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1 Graphite formation by carbonate reduction during subduction Matthieu E. Galvez a,b*, Olivier Beyssac a*, Isabelle Martinez b, Karim Benzerara a, Carine Chaduteau b, Benjamin Malvoisin c,d, Jacques Malavieille e, a IMPMC, CNRS-UPMC-IRD UMR 7590, UPMC Campus Jussieu, Case Courrier 115, 4 place Jussieu, Paris, France b Géochimie des isotopes stables, IPGP, PRES Sorbonne Paris cité, 1 rue Jussieu, Paris, France c Laboratoire de géologie, Département des Géosciences, ENS-CNRS, 24 rue Lhomond, Paris, France d ISTerre, CNRS UMR 5275, Université Grenoble I, BP Grenoble CEDEX 9, France e Géosciences Montpellier, UMR 5243, CNRS-Université Montpellier II, Montpellier CEDEX 05, France Present address: Carnegie Institution of Washington, Geophysical Laboratory, 5251 Broad Branch Road NW, 20015Washington DC, USA. NATURE GEOSCIENCE 1

2 SW NE sediment (qtz-phg-gt-cal) serpentinite S1. Field picture of the Malaspina outcrop (Alpine Corsica, France) showing the serpentinite body (bottom) and the overlying sediment (top) as well as the dark layer corresponding to the reaction zone (indicated by white arrows) at the contact (white line). The scale bar represents 1 meter. 2 NATURE GEOSCIENCE

3 d.t.c* (mm) δ 13 C CM ( ) Precision TOC (wt.%) Precision δ 13 C CAL ( ) TIC (wt.%) Z3A Z3B < <0.01 Z3C < Z3D < Z3E < Z3F < Z3G <0.01 Z3I Z3J < Z3M Z3N < Z4A < Z4B < Z4C < Z4E < Z4F < Z4G < Z4H < * distance to contact calculated using a Monte Carlo analysis. This value incorporates errors associated to each steps of the measurement procedure. TOC is the "total organic carbon", and TIC the total inorganic carbon corresponding to C-calcite. -- not measured S2: Carbon isotopic composition, total organic carbon (TOC) and total inorganic carbon (TIC) in the metasediment along the reaction zone as function of the distance to the contact (d.t.c) with the serpentinite. The precision of the isotopic measurements for CM was calculated using a Monte Carlo analysis (see methods for details). Note that the steady difference between the calcite and CM isotopic composition in PS (δ 13 C cal -δ 13 C CM ~15 ) is consistent with equilibration at the relatively low peak metamorphic temperature (430 C) of this unit 1,2. NATURE GEOSCIENCE 3

4 S3 a ~15m Vertical distance to contact (cm) PS transition RZ Structural ordering of CM (R2 parameter) b First-order region G band Second-order region D1 band D2 band PS_2 PS_1 ~2704cm -1 - RZ_2 RZ_1 2730cm cm Raman shift (cm-1) Raman shift (cm-1) 4 NATURE GEOSCIENCE

5 S3: A. Evolution of the R2 parameter quantifying the structural ordering of CM by Raman spectroscopy along the studied profile. This parameter is the R2=D1/(G+D1+D2) peak area ratio directly extracted from the fitting procedure applied to Raman spectra 3 with G being the graphite band and D1, D2 the defect bands. B. This figure represents the main peaks in the first- and corresponding second-order Raman spectra of graphitic CM in RZ and PS. Two representative spectra for each zones (PS1,PS2, taken ~25mm and ~4mm far from the contact respectively) and (RZ1,RZ2 taken 6cm and ~9cm far from the contact respectively). In the first-order region, the relative intensity of the defect bands is informative on the cristallinity of the graphitic CM. The second-order region provides additional information on the 3D stacking order of graphitic CM. When the three-dimensional structure of graphite is reached (ABAB stacking sequence), the main band at ~2700cm -1 becomes asymmetric and can be fitted using two bands 4-6, centered around 2690 and 2730cm -1 respectively 7. At Malaspina, the D1 and D2 defects bands are weak and the 2700 cm -1 band is asymmetric in RZ which indicates that triperiodic graphite is present with some structural defects. On the other hand, the D1 and D2 bands are much more intense in PS and the main second order band is clearly symmetric and centered around 2700cm -1. This indicates that the graphitic CM in the PS is clearly disordered and turbostratic. The results of the fitting procedure are presented on the second order data (thick curve). The green dotted line is the result of the fitting procedure whereas the fitted 2690 and 2730cm -1 bands are in yellow and red respectively for the RZ. NATURE GEOSCIENCE 5

6 n samples d.t.c (m) R2 T (C) SD SD SE CO CO S4: RSCM temperature data obtained for the Malaspina unit. We provide here the number of spectra (n), mean R2 ratio for n spectra with corresponding standard deviation (SD, 1σ) and calculated temperature with standard deviation and standard error (SE). Standard error is the standard deviation divided by n. The absolute error on temperature is ±50 C. 6 NATURE GEOSCIENCE

7 Pressure (kbar) not realized on earth REDUCING CONDITIONS (reaction 1 at X O eq<1/3) arg cal XO=1/3 CaCO 3 + SiO 2 CaSiO Temperature ( C) S5: This PT diagram displays the calcite-aragonite polymorphic transition curve (brown curve) and the X O =1/3 (O/H=2, neutral fluid composition, see method) isopleths (black curve) of reaction (1). The calculations were made using PerpleX 8, and for sake of clarity, reaction (1) is a generalization of the process evidenced in this study, and conforms to the nomenclature introduced by Connolly 9 by making use of the absolute O atomic fraction of the equilibrium fluid with no bearing as to the speciation. The equilibrium thermodynamic model is justified on geological timescales. The blue domain of the PT diagram corresponds to reducing equilibrium fluids (X O <1/3) whereas the high temperature domain (white) corresponds to oxidizing equilibrium fluids (Xo>1/3) 9. The red area display the range of PT conditions at which reaction 1 might have occurred. In this area, the fluid would be very water rich (x(h 2 O)>0.95), with a very low C atomic fraction (nc/(nc+no+nh)<0.008 in this PT range) in the COH fluid. The carbon bearing species would be mainly composed of CH 4 (x(ch 4 )<0.01) and CO 2 species (x(co 2 )<0.008) with x(i) representing the molar fraction of species i in the COH fluid. NATURE GEOSCIENCE 7

8 Bibliography 1 Kreulen, R. & van Beek, P. C. J. M. The calcite-graphite isotope thermometer; data on graphite bearing marbles from Naxos, Greece. Geochim. Cosmochim. Acta 47, (1983). 2 Valley, J. W. Stable Isotope Thermometry at High Temperatures. Rev. Mineral. Geochem. 43, (2001). 3 Beyssac, O., Goffé, B., Chopin, C. & Rouzaud, J.-N. Raman spectra of carbonaceous material in metasediments: a new geothermometer. J. Metamorph. Geol. 20, (2002). 4 Beyssac, O., Rouzaud, J.-N., Goffé, B., Brunet, F. & Chopin, C. Graphitization in a highpressure, low-temperature metamorphic gradient: a Raman microspectroscopy and HRTEM study. Contrib. Mineral. Petrol. 143, (2002). 5 Lespade, P., Marchand, A., Couzi, M. & Cruege, F. Caracterisation de materiaux carbones par microspectrometrie Raman. Carbon 22, (1984). 6 Ferrari, A. C. et al. Raman Spectrum of Graphene and Graphene Layers. Phys. Rev. Lett. 97, (2006). 7 Cancado, L. G. et al. Measuring the degree of stacking order in graphite by Raman spectroscopy. Carbon 46, (2008). 8 Connolly, J. A. D. Multivariable phase diagrams; an algorithm based on generalized thermodynamics. Am. J. Sci. 290, (1990). 9 Connolly, J. Phase diagram methods for graphitic rocks and application to the system C O H FeO TiO2 SiO2. Contrib. Mineral. Petrol. 119, (1995). 8 NATURE GEOSCIENCE