Primordial neon and the deep mantle origin of kimberlites

doi:10.21203/rs.3.rs-5046180/v1

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Primordial neon and the deep mantle origin of kimberlites

https://doi.org/10.21203/rs.3.rs-5046180/v1

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The genesis of kimberlites – Earth’s deepest-derived melts – remains an unresolved question despite the economic and scientific interest surrounding these diamond-bearing continental magmas. One critical question is whether they tap ancient, deep mantle or the shallow convecting mantle with partial melting triggered by deep-mantle plumes or plate tectonics. To address this question, we report the compositions of He-Ne-Ar isotopes, formidable tracers of the occurrence of primordial material in the mantle, in magmatic fluids trapped in olivine from kimberlites worldwide. We show that two kimberlites have Ne isotopes less nucleogenic than the upper mantle, which unequivocally requires a deep mantle origin. This is corroborated by previous evidence of negative W isotope anomalies and the location of these kimberlites along age-progressive hot-spot tracks. The lack of strong primordial He isotope signatures indicates overprinting by lithospheric and crustal components, which suggests that Ne isotopes are more robust tracers of deep-mantle contributions in intraplate magmas.

Earth and environmental sciences/Solid Earth sciences/Geochemistry

Earth and environmental sciences/Solid Earth sciences/Petrology

kimberlite

olivine

He isotopes

Ne isotopes

mantle plume

chemical geodynamics

Noble gases are robust tracers of the origin of mantle-derived magmas and their study has played a pivotal role in understanding Earth accretion and evolution ^1–3. Elevated primordial to radiogenic/nucleogenic isotope ratios of He and Ne (i.e., high ³He/⁴He and low ²¹Ne/²²Ne) in intra-plate magmas originating from the lower mantle (e.g., many ocean island basalts, or OIBs, and continental flood basalts) compared to upper mantle magmas (mid-ocean ridge basalts or MORBs) implies an origin in the relatively undegassed deep mantle, which is enriched in primordial volatiles ^1–5. Variations in ¹²⁹Xe normalised to a non-radiogenic Xe isotope (e.g., ¹³⁰Xe), where ¹²⁹Xe is produced by radioactive decay of ¹²⁹I within ~ 100 Myr of Earth formation, further point to separation between these lower and upper mantle sources early in Earth’s history ³. The exact mechanism leading to the preservation of primordial noble gas signatures in the deep Earth remains contentious, with proposals ranging from storage in the core ^6–8, dense Fe-rich piles derived from magma ocean crystallisation ⁹ or simply less processed mantle material ¹⁰, perhaps combined with migration and storage of primordial noble gases into geochemically depleted mantle rocks ¹¹. Contributions from geochemically depleted and deeply subducted material to the sources of deep-mantle plumes is supported by combination of noble gas isotopes with other geochemical tracers, such as the radiogenic isotopes of lithophile elements (Sr, Nd, Hf, Pb) in several OIBs ^7,12–14.

The lack of convective homogenisation of primordial noble gases within Earth’s mantle makes He and Ne isotopes powerful tracers of the origin of magmas in the deep mantle. For example, He isotope ratios substantially higher than the MORB value (³He/⁴He = 8 ± 1 × Ra where Ra indicates the atmospheric ratio of 1.384×10^{− 6 1,15} – with lower values reported in some ridge segments) support a deep mantle origin for continental magmas associated with large igneous provinces (LIPs) ^4,5,11. Neon isotopes in LIP magmas, although examined less frequently because their measurement is analytically more challenging due to low concentrations, are less nucleogenic than MORBs, which is consistent with a deep mantle contribution ^5,16,17. In some cases, deep-mantle Ne isotopic signatures do not coincide with high ³He/⁴He reflecting mixing with deeply subducted material ¹⁸, upper convective mantle ^19,20 or sub-continental lithospheric mantle ^21,22.

Helium and Ne isotopes are ideal candidates to investigate the potential occurrence of deep and ancient material in the sources of magmas, including kimberlites, where such contributions are contentious (see below). Kimberlites are carbonate-rich magmas, which are considered to tap similar mantle sources as those of OIBs, based on overlapping Sr-Nd-Hf isotope compositions ^23,24. Simple petrological considerations, including the instability of carbonates in the deep lithosphere combined with the low degrees of melting (< 1%) to generate the observed elevated concentration of mantle-incompatible trace elements, firmly rule out an origin of these melts in the lithospheric mantle ²⁵. Being emplaced on the stable nuclei of continents (i.e. cratons) since at least 2 Ga ^26,27, kimberlites provide a complementary and temporally more extensive perspective on the origin of compositional variations in the convecting mantle than OIBs.

Geographic correspondence between the peripheral zones of large low-shearwave velocity provinces (LLSVPs) above the core-mantle boundary and the reconstructed location of the majority of Phanerozoic kimberlites at the time of eruption has previously been used as evidence to suggest a deep mantle origin for kimberlites ^28,29 – where the LLSVP margins represent the loci of mantle plume generation including the sources of several OIBs and LIPs ³⁰. This view is supported by the spatio-temporal association of some kimberlites with LIPs ³¹. Additional models of kimberlite generation entail partial melting of upper convecting mantle sources in response to extensional tectonics ³² or small-scale convection ³³. An upper-mantle origin might be supported by temporal correspondence between kimberlite formation and episodes of supercontinent reorganisation ^27,32, hence underscoring that no single trigger of kimberlite magmatism exists ²⁵.

The geochemical record of kimberlites similarly includes inconclusive evidence supporting a deep and ancient, rather than shallow mantle origin of these magmas. Long-lived radiogenic isotope systems (⁸⁷Rb/⁸⁷Sr, ¹⁴⁷Sm/¹⁴³Nd, ¹⁷⁶Lu/¹⁷⁶Hf) indicate that the majority of kimberlites tap a long-lived (> 2 Ga) common mantle component that is equivalent to the PREMA (PREvalent MAntle) component documented in OIBs ¹⁴ and is potentially, but not necessarily stored in the deep mantle ^23,34,35. The short-lived ¹⁸²Hf-¹⁸²W system seems to support a deep and/or ancient origin of kimberlites based on very small negative anomalies documented in some ³⁶, but not all kimberlites ³⁷. However, similar anomalies have so far not been detected in the short-lived ¹⁴⁶Sm/¹⁴²Nd system ^36,38. Previous analyses of He and Ne isotopes provide inconsistent and therefore inconclusive results with respect to the origin of kimberlites. While non-nucleogenic, plume-like Ne isotope compositions combined with radiogenic He isotopes (< 8 Ra) were reported for olivines from the Udachnaya-East kimberlite in Siberia ²², very high ³He/⁴He (up to 27 Ra) and MORB-like He isotope values were documented in olivine from two kimberlite provinces in western and southern Greenland (Sarfartoq and Pyramidefjeld, respectively) ³⁹. In contrast, olivine grains from the Holocene Igwisi Hills kimberlite in Tanzania have relatively low ³He/⁴He (~ 5 Ra) ⁴⁰.

The goal of this contribution is to provide new constraints on the genesis of kimberlites by exploring the isotope systematics of noble gases (He, Ne, Ar) in kimberlites worldwide. We present He, Ne and Ar isotope compositions of fluids trapped in 35 olivine separates from 18 kimberlites around the globe with ages ranging from Neoproterozoic (~ 600 Ma) to Cenozoic (14 kyr), including kimberlites from Udachnaya-East and Sarfartoq (Supplementary Table S1). To understand the origin of olivine-hosted fluids and identify potential contributions from crustal components, noble gas data are supplemented by fluid inclusion petrography and determinations of trace element concentrations and Sr-Nd isotope ratios in the residues of olivines that were crushed in vacuo for noble-gas isotope analysis (Methods). This approach, combined with trace element and Sr-Nd-Hf isotope determinations of bulk kimberlite samples from which the olivines were extracted, constrains the roles of source variations, including contributions from ancient lower-mantle and deeply subducted components, interaction with metasomatised lithospheric mantle and crustal contamination in the genesis of kimberlites.

He-Ne-Ar isotopes of olivine in kimberlites

The criteria for sample selection includes wide geographic (9 cratons in 4 continents) and temporal coverage (between 582 Ma – Finland Pipe 1 – and 14 kyr – Igwisi Hills), limited olivine alteration, and existing noble-gas data (see Supplementary Table S1 for details). The Victor kimberlite (Canada) was included because it is located along the Great Meteor hot-spot track ⁴¹ while Letseng (Lesotho), Karowe (Botswana) and Monastery (South Africa) are known to contain sub-lithospheric diamonds ⁴², potentially suggesting very deep origins ⁴³. The kimberlites were also selected to reflect geochemical variations that suggest input from various components including metasomatised lithospheric-mantle wall rocks in micaceous kimberlites from Sierra Leone (Koidu and Tonguma ⁴⁴) or subducted material in those from Lac de Gras (Boa and Leslie ⁴⁵).

All the kimberlites contain abundant macrocrysts (> 1 mm) and microcrysts of olivine (Supplementary Fig. S1) and other less abundant to absent phases (e.g., phlogopite, garnet, clinopyroxene, ilmenite) in a fine-grained groundmass dominated by carbonates and/or serpentine with variable spinel, apatite and perovskite. Regardless of size, olivine is typically zoned between mantle-derived xenocrystic cores and magmatic rims (see details in ⁴⁶). Fluid inclusions are small (generally < 10 µm in size) but abundant and arranged in trails and elongated swarms that cut across olivine zonation (Fig. 1). These inclusions typically host dominant Ca-Mg and alkaline carbonates ^47,48. They are clearly secondary in origin, in that they formed via fluid infiltration after olivine crystallisation at sufficiently high temperature (e.g., ≥ 400–500°C) to allow olivine annealing and entrapment of residual fluids.

To examine the composition of trapped noble gases, olivine grains were separated from crushed kimberlite samples (Supplementary Fig. S2). Noble gas isotope analysis was performed by crushing in vacuum at Woods Hole Oceanographic Institute (WHOI) and Scottish Universities Environmental Research Centre (SUERC) (Methods). Helium isotopes are systematically more radiogenic than the MORB composition with ³He/⁴He values below 6 Ra (Fig. 2; Supplementary Table S2-S3). Analyses undertaken at WHOI and SUERC yield consistent results (Supplementary Fig. S3). ³He/⁴He values show a broad inverse relationship with ⁴He contents (Fig. 2a) indicating that the relative abundance of ⁴He* (i.e. ⁴He in-grown or implanted after kimberlite emplacement) profoundly impacts ³He/⁴He (see discussion below) and is dominant in the oldest kimberlites. Helium isotopes indeed show a broad relationship with age, where Neoproterozoic and Cambrian kimberlites exhibit the lowest ³He/⁴He (≤ 2.3 Ra; Fig. 2b), including all samples from western Greenland. These results are in stark contrast with those of Tachibana et al. ³⁹ who detected extremely high ³He/⁴He (up to 27 Ra) in kimberlites from Sarfartoq, western Greenland. Conversely, new analyses of Udachnaya-East and Igwisi Hills olivine are entirely consistent with those of Sumino et al. ²² and Brown et al. ⁴⁰, respectively. The He isotope compositions of Mesozoic and Cenozoic kimberlites are broadly similar to those measured in Cenozoic alkaline mafic lavas from continental intraplate settings (5.9 ± 1.2 Ra ⁴⁹; Fig. 2a) with the exception of highly to moderately micaceous kimberlites (Koidu-Tonguma and Kimberley, respectively) which exhibit lower ³He/⁴He (< 4 Ra). It is noteworthy that mantle xenoliths entrained by kimberlites worldwide (southern Africa, Siberia, Canada ^50,51) show the same range in He isotopes (0.05 to 6 Ra) as kimberlites in this study, except for a less radiogenic dunite xenolith from Udachnaya-East with ³He/⁴He (9.8 Ra) above the MORB range.

In contrast to He, the isotopes of Ne vary from less to more nucleogenic than the reference MORB value ^52,53 (Fig. 3). Neon isotope ratios are generally resolvable (> 2 × standard deviation) from the air composition, clearly indicating that mantle Ne is present in the majority of olivines. Victor and Udachnaya-East exhibit Ne isotope compositions that are less nucleogenic than MORBs with ²¹Ne/²²Ne_S (extrapolated to ²⁰Ne/²²Ne = 12.5) of 0.046–0.051 (n = 3) and 0.051–0.055 (n = 8), respectively (where such extrapolation is justified by the ubiquitous occurrence of air-related Ne in olivine). The new Udachnaya-East data are similar, although marginally more nucleogenic than those reported by Sumino et al. ²² for olivine separates from the same locality, resulting in a lower slope for the linear regression through the new data in a ²⁰Ne/²²Ne-²¹Ne/²²Ne plot (Fig. 3). These data point to the occurrence of lower-mantle Ne in these kimberlites, as discussed below. ⁴⁰Ar/³⁶Ar are also typically resolvable from the air composition (298.6) and range up to ~ 11,000 in micaceous kimberlites from Koidu with high values recorded in other kimberlites (e.g., up 8,000 in Udachnaya-East; Supplementary Table S2). Linear correlations between ⁴⁰Ar/³⁶Ar and ratios of primordial isotopes (i.e. not affected by radiogenic/nucleogenic processes: ²⁰Ne/²²Ne and ³He/³⁶Ar; Supplementary Fig. S4) in multiple analyses of the same sample indicate a predominant mantle origin for Ar including variable contribution from radiogenic ⁴⁰Ar.

Trace element concentrations and Sr-Nd isotopes in olivine

Noble gas data provide robust evidence for a mantle origin of the fluids trapped in olivine. However, radiogenic helium is clearly present, and the noble gases do not rule out crustal contributions, which is possible considering the secondary origin of fluid inclusions in kimberlitic olivine. To address this issue, the trace element and Sr-Nd isotope composition of crushed olivine were measured after sample dissolution (Methods). The rationale behind this approach is that the concentrations of elements strongly incompatible in olivine, including Rb, Sr, Sm, Nd, Th and U, are completely controlled by the composition and relative abundance of fluid inclusions – solid inclusions in olivine being scarce and dominated by spinel, which is similarly depleted in these elements. This inference is confirmed by the comparison of laser ablation and solution-mode ICP-MS analyses of mantle olivine containing secondary fluid inclusions – ratios of 1/8 and 1/7 for Sr and U, respectively ⁵⁴.

Crushed olivines show high concentrations of compatible elements (e.g., Ni = 1950–2970 ppm; n = 21/22, i.e. one analysis with lower Ni) and, as expected, low concentrations of incompatible trace elements (e.g., Sr = 1.3–6.1 ppm; Ce = 0.12–0.98 ppm; U = 2–21 ppb; n = 15/22; Supplementary Table S3). Exceptions include olivine separates from Boa (Sr = 75–78 ppm; U = 171–224 ppb; n = 2), Igwisi Hills (6.8 ppm; 41 ppb), Ghacho Kué (18 ppm; 27 ppb) and Maniitsoq (≤ 33 ppm; 35–37 ppb; n = 3) where higher contents of Sr, U and other incompatible trace elements are consistent with optical observations of contamination by groundmass material attached to the olivine grains (Supplementary Fig. S1). Compared to bulk rock analyses of the same samples, the olivine separates show broadly similar primitive-mantle normalised patterns of incompatible trace elements although at much lower absolute values (Fig. 4). These characteristics argue for fluid inclusions exerting a dominant control on the trace element budgets of the majority of incompatible elements measured in olivine residues. Positive anomalies of Pb and Sr and flat HREE profiles in some olivine separates compared to negative Pb and Sr anomalies and negatively sloping REE in bulk kimberlites probably stem from localised crustal contribution to the fluids trapped in olivine ⁵⁵. Age-corrected Sr and Nd isotopes of the olivine separates are generally indistinguishable from those of bulk kimberlites (Fig. 5 and Supplementary Fig. S5). Exceptions include Victor and, to a lesser extent, Udachnaya-East, where higher ⁸⁷Sr/⁸⁶Sr in olivine point towards some crustal contamination of the trapped fluids; and Jericho, where the lower ⁸⁷Sr/⁸⁶Sr of olivine rather suggest some crustal contribution to the bulk sample. These data indicate that olivine trapped mostly pristine magmatic fluids with crustal contamination evident in only some cases.

The Udachnaya-East and Victor kimberlites have less nucleogenic Ne isotope compositions compared to the upper mantle source of MORBs (Fig. 3). These data point to lower-mantle contributions for these kimberlites potentially related to plumes from the core-mantle boundary. However, the lack of high ³He/⁴He in Udachnaya-East and Victor (Fig. 2), despite their plume-like Ne isotopic compositions, is intriguing. The lack of plume-like ³He/⁴He in West Greenland is also an interesting finding, even though five kimberlites from two adjacent fields were examined, including Sarfartoq where high ³He/⁴He values had been previously reported ³⁹. The observed relationships between ³He/⁴He and both ⁴He and age (Fig. 2) suggest a likely addition of ⁴He* either in-grown or implanted into the olivine inclusions, at least for the Cambrian and Neoproterozoic samples. Calculations of ⁴He* ingrowth and implantation are highly model-dependent and hence inconclusive (see Methods for details of calculations and related discussion). These data, however, do demonstrate the challenges of interpreting He measurements in ancient Th-U-rich rocks. The measured ³He/⁴He therefore represent minimum values for the kimberlite mantle sources in the Cambrian and Neoproterozoic samples, which might have been further lowered by crustal contamination and interaction with the lithospheric mantle as discussed below. The role of ⁴He* ingrowth and implantation in the younger samples is probably more limited, and effectively negligible in the Holocene kimberlites from Igwisi Hills. Below, the full array of data acquired in this study is employed to assess processes which might have affected noble gases in kimberlites and their olivine during magma ascent and emplacement, followed by potential variations in kimberlite source compositions.

Noble gas modification during kimberlite ascent and emplacement

During ascent to the surface, kimberlites exsolve abundant volatiles, a process which probably governs their very fast ascent and promote their ability to transport xenoliths up to several decimetres in size. The contrasting solubilities of noble gases in melts and volatile-rich fluids affect their relative abundances in the fluid inclusions trapped by olivine as suggested by the direct correlations observed between log(⁴He/⁴⁰Ar*) and log(⁴He/²¹Ne*) (Fig. 6). ⁴He/²¹Ne* values exceeding the mantle production ratio (2.2 × 10^{7 56}) further point to substantial contribution of post-crystallisation ⁴He* in olivine from Neoproterozoic kimberlites. The low He/Ne and He/Ar of olivine suggest variable He loss after kimberlite emplacement. Alternatively, the trapped fluids could represent an exsolved fluid phase, rather than residual melts ^47,48, due to lower solubility of He compared to Ne and Ar in hydrous fluids relative to silicate and carbonate melts ^20,57,58.

Beyond exsolving fluids, kimberlites variably interact with crustal rocks during emplacement ⁵⁹. Crustal contamination, perhaps via mixing with crustal fluids, is evident in the higher Sr isotope compositions of olivine versus bulk rocks from Victor and Udachnaya-East (Fig. 5). It is also supported by positive Pb and Sr anomalies in trace element patterns of these olivines (Fig. 4). Crustal contamination introduces strongly radiogenic He and nucleogenic Ne in mantle-derived magmas due to the scarcity of ³He and ²²Ne in the crust. In addition, the rate of ⁴He* production in contaminated magmas is high due to high abundances of U and Th in continental crust ⁶⁰. The isotopes of He are therefore more substantially affected by crustal contamination than those of Ne because kimberlite fluids trapped in olivine have low He/Ne compared to the ⁴He-rich continental crust – the latter being approximated by the ⁴He/²¹Ne* crustal production rate ⁶⁰ (Fig. 6; see also the ‘crustal contamination’ curve in Fig. 7). The implication is that ³He/⁴He in Victor and Udachnaya-East olivine represent minimum values compared to their sources – a hypothesis explored further below.

Whilst crustal contamination is not ubiquitous, a fundamental role of interaction with lithospheric mantle wall rocks is well established for the petrogenesis of kimberlites worldwide ^25,46,61. For example, a combination of mica enrichment, elevated Sr isotopes and chondritic to marginally super-chondritic Nd-Hf isotopes in micaceous kimberlites from Koidu and Tonguma probably indicates an important contribution from metasomatised, phlogopite-bearing lithologies in the lithospheric mantle that was traversed by kimberlite melts sourced from the convecting mantle ⁴⁴. These kimberlites show the lowest ³He/⁴He (≤ 2.3 Ra) among the samples younger than 500 Ma (Fig. 2) with Ne isotopes typical of the SCLM (Fig. 3) and ⁴He/²¹Ne* close to the mantle production rate (Fig. 6). Although it seems possible that ⁴He is derived in part from implantation from the micaceous groundmass (see Methods), these data suggest that phlogopite-rich lithospheric mantle contributed, at least marginally, to the noble gas and volatile budget of the Koidu and Tonguma kimberlites.

A similar conclusion can be reached for the moderately micaceous kimberlites from Kimberley (Wesselton and Bultfontein) which show broadly similar isotopic compositions to the Koidu and Tonguma samples for the noble gases and lithophile elements Sr, Nd and Hf (Figs. 2–5) combined with a well-established enrichment in mica of the underlying lithospheric mantle ⁶². Mixing models show that the He-Ne isotope composition of the Kimberley kimberlites can be crudely reproduced by combining He and Ne from a melt derived from either an upper-mantle source with MORB-like He-Ne isotope compositions ^1,63, or a lower-mantle source similar to that of the Baffin picrites ^6,11,16, and noble gases from phlogopite-rich lithospheric mantle (Fig. 7; see Methods for details). However, all of these models entail deriving disproportionate amounts of noble gases from the lithospheric mantle (> 70–80%), given that kimberlite melts are likely to have at least one or two orders of magnitude higher concentrations of noble gases than metasomatised lithospheric mantle lithologies (see Methods). Therefore, unless the lithospheric mantle hosts substantially larger amounts of He and Ne than those measured in mantle xenoliths or we have grossly overestimated the concentrations of noble gases in kimberlites, interaction with lithospheric mantle rocks can only have a limited effect on the noble gas isotope composition of kimberlites.

Noble gas constraints on the kimberlite source

The alternative explanation for the He-Ne isotope signature of the Kimberley kimberlites is a contribution from subducted crustal material, a model previously invoked to explain the Sr-Nd-Hf and S isotope systematics of these kimberlites ^23,64. The oceanic crust loses all of its He and most of its Ne budget during subduction ^65,66, but experiences substantial ingrowth of radiogenic He and nucleogenic Ne during mantle residence (see Methods). Mixing models show that addition of subducted material to the convecting mantle can generate He-Ne isotopic signatures that are intermediate between those produced by crustal contamination (substantial decrease of ³He/⁴He at relatively invariant ²¹Ne/²²Ne_S for low degrees of contamination) and interaction with metasomatised lithospheric mantle (moderate decrease in ³He/⁴He with increasing ²¹Ne/²²Ne_S; Fig. 7). Mixing trajectories between an upper mantle source (with or without some lower-mantle influence) and subducted oceanic crust (< 5%) intersect the composition of the Kimberley kimberlites (Fig. 7). The He-Ne isotopes of the Lac de Gras kimberlites may be similarly explained by adding < 2% of subducted crust to an upper mantle source (Fig. 7), which is consistent with the peculiar geochemically-enriched ¹⁴³Nd-Hf isotope compositions ⁴⁵ and the lack of a geodynamic connection with deep-mantle plumes for these kimberlites ^25,29. It is noteworthy that the mixing model requires the subducted crustal material to have higher Ne/He than the convecting mantle source to fit the Kimberley and Lac de Gras data, which is consistent with the more efficient loss of He from subducted materials compared to Ne during subduction via dehydration ^66,67 and/or during mantle storage via diffusion.

Olivine separates and bulk-kimberlite samples from Victor and Udachnaya-East exhibit the highest age-corrected Nd isotope compositions (ε¹⁴³Nd_i; Figs. 5 and 7) with values overlapping the evolution curve of the common, moderately geochemically-depleted (PREMA-like) component identified in kimberlites worldwide ³⁵. Despite some crustal contamination of the trapped fluids based on higher ⁸⁷Sr/⁸⁶Sr_i of olivine compared to their bulk rocks (Fig. 5), these kimberlites have experienced minimal contributions from subducted crustal material or interaction with metasomatised lithospheric mantle, which both lower ε¹⁴³Nd_i. We believe these minimal contributions favoured the preservation of plume-like Ne isotopes in these kimberlites (Fig. 7). A connection between the Victor kimberlite and a deep-mantle plume is consistent with its location along the Great Meteor hot-spot track ⁴¹. High ³He/⁴He (up to 52 Ra) in sub-lithospheric diamonds from Juina (Brazil) were similarly suggested to be linked to the deep-mantle plume that generated Cretaceous kimberlites in Brazil ⁶⁸. Udachnaya-East shows the most negative µ¹⁸²W recorded in kimberlites ³⁶ with olivine in one mantle xenolith and clinopyroxene in three mantle xenoliths from this locality exhibiting ³He/⁴He values marginally above the MORB range ⁵⁰ and Ne isotopes less nucleogenic than MORBs ⁶⁹, respectively. These features, as well as unradiogenic He and non-nucleogenic Ne in kimberlite-related fibrous diamonds from the nearby Nyurbinskaya kimberlite ⁷⁰, point to lower mantle contributions probably related to deep-mantle plumes for the Devonian kimberlites in Siberia, including Udachnaya-East.

Yet, both Victor and Udachnaya-East olivines exhibit moderately radiogenic ³He/⁴He (5.4–5.8 Ra and 5.0-5.7 Ra, respectively) apparently at odds with their low ²¹Ne/²²Ne_S. If He and Ne isotopes reflect the secular evolution of a compositionally homogeneous source, production of ⁴He* and ²¹Ne* should be tightly linked by radioactive decay of U and Th, as it is commonly the case for young oceanic basalts and their olivines (e.g., ^1,52,71). This is clearly not the case for Victor and Udachnaya-East where ‘decoupling’ of He and Ne isotopes requires contribution of at least an additional component, perhaps crustal contamination identified in the olivine Sr isotopes (Fig. 5). Modelling shows that this process can lower ³He/⁴He while leaving Ne isotopes largely unaffected (Fig. 7) with Nd isotopes being similarly unmodified from their mantle values due to low solubility of REE in hydrous fluids and moderately low concentrations of LREEs in the crust ⁷². Crustal contamination is a viable explanation for the relatively low ³He/⁴He only if the ²¹Ne/²²Ne_S of the source was intermediate between the Baffin picrites and MORB values as can be gauged from the mixing trajectories in Fig. 7. Assuming similar ²²Ne (and ³He) contents in the sources, higher ²¹Ne/²²Ne_S (and lower ³He/⁴He) of the kimberlite source compared to typical deep-mantle plumes such as Baffin ¹⁶, Hawaii ⁷³ or Galapagos ⁷¹ is consistent with a more fertile source for kimberlites (or at least more fertile components participating in partial melting) containing higher U and Th concentrations.

Implications

This study shows that kimberlites, which derive from the convective mantle ^24,25,27,45, share similar He isotope compositions to the mantle xenoliths they entrain (0.05 to 6 Ra ^50,51). This overlap confirms previous suggestions that noble gases in mantle xenoliths are dominated by input from entraining and/or precursor magmas which infiltrate the lithospheric mantle not long before eruption ^51,74. This view is consistent with the dominant secondary and, therefore, late origin of fluid inclusions in mantle xenoliths as well as abundant evidence of interaction between mantle xenoliths and their transporting media ^25,75. It is further supported by the occurrence of plume-like He-Ne isotope compositions in lithospheric mantle xenoliths transported to the surface by plume-related magmas at Samoa, Hawaii and in south-eastern Australia ^76,77. Although radiogenic and nucleogenic noble gases produced in situ in the lithospheric mantle might contribute to the percolating sub-lithospheric melts ^49,51, mass balance calculations presented in this work indicate a limited role for indigenous lithospheric-mantle noble gases in the convecting-mantle melts that traverse the lithospheric mantle.

It is clear that at least some kimberlites (e.g., Victor, Udachnaya-East) contain primordial Ne derived from the lower mantle. These results combined with previous noble gas ²², W isotope ³⁶ and ¹⁴³Nd-Hf isotope data ³⁵ as well as geodynamic reconstructions ^28,29,41 underline a link between at least some kimberlites and plumes from ancient domains in the lower mantle ⁴³. It is noteworthy that these kimberlites are not renown hosts of sub-lithospheric diamonds, which underscores a potential dichotomy between kimberlite source regions and origin of entrained sub-lithospheric material, the latter probably entrained from material that underplated continental lithospheric roots ⁷⁸. Conversely, it is evident that some kimberlites (e.g., Lac de Gras) are not related to deep-mantle plumes as shown by the noble-gas data presented herein, while the data are inconclusive for other kimberlites (e.g., Kimberley) where subducted crustal material appear to largely influence the isotopes of He and Ne. In these cases, it is currently not possible to unanimously establish whether the sources of these kimberlites are located in the upper or lower convecting mantle. Application of noble gas isotope analyses, combined with lithophile radiogenic and W isotopes, to a larger number of kimberlites minimally affected by geochemically-enriched components derived from subducted material and/or interaction with metasomatised lithospheric mantle will help elucidate the relative proportions of kimberlites that are related to deep-mantle plumes or upper mantle sources.

A corollary of this work is that Ne isotopes, in samples where mantle contributions can be robustly separated from air contamination, represent more robust tracers of the preservation of early Earth heterogeneities in magmas from the deep mantle compared to He isotopes, especially in continental settings. The likely reason is that plumes from the lower mantle have Ne/He ratios substantially higher than those of upper mantle and crustal contaminants as also noted in some previous studies ^19–21,71, and the importance of radiogenic helium in the older samples, herein demonstrated using kimberlites. Neon isotope measurements of intraplate continental lavas, for which data are restricted to some LIPs ^{5,16,17,19,79}, therefore represent a new avenue of research to detect potential contributions by deep-mantle plumes or material thereof, especially if combined with petrographic, trace element and Sr-Nd isotope analyses of fluid inclusions in olivine. This approach can be extended to ancient rocks where interpretation of He isotopes is complicated by radiogenic ingrowth and implantation, processes that affect Ne to a much lesser extent.

Noble gas analyses

Offcuts of kimberlite samples were fragmented using either a ring mill or a Selfrag, which employs electrostatic discharges to disaggregate rock samples along grain boundaries. This latter method produces cleaner olivine grains, i.e. with less adhered groundmass. After sieving the crushed material, olivine was separated from the 1–2 mm and 0.7-1.0 mm size fractions using a binocular microscope. Separated olivines were then cleaned using various combinations of diluted nitric acid and acetic acid followed by distilled water and acetone to remove carbonates and other groundmass impurities. This step was followed by further olivine purification by picking out olivine grains with evident contamination by extraneous material. For sample BV-1 (Boa, Lac de Gras) and MPV-04-193 (Ghacho Kué) it was not possible to obtain a clean separate, whereas for samples WESK-8 (Wesselton, Kimberley) and 527 (Maniitsoq, West Greenland) both clean and contaminated separates were obtained and measured for comparison (Supplementary Fig. S3).

The analyses of noble gas (He, Ne and Ar) contents and isotopic compositions were conducted at Woods Hole Oceanographic Institutions (WHOI) with a subset of the same samples, including two contaminated olivine separates, measured at the Scottish Universities Environmental Research Centre (SUERC; He only). In both laboratories the analyses were conducted exclusively by in vacuo crushing using a magnetically actuated metallic sphere and a hydraulic crusher equipped with a vertical piston, respectively. At WHOI, He, Ne and Ar were measured sequentially on the same sample aliquot with loaded olivine masses ranging between 0.21 g and 1.16 g, but generally > 0.6 g. Two crushing steps of 10, 20 or 40 strokes each were generally applied to extract the gases trapped in fluid inclusions. The extracted gases were purified on a fully automated extraction line using two SAES ST707 pellet getters, one held at 300° C and the other between room temperature and 300° C. The gases were pre-measured (and split if necessary) using a quadrupole mass spectrometer (QMS) to ensure that gas sample size in the mass spectrometer matched standard size. Following purification and trapping, the noble gases were selectively desorbed from a charcoal trap (helium) and a nude stainless-steel trap (neon and argon). Helium and Ne isotopes were measured using a MAP 215 − 50 mass spectrometer locally referred to as MS5, and Ar isotopes using a dedicated Hiden QMS. Standards and blanks bracketed the samples as closely as possible. The helium standard tank was produced from MORB glass with a ³He/⁴He of 8.34 ± 0.05 Ra (calibrated against air using a separate mass spectrometer); error estimates were dominated by reproducibility of standard aliquots, which was typically 1%. Neon and argon were calibrated with air aliquots from a separate tank. Helium blanks were less than 1 × 10^− 11 cc STP ⁴He; blanks for ²⁰Ne and ⁴⁰Ar during the course of this study were typically less than 1 × 10^− 12 and 2 × 10^− 10 cc STP, respectively. Blanks are negligible for helium and argon, but neon isotope data are considered to be robust only for ²⁰Ne > 1 × 10^− 11 cc STP in the mass spectrometer (approximately 10 times the blank), but are still reported for comparative purpose where ²⁰Ne is higher than 6 × 10^− 12 cc STP (small symbols in Figs. 3, 6 and 7). The mass spectrometer and extraction line has been described previously ^63,71.

For the He analyses at SUERC, approximately 0.30 g of olivine were crushed. The released gas was purified using SAES getters and two liquid-N cooled charcoal traps. Helium concentrations and isotope compositions were measured using a ThermoFisher Helix SFT mass spectrometer tuned to the maximum sensitivity. The average blank levels are 2 ± 0.2 × 10⁸ and 5 ± 2 × 10³ atoms of ⁴He and ³He, respectively. Sensitivity and mass discrimination were determined by repeated analysis of the HESJ standard revealing a reproducibility of ± 0.2%. The crushing procedure releases < 0.05% of the total cosmogenic He present in olivine ⁸⁰ so the analyses presented here are largely unaffected by lattice-hosted He components.

Olivine and kimberlite trace element and Sr-Nd-Hf isotope analyses

Residues of crushed olivine after the noble gas analyses was employed for trace element and Sr-Nd isotope analyses by solution methods at ETH Zurich. For complete sample characterisation, the trace element and Sr-Nd-Hf isotope compositions of bulk kimberlite samples were also characterised following the same analytical procedure outlined by Fitzpayne et al. ⁴⁴, which is summarised below. About 200 mg of crushed olivine and 100 mg of powdered fresh chips of kimberlite were digested during 48 hours in Teflon bomb at 120°C using an HF-HNO₃ mix (3:1). Residues such as oxides or fluorides were subsequently re-dissolved using aqua regia to obtain optically clean solutions. A 10% fraction of each dissolved sample was set aside for trace element analysis and dried down before dilution into 2% HNO₃ − 0.005 M HF. A multi-element spike containing Be, In and Bi was added prior to analysis of the solutions, which was undertaken using an Element XR sector field ICP-MS. The isotopes ⁹Be, ¹¹⁵In and ²⁰⁹Bi were used as internal standards, and the reference materials BIR-1 (USGS basalt) and BE-N (CRPG basalt) were used for calibration of olivine and bulk kimberlite analyses, respectively. USGS basalt BCR-2 and BHVO-2 were analysed as unknowns for quality control and yielded results consistent with published reference values (Supplementary Table S3).

Strontium, Nd, and Hf were separated from the remaining solutions using ion-exchange column-chromatography methods adapted from Münker et al. ⁸¹ and Pin et al. ⁸². Procedural blanks for Sr (45 pg), Nd (20 pg), and Hf (30 pg) were all negligible relative to the sample amounts including Sr and Nd in the olivine separates (e.g., Sr loads generally between 5–10 ng). Strontium isotope analyses of olivine and bulk-rock were carried out using a Thermo-Fisher Triton thermal ionisation mass spectrometer (TIMS) and a Neptune MC-ICP-MS, respectively. Instrumental mass bias was corrected by internal normalization to ⁸⁸Sr/⁸⁶Sr = 8.37521 using the exponential law, and for the TIMS analyses ⁸⁷Sr/⁸⁶Sr additionally normalized to a preferred ⁸⁷Sr/⁸⁶Sr ratio for the NBS SRM987 standard (⁸⁷Sr/⁸⁶Sr = 0.710249) based on the average measured ⁸⁷Sr/⁸⁶Sr ratio for SRM987 in the barrel (for TIMS) or session (for MC-ICP-MS) in which the sample was analysed. The USGS basalt BHVO-2, BIR-1 and BCR-2 were analysed as unknowns alongside the olivine samples, and returned ⁸⁷Sr/⁸⁶Sr values within uncertainty of expected values (Supplementary Table S3). Analyses of Nd and Hf isotopes were carried out using a Nu Plasma II multi-collector inductively-coupled-plasma mass spectrometer (MC-ICP-MS). Instrumental mass bias was corrected by normalization to ¹⁴⁶Nd/¹⁴⁴Nd = 0.7219 and ¹⁷⁹Hf/¹⁷⁷Hf = 0.7325 using the exponential law. ¹⁴³Nd/¹⁴⁴Nd and ¹⁷⁶Hf/¹⁷⁷Hf ratios for unknowns and secondary standards are normalized to La Jolla Nd = 0.511858 and JMC475 = 0.282160, respectively. Analyses of secondary reference materials including the J-Nd solution standard and USGS basalts are consistent with accepted values (Supplementary Table S3). Age corrections were calculated using kimberlite emplacement ages available in the literature (Supplementary Table S1) and ⁸⁷Rb/⁸⁶Sr, ¹⁴⁷Sm/¹⁴⁴Nd and ¹⁷⁶Lu/¹⁷⁷Hf ratios derived from trace element data for the same sample solutions (Supplementary Table S3). ε_Nd and ε_Hf values are calculated relative to the chondritic (CHUR) composition of Bouvier et al. ⁸³. The Rb, Sm and Lu decay constants are 1.397 × 10^− 11/yr, 6.54 × 10^− 12/yr and 1.865 × 10^− 11/yr, respectively.

Olivine selection and assessment of sample contamination

Most of the helium data in the literature is derived from measurements of basaltic glasses and crystals in young lava flows; there are few data from older lava flows, and even fewer from kimberlites. Selection of fresh olivine that is not contaminated by serpentine or attached groundmass (Supplementary Fig. S2) appears to have an important control on measured He isotopes. In order to evaluate the influence of groundmass and serpentinization processes, two samples with sufficient material (Wesselton WESK-8; Maniitsoq 527) were divided into relatively “pure” and “contaminated” olivine fractions. These two fractions showed markedly different ³He/⁴He between the uncontaminated and contaminated fractions, with the contaminated olivines yielding two to three times lower ³He/⁴He ratios (Supplementary Fig. S3 and Supplementary Table S2). It is difficult to definitively attribute this difference to contributions of crustal ⁴He in hydrothermal fluids that crystallise serpentine in kimberlites ⁴⁷ or ⁴He* produced in the U-Th-rich groundmass attached to the contaminated olivine grains. Regardless, the olivine separates used for this study generally contain negligible contamination compared to the contaminated olivines of samples WESK-8 and 527 where olivine was expressly selected to monitor the effect of serpentinisation and contamination by groundmass material.

Modelling of ⁴He* ingrowth and implantation

No attempt was made to estimate or correct for the potential effect of cosmogenic ³He since kimberlite emplacement because all the samples come from underground mining activity with the exception of the West Greenland samples where snow coverage provides screening from cosmic rays for most of the year. Ingrowth and implantation of radiogenic ⁴He* probably affected measured ³He/⁴He and was modelled as follow.

Radiogenic ⁴He generated in olivine fluid inclusions after olivine formation (⁴He*) was calculated using the following Eq. 1:

$$\:{}^{4}He*={}^{238}U\times\:\left\{8\:\times\:\:\left[{e}^{\left(0.155125\times\:t\right)}-1\right]+\frac{7}{137.88}\times\:{[e}^{\left(0.98485\times\:t\right)}-1\right]+6\times\:k\times\:[{e}^{\left(0.049475\times\:t\right)}-1]\}$$

(1)

Where ²³⁸U = U × 0.9927, k = ²³²Th/²³⁸U, and U and Th concentrations are from bulk olivine dissolution analyses (Supplementary Table S3); t is the kimberlite emplacement age (Supplementary Table S1); and both ²³⁸U and ⁴He* are in mol/g. ⁴He* produced by the U and Th measured in dissolved olivine exceeds the measured ⁴He content in 20% of the analyses (n = 5/25; Supplementary Fig. S6). This is probably due to incomplete release of lattice-hosted ⁴He by crushing ^51,84. In addition, it is likely that ⁴He* is not fully retained in fluid inclusions due to alpha particle (i.e. ⁴He*) ejection outside fluid inclusions into olivine lattice ⁸⁵ and loss of ⁴He* via diffusion and through cracks. Only a fraction of ⁴He* is retained in the fluid inclusions because alpha particle recoil distance in olivine (15–20 µm) exceed the typical size (< 10 µm) of fluid inclusions (Fig. 1). However, ⁴He* could have been recoiled from nearby fluid inclusions because fluid inclusions in kimberlitic olivine generally occurs in clusters and swarms of tens to hundreds or more inclusions. Hence, for modelling purpose we arbitrarily assume that 10% of ⁴He* produced in olivine fluid inclusions (± lattice and material in crack and and/or adhering to olivine) was released during crushing, which accounts for alpha particle injection into fluid inclusions from neighbouring fluid inclusions and ⁴He* diffusion from lattice to fluid inclusions. In this scenario, age-corrected ³He/⁴He_i would be similar to measured ³He/⁴He, except for some olivines showing clear evidence of groundmass contamination (e.g., Boa, Ghacho Kué; Supplementary Fig. S6). Hence, unless a substantially higher amount of ⁴He* is retained in the fluid inclusions, ingrowth of ⁴He* does not fully explain the more radiogenic He isotopes of Cambrian and Neoproterozoic kimberlites (Fig. 2).

In alternative, the high U and Th contents of kimberlite groundmass (1.2–6.1 ppm and 5.2–46 ppm for bulk samples in this study; Supplementary Table S3) provides ample opportunity for implantation of ⁴He* into olivine (as previously demonstrated for olivines in Jurassic basalts ⁸⁶). The amount (I) of radiogenic ⁴He* implanted into olivine fluid inclusions from the kimberlite groundmass was calculated using the equation of Dunai and Wijbrans ⁸⁷

$$\:I=\:{}^{4}He*\times\:(\frac{3\times\:S}{4\times\:R}+\frac{{S}^{3}}{16\times\:{R}^{3}})$$

(2)

Where S represents the track length (~ 20 µm) of alpha particles in kimberlite and olivine and R is the olivine radius (1000 µm). To calculate ⁴He* produced in the kimberlite groundmass, we used Eq. (1), kimberlite emplacement ages (Supplementary Table S1) and groundmass U-Th contents equal to twice the U and Th contents of the bulk kimberlite (Supplementary Table S3) to account for the diluting effect of ~ 50% of U-Th-free olivine in kimberlites. Similar results were obtained using the formulation of Lal ⁸⁸

$$\:I=\:{}^{4}He*\times\:\frac{3\times\:S}{4\times\:R}\times\:\frac{{\rho\:}_{k}}{{\rho\:}_{ol}}$$

(3)

where ρ_k and ρ_ol are the densities of kimberlite (2.5 g/cm³) and olivine (3.3 g/cm³), respectively. Implanted ⁴He* can be similar or exceed measured ⁴He even assuming an arbitrary 10% of retention of ⁴He* to account for the fraction of implanted ⁴He* that gets trapped in fluid inclusions (Supplementary Figure S6). The effect of ⁴He* implantation is probably most substantial in the Cambrian and Neoproterozoic kimberlites, which all show low ³He/⁴He coupled with elevated ⁴He contents (Fig. 2), and perhaps also in low-³He/⁴He Mesozoic kimberlites from Koidu and Tonguma, which exhibit the highest concentrations of U (4.7–6.1 ppm) and Th (43–46 ppm) due to their mica-rich groundmass. However, these calculations provide upper estimates because olivine rims are not always fully preserved in kimberlites due to serpentinisation (Fig. 1). In addition, the fraction of implanted ⁴He* that is trapped in the fluid inclusions is unknown, which makes this model inconclusive.

In summary, while both radiogenic ingrowth and implantation act to lower the ³He/⁴He of olivine compared to the original values at the time of kimberlite crystallisation, the effect of these process cannot be accurately quantified. Radioactive decay and implantation are of marginal relevance for Ne isotopes because track lengths of ²¹Ne* are much shorter than for ⁴He*, nucleogenic production of Ne is much slower and Ne diffusion slower than for He ¹⁷.

He-Ne isotope mixing models

Binary mixing calculations were undertaken for ⁴He/³He and ²¹Ne/²²Ne to simulate contamination of exsolved kimberlite fluids by continental crust, interaction between kimberlite melt and metasomatised lithospheric mantle, and contribution of recycled oceanic crust to the kimberlite source (Fig. 7). The compositions employed for each of the mixing endmembers are summarised in Supplementary Table S4 and described below.

For the kimberlite source, two different sets of isotopic compositions are employed for modelling and correspond to sources in the upper mantle (similar to MORB ¹: ⁴He/³He = 9.0 × 10⁴; ²¹Ne/²²Ne = 0.060) and low-³He/⁴He lower mantle (Baffin or proto-Icelandic plume ^6,16: ⁴He/³He = 1.1 × 10⁴; ²¹Ne/²²Ne = 0.035), respectively. The contents of He and Ne in the lower mantle are considered to be similar to those of mantle xenoliths enriched in noble gases (1 × 10^− 5 cm³/g and 1 × 10^− 10 cm³/g, respectively) with half of these concentrations but the same He/Ne ratio employed for the more degassed upper mantle. Using a higher Ne/He ratio in the plume source compared to the upper mantle ^19,20 would not substantially alter the results of these models. These same He-Ne isotopic compositions are employed for kimberlite melt and exsolved fluid, but the concentrations of He and Ne are assumed to be about two orders of magnitude higher than in the source to account for the strongly incompatible nature of noble gases. For the kimberlite melt endmember, He and Ne contents are similar to those of the Atlantic “popping rocks” with the highest noble gas concentrations ^52,63: He = 1 × 10^− 4 cm³/g; Ne = 3 × 10^− 9 cm³/g. The rationale for this choice is that noble gases in volatile-poor tholeiites (MORBs) were shown to have similar solubilities as in carbonate-rich melts (kimberlites) ⁵⁷ perhaps because higher CO₂ contents decrease noble gas solubilities in (silicate) melts ⁸⁹. For the exsolved kimberlite fluid, the He content is halved compared to that in the kimberlite melt to account for the lower solubility of He compared to Ne in C-O-H fluids ^20,57,58.

The He content of the continental crust contaminant is assumed to be 1 × 10^− 3 cm³/g based on typical U-Th contents of the upper crust ⁷² and an age of 2 Ga, with the Ne concentration (1 × 10^− 8 cm³/g) obtained using the employed He content and typical He/Ne ratios for crustal fluids ⁹⁰. ⁴He/³He (7.5 × 10⁷) and ²¹Ne/²²Ne (0.52) for the continental crust are taken from the compilation of Ballentine and Burnard ⁶⁰.

The metasomatised lithospheric mantle is considered to have the same Ne contents as the mantle xenoliths with the highest noble gas concentrations (Ne = 1 × 10^− 10 cm³/g, that is ²²Ne ~ 1 × 10^− 11 cm³/g) and higher He contents than mantle xenoliths (i.e. He = 1 × 10^− 5 cm³/g) resulting from radiogenic ingrowth of ⁴He* in phlogopite-bearing lithologies with elevated U and Th contents (e.g., 4.6 × 10^− 5 cm³/g using U-Th contents from McIntyre et al. ⁹¹, and an age of 2 Ga). The He-Ne isotopic composition is varied between present-day estimates (⁴He/³He = 1.2 × 10⁵; ²¹Ne/²²Ne = 0.071) from Gautheron et al. ⁹² and higher ⁴He/³He and ²¹Ne/²²Ne ratios that account for high time-integrated U and Th contents resulting from ancient metasomatic enrichment (e.g., ⁴He/³He = 5.6 × 10⁵; ²¹Ne/²²Ne = 0.237 for 2 Ga; Supplementary Table S4).

Finally, we model the age-dependent He-Ne isotopic composition of subducted oceanic crust using the following parameters: initial He = 1 × 10^− 7 cm³/g, and Ne = 1 × 10^− 11 cm³/g (or ²²Ne ~ 1 × 10^− 12 cm³/g) corresponding to 1% and 10%, respectively, of He and Ne measured in typical MORBs and oceanic gabbros to account for He and Ne loss during subduction dehydration. ⁴He/³He = 1.2 × 10⁵ and ²¹Ne/²²Ne = 0.029 (that is air-like Ne) as measured by Moreira et al. ⁶⁶ for partially altered oceanic gabbros. The age of subducted crust is varied between 1 and 2 Gyr and ingrowth of radiogenic ⁴He* and nucleogenic ²¹Ne* are calculated using U and Th concentrations of subducted-modified oceanic crust (0.029 and 0.252 ppm, respectively ⁹³). Only 10% of ⁴He* is retained in the subducted oceanic crust to account for diffusive loss of ⁴He to the ambient mantle ⁹⁴. However, if the resulting He concentration of the subducted crustal component becomes lower than He in the kimberlite mantle source, the latter is employed for the subducted crust (because diffusion is prevented due to lack of a He concentration gradient).

While we strive to employ endmembers with compositions as close as possible to natural values, we note that, especially for He and Ne concentrations, no independent constraints exist. Therefore, the binary mixing models should be considered approximations of natural scenarios.

Additional discussion of He-Ne isotope data

Some olivine separates provide anomalous Ne isotope compositions or the data appear to be inconclusive and are hence discussed here rather than in the main text. Jericho (Canada) and Karowe (Botswana) olivines represent special cases in this study where linear regressions through the Ne isotope data do not intersect the air value but rather a “fractionated air” component that probably consists of a mixture of air and crustal Ne (Fig. 3). Although the slopes of regressions through these data are similar to or shallower than that of MORBs, it is not possible to discern potential lower mantle contributions (if present) in these cases due to inputs from multiple components and lack of meaningful ²¹Ne/²²Ne_S.

Of the two analyses of olivines from the Igwisi Hills (Tanzania), one shows atmospheric Ne and the second one a composition similar to those of the Lac de Gras olivines (Figs. 3 and 7). However, with only one available point distinct from air-Ne, any interpretation of these data is necessarily weak. Finally, for Letseng only one Ne isotope analysis is available (i.e. ²⁰Ne > 6 × 10^− 12 cc): in this case ²¹Ne/²²Ne_S cannot be considered robust because the second component could be “fractionated air” as per the olivines from Jericho and Karowe. These considerations help explain why the Karowe and Letseng analyses plot at anomalously high ²¹Ne/²²Ne_S considered their relatively low ⁴He/³He in Fig. 7 where no mixing model can be produced.

Direct correlations observed between log(⁴He/⁴⁰Ar*) and log(⁴He/²¹Ne*) in kimberlite olivine (Fig. 6) suggest variable extents of He loss or trapping of exsolved fluids. Helium loss and/or fluid exsolution must have also impacted ³He/²²Ne_S (where ²²Ne_S reflect extrapolation to a ²⁰Ne/²²Ne of 12.5; Supplementary Fig. S7). Kimberlite olivines plot along a sub-parallel trend in log(³He/²²Ne_S) vs log(⁴He/²¹Ne*) which extends to low ⁴He/²¹Ne* at relatively invariant ³He/²²Ne_S, at odds with data from MORBs and OIBs. The origin of these low ³He/²²Ne_S in kimberlites (1.0-4.2) is unclear and might reflect contributions from fluid exsolution as well as crustal contamination, which all contribute to lower ³He/²²Ne from values perhaps as high as those of MORBs or Baffin picrites, i.e. the proto-Icelandic plume (~ 10) ^6,95.

Acknowledgments

Samples from Maniitsoq in Greenland were collected as part of a mapping project of the Ministry of Mineral Resources, Government of Greenland, and we thank all members of the project who contributed to the fieldwork. Max Schmidt, Richard Brown, Hugh O’Brien and Yaakov Weiss are thanked for providing the material analysed from Letseng, Igwisi Hills, Finland Pipe 1 and Ghacho Kué, respectively. De Beers Group kindly provided the examined sample from the Victor mine.

Funding

This project was funded by the Swiss National Foundation (Ambizione fellowship no. PZ00P2_180126/1 to A.G.).

Competing interests

The Authors declare no competing interests.

Data and materials availability

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

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TableS1samplelist.xlsx
Supplementary Table S1
TableS2HeNeAr.xlsx
Supplementary Table S2
TableS3olivinekimberlitetracesSrNdHf.xlsx
Supplementary Table S3
TableS4mixingmodelendmembers.xlsx
Supplementary Table S4
SupplementaryFiguresAug24final.pdf

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Primordial neon and the deep mantle origin of kimberlites

Status:

Version 1

Abstract

Figures

Introduction

Results

He-Ne-Ar isotopes of olivine in kimberlites

Trace element concentrations and Sr-Nd isotopes in olivine

Discussion

Noble gas modification during kimberlite ascent and emplacement

Noble gas constraints on the kimberlite source

Implications

Methods

Noble gas analyses

Olivine and kimberlite trace element and Sr-Nd-Hf isotope analyses

Olivine selection and assessment of sample contamination

Modelling of ⁴He* ingrowth and implantation

He-Ne isotope mixing models

Additional discussion of He-Ne isotope data

Declarations

References

Additional Declarations

Supplementary Files

Status:

Version 1

Primordial neon and the deep mantle origin of kimberlites

Status:

Version 1

Abstract

Figures

Introduction

Results

He-Ne-Ar isotopes of olivine in kimberlites

Trace element concentrations and Sr-Nd isotopes in olivine

Discussion

Noble gas modification during kimberlite ascent and emplacement

Noble gas constraints on the kimberlite source

Implications

Methods

Noble gas analyses

Olivine and kimberlite trace element and Sr-Nd-Hf isotope analyses

Olivine selection and assessment of sample contamination

Modelling of 4He* ingrowth and implantation

He-Ne isotope mixing models

Additional discussion of He-Ne isotope data

Declarations

References

Additional Declarations

Supplementary Files

Status:

Version 1

Modelling of ⁴He* ingrowth and implantation