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With the past decades of diamond inclusion research, it is now well established that the mantle-derived diamonds are originated either from the lithospheric mantle or sublithospheric mantle. The lithospheric diamonds can be further divided into mainly the peridotitic and eclogitic suites, which can be distinguished based on their inclusion chemistry, carbon, and nitrogen isotopic compositions (1, 2). The parental lithology of sublithospheric diamonds is less well established, partly due to their much lower occurrence relative to the lithospheric diamonds. But there has been growing isotopic evidence for the involvement of subducted materials in the source region of sublithospheric diamonds, such as carbon, boron, oxygen, and iron (3–6). Precipitation of diamonds in the Earth’s mantle has been thought to require the presence of a fluid phase. Being C-O-H, saline, carbonatitic, silicic, or metallic in composition, these fluids were released upon dehydration or partial melting of the parental lithology and migrate through the mantle until they reach diamond saturation point due to either the change in pressure-temperature, or redox conditions. Understanding the parental lithology and fluid composition of different diamonds has primarily relied on their carbon and nitrogen isotope compositions and major/trace element compositions of mineral/fluid inclusions. These tools have been shown to be powerful in many cases but each could have their own disadvantages. Nitrogen isotopes, for example, are less applicable to sublithospheric diamonds due to their low N concentration. Trace element compositions, on the other hand, can be easily manipulated by small mass fractions of low degree-melt that are enriched in incompatible elements. Understanding the diamond-forming fluids and their parental lithology require new tools that can provide a different perspective than the ones discussed above. In this presentation, we show recent developments in adapting Fe, Mg, and K isotope systems to diamond inclusion studies for a better understanding of their formation. These so-called “non-traditional” stable isotope systems were typically developed for large rocks that are not limited by sample amount. In order to adapt them to mineral inclusions tens to hundreds of micrometers in size, we’ve developed dedicated procedures to: 1) clean the diamond surface to remove contamination before extracting individual inclusions; 2) scale down the columns used for chemical purification to minimize blanks; and 3) improving sensitivity on the mass spectrometer to analyze small samples. With a Nu Plasma II at the Carnegie Institution for Science, we have shown to be able to analyze inclusion samples containing as little as 200 ng of Fe (6). With an upgraded Nu Plasma Sapphire at UCLA that is equipped with a collision cell, we are now able to analyze samples with >25 ng Fe. The same strategy has now been expanded to Mg and K isotope systems, for which a low sample limit of 25 ng and 300 ng has been achieved. With examples of Fe and Mg isotopic compositions of ferropericlase in sublithospheric diamond and K isotopic composition of fluid inclusions in fibrous diamonds, we show how isotopic compositions of major elements of mineral/fluid inclusions in diamond bring us new perspectives on their origin. Our tests show promising results to extend existing Mg and Fe protocols to silicate minerals and potentially applying similar strategies to silicon, calcium, and barium isotopes in the future. 

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. 

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.

 

Tschauner et al . (Reports, 11 November 2021, p. 891) present evidence that diamond GRR-1507 formed in the lower mantle. Instead, the data support a much shallower origin in cold, subcratonic lithospheric mantle. X-ray diffraction data are well matched to phases common in microinclusion-bearing lithospheric diamonds. The calculated bulk inclusion composition is too imprecise to uniquely confirm CaSiO 3 stoichiometry and is equally consistent with inclusions observed in other lithospheric diamonds. 

Abstract Detrital chromites are commonly reported within Archean metasedimentary rocks, but have thus far garnered little attention for use in provenance studies. Systematic variations of Cr–Fe spinel mineral chemistry with changing tectonic setting have resulted in the extensive use of chromite as a petrogenetic indicator, and so detrital chromites represent good candidates to investigate the petrogenesis of eroded Archean mafic and ultramafic crust. Here, we report the compositions of detrital chromites within fuchsitic (Cr-muscovite rich) metasedimentary rocks from the Jack Hills, situated within the Narryer Terrane, Yilgarn Craton, Western Australia, which are geologically renowned for hosting Hadean (>4000 Ma) zircons. We highlight signatures of metamorphism, including highly elevated ZnO and MnO, coupled with lowered Mg# in comparison with magmatic chromites, development of pitted domains, and replacement of primary inclusions by phases that are part of the metamorphic assemblages within host metasedimentary rocks. Oxygen isotope compositions of detrital chromites record variable exchange with host metasedimentary rocks. The variability of metamorphic signatures between chromites sampled only meters apart further indicates that modification occurred in situ by interaction of detrital chromites with metamorphic fluids and secondary mineral assemblages. Alteration probably occurred during upper greenschist to lower amphibolite facies metamorphism and deformation of host metasedimentary rocks at ∼2650 Ma. Regardless of metamorphic signatures, sampling location or grain shape, chromite cores yield a consistent range in Cr#. Although other key petrogenetic indices, such as Fe2O3 and TiO2 contents, are complicated in Jack Hills chromites by mineral non-stoichiometry and secondary mobility within metasedimentary rocks, we demonstrate that the Cr# of chromite yields significant insights into their provenance. Importantly, moderate Cr# (typically 55–70) precludes a komatiitic origin for the bulk of chromites, reflecting a dearth of komatiites and intrusive equivalents within the erosional catchment of the Jack Hills metasedimentary units. We suggest that the Cr# of Jack Hills chromite fits well with chromites derived from layered intrusions, and that a single layered intrusion may account for the observed chemical compositions of Jack Hills detrital chromites. Where detailed characterization of key metamorphic signatures is undertaken, detrital chromites preserved within Archean metasedimentary rocks may therefore yield valuable information on the petrogenesis and geodynamic setting of poorly preserved mafic and ultramafic crust. 

Subducting tectonic plates carry water and other surficial components into Earth’s interior. Previous studies suggest that serpentinized peridotite is a key part of deep recycling, but this geochemical pathway has not been directly traced. Here, we report Fe-Ni–rich metallic inclusions in sublithospheric diamonds from a depth of 360 to 750 km with isotopically heavy iron (δ 56 Fe = 0.79 to 0.90‰) and unradiogenic osmium ( 187 Os/ 188 Os = 0.111). These iron values lie outside the range of known mantle compositions or expected reaction products at depth. This signature represents subducted iron from magnetite and/or Fe-Ni alloys precipitated during serpentinization of oceanic peridotite, a lithology known to carry unradiogenic osmium inherited from prior convection and melt depletion. These diamond-hosted inclusions trace serpentinite subduction into the mantle transition zone. We propose that iron-rich phases from serpentinite contribute a labile heavy iron component to the heterogeneous convecting mantle eventually sampled by oceanic basalts.