Sandeep Thapa
email: thapa@ipgp.fr

Frédéric Girault

Damien Deldicque

Rémi Losno

Christian France-Lanord

Chiara Groppo

Franco Rolfo

Shashi Tamang

Thomas Rigaudier

Baptiste Debret

Raj Kabi

Paudyal

Bijaya Lok

Frédéric Adhikari

Perrier

Kabi Raj Paudyal

Adhikari

Frédéric Perrier

to the estimation of carbon stored during Himalayan orogeny

Keywords: Main Central Thrust, Graphite, Carbonaceous material, Raman spectroscopy, Scanning Electron Microscopy, Carbon budget

come

Metric, kilometric and large-scale coherence of metamorphic conditions from graphitic phyllite in the Upper Lesser Himalaya of Nepal: Contribution

Introduction

Elemental carbon is ubiquitous on the Earth's surface and at depth due to the large stability field of its polymorphs [START_REF] Hayes | The carbon cycle and associated redox processes through time[END_REF][START_REF] Beyssac | Graphitic carbon: A ubiquitous, diverse, and useful geomaterial[END_REF]. In metamorphic rocks, the recrystallization of carbonaceous material starts from 350-430 °C [START_REF] Grew | Carbonaceous material in some metamorphic rocks of New England and other areas[END_REF][START_REF] Wang | Carbonaceous material in the Ryoke metamorphic rocks, Kinki district, Japan[END_REF][START_REF] Nakamura | Pressure dependence of graphitization: implications for rapid recrystallization of carbonaceous material in a subduction zone[END_REF] and, depending on pressure, well-ordered graphite crystals form at around 600-700 °C [START_REF] Henry | Raman spectroscopy as a tool to determine the thermal maturity of organic matter: Application to sedimentary, metamorphic and structural geology[END_REF][START_REF] Nakamura | Pressure dependence of graphitization: implications for rapid recrystallization of carbonaceous material in a subduction zone[END_REF]. Regional and contact metamorphism, deformation or fluids contribute to graphite genesis whose crystallinity increases with increasing metamorphic grade [START_REF] Miyashiro | Oxidation and reduction in the Earth's crust with special reference to the role of graphite[END_REF][START_REF] Itaya | Carbonaceous material in pelitic schists of the Sanbagawa metamorphic belt in central Shikoku, Japan[END_REF][START_REF] Wang | Carbonaceous material in the Ryoke metamorphic rocks, Kinki district, Japan[END_REF][START_REF] Pasteris | Raman spectra of graphite as indicators of degree of metamorphism[END_REF][START_REF] Wada | Graphitization of carbonaceous matter during metamorphism with references to carbonate and pelitic rocks of contact and regional metamorphisms[END_REF][START_REF] Beyssac | On the characterization of disordered and heterogeneous carbonaceous materials by Raman spectroscopy[END_REF][START_REF] Kříbek | Graphitization of organic matter and fluid-deposited graphite in Palaeoproterozoic (Birimian) black shales of the Kaya-Goren greenstone belt (Burkina Faso, West Africa)[END_REF][START_REF] Aoya | Extending the applicability of the Raman carbonaceous-material geothermometer using data from contact metamorphic rocks[END_REF][START_REF] Skrzypek | First-and second-order Raman spectra of carbonaceous material through successive contact and regional metamorphic events (Ryoke belt, SW Japan)[END_REF]. In most cases, organic matter is the precursory material originally present in the sedimentary protolith rock from which graphite crystallizes at greenschist to granulite facies conditions [START_REF] Landis | Graphitization of dispersed carbonaceous material in metamorphic rocks[END_REF][START_REF] Diessel | Coalification and graphitization in high-pressure schists in New Caledonia[END_REF][START_REF] Buseck | Conversion of carbonaceous material to graphite during metamorphism[END_REF][START_REF] Beyssac | Peak metamorphic temperature and thermal history of the Southern Alps (New Zealand)[END_REF][START_REF] Kirilova | Textural changes of graphitic carbon by J o u r n a l P r e -p r o o f tectonic and hydrothermal processes in an active plate boundary fault zone, Alpine Fault, New Zealand[END_REF]. Throughout graphitization, pressure impacts the physical changes while temperature is responsible for the chemical transformation [START_REF] Buseck | From organic matter to graphite: Graphitization[END_REF]. During subduction, graphitic-like carbon may be consumed through oxidation or dissolution in fluids, or can be preserved and later exhumed [START_REF] Galvez | Graphite formation by carbonate reduction during subduction[END_REF][START_REF] Bouilhol | Decoupling behavior of inorganic and organic carbon during slab mantle devolatilization[END_REF]. Graphitic-like carbon can also have inorganic origin, when deposited from COH fluids or formed through complex redox reactions (e.g., Connolly and Cesare, different cross-sections in the LHS, confirming the existence of an inverted temperature gradient below the MCT [START_REF] Beyssac | Thermal metamorphism in the lesser Himalaya of Nepal determined from Raman spectroscopy of carbonaceous material[END_REF][START_REF] Bollinger | Thermal structure and exhumation history of the Lesser Himalaya in central Nepal: Thermal structure of the Lesser Himalaya[END_REF]. Thus, the available previous studies demonstrated that graphitic phyllites systematically recorded the highest peak metamorphic temperatures within the LHS, with values ranging from about 550 to 620 °C [START_REF] Beyssac | Thermal metamorphism in the lesser Himalaya of Nepal determined from Raman spectroscopy of carbonaceous material[END_REF][START_REF] Bollinger | Thermal structure and exhumation history of the Lesser Himalaya in central Nepal: Thermal structure of the Lesser Himalaya[END_REF][START_REF] Rawat | Features and characterization of graphite in Almora Crystallines and their implication for the graphite formation in Lesser Himalaya, India[END_REF][START_REF] Mathew | Thermal metamorphism of the Arunachal Himalaya, India: Raman thermometry and thermochronological constraints on the tectono-thermal evolution[END_REF][START_REF] Long | Temperature and strain gradients through Lesser Himalayan rocks and across the Main Central thrust, south central Bhutan: Implications for transportparallel stretching and inverted metamorphism[END_REF][START_REF] Rana | Carbonaceous material in Larji-Rampur window, Himachal Himalaya: Carbon isotope compositions, micro Raman spectroscopy and implications[END_REF]. However, before attempting large-scale estimates, it is important to evaluate whether these results are punctual or can be taken as sufficiently representative. In addition, in the vicinity of the upper-LHS graphitic phyllites, carbon dioxide (CO2) emissions of metamorphic origin have been reported (e.g., [START_REF] Becker | Himalayan metamorphic CO2 fluxes: Quantitative constraints from hydrothermal springs[END_REF][START_REF] Evans | Degassing of metamorphic carbon dioxide from the Nepal Himalaya[END_REF][START_REF] Girault | Large-scale organization of carbon dioxide discharge in the Nepal Himalayas[END_REF][START_REF] Girault | Persistent CO2 emissions and hydrothermal unrest following the 2015 earthquake in Nepal[END_REF]Girault et al., , 2022, in press a,b), in press a,b).

The simultaneous presence of stored carbon (carbonates and graphite) and released carbon (CO2 emissions) at a large spatial scale raises the question as to whether these processes are related and what might be the contribution of graphitic phyllite to the carbon stored during Himalayan orogeny.

In this paper, to address the limitations stated above, we focus on the upper-LHS graphitic phyllites. First, we develop a new technique based on Scanning Electron Microscope and Back-Scattered Electron (SEM-BSE) images to estimate organic carbon (graphite) content directly from thin sections. Second, we study in details five sites located in the Upper Trisuli valley, Central Nepal, a few hundreds of meters south of the MCT. After describing the graphite characteristics, the metamorphic conditions from graphitic phyllites are studied using various analytical methods at increasing spatial scales. Carbon isotopic composition of graphitic phyllites is also studied in relation with total, organic and inorganic carbon content. Based on our additional data from Far-Western to Eastern Nepal, and complemented by published data from other workers, we discuss the possible connection between occurrences of graphite and locations of current CO2 emissions, and the implications of our results on stored carbon for the carbon budget in the Himalaya and in other orogenic belts.

J o u r n a l P r e -p r o o f

Geological background and samples

Geological setting

Resulting from the collision of the Indian plate and the Eurasian plate which started 60-40 Ma ago, the Himalaya is the largest active orogenic belt on Earth [START_REF] Fort | Himalayas: the collided range, present knowledge of the continental arc[END_REF]. The Nepal Himalaya shows various tectonostratigraphic units separated by several thrust and fault systems (Fig. 1a).

Related at depth to the Main Himalayan Thrust (MHT) which accommodates the convergence (e.g., [START_REF] Avouac | Mountain building, erosion, and the seismic cycle in the Nepal Himalaya[END_REF], the Main Central Thrust (MCT) separates low-to medium-grade metamorphic rocks of the Lesser Himalayan Sequence (LHS) to the south from the higher-grade metamorphic rocks of the Greater Himalayan Sequence (GHS) to the north [START_REF] Upreti | An overview of the stratigraphy and tectonics of the Nepal Himalaya[END_REF][START_REF] Searle | Defining the Himalayan Main Central Thrust in Nepal[END_REF]. Consisting of a 10-to-15-km-thick sequence of unmetamorphosed sedimentary units to greenschist-grade metasedimentary units, LHS rocks started deposition about 1880 Ma (e.g., [START_REF] Kohn | The lower Lesser Himalayan sequence: A Paleoproterozoic arc on the northern margin of the Indian plate[END_REF]. They also show an inverted metamorphic series from lower greenschist-grade to the south (P-T conditions: 8 kbar and 340 °C) up to garnet-bearing isograds beneath the MCT (P-T conditions: 9.5-12 kbar and 590 °C) [START_REF] Arita | Origin of the inverted metamorphism of the lower Himalayas, Central Nepal[END_REF][START_REF] Beyssac | Thermal metamorphism in the lesser Himalaya of Nepal determined from Raman spectroscopy of carbonaceous material[END_REF][START_REF] Khanal | In-sequence buoyancy extrusion of the Himalayan Metamorphic Core, central Nepal: Constraints from monazite petrochronology and thermobarometry[END_REF][START_REF] Wang | The Himalayan collisional orogeny: A metamorphic perspective[END_REF][START_REF] Tamang | Implications of garnet nucleation overstepping for the P-T evolution of the Lesser Himalayan Sequence of central Nepal[END_REF]. GHS rocks are younger about 1100 to 440 Ma (e.g., [START_REF] Parrish | Isotopic constraints on the age and provenance of the Lesser and Greater Himalayan sequences, Nepalese Himalaya[END_REF]. Within the MCT shear zone, the Ramgarh Thrust (RT), located southward MCT, separates the oldest LHS rocks from the younger LHS rocks [START_REF] Pearson | Structural geology and regional tectonic significance of the Ramgarh thrust, Himalayan fold-thrust belt of Nepal[END_REF].

This study focusses on graphitic phyllites from the upper-LHS located in the MCT zone of the Nepal Himalaya. The Upper Trisuli valley (Fig. 1b), located about 60 km north of Kathmandu in Central Nepal and crossed by Bhote Koshi, Langtang and Chilime Rivers, exhibits a representative graphitic mica-schist unit between the MCT and RT. Along this transect, GHS rocks encompass, from south to north, the kyanite schist and gneiss unit which includes medium to coarse grained kyanitegneiss and garnet schist, the quartzite unit which includes fine to medium grained quartz with garnet, J o u r n a l P r e -p r o o f and the banded and migmatitic gneiss unit consisting of feldspar, quartz, muscovite, biotite, garnet and tourmaline (Tagaki et al., 2003;[START_REF] Dhital | Geology of the Nepal Himalaya[END_REF][START_REF] Khanal | In-sequence buoyancy extrusion of the Himalayan Metamorphic Core, central Nepal: Constraints from monazite petrochronology and thermobarometry[END_REF]. LHS rocks encompass, from south to north, the garnet schist unit with garnet schist, meta-sandstone and sericitic phyllite also named as "Kuncha formation" [START_REF] Stöcklin | Geology of Nepal and its regional frame: Thirty-third William Smith Lecture[END_REF][START_REF] Kohn | Five generations of monazite in Langtang gneisses: implications for chronology of the Himalayan metamorphic core[END_REF]Girault et al., 2012), the graphitic mica-schist unit which overlies the garnet schist unit and consists of carbonate bands, fine to medium grained garnet schist and graphitic phyllite [START_REF] Kohn | P-T-t data from central Nepal support critical taper and repudiate large-scale channel flow of the Greater Himalayan Sequence[END_REF][START_REF] Dhital | Geology of the Nepal Himalaya[END_REF], and the overlying augen gneiss unit which includes Ulleri-type medium to coarse grained orthogneiss [START_REF] Kohn | Five generations of monazite in Langtang gneisses: implications for chronology of the Himalayan metamorphic core[END_REF][START_REF] Kohn | The lower Lesser Himalayan sequence: A Paleoproterozoic arc on the northern margin of the Indian plate[END_REF][START_REF] Dhital | Geology of the Nepal Himalaya[END_REF].

In the Upper Trisuli valley (Fig. 1b), the graphitic mica-schist unit is well-exposed in the Chilime tributary valley along the road sections of Pajung village and Paragaon village (well-known as Tatopani), and of Tetangchet village nearby the upper Sanjen hydropower site. Mineralogically, the graphitic phyllites mainly consist of quartz (62±12 %), phlogopite (16±3 %), muscovite (12±2 %), graphite (6±1 %) and pyrite (4±1 %) [START_REF] Börner | Anomalous complex electrical conductivity of a graphitic black schist from the Himalayas of Central Nepal[END_REF]. Aluminum-rich minerals such as garnet and staurolite are reported in graphitic phyllites in the same valley [START_REF] Kohn | P-T-t data from central Nepal support critical taper and repudiate large-scale channel flow of the Greater Himalayan Sequence[END_REF][START_REF] Goscombe | Metamorphic response to collision in the Central Himalayan Orogen[END_REF][START_REF] J O U R N A L P R E -P R O O F Rapa | Detection of tectonometamorphic discontinuities within the Himalayan orogen: Structural and petrological constraints from the Rasuwa district, central Nepal Himalaya[END_REF][START_REF] Tamang | Implications of garnet nucleation overstepping for the P-T evolution of the Lesser Himalayan Sequence of central Nepal[END_REF]. Finally, several occurrences of pluridecimetric bands of carbonates (siliceous dolomitic marble) are also observed within the unit.

Description of the outcrops

Within the graphitic mica-schist unit, several outcrops oriented west-east expose the metamorphic rocks along the Chilime River and along the roads from the Bhote Koshi-Chilime confluence to the Paragaon village and to the upper Sanjen hydropower site. Five sites were investigated in January 2020 and January-February 2021 (Fig. 1b): a 400-m-long outcrop located upward Pajung village (site A; Fig. 1c), a 100-m-long outcrop located downward Pajung village (site B; Fig. 1d), a 50-m-long outcrop located near Goljung village (site C; Fig. 1e), a 125-m-long outcrop J o u r n a l P r e -p r o o f located near Sanjen hydropower site (site D; Fig. 1f) and a 90-m-long outcrop located downward Paragaon village (site E; Fig. 1g). At the five sites, fresh and unweathered rock samples were collected, benefitting from fresh scarps made by road construction. For each sample, about 150 g of rock was crushed in powder manually in the field without sieving and placed in a plastic bag, and one to five small representative pieces of 5 cm size maximum were also collected. A total of 77 graphitic phyllite samples were collected with 5 to 10 m spacing in between two samples depending on the considered site: 43 samples at site A (A1-A43) including 25 samples at higher resolution with a sampling interval of 1 m plus one sample of a preliminary survey (ME1), 6 samples at site B (B1-B6), 7 samples at site C (C1-C7), 8 samples at site D (D1-D8) plus two samples of a preliminary survey (ME9 and ME15), and 10 samples at site E (E1-E10). Sampling was done with the utmost caution not to collect samples in the same layer, hence avoiding sampling redundancy. This sampling strategy allows us to study the heterogeneity of the graphitic mica-schist unit of the Upper Trisuli valley at various spatial scales, from meter-scale to formation scale. (Supp. Fig. S2) and April 2021, respectively. In Eastern Nepal, we collected: 8 samples in Leuti area (Supp. Fig. S3) and 8 samples in Akhuwatar area (Supp. Fig. S4) in December 2020, and 2 samples J o u r n a l P r e -p r o o f in Chainpur area in October 2021. A complete list of all the studied samples together with their location is available as Supp. Table S1.

Sample analysis

To study the graphitic phyllite samples, we focused on the determination of the organic, inorganic and total carbon contents and on the characterization of the graphite using different analytical techniques. The methods described below also include the presentation and validation of a new approach based on scanning electron microscopy to estimate graphite content in rock samples.

In the following, every uncertainty associated with a parameter for a sample is given at onesigma (68 % confidence level). The uncertainty of an average value of a parameter calculated for a given group of samples corresponds to the standard error, i.e. the standard deviation divided by the square root of the number of data. All average values are arithmetic means, except stated otherwise.

Analytical methods

Petrography and graphite morphology

Representative samples of graphitic phyllite were selected to study the mineral assemblages: some with an abundant graphite content and others with less graphite associated with carbonate minerals. Thin sections were prepared after induration of the samples. They were studied at University of Turin, Italy under an Olympus BX4 reflected and transmitted polarized light microscope. Photomicrographs were taken with an attached JENOPTIK ProgResC5 digital color camera. Two samples were studied in detail under a Scanning Electron Microscope (SEM) at École Normale Supérieure (ENS), Paris, France to analyze their mineralogical composition. In addition, J o u r n a l P r e -p r o o f several high-resolution SEM photomicrographs were taken to study the morphology of the graphite minerals.

Micro-Raman spectroscopy on carbonaceous material

Principle and sample preparation

The Raman spectrum of carbonaceous material (RSCM) consists of a two-order spectrum depending on the degree of crystallization: the first-order region in the wavelength range of 1100-1800 cm -1 and the second-order region in the range of 2500-3100 cm -1 [START_REF] Tuinstra | Raman spectrum of graphite[END_REF].

Here, we recorded data over the 700-2100 cm -1 range, hence including the first-order region in totality, which depends on the different P-T conditions. All the studied Raman spectra include the three D1, G and D2 bands. The graphite band (G), whose position is between 1580 and 1600 cm -1 , corresponds to in-plane vibration of aromatic carbons. The other bands (mainly D1 and D2) correspond to in-plane defects in the carbon structure. These D1 and D2 bands, occurring respectively at 1350 and 1620 cm -1 , are large and intense in poorly ordered carbonaceous material and decrease in both width and intensity when the crystallinity of graphite increases [START_REF] Rouzaud | Carbon films: Structure and microtexture (optical and electron microscopy, Raman spectroscopy)[END_REF].

To characterize graphite in the phyllite samples, a non-polished millimeter size sample was prepared. Raman spectra were acquired especially on fresh samples with the help of a confocal Raman microscope (inVia™, Renishaw, UK) coupled with an optical microscope (Leica, Germany) at ENS, Paris, France. The microscope allowed focusing the excitation laser beam on the sample at 514.5 nm, due to the dispersive behavior of the defect bands, in order to collect Raman signal. It was equipped with a Kensington controller and adjustment was made with 50x magnification objective lens for proper visualization. The Raman scattered light was dispersed using 100 lines per mm grating and detected by a CDD camera (Philips SPC 900NCPC camera). The laser power on the sample was J o u r n a l P r e -p r o o f maintained lower than 0.2 mW to avoid sample heating. A single acquisition lasted 20 s. Five acquisitions were performed to obtain one spectrum, reducing fluorescence effect (i.e. non-horizontal spectrum; some remain and are shown in Fig. 2a). As the samples appeared relatively homogeneous, stacking only 10 spectra per sample was found sufficient to reduce the standard deviation and thus obtain a representative final spectrum of each sample. This procedure follows common sample preparation details (e.g., [START_REF] Beyssac | Raman spectra of carbonaceous material in metasediments: a new geothermometer[END_REF][START_REF] Aoya | Extending the applicability of the Raman carbonaceous-material geothermometer using data from contact metamorphic rocks[END_REF][START_REF] Nakamura | Microstructural evolution of carbonaceous material during graphitization in the Gyoja-yama contact aureole: HRTEM, XRD and Raman spectroscopic study[END_REF][START_REF] Kouketsu | A new approach to develop the Raman carbonaceous material geothermometer for low-grade metamorphism using peak width[END_REF].

As an example, Fig. 2 shows the ten Raman spectra obtained from sample A28. All the three bands are clearly visible and show some variations between the spectra depending on the considered target (Fig. 2a). After baseline correction, the maxima of each band can be compared (Fig. 2b). Then, the D1, G and D2 bands of each spectrum were analyzed in terms of peak position of band area, band intensity or height and band surface area, as depicted in Fig. 2c, with conventional fitting procedures based on a Voigt function using PeakFit ® software (Systat Software, Inc., USA) (Lünsdorf et al., 2013). The peak positions of the D1, G and D2 bands from all outcrops remain relatively similar and range from 1356.1±0.5 to 1357.9±0.3 cm -1 (mean: 1357.0±0.3 cm -1 ), from 1578.9±0.5 to 1581.1±0.1 cm -1 (mean: 1579.9±0.4 cm -1 ) and from 1617.2±0.2 to 1620.7±0.3 cm -1 (mean: 1619.3±0.6 cm -1 ), respectively. We also determined the full widths at half maximum (FWHM) for each band. The D1-, G-and D2-FWHM are related to the maturity of the rocks [START_REF] Kouketsu | A new approach to develop the Raman carbonaceous material geothermometer for low-grade metamorphism using peak width[END_REF][START_REF] Henry | Raman spectroscopy as a tool to determine the thermal maturity of organic matter: Application to sedimentary, metamorphic and structural geology[END_REF].

On average, relative uncertainty for the values of D1-, G-and D2-FWHM were 4.1±0.3 %, 3.3±0.9 % and 16±1 %, respectively. On average, for a given sample, the mean dispersion of the values obtained from the ten spectra was 2.78±0.01 %.

Determination of peak metamorphic temperature

J o u r n a l P r e -p r o o f

Using the RSCM data, we estimated the peak metamorphic temperature (peak-T) attained by the considered sample. For this purpose, we compared the three well-established different geothermometer relationships. The [START_REF] Beyssac | Raman spectra of carbonaceous material in metasediments: a new geothermometer[END_REF] geo-thermometer is based on temperature conditions reached during regional metamorphism and on the following linear empirical relationship:

𝑇(°C) = 445𝑅2 + 641 , (2)
where

𝑅2 = ( D1 G + D1 + D2 ) A , (3)
where the subscript A refers to the surface area of the D1, G and D2 bands. Eq. ( 2) is applicable for all types of organic precursor, relevant in the temperature range 330-650 °C, and was applied with success to Himalayan metamorphic rocks [START_REF] Beyssac | On the characterization of disordered and heterogeneous carbonaceous materials by Raman spectroscopy[END_REF][START_REF] Beyssac | Thermal metamorphism in the lesser Himalaya of Nepal determined from Raman spectroscopy of carbonaceous material[END_REF][START_REF] Bollinger | Thermal structure and exhumation history of the Lesser Himalaya in central Nepal: Thermal structure of the Lesser Himalaya[END_REF].

To overcome the effects of metamorphic pressure from the lower and higher-temperature rocks, a modified RSCM thermometer based on the R1 and R2 ratios was introduced, with the following empirical equation [START_REF] Rahl | Raman spectroscopic carbonaceous material thermometry of low-grade metamorphic rocks: Calibration and application to tectonic exhumation in Crete, Greece[END_REF]:

𝑇(°C) = 737.3 + 320.9 𝑅1 -1067 𝑅2 -80.638 (𝑅1) 2 , (4)

where

𝑅1 = ( D1 G ) H , (5)
where the subscript H refers to the height or intensity of the D1, G and D2 bands. The reliability of this thermometer extends to the temperature range 100-700 °C.

More recently, [START_REF] Aoya | Extending the applicability of the Raman carbonaceous-material geothermometer using data from contact metamorphic rocks[END_REF] compared the orientation of the materials with the laser beams parallel and normal to the c-axis and concluded that the results are comparable within the R2 ratios. Here, we used the following quadratic empirical equation for regional metamorphism which is valid for the temperature range 340-650 °C [START_REF] Aoya | Extending the applicability of the Raman carbonaceous-material geothermometer using data from contact metamorphic rocks[END_REF]:

𝑇(°C) = 91.4 (𝑅2) 2 -556.3 (𝑅2) + 676.3 . ( 6
)
J o u r n a l P r e -p r o o f

The peak-T of a total of 74 samples was determined by RSCM using the three geothermometers. They are written hereinafter B-, R-and A-RSCM for [START_REF] Beyssac | Raman spectra of carbonaceous material in metasediments: a new geothermometer[END_REF], [START_REF] Rahl | Raman spectroscopic carbonaceous material thermometry of low-grade metamorphic rocks: Calibration and application to tectonic exhumation in Crete, Greece[END_REF] and [START_REF] Aoya | Extending the applicability of the Raman carbonaceous-material geothermometer using data from contact metamorphic rocks[END_REF] method, respectively. For each sample, we used the average values of the parameters determined above based on the ten recorded Raman spectra studied per sample.

Absolute uncertainty on the determination of peak-T remained generally small and ranged from 1.6

to 19 °C for B-RSCM, from 2.0 to 16 °C for R-RSCM and from 2.5 to 22 °C for A-RSCM.

For a given sample, R-RSCM and A-RSCM geo-thermometers show higher peak-T values than the ones obtained using the B-RSCM geo-thermometer. When considering all our samples, the difference in peak-T appears to be linear in the B-RSCM temperature range 518.7-589.9 °C (Supp.

Fig. S5). On average, B-RSCM peak-T is 9.1±0.2 % lower than A-RSCM peak-T and 9.7±0.2 % lower than R-RSCM peak-T. R-RSCM peak-T is lower and higher than A-RSCM peak-T in the range 520-560 °C and 560-650 °C, respectively. While the difference between A-RSCM and B-RSCM methods seems to show a simple shift, the differences with R-RSCM appear more complicated (Supp.

Fig. S5). These systematic differences are important to be considered when working on metamorphic rocks. The A-RSCM peak-T calculation depends on the intensity of bands (R1), that of the B-RSCM on the area of bands (R2) and that of the R-RSCM on both parameters, which may explain the more complicated behavior of R-RSCM compared with the two other methods [START_REF] Mathew | Thermal metamorphism of the Arunachal Himalaya, India: Raman thermometry and thermochronological constraints on the tectono-thermal evolution[END_REF]. In addition, B-RSCM mainly considers that the crystallinity of carbonaceous material depends on metamorphic temperature, while R-RSCM takes into account both metamorphic pressure and temperature. Comparing these peak-T data with other estimated peak metamorphic temperature inferred from thermodynamic modelling on similar graphitic phyllite samples of the same lithostratigraphic unit [START_REF] Tamang | Implications of garnet nucleation overstepping for the P-T evolution of the Lesser Himalayan Sequence of central Nepal[END_REF] suggests that A-RSCM and R-RSCM methods overestimate the peak-T values while the B-RSCM method tends to give more consistent peak-T values. This tentative explanation remains preliminary at this stage and motivates further benchmark J o u r n a l P r e -p r o o f exercises to compare directly the different approaches estimating a peak metamorphic temperature of a given sample. In the following, B-RSCM peak-T calculation is thus used.

We compared for the same samples the calculations of R1 and R2 parameters and peak-T done by the same operator using two different softwares: PeakFit® (Systat Software, Inc., USA) and WiRE (Renishaw, UK). With a total of 11 samples, in the B-RSCM temperature range 530-590 °C, the use of PeakFit® or WiRE software does not show any clear difference (Supp. Fig. S6). Indeed, the mean difference is 6±1 %, hence within the overall experimental uncertainty of the method. In addition, the particular software used does not introduce additional differences (Supp. Fig. S7). In the following, the PeakFit® software is used for the determination of peak-T.

Carbon contents using an elemental analyzer

For each sample, about 70 to 80 g of rock chunks were grounded. For total carbon (TC) content, an amount of about 8 mg was weighted with a microbalance and wrapped in a tin capsule for analysis. For total organic carbon (TOC) content, we used the rinsing method which consists in the decarbonation of the sample using hydrochloric acid to remove the inorganic carbon (IC) from TC content of the sample (Konitzer et. al., 2012;[START_REF] Fujisaki | Pre-treatment methods for accurate determination of total nitrogen and organic carbon contents and their stable Isotopic compositions: Re-evaluation from geological reference materials[END_REF]. This decarbonation method does not affect the carbon isotopic composition [START_REF] Midwood | Soil carbonate decomposition by acid has little effect on δ 13 C of organic matter[END_REF][START_REF] Schubert | Effects of decarbonation treatments on δ 13 C values in marine sediments[END_REF]. Approximately 4 to 6 g of powdered sample was placed into a beaker where 100 mL of 1.5 mol L -1 HCl solution was poured. After 24 h, the solution was diluted by adding 400 mL of distilled water. Again after 24 h, the sample was rinsed two times with distilled water with extra care not to lose some sample powder. Then, the sample was kept for drying to evaporate the remaining water in the beaker. Finally, the dried sample was grounded and weighted to about 8 mg into tin capsules for analysis.

J o u r n a l P r e -p r o o f

The prepared samples were analyzed for TC and TOC using a Flash 2000 organic CHNS Elemental Analyzer (Thermo Scientific™, USA) at Institut de Physique du Globe de Paris (IPGP),

France. The tin capsule containing the sample was introduced into the combustion reactor with oxygen and the resulted gases were facilitated by the helium flow (used as carrier gas) which was swept through a gas chromatography (GC) column. Then, combustion gases were separated by a GC column and detected by a thermal conductivity detector. CHNS calibration was performed using Aspartic Acid having the technical specification of 36 C%. The calibration factor remained relatively stable with a dispersion of about 2 % (n=13) throughout the measurement sequences.

We measured TC content on samples powdered in the field and on cm-size pieces crushed in the laboratory. The dispersion of TC content on pieces of the same sample was considered too large and we retained only the values obtained on the powdered samples. In this paper, we determined TC and TOC contents, as a mass ratio expressed in wt.%, of a total of 77 graphitic phyllite samples from the Upper Trisuli valley and 67 graphitic phyllite samples from other locations in Nepal. Relative experimental uncertainty on the carbon mass ratio, inferred from systematic tests, ranged from 0.5 to 11 % for TC content and from 1.4 to 5.4 % for TOC content. For each sample, IC was inferred by subtracting the TOC content value to the TC content value. Mean relative uncertainty for IC was 29±6 %.

The repeatability of the measurement of carbon content was evaluated independently using repeated measurements of the Aspartic Acid solution. It gave a dispersion of about 2 %, moderately compatible with the value given by the manufacturer (1 %). Four powdered samples were replicated multiple times to estimate the carbon content dispersion within the sample. Relative difference of graphitic phyllite replicates ranged from 1.4 to 5.4 %, with a mean of 4.3±1.0 %. From measurements repeated 3 to 4 times, we obtained a satisfactory repeatability of the results, with a coefficient of variation for a single measurement smaller than 3 % in most cases. A total of 20 powdered samples were also analyzed by the Thermo Scientific elemental analyzer IsoLink IRMS System at Centre de

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Recherches Pétrographiques et Géochimiques (CRPG), Nancy, France. To calculate carbon concentration, the thermal conductivity detector signals were calibrated with internal standards: BFSd (C=0.53 wt.%), CRPG_M2 (C=0.408 wt.%), Eurovector Synthetic Soil Mix #4 (C=2.417 wt.%), and

Eurovector Synthetic Soil Mix #1 (C=3.5 wt.%). Relative uncertainty was better than 5 %. Relative difference with the results obtained at IPGP (Paris, France) ranged from 0.1 to 15 % with a mean of 7±2 %. This blind comparison gave remarkable consistency between the data obtained with the analyzer used at IPGP and those obtained with another analyzer used at CRPG (Supp. Fig. S8). As we had both the effects of sample dispersion (assessed above to be about 2 to 3 %) and absolute uncertainty, we considered a conservative absolute uncertainty of each instrument of 5 %. To compare with the following new method, the experimental uncertainty was added in quadrature with the absolute uncertainty of 5 %.

Organic carbon content using the new SEM-BSE technique

As a new approach to determine TOC content from graphitic phyllite samples, we used a Scanning Electron Microscope (SEM) coupled with the detection of Back-Scattered Electrons (BSE)

and Secondary Electrons (SE). First, a cm-size sample, cut perpendicular to the foliation and parallel to the lineation was prepared in order to fit inside a cylindrical mold. This mold containing the sample was then filled in with resin and epoxy (Supp. Fig. S9a,b). Second, the cylindrical mold containing the sample was incised and polished manually with different grades of polishing paste. Diamond resin polishing discs were used on hand-held machinery maintaining a suitable range of rotations per minute for smoothing the surface. Polishing was first done by 50 µm diamond smoothing disc and followed in decreasing size order by 30, 10, 8 and 5 µm discs (Supp. Fig. S9c). Then, using the twodiamond paste having 1 µm and 0.25 µm sizes, polishing was done manually and carefully on the diamond disc not to lose the soft particles (Supp. Fig. S9d). Finally, the surface of the samples was J o u r n a l P r e -p r o o f coated with gold to make the final sample conductive. The thickness of the coating was maintained to 1-10 nm.

SEM images of 3.4 × 2.3 mm 2 surface area were obtained using a Zeiss Sigma SEM model (Carl Zeiss Microscopy, Germany) at ENS, Paris, France (Fig. 3a,d), with working conditions of 15 kV and distance of ~9 mm. All the images were captured using BSE detector along with a 120 µm diaphragm aperture size. This allowed us to highlight location of elements such as carbon at the surface (Fig. 3b,e). From the obtained SEM image coupled with BSE, the surface area occupied by carbon element was calculated using ImageJ software (National Institutes of Health, USA) on highresolution SEM images. There are different thresholding methods where multiple of colors help in interpreting the results. Manually, the threshold was checked to overcome the over-estimation of the binary image outputs, as shown in Fig. 3c,f. We used blue color threshold as a background, which represents the black or dark areas from the original image, to quantify the surface area. The specific darker color of graphite allowed us to directly discriminate graphite and carbonate (light grey color), as also shown in Fig. 4c,d. We took the utmost caution to avoid micro-scale cracks in the image that may create artifacts during the thresholding step.

The minimum number of SEM images needed to obtain reliable carbon content values was tested. Using two samples with 20 images and all the other samples with 10 images, we found that both a good stability of carbon content values and an acceptable relative uncertainty on carbon content are reached when more than 5 to 7 images are used (Supp. Fig. S10). In addition, we found that 10 different images, selected by order of their acquisition, gave a sufficient representative value of the whole surface area (about 35 % of the whole sample surface studied). Thus, for each sample, 10

images were captured at different locations and the values of elemental carbon (i.e. TOC or graphite in our case) content, expressed in vol.%, were averaged to obtain the mean value and associated

uncertainty. An example of 10 images obtained from a single thin-section is shown in Supp. Fig. S11.

We obtained in this case a mean TOC content (volumetric ratio) of 4.2±0.4 %, with a dispersion of J o u r n a l P r e -p r o o f values of 32±7 %. In total, we measured the TOC content from a total of 15 samples: eight from site A, two from site C, three from site D and one each from sites B and E. The relative uncertainty ranged from 0.5 to 19.0 %, with a mean of 8±2 %.

3.1.5 Carbon isotopic composition (δ 13 C)

Determination of stable carbon isotopic composition (δ 13 C) of the samples was performed online using the Thermo Scientific EA IsoLink IRMS System at CRPG, Nancy, France. Samples were wrapped in tin capsules (approx. 3 mg) and then combusted by flash pyrolysis (≈1400 °C) in a combustion reactor consisting of quartz tube filled with chromium oxide, pure copper and silvered cobalt oxide. Produced gases (N2, CO2 and SO2) were separated on a GC column maintained at 70 °C and carbon isotopic composition of the produced CO2 was then measured with a Thermo Scientific Delta V Advantage continuous flow isotope ratio mass spectrometer. Carbon isotopic composition was determined by comparison with two internal and three international standards routinely included during the analysis: BFSd (δ 13 C=-21.5 ‰), CRPG_M2 (δ 13 C=-24.98 ‰), NBS22 (δ 13 C=-30.03 ‰), IAEA CH6 (δ 13 C=-10.449 ‰) and USGS24 (δ 13 C=-16.1 ‰). Values are quoted in the delta notation in ‰ relative to V-PDB (Vienna Pee Dee Belemnite) and the reproducibility gives an absolute average value of 0.15±0.09 ‰ (n=6). Absolute uncertainty was better than 0.3 ‰. From the five sites in the Upper Trisuli valley, a total of 20 representative graphitic phyllite samples were analyzed for δ 13 C from the bulk (non decarbonated or TC). δ 13 C from some decarbonated samples (or TOC) were also performed, as well as one dolomite sample. Carbonates, reduced organic matter and mantle derived carbon have mean δ 13 C values of about 0 ‰, -25 ‰ and -7 ‰, respectively [START_REF] Dobner | Stable carbon isotopes of graphite from Bogala Mine, Sri Lanka[END_REF].

Results of sample analysis

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Location, description and morphology of graphite

The studied graphitic phyllite samples belong to the graphitic mica-schist unit which can be classified as high-potassium-aluminum metapelites (Fig. 4 and Supp. Fig. S12). Two samples from site A (A1 and A8) and two samples from site C (C4 and C6) were studied.

Sample A1 (Fig. 4a,c) is a fine-grained graphitic phlogopite-bearing phyllite alternated with calcschist, consisting of quartz, carbonates (both calcite and dolomite), phlogopite, abundant graphite, rare kyanite and accessory apatite. It shows a banded structure, characterized by millimetre-thick carbonate-free, quartz-rich layers alternated with carbonate-bearing layers of similar thickness (Fig. 4a). The main schistosity is defined by phlogopite, which occurs in oriented flakes showing a weak pleochroism from colourless to pale orange (Fig. 4a). Kyanite here is rare and limited to the carbonatefree layers; it forms sub-millimetric porphyroblasts partially replaced by late white mica at the rim.

Graphite is equally distributed between the two types of layers and it occurs both as inter-granular and intra-granular phases (e.g., included in quartz and calcite) (Fig. 4c).

Sample A8 (Fig. 4b,d) is a fine-grained graphitic phyllite consisting of quartz, muscovite, phlogopite, abundant graphite and accessory rutile. The pervasive foliation is defined by muscovite and phlogopite, the last being characterized by a weak pleochroism from colourless to pale orange (Fig. 4b). Phlogopite is, on average, less abundant and finer grained than muscovite (Fig. 4d).

Graphite is mostly concentrated in fine-grained aggregates of length up to 100 μm, localized at grain boundaries of the main minerals and elongated along the main schistosity (Fig. 4d).

SEM micrographs of graphite show similar morphologies (Fig. 5). Sample A8 shows a particularly characteristic cluster of flaky graphite plates (Fig. 5a). Intra-granular graphite in carbonates also shows flaky crystals (sample C4, Fig. 5b-d). No fibrous graphite morphology was observed. Some graphite crystals might be hexagonal (Fig. 5d), but those crystals most likely J o u r n a l P r e -p r o o f correspond to other flaky crystals differently oriented or distorted. Thus, besides amorphous carbon, graphite morphology in our samples mainly displays flaky crystals.

Validation of the new SEM-BSE technique

A total of 15 graphitic phyllite samples from the five sites were analyzed for TOC content by both SEM-BSE and CHNS techniques. To compare the mass ratio from the CHNS analyzer with the volume fraction from SEM-BSE, we calculated a volume ratio V based on the measured mass ratio m for TOC content determined using the CHNS analyzer, following:

𝛼 V = 𝛼 m 1 -𝜙 𝜌 g 𝜌 m (1 -𝛼 m ) + 𝛼 m , ( 7
)
where  is the porosity (taken equal to 2 %, as measured on 12 graphitic phyllite samples), and g and m are density of graphite (2200 kg m -3 ) and density of the surrounding mineral matrix (2700 kg m -3 , typical of crustal rocks), respectively.

The calculated TOC content from SEM-BSE ranged from 2.6±0.2 to 7.50±0.04 % which is nearly similar to the volumetric TOC content inferred from CHNS analyzer using Eq. ( 7), with a range from 2.0±0.1 to 8.4±0.5 %. As shown in Fig. 6, a positive correlation is obtained between volumetric TOC content measured by both methods, with a mean relative difference of 7±1 %. Our results show that the SEM-BSE technique is reliable to determine TOC content of graphite-rich rocks.

The SEM-BSE method has the advantage of being faster and does not need a long sample preparation of several days as necessary with the CHNS method for sample decarbonation. This new technique, in addition to giving direct estimates of TOC of a sample, provides information on the microstructural position of elemental carbon and allows us to draw a map of graphite occurrences in a sample. Our

J o u r n a l P r e -p r o o f SEM-BSE technique could thus be compared with the mapping of carbonaceous material obtained by Raman spectroscopy.

Results

The results of each of the five sites (A-E) in the Upper Trisuli valley are shown separately in Figs. 7-11 together with a photograph of the site, and a geological cross-section for sites A and D. All the data are summarized in Table 1 site by site. Additional information, precise location and all available data are presented in Supp. Table S1.

Results of the reference site A in the Upper Trisuli valley

Site A (Fig. 7a) shows a massive graphitic phyllite outcrop from 2002 to 2031 m asl. extending from positions 68 m to the west to 308 m to the east. A shear zone, sandwiched between two quartzite bands, is observed at position 52 m and separates graphitic phyllite from mica-schist. At the eastern end of the profile, debris flow deposits mainly composed of dolomite and soil are present (Fig. 7c).

Based on a total of 26 samples (from sample A1 to sample A34, including the other sample ME1), this site was extensively studied with sampling space of about 10 m. In addition, the two highresolution sampling areas are described in Supp. Fig. S13. The δ 13 C values of carbon from the bulk (i.e. non-decarbonated samples) range from -28.7 to -5.9 ‰, suggesting the occurrence of carbonates in samples with heavy carbon isotope signatures (Fig. 7b). TOC content ranges from 1.8 to 9.0 % with a mean value of 3.9±0.1 % while TC content ranges from 1.8 to 12.6 % with a mean value of respectively, show only small spatial variations, with exceptions in the eastern part of the profile (Fig. 7c). These values suggest the presence of moderately crystalline graphite, consistently with previous studies done in Nepal [START_REF] Beyssac | Thermal metamorphism in the lesser Himalaya of Nepal determined from Raman spectroscopy of carbonaceous material[END_REF][START_REF] Bollinger | Thermal structure and exhumation history of the Lesser Himalaya in central Nepal: Thermal structure of the Lesser Himalaya[END_REF]. Journal Pre-proof 40-cm-thick gouge at position 28 m. To the east of the profile, augen gneiss occurs, while to the west mica-schist crops out. The δ 13 C values of carbon from the bulk, ranging between -16.7 and -2.9 ‰, suggest a significant fraction of IC (Fig. 9b). TOC content shows significantly larger values ranging from 1.0 to 7.0 % with a mean value of 2.5±0.8 %, while TC content shows significantly larger values ranging from 6.2 to 10.9 % with a mean value of 9. to the east to 124 m to the west. The profile is characterized by an alternation of mica-schist with quartz lenses at some locations and graphitic phyllite with a pronounced black color (Fig. 10a,c). The δ 13 C values of carbon from the bulk, ranging between -28.7 and -25.5 ‰, suggest no or little carbonate occurrence (Fig. 10b). TOC content shows values similar to sites A and B, ranging from 1.6 to 4.8 % with a mean value of 3.6±0.3 % (Fig. 10b), while TC content shows values ranging from 1.7 to 4.7 % with a mean value of 3.6±0.4 %. IC content is negligible, except for two samples with a small mean value of 0.69±0.02 %. Calculated B-RSCM peak-T values range from 542 to 581 °C with a mean value of 565.5±4.2 °C. They are higher in the center of the profile (Fig. 10b). While a slight J o u r n a l P r e -p r o o f increase to the west is observed, D1-, G-and D2-FWHM values remain similar along the profile with mean values of 44.1±0.9 cm -1 , 20.5±0.2 cm -1 and 16.6±1.8 cm -1 , respectively.

Site E (Fig. 11a) shows a slightly weathered graphitic phyllite outcrop extending from positions 3 m to the east to 90 m to the west. The outcropping rock is sometimes hidden by debris fall deposits made of soil. The δ 13 C values of carbon from the bulk, ranging between -25.9 and -22.2 ‰, suggest a negligible amount of IC (Fig. 11b). TOC content shows values relatively similar to sites A, B and D, with values ranging from 2.9 to 5.7 % with a mean value of 4.1±0.3 % (Fig. 11b), while TC content shows values ranging from 2.8 to 5.5 % with a mean value of 4.0±0.3 %. TOC content shows higher values above 5 % at positions 3 and 65 m. Calculated B-RSCM peak-T values are relatively homogeneous and range from 549 to 575 °C with a mean value of 563.1±2.7 °C. The highest peak-T value is also observed at position 65 m (Fig. 11b). D1-, G-and D2-FWHM values are higher to the west of the profile and have mean values of 40.3±0.6 cm -1 , 20.6±0.3 cm -1 and 13.7±0.7 cm -1 , respectively.

Overview of carbon contents, peak-T and δ 13 C results in the Upper Trisuli valley

Our extensive data-set obtained from the Upper Trisuli valley allows us to study the distribution of the data obtained (Fig. 12). The TOC content (Fig. 12a) ranges from 0.9 to 9.0 %, with an overall mean of 3.7±0.2 %. The distribution is log-normal, with a median of 3.6±0.4 %, close to the mean. In total, 13 samples (15.4 %) have a TOC content above 5 %. The TC content (Fig. 12b) varies over a larger range, between 1.4 and 12.6 %, with a distribution centered on a mode at 2.5 % and showing a long tail toward higher values. Peak-T (Fig. 12c) ranges between 518.7 and 556.9 °C with a mean of 556.9±5.9 ºC. Peak-T values follow a scattered log-normal distribution centered on the mean, but a long tail toward smaller values is obtained. Regardless the considered site, besides J o u r n a l P r e -p r o o f few outliers, the G-band wavelength and FWHM are gathered around a characteristic mean value of 1579.8±0.2 cm -1 and 20.7±0.2 cm -1 , respectively. Similarly, the data show similar both D1/G FWHM ratio and D1/G intensity ratio, suggesting relatively homogeneous metamorphic conditions for all the studied samples (Supp. Fig. S14). Based on 20 analyses, the δ 13 C values of the non-decarbonated graphitic phyllite samples range between -2.9±0.1 and -28.7±0.1 ‰, with a mean of -22.0±0.1 ‰ (Fig. 12d). δ 13 C values show a main mode at -25 ‰, typical of organic carbon, and a secondary mode around -5 ‰, suggesting mixing with the carbonate source. .1 % and an IC content of about 1 %, suggesting a mixing of organic and inorganic carbon (Fig. 13b,d). When TC content is large, above 7 %, in most of the cases IC content is non-negligible. Decarbonated samples show more negative δ 13 C values (Fig. 13c,d), confirming that most of the carbonates, if not all, were removed. These observations clearly indicate that, despite the fact that the samples were collected in the field with the utmost caution to avoid carbonates, some of them bear carbonates in non-negligible amount. This suggests that even in the case of no visible carbonate band(s) in the field, it appears of the utmost importance to systematically study bulk and decarbonated samples.

Complementary results from Far-Western to Eastern Nepal

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The results obtained on the 67 samples collected from other valleys from Far-Western to Eastern Nepal are summarized in Table 2. Additional information, precise location and all available data are presented in Supp. Table S1.

In other valleys of Central Nepal, in the Ankhu valley to the west and in the Bhote Koshi valley to the east, high TOC content is also obtained with most values above 1 % and the maximum value of 10.4±0.4 %. In Western Nepal, in Kali Gandaki and Marsyandi valleys and at Barpak location (Gorkha district) large TOC contents are obtained with mean values above 3 %, relatively consistent with large values already reported in Marsyandi and Kali Gandaki valleys [START_REF] Evans | Degassing of metamorphic carbon dioxide from the Nepal Himalaya[END_REF][START_REF] Paudel | Carbonaceous schists of the Main Central Thrust zone as a source of graphite: a case study from the Kali Gandaki valley, west Nepal[END_REF]. In Eastern Nepal, in the areas of Akhuwatar and Leuti, TOC contents reach values above 4 to 5 %. By contrast, in Far-Western Nepal, samples from Baitadi-Darchula and Jumla-Humla sections show relatively small TOC contents ranging from <0.1 to 2 %. In Mid-Western Nepal, in Lower Dolpo, TOC content appears even smaller with values from 0.2 to 1.1 %. These data suggest some coherence and heterogeneity at various spatial scales.

Discussion

From the results presented above, we discuss: first, the significance of the data at increasing spatial scales, from metric, kilometric and to larger scales; second, the relationship between carbon content and peak-T and their general patterns; and third, the carbon isotopic signatures. Finally, we discuss the possible implications for our understanding of the carbon reservoirs in the Himalaya.

5.1. Coherence of metamorphic conditions at increasing spatial scales 5.1.1. Sample to metric and decametric scale variations of metamorphic conditions

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In our study, several samples were divided into 3-4 subsamples and analyzed, as detailed in section 3 above ('Sample analysis'). Here, we define the sample-scale dispersion (SASD) as the standard deviation of a parameter obtained over a set of subsamples of the same original sample, and divided by the mean value and expressed in per cent. On average, SASD for TOC content amounts to 46±19 %, while it is 2.90±0.01 % for peak-T (Table 1). The SASD value for TOC is large, slightly larger than other SASD obtained for other parameters from similar samples (Girault and Perrier, 2012), and suggests that large differences can be observed at small spatial scale. At this small spatial scale, the organic matter precursor is probably the same. Albeit difficult to explain, the heterogeneity might be due to non-homogeneous distribution of organic matter in the sedimentary protolith rock, frozen during metamorphism.

At meter-scale, as shown in Supp. Fig. S13 for the two high-resolution sampling areas, larger dispersion of values is obtained for HR1. Here, we define the scarp-scale dispersion (SCSD) as the standard deviation of a parameter obtained over a set of samples located up to a few meters from each other, and divided by the mean value and expressed in per cent. At meter-scale, considering the highresolution sampling area HR2, SCSD values of TOC content and peak-T amount to 24.3±5.5 % and 2.18±0.01 %, respectively (Table 1). The SCSD value for peak-T is small, consistent with a study testing for peak-T representativeness at a scale of 2-3 m [START_REF] Umeda | Testing for robustness on estimation of graphitization degree by Raman spectroscopy[END_REF]. The SCSD value for TOC is smaller than SASD values, but relatively similar to other SCSD values inferred from other parameters (Girault and Perrier, 2012). By contrast, considering the high-resolution sampling area HR1, SCSD values of TOC content and peak-T amount to 48±13 % and 2.86±0.02 %, respectively (Table 1). The SCSD value for TOC is larger than SCSD generally obtained for other parameters in similar context as mentioned above. At such meter scale, similar effects considered for SASD can be considered, and might be even more pronounced. Due to these SCSD values, the collection of one A). However, site C shows a particularly large SCSD value for TOC (81±33 %), due to the occurrence of fault gouge. SCSD appears larger when the number of samples is greater, but remains generally specific for a given site. It is less pronounced for peak-T values (Supp. Fig. S7 and Fig. 14). While TOC content appears mainly dependent on the distribution of organic matter in the protolith, which may differ between outcrops of the same formation, peak-T values are more homogeneous at the scale of an outcrop, and tend to reflect the overall metamorphic conditions. Thus, based on the available data, to obtain a representative peak-T value for a given outcrop, a few samples only appear sufficient.

By contrast, heterogeneities at different spatial scales impede obtaining a representative TOC content value at the outcrop scale when collecting less than 10 samples.

Unit-scale to large scale variations of metamorphic conditions

When considering all the five sites (A-E) of the same unit in the Upper Trisuli valley (Tables 1 and2), we define the unit-scale dispersion (UNSD) as the standard deviation of a parameter obtained over a set of samples located within the same geological unit or formation, and divided by the mean value and expressed in per cent. Over the 7-km-long outcrop of the graphitic mica-schist unit we studied, UNSD values for TOC content (40.8±3.9 %) and peak-T values (2.58±0.01 %) are larger than SASD and, on average, larger than SCSD. Dispersion of values thus depends on the considered spatial scale.

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Increasing again the spatial scale of observation, the graphitic mica-schist unit is encountered in the upper-LHS south to MCT at several locations throughout the Nepal Himalaya. For peak-T values, from Far-Western to Eastern Nepal, we consider our original data-set from the Upper Trisuli valley and from other valleys of Nepal, complemented by published data [START_REF] Beyssac | Thermal metamorphism in the lesser Himalaya of Nepal determined from Raman spectroscopy of carbonaceous material[END_REF][START_REF] Bollinger | Thermal structure and exhumation history of the Lesser Himalaya in central Nepal: Thermal structure of the Lesser Himalaya[END_REF][START_REF] Evans | Degassing of metamorphic carbon dioxide from the Nepal Himalaya[END_REF][START_REF] Paudel | Carbonaceous schists of the Main Central Thrust zone as a source of graphite: a case study from the Kali Gandaki valley, west Nepal[END_REF]. Data are summarized in Table 2 and illustrated in a regional map in Fig. 15. All over this 800-km-long lateral portion of the Himalaya, peak-T values range from 514.3±13.5 °C in Far-Western Nepal to 536.3±9.6 °C in Central Nepal.

UNSD amounts to 3.6±0.7 % for peak-T values. This value is similar to UNSD obtained at km-scale and indicates that the whole unit is representative of the large-scale metamorphic conditions south to MCT. Small along-strike variations in peak-T values at the scale of the Nepal Himalaya suggest similar thermal history of graphitic phyllites. Little carbon content data are available all over the Nepal Himalaya. When considering the available TOC data, with only 58 data from outside Central Nepal, a large UNSD value of 60.5±4.9 % is obtained for TOC and suggests the need for additional measurements.

Outside Nepal, at the scale of the Himalaya, graphitic phyllite is probably encountered from Bhutan to Zanskar, a few kilometers south to MCT. Little is known on graphitic phyllite occurrences, but inferences from several studies focusing on the thermal metamorphism of the LHS rocks south to MCT reported similar ranges of values for peak-T, e.g., in Eastern and Central Bhutan [START_REF] Long | Temperature and strain gradients through Lesser Himalayan rocks and across the Main Central thrust, south central Bhutan: Implications for transportparallel stretching and inverted metamorphism[END_REF][START_REF] Grujic | Deformational temperatures across the Lesser Himalayan Sequence in Eastern Bhutan and their implications for the deformation history of the Main Central Thrust[END_REF] and in Western India [START_REF] Célérier | The Kumaun and Garwhal Lesser Himalaya, India: Part 2. Thermal and deformation histories[END_REF][START_REF] Rawat | Features and characterization of graphite in Almora Crystallines and their implication for the graphite formation in Lesser Himalaya, India[END_REF][START_REF] Mathew | Thermal metamorphism of the Arunachal Himalaya, India: Raman thermometry and thermochronological constraints on the tectono-thermal evolution[END_REF]. Despite the scarcity of available data, TOC content with a mean value of 1.7±0.3 % in the Larji-Rampur window, Himachal Himalaya, India [START_REF] Rana | Carbonaceous material in Larji-Rampur window, Himachal Himalaya: Carbon isotope compositions, micro Raman spectroscopy and implications[END_REF] appears smaller than TOC content obtained in Nepal. These observations denote a large-scale thermal metamorphism that encompasses the whole Himalayan range and suggest a large-scale overall coherence of metamorphic conditions for the Himalayan graphite from the upper-LHS over the orogeny, with possible smooth variations, especially at the edges.

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Carbon content versus peak-T and the highest peak-T attained by upper-LHS rocks

In the studied graphitic phyllite samples, peak-T indicates the thermal history and the carbon content constrains the amount of carbon stored as graphite. Using both complementary parameters will improve our understanding of the relationship between metamorphism releasing carbon and the storage of carbon in the form of graphite. In Fig. 14, TOC content is plotted as a function of peak-T for our sample set. The samples from sites B, C, D and E are clustered and fall into the bottom part of the carbon domain. Site A shows more scattered data. Mean peak-T is relatively similar for all sites. The sites A, B, C, D and E are located at an approximate structural distance to MCT of ca. 2.9, 2.7, 3.2, 2.2 and 3.6 km, respectively. No clear relationship is observed between TOC and peak-T with the distance to the MCT at this spatial scale. From site A and C, higher carbon content tends to imply higher peak-T (Fig. 14). This might depend upon the kinetic of recrystallization of graphite crystals. Indeed, the presence of evident D1 peaks in all our samples suggests that the degree of graphite crystallinity remains low. Thus, graphite in our samples likely derives from progressive recrystallization of carbonaceous material at increasing temperature during prograde metamorphism.

This confirms that the graphitic phyllites are the metamorphic products of sedimentary protoliths rich in organic matter deposited all along the northern margin of the Indian plate. In the context of the MCT zone where no methane has been reported so far, the potential formation of hydrothermal graphite, deposited from COH fluids, remains unlikely at this stage [START_REF] Craw | Graphite reaction weakening of fault rocks, and uplift of the Annapurna Himal, central Nepal[END_REF]. Indeed, if graphite would have been deposited from COH fluids, we would not expect its systematic occurrence within a specific stratigraphic formation, but instead punctual occurrences controlled by fluid pathways.

In our sample set, the lowest and highest peak-T values determined by B-RSCM technique reach 518.7±8.5 °C and 589.9±8.2 °C, respectively. The highest peak-T values are obtained at site A,

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Furthermore, the possible eastward increase of our peak-T values appears similar to the slightly higher temperature towards East as reported by [START_REF] Goscombe | Metamorphic response to collision in the Central Himalayan Orogen[END_REF]. The high inferred peak-T indicates that graphitic phyllites were subjected to higher temperature, while the pressure might have remained relatively similar to other adjacent rocks. This may indicate that the distorted isotherms at depth near the MCT zone (e.g., [START_REF] Herman | Exhumation, crustal deformation, and thermal structure of the Nepal Himalaya derived from the inversion of thermochronological and thermobarometric data and modeling of the topography[END_REF][START_REF] Wang | The Himalayan collisional orogeny: A metamorphic perspective[END_REF] were already there at least 5 Ma years ago when MCT was highly active during Himalayan orogeny. Such modified isotherms may have promoted heating of rocks and fluids, leading to surface manifestations such as hot springs and CO2

degassing areas, similar to or even enhanced while compared with current observations (e.g., [START_REF] Girault | Large-scale organization of carbon dioxide discharge in the Nepal Himalayas[END_REF]Girault et al., , 2018, in press a,b), in press a,b). More data are needed to substantiate this interpretation further, and its consequences in terms of alteration.

Carbon isotopic composition signatures of graphitic phyllites

The bulk carbon isotopic composition (δ 13 C) of the graphitic phyllites is relatively homogeneous, with almost all values around the mean (-22.0±0.1 ‰) and 75 % of our data with δ 13 C value of ca. -26 ‰, suggesting a biogenic carbon origin of the graphite in Central Nepal (Fig. 13), as already reported in Western Nepal (e.g., [START_REF] Evans | Degassing of metamorphic carbon dioxide from the Nepal Himalaya[END_REF]. The outliers have less negative δ 13 C values around -5 ‰ (samples A4, C1, C5 and C7) and one intermediate value around -16 ‰ (sample C4), as also reported in Himachal Pradesh [START_REF] Rana | Carbonaceous material in Larji-Rampur window, Himachal Himalaya: Carbon isotope compositions, micro Raman spectroscopy and implications[END_REF]. Graphitic phyllite after removing carbonates yield more negative δ 13 C values, suggesting the presence of carbonates in the samples.

Analysis of carbon isotopic composition of a dolomite sample belonging to the same lithological unit gave δ 13 C of -1.3±0.1 ‰ (and δ 18 O of -15.0±0.7 ‰), consistent with other published δ 13 C values of ca. -2 ‰ for carbonates in metapelites [START_REF] Evans | Degassing of metamorphic carbon dioxide from the Nepal Himalaya[END_REF].

This variability in our δ 13 C values and the significant negative shift of the carbonate δ 13 C signature remain difficult to explain. However, such inconsistent δ 13 C values have also been reported

J o u r n a l P r e -p r o o f and discussed in other orogenic belts [START_REF] Kiran | Comparison between Raman spectra of carbonaceous material and carbon isotope thermometries in low-medium grade meta-carbonates: Implications for estimation of metamorphic temperature condition[END_REF] and may be more general (Fig. 16). Indeed, while most of δ 13 C values from graphitic rocks in orogenic belts remain generally close to the overall mean value of -23±4 ‰, more negative values [START_REF] Hoefs | The isotopic composition of carbonaceous matter in a metamorphic profile from the Swiss Alps[END_REF][START_REF] Rodas | Graphite occurrences in the low-pressure/high-temperature metamorphic belt of the Sierra de Aracena (southern Iberian Massif)[END_REF][START_REF] Evans | Degassing of metamorphic carbon dioxide from the Nepal Himalaya[END_REF][START_REF] Barrenechea | Graphite morphologies from the Borrowdale deposit (NW England, UK): Raman and SIMS data[END_REF][START_REF] Sharma | Carbon isotopic evidence for the origin of Himalayan graphite from Almora crystallines[END_REF][START_REF] Neubeck | Isotopic signatures of carbon in the 'Los Pobres' graphite mine[END_REF][START_REF] Rana | Carbonaceous material in Larji-Rampur window, Himachal Himalaya: Carbon isotope compositions, micro Raman spectroscopy and implications[END_REF] and outliers with less negative values are sometimes reported [START_REF] Hoefs | The isotopic composition of carbonaceous matter in a metamorphic profile from the Swiss Alps[END_REF][START_REF] Baker | Stable isotope evidence for limited fluid infiltration of deep crustal rocks from the Ivrea Zone, Italy[END_REF][START_REF] Sanyal | Origin of graphite, and temperature of metamorphism in Precambrian Eastern Ghats Mobile Belt, Orissa, India: A carbon isotope approach[END_REF][START_REF] Taner | Origin of graphite in the Southwestern Grenville Province[END_REF][START_REF] Rana | Carbonaceous material in Larji-Rampur window, Himachal Himalaya: Carbon isotope compositions, micro Raman spectroscopy and implications[END_REF][START_REF] Li | 2.0 Ga orogenic graphite deposits and associated 13 C-enriched meta-carbonate rocks from South China Craton: Implications for global Lomagundi event[END_REF].

Carbon isotope fractionation between calcite and graphite, Δ(Cc-Gr), is expected to decrease with increasing temperature (Valley and O'Neil, 1981, Fig. 2;[START_REF] Satish-Kumar | Constraints on the application of carbon isotope thermometry in high-to ultrahigh-temperature metamorphic terranes[END_REF][START_REF] Paudel | Carbonaceous schists of the Main Central Thrust zone as a source of graphite: a case study from the Kali Gandaki valley, west Nepal[END_REF][START_REF] Kiran | Comparison between Raman spectra of carbonaceous material and carbon isotope thermometries in low-medium grade meta-carbonates: Implications for estimation of metamorphic temperature condition[END_REF]. Hence, the low δ 13 C of most of the graphitic phyllites is consistent with the quasi-absence of carbonates at high temperature. However, the high δ 13 C outliers (samples A4, C1, C5 and C7) are characterized by high IC contents (Fig. 13b) and δ 13 C values after removing carbonates are lower than TC values. This suggests re-equilibration during metamorphism between organic carbon and pre-existing sedimentary carbonates. These samples have organic carbon fraction between 0.19 and 0.27. Assuming a protholith with organic carbon fraction of 0.25 with δ 13 C of -25±1 ‰ and carbonate with δ 13 C of -1.3±0.1 ‰ would lead to a bulk composition of ca. -7±1 ‰ using a mass balance approach. This value appears compatible with the range of observed values for three of the outliers.

Sample C7 with a measured δ 13 C value from the bulk of -2.9 ‰ may imply an open system reequilibration in a carbonate-rich environment. This is compatible with our petrographic observations that do not show evidence for equilibria between calcite and graphite in our samples. Similarly, no clear relation between Raman-based peak-T and δ 13 C is observed (Supp. Fig. S15), suggesting that these outliers with high δ 13 C values are not related to deep metamorphic processes (e.g., devolatilization, see [START_REF] Nakamura | Strain-induced amorphization of graphite in fault zones of the Hidaka metamorphic belt, Hokkaido, Japan[END_REF][START_REF] Nakamura | Microstructure and geochemical signatures of metasedimentary origin pseudotachylyte: Implications for fluid activity during paleoseismicity[END_REF][START_REF] Nakamura | Pressure dependence of graphitization: implications for rapid recrystallization of carbonaceous material in a subduction zone[END_REF].

Implications for the carbon reservoirs in the Himalaya

J o u r n a l P r e -p r o o f

Journal Pre-proof

In the Upper Trisuli valley, to the west in the Ankhu valley ("Ruby valley"), and to the east in the Bhote Koshi valley, Central Nepal, a large number of graphitic phyllites from the graphitic mica-schist unit were collected (n=96). They gave relatively high TOC contents with values ranging from 0.6 % to 10.4 % and with a mean value of 3.5±0.2 %. These three valleys are an important starting point to quantify the amount of organic carbon stored as graphite and to compare it with the carbonates reservoir. Here, to simplify, we consider the unit as a whole, characterized by both organic and inorganic carbon. Based on the work by Tamang (2022), we use the average and min-max range of vol.% values for carbonate-bearing lithologies in the unit. Using estimated thicknesses for the unit [START_REF] Decelles | Stratigraphy, structure, and tectonic evolution of the Himalayan fold-thrust belt in western Nepal[END_REF][START_REF] Robinson | Tectonic evolution of the Himalayan thrust belt in western Nepal: Implications for channel flow models[END_REF] and considering also the shortening of the upper-LHS rocks [START_REF] Long | Construction of the Lesser Himalayan-Subhimalayan thrust belt: The primary driver of thickening, exhumation, and high elevations in the Himalayan orogen since the middle Miocene[END_REF], we can give first estimates of the amount of organic carbon and of the fraction of organic carbon compared with total carbon stored in the unit.

In Central Nepal, for the graphitic mica-schist unit, we take 3-4 km thickness of upper-LHS rocks, 50,400±6400 km 2 reconstructed surface area, 26±9 vol.% of carbonates and mean (min-max range) for TOC content of 3.5±0.2 % (0.9-10.4 %). We obtain a total amount of organic carbon of (1.4±0.3)×10 7 Mton, representing about 10±4 % of the total carbon amount in the unit. Considering maximum ranges of values yield an estimated fraction of organic carbon of ca. 20 %. Extrapolating this calculation to Eastern and Western Nepal, using the measured TOC content in this study and other parameters from the aforementioned literature, we can calculate in the same manner the amount and fraction of organic carbon in Eastern and Western Nepal. Smaller amounts of TOC are obtained, (8±5)×10 6 Mton for Western Nepal and (1.5±0.6)×10 6 Mton for Eastern Nepal, which lead to mean TOC fraction of 8 % and 6 % and maximum TOC fraction of ca. 16 % and 11 %, respectively. Thus, at the scale of the upper-LHS rocks in Nepal, the TOC content may represent from 10 to 20 % of the total carbon stored. As similarly evidenced in the case of subduction [START_REF] Plank | Subducting carbon[END_REF], graphite emerges as an important carbon reservoir possibly representing from one-tenth to one-fifth of the total carbon stored during collision.

J o u r n a l P r e -p r o o f

As illustrated in the cross-section, oriented south to north and crossing the Kathmandu Klippe, the graphitic mica-schist unit in the upper-LHS was folded and currently crops out at the surface at three locations (Fig. 17): above the Main Boundary Thrust (MBT), north to the Klippe and in the MCT zone. This particular unit bearing graphite appears to be associated with the major thrust faults of the Himalayan orogeny, and, due to its lubricant behavior, it may control the dip direction of the main thrusts and faults participating to the shortening, govern the underplating and reduce the friction on fault planes (e.g., Parnell and Brolly, 2021;Parnell et al., 2021). In addition, in the MCT zone, and in a smaller part in the MBT zone, numerous hydrothermal systems bearing hot springs, that release varying amounts of CO2 of metamorphic origin, are reported (e.g., [START_REF] Becker | Himalayan metamorphic CO2 fluxes: Quantitative constraints from hydrothermal springs[END_REF][START_REF] Evans | Degassing of metamorphic carbon dioxide from the Nepal Himalaya[END_REF][START_REF] Girault | Large-scale organization of carbon dioxide discharge in the Nepal Himalayas[END_REF][START_REF] Girault | Persistent CO2 emissions and hydrothermal unrest following the 2015 earthquake in Nepal[END_REF]Girault et al., , 2022, in press a,b), in press a,b). The CO2, whose production was initiated as soon as the collision provided the necessary temperature and pressure, is still currently produced by decarbonation reactions at depth, mainly in the LHS rocks beneath the MCT [START_REF] Tamang | Along-strike variations of metamorphic CO2 outputs from the Nepal Himalaya: Comparison between past production and present emissions[END_REF], and in the GHS rocks in a smaller part (e.g., [START_REF] Groppo | Metamorphic CO2 production from calc-silicate rocks via garnet-forming reactions in the CFAS-H2O-CO2 system[END_REF][START_REF] Groppo | CO2 outgassing during collisional orogeny is facilitated by the generation of immiscible fluids[END_REF][START_REF] Rapa | Titanite-bearing calc-silicate rocks constrain timing, duration and magnitude of metamorphic CO2 degassing in the Himalayan belt[END_REF]. The stored graphite-rich layer may act as a barrier for the upward percolation of the deeply produced metamorphic CO2-rich fluids. Conversely, it may channelize the current release of CO2 toward the surface, and probably also the past CO2 emissions [START_REF] Guo | India-Asia collision as a driver of atmospheric CO2 in the Cenozoic[END_REF]. Central Nepal, which can be considered as the generic example, exhibits the highest organic carbon content in the form of graphite, the highest peak-T, and the highest current CO2 emission.

Central Nepal also shows the smallest recurrence time for large earthquakes (e.g., [START_REF] Bollinger | Estimating the return times of great Himalayan earthquakes in Eastern Nepal: evidence from the Patu and Bardibas strands of the Main Frontal Thrust[END_REF]. It was also the rupture zone of the 2015 Mw 7.9 Gorkha earthquake and its aftershocks [START_REF] Adhikari | Seismically active structures of the Main Himalayan Thrust revealed before, during and after the 2015 Mw 7.9 Gorkha earthquake in Nepal[END_REF]. The large-scale coherence of our observations and the systematic association of thick graphitic layers with current CO2 emissions can hardly be coincidental and suggests possible connections between organic carbon stored as graphite and mobile inorganic carbon released as CO2 at the scale of the Himalayan orogenic belt, an important aspect to be investigated further for global J o u r n a l P r e -p r o o f carbon budget estimations [START_REF] Kelemen | Reevaluating carbon fluxes in subduction zones, what goes down, mostly comes up[END_REF][START_REF] Plank | Subducting carbon[END_REF][START_REF] Müller | Evolution of Earth's tectonic carbon conveyor belt[END_REF].

Conclusion

In this paper, using various methods to characterize and quantify graphitic carbon content, Our study is the first multiscale systematic investigation of graphitic phyllites from the upper-LHS, conducted at a scale which is order of magnitude more detailed than that of previous studies.

The new technique based on SEM-BSE for estimating TOC would now benefit from more case studies on other types of metamorphic rocks with more crystalline graphite, such as those formed in hydrothermal veins. Although the graphite in upper-LHS rocks is moderately crystalline, due to its widespread occurrence along the MCT zone, it might represent an important economic asset for the country. However, more effort is needed to better characterize its industrial potential [START_REF] Paudel | Carbonaceous schists of the Main Central Thrust zone as a source of graphite: a case study from the Kali Gandaki valley, west Nepal[END_REF][START_REF] Palosaari | High-quality flake graphite occurrences in a high-grade metamorphic region in Sortland, Vesterålen, northern Norway[END_REF][START_REF] Robinson | Critical mineral resources of the United States-Economic and environmental geology and prospects for future supply[END_REF].

J o u r n a l P r e -p r o o f

We have shown that in the context of the deep carbon cycle, Himalayan graphitic phyllites represent a major reservoir of organic carbon in the Himalaya, in addition to inorganic carbon reservoir of carbonate-bearing lithologies. The fraction of organic carbon may represent up to 20 % of the total carbon stored. The main difference with respect to carbonates, is that graphite is much more stable than carbonates during prograde metamorphism and through weathering, making it a long term sink in the carbon cycle. While carbonates experience decarbonation reactions as temperature and pressure progressively increase, graphite remains passively stable, simply increasing its degree of crystallinity and not being involved in CO2-producing or CO2-consuming reactions. Thus, this graphitic mica-schist unit, encountered from Bhutan to Zanskar, is a remarkable sign of the carbon sink/source duality of the Himalayan orogeny [START_REF] Gaillardet | Himalaya-Carbon sink or source?[END_REF], and emerges as a key marker to study when working on the global carbon budget.

J o u r n a l P r e -p r o o f

Journal Pre-proof is compiled from several studies (see Amatya and Jnawali, 1994;[START_REF] Upreti | An overview of the stratigraphy and tectonics of the Nepal Himalaya[END_REF]Girault et al., 2012 and reference herein;[START_REF] Dhital | Geology of the Nepal Himalaya[END_REF][START_REF] J O U R N A L P R E -P R O O F Rapa | Detection of tectonometamorphic discontinuities within the Himalayan orogen: Structural and petrological constraints from the Rasuwa district, central Nepal Himalaya[END_REF][START_REF] Khanal | In-sequence buoyancy extrusion of the Himalayan Metamorphic Core, central Nepal: Constraints from monazite petrochronology and thermobarometry[END_REF], Girault et al., 2022) and from our own field works. Topography is drawn using the 1 arc-second DEM provided by GMT6 software. The volumetric carbon content determined using the SEM-BSE method is compared with the mass TOC content determined using the CHNS method calculated as a volumetric fraction (see text). The identity line is shown as black dashed line. The fit is shown as solid red line (TOCCHNS=0.914TOCSEM+0.218; R 2 =0.69). The box and the solid vertical bars represent the min/max range and the δ 13 C average at each location.

The dashed vertical bar represents the overall mean δ 13 C value considering all the data plotted. Data are taken from the literature [START_REF] Hoefs | The isotopic composition of carbonaceous matter in a metamorphic profile from the Swiss Alps[END_REF][START_REF] Morikiyo | Carbon isotopic study on coexisting calcite and graphite in the Ryoke metamorphic rocks, northern Kiso district, central Japan[END_REF][START_REF] Morikiyo | Hydrogen and carbon isotope studies on the graphite-bearing metapelites in the northern Kiso district of central Japan[END_REF][START_REF] Baker | Stable isotope evidence for limited fluid infiltration of deep crustal rocks from the Ivrea Zone, Italy[END_REF][START_REF] Large | Poorly crystalline carbonaceous matter in high grade metasediments: implications for graphitization and metamorphic fluid compositions[END_REF][START_REF] Rodas | Graphite occurrences in the low-pressure/high-temperature metamorphic belt of the Sierra de Aracena (southern Iberian Massif)[END_REF][START_REF] Evans | Degassing of metamorphic carbon dioxide from the Nepal Himalaya[END_REF][START_REF] Sanyal | Origin of graphite, and temperature of metamorphism in Precambrian Eastern Ghats Mobile Belt, Orissa, India: A carbon isotope approach[END_REF][START_REF] Barrenechea | Graphite morphologies from the Borrowdale deposit (NW England, UK): Raman and SIMS data[END_REF]Sharma et al., 2011;Martin-Méndez et al., 2016;[START_REF] Taner | Origin of graphite in the Southwestern Grenville Province[END_REF][START_REF] Manoel | On the origin of the Neoproterozoic Peresopolis graphite deposit, Paraguay Belt, Brazil[END_REF][START_REF] Menzies | Carbon dioxide generation and drawdown during active orogenesis of siliciclastic rocks in the Southern Alps, New Zealand[END_REF][START_REF] Wang | Metamorphism, geochemistry, and carbon source on sedimentary-metamorphic graphite deposits in eastern Shandong, China[END_REF][START_REF] Neubeck | Isotopic signatures of carbon in the 'Los Pobres' graphite mine[END_REF][START_REF] Zhu | Carbon isotope and geochemical characteristics of the Paleoproterozoic graphite deposits in the Jiao-Liao-Ji belt, North China Craton: Implications for genesis and depositional environment[END_REF][START_REF] Rana | Carbonaceous material in Larji-Rampur window, Himachal Himalaya: Carbon isotope compositions, micro Raman spectroscopy and implications[END_REF][START_REF] Li | 2.0 Ga orogenic graphite deposits and associated 13 C-enriched meta-carbonate rocks from South China Craton: Implications for global Lomagundi event[END_REF]. emission zones are taken from the literature [START_REF] Becker | Himalayan metamorphic CO2 fluxes: Quantitative constraints from hydrothermal springs[END_REF][START_REF] Evans | Degassing of metamorphic carbon dioxide from the Nepal Himalaya[END_REF][START_REF] Craw | Graphite reaction weakening of fault rocks, and uplift of the Annapurna Himal, central Nepal[END_REF][START_REF] Girault | Large-scale organization of carbon dioxide discharge in the Nepal Himalayas[END_REF][START_REF] Girault | Persistent CO2 emissions and hydrothermal unrest following the 2015 earthquake in Nepal[END_REF]Girault et al., , 2022, in press a,b), in press a,b). The isotherms are taken from [START_REF] Wang | The Himalayan collisional orogeny: A metamorphic perspective[END_REF].

MFT: Main Frontal Thrust; MBT: Main Boundary Thrust; RT: Ramgarh Thrust; MCT: Main Central Thrust; STDS: South Tibetan Detachment System.

J o u r n a l P r e -p r o o f Min/ma x range 2.4/9.0 2.1/6.5 1.9/4.0 1.9/9.0 1.6/5.3 1.0/7.0 1.6/4.8 2.9/5.7 0.9/9.0 a The coordinates indicate point 0 along each profile. b Data from the three samples ME1, ME9 and ME15 are presented in Supp. Table S1.

J o u r n a l P r e -p r o o f J o u r n a l P r e -p r o o f

  To compare with other valleys in Nepal in similar geological contexts, a total of 67 additional graphitic phyllite samples collected during other campaigns in other locations of Nepal were also used. In Far-Western Nepal, we collected: 2 samples from Dharchula area near Khotila and above Pasti in June 2022, 4 samples from Mugu-Karnali valley near Gamgadhi in November 2018 and 5 samples from Humla-Karnali valley along the Jumla-Simikot way in October 2021. In mid-Western Nepal, we collected 4 samples in Lower Dolpo above Rupagad and Tarakot from 2009 to 2011. In Western Nepal, we collected: 10 samples in Kali Gandaki valley near Narchyeng in December 2020 (Supp. Fig. S1), 3 samples in Marsyangdi valley near Ngadi in April 2018 and 2 samples near Barpak in May 2016. In Central Nepal, we collected: 2 samples in Ankhu valley ("Ruby valley") near Lapa in 2017, and 1, 3 and 13 samples in Bhote Koshi valley near Kodari in August 2009, December 2020

  4.2±0.4 %. About two times higher TOC content is observed in the eastern part of the profile near the shear zone rich in gouge with a mean value of 5.2±0.6 % from position 68 to 158 m. IC content is significant in three samples (A1, A4 and A5) with a mean value of 7.2±1.3 %. TOC content decreases J o u r n a l P r e -p r o o f towards the west reaching values smaller than 5 %; IC content remains smaller than 3 % or is negligible. The dispersion of TOC values in the eastern part of the profile (38±10 %; n=10) is larger than in the rest of the profile (20±4 %; n=15) (Fig.7b). Calculated B-RSCM peak-T values range from 519 to 590 °C. While they remain relatively homogeneous, slightly higher peak-T values are obtained in the eastern part of the profile. The values of D1-, G-and D2-FWHM, with mean values of 43.6±0.6 cm -1 , 20.8±0.3 cm -1 and 20.3±1.0 cm -1 ,

4. 2

 2 Results of the two nearby sites B and CSite B (Fig.8a) shows a graphitic phyllite outcrop from 1886 to 1909 m asl. extending from positions 10 m to the west to 99 m to the east. The profile consists mainly of phyllite without occurrence of carbonate bands, as confirmed by decarbonating the samples and the δ 13 C values of carbon from the bulk ranging between -26.6 and -26.1 ‰ (Fig.8b). The eastern part of the profile marks the contact between mica-schist and garnet schist and augen gneiss. TOC content ranges from 1.6 to 5.3 % with a mean value of 3.5±0.6 %, while TC content ranges from 1.4 to 4.4 % with a mean value of 3.0±0.4 %. Calculated B-RSCM peak-T values range from 530 to 574 °C with a mean value of 558.7±6.1 °C. They do not show large spatial variations, besides near the contact (Fig.8b). Only small spatial variations are observed for D1-, G-and D2-FWHM values, with mean values of 40.5±1.0 cm -1 , 21.0±0.3 cm -1 and 16.9±1.7 cm -1 , respectively. Site C (Fig.9a) shows a representative graphitic phyllite outcrop extending from positions 0 m to the east to 50 m to the west. The profile is characterized by a clear anomalous contact with 20-J o u r n a l P r e -p r o o f

  0±0.7 % (Fig 9b). Conversely, IC content values are larger than at the other sites, except for the gouge sample C4, and range from 1.0 to 9.1 % with a mean value of 6.4±1.1 %. The gouge sample C4 has an exceptionally high TOC content of 7.0±0.4 %, separating rocks with similar TOC contents. Calculated B-RSCM peak-T values range from 521 to 581 °C with a mean value of 559.1±8.2 °C. They show large variations in the vicinity of the gouge, with the lowest peak-T value obtained for the gouge sample (Fig.9b). D1-, G-and D2-FWHM values show significant spatial variations in the vicinity of the contact zone and have mean values of 40.3±1.7 cm -1 , 20.1±0.8 cm -1 and 14.0±1.3 cm -1 , respectively.4.3 Results of the two farther sites D and ESite D (Fig.10a) shows a massive graphitic phyllite outcrop extending from positions 30 m

  Considering the whole set of samples from the Upper Trisuli Valley, the presence of IC content is negligible for most of the samples (Fig 13a). Nine samples show IC content above 5 % and relatively low TOC content, most of them belonging to site C. Some samples show the combination of TOC and IC with similar TOC and IC contents. Most of the samples analyzed for δ 13 C from all the outcrops show negligible IC content (Fig. 13b), large TOC content (Fig. 13c) and δ 13 C values lower than -20 ‰ (Fig 13b,c), indicating the predominance of organic carbon. Four samples have large IC content and higher δ 13 C values (mean: -5.1±0.1 ‰). One sample from site C shows an intermediate δ 13 C value of -16.7±0

J

  o u r n a l P r e -p r o o f sample per outcrop only might be risky in terms of larger scale interpretation for carbon content, and poses the question of the representativeness of a given sample. This is emphasized by the SCSD values obtained for 10-m-spaced samples at each of the five studied sites (Figs. 7-11). The dispersion of TOC content is shown in Fig. 14 separately for all the five sites. SCSD values for TOC at the scale of an outcrop remain compatible with SCSD values obtained for 1-m distance with values ranging generally from 20.3±4.7 % (site E) to 41.2±5.1 % (site

  including a new SEM-BSE technique, we have shown that graphitic phyllites of the Upper Lesser Himalaya are significant components when investigating the metamorphic conditions in the MCT zone at the scale of the whole Himalayan orogen. Our data in the Upper Trisuli valley, Central Nepal have shown a significant coherence of the metamorphic conditions at small to large spatial scales (flaky graphite characteristics, peak metamorphic temperature and carbon isotopic composition), but also show some discrepancies between δ 13 C of phyllites, suggesting metamorphic re-equilibration between organic carbon and carbonates in samples with high δ 13 C. The addition of complementary data from other valleys from Far-Western to Eastern Nepal have allowed us to map along strike the metamorphic conditions of upper-LHS rocks. Higher TOC content and peak-T values have been observed in Central Nepal. Our results thus establish a robust framework for the understanding of the carbon stored in orogenic belts, an essential component of the global carbon budget.

Figure 1 .

 1 Figures caption

Figure 2 .

 2 Figure 2. (a) Raw Raman spectra recorded for ten different graphite locations of a graphitic phyllite

Figure 3 .

 3 Figure 3. Protocol for the determination of carbon content from graphitic phyllites using a Scanning

Figure 4 .

 4 Figure 4. Photomicrographs of graphitic phyllites: (a) sample A1 containing carbonates showing the

Figure 5 .

 5 Figure 5. SEM micrographs graphite from graphitic phyllites: (a) sample A8 showing flaky

Figure 6 .

 6 Figure 6. Comparison of the SEM-BSE method with the CHNS analyzer method to determine the

Figure 7 .

 7 Figure 7. Results from site A above Pajung village in the Upper Trisuli valley: (a) photograph of a

Figure 8 .

 8 Figure 8. Results from site B below Pajung village in the Upper Trisuli valley: (a) photograph of the

Figure 9 .

 9 Figure 9. Results from site C near Goljung village in the Upper Trisuli valley: (a) photograph of the

Figure 10 .

 10 Figure 10. Results from site D near Sanjen hydropower site in the Upper Trisuli valley: (a)

Figure 11 .

 11 Figure 11. Results from site E near Paragaon village in the Upper Trisuli valley: (a) photograph of

Figure 12 .

 12 Figure 12. Distribution of (a) total organic carbon (TOC) content, (b) total carbon (TC) content, (c)

Figure 13 .

 13 Figure 13. (a) Inorganic carbon (IC) content as a function of total organic carbon (TOC) content, and carbon isotopic composition (δ 13 C) as a function of (b) IC content, (c) TOC content and (d) total

Figure 14 .Figure 15 .

 1415 Figure 14. TOC content as a function of peak-T of graphitic mica-schists for the five sites of the

Figure 16 .

 16 Figure 16. Distribution of various carbon isotopic composition (δ 13 C) data from the bulk at different

Figure 17 .

 17 Figure 17. Simplified tectonic cross-section of the Nepal Himalaya showing the graphitic mica-schist

Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 Figure 12 Figure 13 Figure 14 Figure 15 Figure 16

 12345678910111213141516 Figure1

Figure 17

 17 Figure17

Table 1 .

 1 Overview of the graphitic phyllites characteristics from the five sites of the Upper Trisuli valley investigated in this study. Statistics of the two high-resolution sampling areas from site A are also given.

	Tables caption

Table 2 .

 2 Overview of carbon content and peak metamorphic temperature from graphitic phyllites in Nepal and in the whole Himalayan orogen. Organized by region, from West to East, data include original values (n=145) and entries from the literature (n=36).

Table 1

 1

	Site		Site A				Site B	Site C Site D Site E	All
											sites
											(A-E)
			Spacin	HR1	HR2 All
			g			sampl
			of 10 m			es
	Locati	Above Pajung village		Below	Near	Near	Near	Upper
	on						Pajung	Goljun	Sanjen	Paraga	Trisuli
							village	g	village	on	valley
								village		village
	Coordi	28.185	28.185	28.185		28.186	28.179	28.215	28.204
	nates		42°N/	07°N/	08°N/		03°N/	73°N/	75°N/	58°N/
	(WGS8	85.323	85.323	85.322		85.331	85.329	85.281	85.298
	4) a		68°E,	02°E,	08°E,		28°E,	54°E,	60°E,	31°E,
			2028 m	2018 m	2024 m		1891 m	1629 m	2277 m	2339 m
	Numbe	25	11	11	43	6	7	10	10	74 b
	r	of
	sample
	s
	TOC
	content
	(%)
			4.2±0.3 3.8±0.6 2.8±0.2 3.8±0.	3.5±0.6 2.5±0.8 3.6±0.3 4.1±0.3 3.7±0.
	Mean					2					2
	±1-σ

Acknowledgments

We thank Som Nath Sapkota, the Department of Mines and Geology, and in particular Mukunda Bhattarai and Rajesh Sharma from the National Earthquake Monitoring and Research Center (DMG/NEMRC) for their support. We thank Sudhan Singh Mahat and Sanjen Jalavidhyut Company Limited for their precious help at site D (Sanjen outcrop). Om Bahadur Tarum, Dawa Tamang, and Niraj Jairu are thanked for assistance in the field. We thank Laurent Bollinger for fruitful discussions. We thank Emmanuelle Raimbault for measurement of carbon content in IPGP and Philippe Blanc (LithoB) for preparation of thin sections. We are thankful to the Editor Balz Kamber, Madhusoodhan Satish-Kumar and an anonymous reviewer for thorough reviews which significantly improved the initial version of the manuscript. Financial support was provided by funding of the IPGP team "Physics of Natural Sites", complemented by a LabEx UnivEarthS Frontier funding "MYSTHIC" (ANR-10-LABX-0023 and ANR-18-IDEX-0001) lead by F.G.

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near the fault zone. At the other sites, high peak-T values are also obtained, with two values above 580 °C at sites C and D. These high peak-T values are below the upper limit of the RSCM method, which has been extended to 640 °C [START_REF] Beyssac | Peak metamorphic temperature and thermal history of the Southern Alps (New Zealand)[END_REF][START_REF] Delchini | Applicability of the RSCM geothermometry approach in a complex tectono-metamorphic context: The Jebilet massif case study (Variscan Belt, Morocco)[END_REF]. These values are among the highest RSCM-based peak-T ever recorded by upper-LHS rocks in Nepal: the highest published values were 571.8±9.0 °C along the Bhote Koshi valley near Kodari, Central Nepal and 568.5±16.0 °C along the Khudi valley near the Marsyandi valley, Western Nepal [START_REF] Beyssac | Thermal metamorphism in the lesser Himalaya of Nepal determined from Raman spectroscopy of carbonaceous material[END_REF]. As shown in the Nepal map (Fig. 15) and summarized in Table 2, both the highest mean peak-T and carbon content values are obtained in Central Nepal. Progressively decreasing carbon content and peak-T values are reported towards Western, Mid-Western and up to Far-Western Nepal. Outside Nepal, peak-T reaches about 568 °C in Kumaun, Western India [START_REF] Rawat | Features and characterization of graphite in Almora Crystallines and their implication for the graphite formation in Lesser Himalaya, India[END_REF]; obtained using B-RSCM), about 574-582 °C in Bhutan [START_REF] Long | Temperature and strain gradients through Lesser Himalayan rocks and across the Main Central thrust, south central Bhutan: Implications for transportparallel stretching and inverted metamorphism[END_REF]; corrected from R-RSCM using our Supp. Fig. S5) and about 600-617 °C in Arunachal Himalaya [START_REF] Mathew | Thermal metamorphism of the Arunachal Himalaya, India: Raman thermometry and thermochronological constraints on the tectono-thermal evolution[END_REF]; obtained using B-RSCM). Thus, peak-T values obtained in Nepal are higher than those obtained in Western India and lower than those obtained in Bhutan and Eastern India. The available data so far might suggest an increase of the highest peak-T of upper-LHS rocks from west to east along the Himalayan belt, but this preliminary inference needs to be confirmed with further data along strike.

We have shown that the graphitic mica-schist unit of the upper-LHS experienced RSCMbased metamorphic temperatures slightly higher than those previously estimated. This suggests that, based on peak-T only, the transition between LHS and GHS rocks is rather narrow, and that it may depend on the location along the strike of the Himalayan orogen. Generally, the peak-T difference between LHS and GHS is about 50 to 100 °C [START_REF] Bollinger | Thermal structure and exhumation history of the Lesser Himalaya in central Nepal: Thermal structure of the Lesser Himalaya[END_REF]; actually, this difference could be smaller, of the order of only 30 °C. More interestingly, at the footwall of the MCT over the Nepal Himalaya, our peak-T data inferred from the RSCM approach, together with published RSCM-based peak-T values, substantially confirm the temperature constraints obtained using the equilibrium thermodynamics approach from Nanga Parbat to Namche Barwa [START_REF] Goscombe | Metamorphic response to collision in the Central Himalayan Orogen[END_REF].

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Declaration of interests

☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☐ The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: