# Boron isotopes in blue diamond record seawater-derived fluids in the lower mantle

Margo Regier, Karen Smit, Thomas B Chalk, Thomas Stachel, Richard Stern, Evan Smith, Gavin Foster, Yannick Bussweiler, Chris Debuhr, Antony Burnham, et al.

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► To cite this version:

Margo Regier, Karen Smit, Thomas B Chalk, Thomas Stachel, Richard Stern, et al., Boron isotopes in blue diamond record seawater-derived fluids in the lower mantle. Earth and Planetary Science Letters, 2023, 602, pp.117923. 10.1016/j.epsl.2022.117923. hal-03911903

HAL Id: hal-03911903

https://cnrs.hal.science/hal-03911903v1

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# Boron isotopes in blue diamond record seawater-derived fluids in the lower mantle

Margo E. Regier<fnref n="1" />, Karen V. Smit<fnref n="1" />,<fnref n="2" />*, Thomas B. Chalk<fnref n="3" />, Thomas Stachel<fnref n="1" />, Richard A. Stern<fnref n="1" />, Evan M. Smith<fnref n="4" />,

Gavin L. Foster<fnref n="3" />, Yannick Bussweiler<fnref n="1" />,<fnref n="8" />, C. Debuhr<fnref n="5" />, Antony D. Burnham<fnref n="6" />, Jeff W. Harris<fnref n="7" /> and D. Graham

Pearson<fnref n="1" />

## Abstract

Boron is a quintessential crustal element but its conspicuous presence in diamond – a mantle mineral – raises questions about potential subduction pathways for boron and other volatiles. It has been a long-standing goal to characterize the isotopic composition of boron in blue, boron-bearing (Type IIb) diamonds to reveal its origin. Mineral inclusions indicate that Type IIb diamonds crystallize at transition zone to lower mantle depths, meaning that if the boron is subducted it would trace a pathway of volatile elements into the deep mantle. Here, using off-line laser ablation sampling, we present the first boron isotope compositions, along with trace element contents and carbon isotope compositions, of a suite of blue diamonds mainly sourced from the Cullinan diamond mine in South Africa. The ten analyzed blue diamonds have a wide range in δ¹¹B, between –9.2 ± 2‰ and –0.5 ± 2‰, compared to the more restricted range for mid-ocean ridge basalts (–7.1 ± 0.9‰). Carbon isotope values for the blue diamonds range between –20.6 and –1.8‰, with a mode at –17‰, significantly more negative than the main mantle mode at –5‰. Combined, the boron and carbon isotope compositions require fluid input from subducted

<footnote n="1">¹Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta T6G 2E3, Canada.</footnote>

<footnote n="2">²School of Geosciences, University of the Witwatersrand, Johannesburg, South Africa (*karen.smit@wits.ac.za).</footnote>

<footnote n="3">³School of Ocean and Earth Science, University of Southampton, National Oceanographic Centre, University of Southampton, Southampton SO14 3ZH, UK.</footnote>

<footnote n="4">⁴Gemological Institute of America (GIA), New York City, NY 10036, USA.</footnote>

<footnote n="5">⁵Department of Geoscience, University of Calgary, Alberta T2N 1N4, Canada.</footnote>

<footnote n="6">⁶Research School of Earth Sciences, Australian National University, Canberra ACT 2601, Australia.</footnote>

<footnote n="7">⁷School of Geographical and Earth Sciences, University of Glasgow G12 8QQ, Scotland.</footnote>

<footnote n="8">⁸Institute of Geology and Mineralogy, University of Cologne, Zülpicher Str. 49b, 50674 Cologne, Germany.</footnote>
oceanic lithosphere down into deep mantle source regions of blue diamonds. This finding highlights a
deep subduction pathway for volatiles to stoke the source regions of deeply-derived magmas.

36

37 **Keywords**

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39 Diamond, Boron, Carbon, Isotopes, Transition Zone, Lower Mantle, Peridotite, Serpentinite

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41 **1. Introduction**

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43 The subduction of seawater-altered oceanic lithosphere across the 660 km seismic discontinuity provides
44 a potential mechanism for deep volatile enrichment (Berovici & Karato, 2003; Creager & Jordan, 1984;
45 Shirey et al., 2021), however the lithologies and mineral phases that retain volatile elements to lower
46 mantle depths are still uncertain. Rare, boron-bearing blue diamonds (classified as Type IIb) provide a
47 unique opportunity to examine the deep water, boron and carbon cycles, because their mineral inclusions
48 and carbon isotope values indicate they formed in subducted oceanic lithosphere at transition zone to lower
49 mantle depths (Smith et al., 2018). Mineral inclusions in blue diamonds have a clear link to subducted
50 lithologies, raising the prospect that boron impurities in blue diamonds may also have a subducted origin
51 (Smith et al., 2018). Geochemical tracers that would fingerprint the phases responsible for transporting
52 boron and other volatile elements to transition zone and lower mantle depths have remained elusive due
53 to the challenges of analyzing these very pure diamond samples.

54

55 Boron isotopes are an ideal tracer of subducted material in the deep mantle due to the large isotopic
56 fractionation that can occur in low-temperature crustal environments during interaction with seawater.
57 Despite the blurring of these distinctive isotopic signatures during the preferential loss of the fluid-mobile
¹¹B isotope (decreasing the residual δ¹¹B) as subducting slabs dehydrate, the isotopic distinction between surface and mantle environments remains strong (de Hoog & Savov, 2018).

Our study of blue diamonds includes the first boron isotope measurements on material directly sourced from the transition zone to lower mantle and is coupled with mineralogical and elemental characterization of the mineral inclusions and crystallographic impurities of blue diamonds. Combined, these data provide us with unique insights into the large-scale cycling and behaviour of volatiles in the deep mantle.

## 2. Samples and Methods

Here we briefly outline our methodology, with full details of each technique given in the Supplementary Text.

Type IIb diamonds are exceedingly rare, comprising less than 0.02 % of all mined diamonds (Smith et al., 2018). Due to the rarity of Type IIb diamonds, the low abundance of mineral inclusions trapped within them, and their high monetary value, the geological origin of Type IIb diamonds has remained elusive until Smith et al. (2018) proved their transition zone / lower mantle origin. Here we studied 23 fragments of blue Type IIb diamonds from the Cullinan mine (exploiting the 1.15 Ga Premier kimberlite; Kjarsgaard et al., 2022) in South Africa and three diamonds of uncertain provenance (Table 1).

Prior to any destructive analytical work, we collected Raman spectra and X-ray microfluorescence (μXRF) data from two diamonds (NL247-2 and Blue2). Confocal Raman spectra were collected at Northwestern University using a custom-built system with an Olympus BX microscope with 100× and 50× objectives and a Melles-Griot solid-state laser. μXRF was completed at synchrotron beamline 13-IDE (GSECARS, APS, Argonne National Laboratory).
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Electron probe microanalysis (EPMA) was completed on an inclusion that was recovered after breaking the enstatite-bearing corner of sample Blue2. The analyses utilized a JEOL JXA-8900R with five wavelength-dispersive spectrometers at the University of Alberta.

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Diamonds were mounted in epoxy and/or indium for cathodoluminescence (CL) imaging and secondary ion mass spectrometry (SIMS) analysis at the Canadian Centre for Isotopic Microanalysis. CL imaging was completed using a Zeiss EVO MA15 scanning electron microscope (SEM), with a broadband photomultiplier detector.

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Boron abundances in diamonds were determined by SIMS analysis at the Canadian Centre for Isotopic Microanalysis. Boron (¹¹B⁻/¹²C⁻) measurements were quantified using the boron concentration of sample NL247-20 as the primary reference material, which was determined by hyperspectral CL (0.185 ± 0.008 µg/g [2σ s.d.]). The hyperspectral CL measurements utilized a FEI Quanta 250 field emission SEM with a Gatan cryo-stage at the University of Calgary. We calculated the boron concentration by monitoring the maximum peak intensity of bound (BE) and free excitons (FE), using a 2400 lines/mm diffraction grating.

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Closed-cell laser ablations of the natural blue diamonds for trace elemental analyses were carried out at the University of Alberta using a 193 nm ArF Excimer laser system (RESOlution M-50-LR, Applied Spectra). Samples were analyzed on a Thermo Scientific Element XR ICPMS, with an APEX- Q high-efficiency sample introduction system. This method is described in detail in Krebs et al. (2019). We only report data that are above the limit of quantification (LOQ), defined as the total procedural blank (TPB) + 7σ. Laser ablations for boron isotope analyses were also conducted using a 193 nm ArF Excimer laser system at the University of Alberta with the ablation product being collected in a custom-made Teflon ablation cell that was capped with a silica glass window. This “offline-ablation” method has been

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described previously (e.g., McNeill et al., 2009). Subsequent solution-based analysis of boron was
completed on a Thermo Neptune MC-ICP-MS with a PFA spray chamber at the University of
Southampton (National Oceanography Centre Southampton) following established analytical protocols
(e.g., Foster, 2008). Details on the analytical techniques are given in the Supplementary Text.

112

113 **3. Results**

114

115 **3.1. Mineral inclusions**

116

117 Inclusions were observed in only four diamonds in this suite: sample 110208425476 was previously
118 reported to contain ferropericlase inclusions along with minor olivine and nyerereite (Na₂Ca(CO₃)₂)
119 associated with healed fractures (Smith et al., 2018). X-ray diffraction-based barometry of ferropericlase
120 in this diamond established a remnant pressure of 1.8 GPa, which was estimated to require entrapment
121 pressures of at least 10–14 GPa (Smith et al., 2018). Sample DVBT was reported to contain inclusions of
122 breyite (walstromite-structured CaSiO₃) and an Fe-Ni-S alloy (Smith et al., 2018). Blue2 and NL247-2
123 have visible bimineralic inclusions of breyite (CaSiO₃) and larnite (Ca₂SiO₄), and Blue2 also has a separate
124 inclusion of enstatite (Figure 1; Data S1). X-ray fluorescence mapping of inclusions in Blue2 and NL247-
125 2 indicates that Fe-Ni rich alloys surround these Ca- and Mg-silicate inclusions (Figure 2). An additional
126 broad graphite peak at ~1580 cm⁻¹ in the Raman spectra suggests that these inclusions are similar to the
127 Fe-Ni-C-S inclusions previously documented in sublithospheric CLIPPIR and blue diamonds (Smith et
128 al., 2016, 2018).

129

130 Electron probe microanalysis of an additional, single inclusion, recovered after breaking the enstatite-
bearing corner of sample Blue2, determined this additional inclusion to be olivine (Data S7). Under
131 equilibrium conditions, the co-presence of breyite and enstatite in this diamond requires formation in the
132
lower mantle, as CaSi- and MgSi-perovskites (bridgmanite; Irifune & Ringwood, 1987). The high NiO content (0.45 wt. %) of the olivine and its close spatial association (possibly touching) with an unrecovered retrogressed bridgmanite (i.e., enstatite) suggest that it formed through retrograde recombination of a composite Ni-rich ferropericlase and high Mg# (where Mg# = 100Mg/(Mg+Fe)) bridgmanite inclusion (Stachel et al., 2000b; Smith et al., 2018). While the high olivine Mg# of 93 implies an ultramafic lithology (bridgmanite must have been > 93), it cannot be used to deduce a depleted or fertile composition, because the initial proportions of ferropericlase and bridgmanite and the final proportions of olivine and orthopyroxene are unknown.

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## 3.2. Internal textures and zoning in blue diamonds

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Polygonized dislocation networks are visible in the panchromatic (350–850 nm) CL images of all the blue diamonds studied here (Figure 3). Polygonized dislocation networks are common in sublithospheric diamonds, being especially pronounced in Type IIa and Type IIb varieties, as a result of deformation and annealing in the hot and dynamic environment of the convecting mantle (Smith et al., 2016, 2018). Deformation likely starts as single lattice dislocations that then polygonize into mosaic networks during annealing at high temperatures in the mantle (Hanley et al., 1977; Sumida & Lang, 1981). Dislocation networks are often associated with 'Band A' emission – a broad peak at 435 nm that remains unassigned (Hanley et al., 1977; Graham et al., 1991). Although polygonized dislocation networks are common only in sublithospheric diamonds, single dislocations and 'Band A' emission are also observed along deformation lamellae in lithospheric diamonds (Smit et al., 2018).

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The majority of our samples show no regular concentric zoning in their CL emission (Figure 3), but ten samples have broad zones with higher CL emission (NL247-2, -9, -10, -12, -14, -15, -16, -17, -21, -22). Secondary ion mass spectrometry analyses determined that regions with low CL emission have ~ 0.2 µg/g
B, similar to the range (0.1 to 8 µg/g B) reported by Gaillou et al. (2012). Zones with higher CL emission generally have lower B concentrations (Figure 3; ~0.02 µg/g B). This may be because higher boron concentrations quench 'Band A' emission in the dislocation networks, or alternatively that boron quenches nitrogen emission (Collins & Williams, 1971). Nitrogen concentrations in all samples were below the detection limit of SIMS, generally less than 0.5 or 0.35 µg/g (Data S3).

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## 3.3. Trace element content of blue diamonds

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Trace element data for blue diamonds are given in Data S2 and the Supplementary Text. The REE<sub>N</sub> patterns (where N denotes normalization to chondrite) in the majority of our blue diamonds show enrichment of LREE over HREE, with La<sub>N</sub>/Ho<sub>N</sub> between 5.4–110 (Figure 4). One sample (NL247-13) has relatively flat LREE<sub>N</sub>-HREE<sub>N</sub> with La<sub>N</sub>/Ho<sub>N</sub> of 0.9 (Figure 4). In three samples where both Tb and Lu were above the LOQ, the MREE<sub>N</sub>-HREE<sub>N</sub> are relatively flat with Tb<sub>N</sub>/Lu<sub>N</sub> = 0.8–1.4. The majority of samples show enrichments in Zr and Hf, when normalized to primitive mantle (Figure 4).

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## 3.4. Carbon and boron isotope composition of blue diamonds

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Average carbon isotope values for our suite of blue diamonds are given in Table 1, and the full dataset is given in Data S3. The carbon isotope composition of individual samples is homogeneous, with within-sample variation ranging from 0.09 to 1.5 ‰ 2σ.

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Carbon isotope values vary considerably across our sample suite (δ<sup>13</sup>C of −20.6 to −1.8 ‰; Data S3), consistent with previously published δ<sup>13</sup>C for six Type IIb diamonds (spanning −20.8 to −1.8 ‰; Milledge et al., 1983; Smith et al., 2018). Carbon isotope values for blue diamonds are similar to the range for eclogitic diamonds from the lithospheric mantle (Figure 5). However, unlike worldwide eclogitic
diamonds, blue diamonds do not display a mode at the canonical mantle value of -5 ‰ (Figure 5). The absence of a δ¹³C mode at -5 ‰ is also seen in sublithospheric diamonds with majoritic garnet inclusions, which crystallize from carbonate-rich slab melts (Thomson et al., 2016b; Regier et al., 2020).

Boron isotope data for blue diamonds are given in Table 1 and in Data S4. Blue diamonds have a range of δ¹¹B_SRM951 values from -9 to 0 ‰ (n = 11; Figure 6). No correlations between δ¹³C and δ¹¹B, or between δ¹¹B and B concentration were observed in our samples (Figure 6).

# 4. Discussion

## 4.1. Blue diamond formation – evidence from mineral inclusions

Given the presence of enstatite and Ca-silicate (touching breyite + larnite) inclusions in a single diamond (Blue2; Table 1), the simplest interpretation is that these minerals were originally entrapped as bridgmanite (MgSiO₃) and CaSiO₃-perovskite in the lower mantle. However, the Ca-silicate inclusions in our study and in Smith et al. (2018), lack Ca-poor, titanite-structured CaSi₂O₅. It is possible that the Ca-silicate inclusions contain a CaSi₂O₅ phase that was not detected by Raman or Synchrotron XRD, which would bring the bulk inclusion stoichiometry in line with an original CaSiO₃ stoichiometry.

Alternatively, it is possible that the observed co-existing breyite (CaSiO₃) and larnite (Ca₂SiO₄) phases represent a bulk inclusion composition with a Ca/Si ratio greater than 1 (Brenker et al., 2005). In this case, the inclusion cannot represent retrogression of an initial single-phase CaSiO₃- perovskite inclusion, and must be derived from a substrate with Ca/Si ratios higher than pyrolite, which has Ca/Si of ~0.1 (Ringwood, 1975).
208 In the case of primary two-phase (Ca₂SiO₄–CaSiO₃) inclusions, high Ca/Si ratios could originate in either
209 of the following environments: 1) carbonatitic melts of carbonate-rich oceanic slabs (Ca/Si ~25;
210 Pal'yanov et al., 2013); 2) serpentinized peridotite, where serpentinization can remove Si and also create
211 Ca-enriched rocks such as rodingites. Both of these scenarios require a slab-influenced lower mantle
212 crystallization environment for the blue diamonds.
213

214 The reduced Fe-Ni rich alloys, preserved as rims around the Ca- and Mg-silicate inclusions (Figure 2),
215 resemble those reported in CLIPPIR diamonds (Cullinan-Like, Inclusion Poor, Pure, Irregular, Resorbed;
216 a variety of sublithospheric diamonds comprising many large, gem-quality Type IIa diamonds) that have
217 iron isotope signatures similar to metallic precipitates in meta-serpentinite (Smith et al., 2016, 2021).
218 Together, the mineral inclusions in our suite of blue diamonds suggest crystallization in an environment
219 similar to CLIPPIR diamonds, i.e., near the boundary of the transition zone and lower mantle.
220

220

221 **4.2. Blue diamond formation – evidence from the trace element and isotope composition of diamond**
222

222

223 Further evidence in support of a slab-influenced crystallization environment for the blue diamonds and
224 their inclusions is provided by trace element and boron isotope measurements of the diamonds themselves.
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225

226 Zr-Hf enrichments in primitive mantle-normalized trace element patterns demonstrate that the boron-
227 bearing fluids could not have derived from carbonated peridotite or other carbonate-bearing lithologies
228 (sediment, altered oceanic crust) in the oceanic lithosphere. Carbonatites and kimberlites are
229 characteristically depleted in Zr-Hf (Nelson et al., 1988; Giuliani et al., 2020), and sublithospheric
230 diamonds from Brazil that have an inferred carbonatitic origin (Walter et al., 2011; Thomson et al., 2016a)
231 exhibit Zr-Hf depletions in their primitive mantle-normalized trace element abundances (Timmerman et
232 al., 2019). Other diamonds that have Zr-Hf enrichments and relatively flat MREE<sub>N</sub>-HREE<sub>N</sub> are Victor
233 monocrystalline diamonds (Krebs et al., 2019), Marange methane-bearing diamonds (Smit et al., 2016, 234 Smit et al., unpublished data) and CLIPPIR diamonds (Smit et al., unpublished data). In the case of the 235 Marange diamonds, reduced hydrous fluids derived from serpentinized peridotite were implicated in their 236 formation (Smit et al., 2016a), and in the case of CLIPPIR diamonds, the Fe isotope signatures of Fe-Ni- 237 C-S melt inclusions indicate derivation from meta-serpentinite (Smith et al., 2021).

238
239 The range of boron isotope values in our blue diamonds (−9 to 0 ‰) is far more variable than that of the 240 convecting mantle as represented by the depleted mid-ocean ridge basalt (MORB)-source mantle (−7.1 ± 241 0.9 ‰) (Marschall et al., 2017). Blue diamonds have boron isotope compositions that range from 242 indistinguishable from MORB to higher values that overlap the range for metamorphosed serpentinite and 243 altered oceanic crust (AOC), and strongly implicate a seawater-altered slab source for the boron (Figure 244 7).

245
246 The measured boron values of the Cullinan blue diamonds are lower than highly metamorphosed 247 sediments and altered oceanic crust (AOC) in the diamond stability field. This is because these lithologies 248 experience large degrees of fluid and ¹¹B loss during subduction-related dehydration (Figure 7A). For 249 instance, models of boron loss during subduction indicate that sediments and AOC lose >90 % of their 250 boron at sub-arc mantle depths (Konrad-Schmolke & Halama, 2014). The remaining boron has δ¹¹B of ≤ 251 −10 ‰ at 2.5 GPa (Marschall et al., 2007), and ≤ −15 ‰ at 5 GPa (Konrad-Schmolke & Halama, 2014). 252 Thus, while the δ¹¹B values of blue diamonds are unambiguously indicative of a seawater-altered slab 253 source, the boron we measure in these blue diamonds is unlikely to derive from the sedimentary or crustal 254 sections of subducting slabs. 255

255
256 While there are very few published B isotope analyses for eclogites, existing data indicate a boron-poor,
257 ¹¹B-depleted nature of subducted and dehydrated AOC and sediments. Thus, an alternative reservoir in
the slab is required to produce blue diamonds that can contain up to 8 µg/g B (Gaillou et al., 2012) and
$\delta^{11}\text{B} \le 0‰$. One plausible source is oceanic slab serpentinite, a significant $^{11}\text{B}$ sink (Martin et al., 2020),
with $\delta^{13}\text{C}$ overlapping the range of our blue diamonds (Figure 5). The seawater-derived $^{11}\text{B}$-enriched
signature of serpentinite is retained under higher pressure conditions at 3–5 GPa (Martin et al., 2020, and
Figure 7A), though the extreme values of the protoliths are moderated by fractionation on B loss, as
indicated by modelling (Cannaò, 2020). Such fractionated B isotope ratios likely persist into the transition
zone and deeper if the serpentine recrystallizes into dense hydrous magnesium silicates (DHMS) along
cool slab trajectories (Scambelluri & Tonarini, 2012) or into metamorphic olivine (de Hoog et al., 2014;
Cannaò, 2020) which stores boron as the hydrous component $\text{Mg}_2\text{BO}_4\text{H}$ (Ingrin et al., 2014).

For typical slab P-T paths (Figure 8), a number of boron-bearing, high-pressure phases – including
ringwoodite, DHMS Phase D and superhydrous Phase B – are likely to destabilize near the upper mantle-
lower mantle boundary (Ohtani et al., 2000, 2001; Harte, 2010; Nishi et al., 2014; Cannaò, 2020; Shirey
et al., 2021). On the basis of mineral inclusions, this is the region proposed for blue diamond formation
(this study; Smith et al., 2018). These phases provide multiple opportunities for transporting seawater-
derived boron into the deep mantle. While the B isotope composition of newly formed diamonds is
unlikely to be exactly transposed from the protoliths, it seems clear that there is a high likelihood of
inheriting a wide range of B isotope compositions that will extend to considerably higher values than
MORB.

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278 **4.3. Model for the formation of blue diamonds**

279

280 There are four observations and associated interpretations that need to be reconciled in any model for blue
diamond formation:
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(1) High Ca:Si ratios in the inclusions; suggesting formation in either a carbonatitic melt or an unusual source rock.

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(2) Elevated Zr and Hf contents in diamond, which is evidence against carbonatite melt as the diamond-forming medium, although we cannot rule out carbonate being present in the earlier prograde subduction path.

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(3) ¹³C-depleted carbon isotope compositions, down to δ¹³C = -21 ‰, that require carbon input from biogenic carbonate or former organic matter (stored as diamond, carbides or in metal).

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(4) Boron isotope compositions (δ¹¹B = -9 to 0 ‰) that vary from MORB-like to significantly more ¹¹B-enriched than MORB mantle (DMM; Marschall, 2018), and likely derived from metaserpentinite (consistent with Fe isotope compositions reported for some sublithospheric diamonds; Smith et al., 2021).

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To satisfy the constraints from boron isotopes, any boron-bearing medium for diamond formation is likely derived directly or indirectly (as fluids) from metaserpentinites. In order to not lose boron entirely during early dehydration stages of the slab, the metaserpentinite that serves as the source for boron-containing diamonds needs to be stored in the deeper inner portions of the slab (Shirey et al. 2021).

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Below we consider three different rock types – *metarodingite*, *metabasalt*, and *metaserpentinite* – that may act as substrates or fluid sources for the formation of blue diamonds and evaluate each with respect to inclusion Ca:Si ratios, the source of ¹³C-depleted carbon, and the Zr-Hf evidence against a carbonatitic diamond-forming medium. We note that any environment for blue diamond formation must be fairly uncommon/unusual in order to account for the rarity of blue diamonds (less than 0.02 % of mined diamonds, and less than 2% of sublithospheric diamonds; Smith et al., 2018).

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**Metarodingite substrate.** Rodingites are Ca-rich metasomatic rocks formed by alteration of mafic to ultramafic rocks. In the oceanic lithosphere and subducting slabs, (meta)rodingites occur at the interface
between serpentinite and AOC (gabbro) or as lenses in serpentinite, where their formation relates to Ca- rich fluids released from serpentinite. Metarodingites are rich in boron – Kobayashi & Kaneda (2010) report vesuvianite from rodingite with up to 1 wt% B₂O₃ – and have elevated Ca/Si.

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For metarodingite to be a plausible environment for blue diamond formation, an external carbon source is required to provide ¹³C-depleted carbon. Such carbon is unlikely to derive from melting of biogenic carbonates, as this cannot be reconciled with the elevated Zr-Hf contents in the diamond; and carbon derived from metaperidotite is unlikely to be ¹³C-depleted. A plausible alternative for the carbon source is oxidative dissolution of former organic matter (stored as diamond, in metal or as carbides), before the carbon-bearing fluid reaches the deeply subducted metarodingite. However, as rodingite is a garnet-rich rock, with bulkrock Al₂O₃ contents that can exceed 30 wt%, blue diamonds forming in a metarodingite environment may be expected to have abundant majoritic garnet inclusions, or inclusions of other highly aluminous phases, which we do not observe in our suite of blue diamonds.

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Verdict: We consider a metarodingite environment to be an unlikely environment for blue diamond formation. This environment can explain the generation of boron-rich rocks with elevated Ca/Si, however, it cannot be reconciled with the lack of abundant majoritic garnet inclusions in blue diamond and requires an external carbon source that typically is located at shallower levels in the AOC.

325

**Metabasalt substrate.** In this model, boron-bearing hydrous fluids from dehydrating serpentinite react with metamorphosed AOC. Within the AOC, the localized environment for diamond formation must be Ca-rich (high Ca:Si), for example through the presence of residual secondary carbonate. The serpentinite-derived fluid must be reducing enough to produce diamond from any carbonate present and transfers boron through interaction with this crust.

331
332 Verdict: We could consider metabasalt a plausible environment for blue diamond precipitation from 333 serpentinite-derived fluid. This environment is consistent with the observed Ca:Si ratios (residual 334 carbonates in the metabasalt), the non-carbonatitic diamond-forming media (fluid derived from 335 metaserpentinite), the 13C-depleted carbon isotope compositions (residual carbon in metabasalt), and the 336 boron isotope distribution (metaserpentinite). The origin of trace element patterns with positive Zr-Hf 337 anomalies is not clear in this model.
338

339 **Metaserpentinite substrate.** In this model, diamond grows in metaserpentinite, where the subducted boron 340 is hosted as a consequence of seawater alteration, rather than transported through a percolating fluid into 341 a second lithology. In the metaserpentinite model, we need to reconcile the high Ca:Si ratios in the 342 inclusions and we require an external carbon source for the 13C-depleted carbon isotope compositions. 343 Accordingly, we consider three possibilities:

344
345 a) Conductive warming of the slab causes melting of residual slab carbonates (see phase equilibria and 346 geotherms presented in Shirey et al., 2021). This carbonated melt moves inwards to colder regions and 347 carries 13C-depleted carbon isotope compositions, i.e., the melt originates from the crustal (AOC) portion 348 of the slab. In a buckled or imbricated slab/megolith situation, former oceanic crust can easily lie below 349 former lithospheric mantle. However, blue diamond formation from carbonatitic melts is precluded based 350 on the Zr-Hf elevations observed in the primitive-mantle normalized trace element patterns, and 351 carbonatitic melts can only explain the high Ca:Si ratios if high quantities of melt were involved.
352

353 b) Ophicalcite veins in metaserpentinites could explain the observed high Ca:Si ratios, but such ophicalcite 354 veins have never been observed to have 13C-depleted signatures and typically have δ13C greater than 0 ‰. 355 (Bach et al., 2011; Alt et al., 2013).
356

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c) Disseminated carbonate can make up to 40 % of serpentinites and could readily explain the elevated Ca:Si ratios. The carbon in serpentinites is likely homogenized by the time it arrives in the deep transition zone and potentially forms part of a megalith at the transition zone-lower mantle boundary (660 km), so different carbon components are likely not relevant but only 'total carbon' needs to be considered. There are no data on δ¹³C for serpentinites from deep suboceanic lithospheric mantle. The only data reported are for tectonically-emplaced serpentinized peridotites in shallow oceanic crust. The total carbon in serpentinites (without calcite veins) often has δ¹³C values extending below -20 ‰ (e.g., Alt et al., 2013, and Data S3). For example, serpentinite from Lost City (Mid Atlantic Ridge) has a δ¹³C distribution from -23.0 to -1.0 ‰ with a median of -13.2 ‰ (Delacour et al., 2008), which provides a good match for the carbon isotope composition in our suite of blue diamonds (Figure 5). Diamond formation in deeply subducted, carbonated metaserpentinites may occur through either physical mixing with adjacent metal-saturated metaperidotites or ingress of reducing dehydration fluids derived from deeper within the slab.

Verdict: We consider a metaserpentinite environment, with disseminated carbonates, as the most likely environment for blue diamond formation. This environment can explain all our key observations: the high Ca:Si ratios in inclusions (disseminated carbonate), the non-carbonatitic diamond-forming media (diamond-forming fluid is derived locally, from metaserpentinite), the ¹³C-depleted carbon isotope compositions of blue diamonds (carbon in metaserpentinites), and the range of boron isotopes in blue diamonds (metaserpentinite). Unlike diamond formation in a metabasalt substrate, the metaserpentinite-derived fluids can precipitate blue diamonds *in situ*, rather than percolate into another lithology for diamond formation. The unresolved question is if, in cold subducting slabs, serpentinite bodies tectonically emplaced into AOC can retain their carbon and boron contents to transition zone and lower mantle depths. Due to low Na contents, carbonate melting in metaserpentinites may be inefficient. But complete dehydration should efficiently remove boron, unless it is transferred into newly formed phases other than wadsleyite and ringwoodite. In detail, boron loss and gain in subducting oceanic lithosphere is
complex, with no B loss observed in some metaserpentinites, possibly due to B influx from fluids derived from breakdown of B-bearing phases in metasediments (Harvey et al., 2014; Clarke et al., 2020). Although the mechanisms for boron transport remain unclear, our results from blue diamonds with recycled carbon and boron isotope signatures, provide the evidence that these elements can indeed be retained to transition zone and lower mantle depths. Since the boron distribution in blue diamonds does not match the full range of B observed in serpentinites, this suggests that the majority of boron was likely lost – as expected during prograde subduction. The fractionation of B isotopes due to hydrous phase breakdown is potentially mediated to mildly positive values by alkaline fluids created from carbonates in the early prograde subduction history (see modelling in Cannaò, 2020). The transport of boron to lower mantle depths in Earth is consistent with the ability of cold subducted slabs to deliver other volatiles such as water and carbon to these depths, as shown by the thermal modelling of slab paths in the context of hydrous phase equilibria and the generation of lower mantle diamonds bearing crustal carbon isotope signatures (Shirey et al., 2021; Walter et al., 2022). In this model, no carbonate-bearing melt is generated during diamond formation, consistent with the trace element systematics.

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## 4.4. Deep subduction of serpentinized oceanic lithosphere

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There is a striking similarity between the boron isotope composition of blue diamonds and those of mantle carbonatites and ocean island basalts (OIB) that are free of late-stage crustal contamination (Figure 7B), which we define here as those with 0.04 to 4.0 µg/g B (Chaussidon & Marty, 1995; Tanaka & Nakamura, 2005; Hulett et al., 2016; Çimen et al., 2018, 2019; Walowski et al., 2019). Given these two types of magmas have long been shown to contain recycled lithospheric components in their source regions (Hofmann, 1997; Hoernle et al., 2002), we suggest that deep-seated carbonatite and possibly OIB magmatism tap a reservoir that was contaminated by the same slab-derived components that contributed boron to lower mantle-derived blue diamonds. The blue diamonds from Cullinan, which must have
crystallized prior to kimberlite eruption at ~1.1 Ga (Kjarsgaard et al., 2022), may have experienced ambient mantle temperatures ~100 °C higher than present (Vlaar et al., 1994). At these temperatures, even the coolest slabs would likely warm to temperatures above the stability of DHMS at uppermost lower mantle depths. This implies that a significant fraction of subducting slabs would have experienced melting of carbonated metabasalt in the transition zone (Thomson et al., 2016b), along with dehydration and associated mobilization of carbonates in metaserpentinite in the uppermost lower mantle, before penetration into the deep lower mantle. While this depletion may have acted as a barrier to volatile subduction and kept the deeper portions of the lower mantle relatively pristine and isolated from incompatible and volatile-rich components in the upper portions of slabs (Figure 9A,B), the lower mantle derived blue diamonds from Premier, erupted at ~ 1.1 Ga, imply that even in this situation, some recycled volatiles made their way at least to the uppermost lower mantle.

Secular cooling of Earth's mantle has likely allowed a greater proportion of modern-day subducting slabs to remain within the stability field of DHMS, including phase H (Figure 8), which is stable to depths greater than 1250 km along cool subduction geotherms (Nishi et al., 2014). Thus, in a cooler modern mantle, hydrous phases may better survive subduction into the deep lower mantle, beyond a depth of 660 km (Figure 9C, D), compared with the pre-Phanerozoic Earth. Intriguingly, mantle-derived carbonatites may only have become systematically ¹¹B-enriched in the recent geologic past (< 250 Ma), around the same time that deep-seated kimberlites begin to show significant slab input to their sources (Hulett et al., 2016; Woodhead et al., 2019). We suggest that these isotopic deviations in deep-seated Mesozoic magmas may reflect the point at which sufficient cooling of the ambient mantle permitted slabs to translate their incompatible element and volatile cargo into the deep lower mantle. The view of the lower mantle boron cycle provided by blue diamonds thus not only highlights the strict thermal and temporal constraints on the formation of Earth's most valuable diamonds, but also identifies seawater-altered serpentinite as a reservoir for chemically diversifying the mantle sources of Earth's deepest-derived melts.
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## 5. Conclusions

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We present new boron isotope and concentration data for a rare suite of sublithospheric boron-bearing blue diamonds. They contain seawater-derived boron that must have been subducted to transition zone to lower mantle depths prior to eruption of the Premier kimberlite 1.1 billion years ago. The diamonds have very negative δ¹³C values that indicate a recycled carbon source.

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The range in δ¹¹B values in blue diamonds does not show the full range of boron observed in serpentinites, likely due to boron loss due to the heating and devolatilization of subducting slabs, with the majority of boron lost in dehydration fluids. The boron isotope compositions of these blue diamonds range from MORB-like to values that are clearly higher, spanning the same range as metamorphosed serpentinized oceanic lithosphere. Only subducting slabs with the coolest PT paths should retain boron, water and carbon to lower mantle depths. This clear view that recycled boron reaches the transition zone and lower mantle requires re-evaluation of the mechanisms through which boron is transported to these depths.

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A similar range of boron isotope signatures to those found in blue diamonds are also found in OIBs and carbonatites. This provides compelling evidence that deep volatile transport by the subduction of serpentinized oceanic lithosphere is essential not only for the formation of highly valuable blue diamonds, but also for the chemical diversity found in Earth's deeply derived intra-plate magmas.

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## CRediT authorship contribution statement

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The analytical work was completed by M.E. Regier, R.A. Stern, T.B. Chalk, G. L. Foster, and C. Debuhr.

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Samples were provided by A. Burnham, E.M. Smith and J.W. Harris. This paper is based on M.E. Regier's PhD thesis.

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All authors contributed ideas and writing to the manuscript.

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**Declaration of competing interest**

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Authors declare no competing interests.

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**Data availability**

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All data are available in the main text or the supplementary materials.

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**Acknowledgements**

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We thank Dan Howell, Steve Shirey, and Terry Plank for discussions that helped shape this research. De

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Beers Group and David Fisher generously provided samples and synthetic diamonds. Heather Goring-

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Harford, Chris Standish, Suzette Timmerman, Chiranjeeb Sarkar, and Sarah Woodland are thanked for

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assistance with ICP-MS analyses. Phil Diggle is thanked for his guidance with hyperspectral CL

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measurements. We acknowledge NSERC Discovery grants to TS & DGP, and Deep Carbon Observatory

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Funding. This research used resources of the Advanced Photon Source, a U.S. Department of Energy

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(DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National

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Laboratory under Contract No. DE-AC02-06CH11357. We acknowledge the support of

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GeoSoilEnviroCARS (Sector 13), which is supported by the National Science Foundation - Earth Sciences

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(EAR-1128799), and the Department of Energy, Geosciences (DE-FG02-94ER14466) and beamline staff
479 scientists Matt Newville and Tony Lanzotti. Horst Marschall, Cees-Jan De Hoog and the editor, Frédéric Moynier, are thanked for their constructive comments that improved the clarity of this manuscript.

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<figure><img src="image_3.png" /><figcaption>Figure 1: Optical photographs and Raman spectra of inclusion-bearing blue diamonds. (A) Photo of sample NL247-2 and a high magnification inset of a selected mineral inclusion. The black ring around the inclusion is composed of an Fe-Ni rich alloy and graphite as determined by µXRF (Figure 2) and a broad Raman peak at 1580 cm⁻¹ (inset). (B) Raman spectra from two bimineralic Ca-silicate inclusions including larnite and breyite. Note that the prominent peak for Ca-poor CaSi₂O₅-titanite phase at ~355 cm⁻¹ is absent in these spectra and in the inclusions reported by Smith et al. (2018). (C) Optical photo of Blue2 with an inset of bimineralic Ca-silicate (top) and enstatite (bottom) inclusions. Dark patterns on the surface of diamond are laser ablation tracks. (D) Selected Raman spectra for inclusions in sample Blue2, including a bimineralic Ca-silicate inclusion, an enstatite inclusion, and a reference enstatite spectrum from the RRUFF database (Lafuente et al., 2016). The CaSi₂O₅-titanite phase is also absent from the Ca-silicate spectrum in this sample. Raman spectral data available in data S1.</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>Figure 2: µXRF data from GSECARS 13-IDE for diamonds Blue2 and NL247-2. (a) Image of Ca-silicate inclusion in diamond NL247-2 with Fe-Ni rich alloy rims. (b) Rim around inclusion is Fe-Ni-rich. Peaks that are not identified are analytical background features due to Bremsstrahlung and x-ray scattering. (c) Ca, Fe, Sr, and Y are visible in the spectra of the Ca-silicate inclusion. (d) Image of enstatite inclusion in Blue2. The central enstatite inclusion appears dark blue due to its low Fe content, but it is identifiable due to the surrounding Fe-Ni rich rim. (e) Fe and Ni spectral peaks are visible around inclusion. (f) Enstatite inclusion shows no visible spectral peaks in this range.</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>Figure 3: Representative panchromatic cathodoluminescence (CL) images of Type IIb diamonds. All samples have CL emissions which show dislocation networks that form during diamond deformation in the deep Earth. A few samples show luminescence zoning that is inversely correlated with boron concentration. Where distinct zones are visible, boron concentrations are shown.</figcaption></figure>

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<figure><img src="image_6.png" /></figure>

Figure 4: A and B) Rare earth element concentrations of blue diamonds, normalized to CI chondrite (Sun & McDonough, 1989). C and D) Trace element concentrations of blue diamonds, normalized to primitive mantle (Sun & McDonough, 1989).

B and D are double-normalized to Ho$_{N}$ = 1 because elemental concentration is a function of nano-sized inclusion density. Only data above the limit of quantification (LOQ; blank + 7σ) are plotted.

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<figure><img src="image_7.png" /></figure>

Figure 5: Carbon isotope histogram for blue diamonds compared to probability density functions for eclogitic diamonds and serpentinite from ophiolites, ultramafic complexes, and active peridotite-hosted hydrothermal systems (Data S3). Analytical precision (2σ) is ~0.1 ‰ for blue diamonds. The histogram mode at approx. −17 ‰ may be an artefact due to oversampling of diamonds from one source. Density function bandwidth is 1.5 ‰ for serpentinite and 0.7 ‰ for eclogitic diamonds. Range of δ¹³C for mid-ocean ridge basalt (MORB) source mantle indicated by bar at approx. −5 ‰ (Marty & Zimmermann, 1999).

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<figure><img src="image_8.png" /><figcaption>Figure 6: Boron and carbon isotope and concentration data of blue diamonds. (a) δ<sup>11</sup>B vs. δ<sup>13</sup>C of blue diamonds from Cullinan mine and those of uncertain origin. Mid-ocean ridge basalt (MORB) is indicated in grey field (Marschall, 2018). (b) Measured δ<sup>11</sup>B vs. the concentration of B in the solution (not normalized to boron concentration in diamond). Although the B concentration of the solution has no correlation with B concentration of the diamond, it is useful to assess that there is not a change in measured B isotopic composition with B concentration. The dashed lines enclose repeat analyses on different sides of the same stone.</figcaption></figure>

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<figure><img src="image_9.png" /></figure>

Figure 7: Boron isotope signature of blue diamonds, slab reservoirs, deeply derived magmas, seawater, and typical upper mantle. Typical upper mantle (grey bars) is represented by mid-ocean ridge basalts (MORB; -7.1 ± 0.9 ‰; Marschall, 2018). (A) δ¹¹B of blue diamonds is compared to ranges for seawater, slab serpentinite, and sediments + altered oceanic crust (AOC). Field for metaserpentinite is from the Cerro del Almirez massif that experienced metamorphism to eclogite-facies conditions (Harvey et al., 2014). The arrow for serpentinite indicates the isotopic evolution during subduction for present-day abyssal serpentinites (Martin et al., 2020), to modeled post-serpentinite phases at >3 GPa (Cannaò, 2020). The arrow for sediment + AOC indicates the isotopic evolution from surface (Data S5) to modeled metasediment and meta-AOC at > 5 GPa (Konrad-Schmolke & Halama, 2014). (B) Box and whisker plot for δ¹¹B of blue diamonds (n = 11; this study), ocean island basalts (OIBs; n = 48) and carbonatites (n = 23; data S5). The typical analytical uncertainty (indicated by the 2σ bar) for blue diamonds includes propagated uncertainty from repeat analysis of secondary reference materials.
<figure><img src="image_10.png" /></figure>

Figure 8: Stability of hydrous meta-peridotitic phases including ringwoodite, wadsleyite, bridgmanite (Brg), and dense hydrous magnesium silicates (DHMS) phases – Phase A (PhA), superhydrous phase B (SHB), phase E (PhE), phase D (PhD). Overlaid on this phase diagram are pressure-temperature (PT) pathways for subducting slabs calculated from deep earthquake seismicity (taken from Shirey et al., 2021): the purple PT pathway represents basaltic lithologies to the top of the oceanic crust, and the green PT pathway represents peridotite mantle lithologies at the slab Moho. Peridotite in subducting slabs can transport water, hosted in DHMS phases, down to the transition zone. As slabs stall and warm up at the transition zone, breakdown of DHMS phases produce H₂O-rich fluids for diamond formation (blue arrows). Figure is modified from Iwamori (2004) and Shirey et al. (2021).
<figure><img src="image_11.png" /><figcaption>Figure 9: Potential modes for the transfer of slab volatiles to the deep Earth in relatively cool slabs in the ancient- and current-day mantle. (A) Cross-section of the mantle depicting a relatively cool slab subducting into the lower mantle. Phase stabilities of serpentinite and dense hydrous magnesium silicates (DHMS), such as Phase H, are indicated in the slab. Formation of Cullinan blue diamonds in the Mesoproterozoic was likely influenced by somewhat higher mantle temperature (~100 °C; Vlaar et al., 1994) than the present-day mantle. At this time, many subducting slabs would produce boron-free diamond after crossing the carbonated metabasaltic solidus in the transition zone (Thomson et al., 2016b), and would also produce boron-rich diamonds during fluid release from metaserpentinite in the uppermost lower mantle. (B) Metaperidotite hosts substantial H₂O, C, and B, until the destabilization of DHMS ('DHMS out') in the Mesoproterozoic mantle. This release of hydrous fluids for blue diamond formation results in volatile and incompatible element-poor subducting slabs entering the deeper lower mantle. (C) While most slab tops will cross a carbonated metabasaltic solidus in the present-day mantle to produce boron-free diamonds, (D) high-pressure DHMS may be retained ('Phase H in') in the current-day deeper portions of the slab, allowing for the transfer of volatile and incompatible elements into the deeper lower mantle within DHMS-rich carbonated metaperidotite (Nishi et al., 2014; Scambelluri et al., 2019).</figcaption></figure>