# Deep carbon through time: Earth's diamond record and its implications for carbon cycling and fluid speciation in the mantle

D. Howell, T. Stachel, R. A. Stern, D. G. Pearson, F. Nestola, M. F. Hardman, J. W. Harris, A. L. Jaques, S. B. Shirey, P. Cartigny, et al.

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D. Howell, T. Stachel, R. A. Stern, D. G. Pearson, F. Nestola, et al.. Deep carbon through time: Earth's diamond record and its implications for carbon cycling and fluid speciation in the mantle. Geochimica et Cosmochimica Acta, 2020, 275, pp.99-122. 10.1016/j.gca.2020.02.011. insu-03584797

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Howell, D. et al. (2020) Deep carbon through time: Earth's diamond record and its implications for carbon cycling and fluid speciation in the mantle. *Geochimica et Cosmochimica Acta*, 275, pp. 99-122.

(doi: 10.1016/j.gca.2020.02.011)

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Deep carbon through time:
Earth's diamond record and its implications for
carbon cycling and fluid speciation in the mantle

D. Howell¹,²,³, T. Stachel¹, R.A. Stern¹, D.G. Pearson¹, F. Nestola², M.F. Hardman¹,
J.W. Harris⁴, A.L. Jaques⁵, S.B. Shirey⁶, P. Cartigny⁷, K.V. Smit⁸, S. Aulbach⁹,
F.E. Brenker⁹, D.E. Jacob¹⁰, E. Thomassot¹¹, M.J. Walter¹², O. Navon¹³

1 Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta T6G 2E3, Canada

2 Department of Geosciences, University of Padova, Via Gradenigo 6, I-35131 Padova, Italy

3 Vrije Universiteit, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands

4 School of Geographical and Earth Sciences, University of Glasgow, Glasgow G12 8QQ, UK

5 Research School of Earth Sciences, Australian National University, 142 Mills Road, Acton, ACT 2601, Australia

6 DTM, Carnegie Institution for Science, Washington, DC20015, USA

7 Laboratoire de Geochimie des Isotopes Stables, Institut de Physique du Globe de Paris, Universite Paris Diderot, Centre National de la Recherche Scientifique, UMR 7154, Sorbonne Paris-Cite, 75238 Paris

8 Gemological Institute of America, 50 West 47th Street, New York City, NY 10036, USA

9 Institut für Geowissenschaften, Goethe-Universität, Frankfurt am Main, Germany

10 Australian Research Council Centre of Excellence for Core to Crust Fluid Systems and Department of Earth and Planetary Sciences, Macquarie University, North Ryde, New South Wales 2109, Australia

11 Centre de Recherches Petrographiques et Geochimiques, Centre National de la Recherche Scientifique, Universite de Lorraine, 54501 Vandœuvre-les-Nancy, France

12 Geophysical Laboratory, Carnegie Institution for Science, Washington, DC20015

13 Institute of Earth Sciences, The Hebrew University, Jerusalem 9190401, Israel

Abstract

Diamonds are unrivalled in their ability to record the mantle carbon cycle and mantle
$f\alpha$O₂ over a vast portion of Earth's history. Diamonds' inertness and antiquity means
their carbon isotopic characteristics directly reflect their growth environment within
the mantle as far back as ~3.5 Ga. This paper reports the results of a thorough
secondary ion mass spectrometry (SIMS) carbon isotope and nitrogen concentration
study, carried out on fragments of 144 diamond samples from various locations, from
~3.5 to 1.4 Ga for P [peridotitic]-type diamonds and 3.0 to 1.0 Ga for E [eclogitic]-
type diamonds. The majority of the studied samples were from diamonds used to
establish formation ages and thus provide a direct connection between the carbon
isotope values, nitrogen contents and the formation ages. In total, 908 carbon isotope
and nitrogen concentration measurements were obtained. The total $\delta^{13}$C data range
from -17.1 to -1.9 ‰ (P = -8.4 to -1.9 ‰; E = -17.1 to -2.1 ‰) and N contents range
from 0 to 3073 at. ppm (P = 0 to 3073 at. ppm; E = 1 to 2661 at. ppm). In general,
there is no systematic variation with time in the mantle carbon isotope record since >3
Ga. The mode in $\delta^{13}$C of peridotitic diamonds has been at -5 (± 2) ‰ since the earliest
diamond growth ~3.5 Ga, and this mode is also observed in the eclogitic diamond
record since ~3 Ga. The skewness of eclogitic diamonds' $\delta^{13}$C distributions to more
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negative values, which the data establishes began around 3 Ga, is also consistent
through time, with no global trends apparent.

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No isotopic and concentration trends were recorded within individual samples, indicating that, firstly, closed system fractionation trends are rare. This implies that diamonds typically grow in systems with high excess of carbon in the fluid (i.e. relative to the mass of the growing diamond). Any minerals included into diamond during the growth process are more likely to be isotopically reset at the time of diamond formation, meaning inclusion ages would be representative of the diamond growth event irrespective of whether they are syngenetic or protogenetic. Secondly, the lack of significant variation seen in the peridotitic diamonds studied is in keeping with modeling of Rayleigh isotopic fractionation in multicomponent systems (RIFMS) during isochemical diamond precipitation in harzburgitic mantle. The RIFMS model not only showed that in water-maximum fluids at constant depths along a geotherm, fractionation can only account for variations of <1 ‰, but also that the principal δ¹³C mode of -5 ± 1 ‰ in the global harzburgitic diamond record occurs if the variation in fO₂ is only 0.4 log units. Due to the wide age distribution of P-type diamonds, this leads to the conclusion that the speciation and oxygen fugacity of diamond forming fluids has been relatively consistent. The deep mantle has therefore generated fluids with near constant carbon speciation for 3.5 Ga.

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Keywords

Mantle carbon; isotope fractionation; mantle oxygen fugacity; diamond growth; deep carbon cycle; subduction.

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Introduction

Carbon and other elements that form volatile species in Earth's mantle, such as nitrogen and hydrogen, are major constituents of our planet and, along with oxygen, dominate the composition of the atmosphere and hydrosphere. The cycling of these elements from the surface into the deep Earth via subduction, and then back to the surface via volcanism has been integral to regulating climate, ocean volume and the habitability of our planet. While many of these volatile cycles in the atmosphere, biosphere and crust are understood, the deeper aspects of these cycles in the mantle

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and core are rather poorly constrained (Hazen & Schiffries, 2013). Of interest is the comparison between constancy of the surficial carbon isotope record (e.g. Des Marais, 2001) and that of the mantle through time because the mantle is - by more than an order of magnitude - a larger reservoir of C than the crust (e.g. Kelemen & Manning, 2015). By studying the fluxes between Earth's reservoirs we gain valuable insights into the geological processes of subduction, partial melting, degassing and metasomatism, and by tracing these processes back through time we gain further insight into the evolution of our planet.

The upper mantle's redox state is intimately linked to the evolution of Earth's atmosphere through volcanic degassing (Kasting et al., 1993). Potential variations in the mantle's oxidation state through time therefore have significant consequences, yet this has been a controversial topic for many decades (McCammon, 2005; Frost & McCammon, 2008; Foley, 2011). Most of the controversy lies in the fact that oxygen fugacity information obtained from the study of volcanic rocks does not agree with that obtained from the study of mantle peridotites (see Figure 1 of Foley (2011) and references therein). Determining mantle $fO_2$ from $Fe^{3+}/\Sigma Fe$ analyses on ancient peridotites and mid-ocean ridge basalts (MORB) is problematic due to the independent or combined effects of alteration and metasomatism. As a result, trace element systematics, such as V, Cr, Sc and Ce, have been employed to estimate oxygen fugacity ($fO_2$; Canil, 1997; Li and Lee, 2004; Trail et al., 2011). Many studies of upper mantle-derived rocks have concluded that mantle $fO_2$ has remained constant, within ~1 log unit of the fayalite-magnetite-quartz (FMQ) oxygen buffer at 1 GPa, since 3.5 Ga (Canil, 1997; Canil and Fedortchouk, 2001; Delano, 2001; Li and Lee, 2004; Berry et al., 2008; Trail et al., 2011; Hibbert et al., 2012; Rollinson et al., 2017; Nicklas et al., 2018). However, the comprehensive review by Foley (2011) along with some recent studies (Aulbach & Viljoen, 2015; Aulbach & Stagno, 2016; Shu et al., 2016; Aulbach et al., 2017; Nicklas et al. 2019) have suggested that the Archean mantle was more reducing than that of the present day. These contrasting results highlight the uncertainty in our knowledge of $fO_2$ through time.

Along with potential temporal variation in the mantle's oxygen fugacity, there are also vertical variations to consider. The effects of pressure cause the mantle to become more reducing with depth (Ballhaus & Frost, 1994). The implications of this
variation for carbon are that at more oxidised (higher fO₂) conditions in the shallow mantle, carbonate and CO₂ would be the stable / dominant species, changing to CH₄ and metallic carbide with decreasing fO₂ (and depth). Therefore variation of mantle fO₂ through time has implications for the speciation of carbon-related volatiles in the upper mantle.

Diamonds are unrivalled in their ability to record the mantle carbon cycle over a vast portion of Earth's history. They are the product of ascending, cooling, carbon-saturated metasomatic fluids-melts and/or redox reactions, predominantly within peridotitic and eclogitic domains in the mantle lithosphere (see Stachel & Luth (2015) and references therein). Their occurrence with inclusions of CO₂ (Schrauder & Navon, 1993), carbonate (e.g. Wang et al., 1996), methane (Smit et al., 2016) and carbide (e.g. Mikhail et al., 2014; Smith et al., 2016), show their formation over a significant range of oxygen fugacities. However, the lack of buffering capacity of peridotitic subcratonic mantle (Luth & Stachel, 2014) suggests that redox reactions may not be responsible for diamond growth in this part of the mantle. Luth & Stachel (2014) showed that the speciation of carbon in mantle fluids is actually the controlling variable when it comes to the oxygen fugacity of the peridotitic lithospheric mantle, which is counter to the traditional view of the fluids adjusting to the prevalent fO₂ rather than buffering them (e.g. Woodland et al. 2006). Understanding the role of carbon-bearing melts in eclogitic domains and their relationship to fO₂ is inherently more complex and the subject of ongoing study.

A key characteristic that makes diamond such a powerful tool for studying the mantle is that once grown, they do not chemically re-equilibrate with their surroundings. This means their carbon isotopic composition, as well as their potential load of nitrogen, hydrogen, boron and nickel impurities, directly reflect their growth environment. Given that diamond growth events have been dated as far back as ~3.5 Ga (Figure 1: see Supplementary Material Table S1 and references therein), no other mineral can provide such detailed insight into mantle fluid processes and the deep carbon cycle throughout Earth's evolution.

The historical database of diamond carbon isotope compositions (e.g., Cartigny et al., 2014) is based on the bulk analysis of diamond fragments. Bulk C-isotopic analyses,
combined with FTIR nitrogen analyses, have been successfully employed in some instances to track Rayleigh fractionation processes during diamond formation (Cartigny et al., 2001; Thomassot et al., 2007; Stachel et al., 2009). The homogeneity of carbon isotope values (± 2 ‰) in the majority of diamonds studied has been shown by comparing data from multiple fragments of the same samples (see figure 5 of Cartigny et al., 2004). Stepped combustion analysis of individual samples also revealed limited heterogeneity in carbon isotopes values (Mikhail et al., 2014) but lacked spatial resolution. The development of secondary ion mass spectrometry (SIMS) methods applicable to measuring carbon isotopes of diamonds *in situ* from very small analytical volumes (Harte & Otter, 1992; Fitzsimmons et al., 1999; Hauri et al., 2002), showed clearly that a few gem diamonds had very heterogeneous carbon isotopic values depending on their growth history (Harte et al., 1999; Bulanova et al., 2002; Zedgenizov & Harte, 2004). Considering the observed diamond internal heterogeneity on the micron scale (e.g. Fitzsimmons et al., 1999), the most accurate approach to determine the possible fluid speciation in diamond-forming fluids is to measure spatially resolved growth-directional trends (i.e. core to rim) in the carbon isotopic and nitrogen concentration systematics in individual diamonds (e.g. Smart et al., 2011; Wiggers de Vries et al., 2013; Smit et al., 2016; 2019a).

This paper reports the results of a major SIMS carbon isotope study, carried out on 144 diamond samples, spanning a large range of geological time (~3.5 to 1.4 Ga for P-type diamonds, 3.0 to 1.0 Ga for E-type diamonds). The sample collection studied provides a direct connection between the ages of diamond formation, obtained from inclusion studies, and the carbon isotope values of the host diamonds. The spatial resolution of the analyses, together with the readily identifiable core to rim growth directions evident in most samples, allows trends in the variability of C isotopic values and nitrogen concentrations to be examined, with the goal to search for possible systematic shifts in the speciation of carbon-bearing mantle fluids through time.

## Samples
183 Fragments of 144 diamond crystals, from nine localities in Canada, Australia,
184 Southern Africa and Russia, have been analysed for their carbon isotopic values and
185 nitrogen contents. Of those, 88 are defined as having a peridotitic association (P-type)
186 based upon their inclusion paragenesis, and 56 formed in eclogitic substrates (E-type).
187 The details of the sample source locations are provided in Table 1, along with
188 references to the original studies that dated the inclusions. With the exception of 18
189 samples from Diavik (Slave Craton, Canada; Donnelly et al., 2007), 11 from Wawa
190 (Superior Craton, Canada; Stachel et al., 2006), 13 from Ellendale (Kimberley Craton,
191 Australia; Jaques et al., 1989; 1994), and 10 from Argyle (Kimberley Craton,
192 Australia; Stachel et al., 2018), the remaining majority of the diamonds studied here
193 are fragments, recovered after breakage to obtain inclusions for dating studies.
194 Therefore, they can be directly linked to the age of the diamond growth event
195 documented in the original dating study.

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197 For peridotitic diamonds from De Beers Pool, Udachnaya and Venetia, diamond
198 formation ages come from radiogenic isotope analysis of pooled garnet inclusions
199 (Richardson et al., 1984; Richardson & Harris, 1997; Richardson et al., 2009). For
200 eclogitic diamonds from De Beers Pool (Bultfontein, De Beers, Dutoitspan, and
201 Wesselton kimberlites), Jwaneng and Orapa, ages were obtained by Re-Os analysis of
202 individual sulphide inclusions (Richardson et al., 2001; 2004; Shirey et al., 2009). In
203 the case of the Diavik, Argyle and Ellendale diamonds, which are not directly derived
204 from age dating studies on these diamond populations (Aulbach et al., 2009;
205 Richardson, 1986; Smit et al., 2010), the inclusion compositions and N aggregation
206 characteristics are similar to the original suites used for geochronology and thus the
207 published ages are applied. The diamonds from Wawa have not been dated directly
208 but are constrained by the age of the rocks they are found in (~2.7 Ga; Stachel et al.,
209 2006) as being Neoarchean, which obviously represents a minimum age. All of the
210 diamonds (with the possible exception of two Wawa diamonds with majoritic garnets)
211 are lithospheric (140 – 250 km) in origin.

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213 Whilst Thomassot et al (2009) argue that at Jwaneng silicate- and sulphide-bearing
214 diamonds form in separate events, the presence of silicate and sulphide inclusions (in
215 part even touching; Richardson et al., 2004) in the same P and E-type diamonds at
216 Jwaneng strongly suggests coeval formation. Even if silicate- and sulphide-bearing
217 diamonds formed during different growth events, this would have very limited
218 implications for this study because all of the peridotitic diamond samples that are
219 directly tied to the dating studies contained only silicate inclusions, while all directly
220 dated eclogitic samples are sulphide-bearing. For the three P-type sample collections
221 with indirect ages (Diavik, Ellendale and Wawa), only 4 of the diamonds contained
222 sulphides (1 from Diavik, 3 from Ellendale). For the E-type sample collections with
223 indirect ages (Diavik and Argyle), all 10 Argyle samples are silicate-bearing (in
224 keeping with the dating study which used silicates) and 4 of the 9 Diavik samples are
225 silicate-bearing (not in keeping with the sulphide derived age). So with the exception
226 of 8 samples, the remainder are all in keeping with the inclusion type used to obtain
227 the age. The peridotitic samples represent silicate-bearing diamond formation events,
228 while excluding Argyle, the remaining eclogitic samples represent sulphide-bearing
229 diamond formation events.
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231 Given that all of the diamonds studied here are fragments, they do not record the
232 crystals' entire growth history. During selection and mounting, every effort was made
233 to discern an outer crystal surface, so that the subsequent analysis would have a
234 context of growth direction and data transects would represent the evolution of the
235 crystal's growth. Data from 96 samples have confirmed growth direction context,
236 supported by cathodoluminescence (CL) imaging of their growth structure.
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238 Fibrous and polycrystalline diamonds are assumed to form during unusually rapid
239 crystal growth driven by strong supersaturation in carbon. Fibrous diamonds are most
240 likely of young (Phanerozoic) geological age (Sunagawa, 1984; Gurney et al., 2010),
241 while no polycrystalline diamonds have ever been dated. Such young and undated
242 diamond types cannot be used to constrain variations in the carbon isotopic
243 composition of diamonds through time, and they are therefore not included in the
244 present study, nor are the literature data derived from those types of samples.
245 Similarly, microdiamonds (generally defined as <0.5 mm) are also not included as age
246 data are not available.
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248 Methods

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Mount preparation, CL imaging and secondary ion mass spectrometry (SIMS) were carried out at the Canadian Centre for Isotopic Microanalysis (CCIM), University of Alberta. Four SIMS mounts were created and analyzed in this study (M1369, M1406, M1410, M1520). The rough diamond fragments were cast together in groups of 20 to 50 in 25 mm epoxy discs, and ground and polished using electroplated diamond pads on rotary equipment to expose flat sections through grain interiors. These epoxy discs containing polished diamonds were trimmed to mm-scale blocks and pressed into SIMS indium mounts (M1369, M1410, M1520) along with a piece of CCIM diamond reference material (RM) S0270 and vitreous carbon S0233A. For epoxy mount M1406, reference materials were pressed into an integrated indium slot within an epoxy mount. In all cases, the RMs and unknowns were spaced closely together on each mount. The SIMS mounts were coated with ~20 nm of Au prior to scanning electron microscopy (SEM) using a Zeiss EVO MA15 instrument, operating at 15 kV and ~3 nA beam current. CL SEM images were obtained using a parabolic mirror coupled to a high-sensitivity, broadband photomultiplier detector. The mounts were subsequently coated with additional Au prior to SIMS analysis.

Isotopes of carbon ($^{13}$C/$^{12}$C) were determined using the IMS-1280 multi-collector ion microprobe at CCIM. Analytical methods and reference materials are detailed in Stern et al. (2014). Primary beam conditions included the use of 20 keV $^{133}$Cs⁺ ions focused to a diameter ~ 15 µm and beam currents of ~ 2.0 nA. The primary beam was rastered across a 20 x 20µm area for 30 s prior to analysis, to clean the surface of Au and contaminants, and implant Cs. The normal incidence electron gun was not utilized, as most diamonds are conducting under ion bombardment. Negative secondary ions were extracted through 10 kV to the grounded secondary column (Transfer section). Automated tuning of the secondary ions in the Transfer section preceded each analysis. Secondary ion collection conditions for C-isotopes included an entrance slit width of 100 µm, field aperture of 5 x 5 mm, a field aperture-to-sample magnification of 100 x, and a fully-open energy slit. Both $^{12}$C⁻ and $^{13}$C⁻ were analyzed simultaneously in Faraday cups (L'2 and FCs using 10¹⁰ Ω and 10¹¹ Ω amplifier circuits) at mass resolutions of 2000 and 2900, respectively. Faraday cup baselines were determined once at the start of the session. Mean count rates for $^{12}$C⁻ and $^{13}$C⁻ were typically 1.1–1.6 x 10⁹ and 1.2–1.8 x 10⁷ counts/s, respectively, determined over a 75 s counting interval, with total analysis time of 210 s. The analytical sequence
interspersed measurements of unknowns with the natural diamond S0270, having
$\delta^{13}C_{VPDB} = -8.88 \pm 0.10$ ‰ in a 4:1 ratio. Instrumental mass fractionation (IMF) for
$^{13}C^{\cdot}/^{12}C^{\cdot}$ is typically about -24 ‰, determined precisely for each session from
utilizing all the replicate analyses of S0270 diamond time-corrected back to the start
of the sub-session. The standard deviation of the $^{13}C^{\cdot}/^{12}C^{\cdot}$ values per session was
$\pm 0.05 - 0.07$ ‰ after correction for systematic within-session IMF drift of $\le 0.5$‰
over several hours. Uncertainties of individual $\delta^{13}C_{VPDB}$ analyses propagate within-
spot ($\sim \pm 0.04$‰, $1\sigma$), between-spot ($\pm 0.05$‰, $1\sigma$), and between-session errors
($\pm 0.01$‰, $1\sigma$), and average $\pm 0.14$ ‰ ($2\sigma$).

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Nitrogen abundances were determined for some diamonds immediately following C-
isotope analysis from the identical spot locations. The primary beam was rastered for
30–60 s prior to analysis to clean the adjacent area. Secondary ion collection
conditions included an entrance slit width of 45 µm, field aperture of 3 x 3 mm, and
energy slit width of 40 eV transmitting low-energy ions. The molecular ion ratios of
$[^{12}C^{14}N^{\cdot}]/[^{12}C^{12}C^{\cdot}]$ or $[^{13}C^{14}N^{\cdot}]/[^{12}C^{13}C^{\cdot}]$ were analyzed simultaneously using either a
Faraday cup–Faraday cup (L’2–FC2) or Faraday cup–electron multiplier combination
(L’2–EM). Mass resolution was ~6300 – 7000 for CN⁻ molecular ions, 2100 for
$^{12}C^{12}C^{\cdot}$, and 6000 for $^{12}C^{13}C^{\cdot}$. Electron multiplier counts were corrected for
background and deadtime (40 ns). Total analysis time was 200 – 240 s, including pre-
analysis raster, secondary ion centering, and peak counting time of 25 – 50 s. The
sensitivity factor for N in diamond was determined by analysis of S0280 diamond
having nitrogen concentration ([N]) = 1670 at. ppm located on a separate mount
(Stern et al., 2014), or analysis of S0270 diamond with ~2150 at. ppm located on the
same mount. Uncertainties in concentration are ±5 % absolute (95% conf.)

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## 311 Results

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In total, 908 combined carbon isotope and nitrogen concentration measurements have
been obtained for peridotitic (n = 504) and eclogitic (n = 404) inclusion-bearing
diamonds. The entire dataset is provided in the **Supplementary Material**, grouped
according to inclusion paragenesis (Table S2). Histograms of the $\delta^{13}C$ data (Figure 2)
are given per location and age, while the statistical characteristics are provided in
Table 2. The δ¹³C data range from -17.1 to -1.9 ‰ (P -8.4 to -1.9 ‰; E -17.1 to -2.1 ‰) and N contents range from 0.4 to 3073 at. ppm (P 0.4 to 3073 at. ppm; E 1 to 2661 at. ppm).

As a different number of analyses were recorded per sample, there is potential for a sample bias to become incorporated in the population data through overrepresentation of one sample's characteristics relative to another sample. For example, in the literature the presence of highly negative (<-15 ‰) portions within a diamond are commonly studied by SIMS in much greater detail than the often more dominant mantle like (~5 ‰) volumes as they are of greater potential interest in defining the source of the carbon, resulting in a strongly bimodal distribution in some datasets, which is proportionally misleading. To check for this within this study, the average of each sample's C and N characteristics were calculated as well as their median values, and then averaged to compare against the whole population per location. It is clear from the values shown in Table 2 that overrepresentation is not a significant issue for the δ¹³C and N concentration data reported here.

From the CL images it is possible to tell that the vast majority of diamonds with sufficient nitrogen content to luminesce are dominated by octahedral growth. Some samples from Diavik (n = 3), Venetia (n = 1), De Beers Pool (n = 1), Jwaneng (n = 4) and Orapa (n = 1) also reveal some portions of cuboid growth, identified by having a CL response consistent with previous studies of cuboid growth (e.g. Welbourn et al., 1989; Howell et al., 2012). However, as there is no evidence of carbon isotopic fractionation between cuboid and octahedral sectors in mixed-habit diamonds (Howell et al., 2013) this observation does not affect the present data. [N.B. No fibrous diamond material was analysed within this study.] The CL images also reveal that the sample collection is quite varied in terms of the growth history of individual diamonds (Figure 3). Nearly half of the diamonds studied here (71 of 144) clearly exhibit either pulsed growth (repetitive subtle variations in CL response; not clear if single or multiple growth events) or clear discrete multiple growth events (noted by a marked difference in CL response as well as resorption features). Complex histories involving multiple growth stages are not necessarily accounted for in the defined ages of the samples, given the rarity of multiple inclusion-bearing diamonds with inclusions in more than one growth zone and that many of the P-type diamond
formation ages were determined from pooled silicate inclusions from many diamonds, as opposed to techniques of dating individual sulphide or silicate inclusions (e.g. Pearson et al., 1998; Timmerman et al., 2017).

Four of the samples exhibiting multiple growth events (all E-type; ddmi-37, JWR7, JWR13, JWR22) show a jump in the δ¹³C values between two layers of >3 ‰; the lowest isotopic values are found in the core of the stone, with subsequent overgrowth having less negative δ¹³C values. If the δ¹³C data for these four samples are removed from the dataset presented in Table 2, then the average Δ values (where Δ represents the variation within individual samples) for E-type diamonds from Diavik (1.9 Ga) and Jwaneng (3 Ga and 2.1 Ga) change to 0.4 ‰ (from 1.0 ‰), 1.5 ‰ (from 3.7 ‰) and 1.4 ‰ (from 2.5 ‰), respectively. This reduces the average Δ value of the E-type population to 1.2 ‰ from 1.6 ‰ (calculated from the average Δ values of the nine diamond growth events from the five localities, as opposed to the average Δ values of all 52 individual samples). Given that the average Δ value of the P-type population is 0.8 ‰, this implies that intra-sample variation is typically slightly higher in E-type diamonds than in the P-type diamonds studied here.

To investigate whether the presence of multiple growth events in half of the diamonds affects the dataset as a whole, the data from samples revealing only a single growth event (Table 3) were plotted as histograms (for the P-type and E-type populations) and compared against the total dataset (Figure 4). The single-phase growth peridotitic data (52 samples out of the 88) show a very similar distribution to the total P-type data set. The mean (-4.7 ‰) and median (-4.8 ‰) values of the single growth data are only slightly higher for than the total dataset (-4.8 ‰ and -4.9 ‰, respectively), with their standard deviations being very similar; 1.1 (total dataset) to 1.2 (single growth). For the E-type single-phase growth data (21 samples out of 56), the second mode in the δ¹³C distribution at -12 to -9 ‰ is much more prominent than in the total dataset. The main reason for this is that the second mode around -10 ‰ is predominantly governed by the Argyle samples, which show less evidence for multiple growth events (4 out of 10 samples), while the Orapa (11 out of 13 samples) and Jwaneng (11 out of 16 samples) data, which are centred around -5 ‰, are greatly reduced (only 51 out of 233 analyses remain). Despite this more prominent bimodality in the single
growth data set, the mean of the single growth data is slightly higher (-6.6 ‰) than for
the total dataset (-6.7 ‰) whilst the median is slightly lower (-5.9 ‰ versus -5.7 ‰).
From this comparison, we conclude that the P-type dataset may be interpreted within
a temporal context without significant concern that the carbon signal could be biased
by multiple growth events being mis-classified into single time intervals. For the E-
type samples, the coupling of the published age data to the new carbon data is clearly
less robust due to a prevalence of multi-stage growth; this will need to be kept in mind
when interpreting the dataset as a whole, although the differences between the mean
and median of the single growth subset compared with the total dataset are small.

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396 Literature Data

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In the discussion section below, the P and E-type data from this study are compared
with that from the literature. Literature data predominantly represent “bulk” (typically
fragments of ≤1 mg) combustion analyses. We compiled a database containing ~2600
monocrystalline samples (i.e. excluding fibrous, polycrystalline and micro-diamonds),
which subsequently is referred to as the “total literature data”.

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Figure 1 shows that in addition to the present work there are more than 20 diamond
growth events (excluding fibrous growth) for which there is a geological age. To
expand both the number of growth events discussed and to compare the data
presented here with published data on the same deposits, we utilized data from the
literature. The key criterion for using literature data was that the mineral inclusion
paragenesis could be defined and clearly tied to a specific dated growth event. For
example, Finsch has an age for P-type diamond growth at 3.3 - 3.5 Ga (Richardson et
al., 1984) and for E-type growth at 1.5 Ga (Richardson et al., 1990), so carbon isotope
data for diamonds from this mine of known paragenesis (e.g. Appleyard et al., 2004;
Palot et al, 2013) can be attributed to either age. Conversely, Premier / Cullinan has
multiple growth ages for peridotitic paragenesis diamonds (see Figure 1) and since the
carbon isotope data cannot be tied to specific growth events, data from Cullinan
diamonds are not considered here.

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A few studies with age constraints have utilized SIMS analysis, with multiple points per sample, namely the Zimmi (Sierra Leone) data from Smit et al. (2019a), the Koffiefontein data (Timmerman et al., 2019a) and some of the Finsch data (Palot et al., 2013), the latter two both from South Africa. In these cases, the number of samples is only 5, 3 and 10, but the number of individual analyses equate to 179, 12 and 113, respectively. The numbers of analyses and data statistics for the literature data discussed below are provided in Table 4, while all of the original data are compiled in the Supplementary Material (Table S3). This subset of literature data (combining ‘bulk’ and SIMS data from age constrained deposits) will be referred to as the “dated literature data”.

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## Discussion

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Carbon isotope systematics

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The range of carbon isotope values recorded in the global bulk diamond database (i.e. total literature data) extends from -41 ‰ to +5 ‰ (see Cartigny et al., 2014 and references therein). The tight clustering of δ¹³C data for peridotitic diamonds (95 ‰ are within -5 ± 2.2 ‰) coincides with δ¹³C data from other mantle sources (e.g. carbonatite and kimberlite carbonates, mid-oceanic ridge basalts, volcanic CO₂ emissions, mantle xenoliths; see Deines (2002) and references therein), leading to a general agreement that a δ¹³C value of around -5 ‰ reflects the carbon isotopic signature of the upper mantle. The negative skewness of δ¹³C data for eclogitic diamonds (~30 ‰ are <-10 ‰) has been the source of debate for decades, and three different explanations have been proposed:

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(1) Primordial heterogeneity within the mantle (Deines et al., 1993).

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(2) The fractionation of carbon isotopes in an open system. This can occur either prior to diamond precipitation, where decarbonation may cause CO₂ enriched in ¹³C to escape, leaving behind a ¹³C-depleted fluid (Cartigny et al., 1998; 2001), or it may be a consequence of carbon isotope fractionation during diamond precipitation in a fluid-limited system (Deines, 1980).
(3) The recycling of crustal carbon via subduction, which has undergone
fractionation by near-surface process to produce a very large range of carbon
isotope values (Sobolev and Sobolev, 1980; Milledge et al., 1983; Kirkley et
al., 1991).

The concept of primordial heterogeneity within the mantle was based on the
observation that a single kimberlite could sample many sub-populations of diamonds,
with no apparent relationship between their physical and chemical characteristics
(Deines et al., 1993). It was therefore suggested that the heterogeneity observed in the
carbon isotope values was primordial, a signature acquired during Earth's accretion
and not subsequently homogenized by mantle convection. While support from this
model comes from similarity in the range of δ¹³C data obtained from carbon in
meteorites and eclogitic diamonds (Deines 1980), it is implausible that ¹³C depleted
primordial heterogeneities should be restricted almost exclusively to metabasaltic
diamond substrates. As such, this hypothesis will be excluded from further discussion
here, and the focus will be on understanding the arguments for and against the second
and third hypotheses.

Fractionation of carbon isotopes, both prior to and during diamond formation, have
been a significant part of the discussion surrounding the negative skewness of the
eclogitic diamond δ¹³C record. Cartigny et al. (1998; 2001) proposed that carbon
isotopic fractionation would be driven by the separation of CO₂ from the diamond-
forming fluid/melt prior to diamond crystallisation, depleting the fluid/melt in ¹³C.
While this process has not been ruled out, it has been calculated that only very small
amounts (<1 %) of the residual fluid/melt reaches a δ¹³C value of <-14 ‰, and almost
none <-20 ‰ (Smart et al., 2011). So, it is thought unlikely that this process alone can
account for the eclogitic diamond δ¹³C record.

The case for fractionation processes occurring during diamond growth are based upon
equilibrium fractionation factors for carbon isotopes at mantle temperatures, which
have been derived from theoretical calculations, experimental determination and
empirical measurements (Chacko et al., 2001; Satish-Kumar et al., 2011; Reutsky et
al., 2012; 2015; Horita & Polyakov, 2015). Diamond growth through redox reactions
(i.e. either the reduction of oxidized carbon species, or the oxidation of reduced
carbon species) produces characteristic δ¹³C trends depending on the redox speciation in the fluid/melt (see Cartigny et al., 2014). Diamonds growing from an oxidized fluid/melt, containing CO₂ or CO₃²⁻, will be offset to lower δ¹³C values relative to the source (the exact amount depends on the isotope fractionation factor between the fluid carbon species and diamond), leading to successive enrichment in ¹³C (Figure 5A, & B). In a fluid-limited system, the resultant diamond population would exhibit a positive skewness in the δ¹³C distribution. Conversely, diamonds growing from a reduced fluid/melt containing CH₄, will be initially offset to heavier δ¹³C values relative to the source, but in a fluid-limited system will be successively enriched in ¹²C (Figure 5C). Empirical support for the fractionation hypothesis has come from carbon isotope data obtained from both individual diamonds by SIMS (oxidized fluid/melt – Zedgenizov et al., 2006; Smart et al., 2011; Palot et al., 2013; Mikhail et al., 2014) and diamond populations via bulk analyses (reduced fluid/melt – Thomassot et al., 2007), while the skewness of population distributions have also been discussed (e.g. Deines, 1980; Stachel & Harris, 2009).

Recently, the effectiveness of redox reactions to account for diamond precipitation in peridotitic substrates has been questioned. Luth & Stachel (2014) showed that the redox buffering capacity of peridotitic mantle is insufficient to account for economic quantities of diamonds to form. CHO fluids within depleted peridotitic mantle with compositions near the water-maximum would not contain just one carbon species but both oxidized and reduced (see Figure 1 of Stachel et al., 2017a; Stachel & Luth, 2015). Rayleigh isotope fractionation in multi-component systems is more complex than in systems with a single carbon species; their modeling showed that regardless of the dominant carbon species, diamond always progresses along ¹³C enrichment paths (Figure 5D). The predicted absence of progression towards more negative δ¹³C values even for fluid mixtures dominated by reduced carbon species (CH₄/CO₂ > 1), was supported by SIMS data for Marange (Zimbabwe) diamonds that contained CH₄ micro-inclusions yet revealed core-to-rim trends towards heavier δ¹³C values (Smit et al., 2016; Stachel et al., 2017a). It is important, however, to highlight the limitations and likely oversimplification of this model. Firstly, it is only truly applicable to harzburgites (in lherzolites and eclogites conditions would be above their solidi, therefore generating more complex melts), and secondly, pure CHO fluids are likely a simplified starting point for the model - they have only been observed as inclusions in
diamonds very rarely. Analysis of more typical micro-inclusions containing diamond-forming fluids/melts shows they contain significant proportions of dissolved material (Weiss et al., 2009), which presently are not considered in this model. However, thermodynamical modelling of hydrous solutions in equilibrium with peridotite or eclogite at high pressures and temperatures up to 1000 °C (Sverjensky and Huang, 2015; Huang and Sverjensky, 2019) does predict the coexistence of a variety of oxidized and reduced species together with high contents of major oxides.

The third hypothesis, which invokes recycled carbon as the source of the varied δ¹³C values, is based around a subducted nature of eclogitic diamond substrates (e.g. Helmstaedt et al., 1972). Altered oceanic crust (AOC; the igneous portion of oceanic crust) shows considerable C isotopic variability. A recent study by Li et al. (2019) suggests the AOC contains two carbon isotopic end-members; (1) a high δ¹³C (~0 ‰) carbonate component that forms during low-temperature alteration of the oceanic crust, and (2) isotopically light biogenic carbonate. However, Valley & O'Neill (1981) have shown that very negative δ¹³C values can be formed through fractionation (~600 – 800 °C) and may not necessarily require biogenic origins (see Etiope & Sherwood Lollar, 2013). The occurrence of low δ¹³C values in sublithospheric diamonds, whose inclusions have been shown to represent a snapshot of deep subduction (e.g. Deines et al., 1991; Tappert et al., 2005; Brenker et al., 2007; Walter et al., 2011) provides empirical support for this subduction model, along with oxygen isotope analyses of eclogitic garnet inclusions that clearly fall outside the mantle range (Ickert et al., 2013; 2015; Schulze et al., 2013; Burnham et al., 2015). Ickert et al. (2013) interpreted their δ¹³C - δ¹⁸O data to represent interaction (not strictly two-component mixing) between fluids carrying 'mantle-like' carbon (i.e. δ¹³C ~-5 ‰) and subducted substrates with non-mantle-like δ¹⁸O; the only portion of former AOC that carries enough carbon to not become overwhelmed in its carbon isotope signature by the mantle-derived fluids are highly altered, and hence carbon-enriched, former pillow basalts, where strongly negative δ¹³C is invariably associated with high δ¹⁸O (δ¹⁸O ≥ 6 ‰; Ickert et al. 2013). Li et al. (2019) propose that this link between altered oceanic crust, mantle fluids, diamonds and their inclusions is also capable of explaining the apparent decoupling of C and N isotopes in diamonds (Mikhail et al., 2014; Hogberg et al., 2016).

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In the present study, we will provide an impartial interrogation of the data reported, along with a comparison with literature data, to evaluate how the above factors fit within the realms of mantle fractionation and subduction models.

## *Paragenetic Variations in the Diamond Record*

The $\delta^{13}\text{C}$ mode of both the P- and E-type paragenetic datasets is $-5.0 \pm 0.5 \text{ ‰}$; 39% of this study's data falls within this tight range (37% for P-type, 41% for E-type) along with 33% of the *total literature data* (obtained through bulk combustion analysis). The proportions of data included increase significantly when only increasing the modal range slightly; 61% of this study's data fall within $-5.0 \pm 1.0 \text{ ‰}$ (68% for P-type, 53% for E-type) along with 52% of total literature data; 81% of this study's data fall within $-5.0 \pm 2.0 \text{ ‰}$ (90% for P-type, 70% for E-type) along with 73% of literature data. The isotopically light tail observed in the data is clearly confined in this study to the E-type diamonds, with the lowest P-type $\delta^{13}\text{C}$ value being only -8.4 ‰.

Our extensive SIMS study (with little or no bias towards individual diamonds that are isotopically heterogeneous and/or depleted in $^{13}\text{C}$) has produced a dataset that is largely in agreement with the *total literature database* obtained through bulk combustion analyses. This provides significant support to the utility of bulk $\delta^{13}\text{C}$ data. Bulk combustion averages out internal variations from multiple growth layers, potentially missing out on reporting the full range of carbon isotope values present. However, given that 66% of our samples showed $<1 \text{ ‰}$ internal $\delta^{13}\text{C}$ variation, with 86% $<2 \text{ ‰}$ and 96% $<3 \text{ ‰}$, it is clear that the bulk analysis of diamond fragments will generally yield a good approximation of their carbon isotopic composition.

While both the P and E-type datasets have a mode around -5 ‰, it is clear from the population statistics, and from previous studies, that eclogitic diamonds show more variation and a greater prevalence of $^{13}\text{C}$ depleted isotopic values. The standard deviation about the mean for the E-type data set ($\pm2.5 \text{ ‰}$) is more than double that for the P-type data ($\pm1.1 \text{ ‰}$) and the E-type distribution is skewed to lower $\delta^{13}\text{C}$ values (-1.3; Pearson's Coefficient of Skewness, see **Table 3**) compared to a negligible
positive skewness of the P-type data (0.2). This is broadly in keeping with the literature data. The P-type literature data are, however, far more tightly distributed around the -5 ‰ mode and nearly 34 % of published E-type δ¹³C data fall below -10 ‰ (only ~16 % in this study), compared to only around 1 % of P-type diamonds (0 % in this study). The average variation observed in individual P-type diamonds in this study is 0.85 ‰, compared to a value of 1.53 ‰ in the E-type diamonds (which includes the 4 samples with highly negative cores). Therefore, E-type diamonds typically do exhibit more internal variation in δ¹³C values compared with P-type diamonds.

The E-type dataset's distribution has a more bimodal appearance than the literature data (see Figure 4 of Cartigny et al., 2014). The main reason for this is the over-representation of data from Argyle. With its prominently ¹³C-depleted distribution (68 % of the Argyle data in this study is lower than -9 ‰, for the literature it is ~86 %) it is over-represented in this study (18 %) compared to the literature (~9 %). However, Argyle is not the only deposit in the world to have a signature of predominantly ¹³C depleted diamond carbon. Guaniamo (Venezuela), Jagersfontein (South Africa), Dachine (French Guiana) and Jericho (Canada), all have population modes that are even lower than seen in Argyle diamonds (see Figure 5 of Cartigny et al. (2014) and references therein), so the bimodality may persist even if the dataset is expanded.

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## *Isotopic Trends within Individual Diamonds*

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The advantage of *in situ* SIMS analysis of diamond is the power to reveal isotopic core-to-rim trends within individual samples, which can provide evidence of Rayleigh fractionation processes and potentially reveal the speciation of the diamond-forming fluids. Diamond core-to-rim fractionation trends have been observed in several diamond suites (Zedgenizov et al., 2006; Thomassot et al., 2007; Smart et al., 2011; Palot et al., 2013; Wiggers de Vries et al., 2013; Petts et al., 2015; Smit et al., 2016; 2019a), however these trends are typically associated with small overall δ¹³C variations <3 ‰. When we consider the data from the present study, there are no convincing trends observed in any of the 144 samples examined in detail. **Figure 6**
shows typical traverses observed in the diamonds studied here, with non-systematic variations in δ¹³C and/or N contents. Two thirds of the samples analysed showed <1 ‰ variation in δ¹³C (one third <0.5 ‰). Overall, our data set shows that diamonds with recognizable fractionation trends are generally very rare. Despite a number of publications reporting clear fractionation trends (see above), our globally representative sample set documents that diamonds with systematic and significant co-variations in δ¹³C and [N] are the exception rather than the rule. This suggests that diamond is a more passive recorder of changing δ¹³C rather than being the cause of δ¹³C variability. The agreement in the modes between the bulk analytical data and our *in situ* data strongly supports this view and we conclude that significant closed system isotopic fractionation did not operate for the majority of individual growth events. This implies that diamond growth typically occurs in temporally and spatially localized systems with high excess of carbon in the fluid, i.e. relative to the mass of growing diamonds.

This conclusion has important implications for the discussion surrounding the syngeneity or protogeneity of inclusions in diamond (see Nestola et al. (2017) and references therein), and the ramifications on diamond growth ages obtained from these inclusions. If diamond inclusions are protogenetic, then in a process with high fluid:rock ratios, the likelihood of the inclusions being isotopically reset at the time of diamond formation is high. This would lead to inclusion ages being representative of the diamond growth event (i.e. being synchronous), irrespective of whether they are syngenetic. Although detailed study has shown that mineral inclusion size and residence temperature play an important role (Nestola et al., 2019). Isotopic resetting has been categorically shown for sulphide inclusions from Zimmi (Smit et al., 2016; 2019b) where inclusions in three different diamonds provide the same isochron ages, yet their Re-Os and sulphur isotopes both show that the sulphides were existing in the lithosphere prior to the diamond formation event.

The work of Stachel et al. (2017a) on Rayleigh isotopic fractionation in multicomponent systems (RIFMS) during isochemical precipitation showed that in water-maximum fluids within harzburgitic domains, fractionation could only account for variations of <1 ‰. This is in keeping with the lack of significant variation seen in the peridotitic diamonds of this and former studies, but is not the only potential
explanation (see following paragraph). In the simplest water-maximum RIFMS model, with only CO₂ and CH₄ as carbon-bearing species, the fluid speciation (X/[CO₂] = CO₂/[CO₂+CH₄]) and the dependent variable oxygen fugacity exert a strong control on the carbon isotope composition of the precipitated diamond, irrespective of growth mode (Figure 5D). For both isochemical and wall-rock buffered redox scenarios, the carbon isotope composition of the first precipitated diamond only depends on the X/[CO₂] of the fluid and its assumed starting composition. Carbon isotopic evolution of the fluid would differ depending on whether diamond formation occurs isochemically or through wall rock-buffered redox reactions and whether the system is fluid-limited or not. However, the variation is limited as the effective fractionation factor is the weighted average of the factors of CH₄ and CO₂, which oppose each other. As both species are consumed in equal amount the variation in fO₂ is also limited. The model shows that the first precipitated diamond varies by 3.7 ‰ in δ¹³C over a range of X/[CO₂] values from 0.9 to 0.1, which corresponds to a total increase in Δ log fO₂ (FMQ) of only about 0.8 log units. Assuming diamond-forming fluids with a constant mantle-like δ¹³C value of -5 ‰, it is, therefore, possible to account for the principal mode of -5 ± 1 ‰ in the global harzburgitic diamond record with a variation in fO₂ of just 0.4 log units, based on modelling of diamond formation at constant depth along a geotherm.

The water-maximum fluids model is somewhat simplified (see earlier discussion in *Carbon Isotope Systematics* section). However, as more realistic thermodynamic models of subsolidus fluids also predict the presence of CO₂, carbonates and more reduced organic species (e.g. Sverjensky and Huang, 2015; Tiraboschi et al., 2018) the conclusions of the above modelling should still be applicable. If dissolved carbonate joins the assemblage, it should act to further limit the fractionation. While subsolidus fluids are expected in harzburgitic rocks along the 40 mW/m² model geotherm, lherzolitic and eclogitic mantle would be above the hydrous solidus. Saline and carbonatitic melts similar to the high-density fluids trapped in fibrous diamonds may also infiltrate all these rocks. Still, as long as the fluids and melts carry both oxidized carbonate and/or CO₂ together with reduced species, fractionation is expected to be rather limited.
If diamond precipitation has only a limited ability to fractionate the carbon isotopic composition of mantle fluids, and if their isotopic composition reflects that of the fluid to within ±1 ‰, then, the pronounced mode of diamond δ¹³C values through time at about -5 ‰ leads to the conclusion that the speciation and consequently oxygen fugacity of diamond forming mantle fluids has been relatively consistent since ~3.5 Ga. This also means that the deep (sublithospheric?) mantle has provided fluids with near constant carbon speciation for the last 3.5 billion years.

At present there is no single model that is widely accepted, to account for the increased variability of δ¹³C values in eclogitic diamonds. The cause of this greater variability in the δ¹³C record of eclogitic diamonds may reflect either (1) diamond-forming fluids with significantly more varied carbon characteristics - stemming from subducted material containing highly variable biogenic and/or abiogenic carbon sources, or (2) a growth environment more prone to significant isotopic fractionation between the diamond and fluid/melt, or (3) a combination of both of these scenarios. We note that an isotopically light carbon surface reservoir was available on Earth for recycling to the mantle depths of diamond growth since 3.5 Ga (e.g. Des Marais, 2001) removing the need to produce this signal solely in the mantle by fractionation.

The Diamond Record Through Time

Figure 7 places the diamond carbon isotope record into a chronological context using data from this study and the *dated literature data*. The restricted nature of peridotitic δ¹³C values (as discussed previously) appears consistent through time, as does the occurrence of isotopically light carbon (< -10 ‰) in eclogitic diamonds. However, the mode of eclogitic δ¹³C values is still at -5 ‰, consistent with that of the peridotitic diamonds. Isotopically light carbon only represents a very small proportion of the data; 6 % of the data from this study and 10 % of the *dated literature data* (excluding the SIMS data) shown in Figure 7. The tail towards ¹³C-depleted carbon in the eclogitic diamonds also shows no signs of systematic variation through time (Figure 7), with its first occurrence in the record from 3 Ga onwards. With the oldest eclogitic diamonds being tied to the onset of plate tectonics (Helmstaedt et al., 2010; Shirey & Richardson, 2011), if the low δ¹³C carbon signature is related to a biogenic carbon
contribution in altered oceanic crust (Li et al., 2019), this would imply that this
contribution, with respect to both amount and δ¹³C values, has been fairly consistent
through 3 Ga. This is supported by the Precambrian marine carbonate isotope
database (Shields and Veizer, 2002). But the data in Figure 7 also shows that
isotopically light carbon, the potential key biomarker in altered oceanic crust, was not
recycled to mantle keel depths as extensively and consistently before 3 Ga as it was
after 3 Ga. Whether this was due to a geodynamic cause such as the absence of
eclogite recycling or a biologic cause such as the absence of biogenic components
deep in the oceanic crust or both is unknown. The important point is that the onset of
the temporal appearance of deeply-sourced, isotopically light carbon had only been
previously inferred from mineral inclusion paragenesis and general E-Type diamond
carbon isotope histograms. For the first time, this study has established the onset and
persistence of deep recycling of potentially biogenic carbon in the rock record by
direct analysis of the geochronologically dated carbon itself.

Looking at the δ¹³C and N data together provides additional insight into potential variations through time. Here we used multidimensional scaling (MDS) and principal component analysis (PCA) to better visualise similarities and differences in the data. MDS was only carried out on the δ¹³C data as a function of their locations, while PCA was carried out on the combined δ¹³C and [N] datasets (see **Supplementary Materials** for detailed information about this data processing methodology).

For the P-type data, MDS of the δ¹³C values shows strong overlap between the
various populations with one exception (Figure 8a). There is clear discrimination
between Wawa and the two older diamond growth events at Diavik and De Beers
Pool (Figure 8a), while the three younger growth events (Venetia, Udachnaya and
Ellendale) exhibit more spread on the x-axis and overlap with all three of the older
populations. This observation is repeated in the PCA plot (Figure 8b). Despite N
concentration being the most significant variable in creating the spread in the data (i.e.
it is weighted most heavily in principal component 1), the discrimination caused by
just the δ¹³C values is still retained in the PCA. This discrimination is the result of
δ¹³C data for Wawa diamonds having a mode at -4 ‰ and its distribution having the
most positive skewness of all the P-type populations, while the older diamonds from
Diavik and De Beers Pool have strong modes at -5 ‰ and distributions with more
negative skewness. Despite this one minor difference in the data from 6 populations, the PCA and MDS strongly reinforce the findings from the earlier interrogation of the data (Figures 2 and 7), that there is no significant variation in δ¹³C with geological time for peridotitic diamonds.

Despite diamond growth at Wawa only being constrained by a minimum age, it is interesting to consider the data in the broader context of the Superior Craton on which the diamonds occur. Diamonds from the Renard (Quebec), Kyle Lake and Attawapiskat (Ontario) kimberlites of the eastern and central Superior Craton, respectively, show a temporal variation in δ¹³C. The Neoproterozoic Renard and T1 (Kyle Lake) kimberlites, along with Wawa (Neoarchean), carry diamond populations with a mode in δ¹³C of -4 ‰ (Hunt et al., 2012; Smit et al., 2014; Stachel et al., 2006) while the Jurassic U2 (Attawapiskat) and Victor pipes exhibit a mode of -5 ‰ (Smit et al., 2014; Stachel et al., 2017b). Between the emplacement of the Kyle Lake and Attawapiskat kimberlites, the southern Superior craton experienced the major Midcontinent Rift event at 1.1 Ga, which is considered to have been diamond-destructive beneath the Attawapiskat area (Smit et al., 2014; Aulbach et al., 2018; Stachel et al. 2018). Globally, our data show no discernible variation of mantle carbon through time; yet, it is clear that detailed studies on the regional / craton scale have the potential to reveal subtle temporal variations. However, it is not possible to discern whether this change beneath the Superior Craton is caused by a small change in the starting carbon isotopic composition, or the carbon speciation (and the dependent variable oxygen fugacity) of the diamond-forming fluid.

While the MDS and PCA plots of the E-type data show more spread (Figure 9a & b) than their corresponding P-type plots, the data from Argyle appear to be a slight outlier. Even though the Argyle data are by no means fully discrete from any other data set, these are the only diamonds with data entirely outside the main cluster. Since the earliest proposals of subducted organic matter playing a role in diamond formation (Sobolev & Sobolev, 1980; Milledge et al., 1983), Argyle has been the primary case study to support this model (e.g., Jaques et al., 1989; Stachel et al., 2018) with its dominance of eclogitic inclusions (~90%) and the normal distribution of δ¹³C data around a mode of -11 ‰. Detailed study of the diamonds and their inclusions have led to the conclusion that eclogitic diamond growth predominantly occurred at unusually
high temperatures (~1250 – 1400 °C), indicating a depth at the very base of the lithospheric mantle (Bulanova et al., 2018; Stachel et al., 2018). Recent noble gas analyses also support a subducted origin for the Argyle eclogitic diamonds which typically have low R/Ra values (<0.5; where R/Ra=$^3$He/$^4$He$_{sample}$ / $^3$He/$^4$He$_{air}$) with the noble gas subduction signatures focused at the base of the lithosphere, suggesting less extensive fluid migration and interaction at higher levels with the sub-continental lithospheric mantle (Timmerman et al., 2019b). Argyle reflects a strong case for subduction-related diamond growth, but what potentially makes Argyle unique is the amount of subducted carbon involved. Whether there was extensive mixing at greater than average depths, of a large amount of isotopically light carbon with either mantle carbon or isotopically heavier subducted carbonates, it created a fairly homogenized diamond-forming fluid with an intermediate δ¹³C signature. This contrasts with the more common observation of a minor isotopically light (~25 ‰) δ¹³C signature coupled with a dominant mantle-like (~5 ‰) δ¹³C signature.

There are two deposits, Orapa and Jwaneng, in our E-type dataset that have multiple dated diamond growth events. This provides some insights into potential variations of carbon through time within small sections of the mantle. The PCA of Orapa data subdivided by age (Figure 9c) reveals some discrimination between the subpopulations, with potential mixing between two end-members. The 2.1 Ga samples have some overlap with the 3 Ga samples, but largely fall in a distinct group, while the 1 Ga samples seemingly spread between the two. Potential endmember (1) has more negative δ¹³C values (-8 to -12 ‰) coupled with lower nitrogen concentrations (<400 ppm), while endmember (2) has more mantle-like δ¹³C values (-4 to -8 ‰) and higher nitrogen concentrations (>400 ppm). This indicates potential variation in the source of diamond forming fluids beneath Orapa through time, in agreement with a similar conclusion by Timmerman et al. (2017) based upon carbon and strontium isotope analyses of Orapa and Letlhakane diamonds. Although Timmerman et al. (2017) discerned three potential endmembers, it is important to bear in mind that only 2 of the 13 Orapa samples in this present study did not show evidence of containing multiple growth events. Therefore, it is possible that the dating of individual growth zones combined with detailed SIMS analyses may reveal a clearer distinction as to identifying the potential sources of carbon through time at Orapa. Interestingly, the PCA and MDS processing of the Jwaneng data reveal no
such subdivision with age. In fact, Orapa, and nearby Letlhakane, (Timmerman et al., 2017) are the only deposits where a variation in the diamonds' carbon isotope record with time has been documented (N.B. this excludes the consideration of fibrous and polycrystalline diamond growth events, as well as those without multiple defined ages at a single location). Despite the subtle variations seen at Orapa and the offset to lower δ¹³C seen at Argyle, the remaining E-type data reveal no systematic variations in δ¹³C through time on a global scale. This applies to the occurrence of both the δ¹³C mode of -5 ‰ as well as the skewness of distributions to the more negative δ¹³C values.

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## Summary

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This comprehensive statistical interrogation of the diamond record for carbon isotope values and nitrogen content through time revealed that, despite potential local fluctuations, in general there is no systematic variation with time in the mantle carbon isotope record over 3 billion years. The mode δ¹³C of peridotitic diamonds has been at -5 (± 2) ‰ since the earliest diamond growth ~3.5 Ga, and it is also observed in the eclogitic diamond record since ~3 Ga. The skewed distribution of the δ¹³C in eclogitic diamonds to more negative values is also consistent through time, with no discernible global trends apparent. However the onset of recycled, isotopically light C that was suggested from Sm-Nd and Re-Os data on dated inclusions (Shirey & Richardson, 2011) is corroborated by a complete C isotope dataset.

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On a local level, it is clear that individual deposits can have carbon isotope distributions that are subtly (Wawa) or more obviously (Argyle) different to the global norm. There is also scope to reveal variations over time in the diamond populations at a single locality (Orapa). What is surprising, however, is that these variations are not more common. The δ¹³C range observed in eclogitic diamonds is a reflection of complex and varied growth environments, potentially involving more significant amounts of fractionation, and/or multiple, isotopically distinct carbon sources. A marked divergence from the global norm is seemingly very limited, with Argyle being the most notable (and most studied) exception. The more limited δ¹³C range observed in peridotitic diamond populations likely reflects a simpler growth environment, associated with the influx of mantle derived CHO-fluids, with less inherent variability
compared to subduction-related scenarios. This more stable and consistent environment potentially allows for more subtle variations in the δ¹³C data to be attributed to minor shifts in the chemistry of the diamond-forming fluids (e.g. Wawa, Kyle Lake and the Attawapiskat diamonds on the Superior Craton).

Despite this potential for variation and heterogeneity, mantle carbon as recorded by diamond has remained remarkably consistent over the course of Earth's history.

## Conclusions

This study has utilized detailed *in situ* SIMS analyses of a large collection of diamonds with established formation ages, providing insight into the deep carbon record over a 3.5 Ga time period, from various cratons worldwide. This work reveals a lack of discernible systematic variations in the global carbon isotope composition of Earth's mantle through time as recorded by peridotitic diamonds. Despite eclogitic diamonds having more varied C isotopic values on an individual crystal basis, on a deposit-scale basis and globally, their C isotopic variability is generally consistent overall and also lacks any discernible systematic temporal variation. While there are clear examples in both the P-type (e.g. Wawa) and E-type (e.g. Argyle) populations of individual deposits whose carbon records sit outside the global norm, the overall consistency of the diamond record is remarkable. Examples of temporal variation within individual diamond deposits (E-type diamond record at Orapa; Timmerman et al., 2017) and on the craton-scale (Archean versus Neoproterozoic P-type diamonds from the Superior Craton) do exist but are the exception. Despite these anomalies, the global δ¹³C diamond record for diamond growth in both peridotitic and eclogitic substrates is remarkably consistent through time.

The fact that the *in situ* dataset reported here provides very good agreement with the global “bulk” diamond record is a very important finding, which confirms earlier combustion studies of internal heterogeneity (Galimov, 1984; Cartigny et al., 2004; Mikhail et al., 2014). Despite single combustion analyses not recording internal heterogeneity within individual diamonds, the global bulk dataset is representative of the diamond record and interpretations based upon it have a solid foundation. A key reason for this agreement is that the typical variation of δ¹³C values in individual
diamond fragments sampled in this study is small, with two thirds of the samples varying by <1 ‰. Additionally, no clear fractionation trends were observed in any of the diamonds studied here, indicating that fluid-limited conditions during diamond growth are very rare and proportionally over-represented in the literature. Instead, diamond growth appears to typically occur in systems with excess of carbon in the fluid, precluding significant Rayleigh fractionation during continued diamond crystallization. A significant implication of this is that inclusions are very likely to be isotopically reset at the time of diamond formation in a fluid-rich environment, either through fluid-assisted equilibration/diffusion or more extensive partial melting, meaning that the radiogenic isotope systematics of diamond inclusions reflect formation ages irrespective of the inclusions being syngenetic or protogenetic with respect to diamond growth (Smit et al., 2016; 2019b; Nestola et al., 2019).

The global literature diamond database has a remarkably consistent mode in δ¹³C at -5 ‰ over 3.5 Ga. While the majority of samples in the database were not directly dated, this present study has provided the temporal context to show that this mode in the δ¹³C is also consistent through time. Focusing on peridotitic diamonds that mostly would have grown under subsolidus conditions (Stachel & Luth, 2015), the data from this study show a tight grouping around this mode (90% fall within -5 ± 2 ‰), which is also consistent with literature data. Modelling of this dataset through growth from water-maximum CHO fluids reveals that this range can be accounted for by only a small variation in the carbon speciation of the fluid (0.4 log units), which in turn reflects a remarkably consistent redox state of these diamond-forming fluids for 3.5 Ga.

Despite eclogitic diamond growth not necessarily occurring in a subsolidus environment (Stachel & Luth, 2015), the E-type diamond record also has a consistent δ¹³C mode of -5 ‰ (70% of data fall within -5 ± 2 ‰). Total δ¹³C variation in individual diamonds is generally minor (typically <2 ‰); when the internal variation is larger, it is often occurring due to multiple discrete growth events. Even when carbon isotope fractionation trends have been recorded in some rare diamond examples, the total variation has always been <3 ‰. This suggests that the environment for diamond growth is not conducive to generating significant variations through fractionation processes alone. In view of the strong negative skewness of the
E-type diamond record, down to values of ~-40 ‰ (De Stefano et al., 2009), this range of δ¹³C values is considered to be predominantly the result of variations in the carbon signal of the primary fluids/melt. The subducted nature of eclogites, containing some isotopically light abiogenic and/or biogenic carbon, is the prevalent model to explain the cause of the negative skew in eclogitic diamond δ¹³C values (Sobolev & Sobolev, 1980; Milledge et al., 1983; Kirkley et al., 1991).

The earliest eclogitic diamonds have been tied to the onset of subduction and plate tectonics (Helmstaedt et al., 2010; Shirey & Richardson, 2011). The appearance of isotopically light carbon at 3 Ga and its persistence in the eclogitic paragenesis after that time affirms a secular change in recycled carbon at this time. If this is indeed the case, then ultimately the net effect of subduction has not altered the ambient signal of the mantle's carbon record. Whether this is due to the net carbon isotopic composition of a subducted plate being about -5 ‰ (Shilobreeva et al., 2011), or the fact that recycled carbon is just a very minor contribution in the overall mantle carbon budget, remains to be proven.

## Acknowledgements

We thank the Deep Carbon Observatory (DCO) for funding the Diamonds and the Mantle Geodynamics of Carbon Group (DMGC) and the research reported in this paper. In particular, Erik Hauri was a long-standing enthusiastic supporter of diamond research within the DCO. Discussions with Andrew Steele among others were an important catalyst for this work. DH wishes to personally thank TS, FN and DGP for their continued support prior to and throughout this project. Many of the diamonds studied here were donated to JWH by the Diamond Trading Company (a member of the De Beers group of Companies); this generosity is greatly appreciated. Suzette Timmerman is thanked for providing her SIMS data for the literature database. Sami Mikhail and an anonymous reviewer, as well as editor Dmitri Ionov, are thanked for their comments which helped to improve the manuscript. TS acknowledges funding through a Natural Sciences and Engineering Research Council of Canada Discovery grant and through the Canada Research Chairs program.

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    <td>Ickert, R. B., Stachel, T., Stern, R. A., & Harris, J.W. (2015). Extreme 18O-enrichment in majorite constrains a crustal origin of transition zone diamonds. Geochemical Perspectives Letters, 1(1), 65-74.</td>
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    <td>1045</td>
    <td>Ickert, R. B., Stachel, T., Stern, R. A., & Harris, J.W. (2015). Extreme 18O-enrichment in majorite constrains a crustal origin of transition zone diamonds. Geochemical Perspectives Letters, 1(1), 65-74.</td>
  </tr>
  <tr>
    <td>1046</td>
    <td>Ickert, R. B., Stachel, T., Stern, R. A., & Harris, J.W. (2015). Extreme 18O-enrichment in majorite constrains a crustal origin of transition zone diamonds. Geochemical Perspectives Letters, 1(1), 65-74.</td>
  </tr>
  <tr>
    <td>1047</td>
    <td>Ickert, R. B., Stachel, T., Stern, R. A., & Harris, J.W. (2015). Extreme 18O-enrichment in majorite constrains a crustal origin of transition zone diamonds. Geochemical Perspectives Letters, 1(1), 65-74.</td>
  </tr>
  <tr>
    <td>1048</td>
    <td>Ickert, R. B., Stachel, T., Stern, R. A., & Harris, J.W. (2015). Extreme 18O-enrichment in majorite constrains a crustal origin of transition zone diamonds. Geochemical Perspectives Letters, 1(1), 65-74.</td>
  </tr>
  <tr>
    <td>1049</td>
    <td>Ickert, R. B., Stachel, T., Stern, R. A., & Harris, J.W. (2015). Extreme 18O-enrichment in majorite constrains a crustal origin of transition zone diamonds. Geochemical Perspectives Letters, 1(1), 65-74.</td>
  </tr>
  <tr>
    <td>1050</td>
    <td>Ickert, R. B., Stachel, T., Stern, R. A., & Harris, J.W. (2015). Extreme 18O-enrichment in majorite constrains a crustal origin of transition zone diamonds. Geochemical Perspectives Letters, 1(1), 65-74.</td>
  </tr>
  <tr>
    <td>1051</td>
    <td>Ickert, R. B., Stachel, T., Stern, R. A., & Harris, J.W. (2015). Extreme 18O-enrichment in majorite constrains a crustal origin of transition zone diamonds. Geochemical Perspectives Letters, 1(1), 65-74.</td>
  </tr>
  <tr>
    <td>1052</td>
    <td>Ickert, R. B., Stachel, T., Stern, R. A., & Harris, J.W. (2015). Extreme 18O-enrichment in majorite constrains a crustal origin of transition zone diamonds. Geochemical Perspectives Letters, 1(1), 65-74.</td>
  </tr>
  <tr>
    <td>1053</td>
    <td>Ickert, R. B., Stachel, T., Stern, R. A., & Harris, J.W. (2015). Extreme 18O-enrichment in majorite constrains a crustal origin of transition zone diamonds. Geochemical Perspectives Letters, 1(1), 65-74.</td>
  </tr>
  <tr>
    <td>1054</td>
    <td>Ickert, R. B., Stachel, T., Stern, R. A., & Harris, J.W. (2015). Extreme 18O-enrichment in majorite constrains a crustal origin of transition zone diamonds. Geochemical Perspectives Letters, 1(1), 65-74.</td>
  </tr>
  <tr>
    <td>1055</td>
    <td>Ickert, R. B., Stachel, T., Stern, R. A., & Harris, J.W. (2015). Extreme 18O-enrichment in majorite constrains a crustal origin of transition zone diamonds. Geochemical Perspectives Letters, 1(1), 65-74.</td>
  </tr>
  <tr>
    <td>1056</td>
    <td>Ickert, R. B., Stachel, T., Stern, R. A., & Harris, J.W. (2015). Extreme 18O-enrichment in majorite constrains a crustal origin of transition zone diamonds. Geochemical Perspectives Letters, 1(1), 65-74.</td>
  </tr>
  <tr>
    <td>1057</td>
    <td>Ickert, R. B., Stachel, T., Stern, R. A., & Harris, J.W. (2015). Extreme 18O-enrichment in majorite constrains a crustal origin of transition zone diamonds. Geochemical Perspectives Letters, 1(1), 65-74.</td>
  </tr>
  <tr>
    <td>1058</td>
    <td>Ickert, R. B., Stachel, T., Stern, R.A., & Harris, J.W. (2013). Diamond from recycled crustal carbon documented by coupled δ18O-δ13C measurements of diamonds and their inclusions. Earth and Planetary Science Letters, 364, 85-97.</td>
  </tr>
  <tr>
    <td>1059</td>
    <td>Ickert, R. B., Stachel, T., Stern, R. A., & Harris, J.W. (2013). Diamond from recycled crustal carbon documented by coupled δ18O-δ13C measurements of diamonds and their inclusions. Earth and Planetary Science Letters, 364, 85-97.</td>
  </tr>
  <tr>
    <td>1060</td>
    <td>Ickert, R. B., Stachel, T., Stern, R. A., & Harris, J.W. (2015). Extreme 18O-enrichment in majorite constrains a crustal origin of transition zone diamonds. Geochemical Perspectives Letters, 1(1), 65-74.</td>
  </tr>
  <tr>
    <td>1061</td>
    <td>Ickert, R. B., Stachel, T., Stern, R. A., & Harris, J.W. (2015). Extreme 18O-enrichment in majorite constrains a crustal origin of transition zone diamonds. Geochemical Perspectives Letters, 1(1), 65-74.</td>
  </tr>
  <tr>
    <td>1062</td>
    <td>Ickert, R. B., Stachel, T., Stern, R. A., & Harris, J.W. (2015). Extreme 18O-enrichment in majorite constrains a crustal origin of transition zone diamonds. Geochemical Perspectives Letters, 1(1), 65-74.</td>
  </tr>
  <tr>
    <td>1063</td>
    <td>Ickert, R. B., Stachel, T., Stern, R. A., & Harris, J.W. (2015). Extreme 18O-enrichment in majorite constrains a crustal origin of transition zone diamonds. Geochemical Perspectives Letters, 1(1), 65-74.</td>
  </tr>
  <tr>
    <td>1064</td>
    <td>Ickert, R. B., Stachel, T., Stern, R. A., & Harris, J.W. (2015). Extreme 18O-enrichment in majorite constrains a crustal origin of transition zone diamonds. Geochemical Perspectives Letters, 1(1), 65-74.</td>
  </tr>
  <tr>
    <td>1065</td>
    <td>Ickert, R. B., Stachel, T., Stern, R. A., & Harris, J.W. (2015). Extreme 18O-enrichment in majorite constrains a crustal origin of transition zone diamonds. Geochemical Perspectives Letters, 1(1), 65-74.</td>
  </tr>
  <tr>
    <td>1066</td>
    <td>Ickert, R. B., Stachel, T., Stern, R. A., & Harris, J.W. (2015). Extreme 18O-enrichment in majorite constrains a crustal origin of transition zone diamonds. Geochemical Perspectives Letters, 1(1), 65-74.</td>
  </tr>
  <tr>
    <td>1067</td>
    <td>Ickert, R. B., Stachel, T., Stern, R. A., & Harris, J.W. (2015). Extreme 18O-enrichment in majorite constrains a crustal origin of transition zone diamonds. Geochemical Perspectives Letters, 1(1), 65-74.</td>
  </tr>
  <tr>
    <td>1068</td>
    <td>Ickert, R. B., Stachel, T., Stern, R. A., & Harris, J.W. (2015). Extreme 18O-enrichment in majorite constrains a crustal origin of transition zone diamonds. Geochemical Perspectives Letters, 1(1), 65-74.</td>
  </tr>
  <tr>
    <td>1069</td>
    <td>Ickert, R. B., Stachel, T., Stern, R. A., & Harris, J.W. (2015). Extreme 18O-enrichment in majorite constrains a crustal origin of transition zone diamonds. Geochemical Perspectives Letters, 1(1), 65-74.</td>
  </tr>
  <tr>
    <td>1070</td>
    <td>Ickert, R. B., Stachel, T., Stern, R. A., & Harris, J.W. (2015). Extreme 18O-enrichment in majorite constrains a crustal origin of transition zone diamonds. Geochemical Perspectives Letters, 1(1), 65-74.</td>
  </tr>
  <tr>
    <td>1071</td>
    <td>Ickert, R. B., Stachel, T., Stern, R. A., & Harris, J.W. (2015). Extreme 18O-enrichment in majorite constrains a crustal origin of transition zone diamonds. Geochemical Perspectives Letters, 1(1), 65-74.</td>
  </tr>
  <tr>
    <td>1072</td>
    <td>Ickert, R. B., Stachel, T., Stern, R. A., & Harris, J.W. (2015). Extreme 18O-enrichment in majorite constrains a crustal origin of transition zone diamonds. Geochemical Perspectives Letters, 1(1), 65-74.</td>
  </tr>
  <tr>
    <td>1073</td>
    <td>Ickert, R. B., Stachel, T., Stern, R. A., & Harris, J.W. (2015). Extreme 18O-enrichment in majorite constrains a crustal origin of transition zone diamonds. Geochemical Perspectives Letters, 1(1), 65-74.</td>
  </tr>
  <tr>
    <td>1074</td>
    <td>Ickert, R. B., Stachel, T., Stern, R. A., & Harris, J.W. (2013). Diamond from recycled crustal carbon documented by coupled δ18O-δ13C measurements of diamonds and their inclusions. Earth and Planetary Science Letters, 364, 85-97.</td>
  </tr>
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      <td></td>
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      <td></td>
    </tr>
    <tr>
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      <td></td>
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    <tr>
      <td>1226</td>
      <td>Figure Captions</td>
    </tr>
    <tr>
      <td>1227</td>
      <td></td>
    </tr>
    <tr>
      <td>1228</td>
      <td>Figure 1: Illustration showing the occurrence of diamond growth events throughout Earth's history. The deposits are grouped based upon their location; Australia (left), Canada, Russia, and Africa (right). White symbols represent deposits studied in this paper. Square symbols represent peridotitic diamond growth events (Hz and Lh denote harzburgitic and lherzolitic classifications where reported), while diamond symbols represent eclogitic diamond growth events. Fibrous diamond growth events (circle symbols) noted here reflect the kimberlite eruption age. A table containing all of the dates and references are provided in the Supplementary Material (Table S1).</td>
    </tr>
    <tr>
      <td>1229</td>
      <td></td>
    </tr>
    <tr>
      <td>1230</td>
      <td></td>
    </tr>
    <tr>
      <td>1231</td>
      <td></td>
    </tr>
    <tr>
      <td>1232</td>
      <td></td>
    </tr>
    <tr>
      <td>1233</td>
      <td></td>
    </tr>
    <tr>
      <td>1234</td>
      <td></td>
    </tr>
    <tr>
      <td>1235</td>
      <td></td>
    </tr>
    <tr>
      <td>1236</td>
      <td></td>
    </tr>
  </tbody>
</table>

Figure Captions

**Figure 1:** Illustration showing the occurrence of diamond growth events throughout Earth's history. The deposits are grouped based upon their location; Australia (left), Canada, Russia, and Africa (right). White symbols represent deposits studied in this paper. Square symbols represent peridotitic diamond growth events (Hz and Lh denote harzburgitic and lherzolitic classifications where reported), while diamond symbols represent eclogitic diamond growth events. Fibrous diamond growth events (circle symbols) noted here reflect the kimberlite eruption age. A table containing all of the dates and references are provided in the Supplementary Material (Table S1).
1237 **Figure 2:** Histograms for all of the analyses of peridotitic (right-hand side) and eclogitic (left-hand side) diamonds studied here. Where available, published literature data (combustion data only, provided in Table S3) are shown in grey. The representation of the two datasets is not additive (one on top of the other) but one is set in front of the other. Note that the scales on the y-axis vary between plots. The histograms use 0.5 ‰ binning.

1243
1244 **Figure 3:** Cathodoluminescence images of a selection of samples showing varied growth histories. (A) JWR 8; Jwaneng, (B) ORS 2; Orapa, (C) DP-4; De Beers Pool, (D) DDMI 199; Diavik, (E) JWR 18; Jwaneng. All scale bars are 200 µm long.

1247
1248 **Figure 4:** Histograms showing the data obtained from all of the samples, compared to those where only diamonds whose CL indicates single growth event are counted, separated based upon inclusion type.

1251
1252 **Figure 5:** Theoretical fractionation plots for diamond growth from a fluid with a starting δ¹³C value of -5 ‰ (yellow star; f_c represents the remaining fraction of carbon-bearing species in the diamond-forming fluid). The offset between the initial diamond growth and the fluid is governed by the carbon species in the fluid. The plots show diamond growth from (a) a CO₂ (oxidized) fluid, (b) a CO₃ (oxidized) fluid, and (c) a CH₄ (reduced) fluid. Plot (d) shows a more complex multicomponent system, with the fluid containing both oxidized and reduced species (namely CO₂ and CH₄) in varying proportions (10 mol% CO₂ - blue, 50 % - black, 89 % - red lines). Note that in these multicomponent calculations, the diamond's δ¹³C values always trend to less negative values irrespective of the dominant carbon species. Plot (d) after Stachel et al. (2017a).

1263
1264 **Figure 6:** CL images of (A) DBP-363_5, (B) V-336, (C) DBP-9, (D) DBP-460_9, with their SIMS δ¹³C analysis locations marked and red arrows showing growth direction. The graphs show the δ¹³C values plotted against N concentration. The lines connect the analyses in order, arrows showing the growth direction.

1268
1269 **Figure 7:** Box and Whisker plot showing the δ¹³C data from this study (bold colours), as well as data from the literature (combustion plus SIMS data; faint colours), in a
chronological order. Where single outliers are more than 3 ‰ from the remaining data, they have been marked as single data points (in Panda, Diavik and Venetia). The literature data plotted here is provided in Table 3 (with the total dataset, including references, provided in Table S3).

**Figure 8:** MDS and PCA plots for peridotitic diamonds. (a) MDS plot showing only δ¹³C data. Note how the Wawa data (red diamonds) do not overlap with the older Diavik (black circles) and De Beers Pool (black squares) data. (b) PCA plot of δ¹³C - N data. Given that MDS is a transformation that assigns input information a unitless (arbitrary unit) value (as it preserves only the relationships between data rather than specific information), we have not placed a scale on either axis of the MDS plot.

**Figure 9:** MDS and PCA plots for eclogitic diamonds. (a) MDS plot showing only δ¹³C data. (b) PCA plot of δ¹³C - N data. The dashed line represents a contour which contains 71% of all the data. Note how the Argyle data falls almost exclusively outside of this cluster. (c) PCA plot showing δ¹³C - N data from only the Orapa samples.

**Table 1:** Details of the diamond samples

Information regarding the source locations and quantities of the diamonds studied in this paper. References provided are the original works that dated the inclusions. In cases where two references are provided, the second notes the study where the diamonds have been sourced from which is separate to the original dating study.

**Table 2:** Summary of the δ¹³C and N data

The statistics generated from the δ¹³C and N datasets for both peridotitic and eclogitic diamonds in this study. The Δ symbol represents the ‘variation per sample’. The ‘Pear Skew’ value is the Pearson Coefficient of Skewness which is calculated as $[3 \times (average - median)] / standard deviation$. The “average of samples’ averages / medians” rows of data have been obtained by taking either the average or median values from each sample individually, and then taking an average of them per deposit. This was done to see if over analysis of an individual diamond was affecting the distribution of the data as a whole. The final rows show the differences between the
1304 data as a whole and of the individual stones. The average of these differences are ~0.1
1305 ‰ for the P-type data, and ~0.2 ‰ for the E-type data. Only the Orapa 3 Ga and 2.1
1306 Ga samples exceed differences of 0.25 ‰, showing that overall individual samples
1307 have not been over-analysed in this study.

1308

1309 **Table 3:** Summary of single growth vs multiple growth data

1310 The statistics of the total P-type and E-type δ¹³C and N datasets compared against the
1311 reduced datasets obtained from samples that exhibit only a single growth event (as
1312 interpreted from their CL imagery).

1313

1314 **Table 4:** Literature δ¹³C data

1315 Table showing the statistics of the literature data presented in Figure 7. *n* represents
1316 the number of diamonds, with each diamond providing a single data point. * Three
1317 locations utilize SIMS data where the first number is the number of diamonds, while
1318 the subsequent number in brackets represents the total number of analyses. The total
1319 dataset summarized here is provided in Table S3, including all relevant references.

1320

1321

1322 SUPPLEMENTARY MATERIAL:

1323

1324 Table S1) References for Figure 1.

1325 This table contains all of the references used to make Figure 1, showing all of the
documented diamond growth events in Africa, Russia, Canada and Australia.

1327

1328 Table S2) Total δ¹³C and N data from this study

1329 This table contains all of the δ¹³C and N data obtained in this study, with the diamond
1330 samples separated based upon their inclusion paragenesis.

1331

1332 Table S3) Literature δ¹³C data

1333 This table contains all of the literature data, referenced in the paper and its figures,
1334 along with the references from which the data were obtained.

1335

1336 Supplementary Material 1: Detailed methodology used for the multi-dimensional
1337 scaling (MDS) and principal component analysis (PCA) presented in this study.
1338

1339 Supplementary Material 2: Reference list for Table S1 and S3.

1340

1341
<table><caption>Table 1</caption><thead><tr><th>Location</th><th>Craton</th><th>Paragenesis</th><th>Number of samples</th><th>Age</th></tr></thead><tbody><tr><td>Argyle</td><td>Kimberley</td><td>Eclogitic</td><td>10</td><td>1.58 Ga</td></tr><tr><td>De Beers Pool</td><td>Kaapvaal</td><td>Eclogitic</td><td>8</td><td>2.9 Ga</td></tr><tr><td>De Beers Pool</td><td>Kaapvaal</td><td>Peridotitic</td><td>21</td><td>3.2 Ga</td></tr><tr><td>Diavik</td><td>Slave</td><td>Eclogitic</td><td>9</td><td>1.86 Ga</td></tr><tr><td>Diavik</td><td>Slave</td><td>Peridotitic</td><td>9</td><td>~ 3.3 Ga</td></tr><tr><td>Ellendale</td><td>Kimberley</td><td>Peridotitic</td><td>13</td><td>1.4 Ga</td></tr><tr><td>Jwaneng</td><td>Kaapvaal</td><td>Eclogitic</td><td>16</td><td>1.0 / 2.1 / 3.0 Ga</td></tr><tr><td>Orapa</td><td>Kaapvaal</td><td>Eclogitic</td><td>13</td><td>1.0 / 2.1 / 3.0 Ga</td></tr><tr><td>Udachnaya</td><td>Siberian</td><td>Peridotitic</td><td>15</td><td>2.0 Ga</td></tr><tr><td>Venetia</td><td>Limpopo Belt</td><td>Peridotitic</td><td>19</td><td>&lt; 2.3 Ga</td></tr><tr><td>Wawa</td><td>Superior</td><td>Peridotitic</td><td>11</td><td>&gt; 2.7 Ga</td></tr></tbody></table>
<table><thead><tr><th></th><th colspan="2">Orapa 3 Ga</th><th colspan="2">Jwaneng 3 Ga</th><th>De Beers P</th></tr><tr><th></th><th>δ13C (VPDB)</th><th>[N] at. ppm</th><th>δ13C (VPDB)</th><th>[N] at. ppm</th><th>δ13C (VPDB)</th></tr></thead><tbody><tr><td>No. of samples</td><td>6</td><td>6</td><td>6</td><td>6</td><td>8</td></tr><tr><td>No. of analyses</td><td>51</td><td>51</td><td>52</td><td>52</td><td>51</td></tr><tr><td>average</td><td>-6.10</td><td>596</td><td>-5.96</td><td>747</td><td>-5.61</td></tr><tr><td>min</td><td>-9.24</td><td>8</td><td>-17.09</td><td>1</td><td>-6.84</td></tr><tr><td>max</td><td>-4.38</td><td>1249</td><td>-2.98</td><td>2661</td><td>-3.30</td></tr><tr><td>average Δ</td><td>0.86</td><td>701</td><td>3.71</td><td>937</td><td>1.42</td></tr><tr><td>min Δ</td><td>0.28</td><td>318</td><td>0.43</td><td>231</td><td>0.53</td></tr><tr><td>max Δ</td><td>1.34</td><td>1166</td><td>12.13</td><td>2516</td><td>2.55</td></tr><tr><td>Q1</td><td>-7.70</td><td>171</td><td>-5.37</td><td>397</td><td>-6.11</td></tr><tr><td>Q3</td><td>-4.92</td><td>920</td><td>-4.83</td><td>1060</td><td>-5.27</td></tr><tr><td>median</td><td>-5.35</td><td>748</td><td>-5.13</td><td>891</td><td>-5.64</td></tr><tr><td>st dev</td><td>1.55</td><td>397</td><td>3.10</td><td>519</td><td>0.71</td></tr><tr><td>Pear Skew</td><td>-1.45</td><td>-1.15</td><td>-0.80</td><td>-5.15</td><td>0.15</td></tr><tr><td>Average of samples'</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>averages</td><td>-6.83</td><td>406</td><td>-6.18</td><td>714</td><td>-5.52</td></tr><tr><td>medians</td><td>-6.84</td><td>408</td><td>-6.52</td><td>606</td><td>-5.44</td></tr><tr><td>Difference between</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>averages</td><td>0.73</td><td>190</td><td>0.23</td><td>33</td><td>-0.09</td></tr><tr><td>medians</td><td>1.49</td><td>340</td><td>1.39</td><td>285</td><td>-0.20</td></tr></tbody></table>

<table><thead><tr><th></th><th colspan="2">Diavik ~3.3 Ga</th><th colspan="2">De Beers Pool 3.2 Ga</th><th>Wawa :</th></tr><tr><th></th><th>δ13C (VPDB)</th><th>[N] at. ppm</th><th>δ13C (VPDB)</th><th>[N] at. ppm</th><th>δ13C (VPDB)</th></tr></thead><tbody><tr><td>No. of samples</td><td>9</td><td>9</td><td>21</td><td>21</td><td>11</td></tr><tr><td>No. of analyses</td><td>56</td><td>57</td><td>135</td><td>135</td><td>65</td></tr><tr><td>average</td><td>-5.12</td><td>132</td><td>-5.45</td><td>76</td><td>-3.58</td></tr><tr><td>min</td><td>-6.29</td><td>0</td><td>-8.39</td><td>2</td><td>-4.48</td></tr><tr><td>max</td><td>-4.19</td><td>1131</td><td>-2.87</td><td>1477</td><td>-1.87</td></tr><tr><td>average Δ</td><td>0.55</td><td>334</td><td>1.20</td><td>230</td><td>0.35</td></tr><tr><td>min Δ</td><td>0.18</td><td>46</td><td>0.19</td><td>4</td><td>0.08</td></tr><tr><td>max Δ</td><td>1.47</td><td>1124</td><td>2.51</td><td>1463</td><td>0.95</td></tr><tr><td>Q1</td><td>-5.39</td><td>11</td><td>-5.72</td><td>10</td><td>-4.16</td></tr><tr><td>Q3</td><td>-4.52</td><td>120</td><td>-4.98</td><td>45</td><td>-2.96</td></tr><tr><td>median</td><td>-5.27</td><td>44</td><td>-5.26</td><td>20</td><td>-3.76</td></tr><tr><td>st dev</td><td>0.55</td><td>236</td><td>0.84</td><td>195</td><td>0.72</td></tr><tr><td>Pear Skew</td><td>0.81</td><td>-0.56</td><td>-0.66</td><td>0.86</td><td>0.75</td></tr><tr><td>Average of samples'</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>averages</td><td>-5.04</td><td>131</td><td>-5.47</td><td>5</td><td>-3.35</td></tr><tr><td>medians</td><td>-5.05</td><td>80</td><td>-5.45</td><td>28</td><td>-3.36</td></tr><tr><td>Difference between</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>averages</td><td>-0.08</td><td>0</td><td>0.03</td><td>71</td><td>-0.23</td></tr></tbody></table>
<table><thead><tr><th></th><th colspan="2">TOTAL P-TYPE DATA</th><th colspan="2">SINGLE GROWTH P-TYPE</th><th>TOTAL E-T</th></tr><tr><th></th><th>δ13C (VPDB)</th><th>[N] at. ppm</th><th>δ13C (VPDB)</th><th>[N] at. ppm</th><th>δ13C (VPDB)</th></tr></thead><tbody><tr><td>No. of samples</td><td>88</td><td>88</td><td>45</td><td>45</td><td>56</td></tr><tr><td>No. of analyses</td><td>504</td><td>506</td><td>244</td><td>245</td><td>404</td></tr><tr><td>average</td><td>-4.79</td><td>221</td><td>-4.73</td><td>171</td><td>-6.74</td></tr><tr><td>min</td><td>-8.39</td><td>0</td><td>-8.39</td><td>2</td><td>-17.09</td></tr><tr><td>max</td><td>-1.87</td><td>3073</td><td>-1.87</td><td>1275</td><td>-2.07</td></tr><tr><td>average Δ</td><td>0.85</td><td>369</td><td>0.49</td><td>147</td><td>1.53</td></tr><tr><td>min Δ</td><td>0.07</td><td>4</td><td>0.07</td><td>4</td><td>0.10</td></tr><tr><td>max Δ</td><td>2.80</td><td>3066</td><td>2.56</td><td>1013</td><td>12.13</td></tr><tr><td>Q1</td><td>-5.42</td><td>19</td><td>-5.38</td><td>21</td><td>-7.90</td></tr><tr><td>Q3</td><td>-4.07</td><td>222</td><td>-4.02</td><td>204</td><td>-5.09</td></tr><tr><td>median</td><td>-4.87</td><td>52</td><td>-4.76</td><td>45</td><td>-5.65</td></tr><tr><td>st dev</td><td>1.12</td><td>369</td><td>1.16</td><td>263</td><td>2.48</td></tr><tr><td>Pear Skew</td><td>0.23</td><td>1.37</td><td>0.08</td><td>1.44</td><td>-1.32</td></tr></tbody></table>
<table><caption>Table 4</caption><thead><tr><th>Location</th><th>PANDA</th><th>DIAVIK</th><th>FINSCH</th><th>DE BEERS POOL</th><th>DE BEERS POOL</th></tr></thead><tbody><tr><td>Age (Ga)</td><td>3.52</td><td>3.5-3.3</td><td>3.3-3.3</td><td>3.3-3.2</td><td>2.9</td></tr><tr><td>Paragenesis</td><td>P-TYPE</td><td>P-TYPE</td><td>P-TYPE</td><td>P-TYPE</td><td>E-TYPE</td></tr><tr><td>n=</td><td>129</td><td>110</td><td>70</td><td>213</td><td>53</td></tr><tr><td>min</td><td>-14.05</td><td>-11.50</td><td>-8.57</td><td>-6.98</td><td>-15.97</td></tr><tr><td>Q1</td><td>-5.45</td><td>-5.44</td><td>-6.59</td><td>-5.42</td><td>-5.62</td></tr><tr><td>median</td><td>-5.12</td><td>-5.13</td><td>-6.12</td><td>-5.18</td><td>-5.13</td></tr><tr><td>Q3</td><td>-4.86</td><td>-4.62</td><td>-5.64</td><td>-4.84</td><td>-4.59</td></tr><tr><td>max</td><td>-2.98</td><td>-2.10</td><td>-2.62</td><td>-0.93</td><td>-3.06</td></tr></tbody></table>
<figure><img src="image_3.png" /><figcaption>Figure 1</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>Figure 2</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>Figure 3</figcaption></figure>
<figure><img src="image_6.png" /></figure>
<figure><img src="image_7.png" /><figcaption>Figure 5</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>Figure 6</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>Figure 7</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>Figure 8</figcaption></figure>
<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /></figure>

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