GSA Data Repository 2016301

Boron isotopic discrimination for subduction-related serpentinites.

Céline Martin1,2, Kennet E. Flores1.3, George E. Harlow1

Appendix 1: Petrological description and P-T conditions

Petrological description

In situ B isotopes were measured in antigorite and lizardite/chrysotile from serpentinites, and in

mica, pyroxene, and amphibole from metabasites and vein-rocks. The precise locations of samples, as

well as the tectonic units they belong to, are presented in Table 1-1.

The serpentine species in serpentinite samples were identified by X-Ray diffraction on 1-inch

diameter polished sections (Rigaku DMAX/RAPID instrument, with JADE (MDI) software and the

International Center for Diffraction Data PDF-2 database for phase identification, AMNH). The major

element analyses of mica, pyroxene, amphibole, feldspar, and axinite-(Mg) were performed with a

Cameca-SX100 electron microprobe (AMNH) with an acceleration voltage of 15 kV, a beam current of

20 nA, a beam size of 1 µm, and a counting time of 20 s except for Na (10 s). Analytical standards were

well-characterized natural and synthetic minerals including diopside (Si, Mg, Ca), jade (Na, Al), K-

feldspar (K), rutile (Ti), olivine (Fe), rhodonite (Mn), chromite (Cr), and barite (Ba).

Serpentinite samples from both mélanges used in this study consist primarily of antigorite (Table

I-2; Fig. 1-1a) with talc, magnetite, ilmenite, chromite, FeNi-sulfides as accessory phases (Table 1-2).

Carbonates (dolomite, ankerite, and magnesite) were encountered mainly in the NMM samples (Fig. 1-

1b) and can be primary (they display intergrowths with antigorite crystals; Fig 1-2a) or secondary (as

vein, Fig. I-2c). Some samples from NMM have veins of secondary antigorite, tremolite (Fig. 1-2b), or

carbonate (Fig. 1-2c), whereas only secondary antigorite veins have been observed in SMM samples (Fig.

1-2d).

All serpentinite samples from the GSZ ophiolites consist primarily of lizardite/chrysotile (Fig. I-

3), and one contains relics of pyroxene and olivine (Table 1-2, Fig. 1-4b, 4b). Veins of chrysotile are

frequently encountered in the ophiolitic serpentinite samples (Fig. 1-4c, 4d). Accessory phases are

clinochlore and magnetite. In all serpentinite samples, the grain size is too small, or the antigorite needles

too narrow, to be analyzed individually under the laser spot. All elemental and isotopic B results therefore

represent a mixture between several antigorite or lizardite grains and chrysotile bundles, and possibly one

or more accessory phases.

The eclogite samples MVE02-15-11, MVE10-58-2, and MVE07B-19-11 are coarse-grained,

partly retrogressed to blueschist (omphacitic pyroxene transformed into glaucophane + albitic feldspar),

locally with an overprint of greenschist facies (chlorite, prehnite). Garnet is euhedral to sub-euhedral,

with abundant inclusions recording different events (magnesio-hornblende and glaucophane, omphacite,

epidote-group species, phengitic mica). Accessory phases are epidote, phengite (Table I-3), and rutile

replaced by titanite. MVE02-15-11 has chalcopyrite crystals and a vein of lawsonite. Grain sizes in

eclogite are large enough to allow analyses of individual crystals for B measurements. Other eclogites

used in this study have been described previously: MVE02-6-3, MVE02-14-6, and MVE04-44-6

(Brueckner et al., 2009, or Harlow et al., 2011).

The diopside-amphibolite sample MVE04-4-3 is a dark-green microcrystalline (grain size = 1-2

µm) rock consisting of ferro-actinolite/tremolite (Table I-3), diopsidic pyroxene, and feldspar with a wide

range of compositions. Both plagioclase and K-feldspar were observed, and the range of composition in

different plagioclase grains is great (Ab17 to Ab99, and An4 to An64) (Table 1-3). The accessory phases are

clinozoisite, titanite, and clinochlore, and the sample is cut by a vein of axinite-(Mg). MVE04-4-3 is too

fine-grained to allow individual analyses of the individual minerals by Laser. All elemental and isotopic B

results therefore represent a mix between diopside, ferro-actinolite, and feldspar. Grains in the axinite-

(Mg) vein are sufficiently large to be analyzed individually.

Jadeitite MVE02-14-7 is a fine- to medium-grained sample (range from 150 to 500 µm in size)

consisting of jadeitic pyroxene (Jd95Ac0Di5) and phengitic mica, chemically zoned (Table 1-3). This

sample was analyzed both by SIMS and LA-MC-ICPMS, and this combination of techniques allows

analyses of individual crystals for B isotopes. Other jadeitites used in this study have been described

previously: MVJ84-51-3 (Harlow, 1994), and JJE01-X-3 and MVE02-8-5 (Harlow et al., 2011).

MVE02-1-1 is coarse-grained mica-rock (crystals up to 1 mm) composed of phengitic mica and

pargasitic to katophoritic amphibole with minor albite, titanite, and zircon. MVJ84-56-3 is a coarse-

grained rock (crystals up to 1 mm) consisting of two micas (phengite and paragonite), albite (Ab98) and

zoisite (some Sr-rich), with minor clinochlore, K-feldspar and zircon. The grains in these two samples are

sufficiently large to permit individual analyses for B concentrations and isotopes. The following mica-

rocks have been described previously: MVJ84-3-2 and MVJ84-29-1 (Harlow, 1994), and MVJ84-52-1

(Harlow, 1995).

Table 1-1: Location of the samples 1

Sample Latitude (N) Longitude (W) Unit Locality Serpentinite from mélanges MVJ87-6-2 14°56'27.66" 89°53'6.36" NMM Estancia de La Virgen MVJ90-11-2 14°57'8.76" 89°52'36.6" NMM Río El Cintillo MVE04-9-1 14°46'12.78" 89°52'55.68" SMM Carrizal Grande- Quebrada del Mico MVE04-18-3 14°48'6.96" 89°46'40.8" SMM La Ensenada MVE06-7-4 14° 51' 7.620" 90° 26' 37.920" NMM Quebrada Los Pescaditos, Rincón Grande MVE12-74-1 14° 55' 33.600" 90° 3' 57.600" NMM Pasasagua Serpentinite from ophiolite MVE10-16-3 15° 18' 30.8340" 89° 7' 48.4399" JDP Quebrada La Pita MVE12-30-4 15° 11' 49.560" 90° 17' 44.623" BVP Quebrada Santo Tomas, La Cebadilla MVE12-38-1 15° 9' 39.497" 90° 17' 20.534" BVP Road to Salamá MVE12-39-1 15° 8' 51.322" 90° 17' 27.140" BVP Road to Salamá Eclogite blocks MVE02-6-3 14°46'36.06" 89°53'16.5" SMM Carrizal Grande-Quebrada El Silencio MVE02-14-6 14°46'33.36" 89°52'5.4" SMM Quebrada El Silencio MVE02-15-11 14°46'10.2" 89°52'26.88" SMM Quebrada Seca MVE04-44-6 14° 51' 7.620" 90° 26' 37.920" NMM Quebrada Los Pescaditos, Rincón Grande MVE07B-19-11 14° 53' 7.971" 90° 35' 41.360" NMM Quebrada Los Pozos, Saltán MVE10-58-2 14° 50' 27.160" 90° 22' 54.777" NMM Rio Los Platanos Diopside-amphibolite block MVE04-4-3 14° 47' 38.040" 89° 54' 32.640" SMM Cerro El Tobón Jadeitite blocks MVJ84-51-3 14°59'24.72" 89°47'14.64" NMM Quebrada El Escorpión JJE01-X-3 14°50'59.52" 89°48'53.1" SMM Río El Tambor MVE02-8-5 14°46'42.9" 89°53'15.72" SMM Quebrada El Silencio MVE02-14-7 14°46'33.36" 89°52'5.4" SMM Quebrada Ell Silencio Mica rock blocks MVJ84-3-2 14°56'31.44" 89°51'6.72" NMM Manzanal MVJ84-29-1 14°57'42.6" 89°45'42.18" NMM La Palmilla MVJ84-52-1 14°57'20.22" 89°46'33.54" NMM north of Usumatlán MVJ84-56-3 14°57'2.7" 89°48'5.58" NMM Cerro Gallinero, Huijó MVE02-1-1 14°55'14.16" 89°55'34.92" NMM km 93.5, Atlantic Highway (C.A.9) Abbreviations: Baja Verapaz Ophiolite (BVP); Juan de Paz Ophiolite (JDP); North Motagua Mélange (NMM); South Motagua Mélange (SMM) 2

3

Table 1-2: Serpentinite samples mineralogy 4

Sample atg ctl lz tlc clc ilm mag FeNiS chr ol opx cpx dol ank mgs cal vein Mélanges SMM MVE04-9-1 + + + + MVE04-18-3 + + + + + atg NMM MVJ87-6-2 + + + + MVJ90-11-2 + + + MVE06-7-4 + + + + + + + + ank ±atg ± clc MVE12-74-1 + + tr ± ctl Ophiolites Juan de Paz MVE10-16-3 + + + + ctl +tlc Baja Verapaz MVE12-30-4 + lz/chr MVE12-38-1 + + + + MVE12-39-1 + + + lz/ctl + mag Abbreviations are from Whitney and Evans, (2010) 5

Table 1-3: Representative major element analyses of metabasites and vein-rocks minerals analyzed for B isotopes 6

Mica Pyroxenerock type MR MR MR MR J J E E J J DA MVE02-

1-1 MVJ84-

56-3 MVJ84-

56-3 MVJ84-

56-3 MVE02-

14-7 MVE02

-14-7 MVE10-

58-2 MVE10

-58-2 MVJ84-

51-3 MVJ84-

51-3 MVE04-

4-3 ph pg ph (rim) ph (core) ph (rim) ph

(core) ph (rim) ph

(core) jd (rim) jd (core) di

SiO2 47.44 43.45 44.43 46.15 54.26 49.46 49.47 49.70 59.36 58.89 52.92 TiO2 0.51 0.11 0.04 0.04 0.27 0.18 0.19 0.13 0.05 0.04 0.01 Al2O3 29.65 36.78 31.34 29.81 23.17 28.37 27.88 27.82 23.13 23.09 0.14 Cr2O3 0.06 0.00 0.00 0.00 0.00 0.00 0.02 0.04 0.00 0.00 0.01 Fe2O3

‡ 1.83 0.00 0.00 0.00 0.00 0.00 0.00 0.00 1.23 0.40 0.29 FeO 0.00 0.12 0.62 0.92 2.09 2.43 2.80 1.88 0.00 0.94 8.04 MgO 2.73 0.14 1.75 2.82 4.04 2.90 3.27 3.59 0.78 0.88 13.6 MnO 0.05 0.00 0.01 0.03 0.00 0.06 0.05 0.00 0.00 0.01 0.33 CaO 0.03 0.14 0.04 0.01 0.02 0.02 0.01 0.00 1.18 1.36 24.86 BaO 1.75 0.22 3.32 1.48 1.97 3.75 0.39 0.70 0.00 0.00 0.00 Na2O 0.90 6.62 0.36 0.82 0.11 0.23 0.69 0.71 14.70 14.28 0.04 K2O 9.36 0.92 9.05 9.40 9.98 9.09 9.99 9.64 0.01 0.00 0.00 H2O* 4.41 4.35 4.22 4.29 4.50 4.45 4.44 4.44 0.00 0.00 0.00 Total 98.79 92.80 95.19 95.77 100.40 100.94 99.22 98.72 100.47 99.88 100.23

Amphibole Feldspar Other

rock type MR DA DA DA DA DA

MVE02-1-1

MVE04-4-3

MVE04-4-3

MVE04-4-3

MVE04-4-3

MVE04-4-3

prg Fe-act ab pl Kfs-pl Mg-Ax

SiO2 47.02 48.48 66.96 55.19 59.61 43.35

TiO2 0.41 0.09 0.05 0.00 0.00 0.00

Al2O3 12.72 3.29 21.13 27.66 22.93 18.26

Cr2O3 0.06 0.00 0.00 0.00 0.00 0.06

Fe2O3‡ 0.00 4.00 0.00 0.00 0.00 0.00

FeO 10.40 20.32 0.32 0.35 0.10 6.90

MgO 12.88 8.97 0.08 0.63 0.09 2.35

MnO 0.21 0.56 0.03 0.07 0.00 1.83

CaO 9.71 11.03 1.22 9.20 4.6 19.84

BaO 0.00 0.01 0.00 0.02 0.10 0.00

Na2O 3.28 0.34 10.65 5.93 1.87 0.02

K2O 0.46 0.16 0.14 0.10 9.62 0.01

H2O* 2.07 1.96 0.00 0.00 0.00 1.62

Total 99.22 99.23 100.62 99.34 99.01 92.64

MR = Mica rock, J = jadeitites, E = eclogite, and DA = Di-amphibolite. Abbreviations are from Whitney and Evans, (2010); ‡ Fe2O3 and H2O calculated from 7

stoichiometry as described by Harlow et al. (2011). 8

9

10

Fig. 1-1. a) X-ray diffraction pattern of mélange sample MVE04-9-1, showing antigorite only, b) X-ray 11

diffraction pattern of mélange sample MVE06-7-4, showing antigorite, talc, and carbonate. 12

13

Fig. 1-2. a) cross-polarized photomicrograph of mélange sample MVJ87-6-2 showing intergrowth of 14

dolomite (dol) and antigorite (atg), b) photograph of the 1-inch polished section of mélange sample 15

MVE12-74-1 showing tremolite (tr) vein, c) photograph of the 1-inch polished section of mélange sample 16

MVE06-7-4 showing dolomite (dol) vein and isolated grains, and d) photograph of the 1-inch polished 17

section of mélange sample MVE04-18-3 showing a vein of fibrous antigorite (atg) crossing antigorite 18

matrix. 19

20

21

Fig. 1-3. X-ray diffraction pattern of ophiolitic sample MVE12-39-1, showing a mixture of lizardite and 22

chrysotile, and magnetite, 23

24

25

Fig. 1-4 a) cross-polarized photomicrograph of ophiolitic sample MVE12-38-1 showing a network of 26

chrysotile (ctl) veins across primary olivine (ol), b) photograph of the 1-inch polished section of 27

ophiolitic sample MVE12-38-1 showing the partial transformation of the peridotite into lizardite (liz) and 28

chrysotile veins (ctl), with relics of primary olivine (ol), c) photograph of the 1-inch polished section of 29

ophiolitic sample MVE12-39-1 with bastite (ba), a serpentine pseudomorph after enstatite and chrysotile 30

veins (ctl), d) photograph of the 1-inch polished section of sample ophiolitic MVE12-30-1 showing 31

completely transformed peridotite into lizardite (liz) and chrysotile veins (ctl). 32

33

34

Pressure – Temperature (P-T) conditions 35

The P-T conditions of formation of serpentinites can only be broadly defined, as there are no 36

minerals other than the serpentine polymorphs to constrain temperature and none for pressure. The 37

serpentine species (lizardite and antigorite) give a broad indication of the temperature of formation 38

(antigorite crystallizes between 300 and ~ 600°C, and lizardite below 300°C; Fig. I-5). Chrysotile, as a 39

polytype of lizardite, is otherwise only an indicator of open space into which the tubules can grow, i.e., 40

veins and fractures. An accepted way to estimate the P-T conditions of serpentinite formation in mélanges 41

is from the estimates for jadeitites and mica-rock veins injected into the mantle wedge or fragments 42

thereof. The SMM records a cold subduction, with an eclogitic peak at ~ 480°C, ~ 2.6 GPa (Tsujimori et 43

al., 2006; Fig. I-5) or ~520ºC, ~2.5 GPa and –Endo et al., 2012, and jadeitites crystallizing in the range 44

250 - 425°C, 0.6 – 2 GPa (Harlow et al., 2015; Fig. I-5). The NMM is warmer with an eclogitic peak at ~ 45

500 - 600 °C, < 2 GPa (Flores et al., 2013; Harlow et al., 2011; Fig. I-5), and jadeitites crystallizing in the 46

range 350 - 500°C, 0.7 – 1.4 GPa (Harlow et al., 2015; Fig. I-5). The P-T conditions of mantle wedge 47

serpentinites are therefore expected to be in the same range as the jadeitites (Harlow et al., 2015 and 48

references therein). 49

50

51

52

Fig. 1-5. P-T diagram showing the conditions encountered for jadeitites and eclogite (Ec) in the SMM (in 53

red; LE = La Ensenada, CG = Carrizal Grande) and the NMM (in purple, dotted line), modified from 54

Harlow et al., (2015). The P-T conditions of eclogites are from Tsujimori et al., (2006) and Endo et al., 55

(2012) for SMM, and from Harlow et al., (2011) and Flores et al., (2013) for NMM. The stability field of 56

antigorite is from Evans et al., (2012), and Scambelluri et al., (2012), and the mineral abbreviations from 57

Whitney and Evans, (2010). Lw-Ec = lawsonite eclogite, Ep-Ec = epidote eclogite, Amp-Ec = amphibole 58

eclogite, HGR = high granulite, GR = granulite, AM = amphibolite, GS = greenschist, BS = blueschist. 59

The dotted lines are geotherms. 60

61

Appendix 2: Boron element and isotopes results 62

63

In situ B contents were analyzed by LA-ICP-MS (ESI New Wave UP-193-FX ArF* (193 nm) 64

excimer laser coupled to a VG PQ ExCell (ThermoScientific) ICP-MS (Lamont Doherty Earth 65

Observatory, Columbia University)). The following isotopes were monitored: 10B, 11B, 27Al, and 29Si. 66

Analyses were performed using traverses (~ 300 µm long, with a beam diameter varying between 40 and 67

150 µm, depending upon the material analyzed) at a repetition rate of 10 Hz and a monitored energy 68

density of 10 J/cm2 on the sample surface. Using 29Si as an internal standard, quantification was 69

performed via external calibration using several glass reference materials (U.S. Geological Survey natural 70

glasses BHVO, BIR and BCR). Based on the standards and settings described, external reproducibility for 71

the elements measured was typically ca. 20% relative standard deviation. 72

Boron isotopes were measured in situ by LA-MC-ICP-MS using an ESI New Wave UP-193-FX 73

ArF* (193 nm) excimer laser coupled to a Neptune Plus (ThermoScientific) MC-ICP-MS (Lamont 74

Doherty Earth Observatory, Columbia University) for B concentrations above 5 ppm (Martin et al., 2015). 75

The measurements were performed with a repetition rate of 10 Hz and a fluence of ~10 J/cm2 on the 76

sample surface, along traverses of ~ 300 µm in length. The spot diameter varied from 10 to 175 µm, 77

depending on the mineral analyzed and whether the collecting mode was Faraday cup or electron 78

multiplier. For B concentrations below 5 ppm, in situ B isotope measurements were performed by SIMS 79

at the North-Eastern National Ion Microprobe Facility (NENIMF) at the Woods Hole Oceanographic 80

Institution following the method of Marschall and Monteleone, (2015), using a Cameca IMS1280 ion 81

microprobe. For the present analyses, the beam current was 30nA, and a beam size of 75 µm, with pre-82

sputtering at 100 µm, was used to perform analyses on pyroxene, serpentine, and amphibole, whereas a 83

beam size of 30 µm, with pre-sputtering at 50 µm, was used for mica. 84

The B isotopes results are presented here using δ notation (see formula below), in which 85

11B/10Bsample is the isotopic ratio measured on the sample, and 11B/10Bstandard the isotopic ratio of the 86

reference material SRM 951 (Catanzaro, 1970), U.S. National Institute of Standards and Technology, 87

NIST). The standards used to monitor the B isotopes measurements by LA-MC-ICP-MS are NIST SRM 88

610 and NIST SRM 612. They yield δ11B of –0.3 ± 2.8 ‰ (n = 100) and of +0.7 ± 3.3 ‰ (n = 262) 89

respectively, using LA-MC-ICP-MS method (Martin et al., 2015). The standards used to monitor The B 90

isotopes measurements by SIMS are GOR132-G and B6. They yield δ11B of +7.7 ± 4.6 ‰ (n = 50) and of 91

–2.5 ± 2.8 ‰ (n = 39) respectively, using SIMS method (Marschall and Monteleone, 2015). 92

93

1 1000

94

As the B contents and the B isotopes cannot be analyzed simultaneously from a single spot (the 95

Laser Ablation Split Stream method, developed by Kylander-Clark et al., (2014) has not yet been 96

developed for light isotopes), we analyzed B contents and isotopes along parallel traverses, during two 97

distinct analytical sessions. However, most of the metamorphic minerals are chemically zoned (Harlow et 98

al., 1994; Table 1-3), and it is almost impossible to link a given B content to a given B isotopic 99

composition. Particularly, the vein rocks display chemically zoned minerals (Harlow, 1994; Table 1-3), 100

and most of them have a great range of B contents coupled with a scattering of δ11B (Table 2-1, Fig. 2c, 101

2d of the manuscript). Where B contents are uniform (e.g., MVE04-4-3), δ11B values are as well (Table 2-102

1, Fig. 2d of the manuscript). Thus, scattered δ11B values may reflect multiple fluid influxes, with the 103

positive values possibly reflecting an influx of fluid(s) from different source(s), like different depths in 104

the subduction channel, or a mixture of fluids. In ophiolite samples, the isotopic and concentration ranges 105

within each sample are narrow (Table 2-1, Fig. 2b of the manuscript), indicating a characteristic fluid 106

responsible for the hydration of the ultramafic protolith, whereas mélange serpentinites are more scattered 107

(Table 2-1, Fig. 2c, 2d of the manuscript), likely reflecting influxes of fluid from different sources or 108

different depths as well. Carbonate-bearing mélange serpentinites (MVJ87-6-2 and MVE06-7-4) also 109

show a significant scattering of δ11B (Table 2-1; Fig. 2c of the manuscript), likely because B isotopic 110

fractionation between carbonate and serpentine is large (Hemming and Hanson, 1992; Wunder et al., 111

2005), leading to significant variations for in situ analyzes, depending on the proportions of serpentine 112

and carbonate analyzed at each point. Thus, we decided to work with the averages of both contents and 113

isotopes for the Fig. 3 of the manuscript, as we cannot link a given concentration to a given isotopic 114

composition with certainty. The vein values are not represented on Fig. 3 of the manuscript, as they 115

represent secondary, possibly disconnected event(s) relative to the subduction. 116

Table 2-1: Boron contents and isotope compositions of GSZ samples 117

method [B] ppm δ11B (‰) location Fig. 2Serpentinites Ophiolites MVE10-16-3 matrix Laser 2.4 ± 0.8 (n=4) +8.2 ± 1.8 (min) Juan de Paz (1) +10.2 ± 1.3 (max) +10 ± 2.5 (av; n=4) MVE10-16-3 vein Laser 2.9 ± 0.3 (n=2) n.d. MVE12-30-4 matrix Laser 15.6 ± 0.7 (n=6) +3.9 ± 2.3 (min) Baja Verapaz (2) +6.2 ±3.0 (max) +5.5 ± 1.8 (av; n=5) MVE12-38-1 ol/px Laser 2.1 ± 1.0 (n=4) +10.6 ± 1.4 (min) Baja Verapaz (4) +10.9 ± 1.4 (max) +10.8 ± 0.5 (av; n=2) MVE12-38-1 vein Laser 7.5 ± 0.9 (n=3) +13.2 ± 1.4 (min) (3) +18.0 ± 2.8 (max) +15.1 ± 5.2 (av; n=3) MVE12-39-1 matrix Laser 1.2 ± 0.4 (n=4) +0.8 ± 1.9 (min) Baja Verapaz (5) +1.8 ± 1.8 (max) +1.5 ± 1.1 (av; n=3) MVE12-39-1 vein Laser 0.7 ± (n=2) n.d. Mélanges MVJ87-6-2 matrix Laser 7.1 ± 0.9 (n=8) – 0.7 ± 1.6 (min) NMM (6) +9.7 ± 1.4 (max) +4.5 ± 9.7 (av; n=7) MVJ87-6-2 matrix SIMS –10.1 ± 1.1 (min) NMM (6) –5.6 ± 0.9 (max) –8.2 ± 2.8 (av; n=7) MVJ87-6-2 matrix all –1.8 ± 14.9 (av; n=14) MVJ90-11-2 matrix Laser 2.3 ± 1.6 (n=6) –12.8 ± 1.6 (min) NMM (7) –5.5 ± 1.3 (max) –8.5 ± 5.5 (av; n=6) MVE12-74-1 matrix Laser 19.1 ± 11.2 (n=5) –14.4 ± 1.9 (min) NMM (9) +1.5 ± 1.9 (max) –5.1 ± 9.1 (av; n=8) MVE12-74-1 tremolite vein Laser 8.6 ± 4.1 (n=3) –13.8 ± 3.9 (min) (8) –8.7 ± 2.2 (max) –11.5 ± 4.9 (av; n=5) MVE06-7-4 matrix Laser 3.2 ± 1.2 (n=4) –8.2 ± 2.5 (min) NMM (10) +0.7 ± 1.8 (max) –2.7 ± 6.5 (av; n=6) MVE06-7-4 matrix SIMS –12.3 ± 2.2 (min) NMM (10) –11.2 ± 1.9 (max) –11.9 ± 1.2 (av; n=3) MVE06-7-4 matrix all –5.8 ± 10.6 (av; n=9) MVE04-18-3 matrix Laser 10.8 ± 1.6 (n=6) –4.4 ± 1.8 (min) SMM (13) +4.9 ±1.9 (max) 0.8 ± 7.0 (av; n=6) MVE04-18-3 matrix SIMS –11.7 ± 1.2 (min) SMM (13)

–2.5 ±1.1 (max) –6.9± 8.0 (av; n=5) MVE04-18-3 matrix all –3.1± 10.7 (av; n=11) MVE04-18-3 antigorite vein Laser 21.4 ± 20.0 (n=7) –11.0 ± 1.4 (min) (12) +3.4 ± 2.1 (max) –3.3± 8.9 (av; n=9) MVE04-18-3 antigorite vein SIMS –12.5 ± 0.9 (min) (12) –1.6 ± 1.0 (max) –5.7± 8.0 (av; n=8) MVE04-18-3 antigorite vein all –3.1 ± 10.7 (av; n=17) MVE04-9-1 Laser 8.2 ± 0.7 (n=11) –7.2 ± 1.1 (min) SMM (11) –3.6 ± 2.0 (max) –5.3 ± 2.7 (av; n=10) Metabasites / veins Eclogites MVE07B-19-11 mica Laser 4.5 – 22 (n=14) –6.4 ± 1.1 (min) NMM (28) +0.8 ± 0.6 (max) –3.0 ± 4.2 (av; n=12) MVE10-58-2 mica Laser <0.7 – 30 (n=8) –8.9 ± 0.8 (min) NMM (29) –1.5 ± 0.8 (max) –5.7 ± 6.2 (av; n=3) MVE04-44-6 mica Laser n.d. –5.7 ± 1.5 (min) NMM (30) +0.4 ± 1.4 (max) –3.0 ± 4.4 (av; n=4) MVE02-15-11 mica Laser 60 – 165 (n=14) –12.7 ± 1.4 (min) SMM (31) –6.4 ± 1.2 (max) –9.4 ± 4.6 (av; n=7) MVE02-14-6 mica Laser 30 – 62 (n=14) –7.5 ± 2.2 (min) SMM (32) –4.7 ± 1.2 (max) –5.8 ± 1.7 (av; n=6) MVE02-6-3 mica Laser 50 – 75 (n=4) –7.2 ± 0.9 (min) SMM (33) +0.5 ± 2.5 (max) –3.6 ± 5.3 (av; n=7) MVE02-6-3 mica SIMS –4.9 ± 2.0 (min) SMM (33) –3.7 ± 2.0 (max) –4.2 ± 1.2 (av; n=5) Diopside-amphibolite

MVE04-4-3 amp+di+fspar Laser 15.8 ± 6.7 (n=4) –15.3 ± 1.2 (min) SMM (14) –13.2 ± 2.2 (max) –13.8 ± 1.7 (av; n=5) MVE04-4-1B ax Laser 12,360 ± 71(n=3) –17.6 ± 1.2 (min) SMM (15) –10.1 ± 1.2 (max) –12.3 ± 5.1 (av; n=7) Jadeitites MVJ84-51-3 mica Laser 17 – 50 (n=) –7.3 ± 1.1 (min) NMM (23) –0.5 ± 1.2 (max) –3.4 ± 4.4 (av; n=6) MVJ84-51-3 cpx Laser 1 – 56 (n=16) –5.5 ± 1.3 (min) (22)

–0.7 ± 1.2 (max) –3.0 ± 4.1 (av; n=4) MVE02-8-5 mica Laser 45 – 150 (n=15) –6.7 ± 1.0 (min) SMM (24) +1.5 ± 3.3 (max) –2.6 ± 5.4 (av; n=10) MVE02-8-5 mica SIMS –4.9 ± 2.0 (min) SMM (24) –3.7 ± 2.0 (max) –2.2 ± 2.2 (av; n=5) MVE02-14-7 mica Laser 20 – 120 (n=18) –7.1 ± 1.2 (min) SMM (25) +4.3 ± 1.3 (max) –0.3 ± 6.6 (av; n=16) JJE01-X-3 mica Laser 90 – 165 (n=8) –7.2 ± 1.1 (min) SMM (27) –4.8 ± 1.2 (max) –6.3 ± 1.8 (av; n=8) JJE01-X-3 mica SIMS –8.3 ± 1.2 (min) SMM (27) –5.5 ± 1.2 (max) –6.7 ± 2.4 (av; n=4) JJE01-X-3 cpx Laser 1.5 – 6.5 (n=6) –6.8 ± 1.3 (min) (26) –3.9 ± 1.3 (max) –4.1 ± 4.1 (av; n=6) JJE01-X-3 cpx SIMS –4.7 ± 2.2 (min) (26) –0.6 ± 2.5 (max) –2.5 ± 3.7 (av; n=4) Mica-rocks MVJ84-52-1 mica Laser 40 – 135 (n=10) –6.8 ± 1.0 (min) NMM (18) –6.4 ± 1.1 (max) –6.6 ± 0.4 (av; n=3) MVJ84-56-3 mica Laser 60 – 115 (n=7) –9.0 ± 1.6 (min) NMM (19) –5.3 ± 1.3 (max) –6.5 ± 8.7 (av; n=4) MVJ84-3-2 mica Laser 80 – 125 (n=6) –14.5 ± 1.4 (min) NMM (20) –7.3 ± 1.3 (max) –10.4 ± 4.3 (av; n=7) MVJ84-29-1 mica Laser 29.5 ± 4.7 (n=12) –9.9 ± 1.8 (min) NMM (21) –7.0 ± 1.7 (max) –8.2 ± 2.4 (av; n=4) MVJ84-29-1 mica SIMS –9.2 ± 1.3 (min) NMM (21) –8.8 ± 1.4 (max) –9.1 ± 0.3 (av; n=4) MVE02-1-1 mica Laser 14 – 70 (n=8) –5.0 ± 1.2 (av; n=1) MVE02-1-1 mica SIMS –8.9 ± 1.4 (min) NMM (17) –6.3 ± 2.1 (max) –7.8 ± 2.2 (av; n=4) MVE02-1-1 amphibole SIMS 1 – 6 (n=4) –8.0 ± 1.7 (min) (16) –5.0 ± 1.6 (max) –6.1 ± 2.8 (av; n=4)

All the data presented here were acquired in situ, by LA-ICP-MS for the concentrations (detection limit is 118

0.5 – 0.7 ppm for all minerals), and by LA-MC-ICP-MS (Laser) or SIMS for the isotopes. In the 119

serpentinite section, matrix means the laser traverses were performed in the fine-grained massive 120

serpentine (antigorite or lizardite/chrysotile ± accessory phases), whereas vein indicates the traverses were 121

performed in individual, well-defined veins. In the metabasites/veins section, the name of the mineral 122

next to the name of the sample indicates the mineral in which the analyses were performed (amp = 123

amphibole, cpx = jadeitic clinopyroxene, ax = axinite-(Mg)). 124

(n=x) indicates the number n of measurements. 125

n.d. is for not determined 126

(min), (max), and (av) indicate respectively the minimum, the maximum and the average isotopic values 127

measured for each sample; details of all measurements are presented on Fig. 2 of the manuscript. The 128

uncertainties on averages are 1 S.D. for concentrations, and 2 S.D. for isotopic values. 129

The last column refers to the sample presented on Fig. 2 of the manuscript. The values underlined in the 130

column δ11B (‰) are the averages used for Fig.3 of the manuscript, together with the averages of contents 131

available in the column [B] ppm. 132

133

Appendix 3: Boron isotope fractionation 134

135

Boron isotopes are known to fractionate easily because of the differences in coordination between 136

the two isotopes: 10B is preferentially tetrahedral (B(OH)4-), and consequently mainly partitioned into the 137

solids, whereas 11B is preferentially trigonal (B(OH)3) and mainly partitioned into fluids (Peacock and 138

Hervig, 1999). Several factors impacts the fractionation, including temperature (Wunder et al., 2005), 139

mineralogy (Marschall et al., 2007; Bebout et al., 2013), or pH (Wunder et al., 2005; Dehyle and Kopf, 140

2005). Boron mainly enters phyllosilicates (micas, clays), and two sets of experiments interpret similar 141

partitioning of B between phyllosilicates (or silicate melt) and neutral aqueous fluid for temperatures 142

ranging from 25 to 1000°C (i) (Wunder et al., 2005, and reference therein). 143

(i) ∆ 10.69 ∗ 3.88, with T in K 144

Wunder et al., (2005) also explored the impact of pH on B partitioning by using a KOH solution to create 145

a basic fluid (although the pH was not measured), and a two-point relation can be inferred from this 146

experiment (ii). 147

(ii) ∆ 13.68 ∗ 12.85, with T in K 148

The δ11B of metamorphic rocks within the slab become increasingly negative with increasing 149

depth (e.g., Bebout and Nakamura, 2003, Marchall et al., 2007, and references therein). Using the two 150

extreme values measured for subduction-related metamorphic rocks (δ11B = –18 ‰ (Halama et al., 2014) 151

and δ11B = +3 ‰ (Xiao et al., 2011), we calculated the δ11B of the fluid released from rocks with equation 152

(i) and (ii), i.e., a neutral and basic fluid, at temperatures ranging between 250 and 600°C (Fig. 3-1). 153

According to these calculations, the δ11B of fluids produced by devolatilization of a slab are expected to 154

decrease with increasing temperature. The fluids released from metamorphic rock with a δ11B of –18 ‰ 155

range from –1.4 to –9.6 ‰ for neutral fluid and from –4.7 to –15.2 ‰ for basic fluid. The fluids released 156

from metamorphic rock with a δ11B of +3 ‰ range from +19.6 to +11.4 ‰ for neutral fluid and from 157

+16.3 to +5.8 ‰ for basic fluid. As pH in serpentinite environment is expected to be basic (e.g., 158

Scambelluri and Tonarini, 2012), the case of an acidic fluid will not be considered here. Another 159

calculation based on B partition coefficients for metamorphic minerals using Perple_X also predicted the 160

decrease of δ11B of the fluid with progressive dehydration (i.e., when the pressure increases) (Marschall et 161

al., 2007). According to this model, the δ11B of fluid released during subduction ranges from +14 to –1 ‰ 162

for pressures between 1.65 and 2.65 GPa in the presence of phengite, and from +6 to –28 ‰ for pressures 163

between 1.80 and 2.65 GPa in the absence of phengite. Thus, both experimentally constrained 164

calculations and the model predict similar results, i.e., the decreasing of δ11B of the fluid released from 165

the slab when the depth increases. Moreover, the values obtained for δ11B from experimentally-derived 166

calculations and modeling overlap. 167

The δ11B values measured in this study of jadeitites and mica-rocks, which are interpreted to 168

crystallize directly from fluid, are also reported in Fig. 3-1, using the temperatures presented in Appendix 169

1. The values range from –14.5 to +4.3 ‰ overall and are in good agreement with those predicted by the 170

model for fluid(s) released from metamorphic rocks. The δ11B of serpentinite from the mantle wedge 171

(SMM and NMM) – also expected to crystallize by reaction with these fluids - reported as well in Fig. 3-1 172

with the values measured in the present study (δ11B = –14.4 to +9.7 ‰) overlap with the predicted values 173

for such fluids. 174

The δ11B of serpentinites in equilibrium with seawater (δ11B = +40 ‰, Spivack and Edmond, 175

1987) were also calculated with equation (i) and (ii) for temperatures ranging from 0 to 250°C and 176

reported in Fig. 3-1, to predict the isotopic signature of serpentinized abyssal peridotites. These 177

serpentinites would crystallize with a δ11B ranging from +4.7 to +23.4 ‰ for a neutral fluid and from +2.7 178

to +26.7 ‰ for a basic fluid. 179

The values measured in the present study of ophiolite serpentine were also reported in Fig. 3-1, taking the 180

stability field of lizardite as the limit for temperature. They range from +0.8 to +18.0 ‰, and are in 181

agreement with the δ11B predicted by the models. 182

183

184

185

186

Fig. 3-1: δ11B -T diagram showing the values modeled for serpentinites in contact with seawater (in 187

black) and fluid released from metamorphic rocks within the slab (in white) using equations (i) and (ii) 188

from Wunder et al., (2005). The range of δ11B in vein-rocks and serpentinites measured in the present 189

study are also shown on the diagram. 190

191

Thus, although B isotopic fractionation depends on several factors, some trends, like the decrease 192

of the δ11B of the fluid released during progressive dehydration (i.e., increase of P and T) of the slab, are 193

constant. The values obtained by modeling the fluid released from a rock with or without phengite, and 194

the calculations made for a neutral or a basic fluid, are in the same range and trend toward negative 195

values. In contrast, calculations for ultramafic rocks serpentinized by seawater show an increase of the 196

δ11B with temperature. Thus, tectonic settings must be considered as factors in producing B isotope 197

fractionation, and it makes sense to discriminate the tectonic origin of serpentinites in subduction 198

environment based on B isotopic signature. 199

200

Appendix: Bibliography 201

202

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temperature mélange along the Motagua fault, Guatemala: a record of Neocomian and 207

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