Matthieu Clog
email: clog@caltech.edu

Cyril Aubaud

Pierre Cartigny

Laure Dosso

The hydrogen isotopic composition and water content of southern Pacific MORB: A reassessment of the D/H ratio of the depleted mantle reservoir

Keywords: hydrogen isotopes, mantle geochemistry, mantle water

In this paper, we re-investigate the isotopic composition of hydrogen in MORB and the possible effects of contamination on δD and water content. A suite of 40 N-MORB from the Pacific-Antarctic ridge, far from any hotspot, was analyzed for chlorine content by electron microprobe and for water content and δD with silica tubes. Cl concentrations (from 29 to 2400 ppm) indicate widespread contamination, more intense with faster spreading rates, while water contents (from 840 to 7800 ppm) are mainly controlled by igneous processes. δD values range from -76 to -48‰, with an average value of -61‰. The lack of correlation between Cl content and either H 2 O/Ce or δD indicate that contamination has a negligible effect on δD for our samples, which is therefore characteristic of the mantle below the Pacific-Antarctic ridge. We suggest that the 20‰ lower δD value reported for the North Pacific and North Atlantic is highly unlikely from geodynamical arguments. We propose that the convecting mantle is characterized by a δD of -60±5‰, as supported by the most recent data from North Atlantic N-MORB.

Highlights

► A suite of 40 MORB was analyzed for δD, water and Cl content. ► Degassing and contamination have negligible effect on D/H. ► Average δD is ∼-60‰, 15‰ higher than earlier studies. ► High regional D are unlikely, analytical artifacts may be responsible. ► We propose an updated value for mantle D/H.

Introduction

Based on the pioneer work of [START_REF] Kyser | Hydrogen isotope systematics of submarine basalts[END_REF], the isotopic composition of hydrogen in the convective mantle is commonly assumed to be characterized by a δD of -80±10 (compared to SMOW). The D/H variations were attributed to secondary processes, especially contamination by seawater and seawater-derived fluids, both with higher δD than the mantle.

Hence, in any given area, the value most likely to characterize mantle hydrogen would be the lowest observed, typically -80 . Several studies [START_REF] Poreda | Helium and hydrogen isotopes in ocean-ridge basalts north and south of Iceland[END_REF][START_REF] Kingsley | D/H ratios in basalt glasses from the Salas y Gomez mantle plume interacting with the East Pacific Rise: water from old D-rich recycled crust or primordial water from the lower mantle?[END_REF][START_REF] Pineau | Deepsea explosive activity on the Mid-Atlantic ridge near 34 • 50' N: a stable isotope (C, H, O) study[END_REF] have however since shown co variations between δD-values (ranging from -80 to -40 ) and tracers of mantle source heterogeneities (either ratios of rare-earth elements or Pb isotopes), demonstrating that source heterogeneity rather than contamination is the main factor controlling the variations of δD and H 2 O in MORB, OIB and seamount chain basalts. This is also consistent with studies of water to light rare earth element ratios of similar incompatibilities [START_REF] Michael | Regionally distinctive sources of depleted MORB: evidence from trace elements and H 2 O[END_REF]Kent et al., 1999b;[START_REF] Danyushevsky | H 2 O abundance in depleted to moderately enriched mid-ocean ridge magmas; part I: Incompatible behaviour, implications for mantle storage, and origin of regional variations[END_REF][START_REF] Le Roux | The effects of variable sources, processes and contaminants on the composition of northern EPR MORB (8-10 • N and 12-14 • N): evidence from volatiles (H 2 O, CO 2 , S) and halogens (F, Cl)[END_REF], which showed that MORB H 2 O/Ce (or /La) are regionally homogeneous and characteristic of the local MORB mantle source. How δD-values may vary remain however typically unaddressed (except for [START_REF] Kingsley | D/H ratios in basalt glasses from the Salas y Gomez mantle plume interacting with the East Pacific Rise: water from old D-rich recycled crust or primordial water from the lower mantle?[END_REF]).

Moreover, a recent study has shown that extracting water by induction heating of basaltic glasses in a Pt-crucible (which was the prefered method in the 80's, [START_REF] Kyser | Hydrogen isotope systematics of submarine basalts[END_REF]) can lower the measured δD by more than 10 [START_REF] Clog | Experimental evidence for interaction of water vapor and platinum crucibles at high temperatures: Implications for volatiles from igneous rocks and minerals[END_REF]. It further illustrates the need to re-evaluate the hydrogen isotopic composition of the mantle and its variability.

In the present study we reinvestigate the current consensus that the δD-value of the MORB mantle is equal to -80 and propose that a value of ≈-60 is more appropriate.

Samples locality background and analytical techniques

The Pacific-Antarctic ridge is ideal to re-investigate the D/H isotope composition of the depleted upper mantle. Two large supersegments (from 65 to 56 • S and from 52 to 41 • S, sampled respectively by the French missions PACANTARCTIC 1 (1996) and 2 ( 2005)) present MORB compositions characterizing a rather homogeneous mantle devoid of hotspot influence (see Vlastélic et al. (2000) for the southern one ; [START_REF] Moreira | Helium isotopes on the Pacific-Antarctic ridge[END_REF] and [START_REF] Hamelin | Sr-Nd-Hf isotopes along the Pacific Antarctic Ridge from 41 to 53[END_REF] for the northern one). They are separated by a region with several fracture zones which record interactions with the Louisville hotspot, as illustrated by the greater variability of MORB compositions in radiogenic isotope ratios [START_REF] Castillo | Sr, Nd and Pb isotopic variation along the Pacific-Antarctic risecrest, 53-57 • S: implications for the composition and dynamics of the South Pacific upper mantle[END_REF][START_REF] Géli | Location of Louisville hotspot and origin of Hollister ridge: geophysical constraints[END_REF]Vlastélic et al., 1998).

Spectral analysis of the variations in isotopic compositions of Pb, Sr and Nd along the Pacific-Antarctic and East-Pacific ridges showed that the composition of MORB from those supersegments reflects subtle variations within the range of N-MORB compositions, consistent with a gradual greater contribution of the HIMU component northwards [START_REF] Hamelin | Geochemical portray of the pacific ridge: New isotopic data and statistical techniques[END_REF].

Except for helium analyses [START_REF] Moreira | Helium isotopes on the Pacific-Antarctic ridge[END_REF][START_REF] Hamelin | Geochemical portray of the pacific ridge: New isotopic data and statistical techniques[END_REF], which show homogeneous isotopic composition typical of the depleted mantle (R/R a =7.29±0.19), there has been no study of volatile elements in this region. [START_REF] Moreira | Helium isotopes on the Pacific-Antarctic ridge[END_REF] also noted limited variations and in particular that the northern supersegment is the most homogeneous ridge portion studied with respect to He isotopic compositions.

The variations in structure and spreading rate in this area are also of importance. The spreading rate increases northwards, from 54 mm/yr at 70 • S to 111 mm/yr at 40 • S (DeMets et al., 1990) and is associated with a change in the structure of the ridge in the southern supersegment, from a central valley typical of slow-spreading ridges to a central dome typical of faster ridges. As noted by Vlastélic et al. (2000), the morphology appears to be primarily controlled by the spreading rate, rather than by the potential mantle temperature. On the whole northern supersegment, the structure is typical of a fast-spreading ridge [START_REF] Klingelhoefer | New structural and geochemical observations from the Pacific-Antarctic ridge between 52 • 45'S and 41 • 15[END_REF][START_REF] Hamelin | Sr-Nd-Hf isotopes along the Pacific Antarctic Ridge from 41 to 53[END_REF].

Faster ridges have typically more important and widespread hydrothermal circulation, leading to a more pronounced chlorine contamination [START_REF] Michael | Influence of spreading rate and magma supply on crystallization and assimilation beneath mid-ocean ridges: Evidence from chlorine and major element chemistry of mid-ocean ridge basalts[END_REF][START_REF] Bonifacie | The chlorine isotope composition of Earth's mantle[END_REF]. Therefore, for water studies, the structure of the ridge could also be of importance, particularly in assessing the effect of contamination of seawater derived-fluids on δD-values.

Sample suite

All samples in this study are glassy rims of pillow lavas dredged at the axis of the ridge, at depths between 1500 and 2800 meters below sea level, except PAC1CV07 which was dredged on an off-axis seamount (figure 1).

Forty basalts were analyzed for water and chlorine contents, and for δD.

All the samples are N-MORB (La/Sm N <1, where N marks normalization to the primitive mantle, McDonough and Sun (1995)), with a slightly higher alkali content for samples collected near the Menard Transform Fault Fracture (samples PAC2 DR7 and PAC2 DR20) due to greater extents of crystal fractionation. Most samples were previously analyzed for their compositions in major (n = 37), trace (n = 35) and radiogenic (Sr, Nd, Pb) elements iso-topic compositions (n = 37) and some (n = 16) for helium concentrations and isotopic compositions (tables 1 and2 , Vlastélic et al. (1999, Vlastélic et al. ( , 2000)); [START_REF] Moreira | Helium isotopes on the Pacific-Antarctic ridge[END_REF]; [START_REF] Hamelin | Sr-Nd-Hf isotopes along the Pacific Antarctic Ridge from 41 to 53[END_REF][START_REF] Hamelin | Geochemical portray of the pacific ridge: New isotopic data and statistical techniques[END_REF]). We report new cerium data measured by NAA (Neutron Activation Analysis) for the samples of the southern supersegment, with the same technique as used for La and Sm by (Vlastélic et al., 2000). Tables 3 and4 report the δD, H 2 O, Cl and Ce concentrations, along with geographic coordinates and depth of collection.

Water content and δD measurements

The technique used in this study has been described in [START_REF] Clog | Experimental evidence for interaction of water vapor and platinum crucibles at high temperatures: Implications for volatiles from igneous rocks and minerals[END_REF] and is summarized here. It is based on the original method described by Vennemann and O'Neil (1993). Glass samples were crushed to shards of 100 to 250 µm, washed in distilled water, ultrasonically cleaned and dried.

Most of the plagioclase, olivine and altered grains were discarded using a Frantz magnetic sorting apparatus. Remaining minerals and altered chips were removed by careful handpicking under a binocular, to select only fresh glass shards.

The mass of sample typically used was 350 mg for each analysis. The sample was introduced into a pre-cleaned sealed quartz tube (2 hours at 1150 • C) connected to a vacuum line and heated to 250 • C for half an hour to eliminate any adsorbed water. Then the samples were heated to 950 [START_REF] Bigeleisen | Conversion of hydrogenic materials to hydrogen for isotopic analysis[END_REF]. The amount of gas is measured by manometry and it is then concentrated into a sampling tube with a Toepler pump. The gas is analyzed with a dual-inlet mass spectrometer (Delta Plus XP). The typical reproducibility on the measurements was ±5% for water content and ±3 for δD (both 2σ, as established from the 10 analysis of the PAC2DR20-1-1 sample). Procedural blanks amount to ≈0.4 µmol. Re-heating a sample that had previously been heated to 900 • C for half an hour produced amounts of water equal to the value of our experimental blanks, demonstrating that we reached quantitative water recovery.

The uranium furnace was calibrated with three standard waters : ROSS (δD=-114.9 ), ) and ORSMOW (δD=+2.4 ). These in-house standards were obtained from the LSCE (Laboratoire des Sciences du Climat et l'Environnement, Saclay, France) where they are regularly calibrated against IAEA standards (VSLAP, δD=-428 and VSMOW, δD=0 ).

During the course of this study, we checked the accuracy of our analytical protocol by regularly measuring the δD-value of the standard NBS-30. We obtain an average value of -63.4±2 (2σ, n=6), in agreement, within analytical uncertainties, with the recommended value of -65.7±0.6 (2σ).

All samples were at least duplicated, reported values representing their averages, and all were within ±3 .

Chlorine content measurements

Fresh glass shards were mounted in epoxy and polished for analysis.

Chlorine concentrations were determined using electron microprobe with a CAMECA SX50 at the CAMPARIS analytical facility in Paris, France, using well-established techniques (e.g. [START_REF] Jambon | Chlorine and bromine abundance in MORB: the contrasting behaviour of the Mid-Atlantic ridge and East Pacific Rise and implications for chlorine geodynamic cycle[END_REF]. Analyses were made using a 20 µm diameter, 500 nA electron beam intensity, at 30 kV acceleration voltage and with a counting time of 60 s for chlorine and for backgrounds.

All four spectrometers were used simultaneously. Calibration was determined using a scapolite standard (Cl content 4.04 wt%). Ten points were measured for each sample along a transect, avoiding visible cracks or imperfection in the polished surface. The detection limit is 13 ppm, as determined on an olivine standard and the reproducibility in chlorine concentration range from 5 to 10 ppm (1 σ).

Results

Water concentrations

Water concentrations range from 840 to 7800 ppm (n = 40), which is within the typical range for MORB (2300 +3500 -1600 ppm, PetDB database [START_REF] Lehnert | A global geochemical database structure for rocks[END_REF] and [START_REF] Kyser | Hydrogen isotope systematics of submarine basalts[END_REF]; [START_REF] Poreda | Helium and hydrogen isotopes in ocean-ridge basalts north and south of Iceland[END_REF]; [START_REF] Jambon | Water in oceanic basalts: evidence for dehydration of recycled crust[END_REF]; [START_REF] Pineau | Strong degassing at ridge crests: The behaviour of dissolved carbon and water in basalt glasses at 14 • N, Mid-Atlantic ridge[END_REF]; [START_REF] Michael | Regionally distinctive sources of depleted MORB: evidence from trace elements and H 2 O[END_REF] [START_REF] Michael | Regionally distinctive sources of depleted MORB: evidence from trace elements and H 2 O[END_REF]). The typical range for Pacific MORB is 180±30 [START_REF] Michael | Regionally distinctive sources of depleted MORB: evidence from trace elements and H 2 O[END_REF][START_REF] Le Roux | The effects of variable sources, processes and contaminants on the composition of northern EPR MORB (8-10 • N and 12-14 • N): evidence from volatiles (H 2 O, CO 2 , S) and halogens (F, Cl)[END_REF], and only samples south of 63 • S present H 2 O/Ce ratios outside this range.

No systematic variation of water content or H 2 O/Ce with latitude was found (figure 2).

Hydrogen isotopic compositions

δD range from -76 to -48 (tables 3 and 4, figure 2a), within the range of previously studied MORB (from -92 to -44 [START_REF] Kyser | Hydrogen isotope systematics of submarine basalts[END_REF][START_REF] Poreda | Helium and hydrogen isotopes in ocean-ridge basalts north and south of Iceland[END_REF][START_REF] Chaussidon | Hydrogen, sulphur and neodymium isotope variations in the mantle beneath the EPR at 12 • 50'N. In: Stable isotope geochemistry: a tribute to Samuel Epstein[END_REF][START_REF] Pineau | Strong degassing at ridge crests: The behaviour of dissolved carbon and water in basalt glasses at 14 • N, Mid-Atlantic ridge[END_REF][START_REF] Kingsley | D/H ratios in basalt glasses from the Salas y Gomez mantle plume interacting with the East Pacific Rise: water from old D-rich recycled crust or primordial water from the lower mantle?[END_REF][START_REF] Pineau | Deepsea explosive activity on the Mid-Atlantic ridge near 34 • 50' N: a stable isotope (C, H, O) study[END_REF]Hauri et al., 2006b;[START_REF] Cartigny | Towards a consistent mantle carbon flux estimate: Insights from volatile systematics (H 2 O/Ce, δD, CO 2 /Nb) in the North Atlantic mantle (14 • N and 34 • N)[END_REF]). The δD values show no significant correlation to either water contents or water to cerium ratios. It is however worth noting that more than >90% of the N-MORB from the North Pacific and Atlantic previously published have δD between -85 and -65 [START_REF] Kyser | Hydrogen isotope systematics of submarine basalts[END_REF][START_REF] Pineau | Deepsea explosive activity on the Mid-Atlantic ridge near 34 • 50' N: a stable isotope (C, H, O) study[END_REF]Hauri et al., 2006b;[START_REF] Cartigny | Towards a consistent mantle carbon flux estimate: Insights from volatile systematics (H 2 O/Ce, δD, CO 2 /Nb) in the North Atlantic mantle (14 • N and 34 • N)[END_REF]. The values measured here are thus more comparable to previously analyzed E-MORB from other oceanic basins [START_REF] Poreda | Helium and hydrogen isotopes in ocean-ridge basalts north and south of Iceland[END_REF][START_REF] Kingsley | D/H ratios in basalt glasses from the Salas y Gomez mantle plume interacting with the East Pacific Rise: water from old D-rich recycled crust or primordial water from the lower mantle?[END_REF], though N-MORB from the East Pacific Rise near 22 • N and the west rift of the Easter microplate have δD close to -65 [START_REF] Kingsley | D/H ratios in basalt glasses from the Salas y Gomez mantle plume interacting with the East Pacific Rise: water from old D-rich recycled crust or primordial water from the lower mantle?[END_REF]. There is also no correlation with Sr, Nd, Hf Pb or He isotope data on theses samples (figure 3b) or with the La/Sm ratio.

Chlorine concentrations

Cl concentrations vary from 23 to 2000 ppm (figure 3c), within the MORB range (from 10 (below the detection level) to more than 4000 ppm [START_REF] Jambon | Chlorine and bromine abundance in MORB: the contrasting behaviour of the Mid-Atlantic ridge and East Pacific Rise and implications for chlorine geodynamic cycle[END_REF][START_REF] Michael | Influence of spreading rate and magma supply on crystallization and assimilation beneath mid-ocean ridges: Evidence from chlorine and major element chemistry of mid-ocean ridge basalts[END_REF][START_REF] Soule | Incorporation of seawater into mid-ocean ridge lava flows during emplacement[END_REF], 90% of measured MORB falling below 800 ppm (PetDB database)). On average, samples from the northern supersegment, where the spreading rate is higher, have higher Cl/K than samples from the southern supersegment (modes 0.32 +0.58 -0.21 vs 0.16 +0.22 -0.09 , figure 3d). Potassium and chlorine having similar bulk partition coefficient during partial melting and crystal fractionation [START_REF] Michael | Influence of spreading rate and magma supply on crystallization and assimilation beneath mid-ocean ridges: Evidence from chlorine and major element chemistry of mid-ocean ridge basalts[END_REF], the variations are thus too large to depend only upon variations in igneous processes, but reflect contamination or assimilation, as discussed in the next section.

Discussion

Degassing

Contrary to CO 2 which can degas at high pressure, water is much more soluble in silicate melts and typically remains under-saturated in typical MORB settings (e.g., [START_REF] Dixon | An experimental study of water and carbon dioxide solubilities in mid-ocean ridge basaltic liquids. Part II: applications to degassing[END_REF][START_REF] Jendrzejewski | Mesures quantitatives de carbone et d'eau dans les verres basaltiques naturels par spectroscopie infrarouge. partie II: l'eau= quantitative measurements of water and carbon concentrations in natural basaltic glasses by infrared spectroscopy. part II: water[END_REF]. [START_REF] Kyser | Hydrogen isotope systematics of submarine basalts[END_REF] suggested that loss of hydrogen could occur either as degassing of H 2 or CH 4 . Subsequent studies have actually shown that MORB are more oxydized than was originally thought [START_REF] Bézos | The Fe 3+ /ΣFe ratios of MORB glasses and their implications for mantle melting[END_REF][START_REF] Kelley | Water and the oxidation state of subduction zone magmas[END_REF], and therefore, H 2 /CH 4 degassing is not significant. Furthermore all samples present very low vesicularity (<1%), are under-saturated with respect to water (figure 4), and can be calculated to be in equilibrium with a gas phase containing less than 5% of water for all but 4 of the samples. The sample PAC1DR10-1 is in equilibrium with a vapor phase containing more than 20% of water, and is thus the most likely to have experienced some degassing. However, its δD is indistinguishable from the δD of the sample PAC1DR10-2 which comes from the same dredge and argues against any significant degassing-induced change in δD values. The homogeneity of the H 2 O/Ce ratios observed along the supersegments is also a compelling argument against degassing. Degassing of water thus appears negligible for all of our samples.

Effects of contamination on volatile contents

It should be first noted that previous studies established that variations in major and trace elements in this sample suite are mostly due to crystal fractionation (Vlastélic et al., 2000;[START_REF] Hamelin | Sr-Nd-Hf isotopes along the Pacific Antarctic Ridge from 41 to 53[END_REF]. This is also illustrated by the evolution of La and Ce content with MgO wt% (figure 5a andb). In this section we investigate to what extent are non-igneous processes responsible for variations in water and chlorine content.

Ratios of Cl and H 2 O to elements of similar incompatibilities (typically K, La or Ce) can be used to quantify the degree of interaction with brines or hydrated rocks during ascent (e.g., [START_REF] Michael | Regionally distinctive sources of depleted MORB: evidence from trace elements and H 2 O[END_REF][START_REF] Michael | Influence of spreading rate and magma supply on crystallization and assimilation beneath mid-ocean ridges: Evidence from chlorine and major element chemistry of mid-ocean ridge basalts[END_REF].

This is because contamination of lavas by seawater-derived fluids results in an increase in chlorine content, while water content appears comparatively less (or not) affected [START_REF] Michael | Influence of spreading rate and magma supply on crystallization and assimilation beneath mid-ocean ridges: Evidence from chlorine and major element chemistry of mid-ocean ridge basalts[END_REF]Kent et al., 1999a,b;[START_REF] Dixon | Volatiles in basaltic glasses from Loihi seamount, Hawaii: evidence for a relatively dry plume component[END_REF][START_REF] Soule | Incorporation of seawater into mid-ocean ridge lava flows during emplacement[END_REF].

Constraining the interaction with brines or hydrated rock on the basis of the δD alone is difficult, as the isotopic composition of the brines deep in the crust is unconstrained, while hydrated rocks can have δD similar to those measured in our samples (e.g., [START_REF] Agrinier | O and H stable isotope compositions of oceanic crust and upper mantle rocks exposed in the Hess Deep near the Galapagos Triple Junction[END_REF]. It is worth noting that experiments led to the suggestion that high-Cl brines should be depleted in D, with ∆D vapor-brine fractionation between 2 and 8 according to [START_REF] Berndt | Hydrogen isotope systematics of phase separation in submarine hydrothermal systems: ex-perimental calibration and theoretical models[END_REF], but the same study also highlighted the difficulty in trying to relate the measured fractionation and the δD of outwards fluxes at hydrothermal vents due to uncertainties in mixing processes, temperature of the reaction, closed versus open system consideration and concurrent processes of interaction with the crustal rocks. In summary, from available studies, the D/H ratio of brines is still difficult to anticipate. Yet, it should also be noted that contamination by either brines or hydrothermally altered rocks cannot buffer the δD of our samples as they are all under-saturated with respect to water.

In the following discussion, we therefore focus on the the H 2 O/Ce and Cl/K ratios.

H 2 O/Ce ratios

Water content increases smoothly with decreasing MgO wt% (figure 5c),

suggesting that water content is primarily controlled by igneous processes rather than contamination. We also observed that the H 2 O/Ce ratios are rather homogeneous on our sample suite. The average H 2 O/Ce ratio is ≈185±30 on the southern supersegment and ≈175±15 on the northern one, which is, within this small range, the opposite of what would be expected from the relationship between ridge structure and degree of assimilation [START_REF] Michael | Influence of spreading rate and magma supply on crystallization and assimilation beneath mid-ocean ridges: Evidence from chlorine and major element chemistry of mid-ocean ridge basalts[END_REF]. All this evidence argues against substantial hydrogen contamination by either seawater-derived fluids or hydrated rocks during ascent (see also figure 6).

Cl/K ratios

Contrary to water, the chlorine content variations cannot be explained solely by crystal fractionation (figure 5d). We observe that the majority of samples have incurred a greater extent of crystal fractionation (MgO<7.5

wt.%) have Cl content much higher than the crystal fractionation trend, which contrasts with samples where MgO>8.0 wt.%. Cl/K ratios are also higher in the northern supersegment (modes 0.32 +0.58 -0.21 in the northern supersegment and 0.16 +0.22 -0.09 in the southern supersegment, figure 3d), which points to more intense contamination with an increasing spreading rate, as has been proposed by [START_REF] Michael | Influence of spreading rate and magma supply on crystallization and assimilation beneath mid-ocean ridges: Evidence from chlorine and major element chemistry of mid-ocean ridge basalts[END_REF] and [START_REF] Bonifacie | The chlorine isotope composition of Earth's mantle[END_REF].

It should be noted that this difference is however mostly driven by a couple of extremely Cl-enriched samples (PAC2 DR34-1 and PAC2 DR28-2).

A recent study [START_REF] Kendrick | Halogen sys-tematics (Cl, Br, I) in Mid-Ocean Ridge Basalts: A Macquarie Island case study[END_REF] (

)). 2012
The contrast between the behavior of water and chlorine suggests that contamination in our sample suite is not due to assimilation of hydrated wall-rock or contribution from seawater or diluted brines, but due to either very concentrated brines or pure halite left in the crust by hydrothermal circulation (figure 6). It should also be noted that samples with low Cl content (<100 ppm, 12 samples) or low Cl/K (<1.6, 12 samples) present a range of δD from -76 to -51 , and an average δD of -63 . Those values are close to those observed for the complete sample suite (δD from -75 to -48 and δD ≈-61 . This is consistent with a very minor effect of contamination on the D/H ratio of our samples.

Incompatibility of water relative to La and Ce

Although it is well described that the bulk partition coefficient of water during mantle partial melting is close to Ce and La [START_REF] Michael | Regionally distinctive sources of depleted MORB: evidence from trace elements and H 2 O[END_REF], the rare-earth element whom behavior is closest to water actually depends on the degree of enrichment of the source in incompatible elements [START_REF] Danyushevsky | H 2 O abundance in depleted to moderately enriched mid-ocean ridge magmas; part I: Incompatible behaviour, implications for mantle storage, and origin of regional variations[END_REF]. In the extreme case of Macquarie Island, where the primitive, weakly fractionated basalts have (La/Sm) N >2, it was recently proposed that Pr was the element whose behavior was the most similar to water (Kamenetsky and Eggins, 2012), emphasizing the need to carefully choose the element for normalization.

For our sample suite, the incompatibility of water is more closer to that of Ce rather than of La, as illustrated on figure 7. The regression line defined by our samples in H 2 O/Ce-Ce space is actually flatter (slope of -0.6±0.7, origin 190±12) than the one in H 2 O/La-La space (slope -13±9, origin 624±54).

This justifies the normalization to Ce rather than La chosen in the previous section as well as in the rest of this article.

Considering the range of (La/Sm) N observed here (between 0.45 and 0.95), our results contrasts with those of [START_REF] Danyushevsky | H 2 O abundance in depleted to moderately enriched mid-ocean ridge magmas; part I: Incompatible behaviour, implications for mantle storage, and origin of regional variations[END_REF] which suggested that La should be more appropriate. It should be noted that there is no requirement for disagreement. Partition coefficient for individual minerals (olivine, garnet, pyroxene) are not similar between water and La/Ce [START_REF] Aubaud | Hydrogen partition coefficients between nominally anhydrous minerals and basaltic melts[END_REF]Hauri et al., 2006a;[START_REF] Kohn | The partitioning of water between nominally anhydrous minerals and silicate melts[END_REF][START_REF] Grant | The partitioning of water between olivine, orthopyroxene and melt synthesised in the system albiteforsterite-H 2 O[END_REF].

Therefore source mineralogy affect the bulk partition coefficient for H 2 O, La or Ce, and as a matter of consequence, the preference for Ce over La in this sample suite.

Crystal fractionation and partial melting effects

In the southernmost area, the H 2 O/Ce ratio is more variable (standard deviation of 50 south of the Saint-Exupery fracture zone versus standard deviation of 15 for the rest of the sample set, see also figure 3). This could mean that the source region is heterogeneous at the scale of sampling (≈100 km between two samples). We also observe that the average MgO% is higher (≈8.1 versus ≈6.9). This is indicative of more primitive melts, and an alternative model to heterogeneities in the source is that we observe un-mixed melting products of an heterogeneous source, as proposed by [START_REF] Langmuir | The geochemistry of oceanic basalts in the vicinity of transform faults: Observations and implications[END_REF], and that northwards, those melts are pooled in magma chambers where they differenciate before eruption.

We calculated the water content of the source by correcting to 8% of MgO [START_REF] Weaver | Calculation of phase equilibrium in mineral-melt systems[END_REF][START_REF] Kelley | Mantle melting as a function of water content beneath back-arc basins[END_REF] and using a simple batch melting model, assuming a constant Na content of the source. The results are illustrated in figure 8, whose caption contains the details of the calculation.

For most (>80%) of the samples, the calculated water content is in the range 150-250 ppm. The obvious limitations of this method are the assumed homogeneity of Na content and the variations of solidus temperature with water content. A posteriori, the influence of the latter is negligible consid-ering the limited range of calculated H 2 O source . The variations observed in water content are thus consistent with a source containing 200±50 ppm of water with an extent of melting between 8 and 10%, followed by various extents of crystal fractionation.

Despite the relative homogeneity of our sample suite, we discuss in the next section the consequence of melting a heterogeneous mantle source in terms of δD-variability.

Hydrogen heterogeneities in the source

As water behaves as a very incompatible element during igneous processes (D peridotite-melt ≈0.012 [START_REF] Aubaud | Hydrogen partition coefficients between nominally anhydrous minerals and basaltic melts[END_REF][START_REF] Kohn | The partitioning of water between nominally anhydrous minerals and silicate melts[END_REF][START_REF] Grant | The partitioning of water between olivine, orthopyroxene and melt synthesised in the system albiteforsterite-H 2 O[END_REF], hydrogen isotopes are not significantly fractionated during melting at ridges (where the extent of melting is typically higher than 5%, [START_REF] Asimow | The importance of water to oceanic mantle melting regimes[END_REF]) and crystal fractionation, even though the isotopic fractionation factor of H between liquid and silicate minerals is higher than 10 at magmatic temperatures [START_REF] Dobson | Hydrogen isotope fractionation between coexisting vapor and silicate glasses and melts at low pressure[END_REF][START_REF] Bell | The isotopic composition of hydrogen in nominally anhydrous mantle minerals[END_REF]. Our data shows that the mantle is not perfectly homogeneous for water concentrations and δD, but north of the Saint-Exupery fracture zone, we mainly observe homogeneous compositions, suggesting that the characteristic size of the heterogeneities is close to the characteristic mixing length along the ridge.

As illustrated by the (La/Sm) N ratio, our samples fall within the field of N-MORB. The gradual increase of 206 Pb/ 204 Pb ratio northwards illustrates the slightly higher amount of HIMU-type component in the source [START_REF] Hamelin | Sr-Nd-Hf isotopes along the Pacific Antarctic Ridge from 41 to 53[END_REF]. Despite the two samples with the lowest 206 Pb/ 204 Pb (PAC1CV04 and PAC1DR12-1) also display low δD, there is no correlation between δD and (La/Sm) N or 206 Pb/ 204 Pb (figure 9), so no enriched source with high (La/Sm) N , δD and H 2 O content can be infered, in contrast with observations near Iceland [START_REF] Poreda | Helium and hydrogen isotopes in ocean-ridge basalts north and south of Iceland[END_REF] or along the Salas y Gomez seamount chain [START_REF] Kingsley | D/H ratios in basalt glasses from the Salas y Gomez mantle plume interacting with the East Pacific Rise: water from old D-rich recycled crust or primordial water from the lower mantle?[END_REF]. This suggests than the heterogeneity in water contents and δD relates to a distinct process, and the lack of relationship with Pb-isotopes (seen as a tracer for a HIMU-type component)

precludes the use of the δD variability to constrain the D/H ratio of the HIMU component. (Saal et al., 2002, among others). These last two characteristics have been interpreted as resulting from a prior melting event of the source [START_REF] Perfit | Recent volcanism in the Siqueiros transform fault: picritic basalts and implications for MORB magma genesis[END_REF][START_REF] Danyushevsky | H 2 O abundance in depleted to moderately enriched mid-ocean ridge magmas; part I: Incompatible behaviour, implications for mantle storage, and origin of regional variations[END_REF][START_REF] Cartigny | Towards a consistent mantle carbon flux estimate: Insights from volatile systematics (H 2 O/Ce, δD, CO 2 /Nb) in the North Atlantic mantle (14 • N and 34 • N)[END_REF], and their δD would suggest that the hydrogen left in the residue after partial melting is depleted in D. However, the apparent correlation is not significant if those two samples are not taken into account, as might be expected if the high TiO 2 /H 2 O ratios reflect a specific process.

Despite having little evidence for significant variability in major and trace elements, we investigate the possibility that the ranges in water content and δD observed reflect a heterogeneous source, containing two distinct components. Without making any a priori hypothesis on the characteristic size or shape of the heterogeneities, we expect that for higher spreading rates, the more abundant production of basaltic liquids and the existence of more permanent magma chambers will lead to more homogeneous lavas erupted on the sea floor. If different water contents are assumed, the most water-rich component would be more fusible, and for low amounts of melting a second episode of melting would be necessary to express the characteristic water content and δD of the more refractory component. Samples PAC1CV04, CV06 and CV07 present low δD and low calculated water in the source, so this explanation looks promising, but we would expect samples with melting rate scaling to water content of the source. Yet the calculated melting rates range for a homogeneous mantle (with respect to major elements) is limited, only from 8 to 11% (figure 8), and previous studies do not suggest a major element concentration contrast in the southernmost section of the sampling area.

The size of possible heterogeneities is limited by the diffusion rate of hydrogen in the conditions of the upper mantle. For the fast axis of olivines at mantle temperatures, it is of the order of 10 -8 m 2 .s -1 [START_REF] Hercule | Hydrogen in diopside; diffusion, kinetics of extraction-incorporation, and solubility[END_REF][START_REF] Ingrin | Diffusion of hydrogen in minerals[END_REF]. For residence times of 10 8 years, heterogeneities must be larger than 5 km, three orders of magnitude greater than the heterogeneities limit for trace elements (about 2 m, [START_REF] Allègre | Implications of a two-component marble-cake mantle[END_REF]). This 5 km figure is smaller than the scale of sampling, although fracture zones can also play the role of barriers between different mantle domains.

So far, the lack of relationship between H 2 O/Ce, δD and Pb isotopes suggest that for this sample suite, the variability observed cannot be accounted for by the presence of an uniquely defined component richer in water and/or D and with a distinct 206 Pb/ 204 Pb.

A reference zone for the study of the N-MORB source

Previous studies on the δD-values of N-MORB have mainly focused on samples from the north Atlantic and Pacific oceans [START_REF] Kyser | Hydrogen isotope systematics of submarine basalts[END_REF][START_REF] Poreda | Helium and hydrogen isotopes in ocean-ridge basalts north and south of Iceland[END_REF][START_REF] Pineau | Strong degassing at ridge crests: The behaviour of dissolved carbon and water in basalt glasses at 14 • N, Mid-Atlantic ridge[END_REF][START_REF] Pineau | Deepsea explosive activity on the Mid-Atlantic ridge near 34 • 50' N: a stable isotope (C, H, O) study[END_REF]Hauri et al., 2006b;[START_REF] Cartigny | Towards a consistent mantle carbon flux estimate: Insights from volatile systematics (H 2 O/Ce, δD, CO 2 /Nb) in the North Atlantic mantle (14 • N and 34 • N)[END_REF]. These studies concluded that the uncontam- N-MORB from the North Atlantic measured with the same protocol as in this study [START_REF] Clog | Experimental evidence for interaction of water vapor and platinum crucibles at high temperatures: Implications for volatiles from igneous rocks and minerals[END_REF] have δD of ≈-60 and do not outline the existence of a distinct δD between the Pacific-Antarctic and the North Pacific or Atlantic mantles. Nonetheless, the dataset is still restricted and we are going to evaluate whether this part of the upper depleted mantle can plausibly have a distinct δD.

Geodynamic model

There are two ways to consider mantle water degassing flux at ridges.

The first is that we observe degassing of some primordial water from the upper mantle (as for [START_REF] Craig | Primordial neon, helium, and hydrogen in oceanic basalts[END_REF]). In this case the apparent contrast with the North Atlantic and North Pacific could be inherited from the processes of crust extraction. This proposition is however unlikely. First, the South Pacific, the North Atlantic and the North Pacific mantle domains have very similar e.g. Sr-isotopes compositions, suggesting an overall similar history and homogeneity resulting from mantle convection (van Keken et al., 2002;Hamelin et al., 2011, among others). Second, water is recycled in the mantle at subduction zones, which is likely to erase any primitive signature. This is illustrated by the high H 2 O/Ce ratios in the North Atlantic due to recent rapid subduction [START_REF] Michael | Regionally distinctive sources of depleted MORB: evidence from trace elements and H 2 O[END_REF] or by the distribution of δD observed in the Manus basin glasses [START_REF] Shaw | Long-term preservation of slab signatures in the mantle inferred from hydrogen isotopes[END_REF]. It is thus unlikely that the southern Pacific mantle present a strong, inherited, δD contrast compared to the rest of the upper mantle.

The alternative, generally accepted model considers the hydrogen cycle as in equilibrium chemically and isotopically (steady state), with water recycled to the convecting mantle at subduction zones [START_REF] Ito | Alteration of oceanic crust and geologic cycling of chlorine and water[END_REF][START_REF] Bebout | The impact of subduction-zone metamorphism on mantle-ocean chemical cycling[END_REF][START_REF] Williams | Hydrogen in the deep earth[END_REF][START_REF] Dixon | Recycled dehydrated lithosphere observed in plume-influenced mid-ocean-ridge basalt[END_REF]. This is supported by the stable continental freeboard in the last half-billion years and analysis of old hydrated rocks [START_REF] Galer | Interrelationships between continental freeboard, tectonics and mantle temperature[END_REF][START_REF] Lécuyer | The hydrogen isotope composition of seawater and the global water cycle[END_REF]. Recent estimates range from 0.2 to 2 ocean masses contained in the mantle, (between 0.3 et 2.8

10 24 g Bolfan- [START_REF] Bolfan-Casanova | Water partitioning between nominally anhydrous minerals in the MgO-SiO 2 -H 2 O system up to 24 GPa: implications for the distribution of water in the earth's mantle[END_REF][START_REF] Bolfan-Casanova | Water in the earth's mantle[END_REF][START_REF] Javoy | Where do the oceans come from?[END_REF][START_REF] Keppler | Thermodynamics of water solubility and partitioning[END_REF][START_REF] Hirschmann | Water, melting, and the deep earth H 2 O cycle[END_REF], with half in the transition zone and very little in the lower mantle (less than 5% of the total water, although this number depends on the oxygen fugacity of the lower mantle). The large range in mantle water budget reflects that several hypotheses are considered.

With the current ridge and hotspot water output (10 14 g/an), the residence time is thus between 3 and 28 Gyr, but fluxes were probably greater during the Hadean and Archean and the equilibrium may have been reached early in Earth history. In this model, changing the composition of the upper mantle is possible by changing how recycling occurs, i.e., how much water is devolatilized, which P-T path the subducting slab follows, or depending on the thickness and extent of serpentinization, for the mantle domains of the South Pacific and North Atlantic.

It should be noted that we have few direct constraints on the inward flux of water in the mantle, and especially on its isotopic composition. The subducting slab must loose more than 80% of its water [START_REF] Nadeau | Fluid inclusion and mineral isotopic compositions (H-C-O) in eclogitic rocks as tracers of local fluid mi-gration during high-pressure metamorphism[END_REF][START_REF] Chaussidon | Boron content and isotopic composition of oceanic basalts: Geochemical and cosmochemical implications[END_REF][START_REF] Dixon | Recycled dehydrated lithosphere observed in plume-influenced mid-ocean-ridge basalt[END_REF] to the arc system, and for the Pacific ocean, flux towards the mantle wedges is not relevant as the subduction zones are directed outwards. The water content of the hydrated oceanic crust is between 0.5 and 5% with a δD between -30 and -50 [START_REF] Kawahata | Strontium, oxygen, and hydrogen isotope geochemistry of hydrothermally altered and weathered rocks in DSDP hole 504B, Costa Rica rift[END_REF][START_REF] Kusakabe | Petrology and isotope characteristics (H, o, s, sr, and nd) of basalts from Ocean Drilling Program hole 504B, leg 111, Costa Rica Rift[END_REF][START_REF] Agrinier | O and H stable isotope compositions of oceanic crust and upper mantle rocks exposed in the Hess Deep near the Galapagos Triple Junction[END_REF][START_REF] Shilobreeva | Insights into c and h storage in the altered oceanic crust: Results from odp/iodp hole 1256d[END_REF], but the sampling is very sparse, especially with respect to depth, and the hydration of the lower oceanic lithosphere is hard to constrain [START_REF] Rüpke | Serpentine and the subduction zone water cycle[END_REF]. To reach isotopic equilibrium of the water cycle, the final average δD of slabs after dehydration should be equal to the average δD of the ridges flux (e.g. -60 as measured in this study), but studies have given possible ranges from -30 to -200 for effectively recycled hydrogen [START_REF] Kingsley | D/H ratios in basalt glasses from the Salas y Gomez mantle plume interacting with the East Pacific Rise: water from old D-rich recycled crust or primordial water from the lower mantle?[END_REF][START_REF] Shaw | Hydrogen isotopes in Mariana arc melt inclusions: Implications for subduction dehydration and the deep-Earth water cycle[END_REF][START_REF] Shaw | Long-term preservation of slab signatures in the mantle inferred from hydrogen isotopes[END_REF].

Changing the δD of the slab

The first obvious answer to obtain a higher δD is to increase the δD of the recycled crust before dehydration, either with a different δD of the ocean or by changing the average temperature of alteration. Both are extremely unlikely for the following reasons.

The first is at odds with results on Phanerozoic and Proterozoic ophiolites suggesting little variability of oceans δD values [START_REF] Lécuyer | The hydrogen isotope composition of seawater and the global water cycle[END_REF]. It would also require, for a difference between the north and south Pacific to be apparent today, that the recycling timescale differs markedly between the different mantle domains.

The second would require a hydrothermal alteration temperature several hundreds of degrees higher (Suzuoki and Epstein, 1976) than the one which produces the altered crust studied by [START_REF] Kawahata | Strontium, oxygen, and hydrogen isotope geochemistry of hydrothermally altered and weathered rocks in DSDP hole 504B, Costa Rica rift[END_REF]; [START_REF] Kusakabe | Petrology and isotope characteristics (H, o, s, sr, and nd) of basalts from Ocean Drilling Program hole 504B, leg 111, Costa Rica Rift[END_REF]; [START_REF] Agrinier | O and H stable isotope compositions of oceanic crust and upper mantle rocks exposed in the Hess Deep near the Galapagos Triple Junction[END_REF]. There is at present no compelling argument suggesting that oceanic basalts could have been altered at such higher temperatures. Moreover, given that present hydration of the ocean crust depends upon spreading rate, it is worth noting that there is again no evidence for significant change in spreading rate at least since 180 Ma [START_REF] Cogné | Temporal variation of oceanic spreading and crustal production rates during the last 180 my[END_REF].

Subduction processes

The H 2 O/Ce ratios on the two supersegments studied here (186±30 and 173±15) are similar to those measured in the rest of the Pacific ridges system (between 150 and 220, [START_REF] Michael | Regionally distinctive sources of depleted MORB: evidence from trace elements and H 2 O[END_REF]; Le Roux et al. ( 2006)). A smaller extent of dehydration during subduction is therefore not a possible explanation for the apparent North Pacific/South Pacific δD contrast. Nevertheless, the exact P-T path followed by the slab depends on the dipping angle of the subduction zone [START_REF] Hager | Subduction zone dip angles and flow driven by plate motion[END_REF][START_REF] Cahill | Seismicity and shape of the subducted Nazca plate[END_REF].

This could be critical for the final δD of the slab, as destabilisation of the water-bearing minerals will occur at different pressures and temperatures [START_REF] Schmidt | Experimentally based water budgets for dehydrating slabs and consequences for arc magma generation[END_REF][START_REF] Kawamoto | Hydrous phases and water transport in the subducting slab[END_REF][START_REF] Smyth | Hydrogen in high pressure silicate and oxide mineral structures[END_REF]. Water solubility in nomimally anhydrous minerals depends on the pressure, so even for the same amount of dehydration [START_REF] Kohlstedt | Solubility of water in the α ,β and γ phases of (Mg,Fe) 2 SiO 4[END_REF][START_REF] Mierdel | Water solubility in aluminous orthopyroxene and the origin of earth's asthenosphere[END_REF], the final δD could differ. Indeed, even at 900 • C, fractionation of hydrogen isotopes between minerals is significant (up to 10 : Suzuoki and Epstein, 1976; [START_REF] Graham | Experimental hydrogen isotope studies: hydrogen isotope exchange between amphibole and water[END_REF][START_REF] Dobson | Hydrogen isotope fractionation between coexisting vapor and silicate glasses and melts at low pressure[END_REF]Vennemann and O'Neil, 1996).

This type of scenario would also create differences for other fluid-mobile elements like Li and B, and could explain the small differences between the two supersegments (δD ≈ -63 in the south and δD ≈ -58 in the north).

Yet this scenario can hardly lead to the 20 contrast between our results and those of [START_REF] Kyser | Hydrogen isotope systematics of submarine basalts[END_REF]. , 1976). This explanation is therefore not satisfactory with the current view of P-T paths and and dehydration of recycled ocean crust.

Revising the δD of the upper depleted mantle

We have shown that previous measurements could suffer from an experimental bias [START_REF] Clog | Experimental evidence for interaction of water vapor and platinum crucibles at high temperatures: Implications for volatiles from igneous rocks and minerals[END_REF], and have not been able to find a satisfactory scenario explaining why the south Pacific would be enriched in D compared to previous data and other locations while being otherwise a perfect example of the upper depleted mantle, unaffected by mantle plumes. Our view is therefore that slight variations in δD (and H 2 O/Ce) can be produced by dehydration and recycling of water, but that the large contrast with previous data reflects analytical artifacts.

This sample suite presents many advantages to characterize the isotopic composition of the depleted mantle. Previous researchers have shown that the source composition is typical of the depleted mantle with limited and gradual variations in compositions in the area. Most of the variations are due to igneous processes, and the variation in spreading rate and ridge structure strengthen the conclusion that contamination is not a major obstacle to the determination of hydrogen isotopic composition of the source of N-MORB.

Finally, the high number of samples (40 in this area compared to less than 70 N-MORB measured in a variety of locations in the literature, [START_REF] Craig | Primordial neon, helium, and hydrogen in oceanic basalts[END_REF]; [START_REF] Satake | Strontium and hydrogen isotope geochemistry of fresh and metabasalt dredged from the Mid-Atlantic ridge[END_REF]; [START_REF] Kyser | Hydrogen isotope systematics of submarine basalts[END_REF]; [START_REF] Poreda | Helium and hydrogen isotopes in ocean-ridge basalts north and south of Iceland[END_REF]; [START_REF] Chaussidon | Hydrogen, sulphur and neodymium isotope variations in the mantle beneath the EPR at 12 • 50'N. In: Stable isotope geochemistry: a tribute to Samuel Epstein[END_REF]; [START_REF] Pineau | Strong degassing at ridge crests: The behaviour of dissolved carbon and water in basalt glasses at 14 • N, Mid-Atlantic ridge[END_REF]; [START_REF] Kingsley | D/H ratios in basalt glasses from the Salas y Gomez mantle plume interacting with the East Pacific Rise: water from old D-rich recycled crust or primordial water from the lower mantle?[END_REF]; [START_REF] Pineau | Deepsea explosive activity on the Mid-Atlantic ridge near 34 • 50' N: a stable isotope (C, H, O) study[END_REF]; [START_REF] Cartigny | Towards a consistent mantle carbon flux estimate: Insights from volatile systematics (H 2 O/Ce, δD, CO 2 /Nb) in the North Atlantic mantle (14 • N and 34 • N)[END_REF]) limits the possibility of sample bias when calculating averages.

On the basis of this study, we propose that the average δD of the upper depleted mantle is rather -60±5 . Although the same average value and a similar range were observed on 8 N-MORB from the Middle Atlantic Ridge [START_REF] Clog | Experimental evidence for interaction of water vapor and platinum crucibles at high temperatures: Implications for volatiles from igneous rocks and minerals[END_REF], further studies on other ridges are necessary to confirm this figure. Variations observed in isotopic composition and water content shows that the upper mantle is not homogeneous, but present both small scales heterogeneities which are averaged by melts pooling when the spreading rate is higher than 7 cm/year, and zonations at a larger scale leading to different average values on the northern and southern supersegments, illustrating the distribution of heterogeneities in the convecting mantle.

Conclusions

Despite widespread Cl contamination, assimilation of seawater-derived fluids or interaction with hydrated rocks did not significantly affect concentrations or isotopic compositions of hydrogen for our suite of samples. An et al., 1999). However the over-enrichment happens for both H 2 O and Ce, as illustrated here and by the constant H 2 O/Ce ratios, and is thus not due to an increase in assimilation with increasing differentiation. [START_REF] Klein | Global correlations of ocean ridge basalt chemistry with axial depth and crustal thickness[END_REF][START_REF] Kelley | Mantle melting as a function of water content beneath back-arc basins[END_REF]. We assumed incompatibilities of 0.012 for water [START_REF] Aubaud | Hydrogen partition coefficients between nominally anhydrous minerals and basaltic melts[END_REF][START_REF] Kohn | The partitioning of water between nominally anhydrous minerals and silicate melts[END_REF][START_REF] Grant | The partitioning of water between olivine, orthopyroxene and melt synthesised in the system albiteforsterite-H 2 O[END_REF] and 0.02 for sodium [START_REF] Kelley | Mantle melting as a function of water content beneath back-arc basins[END_REF].

  inated N-MORB source had a δD of -80±10 (even if the average value measured in those studies is closer to -70 ). Most of those studies also used an extraction technique (induction-heating in Pt crucibles) which can cause biases of more than 10 ([START_REF] Clog | Experimental evidence for interaction of water vapor and platinum crucibles at high temperatures: Implications for volatiles from igneous rocks and minerals[END_REF]. The recent study of[START_REF] Bindeman | Hydrogen and oxygen isotope behaviors during variable degrees of upper mantle melting: Example from the basaltic glasses from macquarie island[END_REF], which does not use crucibles however, found the δD of Macquarie Island basalts to be close to -75±5 . Although falling within the canonical mantle range, it must be noted that those samples are very enriched compared to N-MORB (La/Sm N >1.3 for the most depleted one) and are therefore not representative of the depleted mantle.

  If we model the slab dehydration as an open-system loss of water, the average fractionation of water would need to differ by 10 . Experiments on fractionation factors between water and hydrated minerals indicate that it corresponds to a ≈150 • C change in the range 500-900 • C (Suzuoki and Epstein

  increase in Cl contamination with spreading rate was observed. All samples are N-MORB with no evidence of influence from hotspots. Samples were undersaturated with respect to H 2 O, and variations in concentrations of water are mainly due to igneous processes. The water content in the source ranges from 150 to 250 ppm. The average δD are -63 and -58 and the water to cerium ratios of 186±30 and 173±15 for the southern and northern supersegments respectively. This apparent enrichment compared to previous studies (δD≈-70 ) is not due to difference during the recycling of water. It is rather explained by a combination of a potential experimental bias and different interpretation on the influence of contamination. The δD of the upper depleted mantle is close to -60±5 , with variations in δD and H 2 O/Ce at different length scales resulting from recycling and ridge processes. from the Endeavour segment of the Juan de Fuca ridge. Contributions to mineralogy and petrology 134 (4), 342-363. Suzuoki, T., Epstein, S., 1976. Hydrogen isotope fractionation between OHbearing minerals and water. Geochimica et Cosmochimica Acta 40 (10), 1229-1240. van Keken, P., Hauri, E., Ballentine, C., 2002. Mantle mixing: the generation, preservation, and destruction of chemical heterogeneity. Annual Review of Earth and Planetary Sciences 30 (1), 493-525. Vennemann, T., O'Neil, J., 1993. A simple and inexpensive method of hydrogen isotope and water analyses of minerals and rocks based on zinc reagent. Chemical Geology 103 (1-4), 227 -234. Vennemann, T., O'Neil, J. R., 1996. Hydrogen isotope exchange reactions between hydrous minerals and molecular hydrogen: I. a new approach for the determination of hydrogen isotope fractionation at moderate temperatures. Geochimica et Cosmochimica Acta 60 (13), 2437-2451. Vlastélic, I., Aslanian, D., Dosso, L., Bougault, H., Olivet, J. L., Geli, L., 1999. Large-scale chemical and thermal division of the Pacific mantle. Nature 399 (6734), 345-350. Vlastélic, I., Dosso, L., Bougault, H., Aslanian, D., Géli, L., Etoubleau, J., Bohn, M., Joron, J. L., Bollinger, C., 2000. Chemical systematics of an intermediate spreading ridge: the Pacific-Antarctic ridge between 56 degrees s and 66 degrees s. J. Geophys. Res. Solid Earth 105, 2915-2936.

Figure 1 .

 1 Figure 1. Map of the sampling area, illustrating the position of the samples with respect to the local geographical and geological features. The symbols representing the samples will remain the same in the whole article. The figure was created with the GeoMapApp application and the depths databases included therein.

Figure 2 .

 2 Figure 2. δD (in , top panel) and water content (in ppm, bottom panel) along the sampling area. Error bars are smaller than the symbols for water content.

Figure 3 .

 3 Figure 3. From top to bottom : H 2 O/Ce, 206 Pb/ 204 Pb, Cl content and Cl/K along the sampling area. Except south of Saint-Exupery fracture zone, all H 2 O/Ce ratio are in the range of North Pacific basalts.

Figure 4 .

 4 Figure 4. Water content as a function of eruption pressure and curves illustrating the composition of the vapor in equilibrium with the magma for a given water content and pressure. This figure illustrate that all but sample PAC1DR10-1 were not significantly affected by degassing.

Figure 5 .

 5 Figure 5. Incompatible species content as a function of MgO wt%. From top to bottom, Ce, La, H 2 O and Cl. Continuous lines are best linear fits, while dotted lines illustrate the differentiation trend predicted for a perfectly incompatible species. Note that we observe over-enrichement for H 2 O, La and Ce, as commonly observed in numerous sample suites (e.g. Sours-Page

Figure 6 .

 6 Figure 6. Cl/H 2 O versus Ce/H 2 O and curves resulting from the addition

Figure 7 .

 7 Figure 7. Sodium content as a function of water content, both corrected to 8 wt.% MgO. Continuous lines represent constant water content in the source, and dotted lines correspond to constant melting rate. Both where calculated using a simple batch melting model with 0.29 wt.% of Na in the source (Klein

Figure 8 .

 8 Figure 8. Ratios of water to Ce (top) and La (bottom) as function of Ce and La contents respectively. Straight lines are best linear fits and dotted lines represent the 95% confidence envelope.

Figure 9 .

 9 Figure 9. From top to bottom, δD as a function of La/Sm N , 206 Pb/ 204 Pb and TiO 2 /H 2 O. There are no correlation in the top two panels, while the negative correlation in the bottom panel is controlled by the two samples PAC1CV04 and PAC1CV07 (which may reflect source re-melting, see text).

  Figure 1:

  Figure 4:

  • C for 45 minutes under a pressure of 400 Pa of pure O 2 obtained by thermal decomposition of CuO. The released CO 2 and H 2 O (and other condensable gases such as SO 2 ) are held in a cold trap at -196 • C (liquid nitrogen) and separated by setting the temperature at -140 • C to keep only H 2 O, while incondensable gases and CO 2 are pumped out of the line. Water is then reduced on hot uranium fur-nace held at 800 • C to obtain H 2

  , based on a global compilation of Cl and K concentrations, has proposed that the average Cl/K of uncontaminated MORB is close to 0.08. When comparing to modes of 0.16 and 0.32 on respectively the southern and northern supersegments, this would indicate that our samples are extensively contaminated with respect to Cl.

Conversely, the lowest Cl/K measured on our samples are, for both segments, ≈0.03, illustrating the non-uniformity of the contamination. This would imply, for chlorine budget, that the details of the ascent and eruption scenarii are at least of equal importance to the spreading rate. Lowest Cl/K were observed in both cases where the magmatic activity is less vigorous : a near-axis seamount in the south and a sample from the S2 segment, whose structure is described as poorly defined

[START_REF] Hamelin | Sr-Nd-Hf isotopes along the Pacific Antarctic Ridge from 41 to 53[END_REF]

due to interaction with the Menard Fracture Zone in the north. Assuming that the smallest Cl/K measured correspond to uncontaminated samples, we conclude that more than 80% of Cl comes from contamination in our sample suites (versus more than 50% of Cl from contamination if we use the average value of

Kendrick et al.

Table 1 :

 1 Previous data for the southern supersegment : major elements(Vlastélic et al., 2000) and segment number.

		SiO 2	Al 2 O 3	FeO	MnO	MgO	CaO	Na 2 O	K 2 O	TiO 2	Total	Segment
	PAC1 CV1	49.78	15.70	9.79	0.18	8.45	11.71	2.49	0.04	1.10	99.24
	PAC1 CV2	50.78	14.37	10.49	0.19	7.74	11.25	2.55	0.06	1.67	99.10
	PAC1 CV3	50.70	15.53	8.60	0.16	8.57	11.91	2.53	0.08	1.20	99.28
	PAC1 CV4	50.30	15.46	8.55	0.16	8.78	12.32	2.46	0.04	1.05	99.12
	PAC1 CV6	50.44	15.37	9.31	0.17	8.51	12.01	2.50	0.04	1.23	99.58
	PAC1 CV7	51.06	14.50	9.80	0.19	7.88	12.03	2.71	0.04	1.31	99.52
	PAC1 DR3	51.18	14.51	9.88	0.18	7.30	11.21	3.00	0.15	1.59	99.00
	PAC1 DR4	49.13	15.71	10.08	0.18	7.25	11.83	2.87	0.12	1.35	98.52
	PAC1 DR5-1	51.00	15.45	8.42	0.15	8.33	12.01	2.82	0.09	1.19	99.46
	PAC1 DR6	50.99	13.99	11.20	0.20	6.93	10.58	2.86	0.12	2.02	98.89
	PAC1 DR7-1	50.75	14.43	10.70	0.22	6.83	10.58	3.03	0.19	1.94	98.67
	PAC1 DR7-2	50.83	14.43	10.52	0.18	7.35	11.29	2.91	0.12	1.69	99.32
	PAC1 DR9	50.75	13.42	12.15	0.22	6.36	10.44	3.09	0.12	2.20	98.75
	PAC1 DR10-1	53.79	14.10	10.77	0.21	4.83	8.59	3.43	0.48	1.73	97.93
	PAC1 DR10-2	51.46	14.10	10.87	0.20	6.22	10.36	3.37	0.25	1.75	98.58
	PAC1 DR11-1	50.21	14.32	9.25	0.17	7.33	11.34	2.70	0.06	1.31	96.69
	PAC1 DR12-1	50.94	15.62	8.49	0.15	8.85	12.31	2.32	0.03	1.03	99.74
	PAC1 DR13-1	50.37	14.60	10.89	0.18	7.38	10.76	2.82	0.09	1.64	98.73
	PAC1 DR13-2	50.50	14.52	11.01	0.19	7.42	10.73	2.85	0.09	1.65	98.96

Table 2 :

 2 Previous data for the northern supersegment : major elements[START_REF] Hamelin | Sr-Nd-Hf isotopes along the Pacific Antarctic Ridge from 41 to 53[END_REF] and segment names.

		SiO 2	Al 2 O 3	FeO	MnO	MgO	CaO	Na 2 O	K 2 O	TiO 2	Total	Segment
	PAC2 DR 1-1	50.20	14.60	10.75	0.18	7.30	11.90	2.82	0.20	1.58	99.53	S1
	PAC2 DR3-1											S1
	PAC2 DR4-2	50.40	14.28	11.70	0.20	7.08	11.20	2.83	0.12	1.79	99.60	S1
	PAC2 DR5-2	49.90	13.65	13.45	0.22	6.52	10.40	2.74	0.18	2.13	99.19	S2
	PAC2 DR6-6	51.30	12.56	15.70	0.26	4.34	8.66	3.46	0.31	2.88	99.47	S2
	PAC2 DR7-2											S3
	PAC2 DR8-1	49.40	14.40	12.65	0.20	7.05	11.70	2.87	0.20	1.66	100.13	S3
	PAC2 DR20-1	51.15	13.90	14.20	0.24	4.60	9.35	3.34	0.25	2.40	99.43	N1
	PAC2 DR21-2	50.30	13.88	12.50	0.21	6.73	11.10	2.79	0.15	2.00	99.66	N1
	PAC2 DR22-1	49.90	14.60	11.70	0.20	7.25	11.60	2.78	0.17	1.87	100.07	N1
	PAC2 DR28-2	49.70	15.00	11.15	0.19	7.85	12.10	2.52	0.08	1.39	99.98	N3
	PAC2 DR29-1	49.90	13.37	13.80	0.23	6.16	10.75	2.83	0.19	2.31	99.54	N3
	PAC2 DR30-1	50.00	14.05	12.45	0.21	7.00	11.70	2.64	0.17	1.94	100.16	N3
	PAC2 DR31-3	49.85	14.70	11.70	0.19	7.55	11.45	2.56	0.10	1.74	99.83	N3
	PAC2 DR32-1	50.50	14.00	12.60	0.21	6.80	11.10	2.84	0.16	2.02	100.23	N4
	PAC2 DR33-1	50.20	13.90	12.80	0.21	6.63	10.95	2.84	0.15	2.10	99.78	N4
	PAC2 DR34-1	50.00	15.10	10.44	0.18	8.00	12.08	2.60	0.06	1.30	99.76	N4
	PAC2 DR35-1a	49.50	13.15	15.05	0.23	5.65	9.93	3.05	0.17	2.93	99.66	N5
	PAC2 DR36-1	49.60	14.25	12.50	0.20	7.07	11.30	2.80	0.15	2.09	99.96	N5
	PAC2 DR37-2											N5
	PAC2 DR38-1	49.10	15.7	10.80	0.18	8.35	11.70	2.67	0.09	1.42	99.72	N6

Table 4 :

 4 Data table for the northern supersegment. K 2 O, La, Sm and Ce from Hamelin et al. (2010); H 2 O. Cl and δD from this study.

	Long.	Lat.	Depth	H 2 O	δD	Cl	Ce	H 2 O/Ce	(La/Sm) N	Cl/K
	( • W)	( • S)	(m)	(ppm)	( )	(ppm)	(ppm)

ACKNOWLEDGMENTS

The LSCE provided the water standards used for this study. We wish to thank Michel Fialin for his assitance in performing the chlorine content measurements, Manuel Moreira for access to the PACANTARCTIC2 samples and Cédric Hamelin for early access to geochemical data on those samples.

We acknowledge support from the CNRS via its SEDIT research program. This is IPGP contribution number XX-XXXX.