Frederic Moynier
email: moynier@ipgp.fr

Matthew G Jackson

Ke Zhang

Saemundur Hongming Cai

Ari Halldórsson

Raphael Pik

D Day

Jiubin Chen
email: jbchen@tju.edu.cn

Frédéric Moynier

Saemundur Ari Halldórsson

James M D Day

Frederic Moynier

The Mercury Isotopic Composition of Earth's Mantle and the Use of Mass Independently Fractionated Hg to Test for Recycled Crust

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

Introduction

The composition of Earth's mantle is in part known through the chemical and isotopic analyses of lavas from different tectonic settings such as mid-ocean ridge basalts (MORB) and ocean island basalts (OIB) [e.g. [START_REF] Hofmann | Sampling Mantle Heterogeneity through Oceanic Basalts: Isotopes and Trace Elements[END_REF]. The variable isotopic compositions of OIB are usually interpreted to reflect mantle heterogeneity formed by recycling of surface material back into the mantle through subduction, the contribution of Earth's core into the deep source of certain lavas, or the survival of early-formed heterogeneities in the mantle [e.g. [START_REF] Hauri | Re-Os isotope systematics of EMII and HIMU oceanic basalts from the south Pacific Ocean[END_REF][START_REF] Hofmann | Mantle geochemistry: the message from oceanic volcanism[END_REF][START_REF] Hofmann | Mantle plumes from ancient oceanic crust[END_REF][START_REF] Mukhopadhyay | Noble Gases: A Record of Earth's Evolution and Mantle Dynamics[END_REF][START_REF] Mundl-Petermeier | Anomalous 182W in high 3 He/ 4 He ocean island basalts: fingerprints of Earth's core?[END_REF][START_REF] Rizo | 182 W evidence for core-mantle interaction in the source of mantle plumes[END_REF]. Identifying the geological origin of such mantle sources is paramount to understand mantle dynamics through time.

Mantle heterogeneities have been defined and traditionally traced by using radiogenic isotopes such as Sr, Nd, Hf and Pb [e.g. Blichert-Toft et al., 1999;[START_REF] Blichert-Toft | Hawaiian hot spot dynamics as inferred from the Hf and Pb isotope evolution of Mauna Kea volcano[END_REF][START_REF] Chauvel | HIMU-EM: The French Polynesian connection[END_REF][START_REF] Hofmann | Chemical differentiation of the Earth: the relationship between mantle continental crust, and oceanic crust[END_REF][START_REF] Hofmann | Mantle geochemistry: the message from oceanic volcanism[END_REF][START_REF] White | Sr and Nd isotope geochemistry of oceanic basalts and mantle evolution[END_REF][START_REF] Zindler | Chemical Geodynamics[END_REF], and several end-members have been defined : EM-1 (Enriched-Mantle 1, intermediate 87 Sr/ 86 Sr and low 206 Pb/ 204 Pb), EM-2 (Enriched-Mantle 2, high 87 Sr/ 86 Sr and intermediate 206 Pb/ 204 Pb), and HIMU (defined by low 87 Sr/ 86 Sr and high 206 Pb/ 204 Pb). There is a general consensus emerging for the origin of EM-2 from continental crust materials (e.g. Jackson et al., 2007;[START_REF] White | Sr and Nd isotope geochemistry of oceanic basalts and mantle evolution[END_REF] and HIMU as oceanic crust and lithosphere or marine carbonates (e.g. [START_REF] Cabral | Anomalous sulphur isotopes in plume lavas reveal deep mantle storage of Archaean crust[END_REF][START_REF] Chauvel | HIMU-EM: The French Polynesian connection[END_REF][START_REF] Hofmann | Mantle plumes from ancient oceanic crust[END_REF], but the origin of EM-1 is debated (e.g. continental crust, pelagic sediments, delaminated sub-continental lithosphere or a common origin with EM-2 as terrestrial sediments; [START_REF] Castillo | An alternative explanation for the Hf-Nd mantle array[END_REF][START_REF] Delavault | Sulfur and lead isotopic evidence of relic Archean sediments in the Pitcairn mantle plume[END_REF][START_REF] Eisele | The role of sediment recycling in EM-1 inferred from Os, Pb, Hf, Nd, Sr isotope and trace element systematics of the Pitcairn hotspot[END_REF]Garapic et al., 2015) and understanding its origin represents an important challenge. Stable isotope systems can give complementary information to radiogenic systems, but the potential sources of isotopic fractionation are diverse (e.g. magmatic differentiation, surficial processes), and their interpretation can be somewhat ambiguous.

A few stable isotope systems, the most notable being S, O and Hg, exhibit mass-independent isotopic fractionations (MIF) in naturally-occurring mantle-derived samples that are not strongly modified by high temperature processes [e.g. [START_REF] Cabral | Anomalous sulphur isotopes in plume lavas reveal deep mantle storage of Archaean crust[END_REF][START_REF] Delavault | Sulfur and lead isotopic evidence of relic Archean sediments in the Pitcairn mantle plume[END_REF][START_REF] Moynier | Chondritic mercury isotopic composition of Earth and evidence for evaporative equilibrium degassing during the formation of eucrites[END_REF]. For example, the presence of MIF sulfur in Mangaia (Cook Islands, Polynesia) and Pitcairn lavas demonstrated the presence of an Archean atmosphere-derived sulfur component in their mantle source [START_REF] Cabral | Anomalous sulphur isotopes in plume lavas reveal deep mantle storage of Archaean crust[END_REF][START_REF] Delavault | Sulfur and lead isotopic evidence of relic Archean sediments in the Pitcairn mantle plume[END_REF]. Mercury is the most volatile among the moderately volatile elements [Lodders, 2013] and exhibits large mass-dependent isotopic fractionations (MDF) and MIF. The MIF-Hg signatures of odd isotopes (odd-MIF) are due to magnetic isotope effects [e.g. [START_REF] Bergquist | Mass-dependent and-independent fractionation of Hg isotopes by photoreduction in aquatic systems[END_REF], or a nuclear field shift effect [START_REF] Estrade | Mercury isotope fractionation during liquid-vapor evaporation experiments[END_REF] in surficial environments [e.g. [START_REF] Bergquist | Mass-dependent and-independent fractionation of Hg isotopes by photoreduction in aquatic systems[END_REF][START_REF] Blum | Evidence for methylmercury production below the mixed layer in the central North Pacific Ocean[END_REF][START_REF] Chen | Unusual fractionation of both odd and even mercury isotopes in precipitation from Peterborough, ON, Canada[END_REF][START_REF] Estrade | Isotope tracing of atmospheric mercury sources in an urban area of northeastern France[END_REF][START_REF] Foucher | High-precision measurement of mercury isotope ratios in sediments using cold-vapor generation multi-collector inductively coupled plasma mass spectrometry[END_REF][START_REF] Sherman | Mass-independent fractionation of mercury isotopes in Arctic snow driven by sunlight[END_REF][START_REF] Sonke | Sedimentary mercury stable isotope records of atmospheric and riverine pollution from two major European heavy metal refineries[END_REF], producing > ‰ effects. High temperature processes such as magmatic degassing produce limited (<10ppm) Hg-MIF [START_REF] Moynier | Chondritic mercury isotopic composition of Earth and evidence for evaporative equilibrium degassing during the formation of eucrites[END_REF].

Mercury isotopes are mass-independently fractionated in surface samples and exhibit specific signatures related to different environments, showing potential application to study the nature of recycled materials in the mantle sources of lavas. This is due to the distinct MIF signatures of terrestrial surface environments whereby sub-aerially formed terrestrial reservoirs having generally negative odd-isotopes MIF [e.g. [START_REF] Blum | Mercury Isotopes in Earth and Environmental Sciences[END_REF] while the oceanic environment, including sediments and seawater, shows positive odd-isotope MIF [START_REF] Meng | Mercury isotope variations within the marine food web of Chinese Bohai Sea: Implications for mercury sources and biogeochemical cycling[END_REF][START_REF] Meng | An integrated model for input and migration of mercury in Chinese coastal sediments[END_REF][START_REF] Yin | Identifying the source and processes of mercury in subtropical estuarine and ocean sediments using Hg isotopic composition[END_REF]. In particular, it may be possible to test whether EM1 lavas sample a component of deeply recycled terrestrial sediments [START_REF] Castillo | An alternative explanation for the Hf-Nd mantle array[END_REF][START_REF] Eisele | The role of sediment recycling in EM-1 inferred from Os, Pb, Hf, Nd, Sr isotope and trace element systematics of the Pitcairn hotspot[END_REF]Garapic et al., 2015].

A recent landmark study showed that Hg from gold deposits associated with arc magmatism had positive odd-isotope MIF associated with recycling of marine Hg from the subduction zone into the mantle source of arc lavas [START_REF] Deng | Recycling of mercury from the atmosphere-ocean system into volcanic arc associated epithermal gold systems[END_REF]. This highlights the utility for Hg isotopes to serve as a tracer of deep recycling processes, from subduction zones to hotspots. Here, we test the origin of the material present in various end-member signatures (EM1, EM2, and HIMU) of OIB using Hg isotopes.

Due to the high volatility of mercury, constraints on its isotopic composition within Earth's mantle are critical for evaluating the origin of terrestrial volatile elements by means of comparison with the isotopic composition of primitive meteorites [START_REF] Meier | Mercury (Hg) in meteorites: Variations in abundance, thermal release profile, mass-dependent and mass-independent isotopic fractionation[END_REF][START_REF] Moynier | Chondritic mercury isotopic composition of Earth and evidence for evaporative equilibrium degassing during the formation of eucrites[END_REF]. However, the Hg isotopic composition of the mantle has been estimated from only four basaltic rocks [START_REF] Geng | An optimized protocol for high precision measurement of Hg isotopic compositions in samples with low concentrations of Hg using MC-ICP-MS[END_REF][START_REF] Moynier | Chondritic mercury isotopic composition of Earth and evidence for evaporative equilibrium degassing during the formation of eucrites[END_REF], and these samples may not be ideally suited for evaluating the composition of the primitive mantle. A primary hurdle for Hg isotopic characterization of mantle-derived lavas is their low Hg contents (a few ppb). We have recently developed a method that yields high precision data on less than 5ng of Hg, which enables isotope characterization with just a few grams of material [START_REF] Moynier | Chondritic mercury isotopic composition of Earth and evidence for evaporative equilibrium degassing during the formation of eucrites[END_REF]. This allows targeting of a key set of mantle-derived lavas to evaluate the Hg isotopic composition of the mantle least impacted by crustal recycling, and place key constraints on the origin of the volatile element Hg on Earth.

To evaluate the Hg isotopic composition of the mantle least affected by crustal recycling, we selected lavas with least degassed signatures reflected by their high-3 He/ 4 He (>13 atmospheric ratio, RA), including four samples from Samoa [Jackson et al., 2007], and seven from Iceland [Füri et al. 2010 andHalldórsson et al. 2016b]. To further characterize the composition of the mantle we analysed three additional Icelandic samples [Óskarsson et al. 1982] and six basalts from the Afar (Ethiopia) [START_REF] Deng | Titanium isotopes as a tracer for the plume or island arc affinity of felsic rocks[END_REF][START_REF] Pik | How many mantle plumes in Africa? The geochemical point of view[END_REF]. To search for deep recycling of surface-derived Hg, samples from the three canonical mantle endmembers thought to reflect deep recycling of crustal materials: EM1 (Pitcairn, N=3), HIMU (Tubuai, N=1), EM2 (Samoa, N=1) were selected.

Materials and Methods

Sample descriptions

We report isotopic data on international rock standards to assess data quality and intra-laboratory data comparisons: USGS samples: BCR-2 (basalt, USA), RGM-2 (Rhyolite, USA) and STM-2 (syenite, USA) and three Icelandic samples used as internal standards at the University of Iceland (I-ICE, A-ALK, B-ALK, Óskarsson et al. 1982), all of which are available upon request.

The Hg isotopic composition of BCR-2 has been reported previously [START_REF] Geng | An optimized protocol for high precision measurement of Hg isotopic compositions in samples with low concentrations of Hg using MC-ICP-MS[END_REF]. One trachyandesite from the Samoa hotspot (ALIA-115-21) [START_REF] Adams | Extreme isotopic heterogeneity in Samoan clinopyroxenes constrains sediment recycling[END_REF], three basalt samples from Pitcairn [Garapic et al., 2015], and one basalt sample from Tubuai [START_REF] Hauri | Re-Os isotope systematics of EMII and HIMU oceanic basalts from the south Pacific Ocean[END_REF] were selected to represent the EM2, EM1 and HIMU mantle end-members, respectively.

Four samples from Ofu island (Samoa [Jackson et al., 2007]) and seven samples from several different regions (Eastern Rift Zone, Reykjanes Peninsula, Western Rift Zone, and Northern Rift Zone) of Iceland [Halldórsson et al., 2016ab;Jackson et al., 2020;Macpherson et al., 2005;[START_REF] Rasmussen | Olivine chemistry reveals compositional source heterogeneities within a tilted mantle plume beneath Iceland[END_REF] were taken to represent the high-3 He/ 4 He mantle (>13RA). Six basalt samples from the Stratoid Series in the Afar hotspot [START_REF] Deng | Titanium isotopes as a tracer for the plume or island arc affinity of felsic rocks[END_REF][START_REF] Pik | How many mantle plumes in Africa? The geochemical point of view[END_REF] were manuscript submitted to Geophysical Research Letters selected to further estimate the mantle composition. While the 3 He/ 4 He of the Afar samples have not been characterized, the composition of samples from this region is ~10-13 RA [START_REF] Marty | Helium isotopic variations in Ethiopian plume lavas: nature of magmatic sources and limit on lower mantle contribution[END_REF][START_REF] Medynsky | Controlsonmagmaticcyclesanddevelopmentofrifttopographyofthe Manda Hararosegment(Afar,Ethiopia):Insightsfromcosmogenic 3He investigationoflandscapeevolution[END_REF]. The UM-Almedén cinnabar standard was also analysed for interlaboratory comparison.

Methods

The method follows the same protocol as in [START_REF] Moynier | Chondritic mercury isotopic composition of Earth and evidence for evaporative equilibrium degassing during the formation of eucrites[END_REF]. The Hg isotopic analysis was carried out using a MC-ICP-MS (Nu-Plasma-3D, Nu-Instruments) [START_REF] Chen | Chromatographic pre-concentration of Hg from dilute aqueous solutions for isotopic measurement by MC-ICP-MS[END_REF][START_REF] Huang | An improved dual-stage protocol to pre-concentrate mercury from airborne particles for precise isotopic measurement[END_REF][START_REF] Yuan | Sequential samples reveal significant variation of mercury isotope ratios during single rainfall events[END_REF][START_REF] Zhang | Mercury isotope compositions in large anthropogenically impacted Pearl River, South China[END_REF]. The instrumental mass bias was corrected by an internal NIST-SRM-997-Tl standard using the standard-sample bracketing method. All samples were analyzed in three blocks (33 cycles). A 10-minute washout time in between samples ensured blank levels <0.2% of the preceding sample signals. Signal intensities were 2.5V on 202 Hg (1 ppb solution).

The data for Hg-MDF is reported as d x Hg :

d x Hg (‰)=[( x Hg/ 198 Hg)sample /( x Hg/ 198 Hg)NIST SRM 3133 -1]×1000 (Eq. 1)
where x refers to 199, 200, 201, or 202. d 202 Hg is used for the discussion of MDF. The MIF data are defined as the deviation of the d x Hg from the theoretical value based on kinetic isotopic fractionation [e.g. [START_REF] Blum | Recent developments in mercury stable isotopes analysis[END_REF]:

Δ x Hg(‰)=δ x Hg(‰)-β x ×δ 202 Hg (Eq. 2)
where the mass dependent scaling factor β x is 0.2520, 0.5024 and 0.7520 for 199 Hg, 200 Hg and 201 Hg, respectively.

Errors are reported as twice the standard deviation from the replicates of the same solution, and are typically 0.08‰, 0.03‰, 0.03‰, 0.05‰ for δ 202 Hg, Δ 199 Hg, Δ 200 Hg and Δ 201 Hg, respectively. Long-term measurements of UM-Almadén Hg and CRM-GBW07405 standards give identical values for Hg isotopic composition (both MDF and MIF) to literature [START_REF] Chen | Chromatographic pre-concentration of Hg from dilute aqueous solutions for isotopic measurement by MC-ICP-MS[END_REF][START_REF] Moynier | Chondritic mercury isotopic composition of Earth and evidence for evaporative equilibrium degassing during the formation of eucrites[END_REF][START_REF] Huang | Diel variation in mercury stable isotope ratios records photo-reduction of PM 2.5bound mercury[END_REF], attesting the quality of Hg isotope analysis.

Results

Isotopic data for samples are reported in Table S1 and the D 199 Hg values are reported against d 202 Hg values in Figure 1. The isotopic composition of BCR-2 is identical within error to literature values [START_REF] Geng | An optimized protocol for high precision measurement of Hg isotopic compositions in samples with low concentrations of Hg using MC-ICP-MS[END_REF].

The terrestrial igneous rock samples show variable d 202 Hg values, from -3.23 to -0.01, consistent with literature data [START_REF] Geng | An optimized protocol for high precision measurement of Hg isotopic compositions in samples with low concentrations of Hg using MC-ICP-MS[END_REF][START_REF] Moynier | Chondritic mercury isotopic composition of Earth and evidence for evaporative equilibrium degassing during the formation of eucrites[END_REF] (Figure 1). High 3 He/ 4 He localities -Ofu island and Iceland-do not exhibit any MIF, and the Afar samples also lack MIF-Hg signatures. By contrast, the HIMU sample from Tubuai has a positive odd-isotope-MIF (Δ 199 Hg=+0.23±0.07). The extreme EM2 Samoa ALIA-115-21 sample has a distinct negative oddisotope-MIF signature (Δ 199 Hg=-0.14±0.02). In the Pitcairn lava suite, there is a positive correlation between 87 Sr/ 86 Sr and MIF-Hg: two Pitcairn lavas with the less extreme EM1 signatures have clear negative odd-isotope-MIF-Hg (Δ 199 Hg down to -0.45±0.01), while the Pitcairn sample with the most extreme EM1 signature has no resolvable MIF (Figure 2). No samples exhibit evenisotope-MIF signatures.

4.Discussion

The isotopic composition of the primitive mantle.

Before discussing the potential effects of recycling of surficial material on the Hg isotopic composition of mantle sources, it is critical to establish the composition of the primitive mantle.

OIB with the highest 3 He/ 4 He are inferred to sample mantle sources that are least degassed [START_REF] Craig | Primordial neon, helium, and hydrogen in oceanic basalts[END_REF][START_REF] Mukhopadhyay | Noble Gases: A Record of Earth's Evolution and Mantle Dynamics[END_REF] and the least modified by crustal recycling [Jackson et al., 2020;[START_REF] White | Isotopes, DUPAL, LLSVPs, and Anekantavada[END_REF]. Making the assumption that these samples represent materials with Hg isotopic compositions consistent with the most "pristine" mantle domain, we can then establish the impact of crustal subduction and recycling on the Hg systematics of mantle-derived lavas.

The Ofu island samples have high-3 He/ 4 He (22RA to 34RA, see Table S1) [Jackson et al., 2007] and are inferred to sample an ancient mantle domain [START_REF] Mundl-Petermeier | Anomalous 182W in high 3 He/ 4 He ocean island basalts: fingerprints of Earth's core?[END_REF] less impacted by crustal recycling than other Samoan lavas [Jackson et al., 2020]. Given that the high- Nevertheless, the δ 202 Hg of the 11 high-3 He/ 4 He samples from Ofu island and Iceland show limited variation, with an average -1.7±1.2 (2SD, N=11), which compare well with a previous estimate of the mantle composition based on only a few unrelated igneous rocks (δ 202 Hg <-2.35, [START_REF] Moynier | Chondritic mercury isotopic composition of Earth and evidence for evaporative equilibrium degassing during the formation of eucrites[END_REF], see Figure S1). Furthermore, samples from the Afar ( 3 He/ 4 He ~10-13 Ra, [START_REF] Marty | Helium isotopic variations in Ethiopian plume lavas: nature of magmatic sources and limit on lower mantle contribution[END_REF][START_REF] Medynsky | Controlsonmagmaticcyclesanddevelopmentofrifttopographyofthe Manda Hararosegment(Afar,Ethiopia):Insightsfromcosmogenic 3He investigationoflandscapeevolution[END_REF]) (N=6) have similar Hg isotopic composition within error, suggesting a large-scale homogeneity of Earth's modern mantle.

Our estimate of the primitive mantle Hg isotopic composition falls within the range defined by chondrites (Figure 3). While the large uncertainties on the Hg concentration of the Earth's mantle prevent an accurate estimate the amount of terrestrial Hg that is stored into the Earth's core, the present chondritic isotopic composition of the Earth's primitive mantle is an argument that late accretion of chondritic material delivered the present mantle Hg (and other volatile elements) (see discussion in [START_REF] Moynier | Chondritic mercury isotopic composition of Earth and evidence for evaporative equilibrium degassing during the formation of eucrites[END_REF]). This conclusion is consistent with observations made from C, N and noble gas isotopes [START_REF] Marty | The origins and concentrations of water, carbon, nitrogen and noble gases on Earth[END_REF] and the volatile and chalcophile elements S, Se and Te [START_REF] Wang | Ratios of S, Se and Te in the silicate Earth require a volatilerich late veneer[END_REF][START_REF] Varas-Reus | Selenium isotopes as tracers of a late volatile contribution to Earth from the outer Solar System[END_REF].

Evidence for Hg recycling?

Considering the Hg isotopic composition of high-3 He/ 4 He lavas defined above as representative of the mantle domains the least modified by crustal recycling, we can compare our estimate for the Hg isotopic composition for low-3 He/ 4 He mantle endmembers that are suggested to host recycled surface materials-EM1 Pitcairn (EM1), Samoa (EM2), and Tubuai (HIMU)-and then identify Hg isotope signatures associated with recycling in the mantle sources of these lavas.

Three OIB samples examined in this study that represent mantle end-member compositions-TBA-B3 (Tubuai HIMU), ALIA-115-21 (Samoa EM2), and two Pitcairn lavas (Pit-6 and Pit-8)-show clearly resolvable odd-MIF. The correlation between Δ 199 Hg and Δ 201 Hg values with a slope of ~1 (Figure 4), typical of surface photochemical reactions on Hg [START_REF] Bergquist | Mass-dependent and-independent fractionation of Hg isotopes by photoreduction in aquatic systems[END_REF], suggests that the three samples share likely a similar origin for the MIF signatures in surficial environments. The Hg-MIF signatures in OIB therefore provides evidence for recycling of shallow geochemical reservoirs into the deep sources of hotspots. This correlation between Δ 199 Hg and Δ 201 Hg values also provides further confidence on the quality of the data as isobaric interference would produce isotopic signatures that lie outside of this range.

Photochemical reduction of aqueous Hg(II) to Hg(0) vapor is the major source of odd-MIF, and is the main pathway of transfer of Hg from the ocean (surface) to the atmosphere [e.g. [START_REF] Bergquist | Mass-dependent and-independent fractionation of Hg isotopes by photoreduction in aquatic systems[END_REF]. During the photochemical reduction, a positive Δ 199 Hg [Hg(II)] is retained in the marine environment while the complementary negative Δ 199 Hg [Hg(0)] is released into the atmosphere [e.g. [START_REF] Bergquist | Mass-dependent and-independent fractionation of Hg isotopes by photoreduction in aquatic systems[END_REF]. Given the short residence time of Hg(0) in the atmosphere (~1 year), its quick subsequent deposition confers a globally negative Δ 199 Hg to the terrestrial reservoirs, while marine environments are characterized by positive Δ 199 Hg [e.g. [START_REF] Biswas | Natural mercury isotope variation in coal deposits and organic soils[END_REF][START_REF] Blum | Mercury Isotopes in Earth and Environmental Sciences[END_REF][START_REF] Demers | Mercury isotopes in a forested ecosystem: Implications for air-surface exchange dynamics and the global mercury cycle[END_REF][START_REF] Grasby | Isotopic signatures of mercury contamination in latest Permian oceans[END_REF][START_REF] Jiskra | Source tracing of natural organic matter bound mercury in boreal forest runoff with mercury stable isotopes[END_REF].

How a negative Δ 199 Hg preserved in terrestrial material is then transported into the ocean through time is unclear at present. Further work is required to test the robustness of this negative signature on long timescales necessary for this material to be recyclied within the mantle. This difference between terrestrial and marine settings is well reflected in sediments, with generally negative Δ 199 Hg in coastal sediments (which originate via erosion of nearby terrestrial materials) and positive Δ 199 Hg for open sea marine sediments [e.g. [START_REF] Meng | An integrated model for input and migration of mercury in Chinese coastal sediments[END_REF][START_REF] Yin | Identifying the source and processes of mercury in subtropical estuarine and ocean sediments using Hg isotopic composition[END_REF]. The odd-MIF of ALIA-115-21 (Samoa EM2), TBA-B3 (Tubuai HIMU), and Pit-6 and Pit-8 (Pitcairn) therefore likely reflect a marine Hg isotopic signature in Tubuai, and continental Hg signatures Samoan EM2 lavas (Figure 4). As discussed below, Pitcairn lavas exhibit Hg isotopic evidence for recycled marine and terrestrial materials.

The presence of marine and continental signatures in the mantle sources of Tubuai HIMU and Samoa EM2 lavas, respectively, is consistent with traditional interpretations based on radiogenic isotope signatures. Tubuai exhibits a HIMU signature, which is considered to result from recycling of ancient oceanic crust into the mantle [START_REF] Hofmann | Mantle plumes from ancient oceanic crust[END_REF]. The geochemically-depleted 87 Sr/ 86 Sr and 143 Nd/ 144 Nd signatures in HIMU lavas globally precludes a significant sediment (terrigenous or marine) contribution to their mantle sources, but transfer of positive Δ 199 Hg isotope compositions of seawater to oceanic crust during hydrothermal circulation at a mid-ocean ridge could explain the MIF signatures in Tubuai HIMU OIB. A potential caveat with this model is the low abundance of Hg in seawater compared to fresh basalts that would require a water/rock ratio of ~1000 to transfer the Hg isotopic composition of seawater to oceanic crust. However, the absence of data on Hg behavior during seawater alteration of oceanic crust (e.g., Hg concentration measurements on altered oceanic crust, including serpentinites) limits our ability to quantitatively model this scenario. An additional complexity is that a fraction of the slab Hg budget would be lost during subduction dehydration, but relevant data are not yet available to quantify this. By contrast, Samoan EM2 lavas exhibit geochemically-enriched (high) 87 Sr/ 86 Sr and (low) 143 Nd/ 144 Nd signatures consistent with the input of recycled terrigenous materials [Jackson et al., 2007;[START_REF] Workman | Oxygen isotopes in Samoan lavas: confirmation of continent recycling[END_REF], consistent with the slightly negative Δ 199 Hg terrestrial signature.

The presence of negative Δ 199 Hg in the source of Pitcairn EM1 OIB suggests that, like EM2, EM1 contains a fraction of recycled terrigenous material [START_REF] Castillo | An alternative explanation for the Hf-Nd mantle array[END_REF][START_REF] Delavault | Sulfur and lead isotopic evidence of relic Archean sediments in the Pitcairn mantle plume[END_REF][START_REF] Stracke | Earth's heterogeneous mantle: A product of convection-driven interaction betweencrust and mantle[END_REF]. However, there is complexity in the MIF present in the Pitcairn suite that is not explained by our simple model: the correlation between the Δ 199 Hg and 87 Sr/ 86 Sr for the three Pitcairn samples (Figure 2) is puzzling, as we would expect the sample with the highest 87 Sr/ 86 Sr to host the largest fraction of continent-derived material and, therefore, the lowest Δ 199 Hg. Instead, the Pitcairn lava with the weakest EM1 signature has the strongest negative Δ 199 Hg, and the most extreme EM1 Pitcairn lava has no resolvable Δ 199 Hg signature. The positive correlation between Δ 199 Hg and 87 Sr/ 86 Sr is unlikely to be explained by seawater alteration, a mechanism that would increase both Δ 199 Hg and 87 Sr/ 86 Sr. This is because 1) Δ 199 Hg also exhibit a negative correlation with 143 Nd/ 144 Nd (Figure S2), a radiogenic isotope system immune to seawater influence, and 2) the selected Pitcairn lavas are young (<1.0Ma; [START_REF] Duncan | Pitcairn island-another Pacific hot spot?[END_REF]) and fresh (i.e., no signs of visible alteration). Therefore, we explore a mantle source origin for the correlation between 87 Sr/ 86 Sr and Δ 199 Hg, acknowledging that this relationship is only for three samples.

Unlike Samoan EM2 (which is a melt of a peridotite mantle source that hosts recycled terrigenous material) [START_REF] Jackson | Re-Os isotope systematics in Samoan shield lavas and the use of Os-isotopes in olivine phenocrysts to determine primary magmatic compositions[END_REF] and Tubuai HIMU (which is a melt of a mantle source with recycled oceanic crust) [START_REF] Hauri | Re-Os isotope systematics of EMII and HIMU oceanic basalts from the south Pacific Ocean[END_REF], extreme Pitcairn EM1 is suggested to host both recycled oceanic crust and terrigenous materials [START_REF] Eisele | The 320 kyr Pb isotope evolution of Mauna Kea lavas recorded in the HSDP-2 drill core[END_REF]

Conclusions

To estimate the Hg isotopic composition of the mantle that is least affected by crustal recycling, we analysed high-3 He/ 4 He lavas from Samoa and Iceland. We obtained an average Δ 199 Hg=0.00±0.10, Δ 201 Hg=-0.02±0.0.09 and δ 202 Hg=-1.7±1.2 (2SD, N=11), which is the best estimate for the composition of the primitive mantle.

HIMU lavas from Tubuai exhibit positive Hg-MIF, likely reflecting a transfer of the isotope compositions of seawater to oceanic crust during hydrothermal circulation at a mid-ocean ridge; positive Hg-MIF therefore signals the presence of altered oceanic crust in its mantle source.

The negative MIF observed for EM2 lava from Samoa is consistent with assimilation of continental crust material. The negative MIF for the Pitcairn lavas, and the positive correlation of Δ 199 Hg values with 87 Sr/ 86 Sr, provides an independent argument in favor of mixture between terrestrial and marine materials in the mantle source of EM1.

Table S1

Introduction This table contains the mercury isotopic composition the samples analysed in this study. The full data table can be found in supplementary materials (Table S1) and the Figure S1 (our Hg isotope data plotted against Nd isotope ratios). Helium isotopic data are from [Jackson et al., 2007;Jackson et al., 2020;Macpherson et al., 2005;Furi et al. 2010]. All the data were collected in 2021 on a Nu Instruments Multi-collectors inductively-coupledplasma mass-spectrometer.

3

  He/ 4 He reservoir is least impacted by recycling, and the discovery that some high-3 He/ 4 He lavas preserve early-Hadean signatures identified using several different short-lived isotope systems -129 Xe/ 130 Xe[START_REF] Mukhopadhyay | Noble Gases: A Record of Earth's Evolution and Mantle Dynamics[END_REF], 142 Nd/ 144 Nd[START_REF] Peters | Hadean silicate differentiation preserved by anomalous 142 Nd/ 144 Nd ratios in the Réunion hotspot source[END_REF], and 182 W/ 184 W [[START_REF] Mundl-Petermeier | Anomalous 182W in high 3 He/ 4 He ocean island basalts: fingerprints of Earth's core?[END_REF][START_REF] Rizo | 182 W evidence for core-mantle interaction in the source of mantle plumes[END_REF]-this mantle reservoir is most likely to preserve signatures associated with terrestrial accretion. Indeed, arguments have been made that the high-3 He/ 4 He reservoir is the oldest domain that has survived in Earth's mantle[START_REF] Giuliani | Remnants of early Earth differentiation in the deepest mantle-derived lavas[END_REF][START_REF] Jackson | Evidence for the survival of the oldest terrestrial mantle reservoir[END_REF] but alternative views exist [e.g.[START_REF] Mukhopadhyay | Noble Gases: A Record of Earth's Evolution and Mantle Dynamics[END_REF]. Therefore, the absence of Hg-MIF in the high-3 He/ 4 He samples suggests that Earth's mantle accreted Hg devoid of MIF, and therefore any observed MIF are the consequence of terrestrial fractionation.Combining Hg isotope data from the Ofu (N=4) with Iceland samples for which 3 He/ 4 He are available (N=7, all of which have 13< 3 He/ 4 He<34), return an average Hg isotopic composition for the mantle of Δ 199 Hg=0.00±0.10 and Δ 201 Hg=-0.02±0.0.09 (2SD, N=11). This represents the current best estimate of the terrestrial "primitive mantle" composition. The δ 202 Hg values are more variable within these samples. Since Hg is highly volatile, degassing during magma ascent can induce isotopic fractionation [e.g.[START_REF] Zambardi | Mercury emissions and stable isotopic compositions at Vulcano Island (Italy)[END_REF], and further isotopic fractionation could also occur during igneous processes. Therefore, compared to MIF-Hg signatures, which should not be significantly impacted by high temperature magmatic and degassing processes, the δ 202 Hg values are likely more impacted by the processes leading to the formation of the basalts.

D 199

 199 Hg in this lava can explained by the absence (or near absence) of oceanic crust (which would have apositive D 199 Hg values): the remaining contribution of recycled terrigenous material has strongly negative D 199 Hg, and explains the negative D 199 Hg values determined in lavas. In this way, the relative proportions and absolute abundances of recycled terrigenous and oceanic crust manuscript submitted to Geophysical Research Letters materials can be modulated to explain the magnitude and sign of the D 199 Hg values, and its relationship with radiogenic isotopes.

Figure 1 :

 1 Figure 1: D 199 Hg versus d 202 Hg for all samples analyzed here. Samples from the Afar, Ofu island, and Iceland have similar D 199 Hg that cluster around zero and variable d 202 Hg values. A sample from Tubuai has a positive D 199 Hg value, whereas two Pitcairn samples and one Samoan sample with recycling signatures have negative D 199 Hg.

Figure 3 :

 3 Figure 3: Mercury isotopic composition of the Earth's mantle (red box, defined by high 3 He/ 4 He lavas from Samoa and Iceland) compared to chondrites (CC=carbonaceous chondrites, EC=enstatite chondrites, OC=ordinary chondrites). Mantle-derived lavas exhibit no clear signatures for crustal recycling and fall within the chondritic range..

  . A mixture of both recycled oceanic crust and terrigenous materials can help to explain the correlation between D 199 Hg and 87 Sr/ 86 Sr (Figure2), where Pitcairn's extreme EM1 endmember (highest 87 Sr/ 86 Sr) lacks a clear odd-MIF, and the Pitcairn lava with the weakest EM1 signature (lowest 87 Sr/ 86 Sr) has a negative D 199 Hg. Because recycled oceanic crust and terrigenous materials have complementary positive and negative D 199 Hg signatures, respectively, it is possible for their mixture to generate in a near-zero D 199 Hg value like that observed in the extreme Pitcairn EM1 sample. In this case, the presence of terrigenous materials explains the high 87 Sr/ 86 Sr in the extreme EM1 Pitcairn lava. In contrast, the Pitcairn lava with the lowest 87 Sr/ 86 Sr (weakest EM1 signature) is explained by a smaller fraction of recycled terrigenous material, and the very negative

Table S1 :

 S1 Samplesδ 202 Hg 2SD Δ 199 Hg 2SD Δ 200 Hg 2SD Δ 201 Hg 2SD Δ 204 Hg 2SD

			manuscript submitted to Geophysical Research Letters
	MID-1	-1.54	0.11	0.03	0.13	0.05	0.08	0.02	0.16	0.17	0.07	17
	Northern Rift Zone
	NAL-625	-1.93	0.02	-0.10	0.13	0.06	0.03	-0.08	0.02	0.03	0.03	34
	South Iceland
	Volcanic Zone
	SAL-601	-1.14	0.10	0.06	0.13	0.06	0.08	0.01	0.10	0.04	0.25	19
	Geostandards
	BCR-2	-2.18	0.09	-0.01	0.07	0.02	0.07	0.00	0.10	0.02	0.18
	BCR-2
	(Geng et al.)	-2.08	0.08	0.00	0.14	0.01	0.08	0.00	0.14
	RGM-2	-0.98	0.09	-0.02	0.07	0.02	0.07	-0.03	0.10	0.02	0.18
	STM-2	-1.11	0.09	0.00	0.07	0.01	0.07	-0.01	0.10	-0.12	0.18
	BE-N	-3.08	0.08	0	0.03	0.01	0.03	-0.02	0.05	-0.01	0.09
	UoI internal
	standards I-ICE	-2.12	0.18	-0.01	0.04	0.01	0.03	-0.04	0.02	0.00	0.10	3 He/ 4 He (Ra)
	Afar A-ALK	-1.20	0.10	-0.03	0.13	-0.02	0.08	-0.06	0.10	0.00	0.25
	AF13-339 B-ALK	-0.50 -2.79	0.09 0.10	0.05 0.01	0.07 0.13	0.05 0.08	0.07 0.08	0.08 -0.03	0.10 0.10	-0.02 -0.01	0.18 0.25
	AF13-356	-1.50	0.09	0.03	0.07	0.01	0.07	-0.02	0.10	-0.07	0.18
	AF13-44	-1.09	0.09	0.05	0.07	0.03	0.07	0.05	0.10	-0.03	0.18
	AF13-48	-1.92	0.09	0.02	0.07	0.02	0.07	0.06	0.10	-0.11	0.18
	AF13-56	-1.18	0.09	-0.08	0.07	-0.07	0.07	0.01	0.10	-0.02	0.18
	AF13-90	-2.82	0.09	-0.03	0.07	0.09	0.07	0.05	0.10	-0.05	0.18
	Samoa
	ALIA-115-21	-0.55	0.03	-0.14	0.02	0.02	0.02	-0.12	0.06	-0.04	0.07
	Ofu-04-03	-2.03	0.09	-0.05	0.07	-0.02	0.07	-0.03	0.10	0.03	0.18	24
	Ofu-04-06	-2.68	0.09	0.05	0.07	0.10	0.07	-0.07	0.10	-0.02	0.18	34
	Ofu-04-10	-2.14	0.09	-0.04	0.07	0.01	0.07	-0.07	0.10	-0.22	0.18	22
	Ofu-04-15	-1.46	0.09	-0.02	0.07	0.01	0.07	-0.03	0.10	-0.02	0.18	30
	Pitcairn
	PIT-13	-1.42	0.09	0.05	0.07	0.05	0.07	0.07	0.10	0.00	0.18	9
	PIT-6	-0.74	0.01	-0.45	0.01	-0.01	0.05	-0.38	0.06	0.05	0.05
	PIT-8	-0.01	0.08	-0.11	0.03	0.03	0.03	-0.02	0.05	-0.03	0.09	8
	Tubuai
	TBA-B3	-3.23	0.09	0.23	0.07	0.04	0.07	0.18	0.10	-0.10	0.18	7
	Iceland
	Eastern Rift
	Zone
	A11	-0.51	0.25	0.04	0.01	0.00	0.02	-0.01	0.10	0.00	0.25	24
	A24	-2.07	0.10	0.01	0.13	0.03	0.08	0.01	0.10	-0.16	0.25	26
	Reykjanes Peninsula and Western Rift
	Zone
	STAP-1	-1.87	0.10	-0.02	0.13	0.02	0.08	0.01	0.10	-0.01	0.25	14
	VIF-1	-1.59	0.10	0.02	0.13	0.07	0.08	0.06	0.10	-0.10	0.25	13

Acknowledgments, Samples, and Data

Runsheng Yin and Michael Bizimis are thanked for insightful reviews that have improved this manuscript. This study was supported by the National Natural Science Foundation of China (41625012, 41961144028, U1612442, 41830647) to JB.C and by the China National Space Administration (No. D020205) to ZW. FM thanks Yongsheng Liu for making the visit at CUG Wuhan possible. MGJ thanks IPGP for providing a stimulating academic home during sabbatical, and acknowledges partial support from NSF EAR-1900652 and OCE-1928970.

Open Research section

All the data are presently provided as a supplementary table and will be deposit on the CNRS HAL website as well as on the database Earthchem: https://earthchem.org/