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Osmium isotope and highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, Re) abundance data are reported for picrites and basalts from the ∼132 Ma Etendeka large igneous province (LIP) and the ∼60 Ma North Atlantic Igneous Province (NAIP). Picrite dykes of the Etendeka LIP have HSE abundances and 187Os/188Os (0.1276 to 0.1323; γOsi = -0.5 to +3.1) consistent with derivation from high-degree partial melting (>20 %) of a peridotite source with chondritic to modestly supra-chondritic long-term Re/Os. High-3He/4He NAIP picrites from West Greenland represent large-degree partial melts with similarly elevated HSE abundances and 187Os/188Os (0.1273 to 0.1332; γOsi = -0.2 to +3.9). Broadly chondritic Os isotope ratios have also been reported for the ∼132 Ma Paraná LIP and the ∼201 Ma Central Atlantic Magmatic Province (CAMP). Consequently, LIP associated with Atlantic Ocean opening derive, at least in part, from partial melting of peridotite mantle distinct from the depleted mantle associated with mid-ocean ridge basalt volcanism. Modern locations with high-3He/4He (>25RA) include ocean island basalts (OIB) from Ofu (Samoa), Loihi (Hawaii) and Fernandina (Galapagos) in the Pacific Ocean, and from Iceland, which is considered the modern manifestation of NAIP magmatism. Unlike Etendeka and NAIP picrites, these modern OIB have Sr-Nd-Pb-Os isotopes consistent with contributions of recycled oceanic or continental crust. The lower degree of partial melting responsible for modern high-3He/4He OIB gives higher proportions of fusible recycled crustal components to the magmas, with radiogenic 187Os/188Os and low-3He/4He. The high-3He/4He, incompatible trace element-depleted mantle component in both LIP and OIB therefore also has long-term chondritic Re/Os, which is consistent with an early-formed reservoir that experienced late accretion. Atlantic LIP (CAMP; Paraná-Etendeka; NAIP) provide geochemical evidence for a prominent role for mantle plume contributions during continental break-up and formation of the Atlantic Ocean, a feature hitherto unrecognized in other ocean basin-forming events. 

In the current plate tectonic regime, thermal modeling, petrology, and seismology show that subsurface portions of cold slabs carry some of their volatiles into the deep upper mantle, mantle transition zone, and uppermost lower mantle avoiding the devolatilization occurring with normal arc and wedge subduction. Slab crustal remnants at these depths can melt by intersecting their carbonated solidus whereas slab mantle remnants can devolatilize by warming and metamorphosing to ‘dryer’ mineral assemblages. Since fluid release and earthquake production (“dehydration embrittlement”) operates down to ~300 km depths in all subduction zones, we propose, that deep-focus earthquakes trace the places of fluid release at deeper levels (350 to 750 km). Fluids in faults related to earthquake generation will become diamond-forming as they react with mantle rocks along the fault walls. Diamonds thus formed will record deformation produced by mantle convection and slab buckling during mantle storage. Lithospheric diamonds, stored in static ancient continental keels, lack the connection to this type of geodynamic regime that is evident for sublithospheric diamonds. However, a comparison between the two diamond types suggests a geologic model for lithospheric diamond formation in the ancient past. Lithospheric diamonds and sublithospheric diamonds both contain evidence for the recycling of sediments or surficial rocks that have equilibrated at low temperatures with seawater. The known way to inject these materials into diamond-forming regions is slab subduction. Hence both diamond types may have formed by variants of this same process that differ in depth and style over geologic time. Lithospheric diamonds are different from sublithospheric diamonds in critical ways: higher average N content, ages extending into the Paleoarchean, inclusion assemblages indicating formation at lower pressure, and lack of ubiquitous deformation features. Nitrogen content is the key to relating lithospheric diamonds to the subducting slab. Nitrogen occurs in clays and sediments at the slab surface or uppermost crust. Regardless of whether the slab is hot or cold during subduction, nitrogen will be removed into a mantle wedge if one exists. Additionally, diamonds will not survive in the melts/fluids generated in the wedge under oxidizing conditions. For sublithospheric diamonds, their low to non-existent nitrogen content occurs because they are derived from slab fluids/melts once nitrogen has been largely removed or from rocks deeper in the slab where nitrogen is scarce. The much higher nitrogen in lithospheric diamonds suggests that they formed from fluids/melts derived near the slab surface that contained N. In the Archean, such slabs must have subducted close to the nascent mantle keel with no mantle wedge so the fluids could be directly reduced by the mantle keel. We propose a gradual temporal change from shallow, keel-adjacent, mantle-wedge-poor subduction that produced lithospheric diamonds starting in the Paleoarchean to wedge-avoiding, cold and deep subduction that produced sublithospheric diamonds in the Paleozoic. This temporal change is consistent with many geologic features: an early stagnant lid and a buoyant Archean oceanic lithosphere; the slab-imbrication, advective thickening, and diamond-richness of portions of mantle keels; and anomalously diamond-rich ancient eclogites. 

Over the past decades, dating inclusions in lithospheric diamonds has advanced from analysing tens of pooled inclusions to single sulphide Re-Os analyses and single silicate Sm-Nd analyses, resulting in a fair global coverage of lithospheric diamond ages (Smit et al. 2022) and linking these to tectonomagmatic events. On the other hand, dating of inclusions in sublithospheric diamonds is incredibly limited, mainly due to the rarity of sulphide inclusions and complex retrogressed Ca-silicate phases in already scarce sublithospheric diamonds. Yet, sublithospheric diamonds are important from both a scientific and economic perspective, representing the deepest pristine samples of Earth’s mantle and some of the most valuable diamonds recovered. Understanding their chemical signatures in a broader geological context requires dating such samples. Here we will review the recent progress in dating inclusions in sublithospheric diamonds and discuss their link to the supercontinent cycle and emplacement history. 

Mantle-derived, low-degree melts, such as kimberlites, carbonate-rich olivine lamproites (CROLS), and cratonic olivine lamproites, are the main carriers of diamonds. They are rare ultramafic, volatile-rich volcanic magmas, generally restricted to stable cratons, and are the deepest-sourced magmas erupted onto Earth’s surface. As hybrid magmas, their formation mechanism and mantle sources remain enigmatic and highly debated, especially the nature of the processes leading to their “enriched” isotopic signatures. The often extreme isotopic compositions of Sr, Nd, Pb, and Hf suggest that the mantle sources of these magmas vary between an ancient and geochemically depleted component and various enriched components. The enriched components could include crustal material recycled into the convective mantle or metasomatized lithospheric mantle. For the latter, discriminating between assimilation by sub-lithospheric magmas during the ascent or melting of element-enriched material from within the lithospheric mantle is paramount concerning petrogenesis. As the stable isotope composition of K, and Ba vary between surface and mantle reservoirs, they are well-suited tools for addressing the cause of different radiogenic isotopic signatures and to better constrain the mantle sources of these important magmas. Here, we use collision cell multi-collector inductively-coupled-plasma mass-spectrometry (MC-ICP-MS) and traditional MC-ICP-MS to conduct the first comprehensive whole-rock K and Ba stable isotope study on a wide range of low-degree mantle-derived melts. All the deep-seated, low-degree melts analyzed here show no correlation between melting/differentiation indices and δ41K and δ138Ba compositions, implying that any isotopic fractionation during melting or eruption was limited and that the different mantle and crustal reservoirs affecting these melts dominate their isotopic variability. Overall, kimberlites show limited δ41K and δ138Ba variability, with a median δ41K of -0.40 ± 0.06‰ (2SE) and δ138Ba of 0.00 ± 0.07‰ (2SE), within error relative to an estimated bulk silicate Earth [(BSE: δ41K= -0.42±0.07‰ (2SD) and δ138Ba=0.03±0.04‰ (2SD)], suggesting significant sublithospheric input. While the sample size is small (N=4), Canadian kimberlites from Lake De Gras display a bi-modal distribution with δ41K values slightly higher and lower relative to BSE, ascribed to crustal and lithospheric contamination. Like kimberlites, South African CROLS show limited K isotope variability with a median δ41K of -0.48 ± 0.02‰ (2SE). Their compositions are non-resolvable from two Mica-Amphibole-Rutile-Ilmenite-Diopside (MARID) xenoliths. The δ138Ba of the CROLS also shows limited variation with a median δ138Ba of 0.00 ± 0.07‰ (2SE), plotting within BSE estimations. Compared to the other low-degree mantle-derived melts, cratonic olivine/leucite-bearing lamproites from West Australia show a wide range in δ41K (-0.97‰ to +0.34‰) and δ138Ba (-0.30‰ to +0.27) values. The observed large K isotopic variation in cratonic lamproites is similar to that observed in post-collisional lamproites and is ascribed to sediment recycling. Argyle lamproites define robust correlations between potassium and barium elemental abundances, and their stable isotopes call for significant hydrothermal fluid-assisted leaching and isotopic fractionation. 

The Wombat and Giraffe kimberlite pipes in the Lac de Gras kimberlite field (64°N, 110°W) of the Northwest Territories, Canada, preserve unique post-eruptive lacustrine and paludal sedimentary records that offer rare insight into high-latitude continental paleoclimate. However, depositional timing—a key datum for atmospheric CO2 and paleoclimatic proxy reconstructions—of these maar infills remains ambiguous and requires refinement because of the large range in the age of kimberlites within the Lac de Gras kimberlite field. Existing constraints for the Giraffe pipe post-eruptive lacustrine and paludal maar sedimentary facies include a maximum Rb-Sr age of ca. 48 Ma (Ypresian, Eocene) based on kimberlitic phlogopite and a glass fission-track age of ca. 38 Ma (Bartonian, Eocene). The age of the Wombat pipe lacustrine maar sediments remains unclear, with unpublished pollen-based biostratigraphy suggesting deposition in the Paleocene (66–56 Ma). In this study, we examine distal rhyolitic tephra beds recovered from exploration drill cores intersecting the Wombat and Giraffe maar facies. We integrate zircon U-Pb laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) and chemical abrasion–isotope dilution–thermal ionization mass spectrometry (CA-ID-TIMS) geochronology, glass fission-track dating, palynology, and tephra glass geochemistry to refine chronological frameworks for these sedimentary deposits. The Giraffe maar CA-ID-TIMS tephra zircon U-Pb dating yielded a Bayesian model age of 47.995 ± 0.082|0.087 Ma (Ypresian) for the upper portion of the lacustrine sediments, while a single zircon grain from tephra in the lowermost lacustrine sediments had an age of 48.72 ± 0.29|0.30 Ma. The revised geochronology for the Giraffe maar provides a working age model for the ~50 m record of lacustrine silt and indicates an age ~10 m.y. older than previously thought. The Wombat maar LA-ICP-MS zircon U-Pb dating yielded an age of 80.9 ± 1.0 Ma (Campanian), which indicates deposition during the Late Cretaceous. This first radiometric age for the Wombat maar deposits is substantially older than earlier biostratigraphic inferences of a Paleocene age. This new age suggests that the Wombat maar sediments preserve evidence of some of the oldest known freshwater diatoms and synurophytes and provide key constraints for the paleogeography of the Western Interior Seaway during the Late Cretaceous.

 

Characterizing compositional heterogeneity in Earth’s lower mantle is critical to understanding its dynamics. Three low-nitrogen diamonds from Koffiefontein (South Africa), containing inclusion assemblages of ferropericlase ± orthopyroxene ± magnesite, constrain diamond formation in an Mg-rich lower-mantle environment. Ferropericlase inclusions have Mg# 82.7–88.5 and orthopyroxene inclusions (retrogressed bridgmanite) have Mg# 95.0–95.1 and mantle-like δ18O of +5.6‰ ± 0.2‰. Magnesite included in one diamond implicates carbonated fluids in diamond formation. High Mg# and low Ca, Al, and Na of the assemblage indicate a melt-depleted meta-harzburgitic environment, in contrast to more fertile compositions expected for primitive lower mantle. Extremely low Ca in orthopyroxene inclusions may reflect a combination of melt depletion and low equilibration temperatures at the time of trapping. Inclusion compositions implicate subducted oceanic slab meta-harzburgite as the host for diamond growth. Mantle-like δ18O of the orthopyroxene inclusions indicates unaltered oceanic lithosphere. Similar melt-depleted characteristics in lower-mantle inclusion assemblages worldwide support that meta-harzburgite is the dominant host of lower-mantle diamonds.

 

Subduction related to the ancient supercontinent cycle is poorly constrained by mantle samples. Sublithospheric diamond crystallization records the release of melts from subducting oceanic lithosphere at 300–700 km depths^1,2and is especially suited to tracking the timing and effects of deep mantle processes on supercontinents. Here we show that four isotope systems (Rb–Sr, Sm–Nd, U–Pb and Re–Os) applied to Fe-sulfide and CaSiO₃inclusions within 13 sublithospheric diamonds from Juína (Brazil) and Kankan (Guinea) give broadly overlapping crystallization ages from around 450 to 650 million years ago. The intracratonic location of the diamond deposits on Gondwana and the ages, initial isotopic ratios, and trace element content of the inclusions indicate formation from a peri-Gondwanan subduction system. Preservation of these Neoproterozoic–Palaeozoic sublithospheric diamonds beneath Gondwana until its Cretaceous breakup, coupled with majorite geobarometry^3,4, suggests that they accreted to and were retained in the lithospheric keel for more than 300 Myr during supercontinent migration. We propose that this process of lithosphere growth—with diamonds attached to the supercontinent keel by the diapiric uprise of depleted buoyant material and pieces of slab crust—could have enhanced supercontinent stability.