Christèle Vacquand

Eric Deville
email: eric.deville@ifpen.fr

Valérie Beaumont

François Guyot

Olivier Sissmann

Daniel Pillot

Carlo Arcilla

Alain Prinzhofer

Reduced gas seepages in ophiolitic complexes: Evidences for multiple origins of the H 2 -CH 4 -N 2 gas mixtures

Keywords: ophiolite, serpentinization, hydrogen, abiotic methane, deep nitrogen

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Introduction

Serpentinization generates natural emission of H 2 on Earth. The process is inherent to exposure of reduced mantle rocks to hydration conditions in a wide range of thermal conditions, at least up to the critical temperature of water (374°C). This occurs commonly at mid-oceanic ridges where hydrothermal fluid circulation at high temperature provides conditions for ferromagnesian mineral alteration. Indeed, at mid-oceanic ridges H 2 -rich fluids are associated with N 2 and CH 4 contents and on black smokers with CO 2 [START_REF] Welhan | Methane and hydrogen in East Pacific Rise hydrothermal fluids[END_REF][START_REF] Kelley | Abiogenic methane in deep-seated mid-ocean ridge environments: Insights from stable isotope analyses[END_REF][START_REF] Charlou | Geochemistry of high H 2 and CH 4 vent fluids issuing from ultramafic rocks at the Rainbow hydrothermal field (36°14'N, MAR)[END_REF][START_REF] Kelley | An off-axis hydrothermal field near the Mid-Atlantic Ridge at 30[END_REF][START_REF] Kelley | A serpentinite-hosted ecosystem: the Lost City hydrothermal field[END_REF][START_REF] Gallant | Geochemical controls on hydrothermal fluids from the Kairei and Edmond Vent Fields, 23°-25[END_REF][START_REF] Kumagai | Geological background of the Kairei and Edmond hydrothermal fields along the Central Indian Ridge: Implications of their vent fluids' distinct chemistry[END_REF]and others). Natural H 2 seepages are also reported onshore, in former oceanic rocks present in ophiolite complexes. Notably, H 2 seepages associated with these ultrabasic rocks have been reported in the Sultanate of Oman [START_REF] Neal | Hydrogen generation from mantle source rocks in Oman[END_REF][START_REF] Sano | Origin of hydrogen-nitrogen gas seeps, Oman[END_REF][START_REF] Vacquand | Genèse et mobilité de l'hydrogène naturel: source d'énergie ou vecteur d'énergie stockable ?[END_REF]Boulart et al., 2013;[START_REF] Miller | Modern water/rock reactions in Oman hyperalkaline peridotite aquifers and implications for microbial habitability[END_REF], the Philippines [START_REF] Abrajano | Methane-hydrogen gas seeps, Zambales Ophiolite, Philippines: Deep or shallow origin?[END_REF][START_REF] Abrajano | Geochemistry of reduced gas related to serpentinization of the Zambales ophiolite, Philippines[END_REF] and Turkey [START_REF] De Boer | Homer's chimaera fires (SW of Antalya/Turkey); burning abiogenic methane gases; are they generated by a serpentinization process related to alkalic magmatism[END_REF][START_REF] Hosgörmez | New evidence for a mixed inorganic and organic origin of the Olympic Chimaera fire (Turkey): a large onshore seepage of abiogenic gas[END_REF][START_REF] Etiope | Abiotic methane flux from the Chimaera seep and Tekirova ophiolites ( Turkey ): Understanding gas exhalation from low temperature serpentinization and implications for Mars[END_REF]. These gas seepages are often associated with ultra-basic springs (pH 10-12) that are present in many basic-ultrabasic rock exposures. Such ultra-basic waters have been interpreted as evidences of active serpentinization [START_REF] Barnes | Geochemical evidence of present-day serpentinization[END_REF][START_REF] Barnes | Present day serpentinization in New Caledonia, Oman and Yugoslavia[END_REF][START_REF] Neal | Hydrogen generation from mantle source rocks in Oman[END_REF][START_REF] Abrajano | Methane-hydrogen gas seeps, Zambales Ophiolite, Philippines: Deep or shallow origin?[END_REF][START_REF] Abrajano | Geochemistry of reduced gas related to serpentinization of the Zambales ophiolite, Philippines[END_REF][START_REF] Sano | Origin of hydrogen-nitrogen gas seeps, Oman[END_REF][START_REF] Bruni | Irreversible water-rock mass transfer accompanying the generation of the neutral, Mg-HCO3 and high-pH, Ca-OH spring waters of the Genova province, Italy[END_REF][START_REF] Cipolli | Geochemistry of high-pH waters from serpentinites of the Gruppo di Voltri (Genova, Italy) and reaction path modeling of CO 2 sequestration in serpentinite aquifers[END_REF][START_REF] Deville | Natural flows of H 2 and associated diagenetic processes of atmospheric CO 2 capture and sequestration: a study in the ophiolites of Oman[END_REF][START_REF] Szponar | Geochemistry of a continental site of serpentinization, the Tablelands Ophiolite, Gros Morne National Park: a Mars analogue[END_REF]Etiope et al., 2013;[START_REF] Chavagnac | Characterization of hyperalkaline fluids produced by low-temperature serpentinization of mantle peridotites in the Oman and Ligurian ophiolites[END_REF][START_REF] Cardace | Feasible Metabolisms in High pH Springs of the Philippines[END_REF][START_REF] Meyer-Dombard | High pH microbial ecosystems in a newly discovered, ephemeral, serpentinizing fluid seep at Yanartaş (Chimaera), Turkey[END_REF][START_REF] Woycheese | Out of the dark: Transitional subsurface-to-surface microbial diversity in a terrestrial serpentinizing seep (Manleluag, Pangasinan, the Philippines)[END_REF][START_REF] Deville | The origin of N 2 -H 2 -CH 4 -rich natural gas seepages in ophiolitic context: A major and noble gases study of fluid seepages in New Caledonia[END_REF]. In these cases, H 2 -rich gas is either dissolved or present as free gas bubbling in the ultra-basic water. In other cases, H 2 -rich gas seepages occur in rock fractures and can locally burn spontaneously. This is notably the case of the famous Chimaera near Antalya, place of the first Olympic flame, which are known to be burning since antiquity [START_REF] De Boer | Homer's chimaera fires (SW of Antalya/Turkey); burning abiogenic methane gases; are they generated by a serpentinization process related to alkalic magmatism[END_REF][START_REF] Hosgörmez | Origin of the natural gas seep of Cirali (Chimera), Turkey: Site of the first Olympic fire[END_REF][START_REF] Hosgörmez | New evidence for a mixed inorganic and organic origin of the Olympic Chimaera fire (Turkey): a large onshore seepage of abiogenic gas[END_REF] and the site of "Los Fuegos Eternos" in the Philippines known since the Spanish colonization [START_REF] Abrajano | Methane-hydrogen gas seeps, Zambales Ophiolite, Philippines: Deep or shallow origin?[END_REF][START_REF] Abrajano | Geochemistry of reduced gas related to serpentinization of the Zambales ophiolite, Philippines[END_REF]. H 2 -bearing gases discovered in ophiolitic rocks onshore show lower contents in helium than those discovered in terrestrial context in non-ophiolitic rocks [START_REF] Coveney | Serpentinization and origin of hydrogen gas in Kansas[END_REF][START_REF] Ikorsky | The investigation of gases during the Kola superdeep borehole drilling (to 11.6 km)[END_REF][START_REF] Sherwood-Lollar | Hydrogeologic controls on episodic H 2 release from Precambrian fractured rocks-Energy for deep subsurface life on Earth and Mars[END_REF][START_REF] Larin | Natural molecular hydrogen seepages associated with surficial, rounded depression on the European craton in Russia[END_REF][START_REF] Zgonnik | Evidences for natural hydrogen seepages associated with rounded subsident structures: the Carolina bays (Northern Carolina, USA)[END_REF][START_REF] Guélard | Natural H 2 in Kansas: deep or shallow origin? G 3 Geochemistry[END_REF].

H 2 is generally considered as produced by water reduction that occurs simultaneously to the hydration of ultrabasic or basic rocks, i.e., according to serpentinization reactions. In the case of ophiolitic settings, it is generally supposed to happen at low temperature [START_REF] Moody | Serpentinization: a review[END_REF][START_REF] Neal | Hydrogen generation from mantle source rocks in Oman[END_REF][START_REF] Abrajano | Methane-hydrogen gas seeps, Zambales Ophiolite, Philippines: Deep or shallow origin?[END_REF][START_REF] Etiope | Abiotic methane flux from the Chimaera seep and Tekirova ophiolites ( Turkey ): Understanding gas exhalation from low temperature serpentinization and implications for Mars[END_REF]. The process driving this reduction is the change of valence of transition metals, notably ferrous iron Fe II into ferric iron Fe III , iron being the most abundant transition metal in ultrabasic and basic rocks. Fe II is a major component in ultrabasic rocks in the ophiolitic units within minerals such as olivine and pyroxene. It contributes to the formation of Fe III -bearing minerals, such as magnetite. These mineral assemblages are observed in the field, where peridotites are largely serpentinized, mostly along fracture systems. The question of H 2 provenance in oceanic hydrothermal context has been largely approached by petrographic studies and experimental simulations (Allen and Seyfried, 2003;[START_REF] Yoshizaki | H 2 generation by experimental hydrothermal alteration of komatiitic glass at 300°C and 500 bars: A preliminary result from on-going experiment[END_REF][START_REF] Klein | Iron partitioning and hydrogen generation during serpentinization of abyssal peridotites from 15°N on the Mid-Atlantic Ridge[END_REF][START_REF] Marcaillou | Mineralogical evidence for H 2 degassing during serpentinization at 300°C/300bar[END_REF][START_REF] Neubeck | Formation of H 2 and CH 4 by weathering of olivine at temperatures between 30 and 70°C[END_REF][START_REF] Shibuya | Hydrogen-rich hydrothermal environments in the Hadean ocean inferred from serpentinization of komatiites at 300 °C and 500 bar[END_REF] and numerical modelling [START_REF] Mccollom | Thermodynamic constraints on hydrogen generation during serpentinization of ultramafic rocks[END_REF][START_REF] Klein | Iron partitioning and hydrogen generation during serpentinization of abyssal peridotites from 15°N on the Mid-Atlantic Ridge[END_REF][START_REF] Marcaillou | Mineralogical evidence for H 2 degassing during serpentinization at 300°C/300bar[END_REF]. Fe II is initially provided by different minerals (such olivine and pyroxene) in the first stages of hydration, but also by serpentine or ferrous iron hydroxide, which are produced by the alteration processes, thus allowing the H 2 production to go on after olivine and pyroxene have all been transformed [START_REF] Marcaillou | Mineralogical evidence for H 2 degassing during serpentinization at 300°C/300bar[END_REF]. These studies have shown that depending on thermodynamic conditions, temperature, water chemistry (notably carbonate content, sulphate content and alkalinity), water/rock ratios, different mineral assemblages and sequences are obtained as by-products together with different rates and amounts of H 2 . According to the numerical model proposed by [START_REF] Mccollom | Thermodynamic constraints on hydrogen generation during serpentinization of ultramafic rocks[END_REF], the highest rates of H 2 production are obtained at high temperature (about 315°C) while serpentine and magnetite are the main mineral assemblage of the by-products.

Onshore H 2 -bearing gas seepages yield not only H 2 but also methane and nitrogen, resulting in a three-component gas mixture with proportions varying in a very large range. The origin and the processes of generation of CH 4 and N 2 associated to H 2 are still a matter of discussion in terrestrial ophiolitic units. In the present paper, the geochemical properties of reduced gas seepages from four ophiolite massifs: 1) the Semail ophiolite in the Sultanate of Oman, 2) the Zambales ophiolite in the Philippines, 3) the Antalya ophiolite in southern Turkey and 4) the New Caledonia ophiolite are compared in the light of local geological features to better understand the conditions for di-hydrogen, methane and di-nitrogen production in the geological context of these ophiolite complexes (Fig. 1,2,3).

Geological settings

The ophiolitic units in which this study was conducted correspond to wide ophiolitic massifs (Fig. 1) containing rocks which have not been subjected to high pressure-low temperature (HP-LT) conditions. Peridotites rocks are little serpentinized in wide areas (much less than ophiolites involved in HP-LT conditions). These ophiolites units have been obducted on sediments (Fig. 2) belonging to different paleogeographic domains, either oceanic or continental, which contain, in all cases, clastics and carbonates. The fractures in the peridotite host serpentine and carbonate veins, mainly Mg-bearing carbonates (magnesite, hydromagnesite and dolomite). In the ophiolite-hosted fractured aquifers, groundwater circulations is controlled by fracture hydraulic conductivity. Ultra-basic surface seepages have been found in these ophiolitic massifs and they generally discharge close to the contact between the mantle rocks and overlying former oceanic crust rocks (Moho), and along the basal thrust plane of the ophiolite sequence. This is probably related to favorable drainage conditions in the fractured rocks at the base of the ophiolitic units and to drainage control by the more massive gabbroic units compared to the more fractured peridotites below. They release high pH fluids (commonly higher than 10) which are rich in Ca 2+ , OH -, H 2 and CH 4 .

More specifically, the Oman ophiolite has been extensively studied, as it is the best large exposed massif of this type in the world (see [START_REF] Hopson | Geologic Section Through the Samail Ophiolite and Associated Rocks Along A Muscat-Ibra Transect, Southeastern Oman Mountains[END_REF][START_REF] Ceuleneer | Evidence for a paleo-spreading center in the Oman ophiolite: mantle structures in the Maqsad area[END_REF][START_REF] Nicolas | Variable crustal thickness in the Oman ophiolite: Implication for oceanic crust[END_REF][START_REF] Boudier | Accretion of Oman and United Arab Emirates ophiolite -Discussion of a new structural map[END_REF][START_REF] Python | Nature and distribution of dykes and related melt migration structures in the mantle section of the Oman ophiolite[END_REF]Arai et al., 2006 and many others). This ophiolite corresponds to parts of the oceanic lithosphere of the Arabian Sea which have been obducted on the Arabian plate during late Cretaceous times. H 2 -rich gas seepages associated with Ca 2+ -OH --rich groundwater in the Semail ophiolite of Oman were first studied by [START_REF] Neal | Hydrogen generation from mantle source rocks in Oman[END_REF]. They proposed that H 2 is the by-product of a low temperature (20-50°C) serpentinization that depends on Fe II hydroxide availability and oxidation by meteoric water that occurs in 2 stages: 1) oxidation by atmospheric O 2 dissolved in meteoric water; 2) oxidation by water. [START_REF] Sano | Origin of hydrogen-nitrogen gas seeps, Oman[END_REF] provided a new insight by analyzing noble gases. Their results confirmed that H 2 -rich gas samples are inherited from interaction of meteoric water with Fe II although they propose a high temperature for the reaction (300°C).

In the Philippines, gases were sampled in the Zambales massif, which is located in the northwestern part of the Luzon Island (Nicolas and Violette, 1982;[START_REF] Hawkins | Geology of the Zambales Range, Luzon, Philippines: Ophiolite derived from an island arc-back arc pair[END_REF][START_REF] Abrajano | Methane-hydrogen gas seeps, Zambales Ophiolite, Philippines: Deep or shallow origin?[END_REF][START_REF] Yumul | Translation and docking of an arc terrane: geological and geochemical evidence from the southern Zambales Ophiolite Complex, Philippines[END_REF][START_REF] Encarnacion | Subduction components and the generation of arc-like melts in the Zambales ophiolite, Philippines: Pb, Sr and Nd isotopic constraints[END_REF]. The gas seepages occur either as bubbling in alkaline springs or seeping out from fractured rocks, locally spontaneously burning as, for instance, in Los Fuegos Eternos and Nagsasa. Gas seepages in the Zambales massif were first studied by [START_REF] Abrajano | Methane-hydrogen gas seeps, Zambales Ophiolite, Philippines: Deep or shallow origin?[END_REF]1990). In their paper, they concluded that both mantle origin and serpentinization are consistent with their analytical results and they proposed a temperature of 110-125°C for serpentinization.

The ophiolite of the Antalya region, in southern Turkey, has been extensively studied [START_REF] Juteau | Structural Relationships in Antalya Ophiolite Complex, Turkey -Possible Model for An Oceanic Ridge[END_REF][START_REF] Robertson | Strike slip related sedimentation in the Antalya Complex, SW Turkey[END_REF]Glover and Robertson, 1998a, b). Also, the associated sedimentary sequences have been investigated [START_REF] Bozcu | Correlation of sedimentary units in the Weseter Taurides from the point of petroleum geology[END_REF], while oil seepages have also been observed less than 20 km away from the Chimaera site [START_REF] Hosgörmez | New evidence for a mixed inorganic and organic origin of the Olympic Chimaera fire (Turkey): a large onshore seepage of abiogenic gas[END_REF]. The site of the Chimaera gas seepage has the most spectacular gas outlet since the flames are more intense than those in Zambales, Philippines, and are also more colorful because of a greater content in methane, relatively to the H 2 . H 2 seepages associated with the site of the first Olympic fire was first reported by [START_REF] Hosgörmez | New evidence for a mixed inorganic and organic origin of the Olympic Chimaera fire (Turkey): a large onshore seepage of abiogenic gas[END_REF].

From their chemical and isotopic study, they concluded that gas seepages originate from both thermogenic maturation of organic matter and serpentinization. In a later publication, [START_REF] Etiope | Abiotic methane flux from the Chimaera seep and Tekirova ophiolites ( Turkey ): Understanding gas exhalation from low temperature serpentinization and implications for Mars[END_REF] confirmed previous conclusions and proposed a temperature lower than 100°C for serpentinization.

The ophiolite of New Caledonia is among the largest onshore massifs of ultrabasic rocks preserved on Earth. The peridotites nappe was emplaced during Eocene times over (1) a basement of arc-derived formations of Pre-Cretaceous age which are overlain by basalts and formations of the Central Range that include sandstones, siltstones, graywackes, claystones with coal of Cretaceous age and Tertiary carbonate turbidites and volcaniclastic deposits, and

(2) a tectonic unit of oceanic basalts of Upper Cretaceous to Eocene age, with back-arc or fore-arc affinities which underlies the peridotites nappe (Paris, 1981). H 2 -bearing gas seepages associated to alkaline to hyper-alkaline waters were found in the southern part of the Massif du Sud (bay of Prony) within the peridotite nappe [START_REF] Deville | Peridote-water interaction generating migration pathways of N 2 -H 2 -CH 4 -rich fluids in subduction context: Common processes in the ophiolites of Oman, New-Caledonia, Philippines and Turkey[END_REF][START_REF] Monnin | The low temperature hyperalkaline hydrothermal system of the Prony Bay (New Caledonia)[END_REF][START_REF] Deville | The origin of N 2 -H 2 -CH 4 -rich natural gas seepages in ophiolitic context: A major and noble gases study of fluid seepages in New Caledonia[END_REF], while N 2 -rich seepages were found below the ophiolitic units in the area of La Crouen [START_REF] Deville | The origin of N 2 -H 2 -CH 4 -rich natural gas seepages in ophiolitic context: A major and noble gases study of fluid seepages in New Caledonia[END_REF].

Materials and methods

Gases were sampled in stainless steel tubes with helium-proof valves for chemical composition and noble gas analyses and in glass tubes for stable isotopic composition analyses. Gas sampling devices were evacuated (10 -3 Pa vacuum) before sampling. When possible, stainless steel tubes were swept twice by the sampled gas. When gas was collected in water springs, streams or in the sea (Oman, some places in the Philippines and New-Caledonia), pH, Eh and temperature were measured directly in the field (Table 1).

The chemical compositions of gases were determined by gas chromatography (GC) with a Varian GC3800. Measurement uncertainties are below 0.01% mol for hydrocarbons (FID) and for other gases (TCD). The measurements of the isotopic ratios 13 C/ 12 C (CH 4 and CO 2 ) and D/H in (CH 4 and H 2 ) were performed on a MAT 253 (Thermo Fischer) mass spectrometer coupled with a gas chromatograph (GCC-IR-MS). The results are reported in δ units relative to Pee Dee Belemnite (PDB) for carbon and Standard Mean Ocean Water (SMOW) for hydrogen, respective analytical uncertainties being of 0.5‰ and 5‰. Nitrogen isotopic compositions were measured relative to air with a MAT 253 mass spectrometer. N 2 has been purified in a vacuum line. CH 4 and H 2 were oxidized in a Cu-oxide oven to CO 2 and water vapor, respectively. These produced gases were cryogenically trapped for separation from N 2 .

Analytical uncertainty for the measurement of δ 15 N is lower than 0.1‰.

The noble gases elementary compositions were determined by quadrupole mass spectrometry (Table 2). The QUADRAR line allows determining the contents of 4 He, 20 Ne, 36 Ar and Kr.

The mass spectrometer is a Prisma quadripole QMA/QME200 (Pfeiffer Vacuum) with an open ion source. The analyzer allows measurements of compounds with a mass over charge ratio from 0 to 100. Global relative uncertainties for quantification of noble gases with this method are within the following range: He: ±15%; Ne: ±20%; Ar: ±10%; Kr: ±12%.

Helium isotopic ratios and contents were determined with a high-resolution magnetic sector mass spectrometer Micromass GV 5400 equipped with a modified Nier type electron impact source (Bright). The global relative uncertainty (1σ) on the quantification of 4 He is ±4%. The uncertainty on the quantification of the 3 He/ 4 He ratio is ±2%.

Results

The sites studied in Oman are scattered over Northern Oman and comprise 14 spots of gas bubbling in water (Fig. 1A). Most of the gas samples were taken in ultra-basic springs of the Semail ophiolite and three samples were taken in non-alkaline thermal springs (Rustaq, Nakhal and Al Ali) seeping from formations structurally located below the ophiolite nappe (Table 1). In the Philippines, 3 spots were sampled in the Zambales ophiolite. One site (Mangatarem) corresponds to an ultra-basic thermal spring with abundant bubbling gas, whereas the two others correspond to gas directly seeping from fractures of the peridotites (Table 1). One site corresponds to the Los Fuegos Eternos burning gas close to the Coto Chromite mine [START_REF] Abrajano | Methane-hydrogen gas seeps, Zambales Ophiolite, Philippines: Deep or shallow origin?[END_REF][START_REF] Abrajano | Geochemistry of reduced gas related to serpentinization of the Zambales ophiolite, Philippines[END_REF], the other one is located above the Nagsasa bay [START_REF] Abrajano | Methane-Hydrogen Generation in the Zambales Ophiolite (Philippines) Revisited[END_REF]; Fig. 1B). The gas vents sampled in Turkey were seeping out of the fracture system of the ophiolite unit in the area of Chimaera, south of Antalya (Fig. 1C). In New Caledonia, all gas samples were bubbling in water (some in alkaline springs of the area of the bay of Prony, others in the non-alkaline thermal springs of the area of La Crouen; [START_REF] Deville | The origin of N 2 -H 2 -CH 4 -rich natural gas seepages in ophiolitic context: A major and noble gases study of fluid seepages in New Caledonia[END_REF]; Fig. 1D).

Chemical and isotopic composition of the gas

The gas chemical compositions are listed in Table 1. Gas mixtures contain H 2, N 2 and CH 4 as major components. Figure 4 Carbon and hydrogen isotopic composition (δ 13 C and δD) of H 2 , CH 4 , CO 2 and nitrogen isotopic composition (δ 15 N) of N 2 are reported in Table 2. δD of H 2 values are very negative and comprised in a narrow range from -756 to -699‰, except for the sample from Nagsasa (Philippines) with δD of H 2 = -664‰ (Fig. 6,7). The highest values of δD of CH 4 correspond to the highest contents of methane (Fig. 8A). Relative carbon and hydrogen isotopic compositions of methane are presented in Figure 8B. High values of δ 13 C of CH 4 were obtained, up to +7.9‰, while δD of CH 4 are ranging from -428 to -130‰.

Hydrogen isotopic fractionation between H 2 and CH 4 depends on seepage types. The highest isotopic fractionations between species are observed on dry seepages where δD values of methane are the highest recorded in this study (Table 2). The nitrogen isotopic compositions were also measured for 4 samples. δ 15 N values range from -0.3 to +0.5‰ (Table 2).

Noble gases contents and their isotopic ratios, when measured, are given in Table 2. The samples of the H 2 -rich type are very close to the air end-member with a slight crustal contribution. Those of the N 2 -H 2 -CH 4 type show higher N 2 and CH 4 contents and present either a crustal (Oman), or mantle (Kaoris, New Caledonia) or crust plus mantle contribution (Oman). The samples of the H 2 -CH 4 type present both crustal and mantle contributions, with a very marked mantle contribution for the Nagsasa sample (Zambales, Philippines). N 2 -rich samples are made of a mixing between air and crust end-members.

Gas types

According to the gas compositions, four types can be defined (Fig. 4):

1) The H 2 -rich type shows dominant H 2 contents (61.0 to 87.3% mol) associated with some N 2 (less than 35% mol) and some CH 4 (less than 20 mol%; Fig. 4 and8A). This type is encountered in Oman [START_REF] Neal | Hydrogen generation from mantle source rocks in Oman[END_REF][START_REF] Sano | Origin of hydrogen-nitrogen gas seeps, Oman[END_REF][START_REF] Deville | Peridote-water interaction generating migration pathways of N 2 -H 2 -CH 4 -rich fluids in subduction context: Common processes in the ophiolites of Oman, New-Caledonia, Philippines and Turkey[END_REF][START_REF] Deville | Natural flows of H 2 and associated diagenetic processes of atmospheric CO 2 capture and sequestration: a study in the ophiolites of Oman[END_REF][START_REF] Vacquand | Genèse et mobilité de l'hydrogène naturel: source d'énergie ou vecteur d'énergie stockable ?[END_REF]Boulard et al., 2013;[START_REF] Miller | Modern water/rock reactions in Oman hyperalkaline peridotite aquifers and implications for microbial habitability[END_REF] and this type of gas exclusively seeps as bubbles from ultra-basic springs (pH between 11 and 12; Fig. 5) associated to negative to very negative Eh (from -45 to -780 mV; Fig. 5; table 1). In addition to the high OH -ion concentration responsible for the high pH, the waters are calcium-rich [START_REF] Neal | Past and present serpentinisation of ultramafic rocks; an example from the Semail ophiolite nappe of Northern Oman[END_REF]. Consequently, in the alkaline springs, spectacular precipitations of calcium carbonate (calcite and aragonite) are observed due to the reaction of Ca 2+ and OH -ions with the CO 2 of the atmosphere (see supplementary material). These waters show high electrical conductivities in accordance with their relatively high salinity, essentially related to the presence of Cl -and Na + ions (Cl -between 120 and 380 mg/l; Na + between 150 and 420 mg/l; [START_REF] Neal | Past and present serpentinisation of ultramafic rocks; an example from the Semail ophiolite nappe of Northern Oman[END_REF][START_REF] Kelemen | Rates and Mechanisms of Mineral Carbonation in Peridotite: Natural Processes and Recipes for Enhanced, in situ CO 2 Capture and Storage[END_REF]. The temperatures of these sources are almost ambient to slightly warm (between 22.5 and 35 °C). The δ 13 C values of methane are very high (among the highest values known on Earth, between -13 and +8‰; Table 2; Fig. 8B). The gas composition shows N 2 vs fossil noble gas 36 Ar ratios (Table 2; Fig. 9) comprised between the air and water in equilibrium with air (classically mentioned as ASW for Air Saturated Water).

2) The N 2 -H 2 -CH 4 type is characterized, by higher N 2 values (from 45 to 65 mol%) compared to the previous gas type. Therefore N 2 becomes the main gas. H 2 contents range from 25 to 36 mol%. CH 4 concentrations are similar to those of the H 2 -rich type (below 20 mol%; Fig. 4). This second gas type is mainly found in New Caledonia [START_REF] Deville | The origin of N 2 -H 2 -CH 4 -rich natural gas seepages in ophiolitic context: A major and noble gases study of fluid seepages in New Caledonia[END_REF] but also in some springs of Oman and of the Philippines. This type of gas mixture seeps also in ultra-basic springs. These springs are generally warmer than those of the H 2 -rich type, up to 40.1°C in New Caledonia and pH values are slightly lower, ranging from 10.5 to 11.3. They are, as well, associated with precipitations of calcium carbonate (see supplementary material).

The N 2 / 36 Ar ratios of this gas type are different from the previous type and higher than those recorded for ASW and air (Table 2; Fig. 9). These ratios are also correlated with H 2 / 36 Ar and 3) In the N 2 -rich type, the dominant gas compound is N 2 (over 91 mol%). Gas mixture is H 2free with low CH 4 contents (up to 2.7 mol% in New Caledonia), plus substantial amounts of helium (values above 0.1 mol%). This type of springs was found in Oman [START_REF] Sano | Origin of hydrogen-nitrogen gas seeps, Oman[END_REF][START_REF] Vacquand | Genèse et mobilité de l'hydrogène naturel: source d'énergie ou vecteur d'énergie stockable ?[END_REF] and New Caledonia [START_REF] Deville | The origin of N 2 -H 2 -CH 4 -rich natural gas seepages in ophiolitic context: A major and noble gases study of fluid seepages in New Caledonia[END_REF] where they occur in the same structural position, i.e., at the base of ophiolitic nappes (either in the sole of the nappes or in the sediments immediately below the nappes). δ 13 C values of CH 4 in this type are between -39.2 and -12.1‰. These gases come out in the form of bubbling springs with less alkaline water than previously recorded (pH from 7.5 to 10) and variable Eh (from -230 to +146 mV). The N 2 / 36 Ar ratios are very high (Table 2; Fig. 9). R/Ra ratios of the gases are consistently low (between 0.06 and 0.08, except for the sample collected in Al Ali where it was not possible to avoid air contamination because of access difficulties when sampling the gas and high temperature of the spring, above 66°C; Tables 1 and2; Fig. 10). The dissolved species in the associated waters are very different from those of the waters containing H 2 gases. They show lower concentrations of Na + and K + ions and contain HCO 3 -ions, which is not the case of the ultra-basic springs with H 2 seepages [START_REF] Neal | Past and present serpentinisation of ultramafic rocks; an example from the Semail ophiolite nappe of Northern Oman[END_REF]. In all cases these springs are warm to hot (from 37.9 to above 66°C; Table 1).

4) The H 2 -CH 4 type shows mostly H 2 and CH 4 in variable proportions. This type of gas vent is characterized by focused and relatively high gas flows (> 1 l/s), coming directly out of the rock fractures in the absence of water. This gas type was found in the Philippines and in Turkey where it was mentioned respectively by [START_REF] Abrajano | Methane-hydrogen gas seeps, Zambales Ophiolite, Philippines: Deep or shallow origin?[END_REF]2006) and[START_REF] Hosgörmez | New evidence for a mixed inorganic and organic origin of the Olympic Chimaera fire (Turkey): a large onshore seepage of abiogenic gas[END_REF]. It has not been found in New Caledonia but [START_REF] Neal | Hydrogen generation from mantle source rocks in Oman[END_REF] mentioned locally focused high gas flows seeping out directly from fractures of ultrabasic rocks in Oman (> 10 l/s). The CH 4 /H 2 ratio is commonly higher compared to the types associated to alkaline springs, except for the case of Nagsasa in the Philippines. The gas is able locally to spontaneously ignite as it is the case in Turkey (Chimaera) and the Philippines (Los Fuegos Eternos and Nagsasa) forming visible flames when the CH 4 /H 2 ratio is high. Nitrogen is nearly absent of these dry gas seepages (less than 2 mol%; Fig. 4) whereas since the gas is seeping out from fractures of rocks located above the water table, we would have expected that the gas would be much more easily naturally contaminated by air than in the previous cases which are bubbling in water. Indeed, these H 2 -CH 4 gases do not display predominantly atmospheric features and 20 Ne and 84 Kr concentrations are quite low compared to other types of gas mixtures (Table 2). δ 13 C values of CH 4 are high and fairly constant (between -12 and -5‰; Fig. 8), as well as δD of CH 4 which range from -175 to -130‰. The δD values of CH 4 are higher when compared to the H 2 -rich and N 2 -H 2 -CH 4 gas types (Table 1), while the δD values of H 2 vary between -756 and -664‰, generally in the same order of magnitude as those recorded in the H 2 -rich and N 2 -H 2 -CH 4 gas types (Table 1; Fig. 6). The differences of δD values of CH 4 suggest different processes for CH 4 genesis and probably different sources for hydrogen and carbon (see discussion below).

Discussion

Main characteristics of the different gas types

Comparing the different gas types, several characteristic features can be noticed. Gases bubbling in prings always contain N 2 that is not related to air contamination (see below), while gas seepages in fractures without associated water flows are N 2 -poor. The gas bubbling in springs show CH 4 contents that never exceed 20 mol%. The specificity of the H 2 -CH 4 gas type seeping in fractures without associated water flow is that their CH 4 contents locally reach high levels and that δD of CH 4 is significantly higher compared to the δD of CH 4 in the other gas types (Fig. 8). The water temperature tends to increase in the different types of springs from the H 2 -rich, to the N 2 -H 2 -CH 4 and to the N 2 -rich gas types (Table 1). Gas bubbling in springs is associated to ultra-basic conditions only when H 2 is present, whereas water is less or not basic in the springs of the N 2 -rich gas type. We propose that the chemical characteristics that define gas types reflect differences in origin and transport pathways of gas mixtures.

Hydrogen origin

As mentioned in introduction, H 2 sources of gas seepages in ophiolitic contexts are classically credited to modern hydration processes at low temperature during the weathering of ultrabasic and basic rocks (mostly peridotite and gabbro). At depth, within ophiolitic rocks, biological production of H 2 by fermentation processes is probably a sluggish process due to the paucity of potential organic substrates. High-temperature serpentinization (above 300°C) is a wellknown process but at temperatures below 300°C serpentinization can occur as well [START_REF] Moody | Serpentinization: a review[END_REF]. Indeed, experimental studies simulating land-based peridotite systems have shown that H 2 can be generated during serpentinization at temperatures below 100°C [START_REF] Mayhew | Hydrogen generation from low-temperature water-rock reactions[END_REF][START_REF] Okland | Formation of H 2 , CH 4 and Nspecies during low-temperature experimental alteration of ultramafic rocks[END_REF][START_REF] Neubeck | Formation of H 2 and CH 4 by weathering of olivine at temperatures between 30 and 70°C[END_REF]. On hydrogen-bearing gas seepages associated with high pH waters in ophiolitic context [START_REF] Neal | Hydrogen generation from mantle source rocks in Oman[END_REF][START_REF] Sano | Origin of hydrogen-nitrogen gas seeps, Oman[END_REF][START_REF] Cipolli | Geochemistry of high-pH waters from serpentinites of the Gruppo di Voltri (Genova, Italy) and reaction path modeling of CO 2 sequestration in serpentinite aquifers[END_REF][START_REF] Hosgörmez | New evidence for a mixed inorganic and organic origin of the Olympic Chimaera fire (Turkey): a large onshore seepage of abiogenic gas[END_REF][START_REF] Abrajano | Methane-Hydrogen Generation in the Zambales Ophiolite (Philippines) Revisited[END_REF][START_REF] Vacquand | Genèse et mobilité de l'hydrogène naturel: source d'énergie ou vecteur d'énergie stockable ?[END_REF], H 2 would result from the interaction between ultrabasic rocks and water flows at depth in the fracture system, in anoxic conditions, by reduction of water and oxidation of metals (Fe II , Mn II , Ni II …), Fe II being by far the most abundant electron donor in ultrabasic-basic rocks.

During the serpentinization process, Fe In presence of dissolved inorganic carbon (DIC) in subsurface, Mg 2+ ions are consumed to produce magnesium-bearing carbonates (which are widespread in fractures systems of all the ophiolitic massifs studied here; see below), and (if available) remaining Mg 2+ ions are prone to react with OH -to form brucite, Mg(OH) 2 (which is commonly found in the ultra-basic springs studied here, notably in Oman; [START_REF] Neal | Calcium and magnesium hydroxide precipitation from alkaline groundwaters in Oman, and their significance to the process of serpentinization[END_REF] leaving finally OH -available in solution. In the case of New Caledonia, where alkaline seepages interact with sea water, this results in massive production of brucite due to the reaction between OH -and Mg 2+ present in sea water as it is the case of the seepages studied in New-Caledonia [START_REF] Launay | Les sources thermales de Prony (Nouvelle-Calédonie) et leurs précipités chimiques. Exemple de formation de brucite primaire[END_REF]. More generally, as proposed by different authors, the coupled hydration of Mg-olivine and Ca-bearing pyroxene of peridotite rocks makes the water enriched in Ca 2+ and OH - [START_REF] Barnes | Geochemical evidence of present-day serpentinization[END_REF][START_REF] Neal | Calcium and magnesium hydroxide precipitation from alkaline groundwaters in Oman, and their significance to the process of serpentinization[END_REF][START_REF] Bruni | Irreversible water-rock mass transfer accompanying the generation of the neutral, Mg-HCO3 and high-pH, Ca-OH spring waters of the Genova province, Italy[END_REF][START_REF] Kelemen | Rates and Mechanisms of Mineral Carbonation in Peridotite: Natural Processes and Recipes for Enhanced, in situ CO 2 Capture and Storage[END_REF][START_REF] Miller | Modern water/rock reactions in Oman hyperalkaline peridotite aquifers and implications for microbial habitability[END_REF],

4 Mg 2 SiO 4 + CaMgSi 2 O 6 + 7H 2 O → 3 Mg 3 Si 2 O 5 (OH) 4 + Ca 2+ + 2OH - ( 4
)
Forsterite + Clinopyroxene + water → Serpentine + calcium ions +hydroxyl ions

As such, the interaction between peridotite and water during serpentinization is a potential source of H 2 caused by ferrous iron oxidation and water reduction associated with a rise of pH due to OH -production and associated with the presence of Ca 2+ in the ultra-basic water. The richer in iron is the olivine, the more H 2 and the less OH -are produced.

It has been experimentally demonstrated that highly fractured/altered peridotite is more prone to generate H 2 than a massive unaltered peridotite suggesting a role of the reacting surfaces and possibly that Fe II -bearing secondary formed minerals, such as Fe II -bearing brucite and serpentine, and the presence of N-species may also contribute to form H 2 (experiments at 25°C; [START_REF] Okland | Formation of H 2 , CH 4 and Nspecies during low-temperature experimental alteration of ultramafic rocks[END_REF], see also [START_REF] Klein | Experimental constraints on fluid-rock reactions during incipient serpentinization of harzburgite[END_REF].

The conditions of generation of H 2 can be estimated notably using hydrogen isotopic data. In the studied ophiolitic massifs, whatever are the characteristics of the H 2 -bearing gas types, δD values of H 2 are all very low in the analyzed gas samples (between -756 and -664‰). [START_REF] Abrajano | Methane-hydrogen gas seeps, Zambales Ophiolite, Philippines: Deep or shallow origin?[END_REF] mentioned , δD values of H 2 at -581 and -599‰ in the Los Fuegos Eternos of the Philippines. The lowest values are among the lowest recorded on Earth and they are significantly lower compared to δD of H 2 of hydrothermal sites on mid-oceanic ridges for which the values are more dispersed and generally higher (between -700 and -300‰; Fig. 6).

δD values of H 2 observed in oceanic settings decrease with temperature and appear as a thermometer for H 2 production at isotopic equilibrium with seawater [START_REF] Proskurowski | Low temperature volatile production at the Lost City Hydrothermal Field, evidence from a hydrogen stable isotope geothermometer[END_REF]. Low δD values of H 2 in ophiolitic massifs are classically considered as revealing relatively low temperatures of the generation of H 2 [START_REF] Neal | Hydrogen generation from mantle source rocks in Oman[END_REF]. Indeed, using either (1) the To decipher the conditions of generation of H 2 , it is possible to compare H 2 contents with those of other compounds such as noble gases tracers (Ballentine et al., 2002a and b;[START_REF] Zhou | Noble gas tracing of groundwater/coalbed methane interaction in the San Juan Basin, USA[END_REF][START_REF] Burnard | The Noble Gases as Geochemical Tracers: History and Background[END_REF][START_REF] Prinzhofer | Noble gases in oil and gas accumulations[END_REF][START_REF] Prinzhofer | Origins of hydrocarbon gas seeping out from offshore mud volcanoes in the Nile delta[END_REF]. Notably, the H 2 -rich gas type displays N 2 / 36 Ar ratios between ASW and air (Table 2; Fig. 9). These features are consistent with a genesis process involving surface water charged with dissolved atmospheric components (ASW) as the main fluid reacting with the rock for H 2 production. In this case, H 2 O inherited from meteoric water, initially in equilibrium with atmospheric components, is probably consumed at depth in the fracture system of the ophiolites by water/mineral interactions. Following this interpretation, as a consequence of water consumption and H 2 production, initial gas/water equilibrium with atmosphere is disrupted and atmospheric nitrogen and noble gases are mixed with neo-formed gases (H 2 and CH 4 ) and migrate upwards with no further addition of another gas component. The preservation of atmospheric relative proportions of fossil noble gases ( 20 Ne, 36 Ar, 84 Kr) suggests that these groundwaters reacted at a relatively shallow depth. For deeper depths of gas/water phases interaction, the difference of noble gas solubility from the surface conditions' ones (as P and T are much higher) would fractionate the relative proportions of these compounds. The 4 He contents (< 20 ppm), are in good agreement with [START_REF] Neal | Past and present serpentinisation of ultramafic rocks; an example from the Semail ophiolite nappe of Northern Oman[END_REF] interpretation which asserts that Ca 2+ -OH -water originates from meteoric water recharging at high elevations. The presence of 4 He in subsurface gas is classically interpreted as the result of a lengthy residence time at depth allowing 4 He to accumulate significantly due to natural radioactive decay of 235 U, 238 U and 232 Th present in crustal rocks [START_REF] Ballentine | Production, release and transport of Noble Gases in the continental crust. In "Noble Gases in Geochemistry and Cosmoschemistry[END_REF]. [START_REF] Paukert | Reaction path modeling of enhanced in situ CO 2 mineralization for carbon sequestration in the peridotite of the Samail Ophiolite, Sultanate of Oman[END_REF] considered, from their modelling of CO 2 consumption, that such meteoric waters reached its high pH values after 6500 years. This geological short time frame is consistent with the low 4 He contents of these gases, which are roughly displaying atmospheric signatures.

Considering the N 2 -H 2 -CH 4 and H 2 -CH 4 gas types, the interpretation that H 2 generation is simply a result of interaction between meteoric water and ophiolitic rocks is not sustainable anymore because, notably, N 2 / 36 Ar ratios are not comprised between ASW and air and noble gas contents are different from Air or ASW. This is consistent with a H 2 generation process which occurred deeper than in the H 2 -rich gas type, in higher temperature conditions as already suggested by δD values of H 2 .

Methane origin

Methane generation associated with fluid-rock interaction in ophiolitic contexts remains unclear. In terrestrial ultrabasic-basic contexts at moderate temperatures (below 150°C), methane is interpreted to result from carbon hydrogenation involving mainly sources of inorganic carbon. Methane so formed is termed abiotic (or abiogenic), and is thought to be produced by chemical reactions that do not directly involve organic matter (see discussion in [START_REF] Etiope | Abiotic methane on Earth[END_REF]; with references therein). Eventually, according to the geological conditions, methane produced by fluid-rock interaction can be mixed with biotic methane produced either by microbial processes or by thermogenic degradation of organic matter in sedimentary rocks.

In the case of hydrogenation processes of carbon, considering potential hydrogen sources (H 2 O or H 2 ), hydrogen in CH 4 can be considered as primary (i.e., issued from H 2 O reacting with carbon and preceding H 2 production; [START_REF] Abrajano | Methane-hydrogen gas seeps, Zambales Ophiolite, Philippines: Deep or shallow origin?[END_REF][START_REF] Oze | Have olivine, will gas: Serpentinization and the abiogenic production of methane on Mars[END_REF][START_REF] Suda | Origin of methane in serpentinite-hosted hydrothermal systems: The CH 4 -H 2 -H 2 O hydrogen isotope systematics of the Hakuba Happo hots spring[END_REF][START_REF] Okland | Formation of H 2 , CH 4 and Nspecies during low-temperature experimental alteration of ultramafic rocks[END_REF], or secondary (i.e., issued from an H 2 intermediate by reactions between carbon and H 2 ; [START_REF] Berndt | Reduction of CO2 during serpentinization of olivine at 300°C and 500 bar[END_REF]Foustoukos andSeyfried, 2004: Horita and[START_REF] Horita | Abiogenic methane formation and isotopic fractionation under hydrothermal conditions[END_REF][START_REF] Mccollom | A reassessment of the potential for reduction of dissolved CO2 to hydrocarbons during serpentinization of olivine[END_REF][START_REF] Mccollom | Abiotic methane formation during experimental serpentinization of olivine[END_REF][START_REF] Neubeck | Formation of H 2 and CH 4 by weathering of olivine at temperatures between 30 and 70°C[END_REF].

The experiments of [START_REF] Okland | Formation of H 2 , CH 4 and Nspecies during low-temperature experimental alteration of ultramafic rocks[END_REF] suggest indeed a possible primary production of CH 4 during hydration of ultrabasic rocks without an H 2 intermediate, notably with moderately altered peridotites. In both cases, hydrogen is issued from H 2 O, either primarily or secondarily. [START_REF] Suda | Origin of methane in serpentinite-hosted hydrothermal systems: The CH 4 -H 2 -H 2 O hydrogen isotope systematics of the Hakuba Happo hots spring[END_REF], based on a study of hydrogen isotopic data of CH 4 , H 2 and H 2 O (Fig. 7), defined different domains corresponding to different processes allowing to distinguish CH 4 generation by hydrogenation from water (primary) or from H 2 (secondary).

Primary CH 4 production is regarded as a direct interaction between Fe II -rich minerals and dissolved CO 2 . CH 4 production tied to ferrous mineral reactions with CO 2 has not yet been fully explored [START_REF] Neubeck | Low-temperature dunite hydration: evaluating CH 4 and H 2 production from H 2 O and CO 2[END_REF] but there is some indication from experimental reactions of olivine that passivating layers of silica may coat reactive surfaces and slow related CH 4 generation [START_REF] Garcia | Experiments and geochemical modelling of CO 2 sequestration by olivine: Potential, quantification[END_REF]. However, early production of CH 4 (before H 2 production) has been documented by experimentation on natural peridotite rocks, at low temperature (25°C), especially on fractured and altered peridotite, and these experiments showed a coeval production of H 2 and CH 4 [START_REF] Okland | Formation of H 2 , CH 4 and Nspecies during low-temperature experimental alteration of ultramafic rocks[END_REF], in the same proportions as those measured in this study for the H 2 -rich gas type. The presence of CO 2 is a favorable factor for olivine dissolution [START_REF] Oze | Have olivine, will gas: Serpentinization and the abiogenic production of methane on Mars[END_REF] 7).

As suggested by [START_REF] Suda | Origin of methane in serpentinite-hosted hydrothermal systems: The CH 4 -H 2 -H 2 O hydrogen isotope systematics of the Hakuba Happo hots spring[END_REF] and according to the reactions shown above, a simultaneous production of methane and magnesite can occur. Mg-bearing carbonates (magnesite and dolomite) are indeed abundant in fractures in the peridotites of Oman and they are supposed to have been formed at temperatures in the range 30 to 60°C [START_REF] Kelemen | Rates and Mechanisms of Mineral Carbonation in Peridotite: Natural Processes and Recipes for Enhanced, in situ CO 2 Capture and Storage[END_REF]. Methane and magnesite, have both high and similar δ 13 C values (δ 13 C CH4 between -13 and +8‰, δ 13 C magnesite between +2 and +8‰; see supplementary material). This is compatible with a non-atmospheric carbon origin of CH 4 that could derive from mantellic or metamorphic CO 2 or from carbonate destabilization. These values are different from the carbonates signatures found at the surface in the ultra-basic springs where carbon is interpreted as issued from atmospheric CO 2 which after fractionation during carbonate precipitation give δ 13 C values between -28 and -5‰ [START_REF] Kelemen | Rates and Mechanisms of Mineral Carbonation in Peridotite: Natural Processes and Recipes for Enhanced, in situ CO 2 Capture and Storage[END_REF]; see also supplementary material).

Considering secondary production of methane, via an H 2 intermediate, several pathways of CH 4 generation involving different sources of carbon can be considered, either (1) organic carbon (or reduced carbon including elementary C) found in sediments below the ophiolites notably, or else (2) carbon issued from deep sources from the mantle or from the crust (for example Dissolved Inorganic Carbon issued mantle gas or from carbonates within the sediments). Indeed, H 2 produced by serpentinization is susceptible to be subsequently consumed in a CH 4 producing reaction, involving a source of carbon which can be C°, CO or CO 2 , according to the general reaction [START_REF] Deville | The origin of N 2 -H 2 -CH 4 -rich natural gas seepages in ophiolitic context: A major and noble gases study of fluid seepages in New Caledonia[END_REF],

(2 + X) H 2 + CO X → CH 4 + X H 2 O (8)
In the case x = 0, the source of carbon can correspond to reduced carbon in sediments (graphite or overmature organic matter). In the case x = 1, the source of carbon corresponds to CO. CO has indeed, been mentioned as traces in the gas of the ophiolites of Oman [START_REF] Sano | Origin of hydrogen-nitrogen gas seeps, Oman[END_REF]. It might correspond to an intermediate compound preceding methane generation. In the case x = 2, this corresponds to the classical Sabatier reaction. Considering generation of high δ 13 C methane associated with ultrabasic contexts, it is frequently considered that it corresponds to Fisher-Tropsch Type (FTT) reaction, meaning the abiotic path of the Sabatier reaction [START_REF] Szatmari | Petroleum formation by Fischer-Tropsch synthesis in plate tectonics[END_REF][START_REF] Horita | Abiogenic methane formation and isotopic fractionation under hydrothermal conditions[END_REF][START_REF] Sherwood-Lollar | Methane occurrences in the Canadian shield[END_REF], 1993a&b, 2006[START_REF] Hosgörmez | Origin of the natural gas seep of Cirali (Chimera), Turkey: Site of the first Olympic fire[END_REF][START_REF] Kelley | An off-axis hydrothermal field near the Mid-Atlantic Ridge at 30[END_REF][START_REF] Charlou | Geochemistry of high H 2 and CH 4 vent fluids issuing from ultramafic rocks at the Rainbow hydrothermal field (36°14'N, MAR)[END_REF][START_REF] Foustoukos | Hydrocarbons in hydrothermal fluids: the role of chromium-bearing catalysts[END_REF][START_REF] Seewald | Experimental investigation of single carbon compounds under hydrothermal conditions[END_REF][START_REF] Taran | Carbon isotope effects in the open-system Fischer-Tropsch synthesis[END_REF][START_REF] Proskurowski | Abiogenic hydrocarbon production at lost city hydrothermal field[END_REF]; and others, see references in [START_REF] Etiope | Abiotic methane on Earth[END_REF]. In the Sabatier reaction, CO 2 is reduced to CH 4 through the oxidation of H 2 (so the reaction is controlled by H 2 activity).

The kinetics of FTT reactions are slow at the temperatures considered here (below 150°C) but

Fisher-Tropsch reactions are well-known to be favored by catalytic processes. Heterogeneous catalysis, promoted by minerals present in the ophiolitic rocks, such as chromite, magnetite, sulfides or awaruite can possibly trigger these reactions [START_REF] Mayhew | Hydrogen generation from low-temperature water-rock reactions[END_REF][START_REF] Etiope | Low-temperature catalytic CO2 hydrogenation with geological quantities of ruthenium: a possible abiotic CH4 source in chromitite-rich serpentinized rocks[END_REF] and references therein). For instance, close to Los Fuegos Eternos (in the Philippines), chromite (well-developed in the Coto mines) may promote the catalytic production of CH 4 .

Also, the area of Chimaera in Turkey shows chromite mines (in which an explosion caused the death of tens of underground workers). The area of Prony (New Caledonia) shows also chromite outcrops as well as at many places in the ophiolite of Oman.

Catalytic processes might also be associated with biological activity, which is well-known to reduce the activation energy necessary for chemical reactions. The Sabatier chemical reaction is actually equivalent to one of the classical routes to generate microbial methane (by CO 2 reduction) via hydrogenotroph-methanogen microorganisms. In the case of sedimentary environment with widely available sources of light C provided by organic compounds, it produces methane with δ 13 C generally lower than -50‰, far below the values obtained in the gas seepages of the ultra-basic springs (between -12.8 and +7.9‰; Fig. 8), even if locally in ultra-basic springs δ 13 C of methane are lower than -50‰ [START_REF] Morrill | Geochemistry and geobiology of a present-day serpentinization site in California: The Cedars[END_REF]. However, in some conditions (notably in the case of hyperthermophilic archaea), δ 13 C values of methane can be high [START_REF] Takai | Geochemical and microbiological evidence for a hydrogen-based, hyperthermophilic subsurface lithoautotrophic microbial ecosystem (HyperSLiME) beneath an active deep-sea hydrothermal field[END_REF][START_REF] Takai | Cell proliferation at 122°C and isotopically heavy CH 4 production by a hyperthermophilic methanogen under high-pressure cultivation[END_REF]. The presence of hydrogenotrophic microorganisms has been mentioned in oceanic hydrothermal fields [START_REF] Bradley | Multiple origins of methane at the Lost City hydrothermal field[END_REF] and in ultrabasic springs of ophiolitic units [START_REF] Brazelton | Bacterial Communities Associated with Subsurface Geochemical Processes in Continental Serpentinite Springs[END_REF], and the occurrence of methanogens in ultrabasic contexts [START_REF] Takai | Geochemical and microbiological evidence for a hydrogen-based, hyperthermophilic subsurface lithoautotrophic microbial ecosystem (HyperSLiME) beneath an active deep-sea hydrothermal field[END_REF][START_REF] Brazelton | Methane-and Sulfur-Metabolizing Microbial Communities Dominate the Lost City Hydrothermal Field Ecosystem[END_REF] raise the question, notably in Oman, of the microbial production of methane with elevated δ 13 C in ultra-basic groundwaters [START_REF] Miller | Modern water/rock reactions in Oman hyperalkaline peridotite aquifers and implications for microbial habitability[END_REF]. This interpretation is still challenged (see discussion between [START_REF] Etiope | Methane origin in the Samail ophiolite: Comment on "Modern water/rock reactions in Oman hyperalkaline peridotite aquifers and implications for microbial habitability[END_REF][START_REF] Miller | Reply to ''Methane origin in the Samail ophiolite: Comment on 'Modern water/rock reactions in Oman hyperalkaline peridotite aquifers and implications for microbial habitability[END_REF] but, if it is confirmed, this would suppose that biological activity might contribute to elevated δ 13 C methane from inorganic sources of carbon under extreme carbon limitation. However, methanogens collected in ultra-basic springs at the surface are not necessary representative of the conditions of generation of methane at depth because there is no carbon available in the ultra-basic water flowing from depth [START_REF] Neal | Past and present serpentinisation of ultramafic rocks; an example from the Semail ophiolite nappe of Northern Oman[END_REF] see also below). Methane generation via methanogen microorganisms observed at the surface might be a surficial process resulting from a a reaction between hydrogen issued from depth and a source of carbon provided by atmospheric CO 2 . If the source of deep carbon is important, as in sedimentary environments with fossil organic matter, the use of isotopically light carbon in the widely available organic matter is favored by microbial activity. However, this is not possible in the case of strong restriction of the available carbon as in ophiolitic rocks, especially when the initial sources of carbon are heavy (like carbonates or mantle).

Biological processes generating CH 4 from H 2 and CO 2 with progressively increasing δ 13 C CH4 have been documented in large scale geological gas storages of town gas [START_REF] Buzek | Carbon isotope study of methane production in a town gas storage reservoir[END_REF].

In addition, the presence of methanotrophs in the ultra-basic springs also contribute to increase the δ 13 C values of the residual methane [START_REF] Miller | Modern water/rock reactions in Oman hyperalkaline peridotite aquifers and implications for microbial habitability[END_REF].

In and -32.4‰ [START_REF] Deville | The origin of N 2 -H 2 -CH 4 -rich natural gas seepages in ophiolitic context: A major and noble gases study of fluid seepages in New Caledonia[END_REF]. Consequently, two different types of signatures were observed corresponding initial probably (despite of fractionation processes) to two different carbon sources. The first one (high δ 13 C between -13 and +8‰) would correspond most probably to an inorganic carbon source, while the second one (lower δ 13 C between -39 and -32‰) corresponds probably to an organic source. The high δ 13 C of CH 4 from Oman, the Philippines and Turkey might correspond to a carbon source, probably CO 2 , found as DIC in the reacting waters. Two different origins may be proposed for this inorganic carbon. It may correspond to mantle degassing, as it might be the case in the Philippines in link with the vicinity of the active volcanism of the Pinatubo system related to an active subduction zone, as suggested by noble gas analyses especially for one gas sample with high R/Ra values (Fig. 10), or it may come from the dissolution of carbonate rocks (for instance the carbonate sediments present under the ophiolites), as it might be the case in Oman [START_REF] Boudier | Accretion of Oman and United Arab Emirates ophiolite -Discussion of a new structural map[END_REF] and Turkey [START_REF] Juteau | Structural Relationships in Antalya Ophiolite Complex, Turkey -Possible Model for An Oceanic Ridge[END_REF]. Note that the H 2 -bearing gas samples studied here (as well as those analyzed in previous studies) show only locally tiny traces of CO 2 for which δ 13 C CO2 measurements have to be considered with caution (not reported in table 2). The measurements in the N 2 -rich gas type of Oman (seeping out from below the ophiolites), in which CO 2 contents are more important (data from [START_REF] Sano | Origin of hydrogen-nitrogen gas seeps, Oman[END_REF] this study, Table 1 and2), gave values between -19.4‰ and -9.6‰ which are consistent with an origin from sedimentary rocks below the ophiolites (metamorphism and/or carbonate destabilization).

Methane with δ 13 C values comprised between -39 and -32‰, in New Caledonia, could simply be issued from an organic source [START_REF] Deville | The origin of N 2 -H 2 -CH 4 -rich natural gas seepages in ophiolitic context: A major and noble gases study of fluid seepages in New Caledonia[END_REF]. In agreement with the absence or low contents of C 2+ in the gas mixtures (below 0.01%), methane may originate from a reaction between H 2 and a mature/overmature organic matter or graphite present in the sediments below the ophiolites.

Also note that, in addition to the absence or very low contents of CO 2 in H 2 -bearing gas samples, neither HCO 3 -nor CO 3 2-have been found in the ultra-basic springs, notably in the case of Oman [START_REF] Neal | Past and present serpentinisation of ultramafic rocks; an example from the Semail ophiolite nappe of Northern Oman[END_REF] suggesting that almost the entire stock of available DIC was converted into CH 4 under extreme carbon limitation. Also, no form of organic matter or graphite are known within the peridotites of the areas studied. This limitation probably allows H 2 to be present in the gas, otherwise all the generated H 2 would have been consumed

to generate CH 4 , as it was suggested by [START_REF] Milesi | Contribution of siderite-water interaction for the unconventional generation of hydrocarbon gases in the Solimões basin, north-west Brazil[END_REF] in the Solimes Basin in Brazil.

In both, H 2 -rich and N 2 -H 2 -CH 4 gas types, CH 4 contents being lower than 20 mol%, this suggests a very limited carbon source compared with the H 2 -CH 4 gas type (Fig. 4 and8).

Indeed, if we consider carbonic acid in rain water as the initial main source of carbon, its contribution is regarded as limited and rapidly buffered, whereas if the carbon source corresponds to carbonate rocks or deep CO 2 fluxes, the amount of carbon available is more abundant at depth but the connection with the ophiolites depends on the hydrodynamics and the available carbonate dissolved in circulating water. H 2 -CH 4 gas type might correspond to such a system connected to deep sources of carbon.

Considering oceanic hydrothermal vents, values of δD of CH 4 probably record the isotopic fractionation associated to a primary process involving a carbon compound with water, to produce methane. If open system conditions apply, δD of CH 4 depends on isotopic equilibrium at the given temperature. For dry seepages in fractures, data are in agreement with these conditions. For seepages in water, a different process would provide scattered and very low δD values of CH 4 (Fig. 6), either because the system is closed and isotopic equilibrium is not reached or because a different secondary process (eventually biology) is influencing the isotopic composition of hydrogen in methane. As such, the very low values of δD of methane in spring water raise the question of a possible microbial role in methane generation.

For the H 2 -CH 4 and N 2 -H 2 -CH 4 gas types a remarkable correlation exists between H 2 and CH 4 when normalized to 36 Ar (Fig. 11A) wherever is the site studied in Oman, in the Philippines and New Caledonia. A similar correlation exists between H 2 and CH 4 when normalized to 84 Kr, (Fig. 11B). For the CH 4 -H 2 and N 2 -H 2 -CH 4 gas types, this suggests that, independently of the considered ophiolite and geological context, generations of H 2 and CH 4 are coupled and both related to a deep process which is independent of interactions with ASW shallow aquifers. Conversely, these correlations do not exist for the H 2 -rich gas type which is in agreement with the interpretations proposed above that this type of gas underwent more influences with ASW aquifers. Also, this correlation is not observed in the samples of Turkey possibly due to a mixing with thermogenic methane. Indeed, according to other studies, some samples, notably in Turkey (H 2 -CH 4 type), might comprise a thermogenic component. It has been considered that high methane content cannot be attributed univocally to dissolved carbonates reduction by H 2 , for example in Chimaera (Turkey), up to 40 mol% of the CH 4 has been estimated to be thermogenic gas originating from a source rocks with highly mature kerogen in a Mesozoic limestone nearby [START_REF] Hosgörmez | New evidence for a mixed inorganic and organic origin of the Olympic Chimaera fire (Turkey): a large onshore seepage of abiogenic gas[END_REF][START_REF] Etiope | Abiotic methane flux from the Chimaera seep and Tekirova ophiolites ( Turkey ): Understanding gas exhalation from low temperature serpentinization and implications for Mars[END_REF]. This is in good agreement with occurrences of C2+ (C 2 -C 6 alkanes) in the gas analyzed in Turkey [START_REF] Hosgörmez | New evidence for a mixed inorganic and organic origin of the Olympic Chimaera fire (Turkey): a large onshore seepage of abiogenic gas[END_REF][START_REF] Etiope | Abiotic methane flux from the Chimaera seep and Tekirova ophiolites ( Turkey ): Understanding gas exhalation from low temperature serpentinization and implications for Mars[END_REF] this study, see table 2). However, whatever the geographic location of samples is, dry seepages display higher CH 4 relative contents together with higher δD of CH 4 when compared to gas seepages in water. This suggests that it can be linked to a different process as mentioned above (primary methane production) and probably higher temperature during production of methane. Gas mixtures belonging to the H 2 -CH 4 type correspond probably to highly reactive zones of water reduction at higher temperature with less carbon restriction.

Nitrogen origin

In the subsurface, nitrogen gas can have different origins, either atmosphere via the hydrodynamism of air equilibrated water, or sediments (organic matter, or ammoniumbearing clay minerals, or nitrogen-bearing salt in evaporites present in sedimentary rocks), or else the deep crust or mantle degassing [START_REF] Jenden | Origin of nitrogen-rich natural gases in the California Great Valley: Evidence from helium, carbon and nitrogen isotope ratios[END_REF][START_REF] Zhu | The isotopic compositions of molecular nitrogen : implications on their origins in natural gas accumulations[END_REF]Ballentine and Sherwood-Lollar, 2002b). Nitrogen contents within ophiolitic rocks are low but experimental studies have however shown that adsorbed N-species can be leached out while H 2 and CH 4 are produced from altered peridotites [START_REF] Okland | Formation of H 2 , CH 4 and Nspecies during low-temperature experimental alteration of ultramafic rocks[END_REF].

Considering the geochemical results of this study, N 2 present in the gas of the H 2 -rich type shows N 2 / 36 Ar and N 2 / 84 Kr ratios generally equivalent or slightly higher than those of ASW, suggesting mainly an atmosphere origin for N 2 via a fractured aquifer (Fig. 12A andB).

Samples of the N 2 -H 2 -CH 4 gas type, which show higher N 2 contents than H 2 contents compared to the H 2 -rich gas type display notably a nitrogen enrichment relative to 36 Ar and 84 Kr (Fig. 12A andB), suggesting an addition of non-atmospheric nitrogen. This contributes to dilute the neo-formed H 2 and CH 4 [START_REF] Deville | The origin of N 2 -H 2 -CH 4 -rich natural gas seepages in ophiolitic context: A major and noble gases study of fluid seepages in New Caledonia[END_REF]. Samples of the N 2 -rich gas type show a 4 He enrichment correlated with the N 2 enrichment when compared to atmosphere (Fig. 13). This suggests that non-atmospheric N 2 might be of crustal origin associated to radiogenic helium from depth. The R/Ra ratio ( 3 He/ 4 He ratio of the sample normalized over the same ratio for the air = 1.384 x 10 -6 ) suggests that a crustal component exists in most of the samples and locally a mantle component is observed in the Philippines (Fig. 10).

The differences between the H 2 -rich type and the N 2 -H 2 -CH 4 type are possibly linked to the existence of two different reactive zones for the production of H 2 -rich fluids in peridotitic ground waters: one (H 2 -rich) being more surficial than the other (N 2 -H 2 -CH 4 -rich) and that two different reactive fluids should be considered: (1) a meteoric fluid in the more surficial environment (H 2 -rich) and (2) a deep N 2 -bearing fluid with crustal signature, probably issued from sediment metamorphism from below the obducted ophiolitic units.

Samples of the N 2 -H 2 -CH 4 type could also result from the mixing of H 2 -rich gas and N 2 -rich gas. These N 2 -rich samples contain mostly N 2 and no or little H 2 . Because they were found in hot water springs, displaying pH between 6.9 and 10 and relatively high 4 He contents which suggest a different origin, probably deeper according to water temperature (reaching locally 66°C) and in a crustal environments as suggested by the high 4 He contents. This deep N 2 is probably not directly produced within the peridotite units. The geological context suggests rather an origin from the underthrusted sediments below the ophiolitic units, so in this case N 2 would be issued more probably from the buried sediments, either from the water of hydrated clay (clay dewatering) or from the destabilization of ammonium-bearing clays. Reaction of such deep fluids with peridotite or mixing these fluids with H 2 -rich gas type are susceptible to generate fluids with the chemical signatures of the N 2 -H 2 -CH 4 gas type where N 2 dilutes H 2 and is found in excess relative to ASW or air. Throughout the study of the 4 types of gas, it appears that the presence of nitrogen in the mixture is always linked to the presence of water at the seepage. This gas is almost absent from the dry seepages (H 2 -CH 4 type).

δ 15 N measurements in the nitrogen-dominant gas types (N 2 -H 2 -CH 4 and N 2 -rich), gave values of -0.3‰ in Rustaq (Oman), which is consistent with previous studies [START_REF] Sano | Origin of hydrogen-nitrogen gas seeps, Oman[END_REF]) and -0.1‰ in Mangatarem (Philippines; table 2), whereas δ 15 N measurements in the H 2 -CH 4 gas type gave values comprised between -2.8‰ and -2.1‰ in the Chimaera gas seepages of Turkey [START_REF] Hosgörmez | New evidence for a mixed inorganic and organic origin of the Olympic Chimaera fire (Turkey): a large onshore seepage of abiogenic gas[END_REF] and +0.5‰ in Nagsasa in the Philippines (Table 2). The δ 15 N value for atmospheric nitrogen being 0‰ [START_REF] Sano | Nitrogen isotopes and N2/Ar ratios in cherts: an attempt to measure time evolution of atmospheric δ15N value[END_REF][START_REF] Nishizawa | Speciation and isotope ratios of nitrogen in fluid inclusions from seafloor hydrothermal deposits at ∼ 3.5 Ga[END_REF], it cannot be clearly distinguished from the values measured in the N 2 -H 2 -CH 4 and N 2rich samples considering potential sampling bias plus analytical uncertainties of the measurements. Although these results are compatible with an atmospheric nitrogen component in the N 2 -H 2 -CH 4 and N 2 -rich gas mixtures, this atmospheric component is not a contamination as shown by the N 2 / 36 Ar ratios for instance. This N 2 might thus be issued from pore water trapped in the sediments (initial ASW) below the ophiolites or from the metamorphism of clays from the sediments underthrusted below the ophiolitic units.

However, such a nitrogen isotopic value is not a clear diagnostic. Although it is compatible with an atmospheric value, it does not exclude other origins like deeper components from mantle and/or crust because a multiple origin of nitrogen is indeed supported by the results of the noble gas analyses.

Noble gas contents suggest that there are at least two different sources of N 2 . The plot of R/Ra versus 20 Ne/ 4 He (Fig. 10) shows that the N 2 -bearing gases are ranging on a mixing line between the crust and the atmospheric end-member. In the H 2 -rich type, the N 2 / 36 Ar ratio is close to the air and ASW ratios (Fig. 12A). This implies that N 2 in this gas originates from the atmosphere and was carried by an aquifer equilibrated with the atmosphere or that the main nitrogen component is acquired during the gas migration upwards through the aquifer. The other samples (N 2 -H 2 -CH 4 and N 2 -rich) do not align on these atmospheric ratios. They display a nitrogen enrichment relative to 36 Ar (Fig. 12A) and the contents are much too high to be only due to extraction from an aquifer related to free gas flows toward the surface.

From the plots of figures 12A and B, it can be inferred that N 2 -rich type gases in Oman and New Caledonia consist of a mixture of an atmospheric component and an almost pure nitrogen end-member. The proportion of the nitrogen component varies in a large range (from 10 to 90% mol). 4 He is mainly a radiogenic product issued from the continental crust. It cannot be produced in such amounts by mantle-derived ultrabasic rocks. Thus, the N 2 -rich gases can be interpreted as a crustal gas with a 4 He component. A potential interpretation for the origin of deep N 2 could be that it was produced in the sediments and/or metasediments buried under the ophiolitic units. In this case, the source could be the interstitial water in the sediments and/or the solid matrix of the sediments (organic matter or ammonium in clay sediments).

Conclusion

Integrating the results of previous studies and original results in different areas of the world, this multi-tracer approach provided new insights on gas seepages in ophiolitic contexts. Four types of gas mixtures were defined, three of them being associated with water springs and characterized by the presence of N interacting with the ophiolite. H 2 -CH 4 gas flows are generally more localized (focused) and these flows are generally higher than the other H 2 -bearing gas flows. This is probably due to the fact that they are generated at greater depth and at higher temperature conditions. This probably induces the individualization of a gas phase preventing H 2 to react during the rise toward the surface, notably by preventing any biological consumption of H 2 which required an aqueous media. The N 2 -bearing fluids are probably generated mostly from the sediments located below the ophiolitic units and migrate upwards forming the N 2 -rich gas type when reaching the surface or the N 2 -H 2 -CH 4 type if mixing occurs between the N 2 -rich and the H 2rich types (Fig. 14). (2016). FTT synthesis refer to experiments investigating abiotic methane production in the gas phase (Taran et al., 2010) and aqueous phase (Fu et al., 2007;[START_REF] Mccollom | The influence of carbon source on abiotic organic synthesis and carbon isotope fractionation under hydrothermal conditions[END_REF].

There data were presented in a similar diagram in [START_REF] Suda | Origin of methane in serpentinite-hosted hydrothermal systems: The CH 4 -H 2 -H 2 O hydrogen isotope systematics of the Hakuba Happo hots spring[END_REF] Oman by [START_REF] Neal | Past and present serpentinisation of ultramafic rocks; an example from the Semail ophiolite nappe of Northern Oman[END_REF]values between -11.2 and +10.7‰). Noble gas analyses 4 He, 20 Ne, 36 Ar, 84 Kr (ppmv), and R/Ra (helium isotopic ratios normalized to the air isotopic Ra= 3 He/ 4 He=1.4 10 -6 ). C2+ contents in all the gas samples are very low, below the threshold of analytical precision (< 0.01%). Global relative uncertainties (at 1σ) for quantification of noble gases with the method used is estimated in the range He: ±10%; Ne: ±20%; Ar: ±5%; Kr: ±10%. The global relative uncertainty (1σ) for the R/Ra ratio is in the range ±2%.

  the present study, the different values observed for δ 13 C of CH 4 probably reflect different sources of carbon or different CH 4 generation processes. The present study shows that (a) H 2rich gases from Oman display δ 13 C of CH 4 between -12.8 and +7.9‰, while (b) in the Philippines δ 13 C of CH 4 are ranging between -13.5 and -5.6‰ which is compatible with the results obtained by[START_REF] Abrajano | Methane-hydrogen gas seeps, Zambales Ophiolite, Philippines: Deep or shallow origin?[END_REF], (c) in Turkey gases from H 2 -CH 4 type display a δ 13 C of CH 4 values between -7.9 and -11.9‰(Horgormez et al., 2008 and this study), whereas (d) for New-Caledonia some gases from N 2 -H 2 -CH 4 type values are between -38.5

Figure 1 -

 1 Figure 1 -Structural sketch-maps of the different sites studied in this work in Oman (A), the Philippines (B), Turkey (C) and New-Caledonia (D) (same scale), with the location of the sampling sites and the location of the cross-sections shown in figure 2. The gas seepages are associated with ophiolitic ultrabasic rocks. Gas was sampled from different types of seepages: gas seeping in water which originates from ultrabasic or hot springs and gas seeping from fractures without water flow.

Figure 2 -Figure 3 -

 23 Figure 2 -Geological cross-sections of the different sites studied in this work (same scale;earthquake epicenters from USGS data base). A 2D cross-section for the area of Chimaera in Turkey is not shown because of too complex 3D geometry in a transform fault system.

Figure 4 -

 4 Figure 4 -The major components of the gas mixtures sampled in Turkey, Oman, New Caledonia and the Philippines : H 2 , N 2 and CH 4 in a triangular diagram (% mol). Four different types of gas mixtures can be defined according to the relative contents of H 2 , N 2 and CH 4 . Distinct chemical features correspond to distinct seepages styles : N 2 -containing gas mixtures are associated with water and seep in water streams, whereas N 2 -free mixtures correspond to dry seepages which seep out of fractures of massive rocks and locally ignite spontaneously. The different types of water-associated seepages correspond also to specific water physical properties, notably pH and temperature (see Fig. 5).

Figure 5 -

 5 Figure 5 -Pourbaix diagram pH-Eh of the water where the gas samples were collected in springs. Oblique lines (blue and pink) separate the stability fields of H 2 , H 2 O and O 2 .

Figure 6 -

 6 Figure 6 -Comparison of hydrogen isotopic compositions of CH 4 and H 2 for onshore settings (data from this work and two points, LFE-2 and LFE-3, from[START_REF] Abrajano | Methane-hydrogen gas seeps, Zambales Ophiolite, Philippines: Deep or shallow origin?[END_REF] and oceanic settings[START_REF] Proskurowski | Low temperature volatile production at the Lost City Hydrothermal Field, evidence from a hydrogen stable isotope geothermometer[END_REF]. While a narrow range of CH 4 isotopic composition is observed for oceanic settings these values are lower for onshore settings and cover a large range. The values for the gas seeping out of fractures without associated ultra-basic water (H 2 -CH 4 gas type) are close to those of oceanic setting at Lost City and might indicate a similar generation process. In contrast to the CH 4 isotopic compositions, the H 2 isotopic compositions display a large range in oceanic settings (dependent on temperature) and a narrow range in terrestrial settings for gas seeping from ultra-basic springs.

Figure 7 -

 7 Figure 7 -CH 4 -H 2 -H 2 O hydrogen isotope systematics. Data for biological hydrogenotrophic methanogenesis are from Balabane et al. (1987), Valentine et al. (2004) and Okumura et al.

Fig 8 Figure 8

 88 Fig 8

Figure 9 -

 9 Figure 9 -N 2 content vs N 2 / 36 Ar diagram, depicting nitrogen excess in samples relative to atmospheric 36 Ar. The values from the H 2 -rich type gather along a mixing line between airand air equilibrated water, implying that probably only inherited atmospheric N 2 can be found in the gas mixture, whereas the other types show a N 2 excess, suggesting that nonatmospheric N 2 is present in the gas mixture.

Figure 10 -

 10 Figure 10 -Mixing diagram 20 Ne/ 4 He vs R/Ra ( 3 He/ 4 He ratio of the sample normalized over the same ratio for the air = 1.384 x 10-6 ). The noble gas data show that except the H 2 -rich type which plots around the atmospheric end-member and indicates a single shallow aquifer signature, the other gas mixtures have signatures that result from the interaction of several fluids originating either from the mantle or from the crust. The N 2 -rich type is interpreted as resulting from the mixing of atmospheric and crustal end-members whereas a mantle contribution is recorded for samples of the N 2 -H 2 -CH 4 and H 2 -CH 4 types.

Figure 11 -

 11 Figure 11 -Mixing diagrams comparing H 2 and CH 4 contents normalized over 36 Ar and 84 Kr contents. A. This diagram shows a correlated enrichment in CH 4 vs H 2 when normalized over 36 Ar (linear correlation with R 2 = 0.9894). B. This diagram shows also a correlated enrichment in CH 4 vs H 2 when normalized over 84 Kr (linear correlation with R 2 = 0.9953).

Figure 12 -

 12 Figure 12 -Mixing diagrams comparing H 2 and N 2 contents normalized over 36 Ar and 84 Kr contents. A. The H 2 / 36 Ar vs N 2 / 36 Ar diagram shows different enrichment trends in H 2 vs N 2 between the H 2 -rich gas type and the N 2 -H 2 -CH 4 gas type. Note the good linear correlation between H 2 / 36 Ar and N 2 / 36 Ar for the N 2 -H 2 -CH 4 gas type (linear correlation R 2 = 0.9785). B. The H 2 / 84 Kr vs N 2 / 84 Kr diagram, as the previous diagram, shows different enrichment trends in H 2 vs N 2 between the H 2 -rich gas type and the N 2 -H 2 -CH 4 gas type. Note the good linear correlation between H 2 / 84 Kr and N 2 / 84 Kr for the N 2 -H 2 -CH 4 gas type (linear correlation with R 2 = 0.9739).

Figure 13 -Figure 14 -

 1314 Figure 13 -4 He/ 36 Ar vs N 2 / 36 Ar mixing diagram for the gas samples of the N 2 -rich gas type.This diagram shows a correlated enrichment in 4 He vs N 2 when normalized over 36 Ar (linear correlation with R 2 = 0.9993)

a b 1369 Figure

 1369 Figure S3 a and b -Photos of thin sections showing magnetite aggregates.

Figure S4 -

 S4 Figure S4 -An example of a blue pool in Oman (Sayah). Note the blue color related to the formation of carbonates in water. A precipitate of aragonite and brucite is observed at the bottom of the basin.

Figure S5 -

 S5 Figure S5 -Another example of a blue pool in Oman. The water in these basins shows pH values generally between 11 and 12. A film of calcite develops on the water surface and a precipitate of aragonite and brucite is deposited at the bottom of the basin.

Figure S6 -

 S6 Figure S6 -Calcite film appearing on the surface of ultra-basic water (Oman).

Figure S7 -

 S7 Figure S7 -Aragonite precipitation in ultra-basic water (Oman).

Figure S8 -

 S8 Figure S8 -Aragonite chimneys forming in ultra-basic water (Oman).

Figure S9 -

 S9 Figure S9 -Needles of primary Brucite in the foreshore of the Baie des Kaoris (New Caledonia).

  displays H 2, N 2 and CH 4 molecular contents of all samples in a triangular diagram. Four distinct types of gas mixtures can be identified according to their respective contents in H 2 , CH 4 and N 2 . Three of these types are associated with water seeps: H 2 -rich type; N 2 -H 2 -CH 4 type and N 2 -rich type. The water properties associated to these gas seepages are listed in Table 1 and illustrated on a Pourbaix diagram on figure 5. The last type (H 2 -CH 4 ) corresponds to gas seepages in fractures without associated water flow.

  II -rich minerals, such as olivine [(Mg II ,Fe II ) 2 SiO 4 ] are oxided and form Fe III -bearing minerals, such as magnetite [Fe 2

		III Fe II O 4 ] or Fe III -bearing
	serpentine (Evans et al., 2008), with coeval reduction of water generating H 2 . Olivine is a
	magnesium-iron nesosilicate forming a solid solution series between a Mg-endmember and a
	Fe-endmember. During olivine dissolution, the olivine Fe-endmember (fayalite, Fe 2 SiO 4 )
	tends to react as,
	3 Fe 2 SiO 4 + 2 H 2 O → 2 Fe 3 O 4 + 3 SiO 2 + 2H 2	(1)
	according to the following reaction,
	6 Fe 2 SiO 4 + 7 H 2 O → 3 Fe 3 Si 2 O 5 (OH) 4 + Fe 3 O 4 + H 2	(2)
	Fayalite + water → Fe-Chrysotile + Magnetite + dihydrogen
	The Mg-endmember of olivine (Forsterite, Mg 2 SiO 4 ) reacts as,
	2 Mg 2 SiO 4 + 3 H 2 O → Mg 3 Si 2 O 5 (OH) 4 + Mg 2+ + 2OH -	(3)

Fayalite + water → Magnetite + silica + dihydrogen

At temperature below 150°C, dissolution of the olivine Fe-endmember is even thermodynamically more favorable when Fe-chrysotile is formed

[START_REF] Oze | Have olivine, will gas: Serpentinization and the abiogenic production of methane on Mars[END_REF]

Forsterite + water → Chrysotile + magnesium ions +hydroxyl ions

  than at most of the hydrothermal vents of mid-oceanic ridges. H 2 O-H 2 is seen as the preferred geothermometer but it implies that chemical equilibrium between H 2 and H 2 O has been reached (see discussion below). It is possible that chemical and isotopic equilibrium have been achieved between H 2 and CH 4 (see the following paragraph). Whatever the geothermometer used, either the Horibe and Craig (1995; Fig.7) H 2 O-H 2 and CH 4 -H 2 geothermometers, or the[START_REF] Bottinga | Calculated fractionation factors for carbon and hydrogen isotope exchange in the system calcite-carbon dioxide-graphite-methane-hydrogen-water vapor[END_REF] geothermometer, we obtained relatively narrow windows of temperature conditions in the ranges 17 to 42°C for H 2 of the H 2 -rich type and 24-50°C for H 2 of the N 2 -H 2 -CH 4 gas type, whereas the temperature conditions seem much more heterogeneous and reach much higher temperatures in the H 2 -CH 4 gas type, notably for the samples of the Philippines (up to 136°C, taking into account the δD H2 values from[START_REF] Abrajano | Methane-hydrogen gas seeps, Zambales Ophiolite, Philippines: Deep or shallow origin?[END_REF].

[START_REF] Horibe | D/H fractionation in the system methane-hydrogen-water[END_REF]

H 2 O-H 2 and CH 4 -H 2 geothermometers which consider isotope fractionations between liquid H 2 O or CH 4 and H 2 , or (2) Bottinga's 1969 water vaporhydrogen method, this strongly suggests that the generation of H 2 occurred at lower temperature conditions

  , which releases aqueous Fe II . Primary production of CH 4 from H 2 O and CO 2 (DIC in water) is achieved in a similar way as the H 2 production through the oxidation of iron (see above). The reaction of the Fe-endmember of olivine in presence of secondary CH 4 generated from an H 2 intermediate (gas samples collected in ultra-basic springs showing δD values of CH 4 between -400 et -200‰; Fig.

	dissolved CO 2 can be summarized by the following reaction,
	6 Fe 2 SiO 4 + 2 H 2 O + CO 2 → 4 Fe 3 O 4 + 6 SiO 2 + CH 4	(5)
	Fayalite + water + carbon dioxide → Magnetite + silica + methane
	while a coeval reaction occurs considering the Mg-endmember,
	2 Mg 2 SiO 4 + 2 H 2 O + CO 2 → Mg 3 Si 2 O 5 (OH) 4 + MgCO 3	(6)
	Forsterite + water + carbon dioxide → Serpentine + magnesite
	Over geologic time, in natural environments that include fractured and weathered ultrabasic
	rocks experiencing diverse aqueous geochemical conditions, passivating silica layers may not
	be widespread allowing the folling reaction,.
	3 Mg 2 SiO 4 + 4 H 2 O + SiO 2 → 2 Mg 3 Si 2 O 5 (OH) 4	(7)
	Forsterite + water + silica → Serpentine
	Also, Fe II -bearing brucite-type minerals (for example amakinite) and Fe II -bearing serpentines
	are also good candidates to react with dissolved CO 2 , as suggested by experiments (Okland et
	al., 2014).

Applying the reasoning of

[START_REF] Suda | Origin of methane in serpentinite-hosted hydrothermal systems: The CH 4 -H 2 -H 2 O hydrogen isotope systematics of the Hakuba Happo hots spring[END_REF]

mentioned above to our samples, would suggest that gases from the H 2 -CH 4 type (dry seepages in fractures) contain methane which is considered as primary while, in water-related seepages, the analytical results are more compatible with a

  2 , CH 4 in proportions under 20 mol% and the presence of H 2 within the ultra-basic springs. The fourth type corresponds to focused gas flows seeping out directly from fractures (without water flow) and characterized by high proportions of CH 4 and low N 2 . In all cases, H 2 is interpreted as a consequence of oxidation of Fe II -rich minerals present in the ophiolitic rocks (even though a contribution by microorganisms by dark fermentation processes cannot be ruled out). Taking into account the pH conditions associated with the H 2 generation, Fe III -bearing mineral were formed at the vicinity of the sites of oxidation of Fe II -bearing mineral without an important mobility of Fe 2+ . A relatively shallow H 2 production is substantiated in the H 2 -rich gas type by its association with quasi-The shallower kitchen is interpreted as the H 2 -rich gas type from meteoric water interacting with the ophiolite. The deepest kitchen is interpreted as the H 2 -CH 4 gas type from a deep fluid (carrying CO 2 from the mantle, or from dissolved carbonates or organic carbon)

	atmospheric noble gases. It is consistent with the D/H isotopic data of H 2 that may correspond
	to a fluid rock reaction occurring at low temperature (probably below 50°C), whereas higher
	temperatures are suspected for the H 2 production in the N 2 -H 2 -CH 4 gas type and notably in the
	H 2 -CH 4 gas type (probably above 100°C locally). The preservation of H 2 in the gas is directly
	related to extreme limitation of carbon in the geological setting. It is probably related to the
	carbon capture associated with massive precipitation of carbonates due to high-pH
	conditions. Concerning CH 4 , multiple origins are supported by the carbon isotopic data. The
	carbon and hydrogen sources are various. Both, a primary CH 4 (from H 2 O reacting with DIC)
	and a secondary CH 4 (from an H 2 intermediate reacting with a C source) are suspected for the
	H 2 -CH 4 gas type and for N 2 -H 2 -CH 4 and H 2 -rich gas types, respectively. In the gas mixtures,
	N 2 appears to have two distinct origins: an atmospheric component and a deep crustal
	component, as it was highlighted by the noble gas analyses. The different processes described
	involve different types of fluids, which are: a shallow meteoric fluid, a crustal fluid carrying
	metamorphic N 2 , a deep mantle originating fluid carrying primordial CO 2 . Depending on the

location with respect to the geodynamical context (Fig.

14

), these different fluids interact and mix in different proportions, yielding the types of gas mixture observed at the surface seepages.

Table 2 .

 2 δ 13 C (PDB) values of CO 2 and CH 4 (‰). δD (SMOW) values of H 2 and CH 4 (‰).

He/ 36 Ar ratios[START_REF] Deville | The origin of N 2 -H 2 -CH 4 -rich natural gas seepages in ophiolitic context: A major and noble gases study of fluid seepages in New Caledonia[END_REF]. δ 13 C of methane recorded for this gas type ranges between -38.5 and -32.4‰[START_REF] Deville | The origin of N 2 -H 2 -CH 4 -rich natural gas seepages in ophiolitic context: A major and noble gases study of fluid seepages in New Caledonia[END_REF].

Acknowledgements

The authors would like to thank Georges Ceuleneer for his precious help and knowledge on the field in Oman, Mr Al Attaly for his enthusiasm towards our research and the information and help provided in Oman, Americus Perez and Zaymon Calucin for their help on the field in the Philippines, Christophe Chevillon and François Leborgne from the « Province Sud de Nouvelle-Calédonie, service de la mer et de la protection du lagon » who helped us very efficiently in New-Caledonia, Long Li for the nitrogen isotopes measurements performed at the IPGP. This work was achieved on IFPEN funds.

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