# Complexity of graphite formation in response to metamorphic methane generation and transformation in an orogenic ultramafic body

Antoine Boutier, Isabelle Martinez, Olivier Sissmann, Samuele Agostini, Isabelle Daniel, Mark van Baalen, Sara Mana, Alberto Vitale Brovarone

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## To cite this version:

Antoine Boutier, Isabelle Martinez, Olivier Sissmann, Samuele Agostini, Isabelle Daniel, et al. Complexity of graphite formation in response to metamorphic methane generation and transformation in an orogenic ultramafic body. Geochimica et Cosmochimica Acta, 2024, 364, pp.166-183. 10.1016/j.gca.2023.10.028. insu-04462211

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https://insu.hal.science/insu-04462211v1

Submitted on 16 Sep 2025

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1 Complexity of graphite formation in response to metamorphic

2 methane generation and transformation in an orogenic

3 ultramafic body

4 Antoine Boutier¹,²,³, Isabelle Martinez², Olivier Sissmann⁴, Samuele Agostini⁵,

5 Isabelle Daniel³, Mark Van Baalen⁶, Sara Mana⁷, Alberto Vitale Brovarone⁸,⁹,⁵*

6 *Corresponding author

7 ¹Dipartimento di Scienze della Terra, Università degli Studi di Torino, Via Valperga

8 Caluso 35, 10100 Torino, Italy, ORCID: 0000-0003-4013-0566

9 ²Institut de Physique du Globe de Paris, Sorbonne Paris Cité, Université Paris

10 Diderot, UMR 7154 CNRS, 1 rue Jussieu, F-75005 Paris, France

11 ³Université Claude Bernard Lyon 1, LGL-TPE, CNRS, ENS de Lyon, Université Jean

12 Monnet Saint-Etienne UMR 5276, Villeurbanne, 69622, France

13 ⁴IFP Energies Nouvelles, Rueil-Malmaison Cedex, France

14 ⁵Institute of Geosciences and Earth Resources, National Research Council of Italy,

15 Pisa, Italy

16 ⁶Harvard University, EPS Dept., 20 Oxford St., Cambridge, MA 02138

17 ⁷Department of Geological Sciences, Salem State University, Salem, MA 01970,

18 USA

19 ⁸Dipartimento di Scienze biologiche, geologiche e ambientali, Via Zamboni, 67,

20 Bologna, Italy
21

²⁹Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie (IMPMC),
Sorbonne Université, Muséum National d'Histoire Naturelle, UMR CNRS 7590, IRD
22

23 UR206, 75005 Paris, France

24

25

Abstract

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The fate of carbon in subduction is explored by considering fluid-rock interactions in
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a body of metamorphosed ultramafic rocks from the Appalachian belt as potential
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proxies for metasomatism and carbon recycling (e.g., infiltration, solution,
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precipitation) in the mantle wedge at convergent margins. Ultramafic rocks may
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record intense redox variations affecting the exchanges between solid and fluid
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carbon-bearing phases. The Belvidere Mountain Complex (BMC) in Vermont, USA is
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an ultramafic body that underwent Ordovician Taconic subduction metamorphism up
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to 510-520 °C and 0.9-1.3 GPa. Previous investigations indicate that the BMC
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experienced partial serpentinization and carbon recycling in the subduction zone.
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Here bulk δ¹¹B, Sr⁸⁷/Sr⁸⁶ and δ¹³C, data, in-situ microRaman spectroscopy and bulk
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δ¹³C₄H₄ and δ²H₄H₄ on fluid inclusions, and numerical modeling results aim to
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constrain the origin and formation mechanisms of carbon-bearing solid and fluid
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phases in the BMC. Bulk ⁸⁷Sr/⁸⁶Sr ratios and δ¹¹B of the BMC rocks suggest
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infiltration of metasediment-derived fluids inside the mafic/ultramafic body.
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Carbonates present in the studied ultramafic and mafic rocks have δ¹³C values
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ranging from -7.26 to -1.27‰. This range was found to record progressive reduction
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of an initial isotopically light carbonate to methane under reducing conditions.
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MicroRaman spectroscopy on fluid inclusions confirms CH₄-rich gaseous
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compositions, along with N₂, NH₃, and S-H compounds, which support a
metasedimentary origin for the fluid infiltrating the BMC. Methane-rich fluid inclusions were observed in partially serpentinized peridotites, in carbonate veins, and in amphibolite bodies associated with the ultramafic body, and as well in the surrounding metasedimentary rock units. The δ¹³C and δ²H of methane in fluid inclusions show a wide range of values in ultramafic rocks from -45.2‰ to -12.6‰ for carbon and -226‰ to -140‰ for hydrogen. The methane stable isotope data suggest the presence of two types of methane preserved in studied samples, formed by high-temperature thermogenic processes of metamorphosed graphitic carbon and by abiotic conversion of more oxidized carbon bearing species in the presence of H₂-rich fluids produced during metamorphic serpentinization. The BMC also hosts a remarkable example of graphite deposit, with up to 3.90 wt% graphite, along a ~6 m-thick zone within the serpentinized ultramafic body and along lithologic contacts. The δ¹³C composition of the graphite clusters at -15‰ (VPBD), which is consistent with the composition of graphite precipitating from the mixing of the two types of methane-bearing fluids identified in fluid inclusions. The mixing between metasediment-derived and serpentinization-related methane-rich fluids is proposed as the mechanism leading to graphite precipitation. Further complexity is suggested by carbonate reduction evidenced in the samples, which may have locally contributed to the formation of graphite on the precursor carbonate domains. The BMC rocks highlight the variability of carbon recycling processes, including carbon mobilization from multiple solid sources, and mixing of multiple carbon-bearing fluids in a dynamic fluid-rock system, with carbon mobilization and sink adapting to evolving redox conditions.

68
69 Keywords: Abiotic methane; C fluxes at subductions zones; Natural graphite;
70 Isotopes; Belvidere Mountain Complex

71

72 1 Introduction

73 Subduction is a major actor in the cycling of carbon between surface and deep
74 reservoirs. As material is brought down along the subducted slab, fluid release and
75 interaction with the overlying mantle wedge creates complex pathways for element
76 mobility as well as fluid intake and outtake (Dasgupta, 2013; Dasgupta and
77 Hirschmann, 2010; Manning, 2014). Metamorphic reactions along the subduction
78 path can regulate element mobility as well as fluid speciation (e.g, Evans, 2012;
79 Hayes and Waldbauer, 2006). Carbon in subduction zone rocks can be present
80 under different valence states, from -4 to +4, in various forms, such as carbonate,
81 organic matter, and several fluid species such as CO2, CH4, as well as many others
82 (e.g. Manning et al., 2013; Sverjensky et al., 2020). Carbon mobilization in
83 subduction zones is known to produce different carbonic fluids: CO2-rich fluids
84 resulting from decarbonation reactions (Gorman et al., 2006; Kerrick and Connolly,
85 1998, 2001a, 2001b; Stewart et al., 2019), carbonate dissolution (e.g., Ague and
86 Nicolescu, 2014; Facq et al., 2014; Frezzotti et al., 2011; Piccoli et al., 2021), CH4-
87 rich fluids from serpentinization of preserved peridotite in the slab and of the
88 overlying mantle wedge (Boutier et al., 2021; Peng et al., 2021; Vitale Brovarone et
89 al., 2020a, 2017) or from organic matter-rich metasedimentary rocks (Clift, 2017;
90 Tumiati et al., 2020; Vitale Brovarone et al., 2020b) and mantle derived CO2/CH4
91 fluids (Dasgupta, 2013 and references therein; Green et al., 1987). The precipitation
92 of carbon-bearing minerals, most notably carbonates, graphite, and diamond, as well
as other compounds is largely related to the transformation of these fluids during the
percolation through crustal and mantle rocks. The interplay and interrelations among
processes of carbon mobilization and precipitation –hereafter carbon recycling– at
subduction zones are central in the evolution of Earth and life, and also to assess the
potential habitability of other celestial bodies where subduction is or was once active
(Giovannelli et al., 2022). The formation and migration of CH₄ and associated
reduced fluid species –most notably H₂– is of particular interest in the study of how
life emerged on Earth and potentially elsewhere, and how deep it could extend in the
subsurface biosphere (Colman et al., 2017; Gold, 1992; Ménez et al., 2018; Plümper
et al., 2017; Schrenk et al., 2013).

The fate of metamorphic methane in subduction is strongly dependent upon multiple
fluid-rock interactions that may take place in slab and mantle wedge forming rocks.
For instance, prograde metamorphism of organic-rich metapelitic rocks may release
aqueous fluids rich in CH₄, migration of which may favor the reactivity of more
oxidized carbonate-rich rocks and the precipitation of graphite along lithologic
boundaries (Evans et al., 2002). Or, reducing fluids derived from ultramafic or mafic
rocks may lead to the reduction of carbonate minerals, the formation of abiotic
methane, and/or the precipitation of graphite (Galvez et al., 2013; Tao et al., 2018;
Vitale Brovarone et al., 2017). Ultimately, the migration of metamorphic methane
may support metabolic reactions in the deep subsurface biosphere (Vitale Brovarone
et al., 2020a). However, field examples recording interaction of reduced carbon-rich
fluids remain insufficient to understand the fate of metamorphic methane at depth.
Here we present results from the Belvidere Mountain complex (BMC). The BMC
consists of an ultramafic/mafic body that has been interpreted as a fragment of the
lapetus ocean that was involved in the Taconic orogeny and that recorded HP
metamorphism during the Ordovician (Chew and van Staal, 2014; Gale, 1980; Honsberger et al., 2017). This locality hosts a remarkable example of metamorphic methane formation and migration and fluid-mediated graphite deposition in a subduction zone. This work integrates field, microstructural, fluid inclusion, major and trace element and isotopic geochemical data to investigate the mechanisms of metamorphic methane formation, migration, and transformation in a subduction setting.

## 2 Geologic settings

The Belvidere Mountain Complex (BMC) is a body of mafic/ultramafic rocks located in Northern Vermont and belonging to the Appalachian Mountain system. The complex has been interpreted as a remnant of an ocean-continent transition zone associated with the extension of the Laurentian margin that underwent peak blueschist-facies metamorphism during the Ordovician Taconic subduction (Chew and van Staal, 2014; Gale, 1980). It exhibits variably serpentinized peridotites, mainly dunite and harzburgite, associated with metabasic and metafelsic rocks. The BMC is tectonically embedded within Proterozoic to Cambrian metasedimentary and metavolcanic formations (Fig. 1). Taconic age, peak blueschist-facies metamorphic conditions of the BMC are constrained at 0.9-1.3 GPa and 510-520 °C in metabasic rocks (Honsberger et al., 2017; Laird et al., 1993). A recent study provides additional insight on the PT path of the massif with evidence of mineral assemblages stable at ~1.65-2.0 GPa and ~450-480 °C, and ~1.0-1.4 GPa and ~515-550 °C (Honsberger, 2023). The rocks preserve features of interactions with aqueous fluids and in the subduction zone, leading to variable serpentinization (10-100 vol% serpentine) (Boutier et al., 2021; Labotka and Albee, 1979). This
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tectonometamorphic event was dated at 505-473 Ma by $^{40}$Ar/$^{39}$Ar amphibole and mica geochronology (Castonguay et al., 2012; Laird et al., 1993).

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## 3 Methods

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MicroRaman spectroscopy of minerals and fluid inclusions was performed at the Department of Earth Sciences, University of Turin, with a LabRAM HR (VIS) (HORIBA Jobin Yvon) equipped with a 532.11 nm, solid-state Nd laser, a Super Notch Plus filter with a grating of 600 grooves/mm leading to a spectral resolution of 1 cm⁻¹. The laser emission power was set at 80 mW and focused to 5 µm with a 100x objective with a laser power on the sample < 5 mW. Calibration was performed using the 520.6 cm⁻¹ band of a silicon standard for the 100-2000 cm⁻¹ range, and the 2331 cm⁻¹ band of atmospheric N₂ for the 2000-4000 cm⁻¹ range. Four accumulations of 30-60 s were collected for each spectrum. Raman spectra of fluid inclusions were performed on double-polished thick sections.

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Scanning electron microscopy (SEM) imaging and energy-dispersive X-ray spectroscopy (EDS) compositional analyses were carried out using a Zeiss EVO15 equipped with a LaB6 source, a high-definition backscattered electron detector (HDBSE), and an Oxford Instruments UltimMax 100 detector at the Deep Carbon Lab, (Department of Biological, Geological, and Environmental Sciences, University of Bologna). Additional analyses were performed with a tungsten (W) SEM-JEOL JSM-IT300LV Scanning Electron Microscope combined with an energy dispersive spectrometry (EDS) Energy 200 system and an SDD X-Act3 detector (Oxford Inca Energy) at the Department of Earth Sciences of Turin University. Data were processed with the AztecLive and INCA softwares from Oxford Instruments. Semi-

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quantitative analysis employed 15kV accelerating voltage and a beam current of 50 nA and 20 s to 40 s counting total time. Natural and synthetic mineral and oxide standards were employed. EDS calibration was made using Cobalt standard.

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Quantitative wavelength-dispersive spectrometer (WDS) analyses were carried out using a JEOL 8200 Super Probe with a W source at the Department of Earth Sciences "Ardito Desio" of Milan University. The microprobe operating settings under maximum emission peak were 15kV accelerating voltage under 5 nA, beam diameter 1 µm, counting time 30 s, and background counting time 10 s. Sixteen oxide compositions were measured, using synthetic and natural standards: grossular (Si, Al and Ca), omphacite (Na), K-feldspar (K), fayalite (Fe), forsterite (Mg), rhodonite (Mn), niccolite (Ni), ilmenite (Ti), galena (Pb and S), pure Cr, pure Zn and pure Cu.

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Portions of samples were cut to remove weathered and surface organic contamination, and then crushed and pulverized with Plattner's steel and agate mortars. Considering the very high amount of graphite, the potential effect of organic contaminations by groundwater circulation was considered negligible. Analyses of total organic carbon (TOC) were performed on aliquots of about 1 g of dried, decarbonated samples (HCl, 6 mol/L, attacked at ambient temperature for one night) using a Flash EA1112 elemental analyzer coupled to a Thermo Finnigan DELTA plus XP isotope ratio mass spectrometer via a ConFlo IV interface at IPGP, Paris. Aliquots of the companion, non-decarbonated sample were also analyzed for Total Inorganic Carbon (TIC)+ Total inorganic carbon (TOC). Graphitic C concentrations in samples presented herein correspond to the TOC.

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The δ¹³C and the δ¹⁸O of carbonate were obtained at the Institute of Geosciences and Earth Resources, Pisa, on a GasBench II with a continuous-flow IRMS setup,

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190 after the method of Breitenbach and Bernasconi, (2011). Isotope ratios are reported
191 in permil (‰) using the conventional δ-notation and with respect to the Vienna
192 Standard Mean Ocean Water (V-SMOW) for oxygen and Vienna Pee Dee Belemnite
193 (V-PDB) for carbon. Standard deviation was 0.05‰ for both δ¹³C and δ¹⁸O.

194 The δ¹³C and δ²H of CH₄ was obtained at IFP energies nouvelles by extracting the
195 gas by crushing inclusion-rich samples in a sealed modified planetary mill. The
196 samples were placed in a ZrO jar and sealed by an inlet/outlet-equipped cover. The
197 jar was flushed with N₂ for around 60 seconds, then sealed and crushed at 700 rpm
198 for 240 seconds. The gas was then extracted from the jar by connecting vacuum
199 Exetainer® sample vials. Three vials were collected for each sample. The δ¹³C of
200 each sample was measured by two independent techniques: the first vial of each
201 sample was injected in triplicate in a Picarro G2210-i Isotope Analyzer, to obtain the
202 δ¹³C of CH₄ and the C1/C2 ratio. The two other vials were used to analyze δ²H and
203 δ¹³C of CH₄ with a GC-MS, respectively. Analyses by GC-MS were calibrated using
204 two internal gas standards.

205 Bulk δ¹¹B was analyzed at IGG-CNR, Pisa; for each sample ~0.2 g of sample
206 powder was treated by alkaline fusion in platinum crucibles with a 4:1 flux to sample
207 ratio. Boron was then extracted from the fusion cakes by repeated crushing and
208 centrifuging of the cakes in high pH B-free water. It was further purified by passing
209 the solution through anion and cation exchange columns. Anion columns were
210 packed with Amberlite IRA-743 boron-specific anion exchange resin, while cation-
211 exchange columns were packed with AG 50W-X8 resin. The procedure used an
212 anion column step, followed by a cation column step and then a final (repeat) anion
213 column step to produce the final purified boron solution, as described by (Agostini et
214 al., 2021). Boron isotopic compositions were measured on a Thermo Scientific™
Neptune series multi-collector (MC)-ICP-MS in Pisa, specially tuned for ¹¹B/¹⁰B analysis, following Foster, (2008). Samples were diluted to contain ~20 ppb B and were then bracketed with NBS 951 boric acid standard solution of the same concentration, to correct for instrument-induced mass fractionation. A detailed description of analytical procedure, along with accuracy of several analyses of NIST SRM 951 and other international and in-house standards is available in (Agostini et al., 2021).

Strontium isotope analyses were performed using a Thermo Fisher Scientific Neptune Plus MC-ICP-MS at IGG-CNR of Pisa as described in detail in Agostini et al. (2022). After sample powder digestion in concentrated HF+HNO₃, strontium was extracted from matrix in a class 100 clean room by ion-exchange purification through Sr-spec resin Strontium analyses were performed in 2% HNO₃ solution containing 100-200 ng*g⁻¹ of analyte. Values are corrected for ⁸⁷Sr/⁸⁶Sr of NBS 987 =0.710248.

Whole-rock analyses were performed at the Service d'Analyse des Roches et Minéraux (SARM, Centre de Recherches Pétrographiques et Géochimiques, Nancy, France) by alkali fusion of rock samples (LiBO₂), followed by concentration measurements using an ICP-OES iCAP 6500 (Thermoscientific) for major elements, and an ICP-MS X7 (Thermoscientific) for minor elements (protocol by Carignan et al., 2001). See Supplementary Materials 1 for details on uncertainties and detection limits.

Numerical modeling of isotopic fractionation was done with Rayleigh and batch equations and fractionation factors from Bottinga (1969) using Python. Thermodynamic modeling of COH fluid and associated carbon solid was done using equations from Huizenga (2005) to track the carbon activity using a python code.
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Both models were coupled to investigate and model the interaction of the various

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COH fluids.

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# 4 Results

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## 4.1 Field relationships

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Figure 2 and Table 1 show the location and main features of the selected samples.

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Besides samples of variably serpentinized peridotites, rodingites, and surrounding

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metasedimentary rocks, the most important samples presented in this study were

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collected from two main outcrops, graphite zone A and B hereafter, showing

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evidence of graphite precipitation. Graphite zone A consists of a vertical, ~6 m thick

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graphite-rich shear zone cutting through the ultramafic body (Fig. 2A). This is the

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locality first described in Kerper et al. (2008). Graphite zone A is separated from the

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enclosing serpentinized peridotite by a rather sharp boundary (Fig. 2B). However,

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the host serpentinite next to the graphite zone selvages appears variably

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carbonated. The carbonation is present as veins, mainly on the East side of the

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graphite zone, and micro-scale pervasive carbonation is visible on both sides of the

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graphite zone (Fig. 2C). The second outcrop, referred to as graphite zone B is

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located along the contact between the BMC serpentinized peridotites and

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amphibolites (Fig. 2D). The outcrop forms a vertical wall making the boundary with

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the adjacent amphibolite. The amphibolite next to the contact shows evidence of fluid

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circulation and metasomatism, as indicated by the presence of cross-cutting veins

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(mainly albite, carbonate, and graphite; Fig. 2E) and local rodingitization (mainly

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tremolite, garnet, epidote, carbonate; Fig. 2F).

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Samples V18-1a,c,e correspond to metasedimentary rock of the carbonaceous

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Ottauquechee Formation, from an outcrop located in Hyde Park, Vermont 20 km
south of the BMC. Sample V18-1a consists of fine-grained muscovite, quartz, chlorite, abundant organic matter, and other opaque minerals. Sample V18-1c is quartz-richer, and contains muscovite, chlorite, minor organic matter, coarse-grained isolated and locally disseminated carbonate, coarse-grained sulfides, and detrital zircons. Samples V18-2a,b, V18-3a,b and V18-11 (Fig. 3) consist of slightly serpentinized peridotites. Samples V18-5(a-i), V18-7a,c, and V18-Y(1,b) are from graphite zone A. Samples V18-6a,d, V18-8, V18-9, and V18-10 are variably carbonated serpentinites from the selvage of graphite zone A. Samples V18-12 to V18-15 are from graphite zone B and correspond to variably metasomatized amphibolites and cross-cutting veins. Sample V18-4 is a graphite-carbonate-bearing rodingite.

## 4.2 Petrographic features

Although the petrologic features of the BMC serpentinized peridotites and related mafic rocks have been treated in the literature (Boutier et al., 2021; Labotka and Albee, 1979; Laird et al., 2001), the associated graphite mineralization remains poorly investigated (Carlsen et al., 2015; Chidester et al., 1978; Kerper et al., 2008; Van Baalen et al., 2009). Kerper et al. (2008) described this graphite mineralization and proposed that it was due to fluid mixing between external fluids from underlying carbonaceous metasediments and internal methane-rich fluids within the serpentinite. However, they could not assess the origin of the methane-rich fluids. Here, the main features of graphite zones A and B are presented. Details on samples V18-2a,b, V18-3 and V18-4 (rodingites) can be found in Boutier et al. (2021). The sample series V18-5a to V18-7i corresponds to a high-resolution transect across graphite zone A (Fig. 4). The outcrop comprises two main lithologies, namely tremolite-chlorite-graphite-rich rocks located at the center of the zone (samples V18-
5c,d), and antigorite-graphite-rich rocks located at the selvages (samples V18-5a,b,e,f,g,h,i). Carbonate is present at the edge of the graphite zone as discordant veins (sample V18-5a), or in the matrix of both antigorite-graphite rocks (V18-5i, Fig. 5A). Samples V18-7a,c display graphite associated to carbonate. Samples V18-6(a,d), V18-8, V18-9, and V18-10 have disseminated carbonate-veins and do not contain graphite. Samples V18-Y1 and V18-Yb are two loose blocks collected at the base of the graphite zone showing complementary features, possibly reflecting the early stages of fluid/rock interaction.

V18-5c and V18-5d correspond to samples at the center of graphite zone A. They consist of tremolite (up to 200µm long), antigorite, chlorite, and graphite, and show complex deformation features (Fig. 5B). Graphite is present in the matrix forming coronas around tremolite crystals. Microstructural relationships indicate that at least some of the antigorite present in these samples formed at the expense of former tremolite (Fig. 5B).

Sample V18-5a is composed of antigorite and graphite with discordant carbonate veins (Fig. 5C). However, the former presence of tremolite is clearly indicated by the pseudomorphic growth of antigorite aggregates on euhedral amphibole crystals (Fig. 5C). By comparison with samples V18-5c,d, the microstructures suggest that the graphite rims surrounding these pseudomorphoses formed prior to the replacement of tremolite by antigorite. The graphite deposit coincides with the deformation experienced along the graphite zone A. Similar pseudomorphic microstructure and deformation can be observed in other antigorite samples from V18-5e-i.

Sample V18-7c corresponds to a metasomatized serpentinite to the East of graphite zone A, and consists of antigorite, dolomite, calcite, brucite, and graphite. SEM-
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based BSE imaging suggests at least local conversion of dolomite into calcite, brucite, and graphite (Fig. 5D). This microstructure, however, was not observed in any of the other studied samples.

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V18-Y1 has a breccia-like structure and consists of clasts of antigorite in a matrix of fine-grained foliated tremolite. Post-kinematic diopside porphyroblasts partially overgrew the foliated tremolite matrix and, to minor extent, the serpentine clasts (Fig. 5E). This sample does not contain graphite.

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V18-Yb shares mineralogic features of V18-Y1 with clasts of antigorite or tremolite aggregates in a matrix of fine-grained tremolite (Fig. 5F). Also, in this case, microstructures suggest that at least a part of the antigorite present in the rock formed after the former tremolite. Graphite is abundant in the rock. It is disseminated in the fine-grained tremolite matrix, and forms aggregates around larger tremolite crystals or antigorite aggregates in the clasts.

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Six samples were collected from the graphite zone B (samples V18-12a,b,c, V18-13a,b, V18-14 and V18-15). Samples V18-12a and V18-12c have structures and mineral assemblages similar to samples V18-5c and V18-5d of graphite zone A, with tremolite crystals up to ~200 µm long in a matrix of chlorite (Fig. 6A). Graphite is abundant in the matrix and forms coronas around the tremolite crystals (Fig. 6A).

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Samples V18-12c, V18-13a, V18-13b, V18-14, and V18-15 are mainly composed of brown amphibole – partially to largely replaced by green amphibole –, epidote, and titanite, in agreement with previous description of variably retrogressed amphibolite-facies mafic rocks in the BMC (Labotka and Albee, 1979). In addition, graphite was found as inclusions along the outer rim of epidote and titanite (Fig. 6B). As observed in graphite zone A, field and microstructural relationships indicate that the rock

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recorded fluid overpressure events indicated by crosscutting veins composed of albite, brown mica, carbonate, and graphite (Figs. 2E and 6C). The presence of carbonate along albite-hosted hydrofractures suggests that the carbonate post-dated the formation of the albite veins. Microstructures also suggest that graphite formed by transformation of the vein-filling carbonate rather than coprecipitating with it (Fig. 6D). This is suggested by the preferential distribution of graphite along pressure-solution domains (Fig. 6C,D). The presence of secondary, CH₄-rich fluid inclusions was identified in carbonate minerals by MicroRaman spectroscopy.

Samples V18-12c, V18-13a, V18-13b, V18-14, and V18-15 are mainly composed of hornblende, epidote, titanite, and albite-carbonate veins, consistent with the BMC amphibolite (Labotka and Albee, 1979), but show evidence of fluid infiltration as indicated by the presence of graphite and carbonate formed along grain boundaries or micro-cracks. Particularly V18-15 contains large amounts of carbonate featuring methane-rich fluid inclusions.

## 4.3 Fluid inclusions analysis

Boutier et al., (2021) described the presence of secondary fluid inclusions containing CH₄, N₂, NH₃ and S-H/H₂S in olivine from sample V18-2a in the BMC ultramafic rocks. These reduced fluids were interpreted to have formed through the process of high-pressure serpentinization. Here we provide additional MicroRaman data on fluid inclusions from several other samples from the BMC, and more precisely in olivine from a another weakly serpentinized peridotite (V18-11), in vein calcite (V18-4 and V18-6d), in titanite, plagioclase, and carbonate from amphibolites (V18-12c and V18-14), and in quartz from nearby metasedimentary formations (V18-1b).
The fluid inclusion speciation as derived from MicroRaman spectra analysis is presented in Figure 3. MicroRaman spectra (Supplementary Materials 2) show the presence of marked CH₄ bands (2912 cm⁻¹), as well as N₂ (2327 cm⁻¹), NH₃ (3324 cm⁻¹), and S-H/H₂S (2575 cm⁻¹). This water-free fluid inclusion composition was found in olivine from partially serpentinized peridotites and in calcite from the graphite zone A. The proportion of CH₄ is dominant in olivine-hosted inclusions and decreases in carbonate-hosted inclusions. Olivine-hosted fluid inclusions contain step-daughter hydrous solid phases such as lizardite and brucite, indicating that the initial fluid trapped in the olivine was aqueous. In graphite zone B, calcite-hosted inclusions show similar compositions, whereas plagioclase and titanite-hosted inclusions are water-dominated with less abundant CH₄ and N₂.

In Boutier et al. (2021) we stated that antigorite and chrysotile were the only serpentine polysomes present in the BMC rocks. However, complementary observations in sample V18-2a reveal the presence of veins of lizardite, in particular, brownish lizardite veins that had been misinterpreted as antigorite. The presence of lizardite-vein in only a few samples is not associated with compositional changes in fluid inclusion, Supplementary material 2 shows olivine-hosted inclusions composition for V18-11 similar to the composition for V18-2a presented in Boutier et al. (2021). Therefore, we conclude that this feature does not substantially affect our interpretations of the metamorphic origin of the CH₄-bearing fluid inclusions and their most plausible formation at antigorite-facies conditions as proposed in Boutier et al. (2021). The chronological relationships between antigorite and lizardite in sample V18-2a are not obvious, which possibly suggests conditions close to the antigorite-lizardite transition (380-400 °C, Schwartz et al., 2013).
383 4.4 Bulk rocks trace and major elements compositions

384 Major elements of the BMC ultramafic rocks show that the least serpentinized
385 samples of the BMC (V18-2a,b) share MgO vs Al₂O₃ compositions with both abyssal
386 and orogenic/ophiolitic peridotites (Fumagalli and Klemme, 2015). Tremolite-rich
387 samples located at the center of the graphite zone A display a decrease of SiO₂ and
388 MgO content, and an increase of Al₂O₃ and CaO content with respect to the
389 surrounding ultramafic rocks. Samples most affected by graphite zone B (V18-12b)
390 display a decrease of Al₂O₃, Fe₂O₃ and an increase of SiO and MgO in respect to
391 the less or unaffected mafic rocks (V18-12c, V18-14, V18-15). The major element
392 composition of the samples less affected by the graphite zone B are in range of the
393 mafic rocks composition presented in Spandler et al. (2003).

394 Chondrite-normalized Rare-Earth Element (REE) spider diagrams of the studied
395 samples along with major elements data are presented in Supplementary material 3.
396 Sample V18-1a has a positive REE slope, i.e., LaN > LUN, consistent with terrigenous
397 sedimentary rocks (Ague, 2017; Spandler et al., 2003). Samples of BMC variably
398 serpentinized peridotites (V18-2a,b, V18-3a, V18-5b,c,d,g,h,i, V18-7a, V18-8, V18-
399 10) have very low REE concentrations and patterns consistent with serpentinized
400 ultramafic rocks (Deschamps et al., 2013). Samples from graphite zone A (V18-5a-i)
401 and highly rich in graphite (V18-7c) have REE patterns similar to the variably
402 serpentinized peridotites. Samples from amphibolite and from graphite zone B show
403 patterns typical of gabbro, with the Eu positive anomaly attributed to preferential
404 incorporation in plagioclase (Philpotts and Schnetzler, 1968). REE spectra indicate
405 that the protolith of graphite zone A and B corresponds to depleted and differentiated
406 mantle, respectively.
407 4.5 Carbon content and isotopy

408 Total organic carbon (T ) content is presented in Table 1. The complete details
409 including standard deviation are available in Supplementary material 4. Slightly
410 serpentinized peridotite and serpentinized rocks have TOC content of 0.001-0.02 wt.
411 %. These values correspond to the range of TOC contents in ultramafic rocks from
412 Northern Apennines and ODP samples (>0.1% to 0.7%) (Schwarzenbach et al.,
413 2013). The TOC content of graphite zone A rocks ranges between 1.02 and 3.90 wt.
414 %, showing a peak of TOC in samples close to the center of the shear zone
415 (samples V18-5g and V18-5f). This peak of TOC content is not correlated with the
416 observed the presence or not of tremolite within this graphite zone (see Section 4.1).

417 The TOC content of graphite zone B ranges from 0.2 to 3.69 wt.% (Table 1). These
418 values are higher relative to the TOC content (>0.1 wt%) of mid-ocean ridge crustal
419 rocks and their metamorphic equivalents (Schwarzenbach et al., 2013; Shilobreeva
420 et al., 2011).

421 The TOC contents of samples V18-1a,c,e from the Ottauquechee formation range
422 from 0.17 wt% to 1.99 wt.% and are consistent with the range of TOC contents of
423 black schists (Peltola, 1968; Zhang et al., 2018).

424 Isotopic analyses of δ¹³Corg were conducted to investigate the processes of graphite
425 deposition and its carbon sources. δ¹³Corg data are presented in Table 1 and Figure
426 7, with uncertainties corresponding to the standard deviation. Samples from the
427 Ottauquechee formation (V18-1a-e) exhibit an average δ¹³Corg value of -25.1 ‰
428 (±0.2‰; n=3). These values are consistent with the δ¹³Corg signature of chlorite-
429 biotite zone metapelitic rocks unaffected by fluid-rock interactions from the
430 Appalachian belt (Cook-Kollars et al., 2014; Zhang et al., 2018). The average δ¹³Corg
of least serpentinized peridotites is -27.9 ‰ (±0.4 ‰; n=5), which is consistent with
the signature of serpentinized peridotites of oceanic affinity (Schwarzenbach et al.,
2013). However, it corresponds to a relatively low amount of carbon (< 0.03 wt%)
and could be attributed to recent or biological contamination, as δ¹³C of C3
photosynthesis range around -28 ‰. The δ¹³Corg values of samples from graphite
zone A and B are comparable and correspond to -15.9 ‰ (±1‰; n=9) and -15.2 ‰
(±1.2 ‰; n=3), respectively. These values are clearly different from the δ¹³Corg
signature of regular ultramafic rocks of the BMC. The value of graphite-poor
serpentinite located close to graphite zone A ranges from -29.5 to -20.2 ‰ (Table 1).
A graphite-bearing amphibolite next to graphite zone B has a δ¹³Corg value of -20.5
‰ (Table 1).

The δ¹³Corg values of serpentinites around the graphite zone A show disparity which could correspond to the influence of the graphite zone A. The δ¹³Corg of the samples are not correlated to the direct proximity to the graphite zone A, as the samples directly located near it, such as V18-6a (δ¹³C= -28.6 ‰), do not exhibit δ¹³Corg value close to the graphite zone A (δ¹³C=-15.9). However, the TOC content indicates a correlation with the graphite zones (Fig. 8A), showing that samples with increasing TOC content have values closer to that of the graphite zone A and B. Both samples from the graphite veins in the ultramafic body and the amphibolite contact show δ¹³C signature centered around -15 ‰ and is homogeneous, indicating a common source for graphite zone A and B. However, the variation of the δ¹³C inside the graphite veins appears uncorrelated to the TOC content of the samples (Fig. 8A), as samples with similar TOC content range from -17.7 ‰ to -13.6 ‰.
Carbonates (inorganic carbon) present as veins, associated with serpentine, or in rodingites exhibit a δ¹³Cₗₐₐ₄ signature ranging from -6.49 to -1.27 ‰ (Table 1 and Fig. 7). The carbonate in altered amphibolite V18-14 has a value of -7.62 ‰. These values are lower relative to the average signature of carbonate in metamorphic carbonated ultramafic rocks (Collins et al., 2015), with the exception of rocks with low carbonate/silicate ratios and affected by CO₂-producing metamorphic decarbonation reactions or fluid-deposited carbonates derived from infiltration of fluids sourced from OC-bearing metasedimentary rocks (Bebout, 1991; Bebout and Barton, 1993; Scambelluri et al., 2016). Additional interpretations are discussed in Section 5.2. The oxygen isotopic composition δ¹⁸O of carbonate relative to the SMOW ranges from 8.91 to 16.64 ‰ (Fig. 9). These values are consistent with δ¹⁸O of carbonate in ultramafic rocks metamorphosed to blueschist-facies conditions (Collins et al., 2015). However, the carbonate δ¹⁸O values indicate disequilibrium with the associated BMC antigorite serpentinites. For instance, carbonate values close to 9 ‰ are in equilibrium with serpentine (at 6.4 ‰) for temperatures around 300-400 °C, however the highest value (i.e. 16.64 ‰) would indicate equilibrium T <100 °C (Saccocia et al., 2009; Wenner and Taylor, 1971; Zheng, 1999, 1993) which is inconsistent with the antigorite-facies metamorphic evolution of the BMC (Guillot et al., 2015). From Figure 9, it can be observed that the variability of δ¹³C and δ¹⁸O of carbonate correlates with the distance from the graphite zone. This correlation is discussed in Section 5.2.

Raman spectroscopy of carbonaceous material (RSCM) present in metasedimentary sample V18-1a was conducted to estimate maximum temperature of graphite crystallization according to (Lünsdorf et al., 2014). Spectra collected (n=14) indicate a temperature of 435 +/- 14 °C.
The δ¹³C and δ²H of CH₄ extracted from fluid inclusions from the studied BMC and associated metasedimentary formations are available in Table 1. Fluid inclusions from metasedimentary sample V18-1 display a δ¹³C of -46.3 ‰ and δ²H of -226 ‰ for CH₄ and a δ²H of -812 ‰ for H₂ (Fig. 10A). Partially serpentinized peridotite (olivine-hosted) fluid inclusions from sample V18-2a show values of δ¹³CₚH₄ of -12.6 ‰ and δ²HₚH₄ of -171 ‰ while V18-11 show values of δ¹³CₚH₄ of -45.2 ‰ and δ²HₚH₄ of -226 ‰. Carbonate-hosted fluid inclusions from V18-4, V18-6d and V18-15 show values ranging from δ¹³CₚH₄ of -15.5 ‰ and δ²HₚH₄ of -168 ‰ to δ¹³CₚH₄ of -31.3 ‰ and δ²HₚH₄ of -177 ‰. Graphite-bearing titanite and plagioclase-hosted inclusions from samples V18-12a,c and V18-14 shows values ranging from δ¹³CₚH₄ of -12.6 ‰ and δ²HₚH₄ of -171 ‰ to δ¹³CₚH₄ of -12.6 ‰ and δ²HₚH₄ of -171 ‰. H₂ was present in measurable amounts only in sample V18-12c and has a value δ²H₂ of -680 ‰ (Fig. 10B).

C1/C2 ratios are presented in Table 1 and show a low value of 13.23 for V18-1 typical of thermogenic sources (Martini et al., 2008), whereas samples from the BMC show a wider range of values. Olivine-hosted inclusions in sample V18-2a has a C1/C2 ratio of 1992, while V18-11 have a value of 668. Carbonate-hosted fluid inclusions in V18-4 have a C1/C2 ratio of 6154 and of 635 in V18-6d. The amount of C₂H₆ is below the detection threshold in some samples leading to the absence of C1/C2 ratio.

## 4.6 Sr and B isotopes results

Strontium isotopic analyses were conducted on different BMC rock types and are presented in Table 1 and Figure 8B. The ⁸⁷Sr/⁸⁶Sr of the sample V18-1a from the Ottauquechee formation is 0.7578. The values of the BMC least serpentinized peridotites (V18-2a,b and V18-3a) range from 0.7082 to 0.7095 and ⁸⁷Sr/⁸⁶Sr of least
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altered amphibolite (V18-14) is 0.7093. Serpentinite and samples from the graphite zones have similar $^{87}$Sr/$^{86}$Sr values between 0.7107 and 0.7177. Overall, the $^{87}$Sr/$^{86}$Sr ratios of BMC samples is higher than Basaltic Achondrite Best Initial (BABI), and are not compatible with Mid-Ocean Ridge Basalt (MORB) signature and depleted mantle (DM) signature.

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In a similar way, boron isotopes $\delta^{11}$B were investigated to assess fluid sources in the BMC. $\delta^{11}$B values are negative (Table 1, Fig. 8B). The $\delta^{11}$B of the Ottauquechee formation (V18-1a) is -8.5 ‰. The $\delta^{11}$B of the BMC least serpentinized peridotites (V18-2a,b and V18-3a) ranges from -1.6 to -0.8 ‰ and $\delta^{11}$B of the least altered amphibolite (18-14) is -3.49 ‰. Serpentinite samples (V18-7c and V18-10) and from graphite zone A (V18-5c) have $\delta^{11}$B values ranging from -4.6 to -2.4 ‰.

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# 5 Discussion

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## 5.1 Origin of the Belvidere graphite

The BMC graphite distribution and isotopic signatures suggest a fluid-deposited origin. The graphite concentrations in the BMC up to 3.91 wt% are very high compared to the average organic carbon content of subducted ultramafic and mafic rocks, usually clustering at 0.1 wt % (Alt and Teagle, 1999; Collins et al., 2015; Schwarzenbach et al., 2013; Shilobreeva et al., 2011). The graphite $\delta^{13}$C (-15.9 ‰, ± 1‰) in Zone A and (-15.3 ‰, ± 1.2‰) in zone B differs from the total organic carbon signature of subducted mafic and ultramafic rocks from -26.6 ‰ to -28 ‰ (Alt and Teagle, 1999; Collins et al., 2015; Schwarzenbach et al., 2013; Shilobreeva et al., 2011), and from mantle carbon at -10 ‰ to -4 ‰ (Deines, 1992; Javoy et al., 1986).

The graphite host rocks also confirm the hypothesis of fluid circulation and intense metasomatism. In particular, in graphite zone A, structural and geochemical features
528 indicate that the rock was transiently enriched in Ca, Si and Al, as indicated by the
529 presence of tremolite and chlorite, as observed in sample V18-Y1 with the formation
530 of tremolite matrix sealing serpentine clasts (Fig. 5D). This stage is followed by
531 graphite precipitation, as observed in samples V18-Yb and V18-5c, with the growth
532 of tremolite crystals, the partial preservation of antigorite clasts and the precipitation
533 of graphite coronae around tremolite crystals (Fig. 5(A,E)). Finally, the tremolite was
534 statically replaced by antigorite, indicating a loss of Ca and enrichment in Mg, as
535 observed in sample V18-5a (Fig. 5B). The collected samples suggest that this stage
536 was more intense at the edges of graphite zone A. Carbonate precipitation at zone A
537 selvages may have occurred during the initial stages of fluid infiltration, or earlier.
538 Microstructural features indicate that the initial stage of fluid infiltration was
539 accompanied by fluid overpressure and brecciation, followed by intense syn-
540 tremolite shear deformation, and finally static replacement of tremolite by antigorite.

541 In graphite zone B, graphite precipitated at the interface between ultramafic and
542 mafic rocks. Also, in this case, petrography and microstructural analysis indicates
543 that fluid overpressure was locally achieved in the mafic rocks adjacent to the
544 ultramafic rocks, as indicated by brecciation of albite and precipitation of carbonate
545 along fractures. These mineralogical and structural relationships indicate that the
546 carbonate precipitation event took place at retrograde post-amphibolite greenschist-
547 facies conditions (albite stable). The carbonate stable isotope signature may be
548 consistent with an organic matter-rich metasedimentary fluid source (see also
549 (Spandler et al., 2008) for similar interpretations in New Caledonia). This
550 interpretation would be consistent with the regional setting of the BMC. The
551 microstructural distribution of graphite, and the presence of CH4-rich fluid inclusions
552 in the rock, may suggest a possible H2-mediated carbonate reduction as the origin of
the graphite (Galvez et al., 2013; Giuntoli et al., 2020; Malvoisin et al., 2012; Peng et al., 2022; Vitale Brovarone et al., 2017). This is also suggested by some microstructures in graphite zone A (Fig. 5E) and in other metamorphic settings in the presence of subduction zone serpentinization (Giuntoli et al., 2020; Vitale Brovarone et al., 2017). A prograde, closed system carbonate reduction (e.g., Tao et al., 2018) appears inconsistent with the retrograde timing of the graphite formation (syn-greenschist facies).

## 5.2 Source of carbon for methane formation

The graphite and methane isotopic composition alone does not allow identifying a clear carbon source. In particular, the graphite signature of $\delta^{13}C = -17$ to $-15$ ‰ may be compatible with either organic or inorganic sources, internal or external to the BMC. For example, graphite in metasedimentary rocks from Appalachian metasedimentary rocks shows $\delta^{13}C$ ranging from about $-27$ to $-2$ ‰ (Evans et al., 2002; Zhang et al., 2018). Fluid-deposited graphite from New Hampshire, which is interpreted to derive from mixing of metasediment-derived fluids, shows $\delta^{13}C$ values ranging from $-29$ to $-9$ ‰ (Rumble et al., 1986). The methane isotopic signature of $\delta^{13}C$ and $\delta^{2}H$ from BMC rocks indicates, at least in some samples, the contribution of biogenic organic matter (Fig. 10A). Samples V18-11, with an isotopic composition of $-45.2$ ‰ for $\delta^{13}C_{CH4}$ and $-226$ ‰ for $\delta^{2}H_{CH4}$ has values similar to the methane from graphite-rich metasedimentary sample V18-1 ($-46.3$ ‰ for $\delta^{13}C_{CH4}$ and $-226$ ‰ for $\delta^{2}H_{CH4}$). In other samples of partially serpentinized peridotite petrographically equivalent to V18-11, sample V18-2, the methane signature is very different ($\delta^{13}C_{CH4} -12.9$ ‰; $\delta^{2}H_{CH4} -171$ ‰), which may be consistent with processes involving various
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sources, including organic and inorganic (i.e., carbonate) sources, internal or external to the BMC.

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The combination of ⁸⁷Sr/⁸⁶Sr and δ¹¹B data show a mixing trend between two end members (Fig. 8B), with a metasedimentary pole defined by V18-1 with low δ¹¹B value (-8.48 ‰) and extremely high ⁸⁷Sr/⁸⁶Sr value (0.7578) and on the other end a serpentinized peridotite pole defined by V18-2b and V18-3a (δ¹¹B=-0.83 ‰ and -1.41 ‰, ⁸⁷Sr/⁸⁶Sr= 0.7095 and 0.7082). The ⁸⁷Sr/⁸⁶Sr values of the BMC ultramafic and mafic rocks are either higher relative to their protolith rocks, or seawater which suggest contamination by sedimentary or crustal reservoirs (Cannaò et al., 2016; Yamaoka et al., 2012). The δ¹¹B values of the BMC rocks are negative, which is consistent with serpentinization and metasomatism driven by the infiltration of metamorphic fluids (Martin et al., 2020), rather than seafloor metamorphism, which would result in much higher δ¹¹B.

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The presence of high nitrogen concentrations in the BMC fluid inclusions (enriched in N₂ and NH₃ species) also suggests a metasedimentary source (Boutier et al., 2021 and references therein). Additionally, the presence of abundant sulfur in the BMC fluid inclusions is also consistent with a metapelitic fluid source (Bebout and Fogel, 1992; Evans et al., 2014; Kelemen and Manning, 2015; Plank and Manning, 2019).

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According to the above considerations, at least a part of the carbon in the system was sourced from the surrounding metasedimentary formations. However, the collected data also outline an internal production of CH₄. For example, SEM images suggests carbonates of the BMC as a potential source of carbon for methane production through reduction processes (Sections 4.2). The carbonate δ¹³C and δ¹⁸O data in the BMC ultramafic rocks may be interpreted as a reactional trend (Fig.
9). Graphite-free carbonated serpentinites (V18-6a and V18-14) show low carbonate δ¹³C and δ¹⁸O values while carbonate from graphite-bearing ultramafic rocks (V18-7c and V18-5i) shows higher δ¹³C and δ¹⁸O values. Carbonate-graphite isotopic equilibrium would lead to unrealistic temperatures or gradients, from 150°C to 550°C from the graphite-free selvages to the center of graphite zone A based on fractionation factors from Bottinga, (1968). Instead, owing to the positive carbonate-methane isotopic fractionation during carbonate reduction, it is predicted to produce such a trend in the remaining carbonate fraction (Fig. 9; Galvez et al., 2013; Vitale Brovarone et al., 2017). Alternatively, because of the negative carbonate-carbon dioxide isotopic fractionation, carbonate-silicate reactions producing carbon dioxide with increasing metamorphic conditions is known to induce a decrease in δ¹³C (Cook-Kollars et al., 2014; Piccoli et al., 2016; Valley, 1986) and could explain the reactional trend. However, the observed isotopic shift seems to correlate with the formation of graphite rather than with an increase in carbonate-silicate reactions. Based on the above considerations, the lower δ¹³C carbonates are interpreted as the earliest ones in the studied BMC rocks. These values are low compared to marine carbonate and carbonate in ophicarbonates (Collins et al., 2015). Such low initial values may have been achieved as a result of prograde decarbonation reactions, or from the precipitation of an isotopically light isotopic fluid source such as the above-mentioned graphite-bearing metasediment-derived fluid. A similar process was described for fluid-deposited carbonates found in metasomatized ultramafic rocks in New Caledonia (Spandler et al., 2008).

As outlined in Section 5.3, the evolution of the external metasediment-derived fluid during progressive serpentinization of the ultramafic body and its mixing with methane-rich reduced fluids resulting from carbonate reduction may easily overprint
the original δ¹³C of the infiltrating fluid in favor of the newly formed methane (i.e., the methane formed through carbonate reduction). Figure 10A shows the mixing trend between the metasedimentary source at δ¹³C =-45‰, δ²H =-240‰ and the methane produced through carbonate reduction ranging from δ¹³C = -23‰ to -12‰ and δ²H = -140‰. Figure 11 shows that the model of methanation of carbonate from the BMC is consistent with the isotopic value of the methane ranging outside the thermogenic domain. However, such a trend could have been shifted to lower values by varying mixing trends with the isotopically lighter (δ¹³C= -45 ‰) metasediment-derived fluid.

### 5.2.1 Fluid evolution and mechanism of graphite formation

Graphite precipitation may occur through multiple processes, which depend on the evolution of fluids and/or their interactions with the host rock. Four different mechanisms, which likely all together participated in the precipitation of graphite in the BMC, are considered in the present study, namely fluid dehydration –hereafter desiccation–, fluid mixing, changes in the ambient redox conditions, and carbonate reduction (Connolly, 1995; Duke and Rumble, 1986; Galvez et al., 2013; Holloway, 1984; Luque del Villar et al., 1998; Malvoisin et al., 2012; Vitale Brovarone et al., 2017). Other mechanisms, such as isobaric cooling or isothermal compression, are considered less plausible and will not be discussed further.

In a COH system, the graphite stability field is not only favored by with increasing pressure, and diminishes with increasing temperature (Cesare, 1995). Its stability is also dependent on the fluid component speciation, which is in turn dependent on the redox state of the system, and graphite can precipitate at fixed P-T conditions for varying fluid compositions and redox conditions. The redox state of a ternary COH system has been discussed as a function of the parameter $X_o$ ($X_o = \frac{n_O}{n_O+n_H}$) (Connolly,
649 1995; Connolly and Cesare, 1993). The maximum graphite stability for low carbon
650 concentrations in a COH fluid is observed when the fluid-component speciation is
651 dominantly H₂O, the so called "water maximum" (X₀=1/3). At fixed P and T, fluid
652 infiltration can lead to graphite precipitation if X₀ changes during the fluid-rock
653 interaction. In a similar fashion, desiccation will lead to a change in the ratio of H₂O,
654 CO₂, and CH₄ in the fluid and to the precipitation of graphite according to the
655 following reaction: CH₄ + CO₂ = 2C + 2H₂O (Duke and Rumble, 1986). Fluid
656 interactions have the potential to deposit graphite in amounts varying as a function of
657 the intensity of the X₀ variations. Desiccation leads to graphite precipitation
658 proportional to the intensity of the water loss. A micro-scale example of fluid
659 desiccation is testified by the presence of olivine-hosted gaseous fluid inclusions
660 filled with hydrous step-daughter minerals in the BMC, which indicate that the water
661 present in the original entrapped fluids was progressively exhausted by hydration
662 reactions of the inclusion walls (Boutier et al., 2021; see also Klein et al., 2019). With
663 progressive exhaustion of the H₂O component and residual enrichment of carbon in
664 the remaining fluid, the system may reach carbon saturation and graphite
665 precipitation (see also Andreani et al. (2023) for the precipitation of different
666 carbonaceous complexes). Graphite was found only locally as a step-daughter
667 mineral in the BMC fluid inclusions (Boutier et al., 2021). This indicates that fluid
668 desiccation inside the fluid inclusions cannot account for the precipitation of
669 substantial graphite amounts along shear zones and lithologic boundaries as
670 observed in the BMC. However, the desiccation process may have taken place at
671 larger scales during the progressive infiltration of metamorphic fluids responsible for
672 the partial hydration of the BMC ultramafic rocks.
Based on the collected data, the main fluid infiltration event can be described as an H₂O-rich fluid equilibrated with a carbon-bearing metasedimentary source. Sample V18-1 suggests that this fluid may have had a low δ¹³C signature (Table 1). This initial isotopic composition may be preserved in the fluid inclusions of samples V18-11 and V18-14, as indicated by the δ¹³C and δ²H values of CH₄. A slightly reduced (X₀ ≲ ¹/₃) graphite-saturated COH fluid near water maximum can be dominated by CH₄ with respect to CO₂. For instance, a carbon-saturated COH fluid at X₀=0.33 (QFM -0.5) at 450 °C and 1.1 GPa (conditions estimated for the hydration of the peridotite body and consistent with the thermal metamorphic peak of the surrounding metasedimentary units, Boutier et al., 2021) would consist of 98% (molar) of H₂O with a (CO₂/CH₄+CO₂)=0.32. Such fluid would have plenty of water available to hydrate the peridotite body. This hypothesis is supported by the fluid inclusion composition (Boutier et al., 2021) and bulk-rock isotopic signatures discussed in the previous sections, which all together suggest serpentinization by metasediment-derived fluids. Owing to the formation of hydrous phases such as antigorite and chlorite and to the partial oxidation of iron to form magnetite, the evolution of this externally derived fluid during serpentinization would encounter two important changes, which are progressive desiccation and reduction. The decreasing fO₂ during metamorphic serpentinization of the BMC may have in fact enhanced the conversion of the more oxidized fraction (e.g., carbon dioxide, bicarbonates) of the infiltrating fluid to additional methane. Because a carbon-saturated fluid at water maximum corresponds to the minimum of carbon solubility, desiccation of this fluid toward more reducing conditions would increase the fraction of carbon in the fluid in favor of methane-dominated compositions. This process, described by the model in Figure 12A-D, may precipitate graphite. The model considers desiccation of 100 moles of
metasediment-derived fluid at 450 °C and 1.1 GPa in a closed system. The initial
redox state of the fluid was set slightly below the water maximum (Xo = 0.33) in
order to allow the model to spontaneously proceed toward more reducing conditions
during desiccation (Fig. 12A-C). The model predicts that full desiccation of this fluid
would precipitate 1.24 moles of graphite (per 100 moles of initial fluid, Fig. 12D).
However, this model fails at reproducing the graphite isotopic compositions as
measured in the BMC sample. Considering an infiltrating fluid with δ¹³C = -45‰ (as
measured in samples V18-1A and V18-11), desiccation would lead to the
precipitation of graphite with a too low δ¹³C signature (Fig. 12E). A plausible way to
obtain relative heavier δ¹³C value is the participation of carbonate present in the
BMC mafic and ultramafic rocks. The BMC rocks present evidence of carbonate
reduction, as illustrated by SEM imaging (V18-7c) (Fig. 5D), suggesting reduction of
dolomite into calcite and brucite along with the release of CH₄ and H₂O (Peng et al.,
2021), and formation of graphite around dolomite relicts. The reduction of carbonate
in the ultramafic body, favored by decreasing fO₂ in the evolved infiltrating fluid,
could additionally increase the methane concentration in the fluid and impose a
heavier isotopic composition to the fluid. Isotopically labeled dissolution experiments
of carbonate-graphite mixtures have shown that the isotopic composition of graphite-
derived CO₂ can be buffered by the isotopic composition of carbonate, regardless of
the carbonate/graphite ratios (Tumiati et al., 2022). Those experiments did not
consider reducing conditions. However, considering that the reduction of carbonate
releases more carbon than carbonate dissolution in pure water (Lazar et al., 2012),
and that the initial infiltrating fluid likely was water-rich and carbon-poor (near the
water maximum), it can be expected that the carbon isotopic composition of the
precipitated graphite was controlled more by the carbonate than the desiccated
infiltrating fluid. Moreover, the infiltrating fluid was initially water-dominated, and its desiccation would produce a much smaller amounts of carbon-rich fluid, favoring carbonate as a carbon isotopic buffer. In this case, the transformation of methane at $\delta^{13}C = -23\%$ produced from the reduction of the least altered carbonate in the BMC ultramafic rocks would lead to the precipitation of graphite with $\delta^{13}C = -15\%$ to $-16.5\%$ at 400-450°C. This calculated $\delta^{13}C$ range fits well the range of $\delta^{13}C$ values measured for the BMC graphite and could explain its precipitation mechanism through coupled desiccation and carbonate reduction. Furthermore, the participation of carbonate in the graphite precipitation mechanism may explain the mineralogical variations within graphite zone A, with replacement of antigorite by tremolite.

The distribution of graphite in the BMC along shear zones and lithologic boundaries suggests that these structures acted as major fluid channels. Although the mechanism of graphite precipitation presented above may explain the measured graphite $\delta^{13}C$ values, it represents a static, or instantaneous process, with one single fluid influx from the surrounding metasedimentary rocks. Because a more realistic, dynamic process may imply continuous or multiple fluid influx from the surrounding metasedimentary units and the migration of the evolved fluids outside the BMC, we also consider a graphite precipitation model involving mixing between the metasediment-derived fluid and its evolved (desiccation + carbonate reduction) equivalent.

In this model, presented in Figure 12F-J, the mixing of an external metasedimentary fluid with a $\delta^{13}C$ of methane at -45‰ (sample V18-1) and a methane-rich fluid around $\delta^{13}C = -23\%$ (samples V18-2 and V18-4) formed inside the BMC, would lead to the precipitation of graphite at $\delta^{13}C = -14\%$ to -18 ‰ at 450°C and 1.1 GPa
747

(pressure constrained by Honsberger, 2015; Laird et al., 1993 for the BMC) (Fig.

748

12J). The amount of graphite precipitated through this mechanism is estimated to be

749

1.39 mol for 200 mol of fluid and corresponds to 5% of the carbon in the initial fluid

750

(Fig. 12).

751

This model shows that the observed carbon isotope values of graphite, carbonate,

752

and methane can be explained through the infiltration of a single low δ¹³C fluid, its

753

evolution during serpentinization inside the BMC, and their remixing along major fluid

754

channels. The escape of the strongly reduced fluid formed in response to

755

serpentinization may have encountered redox variations while reaching the

756

surrounding rocks. This may also participate in the formation of graphite-rich rocks in

757

graphite zone B. The BMC would act as an open system with infiltration localized

758

along fluid channels with initial intake of metasedimentary equilibrated water and

759

outtake of serpentinization-related reduced fluid.

760

The above models are also supported by the silicate fraction of the graphite-rich rocks.

761

Samples from graphite zone A show an initial Ca enrichment and Sr isotopic

762

signature consistent with metasediment-derived fluids, followed by a Ca depletion

763

and Mg enrichment, which may be consistent with a reinjection of evolved,

764

serpentinization-related fluids inside a preferential fluid pathway such as graphite

765

zones A and B. The petrological features observed at the edge of the graphite zone

766

A are consistent with the escape of an internal Mg-rich fluid leading to the

767

retromorphosis of tremolite crystals to antigorite (V18-5a)(Fig. 5B).

768

In summary, the precipitation of graphite in the BMC can be summarized in four

769

majors steps, as follows, and is illustrated in an idealized sketch in Figure 13:
1. The BMC was infiltrated by metasediment-derived fluids, resulting in metasomatism (e.g, Ca enrichment; negative $\delta^{11}B$; high $^{87}Sr/^{86}Sr$) along major fluid channels (e.g. graphite zones A and B).

2. This fluid progressively equilibrated with the BMC ultramafic rocks during pervasive serpentinization in the antigorite stability field and may be responsible for the precipitation of isotopically light carbonate in the BMC rocks.

3. The metamorphic serpentinization of the BMC led to progressive desiccation of the infiltrating fluid and decreasing $fO_2$ conditions, which promoted additional methane formation through carbonate reduction and/or respeciation of more oxidized fluids. These processes may have participated in the precipitation of graphite.

4. The CH$_4$-enriched fluids resulting from the BMC serpentinization and carbonate reduction were reinjected along fluid channels, causing additional metasomatism (Mg enrichment and Ca depletion) and favoring fluid mixing and additional graphite precipitation.

### 5.3 Consideration on the origin of metamorphic methane in the BMC

The BMC methane is an example of an increasing number of reports of methane in subduction zone metamorphic rocks. Abiotic methanogenesis in ultramafic rocks has been shown at natural conditions spanning seafloor hydrothermalism, magmatism, and on-land surficial serpentinization (e.g., Etiope and Sherwood Lollar, 2013). This is an important process in deep subsurface biosphere research and geoastrobiology, as it is considered as a source of prebiotic organic compounds and their possible
792

role on the origin of life on Earth and potentially elsewhere (Colman et al., 2017;

793

Holm et al., 2015; Schulte et al., 2006).

794

The initial infiltrating fluid of the proposed evolution may have contained CH₄. If this

795

fluid equilibrates at water maximum, graphite-saturated conditions, a roughly

796

comparable concentration of CH₄ and CO₂ is expected. Sample V18-1 from the

797

Ottauquechee metasedimentary formation contains CH₄ in fluid inclusions. This CH₄

798

can be interpreted as methane formed as a result of graphite-saturated COH

799

thermodynamics, as conventionally proposed for metapelitic rocks, which would be

800

by definition abiotic. However, the analyzed δ¹³C and δ²H data of CH₄ from this

801

sample indicate a thermogenic origin, which is classically considered as biotic.

802

Thermogenic production of CH₄ is generally believed to be limited to about 300 °C

803

(Mangenot et al., 2018; Stolper et al., 2015, 2014). The Ottauquechee

804

metasediments experienced metamorphism to about 434 °C, as indicated by RSCM

805

thermometry. This temperature is well above the conventional limit of thermogenic

806

methane. The interaction of this fluid with the BMC promoted serpentinization and

807

additional reduction of the fluid. During this event, the CO₂ present in the infiltrating

808

fluid (for reference, a graphite saturated fluid at 400 °C, 1.0 GPa, and Xo = 0.333

809

contains a CH₄/CO₂ ratio of 1) is expected to be converted into CH₄ through a

810

process of CO₂ reduction that is, by definition, abiotic. However, its signature would

811

still appear biotic.

812

The reduction of carbonates by the serpentinization-related reducing fluids promoted

813

additional formation of methane. This methane must be considered abiotic in origin

814

and, depending on the degree of mixing with the external fluid, may still record its

815

inorganic source. However, in the case of the BMC, the isotopic signature of the
816

reactant carbonates was negative (-7.26 to -1.43 ‰) and was potentially affected by
previous interaction with fluids recording an organic carbon source.

817

818

Grozeva et al., 2020 report δ¹³C isotopic values for abiotic methane in inclusions from two actives serpentinization systems ranging from -16.7 ‰ to -4.4 ‰ in the Von Damm hydrothermal field and ranging from -12.4 ‰ to -2.8 ‰ for the Zambales ophiolite. In both datasets C1/C2 values range from 163 to 1372 and cluster around 400. The main mechanism proposed for methane formation is the conversion of mantle CO₂ to CH₄ and the carbon isotopic composition of the CH₄ reflects the isotopic composition of mantle CO₂ along with variations caused by the carbon isotopic composition in the host rock. Data from Grozeva et al. 2020 are akin to values of V18-2 and V18-4 fluid inclusions reflecting the abiotic hydrocarbon production of the BMC. However, the fluid composition of the BMC (rich in C, N, and S) strongly differs from the inclusions analyzed by Grozeva et al. 2020, and the collected data point to a different, metamorphic origin in the Taconic subduction zone.

819

820

821

822

823

824

825

826

827

828

829

830

831

The distribution of more biotic or abiotic signatures across the BMC depict the complexity of the process of fluid infiltration and fluid-rock interactions that affected the massif. Also, the collected data stress the complexity of methane generation and evolution in metamorphic settings and call for additional analyses on these metamorphic gases. In particular methane clumped isotope analysis has proven to provide additional insight on the discrimination between biotic and abiotic processes (Ono et al., 2021; Shuai et al., 2018; Young et al., 2017).

832

833

834

835

836

837

838

In conclusion, the results obtained in this work indicate that the origin and chemistry of metamorphic methane can be more complex than previously assumed.

839
840 5.4 Implication on large scale methane production

841 Subducted fresh ultramafic bodies can act as massive converter factories of CO₂ into
842 CH₄ and other reduced carbon phases such as graphite. Because these fluids
843 circulate in the subduction channel, they can have a significant impact on the redox
844 budgets in the subduction zone. Redox barriers, that is an interface between two
845 compartments with a steep difference in redox conditions, imposed at lithological
846 contacts for example, can act as carbon sinks, such as graphite deposition,
847 effectively halting a percentage of the C transfer in metamorphic fluids.

848 When considering next the serpentinizing mantle wedge, which could similarly act as
849 a powerhouse of H₂ and CH₄ formation, the path taken by the resultant fluids will
850 impact significantly on the formation or precipitation of these compounds. This can in
851 turn hamper the quantification of the methane release from subduction zones, as
852 many redox barriers – including the activity of microorganisms within the biosphere –
853 can be expected within the mantle wedge and above inside the crust.

854 6 Conclusions

855 Petrological, Raman, and isotopic data provided additional insights on the fate of
856 carbon at the Belvidere Mountain Complex, Appalachian belt, Northern Vermont.
857 The exceptional graphite deposit observed across the study site highlights limitations
858 on carbon mobility in the subduction zone and the complex mechanism of fluid
859 interaction with evolving redox conditions. The carbon, strontium, and boron isotopes
860 indicate fluid infiltrations evolving from metasedimentary rocks in subduction zones
861 as the initial fluid influx responsible for the high-pressure serpentinization and the
862 first metasomatic event. This is consistent with the model proposed by Kerper et al.
863 (2008). Isotopic investigation of methane in fluid inclusions highlights a complex
864 evolution involving both metasediment-derived, high-temperature thermogenic
865 (biotic) and serpentinization-related (abiotic) CH4-forming processes. Collected data
866 and numerical results point toward graphite precipitation as the results of the mixing
867 of CH4 released by the HP serpentinization of the BMC with more oxidized infiltrating
868 fluid (possibly the same fluid driving the serpentinization). This study outlines that
869 ultramafic bodies in subduction can retain their potential to be abiotic producers of
870 methane during relatively shallow subduction conditions (before antigorite
871 destabilization). However, the redox state of surrounding formations seems to play
872 an important role as it could severely impede fluid mobility of carbon. In this scope,
873 the mantle wedge interacting with the slab as a source of water and carbon could
874 host similar reactions, with implications for the redox state of subduction, carbon
875 cycling in subduction and for the interpretation of biotic vs. abiotic signatures on
876 earth and potentially beyond. A recent study highlights such a phenomenon with the
877 high oxidation potential of the metasedimentary cover of the slab demonstrating
878 capability to convert a high amount of fluid (Ague et al., 2022).

879 7 Acknowledgment

880 The authors wish to thank Leslie White for fundamental support during the field work.
881 IFP Energies Nouvelles and its helpful personnel is thanked for their help in the
882 acquisition on test isotopic data of methane. This work is part of a project that has
883 received funding from the European Research Council (ERC) under the European
884 Union's Horizon 2020 research and innovation program (Grant agreement No.
885 864045; project acronym DeepSeep). A MIUR Rita Levi Montalcini grant and a
886 Richard Lounsbery grant to AVB are also acknowledged. Andrea Risplendente is
887 thanked for assistance during microprobe analysis.
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1081 Manning, C.E., 2014. A piece of the deep carbon puzzle. Nat. Geosci. 7, 333–334.

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1085 Martin, C., Flores, K.E., Vitale Brovarone, A., Angiboust, S., Harlow, G.E., 2020. Deep mantle serpentinization in subduction zones: Insight from in situ B isotopes in slab and mantle wedge serpentinites. Chem. Geol. 545, 119637.

1088 Martini, A.M., Walter, L.M., McIntosh, J.C., 2008. Identification of microbial and thermogenic gas components from Upper Devonian black shale cores, Illinois and Michigan basins. Aapg Bull. 92, 327-339.

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1095 Ono, S., Rhim, J.H., Gruen, D.S., Taubner, H., Kölling, M., Wegener, G., 2021. Clumped isotopologue fractionation by microbial cultures performing the anaerobic oxidation of methane. Geochim. Cosmochim. Acta 293, 70-85.

1098 Peltola, E., 1968. On some geochemical features in the black schists of the Outokumpu area, Finland. Bull Geol Soc Finl. 40, 39-50.

1100 Peng, W., Tumiati, S., Zhang, L., Tiraboschi, C., Vitale Brovarone, A., Toffolo, L., Poli, S., 2022. An Experimental Study on Kinetics-Controlled Ca-Carbonate Aqueous Reduction into CH₄ (1 and 2 GPa, 550° C): Implications for C Mobility in Subduction Zones. J. Petrol. 63.

1104 Peng, W., Zhang, L., Tumiati, S., Vitale Brovarone, A., Hu, H., Cai, Y., Shen, T., 2021. Abiotic methane generation through reduction of serpentinite-hosted dolomite: implications for carbon mobility in subduction zones. Geochim. Cosmochim. Acta 311, 119-140.

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1110 Piccoli, F., Ague, J.J., Chu, X., Tian, M., Vitale Brovarone, A., 2021. Field-based evidence for intra-slab high-permeability channel formation at eclogite-facies conditions during subduction. Geochem. Geophys. Geosystems 22, e2020GC009520.

1113 Piccoli, F., Vitale Brovarone, A., Beyssac, O., Martinez, I., Ague, J.J., Chaduteau, C., 2016. Carbonation by fluid-rock interactions at high-pressure conditions: implications for carbon cycling in subduction zones. Earth Planet. Sci. Lett. 445, 146-159.

1116 Piccoli, F., Vitale Brovarone, A., Ague, J.J., 2018. Field and petrological study of metasomatism and high-pressure carbonation from lawsonite eclogite-facies terrains, Alpine Corsica. Lithos 304-307, 16-37.

1119 Plank, T., Manning, C.E., 2019. Subducting carbon. Nature 574, 343-352.

1120 Plümper, O., King, H.E., Geisler, T., Liu, Y., Pabst, S., Savov, I.P., Rost, D., Zack, T., 2017. Subduction zone forearc serpentinites as incubators for deep microbial life. Proc. Natl. Acad. Sci. 114, 4324-4329.

1123 Rumble, D., Duke, E.F., Hoering, T.L., 1986. Hydrothermal graphite in New Hampshire: Evidence of carbon mobility during regional metamorphism. Geology 14, 452-455.

1125 Saccocia, P.J., Seewald, J.S., Shanks III, W.C., 2009. Oxygen and hydrogen isotope fractionation in serpentine-water and talc-water systems from 250 to 450 C, 50 MPa. Geochim. Cosmochim. Acta 73, 6789-6804.

1128 Scambelluri, M., Bebout, G.E., Belmonte, D., Gilio, M., Campomenosi, N., Collins, N., Crispini, L., 2016. Carbonation of subduction-zone serpentinite (high-pressure ophicarbonate; Ligurian Western Alps) and implications for the deep carbon cycling. Earth Planet. Sci. Lett. 441, 155-166.

1132 Schrenk, M.O., Brazelton, W.J., Lang, S.Q., 2013. Serpentinization, carbon, and deep life. Rev. Mineral. Geochem. 75, 575-606.
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1135 Schwartz, S., Guillot, S., Reynard, B., Lafay, R., Debret, B., Nicollet, C., Lanari, P., Auzende, A.L., 2013. Pressure–temperature estimates of the lizardite/antigorite transition in high pressure serpentinites. Lithos 178, 197–210.

1136 Schwarzenbach, E.M., Früh-Green, G.L., Bernasconi, S.M., Alt, J.C., Plas, A., 2013. Serpentinization and carbon sequestration: A study of two ancient peridotite-hosted hydrothermal systems. Chem. Geol. 351, 115–133.

1137 Shilobreeva, S., Martinez, I., Busigny, V., Agrinier, P., Laverne, C., 2011. Insights into C and H storage in the altered oceanic crust: Results from ODP/IODP Hole 1256D. Geochim. Cosmochim. Acta 75, 2237–2255.

1138 Shuai, Y., Etiope, G., Zhang, S., Douglas, P.M., Huang, L., Eiler, J.M., 2018. Methane clumped isotopes in the Songliao Basin (China): New insights into abiotic vs. biotic hydrocarbon formation. Earth Planet. Sci. Lett. 482, 213–221.

1139 Spandler, C., Hermann, J., Arculus, R., Mavrogenes, J., 2003. Redistribution of trace elements during prograde metamorphism from lawsonite blueschist to eclogite facies; implications for deep subduction-zone processes. Contrib. Mineral. Petrol. 146, 205–222.

1140 Spandler, C., Hermann, J., Faure, K., Mavrogenes, J.A., Arculus, R.J., 2008. The importance of talc and chlorite “hybrid” rocks for volatile recycling through subduction zones; evidence from the high-pressure subduction mélange of New Caledonia. Contrib. Mineral. Petrol. 155, 181–198.

1141 Stewart, E.M., Ague, J.J., Ferry, J.M., Schiffries, C.M., Tao, R.-B., Isson, T.T., Planavsky, N.J., 2019. Carbonation and decarbonation reactions: Implications for planetary habitability. Am. Mineral. J. Earth Planet. Mater. 104, 1369–1380.

1142 Stolper, D.A., Lawson, M., Davis, C.L., Ferreira, A.A., Neto, E.S., Ellis, G.S., Lewan, M.D., Martini, A.M., Tang, Y., Schoell, M., 2014. Formation temperatures of thermogenic and biogenic methane. Science 344, 1500–1503.

1143 Stolper, D.A., Martini, A.M., Clog, M., Douglas, P.M., Shusta, S.S., Valentine, D.L., Sessions, A.L., Eiler, J.M., 2015. Distinguishing and understanding thermogenic and biogenic sources of methane using multiply substituted isotopologues. Geochim. Cosmochim. Acta 161, 219–247.

1144 Sverjensky, D., Daniel, I., Vitale Brovarone, A., 2020. The changing character of Carbon in fluids with pressure: Organic geochemistry of Earth’s upper mantle fluids. Carbon Earths Inter. 259–269.

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1147 Tumiati, S., Tiraboschi, C., Miozzi, F., Vitale Brovarone, A., Manning, C.E., Sverjensky, D.A., Milani, S., Poli, S., 2020. Dissolution susceptibility of glass-like carbon versus crystalline graphite in high-pressure aqueous fluids and implications for the behavior of organic matter in subduction zones. Geochim. Cosmochim. Acta 273, 383–402.

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1212 Zheng, Y.-F., 1999. Oxygen isotope fractionation in carbonate and sulfate minerals. Geochem. J. 33, 109-126.

1214 Zheng, Y.-F., 1993. Calculation of oxygen isotope fractionation in hydroxyl-bearing silicates. Earth Planet. Sci. Lett. 120, 247-263.

1216

1217
Figures captions

1219

1220 **Fig. 1:** A: Simplified geological map of Northeast Vermont, modified from Hibbard et al., (2006). B: Simplified bedrock geologic map of the Belvidere Mountain Complex and the surrounding formations. Modified after Hibbard et al., (2006). Unit descriptions from Hibbard et al. (2006) and Gale (2007).

1224 **Fig. 2:** A-B: Field occurrence of graphite zone A (A). The graphite-enriched shear zone (B) is present inside serpentinized peridotites. C: Carbonate and magnetite veins located on the sides of graphite zone A. D: Graphite zone B. The wall shown in the picture is a graphite-enriched slip plane marking the boundary between amphibolites and serpentinized peridotites. The graphite enrichment can be observed within about 3 meters inside the amphibolite. E: Albite-carbonate veins cutting across the outer margin of the amphibolite next to graphite zone B. The host amphibolite is here intensely affected by greenschist-facies overprinting. F: Example of rodingite-like metasomatism locally affecting the outer portion of the amphibolite at graphite zone B. Gt: garnet; Ep: epidote; Cc: calcite.

1234 **Fig. 3:** Map showing the geology, sample location, δ¹³C (‰ VPDB), of organic carbon content (wt.‰), and ⁸⁷Sr/⁸⁶Sr ratio results. Pie charts indicate the relative composition of different phases in fluid inclusion as determined from Micro-Raman spectra following the method by Frezzotti et al. (2012). The initial presence of water in olivine-hosted fluid inclusions is deduced from the presence of hydrated step daughter minerals. See Fig. 4 for details on graphite zone A.

1240 **Fig. 4:** Field view details of graphite zone A. The location of the studied samples and associated values of δ¹³C (‰ VPDB), wt.% of organic carbon and ⁸⁷Sr/⁸⁶Sr ratios are also shown. The black-colored area represents the location of the graphite-rich shear zone. The shaded yellow-colored area indicates the carbonate-enriched selvages next to the graphite zone. Thin section scans are also shown in order to highlight the distribution of graphite (dark samples) with respect to the enclosing graphite-free ultramafic rocks.

1247 **Fig. 5:** Photomicrographs of graphite zone A. A: Sample V18-5i from graphite zone A. Pervasive carbonate veins are crosscutting the host antigorite + graphite rock. B: Samples V18-5c from graphite zone A. Fine-grained tremolite is replaced by larger tremolite crystals. The tremolite-rich rock has been partially replaced by antigorite. Graphite appears inside antigorite clast and at the rim of tremolite crystals. C: Sample at the edge of graphite zone A (V18-5a). Tremolite crystals have been replaced by antigorite, with graphite rims outlining the pseudomorphic texture. D: SEM backscattered electron image of sample V18-7c. Carbonate reduction is suggested by the transformation of dolomite to calcite + brucite + graphite. E: Clasts of antigorite in a foliated matrix of fine grained tremolite from graphite zone A (V18-Y1). The tremolite foliation is overgrown by static diopside porphyroblasts. F: Edge of an antigorite-rich clast in contact with tremolite matrix (V18-Yb). In this case, the presence of graphite along the edges to tremolite crystals suggest that the graphite precipitation occurred after the serpentinite brecciation and tremolite precipitation in vein (cf. E). with graphite deposit associated with graphite zone A. Atg: antigorite,
1262 Trm: tremolite, Di: diopside, G: graphite, Chl: chlorite, Cc: calcite, Dol, dolomite, Br: brucite.
1263

1264 **Fig. 6:** Photomicrographs of graphite zone B. A: Tremolite-graphite assemblage (V18-12a), analogous to samples observed in graphite zone A. B: Graphite rimming epidote and titanite crystals inside the edge of the amphibolite at graphite zone B (V18-12c). C-D: Brittle deformation inside albite-carbonate veins (V18-14). Note that clasts of albite crystals are sealed by carbonate. Graphite preferentially forms along pressure-solution domains at albite-carbonate boundaries (D; SEM backscattered electron image). Graphite is associated with carbonate. G: graphite, Trm: tremolite, Amph: amphibole, Ttn: titanite, Bt: biotite, Ab: albite, Cc: calcite.
1265
1266
1267
1268
1269
1270
1271

1272 **Fig. 7:** A: Isotopic compositions of the main carbon bearing objects in the BMC.
1273 Error bars do not appear when comprised within the symbol size.

1274 **Fig. 8:** A: Inverse of organic carbon concentration versus δ¹³Cₗ₀ composition of BMC samples. B: δ¹¹B value versus ⁸⁷Sr/⁸⁶Sr ratio with data. Boron data from Martin et al., (2020) and strontium data from Cannaò et al., (2016) (Cima di Gagnone) and Yamaoka et al., (2012)(Oman ophiolite). Metasediment (V18-1a) age corrected at 300 Ma. DM: depleted Mantle.
1275
1276
1277
1278
1279
1280
1281
1282

1279 **Fig. 9:** Carbonate δ¹³C and δ¹⁸O Isotopic compositions from the BMC. Indicative proximities to the graphite zones indicated below samples' name. Reactional trend associated with formation of CO₂ from prograde decarbonation (blue), and carbonate reduction (green) from Piccoli et al. (2018) and Debret et al. (2015).
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292

1283 **Fig. 10:** A: Ranges of δ¹³C and δ²H measured in methane-rich fluid inclusions. A variety of sources are displayed as references from Bradley and Summons (2010).
1284
1285 'Autotrophic' refers to methane produced by H₂/CO₂ methanogenesis, while
1286 'heterotrophic' refers to methane produced by fermentation of acetate or
1287 consumption of methylated organic compounds. 'Thermogenic' refers to cracking of
1288 biologically derived oils, while 'geothermal' refers to cracking of high-molecular-
1289 weight organic compounds. B: δ²H values of CH₄ and H₂ from fluid inclusions.
1290 Methane and hydrogen produced by activity of methanogens are expected to fall
1291 within the grey box and additional reference data from Bradley and Summons,
1292 (2010).
1293
1294
1295

1293 **Fig. 11:** Modeling of equilibrium fractionation for methanation of calcite at 450 °C. A calcite with an initial δ¹³C isotopic composition of -8 ‰ is considered, using fractionation factor from Bottinga (1969).
1294
1295

1296 **Fig. 12:** Modeling at 450°C and 1.1 GPa of the desiccation of a fluid near water maximum toward reducing conditions fluid with an isotopic composition of δ¹³C=-45 ‰ (A-E), and modeling of the mixing of a slightly reduced fluid with an isotopic composition of δ¹³C=-45 ‰ and a reduced fluid with an isotopic composition of δ¹³C=-23 ‰ (F-J). A and F: zoom on a COH diagram of the initial and final position of the involved fluids. B and G: evolution of the CH₄/(CH₄+CO₂) ratio and of the H₂O/(CH₄+CO₂+H₂O) ratio during the modeled fluid interactions. C and H: evolution of the oxygen fugacity of the fluid during the modeled fluid interactions. D and I:
1297
1298
1299
1300
1301
1302
1303
1304 evolution of the total amount of graphite in mole in the system during the modeled
1305 fluid interactions. E and J: evolution of the isotopic composition of the main carbon
1306 bearing species in the system during the modeled fluid interaction. The model does a
1307 titration of one fluid in another and tracks the carbon activity of the system according
1308 to equation form Huizenga (2004). New graphite composition corresponds to the
1309 instantaneous composition of graphite precipitating while total graphite account for
1310 the global composition of graphite in the system.

1311 **Fig. 13: Proposed model of BMC hydration in subduction zone associated with**
1312 graphite deposition linked to CH₄ migration. A: Initial sage of the BMC in subduction,
1313 graphite zone A corresponds to a shear zone and graphite zone B to the lithologic
1314 contact with the amphibolite. B: Infiltration of a fluid from metasedimentary rocks.
1315 This fluid was H₂O dominated, rich in Ca, Si, C, and N, and produced tremolite
1316 metasomatism along the main fluid channels. C: CH₄ released from serpentinization
1317 escapes along main fluid channel and precipitates graphite when encountering redox
1318 barrier. D: As the CH₄ production and escape progresses, the CH₄ becomes
1319 dominant across the study zone and the redox front progresses in the amphibolite.

1320

1321 Table captions

1322 **Table 1: Studied BMC samples and related carbon, strontium and boron data.**

1323

1324

1325

1326
<figure><img src="image_3.png" /></figure>

<figure><img src="image_4.png" /></figure>

Belvidere Mountain Complex

<figure><img src="image_5.png" /></figure>

Ultramafic rocks

<figure><img src="image_6.png" /></figure>

Amphibolite

<figure><img src="image_7.png" /></figure>

Greenstone

Tilloston Peak Complex

<figure><img src="image_8.png" /></figure>

Blueschist Amphibolite
and Green eclogite

Stowe Formation

Late Cambrian

<figure><img src="image_9.png" /></figure>

Grey/green schists

Ottaquechee Formation

Middle Cambrian

<figure><img src="image_10.png" /></figure>

Carbonaceous pyritiferous phyllite

Hazen Notch Formation

Neoproterozoic and Cambrian

<figure><img src="image_11.png" /></figure>

Schist/Gneiss
<figure><img src="image_12.png" /><figcaption>A</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>B</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>C</figcaption></figure>

<figure><img src="image_15.png" /><figcaption>D</figcaption></figure>

<figure><img src="image_16.png" /><figcaption>E</figcaption></figure>

<figure><img src="image_17.png" /><figcaption>F</figcaption></figure>
<figure><img src="image_18.png" /></figure>

Fluid inclusion
composition:

<figure><img src="image_19.png" /></figure>
<figure><img src="image_20.png" /></figure>

5i

5a

2 m

V18-5i

-17 ‰
1.02 %

V18-5h

-17 ‰
1.45 %

V18-5g

-15 ‰
2.76 %

V18-5f

-15 ‰
3.90 %

V18-5e

-17 ‰
1.03 %

V18-5d

-15 ‰
1.08 %

V18-5c

-15 ‰
1.51 %

V18-5b

-15 ‰
1.65 %

V18-5a

-15 ‰
1.39 %

<figure><img src="image_21.png" /></figure>

<figure><img src="image_22.png" /></figure>

<figure><img src="image_23.png" /></figure>

<figure><img src="image_24.png" /></figure>

<figure><img src="image_25.png" /></figure>

<figure><img src="image_26.png" /></figure>

<figure><img src="image_27.png" /></figure>

<figure><img src="image_28.png" /></figure>
<figure><img src="image_29.png" /></figure>

<figure><img src="image_30.png" /></figure>

<figure><img src="image_31.png" /></figure>

<figure><img src="image_32.png" /></figure>

<figure><img src="image_33.png" /></figure>

<figure><img src="image_34.png" /></figure>
<figure><img src="image_35.png" /></figure>

<figure><img src="image_36.png" /></figure>

<figure><img src="image_37.png" /></figure>

<figure><img src="image_38.png" /></figure>
<figure><img src="image_39.png" /></figure>
<figure><img src="image_40.png" /></figure>

<figure><img src="image_41.png" /></figure>
A

<figure><img src="image_42.png" /></figure>

B

<figure><img src="image_43.png" /></figure>
<figure><img src="image_44.png" /></figure>

<figure><img src="image_45.png" /></figure>
<figure><img src="image_46.png" /><figcaption>Fractionation of Calcite to CH₄</figcaption></figure>
## A Dessication

36.1 % of Carbon remaining in the fluid
1.24 mole of graphite precipitated

<figure><img src="image_47.png" /><figcaption>B</figcaption></figure>

<figure><img src="image_48.png" /><figcaption>C</figcaption></figure>

<figure><img src="image_49.png" /><figcaption>D</figcaption></figure>

<figure><img src="image_50.png" /><figcaption>E</figcaption></figure>

<figure><img src="image_51.png" /><figcaption>F</figcaption></figure>

<figure><img src="image_52.png" /></figure>

## F Fluid mixing

94.7 % of Carbon remaining in the fluid
1.39 mole of graphite precipitated

<figure><img src="image_53.png" /><figcaption>G</figcaption></figure>

<figure><img src="image_54.png" /><figcaption>H</figcaption></figure>

<figure><img src="image_55.png" /><figcaption>I</figcaption></figure>

<figure><img src="image_56.png" /><figcaption>J</figcaption></figure>

<figure><img src="image_57.png" /></figure>
<figure><img src="image_58.png" /></figure>

<figure><img src="image_59.png" /></figure>

<figure><img src="image_60.png" /></figure>

<figure><img src="image_61.png" /></figure>
<table><caption>**Table 1:** Studied BMC samples and related carbon, strontium and boron data.</caption><thead><tr><th>Samples</th><th>C<sub>org</sub><br>(wt%)</th><th>δ<sup>13</sup>C<sub>org</sub><br>VPDP</th><th>δ<sup>13</sup>C<sub>inorg</sub><br>VPDP</th><th>δ<sup>18</sup>O<sub>carbonate</sub><br>VSMOW</th><th>δ<sup>13</sup>C<sub>CH4</sub><br>VPDP<br>picarro</th><th>δ<sup>13</sup>C<sub>CH4</sub><br>VPDP<br>IRMS</th><th>δ<sup>2</sup>H<sub>CH4</sub><br>VSMOW</th><th>δ<sup>2</sup>H<sub>H2</sub><br>VSMOW</th><th>C1/C2</th><th><sup>87</sup>Sr/<sup>86</sup>Sr</th><th>δ<sup>11</sup>B</th></tr></thead><tbody><tr><td>V18-1a</td><td>0.58</td><td>-25.3</td><td></td><td></td><td>-46.28</td><td>-46.34</td><td>-226</td><td>-812</td><td>13</td><td>0.7578</td><td>-8.5</td></tr><tr><td>V18-1c metasedimentary rocks</td><td>0.17</td><td>-25</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>V18-1e</td><td>1.99</td><td>-25</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>V18-2a</td><td>0.02</td><td>-27.4</td><td></td><td></td><td>-12.66</td><td>-12.93</td><td>-171</td><td></td><td>1992</td><td>0.7095</td><td>-1.6</td></tr><tr><td>V18-2b mostly preserved peridotite</td><td>0.03</td><td>-28.3</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>0.7095</td><td>-0.8</td></tr><tr><td>V18-3a</td><td>0.03</td><td>-28.1</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>0.7082</td><td>-1.4</td></tr><tr><td>V18-3b</td><td>0.03</td><td>-28.5</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>V18-4 rodingites</td><td>0.01</td><td>-21.7</td><td></td><td></td><td>-15.48</td><td>-17.88</td><td>-177</td><td></td><td>6154</td><td></td><td></td></tr><tr><td>V18-Y1</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>V18-Yb</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>V18-5a</td><td>1.39</td><td>-15.5</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>V18-5b</td><td>1.65</td><td>-15.2</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>0.715</td><td></td></tr><tr><td>V18-5c</td><td>1.51</td><td>-15.6</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>0.7167</td><td>-3.6</td></tr><tr><td>V18-5d graphite zone A</td><td>1.08</td><td>-14.6</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>V18-5e</td><td>1.03</td><td>-17</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>V18-5f</td><td>3.91</td><td>-15.3</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>V18-5g</td><td>2.76</td><td>-15.6</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>0.7177</td><td></td></tr><tr><td>V18-5h</td><td>1.45</td><td>-16.5</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>V18-5i</td><td>1.02</td><td>-17.7</td><td>-1.43</td><td>16.64</td><td></td><td></td><td></td><td></td><td></td><td>0.714</td><td></td></tr><tr><td>V18-6a</td><td>0.06</td><td>-28.6</td><td>-6.49</td><td>8.91</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>V18-6d</td><td>0.03</td><td>-29.5</td><td></td><td></td><td>-31.25</td><td>-29</td><td></td><td></td><td>635</td><td></td><td></td></tr><tr><td>V18-7a</td><td>0.07</td><td>-24.7</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>V18-7c</td><td>0.09</td><td>-20.2</td><td>-1.27</td><td>15.15</td><td></td><td></td><td></td><td></td><td></td><td>0.7138</td><td>-2.4</td></tr><tr><td>V18-8</td><td>0.02</td><td>-27.7</td><td>-4.5</td><td>10.83</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>V18-9</td><td>0.02</td><td>-27.9</td><td>-4.13</td><td>11.81</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>V18-10</td><td>0.37</td><td>-25.1</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>0.7138</td><td>-4.6</td></tr><tr><td>V18-11 mostly preserved peridotite</td><td>0.02</td><td>-27.7</td><td></td><td></td><td>-45.23</td><td>-42.48</td><td>-226</td><td></td><td>668</td><td></td><td></td></tr><tr><td>V18-12a</td><td>3.69</td><td>-15.5</td><td></td><td></td><td>-22.36</td><td>-22.46</td><td>-161</td><td></td><td></td><td></td><td></td></tr><tr><td>V18-12b graphite zone B</td><td>0.96</td><td>-13.6</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>V18-12c</td><td>0.22</td><td>-16.9</td><td></td><td></td><td>-18.59</td><td>-20.66</td><td>-140</td><td>-680</td><td></td><td></td><td></td></tr><tr><td>V18-13</td><td>0.77</td><td>-14.8</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>V18-14</td><td>0.14</td><td>-20.5</td><td>-7.26</td><td>11.41</td><td>-36.95</td><td>-33.45</td><td>-168</td><td></td><td></td><td>0.7093</td><td>-3.5</td></tr><tr><td>V18-15</td><td>0.43</td><td>-15.4</td><td></td><td></td><td>-20.19</td><td>-20.18</td><td>-168</td><td></td><td></td><td>0.7107</td><td></td></tr></tbody></table>