Evaluating mechanisms for eclogitic diamond growth: An example from Zimmi Neoproterozoic diamonds (West African craton)

Chemical Geology 520 (2019) 21–32

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Evaluating mechanisms for eclogitic diamond growth: An example from Zimmi Neoproterozoic diamonds (West African craton)

T

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Karen V. Smita, , Thomas Stachelb, Robert W. Luthb, Richard A. Sternb a b

Gemological Institute of America (GIA), 50 west 47th Street, New York City, NY 10036, USA Canadian Centre for Isotopic Microanalysis, Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta T6G 2E3, Canada

A R T I C LE I N FO

A B S T R A C T

Editor: Balz Kamber

Here we present SIMS data for a suite of Zimmi sulphide-bearing diamonds that allow us to evaluate the origin and redox-controlled speciation of diamond-forming fluids for these Neoproterozoic eclogitic diamonds. Low δ13C values below −15‰ in three diamonds result from fluids that originated as carbon in the oceanic crust, and was recycled into the diamond-stable subcratonic lithospheric mantle beneath Zimmi during subduction. δ13C values between −6.7 and −8.3‰ in two diamonds are within the range for mantle-derived carbon and could reflect input from mantle fluids, serpentinised peridotite, or homogenised abiogenic and/or biogenic carbon (low δ13C values) and carbonates (high δ13C values) in the oceanic crust. Diamond formation processes in eclogitic assemblages are not well constrained and could occur through redox exchange reactions with the host rock, cooling/depressurisation of CHO fluids or during H2O-loss from CHO fluids. In one Zimmi diamond studied here, a core to rim trend of decreasing δ13C (−23.4 to −24.5‰) and decreasing [N] is indicative of formation from reduced CH4-bearing fluids. Unlike mixed CH4-CO2 fluids near the water maximum, isochemical diamond precipitation from such reduced CHO fluids will only occur during depressurisation (ascent) and should not produce coherent fractionation trends in single diamonds that reside at constant depth (pressure). Furthermore, due to a low relative proportion of the total carbon in the fluid being precipitated, measurable carbon isotopic variations in diamond are not predicted in this model and therefore cannot be reconciled with the 1‰ internal core-to- rim variation. Consequently, this Zimmi eclogitic diamond showing a coherent trend in δ13C and [N] likely formed through oxidation of methane by the host eclogite, although the mineralogical evidence for this process is currently lacking.

Keywords: Reduced CHO fluids Eclogitic diamond δ13C Rayleigh fractionation Zimmi West African craton

1. Introduction Traditional models for diamond formation invoke either reduction of carbonate or oxidation of methane during fluid influx and redox exchange with the lithosphere (e.g., Haggerty, 1986; Luth, 1993). Since the diamond-stable subcratonic lithospheric mantle is generally reduced — with fO2 between FMQ-2 and -4 (Stagno et al., 2013) — diamond formation was traditionally thought to occur through reduction of carbonates (e.g., EMOD - Mg2Si2O6 + MgCO3 → Mg2SiO4 + C + O2; Rosenhauer et al., 1977; Eggler and Baker, 1982) already present in the rock or introduced via fluids/melts. Methane oxidation to precipitate diamonds (CH4 + O2 → C + H2O; Deines, 1980; Taylor and Green, 1988) assumes the reverse relation, a reduced ascending fluid interacting with more oxidised lithospheric mantle. This process is thought to be particularly efficient in portions of the lithosphere that have experienced oxidation during previous melt

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metasomatism (e.g., lherzolitic diamond substrates at the Cullinan mine; Thomassot et al., 2007). Redox models for diamond formation are now known to not be applicable in the depleted peridotitic lithosphere, due to the limited ability of such Fe-depleted substrates to oxidise or reduce passing CHO fluids (Luth and Stachel, 2014; Stachel and Luth, 2015). Instead, cratonic peridotite records the oxygen fugacity of the last infiltrating fluid. At oxygen fugacities in the diamond-stable cratonic lithosphere (at 5 GPa, fO2 is approximately FMQ-2 ± 1; Stachel et al., 2017), CHO fluids are predominantly H2O, with variable CH4, subordinate CO2 and/ or C2H6, and negligible H2 and CO. So how do diamonds form in peridotite if traditional redox exchange processes do not work? Luth and Stachel (2014) showed that during isochemical cooling of CHO fluids, carbon becomes oversaturated in the fluid and precipitates during redox-neutral reactions. For CHO fluids near the H2O-maximum that contain small amounts of

Corresponding author. E-mail address: [email protected] (K.V. Smit).

https://doi.org/10.1016/j.chemgeo.2019.04.014 Received 7 December 2018; Received in revised form 8 April 2019; Accepted 13 April 2019 Available online 19 April 2019 0009-2541/ © 2019 Elsevier B.V. All rights reserved.

Chemical Geology 520 (2019) 21–32

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isochron age of 3.4 ± 0.8 Ga (Barth et al., 2002) and Sr isotopic compositions suggesting they are 2.9 Ga (Aulbach et al., 2019). Lower crustal xenoliths from Koidu also record this crustal growth event at 2.9 Ga, along with later metamorphism at 2.1 Ga (Barth et al., 2002). Diamonds at Koidu are only associated with these low-Mg eclogites, and not with any of the high-Mg eclogites and pyroxenites (Hills and Haggerty, 1989). During the Neoproterozoic, subduction and collisional processes occurred along the south- western margin of the Man shield (700–550 Ma Rokelide orogen; Lytwyn et al., 2006). Zimmi eclogitic diamonds have 650 Ma ReeOs crystallisation ages (Smit et al., 2016b), indicating that diamond formation was coeval with the Rokelide orogeny and the associated release of subduction fluids into the West African lithosphere.

both CH4 and CO2, around half of the carbon initially present in the fluid will precipitate during cooling by about 200 °C. Evidence for diamond formation from such H2O-maximum CHO fluids is recorded in Marange diamonds from eastern Zimbabwe (Smit et al., 2016a). Coreto-rim δ13C-[N] trends produced during isochemical cooling of such H2O-maximum fluids are similar to those produced in diamonds precipitating from carbonate-rich fluids and melts (Deines, 1980; Smit et al., 2016a; Stachel et al., 2017). This suggests that the predominance of so-called ‘carbonate trends' in worldwide diamond populations, may actually indicate widespread peridotitic diamond formation during cooling of H2O-maximum CHO fluids. Rocks that are more Fe-rich than depleted cratonic peridotite may, however, have a greater capacity for redox buffering (Luth and Stachel, 2014), implying that diamond formation through redox exchange could still be a valid diamond formation process in Fe-rich fertile peridotites and eclogites. To evaluate eclogitic diamond formation processes, we analysed five gem-quality diamonds from the Zimmi alluvial locality in Sierra Leone (West African craton) by MC-SIMS for their carbon isotopes and nitrogen concentration. Zimmi sulphide-bearing diamonds have Neoproterozoic ReeOs ages (650 Ma; Smit et al., 2016b). This isotopic age overlaps with the timing of continental assembly and subduction along the margin of the Man shield (Lytwyn et al., 2006), suggesting that diamond fluids originated from Neoproterozoic subducting oceanic crust. With our analyses of subtle core-to-rim changes in carbon isotopic compositions and nitrogen concentration of Zimmi diamonds, the aim was to evaluate redox speciation of fluids released during Neoproterozoic subduction, and to assess the applicability of new models of diamond growth involving isochemical cooling of CHO fluids to eclogitic lithologies in the lithospheric mantle.

2.2. Eclogitic origin of Zimmi diamonds There is abundant evidence that the five diamonds analysed here for their δ13C and N concentration (written here as [N]) are from the same diamond population for which results were previously reported, including age constraints (Smit et al., 2016b), spectroscopic characteristics (Smit et al., 2018) and sulphur isotopic compositions of sulphide inclusions (Smit et al., 2019). All 25 samples in our full suite of Zimmi diamonds have identical morphological, compositional and spectroscopic characteristics: 1) all are yellow and have secondary resorbed morphologies (and are not primary octahedral morphologies) (Smit et al., 2016b, 2018); 2) all have similar nitrogen aggregation characteristics i.e., > 60% of nitrogen in Zimmi diamonds occurs as extremely rare isolated substitutional nitrogen (NS) that is preserved in < 0.1% of cratonic diamonds and under very specific conditions (Smit et al., 2016b, 2018); 3) all have abundant defomation-related morphological and spectroscopic characteristics that were interpreted to be due to rapid tectonic exhumation after their formation, and explains the preservation of isolated nitrogen (Smit et al., 2016b, 2018); 4) four garnet inclusions have been observed in four Zimmi diamonds, and from their orange colours are inferred to be eclogitic; and 5) nine diamonds with 16 analysed sulphide inclusions have Ni-poor pyrrhotite-rich compositions typical of eclogitic assemblages (Smit et al., 2016b, 2019), where 10 sulphides in three of these diamonds additionally all have low Re/Os and radiogenic 187Os/188Os (Smit et al., 2016b).

2. Background 2.1. Regional Geology The southern part of the West African craton is comprised of the Archaean Man shield and the Proterozoic Baoule Mossi terrane (Fig. 1; Boher et al., 1992; Barth et al., 2002; Baratoux et al., 2011). Zimmi alluvial diamonds are from the south-eastern Man shield, near the Liberia- Sierra Leone border, and are known for their rare vivid yellow colours and preservation of isolated nitrogen defects (Shigley and Breeding, 2013; Smit et al., 2016b, 2018). The Man shield has a Palaeoarchaean heritage (Thieblemont et al., 2001; Barth et al., 2002) though crustal growth and craton stabilisation occurred during the Mesoarchaean (Barth et al., 2002). The oldest rocks in the Man shield are the 3.5 Ga Guelamata gneisses (Thieblemont et al., 2001) and slightly older 3.6 Ga zircons are preserved in Koidu lower crustal xenoliths (Barth et al., 2002). Eclogites were emplaced into the cratonic lithosphere during the Archaean: low-Mg eclogite xenoliths from Koidu have a ReeOs

Together these characteristics indicate that all the Zimmi diamonds in our suite are eclogitic and share a tightly constrained and unique geological history. Fig. 1. The southern part of the West African craton is divided into the Archaean Man shield and Proterozoic Baoule Mossi terrane. Along the southwestern margin of the Man shield, the Rokelide orogen (RO on map) records subduction and continental assembly associated with the creation of Gondwana (700 to 550 Ma; Lytwyn et al., 2006). Sulphide-bearing Zimmi diamonds (red star) have 650 Ma ReeOs ages suggesting that diamond formation was closely associated with fluids infiltrating the lithosphere from Neoproterozoic subducting slabs. Selected kimberlite and diamond localities are indicated with white stars (locations from Stachel and Harris, 1997; Skinner et al., 2004). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 22

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3. Analytical Techniques

Table 1 MC-SIMS δ13C-[N] data for Zimmi diamonds.

3.1. Plate preparation and imaging of internal growth zones Polished plates were prepared at GIA using a Titanium laser system from OGI Systems Ltd., followed by conventional polishing on a diamond-embedded wheel. Diamonds were cut along a {110} crystallographic plane of each diamond to ensure maximum visibility of any internal growth features (Bulanova et al., 2005). All diamonds contained multiple sulphide inclusions and care was taken to preserve these inclusions within each plate for later analyses. Greyscale panchromatic cathodoluminescence (CL) images of the diamond growth zones, relative to SIMS analytical spots, were obtained at GIA using a Zeiss EVO MA10 scanning electron microscope (SEM) operating in variable pressure mode (with the chamber pressure typically at 20 Pa). Images were collected with a variable pressure secondary electron (VPSE) detector at zero bias, using 15–20 kV accelerating voltage and specimen probe currents between 1 and 20 nA. For this study, CL images were only used to guide SIMS analyses and interpret any observed δ13C and [N] trends. Detailed discussion on the observed CL features, along with other spectroscopic characteristics and their relationship to deformation, can be found in Smit et al. (2018). 3.2. Secondary ion mass spectrometry (SIMS) Five diamond plates from Zimmi were investigated for their carbon isotopic compositions and nitrogen concentration using a Cameca IMS1280 ion microprobe at the University of Alberta. Diamond plates were mounted in epoxy along with reference materials, and coated with 30 nm Au prior to SIMS analyses. Carbon isotopic ratios A 133Cs+ primary beam, with 1.9 to 2.0 nA current was used to measure 13C−/12C– ratios of 179 spots. Sample locations were pre-rastered (20 μm) for 30 s to remove surface contamination, with carbon isotopes measured with a 15 μm spot. Secondary C− ions were collected on dual Faraday cups (12C− on L'2 and 13C− on FC2) with mass resolution > 2800, sufficient to remove any 12C hydride interference on 13C. Total counting time on peak was 75 s (15 cycles of 5 s). Diamond reference material S0270L, with a value of δ13C = −8.88 ± 0.10‰ (Stern et al., 2014), was analysed after every 4 unknowns (with a total of 69 analyses in the analytical session). Propagated uncertainties per spot vary between ± 0.12‰ and ± 0.17‰ (2σ) and include the within-run precision, between spot error and uncertainty related to instrumental mass fractionation. Within run precision (2σ), relevant for assessing relative variations along analytical profiles, is about ± 0.10‰. All results are reported relative to the Vienna Pee-Dee Belemnite standard (V-PDB). Nitrogen concentration A 133Cs+ primary beam, with 1.0 nA current was used to determine nitrogen abundances as 12C13C −/13C14N− on the same analytical spots as the carbon isotope analyses. Sample locations were pre-rastered (20 μm) for 30 s to remove surface contamination, with CN ratios measured with a 15 μm spot. Secondary 12 13 − C C ions were measured on L'2 Faraday cup with mass resolution of 6200, and 13C14N− ions measured on an electron multiplier with mass resolution of 6350. Total counting time on peak was 50 s. A calibration constant is first determined using the reference material (S0280E with homogeneous [N] = 1670 at. ppm ± 5%), which is then applied to the unknowns, along with 5% error propagation. 4. Results All five Zimmi diamonds analysed here by SIMS (Table 1) have low [N] that is generally below 50 at. ppm. Two diamonds have [N] that extends up to around 140 at. ppm in their cores (Figs. 2 and S1). Nitrogen concentration results obtained here by ionprobe are similar to bulk [N] calculated through FTIR mapping of hundreds of spectra across the diamond plates (Smit et al., 2018).

Sample

SIMS alias

Point#

δ13C (‰)

2σ

N (at. ppm)

2σ

Zimmi11 Zimmi11 Zimmi11 Zimmi11 Zimmi11 Zimmi11 Zimmi11 Zimmi11 Zimmi11 Zimmi11 Zimmi11 Zimmi11 Zimmi11 Zimmi11 Zimmi11 Zimmi11 Zimmi11 Zimmi11 Zimmi11 Zimmi11 Zimmi11 Zimmi11 Zimmi11 Zimmi11 Zimmi11 Zimmi14 Zimmi14 Zimmi14 Zimmi14 Zimmi14 Zimmi14 Zimmi14 Zimmi14 Zimmi14 Zimmi14 Zimmi14 Zimmi14 Zimmi14 Zimmi14 Zimmi14 Zimmi14 Zimmi14 Zimmi14 Zimmi14 Zimmi14 Zimmi14 Zimmi14 Zimmi14 Zimmi14 Zimmi14 Zimmi15 Zimmi15 Zimmi15 Zimmi15 Zimmi15 Zimmi15 Zimmi15 Zimmi15 Zimmi15 Zimmi15 Zimmi15 Zimmi15 Zimmi15 Zimmi15 Zimmi15 Zimmi15 Zimmi15 Zimmi15 Zimmi15 Zimmi15 Zimmi15 Zimmi15 Zimmi15 Zimmi15

S4356 S4356 S4356 S4356 S4356 S4356 S4356 S4356 S4356 S4356 S4356 S4356 S4356 S4356 S4356 S4356 S4356 S4356 S4356 S4356 S4356 S4356 S4356 S4356 S4356 S4357 S4357 S4357 S4357 S4357 S4357 S4357 S4357 S4357 S4357 S4357 S4357 S4357 S4357 S4357 S4357 S4357 S4357 S4357 S4357 S4357 S4357 S4357 S4357 S4357 S4358 S4358 S4358 S4358 S4358 S4358 S4358 S4358 S4358 S4358 S4358 S4358 S4358 S4358 S4358 S4358 S4358 S4358 S4358 S4358 S4358 S4358 S4358 S4358

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

−23.05 −23.25 −23.25 −23.31 −23.24 −23.38 −23.46 −23.33 −23.31 −23.25 −23.28 −23.32 −23.26 −23.39 −23.29 −23.37 −23.19 −23.27 −23.16 −23.27 −23.28 −23.30 −23.33 −23.27 −23.34 −23.36 −23.42 −23.56 −23.66 −23.71 −23.72 −23.82 −23.77 −23.94 −23.92 −24.15 −24.09 −24.07 −24.28 −24.18 −24.29 −24.35 −24.23 −24.36 −24.33 −24.48 −24.36 −24.43 −24.51 −24.50 −16.31 −16.48 −16.38 −16.38 −16.29 −16.29 −16.32 −16.39 −16.30 −16.28 −16.46 −16.57 −16.49 −16.46 −16.41 −16.45 −16.43 −16.45 −16.34 −16.18 −16.19 −16.18 −16.18 −16.22

0.16 0.13 0.15 0.14 0.13 0.14 0.16 0.15 0.15 0.14 0.12 0.14 0.14 0.12 0.14 0.14 0.14 0.14 0.13 0.14 0.15 0.15 0.12 0.14 0.14 0.14 0.14 0.14 0.14 0.13 0.13 0.12 0.14 0.13 0.13 0.13 0.14 0.14 0.12 0.13 0.14 0.12 0.13 0.15 0.14 0.15 0.14 0.13 0.13 0.14 0.14 0.14 0.13 0.15 0.13 0.14 0.14 0.17 0.13 0.14 0.14 0.14 0.13 0.15 0.13 0.13 0.14 0.15 0.14 0.13 0.12 0.16 0.12 0.12

118.63 141.29 67.08 58.98 50.08 44.80 38.16 32.01 25.78 19.13 18.18 15.14 13.77 10.98 6.80 4.47 3.46 2.43 4.99 2.66 2.48 2.58 2.02 3.21 3.59 111.79 98.49 84.55 74.01 68.36 55.46 54.47 44.17 34.23 41.78 22.74 24.42 21.73 19.03 19.70 15.78 14.22 13.07 13.74 14.07 11.62 10.66 11.51 7.41 10.71 6.82 7.41 7.04 7.09 6.81 6.74 8.83 7.17 6.42 5.85 7.32 7.36 8.11 7.02 7.04 7.19 8.87 7.98 4.79 5.96 7.02 7.03 7.22 7.07

3.81 4.49 2.31 2.29 1.90 1.57 1.37 1.17 1.19 0.82 0.73 0.72 0.58 0.53 0.40 0.25 0.22 0.17 0.27 0.18 0.18 0.20 0.15 0.29 0.26 3.60 3.26 2.78 2.46 2.28 2.02 2.09 1.64 1.27 1.55 0.90 1.04 0.84 0.75 0.78 0.76 0.61 0.58 0.58 0.60 0.59 0.48 0.51 0.40 0.51 0.34 0.36 0.40 0.35 0.34 0.39 0.41 0.35 0.34 0.31 0.44 0.40 0.39 0.35 0.37 0.39 0.41 0.38 0.26 0.31 0.36 0.35 0.38 0.36

(continued on next page) 23

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Table 1 (continued)

Table 1 (continued)

Sample

SIMS alias

Point#

δ C (‰)

2σ

N (at. ppm)

2σ

Sample

SIMS alias

Point#

δ13C (‰)

2σ

N (at. ppm)

2σ

Zimmi15 Zimmi15 Zimmi15 Zimmi15 Zimmi15 Zimmi15 Zimmi15 Zimmi15 Zimmi15 Zimmi15 Zimmi15 Zimmi15 Zimmi15 Zimmi15 Zimmi15 Zimmi18 Zimmi18 Zimmi18 Zimmi18 Zimmi18 Zimmi18 Zimmi18 Zimmi18 Zimmi18 Zimmi18 Zimmi18 Zimmi18 Zimmi18 Zimmi18 Zimmi18 Zimmi18 Zimmi18 Zimmi18 Zimmi18 Zimmi18 Zimmi18 Zimmi18 Zimmi18 Zimmi18 Zimmi18 Zimmi18 Zimmi18 Zimmi18 Zimmi18 Zimmi18 Zimmi18 Zimmi18 Zimmi18 Zimmi18 Zimmi18 Zimmi18 Zimmi18 Zimmi18 Zimmi18 Zimmi18 Zimmi18 Zimmi18 Zimmi18 Zimmi18 Zimmi18 Zimmi18 Zimmi18 Zimmi18 Zimmi18 Zimmi18 Zimmi20 Zimmi20 Zimmi20 Zimmi20 Zimmi20 Zimmi20 Zimmi20 Zimmi20 Zimmi20 Zimmi20

S4358 S4358 S4358 S4358 S4358 S4358 S4358 S4358 S4358 S4358 S4358 S4358 S4358 S4358 S4358 S4359 S4359 S4359 S4359 S4359 S4359 S4359 S4359 S4359 S4359 S4359 S4359 S4359 S4359 S4359 S4359 S4359 S4359 S4359 S4359 S4359 S4359 S4359 S4359 S4359 S4359 S4359 S4359 S4359 S4359 S4359 S4359 S4359 S4359 S4359 S4359 S4359 S4359 S4359 S4359 S4359 S4359 S4359 S4359 S4359 S4359 S4359 S4359 S4359 S4359 S4360 S4360 S4360 S4360 S4360 S4360 S4360 S4360 S4360 S4360

25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 1 2 3 4 5 6 7 8 9 10

−16.14 −16.22 −16.19 −16.16 −16.24 −16.33 −16.28 −16.28 −16.28 −16.39 −16.35 −16.35 −16.35 −16.35 −16.38 −8.30 −8.31 −8.13 −8.10 −8.06 −8.05 −8.19 −8.32 −8.12 −7.99 −8.00 −8.06 −8.14 −8.03 −8.10 −8.14 −8.10 −8.07 −7.83 −7.84 −7.84 −7.87 −7.81 −7.81 −7.78 −7.73 −7.73 −7.93 −7.88 −7.94 −8.04 −7.98 −7.89 −7.92 −8.00 −8.03 −7.95 −7.95 −7.92 −7.90 −7.91 −8.00 −7.91 −8.03 −7.97 −8.03 −8.07 −7.98 −8.03 −8.01 −7.40 −7.31 −7.42 −7.41 −7.33 −7.38 −7.45 −7.39 −7.49 −7.31

0.12 0.15 0.14 0.15 0.14 0.12 0.14 0.16 0.14 0.14 0.13 0.14 0.15 0.14 0.12 0.13 0.14 0.13 0.12 0.16 0.12 0.12 0.13 0.13 0.14 0.12 0.15 0.13 0.12 0.13 0.15 0.13 0.14 0.14 0.13 0.13 0.13 0.17 0.13 0.14 0.13 0.12 0.13 0.12 0.13 0.14 0.13 0.12 0.15 0.16 0.14 0.16 0.13 0.12 0.16 0.14 0.14 0.16 0.14 0.12 0.15 0.13 0.13 0.14 0.13 0.12 0.13 0.15 0.13 0.13 0.15 0.14 0.14 0.13 0.15

7.94 5.57 6.15 6.55 7.48 9.13 7.39 8.27 5.54 10.66 8.97 8.69 8.68 9.14 8.46 37.32 38.91 44.20 42.87 28.56 33.73 37.08 33.52 18.21 11.25 9.73 14.81 11.53 14.43 12.41 8.52 16.23 5.71 9.16 8.51 7.05 4.76 6.68 5.04 4.22 6.31 5.51 6.50 9.59 7.98 12.22 7.92 12.50 12.24 5.71 7.53 7.71 7.53 7.87 8.37 7.13 9.32 9.27 8.35 8.13 8.45 8.37 8.34 9.54 9.34 28.77 29.21 40.32 27.66 30.49 27.45 27.43 27.85 15.39 25.24

0.42 0.29 0.37 0.33 0.41 0.42 0.36 0.39 0.29 0.49 0.45 0.41 0.42 0.43 0.40 1.33 1.39 1.57 1.57 1.21 1.22 1.32 1.21 1.00 0.50 0.44 0.71 0.51 0.60 0.68 0.40 0.72 0.30 0.44 0.56 0.35 0.26 0.35 0.27 0.24 0.34 0.29 0.39 0.44 0.38 0.53 0.38 0.54 0.64 0.30 0.45 0.38 0.37 0.38 0.40 0.47 0.43 0.43 0.49 0.39 0.40 0.40 0.43 0.44 0.51 1.16 1.11 1.46 1.12 1.15 1.46 1.09 1.03 0.63 0.95

Zimmi20 Zimmi20 Zimmi20 Zimmi20 Zimmi20 Zimmi20 Zimmi20 Zimmi20 Zimmi20 Zimmi20 Zimmi20 Zimmi20 Zimmi20 Zimmi20 Zimmi20 Zimmi20 Zimmi20 Zimmi20 Zimmi20 Zimmi20 Zimmi20 Zimmi20 Zimmi20 Zimmi20 Zimmi20 Zimmi20 Zimmi20 Zimmi20 Zimmi20 Zimmi20

S4360 S4360 S4360 S4360 S4360 S4360 S4360 S4360 S4360 S4360 S4360 S4360 S4360 S4360 S4360 S4360 S4360 S4360 S4360 S4360 S4360 S4360 S4360 S4360 S4360 S4360 S4360 S4360 S4360 S4360

11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40

−7.27 −7.40 −7.34 −7.37 −7.30 −7.42 −7.33 −7.36 −7.40 −7.28 −7.20 −7.15 −7.12 −7.17 −7.00 −7.04 −7.09 −7.05 −7.07 −7.03 −6.93 −7.02 −6.99 −6.96 −6.95 −6.98 −6.88 −6.80 −6.71 −6.78

0.13 0.12 0.13 0.14 0.15 0.14 0.14 0.14 0.12 0.12 0.14 0.12 0.12 0.14 0.13 0.15 0.16 0.12 0.13 0.12 0.15 0.15 0.14 0.14 0.13 0.12 0.13 0.13 0.14 0.13

20.89 22.15 22.84 21.82 22.48 19.58 13.17 20.64 10.42 20.20 7.93 9.71 13.75 13.77 14.36 13.68 16.05 7.17 16.37 16.46 16.23 0.47 15.72 16.76 17.33 16.23 15.79 17.72 17.28 15.02

0.81 0.85 0.87 0.84 0.96 0.77 0.56 0.80 0.47 0.79 0.38 0.44 0.58 0.62 0.60 0.72 0.65 0.39 0.67 0.67 0.66 0.06 0.84 0.72 0.70 0.66 0.65 0.78 0.70 0.73

13

Two diamonds have [N] decreasing outward from core to rim. In Zimmi11, where [N] ranges between 141 and 2 at. ppm, and δ13C between −23.1 and −23.3‰ (n = 25), [N] does not co-vary with δ13C (Table 1, Fig. S1). In Zimmi14, both [N] (112 to 7 at. ppm) and δ13C (−23.4 to −24.5‰) systematically decrease towards the rim (n = 25; Fig. 2, Table 1). One diamond (Zimmi15) has fairly uniform [N] and δ13C from core to rim, although subtle changes are observed between growth zones (Table 1, Fig. S2). Zimmi15 has [N] between 4.8 and 10.7 at. ppm, and δ13C between −16.6 and −16.1‰ (n = 39). The remaining two diamonds (Zimmi18 and Zimmi20) have the highest [N] and lowest δ13C in the core, although no systematic covariations were observed from core to rim (Table 1, Fig. S3 and S4). [N] and δ13C in Zimmi18 varies between 4 and 44 at. ppm and −8.3 and −7.7‰ (n = 50). Zimmi20 has [N] between 0.5 and 40 at. ppm and δ13C between −7.5 and −6.7‰ (n = 40). 5. Discussion 5.1. Origin of carbon in Zimmi diamond-forming fluids Carbon isotopic compositions of diamonds can be used to infer the ultimate origin of carbon, from either the mantle or recycled from the oceanic crust. Zimmi diamonds have 650 Ma ReeOs ages that overlap with Neoproterozoic continental collision along the margin of the West African craton (Smit et al., 2016b). Because subduction occurred during the Rokelide orogeny between 700 and 550 Ma (Lytwyn et al., 2006), it can reasonably be expected that the carbon in the 650 Ma Zimmi diamond-forming fluids may be derived from oceanic crust that was subducted into the lithosphere at this time. Three Zimmi diamonds have δ13C < −15‰ which is outside the range for carbon derived solely from the mantle as determined from analyses of mid-ocean ridge basalts, kimberlites and carbonatites (between −10 and 0‰, with an average around −5‰; Deines and Gold, 1973; Sheppard and Dawson, 1975; Mattey et al., 1984; Marty and Zimmermann, 1999). Zimmi11, Zimmi14 and Zimmi15 have δ13C 24

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Fig. 3. Range in carbon isotopic compositions of the five Zimmi sulphidebearing diamonds analysed for this study. Zimmi18 and Zimmi20 have δ13C that overlap with mantle carbon (between −10 and 0‰, with an average around −5‰). Zimmi11, Zimmi14 and Zimmi15 have δ13C < −15‰ that requires the diamond fluids to contain carbon from recycled oceanic crust and/ or its sediments. Recycled origins for carbon in the diamond-forming fluids supports the temporal link of Zimmi diamond formation with Neoproterozoic Gondwana assembly.

sediments. For example, organic matter in oceanic sediments has δ13C < −10‰ (Thomazo et al., 2009) and serpentinised peridotite has δ13C between −40 and − 6‰ (Etiope and Sherwood-Lollar, 2013). Two Zimmi diamonds have δ13C between −8.5 to −6.5‰ (Zimmi18 and Zimmi20; Fig. 3) that are within the range for mantlederived carbon. In the context of Neoproterozoic subduction mantlelike δ13C can, however, be equally explained by homogenised crustal carbon deriving from oceanic crust that has mantle-like δ13C: altered oceanic crust has average δ13C around −4.7‰ (Shilobreeva et al., 2011), due to mixing between organic (δ13C > −27‰; Javoy et al., 1986; Strauss, 1986) and carbonate components (δ13C ≈ 0‰; Javoy et al., 1986; Schidlowski and Aharon, 1992). Subducted oceanic crust with varying proportions of altered magmatic rocks, serpentinites, and sediments containing organic carbon and carbonates can account for the full range of δ13C values seen in worldwide diamonds (e.g., Kirkley et al., 1991; McCandless and Gurney, 1997; Westerlund, 2005; Cartigny, 2005), and in particular the wide range of δ13C seen in the five Zimmi diamonds analysed here. Recycled origins for carbon in the diamond-forming fluids support the temporal link of Zimmi diamond formation with Neoproterozoic Gondwana assembly (Smit et al., 2016b).

Fig. 2. SIMS profile of N concentration and δ13C from core to rim across the plate cut from Zimmi14. (Top and middle) Cathodoluminescence images showing SIMS analytical spots. (Bottom) Decreasing δ13C and decreasing [N] from core to rim, is indicative of Rayleigh fractionation from a reduced CH4rich hydrous fluid, and is the first time such a trend has been observed in a coreto-rim trend within a single diamond. See Fig. 4 for KN determination.

5.2. Modelling Rayleigh fractionation trends Fluid inclusions trapped in diamond can provide direct evidence of the composition of diamond-forming fluids (e.g., Navon et al., 1988; Izraeli et al., 2004; Weiss et al., 2013; Smit et al., 2016a). In diamonds without fluid inclusions, redox speciation of the diamond-forming fluid can still be inferred through evaluation of δ13C-[N] trends (Deines, 1980; Javoy et al., 1984; Cartigny et al., 2001). Precipitation of diamond in a fluid-limited system results in systematic co-variations in δ13C and [N] from core to rim, allowing for redox speciation determination in individual diamonds.

between −25 and −16‰ (Fig. 3). Smart et al. (2011) demonstrated that it is not possible to evolve to such negative δ13C values from a melt starting with mantle values (i.e., the CO2-escape model proposed by Cartigny et al. (2001)). The only other way to generate the δ13C values observed in Zimmi11, Zimmi14 and Zimmi15 is for the diamond source fluids to contain recycled carbon from altered oceanic crust ± 25

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partition coefficient for a fluid that contains only carbon species (for derivation see Petts et al., 2015). However, since diamond fluids are never pure carbon species (e.g., containing only CH4), any determined partition coefficient is a minimum estimate:

Fractionation from both oxidised and reduced fluids/melts can be modelled using (Rayleigh, 1902):

Rdiamond = Rfluid α and Rfluid = f (α − 1) R 0

(1)

KN − 1 = ∆13 Cdiam − CH4 / m

where:

Δ C diam-CH4 of +1.1‰ (at 1140 °C; Richet et al., 1977; Stachel et al., 2017) and the obtained slope (m) of 0.429, yields KN = 3.5 (Fig. 4). A similar value (KN ≥2.0) was previously obtained for a suite of Cullinan diamonds predicted to form from a CH4-rich fluid (Thomassot et al., 2007). Dilution of the carbon species in the fluid, for example by 90% H2O would increase KN to 76. Such a near watermaximum fluid with about 10% CH4 would exist at 5 GPa and 1140 °C at a Δlog fO2 (FMQ) of −3 (Zhang and Duan, 2010). Our minimum estimate for the N partition coefficient in reduced fluids indicates that, at least for Zimmi14, N is compatible during diamond growth. Importantly, it is comparable to the N partition coefficient obtained for pure carbonate fluid (KN = 4.4; Petts et al., 2015), suggesting that under both oxidising and reducing conditions N is compatible in diamond. In reduced fluids, such as the fluid for Zimmi14, N is likely to be NH3 or NH 4+, whereas in more oxidised fluids N is likely to occur as N2 (Deines et al., 1989; Li and Keppler, 2014; Mikhail and Sverjensky, 2014; Smith et al., 2014). The similarity of the N partition coefficients (KN = 2, 3.5 and 4.4) obtained for both reduced and oxidised fluids shows that regardless of the speciation of N in the diamond fluids (as NH3, NH 4+ or N2), N is similarly compatible in diamond.

Rdiamond = 13C /12C of the precipitated diamond; Rfluid = 13C /12C of the residual fluid/melt; R0 = 13C/12C of the initial fluid/melt; f = fraction of fluid/melt remaining. In delta notation the carbon isotopic composition of the diamond can be calculated as:

δ13C − δ13C0 = ∆C ln(f )

(3)

13

(2)

where: ln(f) = [ln(N) − ln(N0)] / (KN − 1); Δ = 1000 ln α; KN = fractionation factor for N between diamond and the fluid/ melt; α = fractionation factor for C between diamond and the fluid/melt. Since the fractionation factor between diamond and CH4 is positive (Δ = 1.11‰ at 1140 °C; Stachel et al., 2017, using β values from Richet et al. (1977)), diamond is enriched in 13C and the fluid reservoir depleted in 13C, leading to trends of progressive 13C depletion during continued diamond precipitation. This leads to a negatively skewed distribution in δ13C (Deines, 1980). For comparison, the fractionation factors between diamond and oxidised fluid is negative (Δ = −3.7‰ for CO2 and diamond; Δ = −1.7‰ for CaCO3 and diamond; T = 1100 °C; Polyakov and Kharlashina, 1995; Chacko et al., 1991; Smart et al., 2011). This results in progressive 13C enrichment during diamond precipitation and, as a consequence, positively skewed distributions in diamond δ13C (e.g., Deines, 1980). Importantly, coherent fractionation trends and meaningful frequency distributions can only be obtained from individual growth zones within a diamond or for diamonds/diamond populations that grew continuously from a single fluid under fluid-limited conditions. If a diamond suite or individual diamond has several growth events (as seen in many worldwide diamonds, e.g., Bulanova, 1995; Peats et al., 2012; Palot et al., 2013), or if the initial δ13C and fO2 of the fluid varies, a skewed δ13C distribution may be obtained on a histogram that has no direct correlation with the redox conditions of the diamond fluid.

5.3.2. Mechanisms for diamond growth Reduced fluids in the diamond-stable lithosphere are not pure CH4, but are mostly H2O with subordinate amounts of CH4 and minor C2H6 and H2 (Fig. 5; Zhang and Duan, 2010; Luth and Stachel, 2014). Mechanisms for eclogitic diamond formation from such reduced fluids include 1) isochemical depressurisation (ascent), 2) precipitation from a CHO fluid oversaturated in carbon due to H2O loss, and 3) oxidation during reaction with eclogitic host rocks. 1. Isochemical ascent Isochemical ascent of reduced CHO fluids causes diamond precipitation through the reaction: 2C2H6 → 3CH4 + C, resulting in outwardly decreasing δ13C (for fluids below Δlog fO2 (FMQ) of about −3; see Stachel et al. (2017) and Fig. 5). CH4 → C + 2H2 is not a viable diamond-forming reaction since the H2 content of CHO fluids is negligible. During isochemical cooling of less reduced CHO fluids (Δlog fO2 (FMQ) of about −2; Luth and Stachel, 2014; Smit et al., 2016a), diamond formation may occur through the reaction CH4 + CO2 → 2C + 2H2O and would result in outward increasing δ13C, irrespective of the initial CH4 to CO2 ratio. Regardless of the proportions of CH4, C2H6 and H2O, reduced fluids at Δlog fO2 (FMQ) less than −3 do not precipitate diamond during isochemical cooling but are efficient agents of diamond precipitation during ascent through the lithosphere (Fig. 5; Luth and Stachel, 2014). During cooling along the entire diamond-stable lithosphere, such reduced fluids are expected to precipitate 4–6% of their carbon content (amounting to 1–2 wt% diamond precipitated), compared to around 50% (equivalent to 1 wt% carbon precipitated) expected for carbonpoor H2O-maximum fluids (Luth and Stachel, 2014). The Zimmi14 fractionation trend (Fig. 4), requires that around 60% of the initial carbon must have crystallised from the fluid. Such a high percentage of carbon precipitation is not predicted during isochemical ascent of reduced fluids (Fig. 5), and therefore not a suitable model for Zimmi14 diamond growth.

5.3. Eclogitic diamond growth from reduced fluids - Zimmi14 5.3.1. Reduced fractionation trend Diamond Zimmi14 appears to have formed in one continuous episode of growth, with no obvious indications of multiple distinct growth zones or intermittent periods of resorption (Fig. 2a and b). This gives us confidence that the observed continuous core to rim trend (Fig. 2c) can be interpreted as a Rayleigh fractionation trend, rather than being the result of fortuitously perfectly gradual mixing of two end-member fluids with distinct δ13C and [N] in an open-system. On that basis, decreasing δ13C (−23.4 to −24.5‰) along with decreasing [N] is indicative of formation from reduced fluids as opposed to oxidised fluids (for comparison see the oxidised trend for a Jericho eclogitic diamond in: Smart et al., 2011; Petts et al., 2015). Compositions along this fractionation trend in Zimmi14 can be used to calculate the nitrogen partition coefficient during diamond growth from reduced fluids: i.e., whether nitrogen is compatible or incompatible during growth, and how it compares to nitrogen fractionation between diamond and oxidised fluids. The slope (m) of the linear relationship between δ13C and ln(N) (Fig. 4) can be used to derive the

2. Loss of H2O from a carbon-saturated CHO fluid

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Fig. 4. a) Minimum estimate of the nitrogen partition coefficient between diamond and reduced fluid, calculated from core to rim compositions in Zimmi14, using: Δ13C diam-CH4 of +1.1‰ (at 1140 °C; Richet et al., 1977; Stachel et al., 2017) and KN − 1 = Δ13C/slope. A positive nitrogen partition coefficient indicates that N is compatible during diamond growth from reduced fluids, similar to the nitrogen partition coefficient obtained for oxidised fluids (KN ≥ 4.4; Petts et al., 2015). b) Determination of the KN allows for the calculation of idealised reduced fluid and diamond compositional evolution during diamond precipitation in a closed-system. Comparing Zimmi14 core-to-rim compositions to the idealised fractionation trends, shows that around 60% carbon crystallised out of the fluid. Equations for Rayleigh fractionation are given in the text.

diamond formation can occur after the introduction of a hydrous CHO fluid. Conversely, this diamond formation process may not be applicable in harzburgites, since at 5 GPa, the hydrous carbonated harzburgite solidus is much higher, around 1300 °C (Wyllie, 1987). Dissolution of H2O from a CHO fluid into a melt would drive the CHO fluid composition into the “Diamond + Fluid” field, which oversaturates the fluid in carbon and requires diamond growth for the fluid composition to return to the diamond saturation curve (Fig. 6). The amount of melting, H2O removal, and therefore diamond precipitation, is controlled by the starting oxygen fugacity of the CHO fluid as well as the end composition of the fluid. The scenario where the CHO fluid

Alternatively, the Zimmi14 eclogitic diamond could have precipitated from CHO fluids at temperatures above the hydrous solidus when some of its H2O content dissolved into a melt (Luth, 2017). Potentially, diamonds could form in this way when partial melting is promoted by dehydrating subduction slabs. Specifically for the Zimmi diamonds, this process could theoretically have occurred during Rokelide orogen collisional processes (Lytwyn et al., 2006). This diamond formation process could be especially effective in eclogite and lherzolite, since at 5 GPa their hydrous solidi are between 1025 and 1100 °C (Wyllie and Ryabchikov, 2000; Kessel et al., 2005). This gives a wide range of lithospheric conditions where melting and

Fig. 5. a) Speciation and redox state of CHO fluids at 5 GPa and 1140 °C (modified after Luth and Stachel, 2014, calculated using Zhang and Duan (2009)). b) Percentage of carbon precipitation expected from cooling fluids with different O/(O + H) (modified after Luth and Stachel, 2014). CO2-CH4 fluids near the H2Omaximum are expected to precipitate 50% of their low carbon content during cooling along the entire diamond-stable lithosphere. In contrast, reduced C2H6-CH4 fluids such as that required for Zimmi14 (Fig. 4) are expected to only precipitate 4–6% of their much higher carbon content by cooling through the same depth range. Modelling of the Zimmi14 fractionation trend (Fig. 4) shows that around 60% of the initial carbon crystallised from the fluid, so Zimmi14 likely did not form through isochemical cooling of a reduced fluid. Grey bar is the average oxygen fugacity (ƒO2) of the cratonic lithosphere (Stagno et al., 2013). 27

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Fig. 6. a) CHO ternary diagram illustrating the composition of fluids at 5 GPa and 1140 °C, and the location of fluids at the IW buffer and EMOD. Calculated using Zhang and Duan (2009). b) CHO ternary diagram zoomed in around the H2O-maximum. Dissolution of H2O from a CHO fluid into a melt would drive the CHO fluid composition into the”Diamond + Fluid” field, which oversaturates the fluid in carbon and promotes diamond growth. The scenario where the CHO fluid loses the most H2O and undergoes the biggest change in oxygen fugacity will precipitate the most diamond. c) Percentage of diamond that precipitates during H2O extraction from a reduced CHO fluid that contains some amount of CH4. This diagram considers fluids with starting compositions between FMQ-2 and FMQ-3. The two orange curves are for the final CHO fluid composition at either IW or FMQ-3. The maximum amount of initial carbon that can precipitate from a reduced fluid that loses H2O to a melt is around 24%. Since modelling of the Zimmi14 fractionation trend (Fig. 4) shows that around 60% of the initial carbon crystallised from the fluid, this diamond formation process appears to not be suitable.

mantle. Sulphate minerals are not commonly described in lithospheric mantle rocks, and when they are they occur in highly metasomatised phlogopite-containing MARID rocks indicative of high fluid to rock ratios (Giuliani et al., 2013). In particular, Zimmi sulphide inclusions have Archaean mass-independently fractionated sulphur isotope signatures indicating that they resided in the lithosphere for > 2 billion years prior to their inclusion in diamond (Smit et al., 2019). Sulphur could only be present in the form of sulphide in lithospheric mantle that is continually affected by metasomatic processes. So, it seems that sulphur could not have been the driver for redox-mediated formation of Zimmi diamonds. The exact nature of a potential redox process for Zimmi diamonds is currently not fully understood. The provision of oxygen required for the reaction CH4 + O2 → C + H2O in Zimmi14 (and potentially the Orapa diamond studied by Jacob et al., 2016) has to occur through reduction of ferric to ferrous iron and cannot be balanced as an equation based on garnet and clinopyroxene alone but requires that either the starting composition is coesite-eclogite or that the reaction product is olivineeclogite. For reduction of a CO2-bearing fluid, the relationship switches around (SiO2 would be liberated during the reaction). Both associations (coesite- or olivine-bearing) are uncommon in cratonic eclogite xenoliths (Smyth and Hatton, 1977; Fung, 1998; Kopylova et al., 1999; Schulze et al., 2000; Jacob et al., 2003) and only rarely described for diamondiferous eclogites. In addition, coesite represents < 2% of eclogitic inclusions in diamond and eclogitic (Fe-rich) olivine is not observed as an inclusion (Schulze et al., 2003; Stachel and Harris, 2008). So while eclogite may have the capacity to buffer the redox state of diamond-forming fluids, in the Zimmi case we have no constraints on the accessory mineralogy of the eclogitic host rocks.

loses the most H2O and undergoes the biggest change in oxygen fugacity will precipitate the most diamond. For example, a CHO fluid that starts near the H2O-maximum and loses sufficient H2O to drive it to the IW buffer will precipitate nearly all its carbon. Similarly, a CHO fluid that starts around Δlog fO2 (FMQ) = −2.3 (XCO2/XC2H6 = 1) will precipitate around 24% of its initial carbon (Fig. 6). A fluid that starts at XCO2 = 0.1 (green symbols in Fig. 6), will precipitate 13% of its carbon if its composition ends at Δlog fO2 (FMQ) = −3 and 17% of its carbon if its composition ends at IW (Fig. 6). More reduced CHO fluids that have higher mole fractions of CH4, such as the fluid required for Zimmi14, will precipitate even less diamond. Since modelling of the Zimmi14 fractionation trend shows that around 60% of the initial carbon crystallised from the fluid, this diamond formation process is not suitable. 3. Redox exchange processes between fluid and host rock Although diamond formation through redox exchange has been shown to be very inefficient in Fe-depleted peridotitic assemblages (Luth and Stachel, 2014; Stachel and Luth, 2015), this process could potentially still occur in more Fe-rich assemblages such as fertile peridotite and in particular, eclogite. For example, Orapa sulphide-bearing eclogitic diamonds appear to show some evidence for diamond formation through redox processes where, in one example, pyrrhotite inclusions have oxidised rims (Jacob et al., 2016). In the case of this Orapa diamond, it is not clear if the sulphides are protogenetic or syngenetic, or affected by epigenetic oxidation along fractures in the diamond. Potentially the observed oxidation rim is simply an alteration product: since sulphur only occurs in trace amounts in the fertile mantle (250 ppm in pyrolite McDonough and Sun, 1995), and even lower concentrations in the depleted cratonic lithosphere, it is not considered to be an efficient redox buffer for diamond formation (Luth and Stachel, 2014). If however, diamonds form during interaction of a reduced fluid (such as that required for Zimmi14) with an oxidised sulphur-bearing mineral in the eclogitic host rock, that sulphur-bearing mineral would need to be barite or some other oxidised sulphate. Because sulphates are highly fluid mobile, they are not expected to be a stable sulphur species for billions of years in the lithospheric

5.3.3. Other reduced fractionation trends reported for diamond growth Determination of the reduced N partition coefficient allows us to reexamine the reduced trends proposed for Cullinan and Lahtojoki diamonds (Thomassot et al., 2007; Smart et al., 2017). Thomassot et al. (2007) modelled diamond formation from CH4-bearing fluids for a suite of lherzolitic diamonds occurring in a single Cullinan xenolith. These diamonds are likely of similar age to the Cullinan lherzolitic diamonds that formed at around 1.9 Ga (Richardson et al., 1993) after the emplacement of the Bushveld Igneous Complex. Redox exchange processes 28

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Fig. 7. Bulk combustion δ13C-[N] data for two other diamond localities where methane fractionation trends were previously proposed. a and b) δ13C-[N] covariations for multiple diamonds in the DO40 Cullinan xenolith relative to an idealised reduced fractionation trend calculated using the KN ≥ 2 determined by Thomassot et al. (2007), and also a KN ≥ 3.5 determined here from Zimmi14 Fig. 4. The idealised trends for Cullinan diamonds were calculated using Δ13C diam-CH4 of +1.1‰ (at 1140 °C; Richet et al., 1977; Stachel et al., 2017), and the same initial fluid compositions as Thomassot et al. (2007): [N] = 600 at. ppm and δ13C = −0.84‰. The N partition coefficient determined here from Zimmi14 (KN ≥ 3.5) does not appear to be suitable for DO40 diamond growth. This may be because all determined KN values are only estimates since diamond source fluids are not pure CH4. c) Lahtojoki diamonds (Smart et al., 2017) do not have compositional trends that match either the KN ≥ 2 or 3.5 reduced fractionation trends, and can instead be interpreted as multiple growth events or open-system mixing. See Eqs. (1) and (2) in the text for Rayleigh fractionation calculations.

5.4. Lack of fractionation trends in other Zimmi diamonds

have now been discounted as an effective mechanism for diamond formation in depleted Fe-poor peridotite (Luth and Stachel, 2014). So in the context of peridotitic diamond formation by redox exchange, Cullinan diamond growth from reduced fluids suggests that the cratonic lithosphere below Cullinan was locally oxidised and melt enriched due to the emplacement of the Bushveld. Here we plotted the δ13C -[N] covariations for diamonds in the DO40 xenolith relative to an idealised reduced fractionation trend calculated using the KN ≥ 2 determined by Thomassot et al. (2007), and also a KN ≥ 3.5 determined here (Fig. 7). These trends were calculated using Δ13C diam-CH4 of +1.1‰ (at 1140 °C; Richet et al., 1977; Stachel et al., 2017), and the same initial fluid compositions as Thomassot et al. (2007): [N] = 600 at. ppm and δ13C = −0.84‰. The N partition coefficient determined here from Zimmi14 (KN ≥ 3.5) does not appear to be directly applicable for DO40 diamond growth. This may be because any calculated KN values are only applicable for fluids that contain pure CH4. Since diamond-forming fluids are not likely to be pure CH4, but are also expected contain other species, these calculated KN values are only minimum estimates even though the sign (+ or −) of the partition coefficient (indicating compatibility in diamond versus fluid) is still valid. Additionally, other minerals in the eclogitic assemblage may influence nitrogen partitioning during diamond growth, for example, N may substitute into clinopyroxene and garnet with the result that less N is available for incorporation into diamond (Watenphul et al., 2009, 2010; Li et al., 2013). Growth from CH4-rich fluids was also proposed for Lahtojoki eclogitic diamonds, based on the overall negative skewness in the δ13C distribution (Smart et al., 2017). However, no correlation between δ13C and [N] was observed, as would be expected if these diamonds crystallised from one continuously fractionating fluid (Fig. 7). Lack of any δ13C-[N] covariation means that diamond formation from multiple fluids with different compositions cannot be ruled out. Additionally, δ13C-[N] covariations for these xenocrystic diamonds do not match either of the idealised reduced fractionation trends calculated for KN between 2 and 3.5 (Fig. 7). Since δ13C and [N] of Lahtojoki diamonds were obtained through bulk analyses, multiple diamond fluids and growth events within and among diamonds cannot be excluded. A single growth event from a continuously fractionating fluid can only be proposed after careful in-situ analysis and investigation of internal growth features in diamond (Smart et al., 2011).

Uniform co-variations in δ13C and nitrogen that can be attributed to Rayleigh fractionation processes are extremely rare in diamonds. Such a trend was only observed in one out of the five Zimmi diamonds analysed here, and other processes are needed to account for non-systematic variations seen within the other four diamonds. The operation of multiple fluid processes during Zimmi diamond growth, involving variable fluid-to-rock ratios and occasional decoupling of C and N, emphasises that diamond growth even within a single suite of diamonds is not necessarily uniform. 5.4.1. Zimmi11 Decreasing [N] with constant δ13C in Zimmi 11 (Fig. S1) could be due to nitrogen substitution into other phases in the mantle lithosphere. Nitrogen has been shown experimentally to substitute into the pyroxenes, olivine and garnet: as NH4+ substituting for K, as N3− for O2−, or as interstitial N2 (Watenphul et al., 2009, 2010; Li et al., 2013, and references therein). Additionally, Li et al. (2013) showed a redox dependence for nitrogen substitution, where some mantle minerals can incorporate substantially more nitrogen at reducing conditions near the iron-wüstite redox buffer than at oxidising conditions near the Ni-NiO redox buffer. Therefore it can be expected that at the generally more reducing conditions in the diamond-stable lithosphere (Fig. 5) some of the nitrogen in a C-O-H-N-S-bearing diamond source fluid would preferentially partition into other minerals like the pyroxenes rather than diamond. The trend observed in Zimmi11 (Fig. S1) could also occur if the carbon reservoir is being continuously recharged. This open-system behaviour would not result in any variation in δ13C during growth, and without any simultaneous input of N, the N concentration of the diamond would continuously decrease. 5.4.2. Zimmi15 and Zimmi18 Similarly, Zimmi15 and Zimmi18 also show evidence for diamond growth in a system that is not fluid limited. Both these diamonds do not show significant fractionation in either δ13C or [N], and therefore fluid redox state cannot be interpreted (e.g., Fig. S2 and Fig. S3). However, subtle compositional differences can be seen that correlate to growth zones observed in the CL images, which we interpret to represent different fluid compositions during multiple stages of diamond growth. 29

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4) In this small suite of five Zimmi diamonds, the formation processes are not uniform. Multiple fluid processes, including variable fluidto-rock ratios and decoupling of C and N, account for the Zimmi diamonds studied here.

Decoupling of nitrogen and δ13C, which combined with growth interruptions seen in the CL images indicates multiple stages of growth, is commonly observed in worldwide diamonds (Boyd et al., 1987; Harte et al., 1999; de Vries et al., 2013; Peats et al., 2012; Palot et al., 2013; Mikhail et al., 2014; Petts et al., 2016; Hogberg et al., 2016). These growth zones may be separated in time, including perhaps, but not requiring periods of diamond resorption. However, any time separation between different growth zones is unclear and unconstrained.

Acknowledgements Israel Eliezri from Coldiam Ltd. is thanked for providing samples from Zimmi. Adrian Chan (GIA) is thanked for his help with laser cutting and polishing diamond plates. Kate Hogberg assisted with sample preparation during SIMS analyses at the University of Alberta. Many thanks to Evan Smith for spending some time at the Raman hunting unsuccessfully for any trapped reduced volatile phases. Thank you to Sami Mikhail and Kate Kiseeva for their constructive reviews, and Balz Kamber for his editorial handling.

5.4.3. Zimmi20 The inner portions of Zimmi20 have relatively uniform δ13C values and subtly decreasing [N], whereas the outer portions of Zimmi20 show appear to show relatively uniform [N] and subtly increasing δ13C values (Fig. S4). Any Rayleigh fractionation trend requires that [N] continuously decreases with either increasing δ13C values (oxidised fluid; Smart et al., 2011; Petts et al., 2015) or decreasing δ13C values (reduced fluid as seen in Zimmi14; Fig. 4). At first glance the co-variation of δ13C-[N] in the outer portion of Zimmi20 may appear to indicate Rayleigh fractionation from an oxidised fluid. However, when the compositions from Zimmi20 are compared to idealised fractionation trends from an oxidised fluid, the trends do not match (Fig. S5). Instead, we interpret the compositions in Zimmi20 to be the result of multiple growth stages, and decoupling of δ13C and [N] in the diamond source fluids.

Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.chemgeo.2019.04.014. References Aulbach, S., Höfer, H.E., Gerdes, A., 2019. High-Mg and low-Mg mantle eclogites from Koidu (West African craton) linked by Neoproterozoic ultramafic melt metasomatism of subducted Archaean plateau-like oceanic crust. J. Petrol. https://doi.org/10.1093/ petrology/egz011. Baratoux, L., Metelka, V., Naba, S., Jessell, M.W., Gregoire, M., Ganne, J., 2011. Juvenile Paleoproterozoic crust evolution during the Eburnean orogney (~2.2–2.0 Ga), western Burkina Faso. Precambrian Res. 191, 18–45. Barth, M., Rudnick, R.L., Carlson, R.W., Horn, I., McDonough, W.F., 2002. Re-Os and UPb geochronological constraints on the eclogite-tonalite connection in the Archean Man Shield, West Africa. Precambrian Res. 118, 267–283. Boher, M., Abouchami, W., Michard, A., Albarede, F., Arndt, N.T., 1992. Crustal growth in West Africa at 2.1 Ga. J. Geophys. Res. 97, 345–369. Boyd, S.R., Mattey, D.P., Pillinger, C.I., Milledge, H.J., Mendelssohn, M., Seal, M., 1987. Multiple growth events during diamond genesis: an integrated study of carbon and nitrogen isotopes and nitrogen aggregation state in coated stones. Earth Planet. Sci. Lett. 86, 341–353. Bulanova, G.P., 1995. The formation of diamond. J. Geochem. Explor. 53, 1–23. Bulanova, G.P., Varshavsky, A.V., Kotegov, V.A., 2005. A venture into the interior of natural diamond: genetic information and implications for the gem industry. J. Gemmol. 29 (7/8), 377–386. Bulanova, G.P., Walter, M.J., Smith, C.B., Kohn, S.C., Armstrong, L.S., Blundy, J., Gobbo, L., 2010. Mineral inclusions in sublithospheric diamonds from Collier 4 kimberlite pipe, Juina, Brazil: subducted protoliths, carbonated melts and primary kimberlite magmatism. Contrib. Mineral. Petrol. 160, 489–510. Burnham, A.D., Bulanova, G.P., Smith, C.B., Whitehead, S.C., Kohn, S.C., Gobbo, L., Walter, M.J., 2016. Diamonds from the Machado River alluvial deposit, Rondoˆnia, Brazil, derived from both lithospheric and sublithospheric mantle. Lithos 265, 199–213. Cartigny, P., 2005. Stable isotopes and the origin of diamond. Elements 1, 79–84. Cartigny, P., Harris, J.W., Javoy, M., 2001. Diamond genesis, mantle fractionations and mantle nitrogen content: a study of δ13C-N concentrations in diamonds. Earth Planet. Sci. Lett. 185, 85–98. Chacko, T., Mayeda, T.K., Clayton, R.N., Goldsmith, J.R., 1991. Oxygen and carbon isotope fractionations between CO2 and calcite. Geochim. Cosmochim. Acta 55, 2867–2882. de Vries, D.F.W., Bulanova, G.P., de Corte, K., Pearson, D.G., Craven, J.A., Davies, G.R., 2013. Micron-scale couple carbon isotope and nitrogen abundance variations in diamonds: evidence for episodic diamond formation beneath the Siberian Craton. Geochim. Cosmochim. Acta 100, 176–199. Deines, P., 1980. The carbon isotopic composition of diamonds: relationship to diamond shape, colour, occurrence and vapour deposition. Geochim. Cosmochim. Acta 44, 943–961. Deines, P., Gold, D.P., 1973. The isotopic composition of carbonatite and kimberlite carbonate and their bearing on the isotopic composition of deep-seated carbon. Geochim. Cosmochim. Acta 37, 1709–1733. Deines, P., Harris, J.W., Spear, P.M., Gurney, J.J., 1989. Nitrogen and 13C content of Finsch and Premier diamonds and their implications. Geochim. Cosmochim. Acta 53, 1367–1378. Eggler, D.H., Baker, D.R., 1982. Reduced volatiles in the system C-O-H: implications to mantle melting, fluid formation and diamond genesis. In: Manghani, M.H. (Ed.), High Pressure Research in Geophysics, pp. 237–250. Etiope, G., Sherwood-Lollar, B., 2013. Abiotic methane on Earth. Rev. Geophys. 51, 276–299. Fung, A.T., 1998. Petrochemistry of Upper Mantle Eclogites from the Grizzly, Leslie, Pigeon and Sable Kimberlites in the Slave Province, Canada. Extended Abstracts of the 7th International Kimberlite Conference, Cape Town. pp. 230–232.

6. Summary and conclusions 1) We now know that diamonds form from a wider range of compositions than just oxidised carbonate-bearing fluids and melts. Worldwide, many lithospheric and sublithospheric diamonds are known to crystallise from carbonate-bearing fluids and melts, such as fibrous diamonds (Navon et al., 1988; Weiss et al., 2015) as well as gem-quality diamonds from Jericho, Juina, Venetia and Voorspoed (Walter et al., 2008; Bulanova et al., 2010; Smart et al., 2011; Jablon and Navon, 2016; Burnham et al., 2016; Thomson et al., 2016). However, compositions of diamond-forming fluids/melts also include metallic melts (CLIPPIR diamonds; Smith et al., 2016), boron-containing subduction fluids (Smith et al., 2018), CH4-and CO2-bearing hydrous fluids (Marange diamonds; Smit et al., 2016a) and CH4-rich hydrous fluids (Cullinan peridotitic diamonds; (Thomassot et al., 2007) and Zimmi diamonds; this study). 2) The presence of a reduced fractionation trend from core to rim in a single diamond (Zimmi14), allows us to establish a KN ≥ 3.5 for diamond growth from reduced fluids, similar to the previously determined KN ≥ 2 (Thomassot et al., 2007). These partition coefficients for diamond growth from reduced fluids are the same sign and similar magnitude to the KN ≥ 4.4 determined for diamond growth from carbonate-bearing fluids (Petts et al., 2015), and accordingly nitrogen appears to be similarly compatible during growth from both oxidised and reduced fluids. 3) The mechanism for growth of Zimmi14 from a reduced fluid is not well constrained. Diamond formation through either a) isochemical ascent of reduced fluids or b) the removal of H2O from a CHO fluid during partial melting is shown to be unsuitable growth mechanisms. Although diamond formation through redox exchange appears to not be feasible in Fe-poor peridotites, relatively more Fe-rich eclogites could potentially facilitate Zimmi14 diamond formation through oxidation of reduced fluids. However, such a process requires that oxygen be liberated from the host eclogites and the associated mineralogical evidence for such a process is currently absent (either a starting composition of coesite-eclogite or a reaction product of olivine-eclogite is required in the case of a reducing fluid).

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