that the Os isotope values in Samoan rejuvenated lavas are derived from the lithospheric mantle, suggesting melting during Samoan rejuvenation stage occurs close to the lithosphere-asthenosphere boundary (Konter and Jackson, 2012). Additionally, Samoan rejuvenated lavas have lower in 206 S6 and Konter and Jackson, 2012). These later observations are also consistent with sampling of a lithospheric component (Konter and Jackson, 2012). Hawaiian rejuvenated lavas, on the other hand, have highly variable Os isotope compositions than range to more radiogenic values than the main shield lavas, similar to pyroxenite xenoliths (Figure 10), whereas peridotite xenoliths have subchondritic Os ratios (Lassiter et al., 2000;Bizimis et al., 2007;Sen et al., 2011). Hawaiian rejuvenated lavas also have more depleted Sr-(and Nd-) isotope compositions than the main shield lavas (Figure S6). Lassiter et al. (2000) proposed that moderate-degree pyroxenite partial melts in the source of the rejuvenated lavas are most likely the source of the radiogenic Os isotope signature due to correlations between Os isotopes and both major and trace element abundances in the rejuvenated lavas. These results indicate strong similarities in rejuvenated lavas from Juan Fernandez, Samoa and Hawaii.

The Robinson Crusoe basanites also exhibit depleted Sr and Nd isotopes signatures [START_REF] Baker | Geochemical evolution of the Juan Fernandez islands, SE Pacific[END_REF]Farley et al., 1993;[START_REF] Reyes | Contrasting P-T paths of shield and rejuvenated volcanism at Robinson Crusoe Island, Juan Fernández Ridge, SE Pacific[END_REF]Truong et al., 2018)

compared to Robinson

Crusoe main shield lavas, and have lower 143 Nd/ 144 Nd at a given 87 Sr/ 86 Sr than Alexander Selkirk main shield lavas. No distinction is observed between the three types of lavas in terms of 207 Pb/ 204 Pb. However, Alexander Selkirk main shield lavas are less radiogenic in Pb isotopes than Robinson Crusoe main shield and rejuvenated lavas, which have overlapping 206 Pb/ 204 Pb and 208 Pb/ 204 Pb. Thus, the relative sense of isotopic variability in Juan Fernandez main shield and rejuvenated lavas is similar to those in Samoan and Hawaiian main shield and rejuvenated lavas [START_REF] Garcia | Petrology, geochemistry and geochronology of Kaua 'i lavas over 4•5 Myr: Implications for the origin of rejuvenated volcanism and the evolution of the Hawaiian plume[END_REF]Konter and Jackson, 2012), and are consistent with a lithospheric source for rejuvenated lavas [START_REF] Bianco | Secondary Hawaiian volcanism formed by flexural arch decompression[END_REF]Konter and Jackson 2012).

As noted above, Robinson Crusoe rejuvenated lavas have higher 3 He/ 4 He values than Alexander Selkirk main shield lavas and lower than Robinson Crusoe main shield lavas. We propose that the lithosphere was polluted by noble gases from the main shield building phase of Robinson Crusoe Island, during the ascent of the melts through the lithosphere, which then contaminated Robinson Crusoe rejuvenated melts explaining their elevated 3 He/ 4 He signatures.

Model mixtures of a MORB-like depleted component (D) and a high-3 He hotspot endmember (P) are presented in Figure 11 and show that Robinson Crusoe rejuvenated lavas lie between the main shield lavas from Robinson Crusoe and Alexander Selkirk (curves B and C), and correspond to 40 to 60% of contamination of the depleted mantle component by a degassed plume component (curves B and C), corresponding to an open-system where the plume component would have been degassed of its mantle-derived He. This is similar to the mixing model proposed by Farley et al. (1993) between two endmembers Primitive Helium Mantle (PHEM) and the depleted MORB mantle (DMM) to reproduce Juan Fernandez lavas compositions.

Implications for high-3 He/ 4 He and heterogeneous mantle plumes

Amongst OIB, Juan Fernandez lavas span an unusual isotopic compositional range: Juan Fernandez lavas overlap with Samoan lavas that are close to FOZO or C in terms of 187 Os/ 188 Os, 206 Pb/ 204 Pb, 208 Pb/ 204 Pb and 143 Nd/ 144 Nd ratios, but have lower 3 He/ 4 He ratios, intermediate between high-3 He/ 4 He OIB (like Samoan, Icelandic and Hawaiian lavas) and low-3 He/ 4 He OIB (Canary Islands and the Comores) (Figure 12). Overall, Juan Fernandez lavas converge at FOZO or C of the global OIB dataset e.g. [START_REF] Hart | Mantle plumes and entrainment: isotopic evidence[END_REF][START_REF] Hanan | Lead and helium isotope evidence from oceanic basalts for a common deep source of mantle plumes[END_REF]Day et al., 2010;Truong et al., 2018). Juan Fernandez Islands display isotopic characteristics of a considerable range in 3 He/ 4 He (7.8 to 18 R A ), but limited variability in Sr, Nd, Pb and Os isotopic ratios.

Whereas HSE and trace element abundances are controlled by melting and fractional crystallization, isotopic compositions of the Juan Fernandez lavas seem unaffected by fractional crystallization and assimilation processes. In particular, fractional crystallization and other petrogenetic processes cannot account for the high 3 He/ 4 He in Juan Fernandez lavas, as no correlation is observed between 3 He/ 4 He ratios and fractionation indicators like MgO. Thus, it seems that 3 He/ 4 He and bulk He of Juan Fernandez lavas are derived from the mantle plume and the differentiation processes experienced by the parent melts derived from this source, and not from volatiles that permeated shallow magma reservoirs [START_REF] Natland | Capture of helium and other volatiles during the growth of olivine phenocrysts in picritic basalts from the Juan Fernandez Islands[END_REF].

Mantle plume sources can vary significantly in 3 He/ 4 He temporally and spatially, such as Samoa (19.5 to 33.8 R A - Jackson et al., 2007), Hawaii (6 to 28 R A - Mukhopadhyay et al., 2003; up to 29.3 R A [START_REF] Valbracht | Noble gas systematics of deep rift zone glasses from Loihi Seamount, Hawaii[END_REF]) and Iceland (9.6 to 19 R A - [START_REF] Brandon | 186 Os and 187 Os enrichments and high-3 He/ 4 He sources in the Earth's mantle: evidence from Icelandic picrites[END_REF]; up to 37.7 R A - [START_REF] Hilton | Extreme 3 He/ 4 He ratios in northwest Iceland: constraining the common component in mantle plumes[END_REF]; and even 47.5 R A - [START_REF] Harðardóttir | Spatial distribution of helium isotopes in Icelandic geothermal fluids and volcanic materials with implications for location, upwelling and evolution of the Icelandic mantle plume[END_REF]. Variations in 3 He/ 4 He ratios are also observed for Juan Fernandez Islands: starting with inception of high 3 He/ 4 He (Robinson Crusoe main shield lavas) that also impregnates the lithosphere with 3 He (Robinson Crusoe rejuvenated lavas) followed by lower 3 He/ 4 He in Alexander Selkirk. Robinson Crusoe and Alexander Selkirk main shield lavas, however, are distinct in terms of radiogenic (Sr-Nd-Pb-Os) and noble gas (He) isotope compositions. Within the Juan Fernandez Islands, Robinson Crusoe main shield lavas are the most enriched samples with lower 143 Nd/ 144 Nd and higher lavas. The distinction between Robinson Crusoe and Alexander Selkirk main shield lavas suggests an isotopic zonation in the mantle plume. [START_REF] Huang | Geochemical zoning of volcanic chains associated with Pacific hotspots[END_REF] showed that geochemical zoning might be a common feature of mantle plumes beneath the Pacific plate: two subparallel arrays of volcanoes exist at Hawaii, Samoa, and the Marquesas, with the southern trend of volcanoes being more radiogenic in lead and less radiogenic in neodymium. [START_REF] Huang | Geochemical zoning of volcanic chains associated with Pacific hotspots[END_REF] proposed that isotopically enriched material is preferentially distributed in the lower mantle of the Southern Hemisphere, within the Pacific low seismic velocity zone. The distribution of heterogeneity in the Thermal Boundary Layer would result in bilaterally zoned plume conduits at Hawaii and the Marquesas, Samoa being more complex. A bilaterally zoned plume model has previously been proposed for the Hawaiian plume [START_REF] Abouchami | Lead isotopes reveal bilateral asymmetry and vertical continuity in the Hawaiian mantle plume[END_REF]. [START_REF] Weis | Role of the deep mantle in generating the compositional asymmetry of the Hawaiian mantle plume[END_REF] proposed that the geochemical differences between the Kea and Loa trends reflect preferential sampling of two distinct sources of deep mantle material: the Loa trend sampling LLSVP (Large Low-Shear Velocity Province) material, and the Kea trend sampling lower mantle material. Alternatively, [START_REF] Farnetani | Dynamics and internal structure of a lower mantle plume conduit[END_REF], 2010) have proposed that a continuous deep layer overlained by a bilaterally varying layer best reproduce the "bilateral asymmetry" observed for Pb isotopic differences between the Kea and Loa-trends of Hawaiian volcanoes and for the concentric distribution of 3 He/ 4 He [START_REF] Depaolo | Isotopic evolution of Mauna Loa and the chemical structure of the Hawaiian plume[END_REF]. It has also been suggested that high-3 He/ 4 He materials could be distributed as isolated primitive blobs in the lower mantle [START_REF] Becker | Thermal constraints on the survival of primitive blobs in the lower mantle[END_REF], or in an isolated primitive reservoir in the lower mantle [START_REF] Allègre | Constraints on evolution of Earth's mantle from rare gas systematics[END_REF][START_REF] Tolstikhin | Early crust on top of the Earth's core[END_REF] or even in the core [START_REF] Bouhifd | Helium in Earth's early core[END_REF], to explain the isotopic variability observed within individual hotspot tracks.

The Juan Fernandez Islands do not show a double volcanic chain, unlike Hawaii and the other volcanic chains investigated by [START_REF] Huang | Geochemical zoning of volcanic chains associated with Pacific hotspots[END_REF]. Sampling resolution on Juan Fernandez lavas does not allow us to identify two geochemical trends in the isotopic data between Robinson Crusoe and Alexander Selkirk. However, the isotopic variability in the mantle plume observed between the main shield lavas of the two islands could be due to a variety of models including a compositionally heterogeneous mantle source, with heterogeneities being vertically stretched and rising at different rates inside the plume conduit (e.g., [START_REF] Farnetani | Dynamics and internal structure of the Hawaiian plume[END_REF], sampling of lower mantle (e.g. [START_REF] Allègre | Constraints on evolution of Earth's mantle from rare gas systematics[END_REF][START_REF] Tolstikhin | Early crust on top of the Earth's core[END_REF] and LLSVP material (e.g., [START_REF] Weis | Role of the deep mantle in generating the compositional asymmetry of the Hawaiian mantle plume[END_REF].

The origin of the isotopically depleted component in hotspots is debated: it has been proposed as (1) entrained upper mantle (e.g. Lassiter et al., 2000;[START_REF] Regelous | Geochemistry of lavas from the Emperor Seamounts, and the geochemical evolution of Hawaiian magmatism from 85 to 42 Ma[END_REF], (2) metasomatized lithospheric mantle (e.g. [START_REF] Pilet | Metasomatized lithosphere and the origin of alkaline lavas[END_REF][START_REF] Sorbadere | Experimental melting of hydrous peridotite-pyroxenite mixed sources: Constraints on the genesis of silica-undersaturated magmas beneath volcanic arcs[END_REF], (3) intrinsic to the plume (e.g. [START_REF] Yang | Constraints on the source components of lavas forming the Hawaiian North Arch and Honolulu Volcanics[END_REF]Mukhopadhyay et al., 2003;[START_REF] Frey | Origin of depleted components in basalt related to the Hawaiian hot spot: Evidence from isotopic and incompatible element ratios[END_REF][START_REF] Defelice | An isotopically depleted lower mantle component is intrinsic to the Hawaiian mantle plume[END_REF] or (4) subducted ancient lithospheric mantle [START_REF] Castillo | The recycling of marine carbonates and sources of HIMU and FOZO ocean island basalts[END_REF]Truong et al., 2018). The Samoan depleted component is thought to be entrained depleted mantle (e.g. Konter and Jackson, 2012), whereas all the first three sources have been proposed for the depleted component in the Galapagos (e.g. [START_REF] Blichert-Toft | Hf isotope geochemistry of the Galapagos Islands[END_REF][START_REF] Saal | The role of lithospheric gabbros on the composition of Galapagos lavas[END_REF] and Icelandic plumes (e.g. [START_REF] Fitton | Thermal and chemical structure of the Iceland plume[END_REF][START_REF] Hanan | Depleted Iceland mantle plume geochemical signature: Artifact of multicomponent mixing?[END_REF][START_REF] Chauvel | Melting of a complete section of recycled oceanic crust: Trace element and Pb isotopic evidence from Iceland[END_REF]. For the Hawaiian lavas, the depleted component has been proposed to be an isotopically depleted component intrinsic to the plume (e.g., [START_REF] Yang | Constraints on the source components of lavas forming the Hawaiian North Arch and Honolulu Volcanics[END_REF]Mukhopadhyay et al., 2003;[START_REF] Frey | Origin of depleted components in basalt related to the Hawaiian hot spot: Evidence from isotopic and incompatible element ratios[END_REF][START_REF] Defelice | An isotopically depleted lower mantle component is intrinsic to the Hawaiian mantle plume[END_REF], the lithospheric mantle (e.g. Lassiter et al., 2000) or even the Pacific DMM (e.g. [START_REF] Keller | Isotopic evidence for Late Cretaceous plumeridge interaction at the Hawaiian hotspot[END_REF].

The addition of a depleted MORB-like mantle component well explains the isotopic composition of Alexander Selkirk main shield lavas, with the isotopic signature of these lavas being consistent with sampling of FOZO and a more depleted component (Figure 11). However a higher contribution of this depleted component to the compositions of Alexander Selkirk lavas should produce depleted trace element patterns relative to Robinson Crusoe main shield lavas, which is not what we observed (Figure 3). Thus, even if Alexander Selkirk main shield lavas are slightly more isotopically depleted relative to those from Robinson Crusoe, they might still sample a heterogeneous FOZO domain, possibly distinct from the DMM, to produce such trace element compositions.

CONCLUSIONS

We present new major-, trace-and highly siderophile-element abundances (HSE: Re, Pd, Pt, Ru, Ir, Os) and Os isotope compositions for a suite of lavas and olivine separates from the Juan Fernandez Islands. We clarify the previous grouping scheme of Farley et al. (1993) for the basalts after showing that Robinson Crusoe basanites are rejuvenated lavas. The fractionation of an assemblage of 30 modal % olivine and 1-5 modal % spinel, plus a small contribution of the primary melt from olivine melt inclusions (1 to 2% for both main shield lavas, up to 5% for the rejuvenated lavas), recreates HSE patterns for olivine grains in samples and also reproduces the range of HSE compositions observed in Juan Fernandez lavas, consistent with modeling of REE abundances in the olivine separates. Overall, Juan Fernandez lavas sample the convergence zone of all OIB, at the "FOZO" or "C" component. Robinson Crusoe basanite lavas represent rejuvenated volcanism dominantly from a depleted lithospheric mantle source ( 187 Os/ 188 Os <0.13) polluted by high-3 He/ 4 He from the main shield building stage of the island. Robinson Crusoe main shield lavas sample a high-3 He/ 4 He (>18R A ) and enriched mantle source ( 187 Os/ 188 Os = 0.1312) akin to the FOZO or C component, whereas Alexander Selkrik lavas are from a dominant contribution from a depleted mantle component with low-3 He/ 4 He (<10R A ).

Apart from their He signature, these mantle sources of the shield lavas show limited variations for the other radiogenic isotopes, and no clear variations of the HSE or trace elements, likely due to limited differences in the source composition and degree of partial melting. We propose that Robinson Crusoe form during an initial eruption stage of lavas from a primitive high-3 He/ 4 He mantle source ~4 Ma, which also led to noble gas impregnation of the oceanic lithosphere. This first stage was followed by eruption of Alexander Selkirk lavas from a more depleted mantle source at ~ 2 Ma. Mantle heterogeneity preserved in Juan Fernandez lavas over short timescales (<2 Ma) and can impact lithospheric compositions, resulting in formation of rejuvenated lavas with unique isotopic compositions. Ma respectively (e.g. [START_REF] Reyes | Contrasting P-T paths of shield and rejuvenated volcanism at Robinson Crusoe Island, Juan Fernández Ridge, SE Pacific[END_REF]Lara et al., 2018a[START_REF] Lara | 40 Ar/ 39 Ar constraints on the age progression along the Juan Fernández Ridge, SE Pacific[END_REF]. The location of the present hotspot is inferred from the age progression defined by Ar ages [START_REF] Lara | 40 Ar/ 39 Ar constraints on the age progression along the Juan Fernández Ridge, SE Pacific[END_REF]. [START_REF] Garcia | Petrology of lavas from episodes 2-47 of the Puu O'o eruption of Kilauea Volcano, Hawaii: evaluation of magmatic processes[END_REF][START_REF] Garcia | Petrology of lavas from the Puu Oo eruption of Kilauea Volcano: III. The Kupaianaha episode (1986-1992)[END_REF][START_REF] Garcia | Magmatic processes during the prolonged Pu'u'O'o eruption of Kilauea Volcano, Hawaii[END_REF], Mukhopadhyay et al. (2003), [START_REF] Haskins | Scientific drilling reveals geochemical heterogeneity within the Ko'olau shield, Hawaii[END_REF], [START_REF] Frey | The geochemical components that distinguish Loa-and Kea-trend Hawaiian shield lavas[END_REF] and references therein, [START_REF] Sinton | Volcanic evolution of Moloka 'i, Hawai 'i: Implications for the shield to postshield transition in Hawaiian volcanoes[END_REF] and those for the Hawaiian rejuvenated lavas are from [START_REF] Yang | Constraints on the source components of lavas forming the Hawaiian North Arch and Honolulu Volcanics[END_REF], [START_REF] Garcia | Petrology, geochemistry and geochronology of Kaua 'i lavas over 4•5 Myr: Implications for the origin of rejuvenated volcanism and the evolution of the Hawaiian plume[END_REF], [START_REF] Phillips | Isotopic constraints on the genesis and evolution of basanitic lavas at Haleakala, Island of Maui, Hawaii[END_REF]. Diagram after [START_REF] Fitton | Thermal and chemical structure of the Iceland plume[END_REF].

FIGURES AND FIGURE CAPTIONS

Robinson Crusoe Alexander Selkirk Friday

Figure 5 -Plot of 187 Os/ 188 Os ratios versus Os abundances for Juan Fernandez lavas (diamonds and squares) and olivine separates (circles), compared with lavas from Hawaii (Jamais et al., 2008;Ireland et al., 2009), the Comores Islands [START_REF] Class | Osmium isotopes in Grande Comore lavas: a new extreme among a spectrum of EM-type mantle endmembers[END_REF], Réunion Island (Peters et al., 2016), the Canary Islands (Day et al., 2010), Iceland [START_REF] Brandon | 186 Os and 187 Os enrichments and high-3 He/ 4 He sources in the Earth's mantle: evidence from Icelandic picrites[END_REF] Selkirk main shield lavas and olivine; (d) comparison of lavas from Hawaii (Ireland et al., 2009), the Canary Islands (Day et al., 2010) and Réunion Island (Peters et al., 2016). Data are normalized relative to Primitive Mantle (PM) values from [START_REF] Day | 186 Os-187 Os and highly siderophile element abundance systematics of the mantle revealed by abyssal peridotites and Os-rich alloys[END_REF]. et al., 2009), Réunion Island (Peters et al., 2016) and the Canary Islands (Day et al., 2010). RC main shield = Robinson Crusoe main shield lavas; RC rejuvenated = Robinson Crusoe rejuvenated lavas; AS main shield = Alexander Selkirk main shield lavas. [START_REF] Hilton | Helium isotope ratios in mafic phenocrysts and geothermal fluids from La Palma, the Canary Islands (Spain): implications for HIMU mantle sources[END_REF]. The He isotope characteristics of P is taken as 18R A , as the highest ratio measured for Robinson Crusoe main shield lavas (Farley et al., 1993). The plume lead isotopic composition 206 Pb/ 204 Pb is taken as 19.25 in order to be slightly higher than the average isotopic composition for Robinson Crusoe main shield lavas.

Curves are labeled to represent the mixing with variably degassed plume sources: (A) undegassed mantle plume 4 He abundance is taken as 56 µcm 3 STP/g, which is the 4 He content in PIN-8 and the value for the undegassed mantle used by [START_REF] Hilton | Helium isotope ratios in mafic phenocrysts and geothermal fluids from La Palma, the Canary Islands (Spain): implications for HIMU mantle sources[END_REF] Fe-Mn oxide assimilation process is expected to change the MnO content of the melt. There is no indication of Mn enrichment for sample MF-S1 (MnO = 0.17 wt.%; Farley et al., 1993). Instead, this Pt enrichment may be due to contamination during sample preparation and prior to analysis.

c. Magmatic degassing of rhenium

During mantle partial melting and fractional crystallization, La and Yb are incompatible, while Re behaves similarly to Yb [START_REF] Hauri | Rhenium abundances and systematics in oceanic basalts[END_REF]Bennett et al., 2000;[START_REF] Norman | Rhenium and chalcophile elements in basaltic glasses from Ko'olau and Moloka'i volcanoes: Magmatic outgassing and composition of the Hawaiian plume[END_REF]. Additionally, Re, Os and Cu abundances are controlled by residual sulfide and are expected to behave coherently during melting (Bennett et al., 2000). There is a relationship of decreasing Re/Os, Re/Yb and Re/Cu with decreasing Re abundance for the Juan Fernandez lavas (Figure S7). Such a relationship is not observed for Cu/Yb, La/Yb or Os/Yb with decreasing Re abundances. These characteristics suggest Re loss during magmatic degassing (c.f. Day et al., 2010). Nevertheless, it is unlikely that the other HSE experience such significant volatile behavior, as the ratios of these elements versus Yb remain constant or even increase slightly with decreasing Re content (Figure S7), and they correlate positively with indices of magmatic differentiation, such as MgO (Figures 7 andS4).

d. Ingrowth of radiogenic osmium

The samples that we analyzed from the Juan Fernandez Islands are all >1 Ma, offering potential for ingrowth of radiogenic Os from 187 Re decay. The samples have low 187 Re/ 188 Os (<10), however, precluding significant ingrowth (Figure S8) and these are corrected for in Table (Ireland et al., 2009a), Réunion Island (Peters 1175(Peters et al., 2016)), and the Canary Islands (Day et al., 2010). RC main shield = Robinson Crusoe main 1176 S6), assuming a primitive mantle source (garnet peridotite with modal proportions of olivine, orthopyroxene, clinopyroxene and garnet of 0.54/0.17/0.09/0.20; spinel peridotite with modal proportions of olivine, orthopyroxene, clinopyroxene and spinel of 0.54/0.17/0.09/0.20).

(Dy/Yb) (diamonds and squares) and olivine separates (circles), and compared with main shield lavas (Hawaii: Mukhopadhyay et al., 2003;Bryce et al., 2005;Jamais et al., 2008;Samoa Jackson et

Figure 1

 1 Figure 1 -Juan Fernandez Ridge (figure modified from Lara et al., 2018b; map downloaded

Figure 2 -

 2 Figure 2 -Spinel inclusions in Juan Fernandez olivine grains. (a) PF-5 (Robinson Crusoe main

Figure 3 -

 3 Figure 3 -Primitive mantle normalized multi-element variations diagrams for (a) Robinson

Figure 4 -

 4 Figure 4 -Niobium/Y versus Zr/Y for Juan Fernandez lavas (diamonds and squares) and

Figure 6 -

 6 Figure 6 -Highly siderophile elements patterns for Juan Fernandez lavas (diamonds or squares

Figure 8

 8 Figure 8 -(a) Modeled compositions of residual melts after fractional crystallization of 1%, 5%,

Figure 12 -

 12 Figure 12 -Osmium isotope compositions compared with (a) 3 He/ 4 He (R/R a ), (b) 143 Nd/ 144 Nd, (c) 87 Sr/ 86 Sr, (d) 206 Pb/ 204 Pb, (e) 207 Pb/ 204 Pb and (f) 208 Pb/ 204 Pb for Juan Fernandez lavas

  0

Figure

  Figure S2 -(a) W and (b) Mo abundances versus MgO content for Juan Fernandez lavas

Figure S3 -

 S3 Figure S3 -(a) Osmium abundances versus 187 Os/ 188 Os in Juan Fernandez olivine separates,

Figure S4 -

 S4 Figure S4 -Juan Fernandez lavas HSE-MgO trends (diamonds and squares; olivine separates 1174

  RC rejuvenated = Robinson Crusoe rejuvenated lavas; AS main shield = Alexander Selkirk main shield lavas.

Figure S5 -

 S5 Figure S5 -(La/Yb) n versus (Dy/Yb) n for Juan Fernandez lavas. Ratios are normalized to CI-

  Figure S6 -Osmium isotope compositions compared with (a) 3 He/ 4 He (R/R a ), (b) 143 Nd/ 144 Nd, (c) 87 Sr/ 86 Sr, (d) 206 Pb/ 204 Pb, (e) 207 Pb/ 204 Pb and (f) 208 Pb/ 204 Pb for Juan Fernandez lavas

  0

  Pb/ 204 Pb and 208 Pb/ 204 Pb, and intermediate in 87 Sr/ 86 Sr and 143 Nd/ 144 Nd compared to the shield lavas (Figure

  and Samoa(Jackson

	0.15					RC main shield	Hawaii	Canary
						RC rejuvenated	Samoa	Reunion
					O	AS main shield B a s a l t l i v i n e	Iceland	Comores
	0.14
	Os
	Os/ 188
	187
	0.13
								Primitive Mantle
	0.12	0	0.2	0.4	Os (ppb)	0.6	0.8	1

  Composition of olivine after 30% of fractional crystallization from the parent melt of Juan Fernandez lavas, mixed with various amounts of spinel (1% and 5%; black lines) to reproduce the contribution of spinel inclusions to the olivine REE composition. The green lines correspond to the composition of the previous assemblage olivine + 5% spinel, to which we added the contribution of 0.1% to 5% of the composition of the starting material: (a) main shield lava composition, (b) rejuvenated lava composition. Juan Fernandez olivine separates are shown for comparison. As the starting melt composition, we used PF-10 and PF-16, which are the closest to the parent melt composition in terms of MgO content, for Robinson Crusoe main shield and rejuvenated lavas respectively. Partition coefficients for ol/melt are from from[START_REF] Fujimaki | Partition coefficients of Hf, Zr, and REE between phenocrysts and groundmasses[END_REF], and those for sp/melt from[START_REF] Nagasawa | Experimental mineral/liquid partition coefficients of the rare earth elements (REE), Sc and Sr for perovskite, spinel and melilite[END_REF]. Data are normalized relative to Primitive Mantle (PM) values fromMcDonough & Sun (1995).[START_REF] Jackson | Peridotite xenoliths from the Polynesian Austral and Samoa hotspots: Implications for the destruction of ancient 187 Os and 142 Nd isotopic domains and the preservation of Hadean 129 Xe in the modern convecting mantle[END_REF]. RC main shield = Robinson Crusoe main shield lavas and olivine; RC rejuvenated = Robinson Crusoe rejuvenated lavas and olivine; AS main shield = Alexander Selkirk main shield lavas and olivine. The grey field represents the primitive mantle range for 187 Os/ 188 Os from[START_REF] Meisel | Osmium isotopic compositions of mantle xenoliths: a global perspective[END_REF].

	Sample/Primitive Mantle 0.001 10 1 0.1 0.01 Figure 9 -Sample/Primitive Mantle Re RC main shield Pd RC rejuvenated AS main shield a 0.14 10 1 0.1 0.01 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Pt Ru Ir 1% Os 30% +1% sp +5% sp 0.1% 1% 2% 0.5% AS main shield RC rejuvenated RC main shield a 187 0.16 Os/ 188 Os 0.20 0.18 RC main shield Sample/Primitive Mantle 10 1 0.1 0.01 0.001 Re b 0.1 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Pd Pt Ru Ir Os +1 % sp +5 % sp 0.1% 2% 0.01 +1% sp +5% sp 0.1% Sample/Primitive Mantle 10 1 1% 5% 0.5% Hawaii Samoa Main shield Main shield RC rejuvenated Rejuvenated Pyroxenites Peridotites Xenoliths AS main shield O l i v i n e B a s a l t
								Primitive Mantle
	0.12
	0	0.25	0.5	0.75	1 Os (ppb)	1.25	1.5	1.75	2

b Figure 10 -Plot of

187

Os/ 188 Os ratios versus Os abundances for Juan Fernandez lavas (diamonds and squares) and olivine separates (circles), compared with main shield lavas (Hawaii:

Jamais et al., 2008; Ireland et al., 2009; Samoa: Jackson & Shirey, 2011)

, rejuvenated lavas (Hawaii:

Lassiter et al., 2000; Samoa: Konter and Jackson, 2012)

, xenoliths (Hawaii: peridotites from

Lassiter et al., 2000: Bizimis et al., 2007; pyroxenites from Lassiter et al., 2000; Sen et al., 2011;

Samoa: Figure 11 -Plot of He versus Pb isotope variation for Juan Fernandez samples illustrating binary mixing trajectories between depleted component (D) and a high-3 He hotspot endmember (P). Isotopic values for D are from

  FigureS8 -187 Os/ 188 Os ratios versus 187 Re/ 188 Os ratios for Juan Fernandez lavas and olivine grains with Os abundances > 50 ppt. There is no clear correlation with 187 Re/ 188 Os and 187 Os/ 188 Os in the Juan Fernandez Islands dataset. The 187 Os/ 188 Os ratio for the primitive mantle is from[START_REF] Meisel | Osmium isotopic compositions of mantle xenoliths: a global perspective[END_REF].

		RC main shield RC rejuvenated AS main shield B a s a lt O li v in e	Rejuvenated Main shield Pyroxenites Peridotites Hawaii	Samoa Main shield Rejuvenated Xenoliths	0.18 0.17 0.16 Os	d
								0.15 Os/ 188
								187	0.14
									0.13
		b							0.12 18 0.18	e	18.5	19 206 Pb/ 204 Pb 19.5	20	20.5
									0.17
	.16 0.15 0.14 Os/ 188 187 Os							0.14 0.15 Os/ 188 187 0.16 Os
	0.13								0.13
	0.12 0.5126 0.18 c	0.5128 143 Nd/ 144 Nd 0.5130	0.5132	0.12 15.4 15.45 15.5 15.55 15.6 15.65 15.7 207 Pb/ 204 Pb 0.18 f
	0.17								0.17
	0.16 0.15 0.14 Os/ 188 187 Os							0.14 0.15 Os/ 188 187 0.16 Os
	0.13								0.13
	0.12	0.703	0.704	0.705 87 Sr/ 86 Sr	0.706	0.707	0.12 37.5	38	38.5 208 Pb/ 204 Pb 39 39.5	40	40.5

Pb/ 204 Pb ratios (19.163 to 19.292) than Alexander Selkirk basalts (18.939 to 19.221)(Truong et al., 2018). Overall, Sr-Nd-Pb isotopic compositions suggest that all the samples are from a

Pb/ 204 Pb, 208 Pb/ 204 Pb, 187 Os/ 188 Os and 3 He/ 4 He compared to Alexander Selkirk main shield

& Shirey, 2011). RC main shield = Robinson Crusoe main shield lavas and olivine; RC rejuvenated = Robinson Crusoe rejuvenated lavas and olivine; AS main shield = Alexander Selkirk main shield lavas and olivine. The grey field represents the primitive mantle range for 187 Os/ 188 Os from[START_REF] Meisel | Osmium isotopic compositions of mantle xenoliths: a global perspective[END_REF].

(Mukhopadhyay et al., 2003; Bryce et al., 2005; Jamais et al., 2008), the Comores Islands[START_REF] Class | The process of plume-lithosphere interactions in the ocean basins-the case of Grande Comore[END_REF][START_REF] Class | Grand Comore Island: a well-constrained "low 3 He/ 4 He" mantle plume[END_REF][START_REF] Class | Osmium isotopes in Grande Comore lavas: a new extreme among a spectrum of EM-type mantle endmembers[END_REF], the Canary Islands(Day et al., 2010[START_REF] Day | Origin of 3 He/ 4 He ratios in HIMU-type basalts constrained from Canary Island lavas[END_REF], Iceland[START_REF] Brandon | 186 Os and 187 Os enrichments and high-3 He/ 4 He sources in the Earth's mantle: evidence from Icelandic picrites[END_REF]) and Samoa(Jackson et al., 2007, Jackson & Shirey, 2011; Konter and Jackson, 2012). RC main shield = Robinson Crusoe main shield lavas and olivine; RC rejuvenated = Robinson Crusoe rejuvenated lavas and olivine; AS main shield = Alexander Selkirk main shield lavas and olivine.

ACKNOWLEDGEMENTS

We thank Jasper Konter, two anonymous reviewers, and the Associate Editor, Shichun Huang, for their helpful comments. This work was supported by the National Science Foundation (EAR-1116089; EAR-1447130). We are grateful to Emily Chin and Sarah Perry for analytical assistance with the secondary electron microscope.

SUPPLEMENTARY INFORMATION

Supplementary Discussion

a. Data comparison

A comparison of the trace element abundances for Juan Fernandez lavas obtained on unleached powders in this study using a Thermo Fisher Scientific iCAP Qc ICP-MS and those reported by Truong et al. (2018) using a ThermoQuest Element 2 ICP-MS is presented in Figure S1. Overall, our analyses for Juan Fernandez lavas are in good agreement with the measurements published by Truong et al. (2018) and Farley et al. (1993). We report slightly lower abundances of Zr than Truong et al. (2018). This could be due to unaccounted-for interferences of Ar and Ti or Cr on 90 Zr in the study of Troung et al. (2018). In this study, we report new data for the following elements: Sc, V, Cr, Co Ni, Cu, Zn, Rb, Ba, Nb, Sr, Zr, Y, La, Ce Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er Tm, Yb, Lu, Hf, Ta, Pb, Th, U, Ga, Ge, Se, Mo, Sn, Te and W. In particular, Sc, Gd, Tm, Ge, Se, Mo, Sn, Te and W are measured for the first time in Juan Fernandez lavas and olivine separates.

b. Fe-Mn oxide contamination

Iron-Mn oxide coatings have been suggested as potential contaminants during OIB petrogenesis (e.g., Ireland et al., 2009a). They are characterized by radiogenic 187 Os/ 188 Os compositions and high Os abundances. These materials tend to incorporate the HSE from seawater, in particular Pt (e.g., Ireland et al., 2009a and references therein). Sample MF-S1 contains a high concentration of Pt, which could be due to pervasive alteration by Mn-rich material as described by Ireland et al. (2009a) , 2007, Jackson & Shirey, 2011;Konter and Jackson, 2012), rejuvenated lavas (Hawaii: Lassiter et al., 2000;Samoa: Konter and Jackson, 2012), xenoliths (Hawaii: peridotites from Lassiter et al., 2000: Bizimis et al., 2007;pyroxenites from Lassiter et al., 2000;Sen et al., 2011;Samoa: Jackson et al., 2016). RC main shield = Robinson Crusoe main shield lavas and olivine; RC rejuvenated = Robinson Crusoe rejuvenated lavas and olivine; AS main shield = Alexander Selkirk main shield lavas and olivine.